Fouling distribution in forward osmosis membrane process

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1 Journl of Environmentl Sciences 26 ( Aville online t Journl of Environmentl Sciences Fouling distriution in forwrd osmosis memrne process Junseok Lee, Bongchul Kim, Seungkwn Hong School of Civil, Environmentl & Architecturl Engineering, Kore University, Seoul , Kore rticle info Article history: Specil issue: Sustinle wter mngement for green infrstructure Keywords: forwrd osmosis memrne module length orgnic fouling fouling reversiility counter-current flow FO opertion DOI: 1.116/S11-742( strct Fouling ehvior long the length of memrne module ws systemticlly investigted y performing simple modeling nd l-scle experiments of forwrd osmosis (FO memrne process. The flux distriution model developed in this study showed good greement with experimentl results, vlidting the roustness of the model. This model demonstrted, s expected, tht the permete flux decresed long the memrne chnnel due to decresing osmotic pressure differentil cross the FO memrne. A series of fouling experiments were conducted under the drw nd feed solutions t vrious recoveries simulted y the model. The simulted fouling experiments reveled tht higher orgnic (lginte fouling nd thus more flux decline were oserved t the lst section of memrne chnnel, s foulnts in feed solution ecme more concentrted. Furthermore, the wter flux in FO process declined more severely s the recovery incresed due to more foulnts trnsported to memrne surfce with elevted solute concentrtions t higher recovery, which creted fvorle solution environments for orgnic dsorption. The fouling reversiility lso decresed t the lst section of the memrne chnnel, suggesting tht fouling distriution on FO memrne long the module should e crefully exmined to improve overll clening efficiency. Lstly, it ws found tht such fouling distriution oserved with co-current flow opertion ecme less pronounced in countercurrent flow opertion of FO memrne process. Introduction Due to depletion of fresh wter resources, the development of new nd lterntive wter sources, such s sewter deslintion nd wstewter reuse, hs een in gret demnd. Advnced memrne technologies represent one of the most fesile options to meet this criticl chllenge (Boo et l., 212. In recent yers, vrious memrne processes hve een explored nd developed. Such exmples include sewter deslintion y reverse osmosis (RO nd nnofiltrtion (Elimelech nd Phillip, 211; Lee nd Lee, 27; Hong nd Elimelech, 1997, pretretment in sewter deslintion y microfiltrtion nd ultrfiltrtion (Brehnt et l., 22, nd wstewter tretment y memrne iorectors (Meng et l., 29. Forwrd osmosis (FO using osmotic pressure grdient s driving Corresponding uthor. E-mil: skhong21@kore.c.kr force hs gined significnt interest for wide rnge of potentil pplictions which include sewter deslintion, wstewter reclmtion, nd industril wstewter tretment such s shle gs produced wstewter (Kim et l., 212; Boo et l., 213; Hickenottom et l., 213. In FO process, permete wter trnsports from feed solution to drw solution. Therefore, drw solution is diluted nd feed solution is concentrted grdully long the memrne chnnel. Consequently, the difference in osmotic pressure cross the FO memrne decreses, leding to permete flux decline long the memrne chnnel. Such vritions re quite different from typicl RO process in which concentrtion of feed solution is only oserved long the memrne chnnel. Furthermore, unlike highly pressurized RO systems, FO memrne opertes without externl pressure. Thus, it is expected tht the distriution of memrne fouling long the module length could e lso different in FO process, compred to tht of RO

2 Journl of Environmentl Sciences 26 ( process. To dte, no work hs een done to investigte the effect of feed concentrtion nd drw dilution on memrne fouling ehvior long the memrne chnnel length in FO process. In this study, we investigted the distriution of wter flux nd fouling long the module length y conducting theoreticl modeling nd l-scle experiments of FO process. The experimentl wter flux with different chnnel lengths ws first compred with the modeling result to vlidte the suitility of modeling process. Then, fouling long the memrne chnnel length nd susequent fouling reversiility were evluted y systemtic fouling experiments simulted y modeling. Lstly, the effect of flow configurtions on fouling distriution ws ssessed y employing different crossflow directions. Bsed on the results otined, the effects of feed concentrtion nd drw dilution on fouling distriution of FO memrne process were delineted, nd their implictions to scling up FO process nd clening efficiency in rel-scle plnts were discussed. NCl ws used s drw solution to produce the osmotic pressure. All experiments were crried out t ph of 6 7 nd the tempertures of oth solutions were mintined t 2 ± 1. C. To investigte fouling distriution long the memrne chnnel in FO process, fouling tests were conducted with different chnnel lengths. The sodium lginte (Sigm- Aldrich, St. Louis, MO, USA ws used s model orgnic foulnt. Other properties of the lginte used were given in erlier studies (Lee nd Elimelech, 26. The feed wter chemistry ws fixed t totl ionic strength of 5 mmol/l (3 mmol/l CCl 2 nd 41 mmol/l NCl. The cross-flow velocity of oth solutions ws fixed t 8.5 cm/sec nd the ph of oth solutions is 6 7. All fouling experiments performed in this study were operted in FO mode tht the memrne ctive lyer is ginst the feed solution nd the support lyer is fcing the drw solution. Prior to fouling tests, the initil flux ws djusted to the sme vlue for the sme memrne chnnel length to compre fouling ehvior. Then, foulnt ws injected to feed solution nd fouling experiment ws performed for 17 hr. 1 Experimentl method 1.1 FO system A cross-flow FO system with severl memrne cells ws used to investigte the effect of module length on FO memrne performnce. We first confirmed tht ech memrne cell showed sme performnce to eliminte ny effects originted from the difference in memrne cells employed in the following experiments. The FO cell in this system hs two chnnels on oth sides of memrne (i.e., feed nd drw solutions with dimensions of 77 mm long, 26 mm wide, nd 3 mm deep. The FO memrne ws provided y Hydrtion Technology Innovtions (Alny, OR, USA with pure wter permeility coefficient (A of m/(sec tm, nd the solute permeility coefficient (B of m/sec. Vrile speed ger pumps (Cole-Prmer, Vernon Hills, IL, USA were used to flow the solutions in the chnnels nd to chnge the cross-flow velocity. A temperture control ws used to mintin temperture of oth feed nd drw solutions t (2 ± 1. C. The drw solution reservoir ws put on the digitl lnce, nd the chnge of weight ws mesured y computer to determine permete wter flux. 1.2 Wter flux nd fouling distriution experiments with different module lengths Wter flux experiments for vlidting the model were performed with different chnnel lengths (7.7, 15.4, nd 23.1 cm. To counterct the effects of dilution of drw solution nd concentrtion of feed solution, tnk volume of oth solutions ws djusted for different chnnel lengths. The deionized wter ws used s feed solution nd 5 mol/l 2 Modeling 2.1 Wter flux cross FO memrne Theoreticl work for wter flux nd revere solute flux in FO process ws developed nd descried in the previous pulictions (McCutcheon nd Elimelech, 26; Phillip et l., 21; Lee et l., The equtions from these studies were dopted nd modified to simulte the wter flux cross the FO memrne long the chnnel. All simultions performed in this study were operted in FO mode. A schemtic digrm of FO memrne operting in FO mode is shown in Fig. 1. McCutcheon nd Elimelech (26 descried the following equtions, which reflect the effects of internl concentrtion polriztion (ICP nd externl concentrtion polriztion (ECP on wter flux in FO mode. The eqution considering ICP effects in support lyer ws descried: ( ( ( ( 1 B + Aπ C4 1 B + AπC4 K ICP = ln = ln J W B + J W + Aπ C2 J W B + Aπ C3 (1 K ICP = tτ (2 εd where, J W (m/sec is the wter flux, C1 nd C5 re ulk feed nd drw concentrtions, C2 nd C3 re feed nd drw concentrtions t ctive lyer, C4 is drw concentrtion t support lyer, π (tm is osmotic pressure, K (sec/m is the solute resistivity within the porous support lyer, nd t (m, τ, ε nd D (m 2 /sec re the thickness, tortuosity, porosity of the support lyer, nd diffusion coefficient of the solute, respectively. The eqution considering ECP in

3 135 Journl of Environmentl Sciences 26 ( Active lyer Support lyer C 5 = C drw C 4 C 1 = C feed C 2 J W C 3 J S Feed solution Memrne Drw solution π F,1 (C F,1 π F,3 (C F,3 π F,2 (C F,2 π D,2 π D,3 Δ π D,n (C D,2 (C D,3 (C D,n J W1 J W2 J W3 J Wn π D,1 (C D,1 Fig. 1 Theoreticl description of FO memrne process: ( osmotic driving force profile for FO mode considering oth ICP nd ECP, nd ( wter flux distriution in FO module. π F,n (C F,n feed solution chnnel ws descried: π C2 π C1 = C 2 C 1 = exp ( JW k In this eqution, we ssumed tht the rtio of osmotic pressure is pproximtely equl to the rtio of memrne surfce concentrtion to ulk solution concentrtion (Mc- Cutcheon nd Elimelech, 26. k = ShD (4 d h where, k (m/sec is the mss trnsfer coefficient, Sh is the Sherwood numer, nd d h (m is the hydrulic dimeter. A correltion of Sherwood numer is determined y the shpe of memrne chnnel flow such s rectngulr nd tuulr chnnel. The osmotic pressure used in this study ws clculted y OLI nlyzer (OLI Systems Inc., Morris Plins, NJ, USA. The eqution for clirtion curve etween osmotic pressure nd concentrtion is shown elow. π = (3.855 C C (for 2 C (5 The sic eqution of wter flux cross the memrne ctive lyer in FO process is descried: (3 J W,1 = A(π C3 π C2 (6 2.2 Modeling of wter flux distriution long the memrne chnnel To investigte the wter flux distriution, the FO module ws sectioned t regulr intervls (ΔX nd the wter flux ws clculted t ech section considering the chnge of wter flow nd concentrtions of oth feed nd drw solutions. In this study, we set this intervl to.1 m. A schemtic digrm of wter flux distriution in FO module is presented in Fig. 1. FO operting model prmeters re evluted to determine wter flux distriution in FO system. Firstly, the flow rte of oth solutions is chnged s wter permetes through FO memrne from feed to drw solution. The flow rte of feed solution decreses due to the loss of pure wter nd the flow rte of drw solution increses due to the permetion of pure wter. The equtions of the flow rte re descried elow: Q D,n = Q D,n 1 + J W,n 1 W ΔX (7 Q F,n = Q F,n 1 J W,n 1 W ΔX (8 where, Q (m 3 /sec is the flow rte, D is drw side, F is feed side, n is numer of imginry module, nd W (m is width of FO module. Also, the drw solution is diluted nd the feed solution is concentrted y wter permetion nd reverse solute permetion. The equtions of concentrtion were descried s follows: ( Q D,n 1 C D,n 1 B W ΔX exp J W,n 1 k Q D,n 1 C D,n = (9 Q D,n ( Q F,n 1 C F,n 1 + B W ΔX exp JW,n 1 k Q F,n 1 C F,n = (1 Q F,n The wter flux in FO module ws djusted y the chnge of solution concentrtions. The eqution of wter flux t point n ws descried in Eq. (11: J W,n = A(π D,n π F,n (11 The wter flux ws then clculted using solver. To simulte the permete wter flux long the module chnnel nd thus length, the difference of J W in oth Eqs. (1 nd (6 ws djusted to zero t ech section of chnnel length.

4 Journl of Environmentl Sciences 26 ( Wter flux (L/(m 2.hr Experimentl wter flux Modeling flux Reverse solute flux Chnnel length (cm Fig. 2 Experimentl nd modelling wter fluxes, nd reverse solute flux s function of memrne chnnel length. 3 Results nd discussion 3.1 Experimentl vlidtion of FO wter flux modeling To verify the simultion results long the memrne chnnel, series of wter flux experiments were performed with different chnnel lengths. In FO process, the pure wter trnsports from feed to drw solution y osmotic pressure differentil nd the drw solutes diffuse reversely from drw to feed solution, resulting in dilution of drw solution nd concentrtion of feed solution. The flux dt with respect to chnnel length otined during FO tests is depicted in Fig. 2. The reverse slt diffusion ws not vried significntly, within experimentl conditions. The experimentl results lso showed tht modeling flux ws reltively well fitted to the experimentl dt, vlidting the suitility of the FO memrne model developed in this study. Thus, the concentrtions of oth solutions long the memrne chnnel were predicted y this model nd used for the following fouling experiments. The simultion ws further performed to investigte the vritions of FO performnce prmeters including flux, Reverse solute flux (mol/(m 2.hr recovery, drw nd feed concentrtions in FO system. The results re presented in Fig. 3, s function of FO memrne module length. The concentrtions of oth feed nd drw solutions were first clculted using Eqs. (9 nd (1. As shown in Fig. 3, the drw solution ws diluted nd the feed solution ws concentrted grdully long the memrne length y wter permetion nd prtly y reverse solute permetion. Thus, the osmotic pressure differentil cross the memrne decresed with incresing memrne chnnel length. Consequently, s shown in Fig. 3, permete wter flux decresed long the memrne chnnel. The overll recovery of FO system incresed with incresing module length. The simulted concentrtions of oth feed nd drw solutions were used in fouling experiments to evlute the extent of fouling nd reversiility long the memrne module length. 3.2 Memrne fouling distriution Fouling experiments were performed with different chnnel lengths to study fouling distriution long the FO memrne chnnel. The initil flux ws djusted to e identicl in ll fouling experiments to precisely compre fouling ehvior. In these experiments, 5. mol/l NCl ws used s drw solution, nd 1 mg/l sodium lginte ws dded s orgnic foulnts to feed solution. As presented in Fig. 4, more flux decline ws oserved with incresing memrne chnnel length in FO process. The clen wter trnsported from feed to drw solution, nd the drw solutes reversely diffused to feed solution. Consequentilly the concentrtion of feed solution incresed long the memrne chnnel, which creted solution environments fvorle for incresing orgnic fouling. Therefore, the flux reduction rte incresed long the memrne chnnel, implying tht, in rel-scle FO system, fouling could e more severe in the lst elements nd/or stge. To compre the extent of fouling etween the first nd lst sections of rel-scle FO system, we dopted the concentrtions of drw nd feed solutions clculted Drw concentrtion (mol/l Drw concentrtion Feed concentrtion Module length (m Feed concentrtion (mol/l Wter flux (μm/sec Wter flux Recovery Module length (m Fig. 3 Model simultion results: ( concentrtion of drw nd feed solutions long the memrne chnnel length nd ( wter flux nd recovery in FO memrne system Recovery (%

5 1352 Journl of Environmentl Sciences 26 ( cm 15.4 cm 23.1 cm Averge permete volume (ml Fig. 4 Normlized flux decline curves for FO fouling experiments with different memrne chnnel lengths. y modeling considering overll process recovery. The concentrtions of drw nd feed solutions in the first nd lst sections of imginry memrne chnnels designed to chieve vrious FO process recoveries re summrized in Tle 1. It ws ssumed tht feed solution in the first section of this chnnel contined 1 mg/l of lginte, with ckground eletrolytes of 41 mmol/l NCl nd 3 mmol/l CCl 2. The drw solution ws 5. mol/l NCl. As the recovery incresed, feed solution ecme more Tle 1 Concentrtions of drw nd feed solutions clculted y modeling in the first nd lst sections of n imginry memrne chnnel t vrious recoveries Recovery Drw solution Feed solution (% (mol/l Alginte NCl CCl 2 (mg/l (mmol/l (mmol/l Lst section concentrted, resulting higher foulnt (lginte concentrtion nd elevted ckground solute (NCl nd CCl 2 concentrtions in the lst section of the memrne chnnel. For the simplicity of model simultion, the reduction of foulnt concentrtion due to foulnt deposition on FO memrne ws not considered y ssuming tht fouling is reversile nd controlled y simple physicl flushing during opertion (Lee et l., 21; Boo et l., 212, 213 The normlized fluxes of the first nd lst sections of the memrne chnnel re presented in Fig. 5. The flux decline of the lst module section ws more severe thn tht of the first module section. The concentrtion of orgnic foulnts in the feed solution incresed nd consequentilly cused more dsorption, leding to thick fouling lyer formtion on the memrne surfce. The elevted solute concentrtion (e.g., NCl nd CCl 2 t the lst section lso enhnced orgnic dsorption to FO memrne surfce, cusing more compct fouling lyer. The flux reduction rte ws further studied nd compred t vrious recoveries. The normlized flux of the first nd end sections fter fouling t ech recovery is presented in Fig. 5. The flux declined more severely s the recovery incresed due to more foulnts trnsported to memrne surfce with elevted solute concentrtions t higher recovery. It hs een known tht, in RO process, fouling t the first module of pressure vessel is more pronounced primrily due to higher wter production t the first module, nd thus the first module is more frequently clened nd replced in norml prctice (Voutchkov, 21. Similr phenomenon could e oserved in rel scle FO opertion. However, it should e lso noted tht, current FO memrne opertes typiclly t lower flux thn RO process. Furthermore, FO system is not pressurized nd thus fouling is often reversile nd esily controlled, compred to fouling in highly pressurized RO systems. Thus, it my e expected tht the first module of rel-scle FO memrne systems might not e hevily fouled s RO process, nd consequently the extent of fouling in FO process could e simply Lst section Lst section Permete volume (ml Recovery (% Fig. 5 Effect of recovery on fouling ehviour: ( normlized flux otined during the FO fouling runs t 52.8% of recovery, nd ( normlized flux for FO fouling experiments with the different recoveries t the first nd lst sections of memrne chnnel.

6 Journl of Environmentl Sciences 26 ( relted to the concentrtion of foulnts nd solutes in the feed wter. 3.3 Fouling reversiility in FO system The clening experiments were performed immeditely following the fouling experiments to investigte the reversiility of memrne fouling in the first nd lst sections of FO memrne module chnnel t recovery of 52.8%, s presented in Fig. 6. The clening experiments were conducted using deionized wter s clening solution, with flushing t cross-flow velocity of 34.2 cm/sec for 1 hr. The permete wter flux of clened memrnes ws mesured fter the clening experiment to evlute the clening efficiency. The conditions employed to determine the wter flux of clened memrnes were identicl to those used to mesure the initil wter flux. The resulting fouling reversiility of the first nd lst sections of the memrne chnnel is presented in Fig. 6. With incresing cross-flow velocity, the flux t the first section recovered significntly up to 89.6%, while only reltively smller flux ws recovered (i.e., 69.1% t the lst section. This result reveled tht high orgnic nd solute concentrtions t the lst module resulted in more compct nd thicker fouling lyer which ws not effectively removed y physicl clening. Thus, the clening strtegy for lrge-scle FO opertion requires more creful considertion of fouling distriution under vrious operting conditions, including overll process recovery, consequent drw dilution nd feed concentrtion. 3.4 Effect of cross-flow directions To investigte the effect of cross-flow directions on flux nd fouling ehvior, the experiments were performed t counter cross-flow direction, in which feed nd drw solutions flowed oppositely. In counter-current flow opertion, the effects of drw dilution nd feed concentrtion on flux ehvior were less severe thn co-current flow opertion. Therefore, s shown in Fig. 7, the permete flux of counter-current flow opertion ws higher thn tht of co-current flow opertion. Similrly, FO memrne performnce ws investigted in previous studies, vi numericl simultion ccording to the flow direction of feed nd drw solutions, nd it ws found tht higher permete flux ws held y counter-current flow thn co Flux fter fouling Flux fter clening.9.7 Lst section Clening Permete volume (ml 25 3 Lst section Fig. 6 Fouling reversiility: ( normlized flux decline curve nd recovered flux for FO fouling nd clening experiments (recovery 52.8%, nd ( reversiility of lginte-fouled FO memrnes t the first nd lst sections of memrne chnnel. Averge flux (μm/sec Co-current Co-current: Couter-current: Counter-current cm 7.7 cm 15.4 cm 15.4 cm 23.1 cm 23.1 cm Chnnel length (cm Permete volume (ml Fig. 7 Effect of cross-flow directions (i.e., counter-current nd co-current FO opertion. ( flux ehvior with the different memrne chnnel lengths nd ( normlized flux decline curves for FO fouling experiments with the different cross-flow directions.

7 1354 Journl of Environmentl Sciences 26 ( current flow opertion (Xio et l., 212; Jung et l., 211. The normlized flux dt otined during orgnic fouling experiments t the co-current nd counter-current flow opertions is shown in Fig. 7. Although the effect of cross-flow directions on fouling ehvior ws not noticele, flux decline ws slightly less in countercurrent opertion s expected from less feed concentrtion nd drw dilution, prticulrly t smller chnnel lengths. Counter-current flow opertion is more fvorle in lrge scle FO pplictions, nd thus further reserch on fouling distriution is needed for developing n effective fouling control method for rel-scle FO plnt. 4 Conclusions This study simulted fouling distriution in FO memrne process through simple modeling nd corresponding lscle experiments. In FO memrne process, permete wter flux decresed with incresing memrne chnnel length due to the decrese in osmotic differentil cused y feed concentrtion nd drw dilution. The concentrtions of feed nd drw solutions predicted y wter flux distriution model were employed for fouling experiments t vrious operting conditions. More severe fouling ws oserved t the lst section, compred to the first section of the memrne chnnel. Higher foulnt concentrtion nd elevted solute concentrtions t the lst section resulted in the formtion of thicker nd denser fouling lyer, nd reduced the efficiency of physicl clening in the lst module section. The fouling distriution oserved with co-current flow opertion ecme less pronounced in counter-current flow opertion. It should e cutioned tht the findings from this study, however, could hve very limited implictions ecuse of simplified ssumptions employed in the model development nd simulted experimentl conditions. Nonetheless, it is importnt to understnd tht n effective fouling control strtegy should e developed on the sis of fundmentl understnding of fouling distriution in lrge-scle FO memrne process. Acknowledgments This reserch ws supported y the World Clss University Progrm (Cse III through the Ntionl Reserch Foundtion of Kore nd funded y the Ministry of Eduction, Science nd Technology (R nd prtly y grnt from the Fundmentl R&D Progrm for Technology of World Premier Mterils funded y the Ministry of Knowledge Economy, Kore. references Boo, C., Lee, S., Elimelech, M., Meng, Z., Hong, S., 212. Colloidl fouling in forwrd osmosis: Role of reverse slt diffusion. J. Memr. Sci , Boo, C., Elimelech, M., Hong, S., 213. Fouling control in forwrd osmosis process integrting sewter deslintion nd wstewter reclmtion. J. Memr. Sci. 444, Brehnt, A., Bonnelye, V., Perez, M., 22. Comprison of MF/UF pretretment with conventionl filtrtion prior to RO memrnes for surfce sewter deslintion. Deslintion 144(1-3, Elimelech, M., Phillip, W.A., 211. The future of sewter deslintion: energy, technology, nd the environment. Science 333(643, Hickenottom, K.L., Hncock, N.T., Hutchings, N.R., Appleton, E.W., Beudry, E.G., Xu, P., et l., 213. Forwrd osmosis tretment of drilling mud nd frcturing wstewter from oil nd gs opertions. Deslintion 312, Hong, S., Elimelech, M., Chemicl nd physicl spects of nturl orgnic mtter (NOM fouling of nnofiltrtion memrnes. J. Memr. Sci. 132(2, Jung, D.H., Lee, J., Kim, D.Y., Lee, Y.G., Prk, M., Lee, S. et l., 211. Simultion of forwrd osmosis memrne process: Effect of memrne orienttion nd flow direction of feed nd drw solutions. Deslintion 277(1-3, Kim, C., Lee, S., Shon, H., Elimelech, M., Hong, S., 212. Boron trnsport in forwrd osmosis: Mesurements, mechnisms, nd comprison with reverse osmosis. J. Memr. Sci , Lee, K.L., Bker, R.W., Lonsdle, H.K., Memrnes for power genertion y pressure-retrded osmosis. J. Memr. Sci. 8(2, Lee, S., Elimelech, M., 26. Relting orgnic fouling of reverse osmosis memrnes to intermoleculr dhesion forces. Environ. Sci. Technol. 4(3, Lee, S., Lee, C., 27. Effect of memrne properties nd pretretment on flux nd NOM rejection in surfce wter nnofiltrtion. Sep. Purif. Technol. 56(1, 1 8. Lee, S., Boo, C., Elimelech, M., Hong, S., 21. Comprison of fouling ehvior in forwrd osmosis (FO nd reverse osmosis (RO. J. Memr. Sci. 365(1-2, McCutcheon, J.R., Elimelech, M., 26. Influence of concentrtive nd dilutive internl concentrtion polriztion on flux ehviour in forwrd osmosis. J. Memr. Sci. 284(1-2, Meng, F., Che, S.R., Drews, A., Krume, M., Shin, H.S., Yng, F., 29. Recent dvnces in memrne iorectors (MBRs: Memrne fouling nd memrne mteril. Wter Res. 43(6, Phillip, W.A., Yong, J.S., Elimelech, M., 21. Reverse drw solute permetion in forwrd osmosis: Modeling nd experiments. Environ. Sci. Technol. 44(13, Voutchkov, N., 21. Sewter Pretretment, Wter Tretment Acdemy Xio, D., Li, W., Chou, S., Wng, R., Tng, C.Y., 212. A modeling investigtion on optimizing the design of forwrd osmosis hollow fier modules. J. Memr. Sci ,