Impact of dissolved oxygen and loading rate on NH(3) oxidation and N(2) production mechanisms in activated sludge treatment of sewage

Microaerobic activated sludge (MAS) is a one‐stage process operated at 0.5–1.0 mg l(−1) dissolved oxygen (DO) aiming at simultaneous nitrification and denitrification. We used molecular techniques and a comprehensive nitrogen (N)‐transformation activity test to investigate the dominant NH(3)‐oxidizing and N(2)‐producing mechanism as well as the dominant ammonia‐oxidizing bacteria (AOB) species in sludge samples individually collected from an MAS system and a conventional anoxic/oxic (A/O) system; both systems were operated at a normal loading rate (i.e. 1.0 kg chemical oxygen demand (COD) m(−3) day(−1) and 0.1 kg NH(4) (+)‐N m(−3) day(−1)) in our previous studies. The DO levels in both systems (aerobic: conventional A/O system; microaerobic: MAS system) did not affect the dominant NH(3)‐oxidizing mechanism or the dominant AOB species. This study further demonstrated the feasibility of a higher loading rate (i.e. 2.30 kg COD m(−3) day(−1) and 0.34 kg NH(4) (+)‐N m(−3) day(−1)) with the MAS process during sewage treatment, which achieved a 40% reduction in aeration energy consumption than that obtained in the conventional A/O system. The increase in loading rates in the MAS system did not affect the dominant NH(3)‐oxidizing mechanism but did impact the dominant AOB species. Besides, N(2) was predominantly produced by microaerobic denitrification in the MAS system at the two loading rates.