Role of Soluble Microbial Products (SMP) in Membrane Fouling and

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Environ. Sci. Technol. 2006, 40, 969-974

Role of Soluble Microbial Products (SMP) in Membrane Fouling and Flux Decline C H A L O R J A R U S U T T H I R A K * ,† A N D GARY AMY‡ Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520 Thailand, and UNESCO-IHE Institute for Water Education, Delft, The Netherlands

Soluble microbial products (SMP), a significant component of effluent organic matter (EfOM), play an important role in membrane fouling and flux decline in wastewater reclamation/reuse applications. The SMP compounds of a microbial origin are derived during biological processes of wastewater treatment. They exhibit the characteristics of hydrophilic organic colloids and macromolecules. These high molecular weight compounds play an important role in creating high resistance of the membrane, leading to a reduction of permeate flux. The SMP fouling of RO, NF, and tight UF membranes is associated with formation of a cake/gel layer due to size (steric) exclusion. FTIR spectra of SMP- and EfOM-fouled membranes exhibited foulants’ composition, consisting of polysaccharides, proteins, and/or aminosugar-like compounds. This finding reveals the important role of the SMP components as factors in membrane fouling and flux decline associated with EfOM source waters. Solids retention time (SRT) affects the characteristics and amounts of SMP, however, SRT did not affect flux decline trends of RO and NF membranes.

1. Introduction Soluble microbial products (SMP) have been found to be the majority of soluble organic matter in effluents from biological wastewater treatment processes. Such compounds of microbial origin can be derived in solution during substrate metabolism, biomass growth, and biomass decay. The SMP, representing soluble extracellular polymeric substances (EPS), mainly contain small carbonaceous compounds derived from the original substrate during biomass growth and cellular macromolecules generated during the endogenous phase (1-4). The presence of SMP in wastewater effluent not only affects discharge levels of organic compounds, i.e., COD, BOD, and TOC, but also becomes a constraint to wastewater reuse. In a wastewater reclamation plant where membrane filtration processes are employed, a loss of productivity caused by membrane fouling is a major issue to be considered. Previous studies demonstrated that effluent organic matter (EfOM) consists of (i) refractory natural organic matter (NOM) conveyed from drinking water sources, (ii) trace levels of synthetic organic compounds (SOC) produced during domestic use and disinfection byproducts (DBPs) generated during disinfection processes of water and wastewater * Corresponding author phone: (66)2326-4415, ext 6245; fax: (66)2326-4415; e-mail: [email protected]. † King Mongkut’s Institute of Technology Ladkrabang. ‡ UNESCO-IHE Institute for Water Education. 10.1021/es050987a CCC: $33.50 Published on Web 01/04/2006

 2006 American Chemical Society

treatment, and (iii) soluble microbial products (SMP) derived during biological processes of wastewater treatment (5). Thus, the possible foulants of a membrane could be the NOM and/ or the SMP contained in the treated effluent. Many researchers have indicated the effects of the NOM on membrane fouling and flux decline (6-8). However, compared to NOM, the SMP were found to provide high membrane fouling potential during wastewater reclamation/reuse (9). Thus, studies on membrane fouling by SMP are needed. The purpose of this study is to understand the role of SMP in membrane fouling and flux decline for RO, NF, and tight UF membranes. In this study, SMP-feedwaters were generated by bench-scale sequencing batch reactors (SBR), simulating the activated sludge process. The SMP samples were varied by an important operating condition: solids retention time (SRT). Prefiltered SMP samples were characterized and processed through RO, NF, and tight UF membranes. Membrane fouling and flux decline associated with SMP and the effects of SRT on flux decline are described.

2. Theoretical Basis Soluble microbial products (SMP) are defined as “the pool of organic compounds that are released into the solution from substrate metabolism (usually with biomass growth) and biomass decay” (1). They have been classified into two groups based on the bacterial phase from which they were derived: growth associated products and nongrowth associated products (3). Growth associated SMP, also called utilization associated products (UAP), are directly produced from biomass growth and substrate metabolism. The rate of production is proportional to the rate of substrate utilization. Nongrowth associated SMP, known as biomass associated products (BAP), occur as a result of biomass decay and cell lysis during endogenous decay. Typically, cells consume electron-donor substrate to build active biomass. At the same time, they produce bound extracellular polymeric substances (EPS) and UAP. Bound EPS are hydrolyzed to BAP, while active biomass undergoes endogenous decay to form residual dead cells. Therefore, the SMP represent soluble EPS (4). The UAP are mainly small carbonaceous compounds derived from the original substrate, whereas BAP are cellular macromolecules containing carbon and nitrogen (10). The BAP account for most of the SMP rather than the UAP (2). The production rate of the SMP is proportional to the concentration of biomass due to a release of organic materials from cell lysis (11). The SMP consist of proteins, polysaccharides, and some humic-like materials (5, 12). An increase of SRT leads to an accumulation of biomass in the system, and therefore, it may be expected to increase the amount of SMP. Biological reactors with high SRT tend to produce more fractions of high molecular weight (MW) materials, which is a result of cell lysis (13). Such high MW compounds are mixtures of heteropolysaccharides and lipopolysaccharides. Some SMP components can be further biodegraded, therefore, only refractory SMP are discharged as effluent (2). Molecular weight distributions of SMP and of EfOM have been investigated. The SMP exhibited a bimodal distribution with a majority of SMP having a MW less than 1 kDa or greater than 10 kDa, whereas the minority had a MW between 1 and 10 kDa (14). The MW distribution of the EfOM is affected by the operating conditions of the system. At high SRT, higher MW materials become more significant (13). The intense biological activity in an aeration basin could enhance recondensation processes in a short reaction period, leading to a transformation of proteins and sugars to melanoidinlike compounds. These compounds possess high molecular VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Operating Cycles of the SBR time from

to

operation

duration (hr:min)

volume (L)

8:57 am 8:57 am 9:00 am 7:51 pm 8:36 pm

9:00 am 9:00 am 7:51 pm 8:36 pm 8:57 pm

Cycle #1 carbon feed (0.167 L/min) nutrient feed (1.48 L/min) mixed aerobic reaction settling clarified effluent decanting (0.285 L/min)

0:03 0:03 10:51 0:45 0:21

0.5 4.5

9:00 pm 9:00 pm 7:51 am 8:36 am 8:57 am

Cycle #2 carbon feed (0.167 L/min) nutrient feed (1.48 L/min) mixed aerobic reaction settling clarified effluent decanting (0.285 L/min)

0:03 0:03 10:51 0:45 0:21

0.5 4.5

8:57 pm 8:57 pm 9:00 pm 7:51 am 8:36 am

weight, hydrophilic-colloid properties, and refractory character to chemical and biological hydrolysis (15). The SMP may be responsible for membrane fouling. Polysaccharides and aminosugars have been found as dominant fractions on fouled membranes (6).

3. Experimental Section 3.1 SMP-Source (Feed) Waters. Bench-scale sequencing batch reactors (SBR) were employed to simulate the activated sludge process, and to generate soluble microbial product (SMP) used as source (feed) waters for membrane filtration tests. The system was operated in an oxygenated mixed liquor mode. The reactor had a volume of 10 L and another 10-L headspace volume. The SBR was operated on two cycles per day. Each 12-hour cycle provided an operational sequence including filling with nutrients and carbon source, mixed reaction, settling, and decanting. Table 1 summarizes the operating cycle of the SBR. The SBR was operated for longer than twice a given SRT before SMP samples were collected and used in membrane filtration tests. To start up the system, 5 L of activated sludge in the form of mixed liquor was obtained from a local municipal wastewater treatment plant and used to inoculate the SBR. Glucose was used as a sole carbon source for bacterial growth. The initial concentration of dissolved organic carbon in the reactor was approximately 100 mg C/L. Required nutrients derived from NH4Cl, CaCl2, MgSO4, Na2HPO4, and KH2PO4 were provided to the system. Solids retention time, representing an average length of time a microbial cell resides in a bioreactor, is an operating condition possibly affecting the characteristics of SMPs. In this study, the SRT was varied at 2, 5, 10, and 30 days (SMP-2, SMP-5, SMP-10, and SMP-30, respectively) by daily wasting of a specific amount of solids from the SBR mixed liquor. During operation, the MLSS concentration and pH were determined. The mean MLSS concentrations in the SBR with the SRT of 2, 5, 10, and 30 days were 312, 438, 994, and 3160 mg/L, respectively. All SMP-samples were prefiltered through 0.45-µm filters and characterized for DOC, UVA, SUVA, pH, conductivity, TDS, DOC distribution, and molecular weight distribution. The characteristics of SMP samples were compared with those of a bulk EfOM sample, designated as BO-SE, from a local wastewater treatment plant as shown in Table 2. However, it is noted that there are several differences in key operating conditions between the SBR and the continuous stirred tank reactor (CSTR) activated sludge system that may affect the formation and characteristics of SMPs. 3.2 Membrane Filtration Tests. Different membranes were employed in membrane filtration tests, including TFCULP (RO), ESNA (NF), and GM (tight UF) membranes. Each membrane exhibited various trends, resulting in adsorption of different fouling materials and rejection performances. 970

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5

TABLE 2. Characteristics of SMP and EfOM Samplesa DOC SMP/EfOM (mg/L) SMP-2 SMP-5 SMP-10 SMP-30 BO-SE a

3.82 (0.13) 5.10 (0.50) 5.06 (0.48) 4.01 (0.97) 6.84 (1.19)

UVA254 (1/cm)

SUVA (L/mg-m)

pH

0.029 (0.003) 0.036 (0.003) 0.033 (0.006) 0.036 (0.005) 0.119 (0.011)

0.76 (0.07) 0.65 (0.17) 0.66 (0.18) 0.92 (0.17) 1.77 (0.26)

6.61 (0.28) 6.35 (0.15) 6.02 (0.32) 6.00 (0.17) 7.04 (0.25)

TDS conductivity (mg/L) (µS/cm) 223

433

208

445

213

439

204

434

365

768

The values in parentheses indicate standard deviation.

The properties of virgin membrane specimens are listed in Table 3. The ESNA membrane showed a relatively high degree of hydrophobicity based on its contact angle. The GM membrane possessed a high negative surface charge in terms of its zeta potential. The pure water permeability (PWP) of a membrane, defined as capacity for water to pass through the membrane normalized by transmembrane pressure, reflects both pore size and pore density. The TFC-ULP and ESNA membranes were found to be tight membranes, whereas the GM membrane showed a loose structure, based on PWP; however, within a UF classification, the GM is considered a tight UF. Membrane filtration tests were conducted using a stirred cell filtration unit operated in dead-end mode with constant pressure and declining flux. This simulation and mode of operation will result in different fouling trends as compared to cross-flow conditions with constant flux and increasing pressure, however, the stirred cell approach is a simple probe of fouling potential. The SMP sample was continuously fed from a 4-L pressurized feed storage reservoir without recirculation. A magnetic stirrer within the stirred cell unit was operated at a speed of 300 rpm to provide a simulation of cross-flow with a velocity of approximately 50 cm/s. Initial flux was adjusted to 35 L per square m per hour (L/m2/h). The pressure employed to provide the target initial fluxes for RO and NF membranes was 70 psig (4.8 bar) and that for UF membrane was 50 psig (3.5 bar). Typical RO and NF membranes operate at 20 L/m2/h. However, typical pressures are 7-28 bar for NF membrane. These different conditions may reflect different fouling trends in a full-scale installation than in the test unit but, again, the stirred cell test is used as a probe of fouling potential. During the membrane filtration tests, aliquots of approximately 200 mL (70 L/m2) of permeate were collected up to 1.6 L (560 L/m2) of total permeate volume. Permeate flowrates were monitored as a function of permeate volume per unit area. Feedwater,

TABLE 3. Properties of Membranes

membrane

type

material

MWCO (Da)

TFC-ULP ESNA GM

RO NF UF

polyamide polyamide polyamide

n/a 200 8000

aAt

pH 8 and conductivity of 30 mS/m with KCl.

b

contact angle (°)

Zeta potential (mV)

PWP (L/m2/day/kPa)

42.0 60.3 45.5

-28a -11.5b -17.0b

1.32 1.35 2.96

At pH 7 and ionic strength of 0.005 mol/L.

permeate aliquots, and final retentate were analyzed for DOC, UVA254, and conductivity (as surrogate for TDS). Organic matter rejection was determined by mass balance. Membranes fouled by the SMP were characterized using attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) for identification of functional groups of foulants. 3.3 Analytical Methods. 3.3.1 SMP Characterization. (1) Total Organic Carbon (TOC). TOC was determined by a total organic carbon (TOC) analyzer (Sievers 800) with an autosampler. All samples were filtered through 0.45-µm filters prior to measurement. Therefore, the measured TOC values represent dissolved organic matter (DOC) values. (2) Ultraviolet Light Absorbance (UVA254) and Specific UVA (SUVA). UVA254 was measured using a UV/visible spectrophotometer (Shimadzu UV-160A, Japan). The measurement was performed at a wavelength of 254 nm using a 1-cm quartz cell. The specific UVA, defined as a ratio of UVA254 and DOC of a sample, represents an index of aromaticity (humic content) of the organic matter in the sample (3) Molecular Weight Distribution. Molecular weight (MW) distributions of SMP and EfOM in feedwater, retentate, and permeate were determined using high performance size exclusion chromatography (HP-SEC) with on-line DOC detection. The column media of HPSEC, possessing a void volume of 35 000 Da and a salt boundary of ∼100 Da, is a porous gel allowing the separation of molecules according to their molecular size or mass. Polystyrene sulfonates (PSS) with molecular weights of 1800, 4600, 8000, and 35 000 Da, were used as standard solutions. The response as a function of separation time was measured using an on-line total organic carbon (TOC) analyzer (Sievers 800). The traditional UV detector monitors UV-absorbance at the wavelength of 254 nm, indicative of the aromaticity of compounds. However, it has a limitation in detection of low UV-absorbing compounds such as polysaccharides. An on-line TOC detector provides an opportunity to recognize all organic compounds (16). 3.3.2 Membrane Characterization. (1) Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATRFTIR). ATR-FTIR was used to determine the functional group characteristics of clean and fouled membranes. Membrane specimens were examined by FTIR (Nicolet Magna-IR 750 series II, Madison, WI) equipped with an ATR accessory. The optical bench was set to use an IR light source, KBr beam splitter, and DTGS KBr detector, which was set at a gain of 8. Spectra were obtained by averaging 128 scans with a wavenumber resolution of 4.0 cm-1. (2) Contact Angle. Contact angle is an index of the hydrophobicity of a membrane. The measurement of contact angle uses a goniometer (Rame´-Hart Inc.) with a sessile drop method. This method is based on a measure of the contact angle between a water droplet, the membrane surface, and air. Contact angle measurements were performed with clean and fouled membranes to identify the change of hydrophobicity of a membrane surface due to the adsorption of foulants. (3) Zeta Potential. Zeta potential was determined by streaming potential measurements made over a range of electrolyte velocities, using an electrokinetic analyzer ap-

FIGURE 1. SEC chromatograms of DOC transformation during SBR cycle: SRT 30 days (a value in parentheses indicates dilution factor.) paratus (EKA, Brookhaven Instrument Corp, Holtsville, NY). The streaming potential is developed due to the relative motion between the solid phase and surrounding electrolyte solution. Depending on the flow resistance of the channel, a pressure drop (∆P) is detected across the cell. The recirculation of electrolyte leads to the motion of charge (stream current) in the flow direction along the channel. Conductivity, temperature, pressure, and streaming potential (E) are monitored with internal sensors. Each cycle results in a calculation of zeta potential, the change in streaming potential ∆E/∆P, and the correlation coefficient for ∆E/∆P. (17-18).

4. Results and Discussion 4.1 SMP Formation During Biological Processes. The samples from the SBR were collected continuously during a 12-hour cycle. Figure 1 shows SEC chromatograms with DOC detection of samples during a SBR cycle at a SRT of 30 days. The figure illustrates a transformation of organic compounds during biological processes. In the first 2 h, glucose was biodegraded and transformed to intermediates whose molecular weights were higher than 300 Da. The so-called utilization associated products (UAP) were produced during biomass growth and substrate utilization (10). After that, SEC peaks of these compounds decreased while the peaks of larger compounds became more significant. Although glucose (MW 180 Da) was a sole organic carbon source for the reactors, the molecular weight distribution of SMP was similar and primarily composed of organic compounds larger than 10 000 Da. The SEC results of the SMP samples at SRTs of 2, 5, and 10 days exhibited the same trends. These phenomena help explain formation of the SMP, especially biomass associated products (BAP), during biological processes. The results also show a dominance of high molecular weight compounds (>10 000 Da) in the SBR effluent. These compounds are possibly caused by cell lysis during the endogeneous phase. This finding is consistent with the observations of several other investigators (13-15, 19). However, the formation of SMPs is also dependent on several factors, e.g., the type of food source, diversity of microorganisms, and adaptation time of the MLSS to certain SRT. VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Flux decline trends of SMP-samples at different SRT, using TFC-ULP membrane.

FIGURE 3. Flux decline trends of SMP-samples at different SRT, using ESNA membrane. 4.2 Membrane Fouling and Flux Decline by SMP. SMP samples derived from different SRTs were tested with TFCULP (RO), ESNA (NF), and GM (UF) membranes. Figures 2 and 3 show a series of flux decline trends of the TFC-ULP and ESNA membranes, respectively, due to membrane fouling by different SMP-source waters, including SMP-2, SMP-5, SMP-10, and SMP-30. All SMP samples showed similar trends of flux decline compared to that of BO-SE, an EfOMsource water. This finding reflects the major contribution of SMP in membrane fouling and flux decline associated with EfOM. However, flux decline results of the RO and NF membranes did not exhibit significant differences among SMP-samples, suggesting that SRT did not show significant effects on flux decline trends of RO and NF membranes. The BO-SE showed higher flux decline than the SMP samples due to higher organic loading, i.e., delivered DOC (note that the flux decline curves are expressed in terms of cumulative permeate volume, L/m2). The delivered DOC is defined as the amount of organic carbon per unit area delivered to membrane within stirred cell. An increase of feed DOC (i.e., delivered DOC) led to an increase of flux decline. As the BO-SE contained a higher DOC than the SMPs, the greater fouling may simply be a consequence of the higher DOC. Moreover, the later secondary drops on the flux decline trends for BO-SE can be explained by retention of ions. Concentration polarization effects may enhance the possibility of scale formation on the membrane surface, leading to more flux decline (Figure 2). The SRT affects the differences among flux decline trends of the GM (UF) membrane, as shown in Figure 4. Flux decline by SMP-2 was less than those of other SMP-samples with longer SRT. This trend may be explained by the production of SMP during biological processes that produced some macromolecules with characteristics of hydrophilic colloids. 972

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FIGURE 4. Flux decline trends of SMP-samples at different SRT, using GM membrane.

FIGURE 5. Organic matter rejection of SMP-samples by TFC-ULP (RO) membrane.

FIGURE 6. Organic matter rejection of SMP-samples by ESNA (NF) membrane. At shorter SRT, the production of hydrophilic colloids/ macromolecules was small compared with that at longer sludge age, therefore, there was SMP foulant deposited on the UF membrane. However, the formation of SMPs is also dependent on several factors, e.g., the type of food source, diversity of microorganisms, and adaptation time of the MLSS to certain SRT. 4.3 Rejection of Organic Compounds in SMP-Samples by Membranes. Membranes can play a significant role in rejection of bulk organic matter (DOC), precursors of disinfection byproducts (DBP). In these experiments, RO and NF membranes exhibited high performance in removal of DOC from SMP-samples. As shown in Figures 5 and 6, the average percent rejections of the SMP by the TFC-ULP (RO) and ESNA (NF) membranes were found to be 96 and 98%,

FIGURE 7. Organic rejection of SMP-samples by GM (UF) membrane. FIGURE 9. SEC-DOC spectra of SMP-10 feed, permeate, and retentate, using ESNA membrane (dilution factor for the retentate is 1:4).

FIGURE 8. SEC-DOC spectra of SMP-10 feed, permeate, and retentate, using TFC-ULP membrane (dilution factor for the retentate is 1:4). respectively. The major mechanism of organic matter rejection by the RO and NF membranes was size (steric) exclusion associated with the low MWCO of the membranes and high MW of the organic matter in the SMP samples. The SMP retained on RO and NF membranes as a cake/gel layer led to a high resistance on the membrane surface and a reduction in permeate flux. Organic matter rejections of SMP-samples by the GM (UF) membrane are shown in Figure 7. The increasing trend of organic matter rejection can be explained by an accumulation of organic compounds in the retentate and/or the foulant layer of cake/gel formation, acting as a secondary membrane. The average percent organic removal of SMP-samples (∼84%) was higher than that of BO-SE (∼75%), as BO-SE contained not only a high MW fraction, but also humic-like materials and a low MW fraction. The retained SMP was able to block membrane pores and form a cake/gel layer on the membrane surface. The cake/gel formation on the membrane surface provides a porous layer which allows smaller molecules pass through. This cake/gel layer, considered as a secondary membrane, was found to be an effective secondary filtration layer for organic compounds. As a result, a higher percent removal of organic compounds was achieved, as greater flux decline was exhibited. 4.4 Size-Exclusion Chromatography with DOC Detection (SEC-DOC). SEC-DOC analysis provided qualitative information on membrane fouling and organic matter rejection. Figures 8 and 9 show the high capability of the TFC-ULP and ESNA membranes, respectively, in rejection of organic matter from SMP-10, whereas Figure 10 shows significant organic matter in the GM permeate. However, it was found that the high MW fraction (>10 000 Da) of the SMP was virtually completely removed by all tested membranes. The chro-

FIGURE 10. SEC-DOC spectra of SMP-10 feed, permeate, and retentate, using GM membrane (dilution factor for the retentate is 1:2).

FIGURE 11. FTIR spectra of clean and SMP-fouled TFC-ULP (RO) membranes. matograms for retentates, compared to feeds, exhibited a loss of this high MW fraction, reflecting deposition of some of this fraction on membranes as foulants. 4.5 FTIR Spectra of Clean and Fouled Membranes. FTIR spectra of clean and fouled membranes provided some evidence of membrane fouling, as the spectra showed functional groups of foulants covering the membrane surface. Figures 11, 12, and 13 depict the FTIR spectra of fouled TFCULP, ESNA, and GM membranes, respectively, by different SMP-samples and the BO-SE sample compared to the VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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(2) Barker, D. J.; Salvi, S. M. L.; Langenhoff, A. A. M.; Stuckey, D. C. Soluble microbial products in ABR treating low-strength wastewater. J. Environ. Eng. 2000, 126 (3), 239-249. (3) Namkung, E.; Rittmann, B. E. Soluble microbial products (SMP) formation kinetics by biofilms. Water Res. 1986, 20 (6), 795806. (4) Laspidou, C. D.; Rittmann, B. E. A unified theory for extracellular polymeric substances, soluble microbial products, and active and inert biomass. Water Res. 2002, 36, 2711-2720. (5) Drewes, J. E.; Fox, P. Fate of natural organic matter (NOM) during groundwater recharge using reclaimed water. Water Sci. Technol. 1999, 40 (9), 241-248. (6) Cho, J.; Amy, G.; Pellegrino, J. Membrane filtration of natural organic matter: initial comparison of rejection and flux decline characteristics with ultrafiltration and nanofiltration membranes. Water Res. 1999, 33 (11), 2517-2526.

FIGURE 12. FTIR spectra of clean and SMP-fouled ESNA (NF) membranes.

(7) Cho, J.; Amy, G.; Pellegrino, J. Membrane filtration of natural organic matter: factors and mechanisms affecting rejection and flux decline with charged ultrafiltration (UF) membrane. J. Membr Sci. 2000, 164, 89-110. (8) Scha¨fer, A. I.; Mauch, R.; Waite, T. D.; Fane, A. G. Charge effects in the fractionation of natural organics using ultrafiltration, Environ. Sci. Technol. 2002, 36, 2572-2580. (9) Park, N.; Kwon, B.; Kim, I. S.; Cho, J. Biofouling potential of various NF membranes with respect to bacteria and their soluble microbial products (SMP): Characterizations, flux decline, and transport parameters. J. Membr. Sci. 2005, 258, 43-54. (10) Urbain, V.; Mobarry, B.; de Silva, V.; Stahl, D. A.; Rittmann, B. E.; Manem, J. Integration of performance, molecular biology and modeling to describe the activated sludge process. Water Sci. Technol. 1998, 37, 223-229. (11) Grady, C. P. L., Jr.; Daigger, G. T.; Lim, H. C. Biological Wastewater Treatment, 2nd ed.; Marcel Dekker: New York, 1999; pp 282284. (12) Hejzlar, J.; Chudoba, J. Microbial polymers in the aquatic environment-II: isolation from biologically nonpurified and purified municipal wastewater and analysis. Water Res. 1986, 20 (10), 1217-1221.

FIGURE 13. FTIR spectra of clean and SMP-fouled GM (UF) membranes. associated clean membranes. The results show the FTIR peaks exhibited by foulants associated with SMP and EfOM. The peaks at wavenumbers of 1040 and 2940 cm-1 indicate the presence of polysaccharide-like materials, whereas the peaks at 1550 and 1640 cm-1 exhibit the existence of protein-like materials. These compounds, possessing characteristics of hydrophilic organic colloids and macromolecules, correspond to building blocks of peptidoglycans found in the bacterial cell wall. The results were consistent with the FTIR peaks of EfOM-fouled membranes as well as membranes fouled by organic colloid isolates of EfOM in related work performed by the authors (20). This finding provides strong evidence of membrane fouling by SMP.

Acknowledgments We acknowledge partial financial support of the University of Colorado, Center for Membrane Applied Science and Technology (MAST), Boulder, CO, and the King Mongkut’s Institute of Technology Ladkrabang (KMITL), Thailand. We also acknowledge the useful advice of Professor JoAnn Silverstein (University of Colorado) on SBR protocols.

Literature Cited (1) Barker, D. J.; Stuckey, D. C. A review of soluble microbial products (SMP) in wastewater treatment systems. Water Res. 1999, 33 (14), 3063-3082.

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(13) Pribyl, M.; Tucek, F.; Wilderer, P. A.; Wanner, J. Amount and nature of soluble refractory organic produced by activated sludge microorganisms in sequencing batch and continuous flow reactors. Water Sci. Technol. 1997, 35 (1), 27-34. (14) Schiener, P.; Nachaiyasit, S.; Stuckey, D. C. Production of soluble microbial products (SMP) in an anaerobic baffled reactor, composition, biodegradability and the effect of process parameters. Environ. Sci. Technol. 1998, 19, 391-400. (15) Dignac M-F.; Ginestet, P.; Rybacki, D.; Bruchet, A.; Urbain, V.; Scribe, P. Fate of wastewater organic pollution during activated sludge treatment: nature of residual organic matter. Water Res. 2000, 34 (17), 4185-4194. (16) Her, N.; Amy, G.; Foss, D.; Cho, J.; Yoon, Y.; Kosenka, P. An enhanced method for detecting and characterizing NOM by HPLC-size exclusion chromatography (SEC) with UV and online DOC detection. Environ. Sci. Technol. 2002, 36, 1069-1076. (17) Wilbert, M. C.; Delagah, S.; Pellegrino, J. Variance of streaming potential measurements. J. Membr. Sci. 1999, 161, 247-261. (18) Cho, J.; Amy, G.; Pellegrino, J.; Yoon, Y. Characterization of clean and natural organic matter (NOM) fouled NF, and UF membranes, and foulants characterization. Desalination 1998, 118, 101-108. (19) Shin, H.-S.; Kang, S.-T. Characteristics and fates of soluble microbial products in ceramic membrane bioreactor at various sludge retention times. Water Res. 2003, 37, 121-127. (20) Jarusutthirak, C.; Amy, G.; Croue´, J-P. Fouling characteristics of wastewater effluent organic matter (EfOM) isolates on NF and UF membranes. Desalination 2002, 145, 247-255.

Received for review May 26, 2005. Revised manuscript received November 18, 2005. Accepted December 1, 2005. ES050987A