FORWARD OSMOSIS

Jeffrey McCutcheon and Liwei HuangUniversity of Connecticut, Storrs, CT

1 WHAT ARE OSMOTIC PROCESSES?

For decades, aqueous separations have relied on hydraulic pressure to force water acrossmembranes that retain suspended and dissolved solids. This pressure is generated bypumps that can require a large amount of electricity to operate. This can have drawbacks,including increased carbon footprint and cost as energy prices continue to rise. While mostof the membrane science community has attempted to address this problem by makingbetter membranes that have a higher permeability or higher selectivity, few efforts havebeen focused on changing the driving force of separation.

Recently, osmotic pressure has been considered as an alternative driving force becauseosmosis occurs spontaneously without any work needed from an external source. Thishas led to the emergence of a field in membrane science known as engineered osmosis orsalinity-driven processes. This platform technology has garnered much interest in recentyears as a tool for purifying and desalinating water, concentrating/dewatering solutions,and generating electricity. All of these achievements are accomplished through a simple,yet engineered, salinity gradient across a membrane.

First conceived in the 1960s (1, 2), using salinity gradients for separations and powergeneration has found a fierce resurgence as of late. Much of this resurgence hinges notupon the early publications (3) and patents (4–6) on the technology, but rather the recentexplosion of academic publications in the area, starting in 2005 (7). Since then, wellover 150 papers on salinity-driven processes have been published in the peer-reviewedliterature.

Engineered osmosis relies on the spontaneously occurring osmotic flow between twoaqueous solutions of differing osmotic potential. While this is their single unifying theme,harnessing osmotic flow is motivated by different purposes. In some cases, the osmoticflow can be a purified water stream (forward osmosis , FO). In others, the flow might bediscarded as waste (direct osmotic concentration , DOC). In yet others, the water itselfis irrelevant and rather the motion of the flow itself is the product (pressure-retardedosmosis , PRO). In all cases, the choice of the osmotic agent , or draw solution , whichdrives the osmotic flow, is tailored depending on the desired product.

Encyclopedia of Membrane Science and Technology. Edited by Eric M.V. Hoek and Volodymyr V. Tarabara.Copyright © 2013 John Wiley & Sons, Inc.

1

2 FORWARD OSMOSIS

1.1 Forward Osmosis

FO is one of the most commonly used terms to describe any salinity-driven process.However, this vernacular can be misleading. FO refers to an osmotic separations tech-nology where drinking water is the primary product. This requires eventual separation ofwater from the osmotic agent. In FO, saline water is fed to one side of a membrane whilethe draw solution is fed to the other side. Osmotic separation takes place spontaneouslyas water moves from the relatively dilute saline solution into the draw solution. Themembrane, which is ideally semipermeable, retains the solutes on the feed side whilealso preventing crossover of solutes from the draw solution. Thus, we have separatedwater from the original salts without expending any energy except for delivering thewater to the membrane at low pressure.

The next step requires additional effort. The now diluted draw solution is sent toa secondary separation process that is designed specifically for the draw solute chosen(Fig. 1). Volatile solutes, for example, can be stripped out of solution, while others maybe better retained by a second membrane process. The cost of this regeneration is whatdrives process economics and the choice of draw solute requires serious consideration.Types of draw solutes are discussed later in this chapter.

The advantages of FO are dependent on the type of draw solute and draw soluterecovery system chosen. Specific FO processes have the potential to use less costlyenergy (such as waste heat) than reverse osmosis (RO) (8). A draw solute can be chosento provide large driving forces and thus exceed the water recoveries typically garneredwith RO processes (9). For applications with high fouling feeds, such as membranebioreactors or waters with a high scaling potential, FO has the potential for a lowerfouling and scaling propensity (10).

1.2 Direct Osmotic Concentration

DOC is similar to FO except that the concentrated brine stream is the product (Fig. 2).DOC is an osmotic dewatering process that allows for the concentration of products suchas fruit juices (11–13), landfill leachate (14), produced water (15), and proteins (16).The choice of draw solution is dependent on the similar considerations for FO. One

Membrane

Drawsolution

Drawsolute

recovery

Potablewater

EnergyinputSaline water

Brine

FIGURE 1 Illustration of the forward osmosis (FO) process with draw solute recycle. (Pleaserefer to the online version for the color representation of the figure.)

FORWARD OSMOSIS 3

Membrane

Drawsolution

Drawsolute

recovery

Water

EnergyinputSaline water

Brine

FIGURE 2 Illustration of the direct osmotic concentration (DOC) process. (Please refer to theonline version for the color representation of the figure.)

exception is an increased importance of draw solute crossover. Solutes diffusing acrossthe membrane into the feed may spoil the ultimate product. Fouling may also be moreof a concern in DOC given the high concentration of dissolved and suspended solids inthe feed solution as it is dewatered. The low fouling propensity of FO processes mayfind additional value with DOC.

1.3 Osmotic Dilution

Osmotic dilution is a term rarely used, but it does constitute one of the only commercialosmotic processes available at the time of this writing. In osmotic dilution, the dilutedform of the draw solute is utilized directly. The Hydration Technology Innovations (HTI)products known as the X-pack, Sea-Pack, hydropack, Hydrowell, and Village systems allrely on osmotic dilution (17). In these systems, a highly concentrated sugar–electrolytesolution is placed on the inside of the membrane, while the outside of the membrane isexposed to an impaired or saline water source. Water moves across the membrane byosmosis, while the membrane retains salts and contaminants on the feed and preventsthe flux of solutes from the draw. The draw solution becomes diluted (sometimes byfactors of up to 30 times), producing a clean drink immediately ready for ingestion. Themembrane used in these products has been the standard membrane used in many of theFO and PRO studies referenced throughout this chapter. Figure 3 illustrates this process.

Other opportunities for using osmotic dilution exist when concentrates need dilutionbut clean freshwater is in short supply. One such opportunity is in fertigation, whereconcentrated fertilizer is used as the draw solute and otherwise impaired water as thefeed (18). However, any relevant concentrate can be considered for this type of process.

1.4 Pressure-Retarded Osmosis

Much like a gas expands when heated, osmosis is the expansion of a liquid. The solutes insolution wish to move as far away from each other as possible, yet they are restricted bythe phase boundaries of the liquid. Osmotic flow can relieve this “pressure,” allowing thefluid to expand. If this expansion can be captured by a turbine, it can generate electricity.When the fluid expansion is accomplished osmotically, it is referred to as pressure-retarded osmosis (PRO). PRO harnesses the chemical potential difference caused by

4 FORWARD OSMOSIS

Membrane

Drawsolution

Saline water

Brine

Diluteddraw

solution

FIGURE 3 Illustration of the osmotic dilution process. (Please refer to the online version for thecolor representation of the figure.)

naturally occurring and engineered salinity gradients and converts it into electricity usinga hydraulic pressure intermediate (Fig. 4).

PRO was first conceived of in the 1970s, when Sidney Loeb and others identifiedsalinity gradients as a means of generating electricity (19–21). In these cases, salinewater bodies such as the ocean, the Dead Sea (22), or the Great Salt Lake (23) can actas draw solutions. Freshwater rivers and streams that pour into these saline waters canbe diverted and the mixing can be controlled by a membrane. To work, saline water ispressurized to a level below its osmotic pressure, thus retarding the osmotic flow butcreating a resistance to generate work. The subsequent expansion of the diluted salinewater through a hydroturbine generates electricity. Power generated is calculated usingthe following equation.

P = JW �P E

where P is the power (in watts per square meter membrane area), JW is the water flux,�P is the water transmembrane hydraulic pressure (from a pressurized draw solution),and E is the turbine efficiency. As hydraulic pressure is increased, osmosis is furtherretarded and JW is reduced. Efficiencies of hydroturbines are considered quite good(above 90%) and sometimes this term is dropped from the equation. The thought of

Work

Drawsolution

(sea water)

Diluteddrawsolute

Mix anddiscard to

draw source

P

Membrane

Fresh water

X

FIGURE 4 Illustration of pressure-retarded osmosis (PRO) using a naturally occurring salinitygradient. The pressure exchanger (PX) maintains a high pressure region in the element on the drawside. (Please refer to the online version for the color representation of the figure.)

FORWARD OSMOSIS 5

using this entirely untapped renewable energy source was very appealing in the 1970sduring the OPEC oil embargo. While increasing demand for energy independencespurred early research, once the embargo ended and energy prices came down, themotivation for continuing the research was lost. PRO was a mostly dormant field for thenext 20 years. At the turn of the century, interest in PRO was renewed as energy pricessoared (23–26). As a new membrane emerged that promised to make osmotic processesmore feasible, alternative PRO systems were conceived, including a “closed-loop” PROsystem, which uses an engineered draw solute, rather than a natural saline water body.These osmotic heat engines were designed to take low grade heat and convert it intoelectricity with the chemical potential intermediate of a salinity gradient (Fig. 5) (27).

The distinct advantage of using an engineered draw solute is that the osmotic pressure,and therefore the flux, can be increased far beyond what might be available in a naturallyoccurring saline water body. Engineered draw solutes can have osmotic pressures anorder of magnitude higher than seawater, thus resulting in high power densities. Theonly challenge is finding sources of waste heat that have no economic value but can stillbe harnessed for such a process.

Drawsolution

WorkMembrane

PX

Drawsolute

recovery

Energyinput

Pure water working fluid

FIGURE 5 Illustration of an osmotic heat engine. The pressure exchanger (PX) maintains a highpressure region in the element on the draw side. (Please refer to the online version for the colorrepresentation of the figure.)

Water flux

0

Flux reversepoint

FO or DOC(ΔP = 0)

PRO(ΔP < Δπ)

RO(ΔP > Δπ)

ΔPΔπ

FIGURE 6 Illustration of flux versus driving force. The reverse osmosis (RO), pressure-retardedosmosis (PRO), and forward osmosis/direct osmotic concentration (FO/DOC) regimes are indicated.�P is the transmembrane hydraulic pressure and �π is the transmembrane osmotic pressure.

6 FORWARD OSMOSIS

1.5 Summary of Processes

While the purpose of each of these processes is very different, the underlying principlesof osmotic pressure as a driving force are the same. We can describe the flux type usinga figure modified from a review by Cath et al. (28). The region representing RO flux iscommonly seen in studies of cases in which increase in pressure results in increase inwater flux. However, as the transmembrane hydraulic pressure drops below the osmoticpressure of a solution, osmotic flux occurs, but it is retarded by the transmembranehydraulic pressure. This is the PRO regime. When the transmembrane hydraulic pressureis zero, then it is the condition of FO and DOC where water flux is entirely driven byosmosis (Fig. 6).

2 MEMBRANE TRANSPORT

All salinity-driven processes rely on a semipermeable membrane that can retain soluteson the feed side and the draw side, while simultaneously enabling high flux. While ROmembranes exhibit these qualities for pressure-driven flow, osmotic flow requires solutesto directly interact with the membrane. This means that the membrane structure andchemistry play a role in determining the osmotic flux across the membrane.

To better understand this, we begin with first looking at how flux is generated in RO.In RO, flux is calculated by the following equation

JW = A(�P − �π) (1)

where JW is the water flux, A is the water permeance or permeability coefficient, �P isthe transmembrane hydraulic pressure, and �π is the transmembrane osmotic pressure.In osmosis, the �P term is dropped and flux is driven simply by the osmotic pressuredifference between the feed, πF, and draw, πD, solutions.

JW = A(πD − πF) (2)

This is the fundamental equation of osmotic flux. However, the osmotic pressure termsrefer to “effective” osmotic pressures. For instance, the membrane may not be perfectlyselective, meaning that a portion of the solutes from the feed and draw may move freelythrough the membrane and therefore do not contribute to the osmotic pressure differential.Furthermore, as water moves through the membrane by osmosis, solutes are concentratedat the feed side of the membrane and diluted at the draw side of the membrane. Thisresults in boundary layers that form on both sides of the membrane, which lead toa reduction of effective osmotic driving force. This concentration polarization (CP )has been long studied in RO and ultrafiltration processes (29), but CP during osmosisproduces a more dramatic effect on flux. During osmosis, boundary layers are establishedon both sides of the membrane, leading to a reduced driving force as salts are concentratedon the feed side and diluted on the draw side (Fig. 7).

Figure 7 illustrates these “effective” osmotic pressures as being those at the interfaceof the selective membrane (πD,m, πF,m) relative to the osmotic pressures in the “bulk”solution (πD,b, πF,b). We can define a ratio of the membrane interface osmotic pressureto that in the bulk, (πD,m/πD,b, πF,m/πF,b) as the CP modulus. This ratio is greater than1 on the feed side and less than 1 on the draw side.

FORWARD OSMOSIS 7

Δπeff Δπtheo

πD,b

πD,m

πF,m

πF,b

Denselayer

JW

FIGURE 7 Concentration polarization across a dense, selective membrane. (Please refer to theonline version for the color representation of the figure.)

The modulus could be calculated using a simple film theory model proposed byMcCutcheon and others (30) to quantify CP modulus on the feed side as a function ofthe Peclet number

πF,m

πF,b= exp

(JW

kF

)(3)

and on the draw sideπD,m

πD,b= exp

(−JW

kD

)(4)

The Peclet number is a dimensionless quantity that relates advection (flux, JW) anddiffusion (in this case, mass transfer coefficient, k ). Mass transfer coefficient can becalculated using any number of hydrodynamic correlations that are dependent on theflow regime and channel architecture (31). Note that this treatment requires the useof mass transfer film theory , which assumes that mass transfer coefficient has a directproportionality to diffusion coefficient and an inverse proportionality to boundary layerthickness, k = D/δ (32, 33). You will also note that the exponential term is negative forthe draw side, indicating dilution (πm < πb). It is important to note that this treatmentrequires an assumption of proportionality between concentration and osmotic pressure(i.e., Cm/Cb ≈ πm/πb) (30). This assumption is valid for solutes that exhibit ideal solutionbehavior based on the van’t Hoff equation

π = iCRT (5)

where i is the dissociation constant, C is the concentration of the solute, R is the gasconstant, and T is the temperature. In all, Equations 3 and 4 can be combined to give amore accurate version of Equation 2 which is the basis on Figure 7.

JW = A

(πD,b exp

(−JW

kD

)− πF,b exp

(JW

kF

))(6)

Equation 6 demonstrates how flux can be altered by changing hydrodynamic con-ditions on either side of the membrane. One important assumption that needs mention

8 FORWARD OSMOSIS

is that Equations 3, 4, and 6 require an assumption that the membrane is perfectlyselective.

2.1 Concentration Polarization in Asymmetric Membranes

While the theoretical treatment in the preceding section is important to establish the fun-damental relationships between osmotic flux, osmotic driving force, and mass transferresistances, these equations do not represent the osmotic performance of conventionalmembranes. Most of today’s membranes are asymmetric, meaning that they have a selec-tive skin or active layer and a porous support layer. Early asymmetric membranes forRO were made from cellulosic derivatives and comprise a single material usually castby phase inversion. This method creates a skin layer supported by an integrated supportlayer of the same material (31, 34). Thin-film composite (TFC) membranes employ a skinlayer and support layers that are different materials and now dominate the RO markettoday largely because of their superior permselectivity relative to integrated asymmetricmembranes. The improved performance is largely due to an exceedingly thin selectivelayer that is supported by a well-designed porous support layer structure.

These support layers, however, are designed for RO. They have a negligible hydraulicresistance and are chemically, thermally, and mechanically robust. They merely serveto support the of the dense selective layer fragile and highly cross-linked polyamide(35). During osmosis, however, these support layers directly interact with the solutes oneither side of the membrane. Shown in Figure 8, these layers exacerbate concentrationpolarization on either side of the membrane (depending on orientation) by reducingmixing and mass transfer. The two orientations are referred to as the FO and PROmodes (30, 36). In the FO mode, the selective layer is oriented against the feed solution.This is the preferred orientation for FO, DOC, and osmotic dilution as the feed maycontain foulants that would easily clog the porous membrane support layer. In the PROmode, the selective layer is oriented against the draw solution. This is the preferredorientation for PRO as the draw solution is pressurized and the feed side has a spacerto support the membrane. If the selective layer faced the feed, the spacer may damagethe fragile selective layer or the layer may simply delaminate given the applied pressure.Figure 8 depicts both types of orientations.

Δπeff

Δπtheo

πD,b

πD,i

πF,m

πF,b

Poroussupport

(a) (b)

JW

Selectivelayer

Δπeff

Δπtheo

πD,b

πD,m

πF,i

πF,bJW

Poroussupport

Selectivelayer

FIGURE 8 Concentration polarization in asymmetric membranes in the (a) FO and (b) PROmodes. (Please refer to the online version for the color representation of the figure.)

FORWARD OSMOSIS 9

In the FO mode, internal CP is caused on the draw side of the membrane. In this case,we can modify Equation 6 as

JW = A

(πD,b exp

(− JW

kD,eff

)− πF,b exp

(JW

kF

))(7)

where we have not incorporated a new term, kD,eff, into the equation replacing kD. Thischange incorporates the contribution of the membrane support layer properties. We candefine this term as follows:

keff = Deff

δ= DSε

τ t(8)

where we have now defined an effective diffusion coefficient Deff as a function of thesolute diffusion coefficient DS, porosity ε, tortuosity τ , and thickness t . Note that thistreatment assumes that the boundary layer thickness is equivalent to the support layerthickness.

We can take this one step further and extract a structural parameter, S , which representsan effective diffusion distance in the membrane (37).

S = tτ

ε(9)

This is essentially the contribution of the membrane structure to the mass transferresistance. Higher values of S result in more severe internal CP. We can now reformatEquation 7 to incorporate the structural parameter.

JW = A

(πD,b exp

(−JWS

DS

)− πF,b exp

(JW

kF

))(10)

This equation illustrates the deleterious effects of high S values for membranes. AsS increases, osmotic driving force decreases exponentially.

A similar treatment for incorporating S into the flux equation can be taken for mem-branes in the PRO mode. It may be noted that S is now incorporated into the feed sideCP modulus.

JW = A

(πD,b exp

(−JW

kD

)− πF,b exp

(JWS

DS

))(11)

Note that the hydraulic pressure term is often left out of this equation because mem-branes tested in this mode are not often tested under pressure. The equation with thepressure term included would simply be

JW = A

(πD,b exp

(−JW

kD

)− πF,b exp

(JW

DS

)− �P

)(12)

where hydraulic pressure is given an equivalent standing with the effective osmoticpressure terms.

10 FORWARD OSMOSIS

2.2 Support Layer Wetting

A typical RO membrane support may have a thickness of 200 μm, a porosity of 50%,and a tortuosity of 1.5. These membranes would then exhibit a structural parameter ofapproximately 600 μm. However, some studies have reported structural parameters farin excess of that. Intrinsic porosity and tortuosity are not what determine the structuralparameter of a membrane support layer. Since solutes may only transport in water, if asupport layer fails to saturate, those areas that are not wetted will not transfer solutes.When unwetted porosity is no longer available, the tortuosity can increase greatly, thusexacerbating the problem. Therefore support layer wetting should be considered (38).However, this is not as simple as knowing the degree of saturation and adjusting theporosity accordingly. For instance, there may be isolated pockets of water that do notparticipate in transport. The distribution of water throughout the structure may also notbe even in contact with the selective layer, leading to dramatic increases in tortuosity.There may also be cases where there is no interconnected wetted structure in the supportlayer, giving the structure an infinite effective tortuosity.

In any case, wetting phenomenon has not been explored to any great extent in theliterature and membrane designers have tried to get around this simply by force-wettingthe supports with a low surface tension liquid such as isopropyl alcohol (39). However,if the osmotic flux is high or if air bubbles are present in the solution facing the supportlayer, a hydrophobic support layer will drain, leaving a partially unwet structure withincreased S . We sometimes refer to an S value that represents a partially hydrated supportlayer as an effective structural parameter Seff. This takes into account only the wettedporosity and tortuosity:

Seff = tτeff

εeff(13)

In all, it is important for membranes used in any osmotic process to have a supportthat readily wets out upon contact with water. One may consider using an intrinsicallyhydrophilic polymer supports or chemically modify hydrophobic support layer material.More of this is discussed in the next section.

3 MEMBRANE DESIGN

As with any membrane process, performance metrics for membranes in engineered osmo-sis are largely centered on high flux and high selectivity. On the basis of the theoreticaltreatment presented above, we can define a set of criteria to achieve these metrics forsalinity-driven processes:

1. Superior Permselectivity. Essentially, this requires that the membrane perform likea commercial RO membrane.

2. Chemical and Thermal Robustness. The membranes cannot deteriorate in the pres-ence of the draw and feed solutes. RO membranes exhibit this quality as well, withthe notable exception of chlorine tolerance.

3. Thin, Hydrophilic Support Layer with a Low Structural Parameter. These thin,highly permselective, and robust membranes should be supported by a membranesupport with a low effective structural parameter. Commercial RO membranes failto perform in osmotic conditions because of their thick and hydrophobic support

FORWARD OSMOSIS 11

(7, 30, 38), whereas the commercial FO membrane from HTI, with its thin andhydrophilic support, exhibits flux that is an order of magnitude higher. This mem-brane, though, can under certain conditions, fail criteria 2 because it will hydrolyzeunder basic conditions (31).

4. Reasonable Mechanical Strength. It is difficult to make a thin and porous supportlayer while retaining mechanical strength. Fortunately, in salinity-driven processesexcluding PRO, no hydraulic pressure is used and so high pressure tolerance is notrequired.

5. Easily and Inexpensively Manufactured. The materials used in these membranesshould be inexpensive and easy to produce in large quantities. Moreover, the mem-branes should be easy to manufacture on a continuous production line at reasonablespeeds.

6. Tolerate Pressure. When an element or module is used, the membrane may beexposed to modest transmembrane pressure differentials, necessitating that theytolerate some pressure. However, in PRO applications, the membranes may beexposed to pressures up to 12 or 13 bar (or much higher in osmotic heat engines).

The following sections describe some recent advances in membrane technologydesigned specifically for salinity-driven processes.

3.1 Flat Sheet Membranes

The first flat sheet membranes designed for use in osmotic processes based on asymmetricdense membranes were prepared by phase inversion using cellulose acetate polymer (40,41). This membrane is still considered the standard membrane for benchmarking pur-poses, but its lack of high selectivity and its tendency to hydrolyze are drawbacks towardits widespread use. A significant recent advancement was the development of TFC-FOmembranes. The methods for the preparation of TFC-FO membranes are similar as thosefor developing TFC polyamide RO membranes: interfacial polymerization is used toform a thin polyamide active layer in situ directly onto a porous support. However,unlike the relatively thick, hydrophobic, and low porosity supports for RO membranes,supports for TFC-FO membranes are specifically tailored in order to reduce internal CP.Flat sheet TFC-FO membranes were first prepared at Yale University, via interfacialpolymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC) on porouspolysulfone (PSf) substrates that were cast on polyester nonwoven fabrics, where the sub-strate support was optimized to decrease tortuosity without sacrificing the integrity of thepolyamide layer (37, 42). Several groups also considered designing FO membranes withmore hydrophilic supports, such as blending sulfonated polysulfone into polyethersulfonesubstrate (43), or blending sulfonated poly(ether ketone) into polysulfone substrate (44).

More recently, new approaches have been employed in developing supports and selec-tive layers for next-generation of TFC-FO membranes. Bui et al. (45) and Song et al. (46)developed electrospun nanofiber supported TFC-FO membranes through electrospinningfollowed by interfacial polymerization. Compared to conventional phase inversion sup-port, the electrospun nanofiber mat exhibits higher porosity and lower tortuosity, whichhelps to mitigate internal CP by reducing S . Saren et al. developed an alternative methodfor fabricating a high performance FO membrane by employing layer-by-layer (LbL)assembly to form the selective layer (47, 48).

12 FORWARD OSMOSIS

Some chemical modification methods have also been employed to develop new FOmembranes. Arena et al. (49) used polydopamine, a novel bioinspired hydrophilic poly-mer, to modify the hydrophobic support layers of commercial TFC RO membranesfor use in osmotic processes. Setiawan et al. developed a type of flat sheet membranewith a positively charged NF-like selective layer by polyelectrolyte post-treatment of awoven-fabric-embedded substrate using polyethyleneimine (PEI) (50).

3.2 Hollow Fiber Membranes

Similar to flat sheet membranes, the early generation of hollow fiber FO membranes wasasymmetric, dense membranes based on phase inversion (51, 52). Recently, Wang andcoworkers reported the development of hollow fiber TFC-FO membranes (53, 54) andPRO membranes (55). This was accomplished again by in situ interfacial polymerizationof MPD and TMC on the outer or the inner surface of a porous PES substrate. Later, theyalso developed poly(amide-imide) hollow fiber FO membranes with a positively chargednanofiltration-like selective layer (56).

3.3 Summary

Overall, the number of membranes being proposed for use in salinity-driven processesare too numerous to review in such a short article. However, each of these membranestakes its design criteria from the fundamental findings described in Section 2. Manyof the recent publications on osmotic processes have been focused on new membranedesigns or other performance tests. As of this writing, though, far fewer papers haveconsidered the design of the other key aspect of the process: the draw solution.

4 THE DRAW SOLUTION

One of the complexities of osmotic processes is the choice of an appropriate drawsolution. This solute must not only effectively drive the water across the membrane,but then be retained, recycled, used, or discarded as designed in the process. Unlike inRO, where driving force is generated only through a hydraulic pump, the choice of drawsolute gives osmotic process a number of options for generating this driving force. It canbe said, however, that all draw solutes are not created equal. To be an effective osmoticagent, a solute should have the following characteristics:

1. Osmotic Efficiency. This refers to the ability for the draw solution to have a highosmotic pressure which will in turn provide a high flux. Typically, this means thatthe solute will be highly soluble and have a low molecular weight, giving the solutea high molar solubility.

2. Chemically Inert. The draw solute should not degrade the membranes or any ofthe components in the system.

3. Minimal Membrane Crossover. The draw solute should not cross over the mem-brane where it could contaminate the brine or simply need to be replaced orrecovered at additional cost.

FORWARD OSMOSIS 13

4. Removable and Recyclable. The draw solution must be removed and recovered ata very low cost from the dilute draw solution. This criteria is usually reserved forFO or DOC applications.

5. Nontoxicity. The draw solution must be nontoxic. When recycled, some trace levelof the solute will be retained in the product water and must not pose an acute orchronic health risk.

An excellent approach to select draw solutions was promoted by Achilli et al. (57).In this study, draw solutes were evaluated on the basis of a set of merit criteria andcould then be selected for specific applications. This approach can be used for any ofthe processes described here thus far.

4.1 Draw Solute Types

4.1.1 Nonvolatile Inorganic Solutes. Inorganic salts have long been a preferred solutefor testing osmotic flux performance of membranes. Sodium chloride (NaCl) and cal-cium chloride (CaCl2) are both low molecular weight, highly soluble, and dissociatingsolutes. Their dissociation, as with many inorganic solutes, is key to their high osmoticefficiency. However, their removal and recycling is difficult. They can, however, beused for FO applications involving feeds with a high fouling propensity. Some haveconsidered hybridizing RO with FO, using RO for draw solute recycle and FO as a pre-treatment step (58). However, such a system is unlikely to save much energy. If FO hasa lower fouling propensity, as some studies have shown (10, 59), then there could be aneconomic advantage, especially in membrane bioreactor applications. The dual-barrierapproach (54) also could enable wastewater reuse by reducing persistent contaminantconcentrations to acceptable levels.

4.1.2 Nonvolatile Organic Solutes. Nonvolatile organic solutes, such as sugars, areoften used in osmotic dilution applications. The commercial HTI osmotic water purifi-cation systems make use of organic solutes that are mixed with inorganic solutes for thepurposes of providing nutrients in the resulting drink as well as to generate additionalosmotic pressure. Polysaccharides are fairly mediocre draw solutes because they do notdissociate in water, have a low solubility (compared to salts), and at high concentrationresult in high viscosity solutions.

4.1.3 Volatile Inorganic Solutes. Volatile inorganic solutes have been some of the mostpromising draw solutes considered. The one most discussed is the ammonia–carbondioxide (NH3 –CO2) draw solution. When these gases are dissolved in water, they formhighly soluble ammonium salts that can generate large osmotic pressures. These saltscan also be easily stripped from water using low temperature steam. Several studies haveconsidered this draw solute and it has shown promise for both FO and DOC applications(7–9, 60).

4.1.4 Volatile Organic Solutes. Few volatile organic solutes have been considered foruse as draw solutes. They do not have the advantage of dissociation as do organics andthey can have low solubilities. One exception is ethanol, which has been considered (61).However, its molecular similarity to water ensures its relatively easy transport across themembrane, which can cause loss of the solute, contamination of the brine, and a lowerthan expected osmotic pressure.

14 FORWARD OSMOSIS

4.1.5 Functionalized Macromolecules. Adham (62) proposed the use of dendrimers asa novel draw solution. Dendrimers are symmetrical spheroid or globular nanostructuresthat are precisely engineered to carry molecules. These macromolecules consist of ahighly branched treelike structure linked to a central core through covalent bonds. Asmacromolecules, they by themselves do not generate a high osmotic pressure. However,the dissociable functional groups along their chains allow them to generate substantialosmotic pressures. Moreover, they can be readily regenerated by conventional membraneprocesses such as ultrafiltration.

Some stimuli-responsive polymer hydrogels have also been developed as draw solutesby Wang for FO desalination (63). These polymer hydrogels are able to extract andrelease water when there is stimulus by either temperature or pressure, or by light withthe incorporation of light-absorbing carbon particles (64). They can extract water from afeed saline solution in an FO desalination process and then undergo a reversible volumechange when exposed to environmental stimulus.

4.1.6 Switchable Polarity Solvents. Switchable polarity solvents (SPSs) (65) are pre-sented as viable FO draw solutes allowing a novel SPS FO process (66). The transitionof SPSs from water miscibility to water immiscibility is dependent on the presence orabsence of carbon dioxide, respectively, at ambient pressures. These SPS draw solutesprovide high osmotic strength and are easily recycled from the purified water after polarto nonpolar phase shift.

4.1.7 Magnetic Nanoparticles. Hydrophilic magnetic nanoparticles (MNPs) are consid-ered by some to be a promising draw solute (67–69). These functionalized nanoparticleshave highly dissociable surface chemistry that gives them high osmotic efficiency. Regen-eration of MNPs could be achieved by applying an external magnetic field, but thismethod causes agglomeration of MNPs, which decreases their osmotic pressures. Ultra-sonication is suggested to prevent this and allow for resuspension, but this potentiallyweakens the magnetic properties of the MNPs and thus reduces the regeneration effi-ciency (67, 68) while simultaneously requiring additional energy. Recently, Ling andChung (69) used ultrafiltration for the recovery of MNPs.

4.1.8 Polyelectrolytes. Polyelectrolytes of polyacrylic acid sodium (PAA-Na) salts werealso investigated as draw solutes by Ge et al. (70). The species contain multiple dissocia-ble functional groups allowing PAA-Na to potentially outperform conventional ionic salts,such as NaCl, when comparing their FO performance via the same membranes. The recy-cled PAA-Na do not initiate the aggregation problems that exist in MNPs draw solutions.

5 PROCESS DESIGN AND IMPLEMENTATION

At the time of this writing, a number of technical schemes have been proposed, but onlya handful have been demonstrated. Several companies, such as Hydration TechnologyInnovationsTM (17) and StatkraftTM (71) have found increased visibility for their com-mercial products that harness salinity gradients. Other small start-up companies, suchas Oasys WaterTM (72) and Modern WaterTM (73) have been founded to commercial-ize these various processes. For instance, Oasys Water has demonstrated using DOC todewater high salinity produced water in the Permian Basin in Texas at a cost that is 50%lower than distillation (60). They use a form of the NH3 –CO2 draw solution to run the

FORWARD OSMOSIS 15

process. Modern Water has FO pilot plants in Oman and Gibralter, desalinating seawaterusing a proprietary draw solute (74). Statkraft has demonstrated full-scale PRO usingseawater and river water in Norway (75). Others have devised clever schemes involvinghybridizing PRO and RO. PRO and RO can be integrated by reclaiming wastewaterwith a seawater intake to an RO plant (58). Others have claimed to conduct pilot-scaledemonstrations of FO and PRO, but many of these efforts have not been published orlack details because of intellectual property concerns or perhaps lack of promising results.Nonetheless, at the time of this writing, FO and PRO systems are being prototyped aroundthe world, with new membrane and process technologies being built at the pilot scale.

6 FINAL REMARKS

Engineered osmotic processes hold great promise as a next-generation separation andpower generation technology. However, its promotion should be restrained by the realitiesof thermodynamics, process design, and economics. More detailed economic studies arerequired as new membranes and draw solutions are developed. More pilot-scale demon-strations should be conducted and the results of those tests widely disseminated if morepeople are to truly believe in the promise of this emerging technology platform. RO tooknearly over 30 years to go from the laboratory bench to widespread adoption. Engineeredosmosis has only been around in recent times since 2005, where it currently during thestruggles against well-established and commoditized membrane markets. The membranecommunity should understand, however, that engineered osmosis should not be seen as areplacement to existing membrane separations, but rather as a complementary technology.

ACKNOWLEDGMENT

The authors would like to acknowledge funding from the USEPA (#R834872) and theNational Science Foundation (CBET #1067564).

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