Polyelectrolyte-promoted forward osmosis process for dye wastewater treatment – Exploring the feasibility of using polyacrylamide as draw solute

Chemical Engineering Journal 264 (2015) 32–38

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Polyelectrolyte-promoted forward osmosis process for dye wastewater treatment – Exploring the feasibility of using polyacrylamide as draw solute Pin Zhao a, Baoyu Gao a, Shiping Xu a, Jiaojiao Kong a, Defang Ma a, Ho Kyong Shon b, Qinyan Yue a,⇑, Pan Liu a a b

Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, China School of Civil and Environmental Engineering, University of Technology, Sydney (UTS), Post Box 129, Broadway, NSW 2007, Australia

h i g h l i g h t s  Polyacrylamide (PAM, MW  3,000,000) was explored as draw solute in the FO process.  The diluted PAM solution can be used directly to polymer flooding.  PAM had superiority in steadier water flux and less salt leakage.  A high temperature was preferable when using PAM as draw solute.  The application of PAM in FO to treat dye wastewater was proven to be feasible.

a r t i c l e

i n f o

Article history: Received 8 September 2014 Received in revised form 30 October 2014 Accepted 11 November 2014 Available online 18 November 2014 Keywords: Forward osmosis Polyacrylamide Water flux Reverse salt flux Dye wastewater treatment

a b s t r a c t In this study, polyacrylamide (PAM, MW  3,000,000) was applied as draw solute in forward osmosis (FO) process. The influence of temperature, concentration and membrane orientation on FO performance was investigated. The PAM solution was diluted during the FO process, which could maintain the relatively high viscosity and be used directly to polymer flooding in many oilfields to increase oil production. When comparing the FO performance between KCl and PAM via the same membranes, the water flux of PAM (the time of achieving a balance was less than 2 h) was more stable than that of KCl (a sustained downward trend all though the 5 h). The loss of PAM in recovering each liter of water from FS (<0.009 g) was much smaller than that of KCl which was more than 0.2 g. The stable water flux with almost a complete salt rejection in the FO process revealed the superiority of PAM to conventional ionic salts. Furthermore, the application of PAM in FO to treat the Reactive Brilliant Red K-2BP (RBR) dye solution was investigated and proven to be feasible, where the membrane fouling caused by RBR dye solution was slight and the dye rejection was high (almost 1). The overall performance demonstrated that PAM is a promising draw solute. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction As an emerging technology for wastewater reclamation and seawater desalination, forward osmosis (FO) has received renewed research interest all over the world. The FO process is based on the principle of natural osmotic process, driven by the osmotic gradient between two solutions of different osmotic concentrations when they are separated by a semi-permeable membrane. When

⇑ Corresponding author. Tel.: +86 531 88365258; fax: +86 531 88364513. E-mail address: [email protected] (Q. Yue). http://dx.doi.org/10.1016/j.cej.2014.11.064 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.

the highly concentrated draw solution (DS) and the feed solution (FS) are separated by a semi-permeable membrane, pure water moves from the feed side to the concentrated DS due to the osmotic difference generated across the membrane [1–3]. Compared with typical pressure-driven membrane processes, FO occurs spontaneously in the absence of hydraulic pressure, which gives it plenty of potential advantages such as low system energy consumption and membrane fouling propensity [4–6]. Nowadays, FO process is developing well. However, there are still some barriers with the process, in which the draw solute leakage is one of the most notable problems [7]. Boo et al. [8] reported that salts diffused from DS to FS and accumulated within

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the colloidal fouling layer, which would reduce the net osmotic driving force for permeate water flux. Reverse salt diffusion had been proven to be a key mechanism that controlled colloidal fouling behavior as well as fouling reversibility in FO process [9,10]. To enhance FO technology, it is inevitable to develop more suitable draw solutes. So far, there have been a great many researches focusing on the development of different draw agents for FO process [11]. For instance, McCutcheon et al. [12] studied extensively the feasibility of using ammonium bicarbonate as a draw agent which could produce high osmotic pressure and be regenerated easily by distillation at around 60 °C. Ling et al. [13] investigated the potential of polymer-coated magnetic nanoparticles use as a draw solute and verified that it could yield high driving force and subsequent high water flux. Ge et al. [14,15] had developed a series of novel draw solutes based on polyelectrolytes of poly(acrylic acid) sodium (PAA-Na) salts. Their expanded structure was expected to minimize the reverse salt diffusion. Meanwhile, the various structural sizes enabled them to be separated readily from water by low pressure-driven or heating processes determined by the selected system [16]. As a group of water-soluble highmolecular polymer, polyacrylamide (PAM) with average molecular weight (MW) more than 1  107 was widely used as polymer flooding in Daqing Oilfield (Daqing, Heilongjiang, China) to increase oil yield. Due to its good water-solubility and structural configuration, it is expected to be a good candidate for draw solute. Moreover, the diluted PAM solution after FO process would be applied directly to oil extraction. Up to now, there have been few papers on FO performance of PAM as draw solute. In this study, PAM with MW  3,000,000 was selected as the draw solute, while the dye wastewater of Reactive Brilliant Red K-2BP (RBR) was employed as the FS of model wastewater. The test had mainly two parts: (I) investigating the background performance of using draw agent PAM under different concentrations, temperatures and membrane orientations and comparing it with draw agent KCl; (II) studying the application of FO using PAM as draw solute to treat the dye wastewater. It is envisioned that this study may support the fundamentals to the further development of new draw solutes for FO process.

2. Materials and methods 2.1. Feed and draw solutions Both PAM and KCl were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai). They were chemically pure. PAM was highly hydrophilic and non-toxic. The osmosis pressures of both PAM and KCl were measured by Freezing point Osmometer (Germany loser). Unfortunately, the change in osmotic pressure with temperature could not be measured. The viscosities of PAM at different concentrations and temperatures were determined by digital rotary viscosimeter (NDJ-5S, Lunjie Mechanical and Electronic Instrument Co., LTD, Shanghai). The value of viscosity was obtained in the conditions of SPL2 rotor at 30 RPM, which was the apparent viscosity because the PAM solution was the non-Newtonian liquid. Furthermore, the particle size of PAM was

measured by Zetasizer Nano Analyzer and determined to be in the range of 204.1–229.9 nm. Reactive Brilliant Red K-2BP (RBR, Jinan No. 2 Textile Dyeing Mill, China) was selected as a representative of synthetic dyeing wastewater. The simulative wastewater was prepared by dissolving 0.05 g dye in 1 L deionized (DI) water. The pH of raw test water was 6.37, which was within the suitable range of the TFC FO membrane. The maximum absorbance and zeta potential of original RBR dye solution was measured to be 2.44 at kmax (537.5 nm) and 2.1, respectively. The background performance of FO was measured using DI water as FS. The initial volume of DS and FS was maintained at 500 mL and 1000 mL, respectively. 2.2. Forward osmosis system The FO experimental setup in this study was similar to that in previous studies [10,17]. The FO process was conducted using a lab scale FO unit with an effective membrane area of 20.0 cm2 (7.7 cm length  2.6 cm width  0.3 cm depth). The FS and DS flowed concurrently through the lumen and shell sides at the flow rates of 10 cm/s. The performance at temperatures of 25 ± 0.5, 35 ± 0.5 and 45 ± 0.5 °C was measured. A weighing balance (Satorius weighing technology GmbH, Gottingen, Germany) was used to record the variation in the DS weight for water flux computation. 2.3. Forward osmosis membrane The membrane used in our study was PA-based thin-film composite (TFC) FO membrane. It had been studied in previous researches and was determined to be different from both the TFC membrane used widely for reverse osmosis process and the CTA membrane with broad applications in FO system. The membrane exhibited more advantages in terms of FO performance such as higher water permeability, more decent salt rejection, improved selectivity and superior separation properties. The physical and chemical properties of membranes as revealed from various literatures were compared and presented in Table 1 [1,18–21]. Micrographs of the membranes were obtained using a HITACHI S-520 scanning electron microscope. To obtain cross-sections, wet membrane samples were flash-frozen in liquid nitrogen and subsequently cracked. Both membrane surface and cross-section samples were dried overnight. 2.4. Membrane orientation There are two modes of membrane orientation in FO process. Specifically, the first one is porous layer facing the FS and the dense layer facing the DS. This orientation has been used in pressure retarded osmosis (PRO) process, since the porous support layer was required to resist the pressurization of the permeate stream [22]. Therefore it is referred to as the PRO mode. The other mode, the DS is placed against the support layer and the dilute FS is on the active layer, which is the typical orientation as described in previous studies on FO process [23,24]. So it is referred to as the FO mode.

Table 1 The comparison of physical and chemical properties between TFC-FO membrane and CTA-FO membrane. Sample

CTA TFC

Active layer material

Cellulose triacetate Polyamide

Contact angle (°) Active layer

Support layer

76.6 45

81.8 45

Zeta potential at pH 6 (mV) Active layer

Operating pH

Membrane thickness (lm)

Ref.

2.1 86

3–8 2–12

93 116 ± 1

[21] –

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2.5. Theoretical water flux

external concentration polarization (ECP, dilutive) in the active layer. At the same time, salt was diffused from the draw to feed side, leading to internal concentration polarization (ICP, concentrative) in the porous support layer. In the FO mode, dilutive ICP and concentrative ECP occurred on the porous support layer and active layer, respectively [25,26,10].

In an optimal situation, the standard water flux is given by the following equation:

J w ¼ Ar½pD  pF  ¼ ArDp

ð1Þ

where Jw is the water flux through FO membrane which is symmetric; A is the membrane permeability coefficient; r is the reflection coefficient; pD and pF are the osmotic pressures of the DS and FS, respectively, and Dp is the net osmotic gradient. However, all the FO membranes were asymmetric, and they contained thick mechanical support layer which caused the lower r value. This membrane structure had severe CP effect hampering the performance of the FO process. When pure water was used as FS in the PRO mode, water was diffused from feed to draw side due to osmotic pressure gradient across the membrane, resulting in the

3. Results and discussion 3.1. Characteristics of PAM and KCl A more viscous solute was expected to have more resistance when it was transported through the membrane and its water flux would decline accordingly [11]. As the polyelectrolyte viscosity had significant influence on FO performance when used as draw solute, the effects of temperature and concentration on the

Table 2 Basic property parameters of PAM and KCl. DS

PAM

Concentrations (g/L) Osmosis pressure (mOsm/kg H2O) Viscosity (mPa s)

20 366

30 544

40 824

20 508.5

30 814

40 1055.5

180.4 164.4 158.3

244.7 214.6 195.9

314 277.1 253.6

/ / /

/ / /

/ / /

18

(b) 20 g/L 30 g/L 40 g/L 20 g/L 30 g/L 40 g/L

16

Water flux (LMH)

14 12 10 8

16 o

25 C o 35 C o 45 C o 25 C o 35 C o 45 C

14

Water flux (LMH)

(a)

25 °C 35 °C 45 °C

KCl

6

12

10

8

6

4 4

0

1

2

3

4

5

0

6

1

2

3

16 14 12

Water flux (LMH)

5

6

(d) 17 20 g/L 30 g/L 40 g/L 20 g/L 30 g/L 40 g/L

10 8 6

16 15 14 13

Water flux (LMH)

(c)

4

Time (h)

Time (h)

12 11 10 9

o

25 C o 35 C o 45 C o 25 C o 35 C o 45 C

8 7 6 5

4

4 3

2

2 1 0

1

2

3

Time (h)

4

5

6

0

1

2

3

4

5

Time (h)

Fig. 1. Water flux-decline curves under different situations: (a) the PRO mode and at 35 °C; (b) the PRO mode and at 30 g/L; (c) the FO mode and at 35 °C; (d) the FO mode and at 30 g/L. The solid symbol represented the progress using PAM as draw solute; while the hollow symbol represented the progress using KCl as draw solute.

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apparent viscosity of PAM were evaluated. The basic properties of DS such as osmotic potential and viscosity were measured and presented in Table 2. As shown in Table 2, with respect to both PAM and KCl solution, the osmosis pressure varied approximately linearly with the concentration. It is observed that the viscosity increased dramatically with the increase of PAM solution concentration. As shown in Table 2, the viscosity of 20, 30 and 40 g/L PAM at 35 °C was 164.4, 214.6 and 277.1 mPa s, respectively. The larger mass concentration of PAM resulted in more PAM molecules intertwining in the water, and then caused the viscosity increase. Moreover, when the temperature of PAM increased, its apparent viscosity declined. The diluted PAM solution would maintain the high viscosity and be used directly to polymer flooding in many oilfields to increase oil yield, which had been proven in our other studies (The data were provided in Supplemental document). Furthermore, there were many processes such as ultrafiltration, membrane distillation or heating processes for regeneration and recycle of PAM solution [14,15]. 3.2. FO performance: water flux and reverse PAM flux To evaluate the influence of DS concentration, temperatures and membrane orientation on FO performance, FO experiments were

conducted at different DS concentrations (20, 30, and 40 g/L), temperatures (25, 35 and 45 °C) and membrane orientations (FO and PRO mode). Both PAM and KCl were chosen as draw solute to analyze and compare their advantages and disadvantages. The two measures were water flux and reverse salt flux, which were presented in Fig. 1 and Fig. 2, respectively. NOTE: in Fig. 1, the solid symbol represented the progress using PAM as draw solute; while the hollow symbol represented the progress using KCl as draw solute. 3.2.1. Water flux Fig. 1 shows the water flux in the 5 h FO process. It can be obviously seen that, as to both PAM and KCl, the water flux increased with the DS concentration and temperature. The water flux in the PRO mode was higher than that in the FO mode. Although the water flux of PAM was lower than that of KCl, the flux of PAM achieved a balance in 2 h, which was shorter than that of KCl (which declined throughout the 5 h). It indicated that the water flux of PAM was more stable than that of flux. More details are given below. It can be obtained from Fig. 1(a) that although the water flux varied at different concentrations, the variation tendency shared some similarities. Most obviously, the water flux decreased gradually in the initial 1.25 h and thereafter remained constant.

(a)

(a)

40

30

o

45 C o 35 C o 25 C o 45 C o 35 C o 25 C

20

0.00

Concentration ( g/L)

Concentration ( g/L)

40

0.01

0.02

0.03

0.04

0.05

30 o

45 C o 35 C o 25 C o 45 C o 35 C o 25 C

20

0.06

0.000

Reverse salt flux (gMH)

0.002

0.004

0.006

0.008

0.010

The ratio of reverse PAM flux to water flux (g/L)

(b)

(b) o

30 o

45 C o 35 C o 25 C o 45 C o 35 C o 25 C

20

0.0

45 C o 35 C o 25 C o 45 C o 35 C o 25 C

40

Concentration ( g/L)

Concentration ( g/L)

40

0.5

1.0

1.5

2.0

2.5

30

20

3.0

Reverse salt flux (gMH) Fig. 2. RSF during the FO runs using (a) PAM as draw solute and (b) using KCl as draw solute. The box with slashes represented the progress in the PRO mode (the porous layer facing the FS); while the blank box represented the progress in the FO mode (the porous layer was facing the DS).

0.00

0.05

0.10

0.15

0.20

0.25

The ratio of reverse KCl flux to water flux (g/L) Fig. 3. The ratio of reverse PAM flux to water flux during the FO runs using (a) PAM as draw solute and (b) using KCl as draw solute. The box with slashes represented the progress in the PRO mode (the porous layer facing the FS); while the blank box represented the progress in the FO mode (the porous layer was facing the DS).

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Specially, the declined water flux in the first 1.25 h accounted for 96.0% (20 g/L), 79.3% (30 g/L) and 74.5% (40 g/L) of the total decrement of the 5 h, respectively. The gradual decrease of the water flux occurring in the initial 1.25 h mostly resulted from the declined net osmotic pressure difference across the membrane. In addition, water flux improved significantly when PAM concentration increased from 20 to 30 g/L, while it increased slightly when PAM concentration increased from 30 to 40 g/L. In spite of the high osmotic pressure, the water flux at 40 g/L PAM did not increase greatly accordingly. It could be attributed to the viscosity

(a)

of PAM at 40 g/L (277.1 mPa s) which was higher than that of PAM at 30 g/L (214.6 mPa s). The high viscosity ultimately hindered the ability of water transporting through the membrane. As presented in Fig. 2(b), the average water flux was 4.79 L/ (m2 h) (LMH), 5.09 LMH and 5.62 LMH at 25, 35 and 45 °C, respectively. It is clear that the water flux increased by 6.15% when the temperature increased from 25 to 35 °C, while it increased by 10.56% when the temperature increased from 35 to 45 °C. It indicates that the FO performed well at higher temperatures. As to the DS, the kinematic viscosity decreased and the osmotic pressure

(b)

(c)

(d)

(e)

(f)

Fig. 4. SEM images of fouled FO membranes. (a and b) General views of support layer, (c and d) the hole of support layer, (e and f) cross-section.

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3.2.2. Reverse salt flux It is of practical interest to study reverse salt diffusion which is shown in Fig. 2. The results show that the reverse PAM flux increased slightly with temperature and/or PAM solution concentration. Nevertheless, the reverse flux variation was in the range 0.02–0.07 gMH in spite of the temperature, PAM concentration and membrane orientation adopted in this study. Such a small range of variation indicates that the effects of temperature, concentration and membrane orientation on reverse PAM flux were negligible. This observation demonstrates the superiority of PAM draw solutes over KCl. Comparing PAM draw solutes, the reverse salt flux of KCl was much higher (more than about 1.5 gMH) and increased significantly (more than 0.7 gMH) with elevating KCl concentration and temperature. According to the water flux and reverse salt flux values presented in Figs. 1 and 2, the loss of PAM and KCl in recovering each liter of water from FS can be obtained through a simple calculation. The results in Fig. 3 present that the loss of PAM in recovering each liter of water from FS was lower than 0.009 g under all circumstances in this study, while that of KCl was more than 0.2 g. This indicates that, compared with conventional ionic salts, such as KCl, PAM had such a slight loss that the feed stream was uncontaminated and the replenishment cost of the PAM draw solute was extremely low. It presents other advantages of using polyelectrolytes as draw solutes. Given there were high water flux and relative low reverse PAM flux at high temperature, a high temperature was favorable to the FO process using PAM as draw solute. 3.3. Characteristics of TFC membrane To further investigate the fouling of the membrane, membrane coupons after FO run were taken out from the membrane cell and analyzed by SEM, which are shown in Fig. 4. One category (Fig. 4a, c and e) and the other category (Fig. 4b, d and f) presents the SEM images of membrane using PAM and KCl as draw solute in the PRO mode, respectively. The contamination of the membrane used in the FO mode was so slight that it was not presented in this paper. For the same reason, the dense layer of the membrane was not shown. Fig. 4(a) and (b) shows the mechanical support layer of membrane fouled. There were numerous tiny white crystals existing

5.5

5.0

4.5

Water flux (LMH)

increased with increasing temperature. This was likely to enhance the diffusivity of the water through the membrane, thereby positively influencing the water flux. In pressure-based membrane processes, such as RO and NF, the cake-enhanced osmotic pressure (CEOP) formed in the surface of membrane would lead to the water flux decline. The deposited cake layer on the feed side hindered the back diffusion of salt into the bulk feed solution, thus resulting in elevated osmotic pressure near the membrane surface of the feed side. It led to the drop in the net driving force, and thus, resulted in the decline in permeate flux. It had been reported in some other papers that the key mechanism of flux decline in FO was mainly accelerated CEOP [27,4]. In the FO process, salt could diffuse across the membrane from the highly concentrated DS to FS side. Because of this reverse salt diffusion, there would be a greater salt build-up near the membrane surface, thereby accelerating CEOP in FO colloidal fouling. Compared with PAM, KCl would pass through the membrane easier due to its small molecular size. As a result, the CEOP of KCl was higher and led to a faster water flux decline. Moreover, the water flux of PAM declined in the first 2 h gradually and then achieved relative stability, while that of KCl continued to reduce. This was due to the high viscosity of PAM. A certain amount of PAM tended to cling to the surface or pore of membrane much more easily. The CP phenomenon would occur in a relatively shorter time.

4.0

The FO mode The PRO mode Baseline in the FO mode Baseline in the PRO mode

3.5

3.0

2.5

2.0 0

1

2

3

4

5

Time (h) Fig. 5. Variation of permeate flux with time using PAM as DS in the FO process to treat the RBR dye solution.

in the holes of membrane in Fig. 4(b), while there were few in Fig. 4(a). Some of the salt diffused through the membrane from the DS towards the FS was trapped within the porous layer and then accumulated rapidly. Fig. 4(a) demonstrates that the reverse salt diffusion of KCl was higher than that of PAM, which well agreed with the result of Fig. 2. In order to observe the condition of reverse salt diffusion more clearly and directly, the hole and cross-section with enlarged scale are measured and presented in detail in Fig. 4(c) and (d) and Fig. 4(e) and (f), respectively. There was evident fouling as shown in Fig. 4(d) and (f). The crystal was 20 lm in length and 6 lm in width in Fig. 4(d). The SEM pictures of cross-section reveal the process where the salts of the DS in the active dense layer diffused through the membrane towards the FS in the support porous layer. 3.4. Application of PAM in FO to treat the RBR solution PAM at the concentration of 20 g/L is chosen as a reference (DS) to treat the RBR solution (FS) in the FO process at 25 °C. The variation of permeate flux with time in the entire 5 h is presented in the Fig. 5 and compared with the baseline where DI water was used as FS under the same condition (the baseline was the Water flux-decline curves at 25 °C of using 20 g/L PAM and DI water as DS and FS, respectively). As shown in Fig. 5, when the RBR solution was used as the FS, flux values decreased from 3.211 LMH to 2.647 LMH with time in the FO mode, whereas, they decreased from 5.140 LMH to 4.190 LMH in the PRO mode. The water flux in the PRO mode was higher and yet declined faster than in the FO mode. Moreover, the trans-membrane flux in the case of RBR solution as FS was found to be similar to that of the baseline in the both modes. It manifested that the membrane fouling caused by RBR solution was slight and could be neglected. Furthermore, dye concentration of the DS was measured to investigate the dye rejection of membrane. The dye rejections in both the PRO and FO were almost 1, which indicated that the dye rejection of TFC-FO membrane was high. The above results proved that application of PAM in FO to treat the RBR solution was feasible. 4. Conclusions In this study, PAM (MW  3,000,000) was applied as draw solute in FO processes. The influence of temperature, concentration and membrane orientation on the FO performance using PAM as draw solute was studied. Furthermore, 20 g/L PAM was chosen as a representative to treat the RBR wastewater.

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The diluted PAM solution after FO process can maintain high viscosity and be used directly to polymer flooding in many oilfields to increase oil yield. Compared with conventional ionic salts KCl, PAM had superiority in terms of more stable water flux and less salt leakage. Moreover, the effects of temperature, concentration and membrane orientation on reverse PAM flux were negligible. Considering the high water flux and relative low reverse PAM flux at high temperature, a high temperature was favorable to FO process when PAM was used as draw solute. The loss of PAM in recovering each liter of water from FS was low, which would greatly reduce the contamination of the feed stream and the replenishment cost of the PAM draw solute. The SEM images showed that the membrane using KCl suffered more contamination than that using PAM. The application of PAM in FO to treat the RBR dye solution was proven to be feasible, in which membrane fouling caused by RBR dye solution was negligible and the dye rejection of TFC-FO membrane was high. The overall performance indicates that the application of PAM as draw solutes was feasible. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.11.064. References [1] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science 333 (2011) 712–717. [2] S. Zhao, L. Zou, C.Y. Tang, D. Mulcahy, Recent developments in forward osmosis: opportunities and challenges, J. Membr. Sci. 396 (2012) 1–21. [3] L.A. Hoover, W.A. Phillip, A. Tiraferri, N.Y. Yip, M. Elimelech, Forward with osmosis: emerging applications for greater sustainability, Environ. Sci. Technol. 45 (2011) 9824–9830. [4] S. Lee, C. Boo, M. Elimelech, S. Hong, Comparison of fouling behavior in forward osmosis (FO) and reverse osmosis (RO), J. Membr. Sci. 365 (2010) 34–39. [5] H.Y. Ng, W. Tang, W.S. Wong, Performance of forward (direct) osmosis process: membrane structure and transport phenomenon, Environ. Sci. Technol. (2006) 2408–2413. [6] B. Mi, M. Elimelech, Chemical and physical aspects of organic fouling of forward osmosis membranes, J. Membr. Sci. 320 (2008) 292–302. [7] A. Achilli, T.Y. Cath, A.E. Childress, Selection of inorganic-based draw solutions for forward osmosis applications, J. Membr. Sci. 364 (2010) 233–241. [8] C. Boo, S. Lee, M. Elimelech, Z. Meng, S. Hong, Colloidal fouling in forward osmosis: role of reverse salt diffusion, J. Membr. Sci. 390 (2012) 277–284. [9] E. Cornelissen, D. Harmsen, K. De Korte, C. Ruiken, J.-J. Qin, H. Oo, L. Wessels, Membrane fouling and process performance of forward osmosis membranes on activated sludge, J. Membr. Sci. 319 (2008) 158–168.

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