# Exploring Effective Bio-Cover Materials for Mitigating Methane Emission at a Tropical Landfill
## 3 Results and Discussion
### Observed Methane Removals in Bio-Cover  
During the six months of operation, methane loading rates (MLRs) to the bio-cover were found to vary from 8.2-20.3 mol/m\({}^{3}\)/d (Table 3). The MLRs to the biofilters during the dry period (13.7-20.3 mol/m\({}^{3}\)/d) were higher than those during the wet period (8.2-11.5 mol/m\({}^{3}\)/d). It was due to higher methane content in the extracted landfill gas attained during the dry period, i.e., 20.6-29.6% than those during the wet period (12.9-19.2%). [PERSON] et al. [26] reported that higher methane emission during the dry period in tropical landfills could be associated with an increase in gas-filled porosity in cover soil as well as the presence of cracks as soil moisture fell below its shrinkage limit thus facilitating gas transport from the landfill. After bio-cover treatment, the residual methane concentrations observed in the outflow gas during the wet period were not detected (ND) to 0.15% whereas they were 0.6 to 0.7%. during the dry period. Therefore, observed methane removal efficiencies in those bio-covers were >99% during the wet period and 97-98% during the dry period with only small differences between them. In the bio-covers, the majority of methane removals took place at the first 0.2 m from the inlet, i.e., 0.4-0.6 m depth for all media. The average MRRs of SL, C, and SW media in this active zone during the wet period were 10.9, 8.3, and 6.42 mol/m\({}^{3}\)/d which accounted for 94.8%, 79.0%, and 78.0% removal efficiency, respectively. The average MRRs during the dry period in those materials at the same location were 11.4, 10.1, and 7.8 mol/m\({}^{3}\)/d, or 56.2, 66.0, and 56.9% removal efficiency. Based on these results, the MRRs observed in bio-cover materials during the dry period were found to be slightly higher than those during the wet period. Nevertheless, their observed removal efficiencies were considerably lower due to the high MLRs attained during the dry period. The residual methane leftover from the lower part of the bio-cover was subsequently removed in the upper parts of the bio-cover material layer. Especially during the dry period, the remaining methane was largely removed at 0.2-0.4 m depth at the MRRs of 3.9-7.4 mol/m\({}^{3}\)/d and observed removal efficiencies of 75.0-84.5%. Due to effective removals of methane in those active zones, the upper part of the material (0-0.2 m) was responsible for removing only a small amount of residual methane mass with the observed MRRs of 0.2-1.0 mol/m\({}^{3}\)/d. Figure 2 shows the gas profiles observed in different bio-cover materials during wet and dry periods. As methane was supplied to the bio-cover materials, it was reduced along the upward flow through media while oxygen, supplied through surface diffusion at the top of the bio-cover, was utilized for methane oxidation and thus produce carbon dioxide as the end product. From the results, it was found that the SL and C media at 0.40-0.60 m depth were most effective for methane removals. Similar gas profile patterns were also observed in those materials operated during the wet period. For SW material, methane removal was lower than those in the other materials, but the removal could still take place in the upper part thus yielding satisfactory total removal. In all cases, oxygen was found highly available throughout the depth of bio-cover for methane oxidation. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**Period**} & \multirow{2}{*}{ \begin{tabular}{c} **Media Depth** \\ **(m from Top)** \\ \end{tabular}