Mechanistic Insights into the Contrasting Effects of Low DO on Nitrification-Derived N₂O Emissions in Comammox...

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.