Wet landfill decomposition rate determination using methane yield results for excavated waste samples

An increasing number of landfills are operated to accelerate waste decomposition through liquids addition (e.g., leachate recirculation) as a wet landfill. Landfill design and regulation often depend on utilizing landfill gas production models that require an estimate of a first-order gas generation rate constant, k. Consequently, several studies have estimated k using collected gas volumes from operating wet landfills. Research was conducted to examine an alternative approach in which k is estimated not from collected landfill gas but from solid waste samples collected over time and analyzed for remaining gas yield. To achieve this goal, waste samples were collected from 1990 through 2007 at two full-scale landfills in Florida that practiced liquids addition. Methane yields were measured from waste samples collected over time, including periods before and after leachate recirculation, and the results were applied to a first-order decay model to estimate rate constants for each of the sites. An initial, intensive processing step was conducted to exclude non-biodegradable components from the methane yield testing procedure. The resulting rate constants for the two landfills examined were 0.47 yr(-1) and 0.21 yr(-1). These results expectedly exceeded the United States Environmental Protection Agency's rate constants for dry and conventional landfills (0.02-0.05 yr(-1)), but they are comparable to wet landfill rate constants derived using landfill gas data (0.1-0.3 yr(-1)).