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 N 2 O emissions in wastewater treatment, typically enhanced at low dissolved oxygen (DO). The present study revealed that low DO (∼0.2 mg/L) enhanced the N 2 O emission factor (EF) from nitrification by 4.5 times in canonical ammonia-oxidizing bacteria (AOB)-dominated sludge, while it reduced N 2 O EF by 73% in comammox Nitrospira -dominated sludge. During nitrification, the accumulation of intermediate NH 2 OH 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 NH 2 OH and upregulated AOB-NOR gene at low DO promoted NO reduction, thus increasing N 2 O EF while simultaneously decreasing NO emission. In the comammox Nitrospira -dominant sludge, N 2 O was primarily produced via abiotic pathways. Further investigation found that N 2 O production decreased significantly under low DO in inactivated sludge and pure water with the sole addition of NH 2 OH, suggesting that low DO inhibited N 2 O formation via abiotic NH 2 OH oxidation. Therefore, in comammox Nitrospira -dominant sludge, low DO decreased N 2 O production primarily due to the inhibition of low DO to N 2 O formation via abiotic NH 2 OH oxidation and the low NH 2 OH accumulation. These results imply that enriching comammox Nitrospira in the wastewater treatment process can ensure stably low N 2 O emissions regardless of the variations in DO.