Minimization of nitrous oxide emission from anoxic–oxic biological nitrogen removal process: Effect of influent COD/NH4+ ratio and feeding strategy

Nitrous oxide (N2O) is an important greenhouse gas and biological nitrogen removal process of wastewater treatment plant is one of its sources. Mechanisms of N2O emissions from anoxic–oxic biological nitrogen removal process were investigated and minimizations of N2O emissions were carried out from the aspect of organic carbon supplement, i.e., influent COD/NH4+COD/NH4+ ratio (C/N ratio) and feeding strategy. Results showed that during anoxic–oxic biological nitrogen removal process, most of the N2O emissions occurred during the oxic phase, and both nitrifier denitrification and aerobic hydroxylamine oxidation pathways were possible mechanisms responsible for N2O emissions. N2O conversion rate decreased from 6.0% to 1.3% when the influent C/N ratio was increased from 7.5 to 14.5. This was mainly because of decrease in the abundance of Nitrosomonas-like ammonia oxidizing bacteria. Step feeding and external carbon source addition could reduce N2O conversion rate by 66.6% and 12.0%, respectively. Both of them were feasible methods for minimizing N2O emission from wastewater treatment process. The low N2O emission of step feeding was because of its high dissolved oxygen (DO) and low ammonium concentrations during the oxic phase, while the minimization effect of external carbon source addition was ascribed to its high nitrogen removal efficiency.