| Membrane bioreactor (MBR) is a highly efficient combination of membrane separation technology with activated sludge process. However, membrane fouling is always accompanied with the MBR process. Considering the facts that sludge retention time (SRT) has an important impacts on mixed liquors properties such as Zeta potential, hydrophobic, SVI and the EPS concentration directly, and the mixed liquors properties correlated well to membrane fouling, SRT is believed to be one of the most important operation parameters to control membrane fouling. In this study, the effect of SRT to the membrane fouling and sludge properties were investigated in a submerged membrane bioreactor (SMBR) treating the lysine wastewater. Furthermore, the feasibilities of adding flocculant to improve the sludge properties and then control the membrane fouling were also studied. The main research contents and the results were summarized as follows:(1) The effluent water quality, sludge concentration, composition and content of EPS and the change of sludge settling ability were studied under no sludge discharge condition. The results indicated that in the startup stage, the sludge in SMBR grew rapidly, and the COD and ammonia nitrogen removal rate increased constantly. In the stable stage, the COD and the concentration of ammonia nitrogen were below 30 mg/L and 3 mg/L, respectively, and the MLVSS/MLSS was constant at 84.14%. However, as the running time extended, the nutrition in the reactor was short, and the activity of sludge decreased. These facts caused the deterioration of the effluent quality such as the COD and the ammonia nitrogen. The main component of EPS both in the supernatant and sludge were polysaccharide. The EPS concentration in the sludge was higher than that in the supernatant, for instance, the EPS concentration in the sludge was 3.69 and 3.24 times more than that in the supernatant at 30 d and 180 d, respectively. As the running time extended, the sludge activity and absorption flocculation ability of the sludgedeteriorated.(2) The variations of sludge characteristics such as components and contents of EPS, sludge settling ability, relative hydrophobic and its impacts on membrane fouling were investigated at different SRT of 10 d,20 d and 40 d. The results showed that, as the SRT increased, the concentration of EPS decreased, and the ratio between protein and polysaccharide in tightly bound EPS (TBEPS) increased. The settling ability of sludge became worse, while the relatively hydrophobic raised with the increase of SRT. The results of correlation analyses between sludge properties and membrane fouling indicated that the ratio of protein to polysaccharide, sludge settle ability and relative hydrophobic had strong positive correlations with membrane fouling.(3) A dead-end filtration test was conducted to evaluate the effect of different sludge characteristics to PP membrane fouling using the sludge obtained from the MBR under different SRT. The reults showed that the steady flux decreased with an increase of the EPS concentration. The protein adsorption on PP membrane surface was larger than polysaccharides, and the adsorption capacities increased as EPS concentration rised up. The the steady flux increased as both the decrease of the ratio of protein/polysaccharide and the increase of particle size of sludge.(4) The feasibilities of adding different flocculant to improve the properties of the sludge were investigated. The results showed that the adding flocculants could not only decrease the concentrations of water pollutantsbut also reduce the EPS in the mixed liquors caused by the microbial life activities. After adding flocculants, the biological flocculation could increase the size of sludge and then generate a bigger sludge floe, the SVI of sludge also decreased, and the sedimentation ability improved.
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