Understanding membrane fouling: a review of over a decade ...

Understanding membrane fouling: a review of over adecade of research

J.-M. Laîné, C. Campos, I. Baudin and M.-L. Janex

Ondeo Services - CIRSEE, 38 rue du President Wilson, Le Pecq, France (E-mail : [email protected])

Abstract Since the first membrane applications at the end of the 1980s, the water treatment engineeringcommunity has been able to develop reliable low pressure membrane systems that are capable of producinghigh quality drinking water at a competitive price, making membrane technology an attractive solution to bothupgrade existing plants and design new ones. A competitive price means low capital and operating cost,which are inversely proportional to membrane hydraulic performance (permeate flux). Porous membraneslose their hydraulic performance as materials accumulate on their surfaces and/or within their pores, aprocess called membrane fouling. Although a significant effort has been devoted to elucidating the foulingmechanisms of polymeric membranes by natural organic matter (NOM), no single model has yet beenaccepted. In fact, most of the existing literature is contradictory, showing that membrane fouling is far frombeing fully understood. This article reviews over a decade of Ondeo’s experience on characterizing andpreventing fouling of polymeric membranes by natural organic matter and inorganic compounds. The reviewfocuses on the role of NOM size and hydrophobicity, of membrane chemistry, and of solution pretreatment(coagulation and/or adsorption). In addition, the efficacy of some currently used strategies to minimizemembrane fouling is also discussed.Keywords Fouling; membrane; microfiltration; natural organic matter; pretreatment; ultrafiltration

IntroductionThe introduction and rapid development of the membrane technology applications indrinking water production in the last decade represents a major milestone in the waterindustry. It represents a major step in treatment efficacy, greater than that which sand filtersrepresented at the beginning of the 20th century. Since the first developments at the end ofthe 1980s, the water treatment engineering community has been able to develop reliablelow pressure membrane systems, both microfiltration (MF) and ultrafiltration (UF), basedon polymeric porous membrane systems that are capable of producing high quality drink-ing water at a competitive price, making membrane technology an attractive solution toboth upgrade existing plants and design new ones.

Competitive pricing means low capital and operating cost. Both capital and operatingcosts are inversely proportional to membrane permeate flux. Membrane engineers knowthat high fluxes translate into reduced membrane surface installed for a given plantcapacity, and therefore reduced costs. But they also know that porous membranes lose theirhydraulic performance as materials accumulate on their surfaces and/or within their pores,a process called membrane fouling. Cake formation, adsorption of small colloids andnatural organic matter (NOM) and mineral precipitation have been shown to play an impor-tant role. A key issue for design and operating purposes, the impact of membrane fouling onflux reduction is nowadays assessed on a case-by-case basis through time and cost con-suming pilot tests.

Although a significant effort has been devoted to elucidating the fouling mechanisms ofpolymeric membranes by NOM, no single model has yet been accepted. In fact, most of theexisting literature is contradictory. While some authors propose particular size fractions ofthe NOM matrix as the species responsible for membrane fouling, others show that it is

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hydrophilicity/hydrophobicity controlling the fouling extent, independent of size. Solutionconditions, such as calcium concentrations and pH, may completely change the foulingextent. In addition, some membrane chemistries are more susceptible to being fouled thanothers. Furthermore, some pretreatments such as coagulation and/or adsorption onactivated carbon have been shown as beneficial in some cases and detrimental in others.All these observations show that membrane fouling is far from being fully understood. Andunderstanding how membranes foul is the key for developing effective fouling controlstrategies.

It is important to highlight that this paper focuses exclusively on the fouling mechanismsof ultrafiltration cellulosic membranes. While some mechanisms may be similar, theauthors believe that differences might exist between the fouling mechanisms predominantin ultra- and microfiltration membranes.

Fouling theoryDefinition of the phenomenon

From a hydraulic perspective, fouling is the loss of membrane flux, which is the volume ofwater that can be passed through a membrane surface unit per unit of pressure. Low pres-sure membrane systems (either MF or UF) are typically operated either at constant pressureor at constant flux. Figure 1 shows the behavior of transmembrane pressure as a function offiltration time at constant flux. Under these conditions fouling results in an increase of thetransmembrane pressure over time to overcome the loss of membrane productivity. Thisincrease in pressure is the evidence of the accumulation and/or adsorption of materials onthe membrane surface, a phenomenon referred to as fouling. Reversibility of this phenome-non is characterized as based on the backwash efficacy to restore the flux. Thus, the fractionof pressure that can be recovered using a backwash describes the reversible fouling.Irreversible fouling is then determined by the increase of pressure after a backwash.

Impact of NOM characteristics on fouling

Characterization of natural organic matter. In the earliest studies, samples of raw sourcewaters, UF concentrates, UF permeates, and cakes accumulated at the membrane surfacewere analyzed by pyrolysis, gas chromatography and mass spectrometry (Gadel et al.,1987; Mallevialle et al., 1987; Bersillon, 1989; Bruchet et al., 1995). The results of thesecharacterization studies proved the presence of polysaccharides (PS), polyhydroxyaro-matics (PHA), proteins and amino sugars in all types of natural waters.

The impact of the different NOM fractions on membrane fouling was demonstratedthrough various laboratory tests using model compounds. These tests included isotherm

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and kinetics tests of adsorption onto the membrane material, measurement of the adsorbedlayer thickness (viscosimetric method), determination of adhesion forces (hydraulic test)and headloss build-up (percolation test). These tests were conducted using model mole-cules, including Aurin acid (MW = 678 daltons) to represent the PHAs, and Dextrans (MWof 2 × 106 and 104 dalton) to represent the PS. The adsorption of PHA and PS were tested onsiliceous particle surfaces (model for natural particles) and on membrane surfaces(Mallevialle et al., 1989a, 1989b, 1992; Mallevialle, 1993; Clark, 1992; Aptel and Nguyen1994).

Role of the NOM in the fouling mechanism

The differences in the NOM characteristics of the raw and the permeate water indicated thatthe PHA and PS concentrated in the cake deposited onto the membrane surface. In addition,elemental analysis of the cake indicated that around half of the material deposited was inor-ganic (clays, carbonates and hydroxides). Furthermore, the results from scanning electron-ic microscopy confirmed that the fouling cake consisted of a deposit 30 to 50 microns thick,based on clay particles bounded by a NOM-based gel. NOM was thus concentrated onto themembrane surface, which confirmed the adsorptive component of fouling (Marsigny,1990). These laboratory results were further confirmed by pilot- and full-scale experi-ments: waters with a high PHA concentration have an irreversible fouling potential higherthan waters with a high PS concentration. This was observed for comparable total organiccontent waters, hence demonstrating that the characteristics of NOM had an strong impacton fouling.

PHA and PS adsorption nature affects fouling differently. Thus, PHA was observed tohave a quick, high and irreversible affinity for particles and cellulosic membrane surfaces,whereas PS had a rather slow and partially reversible affinity for particles and cellulosicmembranes. In addition, it was found that the adsorbable amount was higher for PHA mol-ecules. PS-type molecules formed a thick non-compact adsorbed layer. Its thicknessdepended on the MW of the PS molecules (ranging 4 to 16 nm). PHA molecules, however,tended to form a denser but thinner (< 0.5 nm) adsorbed layer.

The PHA and PS affinity for both particles and membrane material was confirmed bymeasuring particle adhesion forces. Nature of the adsorbed polymer, its molecular weight,the surface coverage rate and the pH seemed to play a major role on the PHA and PS adhe-sion forces (Baudin et al., 1990, 1992; Baudin, 1991; Cabassud et al., 1992; Gourgues etal., 1990, 1991; Janex, 1994). PHA molecules were found to have the highest adhesionforces.

PS and PHA percolation tests were performed using a liquid chromatography columncontaining silica particles in order to assess the resistance of the cake accumulated on mem-brane surfaces due to NOM adsorption. The pressure increase due to PHA adsorption wasabout 1%, and 10% to 25% for PS with MW 104 and 2 × 106, respectively. This cake resist-ance has an impact on the backwash efficiency in removing the cake accumulated onto themembrane surface. These results confirmed that more frequent backwashes using higherpressures are more efficient to remove a PS-type cake.

Impact of inorganic matter on fouling

Some inorganic compounds, such as aluminum, silica and iron, can be responsible for sig-nificant irreversible fouling under specific conditions of concentration, temperature, andpH.

Results from tests performed on a lake water in Japan containing low NOM concentra-tion and low turbidity have illustrated the fouling potential of certain inorganic compounds(Khatib et al., 1997). In these experiments, a major irreversible fouling was observed

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during winter (temperatures < 10°C), when turbidity and TOC were lower. The fouling ofUF membranes was directly attributed to the formation of both silicium-rich ferric geldirectly deposited on the membrane surface and a secondary amorphous alumino-silicategel layer at a bigger distance. The deposit nature and the membrane/cake interactions wereevaluated using infra-red, X-ray diffraction, Al and Si NMR and Exafs techniques. Theauthors concluded that the low permeability of the Fe-Si gel formed on the membrane sur-face was responsible for fouling. The ferric gel adhesion to the membrane could be relatedto the surface charge of the membrane. The electrokinetic charge of ferric oxide and alumi-no silicates were both negative at pH greater than 7. The electrokinetic potential of the par-ticles and the UF membrane were also found to be negative. Ferric gels can also be formedby aggregates of small ferric clusters with small silicate colloids leading to the formation ofgels with large fractal dimensions and a very dense and compact structure, which translatesinto a irreversible loss of permeability flux.

Biofouling and membrane polarisation

The routine backwash of UF membranes usually includes the use of a sanitizer, generallychlorine. Under these backwash procedures, biofouling is not expected to occur and willnot be further discussed in this paper. It should also be noted that gel polarization may notbe a limiting factor since little dissolved material is removed by the membrane.

Cake structure

Various pretreatments are currently used for either enhancing water quality and/or mem-brane performances. The impact of those treatments on membrane fouling can be drastical-ly different. As different deposits accumulate on the membrane surface, some may act asfoulants and some as a dynamic porous membrane not affecting the production perform-ances of the membrane. For example PAC addition has been reported to enhance membraneperformances. Several hypotheses have been proposed: scour of the adsorbed deposits,adsorption of foulant material on PAC and/or cake structure modification. Two pretreat-ments were evaluated on an Aquasource UF membrane: (1) coagulation with inorganicsalts; and (2) adsorption on powdered activated carbon (PAC). In both cases, a cake accu-mulated onto the membrane surface, as illustrated in Figure 3. The deposit of PAC appearedto be porous and did not affect the membrane performances, even though the layer was fair-ly thick (Jacangelo et al., 1994). On the other hand, the homogeneous cake formed fromdirect coagulation rapidly fouled the UF membrane.

It is interesting to note that traces of polymer such as coagulant aid, had always a nega-tive impact on membrane hydraulic performances. Such compounds favor homogeneous,

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Cake layer

Membranesupportmaterial

Membrane

Alum pretreatment PAC pretreatment

Figure 2 Examples of scanning electron micrographs of cakes deposited onto membrane surface (adaptedfrom Jacangelo et al., 1994)

thick, and sticky deposit onto the membrane surface, and cannot be easily removed by theusual backwash procedures.

Membrane material

Membrane composition has been reported to play a significant role in its fouling (Laîné etal., 1989; Jacangelo et al., 1992; Meyer-Blumenroth et al., 2002). Significant differencescan be observed in two different membrane materials operated under similar conditions ona surface water, as illustrated in Figure 3 (Jacangelo et al., 1992). The hydrophilic mem-brane (cellulosic derivative) did not foul over 20-day operation with increasing operatingpressure, whereas the more hydrophobic material membrane (acrylic polymer) experi-enced flux decline after a few days, even when operated under low operating pressure of 10 psi (0.7 bar).

Cellulose derivative membranes were found to be less susceptible to adsorption oforganic molecules onto the membrane surface, as confirmed by batch adsorption tests(Laîné et al., 1989). In addition, PHA and PS affinity has also been found to be much high-er for polysulfone membranes than for cellulosic membranes. A more hydrophobic and lessnegatively charged membrane (polysulfone) has been shown to adsorb organics and prefer-entially PHA, compounds which as discussed above tend to irreversibly foul UF mem-branes (Baudin, 1991; Crozes et al., 1993; Jucker and Clark, 1994).

Membrane fouling preventionPrevention of inorganic precipitation and scaling

Precipitation of iron, manganese, and carbonate has often been observed as the cause ofmembrane fouling, specially when an oxidant (i.e. chlorine) is applied during the backwash

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Cellulosic derivative UF membrane

Acrylic polymer UF membrane

Figure 3 Flux and transmembrane pressure during testing of two hollow-fiber UF membranes with Boiseriver water – crossflow velocity 3 ft/s and backwash frequency 1/30 min (adapted from Jacangelo et al.,1992)

procedures. This type of fouling can be easily controlled by implementing proper treatmentprocedures.

Dissolved iron and manganese may be present in both ground and surface waters. A pre-treatment installed upstream from the membrane should be applied since ultrafiltrationdoes not remove these compounds in the dissolved form. In the case of the Japanese lakewater mentioned above, iron fouling was efficiently controlled by decreasing the iron con-centration in influent water by using an upstream pretreatment step to avoid gel formationon the membrane.

For most surface waters, dissolved iron and manganese are mostly found in reservoirs,during turnover. In such cases, monitoring reservoir water quality at various depths may becritical, and changing the pumping level may be sufficient to minimize the impact of dis-solved iron and manganese. It is important to highlight that pretreatment of iron by aerationmust require special attention.

Special attention should be paid to the quality of the coagulant salt used for coagulationpretreatment upstream from ultrafiltration. Low-grade ferric chloride often contains signif-icant levels of dissolved manganese which can potentially precipitate on the membrane sur-face. Therefore, dissolved manganese should always be checked when selecting acoagulant in such applications. In addition, the use of chlorine during backwash enhancesmanganese precipitation. Precipitation usually takes a while to be initiated due to the slowreaction with chlorine as opposed to chlorine dioxide. But once manganese starts to precip-itate, usually on the permeate side, the reaction proceeds faster, and precipitation thenoccurs fairly quickly. Since the process may not be immediate, operators may not relate themembrane fouling to the manganese precipitation but to other causes, delaying the applica-tion of the proper corrective actions.

Carbonate precipitation can occur for supersaturated water. Adjustment of pH usuallysolves this problem. This may be enhanced especially when using liquid sodium hypochlo-rite solution; pH of hypochlorite salts is usually high, and acid is added to lower the pH. It isalso recommended to use de-ionized water when preparing sodium hypochlorite solution tominimize Ca concentration. It should also be noted that the use of air during backwash mayenhance precipitation of carbonate due to the CO2 stripping, limiting the efficiency of suchbackwashing. This will be more critical for outside-in types of configuration. Membranefouling due to inorganic precipitation is easily recovered by acidic cleaning (usually citricacid).

Prevention of organic fouling

The results obtained through ultrafiltration experiments and laboratory tests have taught usschematic mechanisms of fouling due to NOM, specially PHA and PS compounds.Pretreatment such as coagulation or adsorption on activated carbon is recommended tominimize the fouling due to high concentration of PHA. On the other hand, oxidation can beefficient for minimizing fouling due to PS by cutting the large molecular chain (Duguet etal., 1993; Baudin et al., 1995; Urbain and Manem, 1995).

Action on the cake structure

Cake structure can be modified in order to generate more porous deposits onto the membrane surface. For example, when using a coagulant pretreatment prior to UF, it is recommended to provide some flocculation time to form the particles before thecoagulant sees the membrane. This results in a porous material that enhances flux instead of causing fouling (Yuasa, 1997; Guigui et al., 2000, 2001; Durand-Bourlier et al., 2000).

The use of powdered activated carbon has been shown to prevent fouling due to both the

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adsorption of organic compounds having a fouling potential and its impact in controllingthe cake structure ( Khatib et al., 1995, 1996; Chang et al., 1996; Clark et al., 1996).

Strategy for fouling control

Several water quality parameters can be used to assess and predict the fouling potential of anatural water: (1) turbidity for the particle content, (2) total organic carbon (TOC) for the totalNOM concentration, and (3) the UV absorbance at 254 nm to characterize the aromaticity ofthe NOM as well as the PHA contents. In addition, the UV 254/ TOC ratio of a given rawwater has been found to be well correlated with membrane fouling (Anselme et al., 1993).

A statistical model was developed to predict UF performances (flux and optimal back-wash frequency) based on the fouling potential of a given source water. Data were collectedin a wide variety of pilot- and full-scale plants treating surface and groundwater. Rawwater quality variations (turbidity, Al, Fe, Si, Ca, TH, pH, temperature, TOC, UV, PHA andPS contents) were correlated with operating UF process parameters (flux, crossflow, back-wash conditions, etc.). The selected discriminating parameters were turbidity, TOC, UVfor the water quality, and flux and backwash for the UF performances. Water quality cate-gories with associated UF performances were then defined from this approach. This classi-fication has been used for the design and the optimization of the operating conditions of theOndeo ultrafiltration plants, a tool that is especially powerful for changing water sources(Mandra et al., 1992).

More sophisticated control strategies include the use of models describing the influenceof certain operating parameters. These models are aimed at conducting short- and/or long-term predictions of the plant behavior. Among these predicting tools, a neural networksapproach has been used (Cabassud et al., 2001a; Vincent et al., 2000). The model predictsthe production of a plant from both water quality and operating parameters, taking intoaccount a minimum number of parameters. A preference is given to parameters that are eas-ily and inexpensively measurable so that the model can be readily applied for simulation orcontrol in full-scale plants. Two different neural networks approaches have been investi-gated based on the number of inlet parameters and the network structure. The best modelconsists of two interconnected neural networks: one aimed at predicting the pressure at theend of the cycle, and a second one aimed at predicting the pressure at the beginning of thenext cycle. These two neural networks are independently trained using a non-recurrent pro-cedure, then connected and used recurrently. The input parameters required for the modelpredictions include the pressure at the end of the filtration cycle, some water quality param-eters, the operating conditions during filtration time and the operating conditions during thebackwashing procedure. So far this approach has been giving satisfactory results (devia-tions less than 20%), even in the case of hard fouling conditions. Future research develop-ments include the implementation of this neural network model into the control andautomation of full-scale facilities.

Conclusions/perspectives The results obtained in the laboratory, together with those obtained in pilot- and full-scaleexperiences, allow us to propose a model to characterize the membrane fouling due toNOM, and specially PHA and PS compounds. Hydrophilic (like cellulosic derivative)membranes, are less susceptible to organic matter adsorption. They have been found to bemore resistant to fouling. In all cases, addition of oxidant such as chlorine during backwashwas necessary to prevent membrane from fouling. This practice also helps in not havingbiofouling occurring on the membrane surface. New membrane materials having morehydrophilic material are now being proposed for application in drinking water treatment tomitigate this effect.

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Selection of pretreatment has an impact not only in the finished water quality in remov-ing contaminants but also in enhancing membrane performances. Pretreatments capable ofeither modifying cake structure and/or removing key foulants (such as PHA fractions)should be evaluated. In our operations, the use of powdered activated carbon was alwaysfound to enhance the UF membrane operation.

Specific ultraviolet absorbance (SUVA), the ratio UV to TOC, was found to be a goodindicator of fouling potential of a given source water. However, this parameter is not foundsufficient to fully characterize nor predict fouling during membrane operation. Additionalparameters are also of importance. Today, more sophisticated models such as neural net-works are available for optimizing operating membrane conditions especially for highlyvariable water quality source waters.

The continuous optimization of the operating procedures is the only way to win the bat-tle against fouling (Aptel et al., 1998; Serra et al., 1999; Cabassud et al., 2001b; Guigui etal., 2001; Masselin et al., 2001). Recent studies have focused on crossflow velocity (Deanvortices) and various backwash procedures (pressure, air scouring). Although not dis-cussed in this paper, these parameters are also critical for minimizing fouling.

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