Field Comparison of Landfill Gas Collection Efficiency Measurements

Gas collection efficiency is important for a variety of environmental, regulatory, and engineering purposes in landfill management. However collection efficiency measurements are typically complex and costly and so are seldom made. Instead conservative collection efficiencies are typically assumed, for example, 75% by the US Environmental Protection Agency (US EPA). In the 1990’s, the Los Angeles County Sanitation Districts (Districts) developed an innovative methodology for measuring gas collection efficiency based on a combination of readily acquired integrated surface methane (ISM) concentration data and modeling using the US EPA’s Industrial Source Complex (ISC) air dispersion model. The Districts previously reported application of this “ISM/ISC” method to their Palos Verdes Landfill using four quarterly ISM monitoring events for fiscal year 2001/2002. The landfill gas collection system at this site is operated for energy recovery and emission control purposes. A collection efficiency of about 95% was measured. This paper presents a recent Districts study comparing the ISM/ISC method with conventional static flux chamber emission measurements at the Palos Verdes Landfill. ISM prescreening was used in Spring 2006 to identify ten flux chamber measurement locations chosen to represent both peak and typical emission areas. The flux chambers were monitored in Summer 2006 for methane at five-minute intervals over 35 minutes. Additionally, chamber samples taken at the start and end of each test were analyzed for toxic constituents by US EPA Method TO-15 using single ion monitoring (SIM) technology for low detection limits. The ISM/ISC method showed +99.2% collection efficiency for the 2006 data set. The increase in efficiency from that measured for the 2001/2002 data set corresponds to improvements in the gas collection system in the intervening time. The flux chambers found no statistically significant methane accumulation. The flux chamber methane levels were not significantly different from the global background for California. The initial and final EPA TO-15 SIM constituent concentrations were not significantly different from one another. Collection efficiencies based on flux chamber measurements were calculated to be essentially 100%. The ISM/ISC results were consistent with those from costlier flux chamber measurements. Methane used by the ISM/ISC method served as a conservative surrogate for EPA TO-15 SIM constituents measured by the flux chambers. The ISM/ISC and flux chamber methods measured collection efficiencies above 99%. These results indicate that commonly assumed collection efficiencies of 75% can be grossly conservative for landfills operated for emission control purposes. INTRODUCTION Methane and carbon dioxide are the end products of solid waste biodegradation under anaerobic conditions and are the primary constituents of landfill gas. Landfill gas may be recovered for a variety of purposes including subsurface migration prevention, odor and emission control, and energy recovery. Additionally national and regional jurisdictions may require landfill gas recovery. For example, the 1996 US EPA Municipal Solid Waste Landfill New Source Performance Standards (NSPS) require landfill gas recovery for air quality purposes. Gas collection efficiency may be defined differently for various environmental, regulatory, and engineering