Aerobic Degradation-stabilization Model In Municipal Solid Waste Landfill And Its Application

The output of municipal solid waste（MSW）in China is large,and many cities are facing the situation of"encircling the city with waste".Landfill is still one of the important disposal methods,but urban land resources are precious and the disposal capacity of landfill sites is limited.The sustainable landfill technology can shorten the stabilization time of waste and reuse the storage capacity through the process of landfill-accelerated stabilization-mining and utilization-relandfill,so as to achieve the strategic goal of"harmlessness,reduction and recycling".The accelerated stabilization is mainly aimed at the aged MSW.At this time,the anaerobic degradation is slow.The ventilation and aeration technology is used to accelerate the MSW degradation and reduce the water content,so as to quickly reach the excavation and screening standard and realize the reuse of the storage capacity.This paper is supported by the national key basic research and development plan（973 Plan）project"Basic research on the breeding of environmental disasters and sustainable prevention and control of municipal solid waste landfill"（2012cb719800）,the National Natural Science Foundation Project（51988101 and 51508504）and the research and development plan project of"top soldiers"and"leading geese"in Zhejiang Province（2022c03095）.Based on the engineering background of accelerated stabilization of MSW by aeration,a set of aerobic biochemical degradation model is established.Combined with waste skeleton deformation,liquid-gas migration,solute migration and heat conduction,a set of thermal water mechanical biological chemical multi field coupling model is established.A set of unit test in aerobic degradation chamber is designed to verify the correctness and effectiveness of the model.The temperature,oxygen concentration,water content The influence of various factors of garbage age on the effect of aerobic accelerated stabilization is studied.The influence laws of well spacing and gas injection rate engineering design parameters on accelerated stabilization are studied.The main conclusions are as follows:（1）A set of aerobic biochemical degradation model was established.According to the law of conservation of charge and mass,the stoichiometric equations of aerobic hydrolysis and simultaneous nitrification denitrification of organic matter were established.The kinetic equations of aerobic degradation were established by using the first-order kinetic equations.These equations considered the effects of temperature,water content,oxygen concentration and carbon nitrogen ratio on the biochemical reaction,as well as the heat release.The unit test of aged garbage was simulated to verify the degradation model.The model accurately reflects the changes of various substances under different degradation conditions,which is consistent with the experimental results of the changes of carbon and nitrogen forms,which shows that aeration has a significant effect on the degradation of solid waste in old age.（2）A multi-field coupling model of aerobic degradation was established.The aerobic degradation model is used to calculate the solid mass loss of all degradable components and calculate the changes of mechanical and hydraulic characteristics when considering the solid mass loss.The macroscopic equilibrium equations considering the deformation of solid skeleton,solute transport,two-phase flow and biochemical degradation,as well as the effects of heat release on biochemical reaction rate and permeability are established,including linear momentum balance equation,mass balance equation,energy balance equation,solute distribution equation and multicomponent gas distribution equation.The constitutive deformation equation including the compressibility and degradation of solid particles is established.The transport equation considering the transport and distribution of multicomponent gas in soil and multicomponent solute in leachate was established.The permeability and saturation equations considering the evaporation and condensation process in the process of fluid migration and the release process of intracellular water are established.The effects of temperature on reaction rate,permeability,soil expansion,pore water vaporization,biochemical reaction,evaporation and condensation were considered.The model can be used to predict the evolution of the main variables in the landfill,including temperature,settlement,solute concentration and the content of degradable solid substances.（3）Taking the MSW of Shanghai Laogang landfill for 3～4 years as the research object,a unit test equipped with forced ventilation system was designed,and the solid,liquid and gas components were monitored to study the reduction of organic pollution load and the migration law of carbon and nitrogen under aerobic conditions.（4）According to the unit test data,the multi-field coupling model is verified,the model parameters are calibrated,and the variation laws of organic matter,ammonia nitrogen concentration,moisture content and temperature during ventilation and oxygen supply are studied,which shows the effectiveness of the multi-field coupling model in simulating the aerobic degradation process.The changes of organic matter in the unit aerobic degradation test were simulated,including the changes of solid-phase degradable organic matter and ammonia nitrogen concentration in liquid-phase.Considering the multi field interaction of evaporation,condensation and heat and seepage,the model can simulate the phenomenon of water content reduction caused by water evaporation loss in the unit aerobic degradation test,and can simulate the variation process of evaporated water content,residual water content and saturation with degradation.The effects of biochemical heat release,evaporation heat absorption,heat taken away by gas discharge and boundary heat dissipation are considered.The simulation results are consistent with the experimental results,which shows the possibility of the model in temperature simulation and prediction,and can provide theoretical and numerical support for field practical engineering.（5）A one-dimensional numerical model of landfill site is established,and the multi-field coupling model is used to study the changes of various engineering characteristics,so as to reveal the one-dimensional aerobic accelerated degradation stabilization law of landfill site.The initial temperature,moisture content,oxygen concentration and age were selected as independent variables,the temperature and stabilization normalized indexesβ,the change quantity and its change rate are used as evaluation indexes,and 8 working conditions are designed.The temperature and stabilization normalization indexβunder different working conditions are analyzed.The sensitivity of the four factors was compared and analyzed.The aerobic degradation of municipal solid waste is sensitive to temperature.The initial temperature is high,which can provide a more suitable environment for microbial biochemical reaction.The aerobic degradation is also started rapidly,which is of great significance for accelerating the degradation and stabilization process.At the same time,too high temperature will also inhibit microbial activity,cause degradation stagnation,and also bring potential fire hazards.Too high water content will cause the temperature to rise slowly,which is not conducive to the degradation of organic matter,but also hides the risk of landslide instability;Too low moisture content will lead to the reduction of microbial activity,and aerobic degradation will not be used.The higher the oxygen concentration,the more sufficient the aerobic degradation reaction,the faster the temperature rises under multi field coupling,the faster the organic matter decreases,and the better the accelerated stabilization effect.The contents of degradable organics in municipal solid waste at different ages are different.In the multi field coupling simulation,the smaller the age,the more degradable substances,the more heat released,and the greater the temperature rise.In turn,the temperature affects the degradation rate of organics.（6）According to the multi-field coupling model of aerobic degradation,a preliminary study on the application of two-dimensional aerobic accelerated stabilization of the landfill was carried out.The in-situ test of Wuhan Jinkou landfill was selected for numerical simulation and reproduction,revealing the change rules of engineering characteristics such as two-dimensional scale settlement,degradable organic matter,temperature and oxygen concentration of the landfill,and calibrating a set of model parameters suitable for practical engineering.Based on this,a two-dimensional engineering case is established by using numerical software to study the influence of well spacing and gas injection rate on aerobic degradation effect.It is revealed that within a certain range,the larger the well spacing,the greater the change of the average value of temperature,DOM and O₂ variables,the greater the influence range,and the less the decrease of the average value of saturation;It is revealed that when the gas injection rate is within a certain range,the greater the value,the greater the change of the average value of temperature,DOM,O₂ and saturation variables,and the greater the influence range.However,these indicators gradually tend to be fixed with the increase of well spacing or gas injection rate,and cannot increase indefinitely.Therefore,it is necessary to comprehensively consider technical factors and economic benefits,and determine the optimal well spacing and gas injection rate in the direction of more aerobic degradation and cost saving.