Benchmarking greenhouse gas emissions from US wastewater treatment for targeted reduction

Here, to assess the national climate impact of wastewater treatment and inform decarbonization, we assembled a comprehensive greenhouse gas inventory of 15,863 facilities in the contiguous USA. Considering location and treatment configurations, we modelled on-site CH4, N2O and CO2 production and emissions associated with energy, chemical inputs and solids disposal. Using Monte Carlo simulations, we estimated median national emissions at 47 million tonnes of CO2 equivalent per year, with on-site process CH4 and N2O emissions exceeding current government estimates by 41%. Treatment configurations with anaerobic digesters are responsible for 16 million tonnes of CO2 equivalent per year of fugitive methane, outweighing benefits achieved through on-site electricity generation. Systems designed for nutrient removal have the highest greenhouse gas emissions intensity, attributable to energy requirements and N2O production, demonstrating current trade-offs between meeting water quality and climate objectives. We analysed key sensitivities and included a geospatial analysis to highlight the scale and distribution of opportunities for reducing life cycle greenhouse gas emissions. Benchmarking greenhouse gas emissions from wastewater treatment plants is an essential step in developing mitigation strategies. This is now achieved for the USA by modelling over 15,000 facilities using Monte Carlo simulations to obtain a national baseline.