Controlling Ethanol Use in Chain Elongation by CO₂ Loading Rate

Area of Science:

• Biotechnology
• Biochemical Engineering
• Microbial Metabolism

Background:

• Chain elongation is an open-culture bioprocess converting volatile fatty acids (VFAs) to medium chain fatty acids (MCFAs) using reduced substrates like ethanol.
• Understanding the role of carbon dioxide (CO2) is crucial for optimizing this bioconversion process.

Purpose of the Study:

• To quantitatively assess the impact of varying CO2 loading rates on ethanol utilization and product formation in a chain elongation process.
• To determine optimal CO2 conditions for upgrading ethanol to caproate versus upgrading VFAs to heptanoate.

Main Methods:

• A continuously stirred anaerobic reactor was fed with ethanol and propionate (a VFA).
• Different CO2 loading rates (0.5 to 2.5 LCO2·L-1·d-1) were applied to evaluate their effect on microbial activity and product yields.
• Ethanol oxidation and MCFAs production (caproate and heptanoate) were quantified.

Main Results:

• High CO2 loading (2.5 LCO2·L-1·d-1) significantly increased excessive ethanol oxidation (EEO) to 29% and caproate production to 10.8 g·L-1·d-1.
• Low CO2 loading (0.5 LCO2·L-1·d-1) reduced EEO to 16% and caproate production to 2.9 g·L-1·d-1.
• Heptanoate production from VFA upgrading remained stable at approximately 1.8 g·L-1·d-1 for CO2 loading rates ≥ 1 LCO2·L-1·d-1.

Conclusions:

• CO2 is essential for the growth of chain elongating microorganisms and stimulates syntrophic ethanol oxidation.
• High CO2 rates are recommended for maximizing caproate production from ethanol, such as upgrading bioethanol.
• Lower CO2 rates are preferable for upgrading VFAs to MCFAs while minimizing costly ethanol consumption.