Bioethanol production performance in a packed bed solid-state fermenter: evaluation of operational factors and intermittent aeration strategies

A mixture of carob pods and wheat bran as filler was evaluated as substrates for bioethanol production using Zymomonas mobilis in a 0.5 L-glass packed bed column fermenter. The experiments were initially performed under the optimum conditions defined previously in a flask-scale fermentation system. Under these conditions, removal of the generated heat and CO2 from the solid medium was more challenging than in the flask-scale system. In addition, the traditional aeration method was not applicable for removing the heat and CO2 since bioethanol production should be conducted under anaerobic conditions. The packed bed column system was modified, and an intermittent aeration method was used to remove the entrapped CO2 from the solid bed, thereby enabling the anaerobic condition to be maintained. The effect of operating temperature and carob particle size on the amount and rate of ethanol produced was also studied. We determined that the optimum operating conditions for this packed bed column fermentation system were 28 °C, a carob particle size of 1 mm, and 0.1 L min−1 intermittent aeration for 15 min each hour during the second 15 h of the fermentation process. Under these conditions 60.9 g ethanol per kilogram dry mixture of carob pod and wheat bran was produced.