Continuous conversion of CO₂/H₂ with Clostridium aceticum in biofilm reactors

A lab-scale stirred-tank bioreactor was reversibly retrofitted to a packed-bed and a trickle-bed biofilm reactor to study and compare the conversion of CO₂/H₂ with immobilised Clostridium aceticum. The biofilm reactors were characterised and their functionality confirmed. Up to 8.6 g of C. aceticum were immobilised onto 300 g sintered ceramic carrier material, proving biofilm formation to be a robust means for cell retention of C. aceticum. Continuous CO₂/H₂-fermentation studies were performed with both biofilm reactor configurations as function of dilution rates, partial gas pressures and gas flow rates. The experiments showed that in the packed-bed biofilm reactor, the acetate space-time yield was independent of the dilution rate, because of low H₂ gas-liquid mass transfer rates (≤17 mmol H₂ L⁻¹ h⁻¹). The continuous operation of the trickle-bed biofilm reactor increased the gas-liquid mass transfer rates to up to 56 mmol H₂ L⁻¹ h⁻¹. Consequently, the acetate space-time yield of up to 14 mmol acetate L⁻¹ h⁻¹ was improved 3-fold at hydrogen conversions of up to 96%.