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NEW PRE-TREATMENT FOR ENERGY EFFICIENT
SEAWATER DESALINATION : (ASSISTED) REVERSE
ELECTRODIALYSIS Aantal woorden: 27675
Astrid Van Houtte Stamnummer: 01201985
Promotor: Prof. dr. 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
Acknowledgements
Eerst en vooral wil ik mijn tutor Marjolein bedanken! Ik denk dat een zelfs een pagina nog niet zou
volstaan om dat te doen. Keer op keer kon ik je bereiken als ik problemen, vragen, zelfvertrouwen
nodig had. Ik had ook nooit het gevoel dat ik je eens lastigviel of ik me teveel voelde. Het was zalig om
te mogen bijleren en fouten te maken. Ik heb vooral heel veel geleerd dit jaar en niet alleen over
ARED/RED! Eveline had het me nog verteld en ik kan het nu alleen maar beamen: jij bent de beste
tutor die er bestaat. Jammer dat er geen nieuwe studenten dat kunnen meemaken volgend jaar.
Zonder jou was die thesis een stuk moeilijker geweest. En het was niet alleen veel werk maar ik heb
me ook nog eens heel goed geamuseerd. Dikke merci voor alles!
Daarnaast wil ik mijn promotor, prof. Dr. ir. Arne Verliefde, van harte willen bedanken voor zijn tijd en
zijn begeleiding. Het was heel erg leuk om deel van de vakgroep en z’n onderzoek te mogen En ook
natuurlijk voor de leuke werksfeer, de mopjes tussendoor en natuurlijk de sponsering van de feestjes.
Content
1.Introduction…………………………………………………………………………………………………………………………………….1
2. Literature review ................................................................................................................................. 6
2.1. Reverse Electrodialysis ............................................................................................................ 6
2.1.1. RED stack ............................................................................................................................... 6
2.1.2. Working principles of RED ..................................................................................................... 7
2.2. Ion exchange membranes ....................................................................................................... 9
2.2.1. Membrane composition ........................................................................................................ 9
2.2.2. Ion exchange membrane properties ................................................................................... 10
2.2.3. Ion and water transport ................................................................................................ 13
2.3. RED performance .................................................................................................................. 15
2.3.1. Maximum power output ............................................................................................... 15
2.3.2. Factors reducing obtainable OCV and RED power output .................................................. 15
2.3.2. Actual potential generated ............................................................................................ 23
2.3.3. Actual power output .................................................................................................... 23
2.4. Assisted reverse electrodialysis ............................................................................................. 25
2.4.1. ARED as pre-treatment for RO ...................................................................................... 25
2.4.2. Working principles of ARED ........................................................................................... 26
3. Objectives of this thesis ................................................................................................................. 28
4. Materials and methods ................................................................................................................. 29
4.1. Experimental (A)RED set-up .................................................................................................. 29
4.2. Chronopotentiometric experiments ..................................................................................... 30
4.2.1. Experimental set-up ...................................................................................................... 31
4.2.2. Protocol ......................................................................................................................... 32
4.3. Batch experiments ................................................................................................................. 33
4.3.1. Stand-alone batch experiments ................................................................................... 33
4.3.2. Combined batch experiments ....................................................................................... 34
4.4. Calculations ........................................................................................................................... 35
4.4.1. Actual stack potential calculations ................................................................................ 36
4.4.2. Power consumption pre-treatment .............................................................................. 38
4.4.3. Degree of desalination .................................................................................................. 39
4.4.4. Cost of pre-treatment ................................................................................................... 39
4.4.5. Total cost of ARED/RED-RO and stand-alone RO .......................................................... 40
5. Results and discussion ................................................................................................................... 41
5.1. Chronopotentiometric experiments ..................................................................................... 41
5.1.1. Preliminary scan experiment ......................................................................................... 41
5.1.2. Effect of the flow velocity .............................................................................................. 43
5.1.3. Effect of real sea- and wastewater composition vs artificial solutions ......................... 44
5.2. Batch desalination experiments ............................................................................................ 50
5.2.1. Stand-alone batch experiments .................................................................................... 51
5.2.2. Combined batch experiments ....................................................................................... 56
5.3. Economic analysis .................................................................................................................. 63
5.3.1. Cost/energy consumption in function of degree of pre-desalination ........................... 64
5.3.2. Cost/energy consumption in function of RO recovery .................................................. 67
6. Conclusion and Prospects .............................................................................................................. 70
6.1. Overall conclusion ............................................................................................................. 70
6.2. Prospects ........................................................................................................................... 71
7. Appendix ........................................................................................................................................ 73
References ............................................................................................................................................. 75
List of Abbriviations
AEM Anion exchange membrane
ARED Assisted reverse electrodialysis
CEM Cation exchange membrane
CP Concentration polarization
DBL Diffusion boundary layer
ED Electrodialysis
EDL Electrical double layer
FCD Fixed charge density
FO Forward osmosis
IEC Ion exchange capacity
IEM Ion Exchange membrane
MDF Multi-stage flash destillation
MED Multi-effect destillation
NF Nanofiltration
OCV Open circuit voltage
PAO Pressure assisted osmosis
PRO Pressure retarded osmosis
RED Reverse electrodialysis
RO Reverse osmosis
SD Swelling degree
SHE Safety, health and environment
List of symbols
𝐴 pm Effective hydrated ion radius
𝐴𝑐 - constant representing change in concentrate
concentration
𝐴𝑑 - constant representing change in diluate concentration
𝐴𝑚 m2 Total membrane area
𝐶𝑏 mol.m-3 Bulk concentration
𝐶𝑏𝑢𝑙𝑘 mol.m-3 Average bulk concentration
𝐶𝑐 mol.m-3 Concentrate concentration
𝐶𝑐,𝑖𝑛 mol.m-3 Concentrate inlet concentration
𝐶𝑐,𝑜𝑢𝑡 mol.m-3 Concentrate outlet concentration
𝐶𝑐𝑜𝑢𝑛𝑡𝑒𝑟−𝑖𝑜𝑛 mol.m-3 Counter-ion concentration
𝐶𝑐𝑜−𝑖𝑜𝑛 mol.m-3 Co-ion concentration
𝐶𝑑 mol.m-3 Diluate concentration
𝐶𝑑,𝑖𝑛 mol.m-3 Diluate inlet concentration
𝐶𝑑,𝑜𝑢𝑡 mol.m-3 Diluate outlet concentration
𝐶𝑚 mol.m-3 Membrane concentration
𝐶𝑡 mol.m-3 Concentration at time t
𝑑 m Thickness of the membrane
𝐷 m2.s-1 Diffusion coefficient
𝑑ℎ m Hydraulic diameter
𝐸 J Electricity consumption
𝐸𝑐𝑒𝑙𝑙 V Cell potential
𝐸𝑂𝐶𝑉 V Open-circuit voltage
𝐹 C.mol−1 Faraday number (96485 C.mol−1)
ℎ m Intermembrane distance
𝐼 A Current
𝑗 A.m-2 Current densisty
L m Cell length
𝑚𝑑𝑟𝑦 g Mass of dry membranes
𝑚𝑤𝑒𝑡 g Mass of wet membranes
𝑁 - Number of cell pairs
𝑁𝐴𝐸𝑀 - Number of anion exchange membranes
𝑁𝐶𝐸𝑀 - Number of cation exchange membranes
𝑁𝑚 - Number of ion exchange membranes
𝑃 W Power density
𝑃𝑔𝑟𝑜𝑠𝑠 W Gross power density
𝑃𝑛𝑒𝑡 W Net power density
𝑃𝑝𝑢𝑚𝑝 W.m-2 Total pumping power needed
𝑞 m2.s-1 Feed flow rate per cell per unit width
𝑄 m3.s-1 Feed flow rate
𝑅𝑔 J.mol-1.K-1 Universal gas constant (8.314 J.mol-1.K-1)
𝑅 Ω.m2 Total stack resistance
𝑅𝐷𝐵𝐿 Ω.m2 Diffusion boundary layer resistance
𝑅𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒𝑠 Ω.m2 Electrode resistance
𝑅𝑒𝑥𝑡 Ω External resistance
𝑅𝑖𝑛𝑡 Ω Internal resistance
𝑟𝑚 Ω.m Specific membrane resistance
𝑅𝑚 Ω Electrical resistance of a membrane
𝑅𝑜ℎ𝑚𝑖𝑐 Ω.m2 Ohmic resistance
𝑅∆𝐶 Ω.m2 Resistance due to changing bulk concentrations
𝑅𝑒 - Reynold number
𝑆 m² Membrane surface
𝑆𝑐 - Schmidt number
𝑆ℎ - Sherwood number
𝑆𝐷 - Swelling degree
𝑇 K Temperature
𝑡𝑏 - Transport number of electrolyte in the bulk phase
𝑡𝑐𝑜𝑢𝑛𝑡𝑒𝑟−𝑖𝑜𝑛𝑏 - Transport number of counter-ion in the bulk phase
𝑡𝑚 - Transport number of electrolyte through the membrane
𝑡𝑐𝑜𝑢𝑛𝑡𝑒𝑟−𝑖𝑜𝑛𝑚 - Transport number of counter-ion through the membrane
𝑇𝑖𝑜𝑛 mol.s-1.compartment-1 Ion transport
t s Time
𝑡𝑟𝑒𝑠 s Residence time
𝑈 V Actual stack potential
𝑈𝑤𝑜𝑟𝑘 V Work potential
𝑣 m.s-1 Feed velocity
𝑉 m-3 Volume
𝑤 m Width flow channel
W W Power output
𝑊𝑚𝑎𝑥 W Maximal power output
𝑋 meq.m-3 Fixed charge density
𝑧 - Electrochemical valence
𝛼 - Ion exchange membrane permselectivity
𝛼𝐴𝐸𝑀 - Anion exchange membrane permselectivity
𝛼𝐶𝐸𝑀 - Cation exchange membrane permselectivity
𝛽 - Mask fraction
𝛾 - Activity coefficient
𝛿 m Thickness of the DBL
휀 - Spacer porosity
𝜂 Pa.S Dynamic viscosity of the solution
𝜅𝑐 S.m-1 Electrolyte conductivity concentrate
𝜅𝑑 S.m-1 Electrolyte conductivity diluate
𝜇𝑖𝑜𝑛 - Ion strength
𝜌 kg.m-³ Density of the solution
ф m3.s-1 Volumetric flow rate of the feed
∆L m Distance between turbulence promoters
∆𝑝 Pa Pressure drop over inlet and outlet
#𝑐𝑜𝑚𝑝 - Amount of flow compartments
Abstract
The accessibility to water that is safe, acceptable and affordable is recognised by the United Nations
as a human right. Commonly, freshwater is the main source for potable water production, despite
earth’s freshwater is only 2.5% of the total water stock on the planet. Besides, only a small part of
earth’s freshwater is readily available as surface water. Moreover freshwater sources are shrinking
and the pressure on them is increasing due to population growth and enhanced living standards. This
implicates the need for drinking water production from alternative resources. Since seawater makes
up 96.5% of the earth’s water reservoir and the supply is free of variations, seawater desalination could
provide the solution. Currently reverse osmosis (RO) is the most applied technique for this process.
The biggest drawback is the high energy demand. This contributes to nuclear waste and greenhouse
gas emissions. Therefore this alternative is not really interesting considering global warming and the
aimed transition to ‘clean’ technologies. The energy efficiency of RO could be improved with
pre-desalination of the seawater. Previous research already investigated reverse electrodialysis (RED)
and assisted reverse electrodialysis (ARED) as pre-desalination. In RED, energy is produced through the
controlled mixing of seawater with a diluted stream. The ions will move spontaneously from the
seawater to the diluted compartment. This energy production can be transformed into electricity and
together with the lower electrolyte concentration for the RO feed, the energy-efficiency of the overall
process increases. The main drawbacks are the low desalination rate and high capital costs due to the
large membrane area needed. On the other hand, ARED has been investigating as an alternative for
RED. In this process ion transport is enhanced by applying an addition voltage in the same direction as
the concentration gradient. This results into higher desalination rates and thus a decreased capital
cost, nevertheless, energy will be consumed instead of produced.
This thesis focusses on the combination of ARED and RED as pre-desalination processes. At the start of
the pre-desalination, the internal stack resistance is high leading to slow desalination. The application
of ARED in that region could enhance the overall transport rate and lower the capital costs. The aim is
to find the ideal combination of both processes, taking both energy efficiency and capital costs in
consideration. Additionally, the same experiments were also performed with real sea- and wastewater,
instead of NaCl, to gain understanding in the practical behaviour of the system. This thesis can be
divided into three parts: chronopotentiometric experiments, batch desalination experiments and an
economic analyses.
The aim of the chronopotentiometric experiments is to examine the effect of flow rate on the ion
transport rate and internal resistance of the system as well as the effect of real sea- and wastewater
when they are used as feed instead of artificial solutions. The system will be evaluated both in ARED
and RED. The mechanisms that affected the outcome most were the presence of a diffusion boundary
layer (or concentration polarization, CP) and the change in feed water and membrane resistance
contributed to variable bulk concentrations during operation. A first conclusion of the scans is that in
RED, the CVC shows a small upward bend which is attributed to CP. In ARED this expected upward
bend was not present, instead a downward bend was observed. This indicates a lower total stack
resistance than was calculated for the ideal situation. This shows that the feed solution resistance and
the membrane resistance, to a lesser extent, decrease as a result of the increased concentrate
concentration by faster ion transport at higher current densities. When the flow rate was increased
from 50 ml.min-1 to 100 ml.min-1, the internal resistance was increased again. From operating the stack
However when the experiment is performed with sea- and wastewater, the CVC is shifted upwards
whereby the energy efficiency becomes smaller. Previous research revealed already that Donnan
dialysis occurs in RED which reduces the driving force for electrolyte transport. In Donnan dialysis
monovalent ions, which move according to the concentration gradient, are exchanged for multivalent
ions from the concentrate. The transport is based on the difference in electrochemical potential
between the solutions and would mainly depend on the presence of these multivalent ions in de
concentrate. However if Donnan dialysis mainly depends on the presence of multivalent ions in the
concentrate, the CVC of 0.5 M NaCl with wastewater should also show a bigger deviation. Since
seawater and wastewater both have a lower conductivity, CP was calculated to see if depletion occurs
at the membrane surface. In the end, no explanation was found for this upward shift. However the
error bars were large which indicates the bad reproducibility. This could be caused by possible
seawater contains polluting components, fouling of the membranes or the inappropriate operation of
the electrolyte rinse solution.
The scope of the batch desalination experiments was to get insight in the combination of ARED and
RED as pre-treatment – and to find the optimal combination for the operation as combined pre-
desalination before RO. The batch desalination experiments were carried out in ARED and RED to
examine resistances and compare the operation modes of the stack. This includes the amount of
energy consumption/production, the desalination rate and the maximal degree of desalination
possible in function of various current densities applied. Afterwards, combined batch experiments
were conducted as well.
The main result obtained from batch desalination experiments in RED is the maximal degree of
desalination. The lower the current density, the higher the maximal degree of desalination. This
decreased desalination capacity is caused by irreversible losses. These losses will increase with
increasing current density. Most likely, the more reversible operation of the stack at lower current
densities is due to the effect of CP at higher current densities. Because the higher the current density,
the lower the concentration gradient over the membranes and thus the lower the driving force.
Through this, the stack potential will sooner equals zero and thus a lower maximal desalination
capacity can be obtained.
To find the optimal combination of ARED-RED as pre-treatment, several assumptions had to be made.
In this thesis, the optimal current density in ARED was based on the minimum pre-treatment cost. For
this a membrane price of 100 €.m-2 was used but this price is highly uncertain and IEM will probably
become cheaper the next years as manufacturers predicted. For the ideal combination found, ARED
would be applied first at a current density of -100 A.m-2. When the concentrate conductivity is equal
to 2.15 mS.cm-1, ARED can be switched to RED. The concentrate conductivity to switch to ARED was
not calculated precisely. The experiments performed showed low stack resistances for concentrate
conductivities between 1.5 and 3 mS.cm-1. Various combined batches with different RED current rates
were performed to find the perfect RED current density. These were also repeated with real sea- and
wastewater but this latter only succeeded for a current density from 0 to -11.13 A.m-2. Additionally,
the maximal degree of desalination varyies between 24 and 28% in the current density range. Current
densities lower than - 11.13 A.m-2 lead to lower desalination rates and this is not wanted.
And last some basic calculations were conducted to make an estimation of the total costs for the
ARED/RED-RO seawater desalination as well as the energy that is saved. Thence, the different
combinations could be compared to each other but also to the total cost of stand-alone RO is order to
test its economic feasibility. From the economic analysis, it appears that none of the ARED/RED-RO
processes is more cost effective than stand-alone RO, due to the high capital costs. Even when the
membrane price changes to 2€.m-2, stand-alone RO would still be cheaper. The cost are the higher for
low current densities applied in RED because of the slower desalination rate and thus the larger
membrane area needed. When ARED/RED-RO with a current density of -11 A.m-2 in RED is compared
to RED-RO with the same current density, the total cost of ARED/RED-RO will be smaller. This proves
that although ARED consumes electricity compared to RED, ARED-RED will still be cheaper due to the
faster desalination and thus less membrane area needed. On the other hand, after a desalination
degree of 7% reached, the electricity production will be lower compared to RO. Half of the energy for
stand-alone RO would be needed if a pre-desalination of 40% would be reached.
However when pre-desalination occurs, higher recovery rates will be possible. Since the biggest cost
of the combined process is the pre-treatment, the total cost per volume of permeate will decrease
with increasing recovery. For a practical pre-desalination of 20%, 60% recovery is possible.
Samenvatting
De toegankelijkheid tot water dat veilig, toelaatbaar en betaalbaar is, wordt door de Verenigde Naties
als een mensenrecht erkend. Algemeen is zoetwater de belangrijkste bron voor drinkwaterproductie,
ondanks dat het aandeel aan zoetwater slechts 2.5% van het totale waterreservoir op aarde bedraagt.
Bovendien is slechts een klein deel van het zoetwater op aarde gemakkelijk beschikbaar, namelijk het
oppervlaktewater. Bovendien neemt de druk op zoetwaterbronnen toe door de continu toenemende
wereldbevolking en de verbeterde levensstandaard. Er is dus behoefte aan alternatieve bronnen voor
de drinkwaterproductie. Aangezien zeewater 96.5% van het waterreservoir van de aarde vormt en de
voorziening vrij is van variaties, kan zeewater ontzouting de oplossing bieden. Momenteel is
omgekeerde osmose (RO) de meest toegepaste techniek voor dit proces. Het grootste nadeel is de
hoge energie vraag. Dit draagt bij tot de uitstoot van kernafval en broeikasgassen. Gezien de
opwarming van de aarde en de gewenste overgang naar duurzame technologieën is ook dit proces niet
echt interessant. De energie-efficiëntie van RO kan echter worden verbeterd met ontzouting van het
zeewater vooraf. Voorafgaand werd al onderzoek uitgevoerd naar omgekeerd elektrodialyse (RED) en
geassisteerde omgekeerde elektrodialyse (ARED) als voorbehandeling. In RED wordt energie
geproduceerd door middel van het gecontroleerd mengen van zeewater met een verdunde stroom.
De ionen zullen spontaan verplaatsen van het zeewater naar het verdunde compartiment. Deze
energieproductie kan worden omgezet in elektriciteit en samen met de lagere elektrolytconcentratie
van de RO-voeding neemt de energie-efficiëntie van het totale proces toe. De belangrijkste nadelen
zijn de lage ontzoutingssnelheid en de hoge kapitaalkosten door het grote membraanoppervlak dat
bijgevolg nodig is. Aan de andere kant werd ARED onderzocht als alternatief voor RED. Bij deze
technologie wordt het ionentransport versneld door het aanbrengen van een extra potentiaalverschil
in dezelfde richting als de concentratiegradiënt. Dit resulteert in hogere ontzoutingssnelheden en dus
een daling van de kapitaalkost. Daar tegenover staat dat energie worden verbruikt worden in plaats
van geproduceerd.
Deze thesis focust op een combinatie van ARED en RED als voorontzouting. Aan het begin van dit
proces, de interne stack weerstand leidt tot trage ontzouting. Het toepassen van ARED in dit gebied
kan de transportkost en investeringskostkost verlagen. Het doel is om de ideale combinatie te vinden
tussen beide processen, en daarbij energie efficiëntie en investeringskost beide in overweging te
nemen. Dezelfde experimenten zijn uitgevoerd met echt zee- en afvalwater, in plaats van NaCl, om
een klare inkijk te krijgen in het gedrag van het systeem. Deze thesis kan in drie delen opgedeeld
worden: chronopotentriometrische experimenten, batch desalination experimenten en een
three parts: chronopotentiometric experiments, batch ontzouting experiments and an economische
analyse.
Het doel van chronopotentiometische experimeten is om het effect van stromingssnelheid op de ion
transportsnelheid en interne weerstand van het system te testen, alsook het effect van echt zee- en
afvalwater als ze gebruikt worden als voedings in plaats van kunstmatige oplossing. Het system zal
geevalueerd worden in ARED en RED. De processen dat de uitkomst het meest beinvloedden waren
de grenslaag effecten (of concentratie polarisatie, CP) and de verandering in voedingswater en
membraan weerstand, te wijten aan de verandelijke bulk concentratie gedurende de werking. Een
eerste conclusie van de scans is dat met RED, de CVC een kleine afbuiging naar boven toont die
toegeschreven is aan CP. In ARED was deze verwachte afbuiging naar boven niet aanwezig, in plaats
daarvan was een afbuiging naar beneden te zien. Dit duidt op een lagere stack weerstand dan
berekend was voor de ideale situatie. Dit toont aan dat de weerstand van de voedings oplossing en de
membraan weerstand, naar een kleinere omvang, verlagen als resultaat van verhoogde concentratie
door sneller ion transport op hogere stroomdichtheid. Als de stroomsnelheid verhoogde van 50
ml.min-1 to 100 ml.min-1 , werd de interne weerstand opneuw verhoogd. Als het experiment echter
met zee- en afvalwater uitgevoerd wordt, wordt de CVC verhoogd, waarbij de energie efficientie
verkleind wordt. Voorgaan onderzoek ttonde al aan dan Donnan dialyse gebeurt bij RED wat de
drijfkracht voor elektroliet transport vermindert. Bij Donnan dialyse zullen monovalente ionen, die
bewegen naargelang de concentratiehelling, vervangen worden door multivalente ionen van het
concentraat. Het transport is gebaseerd op het verschil in electrochemisch potentieel tussen de
oplossingen en zou voor het merendeel afhangen van de aanwezigheid van deze multivalente ionen in
het concenraat. Als Donnan dialyse echter hoofdzakelijk afhangt van de aanwezigheid van multivalente
ionen in het concentraat, dan zou de CVC van 0.5 M NaCl met afvalwater een grotere afwijking moeten
tonen. Omdat zee- en afvalwater een lagere geleidingsvermogen hebben,werd CP berekend om te zien
of er uitputting optreed aan het membraanoppervlak. Uiteindelijk is er geen verklaring gevonden voor
deze opwaardse verschuiving. De foutmarges waren breed wat tot een slechte reproduceerbaarheid
leidt. Dit kan veroorzaakt worden door zeewater die vervuilde componenten bevat, die de membranen
of de ongepaste werking van de elektrolieten spoeling oplossing bevuilen .
Het doel van de batch ontzoutings experimenten was om inzicht te krijgen in de cambinatie van ARED
en RED als voorbehandeling en om een optimale combinatie te vinden voor de verrichting van
gecombineerde ontzouting voor RO. De batch ontzoutings experimenten waren uitgevoerd in ARED en
RED om weerstand te onderzoeken en de verrichtingsmanier van de stack te vergelijken. Dit houdt de
hoeveelheid energieverbruik/productie in, de ontzoutingssnelheid en de maximale graad van
ontzouting die mogelijk is in funtie van verscheidene stromingsdichtheden die aangebracht werden.
Nadien, zijn ook gecombineerde batch experimenten uitgevoerd. Het hoofdresultaat verkregen van
batch ontzoutings experimente met RED is de maximale grad van ontzouting. Hoe lager de
stromingsdichtheid, hoe hoger de maximale ontzoutingsgraad. Deze vermindering van de
ontzoutingscapaciteit is te wijten aan onomkeerbare verliezen. Deze verliezen zullen toenemen met
de stromingsdichtheid. Hoogstwaarschijnlijk is de meer omkeerbare verrichting van de stack bij lagere
stromingsdichtheid te wijten aan het effect van CP bij hogere stromingsdichtheden. Hoe hoger de
stromingsdichtheid, hoe lager de concentratiehelling over het membraan en dus hoe lager de
drijfkracht. Daardoor zal het stack potentieel sneller 0 evenaren en du seen lagere maximale
ontzoutingscapaciteit kunnen verkrijgen.
Om de ideale combinatie te vinden van ARED-RED als voorbehandeling, hebben we verschillende
veronderstellingen moeten maken. In deze thesis was de optimale stromingsdichtheid in ARED
gebaseerd op de kosten van de voorbehandeling. Voor dit werd een membraan voor de prijs van 100
€.m-2 gebruikt, maar deze prijs is zeer onzeker en IEM zal waarschijnlijk goedkoper worden de Jaren
hierop volgend zoals fabrikanten voorspellen. Voor de ideale combinatie zou ARED eerst toegebracht
worden aan een stromingsdichtheid van -100 A.m-2.
Als het concentraat geleidingsvermogen gelijk is aan 2.15 mS.cm-1, ARED kan gewisseld worden met
RED. Het concentraad geleidingsvermogen om ARED om te schakelen is niet precies berekend. De
gedane experimenten toonden lage stack weerstand voor geconcentreerde geleidbaarheid tussen 1.5
and 3 mS.cm-1. Verscheidene gecombineerde batches met verschillende RED stromingssnelheden
werden uitgevoerd om de perfecte RED stromingssnelheid. Deze werden herhaald met echt zee- en
afvalwater, maar dit laatstgenoemde slaagde enkel voor een stromingsdichtheid van 0 to -11.13 A.m-
2. Bovendien varieert de maximale ontzoutingsgraad tussen 24 and 28% in de stromingsdichtheid
omvang. Stromingsdichtheden lager dan - 11.13 A.m-2 leiden tot lagere ontzoutingsgraden en dit is niet
gewild.
Als laatst zijn enkele basis berekeningen uitgevoerd om een schatting te maken van de totale kost voor
de ARED/RED-RO zeewater ontzouting alsook de energie die is gespaard. Vandaar dat de verschillende
combinaties vergeleken kunnen worden, alsook de totale kost van alleenstaande RO om de
economische haalbaarheid te testen. Uit de economische analyse blijkt dat geen enkele van de
ARED/RED-RO processen meer kost dan alleenstaande RO, door de hoge
1
1. Introduction
The accessibility to water that is safe, acceptable and affordable is recognised by the United Nations
as a human right [1]. Despite this, the World Health Organisation estimates in 2015 that still 663 million
people lack access to improved drinking water sources [2].
Furthermore, the pressure on our water resources is increasing due to the population growth and
enhanced living standards. As a consequence, the Food and Agriculture Organisation expects that 1.8
billion people will be living in regions with ‘absolute’ scarcity by 2025 [3]. Absolute scarcity means that
there is insufficient water available to meet human needs and wants. This implies the need for
processes to produce drinking water from alternative resources.
Seawater desalination is a possibility to overcome these issues. Since seawater makes up 96.5% of the
earth’s water reservoir [4], the supply is free of variations. Especially the Middle East depends on this
technique to meet its fresh water supply and its use is still growing. A variety of desalination techniques
exist which can be categorized in thermal and membrane processes. Multi-stage flash (MSF) distillation
and multi-effect distillation (MED) are examples of the first category while reverse osmosis (RO),
nanofiltration (NF) and electrodialysis (ED) belong to the second [5].
The biggest drawbacks of these techniques are the residual concentrate that is discharged and the high
energy demands. Only 25.5% (based on tonnes of oil equivalents) of the total energy production in
Europe is renewable. Other energy production processes lead to nuclear waste or greenhouse gas
emissions [6]. Since global warming is one of the biggest environmental issues we are facing, low
energy technologies should be targeted.
Figure 1: schematic overview of RO (own figure).
Due to its lower cost and simplicity, RO is the most interesting desalination technique [4]. Today, it is
the main technology in desalination processes. In 2009, it accounted for 44% of the desalination
production capacity worldwide [7]. This pressure driven membrane technique creates potable water
by forcing seawater over a semipermeable membrane (figure 1). The membrane ideally only passes
the water: in RO, even the smallest contaminants and ions (99.6 to 99.8%) can be separated.
Eventually, deionized water (permeate stream) will flow through the membrane while the residual
concentrate stream stays behind. The recovery is equal to the permeate flow rate divided by the feed
flow rate. For seawater, a recovery of 60% is the maximum since the salt concentration at the
feedwater side will increase during operation. In order to make the water molecules move against the
osmotic pressure, an external pressure needs to be applied, ranging from 55 to 85 bar. For higher
2
recoveries, higher pressures would have to be applied and the most common RO membrane modules
are not designed for this. Membrane modules that can withstand higher pressures at very expensive
and thus not widely used. This is also the most energy demanding step in the desalination [8].
Crucial steps in this technique are the pre-treatment and post-treatment. The pre-treatment
procedure depends on the feedwater quality and the operational conditions. A poor pre-treatment
could have various effects but mainly decreased lifetime of the membranes and reduced performance
as a result of fouling, biofouling and scaling. Examples of pre-treatments are addition of chemicals
followed by flocculation-coagulation/sedimentation, microfiltration, ultrafiltration,… [8]. Post-
treatment is responsible for assuring the quality of the water meant for distribution by disinfection,
remineralization,… [9].
The theoretical minimal energy for seawater desalination is 1.06 kWh.m-3 (for a feed water containing
35 g.l-1 NaCl, at 50% feed water recovery). With the current technologies, an energy demand within a
factor two of the theoretical one should be possible, though in practise it is still three to four times
higher than the minimum. This is due to the need for extensive pre- and post-treatment steps. Further
investigation in pre- and post-treatment could improve the efficiency of the plant [10]. For the RO, the
minimum energy required has boundaries set by the thermodynamic laws while further.
A solution to reduce the energy demand could be the implementation of electrodialysis (ED) as a pre-
treatment for RO as seen in figure 2 [11]. ED is a voltage-driven membrane process which has been
industrially used since the 1950s. The working principle is based on the fact that the salts dissolved in
water are ions and thus either positively or negatively charged. An electrical voltage is applied over the
electrodes which forces cations to move to the negative electrode and vice versa for anions. To get a
separation between the solvent, mostly water, and the ions, ion exchange membranes (IEM) are used.
They can be divided into two types: anion exchange membranes (AEM) that are positively charged and
thus give passage to anions and cation exchange membranes (CEM) that are negatively charged and
thus give passage to cations. They are alternately positioned in between the electrodes [12]. A
drawback of this desalination technique is that the energy consumption for low salinity water is high,
since resistance plays an important role. In contrast, the power requirements increase for RO when
the salinity of the feedwater is high, which leads to high energy consumption. Thus a combination of
ED as pre-desalination followed by RO (ED-RO) might result in a more energy-efficient system [11].
Figure 2: schematic overview of hybrid ED-RO system (own figure).
3
Reverse electrodialysis (RED) is a desalination technique derived from ED which is based on the same
principles. It also has alternated AEM and CEM positioned between two electrodes but the ion
transport is based on the salinity gradient. The flow channels created by the IEM are alternatingly filled
with a concentrated or diluted salt solution whereby the ions will move spontaneously, driven by the
electrochemical potential difference between the solutions. The ion flow will be converted to an
election flow, generating electrical curunt at the electrodes. This process is also called controlled
mixing. It can be used like ED as a pre-treatment step, but also to produce sustainable energy which is
applied for example at the Afsluitdijk in the Netherlands where fresh water flows into the sea [13].
Theoretically, 1.7 MJ energy can be generated per m3 river water when it is mixed with the same
volume of seawater. When river water is mixed with an excess of sea water the energy production
even can go up to 2.5 MJ per m3 [14]. According to Post et al. (2008), over 80% of this potential energy
can be converted into electricity when the feed water is recirculated [15].
RED can be combined with RO where RED can be used as pre- and/or post-treatment. The
implementation of RED as a pre-treatment step for RO (figure 3A) is a substitute for ED-RO. By pre-
treating the feed solution with RED, the osmotic pressure is reduced which results in lower power
supply needed to perform RO afterwards. Part of the required power for RO can even be generated by
the pre-treatment step. On top of that the concentration of the brine solution, which is related to
environmental issues, will decrease. RED can also be implemented as a post-treatment step for the
concentrated brine solution from the RO. Hereby, the concentration of the brine solution will be
decreased while power will be produced. The produced power will be higher compared to RED as pre-
treatment because of the higher concentration of the brine. However, the osmotic pressure will be
higher which results in a higher power consumption to perform RO. Therefore the energy efficiency is
higher when RED is used as a pre-treatment procedure [16]. A previous study [17] showed that it would
be technically feasible to produce energy-neutral drinking water through the RED-RO hybrid process.
In practice, this hasn’t been achieved due to losses in both RED and RO.
The RED-RO hybrid process could reduce the energy demand though when the RED stack design is
optimised. However, the investment costs need to be kept in mind as well. Since membrane prices are
high, trade-off between required membrane area versus degree of desalination in RED, and thus
energy produced is needed. The price of membranes and fossil fuel is variable, so the economic
situation could be different in the future.
Met opmerkingen [AV1]: Weet niet of je RED een desal techniek kunt noemen
4
Figure 3: schematic overview. A: hybrid RED-RO system, B: hybrid ARED-RO system. The
full lines represent water flow while the dashed lines represent electric current (own
figure).
RED is able to produce energy, however the stack resistance is very high in the beginning. This leads to
slow desalination and thus a large membrane area needed. To overcome the slow desalination rate,
and thus the large membrane area needed, regarding RED as pre-treatment, assisted reverse
electrodialysis (ARED) could be used (figure 3B). In this system, the salt transport rate from the
concentrated to the dilute solution is increased compared with RED. This is done by applying a small
potential difference across the membrane in the same direction as the concentration difference. By
promoting the transport rate, less membrane area is needed. This could be an economic advantage
since IEM are very expensive [18]. The disadvantage is that no electricity is produced. However, the
energy consumption of the ARED-RO combination would still be lower compared to stand-alone RO
[19].
Although RED has been studied for over 60 years, the research has mainly been done with artificial
seawater and river water. Seawater actually contains a variety of salts which can alter the result
obtained by pure sodium chloride. Hong et al (2013) observed a 9–20% lower power density when
magnesium sulphate, sodium sulphate, and magnesium chloride were present in the feed solution
[20]. On top of that, research has been mainly focused on the production of green energy on places
where river water flows into the sea, thus maximizing the power output and the efficiency.
The scope of this thesis is to investigate the use of ARED and RED as pre-treatment techniques for RO
for a more energy efficient drinking water production. RED is able to produce energy, however the
stack resistance is very high in the beginning. This leads to slow desalination and thus a large
membrane area needed. To overcome the high membrane resistance in the beginning, ARED could be
applied first, up to a certain degree of desalination – after which RED could take over. Various current
densities and combinations will be tested in order to seek the most energy efficient/ financially
interesting pre-treatment. Additionally, the switching point from ARED to RED is of great interest since
energy consumption is desired to be as low as possible.
Met opmerkingen [AV2]: Weet de lezer al wat dit is? Je moet een verhaal vertellen, en de lezer daarin meenemen
5
Instead of river water, wastewater will be used as the low salinity water source (and thus ion sink).
Since desalination of seawater with RO has a high energy consumption, it is generally performed in
regions where surface water is scarce. On the contrary, wastewater is omnipresent, and discharged
anyway.
The tests are not only performed with artificial waters but also real sea- and wastewater are tested to
investigate the different outcome.
Met opmerkingen [AV3]: Wel iets zeggen over waarom je niet meteen het afvalwater opnieuw inzet na zuivering
Met opmerkingen [AV4]: Je intro is behoorlijk lang. Mensen kunnen mss de draag kwijt geraken. Mss op einde nog even kernachtig samenvatten in objectieven van dit onderzoek
Met opmerkingen [AV5]: Wat bedoel je daarmee? Wel iets zeggen over wat je verwacht dat ‘”different outcome” zal zijn
6
2. Literature review
2.1. Reverse Electrodialysis
RED is a ‘clean’ technology developed to produce energy from the reversible mixing of two streams
with a different salinity. The Gibbs free energy of mixing is the potential energy that can be recovered,
also referred to as ‘salinity-gradient energy’ or ‘Blue Energy’ [21]. This technology was already
mentioned by Pattle et al. in 1954 [22].
2.1.1. RED stack
In order to understand the working principles, first the design of an RED stack (Figure 4) is explained.
A typical RED stack is made of a variable number of RED cells. Each cell, also called cell pair, consists
of a cation exchange membrane (CEM, see § 2.2.), a compartment for a concentrated salt solution, an
anion exchange membrane (AEM, see § 2.2) and a compartment for a dilute salt solution.
Consequently, the RED stack consists of alternating CEM and AEM placed between an anode and
cathode. Gaskets and spacers are placed between the membranes. While the gaskets regulate the feed
of the concentrated or diluted salt solution to the different compartments, the spacers create the flow
compartments and increase the mixing of the feed.
The outer compartments are filled with electrolyte solution. To separate this solution from the last
flow compartment, an extra CEM is placed after the last cell pair [21].
Figure 4: Design of an RED stack [14].
7
2.1.2. Working principles of RED
RED transforms salinity gradient energy into electricity. The flow compartments are alternately filled
with a concentrated or diluted salt solution while CEM and AEM separate them. CEM and AEM are
negatively and positively charged membranes, respectively. This allows positively charged ions to
transfer through CEM and vice versa for AEM. This means that ions that are oppositely charged to the
fixed ions of the membrane (so-called “counter-ions”) are able to pass through the membrane, co-ions
(mobile ions with the same charge as the membrane) ideally do not pass the membrane [23].
Figure 5: schematic overview of RED where A is an anion exchange membrane, C a cation
exchange membrane, 𝑅𝐿𝑜𝑎𝑑 is the resistance of the external load (Ω), V is the potential
difference over the external load (V) and I is the electrical current (A) [24].
Forced by the salinity gradient across the membranes, ions from the concentrated salt solution
permeate into the dilute salt solution through the corresponding IEM [25]. This feed solution will thus
be diluted and is therefore called the ‘diluate’ while the dilute salt solution will get a higher salt
concentration during operation and is therefore called the ‘concentrate’ [21]. The result of the stack
design will be that the cations move to the cathode and the anions to the anode. From the anode
compartment, an Na-ion will transfer through the CEM which will induce a Cl-ion to transfer to the
same compartment to maintain the electroneutrality. The system thus acts as a ‘cascade system’ until
a Na-ion ends up in the cathode compartments as illustrated in figure 5. The built-up potential
difference across the membranes is composed of two contributors: the chemical and electrical
potential difference. Therefore, it is also called the electrochemical potential difference. It can be
converted into electrical energy in two ways. First, the potential difference drives redox-reactions at
the anode and cathode through the recirculation of an electrode rinse solution (e.g.
Met opmerkingen [AV6]: Wat als je enkel CEM als eindmembranen gebruikt?
Met opmerkingen [AV7]: Okee, mr zeggen hoe het ontstaat – eigenlijk gebruik je allene de mixing entropy, niet de enthalpy
8
K4Fe(CN)6/K3Fe(CN6) or NaCl) in the electrode compartments. This creates an electron flow between
the electrodes when an external load is connected which assures the electroneutrality of the solution
in the electrode compartments. Alternatively, capacitive electrodes can be used which act as a
capacitor. Ion absorbing carbon can convert ionic current into electric current without the need for
redox reactions but inverting the flow channels is required to regenerate the electrodes [26].
The disadvantage of a system with redox reactions is that these redox reactions demand energy but
when operating at an economical scale, hundreds of cells would be used which makes this loss
negligible compared to the generated voltage.
Met opmerkingen [AV8]: Je moet je zinnen veel meer opknippen. Dit zijn 5 zinnen in één.
9
2.2. Ion exchange membranes
2.2.1. Membrane composition
Ion selective membranes have the ability to exchange ions. In desalination processes, we can
distinguish CEM and AEM. Both types of membranes consist of an organic or inorganic backbone on
which charged groups are bound. The amount, type and distribution of the charged ion exchange
groups determine the specific membrane properties [24]. Most IEM are organic-based, although
zeolites, bentonite or phosphate salts are examples of inorganic backbones. The inorganic membranes
can withstand high temperatures, but due to their high cost, and relative bad electrochemical
properties and too large pores, mainly organic polymer membranes are used. Styrene-divinylbenzene
is a widely used polymer for industrial purposes [27].
IEM can be divided into AEM or CEM according to the ion exchange groups. CEM contain negatively
charged groups, such as −𝑆𝑂3−, −𝐶𝑂𝑂−, −𝑃𝑂3
2−, −𝑃𝑂3𝐻−,−𝐶6𝐻4𝑂−, etc., while AEM contain
positively charged groups such as −𝑁𝐻3+, −𝑁𝑅𝐻2
+, −𝑁𝑅2𝐻+, −𝑁𝑅3+, −𝑃𝑅3
+, −𝑆𝑅2+, etc. [27].
This allows positively charged ions to transfer through CEM and vice versa for AEM.
The type of fixed charge divides, in turn, the IEM into strong acid and strong base, or weak acid and
weak base. Strong acid CEM typically contain sulfonic acid functionalities, while weak acid membranes
typically contain carboxylic acid as charged groups. Strong and weak base AEM have, respectivel,y
quaternary and tertiary amines as the fixed positively-charged groups [24].
Based on the structure and preparation procedure, membranes can be classified into homogeneous
and heterogeneous membranes. Homogeneous membranes have a relatively even distribution of the
charged groups by polymerization and polycondensation of functional monomers by via
functionalization of a polymer (chemically bonded) [23]. Heterogeneous membranes are prepared by
melting and pressing of a dry ion exchange resin with a binder polymer (physically mixed) [28]. The
latter leads to a membrane with an uneven distribution of charged groups where parts remain
uncharged [23, 27].
In practise, homogeneous membranes are more suitable for RED [22, 23]. Though the production costs
are lower for heterogeneous membranes, they do not meet the desired low electrical resistance and
high permselectivity (see § 2.2.2.1.) demands for RED. Generally, the binder polymer is hydrophobic
and thus impermeable for ions. The ions can only permeate through the ion exchange resin which is
unevenly distributed and therefore the ion path will be longer. This results in a higher electrical
resistance, and thus also lower power density when using these membranes in RED compared to
homogeneous membranes. In this manner, the volume ratio of ion exchange resin to polymer cannot
be too low if a decent transport rate has to be ensured. If however the volume ratio is too high, the
membrane loses its mechanical stability and water clusters are created in the membrane. This results
in an increased leaking of co-ions through the membrane matrix with a decreased permselectivity as
a result. Both processes can be seen in Figure 4 [22, 29].
Met opmerkingen [AV9]: Wat meer structuur inbrengen Paragraaf 2.2.1.1 AEM vs CEM Paragraaf 2.2.1.2. homogenous vs heterogeneous …
Met opmerkingen [AV10]: In je hele document altijd Figure en Table met hoofdletter schrijven!
10
Figure 6: schematic il lustration. A: cation -exchange membrane with homogeneous
structure, B: ion-exchange membrane with heterogeneous structure.
2.2.2. Ion exchange membrane properties
2.2.2.1. Permselectivity
As mentioned earlier, IEM contain charged groups. When a salt solution is in contact with such a
membrane, the free ions that are oppositely charged (counter-ions) to the fixed groups are able to
pass. Co-ions are mobile ions with the same charge whom ideally do not pass the membrane. This
effect is called Donnan exclusion.
Permselectivity 𝛼 (-) measures the ability of a membrane to discriminate between ions with opposite
charge [30].
𝛼 =
𝑡𝑐𝑜𝑢𝑛𝑡𝑒𝑟−𝑖𝑜𝑛𝑚 − 𝑡𝑐𝑜𝑢𝑛𝑡𝑒𝑟−𝑖𝑜𝑛
𝑏
1 − 𝑡𝑐𝑜𝑢𝑛𝑡𝑒𝑟−𝑖𝑜𝑛𝑏
(4)
Where 𝑡𝑐𝑜𝑢𝑛𝑡𝑒𝑟−𝑖𝑜𝑛𝑚 the counter-ion transport number through the membrane (-), and
𝑡𝑐𝑜𝑢𝑛𝑡𝑒𝑟−𝑖𝑜𝑛𝑏 counter-ion transport number in the bulk phase (-). The transport number of an ion is the
fraction of the total electric current that is carried by the ion [31]. Generally, the membrane transport
number will exceed the bulk transport number because of the higher amount of counter-ions to
compensate for the fixed charges in the membrane (see § 2.2.3.1.).
11
The permselectivity of the membrane depends on the electrolyte concentration in the bulk phase and
the IEC of the membrane. Preferably, the membrane obstructs all the co-ions and is said to be perfectly
permselective [32]. This would result in a transport number of counter-ions equal to 1 and a transport
number of co-ions equal to 0. In other words, a membrane with a permselectivity of 100% is ideal while
a membrane with a permselectivity of 0% is not selective.
2.2.2.2. Ion exchange capacity
IEC quantifies the amount of fixed charged (positive or negative) groups in the membrane. It is
expressed in milliequivalents of fixed groups per gram of dry membrane. This property is important
since it affects almost every other membrane property [23].
2.2.2.3. Swelling degree
The swelling degree (SD) is the water uptake of the membrane for a set of conditions. It depends on
the membrane structure and material, as well as the solution surrounding the membrane [33]. This
property is calculated by comparing the wet and dry weight of the membrane:
𝑆𝐷 =𝑚𝑤𝑒𝑡 − 𝑚𝑑𝑟𝑦
𝑚𝑑𝑟𝑦
(5)
Where 𝑚𝑤𝑒𝑡 and 𝑚𝑑𝑟𝑦 are the mass of the wet and dry membrane, respectively (g) [24].
2.2.2.4. Fixed charge density
The fixed charge density (FCD) is a combination of the two previous membrane properties. IEC the
amount of fixed charges but swelling tends to dilute the concentration of the charged groups [33].
FCD is the ratio between IEC and the swelling degree. It is expressed in milliequivalents of fixed groups
per volume of water in the membrane.
The fixed charges in an ion-exchange membranes are in electrical equilibrium with the counter-ions
[24].
A higher FCD increases the permselectivity and decreases the electrical resistance. As such, a high FCD
is desired for RED. The dependency of the permselectivity on FCD results from the exclusion of the co-
ions by the fixed charge.
According to Długołecki et al. (2008), AEM typically have a lower FCD and consequently a lower
permselectivity than CEM [24]. In general, a high FCD would lead to a low electric resistance but causes
a high degree of swelling and a poor mechanical stability. Nevertheless, as shown in figure 5, CEM
seem to have a higher membrane resistance. This is due to their high degree of crosslinking, which
increases the mechanical strength and thus the membrane resistance.
Heterogeneous IEM have a lower charge density compared to homogeneous membranes due to the
membrane structure and the separation of charged domains in an uncharged polymer matrix.
Met opmerkingen [AV11]: Herformuleren!
Met opmerkingen [AV12]: Zin klopt niet
Met opmerkingen [AV13]: En waarom is dat?
12
Figure 7: Membrane permselectivity (left) and membrane resistance (right) as a function
of the change density, with ∎ representing CEM and 𝑜 AEM [24].
2.2.2.5. Electrical resistance
The electrical resistance of a membrane contributes to the maximum power output in RED. As
mentioned earlier, a low electrical resistance is desired. Electrical resistance of the membrane depends
on the IEC of the membrane, as well as the mobility of the ions through the membrane [24]. This
mobility of the ions is determined by the membrane structure. As discussed earlier, the length of the
path the ions have to travel will be longer for heterogeneous membranes since ions use the unevenly
distributed ion exchange resins to cross the membrane. As CEM typically have a higher FCD than AEM,
CEM would normally have a lower electrical resistance, but due to high crosslinking the resistance
increases, as seen in figure 5.
The electrical resistance of a membrane 𝑅𝑚 (Ω) is defined as follows:
𝑅𝑚 =
𝑟𝑚 . 𝑑
𝑆
(6)
With 𝑟𝑚 as the specific resistance (Ω.m), d the thickness of the membrane (m) and S the surface of the
membrane (m2) [34].
The electrical resistance of the membrane will also decrease when the temperature is increased [23].
Met opmerkingen [AV14]: Because… (oa verhoogde diff.coeff)
13
2.2.3. Ion and water transport
2.2.3.1. Ion transport
In the bulk solution, the concentration of mobile counter-ions and co-ions are equal, as
electroneutrality has to be maintained. However, IEM have fixed charges and when IEM are placed
between electrolyte solutions, ion partitioning will occur at the solution/membrane interface [35]. In
the membrane itself, the concentration of counter-ions will be higher, while the concentration of co-
ions will be lower regarding the bulk solution.
Assuming electro neutrality within the membrane, the concentration of counter-ions can be
considered equal to the sum of the fixed charge density of the membrane and the co-ion concentration
[36].
𝐶𝑐𝑜𝑢𝑛𝑡𝑒𝑟−𝑖𝑜𝑛 = 𝑋 + 𝐶𝑐𝑜−𝑖𝑜𝑛
(7)
Where X is the fixed charge density of the membrane (meq.m-3), 𝐶𝑐𝑜𝑢𝑛𝑡𝑒𝑟−𝑖𝑜𝑛 the concentration of
counter-ions and 𝐶𝑐𝑜−𝑖𝑜𝑛 the concentration of co-ions (mol.m-3).
At the solution/membrane interface, there will be an excess of counter-ions to compensate for the
fixed charges of the membrane and to make sure that electroneutrality is met. This is called the
electrical double layer (EDL) or Donnan layer and it has a thickness of typically a few tens of nanometers
(Debye length) [37]. Across this EDL, there is a large gradient in the concentration of ions and
consequently also in the electrical potential as can be seen in figure 6.
As a result of this Donnan equilibrium, there will be an almost complete exclusion of co-ions in the
membrane while counter-ions will be easily transported [35].
Figure 8: Schematic overview of ion concentration and electrical potential ф in an ion-
exchange mebrane. Bulk solution 1 is the diluate while bulk solution 2 is the
concentrate. The grey arrows indicate the EDL region or Donnan layer [36].
14
There is a second layer on both sides of the membrane next to the EDL, which is called the diffusion
boundary layer (DBL), which typically has a thickness of a few hundreds of micrometres [37]. This layer
is formed by the difference in ion transport numbers between the bulk solution and the ion-exchange
membrane. For counter-ions and co-ions the transport number in the IEM is close to 1 and 0,
respectively, while the transport numbers of both in the bulk solution are almost equal [23]. This
results in a decreased electrolyte concentration near the membrane in the high concentration
compartment and an increased electrolyte concentration in the low concentration compartment [37].
This phenomenon is also called concentration polarization (CP). CP reduces the net power output in
RED, since it generates an additional resistance for ion transport across the membrane [38].
So far, we assumed an ideal IEM in which energy is extracted by the controlled mixing which induces
the permeation of counter-ions. However, the imperfect behaviour of the ion-exchange membranes
results in power loss due to co-ion transport, osmosis and electro-osmosis. These mechanisms will be
discussed in the next paragraphs.
2.2.3.2. Permselectivity
Due to the imperfect behaviour of the IEM, co-ions will be able to cross. The extent to which this occurs
is qualified by the permselectivity 𝛼. The co-ions cross the membrane according to the concentration
gradient, from the diluate to the concentrate, when operating in RED. This mechanism is called co-ion
transport or co-ion leakage [39].
As explained in § 2.1.2. Electroneutrality needs to be met in the bulk solutions. This means that for
every anion that crosses the AEM, a cation should cross the CEM that on the other side of the
compartment as can be seen in figure 5. This effect provides the production of electricity. However,
the leaked co-ions counteract an equal change of counter-ions (i.e., electroneutrality is met without
ions moving in the “right” direction to produce energy). Indeed, this part of the counter-ions won’t
serve to induce the ‘cascade system’ as seen in figure 5. Additionally, the leaking of co-ions from the
high salinity compartments leads to a decreased salt fraction that can be transported as counter-ions.
The result is a lower net ion transport and thus a lower net power output in RED [41].
2.2.3.3. Water transport
When the IEM is positioned between two salt solutions with a different ion concentration, a chemical
potential gradient arises across the membrane, as already stated above. This also induces an osmotic
pressure, resulting in water moving from the concentrate to the diluate, called osmosis. Secondly, in
electrochemical systems, such as ED or RED, water is transported due to electro-osmosis. In electro-
osmosis, water is transported together with the ions as their hydration-sphere of these ions. This
transport happens in the direction opposite to osmosis [35].
According to Veerman et al. (2009), osmosis has a negative effect on the power density in RED [39].
First of all, it counteracts the transport of counter-ions, by drag, and secondly it dilutes the boundary
layer of the membrane on the seawater side.
As in ED both electro-osmosis and osmosis occur in the same direction of ion transport, these
phenomena have a large influence on the separation. However, for RED the electro-osmotic and
osmotic water fluxes move in opposite directions and for typical current densities used in RED their
effects are partially cancelled out. The effect of water transport on the power output is thus negligible
compared to the effect of co-ion transport [35].
Met opmerkingen [AV15]: Kun je veel simpeler zeggen: voor counterions gaat het transport doorheen het membraan veel sneller dan in de bulk, terwijl het voor co-ionen omgekeerd is
Met opmerkingen [AV16]: Eigenlijk omgekeerd. De concentratie-gradient zorgt voor permeatie van counter-ionen, wat energie opwekt
Met opmerkingen [AV17]: Deze moet je nog meer in detail uitleggen
Met opmerkingen [AV18]: So is electro-osmosis positive then?
15
2.3. RED performance
2.3.1. Maximum power output
The Nernst equation can be used to calculate the generated potential over a membrane. The Nernst
potential when the stack is not operational (i.e., when the external electrical circuit is not closed), also
called the open circuit voltage (OCV), in particular is very important since it represents the maximum
voltage produced by the system. The sum of the potential differences over each cell pair gives the
voltage obtained from the whole stack and is called the overall OCV [42,43]. In the case of a
monovalent salt solution such as NaCl, the OCV can be written as
𝐸𝑂𝐶𝑉 = 𝑁𝐶𝐸𝑀 . 𝛼𝐶𝐸𝑀 .
𝑅. 𝑇
𝐹. ln (
𝛾𝑑𝑁𝑎 . 𝐶𝑑
𝛾𝑐𝑁𝑎 . 𝐶𝑐
) + 𝑁𝐴𝐸𝑀 . 𝛼𝐴𝐸𝑀 .𝑅. 𝑇
𝐹. ln (
𝛾𝑑𝐶𝑙 . 𝐶𝑑
𝛾𝑐𝐶𝑙 . 𝐶𝑐
) (8)
In equation 8, 𝑁𝐶𝐸𝑀 and 𝑁𝐴𝐸𝑀 are the amount of CEM and AEM, respectively (-) and 𝛼𝐶𝐸𝑀 and 𝛼𝐴𝐸𝑀
are the permselectivities of the CEM or AEM, respectively (-). 𝛾𝑁𝑎 and 𝛾𝐶𝑙 are the activity coefficients
for Na and Cl, respectively (-), and C stands for the concentration in mol.m-3. The subscript ‘d’
represents diluate while ‘c’ represents concentrate. Furthermore, R is the universal (8.314 J.mol-1.K-1),
T the temperature (K) and F the Faraday number (96 485 C.mol−1).
The activity coefficient 𝛾 (-) of the ions can be calculated with the extended Debye-Hückel equation
[42] :
log(𝛾) =
−0,51 . 𝑧2 . √𝜇𝑖𝑜𝑛
1 +𝐴
305 . √𝜇𝑖𝑜𝑛
(9)
In equation 9, 𝜇𝑖𝑜𝑛 represents the ionic strength of the solution (mol.L-1), 𝑧 the electrochemical
valence (-) and 𝐴 the effective hydrated ion radius (pm).
In case NaCl is used as electrolyte, 𝐴𝑁𝑎+= 450 pm and 𝐴𝐶𝑙−= 300 pm while 𝑧𝑁𝑎 = 𝑧𝐶𝑙 = 1. The ionic
strength 𝜇𝑖𝑜𝑛 in its turn can then be calculated with the following equation:
𝜇𝑖𝑜𝑛 = 0,5 . ∑ 𝑧𝑖2 . 𝐶𝑖
𝑖
(10)
The ionic strength depends on the molar concentration of the ion 𝐶𝑖 (mol.L-1) and on the
electrochemical valence of each ion 𝑧𝑖 (-). The summation is done for every ion in the solution.
2.3.2. Factors reducing obtainable OCV and RED power output
Theoretically, the controlled mixing of one cubic meter of river water with an excess of seawater could
yield about 2.5 MJ of energy [14]. However, in practice, the energy output is only 0.35 MJ.m-3 [42]. This
corresponds with an efficiency of less than 20%. This inefficient energy recovery is determined by
internal and external energy losses as can be seen in equation 11 and 12 [19].
𝐼 =
𝐸𝑐𝑒𝑙𝑙
𝑅𝑖𝑛𝑡 + 𝑅𝑒𝑥𝑡
(11)
Met opmerkingen [AV19]: Deze vergelijkingen geven geen losses aan, ze geven de power output aan. Eigenlijk zou je die hierboven kunenn bespreken, en dan hier moet je enkel de losses besprekn
16
𝑊 = 𝐼2 . 𝑅𝑒𝑥𝑡 = (
𝐸𝑐𝑒𝑙𝑙
𝑅𝑖𝑛𝑡 + 𝑅𝑒𝑥𝑡)
2
. 𝑅𝑒𝑥𝑡
(12)
Where 𝐼 is the current density (A.m-2), 𝐸𝑐𝑒𝑙𝑙 is the cell potential (V), 𝑅𝑖𝑛𝑡 and 𝑅𝑒𝑥𝑡 are the internal and
external resistance, respectively (Ω.m2), and 𝑊 is the power output (𝑊).
External losses are losses that occur outside the stack. Pumping losses are one example and will be
discussed later. Internal losses can be divided into ohmic and non-ohmic resistances. In the next
paragraphs, these different sources of losses will be explained.
2.3.2.1. Non-ohmic resistances
Due to ionic transport, the concentration gradient between diluate and concentrate decreases. This
results in a decreased electromotive force and consequently a lower voltage generated. This is called
the non-ohmic resistance and can be divided again into two processes. The first one is the boundary
layer resistance, 𝑅𝐷𝐵𝐿, also known as concentration polarization. The second resistance, 𝑅∆𝐶, is a result
of the decreased in electromotive force as a consequence of the change in the bulk concentration as
the solution moves in the stack [44].
Concentration polarization
Concentration polarization takes place in the DBL near the membrane surface, as explained in
§ 2.2.3.1. The solute concentrations at the membrane surface 𝐶𝑚 (mol.m-3) can be calculated with
equation 13 [27]. The minus sign stands for the diluate since the electrolyte concentration in the DBL
for that compartment will decrease. The plus sign is used for the concentrate where the opposite
happens.
𝐶𝑚 = 𝐶𝑏 ±
(𝑡𝑚 − 𝑡𝑏) . 𝑗 . 𝛿
𝑧 . 𝐷 . 𝐹
(13)
In which 𝐶𝑏 is the electrolyte concentration in the bulk solution (mol.m-3), 𝑡𝑚 and 𝑡𝑏 are the transport
numbers of the electrolyte in the membrane and bulk solution, respectively (-), 𝑗 is the current density
(A.m-2), 𝛿 is the thickness of the DBL (m), 𝑧 is the electrochemical valence (-), 𝐷 is the diffusion
coefficient of the electrolyte in the DBL (m2.s-1) and 𝐹 is the Faraday number (96485 C.mol−1).
17
Figure 9: Change in concentration gradient du e to concentration polarisation (own
figure).
The thickness of the DBL must be reduced in order to minimise the effect of concentration polarisation.
A good fluid mixing has a positive effect on decreasing CP, and this can be obtained by increasing the
cross-flow velocity of the feed water in the stack [45] as well as by introducing spacers [46]. On the
other hand, these mechanisms also increase the pressure drop (see further) which has a negative
effect on the net power density [46].
The concentration gradient is the driver for the electrolyte transport. Concentration polarisation
reduces the effective concentration gradient, and is consequently responsible for a decreased voltage
output.
Vermaas et al. (2012) proposed and equation to calculate the effect of CP on the resistance. Other
relationships have been suggested but show low correlation to experimental data. The equation of
Vermaas is an empirical formula [45]:
𝑅𝐷𝐵𝐿 =
𝑁𝑚
2(0,62 . 𝑡𝑟𝑒𝑠 .
ℎ
𝐿+ 0,05)
(14)
In which 𝑅𝐷𝐵𝐿 is the DBL resistance (Ω.m2), 𝑁𝑚 is the amount of IEM in the stack (-), 𝑡𝑟𝑒𝑠 is the
residence time of the feed water in the stack (s), h is the intermembrane distance (m) and L is the cell
length (m).
Change in bulk concentration
As the ions move through the membrane from the diluate to the concentrate, the concentrations at
the outlet of the stack will differ from the inlet concentrations. The electrolyte concentration of the
diluate will decrease while the concentration of the concentrate will increase. This decreased salinity
gradient over the membrane results in a decreased electromotive force. Equation 15 and 16 can be
18
used to calculate the outlet concentrations when electrolytes are exchanged under influence of a
direct current, and no leakage is assumed [44].
𝐶𝑑,𝑜𝑢𝑡 = 𝐶𝑑,𝑖𝑛 −
𝑗 . 𝐿
𝐹 . 𝑞𝑑
(15)
𝐶𝑐,𝑜𝑢𝑡 = 𝐶𝑐,𝑖𝑛 +
𝑗 . 𝐿
𝐹 . 𝑞𝑐
(16)
In which 𝐶𝑜𝑢𝑡 and 𝐶𝑖𝑛 are the outlet and inlet concentrations, respectively (mol.m-3 ), 𝑗 is the current
density (A.m-2 ), 𝐿 is the cell length (m), 𝑞 is the flow rate per cell per unit width (m2.s-1) and 𝐹 is the
Faraday number (96 485 C.mol−1).
The resistance due to the change in the bulk concentrations during operation 𝑅∆𝐶 (Ω.m2) is given in
equation 17. In the equation, co-current flow is assumed, the change in electromotive force between
the diluate and concentrate is assumed to be linear, and changes in activity coefficients due to ion
exchange are neglected [45].
𝑅∆𝐶 =
𝑁𝑚
2 . 𝛼 .
𝑅 . 𝑇
𝑧 . 𝐹 . 𝑗 . ln (
𝐴𝑐
𝐴𝑑)
(17)
𝐴𝑐 = 1 +
𝑗 . 𝑡𝑟𝑒𝑠
𝐹 . 휀 . ℎ𝑐 . 𝐶𝑐
(18)
𝐴𝑑 = 1 −
𝑗 . 𝑡𝑟𝑒𝑠
𝐹 . 휀 . ℎ𝑑 . 𝐶𝑑
(19)
Here 𝑁𝑚 is the amount of IEM in the stack (-), 𝛼 the permselectivity of the membrane (-), 𝑅 the
Universal gas constant (8.314 J.mol-1.K-1), 𝑇 the temperature (K), 𝑧 the electrochemical valence (-), 𝐹
the Faraday number (96485 C.mol−1) and 𝑗 the current density (A.m-2). 𝐴𝑐 and 𝐴𝑑 are constants that
represent the change in concentration of the concentrate and diluate (-). Furthermore 𝑡𝑟𝑒𝑠 represents
the residence time of the feed water in the stack (s), 휀 the spacer porosity (-), ℎ is the intermembrane
distance (m) and 𝐶 the concentration of the solution (mol.m-3).
To decrease 𝑅∆𝐶, the flow rates of the fluid can be altered. A high flow rate corresponds to a lower
resistance since fast replenishment of the feedwaters restores the concentration gradient over the
membrane. However, in the scope of desalination, this adjustment is not relevant since a certain
degree of desalination is required (which is not necessarily the case if one aims only at energy
production)[44]. The flow channels could also be made short in order to keep the retention time short.
Another factor is the orientation of the flows. The flows could be orientated in co-flow as illustrated in
figure 10.A. In this case both seawater and wastewater enter at the same side, flowing in the same
direction. Counter-flow means that the feeds flow in opposite directions as illustrated in figure 10.B.
As the electromotive force decreases faster in co-flow operation, counter-flow is preferred. When the
concentrations of the diluate and concentrate become equal, the 𝐸𝑂𝐶𝑉 is 0. As can be seen in equation
19
22 (see § 2.3.2.), a part of the energy is lost due to the internal stack resistance. Therefore in co-flow,
the equilibrium is never reached in practice. The fraction of unused energy is largest in co-flow. In
counter-flow, the feed streams flow opposite so the concentration difference is more equally
distributed over the membrane and the salinity difference can be fully used [47].
Figure 10: Feed water flow orientation in stack. A: co -flow. B: counter-flow (own figure).
2.3.2.2. Ohmic resistances
The decreased voltage due to ohmic losses is related to the ionic transport through the diluate
compartment, concentrate compartment and the membranes [48].
The ohmic area resistance 𝑅𝑜ℎ𝑚𝑖𝑐 (Ω.m2) can be expresses as the sum of the resistances of the
individual stack components [45]. (20)
𝑅𝑜ℎ𝑚𝑖𝑐 =
𝑁𝑚
2 . (
𝑅𝐴𝐸𝑀
1 − 𝛽+
𝑅𝐶𝐸𝑀
1 − 𝛽+
ℎ𝑑
휀2 . 𝜅𝑑+
ℎ𝑐
휀2 . 𝜅𝑐) + 𝑅𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒𝑠
(20)
In equation 20, 𝑁𝑚 is the amount of IEM in the stack, 𝑅𝐴𝐸𝑀 and 𝑅𝐶𝐸𝑀 are the area resistances of the
AEM and CEM, respectively (Ω.m2), ℎ is the intermembrane distance (m), 𝜅 is the electrolyte
conductivity (S.m-1) and 𝑅𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒𝑠 is the (ohmic) resistance of both electrodes at their compartments
(Ω.m2). 휀 and 𝛽 are the spacer porosity and the mask fraction, respectively (-). The mask fraction is the
fraction of the membrane that is covered by the spacer and thus blocks the membrane. This
mechanism is also called the spacer shadow effect. Their values range between 0 and 1. For a
completely open compartment, i.e. no spacers, the values would be 휀 = 1 and β = 0, while it would be
the other way around for solid spacers [15]. As mentioned earlier, 𝑅𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒𝑠 is negligible when
operating on full scale since a large number of membrane pairs is used, and the influence of the
electrode compartment is minimized. The electrode resistance would only contribute to less than 2%
of the total stack resistance [43].
20
Feed water resistance
The resistance of the diluted water compartment represents the largest fraction of the ohmic
resistance [44]. The 3th and 4th term in equation 20 represent the resistance of the diluate and
concentrate, respectively. The resistance of the compartments is inversely proportional to the
electrolyte conductivity 𝜅. This conductivity is low for the concentrate compartment due to its low
electrolyte concentration. Therefore, the resistance of the concentrate will be larger than the one of
the dilute, making its contribution to the ohmic resistance larger. Moreover the intermembrane
distance ℎ and the spacer porosity 휀 affect the resistance. The porosity is related to the resistance
since spacers can block a certain fraction of the compartment creating a higher current density in the
pores and a longer, tortuous path for the electrical current [47]. Small flow channels and completely
open flow compartments result in rapid refreshing. In contrast, at low flow rates the electrolyte
concentration will decrease in the concentrate leading to a lower resistance. However, the resistance
of the DBL will increase due to concentration polarisation [44].
Membrane resistance
Helfferich (1962) described that transport through the membrane (i.e., low resistance) is promoted by
high membrane charge densities, low degrees of cross-linking, small ion sizes, low ion valences, high
external solution concentrations and elevated temperatures [31]. As seen earlier in the part about IEM,
the structure and the membrane properties do have an influence on the resistance. In heterogeneous
membranes, the ions will have to take a longer path to cross the membrane. This results in an increased
resistance.
The membrane resistance is often measured by placing the membrane between external solutions of
equal concentration. However, in RED the membranes are positioned between two solutions with
different salinity. The membrane resistance is mainly determined by the low salinity compartment and
the lower the electrolyte concentration, the higher the resistance. This was examined by Geise et al.
(2014) and Galama et al. (2014) [49, 35]. Galama et al. (2014) measured the membrane resistance of
CEM at different concentrations and the results can be found in Figure 11. Figure 11.A shows the
membrane resistance for the various NaCl concentrations on both sides. From this graph, it can be
noticed that especially the low feed water concentrations result in a high resistance. Figure 11.B shows
the membrane resistance in function of the low concentration compartment. A sharp decrease can be
seen when the concentration rises until the solution reaches a concentration of 0.3 M. Afterwards the
resistance stays more or less constant. This can be explained by the microheterogeneity of the
membrane which divides the membranes in phases. The first phase exists of the polymer matrix with
the fixed charge groups and a charged solution of counter-ions. The second phase is an electroneutral
solution equal to the external solution. Conductivity is controlled by the mobile ions and thus the water
phase of both phases. At a low external concentration, the membrane resistance is limited by the
conductivity of the external solution. The IEM will swell and a larger fraction of the second phase is
present which has a low electrolyte concentration, resulting in a low conductivity. At external solution
concentrations exceeding 0.3 M, the conductivity is limited by the polymer matrix and its fixed charges.
The ion mobility is decreased because of interactions between the ions, water and fixed charges.
Consequently, the conductivity will approximately stay constant [35].
21
Figure 11: membrane resistance of CEM. A: membrane resistance (R M , Ω.cm -1) at varying
external NaCl feed water concentrations (c1 and c2, M). B: Membrane resistance (R,
Ω.cm2) is function of the lowest feed water NaCl concentration (c lo w, M) [35].
The spacers also influence the membrane resistance. The first and second term in Equation 19
represent the resistances of the AEM and CEM, respectively. Both are proportional to the mask fraction
𝛽. The mask fraction is the fraction of the membrane that is covered by the spacer and thus blocks the
membrane. This mechanism is also called the spacer shadow effect [15].
2.3.2.3. Other losses
Fouling
Fouling of RED stacks using natural seawater and river water for a long period is mostly inorganic
colloidal fouling (clay minerals, diatom shells,…) and to a lesser degree scaling and biofouling. Based
on the charge, membranes are susceptible to different types of fouling. AEM are more vulnerable to
remnants of diatoms, clay minerals and organic fouling while the CEM is more vulnerable to scaling.
Also the type of membrane has an influence. Membranes with spacers are more sensitive to fouling
than membranes without spacers (also called ‘profiled membranes’) [44]. This is because colloids can
easily get trapped in the small openings of the spacers. Additionally, the rate and degree of organic
fouling increases with a larger current density. Due to the spacers, parts of the membrane will be
covered and the current density will thus increase on the open parts leading to more organic fouling
[50].
Membrane fouling affects multiple membrane properties and thereby can severely decrease the
power output. The membrane resistance can be increased in two ways. Firstly, dissolved charged
foulants counterbalance the membranes fixed charge. This leads to an increased ohmic resistance and
a decreased permselectivity [50, 51]. This is mainly important for AEM because organic salts are often
composed of a small cation that can easily pass but a large anion which blocks the pores of the
membrane. Secondly, the thickness of the DBL increases when colloids attach to the membrane with
an increased non-ohmic resistance as result [52].
Met opmerkingen [AV20]: Waarom? Scaling is toch gewoon het overschrijden van het oplosbaarheidsproduct?
Met opmerkingen [AV21]: Uitleggen waarom
22
Besides that, fouling will also increase the pumping power needed since colloidal fouling and biofouling
increase the pressure drop over the feed water compartments [21].
To ensure sufficient power output, anti-fouling strategies need to be implemented. First, the feed
waters can be periodically switched. This way the electrical current flows in the opposite direction and
biofouling is reduced [21]. Second, fouling could be reduced by short electrical pulses in this opposite
direction. This is effective for ED (it is called electrodialyis reversal), so it might also work for RED [53].
Lastly, the membranes can be chemically modified by introducing surfactants with fixed negative
charge. The large organic anions will be repelled while other membrane properties such as selectivity
will only be slightly altered [51]. Introducing surfactants also increases the membrane selectivity for
monovalent ions. This is wanted since multivalent ions have a decreasing effect on the power output
[21]. Besides a higher stack resistance in the presence of multivalent ions, experiments performed by
Post et al. (2009) showed that especially the presence of multivalent ions in the low salinity
compartment has a decreasing effect of the power output. This is due to the preferential transport
from the multivalent ions from the concentrate to the diluate, against the concentration gradient. The
multivalents are exchanged by monovalent ions from the diluate at a ratio of 1:2. The driving force for
this ion-exchange process is a difference in electrochemical potential between the ions [54] (the same
electrochemical potential that is used to separate multi- and monovalent ions in so-called “Donnan
dialysis”).
Pumping losses
Pumping losses are an example of external losses, occurring outside the stack (or at least the pumps
are placed outside the stack). The power spent on pumping the feed waters through the stack has to
be deducted from the obtained power. The pumping power can be calculated from the pressure drop
∆𝑝 (Pa) over the stack (from inlet to outlet)and the flow rate using the Darcy-Weisbach equation
(equation 21), assuming that the flow is laminar and fully developed and the channel is infinitely wide
and uniform, which is not the case in practice [55]. Equation 22 represents the total pumping power
needed 𝑃𝑝𝑢𝑚𝑝 (W.m-2) for both feed waters when the thickness of concentrate and diluate
compartments are equal [45].
∆𝑝 =
12 . 𝜂 . 𝐿 . 𝑣
ℎ2=
12 . 𝜂 . 𝐿2
𝑡𝑟𝑒𝑠 . 14 . 𝑑ℎ
2
(21)
𝑃𝑝𝑢𝑚𝑝 =
∆𝑝 . ф
𝐴=
12 . 𝜂 . 𝐿2 . ℎ . 휀
𝑡𝑟𝑒𝑠2 .
14 . 𝑑ℎ
2
(22)
With 𝜂 the viscosity of water (Pa.s), 𝐿 the cell length (m), 𝑣 the feed velocity (m.s-1), ℎ the
intermembrane distance (m), 𝑡𝑟𝑒𝑠 the residence time (s), 𝑑ℎ hydraulic diameter of the flow
compartment (m), ф the volumetric flow rates of the feed solutions (m3.s-1), 𝐴 the total membrane
area (m2) and 휀 the spacer porosity (-).
23
2.3.2. Actual potential generated
The produced voltage will be converted into an electrical current when an external load is connected.
In the previous paragraphs the main contributors to the stack resistance were discussed. Taking into
account these losses, the actual potential U (V) across the stack can be calculated using equations 23
and 24 [45].
𝑈 = 𝐸𝑂𝐶𝑉 − 𝑅 . 𝑗
(23)
𝑈 = 𝐸𝑂𝐶𝑉 − (𝑅𝑜ℎ𝑚𝑖𝑐 + 𝑅∆𝐶 + 𝑅𝐷𝐵𝐿) . 𝑗
(24)
Here, 𝐸𝑂𝐶𝑉 is the open circuit voltage (V) and 𝑗 is the current density in the closed circuit (A.m-2). 𝑅
(Ω.m²) represents the stack resistance (which can be divided into the ohmic area resistance 𝑅𝑜ℎ𝑚𝑖𝑐,
the area resistance due to the change in the bulk concentrations 𝑅∆𝐶 and the area resistance due to
the boundary layer 𝑅𝐷𝐵𝐿).
The actual potential difference across the stack will be maximal when there is no electrical current
applied (which corresponds to the open circuit voltage [45]). When a current is applied, this potential
difference drops due to the resistance of the stack and the external resistance.
2.3.3. Actual power output
2.3.3.2. Maximal power density
According to Kirchhoff's law, the power output for ideal systems can be calculated using equation 25
[33, 56]. The 𝐸𝑂𝐶𝑉 will be used since this represents the maximum voltage that can be generated over
the stack. To calculate the EOCV, equation 8 can be used and the current 𝐼 will be obtained using
equation 11. To calculate the maximal power output 𝑊𝑚𝑎𝑥, 𝑊 needs to be maximal. This can be done
by deriving the equation to one of the resistances to find the maximum. This results in a maximal power
output when the external resistance (Rext) is equal to the stack resistance (Rint) [33]:
𝑊 = 𝐼2 . 𝑅𝑒𝑥𝑡 = (
𝐸𝑂𝐶𝑉
𝑅𝑒𝑥𝑡 + 𝑅𝑖𝑛𝑡)
2
. 𝑅𝑒𝑥𝑡
(25)
𝑊𝑚𝑎𝑥 =
𝐸𝑂𝐶𝑉2
4 . 𝑅𝑖𝑛𝑡
(26)
Where 𝑊 (W) is the power output, 𝑊𝑚𝑎𝑥 is the maximal power output (W), 𝐼 is the current in the
external circuit (A), 𝑅𝑒𝑥𝑡 (Ω) is the external resistance, 𝑅𝑖𝑛𝑡 (Ω) is the internal resistance or overall
stack resistance and 𝐸𝑂𝐶𝑉 (V) is the open circuit voltage.
To calculate the power density 𝑃 (W.m-2), the power output 𝑊 is divided by the membrane area. This
is an important parameter for cost analysis since IEM are expensive [23].
24
The efficiency of the produced power from RED can be calculated using equation 27. The objective is
to produce energy so the efficiency is equal to the power delivered to the external load divided by the
total power consumed by both the stack and the external circuit [56].
𝑒𝑓𝑓 =
𝐼2 . 𝑅𝑒𝑥𝑡
𝐼2 . 𝑅𝑒𝑥𝑡 + 𝐼2 . 𝑅𝑖𝑛𝑡
(27)
Where 𝐼 is the electrical current (A), 𝑅𝑒𝑥𝑡 is the external resistance (Ω) and 𝑅𝑖𝑛𝑡 is the internal
resistance (Ω).
For ideal systems where the power output is maximal (𝑅𝑒𝑥𝑡 = 𝑅𝑖𝑛𝑡), the efficiency will be only 50%. A
higher efficiency can only be achieved when the resistance of the external load is higher than the stack
resistance but this will lead to a decreased power output [56].
2.3.3.2. Power density
The gross power density 𝑃𝑔𝑟𝑜𝑠𝑠 (W) of a stack is obtained by multiplying the actual potential of the
stack U with the current density in the external circuit 𝐼. The net power density is then calculated by
subtracting the power used for pumping [57].
𝑃𝑔𝑟𝑜𝑠𝑠 = 𝑈 . 𝐼 = (𝐸𝑂𝐶𝑉 − 𝑅 . 𝐼) . 𝐼 (28)
𝑃𝑛𝑒𝑡 = 𝑃𝑔𝑟𝑜𝑠𝑠 − 𝑃𝑝𝑢𝑚𝑝
(29)
𝑃𝑝𝑢𝑚𝑝 =
∆𝑝𝑐 . 𝑄𝑐 + ∆𝑝𝑑 . 𝑄𝑑
𝑁𝑚 . 𝐴
(30)
Where ∆𝑝 is the pressure drop (Pa) over the inlet and outlet of the feed water, 𝑄 is the flow rate of
the feed water (m3.s-1), 𝑁𝑚 is the amount of IEM in the stack and 𝐴 is the area of one membrane (m2).
Met opmerkingen [AV22]: So what is the optimal region of operation for RED and for RED as predilution? That will be very different, no?
Met opmerkingen [AV23]: Same question as above: what is your goal for RED and for RED as predesal?
25
2.4. Assisted reverse electrodialysis
As discussed earlier, RO is a very energy demanding seawater desalination technique, resulting in
interest in alternative techniques. One of them is RED, which can be used as a pre-treatment technique
for RO. The energy demand for RO then decreases in two ways. First the electrolyte concentration of
the RO in feed water will be decreased with a reduced osmotic pressure as a result. Secondly, the RED
pre-treatment, energy can even be produced that can be used for the RO. A previous study [17]
showed that it would be technically feasible to produce energy-neutral drinking water through a RED-
RO hybrid process. In practice, this has not been achieved due to losses in both RED and RO. Although
this hybrid system still has a lower energy demand, the salt transport rate is rather slow and therefore
a large membrane surface is needed resulting in high investment costs.
To overcome this issue, other pre-treatment techniques could be used such as pressure retarded
osmosis (PRO) and forward osmosis (FO). PRO and FO are again membrane techniques but here not
ions but water is transported. In FO, the osmotic pressure forces the water to permeate from the dilute
salt solution to the concentrated solution. PRO, works the same as FO but a hydrostatic pressure is
applied in the opposite direction as the osmotic pressure. Therefore the water transport will be
delayed. The transported water will get pressurized by transportation from the low-pressure diluted
solution to the high-pressure concentrated solution. This pressurized water can be used to generate
electric energy using a turbine [58].
RED and PRO are similar techniques since they both predilute the feed for RO, which decreases the
energy demand, and at the same time also produce energy. Since RED has a greater energy efficiency
and is less sensitive to membrane fouling, it has a higher power output and is thus often favoured.
According to Post et al. (2007), RED is more suitable for seawater and river water while PRO is better
for more concentrated solutions such as brine solutions [59]. However, RED transports ions while PRO
transports water. The latter could be an issue since water reuse if often negatively received.
Just like RED, both PRO and FO lead to a limited desalination speed and thus require large membrane
areas. A similar technique to PRO, pressure assisted osmosis (PAO), was designed in the past years.
Here, the water flux is increased by applying a small pressure to the diluted feed stream. Blandin et al.
(2014) demonstrated the economical profit of the hybrid PAO-RO system over FO-RO [60], which is
mainly due to the smaller membrane area required.
2.4.1. ARED as pre-treatment for RO
Just as PAO is an adaptation of PRO, for RED also a similar system can be designed called assisted
reverse electrodialysis (ARED). In this system, the salt transport from the concentrated to the diluted
solution is increased compared to RED. This is done by applying a small potential across the membrane
in the same direction as the concentration potential. According to Vanoppen et al. (2016), the energy
demand of the hybrid ARED-RO would be 1.2 kWh.m-3 [61]. This is less than the 2.9 kWh.m-3 that would
be needed for stand-alone SWRO but higher than for an RED-RO system. In ARED, electrolyte transport
is enhanced by applying a small additional potential across the membrane in the same direction as the
electrochemical potential. ARED is thus using higher current densities, and consequently the effect of
CP will be stronger. Since CP decreases the driving force for ion transport, it would be expected that
the actual ion transport over time would be lower compared to the ideal values. However the potential
Met opmerkingen [AV24]: Moet je nog wat beter uitwerken. Waarom zou transport van ionen dan beter zijn?
26
needed to create a certain current density in ARED, a much lower than expected. This is due to the
faster desalination and thus the faster change in concentrate concentration. The resistance of the
concentrate compartment is the main contribution to the stack resistance when this conductivity is
low. This resistance is inversely proportional to the electrolyte conductivity of the compartment. Due
to the higher ion transport in ARED, the electrolyte conductivity of the concentrate increases and the
resistance drops. This effect apparently largely exceeds the negative effect of CP, causing less energy
consumption for a certain degree of desalination in ARED. As a result, ARED allows faster desalination
(which leads to a smaller membrane area needed and thus lower capital costs) at a lower resistance
than theoretically expected. This confirms the results obtained in previous research by Ella (2015).
This is what make the combination of ARED and RED interesting. While RED produces energy during
desalination, ARED has the ability to overcome high resistances.
2.4.2. Working principles of ARED
The design of an ARED stack can be seen in Figure 12. It is almost the same, except for the external
circuit (Figure 5). The main difference between the two systems is that ARED needs energy while RED
produces energy. Therefor the external load in the external electrical circuit of the RED stack is
replaced by a DC current source. This device is placed between the electrodes to ensure the anode is
positively and the cathode negatively charged, which is opposite to RED. The electrical attraction
between the electrodes and the ions generates the additional driving force.
Just like with RED, the ARED stack consists of alternating CEM and AEM placed between an anode and
cathode. Because of that the negatively charged ions will toward the anode and the positivity charged
ions to the cathode. This transport is a result of both the chemical and the electrical potential
difference.
As in RED, the outer compartments of the stack are filled again with an electrolyte solution to assure
continuous redox reactions at the electrodes.
Generally, ARED and RED have both been investigated as separate pre-treatment steps before RO to
reduce the energy demand. However not a lot of research has been done for the combination of both
to produce energy at acceptable desalination rates. The increased desalination rates decrease the
membrane area which could reduce the capital costs.
27
Figure 12: schematic overview of ARED where A is an anion exchange membrane, C a
cation exchange membrane, 𝐷𝐶 is the current source.
28
3. Objectives of this thesis
Fresh water sources are shrinking and the pressure on our water resources is increasing due to
population growth and enhanced living standards. This implicates the need for drinking water
production from alternative resources. Since seawater makes up 96.5% of the earth’s water reservoir,
seawater desalination could provide the solution. Currently, reverse osmosis (RO) is the most applied
technique. The biggest drawback is the high energy demand. This contributes to nuclear waste and
greenhouse gas emissions. Therefore this alternative is not really interesting considering global
warming and the aimed transition to ‘clean’ technologies. Previous research investigated reverse
electrodialysis (RED) and assisted reverse electrodialysis (ARED) as pre-desalination techniques for RO.
In RED, energy is produced through the controlled mixing of seawater with a diluted stream.
Spontaneous ion transport is obtained from the seawater to the diluted stream, when the flow
compartments are divided by alternating CEMS and AEMs. This produced energy can be transformed
into electricity and at the same time the seawater will be diluted which decreases the energy demand
to perform RO. The result is an increased energy-efficiency of the overall process. At the same time,
the desalination rate is low and capital costs are high due to the large membrane area needed. On the
other hand, ARED has been investigating as an alternative for RED. In this process ion transport is
enhanced by applying an addition voltage in the same direction as the concentration gradient. This
results into higher desalination rates and thus a decreased capital cost, nevertheless, energy will be
consumed instead of produced.
This thesis focusses on the combination of both pre-desalination techniques, ARED and RED. At the
start of the pre-desalination, the internal stack resistance is high leading to slow desalination. The
application of ARED in that region could enhance the overall transport rate and lower the capital costs.
Subsequently, the operation mode could be switched to RED when the internal resistance is lower to
produce energy. The aim is to find the ideal combination of both processes, taking both energy
efficiency and capital costs in consideration. Additionally, the same experiments were also performed
with real sea- and wastewater, instead of NaCl, to gain understanding in the outcome under real
conditions. This thesis can be divided into three parts: chronopotentiometric experiments, batch
desalination experiments and calculations. First of all, the chronopotentiometric experiments give
insight in the behaviour of the system, both ARED and RED. The effect of flow rate and feed solution
compositions on the resistance were examined. Secondly batch desalination experiments were
performed in order the simulate the desalination experiment. From this electrolyte transport and the
internal resistance can be obtained. These experiments were carried out to gain more understanding
in the optimal combination of ARED and RED. And last some basic calculations were conducted to make
an estimation of the total costs for the ARED/RED-RO seawater desalination as well as the energy that
is saved. Thence, the different combinations could be compared to each other but also to the total
cost of stand-alone RO is order to test its economic feasibility.
29
4. Materials and methods
In this thesis ARED and RED are examined as pre-treatment/pre-desalination techniques for seawater
desalination with RO. The aim is to test the optimal combination of ARED and RED as pre-treatment,
i.e., the optimal point in terms of predesalination where ARED is stopped and RED takes over.
Therefore, different types of experiments were conducted: chronopotentiometric experiments, batch
desalination experiments with stand-alone ARED/RED and batch desalination experiments with ARED
and RED combined. All of these experiments were conducted with the same (A)RED stack which is
described in detail below.
4.1. Experimental (A)RED set-up
The design of the (A)RED stack used for all the experiments is the same as for a typical RED stack as
explained in § 2.1.1. The stack is made of plexiglass and consists of 5 cell pairs (11 membranes, 5 AEM
and 6 CEM). The membranes are homogenous ion exchange membranes FUJIIFILM type I (The
Netherlands) with an effective membrane area of 7.8 cm x 11.2 cm (= 87.36 cm2). The main membrane
properties as provided by the distributor are listed in Table 1. Polyamide spacers and silicon gaskets
(Nitex 06-700/53, Sefar, Switzerland) are positioned between the membranes to separate them and
create the flow compartments with a width of 0.485 mm. The most important properties of the spacers
and gaskets are listed in Table 5 in the appendix. The stack was provided with 2 pairs of electrodes,
one pair of working electrodes and one pair of reference electrodes. The working electrodes (Magneto
Special Anodes BV, The Netherlands) have working areas of 100 cm2 and are incorporated in the
plexiglass end plates. The cathode consists of titanium with a ruthenium mixed metal coating and the
anode is made of iridium mixed metal coating. The reference electrodes were made of Ag/AgCl (RE-
1B, ALS Co., Japan) and these are places in the electrode rinse solution. The advantage of these
reference electrodes is the ability to measure the potential difference over the whole stack without
the losses associated with electrode reactions [21]. This potential difference or actual potential U was
discussed in § 2.3.2. and is also referred to as ‘stack voltage’. The pair of reference electrodes is
generally only used in small set-ups since for large set-ups, losses at the working electrodes become
negligible due to the high amount of cell pairs.
The application of a current density or voltage to the stack and the measurement of the output is
regulated by a potentiostat (VMP3, Bio-Logic Science Instruments, France). The pair of working and
reference electrodes are both connected to the device. The working electrodes are controlled while
the reference electrodes are logged. The potentiostat was linked to a computer that logged the data
using EC lab software (Bio-Logic Science Instruments, France).
The solutions in the compartments are pumped around by a peristaltic pump with three pump heads
(520S IP31 pump, Watson-Marlow Fluid Technology Group, United Kingdom). This way an equal flow
rate of the diluate, concentrate and electrolyte solution can be assured. The flow rates were modified
dependent on the different experiments (see further). The feeds were fed at the bottom of the stack
while the outlet was at the top. This way the system is less exposed to air-cavity and potential gas
produced at the electrodes is easily removed.
30
Property Units AEM-type I CEM-type I
Membrane type - Homogeneous Homogeneous
Thickness µm 125 135
Permselectivity % 92 95
Electrical resistance in 2 M NaCl Ω.cm2 0.8 1.3
Water permeation
ml.bar-1.h-1.m-2 6 10
Burst Strength kPa 2.4 2.7
pH stability - 2 - 10 4 -12
Table 1. Membrane properties of the used AEM and CEM provided by the distributor
[62].
4.2. Chronopotentiometric experiments
The first experiments carried out were chronopotentiometric experiments, also called scan
experiments. These experiments give insight in the behaviour of the system such as the stack
resistance or the transport processes near and through ion-exchange membranes [63]. In
chronopotentiometry, a fixed current is applied while the corresponding stack voltage is measured.
The current was gradually imposed to the stack as seen in Figure 13.A. The applied current was
negative since the stack needs to be operated in ARED and RED mode where the ion transport along
the concentration gradient is induced. For all the scan experiments performed, the diluate and
concentrate were not recirculated to ensure a constant inlet concentration. The current-voltage curve
(CVC), as seen in Figure 13.B, can be derived from these experiments. The stack resistance can be
obtained from the slope of the CVC.
31
Figure 13: Typical curves of chronopotentiometric measurement series . A: Course of the
applied current to the stack. B: Theoretical current voltage curve obtained from scan
experiment. The resistance R in the stack equals the inverse of the slope of the curve
(own figure).
4.2.1. Experimental set-up
The set-up used to perform these experiments was explained in § 4.1. and is illustrated in Figure 14.
To ensure the continuous flow of diluate and concentrate, 10 litre barrels were used and refilled on
time. Both streams only ran through the stack once and were not recycled. The electrode rinse solution
had a concentration of 0.25 M NaCl in all the experiments and was constantly recycled in a 1 litre
Erlenmeyer. Prior to each test, the stack was always flushed with one litre per compartment of the
appropriate solution.
RED
ED
ARED
32
Figure 14: Schematic overview of setup for scan experiments (own figure).
4.2.2. Protocol
stack by the potentiostat and the resulting stack voltage was monitored each time. Each current was
applied for 5 minutes to get stable measurements. The applied current density varied from 0 A.m-2 to
-171.18 A.m-2 in a manner as shown in Figure 13.
The first set of experiments were performed with different feed solutions at a fixed flow rate of
50 ml.min-1 (4.4·10-3 m.s-1). To simulate the difference between the theoretical and practical system
performance, both artificial and real feed solutions were used. The concentration of synthetic sea- and
wastewater was 0.5 M NaCl and 0.01 M NaCl, respectively. Also in the following experiments, the same
concentrations will be used when referring to artificial sea- and wastewater. On the other hand, real
sea- and wastewater do not have a fixed composition. To determine the instant concentration,
samples were taken for each batch. The seawater came from a tank that is stored at the Faculty of
Bioscience Engineering (Ghent University) and that is weekly filled with seawater from the North Sea
in Blankenberge. The wastewater is effluent collected at the municipal wastewater treatment plant of
Ossemeersen (Aquafin) in Ghent. Combinations of real seawater with artificial wastewater and real
wastewater with artificial seawater were also tested. Every experiment was repeated to test the
reproducibility of the results.
Subsequently, tests were performed whereby the flow rate was adjusted. For both artificial as real
feed streams, flow rates of 50 ml.min-1 and 100 ml.min-1 were tested. This corresponds to cross flow
velocities of 4.4·10-3 m.s-1 and 8.8·10-3 m.s-1. Compared to RO which has cross flow velocities of 0.2
m.s-1, this flow rate is rather low.
33
4.3. Batch experiments
The next set of experiments performed were batch desalination experiments. In batch experiments
the concentrate and diluate are constantly recirculated leading to concentration changes over time.
This happens gradually until the desired degree of desalination is reached, after which the experiment
is stopped. These experiments were performed in both ARED and RED to compare the two operational
modes. Afterwards, desalination experiments were performed in which a combination of ARED and
RED was tested (i.e., ARED until a certain degree of desalination, after which RED was carried out to
the final desalination degree). During each batch experiment, the current density and flow rate were
kept constant.
4.3.1. Stand-alone batch experiments
4.3.1.1. Experimental set-up
The set-up used to perform these experiments was explained in § 4.1. and is illustrated in figure 15.
The only difference is that the outlet of the diluate and concentrate are sent back to the feed of the
diluate and concentrate, respectively. The conductivity of the wastewater compartment was
continuously monitored with a multi-parameter analyser equipped with conductivity sensor (C3010,
Consort, Belgium) while the weight of the wastewater compartment was monitored using a scale
(OHAUS Defender 5000, OHAUS, USA). The course of the stack potential 𝑈 was measured by the
potentiostat (VMP3, Bio-Logic Science Instruments, France).
Figure 15. Schematic overview of setup for batch desalination experiments (own figure).
34
4.3.1.2. Protocol
Preliminary to each test, the tubing and the flow compartments were again flushed with 1L of the
appropriate solution for diluate and concentrate.
The diluate, concentrate and the electrode rinse solution each had an initial volume of 1 liter. The
solutions were circulated at a flow rate of 100 ml.min-1 (cross flow velocity of 8.8·10-3 m.s-1). A fixed
current density was applied while the corresponding conductivity of the diluate, weight of the
concentrate and the potential of the stack 𝑈 were monitored. This current was interrupted every 15
minutes for 3 minutes (after which it was turned on again). The current interrupt is carried out to keep
track of the course of the OCV since this is indicated by the corresponding stack potential at zero
current density.
Depending on the current density applied, the stack can be operated in ARED or RED mode. These
different regions can be distinguished by performing a chronopotentiometric experiment as explained
in § 4.1. The result of such an experiment can be seen in figure 13.B. In RED, net energy will be
produced. Since the applied current density is negative, net energy will be produced when the stack
potential is positive. For a flow rate of 100 ml.min-1 and the artificial feed water concentrations, the
stack was operated in RED mode ranges when the applied current density ranged between 0 A.m-2 and
-21.37 A.m-2. When a higher current would be applied, the stack would be controlled in ED mode which
is out of scope of this thesis. To operate in ARED mode, a current lower than -21.37 A.m-2 is required.
The current densities applied to the stack in the ARED region are listed in Table 2.
ARED RED
I (A) j (A.m-2) I (A) j (A.m-2)
-0.25 -28.62 -0.05 -5.72
-0.35 -40.06 -0.1 -11.45
-0.45 -51.51 -0.15 -17.17
-0.88 -100.73
Table 2: Overview of the tested current densities in both ARED and RED.
4.3.2. Combined batch experiments
In the next experiments, ARED and RED will be combined as pre-desalination technique. First ARED will
be applied until a certain degree of desalination is reached, after which the stack will be operated in
RED mode until the desired degree of desalination is reached. ARED is applied first to overcome the
high resistance at the beginning. Thus before combining the combined experiments, the switching
point from ARED to RED has to be found.
In order to find the perfect switching point from ARED to RED, the following experiment is performed.
The gross power density obtained by performing RED can be found using Equation 28. The current
where the power density is maximal can be found by deriving the gross power density to the current
as can be seen in Equation 31. When this formula for current density is substituted in Equation 23, the
working potential, for which the gross power density is maximal, is obtained (see Equation 32).
𝑑𝑃𝑔𝑟𝑜𝑠𝑠
𝑑𝐼= 𝐸𝑂𝐶𝑉 − 2 · 𝑅 · 𝐼 = 0
⇒ 𝐼 =
𝐸𝑂𝐶𝑉
2 · 𝑅
(31)
35
𝑈𝑤𝑜𝑟𝑘 =
𝐸𝑂𝐶𝑉
2
(32)
A batch desalination experiment was performed to obtain the course of this maximal power density
for the changing diluate and concentrate concentrations. Both synthetic sea- and wastewater had an
initial volume of 2 liter. The 𝐸𝑂𝐶𝑉 was measured for 1 minute after which half of this potential was
applied to the working electrodes. It was assumed that the course of the stack potential during this
experiment would decline, which leads to the highest resistance, or would show an increase, which
indicates the lowest resistance. The concentrate conductivity where the stack resistance is the lowest
could be a good switching point for ARED to RED.
4.3.2.1. Experimental set-up
The set-up used is exactly the same at the one described in § 4.3.1. and as seen in Figure 15.
4.3.2.2. Protocol
The protocol for the combined batch experiments is almost the same as the protocol described in
§ 4.3.2. However, the diluate and concentrate had an initial volume of 2 litres while the electrode rinse
solution was recycled in its own barrel of 1 litre. This time a volume of 2 liter was chosen because the
decrease or increase in the stack resistance might be very small and thus hardly noticeable when small
volumes were used. Both artificial and real sea- and wastewater were tested. The current was
interrupted every 15 minutes after which the current was turned off for 90 seconds. The current
interrupt period is made shorter because the batch experiments showed that the OCV measurement
is quite stable after that time and to reduce desalination during the OCV measurement. This is
especially important in the beginning since ARED is only applied shortly.
The current density applied in ARED was -100.73 A.m-2 until the conductivity of the wastewater
reached 2.15 mS.cm-1 (see § 5.2.2.1.).
4.4. Calculations
This part of the thesis focusses on the mathematical calculations based on the theory discussed in the
literature review. The aim was to make predictions of the required energy, total cost, maximal degree
of desalination, resulting stack voltage and the required membrane area of the complete process for
different current densities and degrees of pre-desalination. The cost calculations can be divided into
two categories: the operational expenditures (OPEX) and capital expenditures (CAPEX). For these basic
calculations, the OPEX is approximated by the energy costs and the CAPEX by the membrane costs.
The membrane costs of RO were not incorporated since the area is assumed the same for the different
set-ups, only the pressure changes (based on the seawater concentrations after ARED/RED). The
calculations were done for ARED, RED and RO separately and afterwards combined to make a
comparison of the hybrid system with stand-alone RO. Taking these results into consideration, the
optimum current density for ARED and RED was chosen to perform the combined batch desalination
experiments (see § 4.3.). The result of those experiments were then compared to the theoretical
calculations. In addition, calculations were made to test the economic feasibility of the overall process.
36
4.4.1. Actual stack potential calculations
To calculate the stack voltage for the different current densities applied, first the theoretical amount
of ion transport 𝑇𝑖𝑜𝑛 (mol.s-1.compartment-1) per compartment had to be calculated:
𝑇𝑖𝑜𝑛 =
𝑎𝑏𝑠(𝐼)
𝐹
(33)
Where I is the applied current (A) and F the Faraday constant (96 485 C.mol-1). In this formula the
absolute value of the current is used since in ARED and RED the current is always negative. This way,
the concentration of the diluate and concentrate can be calculated over time with Equation 34 and 35.
𝐶𝑐,𝑡1= 𝐶𝑐,𝑡0 + 𝑇𝑖𝑜𝑛 . (𝑡1 − 𝑡0) . #𝑐𝑜𝑚𝑝
(34)
𝐶𝑑,𝑡1= 𝐶𝑑,𝑡0 − 𝑇𝑖𝑜𝑛 . (𝑡1 − 𝑡0) . #𝑐𝑜𝑚𝑝
(35)
With 𝐶𝑐 and 𝐶𝑑 the concentration in the concentrate and diluate (mol.m-3) respectively, 𝑡1 and 𝑡0 the
time at the end and beginning of the time interval (s), #𝑐𝑜𝑚𝑝 the amount of flow compartments and
𝑇𝑖𝑜𝑛 the theoretical amount of ion transport (mol.s-1.compartment-1).
However, the change in bulk concentration as the feeds move through the stack, is not incorporated
in this formula as well as the difference between the bulk concentration and the concentration near
the membrane surface. Therefor the following equations will be incorporated. First of all, the inlet and
outlet concentrations of the stack over time will be calculated. The inlet concentrations of both diluate
and concentrate for each time interval are calculated by taking the average concentration of that
interval. The outlet concentration of the concentrate 𝐶𝑐,𝑜𝑢𝑡 (mol.m-3) can then be calculated by adding
the ions that are transported over the length of the stack. The opposite happens in the diluate.
𝐶𝑐,𝑜𝑢𝑡 = 𝐶𝑐,𝑖𝑛 +
𝑇𝑖𝑜𝑛
𝑄𝑐𝑜𝑚𝑝𝑎𝑟𝑡𝑚𝑒𝑛𝑡
(36)
𝐶𝑑,𝑜𝑢𝑡 = 𝐶𝑑,𝑖𝑛 −
𝑇𝑖𝑜𝑛
𝑄𝑐𝑜𝑚𝑝𝑎𝑟𝑡𝑚𝑒𝑛𝑡
(37)
With 𝐶𝑖𝑛 and 𝐶𝑜𝑢𝑡 the inlet and oulet concentrations, respectively (mol.m-3), and the subscript c and d
represent concentrate and diluate, respectively. 𝑇𝑖𝑜𝑛 is the theoretical amount of ion transport
(mol.s-1.compartment-1) and 𝑄𝑐𝑜𝑚𝑝𝑎𝑟𝑡𝑚𝑒𝑛𝑡 the flow rate in each solution compartment
(m³.s-1.compartment-1).
The average bulk concentration is found by calculating the mean of the input and output
concentrations as seen in Equation 38:
𝐶𝑐/𝑑,𝑏𝑢𝑙𝑘 =
𝐶𝑐/𝑑,𝑖𝑛 + 𝐶𝑐/𝑑,𝑜𝑢𝑡
2
(38)
37
The concentration near the membrane can then be calculated with Equation 13 assuming that the
concentration profile in the diffusion boundary layer is linear as seen in § 2.3.2.1. The permselectivity
of the membranes was given by the manufacturer and can be seen in Table 1. Since the permselectivity
of the AEM and CEM are different, the mean of both is used. The bulk transport number 𝑡𝑏 is equal to
0.5 since the charge in the cell is carried equally by Cl- and Na+. The diffusion coefficient D (m2.s-1) of
NaCl in water depends on the concentration of the feed solutions. Nonetheless it was assumed
constant since E. Criel (2015) found that the effect on the outcome by changing the parameter was not
significant [64]. It was set here to 1.5·10-9 m2.s-1. The diffusion boundary layer thickness δ (m) was
calculated with Equation 39 to 44, described by Mulder et al. (1996) [27].
𝑑ℎ =
2 . ℎ . 𝑤
(ℎ + 𝑤)
(39)
𝑅𝑒 =
𝜌 . 𝑣 . 𝑑ℎ
𝜂
(40)
𝑆𝑐 = 𝜂
𝜌 . 𝐷
(41)
𝑆ℎ = 1,9 . 𝑆𝑐
0,33 . 𝑅𝑒0,5 . (𝑤
∆𝐿)
0,5
(42)
𝑘 =
𝑆ℎ . 𝐷
𝑑ℎ
(43)
𝛿 =
𝐷
𝑘
(44)
To be able to eventually calculate the diffusion boundary layer thickness, various parameters have to
be calculated before. 𝑑ℎ is the hydraulic length (m) and depends on the height and the thickness of
the flow compartment w and h, respectively (m). The Reynolds number 𝑅𝑒 (-) can be found with the
density of the solution 𝜌 (kg.m-³), the cross flow velocity 𝑣 (m.s-1), the hydraulic length and the dynamic
viscosity of the solution 𝜂 (Pa.S). The Schmidt number 𝑆𝑐 (-) is obtained by dividing the dynamic
viscosity of the solution by the solution density and the diffusion coefficient 𝐷 (m².s-1). To calculate the
Sherwood number 𝑆ℎ(-) , the Reynolds number, the height of the flow compartment and distance ∆L
(m) are used. ∆L is equal to the distance between turbulence promoters and thus equal to the mesh
diameter. The mass transfer coefficient in the water compartment 𝑘 (m.s-1) is obtained from the
Sherwood number, the diffusion coefficient and the hydraulic length. Eventually the boundary layer
thickness 𝛿 (m) equals the ratio of the diffusion coefficient and the mass transfer coefficient.
The maximum potential 𝐸𝑂𝐶𝑉 (V) obtainable from the system was afterwards calculated with Equation
8. However, here the concentrations of the solutes at the membrane surface are used to include the
decreased driving force due to concentration polarization.
38
In order to calculate the actual potential generated, the ohmic resistance 𝑅𝑜ℎ𝑚𝑖𝑐 (Ω.m2) and the
resistance due to change in the bulk concentrations 𝑅∆𝐶 need to be calculated. The latter can be done
with Equation 17 to 19. The diluate and concentrate concentrations used were the average bulk
concentration in every time interval.
The ohmic resistance was calculated using Equation 20. The values of 휀 and 𝛽 can be found in Table 4
in the appendix. The electrode resistance is left out of the equation since the reference electrodes are
able to measure the stack voltage. The electrolyte conductivity 𝜅 (mS.cm-1) was calculated with the
empirical Equation 45. Here again the diluate and concentrate average bulk concentrations are used
to approach the existing conditions during every time interval.
𝜅𝑐/𝑑 = 113.85 · 𝐶𝑐/𝑑,𝑏𝑢𝑙𝑘 + 0.171
(45)
The actual stack potential 𝑈 (V) was then calculated with Equation 46 for RED and Equation 47 for
ARED. The equations only differ slightly from Equation 24. 𝑅𝐷𝐵𝐿 is not considered here since it is
already taken into account by using the membrane concentration instead of the bulk concentration to
calculate the 𝐸𝑂𝐶𝑉 .
𝑈𝑅𝐸𝐷 = 𝐸𝑂𝐶𝑉 − (𝑅𝑜ℎ𝑚𝑖𝑐 + 𝑅∆𝐶) · 𝑗
(46)
𝑈𝐴𝑅𝐸𝐷 = 𝐸𝑂𝐶𝑉 + (𝑅𝑜ℎ𝑚𝑖𝑐 + 𝑅∆𝐶) · 𝑗
(47)
4.4.2. Power consumption pre-treatment
The purpose of the pre-treatment ARED/RED is to decrease the power consumption of the overall
seawater desalination. Therefore, for every current density in both ARED and RED, the power
consumption over time was calculated.
The power density of the stack 𝑃 (W) is calculated by multiplying the stack voltage 𝑈 (V) with the
current 𝐼 (A) as seen in equation 28. This will be positive in ARED but negative in RED since ARED
consumes energy while RED produces energy.
The electricity consumption 𝐸 (J) equals the power density multiplied with the time interval ∆𝑡. The
electricity consumption can be converted into kWh with Equation 49.
𝐸 (𝐽) = 𝑃 · ∆𝑡
(48)
𝐸 (𝑘𝑊ℎ) =
𝐸 (𝐽)
3.6 · 103
(49)
39
4.4.3. Degree of desalination
Using the optimal current density in ARED (-100.73 A.m-2, see further), the influence of the desired
degree of pre-desalination (%) on the total cost of pre-desalination was examined. The desired degree
of pre-desalination is the fraction of the salt concentration of the influent seawater that is removed by
the ARED (or RED) process:
𝐷𝑒𝑔𝑟𝑒𝑒 𝑜𝑓 𝑑𝑒𝑠𝑎𝑙𝑖𝑛𝑎𝑡𝑖𝑜𝑛 (%) =
𝑇𝑖𝑜𝑛 · 𝑡 · #𝑐𝑜𝑚𝑝
𝑉 · 𝐶𝑐,𝑖𝑛
(50)
With 𝑇𝑖𝑜𝑛 is the theoretical amount of ion transport (mol.s-1.compartment-1), 𝑡 is the time of operation
(s), #𝑐𝑜𝑚𝑝 is the amount of compartments (-), 𝑉 is the volume of the concentrate and 𝐶𝑐,𝑖𝑛 is the initial
concentration of the concentrate (mol.m-3).
For each current density in RED, the maximum degree of desalination can be measured. This
corresponds to the degree of desalination at the point where 𝑃 (W) becomes negative. Namely, RED
is characterized by the production of energy. The more desalination took place, the lower the
concentration gradient over the membrane will be and thus the lower the remaining driving force will
be.
4.4.4. Cost of pre-treatment
The costs of the pre-treatment can be divided into OPEX and CAPEX. For the OPEX, only energy is
considered. The price of 1 kWh energy was considered 0.1 euro. The amount of kWh was already
calculated with equation 49. In ARED, energy will be consumed while in RED energy will be produced.
The cost of the produced energy is therefore subtracted from the total cost in RED.
For the CAPEX, the membrane price is considered, consequently the required membrane area
𝐴𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 (m2) needs to be calculated. This can be done with equation 51 in which. A membrane
lifetime of 7 years is assumed.
𝐴𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 =
𝑣 . 𝑡2 . 𝐴𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒,𝑠𝑡𝑎𝑐𝑘
𝑉 . (7 . 365 . 24 . 3600)
(51)
With 𝑣 the flow rate (m3.s-1), 𝑡 the time of operation (s), 𝐴𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒,𝑠𝑡𝑎𝑐𝑘 the area of membranes in
the stack (m²) and 𝑉 the volume of the concentrate (mol.m-3).
To calculate the cost of pre-treatment, this needed membrane area is multiplied by the cost of IEM. A
current membrane cost of 100 €.m-2 was used [65]. Since the pre-treatment cost is highly dependent
on the membrane price, and this is expected to decrease over the coming years, the costs will also be
calculated with a membrane cost of 2 €.m-2.
40
The total cost of the pre-treatment consists of both the OPEX and APEX of both ARED and RED. First,
the total cost of pre-desalination by ARED, for a fixed end-concentration, was determined using the
optimal current density at a flow rate of 100 ml.min-1. Then the cost of RED was calculated from the
end-concentration of ARED on for various end-concentrations until the maximum degree of
desalination was reached. This was done for the various current densities in the RED region.
4.4.5. Total cost of ARED/RED-RO and stand-alone RO
Since the ARED/RED-RO system is tested for the application of drinking water, after the pre-treatment
still RO needs to be performed. However, the energy consumption of RO depends on the salinity of
the feed water so the higher the degree of desalination in the pre-treatment step, the lower the energy
costs for RO will be. Therefor this energy cost needs to be incorporated in the total cost. The CAPEX
costs will not be considered here since de membrane area won’t change but only the pressure applied
when the conductivity of the feed changes.
The specific energy consumption for RO 𝑆𝐸𝐶𝑅𝑂 (kWh.m-3 permeate) was calculated with equation 52:
𝑆𝐸𝐶𝑅𝑂 =
2 . 𝑇 . 𝑅𝑔 . 𝐶𝐹𝑒𝑒𝑑 . 3.6 . 106
(1 − 𝑅𝑅𝑂)
(52)
Where 𝑇 is the temperature (K), 𝑅 the universal gas constant (8.314 J.mol-1.K-1), 𝐶𝐹𝑒𝑒𝑑 the
concentration of the RO feed water (mol. m-3) which depends on the pre-treatment step, 𝑅𝑅𝑂 the
recovery of the RO process (-). The recovery is the ratio of the flow rate of the permeate to the flow
rate of the feed. In stand-alone RO systems, a maximal recovery of 0.5 is applied. That’s why this
recovery will be used here to calculate the energy demand.
This was the total energy demand 𝐸𝑡𝑜𝑡 (kWh.m-3 RO permeate) for the overall seawater desalination
process can be calculated with Equation 53. The energy consumption/production of (A)RED was
calculated for the volume of treated feed. For the conversion to energy demand per volume of RO
permeate produced, it is divided by the recovery of RO.
𝐸𝑡𝑜𝑡 = 𝑆𝐸𝐶𝑅𝑂 +
𝐸𝐴𝑅𝐸𝐷
𝑉 · 𝑅𝑅𝑂−
𝐸𝑅𝐸𝐷
𝑉 · 𝑅𝑅𝑂
(53)
Where 𝑆𝐸𝐶𝑅𝑂 is the specific energy consumption for RO (kWh.m-3 permeate), 𝐸𝐴𝑅𝐸𝐷 the energy
consumed to perform ARED in the beginning (kWh ) and 𝐸𝑅𝐸𝐷 the energy produced while performing
RED (kWh), 𝑉 the volume of treated seawater with ARED/RED (m³) and 𝑅𝑅𝑂 the recovery of the RO
process (-).
41
5. Results and discussion
5.1. Chronopotentiometric experiments
The aim of the chronopotentiometric experiments is to examine the effect of flow rate on the ion
transport rate and internal resistance of the system as well as the effect of real sea- and wastewater
when they are used as feed instead of artificial solutions. The system will be evaluated both in ARED
and RED. Ideally, the CVC should be a linear function. When the CVC deviates from the straight line,
this indicates that other resistances than the ohmic resistance, such as CP and feed water resistance,
influence the result. Previous research already indicated the lower practical energy recovery in RED
and the lower energy consumption is ARED compared to the ideal CVC. The expected effect of altered
flow rates on the outcome is unclear because the response of the altered flow rates on the various
resistances is in different directions. For real sea- and wastewater, the energy profit when performing
ARED, is expected to diminish with. Post et al. (2009) demonstrated that Donnan dialysis occurs in RED,
in which monovalent ions, that move according to the concentration gradient, are exchanged for
multivalent ions from the concentrate [21]. This mechanism decreases the stack voltage. A low total
stack resistance is the main aim, because this minimizes the power consumption in the ARED region
and maximizes the power production in the RED region.
5.1.1. Preliminary scan experiment
First, the behaviour of the system when operated in ARED and RED was investigated by performing a
preliminary scan experiment. The set-up and protocol for this experiment were explained in § 4.2. This
scan experiment was performed with artificial sea- and wastewater at a cross flow velocity of
4.4·10-3 m.s-1. The CVC obtained from this experiment was compared to the ideal CVC, in which only
the ohmic resistance is considered. Several factors, such as flow rate and feed concentrations, have an
influence on the stack resistance. Therefore, the effects of adjusted operational conditions are not
unambiguously defined and various scan experiments need to be performed.
The comparison of the experimental to the ideal CVC can be seen in Figure 16. The equation of the
ideal CVC can be constructed using Equation 23. 𝐸𝑂𝐶𝑉 represents the intersection of the CVC with the
x-axis (see Figure 16) and can be calculated with Equation 8. A permselectivity of 0.92 for CEM and
0.95 for AEM was used for the calculations of the ideal CVC, as these values were provided by the
manufacturer (see table 1). The resistance of the ideal CVC only consists of the ohmic resistance which
can be calculated with Equation 20. The inverse of 𝑅𝑜ℎ𝑚𝑖𝑐 represents the slope of the ideal CVC. From
figure 16, it is clear that the RED region in the ideal case stretches from a current density of 0
to -22.91 A.m-2. When the applied current density is lower (i.e. more negative) than -22.91 A.m-2, the
stack is operated in ARED.
42
Figure 16: Experimental CVC compared to ideal CVC. Current density (A. m -²) as a function
of total stack voltage (V) for feed solutions, C c=0.01 M and Cd=0.5 M, and a cross flow
velocity of 4.4·10 -3 m.s -1. For each CVC, the trend line is given with associated equation
and R2 and for the experimental CVC also the error bars (n=2, stdev) were added.
In RED, the only driving force for the electrolyte transport is the electrochemical potential difference.
The higher the concentration difference, the higher the electrolyte transport. As mentioned in
§ 2.3.2.2., the resistance of the diluted water compartment is the main contributor to the ohmic stack
resistance. This resistance will decrease while operating, as ions move from the diluate to the
concentrate. On the other hand, CP will also occur, causing an increase in electrolyte concentration in
the DBL of the concentrate and a decrease in the diluate. This implies a smaller concentration gradient
over the membrane and thus a decreased driving force for the ion transport.
When looking at Figure 16, the practical 𝐸𝑂𝐶𝑉 is 23.8% lower than the ideal 𝐸𝑂𝐶𝑉 . This may be caused
by a lower membrane permselectivity in practice than the value reported by the distributor. Also, in
RED a lower actual stack voltage can be observed compared to the theoretical calculations. This means
that less energy can be produced through the desalination of seawater as was theoretically expected.
This is due to CP which decreases the driving force for the electrolyte transport. However, the deviation
between the practical and the ideal CVC is small, and the slope approaches the ideal behaviour. This
was expected since CP is typically more pronounced at higher current densities.
In ARED, electrolyte transport is enhanced by applying a small additional potential across the
membrane in the same direction as the electrochemical potential. ARED is thus using higher current
densities, and consequently the effect of CP will be stronger. Since CP decreases the driving force for
ion transport, it would be expected that the actual ion transport over time would be lower compared
to the ideal values. However, Figure 16 shows that in the ARED region, a much lower potential is
recorded than expected. The increase in current density, in function of the stack potential, is much
y = -6.06x2 + 33.95x - 20.28R² = 1.00
y = 24.72x - 22.91R² = 1.00
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
20
-7 -6 -5 -4 -3 -2 -1 0 1 2
j (A
.m-2
)
U (V)
Experimental Ideal
43
faster compared to the ideal CVC. That is why the curve shows a downward bend. Whereas the
resistance is inversely proportional to the slope of the CVC (𝑈/𝐼 = 𝑅), the experimental internal
resistance in ARED is much lower than in theory. In fact, as CP is also expected to exert an extra
resistance, this means that the stack resistance in ARED is much lower than theoretically expected, as
it also counteracts CP. This downward bend of the CVC corresponds with the negative coefficient of
the second order in the corresponding equation.
As mentioned earlier, the resistance of the wastewater compartment is the main contribution to the
stack resistance when this conductivity is low. This resistance is inversely proportional to the
electrolyte conductivity of the compartment. Due to the higher ion transport in ARED, the electrolyte
conductivity of the concentrate increases and the resistance drops. This effect apparently largely
exceeds the negative effect of CP, causing less energy consumption for a certain degree of desalination
in ARED. As a result, ARED allows faster desalination (which leads to a smaller membrane area needed
and thus lower capital costs) at a lower resistance than theoretically expected. This confirms the results
obtained in previous research by Ella (2015).
5.1.2. Effect of the flow velocity
The flow rate can alter the outcome by two mechanisms. First of all, there is CP which decreases the
driving force of ion transport. The formation of a DBL can be diminished by good fluid mixing.
Therefore, the effect of CP will decrease with increasing flow velocity . Secondly, there is also the effect
of the bulk concentration solution. When the flow rate is increased, the feed solution will move faster
through the stack and will thus be refreshed more quickly. This means that the same package of water
is in the stack for less time. Consequently, a higher flow rate results in a smaller change in bulk
concentration in the stack, and thus the concentrate concentration will be increased to a lesser extent.
Again, as the feed water resistance and the membrane resistance are inversely proportional to the
concentrate concentration, the resistance will most likely decrease less at higher flow rates compared
to lower flow rates. It is thus clear that flow rate can both have a positive (CP) and negative (smaller
changes in bulk concentration) effect on stack performance in ARED.
In Figure 17, the effect of altered flow rates can be observed. The OCV at different flow rates are equal
and the course in RED approaches a linear trend. This was expected since the OCV depends mainly on
the feed concentrations instead of the flow rate. The experiment with a flow rate of 50 ml.min-1 is the
more energy efficient one of the two. At the same stack potential, the current density will be lower at
a flow rate of 100 ml.min-1 compared to 50 ml.min-1 in ARED. This indicates the reduced decrease in
feed water and membrane resistance because of the smaller retention time of the feed in the stack.
Despite both the curves show a downwards bend which means that the internal resistance is still lower
compared to what would be theoretically expected. This means that the decreased feed water and
membrane resistance still exceeds the increased CP resistance at the higher flow rate.
44
Figure 17: Comparison of CVC at 50 ml.min -1 with CVC at 100 ml.min -1. Current density
(A.m -²) as a function of total stac k voltage (V) for feed solutions, C c=0.01 M and Cd=0.5
M. For each CVC, the trend line is given with associated equation and R 2.
5.1.3. Effect of real sea- and wastewater composition vs artificial solutions
In this part, the CVC of synthetic feeds will be compared to real sea- and wastewater. The energy profit
when performing ARED is expected to diminish compared to the experiment with artificial solutions
because of the presence of multivalent ions. Post et al. [21] already concluded that Donnan dialysis
occurs in RED, in which monovalent ions, that move according to the concentration gradient, are
exchanged for multivalent ions from the concentrate. This happens in such a ratio that electro-
neutrality is met (e.g. 2 Na+ ↔ 1 Ca2+). The driving force for this mechanism is not the concentration
gradient but the difference in electrochemical potential between the solutions. In summary, the stack
voltage depends on the ionic composition of the feed solutions. This mechanism is mainly based on
the presence of these multivalent ions in de concentrate and reduces the driving force for the
electrolyte transport. A shift of the CVC is thus expected when real wastewater is used. Also
membrane fouling could appear, especially in the wastewater compartment. This wastewater was
collected at the wastewater treatment plant nearby before it would be sent to the river and was used
without pre-treatment. Particles and organic material could be observed in the wastewater visually.
On the other hand, especially in the concentrate compartment, scaling could take place, increasing the
stack resistance.
To begin with, the difference between synthetic and real sea- and wastewater was investigated at a
constant flow rate of 50 ml.min-1. The scan experiments were also performed with combinations of
real seawater with artificial wastewater and real wastewater with artificial seawater. The resulting
CVCs are presented in Figure 18. The equations of the trendlines and the correlation coefficient, R2,
are given. OCV’s and feed solution conductivities are given in Table 3.
y = -8.46x2 + 37.32x - 27.86R² = 1.00
y = -4.81x2 + 25.65x - 21.44R² = 1.00
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
20
-4 -3 -2 -1 0 1 2
j (A
.m-2
)
U (V)
50 ml/min 100 ml/min
45
The higher 𝐸𝑂𝐶𝑉 for 0.5 M NaCl - Wastewater can be attributed to the lower conductivity of real
wastewater compared to the artificial solution (0.01 M NaCl). In addition, since the conductivity of real
seawater is lower compared to the artificial solution (0.5 M NaCl), a lower concentration difference
and thus a lower driving force is expected for seawater – 0.01 M NaCl. However, the highest 𝐸𝑂𝐶𝑉 is
reached when real seawater is combined with real wastewater. With the hypothesis mentioned above,
the 𝐸𝑂𝐶𝑉 would be lower than 0.8 V (i.e. the OCV for the 0.5 M NaCl – Wastewater combination).
However in Equation 8 can be seen that 𝐸𝑂𝐶𝑉 not only depends on the electrolyte concentration but
also on the activity coefficients of the ions. The composition of sea- and wastewater differs and
contains multivalent ions (not present in the artificial solutions), which explains the change in OCV.
Figure 18: Comparison of CVCs for the different scenarios at a flow rate of 50 ml.min -1
or a cross flow velocity of 4.4·10 -3 m.s -1. Current density (A.m -²) as a function of total
stack voltage (V). For each CVC, the trend line is given with associated equation and R 2.
The error bars (n=3, stdev) of seawater-wastewater were added as well.
46
Combination 𝐸𝑂𝐶𝑉 (𝑉) Composition Conductivity (mS. 𝑐𝑚−1)
0.5 M NaCl – 0.01 M NaCl 0.68 0.01 M NaCl 1.13
Seawater – Wastewater 0.82 0.5 M NaCl 57
0.5 M NaCl – Wastewater 0.80 Wastewater 0.997
Seawater – 0.01 M NaCl 0.65 Seawater 51
Table 3: In the left-hand table 𝐸𝑂𝐶𝑉 (𝑉) is given for the CVCs of the different scenarios
as seen in Figure 18. The right-hand table contains the various feed solution
conductivities (mS.cm -1).
The CVCs of all the experiments approached again a linear trend in RED. This was expected because
when small current densities are applied (which is the case when operating in RED), effects of CP will
be small. Also the change in bulk concentration between the inlet and outlet will be minimal.
However, in ARED there is a large deviation between the different water types. First of all, there is
almost no deviation between the curves of artificial wastewater, combined with either real or artificial
seawater. As mentioned in § 2.3.2.2., the ohmic resistance is mainly determined by the diluted water
compartment and the ohmic resistance is the main contributor to the total internal stack resistance.
Because of the identical feed in the diluted compartment, the CVCs don’t deviate. They both show a
downward bend, indicating a decreased internal resistance compared to what was ideally expected.
This is the result of the change in concentrate bulk concentration during operation which counteracts
the negative effect of the increased CP in ARED. The curve of 0.5 M NaCl combined with wastewater
shows the same trend as 0.5 M NaCl combined with 0.01 M NaCl and seawater combined with 0.01 M
NaCl, however, the curve is located slightly higher (lower current densities at similar potentials). This
is explained by the lower electrolyte conductivity of wastewater compared to 0.01 M NaCl. While the
latter has a conductivity of about 1.13 mS.cm-1, wastewater has a conductivity of circa 997 µS.cm-1,
resulting in an increase of the wastewater resistance with 13%. Although this conductivity difference
might seem small, it is located in the region where small conductivity differences can have big influence
of the concentrate resistance (see Figure 19).
47
Figure 19: The theoretical resistance of the concentrate (Ω.m2) in function of the
electrolyte conductivity of NaCl solution (mS.cm -1). The graph in the right corner is a
detail of the conductivity range of the concentrate. The squares represent the artificial
and real wastewater conductivity.
The relative difference between the stack potentials of the ‘0.5 M NaCl – Wastewater’ and ‘0.5 M NaCl
– 0.01 M NaCl’ in ARED were measured. This was illustrated in function of the current density in
Figure 20. Despite the different OCV and the different current density where the operation mode
switched from RED to ARED, the relative difference between the stack potential equals 13%. Most of
the difference in stack potential will thus be caused by the change in wastewater conductivity.
48
Figure 20: The relative difference (%) of the stack potential U of ‘0.5 M NaCl –
Wastewater’ compared to ‘0.5 M NaCl – 0.01 M NaCl’ at a flow rate of 50 ml.min -1 (see
curves Figure 18). In the graph, only the ARED region is shown.
Especially the deviation between ‘seawater – wastewater’ from the other CVCs is obvious. The curve
of real sea- and wastewater is shifted upwards approaching a linear trend in ARED. The first part of the
graph represents the mean of three experiments, while the second part only represents one
experiment. From the large error bars, caused by the variability between the repeated experiments,
follows that it is impossible to predict the exact magnitude of the upward shift compared to the other
experiments. Nonetheless, in the second part of the graph a clear change could be observed around a
current density of 115 A.m-2. Where in the beginning a downward trend was still visible, this changed
to an upward bend.
The upward bend of the graph indicates that the positive effect of the changing bulk concentrations
during operation must be counteracted by another mechanism. Most likely this will be the effect of CP
which is increased through the presence of multivalent ions. These ions are more susceptible to CP
because of their lower diffusion coefficients. Another reason for this shift could also be Donnan
dialysis. Hereby monovalent ions, that move according to the concentration gradient, are exchanged
for multivalent ions from the concentrate. The driving force for this mechanism is the difference in
electrochemical potential between the solutions and will occur until electrochemical equilibrium is
reached. It has an increasing effect on the stack voltage for the same current density, so energy
demand increases. Whenever an anion leaves a flow compartment, a cation will leave the same
compartment as well to maintain electroneutrality. This transport leads to desalination of the diluate
when the flow channels are separated with alternating CEM and AEM. But when multivalent ions move
from the concentrate to the diluate, against the concentration gradient, they counteract an equal
charge of monovalent ions. This ion transport does not lead to desalination. The mechanism was
reported to be mainly based on the presence of these multivalent ions in the concentrate. It reduces
the driving force for the electrolyte transport and therefore the required stack potential will increase
0
10
20
30
40
50
60
70
80
-163 -143 -123 -103 -83 -63 -43 -23
Re
lati
ve d
iffe
ren
ce in
U (
%)
j (A.m-2)
49
to reach the same current density. The shift of the CVC was thus expected but if this mechanism would
be the cause, the curve of ‘0.5 M NaCl – wastewater’ would also be subjected to an upward shift. There
is indeed an upward shift but earlier it was discussed to be mainly caused by the higher feed solutions
resistance. Nonetheless also Donnan dialysis could have played a role.
Apparently the desalination rate will be decreased for the combination of sea- and wastewater.
Possibly, the limited concentration gradient could be part of the reason. The conductivity of real
seawater is around 51 mS.cm-1 compared to 57 mS.cm-1 for artificial seawater. The reduced
concentration gradient could lead to faster depletion of ions in the near the membrane. To estimate
this possible depletion, the membrane concentrations were calculated and used to calculate the stack
potential for solutions of NaCl with the same conductivity as sea- and wastewater. This was done for
the different current densities applied. Nonetheless the deviation from 0.5 M NaCl – 0.01 M NaCl is
not significant (see Appendix Figure 34). Also the theoretical CVC of NaCl solutions, with the starting
conductivities equal to the ones of sea- and wastewater, was determined. The course of this curve was
identical to the one of NaCl with starting conductivities of 57 mS.cm-1 and 1.13 mS.cm-1. This means
that also the lower conductivity is not the reason of the shifted graph. Possibly seawater contains
polluting components but in the end, no explanation was found for the upward shift of seawater-
wastewater.
The upward bend of the graph (real sea- and wastewater) at a current density of 115 A.m-2 might be
caused by the increased CP. CP is known at higher current densities but might be even more
pronounced through the presence of multivalent ions.
On the other hand, also the error bars are increased at higher current densities. It is hypothesized that
the redox reactions will occur faster at higher current densities because of the faster desalination. At
the high current densities, a large amount of chlorine gas was visible. If the electrode rinse solution is
not refreshed fast enough, the redox reactions could become limiting, resulting in poorly reproducible
results. Furthermore, high chlorine gas concentrations can damage the membranes. This can result in
a higher stack potential. For this experiment, there was only one pump available, limiting the electrode
rinse solution flow rate to that of the concentrate and dilute. For further research, an electrode rinse
solution velocity higher than the one of the concentrate and dilute, is recommended. In future
research projects, further examination of experiments with real sea- and wastewater is needed. It
would be interesting to take samples and analyse the ion composition to be able to investigate Donnan
dialysis. An extra pre-treatment step of the sea- and wastewater before inserting in the stack could
also have a positive effect on the reproducibility since fouling on the membrane was visible now and
would then be excluded.
The scan experiment with real sea- and wastewater was also executed at a flow rate of 100 ml.min-1
(Figure 21). The CVC of 100 ml.min-1 is predominantly positioned above the one of 50 ml.min-1.
Alteration of the flow rate has an influence on the change in bulk concentration and on CP. Considering
the resistance in the concentrate to be the biggest resistance in the stack in ARED, the upward shift of
CVCs for higher flow rates was expected due to the shorter retention time of the feed, as discussed
before. However, although the difference between both curves is small, the inflection point at higher
current densities, from downward bend to upward bend, is less clear at 100 ml.min-1. If this shift is
indeed caused by CP as discussed earlier, this outcome was expected. Higher flow rates increase
turbulence and result in a smaller DBL. However, since these tests are not repeated and the deviation
50
between both curves is small, these outcomes could also be measurement variation. Especially at
higher current densities because of the large amounts of chlorine gas visible.
Figure 21: Comparison of CVCs for sea- and wastewater at a flow rate of 50 ml.min -1 and
100 ml.min -1 or cross flow velocities of 4.4·10 -3 m.s -1 and 8.8·10 -3 m.s -1.
5.2. Batch desalination experiments
The scope of the batch desalination experiments was to get insight in the combination of ARED and
RED as pre-treatment – and to find the optimal switching point between the two when operating them
as pre-treatment to RO. The batch desalination experiments were carried out in ARED and RED to
examine resistances and compare the operation modes of the stack. This includes the amount of
energy consumption/production, the desalination rate and the maximal degree of desalination
possible in function of various current densities applied. Afterwards, combined batch experiments
were conducted as well.
During batch experiments, the concentrate and diluate were constantly recirculated leading to
concentration changes over time. The aim of the combined pre-desalination before RO is to reach a
certain degree of desalination in order to decrease the electricity demand. Therefor the stack potential
is plotted in function of the charge transported Q from the diluate to the concentrate, expressed in
Coulomb. This is done for the different tested current densities.
y = -0,02x2 + 23,58x - 12,64R² = 0,9948
y = -0,76x2 + 19,23x - 10,92R² = 0,9988
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
-8 -7 -6 -5 -4 -3 -2 -1 0 1 2
j (A
.m-2
)
U (V)
50 ml/min 100 ml/min
51
5.2.1. Stand-alone batch experiments
5.2.1.1. Batch experiments in ARED
The experimental data of the ARED batch experiments can be seen in Figure 22. The data points at the
top of each subfigure show the trend for the 𝐸𝑂𝐶𝑉 , while the lower data points show the trend for the
stack voltage. The 𝐸𝑂𝐶𝑉 decreases during the experiment in a non-linear manner. The decline of 𝐸𝑂𝐶𝑉
is bigger in the beginning of the experiment than at the end. This is the result of the changing
concentration gradient. As a consequence of batch mode operation, the concentration gradient over
the membrane will decrease over time, resulting in a decreased driving force for ion transport. Since
the driving force is bigger at the start, faster desalination will occur in the beginning.
52
Figure 22: Batch desalination experiments in ARED for different current densities
applied.
-0.5
0
0.5
1
0 2000 4000 6000 8000 10000 12000Po
ten
tial
(V
)
Q (C)
-29 A.m-2
-1
-0.5
0
0.5
1
0 2000 4000 6000 8000 10000 12000
Po
ten
tial
(V
)
Q (C)
-40 A.m-2
-1.5
-1
-0.5
0
0.5
1
0 2000 4000 6000 8000 10000 12000 14000 16000
Po
ten
tial
(V
)
Q (C)
-52 A.m-2
-3
-2
-1
0
1
0 5000 10000 15000 20000 25000
Po
ten
tial
(V
)
Q (C)
-101 A.m-2
53
The absolute value of the stack potential first decreases after which it increases again. Equation 23 in
§ 2.3.2. is used for the interpretation of these curves. In the beginning of the experiment, the absolute
value of the stack potential decreases which means that less energy is needed per unit desalination
over time. However, the 𝐸𝑂𝐶𝑉 declines over time while the current density is constant. In other words,
the stack resistance must decrease (most likely due to the increased concentration in the concentrate,
as stated above). After some time in the experiment, both the stack potential and 𝐸𝑂𝐶𝑉 decrease with
the same rate. This determined the point where the concentration of the concentrate becomes large
enough so that the stack resistance does not decrease drastically anymore with increasing
concentration (see Figure 19). At some point, the stack voltage even declines at a higher rate than the
𝐸𝑂𝐶𝑉 , which shows that the overall stack resistance is increasing again. Most likely, this is due to
concentration polarisation which starts to play a major role when the ion concentrations in diluate and
concentrate become more close to each other, further decreasing the driving force.
The graphs of the different applied current densities are very similar in terms of trends. The initial 𝐸𝑂𝐶𝑉
is equal in all cases, as expected since EOCV is measured under the same conditions for each experiment.
Also the trend of the OCV in function of the amount of the ions transported is equal. It looks like the
𝐸𝑂𝐶𝑉 will become zero at the same amount of charged transported for the different current densities
applied, namely around 20 000 Coulomb transported or a concentrate concentration of 0.23 M NaCl.
This was expected since the initial feed concentrations are equal and the 𝐸𝑂𝐶𝑉 depends on the
concentration of diluate and concentrate. Generally, it was expected that ARED would operate until
the concentrations in the concentrate and diluate are equal, i.e. 0.25 M NaCl. In theory this is possible
if the ARED system could operate close to equilibrium, but in reality charge transport in the stack is
subject to irreversible losses.
5.2.1.2. Batch experiments in RED
In order to characterise batch desalination in RED, the same batch desalination experiments are
repeated but now for current densities within the RED range. The resulting curves of the 𝐸𝑂𝐶𝑉 and the
stack potential in function of the amount of charge transported can be seen in Figure 23. The course
of the 𝐸𝑂𝐶𝑉 is shown again by the upper curve while the lower curve shows the course of the stack
voltage. The sign of the stack potential is positive for RED since power is produced instead of
consumed. The course of the 𝐸𝑂𝐶𝑉 during desalination in RED experiments is identical to the
experiments performed in ARED. The 𝐸𝑂𝐶𝑉 decreases and the decline is faster in the beginning of the
experiment. 𝐸𝑂𝐶𝑉 declines due to the desalination and the resulting decreased concentration gradient
over the membranes. The maximal obtained power from the desalination will decrease over time.
The batch desalinations for lower current densities weren’t performed until the stack potential turned
zero because the desalination rate was slow. In the end of these experiments, the stack potential
shows a linear trend and this trend is assumed to continue until the maximal degree of desalination
was reached. This happens when the stack potential turn zero. The maximal degree of desalination for
the various current densities is shown in Table 4.
54
Current density 𝑗 (𝐴. 𝑚−2) Maximal degree of desalination (%)
- 6 21.57
- 11 18.54
- 17 11.50
Table 4: Maximal experimental degree of desalination (%) for the different current
densities applied in RED.
From Table 4 can be observed that the maximal degree of desalination is higher for lower current
densities. This was already demonstrated by Post et al. (2009) [21]. If the RED system could be
operated close to equilibrium, without irreversible losses, ions would be transported until the
concentrations in the concentrate and diluate are equal. In reality, charge transport in the stack is
subject to irreversible losses and these will increase with increasing current density. Therefore, the
maximal amount of charge transported will take place at the lowest current density. Most likely, the
more reversible operation of the stack at lower current densities is due to the effect of CP at higher
current densities. Because the higher the current density, the lower the concentration gradient over
the membranes and thus the lower the driving force. Through this, the stack potential will become
negative sooner and thus a lower maximal desalination capacity can be obtained.
55
Figure 23: Batch desalination experiments in RED for different current densities applied.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 1000 2000 3000 4000 5000
Po
tne
nti
al (
V)
Q (C)
-6 A.m-2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 1000 2000 3000 4000 5000
Po
ten
tial
(V
)
Q (C)
-11 A.m-2
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 2000 4000 6000 8000 10000 12000 14000
Po
ten
tial
(V
)
Q (C)
-17 A.m-2
56
5.2.2. Combined batch experiments
In the next set of experiments, the desalination in ARED and RED are combined. This pre-treatment
process is investigated in order to find a compromise between the energy production but also the rate
of desalination. Energy is produced when water is desalinated with RED but at the same time the
charge transport is rather slow. This is especially true in the early stages of desalination since the low
concentrate concentrations results in a high stack resistance. This results in a larger membrane area
needed for RED which raises the capital costs. On the other hand, desalination with ARED consumes
energy but the ion transport rate is enhanced. Therefore, the use of ARED in the beginning to overcome
the high concentrate resistance, is tested here.
5.2.2.1. Combined batch experiments with artificial feeds
Switching point from ARED to RED
The experiment to find the ideal switching point from ARED to RED was described in § 4.3. The working
potential, for which the gross power density is maximal, can be seen in Equation 32. This will deviate
over time since the feed concentrations in batch mode change. The purpose of this test is to look for
the minimal stack resistance. At that point, the concentrate resistance will have decreased because of
the increased electrolyte concentration and its resistance won’t change largely anymore with small
changes is the concentrate concentration. This high resistance is associated with slow desalination and
the application of ARED serves to overcome this. Considering that the resistance is mainly controlled
by the wastewater compartment, the conductivity of this compartment at that point is a good indicator
to determine the switching point. The results of the experiment can be seen in Figure 24 where the
stack potential is represented in function of the time. The maximal stack potential can be found after
500 seconds and that point corresponds with a concentrate conductivity of 1.24 mS.cm-1.
Figure 24: The stack potential U when the working potential, for which the gross power
density is maximal, is applied. This will change over time because of the altered feed
concentrations during batch experiments. The stack potential U (V) is displayed in
function of the time (s).
0
0.2
0.4
0.6
0.8
1
1.2
0 1000 2000 3000 4000 5000
U (
V)
Time (s)
57
Optimal current density in ARED
The optimal current density applied in ARED, at a feed flow rate of 100 ml.min-1 or a cross flow velocity
of 8.8·10-3 m.s-1, is 100.73 A.m-2. This was found with calculations as explained in § 4.4. and can be seen
in Figure 25. The graph shows the total cost of the desalination of seawater in ARED in function of the
current density applied. The wastewater’s conductivity rises from 1.13 mS.cm-1 to 2.15 mS.cm-1 (see
further). Both the OPEX and APEX are considered to calculate the total costs and a membrane price of
100 €.m-2 was used since this is the current price. The calculations were made for the current densities
used in the experiments but also current densities in the range on the lowest total cost.
The minimal observed cost was chosen as criteria to choose the current density used in ARED.
Figure 25: The total cost of the part of the pre -treatment performed by ARED at a flow
rate of 100 ml.min -1. The total cost is set in function of the current density j. A
membrane price of 100 €.m -2 was used.
Optimal current density in RED
In order to find the optimal current density in RED, both the total energy demand and the cost of the
complete seawater desalination process needs to be considered. In ARED a current density
of -100 A.m-2 will be applied until a concentrate conductivity of 2.15 mS.cm-1 is reached. Afterwards
RED will be applied before the final RO. The current density applied in RED determines the amount of
produced energy and the desalination rate. This desalination rate together with the desired degree of
desalination defines the membrane area needed and thus the CAPEX. The desired degree of
desalination, on its turn, will depend on the change in energy consumption and cost of the RO that will
be applied after RED. In other words, a higher degree of pre-desalination, results in less energy
consumed or a higher recovery possible in RO but also a more expensive pre-treatment will be
consumed or the higher the recovery the Since the optimal current density in RED cannot be defined
unilaterally, some combined batch experiments were performed with different current densities in
RED as well as some theoretical calculations that will be discussed in § 5.3. The obtained results from
the combined batch experiments were compared to the theoretically calculated values. The method
0
0.005
0.01
0.015
0.02
0.025
0.03
0 50 100 150 200 250
Tota
l co
st o
f A
RED
(€
.m-3
)
j (A.m-2)
58
to calculate this was discussed § 4.4. The results can be seen in Figure 26. In all the experiments, the
upper curve represents the theoretically calculated stack potential and the lower curve the measured
one, in function of the change transported.
59
Figure 26: Combined batch desalination experiments with an applied current density of
100.73 A.m -2 in ARED for different current densities applied in RED. The theoretical
values are in shown function of the experimental values.
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
0 5000 10000 15000 20000 25000
U (
V)
Q (C)
-6 A.m-2
Experimental
Reeks2
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
0 5000 10000 15000 20000 25000
U (
V)
Q (C)
-11 A.m-2
Experimental
Theoretical
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
0 5000 10000 15000 20000 25000
U (
V)
Q (C)
-17 A.m-2
Experimental
Theoretical
60
The first part in the graph, where the stack potential is negative, represents the ARED region. In this
part, energy is consumed. The part where the stack potential becomes positive is the RED region. The
switching point between the two of them takes place at a concentrate conductivity of 2.15 mS.cm-1.
The theoretical curve is positioned higher than the experimental curve in all the graphs. This means
that the energy demand in ARED is higher in practice than theoretically expected, while the energy
production in RED is lower. Not only the ohmic resistance but also the resistances of CP, change in bulk
concentration and DBL over time are incorporated in the calculations. Hence, the theoretical curve
should be a good representation of the experimental curve. One reason for the deviations between
theory and practice could be to the values used for the membrane and spacer parameters by the
manufacturers. On top of that, a constant membrane resistance was considered while in practice it
depends on the concentration of diluate and concentrate. Also the influence of the electrolyte
compartment could have let to divergent results. The stack works as an electrolytic cell in which the
electrode rinse solution works to guarantee electro-neutrality. This way, chlorine gas is formed. When
the electrolyte compartment would function properly, this gas formation would be flushed quickly.
However, all the feed solutions here we pumped around with the same pump and only one flow rate
was possible. It was clear during the experiments that the flow rate was too low in the electrolyte
compartment, and a lot of gas formation was visible which hindered the good functioning of the stack.
This was mainly affecting the stack operation in ARED since the higher ion transport in that region has
a consequence of higher chlorine gas production.
When the graphs are compared to each other, the different amount of charge transported until the
stack potential becomes zero, differs. The point where this happens is the endpoint of RED since energy
will be consumed again instead of produced after this point. It can be observed that the maximal
amount of charge transported is higher for lower current densities as discussed in § 5.2.1.1. In reality,
charge transport with RED is subject to irreversible losses and these losses will increase with increasing
current density. Therefore, the maximal amount of charged transported will take place at the lowest
current density. The maximal degree of desalination in function of the current density is measured and
visualized in Figure 27. The trend line was only given for the calculated values because insufficient
experimental values were available to model the trend accurately (the experimental data points are
shown in the graph for reference). Most likely, the more reversible operation of the stack at lower
current densities is due to the effect of CP at higher current densities.
61
Figure 27: Maximum desalination in function of the current density applied in ARED -
RED combined batch desalination experiments with an applied current density of 10 1
A.m -2 in ARED. The trend line and R² are given for the theoretical values and the
experimental points were added as a reference.
It was intended with this thesis to also test these combined batch desalination experiments with real
sea- and wastewater. Initially the aim was to perform this experiment with a current density of -17 A.m-
2 in RED since the produced energy wasn’t that much higher for lower current densities but the
operation time to obtain the same degree of desalination would be significantly higher. As a
consequence, this increases the CAPEX since larger membrane areas would be needed. However,
energy was consumed instead of produced when performing this experiment. This can be explained
with Figure 18. The curve of sea- and wastewater shows an upward shift compared to pure NaCl feed
solutions most likely because of CP. Apparently the driving force to perform RED is not high enough in
real sea- and wastewater. Presumable as a result of the presence of multivalent ions which has a bigger
influence on CP than monovalent ions. Hence the RED range for real sea- and wastewater stretches
from 0 and -11.13 A.m-2 instead of 0 to -22.91 A.m-2 for artificial sea- and wastewater. This is why the
combined batch desalination with real sea- and wastewater has been performed at a current density
of -6 A.m-2 as seen in Figure 28. The upper graph shows the comparison of both batch desalination
experiments until a desalination degree of 12% is reached. The lower graph shows the RED region in
more detail.
At first, the graph with the artificial feeds is positioned higher which indicates its better energy usage
and production. This could indicate a lower internal resistance but this is unclear because the initial
concentrations differs. In Figure 29, the internal resistances are presented in function of the current
densities in RED. This was derived from the chronopotentiometric experiment shown in Figure 18. This
graph is only based on the inlet concentrations instead of the concentration differences over time
during batch experiments. The increased internal resistance for real feed water could demonstrate the
larger effect of CP. The present multivalent ions, with their smaller diffusion coefficients, are more
susceptible to CP. The latter will also influence the 𝐸𝑂𝐶𝑉 because this is determined by the
concentrations at the membrane instead of the bulk concentrations. It also needs to be mentioned
y = 0.52x + 48.89R² = 0.10
0
5
10
15
20
25
30
35
40
45
50
-20 -15 -10 -5 0
Max
imal
de
salin
atio
n (
%)
j (A.m-2)
Theoretical Experimental
62
that the conductivity of both real sea- and wastewater are lower than the artificial sea- and
wastewater. This will influence the resistance and 𝐸𝑂𝐶𝑉 as well.
It is also visible that the stack potential of artificial feeds declines faster and at some point will be equal
to the one of sea- and wastewater. Since the 𝐸𝑂𝐶𝑉 is still bigger than the one with real feeds, the
internal resistance must have increased. A hypothesis could be that the composition of the feeds
influences the outcome, for example the 𝐸𝑂𝐶𝑉 does not only depend on the concentration but also on
the activity coefficient of the different ions (see Equation 9). Since Donnan dialysis takes place, the
compositions of the solutions might vary over time. Monovalent ions, which move from diluate to
concentrate, are partly exchanged for multivalent ions from the concentrate. This does not only lead
to changing concentrations of diluate and concentrate over time but also changing compositions.
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
-100 900 1900 2900 3900 4900 5900
U (
V)
Q (C)
0.5 M NaCl - 0.01 M NaCl
Seawater - wastewater
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
800 1800 2800 3800 4800 5800
U (
V)
Q (C)
0.5 M NaCl - 0.01 M NaCl
Seawater - wastewater
63
Figure 28: Comparison of batch desalination of artificial and real feeds with a current
density of -6 A.m -2. The upper graph shows the comparison until a desalination degree
of 12% is reached while the lower graph shows the RED regio n in more detail.
Figure 29: This graph shows the experimental stack resistance R (Ω.m -2) in function of
the current density j (A.m -2) for RED. It is derived from the chronopotentiometric
experiment shown in Figure 18.
5.3. Economic analysis
In this part of the thesis, an economic analysis is performed on the various combination of the overall
ARED/RED-RO process. The method is explained in § 4.4. A few assumptions were made for the
calculations. The current membrane price for (A)RED is 100 €.m-2 but according to Fujifilm, an
important player in the development and production of ion-exchange membranes, the goal is to target
competitive prices within the next 5 to 10 years. Since the economic feasibility is highly dependent of
the pre-desalination cost, the calculation here are also done for a lower membrane price to take future
prospects in consideration. An alternative membrane price of 2 €.m-2 was chosen since Ella et al. (2015)
highlighted this price to be the maximum IEM price for the hybrid system ARED-RO to become
economically feasible [64]. To calculate RO energy requirements, a recovery of 50% was used. This is
a typical recovery in current seawater desalinations. On the other hand, when the seawater is pre-
desalinated, higher RO recoveries will be feasible. The incoming sea- and wastewater concentrations
were set to 0.5 and 0.01 M NaCl, respectively, and correspond to conductivities of 57 mS.cm-1 and
1.13 mS.cm-1. The first step of the pre-desalination is the operation of ARED at a current density of -
100 A.m-2 until the wastewater has a conductivity of 2.15 mS.cm-1. Afterwards RED is applied. -17.17
A.m-2, - 1.45 A.m-2 and -5.72 A.m-2 are the current densities considered in RED. The water transport in
(A)RED is neglected.
0
0.02
0.04
0.06
0.08
0.1
0.12
-200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 20
R (
Ω.m
-2)
j (A.m-2)
0.5 M NaCl - 0.01 M NaCl Wastewater - seawater
64
5.3.1. Cost/energy consumption in function of degree of pre-desalination
5.3.1.1. Total cost
First, the total cost of the complete seawater desalination process per m³ RO permeate is calculated
(Figure 29), assuming an RO recovery of 50%. The total cost is shown for the various pre-treatment
processes in function of various degrees of pre-desalination. The degree of pre-desalination is shown
up to 40%. In Figure 27 the maximal degree of pre-desalination was already calculated which was be
40 to 45%. When ARED would be operated close to equilibrium, 50% pre-desalination should be
possible. However in these calculations, the resistance of CP and change in bulk concentration are
considered which leads to irreversible losses.
Figure 29: The total cost of the overall system for the production of drinking water from
seawater for different degrees of pre-desalination with a membrane price of 100 €.m -2.
When looking at Figure 29, it is obvious that none of the combined ARED-RED pre-treatments is more
cost effective than stand-alone RO. The biggest contribution to the total price is the CAPEX (i.e. the
membrane cost). As expected, the total costs at lower current densities is higher. The lower ion
transport at a lower current density requires a larger membrane area to attain the same degree of
desalination. The total cost of the ARED-RED pre-treatment with the current membrane price is too
high and will therefore never be applied in full scale. Even with a membrane price of 2 €.m-2 the total
cost is still higher than stand-alone RO, see Figure 30. However, the costs are much lower.
As expected, the total cost for lower current densities be higher to reach the same degree of
pre-desalination. As a result of the Lower current densities applied, the desalination rate will be lower
and thus more membrane area is needed.
On Figure 30, ARED-RO and RED-RO are calculated as well. In ARED-RO, the pre-desalination is only
performed by ARED at a current density of -100 A.m-2. RED-RO represents seawater desalination with
0
5
10
15
20
25
30
35
40
0 5 10 15 20 25 30 35 40
Tota
l co
st (
eu
ro/m
³ p
erm
eat
e)
Degree of pre-desalination (%)
-6 A/m² -11 A/m² -17 A/m² RO
65
RED as pre-treatment at a current density of -11 A.m-2. These two curves were added to see how the
combined ARED-RED performs compared to stand-alone (A)RED as pre-treatment. It can be seen that
stand-alone ARED as pre-treatment is still the most cost effective of all the pre-desalination processes.
However, by comparing the curves of ‘RED-RO’ and ‘-11 A.m-2’, it is obvious that combination of ARED
and RED is beneficial for the total cost. The course of the two curves can be observed with the lower
graph of Figure 30. At 2% pre-desalination, the total cost of ‘-11 A.m-2’ exceeds than the costs of ‘RED-
RO’. But afterwards the total costs of ‘RED-RO’ increases faster and will be equal to the total cost of
‘-11 A.m-2’ after a pre-desalination of 3.2%. This demonstrates the cost saved by applying ARED in the
region where slow desalination occurs because of the high internal resistance.
66
Figure 30: The total cost of the overall system for the production of drinking water from
seawater for different degrees of pre-desalination with a membrane price of 2 € .m -2.
The lower graph is a detail of the upper one.
0.1
0.3
0.5
0.7
0.9
1.1
1.3
1.5
1.7
1.9
0 5 10 15 20 25 30 35 40
Tota
l co
st (
€/m
³ p
erm
eat
e)
Degree of desalination (%)
-6 A/m² -11 A/m² -17 A/m² RO ARED-RO RED-R0
0.1
0.15
0.2
0.25
0.3
2 3 4 5 6 7 8
Tota
l co
st (
eu
ro/m
³ p
erm
eat
e)
Degree of desalination (%)
-6 A/m² -11 A/m² -17 A/m² RO ARED-RO RED-R0
67
5.3.1.2. Energy demand
The actual goal of ARED-RED as pre-desalination technique is to decrease the energy consumption
compared to stand-alone RO. This energy demand is shown for the different combinations in Figure 31.
Between the pre-desalination rate of 0 and 7%, the total energy cost will exceed stand-alone RO
because of the energy consumed in ARED. This energy demand for the desalination in ARED is small,
since it is not applied for a long time. The consumed energy will thus be recovered quickly with RED.
Hence to be more energy efficient than stand-alone RO, a pre-desalination of minimum 7% should be
achieved in (A)RED. The energy efficiency is the higher for the lowest current density applied in RED.
This was expected because the lower the current density, the more RED was operated close to
equilibrium, without irreversible losses.
Figure 31: The total energy demand for the production of drinking water from seawater
for different degrees of pre-desalination.
5.3.2. Cost/energy consumption in function of RO recovery
In the previous calculations, an RO recovery of 50% was used because this is a typical recovery in
seawater desalination. In order to make the water molecules move against the osmotic pressure, an
external pressure needs to be applied. The recovery in RO is limited because the membrane modules
can only withstand pressure of 55 to 85 bar. Therefore a recovery of 50% is typical in seawater
desalination. However, when seawater is pre-desalinated, higher recoveries can be obtained.
Theoretically a desalination of 40% is possible. However, the lab-experiments demonstrated a maximal
desalination of 20% with the combined ARED-RED system (see Figure 27). When the seawater
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 5 10 15 20 25 30 35 40
Ene
rgy
de
man
d (
kWh
/m³
pe
rme
ate
)
Degree of pre-desalination (%)
-6 A/m^2 -11 A/m^2 -17 A/m^2 RO ARED-RO
68
concentration decreases with 20%, the recovery will rise by 10% if the same pressure is applied as for
stand-alone RO.
In the next set of calculations, the total cost and energy demand for the overall seawater desalination
is calculated for RO recoveries ranging from 0 to 60%. The degree of pre-desalination was kept constant
at 20% because this was the maximal degree of desalination at lab-scale. An applied current density of
-17.17 A.m-2 didn’t reach this degree of desalination. On top of that it is not located in the RED range
when the batch experiment is performed with real sea- and wastewater. That is why it is left out of the
next calculations. A projected membrane of 2 €.m-2 was used to investigate the future prospects, since
the process with an IEM membrane price of 100 €.m-2 will never be economic feasible.
5.3.2.1 Total cost
The total cost in function of the RO recovery can be seen in figure 32. The overall price of the combined
pre-desalination followed by RO, is still more expansive than stand-alone RO. However the costs
decrease with increasing RO recovery. The total cost is particularly determined by the cost of the
pre-desalination or in other words, the membranes (CAPEX). When more water is eventually recovered
from this pre-treated water, then the pre-treatment costs can be attributed to a larger volume. That’s
why the higher the RO recovery, the lower the total costs per m³ permeate performed.
Figure 32: The total cost for the production of drinking water from seawater for different
RO recoveries. The seawater was pre-desalination with 20% and a membrane price of 2
€.m -2 was considered.
5.3.2.2. Energy demand
The total energy demand of the complete seawater desalination process can be seen in function of
various RO recoveries in Figure 33. From this graph it seems like the lower the RO recovery, the less
energy will be needed or even produced. On the contrary, lower RO recoveries result in high total costs
(see Figure 32). Before RO is performed, the seawater was desalinated 20% with ARED-RED. This
process costs money but produces energy on the other hand. When looking at the energy demand,
10% RO recovery looks interesting because energy is produced instead of consumed. However the
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
0.00 10.00 20.00 30.00 40.00 50.00 60.00
Tota
l co
st (
€/m
³ p
erm
eat
e)
RO recovery (%)
-6 A/m^2 -11 A/m^2 Stand-alone RO
69
pre-treatment costs do not change if more or less water is recovered afterwards. When more water is
eventually recovered from this pre-treated water, then the pre-treatment costs can be attributed to a
larger volume.
Figure 33: The total energy demand for the production of dr inking water from seawater
for different RO recoveries. The seawater was pre-desalination with 20%.
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
0.00 10.00 20.00 30.00 40.00 50.00 60.00
Ene
rgy
de
man
d (
kWh
/m³
pe
rme
ate
)
RO recovery (%)
-6 A/m^2 -11 A/m^2 Stand-alone RO
70
6. Conclusion and Prospects
The overall goal of this thesis was to investigate the combined ARED-RED process as pre-treatment for
RO. Previous research at the PaIntT research group focused on ARED and RED separately. RED was
found to be more energy efficient than stand-alone RO but not economically feasible due to the high
investment cost which, is a result of the large IEM area needed.
The objective of this thesis was to examine the use of ARED and to investigate the optimal
combinations of ARED-RED. The experiments were performed with both synthetic water and real sea-
and wastewater to gain understanding in the outcome with the real conditions.
6.1. Overall conclusion
The mechanisms that affected the outcome most were the presence of a diffusion boundary layer (or
concentration polarization, CP) and the change in feed water and membrane resistance contributed to
variable bulk concentrations during operation. In the scan experiments, the CVC shows a small upward
bend in RED, which is attributed to CP. This decreases the energy production. In ARED, a downward
bend was observed. This indicates a lower energy consumption for desalination as expected.
All the scan experiments seem to approach a linear trend in RED. However, in ARED there is a lot of
deviation. From the scan experiments performed with real sea- and wastewater and combinations
with artificial feeds, it was clear that the main contributor to the total stack resistance is the
concentrate concentration. When the concentration of low concentrations increases slightly, the
internal resistance might decrease considerably. Since wastewater has a slightly lower conductivity
(0.997 mS.cm-1) then artificial wastewater (1.13 mS.cm-1), experiments performed with wastewater
will be shifted towards higher stack potentials (13% increased).
When a scan is performed with real sea- and wastewater, the CVC is shifted considerably upwards
whereby the lower resistance advantage of ARED decreases. In the end, no explanation was found for
this upward shift. Previous research already revealed Donnan dialysis to occurs in RED which reduces
the driving force for electrolyte transport. But this would mainly depend on the presence of
multivalent ions in de concentrate. However if Donnan dialysis mainly depends on the presence of
multivalent ions in the concentrate, the CVC of 0.5 M NaCl with wastewater should also show a bigger
deviation. Also the lower seawater conductivity (51 mS.cm-1) compared to wastewater (57 mS.cm-1)
was not the cause. However the error bars were large which indicates the bad reproducibility. This
could be caused by polluting components in seawater, fouling of the membranes or the inappropriate
operation of the electrolyte rinse solution.
For the ideal combination of ARED and RED, ARED was be applied first at an ideal current density
of -100 A.m-2. When the concentrate conductivity is equal to 2.15 mS.cm-1, ARED should be switched
to RED. The ideal current density in ARED corresponds with the lowest costs for pre-desalination of
diluate until the concentrate reaches a conductivity of 2.15 mS.cm-1. The concentrate conductivity to
switch to ARED was not calculated precisely. A low stack resistance was noticed around that
concentrate conductivity. For RED various current densities can be applied. When the combined
desalination experiments are performed with real sea- and wastewater, current densities from 0
to -11.13 A.m-2 can be used. Additionally, the corresponding maximal degrees of desalination are 28
and 24%, respectively. These maximal desalination degrees are much lower than the 50% that could
be ideally reached if the stack was operated reversibly instead of irreversibly. These irreversible losses
71
increase with increasing current density. Most likely, the more reversible operation of the stack at
lower current densities is due to the effect of CP at higher current densities. Because the higher the
current density, the lower the concentration gradient over the membranes and thus the lower the
driving force.
From the economic analysis, it appears that none of the ARED/RED-RO processes is more cost effective
than stand-alone RO, due to the high capital costs. Even when the membrane price changes to 2€.m-2,
stand-alone RO would still be cheaper. The cost are the higher for low current densities applied in RED
because of the slower desalination rate and thus the larger membrane area needed. When ARED/RED-
RO with a current density of -11 A.m-2 in RED is compared to RED-RO with the same current density,
the total cost of ARED/RED-RO will be smaller. This proves that although ARED consumes electricity
compared to RED, ARED-RED will still be cheaper due to the faster desalination and thus less
membrane area needed. On the other hand, after a desalination degree of 7% reached, the electricity
production will be lower compared to RO. Half of the energy for stand-alone RO would be needed if a
pre-desalination of 40% would be reached.
However when pre-desalination occurs, higher recovery rates will be possible. Since the biggest cost
of the combined process is the pre-treatment, the total cost per volume of permeate will decrease
with increasing recovery. For a practical pre-desalination of 20%, 60% recovery is possible.
6.2. Prospects
To find the optimal combination of ARED-RED as pre-treatment, several assumptions had to be made.
In this thesis, the optimal current density in ARED was based on the minimum pre-treatment cost. A
membrane price of 100 €.m-2 was used but this price is highly uncertain since IEM will probably become
cheaper the next years like manufacturers predicted. So if the market changes, this optimal
combination needs to be adjusted as well. Therefor this economic analysis only gives some insight in
the current situation.
The outcome of experiments performed with real sea- and wastewater, especially the combination,
deviated from artificial feed waters or combinations between artificial and real feeds. Although some
processes like Donnan dialysis, the increased effect of CP on multivalent ions, … are already knows,
still the behaviour couldn’t all be clarified. Thus further research into the behaviour of real sea- and
wastewater is needed. It could be that the changing ion compositions, because of Donnan dialysis, has
an effect on the overall behaviour of the system. Since not only the total concentration of ions but also
the activity coefficient is important, the outcome could be altered by this. Unfortunately it was not
possible to analyse the composition of the diluate and concentrate over time but this would be
recommended for further research. However the large error bars for the experimental data of real
sea- and wastewater might also assume that the deviation is actually not that big. To assure
reproducibility of the data, an extra pre-treatment step of the sea- and wastewater before inserting in
the stack could also have a positive effects since fouling on the membrane was observed.
Also the electrode rinse solution caused problems during the experiments. At high current densities,
large amounts of chlorine gas was visible. If the electrode rinse solution is not refreshed fast enough,
the redox reactions could become limiting. Furthermore, high chlorine gas concentrations can damage
the membranes. For this experiments, there was only one pump available, limiting the electrode rinse
solution flow rate to that of the concentrate and dilute.
72
For further research, an electrode rinse solution velocity higher than the one of the concentrate and
dilute is recommended.
73
7. Appendix
Property Units Spacer
Fibre material - Polyamide
Weight fabric kg.m-2 0.14
Porosity (ε) - 0.8
Shadow factor (β) - 0.47
Thickness µm 485
Mesh opening width mm 0.7
Open area - 0.53
Wire diameter mm 0.265
Table 5. Spacer properties [63].
Figure 34: CVC of NaCl solutions with the same conductivity as sea - and wastewater.
Compared to the CVC of 0.05 M NaCl – 0.01 M NaCl
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
-6 -5 -4 -3 -2 -1 0 1 2
j (A
.m-2
)
U (V)
0.05 M NaCl - 0.01 M NaCl NaCl with sea- wastewater conc
74
Figure 35: The conductivity of the concentrate at which the internal stack resistance was minimal.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0 0.5 1 1.5 2 2.5 3 3.5
I (A
)
Conductivity (mS.cm-1)
75
References
[1] United nations human rights, [Online.
Available:]http://www.ohchr.org/EN/Issues/WaterAndSanitation/SRWater/Pages/SRWaterIndex.aspx
[2] World Health Organisation, “Health through safe drinking water and basic sanitation.”
[Online]. Available:
http://www.who.int/water_sanitation_health/monitoring/jmp-2015-update/en/
[3] Food and agricultural organisation, [Online]. Available:
http://www.fao.org/nr/water/topics_scarcity.html
[4] P.H. Gleick, in S.H. Schneider (Ed.), Water Resources in Encyclopedia of Climate and Weather, vol. 2,
University Press, New York (1996), pp. 817–823, 1996.
[5] A. D. Khawaji, I. K. Kutubkhanah , and J. Wie, “Advances in seawater desalination technologies,”
Desalination, vol. 221, no. 1–3, pp. 47–69, 2008.
[6] http://ec.europa.eu/eurostat/statistics-
explained/index.php/File:Production_of_primary_energy,_EU-
28,_2014_(%25_of_total,_based_on_tonnes_of_oil_equivalent)_YB16.png
[7] L. F. Greenlee, D. F. Lawler, , B.D. Freeman, , B. Marrot, , P. Moulin, “Reverse osmosis desalination:
Water sources, technology, and today's challenges,” Water Research, vol. 43, no. 9, pp. 2317–2348,
2009.
[8] F. Macedonia, E. Drioli, A. A. Gusev, A. Bardow, R. Semiat, M. Kurihara, “Efficient technologies for
worldwide clean water supply,” Chemical Engineering and Processing: Process Intensification, vol. 51,
pp. 2-17, 2012.
[9] L. Henthorne, B. Boysen, “State-of-the-art of reverse osmosis desalination pretreatment,”
Desalination, vol. 356, pp. 129-139, 2015.
[10] M. Elimelech, W. A. Phillip, “The future of seawater desalination: energy, technology, and the
environment,” Science (New York, N.Y.), vol. 333, no. 6043, pp. 712-717, 2011.
[11] S. Thampy, G.R. Desale, V. K. Shahi, B. S. Makwana, P. K. Ghosh, “Development of hybrid electrodialysis-
reverse osmosis domestic desalination unit for high recovery of product water,” Desalination, vol. 282,
no. 1418, pp. 104-108, 2011.
[12] V. V. Nikonenko, A. V. Kovalenko, M. K. Urtenov, N. D. Pismenskaya, J. Han, P. Sistat, G. Pourcelly,
“Desalination at overlimiting currents: State-of-the-art and perspectives,” Desalination, vol. 342, pp.
85-106, 2014.
[13] http://www.dutchwatersector.com/news-events/news/12388-dutch-king-opens-world-s-first-red-
power-plant-driven-on-fresh-salt-water-mixing.html
[14] J. Veerman, M. Saakes, S. J. Metz, G. J. Harmsen, “Reverse electrodialysis: Performance of a stack with
50 cells on the mixing of sea and river water,” Journal of Membrane Science, vol. 327, pp. 136-144,
2009.
[15] J. W. Post, H. V. M. Hamelers, C. J. N. Buisman, “Energy Recovery from Controlled Mixing Salt and Fresh
Water with a Reverse Electrodialysis System,” Environmental Science and Technology, vol. 42, no. 15,
pp. 5785-5790, 2008.
[16] W. Li, W. B. Krantz, E. R. Cornelissen, J. W. Post, A.R.D. Verliefde, C. Y. Tang, “A novel hybrid process of
reverse electrodialysis and reverse osmosis for low energy seawater desalination and brine
management,” Applied Energy, vol. 104, pp. 592-602, 2013.
76
[17] A. Bakelants, “A novel method for energy-efficient seawater desalination by predilution with impaired
water,” Universiteit Gent, 2013.
[18] J.G. Hong, B. Zhang, S. Glabman, N. Uzal, X. Dou, H. Zhang, X. Wei, Y. Chen, “Potential ion exchange
membranes and system performance in reverse electrodialysis for power generation: A review,”
Journal of Membrane Science, vol. 486, pp. 71-88, 2015.
[19] M. Vanoppen, E. Criel, S. Andersen, A. Prévoteau, A. Verliefde, “Assisted reverse electrodialysis: a novel
technique to decrease reverse osmosis energy demand,” AMTA/AWWA Membrane Technology
Conference, Papers. American Membrane Technology Association (AMTA) ; American Water Works
Association (AWWA), 2016. Print.
[20] J. G. Hong, W. Zhang, J. Luo, Y. Chen, “Modeling of power generation from the mixing of simulated
saline and freshwater with a reverse electrodialysis system: The effect of monovalent and multivalent
ions,” Applied Energy, vol. 110, pp. 244-251, 2013.
[21] J. W. Post, “Blue Energy: electricity production from salinity gradients by reverse electrodialysis,”
Wageningen University, 2009.
[22] R.E. Pattle, “Production of electric power by mixing fresh and salt water in the hydroelectric pile,” Nature, vol. 174, pp. 660, 1954.
[23] H. Strathmann, “Electrodialysis, a mature technology with a multitude of new applications,”
Desalination, vol. 264, no. 3, pp. 268-288, 2010.
[24] P. Długołecki, K. Nymeijer, S. Metz, M. Wessling, “Current status of ion exchange membranes for power
generation from salinity gradients,” Journal of Membrane Science, vol. 319, no. 1-2, pp. 214-222, 2008.
[25] M. Tedesco, A. Cipollina, A. Tamburini, G. Micale, J. Helsen, M. Papapetrou, “REAPower: use of
desalination brine for power production through reverse electrodialysis,” Desalination and Water
Treatment, vol. 53, no. 12, pp. 3161-3169, 2015.
[26] J. Veerman, D. Vermaas, (2016) Reverse electrodialysis: fundamentals. In: Cipollina A, and Micale G,
editors. Sustainable Energy from Salinity Gradients, 1st ed. Woodhead Publishing-Elsevier, London. p.
77–134, 2016.
[27] X. Tongwen, “Ion exchange membranes : State of their development and perspective,” Ion exchange
membranes : State of their development and perspective, vol. 263, no. 2, pp. 1-29, 2005.
[28] J. Schauer, L. Brozova, Z. Pientka, K. Bouzek, “Heterogeneous ion-exchange polyethylene-based
membranes with sulfonated poly(1,4-phenylene sulfide) particles,” Desalination, vol. 200, pp. 632–
633, 2006.
[29] H. Strathmann, A. Grabowski, G. Eigenberger, “Ion-Exchange Membranes in the Chemical Process
Industry,” Industrial & Engineering Chemistry Research, vol. 52, no. 31, pp. 10364-10379, 2013.
[30] A.G. Winger, G.W. Bodamer, R. Kunin, “Some electrochemical properties of new synthetic ion exchange
membranes,” Journal of Electrochemical Society, vol. 100, no. 4 , pp. 178–184, 1953.
[31] F.G. Helfferich, Ion Exchange, McGraw-Hill, New York, 1962.
[32] H. J. Cassady, E. C. Cimino, M. Kuman, M. A. Hickner, “Specific ion effects on the permselectivity of
sulfonated poly(ether sulfone) cation exchange membranes”, Journal of Membrane Science, vol. 508,
pp. 146-152, 2016.
[33] J. Hong, B. Zhang, S. Glabman, N. Uzal, X. Dou, H. Zhang, X. Wei, Y. Chen, “Potential ion exchange
membranes and system performance in reverse electrodialysis for power generation: A review”,
Journal of Membrane Science, vol. 486, pp. 71-88, 2015.
[34] K. Urano, Y. Masaki, M. Kawabata, “Electric resistances of ion-exchange membranes in dilute
solutions,” Desalination, vol. 58, pp. 171–176, 1986.
77
[35] A. H. Galama, D. A. Vermaas, J. Veerman, M. Saakes, H. H. M. Rijnaarts, J. W. Post, K. Nijmeijer,
“Membrane resistance: The effect of salinity gradients over a cation exchange membrane”, Journal of
Membrane Science, vol. 467, pp. 279-291, 2014.
[36] A. H. Galama, J. W. Post, M. A. Cohen Stuart, P. M. Biesheuvel, “Validity of the Boltzmann equation to
describe Donnan equilibrium at the membrane-solution interface”, Journal of Membrane Science, vol.
442, pp. 131-139, 2013.
[37] P. Długołȩcki, A. Gambier, K. Nijmeijer, and M. Wessling, “Practical potential of reverse electrodialysis
as process for sustainable energy generation,” Environ. Sci. Technol., vol. 43, no. 17, pp. 6888–6894,
2009.
[38] P. Długołecki, P. Ogonowski, , S. J. Metz, M. Saakes, K. Nymeijer, M. Wessling, “On the resistances of
membrane, diffusion boundary layer and double layer in ion exchange membrane transport”, Journal
of Membrane Science, vol. 349, no. 1-2, pp. 369-379, 2010.
[39] J. Veerman, R.M. de Jong, M. Saakes, S. J. Metz, G. J. Harmsen, “Reverse electrodialysis: Comparison of
six commercial membrane pairs on the thermodynamic efficiency and power density,” Journal of
Membrane Science, vol. 343, no. 1-2, pp. 7-15, 2009.
[40] M. Tedesco, H.V.M. Hamelers, and P.M. Biesheuvel, “Nernst-Planck transport theory for (reverse)
electrodialysis: I. Effect of co-ion transport through the membranes,” Journal of Membrane Science,
vol. 510, pp. 370-381, 2016.
[41] N. Y. Yip, D. A. Vermaas, K. Nijmeijer, M. Elimelech, “Thermodynamic, Energy Efficiency, and Power
Density Analysis of Reverse Electrodialysis Power Generation with Natural Salinity Gradients,”
Environmental Science and Technology, vol. 48, no. 9, pp. 4925-4936, 2014.
[42] J. Veerman, M. Saakes, S. J. Metz, and G. J. Harmsen, “Reverse electrodialysis: A validated process
model for design and optimization,” Chem. Eng. J., vol. 166, no. 1, pp. 256–268, 2011.
[43] P. Długołecki, A. Gambier, K. Nijmeijer, M. Wessling, “Practical potential of reverse electrodialysis as
process for sustainable energy generation”, Environmental Science and Technology, vol. 43, no. 17, pp.
6888-6894, 2009.
[44] D. A. Vermaas, Energy generation from mixing salt water and fresh water. Twente, p. 254, 2013.
[45] D. A. Vermaas, E. Guler, M. Saakes, K. Nijmeijer, “Theoretical power density from salinity gradients
using reverse electrodialysis”, Energy Procedia, vol. 20, pp. 170-184, 2012.
[46] S. Pawlowski, P. Sistat, J. G. Crespo, S. Velizarov, “Mass transfer in reverse electrodialysis: Flow
entrance effects and diffusion boundary layer thickness,” Journal of Membrane Science, vol. 471, pp.
72–83, 2014.
[47] D. A. Vermaas, J. Veerman, N. Y. Yip, M. Elimelech, M. Saakes, K. Nijmeijer, “High Efficiency in Energy
Generation from Salinity Gradients with Reverse Electrodialysis,” ACS Sustainable Chemistry
Engineering, vol. 1, no. 10, pp. 1295–1302, 2013.
[48] D. A. Vermaas, M. Saakes, K. Nijmeijer, “Doubled power density from salinity gradients at reduced
intermembrane distance”, Environmental Science and Technology, vol. 45, no. 16, pp. 7089-7095, 2011.
[49] G. M. Geise, A. J. Curtis, M. C. Hatzell, M. A. Hickner, B. E. Logan, “Salt concentration differences alter
membrane resistance in reverse electrodialysis stacks,” Environmental science & Technology Letters,
vol. 1, pp. 36-39, 2014.
[50] D. A. Vermaas, D. Kunteng, M. Saakes, K. Nijmeijer, “Fouling in reverse electrodialysis under natural
conditions,” water research, vol. 47, pp. 1289-1298, 2013.
78
[51] V.D. Grebenyuk, R.D. Chebotareva, S. Peters, V. Linkov, V., “Surface modification of anion-exchange
electrodialysis membranes to enhance anti-fouling characteristics,” Desalination, vol. 115, no. 3, 313-
329, 1998.
[52] E. Korngold, F. de Körösy, R. Rahav, M.F. Taboch, “Fouling of anionselective membranes in
electrodialysis,“ Desalination, vol. 8, no. 2, pp. 195-220, 1970.
[53] N. Cifuentes-Araya, G. Pourcelly, L. Bazinet, “Impact of pulsed electric field on electrodialysis process
performance and membrane fouling during consecutive demineralization of a model salt solution
containing a high magnesium/calcium ratio,” Journal of Colloid and Interface Science, vol. 361, no. 1,
79-89, 2011.
[54] J. W. Post, H. V. M. Hamelers, C. J. N. Buisman, “Influence of multivalent ions on power production
from mixing salt and fresh water with a reverse electrodialysis system,” Journal of Membrane Science,
vol. 330, no. 1-2, 65–72, 2009.
[55] G.K. Batchelor, An introduction to fluid dynamics, Cambridge University Press, New York, 2000.
[56] J. Veerman, J. W. Post, M. Saakes, S. J. Metz, G. J. Harmsen, “Reducing power losses caused by ionic
shortcut currents in reverse electrodialysis stacks by a validated model,” Journal of Membrane Science,
vol. 310, no. 1-2, pp. 418-430, 2008.
[57] D.A. Vermaas, M. Saakes, K. Nijmeijer, “Power generation using profiled membranes in reverse
electrodialysis,” Journal of Membrane Science, vol. 385-386, no. 1, pp. 234-242, 2011.
[58] S. Loeb, “Production of energy from concentrated brines by pressure-retarded osmosis : I. Preliminary
technical and economic correlations,” Journal of Membrane Science, vol. 1, pp. 49-63, 1976.
[59] J. W. Post, J. Veerman, H. V. M. Hamelers, G. J. W. Euverink, S. J. Metz, K. Nymeijer, “Salinity-gradient
power: evaluation of pressure-retarded osmosis and reverse electrodialysis,” Journal of Membrane
Science, vol. 288, no. 1-2, pp. 218–230, 2007.
[60] G. Blandin, A. R.D. Verliefde, C. Y. Tang, P. Le-Clech, “Opportunities to reach economic sustainability in
forward osmosis–reverse osmosis hybrids for seawater desalination,” Desalination, vol. 363, pp. 26-
36, 2014.
[61] Vanoppen, M. et al., 2016. Salinity gradient power and desalination. In A. Cipollina & G. Micale, eds.
Sustainable energy from salinity gradients. London: Woodhead Publishing-Elsevier.
[62] A. G. Winger, G. W. Bodamer, and R. Kunin, “Some Electrochemical Properties of New Synthetic Ion
Exchange Membranes,” J. Electrochem. Soc., vol. 100, no. 4, p. 178, 1953.
[63] B.V. Fujifilm Manufacturing Europe, “Product sheet Fujifilm Ion Exchange Membrane Type 2,” The
Netherlands.
[64] E. Criel, “Energy-efficient seawater desalination: a comparison between reverse electrodialysis and
assisted reverse electrodialysis as pre-treatment for reverse osmosis,” University of Ghent, 2015.
[65] A. Daniilidis, R. Herber, and D. a. Vermaas, “Upscale potential and financial feasibility of a reverse
electrodialysis power plant,” Appl. Energy, vol. 119, pp. 257–265, Apr. 2014.
[66] J.G. Hong, B. Zhang, S. Glabman, N. Uzal, X. Dou, H. Zhang, X. Wei, Y. Chen, “Potential ion exchange
membranes and system performance in reverse electrodialysis for power generation: A review,”
Journal of Membrane Science, vol. 486, pp. 71-88, 2015.
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