"Forward Osmosis" In

Home , Forward osmosis, Nanofiltration, Osmotic power

Jeffrey McCutcheon and Liwei Huang University of Connecticut, Storrs, CT

1 WHAT ARE OSMOTIC PROCESSES?

For decades, aqueous separations have relied on hydraulic pressure to force water across membranes that retain suspended and dissolved solids. This pressure is generated by pumps 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 most of the membrane science community has attempted to address this problem by making better membranes that have a higher permeability or higher selectivity, few efforts have been focused on changing the driving force of separation. Recently, osmotic pressure has been considered as an alternative driving force because osmosis occurs spontaneously without any work needed from an external source. This has led to the emergence of a field in membrane science known as engineered osmosis or salinity-driven processes. This platform technology has garnered much interest in recent years 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 power generation has found a fierce resurgence as of late. Much of this resurgence hinges not upon the early publications (3) and patents (4–6) on the technology, but rather the recent explosion of academic publications in the area, starting in 2005 (7). Since then, well over 150 papers on salinity-driven processes have been published in the peer-reviewed literature. Engineered osmosis relies on the spontaneously occurring osmotic flow between two aqueous solutions of differing osmotic potential. While this is their single unifying theme, harnessing osmotic flow is motivated by different purposes. In some cases, the osmotic flow can be a purified water stream (forward osmosis, FO). In others, the flow might be discarded as waste (direct osmotic concentration, DOC). In yet others, the water itself is irrelevant and rather the motion of the flow itself is the product (pressure-retarded osmosis, PRO). In all cases, the choice of the osmotic agent,ordraw solution, which drives the osmotic flow, is tailored depending on the desired product.

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 technology where drinking water is the primary product. This requires eventual separation of water from the osmotic agent. In FO, saline water is fed to one side of a membrane while the draw solution is fed to the other side. Osmotic separation takes place spontaneously as water moves from the relatively dilute saline solution into the draw solution. The membrane, which is ideally semipermeable, retains the solutes on the feed side while also preventing crossover of solutes from the draw solution. Thus, we have separated water from the original salts without expending any energy except for delivering the water to the membrane at low pressure. The next step requires additional effort. The now diluted draw solution is sent to a 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 may be better retained by a second membrane process. The cost of this regeneration is what drives 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 solute recovery system chosen. Speciﬁc FO processes have the potential to use less costly energy (such as waste heat) than reverse osmosis (RO) (8). A draw solute can be chosen to provide large driving forces and thus exceed the water recoveries typically garnered with RO processes (9). For applications with high fouling feeds, such as membrane bioreactors or waters with a high scaling potential, FO has the potential for a lower fouling 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 such as 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 Energy Saline water input

Draw Draw solute solution recovery

Brine Potable water

FIGURE 1 Illustration of the forward osmosis (FO) process with draw solute recycle. (Please refer to the online version for the color representation of the ﬁgure.) FORWARD OSMOSIS 3

Membrane Energy Saline water input

Draw Draw solution solute recovery

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

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

1.3 Osmotic Dilution Osmotic dilution is a term rarely used, but it does constitute one of the only commercial osmotic processes available at the time of this writing. In osmotic dilution, the diluted form 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 all rely on osmotic dilution (17). In these systems, a highly concentrated sugar–electrolyte solution is placed on the inside of the membrane, while the outside of the membrane is exposed to an impaired or saline water source. Water moves across the membrane by osmosis, while the membrane retains salts and contaminants on the feed and prevents the ﬂux of solutes from the draw. The draw solution becomes diluted (sometimes by factors of up to 30 times), producing a clean drink immediately ready for ingestion. The membrane used in these products has been the standard membrane used in many of the FO and PRO studies referenced throughout this chapter. Figure 3 illustrates this process. Other opportunities for using osmotic dilution exist when concentrates need dilution but clean freshwater is in short supply. One such opportunity is in fertigation, where concentrated fertilizer is used as the draw solute and otherwise impaired water as the feed (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 in solution wish to move as far away from each other as possible, yet they are restricted by the phase boundaries of the liquid. Osmotic flow can relieve this “pressure,” allowing the fluid 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 Saline water

Draw Diluted solution draw solution

Brine FIGURE 3 Illustration of the osmotic dilution process. (Please refer to the online version for the color representation of the figure.) naturally occurring and engineered salinity gradients and converts it into electricity using a hydraulic pressure intermediate (Fig. 4). PRO was first conceived of in the 1970s, when Sidney Loeb and others identified salinity gradients as a means of generating electricity (19–21). In these cases, saline water bodies such as the ocean, the Dead Sea (22), or the Great Salt Lake (23) can act as draw solutions. Freshwater rivers and streams that pour into these saline waters can be diverted and the mixing can be controlled by a membrane. To work, saline water is pressurized to a level below its osmotic pressure, thus retarding the osmotic flow but creating a resistance to generate work. The subsequent expansion of the diluted saline water through a hydroturbine generates electricity. Power generated is calculated using the following equation. P = JW PE 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 further retarded 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 Membrane Fresh water

Draw Diluted Mix and P solution draw discard to X (sea water) solute draw source

FIGURE 4 Illustration of pressure-retarded osmosis (PRO) using a naturally occurring salinity gradient. The pressure exchanger (PX) maintains a high pressure region in the element on the draw side. (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 1970s during the OPEC oil embargo. While increasing demand for energy independence spurred early research, once the embargo ended and energy prices came down, the motivation for continuing the research was lost. PRO was a mostly dormant field for the next 20 years. At the turn of the century, interest in PRO was renewed as energy prices soared (23–26). As a new membrane emerged that promised to make osmotic processes more feasible, alternative PRO systems were conceived, including a “closed-loop” PRO system, 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 into electricity 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 naturally occurring saline water body. Engineered draw solutes can have osmotic pressures an order of magnitude higher than seawater, thus resulting in high power densities. The only challenge is finding sources of waste heat that have no economic value but can still be harnessed for such a process.

Work Membrane

Energy input

Draw Draw P solute solution X recovery Pure water working fluid

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

Water flux

RO (ΔP > Δπ) Flux reverse point 0 ΔP Δπ PRO (ΔP < Δπ)

FO or DOC (ΔP = 0)

FIGURE 6 Illustration of ﬂux versus driving force. The reverse osmosis (RO), pressure-retarded osmosis (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 principles of osmotic pressure as a driving force are the same. We can describe the flux type using a figure modified from a review by Cath et al. (28). The region representing RO flux is commonly seen in studies of cases in which increase in pressure results in increase in water flux. However, as the transmembrane hydraulic pressure drops below the osmotic pressure of a solution, osmotic flux occurs, but it is retarded by the transmembrane hydraulic pressure. This is the PRO regime. When the transmembrane hydraulic pressure is zero, then it is the condition of FO and DOC where water flux is entirely driven by osmosis (Fig. 6).

2 MEMBRANE TRANSPORT

All salinity-driven processes rely on a semipermeable membrane that can retain solutes on the feed side and the draw side, while simultaneously enabling high flux. While RO membranes exhibit these qualities for pressure-driven flow, osmotic flow requires solutes to directly interact with the membrane. This means that the membrane structure and chemistry 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 is the 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 pressure difference 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 terms refer to “effective” osmotic pressures. For instance, the membrane may not be perfectly selective, meaning that a portion of the solutes from the feed and draw may move freely through the membrane and therefore do not contribute to the osmotic pressure differential. Furthermore, as water moves through the membrane by osmosis, solutes are concentrated at the feed side of the membrane and diluted at the draw side of the membrane. This results in boundary layers that form on both sides of the membrane, which lead to a reduction of effective osmotic driving force. This concentration polarization (CP) has been long studied in RO and ultrafiltration processes (29), but CP during osmosis produces a more dramatic effect on flux. During osmosis, boundary layers are established on both sides of the membrane, leading to a reduced driving force as salts are concentrated on the feed side and diluted on the draw side (Fig. 7). Figure 7 illustrates these “effective” osmotic pressures as being those at the interface of 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 pressure to that in the bulk, (πD,m/πD,b, πF,m/πF,b) as the CP modulus. This ratio is greater than 1 on the feed side and less than 1 on the draw side. FORWARD OSMOSIS 7

π D,b π D,m

Δπ Δπ eff theo π F,m

π F,b JW

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

The modulus could be calculated using a simple ﬁlm theory model proposed by McCutcheon and others (30) to quantify CP modulus on the feed side as a function of the Peclet number π J F,m = exp W (3) πF,b kF

and on the draw side π J D,m = exp − W (4) πD,b kD

The Peclet number is a dimensionless quantity that relates advection (flux, JW) and diffusion (in this case, mass transfer coefficient, k). Mass transfer coefficient can be calculated using any number of hydrodynamic correlations that are dependent on the flow regime and channel architecture (31). Note that this treatment requires the use of mass transfer film theory, which assumes that mass transfer coefficient has a direct proportionality to diffusion coefficient and an inverse proportionality to boundary layer thickness, k = D/δ (32, 33). You will also note that the exponential term is negative for the draw side, indicating dilution (πm <πb). It is important to note that this treatment requires 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 solution behavior 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 gas constant, and T is the temperature. In all, Equations 3 and 4 can be combined to give a more accurate version of Equation 2 which is the basis on Figure 7. = −JW − JW JW A πD,b exp πF,b exp (6) kD kF Equation 6 demonstrates how ﬂux can be altered by changing hydrodynamic conditions 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 perfectly selective.

2.1 Concentration Polarization in Asymmetric Membranes While the theoretical treatment in the preceding section is important to establish the fundamental relationships between osmotic ﬂux, osmotic driving force, and mass transfer resistances, these equations do not represent the osmotic performance of conventional membranes. Most of today’s membranes are asymmetric, meaning that they have a selective skin or active layer and a porous support layer. Early asymmetric membranes for RO were made from cellulosic derivatives and comprise a single material usually cast by phase inversion. This method creates a skin layer supported by an integrated support layer of the same material (31, 34). Thin-ﬁlm composite (TFC) membranes employ a skin layer and support layers that are different materials and now dominate the RO market today largely because of their superior permselectivity relative to integrated asymmetric membranes. The improved performance is largely due to an exceedingly thin selective layer that is supported by a well-designed porous support layer structure. These support layers, however, are designed for RO. They have a negligible hydraulic resistance and are chemically, thermally, and mechanically robust. They merely serve to 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 on either side of the membrane. Shown in Figure 8, these layers exacerbate concentration polarization on either side of the membrane (depending on orientation) by reducing mixing and mass transfer. The two orientations are referred to as the FO and PRO modes (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 may contain foulants that would easily clog the porous membrane support layer. In the PRO mode, the selective layer is oriented against the draw solution. This is the preferred orientation for PRO as the draw solution is pressurized and the feed side has a spacer to support the membrane. If the selective layer faced the feed, the spacer may damage the fragile selective layer or the layer may simply delaminate given the applied pressure. Figure 8 depicts both types of orientations.

(a) (b) π D,b π D,b

π Δπ Δπ D,m eff theo π Δπ D,i theo π π Δπ F,i F,m eff

π π F,b F,b J W J W

Porous Selective Selective Porous support layer layer support FIGURE 8 Concentration polarization in asymmetric membranes in the (a) FO and (b) PRO modes. (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 − JW JW A πD,b exp πF,b exp (7) kD,eff kF

where we have not incorporated a new term, kD,eff, into the equation replacing kD.This change incorporates the contribution of the membrane support layer properties. We can deﬁne this term as follows: D D ε k = eff = S (8) eff δ τt

where we have now defined an effective diffusion coefficient Deff as a function of the solute diffusion coefficient DS, porosity ε, tortuosity τ, and thickness t. Note that this treatment assumes that the boundary layer thickness is equivalent to the support layer thickness. We can take this one step further and extract a structural parameter, S , which represents an effective diffusion distance in the membrane (37).

tτ S = (9) ε

This is essentially the contribution of the membrane structure to the mass transfer resistance. Higher values of S result in more severe internal CP. We can now reformat Equation 7 to incorporate the structural parameter. = −JWS − JW JW A πD,b exp πF,b exp (10) DS kF

This equation illustrates the deleterious effects of high S values for membranes. As S increases, osmotic driving force decreases exponentially. A similar treatment for incorporating S into the ﬂux equation can be taken for membranes in the PRO mode. It may be noted that S is now incorporated into the feed side CP modulus. = −JW − JWS JW A πD,b exp πF,b exp (11) kD DS

Note that the hydraulic pressure term is often left out of this equation because membranes tested in this mode are not often tested under pressure. The equation with the pressure term included would simply be = −JW − JW − JW A πD,b exp πF,b exp P (12) kD DS

where hydraulic pressure is given an equivalent standing with the effective osmotic pressure 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 of approximately 600 μm. However, some studies have reported structural parameters far in excess of that. Intrinsic porosity and tortuosity are not what determine the structural parameter of a membrane support layer. Since solutes may only transport in water, if a support 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, thus exacerbating the problem. Therefore support layer wetting should be considered (38). However, this is not as simple as knowing the degree of saturation and adjusting the porosity accordingly. For instance, there may be isolated pockets of water that do not participate in transport. The distribution of water throughout the structure may also not be 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 support layer, giving the structure an inﬁnite effective tortuosity. In any case, wetting phenomenon has not been explored to any great extent in the literature and membrane designers have tried to get around this simply by force-wetting the 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 support layer, a hydrophobic support layer will drain, leaving a partially unwet structure with increased S . We sometimes refer to an S value that represents a partially hydrated support layer as an effective structural parameter Seff. This takes into account only the wetted porosity and tortuosity: = tτeff Seff (13) εeff In all, it is important for membranes used in any osmotic process to have a support that readily wets out upon contact with water. One may consider using an intrinsically hydrophilic 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 osmosis are largely centered on high ﬂux and high selectivity. On the basis of the theoretical treatment presented above, we can deﬁne a set of criteria to achieve these metrics for salinity-driven processes:

1. Superior Permselectivity. Essentially, this requires that the membrane perform like a commercial RO membrane. 2. Chemical and Thermal Robustness. The membranes cannot deteriorate in the presence of the draw and feed solutes. RO membranes exhibit this quality as well, with the 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 membrane support with a low effective structural parameter. Commercial RO membranes fail to 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 and hydrophilic support, exhibits ﬂux that is an order of magnitude higher. This membrane, though, can under certain conditions, fail criteria 2 because it will hydrolyze under basic conditions (31). 4. Reasonable Mechanical Strength. It is difﬁcult to make a thin and porous support layer while retaining mechanical strength. Fortunately, in salinity-driven processes excluding PRO, no hydraulic pressure is used and so high pressure tolerance is not required. 5. Easily and Inexpensively Manufactured. The materials used in these membranes should be inexpensive and easy to produce in large quantities. Moreover, the membranes should be easy to manufacture on a continuous production line at reasonable speeds. 6. Tolerate Pressure. When an element or module is used, the membrane may be exposed to modest transmembrane pressure differentials, necessitating that they tolerate some pressure. However, in PRO applications, the membranes may be exposed to pressures up to 12 or 13 bar (or much higher in osmotic heat engines).

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

3.1 Flat Sheet Membranes The first flat sheet membranes designed for use in osmotic processes based on asymmetric dense membranes were prepared by phase inversion using cellulose acetate polymer (40, 41). This membrane is still considered the standard membrane for benchmarking purposes, but its lack of high selectivity and its tendency to hydrolyze are drawbacks toward its widespread use. A significant recent advancement was the development of TFC-FO membranes. The methods for the preparation of TFC-FO membranes are similar as those for developing TFC polyamide RO membranes: interfacial polymerization is used to form 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 interfacial polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC) on porous polysulfone (PSf) substrates that were cast on polyester nonwoven fabrics, where the substrate support was optimized to decrease tortuosity without sacrificing the integrity of the polyamide layer (37, 42). Several groups also considered designing FO membranes with more hydrophilic supports, such as blending sulfonated polysulfone into polyethersulfone substrate (43), or blending sulfonated poly(ether ketone) into polysulfone substrate (44). More recently, new approaches have been employed in developing supports and selective 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 electrospinning followed by interfacial polymerization. Compared to conventional phase inversion support, the electrospun nanofiber mat exhibits higher porosity and lower tortuosity, which helps to mitigate internal CP by reducing S . Saren et al. developed an alternative method for 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 FO membranes. Arena et al. (49) used polydopamine, a novel bioinspired hydrophilic polymer, to modify the hydrophobic support layers of commercial TFC RO membranes for use in osmotic processes. Setiawan et al. developed a type of ﬂat sheet membrane with a positively charged NF-like selective layer by polyelectrolyte post-treatment of a woven-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 was asymmetric, dense membranes based on phase inversion (51, 52). Recently, Wang and coworkers reported the development of hollow fiber TFC-FO membranes (53, 54) and PRO membranes (55). This was accomplished again by in situ interfacial polymerization of MPD and TMC on the outer or the inner surface of a porous PES substrate. Later, they also developed poly(amide-imide) hollow fiber FO membranes with a positively charged nanofiltration-like selective layer (56).

3.3 Summary Overall, the number of membranes being proposed for use in salinity-driven processes are too numerous to review in such a short article. However, each of these membranes takes its design criteria from the fundamental ﬁndings described in Section 2. Many of the recent publications on osmotic processes have been focused on new membrane designs or other performance tests. As of this writing, though, far fewer papers have considered 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 draw solution. 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 in RO, where driving force is generated only through a hydraulic pump, the choice of draw solute gives osmotic process a number of options for generating this driving force. It can be said, however, that all draw solutes are not created equal. To be an effective osmotic agent, a solute should have the following characteristics:

1. Osmotic Efﬁciency. This refers to the ability for the draw solution to have a high osmotic pressure which will in turn provide a high ﬂux. Typically, this means that the solute will be highly soluble and have a low molecular weight, giving the solute a high molar solubility. 2. Chemically Inert. The draw solute should not degrade the membranes or any of the components in the system. 3. Minimal Membrane Crossover. The draw solute should not cross over the membrane where it could contaminate the brine or simply need to be replaced or recovered at additional cost. FORWARD OSMOSIS 13

4. Removable and Recyclable. The draw solution must be removed and recovered at a very low cost from the dilute draw solution. This criteria is usually reserved for FO or DOC applications. 5. Nontoxicity. The draw solution must be nontoxic. When recycled, some trace level of the solute will be retained in the product water and must not pose an acute or chronic 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 and could then be selected for speciﬁc applications. This approach can be used for any of the processes described here thus far.

4.1 Draw Solute Types 4.1.1 Nonvolatile Inorganic Solutes. Inorganic salts have long been a preferred solute for testing osmotic flux performance of membranes. Sodium chloride (NaCl) and cal- cium chloride (CaCl2) are both low molecular weight, highly soluble, and dissociating solutes. Their dissociation, as with many inorganic solutes, is key to their high osmotic efficiency. However, their removal and recycling is difficult. They can, however, be used for FO applications involving feeds with a high fouling propensity. Some have considered 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 has a lower fouling propensity, as some studies have shown (10, 59), then there could be an economic advantage, especially in membrane bioreactor applications. The dual-barrier approach (54) also could enable wastewater reuse by reducing persistent contaminant concentrations to acceptable levels.

4.1.2 Nonvolatile Organic Solutes. Nonvolatile organic solutes, such as sugars, are often 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 the purposes of providing nutrients in the resulting drink as well as to generate additional osmotic pressure. Polysaccharides are fairly mediocre draw solutes because they do not dissociate in water, have a low solubility (compared to salts), and at high concentration result in high viscosity solutions.

4.1.3 Volatile Inorganic Solutes. Volatile inorganic solutes have been some of the most promising draw solutes considered. The one most discussed is the ammonia–carbon dioxide (NH3 –CO2) draw solution. When these gases are dissolved in water, they form highly soluble ammonium salts that can generate large osmotic pressures. These salts can also be easily stripped from water using low temperature steam. Several studies have considered 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 for use as draw solutes. They do not have the advantage of dissociation as do organics and they 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 the membrane, which can cause loss of the solute, contamination of the brine, and a lower than expected osmotic pressure. 14 FORWARD OSMOSIS

4.1.5 Functionalized Macromolecules. Adham (62) proposed the use of dendrimers as a novel draw solution. Dendrimers are symmetrical spheroid or globular nanostructures that are precisely engineered to carry molecules. These macromolecules consist of a highly branched treelike structure linked to a central core through covalent bonds. As macromolecules, they by themselves do not generate a high osmotic pressure. However, the dissociable functional groups along their chains allow them to generate substantial osmotic pressures. Moreover, they can be readily regenerated by conventional membrane processes such as ultrafiltration. Some stimuli-responsive polymer hydrogels have also been developed as draw solutes by Wang for FO desalination (63). These polymer hydrogels are able to extract and release water when there is stimulus by either temperature or pressure, or by light with the incorporation of light-absorbing carbon particles (64). They can extract water from a feed saline solution in an FO desalination process and then undergo a reversible volume change when exposed to environmental stimulus. 4.1.6 Switchable Polarity Solvents. Switchable polarity solvents (SPSs) (65) are presented as viable FO draw solutes allowing a novel SPS FO process (66). The transition of SPSs from water miscibility to water immiscibility is dependent on the presence or absence of carbon dioxide, respectively, at ambient pressures. These SPS draw solutes provide high osmotic strength and are easily recycled from the purified water after polar to nonpolar phase shift. 4.1.7 Magnetic Nanoparticles. Hydrophilic magnetic nanoparticles (MNPs) are considered by some to be a promising draw solute (67–69). These functionalized nanoparticles have highly dissociable surface chemistry that gives them high osmotic efficiency. Regen- eration of MNPs could be achieved by applying an external magnetic field, but this method causes agglomeration of MNPs, which decreases their osmotic pressures. Ultra- sonication is suggested to prevent this and allow for resuspension, but this potentially weakens the magnetic properties of the MNPs and thus reduces the regeneration efficiency (67, 68) while simultaneously requiring additional energy. Recently, Ling and Chung (69) used ultrafiltration for the recovery of MNPs.

4.1.8 Polyelectrolytes. Polyelectrolytes of polyacrylic acid sodium (PAA-Na) salts were also investigated as draw solutes by Ge et al. (70). The species contain multiple dissociable functional groups allowing PAA-Na to potentially outperform conventional ionic salts, such as NaCl, when comparing their FO performance via the same membranes. The recycled 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 only a handful have been demonstrated. Several companies, such as Hydration Technology InnovationsTM (17) and StatkraftTM (71) have found increased visibility for their commercial products that harness salinity gradients. Other small start-up companies, such as Oasys WaterTM (72) and Modern WaterTM (73) have been founded to commercial- ize these various processes. For instance, Oasys Water has demonstrated using DOC to dewater 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 seawater using a proprietary draw solute (74). Statkraft has demonstrated full-scale PRO using seawater and river water in Norway (75). Others have devised clever schemes involving hybridizing PRO and RO. PRO and RO can be integrated by reclaiming wastewater with a seawater intake to an RO plant (58). Others have claimed to conduct pilot-scale demonstrations of FO and PRO, but many of these efforts have not been published or lack 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 around the 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 and power generation technology. However, its promotion should be restrained by the realities of thermodynamics, process design, and economics. More detailed economic studies are required as new membranes and draw solutions are developed. More pilot-scale demonstrations should be conducted and the results of those tests widely disseminated if more people are to truly believe in the promise of this emerging technology platform. RO took nearly over 30 years to go from the laboratory bench to widespread adoption. Engineered osmosis has only been around in recent times since 2005, where it currently during the struggles against well-established and commoditized membrane markets. The membrane community should understand, however, that engineered osmosis should not be seen as a replacement to existing membrane separations, but rather as a complementary technology.

ACKNOWLEDGMENT

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

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