Exogenous addition of H for an in situ biogas upgrading through biological reduction of carbon dioxide into methane.

Biological reduction of CO into CH by exogenous addition of H is a promising technology for upgrading biogas into higher CH content. The aim of this work was to study the feasibility of exogenous H addition for an in situ biogas upgrading through biological conversion of the biogas CO into CH Moreover, this study employed systematic study with isotope analysis for providing comprehensive evidence on the underlying pathways of CH production and upstream processes. Batch reactors were inoculated with digestate originating from a full-scale biogas plant and fed once with maize leaf substrate. Periodic addition of H into the headspace resulted in a completely consumption of CO and a concomitant increase in CH content up to 89%. The microbial community and isotope analysis shows an enrichment of hydrogenotrophic Methanobacterium and the key role of hydrogenotrophic methanogenesis for biogas upgrading to higher CH content. Excess H was also supplied to evaluate its effect on overall process performance. The results show that excess H addition resulted in accumulation of H, depletion of CO and inhibition of the degradation of acetate and other volatile fatty acids (VFA). A systematic isotope analysis revealed that excess H supply led to an increase in dissolved H to the level that thermodynamically inhibit the degradation of VFA and stimulate homo-acetogens for production of acetate from CO and H. The inhibition was a temporary effect and acetate degradation resumed when the excess H was removed as well as in the presence of stoichiometric amount of H and CO. This inhibition mechanism underlines the importance of carefully regulating the H addition rate and gas retention time to the CO production rate, H-uptake rate and growth of hydrogenotrophic methanogens in order to achieve higher CH content without the accumulation of acetate and other VFA.