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

An increasing percentage of US waste methane (CH₄) 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 CH₄ emissions from US wastewater treatment, measuring 63 plants with average daily flows ranging from 4.2 × 10⁻⁴ to 8.5 m³ 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 CH₄ s⁻¹ (0.1-21.6 g CH₄ s⁻¹; 10th/90th percentiles; mean 7.9 g CH₄ s⁻¹), and the median emission factor was 3.4 × 10⁻² g CH₄ (g influent 5 day biochemical oxygen demand; BOD₅)⁻¹ [0.6-9.9 × 10⁻² g CH₄ (g BOD₅)⁻¹; 10th/90th percentiles; mean 5.7 × 10⁻² g CH₄ (g BOD₅)⁻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 CO₂-eq). With increasing urbanization and centralized treatment, efforts to identify and mitigate CH₄ emissions are needed.