Increased production of sugar streams from lignocellulosic materials through enzyme recycling

Abstract

The Canadian pulp and paper mills generate over 1 million tons of primary sludge per year. This sludge mainly contains small cellulosic fibers and contaminants from pulp and paper manufacture and is usually disposed of by landfilling or incineration. Both of these processes are economically and environmentally objectionable. However, the mill sludge could serve as low or negative-cost cellulosic feedstock that requires no pre-treatment and is largely available. The objective of this study was to examine primary sludge as a source of fermentable sugars that are used for production of biofuels and value-added biochemicals. Sludge was hydrolyzed with a commercial enzyme in presence of surfactants that are known to enhance the efficiency of enzymatic hydrolysis. The hydrolysis process was statistically optimized using response surface methodology. In addition, opportunities for enzyme recovery and reuse were also investigated. The primary sludge contained 30% of dry solids of which 51% was glucan. The study of different surfactants revealed that glucose yield from sludge can be improved by up to 12% in presence of polyethylene glycol 4000. Statistical model pointed the solid and enzyme loadings as the most significant factors (p < 0.05) in enzymatic hydrolysis of sludge. Under optimum conditions of 7.4% w/w dry solid loadings, 2.6% enzyme loadings (19.2 FPU g-1dry primary sludge, and 5% w/w surfactant loadings (polyethylene glycol 4000), an 85.6% glucose recovery of the theoretical maximum was attained. Furthermore, 34.7% enzyme was recovered from the sludge hydrolysate using a 3 kDa molecular weight cut off ultrafiltration membrane. The recovered enzyme was reused on fresh sludge sample to produce a glucose yield of 82.3%. Present work suggests that primary sludge can add value in the form of fermentable sugars. It presents an opportunity to reuse nearly 35% of the enzyme, which could significantly reduce the production costs of fermentable sugars. Thus, enzymatic conversion of primary mill sludge can offer the pulp and paper industry an alternative approach for turning waste into value in a cost-efficient and environmentally friendly way.