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A review of draw solutes in forward osmosis processand their use in modern applicationsLaura Chekli a , Sherub Phuntsho a , Ho Kyong Shon a , Saravanamuthu Vigneswaran a , JayaKandasamy a & Amit Chanan aa School of Civil and Environmental Engineering, University of Technology, Sydney (UTS) ,City Campus, Broadway , 2007 , Australia Phone: Tel. +612 9514 2623 Fax: Tel. +612 95142623Published online: 30 Apr 2012.

To cite this article: Laura Chekli , Sherub Phuntsho , Ho Kyong Shon , Saravanamuthu Vigneswaran , Jaya Kandasamy & AmitChanan (2012) A review of draw solutes in forward osmosis process and their use in modern applications, Desalination andWater Treatment, 43:1-3, 167-184, DOI: 10.1080/19443994.2012.672168

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A review of draw solutes in forward osmosis process and their usein modern applications

Laura Chekli, Sherub Phuntsho, Ho Kyong Shon*, Saravanamuthu Vigneswaran,Jaya Kandasamy, Amit Chanan

School of Civil and Environmental Engineering, University of Technology, Sydney (UTS), City Campus,Broadway 2007, AustraliaTel. +612 9514 2623; Fax: +612 9514 2633; email: Hokyong.Shon-1@uts.edu.au

Received 25 December 2011; Accepted 10 February 2012

ABSTRACT

Forward osmosis (FO) is one of the emerging membrane technologies which has gainedrenewed interest recently as a low energy desalination process. The central to FO process isthe draw solution (DS) and the membrane because both play a substantial role on its perfor-mance. Hence, the selection of an appropriate DS is crucial for the process efficiency. ManyDS have been tested so far for a wide range of modern applications and this paper aims toreview the various aspects of the DS in the process performance and provides valuable infor-mation regarding the selection criteria of suitable DS. Several general DS properties such asthe osmotic pressure and the water solubility can affect the process performance. Other intrin-sic properties to specific novel DS such as the emerging magnetic nanoparticles (MNPs) canalso have an impact on the process efficiency and have to be evaluated. Separation and recov-ery of the DS are one of the major challenges facing the development of FO process. Therecovery process should not be energy intensive, otherwise the FO process cannot be compa-rable with other pressure-driven processes. Thermolytic solutions such as ammonia carbon-ates are considered as the promising DS for desalination applications; however, their recoveryprocess efficiency relies on the availability of low-grade heat. MNPs are emerging and effec-tive DS for desalination and can be readily recovered by a magnetic field or conventionalmembrane processes. However, the aggregation of MNPs due to their magnetic propertieshas been issued. The vast numbers of studies on the use of NaCl as DS for the treatment ofimpaired water open up the possibilities of using seawater or reverse osmosis brine streamsas suitable DS for such purpose. Fertilisers were also suggested as DS for seawater and waste-water treatment when the diluted DS can be used directly for irrigation. The development ofan adequate and efficient DS coupled with a low-cost energy recovery system is crucial to theperformance of the process and to achieve success for the large scale of FO.

Keywords: Forward osmosis; Draw solutions; Desalination; Wastewater reuse

1. Introduction

One of the most significant challenges of thiscentury is in meeting the increasing freshwater

demand for drinking water supplies, food productionand other industrial needs to support the enormouspopulation growth [1]. In response to this increasingwater demand, intensive research on finding alterna-tive solutions to supplement insufficient freshwatersources has been carried out, particularly in the field*Corresponding author.

The 4th International Desalination Workshop (IDW4), 1618 November 2011, Jeju Island, South Korea

Desalination and Water Treatmentwww.deswater.com

1944-3994/1944-3986 2012 Desalination Publications. All rights reserveddoi: 10.1080/19443994.2012.672168

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of desalination. Reverse osmosis (RO) is currently themost commonly used desalination technologiesbecause of its merits over other conventional thermaldesalination technologies. Although the performanceof RO desalination plants now consumes significantlylower energy than the RO desalination over severaldecades ago, the energy required for desalination stillremains high due to the thermodynamic limit of themembrane desalination processes. Therefore, in orderto comply with water, energy and environmentalissues, a desalination technology that consumes muchlower energy is essential.

Forward osmosis (FO) (also known as manipulatedosmosis or engineered osmosis) is one of the emergingmembrane technologies as it has the ability to desali-nate seawater or brackish water at low-cost energycompared to traditional processes. The novelty of thisprocess lies in utilising the natural osmotic process fordesalination rather than the hydraulic pressure as inRO. Fig. 1 explains the fundamentals of osmoticprocesses.

When saline feed water and the highly concen-trated solute termed as the draw solution (DS) (otherusual terms are osmotic agent, draw agent, etc., butDS is to be used in this review) are separated by asemi-permeable membrane, water moves from the sal-ine water (lower solute concentration) to the concen-trated DS (higher solute concentration) due to osmoticgradient, while retaining the solutes on both sides ofthe membrane. Hence, the driving force in FO processis created naturally by the difference in osmotic gradi-ent between the feed stream and the DS, and this pro-cess offers many advantages such as lower energycost and significantly lower membrane fouling poten-tial [2]. Therefore, there has been a growing interest instudying the FO process, particularly for desalinationapplications.

Although there are currently some commercialapplications of FO [37], there are still a number ofchallenges that need to be overcome in order toachieve an effective large-scale stand-alone FO pro-

cess. One of these key challenges is in developing asuitable DS that can generate a high osmotic pressureto produce high water flux while being easy to recon-centrate and recover at lower energy cost. The selec-tion and/or development of suitable DS are thereforeone of the big challenges to achieve the commerciali-sation of FO process, especially for desalination fordrinking water production.

Even if the number of research publications on FOhas recently increased, more efforts have been focusedon the development of new membranes and processperformance, but little on the improvement of DS.However, the performance of the FO process greatlydepends on the selection of an appropriate DS. Hence,the aims of the current study are to review the variousaspects of DS in the performance of the FO processfor modern applications such as desalination, waste-water treatment or energy production. This study alsointends to provide valuable information to readers forthe selection of suitable DS. The review begins with adiscussion on DS characteristics that directly influencethe process performance. Then, a classification of dif-ferent types of DS used so far in this process isdescribed and a review on DS separation and recov-ery processes is proposed. Finally, the use of DS invarious modern applications is reviewed and criteriafor the selection of suitable DS are proposed.

2. DS characteristics affecting FO processperformance

There are several factors that can influence the per-formance of the FO process and these factors are, ingeneral, related to the DS characteristics, FO mem-brane properties and operating conditions. Althoughonly the DS characteristics that influence the processperformance are discussed in this section, it is impor-tant to note that membrane properties influence theperformance of some DS and therefore it is alsoimportant to consider how these membrane propertiesalso affect the process performance.

All the general DS characteristics and their impactson the process performance are listed and summa-rised in Table 1, but more details are provided on thenext section.

2.1. Assessment of the performance of FO process

Before discussing the factors influencing the per-formance of the FO process, it is important to under-stand the methods of assessing this performance.Water flux is one of the primary methods used forevaluating any membrane process performance. In

Fig. 1. The principles of osmotic processes: forwardosmosis (FO) and reverse osmosis (RO).

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fact, in any pressure-based membrane process, purewater permeability is one of the basic parameters use-ful for assessing membrane performance. Similarly,the performance of FO process, whether it is specifi-cally for assessing the performance of DS or the per-formance of the membranes, is all assessed in terms ofpure water flux. The latter provides an easy way ofcomparing the performance of FO process under dif-ferent situations.

Reverse draw solute flux is also an importantparameter that has to be assessed when evaluating theperformance of FO process. In fact, several studies[8,9] have demonstrated that this phenomenon canjeopardise the process. Reverse salt diffusion candecrease the net osmotic pressure across the mem-brane which results in flux decline. Moreover, reversesalt transport is not only an economical loss, but canalso complicate concentrate management. In fact,accumulation of DS solutes in the feed solution mayinduce toxicological challenges for sensitive receivingenvironment or affect adjacent treatment processes [8]if contaminants such as nitrate, phosphate or heavymetals present in the feed concentrate. Models todescribe reverse draw solute transport through semi-permeable membrane in FO process have been exten-sively discussed in other studies [8,10]. Phillip et al.[11] showed that the reverse flux selectivitythe ratioof the forward water flux to the reverse solute fluxisa key parameter in the design of the pressure-drivenmembrane processes. The reverse salt transport of asolute can be monitored continuously by electrical

conductivity when pure water is used as feed solutionduring FO performance tests.

Finally, FO process performance can also be evalu-ated by determining the process recovery rates. Forsome specific application (e.g. desalination), otherparameters (e.g. salt rejection) can also be measuredfor assessing the process performance.

2.2. DS properties affecting FO process performance

The performance of FO process greatly dependson the selection of suitable DS as it is the main sourceof the driving force in this process. The primary char-acteristics essential for any DS are high solubility inwater and high osmotic pressure, much higher thanthe feed solution.

The osmotic pressure (p) of the ideal dilute solu-tion is defined based on the theory proposed by VantHoff [12] as shown below.

p n cMW

RT 1

where n is the number of moles of species formed bythe dissociation of solutes in the solution, c is the sol-ute concentration in g/L of solution, MW is themolecular weight of the solute, R is the gas constant(R= 0.0821) and T is the absolute temperature of thesolution. However, this equation is limited to extre-mely dilute solutions and is generally used for thedetermination of large MW [13]. For general solutions,

Table 1General DS characteristics affecting FO process performance and their impact on the process performance

DS characteristics Impact on FO process performance

Osmotic pressure A high DS osmotic pressure and low feed solution osmotic pressure induce high water fluxesacross the membrane

Water solubility High solubility induces high osmotic pressure and therefore can achieve high water flux andhigh recovery rates

Viscosity/diffusivity

A low viscosity combined with high diffusivity leads to high water fluxes

Molecular weight(MW)

Small MW solutes produce higher osmotic pressure than larger MW for equal mass of DS butinduce higher reverse draw solute flux than larger MW DS

Concentration Water flux increases at higher DS concentration but the increase is non-linear. At higher DSconcentration, dilutive CP drastically increases resulting in less effective water flux improvement

Temperature Higher temperature would not only afford higher initial fluxes and higher water recoveries butalso induce more adverse effects on membrane scaling and cleaning

Othercharacteristics

In addition, specific characteristics of a particular draw solute may also influence the FO processperformance. For example, a new class of DS can display unique properties. Such properties canbe particle sizes or particle agglomeration due to special magnetic properties when usingmagnetic nanoparticles (MNPs). Some DS can also act as precursor to scaling and membranefouling during reverse diffusion when DS containing SO24 and Mg

2+ are used, respectively

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the osmotic pressure can be given by the concentra-tion dependence osmotic equation [14], also known asvirial equation, as shown below.

p=cRT 1 Bc Cc2 Dc3 2

where B, C and D are the osmotic virial coefficientsthat can be determined empirically by fitting experi-mental osmotic pressure data, and generally the deter-mination of B and C is sufficient to reproduceobserved data [13].

From the above equations, it is clear that, theosmotic pressure is a function of solute concentration,number of species formed by dissociation in the solu-tion, MW of the solute and the temperature of thesolution and therefore does not depend on the typesof species formed in the solution. A solute with smallMW combined with high water solubility can generatehigher osmotic pressure (on equal mass basis) andtherefore can lead to higher water fluxes [15].

Besides osmotic pressure, the performance of FOprocess is however affected by other properties of theDS such as the diffusion coefficient [12] as shown inthe following equation.

K tseDs

3

where K represents the solute resistance to diffusionwithin the membrane support layer, t, s and repre-sent the thickness, tortuosity and porosity of the mem-brane porous support layer, respectively, and Dsrepresents the diffusion coefficient of the solute. Thevalue of K is an inverse function of the Ds. indicatingthat solutes with higher diffusion coefficient will havelower resistance and can more readily diffuse throughthe membrane support layer and therefore have lowerICP effects. Solutes with lower MW usually havehigher diffusion coefficient compared to those withlarger MW; however, many studies have demon-strated that DS with very small MW showed higherreverse salt diffusion [1620] which could potentiallyhave an adverse impact on the FO performance, espe-cially when high-quality product water is required.Solutes of higher MW have lower diffusion coefficientand therefore tend to cause more severe ICP effects.

The DS concentration also significantly influencesthe performance of the process. Most studies haveshown that higher water fluxes can be achieved byincreasing the DS concentration [2124]. However,contrary to the theoretical solution-diffusion model,which establishes a linear relation between waterfluxes and DS concentration, experiments have shownthat this relation is non-linear. Linear relation is

observed at lower concentrations, but at higher DSconcentration, a logarithmic relationship has beenvisually observed. This is mainly attributed to ICPeffects in the porous support layer which is greater athigher permeate flux resulting in less effective waterflux improvement. Tan and Ng [25] have even demon-strated that the very high increasing DS concentrationcould potentially reduce the water flux to a valuewhich is too low for efficient permeate production.

Finally, similar to pressure-based membrane pro-cess, FO process is also affected by DS temperaturebecause the properties of DS and feed solution suchas osmotic pressure, viscosity and diffusivity areaffected by temperature. Water fluxes in FO processimproved significantly at higher DS temperature asobserved by most studies [24,2628]. These studiesattributed this enhanced water flux due to reducedwater viscosity and therefore enhanced mass transfer.Besides viscosity, the diffusion coefficient of the DSalso increases at higher temperature which conse-quently decreases the value of K (solute resistance todiffusion within the membrane support layer, referEq. (2)) and therefore increases the water flux [24,27].However, this relation between temperature andwater flux is more complex as some recent studieshave demonstrated that higher temperature will alsoinduce more adverse effects on membrane scaling inthe presence of certain scaling species, which mayresult in water flux decline. In fact, Garcia-Castelloet al. [26] and Zhao and Zou [28] observed that, athigher temperature, more compact crystals are depos-ited onto the membrane surface which reduces theefficiency of water cleaning. Hence temperature canenhance water flux to a certain critical point whenmembrane scaling starts to affect process performanceby causing flux decline.

Despite the general characteristics mentionedbefore, other specific DS characteristics can impact onthe process performance, depending on the applica-tion. One good example can be the presence of scaleprecursor ions. In fact, Achilli et al. [18] demonstratedthat when using DS containing scale precursor ions (e.g. Mg2+, Ca2+, Ba2+, SO4

2 and CO32), mineral scaling

will likely occur on the membrane surface when thefeed solution concentration is above the solubilitylimit. Hence, the use of DS which are likely to causescaling (e.g. CaCl2, MgSO4, KHCO3, NaHCO3 andNa2SO4) is strictly limited to application involving theuse of pure feed solution such as the food industry,otherwise it can have potential adverse effects on theprocess performance.

Recent studies on new classes of DS such as MNPsor micelles showed that these novel DS feature uniqueproperties that may also impact on the process effi-

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ciency. For instance, due to their magnetic properties,MNPs have a tendency to agglomerate and this phe-nomenon has to be avoided (using surface coating forinstance) as it could cause membrane fouling andtherefore decrease the process performance bylowering the water flux.

3. Classification of osmotic DS

A wide range of DS have been proposed andtested since the mid-1960s. They can be generally clas-sified as inorganic-based DS, organic-based DS andother compounds including emerging DS such asMNPs or RO brines. The sub-classification wouldinclude such as electrolyte (ionic) solutions and non-electrolyte (non-ionic) solutions depending on whetherthe solution is made up of charged ions or neutral/non-charged solutes, respectively.

3.1. Inorganic-based DS

The majority of FO studies have investigated inor-ganic-based compounds as DS, and they are stillextensively utilised nowadays. Inorganic-based DS is

mainly composed of electrolyte solutions althoughnon-electrolyte solutions could also be possible.

The most recent and comprehensive studies of inor-ganic DS are made by Phuntsho et al. [17], Achilli et al.[18] and Tan and Ng [25]. Achilli and co-workers testedand compared 14 inorganic-based compounds as DSfor FO process. These solutes were chosen from amongmore than 500 inorganic compounds because of theirhigher water solubility, osmotic pressure, lower specificcost and toxicity, which are crucial criteria that canimpact greatly on FO performance and their end use.Tan and Ng [25] proposed a novel hybrid concept bycombining FO and nanofiltration (NF) for seawaterdesalination; seven potential DS were tested at labora-tory scale, including six inorganic-based compounds.Finally, Phuntsho et al. [17] evaluated the performanceof nine commonly used inorganic fertilisers as possibleDS candidates for fertigation. The selected DS exhibitdifferent physical and chemical properties as displayedin Table 2. This table also includes the experimentalwater flux data for comparison.

Numerous studies have used sodium chloride asDS in a wide range of applications. For instance, ithas been applied in food production [26,27] and water

Table 2Physicochemical properties and experimental water fluxes of inorganic compounds tested as DS

DS tested MW Osmotic pressurea

at 2.0M (atm)pHa at2.0M

Max. solubilitya

(M)Scale precursorions

Experimentalwater fluxb

(lm/s)

References

CaCl2 111.00 217.60 6.29 7.4 Yes (Ca2+) 2.64 [18,25]

KBr 119.00 89.70 6.92 4.5 No 2.84 [18]

KHCO3 100.10 79.30 7.84 2.0 Yes (CO23 ) 2.25 [18]

K2SO4 174.20 32.40 7.33 0.6 Yes (SO24 ) 2.52 [18]

MgCl2 95.20 256.50 5.64 4.9 Yes (Mg2+) 2.33 [18,25]

MgSO4 120.40 54.80 6.70 2.8 Yes (Mg2+) 1.54 [18,25]

NaCl 58.40 100.40 6.98 5.4 No 2.68 [18,25]

NaHCO3 84.00 46.70 7.74 1.2 Yes (CO23 ) 2.47 [18]

Na2SO4 142.00 95.20 7.44 1.8 Yes (SO24 ) 2.14 [18,25]

NH4HCO3 79.10 66.40 7.69 2.9 Yes (CO23 ) 2.04 [18]

NH4NO3 80.04 64.90 4.87 84.0 Yes (CO23 ) 4.177 [17]

(NH4)2SO4 132.10 92.10 5.46 5.7 Yes (SO24 ) 5.391 [17,18]

NH4Cl 53.50 87.70 4.76 7.4 No 5.348 [17,18]

Ca(NO3)2 164.10 108.50 4.68 7.9 Yes (Ca2+) 5.022 [17,18]

NaNO3 85.00 81.10 5.98 10.5 No 5.706 [17]

KCl 74.60 89.30 6.80 4.6 No 6.337 [17,18,25]

NH4H2PO4 115.00 86.30 3.93 3.7 No 4.349 [17]

(NH4)2HPO4 132.10 95.00 8.12 6.5 No 3.892 [17]

KNO3 101.10 64.90 5.99 3.3 No 4.429 [17]

aOsmotic pressure, pH and solubility data were calculated by OLI Stream Analyzer.bExperimental water fluxes were taken from [14] for the first ten DS for an osmotic pressure of 2.8MPa, and the other experimentalfluxes were taken from [13] at 2.0M concentration of DS.

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and wastewater treatment [23,2931]. The fact thatNaCl is used as DS for many FO studies is mainlybecause saline water is abundant on earth making sea-water a natural and cheap source of DS. NaCl is alsooften utilised, because it is relatively straightforwardto reconcentrate with RO process without the risk ofscaling and it has high water solubility and exhibitshigh osmotic pressure. Moreover, the thermodynamicproperties of NaCl have been widely investigatedmaking it easier for the study.

The other inorganic DS which are commonly stud-ied include magnesium chloride [32,33] ammoniumbicarbonate mainly as a thermolyte solution that canbe recovered and reused [8,15,16,24] and calcium chlo-ride [18,25,27,34]. Specific application of each of theseDS is discussed in details in a later part of the review.

3.2. Organic-based DS

Over the past decades, organic compounds partic-ularly fructose and glucose solutions have been testedas DS, especially for seawater desalination [16,3537]and food production [27,3840] applications. Althoughorganic DS are usually non-electrolyte compounds,they have the potential to generate high osmotic pres-sure as they generally exhibit high solubility [16] as itis depicted in Table 3.

Other organic DS include polyethylene glycol 400(PEG) to concentrate tomato juice [27], ethanol for therecovery of water from highly impaired sources [41],albumin for dewatering RO concentrate [42] and 2-methylimidazole-based compounds [19].

3.3. Other DS

3.3.1. Magnetic nanoparticles as DS

Nanoparticle research is currently an area ofintense scientific interest due to a wide variety of bio-medical applications such as biocatalysis and drugdelivery. However, some recent studies [4345] have

focused their works on hydrophilic magnetic nanopar-ticles (MNPs). Three different types of MNPs wereinvestigated as potential DS: the polyacrylic acid mag-netic nanoparticles (PAA MNPs), the 2-Pyrrolidone-magnetic nanoparticles (2-Pyrol MNPs) and thetriethyleneglycol magnetic nanoparticles (TREGMNPs). Although they are non-electrolytes, the mainadvantage of MNPs is their extremely high surface-area-to-volume ratio and their bigger sizes comparedto inorganic salts and organic molecules that facilitaterecovery using magnetic fields and low pressuremembrane processes such as microfiltration or NF.Moreover, they are capable of producing very highosmotic pressure, up to 70 atm (for PAA MNPs),which is far higher than the seawater osmotic pres-sure of 26 atm, which make them very attractive fordesalination [44].

3.3.2. Concentrated RO brines as DS

The disposal of concentrated brines from a ROdesalination plant is a significant environmental issue.RO concentrate is made up of waste flow with highlyconcentrated organic and inorganic compounds [46].Hence, there is a need to sustainably manage the ROconcentrate in order to avoid any adverse effects onthe receiving environment. Recent studies havefocused on the potential use of RO brines as DS tosolve concentrate issues. Ling and Chung [43]designed a novel dual-stage FO process where MNPswere used as DS in an up-stage FO process to concen-trate proteins and RO brine was used as DS in adown-stage FO process to reconcentrate MNPs.Bamaga et al. [47] designed a hybrid FO/RO processwhere the first FO process is used as a pre-treatmentfor RO desalination to minimise scaling risk duringthe desalination process. The second FO process,using the RO brine as DS, is utilised to concentratethe impaired water to minimise its volume for furthertreatments. Hence, in this application, the FO process

Table 3Physico-chemical properties and experimental water fluxes of some organic compounds tested as DS

DS tested MW Osmotic pressurea

at 2.0M (atm)pH

a

at2.0M

Max. solubilitya

(M)Experimental waterflux

bReferences

Ethanol 46.07 43.93 7.00 Miscible Not available [41]

Sucrose 342.30 56.81 6.18 6.1 0.35 LMH [27]

Glucose 180.16 55.03 7.01 800.0 0.24 LMH [16,27,35]

Fructose 180.16 55.02 7.01 22.4 7.5 LMH [16,37,42]

Note: MW, molecular weight; LMH, Lm2 h1.aOsmotic pressure, pH and solubility data were calculated by OLI Stream Analyzer.bExperimental fluxes were taken from [3] for fructose at 6M fructose concentration and from [19] for sucrose and glucose at 58.29%(w/w) and 62.86% (w/w), respectively.

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combining with RO brines as DS is used to lowerenergy requirement for desalination. Moreover, thereare several benefits when coupling both FO and ROprocesses. In fact, the RO process was proved to be anefficient reconcentration and recovery process whichis able to produce very high-quality product water.Using this concept of coupling RO and FO processes(Fig. 2), Modern Water [5] successfully designed anddeployed the first commercial FO desalination plantwith a capacity of 100m3/d of produced water inOman on the Arabian Sea.

3.3.4. Ionic polymer hydrogel particles as DS

Li et al. [48] recently studied the development of anew class of DS, the polymer hydrogel particles forFO desalination. Hydrogels are three-dimensional net-works of polymer chains that are linked by eitherphysical or chemical bonds and are able to catch largevolumes of water attracted by the highly concentratedhydrophilic groups. Their sizes generally rangebetween 50 and 150lm. Hydrogels with ionic groupsare able to attract even more amount of water whichincreases their osmotic pressures and make themattractive for desalination applications. One importantand advantageous aspect of polymer hydrogels is thatthey can undergo reversible volume change or solu-tiongel phase transitions in response to environmen-tal stimuli including temperature, light, pressure oreven pH. One particular interesting response to thesestimuli is the change from hydrophilic to hydrophobic(generated by heating or pressure stimuli), whichinduces hydrogel particles to release water. Thisunique characteristic makes also the recovery of this

novel DS very easy and at lower energy cost (com-pared to thermal or membrane processes). In a morerecent study [49], they demonstrated that combiningthe polymer hydrogel particles with light-absorbingcarbon particles enhances heating and dewatering ofthe particles. In both studies, the polymer hydrogelparticles were able to deliver high osmotic pressure ofabout 2.7MPa at 27 C. Water fluxes ranged from 0.55LMH to 1.1 LMH depending on the polymer used.

3.3.5. Micelles close to the Krafft point as DS

Gadelha and Hankins [50] introduced the conceptof using colloidal systems such as micellar solutionsas DS. This new potential DS presents interestingcharacteristics; it exhibits almost constant osmoticpressure above the critical micelle concentration whichallows many dilutions prior to regeneration. More-over, its solubility is highly temperature sensitivearound the Krafft point, which is the critical minimumtemperature under which micelles cannot form. Thisproperty of micellar solutions enables regenerationwith small temperature fluctuations using for instancelow-grade heating and cooling. The surfactant is thenin crystalline form and can be readily separated fromthe product water. Another benefit of micelles is thatthey can act as monomeric concentration bufferswhich minimise the internal concentration polarisation(ICP) effects.

3.3.6. Dendrimers as DS

Adham et al. [42] proposed the use of dendrimersas a novel DS for dewatering RO concentrate. Dendri-mers are symmetrical spheroid or globular nanostruc-tures that are precisely engineered to carry molecules.These macromolecules consist of a highly branchedtree-like structure linked to a central core throughcovalent bonds. Because they are macromolecules,they can provide a high osmotic pressure up to330 psi, much higher than RO concentrate. Moreover,they can be readily regenerated by conventional mem-brane processes such as UF. In this study, two typesof dendrimers were tested: ethylenediaminecore den-drimers with sodium succinamate terminal groupsand pentaerythrityl core dendrimer with sodium car-boxylate terminal groups.

4. Reconcentration and recovery processes for DS

Separation of the DS after it has been diluted forrecovery, regeneration and recycling is one of the big-gest challenges facing the FO process, especially for

Fig. 2. Simplified FO/RO desalination process using RObrines as DS.

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drinking water production when high-quality water isrequired. In order for FO process to compete withother membrane processes, the DS reconcentration andrecovery should operate at low-cost energy. This pro-cess should also provide high recovery of the DS whileproducing high-quality product water. Therefore,attention is now given to find easy and efficient recon-centration and recovery processes for selected DS.

Table 4 summarises some tested DS and their pro-posed reconcentration and recovery methods. Sincethe mid-1960s, attempts have been made to find a DSthat can be easily separated, recovered and regener-ated. For instance, Batchelder [51] was the first to testvolatile solutes (SO2) as DS and recovery was madeby heating and air stripping process. Later, thermo-lytic solutions such as carbonates of ammonia werefound to be readily recovered through distillation pro-cess using low heat energy as this DS can decomposeinto NH3 and CO2 by heating up to only 60 C [24].However, the proximity of low-grade heat from ther-mal power plants for instance is required to ensurethat the recovery process is economically viable.

The regeneration of MNPs has also been investi-gated in many studies. Adham et al. [42] proposedthe regeneration of MNPs by magnetic field separa-tors, but this method caused agglomeration of MNPswhich decreased their osmotic pressures and thereforethe water flux. Ultrasonication was suggested to pre-vent this issue, but this potentially weakens the mag-netic properties of MNPs and thus reduces theregeneration efficiency [43,45]. Recently, Ling andChung [44] used UF for the recovery of MNPs anddemonstrated that PAA-MNPs can be recycled up tofive times only using UF without increasing their sizesor reducing their osmotic pressures while deliveringreasonable water flux and salt rejection.

For some specific applications, however, thediluted DS can be used directly without the need forseparation processes which considerably reduce theenergy cost of the process [52]. Such applicationsinclude emergency water supply [35,37], dilution ofinput stream to RO desalination plant [47,53], dilutionof RO brine before discharging into the environment[29,30], osmotic cleaning of fouled RO membranes

Table 4Summary of tested DS and regeneration methods for FO process. Modified from Gadelha and Hankins [50]

DS tested Reconcentration and regeneration methods References

Volatile solutes (SO2) Heating or air stripping [51]

Alcohol, SO2 Distillation [58]

Al2SO4 Doped Ca(OH)2 to precipitate CaSO4 and Al(OH)2 [59]

Glucose Direct application [35]

Glucose and fructose Direct application [36]

Fructose Direct application [37]

Glucose/fructose RO process [60]

MgCl2 Direct application [10]

NF process [25,32]

KNO3 and SO2 SO2 is removed through standard means [61]

NH4HCO3 Heatingdecomposition into NH3 and CO2 [24]

MNPs Magnetic field separators [42,45]

FO process using RO brines as DS [43]

UF process [44]

Albumin Denatured and solidified upon heating [42]

Dendrimers Wide range of pH values and UF [42]

2-methylimidazole-based compounds FO-MD [19]

NaCl RO process [31]

Distillation/RO process [53]

Direct application [57]

MgSO4 and Na2SO4 NF process [25]

Micelles close to the Krafft point Temperature swing with low-grade heat and crystallisation [50]

RO brines RO process [5]

Ionic polymer hydrogel particles Direct application [47]

Heating or pressure stimuli [49]

Fertilisers Direct application [17]

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[54,55], production of biofuel from algae [32,56,57]and direct irrigation [17].

5. DS for modern applications

Many studies on the potential use of FO processfor both industrial and domestic applications appearin the literature. The following section reviews the useof different types of DS for various FO applicationssuch as desalination, water purification, wastewatertreatment, energy production, biomedical applicationsand food processing.

5.1. DS for potable water production

For the production of potable water by FO desali-nation process, the DS must have special properties.Besides meeting the general criteria such as high solu-bility, high osmotic pressure and pH compatible withthe FO membrane, the DS for potable water shouldhave a low reverse salt flux and be easily separated,recovered and regenerated for reuse with minimumefforts. In fact, any trace concentration of the DS inthe final desalted water should not cause any healthhazard and hence must meet the World HealthOrganisation (WHO) guidelines for drinking waterquality standards [62]. Although FO process canextract water from any saline water sources as long asthe DS can generate osmotic pressure much higherthan the feed water, finding ideal DS meeting all theseproperties for potable water is still a challenge.

Initial works on FO desalination for drinking waterproduction focused on applications where the dilutedDS can be used directly for drinking water without

the need for separation process. Such applicationsinclude production of nutrient drinks, emergencywater supply in life boats and emergency relief situa-tions during natural disasters. For these particularapplications, glucose and fructose were used as DS[3537]. Glycine was also suggested as DS for thispurpose as it is more efficient in osmosis than fruc-tose; however, it can be toxic for human at high con-centrations. Stache [37] showed that fructose contentmust be at least 90% (w/w) which otherwise, thewater flux rapidly decreases and for concentrationbelow 74% (w/w), the water flow may be reversedfrom fructose DS to the saline solution. This also sug-gests that the final nutrient drink shall still containhigh concentration of fructose solution. More lately,Wallace et al. [34] developed a thermodynamic bench-mark for assessing an emergency drinking waterdevice driven by FO process using brackish water asfeed solution and mixtures of salt (MgCl2, CaCl2 andNaCl) and sugar (sucrose and glucose) as DS. HTI hasrecently developed and manufactured the first com-mercial FO membrane material and hydration bagsfor emergency water supply where sugar or beveragepowder are used as DS [3,4]. However, these studiesonly developed batch desalination processes for emer-gency relief situations and not for continuous drinkingwater supply.

In an attempt to develop a continuous FO processinvolving reconcentration and recovery of the DS asshown in Fig. 3, McGinnis [61] suggested the use ofpotassium nitrate (KNO3) and sulphur dioxide (SO2)as DS for seawater desalination. This concept involvedtwo-stage FO process for recovering water from aque-ous solutions by taking advantage of highly tempera-

Fig. 3. Schematic diagram of FO desalination process for potable water with DS recovery system.

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ture dependent solubility of KNO3 and SO2, as well asthe relatively temperature independent solubility ofNaCl, the primary solute present in seawater. How-ever, performance data are not available till now.Later, McGinnis and co-workers [15,24] demonstrateda thermolyte solution using the mixture of ammoniaand carbon dioxide gases in specific ratios that cangenerate high osmotic pressure of up to 238 bars. Themost important feature of this DS is that the mixturecan be easily recovered through distillation process.Water fluxes close to 25 LMH were achieved with adriving force of more than 200 bars using this mixtureas DS. Similarly, Hancock and Cath [8] and Ng andTang [16] tested NH4HCO3 as DS for FO seawaterdesalination. Although adequate water flux can beachieved using NH4HCO3 as DS, the solubility ofNH4HCO3 was found to be one of the lowest in com-parison with other DS tested, whereas its reverse saltdiffusion was one of the highest due to its low MW[18]. DS with high reverse salt flux is not recom-mended for potable water production as discussedearlier since any trace of the DS in the final productmay pose health hazards or result in the final productnot meeting the WHO standards. Finally, Ng andTang [16] found that this DS can also suffer from earlydecomposition if operated at temperature higher than30 C [2] and such temperature is the ambient temper-ature in regions such as in the Middle Eastern coun-tries where desalination for drinking water productionis operated.

Besides thermolytic solutions, other DS have beenproposed for the production of potable water by FOdesalination. Tan and Ng [25] tested seven potential

DS on a new hybrid concept for seawater desalinationby combining FO and NF processes. Their results onFO membrane showed that NaCl and KCl producethe highest flux of more than 25 LMH while MgCl2and CaCl2 exhibited the largest osmotic pressure dueto their high solubility in water. However, waterfluxes were only comparable to those obtained withNaCl and KCl, and these lower-than-expected waterfluxes were attributed to more severe CP effects.Experiments on FO membrane also demonstrated thatsolute rejection for all tested DS was always above99.4% and could achieve 99.99% with MgSO4. NaCland KCl were not selected for tests on the hybrid FO/NF process as previous works demonstrated that bothsolutes exhibit low salt rejection (lower than 65%)with NF membrane. CaCl2 was not selected as wellfor further tests with NF since at high concentration,when exposed to atmosphere, it could form precipi-tates with CO2 and cause membrane fouling. The fourlast compounds (i.e. MgCl2, MgSO4, Na2SO4 andC6H12O6) were tested on the coupled FO/NF process.C6H12O6 was not recommended as DS for this processas it is susceptible to degradation by microorganismsrendering the regeneration of this compound difficult.Tan and Ng concluded that organic DS should beavoided for recycled usage. Results also showed thata single-pass NF used as regeneration process is notable to produce good quality product water in termsof total dissolved solid (TDS) concentration. Experi-ments using the second-pass NF process producedbetter results with permeate qualities having the rec-ommended TDS concentration of 500mg/L whenusing MgSO4 and Na2SO4 as DS.

Fig. 4. Schematic diagram of the fertiliser drawn forward osmosis desalination for direct fertigation.

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A novel innovation has been the use of MNPs forFO desalination by Ling and Chung [44]. Theydesigned and tested a novel integrated FO-UF systemfor desalination using MNPs as DS. This system usesMNPs to induce water across the FO membraneswhile UF membrane was used to regenerate theMNPs DS. Among the MNPs tested, PAA-MNPsshowed the best performance by producing an osmo-tic pressure of about 70 atm at a concentration of0.08mol/L and delivering a water flux of about 17LMH. Moreover, these MNPs can be effectivelyrecovered by several traditional processes as discussedin the previous part.

Another novelty has been the development ofcomposite polymer hydrogel particles (PHP) withlight-absorbing carbon particles for FO desalination byLi et al. [49]. The addition of carbon particles wasproved to increase the swelling pressure of hydrogelsresulting in higher fluxes. Sunlight irradiation wasused as stimulus to separate the produced pure waterfrom the swollen PHP. For some of the polymerhydrogels, more than 98% of water was recoveredafter 60-min exposure to the sunlight with an irradia-tion intensity of 1 kW/m2. However, the use of suchDS could prove a practical challenge especially if itexists in solid form.

5.2. DS for non-potable water production

5.2.1. Fertigation

Due to the limited availability of freshwater, desa-lination of seawater and brackish water could be a

reliable source for irrigation. Since FO was proved tobe more energy-efficient over conventional desalina-tion processes such as RO, this process can be used todesalinate locally available saline water to replacelong-distance diversion of freshwater for irrigation. Arecent study by Phuntsho et al. [17] investigated theuse of fertilisers as DS driven by FO process to extractwater from brackish sources for direct irrigation.There are plenty of advantages when fertilisers areused as DS in FO process and the detail review onthis particular concept can be found in the recentreview by Phuntsho et al. [63]. In fact, the fertiliser DSdoes not require an additional separation process as itcan be directly used for fertigation or fertilised irriga-tion as it is displayed in Fig. 5. The diluted DS is anutrient-rich solution required for the crops, and theFO process has a high salt rejection rate which is alsogood for plants.

Phuntsho et al. [17] studied nine different com-monly used fertilisers as potential DS and evaluatedtheir performance in terms of pure water flux andreverse solute flux. KCl, KNO3 and NaNO3 showedthe highest water flux while NH4H2PO4 and(NH4)2HPO4 exhibited the lowest performance interms of water flux among the selected DS. Theammonium compounds of phosphate and sulphate (i.e. (NH4)2SO4, (NH4)2HPO4 and NH4H2PO4 and Ca(NO3)2) displayed the best performance in terms ofreverse salt diffusion as they have ionic species withlarge hydrated diameter compared to other fertiliserswhich have ionic species with small hydrated diame-ter such as Cl, K+, Na+, NO3 and NH

4 . They also

estimated the theoretical quantity of water that a kilo-

Fig. 5. Schematic diagram of FO/MBR hybrid system (WWTP: wastewater treatment plant; MBR: membrane bioreactor).

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gram of fertiliser mass can extract from sea and brack-ish water. NH4Cl, KCl and NaNO3 were able toextract higher amount of water than other DS tested,while Ca(NO3)2 with largest MW has much lowerwater extraction capacity. All the results discussedpreviously are gathered in Table 5. They concludedthat any soluble fertilisers can be used as DS and theirchoice of the DS will depend on compatibility withthe FO membrane (physical and chemical parameters)and plant nutrient requirements.

5.2.2. RO concentrate management

Past studies have demonstrated that FO can beeffectively used for the treatment of a wide range ofhighly concentrated feed solutions. Martinetti et al.[29] studied the desalination of two RO brine streamswith TDS concentrations of about 7,500 and17,500mg/L using NaCl as DS. The FO processachieved high water recoveries, up to 90% from theRO brines. They found that water recoveries were lim-ited by the precipitation of organic salts on the mem-brane surface. However, after cleaning with Na2EDTAand osmotic backwashes, water fluxes were almostrestored at initial levels.

Adham et al. [42] evaluated the treatment of ROconcentrate using FO with a wide range of DS includ-ing MNPs, albumin and dendrimers. MNPs were ableto generate osmotic pressures ranging from 3 to 25 psiwhich were not adequate for dewatering of RO con-centrate. They explained this due to their high MWand low solubility. However, the authors contem-plated that MNPs may enhance its performance as DSif they can be properly synthesised with a smaller sizeand a more hydrophilic surface. They also observed

that dendrimers could generate osmotic pressure of330 psi which is adequate to dewater RO concentrate.More specifically, it was 20% of a G2-pentaearythiritylsodium carboxylate dendrimer solution which pro-duced this high osmotic pressure. UF was proved tobe an efficient recovery system for this DS with morethan 85% rejection.

5.2.3. Wastewater treatment

FO process is characterised by low fouling poten-tial and therefore a number of studies have beenrecently reported for its potential application inwastewater treatment. Applications of FO process forwastewater treatment include mainly osmoticmembrane bioreactor (OSMBR) [23,64,65], anaerobicdigester concentrate [30] and treatment of TDS fromlandfill leachate [31].

As with many other applications, all the studiesreported on OSMBR so far have been limited to usingNaCl as DS [23,64,65]. This in fact opens up the poten-tial for using seaweater and concentrated brine from anearby RO desalination plants for OSMBR as it is dis-played in Fig. 5. The different studies with NaCl DSindicate that the FO membrane in MBRs could offerthe advantage of higher pollutant rejection with lowerhydraulic pressure in comparison with conventionalMBR. Achilli et al. [23] showed that total organiccarbon (TOC) and NH4

+N removals were muchhigher than those obtained with conventional MBRswith removals greater than 99% compared to 95%with traditional processes. Although reverse salt diffu-sion of DS caused initial flux decline in OSMBR pro-cess, salt concentration in the bioreactor stabilisedafter certain period of operation and then flux decline

Table 5Performance of the fertiliser DS in terms of water flux, reverse salt flux and water extraction capacity [17]

Water flux Reversewater flux

Water extraction capacity

Fertilisers ExperimentalJw (lm/s)

Estimated theoreticalJw (lm/s)

Performanceratio (%)

Js (mmoles/m2 s) From brackish

water(L/kg fertilisers)

From seawater(L/kg fertilisers)

NH4NO3 4.2 27.9 14.9 0.791 32.4 15.2

(NH4)2SO4 5.4 39.7 13.6 0.006 26.3 12.7

NH4Cl 5.3 37.8 14.2 0.333 58.8 29.1

Ca (NO3)2 5.0 46.7 10.8 0.009 23.7 11.5

NaNO3 5.7 34.9 16.3 0.278 34.8 17.7

KCl 6.3 38.4 16.5 0.222 42.4 21.0

NH4H2PO4 4.3 37.2 11.7 0.069 27.6 13.7

(NH4)2HPO4 3.9 40.9 9.5 0.010 30.2 14.4

KNO3 4.4 27.9 15.9 0.486 30.5 13.7

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was only caused by membrane fouling. Besides, thelevel of salinity observed in the bioreactor did notpresent toxic effects on the biological process indicat-ing that NaCl is a suitable DS for this application.

The sludge produced in the effluent fromwastewater treatment facilities is generally treated inanaerobic digesters. After treatment, the sludge isdewatered which produces a centrate (liquid stream)and concentrated solids. The centrate holds high con-centrated nutrients, TDS, TOC, heavy metals, colourand TSS. Sometimes this centrate is used as a soil fer-tiliser, but it is commonly returned to the wastewatertreatment facilities for further treatments. Hollowayet al. [30] investigated the use of FO for the concentra-tion of centrate using NaCl as DS. Although fluxdecline was observed for each trial, due to thedecrease in driving force and membrane fouling;almost complete restoration was obtained after clean-ing. Colour and odour were highly rejected withalmost 100% rejection. Phosphorus rejection exceeded99%, and total Kjeldahl nitrogen and ammonia rejec-tions were about 92 and 87%, respectively. Theyconcluded that if a RO process is combined for DSreconcentration, higher rejection is expected.

Landfill leachate is a complex solution, generallycomposed of organic compounds, nitrogen, TDS andheavy metals. Nowadays, landfill leachates are treatedin wastewater treatment facilities which mainly arefocused on the removal of organic compounds, nutri-ents and heavy metals. TDS is not often treated andsometimes effluents are highly concentrated in TDS.York et al. [31] studied the possibility of using FO asa potential process for treating landfill leachates andespecially TDS. A full-scale system was designedusing a solution of 75 g NaCl/L as DS. In this system,the FO process was used to draw water from theleachate into the DS. The diluted DS was then treatedby a RO process to produce freshwater and reconcen-trate the DS. The combined FO/RO process wasproved to be more efficient than the RO process aloneas RO is less resistant to fouling than the FO process.This system was able to remove the vast majority ofcontaminants present in the feed solution. TDS rejec-tion was almost 98% and most contaminants hadmore than 99% rejection.

Although, in the real application, the selection of DSwould depend on the end use of the final product waterafter treatment of wastewater by FO process, the stud-ies above on wastewater treatment using NaCl as DSindicate the possibilities of using seawater which existsin abundance or RO concentrate as suitable DS for thetreatment of impaired water. Phuntsho et al. [17,63]suggested that fertilisers can also be used as DS fortreating wastewater by FO process and diluted DS can

then be used directly for irrigation purpose. If recovery,regeneration and recycling of DS are necessary, a moresuitable DS could be electrolytes containing multivalentions which can be easily reconcentrated and recycledusing NF or other suitable process.

5.3. DS for other applications

5.3.1. Energy production

Since the concept of osmotic power generation wasfirst reported by Pattle [66], there is a growingresearch interests in power generation by takingadvantage of the osmotic pressure difference betweentwo streams of different salinity specifically at thepoint where freshwater meets the saline water such assea [6770]. The most recent comprehensive review ofthe pressure-retarded osmosis (PRO) for osmoticpower generation can be found in the article byAchilli et al. [71]. In a PRO process, a hydraulicpressure is applied on the opposite direction of theosmotic gradient similarly to RO process, but thewater flow is in the direction of the concentrated DSlike in the FO process. Generally, seawater is used asDS by taking advantage of the salinity gradient thatexists in nature. The osmotic pressure gradientbetween the freshwater from rivers and the highlysaline seawater is converted into a hydrostaticpressure that can run a turbine to generate electricityas it is depicted in Fig. 6. The production of renewableenergy by PRO from seawater is becoming more andmore attractive and in fact seawater is an unexploitedand large resource and its use has very small effect onthe environment [66]. Aaberg [67] estimated that theoverall power production potential from PRO is in theorder of 2000TWh per year.

Although the power generation takes advantage ofthe salinity gradient that exists in nature, there is alsoa potential for generating power by PRO using ROconcentrate as a DS. RO concentrate is one of the mostsignificant issues in a RO desalination plant. Theosmotic gradient between the RO concentrate and theseawater could be significant, and this can be usedadvantageously to generate power which can then beused further for desalination process in the processmaking the desalination plant more energy efficient.

5.3.2. Biomedical applications

Proteins have a wide range of commercial applica-tions, particularly in the pharmaceutical market, butenriching and separating protein is technically andeconomically challenging as most proteins are chemi-cally unstable and heat sensitive. Some studies have

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recently proposed the use of FO process to concentrateproteins. For this specific application, the choice of asuitable DS is crucial since the appropriate DS must beeasy to recover without denaturing the protein.

Yang et al. [33] studied the potential use of MgCl2as DS for protein enrichment by FO process using adual-layer hollow fibre NF membrane. They foundthat MgCl2 can enrich lysozyme product to high pur-ity without changing the conformation or denaturingthe protein due to low reverse solute flux. In a morerecent study, Ling and Chung [43] developed a dual-stage FO system using MNPs as DS to concentrateproteins in an up-stage FO process and RO brines asDS to reconcentrate the MNPs in a down-stage FOprocess as shown in Fig. 7. In their study, they syn-thesised hydrophilic MNPs as DS which can exhibithigh osmotic pressure and can be easily recycledeither by membrane process or magnetic fields [45,72].Moreover, their reverse salt flux was found to be verylow as they are too large to pass across the membranepores which keep proteins intact and stable duringtheir enrichment. They also demonstrated that the useof concentrated salts as DS (e.g. NaCl) may denatureproteins since they will exhibit much reverse salt dif-fusion and inevitable salt leakage will alter proteinintrinsic characteristics.

For long-term treatment of chronic illness, patientsrequire continuous, controlled and targeted release ofdrug for longer period of time (i.e. up to a year).

DUROS [7] has successfully developed and commer-cialised the first osmotic pumps for this purpose.

Fig. 6. Schematic diagram of osmotic power generated by PRO process.

Fig. 7. Schematic diagram of the laboratory-scale dual FOsystem (adapted from [43]).

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Osmotic pump system is made up of a titanium cylin-drical reservoir that protects the drug from bodymoisture, cellular components or enzymes that mayaffect the drug before it has been delivered to thebody. A semi-permeable membrane covers one side ofthe cylinder and the DS is stocked in a small portionof the cylinder, behind the membrane, and separatedfrom the drug compartment by a piston. Generally,the DS is a mixture of NaCl and pharmaceuticalexcipients. The drug outlet is a small orifice locatedon the other side of the cylinder. When implemented,an osmotic gradient is set up between the tissue waterand the DS which induces water to flow across themembrane. The pressure inside the DS compartmentis then continuously increasing which pushes the pis-ton and increases the pressure in the drug reservoirwhich leads to the delivery of the drug into the body.

5.3.3. Food industry

FO process has significant application potential forconcentrating beverages and liquid foods. FO processcan operate at low temperatures and low pressureswhich prevent final products from sensory (i.e. taste,aroma and colour) and nutritional (i.e. vitamins)degradation.

In 1966, Popper et al. [73] were the first to use theFO process to concentrate fruit juices. They used satu-rated NaCl as DS and achieved water flux of 2.5 kg/m2h. Although highly concentrated fruit juices wereproduced, salt was observed in the feed solution dueto reverse diffusion of NaCl. Herron et al. [40]developed a membrane module and a method to con-centrate juices and wines. High water fluxes wereachieved for orange juice and coffee feed solution,probably due to the high turbulence in the FO mem-brane module using 5085 wt.% sugar solution as DS.Adopting the method of Herron et al. [40], Petrotoset al. [27] studied the concentration of tomato juiceand tested six different compounds (i.e. calciumchloride, calcium nitrate, glucose, sucrose and poly-ethylene glycol 400 or PEG400) as potential DS. Theirexperiments showed that the choice of a suitable DSdepends greatly on its mass transport characteristics(i.e. viscosity and diffusivity). Based on this result,NaCl and CaCl2 showed the best FO performance asthey exhibit the highest water flux due to their lowerviscosity. In a further study, Garcia-Castello et al. [26]focused on the performance of the FO process insucrose concentration using NaCl as a surrogate DS.They have found that much higher sucrose concentra-tion factors can be obtained in comparison with ROprocess. However, water fluxes were found to be low

compared to those obtained with RO which is mainlythe consequence of higher concentration factorscombined with ICP effects.

From the above studies, it appears that FO offersplenty of advantages over conventional processes forthe concentration of liquid foods. However, the lackof suitable membranes as well as the development ofan efficient reconcentration and recovery process forthe DS is the major limitation that prevents FO to beimplemented at a full-scale process in the foodindustry.

6. Criteria for the selection of suitable DS

Based on the past and current studies on FO pro-cess, it is clear that the selection of a suitable DSdepends on many criteria. In the following paragraph,useful information for the selection of suitable DS isprovided. The information provided is general to anyFO applications, so it is important to understand thatfor some specific applications, further criteria need tobe evaluated before an appropriate selection of theDS.

Before running any bench-scale experiments, aninitial screening of DS can be carried out. Thermody-namic modelling softwares can be handy in determin-ing some basic properties such as water solubility,pH, speciation and osmotic pressure, which areimportant criteria that can affect greatly the FO pro-cess performance as discussed earlier. Then, it isimportant to ensure that the DS is inert, of near neu-tral pH, stable and non-toxic, especially when FO isused for drinking water production as mentioned pre-viously. Another important factor that has to beassessed is that DS should not alter chemically orphysically the membrane through reaction, adsorp-tion, dissolution or fouling. Finally, to ensure theeconomic viability of the process, the DS should notbe expensive.

Once this preliminary screening has been carriedout, experiments can be conducted to assess DS per-formance in terms of water flux, reverse salt trans-port and water recovery, which are the three mainparameters used for assessing the performance ofFO process. Another important criterion in most FOapplications is the separation and recovery of theDS after it has been diluted. This process should beable to reconcentrate and recover the DS at low-costenergy, otherwise full-scale implementation will notbe financially viable compared to other pressure-dri-ven processes that are already commercialised.

Finally, the FO process should be tested at full-scale with the selected DS and life cycle assessment

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should be conducted in order to ensure that eachstage of the process (from the production of rawmaterials to the treatment of waste) has no or fewimpacts on the environment. A flow diagram that dis-plays the DS selection criteria, as described above, isillustrated in Fig. 8.

7. Concluding remarks

FO is a novel and emerging low energy technologyfor desalination. Its performance relies on the selectionof suitable membrane and DS. In fact, to furtherimprove the FO process performance, efforts have tobe made on the development of new membranes forreducing ICP effects as well as the development ofmethods for the selection of suitable DS. A wide rangeof DS that are used for different applications havebeen reviewed in detail and their limitations are iden-tified and discussed in this particular review. The fol-lowing conclusions may be drawn from this review:

As the main source of the driving force in FO pro-cess, the DS constitutes an integral part of the FOprocess and therefore careful selection of DS isessential as its properties can affect the process per-formance. Although any soluble compounds cangenerate osmotic pressure, the selection of suitableDS depends on the end-use application of the finalproduct water. Besides high solubility and highosmotic pressure, other properties of the DS such asMW, diffusion coefficient and DS viscosity canaffect the performance of FO process. The perfor-mance of each DS varies because they influenceconcentration polarisation effects to varying degree.

Inorganic- and organic-based DS are most com-monly used in the lab-scale FO studies, eitherbecause of their specific application or because theirthermodynamic properties are well understood.However, most candidates have issues related to theseparation of draw solutes from the water for regen-eration and further reuse. Separation and regenera-

DRAW SOLUTION

INITIAL SCREENING

Toxicity Osmotic pressure

Speciation

Membrane compatibility (pH, chemical or physical

reactions)

Specific cost

Physico-chemical properties

FO PERFORMANCE

Reverse salt diffusion

Water flux

Others (e.g. DS recovery system)

Water recovery

Thermodynamic modelling softwares

Others (e.g. plant nutrient requirement)

Based on experimental data

Life Cycle AssessmentFull -scale simulation

APPLICATION

Fig. 8. DS selection criteria flow diagram.

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tion of DS remain a contentious issue as additionalprocess and energy are required, and this process isalso essential especially for FO process for potablewater application. Few emerging DS such as ammo-nia-carbon dioxide, MNPs and hydrogels are consid-ered promising candidates for potable waterapplications and therefore worth further studies.

Because of the issues of energy with the DS separa-tion and regeneration process, FO applications findsuitable for non-potable applications. NaCl or sea-water has been widely used as DS for the wastewa-ter treatment and the diluted DS is eitherdischarged to the sea or, used as RO feed for pota-ble application. RO concentrate has been used asDS for similar applications and recently commercia-lised. Fertilisers are another promising DS candi-dates both for desalination and for treatingimpaired water by FO process where the dilutedDS can be used directly for fertigation.

This review also proposed and discussed the crite-ria for selecting a suitable DS candidate for specificapplication. There is no particular DS that can beconsidered as the most suitable candidate, and thesuitability depends on the types of applicationintended. The success of FO desalination for potablewater will depend on how easily the draw solutecan be separated from the freshwater. Therefore,FO technology still requires extensive research inthe future particularly related to the developmentof more suitable draw solutes. This also includeshow the influence of concentration polarisationeffects can be reduced through process optimisationand improved membrane design.

Acknowledgements

The authors acknowledge the financial support ofthe National Centre of Excellence in DesalinationAustralia (NCEDA), which is funded by the AustralianGovernment through the Water for the Futureinitiative.

References[1] F.A. Ward, M. Pulido-Velazquez, Water conservation in irri-

gation can increase water use, Proc. Natl. Acad. Sci. 105(47)(2008) 1821518220.

[2] J.R. McCutcheon, M. Elimelech, Influence of concentrative anddilutive internal concentration polarization on flux behavior inforward osmosis, J. Membr. Sci. 284(12) (2006) 237247.

[3] HTI Inc., About Humanitarian Water, 2011. Available from:http://www.htiwater.com/divisions/humanitarian/about.html.

[4] HTI Inc., Military Water & Desalination Filters: Case Study,2011. Available from: http://www.htiwater.com/divisions/military_regulatory/case_studies.html.

[5] N.A. Thompson, P.G. Nicoll, Forward osmosis desalination:A commercial reality, IDAWC, Perth, 2011.

[6] P.G. Nicoll, N.A. Thompson, M.R. Bedford, Manipulatedosmosis applied to evaporative cooling make-up waterRev-olutionary technology, IDAWC, Perth, 2011.

[7] J.C. Wright, R.M. Johnson, S.I. Yum, DUROS osmotic pharma-ceutical systems for parenteral & site-directed therapy, DrugDelivery Technol. 3(1) (2003). http://www.drug-dev.com/ME2/dirmod.asp?sid=&nm=&type=Publishing&mod=Publications%3A%3AArticle&mid=8F3A7027421841978F18BE895F87F791&tier=4&id=43487287552E4D019B3EEA3DF2E2D150.

[8] N.T. Hancock, T.Y. Cath, Solute coupled diffusion in osmoti-cally driven membrane processes, Environ. Sci. Technol. 43(17) (2009) 67696775.

[9] S. Lee et al., Comparison of fouling behavior in forwardosmosis (FO) and reverse osmosis (RO), J. Membr. Sci. 365(12) (2010) 3439.

[10] S. Loeb et al., Effect of porous support fabric on osmosisthrough a Loeb-Sourirajan type asymmetric membrane, J.Membr. Sci. 129(2) (1997) 243249.

[11] W.A. Phillip, J.S. Yong, M. Elimelech, Reverse draw solutepermeation in forward osmosis: Modeling and experiments,Environ. Sci. Technol. 44(13) (2010) 51705176.

[12] J.H. Vant Hoff, Die Rolle der osmotischen Druckes in derAnalogie zwischen Losungen und Gasen, Z. Phys. Chem. 1(1887) 481508.

[13] A. Yokozeki, Osmotic pressures studied using a simple equation-of-state and its applications, Appl. Energy 83(1) (2006) 1541.

[14] D. Stigter, T.L. Hill, Theory of the Donnan membrane equilib-rium. II. Calculation of the osmotic pressure and the salt dis-tribution in a Donnan system with highly-charged colloidparticles, J. Phys. Chem. 63 (1959) 551556.

[15] J.R. McCutcheon, R.L. McGinnis, M. Elimelech, A novelammoniacarbon dioxide forward (direct) osmosis desalina-tion process, Desalination 174(2005) (2005) 111.

[16] H.Y. Ng, W. Tang, Forward (direct) osmosis: A novel andprospective process for brine control, Water Environ. Found.(2006) 43454352.

[17] S. Phuntsho et al., A novel low energy fertilizer driven for-ward osmosis desalination for direct fertigation: Evaluatingthe performance of fertilizer draw solutions, J. Membr. Sci.375(2011) (2011) 172181.

[18] A. Achilli, T.Y. Cath, A.E. Childress, Selection of inorganic-based draw solutions for forward osmosis applications, J.Membr. Sci. 364(12) (2010) 233241.

[19] S.K. Yen et al., Study of draw solutes using 2-methylimidaz-ole-based compounds in forward osmosis, J. Membr. Sci. 364(12) (2010) 242252.

[20] K.Y. Wang, R.C. Ong, T.-S. Chung, Double-skinned forwardosmosis membranes for reducing internal concentrationpolarization within the porous sublayer, Ind. Eng. Chem. Res.49(10) (2010) 48244831.

[21] Y. Xu et al., Effect of draw solution concentration and operat-ing conditions on forward osmosis and pressure retardedosmosis performance in a spiral wound module, J. Membr.Sci. 348(12) (2010) 298309.

[22] Y.-J. Choi et al., Toward a combined system of forward osmo-sis and reverse osmosis for seawater desalination, Desalina-tion 247(13) (2009) 239246.

[23] A. Achilli et al., The forward osmosis membrane bioreactor:A low fouling alternative to MBR processes, Desalination 239(13) (2009) 1021.

[24] J.R. McCutcheon, R.L. McGinnis, M. Elimelech, Desalinationby ammoniacarbon dioxide forward osmosis: Influence ofdraw and feed solution concentrations on process perfor-mance, J. Membr. Sci. 278(2006) (2006) 114123.

[25] C.H. Tan, H.Y. Ng, A novel hybrid forward osmosisnanofil-tration (FO-NF) process for seawater desalination: Draw solu-tion selection and system configuration, Desalin. Water Treat.13(2010) (2010) 356361.

[26] E.M. Garcia-Castello, J.R. McCutcheon, M. Elimelech, Perfor-mance evaluation of sucrose concentration using forwardosmosis, J. Membr. Sci. 338(12) (2009) 6166.

L. Chekli et al. / Desalination and Water Treatment 43 (2012) 167184 183

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nloa

ded

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Ein

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en T

echn

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vers

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http://www.htiwater.com/divisions/humanitarian/about.htmlhttp://www.htiwater.com/divisions/military_regulatory/case_studies.htmlhttp://www.htiwater.com/divisions/military_regulatory/case_studies.htmlhttp://www.drug-dev.com/ME2/dirmod.asp?sid=&nm=&type=Publishing&mod=Publications%3A%3AArticle&mid=8F3A7027421841978F18BE895F87F791&tier=4&id=43487287552E4D019B3EEA3DF2E2D150http://www.drug-dev.com/ME2/dirmod.asp?sid=&nm=&type=Publishing&mod=Publications%3A%3AArticle&mid=8F3A7027421841978F18BE895F87F791&tier=4&id=43487287552E4D019B3EEA3DF2E2D150http://www.drug-dev.com/ME2/dirmod.asp?sid=&nm=&type=Publishing&mod=Publications%3A%3AArticle&mid=8F3A7027421841978F18BE895F87F791&tier=4&id=43487287552E4D019B3EEA3DF2E2D150http://www.drug-dev.com/ME2/dirmod.asp?sid=&nm=&type=Publishing&mod=Publications%3A%3AArticle&mid=8F3A7027421841978F18BE895F87F791&tier=4&id=43487287552E4D019B3EEA3DF2E2D150

[27] K.B. Petrotos, P. Quantick, H. Petropakis, A study of thedirect osmotic concentration of tomato juice in tubular mem-branemodule configuration. I. The effect of certain basicprocess parameters on the process performance, J. Membr.Sci. 150(1) (1998) 99110.

[28] S. Zhao, L. Zou, Relating solution physicochemical propertiesto internal concentration polarization in forward osmosis, J.Membr. Sci. 379(12) (2011) 459467.

[29] Martinetti, C.R., A.E. Childress, and T.Y. Cath, High recoveryof concentrated RO brines using forward osmosis and mem-brane distillation. J. Membr. Sci. 331(12) (2009) 3139.

[30] R.W. Holloway, Forward Osmosis for Concentration ofAnaerobic Digester Centrate, University of Nevada, Reno,NV,. p. 90 2006.

[31] R.J. York, R.S. Thiel, E.G. Beaudry, Full-scale experience ofdirect osmosis concentration applied to leachate management,in: Seventh International Waste Management and Landfill,Symposium, 1999.

[32] S. Zou et al., The role of physical and chemical parameterson forward osmosis membrane fouling during algae separa-tion, J. Membr. Sci. 366(12) (2011) 356362.

[33] Q. Yang, K.Y. Wang, T.-S. Chung, A novel dual-layer forwardosmosis membrane for protein enrichment and concentration,Sep. Purif. Technol. 69(3) (2009) 269274.

[34] M. Wallace, Z. Cui, N.P. Hankins, A thermodynamic bench-mark for assessing an emergency drinking water device basedon forward osmosis, Desalination 227(13) (2008) 3445.

[35] R.E. Kravath, J.A. Davis, Desalination of seawater by directosmosis, Desalination 16(1975) (1975) 151155.

[36] J.O. Kessler, C.D. Moody, Drinking water from sea water byforward osmosis, Desalination 18(3) (1976) 297306.

[37] K. Stache, Apparatus for transforming sea water, brackishwater, polluted water or the like into a nutritious drink bymeans of osmosis, US Patent 4, 030, Editor. 1989.

[38] R.E. Wrolstad et al., Composition and sensory characteriza-tion of red raspberry juice concentrated by direct-osmosis orevaporation, J. Food Sci. 58 (1993) 633637.

[39] E.G. Beaudry, K.A. Lampi, Membrane technology for directosmosis concentration of fruit juices, Food Technol. 44 (1990)121.

[40] J.R. Herron et al., Osmotic concentration apparatus andmethod for direct osmosis concentration of fruit juices, USPatent 5 (1994) 430.

[41] P. McCormick et al., Water, salt, and ethanol diffusionthrough membranes for water recovery by forward (direct)osmosis processes, J. Membr. Sci. 325(1) (2008) 467478.

[42] Adham, S., et al., Dewatering Reverse Osmosis concentratefrom water reuse applications using Forward Osmosis,WaterReuse Foundation, 2009.

[43] M.M. Ling, T.-S. Chung, Novel dual-stage FO system for sus-tainable protein enrichment using nanoparticles as intermedi-ate draw solutes, J. Membr. Sci. 372(12) (2011) 201209.

[44] M.M. Ling, T.-S. Chung, Desalination process using superhydrophilic nanoparticles via forward osmosis integrated withultrafiltration regeneration, Desalination 278 (2011) 194202.

[45] M.M. Ling, K.Y. Wang, T.-S. Chung, Highly water-solublemagnetic nanoparticles as novel draw solutes in forwardosmosis for water reuse, Ind. Eng. Chem. Res. (2010),10.1021/ie100438x.

[46] H.Y. Ng et al., Treatment of RO brine-towards sustainablewater reclamation practice, Water Sci. Technol. 58 (2008)931936.

[47] O.A. Bamaga et al., Hybrid FO/RO desalination system: Pre-liminary assessment of osmotic energy recovery and designsof new FO membrane module configurations, Desalination268 (2011) 163169.

[48] D. Li et al., Stimuli-responsive polymer hydrogels as a newclass of draw agent for forward osmosis desalination, Chem.Commun. 47 (2011) 17101712.

[49] D. Li et al., Composite polymer hydrogels as draw agents inforward osmosis and solar dewatering, Soft Matter 7(21)(2011) 1004810056.

[50] G.A.F. Gadelha, N.P. Hankins, Assessment of draw-solu-tion performance in combination with asymmetric mem-branes for forward osmosis, IDAWC, Perth, 2011.

[51] G. Batchelder, Process for the demineralization of water. USPatent 317179903, 1965.

[52] L.A. Hoover et al., Forward with osmosis: Emerging applica-tions for greater sustainability, Environ. Sci. Technol. 45(23)(2011) 98249830.

[53] T.Y. Cath et al., A multi-barrier osmotic dilution process forsimultaneous desalination and purification of impaired water,J. Membr. Sci. 362(12) (2010) 417426.

[54] J.-J. Qin et al., Development of novel backwash cleaning tech-nique for reverse osmosis in reclamation of secondary efflu-ent, J. Membr. Sci. 346(1) (2010) 814.

[55] G. Ramon, Y. Agnon, C. Dosoretz, Dynamics of an osmoticbackwash cycle, J. Membr. Sci. 364(12) (2010) 157166.

[56] Algae systems, Offshore Membrane Enclosure for GrowingAlgae (OMEGA), 2011. Available from: http://algaesystems.com/technology/omega/.

[57] J.D. Trent, et al., Algae bioreactor using submerged enclo-sures with semi-permeable membranes, Patent 0216203, 2010.

[58] D.N. Glew, Process for liquid recovery and solution concen-tration, US Patent 9 (1965) 930.

[59] B.S. Frank, Desalination of seawater, US Patent 367089720,1972.

[60] J. Yaeli, Method and apparatus for processing liquid solutionsof suspensions particularly useful in the desalination of salinewater, US Patent 509857524, 1992.

[61] McGinnis, R., Osmotic desalination process. 2002: US patent7560,029. 1 Feb. 2002.

[62] World Health Organization, WHO guidelines for drinking-water quality, 2011. Available from: http://www.who.int/water_sanitation_health/dwq/guidelines/en/.

[63] S. Phuntsho et al., Fertiliser drawn forward osmosis desalina-tion: The concept, performance and limitations for fertigation,Rev. Environ. Sci. Biotechnol. (2011), 10.1007/s11157011-92592.

[64] W.C.L. Lay, et al., Study of integration of forward osmosisand biological process: Membrane performance under ele-vated salt environment. Desalination 283(1) (2011) 123130.

[65] D. Xiao et al., Modeling salt accumulation in osmotic mem-brane bioreactors: Implications for FO membrane selectionand system operation, J. Membr. Sci. 366(12) (2011) 314324.

[66] R.E. Pattle, Production of electric power by mixing fresh andsalt water in the hydroelectric pile, Nature 174(1954) (1954)660.

[67] R.J. Aaberg, Osmotic powera new and powerful renewableenergy source, ReFocus 4 (2003) 4850.

[68] G.L. Wick, Energy from salinity gradients, Energy 3 (1978)95100.

[69] S. Loeb, Production of energy from concentrated brines bypressure-retarded osmosis: I. Preliminary technical and eco-nomic correlations, J. Membr. Sci. 1 (1976) 4963.

[70] S. Loeb, F. Van Hessen, D. Shahaf, Production of energy fromconcentrated brines by pressure-retarded osmosis: II. Experi-mental results and projected energy costs, J. Membr. Sci. 1(1976) 249269.

[71] A. Achilli, T.Y. Cath, A.E. Childress, Power generation withpressure retarded osmosis: An experimental and theoreticalinvestigation, J. Membr. Sci. 343(12) (2009) 4252.

[72] Q. Ge et al., Hydrophilic superparamagnetic nanoparticles:Synthesis, characterization, and performance in forwardosmosis processes, Ind. Eng. Chem. Res. 50(1) (2011) 382388.

[73] K. Popper et al., Dyalizer concentrates beverages, Food Eng.38 (1966) 102104.

184 L. Chekli et al. / Desalination and Water Treatment 43 (2012) 167184

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