Desalination 261 (2010) 365–372

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Desalination

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Characteristics and potential applications of a novel forward osmosis hollowfiber membrane

Shuren Chou a,b, Lei Shi a, Rong Wang a,b,⁎, Chuyang Y. Tang a,b, Changquan Qiu b, Anthony G. Fane a,b

a Singapore Membrane Technology Centre, Nanyang Technological University, 639798 Singapore, Singaporeb School of Civil and Environmental Engineering, Nanyang Technological University, 639798 Singapore, Singapore

⁎ Corresponding author. School of Civil and EnvironTechnological University, 639798 Singapore, Singapore.6791 0676.

E-mail address: rwang@ntu.edu.sg (R. Wang).

0011-9164/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.desal.2010.06.027

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 May 2010Received in revised form 22 May 2010Accepted 16 June 2010Available online 15 July 2010

Keywords:Forward osmosis (FO)Hollow fiberThin film composite membrane (TFC)Internal concentration polarization (ICP)Seawater desalination

There has been a resurgence of interest in forward osmosis (FO) as a potential means of desalination,dewatering and in pressure retarded osmosis, which Sidney Loeb was advocating over 3 decades ago. Thispaper describes the characteristics and potential applications of a newly developed FO hollow fibermembrane, which was fabricated by interfacial polymerization on the inner surface of a polyethersulfone(PES) hollow fiber. This FO membrane presents excellent intrinsic separation properties, with a water flux of42.6 L/m2 h using 0.5 M NaCl as the draw solution and DI water as the feed with the active layer facing thedraw solution orientation at 23 °C. The corresponding ratio of salt flux to water flux was only 0.094 g/L,which is superior to all other FO membranes reported in the open literature. To evaluate different applicationscenarios, various NaCl solutions (500 ppm (8.6 mM), 1 wt.% (0.17 M) and 3.5 wt.% (0.59 M)) were used asthe feed water to test the performance of the FO membrane. The membrane can achieve a water flux of12.4 L/m2 h with 3.5 wt.% NaCl solution as the feed and 2 M NaCl as the draw solution, suggesting it has goodpotential for seawater desalination.

mental Engineering, NanyangTel.: +65 6790 5327; fax: +65

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Forward osmosis (FO) is a natural process for water transferthrough a semi-permeable membrane under an osmotic pressuregradient across the membrane. Sidney Loeb was an early advocate ofFO processes [1,2], following his pioneering role in the developmentof Reverse Osmosis (RO). Recently there has been a renewed interestin FO as a potential means of desalination, dewatering and in pressureretarded osmosis (PRO), which Sidney Loeb proposed over 3 decadesago. The renewed interest in FO processes comes from its potentialto either reduce energy consumption or produce energy (PRO). How-ever FO requires special membranes because they experienceconcentration polarization on both sides of the membrane. Conse-quentlymembranes with a conventional asymmetric structure (activeskin layer on a porous support layer) do not perform well in FOprocesses due to the phenomena of internal concentration polariza-tion (ICP). This problem causes a diminished driving force and wasdescribed by Sidney Loeb [3]. Similarly, traditional thin film composite(TFC) RO membranes also perform poorly in the FO mode as a resultof ICP in their thick support layer [3,4]. Nevertheless, commercial ROmembranes have been intensively tested for FO applications becauseof their semi-permeable properties [5]. It is now recognized that

an ideal FO membrane requires high water permeability, low saltpermeability and a structure that minimizes ICP. This paper describesthe development of a novel hollow fiber FO membrane with thenecessary characteristics. We also evaluate its potential applications.

To eliminate the ICP phenomenon, the ideal FO membrane shouldbe a semi-permeable thin film without a porous substrate or support,however its lack of mechanical strength would preclude its practicalapplication. Therefore some degree of support is required. There arepotentially two strategies to obtain close-to ideal FOmembranes, 1) tointroduce a backbone or a support into the thin film, or 2) to make thethin film on a highly porous and very thin support layer (includingintegrated and composite structures). The former strategy has beenused by Hydration Technologies Inc. (HTI), whose membranes havebeen tested for various potential applications by several researchers[6–8]. The water flux of the HTI's FO membrane is much higher thanthat of commercial RO membranes under FO conditions. However itswater permeability (A value) is relatively modest and its sodiumchloride permeability (B value) is relatively high which tends to limitits applicability. The second approach has been attempted by severalresearchers (including our group) to develop novel FO membraneswith improved performance [9–11].

In our previous work [9], we described two types of thin filmcomposite (TFC) FOhollowfiber (designated as#Aand#B, respectively)with an ultra-thin RO-like skin layer on either the outer or inner surfaceof a porous hollow fiber substrate. Thesewere successfully fabricated bya two-step preparation — a phase inversion for the hollow fibersubstrate followed by an interfacial polymerization for the RO-like skin

366 S. Chou et al. / Desalination 261 (2010) 365–372

layer. The double-skinned #B-FO membrane exhibited excellentperformance in the FO process. In an extension of our work, animprovedhollowfiber substratewithout anouterUFskin (designated as#C-FO membrane) has been developed. The interfacial polymerization(IP) process has been optimized to achieve an RO-like skin layer withhigher water permeability and salt rejection. This study aims tocharacterize the newly developed #C-FO hollow fiber membrane andexamines its FO performance in potential application scenarios.

2. Experimental

The general details of the FO hollow fiber fabrication and itscharacterization have been described in our previous work [9]. Theprocedure used in the present work for improved hollow fibersubstrate design is described below. In addition details are provided ofa modified FO cross-flow setup with a doping system for the drawsolution control, and the range of experimental conditions (variablesalt concentrations in the feed stream) used in the FO tests.

2.1. Fabrication of FO hollow fibers

Commercial polymer polyethersulfone (PES) was used for sub-strate preparation. The PES hollow fiber substrates (designated as #C-PES) were fabricated by the dry-jet wet spinning method [12]. Inorder to investigate the effect of the membrane structural parameterS (S=τ ⋅ l /ε) on the FO performance [9], a hollow fiber substratewithout an outer UF skin was designed and successfully fabricated byincreasing the air gap in the spinning process. Discussion on the effectof air gap on the membrane structure can been found in the literature[12,13]. The formation of an RO-like skin layer on the inner surfaceof the PES hollow fiber substrate was made based on interfacialpolymerization using m-phenylenediamine (MPD) aqueous solutionand trimesoyl chloride (TMC) hexane solution [14].

2.2. FO cross-flow setup and FO experiments

A schematic diagram of the bench-scale FO setup is shown in Fig. 1.Two variable-speed gear pumps were used to supply the feed (DIwater and NaCl aqueous solutions with various concentrations) anddraw solutions (NaCl aqueous solutions with various concentrations),respectively. The Reynolds numbers of the fluids flowing in the fiber

Fig. 1. Schematic diagram of bench-sc

lumen and the shell of the module were both controlled at around2500 to minimize external (boundary layer) concentration polariza-tion, and to reduce the local feed concentration variation in themembrane module. The permeate flux was determined at predeter-mined time intervals by measuring the weight changes of the feedtankwith a digital mass balance connected to a computer data loggingsystem. The draw solution concentration was maintained constantby a salt doping system. To evaluate whether the permeation was at asteady state, the osmotic pressure difference across the membrane Km

was calculated online to indicate the starting point of the steady state.Prior to each batch of measurements, the FO membrane module waswashed thoroughly with DI water from the fiber lumen side out to theshell under low pressure for at least 2 h. The operation was performedat room temperature of ∼23 °C.

3. Results and discussion

3.1. Characterization of thin film composite hollow fiber membrane

3.1.1. Properties of membrane substratesThe cross-section and surface morphologies of the #C-PES hollow

fiber substrates are shown in Fig. 2. It can be seen that needle-likeporeswith a length of 30–40 μmwere present from both the inner andouter surfaces, and there were macro-voids with thin cell wallsbetween the two layers of the needle-like structure. Since hydraulicpressure is not needed in the FO process, the macro-void structure isconsidered favorable as it contributes to improved substrate porosity.The porosity of the substrate fiber was as high as 82%.Whenmeasuredfrom the lumen side of the hollow fiber substrates, the substrateshowed a low molecular weight cutoff (MWCO) of 39 kDa and a highpure water flux of 278 L/m2h atm. However, when the measurementwas performed from the shell of the membrane module, the MWCOwas much higher (N500 kDa). These results suggest that, as intended,there was an inner UF skin with appropriate pore structure and thatan outer UF-like skin had been successfully avoided. This postulationwas verified by the SEM pictures of the inner and outer surfaces of thesubstrate shown in Fig. 2(c) and (d), respectively. It was observed thatthe inner surface had many crack-like small pores, while much biggerpores with crack-like small pores occurred on the outer surface of thesubstrate.

ale forward osmosis (FO) system.

Fig. 2.Morphology of #C-PES hollow fiber substrates (a) cross-section at 45×, (b) enlarged at 200×; (c) inner surface layer enlarged at 10,000×; and (d) outer surface layer enlargedat 10,000×.

367S. Chou et al. / Desalination 261 (2010) 365–372

The pore size distribution of the #C-PES hollow fiber substratewas calculated based on the MWCO results obtained from the lumenside, as shown in Fig. 3 [15]. The mean pore size in the inner skin was9.6 nm with a small geometric standard deviation (σ) of 1.15 nm,which suggests a narrow pore size distribution. The detailedcharacteristic parameters of the hollow fiber substrates are summa-rized in Table 1. Compared with the #B-PES in our previous work [9],the #C-PES substrate had smaller mean pore size but similar waterflux, thus it was anticipated that the #C-PES substrate would haveadvantages when used in the subsequent interfacial polymerizationprocess to make composite FO membranes.

Fig. 3. Pore size distribution of #C-PES hollow fiber substrate membrane, calculatedby MWCO from lumen side.

Table 2 presents the mechanical strength of the #C-PES hollowfiber substrate and the corresponding FO membrane. It can be seenthat the mechanical strength of both the substrate and FO membranedropped marginally compared with their counterparts (the #B-PES[9]) in terms of tensile modulus (81.0 MPa vs. 88.4 MPa), stretchresistance (tensile stress at break of 3.2 MPa vs. 5.4 MPa) and ductilecapability (strain at break of 48.2% vs. 58.6%). The possible reasons forthis may be the lack of a relatively dense outer skin and the presenceof macro-voids in the #C-PES membranes. Nevertheless, sinceFO processes do not involve applied pressures, it was found that the#C-FOmembranewas able to maintain its integrity in long-term tests.

3.1.2. Properties of FO hollow fibers with an RO-like skinFig. 4 shows the cross-section and surface morphologies of the

#C-FO hollow fiber membranes. The polyamide-based RO-like skinwas synthesized on the inner surface of the #C-PES hollow fibersubstrates by the interfacial polymerization method. The thin filmhad a thickness of ∼300 nm, which was firmly attached to the sub-strate as shown in Fig. 4c. The surface morphology change afterinterfacial polymerization can be observed clearly by comparingFig. 4d with Fig. 2d. The ridge-and-valley structure of the membranerejection skin is typical for polyamide membranes [16].

3.2. Performance in the RO mode — inherent permeability and selectivity

The water permeability A and NaCl salt permeability B of the FOhollow fiber membranes were determined by testing the hollow fibermodule in the RO mode using the pressurized cross-flow filtrationsetup reported previously [9]. The pure water flux and salt rejection

Table 1Characteristics of PES porous hollow fiber substrates.

Sample Dimension Mean pore size Standard deviation Porosityε (%)

Pure water flux MWCO (KDa) Ref.

OD (μm) ID (μm) Thickness (μm) D* (nm) σ (nm) (L/h m2 atm) Lumen side Shell side

Dope formula C (PES) — single-skinned#C-PES 1370 960 205 9.58 1.15 82 278 39 500 Current work

Dope formula B (PES) — double-skinned#B-PES 1 1320 960 180 280 88

12.7 1.08 75 [9]#B-PES 2 1340 1000 170 275 83

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data are presented in Fig. 5. The results show that the pure waterflux Jw increased linearly with increase in transmembrane pressure(TMP,ΔP) in the low pressure range as described by Eq. (1), which canbe extrapolated to a higher pressure range [7].

Jw = A⋅ΔP ð1Þ

The membranes were tested at low pressures to avoid damage.Based on these experiments, the water permeability A of the #C-FOmembrane was found to be 3.50 L/m2h atm (9.72×10−12 m/s Pa),which is as high as some commercial RO membranes [17].

The salt rejection Rs of the #C-FO membrane had a convexincreasing trend with increase in applied hydraulic pressure ΔP, asanticipated by Eq. (2) [9]:

Rs = 1 +B

AðΔP−ΔπÞ� �−1

ð2Þ

where Δπ is the osmotic pressure of the feed solution (500 ppm NaClsolution). Based on the above, the B/A value was 6.4×103 Pa, andthe corresponding B value was 0.22 L/m2h (6.22×10−8 m/s). Theproperties of the thin film polyamide layer of our in-house hollowfiber membranes compare reasonably well with commercial ROmembranes [17].

3.3. Performance in the FO mode

3.3.1. Basic performance with pure water as feedAs a first step, DI water was used as the feed water to evaluate the

basic FO performance. It was important to identify the durationrequired for the ICP to reach steady state. During the initial stageof the FO test, the salt concentration in the membrane substrategradually increased (as illustrated in Fig. 6(a): from curve a to b), andeventually reached steady-state conditions (curve c). To determinewhether the permeation was at steady state [4,7], the osmoticpressure difference across the membrane was calculated online. Atypical curve of Δπ vs. test time in the active rejection layer facingthe draw solution orientation (AL-facing-DS) using DI water as thefeed is presented in Fig. 6(b). Normally, 30 min was sufficient for thebuildup of the salt concentration gradient in the substrate.

Fig. 7a shows the experimental and computed water flux of the#C-FO membranes using DI water as feed in the AL-facing-DS and the

Table 2Mechanical strength of FO membranes.

Sample Tensile modulus(MPa)

Stress at break(MPa)

Strain at break(%)

#C-PES substrate 81.0 3.2 48.2#C-FO membrane 93.0 4.0 47.3#B-PES substrate [9] 88.4 5.4 58.6#B-FO membrane[9] 111.5 5.3 67.5

active rejection layer facing the feed water (AL-facing-FW) orienta-tions, respectively. Theoretically [9],

Jv = Km lnAπdraw−Jv + B

Aπfeed + B

!for AL � facing� DS orientation ð3Þ

Jv = Km lnAπdraw + B

Aπfeed + Jv + B

!for AL � facing� FW orientation ð4Þ

where πdraw and πfeed are the osmotic pressures of the draw solutionand feed water, respectively. The computed FO water flux wasobtained by fitting corresponding experimental data using Eqs. (3)and (4) [9]. For the AL-facing-DS orientation, the water flux was42.6 L/m2h (1.18×10−5 m/s) when using 0.5 M NaCl as the drawsolution, which was 1.3 times higher than that of the B-FO membrane

Fig. 4.Morphology of FO hollow fiber membrane based on #C substrate. (a) lumen sideenlarged at 5000×; and (b) inner skin layer enlarged at 10,000×.

Fig. 5. Water flux and NaCl rejection using a 500 ppm (8.6 mM) NaCl solution as thefeed.

Fig. 7. Experimental and computed results of water and salt flux in the FO process usingDI water as feed. (a) Water flux of #C-FO membrane (●: AL-facing-DS, ■: AL-facing-FW; dash line and dot line: computed; and solid line: ideal); water flux of #B-FOmembrane (○ AL-facing-DS and□ AL-facing-FW) [9]; (b) salt flux of #C-FO membrane(●: AL-facing-DS and■: AL-facing-FW) and slat flux of #B-FOmembrane (○ AL-facing-DS and □ AL-facing-FW) [9].

369S. Chou et al. / Desalination 261 (2010) 365–372

[9]. However, the intrinsic water permeability (reported above inSection 3.2) of the #C-FO membrane was 1.5 higher than that ofthe #B-FO membrane. These results imply that the benefit to FOwater flux achieved by an increase in water permeability A can bemoderated by the ICP effect, as reported previously in the literature[7]. Fig. 7b shows the salt flux as a function of draw solutionconcentration, which was calculated from the increase in feedconductivity and the change of feed volume.

3.3.2. Structural parameter of the FO hollow fiber membranesTo obtain the structural parameter S and the mass transfer

coefficient Km, the water flux was estimated by the theoreticalmodel (Eqs. (3) and (4)), using Km as a fitting parameter to theexperimental data [9]. In Fig. 7a, the computed water flux is presentedas a convex curve deviating from the ideal water flux (where no ICPoccurs) with an increase in the draw solution concentration. Similarphenomena were reported in the literature [4,7,8].

From the resultant Km, the structural parameter S can be calculatedfrom Eq. (5) [6,7].

S =τ⋅lε

=DKm

ð5Þ

Fig. 6. ICP buildup and Δπ vs. test time in AL-facing DS orientation using DI water as the feed.

370 S. Chou et al. / Desalination 261 (2010) 365–372

where l, τ and ε are the thickness, tortuosity and porosity of thesubstrate, respectively; D is the salt diffusion coefficient, assumed tobe 1.61×10−9 m2/s for NaCl. The calculated structural parameter S forthe #C-FO membrane was 5.50×10−4 m, which was better than thatof the #B-FO membrane (5.95×10−4 m) [9]. This S value was alsocomparable to that of the HTI's FO flat membrane (5.75×10−4 m)[18]. The structural parameter is very important in determiningFO performance in situations of raised salt concentrations in thefeed stream, as discussed in the following section.

3.3.3. FO membrane performance in potential applicationsFO processes have several potential applications, such as waste-

water treatment [19,20], food processing [21], and seawater desali-nation [8]. A comprehensive review of FO technology can be found inthe literature [22]. According to these different application scenarios,various salt solutions can be anticipated. To evaluate these scenariosfeeds with NaCl of 500 ppm (8.6 mM), 1 wt.% (0.17 M) and 3.5 wt.%(0.59 M), which correspond roughly to the respective salt concentra-tions in wastewater, food processing and seawater, were used as thefeed streams for FO performance tests. These tests aimed to evaluatethe potential of the newly developed FO hollow fiber membranes forthe range of possible applications.

Following the procedure of the previous section, the steady stateexperimental and computed water fluxes in the FO mode weredetermined and are plotted as a function of draw solution concen-tration in Fig. 8. It can be seen that the water flux decreased to alower level using 500 ppm (8.6 mM) NaCl feed solution compared

Fig. 8. Experimental and computed results of water flux in the FO process. Feed:(a) 500 ppm (8.6 mM) NaCl solution and (b) 1 wt.% (0.17 M) NaCl solution.

with DI water (Fig 7), and the deviation from the ideal flux was alsomore severe. A slight increase in the salt concentration in the feedwater reduced the water flux by nearly 25% (from 42.6 to 32.8 L/m2h)when using 0.5 M NaCl solution as the draw solution. The same trendbut with more severe reduction was observed when using 1 wt.%(0.17 M) NaCl solution as the feed water (Fig. 8(b)). It was also foundthat the curves of water flux obtained using the different orientations(AL-facing-DS and AL-facing-FW) became closer to each other atincreased salt concentrations. Fig. 9 summarizes the water flux versusfeed water concentration data. Consistent with our previous discus-sion, it can be seen that the water fluxes were almost identical for thetwo orientations at a feed concentration of 3.5 wt.% (0.59 M).

The merging of the two curves can be readily explained byconsidering Eqs. (3) and (4). Under conditions of highly concentratedfeed and draw solutions, the numerator and denominator in thelog terms in these equations are dominated by Aπdraw and Aπfeed,respectively. Thus, Eqs. (3) and (4) approach the same limiting form,as reported previously by Sidney Loeb et al. [4]:

Jv = Km lnπdraw

πfeed

!ð6Þ

In other words, regardless of the different orientations used, thetwo curves of water flux tend to merge into one because the terms B/Aand Jv/A are negligible compared to the high values of osmoticpressure of the feed and draw solutions. This implies that under these

Fig. 9. Experimental and computed results of water flux in the FO process. Drawsolution concentration: (a) 2 M and (b) 3 M.

Table 4Performances of various FO membranes.

Sample Water flux(L/m2.h)

Salt flux/water flux(g/L)

Drawsolution

Feed Reference

#C-FO hollowfiber

42.6 0.094 0.5 M NaCl DI water Presentwork32.9 0.088 0.5 M NaCl 500 ppm

(8.6 mM) NaCl24.2 0.082 2 M NaCl 1 wt.% (0.17 M)

NaCl12.4 0.078 2 M NaCl 3.5 wt.% (0.59 M)

NaCl#B-FO hollowfiber

32.2 0.11 0.5 M NaCl DI water [9]

HTI- FO flatsheet

8.5 – 0.5 M NaCl DI water [23]17 0.85 0.5 M NaCl 10 mM NaCl [7]18.6 – 0.5 M NaCl DI water [8]11.2 – 2 M

NH4HCO3

0.5 M NaCl [8]

PBI NF hollow 3.84 – 2 M NaCl DI water [10]

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conditions the water flux is only determined by the membranestructural parameter (S = D

Km

.), and the osmotic pressures of the

feed and draw solutions when the feed contains high concentrationsalts. A further analysis of the mass transfer resistances provides adeeper insight into this.

Considering the resistances of the RO-like skin layer and thesubstrate to water flux, a typical resistance model can be applied:

Jv =πdraw−πfeed

ηRð7Þ

where η is the viscosity of water, and R is the integrated resistance ofthe FO membrane, which is the sum of the resistances of the RO-likeskin layer Rskin and that of the porous substrate Rsubstrate (including thecontribution from ICP). Rskin is an inherent property of the thin film,which can be written as

Rskin =Δπskin

Jvη=

1ηA

ð8Þ

where Δπskin is the transmembrane osmotic pressure difference.However, Rsubstrate depends not only on the physical/geometricalproperties of the substrate but also the applied conditions. Rsubstrateincorporates the effect of ICP and is an ‘apparent’ resistance. Take theAL-facing-DS orientation for an example, substituting Eq. (8) intoEq. (7), and comparing with Eq. (3), we have,

Rskin

Rsubtrate=

πdraw−πfeedeJvKm

� �− B

Ae

JvKm−1

� �

πfeed + BA

� �e

JvKm−1

� � ð9Þ

If DI water is used as the feed, πfeed=0, then Eq. (9) can besimplified to

Rskin

Rsubstrate=

πdraw

BA

eJvKm−1

� �−1 ð10Þ

Table 3 presents the ratio of Rskin to Rsubstrate under variousoperating conditions. Consider the cases with 3 MNaCl solution as thedraw solution. The ratio Rskin /Rsubstratewas 0.05 for a 3.5 wt.% (0.59 M)NaCl solution as the feed. This reveals that the resistance of the RO-like skin was negligible compared with the substrate resistance due tothe severe ICP in this condition, and the mass transfer through thesubstrate is the rate-determining step. However, if DI water and 0.5 MNaCl were used as the feed and draw solution, respectively, theratio Rskin /Rsubstratewas 5.39. This indicates that the resistance ofthe substrate is much less than the RO-like skin, and the waterpermeation through the RO-like skin is the rate-determining step.In the latter case, the water flux is much closer to the ideal linearcurve, indicating that ICP is not a significant factor.

Based on the above discussions, when the concentrations of thefeed and the draw solution are both relatively low, the permeationthrough the RO-like-skin is the rate-determining step, which suggeststhat a higher A value of the FO membrane is more important than

Table 3Various ratios of Rskin to Rsubstrate (AL-facing-DS).

DS 0.5 M 1.0 M 1.5 M 2.0 M 3.0 M

FW

0 5.39 2.89 1.64 1.20 0.84500 ppm 2.08 1.73 1.27 1.12 0.741 wt.% 0.15 0.15 0.13 0.12 0.103.5 wt.% – – – 0.07 0.05

the S value. In contrast, when the concentrations of the feed and thedraw solution are both high (such as in food processing and seawaterdesalination), a lower S value is much more important to the systemperformance. For conditions between these two cases (such as inwastewater treatment), the two functional layers compete with eachother, therefore attention should be paid to both parameters (A and S)of the FO membrane.

Table 4 presents the performances of the #C-FO membranes forvarious applications along with other FO or NF membranes from theliterature. It can be seen that the #C-FO membrane exhibits anexcellent water flux of 42.6 L/m2h using 0.5 M NaCl as the drawsolution and DI water as the feed for the AL-facing-DS orientationat 23 °C. The corresponding ratio of salt flux to water flux is only0.094 g/L, which is superior to all other FOmembranes reported in theopen literature.When the salt concentration in the feedwas increasedto as high as 3.5 wt.% (0.59 M), similar to seawater concentration, the#C-FO membrane was still able to perform well and achieve a waterflux of 12.4 L/m2h using 2 M NaCl as the draw solution, suggestinga good potential of this new FO hollow fiber membrane for seawaterdesalination.

4. Conclusions

A novel type of thin film composite hollow fiber FO membrane,without an outer UF skin, has been successfully developed. The FOhollow fiber membrane presents excellent intrinsic separationproperties (A value: 9.72×10−12 m/s Pa; B value: 6.22×10−8 m/s;S value: 5.50×10−4 m). In the FO process, the water flux of the #C-FOhollow fiber membrane was 42.6 L/m2h using 0.5 M NaCl as the drawsolution and DI water as the feed for the AL-facing-DS orientation at23 °C. The corresponding ratio of salt flux to water flux was only0.094 g/L, which is superior to all other FOmembranes reported in theopen literature.

Based on different application scenarios, various NaCl solutionssuch as 500 ppm (8.6 mM), 1 wt.% (0.17 M) and 3.5 wt.% (0.59 M),were used as the feed water to test the performance of the new FO

fiberPBI NF hollowfiber

9.02 – 2MMgCl2 DI water [10]

2-hourmodifiedPBI

16 – 1MMgCl2 DI water [11]

NF hollowfiber

PBI-PES duallayer

16 0.030 1 M MgCl2 DI water [24]

FO hollowfiber

* in AL-facing-DS configuration at 20–25 °C.

372 S. Chou et al. / Desalination 261 (2010) 365–372

hollow fiber membranes. It was found that if the feed contains a lowconcentration of salts and the concentration of the draw solutionis also relatively low, the water permeation through the RO-like skinis the rate-determining step, where a higher A value of the FO mem-brane is more important than the S value. However, if the salt con-centration in the feed is relatively high, the mass transfer in thesubstrate is the rate-determining step, where a lower S value is muchmore important in determining the FO performance. The newlydeveloped FO hollow fiber membrane can achieve a water flux of12.4 L/m2h for 3.5 wt.% (0.59 M) NaCl feed solution and 2 M NaCldraw solution, suggesting that this new FO hollow fiber membrane ispromising for seawater desalination.

NomenclatureA water permeability coefficient (L/m2h atm)Am effective membrane surface area (m2)B NaCl salt permeability coefficient (L/m2h; m/s)D, Deff solute diffusion coefficient (m2/s)Jw specific pure water flux of the hollow fiber membrane

(L/m2h)Jv volumetric flux of water (L/m2h)Km mass transfer coefficient (L/m2h)ΔP pressure difference across the membrane (atm)Rs salt rejection determined using a feed water containing

500 ppm NaClR, Rskin, Rsubstrate resistance of water permeation (m−1)S membrane structural parameter (m)

Greek lettersε membrane porosityη viscosity of water (Pa s)τ tortuosityπdraw, πfeed osmotic pressures (atm)Δπ osmotic pressure difference across the membrane (atm)σ geometric standard deviation (nm)

Acknowledgements

We would like to thank the Environment and Water IndustrialDevelopment Council of Singapore for funding support under theproject # EWI RFP 08/01. We are also grateful to the SingaporeEconomic Development Board for funding the Singapore MembraneTechnology Centre.

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