Biogas Upgrading via Hydrogenotrophic Methanogenesis in Two-Stage Continuous Stirred Tank Reactors at Mesophilic and Thermophilic Conditions

This study proposes an innovative setup composed by two stage reactors to achieve biogas upgrading coupling the CO₂ in the biogas with external H₂ and subsequent conversion into CH₄ by hydrogenotrophic methanogenesis. In this configuration, the biogas produced in the first reactor was transferred to the second one, where H₂ was injected. This configuration was tested at both mesophilic and thermophilic conditions. After H₂ addition, the produced biogas was upgraded to average CH₄ content of 89% in the mesophilic reactor and 85% in the thermophilic. At thermophilic conditions, a higher efficiency of CH₄ production and CO₂ conversion was recorded. The consequent increase of pH did not inhibit the process indicating adaptation of microorganisms to higher pH levels. The effects of H₂ on the microbial community were studied using high-throughput Illumina random sequences and full-length 16S rRNA genes extracted from the total sequences. The relative abundance of archaeal community markedly increased upon H₂ addition with <i>Methanoculleus</i> as dominant genus. The increase of hydrogenotrophic methanogens and syntrophic <i>Desulfovibrio</i> and the decrease of aceticlastic methanogens indicate a H₂-mediated shift toward the hydrogenotrophic pathway enhancing biogas upgrading. Moreover, <i>Thermoanaerobacteraceae</i> were likely involved in syntrophic acetate oxidation with hydrogenotrophic methanogens in absence of aceticlastic methanogenesis