Greenhouse Gas Emissions from Three Full-Scale Metropolitan Wastewater Reclamation Plants

Domestic and industrial wastewater treatment plants (WWTPs) have been estimated to be the 7th highest contributors to atmospheric concentrations of both nitrous oxide (N2O) and methane (CH4), respectively. This project identified the sources of N2O and CH4 within three conventional activated-sludge plug-flow WWTPs, and quantified their annual total fluxes. Additionally, this project characterized these GHG emissions, using carbon and nitrogen isotope measurements, to investigate the possible biological sources of these gases. Sampling was conducted at the Stickney, North Side, and Egan WWTPs in the Chicago area. The results show that the aeration basins represent the main source (> 85%) of N2O. Methane is produced by a variety of processes where anaerobic conditions develop. A significant contribution to total GHG fluxes from the plants is also constituted by the plant exhausts. The calculated cumulative fluxes from the Stickney WWTP were 5.9 x 105 kg N2O/y (204 g/Pe/y), and 2.8 x 106 kg/y of CH4 (1122 g/Pe/y). The calculated cumulative fluxes for the North Side WWTP were 1.7 x 104 kg/y N2O (12.3 g/Pe/y), and 8.6 x 104 kg/y CH4 (61.1 g/Pe/y). The calculated cumulative fluxes from the Egan WWTP were 1.6 x 104 kg/y N2O (91.8 g/Pe/y), and 6.0 x 104 CH4 (353.6 g/Pe/y). About 0.94%, 0.16%, and 0.97% of the incoming total Kjeldahl nitrogen (TKN) is emitted as N2O at Stickney, North Side, and Egan respectively. The study of the site-specific stable nitrogen isotope distribution in N2O showed a site preference averaging ~0‰, indicating that this GHG is produced mainly by denitrification of ammonia. The study of bulk stable nitrogen isotope ratios of nitrate and ammonia showed a trend that can be modeled with ammonia nitrification plus a variable amount (5-20%) of nitrate denitrification.