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Flow-electrode capacitive deionization enables continuous and energy-efficient brine concentration

Many industrial and agricultural applications require the treatment of water streams containing high concentrations of ionic species for closing material cycles. High concentration factors are often desired, but hard to achieve with established thermal or membrane-based water treatment technologies at low energy consumptions. Capacitive deionization processes are normally assumed as relevant for the treatment of low salinity solutions only. Flow-electrode capacitive deionization (FCDI), on the other hand, is an electrically driven water desalination technology, which allows the continuous desalination and concentration of saline water streams even at elevated salinities. Ions are adsorbed electrostatically in pumpable carbon flow electrodes, which enable a range of new process designs. In this article, it is shown that continuously operated FCDI systems can be applied for the treatment of salt brines. Concentrations of up to 291.5~g/L NaCl were reached in the concentrate product stream. Based on this, FCDI is a promising technology for brine treatment and salt recovery. Additionally, a reduction of the energy demand by more than 70% is demonstrated by introducing multiple cell pairs into a continuous FCDI system. While the economic feasibility is not investigated here, the results show that FCDI systems may compete with established technologies regarding their energy demand.

Ions are adsorbed electrostatically in pumpable carbon flow electrodes, which enable a range of new process designs.

Introduction and motivation
Water treatment technologies should be energy efficient, reliable and economical. While the technologies available for water treatment, such as desalination and concentration have been improved significantly over the last decades, there is no viable water treatment technology available in some application cases, while in many other cases there is still room for improvement [1,2,3].
Significant attention has been focused on capacitive deionization (CDI) technologies in the last decade. In CDI processes, an applied electrochemical potential gradient leads to the transport of dissolved ions towards the oppositely charged electrodes, where the ions are bound via electrostatic adsorption at the electrode-solution interface. High surface areas, and hence high ion adsorption capacities, are usually achieved by employing solid, porous carbon electrodes. CDI processes can be operated in a variety of configu-rations. The use of ion-exchange membranes (IEM) can improve the performance of CDI processes regarding their desalination performance and energy efficiency, when placed in between electrodes and treated solution [2,3,4,5,6,7]. Most CDI processes are based on static carbon-based electrodes and are usually operated discontinuously using charge-discharge cycles. Due to this, the application of CDI processes is usually limited to the treatment of brackish water feed streams. Higher feed salinities would lead to a fast saturation of the electrode material with ions and hence the requirement of very short charge-discharge cycles, which would make CDI processes with static electrodes inefficient due to back-mixing withing the tubing and modules.
An upcoming CDI technology suitable for the treatment of higher salinities is flow-electrode capacitive deionization (FCDI) [8]. In case of FCDI, flow electrodes consisting of pumpable carbon suspensions replace the static CDI electrodes, in which charge percolation leads to a utilization of the suspended carbon particles as electrode material [9,10]. Flow electrodes can be regenerated via mixing [11] or in a separate module or compartment, and enable a fully continuous operation. By now, various continuous FCDI process layouts have been proposed, including a continuous two-module configuration [12], single-module configuration [13], a mixer-settler configuration [14], and a two-step regeneration configuration [15]. In all of these configurations, the electrodes are continuously regenerated and a diluate and concentrate stream is produced continuously with more or less constant salt concentrations.
FCDI technologies are a promising research field due to the high salt ad-sorption capacities [5,12,16,17], good desalination performance, the ability to desalinate high salinity solutions [8], and the possibility of fully continuous operation [12,13,14,18]. A great strength of the FCDI technology is its versatility. The use of suspension-based flow electrodes for desalination and water treatment in general can be seen as a platform technology, which can be operated in many different process layouts for many different applications [19,20]. Lab-scale FCDI studies published until now were, for example, aimed at salt metathesis [21], ammonia recovery from wastewaters [22,23,24], nitrogen and phosphorous recovery [25], nitrate removal [26], copper removal [27], and uranium removal [28]. Apart from a multitude of pure FCDI processes, many combinations with established processes are imaginable, such as the combination of FCDI with nanofiltration [29].
However, most of the prior work on FCDI focuses on low or medium ionic concentration applications, for brackish water and seawater treatment, due to the limited adsorption capacity of the carbon electrodes in discontinuous or semi-continuous operation of FCDI processes.
Several characteristics make FCDI a promising candidate for the treatment of high salinity solutions. Compared to most established thermal or membrane-based water treatment processes, FCDI processes may enable the selective removal of charged species from water streams in the future, and may at the same time become more energy-efficient for many application cases. Even in comparison to other electrically driven processes [30], FCDI has its advantages: Unlike electrodialysis (ED) processes, FCDI relies on the capacitive adsorption of ions in the electrodes at low voltages, and does not require electrochemical reactions in the electrodes. Hence, gas evolution and the use of acids in the electrode compartments is avoided. The elimination of electrochemical reactions in the electrode compartments may also be advantageous for the treatment of sensitive chemicals, which may be altered by the high potentials and side products in case of ED processes.
As shown in our previous article [15], the values regarding the energy demand of FCDI processes cannot yet compete with literature values regarding existing technologies. However, a target-oriented advancement of the FCDI technology may lead to a new alternative solution for many water treatment applications.
This article demonstrates the possibility of concentrating brines in a continuous process based on a single module using FCDI technology. Additionally, it is shown that a significant reduction of the energy demand and an improved productivity can be achieved by using several cell pairs. This approach is similar to the approach recently presented by Ma et al. [31], who presented an FCDI configuration with stacked membranes, which leads to significant productivity improvements, but only enables a semi-continuous water treatment. In our work, we demonstrate the continuous treatment of saline brine using FCDI modules with one or more cell pairs, and therefore an even number of water compartments.
Apart from the investigation of the electrical energy demand for desalination, we also included a comparison to the contribution of the pumping energy to the total energy demand in Section 4.4. Finally, we compare the presented FCDI concept to the established technologies mechanical vapor compression (MVC) and electrodialysis (ED) regarding their energy demand, which presents a complement to the study recently published by Patel et al. [32]. While the experiments presented are specifically aimed at the treatment and concentration of high salinity solutions (brines) using FCDI processes, the findings can be transferred to other potential application areas of FCDI processes.

Reducing the resistance losses in FCDI processes: An analysis
For a better understanding of the contribution of individual electrical resistances in an FCDI cell, theoretical estimations of the individual resistances were made.
The results are plotted in Figure 1, which shows the electrical resistance of specific parts of an FCDI cell depending on the electrical conductivity of the diluate product stream. The calculations are based on the following assumptions: • NaCl feed: 60 g/L NaCl at a diluate feed flow rate of 1 mL/min.
• Resistance of membranes and flow electrodes constant throughout cell.
• The required electrical current is calculated based on the desalination down to the respective diluate concentration, leading to lower energy demands at higher diluate concentrations even at constant resistance.
• (Depleted) laminar boundary layers are not considered.
• Ideal pumping energy: Pump efficiency is not considered.
• Diluate channel resistance: Geometric average between inlet and outlet concentrations.
• A current efficiency of one is assumed.
• Flow-electrode resistance R F e : based on total resistance R total and combined solution/ membrane resistance R S from EIS measurements presented in [33], R F e = R total − R S .
The resistance of the flow electrodes is 0.33-0.4 Ω for an AC content of 15 wt.%, which presents a large fraction of the overall cell resistance. This is in agreement with our previous study [33], in which we observed a significant influence of the carbon slurry quality on the desalination performance.
The second largest resistance in the cell at most diluate concentrations is presented by the membranes. Three ion-exchange membranes of the type fumatech FAB/FKB with PEEK reinforcements, which we currently use in FCDI systems for the treatment of high salinity solutions, account for around 0.12-0.21 Ω of the total cell resistance, based on the manufacturers data. The resistance of the diluate channel varies significantly depending on the outlet concentration, ranging between 0.02-0.1 Ω. The sum of the membrane resistance and the resistance of the diluate spacer channel should be equal to the resistance R S from the equivalent circuit model presented in our previous article [33]. In this, R S was usually found to be in the range of 0.   35,36,37,38]. Here, we present a different approach aiming at the module design, based on the use of multiple cell pairs. The hypothesis is that the number of desalination units per unit flow electrode should be increased in order to reduce the relative contribution of the flow electrodes to the total resistance.

Concept: FCDI with multiple cell pairs
The motivation for introducing additional cell pairs into an FCDI module is illustrated in Figure 2  Apart from the voltage drop, the use of multiple cell pairs will also reduce the specific pumping energy required. In this case, the absolute pumping energy for the flow electrodes will stay the same, while the amount of product stream is multiplied by the number of cell pairs. The contributions of the dif-ferent resistances, as well as the pumping energy, to the total energy demand is further discussed in Section 4.4.
Based on these estimations, the use of multiple cell pairs seems to be a promising approach for FCDI processes, which is in agreement with the observations presented by Ma et al. [31]) regarding a comparable FCDI stack, which is operable in a semi-continuous manner. In the following, the concept of single-module FCDI with multiple cell pairs is implemented, tested, and its performance evaluated. The FCDI experiments were performed in cross-flow mode, as illustrated in Figure 3, which is a simplified 3D representation of the applied cell design. This module design enables the addition of one or more cell pairs into an FCDI module in single-module configuration [13]. Otherwise, the design of the FCDI cells applied in this study is identical to the design described in recent FCDI-related publications by our workgroup [15,33]. using a peristaltic pump (Ismatec MCP process drive, Masterflex two-channel pump head Easy-Load II, Norprene tubing). The feed water was supplied by a second peristaltic pump (Ismatec Reglo ICC peristaltic pump with two independent channels). The pumps, power supplies and conductivity sensors (LTC 0,35/23; Sensortechnik Meinsberg) were connected to a custom-made measuring system "ZUMO-FUMS" (ZUMOLab GbR, Wesseling, Germany), which was used to control the pumps and power supplies as well as record the experimental data. Whenever an FCDI module was newly assembled, fixed amounts of water were recirculated separately through each flow electrode, diluate and concentrate compartment for several hours to rule out internal and external leakages.

End plate
Experiments were performed with one, two and three cell pairs, as indicated in Table 1. In case of the experiments with several cell pairs, the feed flow rates were multiplied by the number of cell pairs, to achieve comparable hydrodynamic conditions within the cell. Hence, overall flow rates of 1, 2 and 3 mL/min were applied for the diluate feed and 0.04, 0.08 and 0.12 mL/min were applied for the concentrate feed.   Table 3. Samples were taken during steady-state operation of the In the first approach, the salt transfer is calculated based on the flow rates and salt concentrations of feed and product water streams, indicated by the green system boundary in Figure 5  This way, the actually applied electrical power, calculated from the applied voltage and the measured current, can be set in relation to the product water streams. Thus, the negative effect of diluate concentration and concentrate dilution due to water crossover is considered in the energy demand, as illustrated in Figure 5 (b).
Due to the influence of the chosen system boundaries, the assumptions and terminology chosen in this article regarding the energy demand and current efficiencies are elucidated in the following.

Electrical energy demand per m 3 diluate product stream
There are several ways to calculate the energy demand for the desalination of one cubic meter of salt water, as indicated above. In addition to the differences between product and full mass balance, disparities between the mass balances around the diluate and around the concentrate stream were observed.
In these equations, I stands for the electric current and V for the cell voltage. c F eed andV F eed are the NaCl concentration and volumetric flow rate of the feed stream, in which the subscripts D and C stand for the diluate and concentrate stream, respectively. n cp stands for the number of cell pairs.
The consideration of the full salt mass balance allows the calculation of the actually transferred salt amount. However, the additional transfer of water due to hydration shells and osmotic effects leads to a reduction of the actual desalination and concentration performance. The concentrate is diluted and the diluate stream concentrated due to the transfer of water.
This effect is not considered when calculating the energy demand via the full salt mass balance.
Consideration of water transfer, E dil,withW T . When calculating the energy demand based on the product salt mass balance, however, the water transfer is considered. By using only the volumetric flow rate of the product streams, the actual energy demand for the production of a specific volume of product (diluate or concentrate) of a certain desired concentration can be calculated, as given in Equation 4 for the energy demand E dil,D,withW T , and in Equation 5 for the energy demand E dil,C,withW T . The average between the two energy demands E dil,D,withW T and E dil,C,withW T is in the following termed E dil,withW T (Equation 6). A full desalination is likely not possible using a single cell and single pass FCDI system at feed concentrations of 60 g/L NaCl. A (nearly) full desalination may be achieved when using an optimized cell design, for example by using ion-conductive spacers, similar to continuous electrodeionization systems. However, a desalination to a low concentration of <1 g/L is possible.
For practical applications, the actual energy demand for a specific separation task should be determined by an experiment in which the desired concentrations are reached. Only this way, or by applying a process model, it can be ensured that interlocking physical and chemical effects are considered.
Mass balances diluate vs. concentrate. Example results for the four different ways to calculate the electrical energy demand are plotted in Figure 6 (a).
The results for the average electrical energy demand per cubic meter diluate without consideration of the water transfer, E dil,noW T , and with consideration of the water transfer, E dil,withW T , are plotted in Figure 6 (b).
When considering the different curves in Figure 6 (a), it is striking that in all cases of this example the energy demand was higher when calculated for the diluate stream, compared to the calculation via the concentrate stream. However, a sighting of data collected over a longer period of time revealed experiments in which also the opposite was the case. The reason for these deviations are most probably experimental and/or measurement errors. An example for a possible systematic error in the data, which leads to a tendency of the energy demand being higher when calculated via the diluate mass balance, may for example be the evaporation of water from the water samples during or after the collection of water samples. This would lead to a concentration of the concentrate (and hence a performance improvement), as well as a concentration of the diluate (and hence a performance reduction and an increased energy demand).
In the following, only the average results between diluate and concentrate mass balance are plotted, similar to Figure 6 (b). This averaging based on two mass balances within the same experiment reduces the impact of experimental and measurement errors and allows a more reliable estimation of the energy demand required per cubic meter diluate product stream.

Electrical energy demand per m 3 concentrate product stream
The results are more realistic when determining the energy demand based on the product mass balance including the water transfer. Due to this, the energy demand per cubic meter concentrate product stream is only plotted including the water transfer in the following sections. To ensure a comparability between the experimental results, the energy demand was recalculated for a concentration up to 260 g/L NaCl concentration in the concentrate, according to the following equation: By using this equation, the additional/ reduced current required to reach exactly the concentration of 260 g/L NaCl in the concentrate is considered.
When normalizing to 260 g/L all concentrations in this case need to be given in g/L and the other units chosen accordingly. The change in resistances and back diffusion due to changes in the concentration gradients are not considered. To identify the actual energy demand for this specific task, an experiment in which these actual concentrations are reached would be required, due to the many interlocking physical and chemical effects.

Current efficiency
Similar to the energy demand, also the current efficiency (CE) can be calculated based on different mass balances. In case of FCDI processes, the current efficiency brings the transferred salt into relation with the required electric current: Or based on the product mass balance: In Equations 9 and 10, M N aCl represents the molar mass of the transferred salt, which in this case is NaCl. The first version of the current efficiency, based on the full mass balance, represents the physically correct current efficiency. The full salt mass balance gives information on how much salt was actually transferred. The CE based on the full mass balance hence represents the actual number of ion pairs transferred per electron and allows conclusions regarding the membrane selectivity. However, as discussed before, this does not represent the apparent current efficiency observed in a real application. In reality, the apparent current efficiency will also be reduced by water transfer, which is why the term "effective current efficiency", CE ef f , is introduced here for the cases when the CE has been calculated based on the product mass balance.

Current-voltage characteristics
For an initial comparison of the performance of different energy demand reduction concepts, the current-voltage characteristics were investigated, as described above. The results for NaCl concentrations of 60 and 120 g/L in feed and flow electrodes are plotted in Figure 7 in form of IV-curves.  Figure 7: IV-curves of FCDI systems at (a) 60 g/L NaCl, comparing the standard system to a system operated at a low concentrate flux and a system with three cell pairs, and at (b) 120 g/L NaCl, comparing the standard system to systems with two and three cell pairs. The active membrane area is 100 cm 2 .
Similar to IV-curves of electrodialysis systems, the FCDI IV-curves show an ohmic region at low voltages, in which the electrical current increases lin-early with increasing potential. At higher potentials, in case of the curves at hand usually around 1-2 V, the curves transcend to a limiting current region. The ohmic resistances and limiting currents differ significantly between the different systems. The resulting ohmic resistance determined from the IV-curves are listed in Table 2.

Experimental parameters
Analysis results In case of the standard FCDI system with one cell pair at 120 g/L, the overall behaviour is similar to the system at 60 g/L, while the increased con-   Table 1. Figure 8  In case of all experimental results plotted in Figure 8, the NaCl concentration achieved in the concentrate stream was higher than 200 g/L NaCl.

Desalination and concentration performance
The maximum NaCl concentration achieved in this study was 291.5 g/L.
The water recovery in the experiments presented here ranges from 70 % to 92 % at NaCl feed concentrations of 60 and 120 g/L NaCl. This is a very promising result regarding the use of FCDI as a potential brine concentration technology.
In case of both 60 and 120 g/L NaCl as feed concentration, the NaCl concentration was reduced by about 30 g/L when applying the same feed flow rate in the FCDI system. This leads to a diluate product stream concen-tration of around 30 g/L NaCl in case of the 60 g/L feed stream and about 90 g/L NaCl in the diluate product stream in case of the 120 g/L feed stream at 1.2 V. The NaCl transfer rates in g NaCl/min calculated for these experiments are in a similar range for both feed concentrations. It is likely that in this concentration range, meaning high salinities of 60-120 g/L NaCl, the additional increase in conductivity has only a small effect. This corresponds well with results presented by Yang et al. [40]. The small effect it has is probably countered by a decreasing membrane selectivity at increasing salt concentrations, which results in a decreasing current efficiency and higher back diffusion rates.
In case of both feed concentrations, the desalination and concentration performance decreases when adding additional cell pairs, due to the increased resistance with increasing number of cell pairs. This trend can be avoided to a certain extent by increasing the applied voltage, as shown in Figure 8

Energy demand
The energy demand corresponding to the results regarding the desalination performance at 60 g/L NaCl in the feedstream, shown in Figure 8 (a), is plotted in Figure 9.
The energy demand is normalized per cubic metre diluate product stream, determined according to the equations described above. The energy demand with and without consideration of the water transfer decreases significantly with increasing number of cell pairs. When comparing these results to the results presented in our previous article [15], indicated by the blue area in Figure 9 (a), it becomes clear that the use of multiple cell pairs can reduce the energy demand of an FCDI process more efficiently than the partial energy recovery achievable using a second regeneration module. With energy recovery, the energy demand for the desalination of a 60 g/L NaCl solution was reduced to around 25 kWh/m 3 diluate, while a value of only 15.5 kWh/m 3 diluate was reached using an FCDI module with three cell pairs.
The same trend is visible in Figure 9 (b), which shows the energy demand per cubic metre concentrate product stream. In this figure, all values consider the water transfer, which leads to a dilution of the concentrate. The use of three cell pairs leads to a reduction of the energy demand by more than 70 %, compared to using only a single cell pair.  the desalination performance improves while at the same time the energy demand increases. However, the overall energy demand is still low compared to a single cell pair system. The right choice of the applied voltage will be an optimization task when designing an FCDI system for an actual application.
While increasing the voltage leads to higher current densities, and hence reduced capital cost due to a lower required membrane area, it also leads to a higher electrical energy demand and hence increased operation costs.
Surprising is that at a constant voltage of 1.4 V, the energy demand for the concentration up to 260 g/L NaCl is nearly the same for a for a 120 g/L feed solution as for a 60 g/L feed solution at 1.2 V. This is a distinct difference to thermally driven processes, in which the feed concentration has a major effect on the energy demand when aiming at the same concentrate concentration. Hence, an advantage of FCDI processes compared to thermally driven processes may be the ability to bridge this wide concentration range with a moderate energy demand.

Cell pair resistances and pumping energy
The use of multiple cell pairs in an FCDI module has several potential advantages, as described above. The theoretical calculations presented above in Figure 2 illustrate the expected advantages. The results presented in Figure 11 are based on the same calculations and illustrate the fractions of the energy demand caused by the different resistances in comparison to the required pumping energy in case of one and three cell pairs. The energy demand calculated for pumping the flow electrodes is based on pressure drop measurements in flow electrodes continuously pumped by a peristaltic pump through a circuit with a dampening vessel to even out pressure fluctuations, and was determined analogue to measurements presented in our previous publication [15]. In Figure 11, the theoretical estimations are compared to actual experimental results. W i t h w a t e r t r a n s f e r E x p e r i m e n t a l r e s u l t R e s i s t a n c e c o n c e n t r a t e c h a n n e l R e s i s t a n c e d i l u a t e c h a n n e l R e s i s t a n c e m e m b r a n e s R e s i s t a n c e c o n c e n t r a t e c h a n n e l R e s i s t a n c e d i l u a t e c h a n n e l R e s i s t a n c e m e m b r a n e s As can be seen from the calculation results plotted in Figure 11 [39]) and Galama et al. [41]) also indicate a more than ten times increased membrane resistance at high concentration gradients across the membrane, which may be an explanation for increased cell resistances. The absolute concentration difference across the membrane is in this case at least 170 g/L NaCl. The question is, whether this increased resistance only occurs when one of the bordering concentrations is very low, as described in the study by Geise et al. [39], or when there is generally a large concentration gradient. The combination of these phenomena may explain why the deviation between experimental and the-oretical results is especially significant in case of the three cell pair system.

C e l l p a i r s 1 5 w t . % A C m L / m i n d i l u a t e
The higher number of membranes in the system may increase the overall effect of the non-idealities which are not considered in this calculation.

Effective current efficiency and diluate concentration
In some cases, extreme concentration gradients occur over a single ionexchange membrane in the above described experiments. In literature, increased membrane resistances were observed for high concentration gradients [39,41]. Apart from the scientific interest, the apparent independency of the cur-rent efficiency on the concentrate product stream concentration is promising for an application of FCDI processes for brine concentration and brine treatment in general.

Energy demand of FCDI compared to mechanical vapor compression
The improvements achieved by the use of more than one cell pair are significant and illustrate the extant potential of the FCDI technology. Figure 13 compares the energy demand achieved in FCDI experiments with one, two and three cell pairs to literature values for mechanical vapor compression [42]) and results achieved by FCDI systems with energy recovery presented in our previous article [15].   [42]) and FCDI with energy recovery [15].
The energy demand for the production of one cubic meter of desalinated water using the different FCDI systems compared to mechanical vapor compression (MVC) is plotted in Figure 13 (a). Figure 13 (b) shows results for the energy demand when normalized per one cubic meter of produced concentrate at 260 g/L NaCl. All values consider the increased energy demand due to water transfer from the diluate to the concentrate channel.
The energy demand scales with increasing feed concentration in case of desalination, which is consistent for all desalination technologies driven by an electrical field. However, in case of FCDI, the energy demand for concentration is fairly independent from the feed concentrations of 60 g/L or 120 g/L NaCl.
To achieve a reduced energy demand it is vital to reduce the cell voltage.
This can be achieved by the use of two or more cell pairs, which leads to a division of the voltage drop over the flow electrode by the number of cell pairs.
The electrical current stays more or less constant, while the product volume stream is doubled or tripled, leading to a significant reduction of the energy demand per product volume. Compared to FCDI with energy recovery, the use of multiple cell pairs is a more efficient approach, as it enables (1) a better desalination and concentration performance, (2) a higher current density, is An open question is, whether FCDI can become competitive with state-ofthe-art technologies, such as MVC, regarding the investment and operating costs.

FCDI compared to electrodialysis
Electrodialysis (ED) is the most obvious technology, which the continuous FCDI processes presented in this work would be compared to. However, based on the data available in literature, a direct comparison is challenging.
Hence, experiments were performed with the aim of collecting directly comparable data for FCDI and ED experiments. The results are described in the following. Figure 14 shows experimental results and theoretical calculations directly comparing an FCDI and an ED system. Figure 14 (a) shows the resulting IVcurves of the elctrodialysis and the FCDI system. The results of theoretical calculations presented in Figure 14 (b) are based on the same assumptions as described above for Figure 1.
The IV-curves plotted in Figure 14 (a) illustrate the main difference between FCDI and ED well. The slopes in the ohmic region are very similar and the systems reach a very similar limiting current density. The difference in the limiting current density likely results from a small deviation in the feed water flow rate setting in the ED system, which is designed for much higher flow rates. However, while the FCDI system produces a linear IVcurve which goes through the origin in the Ohmic region at lower voltages, there is basically no electrical current flowing through the electrodialysis system at low voltages. At around 2 V, the electrical current starts to increase, passing over into a linear increase in the ohmic region. The reason for this is the overpotential required by the electrodes of the electrodialysis system.
In contrast to the FCDI system, which relies on the capacitive adsorption of ions on the flow-electrode surface, the electrodes of the electrodialysis system require faradaic reactions to achieve the transition from ionic current to electrical current. Table 3 shows the results of the desalination experiments directly comparing FCDI and ED. The results show that the FCDI and ED system were able to achieve nearly identical desalination and concentration results and current efficiencies when operated at the same electrical current density. The main difference is, as seen before in the IV-curves, the significantly higher voltage required to run the faradaic reactions at the electrodes. For this specific experiment, the ED system required a voltage increase of nearly 2.5 V in comparison to the FCDI system. This leads to a significant increase in the energy demand.
To reduce the impact of the electrode voltage drop, the electrodialysis stacks used for industrial applications usually consist of dozens or even hundreds of cell pairs. This way, the voltage drop over the electrode does not have such a significant impact on the overall energy demand anymore. However, such large stacks suffer from different restrictions, such as ionic shortcut currents through feed and drain channels [43]. Due to this, it may be advantageous in some cases to be able to operate a desalination/concentration cell efficiently and economically even when using a low number of cell pairs, using a well-designed cell. The plots in Figure 14 (b), which show the results of theoretical calculations of the electrical energy demand for FCDI and ED systems depending on the number of cell pairs, illustrate this. For both systems, a significant reduction of the energy demand is achievable by increasing the number of cell pairs to more than one. In case of FCDI, the voltage drop over the electrodes is much smaller and does not require a starting potential, which leads to the significantly lower energy demand and the lower decrease with increasing number of cell pairs. As per the theoretical calculations, the increase to more than three cell pairs improves the energy demand only to a small extent. In case of electrodialysis, a much higher number of cell pairs is theoretically required to reach the same energy demand as FCDI. At a number of five cell pairs, the energy demand of the electrodialysis system is similar to the FCDI system at one cell pair, not accounting for the pumping energy.
Apart from this, electrodialysis stacks require electrochemical reactions at the electrodes to convert the electrical current in the outside circuits into the ionic current within the cell. Next to gas evolution and the associated safety risks, the chemical reactions occuring at the electrodes can lead to unwanted side reactions, which may be undesired in case of specific applications, e.g.
involving susceptible chemicals or mixtures. In this regard, FCDI processes may be advantageous for certain applications.

Conclusions
In this article, it is shown that continuously operated FCDI systems can be applied for the concentration of salt brines. Concentrations of up to 291.5 g/L NaCl were reached in the concentrate product stream at water recoveries ranging from 70 % to 92 % at NaCl feed concentrations of 60 and 120 g/L NaCl. Hence, FCDI is a promising technology for the treatment and recovery of salts from high salinity water streams (brines).
So far, the energy demand of FCDI systems tested for brine treatment was comparably high. However, the use of multiple cell pairs in FCDI systems, as presented in this article, is a very promising approach to reduce the overall energy demand of FCDI systems. Advantages of this approach are: (1) Reduced influence of the electrical resistance of the flow electrodes and hence a significantly decreased electrical energy demand, (2) reduced flow-electrode pumping energy per volume product stream and per mass of transferred salt, (3) reduced investment costs compared to stacking of single-cell pair systems.
The economic feasibility of FCDI systems still needs to be proven. However, with further application-oriented research, FCDI systems may soon become viable for many industrial and agricultural applications aiming at closing material cycles via salt recovery from waste water streams.