Pre-Treatment of Wastewater Effluent for Seawater/Wastewater Reverse Electrodialysis Application

PRE-TREATMENT OF WASTEWATER EFFLUENT FOR SEAWATER/WASTEWATER REVERSE ELECTRODIALYSIS APPLICATION

Aantal woorden: 23,088

Tom Van Vooren Stamnummer: 0120 4132

Promotor: Prof. Dr. Ir. Arne Verliefde Tutor: Dr. Ir. Marjolein Vanoppen

Masterproef voorgelegd voor het behalen van de graad Master of Science in de bio- ingenieurswetenschappen: milieutechnologie

Academiejaar: 2016 - 2017

ACKNOWLEGDEMENTS

Er is niemand die het meer verdient om bedankt te worden dan Marjolein, de rijzende ster van het Vlaamse televisielandschap. Je was er als het nodig was, als ik een kortstondig dipje had of als er weer eens iets was misgelopen. Je kon oplossingen aanreiken als ik op het punt stond om de stacks uit het raam te gooien. Maar je was er ook om samen te lachen in het labo en om je muzieksmaak op te dringen. Het is nogal een cliché, maar ik meen het als ik zeg dat ik geen betere tutor kon wensen. Bedankt.

Uiteraard kan de grote baas niet vergeten worden in een dankwoordje. Arne, ondanks je overvolle agenda kon ik wel bij je bureau aankloppen als de ideeën even uitgeput waren. Dan maakte je even tijd, vertelde een paar flauwe moppen en gaf mij advies zodat ik weer verder kon. Bedankt om mij te helpen als het nodig was.

A special shout out to my international crew! Gosia, thanks a lot for helping me out with the stack assembling and the analyses. Leo, your experience and knowledge were very useful, but I am most grateful for the life lessons you told me during one of our trips to the wastewater treatment plant. Lingshan, you helped me reassembling the stacks, and although we sometimes were desperately looking for solutions to fix the leakages, you never lost your smile when we had to try again. And again. And again. And again.

Aan iedereen van Paint en Isofys, bedankt voor de fantastische sfeer. Bedankt voor de hulp in het labo. Bedankt voor de feestjes en de avonden in de Koepuur. Bedankt om geen beeldmateriaal te lekken van de feestjes en de avonden in de Koepuur.

Medethesissers, jullie waren een ongelooflijk leuke groep om een gans jaar mee op te trekken. Of het nu op een van de feestjes was, of gewoon in het labo, we hebben veel plezier gemaakt, wat het labowerk een stuk aangenamer maakte. Sommigen onder jullie kende ik nog amper, maar ik ben toch blij dat daar verandering is in gekomen. Astrid, bedankt om de vele uren in lab 3 op te vrolijken, maar vooral voor zoveel meer de afgelopen jaren. We hebben onze momenten gehad, en zeggen dat we het mekaar niet altijd makkelijk maken is een understatement, maar ik ben toch vooral blij dat ik je nu tot mijn beste vrienden kan rekenen.

Dit lijkt mij ook het geschikte moment om mijn ouders bedanken voor de steun in de afgelopen 5 jaar. Het ging niet altijd even vlot, en waarschijnlijk had mama vaak meer examenstress dan mij, maar jullie waren er wel om mij te steunen en dat betekent veel voor mij.

Afsluiten kan natuurlijk alleen met Lotte. Bedankt om al twee jaar lang zoveel geduld met mij te hebben. Bedankt om mijn crisismomentjes te aanvaarden. Bedankt om mij te steunen en om er altijd te zijn voor mij.

CONTENT

I. GENERAL INTRODUCTION ...... 1

II. LITERATURE REVIEW...... 4

II.1. CURRENT STATUS OF RO DESALINATION ...... 4 II.1.1. Principles of the RO process ...... 4 II.1.2. RO as desalination technique ...... 5 II.1.3. Future challenges ...... 6

II.2. REVERSE ELECTRODIALYSIS ...... 9 II.2.1. Potential of salinity gradient energy ...... 9 II.2.2. Components and principles of an RED system ...... 10 II.2.3. Ion exchange membranes...... 12 II.2.4. RED performance & design parameters ...... 18

II.3. MEMBRANE FOULING ...... 21 II.3.1. Fouling in membrane processes ...... 21 II.3.2. Possible anti-fouling strategies ...... 24

III. OBJECTIVES OF THIS THESIS ...... 27

IV. MATERIALS AND METHODS ...... 29

IV.1. Experimental approach ...... 29 IV.1.1. Overview of the experiments ...... 30

IV.2. Design, operation and follow-up of RED stacks ...... 30 IV.2.1. Design and components of the RED stack ...... 30 IV.2.2. Operational approach...... 32 IV.2.3. Cleaning of the stacks during the experimental run ...... 32 IV.2.4. Monitoring of voltage, pressure drop and mixing extent ...... 33

IV.3. Pre-treatment methods for wastewater effluent ...... 34 IV.3.1. Rapid sand filter ...... 35 IV.3.2. 100 µm filter ...... 35 IV.3.3. River bank filter ...... 36

IV.4. Feed water analysis ...... 37 IV.4.1. Ionic composition ...... 37 IV.4.2. TOC content ...... 38

IV.5. Membrane autopsy ...... 38 IV.5.1. Microscopy analysis ...... 39 IV.5.2. Chemical Analysis ...... 40

IV.6. Statistical analysis ...... 41

V. RESULTS AND DISCUSSION ...... 42

V.1. Process efficiency ...... 42 V.1.1. Feed water characteristics, pre-desalination and solute transport ...... 42 V.1.2. Operational aspects ...... 50 V.1.2.2. OCV ...... 54 V.1.2.3. Power density ...... 55

V.2. Analysis of fouling ...... 58 V.2.1. ATP analysis ...... 58 V.2.2. Carbohydrate analysis ...... 61 V.2.3. Microscopy analysis ...... 62

V.3. Economic analysis ...... 64

VI. CONCLUSION AND FUTURE PERSPECTIVES ...... 66

VI.1. Influence of wastewater pre-treatment ...... 66

VI.2. Future research ...... 68

VII. REFERENCES ...... 69

VIII. APPENDIX ...... 75

VIII.1. Protocol for confocal microscopy analysis ...... 75

VIII.2 Feed water salinity ...... 76

VIII.3 Probability of ion transport ...... 77

VII.4 Microscopy pictures of the membrane samples ...... 79

LIST OF FIGURES

Figure II.1 Overview of the osmosis and reverse osmosis processes 4 Figure II.2 A seawater reverse osmosis desalination unit 6 Figure II.3 Energy consumption in seawater reverse osmosis desalination as a function 7 of water recovery Figure II.4 Different reverse osmosis system configurations 8 Figure II.5 Calculated maximum power density for pressure retarded osmosis and 10 reverse electrodialysis Figure II.6 Principle of a reverse electrodialysis system 11 Figure II.7 Basic chemical structure of some of the most frequently used polymers for 12 ion exchange membranes Figure II.8 Power density as a function of permselectivity and membrane cell pair 16 resistance for two membrane pairs with different spacer thickness

Figure III.1 A hybrid system composed of a pre-treatment step, a reversed 27 electrodialysis step and a reverse osmosis step

Figure IV.1 Scheme of the experimental set-up 29 Figure IV.2 Time schedule of the executed fouling experiments 30 Figure IV.3 Reverse electrodialysis stack with 5 cell pairs 30 Figure IV.4 Schematic of the flow patterns in a cross-flow reverse electrodialysis stack 31 Figure IV.5 Set-up of the rapid sand filter 35 Figure IV.6 Set-up of the 100 µm filter 36 Figure IV.7 Set-up of the river bank filter 37 Figure IV.8 Scheme of the different sample locations on a membrane 39

Figure V.1 Relative increase of total organic carbon in the wastewater per batch 48 Figure V.2 Relative decrease of inorganic carbon in the wastewater per batch 49 Figure V.3 Overview of the pressure drop over the wastewater compartments as a 51 function of the time

Figure V.4 Overview of the pressure drop over the seawater compartments as a 53 function of the time Figure V.5 Stack OCV at the beginning of each new batch 54 Figure V.6 Gross power density, pumping power and net power density for the different 56 stacks from the first run Figure V.7 Gross power density, pumping power and net power density for the different 58 stacks from the second run Figure V.8 ATP concentrations at the influent, middle and effluent of the wastewater 59 and seawater compartment Figure V.9 Spacers from the three wastewater compartments and one seawater 60 compartment (as an example) of the stacks running in parallel from the first run Figure V.10 Spacers from the wastewater and seawater compartments of the two 60 stacks running in parallel during the second run Figure V.11 Carbohydrate concentration (expressed as ppm glucose equivalents) for the 62 inlet, middle and outlet of the seawater and wastewater compartments of the different stacks Figure V.12 Reference samples of virgin membranes, analysed with scanning electrode 63 microscopy and confocal microscopy

LIST OF TABLES

Table I.1 Overview of the most common desalination techniques 1

Table II.1 Indicative values for the properties of commercially available cation exchange 15 membranes and anion exchange membranes Table II.2 Typical specifications for a revere electrodialysis stack using spacers and a 21 reverse electrodialysis stack using profiled membranes Table II.3 Composition of deposits on the cation exchange membrane and the anion 23 exchange membrane after operating in a reverse electrodialysis stack fed with natural seawater and river water for 25 days

Table IV.1 Specifications of the membranes (as provided by the manufacturers) used in 31 the reverse electrodialysis stacks Table IV.2 Overview of the different flushing protocols 33 Table IV.3 Stack open circuit voltage before the start of the fouling experiments 34 Table IV.4 Consecutive regimes, as imposed by the potentiostat to the reverse 34 electrodialysis stacks

Table V.1 Average conductivity and standard deviation of the feed waters before the 43 beginning of a batch Table V.2 Average concentrations and standard deviations of the four most important 43 cations in the feed waters before the beginning of a batch Table V.3 Average temperature and standard deviation of the feed waters during both 44 runs Table V.4 Average relative change in conductivity and standard deviation of both feed 44 waters over the time span of one batch (i.e. 3 to 4 days) Table V.5 Average absolute change in conductivity and standard deviation of both feed 45 waters over the time span of one batch (i.e. 3 to 4 days)

Table V.6 Average difference and standard deviation in cation concentration in the feed 46 waters the beginning and the end of a batch, relative to the concentration at the start of the batch Table V.7 Average loss and standard deviation of Ca2+ over the time span of one batch, 47 2+ relative to the total amount of Ca in the beginning of the batch Table V.8 Ion transport, expressed in meq./L, in order to estimate the impact of uphill 47 transport on the potential power output Table V.9 The average total organic carbon increase and inorganic carbon decrease in 50 the wastewater for the different stacks

LIST OF ABBREVIATIONS

AEM anion exchange membrane ATP adenosine triphosphate CAPEX capital expenditures CC-RO closed-circle or closed-loop reverse osmosis CEM cation exchange membrane CIP cleaning in place CP concentration polarization ED electrodialysis EPS extracellular polymeric substances ERD energy recovery device ERS electrode rinse solution FCD fixed charge density FO forward osmosis ICP-OES inductively coupled plasma – optical emission spectroscopy IEC ion exchange capacity IEM ion exchange membrane MED multi-effect distillation MSF multi-stage flash MVC mechanical vapour compression OCV open circuit voltage OM organic material OPEX operating expenditures PBS phosphate-buffered saline PC polycarbonate PMMA polymehtylmethacrylaat PPO poly-(2,6-dimethyl-1,4-phenylene oxide) PRO pressure retarded osmosis PVA polyvinyl alcohol PVC polyvinylchloride RBF river bank filter

RED reverse electrodialysis RO reverse osmosis RSF rapid sand filter SD swelling degree SEM scanning electron microscopy SPEEK sulfonated polyetheretherketone SSF slow sand filter SWRO seawater reverse osmosis TDS total dissolved solids TOC total organic carbon TVC thermal vapour compression WHO World Health Organization

LIST OF SYMBOLS

푋 훾푌 - activity of ion X in solution Y

∆Gmix J free energy of mixing ∆P Pa pressure loss A m² effective membrane area aw - water activity b m width between profiled ridges cs mol/L solute concentration dX m thickness of compartment X E V potential

EAEM V potential over AEM

ECEM V potential over CEM

EOCV V open circuit voltage F 96,485 C/mol Faraday constant FCD me/L fixed charge density h m intermembrane distance i - van ‘t Hoff factor IEC me/g ion exchange capacity j A/m² current density L m average path of the feed water in the compartment L m cell length m g mass N - number of cell pairs ns mol amount of solutes nw mol amount of water

P Pa pressure

R 8.314 J/(mol∙K) universal gas constant 0.0821 L∙atm/(mol∙K)

R∆C Ω resistance due to concentration change in the bulk solution

RAEM Ω∙m² AEM resistance

RBL Ω resistance due to concentration polarization

RCEM Ω∙m² CEM resistance

Rel Ω electrode resistance

Rohmic Ω ohmic resistance

Rstack Ω stack resistance

Rtot Ω total stack resistance

SD % swelling degree T K temperature tres s residence time

U V potential V L volume v m/s velocity vaverage m/s average velocity

VC L/h concentrate flow

VF L/h feed flow

VP L/h permeate flow

Vw L/mol molar volume z - ion valence α - permselectivity

β - mask fraction δ m boundary layer ε - porosity

κX S/m conductivity of solution X

π Pa osmotic pressure atm φ m³/s volumetric flow rate

Nm - amount of membranes

Ppump W/m² pumping power

Pnet W/m² net power density

ABSTRACT

In 2016, two thirds of the global population faced severe water scarcity for at least one month. Due to an increasing global population, an ever increasing global welfare and a decreasing amount of available fresh water, the scale and impact of this problem will only increase in the near future. Research has been performed to use seawater as a cheap and abundant source for the production of potable water through desalination. Reverse osmosis (RO) is at present the most energy-efficient technology for this purpose. In large-scale applications, the RO process itself is able to perform at an energy demand that approaches the thermodynamic minimum. Therefore, the focus of development has to shift towards hybrid systems to decrease the net energy consumption even further and break the thermodynamic limit. A hybrid system consisting of a reverse electrodialysis (RED) step prior to the RO step is in this case a promising new approach. In the preceding RED step, a net transport of ions from the seawater to the fresh water can both pre-desalinate the RO feed seawater and generate power. As a fresh water feed, wastewater can be used. In that way, a low-value waste product can be applied and no valuable fresh water sources have to be depleted.

This master thesis aimed at investigating the influence of different pre-treatment options for secondary wastewater effluent on the performance of a wastewater/seawater RED system. Three different pre-treatment options were tested: a rapid sand filter (RSF), a 100 µm filter and a river bank filter (RBF). For six weeks, seawater and wastewater (whether or not pre- treated) were fed to the lab-scale RED stacks. The fouling behaviour of the different pre- treated wastewater flows and the corresponding stack performances were monitored in order to compare the efficiencies of the pre-treatment methods. After the experiments, the membranes and spacers of the RED stacks were chemically and visually analysed.

Based on the obtained data on stack performance and fouling development, the necessity of adequate wastewater pre-treatment is demonstrated. In the reference stack (fed with no pre-treated wastewater) running in parallel with the stacks fed with wastewater pre-treated by the RSF or the 100 µm filter, the pressure drop over the wastewater compartment increases much faster, resulting in a decreased net power density due to the higher pumping energy needed. The chemical and visual analyses confirm the higher degree of fouling in the reference stack. When comparing the different pre-treatment methods tested, it can be observed that the RBF does not succeed in sufficiently pre-treating the wastewater, while the performance of the 100 µm filter and the RSF seems to be quiet similar. The net power density achieved in the stack fed with wastewater pre-treated with the 100 µm filter is only a fraction higher, and no significant difference in degree of seawater pre-desalination can be observed. Considering the robustness and simplicity of the sand filtration however, this pre- treatment shows to be most promising for upscaling.

SAMENVATTING

Twee derde van de wereldbevolking werd in 2016 minstens een maand lang geconfronteerd met ernstige waterschaarste. De omvang en impact van dit probleem zal in de nabije toekomst alleen maar toenemen door een groeiende wereldpopulatie, een steeds stijgende globale welvaart en een dalende hoeveelheid beschikbaar zoet water. Zeewater kan dienen als een goedkope en overvloedig beschikbare bron om via ontzouting drinkbaar water te produceren. Vandaag de dag is omgekeerde osmose (RO) de meest energie-efficiënte technologie om zeewater te ontzouten. Voor grootschalige toepassingen nadert de energievraag van het RO-proces het thermodynamische minimum. In verdere ontwikkelingen moet de aandacht dan ook verschuiven naar hybride systemen om op die manier het energieverbruik nog verder terug te dringen en de thermodynamische limiet te doorbreken. In deze context is een hybride systeem, opgebouwd uit een omgekeerde elektrodialyse (RED)-stap voor de RO-stap een nieuwe, veelbelovende benadering. In de RED-stap voorafgaand aan RO zal er netto transport van ionen van het zeewater naar het afvalwater plaatsvinden, waardoor het zeewater verdund wordt én er energie kan worden geproduceerd. Door gebruik te maken van afvalwater als de zoet water stroom in het systeem kan een laagwaardig afvalproduct worden aangewend en moet er geen waardevol zoet water worden verbruikt.

In deze master-thesis is de invloed van verschillende voorbehandelingsmethoden voor secundair behandeld afvalwater effluent op de prestaties van een afvalwater/zeewater RED- systeem onderzocht. Drie verschillende voorbehandelingsmethoden werden getest: een snelle zandfilter (RSF), een 100 µm filter en een oeverfilter (RBF). Gedurende zes weken werd er zeewater en (al dan niet voorbehandeld) afvalwater gevoed aan een RED-systeem op labo schaal. Het vervuilingsgedrag van het (op verschillende manieren voorbehandelde) afvalwater en de overeenkomstige prestatie van het RED-systeem werden gemonitord om de efficiëntie van de verschillende voorbehandelingsmethoden te kunnen vergelijken. Na de experimenten werden de membranen en spacers chemisch en visueel geanalyseerd.

Op basis van de verkregen resultaten is duidelijk te zien dat een afdoende voorbehandeling van het afvalwater noodzakelijk is. In de referentie stack (die gevoed werd met niet voorbehandeld afvalwater en in parallel liep met de stacks die gevoed werden met voorbehandeld afvatwater) is duidelijk te zien dat de drukval over het system veel sneller stijgt, wat resulteert in een lager netto vermogen aangezien de pompenergie hoger was. De chemische en visuele analyses bevestigen de hogere mate van vervuiling in de referentie stack. Bij het vergelijken van de verschillende voorbehandelingsmethoden kan duidelijk worden vastgesteld dat de RBF er niet in slaagt het afvalwater voldoende op te zuiveren, terwijl de prestaties van de 100 µm filter en de RSF niet ver uit elkaar liggen. Het netto vermogen van de stack gevoed met afvalwater voorbehandeld met de 100 µm filter ligt maar een fractie hoger, en er is geen significant verschil waar te nemen wat betreft de ontzouting

van zeewater. De eenvoud en robuustheid van de zandfilter maken echter dat deze voorbehandelingsmethode het meest geschikt is voor upscaling.

I. GENERAL INTRODUCTION

In 2016, two thirds of the global population faced severe water scarcity for at least one month [1]. The traditional fresh water sources such as groundwater, lakes and rivers are overexploited, and as a result they are diminishing or enriched with salts and pollutants. The scale and impact of this water scarcity will only increase in the near future. In the 2016 edition of the Global Risks Report, an annual report published by the World Economic Forum, ‘Water crisis’ tops the list of ‘Global Risks of Highest Concern for the next 10 years’. Furthermore, it can easily be seen that other listed global risks such as food crisis and social instability are strongly related to the problem of a mismatch between water supply and demand [2].

The future approach of this problem has to be twofold: a more well thought out, sustainable use and reuse of the available fresh water, combined with the search for new, alternative fresh water sources. Since 97% of the earth’s water can be found in seas and oceans, significant amounts of research have been performed to use this gigantic potential and to turn seawater into potable water through desalination techniques [3]. Desalination is a general term that unites all processes that remove salts from water in view of producing fresh water (Table I.1). Seawater, on average, contains between 30,000 and 45,000 mg TDS/L, while fresh water only contains less than 1,000 mg TDS/L. For potable water, the standard is evidently even more severe with values as low as 500 to 250 mg TDS/L, depending on the ruling legislation [6].

Table I.1 Overview of the most common desalination techniques, with Multi-Stage Flash (MSF), Multi-Effect Distillation (MED) and Thermal Vapour Compression (TVC) the main thermal techniques and Mechanical Vapour Compression (MVC) and Reverse Osmosis (RO) the mechanical techniques [9]

Separation Energy Use Process Desalination Method Water from Salts Evaporation Multi-Stage Flash (MSF)

Multi-Effect Distillation (MED)

Thermal Vapour Compression (TVC)

Solar Distillation (SD) Crystallisation Freezing (FR) Gas Hydrate Processes (GH) Filtration/Evaporation Membrane Distillation (MD) Mechanical Evaporation Mechanical Vapour Compression (MVC) Filtration Reverse Osmosis (RO) Salts from Water Electrical Selective Filtration Electrodialysis (ED) Chemical Exchange Ion Exchange (IE)

The first commercial desalination plants opened in the 1950s and used heat to vaporize water. Countries in the Middle East were pioneers regarding design and implementation of this thermal desalination, first by applying a process called multi-effect distillation (MED) and later with multi-stage flash distillation (MSF) [7]. During the 1960s, research was done into membrane techniques to desalinate water, and soon the first membrane- based commercial plants opened [4]. Nevertheless, thermal techniques remained the most applied techniques for a long time: they were more reliable, there was less need for pre-treatment, the operational requirements were simple and the production capacity was higher [9]. The higher energy cost for the thermal processes was no issue because of the wealth of energy in these Middle Eastern countries at that time.

Since the 1970s, the worldwide desalination capacity has increased exponentially, up to 86.8 million m³ per day in 2016, an increase of more than 100% compared to 2005 and more than 800% compared to 1980 [4,10]. This increase can be explained by the combination of an increased socio-economical pull (the increased worldwide fresh water scarcity) and a developed and developing technology that can push the production to a higher level (more efficient processes, increased membrane quality, reduced costs, etc.). Within the global desalination field, a shift has been going on towards membrane desalination at the expense of thermal desalination, since the membrane processes require less energy [9]. In 2009, thermal and mechanical processes had an equal share in the global desalination capacity [4]. Even in the Middle East, the cradle of thermal desalination processes, more and more effort has been put into the use of membranes, although they are lagging behind in this shift when compared to other regions [14].

In a future where energy supply will be one of the biggest challenges for societies all over the world, membrane technologies in general and reverse osmosis (RO) in specific will only strengthen their position as the most suitable desalination technique. Thermal desalination techniques use on average between 18 and 40 kWh/(m³ of product), even when reusing the invested energy by working in different stages [11]. Although this energy consumption can be decreased in double-purpose plants, where heat from turbine condensing steam is used as a thermal energy source, RO is much less energy demanding. In large scale RO operational plants, the total energy consumption can be as low as 3 to 6 kWh/(m³ of product) [11]. Besides the lower energy consumption, improvements in both membrane and process performance have led to an improved permeate quality. The greatest efficiency gains have arisen from improved membrane performances due to modifications in structure, material and morphology. This has led to an increased permeability, a decreased cost per volume ratio and an increased permeate quality. The salt passage has decreased to beneath 0.2%, which is seven times lower compared to 1978 [13].

Nevertheless, the further development of RO desalination is challenging on several fronts. The management of the produced brine is an issue since uncontrolled discharge

holds a potential ecological risk and the chemical energy stored in the brine gets lost [15]. Besides this challenge, the energy demand is still a key aspect for future development. Since the RO process itself is able to perform at an energy demand that approaches the theoretical thermodynamic minimum (in large-scale applications), the focus of development has to shift towards hybrid systems to decrease the net energy consumption even further [11]. In classical reverse electrodialysis (RED), seawater and fresh water are brought together in different compartments only separated by ion exchange membranes (IEM). Energy can be gained from the difference in salinity between both feed streams, while at the same time the seawater will get diluted. Applying an RED step before desalinating the seawater via RO can thus reduce the overall energy demand both by producing energy and by decreasing the salinity of the seawater. In this type of hybrid system, it is theoretically possible to decrease the energy demand of seawater desalination to the point of energy neutrality [15].

In this master thesis, a next step towards the implementation of a successful RED-RO hybrid system will be investigated. By applying wastewater as the necessary fresh water source in RED, a waste product with limited reuse perspectives can be used and no valuable fresh water sources have to be depleted. However, using wastewater implies the need for pre-treatment in order to ensure a satisfying RED stack performance and an acceptable membrane lifetime. Therefore, different wastewater pre-treatment options will be examined on their performance and feasibility to be applied before RED.

II. LITERATURE REVIEW

II.1. CURRENT STATUS OF RO DESALINATION

II.1.1. Principles of the RO process

Osmosis is the natural movement of a solvent through a semipermeable membrane from the side with a low concentration of solutes to the side with a higher concentration of solutes (Figure II.1a). The driving force for this movement is the existing gradient in chemical potential, and the solvent will tend to migrate until an equilibrium is reached (Figure II.1b). By applying a pressure on the side with the highest concentration of solutes (and thus the highest chemical potential), the solvent can be forced to move against the concentration gradient through the semipermeable membrane (i.e. from a high to a low concentration) (Figure II.1c). This technique is called RO and is mostly applied in the desalination of salty water, but it is also used to produce ultrapure water, as a wastewater treatment step and in several industries such as food and beverage processing and in chemical industries as a separation or concentration technique [16,19].

Figure II.1 Overview of the osmosis and reverse osmosis (RO) processes. (a) Osmosis: a pure solvent (left) and its solution containing a non-volatile solute (right) are separated by a semipermeable membrane through which solvent molecules can pass but solute molecules cannot. The rate of solvent transfer is greater from solvent to solution than vice versa. (b) The system is at equilibrium, there is not net solvent transfer. (c) RO: a pressure greater than the osmotic pressure of the solution is applied which causes a net flow of solvent molecule [23]

In RO desalination, the water is separated from dissolved solutes (including the monovalent ions such as Na+ and Cl-) via a semipermeable membrane that favours the passage of water. All kinds of impurities such as dissolved solids, organics, submicron colloidal matter, colour, nitrate and bacteria are retained by the membrane [20]. The mass transfer through the membrane can be described according to the solution- diffusion model. Permeants dissolve in the membrane material and then diffuse through the membrane. Since RO membranes are hydrophilic and water molecules are very small, water will be able to readily diffuse to the other side [21].

As stated above, a pressure has to be applied on the feed side to impose the water to permeate trough the membrane against the concentration gradient (Figure II.1c). The minimal pressure to apply depends on the difference between the salt concentrations at both membrane sides and can be determined based on the osmotic pressure. The latter is a property of a solution and can be seen as the tendency of the water molecules to move from a hypotonic solution to a hypertonic solution through a semipermeable membrane. Its value in dilute solutions can be calculated by the Van ‘t Hoff equation [23]:

푛푠 휋 = −푅 ∙ 푇 ∙ = 푅 ∙ 푇 ∙ 푐 ∙ 푖 (1) 푉 푠 with π the osmotic pressure (atm), R the universal gas constant (0.0821 L∙atm/(mol∙K)), T the temperature (K), ns the total amount of solutes (mol), V the solvent volume (L), cs the total concentration of solutes (mol/L) and i the Van ‘t Hoff factor (-). The latter is a correction factor for the degree of dissociation and can be calculated as the ratio between the actual and theoretical concentration of dissociated particles. For NaCl, for example, the theoretical amount of particles after dissociation is two: one Na+ cation and one Cl- anion. Because of ion pairing, a phenomena were ions of opposite electrical charge form a pair, there are less particles after dissociation and the i-value for NaCl in practice is only approximately 1.9 [23]. For seawater with an average salt concentration (35 g NaCl/L), the osmotic pressure is approximately 27 atm.

II.1.2. RO as desalination technique

In a seawater reverse osmosis (SWRO) desalination unit (Figure II.2), the feed flow (VF) is brought from its initial pressure (P0) to a high pressure (PH) with a pump that introduces an energy equal to VF∙(PH - P0) in the system. The energy that is introduced by the pump can be split between the energy needed to bring the seawater to a pressure equal to the osmotic pressure (π), and the energy to create the overpressure (PH – π) needed to overcome frictional losses when the water moves through the membrane and to generate reasonable water fluxes. As long as the overpressure is positive (PH - π > 0, or

PH > π), the water will permeate through the membrane.

Figure II.2 A seawater reverse osmosis desalination unit, with VF the feed flow, VC the concentrate flow, FP the permeate flow and P0 and PH respectively the initial pressure and the high pressure [18] Over the past decades, improvements in several aspects of the SWRO desalination process have led to a better performance. Continuous technical development regarding higher-permeable membranes, energy recovery devices, pump efficiencies and other aspects made it possible to decrease the energy consumption significantly [16]. In a controlled pilot-scale experiment on SWRO desalination, an energy consumption rate as low as 1.58 kWh/m³ was reached with a commercially viable recovery (42%) and flux rate (10.2 L/m²∙h) [17]. The theoretical minimal required energy depends on the salinity of the seawater and is in fact the opposite of the free energy of mixing [18]:

−푑(∆퐺푚𝑖푥) = 푅 ∙ 푇 ∙ 푙푛푎푤 ∙ 푑푛푤 = 휋 ∙ 푉푤 ∙ 푑푛푤 (2)

with ∆Gmix the free energy of mixing (J), R the universal gas constant (8.314 J/(mol∙K)), T the temperature (K), aw the activity of water (-), nw number of moles of water (mol), π the osmotic pressure of the water (Pa) and Vw the molar volume of water (L/mol). For seawater with 35,000 ppm salt and at a recovery rate of 50%, the theoretical minimal required energy is with a value of 1.06 kWh/m³ only a fraction lower than the achieved efficiency in the pilot-scale experiment [17]. In practice, these lower limit values will never be reached since a plant is finite in size and operation at thermodynamically reversible conditions will be impractical. Still, RO is at present clearly the most energy- efficient technology for seawater desalination and can be seen as the benchmark for other desalination technologies [18]. For modern SWRO plants, an additional energy cost of more than 1 kWh/m³ should be taken into account for the intake, pre-treatment, posttreatment and brine discharge stages [16].

II.1.3. Future challenges

II.1.3.1. Concentrate and permeate quality

Although membrane desalination in general and RO in specific have been the most applied desalination processes for a while, severe challenges are still to be solved in future development. There are several concerns on adverse environmental impact related to the management of the produced brine. In RO applications, the salt concentration of the brine can be twice the concentration of seawater (at a recovery

rate of 50%), which will strongly increase the effluent density up to 1050 kg/m³ [24]. Besides this increased amount of salts, the concentrate can also contain chemical additives and corrosion products. Antiscalants, antifouling products (often containing Cl- derivatives) and other chemicals such as coagulants may be added during different steps of the RO process [24]. The potential adverse effects of discharging the produced brine depend both on the characteristics of the brine and the ecosystem in which it is discharged.

When it comes to permeate quality, one of the most difficult components to remove is boron, mostly present as boric acid (B(OH)3) in seawater at a concentration of 4.5 to 6 mg/L [25]. Due to its neutral character, only approximately 80% of the boron will be retained at neutral pH in RO, which will lead to a concentration in the permeate about twice the maximal concentration of 0.5 mg/L as imposed by the World Health Organisation (WHO) [26,27]. Several options such as pH adjustment, improved RO membranes, double-pass configurations and electrodeionization are already considered to increase boron removal, but so far all options still have some significant downsides when applied [19].

II.1.3.2. Energy consumption

The decrease in energy consumption is still a key aspect of future development of desalination technologies. Recently constructed SWRO desalination plants consume between 3 and 4 kWh/m³ of energy and produce up to 1.8 kg CO2 per m³ of produced water, since thermoelectric energy is still the main power source [18]. The total energy cost can be split between the energy needed for the RO process and the energy needed for pumping, pre-treatment etc. (Figure II.3).

Figure II.3 Energy consumption in seawater reverse osmosis (RO) desalination as a function of water recovery [11] The total energy consumption shows a minimum around 50% recovery. The higher the recovery ratio, the higher the energy consumption of the RO process due to higher

pressure changes, but the lower the energy needed for pumping and pre-treatment since less water has to be supplied. It can be concluded that major energy savings cannot be expected anymore when only looking at the RO process, which as already stated above is close to its thermodynamic limit. If it would be possible to reduce RO process energy consumption with 30% (e.g. by developing membranes with higher permeability), this would only result in an overall decrease of energy demand in the order of 15% [11]. Although membrane development is useful and needed, other options to reduce the overall energy consumption might lead to a more significant change.

Energy recovery devices (ERD) have been developed to recover the hydraulic energy present in the highly pressurized brine. With pressure exchangers or a turbine system, a major part of this energy can be extracted from the brine and applied to pressurize the feed water [16]. Besides ERD, the energy consumption can also be decreased in a so- called 2-stage RO (Figure II.4b). In this system configuration, 2 high-pressure pumps and 2 membranes modules are placed in series. The first stage operates at a lower applied pressure, and only the concentrate of this first stage is pressurised to a higher level before going to the second membrane module. This can be seen as a step towards a reversible thermodynamic process (which would be reached in a theoretical configuration with an infinite number of stages) [18]. Another possible configuration is a closed-circuit or closed-loop RO (CC-RO) (Figure II.4c). Here, the brine is recycled and mixed with the feed water. As the osmotic pressure of the membrane module influent (i.e. rejected brine mixed with newly added feed water) increases with water recovery, the applied pressure can be increased. This CC-RO configuration mimics the operation of a multi-stage RO without increasing the complexity and capital cost of the installation [28]. Which system is most suited depends on the specific operation. A 1-stage RO has a lower capital cost, a 2-stage RO consumes less energy while still yielding a moderate water flux and a CC-RO has a high operational flexibility.

Figure II.4 Different reverse osmosis (RO) system configurations, with (a) 1-stage RO, (b) 2-stage RO and (c) closed- circuit RO (CC-RO), adapted from Lin & Elimelech (2017) [28] In future development, most gains can be made by combining RO with other processes in a hybrid system. A fresh water source can be used in a preceding dilution step to

decrease the salinity of the seawater and in that way decrease the net energy demand to a value below the thermodynamic limit of seawater desalination. Energy can be produced while diluting the seawater due to the salinity gradient between a fresh water and a seawater stream by means of an RED or a pressure retarded osmosis (PRO) system [15]. As a fresh water feed, wastewater can be used. In that way, a low-value waste product with limited reuse perspectives can be applied instead of depleting fresh water sources suited for drinking water production. A possible downside is the necessary pre- treatment when using wastewater as a fresh water source. The seawater has to be pre- treated either way before it can be applied in RO, but the pre-treatment of the wastewater before RED is an extra cost that has to be taken into account when considering an RED-RO hybrid system. Extensive pre-treated water will result in a better operation (longer membrane lifetime, less fouling, lower pressure drop etc.), but will also increase the pre-treatment cost. A trade-off between the benefits of a specific pre- treatment method and the extra cost that comes with the installation and operation will determine the preferable pre-treatment option.

II.2. REVERSE ELECTRODIALYSIS

II.2.1. Potential of salinity gradient energy

Energy supply, sustainability and climate change are some of biggest challenges the world has to face in the 21th century. In this context, the search for clean and renewable power sources is essential. One of these potential sources is in the mixing of fresh water and seawater. When a high-concentration saline solution is mixed with fresh water, more than 2.2 MJ of free energy is dissipated per cubic meter of fresh water, assuming a large excess of seawater is used [29]. This salinity-gradient energy, often referred to as blue energy, is theoretically available at every estuary where fresh water and seawater meet. With a global river discharge of approximately 1.3∙106 m³/s, the global potential of this energy source can be estimated around 2.8 TW [30]. When fully exploited, this blue energy could account for approximately 15% of the global energy consumption every year [31].

The two most promising techniques for energy generation from a salinity gradient are PRO and RED. In PRO, two solutions with a different salt concentration (such as seawater and wastewater) are separated by a semipermeable membrane that only allows the solvent to pass. The solvent (in most cases water) will move through the semipermeable membrane through osmosis. This movement will increase the hydraulic pressure on the seawater side, and power can be obtained by depressurizing the diluted seawater through a hydroturbine [44]. In RED on the other hand, the membranes in the system are IEM and the salt ions will move instead of the solvent. Energy can be produced by placing the IEM in between electrodes where the ion flow can be converted into an

electron flow. When calculating the theoretical maximum power density, it can be concluded that both techniques have a specific field of operation in which they are the most suited option (Figure II.5) [36]. PRO can yield a higher power density when the concentrated stream is a brine with a salt concentration above 1.5 mol/L. In case of a system with wastewater and seawater, an RED system seems more attractive since the theoretical maximum power density is higher. Besides the higher potential power density, it has been proven that IEM are generally less sensitive to fouling and thus require less pre-treatment of the feed water than the so-called forward osmosis (FO) membranes used in PRO [45].

Figure II.5 Calculated maximum power density for pressure retarded osmosis (PRO) and reverse electrodialysis (RED). The dash-dot-line represents the break-even-line: below this line RED has a higher maximum power density than PRO [36]

In the following section, an in depth view on the RED system will be given. Although RED can operate with any two solutions as long as there is a salinity gradient, focus will be on a system with seawater as the concentrated solution, and fresh water (ranging from river water to impaired water (wastewater)) as the low salinity solution.

II.2.2. Components and principles of an RED system

An RED system consists of consecutive chambers formed by IEM (Figure II.6). Two IEM form a cell pair: a repeating unit which is composed of a cation exchange membrane (CEM), a compartment filled with fresh water, an anion exchange membrane (AEM) and a compartment filled with seawater [33]. To generate an electrical current, a number of these cell pairs are stacked in parallel between a cathode and an anode, which are then connected to each other electrically.

Figure II.6 Principle of a reverse electrodialysis system (CEM = cation exchange membranes, AEM = anion exchange membranes, ERS = electrode rinse solution), adapted from Vermaas et al. (2013) [32] Fresh water and seawater flow through the separate channels in an alternating pattern. Because of the salinity gradient, an electrochemical potential difference over each membrane is created that causes ion transport through the membranes from the solution with the highest concentration to the more diluted solution. For seawater and river water, this potential difference is about 80 to 100 mV for a single membrane [34,46]. The Na+ cations permeate through the CEM towards the cathode, while the Cl- anions permeate through the AEM towards the anode. In an ideal system with perfect membranes, the IEM would only allow transfer of the counter-ions (ions with a charge opposite to that of the IEM) while blocking the transfer of co-ions (ions with the same charge as the IEM) [42]. This so-called permselectivity will be discussed with the other membrane properties in section II.2.3.2.

To generate electricity, the ion flow is converted into an electron flow at the electrodes where the electro-neutrality of the system is maintained by redox-reactions. When connecting the electrodes to each other in an external electric circuit, electrons can move from the anode to the cathode. By connecting an external energy consumer in the electric circuit, power can be generated [34]. In between the electrodes and the membranes of the outer most cell pairs, an electrode rinse solution (ERS) is recirculated to provide the ions for the redox-reactions and protect the electrodes.

II.2.3. Ion exchange membranes

A traditional RED stack consists of alternating CEM and AEM, with spacers in between. These spacers make sure that the membranes stay in a fixed position, ensure defined water compartments and increase turbulence (to improve mass transfer)[53]. The economic feasibility of an RED system is largely determined by the IEM performance. Membranes for RED applications ideally should have a low electrical resistance and a high permselectivity, combined with a long service life time and a low cost [51].

II.2.3.1. Composition and structure of IEM

IEM are composed of a polymer substrate and ion-functionalized groups attached to this backbone. Some of the most frequently used low-cost IEM polymers are polyvinyl alcohol (PVA), poly-(2,6-dimethyl-1,4-phenylene oxide) (PPO) and polyvinylchloride (PVC) (Figure II.7) [50]. These substances are often blended and undergo physical and/or chemical modification. In that way, functional groups can be introduced to allow access of the charged ionic species into the membrane and thus increase the conductivity and decrease the electrical resistance.

Figure II.7 Basic chemical structure of some of the most frequently used polymers for ion exchange membranes, with (a) polyvinyl alcohol, (b) poly-(2,6-dimethyl-1,4-phenylene oxide) and (c) polyvinylchloride [54]

Depending on the type of ionic group, IEM can be classified into CEM and AEM. CEM - 2- contain negatively charged groups such as sulfonic acid (SO3 ), phosphoric acid (PO3 ) - + and/or carboxylic acid groups (RCOO ). Quaternary ammonium cations (NR4 ), + + imidazole cations (C3N2H5 ) and guanidinium cations (CH6N3 ) are some of the most frequently used functional groups for AEM [71]. When an IEM is in contact with an electrolyte, ions with the same charge as the fixed ion groups are retained (so called co- ions), while the ions with the opposite charge (so-called counter-ions) will be able to pass through the membrane. This phenomenon is called Donnan exclusion [46].

A distinction can be made between homogenous IEM, when the fixed charge groups are evenly distributed over the entire membrane matrix, and heterogeneous IEM, which have uncharged polymer domains in the membrane matrix. Heterogeneous IEM are on average cheaper, but only reach a maximum power density of 1.5 W/m² in RED

applications. Homogenous IEM can reach power densities up to 3 to 5 W/m² due to a lower membrane resistance [46].

II.2.3.2. Membrane properties

The specific properties of an IEM can be related to the amount, type and distribution of the charged groups on the membrane and are directly affected by the structure, preparation procedure and chemical composition of the membrane [46,50]. In general, IEM in an electrochemical system such as RED are characterised based on their stability, permselectivity, ion exchange capacity (IEC), swelling degree (SD), fixed charge density (FCD), and electrical resistance.

i) Stability

The thermal, chemical and mechanical stability can be seen as indicators of the durability of the membrane. The thermal stability is influenced by characteristics such as the degree of crosslinking, the properties of the inert polymers and the used reinforcing fabric [50]. Feldheim et al. (2015) also showed an inverse relationship between counter- ion size and thermal stability due to an initial decomposition reaction which is strongly influenced by the polymer-counter-ion interaction [62]. The chemical stability is a measure of the membrane resistance against deterioration by acids, bases, solvents and oxidation. The mechanical strength, strongly influenced by the degree of crosslinking, is necessary to prevent deformation under operational pressures.

Overall, these requirements are rarely challenging in RED due to moderate operational conditions. The common temperature is generally around room temperature with a modest seasonal variation. The membranes do not have to withstand high osmotic pressure differences since the concentration difference of salts is just a bit over an order of magnitude (approximately 0.01 mol/L in fresh water and 0.50 mol/L in seawater). The feeding solutions have a pH close to neutral and the dissociation of water near the electrodes is limited to a negligible extent [50]. As will be discussed later, other priorities can be chosen when designing IEM intended for RED applications.

ii) Permselectivity

The permselectivity (α) reflects the ability of a membrane to distinguish between co-ions and counter-ions. When the permselectivity is high, the counter-ions will be able to transfer through the membrane while co-ions are obstructed. The permselectivity can be seen as the ratio between the apparent potential difference and the potential difference under the same conditions with a perfectly selective/ideal membrane (i.e. α=1)[56] :

퐸 훼 = (3) 퐸𝑖푑푒푎푙

iii) IEC, SD and FCD

The IEC represents the amount of fixed charges inside the IEM per unit of membrane dry weight [46,50]. IEC is expressed as milli-equivalents of fixed groups per gram of dry membrane (meq/g) and it is a crucial characteristic that influences almost all other membrane properties.

푐푡표푡푎푙 퐼퐸퐶 = (4) 푚푑푟푦

The SD is the water content or amount of water uptake and can be calculated as the ratio of the mass of the wet membrane and the membrane dry weight [62].

푚푤푒푡 − 푚푑푟푦 푆퐷 = ∙ 100% (5) 푚푑푟푦

The FCD, expressed in milli-equivalents of fixed groups per volume of water in the membrane (mes/l), depicts the effect of SD on IEC.

퐼퐸퐶 (6) 퐹퐶퐷 = 푆퐷

IEC and permselectivity are positively correlated, but swelling tends to dilute the concentration of the charged groups. Since FCD takes into account both of these membrane characteristics, this is the preferred parameter to characterize IEM.

iv) Electrical resistance

The electrical resistance is a crucial property since it is directly related to the maximum net power output and the energy consumption of an RED process. It is determined by the IEC and the mobility of the ions in the membrane matrix. The resistance is inversely related to temperature and in general, heterogeneous IEM have a higher membrane resistance due to the regions of uncharged polymer matrix structures [46]. In general, less selective membranes have a lower membrane resistance, although this can vary according to the membrane type.

The membrane parameters mentioned above are all linked to each other, and deciding which parameter values are optimal is not straightforward, since they often have opposing effects. The degree of membrane swelling is determined by the type and amount of ion exchange groups, membrane structure (crosslinking and reinforcing fabrics), temperature of the electrolyte solution etc. In general, a higher IEC and a lower degree of crosslinking leads to a higher swelling degree [59]. Although water uptake (and thus swelling degree) decreases the permselectivity, increased swelling does not have to be an adverse effect since it also tends the decrease the membrane resistance,

especially for AEM. Depending on the application, a trade-off between membrane resistance and permselectivity has to be made. By increasing the volume fraction of water from 0.1 to 0.38, it was possible to decrease the ionic resistance of an AEM (with quaternary ammonium as functionalized groups) by more than 3 orders of magnitude, while the permselectivity only decreased from 0.91% to 0.85% [48]. Since the main goal of RED is to produce electricity and not the purity of the products, a moderate permselectivity can be tolerated if the gain in resistance decrease is significant. However, when RED is applied in an RED-RO hybrid system, the ion transport (and thus permselectivity) is an important feature since seawater desalination is also aimed for besides energy production.

In Table II.1, some indicative values of the membrane properties can be found, based on the characteristics of commercially available IEM.

Table II.1 Indicative values for the properties of commercially available cation exchange membranes (CEM) and anion exchange membranes (AEM). Data based on 6 commercial CEM (5 homogeneous, 1 heterogeneous) and 7 commercial AEM (6 homogeneous, 1 heterogeneous) divided over 4 manufacturers (Fumasep, Neosepta, Selemion and Ralex). (IEC = ion exchange capacitiy, SD =swelling degree, FCD = fixed charge density), Adapted from Hong et al. (2015)[50]

Permselectivity IEC SD FCD Resistance Thickness [%] [me/g] [%] [me/g] [Ω∙cm²] [µm]

homogeneous 95.3 1.63 22 7.38 2.34 122

CEM heterogeneous 94.7 2.34 31 7.55 11.33 764

homogeneous 90.08 1.528 22 6.85 2.30 115

AEM heterogeneous 89.3 1.97 56 3.52 3.00 714

II.2.3.3. Spacer influence

On Figure II.8, the theoretical relationship between membrane resistance, permselectivity and power density is shown for two membrane cell pairs with different spacer thicknesses. When the spacer thickness increases, the influence of the membrane properties becomes less important: the power density is almost completely determined by the characteristics of the dilute compartment. When a thinner spacer compartment is used, a power density up to 7 W/m² can be reached when the resistance of IEM is low and permselectivity is high. The lower limit of the spacer thickness is mostly determined by the increasing pressure drop in the channel and thus pumping costs [46].

Figure II.8 Power density as a function of permselectivity and membrane cell pair resistance for two membrane pairs with a different spacer thickness (model calculations based on artificial seawater (0.5 mol/L NaCl) and artificial riverwater (0.05 mol/L NaCl) as feed stream) [46]

Research is going on to reduce the adverse effect of spacers on the performance. An important parameter in this context is the spacer shadow effect, or the portion of the membrane masked by the (non-conductive) spacer. Optimization efforts regarding spacer design can reduce the mask fraction and thus the stack resistance and help reduce the concentration polarization (CP) effect (see section II.2.4.2). Furthermore, the use of ion conductive materials for RED spacers is considered to reduce the stack resistance. In experimental work performed by Dlugolecki et al. (2010), an RED stack with ion conductive spacers was able to reach a power density that was a factor 3 to 4 higher when compared to a stack with the same open area and shape, but with non- conductive spacers [63]. These ion conductive spacers reduce the electrical resistance, but do not reduce the hydraulic friction. As an alternative, profiled membranes can integrate spacer and membrane functionality by means of a structured surface. The membrane profile keeps the membranes separated and provides the necessary channels, thus making the use of spacers unnecessary. These profiled membranes combine a decreased internal resistance with a low hydraulic friction [40]. So far, homogeneous profiled IEM are not yet commercially available [38].

II.2.3.4. IEM development for RED applications

As mentioned before, the overall performance of an RED system is strongly influenced by the use of proper IEM. Despite that, most research and effort on system efficiency so far has gone to stack design and operational parameters, while in most RED studies commercially available IEM essentially designed for other applications such as electrodialysis (ED) were used [50]. In ED, the overall goal is to produce fresh water by separating ions from the feed flow. The IEM are designed for this purpose and a high permselectivity is the most import characteristic; the increased ionic resistance is an inevitable disadvantage. As mentioned before, in RED the ionic resistance is the main hurdle and thus should be low, even though this implies having to settle for a decrease

in permselectivity. It can easily be seen that the contradicting goals will have consequences on the optimal membrane characteristics for each application, and that the design of IEM with well-balanced permselectivity and low resistance is needed to optimize RED salinity gradient power generation [55]. The ideal membranes for RED combine a low resistance and high permselectivity with an easy manufacturing procedure at a competitive price.

Not only the functional groups, but also the backbone polymer materials used in RED IEM need further research and development. The polymers should be easily functionalized with charged groups, the preparation and production process should be as simple and manageable as possible and the polymer materials should be as cheap as possible. Blending inorganic fillers such as Al2O3 or Fe2O3 into the polymer can be a promising new approach to improve the electrochemical properties of the IEM [59]. By 2- blending nano-scale functionalized inorganic fillers (e.g. Fe2O3–SO4 ) with a functionalized polymer (e.g. sulfonated PPO), the membrane can carry more charged functional groups. Several studies can be found with examples of successful implementation of these so-called nanocomposite IEM, where the IEC, FCD and permselectivity of the membranes improved with an increasing amount of nanoparticles, although this positive effect was limited to a certain concentration of inorganic fillers [60,61].

Regarding CEM in commercial applications, perfluorinated or partially fluorinated materials are most frequently used to manufacture the polymeric backbone [50]. These membranes have a high permselectivity and IEC, but also a poor conductivity and a high cost. Non-fluorinated hydrocarbons are often considered as a low-cost alternative material in RED, especially since the high thermal, chemical and mechanical stability of fluorinated materials are not very crucial for RED applications. Güler et al. (2013) experimented with tailor-made sulfonated polyetheretherketone (SPEEK) CEM as an alternative material, and these membranes (with different sulfonation degrees) performed well in an RED operation [56].

Increasing the degree of crosslinking increases the mechanical and chemical stability of a functionalized polymer, but the use of excess crosslinking also decreases the ionic conductivity of the membranes, since it narrows the path of ion transport in the membrane [57]. Therefore, the application of tertiary diamines biquaternization during the amination of the polymers is an interesting future improvement for AEM manufacturing since it brings positively charged groups in the membrane and at the same time a cross-linking reaction takes place [58]. The type and amount of added agent determine the characteristics of the membrane, and in general an excess of diamines with a short chain length of alkyl groups is preferred to achieve high permselectivity and low membrane resistance [50].

II.2.4. RED performance & design parameters

II.2.4.1. Open circuit voltage (OCV)

The difference in salt concentration over a membrane creates an electrical potential difference across this membrane. The theoretical value of the open circuit voltage (EOCV) that is created over a whole stack, consisting of N cell pairs, can be calculated with the Nernst-Plank-equation [35]:

퐸푂퐶푉 = 푁 ∙ (퐸퐶퐸푀 + 퐸퐴퐸푀) (7)

푁푎+ 푁푎+ 퐶푙− 퐶푙− 푅 ∙ 푇 훾푠푒푎 ∙ 푐푠푒푎 푅 ∙ 푇 훾푠푒푎 ∙ 푐푠푒푎 (8) 퐸 = 푁 ∙ [훼 ∙ ∙ ln ( ) + 훼 ∙ ∙ ln ( − − )] 푂퐶푉 퐶퐸푀 푧 ∙ 퐹 푁푎+ 푁푎+ 퐴퐸푀 푧 ∙ 퐹 퐶푙 퐶푙 훾푓푟푒푠ℎ ∙ 푐푓푟푒푠ℎ 훾푓푟푒푠ℎ ∙ 푐푓푟푒푠ℎ

with EOCV the open circuit voltage (V), N the number of cell pairs (-), α the permselectivity of the membrane (-), R the universal gas constant (8.314 J/(mol∙K)), T the 푋 temperature (K), z the valence of the ions (-), F the faraday constant (96,485 C/mol), 훾푌 the activity of ion X in solution Y (-) and c the concentration at membrane-solution interface (mol/liter). This OCV is the summation of the potential over each membrane, and can be seen as the theoretical voltage corrected for the apparent permselectivity [32].

Multivalent ions will induce a lower stack voltage, especially when they are found in the dilute compartment, since they can transfer against their concentration-gradient in an ion exchange process comparable to Donnan dialysis [80]. Divalent ions, such as Mg2+, 2+ 2− Ca and SO4 , have a relative lower electrochemical potential when compared to monovalent ions since their concentration gradient over a membrane is lower. When monovalent ions are transferred towards the dilute compartment, multivalent ions can be transferred from the dilute to the saline compartment due to the potential created by the monovalent ion species. No net electrical current is produced (e.g. an exchange of two Na+ ions for one Mg2+ ion), and the salinity gradient for monovalent ions is decreased [80]. The use of monovalent-selective membranes hampers this transport.

II.2.4.2. Potential losses

The OCV can be calculated based on the concentrations of the ingoing flows and the permselectivity of the membranes. However, once an external circuit is connected and an electrical current is driven by this produced voltage, several phenomena will decrease the voltage to below the OCV [35]:

- the stack itself has an ohmic resistance, which will decrease the voltage over the electrodes - the concentrations in the bulk of the solutions will change due to ion transport from the seawater to the fresh water - the concentration in the boundary layer (i.e. at the membrane-solution interface) will be different than in the inflow, and as a result of this so-called CP the salinity gradient across the membrane will be smaller than in the bulk.

When taking into account these phenomena, the voltage over the electrodes can be calculated [48]:

푈 = 퐸푂퐶푉 − 푅푡표푡 ∙ 푗 (9)

푅푡표푡 = 푅표ℎ푚𝑖푐 + 푅∆퐶 + 푅퐵퐿 (10) with U the voltage (V), Rtot the total stack resistance (Ω∙m²), j the current density (A∙m²),

Rohmic the ohmic resistance inherent to the stack (Ω∙m²), R∆C the resistance due to the concentration change in the bulk solution (Ω∙m²) and 푅퐵퐿the resistance due to the different concentrations in the boundary layer (Ω∙m²).

The ohmic resistance of the stack can be seen as the result of the individual components all contributing to the total resistance:

푁 푅푎푒푚 푅푐푒푚 푑푐 푑푑 푅표ℎ푚𝑖푐 = ∗ ( + + + ) + 푅푒푙 (11) 퐴 1 − 훽 1 − 훽 휀² ∗ 휅푐 휀² ∗ 휅푑

with N the number of cell pairs (-), A the effective membrane area (m²), Raem the AEM resistance (Ω∙m²), Rcem the CEM resistance (Ω∙m²), d the thickness of a compartment

(m), κ the conductivity (S/m), Rel the electrode resistance (Ω∙m²), ε the membrane porosity (-), β the mask fraction (-) and the subscript c and d for respectively the concentrated and diluted solution [37].

The other two phenomena contributing to a decreased voltage are also influenced by the stack design and/or operational variables. In general, the resistance due to the change in bulk concentration increases with an increasing residence time and a decreasing water volume and thus the intermembrane distance. The resistance due to CP in the boundary layer can be decreased by a higher feed water velocity since this creates a higher mixing rate. However, a higher feed flow will increase the necessary membrane area (if a same degree of desalination has to be reached) leading to an increased capital cost.

II.2.4.3. Net power production

Defining the optimal stack design and operational parameters is not straightforward since often both positive and adverse (and thus counteracting) effects have to be taken into account. A smaller channel thickness and the use of spacers can improve the fluid dynamics, but at the same time increase the pressure drop. A reduced flow rate of the feed streams will lead to a better fluid flow distribution and a lower pressure drop, but it might also increase the CP. The net power density is used to characterize the energy obtainable in an RED stack. This parameter can be used to compare different stack designs and operational methods. To calculate the net obtained energy, the gross power production has to be corrected for the power required to pump the feed water through the stack. This pumping power is determined by the flow rate and the pressure drop over the inlet and the outlet of the feed waters [35]:

∆푃 ∙ 훷 (12) 푃 = 푝푢푚푝 퐴

With Ppump the pumping power (W/m²), ∆P the pressure loss (Pa), φ the volumetric flow rate (m³/s) and A the total membrane area (m²). In an ideal case, when the flow is laminar and fully developed in an infinitely wide uniform channel, the pressure drop can be estimated with the Darcy-Weisbach equation. In practice, the pressure drop in the stack is up to 80 times higher than this theoretical value due to the non-ideal circumstances and behaviour. The channels have a finite width, and the flow follows a non-uniform path, especially at the in- and outlet of the stack [41].

The net power density Pnet (W/m²) can be determined by calculating the generated power and subtracting the power spent on pumping [35]:

퐸푂퐶푉 − (푅표ℎ푚𝑖푐 + 푅∆퐶 + 푅퐵퐿) ∙ 퐽 푃푛푒푡 = [ ] ∙ 퐽 − 푃푝푢푚푝 (13) 푁푚

with EOCV the open circuit voltage (V), j the current density (A/m²), Rohmic, R∆C and RBL the different resistance factors (Ω∙m²) and Nm the number of membranes (-). In an optimized RED system, the pumping power can consume up to 25% of the gross produced power [43]. The variables that determine the net power density are the membrane and spacer properties, electrode resistance, feed water concentrations, temperature, cell dimensions, residence time and current density. Some variables, such as the concentrations of the solutions, are hard to manipulate since they are mostly determined by the circumstances of the process. In practice, the intermembrane distance, the residence time and the current density can be varied to obtain the optimum net power density. In Table II.2, an overview of representative values for RED

stacks (with spacers or with profiled membranes) functioning in a large scale operation can be found.

Table II.2 Typical specifications for a revere electrodialysis stack using spacers and a reverse electrodialysis stack using profiled membranes, adapted from Vermaas et al. (2012) [35]

Variable Parameter Spacers Profiled membranes h Intermembrane distance 1-200 µm 1-200 µm tres Residence time 0.5 – 200 s 0.5 – 200 s j Current density 1 – 100 A/m² 1 – 100 A/m²

RAEM = RCEM Membrane resistance 1 Ω∙cm² 1 Ω∙cm² L Cell length 0.1 m 0.1 m

Rel Electrode resistance 2 Ω∙cm² 2 Ω∙cm² α Membrane permselectivity 0.971 0.971

Nm Number of membranes 100 100 csea Concentration of the seawater 0.510 mol/L NaCl 0.510 mol/L NaCl criver Concentration of the river water 0.017 mol/L NaCl 0.017 mol/L NaCl T Temperature 298 K 298 K β Mask factor 0.50 0.1 ε Porosity 0.70 0.90 b Width between profiled ridges - 9∙h

Ssp/Vsp Ratio surface and volume of spacer 8/h 1/m - filaments 1 Depending on the type of membrane, here based on Fumatech FKS/FAS membranes

II.3. MEMBRANE FOULING

II.3.1. Fouling in membrane processes

Membrane fouling is one of the largest challenges in all membrane processes. The amount, type and distribution of foulants depends on the type of membrane and operation. When comparing the fouling tendency, RED is the better choice over PRO to generate power out of a salinity gradient, as IEM have proven to be less sensitive to fouling and clogging than FO membranes, and in RED only the ions pass the membrane in contrast to PRO where water and present foulants are pressed against and through the membrane [64,65]. Nevertheless, fouling is still a problem in RED applications. In an RED set-up on laboratory scale with natural seawater and artificial wastewater, the power density decreased with 67% over a period of 180 days due to biological fouling (leading to a decreased stack performance and an increased pumping cost) [32]. When working with natural fresh water (river water or even wastewater), the risk of membrane fouling and clogging of feed water compartments becomes evidently even bigger.

Overall, fouling studies for RED are scarce, especially with natural water streams, and the conclusions drawn from ED fouling studies are not guaranteed to be valid in RED processes. The fouling behaviour in RED and ED can be different since the processes are

opposite (in ED the concentration is increased in the concentrate channel which can induce scaling), and in ED the typical current densities and intermembrane distances are much higher and only one feed water type is used [65].

II.3.1.1. Membrane performance decline

During long-term membrane applications, the membrane performance can be reduced by several factors. Various chemicals can harm the active layer of the membrane, which can lead to irreversible damage and even destruction of the membrane. This so-called membrane deterioration can be induced by oxidants used in (pre-)treatment, but polymeric membranes are also susceptible to extreme pH values [66].

Membrane fouling is caused by convective and diffusive transport of suspended or colloidal substances present in the feed flow towards the membrane, or by biological growth on the membrane itself (so-called bio-fouling). During operation, a layer of dissolved, colloidal or biological matter can accumulate on the membrane surface which will reduce the performance. A distinction is made between precipitation of inorganic material on the surface and transport of particulate matter to the surface, respectively scaling and fouling. Scaling is especially challenging in RO operations, where the increased salt concentration at the membrane surface leads to supersaturation and precipitation. Even though acid flushing is often a good approach to remove some scaling, pre-treatment in order to stabilize scaling substances is recommended [16].

Another type of fouling linked to IEM in specific is poisoning. Large, multivalent counter- ions can adsorb to the membrane surface and diffuse slowly through the membrane while occupying charged groups. This phenomenon modifies the transport properties of the membrane: the electrical resistance increases while the permselectivity is lowered [8].

II.3.1.2. Fouling in RED applications

Different potential fouling sources relevant to RED processes can be distinguished [68]:

i) Particles that can cause clogging of the spacers. ii) Precipitation of inorganic material on the surface, so-called scaling iii) Organic substances such as humic acids, detergents and oils. These usually involve large negatively charged molecules, resulting in fouling development on the standard (positively charged) AEM iv) Microorganisms (and their associated extracellular polymeric substances (EPS)) that form a biofilm v) Multivalent or large organic ions that can decrease the surface charge (so- called poisoning)

In a fouling experiment conducted by Vermaas et al. (2013) an RED process was run with natural feed waters [32]. Natural seawater and river water were filtered through a series of microfilters (in which the final filter had a median diameter of 20 µm) and fed to the RED stacks for 25 days. After operation, a mixture of deposits was observed on all spacers and IEM. Atom percentages of membrane deposits can be found in Table II.3.

Table II.3 Composition of deposits on the cation exchange membrane (CEM) and the anion exchange membrane (AEM) after operating in a reverse electrodialysis stack fed with natural seawater and river water for 25 days [32]

CEM AEM SW side RW side SW side RW side Carbon (%) 42 ± 28 22 45 51 ± 2 Oxygen (%) 36 ± 18 53 35 31 ±1 Silicon (%) 4 ± 1 6 7 8 ±1 Potassium (%) 0 ± 0 1 1 1 ± 0 Calcium (%) 6 ± 3 4 1 0 ± 0 Aluminium (%) 1 ± 0 4 2 3 ± 0 Phosphorus (%) 7 ± 4 5 2 1 ± 0

Although fouling deposition was found on every membrane, differences can be seen in the composition of the fouling layer. The AEM were more sensitive to fouling due to their charge: negatively charged molecules such as humic acids were absorbed on the membrane surface. The higher amount of Si and O can be attributed to a mixture of clay minerals such as muscovite and amorphous silica from diatoms, which both carry a net negative charge. The CEM at the seawater side has remarkably more Ca and P when comparing to other membrane sides. This can attributed to the precipitation of calcium phosphates (Ca3(PO4)2) as a consequence of increased concentrations due to the preferential exchange of multivalent ions for monovalent substitutes, against the salinity gradient (see section II.2.4.1.)

The development of this fouling layer has a large impact on several operational parameters such as pressure drop, resistance and permselectivity, and although different processes and/or foulants can cause different consequences, they are all more or less linked. A first important consequence of the fouling is an increased pressure drop in the spacer channel, which occurs especially when biofilms start growing in the spacers. In the fouling experiment conducted by Vermaas (2013), the pressure drop over the stack with 11 membranes and 5 spacers increased to 1.5 bar within a week [32]. Since that was the maximum value that the feed water pumps could produce, the pressure drop after the first week stayed constant while the flow rate decreased to only 10% of its original value after 25 days [68].

It can also be noticed that during the first day of operation in the experiments of Vermaas et al. (2013), the apparent permselectivity decreased sharply by approximately 10% and the electrical resistance increased with 40-70%. Large, charged, organic molecules get trapped in the surface layer of the IEM where they will counterbalance

the effective surface charge which leads to a decreased charge density available for ion transport [32]. Besides the influence of these organic foulants, the concentration boundary layer will increase due to the presence of colloids adhered to the membrane surface [70]. This will further increase the resistance.

All these consequences of fouling will lead to a decreased power density. During the first day of operation, the produced power density can decrease to 60% of its theoretical value [32]. It can easily be seen that this will have a huge impact on the economical feasibility of the RED system. Not only is the produced amount of energy much lower than theoretically possible, the lifetime of the system will be endangered and the membrane fouling will entail costs for increased pumping and membrane cleaning and replacement. Nevertheless, research and data about fouling in an RED system is scarce. To make the system economically feasible, more insight is needed regarding the fouling development, less sensitive membranes and possible pre-treatment techniques.

II.3.2. Possible anti-fouling strategies

Several strategies exist in order to control membrane fouling and thus assure a satisfying stack performance and energy production. A distinction can be made between fouling prevention (including membrane and stack design and feed water pre-treatment) and fouling removal (including membrane cleaning in place (CIP), cleaning after demounting and replacement of the membrane pile).

II.3.2.1. Fouling prevention

i) Membrane and stack design

The common design of an RED stack is still comparable to a classical ED design with spacers in between IEM. It is likely that in future designs, these spacers will change drastically or even disappear completely. The spacers contribute significantly to the stack resistance; they can be seen as an undesired insulator [33]. As mentioned in section II.2.3.3., research is ongoing to develop ion conductive spacers to overcome this issue [63], but there are more adverse effects of spacers on the RED system. The spacers can also increase fouling since they are a suited anchor place for biofilm formation [32,68]. Therefore, research is also ongoing towards applying profiled membranes with an open structure and less obstacles [40]. Besides an optimised stack and spacer design, fouling resistant IEM can also contribute to a better and longer stack performance. Coating AEM with negative top layers can both increase the membrane monovalent ion selectivity and decrease its sensitivity towards organic fouling [81].

A more radical design change is the implementation of the ‘breathing cell’, as proposed by Moreno et al. (2016) [38]. In this design, the seawater outlets are periodically opened and closed. When the outlets are closed, a pressure is building up inside the membrane

pile and the IEM will tend to compress the wastewater compartments. This will not only be beneficial for the electrical resistance (due to a periodically thinner wastewater compartment), but it can also decrease formation of permanent fouling due to the shocking movements of the IEM.

ii) Pre-treatment

In the last two decades, technologies for seawater pre-treatment have matured, driven by an increasing demand for SWRO desalination [75]. When applying RED in a hybrid RED-RO process, the pre-treatment of the fresh water feed is therefore more challenging since less is known about the pre-treatment needs and possibilities.

A rapid sand filter (RSF) is nowadays the most common pre-treatment step used in large scale SWRO due to its simplicity, low energy consumption and low operational costs [78]. This filter type can remove suspended solids as small as 0.35 mm by size exclusion and adsorption, which will protect the following membranes from rapid clogging. Besides this physical exclusion, the sand can also serve as a substrate to develop a functional biofilm [78,79]. The microbial organisms present in this biofilm can take up and remove biodegradable compounds from the water. The more nutrients removed from the water before entering the RED stack, the less nutrients remaining for the biofilm development on the IEM.

Considering the pre-treatment of the fresh water feed, a trade-off between pre- treatment cost and treated water quality has to made. When taken into account current energy efficiencies, a power output normally below 0.2-0.5 kWh/(m³ feed solution) can be expected. Therefore, it is targeted to keep the specific energy consumption in the pre-treatment stage lower than 0.05 kWh/m³ [82]. Therefore, more advanced pre- treatment options such as pressurized sand filters or ultrafiltration are not suitable for RED applications. Gravity filters (different types of sand filters including RSF, slow sand filters (SSF) and river bank filters (RBF)) and cartridge filters (for small capacities) seem to be more suitable, but long-term experiments are still to show which pre-treatment option is the preferable option.

II.3.2.2. Membrane cleaning

A possible approach to prevent the IEM from too severe fouling is the periodical switching of the feed waters. In ED, this strategy has already showed to be effective when combined with a switching of the electric field, and according to the results of Post (2009) it can also decrease biofouling in RED [32,73]. Switching streams delivers an osmotic shock to the fouling layer and induces the reversal of the electric field which can reduce fouling of organic acids and charged colloids [77]. Other options for membrane cleaning in RED are applying short electrical pulses or disturbing the system with air bubbles in the feed water compartments (air sparging); both have already proven to be

successful [76,77]. Furthermore, chemical cleaning is an efficient approach to clean IEM since it can remove most of the fouling present. When weakening the formed biofilm with caustic or oxidants, the pressure drop can be decreased with 80% [32]. An import issue regarding chemical cleaning is the compatibility of the membranes and other components to the chemicals. Above that, chemical dosing will bring an extra cost that has to be taken into account.

Overall, it can be concluded that the prevention of fouling is the better choice over cleaning. In practice, both approaches will be necessary since some fouling layer development will always be formed, no matter how efficient the pre-treatment methods [72].

III. OBJECTIVES OF THIS THESIS

To reduce the energy consumption of SWRO desalination, several options can be used, of which using ERD is the most known. However, this only allows to reach the thermodynamical limit of seawater desalination at 50% recovery, namely 1.06 kWh/m³, more closely. In order to further decrease energy consumption, the thermodynamic limit has to be lowered. This can only be done by lowering the salt concentration in the seawater prior to RO. Therefore, in this thesis, a hybrid system composed of a pre- treatment step, an RED step and an RO step is proposed to lower the incoming seawater concentration before the RO (Figure III.1). By combining RED and RO in this hybrid system, it is possible to decrease the energy demand for the desalination of seawater to a value beneath the thermodynamic limit by making use of the salinity gradient between seawater and a fresh water feed. In this way, a hybrid RED-RO process can help to decrease the energy demand of SWRO desalination, one of the big challenges for the future development of RO as desalination technique.

Figure III.1 A hybrid system composed of a pre-treatment step, a reversed electrodialysis (RED) step and a reverse osmosis (RO) step (own figure)

The benefits concerning energy consumption of an RED-RO coupling are twofold. In RED, energy can be produced thanks to the difference in salt concentration between the fresh water and seawater flow. This energy can be used in a final RO step, where the seawater is desalinated. Besides this energy production, the salinity of the feed seawater will decrease in RED which will lower the energy needed to desalinate the water in RO.

Many types of feed water could be suggested to predilute the seawater in the RED step. However, it does not make sense to use fresh river water for this purpose, as in this case drinking water could better be produced straight from the river water. Therefore, wastewater is chosen as a fresh water feed. This low-value product suffers from a bad reputation and drinking recycled wastewater is hard to sell to the public opinion. By using wastewater in RED applications, no other, more valuable, fresh water source has to be exploited. In addition, there is no reuse of wastewater, as the wastewater only serves as sink for the ions from the seawater. As such, negative public perception is overcome.

However, wastewater can lead to fouling in an RED stack. Therefore, a wastewater pre- treatment step should purify the water to protect the RED membranes from immediate fouling, damage etc. A trade-off between pre-treatment performance and investment and operational cost has to be made, however. The more extensive the pre-treatment, the higher the water quality that is fed to the RED stack, the better the RED performance and the longer the IEM lifetime. On the other hand, pre-treatment comes with an extra investment and operational cost. Seawater has to be treated either way before it can be fed to an RO system, but the additional pre-treatment necessary for the wastewater is an extra cost that should be taken into account.

In this master thesis, the influence of different pre-treatment options for secondary wastewater effluent on the performance of a wastewater/seawater RED system will be investigated in long-term fouling experiments. Information on fouling potential of real water streams in RED applications is rare and limited to river water/seawater systems [32]. It can be expected that wastewater will cause more fouling due to a higher amount of organic and inorganic contaminants.

Different pre-treatment options for the wastewater stream are examined. The fouling behaviour of the different pre-treated wastewater flows in the lab-scale RED stacks and the corresponding stack performances are monitored in order compare the efficiency of the pre-treatment methods. After the experiments, the membranes and spacers of the RED stacks are chemically and visually analysed (and fouling is quantified as much as possible) to get a clear view on the differences in structure and composition of the developed fouling layer. In the end, a suggestion is made on the most suitable pre- treatment technique from a technical and economical point of view.

IV. MATERIALS AND METHODS

IV.1. Experimental approach

The influence of different pre-treatment methods on wastewater effluent fed to a seawater/wastewater RED stack, on the performance of this RED stack is investigated. This is done by running long-term fouling experiments with secondary wastewater effluent that underwent different types of pre-treatments to limit fouling in the RED stacks. For 6 weeks, both seawater and wastewater (whether or not pre-treated) were fed to RED stacks running in parallel (Figure IV.1). Inside the stacks, ion transport between seawater and wastewater took place via the IEM. During operation, electrical operational variables (OCV and cell potential) were monitored while a constant current (simulating the RED process) was applied. Twice a week, a fresh batch of (pre-treated) wastewater and seawater was supplied to each stack. Water samples were taken at the beginning and at the end of every batch for analysis, to link water quality to performance. The pressure drop over both the wastewater and seawater compartment was measured every day (except for some days in the weekend) to follow up on the membrane fouling. After 42 days, the operation was stopped and the stacks were opened. The membranes were then thoroughly autopsied. Microscopy, biological and chemical analysis were performed to characterize the fouling on the membranes and spacers.

Figure IV.1 Scheme of the experimental set-up (own figure).

IV.1.1. Overview of the experiments

As can be seen on the time schedule (Figure IV.2), the experiments were carried out during two periods of 6 weeks. During the first period (run 1, November-December), an RSF and a 100 µm filter were tested as wastewater effluent pre-treatment. As the pre- treatment performance of the sand filter and the 100 µm filter tested in the first run showed satisfying results (see further), it seemed most interesting to look for a more rudimentary pre-treatment method that would be economically more interesting during the second trial run. Therefore, in the second period (run 2, March-May) a river bank filtration (RBF) was tested.

Figure IV.2 Time schedule of the executed fouling experiments

IV.2. Design, operation and follow-up of RED stacks

IV.2.1. Design and components of the RED stack

The RED stacks were designed and constructed by REDstack BV (The Netherlands), a spin-off company of Wetsus, European centre of excellence for sustainable water technology (The Netherlands). The membrane pile in the middle of the stack consists of 2 rubber gaskets at the edges, 11 membranes and 10 spacers that together form 5 cell pairs (Figure IV.3).

Figure IV.3 Reverse electrodialysis stack with 5 cell pairs (PMMA = polymethylmethacrylate, CEM = cation exchange membrane, AEM = anion exchange membrane) (own figure)

In between the outer CEM and the polymehtylmethacrylate (PMMA) endplate carrying the working electrode, a rubber gasket is placed to form a compartment for the ERS. The

outer CEM (Fumasep F-10180, Fumatech, Germany) differs from the other CEM to protect the electrolyte compartment from the feed water compartments and vice versa. The other CEM and AEM (IEM Type 10, Fujifilm, The Netherlands) are piled up with 270 micron spacers (Deukum, Germany) in between. The specifics of the membranes can be found in Table IV.1. The effective membrane area of each membrane was 0.01 m².

Table IV.1 Specifications of the membranes (as provided by the manufacturers) used in the reverse electrodialysis stacks

Fumasep F-10180 PTFE [52] Fujifilm Type 10 [6] type CEM CEM & AEM polymer perfluorinated sulfonic acid (not provided) reinforcement PTFE (not provided) thickness µm 150-180 120-140 IEC meq/g 1 2.85 (AEM) or 2.9 (CEM) permselectivity % >99 95-98 Specific area Ω∙cm² <0.5 1.8 (AEM) or 1.9 (CEM) resistance stability pH 1-14 1-13

Around the central membrane pile, 6 PMMA plates are placed: 4 around the membranes, 1 on top of the membrane pile and 1 at the bottom. The top and bottom PMMA plate carry the platinum coated working electrode, as stated above. These PMMA plates are screwed firmly together to keep the water inside and to force the water to follow through the channels towards and along the membranes. There are different inlets for wastewater and seawater. Both feed waters come in at the bottom of the stack, go through the membrane pile in a cross-flow configuration (Figure IV.4) and leave at the top, opposite to their inlet. The alternating orientation of the spacers (as can be seen on Figure IV.3) creates channels for either the wastewater or the seawater.

Figure IV.4 Schematic of the flow patterns in a cross-flow reverse electrodialysis stack (own figure) When reassembling the stacks for the second run, all membranes were renewed. The spacers were cleaned with demineralized water and reused. Only the two most central spacers were replaced, as the spacers used in the first run were cut to analyse deposits (see section IV.4).

IV.2.2. Operational approach

For 42 days, the RED stacks were fed with seawater and wastewater in the alternating compartments. During these six weeks, the water was replaced by a new batch twice a week to mimic the behaviour of a continuous system. Per batch, 60 L of wastewater and 60 L of seawater was circulated. Over a period of six weeks, this amounts to a total volume of 720 L of both streams (spread over twelve batches) treated by the RED unit. The pre-treatment was detached from the RED set-ups, so while one batch was feeding the RED set-ups, the next batch was being pre-treated.

The wastewater effluent is secondary effluent coming from the nearby wastewater treatment plant (Aquafin Ghent, Belgium). The seawater was originally coming from the North Sea and was stored in an underground storage tank (29 m³) at the Faculty of Bioscience Engineering in Ghent. All seawater was automatically filtered over a sand filter, a bead filter (Polygeyser Bead Filter, Aquaculture Systems Technologies, USA) and a UV-filter before storage. Two liters of 0.25 mol/L NaCl solution was circulated through the electrolyte compartments to serve as ERS. The ERS was refilled frequently to assure the volume was always high enough to rule out possible effects of limiting ERS (since some small leakages were unavoidable). All water was pumped around by a peristaltic pump (Watson Marlow 530S,Watson-Marlow Fluid Technology Group, UK) at flow rate of 60 mL/min (0.74 cm/s), which results in a hydraulic residence time of 13.5 seconds.

IV.2.3. Cleaning of the stacks during the experimental run

It can be expected that over time the pressure drop over the wastewater and seawater compartments in RED applications increases due to the build-up of a fouling layer. An increasing fouling layer will lead to an increasing pumping cost and a decreasing stack performance (see section II.3.1.1. and II.3.1.2.). Since it will never be possible to completely avoid membrane fouling (no matter how successful the pre-treatment), membrane flushing will be needed during the experiments from time to time.

During the executed fouling experiments, it was intended to test different cleaning strategies in order to determine the best flushing approach for RED applications (based on the decrease in pressure drop achieved by the executed cleaning). When the pressure drop over one of the compartments exceeded one bar, the membranes were flushed before continuing the experiments. In Table IV.2, an overview of the different flushing protocols can be found. Originally, the stacks were only cleaned with demineralized water and base solution since especially biofouling was expected. Since the results of this first protocol were not satisfying (see further), adjustments were made and a second and third protocol were developed and tested. These different protocols were tested during run 1 when the pressure drop exceeded one bar over one (or more) of the compartments. In run 2, protocol 3 was always followed when flushing was needed since this had shown to be the best approach (see further).

Table IV.2 Overview of the different flushing protocol

PROTOCOL 1 PROTOCOL 2 PROTOCOL 3 flushing First flush with 1L First flush with 2L First flush with 2L solutions demineralized water demineralized water demineralized water

Second flush with 1L 0.1 Second flush with 1L 0.1 Second flush with 1L 0.1 mol/L NaOH mol/L NaOH mol/L HCl

Then recirculate 2L 0.1 Then Recirculate 2L 0.1 Then recirculate 2L 0.1 mol/L NaOH for 30 min mol/L NaOH for 30 min mol/L HCl for 30 min

Final flush with 1L Flush again with 2L Flush again with 2L demineralized water demineralized water demineralized water

Then flush with 1L 0.1 Then flush with 1L 0.1 mol/L HCl mol/L NaOH

Then recirculate 2L 0.1 Then recirculate 2L 0.1 mol/L HCl for 30 min mol/L NaOH for 30 min

Final flush with 2L Final flush with 2L demineralized water and 2L demineralized water 0.01 mol/L NaCl and 2L 0.01 mol/L NaCl flow rate 0.74 cm/s 0.74 cm/s 1.48 cm/s flow direction normal normal reversed

IV.2.4. Monitoring of voltage, pressure drop and mixing extent

As mentioned before, the top and bottom PMMA plate carried the platinum coated working electrode. To minimise the influence of the reactions taking place at the electrode on the measured stack resistance, a four electrode system with another two Ag/AgCl reference electrodes was used. All four electrodes were connected to the potentiostat (BioLogic VSP, Bio-Logic Science Instruments, France), which provided the applied electrical current via the working electrodes and monitored the OCV and cell potential.

Before the fouling experiments started, the OCV over the stack (via the reference electrodes) was measured using artificial seawater (0.507 mol/L NaCl) and artificial fresh water (0.017 mol/L NaCl) (and an ERS of 0.25 mol/L NaCl) at a flow rate of 0.74 cm/s (Table IV.3). Under these conditions, the theoretical OCV over each membrane is 80 mV, so should be 880 mV in total (based on the Nernst-Plank equation).

Table IV.3 Stack open circuit voltage (OCV) before the start of the fouling experiments (RSF=rapid sand filter, RBF=river bank filter)

stack OCV (V) stack OCV (V) run 1 run 2 reference 0.731 reference 0.746 RSF 0.720 RBF 0.721 100 µm filter 0.753

While the wastewater and seawater were fed to the stack, two alternating operating regimes were imposed by specialized software (EC-lab, Bio-Logic Science Instruments, France) driving the potentiostat (Table IV.4).

Table IV.4 Consecutive regimes as imposed by the potentiostat to the reverse electrodialysis stacks

regime 1 regime 2 applied current 0.000 µA 50.000 mA duration 5 min 120 min frequency of data logging 20 s 1.800 s

In the first regime, no current was applied. The potential over the reference electrodes (i.e. the OCV) was logged every twenty seconds during five minutes. After five minutes, the second regime started and a current of 50.000 mA was applied to the stack for two hours. This current was applied to increase transport and in that way mimic the RED process. Every 30 minutes, the voltage over the reference electrodes was logged. After two hours, the applied current was set to zero and the first regime was applied again (to measure OCV). As such, these cycles were repeated until the end of the experiments.

At the beginning and at the end of every batch, water samples were taken. Temperature and conductivity were measured using the C3020 controller equipped with the SK10T conductivity electrode (Consort, Belgium). The pressure drop over the seawater and wastewater compartments was measured daily (except some days in the weekends) using a pressure difference measurements device (Jumo di 308, JUMO GmbH & Co., Germany). The pressure measurements in the last week of the second run were performed with a conventional pressure meter (Jumo Delos SI, JUMO GmbH & Co. Germany) due to malfunctioning (due to short circuiting) of the original pressure difference measurements device.

IV.3. Pre-treatment methods for wastewater effluent

Since seawater has to be pre-treated before it can be desalinated through RO, seawater pre-treatment is already well documented and applied in SWRO desalination plants. Therefore, only the influence of different pre-treatment options for the secondary wastewater effluent on RED performance is investigated. Three different pre-treatment options are tested.

IV.3.1. Rapid sand filter

For the RSF on lab scale, a glass column (length 70 cm, inner diameter 0.0264 cm) was filled with sand grains. The grains (Coeck river sand 0-2 mm, Brico Plan-it Ghent, Belgium) were dried overnight in the oven at 110 °C and sieved to obtain a uniform grain size of 1 to 2 mm. The glass column was filled with the filtered sand grains to a height of 0.5 m, thus creating a filter volume of 0.27 L. To mimic a full-scale RSF with a height of 1.5 m (and thus guarantee good filter quality), the water was recirculated three times over the RSF. Water was pumped from the bottom of the barrel over the RSF, through the filter, and recirculated to the top of the barrel (Figure IV.5a). The exact flow rate varied according to the degree of fouling on the filter. On average, the flow rate was kept high around 9 to 10 L/h (16 to 18 m/h).

Figure IV.5 Set-up of the rapid sand filter, with (a) the normal configuration during operation, and (b) the configuration when the sand medium is flushed (own Figure)

When the fouling on the RSF became too severe and the flow rate dropped to a level below 5 (m/h) with the supernatant water level of 0.5 m, the filter was backwashed (Figure IV.5b). During this backwash, demineralised water was forced through the filter bed bottom to top. The flow rate was gradually increased until visually no pollution could be observed in the supernatant water leaving the RSF.

IV.3.2. 100 µm filter

The second stack from the first run was fed with wastewater that was pre-treated with a commercial filter with a mesh size of 100 µm (Purifo 285235, Van Marcke, Belgium). This cylindrical (candle) filter has a height of 315 mm and a diameter of 133 mm (Figure IV.6).

Figure IV.6 Set-up of the 100 µm filter (own Figure)

The filter works according to the inside-out principle: water enters the filter at the top and is forced to flow from the inside of the filter through the mazes to the outside of the filter. The feed flow rate was 9 to 10 L/h.

IV.3.3. River bank filter

As a third pre-treatment method, it was decided to test an RBF filter. This filter type mimics the behaviour of the natural filtration process when water permeates into the soil. As such, this filter is also relatively easy to implement in practice. The RBF on lab scale consists of a polycarbonate (PC) tube (height 150 cm, diameter 19.2 cm) filled with sand coming from the river bank of the Coupure river. The inlet and outlet were connected to the filter through a polyvinylchloride (PVC) flange.

Before the filter was operational, it was equilibrated with regular tap water and wastewater, which was fed to the RBF for respectively 14 days and 3 days at a very low flow rate (approximately 0.25 m/d). In this way, the biofilm had the time to develop and the pre-treatment performance was representative when, after this start-up period, the filtered water was collected for feeding to the RED stack.

After equilibrating the filter, different batches of 60 L of wastewater were sent bottom to top through the filter (Figure IV.7a). The flow was kept stable around 0.66 L/h (0.55 m/d), although this varied according to the biofilm build-up on the sand grains and the pressure drop in the sand filter. When the pressure drop was higher than 0.00911 bar (9 milli-atmosphere), the filter was washed by flushing the aerobic zone at the bottom with demineralized water. With a 50 mL syringe, demineralized water was injected into the filter via the tube that in normal operational conditions served as the inlet tube. Next, the water was sucked out of the filter again via the same tube. This was done until the water leaving the filter was visually clean.

Figure IV.7 Set-up of the river bank filter, with (a) the bottom to top configuration for the first 18 days and (b) the configuration with the top to bottom flow and the collecting of the leaking water (own Figure)

After 18 days of operation, the filter started leaking at the juncture between the bottom of the column and the flange. Since adding glue and/or silicone did not stop the leakage, it was decided to stop the operation for a week in order to thoroughly restore the filter. The bottom 15 cm of the filter column was removed, and a new flange was placed. The restored column functioned for another 12 days, but again leakages showed up at the bottom flange. It was then decided to change the RBF configuration. The tubes were rearranged so the water would no longer flow bottom to top through the filter, but instead come in at the top and leave at the bottom (Figure IV.7b). The water leaking from the bottom flange was collected and added to the barrel with filtered water (represented by the dotted line on Figure IV.7b).

IV.4. Feed water analysis

Samples of the different feed waters supplied to the RED stacks were taken at the beginning and at the end of every batch (12 batches in total). These samples were analysed on ionic composition and total organic carbon (TOC) content in order to investigate the impact of RED on the water composition.

IV.4.1. Ionic composition

Four cations (Na+, K+, Ca2+ and Mg2+) were analysed using ICP-OES (inductively coupled plasma – optical emission spectroscopy, Varian VISTA-MPX, USA). These major cations include 99% of the total cation concentration in seawater [55]. By analysing the concentration of the divalent cations, the uphill transport of these ions from the wastewater towards the seawater (as described in II.4.1.) can be investigated.

The wastewater samples were diluted 6 times (3 mL sample in 15 mL ultrapure water) whereas the seawater samples were diluted 251 times (80 µL sample in 20 mL ultrapure water) in order to decrease the concentration of all components of interest below 100 ppm. Before analyses were performed, the samples were acidified to 1% HNO3.

IV.4.2. TOC content

The influence of the RED process on (organic) carbon content was determined by performing TOC analyses on the wastewater samples taken at the beginning and at the end of every batch. Vials were filled with 20 mL of sample liquid and analysed by means of catalytic oxidation at 680 °C in a TOC analyser (TOC-VCPN analyser combined with ASI- V autosampler, Shimadzu, Japan). Both inorganic carbon and total carbon were measured, thus making it possible to determine the TOC content.

IV.5. Membrane autopsy

After 42 days of operation, the process was stopped, the stack was dismantled, and analyses were performed. The goal was to get an idea on the composition of the fouling layer, with focus on the developed biofilm. Biological growth on the membranes and spacers, distribution of this biofilm throughout the stack (membranes and compartments) and the activity of the formed biofilm was investigated. Besides the biofilm analysis, the presence of particulate foulants and scaling was investigated.

The fouling and biofilm were harvested at different locations on the middle cell pair (AEM, spacer in the wastewater compartment, CEM and spacer in the seawater compartment). Both sides of the AEM and CEM (seawater and wastewater sides) and the spacers were sampled at the inlet, middle and outlet. On Figure IV.8, a scheme of the different sample locations on a membrane is shown. Both feed waters are represented on one scheme, but in practice, the seawater and wastewater inlet and outlet are located at different sides of the membrane. The middle part of the membrane was also analysed via microscopy since it is assumed that this part is most representative for the largest part of the membrane area during a full-scale operation.

Figure IV.8 Scheme of the different sample locations on a membrane (C = chemical analysis, M = microscopy analysis, WW = wastewater, SW = seawater)

IV.5.1. Microscopy analysis

The central part of the membranes was visually analysed with both confocal and scanning electron (SEM) microscopy. A 2 cm by 2 cm piece was cut with scissors to provide enough sample for both microscopy analyses. A small piece (approximately 1 cm²) was taken apart for the confocal microscopy, while the remainder served as a sample for SEM.

IV.5.1.1. Confocal microscopy

With confocal microscopy, the biofilm was characterized regarding structure, thickness and density. By combining this technique with a live/dead staining, an idea on the activity of the biofilm could be obtained.

The protocol for confocal microscopy analyses was drafted together with Dr. Ir. Jo Philips, who also executed the analyses. The full protocol can be found in appendix VIII.1. In a first step, the membranes were washed twice in phosphate-buffered saline

(PBS) buffer to remove any planktonic cells. Next, the membranes were transferred to a 2.5% glutaraldehyde in PBS buffer for fixation. Afterwards, the membrane samples were transferred to the staining solution (SYT09 and propidium iodide) for 15 minutes. The membrane permeable dye SYT09 identifies the total bacterial population, while propidium iodide can identify bacteria with a compromised membrane (and thus can be considered nonviable). After removing the excess stain with PBS buffer, the membrane pieces were transferred to a microscope slide. After adding the mounting medium, the samples were covered with a coverslip that was glued to the microscope slide.

The confocal microscope analyses were performed with the Nikon A1+ confocal laser scanning microscope (Nikon Instruments Europe B.V., The Netherlands). All pictures were taken with the same lighting setting and at magnification of 10x. As a reference, pieces of virgin membranes (not affected by any fouling) were analysed. For every stack, 4 membrane pieces were analysed: AEM and CEM, both at the wastewater and seawater side.

IV.5.1.2. SEM

As a second microscopy analysis, SEM was performed in order to determine the structure of the top layer and composition of the top layer of the biofilm. The protocol drafted by Arends et al. (2012) was followed [74]. After opening the stacks, the membrane samples were immediately fixed with subsequently 2.5% glutaraldehyde (2 hours) and 1% osmium tetroxide (1 hour) in phosphate buffer. Next, the samples were dehydrated in several steps by dosing ethanol solutions with an increasing concentration. Finally, the samples were dried overnight in a desiccator at room temperature.

SEM pictures were taken on a FEG SEM JSM-7600F (JEOL, USA). An overview picture of the membrane surface was made (300x magnification), and when irregularities were observed, a more detailed SEM analysis was performed (up to 4500x magnification). As the pieces of membrane tended to curl during the fixation and staining process, not all samples were analysed as for some there was no certainty on the orientation (which side was the seawater side and vice versa).

IV.5.2. Chemical Analysis

For the chemical analysis, the biofilm was scraped of the membranes and spacers (pieces of 1.5 by 1.5 cm²) and immersed directly into 50 mL of PBS buffer. This was done for both the seawater and the wastewater compartment, at the inlet, middle and outlet. When opening the stacks and taking apart the membranes, it was clear that it would not be possible to attribute the observed biofilm on a sample location to a specific membrane. The membranes stuck together and when separating the membranes that

form a compartment, the biofilm was disturbed and it was not possible to tell where the biofilm originally developed. Therefore, the deposits from one sample location were scraped off with a scalpel and collected together in the same vial with PBS buffer. In other words, the samples of AEM, CEM and spacer of the same location (e.g. wastewater inlet) were collected together as one sample. The spacer samples were put in the associated sample vials to be sonicated shortly (2 times 30 seconds) in order to release attached material.

IV.5.2.1. ATP analysis

ATP analysis were executed immediately after the samples were sonicated. 100 µL of the samples was transferred into a 96-well plate and equilibrated to room temperature. Next, 100 µL of BacTiter-GloTM (Promega Corporation, USA) reagent was added. The solutions were briefly mixed and incubated for 5 minutes. The absorbance was measured at 600 nm and compared to the absorbance of a standard series (10 pmol/L to 1 µmol/L). All analyses were performed in triplicate.

IV.5.2.2. carbohydrate content

Carbohydrates were analysed with spectrophotometry according to the Dubois method [67]. 1.5 mL of sample was brought together with 0.6 mL of phenol and 3 mL of sulphuric acid in a glass test tube. The samples were vortexed for 30 seconds and incubated at room temperature for 30 minutes. Finally, the absorbance at 487 nm was determined and compared to the absorbance of a standard series (0 mg/L to 50 mg/L glucose).

IV.6. Statistical analysis

When relevant, statistical analysis were performed based on the ANOVA test. P values lower than 0.05 were considered as statistically significant. The calculations were performed using the Analysis ToolPak add-in of the Microsoft Excel software (Microsoft, USA).

V. RESULTS AND DISCUSSION

In order to compare the influence of different pre-treatments of the wastewater on the RED stack performance, three different types of analysis were executed. First of all, operational variables of the RED stacks were monitored: stack voltages, OCV and pressure drop over both sea- and wastewater compartments were measured and logged. These measurements can give insight into the performance and efficiency of the RED process. Secondly, analysis on conductivity and solute concentrations were performed on the collected water samples to assess transport during pre-desalination. Thirdly, membranes were removed from the stacks after operation and analysed to characterize the developed fouling. In the end, a concise economical evaluation is carried out to make a realistic estimate on the feasibility of the pre-treatment methods.

V.1. Process efficiency

V.1.1. Feed water characteristics, pre-desalination and solute transport

V.1.1.1. Variability of the feed water batches

It is important to note that the composition of the feed waters was not constant. Twice a week, water was taken from the nearby wastewater treatment plant to serve as the fresh water feed. It can easily be seen that some variability, caused by e.g. deviations in weather conditions or conditions at the wastewater treatment plant, was inevitable. For seawater, small variations in the composition were also unavoidable since the underground storage tank (where seawater is collected twice a week) was refilled monthly. As the water for all stacks in every batch was always collected at the same time, it can be assumed that the stacks running in parallel were always fed with the same water (whether or not pre-treated). In Table V.1, the average conductivity of the feed waters at the beginning of every batch can be found and in Table V.2, the average cation concentrations of the feed waters at the beginning of every batch is shown. For the second run, no data on ion concentrations are available due to technical issues with the analysis device.

A big, statistically significant (p=2.69∙10-12) difference in seawater conductivity can be noticed between the stacks from run 1 and run 2 due to differences in composition of the seawater delivered to the underground storage tank. Since the difference in salinity determines the potential energy production in an RED system, it is important to keep this deviating conductivity in mind when analysing and comparing the power production of the stacks during the first and second run (section V.1.2.3. ).

Table V.1 Average conductivity and standard deviation of the feed waters (WW = wastewater, SW = seawater) at the beginning of a batch (RSF = rapid sand filter, RBF = river bank filter, n = 12)

stack conductivity (WW) conductivity (SW) [mS/cm] [mS/cm] RUN 1 Reference 1.09 ± 0.24 47.43 ± 6.11 RSF 1.21 ± 0.47 48.96 ± 4.50 100 µm 1.10 ± 0.47 48.65 ± 4.68 RUN 2 Reference 1.04 ± 0.21 58.55 ± 2.94 RBF 1.22 ± 0.34 58.21 ± 2.95

Table V.2 Average concentrations and standard deviations of the four most important cations in the feed waters (WW =wastewater, SW = seawater) before the beginning of a batch (RSF = rapid sand filter, RBF = river bank filter, n = 12). For the second run, no data are available due to technical issues stack Na+ K+ Ca2+ Mg2+ [mg/L] [mg/L] [mg/L] [mg/L] WW SW WW SW WW SW WW SW RUN 1 109.6 11,632.9 22.4 468.4 96.7 582.6 9.7 890.0 Reference ± 32.1 ± 3,849.5 ± 7.9 ± 95.9 ±28.1 ± 80.6 ±2.9 ± 307.9 118.7 11,147.0 19.3 401.9 115.6 569.2 10.6 877.2 RSF ± 34.6 ± 3,270.6 ± 9.4 ± 156.4 ± 71.3 ± 76.8 ± 3.3 ± 302.5 110.0 11,346.5 18.3 468.9 95.7 578.2 9.9 868.5 100 µm ± 31.1 ± 3,788.2 ±9.0 ±98.6 ±27.3 ± 74.5 ± 2.7 ±281.2 RUN 2 Reference NO DATA AVAILABLE RBF

Besides the difference in feed water composition, another uncontrolled variable is the water temperature (Table V.3). In the first run (executed in November and December), the water temperature was significantly lower (p=1.34∙10-19) than in the second run (executed from March until May), due to the lower room temperature in the lab. It can be expected that the biofilm will accumulate faster at higher temperatures [69]. Although it is hard to quantify the effect of the higher temperature in the second run, it is important to keep in mind the different circumstances when comparing results from both runs. Variability in temperature between stacks from the same run can be attributed to the different positioning in the lab (e.g. more solar irradiation on a barrel due to a different positioning towards the window), but is not significant (p=0.10 and p=0.93 for respectively the first and second run).

Table V.3 Average temperature and standard deviation of the feed waters (WW=wastewater, SW =seawater) during the first (Nov-Dec) and second (March-May) run, based on temperature measurements at the beginning and at the end of every batch (RSF = rapid sand filter RBF = river bank filter, n=12)

stack temperature (WW) temperature (SW) [°C] [°C] RUN 1 Reference 19.3 ± 1.1 19.2 ± 1.1 RSF 18.8 ± 1.1 18.9 ± 1.2 100 µm 19.1 ± 1.1 19.0 ± 1.1 RUN 2 Reference 20.6 ± 0.8 20.4 ± 1.0 RBF 20.6 ± 0.9 20.3 ± 1.2

V.1.1.2. RED pre-desalination

Inside the stacks, ions will tend to move between both feed water compartments through the IEM. To follow up on the seawater desalination process, the conductivity of both the seawater and the waste water was measured at the beginning and the end of every batch. As can be seen on the graphs in appendix VIII.2, the evolution of the conductivity shows the expected trend. During one batch, there is net transport of ions from the seawater to the wastewater, which increases the wastewater conductivity and decreases the seawater conductivity. In Table V.4, the average change in conductivity over the time span of one batch for both feed waters can be found.

Table V.4 Average relative change in conductivity and standard deviation of both feed waters over the time span of one batch (i.e. 3 to 4 days), with ∆conductivity calculated as the difference between the conductivity at the beginning and at the end of every batch, relative to the conductivity at the beginning of the batch. (WW = wastewater, SW = seawater, RSF = rapid sand filter, RBF = river bank filter, n=12)

stack relative relative ∆conductivity (WW) ∆conductivity (SW) [%] [%] RUN 1 Reference 135.41 ± 55.18 -3.01 ± 3.18 RSF 105.17 ± 120.78 -3.84 ± 2.88 100 µm filter 158.35 ± 88.64 -3.37 ± 3.77 RUN 2 Reference 140.13 ± 32.53 -6.13 ± 2.55 RBF 116.70 ± 78.04 -5.99 ± 2.57

Overall, the average desalination of the seawater is rather limited with an observed maximum of 6.13% (reference stack of the second run) due to the limited membrane area and the large batch volumes. There is no significant difference in the degree of pre- desalination between stacks running in parallel (p = 0.85 and 0.89 for the first and second run). The stacks from the second run show a significantly higher relative desalination (p=0.002) when compared to the stacks from the first semester. This can be

attributed to an increase in kinetic energy of the ions due to the higher feed water temperature during run 2 (see Table V.3). An increase in kinetic energy results in a higher diffusion coefficient and thus a higher ion mobility.

When looking at the absolute conductivity changes in Table V.5, a general trend over all stacks can be observed despite the high variability (due to the variability in feed water composition as explained in V.1.1.1.). The average conductivity decrease of the seawater is for all stacks higher than the conductivity increase of the wastewater. This can indicate an exchange of multivalent ions towards the seawater compartment at the expense of monovalent ions, since e.g. one Mg2+ cation contributes less to the conductivity than two Na+ cations.

Table V.5 Average absolute change in conductivity and standard deviation of both feed waters over the time span of one batch (i.e. 3 to 4 days), with ∆conductivity calculated as the difference between the conductivity at the beginning and at the end of every batch (WW = wastewater, SW = seawater, RSF = rapid sand filter, RBF = river bank filter, n =12)

stack absolute ∆conductivity absolute ∆conductivity (WW) (SW) [mS/cm] [mS/cm] RUN 1 Reference 1.44 ± 0.40 -1.86 ± 1.00 RSF 0.91 ± 0.75 -1.94 ± 1.63 100 µm filter 1.67 ± 0.56 -1.99 ± 1.89 RUN 2 Reference 1.41 ± 0.34 - 3.59 ± 1.49 RBF 1.20 ± 0.50 -3.51 ± 1.59

V.1.1.3. Ion transport

In order to determine the transport of the most important cations (Na+, K+, Mg2+ and Ca2+), the concentrations of these ions in the feed waters at the beginning and at the end of every batch were measured with ICP-OES. Unfortunately, no data can be shown for the second run, due to technical issues with the ICP-OES device when the analysis were supposed to be carried out.

As different batches were used, the variability in composition and thus in transport is high. This can clearly be seen in Table V.6, where the average relative differences in cation concentrations between the beginning and the end of a batch are shown:

푐퐶퐴푇,푒푛푑 − 푐퐶퐴푇,푠푡푎푟푡 ∆퐶퐴푇 = (14) 퐶퐶퐴푇,푠푡푎푟푡

with cCAT the concentration of the cation (mg/L) at the start or at the end of the batch.

Table V.6 Average relative difference and standard deviation in cation concentration in the feed waters (WW = wastewater, SW = seawater, RSF = rapid sand filter, n = 12)

Stack Feed ∆Na+ ∆K+ ∆Mg2+ ∆Ca2+ water [%] [%] [%] [%] WW 378.7 ± 133.5 69.3 ± 178.7 -42.3 ± 8.4 -86.5 ± 7.1 Reference SW -4.0 ± 6.4 -4.8 ± 6.3 -1.2 ± 2.7 10.6 ± 4.0 WW 280.3 ± 190.3 42.9 ± 57.8 -34.9 ± 29.45 -62.7 ± 31.3 RSF SW -4.0 ± 29.5 -2.3 ± 3.9 0.4 ± 2.4 9.9 ± 0.04 100 µm WW 391.0 ± 129.9 47.9 ± 68.6 -33.2 ± 8.7 -84.1 ± 8.3 filter SW -3.0 ± 26.9 -3.1 ± 2.8 -0.4 ± 1.6 10.1 ± 4.3

Due to the restricted sample size and the high variability between the different batches, it is hard to statistically prove deviations in ion transport for the different stacks. The only significant differences can be observed for the RSF stack: the wastewater Na+ increase (p=0.02) and Ca2+ decrease (p=0.009) are significantly lower. In appendix VIII.3 the results for the wastewater compartment were also given as a probability distribution. In other words, the graphs show the probability of a certain percentage of transport of the ion to occur. Keeping in mind the big variability, some general conclusions can be drawn based on the probability-representations of the ion transport and Table V.6. Transport of monovalent cations (Na+ and K+) from the seawater to the wastewater and of divalent ions (Ca2+ and Mg2+) from the wastewater to the seawater can be observed.

As mentioned before, the Ca2+ concentration of the seawater is already above average. 3- 2- 2- 2+ In the presence of PO4 , S04 , C03 etc. Ca can precipitate when the concentration near the membrane surface increases even further due to the ion transport from the wastewater. This has already been proven to be a source of fouling in an RED system with natural river water as the fresh water feed [32]. Therefore, precipitation of Ca2+ was investigated. A mass balance on the Ca2+ content in the water over the time span of a batch can be made. Since no water transport between both feed waters is observed, a net removal of Ca2+ from the feed waters indicates precipitation. In Table V.7, the average loss of Ca2+ relative to the total amount of Ca2+ in the beginning of the batch can be found:

(푐퐶푎2+ + 푐퐶푎2+ ) − (푐퐶푎2+ + 푐퐶푎2+ ) 2+ 푊푊,푠푡푎푟푡 푆푊,푠푡푎푟푡 푊푊,푒푛푑 푆푊,푒푛푑 ∆퐶푎 = (15) 푐 2+ + 푐 2+ 퐶푎푊푊,푠푡푎푟푡 퐶푎푆푊,푠푡푎푟푡

2+ with cCa2+ the concentration of Ca in a specific feed water at the beginning or at the end of the batch.

Table V.7Average loss and standard deviation of Ca2+ over the time span of one batch, relative to the total amount of Ca2+ in the beginning of the batch (eq. 17, n =12)

stack ∆Ca2+ [%] Reference 3.62 ± 3.98 RSF -0.38 ± 3.47 100 µm 3.19 ± 3.68

The deviations that are shown in Table V.7 are too small to prove whether Ca2+ is precipitated or not. Since the membrane area is rather limited, it has to be questioned if it would be determinable if precipitation took place.

The uphill transport of multivalent ions decreases the potential power output (as explained in section II.2.4.1). In order to estimate the consequences of the uphill transport on the potential power output, the net transport of the monovalent ions (expressed in meq/L) towards the wastewater compartment is corrected for the transport of the divalent ions (expressed in moleq./L) making the inverse movement. In other words, the relative power loss due to the uphill transport is estimated by the transport of the divalent cations, relative to the transport of the monovalent ions, both expressed in moleq./L (Table V.8).

Table V.8 Ion transport, expressed in meq./L, in order to estimate the impact of uphill transport on the potential power output (calculations based on the data of cation transport in wastewater samples)

Stack Transport of Na+ Transport of Ca2+ Loss and K+ and Mg2+ [%] [meq/L] [meq/L] Reference 17.91 4.21 23.50 RSF 10.24 3.42 33.43 100 µm filter 15.31 3.95 25.80

V.1.1.4. TOC analysis

On Figure V.1, the relative TOC increase in the wastewater for every batch can be observed. When looking at the data from the first run (Figure V.1A), a similar pattern for the three stacks can be seen. During the first four batches, the TOC concentration in the wastewater increases. From batch 6 until batch 9, the TOC concentration decreases in every batch but from batch 9 until the end, the TOC concentrations increases again per batch. The stack fed with RSF pre-treated wastewater shows a significantly lower TOC increase (p=0.59). However, the limited amount of samples makes it hard to determine if the stack fed with RSF pre-treated wastewater in fact deviates. In the stacks from the second run (Figure V.1b), the TOC concentration in the wastewater increases in almost every batch. No significant difference (p=0.049) between the reference stack and the RBF stack can be observed.

(A)

(B)

Figure V.1 Relative increase of total organic carbon (TOC) in the wastewater per batch, with (A) the stacks from the first run and (B) the stacks from the second run (RSF = rapid sand filter, RBF = river bank filter)

On Figure V.2, the relative decrease in IC in the wastewater for every batch can be observed. The IC concentration decreases in every batch for all stacks. No significant differences can be observed between stacks from the first run (p=0.004). In the second run, the IC removal is significantly higher (p=0.13) in the reference stack when compared to the RBF stack, although the remarks concerning the limited amount of samples should still be taken into account.

(A)

(B)

Figure V.2 Relative decrease of inorganic carbon (IC) in the wastewater per batch, with (A) the stacks from the first run and (B) the stacks from the second run (RSF = rapid sand filter, RBF = river bank filter)

Based on the available data, it is not possible to state with certainty what causes this IC removal and TOC increase. A possible theory is the conversion of IC to TOC by autotrophic microorganisms present in the biofilm. However, the absolute amount of IC removed is much lower than the absolute increase in TOC, as can be seen in Table V.9. Therefore, another possibility is transport of IC from the wastewater to the seawater. In order to test this hypothesis, a seawater sample was analysed on its IC content. With a concentration of 29.99 mg/L, the IC content was roughly 25% lower than the average IC concentration of the wastewater at the beginning of a batch (42.02 mg/L). Analysing the seawater samples on TOC and IC will show if seawater indeed serves as a sink for IC, but this can only be a part of the explanation. If approximately 5 mg/L IC moves from the wastewater towards the seawater, the concentration gradient drops to zero. With IC removal rates between 17 and 28 mg/L, other factors such as microbiological activity have to be taken into account. Based on the available

data however, it is not possible to estimate to what extent microbiological activity is responsible for the IC removal, and if any other processes play a part in the IC removal in wastewater.

Table V.9 The average total organic carbon (TOC) increase and inorganic carbon (IC) decrease in the wastewater for the different stacks (n=12)

stack ∆TOC ∆IC ∆TOC/∆IC [mg/L] [mg/L] [%] RUN 1 Reference 0.41 ± 3.13 -27.73 ± 6.87 2.1 ± 11.1 RSF 0.16 ± 2.41 -17.26 ± 7.06 1.3 ± 15.9 100 µm filter 0.71 ± 3.68 -23.56 ± 6.12 6.3 ± 19.6 RUN 2 Reference 1.54 ± 1.18 -24.48 ± 9.75 6.8 ± 10.4 RBF 2.69 ± 3.09 -21.48 ± 9.86 32.1 ± 61.1

V.1.2. Operational aspects

V.1.2.1. Pressure drop

The increase in pressure drop is determined by the resistance the water experiences when passing through the membrane module and can thus be related to the fouling that has developed on the membranes and spacers. Therefore, pressure drop development can be seen as an indication of the pre-treatment efficiency. On Figures V.3 and V.4, the pressure drops over respectively the wastewater and seawater compartments of the different stacks from both runs can be found.

As mentioned before, different cleaning protocols were followed when membrane flushing was needed in the first run (Figure V.3a and V.4a). After 13 days of operation, the pressure drop over the wastewater compartment of the reference stack was 1.09 bar (0.74 bar over the seawater compartment). It was decided to only flush the wastewater compartment (at a flow rate of 0.74 cm/s) with water and base solution since especially biofouling was expected (protocol 1 in Table IV.2). However, cleaning protocol 1 only induced a very limited improvement (a small decrease in pressure drop: from 1.09 bar to 0.87 bar over the wastewater compartment) while the pressure drop over the seawater compartment increased to 1.03 bar.

After 15 days of operation in the first run, the wastewater and seawater compartments of all three stacks were flushed (at a flow rate of 0.74 cm/s) with in succession water, base and acid solution in an attempt to remove both biofouling and scaling (protocol 2 in Table IV.2). Cleaning protocol 2 was more successful and the pressure drop over the different compartments decreased to values between 0.13 and 0.30 bar, except for the wastewater compartment of the reference stack. In the latter compartment, the pressure drop after flushing was still 0.64 bar. Therefore, when the pressure drop over this compartment exceeded 1 bar again after 31 days of operation, a different flushing

approach was chosen. The flow was reversed (from outlet to inlet) and doubled in magnitude to 1.48 cm/s, and acid and base were reversed in the flushing sequence (protocol 3 in Table IV.2). Only the wastewater compartment of the reference stack was flushed. Cleaning protocol 3 led to a pressure drop over the wastewater compartment as low as 0.37. Since this approach showed the best results, this was the technique applied when membrane cleaning was needed in the second run of experiments.

(A)

(B)

Figure V.3 Overview of the pressure drop over the wastewater compartments as a function of the time, with (a) the three stacks operational in the first run, and (b) the two stacks operational in the second run. When flushing was performed, the full line on the graph is interrupted (RSF= rapid sand filter, RBF = river bank filter)

When looking at the pressure drop development in the first run (Figure V.3a), a clear difference between the reference stack and the stacks fed with pre-treated wastewater can be noticed. As mentioned before, the wastewater compartment of the reference stack was flushed three times following different protocols (see section IV.2.3.). However, after every flushing, the pressure drop increased again rapidly. The stacks with pre-treated wastewater show results similar to each other with only a small increase in pressure drop over the whole run. When only looking at the pressure drop development, the 100 µm filter performed slightly better (from 0.03 to 0.12 bar) than the RSF (from 0.04 to 0.22 bar).

In the second run, there is less difference in pressure drop build-up over the wastewater compartment between the reference stack and the stack with pre-treated wastewater (Figure V.3b). The stack fed with RBF treated water had to be flushed two times (with subsequently acid and base according to protocol 3 in Table IV.2) to keep the pressure drop below 1 bar. As mentioned before, the RBF had to undergo maintenance after 18 days of operation. After restarting run 2, the pressure drop over the wastewater compartment of the RBF stack increased even faster than the pressure drop in the reference stack. This sudden increase in pressure drop can be attributed to granular particles that got loose during transport and restoration of the RBF. These sand particles were fed to the RED stack and caused clogging of the channels.

The reference stacks of both runs show more or less the same pressure drop development, making it possible to compare the results from the stacks operating in different runs with each other. Keeping in mind the remarks on comparability from section V.1.1. and the disturbance of the RBF halfway run 2, it can still be concluded that the RBF was not capable of achieving the same level of pre-treatment performance when compared to the RSF and 100 µm filter.

The pressure drop over the seawater compartments is shown in Figure V.4. In both runs, the stacks in parallel follow the same general trend since they were fed with the same water. Whereas the pressure drop over the wastewater compartments show a more irregular trend (with temporarily decreases or sudden steep increases), the pressure drop over the seawater compartment shows a steadier increase until 1 bar was reached and a cleaning was executed. It can be observed that the pressure drop over the seawater compartment increases at approximately the same speed (reference stacks, RBF stack) or even faster (RSF stack and 100 µm filter stack) than the wastewater compartment, despite the different filtration steps in the pre-treatment of the seawater. Probably, the bead filter and UV filter used by the university to treat the seawater is not sufficient to remove all micro-organisms present. The underground storage tank is not cooled, which makes regrowth in the tank possible.

(A)

(B)

Figure V.4 Overview of the pressure drop over the seawater compartments as a function of the time, with (a) the three stacks operational in the first run, and (b) the two stacks operational in the second run. When flushing was performed, the full line is interrupted. (RSF= rapid sand filter, RBF = river bank filter)

The only relevant difference between stacks running in parallel can be noticed during the final part of the first run. The stacks receiving pre-treated wastewater show a decreasing pressure drop over the seawater compartment, starting from around 0.60 bar (after 31 days in operation) down to 0.10 bar at the end. Based on the available data, there is no clear explanation for this behaviour. No changes were executed to the pre-treatment or the operation of the RED stacks, and all stacks were checked for integrity but no abnormalities were discovered. If the developed fouling layer would be disturbed by an unknown stimulus (e.g. air bubbles entering the stack, or a sudden change in pH), this would be notable in the other analysis. The change in conductivity of

the feed waters show no deviant behaviour when compared to earlier moments in the run (see appendix VIII.2.), the OCV-values stay more or less stable (see section V.1.2.2.) and no differences in TOC removal can be observed when compared to the reference stack (see section V.1.1.4.). Up until now, no explanation can be given for the pressure drop decrease in the stacks fed with RSF and 100 µm filter pre-treated wastewater.

V.1.2.2. OCV

On Figure V.5, the OCV at the start of a every batch is shown for every stack.

(A)

(B)

Figure V.5 Stack OCV at the beginning of each new batch, with (a) the tree stacks operational in the first run and (b) the two stacks operational in the second run (RSF = rapid sand filter, RBF = river bank filter)

During the first run, the stacks fed with pre-treated wastewater follow the same trend. From the start, the OCV of the pre-treated stacks is lower than the OCV of the reference stack. After 4 batches, the OCV decreases (in absolute values) to reach an equilibrium

around -770 mV after 8 batches (or 28 days in operation). This OCV drop before an equilibrium is reached suggests a decrease in permselectivity, probably caused by organic foulants. These large, charged molecules can be trapped in the membrane surface layer where they counterbalance the effective surface charge of the membranes and in that way decrease the charge density available for ion transport. Due to their negative charge, especially AEM are sensitive to this type of fouling. This causes the decreased apparent permselectivity, resulting in a decreased OCV according to the Nernst-Planck equation. After some time (in this case after 8 batches), the OCV reaches an equilibrium which indicates that the permselectivity does not decrease anymore. No more additional organic foulants are trapped in the membrane surface layer.

In the end, the OCV of the reference stack increases slightly. This might indicate that cleaning protocol 3, executed before the start of the tenth batch, succeeds in removing a considerable amount of biofilm and trapped organic foulants. The temporary increase in OCV during batch 3 and 4 is remarkable and the cause cannot be determined with certainty. According to the conductivity measurements, the feed water compositions did not differ significantly from the previous batches (see appendix VIII.2).

When looking at the stacks from the second run (Figure V.4b), the OCV starts to decrease instantly (in contrast to the OCV of stacks of the first run). It is important to notice that in between batch 6 and 7, the operation was stopped for one week to repair the RBF (see section IV.3.3.). Stacks were stored in a cooling room at 4°C. This interruption (and transport) might have disturbed the developed fouling layer, resulting in a temporary OCV increase in the reference stack when the operation was resumed. However, after the interruption, the stack receiving RBF pre-treated wastewater shows a steady increase in OCV until the end of the run, while the pressure drop was still building and flushing the wastewater compartment was needed four days after the restart of the second run (see Figures V.2 and V.3). A possible explanation for the OCV increase might be found in a disturbance of the biofilm layer by granular particles coming from the RBF. The RBF pre-treated water supplied to the stack contained a high amount of granular particles (that got loose during transportation and restoration of the RBF). These particles might have ruptured and expelled the biofilm. This hypothesis will be confirmed by the results from the membrane autopsy (section V.2).

V.1.2.3. Power density

The gross power density (Pgross) is determined based on the measured voltage and the applied current in the beginning of every new batch. In order to obtain the net power density (Pnet) , the pumping power has to be taken into account:

∆푃푆푊 ∙ Φ푆푊 + ∆푃푊푊 ∙ Φ푊푊 (16) 푃 = 푃 − 푛푒푡 푔푟표푠푠 퐴

with ∆P the pressure drop (Pa) and φ de volumetric flow rate (m³/s) of the feed waters and A the total membrane area (m²). In Figures V.6 and V.7, the different power densities for the stacks from both runs can be seen. (A)

(B)

(C)

Figure V.6 Gross power density , pumping power and net power density for the different stacks from the first run, with (A) the reference stack, (B) the rapid sand filter stack and (C) the 100 µm filter stack

In the power density data for the stacks from the first run (Figure V.6), it can be observed that the gross power density in all three stacks follows the same trend. After a period of gross power density decrease, an equilibrium is reached around 2.6 W/m². This period (i.e. when the gross power density decreases) matches with the decrease in OCV (Figure V.4a) and can thus be contributed to the decrease in charge density available for ion transport due to the trapping of organic foulants. The gross power densities (and the OCV) of the three stacks reach more or less the same equilibrium.

Based on these results, it can be concluded that neither the RSF or the 100 µm filter succeeded in retaining enough organic foulants to avoid a decrease in membrane performance. No conclusive explanation can be given for the alternating gross power density decrease and increase during the first 5 batches in the reference stack (Figure V.6a). Since the OCV is more or less stable in this period (Figure V.4a), no explanation can be found in the adsorption and desorption of organic foulants.

When looking at the pumping energy, a clear difference between the stacks fed with pre-treated wastewater and the reference stack can be observed. For the stack with pre- treated water, the pumping energy increases slightly until the stack compartments are flushed (after batch 5). The decline in the last three batches can be attributed to the unexplainable pressure drop decrease over the seawater compartment, as mentioned in section V.1.2.1. For the reference stack, the pumping energy increases fast from the start due to the pressure drop increase over both the seawater and wastewater compartment (Figures V.3a and V.4a). Only when the stack was flushed (after batch 5 and batch 9), a temporary decrease in pumping energy can be observed.

Based on the gross power densities, it seems as if the stacks all perform at a similar level. However, taking into account the pumping energy and thus calculating the net power densities, the added value of wastewater pre-treatment can be seen. It is hard to quantify the difference in net power density (since only the reference stack is flushed before batch 10 and because of the unexplainable pressure drop decrease in the stacks fed with pre-treated wastewater), but it is clear that the higher pumping energy needed in the reference stack considerably decreases the net power density and thus the stack performance.

The reference stack and the RBF stack from the second run show the same trend in gross power density, pumping energy and net power density for the first six batches. The pressure drop increases fast from the start, which results in a deceasing net power density. Based on these data, it can be concluded that the RBF stack does not succeed in producing a net power density similar to that of the RSF and 100 µm filter stack due to a higher pumping cost. After restarting the second run (batch 7), the gross power density of the RBF stack increases continuously until the end of the operation. This can be attributed to the sand particles entering the stack and disturbing the formed fouling layer. Therefore, it is hard to make valuable and representative conclusions for the last part of the second run.

(A)

(B)

Figure V.7 Gross power density , pumping power and net power density for the different stacks from the second run, with (A) the reference stack and (B) the river bank filter stack

V.2. Analysis of fouling

V.2.1. ATP analysis

Since ATP is an indicator for metabolically active cells, analysis were performed to characterize the activity of the developed biofilm. Scrapings of spacers, AEM and CEM from the inlet, middle and outlet of the sea- and wastewater compartments were collected and brought together in one single sample for each sample location. On Figure V.8, the determined ATP concentrations can be found.

(A)

(B)

Figure V.8 ATP concentrations at the influent, middle and effluent of the wastewater and seawater compartment, with (a) the stacks of the first run and (b) the stacks of the second run

In all stacks (over both runs), the ATP concentration was higher in the seawater compartment when compared to the wastewater compartment. This indicates that, despite the UV treatment executed before storage at the university, the seawater is not free of microorganisms when fed to the RED stack. The underground storage tank has a high volume (29 m³) and has to be refilled monthly. In theory, UV treatment can deal with such a big volumes if it is well designed, but it is plausible that in this case the UV pre-treatment does not succeed in destorying all microorganisms. While stored in the tank, the residual microorganisms have the time to develop in the stagnant water.

In the stacks from the first run (Figure V.8a), the ATP concentrations are always highest at the inlet and lowest in the middle (except for the seawater compartment of the wastewater stack). When comparing the three stacks, the influence of the wastewater pre-treatmens is clear: the ATP concentration is significantly lower when a pre-

treatment is performed, both in the seawater and freshwater compartment. Both findings can also be seen by visually checking the spacers (Figure V.9). Most biofilm can be observed near the inlet of the feedwaters, and the spacer from the reference stack shows more biofilm when compared to the spacers of the other stacks. The spacer from the stack fed with RSF pre-treated wastewater shows more biofilm than the stack fed with wastewater that was pre-treated with the 100 µm filter.

Figure V.9 Spacers from the three wastewater compartments and one seawater compartment (as an example, similar view on the spacers of the seawater compartment of the other stacks) of the stacks running in parallel from the first run (RSF = rapid sand filter)

The data from the stacks that operated in the second run deviate considerably. The overall ATP concentrations are lower (roughly by a factor 10 to 60), and in contrast to the first run, the highest concentrations can be found at the seawater outlet. The latter might be attributed to an unequal thightning of the stack components when the stack were reassembled for the second run. If the stack was somewhat looser at the seawater outlet, a local drop in water flow might have created better circumnstanec for the biofilm to develop. However, this would mean that the stacks from the second run were both thightned in a different way when compared to the stacks from the first run, but equal to each other, which would be quiet coincidal.

As can be seen on Figure V.10, the spacers also show a different fouling type when compared to the first run.

Figure V.10 Spacers from the wastewater and seawater compartments of the two stacks running in parallel during the second run (RBF = river bank filter)

When looking at the spacer from the wastewater compartment (and to a lesser degree the spacer from the seawater compartment) of the RBF stack, dark granular deposits can be seen, whereas the spacers from the first run more show a brown layer with smaller particles. As mentioned before when discussing the OCV data (section V.1.2.2.), the more granular, sand-like deposits in the RBF stack might find their cause in the rebuilding of the RBF itself halfway through the operation (section IV.3.3.). The ATP results together with the pictures on Figures V.9 and V.10 strengthen the idea that the fouling in the RBF stack was significantly different from the fouling occurring in the stacks from the first run, probably due to a disturbance of the biofilm due to sand grains entering the stack.

This can explain the different types of deposits observed and the lower ATP concentrations in the RBF stack, however it does not explain the lower concentration of ATP in the reference stack. Although no certainty on the exact cause can be given, a possible explanation might be found in the irregularities that occurred during the second run. The stacks were stored at 4°C for a week (while the RBF was restored) with demineralized water in the compartments. During the transport or storage, the biofilm might have been damaged and it can not be excluded that the microorganisms did not survive the storage period. The question might arise if the stacks in the second run are representative, but this will be dealt with in the conclusions section.

V.2.2. Carbohydrate analysis

As a second analysis regarding biofilm formation and distribution, carbohydrate content was determined with the Dubois method. On Figure V.11, the carbohydrate concentration at the inlet, middle and outlet of both compartments for all stacks can be found.

These results show clear similarities with the fouling pattern observed on the membranes and spacers (as can be seen on Figure V.9). In every compartment (besides the seawater compartment of the RBS stack), the carbohydrate concentration is highest at the inlet and lowest in the middle of the membrane. When compared with the results of the ATP concentrations, the relative higher amount of carbohydrates at the effluent side can be observed. This presence of the carbohydrates might be explained by EPS that got loose from the biofilm at the inlet and got fixed at the effluent. Near the inlet, where the ATP is highest, the biofilm is most active and microorganisms will secrete EPS to establish and strengthen the biofilm. Due to disturbances (e.g. by small particles) or insufficient adhesion, parts of these EPS might got loose and are transferred with the flow towards the effluent where they are retained by the spacers.

Figure V.11 Carbohydrate concentration (expressed as ppm glucose equivalents) for the inlet, middle and outlet of the seawater and wastewater compartments of the different stacks (Reference 1 = reference stack in the first run, RSF = rapid sand filter, Reference 2 = reference stack in the second run, RBF = river bank filter) When only looking at the stacks operating in the first run, a similar carbohydrate concentration at the inlet of the wastewater compartment can be observed. The biggest differences can be found at the middle and the outlet of the wastewater compartment, where the carbohydrate concentrations in the reference stack are higher when compared to the stacks with pre-treated wastewater. The carbohydrate concentration is lower in the stacks that operated in the second run, although the difference is less pronounced than observed in the ATP concentrations. This contributes to the idea that a biofilm was formed, but did not manage to stay biologically active during the whole run.

In the seawater compartments, the carbohydrate concentration is lower than could be expected based on the ATP results. Apparently, the present microorganisms in the seawater compartment are limited in their EPS production, possibly due to a limited amount of suited carbon source present in the water.

V.2.3. Microscopy analysis

Both SEM and confocal microscopy were executed to investigate the membrane surface. In order to compare the results with a reference, virgin membranes were analysed. These pictured can be found in Figure V.12

SEM AEM CEM

CONFOCAL AEM CEM

Figure V.12 Reference samples of virgin membranes (AEM = anion exchange membranes, CEM = cation exchange membranes), analysed with scanning electrode microscopy (SEM) and confocal microscopy

On the SEM pictures of the virgin membranes, some fibers are clearly visibles. The confocal pictures show a slight green fluorence (normaly indicating living cells) of the AEM and a clear red fluorescence (normally indicating dead cells) of the CEM. In this case, the fluoresence can be attributed to the membrane structure and chemistry (the present functional groups) and not to cells present on the membrane surface. This background reference is important to consider when interpreting the microscopy results of the membrane samples.

The SEM and confocal pictures of the membrane samples can be found in appendix VIII.4 and will be discussed here.

Due to the generally negative nature of organic compounds, most fouling is expected to occur at the AEM. When looking at the wastewater side of the AEM from the stacks in

the first run, denser patches of biofilm can be observed in the reference stack when compared to the stacks with pre-treated wastewater running in parallel. In the RSF and 100 micrometer stack, the observed biofilm is a thin, more evenly distributed film. These results match the higher pressure drop and the higher ATP and carbohydrate content in the reference stack. As expected and based on the SEM and confocal pictures, the wastewater side of the CEM shows much less biofilm and fouling. Besides on some of the fibers (probably where the biofilm could easily attach to the membrane) where some green fluoresence can be observed, no active biofilm can be seen.

The AEM wastewater side from the reference stack of run 2 shows a varried pattern of green, fluorescing areas and more dark spots. However, in the RBF stack a thick green film can be observed on the AEM. This is in contradiction to the observed data on ATP (section V.2.1.), where a much lower concentration was found in the stacks of the second run indicating less active biofilm. In this regard, it is important to point out the limited membrane area that is visualised with SEM and confocal microscopy. Only a small fraction of the membrane is visualised, and microscopy samples were taken in the most central part of the membrane where the differences in ATP were smaller than the differences near the edges (as can be seen in Figure V.8).

The seawater side of both membranes shows a different picture. For the CEM, there is little difference between the reference stack and the 100 µm filtration stack, but the stack pre-treated with sand filtration shows a denser and clustered biofilm formation. For the AEM, the stack with 100 µm filtration pre-treated water shows more biofilm than the reference stack. On all membranes, the biofilm is present as more dense particles when compared to the wastewater compartment where the observed biofilm is more evenly distributed. This can be matched to the ATP and carbohydrate results, where it was concluded that the biofilm in the seawater compartment contains less carbohydrates (and thus less EPS that form the film layer).

The results for the seawater compartment of the stacks from run 2 are less straightforward. Again, a dense green layer can be observed while the SEM show structures that are hard to relate with a biofilm. Up to now, no real explanation can be given for the deviating results from the confocal microsocpy, although it is important to keep in mind the considerations mentioned above regarding the limited membrane area that is visuallised.

V.3. Economic analysis

An extensive economic evaluation of the different pre-treatment methods is beyond the scope of this master thesis. The capital costs (so-called capital expenditures (CAPEX)) and operational costs (so-called operating expenditures (OPEX)) vary according to the design flow, the actual average flow, the quality of the feed water etc. Only a concise comparison in OPEX of the different pre-treatment methods is carried out in order to be

able to make a suggestion on the preferred pre-treatment technique based on both the experimentally determined pre-treatment performance and some economic considerations.

For sand filtration, the running costs are typically very low when compared to other pre- treatment methods due to its simplicity and limited maintenance. The OPEX costs area approximately 0.05 to 0.10 €/m³ in large scale plants [85]. An extra advantage when compared to other filtration methods (such as the 100 µm filter) is the use of a locally available, cheap, abundant and simple filter medium (i.e. sand).

RBF can be seen as a rudimentary, low-cost approach for water treatment. The cost for establishing an RBF system depend mainly on the aquifer characteristics. CAPEX include cost for the abstraction well (construction, pumping devices, control system etc.) while OPEX are primarily cost for pumping electricity. When trying to determine the operation costs of an RBF system, unit prices as low as 0.01 €/m³ were estimated [84].

Reliable data on the OPEX cost of a large-scale equivalent of the 100 µm filter are not obtained. However, it can be expected that the operational costs will be higher when compared to the RSF. In gravitational sand filtration, energy is consumed to pump the water to the top of the filter, but there are in general no additional energy requirements. Filter backwashing is easy and does not require chemicals, so no extra cost has to be taken into account. The sand grains that serve as the filter medium are cheap and can be replaced without entailing excessive costs. Therefore, it can be assumed that the RSF is economically the better choice over the 100 µm filter.

VI. CONCLUSION AND FUTURE PERSPECTIVES

VI.1. Influence of wastewater pre-treatment

The overall goal of this master thesis was to investigate the influence of different pre- treatment options for secondary wastewater effluent on the performance of a wastewater/seawater RED system in view of applying RED as a pre-desalination step in a hybrid RED-RO system for seawater desalination. RED as a unit process is normally evaluated based on its net power output, and although this is still the main criteria, some other aspects should be taken into account when evaluating RED performance as a part of this hybrid system. Besides the energy production, the salinity of the feed seawater will decrease in RED which will decrease the energy needed to desalinate the water in RO. Therefore the degree of desalination in RED also contributes to the evaluation of the system performance, together with the net power production and economic considerations regarding the applied pre-treatment.

Conclusions on the pre-treatment performance of the RBF have to be treated with care. The different analyses show a deviant behaviour of the RED system after the executed maintenance, and as a consequence it has to be questioned whether these results are representative for a well-functioning RBF. Due to the transport and restoration of the filter, sand grains were fed to the RED stack where they disturbed the fouling layer that had developed. The RBF itself was opened during the restoration, which disturbed the anaerobic conditions and due to time constraints, the filter was not equilibrated before it was used again to pre-treat wastewater. Therefore, conclusions regarding the pre- treatment performance of the RBF will only be based on the first 18 days of the fouling experiments (i.e. up until the RBF had to be shut down and restored).

A first conclusion that can be made is the urgency of adequate wastewater pre- treatment before it can be fed to an RED system. From the start, the pressure drop over the wastewater compartment of the reference stack increases fast, indicating a fast development of a fouling layer. The high pressure drop evidently results in a high energy demand for pumping the water through the RED stack. Several in situ membrane cleanings were necessary to reduce the pressure drop, but the flushing only induced a temporary pressure drop decrease. The stacks fed with RSF or 100 µm filter pre-treated wastewater were able to perform for weeks without cleaning while the pressure drop over the wastewater compartments only increased slightly. As the pressure drop in both stacks was still low after six weeks of testing, it is expected that operation is still possible for several weeks before membrane cleaning is needed. Based on the first 18 days of the operation, the RBF does not succeed in adequately pre-treating the wastewater. The pressure drop build-up increases similar to the build-up in the reference stack.

The differences in pressure drop development between the different stacks match the data obtained from fouling analysis. In the stacks fed with RSF or 100 µm filter pre- treated wastewater, the ATP and carbohydrate concentration are lower when compared to the reference stack and visually less biofilm can be observed. The highest ATP concentrations can be found near the inlet of the feed water.

The consequences of the differences in pressure drop development become clear when looking at the net power density for the different stacks. While the gross power density of the different stacks reaches more or less the same equilibrium, the net power density is considerably lower when no wastewater pre-treatment is applied when compared to the stacks fed with RSF or 100 µm filter pre-treated wastewater. Since the gross power density (and the OCV) does not differ much when comparing the reference stack with the stacks fed with pre-treated wastewater, it can also be concluded that the RSF and 100 µm filter do not succeed in removing enough organic foulants to prevent a decrease in membrane performance. The added value of the pre-treatment clearly is in the reduced biofilm formation, resulting in a lower pressure drop and thus a lower pumping energy required.

Although the pre-desalination rate in RED is evidently linked to the power production, in this case it is worth looking at it separately since in an RED-RO hybrid system, the decrease in seawater desalination contributes to a lower energy demand in the following RO step. However, due to the large batch volumes and limited membrane area, the desalination of the seawater in the executed experiments is rather low. No significant differences can be observed between stacks running in parallel, indicating that whether or not the wastewater is pre-treated does not influence the rate of ion transport.

It is clear that an adequate wastewater pre-treatment is necessary, but deciding whether if the RSF or the 100 µm filter is the preferred technique is harder to determine since they show more or less a similar performance. The net power density of the 100 µm filter stack is just a fraction higher at the end of the fouling experiments. The pressure drop in the RSF stack was higher at the end of the fouling experiments and shows a slightly steeper increase over the last batches when compared to the pressure drop development in the 100 µm filter stack. Visual analyses of the biofilm indicate that fouling was slightly more severe in the stack fed with RSF pre-treated wastewater, although exact quantification is difficult. It can be concluded that the pre-treatment performance of the 100 µm filter is slightly better than the RSF. Considering the robustness and simplicity of the sand filtration however, this pre-treatment shows to be most promising for upscaling.

VI.2. Future research

The analyses executed in this master thesis focussed on the link between fouling and stack performance in order to show the influence of wastewater pre-treatment and decide upon the most suited pre-treatment approach. However, more analyses should be performed to get a more complete view on the processes going on in the RED stacks. Analysis of the anion concentrations will be performed similar to the executed analysis of the cations. Furthermore, TOC concentrations in the seawater will be determined. This will give more insight in the TOC-IC dynamics observed in the wastewater samples. Another possible extension of the performed research might include looking into the fate of boric acid present in the seawater. As mentioned before, this is one of the most difficult components to remove in SWRO desalination. Therefore, it is interesting to see how a preceding RED step influences the boron removal.

Another important aspect is the upscaling of the findings to a full-scale plant. As mentioned before, the development and impact of fouling depends on several operational parameters, such as feed flow velocity, membrane area etc. The RSF and 100 µm stacks performed well for 42 days, but it is necessary to evaluate what the obtained results would mean for a full-scale plant (where the feed flow velocity and total membrane area are different).

In future research, more in depth characterization of the fouling layer has to be performed. Genome analysis (e.g. illumine sequencing) can be carried out in order to determine the microbial community in the biofilm. Although it will be challenging to take apart the membranes while keeping the formed fouling layer in a representative state, more diversification between fouling on CEM and AEM can give more insight in the fouling characteristics, development and spatial distribution. These analyses can contribute to a more complete view on the fouling, and thus on the necessary measures to improve RED stack performance.

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VIII. APPENDIX

VIII.1. Protocol for confocal microscopy analysis

Chemicals needed:

- PBS buffer (for 1L): o 8 g NaCl o 0.2 g KCl

o 1.44 g Na2HPO4.2H2O

o 0.24 g KH2PO4 o Dissolve in 800 mL demi-water o Adjust pH with HC lto 7.4 o Adjust volume to 1L with additional demi-water o Sterilize by autoclaving - 2.5% glutaraldehyde in PBS buffer - Staining solution: 1.5 µL SYTO9 (yellow) and 1.5 µL propidium iodide (red) per mL PBS buffer. Cover solution with Al foil to protect from light. - Mounting medium : o 9 mL glycerol o 1 mL 1 mol/L tris-HCl at pH 8,3 (adjust with NaOH) o 0,05 g n-propyl gallate

Procedure:

- Rinse with PBS buffer once or twice to wash away planktonic cells - Transfer membrane to 2.5% glutaraldehyde in PBS buffer and let biofilm fix at 4°C for 30 minutes. - Transfer membrane to staining solution and incubate in the dark (cover with Al foil) for 15 minutes. - Let destain for 15 minutes in PBS buffer to remove excess stain. - Let membrane evaporate for few minutes in the dark. - Cut cover slide to right size using diamond pen. - Cut the top of a yellow tip and apply a little bit of antifade agent on the sample. - Put the coverslip gently over the material. - Wait 30 minutes or longer before sealing the edges with glue - Label samples and store at -20°C.

VIII.2 Feed water salinity

(A)

(B)

Figure VIII.1Evolution of the feed water salinity (SW =seawater, WW = wastewater) over time, with (a) the reference stack, the rapid sand filter (RSF) stack and the 100 micrometer filter stack from the first run, and (b) the reference stack and the river bank filter (RBF) stack from the second run

VIII.3 Probability of ion transport

Figure VIII.2 Probability of a certain amount of transport to occur, with positive transport defined as the transport from seawater to wastewater, for the monovalent ions Na+ and K+ (based on the difference in ion concentration in the wastewater over the timespan of a batch)

Figure VIII.3 Probability of a certain amount of transport to occur, with positive transport defined as the transport from seawater to wastewater, for the divalent ions Mg2+ and Ca2+ (based on the difference in ion concentration in the wastewater over the timespan of a batch)

VII.4 Microscopy pictures of the membrane samples

VIII.4.1. Wastewater side, AEM

Reference stack (run 1)

RSF

NO PICTURE AVAILABLE

100 micrometer filter

Reference stack (run 2)

RBF

VIII.4.2. Wastewater side, CEM

Reference stack (run 1)

RSF

100 micrometer filter

NO PICTURE AVAILABLE

Reference stack (run 2)

NO PICTURES AVAILABLE

RBF

VIII.4.3. Seawater side, AEM

Reference stack (run 1)

RSF NO PICTURE AVAILABLE NO PICTURE AVALAIBLE 100 micrometer filter

Reference stack (run 2)

RBF

VIII.4.4. Seawater side, CEM

Reference stack (run 1)

RSF

100 micrometer filter

Reference stack (run 2)

RBF