Biotic oxidation of methane in landfills in boreal climatic conditions
This study focused on the biotic oxidation of methane in landfill covers as a technology for reducing greenhouse gas emissions from landfills, particularly those located in the boreal climatic zone. First, methane oxidation was studied in laboratory batch assays in a landfill cover soil consisting of a composted mixture of sewage sludge and chemical sludge which had been installed on the landfill surface 4-5 years earlier. Second, methane oxidation was studied using mechanically-biologically treated municipal solid waste (MBT residual) as a material for methane-oxidizing landfill covers both in continuously methane-sparged laboratory columns and in an outdoor pilot lysimeter. Finally, methane oxidation was studied at a closed full-scale landfill with a European Union landfill directive-compliant, multilayer final cover system containing a water impermeable layer, passive gas collection and distribution system, and a soil cover consisting of sludge compost and peat. In the four-year old landfill cover, the methane oxidation rates at moisture of ≥33% of water-holding capacity increased along with temperature (Q₁₀ values 6.5-8.4 at 1-19 °C) while methane oxidation was suppressed at moisture of 17% of water-holding capacity. Methane oxidation (0.2-4.3 μg CH₄ gdw ⁻¹ h⁻¹ at 1-6 °C) and increase in oxidation rate over time were observed even at 1 °C. In MBT residual, high methane oxidation rates were observed in laboratory columns (12.2-82.3 g CH₄ m⁻² d⁻¹ at 2-25 °C) and in batch assays with samples from the columns (up to 104 μg gdw⁻¹ h⁻¹ at 5 °C and 581 μg gdw⁻¹ h⁻¹ at 25 °C). In an outdoor lysimeter filled with MBT residual and containing a cover layer made from the same MBT residual, >96% of the methane produced (<16 g CH₄ m⁻² d⁻¹) was oxidized between April and October, while in January oxidation was lower (<0.6 g CH₄ m⁻² d⁻¹; this was <22% of the methane produced). In the full-scale landfill, of the mean methane flux (2.92-27.3 g CH₄ m-2 d-1) entering the cover layer at the measuring points at the four measuring times, ≥25% was oxidized in October and February, 0% in November and ≥46% in June. At each time, the high methane fluxes into the soil cover at a few points reduced the mean oxidation rate. To conclude, methane-oxidizing landfill biocovers appear feasible for reducing methane emissions in boreal climatic conditions while reduced oxidation rates are likely to occur in wintertime. To maximize the methane oxidation rate at low ambient temperature, the oxidation layer should have a spatially even gas influx, sufficient thickness and suitable characteristics, particularly those related to oxygen transport and thermal insulation.
