METHANE OXIDIZING BIOCOVERS: A LOW-COST ALTERNATIVE TO MINIMIZE EMISSIONS AT OLDER LANDFILL SITES

摘要

A field study in progress at the closed Leon County, Florida Landfill is examining the use of methane-oxidizing biocovers to reduce methane emissions. Typically, a biocover consists of a porous gas distribution layer above the waste which is, in turn, overlain by a compost-amended biocover layer (Huber-Humer, 2004). In preliminary field measurements at the Leon County site (Fig. 1; Chanton and Liptay, 2000), it was observed that 15 cm of yard waste compost above an existing 1 m clay cover enhanced methane oxidation relative to the clay cover alone. The major goal of the current study is to determine the minimum biocover placed above the interim cover that would be capable of substantially mitigating methane emissions without active gas extraction. A second goal of this study is to test the long-term performance of several recycled materials for biocover construction, including glass cullet and crushed fluorescent tubes for the gas distribution layer, as well as the use of ground and composted garden waste for the overlying biocover layer.Two test areas were established at the Leon County Landfill: (1) S1 using a (pre-composted) garden waste biocover over a 15 cm gas distribution layer (crushed glass) over a 40-100 cm interim cover; and (2) S4 using a 30 or 60 cm ground garden waste biocover (not pre-composted) over 15 cm crushed fluorescent tubes over a 15 cm interim cover. For S4, it was hypothesized that the garden waste would compost in place in the warm Florida climate.For the S1 biocover subsites, the monthly data over the first year indicated that median positive fluxes were an order of magnitude less than the control sites. After 6 months placement, more than half of the S1 biocover fluxes were consistently negative, indicating uptake of atmospheric methane. For the S4 deep biocover area (60 cm), initial months of monitoring indicated that the median positive fluxes were at least an order of magnitude less than the controls. The temporal S4 data indicate that up to 75% of the deep biocover fluxes are negative; however, depending on soil moisture and other variables, this percentage can be much lower. Although additional seasonal data are needed to provide definitive design guidance for subtropical climatic conditions, the results to date indicate that methane-oxidizing biocovers can provide a "low tech" strategy to minimize emissions .