Seasonal nitrous oxide emissions and microbial community dynamics: Links to process conditions and operational strategies at a full-scale wastewater treatment plant

Nitrous oxide (N2O) emissions at wastewater treatment plants result from a complex interplay between environmental, operational, and microbial factors. Effective mitigation requires a comprehensive understanding of these interactions. We investigated process variables linked to N2O emissions and microbial community dynamics over 14 months in independent, parallel activated sludge lines at a full-scale treatment plant with ammonia-based aeration control and temperature-based solids retention time (SRT), focusing on a six-month period when N2O increased from 1% to 3% of nitrogen removed. During the seasonal increase, N2O correlated positively with wastewater conductivity, airflow in the swing zones, and negatively with temperature. The same variables were most strongly associated with microbial community compositions. Denitrifiers and aerobic heterotrophs correlated positively and ammonia oxidizers negatively with N2O emissions during the seasonal increase. Nitrifier abundances decreased with temperature, yet nitrification efficiency remained high and nitrite did not accumulate, while N2O accumulated during aeration. Ammonia oxidizers, linked to changes in temperature and salinity, were a likely source of increased N2O emissions. Species increasing with N2O included potentially incomplete denitrifiers, suggesting a contribution from incomplete denitrification. A longer SRT, tested in one line, supported efficient nitrification and denitrification with low net N2O production. Overall, plant-specific microbial and process data can provide additional information for the selection of mitigation strategies by indicating whether ammonia oxidizers or denitrifiers dominate net N2O production.