fouling membran nano.pdf

Separation and Purication Technology 63 (2008) 251263

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Separation and Purication Technology

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Review

Drawb

B. Van dea K.U.Leuven, Db LappeenrantaP.O. Box 20 FI-5

a r t i c l

Article history:Received 26 FeReceived in reAccepted 10 M

Keywords:Membrane ltNanoltrationFoulingConcentratesFractionationWater treatmentDrinking waterWastewater

dealtwith formany applications,whereasmore researchwould result inmanymorepossible applications.It is suggested that these challenges should be among the main priorities on the research agenda fornanoltration. This paper offers an overview of the state-of-the-art in these areas, without going intodetails about specic observations in individual studies, but rather aiming at giving the overall pictureof possible drawbacks. This leads to suggestions which direction the nanoltration research community

Contents

1. Introd2. Memb3. Insuf4. Treatm5. Memb6. Insuf7. Mode8. Concl

AcknoRefere

1. Introduc

The introphenomenabetween be

CorresponE-mail add

mika.manttari1 Tel.: +358 5

1383-5866/$ doi:10.1016/j.sshould follow, and where research questions can be found. Evidently, the six identied challenges areto some extent interrelated; mutual inuences are explained as well as possible solutions, or possiblepathways to solutions.

2008 Elsevier B.V. All rights reserved.

uction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251rane fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252cient separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253ent of concentrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255rane lifetime and chemical resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256cient rejection for individual compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258lling and simulation of nanoltration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259usions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260wledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260nces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260

tion

duction of new technologies always involves transition, such as unexpected start-up problems, discussionslievers and non-believers, and research efforts leading

ding author. Tel.: +32 16 32 23 40; fax: +32 16 32 29 91.resses: [email protected] (B. Van der Bruggen),@lut. (M. Manttari), marianne.nystrom@lut. (M. Nystrom).621 2192; fax: +358 5 621 2199.

to fundamental understanding, suggestions for practical solutionsand technical improvements. Nanoltration was dened as a pro-cess intermediate between reverse osmosis and ultraltration thatrejects molecules which have a size in the order of one nanometer[1]. It was introduced in the late 1980s, mainly aiming at combinedsoftening and organics removal [1]. Since then, the applicationrange of nanoltration has extended tremendously. New possi-bilities were discovered for drinking water production, providinganswers to new challenges such as arsenic removal [27], removalof pesticides, endocrine disruptors and chemicals [811,6,12,13],and partial desalination [1417]. Large plantswere constructed, the

see front matter 2008 Elsevier B.V. All rights reserved.eppur.2008.05.010acks of applying nanoltration and how to avoid them: A review

r Bruggena,, M. Manttari b,1, M. Nystromb,1

epartment of Chemical Engineering, Laboratory for Applied Physical Chemistry and Environmental Technology, W. de Croylaan 46, B 3001 Leuven, BelgiumUniversity of Technology, Department of Chemical Technology, Laboratory of Membrane Technology and Technical Polymer Chemistry,3851 Lappeenranta, Finland

e i n f o

bruary 2008vised form 6 May 2008ay 2008

ration

a b s t r a c t

In spite of all promising perspectives for nanoltration, not only in drinking water production but alsoin wastewater treatment, the food industry, the chemical and pharmaceutical industry, and many otherindustries, there are still some unresolved problems that slow down large-scale applications. This paperidenties six challenges for nanoltrationwhere solutions are still scarce: (1) avoidingmembrane fouling,and possibilities to remediate, (2) improving the separation between solutes that can be achieved, (3) fur-ther treatment of concentrates, (4) chemical resistance and limited lifetime ofmembranes, (5) insufcientrejection of pollutants in water treatment, and (6) the need for modelling and simulation tools.

The implementation of nanoltration in the industry is a success story because these challenges can be/ locate /seppur

252 B. Van der Bruggen et al. / Separation and Purication Technology 63 (2008) 251263

best documented example being theMery-sur-Oise plant in France,which was started in the second half of the 1990s [18,19].

During the last decade, the interest in theuseofmembrane tech-nology in gwastewaterproductiongrowing deto reusewabranes, (4) lmore string

Thenumneverthelesuse of nanowater prodterms of voimpeccableresearch prcerns in theinduced a swhich canbranes. Formembranesextent is ne

The potewater reuseoperation cin which nacarried out;as a potentibut their suinteractionsing organicwastewaterlem [28,29increase ofbrane treatproductionthe yield issource such

The cheindustry arsavings couogy; enviromake nanothis area aremerging ability and liof the procesimulation

The foodatively fastnanoltratitry arehighapplicationproducts suseparationstivalent ionfromoneanHowever, sepotential.

This reviments forproblems. Sor can be dart in nano

are (1) membrane fouling, its causes and possibilities to reme-diate, (2) separation between solutes that can be achieved, (3)further treatment of concentrates, (4) chemical resistance of mem-

, (5)or mappliimprdone

mbra

ling, bux bescale coent,d shis c

ent awou

ate ylantsc soln exltratiingis byof mfoulof o

teracsorped byientmomharae ths, elenentary cnougorces

endioresiclesational fouughtin vabilitsizein nncreaitherrisonrganimemausin, whltraiumhougdyneneral and nanoltration in particular has emerged intreatment as well as drinking water and process water

. This growth can be explained by a combination of (1)mand for water with high quality, (2) growing pressurestewater, (3) better reliability and integrity of themem-ower prices ofmembranes due to enhanced use, and (5)ent standards, e.g., in the drinking water industry.ber of applications for nanoltration increases steadily;s, several challenges remain to be solved to allow theltration in more demanding applications. Drinking

uction, still the largest application of nanoltration inlumes, currently faces new challenges. The notion ofdrinking water developed in the Water Quality 21

ogramme in the Netherlands [20] and the growing con-USA on the presence of emerging contaminants [21]

hift towards less permeable, high rejectionmembranes,be denoted as (low pressure) reverse osmosis mem-tight nanoltration membranes and reverse osmosis, knowledge on which solutes are removed to whateded.ntial for nanoltration in wastewater treatment andis noteworthy [2225], but hindered by unstabilities in

ausedbymembrane fouling. Extensive researchprojectsnoltration was used for water reclamation have beenin themajority of these,membrane foulingwas studiedal problem. Industrial plants may be successful [26,27],ccess depends on a thorough understanding of possiblebetween the feed solution and the membrane, caus-

fouling, scaling, biofouling, or particulate fouling.Whenis tobe treated, the concentrate isusually anotherprob-]. The discharge option is often compromised by theconcentrations in the remaining fraction after mem-ment. This can also be problematic for drinking water, when discharge is not possible or not allowed, or whenconsidered too low for a valuable, permit-protectedas groundwater.mical processing industry and the pharmaceuticale other potential beneciaries of nanoltration. Hugeld be obtained by implementing membrane technol-nmental benets due to reduced energy consumptionltration particularly attractive [30,31]. Drawbacks ine of a different kind; for solvent ltration, one of thepplications, these are mainly related to membrane sta-fetime [32], and the lack of fundamental understandingss performance that can be translated tomodelling andtools [3336].

industry traditionally adopts new technologies rel-. The dairy industry was among the rst users ofon [37]. Nevertheless, the challenges in the food indus-. Standards for foodproducts areveryhighandemergings, such as low fat products, low calories products, anditable for special diets require more and improved. Basedon itspotential to separatemonovalent andmul-s, and to separate organic solutes with different sizeother, nanoltrationcouldbe thepromise for the future.paration factors are often insufcient, which limits the

iew gives a systematic overview of reported imped-nanoltration, and possible solutions to solve theseolutions may be directly suggested from the literature,erived from a critical assessment of the state-of-the-ltration. Challenges that will be covered in this review

branesneed fmanygestedcan be

2. Me

Fouarationcompleat nanonegativtreatmloss, anfoulingtreatmfoulingperme

Fouorgani[45]. Ananodescribanalysstudytion ofFoulingtive inand adreectcoefcdipolebrane cto causpoundcomponecesslarge estatic f[55].

Depbrane pof partpenetrcolloidare thoresultsperme

Thefoulingto an ihave ecompa

Inoon theions, csurfacein theare calcica, altthermoinsufcient rejection in water treatment, and (6) theodelling and simulation tools. It must be stressed thatcations are already running regardless of these sug-ovements.Nevertheless, one shouldbeaware thatmoreif the limitations can be overcome.

ne fouling

is one of the main problems in any membrane sep-t for nanoltration it might be even somewhat morecause of the interactions leading to fouling take placee, and are therefore difcult to understand [3844]. Itsnsequences are obvious and include the need for pre-membrane cleaning, limited recoveries and feed waterort lifetimes of membranes. In that sense, membranelosely related to other problems such as concentratend membrane stability and lifetime: a total control ofld reduce the need for cleaning andwould enhance theield.playing a role for nanoltration membranes can be

utes, inorganic solutes, colloids, or biological solidstensive description of the consequences of fouling inon can be found in the literature [46], including indicesthe feed water fouling potential and the post factummembrane autopsy. Boussu et al. [4749] extended theembrane characteristics to prediction and interpreta-ing caused by organic solutes, colloids and surfactants.rganic solutes is thought to bemainly causedby adsorp-tions with the membrane material [5052]. Foulingtion can be related to component properties, which isthe correlation between the octanolwater partition

(logP) and adsorption; adsorption is also related to theent and thewater solubility [52]. Concerning themem-cteristics, thehydrophobicity of the top layer is believede most ux decline [53,54]. For charged organic com-ctrostatic attraction or repulsion forces between theand the membrane inuence the degree of fouling. Aondition for this is that themembrane surface charge ish; otherwise hydrophobic forces overcome the electro-resulting in more fouling of hydrophobic membranes

ng on the relative size of colloidal particles and mem-, colloidal foulingmay occur either due to accumulationon the membrane surface and build-up of a cake or bywithin themembrane pores [5659]. It is assumed thatling is related tomembrane roughness [60,61]: colloidsto be preferentially transported into the valleys, whichalley clogging. In addition, surface hydrophobicity andy also play a role [6264]., charge and concentration of the colloids also inuenceanoltration. An increase in colloid concentration leadsse in fouling [58,63,6567]; a larger colloid size maya negative [63] or a positive effect [56,68] on fouling inwith smaller colloids.c fouling is related to scaling, i.e., precipitation of saltsbrane surface [46]. Nanoltration membranes retaing an increase of the concentration at the membraneich may exceed the solubility limit at a certain pointtion module. The most common constituents of scalecarbonate, gypsum, barium/strontiumsulphate and sil-h other potential scalants exist [46]. Scaling is a purelyamic process involving a phase change, which requires

B. Van der Bruggen et al. / Separation and Purication Technology 63 (2008) 251263 253

a degree of supersaturation. In general, the point of saturation canbe estimated from the activities of the ions involved in the precipi-tation reaction; nevertheless, it is difcult to determine the criticalpoint of sup

Biofoulinand involveand (in somand involvethe membrtotally prevbiolms wefouling is nscaling andpenetrate inwill remain[69,70] thatstances (EPoccurs.

Classicaltreatmentpretreatmemembrane[7173]; otadsorptionof pretreatm

Cleaningarea on itseing is usuallmaybea siging (backuvibrations aconditions mcleaning, uptreatment creactions susion, chelatoften develotheir own mthat the cleaof the feedtures and prcleaning ma

Membrasolution toinsert hydroverall matto (organic)starting poiby graftingion beam irbranes [91]obtained m

Colloidawith lowerthe foulantto reduce cmembranemembranesurface aremsurface [92e.g., silver n

An intrinbe the concis the maxiwhen opera

ritical: 40 Cpressu.

-destticalsedma

d fromn: thfoulistainoces, theionalle isas ste crit

fci

rnationally, nanoltration has known a breakthrough sincet decade in areas related to water treatment and drink-ter production [97], where it is used for softening andl of pollutants (micropollutants such as pharmaceuticallycompounds, pesticides and other relatively small organic). Nanoltration can also be applied for more challengingtions, involving fractionation rather than purication. It isownthatnanoltrationmembranes canbeused for salt frac-on [98101] since the rejection of monovalent salts is lowerat of multivalent salts. An extreme case of charge-inducedtion is the observation of negative rejections of monovalentthe presence of multivalent ions or polyelectrolytes [102].

lly, the rejection of a divalent ion of the same charge as therane is above 95%, whereas the rejection of amonovalent ionsame charge can be anywhere between 20 and 80% [103].anoltration membranes allow ion fractionation, which iscant advantage and one of the reasons of the fast com-l growth of the process. Nevertheless, the separation factorsed with nanoltration are relatively modest, typically 510.pplicationsof ion separation canbe found [104107]. Awell-application is the separation of peptides based on charge

nces [108]. In the latter case, the solution pH is often the keyrol the desired separation.ersaturation.g is a general problemwithmanymembrane processess all biologically active organisms, mainly bacteriae cases) fungi [46]. Biofouling is a dynamic processs the formation and growth of a biolm attached toane. The biolm may reduce the water ux and evenent water passage. For nanoltration of wastewater, there found to have a thickness of 2030m [69]. Bio-ot a specic problem for nanoltration, in contrast toadsorption of small organic solutes: these may (partly)to the membrane, whereas bacteria are too large andin the supercial biolm. Nevertheless, it is suggestedthe formation and accumulation of exopolymeric sub-S) is the real cause of ux decline when biofouling

solutions to fouling are the optimization of pre-methods and cleaning of membranes. Suggestednt methods often make use of other pressure drivenseparations such as ultraltration and microltrationher options include ozonation or UV/H2O2 oxidation,(PAC) and occulation [74,75]. An extensive overviewent methods can be found in the literature [76].of nanoltration membranes has become a research

lf [7779]. Nevertheless, in practical applications clean-y considered in a very pragmatic way. Physical cleaningnicantpartof thecleaningprotocol and includesush-sh, forwardush, reverseush), scrubbing, air sparging,nd sonication [46,80,81].Membranedesignandprocessay help in this by increasing the efciency of physicalto the point where direct nanoltration without pre-

an be applied [82]. Chemical cleaning involves chemicalch as hydrolysis, saponication, solubilisation, disper-ion, and peptisation [83]. Membrane manufacturersp specic cleaning strategies and products suitable forembranes. However, it should be taken into accountning protocol should also dependon the characteristicssolution. This leads to a wide variety of cleaning mix-otocols in the literature [8486]. In addition, enzymaticy be considered [87].ne modication is potentially the most sustainableobtain fouling-resistant membranes [77]. The idea is toophilic groups into a polymeric structure, so that theerial becomes more hydrophilic and thus less pronefouling. Ultraltration membranes are often taken asnt; a hydrophilic nanoltration membrane is obtained[8890]. Nanoltration membranes can be modied byradiation in view of obtaining fouling-resistant mem-. However, it is not clear to what extent the newlyembranes are stable.l fouling may be reduced by developing membranessurface charge or surface charge similar to that of

. Increasing the hydrophilicity may also be benecialolloidal fouling. Surface roughness may also increasefouling by increasing the rate of attachment onto thesurface [77]; it is accepted thatmembraneswith a roughoreprone to fouling thanmembraneswith a smoother

]. Biological fouling can be reduced by the addition of,anoparticles in the membrane structure [93].sic solution to the problem of membrane fouling couldept of critical or sustainable ux [94]. The critical uxmal ux where fouling interactions remain reversible;ting below the critical ux, ux decline can be reversed

Fig. 1. Cperaturecircles:pressure

by nontheore(disperdensedevolvealisatiorate ofThe suand prthelessoperatexampsure wthat th

3. Insu

Intethe lasing waremovaactivesolutesapplicawell kntionatithan thseparaions inTypicamembof theThus, na signimerciaobtainManyaknowndiffereto contux for paper mill efuent for a at sheet membrane module (tem-, cross-ow velocity 2.7m/s). Open circles: pressure increase; blackre decrease; black squares: pressure decrease after the maximum

ructive measures. The critical ux concept has a soundbasis; it represents the shift from repulsive interactionmatter-polarised layer) to attractive interaction (con-tter-deposit) [94]. The concept of a sustainable ux

the critical ux theory and can be considered a gener-e sustainable ux is dened as the ux abovewhich theng is economically and environmentally unsustainable.able ux depends on hydrodynamics, feed conditionss time, and is therefore difcult to determine. Never-understanding of this principles leads to guidelines forconditions where fouling is minimal [95,96]. A typicalshown in Fig. 1 for paper mill efuent, where the pres-epwise increased and decreased; it can be clearly seenical ux in this case is around 50 l/m2 h.

ent separation


Fig. 2. Typicalwith a nanol

Forunchcharacteriseof molar mbetween ditypical sigmthe separat[110]. Theresize, both bEither the pfraction.

Fractionconsideredtration is pfollowed bythis way ara ner fractrange and b

In the pfractionatioactive com(which makhave to be sing reagentinsufcientsible to retallow a secpass complimpedimening nanolthe overallindividual cwhen produ[115], but isuse of contiwith recyclthermodynter separatof a mixturobtain simuA, and no resize (or anythe membrwith a nanFor gas sepied and areFor liquid sfrom somein reverse onanoltrati

chem6]).

proa

tionranesdustltration for food applications is the second largest after ultra-n [119]. Dairy applications were among the very rst where

ltration was used [120]. Using nanoltration, desalted lac-ntaining whey could be produced with a single process; ca.the salts in whey can be removed. By using dialtration, saltal can be even up to 90%. However, as previously stated, therenicant product loss to permeate (lactose, in this case) whenation is used. Another example is skim milk modication. Acontrol of the milk composition would open new possibil-the area of tailor-made milk products; however, in spite ofharp separation that can be achieved in one step, this seems

he rst and continuing success story of nanoltration [120].he sweetener industry, purication of xylose is an emerg-plication. This requires a challenging separation betweenand glucose, two compoundswith only a slight difference insigmoidal rejection curve obtained for rejection of uncharged solutestration membrane.

argedsolutes, however, (nanoltration)membranesared by a sigmoidal rejection curve (rejection as a functionass) [109], which results in an insufcient separationfferent compounds on the basis of molecular size. Aoidal rejection curve is given in Fig. 2. Furthermore,

ion depends on hydrophobicity and charge interactionsfore, the permeate contains molecules with variableelow and above the claimed pore size of themembrane.ermeate or the retentate is to be considered as a waste

ation using membranes (including nanoltration) isby many authors, but usually in the sense that nanol-receded by either ultraltration or microltration, orreverse osmosis [111114]. The fractions obtained in

e orders of magnitude different in molecular size, andionation (between solutes with size in the nanometerelow) is seldomly reported.harmaceutical industry, many possible applications ofn in the nanorange are to be found. Pharmaceuticallypounds and intermediates are often thermally labilees a distillative separation difcult or impossible), andeparated from smaller or larger side products, remain-s and the solvent. A singlemembrane separation is oftento obtain the desired separation because it is impos-ain one component completely and at the same timeond component, slightly different in size or charge, toetely. The incompleteness of the separation is a majort for awide application ofmembrane processes, includ-tration. A multiple membrane passage may improverejection but not the separation between differentompounds. Dialtration is sometimes a good solutionct recovery is considered, such as in solvent exchangenot applicable for separation of individual solutes. Thenuous counter current integrated membrane cascadese, in analogy with (conventional) separations based on

Fig. 3. Sfrom [11

step ap[118].

Fracmembfood inNanoltrationanotose co40% ofremovis a sigdialtrpreciseities inthe unsto be t

In ting apxyloseamic equilibrium (presented in Fig. 3), may allow bet-ions between individual compounds, or fractionatione. This should allow realising any separation, i.e., toltaneously a nearly complete rejection of componentmoval of component B, slightly different in molecularother parameter playing a role in transport through

ane). To this date the separation that can be attainedoltration cascade has not received much attention.arations with membranes, cascades have been stud-well known as integrated separation processes [116].eparations there is no knowledge on cascades, apartexploratory studies concerning module congurationssmosis [117], which is a more or less similar idea. Foron, cascades have not been considered before; a multi-

molecular sseparation ccult [121]; tbe recovereis impossibbe used moin combinacharides an

Nanolttion of natthat contaiand rebaudrebaudiosid(maximumatic representation of the principle of a membrane cascade (adapted

ch was recently suggested for purication of solvents

ation is also of importance in the food industry. Again,are the key for these separations: the share of the

ry is 2030% of the entire membrane market [119].ize and with similar properties such as, e.g., polarity. Aan be achieved, but it was shown to be extremely dif-heprocess is feasiblewhena single fraction (xylose) is tod with enhanced purity, but again, a sharp separationle using simple one-step solutions. Nanoltration canre easily for separation of oligosaccharides [122124]tion with ultraltration, or even the separation of sac-d salts (in dialtration mode) [125].ration cascades can possibly also be used for purica-ural sweeteners. Stevia rebaudiana Bertoni is a plantns very sweet steviol glycosides, of which steviosideioside A are the most abundant (Fig. 4). Stevioside ande A can be used as a natural sweetener in low doses200300mg/day), without a signicant caloric value. It


is also safeby consumvioside (750metabolic s

It is obviments. Durand sustainbe avoidedAfter extracarate the pr(sweetenertration, buteither a tooand1000.Nnal solutioupon which

4. Treatme

The genintrinsic proing nanoltan unwantedischargedproblem whor a fractiona separationtion of the cincreased cbrane. The cbalance for

Cr,i =(Qf

so that

CF = Cr,iCf,i

=

where REC(mg/l); thepermeate a

Additives such as anti-scalants (polyacrylates, polyacrylic acids,polyphosphates) also end up in the concentrate; the addition ofsulphuric acid or hydrochloric acid inuences the pH of the con-

e. Chofoutivelyals ssod

es, s.sibilifurthor indin grse ishere

he fop prcosts malutiolternm. Thas ain ato

les ine fouis uche

to rs wacludeicallyexamin thuper34].use

ineded fot higtrates a hFig. 4. Chemical structure of stevioside.

for phenylketonuria patients who might be in dangerption of large amounts of aspartame. High doses ste-1500mg/day) might be used in the treatment of theyndrome (hypertension, diabetes type 2) [126].ous that food additives have tomeet very strict require-ing the isolation of the sweeteners from Stevia, safetyable techniques are needed. The use of solvents shouldif possible. The sweeteners can be extractedwithwater.tion, purication of the crude extract is needed to sep-oducts with a molecular mass between 800 and 1000fraction). This could in principle be done with nanol-the separation that can be realised in thisway results inlow purity, or a large loss of the fraction between 800anoltrationcascadesareapossible solution to this. Then can be further concentrated by using reverse osmosis,the sweeteners can be crystallised.

nt of concentrates

eration of a concentrate (or retentate) stream is an

centratand bia (relachemicsuch asacrylatNaOCl)

Posreuse,directcharge

Reucases was in ttion sterelatedavourcient soas an aprobletreatedto obtaappliedexampare to btrationsecondtrationprocesbe conintrins

Anfoundthe rec[1311then beis obtabe reustion (aconcencontainblem for pressure drivenmembrane processes, includ-ration. For aqueous streams, the concentrate is oftend by-product of the purication process and has to beor further treated. This is per denition an unsolveden the feed solution contains unwanted compoundsthat cannot be reused, since membranes only achieveand not a destruction or transformation. The composi-oncentrate is similar to the feed composition, but withoncentrations for components rejected by the mem-oncentration factor CF can be calculated from themasscomponent i [29]:

Cf,i) (Qp Cp,i)Qr

QfQr

[1 (REC Cp,i

Cf,i

],

= recovery, Q=volumetric ow (l/h), C= concentrationsubscripts r, f, p and i refer to the retentate, the feed, thend the component used, respectively.

are almost nadvantage wchemicals tcan be reus

An integreuse in a tconcentrateis entirely[135]. Nanoment, andzero-dischaby a combinbut appearebrane proce[137], a consication. Aindustry is slem of freshthe pollutan

An integremoval ofemical cleaning for removal of scaling, organic foulingling from the membrane surface [79,127] results insmall) additional waste stream, containing cleaninguch as acids (phosphoric acid or citric acid), basesium hydroxide, complexing agents such as EDTA, poly-odium hexametaphosphate) and disinfectants (H2O2,

ties to treat or to discharge the concentrate [29] includeer treatment by removal of contaminants, incineration,irect discharge in surface water, direct or indirect dis-oundwater, and landlling.the most attractive option, but only applicable in fewthe concentrated fraction is the desired product, such

od industry. The beverage industry uses a concentra-ior distribution, allowing to reduce the volume and thes. Water is added at the point of usage; although somey be lost, this method is generally used as themost ef-n.Nanoltration is a cheapconcentrationmethod,usedative for reverse osmosis when salt permeation is not ae permeate is relatively pure water that can be used orwaste fraction. The concentrate is further dehydratedviscous liquid ready for distribution. Nanoltration isthis purpose in several applications [128,129]. Otherfood processing where the concentrate can be reusednd in the dairy industry, as already discussed. Nanol-sed for the recovery of organic nutrients in so-calledese whey [130]. The whey is processed by nanol-ecover a rich lactose fraction in the concentrate and ater with a high salt content in the permeate. It shouldd that the challenge of obtaining a good separation isinterconnected with the concentrate problem.ple of a closed cycle in wastewater treatment can bee tanning industry, where nanoltration is used foration of chromium from exhausted chromium bathsA combination of ultraltration and nanoltration cand to recycle the tanning baths a concentration of Cr (III)in the concentrate fraction; the concentrate can directlyr retaining baths or further concentrated by precipita-

h pH, addition of NaOH required) and dissolution (in ad sulphuric acid solution). The nanoltration permeateigh chloride concentration, because monovalent ionsot retained by the nanoltration membrane. This is anhen the permeate is reused in pickle baths (saving in

o be added). The permeate is then a side product thated as a rinsing water, or discharged.rated treatment system has been considered for waterextile company based on nanoltration, in which thegenerated from purication of exhausted dye baths

recycled by systematically separating all constituentsltration is applied after a classical wastewater treat-produces high-quality process water. The idea of arge system in the textile industrywas already suggestedation of chemical, biological andmembrane processes,d to be quite challenging [136]. A combination ofmem-sses was suggested for design of new productive cyclescept now adopted in the terminology of process inten-t present, the use of membrane processes in the textiletill limited to one-step designs, which solves the prob-water supply but not the waste (water) problem, sincet load is unchanged after concentration.rated approach should comprise two main steps [135]:the organic fraction (dyes, additives), and removal of


the inorganic fraction (salts). After a pretreatment using micro-ltration, the removal of the organic fraction can be done bynanoltration using a membrane with low salt rejection at a hightemperaturmeate fractfraction shoMembranetion from wthe feed. Tremainingenergy contup for the loThe nanolwhere saltsmembraneond nanolThe concendesalinationmembranebrane procrecuperatiocan be foun

If reuse obe necessarcan be distof specicment meththat has tosolidicatioing of conta (treated)charged in sin groundw

Concentspecial caseand surfaceof drinkingalmost excling extensivthe concentsive and tenanoltratibe dischargposal of thecases whertrate disposparametersscalant wer

Other fabe taken inand conditiproximity asurfacewatture; exibiexisting plathe concent

5. Membra

Membramembranesthe need fodemandingtion. These

membrane processes, and their impact is usually studied from apragmatic point of view, i.e., as a solution to specic ltrationproblems. This includes the choice of membranematerials, operat-

ditiond ovaqueon thranegoftntsd inacturcan besses coahe pots arerstobtaiis isg to

dherthe wible bhelesThegedeanicid, phe psaltsthe

themappcicoulants; thand

usuace.lutioot eof cspe

t besualan w46]. Cnto aMemlly wcanc natoncloute forinanpati

c solvoreembof ctivecrosside [e, close to the temperature of the dye bath. The per-ion contains a large fraction of inorganics; the organiculd be low. The concentrate ismainly organic in nature.distillation can be applied to separate the organic frac-ater, taking advantage of the elevated temperature ofhe distillate is recycled to the nishing process; theorganic fraction has an added value by utilizing itsent in an incineration process. The energy yield makesss of energy by losses in the different treatment steps.tration permeate feeds a second nanoltration unit,are retained using a relatively tight nanoltration

with high salt rejection. The permeate from the sec-tration unit is pure enough for reuse as process water.trate is a salt solution, comparable to the brine fromprocesses, and can be used for salt production in a

crystallizer [138]. The combination of all these mem-esses results in a zero-discharge system with energyn. A detailed description and calculation of this systemd in the literature [135].f the concentrate is not feasible, further treatment cany before discharge. Two options for further treatmentinguished: (a) further concentration, and (b) removalcomponents by a proper choice of a selective treat-od. The rst option leads to a sludge or solid wastebe reused (if possible), landlled (if necessary aftern/stabilisation or a similar pretreatment to avoid leach-aminants), or incinerated. The second option leads towastewater, that has to be reused (if possible) or dis-urface water (direct or indirect via sewage systems) orater.rates resulting from drinking water production are a. A distinction should be made between groundwaterwater. Nanoltration is not often used for productionwater from groundwater, because (a) groundwater isusively used when a source of good quality, not requir-e further (membrane) treatment, is available, and (b)rate that is generated is a large waste fraction, expen-chnically challenging to dispose of. For surface water,on is a valuable option when the concentrate can easilyed. A study in the Netherlands [139] revealed that dis-concentrate is a serious problem, especially in those

e no large surface water is present. In general, concen-al as such was feasible, as long as a limited number ofsuch as sulphate, chloride, phosphate, iron and anti-e under control.ctors than the volume and composition that have toto account are legal requirements such as allowancesons; cost of further treatment; local factors such as thend size of awastewater treatment plant, the presence ofer or open land, soil characteristics and geological struc-lity of thedisposalmethod in caseof anexpansionof thent; and public acceptance. Release ofmicropollutants torate was also mentioned as a risk [140].

ne lifetime and chemical resistance

ne lifetime and chemical resistance of nanoltrationis related to the occurrence of fouling (and therefore,r cleaning), and the application of nanoltration incircumstances such as in solvent resistant nanoltra-are well-known problems for nanoltration and other

ing conyield a

Forcantlymembcleanining ageresultemanuftocols[46] iscolloidfrom tfoulanis thepH isate; thallowinfrom aaboveis possNevertbrane.unchanAcid clcitric a(12) Titatedoutsideminescan befor speof bioffoulanozone)This istoleran

A sodoes nimpactfor theIt muster is uthe cletance [taken inised.especia(scale)specicases ction abthe cascontam

Comorganieven mtime. Mdegreealterna[150],polyimns, energy consumption, cleaning chemicals, permeateerall environmental impact.ous applications, membrane lifetime depends signi-e cleaning frequency and the overall strategy againstfouling. Applications where fouling requires frequenten face a fastermembranedeterioration, because clean-also damage the membrane to some extent. This hasvarious cleaning protocols proposed by membraneers, as explainedabove. Examples of these cleaningpro-e found in the literature [46,8487]. Alkaline cleaningntial for the removal of organic foulants, or inorganicted by organics from the surface of the membrane andres of the membrane. It was found that ca. 50% of allorganic in nature [141], therefore, alkaline cleaning

measure to be taken in general. In most cases a highned by using sodium hydroxide and sodium carbon-often combined with a anionic or nonionic surfactantemulsify fat containingparticles and toprevent foulantsing to the surface. Alkaline cleaning requires a pH oftenindow of chemical resistance of the membrane, whichy limiting the contact time with the cleaning solution.s, repetitionof alkaline cleaningmaydamage themem-outcome may be positive as well: increased uxes andrejections were observed after alkaline cleaning [142].ng [46] follows the same principles, using nitric acid,hosphonic acid or phosphonic acid to obtain a low pHurpose in this case is the removal of scale, since precip-are more soluble at low pH. Again, this requires goingapplicable pH window for a short time, which deter-embranes lifetime in the long run. Enzymatic cleaning

lied in more mild conditions, but can only be appliedfoulants (often polysaccharides as excretion productsts). Finally, biocides may be necessary to destroy bio-ese products are mainly based on oxidation (chlorine,therefore also attack the membrane to some extent.

lly indicated by themanufacturer as amaximal chlorine

n where membrane deterioration is completely absentxist. However, a good strategy should minimise theleaning by using well-chosen cleaning agents, tailoredcic application. This usually requires trial-and-error.pointed out that in this procedure, the only parame-ly the cleaning efciency by the water ux recovery,ater ux recovery or the change in membrane resis-onsiderations aboutmembrane lifetime are not usuallyccount, although the importance is known and recog-brane autopsy could help inmaking the optimal choice,hen biofouling is the problem [143]. Inorganic foulantsbe determined by ICP-MS [144]; determination of theure of organic foulants is difcult, although in manyusions can be made, especially when some informa-the possible foulants is available. This is, for example,determination of EPS deposits [145], NOM [146], tracets [147] and even organic deposits in general [148].bility of polymeric membranes with a wide range ofents for solvent resistant nanoltration is a new andchallenging issue in discussions about membrane life-ranes can be made more stable by, e.g., increasing therosslinking of the polymeric top layer [149], by usingmembranematerials such as poly(organophosphazene)linked poly(urethanes) [151], lled PDMS [152], and153], or by improving more common materials such


Table 1Solvent resistant nanoltration membranes and membrane characteristics as specied by the manufacturers

Membrane Manufacturer Material MWCO (Da) Tmax (C) L (l/hm2 bar) R (%)

N30F 95 1.01.8b 7090c

NF-PES-010 95 510b 3050c

MPF-44 40 1.3b 98e

MPF-50 40 1.0f g

Desal-5-DK 90 5.4b 98i

Desal-5-DL 90 9.0b 96i

SS-030505 90 1.0l >90m

SS-169 150 10l 95m

SS-01 150 10l 97n

HITK-1T g 5b g

StarMem-120 60 1.0q g

StarMem-122 60 1.0q g

StarMem228 60 0.26q g

a Nadir Filtrb Pure watec 4% lactosed Koch Meme 5% sucrosef Methanolg Not specih GE Osmoni MgSO4.j SolSep BV,k Covered byl Ethanol pe

m MW500n MW100o Hermsdorp Membraneq Toluene pe

as poly(acrnanoltraticommercia[156].

Solvent rin many apa careful anchoice andtion, deformof up to 17swelling focantly inuplaysa secothe degreeobserved thethanol or ption of alcoinperforma[158] involvswelling. Thpolymeric mit is not clelifetime. Nemembrane

Changesmeric memEven whenand solventPore sizescharacter ohydrophilicmembranes[159], threeble defects

chare toin orlookd toNadira PES 400Nadira PES 1000Kochd PDMS 250Kochd PDMS 700Osmonicsh PA 150300Osmonicsh PA 150300SolSepj k g

SolSepj k g

SolSepj k g

HITKo TiO2 g

METp PI 200METp PI 220METp PI 280

ation GmbH, Wiesbaden, Germany.r permeability.(MW 342).brane Systems, Wilmington, MA, USA.(MW 342).

permeability.ed.ics, Vista, CA, USA.

Apeldoorn, The Netherlands.secrecy and non-analysis agreement.

rmeability.in ethanol.0 in acetone.fer Institut fur Technische Keramik, Hermsdorf/Thuringen, Germany.Extraction Technology, London, UK.rmeability.

ylonitrile) [154]. An overview of solvent resistanton materials can be found in the literature [32,155];lly available membranes are summarised in Table 1

esistant nanoltration is successful even on large scale

and theexposubranesmightchangeplications [31]. Nevertheless, its success depends onalysis of the separation problem, a good membranesmall-scale testing. Recurrent problems are dissolu-ation or swelling of the membrane. Swelling values

0% have been reported [157]. It has been shown thatllows very complex mechanisms and may be signi-enced by pressure, which indicates that compactionndary role. Thepropertiesof the solvent itself determineof swelling for a given membrane. For example, it wasat for mixtures of xylene and heptane with methanol,ropanol, reduced swelling occurred as the concentra-

hol increased [157]. Other studies describe the changesnce as complex solventsolutemembrane interactionsing pore solvation (and solute solvation) rather thanese approaches may explain the dynamic behaviour ofembranes when applied in organic solvents; however,ar to what extent swelling shortens the membranesvertheless, it can be assumed that swelling may lead todeterioration in the long run.in the membrane performance or instability of poly-branes in organic solvents is not always visible [159].there was no apparent interaction between membrane(damage), membrane properties might have changed.

may have changed, or the hydrophobic (hydrophilic)f the membranes may have shifted towards a more(hydrophobic) one. An early study of nanoltrationassumed to be stable in at least some organic solventsout of four rst generation membranes showed visi-

after exposure to one or more organic solvents (Fig. 5),

total loss ofThe dev

applicationchanges inrior chemicmore expen(especiallyHowever, omercially a

Fig. 5. Effectn-hexane, ethnanoltrationracteristics of all fourmembranes changed notably afterthe solvents. Therefore, stability of polymeric mem-

ganic solvents is a very relative concept: themembraneunchanged, but membrane characteristics could havea certain extent, ranging from a slight difference to a

selectivity.elopment of ceramic nanoltration membranes fors inorganic solventsmay solve theproblemsof swelling,performance, and limited lifetime, due to their supe-al resistance. Ceramic membranes are substantiallysive, but this may be compensated by higher uxesat high temperatures) and the prolonged lifetime.nly few ceramic nanoltration membranes are com-vailable today, in spite of the good performance of

of exposure to a range of organic solvents (methylene chloride,yl acetate, ethanol, acetone) of a rst generation solvent resistantmembrane.


these membranes. Hydrophilic ceramic nanoltration membranesin asymmetric multilayer congurations have been successfullydeveloped since the late 1990s [160162]. These consist of an openporous support, mesoporous interlayers, and defectless microp-orous top layers made of (hydrophilic) alumina, zirconia or titania.Attempts have been made to make these ceramic membraneshybrophobic, so that ltration of non-polar solvents would be fea-sible. However, this is work in progress and needs to be furtherdeveloped.

6. Insufcient rejection for individual compounds

Being regarded as low pressure reverse osmosis membranesin the 1980s, nanoltration had the advantage of lower energyconsumption due to higher uxes, resulting from a more loosemembrane structure with more free volume in the polymer.Reduced cost for applicationswhere complete rejection of (mainly)ions was not necessary has paved the way to many applicationsof nanoltration. However, one of the new trends in water treat-ment is the demand for complete absence of all possible pollutants,evenatultra-lowconcentrations. Thismaybea subjective customercriterion not necessarily based on risks or toxicity, but it is a real-ity that has to be recognised. It is to some extent related to newadvances in analytical chemistry, allowing detection of pollutantsat concentrseen for susdards for n are based(methemoga partial relack of oxygnitrite. Nitrcan be obsof unwanteconsideredcan only pafore, revers(nearly) fulthe other htion with, e[163] so thshows nitratypical nano

Table 2Experimental

Membrane

NF90HG19SX10SV10SX01BQ01MX07NF70NF45UTC-20UTC-60MPS44NF70DesalESNA-1 LFNFNF90OPMN-KOPMN-P

depend on the experimental concentrations, it can be concludedthat all membranes with the exception of NF90 have moderatenitrate rejections. Nanoltration for lowering nitrate concentra-tions to somas softenincomplete nipriate proce

An eventant micronrange betwenvironmenacid, whichrejected evepH values btration basefor liquid wa wide speccoal and remAnother stuused high palthough thpared to recan only bemannitol a

ever, buttratiwoulle) porgafromclass; indnol-Aydrol tertoduccleat stuf drintratiA laramonf theova

nt froctionperimviduataination ofmptectedcatio6] ared focut-

phobed fohicandphenol, esompations in the range of ng/l, but the trend can also bepicious compounds such as nitrate. International stan-itrate ranging from 10mg/l (USEPA) to 50mg/l (EU)on possible health effects for infants under 6 monthslobinemia).Havinga stomachpHabove4 (which causesduction of nitrate to nitrite), they might suffer fromen in their blood due to reaction of hemoglobine withate itself does not pose any risk; no clear health effectserved [163]. Nonetheless, nitrates remain on the listd species in (drinking) water; removal techniques arein many studies. Since nitrate is a monovalent ion, itrtially be removed by nanoltration [164168]. There-e osmosis membranes are often preferred to ensure al removal of nitrate. This may be unnecessary, but onand, synergetic toxicity effects may occur in combina-.g., pesticides [170], and some effects may be unknownat the precaution principle can be defended. Table 2te rejection values from the literature obtained withltrationmembranes. Although these rejectionswould

nitrate rejections for typical nanoltration membranes

Nitrate rejection (%) Reference

9498 [169]9 [168]32 [168]28 [168]25 [168]12 [168]8 [168]76 [167]16 [167]32 [167]11 [167]9050 [166]9085 [166]6033 [166]7580 [165]6580 [164]8595 [164]2550 [164]4070 [164]

[171].Npoundconcentrains(possib

Forrangebroadmonesbisphematic h(methycare pralwaysa recenview oconcen[175].foundsome othe remdiffereof rejesive exof indiwas obcombinrejectioan attebe expclassi[174,17ters usweight(hydroexpectmass, weffectsphenylestradithese ce extent, in combination with another objective suchg or NOM removal, is certainly feasible, but if nearlytrate removal iswanted, nanoltration is not the appro-ss.more challenging pollutant is boron. Boron is an impor-utrient for plants, animals and humans, although theeen deciency and excess is narrow [171]. In aqueousts (i.e., neutral pH) boron is mainly present as boricis mostly undissociated and therefore only partiallyn by reverse osmosismembranes. In acid conditions (atetween 3 and 4.5), boron can be removed by nanol-d on charge interactions [172]. This was investigatedaste streams in coal-red power plant, which containtrum of trace elements, most of which originate in theain in theyashorbottomashwhen thecoal is burned.dy, for removal of boron fromchemical landll leachate,H values (around 11) and also obtained good results,e rejection for nanoltration was relatively low com-verse osmosis [173]. Under neutral conditions, boronremoved as a complex. Complexation of boron with

llows a good removal with nanoltration membranestheless, formost applications, boron isnot a target com-it is monitored as a species of interest with a very lowon by preference. A comparison of different treatmentd yield a disadvantage for nanoltration, if boron is aroblem.nic micropollutants, rejections with nanoltration may

high to low. Organic micropollutants are a veryof compounds, comprising natural and synthetic hor-ustrial pollutants such as phthalates, alkylphenols,, PCBs (polychlorinated biphenyls), PAHs (polyaro-carbons), NDMA (N-nitrosodimethylamine) and MTBEiarybutyl ether); pesticides; pharmaceuticals; personalts and disinfection by-products (DBPs) [174]. It is notr which are the most important compounds to look at;dy made an attempt to dene priority compounds inking water production, based on maximum observedons in surface water, toxicity and production volumesge variation of physico-chemical parameters can beg these various types of micropollutants, and becausese parameters have a signicant inuence on rejection,l of micropollutants from aqueous solution can be verym component to component.Modelling and predictions is still difcult (see also next section), so that exten-ental research has to be carried out to assess removall micropollutants. A qualitative appraisal of rejectionsed through a classication of compound/membranens [176]. A further semi-quantitative assessment of theorganic compounds in aqueous solutionwas derived into quantify the range of rejections that can reasonablybased on a limited number of parameters [174]. This

n can be used for practical conclusions. Both studiese based on up to ten classes of compounds. Parame-r this classication are molecular weight, molecularoff of the membrane, pKa (solute charge) and logKowicity). It was shown that the lowest rejection should ber uncharged hydrophobic compounds with low molarh can be explained by the absence of steric hindranceelectrostatic interactions. Examples are 2-naftol, 4-ol, estradiol, ibuprofen, uoranthene and bisphenol-A,trone, atrazine, simazine, diuron, and isoproturon. Forounds, a rejection decrease as a function of time, due


to adsorption in the membrane matrix, was observed, which maylead to misinterpretations: observed rejections may be overesti-mated when the time of measurement was not sufciently long[177].

Micropoidentied athe classicunknown, apore size, dmolar massa pragmaticwrong conchindrance, cther complihydrophilicrejections inplaying a rthe insightscontribute tlutants shoa better undbranes invoof model eqcan be usedpermeates.

7. Modellin

Modellinprises two atwo allowScaling upltration mpermeate yinto accoun

Differention of the porous nanon convectand the Jonfor aqueousmicroltratparameterseter. Nevertin nanoltrDarcys lawcal proportiis not expredone in othsize) are diftration.

Two aspfouling on muchworkFor surfactawas recommposed equaof individuaingwell-knthe feed comthese circumby using essures tomindiscussed in

In contrast with aqueous solutions, the HagenPoiseuille orJonsson and Boesen equation are insufcient to describe theperformance of solvent resistant nanoltration membranes. A

nce-ir thed [18

(c f1 anoltthefacee diffal anlopedescer dn anre foal.mern-difectivse is

ithn w

m)(

moa mid mr mes:

m

ic viol).evid

nt nabefospoed biffusi

P

xdd

to a

1 F Fpermsional poario[185e sizhanllutants from other classes, however, have also beens problematic compounds. It should be understood thatation is not absolute: membrane pore sizes are usuallynd when they are known, they represent an effectiveependent on the determination method. Furthermore,is not a good measure for the size of a solute; it isparameter because of its availability, but may yield

lusions. The (largelyunknown) interplaybetween stericharge repulsion andhydrophobic interactions are a fur-cation. A typical example is NDMA: this is a relativelysolute, but has a low molar mass, which leads to lownanoltration. Research into the removalmechanisms

ole in the nanoltration process can help to improveinto the removal of the organic pollutants, which mayo the development of better barriers, even if other pol-uld arise in the drinking water sources. This requireserstanding of interactions between solutes and mem-lved in transport through the membrane, developmentuations and their translation to a simulation tool thatto provide realistic predictions of concentrations in

g and simulation of nanoltration

g the performance of a nanoltration membrane com-spects: ux prediction and rejection prediction. Thesefull understanding of a lab-scale membrane module.to larger installations requires that changes along theodule are taken into account, i.e., the inuence of theield. This can be done by taking concentration increasest [178].t models have already been proposed for the descrip-ux through a (nanoltration) membrane. For relativelyoltration membranes, simple pore ow models basedive ow can be used. The HagenPoiseuille modelsson and Boesen model, which are commonly usedsystems permeating through porous media, such as

ion and ultraltration membranes, take no interactioninto account, and viscosity is the only solvent param-heless, these expressions are usually sufcient for useation, because they basically express the fundamental(uxproportional to pressure gradient)with an empiri-onality constant, the permeability. The latter parameterssed as a function of membrane characteristics, as it iser processes, because some parameters (porosity, porecult tomeasure or even doubtful as concept in nanol-

ects remain to bemodelled: the inuence ofmembraneux, and prediction of uxes for organic solvents. Nothas beendone onmodelling of fouling in nanoltration.nts, a correlation has been proposed [179], although itended not to replace experimental testing by the pro-

tion. A more general model, based on characteristicsl models, was proposed for aqueous solutions contain-ownorganic solutes [51]. Inmost applications, however,position is unknown, so thatmodels are not helpful. Instances, it may be better to use a pragmatic approach

timates of ux decline to be expected. Practical mea-imise ux decline andmembrane foulingwere alreadya previous section.

resistavent fobe use

J = [

wherethe naneter,cthe surand thmateriis devefor theA furthbinatiomeasumateri

Polysolutioof convthis ca[182].Wequatio

J (

V

ThisVm (asthe solsure fo[184] i

J V

wheredynam(m3/m

It isresistaneeded

Trandescribboth d

Js = L (

Jc = Ps

leading

R = (1

Thebydiffumaximux). Vcientthe porlarger tn-seriesmodel basedonconvective transport of the sol-permeation of pure solvents and solvent mixtures can0]:

P

l) + f1] + f2d f2 are solvent independent parameters characterisingration and ultraltration sublayers, a solvent param-critical surface tension of themembranematerial and ltensionof the solvent. Thismodel takes solvent viscosityerence in surface tension between the solid membraned the liquid solvent into account. However, this modeld for hydrophobic membranes, but seems inadequateription of uxes through hydrophilic membranes [181].isadvantage is that for each solventmembrane com-empirical parameter has to be determined as a

r the interaction between a solvent and the membrane

ic nanoltration membranes can also be described by afusionmechanism, possibly corrected for the inuencee transport [34]. A description of solvent transport innecessarily based on the solution-diffusion (SD) modelrespect to uxmodelling of organic solvents, a possibleould be [183]:

1nm

)

del is based on solvent viscosity, the molar volumeeasure for the molecular size), the surface tension ofembrane material and a sorption value (as a mea-mbranesolvent interactions). An alternative equation

is the difference in surface tension (mN/m), is thescosity (Pa s), and Vm is the solvent molar volume

ent that these models for describing uxes in solventnoltration have not yet converged;more experience isre a translation to a universal model can take place.

rt of solutes through nanoltration membranes can bey the equations of Spiegler and Kedem, which combineve and convective effects:

)

c

x+ (1 )Jsc

n expression for the rejection R:

)with F = exp

(1

PsJs

)

eability Ps is a measure of the transport of a molecule. The reection coefcient of a given component is thessible rejection for that component (at innite solventus models have been proposed for the reection coef-188]. If a lognormal distribution can be assumed fore, a molecule may permeate through every pore that isthe diameter of themolecule [188]. The reection curve


can then be expressed as:

= rc

0

1

Sp

1(

(ln(r) ln(r))2)

with rc =dcwhere Sp isdard deviatis a mean pfor 50%.

This relalready really applicasolvents arcomplex inleading to dsize. The repartitioningassumed tobranes towsorption daion rejectiobut these ctranslationtools to detions or toand in spitenomena, thuse Maxweto a predicttting paramso that furtlation tooltheir own sand ux dagurations,interesting[194]. Neveof new toolnanoltrati

8. Conclus

It is cleaof understareviewed dpropertiesrations thaseparationtion in indof these drally applicaknowledgeplayers. Pramembranethis, membposed; usincan be elaan enhanceplete remoprocess conOn the loning insightmodels intotion have p

would allow prediction of the process performance in all circum-stances.

In summary, it can be stated that further implementation ofltratih obrge s

wled

Funlly aemP

nces

Erikssog. 7 (Brandom drUrasesenic. Wayanes,J. Xia,mova007) 3Kosutinking7144.C. ShocessYoon,ptingembraZhangnolt(20)

J. JungAriji,embraCaussnoltXu, J.En of eembraD. Nghnoltn. Sci.M. Hrooqunew an meM. Ha, A.Fant ba7171.A.K. AationesalinaSemi65.Ventrr-Oise326Ventrale, J.. Vanpplies1188WA

Ps. htFrenzeid wa006) 7. Manuent2160Bellonedwa007) 32 r

exp 2S2p

dr

/2. This equation comprises two variables, Sp and r,the standard deviation of the distribution. This stan-

ion is a measure for the distribution of the pore sizes. rore size, namely the size of a molecule that is retained

atively simple case for uncharged solutes in waterects the difculties in developing a reliable and gener-blemodel for nanoltration.When rejections in organice considered, the problem is even larger, given theteractions between solutes, solvents and membranes,ifferences in solvation and therefore also in effectivejection of organic solutes in water is inuenced byeffects [189,190], and sorption in the membrane was

be one of the factors that govern the selectivity ofmem-ards small organic molecules. Therefore, quantitativeta are crucial for understanding this effect [191]. Forn in water, many models have been developed [192],alculations tend to become so complex that a simpleto a simulation tool is not straightforward.Model-basedsign membrane processes for new industrial applica-optimise existing membrane installations are needed,of all efforts in unravelling ux and separation phe-

ese tools are not yet available. An ambitious attempt tollStefan equations and all current knowledge to comeivemodel led to the conclusion that much depended oneters and not on physically relevant parameters [193],

her research is needed if a generally applicable simu-is envisaged. Membrane manufacturers often developimulation tool, but this is based on empirical rejectionta and cannot be applied for other than standard con-membranes or solutions. For electrolytes in water, anapproach to simulation has been the NANOFLUX toolrtheless, further extension of this tool or developments will be the major key to industrial implementation ofon.

ions

r that nanoltration still has to grow more in termsnding, materials, and process control. Regardless ofrawbacks, NF is widely used in industry and specialof the NF membranes make possible novel sepa-t are difcult or expensive to achieve with othermethods. Furthermore, the potential of nanoltra-ustrial application is still underdeveloped becauseawbacks. In anticipation of new insights and gener-ble solutions, using nanoltration with the currentwill offer a considerable lead to more conservativectical problems requiring a pragmatic solution arefouling and the need for cleaning (and, related torane lifetime). A number of options have been pro-g the expertise of a membranologist, feasible solutionsborated for many applications. Applications whered separation between solutes is required, or a com-val of contaminants, can be solved by using novelgurations or by selecting an adequate membrane.g run, process control can be obtained by translat-s in transport to models, and then translating thesesimulation tools. A number of attempts in this direc-

roven to be successful, and further extension of these

nanoresearcthat la

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[9] Y.na40

[10] Y.A.m

[11] C.na

[12] P.tiom

[13] L.naro

[14] A.FaAtio

[15] A.Sopl15

[16] MtrD

[17] R.54

[18] C.su26

[19] C.sc

[20] J.Csu18

[21] AWPC

[22] I.ac(2

[23] Mef15

[24] C.cl(2on will go hand in hand with the elaboration of thejectives mentioned. In that sense, it can be expectedteps forward will be made during the coming decade.

gements

d for Scientic Research Flanders (FWO-Vlaanderen) iscknowledged for a travel grant (B. Van der Bruggen).ro is acknowledged for nancial support.

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[135] B.in26

[136] HfoTe

[137] E.tio

[138] E.40

[139] Mdi30

[140] L.os(2

[141] S.D

[142] Mnaan

[143] J.Sbi

[144] S.SuatH

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