effluent COD remained stable. The total COD removal efficiency was 72–79% and contribution from MHA was 33–42%. (2) The identified organic pollutants in the influent include benzene, ketones, alcohols, esters, amides, nitriles and phenols, etc. The variety and abundance of organic pollutants reduced significantly by MHA–A/O treatment. (3) The predominant genera in MHA, anoxic and oxic reactors were Anaerolineaceae and Sulfuritalea, Lactococcus and Blastocatella, and Saprospiraceae uncultured and Nitrosomonadaceae, respectively.