Cellulosic ethanol production performance with SSF and SHF processes using immobilized Zymomonas mobilis

Bioethanol converted from lignocellulosic feedstock, such as agricultural waste, is considered one of the most promising biofuels being developed. The raw materials usually need to be pretreated and hydrolyzed by cellulolytic enzymes to produce sugars for subsequent ethanol fermentation. Immobilized bacteria or yeasts have been frequently used for batch or continuous ethanol fermentation due to their feasibility for repeated use with high biomass retention during the process. In this study, Zymomonas mobilis cells immobilized in calcium alginate (CA) and polyvinyl alcohol (PVA) were used to produce ethanol from cellulosic feedstock using SSF (simultaneous saccharification and fermentation) and SHF (separate hydrolysis and fermentation) processes. The performance based on different immobilized cells and different processes was compared. The results show that PVA immobilized cells with the SHF process gave the highest ethanol concentration of 6.24 g/L, with an ethanol yield of 79.09% and a maximum ethanol productivity of 3.04 g/L/h. In contrast, the performance of CA-immobilized cells with SHF was poorer, with the highest ethanol concentration, ethanol yield, and maximum ethanol productivity of 5.52 g/L, 69.96% and 2.37 g/L/h, respectively. For the SSF process, the maximum ethanol concentration, ethanol yield, and maximum ethanol productivity were 5.53 g/L, 70.09%, and of 1.31 g/L/h, respectively, for the PVA immobilized Z. mobilis, while they were 5.44 g/L, 68.95%, and 1.27 g/L/h for CA immobilized cells. The comparison with suspended cells shows that the immobilized cells of Z. mobilis are feasible for ethanol production via SSF and SHF.