In-situ biomethanation of high CO content syngas in agricultural biogas digesters

To meet global energy demands, syngas with high CO content (>50 %) generated from hydrothermal gasification of agricultural residues or wood, could be utilized as a co-substrate in anaerobic digestion (AD) processes for conversion into CH₄. This study investigated in-situ biomethanation using synthetic gas (55 % CO, 35 % H_2, and 15 % CO₂), mimicking wood gasification syngas, during AD of cow manure, the most abundant agricultural waste. A continuously stirred tank reactor (CSTR) served as a control and was fed only cow manure, while the other two reactors were fed syngas with or without gas recirculation at different gas loading rates (GLRs). In-situ syngas injection in AD of cow manure at GLRs ≤0.3 L L_RV⁻¹ d⁻¹ improved overall CH₄ productivity compared to the control (0.35 ± 0.1 vs. 0.29 ± 0.1 L L_RV⁻¹ d⁻¹), though the CH₄ content (45 ± 5 %) was lower than the control reactor (62 ± 3 %), indicating a need for biogas upgrading. Gas recirculation enabled higher CO and H₂ conversion efficiencies of 99–100 % but considerably decreased CH₄ productivity from cow manure. In-situ biomethanation enhanced the relative abundance of hydrogenotrophic methanogens, highlighting their role in syngas conversion to CH₄.