Biofilters for mitigation of landfill methane emissions

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The UK and Europe has a large closed landfill legacy with approximately 20,000 closed landfill sites. Methane generated in landfill is 25x more potent a greenhouse gas than CO2 so prevention of its escape to the atmosphere is a priority for the landfill industry and requires stringent operating conditions to minimise emissions and the risks to the environment. Strumpshaw landfill site near Norwich is a disused quarry of 15 ha filled with ~1,000,000 mcubed of domestic and commercial waste. Waste gas is fed into on- site Stirling engines which manage the landfill gas through combustion and generate electricity. The migration line on site contains 15% methane which is difficult to combust and so needs to be treated to control off-site migration. The Norfolk County Council (NCC) Closed Landfill Team, led by Charles Wright, has been trialling successfully at Strumpshaw the bioxidation of landfill gas using a 58m3 biofilter composed of compost, wood chip, expanded clay and coir, buried in landfill soil, lined with an impermeable gas barrier. Bioxidation of the methane is done by pumping air and landfill gas through the biofilter. Aerobic methane oxidising bacteria (methanotrophs) naturally occurring in soils remove methane by converting it to water and CO2. This biofilter technology is specifically aimed at older closed landfills that are generating gas below the level where electricity generation becomes difficult. A key objective in this multidisciplinary Project is to assess the effectiveness of bio-oxidation as a gas management technique for landfill gas containing 5-20% methane. To achieve this, it is imperative to understand the biology underpinning the effectiveness of the biofilter and to create a robust framework for future design and long term management of landfill methane biofilters.Key questions:How do methanotroph populations and activities change with depth and which are the most effective in consuming methane in the biofilter? Answering these questions would lead to an optimal depth for the biofilter design based on the input gas.Are physico-chemical parameters in the biofilter optimum for methanotrophs. Do they have the nutrients they need (CH4, O2, N, P, Cu, Fe)? This informs biofilter design, to create the ideal matrix to allow gas and nutrients to percolate through and for methanotrophs to thrive.Is moisture content correct, do seasonal differences matter; is temperature important? These questions would answer if the biofilter can be run uncovered/uninsulated/unheated.Methodology:Physico-chemical parameters including moisture content, temperature, matrix permeability, trace element availability and landfill gas composition will be measured. Field analytical techniques used will include surface emissions testing equipment (FID, TDL) and downhole monitoring, and laboratory analysis will be used for collected samples for matrix and trace gas composition analysis.Methane oxidation potential of biofilter samples will be determined using gas chromatography. Distribution and diversity of methanotrophs in the biofilter will be determined by analysis of 16S rRNA genes and genes targeting the key enzyme methane monooxygenase (pmoA, mmoX). Key active methanotrophs present in biofilter samples will be identified by stable isotope probing using 13CH4, a technique pioneered in Murrell's lab and subsequently isolated and characterised at the physiological and molecular level.