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 (N2O) generation from the biological nitrogen removal process under different pH levels. To explore a pH optimum, the online N2O emission and the bacterial composition and function in the anoxic-oxic process were investigated. The mean gaseous N2O 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 N2O, while N2O in the anoxic tank was mainly generated by nitrifier denitrification. No direct correlations were observed between N2O emission and potential nitrifiers and denitrifiers. The impacts of pH on N2O 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 N2O mitigation in anoxic-oxic process.

Exploring the linkage between bacterial community composition and nitrous oxide emission under varied DO levels through the alternation of aeration rates in a lab-scale anoxic-oxic reactor.

Dissolved oxygen (DO) level is crucial in shaping bacterial community and impacts biological nitrogen removal and nitrous oxide (N2O) emission. Online gaseous and off-line dissolved N2O under varying DO levels through aeration rate alternations were measured in lab-scale anoxic-oxic reactors. It showed that sharp changes in DO levels caused immediate N2O emission increase, while the total average gaseous N2O emission stabilized at 0.011%, 0.046%, 0.308% and 0.229% of influent nitrogen as DO in oxic tanks averaged at 0.58, 1.67, 3.2 and 6.12 mg/L, respectively. Process with an average DO concentration of 1.67 mg/L had the highest microbial diversity and relative abundances of potential denitrifers and ammonia-oxidizing bacteria (NOB), while the least ammonia-oxidizing bacteria (AOB) were detected, which contributed to efficient nitrogen removal and minor N2O emission. In conclusion, regulation and control of denitrifiers, AOB and NOB with the determination of a proper DO set point is feasible for N2O mitigation.