Variable contribution of wastewater treatment plant effluents to nitrous oxide emission
Nitrous oxide (N 2O), 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 2O into the atmosphere during the nitrogen removal process. However, the impact of WWTPs on N 2O emissions in downstream aquatic systems remains poorly constrained. By measuring N 2O concentrations at a monthly resolution over a year in the Potomac River Estuary, a tributary of Chesapeake Bay in the eastern United States, we found a strong seasonal variation in N 2O concentrations and fluxes: N 2O 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 2O emissions downstream of WWTPs. N 2O 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 2O from WWTPs into the aquatic system or a higher N 2O production yield in waters influenced by WWTPs. Since wastewater production has increased substantially with the growing population and is projected to continue to rise, accurately accounting for N 2O emissions downstream of the WWTPs would better constrain the global N 2O emissions. Efficient N 2O removal, in addition to dissolved nitrogen removal, should be an essential part of water quality control in WWTPs.
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