RECIRCULATION OF H2, CO2, AND ETHYLENE IMPROVES YIELDS AND CARBON FIXATION OF ANAEROBIC FERMENTATION

Anaerobic fermentation with mixed cultures has been gaining momentum as a bioprocess for its promise to produce platform chemicals from low-value biomass feedstocks. Currently, this technology is limited by 1) low selectivity to the most valuable carboxylates and alcohols; 2) amount of electron donors from the substrate; and 3) competing anaerobic pathways. The most common solutions found for these challenges in lab-scale studies are not always upscalable with the process. In a new approach to tackle these challenges we operated two bioreactors fed with acetate and lactate as a feedstock model with continuously recirculating H2/CO2 to stimulate concomitant autotrophic activity. By 42 days of operation, hydrogenotrophic methanogenesis was predominant and ethylene (=1.3 kPa) was added to one of the reactors, inhibiting methanogenesis completely and allowing net carbon fixation. With methanogenesis inhibited, H2 accounted for 17% (e- equivalent) of the total consumed electron donor. Butyrate/lactate selectivity was 100% (88% in control without ethylene) and caproate/lactate selectivity was 17% (2.3% in control). Community analysis revealed that ethylene caused Methanobaterium to be washed out, giving room to acidogenic genera Eubacterium, Clostridium sensu stricto 12, and Colidextribacter. Culture bottle experiments suggest ethylene to be a more specific methanogenesis inhibitor than 2-bromoethanesulfonate, in particular regarding the non-inhibiting property of ethylene on i-butyrate production. Caproate concentration in the reactor experiment was up to 1.1 g/L, still below extractable titers. However, we believe that longer-term microbial community management, operation with CO, and parameter optimization may enable the concept to become an upscalable strategy to improve efficiency, carbon fixation, and resource conversion of anaerobic fermentation.