Significant differences were observed for the microbial biomass and community composition of the A2O-B and A2O-C process under identical operating conditions. Despite of the lower MLSS and MLVSSconcentrations, the A2O-B process could more efficiently remove organic materials and nutrients than the A2O-C process. Moreover, the activated sludge of the A2O-B process exhibited the better ompressibility and settleability compared to that of the A2O-C process. These can be explained by the higher proportions of microorganisms associated with the removal of organic materials, nutrients (i.e., Zoogloea,Dechloromonas, Nitrospira, and Nitrosomonas) and the reduction of the excess sludge (i.e., Proteobacteria and Bacteroidetes).