Controlling Ethanol Use in Chain Elongation by CO2 Loading Rate

Abstract

Chain elongation is an open-culture biotechnological process which converts volatile fatty acids (VFAs) into medium chain fatty acids (MCFAs) using ethanol and other reduced substrates. The objective of this study was to investigate the quantitative effect of CO₂ loading rate on ethanol usages in a chain elongation process. We supplied different rates of CO₂ to a continuously stirred anaerobic reactor, fed with ethanol and propionate. Ethanol was used to upgrade ethanol itself into caproate and to upgrade the supplied VFA (propionate) into heptanoate. A high CO₂ loading rate (2.5 L_CO2·L^-1·d^-1) stimulated excessive ethanol oxidation (EEO; up to 29%) which resulted in a high caproate production (10.8 g·L^-1·d^-1). A low CO₂ loading rate (0.5 L_CO2·L^-1·d^-1) reduced EEO (16%) and caproate production (2.9 g·L^-1·d^-1). Heptanoate production by VFA upgrading remained constant (∼1.8 g·L^-1·d^-1) at CO₂ loading rates higher than or equal to 1 L_CO2·L^-1·d^-1. CO₂ was likely essential for growth of chain elongating microorganisms while it also stimulated syntrophic ethanol oxidation. A high CO₂ loading rate must be selected to upgrade ethanol (e.g., from lignocellulosic bioethanol) into MCFAs whereas lower CO₂ loading rates must be selected to upgrade VFAs (e.g., from acidified organic residues) into MCFAs while minimizing use of costly ethanol.