**4.4 Comparison between SSF and SHF processes**

Cellulose hydrolysis and fermentation can be achieved through two different process schemes, depending on where the fermentation occurs: separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF) [16].

In SHF, hydrolysis is performed in one reactor and the hydrolysates are fermented in the next second reactor. In SHF, feedstock and utility costs are high due to the cellulosic conversion that shows only about 73% to ethanol in 48 hours, while the remainders are burned. In SSF, hydrolysis and fermentation are carried out in a single reactor, and the operating cost is in general lower than the SHF case. In SSF, yeast ferments the glucose into ethanol as soon as the glucose is produced, which results in preventing the sugars from accumulating/inhibiting the final product.

The SSF system offers a large advantage over SHF processes, because of their reduction of final product inhibition of the cellulase enzyme complex [16]. The SSF process shows a higher yield (88 vs. 73%) and greatly increases product concentrations (equivalent glucose concentration, 10 vs. 4.4%). The most significant advantage is that enzyme loading can be reduced from 33 to 7 IU/g-cellulose, which results in lowering the ethanol cost significantly.

A hybrid hydrolysis and fermentation (HHF) process is also proposed in converting lignocellulosic biomass into ethanol. This process configuration begins with a separate hydrolysis step which involves a higher temperature enzymatic cellular saccharification and ends with SSF step which involves a simultaneous step of mesophilic enzymatic hydrolysis and sugar fermentation.
