A numerical two-phase flow model is presented to determine the moisture distribution and pore water and gas pressures within unsaturated municipal solid waste (MSW) in bioreactor landfills during leachate recirculation. The numerical model used is the Fast Lagrangian Analysis of Continua (FLAC), which is based on finite difference approach. The model governing equations and mathematical formulations is briefly explained. Validation of the model is examined by simulating the published laboratory and field studies and published modeling studies. Overall, the two-phase flow model is found to produce results comparable with those of the published studies. This assures that the model can be used for the prediction of moisture distribution and for the rational design of leachate recirculation systems in bioreactor landfills. 1. Introduction Bioreactor landfills, emerging as a preferred option for the municipal solid waste (MSW) management, essentially involve the recirculation of leachate to increase the moisture content of the MSW and, thus, its biodegradation. For bioreactor landfills to be effective, the uniform and adequate distribution of moisture throughout the MSW is of paramount importance. The proper design of a leachate recirculation system is an important task to ensure the uniform distribution of moisture with the desired saturation levels, which range from 60% to 80% [1]. This range can be achieved if the design of the leachate recirculation systems takes various elements into consideration, including (a) hydraulic properties of MSW (including saturated and unsaturated hydraulic conductivity), (b) selection of the proper type of leachate recirculation system, (c) optimization of the geometric formation and configuration of the selected leachate recirculation system, and (d) spatial variation of the MSW hydraulic properties. The published literature generally considers the leachate routing in a bioreactor landfill by assuming (a) saturated MSW, (b) single phase flow, (c) homogeneous and anisotropic MSW, or (d) improper hydraulic parameters of MSW. Therefore, the accuracy of the literature is often called into question. The current leachate recirculation systems design and operational suggestions are based on limited laboratory studies and field observations and their application has resulted in wide variations in overall bioreactor performance. In addition, the field studies that use geophysical methods (e.g., ERT imaging) have provided useful but limited information on the leachate movement in the landfill. It is of utmost importance to have a
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