# Experimental Characterization of the Engineering Properties of Landfill Compost-Biocover
## 1 Introduction  
Municipal solid waste landfills are the third-largest source of anthropogenic methane (CH\({}_{4}\)) emissions in the atmosphere [1; 2], which contribute 9 to 70 Tg/year of CH\({}_{4}\) to the environment [3; 4], and which therefore have a significant adverse impact on global climate change [5]. Enormous health benefits are associated with climate change mitigation [6]. The occurrence of the anaerobic degradation of municipal solid waste results in the production of two essential gases, namely, carbon dioxide (CO\({}_{2}\)) and CH\({}_{4}\), and CH\({}_{4}\) has 25 to 28 times more global warming potential (GWP) than CO\({}_{2}\) due to its more significant molar absorption coefficient, despite its smaller quantity in the atmosphere according to the IPCC's 4 th and 5 th assessment reports, respectively [7; 8]. CH\({}_{4}\) is second in order after CO\({}_{2}\) for atmospheric concentration [9]. According to [PERSON] [4], landfills in the USA contribute about 30-35% of CH\({}_{4}\) to the atmosphere. Two main approaches can be used to mitigate landfill gas (LFG). From 40% to 90% of LFG can be used for commercial recovery utilizing a network of collection pipes. The remaining gas passes through an intermediate, final cover and, in some cases, through adjoining lateral areas [10]. However, commercial recovery is not economically feasible if the landfills are small or biodegradation is slow. In such scenarios, a layer of biocover is incorporated into the landfill cover system. The biocover is an integral part of the landfill system. The CH\({}_{4}\) gas that passes through a biocover transforms into CO\({}_{2}\). Methanotrophs are bacteria that actively convert CH\({}_{4}\) intoCO\({}_{2}\) under aerobic conditions through microbial oxidation [11]. These bacteria consume methane as their food and release it in carbon dioxide, utilizing their metabolism under aerobic conditions. The process is known as microbial methane oxidation. Different materials, such as soil, compost, and mixtures of sand and compost, can construct landfill biocovers. According to [12; 13; 14; 15], compost material has more oxidation capability than other materials. The basic principle of a landfill biocover is to keep the landfill CH\({}_{4}\) emission at zero, or a negligible amount for a specified thickness, which can be achieved by establishing a balance between the quantities of CH\({}_{4}\) gas generation and consumption. However, biocover design is quite complex due to the number of parameters involved, such as thermal (T; temperature, thermal conductivity, thermal diffusivity, specific heat), hydraulic (H; hydraulic conductivity, water content and degree of saturation, diffusivity, so forth), biological (B; methanotrophs) and chemical (C; organic and inorganic contents) factors that can affect its performance. Figure 1 shows the scheme of a compost-based landfill biocover. Various research studies, e.g., [16; 17; 18; 4; 9; 13], have reported the strong impact of water content variations on the performance of covers for CH\({}_{4}\) oxidation and stressed a need to define a proper range of water content for the microbial oxidation process in a cover medium. The degree of saturation may affect the landfill cover in different aspects such as (i) the degradation phenomenon, (ii) the gas flow through a medium, and (iii) geotechnical stability of the cover. [PERSON] [4] explains that CH\({}_{4}\) oxidation occurs at a certain water content. For high water content, the gaseous phase molecular diffusion is transformed into aqueous phase molecular diffusion due to filling the pores with water, which considerably reduces the oxidation process. Similarly, at low water content, the CH\({}_{4}\) oxidation capability is also reduced due to insufficient microbial activity in the cover materials. The study further reports that aqueous diffusion is 104 times slower than gaseous phase diffusion. [PERSON] et al. [13] show that methanotrophic microorganisms tend to become inactive enzymes at less than 13% water content.