Optimization of mesophilic trickle-bed reactors for enhanced biogas upgrading through ex-situ biological methanation: effect of packing material, gas retention time, nutrient depletion, and pressure

Biological upgrading of biogas to biomethane is a power-to-gas technology with a low environmental footprint, thereby boosting the biomethane economy. This study sys-tematically examined the influence of packing material, nutrient availability, gas re-tention time (GRT), liquid recirculation rate, and operating pressure on the performance of mesophilic trickle-bed reactors (TBRs) for ex situ biological methanation. Plastic packing (Kaldness K1) consistently achieved methane concentrations >96% at short GRTs (1-2 h) at mesophilic conditions. Keeping total ammonium nitrogen above 20 mg 1/L resulted in increased methane content to >97%. Liquid recirculation rates lower than 3 L 1/L 1/h resulted in high variability of methane, despite maintaining a concentration of greater than 95% methane. Mild pressurization (0.1 bar) enhanced productivity, allowing reactor TBR to reach 97.7% methane at a GRT of 1 hour. Microbial analysis revealed strong enrichment of hydrogen-otrophic Methanobacterium (>90% relative abundance), confirming a stable and specialized methanation pathway. Overall, the results highlight key operational and microbial drivers for achieving a methane content above 97% in biomethane at mesophilic temperature under GRT as low as 1 hour.