membranes

Article Comparison of Pressure-Retarded Osmosis Performance between Pilot-Scale Cellulose Triacetate Hollow-Fiber and Polyamide Spiral-Wound Membrane Modules

Yuriko Kakihana 1,2, Nora Jullok 3,4 , Masafumi Shibuya 1, Yuki Ikebe 1 and Mitsuru Higa 1,2,*

1 Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 1677-1 Yoshida, Yamaguchi 753-8511, Japan; [email protected] (Y.K.); [email protected] (M.S.); [email protected] (Y.I.) 2 Blue Energy Center for SGE Technology (BEST), 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan 3 Faculty of Chemical Engineering Technology, Universiti Malaysia Perlis, Kompleks Pusat Pengajian Jejawi 3, Jejawi 02600, Perlis, Malaysia; [email protected] 4 Centre of Excellence for Biomass Utilization, Universiti Malaysia Perlis, Pusat Pengajian Jejawi 3, Jejawi 02600, Perlis, Malaysia * Correspondence: [email protected]; Tel.: +81-836-85-9203

Abstract: Pressure-retarded osmosis (PRO) has recently received attention because of its ability to generate power via an osmotic pressure gradient between two solutions with different salinities: high- and low-salinity water sources. In this study, PRO performance, using the two pilot-scale PRO membrane modules with different configurations—five-inch cellulose triacetate hollow-fiber



 membrane module (CTA-HF) and eight-inch polyamide spiral-wound membrane modules (PA- SW)—was evaluated by changing the draw solution (DS) concentration, applied hydrostatic pressure Citation: Kakihana, Y.; Jullok, N.; difference, and the flow rates of DS and feed solution (FS), to obtain the optimum operating conditions Shibuya, M.; Ikebe, Y.; Higa, M. Comparison of Pressure-Retarded in PRO configuration. The maximum power density per unit membrane area of PA-SW at 0.6 M 2 Osmosis Performance between NaCl was 1.40 W/m and 2.03-fold higher than that of CTA-HF, due to the higher water permeability Pilot-Scale Cellulose Triacetate coefficient of PA-SW. In contrast, the maximum power density per unit volume of CTA-SW at 0.6 M 3 Hollow-Fiber and Polyamide NaCl was 4.67 kW/m and 6.87-fold higher than that of PA-SW. The value of CTA-HF increased Spiral-Wound Membrane Modules. to 13.61 kW/m3 at 1.2 M NaCl and was 12.0-fold higher than that of PA-SW because of the higher Membranes 2021, 11, 177. https:// packing density of CTA-HF. doi.org/10.3390/membranes11030177 Keywords: pressure-retarded osmosis; pilot scale; hollow fiber; spiral wound; power density Academic Editor: Oumarou Savadogo

Received: 1 February 2021 Accepted: 25 February 2021 1. Introduction Published: 28 February 2021 Energy demand is increasing globally, in tandem with economic development and

Publisher’s Note: MDPI stays neutral population growth. Because the majority of primary energy sources comes from fossil with regard to jurisdictional claims in fuels [1], the world is facing crucial challenges in meeting energy-source demands, owing published maps and institutional affil- to the decrease in the accessibility of cheap fossil fuels [2]. Moreover, damaging greenhouse iations. gas emissions ultimately contribute to climate change [3]. Many research efforts have focused on developing efficient alternative energy sources: Solar, wind, tidal, wave, and biomass have been extensively studied as alternative and sustainable energy sources. However, the varying availability of energy sources, complex logistics, and high installation costs still prevent their widespread implementation. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Furthermore, water shortages are among society’s most challenging problems, for This article is an open access article which seawater reverse-osmosis desalination (SWRO) is a promising solution. Although distributed under the terms and the concentrated brine coming from desalination plants sometimes causes environmental conditions of the Creative Commons problems [4], owing to its high salinity, it can be used as a valuable energy resource referred Attribution (CC BY) license (https:// to as salinity gradient energy (SGE) [5–8]. SGE is a renewable energy source that is obtained creativecommons.org/licenses/by/ by mixing two salt solutions with different salinities [9,10]. There are two main membrane- 4.0/). based technologies that convert SGE into electricity: reverse electrodialysis (RED) [9,11,12]

Membranes 2021, 11, 177. https://doi.org/10.3390/membranes11030177 https://www.mdpi.com/journal/membranes Membranes 2021, 11, 177 2 of 21

and pressure-retarded osmosis (PRO) [10,13]. RED generates energy by converting counter- ion permselective transport with ion-exchange membranes to electric current by redox reactions at two electrodes; there have been reports on large-scale RED systems [14–17]. In contrast, in a PRO system, a semi-permeable membrane (SPM) separates a low-salinity solution, the feed solution (FS), from a pressurized high-salinity solution, called the draw solution (DS). The advantages of using SWRO brine as the DS of PRO or RED are as follows: The high salinity of the brine provides high power output for RED and PRO systems and the intake, and the pretreatment of DS supplied to the two systems is associated with a lower cost, as well as increased energy savings. Both the technologies have associated advantages, as well as disadvantages: The gross power density of PRO is higher than that of RED; however, especially in small-scale plants, the low energy conversion efficiency of the high-pressure pumps, pressure exchanger, turbine, and generator of PRO results in a lower net power than in the RED. The effect of the bivalent ions in the seawater and the SWRO brine increases the power output of PRO, while decreasing the output of RED. Currently, the cost of membranes for RED is higher than that for PRO [18]. There was an attempt to compare both PRO and RED, using experimental data obtained from the literature; however, it was unsuccessful, owing to incomplete sets of equal performance data. In 2007, Post et al. [5] developed a model in which each equation was compared to its equivalent for PRO and RED. Based on the developed model, both with respect to power density and energy recovery, it was concluded that each technique has its own field of application; PRO is more suitable for power generation using concentrated saline brines, whereas RED is more effective for power generation using seawater and river water. Details of the comparison between PRO and RED based on the developed model can be found in the literature [5]. In PRO, the osmotic water flow occurs from the FS to the DS through the membrane and propels a turbine connected to a generator to generate electricity. Figure1 shows the water flux and water molecule movement in a dialysis system consisting of SPM, FS, and DS, which explains the principle of forward osmosis (FO), PRO, and reverse osmosis (RO). Here, SPM is assumed to have ideal water permeability. First, there is no hydrostatic pressure difference between the FS and DS sides. In this case, the movement of water molecules from FS to DS (WM1), shown by the dotted line in Figure1, is higher than that from DS to FS (WM2), shown by a broken line, because the thermal mobility of water molecules near the membrane surface at the FS side is higher than that at the DS side because of the presence of non-permeable ions in DS. Therefore, the net water flux defined by subtracting WM1 from WM2 shows a negative value, as shown by the solid line in Figure1. This phenomenon is called forward osmosis. When the hydrostatic pressure difference (∆P) is applied to the DS side, WM2 increases with increasing ∆P, while WM1 is independent of the pressure. Hence, the net water flux increases with increasing ∆P. At the point where WM1 is equal to WM2, the net water flux becomes zero. The ∆P at this point is called the osmotic pressure difference (∆π) between the DS and FS. In the case where ∆P is larger than ∆π, the net water flux becomes a positive value, which is called the reverse osmosis mode. PRO is operated at applied hydrostatic pressures 0 < ∆P < ∆π so that the volume of pressurized DS increases by the net water flux to rotate a turbine to generate energy [19]. Membranes 2021, 11, x FOR PEER REVIEW 3 of 21 , , 177 3 of 21

Figure 1. Water fluxflux andand waterwater moleculemolecule movement movement in in a a dialysis dialysis system system consisting consisting of of semi-permeable semi-perme- ablemembrane membrane (SPM), (SPM), feed solutionfeed solution (FS), and(FS), draw and draw solution solution (DS), to(DS), explain to explain principle principle of forward of forward osmosis osmosis (FO), pressure-retarded osmosis PRO, and reverse osmosis (RO). (FO), pressure-retarded osmosis PRO, and reverse osmosis (RO).

Realizing thethe potentialpotential of of PRO, PRO, intensive intensive studies studies have have been been conducted, conducted, using using laboratory- labora- tory-scalescale PRO PRO analysis, analysis, utilizing utilizing coupon-size coupon-s membranesize membranes [20–26 [20–26]] and smalland small hollow-fiber hollow-fiber (HF) (HF)modules modules [27–33 [27–33].]. Several Several bench-scale bench-scale investigations investigations have demonstrated have demonstrated promising promising results resultsand, to and, some to extent, some extent, the membrane the membrane power densitypower density reaches reaches 60 W/m 602 W/mat a high2 at a pressure high pres- of sure48 bar of with48 bar a 3with M NaCla 3 M DSNaCl using DS ausing thin-film a thin-film composite composite membrane membrane [34]. However,[34]. However, with withregard regard to upscaling, to upscaling, the performance the performance of the of scaled-up the scaled-up prototype prototyp maye differmay differ significantly signifi- cantlyfrom the from laboratory the laboratory data. data. This This is because is because the the real real plant plant scale scale of of the the PRO PRO system,system, for instance,instance, depends depends not not only only on on the the membrane membrane parameters parameters but but also also on the on themembrane membrane ori- entation/membraneorientation/membrane configuration, configuration, and and the the oper operationalational conditions conditions such such as as the the flow flow rates of the DS and FS and the operating pressure [[10,35].10,35]. The need for th thee evaluation of power generation utilizing PRO at the near industrial scale is therefore vital to determine its real potential and to narrow down the gap between theoretical and practical applications. To understand the effect of the abovementionedabovementioned factors on PROPRO performance,performance, evaluationsevaluations using spiral-wound spiral-wound (SW) (SW) modules modules [36,37] [36,37 and] and HF HF modules modules were were investigated. investigated. Saito Saitoet al. [4]et al.performed [4] performed PRO PROtests testsof a of10-inch a 10-inch HF HFmodule module using using high-concentration high-concentration brine brine from from a seawatera seawater RO RO plant plant and and sewage-treated sewage-treated water water from from a regional a regional sewage-treated sewage-treated center center as DS as 2 andDS andFS, FS,respectively, respectively, and and obtained obtained 7.7 7.7W/m W/m2 of a ofmaximum a maximum gross gross power power density density at 2.5 at 2 MPa2.5 MPa of hydraulic of hydraulic pressure pressure difference. difference. Sakai Sakai et al. et [38] al. [obtained38] obtained 13.5 13.5W/m W/m2 of powerof power den- sity,density, using using a 10-inch a 10-inch HF PRO HF PROmodule. module. Experime Experimentsnts and simulation and simulation studies studies of PRO ofperfor- PRO mance,performance, using usingpilot-scale pilot-scale PRO HF PRO modules HF modules under undervarious various operational operational conditions, conditions, have alsohave been also performed been performed [35,39–46]. [35,39 In–46 previous]. In previous studies studies [35,43], [35 the,43 ],module the module performance performance with volumetric-basedwith volumetric-based power power outputs, outputs, as well as as well membrane-area-based as membrane-area-based power power density, density, was investigated,was investigated, to compare to compare the PRO the performanc PRO performancee between between the two the different two different module module config- urations.configurations. However, However, to the tobest the of best the of authors’ the authors’ knowledge, knowledge, there there have have been been no studies no studies on systematicon systematic PRO PRO performance performance evaluation evaluation of ofpilot-scale pilot-scale SW SW and and HF HF membrane membrane modules based on experimental data under the same operating conditions, to compare the PRO performance between SW and HF configurations. configurations. In In this this study, the performance of two types of pilot-scale membrane modules with HF and SW configurationsconfigurations isis investigatedinvestigated under a wide range of PRO operating conditions: DS concentrations, the flow rates of DS under a wide range of PRO operating conditions: DS concentrations, the flow rates of DS and FS, and applied hydraulic pressure differences; furthermore, the PRO performance and FS, and applied hydraulic pressure differences; furthermore, the PRO performance

Membranes 2021, 11, 177 4 of 21

between the two modules is compared to provide useful insights into the design of large- scale PRO plants.

2. Experimental 2.1. Materials and Chemicals Tap water was first treated with activated carbon to remove any traces of chlorine prior to use as the FS and for the preparation of DS. DS was prepared by using analytical-grade sodium chloride (NaCl, Nacalai Tesque Co., Ltd., Kyoto, Japan) dissolved in chlorine-free tap water.

2.2. Cellulose Triacetate Hollow-Fiber (CTA-HF) and Polyamide Spiral-Wound (PA-SW) Membrane Modules An open-ended HF type of cellulose triacetate (CTA) designed for PRO applications from Toyobo Co. Ltd (CTA-HF) and a prototype polyamide thin film composite membrane module (PA-SW) were used. The membrane modules have four open ports: an inlet and an outlet for DS and FS. The specifications of CTA-HF and PA-SW are listed in Table1.

Table 1. Specifications of the cellulose triacetate hollow-fiber (CTA-HF) and polyamide spiral-wound (PA-SW) membrane modules used in this study.

CTA-HF PA-SW Active layer of membrane Cellulose triacetate polyamide Number of ports 4 4 Module diameter (mm) 136 203.2 Module length (mm) 683 1016 Inner diameter of hollow fiber, din (µm) 90 - Outer diameter of hollow fiber, dout (µm) 170 - Number of hollow fibers 187,000 - Membrane area, (m2) 67 15.3 Module volume, (m3) 9.92 × 10−3 3.3 × 10−2 Packing density (m2/m3) 6754 464

2.3. RO Experiment A preliminary analysis in reverse osmosis (RO) mode was conducted to determine the water permeability (A) and salt permeability (B) of the membrane used in the two CTA-HF and PA-SW modules. Figure2 presents a schematic of the evaluation system of a pilot-scale PRO module. The system was the same as in a previous study [35], and was also used for the RO evaluation test in this study. The concentration of FS in the test was 500 ppm NaCl. To operate the system in RO mode, valve V2 was completely closed, so that the permeated water flowed out from the outlet at the FS side of the module. The activated carbon-treated tap water was fed into the DS side of the membrane module with an inlet flow rate of 4.0 L/min. The applied pressure difference between the DS and FS was carefully controlled, using valve V3. The water permeation flux was measured, using a flow meter at the FS outlet. The A (m s−1 Pa−1) value was then determined from the slope of the graph plotted for water flux over the applied pressure according to Equation (1):

Jw = A (∆P − ∆π) (1)

−2 −1 where Jw is the water flux (L m h ), ∆P is the hydraulic pressure difference, and ∆π is the osmotic pressure difference. To determine the value of B, Equation (2) was applied [25]:

A(∆P − ∆π)(1 − R) B = (2) R Membranes 2021, 11, x FOR PEER REVIEW 5 of 21 Membranes 2021, 11, 177 5 of 21

wherewhere the the rejection rejection rate, rate, RR, is, is calculated, calculated, using using Equation Equation (3). (3).

𝐶 Cp 𝑅 =1− R = 1 − (3) (3) CF 𝐶

wherewhere CCp pisis the the concentration concentration at the at the permeate permeate side, side, and and CF isC theF is concentration the concentration at the at feed the solution.feed solution.

Figure 2. Schematic of the pilot-scaleFigure 2. Schematic PRO evaluation of the pilot-scale system used PRO in thisevaluation study. Csy representsstem used conductivityin this study. meter, C represents F is flow con- meter, V is valve, and P is theductivity pressure meter, gauge. F is flow meter, V is valve, and P is the pressure gauge.

2.4.2.4. PRO PRO Experiment Experiment ToTo evaluate evaluate the the performance performance in in PRO PRO mode, mode, a a PRO PRO test test was was performed, performed, using using the the sys- sys- temtem shown shown in in Figure Figure 2.2. Tap Tap water water was was fed fed to to the the system system as as FS, FS, using using a a low-pressure low-pressure pump, pump, andand NaCl NaCl solutions solutions of of various various concentrations concentrations were were fed fed as as DS. DS. The The inlet inlet concentrations concentrations of of FSFS and and DS DS were were measured, measured, using using a a conductivity conductivity meter; meter; the the flow flow rates rates of of the the inlet inlet and and outlet outlet atat the the DS DS and and FS FS sides were measured,measured, usingusing flowflow meters; meters; and and the the pressure pressure at at each each position posi- tion(PFS,in (PFS,, P inDS,in, PDS,inand andP DS,outPDS,out)) waswas measured,measured, usingusing aa pressurepressure gauge. The The inlet inlet flow flow rate rate of of thethe FS FS was was set set between between 2.0 2.0 and and 6.0 6.0 L/min, L/min, while while the the DS DS inlet inlet flow flow rate rate was was between between 2.0 2.0 andand 10.0 10.0 L/min. L/min. The The maximum maximum flow flow rates rates of of FS FS and and DS DS at at the the tests tests were were 6.0 6.0 and and 10 10 L/min, L/min, respectively,respectively, owing owing to to the the pump pump limitation limitation created created by by the the evaluation evaluation system. system. DuringDuring PRO PRO evaluation, evaluation, DS DS was was diluted diluted by by the the permeated permeated water water from from the the FS, FS, and and the the saltsalt concentration concentration in in the FS waswas increasedincreased byby the the reverse reverse salt salt flux flux from from the the DS DS (DS (DS leakage). leak- age).A regeneration A regeneration system system with with RO RO was was added added after after the the PRO PRO unitunit to obtain steady-state steady-state conditionsconditions during during the the PRO PRO test. test. To To regenerate regenerate both both the the DS DS and and FS, FS, the the needle needle valves valves V4 V4 andand V5 V5 were were used used to to control control the the concentration concentration of of the the brine brine and and permeate permeate at at their their respective respective initialinitial FS FS and and DS DS concentrations. concentrations. Thus, Thus, a a steady-state steady-state long-standing long-standing module module operation operation waswas achieved, achieved, using using a a hybrid hybrid PRO PRO and and RO RO regeneration regeneration unit. unit. Each Each data data point point was was logged logged andand recorded recorded on on a a computer computer 5 5 min min after after the the parameter parameter was was adjusted adjusted at at 10 10 s s intervals. intervals. The The PROPRO performances performances of of the the CTA-HF CTA-HF and and PA-S PA-SWW membrane membrane modules modules were were evaluated evaluated under under variousvarious operating operating parameters. parameters. Th Thee permeate permeate water water flow flow rate rate (Qw) ( Qthroughw) through the module the module was determined,was determined, using using the inlet the inletand outlet and outlet flow flowrates rates of FS, of 𝑄 FS,,Q FS,inand 𝑄and,Q,FS,out respectively,, respectively, us- ingusing Equation Equation (4): (4): Qw = QFS,in − QFS,out (4) 𝑄 = 𝑄, −𝑄, (4)

The permeate flux (Jw) through the membrane module was determined, using Equa- tion (5), and is the division of the permeate water flow rate by the membrane area (Sm).

Membranes 2021, 11, 177 6 of 21

The permeate flux (Jw) through the membrane module was determined, using Equation (5), and is the division of the permeate water flow rate by the membrane area (Sm).

Qw Jw = (5) Sm The permeation rate (η) of the FS side was determined by using Equation (6):

Q η ≡ w × 100% (6) QFS,in

When η = 100, all solutions on the FS side permeated to the DS side. The dilution factor (ϕ) on the DS side was defined, using the inlet flow rate of the DS (QDS,in) and Qw, as in Equation (7): Q ϕ ≡ 1 + w (7) QDS,in

When QDS,in » Qw, ϕ is close to unity. This implies that the DS outlet concentration was nearly equal to the DS inlet concentration, indicating that the DS dilution effect was negligible. To estimate the reverse salt flux (Js), Equation (8) was employed:

(QFS,out × CFS,out − QFS,in × CFS,in) Js = (8) Sm Many PRO evaluations in the literature [10,12,13,19–34] are based on the power density per membrane area to characterize the membrane performance. The membrane area-based power density (PDarea) is defined as the effective membrane area-based power density and is calculated by using Equation (9):

area PD = Jw∆P (9)

From an industrial point of view, module volume-based PRO performance is vital because the volume-based size and number of required modules are necessary for designing a full-scale PRO plant [35,43,45]. The module volume-based gross power density (PDvol) used in this study is expressed as follows:

vol vol PD = Jw ∆P (10)

vol Jw is module volume-based water flux expressed as

vol Qw Jw = (11) Vmod

vol area where Vmod is the module volume. From (9) and (10), PD is expressed, using PD as follows: PDvol = βPDarea (12) where β is the packing density of the membrane module and is defined by using the following equation: β = Sm/Vmod (13) The power extracted from the PRO, using SGE, was determined by calculating the conversion efficiency to electrical energy. Electrical energy can significantly contribute to the establishment of the optimum operating conditions. To obtain the optimum operat- ing conditions, the generated net power output (Wnet) must first be determined, using Equation (14) [38]: Wnet = Wgross − (CEDS + CDFS) (14) Membranes 2021, 11, 177 7 of 21

Wgross = PDS,outQwEG (15)
PDS,in − PDS,outEpx QDS,in PEDS = (16) Ep,DS

PFS,inQFS,in PEFS = (17) Ep,FS

where PEDS and PEFS are the pumping energy at the DS and FS sides; Ep,DS, Ep,FS, EG, and Epx are the energy efficiencies of the DS pump, FS pump, electric generator, and pressure exchanger (PX), respectively. In this study, the values of EG = 0.9, Ep,DS =0.88, Ep,FS = 0.88, and Epx = 0.92 were assumed based on the values in the literature [38,46].

3. Results and Discussion 3.1. RO Tests for the Two Modules Figure3a shows the water flux of PA-SW and CTA-HF plotted over the applied hydrostatic pressure (∆P). The water flux increased with an increase in ∆P. The water permeability (A value) was obtained from the slope of the solid line calculated by fitting the data to Equation (1) and gives 0.65 × 10−12 and 4.6 × 10−12 [m s−1 Pa−1] for CTA-HF and PA-SW, respectively. The water permeability of PA-SW was approximately seven times higher than that of the hollow-fiber membrane module (CTA-HF). In general, the water permeability of a PA membrane is higher than that of a CTA membrane because the PA membrane has a thinner active layer than that of the CTA [4,42].

(a)

(b)

Figure 3. (a) Water flux and (b) salt rejection as a function of applied pressure in RO mode. QFS: 4.0 L/min. Tap water and 500 ppm NaCl were used as feed solution in the measurement of water flux and salt rejection, respectively.

1

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The salt rejection as a function of ∆P, using 500 ppm NaCl aqueous as the FS, is shown in Figure3b. It was found that the salt rejection of CTA-HF was higher than that of PA-SW. The salt permeability (B) of CTA-HF and PA-SW was calculated by using Equation (2) from the salt rejection. The value of PA-SW was 2.23 × 10−7 [m s−1], approximately 13 times higher than that of CTA-HF (0.17 × 10−7 [m s−1]). This indicates that the CTA-HF membrane will have a lower DS leakage than the PA-SW membrane.

3.2. PRO Tests for the Two Modules 3.2.1. Effect of FS Inlet Flow Rates on PRO Modules Performance Generally, the performance of a PRO membrane module is lower than the theoretical value calculated in terms of Equation (9), owing to two factors: (a) external concentra- tion polarization (ECP), which governs the water and solute permeation at the inter- face between the salt solution and the membrane surface, and (b) a decrease in between the DS and FS sides in the modules, owing to the water and salt permeation inside the module [10,12,13,25,36,47]. These phenomena decrease the PRO performance of the mod- ule and depend on the flow rates of the DS and FS. Hence, to obtain the optimum operating conditions for the two membrane modules, a systematic investigation of the effect of the inlet flow rate of DS and FS on PRO performance was conducted by measuring the water flux and the outlet concentration of DS and FS during the PRO tests, using 0.6 and 1.2 M NaCl as the DS, and applying a predetermined hydrostatic pressure difference.

Permeate Water Flux and Permeation Rate Versus FS Flow Rate To evaluate the effect of concentrative internal concentration polarization (ICP) inside the support layer, and the increase in salt concentration in FS due to DS leakage (FS up- concentration) on the PRO performance of the two modules, the evaluation of the effect of QFS on Jw, the permeation rate (η), and the concentration of the module outlet at the FS side were investigated and are shown in Figure4. Here, 0.6 M NaCl and tap water were used as DS and FS, respectively. The flow rate of DS (QDS) was set as 10.0 L/min in all the evaluations, which was the maximum DS flow rate of the evaluation system. ∆P was set as 1.2 MPa, which is almost half of the theoretical osmotic pressure difference between the DS and FS. Figure4a shows that Jw of PA-SW gradually increased from 5.6 at 3.0 L/min to 2 6.1 L/m h at 6.0 L/min with increasing QFS, while that of CTA-HF from 1.4 at 2.0 L/min 2 to 1.6 L/m h at 5.0 L/min. These results indicate that Jw of the former module was more than 4 times higher than that of the latter, though the water permeability of the former was more than 13 times higher than that of the latter. The slight increase in Jw of both modules with increasing QFS indicates that the effect of ICP on the PRO performance is almost negligible at high QFS. Figure4b indicates that the permeate rate of the two modules decreased with increasing QFS and the value of CTA-HF decreased from 75% at 2.0 L/min to less than 40% at 5 L/min, and that of PA-SW from 48% at 3.0 L/min to 26% at 6 L/min. Figure4c shows that FS outlet concentration ( QFS,out) of both the modules was less than 0.05 M and decreased with increasing QFS. Increasing QFS helped to flush the salt in the FS that permeated from the DS side to the FS membrane interface. For a CTA-HF module, a permeation rate of less than 70% was required to mitigate the FS up-concentration [35]. CFS,out of the CTA-HF, was slightly lower than that of PA-SW, although the permeate rate of the former was slightly higher than that of the latter. This is because the B value of the former is significantly lower than that of the latter. From the results shown in Figure4b,c, an FS flow rate of more than 5 L/min is sufficient for the two modules to mitigate the effect of FS up-concentration on the PRO performance. Membranes 2021, 11, x FOR PEER REVIEW 9 of 21

meated from the DS side to the FS membrane interface. For a CTA-HF module, a perme- ation rate of less than 70% was required to mitigate the FS up-concentration [35]. 𝐶, of the CTA-HF, was slightly lower than that of PA-SW, although the permeate rate of the former was slightly higher than that of the latter. This is because the B value of the former is significantly lower than that of the latter. From the results shown in Figure 4b,c, an FS Membranes 2021, 11, 177 flow rate of more than 5 L/min is sufficient for the two modules to mitigate the effect of9 of 21 FS up-concentration on the PRO performance.

(a) (b)

(c)

FigureFigure 4. (a) 4.Water(a) Water flux, flux,Jw, andJw ,(b and) FS ( bpermeation) FS permeation rate, η rate,, andη (,c and) the ( cconcentration) the concentration of the ofmodule the module outlet at FS side (CFS,out), respectively, as a function of FS flow rate, QFS. DS: 0.6 M NaCl. FS: treated outlet at FS side (CFS,out), respectively, as a function of FS flow rate, QFS. DS: 0.6 M NaCl. FS: treated tap water. Flow rate of DS (QDS): 10.0 L/min. Hydrostatic pressure (ΔP): 1.2 MPa. tap water. Flow rate of DS (QDS): 10.0 L/min. Hydrostatic pressure (∆P): 1.2 MPa.

FigureFigure 5 shows5 shows Jw andJw ηand, andη, Q andFS,outQ as a functionas a function of QFS ofin Qthe casein the of case1.2 M of NaCl 1.2 M as NaCl Δ FS,out FS DS. Here,as DS. Here,P for CTA-HF∆P for CTA-HF was 2.5 M was Pa, 2.5 which MPa, is which almost is equal almost to Δπ equal. There to ∆ isπ .no There infor- is no mationinformation about the aboutpressure the resistance pressure resistance for PA-SW. for Hence, PA-SW. the Hence, PRO thetest PROusing test PA-SW using was PA-SW performedwas performed at a lower at pressure a lower pressurethan that than using that CTA-HF, using CTA-HF, i.e., 1.6 M i.e., Pa. 1.6 MPa. In the case of 1.2 M DS, Jw of CTA-HF at 6.0 L/min of QFS was 1.56 times higher compared to that in the case of 0.6 M though ∆P increased from 1.2 to 2.5 Mpa, because the increase in ∆π as the driving force of the water flux, as shown in Figure5a. On the contrary, Jw of PA-SW showed a low value of 85% of Jw in the case of 0.6 M NaCl. This is because the increase in the effective osmotic pressure difference in PA-SW did not increase linearly, as mentioned in Section 3.2.2. As shown in Figure5b, the permeation rate in CTA-HF reached 100% at the lowest QFS (<2.5 L/min), and CFS,out of the module had a high value of approximately 0.3 M, as shown in Figure5c. Although the Jw of CTA-HF is lower than that of PA-SW, the total membrane area of the former is more than four times larger than that of the latter; hence, the permeate water flow from the FS to the DS sides in the former module was larger than that in the latter. Therefore, an FS flow rate of 2.5 L/min is not sufficient for CTA-HF to prevent FS up-concentration. CFS,out decreased dramatically as QFS increased, and permeation rate decreased to less than 50% at 6.0 L/min of QFS, while that of PA-SW decreased from 40% to 20% at 6.0 L/min of Q . Hence, these data indicate FS that 5.0 a QFS of is sufficient for both modules to mitigate FS up-concentration even at a high DS concentration of 1.2 M. Membranes 2021, 11, x FOR PEER REVIEW 10 of 21

Membranes 2021, 11, 177 10 of 21

(a) (b)

(c)

FigureFigure 5. (a) 5.Water(a) Water flux, flux,Jw, andJw ,(b and) FS ( bpermeation) FS permeation rate, η rate,, andη (,c and) the ( cconcentration) the concentration of the ofmodule the module outlet at FS side (CFS,out), respectively, as a function of FS flow rate, QFS. DS: 1.2 M NaCl. FS: treated outlet at FS side (CFS,out), respectively, as a function of FS flow rate, QFS. DS: 1.2 M NaCl. FS: treated tap water. QDS: 10.0 L/min. ΔP: 1.6 MPa for PA-SW module and 2.5 MPa for CTA-HF module. tap water. QDS: 10.0 L/min. ∆P: 1.6 MPa for PA-SW module and 2.5 MPa for CTA-HF module.

PermeateIn the case Water of 1.2 Flux M DS, and Jw Dilution of CTA-HF Factor at 6.0 Versus L/min DS of Flow QFS was Rate 1.56 times higher com- Δ pared to thatThe in effect the ofcase the of DS 0.6 flow M though rate (Q DSP )increased on the PRO from performance 1.2 to 2.5 Mpa, was because investigated the by Δπ increasemeasuring in as the the PRO driving performance force of the of water the two flux, modules as shown at variousin FigureQ 5a.DS toOn evaluate the contrary, the effect Jw ofof PA-SW dilutive showed external a low concentration value of 85% polarization of Jw in the (dECP) case of at 0.6 the M DS NaCl. side This and theis because decrease in the increasethe salt concentrationin the effective in osmotic the module pressure inside difference the DS side. in PA-SW Figure6 did shows not increaseJw, dilution line- factor arly,( asϕ), mentioned and the concentration in Section 3.2.2. of the As module shown outlet in Figure at the 5b, DS sidethe permeation (QDS,out) as arate function in CTA- of QDS, HF reachedwhere DS100% and at FSthe are lowest 0.6 M Q NaClFS (<2.5 and L/min), tap water, and CFS,out respectively. of the moduleQFS was had seta high as 5.0 value L/min, of approximatelyand ∆P was 1.2 0.3 MPa, M, as which shown is in nearly Figure equal 5c. Although to the osmotic the J pressurew of CTA-HF difference is lower between than DS that ofand PA-SW, FS. the total membrane area of the former is more than four times larger than 2 that of theFigure latter;6 ahence, shows the that permeateJw increased water from flow 0.53from at the 1.0 FS L/min to the to DS 1.7 sides L/m inh the at10 former L/min of 2 moduleQDS wasfor CTA-HFlarger than and that from in 5.3the at latter. 4.0 L/min Therefore, to 6.5 an L/m FS flowh at 10rate L/min of 2.5 for L/min PA-SW is not as QDS sufficientincreased, for CTA-HF even as to it prevent approached FS up-concentration. the maximum flow CFS,out rate decreased of the pump. dramatically As ϕ approaches as QFS 1, increased,it indicates and permeation that QDS,out rateis equal decreased to the to DS less flow than rate 50% at theat 6.0 inlet. L/min In thisof Q case,FS, whileϕ decreased that of PA-SWfrom 1.58decreased at 1.0 L/minfrom 40% to 1.18 to 20% at 10 at L/min 6.0 L/min (CTA-HF) of QFS. Hence, and from these 1.33 data at 4 indicate L/min to that 1.17 at 5.0 a10 QFS L/min of is sufficient (PA-SW), for as shownboth modules in Figure to6 b.mitigate In the caseFS up-concentration of CTA-HF, CDS,out evenincreased at a high from DS concentration0.38 M at 1.0 of L/min 1.2 M. to 0.51 M at 10 L/min, 63% and 85% of the DS inlet concentration, respectively. In the case of PA-SW, CDS,out increased from 0.45 M and 0.51 M, 75% and 85% Permeate Water Flux and Dilution Factor Versus DS Flow Rate of the DS inlet concentration, respectively, as QDS increased, as shown in Figure6c. This indicatesThe effect that of athe 10 DS L/min flow of rate DS flow(QDS) rate on isthe not PRO sufficient performance for both was modules investigated to be negligible by measuringin the DSthe dilutionPRO performance effect by the of permeatethe two modules water flow at various from the Q FSDS to side. evaluate the effect of dilutive external concentration polarization (dECP) at the DS side and the decrease in the salt concentration in the module inside the DS side. Figure 6 shows Jw, dilution factor (φ), and the concentration of the module outlet at the DS side (QDS,out) as a function of QDS,

Membranes 2021, 11, x FOR PEER REVIEW 11 of 21

where DS and FS are 0.6 M NaCl and tap water, respectively. QFS was set as 5.0 L/min, and Membranes 2021, 11, 177 ΔP was 1.2 MPa, which is nearly equal to the osmotic pressure difference between DS and11 of 21 FS.

(a) (b)

(c)

FigureFigure 6. (a) 6.Water(a) Water flux, flux,Jw, andJw ,(b and) dilution (b) dilution factor, factor, φ, andϕ, ( andc) the (c )concentration the concentration of the of module the module out- outlet let at DS side (CDS,out), respectively, as a function of DS flow rate, QDS. DS: 0.6 M NaCl. FS: treated at DS side (CDS,out), respectively, as a function of DS flow rate, QDS. DS: 0.6 M NaCl. FS: treated tap tap water. QFS: 5.0 L/min. ΔP: 1.2 MPa. water. QFS: 5.0 L/min. ∆P: 1.2 MPa.

2 FigureWhen 6a shows using 1.2that M Jw NaCl increased as DS, from Figure 0.537a shows at 1.0 thatL/minJw toincreased 1.7 L/m fromh at 2.3210 L/min at 4.0 L/minof 2 2 QDS forto 2.9 CTA-HF L/m h and at 10 from L/min 5.3 of atQ DS4.0for L/min CTA-HF. to 6.5 For L/m PA-SW,h at J10w increasedL/min for from PA-SW 5.1 at as 4.0 Q L/minDS 2 increased,to 5.6 L/meven has at it 10approached L/min of QtheDS. maximumSimilar to theflow case rate of of FS the flow pump. rate change,As φ approachesJw of CTA-HF 1, it indicateswas higher that compared QDS,out is equal to that to in the the DS case flow of rate 0.6 M,at the even inlet. though In this∆ Pcase,increased; φ decreased however, fromthat 1.58 of at PA-SW 1.0 L/min had to a 1.18 lower at value.10 L/minϕ decreased (CTA-HF) from and from 1.65 at 1.33 4.0 at L/min 4 L/min to 1.33to 1.17 at 10at L/min10 L/min(CTA-HF) (PA-SW), and as shown from 1.32 in Figure at 4 L/min 6b. In to the 1.14 case at 10of CTA-HF, L/min (PA-SW), CDS,out increased as shown from in Figure 0.38 7b. M atC 1.0DS,out L/minincreased to 0.51 fromM at 0.7310 L/min, M at 4.063% L/min and 85% to 0.90of the M DS at 10inlet L/min concentration, for CTA-HF, respec- 61% and tively.75% In ofthe the case DS of inlet PA-SW, concentrations, CDS,out increased respectively, from 0.45 and M fromand 0.51 0.90 M, M 75% at 4.0 and L/min 85% toof 1.0the M at DS inlet10 L/min concentration, for PA-SW, respectively, 75% and 83% as Q ofDS theincreased, DS inlet as concentration, shown in Figure respectively, 6c. This indicates as shown in that aFigure 10 L/min7c. Even of DS at flow the highestrate is not DS sufficie flow ratent offor the both evaluation modules systemto be negligible (10 L/min), in the especially DS dilutionin the effect case by of the CTA-HF, permeateϕ showed water aflow high from value the (more FS side. than 1.3), and CDS,out was much lower thanWhen the using DS inlet1.2 M concentration. NaCl as DS, ThisFigure is because7a shows the that permeate Jw increased water from flow 2.32 of the at module4.0 L/minfor to CTA-HF 2.9 L/m2 wash at 10 approximately L/min of QDS two for CTA-HF. times higher For thanPA-SW, that J ofw increased PA-SW; although, from 5.1 at the 4.0 water L/minflux to of5.6 the L/m former2h at 10 was L/min approximately of QDS. Similar half to that the ofcase the of latter FS flow at 10 rate L/min, change, indicating Jw of CTA- that the HF wasdilution higher effect compared to reduce to that the PROin the performance case of 0.6 M, cannot even bethough negligible, ΔP increased; especially however, for CTA-HF that underof PA-SW the testhad conditions.a lower value. φ decreased from 1.65 at 4.0 L/min to 1.33 at 10 L/min (CTA-HF) and from 1.32 at 4 L/min to 1.14 at 10 L/min (PA-SW), as shown in Figure 7b. CDS,out increased from 0.73 M at 4.0 L/min to 0.90 M at 10 L/min for CTA-HF, 61% and 75%

Membranes 2021, 11, x FOR PEER REVIEW 12 of 21

of the DS inlet concentrations, respectively, and from 0.90 M at 4.0 L/min to 1.0 M at 10 L/min for PA-SW, 75% and 83% of the DS inlet concentration, respectively, as shown in Figure 7c. Even at the highest DS flow rate of the evaluation system (10 L/min), especially in the case of CTA-HF, φ showed a high value (more than 1.3), and CDS,out was much lower than the DS inlet concentration. This is because the permeate water flow of the module for CTA-HF was approximately two times higher than that of PA-SW; although, the water flux of the former was approximately half that of the latter at 10 L/min, indicating that the Membranes 2021, 11, 177 dilution effect to reduce the PRO performance cannot be negligible, especially for CTA-12 of 21 HF under the test conditions.

(a) (b)

(c)

Figure 7. (a) Water flux, Jw, and (b) dilution factor, φ, and (c) the concentration of the module out- Figure 7. (a) Water flux, Jw, and (b) dilution factor, ϕ, and (c) the concentration of the module outlet let at DS side (CDS,out), respectively, as a function of DS flow rate, QDS. DS: 1.2 M NaCl. FS: treated at DS side (CDS,out), respectively, as a function of DS flow rate, QDS. DS: 1.2 M NaCl. FS: treated tap tap water. QFS: 5.0 L/min. ΔP: 1.6 MPa for PA-SW module and 2.5 MPa for CTA-HF module. water. QFS: 5.0 L/min. ∆P: 1.6 MPa for PA-SW module and 2.5 MPa for CTA-HF module. 3.2.2.3.2.2. PRO PRO Performance Performance of the of Two the Two Modules Modules as a asFunction a Function of Applied of Applied Pressure Pressure w FigureFigure 8a,b8a,b shows shows the the relationship relationship between between J andJw and ΔP∆ ofP ofCTA-HF CTA-HF and and PA-SW, PA-SW, re- respec- spectively, at various inlet DS concentrations (CDS), where the flow rates of DS and FS were tively, at various inlet DS concentrations (CDS), where the flow rates of DS and FS were 10.0 10.0and and 5.0 5.0 L/min, L/min, respectively. respectively. AsAs shownshown inin Equation Equation (1)(1)and andFigure Figure1 ,1, the the measured measured water Membranes 2021, 11, x FOR PEER REVIEWwaterflux flux of both of both modules modules decreased decreased with with increasing increasing∆P because ΔP because∆P facilitates ΔP facilitates water13 waterof movement 21 movement from the DS to FS sides. The measured water flux increased with increasing from the DS to FS sides. The measured water flux increased with increasing CDS, owing to CDSthe, owing increase to the in increase the driving in the force driving of water force permeation of water permeation and the osmotic and the pressure osmotic difference.pres- sure difference.

(a) (b)

Figure 8. Water flux, Jw, as a function of applied hydraulic pressure difference, ΔP, at QDS = 10.0 Figure 8. Water flux, Jw, as a function of applied hydraulic pressure difference, ∆P, at L/min and QFS = 5.0 L/min: (a) PA-SW and (b) CTA-HF membrane module. QDS = 10.0 L/min and QFS = 5.0 L/min: (a) PA-SW and (b) CTA-HF membrane module.

Although relationship between Jw and ΔP, given by Equation (1) indicates a linear relationship, in general, the actual Jw in a PRO system decreases along the parabolic curve with increasing ΔP [29]. However, the obtained water flux of both modules decreased almost linearly with increasing ΔP. This is primarily because the PRO tests in this study were performed at high DS and FS flow rates to minimize the effect of the DS dilution, dECP at the DS side by the permeate water flux, and the concentrated ICP inside the mod- ule by DS leakage, although these effects were not negligible, owing to the limitation of the pump performance, as shown in Figure 7. From the experimental results, the experi- mental water flux, 𝐽,, of the two modules under PRO conditions can be expressed in terms of a simple linear approximation as an empirical line [35]: 𝐽, = 𝐴 ∆𝜋 −𝛥𝑃 (18) Here, the pseudo-water permeability in PRO mode (𝐴) and effective osmotic pres- sure difference (∆𝜋) were obtained from the slope and intersection of the x-axis of the empirical line shown in Figure 8. The relationship between ∆𝜋 and CDS is shown in Fig- ure 9. The broken line represents the theoretical osmotic pressure difference (∆π) calcu- lated by using the van’t Hoff equation. This figure shows that ∆𝜋 increased as CDS in- creased, and ∆𝜋 of the two modules was lower than ∆π. Additionally, PA-SW exhib- ited a greater deviation between ∆𝜋 and ∆π, especially at high CDS. In contrast, CTA-HF shows a small deviation between the two values, and from the relationship between ∆𝜋 and CDS shown in Figure 9, ∆𝜋 inside the module in the PRO test is estimated by using Equation (19), and αmod is calculated as 0.88:

∆𝜋 = 𝛼∆𝜋 (19)

The small deviation between ∆π and ∆𝜋 for CTA-HF despite QDS,out being 85% of the inlet concentration in the case of 0.6 M NaCl shown in Figure 6c indicates that the primary cause of the deviation will be the DS dilution, and the effect of the dilutive ECP on PRO performance will be negligible for the HF type module.

Membranes 2021, 11, 177 13 of 21

Although relationship between Jw and ∆P, given by Equation (1) indicates a linear relationship, in general, the actual Jw in a PRO system decreases along the parabolic curve with increasing ∆P [29]. However, the obtained water flux of both modules decreased almost linearly with increasing ∆P. This is primarily because the PRO tests in this study were performed at high DS and FS flow rates to minimize the effect of the DS dilution, dECP at the DS side by the permeate water flux, and the concentrated ICP inside the module by DS leakage, although these effects were not negligible, owing to the limitation of the pump performance, as shown in Figure7. From the experimental results, the experimental water flux, Jw,exp, of the two modules under PRO conditions can be expressed in terms of a simple linear approximation as an empirical line [35]:

PRO Jw, exp = A (∆πeff − ∆P) (18)

Here, the pseudo-water permeability in PRO mode (APRO) and effective osmotic pressure difference (∆πeff) were obtained from the slope and intersection of the x-axis of the empirical line shown in Figure8. The relationship between ∆πeff and CDS is shown in Figure9. The broken line represents the theoretical osmotic pressure difference ( ∆π) calculated by using the van’t Hoff equation. This figure shows that ∆πeff increased as CDS increased, and ∆πeff of the two modules was lower than ∆π. Additionally, PA-SW exhibited a greater deviation between ∆πeff and ∆π, especially at high CDS. In contrast, CTA-HF shows a small deviation between the two values, and from the relationship between ∆πeff and CDS shown in Figure9, ∆πeff inside the module in the PRO test is estimated by using Equation (19), and αmod is calculated as 0.88:

∆πeff = αmod∆π (19)

The small deviation between ∆π and ∆πeff for CTA-HF despite QDS,out being 85% of the inlet concentration in the case of 0.6 M NaCl shown in Figure6c indicates that the Membranes 2021, 11, x FOR PEER REVIEW 14 of 21 primary cause of the deviation will be the DS dilution, and the effect of the dilutive ECP on PRO performance will be negligible for the HF type module.

FigureFigure 9. 9. EffectiveEffective osmotic osmotic pressure pressure difference, difference, ∆𝜋∆π,eff as, asa function a function of DS of DSconcentration, concentration, CDS.C FS:DS. FS: treatedtreated tap tap water. water. QQDSDS: 10.0: 10.0 L/min. L/min. QFSQ: FS5.0: 5.0L/min. L/min.

TheThe possible possible reasons reasons that the dilutive ECPECP (dECP)(dECP) willwill bebe negligible negligible are are as as follows: follows: A Ahollow-fiber-type hollow-fiber-type membrane membrane element element has has an approximatelyan approximately 10-times-larger 10-times-larger surface surface area area(than (than SW SW type) type) in thein the case case of of an an RO RO module module [42 [42].]. This This impliesimplies thatthat the water flux flux per per membranemembrane area area of of an an HF HF module module is ismuch much lower lower than than that that of an of SW an SWmodule, module, even even if the if twothe modules two modules have havethe same the samewater waterflux per flux mo perdule. module. Hence, Hence,the effect the of effect dECP of on dECP the DS on sidethe DSwill side be lower will be because lower of because the small of the water small flux water per membrane flux per membrane area of the area HF ofmodule. the HF Additionally,module. Additionally, the cross-winding the cross-winding arrangement arrangement of more than of more several than thousands several thousands of hollow of fibershollow in fibersan HF in module an HF moduleallows aallows uniform a uniform flow, which flow, minimizes which minimizes the concentration the concentration polar- izationpolarization in the inshell the side shell (the side DS (the side DS in side this in study) this study) of the ofmodule the module [42]. [42]. The other possible reasons for the deviations ∆π and ∆𝜋 are that the smaller B value of CTA-HF compared to PA-SW will give a lower concentrative ICP inside the sup- port layer of the membrane. Figure 10 shows the apparent water permeation coefficient 𝐴 as a function of CDS. The 𝐴 of CTA-HF decreased gradually from 1.9 ×10 at 0.2 M NaCl to 1.0 10 [L/m2 h Pa] at 1.2 M of CDS, which is similar to the decrease of the CTA-HF reported in a previous study [43] named as M-HF in Table 2. 𝐴of PA-SW decreased drastically from 9.0 at 0.3 M of CDS to 3.7 10 [L/m2 h Pa] at 1.2 M DS. As shown in Figure 9 and Figure 10, the drastic decrease in ∆𝜋 and 𝐴 of PA-SW is due to the concentrated ICP inside the support layer of the PA membrane involving the DS leakage because the membrane has lower salt rejection than the CTA membrane, as shown in Figure 3b. An- other factor may be the complex FS flow channel structure of the PRO SW module in a U- fashion [48]. It may be difficult for the channel structure to flush the DS leakage at a high CDS.

Figure 10. Apparent water permeability coefficient, 𝐴 , as a function of DS concentration, CDS. FS: treated tap water. QDS: 10.0 L/min. QFS: 5.0 L/min.

Membranes 2021, 11, x FOR PEER REVIEW 14 of 21

Figure 9. Effective osmotic pressure difference, ∆𝜋, as a function of DS concentration, CDS. FS: treated tap water. QDS: 10.0 L/min. QFS: 5.0 L/min.

The possible reasons that the dilutive ECP (dECP) will be negligible are as follows: A hollow-fiber-type membrane element has an approximately 10-times-larger surface area (than SW type) in the case of an RO module [42]. This implies that the water flux per membrane area of an HF module is much lower than that of an SW module, even if the two modules have the same water flux per module. Hence, the effect of dECP on the DS side will be lower because of the small water flux per membrane area of the HF module. Additionally, the cross-winding arrangement of more than several thousands of hollow Membranes 2021, 11, 177 fibers in an HF module allows a uniform flow, which minimizes the concentration polar-14 of 21 ization in the shell side (the DS side in this study) of the module [42]. The other possible reasons for the deviations ∆π and ∆𝜋 are that the smaller B value ofThe CTA-HF other possible compared reasons to PA-SW for the will deviations give a lower∆π and concentrative∆πeff are that ICP the inside smaller theB sup-value portof CTA-HFlayer of the compared membrane. to PA-SW will give a lower concentrative ICP inside the support layerFigure of the 10 membrane. shows the apparent water permeation coefficient 𝐴 as a function of CDS. PRO The 𝐴Figure of 10 CTA-HF shows the decreased apparent watergradually permeation from 1.9 coefficient×10 Aat 0.2as M a functionNaCl to of 1.0CDS . PRO −6 −6 10The [L/mA 2 hof Pa] CTA-HF at 1.2 M decreased of CDS, which gradually is similar from to1.9 the× decrease10 at of 0.2 the M CTA-HF NaCl to 1.0reported× 10 2 in[L/m a previoush Pa] study at 1.2 [43] M of namedCDS, which as M-HF is similar in Table to the2. 𝐴 decreaseof PA-SW of the decreased CTA-HF reporteddrastically in a PRO fromprevious 9.0 at study0.3 M [ 43of] C namedDS to 3.7 as M-HF10 [L/m in Table2 h Pa]2. Aat 1.2 ofM PA-SW DS. As decreased shown in drastically Figure 9 and from × −6 2 Figure9.0 at 10, 0.3 the M ofdrasticCDS to decrease 3.7 10 in [L/m∆𝜋 andh Pa] 𝐴 at1.2 of MPA-SW DS. As isshown due toin the Figures concentrated9 and 10 , PRO ICPthe inside drastic the decrease support in layer∆πeff ofand theA PA membof PA-SWrane isinvolving due to the the concentrated DS leakage ICPbecause inside the the membranesupport layer has lower of the salt PA membranerejection than involving the CTA the membrane, DS leakage as because shown in the Figure membrane 3b. An- has otherlower factor salt rejectionmay be the than complex the CTA FS membrane, flow channel as shownstructure in Figureof the 3PROb. Another SW module factor in may a U- be fashionthe complex [48]. It FSmay flow be channeldifficult structurefor the channel of the PROstructure SW moduleto flush inthe a DS U-fashion leakage [ 48at ].a Ithigh may be difficult for the channel structure to flush the DS leakage at a high C . CDS. DS

𝐴 PRO DS FigureFigure 10. 10. ApparentApparent water water permeability permeability coefficient, coefficient, A , as, as a afunction function of of DS DS concentration, concentration, CCDS. . FS: FS: treated tap water. QDS: 10.0 L/min. QFS: 5.0 L/min. treated tap water. QDS: 10.0 L/min. QFS: 5.0 L/min.

3.2.3. Comparison of PRO Module Performance between CTA-HF and PA-SW The volumetric power density of the hollow-fiber-type module (CTA-HF) and spiral- wound type of membrane module (PA-SW) were compared to investigate the feasible operating conditions for generating energy by using the PRO technique. In this section, a detailed comparison was performed to evaluate the volumetric power density as a function of the applied hydrostatic pressure difference, and the maximum gross power, maximum net power output, and pumping energy for feeding DS and FS as a function of the flow rates of the DS and FS.

Volumetric Gross Power Density and Applied Pressure Difference Figure 11a,b shows the volumetric gross power density (PDvol) of PA-SW and CTA-HF in relation to ∆P, respectively. Here, each circle represents the experiments obtained under the operating conditions where QDS and QFS are 10 and 5 L/min, respectively, and each PRO curve is the calculation obtained by substituting A and ∆πeff into Equations (9) and (18). These figures indicate that the PDvol of both modules increased with increasing ∆P, and they reached a maximum value at approximately half of ∆πeff at each CDS. The gross maximum volumetric gross power density (PDmax ) and the applied hydrostatic pressure gross difference at the point that indicates PDmax increased when CDS increased, as predicted gross by Equation (18). Compared to PDmax of PA-SW and CTA-HF, in the case of 1.2 M NaCl gross 3 (SWRO brine level) as DS, PDmax of CTA-HF was 13.6 kW/m at 2.6 MPa of ∆P, and that of 3 gross PA-SW was 1.1 kW/m at 1.5 MPa of ∆P. Hence, CTA-HF showed 12 times higher PDmax gross than PA-SW. In the case of 0.6 M NaCl (seawater level) as DS, PDmax of CTA-HF was 4.7 kW/m3 (∆P = 1.4 MPa), and that of PA-SW is 0.63 kW/m3 (∆P = 1.1 MPa), indicating gross 7.5 times higher PDmax of CTA-HF. This is because of the difference in the packing density Membranes 2021, 11, x FOR PEER REVIEW 15 of 21

3.2.3. Comparison of PRO Module Performance between CTA-HF and PA-SW The volumetric power density of the hollow-fiber-type module (CTA-HF) and spiral- wound type of membrane module (PA-SW) were compared to investigate the feasible op- erating conditions for generating energy by using the PRO technique. In this section, a detailed comparison was performed to evaluate the volumetric power density as a func- tion of the applied hydrostatic pressure difference, and the maximum gross power, max- imum net power output, and pumping energy for feeding DS and FS as a function of the flow rates of the DS and FS. Volumetric Gross Power Density and Applied Pressure Difference Figure 11a,b shows the volumetric gross power density (PDvol) of PA-SW and CTA- HF in relation to ΔP, respectively. Here, each circle represents the experiments obtained under the operating conditions where QDS and QFS are 10 and 5 L/min, respectively, and each curve is the calculation obtained by substituting 𝐴 and ∆𝜋 into Equations (9) and (18). These figures indicate that the PDvol of both modules increased with increasing ΔP, and they reached a maximum value at approximately half of ∆𝜋 at each CDS. The maximum volumetric gross power density (𝑃𝐷 ) and the applied hydrostatic pressure difference at the point that indicates 𝑃𝐷 increased when CDS increased, as predicted by Equation (18). Compared to 𝑃𝐷 of PA-SW and CTA-HF, in the case of 1.2 M NaCl 3 (SWRO brine level) as DS, 𝑃𝐷 of CTA-HF was 13.6 kW/m at 2.6 MPa of ΔP, and that of PA-SW was 1.1 kW/m3 at 1.5 MPa of ΔP. Hence, CTA-HF showed 12 times higher Membranes 2021, 11, 177 𝑃𝐷 than PA-SW. In the case of 0.6 M NaCl (seawater level) as DS, 𝑃𝐷 of CTA-15 of 21 HF was 4.7 kW/m3 (ΔP = 1.4 MPa), and that of PA-SW is 0.63 kW/m3 (ΔP = 1.1 MPa), indi- cating 7.5 times higher 𝑃𝐷 of CTA-HF. This is because of the difference in the pack- ing(the density ratio of (the membrane ratio of membrane area to module area to volume) module between volume) the between two module the two configurations. module con- figurations.The packing The densities packing of densities CTA-HF andof CTA-HF PA-SW wereand PA-SW 6.75 × 10were3 and 6.75 464×10 1/m, and respectively; 464 1/m, respectively;thus, that of thus, CTA-HF that of was CTA-HF 14 times was higher 14 time thans higher that ofthan PA-SW. that of Therefore, PA-SW. Therefore, the permeate the permeatewater flux water of CTA-HF flux of CTA-HF is higher is than higher that than of PA-SW, that of evenPA-SW, though even thethough water the flux water per flux unit permembrane unit membrane area of thearea latter of the is approximatelylatter is approximately three times three higher times than higher that than of the that former, of the as former,shown inas Figureshown6 a,in and Figure CTA-HF 6a, and has CTA-HF only 30% has ofthe only module 30% of volume the module compared volume to PA-SW. com- paredThe total to PA-SW. membrane The areatotal ofmembrane PA-SW (15.3 area m of2) PA-SW is approximately (15.3 m2) is 40% approximately that of a conventional 40% that ofRO a conventional SW module (37 RO m SW2). Thismodule is owing (37 m to2). theThis thicker is owing tricot to fabricthe thicker spacer tricot at the fabric FS side spacer and atlarger the FS dead side membrane and larger areadead due membrane to the presence area due of to additional the presence glue of lines additional along theglue central lines alongline of the the central membranes line of tothe fabricate membranes in a U-shapedto fabricate fashion in a U-shaped [48], compared fashion to [48], a conventional compared toRO a conventional SW module. RO SW module.

(a) (b)

FigureFigure 11. 11. VolumetricVolumetric gross gross power power density, density, PDvolPD, asvol a, asfunction a function of applie of appliedd hydraulic hydraulic pressure pressure dif- ference, ΔP, membrane module: (a) PA-SW and (b) CTA-HF with FS: treated tap water. QDS: 10.0 difference, ∆P, membrane module: (a) PA-SW and (b) CTA-HF with FS: treated tap water. QDS: L/min. QFS: 5.0 L/min. 10.0 L/min. QFS: 5.0 L/min.

gross To investigate the relationship between PDmax and DS concentration in greater detail, gross gross Figure 12 shows the PDmax of CTA-HF and PA-SW as a function of QDS. The PDmax of CTA-HF increased in an approximate quadratic curve with increasing CDS. In the theoretical Equations (1) and (9), the power output of PRO becomes a quadratic curve of CDS because the power output is the product of the water flux and the osmotic pressure difference, and these two factors depend linearly on CDS. Conversely, the degree of the gross increase in PDmax of PA-SW was smaller than that of CTA-HF, indicating that there is a gross large difference in PDmax between the two modules, especially at high DS concentrations. PRO The primary causes of this difference are the difference in ∆πeff and A at high DS concentrations and the packing density between the two modules. Membranes 2021, 11, x FOR PEER REVIEW 16 of 21

To investigate the relationship between 𝑃𝐷 and DS concentration in greater de- tail, Figure 12 shows the 𝑃𝐷 of CTA-HF and PA-SW as a function of QDS. The 𝑃𝐷 of CTA-HF increased in an approximate quadratic curve with increasing CDS. In the theoretical Equations (1) and (9), the power output of PRO becomes a quadratic curve of CDS because the power output is the product of the water flux and the osmotic pressure difference, and these two factors depend linearly on CDS. Conversely, the degree of the increase in 𝑃𝐷 of PA-SW was smaller than that of CTA-HF, indicating that there is a large difference in 𝑃𝐷 between the two modules, especially at high DS concentra- Membranes 2021, 11, 177 tions. The primary causes of this difference are the difference in ∆𝜋 and 𝐴 at 16high of 21 DS concentrations and the packing density between the two modules.

Figure 12. The maximum gross power output (𝑃𝐷 gross) of CTA-HF and PA-SW as a function of Figure 12. The maximum gross power output (PDmax ) of CTA-HF and PA-SW as a function of CDS. CDS. FS: treated tap water. QDS: 10.0 L/min. QFS: 5.0 L/min. FS: treated tap water. QDS: 10.0 L/min. QFS: 5.0 L/min. RelationshipRelationship between between Net Net Power Power Output Output and and Feed Feed Flow Flow Rates Rates of of DS DS and and FS FS TheThe theoretical theoretical power power output output of of the the memb membranerane module module was calculated from thethe waterwa- terflux flux and and applied applied pressure pressure difference. difference. However, However, in in an an actual actual PRO PRO plant, plant, the the efficiency efficiency of ofthe the pumps pumps andand PXPX usedused forfor PRO operation must be be carefully carefully considered. considered. Hence, Hence, in in this this section,section, the the gross gross power, power, net net power power per per module, module, and and pumping pumping energy energy in in the the PA-SW PA-SW and and CTA-HFCTA-HF systems systems were were compared, compared, using using 1.2 1.2M NaCl M NaCl as DS, as at DS, various at various flow flowrates ratesof DS ofand DS FS.and Here, FS. tap Here, water tap was water used was as usedFS, and as Δ FS,P for and PA-SW∆P for and PA-SW CTA-HF and were CTA-HF 1.6 and were 2.5 1.6 MPa, and respectively.2.5 MPa, respectively. Figure 13a,b Figure shows 13 a,bthe showsgross power the gross (Pgross power), net ( Ppowergross), net(Pnet power), and pumping (Pnet), and energypumping (PE)energy per module (PE) perfor PA-SW module and for PA-SWCTA-HF and as a CTA-HF function as of a FS function flow rate. of FSFigure flow 13a rate. showsFigure that 13a P showsgross of thatPA-SWPgross increasedof PA-SW from increased 28 to 35 from W at 28 6 toL/min, 35 W indicating at 6 L/min, a indicating1.25-fold increasea 1.25-fold from increase 3 to 6 L/min from 3Q toFS, 6while L/min CEQ increasedFS, while rapidlyCE increased from 20 rapidly to 44 fromW (2.2-fold 20 to 44 in- W crease).(2.2-fold The increase). Pnet of the The modulePnet ofdecreased the module from decreased 9.3 W and from showed 9.3 Wnegative and showed values negativeat flow values at flow rates of more than 5 L/min. Hence, P had a maximum value of 9.3 W rates of more than 5 L/min. Hence, Pnet had a maximumnet value of 9.3 W at 3.0 L/min of QFS at 3.0 L/min of Q under the test conditions. Meanwhile, as shown in Figure 13b, P under the test conditions.FS Meanwhile, as shown in Figure 13b, Pgross of CTA-HF increasedgross of CTA-HF increased from 93 to 123 W, a 1.3-fold increase with increasing Q from 3 to from 93 to 123 W, a 1.3-fold increase with increasing QFS from 3 to 6 L/min. TheFS CE in- 6 L/min. The CE increased rapidly from 43 to 98 W (2.3-fold increase). P increased from creased rapidly from 43 to 98 W (2.3-fold increase). Pnet increased from 54net W and reached 54 W and reached a maximum value of 55 W at 3 L/min of Q , and then decreased to a maximum value of 55 W at 3 L/min of QFS, and then decreased toFS 23 W at 6 L/min of QFS. Q The23 Wmaximum at 6 L/min net ofpowerFS. Theof the maximum CTA-HF netis more power than of thefive CTA-HF times that is moreof the than PA-SW. five timesThe that of the PA-SW. The changes in Pgross, Pnet, and CE with DS flow rate are shown in changes in Pgross, Pnet, and CE with DS flow rate are shown in Figure 13c,d. The Pgross of PA- Figure 13c,d. The P of PA-SW increased from 33 W at 4 L/min to 36 W at 10 L/min, SW increased from 33gross W at 4 L/min to 36 W at 10 L/min, indicating a 1.09-fold increase indicating a 1.09-fold increase with increasing QDS, while CE increased rapidly from 22 with increasing QDS, while CE increased rapidly from 22 to 32 W (1.45-fold increase). The to 32 W (1.45-fold increase). The Pnet of the module decreased from the maximum value Pnet of the module decreased from the maximum value of 11 W at 4.0 L/min and showed of 11 W at 4.0 L/min and showed negative values at flow rates of more than 9 L/min. negative values at flow rates of more than 9 L/min. For CTA-HF, Pgross increased from 102 For CTA-HF, Pgross increased from 102 W at 4 L/min to 128 W at 10 L/min, indicating W at 4 L/min to 128 W at 10 L/min, indicating a 1.25-fold increase with increasing QDS, a 1.25-fold increase with increasing QDS, while CE increased rapidly from 43 to 63 W while CE increased rapidly from 43 to 63 W (1.46-fold increase). Thus, Pnet of the module (1.46-fold increase). Thus, Pnet of the module increased from 58 W at 4 L/min and reached a maximum of 72 W at 6 L/min and almost a constant value of 71 W. From these results, the dependence of QFS for both modules on CE is higher than that of QDS. The rapid increase in CE with increasing Q for PA-SW is caused by the complex flow channel in the dense FS tricot spacer with high pressure resistance at the FS side mentioned above. For CTA-HF, small-diameter hollow fibers are used in the module. The evaluation indicates that as the flow rates of DS and FS increased for both modules, Pgross increased. However, the rapid increase in CE for PRO operation with increasing flow rates of DS and FS reduced Pnet. PA-SW demonstrates a maximum value of Pnet at lower DS and FS flow rates than the operating conditions of the PRO test in this study. For CTA-HF, the optimal value of QDS and QFS are 6.0 and 3.0 L/min, respectively, under the test conditions. The rapid increase in CE with increasing QFS is the primary cause of the reduction in the net power of the Membranes 2021, 11, x FOR PEER REVIEW 17 of 21

increased from 58 W at 4 L/min and reached a maximum of 72 W at 6 L/min and almost a constant value of 71 W. From these results, the dependence of QFS for both modules on CE is higher than that of QDS. The rapid increase in CE with increasing QFS for PA-SW is caused by the complex flow channel in the dense tricot spacer with high pressure re- sistance at the FS side mentioned above. For CTA-HF, small-diameter hollow fibers are used in the module. The evaluation indicates that as the flow rates of DS and FS increased for both modules, Pgross increased. However, the rapid increase in CE for PRO operation with increasing flow rates of DS and FS reduced Pnet. PA-SW demonstrates a maximum Membranes 2021, 11, 177 value of Pnet at lower DS and FS flow rates than the operating conditions of17 the of 21 PRO test in this study. For CTA-HF, the optimal value of QDS and QFS are 6.0 and 3.0 L/min, respec- tively, under the test conditions. The rapid increase in CE with increasing QFS is the pri- PRO system, especiallymary cause for of CTA-HF. the reduction Hence, in athe flow net channelpower of structurethe PRO system, with a especially low pressure for CTA-HF. Hence, a flow channel structure with a low pressure drop is necessary to increase the PRO drop is necessary to increase the PRO power output. power output.

(a) (b)

(c) (d)

Figure 13. The gross power (Pgross), net power (Pnet), and pumping energy (PE) per a module as a function of flow rates of Figure 13. The gross power (Pgross), net power (Pnet), and pumping energy (PE) per a module as a DS and FS, using PA-SW (a) and (c) and CTA-HF (b) and (d). function of flow rates of DS and FS, using PA-SW (a) and (c) and CTA-HF (b) and (d). 3.2.4. Comparison of PRO Performance between Spiral-wound and Hollow-Fiber Mod- 3.2.4. Comparisonules of PRO Performance between Spiral-Wound and Hollow-Fiber Modules To compare the PRO module performances with different configurations, such as To comparespiral the PRO wound module (SW) performances and hollow fiber with (HF), different a comparison configurations, using volumetric-based such as spiral power wound (SW) andoutputs, hollow as fiber well (HF), as membrane a comparison area-based using power volumetric-based density, is needed power to design outputs, a full-scale as well as membranePRO plant. area-based Table 2 power shows density,the maximum is needed power to density design per a full-scale unit membrane PRO plant.area (𝑃𝐷 ), area Table2 shows the maximum power density per unit membrane area ( PDmax), maximum vol power density per unit volume (PDmax), test conditions, and module dimensions of the modules tested in this study and those in the literature. Kim et al. [37] evaluated the PRO performance of a prototype PRO SW module (M-SW) with an effective membrane area of 29 m2 of the TFC membrane. The dimensions of the module were 0.2 m (8 in.) in diameter −3 3 area and 1 m in length; hence, the module volume was estimated to be 31.4 × 10 m . PDmax 2 vol 3 was reported as 1.0 W/m ; thus, PDmax of the module was estimated as 0.92 kW/m , using 0.6 M NaCl as DS at 0.98 MPa of ∆P. In comparison with M-SW, PA-SW in this area 2 vol 3 study demonstrated 40% higher PDmax (1.40 W/m ) and 74% of PDmax (0.68 kW/m ). The vol smaller PDmax of PA-SW was due to approximately half of the packing density, owing to the smaller effective membrane area of PA-SW compared to M-SW. Higa and Yasukawa et al. [35,43] evaluated the PRO performance of a CTA-HF mod- ule (M-HF). The effective membrane area and volume of the module were 72 m2 and 9.27 × 10−3 m3, respectively. The packing density of M-HF was calculated as 7769 1/m, area the highest value in all the modules listed in Table2. PDmax of the module increased from 2 vol 0.14 at 0.5 M to 0.44 W/m at 0.9 M NaCl as DS, and PDmax changed from 1.09 at 0.5 M to 3.39 W/m2 at 0.9 M NaCl. The effective membrane area of M-HF was 72 m2, which is Membranes 2021, 11, 177 18 of 21

higher than that of CTA-HF. This is because the outer diameter and number of HF using M-HF were 160 µm and 214,000, respectively, while those of CTA-HF were 170 µm and 187000. Hence, the packing density of the former is higher than that of CTA-HF. In a comparison of the power output between the two HF modules, CTA-HF shows approxi- area vol mately three times higher PDmax and PDmax than M-HF. This is owing to higher A value (0.65 × 10−12 [m s−1 Pa−1]) of the former than that of the latter (0.25 × 10−12 [m s−1 Pa−1]) primarily because of the thinner membrane of CTA-HF (40 µm) compared to that of M-HF (50 µm) [35]. In a comparison of the power output between modules with different configurations, area PDmax of PA-SW was 2.03-fold higher at 0.6 M NaCl and 1.16-fold higher at 1.2 M NaCl than that of CTA-HF due to the higher water flux of PA-SW. Conversely, in a comparison vol using the volumetric-based power output, CTA-SW showed 6.87-fold higher PDmax at vol 0.6 M NaCl than PA-SW did. The difference in the PDmax between the two modules vol increased with increasing DS concentration, and CTA-HF had 12.0-fold higher PDmax at 1.2 M NaCl as DS. The higher volumetric-based power density of CTA-HF is due to its higher packing density than that of PA-SW. Moreover, at high DS concentrations, the lower B value of the CTA-HF reduces concentrative ICP due to DS leakage, and the lower water permeate per unit membrane area of HF membranes reduces the effect of dECP at the DS side on the PRO performance. To achieve a high volumetric-based power output for a spiral-wound membrane module, an increase in the packing density to optimize the FS flow channel structure is required. In addition, a low B value to reduce DS leakage and high DS flow rates to decrease dECP are needed to improve the PRO performance of an SW module at high DS concentrations. In this study, we compared the PRO performance between CTA-HF and PA-SW, using NaCl solutions of various concentrations as model DS. In a real PRO application, DS and FS will contain foulant substances, and there will be a difference in the effect of the foulants on the PRO performance between the two module configurations. For example, the more complex channel structure of the U fashion at the FS side of PA-SW will have a greater impact on the long-term stability than CTA-HF. Additionally, DS and FS contain not only monovalent ions such as Na+ and Cl- ions but also bivalent ions such as Mg2+, Ca2+, and 2- SO4 ions. The difference in the reverse flux between the monovalent ions and the bivalent ions will affect the FS up-concentration on the PRO performance and also the difference in the membrane fouling by forming aggregates between the bivalent ions and foulants in FS, such as humic acid [49].

Table 2. Comparison between the dimensions and power densities of PRO modules from the literature and the current study.

S V β DS(M ∆P at PD PDarea PDvol Module Type m m max max max Reference (m2) (m3) × 10−3 (1/m) NaCl) (M Pa) (W/m2) (kW/m3) M-SW SW 29 31.4 923 0.6 0.98 1.0 0.92 [37] 0.5 0.94 0.14 1.09 [35] 0.6 1.1 0.17 1.32 [43] M-HF HF 72 9.27 7769 0.8 1.7 0.35 2.74 [43] 0.9 1.8 0.44 3.39 [43] 0.6 1.2 1.40 0.68 PA-SW SW 15.3 31.4 487 0.8 1.2 1.64 0.80 This study 1.2 1.6 2.33 1.13 0.5 1.6 0.47 3.21 0.6 1.2 0.69 4.67 CTA-HF HF 67 9.92 6770 This study 0.8 2.0 1.09 7.36 1.2 3.0 2.01 13.61 M = module, FS = tap water, SW = spiral wound, HF = hollow fiber. Membranes 2021, 11, 177 19 of 21

4. Conclusions In this study, permeate water flux in PRO mode, using two pilot-scale PRO mem- brane modules with different configurations, a five-inch cellulose triacetate hollow-fiber membrane module (CTA-HF) and eight-inch polyamide spiral-wound membrane modules (PA-SW), was measured by changing the DS concentration, the applied hydrostatic pressure difference, and the flow rates of the DS and FS, to obtain the optimum operating conditions in the PRO configuration. The following conclusions were drawn from this study: The water permeability of PA-SW was approximately seven times higher than that of CTA-HF. In comparison, the salt permeability of PA-SW was approximately 13 times higher than that of CTA-HF, indicating that the CTA-HF membrane will have a lower DS leakage than PA-SW. An FS flow rate of more than 5 L/min is sufficient for the two modules to mitigate the effect of FS up-concentration on PRO performance. However, 10 L/min of DS flow rate cannot make the dilution effect negligible, especially at high DS concentrations (CDS). PA-SW showed a significant deviation between the theoretical and effective osmotic pressure differences obtained from the water flux vs. the applied hydrostatic pressure curves in the PRO test, especially at high CDSs. In contrast, the CTA-HF showed a small deviation between the two values. The FS flow rate’s (QFS) dependence for both modules on the pumping energy for PRO operation is higher than that for DS flow rate. The rapid increase in the pumping energy with increasing QFS is due to the complex U-shaped flow channel in the dense tricot spacer at the FS side for PA-SW and the small diameter of hollow fibers used in CTA-HF. area The maximum power density per unit membrane area (PDmax) of PA-SW was 2.03-fold higher at 0.6 M NaCl and 1.16-fold higher at 1.2 M NaCl than that of CTA-HF, owing to the higher water flux of PA-SW. Conversely, in a comparison using the maximum power vol vol density per unit volume (PDmax), CTA-SW showed a 6.87-fold higher PDmax at 0.6 M. vol The difference in the PDmax between the two modules increased with the increasing DS vol concentration, and CTA-HF had 12.0-fold higher PDmax at 1.2 M NaCl as DS than PA-SW did. This is owing to the high packing density of the CTA-HF.

Author Contributions: Conceptualization, M.H.; methodology, M.H.; investigation, Y.K. and M.H.; data curation, Y.I. and M.S.; writing—original draft preparation, Y.K.; writing—review and editing, N.J. and M.H.; supervision, M.H.; project administration, M.H. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Japan Society for the Promotion of Science (JSPS) through the “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program)”, initiated by the Council for Science and Technology Policy (CSTP). The authors appreciate the support from NEDO for the Mega-Ton Water System project. Conflicts of Interest: The authors declare no conflict of interest.

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