Exploitation of mixed cultures in the fermentation of syngas for the production of biomethane

In the last decades increasing attention has been paid on syngas, produced through the thermal gasification of biomass, for energy recovery. Syngas components, mainly CO and H2, can be used as substrate for the conversion into important chemicals and fuels, in this case specifically methane, by a wide range of microorganisms. This study focused on the exploitation of an anaerobic mixed culture, collected from an anaerobic sludge of the Denmark Technical University (DTU) Bioengineering laboratory, in the fermentation of syngas for the production of CH4. A series of enrichment experiments with different initial partial pressure of CO and H2 were performed, in mesophilic and thermophilic conditions. Both yields and production rates resulted to be higher in thermophilic conditions; experiments with H2 tended to be faster than those without and the mmols of produced methane, over fed carbon (mmol/mmol), were higher than when only CO was present as substrate. A second experimental test has also been executed in thermophilic conditions (since they better performed in the previous experiment) to better evaluate which reactions are those which mainly took place during the syngas fermentation process. Experiments were performed both in the absence and in the occurrence of 2-bromoethanesulfonic acid as an effective inhibitor of the methanogens. Results showed that acetate mainly came from Acetogenesis via CO, being however negligible, while methane was produced either by hydrogenotrophic methanogens either via H2/CO. Production of bio hydrogen was also present, and it is usually further consumed to produce CH4. To conclude, findings of this projects may be considered as a further step towards the knowledge of biogas production through the use of microbial catalysts, giving important information both about the optimal operating conditions and about microbial metabolism.