Mechanism Involved in the Evolution of Physically Irreversible Fouling

Aug 29, 2007 - Effect of Hydration Forces on Protein Fouling of Ultrafiltration Membranes: The Role of Protein Charge, Hydrated Ion Species, and Membr...
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Environ. Sci. Technol. 2007, 41, 6789-6794

Mechanism Involved in the Evolution of Physically Irreversible Fouling in Microfiltration and Ultrafiltration Membranes Used for Drinking Water Treatment HIROSHI YAMAMURA, KATSUKI KIMURA,* AND YOSHIMASA WATANABE Division of Built Environment, Graduate School of Engineering, Hokkaido University, N13W8, Kita-ku, Sapporo 060-8628, Japan

Control of membrane fouling is important for more efficient use of membranes in water treatment. Control of physically irreversible fouling, which is defined as fouling that requires chemical cleaning to be cancelled, is particularly important for reduction of operation cost in a membrane process. In this study, a long-term filtration experiment using three different types of MF and UF membranes was carried out at an existing water purification plant, and the evolution of physically irreversible fouling was investigated. The experimental results demonstrated that the extent of physically irreversible fouling differed significantly depending on the membrane type. Cleaning of the fouled membranes with various chemical reagents demonstrated that organic matter was mainly responsible for physically irreversible fouling. Organic matter that had caused physically irreversible fouling in the long-term operation was desorbed from the fouled membranes and was subjected to Fourier transform infrared and 13C nuclear magnetic resonance analyses. These analyses revealed that carbohydrates were dominant in the membrane foulant regardless of the type of membrane. Based on measurements of molecular weight distribution of organic matter in the feedwater and the permeates from the membranes, a twostep fouling mechanism is proposed to explain the dominance of carbohydrates in the foulant: hydrophobic (humic-like) components with small molecular weight are first adsorbed on the membrane and, consequently, narrow the size of micro-pores of membranes, and then hydrophilic (carbohydrate-like) compounds with larger molecular weight plug the narrowed pores or the hydrophilic compounds are adsorbed onto the membrane surface conditioned by the hydrophobic components.

Introduction Application of membrane technology to drinking water treatment offers many advantages such as strict removal of pathogens (e.g., Cryptosporidium) (1). Many microfiltration (MF)/ultrafiltration (UF) membranes, which can be operated at a relatively low pressure, have been installed in water purification plants all over the world. However, a limitation * Corresponding author e mail: [email protected]. 10.1021/es0629054 CCC: $37.00 Published on Web 08/29/2007

 2007 American Chemical Society

to the use of such membranes in drinking water treatment is high-energy consumption, which is mainly due to deterioration in water permeability of membranes (i.e., membrane fouling). Much effort has been made to control membrane fouling (2, 3). Several physical membrane cleaning methods such as hydraulic backwashing have been developed and used routinely in many existing membrane plants to minimize membrane fouling. Despite routine physical membrane cleaning, membrane filtration resistance gradually increases over a long period of operation, indicating that membrane fouling cannot be completely controlled by physical cleaning. Fouling that cannot be controlled by physical cleaning is defined here as physically irreversible fouling (4, 5). Control of physically irreversible fouling is important for reduction of operation cost in a membrane process because this type of fouling develops even when a very efficient physical cleaning is carried out. Physically irreversible membrane fouling can only be canceled by chemical cleaning. However, chemical cleaning of the membrane should be limited to a minimum frequency because repeated chemical cleaning may shorten the membrane lifetime and disposal of spent chemical reagents poses another problem (6). In a number of previous studies on fouling of membranes used for water treatment, natural organic matter (NOM), composed of a variety of nonbiodegradable organic compounds including humic substances (7), has been shown to be the major constituent causing membrane fouling (8-11). However, it is still not clear which fraction of NOM causes membrane fouling. In early works, hydrophobic fractions of NOM, such as humic substances, were considered to be the major foulants (8). Hydrophobic interaction (8) and electrostatic interaction (9) were the explanations for the binding between hydrophobic NOM and membranes. More recently, hydrophilic NOM with features of carbohydrate or protein has been reported by several researchers to be the major foulant (4, 5, 10, 11). As explanations for the binding between hydrophilic NOM and membranes, van der Waals attraction (10), and hydrophobic interaction (11) between membranes and hydrophobic domains in hydrophilic NOM have been suggested. In addition to NOM, metals and metal-NOM complexes have been reported as the constituents affecting membrane fouling (12). Membrane fouling can be divided by physically reversible fouling, which can be easily cancelled by implementation of physical cleaning such as backwash, and physically irreversible fouling. Physically reversible fouling and physically irreversible fouling were not distinguished in many previous studies. In addition, many previous studies were based on short-term experiments which are not sufficient for observing physically irreversible fouling. As a result, knowledge of physically irreversible fouling occurring in membrane filtration in drinking water treatment is very limited, and further studies need to be carried out with special emphasis on physically irreversible fouling for more efficient use of membranes. In particular, investigation of the characteristics of components that cause physically irreversible fouling would be useful for the establishment of a new protocol of fouling control. In this study, three MF/UF membranes that had been fouled in long-term filtration of surface water used as a drinking water source were investigated in terms of the recovery of water permeability by chemical cleaning and the characteristics of the foulant causing physically irreversible fouling. Based on the results obtained from various analyses, a hypothesis regarding the evolution of physically irreversible fouling is proposed. VOL. 41, NO. 19, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Materials and Methods Pilot-Scale Membrane Filtration Unit. Three different hollow-fiber membranes were used in this study. Two of them were MF membranes and the other was a UF membrane. The two MF membranes had the same nominal pore size of 0.1 µm but were made from different polymers. One MF was made from polyethylene (PE; Mitsubishi Rayon, Tokyo, Japan) and the other was made from polyvinylidenefluoride (PVDF; Asahikasei Chemicals, Tokyo, Japan). The UF membrane had a molecular weight cutoff of 100 000 Da and was made from polyacrylonitrile (PAN; Toray Industries, Tokyo, Japan). Using these three different membranes, pilotscale membrane filtration tests were carried out in parallel from the beginning of October 2005. Filtrations using the two MF membranes were carried out for 50 days, whereas filtration with the UF membrane was terminated after 30 days of operation. The pilot-scale experiments were carried out at the Kami-Ebetsu water treatment plant (Ebetsu, Japan), which receives raw water from Chitose River (5). Effluent from the grit chamber of the plant was directly delivered to the membrane units without any additional treatments. Operating conditions of each membrane unit are summarized in Table S1 (Supporting Information). The PVDF and the PE membranes were submerged in separate tanks and were operated under vacuum. The PAN membrane was housed in a vessel and was operated under pressure. All membranes were operated in the outside-in flow pattern. The three membranes were operated with identical run cycles (filtration: 30 min; air scrubbing: 30 s; hydraulic backwashing: 60 s) at the same constant flux of 0.65 m3/m2/d. Hydraulic backwashing was not accompanied by the addition of chlorine. When membrane fouling became significant in the submerged MF membranes despite the implementation of periodical backwashing, membrane modules were taken out from the tanks and were cleaned by spraying pressurized water on the membrane surface. Chemical Cleaning of the Fouled Membranes. To investigate the features of constituents that were responsible for physically irreversible fouling, the foulant was desorbed from the fouled membranes at the termination of the operation and then analyzed. When the pilot operations were terminated, fouled membranes were taken out from the filtration units. The membrane fibers were immediately brought to the laboratory in a container filled with distilled water. First, each membrane fiber was manually wiped with a sponge and thoroughly rinsed with distilled water, which was carried out to minimize the influence of the accumulated cake causing physically reversible fouling in subsequent tests. By visual inspection, no accumulated cake was found on the membrane after wiping with a sponge. Using the wiped membranes, tiny membrane modules of 40 cm2 in membrane area were assembled and pure water permeability of the fouled membrane was measured by applying 30 kPa of pressure difference. Filtration was continued until a constant permeate flow rate was achieved (typically in 15 min). After measuring the pure water permeability, tiny membrane modules were soaked in various chemical solutions at 20 °C for 24 h. The chemical solutions used for cleaning were Milli-Q water, NaClO (700 ppm as free available chlorine), NaCl (0.1 M), NaOH (pH 12), HCl (pH 2), EDTA (20 mM), and oxalic acid (0.5%) (5). Recoveries in pure water permeability by the chemical cleaning were evaluated, and the chemical solutions containing the foulant desorbed from the membranes were analyzed. Membrane specimens that were not used for assembling the tiny membrane modules were divided into two portions and were soaked in a solution of sodium hydroxide at pH 12 or hydrochloric acid at pH 2. Because a large amount of membrane specimens was available in this study, this process enabled extraction of a sufficient amount of organic matter for advanced analysis (e.g., FTIR and NMR). 6790

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Analytical Methods. Concentrations of total organic carbon (TOC) and dissolved organic carbon (DOC) were determined by a TOC analyzer (TOC-5000, Shimadzu, Kyoto, Japan). UV absorbance was measured by a spectrophotometer (U-2000, Hitachi, Tokyo, Japan). Before UV and DOC measurements, the samples were filtered with 0.45-µm PTFE membranes. Concentrations of metals were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (ICPS-7500, Shimadzu, Kyoto, Japan). Molecular weight distribution of DOC was determined by HPLC-size exclusion chromatography (HPLC-SEC) with UV/TOC detectors (13). Two columns, TSK-gel G3000SW (Tosoh, Tokyo, Japan) and TOYOPEARL HW-65S (Tosoh, Tokyo, Japan), connected in series were used to cover a wide molecular weight range of 107-100 Da. The mobile phase consisted of phosphate buffer (pH 6.8) adjusted to an ionic strength of 0.1 M with sodium sulfate. The HPLC system was operated at a flow rate of 1.0 mL/min and the injection volume of sample was 500 µL. FTIR spectroscopy and 13C NMR spectroscopy were used for investigation of the characteristics of foulant desorbed from the membranes in this study. These techniques have been used in many studies for investigations of the characteristics of isolated NOM (7, 14) but have rarely been used for characterization of a membrane foulant (15). This is probably because small-scale membrane units were used in most previous studies and, consequently, it was impossible to obtain a sufficient amount of organic foulant for conducting FTIR/NMR analyses. Because pilot-scale apparatuses were used in this study, a sufficient amount of organic matter for advanced analysis could be obtained. For FTIR analysis, KBr pellets containing 0.25% of the sample were prepared and examined in an FTIR spectrophotometer (FT/IR-350, Jasco, Tokyo, Japan) at a resolution of 4 cm-1. Solid-state cross polarization magic angle spinning carbon13 (CPMAS 13C) NMR spectra of the membrane foulant were obtained with a Brucker MSL300 spectrometer at 75.47 MHz with a spin rate of 8 kHz and a pulse width of 4.5 µs for the 90° pulse. Contact time was set to 1 ms. Acquisition time and recycle delay were 30 ms and 4 s, respectively.

Results and Discussion Characteristics of the Raw Water and Membrane Permeate. The average quality of the feedwater and that of membrane permeates are shown in Table S2 (see the Supporting Information). In the feedwater, large portions of aluminum (78%) and iron (75%) were present as suspended solids (>0.45 µm), while manganese, calcium and organic matter were mainly present in dissolved forms. Aluminum and iron were effectively removed by the tested membranes due to the strict solid-liquid separation. On the other hand, removal of manganese, calcium and organic matter was not significant in any of the membranes. This implies that the sizes of manganese, calcium, and dissolved organic matter (DOC) were smaller than the pore sizes of the tested membranes. The UF membrane showed slightly higher rates of removal of DOC and UV absorbance than those of the two MF membranes, reflecting the difference between membrane pore sizes of the MF and UF membranes. However, the concentration of aluminum in the PAN membrane was slightly higher than the concentrations in the MF membranes. No reasonable explanation for this is available at present. Changes in Transmembrane Pressure. Figure 1 shows the changes in trans-membrane pressure (TMP) in the three membranes. The rates of increase in TMP in the three membranes were considerably different. As expected, the tightest membrane (PAN) showed the highest rate of increase in TMP. The rates of increase in the two MF membranes were different despite the fact that they had the same nominal pore size. This clearly indicates that the materials of the membrane have a substantial influence on the evolution of

FIGURE 1. Time course changes in trans-membrane pressure difference (TMP) adjusted to 20 °C equivalent value considering the change in water viscosity. membrane fouling (16). Interestingly, the results obtained in this study showing that the PE membrane was less fouled than the PVDF membrane are opposite to the results of a previous study focusing on membrane fouling in membrane bioreactors used for municipal wastewater treatment (16). This implies that characteristics of foulant in the case of drinking water treatment were different from those in the case of wastewater treatment. Further investigation is needed to understand the influence of membrane material on the rate of fouling. In all of the tested membranes, increase in TMP was not constant and rapid increases in TMP were seen several times. After the rapid increases in TMP, however, the value of TMP gradually declined due to the periodical backwashing except for the case of the PVDF membrane. On days 31 and 41, an additional physical cleaning (spraying pressurized water on the membrane surface) was needed to maintain the permeability of the PVDF membrane. This additional physical cleaning worked well and substantial reduction in TMP in the PVDF membrane was seen after cleaning. Chemical cleaning was not carried out at that time. Based on the observations mentioned above, it is assumed that the rapid increases in TMP shown in Figure 1 were caused by the accumulation of cake on the surfaces of the membranes. The three dashed lines shown in Figure 1 are assumed to represent the evolution of physically irreversible fouling in the three membranes, which accumulated and remained despite of the implementation of periodic backwashing and additional physical cleaning. As can be seen in Figure 1, the rates of occurrence of physically irreversible fouling in the three membranes were different. Membrane Cleaning with Various Chemical Cleaning Reagents. Figure 2 shows the degree of restoration of the fouled membranes in terms of pure water flux by chemical cleaning with various reagents. In this figure, the ratio of pure water flux after chemical cleaning (J1) to the flux before

chemical cleaning (J0) is used to express the degree of flux restoration. As described earlier, chemical cleaning was carried out after manually removing reversible cake that had accumulated on the membrane. Therefore, it can be considered that the restoration shown in Figure 2 represents removal of the foulant causing physically irreversible membrane fouling. Actually, manual sponge cleaning carried out prior to chemical cleaning had little effect on the permeability of the fouled membranes, indicating that fouling seen at the termination of the long-term operation could be attributed mainly to physically irreversible fouling. As can be seen in Figure 2, in the case of the PVDF and PAN membranes, NaCl (0.1 M) and EDTA (20 mM) were not effective in mitigation of physically irreversible fouling in this study. Figure 2 also shows that alkaline solution (NaOH) was more efficient than acid solutions (oxalic acid and HCl) for recovery of permeability of the PVDF and PAN membranes. The oxidizing agent (NaClO) exhibited the best cleaning performance in recovery of permeability of the PVDF and PAN membranes. This implies that organic matter was mainly responsible for the evolution of physically irreversible membrane fouling in the PVDF and PAN membranes. In contrast, in the case of the PE membrane, which exhibited the least membrane fouling in the continuous run (Figure 1), the degree of recovery of water permeability following treatment with acid, alkaline, and oxidizing reagents were comparable. This suggests that the contribution of metals to the physically irreversible fouling in the PE membrane was significant (17). Analysis of Foulant Desorbed from the Fouled Membranes. Desorption of membrane foulant was carried out at the termination of the pilot operation. As stated above, to ensure that physically reversible cake was removed from the membrane surface, each membrane fiber was carefully wiped with a sponge prior to desorption tests. Table S3 (Supporting Information) summarizes the results of the principle analyses carried out for the foulant desorbed from the fouled membranes by HCl (pH 2) and NaOH (pH 12) solutions. The value of SUVA was not determined for the constituents extracted by HCl solution because iron desorbed with acid would interfere with the UV absorbance. Although both aluminum and iron in the raw water were effectively removed by the membranes tested (Table S2 in the Supporting Information), only iron was desorbed from the fouled membranes in a significant amount. This suggests that aluminum in the feedwater was rejected or deposited on the membrane surface and subsequently removed by periodic backwashing. In contrast, iron was likely to cause the physically irreversible fouling to some extent. Despite the relatively high concentrations of calcium in the feedwater (Table S2), the amount of calcium desorbed from the fouled membrane was not significant. This might indicate that calcium was less important in the evolution of physically irreversible fouling, although several researchers (8, 9) have claimed that calcium played an important role in the evolution of membrane fouling when it was present with NOM. Rather, calcium seems to be more responsible for the

FIGURE 2. Effect of chemical membrane cleaning (J0: pure water flux before chemical cleaning, J1: pure water flux after chemical cleaning). VOL. 41, NO. 19, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Infrared spectra of membrane foulant desorbed with HCl (pH 2) solution. evolution of reversible fouling (18). However, in the case of PAN membrane, the amount of calcium desorbed from the fouled membrane was comparable to that of iron. This probably indicates that the humic fraction of NOM was involved to a greater extent in the fouling of the PAN membrane, as discussed later. Although manganese has also been pointed out as a foulant causing physically irreversible fouling (5), amounts of manganese desorbed from the fouled membranes were very small. In the cleaning with HCl solution, not only metals but also organic matter were desorbed from the fouled membranes, particularly from the PVDF membrane. Figure 3 shows the FTIR spectra of the foulant desorbed from the fouled membranes by HCl solution. Interestingly, there were significant similarities among the three spectra. All of the spectra had a dominant peak near 1080 cm-1, which is an indication of their carbohydrate character (14, 19). Therefore, the carbohydrate-like organic matter was thought to be the main constituent in the foulant desorbed with HCl solution regardless of membrane type. In a study by KabschKorbutowicz et al. (20), it was shown that a large portion of organic matter desorbed from the fouled membrane by acid or chelate agents formed complexes with metals. Similarly, in the present study, the carbohydrate-like organic matter and metals (mainly iron) desorbed with HCl solution were assumed to form complexes and cause physically irreversible fouling. It has been reported that carbohydrate can form a complex with iron (21). As previously mentioned, NaOH solution restored the membrane permeability to a larger extent and desorbed a larger amount of organic matter from the fouled membranes than did HCl solution. Therefore, analysis of the foulant desorbed from the membrane with NaOH solution would be more useful in understanding the fouling, compared to the case of HCl solution. The value of SUVA is considered to be a surrogate measurement of aromacity of organic matter, and a high SUVA value corresponds to organic matter consisting of a large amount of double-bond or aromatic structures (22). The values of SUVA determined for the foulant desorbed by NaOH solution (shown in Table S3) were much lower than those for the feedwater on average (Table S2). This implies that a relatively hydrophilic fraction of the organic matter in the feedwater was responsible for the physically irreversible fouling. Interestingly, the value of SUVA determined for the foulant were similar among the foulants desorbed from the three membranes. This indicates that the characteristics of the foulants desorbed from the three membranes might be similar, but this turned out to be false as discussed later. FTIR spectra of the foulants desorbed with NaOH solution from the three membranes are presented in Figure 4. There were significant similarities in the spectra obtained for the three membranes. In these spectra, peaks near 1660 and 1540 cm-1 were significant. They are assigned to amido-I and -II bands, respectively (14, 19). In all spectra, a broad peak near 1080 cm-1 was seen. This peak is an indicator of 6792

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FIGURE 4. Infrared spectra of membrane foulant desorbed with NaOH (pH 12) solution.

FIGURE 5. CPMAS 13C NMR spectra of membrane foulants desorbed with NaOH (pH 12) solution. carbohydrate character (14, 19). IR spectra shown in Figure 4 are not similar to those of humic substances (23). This suggests that humic substances were relatively minor components in the foulant responsible for the physically irreversible fouling. CPMAS 13C NMR spectra of the foulants desorbed with NaOH solution from the membranes are presented in Figure 5. A general similarity among the foulants desorbed from the three membranes was found in NMR analysis as well. Although a proteinaceous nature of the foulants form the membranes can be seen by peaks near 175 and 55 ppm (7, 14), carbohydrate (peak at 75 ppm) (7, 14) was dominant in the foulant regardless of the membrane type. The aromatic carbon signal (110-165 ppm) was minor in the spectra for the two MF membranes (PVDF and PE) but was pronounced in the spectrum for the PAN membrane. This indicates that the contribution of the humic fraction of NOM to the evolution of physically irreversible fouling was more significant in the PAN membrane than in the two MF membranes. The humic fraction would be smaller than the carbohydrate, as will be shown later. Thus it is reasonable to assume that the contribution of the small humic fraction would become more significant in a UF membrane (PAN in this case) than in a MF membrane (PVDF and PE in this case). The amount of calcium desorbed with NaOH solution was significant in the case of the PAN membrane (see Table S3 in the Supporting Information). This calcium might have formed a complex with humic substance as suggested by several researchers (8, 9). Nevertheless, carbohydrate was dominant in the foulant desorbed from the PAN membrane as well, as shown in Figure 5. Mechanism Involved in the Evolution of Physically Irreversible Fouling During Continuous Operation. As shown above, both FTIR and NMR analyses demonstrated that the carbohydrate was a dominant component causing physically irreversible fouling regardless of the type of

FIGURE 6. Molecular size distribution of dissolved organic matter in the Chitose river surface water. membrane. Carbohydrate has, however, a hydrophilic nature, and hydrophobic interaction between the membranes and carbohydrate is, therefore, not a reasonable explanation for the participation of the carbohydrate in physically irreversible fouling. To elucidate the fouling mechanisms involved in the continuous operation, changes in rejection rate of both humic acid and carbohydrate in the operation were investigated using HPLC-SEC with UV/DOC detectors. Figure 6 shows the representative molecular weight distribution of organic matter contained in the feedwater used in this study. As can be seen in Figure 6, organic matter contained in the feedwater could be roughly divided into two fractions: large molecules with a hydrophilic nature (little UV absorbance) and small molecules with a hydrophobic nature (high UV absorbance). A similar molecular weight distribution of organic matter was found in previous studies (13, 24). It is thought that large molecules mainly consisted of carbohydrate, whereas small molecules mainly consisted of humic acid (13, 24). Figure 7 shows changes in the removal of the large and small molecules by the three membranes determined by HPLC-SEC with UV/DOC detectors. In the case of the PVDF membrane, about 15% of the fraction of smaller organic molecules mainly composed of humic substances was initially removed. As the operation period became longer, however, the rate of removal of the small organic molecules declined and eventually no removal of small molecules was achieved by the PVDF membrane. The size of the small molecules should be considerably smaller than the nominal pore size of the PVDF membrane (0.1 µm), and therefore the sieving effect was discounted as an explanation for the initial removal of small organic molecules by the PVDF membrane. Rather, the initial removal of the small molecules can be attributed to adsorption on/in the PVDF membrane. In contrast to the small molecules, the rate of removal of the large organic molecules by the PVDF membrane gradually increased during the operation. When the removal of the small molecules declined to a negligible level, the removal of large organic molecules increased by almost 100%. A similar trend was also seen for the other two membranes. Based on these observations, the following hypothesis regarding the evolution of physically irreversible fouling is presented. First, small molecules mainly composed of humic substances are adsorbed on/in membranes by hydrophobic interaction. As a result of adsorption of the small molecules, the sizes of membrane pores decrease and it becomes possible for large molecules mainly composed of carbohydrates to plug the pores and cause physically irreversible fouling. Also, adsorbed humic substances could work as “glue” for carbohydrates and facilitate the capture of carbohydrates on/in membranes. The examined PVDF was assumed to be more hydrophobic than the PE membrane because hydrophilic modification was provided for the PE membrane by the manufacture. It

FIGURE 7. Changes in removal rate of large molecules (carbohydrate) and small molecules (humic acid). is likely that the hydrophobic PVDF membrane adsorbed humic substances more rapidly than did the hydrophilic PE membrane. As a result, the PVDF membrane should achieve complete rejection of carbohydrates earlier than the PE membrane (Figure 7). In the discussion above, it is assumed that foulant causing physically irreversible fouling originated from the feedwater. Another possible origin of the foulant might be biofilms that cannot be removed by backwashing. It was reported that both carbohydrate and humics were excreted by microorganisms (25). Although the possibility that excretion from biofilms was the main source of the foulant cannot be completely eliminated, it would be discounted by the following reasons: (i) evolution of reversible fouling (indication of biofilm formation) did not always dominate in the operation of the membranes as shown in Figure 1, (ii) occasional increases in physically reversible fouling shown in Figure 1 could be explained by increases in turbidity in the feed (data not shown), and (iii) water temperature was low (i.e., 5-10 °C) in the operation. To deal with the issues discussed above more precisely, establishment of the methods that can distinguish the origin of organic matter is indispensable. Table S4 (Supporting Information) shows the zeta potentials of the membranes before and after the long-term operation. The decrease in rejection of small molecules during the operation might be attributable to a decrease in favorable electrostatic interaction (repulsion) since the zeta potential of the tested membranes became slightly less negative after operation as a consequence of carbohydrate deposition. In this study, it was assumed that the decrease in favorable electrostatic interaction was not the principle reason for the decrease in rejection of small molecules both because of the VOL. 41, NO. 19, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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initial zeta potential that was close to neutral and because of the small changes in the zeta potentials after use. However, further investigation is needed to determine the influence of surface conditions of membranes on binding of NOM to membranes.

Supporting Information Available Table S1 (operating conditions of the membrane units), Table S2 (average quality of the feedwater and permeates), Table S3 (concentrations of organic and inorganic substances desorbed with HCl (pH 2) and NaOH (pH 12) solutions). This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review December 7, 2006. Revised manuscript received June 11, 2007. Accepted July 13, 2007. ES0629054