Comparative assessment of endogenous, integrated, and exogenous biomethanation strategies: performance, microbial...

Biological methanation is a promising approach for enhancing methane production rates and upgrading biogas during anaerobic digestion. However, few studies have examined the key biological rate-limiting factors or systematically compared different CO₂ biomethanation strategies. In this study, anaerobic digestion of rice straw was subjected to H₂/CO₂ supplementation to evaluate three CO₂ biomethanation strategies, i.e., endogenous, exogenous, and integrated. All CO₂ biomethanation strategies increased methane production rate and achieved methane content over 83.9 %. Exogenous CO₂ biomethanation achieved the highest methane production rate of 983 ± 68 mL·L^-1·d^-1, which was 121 % and 41 % higher than endogenous and integrated CO₂ biomethanation, respectively. In the integrated strategy, CO₂ addition supplied buffer capacity to limit the pH increase. Rice straw hydrolysis and specific acetoclastic methanogenic activity was partially inhibited. In the CO₂ biomethanation system, Methanobacterium formicicum was enriched, which is the dominant archaeal genus (50 %-69 % relative abundance). The relative abundance of 5,10-methylenetetrahydromethanopterin reductase (EC:1.5.98.2) suggested a potential functional limitation within the hydrogenotrophic methanogenesis pathway. Overall, this comparative assessment of these biomethanation strategies offers valuable references to optimizing full-scale biogas upgrading systems.