Performance analysis and microbial community evolution of in-situ biological biogas upgrading with increasing H2/CO2 ratio

The effect of the amount of hydrogen supplied for the in-situ biological biogas upgrading was investigated by monitoring the process and evolution of the microbial community. Two parallel reactors, operated at 37°C for 211 days, were continuously fed with sewage sludge at a constant organic loading rate of 1.5 gCOD∙(L∙d)-1 and hydrogen (H2). The molar ratio of H2/CO2 was progressively increased from 0.5:1 to 7:1 to convert carbon dioxide (CO2) into biomethane via hydrogenotrophic methanogenesis. Changes in the biogas composition become statistically different above the stoichiometric H2/CO2 ratio (4:1). At a H2/CO2 ratio of 7:1, the methane content in the biogas reached 90%, without adversely affecting degradation of the organic matter. The possibility of selecting, adapting and enriching the original biomass with target-oriented microorganisms able to biologically convert CO2 into methane was verified: high throughput sequencing of 16S rRNA gene, revealed that hydrogenotrophic methanogens, belonging to Methanolinea and Methanobacterium genera were dominant. Based on the outcomes of this study, further optimization and engineering of this process is feasible and needed as a means to boost energy recovery from sludge treatment.