# Exploring Effective Bio-Cover Materials for Mitigating Methane Emission at a Tropical Landfill
## 1 Introduction  
The landfill is the most commonly used municipal solid waste (MSW) disposal method globally [1] due to its flexibility in MSW characteristics, operation simplicity, and low-cost association. Nevertheless, it can generate significant environmental impact in the forms of leachate and landfill gas which can adversely impact human health and the ecosystem if they are not properly managed. Methane (CH4) and carbon dioxide (CO2) are the major landfill gases produced in landfills. Among them, methane is an important greenhouse gas with an infrared activity up to 34 times that of CO2 over a 100-year horizon [2]. Methane from waste accounted for about 18% of total anthropogenic methane emissions in which landfill methane emissions from developed countries have been largely stabilized; however, emissions from developing countries are increasing as more controlled (anaerobic) landfilling practices are implemented [3]. There are several techniques used for mitigating methane emission from landfills such as pre-treatment of organic wastes before landfilling, promotion of aerobic conditions in landfills, landfill gas collection, and utilization of biological methane oxidation in the final cover soil. Among them, biological treatment is the most cost-effective technique for the mitigation of methane emissions through the use of a natural microbial methane oxidation process. Practically, biological oxidation in the bio-filter, bio-window, or bio-cover is a conventional and effective technology for controlling methane emissions at older and smaller landfill sites with low gas potential [4]. During their application, low methane emissions were found in the bio-cover systems operated under passively loaded operation, but some hotspots were identified on actively loaded biofilters [5]. To promote methaneoxidation, several materials have been utilized such as soil, compost, and municipal solid waste [6; 7]. [PERSON] et al. [8] utilized organically labile materials such as compost and natural soil and commonly observed 100% methane oxidation but eventually also observed that the reaction would taper off to a lower level of performance during long-term operation. Effective methane oxidation could be promoted when environmental factors are properly controlled for methanotrophic microorganisms [9]. Methanotrophic bacteria can be naturally grown in landfill cover materials provided that they are maintained under aerobic conditions. Therefore, it can be considered a complementary strategy for minimizing methane emissions from landfills. Methanotrophs are specific microorganisms that can utilize methane as a sole carbon and energy source under aerobic conditions [10]. The capacity of methane oxidation mainly depends on the characteristics of materials such as porosity, water/gas permeability, and nutrients, as well as environmental conditions such as temperature, soil moisture, presence of vegetation, etc. [9; 11; 12]. In tropical climate conditions, maintenance of methanotrophic activities in landfill cover is challenging due to distinct difference in climatic conditions between the monsoon (rainy) season with short-period intensive rain events and the other long dry seasons, i.e., summer and winter with extensive evaporation where moisture content in landfill cover drops significantly. Therefore, the selection of appropriate materials is essential for maintaining moisture content, especially in the dry period for which leachate irrigation can be considered [13]. Meanwhile, ambient temperatures play a less important role in regulating methane oxidation in a tropical landfill because there is not much difference in temperature (30-35 \({}^{\circ}\)C) between the seasons [14]. Our previous investigations in laboratory-scale set-ups simulating tropical climate conditions have demonstrated high methane oxidation capacities, e.g., >80% of methane loaded in landfill cover materials [14; 15].