Research On Biodegradation-consolidation-solute Migration Coupled Behaviors Of Municipal Solid Waste And Landfill Stabilization

Municipal solid waste (MSW) production increased significantly with the rapid development of urbanization in China, which led to the serious situation for MSW disposal. MSW contains biodegradable component, which results in the difference in engineering property between MSW and traditional soil. The landfill is an earth structure as MSW is disposed. At the same time, MSW is degraded under complicated biochemical reactions after being placed in a landfill. Leachate with a substantial amount of pollutants and landfill gas containing greenhouse gases (i.e., methane and carbon dioxide) are generated, making landfill to act as a large bioreactor. There are series of geoenvironmental problems due to complex interactions in landfills, such as: slope failure, settlement, leachate leakage and landfill gas emission even explosion. An improved understanding of these biodegradtion-consolidation-solute migration coupled behaviors will be the key to prevent and control geoenvirmenntal problems, which is important for the design, construct and operation of landfills.The research works in this paper are funded by the National Basic Key Research Program (973), via Grant No.2012CB719800 and No.2012CB719802. Based on numerical simulation method, the biodegradation-consolidation-solute migration coupled analysis of complex interaction mechanisms in MSW landfills were carried out. A series of significant research conclusions are drawn as follow:(1) Based on the biodegradation characteristics of MSW, a two stages anaerobic degradation model by considering the influence of moisture content, substrate content and VFA concentration is proposed. Cellulose is considered as the main degradable substance in MSW. This degradation model is able to predict solid mass loss, intra-particle water release, landfill gas generation and solute (VFA and methanogen) concentration during biodegradation process.(2) Mechanical properties of MSW are summarized including compression property, hydraulic property and solute migration property. The stress-age coupled compression model is proposed, which considers the change of compression property of MSW during biodegradation process. It is found that fresh Chinese MSW has a higher potential compression than that of MSW in developed countries. The water retention characteristic and liqud/gas permeability of MSW are affected by both compression and biodegradation. Van-Genutchen model can be used to describe the water retention curve of MSW and evaluate the liqud/gas permeability of MSW. The transport behaviors of contaminants in landfills includes convection, diffusion, dispersion and adsorption/desorption. Solute migration process is influenced by the velocity of pore water, pore structural properties and concentration gradient of the solute.(3) Based on the biodegradation characteristics and mechanical properties of MSW, a biodegradation-consolidation-solute migration coupled model is established to anaylze the mechanism of complex interactions in landfills. The key variables of coupled model include pore water pressure, pore gas pressure, displacement and concentration of VFA and methane bacteria in leachate. Finite element method is used to numerically solve this coupled model. The biodegradation-consolidation-solute migration coupled model is verificated by simulating an unsaturated soil consolidation problem and stabilization process of a landfill cell respectively. The application of coupled model is extended to degradable soil. The saturated consolidation processes for both degradable soil and traditional soil are compared, the analysis results show that:biochemical reactions in degradable soil leads to solid mass loss and skeleton softening, pore water pressure dissipates slower in degradable soil than that in traditional soil with the assumption of constant permeability, degradable soil has a larger settlement and compression process lag in the dissipation of pore water pressure.(4) Based on the proposed coupled model, the mechanisms of biodegradation-consolidation-solute migration coupled behaviors are analyzed. The results show that MSW degrades during the consolidation process, a large amount of leachate and landfill gas are generated, which change stress state in landfills. Compression of MSW results to increase of volume moisture content, which would accelerate the hydrolysis rate of MSW. Solute concentration distribution is affected by fluid flow. Improved degradation condition in landfill body, which could be achieved via active control technology, has a significant influence in biodegradation rate and consolidation behavior.(5) Based on the proposed coupled model, a series of geoenvironmental problems in landfill are investigated. It is found that typical high kitchen waste content (HKWC) landfills, i.e. landfills in China, produce a large quantity of leachate at early stage after MSW is disposed, which would cause leachate mound and impediment of landfill gas transportation in landfills. Skeleton is weaken more significantly in HKWC landfills than that in low kitchen waste content (LKWC) landfills, which means a larger degradation compression. The pore volume of MSW increased due to biodegradation, which might lead to a sudden collapse. The effect of leachate recirculation in Tianziling landfill is estimated. The results show that leachate recirculation can accelerate stabilization process in typical HKWC landfills. VFA concentration in leachate and the contaminant loading on the bottom barrier of landfills are reduced significantly after leachate recirculation. Acidification inhibition is relieved and methanogenesis starts earlier significantly, which results that landfill gas production and settlement rate increased at the early stage.(6) Based on the research of multiphysics interactions in landfills, a framework of landfill stabilization evaluation indexes, including biodegradation, landfill gas production and settlement, is established and a stabilization analysis method combining field test and numerical simulation is proposed. The stabilization process in Jiangcungou landfill is predicted by using this method. It is found that the stabilization process of HKWC landfills can be divided into three stages obviously. C/L of MSW decreases rapidly during fast degradation stage, solute concentration increases significantly and a large amount of landfill gas is generated at this stage. Some proposals for landfills management at fast degradation stage are given to prevent the potential envriomental disaster and maximize the recycle of MSW.(7) The relationship between settlement and MSW reduction is investigated by using stress-age coupled compression model. It is found that MSW reduction ratio increases with landfilling height. Development of large-scale intensive landfill and expansion landfill can improve use efficiency of land significantly and MSW disposal ability, which would be the future of landfill disposal technology.in China. Parameter analysis are carried out to study the influences of operational practices on landfill storage capacity. The results show that improvement of initial compaction can significantly reduce the operational compression displacement and increase storage capacity. Preferential decomposition conditions in landfills has limit effect on increase storage capacity, but can significantly reduce post-closure settlement, which helps to avoid disable of facilities in landfills. High leachate level reduces the self-weight of the submerged MSW and consequently greatly reduces storage capacity.(8) The proposed coupled model is extend to axis symmetric form. Landfill gas extracting test on Jiangcungou landfill is simulated to study landfill gas transportation with considering the influence of leachate level. The results show that MSW near by the vertical extraction well degradates rapidly, consequently, a large amount of leachate and landfill gas are generated during the test process, which result slight decline of leachate level and pore gas pressure. Transportation of landfill gas is impeded by high leachate mound. The effective range of vertical extraction well is quite small at high saturated aera, so it’s necessary to consider the influence of leachate level on landfill gas collecting efficiency. Parameters analysis indicate that reduction of leachate level can significantly reduce gas pressure and improve effective range of vertical extraction well. When leachate level keeps constant, the influence of water level in well is not obvious. Some operational suggestions are proposed based on analysis results.