Municipal landfill settlement with refuse decomposition and gas generation

Previously, various mathematical models were developed to simulate the liquid and gas flows in municipal solid waste (MSW) landfills. In these models, the landfill is assumed to be a nondeformable medium. To date, there is no published work simulating the complex behavior of landfill settlement with liquid and gas flows.; In this study, a one-dimensional multiphase numerical model is developed to simulate the settlement involving liquid and gas flows in a compressible MSW landfill. The MSW is represented by a chemical composition and a global stoichiometric reaction is used for estimation of maximum yield of gas generation. The MSW is categorized into three representative categories according to their biodegradability, each with a corresponding gas production rate constant. It is assumed that the waste decomposition generates a mixture of gas comprised of methane (CH₄) and carbon dioxide (CO₂). The gas generation causes a change in gas and liquid pressures in the landfill. These changes also affect the porosity, total stress, and degree of liquid and gas saturations. Therefore, a transient change in liquid and gas pressures leads to a settlement of the landfill. First, a gas generation model is developed based on a first-order kinetic single-bioreactor approach. Then, the governing equations are formulated and the Galerkin finite element method is used to discretize the resulting equations. Instead of solving the resulting nonlinear algebraic equations, they are linearized and checked for error in total mass due to the linearization of the equations. This error is used to control the time-step during the simulations. The Gaussian elimination technique is employed to solve the discretized equations with a computer program in FORTRAN.; In the first two chapters, the existing models and information about the MSW landfills are presented. In the third and fourth chapters, differential equations for gas generation and multiphase fluid flow models are developed. Numerical methodology of the operational sequence is given, along with the results and discussions in the fifth chapter. Finally, the summary of the dissertation and conclusions are presented in the sixth chapter.; The developed model can be used to predict the transient and ultimate settlements with waste decomposition and gas generation in MSW landfills. The waste porosity, gas pressure, liquid pressure, gas saturation, liquid saturation, and stress distribution can also be predicted in settling landfills.