Mechanisms And Migration Rules Of Leachate And Landfill Gas Considering Bubble Generation In High Saturated Landfills

Landfill is one of the important ways of waste disposal.The leachate and landfill gas generate during the degradation process of organic matter,which lead to the flow of liquid and gas in the landfill and the settlement and deformation of landfills.Leachate Leakage,landfill gas emission and slope stability are the main environmental disasters for landfills.Water levels of Chinese landfills are commonly high,leading to disaster risks to the landfill.The process of gas generation continues below the water level due to the complex biodegradation,and landfill gas migrates in the form of bubbles inside landfills,leading to the high gas and leachate pressure below the water level.However,existing research basically ignores the effect of gas migration,and the results are generally inconsistent with the field test.Current research on the coupling mechanism in landfills and the process of leachate and gas migration in high-saturated landfills is insufficient.Therefore,it is of great significance for the design and operation of landfills to establish a liquid gas flow model for high water level landfills considering the accumulation of gas bubbles.The main innovations of this paper are listed as follows:(1)For the migration of liquid and gas in the unsaturated zone of landfills,considering the influence of the initial density distribution and volume change in the landfill on the coupling process,a coupled hydro-mechanical-biodegradation(H-M-B)model for municipal solid waste in the unsaturated zone is established.Based on the Open FOAM platform,the H-M-B coupling model solver was compiled by the finite volume method.The change of degradation process,hydraulic properties and settlement of waste are analyzed.The distribution of leachate in the unsaturated zone of the landfill under the condition of rainfall infiltration is simulated,and the influence of the change of void ratio,the compression characteristics of garbage and the biochemical degradation process on the distribution of liquid and gas are analyzed.(2)For the migration of liquid and gas in the saturated zone of landfills,considering the effect of gravity,buoyancy,surface tension and porous medium resistance,the pressure balance equation for bubbles in the porous medium is established to determine the state of bubble movement(gas generation stage or liquid and gas flow stage).In the stage of gas generation,the relationship between leachate and gas pressure is derived from the continuity equation,and the bubble volume growth and the internal pressure change during the bubble generation process is analyzed.In the stage of liquid and gas flow,a discontinuous two-phase flow model for leachate and gas bubble in high saturated zone is established to simulate the migration of leachate and landfill gas,based on a modified Navier-Stokes equation.The resistance boundary of the porous medium is introduced to describe the discontinuous two-phase flow in the porous medium.Based on the equation of motion,the key variables of volume fraction are introduced to establish a two-phase volume fraction of phase model.The results are verified with the analytical model and the model test for saturated sand,which shows good agreements.(3)The saturated-unsaturated dual-domain coupled numerical model framework of the landfill is established.The evolution of the bubble movement state in the high saturated zone and the spacetime distribution are simulated.The rules of bubble accumulation process and two-phase flow process under different soil porosity and gas production conditions are analyzed.The results show that the liquid and gas pressure accumulation is more obvious when the porosity become smaller,and the water level rises faster.The gas production determines the final water level height and the total accumulation of landfill gas,while the rate of gas generation determines the pressure accumulation rate and the critical rising time of bubbles.A simplified calculation method is presented to estimate the part of landfill gas production escaping from the saturated zone of landfill.(4)Based on the saturated-unsaturated dual-domain coupled numerical model framework,the distribution and coupled migration process of liquid and gas under the active control operation are simulated.The numerical calculation model by vertical well dewatering of the landfill and the horizontal short drainage are established.The results shows that the extraction volume of landfill gas accounts for more than 10%.It is necessary to consider the gas generation and migration process below water level.The impact of emergency measures on the stability of the landfill slope is also analyzed.The proportion of gas extraction volume increases with the increase of pumping pressure,while the well distance mainly affects the extraction volume and pumping rate,and has little effect on the proportion of gas extraction volume.For an actual in-service landfill in East China,the redistribution of leachate and landfill gas caused by the gas generation,bubble accumulation and flow of liquid and gas in the landfill are analyzed.The slope instability caused by the high-water level during the design and operation of the landfill is studied,the evaluation method of slope stability for landfill is presented.