Effects of compost biocovers on gas flow and methane oxidation in a landfill cover

Previous publications described the performance of biocovers constructed with a compost layer placed on select areas of a landfill surface characterized by high emissions from March 2004 to April 2005. The biocovers reduced CH4 emissions 10-fold by hydration of underlying clay soils, thus reducing the overall amount of CH4 entering them from below, and by oxidation of a greater portion of that CH4. This paper examines in detail the field observations made on a control cell and a biocover cell from January 1, 2005 to December 31, 2005. Field observations were coupled to a numerical model to contrast the transport and attenuation of CH4 emissions from these two cells. The model partitioned the biocover’s attenuation of CH4 emission into blockage of landfill gas flow from the underlying waste and from biological oxidation of CH4. Model inputs were daily water content and temperature collected at different depths using thermocouples and calibrated TDR probes. Simulations of CH4 transport through the two soil columns depicted lower CH4 emissions from the biocover relative to the control. Simulated CH4 emissions averaged 0.0 g m−2 d−1 in the biocover and 10.25 g m−2 d−1 in the control, while measured values averaged 0.04 g m−2 d−1 in the biocover and 14 g m−2 d−1 in the control. The simulated influx of CH4 into the biocover (2.7 g m−2 d−1) was lower than the simulated value passing into the control cell (29.4 g m−2 d−1), confirming that lower emissions from the biocover were caused by blockage of the gas stream. The simulated average rate of biological oxidation predicted by the model was 19.2 g m−2 d−1 for the control cell as compared to 2.7 g m−2 d−1 biocover. Even though its Vmax was significantly greater, the biocover oxidized less CH4 than the control cell because less CH4 was supplied to it.