Thermodynamic analysis of extracellular electron transfer during ethanol oxidation in anaerobic digestion systems

Abstract

Anaerobic ethanol oxidation is thermodynamically unfavourable under standard conditions, while it can be exergonic with the participation of extracellular electron transfer (EET) through microbial redox-active proteins (RAP). In this study, effects of the fraction of substrate degraded through the EET pathway (EET fraction), product feedback, the distribution of produced electrons from the EET pathway between acetotrophic and hydrogenotrophic methanogenesis, and the redox potential of RAP on anaerobic ethanol oxidation were thermodynamically evaluated. Ethanol oxidation could occur when the redox potential was above −0.408 V with EET, or when the product concentration was below the threshold value. The appropriate redox potential ranges for active acetotrophic and hydrogenotrophic methanogens were below −0.214 V and −0.222 V, respectively. The EET fraction was a key factor in maintaining biomass yields, and the distribution of electrons produced from ethanol oxidation affected the biomass ratio of two types of methanogens. Finally, strategies for one-reactor and zone-separation systems were proposed to optimize system performance and bioenergy recovery from waste resources.