Development of a Novel Submerged Membrane Electro-Bioreactor

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Environ. Sci. Technol. 2010, 44, 3298–3304

Development of a Novel Submerged Membrane Electro-Bioreactor (SMEBR): Performance for Fouling Reduction KHALID BANI-MELHEM* AND MARIA ELEKTOROWICZ Department of Building, Civil and Environmental Engineering, Concordia University, Montreal, Quebec, Canada H3G 1M8,

Received August 5, 2009. Revised manuscript received January 18, 2010. Accepted January 29, 2010.

A novel Submerged Membrane Electro-Bioreactor (SMEBR) was developed to treat wastewater and control the problem of membrane fouling. To validate the new design, experimental work was achieved in a few phases. This paper describes the design constraints and criteria of the new developed SMEBR system, and shows the results of the performance of the SMEBR system to reduce membrane fouling when intermittent direct current (DC) (15 min ON/45 min OFF) was applied using cylindrical iron mesh for both electrodes. Application of the SMEBR system enhanced the membrane filterability by reducing the fouling rate up to 16.3% without any backwashing of the membrane module. The improvement in membrane filterability associated with a decrease in ζ potential of the mixed liquor flocs from -30.5 up to -15.3 mV and a decrease in specific resistance to filtration (SRF) up to 40% was observed.

1. Introduction The decline of permeation flux due to membrane fouling is addressed as a major problem during the operation of submerged membrane bioreactor (SMBR) (1, 2). In general, the membrane fouling is attributed to the particle deposition on the membrane surface or membrane internal clogging (2, 3). The most common three distinct approaches to reduce membrane fouling in SMBR applications are cleaning the membrane module, optimizing the operating parameters, and improving the characteristics of the activated sludge in the reactor (4). The last approach has proven to be effective in reducing the membrane fouling and has received great attention from researchers in recent years (5-9). This approach includes the addition of chemical coagulants such as alum and iron salts (5-8), or the addition of adsorptive materials such as a high concentration of powdered activated carbon and zeolite (8, 9). Coagulation is considered an effective method not only in reducing the fouling of the membrane but also in improving the quality of the effluent (6). However, the addition of chemical coagulants to the wastewater may cause side effects by producing by-products and/or increasing the volume of sludge in the reactor (10). An alternative technology for creating coagulation inside the system, suggested by the authors, is to introduce electrokinetic processes into the same reactor. In this case, one of the major electrokinetic processes is electrocoagulation (EC), which has been proven to be * Corresponding author e-mail: [email protected]. 3298

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effective in wastewater treatment (11). In comparison with the chemical coagulation processes, electrocoagulation (EC) has many advantages: no liquid chemical is added, alkalinity is not consumed, and the EC process does not need coagulation agents thus producing less sludge (12). Therefore, in the proposed design no coagulant is added, leading to the minimization of operation costs and reducing the generation of side-products in the effluent and wasted solids. Contrary to other designs (13, 14), electrocoagulation unit is incorporated inside of MBR, permitting its direct interaction with biological processes and membrane filtration. Subsequently, designed submerged membrane electro-bioreactor (SMEBR) brings a number of advantages (15): smaller footprint (saving in the land needed for construction of treatment facilities by combining all secondary and advanced wastewater treatment units into one hybrid unit); omitting the requirements for chemicals use; reducing the operating costs by reducing the requirements of aeration in conventional SMBR systems; improving sludge dewatering conditions; and initiating investigations on a more sustainable design of a hybrid wastewater treatment system. The SMEBR design is the first attempt to combine electrokinetic principles and SMBR in one reactor vessel. The main objectives of this paper are to introduce the design constraints and criteria of the newly developed SMEBR system and to show the results of the performance of the SMEBR system to reduce membrane fouling.

2. Development of the SMEBR The original design of the SMEBR system appears to lie at the interaction of three fundamental processes: biodegradation, electrochemistry (mainly represented by the electrocoagulation), and membrane filtration integrated into one operation unit. In the following subsections, the design constraints and criteria for development of new SMEBR system are highlighted. 2.1. Considerations for the SMEBR Design. 2.1.1. Electrode Configuration Constraints. Designing the electrodes in the SMEBR system is important for the uniform distribution of the DC field strength inside the electrobioreactor. The design of electrodes should be governed by the following constraints (4, 15): (a) Location of the membrane module in the bioreactor must not affect the DC field distribution; (b) Electrical field cannot affect the longevity of membrane material (i.e., advance oxidation of membrane polymeric materials); (c) Electrodes’ design must maintain an adequate current density within the system; (d) DC field distribution should create homogeneous flocs formation in the mixed liquor (ML) solution; (e) Electrode assembly in the electro-bioreactor’s design will not interfere with the wastewater feeding and flowing toward the membrane module; (f) Selection of electrode material is crucial for electrochemical processes to reduce membrane fouling in the case of SMEBR. Aluminum and iron are readily available materials and proven to be effective for electrocoagulation process (11). Type of dissolved electrode material is also important for microbial community. For example, iron, in the activated sludge culture, is considered as a micronutrient necessary for the microbial growth. It is an important component of many of the enzymes involved in the metabolic pathways of bacteria, or it forms important components with enzymes that are involved in bacterial metabolism (16). However, some higher concentrations of iron might create inhibitory conditions for microbial growth (17, 18). The inhibitory conditions can also be created by aluminum (19). Then, electrode material impact should be tested for the 10.1021/es902145g

 2010 American Chemical Society

Published on Web 03/31/2010

FIGURE 1. Simplified design configuration of the SMEBR system. microbial response. Subsequently, the specific oxygen uptake rate (SOUR) of the activated sludge should be determined in order to know the effect of applying a DC field on microbial activity. 2.1.2. Operation Condition Constraints. (a) The SMEBR design and configuration should take into consideration the requirements of simultaneous biodegradation, electrocoagulation, and sedimentation processes taking place inside the electro-bioreactor; (b) Design should consider the following fluid motions in the SMEBR: supply of feeding wastewater across the anode, airflow upward, electroformation of flocs and their settling, treated water-flow through cathode toward membrane module; (c) Appropriate DC field conditions should be applied for adequate electrolysis; moreover, the selected DC field cannot be supplied in a continuous mode to the mixed liquor (ML) solution due to presence of microbial culture. Al Shawabkeh et al. (20) demonstrated an optimum range of DC, between 0.28 and 1.14 V/cm, which can be applied to aerobic cultures, whereas COD reduction can be enhanced. According to their study, the impact of an applied DC field below 0.28 V/cm may be insignificant, and a DC electric field greater than 1.4 V/cm may be harmful. Therefore, for the operation of the SMEBR system, a preliminary experimental phase was conducted to identify the best conditions in terms of an appropriate DC field and an intermittence of the exposure time to the DC field that might not impede the biological treatment (4); (d) The design of supplied air should take into consideration an adequate amount of oxygen for the microorganisms’ metabolism in the bioreactor; however, an excess of air may break down the flocs’ formation. Therefore, to achieve the overall objectives of the designed configuration, the following five major parameters should be controlled and optimized during the SMEBR operation: applied direct current (DC), exposure time of ML to DC, air supply, hydraulic retention time (HRT), and sludge retention time (SRT). In this study, the impact of the first two parameters (DC and exposure time to DC) was explored. Because the strategy of this study was based on operating the SMEBR at constant transmembrane pressure and long SRT, the impact of variations in HRT and SRT in the SMEBR system was addressed in other investigations. 2.2. Design Criteria. 2.2.1. SMEBR Configuration. To meet the above requirements, the SMEBR system should be designed according to

the following criteria (4, 15): (a) The membrane module must be placed in the center of the bioreactor; (b) The electrodes must be placed around the membrane module at an appreciable distance from the membrane; (c) An accurate distance between the electrodes should be kept in order to minimize the potential effect of an acidic/oxidation zone on microbial community; (d) Such distance should also permit free movement of air and flocs to minimize the sludge shear; (e) Perforated electrodes should be used in the design to facilitate free feed and flow toward the membrane module. Figure 1 represents a schematic diagram of the SMEBR configuration that is in agreement with the above-mentioned criteria. The selected design of the SMEBR system divides the hybrid reactor into two zones: Zone I (electro-bioreactor) is boarded between the external wall of the reactor to the cathode, and Zone II is from the cathode to the membrane module. Generally in Zone I, three processes take place: biodegradation, electrocoagulation, and electrosedimentation; whereas in Zone II, further biodegradation and membrane filtration takes place. 2.2.2. Fluid Motions in the SMEBR System. The designed SMEBR is governed by the following major fluid motions (Figure 2): (a) Supply of wastewater from the outside across the perforated anode in order to undergo biological and electrochemical treatment between the electrodes; (b) Airflow upward. Air is used in SMEBR systems for (i) supplying sufficient oxygen for the microorganism growth; (ii) achieving a good mixing of the ML solution in the electro-bioreactor’s zones; (iii) reducing the fouling rate of the membrane. According to the above explanations, diffusers should be placed in Zone I and Zone II; (c) Electro-formation of flocs and electrosettling due to the principles of electrokinetic phenomena; (d) Treated water flows through the perforated cathode toward the membrane module. Due to the vacuum pump connected to the membrane module, the treated water flows toward the membrane. 2.2.3. Operation Mode. Biodegradation processes are affected by the surrounding environmental conditions (temperature, pH, O2 concentration) in the bioreactor. Applying a DC electrical field onto a mixed liquor solution will affect all of these conditions. For example, when a DC field in terms of a voltages gradient (V/cm) is applied between anode and cathode through mixed liquor solution, oxidation and reduction reactions will take place at the electrodes. Water oxidation generates hydrogen (H+) and oxygen gas at the VOL. 44, NO. 9, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Major types of fluid motions occurring in the SMEBR system. anode (1/2 H2O f e- + H + + 1/4 O2), whereas water reduction produces hydroxyls and hydrogen gas at the cathode (2H2O + 2e- f 2OH- + H2). This will increase the acidity at the anode and reduce the pH; simultaneously, pH will increase in the cathode area. The recommend range of pH for microorganism is 5-9 (20). Subsequently, an interrupted supply of electrical field is needed to preserve the viability of microorganisms. Simultaneously, a DC (at an accurate voltage gradient) is required for an effective electrocoagulation process to create flocs in the bioreactor so the particles can aggregate and prevent membrane fouling. Accordingly, the value of the pH was selected in the preliminary experimental phase as a guiding parameter to optimize the operation mode for the SMEBR system (4).

3. Materials and Methods 3.1. Experimental Setup. A laboratory scale setup was used in this study to validate the design of SMEBR (Figure 1). The experimental setup consisted of the following major units: electro-bioreactor, membrane module, wastewater supply system, effluent collecting tank, aeration system, and DC supply system. The tank of electro-bioreactor had a total volume of 20 L with a working volume of 13.4 L. Cylindrical iron mesh cathode and anode were designed in SMEBR, at a fixed distance of 5.5 cm (Zone I). Due to differences in perforation of electrodes and their circumferences, the effective surface areas for anode and cathode were of 93 and 106 cm2, respectively. The electrodes were connected to a digital external DC power supply. A timer permitted regulating the operation mode, that is, duration of ML exposure to DC field. A hollow fiber membrane module, ZeeWeed-1 (GE/Zenon Membrane Solutions, Canada), consisting of 80 fibers with 0.2 m length and pore size of 0.04 µm was fixed vertically in the center of the electro-bioreactor. Two peristaltic pumps (model 13-876-2, Fisher Scientific, Canada) were used during the operation: a feeding pump and a suction pump from membrane module. A level sensor was connected with the feeding pump to control the constant volume in the electro-bioreactor. Three systems of aeration were applied in the SMEBR: a porous air diffuser was installed just below the membrane module in the bottom center of the reactor in Zone II; a perforated air tube to supply air stream for maintaining the required dissolved oxygen level in the Zone I was located in 3300

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the middle distance between the anode and the cathode; and aeration system was also used for flushing air through the membrane module as designed by the manufacturer to reduce fouling and cake formation. 3.2. Wastewater Feed. The SMEBR system was fed with synthetic wastewater (simulating the municipal wastewater) to maintain the consistent quality of the wastewater composition during the experimental period. The composition of synthetic wastewater contained (in mg/L): glucose (310), peptone (252), yeast extract (300), (NH4)2SO4 (200), KH2 PO4 (37), MgSO4 · 7H2O (40), MnSO4 · H2O (4.5), FeCl3 · 6H2O (0.4), CaCl2 · 2H2O (4), KCl (25), and NaHCO3 (25). The sludge for inoculation was taken from the secondary clarifier in the municipal wastewater treatment plant in the City of SaintHyacinthe (Quebec, Canada). The sludge was acclimatized for two months in order to achieve stable conditions prior to the membrane filtration experiments. The fill-and-draw technique was used to cultivate the activated sludge (21). 3.3. Experimental Procedure. In this study, the feasibility of the designed SMEBR system for reducing membrane fouling has been verified. To operate the SMEBR system, a preliminary experimental phase was conducted to identify the best electrokinetic conditions in terms of an appropriate DC field and an intermittence of the exposure time to the DC field that might not impede the biological treatment (4). First, 200 mL of activated sludge samples exposed to a series of voltage gradients of 1, 2, 4, and 6 V/cm were evaluated. The results showed that the application of DC fields on activated sludge increased the pH at the cathode area whereas the impact was less significant at the anode area. Moreover, the output results demonstrated that a voltage gradient of 1 V/cm was found to be able to correctly preserve pH between 5 and 9. This gradient (1 V/cm) was used in other experiments where the best mode of operation in terms of DC interrupting supply was investigated. The results suggested that the mode of operation of 15 min ON/45 min OFF of DC supply was the best mode for maintaining the pH between 5 and 9. Therefore, a voltage gradient of 1 V/cm and an operating mode of 15 min ON/45 min OFF were determined to be adequate conditions to be applied into SMEBR system which does not inhibit the biological treatment. The experimental period of operating the SMEBR system consisted of two stages: reference stage, Stage I, extended from day 1 to day 26 without supplying DC (simulating conventional SMBR), and Stage II (simulating the SMEBR system) lasting from day 26 to 53 when DC was applied.

TABLE 1. Experimental Conditions of the Submerged Membrane Bioreactor (Stage I) and the SMEBR System (Stage II)

operation time (days) SRT (days) DO in bioreactor (mg/L) DC (V/cm) DC exposure time (minutes) influent temperature (°C)

Stage I

Stage II

26 268 5-8 0 0 19-21

27 268 5-8 1 15 ON/ 45 OFF 19-21

Table 1 shows the different conditions under which the Stages were run and monitored. To study the fouling behavior, the strategy of this study was based on operating the SMEBR at a constant transmembrane pressure (TMP), which was created by withdrawing the effluent via peristaltic pump operated at constant suction pressure and creating a constant level of the ML solution above the membrane module. It was stipulated that a decrease of the permeation flux with time would be mostly due to the fouling phenomenon; then, the fouling rate was evaluated by measuring the decline of permeate flux with time. Subsequently, no backwashing of the membrane module was performed during the operation. However, in order to enhance the recovery of the membrane permeability during the operating period, the membrane module was taken out of the electro-bioreactor and externally washed with tap water for a few minutes to remove the attached sludge cake particles over the membrane surface. Before starting Stage II, and in order to restore most of the membrane’s permeability, the membrane module was removed from the electro-bioreactor and physical and chemical cleaning were applied according to the protocol described by Meng et al. (22). Moreover, the SMEBR was operated at long sludge retention time (SRT) to decrease the amount of sludge wasting from the process. Therefore, during the entire operating period, no sludge was extracted from the electro-bioreactor, except for the purpose of sampling analysis (conventional parameters analysis and SRF tests). On average, a volume of about 350 mL per week of sludge was removed for sampling, corresponding to an average sludge retention time (SRT) of 268 days (complete sludge retention). Other parameters (ζ potential and specific resistance to filtration), which give an indirect indication about the fouling behavior, were also measured. A 50 mL sample of mixed liquor was taken daily from each zone. Furthermore, the sampled supernatant was taken for ζ potential measurement (Zeta-Meter 3.0 +, U.S.) after settling for 30 min. Each sample was measured 10 times, and the average value was taken as ζ potential with a standard deviation of 2-6%. The sample was returned to the electro-bioreactor after taking the measurement. Additionally, about 100 mL of sample were sampled from each zone of the elector-bioreactor every 10 days or at the end of each stage for the specific resistance to filtration (SRF) tests. The SRF tests were performed as described by Ng et al. (23).

4. Results and Discussion 4.1. Permeate Flux (J). To assess the fouling behavior in the SMEBR system during the two stages, the concept of percentage reduction in permeate flux (PRPF) was used according to the following equation: PRPF )

( )

Ji - J × 100% Ji

(1)

where Ji is the initial permeation flux (2.72 × 10-6 m3/m2 · s) measured during the first minute and J is the permeate flux

at any time. Figure 3 shows the percentage reduction in permeate flux (PRPF) during the operating period. During the first operation of Stage I with no DC supply (reference stage), the permeate flux decreased dramatically; the PRPF was 81.2 and 94% after 5 days of operation and at the end of the Stage I, respectively. During the first hours of the SMEBR system application (Stage II), there was no significant influence of the applied electrical field on the coagulation process. However, the effect of the electrocoagulation process began to provide evidence on day 3 of Stage II, when the permeate flux started to decline slowly with time. On day 6 of Stage II, the permeate flux was almost constant, reflecting the effect of applying the DC field into the MLSS solution in the SMEBR system. These results demonstrated that an initiation of electrocoagulation could be recommended to prevent the membrane fouling. During Stage II, the PRPF was, respectively, 57.8 and 68% on day 3 and day 5 of operation. This is the equivalent of a 16.3% improvement in permeation flux compared to conventional submerged membrane bioreactor on day 5 of operation. Finally, the maximum PRPF in Stage II was recorded on day 8 of Stage II and determined to be 73%; (22.3% less than in Stage I). It is worth mentioning that the removal efficiency of the COD in the electro-bioreactor’s zones increased from 75-90% during Stage I to 85-95% in Stage II, which can be attributed to the effect of electro-coagulation phenomenon that occurred during Stage II (4). This result seems to be significant since it was reported by Yamato et al. (24) that some fractions of organic matter in the mixed liquor have higher affinities with the membrane than do other fractions and consequently cause greater irreversible fouling. Therefore, applying a DC filed in the mixed liquor can reduce the contribution load of the dissolved organic matter on membrane fouling. This result may reflect the good performance of the SMEBR system in terms of flux improvement in Stage II where the fouling rate decreased significantly. 4.2. Changes in Zeta Potential. Zeta potential was used by many researchers to provide a quantifiable basis for estimating membrane fouling in SMBR applications. Meng et al. (22) reported a strong correlation between the zeta potential and resistance to membrane fouling, and indicated that the zeta potential is a significant membrane-fouling factor. Ofir et al. (25) reported a strong relationship between the zeta potential and the size of particles in a colloid suspension. They found that the zeta potential increased in absolute values with increases in the particle sizes of the colloid suspension. In this study, the potential for particle aggregation in the electro-bioreactor’s zones can be checked by measuring the zeta potential of floc particles (Figure 4). The magnitude of the zeta potential gives an indication of the stability of the colloidal system. If particles have a large negative or positive zeta potential, the dispersion is stable and it prevents the flocs’ formation. A dividing line between stable and unstable aqueous dispersions is generally taken at zeta potential, either +30 or -30 mV (26). The results of measuring zeta potential demonstrated that during Stage I, no significant difference was observed in both zones, and average zeta potential was approximately -30.5 mV. In Stage II, the zeta potential of the mixed liquor flocs changed from -30.5 mV (on average) to -15.3 mV in Zone I, and from -31 mV to -17 mV in Zone II. Accordingly, the dramatic changes in the zeta potential values in Stage II confirmed the improvement in permeation flux within the SMEBR system. 4.3. Changes in the Specific Resistance to Filtration (SRF). The specific resistance to filtration (SRF) of the activated sludge is an index representing the filterability of VOL. 44, NO. 9, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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the activated sludge when it is dewatered through filter medium. Physically, it gives an indication of the resistance of biomass to filtration (23) and characterizes the fouling (27) as it is related to flocs morphology. It is defined as resistance distributed to the unit filter area when specific weight sludge is filtered at a given pressure. A higher sludge SRF reflects worse filterability. Increase in SRF results in a decline in the flux rate, and an increase in transmembrane pressure (TMP). Ahmed et al. (28) pointed out that the SRF increased as well as membrane biofouling due to the presence of colloidal particles. According to the Carman-Kozeny equation, the specific resistance to filtration (r) of particles is in inverse proportion to the square of particle diameter (eq 2).

r)

180(1 - ε) ε3Fpdp2

(2)

where ε is the cake porosity, Fp is the particle density (kg/ m3), and dp is the particle diameter (m). In this study, all measuring filtration resistances were compared to the SRF of the original activated sludge solution (ro) when no DC was applied to the activated sludge that represents the SRF of the ML solution at the beginning of the operation period. A change of normalized filterability (r/ro) of the activated sludge samples during the operation of SMEBR is shown in Figure 5, where r is the SRF of the mixed liquor solution in both zones measured every 10 days or at the end of each stage during the operation period.

FIGURE 3. Comparison in percentage reduction in permeate flux (PRPF) in submerged membrane bioreactor operation (Stage I) and SMEBR system operation (Stage II).

FIGURE 4. Comparison in zeta potential variations in submerged membrane bioreactor operation (Stage I) and SMEBR system operation (Stage II). 3302

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membrane fouling. This explanation is reasonable because during Stage I, the MLSS concentration was within the range of 2100-3500 mg/L, whereas the fouling rate was higher than the fouling rate in Stage II when the MLSS concentration increased from 3500 to 5000 mg/L. This hypothesis was confirmed in the analysis of the zeta potential and the specific resistance to filtration, as demonstrated in Figures 4 and 5, respectively. In other words, because there were no significant changes in the zeta potential and the specific resistance to filtration of the MLSS solution in Stage I, the development occurring in permeation flux during Stage II would be completely attributable to the changes occurring in the physical/chemical properties of the MLSS solution after applying a DC field. FIGURE 5. Change of specific resistance to filtration of the MLSS solution. The results of SRF tests during Stage I did not show a significant improvement in decreasing the SRF. During Stage II, a higher improvement in decreasing the SRF was observed on day 35 that was attributed to a significant change in floc morphology resulting from the electrocoagulation process occurring in Zone I. Moreover, the results of the SRF tests showed some variations in r/ro between the two zones. The ratio r/ro reduced to 0.67 in Zone I and to 0.74 in Zone II on day 35. At the end of the operation of Stage II, the ratio (r/ro) reduced to 0.60 and 0.66 in Zone I and Zone II, respectively. This indicated a significant improvement in reducing the SRF of the MLSS solution up to 40% and agreed with the results obtained on improving the membrane flux. These results suggest that a strong relationship existed between enhancing the membrane permeability in the SMEBR system and decreasing the SRF of the ML solution in the electro-bioreactor. Referring to eq 2, it seems that the relatively small particles that are the cause of membrane fouling had undergone flocculation due to the electrocoagulation process. Therefore, their effect on membrane fouling was minimized. In fact, particle morphology/size is a key parameter in characterizing the membrane fouling phenomena (29, 30). Generally, particle movement into membrane pores can be reduced by increasing the flocs size, and this enhances the particle back-transport from the membrane surface to the bulk solution (31). Applying a DC through the mixed liquor solution increases the flocs size. This principle was confirmed in the results obtained in this study. 4.4. Influence of the MLSS Concentration on Membrane Fouling. Since the SMEBR system was operated under complete SRT in two sequential stages, the role of the MLSS concentration on membrane fouling should be discussed. According to the literature, it is widely believed that the MLSS concentration is one of the predominant fouling parameters that may affect the MBR performance (1). However, the exact influence of MLSS concentration on membrane fouling is not yet clear. Some of the recent studies reported that MLSS seems to have a mostly negative impact on MBR hydraulic performance represented by a high TMP and a low flux (32), while others have reported a positive impact (33, 34), and some have observed an insignificant impact with small concentration changes (35, 36). It seems that the level of the MLSS concentration plays an important role on membrane fouling. Le-Clech et al. (36) found that there was no significant difference in the concentrations of 4000-8000 mg/L, but that a significant decrease of permeate rate was observed when the MLSS concentration increased to 12,000 mg/L. In this study, the MLSS concentration increased from 2100 mg/L up to 5000 mg/L during the operational period. According to the above discussion, it was assumed that the MLSS concentration did not contribute significantly to

Acknowledgments We gratefully acknowledge the financial sponsorship granted by the Strategic Program of Natural Sciences and Engineering Research Council of Canada (NSERC: STPGP/350666).

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