Transmembrane Pressure Increase Rate Research Articles

The effect of temperature on fouling in anaerobic membrane bioreactor: SMP- and EPS-membrane interactions

Biofouling is the main challenge in the operation of anaerobic membrane bioreactors (AnMBRs). Biofouling strongly depends on temperature; therefore, we hypothesize that the interactions and viscoelastic properties of soluble microbial products (SMP) and extracellular polymeric substances (EPS) vary with temperature, consequently influencing membrane permeability. This study compares the performance of an AnMBR operated at a similar permeate flux at two temperatures. The transmembrane pressure (TMP) rose rapidly after 5 ± 2 days at 25 °C but only after 18 ± 2 days at 35 °C, although the reactor's biological performance was similar at both temperatures, in terms of the efficiency of dissolved organic carbon removal and biogas composition, which were obtained by changing the hydraulic retention time. Using confocal laser scanning microscopy (CLSM), a higher biofilm amount was detected at 25 °C than at 35 °C, while quartz crystal microbalance with dissipation (QCM-D) showed a more adhesive, but less viscous and elastic EPS layer. In situ optical coherence tomography (OCT) of an ultra-filtration membrane, fed with the mixed liquor suspended solids (MLSS) at the two temperatures, revealed that while a higher rate of TMP increase was obtained at 25 °C, the attachment of biomass from MLSS was markedly less. Increased EPS adhesion to the membrane can accelerate TMP increase during the operation of both the AnMBR and the OCT filtration cell. EPS's reduced viscoelasticity at 25 °C suggests reduced floc integrity and possible increased EPS penetration into the membrane pores. Analysis of the structures of the microbial communities constituting the AnMBR flocs and membrane biofilms reveals temperature's effects on microbial richness, diversity, and abundance, which likely influence the observed EPS properties and consequent AnMBR fouling.

The Effect of Heat Sterilization on Key Filtration Performance Parameters of a Commercial Polymeric (PVDF) Hollow-Fiber Ultrafiltration Membrane.

Membrane processes can be integrated with fermentation for the selective separation of the products from the fermentation broth. Sterilization with saturated steam under pressure is the most widely used method; however, data concerning heat sterilization applicability to polymeric ultrafiltration (UF) membranes are scarcely available. In this study, the effect of the sterilization process on the filtration performance of a commercial polyvinylidene difluoride (PVDF) hollow fiber UF membrane was evaluated. Membrane modules were constructed and sterilized several times in an autoclave. Pure water flux tests were performed, to assess the effect of heat sterilization on the membrane’s pure water permeance. Dextran rejection tests were performed for the characterization of membrane typical pore size and its fouling propensity. Filtration performance was also assessed by conducting filtration tests with real fermentation broth. After repeated sterilization cycles, pure water permeance remained quite constant, varying between approx. 830 and 990 L·m−2·h−1·bar−1, while the molecular weight cut-off (MWCO) was estimated to be in the range of 31.5–98.0 kDa. Regarding fouling behavior, the trans-membrane pressure increase rate was stable and quite low (between 0.5 and 7.0 mbar/min). The results suggest that commercial PVDF UF membranes are a viable alternative to high-cost ceramic UF membranes for fermentation processes that require heat sterilization.

Effect of surface-patterned topographies of ceramic membranes on the filtration of activated sludge and their interaction with different particle sizes

The main benefits of surface patterning on filtration membranes are two-fold: (1) surface patterns increase the effective filtration area, and (2) enhance the back diffusion of foulants to minimise membrane fouling. Although the influence of pattern sizes has been revealed, the relationship between pattern sizes and foulant particle sizes has not been systematically studied. In this study, we fabricated ceramic membranes with line patterns of various designed topographies through 3D printing and investigated their differences in fouling alleviation with activated sludge and 5 latex particle sizes with narrow size distributions. Combinations of three pattern heights ( h = 80, 120, 250 μm) and three pattern spacings ( s = 200, 500, 1000 μm) were prepared in this study. Membrane fouling rates were affected by the topography, even after accounting for differences in the effective surface areas. Structures with high height (Pattern HM) performed poorly and had the highest fouling rate in activated sludge filtration compared to the rest of the patterns. The average rate of transmembrane pressure increase for HM was 2.48 ± 0.07 kPa/min, 3 times higher than that of the best performing design (Pattern MM). The same phenomenon was observed for monodispersed latex microspheres. Moreover, we found that the fouling rate varied with h , s and particle size ( a ) and increased with a/(s/h) ratio. For the activated sludge and range of latex particles tested, a combination of h = 120 μm and s = 500 μm (Pattern MM) was the overall best design for fouling alleviation. Our findings provide an effective pathway to achieve maximum benefits of the surface-patterned ceramic membrane according to particle sizes, e.g., the s/h ratio must be designed to increase for the treatment of larger particles. • Appropriate surface patterning on ceramic membrane reduced membrane fouling. • Topographies of line patterns were important considerations for design. • Particle diameter ( a ), pattern height ( h ) and pattern spacing ( s ) studied. • Fouling rate increased with a/(s/h) ratio. • Combination of h = 120 μm and s = 500 μm was best for fouling alleviation.

Applicability of a Combined DAF-MF Process to Respond to Changes in Reservoir Water Quality through a Two-Year Pilot Plant Operation

The optimal operating conditions of a combined dissolved air flotation (DAF)-microfiltration (MF) process to respond to changes in raw water quality were investigated by operating a pilot plant for two years. Without DAF pre-treatment (i.e., MF alone), MF operated stably with a transmembrane pressure (TMP) increase of 0.24 kPa/d when the turbidity of raw water was low and stable (max. 13.4 NTU). However, as the raw water quality deteriorated (max. 76.9 NTU), the rate of TMP increase reached 43.5 kPa/d. When DAF pre-treatment was applied (i.e., the combined DAF-MF process), the MF process operated somewhat stably; however, the rate of TMP increase was relatively high (i.e., 0.64 kPa/d). Residual coagulants and small flocs were not efficiently separated by the DAF process, exacerbating membrane fouling. Based on the particle count analysis of the DAF effluent, the DAF process was optimised based on the coagulant dose and hydraulic loading rate. After optimisation, the rate of TMP increase for the MF process stabilised at 0.17 kPa/d. This study demonstrates that the combined DAF-MF process responded well to substantial changes in raw water quality. In addition, it was suggested that the DAF process must be optimised to avoid excessive membrane fouling.

Investigation of the short-term effects of extracellular polymeric substance accumulation with different backwashing strategies in an anaerobic self-forming dynamic membrane bioreactor

The optimum operation strategy for a side-stream external anaerobic self-forming dynamic membrane bioreactor (AnSFDMBR) was investigated by coupling such a system with an up-flow anaerobic sludge blanket reactor. Time-based backwashing with different intervals and transmembrane pressure (TMP)-based backwashing were compared as the operation strategies of the AnSFDMBR. The system performance, extracellular polymeric substance (EPS) accumulation in the dynamic layer and on the membrane mesh of the AnSFDMBR, and the physical properties of the dynamic layer were closely monitored. Both operation strategies achieved stable operation with effluent turbidity less than 5 nephelometric turbidity units with a slowly increasing TMP. However, with the time-based backwashing strategy, the EPS accumulation rate in the dynamic layer was more than 20 times higher than that on the mesh, indicating that frequent backwashing might have a negative impact on the AnSFDMBR. The impacts of EPS accumulation on the membrane mesh were negligible considering the small amount of EPS residual and the large pore size of the mesh. On the contrary, the EPS accumulation in the dynamic layer changed the layer's physical properties and further impacted on the performance of the AnSFDMBR. The accumulation of polysaccharides in the dynamic layer was the main reason for the layer's compactness, which was negatively correlated with the specific surface area and further led to the TMP increase. The polysaccharides in the dynamic layer-to-sludge ratio increased to around 1.6 with only 5 days of time-base operation. With TMP-based operation, it took more than 10 days for polysaccharides in the dynamic layer-to-sludge ratio reaching 1.6. The low TMP increase rate, high effluent quality, and slow EPS accumulation with TMP-based backwashing indicated TMP-based operation is applicable in the studied AnSFDMBR. Nevertheless, the correlation between TMP and the accumulation of polysaccharides should be further investigated to find the optimum TMP for backwashing.

Treatment of Real Paper-Recycling Wastewater in a Novel Hybrid Airlift Membrane Bioreactor (HAMBR) for Simultaneous Removal of Organic Matter and Nutrients

In this study, a novel integrated Hybrid Airlift Membrane Bioreactor (HAMBR) composed of oxic, anoxic, and anaerobic zones was developed to simultaneously remove organic matter and nitrogen from real paper-recycling wastewater. The removal efficiencies of Chemical Oxygen Demand (COD), ammonium, nitrite, nitrate and Total Nitrogen (TN) for permeate and supernatant were in the range of 88-99%, 54-83%, 70-90%, 65-95% and 61-90%, respectively. In addition, the membrane fouling was evaluated by Trans-Membrane Pressure (TMP) monitoring during the experimental period at a constant flux of 12 L/m2h,and the rate of TMP increase was 1.75 mbar/day. The results showed that the hybrid airlift membrane bioreactor can be applied effectively to the simultaneous removal of organic and nutrient from real wastewater and the performance of the membrane bioreactor was satisfactory in terms of resistance against membrane fouling phenomenon, which is an important parameter during HAMBR operation.

Upflow anaerobic dynamic membrane bioreactor (AnDMBR) for wastewater treatment at room temperature and short HRTs: Process characteristics and practical applicability

An upflow anaerobic dynamic membrane bioreactor (AnDMBR) was set up for real domestic wastewater treatment at room temperature (20–25 °C) and short hydraulic retention time (HRT = 8 h, 4 h, 2 h, and 1 h). Following continuous operation for 93 days with stepwise decreased HRT, stable average chemical oxygen demand (COD) removal was achieved (between 77.3% and 70.6%) when HRT was reduced from 8 h to 4 h, then 2 h with flux varying from 22.5 to 90 L/m2·h. At these three HRTs, the rate of increase in trans-membrane pressure (TMP) was 0.4, 0.38, and 0.57 kPa/d, and average methane (CH4) production was 0.12, 0.10, and 0.08 L/g CODremoved, respectively. Furthermore, decreasing the HRT to 1 h resulted in less COD being removed (60.4%) and lower CH4 production (0.05 L/g CODremoved) as well as a faster rate of TMP increase (2.11 kPa/d). Various analytical methods were applied to characterize the morphology and composition of the dynamic membrane (DM) layers. Organic components analysis revealed that, with reduced HRT, there were apparent increases in soluble microbial products in the liquid phase and accumulation of tryptophan protein-like substances and aromatic protein-like substances in the DM layer, especially when the HRT was shortened to 1 h. Whilst the upflow AnDMBR proved applicable to wastewater treatment at room temperature with short HRTs, 2 h could be the HRT limit for maintaining stable operation.

Development of a moving-bed electrochemical membrane bioreactor to enhance removal of low-concentration antibiotic from wastewater

Removal of low-concentration (ng/L ~ μg/L) antibiotics from water calls for the development of cost-effective treatment technologies. In this study, a novel moving-bed electrochemical membrane bioreactor (MEMBR) was developed for removing sulfamethoxazole (SMX). Results showed that the introduction of external electric field and carbon felt particles could efficiently eliminate SMX (removal efficiency of 88.9%). In contrast, the moving-bed membrane bioreactor (MMBR) took a long time to acclimate microorganism, reaching a removal efficiency of 43.9%. Transmembrane pressure increase rate was much lower in MEMBR (1.06 kPa/d) compared to MMBR (1.72 kPa/d). The presence of carriers increased the generation of reactive oxygen species, contributing to SMX removal. Microbial community analysis revealed that the introduction of electric field could increase microbial community richness/diversity and enrich the phyla of Actinobacteria and Gemmatimonadete, potentially capable of mineralizing SMX. These results clearly demonstrated the potential of this novel MEMBR to be used for enhanced micropollutants removal from water/wastewater.

The Impact of Mechanically-Imposed Shear on Clogging, Fouling and Energy Demand for an Immersed Membrane Bioreactor.

The impact of the application of mechanically-imposed shear on the propensity for fouling and clogging (or “sludging”—the agglomeration of sludge solids in the membrane channel) of an immersed flat sheet (iFS) membrane bioreactor (MBR) was studied. The bench-scale test cell used contained a single flat sheet fitted with a crank and motor to allow the membrane to be oscillated (or reciprocated) vertically at a low rate (20 RPM). The membrane was challenged with sludge samples from a local MBR installation treating petroleum industry effluent, the sludge having previously been demonstrated as having a high sludging propensity. Sludging was measured by direct visual observation of membrane surface occlusion by the agglomerated solids, with fouling being notionally represented by the rate of transmembrane pressure increase. Results demonstrated membrane reciprocation to have a more beneficial impact on sludging amelioration than on suppressing fouling. Compared with the stationary membrane, sludging was reduced by an average of 45% compared with only 13% for fouling suppression at the reference flux of 15 L·m−2·h−1 applied. The specific energy demand of the mechanical shear application was calculated as being around 0.0081 kWh·m−3, significantly lower than values reported from a recent pilot scale study on a reciprocated immersed hollow fibre MBR. Whilst results appear promising in terms of energy efficiency, it is likely that the mechanical complexity of applying membrane movement would limit the practical application to low flows, and a correspondingly small number of membrane modules.

Bismuth Chelate‐Doped Microfiltration Membrane and Its Anti‐Biofouling Performance During a High‐Flux Membrane Bioreactor Operation

The main disadvantage of membrane bioreactor (MBR) technology, which is an important alternative for advanced wastewater treatment, is fouling. In this regard, polyether sulfone (PES) membranes were prepared by adding bismuth 2,3‐dimercapto‐1‐propanol (BisBAL) chelate to the dope solution for biofouling control. BisBAL was synthesized using bismuth metal and a thiol. The bare and BisBAL‐doped PES membranes were tested for their physical properties and biofouling resistance properties using xanthan gum solution as a model foulant and real activated sludge. The results showed that the BisBAL‐doped PES membrane has a lower absorptive surface and fouling tendency when compared to the bare PES membrane. Bismuth release from BisBAL‐doped PES membrane during pure water filtration was measured using inductive‐coupled plasma spectrometry and the results showed that bismuth release from BisBAL‐doped PES membrane was quite low. According to the MBR operation results, it can be said that the rate of transmembrane pressure increase, total fouling and biofilm creation could be decreased with BisBAL‐doped PES membrane usage without any effect on activated sludge structure and microbial community population in the MBR. BisBAL‐doped PES membranes did not lose their durability and anti‐biofouling activities in time and could be chemically washed. The novelty of this study was the application of a microfiltration membrane manufactured with bismuth metal in MBR operation.

Treatment performance and membrane fouling characteristics of inclined-tube anoxic/aerobic membrane bioreactor applied to municipal solid waste leachate

This study investigated the performance of inclined tube anoxic/aerobic membrane bioreactor (itMBR) treating concentrated municipal solid waste leachate over 550 d at organic loading rates of 4.2–31.8 kg BOD/m3 d. At optimum loading of 18 kg BOD/m3 d, the itMBR could achieve high removals of 97% for biochemical oxygen demand, 94% for chemical oxygen demand, and 95% for total Kjeldahl nitrogen. Complete elimination of protein-like substances and partial removal of fulvic and humic acid-like substances were achieved. Major membrane foulants were found to be gel layers containing protein-like and polysaccharide-like substances and Ca-based inorganic scaling. Membrane cleaning using sodium hypochlorite followed by citric acid was found effective for the restoration of transmembrane pressure (TMP) while periodical chemical enhanced back-flushing at every 7–10 d could limit TMP increase rate and sustain the operation in long term.

Filtration behaviour and fouling mechanisms of polysaccharides.

This study investigated filtration behaviors of polysaccharides solutions, both alone and in mixture with proteins, in the short-time constant flux filtration with the focus on factors affecting the transmembrane pressure (TMP) increase rate, the irreversible filtration resistance, and the membrane rejection behavior. The results showed that the TMP increase rates in the short-time constant flux filtration of alginate solutions were significantly affected by the calcium addition, alginate concentration, and flux. Although the addition of calcium resulted in a decrease in the TMP increase rate, it was found that the irreversible fouling developed during the filtration increased with the calcium addition, implying that the double-sided effect of calcium on membrane filtration and that the TMP increase rate observed in the filtration does not always reflect the irreversible membrane fouling development. It was also found that for the filtration of solutions containing mixed alginate and BSA, alginate exerted a dominant effect on the TMP increase rate and the membrane exhibited a reduced rejection to both alginate and BSA molecules compared to that in the filtration of the pure alginate or BSA.

A novel approach to analyzing concentration polarization of polysaccharide solutions

This paper describes a novel approach to analyzing the concentration polarization (CP) in the short-time constant flux filtration of polysaccharide solutions. With application of the pearl-necklace model based polymer solution permeability correlation, a transmembrane pressure (TMP) increase rate model was developed to relate the linear TMP increase rate measured in the short-time constant flux filtration to the solid concentration at the membrane surface, the polymer critical overlapping concentration, and the radius of gyration of the polymer molecules. Establishment of such a theoretical model allows analysis of the gelation propensity of the CP layer formed by polysaccharides based on the experimentally determined linear TMP increase rate.

Development of a Techno-Economic Model to Compare Ceramic and Polymeric Membranes

Ceramic materials for microfiltration and ultrafiltration have a number of potential advantages over polymeric materials including chemical and thermal stability, physical strength, and a longer operational life. In this study an alumina composite ceramic membrane was compared to a polyethersulfone (PES) membrane for the filtration of bentonite and water. The two membranes were operated under equivalent hydrodynamic conditions, described by the nominal Peclet (Pe) number, so that fouling could be attributed to the membrane material differences rather than operational differences. The alumina membrane showed gradual onset of fouling as Pe increased, evidenced by a slow increase in the rate of trans-membrane pressure (TMP) increase over the duration of the experiment. The PES membrane showed negligible TMP increase until a critical Pe was reached, after which point a dramatic increase in the rate of TMP increase was observed. A techno-economic model was used to quantify the performance differences (rate of pressure increase per volume of feed filtered per membrane area) for each membrane at different values of Pe and at different operating conditions in terms of water production costs. The alumina membrane resulted in a higher total water production cost than the PES membrane due to the high membrane material cost. Economic leverage points (i.e., membrane lifespan, membrane material cost, and labor requirements) were identified which make the alumina membrane cost competitive with the PES membrane.

Permeability and clogging in an immersed hollow fibre membrane bioreactor

A pilot-scale hollow fibre immersed membrane bioreactor (HF iMBR) fed with municipal wastewater was operated under conditions pertaining to those at full-scale to assess the impact of clogging by gross solids on permeability. The study proceeded by operating for ∼22h periods at two different fluxes with regular backflushing, and the accumulation of solids within the fibre bundle determined through their mechanical removal and gravimetric measurement. Permeability recovery, along with the fouling rate during sludging, from this complete “declogging” was compared with that from conventional maintenance cleaning from chemically enhanced backflushing (CEB).Results revealed declogging to contribute significantly to permeability recovery, but the comparable impact of the CEB alone revealed fouling to also be important. However the rate of transmembrane pressure increase, normally termed the “fouling rate”, was found to strongly correlate with the mass of accumulated solids, suggesting that clogging suppression is important in maintaining permeability overall during HF iMBR operation.

In-line coagulation prior to UF of treated domestic wastewater – foulants removal, fouling control and phosphorus removal

Abstract The present work investigated fouling control and phosphorus removal by applying in-line coagulation prior to ultrafiltration (UF) of treated domestic wastewater. Experiments were conducted in both lab- and pilot-scale under close to neutral pH condition. Lab-scale foulant removal tests showed that increasing the dosage of FeCl 3 , AlCl 3 and polymeric aluminum chloride (PACl) can improve biopolymer removal. Specifically, PACl reduced preferentially the proteinaceous fraction of biopolymer while the other two coagulants showed no significant preference. The filterability of water samples was improved after coagulation, which is contributed to biopolymer removal and the formation of larger particles. Pilot UF experiments demonstrated that in-line coagulation improved the performance of UF to a large extent. Within 0.037–0.148 mmol Me 3+ /L dosage range, adding more FeCl 3 and AlCl 3 slowed down the development of trans-membrane pressure (TMP) correspondingly, while changing PACl dosage showed little effect on the variation of TMP increase rate. Further investigations indicated that PACl related precipitates contributed to more irreversible fouling than that which the monomeric coagulants made. Fouling control is thus considered as a co-effect determined by foulant removal efficiency, fouling layer structure and the adherence of hydrolysis products/precipitates onto the membrane. With respect to phosphorus removal, dosing FeCl 3 and AlCl 3 achieved higher removal efficiency than using PACl. Based on lab- and pilot-scale results, dosing FeCl 3 and AlCl 3 at a relative dosage of over 2.5 mol Me 3+ per mol total phosphorus (TP) in feedwater is necessarily required to keep the TP concentration under 50 μg/L in UF permeate.

Comparison of ceramic and polymeric membrane permeability and fouling using surface water

The trans membrane pressure (TMP) increase at a constant flux of 150 L m −2 h −1 due to membrane fouling by direct filtration with lake water is investigated for four ceramic (Al 2O 3, ZrO 2, TiO 2, SiC) and a PES/PVP polymeric microfiltration membrane(s). The membrane structures and compositions are characterised with permporometry (pore size, porosity) and XPS. The TiO 2 and SiC membrane have a 5 and 24 times larger average pore size than expected based on supplier information. The TMP increase rate due to fouling inversely follows the measured pore size. Reversible fouling decreases in the order of polymeric ≈ Al 2O 3 ≈ ZrO 2 > TiO 2 > SiC, and for irreversible fouling polymeric > ZrO 2 > Al 2O 3 > TiO 2 > SiC. Removal of non purgeable organic carbon (NPOC) is around 30% for the ceramic membranes, and 13–25% for the polymeric membrane. The reversible NPOC load decreases in the following order (Al 2O 3 > (ZrO 2 ≈ TiO 2)) ≈ SiC > polymeric. The higher degree of fouling on the polymeric membrane is partly due to its lower volume/area ratio, compared to ceramic membranes. Mass balance analysis shows that 85 ± 8% of the NPOC is accounted for. Thus, the used soaking method (pH 12 NaOH, >1 h) does not fully remove the NPOC, suggesting that a more aggressive cleaning solution should be used.

Characterization of fouling in immersed polyvinylidene fluoride hollow fibre membrane ultrafi ltration by particles and natural organic matter

Turbidity-causing particles and natural organic matter (NOM) in surface water are two major contributors to membrane fouling. This study focused on the effects of particles, NOM and particles-NOM contaminated raw water on membrane fouling in ultrafi ltration (UF) and coagulation-UF process, respectively. The kaolinite solution and humic acid (HA) solution was used to simulate the particles and NOM contaminated raw water respectively in the laboratory experiments. It was found that pollutant constituents and concentration of raw water and pre-treatment process had an infl uence on membrane fouling. There was a linear relationship between the total organic carbon (TOC) concentration of raw water and that of effl uent in UF when HA-contaminated raw water was tested. The transmembrane pressure increase rate was in exponential relationship with raw water TOC. HA mainly contributed to the irreversible fouling, while kaolinite mainly brought the reversible fouling. It was verifi ed that coagulation pre-treatment could postpone membrane fouling development in the limited range of coagulant dosage. Additionally, it was also found that combined fouling effects of particles and NOM were not the simple addition of the individual's effect.

Fluorescent dissolved organic matter variations in a submerged membrane bioreactor under different sludge retention times

Abstract A pilot-scale submerged membrane bioreactor (MBR) for the treatment of real municipal wastewater was operated in this study under various sludge retention time (SRT) in order to verify the correlations of fluorescent dissolved organic matter (DOM) with membrane fouling. The fluorescent DOM determined by three-dimensional excitation–emission matrix (EEM) fluorescence spectroscopy and the trans-membrane pressure (TMP) under different SRT operation were investigated, and the correlations between them were analyzed. The results showed that the variations of fluorescent DOM (Peaks B and C) in the sludge supernatants correlated well with the variations of EEM spectra signal of bound extracellular polymeric substances (EPS) even though they were different in nature, i.e., the bound EPS were found to be more non-biodegradable than DOM. The fluorescent intensity (FI) variations of Peaks A and B in the EEM spectra of DOM were closely related to the TMP increase rate (membrane fouling) in the MBR, suggesting that they might be used to monitor the DOM variations and to indicate MBR performance. Through the EEM spectra analysis, it was confirmed in this work that the DOM in the sludge supernatants was not only originated from the bound EPS but also from the influent wastewater.

The Effects of Intermittent Aeration on the Characteristics of Bio-Cake Layers in a Membrane Bioreactor

The effects of a sequencing variation for dissolved oxygen (DO) concentrations on the membrane permeability in a submerged membrane bioreactor (MBR) were studied. An MBR was continuously operated under alternating DO conditions, e.g., 36 h of an aerobic phase, followed by 36 h of an anoxic phase. The rate of increase in transmembrane pressure (TMP) in the anoxic phase was always steeper than that in the aerobic phase, indicating that the fouling rate was higher in the anoxic than in the aerobic condition. Regardless of the phases, the rate of TMP increase became steeper as the cycles were repeated. However, this trend became less important as the cycle numbers increased. Even in identical microbial communities, the number of colloidal particles and soluble extracellular polymeric substances (EPS) in the bulk solution were increased during the anoxic condition, which caused a reduction in the porosity of the bio-cake. During analysis of the bio-cake profile along the cake depth, the temporal variation of the bio-cake structure was attributed to the temporal change in the number of colloidal particles as well as the change in compression forces acting on the bio-cake. The influence of the latter was found to be more important than that of the former, which was verified by comparing the various structures of bio-cake formed in differing DO environments.