Variable contribution of wastewater treatment plant effluents to downstream nitrous oxide concentrations and emissions.

Nitrous oxide (N 2 O), a potent greenhouse gas and ozone-destroying agent, is produced during nitrogen transformations in both natural and human-constructed environments. Wastewater treatment plants (WWTPs) produce and emit N 2 O into the atmosphere during the nitrogen removal process. However, the impact of WWTPs on N 2 O emissions in downstream aquatic systems remains poorly constrained. By measuring N 2 O concentrations at a monthly resolution over a year in the Potomac River estuary, a tributary of the Chesapeake Bay in the eastern United States, we found a strong seasonal variation in N 2 O concentrations and fluxes: N 2 O concentrations were larger in fall and winter, but the flux was larger in summer and fall. Observations at multiple stations across the Potomac River estuary revealed hotspots of N 2 O emissions downstream of WWTPs. N 2 O concentrations were higher at stations downstream of WWTPs compared to other stations (median: 21.2 nM vs. 16.2 nM) despite the similar concentration of dissolved inorganic nitrogen, suggesting the direct discharge of N 2 O from WWTPs into the aquatic system or a higher N 2 O production yield in waters influenced by WWTPs. Meta-analysis of N 2 O measurements associated with WWTPs globally revealed variable influence of WWTPs on downstream N 2 O concentrations and emissions. Since wastewater production has increased substantially with the growing population and is projected to continue to rise, accurately accounting for N 2 O emissions downstream of WWTPs is important for constraining and predicting future global N 2 O emissions. Efficient N 2 O removal, in addition to dissolved nitrogen removal, should be an essential part of water quality control in WWTPs. [ABSTRACT FROM AUTHOR]