Gas transport properties of compost–woodchip and green waste for landfill biocovers and biofilters

Because methane is a potent greenhouse gas emitted from landfills, there is considerable interest in landfill cover materials and biofilters for methane oxidation. Compost and green waste are used as biocovers and biofilters, but the gas transport properties of these materials are not well understood. Large wood particles and high organic matter content are typical for compost and green waste, which will affect air-filled porosity (ɛ), relative gas diffusion coefficient (Dp/D0) and air permeability (ka).Gas transport properties were measured for compost with wood particles >4 mm (compost–woodchip) and green waste using repacked samples and intact cores, collected from a landfill biocover and biofilters. The widely used Millington–Quirk model predicted Dp/D0 poorly, particularly for dry conditions (ɛ > 0.60). Models that assumed an inactive air-filled pore space through which gases cannot diffuse fitted Dp/D0 data much better: the Troeh model was best with root mean square error three to four times smaller than the Millington–Quirk model. Several models were evaluated for describing ka data, but none were able to match the highly variable data trends well. In some intact cores the relationships between Dp/D0 and ɛ, and ka and ɛ suggested the existence of dual-domain porous media with macropores. By examining gas transport properties of compost samples with and without wood particles, wood particles >4 mm were found to increase the total porosity from 0.49 to 0.79 and ka by as much as two orders of magnitude, while Dp/D0 were altered by less than 40%.

► Gas diffusion and gas permeability measured for compost and green waste.
► Intact and repacked cores used to elucidate role of wood particles on measurements.
► Troeh model worked best for describing gas diffusion for both compost and green waste.
► Gas permeability data varied significantly between cores: no model could fit all core data.