Mechanistic Insights into the Contrasting Effects of Low DO on Nitrification-Derived N2O Emissions in Comammox Nitrospira- versus AOB-Dominated Sludge

Nitrification significantly contributes to N2O emissions in wastewater treatment, typically enhanced at low dissolved oxygen (DO). The present study revealed that low DO (∼0.2 mg/L) enhanced the N2O emission factor (EF) from nitrification by 4.5 times in canonical ammonia-oxidizing bacteria (AOB)-dominated sludge, while it reduced N2O EF by 73% in comammox Nitrospira-dominated sludge. During nitrification, the accumulation of intermediate NH2OH in AOB-dominant sludge was much higher and increased more significantly by low DO (from 0.018 to 0.067 mg-N/L) compared to comammox Nitrospira-dominant sludge (from 0.004 to 0.009 mg-N/L). In AOB-dominant sludge, the increased NH2OH and upregulated AOB-NOR gene at low DO promoted NO reduction, thus increasing N2O EF while simultaneously decreasing NO emission. In the comammox Nitrospira-dominant sludge, N2O was primarily produced via abiotic pathways. Further investigation found that N2O production decreased significantly under low DO in inactivated sludge and pure water with the sole addition of NH2OH, suggesting that low DO inhibited N2O formation via abiotic NH2OH oxidation. Therefore, in comammox Nitrospira-dominant sludge, low DO decreased N2O production primarily due to the inhibition of low DO to N2O formation via abiotic NH2OH oxidation and the low NH2OH accumulation. These results imply that enriching comammox Nitrospira in the wastewater treatment process can ensure stably low N2O emissions regardless of the variations in DO.