Impact of biomass- and polymer-based microbial carriers on CO2 biomethanation in trickle-bed reactors: Performance and energy efficiency

Anaerobic digestion is a well-established method for converting sewage sludge into energy. However, the CO₂ in biogas reduces its calorific value and limits its utilization. Trickle-bed reactors (TBRs) present a promising approach for ex-situ CO₂ methanation due to their high gas-liquid mass transfer efficiency and low energy requirements. This study investigated two microbial carriers, biomass-based (BC) and polymer-based (PS), for ex-situ CO₂ methanation in mesophilic TBRs. Laboratory-produced activated biochar was used in TBR for the first time. The maximum CO₂ utilization capacities reached 10.6 ± 1.1 L/L_packed-bed/d for BC-TBR at a gas retention time of 1.16 h and 12.9 ± 0.5 L/L_packed-bed/d for PS-TBR at 1.96 h. Methane concentrations remained above 97.6 % in BC-TBR and 95.7% in PS-TBR, with almost no CO₂ detected during stable periods. The highest biological methane production capacities were 9.4 ± 1.2 L/L_packed-bed/d and 10.2 ± 0.6 L/L_packed-bed/d for BC-TBR and PS-TBR, respectively. Hydrogen utilization efficiencies exceeded 98.2% for both reactors. Integrated analysis of gas composition, liquid-phase chemistry, and microbial community revealed indicators of system destabilization and potential microbial early-warning taxa. Economic analysis demonstrated that integrating TBRs with municipal sewage sludge-to-energy system improved methane quality and revenue potential, and sensitivity analysis identified hydrogen prices as the dominant factor affecting profitability. Overall, the simple configuration, low energy demand and operational robustness TBRs highlight their promise as a cost-effective and scalable technology for sustainable CO₂ biomethanation.