Kinetics of consumption of fermentation products by anode-respiring bacteria

Abstract

We determined the kinetic response of a community of anode-respiring bacteria oxidizing a mixture of the most common fermentation products: acetate, butyrate, propionate, ethanol, and hydrogen. We acclimated the community by performing three consecutive batch experiments in a microbial electrolytic cell (MEC) containing a mixture of the fermentation products. During the consecutive-batch experiments, the coulombic efficiency and start-up period improved with each step. We used the acclimated biofilm to start continuous experiments in an MEC, in which we controlled the anode potential using a potentiostat. During the continuous experiments, we tested each individual substrate at a range of anode potentials and substrate concentrations. Our results show low current densities for butyrate and hydrogen, but high current densities for propionate, acetate, and ethanol (maximum values are 1.6, 9.0, and 8.2 A/m², respectively). Acetate showed a high coulombic efficiency (86%) compared to ethanol and propionate (49 and 41%, respectively). High methane concentrations inside the MEC during ethanol experiments suggest that methanogenesis is one reason why the coulombic efficiency was lower than that of acetate. Our results provide kinetic parameters, such as the anode overpotential, the maximum current density, and the Monod half-saturation constant, that are needed for model development when using a mixture of fermentation products. When we provided no electron donor, we measured current due to endogenous decay of biomass (~0.07 A/m²) and an open-cell potential (−0.54 V vs Ag/AgCl) associated with biomass components active in endogenous respiration.