Comprehensive assessment of the contribution of wastewater treatment to urban greenhouse gas and ammonia emissions

Population growth and urbanization are driving the demand for centralized wastewater treatment, a primary source of N2O and CH4 emissions. We have conducted the first comprehensive assessment of CH4, N2O and NH3 emissions across diurnal, day-to-day and seasonal scales at 96 US water resource recovery facilities (WRRFs) that collectively treat 9% of US centralized wastewater. Facility-level emissions were scaled to the national level using a probabilistic approach. Here we show that the measured emissions were 1.9 times higher for N2O (95% confidence interval (CI): 1.3–2.6) and 2.4 times higher for CH4 (CI: 1.9–2.9) than current US inventories. Considering the cumulative climate impacts of CH4 and N2O, the top 10% of emitters contributed 74% of the carbon footprint, with the top half contributing 98%, highlighting priorities for mitigation. Although detected at only a small fraction of facilities, measurements of NH3 emissions (86 kt yr−1 in the USA) suggest WRRFs are an overlooked source of urban NH3. Finally, the contribution of centralized wastewater treatment to global greenhouse gas emissions will increase 2- to 17-fold by 2100 under future scenarios. Overall, greater consideration of wastewater treatment emissions is needed to reach sustainability targets. Extensive measurements of the emissions of methane, nitrous oxide and ammonia from wastewater treatment facilities in the USA present higher values than are currently stated in national inventories. The results of this analysis show that greenhouse gas and nitrogenous emissions from the wastewater sector are often overlooked and that their impact on climate should be reassessed.