protein-based conditioners for enhancing biosludge ... · proteins as conditioners for enhancing...

Protein-Based Conditioners for Enhancing Biosludge Dewaterability

By

Sofia Bonilla

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Department of Chemical Engineering and Applied Chemistry University of Toronto

© Copyright by Sofia Bonilla 2017

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Protein-Based Conditioners for Enhancing Biosludge

Dewaterability

Sofia Bonilla

Doctor of Philosophy

Department of Chemical Engineering and Applied Chemistry

University of Toronto

2017

Abstract

Synthetic organic polymers are commonly used conditioners to enhance sludge dewaterability.

However, these polymers are petroleum-derived, costly and can be toxic to aquatic systems. There have

been limited studies on the potential use of enzymes for enhancing the dewaterability of sludge and little

is known about the mechanisms for such enhancement. This thesis investigated the potential of using

proteins as conditioners for enhancing biosludge dewaterability.

Cationic proteins can enhance biosludge dewaterability. This was demonstrated by the conditioning

effect on biosludge of lysozyme and protamine. After screening several enzymes, lysozyme was the only

enzyme that showed dewatering improvements, increasing the cake solids content of biosludge by up to

5.8%. Active and inactive lysozyme exhibited a similar ability for enhancing sludge dewaterability

suggesting a non-enzymatic mechanism. The mechanism by which cationic proteins, such as lysozyme,

enhance biosludge dewaterability appears to be charge neutralization. In agreement with this proposed

mechanism, it was found that the surface charge of a protein largely determines its potential as a

conditioner. Synthetic polymers consistently outperformed cationic proteins increasing biosludge cake

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solids by up to 7.4%. However, protamine showed a higher flocculating activity than synthetic

polymers on kaolin suspensions (up to 36% higher at pH 7 and pH 9). Cationic proteins are biodegradable

and can potentially be extracted from waste which supposes them an advantage over synthetic polymers.

Although the enzymes in this study were not found to positively affect the dewaterability of

biosludge, enzymes can improve the anaerobic digestibility of biosludge as a result of their enzymatic

activity. Treatment with proteases and glycosidases increased biogas yields by 10% after 62 days of

anaerobic digestion.

Taken together, the findings of this thesis improved the current understanding of how enzymes and

proteins can change biosludge, and how these changes affect its dewaterability and anaerobic

digestibility.

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Acknowledgments

I would like to express my sincere gratitude to Prof. Grant Allen for being a great supervisor and

mentor. His continuous support, guidance, and encouragement were instrumental in this project and my

professional development.

I am thankful to Prof. Edgar Acosta and Prof. Ramin Farnood, for their helpful advice which I am

certain made my project better; to Sasha (Prof. Alexander Yakunin), for his valuable input on all things

enzyme-related in my project; to Prof. Elizabeth Edwards for providing feedback on all things related to

anaerobic digestion and to Prof. Honghi Tran for always providing me with an industrial perspective.

Special thanks to all the Administrative Staff in the Chemical Engineering and Applied Chemistry

Department and BioZone, including Leticia, Gorette and Line. Every time I needed help, information or

anything, you were all willing and happy to help. My special appreciation to Mary Butera for great

conversations and delicious snacks prior to or after meetings with Prof. Allen.

I would also like to thank Susie (Endang Susilawati) for her constant willingness to help and

contagious positive attitude. I really enjoyed all our equipment-hunting adventures. Special thanks to Paul

Jowlabar for helping me out with the constant troubleshooting of the 80L reactor and for being so

generous with his time, tools and knowledge.

There are too many names to list them all but I would like to convey great appreciation to all my

colleagues in the Allen Lab and BioZone for providing feedback to my project, great conversations,

training and generosity with their time and technical knowledge; to Summer Students and M.Eng.

Students who worked hard conducting research in many projects related to my thesis.

I am thankful for the financial support of the Natural Science and Engineering Research Council of

Canada (NSERC), the Ontario Government and the Energy Recovery Consortium.

Por último, nada de esto hubiera sido posible sin mi familia. Magdis, no hay palabras para

agradecerte por tu amor incondicional y por ser el mejor ejemplo de integridad y trabajo. Sergio, el amor

de mi vida, mil gracias por siempre creer en mí y hacer de nuestra familia un sueño. Tu y yo somos el

mejor equipo.

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Table of Contents

Acknowledgments ....................................................................................................................................... iv

Table of Contents ......................................................................................................................................... v

List of Tables ................................................................................................................................................ x

List of Figures ............................................................................................................................................. xi

List of Abbreviations .............................................................................................................................. xvii

1 Chapter 1 - Introduction ........................................................................................................................ 1

Research Statement .......................................................................................................................... 1

Research Motivation ......................................................................................................................... 1

Hypotheses ....................................................................................................................................... 4

Objectives ......................................................................................................................................... 4

General Approach ............................................................................................................................. 4

Thesis Outline................................................................................................................................... 7

Contributions .................................................................................................................................... 8

Publications ......................................................................................................................... 8

Invention Disclosures .......................................................................................................... 8

Non-Refereed Contributions ............................................................................................... 8

References ...................................................................................................................................... 10

2 Chapter 2 – Literature Review ............................................................................................................ 13

Biosludge in Pulp and Paper Mills ................................................................................................. 13

Biosludge - Properties and their Effect on Dewaterability ............................................................. 15

Bound Water ..................................................................................................................... 16

Extracellular Polymeric Substances (EPS) ........................................................................ 17

Surface Charge .................................................................................................................. 18

Particle Size ....................................................................................................................... 18

Cations ............................................................................................................................... 19

Compressibility ................................................................................................................. 20

Dewaterability Assessment ............................................................................................................ 21

Capillary Suction Time (CST) .......................................................................................... 22

Crown Press®.................................................................................................................... 23

Polymer Demand ............................................................................................................... 24

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Conditioning ......................................................................................................................... 24

Chemical Conditioners ...................................................................................................... 25

Natural Flocculants............................................................................................................ 27

Enzymes for Enhancing Biosludge Dewaterability ........................................................... 29

Enzymes for Enhancing Biosludge Anaerobic Digestion ................................................. 31

Summary and Significance of this Research .................................................................................. 32

References ...................................................................................................................................... 34

3 Chapter 3 - Enhancing Pulp and Paper Mill Biosludge Dewaterability using Enzymes ............... 44

Introduction .................................................................................................................................... 44

Materials and Methods ................................................................................................................... 47

Sludge Samples ................................................................................................................. 47

Enzymes ............................................................................................................................ 48

Capillary Suction Time ..................................................................................................... 49

Effect of Concentration and Enzymatic Incubation Conditions ........................................ 50

Lysozyme Inactivation ...................................................................................................... 50

Particle Size Distribution................................................................................................... 51

Polymer Demand ............................................................................................................... 51

Mechanical Dewatering ..................................................................................................... 52

Results and Discussion ................................................................................................................... 53

Enzyme Screening for Improved Biosludge Dewaterability ............................................. 53

Effect of Incubation Time of Lysozyme’s Conditioning Treatment ................................. 55

Effect of Incubation Temperature and Mixing on Biosludge Conditioning with

Lysozyme .......................................................................................................................... 55

Effect of the Enzymatic Activity of Lysozyme on Biosludge Conditioning with

Lysozyme .......................................................................................................................... 56

Effect of Lysozyme on the Particle Size Distribution of Biosludge .................................. 59

Polymer Demand after Lysozyme Treatment .................................................................... 60

Mechanical Dewatering after Lysozyme Conditioning ..................................................... 61

Lysozyme Mechanism ....................................................................................................... 62

Effect of Lysozyme on the Dewaterability of Sludge Mixtures ........................................ 62

Conclusions .................................................................................................................................... 65

References ...................................................................................................................................... 65

4 Chapter 4 - Novel Enzymes for Enhancing Biosludge Dewaterability ............................................ 69

Introduction .................................................................................................................................... 69


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Enzyme Production ................................................................................................. 70

Enzyme Purification .......................................................................................................... 72

Chemical Composition of Biosludge during Enzymatic Treatment .................................. 73

Dewaterability Assessment – Capillary Suction Time ...................................................... 74

Results and Discussion ................................................................................................................... 74

Effect of Incubation Time on the Dewaterability of Biosludge treated with Novel

Enzymes ............................................................................................................................ 74

Effect of Enzyme Dose on Soluble COD, Protein and Carbohydrate Content ................. 79

Conclusions .................................................................................................................................... 81

References ...................................................................................................................................... 82

5 Chapter 5 - Addressing the Challenges Associated with Evaluating the Effect of Enzymatic

Pretreatment on the Anaerobic Digestibility of Biosludge ............................................................... 84

Introduction .................................................................................................................................... 84


Biosludge Samples ............................................................................................................ 88

Anaerobic Inoculum (Granules) ........................................................................................ 88

Enzyme Preparations ......................................................................................................... 89

Commercial Enzymes Preparation .................................................................................... 89

Cloning, Overexpression and Purification of Novel Enzymes .......................................... 89

Enzymatic Assays ............................................................................................................. 90

Biosludge Pretreatment ..................................................................................................... 92

Chemical Analyses ............................................................................................................ 92

Biochemical Methane Potential (BMP) Assays ................................................................ 94

Results ............................................................................................................................................ 96

Set up Conditions of BMP Assays .................................................................................... 96

Effect of Enzymatic Pretreatment of Biosludge on Biogas Production ............................ 97

Effect of Enzymatic Pretreatment of Biosludge on Biogas Composition ....................... 100

Effect of Inoculum, Substrate and ISR on Biogas Composition and Biogas Production 101

Effect of Enzymatic Treatment of Biosludge on Soluble COD ...................................... 103

Biogas Production from Enzyme Solutions Alone .......................................................... 105

Potential of Enzymatic Activity Assays to Predict Effect of Enzymes on Biosludge

Digestibility, Inhibition and Inactivation ........................................................................ 107

Conclusions .................................................................................................................................. 109

References .................................................................................................................................... 110

6 Chapter 6 - Flocculating Activity of Lysozyme: A Non-Enzymatic Application .......................... 113

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Introduction ........................................................................................................................ 113

Material and Methods ................................................................................................................... 115

Lysozyme ........................................................................................................................ 115

Kaolin .............................................................................................................................. 115

Polymer – Polyacrylamide (PAM) .................................................................................. 116

Cation Supplements ......................................................................................................... 116

Additional Substrates for Flocculation ............................................................................ 117

Flocculating Activity ....................................................................................................... 117

Zeta Potential ................................................................................................................... 117

Gel Electrophoresis ......................................................................................................... 118

Results and Discussion ................................................................................................................. 118

Effect of Lysozyme Concentration and pH ..................................................................... 118

Effect of Cation Concentration ........................................................................................ 120

Zeta Potential and Lysozyme’s Flocculating Activity .................................................... 122

Flocculation of Algae and Activated Carbon .................................................................. 122

Lysozyme Active vs. Inactive ......................................................................................... 124

Lysozyme Flocculating Mechanisms .............................................................................. 126

Conclusions .................................................................................................................................. 127

References .................................................................................................................................... 127

7 Chapter 7 - A Look into the Potential of Cationic Proteins and Cationic Fractions to

Enhance Solid-Liquid Separations.................................................................................................... 130

Introduction .................................................................................................................................. 130

Materials and Methods ................................................................................................................. 131

Sludge Samples ............................................................................................................................ 132

Cationic Proteins ............................................................................................................. 132

Chemical Composition of Biosludge .............................................................................. 133

Dewaterability Assessment - Capillary Suction Time (CST) .......................................... 133

Flocculating Activity of Kaolin Suspensions .................................................................. 134

Cationic Fractions from Biosludge .................................................................................. 134


Effect of Protamine on Biosludge Dewaterability ........................................................... 135

Effect of Protamine on Soluble COD, Protein and Carbohydrate ................................... 136

Flocculating Activity of Protamine on Kaolin Suspensions ............................................ 138

Cationic Extractions from Biosludge - Effect of Incubation Conditions on the Extract

Yield ................................................................................................................................ 140

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Conclusions ........................................................................................................................ 142

References .................................................................................................................................... 142

8 Chapter 8 - Cationic Proteins for Enhancing Biosludge Dewaterability: A comparative

Assessment of Surface and Conditioning Characteristics of Synthetic Polymers, Surfactants

and Proteins ........................................................................................................................................ 144

Introduction .................................................................................................................................. 144

Materials and Methods ................................................................................................................. 146

Biosludge ......................................................................................................................... 146

Conditioners .................................................................................................................... 146

Surface Properties Analyses ............................................................................................ 148

Dewaterability Assessment ............................................................................................. 150


Effect of conditioners on CST, cake and filtrate solids content ...................................... 151

Effect of conditioners on filtration rate during gravity thickening .................................. 157

Effect of Surface Charge, Surfactant Activity and Wettability on Conditioning of

Biosludge ......................................................................................................................... 159

Conclusions .................................................................................................................................. 162

References .................................................................................................................................... 162

9 Chapter 9 – Overall Discussion ......................................................................................................... 165

Enzymes and Their Effect on Biosludge Dewaterability ............................................................. 165

Enzymes and Their Effect of Anaerobic Digestion of Biosludge ................................................ 166

Proteins and Surfactants as Conditioners for Improved Dewaterability ...................................... 166

Cationic Proteins as Potential Flocculants ................................................................................... 168

Flocculation Mechanisms of Cationic Proteins and Polymers ..................................................... 168

Significance of Findings ............................................................................................................... 171

Scientific Significance ..................................................................................................... 171

Industrial Significance ..................................................................................................... 172

References .................................................................................................................................... 174

10 Chapter 10 - Conclusions and Recommendations for Future Work ............................................. 175

Recommendations for Future Work ............................................................................................. 176

Appendices………………………………………………………………………………………………179

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List of Tables

Table 3-1 Enzymes used in the screening tests of biosludge conditioning for improved dewatering .........49

Table 3-2 Mean diameter of sludge fractions after treatment.....................................................................60

Table 4-1 Novel enzymes used in the screening of biosludge conditioning for improved dewatering .......71

Table 4-2 General properties of enzymes included in this study ................................................................79

Table 5-1 General information of enzymes used in this study ....................................................................89

Table 5-2 Characteristics of raw biosludge, inoculum, and inoculum-to-substrate ratios based on COD

used in the three biochemical methane potential (BMP) assays performed in this study. ..........................97

Table 5-3 Effect inoculum-to-substrate ratio (ISR) on total biogas production (TBP), specific biogas

yields (SBY) and methane concentration ..................................................................................................102

Table 8-1 Conditioners used in this study to compare their surface properties and effect on biosludge

dewaterability. ...........................................................................................................................................148

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List of Figures

Figure 1-1 An outline of the experimental approach taken to study the effect of proteins and enzymes on

biosludge dewatering properties ....................................................................................................................6

Figure 2-1 Simplified schematic of a typical wastewater treatment. ...........................................................14

Figure 2-2 Simplified schematic representation of flocs from biosludge. ...................................................15

Figure 2-3 Schematic representation of free and bound (vicinal, interstitial and hydration) water in flocs.

Based on the definitions of bound water presented in Vaxelaire & Cézac, 2004........................................16

Figure 2-4 Characteristics of an improved sludge dewatering process and their relation to assessment

methods used in this project: capillary suction time (CST), Crown Press and polymer demand. ...............21

Figure 2-5 Capillary Suction Time (CST), a) apparatus; b) schematic of cross-sectional of CST apparatus

sample column and plates. ...........................................................................................................................22

Figure 2-6 Crown Press – A bench-scale simulator of full-scale presses used to assess the dewaterability

of biosludge in this project. Gravity thickening and active mechanical pressing are separated in two steps.

.....................................................................................................................................................................23

Figure 3-1 Biosludge dewaterability assessment using capillary suction time (CST) after different

enzymatic treatments over a range of enzyme doses (0.05-1.5%). Lower CST means better

dewaterability. CST values correspond to incubation times of 90 min. Note the break in the X axis due to

log scale. Error bars (not always visible within the symbol) show standard deviation of triplicates. .........53

Figure 3-2 Effect of incubation time on biosludge dewaterability using different doses (%) of lysozyme.

Note the break in the X axis. Error bars show standard deviation of triplicates. ........................................55

Figure 3-3 Effect of lysozyme treatment conditions on biosludge dewaterability as capillary suction time

(CST); a) effect of temperature; b) effect of mixing rate. Lysozyme was added at a dose of 0.5% and CST

was measured after 2 hours of treatment. ....................................................................................................56

Figure 3-4 Capillary suction time of biosludge conditioned with active and inactive lysozyme as a

function of time, a) Pulp and paper mill biosludge and b) Municipal biosludge. .......................................57

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Figure 3-5 Effect of enzymatic active units on the treatment of lysozyme for improved sludge

dewaterability measured via capillary suction time (CST). Error bars show standard deviation of

triplicates. ....................................................................................................................................................58

Figure 3-6 Capillary suction time of biosludge with different doses of active and inactive lysozyme after

90 min of incubation. Error bars show standard deviation of triplicates. Two x-axis to show what units in

w/v % translate to kg of enzyme / DT sludge..............................................................................................58

Figure 3-7 Particle size distributions of sludge fractions before and after treatment with active and

inactive lysozyme; a) Fraction 1 (25-32 µm); b) Fraction 2 (32-75 µm); c) Fraction 3 (75-105 µm) and d)

Fraction 4 (> 105 µm). ................................................................................................................................59

Figure 3-8 Supernatant of biosludge after centrifugation. Left to right correspond to conditioning

treatments with: no enzyme, active lysozyme and inactive lysozyme. .......................................................60

Figure 3-9 Polymer demand after treatment with no enzyme, active and inactive lysozyme. Lowest

polymer dose to obtain lower CST values indicate the optimum. Polymer doses (%) are from a 1% stock

solution. Error bars show standard deviation of triplicates. ........................................................................61

Figure 3-10 Cake solids after mechanical dewatering using the crown press (Left Y axis). Capillary

suction time before mechanical dewatering (Right Y axis). Error bars show standard deviation of

triplicates. ....................................................................................................................................................62

Figure 3-11 Optimal lysozyme doses (kg/DT) for biosludge and sludge mixtures with primary sludge

determined by capillary suction time. Error bars show standard deviation over at least 3 experiments. ....63

Figure 3-12 Dry solids content after mechanical pressing of sludge mixture (50% primary, 50%

biosludge) after different conditioning treatments. Error bars show standard deviation of triplicates. .......64

Figure 3-13 Capillary suction time of mixed sludge (50% primary and 50% Biosludge) with different

doses of polymer. A sample with no enzyme and a sample with 12 kg/DT of lysozyme. ..........................64

Figure 4-1 Effect of incubation time on the dewaterability of biosludge treated with different

concentrations (0, 0.05, 0.1 and 0.5 %) of lysozyme. Error bars represent the standard deviation of

triplicates. ....................................................................................................................................................75

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concentrations of enzymes; a) BSU3124, b) PP1034, c) BSU3441, d) OLEI4758, e) NE1796 and f)

ATC1791. Error bars represent the standard deviation of triplicates. .........................................................76

Figure 4-3 Effect of enzyme dose of OLEI4758 and lysozyme on the dewaterability of anaerobically

digested biosludge. Error bars represent the standard deviation of triplicates ............................................77

Figure 4-4 Effect of enzyme dose on the dewaterability of biosludge after 3.5 h of enzymatic

conditioning. Lines show trend of positive, neutral and negative effect. Error bars represent the standard

deviation of triplicates. ................................................................................................................................78

Figure 4-5 Effect of enzyme dose on the soluble COD, protein and carbohydrate content of sludge after

3.5 h of incubation. Error bars represent standard deviation of triplicates. .................................................80

Figure 5-1 General approach for investigating the effect of enzymatic pretreatment on biosludge

anaerobic digestibility .................................................................................................................................87

Figure 5-2 Total biogas production, TBP, of biosludge pretreated with enzymes over 62 days of anaerobic

digestion. a) protease from A. oryzae; b) lysozyme; c) protease from B. licheniformis; d) glycosidase

SCO6604; e) BCE_2078 and f) CTec 2. Untreated (control) had phosphate buffer instead of enzyme

solution. Range differences between BMP 1 (a, c, e) and BMP 2 (b, d, f) are due to differences in

biosludge and granules, inoculum to substrate ratios and soluble chemical oxygen demand (sCOD). ......98

Figure 5-3 Specific biogas yield normalized against the untreated sample (control). Assuming untreated

sample of each BMP as 100%, the yield of each of the enzyme-treated samples was calculated after 62

days of anaerobic digestion. Circles represent the concentration of methane in the biogas produced at day

62 of the BMP assay. .................................................................................................................................101

Figure 5-4 Soluble chemical oxygen demand (COD), protein and carbohydrate content -during enzymatic

pretreatment of gamma irradiated biosludge for 24 hours. Proteases are shown on the left and

glycosidases on the right; a) and b) soluble COD (sCOD), c and d) soluble carbohydrates

(sCarbohydrates) and e and f) soluble protein (sProtein) content. Error bars (not always visible) represent

the standard deviation of triplicates. ..........................................................................................................104

Figure 5-5 Biogas production from enzyme solutions. Total biogas production (TBP) are presented for

BMP 3, samples that contained enzyme solutions and inoculum. a) protease from A. oryzae; b) lysozyme;

c) protease from B. licheniformis; d) glycosidase SCO6604 Inoculum only is the control, i.e. no enzyme

added. Error bars show standard deviation of triplicates. ..........................................................................106

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Figure 5-6 Enzymatic assays. a) protease activity assays for enzymes studied in BMP 1. Casein was

used as the standard substrate. b) glycosidase activity assays for enzymes studied in BMP 2 (except

lysozyme). Carboxymethyl cellulose (CMC) was used as the standard substrates, biosludge and a

combination of them. Active and inactive enzymes were included. Note the two vertical axis in part b are

in the same units but ranges are different. Error bars show standard deviation of triplicates. ..................108

Figure 6-1 Effect of the concentration of lysozyme on the flocculation of kaolin solutions under different

pH conditions; a) pH 3, b) pH 5.1 (Non-adjusted), c) pH 7 and d) pH 9. Error bars represent the standard

deviation of triplicates. ..............................................................................................................................119

Figure 6-2 Effect of cation addition on the flocculating activity of lysozyme after 180min of treatment

with a lysozyme dose of 10 mg/L. a) CaCl2, b) MgSO4 and c) Fe2 (SO4)3. Error bars represent the standard

deviation of triplicates. ..............................................................................................................................121

Figure 6-3 Zeta potential of kaolin suspensions at pH 5 and pH 7 with doses of PAM and lysozyme that

resulted in significant flocculating activity. Error bars show standard deviation of duplicates. ...............123

Figure 6-4 Flocculating activity of lysozyme active and inactive on left: powdered activated Carbon and

right: microalgae. Error bars represent the standard deviation of triplicates. ............................................124

Figure 6-5 Gel electrophoresis of active and inactive lysozyme. Samples were treated with and without a

reducing agent (2-mercaptoethanol) to visualize intermolecular disulfide bonds .....................................125

Figure 6-6 Proposed Mechanism of Lysozyme Flocculation. Not to scale. ..............................................126

Figure 7-1 Experimental approach to investigate the potential of cationic proteins as flocculants ..........131

Figure 7-2 Effect of protein dose on biosludge dewaterability after 2 h of treatment. Error bars represent

standard deviation of duplicates. ...............................................................................................................135

Figure 7-3 Effect of low doses of protamine on the CST of biosludge. Error bars represent standard

deviation of duplicates. ..............................................................................................................................136

Figure 7-4 Effect of protein dose on the a) chemical oxygen demand (COD); b) soluble protein and c)

soluble carbohydrate content of biosludge after 2 h of treatment with protamine and lysozyme. Error bars

represent standard deviation of duplicates. ...............................................................................................137

Figure 7-5 Flocculating activity of protamine, lysozyme and a synthetic polymer (PAM) on kaolin

suspensions at their optimum doses and three different pH values: a) pH 5, b) pH 7 and c) pH 9. Error

xv

bars show standard deviation of triplicates. Note that for a) PAM dose is 1 mg/ml and for b) and c),

PAM dose is 10 mg/ml. .............................................................................................................................139

Figure 7-6 Soluble protein in biosludge before and after sonication under different overnight incubation

conditions. Error bars represent standard deviation of triplicates. ............................................................140

Figure 7-7 a) Capillary suction time of biosludge treated with lysozyme and a cationic fraction extracted

from biosludge; b): Capillary suction time of anaerobically digested sludge treated with lysozyme and a

cationic fraction extracted from biosludge. Conditioner dose for both experiments was 0.1%. Error bars

show standard deviation of triplicates. ......................................................................................................141

Figure 7-8 Effect of cationic fractions on the settling of biosludge after 2h of treatment at 37 ˚C and 150

rpm. ...........................................................................................................................................................141

Figure 8-1 Effect of different doses of proteins on biosludge dewaterability. a) active lysozyme; b)

inactive lysozyme; c) protamine; d) bovine Serum albumin (BSA). Dewaterability was assessed by

capillary suction time (CST) (left axis) and solids content (%) in the cake after pressing and in the filtrate

solids after gravity thickening (i.e. crown press) (right axes). Note different range in X-axis (i.e. lower

doses) for c and d. Error bars represent standard deviation of replicates. .................................................152

Figure 8-2 Effect of different doses of surfactants on biosludge dewaterability. a) CTAB; b) Triton X-100;

c) SDS. Dewaterability was assessed by capillary suction time (CST) (left axis) and solids content (%) in

the cake after pressing and in the filtrate solids after gravity thickening (i.e. crown press) (right axes).

Note different range in X-axis, 10 fold higher for CTAB vs Triton X-100 or SDS. .................................153

Figure 8-3 Effect of different doses of polymers on biosludge dewaterability. a) Zetag 8165; b) AF9645;

c) Organopol; d) Zetag 8185. Dewaterability was assessed by solids content (%) after mechanical

dewatering (i.e. crown press) (left axis) and capillary suction time (right axis). Increased solids content

and reduced capillary suction time are indicative of improved dewatering properties. Error bars represent

standard deviation of triplicates. ................................................................................................................154

Figure 8-4 Effect of conditioners on CST at their optimum dose. Bar graph represents the capillary

suction time (left axis), the corresponding dose (i.e. optimum) is presented as orange diamonds (right

axis). Dashed line represents the CST of deionized water. Water was added to biosludge as a control and

is represented by the grey bar. Error bars represent standard deviation of triplicates. ..............................155

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Figure 8-5 Correlation of capillary suction and dry solids content data for the three groups of

conditioners at their optimum dose. Error bars represent standard deviation of triplicates. .....................156

Figure 8-6 Filtration curves of biosludge conditioned during gravity thickening in the Crown Press. The

control was biosludge with the same volume water added instead of conditioner; a) polymers; b) proteins

and c) surfactants. The dose of each conditioner in g/g TSS of biosludge is in parentheses. These doses

were selected because they led to the highest dry solids content after testing various doses of each

conditioner. Error bars show standard deviation of duplicates..................................................................158

Figure 8-7 Effect of surface charge on the effect of conditioner on capillary suction time of biosludge at

their optimal dose. Trend line equations, r2 and P values are shown for three cases: all conditioners,

proteins and surfactants, and polymers. ....................................................................................................160

Figure 8-8 a) Effect of surface tension of conditioners and biosludge (conditioned) on the dewaterability

of biosludge; b) effect of wettability (contact angle) on the dewaterability of biosludge as measured with

capillary suction time. ...............................................................................................................................161

Figure 9-1 Simplified schematic illustrating the various mechanisms of cationic proteins and synthetic

polymers for inducing biosludge flocculation. Note: mechanisms are shown separately but they may

happen simultaneously. .............................................................................................................................169

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List of Abbreviations

µg, mg, g, kg Mass units

µL, mL, L Volume units

AD Anaerobic digestion

BCA Bicinchoninic acid

BCTMP Bleached-chemi-thermomechanical pulp

BMP Biochemical methane potential

BSA Bovine serum albumin

CMC Carboxylmethyl cellulose

Co-60 Cobalt 60

COD Chemical oxygen demand

CST Capillary suction time

CVI Colloidal vibration current

Da, kDa Atomic mass units

DNA Deoxyribonucleic acid

DNS Dinitrosalicylic acid

EPS Extracellular polymeric substances

IPTG Isopropyl β-D-1-thiogalactopyranoside

ISR Inoculum-to-substrate ratio

LB Luria broth

OD Optical density

P&P Pulp and paper

PAM Synthetic organic polymer, polyacrylamide

pI Isoelectric point

RNA Ribonucleic acid

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rpm Revolutions per minute

s, min, h Time units

SBY Specific biogas yield

sCOD Soluble chemical oxygen demand

SRF Specific resistance to filtration

TB Terrific broth

TBP Total biogas yield

TCA Trichloroacetic acid

TCD Thermal conductivity detector

tCOD Total chemical oxygen demand

TS Total solids

TSS Total suspended solids

USDA United States Department of Agriculture

VS Volatile solids

VSS Volatile suspended solids

WAS Waste activated sludge

WWTP Wastewater treatment plant

1

1 Chapter 1 - Introduction

Research Statement

Biosludge dewatering is an energy and chemical intensive process in wastewater treatment plants

(WWTP) (Novak et al., 1999; Chu et al., 2005). The addition of synthetic organic polymers is standard in

the pulp and paper industry for conditioning biosludge (or mixtures of sludges) and improving its

dewaterability (Amberg, 1984; Arcand, 1991; Dorica et al., 1999; Velema, 2004). However, polymers

represent a significant cost in dewatering processes and have a substantial impact in the overall economic

performance of WWTPs. Synthetic polymers are typically petroleum-derived which may affect their

availability and cost in the near future (Lee et al., 2014). Moreover, polymers have been reported to be

toxic (Liber et al., 2005; Bolto & Gregory, 2007; Harford et al., 2011). As an alternative, we propose that

the use of protein-based conditioners could improve the current practices in biosludge dewatering.

Unlike polymers, protein-based conditioners can be produced from renewable sources and/or from

waste. Due to their biodegradability, the toxicity from protein-based conditioners is expected to be low.

There are previous reports of enzymes for improving biosludge dewaterability (Ayol, 2005; Ayol &

Dentel, 2005; Dursun et al., 2006). However, the mechanisms for this improvement are not well

understood which prevents the implementation of such technology. Overall, a better understanding of the

mechanisms and the changes that biosludge undergoes during enzymatic and/or protein treatment could

potentially result in better sludge conditioning strategies.

Research Motivation

Sludge processing and disposal is a challenge due to the variability, gel-like structure and high

moisture content of biosludge (Jin et al., 2004). Activated sludge treatment is based on the ability of

microbial aggregates to remove soluble organic matter in wastewater and it has been widely used for its

flexibility, reliability and high effluent quality (Nguyen et al., 2007). The main disadvantage of activated

2

sludge systems is the generation of biosludge (Pérez-Elvira et al., 2006); its processing and disposal

accounts for up to 60% of the overall costs in a wastewater treatment plant (Mahmood & Elliott, 2006).

Biosludge management typically includes dewatering (i.e. liquid-solid separation) prior to disposal and

this process is energy and chemical intensive (Novak et al., 1999; Chu et al., 2005). Understanding the

interaction water-solids in biosludge in order to improve dewatering process is difficult due to the

variability among sludges and the presence of gel-like material. Bound water has been reported to affect

dewaterability (Amberg, 1984; Katsiris & Kouzeli-Katsiri, 1987; Lee, 1994; Lee & Hsu, 1995; Wu et al.,

1998; Chih et al., 1998; Ayol, 2005). However, reports indicate that bound water accounts for only 3 -8%

of the total water content in biosludge (Katsiris & Kouzeli-Katsiri, 1987; Colin & Gazbar, 1995; Chih et

al., 1998). Therefore, other factors such as compressibility and blinding effects may be hindering the

ability to mechanically remove the so called “free” water (Novak et al., 1988; Curvers et al., 2011;

Raynaud et al., 2012). Many factors have been reported to affect sludge dewaterability, including particle

size distribution, extracellular polymeric substances (EPS), divalent cations, and particle surface

properties (Mikkelsen & Keiding, 2002; Fargues & Turchiuli, 2003; Jin et al., 2003; Ayol, 2005; Shao et

al., 2009).

Chemical conditioners are commonly used to enhance sludge dewaterability prior to thickening and

mechanical dewatering processes (Bolto, 2006). This conditioning steps makes dewatering and

subsequent disposal possible. However, there are some associated disadvantages with the use of these

conditioners (Bolto & Gregory, 2007; Lee et al., 2014). For example, the addition of inorganic chemicals

increases the final sludge mass and reduces its heating value; therefore, it is not the best option when

sludge is incinerated (Albertson et al., 1987; Bolto, 2006). Alternatively, synthetic organic polymers (also

known as polyelectrolytes) provide versatility, significantly increase dewaterability at low doses and do

not reduce the heating value of biosludge (Bolto & Gregory, 2007). However, these polymers represent a

significant cost e.g. 2.7M (5% of operating costs) in a wastewater treatment plant in the City of Toronto

(Ashbridges Bay Wastewater Treatment Plant, Annual Report. 2015), and are sensitive to dose rate (Lee

3

et al., 2014). Therefore, finding suitable replacements and/or alternatives to reduce the use of synthetic

organic polymer is attractive. Furthermore, any improvement to mechanical dewatering efficiencies

represent cost savings, both in energy and chemicals.

Enzymes have been reported in the literature as conditioners for enhancing sludge dewaterability

(Ayol, 2005; Ayol & Dentel, 2005; Lu et al., 2011). However, it is not fully understood how enzymes

change biosludge properties improving its dewatering properties (Ayol, 2005). Moreover, enzymes

reported for improving sludge dewaterability represent a small sample of enzymatic activities available

either commercially and in novel-enzyme libraries. Therefore, a better understanding of how enzymes

improve biosludge dewaterability is needed. Our group has access to a library of enzyme that could be

explored for enzymatic conditioners potentially leading to improved regimes for enhancing sludge

dewaterability.

Bioflocculants can improve the dewaterability of biosludge. Given the abundance of proteins in

renewable materials, and waste, it is conceivable that proteins could be, in the near future, a practical

alternative to current chemical conditioners (Piazza & Garcia, 2010). However, a lack of understanding of

the mechanisms and the properties of interest for selecting proteins (and/or protein fractions) hinders the

development of protein-based conditioners.

The overall aim of this project is to gain a better understanding of how enzymes affect biosludge

dewatering properties. Understanding the effect of enzymes may bring about improvements in sludge

management technologies. In addition, as proteins have shown potential to be used as flocculants, it is our

objective to understand the mechanisms by which proteins can flocculate colloidal suspensions such as

biosludge. Identifying the mechanism and characteristics of protein-based flocculants will further our

knowledge and may potentially help us finding new, environmentally friendly, alternatives to synthetic

flocculants.

4

Hypotheses

The overall hypothesis of this project is that novel protein-based conditioners can improve biosludge

dewaterability.

The specific hypotheses set out to test in this project are:

i. Enzyme-based conditioners can affect sludge dewatering properties due to their enzymatic

activity.

ii. Protein-based conditioners enhance sludge dewatering properties by promoting charge

neutralization.

iii. Enzyme-based conditioners affect anaerobic digestibility of biosludge by hydrolysing proteins

and/or carbohydrates present in biosludge.

Objectives

The main goal of this project is to propose novel protein-based conditioners to improve the

dewaterability of biosludge. The following specific objectives were established to achieve this goal and to

gain a better understanding of how enzymes and proteins can affect biosludge.

i. Identify commercial and novel proteins and enzymes with the ability to enhance biosludge

dewatering.

ii. Investigate the mechanism(s) by which enzymes and cationic proteins enhance the dewaterability

of biosludge.

iii. Assess the effect of enzyme-based conditioners on anaerobic digestibility of biosludge.

General Approach

This project was divided into five experimental phases (I-V) to meet the previously discussed

objectives and test the hypotheses. A general illustration of the approach can be seen in Figure 1-1.

5

Phase I. Commercial enzymes for improved biosludge dewaterability: This phase involved the

screening and selection of commercial enzymes for their ability to improve biosludge dewatering. The

screening consisted in assessing sludge dewaterability after enzymatic treatment with different enzyme

doses. The assessment was based on a rapid method based on the movement of water through a filter called

capillary suction time (CST). From the six commercial enzymes screened, only one i.e., lysozyme, resulted

in improved dewaterability. The effect of treatment conditions such as enzyme dose, temperature and

mixing rate was investigated.

Phase II. Novel enzymes for improved biosludge dewaterability: This phase comprised the screening

of non-characterized (novel) enzymes for their ability to improve biosludge dewatering. New enzymes are

discovered daily and BioZone, a research centre in the Department of Chemical Engineering and Applied

Chemistry at the University of Toronto, has a library of novel enzymes to be explored. In this phase,

enzymes from the BioZone library were selected based on the hydrolytic activities that could potentially

act on the extracellular polymeric substances (EPS) of biosludge as well as the expression levels in

Escherichia coli for facilitating the in-house production of protein to conduct all the experiments needed.

Different doses of six enzymes were evaluated via CST. Chemical oxygen demand (COD), protein and

carbohydrate content were measured during enzymatic treatment in an effort to detect compositional

changes during enzymatic treatment and identify possible mechanism (s) of action.

Phase III. Cationic proteins as flocculants and biosludge conditioners: This phase included the

study of proteins with high isoelectric point (>9) as conditioners for improved dewaterability. This phase

was created as a result of our findings during Phase I and II. Lysozyme was found to significantly

improve sludge dewaterability and all the evidence suggests that its mechanism was based on its cationic

charge which allowed it to neutralize the negative charge present in biosludge particles. Therefore,

finding other cationic proteins that would enhance biosludge dewaterability could confirm lysozyme’s

mechanism and lead us to better protein-based conditioners.

6

Within Phase 3, a comparative assessment of different chemical conditioners (synthetic polymers,

surfactant, and proteins) was carried out to get a better understanding of the properties of conditioners.

More specifically, surface characteristics of the conditioner and the mechanisms that result in improved

biosludge dewaterability. In addition, the effect of these conditioners on dewaterability was assessed with

a lab-scale belt press which simulates industrial belt presses (i.e. Crown press®) and CST.

Figure 1-1 An outline of the experimental approach taken to study the effect of proteins and

enzymes on biosludge dewatering properties

Phase 5

Flocculating

potential of

proteins

(Chapter 6-7)

Commercial

enzymes

screening

(Chapter 3)

Novel

enzymes

screening

(Chapter 4)

Cationic

protein as

conditioners

(Chapter 7-8)

Dewatering

assessment and

characterization

(Chapter 3)

Dewatering

assessment and

characterization

(Chapter 4)

Anaerobic

digestibility studies

(Collaboration with

Zahra Choolaei)

(Chapter 5)

Phase 1 Phase 2 Phase 3

Phase 4

Novel enzymes

production

(Chapter 4)

Dewatering

assessment

(Chapter 8)

Mechanistic

understanding of how

enzymes and cationic

proteins affect sludge

for the purpose of

enhancing its

dewaterability and

anaerobic digestibility

Propose

Novel Protein-Based

Conditioners for

Enhancing the

Dewaterability of

Biosludge

Surface

properties

analysis

(Chapter 8)

7

Phase IV. Enzymatic pre-treatment for enhancing the anaerobic digestibility of biosludge: This

phase comprised the study of enzymes, namely proteases and glycosidase, for improving the anaerobic

digestibility of biosludge. Biological methane potential (BMP) assays, widely-used, lab-scale microcosms

to detect the anaerobic degradability of various substrates, were conducted to evaluate the impact of

enzymatic treatment on anaerobic digestibility. As expected, substrate specificity seems to determine their

potential. Using an approach that takes into account the organic load of the enzymes and their catalytic

activity, it was found that enzymes can enhance the anaerobic digestibility of biosludge and a distinction

between the effect of the organic matter added with the enzyme solutions and their catalytic activity was

possible.

Phase V. Flocculating potential of cationic proteins: The flocculating activity of lysozyme and

protamine on kaolin suspensions was investigated in this phase. These proteins were found to improve

biosludge dewaterability. Evidence of flocculation during the conditioning of biosludge suggests that

these proteins could be used as flocculants. Kaolin suspensions were used as the standard in flocculation

tests. The effect of dose and pH were evaluated. Lysozyme and protamine both have flocculating

potential. The required dose for protamine is lower than for lysozyme.

Thesis Outline

This thesis is divided into ten chapters. Chapter 1 provides an introduction of the research project,

hypotheses and objectives, followed by a literature review in Chapter 2. The following six chapters

(Chapter 3 to Chapter 8) contain the main findings of this research and are all presented in a paper format.

Each chapter is sectioned into introduction, methods, results and discussion, followed by the conclusions.

Chapter 3 reports the study of commercial enzymes for enhancing biosludge dewaterability. Thereafter,

the study of novel enzymes for enhancing biosludge dewaterability is presented in Chapter 4. Chapter 5

closes the research dedicated to enzymes, describing the effect of enzymatic pretreatment of biosludge on

its anaerobic digestibility. Chapter 6 describes the study of lysozyme as a flocculant using kaolin

8

suspensions. A further investigation on the potential of cationic proteins as flocculants is reported in

Chapter 7. Chapter 8 presents the study of the surface properties of various conditioners and their effect

on biosludge dewaterability. Following, results obtained in this project are discussed in Chapter 9, as well

as their scientific and industrial significance. Conclusions and recommendations for future work that arise

from this work are presented in Chapter 10. A series of appendices can be found at the end of this

document.

Contributions

Publications

1. Bonilla, S., Tran, H. and Allen, D. G. (2015), Enhancing the Dewaterability of Biosludge Using

Enzymes. Water Research, 68: 692-700.

2. Bonilla T., S. and Allen, D. G. (2016), Flocculation with Lysozyme: A Non-Enzymatic

Application. Canadian Journal of Chemical Engineering, 94: 231–237.

3. Bonilla, S., Choolaei, Z., Meyer, T., Yakunin, A. F., Edwards, E.A and Allen, D.G. (Submitted),

Addressing the Challenges Associated with Evaluating the Effect of Enzymatic Pretreatment on the

Anaerobic Digestibility of Biosludge.

4. Bonilla T., S. and Allen, D. G. (Submitted), Cationic Proteins for Enhancing Biosludge

Dewaterability: A comparative Assessment of Surface and Conditioning Characteristics of

Synthetic Polymers, Surfactants and Proteins.

Invention Disclosures

1. Bonilla-Tobar, I.S. and Allen, D.G. "Enhancing the dewaterability of secondary wastewater sludge

with proteins", Invention Disclosure MI 2012-140, UT10002467, August 27, 2012.

Non-Refereed Contributions

1. Choolaie, Z.*, Bonilla, S., Yakunin, A. F., Allen, D.G., Edwards, E. A. (2016) Enzymatic

Pretreatment of Pulp and Paper Mill Biosludge for Enhancing its Anaerobic Digestibility. 16th

9

International Symposium on Microbial Ecology. Aug 21-26 (PhD work- Poster Presentation).

International

2. Hamemeh, R.*, Loo-Yong-Kee, S., Bonilla, S., Allen, D.G. (2016) Scaling-Up Protein Production

on Escherichia coli: Effect of Induction Temperature Profile on Cell Yield in a Pilot-Scale

Fermentation Unit. 18th Canadian Society for Chemical Engineering Ontario-Quebec

Biotechnology Meeting. May 26 (PhD work- Poster Presentation). Regional

3. Bonilla, S.* Tran, H. and Allen, D.G. (2014) Enhanced Dewaterability of Biosludge using

Enzymes. 16th Canadian Society for Chemical Engineering Ontario-Quebec Biotechnology

Meeting. May 16 (PhD work – Oral Presentation). Regional

4. Bonilla, S., Amin, P., Tran, H. and Allen, D. G. * (2013) Enhancing Biosludge Dewaterability and

Combustion through Treatment with Novel Biopolymers and the Addition of Primary Sludges.

World Congress of Chemical Engineering, Seoul, Korea, Aug 18-23 (PhD work – Poster

Presentation). International

5. Bonilla, S.*, Tran, H. and Allen, D. G. (2013) Bio-Conditioners for Enhancing Biosludge

Dewaterability Industrial Biotechnology Congress, Montreal, Quebec, Jun 17-19, (PhD work –

Poster Presentation). International

6. Bonilla, S.*, Tran, H. and Allen, D. G. (2013) Enhancing the Dewaterability of Pulp and Paper

Mill Biosludge Using Enzymes" Paper Week Canada 2013 Conference, Montreal, Quebec, Feb 5-6

(PhD work – Poster Presentation). National

7. Bonilla, S.* and Allen, D.G. (2012) Enhancing the Dewaterability of Biosludge through Enzymes:

The case of Lysozyme. 14th Canadian Society for Chemical Engineering Ontario-Quebec

Biotechnology Meeting. May 31, (PhD work – Poster Presentation). Regional

8. Bonilla, S.*, Yakunin, A. and Allen, D. G. (2012) Enhancing Biosludge Dewaterability using

Biomolecules: The case of lysozyme. 62nd Canadian Chemical Engineering Conference.

Vancouver, BC, Oct 17 (PhD work - Oral Presentation). National

10

References

Ashbridges Bay Wastewater Treatment Plant, Annual Report. (2015)

Albertson, O. E., Burris, B., Reed, S., Semon, J., Smith, J., & Wallace, A. T. (1987) Design Manual:

Dewatering of Municipal Wastewater Sludges.

Amberg, H. R. (1984) Sludge Dewatering and Disposal in the Pulp and Paper Industry. Journal (Water

Pollution Control Federation) 56: pp. 962.

Arcand, G. (1991) Dewatering pulp and paper mill sludge using the Kamyr ring press. Ottawa,

Environment Canada.

Ayol, A. (2005) Enzymatic treatment effects on dewaterability of anaerobically digested biosolids-I:

performance evaluations. Process Biochemistry 40: 2427.

Ayol, A., & Dentel, S. K. (2005) Enzymatic treatment effects on dewaterability of anaerobically digested

biosolids-II: laboratory characterizations of drainability and filterability. Process Biochemistry 40: 2435.

Bolto, B. (2006) Coagulation and flocculation with organic polyelectrolytes. In Interface science in

drinking water treatment. G. Newcombe and D. Dixon (ed). Elsevier Ltd., pp. 63.

Bolto, B., & Gregory, J. (2007) Organic polyelectrolytes in water treatment. Water Res 41: 2301.

Chih, C. W., Chihpin, H., & Lee, D. J. (1998) Bound water content and water binding strength on sludge

flocs. Water Res 32: 900.

Chu, C. P., Lee, D. J., & Chang, C. Y. (2005) Energy demand in sludge dewatering. Water Res 39: 1858.

Colin, F., & Gazbar, S. (1995) Distribution of water in sludges in relation to their mechanical dewatering.

Water Res 29: 2000.

Curvers, D., Saveyn, H., Scales, P. J., & Van der Meeren, P. (2011) Compressibility of biotic sludges â€“

An osmotic approach. Chem Eng J 166: 678.

Dorica, J., Harland, R., & Kovacs, T. (1999) Sludge dewatering practices at Canadian pulp and paper

mills [Survey]. Pulp Pap Can 100: 19.

11

Dursun, D., Turkmen, M., Abu-Orf, M., & Dentel, S. K. (2006) Enhanced sludge conditioning by enzyme

pre-treatment: comparison of laboratory and pilot scale dewatering results. Water science and technology

54: 33.

Fargues, C., & Turchiuli, C. (2003) Structural Characterization of Flocs in Relation to Their Settling

Performances. Chem Eng Res Design 81: 1171.

Harford, A. J., Hogan, A. C., Jones, D. R., & van Dam, R. A. (2011) Ecotoxicological assessment of a

polyelectrolyte flocculant. Water Res 45: 6393.

Jin, B., Wilén, B., & Lant, P. (2004) Impacts of morphological, physical and chemical properties of

sludge flocs on dewaterability of activated sludge. Chem Eng J 98: 115.

Jin, B., Wilén, B., & Lant, P. (2003) A comprehensive insight into floc characteristics and their impact on

compressibility and settleability of activated sludge. Chem Eng J 95: 221.

Katsiris, N., & Kouzeli-Katsiri, A. (1987) Bound water content of biological sludges in relation to

filtration and dewatering. Water Res 21: 1319.

Lee, C. S., Robinson, J., & Chong, M. F. (2014) A review on application of flocculants in wastewater

treatment. Process Saf Environ Prot 92: 489.

Lee, D. J., & Hsu, Y. H. (1995) Measurement of Bound Water in Sludges: A Comparative Study. Water

Environ Res 67: 310.

Lee, D. (1994) Measurement of bound water in waste activated sludge: Use of the centrifugal settling

method. J Chem Technol Biotechnol 61: 139.

Liber, K., Weber, L., & Levesque, C. (2005) Sublethal toxicity of two wastewater treatment polymers to

lake trout fry (Salvelinus namaycush). Chemosphere 61: 1123.

Lu, J., Rao, S., Le, T., Mora, S., & Banerjee, S. (2011) Increasing cake solids of cellulosic sludge through

enzyme-assisted dewatering. Process Biochemistry 46: 353.

Mahmood, T., & Elliott, A. (2006) A review of secondary sludge reduction technologies for the pulp and

paper industry. Water Res 40: 2093.

12

Mikkelsen, L. H., & Keiding, K. (2002) Physico-chemical characteristics of full scale sewage sludges

with implications to dewatering. Water Res 36: 2451.

Nguyen, T. P., Hankins, N. P., & Hilal, N. (2007) A comparative study of the flocculation behaviour and

final properties of synthetic and activated sludge in wastewater treatment. Desalination 204: 277.

Novak, J., Agerbæk, M., Sørensen, B., & Hansen, a. (1999) Conditioning, Filtering, and Expressing

Waste Activated Sludge. J Environ Eng 125: 816.

Novak, J. T., Goodman, G. L., Pariroo, A., & Huang, J. (1988) The Blinding of Sludges during Filtration.

Journal (Water Pollution Control Federation) 60: pp. 206.

Pérez-Elvira, S. I., Nieto Diez, P., & Fdz-Polanco, F. (2006) Sludge minimisation technologies. Rev

Environ Sci Biotechnol 5: 375.

Piazza, G. J., & Garcia, R. A. (2010) Proteins and peptides as renewable flocculants. Bioresour Technol

101: 5759.

Raynaud, M., Vaxelaire, J., Olivier, J., DieudÃ©-Fauvel, E., & Baudez, J. (2012) Compression

dewatering of municipal activated sludge: Effects of salt and pH. Water Res 46: 4448.

Shao, L., He, P., Yu, G., & HE, P. (2009) Effect of proteins, polysaccharides, and particle sizes on sludge

dewaterability. Journal of Environmental Sciences 21: 83.

Velema, G. (2004) Management and benefits of pulp and paper mill residuals at Domtar Cornwall. Pulp

Pap Can 105: 26.

Wu, C. C., Huang, C., & Lee, D. J. (1998) Bound water content and water binding strength on sludge


13

2 Chapter 2 – Literature Review

Among the sludges produced in wastewater treatment, biosludge is the most difficult to dewater

(Goodwin & Forster, 1985; Katsiris & Kouzeli-Katsiri, 1987; Wu et al., 1998; Legrand et al., 1998;

Krishnamurty & Viraraghavan, 2005; Mahmood & Elliott, 2006; Sveegaard et al., 2012). Activated

sludge treatment uses the ability of biological aggregates to remove soluble organic matter in wastewater

(Ramdani et al., 2010). A typical wastewater treatment is illustrated in Figure 2-1. While activated sludge

has been successfully used for its flexibility, reliability and high quality in numerous plants (Nguyen et

al., 2007), its main disadvantage is the production of waste activated sludge, also known as biosludge.

Disposal of excess biosludge results in high costs for wastewater treatment plants. It is widely-

acknowledged that improving the dewaterability of biosludge is valuable because it would reduce the

environmental impact of sludge disposal and can potentially improve the economics by reducing

processing and transportation costs (Benítez et al., 1994; Vaxelaire & Cézac, 2004; Ayol & Dentel, 2005;

Ayol, 2005; Wood et al., 2009).

Biosludge in Pulp and Paper Mills

The pulp and paper industry uses high volumes of water in the pulping process and requires

wastewater treatment to meet environmental regulations in its effluent. It has been reported that between

82 and 292 cubic meters of water are use per tonne of product (75-285 gal/ton product) (Dorica et al.,

1999). Wastewater treatment, in the majority of pulp and paper mills, requires primary and secondary

(biological) treatment (Pokhrel & Viraraghavan, 2004). Biological treatment can be performed through

different systems which can be aerobic or anaerobic systems. Aerobic treatments are the most commonly

used, in particular activated sludge. According to a Canadian survey, activated sludge is used by

approximately 61% of pulp and paper mills in Canada (Dorica et al., 1999) and this is due to their high

quality effluent and smaller footprint when compared with lagoons. In most cases, biosludge is mixed

with primary sludge (mainly composed of wood fibres) to boost biosludge’s dewaterability. Once the

14

mixed sludge is dewatered, disposal is carried out through incineration, land application and/or landfilling

or a combination of these.

Figure 2-1 Simplified schematic of a typical wastewater treatment.

Finding new opportunities to improve biosludge dewaterability can lead to cost reductions and

potentially, energy recovery. Biosludge dewatering is more important now than it has ever been due to

changes in sludge production ratios and environmental regulations. Biosludge is usually combined with

primary sludge and/or wood residues to make the combustion process viable but sludge can be dewatered

only up to 40% solids (Dorica et al., 1999). There is a high cost associated with the chemical and energy

demand to take the sludge to 40% dry solids content. However, adding primary sludge to biosludge to

improve the latter’s dewaterability may become problematic because biosludge production is expected to

increase as a result of more stringent effluent environmental regulations and primary sludge is constantly

reduced as a result of better pulping processes (Mahmood & Elliott, 2006). The current practice is to

dewater sludge mixtures of approximately 70% primary sludge and 30% biosludge which may become

unsustainable.

Primary Treatment

Primary

Sludge

Clarifier Aerated Basin

Return Activated Sludge

Clarifier

Waste Activated

Sludge

(Biosludge)

DewateringConditioning

Wastewater

Effluent

Disposal

Secondary Treatment

15

In recent years, sludge management strategies in the pulp and paper industry have been affected by

increasing environmental regulations. Until recently, landfills had been the most commonly used disposal

method. For example, in 1995, 62% of the total sludge was disposed of in landfills (Scott & Smith, 1995).

Incineration is now broadly used as the final disposal method of biosludge while land application

accounts for only 5% of the disposal approaches (Mahmood & Elliott, 2006). The advantages of

incineration over other disposal strategies are that it fits well with other processes in a pulp mill and it is

carried out in-situ which eliminates the costs of transportation. However, the main challenge of sludge

incineration is the high moisture content of sludge which results in low heating values. Therefore,

additional fuel is added to sustain the combustion of sludge which is costly (Scott & Smith, 1995).

Biosludge - Properties and their Effect on Dewaterability

Sludge is a complex mixture of microorganisms, organic, and inorganic matter. It has a gel-like

structure due to the presence of extracellular polymeric substances (EPS) that are produced by the

microorganisms. EPS assist in the aggregation of particles in sludge and these aggregates are called flocs

(Legrand et al., 1998). A schematic representation of a floc is depicted in Figure 2-2.

Figure 2-2 Simplified schematic representation of flocs from biosludge. Based on microscopy

observations, Legrand et al., 1998 and de Kreuk et al., 2010.

Floc properties and their impact on sludge dewaterability have been evaluated previously without a

consensus of the key properties and the degree of their impact on the dewaterability of biosludge. In the

Bacteria

Inorganic

Particles

Organic

Fibres

Filamentous

Bacteria

Organic

Particles

Extracellular

Polymeric

Substances

(EPS)

16

following sections, a literature review of the properties with most relevance to this project will be

presented.

Bound Water

Bound water has been reported as a key property impeding the solid-liquid separation of biosludge.

Within a floc, water behavior differs due to its interaction with the different solids present in it (Vaxelaire

& Cézac, 2004). Water in sludge is generally classified as either free or bound water. Free water is

defined as the water that can be removed by mechanical force (Vaxelaire & Cézac, 2004). Bound water is

classified into three groups, interstitial, vicinal, and hydration water, depending on the specific interaction

with solids in sludge. The water contained within microbial cells can be grouped in the vicinal and

interstitial water categories (Vesilind, 1994). As seen in Figure 2-3, interstitial water is located inside the

flocs and held by capillary forces; surface or vicinal water is associated and bound to particles. Hydration

water is chemically bound to particles and it can only be released by thermo-chemical treatment

(Vaxelaire & Cézac, 2004).

Figure 2-3 Simplified schematic representation of free and bound (vicinal, interstitial and

hydration) water in flocs. Based on the definitions of bound water presented in Vaxelaire & Cézac,

2004.

The moisture distribution in sludge has been studied extensively but the literature is contradictory

and hard to compare for two main reasons: the lack of a standard method and the lack of a universal

Free

water

Hydration

water

Vicinal

water

Interstitial

water

17

bound water definition. There are several methods to quantify bound water in the literature. These include

variations of drying, centrifugal and dilatometric tests (Katsiris & Kouzeli-Katsiri, 1987; Lee, 1994; Lee

& Hsu, 1995; Smith & Vesilind, 1995; Chih et al., 1998; Vaxelaire & Cézac, 2004). Due to the various

methods, bound water results are difficult to compare mainly because bound water data available is

operationally defined (Robinson & Knocke, 1992; Lee, 1994; Lee & Hsu, 1995; Vaxelaire & Cézac,

2004).

Although bound water seems to be the obstacle impeding removal of water from sludge and

obtaining dryer cakes, it has not been proven a good indicator of dewaterability. In other words, less

bound water does not always result in better dewaterability. Researchers have found bound water to be

only 3-8% of the total water content (Katsiris & Kouzeli-Katsiri, 1987; Colin & Gazbar, 1995; Chih et al.,

1998; Liao et al., 2000). Chemical conditioners were found to decrease bound water but increase the

specific resistance to filtration (i.e. worsening filtration) (Katsiris & Kouzeli-Katsiri, 1987). Therefore,

other properties, and more likely a combination of various properties determines the dewatering

properties of biosludge. This is consistent with reports that suggest that several properties simultaneously

affect sludge dewaterability (Karr & Keinath, 1978; Katsiris & Kouzeli-Katsiri, 1987; Murthy & Novak,

1999; Mikkelsen & Keiding, 2002; Vaxelaire & Cézac, 2004). Thus, changing the structure and properties

of flocs could potentially enhance sludge dewaterability.

Extracellular Polymeric Substances (EPS)

The quantity and quality of extracellular polymeric substances (EPS) in sludge affect its

dewaterability. EPS are the result of biological synthesis and lysis during biological treatment and are

essential in the formation of flocs (Morgan et al., 1990; Shao et al., 2009). The importance of EPS in the

aggregation of particles in sludge is well-acknowledged. However, an excess of EPS can lead to poor

dewatering properties (Novak et al., 2003). Similar findings have been reported by other researchers.

EPS, and in particular proteins, have been linked to poor dewatering properties (Novak et al. 2003 and

18

Morgan, 1990). Water retention in sludge has been attributed to high protein content without an accepted

mechanism. Jin et al. (2003) proposed that this phenomenon may be due to high water uptake by the

polymer networks with negative charges surrounded by counter ions which leads to an osmotic gradient.

Nonetheless, flocs formation is not possible without EPS and the lack of these polymers has been linked

to floc breakage (Jarvis et al., 2005a), which can also result in poor dewaterability (Rasmussen et al.,

1994). Since both, low and high concentrations of EPS have been linked to poor dewatering properties, an

optimum quantity (and likely quality) of EPS may exist for improved dewaterability. However, a lack of

standard in EPS extraction methods in the literature currently makes the comparison between studies

difficult and hinders a further understanding of the effect of EPS on biosludge dewaterability.

Surface Charge

Surface charge affects the liquid-solid separation properties of biosludge. Microorganisms are

considered colloids with a predominantly negative charged surface (McKinney & Edwards, 1952;

Morgan et al., 1990) and biosludge is mainly comprised of microorganisms. The chemical composition of

biosludge largely determines its surface charge. The ratio of proteins to carbohydrates in activated sludge

has been shown to be correlated to its surface charge (Morgan et al., 1990; Liao et al., 2001). At the

close-to-neutral pH of sludge, most proteins carry a net negative charge. Thus, an increase in protein in

the EPS, results in more negatively charged particles. This in turn leads to the repulsion of particles in

biosludge, hindering settling and negatively affecting dewaterability. To improve the dewaterability of

biosludge, widely-used conditioners (i.e. synthetic polymers) reduce particle repulsion, neutralize charges

and bridge particles.

Particle Size

Particle size is arguably the most important property affecting the dewatering properties of sludge.

Several studies have been conducted to investigate the effect of particle size on the dewaterability of

biosludge (Karr & Keinath, 1978; Knocke & Zentkovich, 1986; Vesilind, 1994; Feitz et al., 2001; Chu et

19

al., 2001; Fitria et al., 2014). There is consensus in the literature as reports agree that smaller particles

negatively affect the dewatering properties of biosludge dewaterability. The supracolloidal fraction (1-100

µm) appears to have the largest impact meaning that more particles in this range had a negative effect on

dewaterability (Karr & Keinath, 1978). Studies suggest that the water attached to the surface of particles

(i.e. vicinal water) can impede a successful dewatering process (Vesilind, 1994), thus, changing the

surface available for water to adhere to by increasing particle size can improve the dewaterability of

biosludge.

In addition to the size of particles in biosludge, the particle size distribution range affects the

dewatering properties of biosludge because it determines its packing characteristics. Blinding of filter

media and cake is common when there are wide particle size distributions since it leads to smaller

particles blocking the pores produced by larger particles (Novak et al., 1988; Qi et al., 2011). The

porosity of the cake and the filter media is more affected by blinding (i.e. pore blockage) when particles

are smaller than 40 microns (Novak et al., 1988).

Cations

Cations within sludge play an important role in the aggregation of particles interacting with EPS and

other negatively charged particles (Higgins & Novak, 1997). Divalent cations, such as Ca2+ and Mg2+ act

like bridging agents inside the organic extracellular matrix (i.e. EPS) of flocs (Nguyen et al., 2008). It is

also known that Ca2+ has higher binding capacity than Mg2+ in activated sludge (Park 2002, Jin, et al.

2004, Guan, et al. 2012), making calcium ions more important players in floc strength and breakage.

Removal of Ca2+ ions has been related to smaller flocs and poor dewatering properties (Bruus et al.,

1992). Monovalent cations also affect biosludge dewaterability. For example, an excess of monovalent

cations to divalent cations in activated sludge treatment plants leads to poor sludge dewatering properties

and poor effluent quality (Park, 2002; Nguyen et al., 2008, Murthy & Novak, 1999). The effect of cations

20

on the dewatering properties has been widely studied and divalent cations are well-known enhancers of

biosludge dewaterability.

Compressibility

Sludge compressibility is a key property to consider for enhancing biosludge dewaterability. For

highly compressible sludge such as biosludge, there is a critical point where liquid drainage is

independent of the pressure applied during mechanical dewatering (Qi et al., 2011; Sveegaard et al.,

2012). As a result of the high compressibility of biosludge, an increase in mechanical stress leads to: a)

reduction of porosity and b) increase in the resistance to flow. Thus, hindering the flow of “free water”

through the cake and the filter (Tiller & Kwon, 1998; Qi et al., 2011). Therefore, drainage and low

pressure filtration are often used to dewater sludge. High-solid cakes could be produced if a sufficient

amount of filtration time is given. However, the time to reach the maximum solids content has been

reported to be over 30 hours which would be impractical in industrial processes (Qi et al., 2011).

Overall, while several properties have been studied and associated with biosludge dewaterability, the

interactions between floc properties remain poorly understood. Several studies have been carried out to

assess the impact of different sludge properties including, particle size, cations, EPS, surface charge,

bound water and compressibility. As previously discussed, most studies have concluded that these

properties somehow affect dewaterability, but the degree of the effect is still unknown. Moreover,

simultaneous effects of these properties occur during treatment because these properties affect each other.

Therefore, a clear understanding of how these properties affect sludge dewaterability is difficult.

Nonetheless, a better understanding of which properties should be considered first for improving liquid-

solid separations is key for the design and assessment of conditioning treatments that enhance biosludge

dewaterability.

21

Dewaterability Assessment

Several methods have been used to assess the dewaterability of biosludge in the laboratory.

However, capillary suction time (CST) and specific resistance to filtration (SRF) are the most commonly

used in the literature to assess the dewaterability of sludges. Specific resistance to filtration (SRF)

indicates the rate at which the fluid passes through the cake during vacuum or pressure filtration. A

comprehensive description of SRF can be found elsewhere (Christensen & Dick, 1985a; Christensen &

Dick, 1985b). The main limitations of SRF are that the sample size per measurement is ~100 mL and the

method is time-consuming.

Figure 2-4 illustrates the five characteristics of an enhanced dewatering process and how assessment

methods can relate to these characteristics. A conditioning treatment should improve at least one of the

characteristics. The three methods used in this project are CST, Crown Press and polymer demand, and

will be discussed in this section.

Figure 2-4 Characteristics of an improved sludge dewatering process and their relation to

assessment methods used in this project: capillary suction time (CST), Crown Press and polymer

demand.

Enhanced

Dewatering

Process

Increased Solids Capture (Crown Press)

Reduced Moisture Content

(Crown Press)

Reduced Energy Required for Mechanical Dewatering

Reduced Chemical Demand

Reduced Time Needed for Dewatering

(CST and Crown Press)

22

Capillary Suction Time (CST)

Capillary suction time (CST) is a well-known method used to assess sludge dewaterability. CST is

based on the capillary suction pressure generated by a filter paper. The CST apparatus has two electrodes

to measure the time that water takes to travel a given distance through the filter paper. A lower CST

indicates better dewaterability. Figure 2-5 shows a picture and a schematic of the CST apparatus. Because

CST does not have a mathematical basis and does not fundamentally explain dewaterability, the use of

CST in research is somehow controversial (Vesilind, 1988). Nonetheless, numerous researchers have used

CST to report effects on sludge dewaterability (Vesilind, 1988; Dentel, 1993; Lee & Liu, 2000; Chang et

al., 2001; Jin et al., 2004; Krishnamurty & Viraraghavan, 2005; Ayol, 2005; Dursun et al., 2006; Sawalha

& Scholz, 2007; Feng et al., 2009; Yuan et al., 2011) because it provides a reliable dewaterability

assessment. In some cases, SRF can be predicted from CST data (Sawalha & Scholz, 2010).

Figure 2-5 Capillary Suction Time (CST), a) apparatus; b) schematic of cross-sectional of CST

apparatus sample column and plates.

In order to assess dewaterability using CST, it is important to considered its limitations. Firstly, the

solids content of sludge affects CST so samples need to have the same solids content if they are going to

be compared (Vesilind, 1988). Secondly, the temperature needs to be considered when conducting the

23

experiments. Samples need to be evaluated under the same temperature conditions, to avoid unwanted

viscosity effects on CST values (Sawalha & Scholz, 2007). Even taking these considerations, CST values

cannot be compared between different types of sludge. Thus, it is recommended to use the appropriate

experimental controls every time that CST will be used to assess effect on dewaterability and use at least

two dewatering assessment methods to validate results.

Crown Press®

The Crown Press provides a more realistic assessment of the dewaterability of biosludge in

comparison with CST. The Crown Press is a bench-scale apparatus that has shown a good correlation

with full-scale belt presses (Severin et al., 1998). It has been used to determine the potential of several

conditioning treatments (Hartong et al., 2007; Erden et al., 2010; Erden & Filibeli, 2010; Amin, 2015;

Bouchard, 2015; Singh, 2015), including enzymatic and protein conditioning of sludge (Ayol, 2005; Ayol

& Dentel, 2005; Dursun et al., 2006; Lu et al., 2011; Banerjee, 2014).

Gravity

Thickening

Mechanical

Dewatering

Figure 2-6 Crown Press – A bench-scale simulator of full-scale presses used to assess the

dewaterability of biosludge in this project. Gravity thickening and active mechanical pressing

are separated in two steps.

24

The Crown press operation is divided into two steps: gravity thickening and mechanical dewatering.

The apparatus used in this project is shown in Figure 2-6. This two-step process is in agreement with

current industrial practices. In the Crown Press, a sludge sample is first added to the gravity thickening

step where the sample is gravity filtered for a predetermined amount of time. The produced cake is then

transferred to the belt filter in the “mechanical dewatering” part of the apparatus and exposed to a certain

pressure. From these two steps it is possible to evaluate three characteristics of a good dewatering

process: Filtrate rate and solids capture in the gravity thickening step, and final cake solids in the

mechanical dewatering step.

Polymer Demand

Polymers are used to improve the dewaterability of sludges. It is generally accepted that a sludge

with fairly good dewatering properties will need less polymer than a sludge with poor dewatering

properties. Thus, reducing the polymer demand while achieving similar dewatering properties is

indicative of a positive effect on the dewaterability of biosludge. Polymer demand has been previously

used to assess the effectiveness of a conditioning treatment (Ayol, 2005; Ayol & Dentel, 2005; Novak,

2006).

Conditioning

Biosludge dewatering today is chemical and energy intensive. In addition to the energy used in

mechanical dewatering devices such as belt or screw presses, the high moisture content of biosludge, even

after mechanical dewatering means that during incineration, fuel needs to be added because sludge

combustion is not self-sustainable (Murakami et al., 2009). There are several, available and in-research

stage, treatments to condition sludge prior to mechanical dewatering for enhanced dewaterability. The

main categories where the literature is concentrated are: thermal (Neyens & Baeyens, 2003), electric

(Aziz et al., 2006; Mahmoud et al., 2010; Mahmoud et al., 2011), ultrasonic (Yin et al., 2004) and

chemical conditioning (Roberts & Olsson, 1975; Matsumoto et al., 1980; Arcand, 1991; Lee & Liu, 2000;

25

Bolto & Gregory, 2007; Ahmad et al., 2008; Ariffin et al., 2012). Chemical conditioners are widely used

to improve biosludge dewaterability. The following section will discuss chemical conditioners with a

focus on synthetic polymers and the emerging natural flocculants.

Chemical Conditioners

Chemicals are the most widely used conditioning treatment in wastewater plants (Lee & Liu, 2000).

Biosludge is usually mixed with primary sludge and the mixture is then conditioned with synthetic

polymers (flocculants). Polymer addition is used for thickening and dewatering of sludge and it represents

a significant cost in wastewater treatment plants. Up to 5% of the total operation costs of a wastewater

treatment plant in the City of Toronto was used for flocculants in 2015 (Ashbridges Bay Wastewater

Treatment Plant, Annual Report. 2015). In the pulp and paper industry, synthetic organic polymers, also

known as polyelectrolytes, are preferred because they do not affect the heating value of sludge when

incineration is used as the final disposal method (Albertson et al., 1987).

Synthetic Organic Polymers (Synthetic Flocculants)

Synthetic organic polymers, also known as polyelectrolytes, have been used as conditioners for more

than 30 years and their use is constantly growing because of the advantages they offer over inorganic

chemicals during incineration (Novak & Haugan, 1980). Additionally, the versatility in charge and chain

length of polymers offers the possibility to optimize polymers and increase their performance under

specific treatment conditions (Bolto & Gregory, 2007). For the group of synthetic organic polymers,

polyacrylamide-based polymers are the most commonly used. Polymers improve the dewaterability of

sludges through flocculation.

Flocculation is the mechanism where particles are aggregated. In the case of synthetic polymers,

Small flocs are first formed as a result of charge neutralization. Particles are further aggregated by

polymer bridging. There are different mechanisms that can result in bridging of particles: electrostatic

26

attraction, hydrogen bonds and salt linkages (Bolto, 2006). Electrostatic attraction happens when a

polymer with an opposite charge is used. In the case of sludge, cationic polymers are used, hydrogen

bonding is common with non-ionic polymers where exposed hydrogen atoms have a weak positive charge

and can attract negatively charged particles. Although an individual bond is weak, the sum of the bonds

can result in very stable aggregates (Bolto, 2006). Thus, for non-ionic polymers, a longer chain is better

(Moodey, 2007). Cations promote a different flocculation mechanism by creating bonds with anionic

polymers. The chemical versatility of polymers means that all these mechanisms can be contributing to

the aggregation of particles when sludge is conditioned.

Although the mechanisms of polymer conditioning are known and fairly understood, in practice,

polymer selection and optimum dose is empirical. Many factors affect polymer conditioning and thus,

each plant requires empirical testing (Dentel, 1993). The optimal dose is known to depend on solids

concentrations, mixing speed, mixing time and on the mechanical device to be used for dewatering

(Novak & Haugan, 1980). Mixing and dose are critical parameters for the optimal conditioning of sludge

with polymers. For example, high mixing rates are needed to obtain a sludge with good dewaterability

and to maintain it overtime (Novak & Haugan, 1980). Alternatively, higher doses of polymer may be

needed. In general, high molecular weight polymers appear to perform better on biosludge (Novak &

Haugan, 1980).

There are disadvantages associated with the use of synthetic polymers as conditioners. Synthetic

organic polymers represent a significant cost and are sensitive to dose rate. If the optimum dose is

surpassed, sludge dewatering becomes even more difficult which is a setback due to the variable nature of

sludge (Bolto, 2006). For organic polymers, overdosing is common, but its mechanisms are poorly

understood. An increase in viscosity has been associated with overdosing (Christensen et al., 1993). One

of the main disadvantages of using polymers today is that environmental regulations have become more

stringent and synthetic cationic polymers can be toxic to aquatic systems (Bolto, 2006; Bolto & Gregory,

27

2007). For example, they have been banned in Japan and Switzerland from drinking water treatment

(Bolto, 2006). Therefore, finding suitable replacements and/or alternatives to reduce synthetic polymer

use is attractive. Additionally, any improvement to mechanical dewatering efficiencies could result in cost

and/or energy savings.

Natural Flocculants

Due to the disadvantages of synthetic polymers and the increased interest in “natural” approaches for

solving industrial problems, natural flocculants have recently received special attention. The following

section broadly describes the research landscape for the use of naturally-derived and “environmentally

friendly” options for different liquid-solid separations.

Chitosan

Chitosan is the most studied and promising naturally-derived flocculant. Natural flocculants have

been extensively studied in recent years as a “green” alternative to synthetic polymers. Biopolymers such

as chitosan, tannin and guam have receive special attention (Sharma et al., 2006; Lee et al., 2014).

Chitosan is the result of the alkaline deacetylation of chitin which is the second most abundant

biopolymer on earth. It has been extensively studied for coagulation and flocculation of wastewater and

for the recovery of suspended solids in a variety of industries (Sharma et al., 2006; Renault et al., 2009;

Lee et al., 2014). However, limited studies were found on the use of chitosan as a conditioner for

enhancing sludge dewaterability (Zemmouri et al., 2015). One of the main disadvantages of chitosan is

that is insoluble in water and it needs to be dissolved in acids. Moreover, only at an acidic pH, chitosan

exhibits a cationic charge and would be able to neutralize the repelling particles in biosludge. Chemical

modifications of chitosan are being studied for overcoming its current limitations as a flocculant and also

to use chitosan in other processes and industries (Alves & Mano, 2008).

28

Microbial Flocculants i.e. Bioflocculants

Bioflocculants have shown potential to improve liquid-solid separations but the area is still in its

infancy. The term bioflocculant in this document will refer to flocculants produced from microbial strains

in the form of extracellular polymeric substances. Research efforts for the discovery, characterization and

use of bioflocculants have increased dramatically in the past 15 years. Several bacterial strains have been

used for this purpose and numerous bacterial extracts have shown flocculating activity (Salehizadeh &

Shojaosadati, 2001; More et al., 2014; Salehizadeh & Yan, 2014). Bioflocculants seem to be mainly

composed of carbohydrates and are in a wide molecular weight range (103 to 106 Daltons) (Salehizadeh &

Yan, 2014). Surprisingly, although studies have looked at chemical composition, effect of ions,

temperature, among other factors, little is known about the surface charge of bioflocculants. Given that

charge is a key factor in flocculation theory, a distinct gap in the literature refers to the mechanism of

bioflocculants and to what extent their effectiveness is associated with their surface charge.

Waste Extracts and Others

There has been limited interest in extracting flocculants from wastes. One of the groups leading this

area of research is the Agricultural Research Service from the United States Department of Agriculture

(USDA). Extracts from blood, feather, meat and bone meal have potential as flocculants (Piazza &

Garcia, 2010a; Piazza & Garcia, 2010b; Piazza et al., 2011; Piazza et al., 2012; Piazza et al., 2014; Garcia

et al., 2014; Piazza et al., 2015; Piazza et al., 2016; Xu et al., 2016; Garcia et al., 2016). Particular

attention has been given to proteins as alkaline extraction led to extracts with significant flocculating

activity a pH 5.5 (Piazza & Garcia, 2010b). However, this potential has not been comprehensively studied

and the flocculating mechanisms are poorly understood. It is unknown which of the properties in these

extracts play a key role for the reported flocculating activity. Moreover, all these studies were conducted

on kaolin suspensions. A more complex suspension such as biosludge may yield different results. A study

reported that soy protein could improve fibrous sludge dewaterability (Banerjee, 2014). Soy protein is

29

negatively charged so an improvement was not expected unless the protein was cationized. Nonetheless,

the authors reported significant improvements and potential costs reductions. Overall there is potential for

using proteins as flocculants but there is a limited understanding of the mechanisms involved and the

protein properties that result in better liquid-solid separations.

Enzymes for Enhancing Biosludge Dewaterability

Enzymes (proteins with catalytic activity) have been reported to enhance sludge dewaterability.

Table 2-1 provides a summary of the reports found in the literature that investigate the effect of enzymes

on sludge dewaterability. There is great variability in the improvements reported in the literature. From

3% improvements to up to 80% improvements in cake solids after enzymatic treatment (Ayol & Dentel,

2005; DeLozier & Holmes, 2008). This is not surprising given that different enzymatic products and

conditions have been studied. There is a limited understanding about the mechanisms involved during

enzymatic treatment that result in improved dewatering properties. Enzymatic hydrolysis of particles in

flocs has been the focus of previous studies, where in general, the mechanisms suggested are based on the

ability of enzymes to break molecules (extracellular polymer substances (EPS) and other polymers such

as cellulose), therefore, releasing water trapped in flocs (Ayol, 2005; Ayol & Dentel, 2005; Dursun et al.,

2006; Ayol et al., 2007; Ayol et al., 2008). However, little evidence has been provided to support this

proposed mechanism.

There has been scientific and industrial interest in using enzymes to improve biosludge

dewaterability. Besides the scientific studies that report on biosludge dewaterability (Thomas et al., 1993;

Ayol, 2005; Ayol & Dentel, 2005; Dursun et al., 2006; Ayol et al., 2007; Ayol et al., 2008), there are

three filed patents where enzymes are shown to improve sludge dewaterability (Sarkar et al., 2003; Sarkar

et al., 2005; DeLozier & Holmes, 2008). Reported enzymes are mostly of the following classes:

cellulases, alpha amylases and peroxidases and have been used in the majority of studies as cocktails. The

30

fact that enzyme companies such as Novozymes (DeLozier & Holmes, 2008) are looking at this type of

product also suggests the importance of the potential market.

Table 2-1 Summary of enzymes and their reported effect on sludge dewaterability

Sludge Industry Enzyme Dewatering

Improvements Reference

Digested Municipal DEGOMMA 7083

At 2 ppm enzyme dose a

CST reduction from 29.2

to 15.4 s. Increase in dry

solids content of 2-5%

(Thomas et al.,

1993)

Anaerobically

Digested Brewery

Alpha-amylase and

Beta-glucanase

Polymer demand was

reduced from 1.5 kg/ton to

0.5-1 kg/ton

(Ayol et al.,

2007)

Anaerobically

Digested Municipal Endo-glucanase

Filtration rate was

increased from 94.4 mL to

138.3 mL per 10 s.

(Sarkar et al.,

2005)**

Anaerobic and

Aerobically digested Municipal

Alpha-amylase and

Beta-glucanase

Enzyme addition

improved CST in sludge

during anaerobic digestion

e.g. Day 2: Control

CST=1250 s, enzyme

treatment CST=1100 s.

(Ayol et al.,

2008)

Anaerobically

Digested Municipal Cellulases

Filtration rate increased

from 63.2 ml to 83.6 ml

per 30 sec.

(Sarkar et al.,

2003)**

Anaerobically

Digested Municipal Envirozyme*

Lab-scale: Cake solids

content was increased

from 18% to 27% TS;

pilot Scale: No effect

(Dursun et al.,

2006)

Anaerobically

Digested Municipal Envirozyme*

Cake solids increased

from 26.6% to 48.6% DS (Ayol, 2005)

Anaerobically

Digested Municipal

AQUAZYME

ULTRA 1200

Cake solids increased by

at least 3.2%

(Delozier et al,

2015)**

*protease, lipidase, anaerobic bacteria, Aspergillus oryzae, and an enzyme complex mixture (other

hydrolytic enzymes).

** Patent

The three main limitations of current literature regarding enzymatic treatment for enhanced sludge

dewaterability are: i. Enzymatic treatment has been conducted through addition of enzyme mixtures and

although enzyme cocktails are promising, the use of mixtures hinders the understanding of the

mechanisms involved because it is unknown which of the enzymes(s) or additives in the mixture is

responsible for the improvement; ii. As can be seen in Table 2-1, research is concentrated on

31

anaerobically digested sludge from municipal wastewater treatment; iii. The influence of variables such

as, temperature, mixing rate and concentration needs further research. Overall, a better understanding of

the mechanisms involved is needed in order to find promising and low cost enzymatic treatment

alternatives for enhanced dewaterability.

Enzymes for Enhancing Biosludge Anaerobic Digestion

Anaerobic digestion of sludge is widely used in the municipal wastewater industry (Elliott &

Mahmood, 2007). It reduces pathogens, avoids potential odors, reduces the volume of sludge and recovers

energy in the form of methane (Appels et al., 2008). However, the use of anaerobic treatment is limited in

the pulp and paper mill industry due to lower digestibility and low methane yields (Meyer & Edwards,

2014). To improve the digestibility of biosludge, different pretreatments have been studied: thermal,

ultrasound, ozone, mechanical, alkaline and enzymatic pre-treatments (Elliott & Mahmood, 2007; Monte

et al., 2009).

Proteases and glucosidases have been the enzymes most studied for testing an enzymatic pre-

treatment for enhancing the digestibility of biosludge. Proteins and carbohydrates are the main

components of cell walls and in biosludge they can represent up 70% of its dry weight (Meyer &

Edwards, 2014). However, there is conflicting evidence about the positive effect of enzymatic

pretreatment on biosludge. While some authors have reported a substantial improvement in biogas

production, methane yield, and/or chemical oxygen demand (COD) solubilization (Barjenbruch &

Kopplow, 2003; Wawrzynczyk, 2007; Recktenwald et al., 2008; Yang et al., 2010), others reported

improvements only in lab-scale experiments (Karlsson et al., 2011) and others found no improvement

(Bayr et al., 2013). Thus, more research is needed to assess better the potential of enzymatic treatment for

enhancing biogas production.

Anaerobic digestion has implications on biosludge dewaterability. Anaerobically digested sludge can

exhibit poor dewatering properties when compared with aerobically digested sludge (Novak et al., 2003;

32

Ayol, 2005; Ayol & Dentel, 2005; Dursun et al., 2006). This detrimental effect has been explained by an

excess of EPS in sludge and a reduction in particle size. The excess was hypothesized to be a result of the

lack of enzymatic activity during digestion (Novak et al., 2003; Ayol & Dentel, 2005; Ayol, 2005).

However, as mentioned previously, the degree of the effect of EPS on dewaterability remains unclear.

Enzymes have also been reported to improve anaerobic digestion and the hypothesis behind this

effect is that enzymes hydrolyse compounds which results in more accessible substrates. Therefore, it is

expected that after hydrolysis, particle size in sludge will be reduced also affecting the dewaterability and

the blinding potential of sludge (Novak et al., 1988). A dual- enzymatic treatment could be proposed,

where enzymes are first added to solubilise sludge and facilitate anaerobic digestion. Then the digested

sludge will undergo a conditioning step with a second enzymatic product that will improve the dewatering

properties of sludge.

Summary and Significance of this Research

Significant progress has been made to improve the dewaterability of biosludge. Synthetic polymers

make possible to dewater sludges up to 40% with mechanical dewatering equipment. However, the

current reliance on synthetic polymers for enhancing solid-liquid separations is not desirable. Polymers

are petroleum-derived and known to be toxic to aquatic systems. Operationally, polymers are dose

sensitive and have special mixing requirements. Therefore, developing alternative conditioning treatments

that overcome these disadvantages is of interest.

There is great potential for using biomolecules for enhancing biosludge dewaterability but little is

known about their mechanism. Carbohydrates and proteins have shown flocculating activity on various

substrates and can be produced from microorganisms or extracted from waste sources. Only a few studies

of bioflocculants have reported their effect on biosludge dewatering. On the other hand, enzymes can

change the structure of biosludge by attacking molecules in flocs through their catalytic activity. These

33

changes can potentially result in improved dewaterability and anaerobic digestibility of biosludge.

However, our current knowledge on how to use proteins for enhancing biosludge dewaterability and other

liquid-solid suspensions is limited. The research described in this thesis advances the knowledge by

addressing the following knowledge gaps and research opportunities:

Studies on the use of enzymes for improved sludge dewaterability are concentrated in municipal and

anaerobically digested sludge. Although municipalities have moved towards anaerobic digestion of

biosludge prior to dewatering to reduce solids content. Pulp mills and other industries are still

dewatering biosludge (waste activated sludge). Thus, testing enzymes in different sludges such as

biosludge from pulp and paper mills will contribute to the current knowledge and understanding of

enzyme-based conditioners.

Enzymes have been found to improve sludge dewaterability. However, the mechanisms involved in

this improvement are not understood and only just a few enzymatic activities have been tested.

Moreover, enzymes have not been tested individually. This limits the ability to identify the specific

activity (ies) responsible for sludge dewaterability improvements. Thus studying more enzymatic

activities and doing so individually will allow a better understanding of the mechanisms involved

during enzymatic treatment.

There are limited studies on the use of proteins as flocculants. Several bioflocculants have been

isolated from microorganisms but the key properties that affect their flocculating potential are not

understood. For example, the effect of charge has not been evaluated.

The changes that sludge needs to undergo to see an improvement in anaerobic digestion and/or

dewaterability are not well understood. The solubilization of organic matter (i.e. floc breakage) is

important for anaerobic digestion while flocculation is currently used as the mechanism to improve

dewaterability (i.e. floc aggregation). A correlation of how these changes would affect anaerobic

digestion and dewaterability after enzymatic or protein conditioning could provide information about

the mechanisms involved.

34

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Amin, P. (2015) Primary Sludge Addition for Enhanced Biosludge Dewatering.

Appels, L., Baeyens, J., Degrève, J., & Dewil, R. (2008) Principles and potential of the anaerobic

digestion of waste-activated sludge. Progress in Energy and Combustion Science 34: 755.

Arcand, G. (1991) Dewatering pulp and paper mill sludge using the Kamyr ring press. Ottawa,

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44

3 Chapter 3 - Enhancing Pulp and Paper Mill Biosludge Dewaterability using Enzymes

This chapter is mainly based on the following article: Bonilla, S., Tran, H. and Allen, D. G. (2015),

“Enhancing the Dewaterability of Biosludge Using Enzymes”. Water Research, 68: 692-700. Some

additions were made since the publication of this article because new experiments were conducted. The

items added relate to results presented in section 3.3.9 and 3.3.10.

Accreditations:

Sofia Bonilla designed and conducted all the experiments, collected, analyzed and interpreted data, and

prepared the first draft of the manuscript.

D. Grant Allen provided advice on experimental design analysis and interpretation of data and editing of

the manuscript.

Honghi Tran provided advice on interpretation of data and editing of the manuscript.

Introduction

Biosludge, also known as waste activated sludge (WAS), is the most difficult to dewater among the

sludges produced in wastewater treatment plants in pulp and paper mills (Goodwin and Forster, 1985;

Mahmood and Elliott, 2006). To improve dewaterability, biosludge is commonly combined with primary

sludge (Dorica et al., 1999). However, this practice will become problematic given an industry-wide

tendency to reduce primary sludge production as pulping processes become more efficient, and to

produce a larger amount of biosludge as regulations become more stringent (Mahmood and Elliott, 2006).

Moreover, sludge management represents up to 60 % of the total cost of wastewater treatment, with the

liquid-solid separation efficiency during sludge dewatering defining the energy and overall costs

associated with sludge management and disposal (Ayol and Dentel, 2005; Ayol, 2005; Benítez et al.,

1994; Vaxelaire and Cézac, 2004; Wood et al., 2009). Thus, there is interest in finding new approaches

for improving biosludge dewaterability.

The high moisture content of sludge affects its downstream processing and disposal, and reduces the

possibility of recovering energy or chemicals from biosludge. In pulp and paper mills, incineration is

45

considered to be the last resort for sludge disposal (Dorica et al., 1999). While biosludge incineration can

be carried out in existing boilers, eliminating the costs of transportation, it is not cost-effective due to the

high energy cost with drying a large amount of water in biosludge. Biosludge is increasingly

acknowledged as valuable in terms of energy and chemical recovery. Overall, any improvement in

biosludge dewatering would lower the disposal cost and increase energy recovery.

Chemicals can be used to improve the solid-liquid separation of biosludge; however, the use of

chemicals has some disadvantages. For example, the addition of inorganic chemicals increases the final

sludge mass and reduces its heating value; thus, it is not a good option when sludge is to be incinerated

(Albertson et al., 1987; Bolto, 2006). Alternatively, synthetic organic polymers (polyelectrolytes) are

required in lower doses and do not reduce the heating value of biosludge. However, these polymers

represent a significant cost and are sensitive to dose rate. If the optimum dose is surpassed, sludge

dewatering becomes even more difficult, especially when considering the variable nature of sludge

(Bolto, 2006). Moreover, there are environmental concerns related to the use of synthetic polymers, as

some of these have been reported to be toxic to aquatic organisms (Bolto, 2006; Bolto and Gregory,

2007). The combination of chemical conditioning treatments and mechanical aids helps dewater

biosludge up to 40% dry solids (in exceptional cases).

Biosludge is a complex mixture of microorganisms, organic and inorganic matter (Keiding et al.,

2001; Sheng et al., 2010; Yang and Li, 2009). It has a gel-like structure due to the presence of

extracellular polymeric substances (EPS) that are produced by bacteria. The EPS assist in the aggregation

of particles in biosludge producing aggregates called flocs (Legrand et al., 1998). Flocs in biosludge are

known to carry a net negative charge making biosludge a stable suspension and hence impeding a natural

solid-liquid separation. Changing the structure of flocs could potentially improve biosludge dewatering

properties. This may include releasing the water trapped inside the flocs (Vaxelaire and Cézac, 2004)

and/or increasing the particle size of flocs to reduce the surface area available for binding of water

46

molecules. Although floc properties and their impact on biosludge dewaterability have been studied

previously, the key properties and the degree of their impact on dewaterability are still not well

understood (Jarvis et al., 2005a; Jarvis et al., 2005b; Park, 2002; Wu et al., 1998).

Enzymes are proteins with a catalytic activity and have been previously reported as conditioners for

improved sludge dewaterability. Enzymes can break EPS reportedly releasing water trapped in flocs

(Ayol, 2005; Dursun et al., 2006; Thomas et al., 1993). Poor dewatering properties in biosludge have

been attributed to the lack of enzymatic activity after sludge digestion resulting in excess EPS which can

trap water in their gel-like structure (Ayol, 2005; Novak et al., 2003). Thomas et al. (1993) used a

product with carbohydrase, lipase and protease activities on digested sludge. Their study suggested that

water-binding molecules were hydrolyzed resulting in better dewaterability. The effectiveness of another

enzymatic product which contained protease, lipidase, anaerobic bacteria, Aspergillus oryzae and other

hydrolytic enzymes was evaluated in two companion papers (Ayol and Dentel, 2005; Ayol, 2005). A

reduction of proteins and polysaccharides in sludge was also noted after enzymatic treatment. Laboratory

and pilot scale experiments have been carried out by Dursun et al., (2006) using enzymes on

anaerobically digested sludge. They found dewatering improvements in lab-scale experiments but not in

pilot experiments.

The previous studies illustrate the potential of using enzymes for improving dewatering, however, an

understanding of how enzymes change sludge structure and properties is lacking in the literature.

Enzymatic conditioners have been studied as mixtures and although these “cocktails” are promising, their

use hinders the understanding of the mechanisms involved since it is difficult to identify which of the

enzymes(s) in the mixture is contributing to a given effect. Research to date has mostly been focused in

enzymatic conditioners for improved dewaterability of anaerobically digested sludge. Little has been

studied for the use of enzymes on waste activated sludge. Moreover, the effect of conditions such as

temperature, time and mixing on the effectiveness of these conditioners has not been explored. A better

47

understanding of the changes in the physical and chemical properties of sludge during enzymatic

treatment and the associated mechanisms is important to identify key properties for improving

dewatering.

The objectives of this study were:

• To carry out an enzymatic screening to identify enzymes with potential for improving biosludge

dewatering;

• To determine the effect of enzyme treatment conditions such as concentration, time, temperature

and mixing conditions on improving biosludge dewaterability; and

• To characterize dewaterability improvements when using enzymes for biosludge conditioning.

Materials and Methods

A series of screening tests was performed to identify enzymes that have a positive effect on

biosludge dewaterability using capillary suction time (CST) as the dewatering assessment method. Of the

enzymes tested, lysozyme was the only enzyme that showed a positive effect on biosludge dewaterability.

Different concentrations of lysozyme, mixing intensities and temperatures were evaluated to identify the

optimal conditions to achieve maximal biosludge dewatering. The effect of enzymes on sludge

dewaterability was further evaluated with a bench-scale belt press and compared with the CST results.

The effect of enzymes on reducing the demand of synthetic polymer used to enhance dewatering was also

measured. Particle size distribution analysis was used to investigate structural changes in flocs due to

enzymatic treatment.

Sludge Samples

Biosludge from a secondary clarifier was obtained from a Canadian pulp and paper mill which

produces a variety of pulp, paper and specialty products using sulfite pulping and mechanical pulping

48

(bleached chemi-thermomechanical pulp- BCTMP). The sludge was kept at 4°C in the laboratory prior to

analysis for a maximum of two weeks. Biosludge was left to settle for at least 2 hours and the supernatant

discarded to obtain a thickened sludge. To re-activate the microbial community present in sludge,

thickened sludge was aerated for 1 hour and brought to room temperature before running experiments.

Total suspended solids (TSS) and pH were measured to be 15.9 (± 3.5) g/L and 7.2-7.6, respectively.

Unless otherwise stated, all experiments were carried out with this biosludge.

Additionally, a set of experiments was run on biosludge produced in a wastewater treatment plant

from the municipality of Toronto to compare the effect of lysozyme on a different sludge, and to validate

the reproducibility of the results obtained with the pulp and paper mill sludge. The sludge was kept at 4°C

and used within 4 hours of sampling. The same thickening and aeration process was used and the TSS

content was 12.1 g/L.

Experiments with sludge mixtures were conducted with primary sludge obtained from the same mill,

this sludge contained mostly hardwood BCTMP process residues. Total suspended solids ranged from 18-

42 g/L (±0.5). Mixtures of biosludge (B) and primary sludge (P) were prepared at different dry mas ratios

P:B; (1:1); (1.5:1), (2.3:1) and the volume of each mixture were adjusted to keep all sample volumes

constant.

Enzymes

A screening of commercially available enzymes was carried out to select enzymes with potential for

conditioning biosludge to improve its dewatering properties. All enzymes used in this study (described in

Table 3-1) were hydrolases (Enzyme Class 3) and represent a wide range of substrate-specificity for

biopolymers present in biosludge.

Enzyme stock solutions of 10 mg/mL were prepared with deionized water (18.2 MΩ cm) and added

to sludge to achieve the desired final concentration. For negative controls, deionized water was added to

49

the sludge instead of the enzyme solution. The same volume was added to all sludge samples treated with

and without enzymes. In the results section, Protease 1 is the enzyme from Bacillus licheniformis and

Protease 2 from Bacillus sp.

Table 3-1 Enzymes used in the screening tests of biosludge conditioning for improved

dewatering

Common Name

Enzymatic Activity*

Enzyme Class

Catalized Reactiona Organism Supplier - Cat #

Cellulase ≥0.7

units/mg 3.2.1.4

Endohydrolysis of (1→4)-β-D-glucosidic linkages in cellulose, lichenin and cereal β-D-glucans

Trichoderma reesei

Sigma-Aldrich C2730

α-Amylase ≥0.25

units/mg 3.2.1.1

Endohydrolysis of (1→4)-α-D-glucosidic linkages in

polysaccharides containing three or more (1→4)-α-linked D-glucose

units

Bacillus amyloliquefaciens

Sigma-Aldrich A7595

Protease ≥0.0024 units/mg

3.4.21.62

Hydrolysis of proteins with broad specificity for peptide bonds, and a preference for a large uncharged residue in P1. Hydrolyses peptide

amides

Bacillus licheniformis

Sigma-Aldrich P4860

Protease b ≥0.016

units/mg _ _ Bacillus sp.

Sigma-Aldrich P3111

Lysozymec ≥40,000 units/mg

3.2.1.17

Hydrolysis of (1→4)-β-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in

a peptidoglycan and between N-acetyl-D-glucosamine residues in

chitodextrins. Breaks peptidoglycan in bacteria cells.

Gallus gallus Sigma-Aldrich

L6876

Lysozyme ≥23,000 units/mg

As previous

As previous Gallus gallus Sigma-Aldrich

62970

Lysozyme ≥70,000 units/mg

As previous

As previous Gallus gallus Bioshop LYS702

a From IUBMB Enzyme Database

b No information available

c Unless otherwise stated, the lysozyme used for all the experiments was from chicken egg white (Sigma-Aldrich L6876).

* According to manufacturer’s specifications

Capillary Suction Time

Capillary Suction Time (CST) was used to evaluate the conditioning treatment of biosludge with

enzymes. CST has been widely used as a method to assess sludge dewaterability due to its correlation

with filterability and mechanical dewatering (Dentel, 1993; Jin et al., 2004; Krishnamurty and

Viraraghavan, 2005). The instrument consists of two electrodes: once the water reaches the first electrode,

50

a timer counts the seconds until the water reaches the second electrode where the timer stops. The time

required for water to travel from the first to the second electrode is the CST. A detailed description of the

CST apparatus and method can be found in (Vesilind, 1988). A lower CST implies better dewaterability.

As a baseline, the CST of pure water was found to be 5.4 (± 0.2) s.

Effect of Concentration and Enzymatic Incubation Conditions

To determine the effect of concentration on the enzymatic conditioning of biosludge, different

concentrations of enzyme ranging from 0.05% to 1.5% (w/v) were evaluated. Sludge was transferred to

50 mL falcon tubes and once the enzyme or water was added, the tubes were incubated at 37˚C and 150

rpm using an orbital shaker incubator (Amerex Gyromax 747R), and the CST was measured over time for

24 h. The optimum concentration was used for further experiments unless otherwise stated.

The effect of mixing and temperature on the enzymatic conditioning was evaluated using CST to

define the optimal conditions of the treatment. Experiments were carried out to investigate the effect of

mixing at 37˚C with the optimal concentration. Four different mixing speeds (0, 75, 150 and 200 rpm)

were used in a shaker incubator. Similarly, the effect of temperature on enzymatic treatment was

evaluated with four different temperatures (4, 23, 37 and 50°C) at a fixed mixing intensity of 150 rpm and

the optimum enzyme concentration.

Lysozyme Inactivation

To investigate the extent of the effect of enzymatic activity on the conditioning treatment, lysozyme

was added in an inactivated state to compare with results from the active enzyme. The measurement of

the activity of lysozyme was based on the change in absorbance of a Micrococcus lysodeikticus cell

suspension (Chipman and Sharon, 1969; Gorin et al., 1971; Meyer et al., 1936). The inactivation of

lysozyme was achieved by exposing the lysozyme solution to 103°C for 6 hours followed by immediate

exposure to -20°C until frozen. The enzymatic activity of lysozyme was analyzed in parallel with other

51

experiments. Absorbance values of the M. lysodeikticus cell suspension with lysozyme active and inactive

were used to confirm that the inactivation of lysozyme was successful. No reduction of absorbance was

considered to be indicative of the inactivity of the enzyme. A typical absorbance curve for active and

inactive solutions of lysozyme are shown in Appendix I.

Particle Size Distribution

The particle size distribution of biosludge after lysozyme conditioning was measured to investigate

the effect of the conditioning on the physical properties of biosludge since particle size distribution of

sludge is known to affect sludge dewatering properties. Chemical conditioners usually result in larger

particles. The particle size distribution was analyzed using a laser diffraction-based instrument

(Mastersizer S, Malvern, UK). The obscuration, which is a measure of the concentrations of particles per

analysis, was maintained at 20 (±3) % to reduce instrumental error as previously described by Guan et al.

(1998).

To detect if lysozyme was interacting with a particular size range of particles, sludge samples were

screened into four different fractions: 25-32 µm, 32-75 µm, 75-105 µm and >105 µm. Before lysozyme

treatment, sludge was fractionated using standard sieves (U.S.A. Standard Testing Sieve) following the

method previously described by Yuan et al. (2009). Each size fraction was then conditioned with active

and inactive lysozyme at its optimum dose to evaluate the changes in particle size distribution after

treatment. The particle sizer produces volume-based results which are the volume diameters assuming

that particles are spheres.

Polymer Demand

Polymer demand of enzyme-conditioned biosludge was evaluated as a measurement of

dewaterability and to investigate if a dual treatment of enzyme-polymer had a synergistic effect on the

dewaterability of biosludge. In industrial practice, biosludge is commonly conditioned with cationic

52

polymers to improve its dewatering properties. Polymer demand has been used previously as a

measurement of dewaterability (Ayol, 2005). Polymer demand tests with biosludge were performed using

a cationic, water-soluble polymer in emulsion (AXCHEM AF 4850). In accordance with the

manufacturer’s instructions, a 1% (v/v) stock solution was prepared by adding polymer to Milli-Q water

while vortexing. The solution was further mixed for 1 minute and allowed to sit for 1 hour prior to the

experiments. Lysozyme or water-only (control) treated samples were incubated for 2 hours and then

treated with different polymer doses. Sludge samples were exposed to rapid mixing using a magnetic

stirrer. Once a vortex was created, the polymer solution was added into the vortex and further mixed for

30 s. CST measurements were taken in triplicates and the optimum polymer dose was selected as the

lowest dose that resulted in the lowest CST.

Polymer demand tests on sludge mixtures were conducted with a different polymer because the

previous polymer was no longer available in the laboratory. Zetag 8185, a cationic, water-soluble polymer

had been studied in the laboratory and had shown good performance on biosludge. A 0.5% (w/v) stock

solution was prepared by adding polymer to Milli-Q water while vortexing. The solution was further

mixed for 1 minute and allowed to sit for 1 hour prior to the experiments

Mechanical Dewatering

To test the applicability of the results obtained from CST measurements to industrial practice, a

bench-scale belt press was used to assess the mechanical dewaterability of sludge samples. Sludge was

first treated with the conditioner solution (i.e. lysozyme or polymer). In the case of a combination,

lysozyme was added first and the polymer followed. Then samples were transferred to a Crown press, an

instrument that has been used by others to simulate industrial belt presses (Ayol and Dentel, 2005;

Severin et al., 1998). The sample was allowed to drain through gravity thickening for 5 min. The filtrate

was collected and the total suspended solids content (TSS) was measured. The cake formed during the

gravity thickening was then transferred to the pressing area where a schedule of 120, 150 and 200 lbs

53

(6.3, 7.9 and 10.5 psi, respectively) was applied to all samples. Each pressure was sustained for 10

seconds followed by a fast release. The total solids content (TS) of the cake was measured.

Results and Discussion

Enzyme Screening for Improved Biosludge Dewaterability

Enzymatic conditioning of biosludge with cellulase, proteases and amylase resulted in similar poor

dewaterability (Figure 3-1). Dewaterability was not significantly affected at low doses (0.05-0.5%) and

increasing doses resulted in a negative effect (increase in CST) on dewaterability. When compared with

the control (no enzyme) and at a dose of 0.5%, a significant negative effect was observed for cellulase,

amylase and protease 1 (p <0.05). For protease 2, no significant effect was observed at 0.5% (p= 0.1).

Figure 3-1 Biosludge dewaterability assessment using capillary suction time (CST) after

different enzymatic treatments over a range of enzyme doses (0.05-1.5%). Lower CST means

better dewaterability. CST values correspond to incubation times of 90 min. Note the break in the

X axis due to log scale. Error bars (not always visible within the symbol) show standard deviation

of triplicates.

0

5

10

15

20

25

30

35

0.0001 0.001 0.01 0.1 1 10

Capill

ary

Suction T

ime (

s)

Enzyme dose (%)

Cellulase Alpha-amylase

Protease 1 Protease 2

Lysozyme Control

H2O

54

This is contrary to previous reports by DeLozier and Holmes (2008) and Sarkar et al. (2005); which

found that conditioning with amylase and cellulase resulted in an improvement on sludge dewaterability.

Ayol and Dentel (2005) reported an improvement in sludge dewaterability after conditioning with an

enzymatic product. The differences in the results observed between previous reports and this study are

probably due to the different conditions used e.g. enzymatic product, incubation time, dose and sludge

type. It is also possible that the substrates of the enzymes investigated are not available in the sludge.

Some of the enzymes that did not show positive results in this study could potentially enhance dewatering

in combination with other enzymes, treatments or under different treatment conditions. Moreover, there

are several enzymes that have not been studied as enzymatic conditioners which could potentially

enhance sludge dewaterability.

Lysozyme was the only enzyme in the screening that resulted in improved biosludge dewaterability

(Figure 3-1). The first improvement in dewaterability was evident at a dose of 0.05% reducing the CST

from 16 to 14.4 s. The optimum concentration was found to be 0.5% where lysozyme treatment reduced

the CST to 10 s and was significantly different from the control (p <0.001). A higher concentration

(1.5%) resulted in a reduction of the positive effect observed at lower doses, resulting in a CST of 12.9 s

which was comparable to the CST obtained with a dose of 0.15%. This overdose effect is consistent with

previous reports where high enzyme doses resulted in a negative effect on sludge dewaterability (Ayol,

2005; Thomas et al., 1993). It was hypothesized in those reports that the enzyme overdose was the result

of excess hydrophilic groups which had a detrimental effect on the dewatering properties of sludge. A

similar overdose effect is also typical of synthetic organic conditioners (polyelectrolytes). Synthetic

polymer overdoses have been mainly attributed to two mechanisms: an excess in the charge needed to

neutralize taking the particles to their initial state of repulsion, also known as, charge reversal and/or an

increase in viscosity (Dentel, 1993).

55

Effect of Incubation Time of Lysozyme’s Conditioning Treatment

Most of the effect of lysozyme on biosludge dewaterability occurred rapidly and no significant

change was observed after 90min of treatment (Figure 3-2). The maximum change (i.e. reduction in CST)

occurred during the first 30 min of treatment; for lysozyme CST was reduced from 20.3 to 11.3 s at its

optimum dose (0.5%). After 90 min, 88% of the total reduction (CST = 10 s) was achieved, no further

significant change was observed despite an additional 20 hours of incubation. Previous enzymatic

conditioning reports have used incubation times of 16 hours (Ayol, 2005; Dursun et al., 2006; Thomas et

al., 1993). Our results, therefore, represent a significant reduction in conditioning time, since they suggest

that only 90 min were required to obtain the maximum effect of lysozyme as a biosludge conditioner.

Effect of Incubation Temperature and Mixing on Biosludge Conditioning with Lysozyme

Lysozyme conditioning for improving the dewaterability of biosludge was positively affected by

increasing temperatures (i.e. higher temperatures resulted in reduced CST) using the optimum dose, i.e.

0.5% (Figure 3-3). The effect of temperature on the sludge dewatering with no enzyme was not

0

5

10

15

20

25

0 100 200 300 400

Capill

ary

Suction T

ime (

s)

Incubation Time (min)

0.005 0.05 0.15 0.5 1.5 0

H2O

1200 0 100 300 200

Figure 3-2 Effect of incubation time on biosludge dewaterability using different doses (%) of

lysozyme. Note the break in the X axis. Error bars show standard deviation of triplicates.

56

significant at the 95% confidence (p = 0.1). However, for the lysozyme treated biosludge, increasing

incubation temperature resulted in a significant decrease in CST values (p < 0.001). For the treatment at

4˚C, the CST was reduced from 15.7 to 12.8 s, while the lysozyme treatment at room temperature (23˚C)

reduced the CST from 15.5 to 9.9 s. As shown in Figure 3-3, at 37 and 50˚C, lysozyme treatment reduced

CST from 15.7 to 9.4 s and 16.3 to 8.3 s, respectively. Mixing had a very limited effect on the CST, being

slightly negative with no enzyme (p < 0.005) and slightly positive with lysozyme (p < 0.001).

Figure 3-3 Effect of lysozyme treatment conditions on biosludge dewaterability as capillary suction

time (CST); a) effect of temperature; b) effect of mixing rate. Lysozyme was added at a dose of

0.5% and CST was measured after 2 hours of treatment.

The small effect of mixing and temperature over the ranges studied has practical and mechanistic

consequences. From a practical perspective, the results suggest that one can provide a consistent dose of

enzyme regardless of fluctuating operating temperatures and that energy intensive mixing is not required.

The absence of a substantial mixing effect suggests that the changes that sludge undergoes with lysozyme

addition are neither reversible nor enhanced by the shear forces during mixing.

Effect of the Enzymatic Activity of Lysozyme on Biosludge Conditioning with Lysozyme

Surprisingly, the activity of the enzyme had a negligible effect on the degree of enhanced

dewaterability. Active and thermo-inactive preparations of lysozyme at a dose of 0.5% showed similar

0

5

10

15

20

0 20 40 60

Capill

ary

Suction T

ime (

s)

Temperature (˚C)

Lysozyme

No Enzyme 0

5

10

15

20

25

0 100 200 300

Capill

ary

Suction T

ime (

s)

Mixing Rate (rpm)

Lysozyme

No Enzyme

ba

a

)

b

)

57

results (Figure 3-4). The same trend was observed for both sludges studied. Similarly, adding the same

mass of enzymes but using different concentrations of active units (i.e. 23,000, 40,000 and 70,000

units/mg) had a negligible effect on the improvement in dewaterability (Figure 3-5). Thus, the

improvement on biosludge dewaterability does not appear to be the result of an enzymatic reaction, rather

it is the result of other physicochemical interactions between lysozyme molecules and particles present in

sludge.

Figure 3-4 Capillary suction time of biosludge conditioned with active and inactive lysozyme as a

function of time, a) Pulp and paper mill biosludge and b) Municipal biosludge. Error bars (not

always visible) show standard deviation of triplicates.

Different doses of active and inactive lysozyme solutions were further used to evaluate the change in

biosludge dewaterability (Figure 3-6). Inactive lysozyme solutions resulted in equal or better conditioning

performance at most concentrations. Active and inactive preparations of lysozyme resulted in similar

dewaterability improvements when doses of 0.2 to 0.6% were used. Inactive lysozyme does not show an

“overdose” effect where decreased dewaterability is observed, but rather seems to stabilize at a point

where no extra improvement occurs. Although lysozyme does not improve sludge dewatering due to its

enzymatic activity, it is very likely that other enzymes affect sludge (enzymatically) enhancing its

0

5

10

15

20

25

30

35

0 50 100 150 200

Capill

ary

Suction T

ime (

s)


No Enzyme

Active Lysozyme

Inactive Lysozyme0

5

10

15

20

25

30

35

0 50 100 150 200

Capill

ary

Suction T

ime (

s)


No Enzyme

Active Lysozyme

Inactive Lysozyme

58

dewatering properties. Thus, future studies of enzymatic conditioners for enhanced sludge dewaterability

should include the use of inactive enzymes in order to identify their mechanism of action on sludge.

Figure 3-5 Effect of enzymatic active units on the treatment of lysozyme for improved sludge

dewaterability measured via capillary suction time (CST). Error bars (not always visible) show

standard deviation of triplicates.

Figure 3-6 Capillary suction time of biosludge with different doses of active and inactive lysozyme

after 90 min of incubation. Error bars (not always visible) show standard deviation of triplicates.

Two x-axis to show what units in w/v % translate to kg of enzyme / dry tonne (DT) sludge.

0

5

10

15

0 0.5 1 1.5 2

Capill

ary

Suction T

ime (

s)

Incubation Time (h)

No Enzyme

23,000 U/mg

40,000 U/mg

70,000 U/mg

0

2

4

6

8

10

12

14

0 0.2 0.4 0.6 0.8 1 1.2

Capill

ary

Suction T

ime (

s)

Enzyme Dose (%)

Active Lysozyme

Inactive Lysozyme

0 50 100 150 200 250 300 350 400 450

Capill

ary

Suction T

ime (

s)

Enzyme Dose (kg/DT)

0

2

4

6

8

10

12

14

0 0.2 0.4 0.6 0.8 1 1.2

Capill

ary

Suction T

ime (

s)

Enzyme Dose (%)

Active Lysozyme

Inactive Lysozyme

59

Effect of Lysozyme on the Particle Size Distribution of Biosludge

Lysozyme conditioning changed the particle size distribution of biosludge to a larger size range

(Figure 3-7). The volume mean in all the fractions studied was higher for lysozyme-treated sludge

independent of the activity of the lysozyme (Table 3-2). Similar differences were observed for active and

inactive lysozyme suggesting that the aggregation of particles was related to the dewaterability

improvements. Lysozyme treatment clearly results in a less turbid supernatant independent of the

enzymatic activity (Figure 3-8).

Figure 3-7 Particle size distributions of sludge fractions before and after treatment with active and

inactive lysozyme; a) Fraction 1 (25-32 µm); b) Fraction 2 (32-75 µm); c) Fraction 3 (75-105 µm)

and d) Fraction 4 (> 105 µm).

0

5

10

15

0 50 100 150

Volu

me (

%)

Particle Diameter (µm)

No Enzyme

Active Lysozyme

Inactive Lysozyme

0

5

10

15

0 100 200 300

Volu

me (

%)


No Enzyme

Active Lysozyme

Inactive Lysozyme

0

5

10

15

0 100 200 300 400 500

Volu

me (

%)


No Enzyme

Active Lysozyme

Inactive Lysozyme

0

5

10

15

0 100 200 300 400

Volu

me (

%)


No Enzyme

Active Lysozyme

Inactive Lysozyme

a) b)

c) d)

60

Table 3-2 Mean diameter of sludge fractions after treatment.

Mean Diameter (µm)

Fraction # Size Range µm No Enzyme Active Lysozyme Inactive Lysozyme

1 25-32 µm 30.84 33.5 33.4

2 32-75 µm 49 61.5 60.1

3 75-105 µm 67.8 88.3 85.5

4 > 105 µm 71.4 87.2 86.8

Figure 3-8 Supernatant of biosludge after centrifugation. Left to right correspond to conditioning

treatments with: no enzyme, active lysozyme and inactive lysozyme.

Polymer Demand after Lysozyme Treatment

Lysozyme conditioning reduced the polymer demand of biosludge (Biosludge without lysozyme

conditioning needed a polymer dose of 11% (v/v) to achieve the lowest CST while the addition of active

and inactive lysozyme achieved the lowest CST with a polymer dose of 6% (v/v). After a polymer

addition of 6%, the active and inactive lysozyme reached a CST of 6.4 and 5.7 s, respectively. The CST

of biosludge with no lysozyme decreased from 16.3 s to 6 s after a dose of 11% of the same polymer

solution. No synergistic effects were observed with the dual conditioning of lysozyme and polymer when

compared with polymer-only treatment. There is a clear overdose effect with polymer addition, after

which dewaterability becomes poorer (higher CST). This overdose is also evident in the case of lysozyme

treated sludge.

No

Lysozyme

Active

Lysozyme Inactive

Lysozyme

61

Figure 3-9 Polymer demand after treatment with no enzyme, active and inactive lysozyme. Lowest

polymer dose to obtain lower CST values indicate the optimum. Polymer doses (%) are from a 1%

stock solution. Error bars show standard deviation of triplicates.

Mechanical Dewatering after Lysozyme Conditioning

The concentration of total suspended solids (TSS) present in the filtrate after gravity thickening was

found to be significantly lower for sludge treated with lysozyme than for untreated sludge. Gravity

thickening was carried out using the gravity filtration set up in the crown press apparatus with the filtrate

collected for TSS analysis. In practice, this filtrate is recycled to the wastewater treatment process and

thus, reducing the organic load in the filtrate is important to minimize organic load returning to the

system. TSS in the filtrate of sludge with no enzyme after gravity thickening were 4.9 g/L and for sludge

treated with active and inactive lysozyme were 3.5 and 3.1 g/L, respectively. This reduction of TSS in the

filtrate by 29 and 38% for active and inactive lysozyme treated sludge, respectively, is further evidence of

the potential of lysozyme as a biosludge conditioner.

The dry solids content of the cake was significantly improved from 5.8% with no enzyme to 8.9%

with active lysozyme and 9.4% with inactive lysozyme and no significant difference was found between

active and inactive lysozyme with a confidence level of 95% (Figure 3-10). Additionally, lysozyme

treatment resulted in the same increase in dry solids as with the polymer treatment. Results from CST and

0

10

20

30

40

50

60

0 5 10 15 20

Capill

ary

Suction T

ime (

s)

Polymer Dose (%)

No Enzyme

Active Lysozyme

Inactive Lysozyme

62

dry solids after mechanical pressing were compared to assess the validity of using CST for screening. Dry

solids content after mechanical dewatering was found to be consistent with CST assessments. Both results

confirm the dewaterability improvement when biosludge is conditioned with lysozyme and that there is

no significant difference in this effect between active and inactive lysozyme.

Figure 3-10 Cake solids after mechanical dewatering using the crown press (Left Y axis). Capillary

suction time before mechanical dewatering (Right Y axis). Error bars show standard deviation of

triplicates.

Lysozyme Mechanism

It is hypothesized that the effect of lysozyme on sludge is a result of lysozyme’s cationic net charge.

The isoelectric point (pI) of lysozyme is 10.5-11 (Salton, 1957). At the pH of biosludge, lysozyme carries

a positive charge which can interact with net negative charged particles in sludge. This interaction would

reduce repulsion and aid the aggregation of particles, as observed in the particle size distribution data and

turbidity experiments. Thus, lysozyme appears to act similarly to a cationic polymer.

Effect of Lysozyme on the Dewaterability of Sludge Mixtures

In primary sludge/biosludge mixtures, lysozyme shows conditioning potential at significantly lower

doses than the doses needed to achieve optimum results for biosludge-only samples. The optimum dose of

lysozyme in biosludge-only samples is ~ 0.5% w/v (as shown in Figure 3-6) (equivalent to 200 (±50) kg

0

2

4

6

8

10

12

14

16

0

2

4

6

8

10

12

No Enzyme Polymer ActiveLysozyme

InactiveLysozyme

Cap

illa

ry S

uctio

n T

ime

(s)

Dry

So

lids a

fte

r B

elt P

ress (

%)

63

of lysozyme/ DT of biosludge). An effective dose of lysozyme is cut by approximately 1/5 (45 kg of

lysozyme/ DT of biosludge) when lysozyme is used to condition primary/biosludge mixtures (Figure 3-

11).

Our results suggest that by adding primary to biosludge sample the need for lysozyme conditioning

is reduced which has economic implications. Primary sludge is currently used in industry to improve the

dewatering properties of biosludge. A dose reduction is in agreement with the proposed mechanism of

proteins (i.e. charge neutralization). Since the dose reduction appears to not only be the result of less

biosludge in a given sample (e.g. 50 % biosludge in mixture leads to 50% dose reduction), these results

suggest that there may be some synergies when using primary sludge and protein to enhance sludge

dewaterability. However, results shown in Figure 3-11 are based on CST and is important to note that

these values could be misleading because samples are different sludge mixtures and have different solids

content which affects CST values.

A combination of lysozyme and synthetic polymer (Zetag 8185) resulted in similar dewatering

properties (CST values and dry solids) as the sample treated with polymer only (Figure 3-12 and 3-13).

These results demonstrate that the polymer demand is also reduced in sludge mixtures. More importantly,

0

50

100

150

200

250

300

100% 50% 40% 30%

Lys

ozym

e D

ose (

kg/D

T)

Biosludge in Mixture

Figure 3-11 Optimal lysozyme doses (kg/DT) for biosludge and sludge mixtures with primary

sludge determined by capillary suction time. Error bars show standard deviation over at least 3

experiments.

64

the total dose of conditioner added (25 kg/DT) is comparable to the dose of polymer only (20 kg/DT).

Dry solids content after mechanical dewatering showed that this effect is also observed in the cake solids

content where samples treated with 40 kg/DT of lysozyme, 20 kg/DT of polymer or a combination of

12kg/DT of lysozyme and 13 kg/DT of polymer yielded similar dry solids content of 21 (±1) %. These

results show that one could reduce the use of polymer by adding lysozyme. A combination of lysozyme

and polymer could be a more environmental friendly solution.

Figure 3-13 Capillary suction time of mixed sludge (50% primary and 50% Biosludge) with

different doses of polymer. A sample with no enzyme and a sample with 12 kg/DT of lysozyme.

0

5

10

15

20

25

Untreated Lysozyme (40kg/DT)

Polymer(20 kg/DT)

Lys+Polymer(12 kg Lys + 13

kg Pol/DT)

% D

ry S

olid

s C

onte

nt

b

0

5

10

15

20

0 5 10 15 20 25 30 35

Capill

ary

Suction T

ime (

s)

Polymer Dose (kg/DT)

No Lysozyme

Lys 12 kg/DT

Figure 3-12 Dry solids content after mechanical pressing of sludge mixture (50% primary, 50%

biosludge) after different conditioning treatments. Error bars show standard deviation of

triplicates. Different letter show statistically significant differences.

a

b b b

65

Conclusions

A comparison of the potential of five hydrolases (cellulase, amylase, two proteases and

lysozyme) as sludge conditioners for improving dewatering showed that only lysozyme

improved sludge dewaterability.

Lysozyme addition resulted in an increase of dry solids content after mechanical dewatering.

A dual-conditioning with lysozyme and polymer showed similar dewatering properties as

polymer treated sludge.

Both active and inactive lysozyme increased the particle size in sludge suggesting a

mechanism similar to that of flocculants, such as cationic polymers.

There is an optimum concentration of lysozyme for enhancing sludge dewaterability.

Within the ranges studied, different mixing and temperature conditions did not seem to have a

major role in the conditioning treatment with lysozyme.

For sludge mixtures (primary sludge/biosludge), the optimum dose is dramatically reduced

from ~200 kg/DT for biosludge only to ~45kg/DT for sludge mixtures.

A polymer/lysozyme dual treatment of sludge mixtures shows similar improvements in

dewaterability as polymer-only conditioning. The total dosage needed to achieve a 21% dry

solid cake is 25 kg/DT for polymer/lysozyme conditioning (12 kg/DT lysozyme and 13 kg/DT

polymer) and 20 kg/DT for polymer-only conditioning.

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66



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Severin, B. F., Prindle, G., & Traynor, G. (1998) Belt Press Dewatering: Laboratory Simulation of the

Pressure Rollers. Environ Technol 19: 697.

Sheng, G., Yu, H., & Li, X. (2010) Extracellular polymeric substances (EPS) of microbial aggregates in

biological wastewater treatment systems: A review. Biotechnol Adv 28: 882.

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Thomas, L., Jungschaffer, G., & Sprossler, B. (1993) Improved Sludge Dewatering by Enzymatic

Treatment. Water Science and Technology 28:

Vaxelaire, J., & Cézac, P. (2004) Moisture distribution in activated sludges: a review. Water Res 38:

2215.

Vesilind, P. A. (1988) Capillary Suction Time as a Fundamental Measure of Sludge Dewaterability.

Journal (Water Pollution Control Federation) 60: 215.





Yang, S., & Li, X. (2009) Influences of extracellular polymeric substances (EPS) on the characteristics of

activated sludge under non-steady-state conditions. Process Biochemistry 44: 91.

Yuan, Y., Ndoutoumve, J. F., Siew, M., Vo, O., & Farnood, R. (2009) Sizing of Wastewater Particles

Using the Electrozone Sensing Technique. Particul Sci Technol 27: 50.

69

4 Chapter 4 - Novel Enzymes for Enhancing Biosludge Dewaterability

Introduction

Enzymes have shown potential as conditioners for improving biosludge dewaterability but the vast

majority of enzymes available remains unexplored. While in nature there is an immense diversity of

enzymes, only a few enzymatic activities (i.e. cellulase, amylase, protease and lysozyme) have been

studied as potential biosludge conditioners (Sarkar et al., 2003; Ayol & Dentel, 2005; Sarkar et al., 2005;

Dursun et al., 2006; DeLozier & Holmes, 2008; Bonilla et al., 2015). Recent molecular biology advances

have made possible the discovery of numerous new, uncharacterized enzymes; some of which could have

potential for improving biosludge dewaterability. These new, uncharacterized enzymes will be referred in

this document to as “novel enzymes”. Our research group is part of BioZone, a research Centre in the

Department of Chemical Engineering and Applied Chemistry at the University of Toronto. BioZone has

access to a library of novel enzymes that could be explored to find enzymes with the potential to improve

biosludge dewatering.

Because novel enzyme production takes place in our laboratory, potential secondary effects of

unknown chemicals present in commercial preparations could be removed. Commercial enzymes are a

reasonable first approach for investigating the potential of enzymatic treatment to improve biosludge

dewaterability. This has been the case both in the literature and in previous studies in this thesis (Chapter

3). However, these preparations contain unknown chemicals that could have secondary effects on

biosludge dewaterability and hinder our ability to discern between enzymatic effects and other possible

effects. Thus, to fully understand the effect of enzymatic activity on biosludge dewaterability and the

changes that biosludge undergoes during treatment, the use of novel enzymes, produced in our laboratory,

can provide advantages over commercial preparations.

70

The main objective of this study was to assess the potential of novel enzymes for enhancing

biosludge dewaterability and simultaneously, potential secondary effects from buffers and unknown

chemicals in the enzyme solutions can be reduced. The specific objectives were:

Perform a screening of various novel hydrolases for their potential to improve biosludge

dewaterability.

Scale-up production of the novel enzymes with potential for improving biosludge dewatering.

Develop a new methodology to reduce the effect of chemical additives in enzyme solutions.

Evaluate the effect of novel enzymes on the protein, chemical oxygen demand and carbohydrate

content of biosludge during enzymatic treatment.


Enzyme Production

From a preliminary screening, six novel enzymes were selected to conduct further experiments and

assess their potential as biosludge conditioners. Novel enzymes were produced “in-house” using

recombinant strains of Escherichia coli containing the gene that expresses the enzyme of interest; these

clones were available in BioZone. The production of novel enzymes was carried out as described by

Gonzalez et al., (2006), with a few modifications. The recombinant plasmid (p15TvL) containing the

coding gene for the His-tagged proteins was transformed into Escherichia coli strains (BL21) for

overexpression. Enzymes were first produced using the E. coli clones and growing them in small scale (1

L flasks) to test their expression and feasible protein purification. Once the feasibility of the protein

expression and purification was verified, large scale batches of E. coli were grown in an 80 L fermenter

followed by large scale protein purification. More information about the novel enzymes included in this

study are presented in Table 4-1.

71

Table 4-1 Novel enzymes used in the screening of biosludge conditioning for improved

dewatering

Enzyme ID Enzymatic Activity Swissprot -

Annotation # Organism

NE1796 Esterase/lipase/thioesterase Q82TL1 Nitrosomonas europaea

OLEI4758 Esterase n/a Oleispira antarctica

ATC1791 Esterase (Alpha/beta

hydrolase) A9CIK7

Agrobacterium tumefaciens

C58

BSU3124 Oligo-1,6-glucosidase 3 O05242 Bacillus subtilis

BSU3441 Protease P32959 Bacillus subtilis

PP1043 Phosphatase Q88P10 Pseudomonas putida

Pilot Scale – 80L Reactor

To facilitate the large scale production of the enzymes, E. coli was grown using an 80 L bioreactor.

Using Luria broth (LB) as the starter medium, an inoculum of the strain in question was grown overnight

in the presence of ampicillin and kanamycin (0.1 g/L and 0.05 g/L, respectively) at 37 ˚C and 200 rpm.

On the next day, the fermenter containing sterile Terrific broth medium (TB), and the same concentration

of antibiotics, was inoculated with 1% of the total volume. The pH of the reactor was kept between 6.8

and 7.2, it was adjusted when necessary by automatic addition of 6 N hydrochloric acid and 6 N sodium

hydroxide. The dissolved oxygen of the reactor was kept at a minimum of 30% air saturation when

possible by manually adjusting the incoming air flow. Antifoam was added at the beginning of the run at

a concentration of 0.01% v/v and it was further added in 1 ml doses if necessary during the fermentation.

Incubation conditions were kept at 37 ˚C and 200 rpm and optical density (OD) was monitored overtime.

When the OD of the culture reached 0.8-1, 0.4 mM of Isopropyl β-D-1-thiogalactopyranoside (IPTG) was

added to induce the culture to express the protein of interest. At this point, the incubation temperature was

set at 16 ˚C to reduce the action of proteases and ensure proper protein folding. The culture was grown

overnight and on the next day it was harvested using a continuous centrifuge. The pellet collected after

centrifugation was stored at -20 ˚C for subsequent protein purification. Additional information on the

large-scale fermentation process can be found in Loo-Yong-Kee (2015) and Hamemeh (2016).

72

Enzyme Purification

The protein of interest was purified using affinity chromatography. The His-tag in the protein of

interest allowed for a selective and robust purification process as previously reported (Lichty et al., 2005).

First, frozen cell pellets were thawed and re-suspended in a “binding” buffer containing 50 mM HEPES,

pH 7.5, 250 mM NaCl, 5 mM imidazole and 5% glycerol. The cell suspension was then sonicated to

disrupt the cell membrane and release the intracellular material where the protein of interest would be

found. Sonication was carried out at an amplitude of 100 for 25 minutes using a pulse of 5 seconds on and

5 seconds off to avoid overheating the sample. Cell suspensions were always on ice to avoid denaturation

and reduce any proteolytic activity. The disrupted cell suspension was immediately centrifuged at 21,000

g for 45 min at 4 ˚C. The supernatant was kept on ice and 50% Ni-NTA resin (affinity chromatography

media, Qiagen) was added (4ml of resin per 1L of initial cell culture). A sample of the supernatant was

saved for gel electrophoresis. After 30 min of resin-supernatant contact, the mixture was transferred to a

chromatography column and drained. A sample of the flow-through was saved for gel electrophoresis.

The resin was then washed with 5 column volumes of a buffer containing 50 mM HEPES, pH 7.5, 250

mM NaCl, 30 mM imidazole and 5% glycerol. The purified protein was eluted by adding small amounts

of elution buffer (50 mM HEPES, pH 7.5, 250 mM NaCl, 250 mM imidazole and 5% glycerol) and

monitoring protein concentration by a visual Bradford assay (qualitatively). Once all the protein was

eluted and collected, the protein concentration was measured quantitatively using the Bradford reagent

and the protein was stored at -80 ˚C after the drop flash-freeze method.

To confirm that the protein of interest was purified and the purification process was successful, the

eluted protein sample was run on a SDS gel along with the first supernatant and the flow through

collected during the purification process. A single protein band with the expected protein size in the

eluted sample was indicative of a successful purification process.

73

Protein Dialysis

Protein solutions were dialyzed to reduce the concentration of organics and salts present in the

protein solution due to the elution buffer. These chemicals could affect the dewaterability of sludge and

therefore, our ability to identify potential novel enzymes that improve biosludge dewatering. Protein

solutions (7.5 ml) were added to Vivaspin® 15R centrifugal concentrator with molecular weight cut-off

of 5kDa. Each tube was filled with dialysis buffer (10 mM HEPES and 50 mM NaCl). The tubes were

centrifuged at 5,000 g for 30 min. The control was prepared by adding the elution buffer used in the

purification process instead of the protein solution. Protein and chemical oxygen demand (COD) were

measured of the solutions before and after dialysis.

Dialyzed enzyme solutions were added to biosludge at different concentrations to evaluate their

potential as conditioners for enhancing dewaterability. Doses of each enzyme ranging from 0.01% to 0.5

% w/v were tested on biosludge. Samples were incubated at 37 ˚C and mixed at 100 rpm. Capillary

suction time (CST) was measured over time and samples were taken during the experiment to evaluate

changes in soluble COD, protein and carbohydrate content. The latter analyses were carried out to

identify changes in the biosludge that could be related to enhanced dewaterability.

Chemical Composition of Biosludge during Enzymatic Treatment

To understand if the changes that sludge undergoes during enzymatic treatment were consistent with

the known activities of the enzymes studied, chemical oxygen demand, protein and carbohydrate content

in the soluble portion of biosludge were measured during the experiments. Biosludge samples were

filtered using a syringe filter with a pore size of 0.45 µm. The filtrate was considered the soluble portion

and was used for all analyses. Lysozyme was added to this set of experiments as it was the only enzyme

that showed improved dewaterability from the screening with commercial enzymes (Chapter 3), thus

acted as a positive control for our experiments.

74

Chemical Oxygen Demand (COD)

Chemical oxygen demand (COD) analysis was carried out according to the Standard Methods for the

Examination of Water and Wastewater closed reflux, colorimetric method (5220 D).

Proteins and Carbohydrates

The soluble fraction was then used to determine the protein content using the bicinchoninic acid

(BCA) method with a kit from Sigma-Aldrich and carbohydrate content in samples was evaluated using

the phenol sulfuric method (Dubois, et al. 1956). Calibration curves were prepared using Bovine Serum

Albumin (BSA) for the BCA method and glucose for the phenol-sulfuric method.

Dewaterability Assessment – Capillary Suction Time

Capillary Suction Time (CST) was used to evaluate the conditioning treatment of biosludge with

enzymes. In the CST apparatus, sludge is poured into a reservoir and water travels through a filter paper.

The instrument consists of two electrodes: once the water reaches the first electrode, a timer counts the

seconds until the water reaches the second electrode where the timer stops. The time required for water to

travel from the first to the second electrode is the CST. A lower CST implies better dewaterability. As a

baseline, the CST of pure water was found to be 5.4 (± 0.2) s.


Effect of Incubation Time on the Dewaterability of Biosludge

treated with Novel Enzymes

The methodology used to assess the effect of novel enzymes on biosludge dewaterability was

successfully validated using lysozyme. Lysozyme showed an improvement in biosludge dewaterability as

has been previously reported (Bonilla et al., 2015) (Figure 4-1). The maximum effect of lysozyme was

found after 90 min of incubation and no significant change was observed thereafter.

75

Treating biosludge with BSU3124 (glucosidase), PP1034 (phosphatase) and BSU3441 (protease) did

not improve its dewaterability but instead, at some doses, it had a negative effect (Figure 4-2). Treatment

with BSU3124 at a dose of 0.5%, increased the CST to 15.1 s while the control had a CST of 9.5 s.

Similar results were observed for PP1034 at a dose 0.5%. After 60 min, samples showed a CST of 14.4 s

while the control was only 12.4 s. The negative effect on biosludge dewaterability of BSU3441 was

observed at a dose of 0.4% with a CST of 14.2 s while the control had a CST of 10.6 s.


concentrations (0, 0.05, 0.1 and 0.5 %) of lysozyme. Error bars represent the standard deviation of

triplicates.

None of the esterases (OLEI4758, ATC1791 and NE1796) showed a significant effect on the

dewaterability of biosludge under the conditions studied (Figure 4-2). Five minutes after adding the

enzymes, the CST increased with increasing enzyme concentration, after this period, there was no

significant difference between the enzyme treated samples and the control (no enzyme). Only OLEI4758

with a concentration of 0.1% showed a slight decrease in CST, but practically, a reduction of CST of 1.5

seconds may be the result of the instrument’s variability and not a real improvement in biosludge

dewaterability.

0

5

10

15

20

0 100 200 300 400

Cap

illa

ry S

uctio

n T

ime

(s)

Incubation time (min)

No Enzyme

Lys 0.05

Lys 0.1

Lys 0.5

76

0

5

10

15

20

0 100 200 300

Ca

pill

ary

Su

ctio

n T

ime

(s)


No Enzyme

BSU3124 0.01

BSU3124 0.05

BSU3124 0.1

BSU3124 0.50

5

10

15

20

0 100 200 300

Ca

pill

ary

Su

ctio

n T

ime

(s)


No Enzyme

PP1034 0.01

PP1034 0.05

PP1034 0.1

PP1034 0.5

0

5

10

15

20

0 100 200 300 400

Cap

illa

ry S

uctio

n T

ime

(s)


No Enzyme

BSU3441 0.2

BSU3441 0.410

15

20

25

0 100 200 300 400

Cap

illa

ry S

uctio

n T

ime

(s)


No Enzyme

OLEI4758 0.01

OLEI4758 0.05

OLEI4758 0.1

OLEI4758 0.5

0

5

10

15

20

25

0 100 200 300 400

Cap

illa

ry S

uctio

n T

ime

(s)


No enzyme

NE1796 0.01

NE1796 0.025

NE1796 0.05

NE1796 0.1

0

5

10

15

20

0 100 200 300 400

Cap

illa

ry S

uctio

n T

ime

(s)


No Enzyme

ATC1791 0.01

ATC1791 0.05

ATC1791 0.1

ATC1791 0.5

a) b)

c) d)

e) f)

Figure 4-2 Effect of incubation time on the dewaterability of biosludge treated with

different concentrations of enzymes; a) BSU3124, b) PP1034, c) BSU3441, d) OLEI4758, e)

NE1796 and f) ATC1791. Error bars represent the standard deviation of triplicates.

77

Nonetheless, the slight decrease in CST with OLEI4758 (0.1%) was further investigated using

anaerobically digested sludge. One of the limitations of CST is that if initial CST values are low, the

method becomes less sensitive for detecting potential improvements in dewaterability. This could have

been the case of biosludge treated with OLEI4758 (Figure 4-2d). Therefore, anaerobically digested (AD)

sludge from the City of Toronto was used to verify the results obtained during enzymatic treatment with

OLEI4758. AD sludge had an initial CST of 179 s which would likely allow us to detect improvements

(i.e. CST reduction). However, no significant improvement was observed with OLEI4758, on the

contrary, CST steadily increased with increasing enzyme concentrations. Samples treated with lysozyme

resulted in a CST reduction to 103 s at a dose of 0.5%. At the same dose, OLEI4758 increased CST to

242 s (i.e. worsening dewaterability).

Figure 4-3 Effect of enzyme dose of OLEI4758 and lysozyme on the dewaterability of anaerobically

digested biosludge. Error bars represent the standard deviation of triplicates

The effect of enzymes in this study can be classified into three groups: positive effect on

dewaterability, negative effect on dewaterability and no effect on dewaterability (Figure 4-4). For

lysozyme, a negative correlation between enzyme dose and CST was significant with p < 0.05. The

0

50

100

150

200

250

300

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Capill

ary

Suction T

ime (

s)

Enzyme dose (%)

OLEI4758

Lysozyme

78

enzymes that showed a negative effect on biosludge dewaterability showed positive correlation of enzyme

dose and CST with p < 0.05. Three of the enzymes studied, from the group of esterases which did not

appear to affect the dewaterability of biosludge, did not show any significant correlation between enzyme

dose and CST.

.

Figure 4-4 Effect of enzyme dose on the dewaterability of biosludge after 3.5 h of enzymatic

conditioning. Lines show trend of positive ( ), neutral ( ) and negative ( ) effect.

Error bars represent the standard deviation of triplicates.

As shown in Table 4-2, enzymes included in this study have different characteristics as a result of

their amino acid sequences. Lysozyme, the only enzyme that improved biosludge dewatering has a unique

property among this group of enzymes i.e. high isoelectric point (pI). On the other hand, the enzymes that

showed a negative effect on dewaterability have the low pI. Proteins with low pI would carry a negative

charge which is known to have a detrimental effect, however, the extent of this potential negative effect is

unknown.

0

2

4

6

8

10

12

14

16

18

0 0.1 0.2 0.3 0.4 0.5 0.6

Ca

pill

ary

Su

ctio

n T

ime

(s)

Enzyme Dose (%)

BSU3441 Lysozyme BSU3124

ATC1791 NE1796 OLEI4758

PP1034

79

Table 4-2 General properties of enzymes included in this study

Enzyme Activity Size (kDa) * pI* Hydrophobicity**

Lysozyme Glucosidase 14.3 10.7 -0.15

BSU3124 Glucosidase 63.9 5.1 -0.51

PP1034 Phosphatase 74.3 4.7 -0.45

OLEI4758 Esterase 44.5 6.4 -0.25

NE1796 Esterase 23.5 6.7 0.03

ATC1791 Esterase 25.0 6.9 -0.34

BSU3441 Protease 51.4 5.0 -0.39

*Calculated based on their amino acid sequence.

** Hydrophobicity values were calculated from the amino acid sequence of the enzymes (Kyte and Doolittle, 1982).

Higher numbers represent more hydrophobicity.

Effect of Enzyme Dose on Soluble COD, Protein and Carbohydrate Content

Changes in soluble COD during enzymatic treatment of biosludge were enzyme-dependent (Figure

4-5). Lysozyme, NE1796 and BSU3441 were selected because they represent the three different effects

that enzymes have on biosludge dewaterability i.e. positive, neutral and negative effect, respectively. As

can be seen in Figure 4-5, the soluble COD content was reduced with increasing enzyme doses of

lysozyme. There was a negative and significant correlation in the case of lysozyme with a p- value <

0.001. This reduction in soluble COD is in accordance with the flocculating mechanism previously

reported (Bonilla et al., 2015). As lysozyme flocculates biosludge, less particles remain in suspension and

pass through the filter used, thus reducing the soluble COD. The reduction of COD after NE1796

addition seemed to be significant only when the first dose of enzyme was added. No further reduction was

observed after 0.05% which suggests that the change in COD is not due to the enzyme itself but the

addition of the enzymatic solution and possibly due to the effect of the “suspending” buffer. On the other

hand, BSU3441, the enzyme that showed a significant negative effect on dewaterability, resulted in an

increase in soluble COD with increasing enzyme concentrations (P value < 0.002). These results suggest

that COD solubilization may be related to a negatively effect on biosludge dewaterability.

80

The soluble protein content was significantly reduced with increasing concentrations of lysozyme

(Figure 4-5). This is in agreement with the hypothesis that lysozyme molecules are interacting with

particles in biosludge and precipitating other proteins that were initially in solution. The effect of NE1796

and BSU3441 on soluble protein was the opposite, higher protein content was observed with increasing

concentrations of these enzymes.

0

500

1000

1500

2000

2500

CO

D (

mg

/L)

Lysozyme

NE1796

BSU3441

0

500

1000

1500

2000

Pro

tein

(m

g/L

)

Lysozyme

NE1796

BSU3441

0

50

100

150

200

250

0 0.1 0.2 0.3 0.4 0.5 0.6

Carb

oh

yd

rate

s (m

g/L

)

Enzyme Dose (%)

Lysozyme

NE1796

BSU3441

a)

b)

c)

Figure 4-5 Effect of enzyme dose on the soluble COD, protein and carbohydrate content of

sludge after 3.5 h of incubation. Error bars represent standard deviation of triplicates.

81

Unlike COD and proteins, carbohydrate content trends were not clear (Figure 4-5). Increasing doses

of NE1796 and lysozyme slightly reduced the content of carbohydrates in the soluble portion of

biosludge. No correlation was observed for enzyme dose and carbohydrate concentration with NE1796

and BSU3441.There were no trends between the effect of enzymes on soluble carbohydrates and

dewatering properties of biosludge.

Literature evaluating the effect of enzymes on the protein and carbohydrate composition of biosludge

is limited. A study that reported enhanced biosludge dewaterability after using enzymatic conditioners

measured the concentration of protein and carbohydrate and reported a decrease in both after enzymatic

addition (Ayol, 2005). However, their measurement excluded the soluble portion of the sludge. Thus,

there is no comparison to be made with our results. Another study reportedthat an increase in soluble

protein resulted in higher CST values (Novak et al., 2003). This is consistent with our results after

treatment with lysozyme which show a significant decrease in soluble protein, COD and carbohydrate.

Also, BSU3441, the enzyme that had a negative effect on biosludge dewaterability (i.e. higher CST)

showed the opposite, an increase in soluble protein. This negative effect on biosludge dewaterability can

also be explained by the negative charges that most proteins carry at neutral pH. Settling and dewatering

properties would be negatively affected with increasing negative charge entering the system.

Conclusions

A methodology to reduce the effect of buffers present in enzyme solutions on the dewaterability of

biosludge was successfully validated. None of the novel enzymes tested appeared to improve biosludge

dewaterability at the conditions studied. On the contrary, some of the novel enzymes resulted in

worsening of dewatering properties. Soluble proteins, carbohydrates and COD were affected by the

addition of novel enzymes to biosludge but the effect on proteins was greater possibly as a result of the

novel enzymes added to the system.

82

References







Bonilla, S., Tran, H., & Allen, D. G. (2015) Enhancing the dewaterability of biosludge using enzymes.

Water Res 68: 692.


Amylase Treatment. PCT/US2009/064788:


ISP-5. Alexandria, VA, Water Environment Research Foundation.

Dubois, M., Gilles, K., Hamilton, J., Rebers, P., & Smith, F. (1956) Colorimetric Method for

Determination of Sugars and Related Substances. - Anal Chem 28: 350-356. Dursun, D., Turkmen, M.,

Abu-Orf, M., & Dentel, S. K. (2006) Enhanced sludge conditioning by enzyme pre-treatment: comparison

of laboratory and pilot scale dewatering results. Water science and technology 54: 33.

Gonzalez, C. F., Proudfoot, M., Brown, G., Korniyenko, Y., Mori, H., Savchenko, A. V., & Yakunin, A.

F. (2006) Molecular Basis of Formaldehyde Detoxification: A. F. (2006) Molecular Basis of

Formaldehyde Detoxification: Characterization of two S-formyglutathione hydrolases from Escherichia

coli, FrmB and YeiG. J Biol Chem 281: 14514.

Hamameh, R. (2016) Effect of induction temperature profile on Escherichia coli cell yield and protein

production in shake flasks, 5-L minifors reactors and a pilot-scale Fermentation Unit. Master of

Engineering Report. University of Toronto.



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Kyte J, Doolittle RF. 1982. A simple method for displaying the hydropathic character of a protein. J Mol

Biol 157:105.



Lichty, J. J., Malecki, J. L., Agnew, H. D., Michelson-Horowitz, D. J., & Tan, S. (2005) Comparison of

affinity tags for protein purification. Protein Expr Purif 41: 98.

Loo-Yong-Kee, S. (2015) Optimizing the pilot scale fermentation unit for the production of valuable

protein. Master of Engineering Report. University of Toronto.




10/764,684.

Sarkar, J., Shah, J., & Ramesh, M. (2003) Method of dewatering sludge using enzymes.

84

5 Chapter 5 - Addressing the Challenges Associated with Evaluating the Effect of Enzymatic Pretreatment on the Anaerobic Digestibility of Biosludge

This chapter is based on the manuscript submitted: Bonilla, S., Choolaei, Z., Meyer, T., Yakunin, A. F.,

Edwards, E.A and Allen, D. G. Enzymatic Pretreatment of Pulp and Paper Mill Biosludge for Enhancing

its Anaerobic Digestibility. Submitted to Water Research on October 23, 2016.

Accreditations:

Sofia Bonilla formulated the research questions, designed and conducted half of the experiments,

analyzed and interpreted data, and prepared the first draft of the manuscript.

Zahra Choolaei designed and conducted half of the experiments and helped editing the manuscript.

D. Grant Allen, Alexander Yakunin and Elizabeth Edwards provided advice on experimental design

analysis and interpretation of data and editing of the manuscript.

Introduction

There is increasing interest in developing technologies to reduce biomass produced during

wastewater treatment processes in pulp and paper (P&P) mills. Sludge management accounts for up to

60% of treatment costs (Mahmood & Elliott, 2006). Typically, primary sludge is mixed with biosludge to

enhance the latter’s dewaterability. Primary sludge is mainly composed of wood fibres wasted during the

pulping process and is relatively easy to dewater. On the other hand, dewatering of biosludge (secondary

or waste activated sludge), which is a complex mixture of microorganisms, organic, and inorganic

particles, is a challenge due to its high moisture content and poor solid-liquid separation properties

(Mahmood & Elliott, 2006). The mixture is then conditioned with chemicals, usually synthetic

flocculants, and it is mechanically dewatered prior to its final disposal via incineration, land application

and/or landfilling. A reduction in the production of primary sludge and an increase in biosludge are

expected to result from more efficient pulping processes and higher regulatory standards (Elliott &

Mahmood, 2007). This new sludge production ratio will favour different technologies for sludge

processing and disposal (Meyer & Edwards, 2014). In addition, a general recognition of the potential of

85

biosludge as a source of value-added products has motivated the consideration of other sludge

management technologies in P&P mills, such as anaerobic digestion.

While anaerobic digestion of biosludge has been extensively used in municipal wastewater

treatment, its implementation in pulp mills is still limited. A clear advantage of anaerobic treatment over

traditional aerobic systems is the recovery of fuel (i.e. methane). Additionally, the mass and volume

reduction after anaerobic digestion translates into savings associated with handling, processing and

disposal of sludges. Use of anaerobic digestion for P&P mill biosludge has not been industrially

established because of low methane yields, reportedly due to the complexity and recalcitrance of pulp and

paper mill biosludge, and the presence of toxic chemicals (Meyer & Edwards, 2014). As discussed in

recent reviews, several biosludge pretreatment approaches have been investigated for improving the

feasibility of anaerobic digestion of biosludge in P&P mills (Elliott & Mahmood, 2007; Meyer &

Edwards, 2014).

Enzymatic pretreatment of biosludge can potentially enhance methane yields. Hydrolysis is widely

accepted as the limiting step in the anaerobic conversion of the complex organic matter in biosludge.

Enzymes that can speed-up hydrolysis are gaining the attention of industry because of their catalytic

activity and potential to be produced from renewable and/or waste sources (Ben Rebah & Milled, 2013).

Discovery of novel enzymes, enzyme engineering, and the reduction of production costs is driving the

development of many enzyme-based technologies. As discussed in Parawira (2012), enzymes are

recognized for their potential to hydrolyze biosludge, resulting in improved anaerobic digestion.

However, the effects of enzymatic pretreatment are poorly understood. To date, studies have concentrated

primarily on municipal biosludges with conflicting findings. While some authors have reported a

substantial improvement in biogas production, methane yield, and/or chemical oxygen demand (COD)

solubilization (Barjenbruch & Kopplow, 2003; Wawrzynczyk, 2007; Recktenwald et al., 2008; Yang et

86

al., 2010b), others reported improvements only in lab-scale experiments (Karlsson et al., 2011) and others

found no improvement (Bayr et al., 2013).

Proteases and glycosidases are the obvious first enzyme candidates for pretreatment, because

biosludge is mainly composed of microbial biomass whose main cellular components are proteins and

complex carbohydrates. In addition, the particles in biosludge are embedded in a gel-like matrix of

extracellular polymeric substances (EPS) comprising different biopolymers, including proteins,

carbohydrates, lignin, DNA, and RNA (Li & Ganczarczyk, 1990; Frølund et al., 1996). Proteins and

carbohydrates account for up to 70% of the organic matter present in P&P biosludge (Meyer & Edwards,

2014). Accordingly, previous studies mainly tested proteases, glycosidases, or a combination thereof for

biosludge treatment (Wawrzynczyk, 2007; Yang et al., 2010b; Karlsson et al., 2011; Bayr et al., 2013).

Previous studies on the enzymatic pretreatment of biosludge have revealed three main problems

which we address in this study. Firstly, in only two studies does the chemical oxygen demand (COD)

contributed by the enzymes appear to be taken into account (Karlsson et al., 2011; Bayr et al., 2013).

Secondly, enzymes are polymers of amino acids (polypeptides) and, as such, could have an effect on

biosludge digestibility that is not related to their enzymatic activity (Bonilla et al., 2015). Accordingly,

the use of inactivated enzyme controls is needed to investigate the enzymatic (catalytic) effect of a

pretreatment. Lastly, biosludge and anaerobic granules (inoculum) are complex microbial communities

and both produce biogas under anaerobic conditions. Isolating the biogas produced from biosludge and

the biogas produced by the inoculum is important to quantify the effect of enzymes on biogas yields.

Reducing the “background” biological activity of the system (i.e. biogas from inoculum) could facilitate

the quantification of the changes that biosludge undergoes during enzymatic treatment. Addressing these

issues will lead to a better assessment of the potential of enzymatic pretreatment for enhanced anaerobic

digestibility of biosludge. Based on the previous discussion, the specific objectives of this study were:

87

To develop an experimental methodology that allows the evaluation of the effect of enzymatic

pretreatment on anaerobic digestibility while isolating the effect of the enzymes as organic additives.

To test hydrolytic enzymes from two groups, proteases and glycosidases, for their potential to

enhance the anaerobic digestibility of biosludge.

To measure enzymatic activity using standard substrates in biosludge, to detect possible inhibitions

or synergies.

To investigate the changes in COD, protein and carbohydrate content during enzymatic pretreatment

of biosludge to better characterize the process.


The approach used to meet the objectives stated above involved three biochemical methane potential

(BMP) assays, enzymatic and compositional analyses. A flow diagram of the general approach can be

seen in Figure 5-1.

Enzyme production and preparation (Sections 5.2.3 and 5.2.4)

Enzymatic pretreatment of raw biosludge

(Section 5.2.5)

Biochemical methane potential (BMP) assays (Section 5.2.7)

BMP 1 BMP 2 BMP 3

Effect of proteases on raw biosludge

and biogas production

Effect of glycosidases

on raw biosludge and

biogas production

Effect of enzymes on the inoculum and biogas

production

Enzymatic pretreatment of gamma irradiated biosludge

(Section 5.2.5 and 5.2.6)



88

Biosludge Samples

Biosludge (waste activated sludge) from a secondary clarifier was obtained from a Canadian P&P

mill which produces a variety of pulp, paper and specialty products using sulfite pulping and mechanical

pulping (bleached chemi-thermomechanical pulp - BCTMP). Biosludge is the by-product of the aeration

stage in the wastewater plants treating mill effluents. Samples were kept at 4˚C in the laboratory prior to

the experiments and for a maximum of two weeks. Before use in experiments, the biosludge sample was

allowed to settle overnight and the supernatant was discarded to obtain a thickened sludge.

Gamma irradiated sludge was used for BMP 3 to inactivate microbial processes in the biosludge to

enable testing the enzyme’s activity on protein, carbohydrate and COD content and quantification of

compositional changes in biosludge as a result of the enzymatic pretreatment only, without confounding

effects from microbial activity inherent to biosludge. Sludge was irradiated at a dose of 25kGy produced

from a cobalt source (Co-60) using the Gamma Cell (G.C. 220). Previous studies have reported a >99%

inactivation of common pathogens present in sewage sludge at a dose of 5kGy (Farooq et al., 1993).

Anaerobic Inoculum (Granules)

Anaerobic granules were used as the inoculum for the BMP assays described in section 2.6. Granules

were obtained from the anaerobic digester of a Canadian pulp and paper mill and were maintained in the

laboratory under anaerobic conditions at 4 ˚C. Two weeks before the BMP set up, anaerobic granules

were diluted (1:2) in a synthetic medium described in (Edwards & Grbić-Galić, 1994). The diluted

granules suspension was then incubated at 37˚C and fed with the synthetic feed (0.4% v/v) previously

reported by Yang et al. (2010a). The anaerobic activity of the inoculum was confirmed by measuring

biogas production. The inoculum was left incubating until the day of the experiment. This two-week

incubation period reduced the easily digestible COD minimizing the background biogas produced in the

BMP assays.

89

Enzyme Preparations

The enzymes used in this study were hydrolases from two subgroups: proteases (EC 3.4) and

glucosidases (EC 3.2.1). Four of the enzymes were available commercially and two were produced in our

laboratory. Information about the enzymes used in this study is presented in Table 5-1.

Table 5-1 General information of enzymes used in this study

* Merz et al., (2015)

** Rodrigues et al., (2015)

Commercial Enzymes Preparation

Solutions of commercial enzymes (25 % v/v) were prepared in 50 mM phosphate buffer at pH 7. The

solutions were then dialysed overnight against the same buffer using a Pur-A-Lyzer™ Mega Dialysis Kit

(Sigma-Aldrich, St. Louis, USA). After dialysis, half of the enzyme solution was taken to prepare the

inactive enzyme solution by placing it in an oven at 103˚C for 6 hours followed by immediate exposure to

-20˚C for at least 2 hours. This temperature shock resulted in the irreversible inactivation of the enzyme

as verified in the enzymatic assays described in section 5.2.6.

Cloning, Overexpression and Purification of Novel Enzymes

The production of novel enzymes was carried out as described by (Gonzalez et al., 2006), with a few

modifications. The recombinant plasmid (p15TvL) containing the coding gene for the His-tagged proteins

(BCE_2078 or SCO6604) was transformed into Escherichia coli strains (BL21) for overexpression. Cells

Enzymes EC Number Activities Source

Protease from

Bacillus licheniformis 3.4.21.62 Serine protease (subtilisin)

Sigma-Aldrich

(P4860)

Protease from

Aspergillus oryzae 3.4.-

Mixture of seven peptidases and one

α-amylase*

Sigma-Aldrich

(P6110)

BCE_2078 from

Bacillus cereus (Q739R2) 3.4.21.- Serine protease

Produced

in-house

Lysozyme from

chicken egg white 3.2.1.17 Glycosidase

Bioshop

(LYS702)

Cellic® CTec 2 3.2.1.- Mixture of cellobiohydrolase I,

endoglucanase, and β-glucosidase ** Novozymes

SCO6604 from

Streptomyces coelicolor (Q8CJM3) 3.2.1.21 β-glucosidase

Produced

in-house

90

were grown in terrific broth (TB) to an OD600 of approximately 1 and protein expression was induced

with 0.4 mM isopropyl-D-thiogalactopyranoside. After induction cells were incubated overnight at 16˚C.

The harvested cells were resuspended in buffer A (50 mM HEPES, pH 7.5, 5 mM imidazole and 5% v/v

glycerol) and sonicated. The cell debris was then pelleted by centrifugation at 21,000g for 45 min in a

Beckman-coulter centrifuge (Avanti JE, rotor JLA 16.250). BCE_2078 and SCO6604 were affinity

purified from the soluble fraction using Ni-NTA resin (Qiagen, Hilden, Germany), followed by washing

the column with buffer B (same as buffer A but 50 mM imidazole) and elution was carried out with buffer

C (same as buffer A but 250 mM imidazole). SDS-gel electrophoresis was used to verify the purification

of the enzyme of interest. The eluted enzymes were dialysed overnight and further processed as described

in section 5.2.4 of this document.

Enzymatic Assays

Enzymatic assays were conducted to confirm the activity of the enzymes prior to biosludge treatment

and, to potentially correlate these enzymatic activities to the effect of enzymatic pretreatment on

biosludge anaerobic digestibility. For proteases and glycosidases (except lysozyme), assays with standard

substrates, biosludge, and a combination thereof, were used to evaluate enzymatic inhibition by

biosludge. Lysozyme’s activity on biosludge could not be measured because biosludge interferes with the

basis of the lysozyme activity assay (i.e. cell optical density). The specific details of the enzymatic assays

are described in Section 5.2.4.1 and 5.2.4.2).

Protease activity

Protease activity assays were used to assess the activity of the proteases used in this study. A

modified version of “Sigma's non-specific protease activity assay using casein as the substrate” was used

for this purpose. In 96-well plates, 200 μg of enzyme was incubated with 25 μL of a 40 g/L casein

(standard substrate) solution at 37°C for 30 min, final volume of all wells was maintained at 185 μL. The

reaction was stopped by adding 185 μL of a trichloroacetic acid (TCA) solution (20% w/w) and incubated

91

at 37°C for 30 min. Plates were centrifuged at 13,000 rpm (Eppendorf centrifuge 5417C) and the

supernatant was recovered. For the colorimetric detection, sodium carbonate (310 mM) was added to 88

μL of the supernatant followed by the addition of 60 mM Folin-Ciocalteau phenol reagent. After 30 min

of incubation at 37°C, the absorbance was read at 660 nm. Blanks were prepared by adding the TCA

before enzyme addition. In addition to casein as the standard substrate, samples with biosludge only, and

biosludge and casein were used to assess enzymatic activity, potential synergies and inhibitions. Protease

activity is presented as μmolar tyrosine equivalents released per μg of enzyme per min (mM Tyr/mg

enzyme/min) using a tyrosine calibration curve. All assays were carried out in triplicate for active and

inactive proteases.

Glycosidase Activity

Glycosidase activity assays were conducted based on the use of dinitrosalicylic (DNS) acid reagent

for the measurement of reducing glucose. The DNS reagent was prepared by dissolving 5 g of 3,5-

dinitrosalicylic acid in 200 mL of ddH2O while heating at around 50°C. To this solution, 50 mL of 4 N

sodium hydroxide and 150 g sodium potassium tartrate were added, and the volume was adjusted to 500

mL. The assay was started by incubating 200 μg of enzyme with 1% carboxymethyl cellulose (CMC) in

96 well plates, at a total volume of 200 μL, for 1 h at 37°C. One volume of this sample was mixed with

one volume of DNS reagent, and incubated at 100°C for 10 minutes. Afterwards, the plate was cooled

down at room temperature, and the absorbance was recorded at 540 nm against a blank (containing

phosphate buffer instead of enzyme solution). As with proteases, samples with biosludge only, and

biosludge and CMC, were used to assess enzymatic activity on biosludge, potential synergies and

inhibitions. Glucose concentration was calculated using a glucose standard curve. All assays were carried

out in triplicate for active and inactive enzymes.

Lysozyme activity was measured using the standard method described by Sigma-Aldrich. A

suspension containing Micrococcus lysodeikticus (0.01% w/v) purchased from the same company in

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potassium phosphate monobasic (66 mM, pH 6.2) was prepared. In cuvettes with 1 mL of the M.

lysodeikticus cell suspension, the absorbance was measured and used as the blank. Lysozyme solution

was added (0.1 mL) and the change in absorbance was monitored overtime for 5 min. All assays were

carried out in triplicate for active and inactive enzyme. However, for lysozyme, assays on biosludge or

biosludge and cells could not be performed since the assay used the absorbance of cells and no distinction

could be made between cells from biosludge and cells from M. lysodeikticus.

Biosludge Pretreatment

Solutions of active and inactive enzymes were added to the thickened biosludge and incubated for 6

h at 37˚C and shaken using an orbital shaker incubator (Amerex Gyromax 747R) at 100 rpm. Final

enzyme concentrations were adjusted to 1% (protein/TSS biosludge). Protein concentrations were

measured using the Bradford Reagent (Biorad, California, USA) and a bovine serum albumin (BSA)

calibration curve was used to determine the amount of enzyme solution to be added. Biosludge with

deionized water and biosludge with phosphate buffer were used as controls. The volume of biosludge,

enzymes, water or buffer was maintained constant for all the samples. At the end of the incubation period,

the final COD concentration was used to calculate the amount of biosludge to be added to the BMP

assays. In all cases, enzymes contributed less than 3% of the total COD added, except for the protease

from A. oryzae which contributed 5% and lysozyme which contributed 12%. For BMP 3, the enzymatic

pretreatment was carried out for 24 h instead of 6 h to measure the effect of enzymatic treatment over a

longer period of time. Chemical analyses were carried out on samples taken at 0, 4, 7 and 24 h.

Chemical Analyses

Solid analyses

Total suspended solids (TSS) and volatile suspended solids (VSS) for biosludge and anaerobic

granules samples were quantified according to the APHA Standard Methods (APHA, 1992).

93

Chemical Oxygen Demand (COD)

Total chemical oxygen demand (tCOD) was analysed following the Standard Methods for the

Examination of Water and Wastewater (APHA, 1998). For soluble chemical oxygen demand (sCOD)

measurements, the samples were first centrifuged at 13,000 rpm for 10 min using an Eppendorf

microcentrifuge (5417C). In this study, the supernatant was considered as the soluble fraction and was

further analysed using TNTplus™ vials, Hach Method 8000 with range 3-150 mg/L COD (Hach Co.,

USA).

Protein Content

Soluble protein content of biosludge was analysed using a modified version of the Lowry Method

(Lowry et al., 1951; Zhang, 2008). In 96-well plates, 40 μL of biosludge or BMP samples were mixed

with 36 μL of Solution A (KNaC4H4O6·4H2O and Na2CO3) followed by the addition of 4 μL of Solution

B (KNaC4H4O6·4H2O CuSO4.5H2O). Folin-Ciocalteau phenol reagent (0.5 N) was then added (120 μL) to

each well for colour development. Bovine Serum Albumin (BSA) was used to prepare a calibration curve.

Samples were centrifuged as described in section 5.2.8.2 and the supernatant was analysed for soluble

protein content.

Carbohydrate Content

Soluble carbohydrate content of biosludge was analysed using the Anthrone method for

quantification of sugars (Trevelyan et al., 1952). Biosludge samples were centrifuged as described in

section 5.2.8.2 and the supernatant analysed for soluble protein content. A modified version to perform

the analysis in 96-well plates was used. Anthrone reagent (Sigma-Aldrich, St. Louis, USA) was dissolved

in concentrated sulphuric acid (0.2% w/v), 150 μL of the Anthrone solution was added to 50 μL of

sample. The plate was incubated for 10 min at 4°C followed by a second incubation at 103°C for 20 min.

Absorbance was read at room temperature at 620nm. A calibration curve of glucose was used to quantify

carbohydrates in the samples.

94

Biochemical Methane Potential (BMP) Assays

The biochemical methane potential (BMP) assays first described in (Owen et al., 1979) were

modified to evaluate the anaerobic digestibility of enzymatically-pretreated biosludge. The assays were

prepared in an AtmosBag with Zipper-lock closure (Sigma-Aldrich, St. Louis, USA) supplied with a gas

mixture with composition 80% N2, 10% CO2, and 10% H2 by volume. All samples were prepared in

triplicates in 160 mL serum bottles. In each set of experiments, all bottles contained the same amount of

anaerobic granules (10 mL) and synthetic anaerobic medium (60 mL). The volume of biosludge added

was adjusted to maintain the same COD in all the bottles. The liquid volume was maintained at 80 mL

using deionized, sterile, anaerobic water. Once prepared, bottles were incubated at 37˚C and 100 rpm for

at least 60 days.

Inoculum to substrate ratios (ISRs) used in this study were 0.4 and 0.8 based on total COD,

equivalent to 0.4 and 1.0 based on volatile solids (VS). It has been previously reported that ISRs affect the

rate of anaerobic digestion and if the ISR is <0.5 (VS basis), acidification due to volatile fatty acids

accumulation may delay or inhibit methane production (Raposo et al., 2009; González-Fernández &

García-Encina, 2009). However, for the purpose of this study, high ISRs ratios are not advisable because

they result in large amounts of biogas produced from the inoculum compared to the biogas produced from

the actual samples of interest (i.e. biosludge). This hinders our ability to compare the effect of different

enzymes. Potential effects from the ISRs used in this study were also considered.

Controls were added to BMP assays to investigate the effect of inactive enzymes, biosludge and

granules on biogas yields. In addition, in each assay, the synthetic feed used in section 5.2.1 was used

instead of biosludge, maintaining the same COD/bottle, to evaluate the methanogenic activity of the

granules with easily digestible substrates (i.e. a mix of glucose, sodium acetate, sodium propionate and

methanol), these samples will be referred throughout this document as “positive controls”. Samples

named “inoculum only” were used as the experimental blank, they represent the background

95

methanogenic activity from the inoculum. When the biogas and the specific biogas yield (SBY) are

reported, the amount of biogas produced from these inoculum-only bottles, is subtracted from all the

samples that contained inoculum (see Equation 5-1). Samples named “biosludge only”, i.e. without the

inoculum, were used to evaluate the self-digestibility of biosludge. BMP assays were also carried out on

biosludge pre-treated with inactive enzymes to account for COD contributions from the enzymes

themselves. To evaluate the digestibility and gas production from the enzyme solutions specifically, BMP

3 included bottles where enzyme solutions were added with inoculum and synthetic medium (without

biosludge).

Biogas Production

Biogas production was measured using a water-lubricated glass syringe (Owen et al., 1979). Since

the BMP assays were prepared in a glove bag at room temperature, and then sealed bottles were moved to

an incubator at 37˚C, initial biogas samples will include the volume of gas associated with expansion

caused by the increase in temperature. To correct for this effect, the amount of biogas produced after 24

hours in the negative controls (biosludge only) was subtracted from all the samples at that time point. For

data analysis and treatment comparison, both specific biogas yield (SBY) and total biogas production

(TBP) were computed, as per Equations 5-1 and 5-2 below:

Equation 5-1

𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐵𝑖𝑜𝑔𝑎𝑠 𝑌𝑖𝑒𝑙𝑑 (𝑆𝐵𝑌)(𝑚𝐿 𝑔−1𝐶𝑂𝐷) = 𝐶𝑢𝑚𝑢𝑙𝑎𝑡𝑖𝑣𝑒 𝐵𝑖𝑜𝑔𝑎𝑠 𝑠𝑎𝑚𝑝𝑙𝑒(𝑚𝐿) − 𝐶𝑢𝑚𝑢𝑙𝑎𝑡𝑖𝑣𝑒 𝐵𝑖𝑜𝑔𝑎𝑠 𝑖𝑛𝑜𝑐𝑢𝑙𝑢𝑚(𝑚𝐿)

𝐶𝑂𝐷𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 (𝑔)

Where CODsubstrate is the COD added from the biosludge sample. The COD was measured after

enzymatic treatment. Specific biogas yield represents the final BMP yield. Thus, cumulative biogas at the

end of each BMP were used.

Equation 5-2

𝑇𝑜𝑡𝑎𝑙 𝐵𝑖𝑜𝑔𝑎𝑠 𝑃𝑟𝑜𝑑𝑢𝑐𝑡𝑖𝑜𝑛 (𝑇𝐵𝑃) (𝑚𝐿 𝑔−1𝐶𝑂𝐷) =𝐵𝑖𝑜𝑔𝑎𝑠𝑠𝑎𝑚𝑝𝑙𝑒(𝑚𝐿)

𝐶𝑂𝐷𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 (𝑔) + 𝐶𝑂𝐷𝑖𝑛𝑜𝑐𝑢𝑙𝑢𝑚 (𝑔)

96

In addition, the theoretical biogas potential was calculated and used as a benchmark for

complete conversion of organic matter to methane and carbon dioxide. This full conversion is expected

for the synthetic feed which is composed of easily digestible compounds (glucose, propionate, acetate and

methanol) (Yang et al., 2010a). Using the equivalence of 1 g COD to 397 mL CH4 at 37 ˚C (Khanal,

2008) and the CH4 concentration in biogas was calculated. The theoretical biogas production was

calculated with Equation 5-3

Equation 5-3

𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝐵𝑖𝑜𝑔𝑎𝑠 𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 (𝑚𝑙) = 𝐶𝑂𝐷 𝑎𝑑𝑑𝑒𝑑 (𝑔) 𝑥 397 𝐶𝐻4 (𝑚𝐿/𝑔𝐶𝑂𝐷)

𝐶𝐻4 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 (%)

Methane Analysis

Using a 500 µL glass-tight syringe, 200 µL of the headspace were removed and injected into a

Hewlett Packard 5890 equipped with CTR I column and a thermal conductivity detector (TCD). The

column head pressure was maintained at 22-24 psi with Helium as the carrier gas. The oven temperature

was isothermal at 50˚C. The injector and detector temperature was 200˚C for both. Methane standards

were used to prepare a calibration curve and methane was eluted at 8.3 min. Methane production was

calculated for every sampling day using Equation 5-4.

Equation 5-4

𝑀𝑒𝑡ℎ𝑎𝑛𝑒 𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑑 (𝑚𝐿) = 𝐵𝑖𝑜𝑔𝑎𝑠 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 (𝑚𝐿) 𝑥 𝑚𝑒𝑡ℎ𝑎𝑛𝑒 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑓𝑟𝑜𝑚 𝐺𝐶 (%)

100

Results

Set up Conditions of BMP Assays

Biosludge and anaerobic granules used in the three BMPs conducted in this study were collected at

different times in the mill and used at different times in the laboratory; thus, there is variability in their

composition. Total suspended solids (TSS), volatile suspended solids (VSS), and chemical oxygen

demand (COD) of the different sludge samples and anaerobic granules are shown in Table 5-2. Given the

97

conditions of each BMP, inoculum to substrate ratios were different and defined based on the COD

content as per described in Table 5-2.

Table 5-2 Characteristics of raw biosludge, inoculum, and inoculum-to-substrate ratios based on

COD used in the three biochemical methane potential (BMP) assays performed in this study.

BMP 1

(Feb 02/2015)

BMP 2

(Mar 3/2015)

BMP 3*

(Jul 07/2015)

Raw characteristics

Biosludge (3 distinct samples for each BMP) Sample 1 Sample 2 Sample 3

TSS (g/L) 18.8 (±0.7) 18.6 (±0.3) 20.1 (±0.8)

VSS (g/L) 16.1 (±0.6) 15.8(±0.1) 17.6 (±0.8)

COD (g/L) 24.2 (±0.4) 25.2 (±1.8) 27.8 (±2.8)

Granules (3 distinct samples for each BMP)

TSS (g/L) 17.7 (±0.2) 26.1(±1.3) 19.9 (±1.7)

VSS (g/L) 15.6 (±0.1) 25.6 (±1.1) 17.9 (±1.4)

COD (g/L) 26.2 (±1.4) 33.1 (±1.3) 21.6 (±2.4)

COD contribution in BMP bottles (mg COD/bottle)

Granules (inoculum) 86 122 151

Biosludge (substrate)** 200 150 200

Inoculum to substrate ratio*** 0.4 0.8 0.8

* Biosludge in BMP 3 was gamma irradiated

** COD was measured after enzymatic treatment

*** Ratio was calculated based on COD

Effect of Enzymatic Pretreatment of Biosludge on Biogas Production

Enzymatic pretreatment with proteases enhanced biogas production; of the three proteases tested two

showed a significant increase in total biogas production (TBP) when compared to biosludge treated with

inactive proteases (Figure 5-2a). Biosludge samples pretreated with active protease from B. licheniformis

showed a total biogas production (TBP) of 166 ±3 mL g-1 COD after 62 days of anaerobic treatment while

the control (untreated) produced 150 ±5 mL g-1 COD (Figure 5-2a). The yield for the biosludge treated

with inactive protease from B. licheniformis was lower (131 ±8 mL g- 1 COD) over the same period.

Similarly, active protease from A. oryzae produced 168 ±5 mL g-1 COD while the inactive protease

produced 147 ±7 mL g-1 COD. The positive effect for these two proteases was evident from the first

sampling day and maintained over the 62 days of BMP assay. On the other hand, biosludge pretreatment

with the BCE_2078 protease did not show any improvement on biogas production; active and inactive

98

BCE_2078 resulted in similar biogas yields (138 ±2 mL g-1 COD) at the end of the BMP assay suggesting

that the effect of enzymatic pretreatment with proteases depends on the type of protease used.

0

20

40

60

80

100

120

140

160

180

0 20 40 60

Tota

l B

iogas P

roductio

n (

mL/g

of C

OD

)

Time (d)

Untreated (Control)

Protease A. oryzae (Active)

Protease A. oryzae (Inactive)0

50

100

150

200

250

0 20 40 60

Tota

l B

iogas P

roductio

n (

mL/g

of C

OD

)

Time (d)

Untreated (Control)

Lysozyme (Active)

Lysozyme (Inactive)

0

20

40

60

80

100

120

140

160

180

0 20 40 60

Tota

l B

iogas P

roductio

n (

mL/g

of C

OD

)

Time (d)

Untreated (control)

Protease B. licheniformis (Active)

Protease B. licheniformis (Inactive)0

50

100

150

200

250

0 20 40 60

Tota

l B

iogas P

roductio

n (

mL/g

of C

OD

)

Time (d)

Untreated (Control)

SCO6604 (Active)

SCO6604 (Inactive)

0

20

40

60

80

100

120

140

160

180

0 20 40 60

Tota

l B

iogas P

roductio

n (

mL/g

of C

OD

)

Time (d)

Untreated (Control)

Protease BCE_2078 (Active)

Protease BCE_2078 (Inactive)0

50

100

150

200

250

0 20 40 60

Tota

l B

iogas P

roductio

n (

mL/g

of C

OD

)

Time (d)

Untreated (Control)

CTec 2 (Active)

CTec 2 (Inactive)

a) b)

c) d)

e) f)

Figure 5-2 Total biogas production, TBP, of biosludge pretreated with enzymes over 62 days of

anaerobic digestion. a) protease from A. oryzae; b) lysozyme; c) protease from B. licheniformis; d)

glycosidase SCO6604; e) BCE_2078 and f) CTec 2. Untreated (control) had phosphate buffer

instead of enzyme solution. Range differences between BMP 1 (a, c, e) and BMP 2 (b, d, f) are due

to differences in biosludge and granules, inoculum to substrate ratios and soluble chemical oxygen

demand (sCOD).

99

Enzymatic pretreatment of biosludge with glycosidases enhanced biogas production during

anaerobic digestion (Figure 5-2b). After 62 days of anaerobic digestion, lysozyme treatment resulted in

higher biogas production (323 ±6 mL g-1 COD) than the control (226 ±7 mL g-1 COD) over the BMP

period (62 days). Biogas measurements were taken in triplicates and the TBP of biosludge after treatment

with active lysozyme was higher than the yield obtained when biosludge was treated with inactive

lysozyme (i.e. 305 ±9 mL g-1 COD). This difference is statistically significant (P<0.001), suggesting that

lysozyme’s enzymatic activity can increase the biogas production of biosludge.

Furthermore, pretreatment with glycosidase SCO6604 increased the TBP of biosludge (277 ±10 ml

g-1 COD) when compared to the control after 62 days of anaerobic digestion. When compared against the

inactive enzyme (259 ±3 mL g-1 COD), there was a statistically significant improvement during the same

period (P < 0.05). Pretreatment with CTec2 resulted in an TBP of 318 ±4 mL g-1 COD. There was no

improvement against the inactive CTec2 (318 ±9 mL g-1 COD) over the BMP assay. Lysozyme and

SCO6604 do no attack similar substrates. Lysozyme hydrolyses the bonds between N-acetylmuramic acid

and N-acetyl-D-glucosamines while SCO6604 is a confirmed b-glycosidase. The fact that enzymes with

different activities can have a positive effect on biosludge digestion is not surprising given the various

molecules that can be present in biosludge.

The addition of proteases and glycosidases showed a significant increase in biogas production in

comparison to the untreated biosludge control. These improvements are likely the sum of two effects:

more soluble COD from the enzyme solution, and hydrolysis of organic matter. In order to isolate these

two effects, we compared active and inactive enzymes. The results of this comparison can be interpreted

as the effect of enzymatic pretreatment caused by enzymatic hydrolysis alone. Since only proteases from

B. licheniformis, A. oryzae, the glycosidase from SCO6604 and lysozyme showed significant

improvements over their inactivated controls, it is proposed that the increase in biogas production of

biosludge pretreated with these enzymes is due to their hydrolytic activity.

100

Results show that proteases and glycosidases used for enzymatic pretreatment can improve the

anaerobic digestibility of biosludge. However, not all enzymes from these groups resulted in increased

biogas yields. The success of the enzymatic pretreatment for enhancing anaerobic digestibility of

biosludge can be affected by several factors. Firstly, limited conditions were studied; it is conceivable that

the enzymes used in this study, in particular the ones that not showed an increase in biogas production

(BCE_2078 and CTec 2) or that showed marginal improvements (SCO6604 and lysozyme), could

perform better under different conditions (e.g. enzyme dose, temperature, pH, time). Even the enzymes

that showed a significant positive improvement might result in greater improvements under different

conditions. Secondly, the substrates for these enzymes were possibly not readily available for catalysis

and/or if available, the products of catalysis did not significantly enhance the anaerobic digestibility of

biosludge. Thirdly, it is possible that there was significant denaturation and/or inhibition of these

enzymes, and thus no significant hydrolysis was achieved.

Effect of Enzymatic Pretreatment of Biosludge on Biogas Composition

Enzymatic pretreatment of biosludge resulted in higher methane production as a result of increased

biogas production. Normalized specific biogas yields as a percentage of the untreated samples (control)

for each BMP and methane concentration in biogas are shown in Figure 5-3. The difference in yields for

the untreated biosludge (control) samples in BMP 1 and 2 is attributed to the different biosludge, and

inoculum, as well as the higher ISR used in BMP 2. Moreover, methane concentration was similar for all

samples within each BMP assay. For BMP 1, methane ranged from 74-76% while for BMP 2, it was 69-

75% (Figure 5-3). No statistical analyses could be performed to evaluate if there were significant

differences within the samples in the same BMP due to the lack of replicates in methane analysis.

Nonetheless, Figure 5-3 shows that enzymatic treatment increases the specific biogas yield and does not

seem to affect the methane concentration of biogas produced in BMP assays.

101

Effect of Inoculum, Substrate and ISR on Biogas Composition and Biogas Production

Biogas production is affected by inoculum to substrate ratios (ISR), biosludge and inoculum quality.

As can be seen in Table 5-3, yields (SBY and TBP) for the controls used in this study differ with different

ISRs. Biogas production yields were generally lower with the lowest ISR (0.4), except for bottles that

contain inoculum only. The quality and anaerobic activity of the inoculum and the biosludge also play a

role. As can be seen in table 3, there were significant differences in the biogas production obtained from

biosludge only and inoculum only samples, even though ISR has no impact on the SBP or TBP of these

controls. Comparisons within a BMP, with the relevant controls, can be made in the case of this study

Figure 5-3 Specific biogas yield normalized against the untreated sample (control). Assuming

untreated sample of each BMP as 100%, the yield of each of the enzyme-treated samples was

calculated after 62 days of anaerobic digestion. Circles represent the concentration of methane

in the biogas produced at day 62 of the BMP assay.

102

because enzyme pretreated samples were compared against individual control (biosludge without

enzymatic treatment and inoculum) or biosludge with inactive enzyme and inoculum. However,

comparisons between BMP assays must be made with caution given that the inoculum (i.e. granules),

substrate (i.e. biosludge) and/or ISRs are different.

Table 5-3 Effect inoculum-to-substrate ratio (ISR) on total biogas production (TBP), specific biogas

yields (SBY) and methane concentration

All values were calculated based on the last biogas sample of each BMP assay i.e. 62 days of anaerobic digestion for BMP

1 and 2 and 50 days for BMP 3.

1 Biogas produced per total chemical oxygen demand (COD) in the bottle.

2 Biogas produced per chemical oxygen demand (COD) fed. Biogas produced from the inoculum was subtracted from all

samples with inoculum.

3 For reference, the theoretical maximum biogas production for the synthetic feed is 532 (±35) ml/g COD fed and methane

content in the biogas should be between 70-80%)

Although the background biogas produced by the inoculum should be minimized, using a low ISR

such as 0.4 may hinder the biogas production obtained during the BMP, as it is shown in Table 5-3, where

the theoretical maximum was not achieved even after 62 days of digestion. It is shown that an ISR of 0.8

sufficiently reduces background biogas production while allowing maximum biogas production.

Sample ISR

TBP

(ml/g COD

total)1

SBY

(ml/g COD

fed) 2

Methane

Concentration

(%)

BMP1 Inoculum only 0.4 153 (±6) N/A 76



BMP1 Biosludge + Inoculum 0.4 150 (±3) 148 (±3) 74

BMP2 Biosludge + Inoculum 0.8 171 (±4) 225 (±9) 75

BMP3 Biosludge (gamma irradiated) + Inoculum 0.8 164 (±16) 150 (±9) 75

BMP1Synthetic feed + Inoculum

(Positive Control)3 0.4 344 (±8) 425 (±11) 74

BMP2 Synthetic feed + Inoculum


BMP3 Synthetic feed + Inoculum


103

Differences between BMP assays are most likely due to the variability in the composition and structure of

biosludge and granules between batches. It is important to highlight that biogas yields are not additive

i.e., the samples with biosludge and inoculum do not show yields equivalent to the sum of yields for

biosludge only and inoculum only (Table 5-3).

Effect of Enzymatic Treatment of Biosludge on Soluble COD

Improvements in biogas production with enzymatic pretreatment do not seem to be the result of

COD solubilization. Soluble COD (sCOD) was measured over time during the enzymatic pretreatment

(Figure 5-4a and 5-4b). As shown, sCOD increased over time for all samples, including the control (no

enzyme). It has been previously reported that COD is solubilized during gamma irradiation and, although

the native enzymatic activity in sludge may be reduced, it is not completely removed (Farooq et al., 1993;

Chu et al., 2011). Thus, the changes over time in sCOD may be the result of remaining enzymatic and

even microbial activity. Treatment with proteases appear to result in higher sCOD values over time when

compared with the control (Figure 5-4a) while glycosidases remain close to the sCOD values of the

control after the 24 hours (Figure 5-4b). Regardless, active and inactive enzymes show similar trends over

time and higher sCOD values do not correlate with higher biogas yields, suggesting that the positive

effect in biogas production from the enzymes in this study is not the result of COD solubilization.

The effect of the enzymatic pretreatment of biosludge on soluble carbohydrate content is shown in

Figure 5-4b. All treatments, including the control, have a similar trend except for the samples treated with

the protease from A. oryzae. At time 0, samples with protease from A. oryzae, active and inactive

versions, showed higher soluble carbohydrate content 0.37 (± 0.04) mg/mL compared to other samples

(0.11-0.16 mg/mL). This higher carbohydrate content is likely the result of additives in the enzyme

solution that were not removed by the dialysis. Most samples showed an increase in the soluble

carbohydrate content over time during the enzymatic treatment while biosludge treated with protease

from A. oryzae showed a decrease in the soluble carbohydrate content.

104

Figure 5-4 Soluble chemical oxygen demand (COD), protein and carbohydrate content -during

enzymatic pretreatment of gamma irradiated biosludge for 24 hours. Proteases are shown on the

left and glycosidases on the right; a) and b) soluble COD (sCOD), c and d) soluble carbohydrates

(sCarbohydrates) and e and f) soluble protein (sProtein) content. Error bars (not always visible)

represent the standard deviation of triplicates.

0

1

2

3

4

0 5 10 15 20 25

sC

OD

(m

g/m

l)

Time (h)

ControlA. oryzae ActiveA. oryzae InactiveB. licheniformis ActiveB. licheniformis Inactive

0

1

2

3

4

0 5 10 15 20 25

sC

OD

(m

g/m

l)

Time (h)

ControlSCO6604 ActiveSCO6604 InactiveLys ActiveLys Inactive

0.0

0.1

0.2

0.3

0.4

0 5 10 15 20 25

sC

arb

oh

ydra

te (m

g/m

l)

Time (h)


0.0

0.5

1.0

1.5

2.0

0 5 10 15 20 25

sP

rote

in (m

g/m

l)

Time (h)


0.0

0.1

0.2

0.3

0.4

0 5 10 15 20 25

sC

arb

oh

ydra

te (m

g/m

l)

Time (h)


0.0

0.5

1.0

1.5

2.0

0 5 10 15 20 25

sP

rote

in (m

g/m

l)

Time (h)


a) b)

c) d)

e) f)

105

While moderate changes were observed in COD and carbohydrate content, there was no significant

change in soluble protein content over the same 24 h period (Figure 5-4e and 5-4f). Soluble protein in

samples treated with lysozyme (active and inactive) showed lower soluble protein than the control from

the start of the treatment, t=0 (Figure 5-4f). This can be attributed to the effect of lysozyme as a flocculant

(Bonilla T. & Allen, 2016). No significant difference in the soluble protein content was observed between

the control, SCO6604, active and inactive (Figure 5-4f). Biosludge treated with proteases (active and

inactive) showed higher soluble protein content (Figure 5-4e). However, these values do not correlate

with an increase in biogas yields. Soluble COD, protein and carbohydrate data were plotted against biogas

yields from the BMP assays using Pearson’s r for assessing linear correlations, and no significant

correlation was found.

Enzymes can enhance the anaerobic digestibility of biosludge but the increase in biogas yield

associated with proteases from B. licheniformis and A. oryzae, SCO6604 and lysozyme, cannot be

explained by changes in soluble COD, protein and carbohydrates. Previous reports show COD

solubilisation as the mechanism for enhanced anaerobic digestibility (Wawrzynczyk, 2007; Yang et al.,

2010b). However, COD solubilisation cannot be identified as the mechanism for such improvement in

this study since no evidence of COD solubilisation was observed (Figure 5-4a and 5-4b). It is

hypothesized that the enzymes that increase biogas production (as a result of their enzymatic activity) are

hydrolyzing substrates that are present in the soluble portion of biosludge.

Biogas Production from Enzyme Solutions Alone

The COD contributed by the enzymes is not completely converted to biogas by the inoculum. Total

biogas yields from the samples with enzyme and inoculum (no biosludge) from BMP 3 are shown in

Figure 5-5. The expectation was that samples with enzymes would produce more biogas than the control

106

(inoculum only) because there was more COD present (i.e. COD from the inoculum and COD from the

enzyme solution); however, in most cases the opposite was observed (Figure 5-5).

Figure 5-5 Biogas production from enzyme solutions. Total biogas production (TBP) are

presented for BMP 3, samples that contained enzyme solutions and inoculum. a) protease from A.

oryzae; b) lysozyme; c) protease from B. licheniformis; d) glycosidase SCO6604 Inoculum only is the

control, i.e. no enzyme added. Error bars show standard deviation of triplicates.

Enzyme solutions are not used by the inoculum as a source of COD (i.e. substrate). In other words,

the COD contributed by the enzyme solutions is not always converted to biogas (Figure 5-5). In fact, in

most cases, enzymes negatively affected the biogas yield of the inoculum and the effect of enzymes on

0

50

100

150

200

0 20 40 60

Tota

l B

iog

as P

rod

uction

(m

l/g

CO

D)

Time (d)

Inoculum only

A. oryzae (Active)

A. oryzae (Inactive)

0

50

100

150

200

0 20 40 60T

ota

l B

iog

as P

rod

uction

(m

l/g

CO

D)

Time (d)

Inoculum only

Lysozyme (Active)

Lysozyme (Inactive)

0

50

100

150

200

0 20 40 60

Tota

l B

iog

as P

rod

uction

(m

l/g

CO

D)

Time (d)

Inoculum only

B. licheniformis (Active)

B. licheniformis (Inactive)

0

50

100

150

200

0 20 40 60

Tota

l B

iog

as P

rod

uction

(m

l/g

CO

D)

Time (d)

Inoculum only

SCO6604 (Active)

SCO6604 (Inactive)

a) b)

c) d)

107

the inoculum was found to be enzyme-dependent. For example, active lysozyme resulted in higher yields

than the control during the first 40 days, while protease from B. licheniformis resulted in equal or reduced

biogas yield throughout the BMP assay (Figure 5-5). Thus, using theoretical biogas production based on

the conversion of COD to CH4 to account for the effect of COD contributed by the enzyme is not

recommended.

Potential of Enzymatic Activity Assays to Predict Effect of Enzymes on Biosludge Digestibility, Inhibition and Inactivation

Enzymatic assays were performed with standard substrates, biosludge, and a combination of both

(Figure 5-6). When exposed to casein, the proteases from B. licheniformis, A. oryzae, and BCE_2078

showed enzymatic activities (Figure 5-6a). As predicted, inactive enzymes showed almost no activity in

the presence of casein which confirmed the heat-inactivation process was successful. Proteases exhibited

low enzymatic activities when exposed to biosludge as the only substrate. BCE_2078 showed the highest

activity in biosludge compared to the other proteases. Active proteases in the presence of biosludge and

casein showed significant enzymatic activity. Thus, inhibition of proteases by biosludge or denaturation is

not likely in the conditions studied. The reduction in activity when compared to casein could be the result

of minor enzymatic inhibition but more likely because when casein is mixed with biosludge it may not be

as readily available for enzymatic hydrolysis (Figure 5-6a).

The results observed in these enzymatic assays do not correlate with the biogas yields obtained

during BMP assays. Proteases from A. oryzae and BCE_2078 showed the same enzymatic activity but

BCE_2078 did not show any improvement in biogas production during BMP assays, while protease from

A. oryzae showed significant potential for enhancing anaerobic digestion of biosludge. It is possible that

while protease from B. licheniformis and A. oryzae found suitable substrates for hydrolysis in biosludge,

BCE_2078 did not, thus, the difference in biogas production during the BMP assay. In addition, it is

conceivable that the products of enzymatic hydrolysis are being consumed or transformed by the active

108

microbial community in the BMP assays (i.e. biosludge and granules), and the net change during

enzymatic treatment does not result in more easily digestible substrates, which could explain the lack of

effect from BCE_2078 in BMP assays.

Figure 5-6 Enzymatic assays. a) protease activity assays for enzymes studied in BMP 1. Casein was

used as the standard substrate. b) glycosidase activity assays for enzymes studied in BMP 2 (except

lysozyme). Carboxymethyl cellulose (CMC) was used as the standard substrates, biosludge and a

combination of them. Active and inactive enzymes were included. Note the two vertical axis in part

b are in the same units but ranges are different. Error bars show standard deviation of triplicates.

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

Active +substrate

Inactive +substrate

Active +biosludge

Inactive +biosludge

Active +substrate +biosludge

Inactive +substrate +biosludge

mM

glu

cose/m

g e

nz/m

in

mM

glu

cose/m

g e

nz/m

in

SCO6604 Ctec 2

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

Active +substrate

Inactive +substrate

Active +biosludge

Inactive +biosludge

Active +substrate +biosludge

Inactive +substrate +biosludge

mM

Tyro

sin

e/g

en

z/m

in

A. oryzae B. licheniformis BCE_2078

a)

b)

109

As seen in Figure 5-6b, glycosidases were active on CMC and no significant activity was measured

for their inactive counterparts. SCO6604 showed far less activity than CTec 2, 0.341 (±0.002) and 0.013

(±0.002) mM glucose/mg enzyme/min, respectively. When incubated with biosludge as a substrate,

neither SCO6604 nor CTec 2 showed any activity. It is possible that because the assay relies on glucose

released, microorganisms in biosludge consume the glucose and it cannot be measured by the colorimetric

assay used in this study. When biosludge and CMC are added together, only CTec 2 shows significant

activity. However, biosludge pretreatment with CTec 2 did not show any significant increase in biogas

during the BMP assays. SCO6604 showed no glycosidase activity in the presence of biosludge which

suggests a possible inhibition or denaturation of the enzyme. There seems to be an interference of the

inactive SCO6604 and/or biosludge with the DNS assay. Inactive SCO6604, in the presence of CMC and

biosludge, showed “negative” enzymatic activity (Figure 5-6b). The activity of lysozyme was measured

using a standard substrate (M. lysodeikticus cells) and the inactivation was confirmed.

Conclusions

Enzymes can enhance the anaerobic digestibility of biosludge as measured by BMP assays

All enzymes included were found to increase biogas production. Proteases from B. licheniformis and

A. oryzae, a novel glycosidase (SCO6604) and lysozyme from chicken egg white, do so as a result of

their enzymatic activity.

COD solubilisation could not be identified as the mechanism for enhancing anaerobic digestibility of

biosludge. Alternatively, hydrolysis of soluble material is proposed as the reason for enhancing

anaerobic digestibility of biosludge.

Unexpectedly, it was found that the inoculum does not completely convert the COD of the enzyme

solution to biogas; in some cases, enzyme solutions negatively affect the inoculum, and decrease

biogas production.

110

Enzymatic assays showed low activity of the enzymes on biosludge but there was no significant

inhibition or denaturation.

No correlation was found between the enzymatic activities on standard substrates or biosludge, and

the effect of enzymes on biogas production during BMP assays.

A new approach for studying enzymatic treatment for enhanced anaerobic digestibility is proposed

here, where the COD contributed by the enzyme solutions and the effect of enzymatic activity, are

isolated by including inactive enzymes in each assay.

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6 Chapter 6 - Flocculating Activity of Lysozyme: A Non-Enzymatic Application

This chapter is based on the following article: Bonilla, S. and Allen, D. G. (2016), “Flocculation with

lysozyme: A non-enzymatic application”. Canadian Journal of Chemical Engineering, 94: 231–237.

Accreditations:

Sofia Bonilla designed and conducted all the experiments, analyzed and interpreted data, and prepared the

first draft of the manuscript.

D. Grant Allen provided advice on experimental design, analysis and interpretation of data and editing of

the manuscript.

Introduction

Many industries, including pulp and paper mills, wastewater treatment, and food processing plants,

require the addition of flocculants which enhance solid-liquid separation. It is widely acknowledged that

these separations are challenging and costly. Thus, it is not surprising to find a vast variety of flocculants

available which, in general, can be categorized into two groups: i: inorganic salts and ii: organic synthetic

polymers. A description of flocculation mechanisms can be found in the literature (Bolto, 2006; Sharma

et al., 2006; Gregory & Barany, 2011). Although widely used, inorganic and synthetic organic flocculants

present numerous disadvantages. The main drawbacks of inorganic salts are high dosage requirements,

pH sensitivity, and the potential for negatively for negatively affecting downstream processes (Sharma et

al., 2006). More recently, synthetic organic flocculants, also known as polyelectrolytes, have been widely

used and frequently preferred over inorganic salts. The main advantages of synthetic organic polymers

over inorganic salts are process-specific optimization, low dosage requirements, and ionic strength

flexibility (Bolto, 2006). Nevertheless, these flocculants are costly, their precursors are petroleum-based

(non-renewable) and there are concerns regarding their impact on the environment (Liber et al., 2005;

Bolto & Gregory, 2007). For example, polyacrylamide, the most widely used flocculant, has been

114

reported to have a toxic effect on aquatic systems, and its monomer, acrylamide, is known to be a

neurotoxin (Harford et al., 2011).

Bioflocculants have been studied as an environmentally-friendly alternative to inorganic salts and

synthetic organic polymers due to their biodegradability and renewable sourcing. Moreover, it is known

that microorganisms produce natural polymers that aid in the aggregation of organic material and cells.

Several researchers have explored the extraction of bioflocculants from microbial strains including pure

cultures and communities (Kurane et al., 1986; Salehizadeh & Shojaosadati, 2001; Shih et al., 2001; Wu

& Ye, 2007). Reports suggest that macromolecules, mixtures mainly of polysaccharides and proteins, are

responsible for the flocculating properties of such reported bioflocculants (Yokoi et al., 2002; Wu & Ye,

2007; Piazza & Garcia, 2010; Zhang et al., 2013).

Semi-pure and crude extracts from cultures have been reported as bioflocculants. In the case of semi-

purified bioflocculants, doses of 0.7 mg/g of kaolin for a bacterial-produced homopolymer of glutamic

acid (Shih et al., 2001) and 3.5–4.5 mg/g of kaolin for an unknown bioflocculant from Serratia sp. were

used (More et al., 2012). For “cruder” extracts, a higher dose was needed for partially-hydrolyzed protein

extracts from animal meal, e.g. 100–500 mg/g of kaolin (Piazza & Garcia, 2010) and 700 mg/g of kaolin

for an unknown bioflocculant extracted from sludge (Zhang et al., 2013). Research on biopolymers with

novel properties such as flocculating activity is still in its early stages and a better understanding of the

key properties for screening biopolymers with flocculating potential is needed. Particularly, the study of

the potential of proteins and enzymes as flocculants is limited.

We have recently discovered that lysozyme from chicken egg white, a widely-available and well-

studied enzyme, shifted the particle size distribution of wastewater secondary sludge towards larger

particle sizes, which suggested a flocculating activity (Bonilla et al., 2015). Additionally, the use of the

same enzyme without its catalytic activity (after heat-induced inactivation) also resulted in a similar

effect, indicating that the mechanism was not enzymatic. The only report of lysozyme’s flocculating

115

activity in the literature is by Kamaya, (1969) on yeast (Candida Albicans) and a more recent study

explored the adsorption of lysozyme on silica nanoparticles and noted the aggregation of such particles by

lysozyme (Bharti et al., 2011). However, the flocculation activity of lysozyme on inorganic and other

organic substrates has not been further studied nor quantified.

The overall objective of this study was to more fully characterize the flocculating characteristics of

lysozyme in its active and inactive form and elucidate its mechanism of action. This was achieved by:

comparing the flocculating activity of active and inactive lysozyme to that of a known flocculant;

determining the optimal concentration range for lysozyme as a bioflocculant in kaolin suspensions;

evaluating the effect of pH on the flocculating activity of active and inactive lysozyme; and exploring

synergies with the flocculating activity of lysozyme in the presence of cations.

Material and Methods

Lysozyme

Lysozyme from chicken egg white was purchased from Bioshop, Canada. Stock solutions at a

concentration of 1 g/L were prepared fresh with deionized water (18.2 MΩ cm). To compare the

flocculating properties of active and inactive lysozyme, the enzyme was inactivated by exposing the stock

solution to 103 °C for 6 hours followed by immediate exposure to -20 °C until the solution was fully

frozen (approximately 2 hours). An inactivated lysozyme solution was then thawed and a fresh solution of

the active lysozyme was prepared. The enzyme was irreversibly inactivated when exposed to 103 ˚C

followed by immediate storage at -20 ˚C. The inactivation was confirmed using the change in turbidity of

a Micrococcus lysodeikticus suspension as a standard method described in the literature (Chipman &

Sharon, 1969; Gorin et al., 1971; Lesnierowski & Kijowski, 2007).

Kaolin

Kaolin, a natural clay with chemical formula Al2O 3·2SiO2 ·2H2O has been widely used as a model

suspension and indicator of flocculating activities (Song et al., 2000; Shih et al., 2001; Divakaran &

116

Pillai, 2001; Yokoi et al., 2002; Wu & Ye, 2007; Piazza & Garcia, 2010; Zhang et al., 2013). Kaolin

carries a permanent negative charge which results in a stable suspension, ideal for flocculation studies.

Natural kaolin was purchased from Sigma Aldrich. A stock solution of 1g/L was prepared with deionized

water and the pH was found to be 5.1(±0.2). Additional stock solutions were prepared with the same

kaolin concentration and the pH was adjusted to 3, 7 and 9 using 1M NaOH and HCl solutions. The stock

solution was mixed vigorously before retrieving samples to ensure homogenization.

Polymer – Polyacrylamide (PAM)

A cationic polymer was used to compare the flocculating activity, pH range and dose of lysozyme to

those of a standard flocculant. A stock solution of a water-soluble polyacrylamide -based polymer

(GB1000 from SNF, Canada) was prepared fresh on the day of the experiment adding 0.5g in 0.2 L of

pure deionized water. The polymer was added to water while vortexing and further mixed for 2 hours.

The polymer solution was allowed to sit for one hour before the experiments as per manufacturer’s

recommendations.

Cation Supplements

Since divalent cations actively participate in flocculating phenomena and can stimulate the

flocculating activity of colloids by charge neutralization and bridging of particles (Salehizadeh &

Shojaosadati, 2001; Li et al., 2012), the effect of cations on the flocculating activity of lysozyme was also

evaluated. Three different cation stock solutions (0.5M MgSO4, FeSO4 and CaCl2) were prepared in

deionized water to supplement the flocculation activity of lysozyme solutions. Cation stock solutions

were added to the kaolin suspensions to achieve final concentrations of 0.05mM, 0.5mM, 5mM and

50mM. Experiments with cations were carried out using the optimum lysozyme concentration for

flocculation at the natural pH of kaolin.

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Additional Substrates for Flocculation

Microalgae and powdered activated carbon (PAC) suspensions were used as examples of potential

industrial applications of lysozyme as a bioflocculant (Hamada & Miyazaki, 2004; Ayotamuno et al.,

2007; Vandamme et al., 2013). For microalgae, a pure culture of Scendesmus obliquos grown on a

synthetic wastewater medium was obtained and adjusted to a concentration of 1 g/L. Lysozyme was

added at doses of 10 and 100 mg/L. For the PAC, a suspension was prepared by crushing 1 g of activated

carbon and dissolving it in a liter of deionized water. For both the microalgae and PAC the flocculating

activity experiments were carried out as described in section 6.2.6.

Flocculating Activity

Nine ml of kaolin solution (1g/L) were dispensed into each glass tube (16 x 100mm), the appropriate

amount of polymer, active and inactive lysozyme solutions were subsequently added. Tubes were mixed

by inverting them and then were left undisturbed at 22 (±2) ˚C during the experiment. Absorbance

measurements of the tubes were carried out using a DR3900 spectrophotometer (Hach, US) at 550 nm.

The instrument read the absorbance directly from the tubes, resulting in minimal disturbance of the

samples over time. All absorbance measurements were below 1.2. Controls were identically prepared but

instead of the lysozyme solutions, the same volume of deionized water was added. The flocculating

activity was calculated as expressed in the equation below after Kurane et al., (1986); Yokoi et al., (2002)

and More et al., (2010):

Equation 6-1 Flocculating activity (%) = (𝑨𝒃𝒔𝑪𝒐𝒏𝒕𝒓𝒐𝒍 −𝑨𝒃𝒔𝑺𝒂𝒎𝒑𝒍𝒆

𝑨𝒃𝒔𝑪𝒐𝒏𝒕𝒓𝒐𝒍) × 𝟏𝟎𝟎

Zeta Potential

The zeta potential of kaolin suspensions was measured to identify if charge neutralization was

playing a role in the flocculating activity of lysozyme observed at pH 5 and pH 7. Zeta potentials were

obtained using an acoustic and electroacoustic spectrometer (DT1200, Dispersion Technologies, Bedford

118

Hills, USA) equipped with conductivity and electroacoustic probes. The instrument measures the particle

size distribution and the colloidal vibration current (CVI) and with these values is able to calculate the

zeta potential (Dukhin & Goetz, 2002).

Gel Electrophoresis

To confirm the purity and size of active and inactive lysozyme, gel electrophoresis was carried out

using a Mini-PROTEAN® TGX™ precast gel, Laemmli sample buffer and SDS running buffer with a

constant voltage of 160V. Each sample was prepared with and without 2-mercaptoethanol. This reducing

agent is widely used in gel electrophoresis to denature proteins to their primary structure by breaking

disulfide bonds. The original Laemmli buffer recipe includes the addition of 2-mercaptoethanol to break

disulfide bonds. Complete recipe of sample buffer and running buffer is shown in Appendix IV.


Effect of Lysozyme Concentration and pH

Lysozyme has a significant flocculating activity on kaolin suspensions and its effect is pH dependant

(Figure 6-1). There was significant flocculating activity of active and inactive lysozyme on kaolin

suspensions at pH 5 and 7 (Figures 6-1b–c). For kaolin suspensions at pH 5, flocculation with the cationic

synthetic polyacrylamide (PAM) occurred faster than with lysozyme solutions. The flocculating activity

of PAM was significantly faster than the effect of lysozyme; however, both reached a flocculating activity

of approximately 60 % after 180 min. At pH 7, lysozyme, independently of its activity, resulted in better

flocculation than PAM (Figure 6-1c). This is possibly explained by the folding changes that polymers

such as PAM undergo at different pH levels and which affect the flocculating potential of polymers

(Gregory & Barany, 2011).

Lysozyme exhibited similar flocculating activity on kaolin suspensions independent of its enzymatic

activity. At pH 5 and after 180 min of treatment, active and inactive lysozyme solutions at a concentration

119

of 10 mg/L reached the same level of flocculation as PAM (1 mg/L) with a flocculating activity of

80 ± 4 %. For kaolin suspensions at pH 7 the trend was similar to pH 5 suspensions. The flocculation

appears to occur faster at pH 7 than at pH 5 (Figure 6-1c). No flocculating activity was observed on

experiments with lysozyme concentrations of 1 and 100 mg/L, suggesting that there is an optimum

concentration for flocculation close to 10 mg/L.

Figure 6-1 Effect of the concentration of lysozyme on the flocculation of kaolin solutions under

different pH conditions; a) pH 3, b) pH 5.1 (Non-adjusted), c) pH 7 and d) pH 9. Error bars


-800

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a) – pH 3 b) – pH 5

c) – pH 7 d) – pH 9

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0 20 40 60 80 100 120 140 160 180 200

Flo

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Time (min)

Lys Active 1 mg/L Lys Active 10 mg/L Lys Active 100 mg/L

Lys Inactive 1 mg/L Lys Inactive 10 mg/L Lys Inactive 100 mg/L

PAM 1mg/L PAM 10 mg/L PAM 100 mg/L

120

The mechanisms of lysozyme and PAM flocculation appear to be different. The mechanism of

lysozyme seems to be charge neutralization of negatively-charged particles, while the flocculating activity

of PAM seems to be charge neutralization due to its cationic charge and bridging of particles. At pH 5 and

7, both flocculants perform similarly. The mechanistic difference becomes apparent at pH 3 and 9, where

large differences could be observed between the two flocculants. Surfaces of kaolin at pH 5 expose

hydroxide groups, which results in a stable colloidal suspension. At low pH, protonation of those surfaces

can reduce the stability of the suspension, reducing the repulsive forces and allowing the particles to

settle. Lysozyme has a high isoelectric point of 10.5–11 (Salton, 1957) at a pH below this point a

molecule of lysozyme has a net positive charge.

Lysozyme does not improve the flocculation of kaolin suspensions at low pH; instead, our results

suggest that it reduces flocculation due to its cationic charge and its repulsion to the protonated surfaces

of kaolin particles, resulting in a re-stabilized suspension. In contrast, PAM showed significant

flocculating activity at pH 3, suggesting that its mechanism is not only charge neutralization but possibly

the bridging of kaolin particles due to its size. It has been noted that particle bridging is the major

flocculation mechanism when high molecular mass polymers such as PAM are used, even when the

particles and polymer carry the same charge (Gregory & Barany, 2011). When the suspension was at pH

9, the enzyme was approaching its isoelectric point which reduces its cationic charge, and thus no

significant flocculating activity was observed. PAM showed significant flocculating activity but required

a higher dose to achieve similar flocculation.

Effect of Cation Concentration

The flocculating activity of lysozyme was not affected by the addition of Mg2+ and Ca2+. Kaolin

suspensions supplemented with Ca2+ in the form of CaCl2 showed no significant effect on the flocculating

activity of active and inactive lysozyme at various cation concentrations (0.05 mM to 5 mM) (Figure 6-

2a). A supplement of cations only (in the absence of lysozyme) had a negative effect on the flocculating

121

activity at a concentration of 0.05 mM, and the flocculating activity was increasing with the cation

concentration until reaching the same flocculating activity of lysozyme solutions with cations (Figure 6-

2a Figure 6-2b). This effect could be due to the ability of cations to neutralize residual carboxyl groups in

lysozyme and bridge with kaolin or directly bind to negatively charge groups in kaolin. No significant

difference was observed between the addition of Mg2+ and the addition of Ca2+ ions.

Figure 6-2 Effect of cation addition on the flocculating activity of lysozyme after 180min of

treatment with a lysozyme dose of 10 mg/L. a) CaCl2, b) MgSO4 and c) Fe2 (SO4)3. Error bars


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122

In contrast, adding Fe2(SO4)3 had a negative effect on the flocculating activity of lysozyme. Low

concentrations of Fe2+ ions (0.05 and 0.5 mM) resulted in a reduction of lysozyme’s flocculating activity

to almost half the activity observed in the absence of cations (Figure 6-2c). Cations can inhibit the

adsorption of polymers and it seems to be the case of Fe2+ since low concentrations of these ions result in

a significant negative effect on the flocculating activity of lysozyme.

The flocculating activity of lysozyme is not generally affected by the addition of cations. Ca2+ and

Mg2+ did not have an effect and Fe2+ ions showed some flocculation inhibition at low concentrations.

Lysozyme was found to flocculate kaolin suspensions in the absence of cations. At high cation

concentration, when lysozyme is present, it is not clear whether lysozyme, the cations or both are

contributing to the flocculating activity observed. However, these results are evidence that there are no

positive synergistic effects of cations and lysozyme solutions as it is the case of other bioflocculants

(Salehizadeh & Shojaosadati, 2001).

Zeta Potential and Lysozyme’s Flocculating Activity

As expected, lysozyme and polymer addition to kaolin suspensions resulted in some destabilization

(Figure 6-3). At pH 5, polymer addition resulted in a larger effect taking the zeta potential from -44.2 mV

for the kaolinite suspension to -26.1mV after the addition of polymer at a concentration of 1 mg/L.

Consistent with the observations of flocculating activity at pH 5 (Figure 6-1b), lysozyme also reduced the

stability of the suspension but in a lesser degree, resulting in -37.5 and -33.4 mV, for active and inactive

lysozyme, respectively, at a concentration of 10 mg/L. At pH 7, the destabilization of the suspension was

similar for the polymer, active and inactive lysozyme.

Flocculation of Algae and Activated Carbon

Active and inactive lysozyme showed a significant flocculating activity on activated carbon. All

samples resulted in a flocculating activity of 30 (±4.5) % after 180min. No significant difference was

123

found between active and inactive solutions, or between the two concentrations studied, see Figure 6-4.

Overall, the effect of lysozyme on activated carbon was positive; however, the flocculating activity of

lysozyme on the activated carbon solution was significantly lower than the activity observed on kaolin

solutions. Different concentrations, pH or other conditions could potentially increase the flocculating

activity of lysozyme on activated carbon.

Figure 6-3 Zeta potential of kaolin suspensions at pH 5 and pH 7 with doses of PAM and lysozyme

that resulted in significant flocculating activity. Error bars show standard deviation of duplicates.

The initial negative flocculating activity observed for algae suspensions (Figure 6-4) can be

explained by the multi-step flocculation process. During the first 90 min, it is suspected that algae cells

and lysozyme are slowly aggregating but settling is not sufficient to reduce the absorbance of the

suspension. Once significant aggregation has occurred, settling happens, and the flocculating activity can

be measured. For charge neutralization mechanisms, the particles in the suspension are first destabilized

by the charge of the enzyme, allowing them to overcome their repulsion state. Once that barrier is broken,

the flocculant can be adsorbed onto the surface of particles in a process called the “patch mechanism.”

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Control ActiveLys 10mg/L

InactiveLys 10mg/L

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PAM 1mg/ L

Kaolin pH 5 Kaolin pH 7

Zeta

Pote

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V)

124

When cationic patches are created, these can interact with negative surfaces in other particles which

subsequently results in flocculation, and eventually the settling of particles.

Figure 6-4 Flocculating activity of lysozyme active and inactive on left: powdered activated Carbon

and right: microalgae. Error bars represent the standard deviation of triplicates.

Lysozyme Active vs. Inactive

The inactivation of lysozyme with heat treatment is the result of new inter-molecular disulfide bonds.

Lysozyme from chicken egg-white has 129 amino acid residues and a molecular weight 14,400Da

(Lesnierowski & Kijowski, 2007). The ladder on the left of Figure 6-5 was used to compare the relative

size of lysozyme to literature reports. An approximate size of 14 kDa was confirmed for both active and

inactive lysozymes in the presence of the reducing agent. However, without the reducing agent, i.e. with

the presence of disulfide bonds, the inactive lysozyme showed various bands (sizes). After the heat

treatment, when lysozyme molecules have been denatured and are re-folding, they appear to form

intermolecular disulfide bridges resulting in larger molecules. In other words, the inactive lysozyme is a

polymer of lysozyme molecules bound by disulphide bonds.

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125

The polymer size did not play a major role in the flocculating performance of lysozyme. The size

and charge of polymers are key properties that are known to affect their flocculating properties. Charged

polymers reduce the repulsion in a colloidal suspension, allowing polymer adsorption and subsequent

flocculation. Bridging mechanisms of flocculation are more likely in larger polymers such as

polyacrylamides (Kitchener, 1972). Figure 6-5 shows that the solution with inactive lysozyme consisted

of polymers ranging from 14 to approximately 100 kDa. This increase in polymer size of the inactive

lysozyme did not result in significant changes in the flocculating activity of lysozyme, since in most cases

active and inactive lysozyme resulted in similar flocculating profiles.

Figure 6-5 Gel electrophoresis of active and inactive lysozyme. Samples were treated with and

without a reducing agent (2-mercaptoethanol) to visualize intermolecular disulfide bonds

Lysozyme (14kDa)

250 kDa

50 kDa

10 kDa

150 kDa

100 kDa

75 kDa

37 kDa

25 kDa

20 kDa

15 kDa

126

Lysozyme Flocculating Mechanisms

The flocculating properties of lysozyme on kaolin, powdered activated carbon, and algae

suspensions seem to be the result of the cationic charge of lysozyme at close-to-neutral pH values. Our

results suggest that the main mechanism associated with lysozyme as a bioflocculant is charge

neutralization by patch adsorption. A schematic of the flocculating mechanisms of active and inactive

lysozyme and their interaction with kaolin particles is shown in Figure 6-6. In the inactive lysozyme

solution, molecules are larger due to intermolecular disulphide bonds, and therefore a bridging

mechanism is more likely than for the active lysozyme. However, the lack of significant differences in

flocculating activity in most experiments suggests that the bridging mechanism is not relevant for the

conditions studied. However, it is also possible that some of the larger polymers present in the inactive

lysozyme solution could potentially be adsorbed onto more than one particle.

Figure 6-6 Proposed Mechanism of Lysozyme Flocculation. Not to scale.

Active Lysozyme

(129 amino acids,

intra-molecular

disulfide bonds)

Inactive Lysozyme

(>129 amino acids,

inter-molecular

disulfide bonds)

Disulfide Bonds Negatively Charged

Kaolin Particle

Repelled particles in kaolin

suspension

Lysozyme

molecules

adsorbed onto a

kaolin particles

127

Conclusions

The flocculating potential of lysozyme was demonstrated across various substrates. Active and

inactive lysozyme showed a flocculating activity comparable to that of a cationic polyacrylamide at pH 5

and a higher flocculating activity at pH 7. At pH 3 there was a negative effect on flocculation while at pH

9 no significant effect could be observed. Ca2+ and Mg 2+ divalent cations did not have a significant effect

on lysozyme’s flocculating properties The flocculating potential of lysozyme was also confirmed with

algae and powdered activated carbon suspensions. Our results suggest that there is potential for using

cationic proteins as flocculants.

References

Ayotamuno, M. J., Okparanma, R. N., Ogaji, S. O. T., & Probert, S. D. (2007) Chromium removal from

flocculation effluent of liquid-phase oil-based drill-cuttings using powdered activated carbon. Appl Energ

84: 1002.

Bharti, B., Meissner, J., & Findenegg, G. (2011) Aggregation of Silica Nanoparticles Directed by

Adsorption of Lysozyme. Langmuir 27: 9823.


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Water Res 68: 692.

Chipman, D. M., & Sharon, N. (1969) Mechanism of Lysozyme Action. Science 165: pp. 454-465.

Divakaran, R., & Pillai, S. (2001) Flocculation of kaolinite suspensions in water by chitosan. Water Res

35: 3904.

A. Dukhin, P. Goetz., Ultrasound for characterizing colloids particle sizing, zeta potential, rheology.

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Gorin, G., Wang, S. F., & Papapavlou, L. (1971) Assay of lysozyme by its lytic action on M.

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Gregory, J., & Barany, S. (2011) Adsorption and flocculation by polymers and polymer mixtures. Adv

Colloid Interface Sci 169: 1.

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membrane aided by flocculation and activated carbon treatments. Desalination 169: 257.

Harford, A. J., Hogan, A. C., Jones, D. R., & van Dam, R. A. (2011) Ecotoxicological assessment of a

polyelectrolyte flocculant. Water Res 45: 6393.

Kamaya, T. (1969) Flocculation phenomenon of Candida albicans by lysozyme. Mycopathologia 37: 320.

Kitchener, J. A. (1972) Principles of action of polymeric flocculants. Brit Polym J 4: 217.

Kurane, R., Takeda, K., & Suzuki, T. (1986) Screening for and Characteristics of Microbial Flocculants.

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Li, H., Wen, Y., Cao, A., Huang, J., Zhou, Q., & Somasundaran, P. (2012) The influence of additives

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More, T. T., Yan, S., Tyagi, R. D., & Surampalli, R. Y. (2010) Potential use of filamentous fungi for

wastewater sludge treatment. Bioresour Technol 101: 7691.

More, T. T., Yan, S., Hoang, N. V., Tyagi, R. D., & Surampalli, R. Y. (2012) Bacterial polymer

production using pre-treated sludge as raw material and its flocculation and dewatering potential.


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Salehizadeh, H., & Shojaosadati, S. A. (2001) Extracellular biopolymeric flocculants. Biotechnol Adv 19:

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Salton, M. R. J. (1957) The properties of lysozyme and its action on microorganisms. Bacteriol Rev 2: 82.

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Yokoi, H., Obita, T., Hirose, J., Hayashi, S., & Takasaki, Y. (2002) Flocculation properties of pectin in

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7 Chapter 7 - A Look into the Potential of Cationic Proteins and Cationic Fractions to Enhance Solid-Liquid Separations

Introduction

There is recent interest in using biopolymers to reduce or replace the use of synthetic polymers for

facilitating otherwise challenging liquid-solid separations. The potential of enzymes to improve biosludge

dewaterability has been previously evaluated. Lysozyme was the only enzyme found to improve the

dewatering properties of biosludge after screening several enzymes. This improvement appears to be the

result of lysozyme’s cationic charge and is independent of lysozyme’s catalytic activity (Bonilla et al.,

2015). In addition to enhancing biosludge dewaterability, lysozyme flocculates kaolin, microalgae and

powdered activated carbon suspensions (Bonilla T. & Allen, 2016). These applications are clear examples

of the potential of lysozyme to improve liquid-solid separation processes. Moreover, since the cationic

charge appears to play an important role in the mechanism of lysozyme, looking for other cationic

proteins with potential as flocculants is of interest.

The discovery of proteins that perform better and/ or are less expensive than currently used synthetic

polymers can improve the feasibility of using biopolymers as flocculants in industrial processes.

Currently, synthetic polymers are the most commonly used conditioners in industry because they are

effective flocculants and require low dosages (Bolto, 2006). Lysozyme has shown potential as a

flocculant but its doses remain higher than most synthetic polymers (Bonilla et al., 2015, Bonilla T. &

Allen, 2016). Therefore, to compete in the future with synthetic polymers, the limitations of lysozyme as

a flocculant must be addressed by: a) increasing the effectiveness of proteins as flocculants and/or b)

finding low cost sources of cationic proteins. An approach to solve these issues is to find commercially

available cationic proteins that have high flocculating activity, alternatively, finding high flocculating

131

proteins in waste sources could reduce the costs of cationic proteins while adding value to a waste stream.

Based on this, the main objective of this study is to further assess the potential of cationic proteins and

cationic fractions as flocculants. To this end, the following sub-objectives were set:

Evaluate the effect of protamine, a commercially available cationic protein, on biosludge

dewaterability.

Characterize the effect of protamine on chemical oxygen demand (COD), protein and

carbohydrate content of biosludge.

Investigate the flocculating potential of protamine on kaolin suspensions under different pH

conditions.

Determine if cationic fractions can be extracted from biosludge and if this fractions can enhance

solid-liquid separations.


To investigate the potential of cationic proteins and meet the objectives, the approach taken in this

study is summarized in Figure 7.1

Figure 7-1 Experimental approach to investigate the potential of cationic proteins as flocculants

Investigating the potential of cationic proteins as flocculants

Commercial cationic proteins (i.e. protamine)

Cationic fractions from biosludge

Effect of protamine on biosludge dewaterability

Effect of protamine on chemical

composition of

biosludge

Effect of protamine on kaolin

suspensions

Effect of cationic fractions on the dewaterability of

biosludge and anaerobic

digested sludge

Effect of incubation

conditions on

extraction process

132

Sludge Samples


produces a variety of pulp, paper and specialty products using BCTMP (bleached chemi-

thermomechanical pulp) processes. The sludge was kept at 4°C in the laboratory prior to analysis for a

maximum of two weeks. Biosludge was left to settle for at least 2 hours and the supernatant discarded to

obtain a thickened sludge. To re-activate the microbial community present in sludge, thickened sludge

was aerated for 1 hour and brought to room temperature before running experiments. Total suspended

solids (TSS) and pH were measured to be 25.2 (± 0.3) g/L and 7.2, respectively.

Cationic fractions extracted from pulp and paper mill biosludge were also tested on anaerobically

digested (AD) sludge. The effect of cationic fractions on AD sludge was evaluated to validate the

reproducibility of the results obtained with the pulp and paper mill biosludge. AD sludge was collected

from a wastewater treatment plant in the municipality of Toronto (Ashbridges Bay). Samples were taken

to the laboratory, kept at 4°C and used within 4 hours of sampling. The same thickening and aeration

process previously described for pulp and paper mill sludge was used. The solids content of the AD

sludge was 22.4 (±1.1) g/L.

Cationic Proteins

Protamine was obtained from Sigma Aldrich to test its potential as a conditioner for enhanced

biosludge dewaterability. Protamines are small proteins (5-10 kDa) and have a net cationic charge at most

pH values (pI > 12) as a result of the high concentration of arginine residues in its amino acid sequence.

Various doses (w/v) of protamine were added to biosludge and incubated at 37 ˚C, 100 rpm for 2 hours.

Different doses of protamine were also added to kaolin suspensions to evaluate their potential as

flocculants. Lysozyme was also included in the experiments as a positive control and was prepared as

previously described in Bonilla et al, (2015).

133

Chemical Composition of Biosludge

To understand the changes that sludge undergoes during protamine conditioning, chemical oxygen

demand (COD), protein and carbohydrate content in the soluble portion of biosludge were measured

during the experiments.

Samples of biosludge during conditioning treatment with protamine and lysozyme were filtered

using a syringe filter with a pore size of 0.45 µm. The filtrate was used as the soluble fraction and

analysed for COD content, protein and carbohydrate. COD analyses were carried out according to the

Standard Methods for the Examination of Water and Wastewater closed reflux, colorimetric method

(5220 D).

The soluble protein content in biosludge was measured using the bicinchoninic acid (BCA) method

with a kit from Sigma-Aldrich. A calibration curve was prepared with Bovine Serum Albumin (BSA) as

the protein standard. The soluble carbohydrate content was evaluated using the phenol-sulphuric method

(Dubois et al., 1956). A calibration curve was prepared using glucose as the standard.

Dewaterability Assessment - Capillary Suction Time (CST)

Capillary Suction Time (CST) was used to evaluate the effect of protamine on the dewaterability of

biosludge. Biosludge samples were also conditioned with lysozyme to compare protamine’s performance.

A Type 304M Laboratory CST Meter (Triton Electronics Ltd.) was used and tests were performed in at

least triplicates at 22°C ±2°C as described in (Bonilla et al., 2015). A lower CST implies better

dewaterability. As a baseline, the CST of pure water was 5.4 (± 0.2) s. In falcon tubes, biosludge was

conditioned with different doses of protamine and lysozyme. All CST measurements were done at least in

triplicate. To test the effect of cationic fractions extracted from biosludge, biosludge from a pulp and

paper mill and anaerobically digested sludge from the city of Toronto was treated with cationic fractions

(0.1% w/w), see Section 7.3.5. The dewaterability was assessed via capillary suction time (CST) after 2

hours of treatment at 37˚C.

134

Flocculating Activity of Kaolin Suspensions

The flocculating activity of protamine was evaluated on kaolin suspensions following the same

procedure described in Bonilla T. & Allen, (2016). Kaolin suspension at pH 5, 7 and 9 were used to

assess the effect of pH and the equation used to calculate flocculating activity is as follows:

Equation 7-1 Flocculating activity (%) = (𝑨𝒃𝒔𝑪𝒐𝒏𝒕𝒓𝒐𝒍 −𝑨𝒃𝒔𝑺𝒂𝒎𝒑𝒍𝒆

𝑨𝒃𝒔𝑪𝒐𝒏𝒕𝒓𝒐𝒍) × 𝟏𝟎𝟎

Cationic Fractions from Biosludge

Biosludge was used as a potential source of cationic proteins to be used as flocculants. Up to 35% of

pulp and paper mill biosludge dry mass is proteins (Wood et al., 2009). Thus biosludge was used to if

cationic fractions could be extracted and if these fractions could be subsequently used as conditioners for

enhanced dewaterability. These fractions can potentially be a less-expensive alternative to commercial

products such as lysozyme or protamine and could be used in a portion of the biosludge or simply used as

a value-added product with a significant market potential. In previous studies, alkaline and acid extraction

have been used to extract potential flocculants from biosludge (More et al., 2012). In this study, the

extraction process will be based on charge using cation exchange chromatography.

First, it was investigated whether an incubation step prior to the extraction process would affect the

yield of protein extracted from biosludge. Biosludge was incubated overnight at four different

temperatures and mixing conditions: a) 4˚C/static, b) 37˚C/static, c) 25˚C/150 rpm and d) 37˚C and 150

rpm. After the overnight incubation, biosludge was sonicated for 25 min, a 5 second on/off pulse and

amplitude of 100. Sludge was subsequently centrifuged at 21,000 g for 40 min and the supernatant was

kept for further separation. A cation exchange resin (Macroprep S from biorad) was used to extract the

cationic fraction from the supernatant. Resin was added to the supernatant and was left in contact for 30

min. The mixture was added to a chromatography column and drained. The cationic fraction was then

135

eluted using three buffers: 200 mM, 500 mM and 1 M NaCl. The protein concentration was measured and

the solutions were concentrated in 15R vivaspin centrifugal concentrators for 30 min at 5,000 g and 4˚C.


Effect of Protamine on Biosludge Dewaterability

Protamine enhanced biosludge dewaterability and it required a lower dose than lysozyme to achieve

a similar dewaterability improvement (i.e. CST reduction) (Figure 7-2). A dose of 0.5% of lysozyme was

needed to achieve a CST of 6.9 (±0.1) s while a dose of 0.15% of protamine resulted in CST values of 6.5

(±0.4) s. An overdose was observed with protamine at a dose of 0.5%. A similar overdose was also

observed at higher doses of lysozyme (> 0.5%) (Bonilla et al., 2015). Lower doses of protamine were

tested to find out the dose that resulted in the lowest CST (i.e. optimum dose) given that the dose range

presented in Figure7-2 did not include doses from 0 to 0.15%. As can be seen in Figure 7-3, the optimum

dose of protamine was somewhere between 0.13 and 0.17%. Protamine has a higher cationic charge (pI ~

12.5) than lysozyme (pI ~ 10.7) at the pH of the sludge (i.e. pH 7.2) which explains the lower optimum

dose of protamine. Our results support that charge plays an important role in lysozyme’s and protamine’s

effect on biosludge dewaterability.

Figure 7-2 Effect of protein dose on biosludge dewaterability after 2 h of treatment. Error bars

represent standard deviation of duplicates. Asterisk indicates statistically significant differences

between lysozyme and protamine at 0.15% w/v.

0

2

4

6

8

10

12

14

0.0 0.1 0.2 0.3 0.4 0.5

Capill

ary

Suction T

ime (

s)

Protein Dose (% w/v)

Protamine

Lysozyme

*

136

Figure 7-3 Effect of low doses of protamine on the CST of biosludge. Error bars represent standard

deviation of duplicates.

Effect of Protamine on Soluble COD, Protein and Carbohydrate

The trends observed in soluble COD and protein content during protein conditioning share

similarities with the trends observed in CST (Figure 7-4). Protamine shows a significant reduction of

soluble COD at its optimum dose (i.e. 0.15%) and as the dose of protamine increases there is a substantial

increase in soluble COD (Figure 7-4a). The lowest COD observed with lysozyme and protamine was

similar, 400 (± 10) mg/L at a dose of 0.5 and 0.15%, respectively. These doses also resulted in the lowest

CST values, suggesting a possible cause and effect relationship between soluble COD and CST (Figure 7-

2). The same trend was observed with soluble protein (Figure 7-4b). A decrease in soluble proteins was

observed at the doses at which protamine and lysozyme had a positive effect on dewaterability. Once the

optimum dose of protamine is surpassed (i.e. overdose), soluble protein sharply increases.

Soluble COD and protein content in biosludge are affected in two opposite ways during treatment

with lysozyme and protamine. The amount of protein in the system increases by the addition of cationic

proteins. On the other hand, there is flocculation of particles in biosludge leading to an overall reduction

of organic material in the soluble fraction of biosludge. Our results suggest that an overdose is

accompanied by an increase in soluble protein, which in turn increases the soluble organics (i.e. soluble

0

2

4

6

8

10

12

14

0.00 0.05 0.10 0.15 0.20

Capill

ary

Suction T

ime (

s)

Protein Dose (% w/v)

Protamine

137

COD). At a dose of higher than 0.15%, protamine molecules may not be effectively interacting with

biosludge particles for two reasons: a) the negative charge of biosludge was neutralized and/or b)

adsorbed molecules are physically impeding interaction of new protamine molecules and biosludge

particles.

Figure 7-4 Effect of protein dose on the a) chemical oxygen demand (COD); b) soluble protein and

c) soluble carbohydrate content of biosludge after 2 h of treatment with protamine and lysozyme.

Error bars represent standard deviation of duplicates.

0

500

1,000

1,500

2,000

2,500

0 0.1 0.2 0.3 0.4 0.5 0.6

CO

D (

mg

/L)

Lysozyme

Protamine

0

200

400

600

800

0 0.1 0.2 0.3 0.4 0.5 0.6

Pro

tein

(m

g/L

)

Protamine dose (%w/v)

Lysozyme

Protamine

0

50

100

150

200

0 0.1 0.2 0.3 0.4 0.5

Ca

rbo

hyd

rate

s (

mg

/L)

Protein dose (% w/v)

Lysozyme

Protamine

a)

b)

c)

138

In comparison with the effect of cationic proteins on the soluble protein and COD content of

biosludge, carbohydrates were less affected. As seen in Figure 7-4c, a significant reduction in

carbohydrates was observed with both cationic proteins. However, the effect of protamine was greater,

reducing the concentration of carbohydrates from 176 (±12) mg/L for the control to 106 (±11) mg/L. At

its optimum dose, the carbohydrate content of biosludge treated with lysozyme was 129 (±12) mg/L.

Above the optimum dose there was no substantial increase in carbohydrate content which supports that

the increase in COD could be due to extra protamine in the system.

The compositional changes in biosludge during treatment with cationic proteins are in agreement

with flocculation as the proposed mechanism by which cationic proteins improve biosludge

dewaterability. Cationic proteins and biosludge particles are attracted due to their opposite charges. This

results in larger particles that are no longer in the “soluble” fraction of biosludge as it is validated with the

effect of cationic proteins on soluble COD, protein and carbohydrates.

Flocculating Activity of Protamine on Kaolin Suspensions

The flocculating activity of protamine was compared with results from previous studies of lysozyme

and PAM (cationic synthetic polymer) on kaolin suspensions (Bonilla T. & Allen, 2016 and Chapter 6).

Figure 7-5 shows the flocculating activity of: protamine, active lysozyme, inactive lysozyme and PAM on

kaolin suspension under different pH conditions.

Protamine showed substantial potential as a flocculant. It has a higher flocculating activity than

lysozyme and at pH 7 and 9, protamine showed a higher flocculating activity than the synthetic polymer

(PAM). As shown in Figure 7-5, protamine showed an activity >70% after 90 min at pH 5, 7 and 9. The

performance of lysozyme was only comparable at pH 7 but still the rate and extent of flocculation was

higher for kaolin suspensions treated with protamine. When compared to PAM, at pH 5, protamine

showed similar performance but the dose needed to achieve similar flocculation is higher (3.5 mg/ml vs. 1

mg/ml) and the rate of flocculation with PAM was higher. However, at pH 7 and 9, protamine is overall

139

better than PAM. The dose needed for PAM is higher (10mg/ml) and protamine’s optimal dose remains at

3.5 mg/ml (the effect of different doses of protamine on kaolin suspensions can be found in the

Appendices). The performance of synthetic polymers as flocculants is greatly affected by the pH and

ionic strength of the suspensions (Gregory & Barany, 2011). Our results show a clear advantage of

protamine over synthetic cationic polymers being an effective flocculant at a wide pH range.

Figure 7-5 Flocculating activity of protamine, lysozyme and a synthetic polymer (PAM) on kaolin

suspensions at their optimum doses and three different pH values: a) pH 5, b) pH 7 and c) pH 9.

Error bars show standard deviation of triplicates. Note that for a) PAM dose is 1 mg/ml and for b)

and c), PAM dose is 10 mg/ml.

-20

0

20

40

60

80

100

0 50 100 150 200

Flo

ccu

latin

g A

ctivi

ty %

Time (min)

PAM 1 mg/mlProtamine 3.5 mg/mlLys Active 10 mg/mlLys Inactive 10 mg/ml

0

20

40

60

80

100

0 50 100 150 200

Flo

ccula

ting A

ctivi

ty %

Time (min)

PAM 10 mg/mlProtamine 3.5 mg/mlLys Active 10 mg/mlLys Inactive 10 mg/ml

-20

0

20

40

60

80

100

0 50 100 150 200

Flo

ccula

ting A

ctivi

ty %

Time (min)

PAM 10 mg/ml

Protamine 3.5 mg/ml

Lys Active 10 mg/ml

Lys Inactive 10 mg/ml

a)

b)

c)

140

Cationic Extractions from Biosludge - Effect of Incubation Conditions on the Extract Yield

Overnight incubation prior to the extraction of cationic fractions affected the yield of protein

recovered from biosludge (Figure 7-6). Sludge was incubated at four different conditions i.e. a) 4˚C,

static, b) 25˚C, 150rpm, c) 37˚C, static and d) 37˚C, 150 rpm. Re-activating the biomass in biosludge

using temperature and aeration resulted in higher protein yields after sonication. From the conditions

studied, overnight incubation at 37˚C and 150 rpm results in the highest protein recovery from biosludge.

Figure 7-6 Soluble protein in biosludge before and after sonication under different overnight

incubation conditions. Error bars represent standard deviation of triplicates.

The cationic fraction (CF) extracted from biosludge resulted in dewaterability improvements for both

sludges, biosludge from pulp and paper mill and anaerobically digested sludge from the city of Toronto

(Figure 7-7). The slight reduction in CST observed with lysozyme treatment is consistent with previous

results which suggest that a 0.1% dose of lysozyme has a minor effect on dewaterability and that the

optimum dose is closer to 0.5%. However, the CF at the same dose showed better results than lysozyme.

These results confirm the potential of using CF as sludge conditioners and flocculants.

0

0.5

1

1.5

2

2.5

3

3.5

Initial Sludge 4°C 25°C, 150 rpm 37°C 37°C, 150 rpm

Pro

tein

Co

nce

ntr

atio

n (

mg

/mL

)

After

Sonication

Before

Sonication

141

Figure 7-7 a) Capillary suction time of biosludge treated with lysozyme and a cationic fraction

extracted from biosludge; b): Capillary suction time of anaerobically digested sludge treated with

lysozyme and a cationic fraction extracted from biosludge. Conditioner dose for both experiments

was 0.1%. Error bars show standard deviation of triplicates.

Cationic fractions also resulted in improved settling of solids present in biosludge as supported by

visual observations (Figure 7-8). Biosludge treated for 2 hours and incubated at 37˚C, 150 rpm showed

differences in the settling of solids when treated with CF. From 13ml of sample added to each tube, the

controls (biosludge + H2O; biosludge + elution buffer), showed the solids to have settled to a volume of

10 ml. On the other hand, cationic fractions at a dose of 0.05 and 0.1% resulted in solids settled to a

volume of 9 and 8 ml, respectively.

0

5

10

15

Control Lysozyme CationicProteinFraction

Capill

ary

Suction T

ime (

s)

0

100

200

300

400

500

Control Lysozyme CationicProteinFraction

Capill

ary

Suctio

n T

ime (

s)

a) b)

H2O Elution

Buffer

0.1% 0.05%

Figure 7-8 Effect of cationic fractions on the settling of biosludge after 2h of treatment at 37

˚C and 150 rpm.

142

Conclusions

Protamine improves the dewaterability of biosludge. This cationic protein performs better than

lysozyme likely due to its higher isoelectric point (i.e. higher cationic charge). Soluble protein,

carbohydrates and chemical oxygen demand (COD) were affected by the addition of protamine. Overdose

of protamine was accompanied by a sharp increase in COD and protein content that could be the result of

saturation of protamine in the system.

In kaolin suspensions, protamine showed a high flocculating activity (>80%). This activity was

stable over pH values of 5, 7 and 9. More importantly, protamine performs significantly better at pH 7

and pH 9 than lysozyme and the synthetic polymer (PAM). At pH 5, the rate and the extent of

flocculation in kaolin suspensions with PAM is higher than for protamine.

Preliminary studies of cationic extraction from biosludge show that is possible to find fractions with

conditioning potential. Overall, there is great potential for using proteins and/or cationic fractions as

flocculants to enhance different liquid-solid separations. More research is needed to find other sources to

extract cationic fractions, optimize the extraction process from wastes and/or produce cationic proteins

using recombinant protein technology.

References






Water Res 68: 692.

Dubois, M., Gilles, K., Hamilton, J., Rebers, P., & Smith, F. (1956) Colorimetric Method for

Determination of Sugars and Related Substances. - Anal Chem 28: 350.

143


Colloid Interface Sci 169: 1

More, T. T., Yan, S., Hoang, N. V., Tyagi, R. D., & Surampalli, R. Y. (2012) Bacterial polymer

production using pre-treated sludge as raw material and its flocculation and dewatering potential.




144

8 Chapter 8 - Cationic Proteins for Enhancing Biosludge Dewaterability: A comparative Assessment of Surface and Conditioning Characteristics of Synthetic Polymers, Surfactants and Proteins

This chapter is based on the manuscript submitted to Separation and Purification Technology: Bonilla, S.,

and Allen, D. G. Cationic Proteins for Enhancing Biosludge Dewaterability: A comparative Assessment

of Surface and Conditioning Characteristics of Synthetic Polymers, Surfactants and Proteins.

Accreditations:

Sofia Bonilla designed and conducted all the experiments, analyzed and interpreted data, and prepared the

first draft of the manuscript.

D. Grant Allen provided advice on experimental design analysis, interpretation of data and editing of the

manuscript.

Introduction

Biosludge dewatering is a challenge in wastewater treatment plants. Biosludge, also known as waste

activated sludge, is a colloidal suspension of microbial aggregates with high moisture content (>98%) and

a gel-like matrix of extracellular polymeric substances that hinders the removal of water, making

biosludge particularly difficult to dewater (Li & Ganczarczyk, 1990; Frølund et al., 1996; Nielsen et al.,

2012). Several pretreatment and conditioning strategies are used to improve biosludge dewaterability.

Chemicals that improve biosludge dewaterability, also known as conditioners, are widely employed

in wastewater treatment plants. Synthetic, water-soluble polymers are the most commonly used. Cationic

polymers are preferred for negatively-charged colloidal suspensions, such as biosludge. The cationic

charge reduces the repulsion between polymer molecules and biosludge particles which destabilizes the

suspension and facilitates bridging of particles (Gregory & Barany, 2011). Bridging leads to large, strong

flocs and is the main mechanism by which polymers improve biosludge dewaterability (Kitchener, 1972;

Bolto, 2006). It has been reported that polymers that carry the same charge as the suspension can also lead

to flocculation with bridging as the sole mechanism (Zhou & Franks, 2006). It is acknowledged that while

145

charge neutralization aids particle bridging, it is not a requirement. Polymers are effective at low doses

but there are some disadvantages associated with their use as conditioners. They represent a major portion

of the overall cost of the treatment, are petroleum-derived, dose-sensitive and can be toxic to aquatic

systems (Dentel, 1993; Bolto, 2006). Moreover, high moisture content in the cake after dewatering has

been associated with the hydration of high molecular weight polymers (Dentel, 1993; Besra et al., 2002).

In addition to synthetic polymers, surfactants have also been proposed as potential conditioners of

biosludge. Their use has been extensively reported for enhancing liquid-solid separations in the mineral

industry. A review of studies in fine particle suspensions was prepared by Besra et al, (1998). Surfactant

addition is thought to complement polymer conditioning, when the end-goal is to reduce the moisture

content in cakes after mechanical dewatering (Sun et al., 2014). Reducing the surface tension of the

suspension facilitates movement of water through cake pores (Stroh & Stahl, 1990). Dual conditioning

(i.e., surfactant-polymer) has been reported on various suspensions and improvements were found

regardless of the iconicity (i.e. charge) of the surfactants studied (Chitikela & Dentel, 1998; Huang et al.,

2002; Besra et al., 2003). However, when surfactants have been used on biosludge, and as a single

conditioner step (i.e. in the absence of polymer), only cationic surfactants have shown improvements on

biosludge dewaterability (Yuan et al., 2011; Sun et al., 2014; Wang et al., 2014). The effect of surfactant

activity on biosludge dewaterability is still unknown.

Proteins have shown potential as a ‘greener’ alternative to enhance liquid-solid separations. Proteins

can improve the dewaterability of biosludge and promote the solid-liquid separation of kaolin

suspensions. Given the abundance of proteins in renewable materials and organic waste, it is conceivable

that proteins could be a feasible alternative to chemical conditioners in the near future. However, a lack of

understanding of the mechanisms and the key properties that affect the potential of proteins as

conditioners hinders the development of protein-based conditioners and treatments. Previous studies of

lysozyme on biosludge and kaolin suspensions suggest that charge neutralization is the main mechanism

146

for such enhancement (Bonilla et al., 2015; Bonilla T. & Allen, 2016). However, proteins have also been

reported to have surfactant activity (Possmayer et al., 2001). Thus, it is currently unknown if the protein’s

cationic surface charge and/or its surfactant activity is responsible for the improvement of biosludge

dewatering properties.

The aim of this study was to evaluate the effect of various conditioners representing the three

chemical groups previously discussed, i.e., polymers, surfactants, and proteins, on biosludge

dewaterability, to get a better understanding of their effect on dewatering properties. Surface charge,

surface tension and contact angles of conditioners were evaluated to investigate the effect of surface

properties on their potential to improve dewatering.


Biosludge


produces a variety of pulp, paper and specialty products using sulfite pulping and mechanical pulping

(bleached chemi-thermomechanical pulp- BCTMP). Biosludge is the by-product of the aeration stage in

the wastewater treatment plant treating mill effluents. Samples were kept at 4˚C in the laboratory prior to

the experiments and for a maximum of three weeks. All the experiments were carried out with the same

batch of biosludge which had a total suspended solids (TSS) content of 12.4 (±0.3) g/L and volatile

suspended solids (VSS) content of 10.5 (±0.3) g/L.

Conditioners

Synthetic Organic Polymers

Different cationic polymers were used to evaluate their surface properties and compare their effect

on dewaterability with surfactants and proteins. Polymers represent the benchmark as conditioners for

improving biosludge dewaterability since they are used in virtually all wastewater treatment plants. A

147

stock solution (0.5% w/v) of each polymer was prepared a day in advance of the experiment with pure

deionized water. Polymers were added to water while vortexing to facilitate dispersion. The suspension

was further mixed for 1 h and allowed to sit undisturbed until the next day when the experiments were

conducted. Four polymers with different characteristics were used in this study (Table 8-1).

Dewaterability assessment was conducted as described in section 6.2.4.

Surfactants

To test the effect of surfactants on the dewaterability of biosludge and investigate the effect of

surfactant activity on the potential of conditioners, three surfactants with different ionicity were selected.

Triton X-100, CTAB and SDS represent non-ionic, cationic and anionic surfactants, respectively, and

have been previously studied for enhancing biosludge dewaterability (Chitikela & Dentel, 1998; Huang et

al., 2002; Besra et al., 2003). See Table 8-1 for more information on the surfactants used in this study.

A stock solution of 8 g/L was prepared for each of the surfactants in deionized water. In each of the

experiments, the surfactants were added, mixed three times by inversion and left for 60 min before CST

measurements. Dewaterability assessment was conducted as described in section 6.2.4.

Proteins

Cationic proteins (active and inactive lysozyme, and protamine) were selected to investigate their

surface properties and their effect on dewaterability. In addition to cationic proteins, bovine albumin

serum (BSA) was added as a control since it does not carry a net cationic charge at the close-to-neutral

pH values of biosludge (Table 8-1). Active and inactive stock solutions of lysozyme (50 g/L) were

prepared as previously described in Bonilla T. & Allen, (2016). Stock solutions of protamine (20 g/L) and

BSA (65 g/L) were prepared in deionized water and mixed using a vortex until dissolved. Proteins were

added to biosludge and samples were mixed three times (by inversion) and left for 60 min before CST

measurements. Dewaterability assessment was conducted as described in section 6.2.4.

148

Table 8-1 Conditioners used in this study to compare their surface properties and effect on


Conditioners Supplier Charge*

Polymers

Zetag 8165 BASF Cationic (Medium-high)

Zetag 8185 BASF Cationic (High)

Organopol 5400 BASF Cationic (Low)

AF 9645 AXCHEM Cationic (High)

Surfactants

Triton X-100 Sigma Non-ionic

Sodium dodecyl sulfate (SDS) Sigma Anionic

Cetyltrimethylammonium bromide (CTAB) Sigma Cationic

Proteins

Lysozyme Bioshop pI** ~10.7

Protamine Sigma pI** ~12.5

Bovine Albumin Serum (BSA) Sigma pI** ~4.8

* Information provided by vendor

** pI : Isoelectric point

Surface Properties Analyses

Surface Charge

Surface charge measurements were performed with colloidal titration using the principles reported

by Kawamura et al., (1967). In a 50 ml beaker, 5 ml of conditioner sample, 2 ml of poly

(diallyldimethylammonium chloride) solution (3% w/w) and 2 drops of 0.1% (w/v) toluidine blue were

added and gently mixed. The mixture was then back-titrated by adding potassium salt of polyvinyl

sulphate (PVSK) (0.0025N) until the neutral endpoint, indicated by a change of color from blue to purple,

was maintained for at least 10 seconds. The milliequivalent charge of the samples was then compared to

that of pure water to find out the surface charge of conditioners.

Surface Tension

Surface tension of the conditioner and the conditioned sludge were measured with a Sigma 700

tensiometer (KSV Instruments, Helsinki, Finland) using the Wilhelmy plate method. Measurements were

carried out at 22 (±2) ºC, using a stabilization time of 10 min. Before every experiment, and under the

same conditions, the surface tension of deionized water was measured to confirm a value of 72 (±1)

149

mN/m for deionized water and ensure the accuracy of the instrument. Samples were measured at least 5

times and a maximum variability within replicates of ±2 mN/m was observed.

Contact Angle Measurements

Contact angles of the conditioners were measured on glass and on biosludge to investigate if their

wetting properties affected their potential for enhancing biosludge dewaterability.

8.2.3.3.1 Contact angle on biosludge

The biosludge surface was prepared as described by Liao et al., (2001). Using a goniometer, a drop

of conditioner was placed onto the glass surface while a video recorded. Drop images were later extracted

from the video. At least 5 drops were used to find the contact angle of the conditioners on biosludge.

Hence, 10 angle values (2 sides per drop) were used for each of the samples analyzed. The maximum

standard deviation observed was ±4º. Except for the polymeric flocculants, all the conditioners studied

were absorbed relatively quickly (< 10 s) into the biosludge surface. Thus, all the measurements were

taken from images taken after 5 s of contact when there was no observable absorption.

8.2.3.3.2 Contact angle on glass

As a result of the challenges associated with using biosludge as a surface for contact angle

measurements (e.g. surface roughness, variability and absorption of conditioners), the contact angle of

conditioners was also measured on glass to compare the results obtained on biosludge surfaces. Glass

microscope slides were cleaned with 70% ethanol, rinsed with deionized water and oven- dried before the

experiments. Unlike biosludge surfaces, conditioners were not absorbed into glass, thus, angles were

measured when no further expansion of the drop was observed and evaporation was not significant. At

least 5 drops were used to find the contact angle of glass for each of the conditioners. Hence, 10 angle

values were used for each of the samples analyzed.

150

Dewaterability Assessment

Capillary Suction Time (CST)

Capillary Suction Time (CST) was used to evaluate the effect of the conditioners on the

dewaterability of biosludge. The instrument consists of two electrodes: once the water reaches the first

electrode after travelling through a filter paper from the sludge reservoir, a timer counts the seconds until

the water reaches the second electrode where the timer stops. The time required for water to travel from

the first to the second electrode is the CST. A lower CST implies better dewaterability. As a baseline, the

CST of pure water was 5.4 (± 0.2) s. A Type 304M Laboratory CST Meter (Triton Electronics Ltd.) was

used and tests were performed in at least triplicates at 22°C ±2°C. In falcon tubes, biosludge was

conditioned with at least five different doses for each of the conditioners studied. All CST measurements

were done in triplicate.

To test if the conditioners had an effect on CST, the CST of aqueous suspensions for each of the

conditioners in this study was assessed. Aqueous suspensions of the conditioners were added to water at

the same volumes that were added to biosludge to achieve the greater effect on dewaterability and CST

was measured. Results of the conditioners in aqueous suspensions can be found in the Appendices.

Crown Press

To compare the CST results and obtain a dewaterability assessment more applicable to industrial

practice, a bench-scale belt press (Crown® press from Phipps & Bird Inc.) was used. Samples of 200 mL

of biosludge and the respective conditioner were first transferred to the gravity thickening component of

the equipment for 10 min. During gravity thickening, the filtration rate was measured and after 10 min,

the filtrate was collected and analyzed for total solids content. The resulting cake was transferred to the

belt press area where a pressure schedule of 120, 150 and 200 lbs (6.3, 7.9, 10.5 psi, respectively) was

used for all samples. Each pressure was sustained for 10 s followed by a fast release. The total solids

content in the cake was measured to assess the effect of the conditioners on mechanical dewatering of

151

biosludge. Crown press experiments were prepared in duplicates and the total solids for each replicate

was measured in triplicate.

Where an “optimum” dose was evident from the CST data, the doses before and after such dose, as

well as the “optimum” were used for further dewaterability assessment with the crown press. Three doses

were tested per conditioner. When there was no clear optimum from the CST data, three consecutive

doses that had shown a significant effect on CST were selected. The control for these experiments was a

sample of biosludge with the same volume of deionized water added instead of conditioner.


Effect of conditioners on CST, cake and filtrate solids content

From the four proteins tested, only the proteins with cationic charge (active lysozyme, inactive

lysozyme and protamine) resulted in improved dewatering in both assessment methods, i.e. cake solids

after mechanical dewatering and CST (Figure 8-1). Active and inactive lysozyme showed similar trends

as conditioners as has been previously reported (Bonilla et al., 2015). Active lysozyme increased the cake

solids from 8.1(±0.7) to 13.9 (±0.3) % while inactive lysozyme increased it to 12.2 (±0.7) % with a dose

of 0.3 g/g TSS. Protamine increased the cake solids after mechanical pressing to 11.2 (±0.6) % with a

dose of 0.1 g/g TSS and even with half of that dose (i.e. 0.05 g/g TSS), solids were significantly increased

to 10.8 (±0.3) %. These results confirm the potential of cationic proteins as conditioners for enhancing


Only the surfactant with cationic charge, CTAB, improved biosludge dewaterability. Triton X-100

and SDS had a negative effect on CST and dry solids (Figure 8-2). The optimum dose for CTAB was 0.35

g/g TSS, at which CST was reduced to 8 s. CTAB increased the cake solids content of biosludge after

mechanical pressing from 8.1 (±0.7) % to 14.8 (±0.7) % with a dose of 0.5 g/g TSS. At this dose, CST

data showed an overdose effect (i.e. increase in CST values from 8.0 with 0.3 g/g TSS to 9.2 at 0.5 g/g

152

TSS); however, this overdose was not observed in cake solids data (Figure 8-2a). Even at low doses,

anionic and non-ionic surfactants (SDS and Triton X-100, respectively) had a detrimental effect on

dewaterability.

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Figure 8-1 Effect of different doses of proteins on biosludge dewaterability. a) active lysozyme; b)

inactive lysozyme; c) protamine; d) bovine Serum albumin (BSA). Dewaterability was assessed

by capillary suction time (CST) (left axis) and solids content (%) in the cake after pressing and in

the filtrate solids after gravity thickening (i.e. crown press) (right axes). Note different range in

X-axis (i.e. lower doses) for c and d. Error bars represent standard deviation of replicates.

153

Figure 8-2 Effect of different doses of surfactants on biosludge dewaterability. a) CTAB; b) Triton

X-100; c) SDS. Dewaterability was assessed by capillary suction time (CST) (left axis) and solids

content (%) in the cake after pressing and in the filtrate solids after gravity thickening (i.e. crown

press) (right axes). Note different range in X-axis, 10 fold higher for CTAB vs Triton X-100 or

SDS.

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154

Figure 8-3 Effect of different doses of polymers on biosludge dewaterability. a) Zetag 8165; b)

AF9645; c) Organopol; d) Zetag 8185. Dewaterability was assessed by solids content (%) after

mechanical dewatering (i.e. crown press) (left axis) and capillary suction time (right axis).

Increased solids content and reduced capillary suction time are indicative of improved dewatering

properties. Error bars represent standard deviation of triplicates.

Unlike proteins and polymers, all surfactants resulted in increased filtrate solids with increasing

doses (Figure 8-2). Results suggest that surfactants disrupt biosludge flocs, resulting in smaller particles

that can pass through the gravity filter which increases the solids content in the filtrate. The detrimental

effect of surfactants on biosludge dewatering properties is not surprising. Surfactants are widely used to

disrupt cell membranes (Jones, 1999) and breakage of particles has been previously reported to be

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155

detrimental to the dewatering properties of biosludge (Feng et al., 2009; Prorot et al., 2011). Therefore,

even though surfactants have shown to be a promising conditioning treatment for inorganic suspensions,

they may result in worsening dewatering properties for biosludge and potentially other biological

suspensions.

Figure 8-4 Effect of conditioners on CST at their optimum dose. Bar graph represents the capillary

suction time (left axis), the corresponding dose (i.e. optimum) is presented as orange diamonds

(right axis). Dashed line represents the CST of deionized water. Water was added to biosludge as a

control and is represented by the grey bar. Error bars represent standard deviation of triplicates.

Considering both, their effect on dewaterability and the required dose, polymers remain the best

conditioners. With a significantly lower dose than other conditioners, polymers were able to reduce CST

to water-like values (~ 6 s) (i.e. lowest possible CST) (Figure 8-3). Three of the four polymers studied

(Zetag 8165, Zetag 8185 and AF9645) had an optimum dose of 0.03 g/g TSS. After this dose, a sharp

increase was observed in CST, indicative of an overdose. This overdose was also observed in the dry

solids content after mechanical pressing. Organopol’s performance had significant difference when

compared to the other polymers. For this polymer, a lower optimum dose (0.005 g/g TSS) was observed

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156

but the CST was only reduced to 9.6s. Cationic proteins (lysozyme active, lysozyme inactive and

protamine) and the cationic surfactant (CTAB) also showed significant improvements but higher doses

were needed and the improvements observed on dewaterability were not as large as with polymer

conditioning (Figure 8-4).

Regardless of the conditioner group, cationic conditioners resulted in reduced CST (i.e. better

dewaterability) while anionic conditioners increased CST (i.e. worse dewaterability). Polymers,

surfactants and proteins can enhance the dewaterability of biosludge and their effect appears to be mainly

affected by their cationic charge. In Figure 8-4, conditioners used in this study are organized by their

effect on dewaterability and only cationic conditioners showed dewaterability improvements (i.e. reduced

the CST of biosludge).

Figure 8-5 Correlation of capillary suction and dry solids content data for the three groups of

conditioners at their optimum dose. Error bars represent standard deviation of triplicates.

Results from mechanical dewatering assessment (i.e. Crown press) are consistent with CST. As

shown in Figure 8-5, high CST values tend to result in high cake solids and vice versa. A strong linear

correlation was observed between dry solids content and CST data for the eleven (11) conditioners

y = -1.37x + 23.2R² = 0.88

P < 0.0001

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157

studied at their optimum dose. The limitations of CST are well-known and have been discussed elsewhere

(Vesilind, 1988, Chen et al., 1996; Sawalha & Scholz, 2007). Nonetheless, these results demonstrate that

CST can be used to infer trends in the effect of conditioners on dewaterability. Determining trends is

particularly important for screening potential future conditioners. From the eleven conditioners studied,

there were two conditioners that showed slight inconsistencies between dry solids content and CST data

around the “optimum dose” i.e. active lysozyme and CTAB. In the case of active lysozyme, while CST

appeared to remain relatively constant with doses 0.1-0.3 g/g TSS, the dry solids content increased from

10.4 to 13.9% in the same dosage range (Figure 8-1a). In the case of CTAB, CST data showed a clear

overdose at the highest dose (0.5 g/g TSS) while crown press data showed an increase in dry solids

content from 12.2 (±0.2) to 14.8 (±0.7) % with increasing doses from 0.35 to 0.5 g/g TSS suggesting that

an optimum had not been reached (Figure 8-2a).

Capillary suction time is affected by all solids present in biosludge, while the crown press is

separated in two stages, gravity thickening (i.e. filtrate solids) and belt pressing (i.e. cake solids). In the

crown press, small particles solids are removed prior to mechanical dewatering and they do not affect

cake solids content. It is recommended that when gravity thickening and mechanical pressing are

separated in two-stages, as in the case of the crown press, both measurements are taken into account to

assess the effect of conditioners. Furthermore, filtrate solids are of great importance to wastewater

treatment efficiencies as these solids would be recycled back to the aeration tank, unnecessarily

increasing the organic load of the plant.

Effect of conditioners on filtration rate during gravity thickening

Conditioning of biosludge with polymers resulted in improved gravity filtration rates. AF9645, Zetag

8165 and 8185 showed overlapping filtration curves (Figure 8-6a) where gravity thickening was virtually

complete after 1 minute with 138 (±5-16) mL filtered. On the other hand, for the control (biosludge with

water instead of conditioner), only 54 ±4 mL were filtered after 1 min. Organopol, at a dose of 0.01 g/g

158

TSS improve the filtration rate (63 ±4 mL) compared to the control, but in agreement with other

dewatering assessment methods (i.e. cake solids % and CST) it showed a limited effect when compared to

the other polymers studied. More water is removed during the thickening step with polymer conditioning.

Figure 8-6 Filtration curves of biosludge conditioned during gravity thickening in the Crown Press.

The control was biosludge with the same volume water added instead of conditioner; a) polymers;

b) proteins and c) surfactants. The dose of each conditioner in g/g TSS of biosludge is in

parentheses. These doses were selected because they led to the highest dry solids content after

testing various doses of each conditioner. Error bars show standard deviation of duplicates.

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159

Cationic proteins slightly improved the filtration rate of biosludge during gravity thickening.

Protamine and active lysozyme showed better filtration rates than the control (Figure 8-6b). After 2

minutes, both proteins showed an increase in filtrate volume with a maximum of 112 (±1) and 113 (±2)

mL for active lysozyme and protamine respectively, while the control had only filtered 95 (±10) mL after

4 min. Inactive lysozyme and BSA showed no improvement on the filtrate rate. Data for the other doses

evaluated, but not presented in Figure 8-6, can be found in the Appendices.

While only the cationic surfactant CTAB increased the cake solids content of biosludge after

mechanical dewatering (extent), the rate of filtration was positively affected by CTAB and Triton X-100

(non-ionic). There was no improvement on the filtration rate after SDS conditioning. CTAB and Triton

X-100 increased the filtrations rates and the final amount of water removed during gravity thickening

(Figure 8-6c). The effect of Triton X-100 on biosludge dewaterability is the opposite of what has been

reported previously in the literature. Surfactants were proposed to improve cake solids content but do not

have a significant effect on filtration rate (Besra et al., 2003).

Effect of Surface Charge, Surfactant Activity and Wettability on Conditioning of Biosludge

The results consistently indicate that more cationic surface charge in the conditioners results in

improved biosludge dewaterability. Surface charge (charge equivalents/g TSS) and CST data show a

strong negative correlation (Figure8-7). If all the conditioners are considered as a group, the correlation

between surface charge and CST is moderate (r = 0.69, p <0.0001). However, if the conditioners are

separated into two groups: polymers and, surfactants and proteins, the correlation in each data set is

stronger, r = 0.94, p <0.001 and r = 0.88, p <0.0001, respectively. Thus, two different relationships

appear to describe the effect of surface charge on biosludge dewaterability for the three groups

conditioners studied. This suggests that the effect of surface charge, although significant for all the

conditioners, is stronger for surfactants and proteins than for polymers. This is in agreement with the

160

mechanism of polymers where bridging plays the main role and is only promoted by charge neutralization

(Kitchener, 1972). Charge neutralization is possibly the main mechanism of surfactants and proteins for

improving biosludge dewaterability as their effect is greatly affected by their surface charge.

Contrary to what has been proposed in the literature, increased surfactant activity (i.e. reducing the

surface tension) does not improve the effect of the conditioners on the dewaterability of biosludge. On the

contrary, increasing surfactant activity of the conditioners appears to result in worsening dewaterability

(Figure 8-8a). If surface tension of the conditioners increased, CST decreased (i.e. better dewaterability.

This linear correlation was significant (p < 0.05). The trend observed for the effect of surface tension on

biosludge dewaterability could be attributed to floc breakage as a result of surfactant activity on biosludge

flocs. This suggests that surfactants may not be good conditioners for biosludge since smaller particles are

generally not desirable in liquid-solid separation processes.

Figure 8-7 Effect of surface charge on the effect of conditioner on capillary suction time of

biosludge at their optimal dose. Trend line equations, r2 and P values are shown for three cases: all

conditioners, proteins and surfactants, and polymers.

y = -0.78x + 19.1R² = 0.47p<0.0001

y = -0.40x + 10.3R² = 0.87p <0.001

y = -0.73x + 21.9R² = 0.82

p < 0.0001

0

5

10

15

20

25

30

-5 0 5 10 15 20

Capill

ary

Suction T

ime (

s)

Surface Charge (meq/ g TSS)

Surfactants and Proteins

All conditioners

Polymers

161

Figure 8-8 a) Effect of surface tension of conditioners and biosludge (conditioned) on the

dewaterability of biosludge; b) effect of wettability (contact angle) on the dewaterability of

biosludge as measured with capillary suction time.

Neither surfactant activity nor wettability appears to be a good property for screening potential

biosludge conditioners. As shown in Table 8-2, linear correlation and significance values are weak. The

correlations observed in Figure 8-8 appear to be more the result of intrinsic properties for each group of

conditioner rather than surfactant activity or wettability determining the effect of conditioners on


Table 8-2 Pearson coefficients (r2) and significance (p-value) for assessing the strength of the

correlation between surface tension and contact angle, and the effect of conditioners on

dewaterability (i.e. capillary suction time, CST). Correlations were evaluated for all the

conditioners as one group and separately per group of conditioner.

Pearson Correlation r2 p-value

Surface Tension

All conditioners 0.5 < 0.01 Polymers 0.6 0.2 Surfactants 0.6 0.6 Proteins 0.2 0.7

Contact Angle

All conditioners 0.8 < 0.01 Polymers 0.1 0.6 Surfactants 0.9 0.2 Proteins 0.3 0.3

y = -0.29x + 30.3R² = 0.51P<0.01

0

5

10

15

20

25

30

-5 15 35 55 75 95

Ca

pill

ary

Su

ctio

n T

ime

(s)

Surface Tension (mN/m)

Allconditioner

y = -0.77x + 34.4R² = 0.76P <0.01

0

5

10

15

20

25

30

-5 15 35 55

Ca

pill

ary

Su

ctio

n T

ime

(s)

Contact Angle (˚)

All conditioners on glass

a) b)

162

Conclusions

All the cationic chemicals in this study improved the dewaterability of biosludge. Within each group

of conditioners, increased cationic charge resulted in better conditioning performance. While charge plays

a major role in the efficacy of all conditioners, this effect is greater for proteins and surfactants. Polymers

were significantly better as conditioners because they improve dewaterability and increased dry solids

content with low dosages. Protamine showed significant improvements and was the best conditioner from

the group of proteins and surfactants but the dose required was three times more than for polymers.

Nonetheless, cationic proteins have potential as conditioners of biosludge and as flocculants.

Proteins are abundant in nature, can be extracted from wastes and are biodegradable which can be a

potential advantage over polymers. Moreover, finding cationic proteins and their sources can improve the

feasibility of using them in industrial processes. Moreover, proteins are an alternative when synthetic

polymers are not desirable, e.g. food processing. Since surface charge is a key property of proteins and

determines their potential, it is proposed as the first cut-off to screen proteins for enhancing biosludge

dewaterability. Surfactant activity or the wettability of conditioners was not found as a consistent

indicator of conditioning performance. The use of these properties for screening purposes is not

recommended.

References

Besra, L., Sengupta, D. K., Roy, S. K., & Ay, P. (2003) Influence of surfactants on flocculation and

dewatering of kaolin suspensions by cationic polyacrylamide (PAM-C) flocculant. Separation and

Purification Technology 30: 251.

Besra, L., Sengupta, D. K., Roy, S. K., & Ay, P. (2002) Polymer adsorption: its correlation with

flocculation and dewatering of kaolin suspension in the presence and absence of surfactants. Int J Miner

Process 66: 183.



163




Water Res 68: 692.

Chitikela, S., & Dentel, S. K. (1998) Dual-Chemical Conditioning and Dewatering of Anaerobically

Digested Biosolids: Laboratory Evaluations. Water Environ Res 70: 1062.


ISP-5. Alexandria, VA, Water Environment Research Foundation,

Feng, X., Deng, J., Lei, H., Bai, T., Fan, Q., & Li, Z. (2009) Dewaterability of waste activated sludge

with ultrasound conditioning. Bioresour Technol 100: 1074.

Frølund, B., Palmgren, R., Keiding, K., & Nielsen, P. H. (1996) Extraction of extracellular polymers from

activated sludge using a cation exchange resin. Water Res 30: 1749.


Colloid Interface Sci 169: 1.

Huang, C., Ruhsing Pan, J., Fu, C., & Wu, C. (2002) Effects of Surfactant Addition on Dewatering of

Alum Sludges. J Environ Eng 128: 1121.

Jones, M. N. (1999) Surfactants in membrane solubilisation. Int J Pharm 177: 137.

Kawamura, S., Hanna, G. P., & Shumate, K. S. (1967) Application of Colloid Titration Technique to

Flocculation Control. Journal (American Water Works Association) 59: 1003.

Kitchener, J. A. (1972) Principles of action of polymeric flocculants. Brit Polym J 4: 217.

Li, D., & Ganczarczyk, J. J. (1990) Structure of activated sludge flocs. Biotechnol Bioeng 35: 57.

Liao, B. Q., Allen, D. G., Droppo, I. G., Leppard, G. G., & Liss, S. N. (2001) Surface properties of sludge

and their role in bioflocculation and settleability. Water Res 35: 339.

164

Nielsen, P. H., Saunders, A. M., Hansen, A. A., Larsen, P., & Nielsen, J. L. (2012) Microbial

communities involved in enhanced biological phosphorus removal from wastewater — a model system in

environmental biotechnology. Energy biotechnology • Environmental biotechnology 23: 452.

Possmayer, F., Nag, K., Rodriguez, K., Qanbar, R., & Schürch, S. (2001) Surface activity in vitro: role of

surfactant proteins. Comparative Biochemistry and Physiology Part A: Molecular & Integrative

Physiology 129: 209.

Prorot, A., Julien, L., Christophe, D., & Patrick, L. (2011) Sludge disintegration during heat treatment at

low temperature: A better understanding of involved mechanisms with a multiparametric approach.

Biochem Eng J 54: 178.

Stroh, G., & Stahl, W. (1990) Effect of surfactants on the filtration properties of fine particles. Filtration

Sep 27: 197.

Sun, Y., Zheng, H., Zhai, J., Teng, H., Zhao, C., Zhao, C., & Liao, Y. (2014) Effects of Surfactants on the

Improvement of Sludge Dewaterability Using Cationic Flocculants. PloS one 9.

Wang, L., He, D., Tong, Z., Li, W., & Yu, H. (2014) Characterization of dewatering process of activated

sludge assisted by cationic surfactants. Biochem Eng J 91: 174.

Yuan, H., Zhu, N., & Song, F. (2011) Dewaterability characteristics of sludge conditioned with

surfactants pretreatment by electrolysis. Bioresour Technol 102: 2308.

Zhou, Y., & Franks, G. V. (2006) Flocculation Mechanism Induced by Cationic Polymers Investigated by

Light Scattering. - Langmuir 6775.

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9 Chapter 9 – Overall Discussion

The overall goal of this project was to propose novel protein-based conditioners to improve the

dewaterability of biosludge. The three main contributions from this project are: a) identification of

proteins that can improve the dewaterability of biosludge (i.e. lysozyme and protamine); b) determining

charge neutralization as the main mechanism of protein conditioning for improved biosludge dewatering;

c) identification of enzymes that improve the anaerobic digestion of biosludge while isolating the effects

of additional organic matter with enzyme solutions and the effect from enzymatic activity on biosludge.

Based on the literature review (Chapter 2), this work is the first to study the effect of enzymatic activity

during enzymatic conditioning of biosludge and to determine that enzymatic activity, in the case of

lysozyme, did not have any effect on sludge dewaterability. Moreover, this is the first study to evaluate

surface properties of proteins and surfactants and identify the cationic charge as the key property that

results in biosludge dewatering improvements. The results from this project assist in understanding the

changes that sludge undergoes during enzymatic and protein treatment and how these changes can

improve the dewaterability and anaerobic digestion of biosludge

Enzymes and Their Effect on Biosludge Dewaterability

Lysozyme, had a positive effect on the dewaterability of biosludge regardless of its enzymatic

activity. Enzymes have been previously reported to enhance biosludge dewaterability. In this study, after

screening 27 enzymes (commercial and novel), only lysozyme resulted in improved biosludge

dewaterability (Chapter 3 and 4). As discussed throughout this document, previous reports had suggested

that enzymes could improve biosludge dewaterability by hydrolyzing extracellular polymeric substances

(EPS) in biosludge. However, this mechanism was not confirmed in this study. Breaking sludge particles

does not seem to improve biosludge dewaterability. In fact, the opposite effect was found, the cationic

charge of lysozyme neutralizes the predominant negative charges of biosludge, facilitating the

aggregation of particles in biosludge and this leads to better solid-liquid separations.

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Enzymes and Their Effect of Anaerobic Digestion of Biosludge

Enzymes can improve the anaerobic digestion of biosludge as a result of their enzymatic activity.

Previous studies on enzymatic pretreatment for enhanced anaerobic digestibility of biosludge showed the

potential of using enzymes. However, these studied failed to account for the effect of organic load within

the enzyme solutions entering the system. In this thesis, the effect of enzymatic activity on the anaerobic

digestibility of biosludge was isolated from the effect of the enzyme’s organic load. Proteases and

glycosidases were found to improve anaerobic digestion resulting in higher biogas and methane

production as a result of the enzyme’s catalytic activity.

A dual-treatment that includes solubilization of chemical oxygen demand followed by enzymatic

hydrolysis is proposed for improving the anaerobic digestion of biosludge. Previous reports suggest that

chemical oxygen demand (COD) solubilization is the main mechanism by which enzymes improve

biosludge anaerobic digestibility. However, enzymes are more likely to hydrolyze soluble organic

material in biosludge as was shown in this study (Chapter 5). Thus, to maximize the effect of enzymes,

the soluble COD in biosludge could be increased in a first “solubilization” treatment, followed by

enzymatic treatment to speed-up the hydrolysis of the soluble material. This would potentially make more

available the precursors for methanogenesis, increasing biogas yields.

Proteins and Surfactants as Conditioners for Improved Dewaterability

Surface charge determines the potential of proteins for conditioning biosludge. Lysozyme and

protamine, both cationic proteins, resulted in improved biosludge dewaterability. In this thesis, the effect

of surface properties on the conditioning potential of different chemicals (i.e. polymers, surfactants and

proteins) was investigated (Chapter 6). Surface charge can be used to screen proteins and likely other

biopolymers to assess their potential as conditioners of biosludge.

167

Surfactants, although used to enhance separation of inorganic suspensions, appear to break flocs in

biosludge which results in poor dewatering characteristics (Chapter 6). While the effect of surface charge

and particle size on the dewaterability of biosludge has been widely studied and is fairly well understood

(Chapter 2), the effect of surfactant activity is still poorly understood.

Surfactants may lead to opposite effects on biosludge dewaterability. For water to flow through the

cake and filter in a dewatering process, a pressure differential, ∆p, needs to be surpassed. ∆p can be

described by capillary action theory with the following equation:

Equation 9-1 ∆p =2γcosθ

𝑟

where γ, is the surface tension at the water – air interface, θ is the water contact angle of the inner walls of

the capillary and r is the radius of the capillary. The relationship between ∆p and γ shows that reducing

the γ is one way to reducing ∆p. By reducing ∆p, higher solids content in cake could be achieved. This

supports the rationale behind the use of surfactants to improve dewatering processes. However,

surfactants also break sludge particles possibly reducing the radius of the capillary r. Our results show

that floc breakage by the action of surfactants did not improve cake solids. In fact, surfactant activity

resulted in poor dewatering properties likely due to filter and cake blinding caused by smaller particles.

Only the cationic surfactant, CTAB, showed improvements in biosludge dewaterability, thus confirming

the importance of charge when evaluating the potential of conditioners. Larger particles produced as a

result of the cationic charges in the conditioner can also increase the radius of the capillary r, improving

the dewatering process by reducing de pressure differential, ∆p.

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Cationic Proteins as Potential Flocculants

In addition to being a potentially “greener” alternative than synthetic polymers, proteins can

sometimes perform better as flocculants. This is particularly important for promoting the use of proteins

as an alternative to synthetic polymers in the future. The potential of lysozyme and protamine to facilitate

other solid-liquid separations was evaluated in this study (Chapter 7-8). Experiments on kaolin

suspensions showed that both cationic proteins (i.e. lysozyme and protamine) can flocculate kaolin

particles. However, protamine performance is better than lysozyme’s and in some cases, protamine is

better than synthetic polymers. For example, at pH 7 and 9, the rate and extent of flocculation promoted

by protamine in kaolin suspensions was higher than for a synthetic polymer. Finding economical sources

of cationic proteins (e.g. wastes) can determine the prospects of using proteins as flocculants in the future.

Flocculation Mechanisms of Cationic Proteins and Polymers

This thesis has provided a better understanding of the effect of enzymes and proteins on biosludge.

In the schematic below (Figure 9-1), the proposed mechanisms by which cationic proteins flocculate

biosludge particles resulting in improved biosludge dewatering are illustrated along with the flocculating

mechanisms promoted by synthetic polymers. It is important to note that although these mechanisms are

presented individually, they are not mutually exclusive. On the contrary, all the mechanisms are likely to

simultaneously play a role in biosludge flocculation.

Proteins and synthetic polymers can promote flocculation of biosludge particles through mechanisms

that rely on surface charge. Using their cationic charge, proteins and polymers can result in a) double

layer compression and/or, b) charge neutralization (Figure 9-1). Double layer compression is the

mechanism where the repulsion of negatively charged particles in biosludge is reduced. In a stable

colloidal suspension, such as biosludge, reducing the repulsive state of particles is referred to as

destabilization. The double layer caused by the electrostatic forces is compressed, this allows attractive

169

forces (such as van der Waals) between the particles to become predominant and bonds between

biosludge particles can be formed. The double layer compression mechanism does not involve any

physical interaction between proteins or synthetic polymers with biosludge particles, it only considers

how charge and ionic strength entering the system affect the electrostatic forces in biosludge.

Figure 9-1 Simplified schematic illustrating the various mechanisms of cationic proteins and

synthetic polymers for inducing biosludge flocculation. Note: mechanisms are shown separately but

they may happen simultaneously.

Polymer conditioning mechanisms

-

+

+

+

++

+

+

+

--- -

-- - -

-

-

--

--

-

+

+

+

++

+

+

+

--- -

-- -

--

-

--

-+

+

++

-

-

Protein conditioning mechanisms

-+

+

--- -

-- - -

-

-

-

-

--

-

+ + ++

+

+

-+

+

--- -

-- - -

-

-

-

-

- -+ + +

+

+

+-

Bridging

Charge neutralization

Double layer compression

Charge neutralization

Double layer compression

Biosludge

Particles

Synthetic

Polymer

Proteins

170

Charge neutralization is likely the predominant mechanism for proteins. As shown in Figure 9-1,

charge neutralization involves physical interactions between the conditioners and biosludge. There are

attractive forces between cationic proteins and polymer molecules and, biosludge particles. Cationic

proteins are likely adsorbed onto the surface of biosludge particles creating “patches” of net cationic

charge (Gregory, 1973). These patches can interact with other particles in biosludge and aggregation of

particles is possible. In the case of cationic proteins, the fact that an increase in soluble protein content is

not observed during conditioning treatment indicates that the cationic protein is physically interacting

with biosludge particles. Additionally, at optimal doses, the charge delivered by the polymers used in this

study (with the exception of organopol) was 9.4 – 11.4 meq/g TSS and by the cationic proteins 8-15.3

meq/g TSS (data shown in table 11.4 of Appendices), is in the range of the surface charge of biosludge

(i.e. -12.8 ±0.5 meq/g TSS). All this suggests that charge neutralization plays a major role in the effect of

cationic proteins and polymers on biosludge dewaterability. Nonetheless, most polymers delivered a

similar cationic charge as lysozyme but their effect on the dewaterability of biosludge was significantly

better suggesting a mechanistic difference between proteins and polymers.

The size of synthetic polymers largely determines their effectiveness as conditioners. Bridging of

particles is known to be the main flocculating mechanism of polymers. Once polymers are adsorbed onto

biosludge particles, their length allows them to become attached to surfaces in other particles (when their

adsorbed configuration permits), bridging particles in biosludge (Figure 9-1). Bridging is aided by charge

but not required. As a result of their small size when compared to most cationic polymers, cationic

proteins (lysozyme and protamine) are not likely able to bridge particles. Lysozyme with dimensions 3.0

nm × 3.0 nm × 4.5 nm (Kim et al., 2002) and protamine’s length is not known but its amino-acid

sequence is 2-3 times smaller than lysozyme. Polymers, on the other hand, can be up to 10 µm (Wills,

2016), thus their size allows them to interact with more than one particle are a time. Biosludge particles

171

have a wide size range but particles below 100 µm are considered the most problematic. It is widely

accepted that flocs formed by bridging are larger and stronger than flocs formed by other mechanisms.

Therefore, it is possible to suggest that the mechanistic differences between cationic proteins and

polymers are the cause of the differences observed in their performance as conditioners (polymers

generally perform better). Proteins and polymers both seem to rely on charge neutralization as a

mechanism for improving biosludge dewaterability. However, polymers show a better performance than

proteins as enhancers of biosludge dewaterability suggesting that in addition to charge neutralization,

polymer bridging plays a significant role. More importantly, this provides possible guidance for future

research since larger cationic proteins could be better flocculants than the proteins studied in this thesis.

Significance of Findings

Scientific Significance

The scientific significance of this research can be divided in three aspects:

1. Identification of challenges associated when evaluating the potential of enzymatic treatment

on biomass. We have demonstrated the importance of considering the chemistry and physical properties

of enzymes when assessing enzymatic treatments and have developed better methods to accurately assess

the effect of enzymes on biosludge. Throughout the work conducted on biosludge dewatering and

anaerobic digestion in this project, we have identified and addressed the challenges of evaluating

enzymatic pretreatments on biomass. Surprisingly, these challenges have not been factored in when

evaluating enzymatic treatment on biosludge in the literature available previous to this study. Firstly, the

effect of inactive enzymes should always be considered to determine if enzymatic activity plays a role or

if the chemistry of enzymes is responsible for any effect observed. Secondly, the effect of chemical

additives in enzymes preparations should also be considered. This is especially true for commercial

enzymes, where unknown chemicals are present in enzyme preparations. Not considering the

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physicochemical properties of enzymes and enzyme products has hindered advances in enzymatic

treatment technologies due to a lack of understanding of the mechanism(s) involved.

2. Identification of mechanisms during protein conditioning. Cationic proteins flocculate

biosludge particles as a result of the interactions between the cationic charges of protein molecules and

the anionic charges in the surface of biosludge particles. Flocculation of biosludge improves its

dewatering properties.

3. Surface charge is the key property by which proteins can enhance solid-liquid separations.

This is important because it facilitates the future screening of proteins and other biomolecules with

potential as flocculants.

Industrial Significance

The work conducted in this project was mainly motivated by the limitations of current technologies

for improving sludge dewatering processes in industry. In particular, the reliance on synthetic polymers

which are non-renewable, costly and toxic, for improving liquid-solid separations. Based on this, the

industrial significance of this thesis can be grouped in three areas:

1. Cationic proteins can be used to improve sludge dewatering. Our results on biosludge

dewaterability after treatment with lysozyme and protamine suggest that these proteins can significantly

improve dewatering. The main drawback for using proteins on biosludge is that the doses needed are up

to 10 times greater (100-300 kg/DT) than the doses currently used with synthetic polymers (~20 kg/DT).

However, using proteins on primary sludge and biosludge mixtures (Chapter 3) can reduce the dose

needed to observe dewatering improvements. For example, a dose of 40 kg/DT of lysozyme was needed

to improve the dewaterability of a sludge mixture containing 50% primary sludge, 50% biosludge.

Furthermore, when considering a combination of lysozyme and polymer to condition the same sludge

mixture, the total dose needed was only 25 kg/DT (i.e. lysozyme 12 kg/DT and polymer 13 kg/DT).

173

The economic feasibility for using proteins as conditioners is conceivable in the future. During the

course of this study we have found a reduction in the doses needed from cationic proteins as described

previously (Chapter 3 and Chapter 7). Further studies with sludge mixtures are needed to evaluate the

synergistic effects of synthetic polymers and proteins and get a better understanding of the potential

economics of cationic proteins as conditioners. Synthetic polymers and current bulk enzymes (e.g.

cellulase) are currently in the same price range $2-4 per kilogram and proteins can be less expensive at

$0.5-2 per kilogram. However, cationic proteins (including cationic enzymes) are not currently produced

in “bulk”. Their current cost is prohibitive for using them in the wastewater industry (e.g. lysozyme $30-

150 per kg). Therefore, a new method for producing low cost cationic proteins is needed. This process

could be addressed through the production of cationic recombinant proteins with low purity to reduce

purification costs or extraction of cationic fractions from wastes sources as it was briefly demonstrated in

this thesis (Chapter 8).

2. Proteins and their potential as flocculants. The previous analysis is based on the potential use

of cationic proteins to improve biosludge dewaterability, which was the main focus of this project.

However, our results on other suspensions (kaolin, microalgae and powdered activated carbon) show that

cationic proteins can be used as flocculants for a variety of solid-liquid separation processes (Chapter 7

and 8). As a result of their toxicity, the use of synthetic polymers is not recommended in some industries.

Therefore, the research presented in this thesis can be of great interest to other industries, including food

processing and future biorefineries, as it provides an alternative to current separation technologies.

Findings in this thesis open up a whole new range of potential applications for cationic and even anionic

proteins as flocculants.

3. Enzymes for improving anaerobic digestion. Enzymes have been previously shown to improve

anaerobic digestion and reports suggested chemical oxygen demand (COD) solubilization as the main

mechanism for this improvement. In order to make enzymatic treatment a cost-efficient treatment we first

174

need to understand the changes that sludge undergoes during enzymatic pretreatment. Results from this

thesis showed that enzymes may be attacking the substrates that are already soluble (Chapter 5). We

propose then, that a dual treatment, that involves first solubilization of COD (e.g. thermal treatment) and

then enzymatic treatment to hydrolyze the solubilized material, would result in further improvements.

References

Gregory, J. (1973) Rates of flocculation of latex particles by cationic polymers. Journal of Colloid

Interface Science 42: 448.

Kim, D. T., Blanch, H. W., & Radke, C. J. (2002) Direct Imaging of Lysozyme Adsorption onto Mica by

Atomic Force Microscopy. Langmuir 18: 5841.

Wills, B. A. (2016) Wills' mineral processing technology: an introduction to the practical aspects of ore

treatment and mineral recovery. Amsterdam, [Netherlands], Butterworth-Heinemann.

175

10 Chapter 10 - Conclusions and Recommendations for Future Work

This Ph.D. thesis has provided a mechanistic understanding of how enzymes and cationic proteins

can affect sludge for the purpose of enhancing its dewaterability and anaerobic digestibility. The main

conclusions that arise are:

1. Enzyme-based conditioners can affect the dewaterability of biosludge. Although enzymes

were initially considered for their potential to improve dewaterability as a result of their hydrolytic

activity, none of the enzymes showed such improvement. In contrast, some of the enzymes showed a

negative effect on sludge dewaterability.

2. Proteins (including enzymes) can improve biosludge dewaterability as a result of their

surface charge. Lysozyme and protamine, both cationic proteins improved biosludge dewaterability.

Charge neutralization is the proposed main mechanism to cause biosludge dewatering improvements with

cationic proteins.

3. Cationic proteins can be used as flocculants. The significant flocculating activity of cationic

proteins on kaolin suspensions, microalgae and powdered activated carbon demonstrates their potential as

flocculants. Protamine, performed better than synthetic polymers at pH 7 and pH 9, suggesting a key

advantage over synthetic polymers to be further explored.

4. Surface charge determines the potential of proteins as conditioners. A study of surface

charge, surface tension and hydrophobicity revealed that surface charge has a strong correlation with the

effect of proteins on biosludge dewaterability. This is in agreement with the proposed mechanism of

proteins for improving biosludge dewaterability. In contrast with literature reports, surfactant activity did

not improve biosludge dewaterability.

176

5. Enzymes can improve the anaerobic digestibility as a result of their enzymatic activity. All

the enzymes tested could increase biogas yields due to the additional organic (and easily digestible) load

entering the system with enzyme solutions. However, two proteases and two glycosidases showed

improvements on biogas production that could only be attributed to their enzymatic activity.

6. Soluble organics in biosludge appear to be the preferred substrates for enzymes. Enzymes

do not appear to solubilize particulate matter in biosludge. Their effect on anaerobic digestibility seems to

be the result of enzymes attacking soluble organic material.

7. Cationic fractions extracted from waste sources can be used to improve the dewaterability

of biosludge. Cationic fractions extracted from biosludge improved the dewaterability and settling

properties of biosludge and anaerobically digested sludge.

Recommendations for Future Work

This research is part of a recent interest in using environmentally-friendly flocculants for improving

biosludge dewaterability and other liquid-solid separations. This thesis has advanced the knowledge in the

area of enzyme and proteins for improving dewaterability and anaerobic digestibility of sludge, but there

are still many unknowns. Several recommendations for future work are proposed below:

Evaluate combination of proteins and synthetic polymers to improve the dewaterability of

primary sludge-biosludge mixtures. Results obtained in this study suggest that a combination of

proteins and synthetic polymers could have a synergistic effect on the dewaterability of biosludge/primary

sludge mixtures. To this end, sludge mixtures at different primary-biosludge ratios could be treated with

cationic proteins, polymers and a combination thereof. Using capillary suction time (CST) to assess

dewaterability is not recommended for this purpose because the different characteristic of sludge mixtures

can affect CST data making its interpretation difficult. Dewaterability assessment could be made with the

crown press and or centrifugation.

177

Investigate the effect of enzymes that crosslink proteins (e.g. transglutaminase). Increasing the

particle size appears to be the main mechanism by which one can improve the dewaterability of

biosludge. Hydrolases used in this study appear to either have no effect or a negative on dewaterability.

Enzymes that can catalyze reactions that lead to larger biosludge particles would potentially improve

biosludge dewatering properties. Transglutaminases catalyze a reaction that binds protein molecules (also

known as cross-linking). Proteins are abundant in sludge; thus such reaction could result in particle

aggregation which it is known to improve biosludge dewatering. Transglutaminases are provided as one

example but other enzymes that could potentially result in aggregation of biosludge could also be studied.

Investigate the effect hydrolases in combination with polymers on the dewaterability of

biosludge. Breakage of particles as a direct (and only) treatment does not result in improved dewatering

properties. It is possible, however, that breaking sludge flocs and using polymer subsequently could result

in large, more compact flocs. More compact flocs have been associated with better dewatering properties.

Therefore, using an enzymatic treatment to first “digest” sludge and later aggregate particles with a

flocculant could result in improved dewatering properties.

Investigate the effect of protein size on their potential as flocculants. Currently used synthetic

polymers provide versatility for different processes and conditions due to the vast options available based

on charge, chemistry and size of polymers. The large size of polymers is responsible for bridging which is

their main mechanism. Thus, it is expected that variations in the size of proteins will have an impact on

their ability to flocculate suspensions. Potentially allowing proteins to flocculate biosludge by bridging of

particles. This can be studied with naturally produced proteins, but in order to control the effect of amino

acid sequence and only evaluate the size, synthetically produced or recombinant proteins would be

preferred for experimental purposes. Short and long protamines can also be used to evaluate the effect of

protein size.

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Develop methods to extract cationic fractions from waste sources and characterize their

potential as flocculants. There are various sources of protein-rich “wastes”. It is conceivable that the

cationic fraction from these wastes could be extracted and used to neutralize negatively charged

suspensions to improve solid-liquid separations. We have briefly explored this research avenue but more

research is needed. Research efforts should be focused on finding waste sources for extracting cationic

fractions and to find (or develop) an appropriate extraction method.

Determine a more realistic “maximum” potential of enzymes for improving anaerobic

digestion. The effect of enzymes on biogas production described in Chapter 5 showed that enzymes could

increase methane production by ~10%. However, the conditions of the study conducted in this thesis

were not optimal for the enzymes studied. Therefore, to determine the maximum potential of proteases or

glycosidases for increasing methane yields, a follow-up study with the enzymes that showed potential in

this thesis but using optimal conditions is advised. This would provide a more “realistic” preview of the

potential of using enzymes for improving anaerobic digestion of biosludge.

Investigate a dual treatment on biosludge for anaerobic digestion. As previously discussed, our

result suggests that a dual treatment that first solubilizes COD and then uses enzymes to hydrolyze that

soluble COD can improve the impact of enzymatic treatment on biosludge. Although chemical or thermal

treatment have been previously used to solubilize COD, thermal treatment would be preferred as it does

not add chemicals to the system with potential secondary effects. Once COD is solubilized different

enzymes can be used to assess their hydrolytic activity.

179

Appendices

Appendix I - Inactivation of lysozyme (Chapter 3)

Figure A1. Enzymatic activity of lysozyme. Change in absorbance at 450 nm of a suspension of

Micrococcus lysodeikticus. A reduction in absorbance of the suspension is indicative of enzymatic

activity as observed in the blue line (active lysozyme). No reduction in absorbance is observed with

Inactive lysozyme (green line).

Note: Initial absorbance was 0.541 (±0.014). There is a continuous increase in absorbance in the sample

with inactive lysozyme likely as a result of cell aggregation. This trend was also observed when lysozyme

(active and inactive) was added to algae cells (See Figure 6.4).

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.5 1 1.5 2 2.5

Mic

roc

oc

cu

s L

ys

od

eik

tic

us

Su

sp

en

sio

n,

ΔAbs

450

Time (min)

Enzyme addition

Active Lysozyme

Inactive Lysozyme

180

Appendix II - Conditioning of primary sludge and biosludge

mixtures with lysozyme and polymer (Chapter 3)

Capillary suction time (CST) of sludge mixtures at different polymer (Zetag 8185) doses. Legend

shows the primary sludge content (by mass) in each mixture, the remaining was biosludge. Error

bars show standard deviation of triplicates

Capillary suction time (CST) of sludge mixtures conditioned with different doses of lysozyme.

Legend shows the primary sludge content (by mass) in each mixture, the remaining was biosludge.

Error bars show standard deviation of triplicates.

0

10

20

30

40

50

0 10 20 30 40 50 60

CS

T (

s)


70% Primary Sludge60% Primary Sludge50% Primary Sludge

0

2

4

6

8

10

12

14

16

18

20

0 20 40 60 80 100

CS

T (

s)

Lysozyme dose (kg/ DT)

70% Primary Sludge

60% Primary Sludge

50% Primary Sludge

181

Capillary suction time (CST) during dual conditioning of sludge mixtures (50% primary, 50%

biosludge) with lysozyme and polymer (Zetag 8185). Error bars show standard deviation of

triplicates.



triplicates.

0

2

4

6

8

10

12

14

0 10 20 30 40

Capill

ary

Suction T

ime (

s)


Lysozyme 12.6 kg/DT

Lysozyme 25.3 kg/DT

Lysozyme 37.9 kg/DT

0

2

4

6

8

10

12

14

16

18

0 10 20 30 40 50 60

Capill

ary

Scution T

ime (

s)

Polymer dose (kg/DT)

Lysozyme 12.9 kg/DT

Lysozyme 19.3 kg/DT

Lysozyme 25.8 kg/DT

182



triplicates.

0

2

4

6

8

10

12

0 5 10 15 20 25

Capill

ary

Suction T

ime (

s)

Polymer dose (kg/DT)

Lysozyme 7.4 kg/DT

Lysozyme 11.1 kg/DT

Lysozyme 14.8 kg/DT

183

Appendix III - Methane production of BMP assays (Chapter 5)

Methane concentration (%) for BMP 1 bottles, measured using gas chromatography (GC).

BMP 1 Methane concentration (%), from GC measurements

Time (day) 0 1 4 8 14 22 29 42 62

Biosludge only 0.0% 0.0% 1.3% 0.2% 12.2% 16.9% 20.1% 22.0% 26.3%

Inoculum only 0.0% 0.0% 1.7% 0.2% 0.2% 8.5% 10.9% 13.2% 16.6%

Positive control 0.0% 23.6% 43.0% 45.0% 40.8% 46.7% 51.7% 47.4% 52.3%

Untreated (control) 0.0% 2.1% 6.4% 11.9% 17.1% 21.2% 25.7% 26.9% 33.8%

Protease A. oryzae (Active) 0.0% 3.4% 8.5% 14.3% 20.7% 23.9% 27.8% 31.1% 35.2%

Protease A. oryzae (Inactive) 0.0% 3.0% 7.4% 13.9% 18.4% 22.8% 25.4% 27.1% 33.5%

Protease B. licheniformis (Active) 0.0% 4.3% 10.4% 15.3% 20.4% 24.5% 28.1% 31.3% 36.0%

Protease B. licheniformis

(Inactive) 0.0% 2.6% 7.1% 13.0% 18.5% 21.4% 25.2% 28.6% 32.9%

Protease BCE2078 (Active) 0.0% 2.2% 6.7% 13.0% 18.3% 20.7% 25.6% 27.8% 32.8%

Protease BCE2078 (Inactive) 0.0% 2.0% 7.4% 12.9% 18.9% 22.6% 24.9% 28.2% 32.1%

184


BMP 2 Methane concentration (%) from GC

Time (day) 0 1 4 8 15 50 62

Biosludge only 0.0% 0.2% 0.2% 4.5% 12.7% 23.4% 24.8%

Inoculum only 0.0% 0.2% 0.2% 1.8% 0.2% 9.8% 12.5%

Positive Control 0.0% 12.6% 37.1% 41.3% 40.6% 46.0% 46.6%

Untreated (control) 0.0% 2.4% 5.5% 10.6% 16.7% 30.3% 32.2%

CTec 2(Active) 0.0% 4.0% 8.8% 16.5% 22.5% 36.3% 38.0%

CTec 2 (Inactive) 0.0% 4.0% 9.0% 16.7% 21.4% 36.7% 38.4%

Cellulase SCO6604 (Active) 0.0% 4.1% 7.5% 14.9% 22.0% 33.7% 35.7%

Cellulase SCO6604 (Inactive) 0.0% 2.7% 6.8% 14.5% 18.7% 32.4% 34.3%

Lysozyme (Active) 0.0% 3.1% 13.4% 20.0% 23.7% 36.4% 37.7%

Lysozyme (Inactive) 0.0% 3.8% 9.6% 18.4% 22.7% 35.5% 36.8%

185


BMP 3 Methane concentration (%) from GC

Time (day) 0 1 4 15 50

Biosludge only 0.0% 0.2% 0.2% 2.2% 0.2%

Inoculum only 0.0% 1.3% 4.4% 9.8% 27.0%

Positive control 0.0% 26.3% 56.3% 50.9% 65.7%

Untreated control 0.0% 9.7% 18.4% 24.1% 41.0%

Protease A. oryzae (Active), no biosludge 0.0% 2.3% 5.5% 8.8% 18.4%

Protease A. oryzae (Inactive), no biosludge 0.0% 2.8% 6.1% 12.5% 28.1%

Protease B. licheniformis (Active), no biosludge 0.0% 2.3% 5.1% 8.4% 19.4%

Protease B. licheniformis (Inactive), no biosludge 0.0% 2.2% 4.1% 10.4% 24.4%

Cellulase SCO6604 (Active), no biosludge 0.0% 1.5% 3.5% 8.2% 17.6%

Cellulase SCO6604 (Inactive), no biosludge 0.0% 0.2% 3.8% 5.9% 17.4%

Lysozyme (Active), no biosludge 0.0% 1.9% 10.0% 13.5% 24.7%

Lysozyme (Inactive), no biosludge 0.0% 1.5% 8.8% 12.4% 21.8%

186

Appendix IV – Sample and Running Buffer Recipes (Chapter 6)

Laemmli Sample Buffer

33 mM Tris-HCL, pH 6.8

13 % (w/v Glycerol)

1% SDS

0.005% Bromophenol Blue

For samples with 2-mercaptoethanol

355 mM 2-mercaptoethanol

Note: SDS-PAGE gel eltrophoresis is based on the denaturing effect of SDS on proteins. However, to break the disulfide bonds present in some proteins and ensure the "linearity" of a protein molecule and its proper migration on a eletrophoresis gel, a reducing agent such as 2-mercaptoethanol is added to the sample buffer.

Running Buffer

25 mM Tris

192 mM Glycine

0.1% SDS

pH 8.3

187

Appendix V - Effect of protamine dose on the flocculation of kaolin

suspensions (Chapter 7)

Flocculating activity over time of various doses of protamine (mg/ml) on kaolin suspension at

different pH values, a) pH 5, b) pH 7 and c) pH 9. Error bars show standard deviation of triplicates

-20

0

20

40

60

80

100

0 100 200 300

Flo

ccula

ting

Activity

(%)

Time (min)

0.7 mg/ml 1.1 mg/ml

1.4 mg/ml 3.5 mg/ml

0

20

40

60

80

100

0 100 200 300

Flo

ccula

ting A

ctivity

%

Time (min)

0.7 mg/ml 1.1 mg/ml

1.4 mg/ml 3.5 mg/ml

-20

0

20

40

60

80

100

0 100 200 300

Flo

ccula

ting A

ctivity

%

Time (min)

0.7 mg/ml 1.1 mg/ml

1.4 mg/ml 3.5 mg/ml

a) pH 5

b) pH 7

c) pH 9

188

Appendix VI - Capillary suction time of conditioners as aqueous

suspensions (Chapter 8)

Capillary suction time (CST) of conditioners in aqueous solutions. Each conditioner was prepared

at the optimum dose and instead of biosludge, conditioners were added to water. CST of pure water

was 5.9 (±0.2). Error bars show standard deviation of triplicates.

189

Appendix VII - Summary of surface properties of conditioners and

treated biosludge (Chapter 8)

Capillary suction time and surface properties of conditioners and sludge conditioned. Top of table

present the average and bottom, shows the standard deviation.

Conditioner

Capillary

Suction Time

(s)

Surface

Tension

Conditioner

(mN/m)

Surface Tension

WAS

Conditioned

(mN/m)

Contact

Angle on

Glass (˚)

Contact

Angle on

WAS (˚)

Surface

Charge

(meq/g

TSS)

Zetag 8165 5.7 76.9 64.7 32.7 111.9 9.4

Zetag 8185 6.0 68.8 56.5 32.3 112.0 11.4

AF9645 6.5 75.1 62.4 38.6 112.2 10.6

Organopol 9.7 64.5 63.2 31.0 30.7 0.4

SDS 22.4 36.6 30.4 13.1 12.8 -4.1

Triton X-100 25.1 29.9 31.2 13.9 10.5 0.0

CTAB 11.2 36.0 41.2 25.1 22.5 13.8

Water 16.6 71.6 68.2 25.8 29.8 0.0

Protamine 11.7 67.3 66.9 26.9 30.3 15.3

Lys Active 13.7 64.8 64.3 29.0 27.1 8.0

Lys Inactive 13.8 52.0 64.9 30.8 38.8 9.4

BSA 20.7 56.2 59.6 25.8 27.0 5.2

Standard Deviation of Triplicates

Conditioner

CST WAS

conditioned

(s)

Surface

Tension

Conditioner

(mN/m)

Surface Tension

WAS

conditioned

(mN/m)

Contact

Angle on

glass (˚)

Contact

Angle on

WAS (˚)

Surface

Charge

(meq/g

TSS)

Zetag 8165 0.1 1.2 0.5 2.7 1.1 0.7

Zetag 8185 0.1 0.2 0.1 3.7 4.2 0.3

AF9645 0.2 2.1 0.1 5.0 3.9 1.3

Organopol 0.2 0.2 0.2 2.5 5.3 0.0

SDS 0.2 0.0 0.1 2.5 1.9 0.2

Triton X-100 0.8 0.0 0.1 1.9 1.9 0.0

CTAB 0.4 0.0 0.6 2.1 1.0 0.3

Protamine 0.2 0.2 0.5 2.6 2.4 0.9

Lys Active 0.8 0.7 0.6 3.5 2.9 0.5

Lys Inactive 0.8 0.2 1.0 2.5 4.5 0.4

BSA 0.5 0.4 1.5 3.2 1.0 1.2

Water 0.2 0.1 0.8 2.3 2.9 N/A

190





Appendix VIII - Effect of Particle Size of Acrylic Beads on Capillary

Suction Time

Effect of spherical acrylic particles on capillary suction time (CST). Two different concentrations

of particles were investigated 100 mg/ml and 250 mg/ml. Error bars show standard deviation of

triplicates

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60 70

Cap

illar

y Su

ctio

n T

ime

(s)


100 mg/ml 250 mg/ml

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