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

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.