Continuous biogas production and ex-situ biomethanation in a trickling bed bioreactor under mesophilic and thermophilic conditions

Abstract

The utilization of biogas as a renewable energy source necessitates a reduction in its CO2 content. "Ex-situ biomethanation" is employed to convert CO2 in biogas to CH4 through hydrogenotrophic methanogens. In this study, a biogas stream (32–36 % CO2) originating from cow manure digestion was subjected to treatment in a trickle bed bioreactor (4 liters). The presence of microorganisms on the packing material was confirmed by using a 16 S rRNA test and SEM images. This study stands out by considering all four combinations of thermophilic and mesophilic conditions, monitoring the transition and steady-state phases in a two-reactor series setup. The mesophilic-thermophilic mode yielded the highest purity (92 %) with an 86 % CO2 conversion rate and an energy density of 13,870 kJ/m3. Additionally, the performance consistency was also assessed using a larger TBB (8 liters). Extending the residence time, the thermophilic-thermophilic mode yielded the highest CH4 concentration at 98 %. The 16 S rRNA results revealed that, influenced by the designated growth conditions encompassing culture media, pH, temperature, and reactor retention time, an enrichment of hydrogenotrophic methanogens occurred. Furthermore, promising results in terms of CH4 concentration and process efficiency were demonstrated by the trickled bed bioreactor employing liquid-in-gas dispersion.