Modeling Coupled Hydro-Bio-Mechanical Processes in Bioreactor Landfills: Framework and Validation

Abstract

Bioreactor landfills using leachate recirculation in municipal solid waste (MSW) undergo complex interrelated hydraulic, mechanical, and biochemical processes. In this study, the authors present a mathematical framework that combines and solves together a hydraulic model, a biodegradation model, and a mechanical model to simulate and predict the coupled hydro-bio-mechanical behavior of MSW in bioreactor landfills. The mathematical framework was implemented in Fast Lagrangian Analysis of Continua (FLAC), a finite difference code. The hydraulic model was a two-phase flow model where the fluid flow was governed by Darcy’s law while the saturated–unsaturated hydraulic behavior was governed by a soil-water retention model. The mechanical behavior of the MSW was evaluated by a plane-strain explicit formulation of the Mohr-Coulomb constitutive model, which accounted for concurrent changes in the MSW properties as a result of the MSW degradation. The MSW decomposition was modeled as a first-order decay kinetics biodegradation model similar to the LandGEM model used by the United States Environmental Protection Agency. The coupled mathematical framework was validated with a published large-scale laboratory experiment and a field-scale experiment. It was applied to simulate the coupled behavior of a typical bioreactor landfill cell undergoing hydro-bio-mechanical interactions. Overall, the study shows that the proposed mathematical framework accounted for both the spatial and temporal changes in the geotechnical properties of the MSW, including the extent of degradation, and predicted the landfill settlement and MSW stabilization time for a typical bioreactor landfill configuration subjected to coupled hydro-bio-mechanical processes. In addition, some of the limitations and key challenges associated with the numerical model are highlighted.