Effect of pressure on biomethanation process and spatial stratification of microbial communities in trickle bed reactors under decreasing gas retention time

The current study investigated the effect of elevating gas pressure on biomethanation in trickle-bed reactors (TBRs). The increased pressure led to successful biomethanation (CH4 > 90 %) at a gas retention time (GRT) of 21 min, due to the improved transfer rates of H2 and CO2. On the contrary, the non-pressurized TBR performance was reduced at GRTs shorter than 40 min. Metagenomic analysis revealed that the microbial populations collected from the lower and middle parts of the reactor under the same GRT were more homogeneous compared with those developed in the upper layer. Comparison with previous experiments suggest that microbial stratification is mainly driven by the nutrient provision strategy. Methanobacterium species was the most dominant methanogen and it was mainly associated with the bottom and middle parts of TBRs. Overall, the increased pressure did not affect markedly the microbial composition, while the GRT was the most important parameter shaping the microbiomes.