# Exploring Effective Bio-Cover Materials for Mitigating Methane Emission at a Tropical Landfill
## 3 Results and Discussion
### Methanatrophi Population and Microbial Consortium  
The methanatrophi population in the bio-cover material was quantified as shown in Table 5. Generally, methane, oxygen, and nutrient contents in the cover material are the regulator of the methanatrophic population. Previous research [30] reported that the growth of methanatroph type I was enhanced under high oxygen conditions in the landfill cover. In this study, both methanatroph type I and type II were observed abundant under both wet and dry conditions. During the wet period, a greater population of methanatroph type II than type I was found at all depths in the C and SL materials whereas they were quite similar in the SW material. During the dry period, the methanatroph type I population increased in the C and SL materials while type II decreased. The populations of both methanatroph types increased in SW material when the bio-cover was operated continuously from the wet period to the dry period. These results suggest that the methanatroph population in the bio-cover varied seasonally and the population also varied between the materials. The predominant groups of methanatrophs were also influenced by the environmental condition. In C material, methanotrophs of type II grew well during the wet period, especially in the upper part while they were found to be more prevalent at the middle and bottom parts of the material bed during the dry season. For SL material, the predominant methanatroph population shifted from type II during the wet period to type I during the dry period. Its methanatroph population was quite consistent along the bed depth except  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Media**} & \multirow{2}{*}{**Depth (m)**} & \
\begin{tabular}{c} **MOR** \\ **(\(\mu\)mol/g Dry** \\ **Solids/h)** \\ \end{tabular} &
\begin{tabular}{c} **CPR** \\ **(\(\mu\)mol/g Dry** \\ **Solids/h)** \\ \end{tabular} & \multirow{2}{*}{**MOR:CPR**} \\ \hline SL & 0–0.20 & 1.31 & 0.74 & 1.77 \\  & 0.20–0.40 & 0.50 & 1.03 & 0.48 \\  & 0.40–0.60 & 0.60 & 1.32 & 0.45 \\ \hline C & 0–0.20 & 2.49 & 2.1 & 1.19 \\  & 0.20–0.40 & 2.08 & 2.4 & 0.87 \\  & 0.40–0.60 & 2.03 & 2.5 & 0.81 \\ \hline SW & 0–0.20 & 0.13 & 0.44 & 0.3 \\  & 0.20–0.40 & 0.12 & 0.17 & 0.69 \\  & 0.40–0.60 & 0.20 & 0.27 & 0.74 \\ \hline \hline \end{tabular}  
* Remark: MOR: Methane oxidation rate; CPR: Carbon dioxide production rate.  
\end{table}
Table 4: Methanotrophic activity in the bio-cover materials.  
for type II during the dry period. For SW material, the methanotroph population was found to be less than in other materials while it was greater at the deeper zone for all conditions.  
The PCR-DGGE profiles of microbial communities (shown in Figure S4) indicate the presence of five species of methanotroph type I and one methanotroph type II in the biofilter media. They are _Methylococcus_, _Methylomicrobium_, _Methylolometer_, _Methylomonas_, and _Methylololatum_ for type I and _Methylolystic_ for type II. Other non-methanotroph species were also detected in SW media which may relate to plastic wastes containing the media, e.g., _Alcanivorax dieselolei_ (an alkane degrading bacterium) and _Nittrafficater salsuginis_ (a denittiring bacterium). In previous research, it was also reported that the growth of _Methylobacter_ and _Methylococcus_ had a positive correlation with the oxygen available while that of _Methylolystic_ was positively correlated with methane [30]. A high abundance of methanotroph type I over type II suggests a high oxygen environment available for methane oxidation in the media examined in this study which is well supported by the gas profile observed along the biofilter media bed (Figure 2).