Comparative Study of Mesophilic Biomethane Production in Ex Situ Trickling Bed and Bubble Reactors

Abstract

Biomethane production via biogas upgrading is regarded as a future renewable gas, further boosting the biogas economy. Moreover, when upgrading is realized by the biogas CO2 conversion to CH4 using surplus renewable energy, the process of upgrading becomes a renewable energy storage method. This conversion can be carried out via microorganisms, and has attracted scientific attention, especially under thermophilic conditions. In this study, mesophilic conditions were imposed using a previously developed enriched culture. The enriched culture consisted of the hydrogenotrophic Methanobrevibacter (97% of the Archaea species and 60% of the overall population). Biogas upgrading took place in three lab-scale bioreactors: (a) a 1.2 L bubble reactor (BR), (b) a 2 L trickling bed reactor (TBR) filled with plastic supporting material (TBR-P), and (c) a 1.2 L TBR filled with sintered glass balls (TBR-S). The gas fed into the reactors was a mixture of synthetic biogas and hydrogen, with the H2 to biogas CO2 ratio being 3.7:1, lower than the stoichiometric ratio (4:1). Therefore, the feeding gas mixture did not make it possible for the CH4 content in the biomethane to be more than 97%. The results showed that the BR produced biomethane with a CH4 content of 91.15 ± 1.01% under a gas retention time (GRT) of 12.7 h, while the TBR-P operation resulted in a CH4 content of 90.92 ± 2.15% under a GRT of 6 h. The TBR-S operated at a lower GRT (4 h), yielding an effluent gas richer in CH4 (93.08 ± 0.39%). Lowering the GRT further deteriorated the efficiency but did not influence the metabolic pathway, since no trace of volatile fatty acids was detected. These findings are essential indicators of the process stability under mesophilic conditions.