Pilot-scale membrane capacitive deionisation for effective bromide removal and high water recovery in seawater desalination

Desalination 479 (2020) 114309

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Pilot-scale membrane capacitive deionisation for effective bromide removal and high water recovery in seawater desalination ⁎

Pema Dorjia, David Inhyuk Kima,b, Seungkwan Hongb, Sherub Phuntshoa, , Ho Kyong Shona, a b

T

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School of Civil and Environmental Engineering, University of Technology, Sydney (UTS), 15 Broadway, NSW 2007, Australia School of Civil, Environmental & Architectural Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Bromide Disinfection by-products Membrane capacitive deionisation Desalination Short-circuit Water recovery Energy

Although seawater desalination is becoming an important technology for freshwater production, the presence of a high concentration of bromide in the seawater presents a major challenge. Bromide is one of the major inorganic precursors for the formation of disinfection by-products such as bromate, which is highly regulated due to its toxicity and carcinogenicity. Hence, a significant reduction of bromide ions is required prior to water disinfection. In Australia, all the desalination plants have to operate a two-stage reverse osmosis system to ensure effective bromide removal, which adds significant cost to the desalination system. In this study, a pilot-scale membrane capacitive deionisation (MCDI) was investigated as a potential alternative to the 2nd stage RO in seawater desalination. Moreover, strategies to enhance water recovery in MCDI was also carried out by using lower flow rates and shorter duration during the desorption stage. In order to reduce energy consumption in MCDI, a combined short-circuit and reverse polarity desorption is introduced. The results showed that MCDI can effectively remove bromide and dissolved salt at a much lower energy consumption compared with membrane process and that MCDI can be operated to achieve high water recovery without increasing the total energy consumption.

1. Introduction Safe drinking water is an essential requirement for the safety and

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well-being of human society. However, it is becoming a scarce resource, and more than four billion people experience some form of water scarcity [1]. Globally, freshwater accounts for only about 2.5% of

Corresponding authors. E-mail addresses: [email protected] (S. Phuntsho), [email protected] (H.K. Shon).

https://doi.org/10.1016/j.desal.2020.114309 Received 7 September 2019; Received in revised form 30 December 2019; Accepted 1 January 2020 0011-9164/ © 2020 Published by Elsevier B.V.

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membranes in CDI, commonly known as MCDI has significantly addressed this problem [33,36]. The earliest demonstration of the use of MCDI was on the treatment of wastewater from a thermal power plant in 2006 [38]. Since then, MCDI has generated lots of interest from research groups around the world. Several comparative studies between CDI and MCDI reported MCDI to have better performance than CDI, both in terms of desalination efficiency and energy consumption [38–45]. From an energy perspective, MCDI was also found to be more favourable for energy recovery where 47%–83% of the energy spent in charging the MCDI cell can be recovered, which has significant potential to reduce the overall energy consumption in MCDI [46,47]. While CDI is vulnerable to organic fouling [48,49], MCDI was less prone to organic fouling compared to CDI [50,51]. In the early stage of CDI development, MCDI was considered to be an expensive alternative mainly due to the use of expensive stand-alone ion-exchange membranes, however, commercial entities have started commercial production of composite electrodes where a thin layer of ion-exchange polymer is coated directly on the surface of the carbon electrode, similar to the one used in this study. Such incorporation of ion-exchange polymers reduces circuit resistance, allows for easy assembly of the electrodes and most importantly, reduce significant cost compared with MCDI that uses separate layers of ion exchange membranes. CDI/MCDI operation is a two-stage process: adsorption followed by desorption, and a typical operation normally have the same flow rates and same duration for both adsorption and desorption, restricting the water recovery (WR) at around 50% [52,53], which is very small compared with membrane technology. One of the ways to increase water recovery is by optimizing the desorption stage such that adequate regeneration of the electrodes is achieved with a significant reduction in brine volume. Currently, the most common desorption method is either through polarity reversal or short circuit [53] or it can also be a combination of short-circuit followed by polarity reversal as introduced in this study to reduce energy input during the desorption stage. Polarity-reversal during desorption is found to regenerate the electrodes at a faster rate [54] but at the expense of significant energy input since it contributes about 50% of the total energy consumption in MCDI [55]. The short-circuit method, on the other hand, is a zero-energy desorption process, which is found to accelerate ion desorption from the electrodes, but it requires a much longer time for regeneration of the electrodes, thereby, making the process inefficient for practical application. Therefore, there is a need to investigate an effective desorption method to reduce energy consumption and at the same time to increase water recovery in MCDI operation. In this paper, the application of pilot MCDI for bromide removal and strategies to increase water recovery and reduce energy consumption was systematically evaluated. This study builds on our previous study [11] where we showed the feasibility of bromide removal in MCDI through series of fundamental investigations related to bromide removal for different water quality and MCDI operating parameters at a lab-scale MCDI. The focus of this pilot study is to develop strategies to increase water recovery and to reduce energy consumption for practical application. Firstly, in order to ensure the operational reliability of the pilot MCDI unit, the total dissolved salt (TDS) removal of the feed water with varying TDS and flow rates were conducted. Secondly, a feed water TDS of 150 mg/L with spiked bromide was used as a representative 1st pass SWRO permeate to determine the optimum flow rates and also to determine the effect of using lower desorption flow rates and shorter desorption time to increase the water recovery. A detailed analysis of TDS and bromide removal, energy consumption and salt desorption rates in each experiment were assessed. A combined short-circuit followed by polarity reversal method is introduced for the desorption stage to reduce the overall energy consumption per cycle.

global water resource, much of which is inaccessible for human use, and the rest 97.5% exist as seawater [2]. In order to meet the rising demand for freshwater, and to adapt to changing climate, many countries started to make a substantial investment in seawater desalination system. Currently, more than 15,906 desalination plants operate worldwide with a total freshwater production capacity of 95 million m3/d [3]. Seawater Reverse Osmosis (SWRO) technology accounts for 70% of the global desalinated water, which clearly shows that SWRO is a preferred desalination technology [3]. In Australia, since the “millennium drought’, more than AUD 10 billion has been invested in SWRO desalination system with a current capacity to produce 1.4 million m3/day of freshwater for municipal water supply [4–6]. However, the desalination system such as SWRO is the most expensive option for freshwater production compared with the treatment of other conventional water sources mainly due to high capital investment and high operational cost [6–8]. Besides, even the state-ofthe-art SWRO has limitations for effective removal of target ions such as bromide, as a result, a significant change in the SWRO configuration is required. For example, SWRO desalination plants in Australia have to be designed as a two-stage RO process where the desalinated water from the 1st stage SWRO has to be treated again in the 2nd stage brackish water RO (BWRO) for the production of high-quality drinking water. This requirement is mainly to comply with the design requirement to maintain bromide concentration of less than 100 μg/L in the desalinated water so that the risk of production of toxic disinfection byproducts during water disinfection can be reduced [9]. Such addition of the 2nd stage BWRO adds significant additional capital cost and operation cost, which further increases the overall cost of the technology [10–12]. Therefore, alternative options for effective bromide removal in seawater desalination have to be investigated. While water disinfection ensures that water is free from harmful pathogens, it is also a source of a significant quantity of toxic disinfection by-products (DBPs) when water-containing bromide is disinfected with strong oxidising agents [13,14]. Currently, water disinfection is achieved by chlorination, chloramination, ozonation and ultraviolet radiations [13,15]. More than 600 types of DBPs have been identified [15,16] of which bromide related DBPs such as bromate are highly regulated due to their high toxicity and carcinogenicity [16,17]. The Australian drinking water standard for bromate is 20 μg/L whereas WHO and other countries have a stricter standard of 10 μg/L [18]. Bromide salts occur naturally, but industrial use of brominated pesticides and fuel additives can also contribute bromide salts into the environment [19]. Bromide is one of the major inorganic precursors for the formation of DBPs during disinfection process [13,20–24]. Therefore, bromide concentration has to be significantly reduced prior to disinfection. Thus far, RO is found to have excellent bromide removal efficiency compared to other technologies although it is generally considered to be an expensive option due to high cost of the technology whereas electrochemical and adsorption processes have technical limitations for large-scale application [25]. The conventional treatment system such as enhanced coagulation and treatment using powdered activated carbon are also not effective for bromide removal [26–28]. Interestingly, a detailed review of halide removal technologies [25] identifies that optimisation in capacitive deionisation (CDI) could potentially rival membrane technology. Capacitive deionisation is an emerging desalination technology, which removes ions through electrosorption by the charged electrodes [29]. Compared to other common technologies such as membrane processes, CDI is a chemical-free and energy-efficient process for desalination of low-saline water sources. In recent years, its applications have expanded to include selective removal of ions, resource recovery and nutrient removal [30–35]. One of the major issues experienced by CDI is the occurrence of simultaneous adsorption of counterions and expulsion of co-ions from the electrode region during charging and discharging phases, which reduces the desalination performance [36,37]. As a result, the incorporation of cation and anion exchange 2

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2. Materials and methods

concentration in the feed water was adjusted by using NaBr (Sigma Aldrich, Israel). Firstly, to assess the stability of the MCDI module, feed water with different TDS (200, 300 and 400 mg/L) was treated for at least 5 consecutive cycles at different flow rates, and the performance of the system was evaluated based on TDS removal and monitoring the conductivity profiles. Secondly, to simulate the application of pilot MCDI as an alternative to 2nd pass BWRO in seawater desalination, recycled domestic wastewater was again used where, the total TDS was adjusted to about 150 mg/L, which is a typical TDS of first-pass SWRO permeate. Since a lot of water is required for the pilot unit that operated at flow rates of up to 5 L/min for multiple cycles, it would not be economical to use synthetic water prepared by dissolving NaCl, which is normally the case. It was also not possible to use actual SWRO permeate because the volume required for the study was very high, and there was no SWRO desalination plant in operation nearby. The recycled domestic wastewater may be slightly different in terms of water characteristics compared with SWRO permeate, however, its overall effect on bromide removal would be insignificant. A bromide concentration of about 0.3 mg/L was maintained in the feed water based on our earlier works [11].

2.1. Pilot-scale MCDI and operation sequence The pilot-scale MCDI system consisted of two MCDI modules (Siontech Co., Korea) connected in series with each module consisting of 50 pairs of 100 mm × 100 mm activated carbon electrode (activated carbon P-60, Kuraray Chemical Co., Japan) with cation and anion exchange polymers coated directly on the electrode surface. Each electrode pair was separated by a non-conductive nylon spacer (200 μm) to prevent electrode short-circuit, which also used as a flow channel within the MCDI modules. The feed water flowed from the periphery of the module and exited from the centre of the module. The cathodes and anodes were arranged alternately in the module, and all the graphite current collectors from cathodes and anodes were connected to two separate copper terminals from which the voltage was regulated. The BET surface area and the weight of the activated carbon as per the manufacturer were 1689.5 m2/g and 1.6 g per pair of electrodes, respectively. The pilot unit operated on a single-pass operation mode that is ideal for practical application where the feed water passes through the module only once. The feed water was pumped at various flow rates using a pressure pump (Walrus pump Co., Taiwan). As a basic pretreatment, the feed water passed through a 10-μm filter cartridge attached to the MCDI system. The voltage and current were monitored using the onboard GL 220 midi logger (Graphtec Corporation., Japan). The unit was programmed to operate in a certain sequence: the unit always started with a desorption stage followed by adsorption depending on the pre-determined settings. After adsorption (1.3 V), desorption phase starts immediately through combined short-circuit for the initial 30 s followed by polarity reversal (−1 V). For the safety of the pilot MCDI unit operation, during the polarity reversal, the negative voltage application is gradually applied from 0 to −1 V, which takes about 15 s before a stable desorption voltage (−1 V) is maintained during the active desorption phase. Also, during the desorption stage, irrespective of desorption time, the final 20 s of desorption was allowed for flushing the MCDI cell with the feed water without any applied voltage. Throughout the experiment, the conductivity and pH were continuously monitored through the onboard pH and conductivity meter every second. After each cycle, the average TDS and conductivity were recorded and 10 mL sample was collected for cumulative bromide and TDS measurement in the treated water. A single switch in the MCDI system controlled the entire process from operating the pump to the application of the voltage (adsorption, short-circuit and polarity reversal) for the pre-determined adsorption and desorption settings. The maximum time that can be set in the pilot system was 4 min for adsorption, 2 min for desorption and 30 s for short-circuit. In all the experiments, feed water was used during desorption to simulate a realworld MCDI application instead of using purified water, which is normally the case in most lab-scale studies. The required flow rates were manually adjusted for each set of experiment based on the onboard flow meter reading. All the data reported in this study are averages based on five consecutive cycles for each set of experiments. The pilot MCDI unit was earlier used for our study on the treatment of domestic wastewater where several hundred adsorption and desorption cycles were performed including long-term experiments where the unit was operated continuously for 15 days. Therefore, before starting this study, mild acidic and basic solutions were used to flush the system of any organic and other pollutants from an earlier operation. The schematic of the pilot MCDI unit is presented in Fig. 1.

2.3. Sample analysis and data treatment The bromide concentrations were analysed using ICP-MS 7900 (Agilent Technologies, Japan). All the tests were done for five consecutive cycles unless otherwise stated, and average values are presented. The salt adsorption capacity was calculated using Eq. (1):

SAC (mg/g) =

C0 − C ∗V M

(1)

where, C0 and C refer to initial and final TDS concentration (mg/L), M is the total mass of activated carbon in the electrode (g), and V is the volume of treated water (L). The TDS and bromide removal efficiency was calculated using Eq. (2):

Removal efficiency (%) =

C0 − C ∗ 100 C0

(2)

where C0 and C represent initial and final concentrations (mg/L) in the feed water and treated water, respectively. The energy consumption was calculated using Eq. (3): t

Energy (kWh/m3) =

t

Eads ∫0 Iads (t ) dt + Edes ∫0 Ides (t ) dt V

(3)

where, E, I and t represent voltage, current and time respectively. The subscripts ads and des refer to adsorption and desorption stages, respectively and V is the amount of treated water produced per cycle. The energy consumption is calculated only for the active adsorption and desorption phases where external voltages were applied for adsorption and desorption phases, and that the energy for pumping the water is not included since it is considered negligible compared to the energy required for ion separation in CDI/MCDI [56]. The water recovery was calculated as a ratio between the volume of treated water produced and the total volume of feed water used per cycle. The required water recovery was obtained by varying the flow rates and duration for adsorption and desorption stages. 3. Results and discussions 3.1. Operational stability of the pilot MCDI unit for different feed TDS and varying flow rates

2.2. Feed water preparation

Fig. 2 shows the performance of the MCDI system for TDS removal for different feed water TDS operated at several flow rates. As indicated, the saturation of the electrodes can be observed from the conductivity profiles when the feedwater TDS and flow rates were increased. However, the conductivity profiles for each set showed a consistent

Feed water was prepared using recycled domestic wastewater, and analytical grade NaCl (AnalaR, MERCK Pty. Limited, Australia) was added to obtain the required TDS for the feed water. The bromide 3

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Fig. 1. Process schematic of pilot-scale MCDI.

performance of the MCDI system over five consecutive cycles when operated at 50% water recovery (Fig. 2(a)). The average TDS of the treated water for 200 mg/L TDS increased from about 8.5 mg/L to 17.7 mg/L when the flow rates were varied from 2 to 4 L/min as shown in Fig. 2(b). Similar trends were observed for feed TDS of 300 and 400 mg/L where increasing the flow rates resulted in higher TDS in the treated water. For example, the treated water TDS for the feed water containing 300 and 400 mg/L increased from 14.1 to 37.1 and 26.2–74.5 mg/L respectively, when the flow rates were increased from 2 to 4 L/min. The corresponding TDS removal efficiencies were higher than 91% for the feed water containing 200 mg/L for the range of flow rates; however, there was a significant reduction in TDS removal efficiencies when the MCDI was operated with water containing higher TDS. Further, the effect of operating the unit at higher flow rates for feed water containing high TDS such as 300 and 400 mg/L was also evident where the quality of the treated water deteriorated as the flow rates increased as observed in other studies [57]. The overall salt adsorption capacity of the electrodes ranged between 4.5 and 12.1 mg/g of activated carbon with the highest adsorption capacity achieved at 400 mg/L feed TDS at the highest flow rate of 4 L/min (Fig. 3). The increasing trend of the salt adsorption

Fig. 3. The salt adsorption capacity of the carbon electrodes at different feed TDS and flow rates. [Operating parameters: adsorption time & desorption time: 2 min each; adsorption voltage: 1.3 V; Desorption: short-circuit + polarity reversal (−1 V); water recovery: 50%].

Fig. 2. (a) Effluent conductivity profiles for five consecutive adsorption stages (b) TDS of treated water and TDS removal efficiencies at different feed water TDS and flow rates. [Operating parameters: adsorption time & desorption time: 2 min each; adsorption voltage: 1.3 V; Desorption: short-circuit + polarity reversal (−1 V); Water recovery: 50%]. 4

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Fig. 4. (a) Conductivity profiles and (b) TDS of treated water and TDS & bromide removal efficiencies at different flow rates. The spiked bromide concentration in the feed was 0.3 mg/L [Operating parameters; adsorption time & desorption time: 2 min each; feed TDS: 150 mg/L, adsorption voltage: 1.3 V, desorption: shortcircuit + polarity reversal (−1 V)].

capacity with the increase in feed TDS and the increase of flow rates is mainly due to improved diffuse double-layer capacity and more salt transport at higher flow rates [57]. 3.2. Water quality, salt balance and energy consumption at different flow rates at 50% water recovery (feedwater TDS: 150 mg/L) As mentioned above, recycled domestic wastewater was used as feed water with its TDS adjusted to about 150 mg/L to simulate a 1st pass SWRO permeate. To be able to make a direct comparison with 2nd pass BWRO, the threshold TDS in the treated water by the pilot MCDI unit is set at 10 mg/L. Fig. 4(a) shows the conductivity profile of five consecutive cycles at different flow rates from 2 to 5 L/min at 50% water recovery, which is a typical MCDI operation. It can be observed from the conductivity profiles that there is good replicability in the performance of the MCDI unit. The water quality is severely dependent on the flow rates where better water quality is achieved at lower flow rates compared to higher flow rates. This is typical of MCDI operation in a single-pass mode, where the hydraulic residence time is a determining factor for water quality, unlike batch operation where flow rates have a negligible effect on desalination. Fig. 4(b) shows the average TDS for five consecutive cycles at a flow rate of 2–5 L/min. Except for flow rate of 5 L/min, the TDS of the treated water for all the other flow rates meet the threshold TDS of 10 mg/L whereas, at the flow rate of 5 L/min, the TDS of the treated water was in the range of 13–14 mg/L, which is beyond the threshold TDS of 10 mg/L. The corresponding average TDS removal efficiencies range between 90.9 and 96.2% with higher TDS removal efficiencies observed at lower flow rates compared to higher flow rates. It has to be noted that the desalination efficiency at lower flow rates was much higher, however, the volume of treated water per cycle is also less at lower operating flow rates. For practical application, higher flow rates are preferred as long as the treated water quality achieves the desired TDS threshold. In this case, the operation of the pilot MCDI pilot at the 4 L/min produced treated water which meets the threshold requirement of 10 mg/L. As a result, 4 L/min is used as a benchmark based on which other operational factors are evaluated as reported in the subsequent sections. The bromide removal efficiency for different flow rates was between 68.7 and 70.3% with final bromide concentration between 0.086 and 0.099 mg/L, which is lower than the design requirement of 0.1 mg/L in SWRO plants in Australia. Fig. 5 further quantifies the average salt adsorbed and desorbed from the MCDI modules at different flow rates to determine the electrode regeneration capacity. The salt desorption rates for different flow

Fig. 5. Average TDS balance for adsorption and desorption stages at different flow rates. [Operating parameters; adsorption time & desorption time: 2 min each; feed TDS: 150 mg/L, adsorption voltage: 1.3 V, desorption: short-circuit + polarity reversal (−1 V)].

rates was between 94 and 97% of the total adsorbed salt, which shows effective regeneration of electrodes when a typical MCDI operation with 50% water recovery (WR) is implemented by using the same adsorption and desorption time (2 min each), and also using the same flow rates for both adsorption and desorption stages. However, for MCDI to be competitive with matured technology such as RO, which normally have high water recovery, it is important to devise strategies to increase water recovery in MCDI operation. In the following sections, two important methods have been investigated in detail: using lower desorption flow rates compared to adsorption, and shorter duration for desorption phase compared to adsorption so that in both the cases less brine is generated per cycle. The energy consumption at different flow rates was between 0.15 and 0.21 kWh/m3 which is much lower than the BWRO energy consumption in Perth desalination plant. The energy consumption tends to be higher at lower flow rates compared to higher flow rates (Fig. 6(a)) mainly due to the low volume of treated water produced per cycle at lower flow rates [57]. The current profiles (Fig. 6(b)), especially during desorption shows two different profiles for short-circuit phase and active desorption phase, where −1 V was applied to further remove the ions from the electrode surface. It can be seen that short-circuiting at the initial phase of desorption can be an effective technique for rapid 5

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Fig. 6. (a) Energy consumption at different flow rates and (b) current profiles during the adsorption and desorption stages. [Operating parameters; adsorption time & desorption time: 2 min each; feed TDS: 150 mg/L, adsorption voltage: 1.3 V, desorption: short-circuit + polarity reversal (−1 V)].

lower desorption flow rates resulting in increased water recovery was also highlighted as a possible explanation in CDI [58]. The average TDS in the treated water was between 9.4 and 10.3 mg/ L for the four different desorption flow rates (Fig. 7(b)). It can be concluded that using lower desorption flow rates can be an effective method to increase water recovery without compromising on the quality of the treated water. By changing the desorption flow rates, water recovery of 80, 67, 57 and 50% can be obtained at the desorption flow rates of 1, 2, 3 and 4 L/min respectively, while maintaining the same adsorption flow rate of 4 L/min. The TDS removal efficiencies of greater than 93% were observed in all the cases, which further illustrates that the lower desorption flow rates in MCDI do not have a major effect on the electrode regeneration although desorption flow closer to zero is not practical that will give significantly low charge and thermodynamic efficiency [58]. The average bromide removal was about 69.33% with a final bromide concentration range of 0.084–0.106 mg/L. The salt balance for the adsorption and desorption are presented in Fig. 8. As illustrated, the salt desorption rate increased from about 84% at a desorption flow rate of 1 L/min (WR = 80%) to more than 94% when the desorption flow rates were 2, 3 and 4 L/min with a corresponding water recovery of 67, 57 and 50% respectively. Although there is a significant increase in water recovery at lower desorption flow rates, the electrode regeneration capacity at 84% can still produce product water which meets the TDS threshold of about 10 mg/L. The results clearly indicate the suitability of using much lower desorption flow rates compared to adsorption to significantly improve water recovery in MCDI. However, it is important to determine the optimum desorption flow rates based on the required water quality, which shows MCDI operation can be highly tuned for specific requirements. The energy consumption at different desorption flow rates showed some interesting results as observed in Fig. 9(a). Despite achieving significant improvement in water recovery using lower flow rates compared to adsorption flow rates, the difference in energy consumption across different desorption flow rates was rather insignificant. The energy consumption was between 0.15 and 0.16 kWh/m3 of treated water for different desorption flow rates, which is significantly lower than the energy consumption in Perth desalination plant for 2nd pass BWRO. It is observed that increasing water recovery in MCDI does not necessarily compromise on the quality of treated water as discussed above and also does not increase overall energy consumption. As stated above, the use of an ion-exchange membrane is contributing to faster desorption even at lower desorption flow rates because the ion exchange membranes prevent the re-adsorption of counter ions on the electrodes, which is not the case with typical CDI system. It has to be noted that the energy consumption comparison between pilot-MCDI and Perth desalination plant may be slightly biased because of the scale

discharge of ions without transferring any charge from an external source as indicated by high current flows. This is mainly due to lower resistance of the electrolyte, and also the absence of electrostatic attraction to hold the ions during short-circuit phase. The polarity reversal at the later stage of desorption further enhanced ion desorption to achieve high ion desorption rate since short-circuit alone can take much longer time, which may not be of practical significance. During the active desorption, a plateau-like current profile is observed at the initial stage because the reverse voltage application is gradually applied from 0 to (−1 V) in the initial 15 s after the start of the active desorption phase as a safety feature of the pilot MCDI operation. 3.3. Water quality, salt balance, water recovery and energy consumption at lower desorption flow rates (feedwater TDS: 150 mg/L) As discussed above, water recovery is an important parameter in water desalination, and current membrane technologies are able to achieve water recovery of higher than 75% for brackish water treatment. For MCDI to compete with membrane process, strategies to increase water recovery have to be developed. In a conventional MCDI operation, typical water recovery of about 50% is normally achieved, which is highlighted as one of the limitations of MCDI technology since a large quantity of water is used during the desorption stage [52]. One of the ways to increase water recovery in MCDI is to use lower desorption flow rates compared to adsorption flow rates. In this section, the desorption flow rates were adjusted to 1 to 4 L/min, whereas the flow rate used during adsorption stage was fixed at 4 L/min and with same adsorption and desorption time of 2 min each. This way, water recovery of 50%, 57%, 67% and 80% could be obtained at four different desorption flow rates as discussed later under Fig. 9(a). The production of less volume of highly concentrated brine during desorption stage also allows MCDI application in resource recovery of target ions. Fig. 7 shows the effect of using lower desorption flow rates on the quality of treated water. As observed, for all the four different desorption flow rates, there are no noticeable changes in the adsorption profile, which indicates that the electrode regeneration was not affected by the lower flow rates used in the desorption stage (Fig. 7(a)). Some studies observed that increasing the desorption flow rates reduced the regeneration time of the electrodes quite significantly [54], however, increasing the desorption flow rates can reduce water recovery, therefore, an optimum desorption flow rate has to be determined which meets the water quality requirement. In this study, however, the low desorption flow rate was effective for regeneration because of the presence of the ion-exchange membranes which prevented adsorption of counter-ions unlike in the studies conducted using conventional CDI [54]. The improved thermodynamic efficiency of CDI by 2 to 3 fold at 6

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Fig. 7. Effect of lower desorption flow rates on water quality (a) Effluent conductivity profile and (b) TDS and bromide removal efficiency. [Experimental conditions; adsorption flow rate: 4 L/min, adsorption time and desorption time: 2 min each; feed TDS: 150 mg/L; adsorption: 1.3 V, desorption: short-circuit + reverse polarity].

different desorption flow rates, the current intensity is similar, which indicates effective ion removal even at lower desorption flow rates. 3.4. Water quality, salt balance, water recovery and energy consumption with shorter desorption time (feedwater TDS: 150 mg/L) Another strategy to increase water recovery in MCDI is to use shorter desorption time because the presence of ion-exchange membrane allows for faster regeneration of the electrode capacity by preventing the migration of ions from one electrode to the other [38]. In this section, desorption times of 1, 1.5 and 2 min were used whereas adsorption time was fixed at 4 min (the maximum duration that can be set for adsorption in the pilot unit), and the flow rate of 4 L/min was used for both adsorption and desorption stage. Fig. 10(a) shows the conductivity profiles of five consecutive adsorption cycles for the three different desorption times. What is noticeable is that, at shorter desorption time of 1 min, the subsequent conductivity profiles seem to indicate incomplete regeneration of the electrodes as indicated by the gradual increase in the lowest minimum conductivity values of the effluent water in the subsequent cycle. This phenomenon deteriorated the overall treated water quality resulting in higher TDS in the treated water. However, at the desorption time of 2 min, which is 50% of adsorption time of 4 min, the MCDI unit showed a stable performance as illustrated by consistent conductivity profiles, which indicates improved regeneration of electrode capacity. The average TDS of treated water was 23.49, 17.1 and 13.4 mg/L

Fig. 8. Average TDS balance for adsorption and desorption stages for different desorption flow rates. [Experimental conditions; adsorption flow rate: 4 L/min, adsorption time and desorption time: 2 min each; feed TDS: 150 mg/L; adsorption: 1.3 V, desorption: short-circuit + reverse polarity].

of operation, however, the pilot-MCDI operation was designed such that the final TDS and bromide concentration of the treated water was similar to that of 2nd pass BWRO. The current profiles in Fig. 9(b) show similar profiles for adsorption and interestingly, even for desorption at

Fig. 9. (a) Energy consumption at different desorption flow rates and different water recovery (WR) (b) current profiles for adsorption and desorption stages. [Experimental conditions; adsorption flow rate: 4 L/min, adsorption time and desorption time: 2 min each; feed TDS: 150 mg/L; adsorption: 1.3 V, desorption: shortcircuit + reverse polarity]. 7

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Fig. 10. Effect of shorter desorption time (a) effluent conductivity profiles and (b) TDS and bromide removal efficiency. [Experimental conditions: adsorption time: 4 min, adsorption and desorption flow rates: 4 L/min; feed TDS: 150 mg/L; adsorption: 1.3 V, desorption: short-circuit + reverse polarity (−1 V)].

for the desorption time of 1, 1.5 and 2 min respectively within five consecutive cycles as shown in Fig. 10(b). However, as observed with the conductivity profiles, the treated water quality showed much higher variation between 13 and 29 mg/L over five cycles when the desorption time was 1 min, which is a clear indication of incomplete desorption of ions from the preceding cycle due to shorter desorption time. This led to a gradual accumulation of ions on the electrodes, which reduced the overall electrode capacity for the subsequent adsorption. The variation in the TDS was reduced between 11 and 14 mg/L when desorption time was increased to 2 min, thereby improving electrode regeneration capacity compared to lower desorption time of 1 min. The average TDS removal efficiencies for the three different desorption times were 84.4%, 88.6% and 91.1%. The overall bromide removal was 77.2% with final bromide concentration in the treated water with between 0.052 and 0.094 mg/L. The effect of using shorter desorption time and ion desorption rate is illustrated in Fig. 11. It is obvious that lower desorption rate of 71% was observed when desorption time is 1 min, however, when the desorption time was increased to 1.5 min and 2 min, more than 90% ion desorption rate was determined. This result shows that it is possible to increase water recovery by operating the MCDI unit with shorter desorption time compared to adsorption time. However, it is important to determine optimum desorption time in conjunction with other operating parameters to ensure that the final treated water meets the required water quality standards. The main advantages of using shorter

desorption time are that the unit productivity of freshwater can be increased and a small volume of brine water will be generated per cycle without significant compromise on product water quality. This result also shows that ion desorption can be quite rapid in MCDI. The energy consumption for the three different desorption time did not vary much and ranged between 0.11 and 0.14 kWh/m3 with slightly higher energy consumption at longer desorption time (Fig. 12(a)). This is expected because the energy used during polarity reversal can contribute to the total energy demand, and longer desorption time means more energy is spent during the desorption stage. Although there is not much difference in the energy consumption at different desorption time, the effect on water quality is noticeable, where longer desorption time produced better water quality in a consistent manner as reported from the conductivity profiles and TDS in the treated water. Therefore, unlike desorption flow rates, the duration of the desorption stage seems to affect the water quality quite significantly, although an optimum desorption time can be determined based on the final water quality requirements. The adsorption and desorption current profiles are illustrated in Fig. 12(b), which show same adsorption current profiles, and the increase in energy consumption at longer desorption time of 1.5 and 2 min are mainly from the additional energy invested during the reverse polarity desorption.

4. Conclusions and recommendations In this work, a pilot MCDI unit consisting of commercial composite activated carbon electrode was used as an alternative to the 2nd pass BWRO for further TDS and bromide removal in seawater desalination. More importantly, strategies for improving water recovery and energy consumption in MCDI were evaluated and discussed. The pilot MCDI unit showed consistent performance in bromide and TDS removal with the final bromide concentration of less than 0.1 mg/L and TDS of about 10 mg/L in the treated water, which is comparable to the performance of the 2nd pass BWRO in seawater desalination. The use of lower desorption flow rates compared to adsorption flow rates was effective for electrode regeneration leading to acceptable water quality and increased water recovery. The use of shorter desorption time showed some deterioration in the treated water quality due to incomplete electrode regeneration. However, desorption time of 2 min against adsorption time of 4 min (50% of adsorption time) still produced treated water with average TDS of 13.4 mg/L with overall TDS removal of higher than 91% and final bromide concentration of less than 0.1 mg/L in the treated water. The energy consumption in MCDI decreases with the increase in flow rates but also produced treated water quality, which is inferior compared to lower flow rates. From the results, it is possible to obtain water recovery of greater than 90% by

Fig. 11. Average TDS balance for adsorption and desorption stages for different desorption time. [Experimental conditions: adsorption time: 4 min, adsorption and desorption flow rates: 4 L/min; feed TDS: 150 mg/L; adsorption: 1.3 V, desorption: short-circuit + reverse polarity (−1 V)]. 8

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Fig. 12. (a) Energy consumption for different desorption time (b) current profiles for adsorption and desorption stages. [Experimental conditions: adsorption time: 4 min, adsorption and desorption flow rates: 4 L/min; feed TDS: 150 mg/L; adsorption: 1.3 V, desorption: short-circuit + reverse polarity (−1 V)].

optimizing the desorption process, mainly by combining lower desorption flow rates and shorter desorption time. Overall, this study demonstrated that MCDI could be more favourable for effective bromide and TDS removal compared to conventional 2nd pass BWRO in seawater desalination due to its lower energy consumption. While the pilot MCDI unit had some flexibility to tune the operational parameters, for practical application, it would be ideal to have additional features to automatically control the adsorption and desorption flow rates based on the conductivity threshold so that a consistent effluent can be produced through simple automation. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[15]

CRediT authorship contribution statement

[16]

Pema Dorji: Conceptualization. Writing - review & editing. Ho Kyong editing.

[10]

[11]

[12]

[13]

[14]

Writing - original draft, Methodology, David Kim: Investigation. Seungkwan Hong: editing. Sherub Phuntsho: Writing - review & Shon: Conceptualization, Writing - review &

[17]

[18]

Acknowledgements

[19]

This work was supported by Korea Environment Industry & Technology Institute (KEITI) through Industrial Facilities & Infrastructure Research Program, funded by Korea Ministry of Environment (MOE) (1485016424).

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