Two-Phase Modeling of Leachate Recirculation Using Vertical Wells in Bioreactor Landfills

Abstract

Leachate recirculation or liquid injection is an established practice for operating landfills as bioreactors to enhance the biodegradation of municipal solid waste (MSW). Among other subsurface liquid injection methods, vertical wells (VWs) represent one of the most common methods used for active or closed landfills. The current design and operation of VW systems does not consider the effect of additional liquid injection on the increase in gas pressures. In this study, a two-phase model that assumes landfill leachate and gas as immiscible phases was used to predict the moisture distribution and pore water and pore-gas pressures in a typical bioreactor landfill that uses VWs as its leachate injection or recirculation system. The unsaturated liquid and gas properties of MSW were simulated based on the van Genuchten model. The study evaluates the effect of the unsaturated hydraulic conductivity of MSW, the heterogeneous and anisotropic nature of the MSW, and the geometric configuration of VWs on moisture distribution and pore water and gas pressures. The unsaturated hydraulic properties of MSW significantly influence the wetted area and pore-water pressures during the initial stages of leachate injection and during the gravity drainage that follows. The numerical modeling results show that the gas pressures in the landfill will be of the order of 60 to 150 kPa, which are far above the liquid pressures for most typical liquid injection rates when no gas accumulation in the landfill is assumed due to an active gas extraction system. Hence, the slope stability of bioreactor landfills needs to be assessed by using the effects of gas and liquid pressures, especially when the gas extraction system is operating at below optimum efficiency.