Measurement Of Gas Permeability And Methane Oxidation Capacity For Loess And Their Effect On Landfill Cover Performance

The northwestern region of China is mainly semi-arid to arid and loess is ubiquitous. This natural resource has considerable potential to be transformed into earthen final covers for local landfills. The gas permeability and methane oxidation capacity of loess will affect the landfill gas emission and methane reduction directly. Therefore, studying on the gas permeability and methane oxidation capacity of loess, as well as gas migration and methane oxidation in loess cover, has an important practical significance for the design of the methane oxidation function landfill cover.Laboratory unit test and model tests, numerical simulations and analytical methods were carried out to study the issues. The main research works and conclusions are as follow.(1) A device was developed to measure the gas permeability of unsaturated compacted loess specimens. Experiments were carried out to investigate the effects of the compaction dry density, the compaction water content, the soil clod size and the service water content of the compacted loess. It was found that the gas permeability exponentially decreased with increasing compaction dry density. The compaction water content and soil clod size exerted a combined effect on the gas permeability. For specimens with identical soil structure, a nonlinear relationship was measured between the gas permeability and the service water content after compaction.(2) Batch tests with duration up to22days were carried out on the loess samples to measure their methane oxidation capacity. The influences of placement duration, moisture content and mix proportion of compost on methane oxidation were investigated. The relationships among the volume changes of different gas compositions during methane oxidation process were analyzed. The test results demonstrated that the methane oxidation capacity of the cover loess varied widely. The fresh loess had no methane oxidation capacity within22days of incubation. Mixing compost in loess significantly enhanced the methane oxidation ability of the loess. The moisture content of loess had a significant influence on the methane oxidation, and the optimum moisture content of loess for methane oxidation is about20%～30%. In methane oxidation process, about44%of methane was oxidized to carbon dioxide, the rest carbon was absorbed by methanotrophic bacteria.(3) A simulation landfill cover column test device was developed to simulation of the loess landfill cover, the gas migration and methane oxidation in soil column and the influence of rainfall were studied. The experiment results show that the gas pressure in the two layers of column increased linearly with depth increasing. The depth of air diffusion into the soil was about30～40cm. The methane oxidation capacity of column was increased from zero to a stable value after a training time. The maximum methane oxidation efficiency and the maximum methane oxidation rate during the experiment were27.8%and68.4g CH4· m-2· d-1respectively. The area which the methane oxidation activity most intense was the depth of5～25cm in column. The gas permeability and methane oxidation capacity of column upper soil decreased rapidly because of simulate rainfall and gradually restored as water infiltration.(4) One-dimensional steady-state model of gas migration in waste layers and final cover is established on the basis of the flow theory in porous media. The influences of gas permeability of the loess cover and gas extraction rate in the diffusion layer on landfill gas emission were investigated. The methane oxidation capacity of landfill cover was discussed and some suggest of landfill cover design were put forward. The assessment showed that the gas pressure at the bottom of the loess cover increased as a result of a decrease in the gas permeability. Extracting landfill gas from the gas diffusion layer of the final cover was able to reduce the gas pressure and the landfill gas emission rate. In the design of landfill final cover, the mix proportion of compost should be determined according to the methane generation and the gas permeability of landfill cover..