Microbial aerobic methane oxidation (MAMO) has been considered an environmental-friendly method for mitigating methane emission from municipal landfill. In landfill cover, MAMO is affected by water, gas and heat reactive transfer. The coupled process is complex and its influence on methane oxidation efficiency is not clear, especially in steep covers where spatial variations of water, gas and heat are significant. Many experimental studies have demonstrated that vegetation affects MAMO. However, effect of root architecture on MAMO is not fully understood. Furthermore, although it is well known that MAMO is an exothermic process, effect of heat generation on MAMO is generally ignored in current theoretical and experimental studies. The principal objectives of this research are to investi...[ Read more ]

Microbial aerobic methane oxidation (MAMO) has been considered an environmental-friendly method for mitigating methane emission from municipal landfill. In landfill cover, MAMO is affected by water, gas and heat reactive transfer. The coupled process is complex and its influence on methane oxidation efficiency is not clear, especially in steep covers where spatial variations of water, gas and heat are significant. Many experimental studies have demonstrated that vegetation affects MAMO. However, effect of root architecture on MAMO is not fully understood. Furthermore, although it is well known that MAMO is an exothermic process, effect of heat generation on MAMO is generally ignored in current theoretical and experimental studies. The principal objectives of this research are to investigate coupled bio-chemo-hydro-thermal reactive transport considering methane oxidation and vegetation.

A fully coupled model for water-gas-heat reactive transport considering methane oxidation and vegetation in unsaturated landfill covers was developed. The model considers effects of root architecture, microbial oxidation-generated water and heat. The model was implemented in a finite element software, COMSOL. To investigate effect of heat generation by methane oxidation on MAMO with ambient temperature ranging from 15 to 30 ℃, two soil columns and batch incubation tests were carried out. The lateral wall of one soil column was covered with thermal insulation material in order to achieve one-dimensional heat transfer. The measured data from soil column test were used to verify the newly proposed theoretical model. In addition, a series of axisymmetric and two-dimensional numerical parametric studies were performed based on published data from a laboratory soil column test and flume model investigating MAMO, respectively. Furthermore, one-dimensional numerical simulations were conducted to investigate effect of different root architectures on MAMO.

Experimental results show that heat generation enhanced MAMO. Ignoring heat generation by MAMO can result in a significant difference in methane oxidation efficiency by 100%. Model predictions are consistent with experimental results, including suction, gas concentrations, temperatures and methane oxidation efficiency. The new model can capture the variation of methane oxidation at different ambient temperature.

Two dimensional numerical simulations showed that a steeper cover has a lower oxidation efficiency due to enhanced downslope water flow, during which the soil desaturation promoted methane transport and hence methane emission. This effect is magnified as the cover angle and landfill gas generation rate at the bottom of the cover increase. Assuming steady-state methane concentration in a cover would result in a non-conservative overestimation of oxidation efficiency, especially when a steep cover is subjected to rainfall infiltration. By considering the transient changes in methane concentration, a new method is proposed and gives a more accurate oxidation efficiency.

Numerical investigation on effect of vegetation on MAMO illustrated that exponential and triangular root architectures have higher methane oxidation efficiency than uniform and parabolic, due to higher water content retained relieves water shortage for MAMO. When the soil is initially wet, optimum transpiration rate exists for methane oxidation. On the contrary, oxidation efficiency decreases as transpiration increases when soil is initially dry. Under continuous drying, methane oxidation efficiency decreases due to water shortage for MAMO.