Emerging forward osmosis (FO) technologies and challenges ahead for clean water and clean energy applications

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Emerging forward osmosis (FO) technologies and challenges ahead for clean water and clean energy applications Tai-Shung Chung1,2, Xue Li2, Rui Chin Ong1, Qingchun Ge1, Honglei Wang1 and Gang Han1 The purpose of this short review is to share our understanding and perspectives with the chemical, environmental, water and osmotic power communities on FO processes in order to conduct meaningful R & D and develop effective and sustainable FO technologies for clean water and clean energy. Addresses 1 Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117576, Singapore 2 NUS Graduate School for Integrative Science and Engineering, National University of Singapore, Singapore 117456, Singapore Corresponding author: Chung, Tai-Shung ([email protected])

Current Opinion in Chemical Engineering 2012, 1:246–257 This review comes from a themed issue on Energy and environmental engineering Edited by Rakesh Agrawal and Subhas K Sikdar For a complete overview see the Issue and the Editorial Available online 26th July 2012 2211-3398/$ – see front matter, # 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.coche.2012.07.004

Introduction Technologies to produce clean water and clean energy have received global attention owing to water scarcity, resource depletion, and global warming [1,2,3– 5,6,7,8,9,10]. Forward osmosis (FO) has emerged as one of potential technologies to mitigate water and energy shortage [6,7,10,11]. Not only can it produce clean water but also clean energy by using the osmotic gradient across a semi-permeable membrane as the driving force for water production and power generation. Basically, FO takes the advantage of naturally induced water diffusion across a semi-permeable membrane from a low concentration solution to a high concentration solution [12]. Ideally, the semi-permeable membrane performs as a barrier that allows water to pass through but rejects salts or unwanted elements. The high concentration solution acts as a draw solution, which has a higher osmotic pressure than the feed solution, to draw water from the feed across the membrane to itself. Thus, FO processes take place only if (1) there is a semi-permeable membrane partitioning the feed from the draw solution and (2) there is an osmotic pressure difference across the membrane. A typical FO process is shown in Figure 1(a). Current Opinion in Chemical Engineering 2012, 1:246–257

For water reuse and desalination, FO requires much less energy to induce a net flow of water across the membrane compared to traditional pressure-driven membrane processes such as reverse osmosis (RO). However, different from RO, the permeate of FO is not a product water ready for consumption but a mixture of drawn water and draw solution. As a result, a second step of separation must be employed to produce clean water and to regenerate the draw solution. The second step of separation might be energy intensive if inappropriate draw solutes and recycle processes were utilized. Therefore, one must take both costs of FO membranes and draw solute recycle into consideration in order to have a fair comparison of a FO technology with other water production technologies. Otherwise, the conclusion could be biased and misleading. The concept of osmotic energy generation was proposed about 60 years ago, but most of the early works were discontinued owing to the lack of effective membranes [13–18], which are the heart of osmotic power systems. Statkraft of Norway is the first company that pioneered serious research on osmotic power and built the first prototype plant in 2009 by mixing river water and seawater across a semi-permeable membrane [11,19,20,21]. Figure 1(b) illustrates schematic diagrams for osmotic power generation. Theoretically, the continuous operation pressure of the seawater compartment may increase up to 13.5 bar that can drive a hydro-turbine and generate electricity consequently. A pressure of 13.5 bar is equivalent to a water column of about 135 m in a hydropower plant. The estimated global osmotic energy that can be derived from mixing ocean and river waters is in the order of about 1750–2000 TWh per year [11,19,20]. Considering the great potential of FO for various applications, the purposes of this short review are to (1) elaborate the challenges ahead for its applications to clean water and clean energy production and (2) summarize the state-of-art FO technologies in terms of membranes and draw solutes.

Limitations and challenges of FO for clean water and clean energy Currently, the major challenges of FO technologies are (i) ineffective membranes that are heart of most FO-based processes; (ii) lack of cost effective draw solutes that can be easily recycled; and (iii) limited studies on fouling [6]. Compared to the state-of-art RO, FO for desalination needs significant breakthroughs on the development of www.sciencedirect.com

Current forward osmosis technology development Chung et al. 247

Figure 1

(a)

Seawater

(b)

Pressure exchanger Feed

Draw solution

Membrane

Draw solution regeneration FO membrane

Diluted seawater

RO

Clean water

Fresh water

Concentrated feed

Turbine

River water

Seawater

Pressure exchanger

Diluted draw solution

Membrane Turbine

Diluted seawater

Retentate of recycled water Current Opinion in Chemical Engineering

Schematic diagrams of (a) a typical forward osmosis (FO) process and (b) osmotic power generation from the mixing of seawater and fresh water (top) and from the mixing of RO and recycled water retentates (bottom).

FO membranes and cost-effectively recyclable draw solutes. However, for direct fertigation [22] and osmotic power generation, fertilizer and seawater are natural draw solutes, respectively. Therefore, molecular design of FO membranes with high flux and power density has received major attention in the R & D of these two applications. For osmotic membrane bioreactor (MBR), the required membrane performance may not be as stringent as those for osmotic power and desalination [23,24], but finding low cost and easy recyclable draw solutes for osmotic MBR is still quite challenging unless RO retentate is

readily available to be used as the draw solute as RO retentate may provide adequate osmotic pressure and can be obtained at low or no cost if available. Although the fouling behavior of FO membranes is more reversible than RO membranes [25,26,27], the removal of foulants in the former is more complicated than the latter because of the internal concentration polarization when the feed stream faces the porous sublayer [24,28– 32]. In addition, owing to the high hydraulic pressure in the high-pressure compartment, it is believed that the

Table 1 Benefits and challenges of different applications of FO Applications of FO

Benefits

Challenges Ineffective membranes; lack of cost effective draw solutes

Osmotic power generation

Low energy consumption for water transport across the semi-permeable membrane Fertilizers are natural draw solutes; diluted draw solution is useful for irrigation Seawater is a natural draw solute

Osmotic membrane bioreactor

Low fouling and low energy consumption

Desalination Direct fertigation

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Limited application sites Pretreatments of seawater and river water; complicated fouling phenomenon owing to the high pressure in the seawater compartment Need to find low cost and easy recyclable draw solutes

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Table 2 A comparison of FO and RO processes Advantages

Disadvantages

Challenges

FO

Less energy intensive for water transport across the semi-membranes; more reversible fouling

Permeate water 6¼ product; requires a second separation step

RO

Permeate water = high quality product

High energy consumption; some irreversible fouling

Ineffective membranes; lack of cost effective draw solutes; limited studies on fouling How to improve energy recovery efficiency; How to mitigate membrane fouling

Process

fouling behavior during osmotic power generation may be quite different from that in low-pressure FO processes. So far, apart from Statkraft patents [21], almost no academic studies have touched this interesting subject. Table 1 summarizes benefits and challenges for each application of FO. Table 2 and Figure 2 show a comparison of FO and RO processes.

by Cadotte [34]; (iii) the layer-by-layer (LbL) deposition of nanometer-thick polycations and polyanions on porous charged substrates [35]; and (iv) aquaporin (Aqp) incorporated biomimetic membranes [36]. Wholly integrated asymmetric FO membranes made of cellulose triacetate (CTA) [37,38], polybenzimidazole (PBI) [39–42], cellulose acetate [43,44,45,46] and polyethersulfone [47] are typical examples of the 1st approach (as shown in Figure 3(a)–(d)), while FO membranes made of polyamide via interfacial polymerization on polysulfone based substrates [48,49], sulfonated substrates [50,51], cellulose acetate propionate (CAP) substrates [52] and nanofibers [53,54] belong to the second approach (as shown in Figure 3(e), (f)). Examples of LbL FO membranes can be found elsewhere [55,56].

The state-of-the-art FO membranes for low pressure processes The desired FO membranes must have (i) high salt retention and high water flux; (ii) low concentration polarization; and (iii) resistance to chlorine and wide range of pH plus long-term stability in separation performance and mechanical strength [6]. Up to the present, four approaches have been adopted to prepare polymeric FO membranes by using (i) the non-solvent phase inversion method developed by Loeb and Sourirajan [33]; (ii) the thin-film composition (TFC) method via interfacial polymerization on porous substrates invented

Usually, membranes derived from the phase inversion method have relatively low fluxes compared to those membranes made from TFC approach. In addition to their inherent differences in water permeability and salt

Figure 2

RO vs. FO Hydraulic pressure gradient (RO)

Osmotic pressure gradient (FO) No hydraulic pressure gradient

Water = Product

Seawater (feed)

NaCl

Water

Product

Seawater (feed)

Draw solution

NaCl Reverse flux of draw solutes Thick sub-layer The RO membrane is densified under high pressures

Thin sub-layer The FO membrane is loose under no or low pressures Current Opinion in Chemical Engineering

A comparison of (a) RO and (b) FO processes. Current Opinion in Chemical Engineering 2012, 1:246–257

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Current forward osmosis technology development Chung et al. 249

Figure 3

(a)

(b)

(c)

OL

40 µ 1.53µm

10.61µm

1 µm Single-layer PBI membrane (d)

Dual Layer PBI/PES membrane (e)

CA flat sheet membrane

(f)

200 µm

HTI CTA membrane

Thin-film interfacial polymerized flatsheet FO membrane

Thin-film interfacial polymerized FO hollow fiber Current Opinion in Chemical Engineering

Some of typical FO membranes for water reuse and desalination. (a) Single-layer polybenzimidazole (PBI) membrane1; (b) Dual Layer PBI/ polyethersulfone (PES) membrane2; (c) CA flat sheet membrane3; (d) Hydration Technology Innovations (HTI) CTA membrane; (e) Thin-film interfacial polymerized flat-sheet FO membrane4; and (f) Thin-film interfacial polymerized FO hollow fiber5. 1 Reprinted from ref. [40] with permission from Elsevier. 2 Adapted from ref. [42] with permission from Elsevier. 3 Adapted with permission from ref. [43]. Copyright (2010) American Chemical Society. 4 Reprinted from ref. [50] with permission from John Wiley and Sons. 5 Reprinted with permission from ref. [111]. Copyright (2012) American Chemical Society.

rejection [57,58,59], the phase inversion membrane also tends to have a greater sublayer resistance and internal concentration polarization (ICP) owing to the difficulties in controlling the selective skin and sublayer morphology simultaneously during the rapid phase inversion process, while the TFC membrane has more design flexibility by separately tuning selective skins and sublayers with the aid of using more porous or less tortuous membranes as the TFC substrates. Furthermore, by applying the duallayer co-extrusion technology or electrospun nano-fibers, one may have greater capabilities to effectively manipulate the sublayer morphology and significantly mitigate the low flux issue [42,45,53,54]. To reduce the ICP effects, Wang et al. were the first in inventing double-skinned FO membranes consisting of a less selective nano-filtration (NF) skin layer, a fully porous cross-section, and a highly selective RO skin layer [43]. Subsequent theoretical and experimental works have confirmed the unique characteristics and advantages of this type of membrane morphology such as low fouling and low ICP [60,61]. In addition, Fang et al. [62] and Su et al. [63] www.sciencedirect.com

also extended the basic principle of double skins to fabricate double-skin FO hollow fibers consisting of a NF and a RO skins. On the contrary, Wang et al. [50] and Widjojo et al. [51] adopt another scheme to circumvent the ICP. They reported that the hydrophilicity of porous substrates plays an important role on TFC FO membranes. TFC membranes that are interfacially polymerized on hydrophilic porous substrates not only show reduced ICP effects but also have a very high water flux (as shown in Figure 4). So far, 22 LMH (L m2 h1) is the highest ever reported water flux for TFC FO membranes in seawater desalination using 2.0 M NaCl as the draw solution (DS) by Widjojo et al. [64]. Consistent with Wang et al. [50] and Widjojo et al. [51] observation, Arena et al. surface modified the support layers of commercially available RO TFC membranes with polydopamine (PDA) to improve the membranes’ hydrophilicity for pressure retarded osmosis (PRO) [65]. Following the similar principle, Han et al. [66] surface modified hydrophobic polysulfone (PSf) substrates with polydopamine before conducting interfacial polymerization. Results show effective enhancements in both water flux and salt rejection of the resultant TFC membranes. Current Opinion in Chemical Engineering 2012, 1:246–257

250 Energy and environmental engineering

Figure 4

(a)

(b) 60

Pressure retarded osmosis (PRO) mode

Water flux (LMH)

50

20 µm

Cross section Macrovoid free

40

Hydrophilic substrate by using BASF materials

30

Forward osmosis (FO) mode

20

Sponge-like cross-section

Porous bottom surface

Cross section 40k thin film

10 0

1

2

3

4

5

Draw solution concentration, NaCl (M) Feed (DI water): flux 33 LMH (DS: 2 M NaCl), salt reverse flux 3.6 gMH Seawater (3.5 wt% NaCl): 15 LMH (DS: 2 M NaCl) Current Opinion in Chemical Engineering

TFC FO membrane with macrovoid-free substrate. (a) Water flux as a function of draw solution concentration, and (b) SEM images of TFC membrane. Adapted from ref. [51] with permission from Elsevier.

So far, FO membranes made from LbL have shown a trade-off between the water fluxes and the reverse salt leakages. LbL membranes reported either have low reverse salt leakages but low water fluxes (for the cross-linked ones) [56] or high water fluxes but with high reverse salt leakages (for the un-cross-linked ones) [55]. Since most data were obtained by using MgCl2 as the draw solute, no experimental data have proven that FO membranes prepared from LbL have good rejections to NaCl. A better design of LbL morphology and appropriate choices of electrolytes and cross-linkers are essential to advance LbL FO membranes for real applications in water reuse and desalination. FO membranes made from TFC/nano-fibers [53,54] also show significant differences in performance; observed water fluxes of 66 vs. 26 LMH have been reported using 1.5 M NaCl as the draw solution. A better understanding of the causes of the differences is essential for the advancement of this technology. Novel Aquaporin (Aqp) incorporated biomimetic FO membranes have recently been developed by Wang et al. [36]. The membranes were prepared by rupturing the AqpZ-embedded triblock copolymer vesicles on the acrylate-functionalized polycarbonate tracked-etched (PCTE) substrates. The planar porespanning biomimetic membrane displays the highest water flux of 142 LMH ever reported with very low reverse salt leakage using 2.0 M NaCl as the draw solution. However, the Aqp embedded membranes are not mechanically strong because the selective layer is only 10 nm in thickness. Current Opinion in Chemical Engineering 2012, 1:246–257

FO membranes for osmotic energy under PRO Theoretically, the hydraulic pressure difference in the seawater compartment during the mixing of river water and seawater across a semi-permeable membrane under PRO is preferred to operate at about 13.5 bars for seawater consisting of 3.5 wt% NaCl in order to generate the maximal energy output [19,20,21]. Since most conventional FO membranes are designed for no-pressure or low-pressure operation environments, currently available FO membranes are likely to be damaged under this high pressure condition. For example, based on a recent visit to Statkraft, the latest membranes used in Statkraft are only operated at about 6 bar because of membrane limitations [67]. Han et al. have recently developed flat asymmetric membranes with osmotic power density in the range of 6– 10 W/m2 that can withstand up to 15 bar using model seawater (0.59 M NaCl) and DI water [68,69,70]. To the best of our knowledge, among the available membranes for osmotic power generation [19,20,21,68,71–74], this is the first FO membrane that can withstand a hydraulic pressure difference over 13.5 bar and also produce a high energy output. It is worth mentioning that the experiments to estimate membrane’s power density must be conducted in actual PRO setup in which the hydraulic pressure varies in the high pressure compartment. As the real power density usually deviates a lot from the power density calculated from an extrapolation of water flux vs. pressure from the initial water flux under no hydraulic pressure difference. As a result, any conclusion derived from ideal theoretical predictions could be misleading. www.sciencedirect.com

Current forward osmosis technology development Chung et al. 251

Although an increase in membrane thickness and polymer concentration during casting or spinning may improve membrane’s mechanical strengths, it also results in a much lower water flux and power density. Therefore, the FO membrane design for power generation under high salinity gradients will be significantly different from those used in conventional low-pressure environments. One must consider membrane’s physicochemical properties in the wet state as well as their changes under tensile, elongation, compression, and bending stresses [75,76]. If RO retentate is to be used as the draw solution while retentate of recycled water (i.e. retentate from wastewater reclamation processes) as the feed solution as proposed elsewhere [6] and shown in Figure 1(b), as the salinity of RO retentate is much greater than that of seawater (about 7.9–8.5 vs. 3.5 wt%), it will then increase the salinity gradient and generate higher power output. The former can result in a much higher osmotic pressure (about 70–77 vs. 28 bar at 22.5 8C) and osmotic energy than the latter, but also create tremendous challenges for membrane scientists to design high flux FO membranes with super high mechanical strengths. However, if osmotic power generation and RO plants can be successfully integrated, not only can it make seawater desalination less energy dependent and more sustainable, but also significantly

alleviate the disposal and environmental issues of waste RO retentate. In addition, since the RO retentate has been well pre-treated in its previous processes, it can significantly reduce the membrane fouling in the high pressure compartment. As a result, the integration may save some of expensive pre-treatment costs originally required for seawater before PRO. In addition, the integration of RO and osmotic power generation will significantly alleviate the disposal of highly concentrated brine back to ocean. Therefore, from the environmental standpoint, the integration may provide a better ecosystem for habitats and species, water composition, and landscape.

The development of draw solutes Compared to FO membranes, the progress in draw solutes is much slower. This is owing to the fact that it is not trivial to design draw solutes with characteristics of (i) good water solubility; (ii) high osmotic pressures; (iii) low leakages or reverse fluxes; (iv) easy recovery; and (v) membrane compatibility and (vi) zero toxicity. Since the 1960s, many efforts have been devoted to discover suitable draw solutes such as sulfur dioxide [77], aluminum sulfate [78], glucose [79,80], fructose [80,81], sucrose [63], fertilizers [22], and inorganic salts [38–56,60–62,82]. Prof. Elimelech and his colleagues at

Figure 5

2-Pyrol-MNP:

TREG-MNP:

Fe(acac)3+triethylene glycol

PAA-MNP:

O

Fe(acac)3+2-pyrrolidine

Fe(acac)3+triethylene glycol+ polyacrylic acid

245ºC reflux

280ºC reflux

280ºC reflux

O O

Fe O

O

Structure of Tris(acetylacetonato) Iron: Fe(acac)3

O

Current Opinion in Chemical Engineering

Schematic diagram of syntheses of water soluble magnetic nano-particles (MNP): 2-Pyrol-MNP, TREG-MNP, and PAA-MNP. Reprinted with permission from ref. [86]. Copyright (2010) American Chemical Society. www.sciencedirect.com

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252 Energy and environmental engineering

Yale University developed the first generation of draw solution from water and ammonium bicarbonate (NH4HCO3) mixtures for desalination in the early 2000s. The draw solute of NH4HCO3 decomposes to ammonia and carbon dioxide upon heating at about 65 8C and it can be regenerated by re-dissolution [2,83]. However, using compounds with small molecules as draw solutes may not be economic and practical because of high energy consumption in their recycles and significant reverse fluxes in FO processes. Small molecular salts may also induce clogging in the supporting layer and lead to severe fouling and internal concentration polarization [30,31,84,85]. By taking advantages of the characteristics of high surface area and high osmotic pressure, hydrophilic magnetic nanoparticles were developed by Ling et al. [86] and Ge et al. [87] as draw solutes. The original idea was to produce pure water as well as to recapture nanoparticles by using a magnetic separator. Figure 5 shows syntheses of water soluble magnetic nano-particles [86] and Figure 6 illustrates the draw solution regeneration of water soluble magnetic nano-particles in FO processes. However, the nanoparticles gradually clumped together owing to the strong magnetic field. As a result, the osmotic pressure of draw solutions reduced after regeneration and so did the yield of fresh water. Ling and Chung demonstrated that the use of an ultrafiltration (UF) process can eliminate the magnetic field induced agglomeration [88]. To enhance the separation efficiency of nanoparticles from water and minimize the loss of nanoparticles during the UF recycle process, Ling et al. designed the nanoparticles comprising an outer layer of a temperature sensitive amphiphilic polymer [89]. Below 34 8C, the

nanoparticles performed as draw solutes because of strong hydrogen bonding interactions with water, while above 37 8C, the nanoparticles clumped together as hydrophobic globules, making them easier to be captured by means of UF. Recently, a series of novel draw solutes based on polyelectrolytes of PAA-Na salts were developed by Ge et al. [90]. The characteristics of high solubility in water and flexibility in structural configuration enable this type of draw solutes to generate high water fluxes yet with insignificant reverse salt fluxes in the FO process. These unique properties not only ensure high efficiency in water reclamation and high quality in water product, but also lower the replenishment cost of draw solutes. In addition, PAA-Na salts have good stability and show repeatable performance after many recycles. Figure 7 shows some common draw solutes and preparation of poly(acrylic acid sodium) (PAA-Na).

Integrated systems for clean water production and draw solute regeneration Sustainable integrated systems for water production and draw solute recycle must be developed in order to successfully market FO technologies. For seawater desalination, researchers have proposed the integration of FO and RO/NF processes for draw solute recovery and clean water production [91–93]. They are technically feasible but economically and industrially unpractical because of high energy costs to operate RO and NF for draw solute recycles. If waste heat or cold energy is available, an integrated FO–MD (forward osmosis–membrane distillation) system (as shown in Figure 8(a)) is a promising process for seawater desalination [94]. The ‘cold energy’

Figure 6

Feed (seawater)

Concentrated draw solution

Draw solution regeneration Magnetic field N

S

FO membrane Product water Magnetic nano-particles recycled back to FO

Concentrated brine

Diluted draw solution Current Opinion in Chemical Engineering

Schematic diagram of water soluble magnetic nano-particles draw solutes for FO processes. Current Opinion in Chemical Engineering 2012, 1:246–257

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Current forward osmosis technology development Chung et al. 253

Figure 7

(a)

Na+ Cl–

CH2OH O

CH2OH

OH

O

OH

Mg2+ Cl2–

O

OH OH

NH4HCO3 or NH3/CO2 Salts

CH2OH OH

Magnetic nanoparticles

Sucrose

(b)

H

H

C

C

H

NaOH

H

C

C

H

C O

H

C O

OH

O- Na+ n

n PAA

PAA-Na Current Opinion in Chemical Engineering

(a) Some common draw solutes and (b) preparation of poly(acrylic acid sodium) (PAA-Na).

refers to the heat absorption effect from the ambient surrounding when liquefied natural gas (LNG) is regasified at the LNG terminals [95]. However, the provision of low-cost waste heat for MD is the pre-condition for the success of the integrated FO–MD process [96,97]. By using highly hydrophilic nano-particles as draw solutes, one may minimize the fouling issues including scaling and crystallization in MD. Several attempts have

been made. Yen et al. [98] and Wang et al. [99] were one of the firsts demonstrating the FO–MD process for water reuse and protein enrichment applications, respectively. Su et al. extended their works by using a novel CAP polymer as the FO membrane material and 0.5 M MgCl2 as the draw solution for wastewater reclamation [100], while Ge et al. developed a polyelectrolyte-promoted FO–MD hybrid system for the recycle of wastewater

Figure 8

(a)

FO

(b)

MD

FO

RO

Draw solution feed solution

Feed

Draw solution

Draw solution

Draw solution

MD

Clean water

feed solution

Draw solution

Clean water

Feed Current Opinion in Chemical Engineering

Integrated (a) FO–MD and (b) FO–RO systems to regenerate the draw solution and produce water. www.sciencedirect.com

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254 Energy and environmental engineering

and an acid dye [101]. High performance home-made polyvinylidene fluoride (PVDF) membranes [102–104] were used in their studies to recycle draw solute and produce product water. Many useful design principles and membrane modifications for MD can be found elsewhere [105–107]. However, to the best of our knowledge, no demonstration of FO–MD process is available for seawater desalination. This is owing to the fact that we still lack (1) high performance FO membranes with high fluxes and high salt rejections; and (2) cost effective draw solutes with high osmotic pressures and minimal reverse fluxes. For water reuse, FO–UF, FO–NF, FO–MD and FO–RO integrated systems may have great potential depending on the quality of feed solutions, physicochemical properties of draw solutions, and applications [63,99,100,101,108,109]. UF is the most preferred because it is a well-established low energy filtration process compared to RO and NF [110]. In addition, various types of UF membranes and modules are commercially available. As a result, the overall system development and operation cost for FO–UF are easier and more affordable compared to other integrated systems. On the contrary, if waste but clean RO retentate is employed as the draw solute, a FO–RO integrated system (as shown in Figure 8(b)) is recommended to remove mono-valent ions. A combined system comprising FO, UF and magnetic separators may be also a good choice for water reuse. However, most existing magnetic separators possess high magnetic fields because they are designed for other purposes. To avoid particle aggregation, tailored magnetic separators with tunable magnetic strengths are needed to recycle different magnetic nanoparticles in the FO process.

Conclusion It took about 40 years (from about 1960 to about 2000) for RO to surpass thermal multi-effect evaporation technologies as the dominant technology in seawater desalination. Technology evolutions on both RO membranes and process design have been continuously taking place to increase membrane performance and achieve better energy efficiency and mitigate fouling. Similarly, FO technologies may appear promising but are still in the infancy stage. Time and more R & D efforts are needed in order to have significant breakthroughs on FO membranes, draw solutes and their regeneration methods so that the FO technologies can compete effectively with the well-established RO technologies for seawater desalination. Commercialization of FO for fertigation appears promising, while cost effective and easily recyclable draw solutes must be found for water reuse. The use of RO retentate as the draw solute for water reuse may lower the operation cost and bring FO closer to commercialization. A successful integration of osmotic power generation and RO desalination plants will entirely revolutionize the future power and desalination industries. However, Current Opinion in Chemical Engineering 2012, 1:246–257

membrane scientists must overcome the challenges to design high flux FO membranes with extremely robust mechanical properties to withstand the operating pressure in the high pressure PRO process. Encouragingly, a few breakthroughs on high flux and high strength FO membranes, draw solutes with high osmotic pressures, and advanced integrated systems for water production and draw solute recycle have been recently demonstrated.

Acknowledgements This research was funded by the Singapore National Research Foundation under its Competitive Research Program for the project entitled, ‘‘Advanced FO Membranes and Membrane Systems for Wastewater Treatment, Water Reuse and Seawater Desalination’’ (grant numbers: R279-000-336-281 and R-279-000-339-281). The authors also thank Miss Sui Zhang, Dr. Jincai Su, Dr. Natalia Widjojo, Miss Sicong Chen, Miss Yue Cui for their help and suggestions. Special thanks are due to BASF, Eastman Chemicals and Mitsui Chemicals for their financial supports as well as Prof. Donald R. Paul, University of Texas at Austin, Dr. J.J. Qin, Public Utilities Board (PUB, Singapore), Prof. D. Bhattacharyya, University of Kentucky, Dr. Subhas Sikdar, National Risk Management Research Laboratory, US EPA, Prof. Gary Amy, KAUST as well as the editorial team of COCHE (Prof. Sirkar and Prof. Agrawal) for their valuable suggestions.

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5.

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6. 

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8. 

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19. Gerstandt K, Peinemann K-V, Skilhagen SE, Thorsen T, Holt T:  Membrane processes in energy supply for an osmotic power plant. Desalination 2008, 224:64-70. This provides the fundamental knowledge of energy production via PRO.

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39. Wang KY, Chung TS, Qin JJ: Polybenzimidazole (PBI) nanofiltration hollow fiber membranes applied in forward osmosis process. J Membr Sci 2007, 300:6-12. 40. Wang KY, Yang Q, Chung TS, Rajagopalan R: Enhanced forward osmosis from chemically modified polybenzimidazole (PBI) nanofiltration hollow fiber membranes with a thin wall. Chem Eng Sci 2009, 64:1577-1584. 41. Hausman R, Digman B, Escobar IC, Coleman M, Chung TS: Functionalization of polybenzimidizole membranes to impart negative charge and hydrophilicity. J Membr Sci 2010, 363:195-203. 42. Yang Q, Wang KY, Chung TS: Dual-layer hollow fibers with enhanced flux as novel forward osmosis membranes for water reclamation. Environ Sci Technol 2009, 43:2800-2805. 43. Wang KY, Ong RC, Chung TS: Double-skinned forward osmosis  membranes for reducing internal concentration polarization within the porous sublayer. Ind Eng Chem Res 2010, 49:4824-4831. This is the first work on the invention of double skinned forward osmosis membranes. 44. Su JC, Yang Q, Teo JF, Chung TS: Cellulose acetate nanofiltration hollow fiber membranes for forward osmosis processes. J Membr Sci 2010, 355:36-44. 45. Su JC, Chung TS: Sublayer structure and reflection coefficient  and their effects on concentration polarization and membrane performance in FO processes. J Membr Sci 2011, 376:214-224. This paper covers a holistic study on the effects of membrane structural parameters on the forward osmosis performance. 46. Zhang S, Wang KY, Chung TS, Jean YC, Chen HM: Molecular design of the cellulose ester-based forward osmosis membranes for desalination. Chem Eng Sci 2011, 66:2008-2018. 47. Yu Y, Seo S, Kim IC, Lee S: Nanoporous polyethersulfone (PES) membrane with enhanced flux applied in forward osmosis process. J Membr Sci 2011, 375:63-68. 48. Yip NY, Tiraferri A, Phillip WA, Schiffman JD, Elimelech M: High  performance thin-film composite forward osmosis. Environ Sci Technol 2010, 44:3812-3818. This study is one of the firsts to report the achievement of high water flux by inventing thin film composite membranes specially designed for forward osmosis. Current Opinion in Chemical Engineering 2012, 1:246–257

256 Energy and environmental engineering

49. Wang R, Shi L, Tang CY, Chou S, Qiu C, Fane AG:  Characterization of novel forward osmosis hollow fiber membranes. J Membr Sci 2010, 355:158-167. This study is one of the firsts to report the achievement of high water flux by inventing thin film composite hollow fiber membranes specially designed for forward osmosis. 50. Wang KY, Chung TS, Amy G: Developing thin-film-composite forward osmosis membranes based on the PES/SPSf substrate through interfacial polymerization. AIChE J 2012, 58:770-781. 51. Widjojo N, Chung TS, Weber M, Maletzko C, Warzelhan V: The  role of sulphonated polymer and macrovoid-free structure in the support layer for thin-film composite (TFC) forward osmosis (FO) membranes. J Membr Sci 2011, 383:214-223. This study reported the highest water flux performance for forward osmosis desalination to date. 52. Li X, Wang KY, Helmer B, Chung TS: Thin-film composite membranes and formation mechanism of thin-film layers on hydrophilic cellulose acetate propionate substrates for forward osmosis processes. Ind Eng Chem Res, http:// dx.doi.org/10.1021/ie2027052. 53. Song X, Liu Z, Sun DD: Nano gives the answer: breaking the  bottleneck of internal concentration polarization with a nanofiber composite forward osmosis membrane for a high water production rate. Adv Mater 2011, 23:3256-3260. This study is one of the firsts to report using a very porous nanofiber membrane as the support for TFC membranes with reduced ICP. 54. Bui NN, Lind ML, Hoek EMV, McCutcheon JR: Electrospun  nanofiber supported thin film composite membranes for engineered osmosis. J Membr Sci 2011, 385–386:10-19. This study is one of the firsts to report using a very porous nanofiber membrane as the support for TFC membranes. 55. Qi S, Qiu CQ, Tang CY: Synthesis and characterization of novel forward osmosis membranes based on layer-by-layer assembly. Environ Sci Technol 2011, 45:5201-5208. 56. Qiu C, Qi S, Tang CY: Synthesis of high flux forward osmosis membranes by chemically crosslinked layer-by-layer polyelectrolytes. J Membr Sci 2011, 381:74-80. 57. Sagle A, Freeman B: Fundamentals of membranes for water treatment.The Future of Desalination in Texas. Austin, TX: Texas Water Development Board; 2004: 137–154.

enhanced performance for forward osmosis (FO), submitted for publication. 65. Arena JT, McCloskey B, Freeman BD, McCutcheon JR: Surface modification of thin film composite membrane support layers with polydopamine: enabling use of reverse osmosis membranes in pressure retarded osmosis. J Membr Sci 2011, 375:55-62. 66. Han G, Zhang S, Li X, Widjojo N, Chung TS: Thin film composite  forward osmosis membranes based on polydopamine modified polysulfone substrates with enhancements in both water flux and salt rejection. Chem Eng Sci 2012, 80:219-231. This paper demonstrates the improvement of separation performance of thin-film composite FO membranes. 67. Chung TS: Personal communication during a recent visit to Statkraft in Oct 13, 2011. 68. Han G, Zhang S, Li X, Chung TS: High performance pressure  retarded membranes: break the power output bottleneck of harvesting renewable osmotic power from nature salinitygradient resources, in preparation. The membranes show a very high power density and can withstand high pressures that may provide important insights to the design of membranes for osmotic power generation. 69. Li X, Zhang S, Han G, Sun S, Chung TS: Molecular design of membranes for osmotic power generation. 22nd Annual Meeting 2012; North American Membrane Society, New Orleans: 2012. 70. Zhang S, Li X, Han G, Sun S, Sukitpaneenit P, Chung TS: Membrane development for osmotic power generation at National University of Singapore (NUS). 3rd Osmosis Summit 2012; Barcelona: 2012. 71. McGinnis RL, McCutcheon JR, Elimelech M: A novel ammoniacarbon dioxide osmotic heat engine for power generation. J Membr Sci 2007, 305:13-19. 72. Achilli A, Cath TY, Childress AE: Power generation with pressure retarded osmosis: an experimental and theoretical investigation. J Membr Sci 2009, 343:42-52. 73. Yip NY, Elimelech M: Performance limiting effects in power generation from salinity gradients by pressure retarded osmosis. Environ Sci Technol 2011, 45:10273-10282. 74. Chou S, Wang R, Shi L, She Q, Tang CY, Fane AG: Thin-film composite hollow fiber membranes for pressure retarded osmosis (PRO) process with high power density. J Membr Sci 2012, 389:25-33.

58. Geise GM, Lee HS, Miller DJ, Freeman BD, McGrath JE, Paul DR:  Water purification by membrane: the role of polymer science. J Polym Sci Part B: Polym Phys 2010, 48:1685-1718. This paper is an in-depth study on polymer science to relate how the polymer basic characteristics affect membranes’ water transport properties.

75. Zhang S, Fu FJ, Chung TS: Substrate modifications and alcohol treatment on thin film composite membranes for osmotic power, submitted for publication.

59. Zhang S, Zhang RW, Jean YC, Paul DR, Chung TS: Cellulose esters for forward osmosis: characterization of water and salt transport properties and free volume. Polymer 2012, 53:2664-2672.

76. Li X, Zhang S, Fu FJ, Chung TS: Deformation and reinforcement of thin-film composite (TFC) polyamide-imide (PAI) membranes for osmotic power generation, submitted for publication.

60. Zhang S, Wang KY, Chung TS, Chen HM, Jean YC, Amy G: Wellconstructed cellulose acetate membranes for forward osmosis: minimized internal concentration polarization with an ultra-thin selective layer. J Membr Sci 2010, 360:522-535.

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61. Tang CY, She Q, Lay WCL, Wang R, Field R, Fane AG: Modeling double-skinned FO membranes. Desalination 2011, 283:178-183. 62. Fang W, Wang R, Chou S, Setiawan L, Fane AG: Composite forward osmosis hollow fiber membranes: integration of ROand NF-like selective layers to enhance membrane properties of anti-scaling and anti-internal concentration polarization. J Membr Sci 2012, 394–395:140-150. 63. Su JC, Chung TS, Helmer BJ, de Wit JS: Enhanced doubleskinned FO membranes with inner dense layer for wastewater treatment and macromolecule recycle using Sucrose as draw solute. J Membr Sci 2012, 396:92-100. 64. Widjojo N, Chung TS, Weber M, Maletzko C, Warzelhan V: A sulfonated polyphenylenesulfone (sPPSU) as the supporting substrate in thin film composite (TFC) membranes with Current Opinion in Chemical Engineering 2012, 1:246–257

78. Frank BS: Desalination of sea water. US Patent 3,670, 897; 1972. 79. Kravath RE, Davis JA: Desalination of sea-water by direct osmosis. Desalination 1975, 16:151-155. 80. Kessler JO, Moody CD: Drinking-water from seawater by forward osmosis. Desalination 1976, 18:297-306. 81. Stache K: Apparatus for transforming seawater, brackish water, polluted water or the like into a nutritious drink by means of osmosis. US Patent 4,879,030; 1989. 82. Achilli A, Cath TY, Childress AE: Selection of inorganic-based draw solutions for forward osmosis applications. J Membr Sci 2010, 364:233-241. 83. McCutcheon JR, McGinnis RL, Elimelech M: A novel ammonia carbon dioxide forward (direct) osmosis desalination process. Desalination 2005, 174:1-11. This paper reported an ammonium bicarbonate solution as the draw solution that can be a viable desalination process. www.sciencedirect.com

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84. Hancock NT, Cath TY: Solute coupled diffusion in osmotically driven membrane processes. Environ Sci Technol 2009, 43:6769-6775.

98. Yen SK, Haja NFM, Su ML, Wang KY, Chung TS: Study of draw solutes using 2-methylimidazole based compounds in forward osmosis. J Membr Sci 2010, 364:242-252.

85. Phillip WA, Yong JS, Elimelech M: Reverse draw solute permeation in forward osmosis: modeling and experiments. Environ Sci Technol 2010, 44:5170-5176.

99. Wang KY, Teoh MM, Nugroho A, Chung TS: Integrated forward osmosis-membrane distillation (FO-MD) hybrid system for the concentration of protein solutions. Chem Eng Sci 2011, 66:2421-2430.

86. Ling MM, Wang KY, Chung TS: Highly water soluble magnetic  nanoparticles as novel draw solutes in forward osmosis for water reuse. Ind Eng Chem Res 2010, 49:5869-5876. The application of highly water-soluble magnetic nanoparticles as draw solutes was reported for the first time in the work.

100. Su JC, Ong RC, Wang P, Chung TS, Helmer BJ, de Wit JS: Advanced FO membranes from newly synthesized CAP polymer for wastewater reclamation through an integrated FO-MD hybrid system. AIChE J, in press.

87. Ge QC, Su JC, Chung TS, Amy G: Hydrophilic  superparamagnetic nanoparticles: synthesis, characterization, and performance in forward osmosis processes. Ind Eng Chem Res 2011, 50:382-388. The paper gives an investigation for the synthesis of PEG based magnetic nanoparticles and their application in FO process. The draw solutes exhibited high osmotic pressures and good recovery behavior.

101. Ge QC, Wang P, Wan CF, Chung TS: Polyelectrolyte-promoted  forward osmosis-membrane distillation (FO-MD) hybrid process for dye wastewater treatment. Environ Sci Technol 2012, 46:6236-6243. This paper firstly demonstrates the concept of a polyelectrolyte-promoted FO–MD system. The wastewater was efficiently dehydrated and hybrid technology showed potential in other separation processes.

88. Ling MM, Chung TS: Desalination process using super hydrophilic nanoparticles via forward osmosis integrated with ultrafiltration regeneration. Desalination 2011, 278:194-202.

102. Su ML, Teoh MM, Wang KY, Su JC, Chung TS: Effect of innerlayer thermal conductivity on flux enhancement of dual-layer hollow fiber membranes in direct contact membrane distillation. J Membr Sci 2010, 364:278-289.

89. Ling MM, Chung TS, Lu XM: Facile synthesis of thermosensitive magnetic nanoparticles as ‘‘smart’’ draw solute in forward osmosis. Chem Commun 2011, 47:10788-10790. 90. Ge QC, Su JC, Amy G, Chung TS: Exploration of polyelectrolytes  as draw solutes in forward osmosis processes. Water Res 2012, 46:1318-1326. This paper firstly explored polyelectrolytes as draw solutes with low salt leakages, high salt rejection, repeatable recycling performance, no aggregation problems.

103. Teoh MM, Chung TS, Yeo YS: Dual-layer PVDF/PTFE composite hollow fibers with a thin macrovoid-free selective layer for water production via membrane distillation. Chem Eng J 2011, 171:684-691. 104. Wang P, Teoh MM, Chung TS: Morphological architecture of dual-layer hollow fiber for membrane distillation with higher desalination performance. Water Res 2011, 45:5489-5500.

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108. Zhao S, Zou L, Mulcahy D: Brackish water desalination by a hybrid forward osmosis-nanofiltration system using divalent draw solute. Desalination 2012, 284:175-181. 109. Lutchmiah K, Cornelissen ER, Harmsen DJH, Post JW, Lampi K, Ramaekers H, Rietveld LC, Roest K: Water recovery from sewage using forward osmosis. Water Sci Technol 2011, 64:1443-1449. 110. Zeman LJ, Zydney AL: Microfiltration and Ultrafiltration: Principles and Applications. Marcel Dekker, Inc.; 1996. 111. Sukitpaneenit P, Chung TS: High performance thin-film  composite forward osmosis hollow fiber membranes with macrovoi-free and highly porous structure for sustainable water production. Environ Sci Technol 2012, 46:7358-7365. This paper presents well-constructed thin-film composite (TFC) hollow fiber membranes with high-performance.

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