Environ. Sci. Technol. 2005, 39, 6293-6299
Membrane Fouling in Pilot-Scale Membrane Bioreactors (MBRs) Treating Municipal Wastewater KATSUKI KIMURA,* NOBUHIRO YAMATO, HIROSHI YAMAMURA, AND YOSHIMASA WATANABE Department of Urban and Environmental Engineering, Hokkaido University, N13W8, Kita-ku, Sapporo 060-8628, Japan
The main obstacle for wider use of membrane bioreactors (MBRs) for wastewater treatment is membrane fouling (i.e., deterioration of membrane permeability), which increases operating costs. For more efficient control of membrane fouling in MBRs, an understanding of the mechanisms of membrane fouling is important. However, there is a lack of information on membrane fouling in MBRs, especially information on features of components that are responsible for the fouling. We conducted a pilot-scale experiment using real municipal wastewater with three identical MBRs under different operating conditions. The results obtained in this study suggested that the food-microorganisms ratio (F/M) and membrane filtration flux were the important operating parameters that significantly influenced membrane fouling in MBRs. Neither concentrations of dissolved organic matter in the reactors nor viscosity of mixed liquor, which have been thought to have influences on fouling in MBRs, showed clear relationships with membrane fouling in this study. Organic substances that had caused the membrane fouling were desorbed from fouled membranes of the MBRs at the termination of the operation and were subjected to Fourier transform infrared (FTIR) and 13C nuclear magnetic resonance (NMR) analyses. These analyses revealed that the nature of the membrane foulant changes depending on F/M. It was shown that high F/M would make the foulant more proteinaceous. Carbohydrates were dominant in membrane foulants in this study, while features of humic substances were not apparent.
Introduction As an efficient technology for municipal and industrial wastewater treatment, membrane bioreactors (MBRs) have gained significant popularity in the past decade. MBRs, in which biomass is strictly separated by a membrane, offer several advantages over the conventional activated sludge process, including a high biomass concentration, reduced footprint, low sludge production, and better permeate quality (1). MBRs can be generally classified into two categories: recirculated MBRs and submerged (integrated) MBRs (2). Recently, submerged MBRs have been preferred since energy consumption can be significantly reduced (3, 4). Membrane fouling is a major obstacle for wider application of MBRs. Membrane fouling results in reduced perfor* Corresponding author phone:
[email protected]. 10.1021/es0502425 CCC: $30.25 Published on Web 07/15/2005
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2005 American Chemical Society
mance, severe flux decline, high-energy consumption, and frequent membrane cleaning or replacement. (1, 2, 5). To establish strategies for controlling membrane fouling, understanding of the mechanism of membrane fouling is indispensable. Many factors that might influence membrane fouling in MBRs have been reported (6, 7). Attention has been given to various design and operating parameters such as airflow rate in the reactor (8-10), membrane configuration (11), membrane flux (12, 13), concentrations of mixed liquorsuspended solids (MLSS) (14-16), and solid retention time (10, 17-19), which is directly related to the food-tomicroorganism ratio (F/M) (20). However, there is a lack of information on the role of polymeric membrane materials in membrane fouling in MBRs (7). Indexes representing characteristics of mixed liquor of biomass in the reactor such as the concentration of dissolved organic carbon (16, 21) and viscosity (14-16) also have been suggested to have relationships with membrane fouling in MBRs. Extracellular polymeric substance (EPS) have recently been identified as the most significant biological factor responsible for membrane fouling (15, 17, 22). EPS is a complex mixture of carbohydrates, proteins, humic compounds, uronic acids, and DNA (23, 24). Despite those intensive efforts mentioned previously, accumulation of knowledge to establish a general rule that can be used for prevention of membrane fouling in full-scale MBRs is insufficient. This is partly because synthetic wastewater was used in many previous studies. It is thought that membrane fouling in MBRs is mainly caused by organic substances that are associated with microbial metabolic activities and/or organic substances that originate from unmetabolized wastewater components. At present, however, very little is known about characteristics of the components that could cause membrane fouling in MBRs. Fourier transform infrared (FTIR) spectroscopy and solidstate 13C-nuclear magnetic resonance (NMR) spectroscopy are powerful analytical tools for investigation of the characteristics of organic substances. These techniques have been used in many studies for investigation of the characteristics of isolated natural organic matter (NOM) (25-33) but not for characterization of membrane foulants in MBRs. This is probably because small-scale membrane units were used in most previous studies, and sufficient amounts of organic foulants for conducting FTIR/NMR analysis could not be obtained. Information on foulant in MBRs obtained by using those analytical techniques, however, should be very useful for establishing a strategy for prevention of membrane fouling. In this study, we conducted a pilot-scale MBR operation at an existing wastewater treatment facility to obtain sufficient amounts of organic foulants from fouled membranes for FTIR/NMR analysis. The results obtained in this study should be realistic, and they provide new insights into characteristics of foulants in MBRs. Three identical MBRs were operated in parallel under different operating conditions in the pilot run. The effects of operating conditions on membrane fouling in MBRs are also discussed on the basis of observed differences.
Materials and Methods Operation of MBRs. Continuous operation of MBRs was carried out at the Soseigawa Municipal Wastewater Treatment Facility, Sapporo, Japan. Three identical MBRs were operated in parallel with the same feed wastewater delivered from the primary sedimentation basin of the facility. The average feed concentrations are given in Table 1. The examined wastewater is classified as weak (20). VOL. 39, NO. 16, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Mean Concentrations of the Feeda
mean concentration a
TOC (mg/L)b
DOC (mg/L)b
UV absorbance at 260 nm (cm-1)b
NH4+-N (mg/L)b
T-P (mg/L)c
pHb
temperature (°C)b
79.6 ( 24.3
40.0 ( 14.2
0.239 ( 0.040
16.5 ( 3.8
2.3 ( 2.0
7.8 ( 0.3
12.6 ( 1.5
Values are given ( standard deviation.
b
n ) 29. c n ) 7.
TABLE 3. Average Quality of Treated Watera
TABLE 2. Operating Conditions of the MBRs membrane MLSS hydraulic F/M ratio solid permeate flux concentration retention (mg of C/mg retention 3 2 (m /m /day) (g/L) time (h) of MLSS/day) time (days)a MBR-A MBR-B MBR-C a
0.4 0.2 0.2
10 10 5
4.4 8.8 8.8
0.044 0.022 0.044
42 83 32
MBR-A MBR-B MBR-C a
TOC (mg/L)b
NH4+-N (mg/L)b
T-P (mg/L)c
3.9 ( 1.4 3.5 ( 0.8 3.8 ( 1.1
0.5 ( 0.6 0.3 ( 0.4 0.5 ( 0.5
0.5 ( 0.3 0.5 ( 0.4 0.4 ( 0.3
Values are given (standard deviation.
b
n ) 26. c n ) 7.
Experimental results.
After the MBRs had been preliminarily operated for 2 months to acclimatize biomass, continuous monitoring was initiated with new membranes on January 5, 2004. Each MBR was equipped a hollow-fiber microfiltration (MF) membrane module made of polyethylene that had a total surface area of 3 m2 and a nominal pore size of 0.2 µm (Mitsubishi Rayon, Tokyo, Japan). In each reactor, aeration was continuously carried out at the flow rate of 3.8 m3/h. The volume of each reactor was 175 L. Filtration was carried out with the constant flow rate mode of operation using suction pumps. Intermittent filtration (12 min filtration and 3 min pause) was also carried out. For submerged aerobic MBRs, intermittent suction is effective for suppression of fouling (34, 35). Operating conditions for each MBR are summarized in Table 2. Conditions for MBR-A and -C were determined to provide the same food-microorganism ratio (F/M), which has been reported to have an impact on the evolution of fouling (10, 17-19). MBR-B was operated with a low F/M and a low membrane flux. MBR-B was consequently expected to show the least membrane fouling. When membrane fouling became significant, membrane modules were taken out from the reactor and were cleaned physically and/or chemically. Physical membrane cleaning was carried out by spraying pressurized water on the membrane surface. Chemical membrane cleaning was carried out by submerging the membrane module in a solution of hydrochloric acid (pH 2) and sodium hypochloride (500 ppm). The degree of membrane fouling was evaluated by membrane filtration resistance calculated by the following equation:
J)
∆P µRt
where J is the membrane permeate flux (m3/m2/s), ∆P is the transmembrane pressure difference (Pa), µ is the water viscosity (Pa s), and Rt is the total membrane filtration resistance. Desorption of Organic Matter from Fouled Membranes. To investigate features of constituents responsible for membrane fouling that occurred in the pilot run, organic matter was desorbed from the fouled membranes at the termination of the operation and was then analyzed. When the pilot operation was terminated, membrane modules were taken out from the reactors and were disassembled. Each membrane fiber was manually wiped with a lab paper wipe to remove accumulated cake that can be physically removed. Desorption of organic matter from the fouled membranes was carried out by soaking the membranes in an alkaline solution (sodium hydroxide) for 24 h. The solution pH was 6294
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set at 10. After desorption, measurements of total organic carbon (TOC) and ultraviolet (UV) absorbance at a wavelength of 260 nm were carried out. The remaining solutions were subsequently processed with electric dialysis for desalination and lyophilized for advanced analyses (e.g., FTIR and NMR). Analytical Methods. Concentrations of TOC and dissolved organic carbon (DOC) were determined by using a TOC analyzer (TOC-5000, Shimadzu, Kyoto, Japan). UV absorbance was measured by a spectrophotometer (U-2000, Hitachi, Tokyo, Japan). The measurement of proteins was carried out by Lowry methods (36). BSA was used as a standard. The phenol-sulfuric acid method of Dubois et al. (37) was used for carbohydrate determination. Glucose was used as a standard. The viscosity of mixed liquor was measured by a viscosity meter (Model BL, Tokimec, Tokyo, Japan) at a shear rate of 16.8 s-1. Molecular weight distribution of dissolved organic matter in mixed liquor was determined by gel permeation chromatography using a column filled with Superose 6 pg (Amersham Biosciences, Piscataway, NJ). The diameter and length of the column were 2.6 and 100 cm, respectively. Samples were filtered with a 0.5 µm PTFE membrane filter and were then concentrated 10-fold using a vacuum evaporator prior to applying to the gel permeation system. Ten milliliters of the concentrated sample was applied to the system and eluted by a 0.04 M phosphorus buffer (pH 7) at the flow rate of 3.0 mL/min. Each 6 mL of eluted fraction was analyzed by TOC and UV measurement. Pullulans were used as model compounds for correlating molecular weight with elution volume. For FTIR studies, KBr pellets containing 0.25% (foulants collected from MBR-A and -C) or 1% (foulant from MBR-B) 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 carbon-13 (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 All three MBRs could be operated very stably in terms of quality of treated water. Table 3 summarizes average water quality of treated water in the three MBRs. In this study, all of the treated water passed through membranes with a pore size of 0.2 µm, and TOC in the treated water can be considered as DOC. Increases in Filtration Resistance. Figure 1 shows changes in filtration resistance determined for the three MBRs. Data plotted in Figure 1 were adjusted to 20 °C equivalent values considering the influence of water viscosity on filtration resistance.
FIGURE 1. Change in total filtration resistance during the pilot run. Over a long period of operation, the rates of increase in filtration resistance in MBR-B and -C were fairly constant, while the rate of increase in filtration resistance gradually declined in MBR-A. On day 32, the membrane in MBR-A was considerably fouled, and physical membrane cleaning was carried out. However, the physical cleaning did not restore membrane permeability. Chemical cleaning was therefore carried out, and it considerably reduced the filtration resistance. This implies that fouling was not caused by deposition of suspended solids on the membrane but by irreversible adsorption of foulants on/within the membrane. The membrane in MBR-A exhibited relatively rapid fouling after the first cleaning, and a second cleaning was carried out on day 50. On day 50, the feedwater pump did not work well, and the liquid level in MBR-A was significantly lowered. This caused considerable adhesion of suspended solids on the membrane. Physical cleaning was effective to some extent at that time, but it did not reduce filtration resistance to a value less than that recorded just before pump failure. Chemical cleaning was therefore carried out, and it worked well. After the second cleaning, the filtration resistance of MBR-A suddenly increased on day 58. This sudden increase in filtration resistance of MBR-A was again attributed to failure in operation of the feedwater pump. At this time, physical cleaning was sufficient to restore membrane permeability, and the operation was continued. The rate of increase in filtration resistance in MBR-A was generally constant after the second cleaning. Operations of MBR-B and -C were generally stable and were disturbed only once, on the same day (day 75). Sudden increases in filtration resistance were observed in both MBR-B and -C at that time. The disturbance was also caused by feed pump failure. Physical cleaning reduced the filtration resistance to the levels observed just before the pump failure, and the operations were continued. From Figure 1, it is obvious that operating conditions in MBRs had great influences on the evolution of filtration resistance. As expected, MBR-B, which was operated with low F/M and low membrane permeate flux, exhibited the least membrane fouling. MBR-A and -C were operated with the same F/M but with different membrane fluxes. Membrane flux of MBR-A was 2 times higher than that of MBR-C. The rate of increase in filtration resistance observed for MBR-A was more than 2 times higher than that of MBR-C, suggesting that membrane flux is an important factor in evolution of
FIGURE 2. Changes in DOC (top), dissolved carbohydrate (middle), and dissolved protein (bottom) in the membrane chambers during the pilot run. Squares, MBR-A; circles, MBR-B; and crosses, MBR-C. membrane fouling (as discussed later). At the end of the operation, the membrane modules were taken out from the three MBRs for extraction of organic matter that had caused membrane fouling. When the operations were terminated, visual inspection revealed that accumulation of cake on the membrane surface was not significant in any of the MBRs, indicating that aeration carried out in the reactors was effective enough for prevention of cake accumulation, which is sometimes the main cause of membrane fouling in MBRs (17). Relationship between Membrane Fouling in MBRs and Indexes Representing Mixed Liquor Properties. There have been many reports on relationships between membrane fouling in MBR and indexes representing mixed liquor properties (e.g., DOC, EPS, and viscosity) (14-17, 21, 22). These relationships were verified by the results obtained from the pilot run using real wastewater. Figure 2 shows changes in dissolved organic carbon (DOC), dissolved carbohydrate, and dissolved protein measured for mixed liquors of the three MBRs. Carbohydrate and protein are representative components of EPS (23). Referring to Figures 1 and 2, it is difficult to establish a clear relationship between changes in these water quality indexes measured for mixed liquor and membrane fouling of the three MBRs. As mentioned previously, MBR-A exhibited the fastest membrane fouling, while MBR-B showed the slowest evolution of fouling. Concentrations of DOC, dissolved carbohydrate, and dissolved protein in MBR-A were not always the highest among the three MBRs. In addition, as it can be seen from Figure 2, concentrations of dissolved organic matter measured for mixed liquor in the three MBRs considerably fluctuated, while the rates of increase in filtration resistance were fairly constant in MBR-B and -C. It is therefore thought that those indexes are not likely to be directly related VOL. 39, NO. 16, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Changes in viscosity of the mixed liquor of the three MBRs. Squares, MBR-A; circles, MBR-B; and crosses, MBR-C. to membrane fouling in real MBRs. We did not measure the quantity of bound EPS represented as mg of EPS/g of MLSS, which requires an extraction procedure (23, 38) to determine. Several researchers (15, 17) have suggested a relationship between bound EPS and membrane fouling in MBRs. Bound EPS was not considered in the present study and is currently under investigation using the same MBRs in our laboratory with respect to a relationship with fouling in MBRs. Figure 3 shows changes in viscosity of the mixed liquor. As expected, the viscosity of the mixed liquor of MBR-C, in which MLSS was maintained at a lower value (5000 mg/L), was the lowest among the three MBRs. The lowest viscosity observed for MBR-C, however, did not lead to the greatest suppression of membrane fouling as can be seen from Figure 1. The measured viscosity of the mixed liquor of MBR-A also does not indicate a specific relationship between viscosity and membrane fouling in MBRs. During the 4 month operation period, the viscosity of the mixed liquor of MBR-A initially declined and then started to increase, while the rate of membrane fouling in MBR-A continuously declined. On the basis of these observations, the importance of viscosity on membrane fouling cannot be claimed at least in the examined conditions. It should be noted here that the situation would be different when a very high MLSS concentration was examined. Itonaga et al. (39) reported that viscosity would exponentially increase when the MLSS concentration exceeds about 10 000 mg/L. At very high MLSS concentrations, the influence of viscosity on membrane fouling would be significant and should be taken into account. Gradual Decline in Membrane Fouling Rate Observed in MBR-A. As shown in Figure 1, the membrane fouling rate in MBR-A declined over the period of continuous operation. Indexes measured for mixed liquor in MBR-A, such as DOC, dissolved carbohydrate, dissolved protein, and viscosity (Figures 2 and 3), did not show synchronous changes corresponding to the decline in the fouling rate, although those indexes have been reported to be related to membrane fouling (14-17, 21, 22). A linkage, however, might be found between membrane fouling and molecular weight distribution of organic matter determined by gel permeation chromatography. Figure 4 shows the molecular weight distribution of dissolved organic matter measured for the mixed liquor of MBR-A on different dates. Two distinct peaks are seen near molecular weights of 1 000 000 Da and 5000 Da. On day 17, when the fouling rate was high, on the basis of TOC measurement, the relative weight of large components with molecular weights of around 1 000 000 was significant in the mixed liquor of MBR-A. These large organic molecules were found to be less UV-sensitive than were molecules with molecular weights of around 5000 Da. On day 37, when the fouling rate was still high, similar results were obtained, although the baseline TOC signal was relatively high. In contrast, on day 74, when the fouling rate was relatively low, 6296
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the intensity of the peak due to large molecules had clearly declined. The appearance/disappearance of dissolved organic matter with a large molecular weight as well as less UV absorbance might be related to the rate of membrane fouling in MBRs. Figure 4 also shows the molecular weight distribution of organic matter measured for the mixed liquor of MBRB. In contrast to MBR-A, intensities of the peak for a molecular weight of 1 000 000 Da were fairly constant and weak in MBR-B. This might be in accordance with the fouling tendency observed for MBR-B (Figure 1). Further research is needed to investigate this point. Relative Importance of F/M and Membrane Flux on Membrane Fouling in MBRs. Among the three tested MBRs, only one MBR (MBR-B) was operated at a lower F/M. This reactor exhibited the slowest evolution of membrane filtration resistance, indicating the importance of F/M for control of membrane fouling in an MBR. Low F/M (i.e., long SRT) is likely to suppress membrane fouling in an MBR, as was suggested by results of previous studies (10, 17-19). There are two ways for reducing F/M: reducing F or increasing M. An increase in M (microorganisms) can be achieved by increasing the MLSS concentration associated with longer SRT. Reducing F in the operation of an MBR can be achieved by some pretreatment or lowering the membrane permeate flux. The effectiveness of pretreatment for mitigation of membrane fouling in MBRs is discussed elsewhere (39, 40). Lowering the membrane permeate flux also decreases F/M. Lowering the membrane flux is, however, likely to be effective in mitigation of membrane fouling in a different way: lowering the membrane flux means the reduction of transport of fouling components toward membranes. The rates of increase in filtration resistance observed in the pilot run can be determined from the slopes of curves in Figure 1. For MBR-B and -C, determination of the rate is relatively straightforward since filtration resistance increased steadily in the two MBRs. For MBR-A, however, the rate declined over the period of operation. The slowest fouling rate in MBR-A was determined from the period after the second cleaning and will be used in the following discussion. The rates of increase in filtration resistance (unit: m-1 day-1) in MBR-A, -B, and -C were 0.076, 0.019, and 0.030, respectively. A comparison of MBR-B and -C should reveal the influence of F/M on membrane fouling since membrane flux was set at the same value for the two MBRs, and a comparison of MBR-A and -C should reveal the importance of membrane flux since the F/M values for the two MBRs were the same. The degree of difference between MBR-B and -C in terms of F/M and the degree of difference between MBR-A and -C in terms of membrane flux were the same: a factor of 2. Response to these differences (i.e., evolution of membrane fouling) was, however, more emphasized when the membrane flux was altered. The ratio of fouling rate in MBR-A to that in MBR-C, which should represent the influence of membrane flux on membrane fouling, was determined to be 2.5. Similarly, the ratio of fouling rate in MBR-C to that in MBR-B, which should represent the influence of F/M on membrane fouling, was determined to be 1.6. This difference suggests that membrane fouling is a more influential operating factor than is F/M, in agreement with the results reported by Nagaoka et al. (22). MBR-A was operated with a higher F/M and higher membrane flux, while MBR-B was operated with a lower F/M and lower membrane flux. Difference in the rate of membrane fouling was greatest between MBR-A and -B. The ratio of the fouling rate in MBR-A to that in MBR-B was 4.0, reflecting a mixed effect of F/M and membrane flux. Often, in the design process, a large membrane area in an MBR is avoided because it increases capital costs. On the basis of the previous arguments, however, the increase in membrane area in an MBR may be more economically feasible than the current design concept
FIGURE 4. Molecular weight distribution of dissolved organic matter collected from MBR-A and -B at different dates. Bold lines: TOC response and thin lines: UV response. because expected mitigation of membrane fouling associated with increased membrane area (i.e., lower flux) will significantly reduce operating costs and may compensate the initial costs needed for membranes. Since the costs of membrane are decreasing, that would be a reasonable design approach for MBRs soon. Analysis of Organic Matter Desorbed from Fouled Membranes. As stated previously, in the continuous operation, physical cleaning was carried out when failure in the feed pump caused considerable adhesion of suspended solids on the membrane. However, it did not reduce the filtration resistance to a value less than that recorded just before pump failure. In addition, accumulation of cake on the membrane surface was not significant in any of the MBRs when the operations were terminated. These findings imply that fouling was mainly caused by irreversible adsorption of foulant (probably organic substance) on/within the membrane, not by deposition of cake. Detailed information on characteristics of organic matter that causes irreversible membrane fouling in MBRs is valuable for establishing strategies for prevention of it. However, information has been very limited. Powerful tools for the analysis of organic matter such as FTIR or NMR require certain amounts of samples. In studies using smallscale membrane units, the required amount of organic matter that is responsible for irreversible membrane fouling cannot be simply prepared because the membrane area is too small. In the present study, however, a pilot-scale apparatus was used, and sufficient amounts of organic matter for advanced analytical techniques could therefore be obtained. Desorption of membrane foulant was carried out at the termination of the pilot operation. To make sure that invisible accumulated cake was removed from the membrane surface, each membrane fiber was carefully wiped with a paper lab wipe prior to the desorption tests. Table 4 summarizes the results of the desorption tests. The amount of desorbed organic matter followed the order: MBR-A > MBR-C > MBR-B, corresponding to the
TABLE 4. TOC Concentration and UV Absorbance of Organic Matter Desorbed from 0.8 m2 of Fouled Membranes with 1.2 L of Alkaline Solution MBR-A MBR-B MBR-C
TOC (mg/L)
UV absorbance (1/cm)
SUVA (1/m/mg/L)
7.4 2.8 6.1
0.077 0.040 0.096
1.04 1.42 1.57
degree of membrane fouling (Figure 1). However, a linear quantitative relationship between the degree of filtration resistance and the desorbed amount of organic matter could not be established. Specific ultraviolet adsorption (SUVA) represents the aromaticy of organic matter (41). The values of SUVA are also shown in Table 4 and suggest that organic matter causing the fouling in MBR-B and -C had similar features, which turned out to be false as subsequently discussed. FTIR spectra of the foulants obtained from the three MBRs are presented in Figure 5. There were significant similarities in the spectra obtained for MBR-A and -C. In these spectra, peaks near 1660 and 1540 cm-1 were significant. They are assigned to the amido-I and -II bands, respectively, (31) and therefore indicate the presence of protein in the foulants. In contrast, the spectrum obtained for MBR-B had a peak near 1620 cm-1, which was assigned to aromatic CdC (33), instead of the peaks that represented proteinaceous features in the spectra for MBR-A and -C. In all three spectra, a broad peak near 1100 cm-1 was seen. This peak is an indication of carbohydrate character (31). Relative signal intensity associated with the presence of carbohydrate was greater in MBR-B than in MBR-A and -C. IR spectra shown in Figure 5 are not similar to those of humic substances (42). This suggests that humic substances were minor components in foulants in MBRs. CPMAS 13C NMR spectra of the foulants desorbed from the MBRs are presented in Figure 6. The spectrum obtained VOL. 39, NO. 16, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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more proteinaceous, which might be related to more severe fouling (see Figure 1). It should be noted that organic matter desorbed in this work might not represent all of the membrane fouling. It was not assumed that all of the organic foulant could be desorbed by the process used in this study. It has been reported that an oxidation reagent (e.g., sodium hypochrolide) was more effective than an alkali (e.g., sodium hydroxide) in canceling membrane fouling (44). An oxidation reagent would have removed larger amounts of organic matter from fouled membranes but was not employed as a desorption reagent in this study. This is because an oxidation reagent is more likely to alter the nature of organic matter during the desorption process. Further investigation and modification of the desorption procedure is needed. It would be interesting to determine whether membrane material causes changes in the nature of organic matter that adsorbs on/into membrane and causes membrane fouling.
Acknowledgments FIGURE 5. Infrared spectra of membrane foulants.
We thank Dr. Takeshi Sasaki of the Department of Molecular Chemistry in Hokkaido University for his assistance in FTIR analysis. NMR analysis was done by Dr. Eiji Yamada of the NMR laboratory in Hokkaido University.
Literature Cited
FIGURE 6. CPMAS 13C NMR spectra of membrane foulants. for MBR-B was considerably scattered probably due to insufficient content of carbon in the analyzed sample. A similarity between MBR-A and -C was recognized in NMR analysis as well. Proteinaceous nature in the foulants from MBR-A and -C can be seen by peaks near 175 and 55 ppm (25). Also, it was found that the foulants of MBR-A and -C were rich in carbohydrate (peaks at 75 and 105 ppm) (25). The aromatic carbon signal (110-165 ppm) was minor in the spectra for MBR-A and -C. It was difficult to characterize the foulant from MBR-B based on the NMR data because of considerable fluctuation in the spectrum. Nevertheless, the relative signal intensity representing aromatic carbon (110165 ppm) in the spectrum for MBR-B seemed to be more pronounced than those for MBR-A and -C. The foulant from MBR-B might contain a larger amount of aromatic carbon than those from MBR-A and -C, which is in accordance with results of FTIR analysis described previously. NMR spectra shown in Figure 6 are not similar to those for humic substances (43). As stated previously, results of FTIR and NMR analyses suggested that a difference in F/M in the operation of MBRs causes difference in the nature of foulants. Although fouling in MBRs is caused by a heterogeneous mixture of organic matter, both FTIR and NMR analyses demonstrated that the carbohydrate was a dominant component in the foulants. High F/M ratios would change the nature of the foulant to 6298
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Received for review February 4, 2005. Revised manuscript received June 3, 2005. Accepted June 8, 2005. ES0502425
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