Effects Of Leachate Recirculation On Landfill Gas Production And Heavy Metal Leaching During The Co-Landfill Process Of MSW And Stabilized Fly Ash

The rapid growth in the scale of municipal solid waste incineration（MSWI）treatment has led to an annual increase in the production of incineration fly ash in China.The fly ash contains a wide variety of heavy metal pollutants,which require solidification/stabilization pre-treatment before entering the MSW landfill for separate zone landfilling.However,in the real environment,due to factors such as nonstandard landfill operations and imperfect management systems,there are some situations where stabilized fly ash and MSW are co-landfilled.Currently,the laws and mechanisms of the impact of leachate recirculation and aeration operations on leachate quality,landfill gas production and discharge,heavy metal leaching,and microbial community structure changes in co-landfill systems are unclear.This study focuses on exploring the changes in the production and discharge of landfill gas（CO₂/CH₄）and the leaching of heavy metals（Pb/Zn/Cu/Cd/Cr/Ni）in the co-landfill system under simulated rainfall,landfill leachate recirculation,and other operating conditions through the construction of anaerobic and semi-aerobic co-landfill columns loading with raw MSW and stabilized FA,and analyzes the relevant impact mechanisms.The experiment has been running continuously for 245 days,and the main results are as follows:（1）Compared to the co-landfill column with leachate recirculation,the anaerobic/semi-aerobic co-landfill column with simulated rainfall achieved faster stabilization of indicators such as COD and NH₄⁺-N in the leachate,accelerating the stabilization process of the co-landfill system.Compared to the two types of co-landfill columns with leachate recirculation,the frequency of recirculation once every two days leads to the accumulation of organic matter in the leachate of the anaerobic co-landfill system,which inhibits the stabilization process of the waste in the co-landfill system.The rule of semi-aerobic landfill column is opposite.（2）The production and emission laws of CO₂ and CH₄ from co-landfill gas are significantly influenced by aeration operating conditions.Among them,leachate recirculation conditions and simulated rainfall conditions have the most significant differences in the production and emission changes of CO₂ and CH₄ in the anaerobic co-landfill column and the semi-aerobic co-landfill column,respectively.Compared to the co-landfill column with simulated rainfall,the co-landfill column with leachate recirculation has an inhibitory effect on CH₄ production;The frequency of recirculation once every two days can accelerate the process of CH₄ production and discharge in a semi-aerobic co-landfill environment and promote waste stabilization.（3）The leachate recirculation operation can reduce the concentration of heavy metals in the leachate at the early stage of co-landfill to a certain extent,and reduce the risk of release of heavy metals in the co-landfill system.However,compared to the co-landfill column with simulated rainfall,the leaching concentration of heavy metals in the co-landfill column with leachate recirculation is still higher.Pb,Cu,Cr,and Ni in the leachate of anaerobic co-landfill column and Pb,Zn,Cu,Cd,and Ni in the leachate of semi-aerobic co-landfill column maintain relatively low leaching concentrations under operating conditions with a recirculation frequency of once a week and once a day,respectively.However,changes in recirculation frequency can only temporarily inhibit or accelerate the leaching behavior of heavy metals in the co-landfill system.With the extension of co-landfill time,the impact of changes in recirculation frequency on the leaching behavior of heavy metals will gradually weaken.（4）The carbonation of fly ash caused by the production and discharge of landfill gas runs through the entire co landfill period,and there is a negative correlation between the CO₂ production and discharge rules and most heavy metal leaching behaviors.This indicates that the period when the concentration of heavy metals in the leachate in the co-landfill system is relatively low is to some extent related to the indirect carbonation effect of CO₂ on the stabilized fly ash matrix,especially during the period when the acid neutralization ability of the stabilized fly ash matrix is strong.（5）The bacterial community changes in the co-landfill system simulated in this study mainly include Spirochaetes and Actinobacteria at the gate level,Spirochaetia,Bacilli,and Actinobacteria at the class level,and Sphaerochaeta and Propionibacterium at the generic level,all of which are affected by co-landfill operating conditions（aeration,recirculation,etc.）,and are positively correlated with changes in the concentration of COD,TOC,NH₄⁺-N indicators in the leachate of the co-landfill system,as well as changes in the concentration of some heavy metals.The dominant methanogenic bacteria in the co-landfill columnd with leachate recirculation are mainly hydrogen trophic bacteria（Metanobacteriun,Metanoculleus）,while the methanogenic bacteria in the co-landfill columns with simulated rainfall are mainly acetic acid trophic bacteria（Metanosarcina）.The changes in EC and TOC indicators of leachate are the main environmental factors that affect the changes in the structure of archaea and bacterial communities in each co-landfill system.The research results of this thesis can provide a certain scientific basis and theoretical reference for the optimization of operating conditions（aeration,recirculation,etc.）of MSW landfills with historical or current co-landfill problems,the design of landfill gas or/and leachate drainage paths,and environmental risk management for heavy metal leaching in China.