Biological control

Techniques to optimize biological CH4 oxidation in landfill cover soils were recently reviewed by Hilger and Humer (2003). Efforts to optimize microbial CH4 oxidation from landfill gas are concentrating on two approaches: (i) optimizing the cover soil itself (the biocap); and (ii) sending the landfill gas through a separate biofilter. Optimization of CH4 oxidation by using compost as a landfill biocap was studied extensively by Humer and Lechner (1999, 2001) in both the laboratory and field scales. They found that compost from municipal solid waste (MSW) and fully matured sewage sludge compost were optimal substrates for maintaining a high CH4 oxidation efficiency in the cover. Thanks to the high porosity of MSW compost, a 60 cm layer can remove up to 350 g CH4/m2/day in laboratory-scale experiments. Field trials confirmed that CH4 oxidation in MSW compost covers can be 100% effective. Wilshusen et al. (2004b) investigated CH4 oxidation by various composts in laboratory-scale set-ups and found the highest removal efficiency in the case of leaf compost. Barlaz et al. (2004) reported that compost covers oxidized more CH4 than conventional clay covers in field trials, but warned that compost covers can also produce CH4 if the moisture content is too high. Berger et al. (2005) warned that a clear boundary between two cover layers with different hydrologic properties can lead to water accumulation.

Oonk et al. (2004) investigated different types of cover material on two pilot-scale sites in the Netherlands. They found that spatial homogeneity is an important requirement for efficient CH4 oxidation. When a mixture of 70% (weight) uniform sand and 30% (weight) garden waste compost is used, the CH4 flux that can be oxidized is 30-50 g/ m2/day. It is expected that emission reductions are possible for e3.5-5/t CO2 equivalents, provided that sufficiently high fluxes are present.

CH4 oxidation in separate biofilters has been investigated by Gebert et al. (2003, 2004) and by Streese and Stegmann (2003). Gebert et al. (2003) experimented with expanded clay as biofilter material, which had good moisture-holding characteristics for CH4 oxidation at high air-filled pore space, but contained inhibiting amounts of salt. The CH4 oxidizing capacity increased after leaching of the salts. Streese and Stegmann (2003) tested pilot-scale biofilters containing finegrained compost and found CH4 degradation rates of up to 63 g CH4/m3/h. These results indicate that separate biofilters can be a feasible alternative solution when regulations do not allow for biocaps.

As has been pointed out, homogeneity is a requirement for high CH4 oxidation efficiencies, because heterogeneities such as cracks lead to bypassing of the landfill gas. This conclusion has been confirmed by modelling studies (De Visscher, 2001). For this reason, it is important that efforts be made to keep landfill cover soils homogeneous, even after settling of the waste. Patches of reduced vegetation growth are indicative of landfill gas hot spots.

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