Experimental And Numerical Study On Gas Migration In Landfill Of Municipal Solid Waste

With the rapid economic development and urbanization, the generated quantity ofmunicipal solid waste (MSW) increased at high speed in China. Landfill has becomethe main means to deal with MSW in this country. Pollution control and resourcerecycle of landfill gas increasingly get more attention. Unordered emission of landfillgas pollutes the environment and gives health hazard to the residents around landfillsites. Also the methane in landfill gas easily raises blast and fire hazard. When landfillgas is not vented effectively, the gas pressure in landfill will increase, which impactsseriously settlement and slope stability of waste piles. Therefore, it is of practical andtheoretical signification to study the gas migration in landfills.The issues of gas migration in landfills mostly consist of gas generation rate, gaspermeability and gas migration model. Focused on the issues, laboratory tests, fieldmonitoring, and simulation analyses were carried out. The main contents andconclusions are as follows.Against that the MSW in China contains much food waste and water, the gasgeneration rule of the MSW was investigated. Basing on the test results of gasgeneration of the MSW, the Findikakis-Leckie model of gas generation was chosen,and the gas generation parameters for the MSW in China are made out. Basing on theFindikakis-Leckie model, the calculation method of gas generation rate for multi-layer MSW was presented. The soil water characteristics curve of the MSW wasmeasured by the use of Tempe cell and pressure plate. The study showed that the gasgeneration rate was between 5.8×10^-7 and 4.4×10^-6 kg m^-3s^-1 and the peak rate wasbetween 1.9×10^-6 and 8.7×10^-6 kg m^-3s^-1. Considering fill ages of the MSW, the gasgeneration rate was between 1×10^-7 and 8.7×10^-6 kg m^-3s^-1.Gas permeability of the MSW taken from the Qizishan landfill in Suzhou of Chinawas measured by the use of a purposely-designed lysimeter. The influences of voidratio, degree of saturation and MSW composition on gas permeability wereinvestigated. The test results showed that intrinsic permeability of the MSW rangedfrom 10^-13 to 10^-10 m². For a given composition and degree of saturation, gaspermeability was found to increase exponentially with void ratio of the MSW. For agiven composition and void ratio, a bilinear relationship was observed between gaspermeability and degree of saturation. The degree of saturation corresponding to thedivision point (i.e., Sr=50%) was slightly less than that corresponding to the fieldcapacity of the waste. When the degree of saturation was greater than the division point, gas permeability decreased exponentially with an increase in the degree ofsaturation. However, at the low range of degree of saturation (i.e., Sr≤50%), theinfluence of Sr on gas permeability was not so significant. Predictor formula of thegas permeability with degree of saturation and void ratio of MSW was established.The one-dimension model and the axis-symmetric of gas migration are establishedby using the theory of porous media flow. Basing on the 1D model, the influences ofgas generation rate, gas permeability and waste thickness on the gas pressure in wastepiles are investigated. The influences of gas permeability and thickness of final/interim cover on the gas pressure are analyzed. The calculation method of gas pressurein multi-layer MSW was presented. Basing on the axis-symmetric model, the bleed gasflow and gas pressure field of different work conditions are analyzed. The term'reduction pressure ratio' was presented to determine the effect radius of bleed gas well.The calculation method of the length and spacing of bleed gas well was presented.The 1D model analysis showed that the gas pressure in landfill increases with anincrease in thickness of waste piles. For the uncovered waste piles, the gas pressureincreases with an increase in the ratio of gas generation rate to gas permeability. Forthe covered waste piles, the influence of gas generation rate on the gas pressure ismore than that of gas permeability. The gas pressure increases with an increase in thecover thickness or with a decrease in the gas permeability of the cover. The max gaspressure could reach 45 percent of the total self-weight stress of MSW piles, whichaffected the stability of MSW piles. The max gas pressure under the final cover couldreach decades' kPa and exceed the self-weight of the final cover, which induced failureby piping. So it is advised that the gas pressure in MSW piles should be reduced bysome measures.The axis-symmetric model analysis showed that the bleed gas flow and the effectradius increased with an increase in the length of bleed gas well and the suctionpressure. To effectively extract the gas out of the whole waste piles, the length of thewell must be larger than 65 percent of the MSW thickness, and the spacing of the wellis less than 1.5～2.5 times the length of the well.The monitoring results and the simulation analysis showed that the max gaspressure in the Qizishan landfill in Suzhou was 10 and 20 kPa. When the gas permea-bility of MSW is very low, the max gas pressure can add up to 50～60 kPa. As theMSW thickness in the Laogang ecological demonstration landfill in Shanghai is only8m, the present spacing of the gas well (i.e., 40～50 m) is oversize, and the adequateis about 30 m.