Nitrous oxide emission and microbial community of full-scale anoxic/aerobic membrane bioreactors at low dissolved oxygen setpoints

Abstract Low dissolved oxygen operation has been adopted by water resource recovery facilities to reduce operating costs of activated sludge processes. However, the effects of low dissolved oxygen setpoint in aeration tanks on nitrous oxide emission and microbial community have rarely been addressed at full-scale activated sludge systems. This study for the first time monitored nitrous oxide emission as an anoxic/aerobic membrane bioreactor facility decreased dissolved oxygen setpoint gradually from 3.5 to 0.5 mg/L. Low dissolved oxygen operation retained efficient ammonia removal and increased total nitrogen removal efficiency from 21.4% to 50.1%. Nitrous oxide emission decreased from 0.65% to 0.15% of total nitrogen removal due to uninhibited last stage of heterotrophic denitrification and one-step nitrification. Low dissolved oxygen operation is hence an energy-saving strategy that promotes climate-friendly, efficient biological nitrogen removal. Moreover, this study diagnosed microbial communities in anoxic/aerobic membrane bioreactors of another full-scale plant which had been operated efficiently for years at an ultra-low dissolved oxygen level (0.2 mg/L). Microbial community was similar in all the bioreactors and simultaneous nitrification and denitrification was the main mechanism of nitrogen removal. Nitrospira, which may include complete ammonia oxidizing bacteria, was the only genus of autotrophic nitrifying bacteria. Simultaneous nitrification and denitrification and one-step nitrification by comammox bacteria in low dissolved oxygen operation led to low nitrite concentration and less nitrous oxide emissions from both anoxic and aeration tanks.