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,H- 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 H-2 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.