Degradation-Consolidation Behaviors And Treatment Methods For The Liquid-Gas Induced Environmental Disaster Of High-Food-Waste-Content MSW Landfill

According to statistics,there are currently about 27,000 informal landfills and1800 sanitary landfills in China's cities and surrounding areas.The informal landfills are lack of the facilities for preventing the leachate and LFG(LFG)pollution,which result in the odor nuisance and the pollution of groundwater and soil.So it is urgently necessary to carry out treatment measures to achieve the land resource reuse.The sanitary landfills in service are in dangerous state due to long-term overload operation,so the environmental disasters such as landfill landslides,leachate leakage and LFG diffusion are tend to happen frequently,which poses a huge threat to the city safety.It is necessary to take measures for improving the landfill storage capacity.Different from traditional soil,the food waste content and water content of Chinese municipal solid wastes(MSW)are 57%and 55%,respectively.After landfilling,there will be mass loss,solid skeleton decay due to time,LFG and leachate production in the degradation-consolidation process of MSW,which result in engineering characteristics variation and mechanism of the landfill disasters to be more complicated.There is still lack of systematic and comprehensive research on the evolution of leachate production,LFG production and engineering characteristics during the whole degradation-consolidation process.At present few scholars have conducted the completely continuous degradation-consolidation experiments to monitor and quantitatively analysis the changes of engineering characteristic.The MSW degradation-consolidation test device was developed,and the laboratory-scale test of high food waste content(HFWC)MSW with different stress-degradation paths was carried out,and the prediction model of soil mechanical parameters under the combined action of stress and degradation and the prediction model of leachate and gas production of HFWC MSW were established.Then through the degradation-mass transfer-seepage coupled model,the mechanism of liquid-gas accumulation in the HFWC MSW landfill was analyzed.Finally,the LFG-induced environmental disaster prevention and control technology was developed,and a complete set of technologies for degradation-stabilization accelerating and landfill mining based on the evaluation of degradation stabilization degree was developed,which has been successfully applied in many landfills projects in China.The main research work and the conclusions obtained were as follows:(1)Using the MSW degradation-consolidation test device design,4 sets of HFWC MSW laboratory-scale tests were carried out,revealing the leachate and gas production and soil mechanical properties of HFWC MSW under different stress-degradation conditions.The prediction model of soil mechanics parameters under the combined action of stress and degradation was established.The research results show that in the semi-logarithmic coordinate system,the leachate production rate increases linearly with the increase of the vertical load.The slope of the curve gradually decreases with the increase of the degradation stabilization degree.When the load reaches 1000 k Pa,the final leachate production rate is about 50%,with the corresponding field capacity25%.In the semi-logarithmic coordinate system,the void ratio and vertical strain of the MSW increase linearly with the vertical load.The immediate compression index and the modified immediate compression index gradually decrease as the degradation stabilization degree increases,and the immediate compression curves of MSW with different degradation stabilization degrees approximately intersect when the vertical stress is 1000 k Pa,and the corresponding void ratio and vertical strain are 0.75 and 0.76,respectively.The intrinsic permeability coefficient of MSW obtained through the rigid wall permeability test gradually decreases with the improvement of the degradation stabilization degree,but the variation range is within an order of magnitude.The saturated hydraulic conductivity of MSW and gas permeability under field capacity are mainly controlled by vertical stress.(2)A modified two-stage LFG production prediction model suitable for HFWC MSW was established.Based on the physical and chemical composition of the MSW and on-site pumping tests,the calculation method and recommended value range of the required parameters for the model were given.The results of the study show that the proportion of LFG production potential(approximately 89%)of the rapidly degradable compositions contained in the unit wet-base quality HFWC MSW is significantly higher than that of the LFWC MSW(49%).After the acid inhibition phenomenon is relieved,during the stable methanogenesis period,and the LFG production rate reaches the peak at about 100 days,and then decreases rapidly.The LFG production rate constant at methanogenesis period is about 0.99-1.97 a^-1 under different degradation conditions.A back analysis method of LFG production rate based on field pumping test is proposed.The LFG production rate constant of rapidly degradable compositions ranges from 0.40 to 1.90 a^-1,which is about 4 times that of slowly degradable compositions(0.09-0.55 a^-1).The LFG production lag time of rapidly degradable compositions is about 1 to 6 months.The LFG production lag time of slowly degradable compositions is as same as that of LFWC MSW in Europe and the United States,which is 12 months.Comparing the LFG production law of HFWC MSW and LFWC MSW,it shows that the LFG production peak of HFWC MSW was about 6.3 times of LFWC MSW.After reaching the peak,the LFG production rate of HFWC MSW decayed rapidly,and it decreased to 50%of the peak after about 8 months.The decay process of LFWC MSW is more gradual,and it takes about 63 months to reduce to half of the peak value.(3)According to the characteristics of the degradation stabilization process of HFWC MSW revealed by the laboratory-scale experiments,a biochemical-mass transfer-seepage coupling model was established to verify the increase in pore pressure induced by the accumulation of liquid and gas in the HFWC MSW landfill.The mechanism and the influence of factors such as leachate level,LFG collection conditions,leachate drainage conditions,and landfill age of the underlying MSW layer on the distribution of liquid-gas pressure are analyzed.The research results show that LFG is easy to accumulate at the interface between the new and old MSW layers or the middle soil layers.The accumulated gas pressure will reach a peak within 30 days after the completion of the new MSW layer,and then it will gradually decay until it reaches the balanced state with the head pressure of the leachate level.When high VFA concentration leachate generated from the the overlying new MSW layer infiltrates into the old MSW layer,LFG will be generated due to the VFA digested by the methane bacteria in underlying old MSW layer,which is the main source of accumulated air pressure.When the age of the underlying MSW layer is 3,5,and 7 years,the proportion of LFG generated from the infiltrated leachate of the overlying MSW layer is 50%,70%and 80%,respectively.A method for determining the warning gas pressure value in the HFWC MSW landfill is proposed.The warning gas pressure can not exceed the head pressure corresponding to the warning water level at the monitoring point.The liquid and gas pressure control measures such as emergency small-diameter drainage shafts and downslope long and short blind ditches have been applied at the Guangzhou Xingfeng Landfill,and the overall slope safety factor increased from 1.25 to 1.67.(4)According to the characteristics of LFG production for HFWC MSW during the rapid and slow degradation stages,high efficient LFG collection technology and ecological soil final-cover technology are proposed,which were applied in Shenzhen Xiaping landfill.The results show that the high efficient LFG collection technology includes two key technical contents:enhanced temporary cover and multi-level LFG collection-transmission pipeline network.The high efficient LFG collection technology has been applied at Shenzhen Xiaping Landfill,the amount of LFG collected Increased from 22,000 m³/h to 45500 m³/h during 2014-2019,and the LFG collection rate increased from 24%to over 95%.The lateral diversion length of the ecological soil final-cover can be maintained at about 10 m during continuous heavy rainfall.According to the maximum annual LFG flux(49.0 m³ LFG m^-2 a^-1)determined by the safety control pressure under the final-cover,Shenzhen Xiaping landfill can be closed with ecological soil final-cover in the 14th month after the landfill is stopped.(5)According to the test data of multiple scales such as field test,large model test and laboratory test,the evaluation method of degradation stabilization degree of landfill based on the normalized stabilization index(?)is proposed.For landfills in the slow degradation stage(0.6<??0.9)and post-stabilization stage(0.9<??1.0),a complete set of technologies for landfill mining and safety risk control are proposed,including accelerating stabilization technology based on three-dimensional liquid-gas pumping and injection,landfill excavation and safety risk control technology,odor emission control technology during the excavation process,landfill waste screening and utilization technology.This technology has been successfully applied to the Hangzhou Xianghu Simple Landfill Treatment Project.The degree of separation for fine fraction of MSW after screening was 63.7%.