Ethanol production from cheese whey powder solution in a packed column bioreactor at different hydraulic residence times

Cheese whey powder (CWP) solution containing 50 g L−1 total sugar was fermented to ethanol in a continuously operated packed column bioreactor (PCBR) using olive pits as support particles for cell attachment. Pure culture of Kluyveromyces marxianus (DSMZ 7239) was used in the PCBR for ethanol formation from lactose content of CWP solution. Sugar utilization and ethanol formation were investigated as function of the hydraulic residence time (HRT) between 17.6 and 64.4 h. Sugar concentration decreased with increasing ethanol concentration along the height of the column. Percent sugar utilization increased while effluent sugar concentration was decreasing with HRT between 17.6 and 50 h. Similarly, effluent ethanol concentration increased while ethanol productivity was decreasing with increasing HRT up to 50 h. Further increases in HRT above 50 h resulted in decreases in effluent ethanol concentration. The ethanol yield coefficient also increased with increasing HRT and reached the highest level of 0.54 gE g−1S at an HRT of 50 h. Due to cell settling to the bottom of the column, high fermentation rates were obtained in the lower section of the system. Therefore, the system can be operated with a height of 36 cm from the inlet to obtain high ethanol contents in the effluent with an HRT of 18 h.