The performances of anaerobic-anoxic-oxic processes with (A(2)O-B) and without (A(2)O-C) beneficial micro-organisms were compared to provide valuable insights on how they are affected by changes in the microbial biomass and community composition. Although the A(2)O-B process showed lower concentrations of mixed liquor suspended solids and mixed liquor volatile suspended solids than the A(2)O-C process under identical operating conditions, the A(2)O-B process was more effective for the removal of organic materials and nutrients compared with the A(2)O-C process. Furthermore, the compressibility and settleability of the activated sludge were significantly better in the A(2)O-B process than in the A(2)O-C process due to the enhanced decomposition of extracellular polymeric substances. These results indicated that the inoculation of beneficial microorganisms may increase the proportions of microorganisms in relation to 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).