Nitrous oxide emission in a laboratory anoxic-oxic process at different influent pHs: Generation pathways and the composition and function of bacterial community.

This study focused on the nitrous oxide (NO) generation from the biological nitrogen removal process under different pH levels. To explore a pH optimum, the online NO emission and the bacterial composition and function in the anoxic-oxic process were investigated. The mean gaseous NO emission accounted for 0.329%, 0.103%, 0.085%, and 0.793% of the influent total nitrogen at pH of 5, 6, 8, and 9, respectively. Incomplete oxidation in oxic tanks was the primary source of NO, while NO in the anoxic tank was mainly generated by nitrifier denitrification. No direct correlations were observed between NO emission and potential nitrifiers and denitrifiers. The impacts of pH on NO generation were more likely related to the response of bacterial enzymes and nitrogen compounds, rather than the feedback of bacterial community structure itself. Above all, an influent pH range of 6-8 is recommended for nitrogen removal and NO mitigation in anoxic-oxic process.