Methane Emissions from Municipal Wastewater Collection and Treatment Systems.

Municipal wastewater collection and treatment systems are critical infrastructures, and they are also identified as major sources of anthropogenic CH 4 emissions that contribute to climate change. The actual CH 4 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 CH 4 emissions from different types of sewers and water resource recovery facilities (WRRFs). We examined variations in CH 4 emissions associated with methods applied in different monitoring campaigns, and identified main CH 4 sources and sinks to facilitate carbon emission reduction efforts in the wastewater sector. We found plant-wide CH 4 emissions vary by orders of magnitude, from 0.01 to 110 g CH 4 /m 3 with high emissions associated with plants equipped with anaerobic digestion or stabilization ponds. Rising mains show higher dissolved CH 4 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 CH 4 emission from the U.S. centralized, municipal wastewater treatment to be approximately 10.9 ± 7.0 MMT CO 2 -eq/year, which is about twice as the IPCC (2019) Tier 2 estimates (4.3-6.1 MMT CO 2 -eq/year). Given CH 4 emission control will play a crucial role in achieving net zero carbon goals by the midcentury, more studies are needed to profile and mitigate CH 4 emissions from the wastewater sector.