ENG 470: ENGINEERING HONOURS THESIS

FINAL REPORT

BIOFOULING IN REVERSE OSMOSIS PROCESSES

BY

AMAR ZAGHY

BACHELOR OF ENVIRONMENTAL ENGINEERING

HONOURS

SCHOOL OF ENGINEERING AND INFORMATION

TECHNOLOGY 2016

SUPERVISORS

DR. LINDA LI EMERITUS PROFESSOR GOEN HO

A report submitted to the school of Engineering and Information Technology in partial fulfilment of requirements for the unit ENG470 Engineering Honours Thesis, at Murdoch University, Semester 1, 2016.

i

Author’s declaration

I declare that this submission is my own account of my research unless stated otherwise and

excluding references and appendices.

Amar Zaghy

ii

Executive Summary

Reverse Osmosis (RO) is a water purification technology that uses a semi-permeable membrane

to remove salt and other particles from drinking water. It is the dominant technology which has

overtaken many conventional systems in recent years. Membrane biofouling is the main

disadvantage of using RO technology which can result in reducing the system’s efficiency. The

rejected microorganisms on the surface of the membrane form a fouling layer (biofouling) which

leads to a decline in permeate flux, increase of hydraulic resistance, increase in operating

pressure, and shortening of the membrane life. Polysaccharides, produced by microorganisms,

are the main substances responsible for membrane biofouling. In this study, two types of

polysaccharides (alginate and pullulan) were used to investigate their individual fouling effects as

well as their fouling effects coupled with sodium chloride and calcium chloride. 50 mM of ionic

strength (27.5 g NaCl + 1.47 g CaCl2) and 0.2 g/L of polysaccharides were used in the fouling

experiments conducted with a laboratory-scale reverse osmosis system.

It was found that alginate lead to more reduction in system’s efficiency in comparison with

pullulan. The effect of alginate on the efficiency of the system was much more severe in the

presence of salt, namely sodium chloride and calcium chloride, compared to its individual effect

in the absence of salt. The addition of salt led to an increase in membrane fouling and a decrease

in system’s efficiency. On the other hand, it was found that pullulan enhanced the system’s

efficiency when it is combined with salt.

To support the above findings, a Confocal Laser Scanning Microscopy (CLSM) analysis, a Total

Organic Carbon (TOC) test, and an estimation of the weight of produced fouling layers were

performed. In general, analysing the results of the tests supported the findings.

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Acknowledgement

I would like to acknowledge Emeritus Professor Goen Ho, and Dr. Linda Li who made this work

possible with their continuous support throughout the course of this project.

I also appreciate the help and support provided by PhD candidate Zhangwang Xie (John) on the

instruction of experimental setup and supervision of this project.

Lastly, I would like to show gratitude to Murdoch University institution for giving me the

opportunity to work on this project.

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List of Abbreviation & acronyms DI: Deionised

RO: Reverse Osmosis

EPS: Extra-cellular Polymeric Substances

MSF: Multi-Stage Flash

CA: Cellulose Acetate

TFC: Thin Film Composite

NaCl: Sodium Chloride

CaCl2: Calcium Chloride

CLSM: Confocal Laser Scanning Microscopy

CP: Concentration Polarization

TOC: Total Organic Carbon

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Contents

Executive Summary ...................................................................................................... ii

Acknowledgement ...................................................................................................... iii

List of Abbreviation & acronyms .................................................................................. iv

List of Figures ............................................................................................................. vii

List of Tables ............................................................................................................. viii

1. Introduction .......................................................................................................... 1

1.1. Objectives....................................................................................................... 2

1.2. Project Management Plan ............................................................................... 2

2. Background ........................................................................................................... 3

2.1. Reverse Osmosis System ................................................................................. 3

2.2. Membrane Fouling ......................................................................................... 5

2.2.1. Concentration Polarisation .............................................................................. 6

2.2.2. Back Diffusion .................................................................................................. 7

2.2.3. Biofilm Formation ............................................................................................ 8

2.3. Polysaccharides .............................................................................................. 9

2.3.1. Alginate ............................................................................................................ 9

2.3.2. Pullulan .......................................................................................................... 11

3. Methodology & Research Design ......................................................................... 11

3.1. Experimental Methods.................................................................................. 12

3.1.1. System Design ................................................................................................ 12

3.1.2. Chemical Parameters and Operating Conditions .......................................... 16

3.2. Confocal Laser Scanning Microscopy (CLSM) Methodology ............................ 17

3.3. Total Organic Carbon (TOC) measurement .................................................... 18

3.4. Polysaccharides weighing methodology ........................................................ 19

3.5. Salt Concentration using “The Standard Curve” Method ................................ 19

4. Modelling ............................................................................................................ 20

4.1. Modelling permeate flux ............................................................................... 20

4.2. Modelling Hydraulic Resistances at the membrane surface ........................... 21

4.3. Modelling Osmotic Pressure & Concentration Polarisation at the membrane

surface .................................................................................................................... 23

4.4. Salt Rejection Rate ........................................................................................ 24

4.5. Back Diffusion Rate ....................................................................................... 24

5. Results and Data Analysis .................................................................................... 25

5.1. Permeate Flux............................................................................................... 25

5.1.1. Concentration Polarisation Effects ................................................................ 26

5.1.2. Polysaccharides Effects.................................................................................. 27

5.1.3. Concentration Polarisation and polysaccharides Combined Effects ............. 29

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5.1.4. Flux Comparison ........................................................................................... 31

5.2. Total Hydraulic Resistances ........................................................................... 33

5.2.1. Comparison of Total Resistance due to Alginate Fouling ............................. 34

5.2.2. Comparison of Total Resistance due to Pullulan Fouling ............................. 35

5.3. Concentration Polarisation Vs Osmotic Pressure at the Membrane Surface ... 36

5.4. Conductivity ................................................................................................. 38

5.5. Fouling Layers Weight Estimation ................................................................. 39

5.6. CLSM Analysis ............................................................................................... 41

5.7. Results Summary and Comparison ................................................................ 45

6. Conclusion ........................................................................................................... 47

7. Future Research Opportunities ............................................................................ 48

8. References .......................................................................................................... 50

9. Appendices: ......................................................................................................... 54

Appendix A: Gantt chart .......................................................................................... 54

Appendix B: HTI OsMEMTM CTA-NW Membrane Specification Sheet ...................... 55

Appendix C: Performed RO experiments.................................................................. 56

Appendix D: TOC results .......................................................................................... 57

Appendix E: Estimation of the weight of produced fouling layers ............................. 58

Appendix F: Results of “The Standard Curve” method ............................................. 58

Appendix G: Comparison of permeate fluxes with highest and lowest membrane

resistances .............................................................................................................. 59

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List of Figures Figure 1: Reverse osmosis principle. Left: osmosis; Right: reverse osmosis (Fritzmann et al. 2007).4

Figure 2: Concentration Polarisation (CP) phenomenon in RO processes. ........................................ 7

Figure 3: Structural formula of sodium alginate molecule (Katsoufidou, Yiantsios and Karabelas 2007). ............................................................................................................................................... 10

Figure 4: The egg-box model for binding divalent cations to sodium alginate molecules (Katsoufidou, Yiantsios and Karabelas 2007). .................................................................................. 10

Figure 5: Schematic chemical structure of pullulan with maltotriose as repeating unit (Singh, Saini and Kennedy 2008). ......................................................................................................................... 11

Figure 6: Laboratory-sale reverse osmosis system used to conduct the experiments. ................... 13

Figure 7: Schematic Diagram of the laboratory-Scale RO system used to conduct the experiments. .......................................................................................................................................................... 13

Figure 8: Sterlitech CF042 membrane cell with its components.Experimental Setup and procedure .......................................................................................................................................................... 14

Figure 9: The procedure of RO experiments conducted in this study. A) Control experiments; B) Fouling experiments without salt; and C) Fouling experiments in the presence of salt.................. 15

Figure 10: Picture of the Nikon C2 Confocal Microscope. ............................................................... 18

Figure 11: Effects of concentration polarisation (NaCl and CaCl2) on permeate flux. .................... 26

Figure 12: Effects of sodium alginate on permeate flux. ................................................................. 27

Figure 13: Effects of pullulan on permeate flux. .............................................................................. 28

Figure 14: Effects of alginate on permeate flux in the presence of NaCl and CaCl2. ...................... 29

Figure 15: Effects of pullulan on permeate flux in the presence of NaCl and CaCl2. ...................... 30

Figure 16: Comparison of permeate fluxes caused by individual and combined effects of salt and alginate foulant. ............................................................................................................................... 31

Figure 17: Comparison of permeate fluxes caused by individual and combined effects of salt and pullulan foulant. ............................................................................................................................... 32

Figure 18: Comparison between total hydraulic resistances in alginate experiments. ................... 34

Figure 19: Comparison between total hydraulic resistances in pullulan experiments. ................... 35

Figure 20: Correlation between concentration polarisation and osmotic pressure at the membrane surface. .......................................................................................................................... 37

Figure 21: Confocal image of the baseline foulaed membrane (NaCl + CaCl2). .............................. 41

Figure 22: Confocal image of the fouled membrane due to alginate fouling. ................................. 42

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Figure 23: Confocal image of the fouled membrane due to pullulan fouling. ................................ 42

Figure 24: Confocal image of a fouled membrane due to the interaction of salt and alginate. .... 43

Figure 25: Confocal image of a fouled membrane due to the interaction of salt and pullulan. ..... 44

List of Tables

Table 1: The average measured conductivity of the permeate and the feed solution during all performed experiments. .................................................................................................................. 38

Table 2: Estimation of mass and density of the fouling layers in the performed experiments. ...... 39

Table 3: Comparison between the performed RO experiments. .................................................... 45

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1. Introduction

Today, reverse osmosis (RO) membrane technology is considered as an important tool for

sustainable growth in desalination industry because of its simplicity in concept and operation.

However, like most other technologies is not free of some concerns (Matin et al. 2011).

Membrane biofouling is an inevitable phenomenon in reverse osmosis systems which can cause a

decrease in system’s efficiency (NG and Elimelech 2004). It results in several deleterious effects,

including a decrease in water production because of a gradual decline in flux, an increase in applied

pressure required for a constant rate of water production, a gradual membrane degradation which

reduces membrane life, and decreases the permeate quality (NG and Elimelech 2004). Despite the

enormous research and investigation, membrane biofouling is still a major technical hurdle that

needs to be addressed to enhance the efficiency and cost-effectiveness of operating reverse osmosis

system (Zhao, Song and Ong 2010). Biofilms, the major type of fouling in RO membranes, are formed

by bacterial extra-cellular polymeric substances (EPS) with polysaccharides play a significant role in

EPS (Vu et al. 2009). For this main reason the individual fouling effects of sodium alginate, as the

most common polysaccharide used in membrane biofouling research, along with less used

polysaccharide pullulan, have been studied in this work. The interaction between Sodium Chloride

(NaCl) and Calcium Chloride (CaCl2) with polysaccharides (Sodium alginate and pullulan) also have

been investigated. Previous studies have indicated that the presence of cations such as Ca2+ and Na+

and their interactions with polysaccharides can significantly aggravate the fouling process (Alazmi,

Nassehi and Wakeman 2010). Also, the physical properties of polysaccharides and their viscosity in a

solution are highly affected by ionic strength (Lee and Mooney 2012).

In this study, a laboratory-scale reverse osmosis system has been employed to conduct the fouling

experiments and demonstrate the fouling effects on RO processes.

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1.1. Objectives

With guidance from the assigned supervisors, a set of objectives has been developed to elucidate the

mechanisms of biofouling in reverse osmosis processes. The purpose of this study is to investigate

the following main objectives systematically.

1. To discover the effects of membrane biofouling on permeate flux and hydraulic resistance

and consequently the system’s efficiency.

2. To investigate and compare the fouling behaviour of alginate and pullulan, both individually

and when coupled with salt (NaCl and CaCl2).

3. To generate a model which would explain cake layer formation, cake-enhanced

concentration polarisation as well as the relationship between osmotic pressure and

concentration polarisation?

4. To determine if variation in cross-flow velocities has any effect on membrane biofouling.

Three different cross-flow velocities have been applied to address any knowledge gaps in RO

membrane studies.

These objectives have been carefully developed to maintain and possibly improve the performance

of reverse osmosis systems.

This study also aims to demonstrate the following competencies for engineering students:

To demonstrate the ability to undertake the necessary research and design practice to

produce an outcome for the assigned task; and

To be able to inspect, analyse and deliver the findings of the subject area.

1.2. Project Management Plan

During this project, a management plan has been devised to keep track of all activities for this

project. Consequently, a Gantt chart has been constructed using Microsoft Excel spreadsheet to

illustrate the progression of all relevant tasks involved throughout the course of this project. This

chart provides a means of organising weekly tasks and its time scale and identifies the order in which

tasks are to be completed (Gantt.com 2016). The progress of some of the listed tasks has been varied

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due to the rising of unexpected problems and therefore, the chart has been amended and modified

from its original version (refer to Appendix A).

2. Background

2.1. Reverse Osmosis System

Reverse osmosis is a common technology used for changing saline or contaminated water to a

drinkable water source (Li, Xu and Pinnau 2007). This technology has become increasingly popular

since it is the most efficient tool for wastewater reclamation (tertiary treatment) and is one of the

best performing technologies for desalting brackish water and seawater (Matin et al. 2011).

Membrane-based reverse osmosis is today the leading desalination technology which has overtaken

conventional thermal technology such as multi-stage flash (MSF) desalination and is expected to

maintain its leadership in the near future, though new technologies such as membrane distillation

and forward osmosis have been recently proposed (Lee, Arnot and Mattia 2011). It is a process that

is inherently easy to design and operate compared with many traditional separation methods and

considered as the simplest and most efficient technique for water purification purposes (Prakash

Rao, Desai and Rangarajan 1997). Reverse osmosis is the process of forcing a solvent from a high

solute concentrated region (feed side) through a membrane to a low solute concentrated region

(permeate side) by applying a pressure excess of the osmotic pressure (Figure 1) (Rathore et al.

2013). The applied pressure in excess of osmotic pressure reverses water flow direction and

due to this reason it is referred as “Reverse Osmosis”.

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Figure 1: Reverse osmosis principle. Left: osmosis; Right: reverse osmosis (Fritzmann et al. 2007).

This process is the reverse of normal osmosis process in which natural movement of solvent from an

area of low solute concentration, through a membrane, to an area of high solute concentration. The

RO membrane is the heart of the process in separating the undesired constituents from the feed to

obtain the desired pure product (Matin et al. 2011). It acts as a semi-permeable barrier that allows

the passage of a particular species (solvent, usually water) while partially or completely blocking

other species (solutes, such as salt) (Matin et al. 2011). In practice, as the saline/contaminated feed

water is pumped and pressurised against the membrane, a portion of water passes through the

membrane while the remaining water is diffused back causing an increase in salt content of the feed

water. While no heating or phase change is necessary for this separation, the major energy required

for desalting is for pressurising the feed water (Buros 2000).

Reverse osmosis membrane technology is relatively new which has developed over the past 40 years

to a 44% share in world desalting production capacity, and an 80% share in the total number of

desalination plants installed worldwide (Greenlee et al. 2009). Commercial interest in RO technology

has increased globally due to continuous process improvements, which in turn lead to significant cost

reductions (Lee, Arnot and Mattia 2011). The greatest efficiency gains have arisen from the

improvement of the membranes. The first major breakthroughs in membrane improvement came

from Loeb- Sourirajan in early 1960s when he developed the first asymmetric reverse osmosis

membranes which showed up to 100 times higher flux than any other known membranes (Loeb and

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Sourirajan 1964). In early 1970s, the first commercially available reverse osmosis membrane,

cellulose-acetate (CA) membrane was introduced (Li 2008). A CA asymmetric membrane was formed

with a dense 200nm thin layer over a thick micro-porous body. This new morphology produced a

water flux of at least an order of magnitude higher than the initial symmetric membrane (Liu 1981).

Later on, in the 1980s, composite membranes with more stability in physical and chemical properties

were introduced (Lee, Arnot and Mattia 2011). In recent years, the structure, material, and

morphology of RO membranes have improved functionally (permeability and selectivity) and

applicability (mechanical, chemical and biological stability (Lee, Arnot and Mattia 2011). Among the

existing membranes currently available, thin film composite (TFC) membranes are considered the

most efficient for desalination (Ismail et al. 2015). These membranes enhance both selectivity and

productivity with less energy consumption as compared to typical asymmetric membranes. The

continuous enhancement of membranes over the past decade has resulted in substantial reduction

of both membrane cost and energy consumption in RO systems.

The overall efficiency of a reverse osmosis system also depends on how well the system is designed.

Several factors including feed water characterisation, pre-treatment options, membrane choice,

operating conditions, and system cleaning can significantly affect the efficiency of RO systems

(Fritzmann et al. 2007). Types of elements exist in the feed water such as salt ions, or organic

elements can assist in choosing the suitable pre-treatment option. The feed water is diluted by pre-

treatment processes prior entering the RO system to prevent severe membrane fouling (Henthorne

and Boysen 2015). Using an appropriate membrane type is important in achieving high efficiency in

RO systems. Furthermore, applying optimal operating conditions such as temperature, pressure, and

cross-flow velocity can extend the lifetime of RO systems (Mickols et al. 2005).

2.2. Membrane Fouling

Membrane fouling is an inevitable phenomenon in reverse osmosis systems and causes a decrease in

water production because of gradual decline in permeate flux, an increase in applied pressure

required for a constant rate of water production, a gradual membrane degradation which results in a

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shorter membrane life, and a decrease in salt rejection (NG and Elimelech 2004). It occurs as a result

of the accumulation of particles on the surface of the membrane or within the pores of the

membrane through the filtration of feed water (Matin et al. 2011).It is recognised that there are four

major types of reverse osmosis membrane fouling: inorganic or crystalline salt precipitation

(concentration polarisation), organic fouling caused by deposition of organic substances such as oil or

proteins, colloidal fouling (build-up of colloidal cake layers caused by deposition of clay, silt, debris,

or silica), and microbiological (adhesion and accumulation of bacteria forming biofilms) (Herzberg

and Elimelech 2007). Generally, three factors affecting the membrane fouling: (i) the membrane

properties including surface properties, membrane material, and morphological structure; (ii) the

feed solution chemistry and composition such as concentration of the substances and pH; and (iii)

the operating conditions including applied hydraulic pressure, temperature, and cross-flow rate (Li

and Chen 2010).

Membrane fouling imposes a large economic burden and is still a major technical problem in RO

operations that needs to be addressed to improve the cost-effectiveness of the system (Zhao, Song

and Ong 2010). For this reason, one of the major goals of membrane research has been to enhance,

or at least maintain, water flux and ultimately increase efficiency and reduce operational cost in RO

systems.

2.2.1. Concentration Polarisation

In all membrane separation processes, as water from the feed area reaches the membrane surface,

the dissolved material (salt ions) are rejected by the semi-permeable membrane resulting in

accumulation of these substances in front of the membrane. Consequently, the concentration in this

boundary layer exceeds the concentration of the bulk water (feed side) (Fritzmann et al. 2007). This

phenomenon is called concentration polarisation (CP) (Figure 2).

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Figure 2: Concentration Polarisation (CP) phenomenon in RO processes.

Concentration polarisation increases the potential for membrane fouling by increasing the osmotic

pressure at the membrane surface leading to (i) reduction in permeate flux (ii) decrease in rejection

because of increased salt concentration at the membrane surface; and (ii) hindrance of mass transfer

through the membrane due to precipitation layer on the membrane surface (Fritzmann et al. 2007).

In addition, the build-up of cake layer on the membrane surface due to the accumulation of rejected

particles reduces the efficiency in RO processes (Hoek and Elimelech 2003). The severity of

concentration polarisation in RO processes is not only determined by the size of the particles that

exist in the feed solution, but it also depends on the applied cross flow rate in the system (Hoek and

Elimelech 2003). Therefore, increasing the feed velocity is considered as the primary method of

minimising the influence of concentration polarisation in membrane based RO systems.

2.2.2. Back Diffusion

Diffusion is defined as the movement of a solution or air from a higher concentrated region to a

lower concentrated region and is influenced by the kinetic properties of the particles (Marie

Helmenstine 2016). The diffusion process will continue until an equilibrium concentration is achieved

in the two regions. In membrane-based reverse osmosis systems, the unavoidable phenomenon of

concentration polarisation can severely reduce the permeate flux due the gel layer formation. In such

8

systems, back diffusion occurs from the membrane surface back to the bulk (feed solution)

(Abdelrasoul et al. 2015). As solution from the feed side flows towards the membrane, a fraction of

solute (salt) starts to accumulate at the membrane surface while the remaining flows with the bulk

solution back to the feed with the help of back diffusion. The amount of salt runs back to the feed

during this period is usually lower than the amount of salt accumulates at the membrane surface (Ye

et al. 2005). Once sufficient number of salt ions accumulate at the membrane surface, the number of

salt ions return to the bulk solution due to back diffusion will equal the number of salt ions

accumulate at the membrane surface. At this time point, the system reaches a steady state (Ye et al.

2005). Back diffusion is hindered as the thickness of the fouling layer on the membrane surface

increases. Therefore, the thicker the fouling layer, the lower back diffusion is, and it is insignificant

once the fouling layer reaches certain thickness (Ye and Wang 2007). The theoretical principles of

cross-flow filtration in RO processes are derived from Fick’s law of diffusion which addresses the

migration of suspended solids/macromolecules in a flowing stream towards a filtration surface, and

the potential back-diffusion into the bulk stream (Abdelrasoul et al. 2015).

2.2.3. Biofilm Formation

Biofouling, a major type of membrane fouling, is caused by the adhesion and accumulation of

microorganisms and biological matter on the membrane surface resulting in biofilm formation (Matin

et al. 2011). Biofilms are a group of surface-associated microbial cells that are irreversibly linked to

the surface and enclosed in a matrix of extracellular polymeric substances (EPS), namely

polysaccharides (Flemming 1997). The biofouling phenomenon in RO processes leads to the use of

higher operating pressure, more frequent chemical cleaning, and shorter membrane life and, can

never be prevented through pre-treatment alone (Fritzmann et al. 2007). In the Middle East, the

region which produces the largest amount of desalted water in the world, about 70% of the seawater

RO membrane installations suffer from biofouling problems (Matin et al. 2011). Donlan (2002)

explains that the formation of a biofilm in RO processes starts with the transport and attachment of

the microorganisms on the membrane surface. The attachment of microorganisms on any surface is

facilitated by physical, chemical, and biological factors and, it is more likely for microorganisms to

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attach to hydrophobic and non-polar surfaces (Donlan 2002). Surface irregularity, hydrodynamics,

and feed water characteristics also influence the formation of biofilms.

2.3. Polysaccharides

One of the main causes of membrane fouling in RO processes are the extracellular polymeric

substances (EPS) (Tarnacki et al. 2005). They are produced by microorganisms as part of the

metabolism and due to biological stress and they are mainly composed of polysaccharides, nucleic

acids and proteins (Katsoufidou, Yiantsios and Karabelas 2007). Polysaccharides are a form of

carbohydrates within which there are simple sugars and its derivatives, cellulose, starches, gums and

chitosan (Talens, Fabra, and Chiralt 2010). Polysaccharides, as the main constituents of EPS, play an

important role in membrane fouling and because of this reason; they are used as model substances

of EPS in membrane fouling research (Frank and Belfort 2003). In addition, studies have revealed that

these substances seem to simulate EPS behaviour during filtration better than other EPS constituents

(Ye, Chen and Fane 2006). Furthermore, many studies have shown that polysaccharides exhibit

enhanced binding for calcium (divalent cation exist in seawater and wastewater) leading to the

formation of a stable gel network (Katsoufidou, Yiantsios and Karabelas 2007). In this project, two

main types of polysaccharides have been studied. Sodium alginate was chosen because previous

studies have shown that alginate forms a thick fouling layer on the surface of the membrane

particularly, in the presence of salt ions such as calcium and sodium. Pullulan was also used in this

study since its behaviour on the membrane fouling has not been fully understood.

2.3.1. Alginate

Sodium alginate is a hydrophilic microbial polysaccharide, mainly produced by brown algae, which is

often used as a model for EPS (Lee, Ang and Elimelech 2006). It consists of a linear copolymer

composed of 1-4 linked β-D-mannuronic acid, C-5 epimer and α-L-guluronic acid in varying properties

as shown in Figure 3 (Ye et al. 2005).

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Figure 3: Structural formula of sodium alginate molecule (Katsoufidou, Yiantsios and Karabelas 2007).

Several studies have indicated that in the presence of cations such as sodium and calcium, alginate

forms complexes of unique structure, resulting in a highly compacted gel network explained by “egg-

box” model as it is seen in Figure 4 (Katsoufidou, Yiantsios and Karabelas 2007).

Figure 4: The egg-box model for binding divalent cations to sodium alginate molecules (Katsoufidou, Yiantsios and

Karabelas 2007).

In this model, calcium ions bind to the carboxylic groups of alginate in a highly organised manner and

form bridges between adjacent alginate molecules, producing egg -box-shaped gel network

(Steinbuchel and Rhee 2005). In general, alginate is broadly used for various applications including

food industry and medical products due to its high viscosity (Lee and Mooney 2012).

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2.3.2. Pullulan

Pullulan is a linear homopolysaccharide of glucose that is often described as α-(1-6) linked

maltotriose, secreted primarily by strains of the polymorphic fungus A. Pullulans (Figure 5) (Leathers

2003). Although this unique biopolymer has a broad range of commercial and industrial applications

in many fields like food science, health care and pharmacy however, only a few of these potential

uses have been widely adopted due to its relatively high cost (Singh, Saini and Kennedy 2008).

Pullulan has a low viscosity and is unaffected by heating, changes in pH, and most metal ions

including sodium chloride (Leathers 2003). To the best of my knowledge, there has been little study

on pullulan behaviour in membrane separation processes particularly in the presence of cations such

as calcium and sodium.

Figure 5: Schematic chemical structure of pullulan with maltotriose as repeating unit (Singh, Saini and Kennedy 2008).

3. Methodology & Research Design This chapter has been set out to provide detailed information of all necessary materials, techniques,

and procedures used in this study to understand the effects of biofouling in reverse osmosis systems.

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3.1. Experimental Methods

3.1.1. System Design

For this study, a laboratory-scale reverse osmosis system has been pre-arranged to conduct the

fouling experiments as shown in figure 6 with its schematic diagram in figure 7. The system is made

up of a feed tank where the feed solution is placed, an electrical pressure pump to push the solution

through the membrane, a pressure gauge to maintain the assigned pressure value, membrane cell,

and a temperature control system (water-bath). It also includes a flow meter to maintain the

assigned cross flow velocity, a bucket for collecting treated water, a balance for weighing the water

of the permeate, and finally a computer program where it records the weight of the permeate in a

one-minute interval. A commercial forward osmosis membrane supplied by Hydration Technologies

Inc. was used to conduct the fouling experiments. The membrane was made up of cellulose

triacetate (CTA) and cast onto a non-woven backing consisting of polyethylene-coated polyester

fibres. As illustrated in figure 7, while the system is running, the feed solution is pushed through the

membrane cell by the pressure pump after entering the water-bath. Once the solution reaches the

membrane surface, the solvent (water) passes through the membrane into the permeate side and is

collected in a bucket placed on a balance. The semi-permeable membrane rejects other particles

(solute) where some accumulate on the membrane surface, while the rest flow back into the feed

solution.

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Figure 6: Laboratory-sale reverse osmosis system used to conduct the experiments.

Figure 7: Schematic Diagram of the laboratory-Scale RO system used to conduct the experiments.

The membrane cell used in the designed system was supplied by Sterlitech Corporation. A detailed

design specification of the membrane cell is available in Appendix B. The membrane cell consists of

Pressure gauge

Membrane Cell

Feed Tank

Water-bath

Pressure pump

Flow meter

Balance

Permeate flux

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an upper component (a porous stainless steel plate) and a base component (feed inlet and

concentrate outlet) as shown in figure 8. The membrane is placed between these two components

with non-active side facing the upper part of the cell. Before use, the membrane was cut into a

sample with a size suitable to fit the cell and stored in DI water for a minimum of 24 hours.

Figure 8: Sterlitech CF042 membrane cell with its components.

3.1.2. Experimental setup and procedure

At the start of this project and with help from research assistant (Zhangwang Xie), preliminary

experiments were conducted to gain an insight of the subject matter and become familiar with the

way the designed RO system works and operates. Following the completion of the preliminary

experiments, a set of designed experiments was carefully formulated. They were divided into three

main categories. The procedure of each type is illustrated in figure 9.

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A) Control experiment (baseline)

B) Fouling experiment without salt

C) Fouling experiments in the presence of salt

Figure 9: The procedure of RO experiments conducted in this study. A) Control experiments; B) Fouling experiments without salt; and C) Fouling experiments in the presence of salt.

Control experiments were designed to investigate the effects of concentration polarisation by adding

merely salt to the feed solution. On the other hand, fouling experiments were set to study the effects

of polysaccharides (alginate/pullulan) by adding alginate and pullulan to the feed solution. Lastly, to

examine the behaviour of the enhanced fouling layer on the membrane surface, salt (calcium

chloride and sodium chloride) and foulants (alginate and pullulan) were added to the system as

illustrated in Figure 9 (C).

Every experiment lasted for 28 hours with only DI water was used as a feed in the first two hours of

experiments to allow for membrane compaction and ensure the stability of all operating conditions.

Following the addition of salt/foulant to the feed at time= 2 hours, the system operated for another

Compaction

Addition of salt

Cross-flow velocity

12.75cm/s

Cross-flow velocity

5.23cm/s Stabilisation

Start (0h) End (28h) 4

h

12h 20h

Cross-flow velocity

8.5cm/s

2h

Compaction

Addition of foulant

Compaction

Addition of salt

Cross-flow velocity

12.75cm/s

Cross-flow velocity

5.23cm/s Stabilisation

Start (0h) End (28h) 4

h

12h 20h

Cross-flow velocity

8.5cm/s

2h

Addition of foulant

Cross-flow velocity

12.75cm/s

Cross-flow velocity

5.23cm/s Stabilisation

Start (0h) End (28h) 4

h

12h 20h

Cross-flow velocity

8.5cm/s

2h

16

2 hours with the same initial conditions to allow for membrane stabilisation and equilibrium. Cross-

flow velocity was manually set at 8.5 cm/s (identical to initial cross flow velocity) at time= 4 hours

before it was increased to 12.75 cm/s at time= 12 hours and finally it was dropped to 5.23 cm/s at

time= 20 hours. The system operated for eight hours under each assigned cross-flow velocities.

A total of 12 experiments were conducted over the course of this project with every type of

experiment was performed at least twice to ensure reliability and reproductively of the results (Refer

to Appendix C for details of the performed RO experiments). The conductivity of both feed solution

and permeate was measured periodically in all experiment at time= 4 hours, time= 12 hours, time=

20 hours, and time= 28 hours for determination of salt rejection. Membranes that showed significant

deviations from the measured average values of clean permeate flux and salt rejection in the

compaction stage were considered defective and discarded. At the end of every experiment, the

system was back washed with 20 L of DI water using a recyclable membrane to ensure the removal of

foulant or salt from the system.

3.1.2. Chemical Parameters and Operating Conditions

50 mM of ionic strength was added to the feed solution to investigate the behaviour of

concentration polarisation in RO processes. This amount of concentration was achieved by adding

27.5 g sodium chloride (NaCl) and 1.47 g calcium chloride (CaCl2) (with a similar ratio in artificial

seawater) (Demicco et al. 2005). Two types of foulant- sodium alginate and pullulan were also used

to study the behaviour of polysaccharides on membrane fouling. As mentioned in chapter 2,

polysaccharides are used as common model substances of EPS in membrane fouling research. A

concentration of 0.2 g/L of the available polysaccharides, in the form of powder, was applied in

fouling experiments. This concentration was prepared by adding the foulant to a small amount of DI

water (approximately 300 mL) and stirring until dissolved before adding the solution into the feed. In

experiments involving pullulan, two different types of pullulan products supplied by SIGMA

(Germany) and RONGSHENG Biotechnology Co. LTD (China) was used in the system. Consequently,

three fouling experiments (pullulan with no salt- two experiments using SIGMA products and one

17

experiment using RONGSHENG product) were conducted, and the behaviour of the two products was

compared (refer to “Results and Data Analysis” section).

All operating conditions were kept consistent throughout all experiments with the following

parameters:

Initial feed volume: 10L of DI water

Hydraulic pressure difference: 400 psi

Temperature: 25 ± 1 °C

Initial cross-flow velocity: 8.5 cm/s

3.2. Confocal Laser Scanning Microscopy (CLSM) Methodology

At the end of each type of experiments (experiments 2 (baseline), 5 (alginate only), 7 (pullulan only),

9 (salt+ alginate), and 10 (salt+ pullulan)), fouled membranes were carefully removed from the

membrane cell. The samples were then stored in containers with sufficient moisture levels using DI

water and kept in the fridge for CLSM analysis. With guidance from Dr. Lucy Skillman (staff technical

specialist in Murdoch University), fouled membranes were carefully removed from containers and at

a randomly chosen area was cut into <2 cm x 2 cm pieces. All samples were then put on clean

microscope glass slides, cautiously stained using a DTAF solution (approximately 75 mL), and secured

with cover slips to prevent the transportation of the stain. A pH value of 9 was used in the pre-

prepared stain solution provided by Dr. Lucy for the purpose of highlighting the fouling layer under

the microscope (Ahmed, Stal and Hasnain 2010). The samples were then put in a container, covered

with an aluminium foil to avoid the penetration of light intensity into the stain, and left overnight. All

samples were analysed by a Nikon C2 Confocal Laser Scanning Microscope (CLSM) (Figure 10) at 20x

magnification using associated software (NIS-Elements) to collect z-stacks of at least three areas per

membrane.

18

Figure 10: Picture of the Nikon C2 Confocal Microscope.

A series of 3D images was captured from the microscope so the thickness and the uniformity of the

fouling layer on the membrane surface could be analysed.

3.3. Total Organic Carbon (TOC) measurement

Polysaccharides, the main components of EPS, may account for 50-90 % of the total organic carbon

of biofilms and can be considered the primary matrix material of the biofilm (Matin et al. 2011).

Therefore, the total organic carbon (TOC) measurement helps to identify the concentration of these

foulants at various times of the experiment. The results from this test provide a better understanding

of the amount of polysaccharides being deposited on the membrane surface in fouling experiments.

In experiments involving foulant, samples (approximately 100 mL) from the feed solution were

collected at predetermined times (at times 4 h, 12 h, 20 h, and 28 h) for analysis of TOC. The samples

were stored in the refrigerator under a controlled temperature (between 0-4 :C) prior to the test.

The TOC analysis was completed by an external source in “Marine and Freshwater Research

Laboratory” located at Murdoch University. The result of this test is found in Appendix D. In addition,

19

two extra samples from the feed and permeate were collected at the end of the fouling experiment

involving combined alginate and salt to ensure the reproductively of the used analytical method.

3.4. Polysaccharides weighing methodology

Determining the weight of polysaccharides on the membrane surface provides a good indication of

the extent of the fouling layers in fouling experiments. Stored fouled membranes were cut to 2.2 cm

x 2.2 cm pieces (same size as the microscope cover glass) and left in an incubator with a controlled

temperature at 55 :C overnight to remove the excess of moisture. Prior to this practice, every

microscope cover glass used in the procedure was weighted on the available scale (W1). A new,

unused membrane was also identically cut and stored as a control (W2) for comparing the difference

in weight with fouled membrane samples. All dried samples were then measured (W3) to determine

the weight of biofilms deposited on the membrane surface. The procedure applied for calculating the

weight of biofilms on the membrane surface is as follows:

W2 – W1: weight of fresh membrane without glass

W3 – W1: weight of sample membrane without glass

(W3 – W1) – (W2 – W1): weight of fouling layers and salt particles

The result of above procedure can be found in Appendix E.

3.5. Salt Concentration using “The Standard Curve” Method

The measured conductivity during experiments was used to determine the concentration of salt in

the feed, permeate, and ultimately at the membrane surface. To obtain the corresponding salt

concentration from the measured conductivity, “The Standard Curve” method was applied. The

curve was constructed by diluting the initial salt concentration used in experiments (50 mM) which

was achieved by mixing 27.5g NaCl and 1.47g CaCl2 to a small portion of DI water. As this ratio was

added to 10L of DI water, it was scaled down to 0.275g NaCl and 0.015g CaCl2 dissolved in 100mL DI

water to meet the purpose. The dissolved solution was further diluted using the following steps.

The 100mL solution was put in a 100mL volumetric flask and labelled as 50mM in

concentration.

20

The solution in step 1 was put into a 50mL volumetric flux using a funnel. The excessive of

the solution was then put into an empty 100mL volumetric flux and supplemented with DI

water to reach 100mL volume. The flux was then labelled as 25mM in concentration.

The procedure was repeated to achieve 10mM and 5mM concentration.

The conductivity of each solution was recorded, and a graph of conductivity vs.

Concentration was produced.

Finally, the measured conductivity during experiments was applied to the curve so

corresponding concentration could be estimated.

The result of this process is found in Appendix F.

4. Modelling

The permeability, the hydraulic resistances, the concentration polarisation, and the salt rejection rate

and back diffusion rate in the performed experiments have been assessed on the basis of following

considerations.

4.1. Modelling permeate flux

To study the influence of fouling experiments on permeate flux (J), a model of the permeate flux has

been constructed. This model was based on a model developed by PhD candidate Zhangwang Xie

(John). As the weight of permeate flux (in grams) was recorded on a computer at a one-minute

interval, the difference in weight over ten minutes was calculated. The produced values, in grams,

were then converted to litres per square meter per hour (LMH). Lastly, the corresponding values to

every thirty minutes were used to develop the permeate flux model.

𝐽 =(𝑉𝑛+10 –𝑉𝑛 )

𝐴.𝑇 (Eq. 1)

Where J is the permeate flux over ten-minute interval (LMH or L/m2.h), Vn+10 is the recorded volume

of permeate (L) at time n+10, Vn is the recorded volume of permeate (L) at time n, A is the effective

membrane area (42x10-4 m2), and T is the time (h).

21

4.2. Modelling Hydraulic Resistances at the membrane surface

Based on John’s modelling structure, a model of the hydraulic resistances at the membrane surface

has been developed to investigate the effects of resistances on the resulting permeate flux. Using

Darcy’s Law (the flow of fluid through a porous medium) as a basis, the hydraulic resistances caused

by the fouled membranes can be calculated during the process (Dreszer et al. 2013).

𝐽 = ∆𝑃

𝜇 .𝑅𝑡 (Eq. 2)

Where J is the water permeate flux (in m/s), ΔP is the applied transmembrane pressure (in Pa), 𝜇 is

the viscosity of DI water (in Pa.s), and Rt is the total hydraulic resistance (in m-1). The following

procedure was then applied to obtain the membrane resistance (Rm), the resistance due to

concentration polarisation (Rcp), and the resistance of the cake layer (Rc).

A. Membrane Resistance (Rm)

During the membrane compaction between time= 0h and time= 2h when only DI water was

presented in the feed, the resistance of the membrane was calculated. As no foulant or salt was

added to the feed, the only hydraulic resistance exists in the system was the membrane resistance

(Rm).

𝑅𝑚 = 𝑅𝑡 = ∆𝑃

𝜇 .𝐽 (Eq. 3)

In every experiment, Rm was measured four times during membrane compaction (every half an hour)

however, only the third or fourth value was assumed as the constant (Rm).

B. Resistance due to Concentration Polarisation (Rcp)

Once the salt is introduced to the system, the accumulation of salt ions at the boundary of the

membrane surface causes an increase in concentration polarisation which in turn adds to the

22

hydraulic resistance in the process. Hence, the concentration polarisation resistance was calculated

by altering Eq. 2.

𝑅𝑡 = 𝑅𝑐𝑝 + 𝑅𝑚 → 𝑅𝑐𝑝 = 𝑅𝑡 − 𝑅𝑚 = ∆𝑃

𝐽 ∙𝜇− 𝑅𝑚 (Eq. 4)

C. Cake Resistance (Rc)

In experiments where foulant (alginate or pullulan) is introduced to the system, the gradual

formation of the fouling layer on the membrane surface results in additional non-membrane

resistance. The resistance of this fouling layer was obtained by:

𝑅𝑡 = 𝑅𝑐 + 𝑅𝑚 → 𝑅𝑐 = 𝑅𝑡 − 𝑅𝑚 = ∆𝑃

𝐽 ∙𝜇− 𝑅𝑚 (Eq. 5)

D. Combined Cake resistance and Concentration Polarisation Resistance (Rc + Rcp)

In the case where foulant and salt are added to the system, both the cake layer resistance and

concentration polarisation resistance contribute to the non-membrane resistance in the process.

Sometimes, this combined resistance is referred as cake-enhanced layer resistance. The cake-

enhanced layer resistance is calculated as follows:

𝑅𝑡 = (𝑅𝑐 + 𝑅𝑐𝑝 ) + 𝑅𝑚 → (𝑅𝑐 + 𝑅𝑐𝑝 ) = 𝑅𝑡 − 𝑅𝑚 = ∆𝑃

𝐽 ∙𝜇− 𝑅𝑚 (Eq. 6)

The above equations have been developed on the basis of the following considerations to reduce the

complicity of the produced model.

Membrane blockage resistance (Rb) was not considered in the calculation as it was concluded

to be insignificant in the performed experiments.

The resistance of membrane was assumed to remain steady after membrane compaction in

all experiments.

The increase in osmotic pressure due to the addition of foulant was negligible and hence was

ignored.

23

4.3. Modelling Osmotic Pressure & Concentration Polarisation at the membrane surface

In experiments where salt has been used, as the salt ions start to accumulate at the membrane

surface, the osmotic pressure (𝜋cp) begins to increase resulting in a reduction of permeate flux. The

increase of the osmotic pressure is therefore deducted from the applied transmembrane pressure.

∆𝑃 = 𝑃 − 𝑃0 − 𝜋𝑐𝑝 (Eq. 7)

The calculation of the osmotic pressure at the membrane surface has been illustrated by using Eq.2

and Eq.6.

𝐽 = ∆𝑃− 𝜋𝑐𝑝

𝜇 .𝑅𝑚 → 𝜋𝑐𝑝 = 𝛥𝑃 − (µ. 𝐽. 𝑅𝑚) (Eq. 8)

By applying De Van’t Hoff’s Law for the osmotic pressure, the concentration polarisation at the

membrane surface has been estimated (Fritzmann et al. 2007).

𝜋𝑐𝑝 = 𝑖 ∙ 𝐶𝑐𝑝 ∙ 𝑅𝑔 ∙ 𝑇 → 𝐶𝑐𝑝 =𝜋𝑐𝑝

𝑖 .𝑅𝑔 ∙𝑇 (Eq. 9)

Where 𝑖 represents the van’t Hoff’s factor which is equal to the number of ions per mole of solute (2

was used in the above equation), 𝐶𝑐𝑝 is the salt concentration at the membrane surface (mol/L), 𝑅𝑔

is the gas constant (0.08206 L.atm/mol.K), and 𝑇 is the temperature in Kelvin.

The estimation of concentration polarisation has also been modelled based on the measured

conductivity at a different time interval during the experiments. Referring to “The Standard Curve”

graph, the concentration polarisation in the feed and permeate was estimated from the

corresponding measured conductivity. The total mass in the permeate and feed can be calculated by

multiplying the concentration with related volume. Using the effective membrane area and the

estimated thickness of the deposition of salt at the membrane surface (from CLSM images), the

volume at the membrane surface can be estimated.

By applying the total mass equation in the process, the salt concentration at the membrane surface

was determined.

24

𝑇𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 = 𝑇𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 𝑖𝑛 𝑡𝑕𝑒 𝑓𝑒𝑒𝑑 + 𝑇𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 𝑖𝑛 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒 +

𝑇𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 𝑎𝑡 𝑡𝑕𝑒 𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 (Eq. 10)

4.4. Salt Rejection Rate

The rate of salt rejection has been estimated in experiments where salt was added to the system. It

was calculated based on the measured conductivity to determine the efficiency of the membranes

used in experiments.

𝑅𝑒𝑗𝑒𝑐𝑡𝑖𝑜𝑛 % = 1 − 𝐶𝑝

𝐶𝑓 × 100% (Eq. 11)

Where 𝐶𝑝 is the permeate conductivity and 𝐶𝑓 is the conductivity of the feed.

4.5. Back Diffusion Rate

In the absence of fouling, the rejected salt ions result in high concentration of the salt at the surface

of the membrane. Consequently, a back diffusion of these ions occurs from the layer adjacent to the

membrane back to the feed solution (Abdelrasoul et al. 2015). The rate of back diffusion can be

explained by the Fick’s Law. This law relates the diffusive flux to the concentration under the

assumption of steady state (Abdelrasoul et al. 2015).

𝐽 = 𝐷𝛥𝐶

𝑕 (Eq. 12)

Where 𝐽 is the rate of back diffusion (mol/m2.s), D is the diffusion coefficient (1.58 x 10-4 m2/s), 𝛥𝐶 is

the difference in permeate concentration and feed concentration (mol/m3), and h is the thickness of

the concentration polarisation layer (m).

25

5. Results and Data Analysis

Based on the developed model and experimental procedures, results have been obtained and

analysed. In order to make the results comparable, a normalised flux was used based on the

following formula.

𝐽𝑁 = 𝐽

𝐽0 (Eq. 13)

Where JN is the normalised flux, J is the flux at a given time, and J0 is the initial flux.

Moreover, for the baseline experiments, the rate of salt rejection was calculated based on Eq. 11.

A high rejection rate ranging between 97.8 and 99.3 percent was achieved by the membranes

used in the experiments. The high salt rejection rate is an indication of an excellent efficiency of

the used membranes in the system.

5.1. Permeate Flux

The performed experiments in this study have been divided into three main categories as

mentioned previously. The baseline experiments were set to study the effects of concentration

polarisation, the fouling experiments were designed to investigate the effects of polysaccharides

(alginate and pullulan), and the fouling experiments in the presence of salt aimed to explore the

effects of salt and polysaccharides in combination. The addition of salt and polysaccharides in RO

processes usually cause a flux decline. Therefore, in his section, the effects of conducted

experiments on the resulting permeate flux have been analysed.

26

5.1.1. Concentration Polarisation Effects

Figure 11: Effects of concentration polarisation (NaCl and CaCl2) on permeate flux.

Three baseline experiments, with 50mM of salt concentration (NaCl and CaCl2) added to each

experiment, have been conducted during this study. These experiments have been conducted to

study the effects of concentration polarisation individually so it could be compared with the

fouling experiments. Figure 11 is demonstrates the fluctuation of normalised permeate flux

during the experiments. As it is seen from the graph, the permeate flux expresses a sudden

decline at time= 2h soon after the salt was added to the feed solution. After membrane

compaction, at time=4h, the flux remains relatively steady until time= 12h although a minor

increase in permeate flux was noticed at some points due to the increase in applied pressure

which was observed during the experiments. At time= 12h when the cross-flow velocity was

increased to 12.75 cm/s, a flux showed a slight decrease before it levelled until time= 20h. A

similar trend was observed in the final 8 hours when the system was operating with a cross-flow

velocity of 5.23 cm/s. Overall, the addition of salt caused an average of 15.9% decline on the

resulting permeate flux. Clearly, concentration polarisation due to the addition of NaCl and CaCl2

to the system leads to a decline in the permeate flux. Also, it appears that variation in cross-flow

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Control Exp

Flow rate 5.23 cm/s

27

velocities had an insignificant effect on the resulting permeate flux as the increase or the

decrease in cross-flow velocities resulted in permeate flux decline.

5.1.2. Polysaccharides Effects

Two main polysaccharides- alginate and pullulan have been used to study the fouling effects

on the resulting permeate flux.

I. Alginate Fouling

Figure 12: Effects of sodium alginate on permeate flux.

Two fouling experiments have been conducted with alginate polysaccharide to explore its effect

on permeate flux. The average of the two experiments has been graphed as it is seen in Figure 12.

As the error bars in the graph suggest, the result shows a close correlation between the two

experiments. According to the figure, soon after the addition of alginate to the feed solution at

time= 2h, a fast decline in permeate flux was observed. The effect of alginate on permeate flux

remained relatively steady after time= 4h until the end of experiments despite the change in

cross-flow velocities. Overall, alginate has resulted in an average of 16.8% decline in permeate

flux. It is suggested that after the addition of alginate to the system, a fouling layer is built on the

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Stage 2

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Time (h)

DI + Alginate

Flow rate 5.23 cm/s

28

membrane surface causing a rapid decline in permeate flux. The effects of this layer on permeate

flux decreases as the time increases during the experiments.

II. Pullulan Fouling

Figure 13: Effects of pullulan on permeate flux.

Two fouling experiments where pullulan was used have been performed to study the effect of

this type of polysaccharide on permeate flux. The average of the two experiments is shown in

Figure 13. An average decline of 8.9% in permeate was recorded from the normalised data which

indicates an insignificant reduction caused by pullulan. The major permeate decline was noticed

in the final step of the experiments where the system was operating with a cross-flow velocity of

5.23 cm/s. It is suspected that the reduction in permeate flux during this period was due to the

decrease in the applied pressure which was observed to be lower than the assigned value

(approximately 360psi).

Due to limited availability of pullulan product supplied by SIGMA (Germany), a different

pullulan brand supplied by RONGSHENG Biotechnology Co. LTD (China) was used for the

remaining fouling experiments involving pullulan and salt in combination. Thus, to compare the

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29

effects of the new pullulan product on permeate flux with the old product, an additional

experiment (with same conditions and parameters) was conducted. The result of the additional

experiment indicated that the fouling behaviour of the new pullulan product was closely related

to experiments where the old pullulan product was used.

5.1.3. Concentration Polarisation and polysaccharides Combined Effects

In this section, the fouling behaviour of alginate and pullulan in combination with salt has been

analysed.

I. Alginate and salt

Figure 14: Effects of alginate on permeate flux in the presence of NaCl and CaCl2.

Two experiments with alginate and salt in combination have been conducted to study the

effects of NaCl and CaCl2 interaction with alginate on the permeate flux. According to Figure 14,

as salt was introduced to the system at time= 2h, a fast decline in flux was noticed. After adding

the alginate at time= 4h, the permeate flux continued to decrease gradually until the cross-flow

velocity was increased to 12.75 cm/s. A small increase in flux was noticed during the period

between time= 12h and time = 20h due to the slight increase in the applied pressure. The

permeate flux was further reduced in the last period where the system was running with low

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Time (h)

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30

cross-flow velocity (5.23 cm/s). Overall, the interaction of salt and alginate resulted in an average

decline of 36.9% in the permeate flux.

II. Pullulan and salt

Figure 15: Effects of pullulan on permeate flux in the presence of NaCl and CaCl2.

The final two experiments conducted during this study were the combined salt and pullulan

together. The effects of the interaction of salt and pullulan on permeate flux have been graphed

and shown in Figure 15. It is seen from the figure that as the salt was introduced to the system at

time= 2h, a rapid reduction in flux was observed. As the pullulan was added to the system at

time= 4h, the permeate flux remained relatively steady for the entire duration of experiments

despite the changes in cross-flow velocities. It seems that pullulan has a minimum effect on

permeate flux particularly in a case where it interacts with salt. Overall, the fouling caused by

pullulan and salt together resulted in an average reduction of 18.1% in permeate flux.

0.6

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0.8

0.9

1

1.1

1.2

00

.5 11

.5 22

.5 33

.5 44

.5 55

.5 66

.5 77

.5 88

.5 99

.5 10

10

.5 11

11

.5 12

12

.5 13

13

.5 14

14

.5 15

15

.5 16

16

.5 17

17

.5 18

18

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20

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21

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23

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24

.5 25

25

.5 26

26

.5 27

27

.5

Stage 1

Stage 2

Flow rate 8.5cm/s Flow rate 12.75cm/s Flow rate 4.25cm/s

No

rmal

ise

d P

erm

eat

e F

lux

Time (h)

DI + Salt + Pullulan

Flow rate 5.23 cm/s

31

5.1.4. Flux Comparison

5.1.4.1. Alginate Fouling Comparison

Figure 16: Comparison of permeate fluxes caused by individual and combined effects of salt and alginate foulant.

In Figure 16, the resulting permeate flux due to individual effects of concentration polarisation

(Control experiments), the resulting permeate flux due to individual effects of alginate fouling (DI

+ Alginate), and the resulting permeate flux due to the combined effects of concentration

polarisation and alginate fouling (DI + salt + Alginate) are plotted. Concentration polarisation has

resulted in 15.9% decrease in flux followed by alginate fouling which resulted in 16.8% reduction,

and the interaction of salt and alginate have resulted in 36.9% decline in flux. Based on the

produced results, although both concentration polarisation and alginate have affected the

permeate flux and consequently the efficiency of the system however, the interaction of salt and

alginate have resulted in a major deficiency in the system. It is suggested that the presence of salt

(NaCl and CaCl2) in the feed solution may enhance the formation of the fouling layer on the

membrane surface. Van den Brink et al. (2009) explains that the presence of ionic strength in the

solution promotes alginate gel formation by compaction of the electric double layers on the

membrane and the alginate molecule, which results in a lower electrostatic repulsion and a

denser fouling layer (van den Brink et al. 2009). In another study conducted by Alazmi, Nassehi

0.6

0.7

0.8

0.9

1

1.1

1.20

0.5 1

1.5 2

2.5 3

3.5 4

4.5 5

5.5 6

6.5 7

7.5 8

8.5 9

9.5 10

10

.5 11

11

.5 12

12

.5 13

13

.5 14

14

.5 15

15

.5 16

16

.5 17

17

.5 18

18

.5 19

19

.5 20

20

.5 21

21

.5 22

22

.5 23

23

.5 24

24

.5 25

25

.5 26

26

.5 27

27

.5

Stage 1

Stage 2

Flow rate 8.5cm/s Flow rate 12.75cm/s Flow rate 4.25cm/s

No

rmal

ise

d P

erm

eat

e F

lux

Time (h)

Control Exp

DI + Alginate

DI + Salt + Alginate

Flow rate 5.23 cm/s

32

and Wakeman (2010), it was shown that the binary solution of alginate-calcium caused the

highest irreversibility among the tested solutions and it was due to the ‘egg-box’ structure of the

alginate-calcium and its deposition on and interaction with the membrane surface (Alazmi,

Nassehi and Wakeman 2010). In the presence of divalent cations, such as Ca2+, alginate forms

complexes of unique structure, resulting in a highly compacted gel network (Egg-box)

(Katsoufidou, Yiantsios and Karabelas 2007).

5.1.4.2. Pullulan Fouling Comparison

Figure 17: Comparison of permeate fluxes caused by individual and combined effects of salt and pullulan foulant.

In Figure 17, the resulting permeate flux due to the individual effects of concentration

polarisation (Control experiments), the resulting permeate flux due to individual effects of

pullulan fouling (DI + Pullulan), and the resulting permeate flux due to the combined effects of

concentration polarisation and pullulan fouling (DI + salt + Pullulan) are plotted. As mentioned

previously, concentration polarisation has resulted in 15.9% decrease in flux. The effects of

pullulan fouling have resulted in 8.9% reduction, and the interaction of salt and pullulan have

resulted in 18.1% decline in flux. Based on illustrated results, while salt and pullulan affect the

permeate flux and in turn the efficiency of the system, the interaction between the salt and

0.6

0.7

0.8

0.9

1

1.1

1.2

00

.5 11

.5 22

.5 33

.5 44

.5 55

.5 66

.5 77

.5 88

.5 99

.5 10

10

.5 11

11

.5 12

12

.5 13

13

.5 14

14

.5 15

15

.5 16

16

.5 17

17

.5 18

18

.5 19

19

.5 20

20

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21

.5 22

22

.5 23

23

.5 24

24

.5 25

25

.5 26

26

.5 27

27

.5

Stage 1

Stage 2

Flow rate 8.5cm/s Flow rate 12.75cm/s Flow rate 4.25cm/s

No

rmal

ise

d P

erm

eat

e F

lux

Time (h)

Control Exp

DI + Pullulan

DI + Salt + Pullulan

Flow rate 5.23 cm/s

33

pullulan result in less effect on permeate flux in comparison with the sum of individual effect. As

the effects of pullulan on membrane fouling has not been extensively studied in the past, it is

suspected that the improve in permeability caused by the interaction of salt and pullulan is due to

the chemical and physical characteristics of pullulan and the low binding strength between

pullulan and salt ions. It is suggested that pullulan in the presence of salt may enhance the

permeability of the solution.

5.2. Total Hydraulic Resistances

In this section, the total hydraulic resistances due to concentration polarisation, the total

resistances caused by polysaccharides (alginate and pullulan), and the total resistances due to the

interaction of salt and polysaccharides have been analysed and compared. The membrane

resistance was calculated during the compaction stage of each experiment (first two hours) and

assumed to remain consistent throughout the course of the experiments. Interestingly, the two

fouling experiments involving salt and alginate had the lowest and the highest membrane

resistance values (8.49 x 1014m-1 and 1.03 x 1015m-1 respectively). It is expected that the higher

the membrane resistance is, the more water is permeating through the membrane into the

permeate flux. This was later approved after the permeate flux in the referred experiments was

calculated (Refer to Appendix G for further details).

34

5.2.1. Comparison of Total Resistance due to Alginate Fouling

Figure 18: Comparison between total hydraulic resistances in alginate experiments.

In above figure, the total hydraulic resistances due to the individual effects of concentration

polarisation (salt) and alginate, and the total resistances due to the combined effects of

concentration polarisation and alginate have been compared. As illustrated, the interaction

between salt and alginate resulted in higher resistances (31.7%) in the system in comparison with

their individual effects. Concentration polarisation due to the addition of NaCl and CaCl2 to the

feed solution caused an average increase of 9.7% in total resistances, followed by the alginate

fouling in the absence of salt which caused an increase in total resistances by 15.2%. As expected,

the formation of a thick fouling layer in a case where alginate and salt are used in combination

resulted in greatest total resistances in the system. Matin et al. (2011) stated that in water

purification membrane systems, biofilms act as a secondary membrane and give rise to many

deleterious consequences that include increased hydraulic resistance resulting in reduced

permeate flux, and enhanced concentration polarisation resulting in decreased salt rejection

(Matin et al. 2011). It was also found that alginate fouls the membrane quickly and the fouling

process is aggravated in the presence of divalent cations such as Ca2+ and Mg2+ (Li, Xu and Pinnau

2007). In addition, the formation of the fouling layer on the membrane surface caused by alginate

0.8

0.9

1

1.1

1.2

1.3

1.4

1.52

2.5 3

3.5 4

4.5 5

5.5 6

6.5 7

7.5 8

8.5 9

9.5 10

10

.5 11

11

.5 12

12

.5 13

13

.5 14

14

.5 15

15

.5 16

16

.5 17

17

.5 18

18

.5 19

19

.5 20

20

.5 21

21

.5 22

22

.5 23

23

.5 24

24

.5 25

25

.5 26

26

.5 27

27

.5

Rt

(x1

0^1

5 m

-1)

Time (h)

Salt

Alginate

Salt + Alginate

35

reduces back diffusion of the ions salt at the membrane surface which in turn increases the

concentration polarisation and osmotic pressure at the membrane surface (Kim et al. 2006).

Alazmi, Nassehi and Wakeman (2010), in their research on calcium cation interactions with

polysaccharides, found that almost all the calcium cations were absorbed by the alginate in the

solutions. They also concluded that the highest increase in membrane resistance was due to high

calcium concentrations and low alginate concentration in the feed (Alazmi, Nassehi and

Wakeman 2010).

5.2.2. Comparison of Total Resistance due to Pullulan Fouling

Figure 19: Comparison between total hydraulic resistances in pullulan experiments.

In figure 19, the “salt” represents the total hydraulic resistance due to concentration

polarisation; the “pullulan” represents the total hydraulic resistance due the pullulan fouling; and

the “salt + pullulan” illustrates the total hydraulic resistance due to pullulan and salt interaction.

The comparison between the plots shows that NaCl and CaCl2 have increased the total resistance

in the system by 9.7%. Pullulan in the absence of salt have resulted in an increase in total

resistance by 14.6% and the increase in resistance due to the interaction of salt and pullulan was

at 12.4%. Surprisingly, the results show that pullulan reduces the total resistance in the system

when combines with the salt. Previous studies have shown that membrane fouling caused by

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

1.3

22

.5 33

.5 44

.5 55

.5 66

.5 77

.5 88

.5 99

.5 10

10

.5 11

11

.5 12

12

.5 13

13

.5 14

14

.5 15

15

.5 16

16

.5 17

17

.5 18

18

.5 19

19

.5 20

20

.5 21

21

.5 22

22

.5 23

23

.5 24

24

.5 25

25

.5 26

26

.5 27

27

.5

Rt

(x1

0^1

5 m

-1)

Time (h)

Salt

Pullulan

Salt + Pullulan

36

polysaccharides is a major hurdle in RO processes which results in the increase of hydraulic

resistances (Zhao, Song and Ong 2010). It is suspected that pullulan does not form a strong

binding with the salt ions on the membrane surface. The thin fouling layer produced by pullulan

in the presence of salt was evident when the fouled membrane was removed from the

membrane cell at the end of experiments. The behaviour of pullulan in the presence of salt and

its effect on permeate flux and hydraulic resistances in RO processes require further investigation

and analysis to support the produced results in this study.

Based on the results from figures 18 and 19, it was concluded that the interaction of salt

and alginate in the solution lead to the highest increase in total resistance (31.7%). Therefore,

alginate in the presence of salt poses a great deficiency in RO processes.

5.3. Concentration Polarisation Vs Osmotic Pressure at the Membrane Surface

Where salt has been used in experiments, the increase in concentration polarisation and osmotic

pressure has been analysed. Based on the developed model (Eq. 8, and 9), the osmotic pressure

(𝜋𝑐𝑝 ) and concentration polarisation (𝐶𝑐𝑝 ) at the membrane surface was estimated as it is seen in

figure 20. The figure indicates the progression of osmotic pressure and concentration

polarisation of NaCl and CaCl2 at the membrane surface over a 28 hours period. It is noted from

the figure that both osmotic pressure and concentration polarisation have increased

proportionally over time. As more salt ions deposit at the membrane surface due to the natural

membrane separation process, the osmotic pressure at the membrane surface would also

increase. The increase would suggest that less water would permeate through the membrane due

to the accumulation of salt which reduces the resulting permeate flux over time and in turn

increases the total hydraulic resistance in the system. It has been established (in sections 5.1.1.

and 5.2.1.) that the addition of NaCl and CaCl2 resulted in 15.9% reduction in permeate flux and

9.7% increase in total hydraulic resistance in the system. The growth of concentration

polarisation and osmotic pressure seem to become relatively steady in the last 8 hours of the

37

experiments. The trend in the final hours suggests that the amount of salt accumulating at the

membrane surface is close to the amount of salt being diffused from the membrane surface to

the bulk solution.

Figure 20: Correlation between concentration polarisation and osmotic pressure at the membrane surface.

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

22

.5 33

.5 44

.5 55

.5 66

.5 77

.5 88

.5 99

.5 10

10

.5 11

11

.5 12

12

.5 13

13

.5 14

14

.5 15

15

.5 16

16

.5 17

17

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18

.5 19

19

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20

.5 21

21

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22

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23

.5 24

24

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25

.5 26

26

.5 27

27

.5

CP

(m

ol/

L)

πfe

ed

,m (

x10

^6 P

a)

Time (h)

πfeed,m CP

38

5.4. Conductivity

Table 1: The average measured conductivity of the permeate and the feed solution during all performed experiments.

Measured Conductivity

time Baseline exp Alginate exp Alginate+salt exp Pullulan exp Pullulan+salt exp

Feed Permeate Feed Permeate Feed Permeate Feed Permeate Feed Permeate

(h) (µs)

4 2910.00 30.15 32.00 4.13 3115.00 23.11 1.46 1.16 3025.00 20.33

12 3240.00 39.95 34.80 1.58 3480.00 42.90 2.20 1.41 3485.00 40.85

20 3495.00 46.10 34.50 1.32 3575.00 50.10 2.51 1.65 3600.00 43.15

28 3600.00 49.95 36.70 1.25 3640.00 56.90 2.96 1.90 3715.00 47.65

Average feed and permeate conductivity have been measured periodically at times= 4h, 12h, 20h,

and 28h in all conducted experiments. The results of the measured conductivity have been

summarised in Table 1. As illustrated in the table, all the measured conductivity in the feed

solution and the permeate flux have increased over the course of 28h except in alginate

experiment where a high permeate conductivity was recorded at time= 4h. The high measured

conductivity in alginate experiment was due to the salt left in the system from the previous set of

experiments (baseline experiments with salt added in the system). The increase of feed and

permeate conductivity is an indication of an increase in salt concentration in the permeate and

the feed solution. As water moves through the semi-permeable membrane to the permeate side,

the volume of the feed solution declines which leads to an increase of the salt concentration in

the feed solution explaining the increase of measured conductivity. The salt concentration in the

permeate flux also increases over time because of the build-up of concentration polarisation and

the fouling layer on the membrane surface which allow a small proportion of salt ions to go

through the membrane into the permeate. The permeate conductivity has increased over time in

all experiments which would suggest that the water quality in the permeate decreases over time.

The interaction of salt with alginate has resulted in the highest reduction in the quality of the

treated water due to the formation of a compact fouling layer on the membrane surface which

allow more salt to go through the membrane into the permeate side.

39

5.5. Fouling Layers Weight Estimation

Table 2: Estimation of mass and density of the fouling layers in the performed experiments.

Experiment type Deposit on the

membrane (g)

Thickness (cm) Density of deposit

(g/cm3)

Baseline (salt) 0.026 0.0047 0.13188

Alginate 0.035 0.034 0.02431

Pullulan 0.026 0.018 0.03444

Salt + Alginate 0.095 0.062 0.03666

Salt + Pullulan 0.009 0.021 0.00984

Based on the methodology explained in section 3.4 of this report, the weight of the fouling

layers caused by concentration polarisation, polysaccharides, and combined polysaccharides and

concentration polarisation has been estimated. Table 2 summarises the weight, the thickness

(using CLSM images), and the density of each fouling layer. The density of the deposit was

estimated by applying the following formula:

𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = 𝑚𝑎𝑠𝑠

𝑎𝑟𝑒𝑎 ∙𝑡𝑕𝑖𝑐𝑘𝑛𝑒𝑠𝑠 (Eq. 14)

40

Where density is estimated in g/cm3, “mass” is the mass of deposit on the membrane (g), “area”

is the effective membrane surface area (cm2), and “thickness” is the estimated fouling layer from

CLSM images (cm). As illustrated, the fouling layer in salt+ alginate experiment has the highest

mass (0.095g) as opposed to the fouling layer in salt+ pullulan experiment with a small mass of

0.009g. The interaction between salt and alginate also resulted in a thick fouling layer (0.062 cm)

while concentration polarisation resulted in a thin layer at the membrane surface (0.0047cm).

These findings support the earlier discovery in sections 5.1.3. and 5.2.1. about the reduction in

flux and increase in total resistance caused by the cake-enhanced fouling layer due to the

interaction of salt and alginate. Due to the high density of salt ions, the baseline experiment

recorded the highest density (0.1319 g/cm3). The small mass (0.009g) and low density

(0.00984g/cm3) of the fouling layer in the salt+ pullulan experiment would suggest that pullulan

does not bind well with NaCl and Cacl2 resulting in a thin fouling layer on the membrane surface.

41

5.6. CLSM Analysis

The stored membranes have been examined by Nikon C2 microscopy, and 3D images of the

fouled membranes were produced. The images were analysed with a purpose of estimating the

thickness and the uniformity of the fouling layers deposited on the membrane surface.

Figure 21: Confocal image of the baseline foulaed membrane (NaCl + CaCl2).

Figure 21 is a confocal image of a fouled membrane due to concentration polarisation resulted by

the addition of NaCl and CaCl2 to the system. The green colour represents the fouling due to

concentration polarisation, and the white colour is the surface of the membrane. The thickness of

the fouling layer is estimated to be 47 µm based on the scale provided in the image. Since the

surface of the membrane is clearly visible (white colour), it is unlikely that the fouling layer is

evenly distributed across the membrane surface. Also, the salt particles seem to be compact and

dense as the dark green colour suggest. It is predicted that the darker the colour of the fouling

layer is, the more compact and dense the fouling layer becomes.

42

Figure 22: Confocal image of the fouled membrane due to alginate fouling.

Figure 22 is the confocal image of the fouled membrane due to alginate fouling. Based on the

provided scale in the image, the thickness of the fouling layer is estimated to be 340 µm. Only a

few white points can be seen in the image which is an indication of a uniform distribution of the

fouling layer across the membrane surface. The alginate fouling layer seems to be sparse with low

density as the light green colour indicates.

Figure 23: Confocal image of the fouled membrane due to pullulan fouling.

43

Figure 23 is a confocal image of the fouled membrane resulted from pullulan fouling. The

thickness of the fouling layer is approximately 180 µm. The fouling layer seems to be uniformly

distributed across the surface of the membrane as very few white patches are seen in the image.

The colour of the image suggests that the resulted fouling layer has a uniform density.

Figure 24: Confocal image of a fouled membrane due to the interaction of salt and alginate.

Figure 24 shows the resulted fouled membrane caused by the interaction of salt and alginate in

RO processes. Based on the provided scale in the image, the fouling layer seems to have the

greatest thickness compared to other images (approximately 620 µm). The fouling layer is evenly

distributed across the surface of the membrane. Although the fouling layer seems to be very thick

however, the density of the fouling layer seems to be low as the bright green colour suggests. It is

observed from the image that the top of the fouling layer is level due to the reason that the

fouling layer was covered by the cover glass at the time of inspection under the microscope.

44

Figure 25: Confocal image of a fouled membrane due to the interaction of salt and pullulan.

Figure 25 represents a confocal image of the fouled membrane due to the addition of salt and

pullulan to the feed solution. It is estimated that the fouling layer has a thickness of 210 µm. An

evenly distributed fouling layer across the membrane surface is observed from the image. Also,

the bright green colour is an indication of a thin fouling layer being formed on the membrane

surface.

45

5.7. Results Summary and Comparison

Table 3: Comparison between the performed RO experiments.

Comparison between Performed RO experiments

Experiment Type Experiment

number

Initial flux

Final Flux

Flux reduction

Flux reduction

Rm Rt

increase CLSM

thickness Deposit on membrane

Density of deposit

(LMH) (LMH) (LMH) % (x1015

m-1

) (x1015

m-1

) (µm) (g) (g/cm3)

(DI Water + Salt) Exp 2 9.41 8.14 1.27 13.5% 0.95 6.1% 47 0.026 0.1319

(DI Water + Alginate)

Exp 5 10.71 8.89 1.83 17.1% 0.90 15.9% 340 0.035 0.0243

(DI Water + pullulan)

Exp 7 10.01 9.29 0.73 7.3% 0.91 14.1% 180 0.026 0.0344

(DI Water + Salt + Alginate)

Exp 9 9.27 5.99 3.29 35.4% 1.03 27.9% 620 0.095 0.0367

(DI Water + Salt + Pullulan)

Exp 10 9.41 7.70 1.71 18.2% 1.01 12.0% 210 0.009 0.0098

Table 3 summarises different type of RO experiments conducted during this study. It only includes experiments which had their fouled membranes

stored for the CLSM analysis. The table shows the initially recorded flux (LMH), the final recorded flux (LMH), the reduction of the fluxes, the membrane

resistance, and the total hydraulic resistance in each performed experiment. It also demonstrates the estimated thickness of each fouled membrane and

the approximation of weight and density of the substances deposited on the membrane surfaces. As illustrated, the initial flux starts between 9.27 to

10.71 LMH, mainly depending on the resistance of the used membrane. Higher membrane resistance in experiment 9 (1.03 x 1015m-1) has resulted in a

lowest initial flux (9.27 LMH) for the same experiment. In contrast, the lowest measured membrane resistance (0.90 x 1015m-1) in experiment 5 has led to

the highest initial flux (10.71 LMH) for the same experiment. Experiment 7 (DI water+ pullulan) recorded the lowest flux reduction (0.73 LMH or 7.3%) in

46

opposed to experiment 9 (DI water+ salt+ alginate) which expressed the highest flux reduction

between all experiments (3.29 LMH or 35.4%). The lowest total resistance occurred in baseline

experiment, where NaCl and CaCl2 were used, due to the reason that the accumulation of salt

molecules at the membrane surface has an insignificant influence on total resistance of the

system. On the other hand, in the experiment where salt and alginate were used in combination,

the highest total resistance was recorded. Based on Darcy’s Law used in the calculation, the

permeate flux inversely proportional to the total hydraulic resistance. Therefore, it was expected

that experiment 7 with the lowest permeate flux reduction (7.3%) would have the least total

resistances. The total resistance of the system also depends on the resistance of the membrane,

the thickness of the fouling layer, and the amount and density of the deposited substances on the

membrane surface. Baseline experiment (Experiment 2) recorded the lowest fouling layer

thickness (47 µm), the least deposit of salt (0.026 g), and the highest density (0.1319 g/cm3)

among all RO experiments. The interaction of salt and alginate not only resulted in the greatest

flux reduction (35.4%) and the highest total resistance (27.9%) among all experiments but, it also

resulted in a formation of a thick (620 µm), heavy (0.095 g), and relatively dense (0.0367 g/cm3)

fouling layer on the membrane surface. The correlation between these findings support are

consistent and support the previous studies on the salt and alginate interaction in membrane

separation processes. Conversely, the interaction of salt and pullulan (Experiment 10) has

resulted in a relatively thin (210 µm), light (0.009 g), and a very sparse (0.0098 g/cm3) fouling

layer on the surface of the membrane. Pullulan in combination with salt seems to enhance the

weight and the density of the fouling layer based on the calculation. In general, alginate results in

more fouling on the membrane surface compared with pullulan.

47

6. Conclusion

This study aimed to investigate the behaviour of two types of polysaccharides, alginate and

pullulan, in RO processes. Their individual effects, as well as their effects in the presence of

sodium chloride and calcium chloride, were analysed and compared.

Following the analysis of the results obtained from the performed experiments by using the

developed model, it was found that concentration polarisation due to the accumulation of salt

ions at the surface of RO membranes slightly affected the overall efficiency of the system by

increasing the hydraulic resistances and reducing the permeate flux.

Furthermore, pullulan fouling insignificantly reduced the permeate flux, slightly increased the

total hydraulic resistance in the system, and ultimately had a minor effect on the efficiency of the

system. The total hydraulic resistance was further improved when pullulan interacted with the

salt. As it was clear from the conducted experiments that increase of hydraulic resistances leads

to a major reduction of system efficiency, it is concluded that pullulan enhances the efficiency in

RO systems.

In contrast, alginate fouling of RO membranes resulted in more deficiency in the system due to

higher flux reduction and greater hydraulic resistances. In the presence of sodium and calcium

ions, alginate had much greater effect on fouling. The severe alginate fouling in the presence of

salt resulted in a formation of a thick, dense, and heavy fouling layer at the surface of the

membrane. The estimated fouling thickness using CLSM analysis and the weight estimation

methodology of fouled membranes supported these findings. It is concluded that alginate causes

more fouling and higher deficiency in RO processes in comparison to pullulan particularly when it

is mixed with sodium chloride and calcium chloride. These findings indicate that alginate has a

much stronger tendency to bond with sodium and calcium ions, compared to pullulan, resulting in

a formation of a thicker fouling layer on the membrane surface.

48

Variation in the applied cross-flow velocities during each experiment had little effect on the

extent of the resulted fouling. As the cross-flow velocity increased during the experiments, higher

flux decline and greater hydraulic resistance were observed.

By analysing the Confocal Laser Scanning Microscopy (CLSM) images, it was found that the thicker

the fouling layer is, the greater hydraulic resistance in the system becomes. For instance, in the

experiment where salt interacted with alginate, the estimated fouling layer thickness was closely

correlated to the hydraulic resistance in the system.

The findings in this study provide an indication of improving the membrane fouling in RO

processes.

7. Future Research Opportunities

After a comprehensive study on the behaviour of membrane fouling in RO system using salt

including NaCl and CaCl2, polysaccharides including alginate and pullulan, and the interaction of

salt and polysaccharides, the following research areas have been identified for further study:

The interaction of salt and alginate causes a major deterioration of the efficiency in RO

systems. Therefore, research on ways to improve or at least maintain the same efficiency

in RO system is vital. In particular, research on reducing the salt concentration before

using RO treatment through pre-treatment processes can help with improvement of

membrane fouling in RO system.

Forward Osmosis system is a growing technology which has been used recently for water

purification purposes. Similar to RO system, forward osmosis uses a semi-permeable

membrane to treat contaminated water (HTI 2010). The performed experiments in this

study can be reapplied to forward osmosis system with identical experimental conditions

and chemical parameters so a comparison between the two technologies can be made.

49

Previous studies have found that the divalent calcium cation in the presence of alginate

intensifies the formation of the fouling layer on the membrane surface. Since a mixture of

sodium chloride and calcium chloride was used in this study, the effect of sodium ion on

membrane fouling is ambiguous. In future, sodium chloride can be used solely so the

effect of sodium on membrane fouling can be further investigated.

Factors such as membrane type, applied pressure, pH, temperature, cross-flow velocity

and chemical concentration play an important part on the severity of the produced

fouling layer on the membrane surface. Conducting the same experiments with different

variables can provide answers on applying optimal conditions in RO processes to prevent

worsening the membrane fouling.

Magnesium ions are more abundant in seawater and according to previous studies; they

can trigger the gel formation on the surface of the membrane once they are mixed with

polysaccharides. On the other hand, xanthan is an important polysaccharide used in

membrane fouling research. It would be motivating to study the behaviour of these

chemicals both individually and when they are mixed together on the membrane fouling.

50

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54

9. Appendices:

Appendix A: Gantt chart

21

91

12

42

1425

121

4326

71

31

4614

101

38

45

22-Feb 7-Mar 21-Mar 4-Apr 18-Apr 2-May 16-May 30-May 13-Jun 27-Jun

Review & confirm thesis topicOrganise meeting with Supervisors for Topic Brief

Conduct Preliminary researchAttend the Safety in Research & Teaching Induction

Laboratory introduction & system design familiarisationDesign & conduct preliminary experiments

Systematic calculation method to obtain experiments result (ongoing)RAMP form submission

Conduct in depth researchCreate Gantt Chart

Start project plan write upDraft Project Plan Submission

Draft review and editFinalised Project Plan Submission

Design and conduct experiments (ongoing)Conduct Further research

Start Progress Report write upDraft Progress Report Submission

Draft review and editFinalised Progress Report Submission

Start Final Report Write UpData Compilation and analysis

Compile Draft Final ReportDraft Final Report Submission

Draft review and editPresentation Preparation

PresentationFinal Report Amendment

Final Report Submission

Date (2016)

Task

s

Gantt Chart

55

Appendix B: HTI OsMEMTM CTA-NW Membrane Specification Sheet

Features:

The OsMemTM CTA-NW Membrane is HTI’s fouling resistant and most chlorine resistant FO membrane cast on a weldable nonwoven support.

The OsMemTM CTA-NW Membrane is used in all hydration pouches (HydroPack, LifePack, X-Pack, etc.).

The OsMemTM CTA-NW Membrane is cast on 40’’ (1-m) wide rolls and “dried,” where vegetable-based glycerine replaces the water.

The OsMemTM CTA-NW Membrane coupons are shipped “dry,” where vegetable-based glycerine replaces the water

Typical FO Performance (Rejection Layer Contacting Feed):

Water Permeation : 2.4GFD (gallons per square foot each day) (4.0 LMH – litres per square meter each hour)

Salt Rejection: 99% as defined in Test Conditions Test Conditions:

Feed: 1 gpm (4 1 pm) tap water feed at 77F (25C) fed at the bottom into a 4” (100 mm) by 0.2 “ (5 mm) open channel with an initial volume of 0.40 gal (1.5 L) and an exit pressure of 5 psi (35 kPa).

Draw: 7 gph (26 lph) 1 M NaCl (58.5 g/L) at the bottom at 2 psi (15 kPa) feed into a 4” (100 mm) by 0.055” (1.4 mm) channel of two 30-mil (0.76 mm) diamond-type polypropylene feed spacers (strands spaced at 11 strands per inch (25.4 mm)) with an initial volume of 0.13 gal (0.5 L)

Membrane Area: 0.22 ft2 (0.020 m2) Typical uPRO* Performance (Rejection Layer Contacting Draw Solution):

Water Permeation: 5.3 GFD (gallons per square foot each day) (9.0 LMH – litres per square meter each hour)

Salt Rejection: 99% as defined in Test Conditions Test Conditions:

Feed: 7 gpm (26 1 pm) tap water at the bottom at 2 psi (15 kPa) feed into a 4” (100 mm) by 0.055” (1.4 mm) channel of two 30-mil (0.76 mm) diamond-type polypropylene feed spacers (strands spaced at 11 strands per inch (25.4 mm)) with an initial volume of 0.26 gal (1.0 L).

Draw: 7 gph (26 lph) 1 M NaCl (58.5 g/L) at 77F (25C) fed at the bottom into a 4” (100 mm) by 0.2” (5 mm) open channel with an initial volume of 0.2 gal (0.8 L) and an exit pressure of 5 psi (35kPa).

Membrane Area: 0.22 ft2 (0.020 m2)

Rejection: {1 – [(mol NaCl transferred to feed )/(L water removed)/(1 M]}

*uPRO: unpressurized Pressure Retarded Osmosis membrane orientation Operating Limits and Guildlines:

Membrane Requirements: Membrane coupons are shipped in glycerine. Should be soaked in water for 30 minutes prior to use. After glycerine extraction, membrane must be kept moist at all times. Do not allow to freeze. Exercise care in handling

Membrane Type: Cellulose Triacetate (CTA) on heat- or RF-weldable nonwoven support

Maximum Operation Temperature: 160F (71C)

Maximum Transmembrane Pressure: 10 psi (70 kPa), if supported

pH Range: 3 to 8

Maximum Chlorine: 2ppm

Cleaning Guidelines: use only cleaning chemicals approved for CA/CTA RO membranes

56

Storage Guidelines: Store out of direct sunlight with a couple of mL of water FO Membrane Notes The membrane in initially cast on rolls. On a roll, the rejection layer is to the inside of the roll is the shiny side away from the nonwoven backing. FO membranes behave similarly to RO membranes in that dissolved gases are not rejected well. Their ions are rejected, but the (often small) fraction that exists as a dissolved gas is not rejected. Small polar, water-soluble organics, such as urea, methanol, and ethanol, are also not rejected well. Brief Start-up Description If the process is being run with the draw solution contacting the rejection layer (uPRO), make sure that there is water in the cell on the supported side to draw from. Start the pump on the unsupported side. Adjust the flowrate with the inlet valve and the exit pressure to 5 psi (35 kPa) with the exit valve. Start the side with the membrane support and adjust to the desired inlet pressure of 2 psi (15 kPa). Monitor volume or weight changes, temperature, and concentrations with time. Brief Shutdown descriptions Turn off the pumps and drain the high osmotic pressure solution first. Then drain the low osmotic pressure solution. Rising is recommended. The membrane can be stored in the cell – preferably drained.

Source: OsMemTM membrane specification sheet

Appendix C: Performed RO experiments

Laboratory-scale Reverse Osmosis Experiments

Category Description Experiments Start Date End Date

Control Experiments

DI Water + Salt

Exp 1 31-03-2016 01-04-2016

Exp 2 04-04-2016 05-04-2016

Exp 3 07-04-2016 08-04-2016

Fouling Experiments Without salt

DI Water + Alginate Exp 4 11-04-2016 12-04-2016

Exp 5 14-04-2016 15-04-2016

DI Water + Pullulan (old)

Exp 6 18-04-2016 19-04-2016

Exp 7 21-04-2016 22-04-2016

DI Water + Pullulan (new)

Exp 12 09-05-2016 10-05-2016

Fouling Experiments in the Presence of Salt

DI Water + Salt + Alginate

Exp 8 25-04-2016 26-04-2016

Exp 9 28-04-2016 29-04-2016

DI Water + Salt + Pullulan (new)

Exp 10 01-05-2016 03-05-2016

Exp 11 03-05-2016 04-05-2016

57

Appendix D: TOC results

58

Appendix E: Estimation of the weight of produced fouling layers

Appendix F: Results of “The Standard Curve” method

Sample number FO/RO Experiment type

Glass weight, W1

(g)

Glass + fresh membrane

weight, W2 (g)

Glass + sample,

W3 (g)

Fresh membrane

weight, W2 - W1

Sample membrane

weight, W3 - W1

Gross fouling layer weight,

(W3 - W1) - (W2 - W1)

1 FO Baseline 0.194 0.248 0.255 0.048 0.061 0.013

Trial 1 0.197 0.248 0.253 0.048 0.056 0.008

Trial 2 0.2 0.248 0.261 0.048 0.061 0.013

3 FO Salt + alginate 0.196 0.248 0.264 0.048 0.068 0.02

4 FO Xanthan 0.198 0.248 0.269 0.048 0.071 0.023

5 FO Salt + xanthan 0.196 0.248 0.283 0.048 0.087 0.039

6 FO Pullulan 0.197 0.248 0.258 0.048 0.061 0.013

7 RO Baseline 0.192 0.248 0.243 0.048 0.051 0.003

8 RO Alginate 0.18 0.248 0.232 0.048 0.052 0.004

9 RO Pullulan 0.201 0.248 0.252 0.048 0.051 0.003

10 RO Salt + alginate 0.2 0.248 0.259 0.048 0.059 0.011

11 RO Salt + pullulan 0.195 0.248 0.244 0.048 0.049 0.001

12 - Fresh membrane 0.2 0.248 - 0.048 - -

AlginateFO2

y = 0.0168xR² = 0.9951

0

10

20

30

40

50

60

0 500 1000 1500 2000 2500 3000 3500

Co

nce

ntr

atio

n (

mM

)

Conductivity (us)

Standard Curve of Salt Concentration Vs Conductivity

59

The R2 value in above figure is greater than 0.995 which indicates a close correlation between concentration polarisation and the measured conductivity. The relationship between concentration polarisation and conductivity has been illustrated by the following equation.

𝑦 = 0.0168𝑥

Where x is the measured conductivity and y is the concentration of salt in the solution.

Appendix G: Comparison of permeate fluxes with highest and lowest membrane resistances

5

6

7

8

9

10

11

12

13

0 1 2 3 4 5 6 7 8 9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

Pe

rme

ate

Flu

x (L

MH

)

Time (h)

Alginate fouling experiemtns in the presence of salt

lowest Rm

Highest Rm