Two stirred-tank bioreactors in series enable continuous production of alcohols from carbon monoxide with Clostridium carboxidivorans

Abstract

Microbial batch production of alcohols by fermentation of CO-rich gases with Clostridia is limited by low volumetric productivities due to the need for formation of organic acids first (acidogenic phase) followed by re-consumption of the acids to form alcohols (solventogenic phase). Continuous autotrophic production of alcohols was made possible with C. carboxidivorans by use of two continuously operated stirred-tank bioreactors in series without cell retention. The pH in the first reactor was controlled to pH 6.0 for continuous growth of the cells. Steady-state concentrations of 3.0 g L⁻¹ acetate and 0.1 g L⁻¹ butyrate were measured at a mean hydraulic residence time of 8.3 h. The pH in the second reactor was controlled to pH 5.0 for enhancing continuous formation of alcohols resulting in steady-state concentrations of 6.1 g L⁻¹ ethanol, 0.7 g L⁻¹ butanol, and 0.1 g L⁻¹ hexanol at a mean hydraulic residence time of 12.5 h. Continuous formation of alcohols from CO was already observed in the first stirred-tank reactor parallel to the formation of acids, whereas re-consumption of acids as well as de-novo syntheses of alcohols from CO was shown in the second stirred-tank reactor. Thus, high final alcohol-to-acid ratios of 3.9 g_ethanol g_acetate⁻¹ and 4.4 g_butanol g_butyrate⁻¹ were achieved in the continuous syngas-fermentation process with C. carboxidivorans.