# Experimental Characterization of the Engineering Properties of Landfill Compost-Biocover
## 1 Introduction
According to [PERSON] [4], oxidation tends to become zero at less than 6% water  
Figure 1: Schematic sketch of compost-based landfill biocover. content and increases up to 56% under saturation conditions. The optimal oxidation water content is different for different cover materials such as soil, compost, and a combination of compost and soil. Gas transport is another crucial process that may considerably affect the CH4 oxidation rate, as the availability of O2 is essential for CH4 oxidation purposes [19]. According to [PERSON] et al. [14], the CH4 oxidation process is directly associated with the O2 penetration potential of landfill cover soil, and [PERSON] et al. [20] report that O2 is the primary element that maintains the respiratory and metabolic activities of microbes, which actively take part in the CH4 oxidation process. [PERSON] [21] highlights the importance of O2 for a compost-based cover, reporting that methane oxidation potential is directly related to water content and free air space. According to studies, such as [9; 21; 22], the free air space ranging from 20% to 35% establishes the maximum oxygen consumption rate, i.e., 95%. [PERSON] [9] suggests that a water content between 25% and 75% in the landfill cover provides maximum CH4 oxidation potential. The optimum conditions for microbial biodegradation occur at a water content level and free air space level of 60% and 22%, respectively. Gas transport is generally governed by diffusion. The diffusion coefficient is employed in transport and reaction-based mathematical models to predict gas transport and microbial degradation [23], directly linked with the free air space [3; 24]. It means that it is essential to identify optimum conditions for water content, free air space, and relative diffusion coefficient regarding the CH4 oxidation potential of landfill biocovers, as inadequate free air space and low or high water content/degree of saturation may undoubtedly reduce the CH4 oxidation process in landfill biocover, due to insufficient supply of O2 to microbial organisms. The hydraulic conductivity of traditional (soil cover) and biocover is another critical parameter for a cover design regarding its structural suitability and methane oxidation potential. It is well understood that a low permeable cover reduces rainfall infiltration, which results in the saturation of cover material during the rainy season [25]. Studies have shown that when saturation reaches approximately 85%, gas diffusion should be completed in the liquid phase, much slower than in the gas phase [14; 26]. It can radically decrease CH4 oxidation due to the lack of both O2 and CH4 in the biocover [14]. It means that optimum hydraulic conductivity of biocover can support an appropriate gas exchange process and minimize rainfall infiltration and subsequent biocover saturation, which can drastically reduce the CH4 oxidation rate. Furthermore, the research studies [4; 7; 27; 28] state that CH4 oxidation in biocovers is considerably correlated with temperature. [PERSON] [21] states that compost material's chemical reaction and biological activity occur within a specific temperature range. The temperature beyond this range produces inactive enzymes, insufficient to generate the desired response for microbes. According to [PERSON] [8], CH4 oxidation of various media is different at different temperatures, and the optimal temperature for CH4 oxidation varies from 20 \({}^{\circ}\)C to 35 \({}^{\circ}\)C. [PERSON] et al. [27] report that the optimum temperature for CH4 oxidation is 25 \({}^{\circ}\)C in landfill cover material, and according to [PERSON] [9], the growth rates of microbial species are highly related to thermal properties along with some other parameters, such as water content, porosity, and permeability. Other studies such as [PERSON] and [PERSON] [16], [PERSON] et al. [27], [PERSON] [29], [PERSON] et al.