Understanding Membrane Fouling: a Review of Over a Decade of Research

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Understanding membrane fouling: a review of over a Water Science and Technology: Supply decade 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 engineering community has been able to develop reliable low pressure membrane systems that are capable of producing high quality drinking water at a competitive price, making membrane technology an attractive solution to both upgrade 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 membranes lose their hydraulic performance as materials accumulate on their surfaces and/or within their pores, a process called membrane fouling. Although a significant effort has been devoted to elucidating the fouling Vol 3 No 5–6 pp 155–164 mechanisms of polymeric membranes by natural organic matter (NOM), no single model has yet been accepted. In fact, most of the existing literature is contradictory, showing that membrane fouling is far from being fully understood. This article reviews over a decade of Ondeo’s experience on characterizing and preventing fouling of polymeric membranes by natural organic matter and inorganic compounds. The review focuses 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 minimize membrane fouling is also discussed. Keywords Fouling; membrane; microfiltration; natural organic matter; pretreatment; ultrafiltration © 2003 IWA Publishing and the authors Introduction The introduction and rapid development of the membrane technology applications in drinking water production in the last decade represents a major milestone in the water industry. It represents a major step in treatment efficacy, greater than that which sand filters represented at the beginning of the 20th century. Since the first developments at the end of the 1980s, the water treatment engineering community has been able to develop reliable low pressure membrane systems, both microfiltration (MF) and ultrafiltration (UF), based on polymeric porous membrane systems that are capable of producing high quality drinking water at a competitive price, making membrane technology an attractive solution to both upgrade existing plants and design new ones. Competitive pricing means low capital and operating cost. Both capital and operating costs are inversely proportional to membrane permeate flux. Membrane engineers know that high fluxes translate into reduced membrane surface installed for a given plant capacity, and therefore reduced costs. But they also know that porous membranes lose their hydraulic performance as materials accumulate on their surfaces and/or within their pores, a process called membrane fouling. Cake formation, adsorption of small colloids and natural organic matter (NOM) and mineral precipitation have been shown to play an important role. A key issue for design and operating purposes, the impact of membrane fouling on flux 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 of polymeric membranes by NOM, no single model has yet been accepted. In fact, most of the existing literature is contradictory. While some authors propose particular size fractions of the NOM matrix as the species responsible for membrane fouling, others show that it is 155 hydrophilicity/hydrophobicity controlling the fouling extent, independent of size. Solution conditions, such as calcium concentrations and pH, may completely change the fouling extent. In addition, some membrane chemistries are more susceptible to being fouled than others. Furthermore, some pretreatments such as coagulation and/or adsorption on activated 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. And

J.-M. Laîné understanding how membranes foul is the key for developing effective fouling control strategies. It is important to highlight that this paper focuses exclusively on the fouling mechanisms et al. of ultrafiltration cellulosic membranes. While some mechanisms may be similar, the authors believe that differences might exist between the fouling mechanisms predominant in ultra- and microfiltration membranes.

Fouling theory Deﬁnition of the phenomenon From a hydraulic perspective, fouling is the loss of membrane flux, which is the volume of water that can be passed through a membrane surface unit per unit of pressure. Low pressure membrane systems (either MF or UF) are typically operated either at constant pressure or at constant flux. Figure 1 shows the behavior of transmembrane pressure as a function of filtration time at constant flux. Under these conditions fouling results in an increase of the transmembrane pressure over time to overcome the loss of membrane productivity. This increase in pressure is the evidence of the accumulation and/or adsorption of materials on the membrane surface, a phenomenon referred to as fouling. Reversibility of this phenomenon is characterized as based on the backwash efficacy to restore the flux. Thus, the fraction of 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 source waters, UF concentrates, UF permeates, and cakes accumulated at the membrane surface were 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 these characterization 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 demonstrated through various laboratory tests using model compounds. These tests included isotherm

156 Figure 1 Schematic of membrane fouling (adapted from Laîné et al., 1991) and kinetics tests of adsorption onto the membrane material, measurement of the adsorbed layer thickness (viscosimetric method), determination of adhesion forces (hydraulic test) and headloss build-up (percolation test). These tests were conducted using model molecules, including Aurin acid (MW = 678 daltons) to represent the PHAs, and Dextrans (MW of 2 × 106 and 104 dalton) to represent the PS. The adsorption of PHA and PS were tested on siliceous particle surfaces (model for natural particles) and on membrane surfaces

(Mallevialle et al., 1989a, 1989b, 1992; Mallevialle, 1993; Clark, 1992; Aptel and Nguyen J.-M. Laîné 1994).

Role of the NOM in the fouling mechanism et al. The differences in the NOM characteristics of the raw and the permeate water indicated that the 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 inorganic (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 the membrane surface, which confirmed the adsorptive component of fouling (Marsigny, 1990). These laboratory results were further confirmed by pilot- and full-scale experiments: waters with a high PHA concentration have an irreversible fouling potential higher than waters with a high PS concentration. This was observed for comparable total organic content waters, hence demonstrating that the characteristics of NOM had an strong impact on fouling. PHA and PS adsorption nature affects fouling differently. Thus, PHA was observed to have 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 cellulosic membranes. In addition, it was found that the adsorbable amount was higher for PHA molecules. PS-type molecules formed a thick non-compact adsorbed layer. Its thickness depended 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 by measuring 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 adhesion forces (Baudin et al., 1990, 1992; Baudin, 1991; Cabassud et al., 1992; Gourgues et al., 1990, 1991; Janex, 1994). PHA molecules were found to have the highest adhesion forces. PS and PHA percolation tests were performed using a liquid chromatography column containing silica particles in order to assess the resistance of the cake accumulated on membrane surfaces due to NOM adsorption. The pressure increase due to PHA adsorption was about 1%, and 10% to 25% for PS with MW 104 and 2 × 106, respectively. This cake resistance has an impact on the backwash efficiency in removing the cake accumulated onto the membrane surface. These results confirmed that more frequent backwashes using higher pressures 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 significant irreversible fouling under specific conditions of concentration, temperature, and pH. Results from tests performed on a lake water in Japan containing low NOM concentration 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 157 during winter (temperatures < 10°C), when turbidity and TOC were lower. The fouling of UF membranes was directly attributed to the formation of both silicium-rich ferric gel directly deposited on the membrane surface and a secondary amorphous alumino-silicate gel layer at a bigger distance. The deposit nature and the membrane/cake interactions were evaluated using infra-red, X-ray diffraction, Al and Si NMR and Exafs techniques. The authors concluded that the low permeability of the Fe-Si gel formed on the membrane sur-

J.-M. Laîné face was responsible for fouling. The ferric gel adhesion to the membrane could be related to the surface charge of the membrane. The electrokinetic charge of ferric oxide and alumino silicates were both negative at pH greater than 7. The electrokinetic potential of the par- et al. ticles and the UF membrane were also found to be negative. Ferric gels can also be formed by aggregates of small ferric clusters with small silicate colloids leading to the formation of gels with large fractal dimensions and a very dense and compact structure, which translates into a irreversible loss of permeability flux.

Biofouling and membrane polarisation The routine backwash of UF membranes usually includes the use of a sanitizer, generally chlorine. Under these backwash procedures, biofouling is not expected to occur and will not be further discussed in this paper. It should also be noted that gel polarization may not be 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 membrane 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 as foulants and some as a dynamic porous membrane not affecting the production performances of the membrane. For example PAC addition has been reported to enhance membrane performances. Several hypotheses have been proposed: scour of the adsorbed deposits, adsorption of foulant material on PAC and/or cake structure modification. Two pretreatments were evaluated on an Aquasource UF membrane: (1) coagulation with inorganic salts; and (2) adsorption on powdered activated carbon (PAC). In both cases, a cake accumulated onto the membrane surface, as illustrated in Figure 3. The deposit of PAC appeared to be porous and did not affect the membrane performances, even though the layer was fairly thick (Jacangelo et al., 1994). On the other hand, the homogeneous cake formed from direct coagulation rapidly fouled the UF membrane. It is interesting to note that traces of polymer such as coagulant aid, had always a negative impact on membrane hydraulic performances. Such compounds favor homogeneous,

Cake layer

Membrane

Membrane support material Alum pretreatment PAC pretreatment

Figure 2 Examples of scanning electron micrographs of cakes deposited onto membrane surface (adapted 158 from Jacangelo et al., 1994) thick, and sticky deposit onto the membrane surface, and cannot be easily removed by the usual backwash procedures.

Membrane material Membrane composition has been reported to play a significant role in its fouling (Laîné et al., 1989; Jacangelo et al., 1992; Meyer-Blumenroth et al., 2002). Significant differences can be observed in two different membrane materials operated under similar conditions on J.-M. Laîné a surface water, as illustrated in Figure 3 (Jacangelo et al., 1992). The hydrophilic membrane (cellulosic derivative) did not foul over 20-day operation with increasing operating pressure, whereas the more hydrophobic material membrane (acrylic polymer) experi- et al. 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 of organic 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 higher for polysulfone membranes than for cellulosic membranes. A more hydrophobic and less negatively charged membrane (polysulfone) has been shown to adsorb organics and prefer- entially PHA, compounds which as discussed above tend to irreversibly foul UF membranes (Baudin, 1991; Crozes et al., 1993; Jucker and Clark, 1994).

Membrane fouling prevention Prevention of inorganic precipitation and scaling Precipitation of iron, manganese, and carbonate has often been observed as the cause of membrane fouling, specially when an oxidant (i.e. chlorine) is applied during the backwash

Cellulosic derivative UF membrane

Acrylic polymer UF membrane

Figure 3 Flux and transmembrane pressure during testing of two hollow-ﬁber UF membranes with Boise river water – crossﬂow velocity 3 ft/s and backwash frequency 1/30 min (adapted from Jacangelo et al., 1992) 159 procedures. This type of fouling can be easily controlled by implementing proper treatment procedures. Dissolved iron and manganese may be present in both ground and surface waters. A pretreatment installed upstream from the membrane should be applied since ultrafiltration does not remove these compounds in the dissolved form. In the case of the Japanese lake water mentioned above, iron fouling was efficiently controlled by decreasing the iron con-

J.-M. Laîné centration in influent water by using an upstream pretreatment step to avoid gel formation on the membrane. For most surface waters, dissolved iron and manganese are mostly found in reservoirs, et al. during turnover. In such cases, monitoring reservoir water quality at various depths may be critical, and changing the pumping level may be sufficient to minimize the impact of dissolved iron and manganese. It is important to highlight that pretreatment of iron by aeration must require special attention. Special attention should be paid to the quality of the coagulant salt used for coagulation pretreatment upstream from ultrafiltration. Low-grade ferric chloride often contains significant levels of dissolved manganese which can potentially precipitate on the membrane surface. Therefore, dissolved manganese should always be checked when selecting a coagulant in such applications. In addition, the use of chlorine during backwash enhances manganese precipitation. Precipitation usually takes a while to be initiated due to the slow reaction with chlorine as opposed to chlorine dioxide. But once manganese starts to precipitate, usually on the permeate side, the reaction proceeds faster, and precipitation then occurs fairly quickly. Since the process may not be immediate, operators may not relate the membrane fouling to the manganese precipitation but to other causes, delaying the application of the proper corrective actions. Carbonate precipitation can occur for supersaturated water. Adjustment of pH usually solves this problem. This may be enhanced especially when using liquid sodium hypochlorite solution; pH of hypochlorite salts is usually high, and acid is added to lower the pH. It is also recommended to use de-ionized water when preparing sodium hypochlorite solution to minimize Ca concentration. It should also be noted that the use of air during backwash may

enhance precipitation of carbonate due to the CO2 stripping, limiting the efficiency of such backwashing. This will be more critical for outside-in types of configuration. Membrane fouling due to inorganic precipitation is easily recovered by acidic cleaning (usually citric acid).

Prevention of organic fouling The results obtained through ultrafiltration experiments and laboratory tests have taught us schematic mechanisms of fouling due to NOM, specially PHA and PS compounds. Pretreatment such as coagulation or adsorption on activated carbon is recommended to minimize the fouling due to high concentration of PHA. On the other hand, oxidation can be efficient for minimizing fouling due to PS by cutting the large molecular chain (Duguet et al., 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 the coagulant 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). 160 The use of powdered activated carbon has been shown to prevent fouling due to both the adsorption of organic compounds having a fouling potential and its impact in controlling the 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 a natural water: (1) turbidity for the particle content, (2) total organic carbon (TOC) for the total

NOM concentration, and (3) the UV absorbance at 254 nm to characterize the aromaticity of J.-M. Laîné the NOM as well as the PHA contents. In addition, the UV 254/ TOC ratio of a given raw water 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- et al. wash frequency) based on the fouling potential of a given source water. Data were collected in a wide variety of pilot- and full-scale plants treating surface and groundwater. Raw water quality variations (turbidity, Al, Fe, Si, Ca, TH, pH, temperature, TOC, UV, PHA and PS contents) were correlated with operating UF process parameters (flux, crossflow, backwash conditions, etc.). The selected discriminating parameters were turbidity, TOC, UV for 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 the Ondeo 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 influence of 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 networks approach has been used (Cabassud et al., 2001a; Vincent et al., 2000). The model predicts the production of a plant from both water quality and operating parameters, taking into account a minimum number of parameters. A preference is given to parameters that are easily and inexpensively measurable so that the model can be readily applied for simulation or control 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 model consists of two interconnected neural networks: one aimed at predicting the pressure at the end of the cycle, and a second one aimed at predicting the pressure at the beginning of the next cycle. These two neural networks are independently trained using a non-recurrent procedure, then connected and used recurrently. The input parameters required for the model predictions include the pressure at the end of the filtration cycle, some water quality parameters, the operating conditions during filtration time and the operating conditions during the backwashing 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 developments include the implementation of this neural network model into the control and automation of full-scale facilities.

Conclusions/perspectives The results obtained in the laboratory, together with those obtained in pilot- and full-scale experiences, allow us to propose a model to characterize the membrane fouling due to NOM, and specially PHA and PS compounds. Hydrophilic (like cellulosic derivative) membranes, are less susceptible to organic matter adsorption. They have been found to be more resistant to fouling. In all cases, addition of oxidant such as chlorine during backwash was necessary to prevent membrane from fouling. This practice also helps in not having biofouling occurring on the membrane surface. New membrane materials having more hydrophilic material are now being proposed for application in drinking water treatment to mitigate this effect. 161 Selection of pretreatment has an impact not only in the finished water quality in removing contaminants but also in enhancing membrane performances. Pretreatments capable of either 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 always found to enhance the UF membrane operation. Specific ultraviolet absorbance (SUVA), the ratio UV to TOC, was found to be a good

J.-M. Laîné indicator of fouling potential of a given source water. However, this parameter is not found sufficient to fully characterize nor predict fouling during membrane operation. Additional parameters are also of importance. Today, more sophisticated models such as neural net- et al. works are available for optimizing operating membrane conditions especially for highly variable 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 et al., 2001; Masselin et al., 2001). Recent studies have focused on crossflow velocity (Dean vortices) and various backwash procedures (pressure, air scouring). Although not discussed in this paper, these parameters are also critical for minimizing fouling.

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