Influence of Increased Cell Densities on Product Ratio and Productivity in Syngas Fermentation

Synthesis gas fermentation with anaerobic microorganisms such as Clostridium ljungdahlii enables highly selective conversion of substrate gases such as CO, CO2, and H2 to high value intermediates like organic acids and alcohols. By using a biomass retention system with the aid of cross-flow ultrafiltration in an external circuit for biomass retention at a CSTR, we have succeeded in retaining the biocatalyst in the reactor and in increasing the cell density by more than 160% and the space-time yield of C2 products by 46%. In addition, we found that the use of a cell retention system promoted ethanol formation and decreased the acetic acid to ethanol product ratio. Low partial pressures of CO in the exhaust gas were found to promote hydrogen uptake and ethanol formation. The space-time yield of ethanol, 8.71 mmol L–1 h–1, is the highest yield measured to date with an unmodified strain of C. ljungdahlii.