Influence of nutrient medium supply rate and liquid recirculation regime on syngas biomethanation in thermophilic trickle-bed reactor

Syngas biomethanation enables the use of recalcitrant biomass or waste for methane production. To reveal knowledge on the importance of nutrient medium supply rate (NMSR) and liquid recirculation regime, a thermophilic 5 L trickle-bed reactor was operated for 283 days. Efficient and stable operation with >99 % H 2 and CO conversion rates was achieved at a minimum NMSR of 14 mL/(L pbv ·d) and 1 h gas retention time, yielding a maximum methane evolution rate (MER) of 4.3 L/(L pbv ·d). Reduced intermittent liquid recirculation resulted in lowered MERs (max. 3.4 L/(L pbv ·d)) with CO conversion more affected by low recirculation frequencies than H 2 conversion. The microbial analysis revealed a similar microbial community structure across all experimental phases, dominated by Methanothermobacter in both liquid and carrier biofilm. CO was likely converted to methane and acetate, with syntrophic acetate-oxidizing bacteria metabolizing acetate to H 2 and CO 2 , supporting efficient hydrogenotrophic methanogenesis. • >99 % H 2 and CO conversion rates at a minimum nutrient supply rate of 14 mL/(L pbv ·d) • High Methane Evolution Rate (4.3 L/(L pbv ·d)) with digestate as nutrient source • Reduced liquid recirculation caused decreased MER (3.4 L/(L pbv ·d)) • Methanothermobacter dominated in liquid and on biocarriers. • Syntrophic acetate oxidation was a key to efficient gas conversion.