Enrichment of hydrogenotrophic methanogens by means of gas recycle and its application in biogas upgrading

Biomethanation by hydrogenotrophic methanogens has been proven as a potential process for managing renewable power intermittency and upgrading biogas. The present work aimed to enrich hydrogenotrophic methanogens under different mixing conditions (gas recycle vs. mechanical mixing) and temperatures (mesophilic vs. thermophilic conditions) for biogas upgrading. The synthetic gas (H2:CO2 = 4:1) was fed to the reactor bottom at a hydrogen injection rate (HIR) of 1.6 L H2 L−1 d−1. The gas recycle (100 L L−1 d−1) under thermophilic condition was found to be the most effective, reaching over 96% H2 conversion to CH4 within 15 d of operation. Archaea community analysis performed by 454 pyrosequencing showed that the sequence of Methanosaeta sp. decreased while obligate-hydrogenotrophic methanogens increased: Methanoculleus chikugoensis (19.5%) and Methanothermococcus thermolithotrophicus (28.1%) under mesophilic and thermophilic condition, respectively. To the thermophilic enriched culture, the biogas produced from an up-flow anaerobic sludge blanket reactor with additional hydrogen (four times of CO2) was fed at various HIRs for 200 d. As HIR increased, H2 consumption rate also increased with CO2 removal contained in the biogas. Up to an HIR increase to 19.2 L H2 L−1 d−1, the high calorific biomethane (96% of CH4) could be obtained at gas recycle rate of 200 L L−1 d−1.