Ex-situ biological hydrogen methanation: Integration into biogas production facilities for CO2 and nutrient recycling

Biological hydrogen methanation (BHM) in trickle bed reactors (TBRs) is a sustainable biocatalytic process for biogas upgrading. Besides increase in CH4 productivity, integration of Ex-situ BHM processes into biogas facilities has the advantage of using inoculum, CO2, and nutrients directly from the existing anaerobic digestion (AD) processes. The indirect start-up of TBRs which has been widely done in laboratory using already enriched or conditioned microbial communities might not be possible for the commercialization application. In this current study, the feasibility, and reproducibility of Ex-situ BHM in TBRs for integration into biogas facilities was done in a systematic approach using three discrete phases. For start-up phase digestate from a biogas facility was used for direct inoculation of TBRs using synthetic media. Process was then adapted to digested biowaste reject water as the nutrient source. During Experimental phase hydrogen loading rates were optimized. Reactor start-up with digestate as inoculum required 108 days and enabled enrichment of hydrogenotrophic methanogens under thermophilic conditions. Process adaptation to reject water was done successfully. More than 95% H2 conversion was achieved for gas retention time (GRT) as low as 1.8 hours, corresponding to H2 loading rate of 10.8 LH2 L-1Rv d-1 showing the highest CH4 productivity of 2.6 L L-1Rv d-1. Supply of trace elements is required for a stable process with increased H2 loading rate when reject water is used. A visible biofilm was formed on carrier material and reject water as a nutrient source directed the evolution of microbial communities both in planktonic and biofilm, promoting the predominance of archaeal genus Methanothermobacteriaceae. In conclusion, thermophilic Ex-situ BHM in TBRs appears feasible for integration into biogas plants.