Renewable energy powered membrane technology: Impact of solar irradiance fluctuation on direct osmotic backwash

Journal of Membrane Science xxx (xxxx) xxx

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Renewable energy powered membrane technology: Impact of solar irradiance fluctuation on direct osmotic backwash €fer * Yang-Hui Cai, Andrea Iris Scha Membrane Technology Department, Institute of Functional Interfaces (IFG-MT), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany

A R T I C L E I N F O

A B S T R A C T

Keywords: Decentralized water treatment Solar energy fluctuation Concentration polarization Periodic solar irradiance tests Osmotic backwash

Direct osmotic backwash (OB) during solar energy fluctuations is an important observation in directly coupled, battery-less solar energy powered nanofiltration/reverse osmosis (NF/RO) membrane systems. OB occurs spontaneously and has the potential to induce membrane self-cleaning. The impact of controlled solar irradiance fluctuation conditions, feed water salinity (1–10 g/L) and membrane type (NF membrane NF270, RO membrane BW30) on OB were investigated by periodic step-response tests. The objective of this research was to investigate the OB mechanism under solar irradiance fluctuations. The results show that solar energy fluctuations lead to a change of filtration hydrodynamic conditions (feed flow rate, applied pressure), operating time, and permissible backwash time. This results in variations of OB driving force which is determined by the osmotic pressure dif­ ference during filtration. High feed water salinity and solar irradiance before fluctuation enhance the OB process, while relative high solar irradiance during fluctuation weakens the OB process. Furthermore, the BW30 mem­ brane shows a higher OB flow rate and less accumulated volume than NF270. This was attributed to the higher salt retention of BW30 resulting in a larger backwash driving force and much lower flux of BW30 which in turn caused a thinner CP layer. These findings indicate that solar irradiance fluctuation conditions may potentially delay fouling and hence enhance the reliability and robustness of battery-less photovoltaic-membrane systems.

1. Introduction Directly coupled renewable energy (RE) with nanofiltration/reverse osmosis (NF/RO) membrane desalination technology is a promising decentralized solution to water supply [1,2]. This technology uses a synergy of water and energy in remote regions, where electricity is typically unavailable or unreliable. Direct coupling without energy storage promises to increase system robustness, simplicity, efficiency and saves expensive infrastructure [3,4]. In short term research trials, the good performance battery-less RE- NF/RO system performance and feasibility have been demonstrated [5–7]. Typical solar irradiance fluctuation over the course of a day in Karlsruhe (Germany) and Mdori (Tanzania) is shown in Fig. 1. Fluctu­ ating energy in such directly coupled systems causes variation of hy­ drodynamic conditions (feed flow rate, applied pressure), which leads to a temporary permeate quality decline, as small volume water was pro­ duced during cloudy periods. However, the cumulative permeate quality normally meets the World Health Organization (WHO) guideline [8]. When high solar irradiance (Ihigh) drops to a low level (Ilow), applied

pressure of the system is less than the osmotic pressure of the feed so­ lution, resulting in a reverse driving force that drives osmotic backwash (OB). OB is the phenomenon where water flows from the permeate side to the feed side due to an osmotic pressure gradient caused by differences in salt concentration [9]. This process can occur either by increasing the permeate pressure to a level that allows backflow, or by reducing operating pressure to less than the osmotic pressure of the feed solution, which is likely to occur naturally in a battery-less RE-membrane system [9]. OB may have two consequences; the first is the potenital delami­ nation of the active layer of thin film composite membranes may occur when the permeate backpressure (permeate static pressure > feed static pressure) is greater than 0.3 bar. This typically leads to the loss of salt retention [10–12]. In this study, there is no additional permeate pres­ sure applied to cause the OB process. Therefore, delamination of the membranes is less likely due to the absence of a permeate backpressure. The second possible consequence is a desired disturbance of a boundary layer where concentration polarization (CP) and fouling/scaling may occur [9,13]. This suggests a RE-membrane system may either

* Corresponding author. E-mail address: [email protected] (A.I. Sch€ afer). https://doi.org/10.1016/j.memsci.2019.117666 Received 23 July 2019; Received in revised form 12 November 2019; Accepted 13 November 2019 Available online 15 November 2019 0376-7388/© 2019 Published by Elsevier B.V.

Please cite this article as: Yang-Hui Cai, Andrea Iris Schäfer, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.117666

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Fig. 1. Solar irradiance fluctuations of a real solar day in Karlsruhe (Germany) in May, 2016 and Mdori (Tanzania) in February 2014 [8].

Fig. 2. Schematic of the reverse osmosis process (A) and two periods of OB process: (B) dilution of CP layer and (C) dilution of bulk by permeate.

experience a long term degradation of performance or a benefit from operation with a directly coupled fluctuating energy source [14,15]. The OB phenomenon was first observed and developed in 1981 to clean fouled cellulose acetate RO membranes by Spiegler and Macleish [16]. In 1997, Rolf and Eckehard patented it as a “suck back effect” with the potential to exploit OB for self-cleaning of membranes in the second stage of NF/RO membrane system operation [17,18]. A simple pipe was designed to make use of the permeate from the first stage for osmotic backwashing the second stage to save the cost of the additional reser­ voir. The suck back effect takes place in NF and RO plants when the pump is halted over a period of several minutes and OB has been shown to disrupt fouling in such situations [17,18]. Two distinct periods of OB flow rate vs. time were found by Sagiv and Semiat [9]. The schematic of the RO process and two periods of the OB process are shown in Fig. 2. When the applied pressure is higher than the osmotic pressure of feed solution, desalination/filtration occurs, and a CP layer is formed. When the applied pressure is lower than the feed osmotic pressure or drops altogether, two periods of OB occur. The backflow of permeate is, particularly in full scale systems, expected to be dependent on the CP layer profile on the membrane surface. During the OB period 1 (OB1), the backwash flow rate is the highest at the begin­ ning and declines sharply with time due to the quick dilution of the CP layer until the entire CP layer is removed, which reduces the driving force. More permeate is expected to backflow at the module outlet (brine) compared to the inlet (feed) area due to enhanced CP layer in the brine outlet area, as shown in Fig. 2B. In the second period (OB2), the driving force slowly declines due to bulk dilution (feed side), until it levels off [9]. Thus, the nature of the CP boundary layer and the initial driving force, which is determined by the salt concentration difference across the membrane, are key for the OB process. As suggested in Sagiv and Semiat’s work [9], the OB1 period plays an important role in the OB process, which should be controlled by limiting the backwash frequency and volume to avoid excessive loss of clean water for OB. The possible

solution to limit the backwash volume is to place a control valve on permeate side and close this valve once the OB1 period is completed, while the frequency control requires short term energy buffering such as supercapacitors. The main characteristics of OB are (i) osmotic backwash flow rate (OBQ), (ii) accumulated backwash volume (ABV) and (iii) effective backwash time (EBT). These are mainly dependent on the backwash driving force and the CP boundary layer on the membrane surface [19]. To allow the OB process to complete, it is important that an adequate permeate volume is available [9]. The parameters that affect OB are the parameters influencing the CP layer and the driving force. Those include system hydraulic conditions (such as applied pressure, feed flow rate, operating time and permissible backwash time), feed water salinity and membrane characteristics (salt retention, permeability) [19–21]. In several studies, Semiat et al. [9,19, 20] have evaluated the impact of those RO feed variables (feed salt concentration, applied pressure and feed flow rate) on the OBQ and ABV with osmotic pressure difference as the only driving force and no additional pressure on the permeate side. It was found that high feed concentration increased initial OBQ and ABV, and increased the initial flux drop. A high driving force was attributed to high feed concentration and yielded a high initial OBQ and reaches a low concentration on the membrane surface earlier than a low initial driving force. Consequently, a lower driving force later yields a low later OBQ. The effects of the applied pressure (40–60 bar) on OB were also examined [19]. Results showed that an increase of ABV profiles with an increase of applied pressure occurred due to a greater boundary layer thickness under higher applied pressure. ABV was inversely proportional to the feed flow rate (0.2–0.6 m3/h), which was probably due to the thinner CP layers with increased velocity. Compared to the obvious effect of feed con­ centration, the applied pressure and feed flow rate influence OB to a lesser extent. In the case of PV-membrane system without energy storage, the 2

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are naturally more difficult to implement in decentralized systems. OB has been investigated as a potential technique for fouling control and cleaning of NF and RO in seawater desalination, industrial water pro­ duction, and wastewater reclamation [17,26–30]. Such a cleaning mechanism during intermittent operation in a direct photovoltaic (PV)-RO system was also proposed by Freire-Gormaly and Bilton [15] to explain membrane permeability recovered at the start of each day of filtration. Since OB occurs simultaneously and naturally during the solar energy fluctuation [14], it is worth studying OB as a potential membrane self-cleaning method in the battery-less PV-NF/RO system. In a directly coupled renewable energy system operation, solar irradiation variation through passing clouds can result in a non-steady feed flow and applied pressure. This results in OB. Such fluctuation effects on membrane sys­ tem performance for brackish water (total dissolved solids (TDS) � 10 g/L) treatment have been investigated systematically for both solar and wind energy [14,31]. However, even though OB was observed, the impact of the OB process on system operation and fouling during solar energy fluctuation has yet to be investigated and quantified [32]. This article is intended to carefully examine OB mechanisms before exam­ ining the effect of solar energy fluctuation on scaling and fouling. The scaling experiments are the next part of this research with solar energy fluctuations. The same system as described in this paper will be used for scaling and fouling study. In this study, the impacts of controlled solar irradiance fluctuations on OB characteristics under zero additional permeate pressure condi­ tions (atmospheric pressure allowed on both permeate and concentrate sides) were investigated. The impact of feed water salinity, operational parameters of the system, and membrane type on OB characteristics were examined. Specifically, feed salt concentration in the brackish water range (TDS 1–10 g/L) and membrane type (BW30 and NF270) on OB were investigated. Solar irradiance fluctuation was implemented using periodic solar irradiance tests with a solar array simulator. The main objective of this study was to quantify OB performance parameters under solar irradiance fluctuations and understand the OB oparation for subsequent fouling control in RE-membrane systems.

Table 1 Comparison between UF backwash and OB of RO. 2

Flux (L/m h) Backwash flux (L/m2h) Specific backwash volume (L/m2)

UF [25]

ROa [19]

90–120 230–300 2.5–4

49–62 36–104 1–5

a Spiral wound seawater RO membrane system with a NaCl feed concentration of 18.3–52.9 g/L.

Fig. 3. Schematic of the solar energy powered cross-flow membrane filtration system and design of crossflow cell and permeate side in detail. The sensors include the pressure sensor (P), flow sensor (F) and conductivity sensor (C).

2. Materials and methods

status of OB is more complicated. Due to energy fluctuations, the different operating parameters of the system vary simultaneously. For instance, both applied pressure and feed flow rate will increase with an increase of solar irradiance [14]. The effect of such variations on OB is not clear and has not yet been investigated. CP is further affected by membrane type in addition to operating parameters [22] and this re­ quires investigation. OB results in the dilution of the CP layer and detachment of some foulants from the membrane surface [19]. As such, this process is comparable to permeate backwash in micro- and ultrafiltration (MF/UF), where particle deposits are washed away from the membrane surface and the flux is stable over extended periods [23,24]. The com­ parison of backwash parameters between UF and RO is shown in Table 1. For UF membranes, backwash flux is usually set at 230–300 L/m2h and 2.5–4 L/m2 specific backwash volume compared to a flux of 90–120 L/m2h during operation [25]. In a spiral wound seawater RO membrane system, Sagiv et al. [19] observed an initial OB flux of 36–104 L/m2h and an accumulated specific volume of 1–5 L/m2 back­ wash when the flux was 49–62 L/m2h and the feed NaCl concentration 18.3–52.9 g/L. OB can form a strong driving force to lift and sweep the foulant from the membrane surface, which will then be carried over to brine [13,26]. Thus, OB can reduce the amount of the accumulated foulants and hence potentially prevent fouling in an early stage of formation. This means that OB may delay the occurrence of fouling and reduce chemical cleaning requirements. While such investigations are the topic of future work, fouling normally needs to be detected early to ensure that the OB process will be most effective. When the flux cannot be recovered by OB, fouling control methods like chemical cleaning are required and these

2.1. PV-membrane cross-flow system set-up Fig. 3 shows the bench scale PV-membrane cross-flow system set-up. As shown in Fig. 3, the cross flow cell (MMS Membrane Systems, Switzerland) is composed of two stainless steel plates held together by eight screws. The dimensions of flow channel are 0.19 m in length, 0.025 m in width and 0.7 � 10-3 m in height with an active membrane area of 4.7 � 10-3 m2. No feed spacer was used to avoid interference with CP, while a porous stainless steel plate is placed on the permeate side in order to support the membrane. The diaphragm pump Hydra Cell P200 (Wanner Engineering, Germany) combined with a 370W DC motor (Baldor Electric, model VP3428D, Germany) was used to provide the operating pressure as a driving force for desalination. A chiller (LaudaBrinkmann, MC250, Germany) was used to maintain a stable feed water temperature of 21 � 1 � C. The pulsation damper (Speck Triplex Pumpen, MS 160C, Germany) was filled with 50–60% of the system pressure to maintain stable pressure conditions. Feed salt solution was pumped from a feed tank (maximum 10 L) to the membrane flow cell. The concentrate was recycled to the feed tank. The permeate was recycled to the feed tank when carrying out membrane compaction and permeability mea­ surement. When carrying out the periodic solar irradiance fluctuation OB tests (presented in section 2.3), the permeate was collected in the permeate container. A stainless-steel pressure relief valve (Swagelok Co., SS-4R3A, Germany) was installed for safety, in case the system pressure accidentally exceeded 23 bar. The applied pressure and feed flow rate are determined by the power from SAS and the opening of pressure control valve (Pump Engineering, Badger RC200, Germany) which can be controlled through LabVIEW (Version 2014, 32bit, 3

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Table 2 Tested membrane characteristics. Membrane

Active layer

Thickness of active layer (nm)

MWCO (Da)a

Salt retentiona

Permeability (L/m2h/bar)

NF270

Semiaromatic piperazine –based polyamide Fully aromatic polyamide

57 � 2 [33]

200–400

MgSO4 � 97% NaCl 40–60%

16.2-19 [34, 35]

233 � 88 [36]

~ 100

NaCl 99.5%

2.45–4.4 [35, 37,38]

BW30

a From supplier, the retention of MgSO4 was measured using a feed concen­ tration of 2 g/L, 4.8 bar, 25 � C and 15% recovery; the retention of NaCl was measured using a feed concentration of 2 g/L, 15.5 bar, 25 � C and 15% recovery.

Fig. 6. The OB flow rate of different membranes (BW30 and NF270) during the first 60 s. C(NaCl) ¼ 5 g/L, Ihigh¼¼ 600 W/m2, Ilow ¼ 0 W/m2, cycle time ¼ 10 min, thigh: tlow ¼ 1:1 and T ¼ 21 � 1 � C. Table 3 OB characteristics, salt retention, CP modulus and Δπ of BW30 and NF270 from the experiments in Fig. 6. Membranes

ABV (mL)

OBQ1 (mL/s)

EBT (s)

Salt retention (%)

CP modulus

Δπ(bar)

BW30 NF270

1.91 3.80

0.21 0.17

23 22

76 40

1.01 1.07

3.23 1.96

stainless steel turbine wheel flow meters (Kobold Messring, Model FELLMX, � 2% accuracy, Germany). Permeate flow rate and OBQ were measured bi-directionally by a liquid flow sensor (Sensirion, SLS-1500, �5% accuracy, Switzerland). An electrical conductivity (EC) sensor (Jumo, Blackline CR-GT 00430770, Germany) in the feed tank was used to monitor the conductivity and temperature of feed solution. Another EC sensor (Jumo, Blackline CR-EC 00418069, Germany) on the permeate side was used to monitor the permeate conductivity. All data from sensors were recorded continuously in an interval of 1 s by a data acquisition platform (National Instruments, cDAQ chassis 9184) and processed by using LabVIEW 2014 (32bit, National Instruments, Ger­ many). A solar array simulator (SAS, Chroma, 62050H–600S) was used to simulate different levels of solar irradiance (up to 1200 W/m2) by varying power from two 150 W PV modules (BP Solar, model BP 3150, 1.6 m � 0.79 m) which were employed in the real field work with a 400 module (see Richards et al. [5]). Such a SAS setting would produce similar phase, voltage and current output to drive the DC pump as real PV modules. In this crossflow system (Fig. 3), the feed flow rate was up to 200 L/h (namely feed flow velocity up to 3.3 m/s) when the DC pump was supplied by the SAS (simulating full power output of two PV modules) with up to 1200 W/m2 solar irradiance at a specific control valve opening (set point). This results in a Reynolds number (Re) of up to 5000. Since the feed velocity and Re number in this setting were significantly higher than normal membrane module operation resulting from the high power output (300 W) of the full area of two PV modules, the area of PV modules was reduced by two-thirds (namely the power output was reduced by two-thirds, 100 W). This resulted in a more realistic velocity for this small membrane crossflow cell (0.1–0.6 m/s) and low Re number (159–881). An overview of resulting feed flow ve­ locities is provided in Table 4 in the Supporting Information. The pur­ pose is to investigate the effect of power variation which results in different velocity on OB process. The detailed operation is shown in section 2.3. The OB was implemented in the form of a stainless-steel loop that provides a suitable permeate volume (2.1 m length, 1/8”, volume 6.8

Fig. 4. Controlled solar irradiance fluctuation (Ihigh ¼ 100–1200 W/m2, Ilow ¼ 0 and 250 W/m2, cycle time ¼ 10 min, thigh: tlow ¼ 1 : 1, the number of cycle is 3, pump stops when solar irradiance �100 W/m2).

Fig. 5. Typical OB flow rate curve (mL/s) as a function of time (the first minute). Experimental conditions: C(NaCl) ¼ 5 g/L, Ihigh ¼ 1000 W/m2, Ilow ¼ 0 W/m2, cycle time ¼ 10 min, thigh: tlow ¼ 1:1, and T ¼ 21 � 1 � C.

National Instruments, Germany). Before the tests, the opening of the valve will be fixed by LabVIEW, so that the pressure and feed flow rate is controlled by the PV fluctuations during the tests at fixed valve set point. The detailed operation is shown in section 2.3. The system was equipped with two pressure sensors (Wika Anlexander Wiegand, Type A-10, �0.5% accuracy, Germany) to monitor feed and concentrate pressure. The feed flow rate was measured by 4

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Fig. 7. Effect of Ihigh on system hydrodynamics and membrane performance: (A) applied pressure, (B) feed flow rate, (C) permeate flux, and (D) observed salt retention for NF270 and BW30. C(NaCl) ¼ 5 g/L, Ilow ¼ 0 W/m2, cycle time ¼ 10 min, thigh: tlow ¼ 1:1, T ¼ 21 � 1 � C, and system supplied by full solar power output of two PV modules (high feed velocity).

mL). The total permeates side volume was estimated to be 10 mL including permeate side pipe, loop and the dead volume of the con­ ductivity sensor cell. Flow sensor, loop, and EC sensor were placed in the same horizontal line to avoid a hydraulic pressure difference. The permeate volume of 10 mL was determined based on membrane area and ABV in previous research [19]. No noticeable effect of stainless-steel loop length on OB process was observed when varying lengths from 0.5 to 2.1 m. This result is presented in the Supporting Information.

filtration at 10 bar for 30 min. Fourthly, DI water was replaced by 5 L specific concentration (1, 2.5, 5, 7.5, 10 g/L) NaCl (VWR chemicals, purity � 99.9%, Germany) solution as the synthetic brackish water with 10 mM NaHCO3 (Bernd Kraft, purity �99.7%, Germany) as the back­ ground solution in the feed tank. The feed water pH was 7.0 � 0.5, and pH was not adjusted. In the case of simulating full power output of two PV modules, the opening of the concentrate side pressure control valve was set to about 32% to achieve a 10 bar applied pressure under 1000 W/m2. In the case of operating at a third of the power output of the PV modules, the opening of the valve was set at about 23% to achieve the same pressure. The purpose of this ‘set point’ is to achieve 10 bar applied pressure for the system when solar irradiance is at the peak perfor­ mance. Finally, a periodic solar irradiance fluctuation step-response test was carried out. Four solar irradiance fluctuation characteristics were investigated in this study: high-level solar irradiance before dropping Ihigh, low-level solar irradiance after dropping Ilow, time of high-level solar irradiance thigh (namely system operating time), time of low-level solar irradiance tlow (namely permissible backwash time). Cycle time is defined by the total time thigh plus tlow. The various Ihigh increased gradually from small to extreme (100–1200 W/m2) as an experiment in order to determine the effect of increasing solar irradiance dropping to zero (Ilow ¼ 0 W/m2) on the OB process under a 10 min cycle time period. A 1:1 thigh: tlow ratio was chosen to ensure that the equilibrium of both CP and the OB process had been reached. The Ioff was increased from zero to 250 W/m2 as an experiment in order to determine the effect of cloud cover (600 W/m2 dropping to less than 250 W/m2) in a 10 min cycle time period and 1:1 thigh: tlow ratio. The effect of rapid or slow cyclic variations in solar irradiance (cycle time) on OB was investigated by varying from 0.5 to 20 min under Ihigh level (600 W/m2) and Ilow level (zero W/m2) and thigh: tlow ¼ 1:1. The effect of different ratios of thigh and tlow (1:4, 2:3, 1:1, 3:2, 4:1) namely operating time and permissible backwash time ratio on the OB process were investigated. An example of solar irradiance variation during such a test is presented in Fig. 4. As shown in Fig. 4, Ihigh is 100–1200 W/m2, Ilow is zero and 250 W/m2, cycle time is 10 min, and

2.2. Membrane choice Flat sheet membranes of high salt retention/low permeability (BW30) and low salt retention/high permeability (NF270) (Dow Chemical, Filmtec™) were selected to cover a wide range of membrane performance. The characteristics of tested membranes are shown in Table 2. 2.3. Experimental protocol Each experiment included five different steps, namely membrane pre-conditioning, compaction, pure water flux measurement, setting the feed pressure and a periodic solar irradiance step-response test. Firstly, a new membrane coupon was soaked in a 10 mM NaCl (VWR chemicals, purity � 99.9%) solution for 1 h prior to use. The NaCl solution can enhance the opening of pores and the swelling of the active layer due to the interaction of electrolytes with the polyamide layer [39]. Secondly, after the PV-membrane system (Fig. 3) was cleaned by deionized (DI) water (conductivity < 1 μs cm-1, pH ¼ 7 � 0.4; used throughout this study unless otherwise stated), the membrane coupon was placed inside the cross-flow cell and then compacted by recycling 5 L DI water in the feed tank at 10 bar for 1 h. It was controlled by setting a constant power output (60 V, 1.5A) from the SAS and fixing the opening of the pressure control valve (about 25–26%). The chiller was kept running to maintain the feed water at room temperature 21 � 1 � C during the experiment. Thirdly, pure water flux (L/m2h/bar) was measured at a stable state of 5

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the ratio thigh: tlow is 1:1. The pump is not operating when the system is supplied by the full power output of two PV modules and Ihigh � 100 W/ m2, while the pump is not running when the system is supplied by one third of the power output of PV modules and Ihigh < 400 W/m2. In all tests, the solar irradiance fluctuation data and output of power setting (69 V þ 300 W or 69 V þ 100 W) were input to the SAS “Dynamic MPPT Test” interface. Three cycles were carried out in repeat to ensure the results were reproducible. Feed water temperature was maintained at 21 � 1 � C (room temperature) by the chiller. Additionally, the effect of feed salt concentration (1, 2.5, 5, 7.5, 10 g/L) on the OB process were investigated under the solar irradiance conditions Ilow ¼ 0 W/m2, Ihigh ¼ 600 W/m2, cycle time ¼ 10 min, the thigh: tlow ¼ 1:1. 2.4. Determination of osmotic backwash characteristics In this study, the main OB characteristics are initial OBQ, ABV and EBT [9,19,20]. The OBQ was measured by a permeate flow sensor each second. The typical OBQ (mL/s) as a function of time (s) is shown in Fig. 5. OBQ decreased with time until zero mL/s, and OBQ in the first second (OBQ1) is considered as main OB characteristic in this study because it is related to initial OB driving force according to Sagiv et al. [19]. ABV (mL) is another important characteristic, because it is related to CP layer thickness/volume, which is also indicated by Sagiv et al. [19]. In this case, it was calculated by integrating the area of OBQ (mL/s) over time (s) in Software OriginPro 2017 (64-bit, SR2, OriginLab Corporation). The integration tool in OriginPro 2017 performs numer­ ical integration on the active data plot using the trapezoidal rule [40]. The effective backwash time (EBT) is the time of OBQ dropping from the beginning to zero, which is related to the CP layer and driving force. In the case of Fig. 5, the OBQ1 is 0.234 mL/s, the EBT is about 21 s and the ABV is 2.026 mL. 2.5. Concentration polarization (CP) modulus and OB initial driving force calculation Salt polarization occurs on the membrane surface during an effective membrane filtration process due to water permeation and salt rejection [22]. The ratio between salt concentration at the membrane surface and in the bulk, referred to as CP modulus, is used to represent the extent of CP. It can be calculated as described in Eq. (1) [22]: 1 0 Fig. 8. Effect of Ihigh on OB characteristics: (A) ABV, (B) OBQ1 and (C) EBT for NF270 and BW30. C(NaCl) ¼ 5 g/L, Ilow ¼ 0 W/m2, cycle time ¼ 10 min, thigh: tlow ¼ 1:1, T ¼ 21 � 1 � C, and system supplied by full solar power output of two PV modules (high feed velocity) and one third power output of two PV modules (low feed velocity).

CP ¼

Jv Cm B Jv ¼ ð1–Robs Þ þ ​ Robs ​ ekm ¼ 1 þ Robs @ekm Cb

C 1A

(1)

where Cm is the salt concentration at the membrane surface (g/L), Cb is the salt concentration in the bulk (feed NaCl concentration, g/L), Robs is the observed salt retention of membrane, Jv is permeate volumetric flux

Fig. 9. (A) CP modulus and (B) driving force Δπ of NF270 and BW30 as a function of Ihigh. Conditions are the same as in Fig. 8. 6

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Fig. 10. The feed flow rate (A), applied pressure (B), permeate flux (C), OB characteristics (D) ABV, (E) OBQ1, and (F) EBT during OB period as a function of different Ilow level (0, 100, 150, 200, 250 W/m2) dropping from Ihigh (600 W/m2). BW30 membrane, C(NaCl) ¼ 5 g/L, cycle time ¼ 10 min, thigh: tlow ¼ 1:1, and T ¼ 21 � 1 � C.

(m/s), and km is the mass transfer coefficient (m/s). The equations to calculate those parameters are shown in Appendix A. According to Eq. (1), the CP modulus is dependent on salt retention, permeate flux and the hydrodynamics of the system. It is worth noting that the CP modulus reflects the conditions right before the applied pressure drops to zero. The driving force for OB is the net force across membrane when applied pressure is lower than osmotic pressure. To some extent, the osmotic pressure difference, Δπ, between membrane surface (πm ) and permeate (πp ) can represent the initial driving force for OB just after the applied pressure drops to zero, as it is the only determining factor in this study. Δπ is calculated by Eq. (2): 3 2 � iRT Jv 7 iRT iRT 6 ½Cb * ðCP 1 þ Robs Þ� ¼ Δπ ¼ π m π p ¼ Cm Cp ¼ 4Cb * Robs ​ ekm 5 M M M (2) where i is the Van’t Hoff factor (i ¼ 2 for NaCl in this study), R is the ideal gas constant (8.314 � 103 L⋅N⋅m-2⋅K 1 mol 1), T is the absolute temperature (K, ¼ 273.15 þ t (� C)), M is molecular weight of NaCl (58.44 g/mol). According to Eq. (2), the high feed concentration, permeate flux, high salt retention and low mass transfer coefficient will result in a high initial driving force for OB.

7

3. Results and discussion This section will analyze the main factors affecting OB in a batteryless PV-NF/RO system: membrane type, solar irradiance fluctuation characteristics and feed water salinity. 3.1. Impact of membrane type on osmotic backwash under solar irradiance fluctuation To investigate the impact of different membranes on OB performance under solar irradiance fluctuation, steps in the solar irradiance from 600 W/m2 at 5 min operating period to zero W/m2 (pump off) at 5 min permissible backwash period with BW30 and NF270 were applied to the system, respectively. The research question addressed here is; how does membrane type affect OB performance, particularly in terms of OB flow rate changing with time, accumulated backwash volume (ABV), initial OB flow rate (OBQ1), effective backwash time (EBT), CP modulus and initial driving force? The OB performance with BW30 and NF270 under identical solar fluctuation conditions is presented in Fig. 6. According to Eq. (1) and Eq. (2), the CP layer and Δπ is dependent on hydrodynamics and membrane characteristics. Thus, it is expected that BW30 has different OB performance from NF270. Fig. 6 shows different OB flow rate behaviors of two membranes: BW30 had a higher OBQ but a very short backwash time in the first backwash period followed by a

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applied pressure, which are determined and affected by the solar irra­ diance fluctuations, will be discussed. 3.2. Effects of high-level solar irradiance before fluctuation (Ihigh ) To investigate the effect of Ihigh irradiance on OB, steps in the solar irradiance from high-level (100–1200 W/m2) at 5 min operating periods to low-level solar irradiance (zero W/m2) at 5 min periods were applied to the system. The system was supplied by simulating full power output from two PV modules (high feed velocity) and one third of full power output (low feed velocity), respectively. The research questions addressed here are, i) how does a gradual increasing Ihigh affect the OB process? and ii) how does different solar power output setting affect the OB process? When increasing Ihigh, the system hydrodynamics change simulta­ neously which affects the CP layer and the driving force for back­ washing. Increasing feed flow rate would weaken CP and reduce Δπ via increasing the mass transfer coefficient. An increase of applied pressure enhances CP and Δπ due to an increase in permeate flow and salt retention according to Eqs. (1) and (2). The overall effects of hydrody­ namics and membrane performance on CP and driving force on back­ washing are not clear. Therefore, this is investigated in this section. Firstly, the hydrodynamics of system (feed flow rate and applied pres­ sure) and membrane performance (permeate flux and salt retention) under full power output setting as a function of Ihigh are presented in Fig. 7A–D. As shown in Fig. 7A–B, the same feed flow rate and applied pressure increased simultaneously with an increase of Ihigh for both membrane as expected. This indicates that the hydrodynamic conditions were similar when different membranes were applied. However, Fig. 7C–D shows that the permeate flux and salt retention of two membranes increased differently under similar hydrodynamics, which were dependent on membranes characteristics (NF270 has much higher permeability while BW30 has higher salt retention). There is a sharp transition in permeate flux during 200–400 W/m2, because the applied pressure increased with a sudden increase of solar power. It is worth noting here that there is also a sharp increase in salt retention between 200 and 400 W/m2, followed by a slow increase. A similar observation was previously reported by Richards et al. [14]. The increase of salt retention can be explained by an increase of permeate flux via increase of pressure [22,42,43]. The results of membrane performance under one third power output setting were not shown here, due to a very similar performance except for the low feed velocity (0.1–0.6 m/s) during 400–1200 W/m2. Secondly, all OB characteristics as a function of Ihigh under full power output setting and one third power output setting are presented in Fig. 8. As shown in Fig. 8A–C, all OB characteristics increased with an increase of Ihigh for both membranes. ABV and OBQ1 in particular increased sharply at the beginning, followed by a slower increase. This means that an increase in Ihigh enhances the OB process, especially when Ihigh is between zero and 400 W/m2. It should be noted that the low velocity caused by low power output setting led to a higher ABV, a higher OBQ1 and especially a longer EBT than the full power output setting. This means the low power output setting is more suitable for this cross-flow system to reflect more realistic membrane system hydrodynamics [44]. Besides, Fig. 8 suggests that CP layer and the driving force are probably enhanced with Ihigh. To confirm that, CP modulus and Δπ (initial driving force) were calculated and shown in Fig. 9A–B. Fig. 9A shows that CP modulus of NF270 is higher and more sensitive to an increase of Ihigh after 300 W/m2, compared to the BW30 mem­ brane. This means that the CP layer of the NF270 is more enhanced by Ihigh than that of the BW30 membrane. The effect of Ihigh on CP modulus of BW30 at high feed velocity is negligible, since the CP valve was in a very small range 1.001–1.02. In contrast, the increase of Ihigh led to obvious increase of CP modulus of both membranes under low velocity, which confirms that the Ihigh and low feed velocity enhance CP [19]. On the other hand, Fig. 9B shows a rapid increase in Δπ for both membranes

Fig. 11. OB characteristics (A) ABV, (B) OBQ1, (C) EBT as a function of a wide range of cycle time (0.5, 1, 2, 3, 4, 5, 10, 15 and 20 min) with different membranes: NF270 and BW30. Experimental conditions: C(NaCl) ¼ 5 g/L, Ihigh is 600 W/m2, Ilow ¼ 0 W/m2, thigh: tlow ¼ 1:1, and T ¼ 21 � 1 � C.

sharp decrease, while the NF270 had a low OBQ and a longer backwash time in the first period. Table 3 shows that higher initial OBQ of BW30 compared to NF270 was due to the higher initial driving force. In this case, the feed solution concentration and hydrodynamic condition (as shown in Fig. 7) are the same. According to Eq. (2), the only reason why BW30 has higher initial driving force is that BW30 has a higher salt retention and induces a higher osmotic pressure difference than NF270, even though NF270 has higher permeate flux. However, NF270 had a higher ABV and longer OB time in the first period. This could be related to higher permeability and thicker CP layer of NF270, because more water is available to backflow and cause the high ABV according to Sagiv et al. [19]. The CP layer thickness of BW30 and NF270 has been investigated by Onorato et al. [41]. A computational fluid dynamics model was developed and the results showed that NF270 had thicker CP layer (30–40 μm) than BW30 (8–10 μm) under the identical solar irra­ diance and operation conditions. Overall, the results suggest that the high salt retention membrane causes a high initial OB flow rate, while the high permeability mem­ brane results in a high OB volume. In addition, the results confirm that selection of membrane is indeed important for the OB process. In next section, the impact of system hydrodynamics, namely feed flow rate and 8

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Journal of Membrane Science xxx (xxxx) xxx

Fig. 12. OB characteristics ABV (A and D), OBQ1 (B and E) and EBT (C and F) as function of thigh: tlow ratio (1:4, 2:3, 1:1, 3:2, 4:1) in different cycle time (1, 2 and 10 min) with different membranes: (A–C) NF270 and (D–F) BW30. C(NaCl) ¼ 5 g/L, Ihigh is 600 W/m2, Ilow ¼ 0 W/m2, thigh: tlow ¼ 1:1, and T ¼ 21 � 1 � C.

when Ihigh was between 200 and 400 W/m2, followed by a slower in­ crease. This is similar to the behaviors of ABV and OBQ1 (Fig. 8A–B), which means that the initial driving force determines the OB process. This confirms the main finding of Sagiv et al. [19] in that the initial driving force is the dominant RO parameter of the OB process. It is worth noting that the curve of Δπ (Fig. 9B) is very similar to the increasing curve of salt retention (Fig. 7D). This indicates that the increased salt retention is more critical for the OB process than the CP layer in this case. This finding may provide a new perspective to investigate the OB process, because pervious literature [9,19,20,45] overemphasized the importance of the CP layer somewhat. In summary, the OB process is most enhanced with increasing highlevel solar irradiance before fluctuation via enhanced CP and driving force, especially under low velocity. In the next section, the effect of lowlevel solar irradiance during fluctuation on OB will be discussed.

rate, applied pressure), permeate flux and OB characteristics after solar irradiance dropping from 600 W/m2 to low-level are shown in Fig. 10A–F. As shown in Fig. 10A–C, applied pressure and feed flow rate during the OB period increased from zero to 1 bar and zero to 35 L/h, respec­ tively, with an increase of Ilow. The flux remained zero because the applied pressure was lower than the osmotic pressure. Fig. 10D–F shows that when high-level solar irradiance drops to a low-level � 100 W/m2 (more cloud cover), the pump turns off due to lack of power and all OB characteristics are constant. When solar irradiance drops to a low-level above 100 W/m2 (less cloud cover), the pump continues to run and all OB characteristics decrease with increasing Ilow. This suggests that the increasing Ilow weakened the OB process and caused less backwash volume, lower initial OBQ and shorter EBT. This means that the driving force for OB was weakened. As shown in Fig. 10A, a high level of Ilow (100–250 W/m2) leads to a higher feed flow rate (up to 35 L/h) during the backwash period, which increases km and subsequently reduces the Δπ according to Eq. (2), thus weakening the OB process. This result confirms the finding of Sagiv and Semiat [20] that the OB process can be weakened with an increase of feed flow rate. When Ilow was above 100 W/m2, no OB process was observed when the same experiment was carried out with the NF270 membrane. This means the feed flow rate had a stronger disturbing effect on OB process with the more open

3.3. Effects of low-level solar irradiance during fluctuation (Ilow ) To investigate the effect of Ilow (cloud cover) on OB process, alter­ nating 5 min periods of high-level (600 W/m2) and 5 min periods of different low-level solar irradiance (0, 100, 150, 200, 250 W/m2) were applied to the system. The system was supplied by simulated full power output of PV modules. The results of system hydrodynamics (feed flow 9

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Journal of Membrane Science xxx (xxxx) xxx

NF270 membrane. In summary, the OB process is weakened with an increase in low-level solar irradiance during fluctuation due to an enhanced mass transfer coefficient caused by the increased feed flow rate. This suggests that the sudden less cloud cover weakens the OB process more than sudden more cloud cover. 3.4. Effects of cycle time To study the effect of rapid or slow cyclic variations in solar irradi­ ance (cycle time ¼ thigh þ tlow) on OB, steps in the solar irradiance from high-level (600 W/m2) dropping to Ilow level (zero W/m2, pump off) at periods ranging from 0.5 to 20 min cycle time were applied to the PVmembrane system. The system was supplied by simulated full power output of PV modules. In this case, all the ratio between thigh and tlow was set 1:1, which means that operating time (thigh) and permissible backwash time (tlow) are the same (a half of cycle time). The result of OB characteristics as a function of cycle time is presented in Fig. 11A–C. As shown in Fig. 11A–C, for both membranes, all OB characteristics increased sharply with an increase of cycle time in the first few minutes (0.5–4 min) and then increased very slowly or kept constant (5–20 min). This means that at shorter cycle times (below 4 min), the OB process is weakened; when the cycle time becomes longer, the OB process is enhanced or kept constant. At shorter cycle times (shorter thigh and tlow), two possibilities may occur: i) the observed salt retention does not reach steady-state in this short time (resulting in a higher permeate concen­ tration and filling the backwash loop with low concentration permeate requires more time), and ii) the OB process is forced to end earlier than it would naturally (due to a short tlow, for example below 20 s). Both possibilities can weaken the OB process. This means that the highfrequency solar irradiance is interfering with the OB process, and hence it should be avoided. This suggests that the use of supercapacitors to buffer high-frequency solar irradiance fluctuation would be beneficial [14]. In contrast, when the cycle time becomes longer, the observed salt retention achieves a steady-state and the OB process ends naturally so that the OB process can achieve equilibrium. Thus, the OB process is either enhanced or kept constant in this case, compared to the weakened effect of shorter cycle time. In summary, the results emphasize that the OB process is weakened in the case of high-frequency solar irradiance fluctuation, probably due to an unstable CP and salt retention status and not enough backwash time. The effect of operating time and permissible backwash time on the OB process will be discussed in next section.

Fig. 13. OB characteristics (A) ABV, (B) OBQ1 and (C) EBT as function of feed NaCl concentration (1, 2.5, 5, 7.5, 10 g/L) with different membranes (NF270 and BW30). Ilow ¼ 0 W/m2, Ihigh ¼ 600 W/m2, cycle time ¼ 10 min, the thigh: tlow ¼ 1:1, T ¼ 21 ± 1 � C and system was supplied by simulated full power output of PV modules.

3.5. Effects of the ratio between operation time and permissible backwash time The research questions addressed here are: i) how does the ratio between thigh and tlow affect the OB process and ii) which one (shorter

Fig. 14. (A) Cm and (B) initial driving force as functions of feed NaCl concentration. 10

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Journal of Membrane Science xxx (xxxx) xxx

thigh or tlow) makes a lesser contribution to the OB process. The steps in solar irradiance 600 W/m2 dropping to zero W/m2 for 1, 2 and 10 min cycle times with different thigh: tlow ratio (1:4, 2:3, 1:1, 3:2, 4:1) were applied into the system. The system was supplied by simulated full power output of PV modules. thigh is the time in which the observed salt retention can achieve a steady-state, and tlow is the time for the OB process. When this ratio is low, it means short thigh and long tlow. As discussed in section 3.4, when thigh is too short, the observed salt retention could not achieve a steady-state, which can weaken the OB process. If tlow is shorter than the effective backwash time, the OB is forced to end earlier, which also weakens the OB process. Therefore, it is expected that when the ratio is smaller or larger, the OB process will be weakened during a short cycle time, whereas the OB process is not affected significantly by the ratio during long cycle time. Results are presented in Fig. 12A–F. As shown in Fig. 12A–F, in the case of longer cycle times (10 min), OB characteristics were relatively constant, which can be attributed to the longer ton and toff allowing a stable salt retention status to form and the OB process to conclude fully. In the case of shorter cycle times (1 and 2 min), ABV and initial OBQ increased and then decreased from 1:4 to 4:1, which means both short ton and short toff weakened OB process. Particularly, ABV and EBT decreased sharply in the ratio of 4:1 while initial OBQ decreased sharply in a ratio of 1:4, which means that accumulated volume and effective backwash time are more sensitive to shorter permissible times, while the initial OB flow rate is more sensitive to shorter operating time. In summary, the OB process is enhanced in the case of less solar irradiance fluctuations (long permeate time and backwash time) and is weakened in sudden and short fluctuation.

the case of BW30 (Fig. 6). In summary, the results suggest that the OB process is enhanced by high feed water salinity, and the high retention membrane has a higher initial OB flow rate. 4. Conclusions In order to investigate the effects on OB in a bench scale battery-less photovoltaic-membrane system, systematic step-response tests were carried out over a wide range of fluctuating conditions and feed water salinities. Solar energy fluctuations lead to variations of system hydro­ dynamics, operating time thigh and permissible backwash time tlow, which influences the extent of OB process by affecting the driving force. More specifically, higher high-level solar irradiance before fluctuating Ihigh enhances the OB process due to the enhanced effect of driving force by applied pressure. In contrast, higher low-level solar irradiance during fluctuating Ilow weakens the OB process because the driving force was weakened by the increased mass transfer coefficient due to the increasing feed flow rate. When the cycle time of fluctuation was varied, poor OB performance was observed at shorter cycle times (higher fre­ quency fluctuation), while better OB performance was achieved at longer cycle times. This was due to the enhancement of the OB process by longer operating time and permissible backwash time. When the ratio between operating time and permissible backwash time was varied, poor OB performance was observed at sudden and short intervals. Those results show that longer operating times allow for a steady-state of salt retention, and a longer permissible backwash time allows the OB process to finish naturally. When the feed water salinity was increased, better OB performance was observed due to the enhanced driving force resulting from the higher feed water concentration. Compared to the high permeability membrane NF270, the high salt retention membrane BW30 causes higher initial OB flow rate but less accumulated volume. This was attributed to the higher salt retention of the BW30 membrane causing a larger backwash driving force and much lower flux of BW30, resulting in a thinner CP layer. As far as system design and operation under fluctuating conditions is concerned, there are several important messages. Firstly, the OB process during fluctuating operation indeed exists and is dependent on the system hydrodynamics of the fluctuating conditions. Secondly, this process may help to clean membranes and delay fouling and scaling, which warrants further research. Thirdly, the application of super­ capacitors for buffering sudden and short fluctuations for several mi­ nutes may be helpful for membrane self-cleaning via the OB process.

3.6. Impact of feed salt concentration on osmotic backwash process under fluctuation Feed salt concentration is an inherent variable in brackish water treatment. A membrane is typically chosen in terms of salt retention such that the water quality guideline is met. Higher than necessary salt retention normally results in higher energy consumption. A typical brackish water TDS ranges from about 1 to 10 g/L. This range was used in this study to determine the effect of the feed water salinity on the OB process under fluctuations. It should be noted that membranes with both low (NF270) or high (BW30) retention (Table 2) are chosen in this study irrespective of the fact if with a particular TDS the water quality guideline can be reached. The results of OB characteristics as a function of feed NaCl concen­ tration are presented in Fig. 13A–C. More backwash volume, higher backwash flow rate and longer backwash time were observed with an increase of feed NaCl concentration for both membranes in Fig. 13A–C. In addition, a similar result was observed again in a comparison of BW30 and NF270 with regards to the OB process. The BW30 membrane had higher initial OB flow rate but less accumulated backwash than NF270. Overall, the results show that increasing feed salt concentration en­ hances the OB process. This is probably due to the enhancing effect of the feed salt concentration on the initial OB driving force according to Eq. (2). Sagiv et al. [19] found the feed salinity had a strong effect by enhancing the initial OB driving force. To confirm this hypothesis, the salt concentration on the membrane surface Cm and the initial driving force were calculated and the results are presented in Fig. 14A–B. As shown in Fig. 14A–B, with an increase of feed NaCl concentration, firstly the salt concentration at the membrane surface Cm and initial driving force increased. This confirms that the OB process is strength­ ened due to the enhanced driving force. Secondly, Cm of the NF270 is a very similar to that of the BW30, while the initial driving force of BW30 is significantly higher than that of NF270. This is due to the higher salt retention of BW30 caused much lower permeate concentration than for NF270 and in turn a much higher osmotic pressure difference. Higher initial OB driving force of BW30 can attribute to higher OB flow rate in

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. Acknowledgments The authors would like to thank China Scholarship Council (CSC) for granting a Ph.D. scholarship to Y.H. Cai, and the Helmholtz Recruitment Initiative for funding the research at IFG-MT. The authors also would like to thank Prof. Raphael Semiat for helpful discussion on OB mech­ anism and Prof. Jack Gilron for detailed comments. Prof. Dr. Bryce S. Richards (KIT, IMT) is thanked for temporary laboratory use and continuous support with renewable energy related discussions. Dr. Alessandra Imbrogno helped with CP and mass transfer calculation. Sheying Li is thanked for help with using SAS and providing the solar irradiance data of Karlsruhe. Manuel Kulmus is thanked for the cross­ flow filtration system design and construction, and Jürgen Benz for system safety checking. DOW Chemical Company provided membrane samples. Irene Steves proof read the manuscript.

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Appendix A The observed salt retention, Robs , is calculated by � � Cp Robs ¼ 1 � 100% Cb

(A1)

where Cp is salt concentration in the permeate and Cb is salt concentration in feed tank. The actual steady-state observed salt retention, Ract , is calculated by Ref. [42]. Ract ¼

Jv Jv

Jv þ B*ekm

(A2)

� 100%

where B is the salt permeability, Jv is the permeate volumetric flux, and km is mass transfer coefficient. The salt permeability B (m/s) is calculated by Ref. [46]. B¼

Js cb

(A3)

cp

where Js is the salt flux, the amount of TDS that has passed through a given area of membrane per unit of time (g/m2h). The permeate volumetric flux, Jv (m/s), is calculated by Jv ¼

Qp A

(A4)

where Qp is the measured permeate flow rate (m3/s), A is the surface area of membrane (m2), in this case, 4.7 � 10-3 m2. The mass transfer coefficient km (m/s), can be calculated using Sherwood number (Sh) correlation for a cross-flow membrane system in different flow regimes as described in Eq. (A4) and Eq. (A5). Sh ¼

Km d h ¼ 0:023Re0:875 Sc0:25 D

Sh ¼

� �0:33 ​ Km dh dh for ​ Re ​ < ​ 2000 ​ ðlaminar ​ flow ​ regimeÞ ¼ 1:86 ReSc D L

(A5)

for ​ Re ​ > ​ 2000 ​ ðturbulent ​ flow ​ regimeÞ

(A6)

where dh is the hydraulic diameter (m) of rectangular channel of membrane flow cell, calculated by Eq. (A6), D is the diffusion coefficient of 1–10 g/L NaCl solution (about 1.5 � 10-9 m2/s, obtained from Vitagliano and Lyons [47]), Sc is the Schmidt number and Re is Reynolds number, calculated by Eq. (A7) and Eq. (A8) respectively. (A7)

dh ¼ 2WH=ðW þ HÞ where W is the width of the channel (2.5 � 10-2 m) and H is the height of the channel (0.7 � 10-3 m). Sc ¼

v D

(A8)

Re ¼

dh u v

(A9)

where v is the kinematic viscosity of NaCl solution in 21 � 1 � C (m2/s, obtained from Kestin et al. [48]), and u is the feed flow velocity (m/s), which is calculated by Eq. (9): u¼

QF Ac

(A10)

where QF is the feed flow rate (m3/s), and Ac is cross-section area of the membrane flow cell (2.5 � 10-5 m2). The mass transfer coefficient km depends strongly on the hydrodynamics of the system (the feed flow velocity), the diffusion coefficient and viscosity of solute, and the channel shape and dimensions according to Eq. (A4)-Eq. (A9). Appendix B. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.memsci.2019.117666.

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