A economically viable method for producing sugars using concentrated acid hydrolysis of biomass containing cellulose and hemicellulose is disclosed. The cellulose and hemicellulose in the biomass is first decrystallized and then hydrolyzed to produce a hydrolysate containing both sugars and acid. Silica present in the biomass can then be removed for further processing. The remaining solids are then subjected to a second decrystallization and hydrolyzation to optimize the sugar yields. An improved method for separating the sugars from the acid in the hydrolysate is also disclosed. The resulting sugar stream can then be fermented, using an improved method which allows both hexose and pentose sugars to be fermented simultaneously.