One-Dimensional Gas Flow Model for Horizontal Gas Collection Systems at Municipal Solid Waste Landfills

Extraction of biogas from horizontal layers above, below, and within municipal solid waste landfills is becoming more commonplace. A steady-state one-dimensional analytical landfill gas model was developed to assist in the assessment and design of such collection systems. The model simulates the distribution of gas pressure within a layer of landfill waste under a variety of operating conditions that include upper and lower boundaries specified at given fluxes or pressures. The model can be used to predict where maximum pressures will build up within the landfill and what vacuum pressures must be applied to achieve specific gas collection efficiency in a horizontal collection layer. The utility of the model was illustrated for several scenarios of interest. In the absence of gas collection from a landfill’s leachate collection system, considerable gas pressures can build up at the bottom of the landfill. The design of leachate collection systems for landfill gas removal should be considered from the outset. An evaluation of the parameters that impact vacuum requirements—waste depth, gas generation rate, and waste permeability—suggests that it may not be feasible to rely solely upon the leachate collection system for the removal of landfill gas. The model was thus used to illustrate cases where a horizontal collection layer underneath the landfill cap is used in conjunction with gas extraction from the bottom of the landfill. Several recommendations are proposed to improve the gas collection efficiencies for landfills utilizing horizontal gas collection layers.