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.

Membrane fouling is the major problem that hinders the practical application of membrane bioreactor (MBR) systems. Since the MBR system includes living microorganisms and their metabolites, the fouling mechanism is even more complex than that of conventional membrane separation processes. This paper provides an improved understanding of the influence of filamentous bacteria on membrane fouling in microfiltration of activated sludge wastewater. In order to make clear the membrane fouling mechanism of filamentous bacteria, the physicochemical characteristics of sludge flocs, including extracellular polymeric substance (EPS), zeta potential, relative hydrophobicity (RH) and floc morphology were systematically investigated. The results showed that the sludge flocs with negligible filamentous bacteria led to severe membrane pore blocking, and the sludge flocs with filamentous bacteria created the formation of a non-porous cake layer on the membrane surface. The excess growth of filamentous bacteria resulted in much more release of EPS, lower zeta potential, higher hydrophobicity of sludge flocs and more irregularly shaped flocs, which did great harm to membrane filtration. The sludge flocs with a small quantity of filamentous bacteria had a positive effect on membrane permeation. It is important to control filamentous bacteria concentration in the operation of MBRs.

采用统计分析方法研究了16种不同性质活性污泥混合液对膜生物反应器膜污染的影响机理。结果表明，胞外聚合物 （extracellular polymeric substances, EPS）、溶解性有机物（soluble microbial products, SMP）、上清液胶体颗粒（suspended solids in supernatant, SSs）、污泥混合液粘度（μ）、相对疏水性（relative hydropho bicity, RH）、Zeta 电位（Zeta potential） 均对膜生物反应器膜渗透性能有显著的影响作用，其与膜污染阻力的皮尔逊相关系数rp分别为：01898、01712、01810、01691、01837、-01881；同时发现，胞外聚合物是影响活性污泥中溶解性有机物含量（rp=01682）、污泥粘度大小（rp=01633）、上清液胶体颗粒含量（rp=01783）、Zeta 电位（rp=-01953） 及相对疏水性大小（rp=01877） 的主要因素；在活性污泥性质中胞外聚合物是影响膜污染的根本原因，是膜生物反应器膜污染的重要影响因素.

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.