Biogas upgrading from anaerobic digestion of waste frying oils (WFO) was accessed in this study. For that, two bioreactors (Rb-biogas-lift bioreactor with gas and liquid recirculation, Rc-control reactor with liquid recirculation) were fed three times per week, with a mixture of WFO, glycerol and volatile fatty acids (VFA). Rb produced 1.4 times more biogas with higher methane content (79%) than Rc (67%). Higher relative abundance of hydrogenotrophic methanogens (34%e39%) was observed in Rb, when compared to Rc (16%e21%). The relative abundance of Sprochaetia class, which includes some homoacetogens/ syntrophic acetate oxidizing genera, was also higher in Rb. This work shows that biogas recirculation applied in the biogas-lift bioreactor facilitated WFO degradation, most probably due to the selective enrichment of hydrogenotrophic methanogens. Recirculation of CO 2 present in the biogas and reverse homoacetogenesis (i.e. syntrophic acetate oxidation) seem to be the main factors involved in the stimulation of hydrogenotrophic methanogenesis.

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