The overall performance of the Membrane Bioreactor (MBR) process for municipal wastewater treatment was studied to determine the characteristics of activated sludge under different Sludge Retention Times (SRT) and Hydraulic Retention Times (HRT). The experiment lasted over a period of 300 days, which included 4 runs. The effluent quality of the MBR process in terms of COD and suspended solids, was excellent under all conditions tested. Specific nitrification rates of the activated sludge were measured at steady state in each run. Similar maximum nitrification rate values were obtained through batch experiments with either only NH4Cl or raw wastewater as substrate. Mass balances of the process in terms of COD, nitrogen and inorganic suspended solids were made, and it was found that 28%, 42%, and 48% of influent COD were converted into activated sludge at SRTs of 20, 10, and 5 days, respectively. The COD/VSS ratio of the activated sludge seems to be dependent on mass loading rate. The estimated true yield and decay rate coefficients of the activated sludge were 0.61 kgCOD/kgCOD and 0.050 d−1, respectively. In the completely aerobic system, nitrogen balances were always close to 100%.

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