Study On The Mechanism And Control Technology Of Clogging Of Leachate Drainage Layer In Landfill Site

Today,sanitary landfill is still one of the main disposal methods of urban domestic waste.In the sanitary landfill,the clogging of the leachate drainage system is common.The mechanism and prevention and control technology of leachate clogging in the drainage layer have always been research hotspots of environmental and municipal engineers.In China,compared with developed countries,domestic landfill leachate has a large output and high organic content,and the limitations of landfill zoning,in the drainage layer is the mixed leachate produced by various stages of waste.Therefore,this thesis combined with the characteristics of the production and drainage of leachate in domestic landfills,by laboratory simulation experiments,using physical and chemical index analysis,high-throughput gene sequencing,scanning electron microscopy（SEM）,X-ray diffraction（XRD）,X-ray fluorescence spectroscopy（XRF）and other analysis and characterization methods,the mechanism of leachate forming precipitation clogging in the drainage layer was revealed,the process and mechanism of using the aged refuse layer to control the precipitation and clogging of the leachate in the drainage layer was studied,a method to control the clogging of the drainage layer was proposed by combining the aged refuse layer as the drainage layer protection layer and the scientific and standardized management of the landfill.The research results of this thesis can provide theoretical basis and technical support for the prevention and control of the clogging of the leachate drainage system of domestic landfills.The main achievements of this research were the following:（1）Using static anaerobic culture experiments,the role of microbial flora in the formation of precipitation of leachate was analyzed,and the precipitation performance of mixed leachate was studied.The study found that the Chemical Oxygen Demand（COD）and Ca²⁺in the simulated leachate added with 5 m L of actual leachate decreased by 53%and80%,respectively,while the COD content of pure simulated leachate didn’t change significantly,indicating that microbial flora played an important role in the formation of leachate precipitation process.The static anaerobic culture experiments on the mixed leachate found that the COD and Ca²⁺contents of the mixed leachate with the volume ratio of young leachate of 10%,20%and 30%decreased by 12%,38%,24%and 50%,84%and50%,respectively.The mixed leachate is easy to form precipitation,and the inorganic precipitate is mainly calcium-bearing carbonate,accounting for 38%,78%and 84%of the inorganic precipitate,respectively.（2）Using the column experiments,the changes of main physical and chemical indexes and the law of formation of precipitation when leachate flows through the drainage layer were deeply studied.When the leachate flowed through the saturated drainage layer,the content of COD and Ca²⁺decreases by 28%～71%and 18%～59%,respectively,and the content reduction was mainly in the upper part of the drainage layer.The degradation of organic matter in the leachate by microbial flora was the main reason for the reduction of COD content,and the reduction of Ca²⁺content was mainly due to the formation of carbonate precipitation.The leachate formed more clogging in the saturated drainage layer,which caused serious clogging of the drainage layer.The clogging was the most serious in the upper section of the drainage layer that first received the leachate,and the drainable porosity decreased by 53%,indicating that the longer the hydraulic retention time（HRT）of the leachate and the higher the content of organic matter and calcium ions,the easier the formation of precipitation.The clogging degree of the unsaturated gravel layer is lighter,but the drainable porosity is also reduced to a certain extent.Research shows that maintaining the leachate in the unsaturated state of the drainage layer can reduce the clogging degree.In the unsaturated and saturated gravel drainage layers,the inorganic and organic components of the precipitates are 65%～68%,32%～35%and 53%～57%,43%～47%,respectively.The content of the inorganic components is greater than that of the organic components,and the main inorganic precipitates are calcium carbonates,accounting for 78%～89%and 61%～91%of the inorganic precipitates respectively.It indicated that the inorganic precipitates mediated by the microbial flora played an important role in the clogging of the drainage layer.（3）Using high-throughput gene sequencing technology and a variety of analysis methods,the mechanism of microbial flora-mediated precipitate and clogging of leachate in the guide drainage layer was revealed.There were 47 phyla,606 genera,880 species and 42phyla,716 genera,and 1083 microbial flora in the precipitates of the saturated and unsaturated experimental columns,respectively.The microbial flora in the precipitate formed by the saturated and the unsaturated drainage layer had better diversity and rich biological functions.Part of the microbial flora can form extracellular polymeric substance（EPS）,which is beneficial for the microbial flora to attach to the gravel surface to form a biofilm.The biofilm adheres to the gravel surface,the rich microbial flora in the biofilm degrades the organic matter in the leachate,which causes a great change in the physical and chemical properties of the leachate,and bio-chemical reactions occur in the leachate to form inorganic precipitates mainly containing calcium carbonate.It is deposited on the drainage layer,causing chemical clogging of the drainage layer.Through the SEM observation,the precipitate on the gravel surface of the drainage layer contains abundant microbial flora,the morphology of the precipitate is a dense porous structure,the precipitate and the microbial flora are closely combined and attached to the gravel surface to form a complex biological inorganic precipitates.The precipitation clogging of leachate in the drainage layer is mainly the result of the combined effect of biological and chemical clogging.（4）Through the analysis of the research results that the aged refuse layer controls the leachate to form precipitation in the drainage layer,the mechanism and action process of the aged refuse layer controlling the clogging of the drainage layer are as follows.The special structure and properties of the aged refuse layer have strong interception,adsorption and degradation functions for the suspended solids（SS）in the leachate,so that the SS can be effectively removed,the maximum removal rate is about 87%.Leachate is partially saturated in the aged refuse layer,and the HRT of the leachate in this layer is longer,so that the rich microbial community in this layer can degrade the organic matter in leachate fully,the maximum removal rate is about 85%.Due to the fluctuation of the properties of the actual leachate,and the aged refuse also contains more calcium and magnesium precipitates,under the CO₂ mediation,there is a dynamic precipitation and dissolution balance between the dissolved Ca²⁺in the leachate and the calcium salt precipitated in the aged refuse.When the leachate flows through the aged refuse layer,its higher content of Ca²⁺will form precipitate with the carbonate in the aged refuse layer,thereby preventing the migration of calcium salts into the drainage layer.After the leachate enters the drainage layer,the biodegradable organic matter in the leachate has been completely degraded in the aged refuse layer,and the drainage layer is not suitable for the mass growth and reproduction of microorganisms.In addition,the main physical and chemical properties of the leachate are basically stable,so it will not form obviously precipitate and clogging in the drainage layer.The measurement of the drainage porosity of the drainage layer showed that the index didn’t change significantly before and after the experiment,which proved that the drainage layer didn’t form obviously clogging.Laying the cleaned aged refuse on the drainage layer and on the outside of the graded gravel layer of the vertical drainage well as a protective layer,combined with the scientific and standardized management of the landfill,can better control the leachate in the drainage layer to forme of precipitates and clogging.