N2O Emissions from Activated Sludge Processes, 2008-2009: Results of a National Monitoring Survey in the United States

Despite recognition of the possible role of biological nitrogen removal (BNR) processes in nitrous oxide (N2O) emission, a measured database of N2O emissions from these processes at the national scale does not currently exist This study focused on the quantification of N2O emissions at 12 wastewater treatment plants (WWTPs) across the United States using a newly developed U.S. Environmental Protection Agency (USEPA) reviewed protocol. A high degree of variability in field-scale measurements of N2O was observed, both across the WWTPs sampled and within each WWTP. Additionally, aerobic zones, which have hitherto not been considered in the USEPA approach of estimating N2O emissions, generally contributed more to N2O fluxes than anoxic zones from BNR reactors. These results severely qualify the conventional use of a single emission factor to "estimate" N2O emissions from BNR processes, solely by virtue of denitrification. Upon subjecting the nationwide data set to multivariate regression data mining, high nitrite, ammonium, and dissolved oxygen concentrations were positively correlated with N2O emissions from aerobic zones of activated sludge reactors. On the other hand, high nitrite and dissolved oxygen concentrations were positively correlated with N2O emissions from anoxic zones. Based on these results, it can be argued that activated sludge processes that minimize transient or permanent build up of ammonium or nitrite, especially in the presence of dissolved oxygen, are expected to have low N2O emissions.