The physical and biological characteristics of a tangential-flow membrane bioreactor (MBR) for the treatment of municipal wastewater are presented in this paper. It was found that the average removal of chemical oxygen demand, ammonia nitrogen, and suspended solids were 95,96.2 and 100%, respectively. To explore microbial diversity under different hydraulic conditions, the concept of nominal recirculation period (R-p) was introduced. The relation between sludge concentration and sludge viscosity was found to be linear. The minimum surface velocity (u(min)) was defined and then simulated at different sludge concentrations. The overall oxygen transfer coefficient and total oxygen uptake rate were also studied. Finally, the effect of membrane separation on the bioreactor was theoretically discussed.