Low electric current in a bioelectrochemical system facilitates ethanol production from CO using CO-enriched mixed culture.

Fossil resources must be replaced by renewable resources in production systems to mitigate green-house gas emissions and combat climate change. Electrofermentation utilizes a bioelectrochemical system (BES) to valorize industrial and municipal waste. Current electro-fermentation research is mainly focused on microbial electrosynthesis using CO₂ for producing commodity chemicals and replacing petroleum-based infrastructures. However, slow production rates and low titers of metabolites during CO₂ -based microbial electrosynthesis impede its implementation to the real application in the near future. On the other hand, CO is a highly reactive gas and an abundant feedstock discharged from fossil fuel-based industry. Here, we investigated CO and CO₂ electro-fermentation, using a CO-enriched culture. Fresh cow fecal waste was enriched under an atmosphere of 50% CO and 20% CO₂ in N₂ using serial cultivation. The CO-enriched culture was dominated by Clostridium autoethanogenum (≥89%) and showed electroactivity in a BES reactor with CO₂ sparging. When 50% CO was included in the 20% CO₂ gas with 10  mA applied current, acetate and ethanol were produced up to 12.9  ±  2.7  mM and 2.7  ±  1.1  mM, respectively. The coulombic efficiency was estimated to 148%  ±  8% without an electron mediator. At 25  mA, the culture showed faster initial growth and acetate production but no ethanol production, and only at 86%  ±  4% coulombic efficiency. The maximum optical density (OD) of 10  mA and 25  mA reactors were 0.29  ±  0.07 and 0.41  ±  0.03, respectively, whereas it was 0.77  ±  0.19 without electric current. These results show that CO electro-fermentation at low current can be an alternative way of valorizing industrial waste gas using a bioelectrochemical system. [ABSTRACT FROM AUTHOR]