IN DEGREE PROJECT TECHNOLOGY,FIRST CYCLE, 15 CREDITS

, STOCKHOLM SWEDEN 2019

Validation of a novel water purification technology

ALGOT RICKMAN

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF ENGINEERING SCIENCES

www.kth.se

IN DEGREE PROJECT TEKNIK,FIRST CYCLE, 15 CREDITS

, STOCKHOLM SWEDEN 2019

Validering av ny vattenrenings teknik

ALGOT RICKMAN

KTH ROYAL INSTITUTE OF TECHNOLOGYSKOLAN FÖR TEKNIKVETENSKAP

www.kth.se

1 Abstract

Access to clean water is the fundamental human right. With climate change respon-sible for the alterations of patterns of rain across the globe, there is an increasingshortage of drinking water during summer in Sweden. Water desalination of theBaltic sea could solve many of the problems associated with shortages but reverseosmosis processes are not very efficient and have all associated problems of foulingdue to the membranes, apart from being quite expensive. Membrane free capaci-tive desalination can be a viable alternative alleviate to many of these problems.However, the operations of the devices at varying temperatures need to be studiedto find out the suitability of such systems for implementation in field applicationsacross the world. Thus, the main focus of this project is to evaluate the practical op-eration of capacitive deionization (CDI) devices at different operating temperaturesand to analyse the resulting behavior obtained. The aim was to setup a test benchto automatically record desalination (through the monitoring of ion-conductivitychanges in water) and analyse the data so obtained. Experiments were performedwith saline water (1000 ppm sodium chloride) at 20 ◦C, 40 ◦C and 60 ◦C using aflow through/flow between CDI system utilizing activated carbon flexible electrodes.Experiments were both carried out with 1.6 V applied potential, representing ex-pected real-world use, and 1 V in an effort to minimise electrolytic reactions (sincewater splits at 1.23 V). Maximum salt removal per pass through the capacitor wasmeasured to be 1% (20 ◦C), 1.4% (40 ◦C) and 1.1% (60 ◦C) at 1.6 V and 0.52% (20◦C), 0.51% (40 ◦C) and 0.45% (60 ◦C) at 1 V, respectivly. Rate of salt removal wasalso measured, resulting in higher temperature giving higher rates of absorption,which indeed could be an advantage for the membrane less-CDI since desalinationefficiency dramatically reduces in membrane based processes like reverse osmosis athigher temperatures.

2 Sammanfattning

Tillg̊ang till rent vatten är en mänsklig rättighet. Med klimat förändringar somorsakar en förändring i regn mönster över hela planeten skapas underskott av dricks-vatten under svenska somrar. Avsaltning av Östersjövatten kan lösa m̊anga av deproblem som har orsakats av vatten brist. Tyvärr är omvänd osmos processer intealltid effektivt och lider av problem igensättning av membran, utöver att de är rela-tivt dyra. Membranfri kapacitiv dejonisering kan vara ett alternativ för att lösa fleraav dessa problem. Dock s̊a behöver användningen av tekniken vid olika tempera-turer studeras för att avgöra om den är användbar över hela världen. S̊aledes varhuvudfokusen av projektet att evaluera den praktiska applikationen av av kapasitivdejonisering (CDI) enhet vid olika temperaturer och att analysera det resulterandebeteendet. Målet var att sätta upp en experiment uppställning som automatiskt

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mätte avsaltning (genom att mäta förändringar jol-konductiviteten av vattnet) ochanalysera den uppmätta datan. Experimentet var genomfört saltvatten vid 20 ◦C,40 ◦C och 60 ◦C och använde en genomflödes/kringflödes CDI system som utnytt-jade flexibla elektroderavaktiverat kol. Experimenten var b̊ade genomförda vid 1.6V applicerad potential, vilket representerar värkliga förh̊allanden och 1 V för attminimera electrolytiska reaktioner (somstartar vid 1.23 V). Den maximala avsalt-ningen per passering genom kondensatorn uppmättes till 1% (20 ◦C), 1.4% (40 ◦C)och 1.1% (60 ◦C) vid 1.6 V och 0.52% (20 ◦C), 0.51% (40 ◦C) och 0.45% (60 ◦C)vid 1 V. Hastigheten med vilket saltet avlägsnades mättes ocks̊a, resulterande i atthögre temperatur uppvisade snabbare salt upptagningsförm̊aga, vilket kan visa sigvara en stor fördel för membranfri CDI eftersom att avsaltnings effektivitet drastisktförsämras för membran baserade processer s̊a som omvänd osmos vid högre temper-aturer.

3 Introduction

Access to safe drinking water is a pre-requisite to human health and sustainablesocial and economic development of the world. Worldwide shortages in water due tothe exponential growth of human population, rapid industrialization and effects ofclimate change is driving active research in the development of technologies to desali-nate abundantly available seawater. Freshwater sources in the world (2.5–2.75%),1.75–2% in frozen glaciers, ice and snow, 0.5–0.75% as fresh groundwater, and lessthan 0.01% of it as surface water in ponds, lakes, rivers and streams [1]. Freshwater contains low concentrations of dissolved salts and other total dissolved solids.Brackish water is characterized by water with total dissolved salts less than 10,000ppm(parts per million). Baltic Sea is enclosed by the Nordic countries includingSweden and the northeast Germany, Poland, Russia and the North and Central Eu-ropean Plain is a major reserve of brackish water in the world. This brackish watercan be our source for increasing needs of fresh water supply to satisfy not only theneeds of our growing population but also for agriculture and animal husbandry. Ingeneral, several water desalination techniques such as distillation, reverse osmosis(RO) and electrodialysis have been developed but they are either very expensive toimplement and/or are energy-inefficient.

Capacitive Deionization (CDI) is an emerging technology in the field of wa-ter desalination that is a socio-technical solution for providing safe drinking watertechnology through transformative change in water provision and water reuse andaffordable decentralized treatments.

CDI has several advantages over other desalination techniques: (i) it works withelectroadsorption, and is energy-efficient since it does not require any high-pressurepumps (ii) Electrosorption can efficiently occur from in 0.8–2.0 V, which renders

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it suitable to be integrated with decentralized installations powered by solar/windenergy and thus can be operational in remote areas (iii) Rejected (wastage) water ismuch lower compared to other desalination techniques such as RO (iv) carbon basedelectrodes can withstand much higher temperatures than membranes and thus arebetter suited for field applications [2].

Typically, a CDI cell consists of a pair of highly porous electrodes (usually madewith carbon), separated by a nonconducting ion-porous separator. Upon the appli-cation of an electrical potential (≤ 2.0 V), a potential difference is generated acrossthe electrodes leading to energy storage through the adsorption of ions on theseelectrodes. Dissolved salt is thus effectively removed from the water and upon thesaturation of the electrodes, usually the electrical potential is removed while flowingwater through the cells to redissolve the salt ions in water, thus regenerating the elec-trodes and making them ready for fresh ion adsorption while rejecting the washingwater. The adsorption and desorption of ions take place simultaneously and usuallythe amount of salt which is adsorbed in the first cycle is not desorbed completelyduring regeneration cycle. In order to circumvent this problem, researchers haveintroduced ion-exchange (IEX) membranes in CDI systems on the porous carbonelectrodes (called membrane-CDI (MCDI)). This leads to increase in price of CDIsystems and brings back all the other problems associated with membrane baseddesalination systems, including biofouling [3]. Functional Materials Group at KTHhas developed a novel CDI technology using flexible activated carbon cloth materials(as electrodes) rolled up into cylindrical cells and do not add any IEX membranes.Instead the group has worked on switching between adsorption and desorption pro-cesses (before total saturation of electrodes occur) and have found ways of improvingdesalination efficiency[4].

There are however several aspects of this technology that so far have not beensufficiently explored in order to establish a full understanding of its complete func-tion nor to make it a technology to seriously challenge established methods com-mercially. One such aspect being operating temperature. This being particularlyimportant once one starts to consider operation year round, in different environ-ments and especially in different nations. This project therefore aimed to establishan understanding of the basic workings of the CDI technology, with a combinedunderstanding of its theoretical aspects as well as its practical ones. And to developand perform experiments to gain a deeper understanding of the temperature’s effecton the operation of a CDI cell.

4 Background

The process that is used to remove salt from a solution using electrosorption apply-ing low dc potential, called capacitive deionization, which will be referred to as CDI

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in this report has been carried out with a cylindrical cell fabricated as reported inthe patent using activated carbon cloth electrodes [6]. The basic working principleof CDI involve salt ion adsorption and desorption from the electrodes, of a capac-itor. Oppositely charged ions will stick to either capacitor plates (depending onthe polarity) upon the application of a voltage (electrosorption). When the voltageis turned off, the ions can be easily released from the electrodes, thus resulting inwaste water during the regeneration phase. These two distinct modes of operationare the backbone of CDI technology and will in this report be referred to as thedesalination phase, where salt is removed from the water and collected by the elec-trodes in a cell followed by the process of regeneration of the electrodes where saltis removed from the cell. During normal operation desalination and regenerationphases are periodically switched back and forth, removing a small amount of salt inevery cycle, cumulatively contributing to finally remove all the salt ions up to waterthat would be suitable for drinking.

(a) (b)

Figure 1: Schematic representation of (a) ion adsorption from a solution of sodiumchloride upon the application of an external potential (desalination) (b) removalof ions from the electrodes upon removal of external voltage (regeneration of theelectrodes)

The factor governing the amount of salt a cell can hold is the charge q over thecapacitor. This is governed by its capacitance which is defined as, C = qU , where Uis the applied voltage. For a simple parallel plate capacitor, the capacitance, C, canbe estimated by, C = Ad , where ‘A’ is the surface area of the electrodes and ‘d’ isthe distance between the two plates. This provides the charge stored on the plates,roughly estimated as, q = A∗Ud . In order to maximize the effectiveness of the cellfor charge storage, the distance ‘d’ between the plates should preferably be set tothe lowest value that does not cause a short circuit through the spacer. Optimizingthe process by increasing the voltage ‘U’ over the electrodes is limited due to theelectrolysis of water into oxygen and hydrogen gases due to the passage of an electriccurrent beyond 1.23 V. This fundamentally requires increasing the electrode areato enable effective charge storage that is acheived using nano-materials like the

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activated carbon cloth used in our cylindrical cell. The cell used in this experimentutilises a cloth made of activated carbon fibres as electrodes, with specific surfacearea per weight of roughly 1000m

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g .In order to maximise the effectiveness of the cell the distance d between the

plates can simply be set to the lowest value that does not cause a short circuit witha spacer. Optimising the process by increasing the voltage U over the electrodesis counterproductive due to its increasing power consumption substantially thusleaving the voltage with the need to be tuned precisely. This only leaves increasingthe electrode area to enable effective CDI technology. As it turns out modern nano-materials fills the necessary requirements to do this. The cell used in this experimentutilises a cloth made of activated carbon Fibres as electrodes, these are structuredin such a way that the surface area is roughly 1000m

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g .

5 Experimental setup

The experimental setup consist of a CDI cell connected to a peristaltic pump drivingwater from a reservoir to and from the cell and essentially an Arduino control unitconnected to the cell and measuring equipment. At the outlet of the water fromthe cell, a online conductivity meter (eDaq) was connected. The system uses aperistaltic pump to set a flow rate of 250 ml/m throughout for all the experiments.The reservoir was fixed at a constant volume of 4L for the entirety of the experiments,and was placed on a heating plate with an inbuilt stirrer. Insulation of the waterpipes and the complete system was done and care was taken so that water vapourdid not leave the closed system, especially at higher temperatures. Temperature inthe system was measured at two points. In the reservoir, there was three separatemeasurements taken from one digital thermometer connected to the hotplate, onetraditional glass thermometer and one temperature probe connected to a multimeter.Connected to the end of the loop there was a second measuring point to determinethe temperature loss of the entire system, only a multimeter probe was used at thispoint. At the outlet of the reservoir a e-daq conductivity meter was fixed to obtaininformation about the water salinity levels.

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5.1 Cell construction

Figure 2: Figure 4a illustrates a perspective view of CDI device. Figure 4b schemat-ically illustrates a cross-sectional view along the plane B-B of figure 4a [5].

The setup of the CDI cell that was used was a cylindrical variant. It was constructedby laying the electrode sheets on top of each other separated by a filter paper actingas a separator. The arrangement of sheets was then rolled up forming a multilayeredcylinder shell (figure 4b). The cell was then placed within a plastic enclosure withan input valve on the top connected to the inside of the electrodes and output valvesconnected to a space outside the electrode (figure 4a). During operation, this givesa flow through the electrodes. The particular cell used to perform the experimentsutilised electrodes with dimensions of 35 cm x 15 cm and a rough weight of 12 ggiving it an effective surface area of circa 1.2104m2.

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5.2 Mode of operation

When doing experiments with the cell there are two distinct modes of operation.The first resembles the real-world use of CDI. It is generally called continuous modeand simply works by putting in new water in to the cell that upon exit is measuredand discarded. The point of this method is to mimic realistic use, it unfortunatelyuses a lot of water which becomes a problem when the input water needs prepara-tion to get valid experimental results. To solve this a different mode of operationmight be used where the water that would otherwise be discarded instead is putback into the same container that new water is pulled from. This method is calledbatch mode operation, as it only uses a single sample batch over and over. Givenappropriate control of certain parameters, such as ensuring low variation in ioniccontent, batch mode is in principle equivalent to continuous mode operation, differ-ing only by instead giving the integral of corresponding continuous operation. Forall our experiments, we monitored that the variation of ionic content is kept within2% of starting concentration, which was achieved by using a large volume of startingsolution, namely 4 liters of saline water.

5.3 Asymmetry of the cell

When the first few experiments were performed a somewhat problematic behaviourwas observed. It was found that the cell exhibited asymmetrical behaviour in respectto current. In effect this results in a difference in desalination and regenerationcharacteristics when current is reversed.

Figure 3: Two different runs both at 20 ◦C and 1.6 V

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In the first run, it can be observed that the conductivity increases during thedesalination process which implies an increase in salt concentration, only stabilisingat the baseline. This behaviour is called co-ion expulsion as it can be explainedby considering the applied voltage expelling cations that passively stick to the elec-trode. This degradation might be explained by wear on the cell from long use beforeexperiments started for this project. The second run however, behaved as predictedand according to previous work, this is how all following experiments were con-ducted. This deteriorating effect is further explained by Cohen [7]. It also explainsdeterioration with long use, the order of each specific experiment will therefore betaken in to account.

5.4 Stability

Stability of the system was one of the main considerations necessary to get a goodexperimental result. The main issue was the requirement for several desalinationcycles before stability was reached. The solution to this problem was simply to fa-cilitate longer experiments, this was achieved by implementing and programming anArduino UNO control chip. With the Arduino in place, automation could be imple-mented, allowing for both unsupervised experiments and more consistent switchingbetween desalination and regeneration.

At higher operating temperature, a different problem arose that could not becompensated for by extending the time of the experiment, namely, evaporation. At20 ◦C this was not an issue, but when heated to 60 ◦C a substantial portion of thewater was lost during the experiment, increasing the concentration due to the saltin the solution not being affected. This problem was solved by closing of any opensurfaces of water and to insulate the entirety of the system. This had the pleasantside effect that variations in temperature also lessened, solving another penitentialstability issue.

6 Results

Desalination/regeneration runs was made with 1000 PPM saline water kept at aconstant temperature of 20 ◦C, 40 ◦C and 60 ◦C, corresponding to room tempera-ture, the highest expected natural temperature and a high-point. Experiments wereperformed with a dc voltage of 1 V and 1.6 V lasting 80 respectively 45 minutesfor desalination and 60 respectively 45 minutes for regeneration. Concentration inthe solution was 1 g/L or 17 mMol of NaCl. All measurements were of conductivitythat has a linear relationship with salt concentration, all graphs present percentilesalt removed with the 0 point at unaltered concentration and the -100 point as 0%concentration.

The difference between 1V and 1.6V can be seen in figure 4.

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(a) (b)

Figure 4:

1 V was used to avoid electrolytic reactions that occur at 1.23 V [8], though,this demanded longer experiments. In contrast 1.6V exhibit faster absorption whilstrisking problematic effects from the splitting of water. Both were used for furthertests.

6.1 Temperature dependence

The temperature of the solution was kept constant at 20 ◦C, 40 ◦C and 60 ◦C andthe test was given several cycles of desalination in order to stabilise before readingfrom the measurements were considered. Comparing the desalination at differenttemperatures the results are presented in figure 5.

(a) (b)

Figure 5: Tree runs at different temperatur: a) 1 V b) 1.6 V

The resulting graphs reveal an interesting pattern with both improvement and

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worsening with higher temperature. The difference between 1 V and 1.6 V turnsout to be significant for further investigation.

6.2 experiments at 1 V

The above graphs give a good indication of the effect the operating temperaturehas on the CDI process. However, to get a more detailed picture, the averages ofseveral separate experiments was constructed into a graph of a single cycle in figure6. Here, only focusing on experiments at 1V. A special consideration was taken tothe maximum adsorption of the different temperatures also in figure 6.

(a) (b)

Figure 6: a) Salt concentration when operating at different temperatures at 1 V. b)Average maximum adsorption at different temperature also at 1 V with a maximumvariation for the different experiments as error bars.

Temperature 20 ◦C 40 ◦C 60 ◦C

salt removal 21 mg (0.52%) 20 mg (0.51%) 18 mg (0.45%)

improvement over 20 ◦C N/A -1% -13%

rate of removal (1/min) 0.017% 0.024% 0.025%

Max conductivity decrease(mS) 0.0096 0.09 0.00825

Table 1:

These tabulated values show that there was no improvement in desalination for40 ◦C compared 20 ◦C instead remaining at roughly the same level. They also showa decreased adsorption of 13% at 60 ◦C in comparison to 20 ◦C.

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6.3 Experiments at 1.6 V

With results from the 1 V experiments established, 1.6 V was investigated in thesame manner.

(a) (b)

Figure 7: a) Salt concentration when operating at different temperatures at 1.6V. b) Average maximum adsorption at different temperature also at 1.6 V with amaximum variation for the different experiments as error bars.

Temperature 20 ◦C 40 ◦C 60 ◦C

salt removal 40mg (1%) 52mg (1.4%) 44mg (1.1%)

improvement over 20 ◦C N/A 10% 40%

rate of removal (1/min) 0.047% 0.066% 0.065%

Max conductivity decrease(mS) 0.019 0.027 0.021

Table 2:

For this experiment, in contrast to the previous, there was a small improvementin maximum ion absorption when the solution was heated to 60 ◦C. They also showan increased absorption of 40% at 40 ◦C in comparison to 20 ◦C. These resultssuggest that there is no significant decrease in maximum absorption in the cell whenoperating at temperatures at up to 60 ◦C at 1.6 V. It also suggests that there exist anoptimal temperature in the span between 60 ◦C and 20 ◦C that provides a significantincrease in maximum absorption.

6.4 Short desalination time

When operating a CDI cell in the continuous mode it is expected to reach a steadystate with only minor deviations. A qualitative look at the 1.6V figures shows that

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this is not the case. this is likely due to electrolytic reactions at higher chargedensity as discussed by Choi [8]. There is however an important difference in thelength of desalination that was used in the experiments and that which is used inreal-world applications, which is much shorter. Therefore, the initial portion of theexperiments was separated for further evaluation.

(a) (b)

Figure 8: shortened experiments to represent functional use a) first 50 minutes at 1V b) first 30 minutes at 1.6 V

When limiting the desalination time a pattern of higher adsorption rate for highertemperature emerges. This effect is not very pronounced in the 1V run but is rathernoticeable in the 1.6 one. It is noteworthy that the difference between 40 ◦C and 60◦C at 16V is very minor up until 20 minutes of desalination.

7 Discussion

As CDI technology rapidly approaches real-world use and leaving the safety andcertainty of the laboratory scenarios beyond the ideal and controlled needs to beunderstood. In applied use with current membrane based desalination techniques,temperature is most often inconvenient or too costly to control. This is further em-phasised by big variations in environments where there is a need for the technology,from the cold waters of the Swedish archipelago to the scorching heat of deserts. Anexperiment was setup and incrementally improved to find an answer to this ques-tion. Focus was put on gaining knowledge of the inherent process to achieve the bestaccuracy. What was found was that there is no a simple correlation between temper-ature and absorption, rather, there exists an optimal temperature for a given setup,specifically depending on the applied potential as seen in these experiments. It canhowever be stated that higher temperature provides faster adsorption, voltage being

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inconsequential for this, at least throughout these somewhat limited experiments.It can be concluded that taking the temperature into account can help optimising

absorption. For usage at 1.6 V total absorption at 60 ◦C is comparable to 20 ◦C, ifthe desalination process is instead shortened a significant improvement of roughly50% efficiancy can be gained.

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8 Acknowledgements

Finally, I wold like to thank my supervisor Joydeep Dutta for support during theproject. I wold also like to thank Johan Nordstrand and Karthik Laxman for adviseand discussions as well as the team at functional materials.

References

[1] https://en.wikipedia.org/wiki/Fresh water,downloaded on 9th May 2019.

[2] Karthik Laxman, Afzal Husain, Asma Nasser, Mohammed Al Abri and JoydeepDutta. Tailoring the pressure drop and fluid distribution of a capacitive deion-ization device.. Desalination 449 (2019) 111–117.

[3] Mohammed Al abri, Albert Boretti, Buthayna Buthayna Al-Ghafri, TanujjalBora, Sergey Dobretsov, Joydeep Dutta, Stefania Castelletto, and Lorenzo Rosa.Chlorination Disadvantages and Alternative Routes for Biofouling Control inReverse Osmosis Desalination.. NPJ CLEANWATER, 2 (2019) 2.

[4] http://www.diva-portal.org/smash/record.jsf?pid=diva2%3A1294047dswid=-6191.

[5] Karthik Laxman Kunjali and Joydeep Dutta Device for capacitive deionizationof aqueous media and method of manufacturing such a device. Swedish Patent:2018 (appl. no. 1750797-1)

[6] KL Kunjali, J Dutta Three-electrode structure for capacitive deionization desali-nation. (2017) US Patent 9,751,779

[7] Izaak Cohen, Eran Avraham, Malachi Noked, Abraham Soffer, Doron Aur-bach. Enhanced Charge Efficiendy in Capacitive Deionization Achied by Surface-Treated Electrodes and by Means of a Third Electrode.. J.phys.chem.(2011)19856-19863.

[8] Jae-Hwan Choi, Duck-Jin Yoon. The maximum allowable charge for operatingmembrane deionization without electrode reactions. Separation and PurificationTechnology 215 (2019) 125–133.

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