Mechanistic Insights into the Contrasting Effects of Low DO on Nitrification-Derived NO Emissions in Comammox - versus AOB-Dominated Sludge.

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