Comparison of Single-Phase and Two-Phase Modeling of Leachate Distribution in Bioreactor Landfills

During initial stages of post-closure in bioreactor landfills, the buried municipal solid waste (MSW) experiences unsaturated flow conditions and relatively high suction mainly due to low degree of saturation. However, previous studies have only focused on leachate phase and neglected the landfill gas phase in MSW. In this study, a comparison is made between single-phase flow and two-phase flow modeling studies to signify the importance of considering immiscible gas phase in MSW alongside with liquid phase. Leachate was recirculated through a horizontal trench both continuously and intermittently under typically operated injection pressure. In two-phase flow model, unsaturated hydraulic properties of MSW were accounted by the van Genuchten functions. The results from both models were compared in terms of moisture distribution, pore fluid pressures, and their respective influence(s) on the stability of bioreactor landfill slope. It was found that lack of consideration of unsaturated state of MSW in the case of single-phase flow model led to overestimation of leachate distribution in MSW, underestimation of pore fluid pressure (no pore gas pressure), and therefore, overestimation of resulting stability of landfill slope than a two-phase flow mathematical model. The two-phase flow model accurately represents the development of unsaturated MSW condition during the period of gravity drainage after leachate injection (intermittent injection mode of injection) and during initial stages after the waste placement. Based on this study, it was recommended that numerical two-phase flow model be used for simulations of leachate flow and distribution and resulting slope stability evaluation in bioreactor landfills for safe and effective design of landfills.