Municipal wastewater collection and treatment systems are critical infrastructures, and they are also identified as major sources of anthropogenic CH₄ emissions that contribute to climate change. The actual CH₄ emissions at the plant- or regional level vary greatly due to site-specific conditions as well as high seasonal and diurnal variations. Here, we conducted the first quantitative analysis of CH₄ emissions from different types of sewers and water resource recovery facilities (WRRFs). We examined variations in CH₄ emissions associated with methods applied in different monitoring campaigns, and identified main CH₄ sources and sinks to facilitate carbon emission reduction efforts in the wastewater sector. We found plant-wide CH₄ emissions vary by orders of magnitude, from 0.01 to 110 g CH₄/m³ with high emissions associated with plants equipped with anaerobic digestion or stabilization ponds. Rising mains show higher dissolved CH₄ concentrations than gravity sewers when transporting similar raw sewage under similar environmental conditions, but the latter dominates most collection systems around the world. Using the updated data sets, we estimated annual CH₄ emission from the U.S. centralized, municipal wastewater treatment to be approximately 10.9 ± 7.0 MMT CO₂-eq/year, which is about twice as the IPCC (2019) Tier 2 estimates (4.3-6.1 MMT CO₂-eq/year). Given CH₄ emission control will play a crucial role in achieving net zero carbon goals by the midcentury, more studies are needed to profile and mitigate CH₄ emissions from the wastewater sector.

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.