Oversimplification and misestimation of nitrous oxide emissions from wastewater treatment plants

Wastewater treatment is a major source of anthropogenic nitrous oxide (N2O) emissions. However, the current emission estimations rely on a uniform emission factor (EF) proposed by the Intergovernmental Panel on Climate Change based on a limited database suffering from large uncertainties and inaccuracies. To address this limitation, this study expands the database 12-fold and develops a tier-based approach. Our method considers emission variations across spatial scales, treatment processes and monitoring techniques, enabling more-precise estimations. Here, applying this approach to the US database, we highlight the limitations of current estimations based on uniform EFs and quantified the mean wastewater N2O emission in the United States to be 11.6 MMT CO2-eq. The results also reveal the diverse nature of wastewater N2O emissions and underscore the need for a customized approach to inform facility-level N2O emission estimation as well as inform national- and sector-wide greenhouse gases inventories with emphasis on site-specific considerations. Overall, this study provides a tool to recalibrate the estimations of wastewater N2O emissions, which form the foundation of carbon footprint reduction in wastewater treatment.