Greenhouse Gas Emissions from Three Chicago Wastewater Treatment Plants

Wastewater reclamation plants (WRPs) have recently been recognized as significant emission sources of nitrous oxide (N2O) and methane (CH4). A comprehensive database of measured emissions from various liquid and solids treatment processes does not currently exist. Annual N2O and CH4 emissions from select processes at three activated-sludge Chicago WRPs were measured for 2009; additionally, CO2 emissions were measured for a subset of these treatment processes. According to the flux estimates, the aeration basins were the dominant source of N2O, while primary settling tanks, exhausts, and aeration basins were the main sources of CH4 among the three plants investigated. Total N2O fluxes were calculated to be 5.2×105 kg/y for the Stickney WRP, 1.7×104 kg/y for the North Side WRP, and 1.6×104 kg/y for the Egan WRP. Total CH4 fluxes, not including CH4 captured for beneficial use, were calculated to be 2.8×106 kg/y for the Stickney WRP, 8.6×104 kg/y for the North Side WRP, and 6.0×104 kg/y for the Egan WRP. The aeration basins at the Stickney WRP also emitted 1.1×108 kg/y of CO2, which is a significantly larger contribution than the aerated grit chamber (3.8×106 kg/y) and floating cover anaerobic digesters (3.9×104 kg/y). CH4 emissions were also observed in aerobic processes, possibly in small anaerobic pockets.