An operation of a membrane bioreactor in sequencing batch mode named a sequencing batch membrane bioreactor (SBMBR) was investigated for enhancing nitrogen and phosphorus removal. Its performance was compared with a conventional membrane bioreactor (CMBR) at various influent COD/TN ratios of 3.4-28.2. The operational parameters were optimized to increase the treatment efficiency. COD removal averaged at 94.9 and 97.7%, respectively, for SBMBR and CMBR during the 8 months experimental period. The SBMBR system demonstrated good performance on nitrogen and phosphorus removal at different COD/TN ratios. When COD/TN was 6.3 and the total nitrogen (TN) load was 0.22kg/(m3 days), the TN and ammonium nitrogen removals of the SBMBR were maintained over 65 and 90%, respectively. Total phosphorus (TP) removal of the SBMBR was approximately 90% during most of the experimental time. In comparison, the CMBR did not perform so well. Its effluent TN concentration was close to that in the influent at COD/TN=6.3 and TP removal was not stable. The specific nitrification rate test showed that pH value affected the activity of nitrifiers but no irreversible harm was induced. Furthermore, the sequencing batch mode operation of MBR retarded membrane fouling according to the monitoring of trans-membrane pressure (TMP).

Cake layer formation on the membrane surface has been a major challenge in the operation of membrane bioreactors (MBRs). In this study, the cake layer formation mechanism in an MBR used for synthetic wastewater treatment was investigated. The major components of cake layer were systematically examined by particle size analyzer (PSA), scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), X-ray fluorescence (XRF), energydiffusive X-ray analyzer (EDX), and Fourier transform infrared (FTIR) spectroscopy. The results indicate that the small particles in sludge suspension had a strong deposit tendency on the membrane surface. The SEM and CLSM analysis exhibited that bacterial clusters and polysaccharides were significant contributors to membrane fouling. The main components of biopolymers were identified as proteins and polysaccharide materials by the FTIR. The examination by EDX and XRF demonstrated that Mg, Al, Ca, Si, and Fe were the major inorganic elements in fouling cake. Furthermore, the results suggest that bridging between deposited biopolymers and inorganic compounds could enhance the compactness of fouling layer. During the operation of MBRs, the biopolymers and inorganic elements in the bioreactor should be controlled to minimize membrane fouling.

This study is to evaluate the utility of a strategy of stepwise increased selection pressure for cultivating aerobic granules. Three sequencing batch reactors (SBR; R1-R3) were used. Substrate ammonium-nitrogen (NH4+-N) concentration was a microbial selection pressure for selecting nitrifying bacteria. It was stepwise increased from 50 to 200mg l−1 in R2 and fixed at 50mg l−1 in R1 and 200mg l−1 in R3. Results showed that aerobic granules failed to be developed in R3 due to free ammonia (FA) inhibition, while they formed quickly in both R1 and R2. Filamentous dominated granules in R1 exhibited poor performance and started to disintegrate from day 131. Stepwise increased selection pressure in R2 selected slow-growing nitrifying bacteria gradually, and this suppressed filamentous growth and improved the stability of aerobic granules. During the 283 days operation, R2 produced aerobic granules with incrementally higher biomass concentration, better settleability, and larger granule size. Larger granules further favored denitrification and the optimal granule size is suggested to be controlled within 2-3mm. This study demonstrates the successful application of the strategy of stepwise increased selection pressure for avoiding the failure of aerobic granulation and improving the stability and performance of aerobic granules.

Membrane bioreactor (MBR) is an important solid–liquid separation technology employed widely in the wastewater treatment. However, membrane permeability decline rapidly due to membrane fouling, which limited the application of MBR. In order to obtain understanding of the complex membrane fouling mechanisms, analysis of the effect of the microstructure on the cake permeability is necessary. In this regard, study of the distribution and porosity of the cake is very necessary. The pore distribution is strongly influenced by cake compressibility but information on how to describe this effect is very limited. In this paper, fractal theory was used to gain knowledge of cake icrostructure and its correlation to macroscopic cake properties. Scanning electron microscopy (SEM) and image analysis were utilized to estimate pore size and pore size distributions. The two-dimensional fractal dimension was determined. The results show that the cake layer had a good fractal characteristic, and the fractal dimension (Ds) was linearly correlated with porosity (R2=0.929). The fractal dimension had some relation with cake resistance and cake specific resistance. It indicates that fractal dimension provides an approach for quantification of cake structure and cake permeability.

Cake layer formed on membrane surface presents a major challenge to membrane permeation, and it can be considered as a porous media. Determination of the cake layer permeability is critical for an accurate analysis and design of membrane filtration. A permeation model, based on fractal theory and Darcy's law, for evaluating cake layer permeability in microfiltration of activated sludge wastewater was developed. The cake layer permeability was derived and found to be a function of the pore-area fractal dimension and microstructural parameters. The validity of the model was studied systematically in this paper. The permeation model was applied to study the effect of mixed liquid suspended solids (MLSS) concentration, particle size distribution (PSD) and extracelluler polymeric substance (EPS) on cake layer permeability in a submerged membrane bioreactor. Results showed that the permeation model was a useful tool to study the micromechanism of membrane fouling. There was a close correlation between MLSS concentration and cake layer permeability. There were a slight and a distinct decrease of the cake layer permeation as MLSS increased less and larger than 10,000mg/L, respectively. PSD and EPS are two significant factors affecting cake layer permeability in membrane bioreactor. Effect of activated sludge on cake layer permeability was mainly caused by EPS and small particles attached on membrane surface.