Enzyme cost is expected to account for more than 10% of ethanol production.  Therefore, a reduction in enzyme cost by recycling can be important to make the overall conversion process more economically feasible.  A substantial amount of work has been done with the recycling of hydrolysate stream back into enzyme hydrolysis process.  However, recycled hydrolysate is usually with high concentration of sugar and the activity of new enzymes can be inhibited.  Furthermore, a great amount of enzymes still remains on solid residue and cannot be recycled.  Surfactant was used to improve the amount of recycled enzymes.  However, it was found that surfactant accumulation also decreased sugar conversion in enzymatic hydrolysis.

To remove sugar and/or surfactant, filtration and wash step was introduced in this study, which mimics a wet press and washer in the commercial plant such as a pulping mill.  Six recycling processes were developed and their enzyme recycling rate and enzymatic hydrolysis efficiency were evaluated using green-liquor pretreated softwood and hardwood.  It was found that 10% to 80% of enzyme could be recycled depending on the process design and high enzyme recycle decreased the conversion efficiency due to the ambulation of sugar and surfactant.  When two-stage washing stage was applied with the addition of surfactant, hydrolysis efficiency increased about 40% for softwood at 20 FPU (8 mg/g) enzyme dosage and about 25% for hardwood at 5 FPU (2 mg/g).