The Modification Of The Intermediate Covering And Its Effect On Upgrading Of CH₄ Capacity In Landfill

The main components of landfill gas(LFG)are CO2(30%~50%)and CH4(40%~60%),which have great energy potential.China’s"Twelfth Five-Year Plan"energy development plan proposes to vigorously develop unconventional natural gas such as municipal landfill gas.Therefore,it is of great significance to improve landfill treatment technology,strengthen the biodegradation process of MSW,and improve the gas production potential of MSW.Intermediate cover is often used in parts of landfills or when extended extensions require a certain amount of time to maintain openness and is an important part of sanitary landfills.Traditional waste sanitary landfills generally use soil as an intermediate cover,especially small and medium-sized landfills,which consume a lot of soil resources,and soil permeability and viscosity also affect the transport and migration of gas-liquid two phase in the landfill.This paper studied the main influencing factors of landfill degradation gas production.The construction waste soil was modified with lime,sawdust and polyacrylamide(PAM),and its engineering properties(compressive strength,permeability coefficient,water retention performance)were determined.The intermediate covering layer with bio-enhancement function is prepared by using waste such as construction waste soil and waste sawdust.The effects of different intermediate covering materials on landfill gas yield and composition were analyzed.The relationship between leachate characteristics and methane yield and microbial community structure during landfill degradation were studied.The main conclusions are as follows:(1)The effects of three modifiers on the engineering properties of the covered soil were compared.It was found that the gelation of lime,sawdust and PAM could enhance the compressive strength of the specimen.The permeability of the lime modified group was enhanced.In contrast,the permeability of the sawdust and PAM modified groups was significantly reduced.With the increase of the amount of sawdust,the water holding capacity of the modified soil samples increased,and the water holding capacity ranged from 39.46%to 65.19%.In alkaline soil,the increase of PAM blending ratio had no significant effect on the water holding capacity of modified soil samples.The leaching test results showed that the modified intermediate covering could be used as an effective means to adjust the pH value in the reactor.Among them,the 5%lime modified group and the 5%wood chip modified group could adjust the leachate pH values of 7.78 and 8.8,respectively,which is beneficial to the biodegradation and stability of MSW.In addition,the modified intermediate cover had a significant effect on reducing the organic pollution load and heavy metals in the leachate.According to the comparison,the modified intermediate cover layer of 5%lime group,5%sawdust group and 0.1%PAM group had the best comprehensive performance,stable structure,moderate permeability,could save storage capacity,and its technical and economic feasibility was better than that of traditional clay covering material.(2)The effects of intermediate cover,biological microbial agent,landfill density and initial moisture content on the biogas production and methanogenic concentration were studied.The test results showed that the significant order of factors affecting the biogas production during the sedimentation period was:intermediate cover>compaction density>bio-agent>moisture content.The significant order of the factors affecting the total gas production and methanogenic concentration of the whole MSW digestion stage was:bio-agent>intermediate cover>compaction density>moisture content.The addition of Cytophaga and Trichoderma could promte the biogas production and increase the abundance of Methanomicrobia in the digestive system,which was beneficial to the methanogenic capacity of the system.The methanogenic concentration of the system was higher,ranging from 20.1%to 46.9%.However,the abundance of methanogens in the test group with acid-producing bacteria was low,the system produced acidification,and the gas production and methanogenic concentration decreased,which was lower than the blank group without added microbial agents.The methanogenic concentration was 10.6%~24.3%.The sawdust modified intermediate cover would hinder the release of landfill gas.A compaction density of 800 kg/m~3 is advantageous for the conductivity of the landfill gas.The addition of sawdust or PAM to the over soil was beneficial to the methanogenic capacity of the anaerobic digestive system.(3)Comparing the gas production law of anaerobic digestion of MSW in five bioreactors,it is found that stratified landfill with different intermediate cover layers cloud improve the gas production rate and gas yield of MSW.The model fits the gas production rate constant size:PAM group>lime group>sawdust group>soil group>blank group.The cumulative gas production in the PAM group was the highest,which was65.5%higher than that in the blank group.The lime group and the PAM group produced biogas quickly and stably.The sawdust group has a strong adsorption effect on the gas due to the sawdust in the cover,resulting in a decrease in gas conduction capacity and a low gas production.Analysis of the gas components of the landfill gas showed that the modified intermediate cover could significantly improve the hydrogen production and methane production capacity of the MSW digestion system,and accelerate the transition from the hydrolysis acidification stage to the methanogenesis stage and shorten the gas production cycle.The maximum concentrations of H2 in the blank group,soil group,lime group,sawdust group and PAM group were 6.78%,8.94%,8.79%,8.65%and9.66%,respectively.The H2 concentration in landfill gas was maintained at a low level(about 0.2%)during the whole process of methane digestion and no accumulation.H2would be preferentially used by methanogens to form methane or react with CO to form acetic acid.In the methanogenesis period,the highest concentrations of methane concentration in the soil group,lime group,sawdust group and PAM group were63.25%,64.16%,62.09%,and 70.83%,respectively,while the highest concentration of CH4 in the blank group was only 39.14%.The methanogenesis lag time of the blank group,the soil group,the lime group,the sawdust group and the PAM group were 95days,85 days,70 days,90 days and 60 days,respectively.The length of methanogenesis lag time was as follows:blank group>sawdust group>soil group>lime group>PAM group.In the PAM group,the methane concentration reached 10%on the 67th day of filling,and entered the fast methanogenesis stage 80 days earlier than the blank group.As the concentration of CH4 in the landfill gas increased during methanogenesis stage,the concentration of impurity gases decreased.The intermediate cover layer has a certain removal effect on the trace impurity gas in the landfill gas,and the concentration of the impurity gas component decreases by:sawdust group>PAM group>lime group>soil group>blank group.(4)The relationship between leachate characteristics and methane yield in soil,lime and PAM groups was analyzed.In the process of anaerobic digestion of MSW,the relationship between the change of pH value of leachate and the methanogenesis process of the system was lagging.,not alone as an indicator.The HAc/HVa value in the leachate is first decreased and then fluctuated.At the first inflection point,the digestive system could be predicted to enter the methanogenesis stage.The TOC/TN value of leachate has a certain indication effect on the stability of MSW methane production system.When the TOC/TN≥13 in the leachate,the MSW anaerobic fermentation system stabilizes normal gas production.When the TOC/TN<13 in the leachate,the fermentation system was unstable and biogas production was small.At the end of the reaction,a higher concentration of acetic acid still existed in the system,and acid accumulation lead to the termination of gas production.(5)The 16S rRNA gene tag pyrosequencing technique was used to evaluate the microbial community of leachate samples at different reaction stages and the end-of-test MSW samples and cover soil samples in the PAM group reactor.The microbial community diversity was found to be the most abundant in the MSW samples,followed by the cover samples.Cluster tree analysis showed that the MSW samples were closely related to the microbial species in the leachate samples,and the longer the reaction period,the higher the similarity was.Nitrospira was lacking in the MSW anaerobic digestion system.The intermediate cover layer was the main place for the system to carry out the nitrification reaction.The cover samples contained the dominant species that could enhance the degradation of waste,which could promote the anaerobic digestion methane production.The methanogenic bacteria in the reactor were mainly hydrotrophic methanogens with strong tolerance to ammonia and salt,and the metabolism type was syntrophic acetate oxidation and methanogenesis.Ammonia accumulation inhibited the activity of the acetolytic methanogens,which was an important cause of the decrease in gas production capacity of anaerobic digestion of MSW.