Collection-system-wide methane emission estimates are being developed from field sampling in DeKalb County, GA. This effort is part of a Water Environment Research Foundation (WERF) project entitled “Methane Evolution from Wastewater Treatment and Conveyance“ under WERF's newly formed Climate Change Program.
Methane (CH4) production from sewers is a suspected, yet relatively undocumented source of greenhouse gases (GHGs). The Intergovernmental Panel on Climate Change (IPCC) published the “2006 IPCC Guidelines for National Greenhouse Gas Inventories“ which states 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.“ 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.
The goal of the first phase of the investigation was to determine if CH4 could be detected in the wetwells and forebays of a sanitary wastewater collection system. During that initial phase, methane emissions were quantified from DeKalb County's 65 pumping stations. The results of that investigation documented that approximately 1,000 MT of carbon dioxide equivalents (CO2e) are emitted each year from CH4 evolution at pumping stations serving DeKalb's roughly 600,000 citizens in moving the County's 100 mgd of wastewater. While 1,000 MT CO2e/yr were quantified, significant sources of under reporting are thought to exist, potentially masking CH4 emissions estimated at between 2 and 5 times greater than those quantified under Phase 1.
During Phase 2 of the WERF project, continuous gaseous- and liquid-phase were conducted at the discharge of a 16-inch, 3.3-mile-long force main from the Honey Creek Lift Station. Continuous monitoring showed that 8.4 kg of CH4 were emitted per day during the sampling, which was conducted in the summer. The continuous monitoring data was used to calibrate a process model developed by the University of Queensland (Guisasola, 2009) to predict CH4 and hydrogen sulfide (H2S) evolution by simulating a forcemain as a plug-flow, fixed-film reactor. The model was then used to simulate forcemain CH4 emissions over a calendar year using historical flow and monthly average temperatures. This effort calculated emissions of 52 MT of CO2e/year.
This paper presents the results from Phase 2 of the collection-system-methane portions of this WERF investigation and compare and contrast those results to the Phase-1 estimates. The University of Queensland Sewer model was used to predict annual CH4 emissions as a function of wastewater flow and temperature in the Honey Creek forcemain.
