Efficiency of gas collection systems at Danish landfills and implications for regulations.

Globally, landfills are an important source of anthropogenic methane emissions. Regulations require landfill gas be managed to reduce emissions, and some landfills have therefore installed gas collection systems to recover energy and mitigate methane emissions. However, the efficiency of such systems is seldom evaluated. This paper presents the gas collection efficiencies of 23 Danish landfills and suggests how these values could be used to regulate landfill methane emissions in Denmark. Methane emissions from all sites were measured using the tracer gas dispersion method, and gas collection efficiencies were calculated using the ratio of the methane collection rate to the sum of the collection and emission (and oxidation) rates. Gas collection efficiencies ranged between 13 and 86% with an average of 50% - a value lower than for Swedish (58%), UK (64%) and US (63%) landfills. Possible reasons for the inefficiency of gas collection systems in Denmark include shallow gas collection pipes, leakage from installations (e.g. leachate wells, gas engines), low gas recovery due to minimal gas production or a lack of gas collection in active waste cells. It is suggested to use gas collection efficiency to regulate landfills and help them reach a particular methane mitigation goal. Gas collection efficiency that falls below the target mitigation rate would in turn trigger actions to reduce landfill methane emissions. At sites where the quality of the collected gas is too low to operate a gas engine, the installed gas collection system could be retrofitted to a biocover system designed for methane oxidation.