# Exploring Effective Bio-Cover Materials for Mitigating Methane Emission at a Tropical Landfill
## 3 Results and Discussion
### Observed Methane Removals in Bio-Cover
 } & \multicolumn{4}{c}{**MLR (mol/m\({}^{3}\)/d)**} & \multicolumn{4}{c}{**MRR (mol/m\({}^{3}\)/d)**} \\ \cline{3-8}  & & **SL** & **C** & **SW** & **SL** & **C** & **SW** \\ \hline \multirow{3}{*}{Wet} & 0–0.20 & 0.4 \(\pm\) 0.1 & 0.4 \(\pm\) 0.2 & 0.2 \(\pm\) 0.1 & 0.3 \(\pm\) 0.1 & 0.3 \(\pm\) 0.2 & 0.2 \(\pm\) 0.1 \\  & 0.20–0.40 & 0.6 \(\pm\) 0.2 & 2.2 \(\pm\) 0.7 & 1.7 \(\pm\) 0.6 & 0.2 \(\pm\) 0.1 & 1.8 \(\pm\)The overall MRRs in SL, C, and SW materials (0.6 m layer depth) observed during the wet period in this study were 11.4, 10.4, and 8.2 mol/m\({}^{3}\)/d which are equivalent to 109.4, 99.8, and 78.7 g/m\({}^{2}\)/d. The removals were increased to 19.8, 15.0, and 13.3 mol/m\({}^{3}\)/d or 190.1, 144.0, and 127.7 g/m\({}^{2}\)/d during dry period in those materials, respectively. Comparatively, a wide range of methane removals in bio-cover were reported in the literature. [PERSON] et al. [27] reported a much lower removal rate of 2.7 g/m\({}^{2}\)/d in compost bio-cover whereas [PERSON] et al. [28] reported as high as 237 g/m\({}^{2}\)/d in a bio-window filled with biosolids compost supplemented with yard waste and leaf compost. Similarly, [PERSON] et al. [29] reported a maximum methane oxidation rate of 460 g/m\({}^{2}\)/d in an active loaded open compost filter treating diluted landfill gas. Meanwhile, [PERSON] et al. [5] reported a comparable methane oxidation rate of 60.6-70.6 g/m\({}^{2}\)/d when Danish biofilters were loaded at 60.7-70.7 g/m\({}^{2}\)/d with an observed oxidation efficiency of 98.9-99.9%. The average MER determined by the close flux chamber measurement at the surface of bio-cover material layers (weekly MER variation shown in Figure S3) were 0.007, 0.016, and 0 mol/m\({}^{2}\)/d for SL, C, and SW materials during the wet period and 0.092, 0.080, and 0.069 mol/m\({}^{2}\)/d for those respective materials during the dry period. Those emissions  
Figure 2: Gas concentration profiles observed in sandy loam (SL), compost (C), and stabilized waste (SW) media during wet and dry periods. accounted for 0-0.8% and 1.4-1.6% of methane loading to the bio-cover material layers during wet and dry periods, respectively. The results suggested that there was only a small amount of residual methane leaving as fugitive emission at the surface of the bio-cover material layer. Thus, more than 98% of methane mass entering the bio-cover unit was considered removed mainly through bio-oxidation. It should also be noted that the bio-cover system in this study was operated at a low loading rate to simulate the natural condition of tropical landfill emission. Therefore, the landfill gas was highly diluted as it entered the bio-cover material bed, noticeable from the significant reduction in methane and carbon dioxide concentrations especially at 0.4-0.6 m depth of the materials (Figure 2). Figure 3 shows the relationship between MLRs and MRRs observed in bio-cover materials. It was clearly observed that higher MRR was achieved under higher MLR conditions. Nevertheless, MRRs were found to be more varied under high MLR conditions. Among them, the MRRs observed in C material were found to fluctuate less than the others at the highest MLR range. Meanwhile, SL material performance varied only under high MLR conditions (>15 mol/m\({}^{3}\)/d) whereas MRRs in SW material fluctuated even at a MLR of 8-15 mol/m\({}^{3}\)/d. These differences could be due to the heterogeneous nature of bio-cover materials which can possibly be higher for stabilized waste than sandy loam and compost materials. Fewer variations were mostly observed in all materials operated under lower MLRs (<8 mol/m\({}^{3}\)/d).