Estimation of mass transport parameters of gases for quantifying CH4 oxidation in landfill soil covers

Abstract

Methane (CH4), which is one of the most abundant anthropogenic greenhouse gases, is produced from landfills. CH4 is biologically oxidized to carbon dioxide, which has a lower global warming potential than methane, when it passes through a cover soil. In order to quantify the amount of CH4 oxidized in a landfill cover soil, a soil column test, a diffusion cell test, and a mathematical model analysis were carried out. In the column test, maximum oxidation rates of CH4 (Vmax) showed higher values in the upper part of the column than those in the lower part caused by the penetration of O2 from the top. The organic matter content in the upper area was also higher due to the active microbial growth. The dispersion analysis results for O2 and CH4 in the column are counter-intuitive. As the upward flow rate of the landfill gas increased, the dispersion coefficient of CH4 slightly increased, possibly due to the effect of mechanical dispersion. On the other hand, as the upward flow rate of the landfill gas increased, the dispersion coefficient of O2 decreased. It is possible that the diffusion of gases in porous media is influenced by the counter-directional flow rate. Further analysis of other gases in the column, N2 and CO2, may be required to support this hypothesis, but in this paper we propose the possibility that the simulations using the diffusion coefficient of O2 under the natural condition may overestimate the penetration of O2 into the soil cover layer and consequently overestimate the oxidation of CH4.