Influence of solid retention time on sludge characteristics and effluent quality in immersed membrane bioreactor

Here the effect of solid retention time (SRT) on the concentration of the mixed liquor suspend solid (MLSS), the sludge characteristics, the content of extra-cellular polymeric substances (EPS), the viscosity of mixed liquor and effluent quality in the immersed membrane bioreactor (IMBR) was investigated. The results indicate that the increase of the EPS content is the main reason for the increase of mixed liquor viscosity, the former is positively correlated with the latter (R ² = 0.9751). The size distribution profile of particles in the mixed liquor presents double-peak shape at SRT more than 30 days. The filtration resistance of membrane in IMBR is mainly caused by the tiny particles and the viscosity of the mixed liquor. In this study, the extension of SRT can hardly affect the removal efficiency of Chemical Oxygen Demand (COD) and NH₄ ⁺-N, and when SRT is below 30 days, silt density index (SDI₁₅) is less than 3, the effluent can be deeply treated by using reverse osmosis system (RO) or nano-filtration system (NF). The method of controlling SRT is put forward by analyzing the relationship between SRT and the minimum generation-time of dominant bacteria (at the maximum specific growth rate under the operation temperature). Supported by Natural Science Foundation of Tianjin (Grant No. 08JC2DJC24300)