novel alternative for aeration in the treatment of …zeeshan bilal student no. 32024 msc thesis mwi...

Novel Alternative for Aeration in the Treatment of Wastewater by Membrane Bioreactors Operated at High Biomass

Concentrations in the Context of Emergency Sanitation

Zeeshan Bilal Student No. 32024 MSc Thesis MWI 2013-02 April 2013

[Month year]

Novel Alternative for Aeration in the Treatment of

Wastewater by Membrane Bioreactors Operated at High

Biomass Concentrations in the Context of Emergency

Sanitation

Master of Science Thesis by

Zeeshan Bilal

Supervisors Prof. Damir Brdjanovic, PhD, MSc (UNESCO-IHE)

Mentors Hector Garcia Hernandez, PhD, MSc (UNESCO-IHE)

Carlos Lopez-Vazquez, PhD, MSc (UNESCO-IHE)

Examination committee Prof. Damir Brdjanovic, PhD, MSc (UNESCO-IHE)

Hector Garcia Hernandez, PhD, MSc (UNESCO-IHE) C.B. Milligan, MSc (BlueInGreen)

This research is done for the partial fulfilment of requirements for the Master of Science degree at the

UNESCO-IHE Institute for Water Education, Delft, the Netherlands

Delft April 2013

©Yearby Zeeshan Bilal . All rights reserved. No part of this publication or the information contained herein may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, by photocopying, recording or otherwise, without the prior permission of the author. Although the author and UNESCO-IHE Institute for Water Education have made every effort to ensure that the information in this thesis was correct at press time, the author and UNESCO-IHE do not assume and hereby disclaim any liability to any party for any loss, damage, or disruption caused by errors or omissions, whether such errors or omissions result from negligence, accident, or any other cause.

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Abstract After disasters, the provision of adequate sanitation is a vital component of humanitarian relief works to avoid the spread of diseases in affected areas. The selection of sanitation technology during emergency depends upon many factors and the MBR technology can be one option.

The use of membrane bioreactors (MBRs) has been widely expanded during the last two decades; new applications of these systems are being considered as in the field of emergency sanitation. The inherent properties of MBRs are the main drivers for observing that trend including: compactness, lower foot print, high quality effluent, less sludge production, and aptness for different wastewater characteristics. Constraints regarding this wastewater treatment technology include: high capital cost, membrane fouling, and high operational and maintenance costs (mainly aeration cost, and membrane maintenance and replacement related costs). Aeration is the largest portion of energy costs in most aerobic wastewater treatment systems such as membrane bioreactors (MBRs), and conventional activated sludge (CAS) processes. The energy requirement for aeration are higher for MBRs compared to CAS processes, given that MBRs are operated at high mixed liquor suspended solids (MLSS) concentrations and that an extra amount of air is required for membrane scouring purposes. Air is mainly provided through coarse and fine air diffusers. A gas transfer process occurs between the gas (bubbles) and the liquid phase of the system. The gas transfer process is negatively affected by many parameters such as MLSS concentrations, wastewater characteristics, biomass characteristics, and features of the aeration systems and their configurations. The MLSS concentration produces the highest negative impacts on the gas transfer process. An increase in the MLSS concentration has negative effects on the oxygen transfer process; there is a "negative" exponential relation between oxygen transfer efficiency and the MLSS concentration. The oxygen transfer process limits the attainment of higher MLSS concentrations in MBRs. The design of MBRs at high MLSS concentration may result in smaller footprint (compact systems) one of the most desirable goals when designing MBR systems (besides an extremely high quality treated effluent). Moreover, the low oxygen transfers efficiency at high biomass concentrations directly impact on the high energy requirements (and operational costs). Therefore, innovations are required in the field of aeration to overcome the disadvantages of conventional aeration systems (coarse and fine bubbles diffusers).

In this research an innovative bubble free technology, super saturated dissolved oxygen (SDOX) - BlueInGreen injection system, was assessed for supplying dissolved oxygen to MBRs. The SDOX technology supersaturates a stream of liquid by spraying it into a pressurized gaseous headspace (increasing the degree of saturation compared to atmospheric pressure) and mixes the supersaturated effluent stream with the main liquid body. A laboratory scale SDOX unit was assessed for supplying dissolved oxygen in tap water as well as in sludge at different MLSS concentrations ranging from 3.2 to 34.2 g/l. The SDOX technology successfully supplied oxygen at all the different MLSS concentrations evaluated in this research. The oxygen transfer efficiency was observed to be less sensitive to high MLSS concentration in the SDOX system compared to commonly used air coarse and fine bubble diffusers. The oxygen transfer efficiency observed with the SDOX system was not very much affected by the suspended solids present in the sludge up to MLSS concentrations of 24.0 g/l. Above 24 g/L MLSS concentrations, a reduction in the oxygen transfer efficiency (at the experimental conditions evaluated with the laboratory SDOX unit) was observed. The comparison of the most commonly applied coarse bubble aeration diffusers (at an extremely high air flow rate) with the SDOX technology at a relatively high MLSS concentration of approximately 16.0 g/l in a MBR system revealed that the oxygen transfer rate is much higher for SDOX technology (even at the very high air flow rate applied to the coarse bubble diffuser system). The presence of suspended solids reduced the oxygen transfer capability of the SDOX unit by 12%. However, the oxygen transfer capability of the bubble aeration diffuser was reduced by approximately 40%. Generally, an increase in the oxygen uptake rate (OUR) of the sludge was observed after the exposure of different MLSS concentrations to the high pressure conditions of SDOX technology. In addition, a specific

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experiment was performed with sludge at a MLSS concentration of 5.7 g/l. The sludge was exposed to the SDOX system for a continuous ninety (90) minutes period to assess the effects of the high pressure conditions of SDOX technology on the activity of the microorganisms. The OUR of the sludge exposed to the SDOX system was evaluated and compared to the OUR of the same sludge not exposed to the SDOX system. The results demonstrated that the exposure of the microorganisms at the high pressure conditions of the SDOX pressurized chamber has not a detrimental effect on the activity of the sludge. Moreover, an increase in the soluble chemical oxygen demand (COD) and a potential breakup of flocs of the sludge were observed. Adverse effect on the activity of the sludge such as biomass inactivation was not observed. Given the effective and efficient oxygen transfer efficiency exhibited by the SDOX system at such high MLSS concentrations, the SDOX technology can be proposed as a promising technology for supplying dissolved oxygen in MBR systems operated at high biomass concentrations (in the order of 30 g/L or higher). However, further investigations are required to evaluate the performance of the system (MBR-SDOX system) operated at high OURs conditions. Moreover, the effects of the high pressure conditions at which microorganisms are exposed in the pressurized chamber of the SDOX chamber, as well as the effects of the SDOX system on membrane fouling need to be further evaluated. Further investigations are also required to determine the fecal sludge digestion and inactivation of pathogens by the SDOX technology for the safe disposal of fecal sludge in case of emergencies. Keywords: Membrane bioreactor (MBR), super saturated dissolved oxygen (SDOX) injection system, aeration, KLa, α - factor

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Acknowledgements I would like to express my deepest sense of gratitude to my supervisor Prof. Damir Brdjanovic and all my mentors; Dr. Hector Garcia Hernandez, Dr. Tineke Hooijmans, Dr.Carlos Lopez-Vazquez and Peter Mawioo for their valuable comments, guidance and encouragement through my research period. I want to extend my gratitude to the laboratory staff; Fred Kruis, Ferdi Battes, Peter Heerings, Don van Galen, Lyzette Robbemont, Frank Wiegman for their generous support, assistance and help throughout my laboratory work. I am indebted to my family especially my mother who always gives me love, affection and prayers. I am also greatly thankful to my wife Shehwar for her understanding, patience, support and love. Living away from my family and especially from my daughter Zunaira Bilal was the toughest challenge. I would also like to express thanks to all UNESCO-IHE staff especially the core staff of Municipal Water and Infrastructure (MWI). I once again want to mention the names of Dr. Hector Garcia Hernandez and Dr.Carlos Lopez-Vazquez for their constructive comments, prompt guidance and criticism throughout my research work that made possible to present this work in this shape. I am sincerely thankful to the Bill and Melinda Gates foundation for sponsoring my education and stay in the Netherlands. Moreover, I would like to extend my thankfulness to BlueInGreen Corporation for providing the SDOX laboratory scale unit, additional equipment such as a high pressure pump and an air flow meter, and the technical support provided by their technical staff. Specially, I want to express my gratitude to Christopher B. Milligan for his support setting up the laboratory scale SDOX unit and for being always available to answer my questions. I also want to acknowledge the love, care and support my friends gave me here. All the great moments, I shared with them here, will remain special for the rest of my life. Above all, my thanks and praise to almighty GOD, Allah Subhana Tallah, where all wisdom and love come from.

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Table of Contents

Abstract i

Acknowledgements iii

List of Figures vii

List of Tables ix

Abbreviations x

1. Introduction 1

1.1. Background and justification .................................................................................................. 1 1.2. Super saturated dissolved oxygen (SDOX) Injection system ................................................. 2

1.3. Problem Statement ................................................................................................................. 3 1.4. Goal and objectives ................................................................................................................ 4

1.4.1. Main objective ........................................................................................................... 4 1.4.2. Specific objective ...................................................................................................... 4

2. Literature Review 5

2.1. Emergency sanitation ............................................................................................................. 5 2.2. Conventional activated sludge ................................................................................................ 6 2.3. Membrane bioreactor (MBR) ................................................................................................. 7 2.4. Membrane bioreactor aeration ................................................................................................ 7 2.5. Oxygen transfer mechanism ................................................................................................... 8 2.6. Factors affecting oxygen transfer ......................................................................................... 10

2.6.1. Biomass characteristics............................................................................................ 10 2.6.2. Operational conditions ............................................................................................. 14

2.7. The SDOX technology ......................................................................................................... 15 2.7.1. Benefits of the SDOX technology ........................................................................... 17 2.7.2. Components of the laboratory scale SDOX unit ..................................................... 17

3. Methodology 23

3.2. Tap water testing ..................................................................................................................... 25

3.2.1. Materials ..................................................................................................................... 25

3.2.2. Methodology .............................................................................................................. 25 3.3. Sludge Testing ...................................................................................................................... 27

3.3.1. Materials .................................................................................................................. 27 3.3.2. Methodology ............................................................................................................ 28

3.4. Comparison of conventional bubble aeration diffusers with the SDOX technology ........... 32

3.5. The effect of the SDOX technology on active microorganisms ........................................... 35

3.5.1. Methodology ............................................................................................................ 36

4. Results and Discussion 39

4.1. Tap water experiments ......................................................................................................... 39 4.1.1. Orifice size 0.70 mm ............................................................................................... 39 4.1.2. Orifice size1.14 mm ................................................................................................ 40

vi

4.1.3. Orifice size 1.20 mm ............................................................................................... 42 4.1.4. Summary of the tap water tests ................................................................................ 43 4.1.5. Discussion ................................................................................................................ 45

4.2. Sludge testing ....................................................................................................................... 45

4.2.1. Sludge concentration 3.2 g/l .................................................................................... 45 4.2.2. Sludge concentration 6.3 g/l .................................................................................... 46 4.2.3. Sludge concentration 12.7 g/l .................................................................................. 47 4.2.4. Sludge concentration 19.7 g/l .................................................................................. 48 4.2.5. Sludge concentration 24.0 g/l .................................................................................. 49 4.2.6. Sludge concentration 34.2 g/l .................................................................................. 50 4.2.7. Summary of the sludge experiments ....................................................................... 51 4.2.8. Discussion ................................................................................................................ 56

4.3. Comparison of bubble aeration diffusers with the SDOX technology ................................. 57

4.3.1. Bubble aeration diffusers ......................................................................................... 57 4.3.2. SDOX technology ................................................................................................... 58 4.3.3. Discussion ................................................................................................................ 60

4.4. SDOX technology effects on microorganisms ..................................................................... 62 4.4.1. Discussion ................................................................................................................ 65

4.5. Potential applications of SDOX technology in emergency sanitation ................................. 65

5. Conclusions and recommendations 67

5.1. Conclusions .......................................................................................................................... 67

5.2. Recommendations ................................................................................................................ 67

References 69

Appendices 72

vii

List of Figures Figure 1.1 General Schematic of the SDOX Unit .................................................................................. 3 Figure 2.1 Schematic diagram of CAS ................................................................................................... 6 Figure 2.2 A submerged MBR ............................................................................................................... 7 Figure 2.3 Aeration impacts in an iMBR ............................................................................................... 8 Figure 2.4 Gas transfer at gas liquid interface ........................................................................................ 9 Figure 2.5 Steps and resistances for oxygen transfer from gas bubble to cell ........................................ 9 Figure 2.6 Relation between Alpha factor and MLSS .......................................................................... 11 Figure 2.7 Free water content and bound water/solid matter content at the bubble interface at high

(Left) and Low (Right) MLSS ......................................................................................... 12 Figure 2.8 Alpha Factor as function of MLSS conc. in different studies ............................................. 12 Figure 2.9 Alpha factor as function of MLVSS conc. in different studies ........................................... 13

Figure 2.10 SDOX oxygen delivery ..................................................................................................... 16 Figure 2.11 SDOX unit applied to oxygenation of a river ................................................................... 16 Figure 2.12 Components of the SDOX unit ......................................................................................... 17 Figure 2.13 Control panel's main screen .............................................................................................. 18 Figure 2.14 Control screen ................................................................................................................... 19

Figure 2.15 Inflow pump ...................................................................................................................... 20

Figure 2.16 Spray nozzle ...................................................................................................................... 21

Figure 2.17 Inside view of pressurized saturation tank ........................................................................ 21 Figure 3.1 Research framework ........................................................................................................... 24 Figure 3.2 Experimental setup for tap water testing ............................................................................. 25 Figure 3.3 Sampling point of sludge at the local WWTP ..................................................................... 28 Figure 3.4 Solids retained after sieving the sludge through a 500 µm sieve ........................................ 28 Figure 3.5 Settled sludge ...................................................................................................................... 29

Figure 3.6 Concentrating sludge with membrane ................................................................................. 30 Figure 3.7 Concentrated sludge (34.2 g/l) ............................................................................................ 30 Figure 3.8 Experimental setup for sludge experiments ........................................................................ 31 Figure 3.9 Schematic sequence of the sludge experiments for the SDOX unit .................................... 32

Figure 3.10 Coarse bubble aeration ...................................................................................................... 33 Figure 3.11 Experimental setup to measure KLa with tap water .......................................................... 33 Figure 3.12 Experimental setup to measure KLa in sludge ................................................................... 34

Figure 3.13 Experimental setup to supply oxygen by SDOX unit in a membrane reactor .................. 35

Figure 3.14 Respirometry setup to measure OUR ................................................................................ 36 Figure 4.1 DO vs. Time - Orifice size 0.70 mm ................................................................................... 39 Figure 4.2 DO vs. Time - Orifice size 1.14 mm ................................................................................... 41 Figure 4.3 DO vs. Time - Orifice size 1.20 mm ................................................................................... 42 Figure 4.4 Calculated CSDOX for tap water at different air pressures with different orifice sizes

(diameters) ....................................................................................................................... 43

Figure 4.5 DO vs. Time for tap water at 30 psi air pressure for different orifice sizes ........................ 44



Figure 4.8 DO vs. Time for sludge at a MLSS concentration of 3.2 g/l ............................................... 46 Figure 4.9 DO vs. Time for the sludge at a MLSS concentration of 6.3 g/l ......................................... 47

Figure 4.10 DO vs. Time for the sludge at a MLSS concentration of 12.7 g/l ..................................... 48

Figure 4.11 DO vs. Time for sludge at a MLSS concentration of 19.7 g/l ........................................... 49



Figure 4.14 DO vs. Time for different sludge (MLSS) concentrations ................................................ 52

viii

Figure 4.15 Calculated CSDOX values for different MLSS concentrations ............................................ 53

Figure 4.16 Calculated average CSDOX values with positive negative error for different MLSS concentrations .................................................................................................................. 54

Figure 4.17 CSDOX sludge to water ratio for different MLSS concentrations ....................................... 54

Figure 4.18 Change in specific oxygen uptake rate (SOUR) for different sludge concentrations ....... 55

Figure 4.19 Comparison of SDOX and diffused aeration alpha values................................................ 57

Figure 4.20 DO vs. Time - Oxygen supply by bubble aeration diffusers in membrane reactor (tap water and sludge) ............................................................................................................. 58

Figure 4.21 DO vs. Time - Oxygen supply by the SDOX unit in membrane reactor (tap water and sludge) ............................................................................................................................. 60

Figure 4.22 DO vs. Time - Bubble aeration and oxygen supply by the SDOX for sludge concentration 15.43 g/l .................................................................................................... 60

Figure 4.23 α - factor vs. volumetric air flow rate ................................................................................ 61 Figure 4.24 SOUR values for different sludge samples ....................................................................... 63 Figure 4.25 Living organisms (clockwise from top left) (i) no exposure sludge 20x (ii) no

exposure sludge 20x (iii) 30 minute exposure sludge 10x (iv) 90 minute exposure sludge 20x ........................................................................................................................ 64

Figure 4.26 Floc size (clockwise from top left) (i) no exposure sludge 10x (ii) no exposure sludge 20x (iii) 30 minute exposure sludge 20x (iv) 90 minute exposure sludge 20x ............... 64

ix

List of Tables Table 3-1 The SDOX's operational conditions for the tap water experiments ..................................... 26

Table 3-2 Sludge concentrations evaluated .......................................................................................... 29 Table 3-3 Operational conditions for SDOX to assess impact on microorganisms.............................. 37

Table 3-4 Test conditions for OUR tests .............................................................................................. 37 Table 4-1 Test conditions for tap water test with orifice size 0.70 mm ................................................ 39 Table 4-2 Calculated CSDOX values for the orifice size 0.70mm ........................................................... 40

Table 4-3 Test conditions for tap water test with orifice size 1.14mm ................................................. 40 Table 4-4 Calculated CSDOX values for the orifice size 1.14mm ........................................................... 41

Table 4-5 Test conditions for tap water test with orifice size 1.20 mm ................................................ 42 Table 4-6 Calculated CSDOX values for the orifice size 1.20mm ........................................................... 42

Table 4-7 Test conditions for sludge at a MLSS concentration of 3.2 g/l ............................................ 45 Table 4-8 Test conditions for sludge at a MLSS concentration of 6.3 g/l ............................................ 46 Table 4-9 Calculated CSDOX values for sludge at a MLSS concentration of 6.3 g/l .............................. 47

Table 4-10 Test conditions for sludge at a MLSS concentration 12.7 g/l ............................................ 47 Table 4-11Calculated CSDOX values for sludge at a MLSS concentration of 12.7 g/l ........................... 48

Table 4-12 Test conditions for sludge at a MLSS concentration of 19.7 g/l ........................................ 48

Table 4-13 Calculated CSDOX values for sludge at a MLSS concentration of 19.7 g/l .......................... 49 Table 4-14 Test conditions for sludge at a MLSS concentration 24.0 g/l ............................................ 50 Table 4-15 Calculated CSDOX values for sludge at a MLSS concentration of 24.0 g/l .......................... 50 Table 4-16 Test conditions for sludge at a MLSS concentration 34.2 g/l ............................................ 51 Table 4-17 Calculated CSDOX values for sludge at a MLSS concentration of 34.2 g/l .......................... 51 Table 4-18 CSDOX sludge to water ratio for different MLSS concentrations ......................................... 53

Table 4-19 Test conditions for bubble aeration (tap water & sludge) .................................................. 58 Table 4-20 Test conditions for oxygen supply by the SDOX in membrane reactor (tap water and

sludge) ............................................................................................................................. 59

Table 4-21 calculated CSDOX for tap water and sludge in membrane reactor ........................................ 59

Table 4-22 Soluble COD, NH4-N and TSS & VSS values for sludge with different exposure time to the SDOX .................................................................................................................... 62

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Abbreviations AE Aeration Efficiency in clean water

CAS Conventional Activated Sludge

COD Chemical Oxygen Demand

DO Dissolved Oxygen

EPS Extracellular Polymeric Substances

EPSC Carbohydrate fraction of Extracellular Polymeric Substances

HRT Hydraulic Retention Time

iMBR Immersed/Submerged Membrane Bioreactor

MBR Membrane Bioreactor

MLSS Mixed Liquor Suspended Solids

MLVSS Mixed Liquor Volatile Suspended Solids

OUR Oxygen Uptake Rate

OTE Oxygen Transfer Efficiency in clean water

OTR Oxygen Transfer Rate in clean water

RBCOD Readily Biodegradable COD

SRT Sludge Retention Time

SMP Soluble Microbial Products

SMPCOD COD fraction of Soluble Microbial Products

SOUR Specific Oxygen Uptake Rate

SOTR Oxygen Transfer Rate at Standard conditions in clean water

SAE Aeration Efficiency at Standard conditions in clean water

SOTE Oxygen Transfer Efficiency at Standard conditions in clean water

SDOX Supersaturated Dissolved Oxygen Injection system

TSS Total Suspended Solids

VSS Volatile Suspended Solids

WWTP Waste Water Treatment Plant

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List of Symbols CSDOX Constant dissolved oxygen concentration supplied by the SDOX unit

CSDOX(water) Constant dissolved oxygen concentration supplied by the SDOX unit in tap water

CSDOX(sludge) Constant dissolved oxygen concentration supplied by the SDOX unit in certain sludge

concentration

K L Liquid film coefficient

a Interfacial area / unit of liquid volume

K La Liquid side mass transfer coefficient

K La(T) Liquid side mass transfer coefficient at given temperature

K La(20) Liquid side mass transfer coefficient at temperature 20oC

α Ratio of process to clean water mass transfer

β Wastewater correction factor for saturation

C Dissolved oxygen concentration

C* Dissolved oxygen saturation concentration

Ci Initial dissolved oxygen concentration

Q Flow rate

Introduction 1

1.1. Background and justification

Population growth, industrial development and the rise in food requirements increase the load on existing water resources and necessitate the protection of the resources for sustainable use. Pollution prevention for water resources is a major concern as it not only threatens the general public health, but also increases the treatment cost tremendously for its further use. Environmental regulations have strengthened to protect the water resources. At the same time this environmental regulations demand for advancement in wastewater treatment technology. With the development of wastewater treatment technology different new systems have emerged such as the conventional activated sludge (CAS) processes, anaerobic treatment, membrane bioreactor (MBR), among others.

Currently, the CAS process is the most extensively used all around the world. The CAS process involves the treatment of wastewater by active microorganisms in a controlled and engineered environment that is suitable for microorganism's growth and the removal of pollutants. The classic CAS treatment configuration includes a primary clarifier, a bioreactor, a secondary clarifier and a sludge processing line. The secondary clarifier performs the liquid solid separation by gravity to separate the effluent and sludge. The secondary clarifier limits the mixed liquor suspended solid (MLSS) concentration at which the CAS can be designed. CAS systems work at MLSS concentration (ranging from 1,500 to 6,000 mg TSS/L). Such low MLSS concentrations along with the use of primary and secondary clarifiers introduce the need of a relatively high surface area for these systems (high footprint). Furthermore, the gravity liquid solid separation performance of the secondary clarifiers of a CAS system provides a treated effluent quality that needs further treatment for wastewater reuse applications.

The replacement of the secondary clarifier by filtration membranes in MBRs enables the design of the aerobic tank of a wastewater treatment system at high MLSS concentrations (ranging from 10,000 to 20,000 mg TSS/l). Higher biomass concentration in the aeration tank reduced the space requirement of the treatment systems (Germain and Stephenson 2005). Moreover, the system can work at much higher volumetric loading rate (or influent load) in comparison to the CAS system, and would require a lesser area at the same time. Micro- or ultra-filtration membrane filtration processes provide a high quality effluent eliminating most of the suspended solids in the effluent, reducing the concentration of pathogenic microorganisms. Thus, wastewater treated by the MBR has more potential for reuse application compared to the CAS systems (Côté et al. 2004). Moreover, features such as portability, compactness and high quality effluent makes MBRs a suitable technology for emergency sanitation applications. The technology can be use for treatment of high strength black

CHAPTER 1

Introduction

Novel alternative for aeration in the treatment of wastewater by MBR operated at high biomass concentrations in the context of emergency sanitation 2

water generated from emergency camps (Rusliana 2012). MBR systems also have a greater potential for treating industrial wastewater considering the compactness of the plant, complexity of industrial effluent, and suitability for the reuse of treated wastewater (Hoinkis et al. 2012). In short, the MBR system has the following advantages: better removal of pollutants, lesser sludge production, complete solid liquid separation, and pathogen removal. The major disadvantages associated with MBRs include: high capital and operational costs, membrane fouling, and aeration requirements (Henze et al. 2008). From the above outlined disadvantages of the MBR systems, the aeration requirements and its associated operational cost is a major constraint. Aeration is the major component of the MBR's operational cost. Air is supplied both to provide the necessary oxygen required by microorganism to grow, and to clean the membrane in a submerged MBR (membrane scouring) in order to avoid membrane fouling (Germain and Stephenson 2005).

Oxygen is majorly supplied to the MBRs by diffused aeration, mostly through fine bubble diffusers. However, coarse bubbles diffusers are also used to control fouling. Many previous studies have shown the negative effects of the MLSS concentration on the oxygen transfer efficiency for bubble aeration diffusers (Krampe and Krauth 2003). That is, MLSS concentration limits the design of MBR systems. Moreover, high MLSS concentrations and the need of aeration for fouling control exert an energy demand much higher for MBRs compared to conventional activated sludge treatment systems (P Cornel et al. 2003). Foregoing above, new innovative technologies are required to supply the oxygen; especially, for MBRs systems to overcome the needs related to oxygen supply at high MLSS concentrations, and to reduce the aeration cost.

In this research, an innovative supersaturated dissolved oxygen (SDOX) injection system technology was assessed to explore the possibility for the attainment of high MLSS concentration in MBRs overcoming the limitation of oxygen supply.

1.2. Super saturated dissolved oxygen (SDOX) Injection system

The innovative BlueInGreen SDOX technology is different from other aeration devices. Rather than attempting to get gas bubbles to dissolve into a body of liquid, the SDOX unit injects and distributes the stream, supersaturated with dissolved oxygen, into the mixed liquor that contains (the mixed liquor) a dissolved oxygen concentration below the saturation concentration. That is, the dissolved oxygen (DO) does not have the conditions to escape from the solution. Figure 1.1 shows a general scheme of the SDOX unit.

The SDOX unit sprays the incoming stream of mixed liquor in a pressurized tank into a large body of gas under conditions that are ideal for dissolving oxygen. Later on, the SDOX unit injects this stream of the mixed liquor, supersaturated with dissolved oxygen, into the body of mixed liquor where oxygenation is required. The mixed liquor coming from the SDOX unit is supersaturated with dissolved oxygen and as such the supersaturated mixed liquor cannot exist at atmospheric pressure conditions without oxygen leaving the system (that is, being released to the atmosphere). As soon as the supersaturated mixed liquor leaves the SDOX unit, the stream is mixed into the target mixed liquor body in such a way that the supersaturated mixed liquor is rapidly diluted in the mixed liquor body to a certain concentration level (below that of saturation). Therefore, the dissolved oxygen is not leaving the mixed liquor body, unless the bulk mixed liquor DO concentration is above the saturation level.

According to the manufacturer, if pure oxygen is used as the oxygen supply source to the SDOX unit, a DO of up to 400 mg/l can be achieved in the SDOX effluent stream depending upon temperature and pressure inside the pressurized chamber of the SDOX unit (BlueInGreen 2012).

Introduction 3

Figure 1.1 General Schematic of the SDOX Unit

[Source : Operations and maintenance manual for SDOX, 2012]

1.3. Problem Statement

Oxygen is majorly supplied into MBR systems by diffusing atmospheric air / pure oxygen through air bubble diffusers. Air bubbles transfer oxygen to the bulk liquid via a liquid / gas interface. The other purposes of aeration are for mixing the tank content, and for cleaning the membranes in the MBRs.

Oxygen transfer in the MBR systems is a complex phenomenon and depends on many factors such as the aeration system, the tank geometry, operational conditions, and biomass characteristics. The interrelationship between aeration system and biomass characteristics is complex, especially in the submerged MBRs where aeration is also used for membrane scouring (Simon Judd 2006).

MBRs work at higher MLSS concentrations than CAS systems; therefore, the oxygen transfer efficiency is lower in MBR systems. The ratio of oxygen transfer efficiency (OTE) between process mixed liquor and clean water is often expressed as α (the "alpha factor ") (Henze et al. 2008). The alpha factor is the ratio of oxygen mass transfer in process and clean water; and thus, it gives only a measure of the oxygen transfer efficiency for a system accounting for the complexity related to oxygen transfer. The alpha factor decreases exponentially (E. Germain, 2006) with the increase of MLSS concentration. The decrease of oxygen transfer efficiency with the increase of MLSS concentration limits the operation of MBR systems at higher MLSS concentrations than 18 g/L. Moreover, the decrease in the oxygen transfer efficiency impacts on the energy requirement for aeration for MBR systems. The evaluation of the innovative SDOX system for supplying dissolved oxygen at high suspended solids concentration in MBR may enable the operation of MBR systems at higher MLSS concentrations, reducing MBR footprint and aeration cost.

Foregoing above, the oxygen transfer efficiency decreases with increasing the MLSS concentrations. Thus, the oxygen transfer efficiency limits the MLSS concentration at which MBR systems can be designed. Moreover, the reduction of the oxygen transfer efficiency at high MLSS concentration increases the energy costs for aeration, which is a major component of the total MBR operational cost. There is a clear demand for innovate aeration technologies to overcome oxygen transfer problems at high MLSS concentrations, reducing MBR footprint and operational costs.


1.4. Goal and objectives

1.4.1. Main objective The main objective of this study is to assess the performance of a laboratory scale SDOX unit for supplying dissolved oxygen in sludge at different MLSS concentrations to operate membrane bioreactors at high biomass concentrations in the context of emergency sanitation. If dissolved oxygen can be successfully supplied at high MLSS concentrations, the SDOX technology may lead to the re-design of MBR systems reducing both MBR footprint and aeration costs. 1.4.2. Specific objective • To evaluate the oxygen transfer capabilities of a SDOX unit in tap water. • To investigate the effects of MLSS concentrations on the oxygen transfer efficiency of the SDOX

unit. • To compare the oxygen transfer efficiency of the SDOX technology with the oxygen transfer

efficiency of coarse bubble aeration diffusers. • To study the effects on microorganisms, if any, due to their exposure to the high pressure

conditions of the pressurize chamber of the SDOX unit.

Literature Review 5

2.1. Emergency sanitation

In recent past, the frequency of natural disasters has increased. Tsunamis in Indonesia, earth quake in Haiti, floods in Pakistan are recent incidents of major natural disasters. Environmental degradation and climate change are among the main factors for the frequent occurrence of the natural disaster.

After disaster, the emergency situation causes displacement of people to emergency camps. Apart from natural disasters the war situations also lead to large numbers of internal displace people (IDP). Along with others, sanitation is a vital life-support necessity in emergency relief camps. The people affected by the disasters are more vulnerable to diseases due to lack of clean drinking water, lack of sanitation and unhygienic conditions. The Inadequate sanitation can lead to outbreak of diseases. In case of Haiti earth quake 2010, the inadequate provisions of sanitation facilities lead to outbreak of cholera and consequently death of several people.

During emergency situations the provision of adequate sanitation is essential in order to control the spread of water and sanitation related diseases. Wastewater and excreta management is a major challenge during emergencies. Excreta management is vital in emergencies due to the presence of large amount of pathogens in human excreta.

In cases of emergency, the selection of sanitation technology depends upon many factors; nature of disaster, availability of existing sanitation infrastructure, ground water table, and availability of resources are among others. There are many sanitation technologies available from simple and conventional technology e.g. pit latrine to more complex and advance technology e.g. membrane bioreactors. As earlier stated, the selection of most feasible sanitation intervention depends upon many factors. Membrane bioreactor can be one option due to its compactness and high quality treated effluent. Given the ease of deployment, MBRs can be used in mid-term and long-term stages of emergency. However, as discussed in first chapter, the oxygen supply limitations at higher biomass concentrations limits the compactness of the MBR system and increase the operational cost as well. However, attainment of high MLSS concentrations and reduction in operational cost can make MBRs a suitable option for emergency relief.

Hereunder, the comparison of conventional activated sludge and MBRs and the major factors influencing the aeration in MBRs are discussed.

.

CHAPTER 2

Literature Review

Novel alternative for aeration in the treatment of wastewater by MBR operated at high biomass concentrations in the context of emergency sanitation

2.2. Conventional a

The activated sludge process was discovered in UK: experiments on treated sewage in sequence batch reactor produced effluent of high quality. The process was named as "Activated Sludge" by believing that sludge is active similar to ac

In a conventional activated sludge process wastewater is treated by microorganisms that remove the organic pollutants by utilizing them. It results in an increase in biomass and COwater. The biomass and other pollutants enmeshed within the biomass (sludge) are separated, and the treated water is discharged to a water course. A fraction of the sludge sustain a certain concentration of biomass in the system. The remaining fraction is discharged and further treated in the sludge line before its final disposal. After the realization of eutrophication problems and its major cause as nitrogen and phosphorous, the extensive research in this area led to biological nutrient removal (BNR) activated sludge system. In this system the nitrogen and phosphorous are also removed by the aid of different groups of microorganisms along wiorganic pollutants. Figure 2.1 shows a schematic diagram of a typical activated sludge system.

Source: [http://www.emeraldinsight.com/content_images/fig/2490040104001.png2013.

Preliminary treated wastewater first goes to a primary sedimentation for the removal of settleable solids. The effluent from the primary treatment is treated in a biological reactor where aeration is provided for the biological process. Later on, the effluent from the biological reactor goes to a secondary clarifier where the sludge is separate

As the secondary clarifier separates the sludge and treated water, the area of the clarifier depends on the concentration of the sludge coming from bioreactor. Due to this limitation the CAS works on MLSS concentration of between 1500increases the foot prints of CAS systems.

Major problems associated with CAS• Large footprint • Settlement issues (Bulking Sludge)• Tertiary treatment required for disinfection / polishing


activated sludge

The activated sludge process was discovered in UK: experiments on treated sewage in sequence batch reactor produced effluent of high quality. The process was named as "Activated Sludge" by believing

tivated carbon (Henze et al. 2008). In a conventional activated sludge process wastewater is treated by microorganisms that

anic pollutants by utilizing them. It results in an increase in biomass and COwater. The biomass and other pollutants enmeshed within the biomass (sludge) are separated, and the treated water is discharged to a water course. A fraction of the sludge is recycled to the CAS system to sustain a certain concentration of biomass in the system. The remaining fraction is discharged and further treated in the sludge line before its final disposal. After the realization of eutrophication

cause as nitrogen and phosphorous, the extensive research in this area led to biological nutrient removal (BNR) activated sludge system. In this system the nitrogen and phosphorous are also removed by the aid of different groups of microorganisms along wi

shows a schematic diagram of a typical activated sludge system.

Figure 2.1 Schematic diagram of CAS

http://www.emeraldinsight.com/content_images/fig/2490040104001.png

wastewater first goes to a primary sedimentation for the removal of ttleable solids. The effluent from the primary treatment is treated in a biological reactor where

aeration is provided for the biological process. Later on, the effluent from the biological reactor goes to a secondary clarifier where the sludge is separated by gravity and the treated water is discharged.

As the secondary clarifier separates the sludge and treated water, the area of the clarifier depends on the concentration of the sludge coming from bioreactor. Due to this limitation the CAS

ration of between 1500 to max. 6000 mg/l. This concentration limitation increases the foot prints of CAS systems.

Major problems associated with CAS are listed below:

Settlement issues (Bulking Sludge) Tertiary treatment required for disinfection / polishing

Novel alternative for aeration in the treatment of wastewater by MBR operated at high biomass 6

The activated sludge process was discovered in UK: experiments on treated sewage in sequence batch reactor produced effluent of high quality. The process was named as "Activated Sludge" by believing

In a conventional activated sludge process wastewater is treated by microorganisms that anic pollutants by utilizing them. It results in an increase in biomass and CO2 and

water. The biomass and other pollutants enmeshed within the biomass (sludge) are separated, and the is recycled to the CAS system to

sustain a certain concentration of biomass in the system. The remaining fraction is discharged and further treated in the sludge line before its final disposal. After the realization of eutrophication

cause as nitrogen and phosphorous, the extensive research in this area led to biological nutrient removal (BNR) activated sludge system. In this system the nitrogen and phosphorous are also removed by the aid of different groups of microorganisms along with other

shows a schematic diagram of a typical activated sludge system.

http://www.emeraldinsight.com/content_images/fig/2490040104001.png. Accessed on March

wastewater first goes to a primary sedimentation for the removal of ttleable solids. The effluent from the primary treatment is treated in a biological reactor where

aeration is provided for the biological process. Later on, the effluent from the biological reactor goes d by gravity and the treated water is discharged.

As the secondary clarifier separates the sludge and treated water, the area of the clarifier depends on the concentration of the sludge coming from bioreactor. Due to this limitation the CAS

000 mg/l. This concentration limitation

Literature Review 7

2.3. Membrane bioreactor (MBR)

In a membrane bioreactor, the bio treatment process is similar to that of CAS but in the MBR the solid-liquid separation is carried out with a membrane. Thus, in the MBR the solid-liquid separation process is independent of floc formation and settleability. Rather, it depends upon membrane efficiency and performance to retain the solids. Generally, it results in higher clarification and disinfection (depending upon the membrane pore size). Figure 2.2 shows configuration of a submerged MBR.

Figure 2.2 A submerged MBR

Other advantages related to MBR are listed down; (Henze et al. 2008): • Production of high quality and largely disinfected treated water suitable for reuse in single stage • Uncoupling of HRT and SRT • Operation at higher MLSS compare to CAS which allows to have lesser footprint • Operation at long SRT gives opportunity to select the slow growing bacterial population with

possible enhanced treatment • Lesser sludge production

Major disadvantages related to MBR are as follows; • Process complexity • Higher capital and operational cost • Membrane fouling • Less dewaterable sludge • Higher aeration requirement due to limitation of air supply at higher MLSS concentration and

membrane fouling control. From the above list, the main disadvantages are membrane fouling and aeration cost. These are major constraints to adopt MBR technologies. Extensive research has been done to understand membrane fouling and control. But as this work is more focus on oxygen transfer at higher MLSS concentrations, so membrane fouling will not be discussed here.

2.4. Membrane bioreactor aeration

Aeration is required to supply dissolved oxygen (DO) for metabolism and to maintain the solids in suspension in activated sludge systems. Due to the high biomass concentration the oxygen requirements are higher in MBR than the CAS. Furthermore, aeration is also required for cleaning of


membrane to control fouling in submerged MBR aerators are generally used to provide oxygen and keep the tank well mixed. In addition, coarse bubble aerators situated under membrane modules are used to scour and/or gently agitate the membrane in order to control membrane fouling with MBR at higher biomass concentration which increases the operational costs of MBR.

In MBR, aeration system characteristics, biomass characteristics and minterrelated to each other. Figure

Source: (Germain, 2004)

Oxygen transfer rate is dependent on aeration system but at the same time it affects the biomass properties which not only affect the oxygen transfer but also the membrane flux. As this literature review is more focused on oxygen transfer in MBR, the membrane fluand its relation with aeration system and biomass characteristics will not be discussed here.

In order to understand the key factors that limit the oxygen transfer in MBR, it is necessary to understand the mechanisms of oxygen transfe

2.5. Oxygen transfer mechanism

Oxygen molecules in a bubble are first transferred from the gas phase to the surface of liquid and equilibrium is established at the gas body of the liquid from the gas (Figure 2.4).


membrane to control fouling in submerged MBR (Germain and Stephenson 2005)aerators are generally used to provide oxygen and keep the tank well mixed. In addition, coarse bubble aerators situated under membrane modules are used to scour and/or gently agitate the membrane in order to control membrane fouling (Germain et al. 2007). Higher aeration requirements are associated with MBR at higher biomass concentration which increases the operational costs of MBR.

In MBR, aeration system characteristics, biomass characteristics and membrane flux, are all Figure 2.3 shows this interrelation.

Figure 2.3 Aeration impacts in an iMBR

transfer rate is dependent on aeration system but at the same time it affects the biomass properties which not only affect the oxygen transfer but also the membrane flux. As this literature review is more focused on oxygen transfer in MBR, the membrane flux or membrane fouling and its relation with aeration system and biomass characteristics will not be discussed here.

In order to understand the key factors that limit the oxygen transfer in MBR, it is necessary to understand the mechanisms of oxygen transfer.

Oxygen transfer mechanism

Oxygen molecules in a bubble are first transferred from the gas phase to the surface of liquid and equilibrium is established at the gas - liquid interface. Then, these oxygen molecules move to the main

om the gas - liquid interface (Mueller et al. 2002). The process is explained in


(Germain and Stephenson 2005). Fine bubbles aerators are generally used to provide oxygen and keep the tank well mixed. In addition, coarse bubble aerators situated under membrane modules are used to scour and/or gently agitate the membrane in

. Higher aeration requirements are associated with MBR at higher biomass concentration which increases the operational costs of MBR.

embrane flux, are all

transfer rate is dependent on aeration system but at the same time it affects the biomass properties which not only affect the oxygen transfer but also the membrane flux. As this

x or membrane fouling and its relation with aeration system and biomass characteristics will not be discussed here.

In order to understand the key factors that limit the oxygen transfer in MBR, it is necessary to

Oxygen molecules in a bubble are first transferred from the gas phase to the surface of liquid and liquid interface. Then, these oxygen molecules move to the main

. The process is explained in

Literature Review 9

Figure 2.4 Gas transfer at gas liquid interface

[Source : (Mueller et al. 2002)

Furthermore, the oxygen transfer from the bubble to the cell can be represented by a number of steps and resistance as shown in Figure 2.5.

Figure 2.5 Steps and resistances for oxygen transfer from gas bubble to cell

Source: (Garcia-Ochoa et al., 2008)

According to Garcia-Ochoa and Gomez (2009) the gas-liquid mass transfer is explained by

different theories. The simplest one is the two film theory (Whitman, 1923) which describes the flux of gas (oxygen) through each film (i.e. gas and liquid film) equal to the product of the local mass transfer coefficient and the driving force (i.e. concentration gradient). Because the interfacial concentrations are not directly measurable the overall mass transfer coefficient is considered to express the flux:


� = �� . �� − �∗� = �� . � ∗ − ��

(2.1)

Where J = molar flux of oxygen through gas-liquid interface; kG and kL are overall mass transfer coefficient; PG and CL are oxygen partial pressure in gas bubble and dissolved oxygen concentration in bulk liquid respectively; P* is the oxygen pressure in equilibrium with liquid and C* is oxygen saturation concentration in bulk liquid in equilibrium to bulk gas phase.

The greatest resistance for the mass transfer lies on the liquid side and therefore the expression on the gas side can be neglected. This gives:

� = �� . � ∗ − �� (2.2)

Thus, oxygen mass transfer rate per unit of liquid volume is obtained by multiplying the flux with the gas - liquid interfacial area per unit of liquid volume (a).

�� =��. � ∗ − � (2.3)

Where; OTR = Oxygen transfer rate (C* - C) = Difference of saturated oxygen concentration and dissolved oxygen concentration.

Both K L and a are difficult to measure individually therefore the combined effect of both is measured and called the volumetric mass transfer coefficient K La, which is used as parameter to measured and characterize the transfer efficiency from the gas to the liquid, (Simon Judd 2006).

The other parameter which is used to define the oxygen transfer in biological aerated system is the Alpha - factor (α), that is defined as the ratio between KLa in process water and KLa in clean water (Germain and Stephenson 2005).

2.6. Factors affecting oxygen transfer

Oxygen transfer in MBR systems through aeration is related to many factors, among others, biomass characteristics, influent characteristics, aeration system, tank geometry and operational conditions. The aeration, biomass characteristics and membrane operation (especially for iMBR where aeration is used for scouring) are interlinked with each other. Hereunder the effect of biomass characteristics, influent impurities and operational conditions on oxygen transfer is discussed. 2.6.1. Biomass characteristics The biomass is a heterogeneous mixture of particles, microorganisms, colloids, organic polymers and ions, which all have different shapes, sizes and densities. All these parameters affect the oxygen transfer in activated sludge. As already mentioned, the oxygen mass transfer is dependent on the contact area size between the gas and liquid phase. Therefore, the bubble size and solid concentration impacts the oxygen transfer. Bubble characteristics depend on the aeration system and bubble coalescence. Whereas, bubble coalescence is a function of the biomass characteristics. Generally, fine bubble aerators are employed in the MBR to increase the oxygen transfer and for mixing. But, in submerged MBR also coarse bubble aerators are used to scour and / or gently agitate the membrane to control the membrane fouling.

The main parameters to characterize the biomass are particle concentration, particle size and viscosity. These three parameters are known to have an effect on the oxygen transfer and therefore, are interrelated with aeration. Aeration intensity has an impact on particle size, while viscosity depends

Literature Review

upon particle concentration and impacts oxygen transfer. The individual impact on oxygthese parameters can be modified by the added effect of another parameter, particularly for particle size and concentration (Germain and Stephenson 2005) 2.5.1.1. MLSS concentrationBiomass concentration (MLSS) is assumed to have an adverse effect on oxygen transfer rate in activated sludge system. Muller et al. (1995)many others investigate the effect of MLSS on oxygen transfer and all concluded that with increase in MLSS concentration, the alpha-factor is reduced. An exponential relationship between the alphaand the MLSS concentration has been MLSS concentration by various studies.

Figure

Source : (Germain et al. 2007)

According to Henkel et al. (2009)parameter which influence the oxygen transfer rather than MLSS concentration. At hconcentration. With increasing MLVSS concentration the free water content of the sludge decreases and the interfacial area between the free water and the bubble is hindered by the bound water/solidmatter fraction (Figure 2.7).

upon particle concentration and impacts oxygen transfer. The individual impact on oxygthese parameters can be modified by the added effect of another parameter, particularly for particle

(Germain and Stephenson 2005).

oncentration Biomass concentration (MLSS) is assumed to have an adverse effect on oxygen transfer rate in

Muller et al. (1995) , Germain et al. (2007) , Krampe and Krauth (2003)many others investigate the effect of MLSS on oxygen transfer and all concluded that with increase in

factor is reduced. An exponential relationship between the alphaS concentration has been observed. Figure 2.6 shows the relationship of alpha factor and

MLSS concentration by various studies.

Figure 2.6 Relation between Alpha factor and MLSS

Henkel et al. (2009), the MLVSS concentration seems to be the decisive parameter which influence the oxygen transfer in activated sludge system operating at a high SRT rather than MLSS concentration. At high SRT the alpha-factor is mainly influenced by the MLVSS concentration. With increasing MLVSS concentration the free water content of the sludge decreases and the interfacial area between the free water and the bubble is hindered by the bound water/solid

11

upon particle concentration and impacts oxygen transfer. The individual impact on oxygen transfer by these parameters can be modified by the added effect of another parameter, particularly for particle

Biomass concentration (MLSS) is assumed to have an adverse effect on oxygen transfer rate in Krampe and Krauth (2003) and

many others investigate the effect of MLSS on oxygen transfer and all concluded that with increase in factor is reduced. An exponential relationship between the alpha-factor

the relationship of alpha factor and

MLVSS concentration seems to be the decisive activated sludge system operating at a high SRT

factor is mainly influenced by the MLVSS concentration. With increasing MLVSS concentration the free water content of the sludge decreases and the interfacial area between the free water and the bubble is hindered by the bound water/solid


Figure 2.7 Free water content and bound water/solid matter content at the bubble interface at high (Left) and Low (Right)

Source: (Henkel et al. 2009)

When correlating the alpha factor with MLSS and MLVSS concentration from

authors a clear tendency was found between alpha factor and MLVSS concentration irrespective of the diverse origin of the sludge (Figure

Figure 2.8 Alpha Factor as function of MLSS conc. in different studies


Free water content and bound water/solid matter content at the bubble interface at high (Left) and Low (Right) MLSS

When correlating the alpha factor with MLSS and MLVSS concentration from authors a clear tendency was found between alpha factor and MLVSS concentration irrespective of the

Figure 2.8 & Figure 2.9 ).

Alpha Factor as function of MLSS conc. in different studies


Free water content and bound water/solid matter content at the bubble interface at high (Left) and Low (Right)

When correlating the alpha factor with MLSS and MLVSS concentration from different authors a clear tendency was found between alpha factor and MLVSS concentration irrespective of the

Alpha Factor as function of MLSS conc. in different studies

Literature Review

Figure 2.9 Alpha factor as function of MLVSS conc. in different studies

Source : (Henkel et al. 2009) 2.5.1.2. SMP and EPS Other characteristics of biomass which systems are soluble microbial products (SMP) and Extracellular polymeric substances (EPS). SMP is soluble and EPS is bounded with cells therefore SMP and EPS are part of liquid phase and part of solid phase respectively. Oxygen contained in the bubble cells need to reach inside the bacterial cell and in order to do so, it has to go through the SMP surrounding the flocs and then diffuse through the floc matrix.(Germain and Stephenson 2005)on concentration gradient.

In view of Germain et al. (2the EPS (EPSC) have an impact on oxygen transfer. Furthermore, SMPoxygen, whereas, the EPSc has positive impact on the oxygen that SMPCOD has negative effects might be due to the presence of surfactants in the biomass that accounts for COD measurements. EPSc support the aggregation ofand therefore diffusivity. However, diffusivity will not have an impact on oxygen transfer from gaseous phase to liquid phase rather it can affect the oxygen uptake rate by the microorganisms present in the flocs. 2.5.1.3. Viscosity Viscosity and solid concentration are correlated exponentially. With the increasing MLSS concentrations viscosity increases in bioreactor and show negative effect on oxygen transfer rate (Francisco a. Rodríguez et al. 2012)

Different explanations concerning this phenomenon have been given in the literature. According to Germain and Stephenson (2005)in modifications in bubble size distribution. 2.5.1.4. Particle size distribution The particle size distribution varies in different MBR systems. Large particle size found in a MBR system where particle sizes ranges from a few distribution in an MBR system is depended upon aeration system and intensity. Also particle size is depended upon the membrane system (i.e. side stthe activated sludge is re-circulated through pump. To avoid the cake layer built up on membrane the cross flow velocity is kept high and constant permeate flux is maintained through membrane by high

Alpha factor as function of MLVSS conc. in different studies

Other characteristics of biomass which appear to affect the oxygen transfer rate in activated sludge systems are soluble microbial products (SMP) and Extracellular polymeric substances (EPS). SMP is soluble and EPS is bounded with cells therefore SMP and EPS are part of liquid phase and part

tively. Oxygen contained in the bubble cells need to reach inside the bacterial cell and in order to do so, it has to go through the SMP surrounding the flocs and then diffuse through the

(Germain and Stephenson 2005). Thus, SMP and EPS effect the OUR, that has an impact

Germain et al. (2007) COD fraction of SMP (SMPCOD) and carbohydrate fraction of ) have an impact on oxygen transfer. Furthermore, SMPCOD has negati

hereas, the EPSc has positive impact on the oxygen transfer. Germain et al. (2007)has negative effects might be due to the presence of surfactants in the biomass that

accounts for COD measurements. EPSc support the aggregation of cell that lead to large porous flocs

However, diffusivity will not have an impact on oxygen transfer from gaseous phase to liquid phase rather it can affect the oxygen uptake rate by the microorganisms present in the flocs.

Viscosity and solid concentration are correlated exponentially. With the increasing MLSS concentrations viscosity increases in bioreactor and show negative effect on oxygen transfer rate (Francisco a. Rodríguez et al. 2012), (Krampe and Krauth 2003) and (Martı et al. 2010)

Different explanations concerning this phenomenon have been given in the literature. Germain and Stephenson (2005) bubble coalescence is influenced by viscosity, resulting

in modifications in bubble size distribution.

2.5.1.4. Particle size distribution The particle size distribution varies in different MBR systems. Large particle size found in a MBR system where particle sizes ranges from a few µm to 500 distribution in an MBR system is depended upon aeration system and intensity. Also particle size is

membrane system (i.e. side stream or submerged system). In a side stream system circulated through pump. To avoid the cake layer built up on membrane the

cross flow velocity is kept high and constant permeate flux is maintained through membrane by high

13

Alpha factor as function of MLVSS conc. in different studies

oxygen transfer rate in activated sludge systems are soluble microbial products (SMP) and Extracellular polymeric substances (EPS). SMP is soluble and EPS is bounded with cells therefore SMP and EPS are part of liquid phase and part

tively. Oxygen contained in the bubble cells need to reach inside the bacterial cell and in order to do so, it has to go through the SMP surrounding the flocs and then diffuse through the

. Thus, SMP and EPS effect the OUR, that has an impact

) and carbohydrate fraction of has negative effect on the

Germain et al. (2007) assumed has negative effects might be due to the presence of surfactants in the biomass that

cell that lead to large porous flocs

However, diffusivity will not have an impact on oxygen transfer from gaseous phase to liquid phase rather it can affect the oxygen uptake rate by the microorganisms present in the flocs.

Viscosity and solid concentration are correlated exponentially. With the increasing MLSS concentrations viscosity increases in bioreactor and show negative effect on oxygen transfer rate

(Martı et al. 2010) . Different explanations concerning this phenomenon have been given in the literature.

ubble coalescence is influenced by viscosity, resulting

The particle size distribution varies in different MBR systems. Large particle size distribution can be m to 500 µm. Particle size

distribution in an MBR system is depended upon aeration system and intensity. Also particle size is ream or submerged system). In a side stream system

circulated through pump. To avoid the cake layer built up on membrane the cross flow velocity is kept high and constant permeate flux is maintained through membrane by high


pressure. This results in higher shear stress on flocs and reduction in particle size. In submerged membrane system aeration provides the cross flow velocity and it allows lower suction pressure. Thus, force applied on flocs is weaker and result in bigger particle. (Germain and Stephenson 2005).

The variation in airflow not only impacts the particle size distribution but also the oxygen concentration and both influence the particle size. Balme and Wile (1999) concluded that there has been no clear co relation between Dissolved Oxygen (DO) concentration and average floc diameter. According to Abbassi et al. (1999) an increase in airflow to have more oxygen concentration also increases mixing intensity. Consequently, shear stress on flocs increases which result in floc breakup and formation of particles of different sizes. Therefore, when air flow is increased the differences in floc size not only result from the increase in oxygen concentration but also from the increase in mixing intensity.

The presence of solid affects K La negatively decreasing both K L and a but the effect is more pronounced for smaller particle. According to Mena et al. (2005) in case of small particles the variation in volumetric gas transfer coefficient is due to effect of smaller particles on both KL and a; whereas, in case of large particles such variation is due to KL mainly. Similarly, according to Germain and Stephenson (2005) bubble coalescence may occur when smaller particles are present at higher concentration (viscosity), whereas, smaller bubbles can be formed in the presence of large particles. 2.5.1.5. Influent Impurities Wastewater contains many impurities like salt, surfactants etc. which have effect on oxygen transfer. The effect of such constituents is accounted for by the factor β (Simon Judd 2006), which is defined as:

� = ∗ ��∗ (2.4)

Part of readily biodegradable COD (RBCOD) is composed of surface active agents (i.e. surfactants). The main sources of surfactants are oil, soaps and detergents (Henze et al. 2008). These surfactants accumulate at air-water surface of bubble thus causing the increase in interfacial rigidity, reduction of internal gas circulation, and overall gas transfer rate (Rosso and Stenstrom 2006). Jamnongwong et al. (2010) conducted comparative study of impurities i.e. salt (NaCl), sugar (glucose) and surfactants and found out that surfactants affect the liquid side mass transfer coefficient (KL) factor considerably in comparison to the salt and glucose. Similarly Chern et al. (2001) studied the effect of different impurities (i.e. surfactants , soybean oil) and found that they have a negative effect on the volumetric oxygen mass transfer coefficient.

However, MBR systems work on high SRT which allows the degradation of surfactant (RBCOD) and has little effect on oxygen transfer. According to Henkel et al. (2009) surfactants influent have little effect on alpha factor with high SRT, because higher SRT allows the degradation of surfactants completely. 2.6.2. Operational conditions 2.5.2.1. Hydraulic retention time Due to membrane separation the MBR system can work at long SRT which result in low food-to-microorganisms ratio (F / M) which give chance to reduce the HRT but with reduction in HRT the MLSS concentration increases due to the rise in influent flow. Rodríguez et al. (2012) studied the effect of different MLSS concentrations at different HRT on alpha- factor and found out that the oxygen transfer efficiency is highly influenced by MLSS and HRT. With the increase in MLSS concentration, corresponding to two different HRTs, it was found that the alpha factor decreases more significantly with an increase in MLSS concentration at low HRT than an increase in MLSS concentration at high HRT. HRT determines the contact time of a bubble

Literature Review 15

and liquid, therefore high HRT will have more contact time and vice versa. Furthermore While studying the effect of HRT on membrane fouling and biomass characteristics Meng et al. (2007) concluded that the low HRT would result in high MLSS concentration and sludge viscosity. In preceding part of this document it has already been discussed that the increase in MLSS concentration and sludge viscosity has negative effect on oxygen transfer. Similarly, Yoon et al. (2004) observed that the operational costs of the MBR operated at lower HRT is higher than the MBR operated at higher HRT. Because at lower HRT the higher steady state MLSS cause lower oxygen transfer efficiency. 2.5.2.2. Use of pure oxygen Instead of atmospheric air, pure oxygen can be used in MBR to increase the concentration of oxygen for better transfer but it is more costly. With the use of pure oxygen, the MBR can be designed at higher MLSS concentration than if air is used for aeration. But still with the increase in MLSS concentration the oxygen transfer efficiency (Alpha factor) decreases even with use of pure oxygen. Rodríguez et al. (2012) showed that with the use of pure oxygen the nitrogen removal efficiency of MBR improved due to changes in kinetic coefficients related to autotrophic bacteria. Kinetic coefficient show changes when pure oxygen is used and kinetics are favored with the use of pure oxygen and further improvement in kinetics can be achieved at higher HRT. Similarly , Martı et al.( 2010) observed that use of pure oxygen is efficient in establishing aerobic conditions with high efficiency of organic removal in MBR. Rodríguez et al. (2011) observed that use of pure oxygen in MBR allowed achieving the specific growth rate at higher MLSS concentration and same specific growth rate was achieved in other studies at lower MLSS concentration when air was used.

However, in all the above mentioned studies the alpha factor decreased at higher MLSS concentration even with the use of pure oxygen for providing aerobic conditions. 2.5.2.3 Sludge retention time Sludge retention time is the most important parameter in CAS and its selection depends upon many factors such as nutrient removal, quality of effluent, oxygen demand, sludge treatment etc. In the MBR system uncoupling of SRT and HRT by membrane filtration give chance to operate MBR at high sludge age. But high sludge age effects biomass characteristics and indirectly affects the oxygen transfer in the MBR system. SRT changes the composition of MLSS and influence the oxygen transfer rate.

According to Pollice et al. (2004, 2008), Huang et al. (2001) and Massé et al. (2006) MBR working at long sludge age results in high biomass concentration but without any significant drawback in terms of biodegradation activities and removal efficiency. With the increase in biomass concentration the KLa decreases thus reducing the oxygen transfer rate. 2.5.2.4. Temperature Temperature effect for oxygen transfer is accounted by the factor θ and KLa is corrected by the equation 2.5:

�� =�� !��

(2.5)

Where T is the temperature (oC) and θ is constant. Typical θ values are between 1.015 and 1.040, with 1.024 being the ASCE standard.

2.7. The SDOX technology

It has been discussed in the introduction section that in a SDOX unit oxygen gas is pre-dissolved into the liquid inside a pressurized saturation tank. The liquid, super saturated with oxygen, is then mixed in the main body of the liquid where oxygen is required (Figure 2.10).


[Source: Adapted from http://www.blueingreen.com/technology/sdox/

The use of SDOX unit is not limited to wastewater treatment. Its application includes, but not limited to, post aeration, force mains, drinking water reservoirs, oxidation ditches, oxygenation of lakes and lagoons. Figure 2.11 shows

Figure 2

[Source: BlueInGreen SDOX® System applied to oxygenation of a rstudy/noland-wastewater-treatment-

A. Oxygen / air passes from the source through a pressure regulator and into the gaseous oxygen

headspace in the pressurized saturation tank.B. Then, the water is sprayed from a nozzle through area of the pressurized oxygen/air. There are

two mechanism involved in supersaturation of oxygen in this tank. First the nozzle sprays the water in tiny droplets. This allows the oxygen to dissolve in the tiny droplets neinstantaneously. The other mechanism that is responsible for super saturation of the water is pressure in the tank. In the pressurized saturation tankwater droplets is under a high pressure that isit can hold at normal atmospheric pressure. These two mechanisms make the water


Figure 2.10 SDOX oxygen delivery

http://www.blueingreen.com/technology/sdox/ Accessed October 2012]

The use of SDOX unit is not limited to wastewater treatment. Its application includes, but not limited to, post aeration, force mains, drinking water reservoirs, oxidation ditches, oxygenation of

shows the use of SDOX unit in a river or channel (BlueInGreen n.d.)

2.11 SDOX unit applied to oxygenation of a river

BlueInGreen SDOX® System applied to oxygenation of a river (n.d) http://www.bluein-facility-fayetteville-ar/. Accessed October 2012]

Oxygen / air passes from the source through a pressure regulator and into the gaseous oxygen headspace in the pressurized saturation tank.

is sprayed from a nozzle through area of the pressurized oxygen/air. There are two mechanism involved in supersaturation of oxygen in this tank. First the nozzle sprays the water in tiny droplets. This allows the oxygen to dissolve in the tiny droplets neinstantaneously. The other mechanism that is responsible for super saturation of the water is

tank. In the pressurized saturation tank the oxygen interacting with the tiny is under a high pressure that is why the water can hold more oxygen than what

it can hold at normal atmospheric pressure. These two mechanisms make the water


2012]

The use of SDOX unit is not limited to wastewater treatment. Its application includes, but not limited to, post aeration, force mains, drinking water reservoirs, oxidation ditches, oxygenation of

(BlueInGreen n.d.).

http://www.blueingreen.com/case-

Oxygen / air passes from the source through a pressure regulator and into the gaseous oxygen

is sprayed from a nozzle through area of the pressurized oxygen/air. There are two mechanism involved in supersaturation of oxygen in this tank. First the nozzle sprays the water in tiny droplets. This allows the oxygen to dissolve in the tiny droplets nearly instantaneously. The other mechanism that is responsible for super saturation of the water is

the oxygen interacting with the tiny an hold more oxygen than what

it can hold at normal atmospheric pressure. These two mechanisms make the water

Orifice


supersaturated with oxygen. Finally, the supersaturated water is collected at the bottom of the tank.

C. The SDOX unit adds this supersaturated stream of water to a large body of water where oxygenation is required. The supersaturated water is mixed with the target water faster than oxygen gas can exit the water at normal atmospheric pressure. This is achieved by the design of injection orifice. The energy required for adding the side stream of supersaturated oxygen through an orifice, without degasifying, is recovered from the compressed gas pressure in pressurized saturation tank. The mixing rate is controlled to achieve the certain dissolved oxygen concentration. For example, mixing 1 liter supersaturated water (DO 400 mg/l) with 100 liters of water at 2mg/l will give an overall dissolved oxygen concentration of 6 mg/l.

2.7.1. Benefits of the SDOX technology According to the BlueInGreen ( n.d.) the SDOX technology has many benefits over other aeration technologies.

1. Lower operating cost because of the high efficiency of oxygen delivery. 2. Increase flexibility in oxygen delivery location. This enables to supply oxygen at critical locations. 3. The SDOX technology can be used in a case of emergency like oil spills to provide oxygen. 4. The SDOX technology is bubble free. So oxygen delivered by the SDOX technology remains bio

available. 5. Due to portability, the technology can be used in the remote locations for treatment.

2.7.2. Components of the laboratory scale SDOX unit Major components of the SDOX unit used in the laboratory test are shown in Figure 2.12 (BlueInGreen 2012).

Figure 2.12 Components of the SDOX unit

1. Control Panel

2

3

4

5

7

6 1

8


SDOX unit is equipped with a touch screen control panel to operate it. Figure 2.13 shows the main screen of the control panel.

Figure 2.13 Control panel's main screen

a) Gives the value of DO concentration from a DO meter. The SDOX unit did not come with a DO meter. If the unit is connected to a DO meter then the operation of the unit can also be controlled on basis of predefined DO set point in the "control screen" present at the bottom right corner of the main screen (Figure 2.14). b) Shows the pressure in the pressurized saturation tank. However, the pressure inside the pressurized saturation tank can also be measured from the dial pressure gauge fitted at the top of tank. c) This shows the pump flow rate as measured by a liquid flow meter attach to the influent pipe system for the pressurized saturation tank. However, when carrying out the experimental phases of this research, it was not possible to measure the flow rate as the minimum flow rate that could be measured from the liquid flow meter was higher than the flow rates used during different tests. d) Shows the level of liquid in the pressurized saturation tank in percentage of the total height of the tank. This level depends upon the gas pressure, incoming liquid's flow rate and end orifice size.

Different orifice sizes can be used for the SDOX unit's effluent pipe. As already mentioned in section 2.6 of this document that gas pressure inside the pressurized saturation tank is used to deliver the saturated stream of the liquid through an orifice, therefore, the flow rate through an orifice is dependent on the orifice size and gas pressure. To have a stable level of the liquid in the pressurized saturation tank a certain flow is required corresponding to the air pressure and end orifice size. This flow can be determined by trial and error method for a certain air pressure and orifice size. However, high flow rate is required for higher gas pressure and bigger orifice size and vice versa. e) SDOX unit can be operated in three modes i.e. DO mode, Auto mode and Manual mode.

• DO MODE - In DO MODE the system is controlled on the basis of a set DO value in control screen.

• AUTO MODE - In AUTO MODE the system is automatically controlled on the basis of different parameters defined in control screen.

• MANUAL MODE - In MANUAL MODE system can be operated manually by using the red color "Manual" button (see Figure 2.13) for different parameters.

f) Figure 2.14 shows the control screen that is accessible through the bottom right corner of the main screen. The control screen contains set points for different operational parameters of the unit.

b

ad

c

efg


Figure 2.14 Control screen

• dP Low - Level SP (%) & dp High - Level SP (%) - These values is the allowable range in terms of % height of the tank for the liquid's low level and high level. At defined low level, the gas flow will stop by closure of gas solenoid. Also, when liquid will reach the set point high level the gas solenoid will open, allowing the gas flow in the tank. The main purpose of these set points to avoid emptying or filling up of the tank.

When the unit is started, it starts pumping the liquid in the tank until it reaches to set point high level and gas inflow start at this point. Later on, the tank continuously receives gas flow until unless liquid level goes to the set point low level where gas flow will be shut down by the system automatically. In present study the Low-Level and High-Level were 25 and 50 respectively.

• dP Tank Low- Level Alarm SP (%) & dP Tank High- Level Alarm SP (%) - These are liquid's low level and high level in the tank where the alarm will shut down the system. dP Tank Low-Level Alarm Delay (sec) & dP Tank High-Level Alarm Delay (sec) - This values defines the time in seconds that tank level must remain on high or low level set points to open or close the gas solenoid.

• DO Setpoint (mg/l) - This value control the system based on the defined DO value (mg/l) in DO MODE. However, it is not possible currently to operate the unit in DO MODE.

• High-Pressure Alarm SP (psi) - This value defines the highest gas pressure in psi at which the alarm will activate and shut down the system.

• DO Alarm Delay (sec) - Delay time in seconds to activate the DO Alarm when DO deviates from the set DO point.

• High-Pressure Alarm Delay (sec) - Delay time in seconds to activate the high pressure alarm when gas pressure in tank reaches or go above the high pressure set point. Refer back to the Figure 2.13:


2) Pump - Inflow pump for the SDOX unit. The pump was replaced with a lab scale pump. The new pump used in this research is shown in Figure 2.15

Figure 2.15 Inflow pump

3) Liquid inlet valve - Valve to control the inflow of liquid. Not used after the pump

replacement. 4) Oxygen/air inlet valve - Valve to open or close the gas flow to the unit. 5) Gas flow meter - To measure the flow of gas to the pressurized saturation tank. The gas flow

meter was under capacity to measure the gas flow to the unit during tests. 6) Pressurized saturation tank - The pressurized saturation tank is fitted with a nozzle at top of

tank to spray the liquid in tiny droplets. There is an effluent pipe fixed at the bottom of the tank to deliver the saturated stream of liquid. Figure 2.16 and Figure 2.17 shows the nozzle and inner view of the tank respectively. The volume of saturation tank is approximately 1.90 L.


Figure 2.16 Spray nozzle

Figure 2.17 Inside view of pressurized saturation tank

7) Pump pressure gauge - Shows the pressure in psi for inflow pump 8) Pressurized saturation tank's pressure gauge. Shows the pressure in psi for the pressurized

saturation tank.

Spray Nozzle

Methodology 23

The present study was divided in four phases. The first phase involves the evaluation of the SDOX unit to supply dissolved oxygen in tap water. The second phase involves the evaluation of the SDOX unit at different sludge or MLSS concentrations. Comparison between the results of tap water test and sludge test was carried out to evaluate the potential effects of suspended solids on the oxygen supply capability of the SDOX unit. The third phase of experiments compares the oxygen delivery capabilities of the SDOX at a specific MLSS concentration with the oxygen delivery capabilities of conventional coarse aeration diffusers at the same MLSS concentration. In the last phase of experiments, the effect of exposing the microorganisms to the high pressure conditions of the SDOX unit was evaluated.

The general framework of the research activities is illustrated in the Figure 3.1.

CHAPTER 3

Methodology


3.1. Startup of the SDOX unit

Conclusions and

recommendations

SDOX tap water experiments

SDOX Sludge experiment (at

different sludge concentrations)

Comparison of conventional

aeration diffusers with SDOX unit at

a set MLSS concentration

Effects of the SDOX system on

active microorganism

Data analysis

Startup of the SDOX unit

Figure 3.1 Research framework

Methodology 25

The laboratory scale SDOX unit was set up in the laboratory facility of the UNESCO-IHE. The setup included the appropriate placement of the unit and pressurized air connection. Later on, several preliminary experiments were carried out to get familiar with the operational conditions of the unit. The unit came with a built-in pumping system, but during the start up phase it was realized that the pump was too large for the flow rates to be evaluated and introduced an unnecessary recirculation of the flow that heat up the water that passes through the SDOX unit. Therefore, the pump was replaced with a new pump that can work at the desired flow rate.

After the starting up of the unit, experiments on tap water were performed.

3.2. Tap water testing

3.2.1. Materials The following materials are used in the tap water testing.

1. The SDOX unit. 2. End orifices with the following diameters: 0.7 mm, 1.14 mm and 1.2 mm placed at the SDOX

effluent pipe. 3. Sodium sulfite to decrease the dissolved oxygen concentration. 4. Mechanical stirrer. 5. A calibrated Hach HQ30d DO probe to measure DO concentrations and temperatures. 6. Stop watch to keep track of the time at each experiment. 3.2.2. Methodology It is essential to know the oxygen supply capabilities of the SDOX unit in tap water, so that the oxygen transfer efficiency at different MLSS concentrations can be later determined. Several experiments were conducted to evaluate the oxygen delivery capabilities of the SDOX unit in tap water.

The experimental setup is shown in Figure 3.2. The tap water from a container was circulated through the SDOX unit and the increase in the DO concentration versus time was measured with a calibrated DO probe. Complete mixing conditions were achieved by using a mechanical stirrer. `

Figure 3.2 Experimental setup for tap water testing

Operational conditions • Air pressure 30, 60 and 90

psi • Orifice size (0.7mm ,

1.14mm and 1.20mm)

System Boundary

SDOX Unit

Completely Mixed water Water Tank


The tap water from the laboratory was collected in a container with total capacity of fifty (50) liters. The tap water had a high dissolved oxygen (DO) concentration. In order to decrease the dissolved oxygen concentration, sodium sulfite was added to the system. Sodium sulfite is often used as an oxygen scavenger as it reacts with the DO to produce sodium sulfate. The dosage of the required sodium sulfite was calculated on the basis of the balanced equation 3.1.

2#��$�% +�� → 2#��$�( (3.1)

Equation 3.1 shows that two moles (126g/mole) of sodium sulfite requires one mole (32 g/mole) of oxygen. So for every one part of oxygen the theoretical demand of the sodium sulfite is 7.88. To lower down the DO concentration up to the desirable concentration (DO concentration below 1 mg/L), the theoretical demand of sodium sulfite was calculated. Complete removal of the dissolved oxygen from the tap water was avoided to make sure that there is no a residual sodium sulfite concentration in the system. A residual concentration of sodium sulfate may affect the results by reacting with the dissolved oxygen supplied by the SDOX unit. After each test, the tap water was replaced with new tap water to avoid the built-up of the sodium sulphate chemical in the container.

In order to assess the oxygen delivery capabilities of the SDOX unit at different operational conditions, different experiments were conducted at different orifice sizes and air pressures. Each orifice size was tested at three different pressures of approximately 30, 60 and 90 psi (see Table 3-1). At each experimental pressure, the flow rate deliver by the SDOX unit will depend on the orifice size. Larger orifice sizes deliver higher flow rates since the SDOX unit operates at a constant pressure.

Table 3-1 The SDOX's operational conditions for the tap water experiments

Orifice size (mm) Air pressure (Psi)

0.7

30

60

90

1.14

30

60

90

1.20

30

60

90

As observed in Table 3-1 all three orifice sizes (0.7mm, 1.14mm and 1.2mm) were tested at

three different air pressures of approximately 30, 60 and 90 psi. The experiment for the air pressure at 60 psi was repeated three times to establish the consistency of the oxygen supply by the unit at similar operational conditions. Furthermore, it was decided that the experiments in the second phase (sludge experiments) will be carried out at an air pressure of 60 psi.

Methodology 27

A mass balance for oxygen was carried out across the boundary of the container to calculate the value of the dissolved oxygen concentration supplied by the SDOX unit (CSDOX). The CSDOX was assumed constant during the test. Where; Q = constant flow rate through the SDOX unit CSDOX = Constant DO concentration supplied by the SDOX unit V = Volume of the container C = DO concentration in the container = DO concentration going out from the container (Complete mixing conditions)

) *+*, = -. ./01 − -.

(3.2)

By integrating and rearranging;

= ./01 21 −45! 67�89 + : ∗ 45! 6

7�8

(3.3)

Where; Ci = Initial tap water DO concentration.

The DO concentration provided by the SDOX unit (CSDOX) was calculated by using equations 3.2 and 3.3 using both AQUASIM software and Microsoft Excel solver. In order to estimate the (CSDOX), DO concentrations below the saturation concentration at the experimental conditions were considered. After reaching saturation some dissolved oxygen may escape from the container, condition that was not consider in the equations 3.2 and 3.3. The experimental conditions for each experiment are provided in Section 4.1.

3.3. Sludge Testing

3.3.1. Materials The following materials were used in the sludge testing. 1. The SDOX unit. 2. End orifice size of 1.2 mm 3. Sludge at different MLSS concentrations 4. Reactor provided with flat sheet membrane 5. Pumps 6. A calibrated Hach HQ30d DO probe to measure DO concentrations and temperatures 7. Stop watch 8. 500 µm sieve 9. Mechanical stirrer

V = Volume of tank

C = DO conc. in container

Q , C CSDOX, Q


3.3.2. Methodology The oxygen supply by the SDOX unit at different mixed liquor suspended solids (MLSS) concentrations was assessed using the same methodology as previously described in Section 3.2. The experiments were performed on sludge rather than on tap water. Several experiments were performed to assess the oxygen delivery capabilities of the SDOX in sludge. The sludge used in these experiments was taken from the local (Harnaschpolder) wastewater treatment plant (WWTP). Except for the experiments at MLSS concentration of 34 g/L, the MLSS samples were taken from the aerobic bioreactor on a weekly basis and stored in the cold storage room at 4oC. Figure 3.3 shows the sampling point of the sludge at the WWTP. The local wastewater treatment plant operates as a conventional activated sludge process.

Figure 3.3 Sampling point of sludge at the local WWTP

The local wastewater treatment plant works at a sludge retention time of approximately twenty (20) days. Before carrying out each single experiment, the sludge was removed from the cold storage room and brought up to room temperature. The sludge was filtered through a 500 µm sieve to avoid any potential clogging of the SDOX unit; especially, at the orifice end of the SDOX effluent tube/pipe. Figure 3.4 shows an example of the solids retained in the sieve after passing the sludge through the 500 µm sieve. After filtering the sludge, it was aerated overnight to make sure that endogenous decay respiration status was achieved.

Figure 3.4 Solids retained after sieving the sludge through a 500 µm sieve

Methodology 29

Six different sludge concentrations (see Table 3-2) were evaluated to determine the oxygen delivery capabilities of the SDOX unit. A complete set of experimental conditions for each experiment is described in Section 4.2.

Table 3-2 Sludge concentrations evaluated

Exp # Sludge concentration (g/l)

1 3.20

2 6.3

3 12.7

4 19.7

5 24.0

6 34.2

The different sludge concentrations were achieved by thickening the sludge mainly by gravity.

3.3.2.1. Sludge thickening The sludge was thickened to achieve the desired experimental sludge concentrations. The sludge taken from the local wastewater treatment plant aerobic reactor had a MLSS concentration of approximately 3.5 g/l. The sludge was allowed to settle down in a container, and the supernatant was removed from the top of the container to concentrate the sludge up to the desired concentration (See Figure 3.5).

Figure 3.5 Settled sludge

However, to achieve sludge concentrations higher than 20 g/L a membrane bioreactor was used to concentrate the sludge. The sludge was filtering through flat sheet microfiltration membranes with a pore size of 0.45 µm. (See Figure 3.6)


Figure 3.6 Concentrating sludge with membrane

To achieve the final sludge concentration point at 34 g/l, sludge from the secondary settler (rather than form the aeration basin) with a concentration of approximately 8.5 g/l was taken from the local WWTP. The sludge was concentrated by filtering through a microfiltration membranes (as previously described). Figure 3.7 shows the sludge concentrated up to a final concentration of approximately 34 g/L.

Figure 3.7 Concentrated sludge (34.2 g/l) 3.3.2.2. Experimental setup The experimental setup for the experiments performed on sludge at all the different MLSS concentrations is shown in Figure 3.8. Before evaluating the sludge through the SDOX unit, the total suspends solids (TSS) and volatile suspended solids (VSS) concentrations were determined according to standard methods for examination of water and wastewater SM-2540D (APHA 1999). The first step consisted on determining the sludge oxygen uptake rate (OUR). Oxygen was supplied to the container by bubbling air into the sludge. The OUR at the biomass endogenous respiration rate was assessed. After determining the OUR, the sludge was re-circulated through the SDOX unit and the increase in the DO concentration in the complete mixed container was recorded as a function of time using a calibrated DO probe. After concluding the experiment the OUR test was measured again. All the sludge concentrations were tested with the SDOX unit operated at a constant air pressure of approximately 60 psi and using the orifice at a 1.20 mm diameter.

Methodology 31

As described in previous Section 3.2, a mass balance on oxygen was carried out with boundaries across the container. The DO concentration supplied by the SDOX unit (CSDOX) was assumed constant during the testing. Where; Q = constant flow rate through the SDOX unit CSDOX = Constant DO concentration supplied by the SDOX unit V = Volume of the container C = DO concentration in the container = DO concentration going out from the container (complete mixing conditions) OUR = Oxygen uptake rate of microorganisms at the endogenous state

) *+*, = -. ./01 − -. − �;�. )

(3.4)


= 2 ./01 −�;�. )- 9 21 −45! 67�89 + :. 45! 6

7�8

(3.5)

Where; Ci = Initial DO concentration in the tank

V = Volume of container

C = Conc. of DO in container OUR = Oxygen uptake rate

Q , C CSDOX, Q

Figure 3.8 Experimental setup for sludge experiments

Operational Conditions • Air pressure 60 psi

• Orifice size 1.20mm

System Boundary

SDOX Unit

Completely

Mixed Sludge


The DO concentration provided by the SDOX unit (CSDOX) was calculated by equations 3.4

and 3.5 using both AQUASIM software and Microsoft Excel solver. In order to estimate the (CSDOX), DO concentrations below the saturation concentration at the experimental conditions were considered.

Three replicates of each experiment were performed. The average of the oxygen uptake rate

(OUR) experiments measured before each SDOX experiment was used to determine CSDOX by equation 3.5. A schematic showing the sequence of the experiments is described in Figure 3.9, and described as follows:

As described in Figure 3.9, the oxygen uptake rate (OUR 1) was measured by supplying air

through diffusers (that is, bubbling the air into the sludge). Then, the sludge was circulated through the SDOX unit (SDOX 1st run) to evaluate the SDOX oxygen deliver capabilities at such MLSS concentration. Afterwards, the oxygen uptake rate (OUR 2) was again determined. The rest of the sequence was performed as observed in Figure 3.9. The results from the oxygen uptake rate experiments (OUR 1, OUR 2, and OUR 3) and from the oxygen delivery capabilities of the SDOX unit experiments (SDOX (1st run), SDOX (2nd run) and SDOX (3rd run)) were fit to the equation 3.5 to determine the CSDOX value.

However, at MLSS concentrations higher than 19 g/L (that is, for experiments performed at MLSS concentrations of 19.7 g/l, 24.0 g/l and 34.2 g/l) it was not possible to supply the required oxygen demand for the determination of the oxygen uptake rate experiments by using the air diffusers (that is by bubbling air into the container). Consequently, the dissolved oxygen to evaluate the oxygen uptake rate was supplied by means of the SDOX unit. It is important to highlight that before and after performing each test as described in Figure 3.9, the OUR was measured by doing triplicates. The OUR experiments were performed by first increasing the DO concentrations, and then recording the decrease in the DO concentration values as a function of time. The DO concentration needed to be always higher than 2.0 mg/l.

3.4. Comparison of conventional bubble aeration diffusers with the SDOX technology

The comparison of the conventional coarse bubble aeration diffusers with the SDOX technology was carried out to determine the possible advantages for supplying the oxygen by the SDOX technology. A membrane reactor with two submerged Kubota flat sheet membranes (nominal pore size of 0.4µm and an effective filtration area of 0.10 m2 per membrane) was used. This unit was provided with a coarse bubble aeration diffuser (see Figure 3.10). Batch experiments to evaluate the oxygen delivery capabilities of the coarse bubble aeration and of the SDOX technology were performed at a MLSS concentration of 15.4 g/l. The sludge as explained in Section 3.3 was taken from the local WWTP and concentrated up to the desired concentration. The sludge was aerated over night until a constant OUR was observed to make sure that there was no residual soluble COD left in the sludge.

Figure 3.9 Schematic sequence of the sludge experiments for the SDOX unit

OUR1

SDOX 1st Run

OUR2

SDOX 2nd Run

OUR3

SDOX

3rd Run

Methodology 33

Figure 3.10 Coarse bubble aeration

The alpha factor value for the bubble diffuser was determined by measuring the KLa value in tap (clean) water and compared to the same value in sludge at the desired concentration and at the same reactor volume of 8.5 L. The air was supplied at a flow rate of 690 l/hr. The minimum air flow rate required was determined by a progressive increase in the air flow rate until a increase in the DO concentration in the reactor was fast enough to determine the KLa values in a reasonable period of time.

The KLa(T) values obtained for the experiment performed both in tap water and sludge were converted to KLa (20

oC) in order to be able to compare those values and determine the α(20) - factor. The

conversion of KLa (T) to KLa (20oC) was carried out by using the equation 3.6. The value of θ was

considered equal to 1.024. The experimental setup to measure the KLa in tap water is shown in Figure 3.11. The detailed experimental conditions for this experiment are described in Section 4.3.

�� =�� !��

(3.6)

Figure 3.11 Experimental setup to measure KLa with tap water

8.5 L of tap water

Air supply

DO Probe N2 source


The non-steady state absorption method was used to determine the KLa value in the tap water (He et al. 2003). Experiments were performed by first reducing the existing DO concentration in the reactor by sparging N2 gas. Later on, oxygen was supplied by bubbling air into the reactor filled with tap water. The increase of the DO concentration as a function of time was recorded with a calibrated DO probe placed at a half water depth. The oxygen transfer rate (OTR) can be calculated by using the following equation:

* *, = �� ∗ − � (3.7)


= ∗�1 − 4!<=�� + : ∗ 4!<=�� (3.8)

Where; C* = saturated DO concentration at given temperature Ci = Initial DO concentration

The KLa value was calculated by fitting the experimental data to the Equation 3.8. Both AQUASIM, and Microsoft Excel solver were used to fit the experimental data to the equation 3.8.

The KLa value on sludge was determined by applying the non-steady absorption method (Mueller et al. 2002). The experimental setup is described in Figure 3.12. A more detail description if the experimental conditions are described in Section 4.3.

To determine the KLa values in sludge, the oxygen uptake rate of the mixture needs to be determined. The OUR experiments were carried out in a biological oxygen measurement cell (BOM) equipped with a dissolved oxygen sensor to measure the changes in the DO concentration as a function of time. To determine the OUR, the sludge was first aerated to increase the DO concentration. Then, the sludge was circulated through the BOM, and the DO concentrations were recorded as a function of time at a pre defined time intervals.

Magnetic stirrer

Figure 3.12 Experimental setup to measure KLa in sludge

Air supply

8.5 L of sludge

P

DO Probe

BOM

DO Probe

Methodology 35

After the OUR values were determined, air was supplied into the reactor, and the increase in DO concentration as a function of time was measured with a calibrated DO probe. Incorporating the OUR value in the mass balance of the equation 3.7 gives the following equation:

*+*, = �� ∗ − � − �;�

(3.9)


= ∗�1 − 4!<=�� − �;�� 1 − 4!<=�� (3.10)

The KLa and C* values were determined by fitting the experimental data to Equation 3.10

using both AQUASIM software package as well as Microsoft Excel solver.

The oxygen delivery capability of the SDOX unit in the batch membrane reactor was carried out with the same sludge used for determining the oxygen transfer efficiency of the bubble diffusers. The methodology for the SDOX experiments is exactly the same as previously explained in Section 3.3.2. The experimental setup is shown in Figure 3.13. A more detailed explanation of the experimental conditions is shown in Section 4.3.

3.5. The effect of the SDOX technology on active microorganisms

The SDOX unit works at high pressure conditions as discussed in Section 2.6, while the container from where the sludge is re-circulated to the SDOX unit is open to the atmosphere. That is, the microorganisms alternate between being exposed to high pressure conditions and to atmospheric pressure conditions. This process is cyclic, and the retention time of the microorganisms allocated to

8.5 L of sludge

P

BOM

DO Probe

DO Probe

SDOX

Magnetic stirrer

Figure 3.13 Experimental setup to supply oxygen by SDOX unit in a membrane reactor


each pressure (high and atmospheric) depends on process conditions and oxygen requirements. That is, microorganisms have to face the sudden pressure drop across the delivery orifice of the SDOX unit, and the other way around. In addition, microorganisms are exposed to high pressure conditions for a certain period of time equal to the residence time in pressurized saturation tank of the SDOX unit. Some effects on active microorganisms may be observed, and to determine these effects, if any, the following experiments were proposed. 3.5.1. Methodology The sludge taken from the local wastewater treatment plant was aerated with air for five hours before performing the experiments. A five liters volume of that sludge was continuously circulated through the SDOX unity for ninety (90) minutes, and samples of the sludge mixture were taken at thirty (30) and ninety (90) minutes. The sludge samples (not exposed to the SDOX, after 30 minutes of exposure time to the SDOX, and after 90 minutes of exposure time to the SDOX) were analyzed to determine the following parameters: soluble COD, TSS and VSS and ammonia NH4

- N. TSS, VSS, and NH4 - N were determined in according to the standard methods for

examination of water and wastewater 2540D and 4500-NH4. Soluble COD was measured according to standard methods for examination of water and wastewater 5220C (open reflux, titrimetric) after filtering the samples through a 0.45µm filter (APHA 1999).

Oxygen uptake rate experiments were carried out for the sludge samples as described in Table 3.4. The OUR values for the sludge not exposed to the SDOX system were used as a baseline. The sludge that was exposed to the SDOX unit for 30 and 90 minutes was evaluated by combining with an equal volume of sludge that was not exposed to the SDOX unit. This was done to observe if there was any change in the OUR due to availability of some soluble COD produced from the exposure of the sludge to the high pressure conditions of the SDOX unit. The OUR tests were performed in a respirometry setup (see Figure 3.14). The constantly stirred sludge was aerated. Once the desired dissolved oxygen concentration was achieved, the aeration was turned off and the decrease in DO concentration as a function of time was recorded using a calibrated DO probe attached to a computer. The temperature was controlled at 20oC. The operational conditions for the SDOX and respirometry experiments are presented in Tables 3.3 and 3.4.

Figure 3.14 Respirometry setup to measure OUR

Methodology 37

Table 3-3 Operational conditions for SDOX to assess impact on microorganisms

Volume of Sludge (Liters)

Air pressure for SDOX (psi)

Orifice size (mm) Flow rate through

SDOX unit (ml/min) 5 33 1.0 650

Table 3-4 Test conditions for OUR tests

Description Volume (ml) Temperature (oC)

Sludge without exposure 200 20 Sludge without exposure + sludge with 30 minutes exposure 100 + 100 = 200 20

Sludge with 30 minutes exposure 200 20 Sludge without exposure + sludge with 90 minutes exposure 100 +100 = 200 20

The test was performed at an air pressure of 33 psi. Due to operational limitation, it was not

possible to achieve higher pressure conditions to perform these experiments. All three sludge samples (that not exposed to the SDOX, continuously exposed for 30

minutes, and continuously exposed for 90 minute)s were investigated under the microscope.

Results and Discussion 39

4.1. Tap water experiments

4.1.1. Orifice size 0.70 mm As described in section 3.2, tap water experiments at an orifice size of 0.70 mm were

performed at three different air pressures 30, 60, and 90 psi. The experimental conditions of the experiments are provided in the Table 4-1. The DO concentrations as a function of time intervals are given in the Figure 4.1.

Table 4-1 Test conditions for tap water test with orifice size 0.70 mm

Exp # Average air pressure (psi)

Flow through SDOX

(ml/min)

Average water temperature (oC)

Initial volume of water (Liters)

1 34 400 18.0 40

2 63 495 13.0 40

3 93 575 13.4 40

CHAPTER 4

Results and Discussion

0

2

4

6

8

10

12

14

16

0 20 40 60 80 100

DO

(m

g/l)

Time (minutes)

DO vs. Time - Orifice 0.7 mm

30 PSI

60 PSI

90 PSI

Figure 4.1 DO vs. Time - Orifice size 0.70 mm


Figure 4.1 shows that the higher the pressure, the higher the rate at which the DO concentration increases. The SDOX unit was able to supply more dissolved oxygen at higher pressures because of two reasons: i) the increase in the flow rate through the SDOX unit as the pressure increases; and ii) the increase in the dissolved oxygen concentration supplied by the SDOX unit (CSDOX). The CSDOX values calculated in accordance to the procedure presented in Section 3.2.2 are shown in the Table 4-2.

Table 4-2 Calculated CSDOX values for the orifice size 0.70mm

Air pressure (psi) CSDOX (mg/l)

30 16.0

60 27.8

90 34.3

4.1.2. Orifice size1.14 mm

Experiments performed on tap water using the 1.14 mm orifice size showed similar results as the previous experiments performed on tap water with the 0.70 mm orifice size. The 1.14 mm orifice was evaluated at the same three air pressures (30, 60, and 90 psi) previously described. As described in Section 2.6.2, the flow rates through the SDOX unit needed to be adjusted accordingly to achieve a constant water level in the pressurized chamber of the SDOX unit. The experimental conditions of the present experiments are described in Table 4-3. As indicated in Figure 4.2, the higher the air pressure in the SDOX unit, the higher the oxygen supply rate (see Figure 4.2 ).

Table 4-3 Test conditions for tap water test with orifice size 1.14mm


Flow through SDOX

(ml/min)

Average water temperature (oc) Initial volume of

water (Liters)

1 34 800 22.0 40

2 61 1000 16.5 40

3 64 1000 18.0 40

4 63 1000 16.0 40

5 99 1160 20.0 40



The DO concentration (CSDOX) deliver by the SDOX unit was determined by carrying out the mass balance across the boundaries of the container described in Section 3.2. The calculated CSDOX

values are presented in Table 4-4.



30 15.4

60 27.7 ± 1.1

90 32.2

Initially, it was decided to use the same orifice size (1.14mm) to evaluate the SDOX

performance in sludge as well. The cap that contains the orifice at 1.14 mm is made of PVC material. The tap water experiments at an air pressure of 60 psi were repeated three times to achieve an average and more precise value of the concentration deliver by the SDOX unit (CSDOX). However, in the second phase of the research (experiments on sludge) during a sludge test using the same cap with an orifice size of 1.14 mm a sudden drop on the level of sludge/liquid in the pressurized saturation tank of the SDOX unit was observed. From trouble shooting the system, it was observed that the orifice of the PVC cap was increased its diameter due to the friction between the solids of the sludge and the PVC material of the cap. This caused the emptying of the pressurized saturation tank because a higher flow rate was needed to stabilize the level of sludge in the pressurized saturation tank. Thereby, it was decided to use a metal orifice rather than a PVC orifice to ensure a constant orifice size during the whole duration of the experiments. Therefore, a metal orifice with a diameter of 1.2mm was made, and all the experiments with sludge at different MLSS concentrations were performed with that orifice. To compare the experimental results on the sludge with the orifice size of 1.2 mm, tap water experiments were also repeated, but now using the 1.2 mm orifice.

0

2

4

6

8

10

12

14

0 20 40 60 80

DO

(m

g/l)

Time (minutes)

DO vs. Time - Orifice 1.14mm

30 PSI 60 PSI, 1st Run 60 PSI, 2nd Run 60 PSI, 3rd Run 90 PSI


4.1.3. Orifice size 1.20 mm The tap water experiments with a 1.20 mm orifice size were carried out at the test conditions

described in Table 4-5. Figure 4.3 shows the increase in the DO concentration as a function if time at the different pressures.

Table 4-5 Test conditions for tap water test with orifice size 1.20 mm


Flow (ml/min)

Average water temperature (oc)

Initial volume of water (Liters)

1 34 1100 12.6 40

2 63 1350 14.0 40

3 63 1350 12.0 40

4 63 1350 15.6 40

5 96 1600 16.8 40


As in previous experiments at different orifice diameters (0.7mm and 1.14 mm), the DO concentration supplied by the SDOX unit (CSDOX) was determined (Table 4-6).



30 19.4

60 29.5 ± 0.4

90 33.1

0

2

4

6

8

10

12

14

16

0 10 20 30 40

DO

(m

g/l)

Time (minutes)

DO vs. Time - Orifice 1.2 mm

30 PSI 60 PSI, 1st Run 60 PSI, 2nd Run 60 PSI, 3rd Run 90 PSI


4.1.4. Summary of the tap water tests The tap water experimental results showed that DO concentration supplied by the SDOX unit (CSDOX) depends on the pressure conditions in the pressurized saturation chamber of the SDOX unit. The higher the pressure in the pressurized chamber of the SDOX unit, the higher the concentration of dissolved oxygen (CSDOX) that is delivered by the system. The concentration of dissolved oxygen (CSDOX) deliver by the SDOX unit does not depend on the orifice size (diameter), as can be observed in Figure 4.4.

Figure 4.4 Calculated CSDOX for tap water at different air pressures with different orifice sizes (diameters)

Moreover, at the same air pressures on the pressurized chamber of the SDOX unit, the different orifices showed different oxygen supply rates. Figure 4.5, Figure 4.6 and Figure 4.7 show the rate of increase of the DO concentration at similar pressure with different orifice sizes. The CSDOX

is independent of the orifice size and only depends on the air pressure in the pressurized chamber. However, as discussed in Section 2.6.2, the flow rate is dependent on the orifice size. That is, the flow rate increases with the orifice size. Thereby, the bigger the orifice size results in more flux of oxygen being delivered from the SDOX unit to the system. Moreover, the consistency of the SDOX unit is evident from observing the experimental results summarized in Figure 4.6 under similar experimental conditions.

16.9

28.733.2

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

30PSI 60PSI 90PSI

DO

(m

g/l)

CSDOX for different pressures - tap water


Figure 4.5 DO vs. Time for tap water at 30 psi air pressure for different orifice sizes


0

2

4

6

8

10

12

14

0 10 20 30 40 50

DO

(m

g/l)

Time (minutes)

DO vs. Time at air pressure 30 psi

0.7mm orifice 1.14mm orifice 1.20mm orifice

0

2

4

6

8

10

12

14

0 10 20 30 40 50

DO

(m

g/l)

Time (minutes)

DO vs. Time at air pressure 60 psi

orifice 0.70mm orifice 1.14mm(1) orifice 1.14mm(2)

orifice 1.14mm(3) orifice 1.20mm(1) orifice 1.20mm(2)

orifice 1.20mm(3)



4.1.5. Discussion The oxygen transfer rate exhibited by the SDOX unit depends on several factors such as the

pressure inside the pressurized chamber, the orifice size (diameter), and the performance of the spray nozzle inside the pressurized saturation chamber. Increasing the pressure in the pressurized tank, results in a higher dissolved oxygen concentration delivered by the SDOX unit. An increase in the orifice size (diameter) results in an increase of the flux of oxygen delivered by the SDOX unit, due to an increase in the flow rate through the SDOX unit. A decreases in the spray nozzle performance (decrease in size), increases the oxygen saturation in the pressurized chamber due to an increase in the interfacial contact area between the sprayed liquid and the pressurized gas. In the present study, all the experiments were carried out using the same nozzle size that came with the laboratory scale SDOX unit.

4.2. Sludge testing

After performing the tap water experiments, different sludge concentrations were evaluated. The goal of these set of experiments was to elucidate how different MLSS concentrations impact the performance of the SDOX unit. 4.2.1. Sludge concentration 3.2 g/l

The SDOX unit was able to successfully supply dissolved oxygen into sludge with a MLSS concentration of 3.2 g/l. The experimental conditions are presented in Table 4-7.

Table 4-7 Test conditions for sludge at a MLSS concentration of 3.2 g/l

Test Run

Avg. OUR before test (mg/l.hr)

Average air pressure (psi)

Flow through SDOX

(ml/min)

Average sludge

temperature (oc)

Initial volume of sludge (Liters)

1st 14.6 64 1360 12.6 35.5

0

2

4

6

8

10

12

14

0 10 20 30 40 50

DO

(m

g/l)

Time (minutes)

DO vs. Time at air pressure 90 Psi

orifice 0.70mm orifice 1.14mm orifice 1.20mm


The constant DO concentration delivered by the SDOX unit (CSDOX) was calculated as 30.1 mg/l. The experimental data observed in Figure 4.8 was fit to the Equation 3.5 described in Section 3.3 and the DO concentration delivered by the SDOX unit (CSDOX) was determined.

Figure 4.8 DO vs. Time for sludge at a MLSS concentration of 3.2 g/l

4.2.2. Sludge concentration 6.3 g/l Next, sludge at a MLSS concentration of 6.3 g/l was evaluated to determine the dissolved oxygen concentration delivered by the SDOX unit. The experimental test conditions are described in the Table 4-8.


Test Run



Flow through SDOX

(ml/min)

Average sludge temperature

(oc)


1st 5.5 63 1350 18.8 30

2nd 8.6 63 1350 20.2 30

3rd 10.8 63 1350 20.8 30

The SDOX unit successfully supplied dissolved oxygen to the sludge at the MLSS

concentration of 6.3 g/l. The oxygen supply rate is shown in the Figure 4.9

0

2

4

6

8

10

12

0 5 10 15 20 25

DO

(m

g/l)

Time (minutes)

DO vs. Time - Sludge concentration 3.2 g/l


Figure 4.9 DO vs. Time for the sludge at a MLSS concentration of 6.3 g/l

The experimental data observed in Figure 4.9 was fit to the Equation 3.5 described in Section 3.3 and the DO concentration delivered by the SDOX unit (CSDOX) was determined and it is presented in Table 4.9.

Table 4-9 Calculated CSDOX values for sludge at a MLSS concentration of 6.3 g/l

Test Run Average CSDOX (mg/l)

Standard deviation 1st 2nd 3rd

29.7 28.9 28.6 29.1 ± 0.6

4.2.3. Sludge concentration 12.7 g/l Sludge at a MLSS concentration of 12.7 g/l was evaluated. Two experiments were performed

at these experimental conditions. Table 4-10 shows the test conditions. Figure 4.10 describes the increase in the DO concentration as a function of time.

Table 4-10 Test conditions for sludge at a MLSS concentration 12.7 g/l

Test Run



Flow through SDOX

(ml/min)


(oc)


1st 19.2 64 1360 16.7 27

2nd 21.2 64 1360 18.8 27

0

2

4

6

8

10

12

0 5 10 15

DO

(m

g/l)

Time (minutes)

DO vs. Time - Sludge concentraion 6.3 g/l

1st Run 2nd Run 3rd Run




Table 4-11Calculated CSDOX values for sludge at a MLSS concentration of 12.7 g/l

Test run Average CSDOX (mg/l)

Standard deviation 1st 2nd

21.1 26.8 23.9 ± 4.0

4.2.4. Sludge concentration 19.7 g/l As stated in Section 3.3.2.1, a MLSS concentration of 19.7 g/l was achieved by filtering the

sludge through microfiltration membranes. The concentrated sludge was circulated through the SDOX unit at the experimental conditions described in the Table 4-12 to ascertain the capability of the SDOX unit to supply oxygen at high suspended solids concentration. As observed in Figure 4.11, the SDOX unit was able to deliver dissolved oxygen. The increase in DO concentration as a function of time can also be observed in the same figure.


Test Run

OUR before test

(mg/l.hr)


Flow through SDOX

(ml/min)

Average sludge

temperature (oc)


1st 21.0 63 1350 20.2 25

2nd 21.2 63 1350 21.1 25

3rd 23.6 63 1350 21.6 25

0

1

2

3

4

5

6

7

8

9

0 5 10 15

DO

(m

g/l)

Time (minutes)


1st Run

2nd Run


Figure 4.11 DO vs. Time for sludge at a MLSS concentration of 19.7 g/l





19.9 19.1 24.0 21.0 ± 2.6

4.2.5. Sludge concentration 24.0 g/l

After evaluating the sludge at a MLSS concentration of approximately 20 g/l, the SDOX system was evaluated at a higher MLSS concentration of 24.0 g/l. The first experimented was carried out in a tank containing 20 L of sludge. The sludge was circulated through the SDOX unit at a MLSS concentration of approximately 24.0g/l. It was observed that the SDOX unit was not able to deliver the necessary amount of dissolved oxygen to increase the DO concentration in the tank. The amount of oxygen delivered by the SDOX unit, (air pressure 60 psi, and orifice size of 1.20 mm), was not enough to cope with the oxygen demand of the sludge. Therefore, it was decided to decrease the working volume of the reactor (container) to be able to supply the required amount of dissolved oxygen keeping the same operational conditions of previous experiments. The working volume of the reactor was gradually reduced to cope with the oxygen uptake rate. The experimental conditions of are shown in Table 4-14.

0

1

2

3

4

5

6

7

8

0 5 10 15 20

DO

(m

g/l)

Time (minutes)





Test Run

OUR before test

(mg/l.hr)


Flow through SDOX

(ml/min)

Average sludge

temperature (oc)


1st 154.1 62 1500 20.3 7

2nd 90.4 62 1500 20.7 7

3rd 52.9 62 1500 20.9 7






23.1 22.7 22.7 22.8 ± 0.2

4.2.6. Sludge concentration 34.2 g/l Similar to the experiments on sludge carried out at MLSS concentration of 24 g/l, experiments at MLSS concentration of 34.2 g/l could only be carried out in a five liters reactor at the operational conditions described in Table 4-16.

0

1

2

3

4

5

6

7

8

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50

DO

(m

g/l)

Time (minutes)





Test Run

OUR before test

(mg/l.hr) Average air

pressure (psi)

Flow through SDOX

(ml/min)

Average sludge

temperature (oc)


1st 54.7 62 1500 20.3 5

2nd 86.0 62 1500 20.6 5

Although the oxygen supply profiles shown in Figure 4.13 were not as smooth as the observed at lower MLSS concentration, the SDOX was able to successfully supply dissolved oxygen at such high biomass concentrations of approximately of 34.2 g/l.




Test run Average CSDOX (mg/l)

Standard deviation 1st 2nd

9.7 9.7 9.7 ± 0.0

4.2.7. Summary of the sludge experiments

The SDOX unit successfully supplied dissolved oxygen on sludge at different MLSS concentration, ranging from 3.2 g/l to 34.0 g/l. All the MLSS concentrations were evaluated at similar

0

1

2

3

4

5

6

7

0 1 2 3 4 5 6

DO

(m

g/l)

Time (minutes)


1st Run 2nd Run


operational conditions for the SDOX unit with little change in the flow rate through the unit. The Figure 4.14 presents the increase in DO concentrations as a function of time for all the different MLSS concentrations evaluated. The rate of increase of DO concentration was dependent on the volume of the reactor where the experiment were performed, on the oxygen uptake rate of the microorganisms, and on the concentration of dissolved oxygen delivered by the SDOX unit (CSDOX). That is, as observed in Figure 4.14, the rate of increase of DO concentration at low MLSS concentrations are lower than at high MLSS concentrations.

Figure 4.14 DO vs. Time for different sludge (MLSS) concentrations

Comparing the values of the dissolved oxygen concentration delivered by the SDOX unit (CSDOX) on sludge to the dissolved oxygen concentration delivered by the SDOX on tap water at the same operational conditions, the following remarks can be concluded: (i) the performance of the SDOX unit was not highly affected at MLSS concentrations of 3.2 g/l, and 6.3 g/l; (ii) it was slightly affected at MLSS concentrations of 12.7 g/l, 19.7 g/l and 24.0 g/l, and (iii) it was highly affected at MLSS concentrations of 34.2g/l. Table 4-18, and Figure 4.15 and Figure 4.16 describe that trend more precisely.

0

2

4

6

8

10

12

0 2 4 6 8 10 12 14 16

Do

(mg/

l)

Time (minutes)

DO vs. Time - Different sludge concentratios

3.2 g/l 6.34g/l 12.72g/l 20g/l 24g/l 34


Table 4-18 CSDOX sludge to water ratio for different MLSS concentrations

MLSS concentration

(g/l) CSDOX(mg/l)

Average CSDOX

(mg/l)

Standard Deviation

CSDOX for tap water at similar

operational conditions of

SDOX

(Ratio) CSDOX(sludge)

/ CSDOX(water)

3.2 30.1 30.1 0.0

29.5

1.0

6.3 (1st run) 29.7

29.1 0.5 1.0 6.3 (2nd run) 28.9

6.3 (3rd run) 28.6

12.7 (1st run) 21.1 23.9 4.0 0.8

12.7 (2nd run) 26.8

19.7 (1st run) 19.9

21.0 2.6 0.7 19.7 (2nd run) 19.1

19.70 (3rd run) 24.0

24.0 (1st run) 23.1

22.8 0.2 0.8 24.0 (2nd run) 22.7

24.0 (3rd run) 22.7

34.2 (1st run) 9.7 9.7 0.1 0.3

34.2 (2nd run) 9.7

Figure 4.15 Calculated CSDOX values for different MLSS concentrations

30.1 29.7 28.9 28.6

21.1

26.8

19.9 19.1

24.0 23.1 22.7 22.7

9.7 9.7

0

5

10

15

20

25

30

35

DO

SD

OX

(m

g/l)

DO SDOX (Csdox) for different MLSS ConcentrationsCSDOX for tap water


Figure 4.16 Calculated average CSDOX values with positive negative error for different MLSS concentrations

Experiments were carried out both on tap water and on sludge mainly to evaluate the performance of the SDOX unit with the presence of the suspended solids. Figure 4.17 shows the ratio between the calculated DO concentration for the unit (CSDOX) at different sludge concentrations and the calculated DO for the unit with tap water at similar operating conditions of the SDOX unit.

Figure 4.17 CSDOX sludge to water ratio for different MLSS concentrations

The performance of the SDOX unit was not very much affected by the presence of solids even at MLSS concentrations as high as 24g/l. However, the system seems to be affected at solid concentrations higher than 30 g/l.

According to the equation 3.5 in Section 3.2, the oxygen uptake rate is a key parameter in order to determine the concentration of dissolved oxygen supplied by the SDOX system. The OUR values obtained before carrying out the SDOX experiments were considered for calculating the CSDOX value. Every time the sludge passed through the SDOX unit, an increase in the OUR was observed for most of the MLSS concentrations except at of 3.2g/l and 24.0g/l where the OUR slightly decreased after each test (see Figure 4.18).

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

3.2 g/l 6.34 g/l 12.72 g/l 19.7 g/l 24 g/l 34 g/l

CS

DO

X (m

g/l)

Average CSDOX for different sludge concentrations

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 5 10 15 20 25 30 35 40

CS

DO

Xsl

udge

to w

ater

rat

io

Sludge concentration (g/l)

CSDOX for tap water


SOUR 1

6.42 mg/(g-vss.h)

SDOX

(1st Run)

SOUR 1

4.70 mg/(g-vss.h)

Sludge concentration 3.2 g/l

SOUR 1

1.23 mg/(g-vss.h)

SDOX

(1st Run)

SOUR 2

1.92 mg/(g-vss.h)

SDOX

(2nd Run)

SOUR 3

2.4 mg/(g-vss.h)

SDOX

(3rd Run)

Sludge Concentration 6.3 g/l

SOUR 1

8.87 mg/(g-vss.h)

SDOX

(1st Run)

SOUR 2

5.20 mg/(g-vss.h)

SDOX

(2nd Run)

SOUR 3

3.04mg/(g-vss.h)

SDOX

(3rd Run)


SOUR 1

2.13 mg/(g-vss.h)

SDOX

(1st Run)

SOUR 2

3.35 mg/(g-vss.h)

SDOX

(2nd Run)

Sludge concentration 34.2

SOUR 1

1.54 mg/(g-vss.h)

SDOX

(1st Run)

SOUR 2

1.56 mg/(g-vss.h)

SDOX

(2nd Run)

SOUR 3

1.74 mg/(g-ss.h)

SDOX

(3rd Run)



SOUR 1

2.24 mg/(g-vss.h)

SDOX

(1st Run)

SOUR 2

2.48 mg/(g-vss.h)

SDOX

(2nd Run)

Figure 4.18 Change in specific oxygen uptake rate (SOUR) for different sludge concentrations


Even though an increase in the temperature was observed every time the sludge passed through the SDOX unit, the increase in temperature is not significant to have a strong impact on the OUR of the mixture. 4.2.8. Discussion

All the previous experiments on sludge at different MLSS concentrations were carried out at similar operational conditions (same pressure - 60 psi, and same orifice - 1.20 mm). That is, the results obtained can be directly compared and a trend can be determined.

Although the sludge was filtered through a 500 µm sieve, clogging issues of the orifice was observed a couple of times during the sludge tests through the SDOX; especially, at high sludge concentrations. It is important to highlight that a laboratory scale SDOX unit was operated during this experiments, and that a full scale SDOX unit may not experience clogging issue since they are provided with larger diameter orifices. However, it may be required to incorporate some pre-treatment steps (filtration, or grinding) prior to the intake of the MLSS into the SDOX unit.

As described in the methodology section, (Sections 3.2 and 3.3) the mass balance for DO oxygen concentration was carried out across the boundaries of the container. Consequently, a little volume of tap water or sludge that was in the pressurized saturation tank and in the piping system of the SDOX unit was not included in the mass balance or parameter estimation for CSDOX. It was observed that this volume was approximately 1.5 L. This might have resulted in an underestimation of the calculated CSDOX values; especially, in the case of the sludge where microorganisms may consume oxygen in the SDOX system as well. However, the volume in the SDOX system was comparatively low to the volume in the container; except for sludge tested experiments at MLSS concentrations of 24.0 g/l and 34.2 g/l, where the initial sludge volumes were only 7 L and 5L, respectively. Therefore, in case of sludge concentrations of 24.0 g/l and 34.2 g/l, this volume might have impacted the calculated values of CSDOX.

The OUR values prior to the batch tests were used for estimating the values of CSDOX. However, changes in the OUR were observed after each batch experiment indicating that the OUR values did not remain constant throughout the experiments. Since it was not possible to measure the changes in the OUR during the experiments, a constant value of OUR, prior to batch test, was considered for calculating the value of CSDOX.

The SDOX unit showed the capability of transferring oxygen efficiently not only in tap water, but also in sludge at high suspended solid concentrations where conventional bubble aeration diffusers showed several limitations. Even at high MLSS concentrations of 24.0 g/l, the SDOX system exhibited a relatively high ratio of CSDOX for sludge to CSDOX for water [CSDOX(sludge)/ CSDOX(water)] approximately of 0.80. The impact of MLSS on the oxygen transfer capability is not as pronounced for the SDOX unit compared to bubble diffusers.

The Figure 4.19 shows the comparison of the ratio of CSDOX for sludge to CSDOX for water [CSDOX(sludge)/ CSDOX(water)], for all the experiments performed on sludge at the following MLSS concentrations 3.2 g/l, 6.3g/l, 12.7g/l, 19.7g/l, 24.0g/l, and 34.2g/l. The figure also shows the corresponding alpha factor values for conventional diffusers from two previous different studies carried out by Krampe and Krauth (2003) and Germain et al. (2007). The alpha factor values, corresponding to the MLSS concentrations of 3.2 , 6.3, 12.7, 19.7, 24.0, and 34.2 g/l, in the studies carried out by Krampe and Krauth (2003) and Germain et al. (2007) were calculated on the basis of the negative exponential relation between alpha factor and MLSS concentrations found in those studies.


Figure 4.19 Comparison of SDOX and diffused aeration alpha values

It is evident from Figure 4.19 that the oxygen supply capability of the SDOX technology is less affected by the higher MLSS concentrations, whereas the higher MLSS concentration have a severe negative effect on conventional diffused aeration.

Therefore, by using this innovate oxygen supply technology MBRs can be operated at much higher concentrations and reductions in footprint and operational cost may be achieved. After performing several experiments on sludge at different MLSS concentrations, the comparison of conventional coarse diffuser aeration (bubble aeration) with the SDOX technology was performed, and it is discuss in the following section.

4.3. Comparison of bubble aeration diffusers with the SDOX technology

Oxygen was supplied by the bubble aeration diffusers and by the SDOX unit at the same volume of the sludge and water in a membrane reactor to perform the comparison. 4.3.1. Bubble aeration diffusers First, 8.5L of sludge at a MLSS concentration of 15.4 g/l was aerated by means of bubble aeration diffusers. Later on, the same volume of tap water was aerated with the same bubble aeration diffusers. The airflow rate was the same for both water and sludge aeration experiments. The experimental conditions for both experiments are presented in Table 4-19.

CSDOX(sludge / CSDOX(water) = 1.3632e-0.033 MLSS

α = e-0.088 MLSS

α = 6.77e-0.26MLSS

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 5 10 15 20 25 30 35 40

Alp

ha fa

ctor

CS

DO

Xsl

udge

to w

ater

rat

io

MLSS (g/l)

SDOX Krampe and Krauth 2003 Germain 2005


Table 4-19 Test conditions for bubble aeration (tap water & sludge)

Test run Volume (Liters) Air supply rate (l/hr) Temperature (oC) Tap water

1st 8.5 690 10.5 2nd 8.5 690 11.4

Sludge concentration 15.4 g/l 1st 8.5 690 18.0 2nd 8.5 690 18.1 3rd 8.5 690 18.1

The experimental data was fit to the Equations 3.8 (for tap water) and to the Equation 3.10

(for sludge) and the KLa values for the water experiments, and the KLa and C* values for the sludge were determined.

KLa (20oC) (tap water) = 0.728 ± 0.003 minute -1

KLa (20

oC) (sludge) = 0.458 ± 0.011 minute -1

C*(sludge) = 8.06 ± 0.49 The alpha value at 20oC was obtained from the KLa values of water and sludge.

α(20) = 0.458 / 0.728 = 0.629 The Figure 4.20 shows the changes in the DO concentration as a function of time for the experiments performed in tap water and in sludge. The oxygen was supplied by the coarse bubble aeration diffuser. The effect of solids on oxygen transfer is very evident.

Figure 4.20 DO vs. Time - Oxygen supply by bubble aeration diffusers in membrane reactor (tap water and

sludge)

4.3.2. SDOX technology Batch experiments were conducted to evaluate the performance of the SDOX unit with both the tap water and with the same sludge used in the previous bubble aeration diffuser experiments in the

0

2

4

6

8

10

12

0 1 2 3 4 5 6

DO

(m

g/l)

Time (minutes)

DO vs. Time

Tap water

Sludge


membrane reactor. The sludge and tap water were tested at very similar operational conditions as described in Table 4-20.

Table 4-20 Test conditions for oxygen supply by the SDOX in membrane reactor (tap water and sludge)

Test Run


Flow through SDOX (ml/min)


(oc)


Water

1st 63 1350 11.0 8.5

2nd 63 1350 12.0 8.5

Sludge

1st 62 1500 18.5 8.5

2nd 62 1500 19.1 8.5

3rd 62 1500 20.2 8.5

The CSDOX value supplied by the SDOX unit for both tap water and sludge were determined by fitting the experimental data to Equations 3.3 and 3.5 for water and sludge, respectively. The CSDOX values are described in Table 4-21.

Table 4-21 calculated CSDOX for tap water and sludge in membrane reactor

Description CSDOX (mg/l)

Tap water 27.275 ± 0.007

Sludge 23.867 ± 4.942

CSDOX for sludge / CSDOX for tap water = 0.88

The presence of solids reduced the oxygen supply capability of the SDOX unit by 12%. However, for bubble aeration diffusers, the oxygen transfer efficiency was reduced by approximately 38%. That is, the diffusers exhibit a stronger impact on the oxygen transfer efficiency due to the presence of solids. Figure 4.20 and Figure 4.21 clearly indicate the impact of solids in both systems.


Figure 4.21 DO vs. Time - Oxygen supply by the SDOX unit in membrane reactor (tap water and sludge)

Moreover, Figure 4.22 shows that the SDOX unit has higher oxygen transfer rate than the conventional bubble aeration diffusers on sludge at the experimental MLSS concentration.

Figure 4.22 DO vs. Time - Bubble aeration and oxygen supply by the SDOX for sludge concentration 15.43 g/l

4.3.3. Discussion In the present study, a very high air flow rate of 690 l/hr was used for bubble aeration.

Working at such high air flow rate maximizes the value of the alpha factor to better assess the limitations of the coarse bubble aeration diffuser compared to the SDOX technology.

Different previous studies have reported different values of alpha factor for bubble aeration diffusers. These studies were carried out in different tanks geometries using different aeration devices and air flow rates. Germain et al. (2007) studied the effects of different MLSS concentrations ranging from 7.2 to 30.2 g/l at different air flow rates on alpha factors. The author concluded that the alpha factor increases with air flow rate. The Figure 4.23 shows the impact of air flow rate on alpha factors as reported by Germain et al. (2007)

0

2

4

6

8

10

12

0 1 2 3 4 5 6

DO

(m

g/l)

Time (minutes)

DO vs. Time

Tap water

Sludge

0

2

4

6

8

10

12

0 1 2 3 4 5 6

DO

(m

g/l

)

Time (Minutes)

Bubble aeration vs. SDOX

Bubble Aeration

SDOX Aeration

Results and Discussion

Figure

Source : (Germain et al. 2007) Figure 4.23 indicates that for a MLSS concentration of approximatelyconcentration used in this research) an alpha factor of approximately 0.2 can be observed at a volumetric air flow rate of 6 approximately 81 (m3m3.h-1) was usedtimes higher than the airflow rate at which an alpha factor of 0.2 was reportedmore commonly accepted) air flow rate a much lower flow rate was fixed at a highmicroorganism (OUR), so that oxygen consumption by microorganismtransfer rate. The average measured OUR(mg/l.hr). If the oxygen transfer air are assumed to be 10%, 0.5 enough to satisfy the oxygen requirements approximately thirty times higher than the oxygen requirements for microorganisms. factor value reported in the present studies (see Figure 4.19).

Even at such extremely comparison of both technologies SDOX technology, and the effects of the presSDOX technology.

In addition, the oxygen transfer rate (OTR) only equipment to provide oxygen without considering its efficiency. Mostlydescribed by the standard aeration efficiency (SAE)consumed (KgO2/KWh) at standard conditions.coarse bubbles diffusers is usually expressed asamount received in the liquid at standard condition

The SDOX system delivers dissolved oxygen at a higher conventional bubble diffusers. Moreover, the SDOX technology is also of the suspended solids. In the present experiment determined (that is how much oxygen is putting into the SDOX unit, and how much of that oxygen is retained in the system) due to the lack of the righsystem. There is a low probability can only occur in the bulk liquid after through the nozzle passed through a gas head space and form a pool at the bottom of

Figure 4.23 α - factor vs. volumetric air flow rate

for a MLSS concentration of approximately 16.8 g/lresearch) an alpha factor of approximately 0.2 can be observed at a

air flow rate of 6 (m3m3.h-1). In the present study, a volumetric air flow rate was used. That is, a volumetric air flow rate approximately

than the airflow rate at which an alpha factor of 0.2 was reported. That is, at a lower (and more commonly accepted) air flow rate a much lower alpha factor is expected.

higher value than the endogenous decay oxygen so that oxygen consumption by microorganisms does not limit the oxygen e measured OUR, before the bubble aeration test, was approximately 45

efficiency of the diffuser, the alpha factor, and the and 20%, respectively, an air flow rate of 24 l/h

enough to satisfy the oxygen requirements of the microorganisms. The air flow rate of 690 l/hr was approximately thirty times higher than the oxygen requirements for microorganisms.

present study of 0.629 is much higher than the value reported in

such extremely high air flow rate for the coarse bubble aerationcomparison of both technologies still showed that the rate of oxygen supply is

and the effects of the presence of suspended solids is less pronounced for the

xygen transfer rate (OTR) only indicates the capacity of without considering its efficiency. Mostly, the energy efficiency is

described by the standard aeration efficiency (SAE); that is, the oxygen transfer rate per unit of energy /KWh) at standard conditions. The oxygen transfer efficiency (OTE)

coarse bubbles diffusers is usually expressed as the amount of oxygen supplied amount received in the liquid at standard condition (Henze et al. 2008).

system delivers dissolved oxygen at a higher transfer rate conventional bubble diffusers. Moreover, the SDOX technology is also less affected b

In the present experiment the efficiency of the SDOX unit was not determined (that is how much oxygen is putting into the SDOX unit, and how much of that oxygen is

the lack of the right flow meter to measure the amount of air entering the here is a low probability that the dissolved oxygen escape out of the system. Oxygen losses ccur in the bulk liquid after the saturated stream leaves the end orifice.

through the nozzle passed through a gas head space and form a pool at the bottom of

61

16.8 g/l (similar to the research) an alpha factor of approximately 0.2 can be observed at a

a volumetric air flow rate of approximately fourteen That is, at a lower (and

alpha factor is expected. Furthermore, the air oxygen requirements of the

not limit the oxygen was approximately 45 the oxygen content in

te of 24 l/hr would have been The air flow rate of 690 l/hr was

approximately thirty times higher than the oxygen requirements for microorganisms. That is, the alpha value reported in previous

coarse bubble aeration diffuser, the showed that the rate of oxygen supply is much higher for the

less pronounced for the

the capacity of the aeration the energy efficiency is

oxygen transfer rate per unit of energy he oxygen transfer efficiency (OTE) for fine and

the amount of oxygen supplied to the system to the

transfer rate compared to the less affected by the presence

efficiency of the SDOX unit was not determined (that is how much oxygen is putting into the SDOX unit, and how much of that oxygen is

to measure the amount of air entering the that the dissolved oxygen escape out of the system. Oxygen losses

the end orifice. The liquid sprayed through the nozzle passed through a gas head space and form a pool at the bottom of the pressurized


saturation tank that act as a seal which do not let any incoming gaseous air/oxygen to leave the system without being dissolved in the liquid. According to information provided by BlueInGreen Corporation, if the supersaturated SDOX effluent stream is properly distributed in the bulk liquid, the SDOX unit may give an oxygen transfer efficiency of 95%. The remaining 5% accounts for inefficiencies related to the mixing of the super saturated stream injection and occasional the need for venting of gas during shutting down of the equipment. Krampe and Krauth (2003) evaluated the oxygen transfer efficiency for the different aeration systems for a suspended solids range up to 18g/l and reported that the fine bubble aeration is the most efficient aeration system. The oxygen transfer efficiency in fine bubble aerators is not very high at higher suspended solids concentrations, where the oxygen transfer efficiency is negatively affected by both the presence of suspended solids, and due to the rise of bubbles towards the surface. After going through different sources and manufacturers of diffused aeration systems, the maximum SOTE in clean water was found to be equal to 60%, depending upon the aeration system configuration, submergence and airflow rate (ovivo n.d.), (Mequipco Ltd. n.d.), (Alita Industries n.d.), (Xylem n.d.) and (Mueller et al. 2002). However, this value decreases significantly, depending on alpha factor for suspended solids concentrations (�SOTE).

The rate of oxygen supply per unit of energy consumed (aeration efficiency) may be higher in case of SDOX technology because of higher oxygen transfer efficiency. The pressure inside the pressurized saturation tank is used to deliver the supersaturated stream of liquid and the mixing of the stream into the bulk liquid. That is, some of the energy required by the SDOX system is recovered at some extent through mixing energy. The other energy requirement for the SDOX technology is from pumping the liquid through the SDOX unit at high pressure. The energy requirements depend on the pressure inside the pressurized saturation tank and design of the spray nozzle. The higher the pressure inside the pressurized saturation tank, the more oxygen will be dissolved in liquid due to an increase in partial pressure of oxygen in tank. Moreover, the lower the size of the nozzle, the higher the oxygen saturation since the liquid spray is more atomized. However, an increase in the pressure and a decrease in the nozzle size increase the backpressure on pump, which will increase the energy cost. The possible effects on microorganisms, due to their exposure to the SDOX environment, were investigated in the last phase of this research.

4.4. SDOX technology effects on microorganisms

As mentioned in Section 3.5, to determine the potential effects of the SDOX system on active microorganisms, the sludge was continuously circulated through the SDOX unit and samples were collected after a time interval of 30 and 90 minutes. The samples with no exposure to the SDOX unit, and exposed for 30 and 90 minutes to the SDOX unit were analyzed for the following parameters: soluble COD, NH4 - N, TSS, and VSS. The results are described in Table 4-22. Table 4-22 Soluble COD, NH4-N and TSS & VSS values for sludge with different exposure time to the SDOX

Description Soluble COD (mg/l) NH4- N(mg/l)

TSS (g/l) VSS (g/l)

Sludge without exposure to the SDOX

54.4 ± 4.5 1.0 ± 0.1 5.73 ± 0.02 4.34 ± 0.04

Sludge with 30 minute exposure to the SDOX

27.7 ± 4.9 0.3 ± 0.1 5.74 ± 0.09 4.24 ± 0.10

Sludge with 90 minute exposure to the SDOX

73.6 ± 0.0 0.3 ± 0.0 5.43 ± 0.08 4.12 ± 0.08


The data showed that there was some soluble COD present in the sludge even before circulating the sludge through the SDOX unit. After thirty minutes the concentration of the soluble COD was reduced, but an increase on the soluble COD concentration was observed after ninety minutes exposure of the sludge to the SDOX unit.

The analysis for NH4 - N showed that there was not much NH4 - N present before and after the circulation of the sludge through the unit.

TSS and VSS analysis revealed that there was a little decrease in TSS and VSS after thirty minutes and 90 minutes exposure to the SDOX.

Figure 4.24 shows the specific oxygen uptake rate (SOUR) for the sludge that was not circulated through the unit (No exposure), the SOUR for the sludge with 30 minute exposure (30minute exposure), combined equal volume of unexposed and 30 minute exposed sludge (30 minute exposure+ no exposure), and combined equal volume of unexposed and 90 minute exposed sludge (90 minute exposure + no exposure), respectively. An average value of VSS is considered to calculate SOUR for sludge samples of (30 minutes + no exposure) and (90 minutes + no exposure).

Figure 4.24 SOUR values for different sludge samples

The microscopic investigation of sludge samples revealed that the presence of living organisms in sludge at different exposure times to the SDOX environment (see Figure 4.25). As observed in Figure 4.26 a decrease in the floc size was observed after passing the sludge through the SDOX unit.

0.00

0.02

0.04

0.06

0.08

0.10

0.12

No expsoure 30 minute

exposure

30 minute

expsoure + no

expsoure

90 minute

exposure + no

exposure

SO

UR

(m

g O

2 /g

-vss

.min)

Specific oxygen uptake rates


Figure 4.25 Living organisms (clockwise from top left) (i) no exposure sludge 20x (ii) no exposure sludge 20x (iii) 30 minute exposure sludge 10x (iv) 90 minute exposure sludge 20x

Figure 4.26 Floc size (clockwise from top left) (i) no exposure sludge 10x (ii) no exposure sludge 20x (iii) 30 minute exposure sludge 20x (iv) 90 minute exposure sludge 20x


4.4.1. Discussion

A decrease in the soluble COD concentration after thirty minutes exposure time to the SDOX, and an increase in the soluble COD after ninety minutes exposure to the SDOX unit were observed. The OURs for thirty and ninety minutes samples do not show large variation from the unexposed sludge. The TSS and VSS values showed little change with the progression of the exposure time. The increase in soluble COD concentrations may have resulted from the flocs breakup and due to release of intracellular matter due to the applied stress due to the continuous cyclic exposure of the microorganisms to the high pressure and ambient pressure (frictional stresses while passing through the spray nozzle and exposure to the high pressure for a time equal to the residence time in the pressurized saturation tank). Generally, an increase in oxygen uptake rate was observed every time the sludge was circulated through the SDOX unit during the sludge tests. That increase might have resulted from the increase of soluble COD during the tests. The technology did not show any adverse effects on microorganism or inactivation of microorganisms. However, it may affect the biomass characteristic. The change in biomass characteristics can impact the fouling in MBRs. Membrane fouling is one of the main hindrances in the adoption of the MBR technology. Membrane fouling is a complex phenomenon and studies have shown that it depends upon many factors, such as MLSS concentration, floc characteristics, EPS, SMP, viscosity, and aeration among others (Le-Clech et al. 2006). The use of SDOX technology may change these parameters significantly. The microscopic investigation of the sludge circulated through the SDOX unit, revealed a change in the floc size. The breakup of flocs can release the EPS bounded in flocs. Furthermore, viscosity has negative impact on fouling (Lee and I T Yeom 2007) and shear stresses in SDOX can decrease the viscosity. However, considering the complexity related to the fouling phenomena, further study is required to establish whether the use of SDOX technology with a MBR system can have an impact on membranes fouling. The potential usage of the SDOX technology in case of emergency sanitation is discussed below.

4.5. Potential applications of SDOX technology in emergency sanitation

As discussed previously, the provision of adequate sanitation is one of the requirements in the aftermath of the disaster to reduce the risk of epidemics and enteric diseases. A wide range of potential technologies are available for emergency sanitation and selection of suitable technology depends upon case specific factors. Membrane bioreactor is an advance treatment technology but properties like compactness, high quality effluent and portability make it suitable option for emergency sanitation.

The experimental results of present research showed that the SDOX technology can supply oxygen at high concentrations of suspended solids and also the technology is more efficient in comparison to the conventional diffused aeration. Thus, the SDOX technology coupled with the MBRs, operating at very high biomass concentrations, may lead to further compaction of the MBRs. However, further investigations are required in this regard. Also the availability and requirements of energy is one of factor for selection of treatment technology in emergencies. In comparison to the diffused aeration, the SDOX technology may lead to reduction in aeration cost significantly, especially, for MBRs operated at high biomass concentrations. The SDOX is a portable technology. The pressurized oxygen gas cylinder or other portable mean of gas supply may be used with the SDOX technology for supply of oxygen. Thus, the SDOX technology provides a portable and efficient mean of oxygen supply for offsite wastewater treatment facilities. The digestion of fecal sludge and inactivation of pathogens is another potential use of SDOX technology in case of emergencies. The safe disposal of fecal sludge, generated from the latrines in emergency camps, is one of the requirements for provision of adequate sanitation due to presence of large number of pathogens in human excreta. The use of SDOX technology for the treatment of fecal


sludge and inactivation of pathogens present in the human excreta can be investigated. The SDOX technology can be used for fecal sludge treatment before emptying of latrines due to flexibility of its delivery. The fecal sludge can be circulated from the SDOX unit, before or after emptying the pit latrines, for its treatment. However, further investigations are required in this regard to assess the capability of the technology for the treatment of fecal sludge and inactivation of pathogens.

Conclusions and recommendations 67

5.1. Conclusions

1. The SDOX technology effectively delivers dissolved oxygen in tap water and the delivery rate of the dissolved oxygen can be controlled by the spray nozzle, end orifice size, and air/oxygen pressure in the pressurized saturation chamber of the SDOX unit.

2. The SDOX technology is able to deliver dissolved oxygen at concentrations higher than 30 g/L where other conventional aeration systems are not effective. The SDOX technology may change the direction in which aerobic processes (particularly the aeration tank of MBRs) are designed. Biomass concentrations in the aeration tank higher than 30 g/L can be achieved. The MBR-SDOX at high biomass concentrations may lead to significant reduction in footprint and aeration cost that makes it a suitable option for emergency relief works.

3. The SDOX technology supplied oxygen successfully to different MLSS concentrations ranging from 3.2 to 34.2 g/l. The impact of suspended solids at the oxygen delivery capability is found lower in comparison to the bubble aeration. This fact makes the SDOX a promising technology that may be used in the MBRs to supply oxygen for the attainment of high MLSS concentrations that can translate in reduction of footprint. However, further investigations are required in this regard, especially for the effects of technology on microorganism and fouling of the membranes.

4. The comparison of the conventional bubble aeration and the SDOX technology, for a suspended solids concentration of approximately 16 g/l, shows that the SDOX technology exhibits a higher oxygen supply rate and is less affected by the presence of suspended solids.

5. The SDOX technology shows some effects on microorganism including: an increase in the oxygen

uptake rate of microorganisms, release of soluble COD, and the reduction in floc size.

5.2. Recommendations

1. Further research should be carried out to study the impact of SDOX technology on microorganisms to explore the possibility of its usage couple with a MBR system. It would be interesting to evaluate the performance of the SDOX unit for supplying dissolved oxygen to a continuous MBR system operated at biomass concentrations higher than 30 g/L.

CHAPTER 5

Conclusions and recommendations


2. Further investigations are required to assess the fecal sludge digestion and inactivation of pathogen by aeration with the SDOX unit. This may give further applications of the technology in case of emergency for the safe disposal of fecal sludge.

3. A study can be carried out to investigate the potential use of the technology for aerobic digestion

of sludge. Particularly, it would be interesting to evaluate the reduction of VSS in the sludge as well as the pathogen inactivation.

4. Further study should be carried out to compare the oxygen transfer efficiency and aeration

efficiency of the SDOX technology and conventional bubble aeration. 5. A detail study is required to investigate the impact of SDOX technology to different factors that

are related to membrane fouling.

References 69

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Appendices

Appendices The figures below show the parameter estimation through AQUASIM software packaThe solid line shows the Where more than one trial was carried out the 1st trial results are shown.regarding all tests is also Tap water test - orifice size 0.70mm

The figures below show the parameter estimation through AQUASIM software packaThe solid line shows the modeled, whereas, the rectangle points show the measured data.Where more than one trial was carried out the 1st trial results are shown.

also submitted in electronic files to Dr. Hector Garcia H

orifice size 0.70mm

72

The figures below show the parameter estimation through AQUASIM software package. modeled, whereas, the rectangle points show the measured data.

Where more than one trial was carried out the 1st trial results are shown. All data . Hector Garcia Hernandez.

Appendices

Tap water test - orifice size 1.14

orifice size 1.14mm

73

Appendices

Tap water test - orifice size 1.20mm

orifice size 1.20mm

74

Appendices

Sludge Tests

75

Appendices

76

Appendices

77

Appendices

Comparison of SDOX technology and co Bubble aeration

• KLa for tap water

• KLa for MLSS concentration 15.43g/l

Comparison of SDOX technology and coarse bubble aeration

KLa for MLSS concentration 15.43g/l

78

Appendices

SDOX aeration

• tap water

• MLSS concentration 15.43 g/l

MLSS concentration 15.43 g/l

79

novel alternative for aeration in the treatment of …zeeshan bilal student no. 32024 msc thesis mwi...

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