Research On The Biochemical And Mechanical Behaviors Of High Food Content MSW

In 2015,over 110 million tons of municipal solid wastes(MSW)were disposed of in landfills in China.Compared with the MSW in developed countries,the MSW in China has high food content(HFC).Studying the biochemical reactions and mechanical behaviors of HFC MSW will guide the design and operation of landfills in China,and hence help them reduce environmental risks and use the field more effectively.This paper is funded by the National Basic Key Research Program(973)(Grant No.2012CB719800).The biochemical and mechanical behaviors of HFC MSW were studied.The research was mainly based on theoretical analysis and numerical simulation.Main conclusions are given in the below.(1)The data of physical composition of MSW in representative Chinese cities and the moisture content and chemical composition of waste components were collected.The chemical composition characteristics of Chinese MSW were further calculated and analyzed.A numerical model was developed to describe the degradation of HFC MSW.In this model,degradations of both cellulose and non-cellulose compounds are considered,the waste components are grouped into easily and hardly degradable categories based on their hydrolysis rates under no water inhibition conditions,a hydrolysis-methanogenesis reaction description is used,and the inhibitions of pH and unionized VFA(volatile fatty acids)are considered.A method based on the variation of cellulose content/lignin content(C/L)with time of MSW was proposed to determine hydrolysis rate constants.The degradations of HFC MSW in both column and large-scale experiments were simulated,and the proposed degradation model was verified.The inhibitions under different acidification levels were simulated successfully,and the results showed that when the VFA accumulation is limited,the methanogenesis is inhibited by pH to some extent,and where there is a serious VFA accumulation,unionized VFA cause severe inhibitions on both hydrolysis and methanogenesis.(2)By analyzing related experiments,it was reveald that under the same stress,the(field)water holding capacity of MSW is proportional to the percentage(wt/wt,dry basis)of those components(kitchen waste,paper,wood and grass,textile and dust)that can easily absorb water.A method to calculate the leachate generation of MSW was proposed.Calculation results showed that the HFC MSW itself can generate large quantities of leachate after being landfilled,which is mainly attributed to degradation effect rather than stress.(3)A coupled analysis model was estabilished for MSW,which is able to consider the mutual effects between degradation reactions and mechanical behaviors in a landfill.By calculating the leachate generation of MSW elements and the variations caused by the transfer of leachate,the simplified simulation of migrations of water and solute in a landfill is achieved.The coupled model uses the mechanistic degradation model proposed in this paper to simulate the MSW degradation,which considers the influences of various environmental factors.The leachate generation and compression models used,respectively,correlates the variation of MSW's water holding capacity and secondary compression under the same stress directly to its solid mass loss.The solid mass loss is calculated via the degradation model's simulation.(4)The degradation and mechanical behaviors of an HFC MSW landfill unit in early landfilling period was simulated and analyzed.Results showed that the unevenly distribution of MB in MSW is very important for the acidification mitigation in a landfill unit;the settlement and leachate generation of landfill unit is highly correlated to its degradation extent;the injection of water can hardly enhance the degradation of HFC MSW,but the injection of liquid with high MB concentration can ease the acid inhibition.The simulation of the long-term biodegradation and mechanical behaviors of the landfill unit showed:for an HFC MSW landfill unit,most of the degradation,settlement and leachate generation can take place relatively quickly;the whole stabilization process of HFC MSW can be divided into rapid stabilization stage(0-2 year),slow stabilization stage(2-10 year)and after stabilization stage(10?year);the C/L of MSW can be used to judge its stabilization stage.(5)A geochemical model was estabilished to calculate the CaCO3 precipitation in leachate collection systems,and was verified.Simulation and analysis showed that the relationship between CaCO3 precipitation and VFA fermentation varies with the water quality:when the CaCO3 is undersaturated,the ratio of[Ca2+]loss to[VFA]loss(d[Ca2+]/d[VFA])is zero;for the leachate with saturated CaCO3,when Ca2+ and VFA concentrations are high,d[Ca2+]/d[VFA]also has a big value,and after that with VFA consumption,Ca2+ concentration decreases,and d[Ca2+]/d[VFA]also decreases.When Ca2+ concentration is below a certain value,the decrease of d[Ca2+]/d[VFA]with the decrease of Ca2+ concentration is very rapid.With a higher aqueous concentration of carbonic acid,the rapid decrease of d[Ca2+]/d[VFA]appears at a higher Ca2+ concentration.MSW degradation result in a high CO2 partial pressure in landfills,and hence the rapid decrease of d[Ca2+]/d[VFA]normally appears when there is still a relatively high Ca2+ concentration in leachate.