Passive methane oxidation biosystems (PMOBs) are often proposed as a means to reduce fugitive landfill CH4 emissions, i.e. emissions not captured by gas collection systems.However, current designs may lead to the formation of a capillary barrier along the interface between the two main layers constituting passive biosystems, namely the methane oxidation layer and gas distribution layer. The formation of a capillary barrier may result in restricted upward flow of biogas at the base of methane oxidation layer, thereby leading to concentrated biogas emissions in regions known as hotpots, where passive oxidation of biotic methane is failing, if not absent. In this study, design criteria are introduced to assess the ease of biogas flow across the gas distribution-methane oxidation layers' interface. Laboratory experiments were conducted to obtain the water retention curve, air permeability function and line of optima (on Standard Proctor curve) of the materials used to construct the methane oxidation layer of two experimental PMOBs at the St-Nicephore (Quebec, Canada) landfill. In addition, the main characteristics for other materials were obtained from the literature. Design criteria were then defined based on the degree of water saturation at the lines of optima and the pattern of air permeability functions and water retention curves. Considering these criteria in the design of PMOBs is fundamental to reduce the risk of creating hotspots when implementing PMOBs.

The design process of passive methane oxidation biosystems needs to include design criteria that account for the effect of unsaturated hydraulic behavior on landfill gas migration, in particular, restrictions to landfill gas flow due to the capillary barrier effect, which can greatly affect methane oxidation rates. This paper reports the results of numerical simulations performed to assess the landfill gas flow behavior of several passive methane oxidation biosystems. The concepts of these biosystems were inspired by selected configurations found in the technical literature. We adopted the length of unrestricted gas migration (LUGM) as the main design criterion in this assessment. LUGM is defined as the length along the interface between the methane oxidation and gas distribution layers, where the pores of the methane oxidation layer material can be considered blocked for all practical purposes. High values of LUGM indicate that landfill gas can flow easily across this interface. Low values of LUGM indicate greater chances of having preferential upward flow and, consequently, finding hotspots on the surface. Deficient designs may result in the occurrence of hotspots. One of the designs evaluated included an alternative to a concept recently proposed where the interface between the methane oxidation and gas distribution layers was jagged (in the form of a see-saw). The idea behind this ingenious concept is to prevent blockage of air-filled pores in the upper areas of the jagged segments. The results of the simulations revealed the extent of the capability of the different scenarios to provide unrestricted and conveniently distributed upward landfill gas flow. They also stress the importance of incorporating an appropriate design criterion in the selection of the methane oxidation layer materials and the geometrical form of passive biosystems.

The efficiency of methane oxidation in passive methane oxidation biosystems (PMOBs) is influenced by, among other things, the intensity and distribution of the CH loading at the base of the methane oxidation layer (MOL). Both the intensity and distribution are affected by the capillary barrier that results from the superposition of the two materials constituting the PMOB, namely the MOL and the gas distribution layer (GDL). The effect of capillary barriers on the unsaturated flow of water has been well documented in the literature. However, its effect on gas flow through PMOBs is still poorly documented. In this study, sets of numerical simulations were performed to evaluate the effect of unsaturated hydraulic characteristics of the MOL material on the value and distribution of moisture and hence, the ease and uniformity in the distribution of the upward flow of biogas along the GDL-MOL interface. The unsaturated hydraulic parameters of the materials used to construct the experimental field plot at the St-Nicephore landfill (Quebec, Canada) were adopted to build the reference simulation of the parametric study. The behavior of the upward flow of biogas for this particular material was analyzed based on its gas intrinsic permeability function, which was obtained in the laboratory. The parameters that most influenced the distribution and the ease of biogas flow at the base of the MOL were the saturated hydraulic conductivity and pore size distribution of the MOL material, whose effects were intensified as the slope of the interface increased. The effect of initial dry density was also assessed herein. Selection of the MOL material must be made bearing in mind that these three parameters are key in the effort to prevent unwanted restriction in the upward flow of biogas, which may result in the redirection of biogas towards the top of the slope, leading to high CH fluxes (hotspots). In a well-designed PMOB, upward flow of biogas across the GDL-MOL interface is unrestricted and moisture distribution is uniform. This paper tries to show how to obtain this.