Effect of ultrafiltration membranes properties and channel configuration on membrane fouling and performance

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1 Effect of ultrafltraton membranes propertes and channel confguraton on membrane foulng and performance Dmtry Sptsov 1, and Alexey Andranov 1,* 1 Moscow State Unversty of Cvl Engneerng, Yaroslavskoe shosse, 26, Moscow, , Russa Abstract. The work descrbes the exstng vews on collodal, suspended and organc foulng n ultrafltraton (UF) membrane modules usng for natural water treatment. The paper presents the results of the expermental research allowng for a quanttatve determnaton of sorpton and pluggng rates, and for evaluaton of ther effect on membrane performance loss over tme. Small collodal partcles pose a rsk to UF unt due pore pluggng; especally when membranes treat water wth a very low content of suspended solds. Ths effect often cannot be detected durng short-tme plot tests, but t can have a decsve nfluence on the operaton of a membrane unt. Study of the flterablty characterstc of water contamnants enables us to forecast the fltraton type on UF membranes and to evaluate ther performance losses. The paper also descrbes the effect of a feed channel structure n partcular, a presence of a feed spacer n spral wound membrane modules on channel hydraulc resstance growth. The technque of channel resstance estmaton, dependng on suspended partcles concentraton n the source water, s presented. 1 Effect of membrane surface propertes on organc foulng Membrane ultrafltraton technque s wdely used nowadays for natural water treatment, but generally membrane facltes face thechallengeof ther collodal, organc and bologcal foulng and, as a result, the deteroraton of membrane characterstcs [1, 2]. The mechansm of organc foulng of reverse osmoss and ultra fltraton membranes has been well studed[3-5]. Accordng to [6], organc substances generate an adsorbed flm on membrane surface, whch has vrtually no detrmental effect on membrane characterstcs. However, experence shows that organc foulng does result n selectvty and performance loss over long tme. Ths s, obvously, due to the fact that dssolved organc matter has very varous propertes and a complex pcture of molecular-mass dstrbuton. Organc molecules are adsorbed n the pores and on the surface of a membrane, changng ts degree of hydrophlcty,.e., "modfyng" membrane surface. Accordng to Fane [6], the concentraton of hgh-molecular substance n the near membrane layer ncreases wth tme. In [7] authors suggest a model based on the assumpton that membrane flux declne durng fltraton of water contanng natural organc compounds s accounted for by gel layer formaton on the membrane surface, and by sorpton occurrng as a result of * Correspondng author: AndranovAP@mgsu.ru The Authors, publshed by EDP Scences. Ths s an open access artcle dstrbuted under the terms of the Creatve Commons Attrbuton Lcense 4.0 (

2 nteracton of organc substances wth one another and wth the membrane materal. Ths model can be descrbed by equaton: P J ( R R R ), (1) 2 m ads1 ads where R m s membrane hydraulc resstance; R ads1 s resstance of the gel layer plus resstance nduced by sorpton of organc substances on the membrane surface; R ads2 s resstance caused by nteracton of organc substances located on the membrane surface and n the soluton. The value R ads1 descrbes a change n membrane resstance durng the ntal perod and grows over tme to reach a certan equlbrum value R eq. The value R ads2 ncreases durng the entre fltraton perod, but ts contrbuton to the total resstance s mnor. The resstance R eq, as shown by experments descrbed n [7] s drectly proportonate to the amount of substances adsorbed on the membrane. The amount of adsorbed substances and, consequently, the value R eq, are descrbed by Langmur and Freundlch equatons: R ac m eq ; 1 bcm R q p ( C, (2) eq m) where a, b, p, q are adsorpton sotherm coeffcents, C m concentraton of organc substance near membrane surface. A more smplfed outcome of the adsorpton-nduced foulng model s gven n [8]: R ads max pt R (1 e ), (3) ads R max ads Cm const C m. (4) Substances adsorbed by the surface of an UF membrane can change ts propertes: e.g., ts surface charge or hydrophlcty. In a number of cases, membrane selectvty may ncrease due to pore constrcton or surface charge change. The ntensty of organc matter adsorpton on membrane surface has been studed by a number of authors [9, 10] usng a crculaton-mode experment. In ths method, a gven volume of feed soluton (natural water or a peat extract-based model soluton) was crculated through a membrane module whle the fltrate and concentrate were returned back to the feed soluton. Durng the experment the concentratons of organc compounds were measured usng varous methods (TOC determnaton [9],photoelectrc colormetry [10], gas-lqud chromatography [10] etc.). The decrease n the organc substance concentraton n the crculatng solutons observed durng the experment proves precptaton (or sorpton) of organc compounds on the membrane surface. Multple repettons of the crculaton-mode experments made t possble to expermentally determne the laws of organc compound accumulaton on membrane surface. Organc compounds adsorbed on the membrane surface form a thn flm, whch has vrtually no effect on the membrane's performance or selectvty. After spreadng over the entre membrane surface, the organc flm stops growng and the rates of organc substance adsorpton dramatcally decrease. Expermental determnaton of organc substance adsorpton rates enables us to measure the sorpton capacty of the membrane surface. Determnaton of the adsorpton capacty of acetate UF membranes (UAM type, Russa) was performed for a mxture of humc acds (peat extract), smulatng a natural organc matter (water colour). The prelmnary fndng was that the color of the prepared peat 2

3 extract soluton was drectly proportonal to the concentraton of humc substances. The humc acd soluton was prepared by dssolvng peat extract n dstlled water alkalzed to ph = 10, and then was dosed nto the feed water to obtan a specfed concentraton (color). The feed soluton was crculated through a membrane module at hgh crossflow velocty for several hours wthout fltrate passage, to exclude foulng and pore pluggng. Durng the experment, organc substances are absorbed by the membrane materal and the color of the crculatng soluton contnuously decreases (Fg. 1a). Based on the drop n the organc substance concentraton n the crculatng soluton, one can calculate the amount of substance adsorbed on the membrane (Fg. 1b). The plot n Fg. 1b s used to determne the coeffcent р n the adsorpton equaton (3) for an arbtrary tme t. The sorpton rate decreases as the membrane materal becomes saturated wth organc substances. Several seres of experments were conducted to determne the membrane sorpton capacty n terms of humc acds. A freshly prepared peat extract soluton was used for each seres. Fg. 1.Determnaton of humc substances adsorpton on UAM acetate membranes: color n the crculatng soluton (a) andmass of adsorbed humc acds (b) by tme (р = 0,46 h -1 ) Fg.2.Influence of adsorbed humc substances on UF membrane characterstcs: dependences of membrane performance (a) and addtonal membrane resstance (b) on the mass of humc acds adsorbed on the membrane (k ads = 0.917) Membrane performance n clean water (Fg. 2a) was measured durng the experments to determne the value R max ads (Fg. 2b). Despte a relatvely small amount of organc substances precptatng on the UF membrane (~ 600 mg/m 2 ), ts performance decreased almost by two tmes (Fg. 2a), whch proves that sorpton occurs nsde pores as well. The dependences n Fg 1b and 2a allow to predct the performance loss n UF membranes over tme and to determne the amount of accumulated organc substances for treatng water of a specfed color. 3

4 As descrbed n [3, 11] the membrane materal affects the organc adsorpton rate and adheson of collodal and suspended partcles. Experments showed that acetate membranes are less subject to organc and collodal foulng [12]. Long-term developments were targeted at creatng new types of membranes wth modfed surfaces that could "reject" a number of foulants contaned n water (bactera, collods, and organcs) and, thanks to ths, could ensure a long servce lfe of a membrane wth mnmum foulng and effcent removal of foulants durng hydraulc flushng. However recent research has demonstrated that membrane organc foulng s not only determned by membrane polymer propertes, but rather on the whole of the surface chemstry determnng the amount and strength of the possble foulant membrane nteractons [3, 5]. 2 The mpact of foulng on feed channel resstance ncrease n spral wound membrane module Lke n deep bed fltraton wheren suspended and collodal substances are deposted n the pores of the flter bed formng "arches" n the mouths of pore channels, a smlar pcture can be observed at places where feed spacer nodes contact the membrane surface n spral wound elements. At hgh veloctes, the "arches" are destroyed by hydrodynamc forces. The amount of suspended solds trapped n the bed due to partcle adheson to the bed surface and carred out can be descrbed by standard equatons for deep-bed fltraton [13]. Smlar to flterng through a granulated bed, one can use expermentally obtaned dependences for spral wound module cross flow resstance calculatons. Fg.3.Example of resstance ncrease over tme n an 1812-type spral wound membrane module Fg. 3 shows an example of feed channel resstance determnaton n an 1812-type spral wound element. Fg. 3a presents the results of determnaton of head loss from cross flow velocty for dfferent quanttes of cake accumulated n membrane module. Based on the determnaton of foulng rates, the cake amount on a membrane was forecasted dependng on the perod of module operaton wth feed water of a specfed composton (Fg 3b). The forecast shown n Fg. 3c s for rver water and for rver water concentrated by 2 and 3 tmes. Based on the data n Fg. 3b, each cake quantty can be correlated wth the resstance n fouled membrane module and depends on water qualty, tme and cross flow flux. 4

5 3 The mpact of pore pluggng on UF membranes performance As we know, membrane performance loss over tme can be explaned by cake growth and membrane resstance ncrease due to partal pore pluggng. The pore pluggng effect, unfortunately, s not taken nto account by the majorty of models descrbng the membrane fltraton process. Nevertheless, ths process s of a very sgnfcant mportance, snce small partcles (smaller than membrane pore szes) present n water turn out to be deleterous for membrane unts. It s the presence of small partcles n water and the rsk of ther penetraton nto the membrane body that explans the need for pre-coagulaton and bndng of small partcles nto larger agglomerates, whch underles the operatng specfcatons related to Nort, Inge and other membrane unts. Small partcles are especally dangerous durng operaton n a dead-end fltraton mode at hgh operatng pressures, when small partcles may thoroughly plug the pores and be dffcult to remove durng backflushng. Probably, t s ths fact that explans an often unsuccessful experence of surface water treatment n a dead-end fltraton mode wth coagulaton n drect flow. Ineffcent coagulaton often results n up to 12% of water consumpton for UF membranes backwash. One of effectve ways to reduce the pluggng effect by small partcles s transton from a dead-end fltraton to a cross flow recrculaton mode wheren hgh flux velocty over the membrane removes partcles from ts surface. Such mode s used both for water wth a hgh suspended sold content and for treated water. E.g., at Moscow South-West WTP, UF faclty s used for post-treatment of water that has already been subjected to coagulaton, sedmentaton and flterng, but UF unts most of the tme operate n a recrculaton mode. Smultaneously, as was already demonstrated n a number of works [14, 15], water subjected to coagulaton and flterng treatment contans small partcles most dangerous n terms of membrane performance loss. In order to reduce the membrane pore pluggng rsk, t seems approprate to use membranes wth smaller pore szes, but unfortunately they have reduced performance. Evdently, provded by low cost UF modules wth capllary membranes, manufacturers consdered that t s uneconomcal to use membranes wth the molecular cutoff below 100 kda. The use of a recrculaton mode (wth cross flow veloctes m/s) results n a hgh energy consumpton. Therefore, developers of UF technologes lmted themselves to the dead-end fltraton mode wth a mandatory precoagulaton of feed water. Meanwhle, t s absolutely obvous that the use of membranes wth smaller pore szes (20-30 kda) workng n recrculaton mode reduces the concentraton polarzaton level and mproves the qualty of treated water n terms of organc content, reduces the rsk of membrane pluggng and formaton of collodal cake. The narrow spread of UF n water treatment practce can, probably, be explaned by hgh captal and operatonal costs. The exstng, wdely mplemented capllary membrane UF technology s a sort of compromse at the current stage of ndustral development of the membrane technology. Cheapenng of membrane modules and mprovement of spral wound membrane modules, less costly than capllary and tubular ones, wll lead to a change n ther applcaton technology. When UF unt operates n a dead-end fltraton mode, one can observe a slow, run-torun declne n membrane performance. An rreversble performance loss s caused by cake accumulaton n dead areas of membrane module and pluggng of part of the pores. Membrane performance declne durng one flter run s shown n Fg. 4a, where the data were obtaned durng fltraton of rver water n a dead-end mode through an 1812-model spral wound module usng MGA-100 UF membranes (200 kda) at pressure 2.0 bar. 5

6 Fg.4.Dependency of performance loss n an 1812-type UF module durng one flter run and processng of ths dependency n the coordnates V t/v and t t/v Snce the process of membrane performance declne s descrbed by an equaton for an ncompressble cake fltraton, the plot of the value t/v (relaton of the fltraton tme to the volume of fltered water, mn/l) aganst the volume of fltered water V s presented as a straght lne (Fg. 4b) [12].Due to ts hgh resstance, the formed cake obvously entraps small partcles whch cause pluggng. As can be seen from the plots n Fg. 4a, a sharp drop n UF membrane performance s preceded by a short straght secton, a perod when the membrane performance remans at the ntal level, and the cake resstance s not hgh enough to cause a performance drop. On the plot n coordnates t/v t (Fg. 4a) a sloped secton s preceded by a secton parallel to the X-axs. Evdently, durng ths perod small partcles seep through the membranes and plug the pores. Membrane pluggng reveals wth tme and s notced by performance loss after qute a long perod. In order to forecast membrane performance loss, t s necessary to determne the effect of membrane pluggng on the performance rate,.e., to reveal the dependency of performance on the volume of fltered water (for varous types of feed water). If pluggng occurs slowly as pores are clogged, then the process s descrbed by a straght lne n the coordnates t/v t(where t s fltraton tme, V s volume) [12]. Several operatons are requred n order to determne the tme dependency of membrane performance loss due only to pore pluggng. 1) Frst we determne the share of water volume passed through the membrane, whch caused pluggng. In the plot, ths volume corresponds to the ntal perod of fltraton (the pluggng process) untl cake layer s buld up and can be ntroduced as a coeffcent k: V f n = k Q f avg Т f, where Q f avg s the average performance durng the fltraton cycle, Т f s the fltraton cycle tme, V f n s the volume correspondng to the notceable membrane performance loss occurs durng the fltraton cycle. 2) The entre unt operaton perod T, durng whch membrane productvty loss s observed, can be dvded nto several perods ( = 4 to 6) wthn whch the average performance for such perod can be determned as: Q avg V T 3) Wthn each perod of tme T the total volume of fltered water V f and the average fltrate flow rate Q avg can be determned. f 6

7 4) Then we determne the share of fltrate volume correspondng to plugged fltraton for each volume V f : V f.n. = k V f, and the tme of fltraton durng whch pluggng occurs: T з t V Q 5) Knowng the value of the fltraton tme, the volume of fltered water and the performance rate for each perod, we can plot the value t/v aganst the fltraton tme t and aganst volume V, whch wll allow us to forecast membrane performance loss durng pluggng. In order to forecast performance loss n membranes due to pore pluggng, one can determne the tme dependency of performance usng the cell for determnaton of the Modfed Foulng Index MFI [16]. To obtan the volume of water to examne, we can use frst portons fltered through mcrofltraton membranes before cake formaton begns. By plottng a chart n the coordnates t t/v, we can determne the values of volume V n and perods T n by the slope angle. After studyng the operaton of membranes wth varous types of feed water, one can draw the concluson that partcle sze dstrbuton plays a sgnfcant role. A specfc problem s posed by small partcles (<0.1 um): they are more dffcult to remove from water. Besdes that, cake formed by such foul ants has a sgnfcantly hgher resstance than that of large partcles. Small partcles are especally dangerous when they plug the pores of UF membranes. The treatment practce often shows that the more thorough pre-treatment was appled to water, the greater nfluence small partcles exert on membrane foulng. Small partcles can be found n norganc collodal foul ants. These are alumnoslcates (clay partcles) 0.3 to 1 um n sze, and collodal partcles of ron, alumnum; slcates. Components of organc collodal partcles nclude protens, carbohydrates, fats, ols, and surfactants. Aromatc acds, lke, e.g., humc acds, n most cases exst n the form of very small collods (< 0.01 um), whch often appear unstable and, dependng on the ph value, cause membrane foulng. The revew [17] states that modern methods of water qualty (degree of removal of collodal and organc substances) evaluaton based on water mcrofltraton and on study of expermentally obtaned dependences of cake resstance on the volume of fltered water often yeld unexpected results. Ths can be explaned by presence n coagulated and fltered water of partcles, whch cause pluggng of mcro flters used n tests and, as a result, loss of ther performance, whch s often greater than when they flter untreated water. That s why for water treatment usng ultra fltraton, one should take nto account the composton of the feed water, as well as the nature and szes of partcles contaned theren n order to avod the rsk of pluggng and premature loss of membrane performance. f.n. avg 4 Conclusons 1. Performance loss n UF membranes n water treatment unts occurs due to sedment layer formaton and lnear gan of resstance, whle performance drop n NF elements occurs due to ncrease n membrane module resstance and drop of operatng pressure. 2. Research shows that organc compounds exert an nsgnfcant effect on performance and selectvty of UF membranes, and the degree of characterstc degradaton n the latter depends on the membrane materal and the degree of ts hydrophobcty. 7

8 3. The degree of performance loss n UF membranes s naturally dependent on the concentraton of suspended solds n water beng processed: the more numerous they are, the faster performance declnes due to formaton of a cake layer on the membrane surface. In addton, another phenomenon can be observed, especally n a transton flow mode: cake accumulaton occurs n the nodes of the feed spacer causng an addtonal ncrease of the general hydraulc resstance. 4. Durng treatment of water wth a very low content of suspended solds, membrane pores are plugged resultng n a major performance loss. Ths effect often cannot be detected durng short-tme plot tests, but t can have a decsve nfluence on the operaton of a membrane unt. Study of the flterablty characterstc of water under treatment enables forecastng of the fltraton type usng ultra fltraton membranes and enables evaluatng the drop n ther performance. References 1. W. Gao, H. Lang, J. Ma, M. Han, Z. Chen, Z. Han, G. L, Desalnaton 272, 1-8 (2011). 2. X. Sh, G. Tal, N.P. Hankns, V.Gts, J. of Water Process Eng. 1, (2014). 3. G. Mustafa, K. Wyns, A. Buekenhoudt, V. Meynen, Water Res. 93, (2016) 4. H. L, H. Xa, Y. Me, Desalnaton 386, (2016) 5. N. Shamsuddn, D.B. Das, V.M. Starov, Chem. Eng. J. 276, (2015) 6. A.I. Shafer, A.G. Fane, T.D. Wate, Desalnaton 131, (2000) 7. J.Cho, G.Amy, Y.Yoon, J.Sohn, Desalnaton142, (2002) 8. J.G. Grespo, K.W. Buddeken (ed.) Membrane processes n separaton and purfcaton (NATO ASI Seres) 272, (1994) 9. T.L. Champln, Proc. of the Conf. on Membranes n Drnkng and Industral Water Producton(Pars, France)1, (2000) 10. A.G. Pervov, Desalnaton126, (1999) 11. C. Combe, E.Mols, P.Lucas, R.Rley, M.M.Clark., J. of Membrane Sc. 154, (1999). 12. M.Elmelech, X. Zhu, A.E. Chldress, S. Hong, J. of Membr. Sc., 127 (1997) C. Ten (ed.) Prncples of Fltraton(Elsever, Amsterdam, 2012) 14. W.Yuan, A.Kocc, A.L.Zydney, J. of Membrane Sc. 198, (2002) 15. E.Aoustn, A.I.Schäfer, A.G.Fane, T.D.Wate, Separaton and Purfcaton Tech.22-23, (2001) 16. S.F.E. Boerlage, M.D. Kennedy, M.P.Anye, E.Abogrean, Z.S. Tarawneh, J.C. Schppers, J. Membrane Sc. 211, (2003). 17. A.G.Fane, C.J.D.Fell, Desalnaton 62, (1987) 8