Syngas fermentation and microbial electrosynthesis integration as a single process unit.

Industrially relevant syngas (15 % CO, 15% H2, 20% N2 in 50% CO2) fermentation and microbial electrosynthesis were integrated as a single process unit in open and closed-circuit modes. This study examined the impact of electrochemical reducing power from -50 to -400 mV on the acetic acid synthesis and CO inhibition on fermentation. -150 mV vs. Ag/AgCl (3.0 NaCl) was identified as the lowest benchmark potential for improved acetic acid synthesis rate (0.263 mmol L-1h-1), which is 15-fold higher than the open circuit mode's rate. No significant inhibition by CO in the fermentation was observed, while 60% of the gas was consumed. Anodic potential above 2.0 V substantially lowered the product formation. Superseding the fermentation medium with fresh inoculum through a fed-batch operation helped lower the anodic potential.

Impact of electrochemical reducing power on homoacetogenesis.

Homoacetogenesis was performed in a microbial electrosynthesis single-chamber reactor at open and closed circuits modes. The aim is to investigate how an applied reducing power affects acetic acid synthesis and H2 gas-liquid mass transfer. At a cathode voltage of -175 mV vs. Ag/AgCl (3.0 NaCl), the acetic acid synthesis rate ramped up to 0.225 mmol L-1h-1 due to additional electrons and protons liberation from carbon-free sources such as water and ammonium via anodic oxidation. The study sets a new lowest benchmark that acetic acid can be bioelectrochemical synthesized at - 175 mV. The applied reducing power did not increase the H2 gas-liquid mass transfer because the direct electron transfer from cathode to microorganisms reduced the demand for H2 in the fermentation medium. Microbial analysis shows a high presence of Veillonellaceae spore-forming clostridia, which are identified as homoacetogens.