Oversimplification and misestimation of nitrous oxide emissions from wastewater treatment plants

Abstract Wastewater treatment is a major source of anthropogenic nitrous oxide (N 2 O) 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 N 2 O emission in the United States to be 11.6 MMT CO 2 -eq. The results also reveal the diverse nature of wastewater N 2 O emissions and underscore the need for a customized approach to inform facility-level N 2 O 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 N 2 O emissions, which form the foundation of carbon footprint reduction in wastewater treatment.