Effects of mass retention of dissolved organic matter and membrane pore size on membrane fouling and flux decline

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1 ARTICLE IN PRESS wter reserch xxx (8) 1 Aville t journl homepge: Effects of mss retention of dissolved orgnic mtter nd memrne pore size on memrne fouling nd flux decline Cheng-Fng Lin*, Angel Yu-Chen Lin, Pnchngm Sri Chndn, Cho-Yun Tsi Grdute Institute of Environmentl Engineering, Ntionl Tiwn University, 71 Chou-Shn Rd., Tipei 1, Tiwn rticle info Article history: Received 17 My 8 Received in revised form August 8 Accepted 1 Octoer 8 Pulished online - Keywords: Ultrfiltrtion Dissolved orgnic mtter (DOM) Memrne fouling Memrne resistnce DOM retention strct Ultrfiltrtion (UF) fouling hs een ttriuted to concentrtion polriztion, gel lyer formtion s well s outer nd inner memrne pore clogging. It is elieved tht mss of humic mterils either retined on memrne surfce or ssocited with memrne inner pore surfce is the primry cuse for permete flux decline nd filtrtion resistnce uildup in wter supply industries. While iofilm/iofouling nd inorgnic mtter could lso e contriuting fctors for permeility decline in wstewter tretment prctices. The present study reltes UF fouling to mss of dissolved orgnic mtter (DOM) retined on memrne nd quntifies the effect of retined DOM mss on filtrtion flux decline. The results demonstrte tht lrger pore memrnes exhiit significnt flux decline in comprison with the smller ones. During -h period, dissolved orgnic cron mss retined in 1 kd memrnes ws out 1. g m nd tht in 1 kd memrnes ws more thn 3 times higher (3. g m ). The ccumultion of retined DOM mss significntly ffects permete flux. It is highly likely tht some DOMs ind or ggregte together to form surfce gel lyer in the smller 1 kd UF system; those DOMs lrgely present in inner pore nd serving s pore lockge on loose memrne (1 kd) re responsile for severe flux decline. ª 8 Elsevier Ltd. All rights reserved. 1. Introduction Memrne ppliction for wter purifiction hs gined much populrity in recent yers. Wter supply industries hve successfully used nnofiltrtion (NF) or ultrfiltrtion (UF) technologies to remove nturl orgnic mtter (NOM) nd simultneously control disinfection yproduct precursors. The criticl prolem encountered in NF or UF of nturl wter contining dissolved orgnic mtter (DOM) is memrne fouling which cuses permete flux decline nd thus shortens the UF opertion life, increses the mintennce cost nd eventully dds dditionl cpitl cost for memrne replcement (Mrtens et l., 1998). Pulictions on memrne fouling re quite undnt (e.g., Aoustin et l., 1; Howe nd Clrk, ; Zhng et l., 3; Kimur et l., ; Kweon nd Lwler, ; Lee et l., ). In generl, UF fouling resulting due to NOM hs een ttriuted to concentrtion polriztion, gel or cke lyer formtion s well s outer nd inner pore clogging. Aoustin et l. (1) reported tht internl deposition of NOM resulted in irreversile fouling with lrger moleculr frction of humic mterils responsile for pore clogging. Fouling due to concentrtion polriztion is rther complex to quntify. Thorsen () studied the concentrtion polriztion cused y NOM in NF nd UF nd concluded tht dsorption of NOM onto memrne surfce plyed little role on memrne fouling. Although memrne fouling cn theoreticlly e rtionlized with the physicl nd chemicl phenomen, quntittive * Corresponding uthor. Tel.: þ ; fx: þ E-mil ddress: cflin@ntu.edu.tw (C.-F. Lin) /$ see front mtter ª 8 Elsevier Ltd. All rights reserved. doi:1.11/j.wtres.8.1. on memrne fouling nd flux decline, Wter Reserch (8), doi:1.11/j.wtres.8.1.

2 ARTICLE IN PRESS wter reserch xxx (8) 1 differentition mong concentrtion polriztion, gel or cke lyer formtion nd pore clogging is difficult. However, it is elieved tht mss of humic mterils deposited on memrne surfce ssocited with memrne inner pore surfce or pore lockge re the primry resons for permete decline nd filtrtion resistnce uild-up in wter supply industries. While iofilm/iofouling nd inorgnic mtter could lso e contriuting fctors for permeility decline in wstewter tretment prctices. Hong nd Elimelech (1997) studied the interction etween divlent ions nd NOM nd concluded tht NOM deposition on the memrne surfce nd the formtion of fouling lyer were due to the moleculr ssocitions etween NOMs. At deposited NOM less thn gm, the memrne flux could e mintined t out 1 1 m 1 s. A similr work ws conducted y Ktsoufidou et l. (5) on the effect of divlent ions on memrne fouling y humic cids. The mounts of humic cids deposited on memrne surfce vried from 1 to 17 mg m nd depended on solution C þ concentrtions. Lee et l. () used field wters to study the fouling of microfiltrtion (MF) nd UF y DOM nd reported tht the dissolved orgnic cron (DOC) t 5 mg m would cuse 11 71% of permete flux decline, depending on the wter chemistry of the feed wters. These studies elucidte tht memrne fouling is consequence of NOM ccumultion nd deposition, ut provided very little quntittive informtion on the fouling lyer. Consequently, the present work is intended to present quntittively the effects of DOM on UF fouling through mss lnce of humic mterils on UF permetion. Specificlly, two types of memrne size, operting pressure nd feed DOC concentrtion were evluted to quntify their effects on flux decline. The role of memrne pore size on surfce gel lyer formtion nd inner pore ssocition or pore lockge ws lso evluted.. Experimentl section Humic cid (sodium slt) from Aldrich ws used to prepre stock humic cid feed solution ( nd 9 mg L 1 DOC) y dissolving 1 g of humic cid in 1 L deionized wter (Milli-Q), nd filtering through.5 mm memrne filter. The feed solutions were supplemented with NCl to mintin conductivity ner 3 mscm 1 nd ph ws djusted to 7 with 1 N NOH nd/or 1 N HCl. The stock humic cid solution nd feed solutions were stored t C for susequent UF experiments. A single hollow fier module (hydrophoic with negtively chrged polysulfone: A/G Technology) with length of 9 cm nd n re of pproximtely 9 cm ws used. Two types of UF memrnes with the nominl moleculr weight cutoff (MWCO) of 1 nd 1 kd were employed. The crossflow mode in the UF system ws operted without recirculting the concentrted retentte to void chnges in feed solution composition. Fig. 1 shows the schemtic of the crossflow experimentl setup which includes high pressure N gs chmer tht provides the driving pressure to deliver feed solution, hollow fier UF module, pressure guges nd frction collector for flux mesurement. The trnsmemrne pressure (TMP) ws controlled t four different levels: ner 7 kp (1 psi), 15 kp (15 psi), 1 kp ( psi) nd (5 psi). In Fig. 1 Schemtic digrm of UF hollow fier module employed in the permetion experiments. ll the UF opertions, tngentil flow velocity ws controlled t.1 m s 1. For ech experiment, new hollow fier UF ws used to ensure no interference of humic mterils tht were remining from previous experiments. The hollow fier ws initilly pre-wshed with lcohol for 5 min nd then flushed with Milli-Q wter for 8 h efore the designted experiments. Throughout the experiments (T ¼ 5 C), the TMP ws mintined constnt nd permete flux monitored. The permete ws collected using frction collector t vrious time intervls: 3 (t the eginning), 1, 15, 3, 5 min (fter h). Permete volume ws mesured using electric lnce with the detection level in mg. The DOC concentrtions in retentte nd permete were mesured t the pre-designted time intervls. The DOM mss (retentte nd permete) t time t n ws the verge of time t n 1 nd t n. DOC concentrtion of feed solution for ech experiment ws quntified in n orgnic cron nlyzer (O.I. Corportion, model 7). The mss retined on memrne ws clculted sed on the mss lnce of humic mterils in DOC. During the UF opertion, the retined mss of humic mterils on memrne is equl to humic mteril mss in feed minus humic mteril mss out in oth retentte nd permete. The humic mteril mss is then summed throughout the entire permetion period to represent the ccumulted DOM. The memrne permetion resistnce during opertion is clculted sed on resistnce-in-series model which hs originted from the hydrulic grdient theorem. It stipultes tht the energy loss to mintin constnt fluid velocity is due to the friction on porous medi s (Ching nd Cheryn, 198): P T P T J ¼ ¼ (1) R M þ R I þ R G R M þ R I þ FP T where J is permete flux (m 3 m s 1 ), P T TMP (P), R M memrne intrinsic resistnce (P s 1 m 1 ), R I interction resistnce (P s 1 m 1 ) due to the interction etween DOC nd inner memrne mteril, nd R G gel lyer/concentrtion on memrne fouling nd flux decline, Wter Reserch (8), doi:1.11/j.wtres.8.1.

3 ARTICLE IN PRESS wter reserch xxx (8) 1 3 polriztion resistnce (P s 1 m 1 ). R G is considered to e proportionl to the mss of deposited DOC nd the specific resistnce of the gel lyer. As the gel lyer is compressed, the resistnce tends to increse with pplied pressure (McCrthy et l., ). Therefore R G is expressed s function of TMP with R G ¼ FP T, where F is dependent on the specific memrne system nd cn e determined from experimentl dt (Yeh nd Wu, 1997). The R M cn e esily determined in deionized wter UF system with fresh hollow fier module. By vrying TMP nd plotting (1/J ) versus (1/P T ), the slope of the stright line yields the R M. To determine R I nd F, the flux s function of TMP nd feed DOC concentrtion ws monitored. The plots of (1/J ) t versus (1/P T ) re constructed nd the slope nd the intercept yield (R M þ R I ) nd F, respectively, s: 1 ¼ F þ R M þ R I () J t P T where t represents the filtrtion time. R I is then otined y sutrcting R M vlue from R M þ R I (Eq. ()). Since R G nd R I were not experimentlly differentile t this stge, we rther used fouling resistnce, R F (due to retined DOM mss on memrne) to represent the confounding results of oth gel lyer resistnce nd inner pore dsorption/clogging resistnce. In this study, R F refers to the resistnce resulting from DOC retined during the filtrtion. Numericlly, R F is equted to the difference etween totl resistnce nd the memrne intrinsic resistnce (¼(P T/J ) R M ) kd, mg L -1 1 kd, mg L TIME (min) 7 kp 15 kp 1 kp 7 kp 15 kp 1 kp Fig. UF permete flux evolution under four trnsmemrne pressures t feed DOC mg L L1. () 1 kd UF, () 1 kd UF. 3. Results nd discussion A typicl UF permete flux evolution t four TMPs with time is presented in Fig.. Two MWCO UF (1 nd 1 kd) were fed with mg L 1 DOC to induce memrne fouling. The initil permete flux rnged from 3 to 7 mms 1 for 1 kd UF nd to 1 mms 1 for 1 kd UF. Initil flux vlues re seen to e clerly dependent on memrne size s well s TMP. Permete flux decline ws fster nd more severe in the loose pore UF system (1 kd), prticulrly during the first min filtrtion (Fig. ). On the other hnd, there ws only slight flux chnge within -h filtrtion period for 1 kd UF system, or from 1 to 3 mms 1 (Fig. ). Bsed on these results, it is justifile to stte tht the loose pore memrne exhiits more significnt flux decline thn the tight pore memrne. In ddition to MWCO size, the TMP lso significntly ffects the flux decline. For exmple, for 1 kd system, flux dropped from 3 to 5 mms 1 t TMP of 7 kp fter h, wheres for n Tle 1 DOM mss retention (g m L ) on 1 kd nd 1 kd memrnes during -h opertion. Trnsmemrne mgl 1 9mgL 1 pressure (kp) 1 kd 1 kd 1 kd 1 kd P (.5.8) P (1. 1.) P (..9) P (. 5.5) R (3.) R (.) R (1.7) R (1.1) P (.5 1.) P (1. 1.5) P (.5 1.3) P (..3) R (3.5) R (.) R (11.9) R (1.) P (..8) P (1. 1.5) P (.9 1.) P (.5.) R (3.) R (.5) R (11.1) R (1.) P (.7 1.) P (1. 1.5) P (1.1.) P (..) R (.9) R (.3) R (1.1) R (1.1) P ($): the rnge of permete concentrtions in mg L 1. R ($): the verged retentte concentrtion in mg L 1. on memrne fouling nd flux decline, Wter Reserch (8), doi:1.11/j.wtres.8.1.

4 ARTICLE IN PRESS wter reserch xxx (8) 1 incresed TMP of the flux decline ws from 155 to mms 1. The quntittive results of flux evolution presented in Fig. were converted to the mss of DOM retined on memrne for the entire -h opertion s shown in Tle 1. The permete DOC nd verge DOC in retentte re lso shown in Tle 1. The dt gin revel some common phenomen: (1) higher TMP results in more ccumulted DOM on memrne, () higher DOC feed concentrtions ring more DOM on memrne, nd (3) lrger pore size memrne retins more DOM during the sme opertion period. For exmple, the ccumulted DOC on 1 kd memrne t feed DOC of mgl 1 hs incresed from.7 to. g m when TMP ws incresed from 7 to. For the 1 kd UF with TMP of, DOC retined on memrne incresed from to 11 g m s feed DOC concentrtion ws incresed from to 9mgL 1. Under sme operting conditions, 1 kd memrne retined up to times more humic mterils thn 1 kd memrne. According to the work y McCrthy et l. (), gel lyer thickness ws considered to e ffected y TMP. Therefore it ws ssumed in our study tht surfce gel lyer would hve sme thickness for the sme permetion re (i.e., sme mount of DOC); the extr mount of retined DOC on 1 kd memrne should e deposited inside the inner pores. In other words, most of the foulnt (DOM) should e ssocited with inner pore surfces. The deposition of DOM on memrne cuses permetion flux decline nd lso increses permetion resistnce. Figs. 3 (1 kd) nd (1 kd) present the dvncement of UF permetion resistnces s function of DOM mss retention. In oth systems, the permetion resistnces were reltively Fouling Resistnce (MP s -1 mm -1 ) Fouling Resistnce (MP s -1 mm -1 ) 1 kd, mg L -1 1 kd, 9 mg L -1 7 kp 15 kp 1 kp 7 kp 15 kp 1 kp DOC Accumultion (g m - ) Fig. 3 Advncement of UF permetion resistnces s function of DOC mss retention for 1 kd UF. () Feed DOC mg L L1, () feed DOC 9 mg L L1. Fouling Resistnce (MP s -1 mm -1 ) Fouling Resistnce (MP s -1 mm -1 ) 1 kd, mg L -1 1 kd, 9 mg L DOC Accumultion (g m - ) 7 kp 15 kp 1 kp 7 kp 15 kp 1 kp Fig. Advncement of UF permetion resistnces s function of DOC mss retention for 1 kd UF. () Feed DOC mg L L1, () feed DOC 9 mg L L1. linerly dependent on DOC mss ccumultion on memrne. The initil slopes of fouling resistnces versus retined DOM (Figs. 3 nd ) were clculted with the regression method of lest squres nd the corresponding results re presented in Tle. The correltion coefficients were lwys >.9. Interestingly, the slopes representing the resistnce chnge per unit retined DOM mss re steeper in mg L 1 systems thn those in 9 mg L 1 systems, prticulrly for 1 kd memrnes. The higher humic cid concentrtion (9 mg L 1 ) might fvor moleculr gglomertion which cn result in the formtion of lrger molecules. It ppers tht lrger molecules cn esily get deposited on memrne surfce rther thn moving into the inner pores. Therefore it is elieved tht the effect of inner pore interction on the fouling resistnce per unit retined DOM mss t mg L 1 feed concentrtion ws less thn 9 mg L 1 feed concentrtion. Tle Fouling resistnces per unit retined DOM mss (MP s L1 m L1 mm L1 g L1 m ). Trnsmemrne mgl 1 9mgL 1 pressure (kp) 1 kd 1 kd 1 kd 1 kd The regression coefficients of the slopes (R ) re >.9 in ll cses. on memrne fouling nd flux decline, Wter Reserch (8), doi:1.11/j.wtres.8.1.

5 ARTICLE IN PRESS wter reserch xxx (8) 1 5 The effect of TMP on the fouling resistnces per DOM mss retined ws different for 1 nd 1 kd systems. For exmple, the fouling resistnce per unit ccumulted DOM decresed s TMP ws incresed from 7 to in the 1 kd UF system, wheres it incresed stedily with incresing TMP in the loose 1 kd UF system (Tle ). TMP cts s the min driving force through memrne for permete flux nd leves the DOM molecules on memrne either s gel lyer or inner pore ccumultion. For 1 kd memrne, higher TMP might push nd deliver more DOM molecules into memrne pores resulting in severe inner pore dsorption/clogging; therefore, the fouling resistnce per unit mss of retined DOM incresed s the operting TMP incresed resulting in severe flux decline. As for the 1 kd systems, fouling resistnce decresed with incresing TMP. The totlly contrst results oserved in tight UF might e ecuse of the size exclusive effect nd the surfce gel lyer effect tht prevent pushing of DOM molecules to cuse the inner pore dsorption/clogging. Indeed, n erlier report shows tht lrger humic mterils re responsile for the surfce lyer clogging (Aoustin et l., 1). Quntittive effects of retined DOM on permete flux decline over -h filtrtion period re profiled in Figs. 5 nd. Agin, the influent with mg L 1 DOC cused more severe flux decline per unit retined DOM mss thn tht of 9 mg L 1 systems, especilly during the eginning of filtrtion stge. The slopes in Figs. 5 nd represent the effect of unit mss of retined DOM on chnges of permete flux. In Fig. 5, the trend of slopes of the curves ws similr indicting tht the effect of TMP on unit flux chnge per unit retined DOM ws not significnt in the kd, mg L -1 1 kd, 9 mg L DOC Accumultion (g m - ) 7 kp 15 kp 1 kp 7 kp 15 kp 1 kp Fig. 5 Effects of DOC mss retention on memrne flux decline of 1 kd UF. () Feed DOC mg L L1, () feed DOC 9mgL L kd, mg L -1 1 kd, 9 mg L -1 7 kp 15 kp 1 kp 7 kp 15 kp 1 kp DOC Accumultion (g m - ) Fig. Effects of DOC mss retention on memrne flux decline of 1 kd UF. () Feed DOC mg L L1, () Feed DOC 9mgL L1. cse of 1 kd UF. Wheres for 1 kd UF (Fig. ), higher TMP (1 nd in mg L 1 system) ppers to cuse more flux decline y the retined DOM. The initil flux chnges with the unit DOM mss retined were nonliner nd flux then grdully reched the stedy-stte fouling condition. The two-stge flux phenomenon ws est illustrted for 1 kd t mg L 1 with higher TMP (Fig. ). The initil rpid drop of flux with unit DOM mss retined my e due to pore lockge for this loose memrne. Therefter, DOM ccumultion on the memrne surfce my e responsile for grdul flux decline. It ws construed tht the retined DOM foulnt indeed results in permete flux decline. The effects of the retined DOM on permete flux decline ws further shown to e dependent on the memrne pore size in conjunction with the feed nd the opertionl conditions. The oserved condition for the most severe impct on permete flux decline per unit retined DOM ws low feed concentrtion nd loose memrne with higher TMP. Hypotheticlly, we hve proposed two clsses of fouling schemes s depicted in Fig. 7. Fouling on tight pore memrne is primrily due to surfce gel lyer formtion with very little inner pore dsorption or clogging (Fig. 7). This is prtilly supported y the fct tht the frction of <1 kd size humic mterils is less thn 5% of totl DOC sed on the gel filtrtion chromtogrphy experiments of humic mterils (Syfei, 7). Memrne fouling in the 1 kd UF, on the other hnd, is elieved to e inner pore dsorption/lockge (Fig. 7). A lrge frction of the humic mterils hving the moleculr size <1 kd is le to enter memrne pore nd susequently ggregte nd stick together in the pore on memrne fouling nd flux decline, Wter Reserch (8), doi:1.11/j.wtres.8.1.

6 ARTICLE IN PRESS wter reserch xxx (8) 1 per unit retined DOM. Depending on the permetion conditions, memrne flux due to ccumultion of DOM could e clssified into n initil rpid decline nd eventul grdul decrese efore reching stedy-stte condition. Acknowledgement The uthors would like to thnk the Ntionl Science Council of the Repulic of Chin for finncilly supporting this reserch under Contrct No. NSC95-1-E--1. references Fig. 7 The proposed fouling mechnisms for () tight memrne (surfce gel lyer) nd () loose memrne (inner pore dsorption nd clogging). chnnel. Indeed, study of Ktsoufidou et l. (5) supports this hypothesized fouling mechnism on tight nd loose pore memrnes. They further stted tht the irreversile fouling ws ttriuted to internl pore dsorption with fouling cke deposited on memrne surfce tht cn esily e removed y ckwshing. It is nticipted tht the results of our experiments nd the proposed hypotheticl fouling mechnism will e of gret help for the memrne modeling reserches.. Conclusions UF memrne is esily fouled y feed wter contining DOM. The fouling is primrily originted from the gel lyer formtion nd inner pore dsorption or pore lockge. A 1 kd UF feeding with mg L 1 DOC will ccumulte.7. g m DOM in -h opertion period, while 9 mg L 1 DOC feed results in the ccumulted g m DOM. In the cse of 1 kd UF, the ccumulted DOM is 3.3. g m ( mg L 1 ) nd 8 11 g m (9 mg L 1 ). We proposed tht those DOMs re lrgely present in inner pore in loose memrne resulting in severe flux decline. The memrne surfce gel lyer formtion is more crucil in the tight pore memrne. The effects of retined DOM on permete flux decline depend on the memrne pore size s well s the feed nd opertionl conditions. Low feed concentrtion, loose memrne with higher TMP will cuse stern impct on permete flux decline Aoustin, E., Schfer, A.I., Fne, A.G., Wit, T.D., 1. Ultrfiltrtion of nturl orgnic mtter. Seprtion nd Purifiction Technology 3 (1), Ching, B.H., Cheryn, M., 198. Ultrfiltrtion of skim milk in hollow fiers. Journl of Food Science 51 (), 3 3. Hong, S., Elimelech, M., Chemicl nd physicl spects of the nturl orgnic mtter (NOM) fouling of nnofiltrtion memrnes. Journl of Memrne Science 13 (), Howe, K.J., Clrk, M.M.,. Fouling of microfiltrtion nd ultrfiltrtion memrne y nturl wters. Environmentl Science nd Technology 3 (17), Ktsoufidou, K., Yintsios, S.G., Krels, A.J., 5. A study of ultrfiltrtion memrne fouling y humic cids nd flux recovery y ckwshing: experiments nd modeling. Journl of Memrne Science (1 ), 5. Kimur, K., Hne, Y., Wtne, Y., Amy, G., Ohkum, N.,. Irreversile memrne fouling during ultrfiltrtion of surfce wter. Wter Reserch 38 (1 15), Kweon, J.H., Lwler, D.F.,. Fouling mechnisms in the integrted system with softening nd ultrfiltrtion. Wter Reserch 38 (19), Lee, N.H., Amy, G., Croue, J.P., Buisson, H.,. Identifiction nd understnding of fouling in low-pressure memrne (MF/UF) filtrtion y nturl orgnic mtter (NOM). Wter Reserch 38 (), Mrtens, A., Swrt, P., Jcos, E.P., Humic memrne foulnts in nturl rown wter: chrcteriztion nd removl. Deslintion 115 (3), McCrthy, A.A., Wlsh, P.K., Foley, G.,. Experimentl techniques for quntifying the cke mss, the cke nd memrne resistnces nd the specific cke resistnce during crossflow filtrtion of microil suspension. Journl of Memrne Science 1 (1 ), Syfei, A.D., 7. TiO coted memrne coupled with UV irrdition for nturl orgnic mtter removl. Mster Thesis, Ntionl Tiwn University. Thorsen, T.,. Concentrtion polriztion y nturl orgnic mtter (NOM) in NF nd UF. Journl of Memrne Science 33 (1 ), Yeh, H.M., Wu, H.H., Memrne filtrtion in comined hollow-fier module systems. Journl of Memrne Science 1 (1), Zhng, M., Li, C., Benjmin, M.M., Chng, Y., 3. Fouling nd nturl orgnic mtter removl in dsorent/memrne systems for drinking wter tretment. Environmentl Science & Technology 37 (8), on memrne fouling nd flux decline, Wter Reserch (8), doi:1.11/j.wtres.8.1.