Influence of Hydrogen and Ethanol Addition in Methanogen-Free Mixed Culture Syngas Fermentations in Trickle Bed Reactors

The use of mixed cultures in gas fermentations could reduce operating costs in the production of liquid chemicals such as alcohols or carboxylic acids. However, directing reducing equivalents towards the desired products presents the challenge of co-existing competing pathways. In this study, two trickle bed reactors were operated at acetogenic and chain elongating conditions to explore the fate of electron equivalents (ethanol, H₂, and CO) and test pH oscillations as a strategy to target chain-elongated products. Hereby, the use of a H₂-rich syngas increased gas conversion rates and the specificity towards acetic acid (86% of C-mol production, 9.0 g L_EBV^-1 day^-1, with EBV referring to empty bed volume), while preliminary experiments with CO-rich syngas show promising results in increasing the ethanol production necessary to target chain-elongated products. On the other hand, ethanol supplementation hindered the endogenous ethanol production of the acetogenic culture but promoted butanol production (1.0 g L_EBV^-1 day^-1) at high ethanol concentrations (9.6 g L^-1) in the fresh media. Finally, pH oscillations improved chain elongation yields but negatively affected acetogenic growth, reducing production rates.