Quantifying Methane Evolution from Sewers: Phase 1 Results of the WERF/DeKalb County, Georgia, Case Study

Collection-system-wide methane emission estimates are being developed from field sampling of pumping stations in DeKalb County, GA. This effort is part of a Water Environment Research Foundation (WERF) project entitled “Methane Evolution from Wastewater Treatment and Conveyance” (number U2R08) under WERF's newly formed Climate Change Program.Methane (CH4) production from sewers is a suspected, yet relatively undocumented source of greenhouse gases (GHG). The Intergovernmental Panel on Climate Change (IPCC) published the “2006 IPCC Guidelines for National Greenhouse Gas Inventories” (IPCC, 2006) which says that “In most developed countries and in high-income urban areas in other countries, sewers are usually closed and underground. Wastewater in closed underground sewers is not believed to be a significant source of CH4.” In contrast to this statement, a more recent publication (Guisasola, 2008) suggests that “relatively large amounts of dissolved CH4 can occur in the liquid phase of a sewer system, with the possibility of CH4 being supersaturated. In reality, CH4 produced in the sewer will remain dissolved until there is a point of release to the atmosphere and may thus contribute significantly to the greenhouse effect.” CH4 is a greenhouse gas that has a global warming impact that is 21 times that of carbon dioxide (CO2) and, as such, is of heightened interest in GHG modeling and inventories.This paper summarizes the results of the first phase assessment of DeKalb pumping station methane emissions using instantaneous gas-phase samples and a wet-well-breathing cycle approach to approximate the volume and mass of CH4 emissions. CH4 was detected using this approach. While the Phase 1 approach is considered to underreport the mass evolved, follow-on efforts are underway to better quantify these emissions.