Dependence of Gas Collection Efficiency on Leachate Level at Wet Municipal Solid Waste Landfills and Its Improvement Methods in China

Abstract

Landfill gas (LFG) collection efficiency is low at many municipal solid waste (MSW) landfills in China. The relevant mechanism and potential solution for this problem are studied through laboratory gas permeability tests on MSW, field LFG extraction tests at a landfill subjected to leachate drawdown, numerical assessment of the LFG collection efficiency at the landfill with different leachate levels, and engineering application of the measures to improve LFG collection. The research work outcomes are as follows: High water content in the food-rich MSW is one of the major reasons causing the high leachate level at many Chinese landfills. High leachate mounds tend to result in a high degree of saturation in waste, and hence a low gas permeability of the waste. Therefore, the LFG collection efficiency at Chinese landfills with high leachate level is lower than expected. A drawdown of leachate level at a Chinese landfill by pumping resulted in a significant increase in the LFG collection rate and the influence radius of LFG extraction wells. The dependence of LFG collection efficiency on the leachate level relative to the total waste thickness was verified and quantified. The leachate level is suggested to be controlled to less than 30% of the total waste thickness to achieve a high LFG collection efficiency. Engineering application at a Chinese landfill indicated that the retrofitted facilities, including deep vertical wells and horizontal drainage trenches, were effective in lowering high leachate level. The drawdown of leachate level resulted in an increase of LFG collection efficiency from 10–20 to 60–90%.