Biological methanation of H₂ and CO₂ in a continuous stirred tank reactor

The biological methanation of H₂ and CO₂ is beneficial to industrial upgrading of anaerobic digestion, efficient utilization of surplus electricity and H₂, energy storage and peak regulation, as well as CO₂ emission reduction. Applying well-developed continuous stirred tank reactor (CSTR) for methanation is economical and convenient. In this study, the biological methanation process was conducted in a CSTR for 151 days. H₂ and CO₂ were the substrates and no external additives were added. The maximum CH₄ yield was 355.8 mL/(L·d) at a CH₄ content of 94.8%, and after double loading shock and recovery, the highest CH₄ content of 99.5% was achieved at a CH₄ yield of 249.3 mL/(L·d). The reactor could run stably at a pH around 8.5, and CO₂ flow was adjusted for pH control. As the experiment proceeded, hydrogenotrophic methanogens were enriched, and the microbial community in different stages had significant succession. Acetate was accumulated after loading shock, causing proportion increase of Natronincola, a genus of homoacetogen. Pseudomonas, an acetate-consuming and electroactive bacterium, was dominant in later period. Methanosarcina and Methanoculleus were in the majority of archaea, accounting for 80%–90%. In stable phase, 72.8% of the input CO₂ flowed to CH₄, 17.8% consumed in rising pH, and 8.9% increased VFAs and microbes. As a complex system with competition and synergy among different reactions, the scientific design and regulation of the system to achieve an efficient methanation process is an important direction for future research.