Abstract:A simple model based on mass balance was constructed for the prediction of fine sediment migration in activated sludge reactor under different influent fine sediment load. The results shown that, during the influence period, the predicted fine sediment concentration was mainly effected by concentration of influent fine sediment, proportions of suspended fine sediment, sludge age, hydraulic retention time and impact time. During the recovery period, however, it was mainly effected by initial fine sediment concentration, sludge age, and recovery time. The relative errors of the predicted fine sediment concentration during the influence and recovery period were between 11.7% and 24.6% , 29.1% and 46.0%, respectively. The relative errors of volatile suspended solids ratio in sludge during the influence and recovery period were between 0.9%and 7.3%, 2.3%and 34.4%, respectively. The maximum fine sediment concentration in mixed liquor and deposition rate of fine sediment are proportional to influent fine sediment load.