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

[Image: see text] 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.