Air Permeability of Waste in a Municipal Solid Waste Landfill

The permeability of compacted municipal solid waste in a landfill with respect to air (or gas) flow was estimated using a shortterm air injection test. Air was added to 134 vertical wells installed at three different depths at flow rates in the range of 0.14  -1.4  m3  min-1 and the corresponding steady state pressures were recorded. The permeability of the waste with respect to airflow (described here as the air permeability) was estimated for different anisotropy ratios (kr/kz=1, 10, and 100) using a steady state, two-dimensional, axisymmetric analytical fluid flow model in conjunction with the measured flow and pressure data. The air permeability of landfilled municipal solid waste modeled as an isotropic medium was found to range from 1.6*10^-13 to 3.2*10^-11  m2. The estimated air permeability results were on the low end of values previously applied to model landfill gas flow. Estimated air permeability decreased significantly with increasing waste depth. The lower permeability encountered in the deeper layers was primarily attributed to the lower porosity of the waste caused by higher overburden pressures and higher moisture content of waste in deeper layers of the landfill than in shallow layers. The results suggest that multiple wells screened at different depths provide greater control of air distribution within the landfill. Leachate recirculation was documented to impact the ability to add air. In addition to limitations posed by standing water in many of the deeper wells, waste exposed to leachate recirculation was found to be significantly less permeable to air when compared to original conditions.