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A new insight into membrane fouling mechanism during membrane filtration of bulking and normal sludge suspension
Journal of Membrane Science 285 (2006) 159–165
A new insight into membrane fouling mechanism during membrane filtration of bulking and normal sludge suspension Fangang Meng, Fenglin Yang ∗ , Jingni Xiao, Hanmin Zhang, Zheng Gong School of Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, China Received 19 March 2006; received in revised form 14 August 2006; accepted 14 August 2006 Available online 26 August 2006
Abstract Membrane bioreactor (MBR) is an important solid–liquid separation technology employed widely in the wastewater treatment. However, membrane fouling is the major problem that hinders the practical application of membrane bioreactor (MBR) systems. A major obstacle for the application of MBRs is the rapid decline of the permeation flux as a result of sludge filamentous bulking. In this paper a series of membrane filtration tests were performed to evaluate the fouling behaviors of bulking sludge and normal sludge. In order to specify the fouling mechanism of bulking sludge, the floc morphology of bulking sludge and normal sludge was studied based on fractal theory and image analysis. The sludge characteristics was also evaluated in terms of extracellular polymeric substances (EPS), relative hydrophobicity (RH) and sludge viscosity. The results showed that the bulking sludge resulted in a greater flux decline rate than normal sludge. The bulking sludge could cause a severe cake fouling, which was induced by the fixing action of filamentous bacteria. The results also showed that the sludge flocs had a good fractal characteristic, and the boundary fractal dimension of bulking sludge flocs was much larger than that of normal sludge. The flocs of bulking sludge had more irregular shape than the flocs of normal sludge. The irregular shape of bulking sludge would do great harm to membrane filtration process. The higher EPS concentration, RH and sludge viscosity of bulking sludge would speed up membrane fouling further. © 2006 Elsevier B.V. All rights reserved. Keywords: Membrane bioreactor; Membrane fouling; Filamentous bacteria; Sludge floc morphology; Fractal theory
1. Introduction In recent years, membrane bioreactors (MBRs) have been widely used in wastewater treatment to achieve higher effluent quality, which is often difficult to be effectively met by conventional activated sludge process. The advantages of MBR are a high mixed liquid of suspended solids (MLSS) concentration, up to 35,000 mg/L, producing higher rate of removal of biological oxygen demand (BOD) and chemical oxygen demand (COD), a lower excess sludge production and the production of treated water can be reused [1]. In addition, the space occupied by MBR systems is greatly reduced due to the absence of settling tanks and the reduction in bioreactor volume made possible by the higher biomass concentration. But a major obstacle for the application of MBRs is the rapid decline of the permeation
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Corresponding author. Tel.: +86 411 84706172; fax: +86 411 84708084. E-mail addresses: [email protected] (F. Yang), [email protected] (F. Meng). 0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.08.020
flux as a result of membrane fouling [2]. That is to say, membrane fouling reduces the productivity of MBRs and enhances its maintenance and operation fee, thus membrane fouling is the key problem should be solved. The overgrowth of filamentous bacteria in sludge suspension could result in severe membrane fouling due to formation of a thick and non-porous cake layer [3]. It also showed that filamentous bacteria had significant influence on sludge floc morphology [3,4]. As filamentous bacteria grew excessively in activated sludge, the sludge flocs became big and irregular, which would do great harm to membrane permeation [3]. It indicates that the floc morphology of sludge suspension has significant influence on membrane performance in MBR systems. The excessive growth of filamentous microorganisms could lead to large floc size and loose floc structure [5,6]. Fractal dimension is often used to describe the geometric characteristics of the multi-leveled floc structure, e.g., activated sludge flocs [6–8]. Euclidean geometry describes regular objects such as points, curves, surfaces, and cubes using integer dimensions 0–3, respectively. Associated with each dimension is a
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measure of the object such as the length of a line, area of a surface and volume of a cube. Since sludge floc aggregates are disordered and irregular, the floc structure cannot be described by Euclidean geometry due to the scale-dependent measures of length, area and volume. These objects are called fractals, and their dimensions are non-integral and defined as fractal dimensions. A fractal object can be divided into parts, each of which is similar with the whole. Fractal theory has been used widely in the study of membrane fouling during membrane filtration of colloids [9]. Khatib et al. [10] studied chemically assisted UF of raw water and suggested that the fractal properties of the aggregates formed may have influence on the membrane filtration behavior. In previous work [11–13], it was found that reaction-limited aggregation (RLA) leads to compact floc (high fractal dimension) and diffusionlimited aggregation (DLA) produces more open floc (lower fractal dimension) with the actual values of fractal dimension dependent upon the mode of flocculation (e.g., cluster–cluster, particle–cluster, etc.), and showed that the cake layer formed by RLA was more compact than the cake layer formed by DLA. These previous literatures provide us a new approach to study fouling mechanism of filamentous bacteria from the viewpoint of floc morphology analysis. In the present work, batch membrane filtration tests were carried out to study the effect of sludge floc morphology on membrane fouling. These tests were based on two kinds of classical sludge: bulking sludge (bulking sludges 1 and 2) and normal sludge (normal sludges 1 and 2). The objective of this study is to investigate fouling mechanism of filamentous bacteria by characterizing floc shape. In this paper, membrane fouling behavior of these sludge samples were studied, the fractal dimension (DP ), roundness (Ro), form factor (FF), and aspect ratio (AR) of sludge flocs were analyzed to comparison the impacts of bulking sludge and normal sludge on membrane fouling. 2. Materials and methods 2.1. Batch filtration tests A hollow fiber membrane module made of polyethylene (DAIKI, Japan) that had a total area of 0.1 m2 and a normal pore size of 0.1 m was used in the tests. The membrane module was submerged in 12 L of mixed liquor with aeration of 200 L/h, and membrane filtration was carried out by applying 4.0 kPa of trans-membrane pressure with a suction pump. Filtration was continued for 240 min, which allowed the permeation flow rate to become stable, and then the fouling resistance was estimated by Darcy’s equation as follows TMP Rt = Rm + Rf = Rm + Rp + Rc = μJ
(1)
where Rt is total hydraulic resistance, Rm the membrane resistance, Rp the pore blocking resistance, Rc the cake layer resistance, TMP the transmembrane pressure, μ the dynamic viscosity, and J is the membrane flux. The experimental procedure to get each resistance value was as follows [14,15]: (1) Rm was estimated by measuring the water flux of de-ionized (DI) water; (2)
Rt was evaluated by the final flux of biomass microfiltration; (3) the membrane surface was then flushed with water and cleaned with a sponge to removal the cake layer. After that, the DI water flux was measured again to get the resistance of Rm + Rp . The pore blocking resistance (Rp ) was calculated from steps (1) and (3) and the cake resistance (Rc ) from (2) and (3). To ensure that the condition and performance of the membrane module was almost the same in all experiments, postcleaning was performed after every experiment to remove the fouling cake and the membrane module was immersed in 0.03% NaClO solution for 24 h to obtain a permeability recovery more than 96%. 2.2. Analytical methods 2.2.1. Sludge suspension The sludge samples for batch filtration tests were taken from different stages of two large MBRs, which were applied for wastewater treatment. It indicates that the sludge samples had been acclimatized by membrane bioreactor systems. Because the sludge suspension used in our tests was taken from other MBR systems, this research can reflect the real fouling mechanism of membrane bioreactor. The MLSS concentration of each sludge suspension was adjust to about 6000 mg/L with its supernatant prior to the membrane filtration test in order to exclude the concentration effect on membrane flux. 2.2.2. Microscopy and image analysis The sludge flocs were characterized by means of microscopic observation (Olympus, BH2-RFCA). The structure parameters of sludge flocs were determined using a Leica Q500IW image processing and analysis system (Leica Cambridge Ltd., Cambridge, UK). Images were acquired using microscopic observation are JEPG format coded with True Color (Fig. 3a and b). Images were transferred to gray-scale formation (256 grey-scale levels) with Adobe Photoshop 7.0 software. Before analyzing an image, a threshold has to be determined in order to distinguish flocs from the background, and obtain a binary image (Fig. 3c and d). For each binary image, threshold was estimated as the grey level value that corresponded to that maximum of the grey level histogram second derivation [16]. For the binary images, pixels have a value of either one or zero, enabling easy processing of the images using the computer. A series of at least 40 images were analyzed for each sludge sample to obtain accurate data. The floc area, floc perimeter and morphological parameters can be determined by analysis the binary images. 2.2.3. Boundary fractal dimension of sludge flocs A perimeter–area relationship of fractal dimension analysis has been developed for sludge floc analysis [17]. The perimeter P follows a power law of the area A according to the fractal law described by Feder [18]: P ∼ ADP /2
(2)
where DP is the fractal dimension of the sludge flocs. This implies that the fractal nature of the contour of the flocs. The
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boundary dimensions for regular objects, such as circle or square, are equal to one (DP = 1). The perimeter and area of sludge flocs were determined by image analysis system. Thus, a Richardson plot of ln P versus ln A yields a straight line of slope DP /2, enabling the fractal dimension, DP , of the sludge flocs to be determined. 2.2.4. Shape characterization The sludge floc size was determined by focused beam reflectance measurement (Model M400L, Lasentec, Redmond, USA). The floc size distribution was adopted to characterize its effect on membrane fouling. Image analysis techniques have been used widely in the study of sludge floc structure [19]. In order to compare floc morphology of bulking sludge and normal sludge, the following shape parameters were examined. Roundness (Ro): Ro was defined as the ratio between the object area (A) to the area of a circle with a length equal to the object length (L), which indicates to what an extant the measured floc is similar to the true circle [4]. For a circle Ro = 1 and it decreases as more elongated the object is. Ro was calculated as follows Ro =
4πA L2
(3)
The form factor (FF) describes the deviation of an object form a circle. It is particularly sensitive to the “roughness” of the boundaries [20]. Ro was defined as the ratio between the object area (A) to the area of a circle with a perimeter equal to the object perimeter (P). For a circle FF = 1. FF was defined as follows FF =
4πA P2
(4)
The three-dimensional aspect ratio (AR) is applied to describe the extension of an object [20]. The more elongated it is, the larger the value of this parameter is. For a circle AR = 1. AR was evaluated as follows 4 length AR = 1.0 + − 1.0 (5) π width 2.2.5. Analysis of sludge characteristics For each sludge sample the activated sludge properties including EPS, dynamic viscosity and RH were analyzed to investigate the correlation between these properties and membrane fouling resistance. The extraction of EPS was based on a cation ion exchange resin (Dowex-Na form) method [21]. EPS was normalized as the sum of carbohydrate and protein, which were analyzed using phenol/sulfuric-acid method and folin method [22], respectively. The sludge floc size was determined by focused beam reflectance measurement (Model M400L, Lasentec, Redmond, USA). The dynamic viscosity was determined using a rotational viscosity meter (Model NDJ-7, Shanghai, China). The relative hydrophobicity (RH) was evaluated similar to Wil´en et al. [23]. Each sample was measured three times with a standard deviation 1–8%, 2–7%, 2–5% for EPS, dynamic viscosity and RH, respectively. For the sludge samples the membrane fouling
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tests and activated sludge analysis were performed within 10 h to avoid changes in sludge characteristics. In order to give exact information about sludge properties, the sludge suspension for analysis was sampled during the membrane filtration test. 2.2.6. Scanning electron microscope The fouled membrane surface was observed with the help of a scanning electron microscope (SEM) (KYKY-2800B, Beijing, China). As the membrane filtration was stopped, a piece of membrane fiber was cut from the middle of the membrane module. The sample was fixed with 3.0% glutaraldehyde in 0.1 M phosphate buffer at pH 7.2. The sample was dehydrated with ethanol, silver-coated by a sputter and observed in the SEM. 3. Results and discussion 3.1. Comparison of membrane fouling behavior Since the typical operating conditions of MBR systems, such as HRT, influent feed, and F/M ratio, are more likely to induce excessive growth of filamentous bacteria in sludge suspension, a comparison study was made between sludge suspension, normal sludge and bulking sludge, with respect to membrane fouling. Fig. 1 shows the decline behavior of membrane flux during membrane filtration of normal sludge suspension and bulking sludge suspension. It was observed that the bulking sludge suspension showed a much greater flux decline than the normal sludge suspension. This result indicates that an excessive growth of filamentous bacteria has serious negative effect on membrane permeation. Our previous study also implied that the sludge flocs with a small quantity of filamentous bacteria (normal sludge suspension) could benefit membrane permeability [3]. The decline of membrane flux could be caused by two factors: the thickness of the cake layer and the compactness of the cake layer. To examine the fouling tendencies, each resistance term was calculated (Table 1). As more activated sludge particles were deposited on the membrane surface, the cake resistance increased dramatically and became the dominant resistance. Table 1 indicates that the bulking sludge showed a higher cake layer resistance than normal sludge. The cake resistance of
Fig. 1. Comparison of flux decline behavior during membrane filtration of normal sludge suspension and bulking sludge suspension.
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Table 1 A series of resistances during microfiltration of different sludge samples Items (%)
Normal sludge 1 Normal sludge 2 Bulking sludge 1 Bulking sludge 2
Rm (×1011 m−1 )
Rp (×1011 m−1 )
Rc (×1011 m−1 )
Rt (×1011 m−1 )
1.05 (21.97) 1.05 (20.51) 1.05 (9.55) 1.05 (4.61)
1.27 (26.57) 1.14 (22.27) 1.53 (13.91) 2.01 (8.83)
2.46 (51.46) 2.93 (57.23) 8.42 (76.55) 19.70 (86.56)
4.78 5.12 11.00 22.76
Fig. 2. SEM images showing the surfaces of fouled membrane: (a) normal sludge and (b) bulking sludge.
bulking sludge (8.42, 19.70 × 1011 m−1 ) was more than two times of normal sludge (2.46, 2.93 × 1011 m−1 ), and its proportion shifted from 51.46% and 57.23% to 76.55% and 86.56%. But, the over growth of filamentous bacteria had little influence on pore blocking resistance. In general, Membrane pore blocking mainly results from the adsorption of colloids, macro-
molecules, biologically active substances, various ions, etc. In bulking sludge suspension, the filamentous bacteria can capture these matters, so there were few colloids and macromolecules in the supernatant of bulking sludge. Thus, here bulking sludge did not induce severe pore blocking. Therefore, with respect to bulking sludge suspension, the formation of a cake layer on the
Fig. 3. Microscopic images of sludge flocs: (a) initial true color image of normal sludge, (b) initial true color image of bulking sludge, (c) binary image of normal sludge, and (d) binary image of bulking sludge, respectively (1000×).
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membrane surface was the major factor that affects membrane fouling behavior. SEM images of fouled membrane surface were taken at the end of the operation (Fig. 2). The membrane fouling was mostly attributed to the pore blocking as well as the formation of a cake layer. From Fig. 2 it can be seen that the cake layer formed with normal sludge (Fig. 2a) had obvious pores though it appeared to be less porous than clean membrane. The cake layer formed with bulking sludge was dense in appearance, which results from the deposition of fiber-shape bacteria and other materials on the membrane surface (Fig. 2b). Filamentous bacteria acted as the framework in the cake layer. In the membrane filtration, the deposition of sludge flocs is determined by two factors: the suction force, which is generated by transmembrane pressure, and the shear force, which is generated by aeration. With the suction force the bulking flocs can easily accumulate on the membrane surface due to their irregular morphology. The filamentous bacteria had a fixing action on the membrane foulants, which adhere and penetrate between the membrane and membrane foulants (Fig. 2b). Thus, the bulking sludge could result in a severe cake fouling. 3.2. Evaluation of fractal dimension It can be seen that the sludge flocs of normal sludge were much smaller than that of bulking sludge in appearance (Fig. 3a). The sludge floc structure, however, became irregular as filamentous bacteria grew excessively (Fig. 3b). As the floc size increased, the density decreased, and the porosity increased [24].
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Table 2 Comparison of the sludge floc morphology between normal sludge and bulking sludge Items
Normal sludge 1 Normal sludge 2 Bulking sludge 1 Bulking sludge 2
DP
Ro
1.118 1.210 1.534 1.475
0.73 0.68 0.25 0.31
FF ± ± ± ±
0.18 0.20 0.14 0.09
0.87 0.79 0.61 0.57
AR ± ± ± ±
0.11 0.15 0.10 0.16
2.23 2.35 3.40 3.13
± ± ± ±
0.41 0.22 0.55 0.64
Analyzing a sludge floc image with image analysis system, the overall area of the image Sc was about 40,000 m2 . The perimeter and area of sludge flocs were also evaluated by image analysis. A double logarithmic plot of the floc area and floc perimeter of the sludge is shown in Fig. 4. Good linearity with high correlation coefficient (R2 = 0.898, 0.886) was observed, suggesting the floc structure of sludge flocs was similar with each other. Thus, the floc structure characterized by the floc area and floc perimeter was a typical fractal structure. From Fig. 4 and Table 2 it can be seen that the boundary fractal dimensions of bulking sludge (DP = 1.534, 1.475) were much larger than those of normal sludge (DP = 1.118, 1.210). These values are different from the recent result of Zartarian et al. [17] who found an average value of 1.31. The results indicate that the bulking sludge had a more irregular shape than normal sludge. This result also implies that flocs with excessive filaments were generally less strong. With a great amount of filaments growing inside, the shape and structure of flocs became very open and irregular (Fig. 3b). The filamentous bacteria are believed to form the “backbone” of activated sludge flocs to which floc-forming bacteria attach by means of EPS and form strong flocs. In the absence of filaments, “pin-floc” will be formed. These tiny and weak flocs will tend to contribute to membrane pore blocking dramatically [3]. However, an excess of filamentous bacteria would produce an abundance of filaments extending from the flocs into the bulk solution, producing a bridging lattice, which prevents the agglomeration of floc particles [25]. Therefore, the flocs with excess filamentous bacteria are big and irregularly shaped. Additionally, from previous studies it also can be seen that the bulking sludge has a very poor settleability [26]. 3.3. Characterization of floc shape
Fig. 4. Determination of fractal dimension (2 × slope) plotting double logarithmic of P vs. A: (a) normal sludge flocs and (b) bulking sludge flocs.
The floc size distributions of each sludge sample were measured, which are shown in Fig. 5. It indicates that the sludge floc size of bulking sludge was larger than that of normal sludge (Fig. 5). The over growth of filamentous bacteria in sludge suspension could cause an increase of floc size [3]. Usually, the sludge suspension with larger floc size would benefit membrane filtration process [2,27]. However, in our case the bulking sludge with higher floc size resulted in severe membrane fouling. The data obtained from the current investigation, together with previous work in the literature, indicate that floc size distribution just is one of the factors that affect membrane fouling. Other factors
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Table 3 Comparison of the sludge characteristics between normal sludge and bulking sludge EPS
Normal sludge 1 Normal sludge 2 Bulking sludge 1 Bulking sludge 2
Carbohydrates (mg/g MLSS)
Proteins (mg/g MLSS)
P/C
Total (mg/g MLSS)
16.07 12.79 41.27 52.00
29.55 22.59 97.52 112.24
1.84 1.77 2.36 2.16
45.62 35.38 138.79 164.24
such as floc shape may also have certain impacts on membrane fouling process. Therefore, it is necessary to investigate the effect of floc morphology on membrane fouling mechanism. The fractal dimension of sludge flocs has been discussed in Section 3.2. As shown in Table 2, the roundness of bulking sludge was smaller than that of normal sludge, which indicates that the shape of normal sludge is much closer to a circle. It has been shown that FF and AR seem to be the most suited morphological parameters to estimate the settleability of activated sludge [20]. From Table 2, it can be seen that the form factor of bulking sludge was smaller than that of normal sludge; however, the aspect ratio of bulking sludge was larger than that of normal sludge. These results also indicate that the bulking sludge flocs had more irregular shape than normal sludge floc. The irregular-shaped sludge flocs had a tendency to adhere onto the membrane surface, and there easily intertwisted on the membrane fibers. Moreover, as a role of fixing substance, the filamentous bacteria resulted in more foulants adhering to membrane and enhance their clinging intensity, which worsen the membrane permeability seriously. 3.4. Comparison of sludge characteristics Except for floc morphology, another difference between bulking sludge and normal sludge is the physicochemical characteristics of sludge suspension. The bulking sludge had higher EPS concentration than normal sludge (Table 3). From Table 3, it also can be seen proteins appeared to be the major component of EPS. Many previous studies have demonstrated that the over growth of filamentous bacteria could result in much more release of EPS, and did severe harm to membrane permeation [3,28]. As shown in Table 3, the bulking sludge had higher RH values than those of normal sludge. It has been reported that
RH (%)
Viscosity (mPa s)
37.00 40.33 84.91 93.66
1.23 1.45 3.05 3.71
attached bacteria were much more hydrophobic than their suspended counter parts [29,30]. These results demonstrated that the microbial flocs with higher RH deposit on membrane surface much easier. Lee et al. [31] found that the proteins in EPS had a strong positive influence on the hydrophobicity of microbial flocs, while carbohydrate had no remarkable influence. It was also observed that the proteins in the EPS are mainly made up of hydrophobic amino acids [32]. As shown in Table 3, the proteins/carbohydrates (P/C) ratios of bulking sludge were larger than that of normal sludge, indicating that the change of P/C may impact the RH of sludge suspension. The dynamic viscosity is a reflection of the magnitude of viscous substances, which may be considerably contributed by the polymers such as protein, carbohydrate, and so on. An increase of biopolymers in the sludge suspension will increase its viscosity, and hence reduce the MBRs permeate flux [33,34]. Thus, EPS has great contribution on the increase of the dynamic viscosity of the mixed liquor. In our previous work, we found that the sludge suspension with excessive filamentous bacteria had high viscosity for the presence of too much EPS [3]. With the suction force the sludge flocs presenting high viscosity can easily accumulate on the membrane surface and the filamentous bacteria has a fixing action on the membrane foulants, which adhere and penetrate between the membrane and membrane foulants. As to the detailed discussion on sludge characteristics, please see our previous report [3]. It must be pointed out that sludge suspension has a very complex impact on membrane fouling. Therefore, maybe it is interesting to study the separate influence of sludge characteristics and floc structure on membrane fouling. A further work will be carried out to determine the contribution of floc structure and sludge characteristics on membrane fouling, and determine if there were coactions between floc structure and sludge characteristics. 4. Summary This paper presents a comparative and correlative study of membrane fouling mechanisms during membrane filtration of normal sludge suspension and bulking sludge suspension. On the basis of floc morphology analysis and sludge characteristics analysis, the membrane fouling mechanisms of these two sludge suspensions were investigated. From the results reported here, the main contribution of this study can be summarized as:
Fig. 5. Floc size distributions of normal sludge and bulking sludge.
• The membrane fouling behavior was severe during membrane filtration of bulking sludge suspension; however, the mem-
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brane fouling was slight during membrane filtration of normal sludge suspension. The bulking sludge resulted in a thick and dense cake layer on the membrane surface. The filamentous bacteria had a fixing action on the membrane foulants, which adhere and penetrate between the membrane and membrane foulants. • In our case, the floc structure characterized by the floc area and floc perimeter was a typical fractal structure. The boundary fractal dimensions of bulking sludge were much larger than those of normal sludge. The floc size of bulking sludge suspension was much larger than normal sludge suspension. Moreover, the shape and structure of bulking flocs were open and irregular. The flocs of bulking sludge had more irregular shape than the flocs of normal sludge. The irregular shape of bulking sludge would do great harm to membrane filtration process. • The over growth of filamentous bacteria could induce the increase of EPS concentration. The RH and sludge viscosity of bulking sludge were larger than those of normal sludge. The higher EPS concentration, RH and sludge viscosity of sludge suspension would do great harm to membrane filtration. Acknowledgements The project supported by National Natural Science Foundation of China, Grant Nos. 50578023 and 50578024. References [1] L. van Dijk, G.C.G. Roncken, Membrane bioreactors for wastewater treatment: the state of the art and new developments, Water Sci. Technol. 35 (1997) 35–41. [2] A.L. Lim, R. Bai, Membrane fouling and cleaning in microfiltration of activated sludge wastewater, J. Membr. Sci. 216 (2003) 279– 290. [3] F.G. Meng, H.M. Zhang, F.L. Yang, Y.S. Li, J.N. Xiao, X.W. Zhang, Effect of filamentous bacteria on membrane fouling in submerged membrane bioreactor, J. Membr. Sci. 272 (2006) 161–168. [4] E.M. Contreras, L. Giannuzzi, N.E. Zaritzky, Use of image analysis in the study of competition between filamentous and non-filamentous bacteria, Water Res. 38 (2004) 2621–2630. [5] B. Jin, B.-M. Wil´en, P. Lant, Impacts of morphological, physical and chemical properties of sludge flocs on dewaterability of activated sludge, Chem. Eng. J. 98 (2004) 115–126. [6] B. Wil´en, P. Jin, P. Lant, Impacts of structural and microbial characteristics on activated sludge floc stability, Water Res. 37 (2003) 3632–3645. [7] D.J. Lee, G.W. Chen, Y.C. Liao, C.C. Hsieh, On the free-settling test for estimating activated sludge floc density, Water Res. 30 (1996) 541–550. [8] D. Lee, T. Waite, R. Wu, Multilevel structure of sludge flocs, J. Colloid Interface Sci. 252 (2002) 383–439. [9] S.A. Lee, A.G. Fane, T.D. Waite, Impact of natural organic matter on floc size and structure effects in membrane filtration, Environ. Sci. Technol. 39 (2005) 6477–6486. [10] K. Khatib, J. Rose, O. Barres, W. Stone, J.-Y. Bottero, Physicochemical study of fouling mechanisms of UF membranes on Biwa Lake (Japan), J. Membr. Sci. 130 (1997) 53–62.
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A new insight into membrane fouling mechanism during membrane filtration of bulking and normal sludge suspension Fangang Meng, Fenglin Yang ∗ , Jingni Xiao, Hanmin Zhang, Zheng Gong School of Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, China Received 19 March 2006; received in revised form 14 August 2006; accepted 14 August 2006 Available online 26 August 2006
Abstract Membrane bioreactor (MBR) is an important solid–liquid separation technology employed widely in the wastewater treatment. However, membrane fouling is the major problem that hinders the practical application of membrane bioreactor (MBR) systems. A major obstacle for the application of MBRs is the rapid decline of the permeation flux as a result of sludge filamentous bulking. In this paper a series of membrane filtration tests were performed to evaluate the fouling behaviors of bulking sludge and normal sludge. In order to specify the fouling mechanism of bulking sludge, the floc morphology of bulking sludge and normal sludge was studied based on fractal theory and image analysis. The sludge characteristics was also evaluated in terms of extracellular polymeric substances (EPS), relative hydrophobicity (RH) and sludge viscosity. The results showed that the bulking sludge resulted in a greater flux decline rate than normal sludge. The bulking sludge could cause a severe cake fouling, which was induced by the fixing action of filamentous bacteria. The results also showed that the sludge flocs had a good fractal characteristic, and the boundary fractal dimension of bulking sludge flocs was much larger than that of normal sludge. The flocs of bulking sludge had more irregular shape than the flocs of normal sludge. The irregular shape of bulking sludge would do great harm to membrane filtration process. The higher EPS concentration, RH and sludge viscosity of bulking sludge would speed up membrane fouling further. © 2006 Elsevier B.V. All rights reserved. Keywords: Membrane bioreactor; Membrane fouling; Filamentous bacteria; Sludge floc morphology; Fractal theory
1. Introduction In recent years, membrane bioreactors (MBRs) have been widely used in wastewater treatment to achieve higher effluent quality, which is often difficult to be effectively met by conventional activated sludge process. The advantages of MBR are a high mixed liquid of suspended solids (MLSS) concentration, up to 35,000 mg/L, producing higher rate of removal of biological oxygen demand (BOD) and chemical oxygen demand (COD), a lower excess sludge production and the production of treated water can be reused [1]. In addition, the space occupied by MBR systems is greatly reduced due to the absence of settling tanks and the reduction in bioreactor volume made possible by the higher biomass concentration. But a major obstacle for the application of MBRs is the rapid decline of the permeation
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Corresponding author. Tel.: +86 411 84706172; fax: +86 411 84708084. E-mail addresses: [email protected] (F. Yang), [email protected] (F. Meng). 0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.08.020
flux as a result of membrane fouling [2]. That is to say, membrane fouling reduces the productivity of MBRs and enhances its maintenance and operation fee, thus membrane fouling is the key problem should be solved. The overgrowth of filamentous bacteria in sludge suspension could result in severe membrane fouling due to formation of a thick and non-porous cake layer [3]. It also showed that filamentous bacteria had significant influence on sludge floc morphology [3,4]. As filamentous bacteria grew excessively in activated sludge, the sludge flocs became big and irregular, which would do great harm to membrane permeation [3]. It indicates that the floc morphology of sludge suspension has significant influence on membrane performance in MBR systems. The excessive growth of filamentous microorganisms could lead to large floc size and loose floc structure [5,6]. Fractal dimension is often used to describe the geometric characteristics of the multi-leveled floc structure, e.g., activated sludge flocs [6–8]. Euclidean geometry describes regular objects such as points, curves, surfaces, and cubes using integer dimensions 0–3, respectively. Associated with each dimension is a
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measure of the object such as the length of a line, area of a surface and volume of a cube. Since sludge floc aggregates are disordered and irregular, the floc structure cannot be described by Euclidean geometry due to the scale-dependent measures of length, area and volume. These objects are called fractals, and their dimensions are non-integral and defined as fractal dimensions. A fractal object can be divided into parts, each of which is similar with the whole. Fractal theory has been used widely in the study of membrane fouling during membrane filtration of colloids [9]. Khatib et al. [10] studied chemically assisted UF of raw water and suggested that the fractal properties of the aggregates formed may have influence on the membrane filtration behavior. In previous work [11–13], it was found that reaction-limited aggregation (RLA) leads to compact floc (high fractal dimension) and diffusionlimited aggregation (DLA) produces more open floc (lower fractal dimension) with the actual values of fractal dimension dependent upon the mode of flocculation (e.g., cluster–cluster, particle–cluster, etc.), and showed that the cake layer formed by RLA was more compact than the cake layer formed by DLA. These previous literatures provide us a new approach to study fouling mechanism of filamentous bacteria from the viewpoint of floc morphology analysis. In the present work, batch membrane filtration tests were carried out to study the effect of sludge floc morphology on membrane fouling. These tests were based on two kinds of classical sludge: bulking sludge (bulking sludges 1 and 2) and normal sludge (normal sludges 1 and 2). The objective of this study is to investigate fouling mechanism of filamentous bacteria by characterizing floc shape. In this paper, membrane fouling behavior of these sludge samples were studied, the fractal dimension (DP ), roundness (Ro), form factor (FF), and aspect ratio (AR) of sludge flocs were analyzed to comparison the impacts of bulking sludge and normal sludge on membrane fouling. 2. Materials and methods 2.1. Batch filtration tests A hollow fiber membrane module made of polyethylene (DAIKI, Japan) that had a total area of 0.1 m2 and a normal pore size of 0.1 m was used in the tests. The membrane module was submerged in 12 L of mixed liquor with aeration of 200 L/h, and membrane filtration was carried out by applying 4.0 kPa of trans-membrane pressure with a suction pump. Filtration was continued for 240 min, which allowed the permeation flow rate to become stable, and then the fouling resistance was estimated by Darcy’s equation as follows TMP Rt = Rm + Rf = Rm + Rp + Rc = μJ
(1)
where Rt is total hydraulic resistance, Rm the membrane resistance, Rp the pore blocking resistance, Rc the cake layer resistance, TMP the transmembrane pressure, μ the dynamic viscosity, and J is the membrane flux. The experimental procedure to get each resistance value was as follows [14,15]: (1) Rm was estimated by measuring the water flux of de-ionized (DI) water; (2)
Rt was evaluated by the final flux of biomass microfiltration; (3) the membrane surface was then flushed with water and cleaned with a sponge to removal the cake layer. After that, the DI water flux was measured again to get the resistance of Rm + Rp . The pore blocking resistance (Rp ) was calculated from steps (1) and (3) and the cake resistance (Rc ) from (2) and (3). To ensure that the condition and performance of the membrane module was almost the same in all experiments, postcleaning was performed after every experiment to remove the fouling cake and the membrane module was immersed in 0.03% NaClO solution for 24 h to obtain a permeability recovery more than 96%. 2.2. Analytical methods 2.2.1. Sludge suspension The sludge samples for batch filtration tests were taken from different stages of two large MBRs, which were applied for wastewater treatment. It indicates that the sludge samples had been acclimatized by membrane bioreactor systems. Because the sludge suspension used in our tests was taken from other MBR systems, this research can reflect the real fouling mechanism of membrane bioreactor. The MLSS concentration of each sludge suspension was adjust to about 6000 mg/L with its supernatant prior to the membrane filtration test in order to exclude the concentration effect on membrane flux. 2.2.2. Microscopy and image analysis The sludge flocs were characterized by means of microscopic observation (Olympus, BH2-RFCA). The structure parameters of sludge flocs were determined using a Leica Q500IW image processing and analysis system (Leica Cambridge Ltd., Cambridge, UK). Images were acquired using microscopic observation are JEPG format coded with True Color (Fig. 3a and b). Images were transferred to gray-scale formation (256 grey-scale levels) with Adobe Photoshop 7.0 software. Before analyzing an image, a threshold has to be determined in order to distinguish flocs from the background, and obtain a binary image (Fig. 3c and d). For each binary image, threshold was estimated as the grey level value that corresponded to that maximum of the grey level histogram second derivation [16]. For the binary images, pixels have a value of either one or zero, enabling easy processing of the images using the computer. A series of at least 40 images were analyzed for each sludge sample to obtain accurate data. The floc area, floc perimeter and morphological parameters can be determined by analysis the binary images. 2.2.3. Boundary fractal dimension of sludge flocs A perimeter–area relationship of fractal dimension analysis has been developed for sludge floc analysis [17]. The perimeter P follows a power law of the area A according to the fractal law described by Feder [18]: P ∼ ADP /2
(2)
where DP is the fractal dimension of the sludge flocs. This implies that the fractal nature of the contour of the flocs. The
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boundary dimensions for regular objects, such as circle or square, are equal to one (DP = 1). The perimeter and area of sludge flocs were determined by image analysis system. Thus, a Richardson plot of ln P versus ln A yields a straight line of slope DP /2, enabling the fractal dimension, DP , of the sludge flocs to be determined. 2.2.4. Shape characterization The sludge floc size was determined by focused beam reflectance measurement (Model M400L, Lasentec, Redmond, USA). The floc size distribution was adopted to characterize its effect on membrane fouling. Image analysis techniques have been used widely in the study of sludge floc structure [19]. In order to compare floc morphology of bulking sludge and normal sludge, the following shape parameters were examined. Roundness (Ro): Ro was defined as the ratio between the object area (A) to the area of a circle with a length equal to the object length (L), which indicates to what an extant the measured floc is similar to the true circle [4]. For a circle Ro = 1 and it decreases as more elongated the object is. Ro was calculated as follows Ro =
4πA L2
(3)
The form factor (FF) describes the deviation of an object form a circle. It is particularly sensitive to the “roughness” of the boundaries [20]. Ro was defined as the ratio between the object area (A) to the area of a circle with a perimeter equal to the object perimeter (P). For a circle FF = 1. FF was defined as follows FF =
4πA P2
(4)
The three-dimensional aspect ratio (AR) is applied to describe the extension of an object [20]. The more elongated it is, the larger the value of this parameter is. For a circle AR = 1. AR was evaluated as follows 4 length AR = 1.0 + − 1.0 (5) π width 2.2.5. Analysis of sludge characteristics For each sludge sample the activated sludge properties including EPS, dynamic viscosity and RH were analyzed to investigate the correlation between these properties and membrane fouling resistance. The extraction of EPS was based on a cation ion exchange resin (Dowex-Na form) method [21]. EPS was normalized as the sum of carbohydrate and protein, which were analyzed using phenol/sulfuric-acid method and folin method [22], respectively. The sludge floc size was determined by focused beam reflectance measurement (Model M400L, Lasentec, Redmond, USA). The dynamic viscosity was determined using a rotational viscosity meter (Model NDJ-7, Shanghai, China). The relative hydrophobicity (RH) was evaluated similar to Wil´en et al. [23]. Each sample was measured three times with a standard deviation 1–8%, 2–7%, 2–5% for EPS, dynamic viscosity and RH, respectively. For the sludge samples the membrane fouling
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tests and activated sludge analysis were performed within 10 h to avoid changes in sludge characteristics. In order to give exact information about sludge properties, the sludge suspension for analysis was sampled during the membrane filtration test. 2.2.6. Scanning electron microscope The fouled membrane surface was observed with the help of a scanning electron microscope (SEM) (KYKY-2800B, Beijing, China). As the membrane filtration was stopped, a piece of membrane fiber was cut from the middle of the membrane module. The sample was fixed with 3.0% glutaraldehyde in 0.1 M phosphate buffer at pH 7.2. The sample was dehydrated with ethanol, silver-coated by a sputter and observed in the SEM. 3. Results and discussion 3.1. Comparison of membrane fouling behavior Since the typical operating conditions of MBR systems, such as HRT, influent feed, and F/M ratio, are more likely to induce excessive growth of filamentous bacteria in sludge suspension, a comparison study was made between sludge suspension, normal sludge and bulking sludge, with respect to membrane fouling. Fig. 1 shows the decline behavior of membrane flux during membrane filtration of normal sludge suspension and bulking sludge suspension. It was observed that the bulking sludge suspension showed a much greater flux decline than the normal sludge suspension. This result indicates that an excessive growth of filamentous bacteria has serious negative effect on membrane permeation. Our previous study also implied that the sludge flocs with a small quantity of filamentous bacteria (normal sludge suspension) could benefit membrane permeability [3]. The decline of membrane flux could be caused by two factors: the thickness of the cake layer and the compactness of the cake layer. To examine the fouling tendencies, each resistance term was calculated (Table 1). As more activated sludge particles were deposited on the membrane surface, the cake resistance increased dramatically and became the dominant resistance. Table 1 indicates that the bulking sludge showed a higher cake layer resistance than normal sludge. The cake resistance of
Fig. 1. Comparison of flux decline behavior during membrane filtration of normal sludge suspension and bulking sludge suspension.
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Table 1 A series of resistances during microfiltration of different sludge samples Items (%)
Normal sludge 1 Normal sludge 2 Bulking sludge 1 Bulking sludge 2
Rm (×1011 m−1 )
Rp (×1011 m−1 )
Rc (×1011 m−1 )
Rt (×1011 m−1 )
1.05 (21.97) 1.05 (20.51) 1.05 (9.55) 1.05 (4.61)
1.27 (26.57) 1.14 (22.27) 1.53 (13.91) 2.01 (8.83)
2.46 (51.46) 2.93 (57.23) 8.42 (76.55) 19.70 (86.56)
4.78 5.12 11.00 22.76
Fig. 2. SEM images showing the surfaces of fouled membrane: (a) normal sludge and (b) bulking sludge.
bulking sludge (8.42, 19.70 × 1011 m−1 ) was more than two times of normal sludge (2.46, 2.93 × 1011 m−1 ), and its proportion shifted from 51.46% and 57.23% to 76.55% and 86.56%. But, the over growth of filamentous bacteria had little influence on pore blocking resistance. In general, Membrane pore blocking mainly results from the adsorption of colloids, macro-
molecules, biologically active substances, various ions, etc. In bulking sludge suspension, the filamentous bacteria can capture these matters, so there were few colloids and macromolecules in the supernatant of bulking sludge. Thus, here bulking sludge did not induce severe pore blocking. Therefore, with respect to bulking sludge suspension, the formation of a cake layer on the
Fig. 3. Microscopic images of sludge flocs: (a) initial true color image of normal sludge, (b) initial true color image of bulking sludge, (c) binary image of normal sludge, and (d) binary image of bulking sludge, respectively (1000×).
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membrane surface was the major factor that affects membrane fouling behavior. SEM images of fouled membrane surface were taken at the end of the operation (Fig. 2). The membrane fouling was mostly attributed to the pore blocking as well as the formation of a cake layer. From Fig. 2 it can be seen that the cake layer formed with normal sludge (Fig. 2a) had obvious pores though it appeared to be less porous than clean membrane. The cake layer formed with bulking sludge was dense in appearance, which results from the deposition of fiber-shape bacteria and other materials on the membrane surface (Fig. 2b). Filamentous bacteria acted as the framework in the cake layer. In the membrane filtration, the deposition of sludge flocs is determined by two factors: the suction force, which is generated by transmembrane pressure, and the shear force, which is generated by aeration. With the suction force the bulking flocs can easily accumulate on the membrane surface due to their irregular morphology. The filamentous bacteria had a fixing action on the membrane foulants, which adhere and penetrate between the membrane and membrane foulants (Fig. 2b). Thus, the bulking sludge could result in a severe cake fouling. 3.2. Evaluation of fractal dimension It can be seen that the sludge flocs of normal sludge were much smaller than that of bulking sludge in appearance (Fig. 3a). The sludge floc structure, however, became irregular as filamentous bacteria grew excessively (Fig. 3b). As the floc size increased, the density decreased, and the porosity increased [24].
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Table 2 Comparison of the sludge floc morphology between normal sludge and bulking sludge Items
Normal sludge 1 Normal sludge 2 Bulking sludge 1 Bulking sludge 2
DP
Ro
1.118 1.210 1.534 1.475
0.73 0.68 0.25 0.31
FF ± ± ± ±
0.18 0.20 0.14 0.09
0.87 0.79 0.61 0.57
AR ± ± ± ±
0.11 0.15 0.10 0.16
2.23 2.35 3.40 3.13
± ± ± ±
0.41 0.22 0.55 0.64
Analyzing a sludge floc image with image analysis system, the overall area of the image Sc was about 40,000 m2 . The perimeter and area of sludge flocs were also evaluated by image analysis. A double logarithmic plot of the floc area and floc perimeter of the sludge is shown in Fig. 4. Good linearity with high correlation coefficient (R2 = 0.898, 0.886) was observed, suggesting the floc structure of sludge flocs was similar with each other. Thus, the floc structure characterized by the floc area and floc perimeter was a typical fractal structure. From Fig. 4 and Table 2 it can be seen that the boundary fractal dimensions of bulking sludge (DP = 1.534, 1.475) were much larger than those of normal sludge (DP = 1.118, 1.210). These values are different from the recent result of Zartarian et al. [17] who found an average value of 1.31. The results indicate that the bulking sludge had a more irregular shape than normal sludge. This result also implies that flocs with excessive filaments were generally less strong. With a great amount of filaments growing inside, the shape and structure of flocs became very open and irregular (Fig. 3b). The filamentous bacteria are believed to form the “backbone” of activated sludge flocs to which floc-forming bacteria attach by means of EPS and form strong flocs. In the absence of filaments, “pin-floc” will be formed. These tiny and weak flocs will tend to contribute to membrane pore blocking dramatically [3]. However, an excess of filamentous bacteria would produce an abundance of filaments extending from the flocs into the bulk solution, producing a bridging lattice, which prevents the agglomeration of floc particles [25]. Therefore, the flocs with excess filamentous bacteria are big and irregularly shaped. Additionally, from previous studies it also can be seen that the bulking sludge has a very poor settleability [26]. 3.3. Characterization of floc shape
Fig. 4. Determination of fractal dimension (2 × slope) plotting double logarithmic of P vs. A: (a) normal sludge flocs and (b) bulking sludge flocs.
The floc size distributions of each sludge sample were measured, which are shown in Fig. 5. It indicates that the sludge floc size of bulking sludge was larger than that of normal sludge (Fig. 5). The over growth of filamentous bacteria in sludge suspension could cause an increase of floc size [3]. Usually, the sludge suspension with larger floc size would benefit membrane filtration process [2,27]. However, in our case the bulking sludge with higher floc size resulted in severe membrane fouling. The data obtained from the current investigation, together with previous work in the literature, indicate that floc size distribution just is one of the factors that affect membrane fouling. Other factors
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Table 3 Comparison of the sludge characteristics between normal sludge and bulking sludge EPS
Normal sludge 1 Normal sludge 2 Bulking sludge 1 Bulking sludge 2
Carbohydrates (mg/g MLSS)
Proteins (mg/g MLSS)
P/C
Total (mg/g MLSS)
16.07 12.79 41.27 52.00
29.55 22.59 97.52 112.24
1.84 1.77 2.36 2.16
45.62 35.38 138.79 164.24
such as floc shape may also have certain impacts on membrane fouling process. Therefore, it is necessary to investigate the effect of floc morphology on membrane fouling mechanism. The fractal dimension of sludge flocs has been discussed in Section 3.2. As shown in Table 2, the roundness of bulking sludge was smaller than that of normal sludge, which indicates that the shape of normal sludge is much closer to a circle. It has been shown that FF and AR seem to be the most suited morphological parameters to estimate the settleability of activated sludge [20]. From Table 2, it can be seen that the form factor of bulking sludge was smaller than that of normal sludge; however, the aspect ratio of bulking sludge was larger than that of normal sludge. These results also indicate that the bulking sludge flocs had more irregular shape than normal sludge floc. The irregular-shaped sludge flocs had a tendency to adhere onto the membrane surface, and there easily intertwisted on the membrane fibers. Moreover, as a role of fixing substance, the filamentous bacteria resulted in more foulants adhering to membrane and enhance their clinging intensity, which worsen the membrane permeability seriously. 3.4. Comparison of sludge characteristics Except for floc morphology, another difference between bulking sludge and normal sludge is the physicochemical characteristics of sludge suspension. The bulking sludge had higher EPS concentration than normal sludge (Table 3). From Table 3, it also can be seen proteins appeared to be the major component of EPS. Many previous studies have demonstrated that the over growth of filamentous bacteria could result in much more release of EPS, and did severe harm to membrane permeation [3,28]. As shown in Table 3, the bulking sludge had higher RH values than those of normal sludge. It has been reported that
RH (%)
Viscosity (mPa s)
37.00 40.33 84.91 93.66
1.23 1.45 3.05 3.71
attached bacteria were much more hydrophobic than their suspended counter parts [29,30]. These results demonstrated that the microbial flocs with higher RH deposit on membrane surface much easier. Lee et al. [31] found that the proteins in EPS had a strong positive influence on the hydrophobicity of microbial flocs, while carbohydrate had no remarkable influence. It was also observed that the proteins in the EPS are mainly made up of hydrophobic amino acids [32]. As shown in Table 3, the proteins/carbohydrates (P/C) ratios of bulking sludge were larger than that of normal sludge, indicating that the change of P/C may impact the RH of sludge suspension. The dynamic viscosity is a reflection of the magnitude of viscous substances, which may be considerably contributed by the polymers such as protein, carbohydrate, and so on. An increase of biopolymers in the sludge suspension will increase its viscosity, and hence reduce the MBRs permeate flux [33,34]. Thus, EPS has great contribution on the increase of the dynamic viscosity of the mixed liquor. In our previous work, we found that the sludge suspension with excessive filamentous bacteria had high viscosity for the presence of too much EPS [3]. With the suction force the sludge flocs presenting high viscosity can easily accumulate on the membrane surface and the filamentous bacteria has a fixing action on the membrane foulants, which adhere and penetrate between the membrane and membrane foulants. As to the detailed discussion on sludge characteristics, please see our previous report [3]. It must be pointed out that sludge suspension has a very complex impact on membrane fouling. Therefore, maybe it is interesting to study the separate influence of sludge characteristics and floc structure on membrane fouling. A further work will be carried out to determine the contribution of floc structure and sludge characteristics on membrane fouling, and determine if there were coactions between floc structure and sludge characteristics. 4. Summary This paper presents a comparative and correlative study of membrane fouling mechanisms during membrane filtration of normal sludge suspension and bulking sludge suspension. On the basis of floc morphology analysis and sludge characteristics analysis, the membrane fouling mechanisms of these two sludge suspensions were investigated. From the results reported here, the main contribution of this study can be summarized as:
Fig. 5. Floc size distributions of normal sludge and bulking sludge.
• The membrane fouling behavior was severe during membrane filtration of bulking sludge suspension; however, the mem-
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brane fouling was slight during membrane filtration of normal sludge suspension. The bulking sludge resulted in a thick and dense cake layer on the membrane surface. The filamentous bacteria had a fixing action on the membrane foulants, which adhere and penetrate between the membrane and membrane foulants. • In our case, the floc structure characterized by the floc area and floc perimeter was a typical fractal structure. The boundary fractal dimensions of bulking sludge were much larger than those of normal sludge. The floc size of bulking sludge suspension was much larger than normal sludge suspension. Moreover, the shape and structure of bulking flocs were open and irregular. The flocs of bulking sludge had more irregular shape than the flocs of normal sludge. The irregular shape of bulking sludge would do great harm to membrane filtration process. • The over growth of filamentous bacteria could induce the increase of EPS concentration. The RH and sludge viscosity of bulking sludge were larger than those of normal sludge. The higher EPS concentration, RH and sludge viscosity of sludge suspension would do great harm to membrane filtration. Acknowledgements The project supported by National Natural Science Foundation of China, Grant Nos. 50578023 and 50578024. References [1] L. van Dijk, G.C.G. Roncken, Membrane bioreactors for wastewater treatment: the state of the art and new developments, Water Sci. Technol. 35 (1997) 35–41. [2] A.L. Lim, R. Bai, Membrane fouling and cleaning in microfiltration of activated sludge wastewater, J. Membr. Sci. 216 (2003) 279– 290. [3] F.G. Meng, H.M. Zhang, F.L. Yang, Y.S. Li, J.N. Xiao, X.W. Zhang, Effect of filamentous bacteria on membrane fouling in submerged membrane bioreactor, J. Membr. Sci. 272 (2006) 161–168. [4] E.M. Contreras, L. Giannuzzi, N.E. Zaritzky, Use of image analysis in the study of competition between filamentous and non-filamentous bacteria, Water Res. 38 (2004) 2621–2630. [5] B. Jin, B.-M. Wil´en, P. Lant, Impacts of morphological, physical and chemical properties of sludge flocs on dewaterability of activated sludge, Chem. Eng. J. 98 (2004) 115–126. [6] B. Wil´en, P. Jin, P. Lant, Impacts of structural and microbial characteristics on activated sludge floc stability, Water Res. 37 (2003) 3632–3645. [7] D.J. Lee, G.W. Chen, Y.C. Liao, C.C. Hsieh, On the free-settling test for estimating activated sludge floc density, Water Res. 30 (1996) 541–550. [8] D. Lee, T. Waite, R. Wu, Multilevel structure of sludge flocs, J. Colloid Interface Sci. 252 (2002) 383–439. [9] S.A. Lee, A.G. Fane, T.D. Waite, Impact of natural organic matter on floc size and structure effects in membrane filtration, Environ. Sci. Technol. 39 (2005) 6477–6486. [10] K. Khatib, J. Rose, O. Barres, W. Stone, J.-Y. Bottero, Physicochemical study of fouling mechanisms of UF membranes on Biwa Lake (Japan), J. Membr. Sci. 130 (1997) 53–62.
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