Fortifying spent sulfite pulping liquor with hydrolyzed reject knots

A limitation for spent sulfite liquor (SSL) fermentation, particularly hardwood SSL fermentation is the low sugar content. The objective of this work was to increase the sugar content of SSL through the addition and hydrolysis of reject knots from the pulp line. Factorial experiments indicated that hydrolysis yield was significantly affected by cellulase and β-glucosidase loading, concentration of SSL, knot concentration, pH and temperature. Spent sulfite liquor reduced the hydrolysis rate and yield, however this inhibition decreased with time. The resultant hydrolysates were readily fermented alone or in combination with 10% SSL. In 22% SSL, fermentation yields were reduced. Economic analysis indicated that knots would be an economically-viable source of supplemental sugars at a cellulase cost of up to $5 CDN per million FPU.