Underestimation of Sector-Wide Methane Emissions from United States Wastewater Treatment.

An increasing percentage of US waste methane (CH 4 ) emissions come from wastewater treatment (10% in 1990 to 14% in 2019), although there are limited measurements across the sector, leading to large uncertainties in current inventories. We conducted the largest study of CH 4 emissions from US wastewater treatment, measuring 63 plants with average daily flows ranging from 4.2 × 10 -4 to 8.5 m 3 s -1 (<0.1 to 193 MGD), totaling 2% of the 62.5 billion gallons treated, nationally. We employed Bayesian inference to quantify facility-integrated emission rates with a mobile laboratory approach (1165 cross-plume transects). The median plant-averaged emission rate was 1.1 g CH 4 s -1 (0.1-21.6 g CH 4 s -1 ; 10th/90th percentiles; mean 7.9 g CH 4 s -1 ), and the median emission factor was 3.4 × 10 -2 g CH 4 (g influent 5 day biochemical oxygen demand; BOD 5 ) -1 [0.6-9.9 × 10 -2 g CH 4 (g BOD 5 ) -1 ; 10th/90th percentiles; mean 5.7 × 10 -2 g CH 4 (g BOD 5 ) -1 ]. Using a Monte Carlo-based scaling of measured emission factors, emissions from US centrally treated domestic wastewater are 1.9 (95% CI: 1.5-2.4) times greater than the current US EPA inventory (bias of 5.4 MMT CO 2 -eq). With increasing urbanization and centralized treatment, efforts to identify and mitigate CH 4 emissions are needed.