Methane Evolution from Wastewater Conveyance

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. Phase 1 of this project entailed instantaneous methane emissions measurements at 64 raw sewage lift stations throughout the collection system in the DeKalb County, Georgia. During Phase 2 of this project, continuous gaseous-and liquid-phase monitoring were conducted at the discharge of a 16-inch, 3.3-mile-long force main from the Honey Creek Pumping Station. Liquid phase sampling during this phase showed that 8.87 kg of CH4 were emitted per day at the force main receiving manhole during the summer months. The continuous monitoring data was used to calibrate a process model developed by the University of Queensland to predict CH4 and hydrogen sulfide evolution by simulating a force main as a plug-flow, fixed-film reactor. The model was then used to simulate force main CH4 emissions over a calendar year using historical flow and monthly average temperatures. This effort calculated emissions of 52 MT of CO2e/year. Overall this exploratory research project provides following benefits and outcomes.