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Membrane fouling in a submerged membrane bioreactor (MBR) under sub-critical flux operation: Membrane foulant and gel layer characterization
Journal of Membrane Science 325 (2008) 238–244
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Membrane fouling in a submerged membrane bioreactor (MBR) under sub-critical flux operation: Membrane foulant and gel layer characterization Zhiwei Wang ∗ , Zhichao Wu, Xing Yin, Lumei Tian State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China
a r t i c l e
i n f o
Article history: Received 28 May 2008 Received in revised form 15 July 2008 Accepted 19 July 2008 Available online 26 July 2008 Keywords: Critical flux Gel layer Membrane fouling Membrane bioreactor (MBR) Wastewater treatment
a b s t r a c t Membrane foulants and gel layer formed on membrane surfaces were systematically characterized in a submerged membrane bioreactor (MBR) under sub-critical flux operation. The evaluation of mean oxidation state (MOS) of organic carbons and Fourier transform infrared (FT-IR) spectroscopy demonstrated that membrane foulants in gel layer were comprised of not only extracellular polymeric substances (EPS) (proteins, polysaccharides, etc.) but also other kinds of organic substances. It was also found that fine particles in mixed liquor had a strong deposit tendency on the membrane surfaces, and membrane foulants had much smaller size than mixed liquor in the MBR by particle size distribution (PSD) analysis. Gel filtration chromatography (GFC) analysis showed that membrane foulants and soluble microbial products (SMP) had much broader distributions of molecular weight (MW) and a larger weight-average molecular weight (Mw ) compared with the influent wastewater and the membrane effluent. Scanning electron microscopy (SEM) and energy-diffusive X-ray (EDX) analysis indicated that membrane surfaces were covered with compact gel layer which was formed by organic substances and inorganic elements such as Mg, Al, Fe, Ca, Si, etc. The organic foulants coupled the inorganic precipitation enhanced the formation of gel layer and thus caused membrane fouling in the MBR. © 2008 Elsevier B.V. All rights reserved.
1. Introduction As an efficient technology for municipal and industrial wastewater treatment, submerged membrane bioreactors (MBRs) have gained increasing popularity in recent years. MBRs, in which solid/liquid separation is performed by membranes, offer several prominent advantages over conventional activated sludge (CAS) system including a smaller footprint, less sludge production and better effluent quality, etc., [1–3]. However, membrane fouling and its consequences in terms of plant maintenance and operating costs limit the widespread application of MBRs [4,5]. In order to control membrane fouling and maintain sustainable operation, the concept of critical flux was proposed by Field et al. [6] in MBRs. They defined the critical flux as the flux below which the increase of trans-membrane pressure (TMP) with time under constant flux operation does not occur; however, above that level, fouling is observed. Since then, the critical flux has been extensively applied to membrane processes from microfiltration (MF) to reverse osmosis (RO) [7]. In MBRs, the critical flux is often determined by step-wise method in short-term experiments [8,9].
∗ Corresponding author. Tel.: +86 21 65980400; fax: +86 21 65980400. E-mail address: zhiweiwang [email protected] (Z. Wang). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.07.035
However, it has been reported by a number of authors that even at very low fluxes an increase of TMP with time takes place. This has led to the introduction of so-called sustainable flux, which represents the flux value at which the fouling rate is operationally and economically acceptable for MBR operation. Flux sustainability is typically assessed by long-term trials in which a flux lower than the short-term critical one, also known as sub-critical flux, is employed and TMP is continuously monitored [10,11]. Under sub-critical flux operation, membrane fouling characteristics in MBRs have been studied by many researchers. It is generally characterized by a twostep fouling phenomenon, i.e., a gradual increase of TMP is followed by a sudden increase. This behavior has been explained with different perspectives based on local flux distribution [8,11], percolation theory [12] and in-homogeneity of fiber bundles [13], etc. It is also widely accepted that sub-critical fouling in MBRs is mainly caused by organic macro-molecules such as soluble microbial products (SMP), extracellullar polymeric substances (EPS) and possibly other substances that are released during cell lysis, diffuse through the cell membrane, and/or are lost during synthesis or are excreted for some purpose [14,15]. Many authors further dedicated to classifying the contribution of different fractions of sludge including soluble substances, colloids and suspended solids to membrane fouling [16–18]. Although those intensive efforts mentioned above are very helpful to understand the characteristics of sub-critical fouling in
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screen, raw wastewater from the WWTP was supplied into the anoxic zone, and the mixed liquor of anoxic zone then flowed into oxic zone by gravity. The characteristics of the raw wastewater are listed in Table 1. The influent pump was controlled by a water level sensor to maintain a constant water level in the bioreactor over the experimental system. The membrane-filtered effluent was then obtained by suction using a pump connected to the modules. The effluent flow rate and the TMP were monitored by a water meter and a pressure gauge, respectively. 2.2. Operating conditions
Fig. 1. Flow diagram of the pilot-scale MBR.
MBRs, there is a lack of sufficient information on the properties of gel-like layer (called as gel layer in this study) which is formed on membrane surfaces under sub-critical flux operation. The formation of gel layer (mainly caused by SMP, colloids, solutes, etc.) is different from the deposition of sludge cake (attributed to the adhesion and deposition of sludge flocs, large sludge particles, etc.) on the membrane which can be readily removed by physical washing. A detailed characterization of membrane foulants and the gel layer will be conducive to a better understanding of the sub-critical fouling, and it will also facilitate to develop membrane fouling control measures and to optimize operational parameters in MBRs. In this study, a pilot-scale MBR for the treatment of real municipal wastewater at an existing wastewater treatment plant (WWTP) was operated in order to investigate the gel layer properties formed on membrane surfaces under sub-critical flux operation. The properties of membrane foulants in the gel layer were critically examined by the evaluation of mean oxidation state (MOS) of organic carbon, Fourier transform infrared (FT-IR) spectroscopy, gel filtration chromatography (GFC), particle size distribution (PSD) analyzer, scanning electron microscopy (SEM) and energy-diffusive X-ray (EDX) analyzer, etc. The results obtained in this study are expected to provide a sound understanding of the gel layer properties and membrane fouling in MBRs under sub-critical flux operation. 2. Materials and methods 2.1. Experimental setup The pilot-scale submerged MBR as shown in Fig. 1, which was located in Quyang Municipal WWTP of Shanghai, consisted of an effective volume of 160 L anoxic and 480 L oxic zone. Nine flat sheet membrane modules (SHZZ-MF, Zizheng Environment Incorporated, Shanghai, China) were mounted vertically between two baffle plates located above an air diffuser in oxic zone. The membranes were made of polyvinylidene fluoride (PVDF) membrane with mean pore size 0.20 m. The effective filtration area for each module was 0.7 m2 . Air was monitored by a flow-rate meter and supplied through the air diffuser which was below the membrane modules in order to supply oxygen demanded by the microorganisms and to induce a cross-flow velocity (CFV) along membrane surfaces. After passing through a 0.9-mm pore-sized stainless bar
The membrane flux of the pilot-scale MBR was kept constant at about 25 L/(m2 h) which was lower than the critical flux 32–38 L/(m2 h), and a suction cycle of 10 min followed by 2 min relaxation (no suction) was applied. The anoxic and oxic hydraulic retention times (HRT) were about 1.3 h and 3.9 h, respectively. Sludge was wasted from the oxic zone to maintain a sludge retention time (SRT) of 40 d. Recycled sludge from oxic zone to anoxic zone was controlled at 3 times of the influent flow rate. The CFV along membrane surfaces was kept at about 0.3 m/s by aeration with an average air flow rate of 6 m3 /h. Dissolved oxygen (DO) concentration in the oxic zone was in the range of 1–3 mg/L while it was maintained at about 0.2 mg/L in the anoxic zone. The MBR was operated under ambient temperature in the range of 15–20 ◦ C. Chemical cleaning-in-place procedure (0.5% (v/w) NaClO solution, 2 h duration) would be carried out if TMP reached about 30 kPa during the operation. 2.3. Analytical methods 2.3.1. Membrane foulants collection and pretreatment The fouled membrane modules were taken out from the bioreactor at the end of each operation cycle when the TMP reached about 30 kPa. Gel layer on about 0.35 m2 membrane surface was carefully scraped off by a plastic sheet and simultaneously flushed with pure water. The collected sample was placed on a magnetic blender (Model JB-2, Leici Instrument Incorporated, Shanghai, China) and well mixed. Then the collected sample was prepared and could be further treated according to the requirements of specific analysis items. 2.3.2. Characterization of mean oxidation state of organic carbons 100 mL of the collected sample as described in Section 2.3.1 was filtered through a 0.45 m-pore-size-filter paper, and the filtrate was used to analyze chemical oxygen demand (COD) according to Chinese NEPA standard methods [19] and total organic carbon (TOC) by a TOC analyzer (TOC-VcPN, SHIMADZU, Japan). The MOS of the organic carbon can be computed through Eq. (1) [20]. The oxidation states of organic molecules change according to their chemical structures. Therefore, if the oxidation state of organic carbons is determined, rough estimation of the main substrates in the samples could be made. EPS were also extracted from the biomass in Table 1 Influent wastewater characteristics of the MBR (n = 43) Items
Mean concentration
COD (mg/L) TN (mg/L) NH3 -N (mg/L) TP (mg/L) SS (mg/L) pH
361.0 45.6 27.4 8.8 280.0 6.9
Values are given ± standard deviation.
± ± ± ± ± ±
221.0 20.6 11.6 3.6 220.0 0.6
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the MBR according to the thermal treatment method described by Chang and Lee [21], and then COD together with TOC of EPS sample was assayed in order to obtain the MOS of EPS. MOS of organic carbon =
4(TOC − COD) TOC
(1)
where COD is expressed in mol O2 /L and TOC in mol C/L. 2.3.3. Fourier transform infrared (FT-IR) spectroscopy Another 200 mL of the collected sample as described in Section 2.3.1 was dried at 70 ◦ C for 48 h. The dry matter was analyzed by a FT-IR spectrometer (Nicolet 5700, Thermo Electron Corporation, USA) in order to obtain the organic foulant information in the gel layer. EPS extracted from biomass were also analyzed in this study according to the procedures mentioned above. 2.3.4. Gel filtration chromatography analysis The filtrate, which was obtained by filtering 100 mL of the collected sample as described in Section 2.3.1 with a 0.45 m-poresize-filter paper, was fractionated by GFC analyzer. The GFC system consisted of a TSK G4000SW type gel column (TOSOH Corporation, Japan) and a liquid chromatography spectrometer (LC-10ATVP, SHIMADZU, Japan). Polyethylene glycols (PEGs) with molecular weight (MW) of 1,215,000 Da, 124,700 Da, 11,840 Da and 620 Da (Merck Corporation, Germany) were used as standards for calibration. It is well known that solution environments have significant effects on MW fractionation of the samples [22,23]. Therefore, pure water was used as eluent. The elution at different time intervals was collected by an automatic fraction collector and automatically analyzed by using UV spectroscopy and dissolved organic carbon (DOC) analyzer to obtain a MW distribution curve. The MW distributions of influent wastewater, MBR effluent and the SMP in the mixed liquor (SMP sample was obtained according to the treatment principle described by Laspidou and Rittmann [24]) were also carried out to elucidate the MW variations in the system. 2.3.5. Particle size distribution analysis PSD of the collected sample as described in Section 2.3.1 was carried out by a focused beam reflectance measurement (Accusizer 780, Santa Barbara Corporation, USA). Same measurement of the mixed liquor in the MBR was conducted in order to verify the differences between them. 2.3.6. Scanning electron microscopy and energy-diffusive X-ray measurement For SEM analysis, a panel of flat-sheet membrane covered with gel layer was taken out from the MBR and a piece of the membrane was cut form the middle of the fouled membrane module after one operation cycle. The sample was fixed with 2% (v/v) glutaraldehyde in 0.1 M phosphate buffer at pH 7.2 for 2 h and then washed twice for 10 min and again immersed for 1 h in 0.1 M phosphate buffer. The fixed sample was dehydrated with ethanol and coated with aurum–platinum alloy (with coating depth 10 nm) for 2 min. The coated sample was examined under a SEM (Model XL-30, Philips, Netherland). The fouled membrane was then submerged in 0.5% (v/w) NaClO solution to remove the foulants for 2 h, and SEM analysis of the cleaned membrane was also conducted according to the procedures mentioned above. The SEM coupled an EDX analyzer (Phoenix, EDAX Incorporated, USA) was employed to determine the inorganic components in the gel layer. 2.3.7. Other item analysis Measurements of COD, total nitrogen (TN), total phosphorus (TP), ammonia (NH3 -N), and pH in the influent and membrane effluent, mixed liquor suspended solids (MLSS) and mixed liquor
Table 2 Average characteristics of treated water (n = 43) Items (mg/L)
Mean concentration
COD TN NH3 -N TP SS
22.0 ± 16.0 15.0 ± 5.6 1.9 ± 1.1 4.0 ± 1.8 0.0
Values are given ± standard deviation.
volatile suspended solids (MLVSS) in the system were performed according to Chinese NEPA standard methods [19]. DO concentration in the reactor was measured by a dissolved oxygen meter (Model YSI 58, YSI Research Incorporated, OH, USA). CFV was determined using Cup-type Current Meter (Model LS45A, Chongqing Hydrological Instrument Incorporated, Chongqing, China). 3. Results and discussion 3.1. Process performance During the operation, MLSS concentration in anoxic zone and oxic zone of the MBR was maintained at about 15 g/L and 18 g/L, respectively, by extraction of excess sludge. Table 2 summarizes the average characteristics of treated water in the MBR. It can be seen that the removal of COD, ammonia, TN and suspended solids (SS) was quite successful. About 60% TP removal was also achieved in the MBR during the experiment. The variations of membrane flux and TMP with operation time are demonstrated in Fig. 2. It can be observed that the TMP increased with operation time as membrane flux was kept at about 25 L/(m2 h) (sub-critical flux) during the experiment. The TMP variations were characterized by a two-step fouling phenomenon, i.e., a slow increase of TMP followed by a rapid increase. When TMP reached about 30 kPa, chemical cleaning procedure was carried out to remove membrane fouling and to recover the membrane permeability. Before each cleaning process, membrane modules were taken out from the system in order to observe and examine the membrane foulants formed on the membrane surfaces. It is worth noting that there was no obvious sludge cake on membrane surfaces but slime gel layer. The fouling behavior demonstrated that sludge cake fouling mainly caused by suspended sludge flocs, to a great extent, could be controlled by adopting sub-critical flux operation [25], and the gradual formation of gel layer on membrane surfaces led to the gradual increase of TMP in the MBR. Many researchers [2,26] argued that sludge cake was found on membrane
Fig. 2. Variations of membrane flux and TMP during the experiment. (An arrow indicates that chemical cleaning of membrane was performed on that day.)
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Table 3 Mean oxidation state of organic carbon of different substances (n = 6) Organic substances
COD/TOC
Mean oxidation state of carbon
Estimation of components
EPS Gel layer
1.06 ± 0.02 1.60 ± 0.21
−0.23 ± 0.18 −2.45 ± 0.78
Proteins and polysaccharides –
Values are given ± standard deviation.
surface when they studied membrane fouling in MBRs, which might be due to the fact that they did not employ the sub-critical flux operation. In our study, it was verified that under sub-critical flux operation the sludge cake fouling was significantly controlled while the fouling caused by macromolecules, colloids and SMP, etc., was inevitable to form a gel layer in the MBR. 3.2. MOS of organic carbons of gel layer and that of EPS The MOS of organic carbons in gel layer and that in EPS, computed according to Eq. (1), is listed in Table 3. It can be seen that the MOS of organic carbons of membrane foulants is different from that of EPS, which means that the organic foulants in the gel layer were not completely formed by EPS. Although it has been believed that EPS play an important role in membrane fouling process of MBRs and might be a vital factor to cause the formation of fouling layer [27–29], further research on the organic components in the gel layer needs to be carried out based on the results of MOS of organic carbons. Besides EPS, there might be other organic substances in gel layer. From the estimation protocol of Stumm and Morgan [20], it can be easily predicted that major components of EPS are proteins and polysaccharides while it could hardly estimate the existing organic substances in gel layer. It might indicate that the gel organic material had a relatively high organic structural complexity. In the
following study, other technical measures were employed to characterize the organic components in gel layer. 3.3. FT-IR analysis The FT-IR spectra of membrane foulants in gel layer and EPS are illustrated in Fig. 3. In Fig. 3(a), the spectrum shows a broad region of adsorption around a peak at 3421 cm−1 , which is attributed to stretching of the O H bond in hydroxyl functional groups, and a sharper peak at 2932 cm−1 , which is due to stretching of C H bonds [30]. Two sharp peaks (1653 cm−1 and 1558 cm−1 ) are also observed in the spectrum, which are unique to the protein secondary structure, namely amides I and II [31]. It indicates that proteins were one of components of the gel layer. The peaks at 1378 cm−1 and 1247 cm−1 might be due to the presence of methyl and C O (ester) bonds, respectively. In addition, a broad peak at 1045 cm−1 exhibits the character of polysaccharides or polysaccharides-like substances [32], which indicates that polysaccharides were presented in the gel layer. Another broad peak appears at 600 cm−1 in the finger print region of the spectrum. In comparison with Fig. 3(a), the spectrum of EPS in Fig. 3(b) shows similarities on the peaks of 3427 cm−1 , 1638 cm−1 and 1082 cm−1 , which demonstrates the presence of O H bonds, proteins and polysaccharides in EPS. However, it could be observed that the number of peaks in EPS spectrum is less than that of membrane foulant spectrum especially between 1000 cm−1 and 1600 cm−1 , which illustrates the organic substances in membrane foulants were more complicated than those in EPS. From the FTIR spectra, it could be seen that membrane foulants include not only EPS (proteins, polysaccharides, etc.) but also other organic substances featured by the functional groups as mentioned above. The results are also supported by the evaluation of MOS of organic carbons in gel layer and that in EPS as described in Section 3.2, and gel organic material might have high organic structural complexity compared with EPS. 3.4. MW distribution analysis by GFC technology In principle, large MW molecules are excluded earlier than smaller ones because they are unable to travel through the gel
Fig. 3. FT-IR spectra of membrane foulants and EPS. (a) Membrane foulants; (b) EPS.
Fig. 4. Relationship between log(MW) and exclusion time.
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Fig. 5. GFC chromatograms of influent wastewater, membrane effluent, SMP in mixed liquor and membrane foulants. (a) Chromatograms of influent wastewater and membrane effluent; (b) chromatograms of SMP and membrane foulants.
pores, and then high correlation between the exclusion time and the MW, as shown in Fig. 4, was obtained. The GFC chromatograms of the influent wastewater, membrane effluent, SMP in mixed liquor and membrane foulants are shown in Fig. 5. It could be observed that before 12th min there are peaks in chromatograms of SMP and membrane foulants (see Fig. 5(b)) while no peaks appear in Fig. 5(a) during this period. It indicates that high MW molecules, according to the correlation between exclusion time and MW as illustrated in Fig. 4, existed in SMP and membrane foulants while they were absent in the influent and membrane effluent. The high MW molecules are produced by microorganisms during substrate metabolism and/or released during cell lysis and decay, and they have been widely considered and accepted as a major factor resulting in membrane fouling [14,15,21,24]. The intermediate MW molecules were also major components of membrane foulants which could be seen from the sharp peaks presented from 12 min to 20 min. The absence of high MW molecules (before 12th min in terms of exclusion time) in membrane effluent was attributed to the fine pores of membrane and to the gel layer formed on membrane surfaces. However, the low MW molecules existing in the SMP were able to permeate through the gel layer and membrane pores, thus resulting in the soluble COD in the effluent of MBR. It was also reported that the presence of low MW fraction was observed in a MBR for the treatment of landfill leachate [33]. In order to better understand MW distributions of the samples, number-average molecular weight (Mn ), weight-average molecular
Fig. 6. Particle size distribution of membrane foulants and mixed liquor in the MBR. (a) Membrane foulants; (b) mixed liquor in the MBR. (X-coordinate represents the particle size (m) while Y-coordinate indicates particle number percentage (%).)
weight (Mw ) and the coefficient of MW distribution (Mw /Mn ) were used in this study. A low coefficient of Mw /Mn indicates that the organic substance has a narrow distribution of MW. Table 4 presents the MW distributions of the influent, membrane effluent, SMP and membrane foulants. Compared with the influent wastewater and membrane effluent, membrane foulants and SMP had much broader distributions of MW, of which the Mw /Mn was 426.54 and 243.62, respectively. It demonstrated that microorganisms generated SMP with a broad MW distribution besides consuming influent wastewater (with a relative narrow MW distribution) and making new biomass. During the filtration process, the SMP and other organic substances such as colloids, solutes, etc., existing in mixed liquor would block membrane pores and/or deposit on membrane surfaces to form a fouling layer (called as gel layer in this study) due to the fine pores of membranes. The formation of gel layer, in turn, would retain macromolecules and large particles (making the layer more compact simultaneously) but allow the substances with low MW substances passing through the layer and through membrane pores to enter effluent water. Therefore, the membrane effluent had a narrow distribution of MW, with Mw /Mn coefficient 7.07 as shown in Table 4.
Table 4 MW distributions of the influent, membrane effluent, SMP and membrane foulants Items
Influent wastewater
Membrane effluent
SMP
Membrane foulants
MW (kDa) Mw (kDa) Mn (kDa) Mw /Mn
16.8–7426.1 228.2 40.5 5.63
0.9–1215.0 142.1 20.1 7.07
0.9–35450.6 4092.7 16.8 243.61
0.2–201075.0 6824.6 16.0 426.54
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Fig. 7. SEM photographs of cleaned membrane surface and fouled membrane surface. (a) Cleaned membrane; (b) fouled membrane.
3.5. Particle size distribution analysis The PSD of membrane foulants and mixed liquor in MBR is presented in Fig. 6(a) and (b), respectively. From Fig. 6(a), it can be observed that the membrane foulants had a narrow range profile of size distribution, which mainly distributed between 0.5 m and 2.0 m. To the contrary, the mixed liquor in the MBR had a broader size distribution profile ranging from 0.5 m to over 100 m, which was featured with two sharp peaks at 0.57 m and 20.0 m. Statistical analysis of particle distributions demonstrated that about 88% of the membrane foulants distributed in a size range of 0–2.0 m while there are only 19% of the particles in mixed liquor distributed in the size range of 0–2.0 m. The results showed that particle size of membrane foulants was much smaller than that of mixed liquor in the MBR. In membrane filtration process, it is known that the particle transport is mainly dependent on two forces, i.e., the permeation drag force and the shear stress induced by CFV. During the operation of MBR, the small particles in the mixed liquor including colloidal substances and macromolecules of SMP could easily deposit on the membrane surfaces by permeation drag to form a gel layer fouling, and not easily detached by the CFV because of their low back transport velocity [34]. Large particles in the mixed liquor like suspended microbial flocs could be detached from membrane surfaces due to the shear stress induced by aeration and thus under sub-critical flux operation, membrane fouling caused by large sludge particles, to a great extent, could be controlled. Colloidal substances, SMP and other fine particles were predominant foulants to cause the formation of gel layer in MBR under sub-critical flux operation. Rosenberger et al. [35] have also reported that macromolecules and
organic colloids with a MW of >120 kPa contributed significantly to membrane fouling and higher fouling rates were observed at higher concentrations of these substances. 3.6. SEM and EDX measurement SEM images of cleaned membrane and fouled membrane are presented in Fig. 7(a) and (b), respectively. It reveals that the fouled membrane was covered with slime gel layer. Once the gel layer was developed, it would become difficult to remove the layer from the membrane surfaces by routine aeration. Consequently, the formation of gel layer resulted in the increase of TMP and caused severe irreversible membrane fouling in this study. Element analysis was further performed on the surface layer in order to identify the chemical components of the layer by EDX analysis. The elements of C, O, N, Cl, P, Na, Mg, Fe, Al, K, Ca and Si were detected and shown in Fig. 8. It should be pointed out that the sharp peak of Au element in this diagram is caused by the coated aurum on the sample and irrelevant to the elemental analysis of the membrane foulants. Mg, Al, Fe, Ca and Si had significant effects on the formation of gel layer though the relative contents of these elements were small. The biopolymers contain anion groups such as SO4 2− , CO3 2− , PO4 3− and OH− , and the cations such as Mg2+ , Al3+ , Fe3+ , and Ca2+ could be easily precipitated by these negative ions [36]. It has been identified that the major components of inorganic foulants were MgNH4 PO4 ·6H2 O (struvite) in the membrane-coupled an anaerobic bioreactor for alcohol-distillery wastewater treatment [37]. It was also reported by You et al. [38] that the MBR with internal membrane system had severe inorganic fouling problem with calcium addition to the system. The organic foulants as mentioned in above sections coupled the inorganic precipitation would enhance the formation of gel layer and thus caused the fouling behavior in the MBR. In order to control membrane fouling caused by gel layer in submerged MBR under sub-critical flux operation, the organic substances such as SMP, colloids and solutes, etc., together with inorganic matters in the bioreactor should be controlled. The inorganic matters should be adjusted to a proper concentration by pretreating the influent wastewater. The concentration of colloidal and soluble organic materials in mixed liquor could be, to some extent, eliminated and controlled by optimizing operational parameters such as SRT, HRT, volumetric organic load (VOL) and so on [4,13,21,27–29]. 4. Conclusions
Fig. 8. EDX analysis of membrane foulants in gel layer.
The characteristics of membrane foulants and gel layer were studied by using the evaluation of MOS of organic carbons, GFC, FT-
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IR, PSD, SEM and EDX analysis in a submerged membrane bioreactor under sub-critical flux operation, and the following conclusions could be drawn. (1) Through the prediction of MOS of organic carbons, it was found that EPS and membrane foulants had different MOS of organic carbons. EPS could be evaluated as mainly proteins and polysaccharides while membrane foulants might be comprised of high organic structural complexity or other organic matters besides EPS. FT-IR spectra of membrane foulants had more peaks than that of EPS, and it was further verified that membrane foulants consisted of not only EPS (proteins, polysaccharides, etc.) but also other organic substances. (2) Particle size distribution showed that fine particles in mixed liquor had a strong deposit tendency on the membrane surfaces, and membrane foulants had much smaller size than mixed liquor in the MBR. (3) GFC analysis demonstrated that membrane foulants and SMP had much broader distributions of MW and larger Mw compared with the influent wastewater and membrane effluent. The membrane effluent had a narrow distribution of MW due to the separation function of gel layer and membrane pores. (4) SEM and EDX analysis demonstrated that membrane surfaces were covered with compact gel layer formed by organic substances and inorganic elements such as Mg, Al, Ca, Si, Fe, etc. Acknowledgements Financial support of this work by the Independent Research Fund of Chinese State Key Laboratory of Pollution Control and Resource Reuse for Young Scholars (Grant No. PCRRY08005) and by Science and Technology Commission of Shanghai Municipality (STCSM) (Grant No. 052312046) is gratefully acknowledged. References [1] P. Cote, H. Buisson, M. Praderie, Immersed membranes activated sludge process applied to the treatment of municipal wastewater, Water Sci. Technol. 38 (1998) 437–442. [2] Y. Miura, Y. Watanabe, S. Okabe, Membrane fouling in pilot-scale membrane bioreactors (MBRs) treating municipal wastewater: Impact of biofilm formation, Environ. Sci. Technol. 41 (2) (2007) 632–638. [3] Z.W. Wang, Z.C. Wu, G.P. Yu, J.F. Liu, Z. Zhou, Relationship between sludge characteristics and membrane flux determination in submerged membrane bioreactors, J. Membr. Sci. 284 (2006) 87–94. [4] P. Le-Clech, V. Chen, T.A.G. Fane, Fouling in membrane bioreactors used in wastewater treatment, J. Membr. Sci. 284 (2006) 17–53. [5] K. Kimura, N. Yamato, H. Yamamura, Y. Watanabe, Membrane biofouling in pilot-scale membrane bioreactors (MBRs) treating municipal wastewater, Environ. Sci. Technol. 39 (2005) 6293–6299. [6] R.W. Field, D. Wu, J.A. Howell, B.B. Gupta, Critical flux concept for microfiltration fouling, J. Membr. Sci. 100 (1995) 259–272. [7] G. Guglielmi, D. Chiarani, S.J. Judd, G. Andreottola, Flux criticality and sustainability in a hollow fiber submerged membrane bioreactor for municipal wastewater treatment, J. Membr. Sci. 289 (2007) 241–248. [8] B.D. Cho, A.G. Fane, Fouling transients in nominally sub-critical flux operation of a membrane bioreactor, J. Membr. Sci. 209 (2002) 391–403. [9] P. Le-Clech, B. Jefferson, I.S. Chang, S.J. Judd, Critical flux determination by the flux-step method in a submerged membrane bioreactor, J. Membr. Sci. 227 (2003) 81–93. [10] G. Guglielmi, D.P. Saroj, D. Chiarani, G. Andreottola, Sub-critical fouling in a membrane bioreactor for municipal wastewater treatment: experimental investigation and mathematical modeling, Water Res. 41 (2007) 3903–3914. [11] S. Ognier, C. Wisniewski, A. Grasmick, Membrane bioreactor fouling in subcritical filtration conditions: a local critical flux concept, J. Membr. Sci. 229 (2004) 171–177.
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Wang, Long-term study on operating charateristics of submerged flat-sheet membrane bioreactor, Ph.D. Thesis, Tongji University, Shanghai, China, 2007. [24] C.S. Laspidou, B.E. Rittmann, A unified theory for extracellular polymeric substances, soluble microbial products, and active and inert biomass, Water Res. 36 (2002) 2711–2720. [25] M. Matosic, M. Vukovic, M. Curlin, I. Mijatovic, Fouling of a hollow fiber submerged membrane during long-term filtration of activated sludge, Desalination 219 (2008) 57–65. [26] X.M. Wang, X.Y. Li, X. Huang, Membrane fouling in a submerged membrane bioreactor (SMBR): characterisation of the sludge cake and its high filtration resistance, Sep. Purif. Technol. 52 (2007) 439–445. [27] D. Al-Halbouni, J. Traber, S. Lyko, T. Wintgens, T. Melin, D. Tacke, A. Janot, W. Dott, J. Hollender, Correlation of EPS content in activated sludge at different sludge retention times with membrane fouling phenomena, Water Res. 42 (2008) 1475–1488. [28] Z.C. Wu, Z.W. Wang, Z. Zhou, G.P. Yu, G.W. Gu, Sludge rheological and physiological characteristics in a pilot-scale submerged membrane bioreactor, Desalination 212 (2007) 152–164. [29] S.H. Hong, W.N. Lee, H.S. Oh, K.M. Yeon, B.K. Hwang, C.H. Lee, I.S. Chang, S. Lee, The effects of intermittent aeration on the characteristics of bio-cake layers in a membrane bioreactor, Environ. Sci. Technol. 41 (2007) 6270–6276. [30] M. Kumar, S.S. Adham, W.R. Pearce, Investigation of seawater reverse osmosis fouling and its relationship to pretreatment type, Environ. Sci. Technol. 40 (2006) 2037–2044. [31] T. Maruyama, S. Katoh, M. Nakajima, H. Nabetani, T.P. Abbott, A. Shono, K. Satoh, FT-IR analysis of BSA fouled on ultrafiltration and microfiltration membranes, J. Membr. Sci. 192 (2001) 201–207. [32] J.P. Croué, M.F. Benedetti, D. Violleau, J.A. 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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Membrane fouling in a submerged membrane bioreactor (MBR) under sub-critical flux operation: Membrane foulant and gel layer characterization Zhiwei Wang ∗ , Zhichao Wu, Xing Yin, Lumei Tian State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China
a r t i c l e
i n f o
Article history: Received 28 May 2008 Received in revised form 15 July 2008 Accepted 19 July 2008 Available online 26 July 2008 Keywords: Critical flux Gel layer Membrane fouling Membrane bioreactor (MBR) Wastewater treatment
a b s t r a c t Membrane foulants and gel layer formed on membrane surfaces were systematically characterized in a submerged membrane bioreactor (MBR) under sub-critical flux operation. The evaluation of mean oxidation state (MOS) of organic carbons and Fourier transform infrared (FT-IR) spectroscopy demonstrated that membrane foulants in gel layer were comprised of not only extracellular polymeric substances (EPS) (proteins, polysaccharides, etc.) but also other kinds of organic substances. It was also found that fine particles in mixed liquor had a strong deposit tendency on the membrane surfaces, and membrane foulants had much smaller size than mixed liquor in the MBR by particle size distribution (PSD) analysis. Gel filtration chromatography (GFC) analysis showed that membrane foulants and soluble microbial products (SMP) had much broader distributions of molecular weight (MW) and a larger weight-average molecular weight (Mw ) compared with the influent wastewater and the membrane effluent. Scanning electron microscopy (SEM) and energy-diffusive X-ray (EDX) analysis indicated that membrane surfaces were covered with compact gel layer which was formed by organic substances and inorganic elements such as Mg, Al, Fe, Ca, Si, etc. The organic foulants coupled the inorganic precipitation enhanced the formation of gel layer and thus caused membrane fouling in the MBR. © 2008 Elsevier B.V. All rights reserved.
1. Introduction As an efficient technology for municipal and industrial wastewater treatment, submerged membrane bioreactors (MBRs) have gained increasing popularity in recent years. MBRs, in which solid/liquid separation is performed by membranes, offer several prominent advantages over conventional activated sludge (CAS) system including a smaller footprint, less sludge production and better effluent quality, etc., [1–3]. However, membrane fouling and its consequences in terms of plant maintenance and operating costs limit the widespread application of MBRs [4,5]. In order to control membrane fouling and maintain sustainable operation, the concept of critical flux was proposed by Field et al. [6] in MBRs. They defined the critical flux as the flux below which the increase of trans-membrane pressure (TMP) with time under constant flux operation does not occur; however, above that level, fouling is observed. Since then, the critical flux has been extensively applied to membrane processes from microfiltration (MF) to reverse osmosis (RO) [7]. In MBRs, the critical flux is often determined by step-wise method in short-term experiments [8,9].
∗ Corresponding author. Tel.: +86 21 65980400; fax: +86 21 65980400. E-mail address: zhiweiwang [email protected] (Z. Wang). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.07.035
However, it has been reported by a number of authors that even at very low fluxes an increase of TMP with time takes place. This has led to the introduction of so-called sustainable flux, which represents the flux value at which the fouling rate is operationally and economically acceptable for MBR operation. Flux sustainability is typically assessed by long-term trials in which a flux lower than the short-term critical one, also known as sub-critical flux, is employed and TMP is continuously monitored [10,11]. Under sub-critical flux operation, membrane fouling characteristics in MBRs have been studied by many researchers. It is generally characterized by a twostep fouling phenomenon, i.e., a gradual increase of TMP is followed by a sudden increase. This behavior has been explained with different perspectives based on local flux distribution [8,11], percolation theory [12] and in-homogeneity of fiber bundles [13], etc. It is also widely accepted that sub-critical fouling in MBRs is mainly caused by organic macro-molecules such as soluble microbial products (SMP), extracellullar polymeric substances (EPS) and possibly other substances that are released during cell lysis, diffuse through the cell membrane, and/or are lost during synthesis or are excreted for some purpose [14,15]. Many authors further dedicated to classifying the contribution of different fractions of sludge including soluble substances, colloids and suspended solids to membrane fouling [16–18]. Although those intensive efforts mentioned above are very helpful to understand the characteristics of sub-critical fouling in
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screen, raw wastewater from the WWTP was supplied into the anoxic zone, and the mixed liquor of anoxic zone then flowed into oxic zone by gravity. The characteristics of the raw wastewater are listed in Table 1. The influent pump was controlled by a water level sensor to maintain a constant water level in the bioreactor over the experimental system. The membrane-filtered effluent was then obtained by suction using a pump connected to the modules. The effluent flow rate and the TMP were monitored by a water meter and a pressure gauge, respectively. 2.2. Operating conditions
Fig. 1. Flow diagram of the pilot-scale MBR.
MBRs, there is a lack of sufficient information on the properties of gel-like layer (called as gel layer in this study) which is formed on membrane surfaces under sub-critical flux operation. The formation of gel layer (mainly caused by SMP, colloids, solutes, etc.) is different from the deposition of sludge cake (attributed to the adhesion and deposition of sludge flocs, large sludge particles, etc.) on the membrane which can be readily removed by physical washing. A detailed characterization of membrane foulants and the gel layer will be conducive to a better understanding of the sub-critical fouling, and it will also facilitate to develop membrane fouling control measures and to optimize operational parameters in MBRs. In this study, a pilot-scale MBR for the treatment of real municipal wastewater at an existing wastewater treatment plant (WWTP) was operated in order to investigate the gel layer properties formed on membrane surfaces under sub-critical flux operation. The properties of membrane foulants in the gel layer were critically examined by the evaluation of mean oxidation state (MOS) of organic carbon, Fourier transform infrared (FT-IR) spectroscopy, gel filtration chromatography (GFC), particle size distribution (PSD) analyzer, scanning electron microscopy (SEM) and energy-diffusive X-ray (EDX) analyzer, etc. The results obtained in this study are expected to provide a sound understanding of the gel layer properties and membrane fouling in MBRs under sub-critical flux operation. 2. Materials and methods 2.1. Experimental setup The pilot-scale submerged MBR as shown in Fig. 1, which was located in Quyang Municipal WWTP of Shanghai, consisted of an effective volume of 160 L anoxic and 480 L oxic zone. Nine flat sheet membrane modules (SHZZ-MF, Zizheng Environment Incorporated, Shanghai, China) were mounted vertically between two baffle plates located above an air diffuser in oxic zone. The membranes were made of polyvinylidene fluoride (PVDF) membrane with mean pore size 0.20 m. The effective filtration area for each module was 0.7 m2 . Air was monitored by a flow-rate meter and supplied through the air diffuser which was below the membrane modules in order to supply oxygen demanded by the microorganisms and to induce a cross-flow velocity (CFV) along membrane surfaces. After passing through a 0.9-mm pore-sized stainless bar
The membrane flux of the pilot-scale MBR was kept constant at about 25 L/(m2 h) which was lower than the critical flux 32–38 L/(m2 h), and a suction cycle of 10 min followed by 2 min relaxation (no suction) was applied. The anoxic and oxic hydraulic retention times (HRT) were about 1.3 h and 3.9 h, respectively. Sludge was wasted from the oxic zone to maintain a sludge retention time (SRT) of 40 d. Recycled sludge from oxic zone to anoxic zone was controlled at 3 times of the influent flow rate. The CFV along membrane surfaces was kept at about 0.3 m/s by aeration with an average air flow rate of 6 m3 /h. Dissolved oxygen (DO) concentration in the oxic zone was in the range of 1–3 mg/L while it was maintained at about 0.2 mg/L in the anoxic zone. The MBR was operated under ambient temperature in the range of 15–20 ◦ C. Chemical cleaning-in-place procedure (0.5% (v/w) NaClO solution, 2 h duration) would be carried out if TMP reached about 30 kPa during the operation. 2.3. Analytical methods 2.3.1. Membrane foulants collection and pretreatment The fouled membrane modules were taken out from the bioreactor at the end of each operation cycle when the TMP reached about 30 kPa. Gel layer on about 0.35 m2 membrane surface was carefully scraped off by a plastic sheet and simultaneously flushed with pure water. The collected sample was placed on a magnetic blender (Model JB-2, Leici Instrument Incorporated, Shanghai, China) and well mixed. Then the collected sample was prepared and could be further treated according to the requirements of specific analysis items. 2.3.2. Characterization of mean oxidation state of organic carbons 100 mL of the collected sample as described in Section 2.3.1 was filtered through a 0.45 m-pore-size-filter paper, and the filtrate was used to analyze chemical oxygen demand (COD) according to Chinese NEPA standard methods [19] and total organic carbon (TOC) by a TOC analyzer (TOC-VcPN, SHIMADZU, Japan). The MOS of the organic carbon can be computed through Eq. (1) [20]. The oxidation states of organic molecules change according to their chemical structures. Therefore, if the oxidation state of organic carbons is determined, rough estimation of the main substrates in the samples could be made. EPS were also extracted from the biomass in Table 1 Influent wastewater characteristics of the MBR (n = 43) Items
Mean concentration
COD (mg/L) TN (mg/L) NH3 -N (mg/L) TP (mg/L) SS (mg/L) pH
361.0 45.6 27.4 8.8 280.0 6.9
Values are given ± standard deviation.
± ± ± ± ± ±
221.0 20.6 11.6 3.6 220.0 0.6
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the MBR according to the thermal treatment method described by Chang and Lee [21], and then COD together with TOC of EPS sample was assayed in order to obtain the MOS of EPS. MOS of organic carbon =
4(TOC − COD) TOC
(1)
where COD is expressed in mol O2 /L and TOC in mol C/L. 2.3.3. Fourier transform infrared (FT-IR) spectroscopy Another 200 mL of the collected sample as described in Section 2.3.1 was dried at 70 ◦ C for 48 h. The dry matter was analyzed by a FT-IR spectrometer (Nicolet 5700, Thermo Electron Corporation, USA) in order to obtain the organic foulant information in the gel layer. EPS extracted from biomass were also analyzed in this study according to the procedures mentioned above. 2.3.4. Gel filtration chromatography analysis The filtrate, which was obtained by filtering 100 mL of the collected sample as described in Section 2.3.1 with a 0.45 m-poresize-filter paper, was fractionated by GFC analyzer. The GFC system consisted of a TSK G4000SW type gel column (TOSOH Corporation, Japan) and a liquid chromatography spectrometer (LC-10ATVP, SHIMADZU, Japan). Polyethylene glycols (PEGs) with molecular weight (MW) of 1,215,000 Da, 124,700 Da, 11,840 Da and 620 Da (Merck Corporation, Germany) were used as standards for calibration. It is well known that solution environments have significant effects on MW fractionation of the samples [22,23]. Therefore, pure water was used as eluent. The elution at different time intervals was collected by an automatic fraction collector and automatically analyzed by using UV spectroscopy and dissolved organic carbon (DOC) analyzer to obtain a MW distribution curve. The MW distributions of influent wastewater, MBR effluent and the SMP in the mixed liquor (SMP sample was obtained according to the treatment principle described by Laspidou and Rittmann [24]) were also carried out to elucidate the MW variations in the system. 2.3.5. Particle size distribution analysis PSD of the collected sample as described in Section 2.3.1 was carried out by a focused beam reflectance measurement (Accusizer 780, Santa Barbara Corporation, USA). Same measurement of the mixed liquor in the MBR was conducted in order to verify the differences between them. 2.3.6. Scanning electron microscopy and energy-diffusive X-ray measurement For SEM analysis, a panel of flat-sheet membrane covered with gel layer was taken out from the MBR and a piece of the membrane was cut form the middle of the fouled membrane module after one operation cycle. The sample was fixed with 2% (v/v) glutaraldehyde in 0.1 M phosphate buffer at pH 7.2 for 2 h and then washed twice for 10 min and again immersed for 1 h in 0.1 M phosphate buffer. The fixed sample was dehydrated with ethanol and coated with aurum–platinum alloy (with coating depth 10 nm) for 2 min. The coated sample was examined under a SEM (Model XL-30, Philips, Netherland). The fouled membrane was then submerged in 0.5% (v/w) NaClO solution to remove the foulants for 2 h, and SEM analysis of the cleaned membrane was also conducted according to the procedures mentioned above. The SEM coupled an EDX analyzer (Phoenix, EDAX Incorporated, USA) was employed to determine the inorganic components in the gel layer. 2.3.7. Other item analysis Measurements of COD, total nitrogen (TN), total phosphorus (TP), ammonia (NH3 -N), and pH in the influent and membrane effluent, mixed liquor suspended solids (MLSS) and mixed liquor
Table 2 Average characteristics of treated water (n = 43) Items (mg/L)
Mean concentration
COD TN NH3 -N TP SS
22.0 ± 16.0 15.0 ± 5.6 1.9 ± 1.1 4.0 ± 1.8 0.0
Values are given ± standard deviation.
volatile suspended solids (MLVSS) in the system were performed according to Chinese NEPA standard methods [19]. DO concentration in the reactor was measured by a dissolved oxygen meter (Model YSI 58, YSI Research Incorporated, OH, USA). CFV was determined using Cup-type Current Meter (Model LS45A, Chongqing Hydrological Instrument Incorporated, Chongqing, China). 3. Results and discussion 3.1. Process performance During the operation, MLSS concentration in anoxic zone and oxic zone of the MBR was maintained at about 15 g/L and 18 g/L, respectively, by extraction of excess sludge. Table 2 summarizes the average characteristics of treated water in the MBR. It can be seen that the removal of COD, ammonia, TN and suspended solids (SS) was quite successful. About 60% TP removal was also achieved in the MBR during the experiment. The variations of membrane flux and TMP with operation time are demonstrated in Fig. 2. It can be observed that the TMP increased with operation time as membrane flux was kept at about 25 L/(m2 h) (sub-critical flux) during the experiment. The TMP variations were characterized by a two-step fouling phenomenon, i.e., a slow increase of TMP followed by a rapid increase. When TMP reached about 30 kPa, chemical cleaning procedure was carried out to remove membrane fouling and to recover the membrane permeability. Before each cleaning process, membrane modules were taken out from the system in order to observe and examine the membrane foulants formed on the membrane surfaces. It is worth noting that there was no obvious sludge cake on membrane surfaces but slime gel layer. The fouling behavior demonstrated that sludge cake fouling mainly caused by suspended sludge flocs, to a great extent, could be controlled by adopting sub-critical flux operation [25], and the gradual formation of gel layer on membrane surfaces led to the gradual increase of TMP in the MBR. Many researchers [2,26] argued that sludge cake was found on membrane
Fig. 2. Variations of membrane flux and TMP during the experiment. (An arrow indicates that chemical cleaning of membrane was performed on that day.)
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Table 3 Mean oxidation state of organic carbon of different substances (n = 6) Organic substances
COD/TOC
Mean oxidation state of carbon
Estimation of components
EPS Gel layer
1.06 ± 0.02 1.60 ± 0.21
−0.23 ± 0.18 −2.45 ± 0.78
Proteins and polysaccharides –
Values are given ± standard deviation.
surface when they studied membrane fouling in MBRs, which might be due to the fact that they did not employ the sub-critical flux operation. In our study, it was verified that under sub-critical flux operation the sludge cake fouling was significantly controlled while the fouling caused by macromolecules, colloids and SMP, etc., was inevitable to form a gel layer in the MBR. 3.2. MOS of organic carbons of gel layer and that of EPS The MOS of organic carbons in gel layer and that in EPS, computed according to Eq. (1), is listed in Table 3. It can be seen that the MOS of organic carbons of membrane foulants is different from that of EPS, which means that the organic foulants in the gel layer were not completely formed by EPS. Although it has been believed that EPS play an important role in membrane fouling process of MBRs and might be a vital factor to cause the formation of fouling layer [27–29], further research on the organic components in the gel layer needs to be carried out based on the results of MOS of organic carbons. Besides EPS, there might be other organic substances in gel layer. From the estimation protocol of Stumm and Morgan [20], it can be easily predicted that major components of EPS are proteins and polysaccharides while it could hardly estimate the existing organic substances in gel layer. It might indicate that the gel organic material had a relatively high organic structural complexity. In the
following study, other technical measures were employed to characterize the organic components in gel layer. 3.3. FT-IR analysis The FT-IR spectra of membrane foulants in gel layer and EPS are illustrated in Fig. 3. In Fig. 3(a), the spectrum shows a broad region of adsorption around a peak at 3421 cm−1 , which is attributed to stretching of the O H bond in hydroxyl functional groups, and a sharper peak at 2932 cm−1 , which is due to stretching of C H bonds [30]. Two sharp peaks (1653 cm−1 and 1558 cm−1 ) are also observed in the spectrum, which are unique to the protein secondary structure, namely amides I and II [31]. It indicates that proteins were one of components of the gel layer. The peaks at 1378 cm−1 and 1247 cm−1 might be due to the presence of methyl and C O (ester) bonds, respectively. In addition, a broad peak at 1045 cm−1 exhibits the character of polysaccharides or polysaccharides-like substances [32], which indicates that polysaccharides were presented in the gel layer. Another broad peak appears at 600 cm−1 in the finger print region of the spectrum. In comparison with Fig. 3(a), the spectrum of EPS in Fig. 3(b) shows similarities on the peaks of 3427 cm−1 , 1638 cm−1 and 1082 cm−1 , which demonstrates the presence of O H bonds, proteins and polysaccharides in EPS. However, it could be observed that the number of peaks in EPS spectrum is less than that of membrane foulant spectrum especially between 1000 cm−1 and 1600 cm−1 , which illustrates the organic substances in membrane foulants were more complicated than those in EPS. From the FTIR spectra, it could be seen that membrane foulants include not only EPS (proteins, polysaccharides, etc.) but also other organic substances featured by the functional groups as mentioned above. The results are also supported by the evaluation of MOS of organic carbons in gel layer and that in EPS as described in Section 3.2, and gel organic material might have high organic structural complexity compared with EPS. 3.4. MW distribution analysis by GFC technology In principle, large MW molecules are excluded earlier than smaller ones because they are unable to travel through the gel
Fig. 3. FT-IR spectra of membrane foulants and EPS. (a) Membrane foulants; (b) EPS.
Fig. 4. Relationship between log(MW) and exclusion time.
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Fig. 5. GFC chromatograms of influent wastewater, membrane effluent, SMP in mixed liquor and membrane foulants. (a) Chromatograms of influent wastewater and membrane effluent; (b) chromatograms of SMP and membrane foulants.
pores, and then high correlation between the exclusion time and the MW, as shown in Fig. 4, was obtained. The GFC chromatograms of the influent wastewater, membrane effluent, SMP in mixed liquor and membrane foulants are shown in Fig. 5. It could be observed that before 12th min there are peaks in chromatograms of SMP and membrane foulants (see Fig. 5(b)) while no peaks appear in Fig. 5(a) during this period. It indicates that high MW molecules, according to the correlation between exclusion time and MW as illustrated in Fig. 4, existed in SMP and membrane foulants while they were absent in the influent and membrane effluent. The high MW molecules are produced by microorganisms during substrate metabolism and/or released during cell lysis and decay, and they have been widely considered and accepted as a major factor resulting in membrane fouling [14,15,21,24]. The intermediate MW molecules were also major components of membrane foulants which could be seen from the sharp peaks presented from 12 min to 20 min. The absence of high MW molecules (before 12th min in terms of exclusion time) in membrane effluent was attributed to the fine pores of membrane and to the gel layer formed on membrane surfaces. However, the low MW molecules existing in the SMP were able to permeate through the gel layer and membrane pores, thus resulting in the soluble COD in the effluent of MBR. It was also reported that the presence of low MW fraction was observed in a MBR for the treatment of landfill leachate [33]. In order to better understand MW distributions of the samples, number-average molecular weight (Mn ), weight-average molecular
Fig. 6. Particle size distribution of membrane foulants and mixed liquor in the MBR. (a) Membrane foulants; (b) mixed liquor in the MBR. (X-coordinate represents the particle size (m) while Y-coordinate indicates particle number percentage (%).)
weight (Mw ) and the coefficient of MW distribution (Mw /Mn ) were used in this study. A low coefficient of Mw /Mn indicates that the organic substance has a narrow distribution of MW. Table 4 presents the MW distributions of the influent, membrane effluent, SMP and membrane foulants. Compared with the influent wastewater and membrane effluent, membrane foulants and SMP had much broader distributions of MW, of which the Mw /Mn was 426.54 and 243.62, respectively. It demonstrated that microorganisms generated SMP with a broad MW distribution besides consuming influent wastewater (with a relative narrow MW distribution) and making new biomass. During the filtration process, the SMP and other organic substances such as colloids, solutes, etc., existing in mixed liquor would block membrane pores and/or deposit on membrane surfaces to form a fouling layer (called as gel layer in this study) due to the fine pores of membranes. The formation of gel layer, in turn, would retain macromolecules and large particles (making the layer more compact simultaneously) but allow the substances with low MW substances passing through the layer and through membrane pores to enter effluent water. Therefore, the membrane effluent had a narrow distribution of MW, with Mw /Mn coefficient 7.07 as shown in Table 4.
Table 4 MW distributions of the influent, membrane effluent, SMP and membrane foulants Items
Influent wastewater
Membrane effluent
SMP
Membrane foulants
MW (kDa) Mw (kDa) Mn (kDa) Mw /Mn
16.8–7426.1 228.2 40.5 5.63
0.9–1215.0 142.1 20.1 7.07
0.9–35450.6 4092.7 16.8 243.61
0.2–201075.0 6824.6 16.0 426.54
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Fig. 7. SEM photographs of cleaned membrane surface and fouled membrane surface. (a) Cleaned membrane; (b) fouled membrane.
3.5. Particle size distribution analysis The PSD of membrane foulants and mixed liquor in MBR is presented in Fig. 6(a) and (b), respectively. From Fig. 6(a), it can be observed that the membrane foulants had a narrow range profile of size distribution, which mainly distributed between 0.5 m and 2.0 m. To the contrary, the mixed liquor in the MBR had a broader size distribution profile ranging from 0.5 m to over 100 m, which was featured with two sharp peaks at 0.57 m and 20.0 m. Statistical analysis of particle distributions demonstrated that about 88% of the membrane foulants distributed in a size range of 0–2.0 m while there are only 19% of the particles in mixed liquor distributed in the size range of 0–2.0 m. The results showed that particle size of membrane foulants was much smaller than that of mixed liquor in the MBR. In membrane filtration process, it is known that the particle transport is mainly dependent on two forces, i.e., the permeation drag force and the shear stress induced by CFV. During the operation of MBR, the small particles in the mixed liquor including colloidal substances and macromolecules of SMP could easily deposit on the membrane surfaces by permeation drag to form a gel layer fouling, and not easily detached by the CFV because of their low back transport velocity [34]. Large particles in the mixed liquor like suspended microbial flocs could be detached from membrane surfaces due to the shear stress induced by aeration and thus under sub-critical flux operation, membrane fouling caused by large sludge particles, to a great extent, could be controlled. Colloidal substances, SMP and other fine particles were predominant foulants to cause the formation of gel layer in MBR under sub-critical flux operation. Rosenberger et al. [35] have also reported that macromolecules and
organic colloids with a MW of >120 kPa contributed significantly to membrane fouling and higher fouling rates were observed at higher concentrations of these substances. 3.6. SEM and EDX measurement SEM images of cleaned membrane and fouled membrane are presented in Fig. 7(a) and (b), respectively. It reveals that the fouled membrane was covered with slime gel layer. Once the gel layer was developed, it would become difficult to remove the layer from the membrane surfaces by routine aeration. Consequently, the formation of gel layer resulted in the increase of TMP and caused severe irreversible membrane fouling in this study. Element analysis was further performed on the surface layer in order to identify the chemical components of the layer by EDX analysis. The elements of C, O, N, Cl, P, Na, Mg, Fe, Al, K, Ca and Si were detected and shown in Fig. 8. It should be pointed out that the sharp peak of Au element in this diagram is caused by the coated aurum on the sample and irrelevant to the elemental analysis of the membrane foulants. Mg, Al, Fe, Ca and Si had significant effects on the formation of gel layer though the relative contents of these elements were small. The biopolymers contain anion groups such as SO4 2− , CO3 2− , PO4 3− and OH− , and the cations such as Mg2+ , Al3+ , Fe3+ , and Ca2+ could be easily precipitated by these negative ions [36]. It has been identified that the major components of inorganic foulants were MgNH4 PO4 ·6H2 O (struvite) in the membrane-coupled an anaerobic bioreactor for alcohol-distillery wastewater treatment [37]. It was also reported by You et al. [38] that the MBR with internal membrane system had severe inorganic fouling problem with calcium addition to the system. The organic foulants as mentioned in above sections coupled the inorganic precipitation would enhance the formation of gel layer and thus caused the fouling behavior in the MBR. In order to control membrane fouling caused by gel layer in submerged MBR under sub-critical flux operation, the organic substances such as SMP, colloids and solutes, etc., together with inorganic matters in the bioreactor should be controlled. The inorganic matters should be adjusted to a proper concentration by pretreating the influent wastewater. The concentration of colloidal and soluble organic materials in mixed liquor could be, to some extent, eliminated and controlled by optimizing operational parameters such as SRT, HRT, volumetric organic load (VOL) and so on [4,13,21,27–29]. 4. Conclusions
Fig. 8. EDX analysis of membrane foulants in gel layer.
The characteristics of membrane foulants and gel layer were studied by using the evaluation of MOS of organic carbons, GFC, FT-
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IR, PSD, SEM and EDX analysis in a submerged membrane bioreactor under sub-critical flux operation, and the following conclusions could be drawn. (1) Through the prediction of MOS of organic carbons, it was found that EPS and membrane foulants had different MOS of organic carbons. EPS could be evaluated as mainly proteins and polysaccharides while membrane foulants might be comprised of high organic structural complexity or other organic matters besides EPS. FT-IR spectra of membrane foulants had more peaks than that of EPS, and it was further verified that membrane foulants consisted of not only EPS (proteins, polysaccharides, etc.) but also other organic substances. (2) Particle size distribution showed that fine particles in mixed liquor had a strong deposit tendency on the membrane surfaces, and membrane foulants had much smaller size than mixed liquor in the MBR. (3) GFC analysis demonstrated that membrane foulants and SMP had much broader distributions of MW and larger Mw compared with the influent wastewater and membrane effluent. The membrane effluent had a narrow distribution of MW due to the separation function of gel layer and membrane pores. (4) SEM and EDX analysis demonstrated that membrane surfaces were covered with compact gel layer formed by organic substances and inorganic elements such as Mg, Al, Ca, Si, Fe, etc. Acknowledgements Financial support of this work by the Independent Research Fund of Chinese State Key Laboratory of Pollution Control and Resource Reuse for Young Scholars (Grant No. PCRRY08005) and by Science and Technology Commission of Shanghai Municipality (STCSM) (Grant No. 052312046) is gratefully acknowledged. References [1] P. Cote, H. Buisson, M. Praderie, Immersed membranes activated sludge process applied to the treatment of municipal wastewater, Water Sci. Technol. 38 (1998) 437–442. [2] Y. Miura, Y. Watanabe, S. Okabe, Membrane fouling in pilot-scale membrane bioreactors (MBRs) treating municipal wastewater: Impact of biofilm formation, Environ. Sci. Technol. 41 (2) (2007) 632–638. [3] Z.W. Wang, Z.C. Wu, G.P. Yu, J.F. Liu, Z. Zhou, Relationship between sludge characteristics and membrane flux determination in submerged membrane bioreactors, J. Membr. Sci. 284 (2006) 87–94. [4] P. Le-Clech, V. Chen, T.A.G. Fane, Fouling in membrane bioreactors used in wastewater treatment, J. Membr. Sci. 284 (2006) 17–53. [5] K. Kimura, N. Yamato, H. Yamamura, Y. Watanabe, Membrane biofouling in pilot-scale membrane bioreactors (MBRs) treating municipal wastewater, Environ. Sci. Technol. 39 (2005) 6293–6299. [6] R.W. Field, D. Wu, J.A. Howell, B.B. Gupta, Critical flux concept for microfiltration fouling, J. Membr. Sci. 100 (1995) 259–272. [7] G. Guglielmi, D. Chiarani, S.J. Judd, G. Andreottola, Flux criticality and sustainability in a hollow fiber submerged membrane bioreactor for municipal wastewater treatment, J. Membr. Sci. 289 (2007) 241–248. [8] B.D. Cho, A.G. Fane, Fouling transients in nominally sub-critical flux operation of a membrane bioreactor, J. Membr. Sci. 209 (2002) 391–403. [9] P. Le-Clech, B. Jefferson, I.S. Chang, S.J. Judd, Critical flux determination by the flux-step method in a submerged membrane bioreactor, J. Membr. Sci. 227 (2003) 81–93. [10] G. Guglielmi, D.P. Saroj, D. Chiarani, G. Andreottola, Sub-critical fouling in a membrane bioreactor for municipal wastewater treatment: experimental investigation and mathematical modeling, Water Res. 41 (2007) 3903–3914. [11] S. Ognier, C. Wisniewski, A. Grasmick, Membrane bioreactor fouling in subcritical filtration conditions: a local critical flux concept, J. Membr. Sci. 229 (2004) 171–177.
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