Critical analysis of literature on landfill gas collection efficiency and its application to emissions estimates

Landfills are considered the third largest source of anthropogenic methane emissions in the U.S. and there is considerable interest in estimating emissions from individual landfills. Collection efficiency (CE) or methane collection index (MCI) is used in models to estimate emissions. The objective of this review is to critically analyze literature describing measured CEs and MCIs. Reported values ranged from 14 to 95% for intermediate covers and 53-100% for final covers. Many factors contribute to the ranges including: covers were classified as intermediate or final which is overly simplistic; variability in operational factors (e.g., well coverage, maintenance quality); waste characteristics such as age, composition, and mass in place which influence methane generation and likely collection; and uncertainty in emissions measurements and sometimes collected methane. A desired outcome of this review was to recommend values or ranges for intermediate and final covers that could be used in methane emissions models as applied to U.S. landfills. Such a recommendation is difficult considering the ranges reported in the literature and numerous site-specific factors that influence methane collection. Ultimately, each reported measurement is valid for a specific landfill at a specific time. Models that relate cover type and operational variables to MCI would best support estimation of a site-specific MCI, but such models are not available. Based on this review, the authors do not recommend a specific value for the MCI for an intermediate or final cover but rather expect the operator to select a value considering factors applicable to their landfill and the information presented here.