Bioconversion of knot rejects from a sulphite pulp mill to ethanol

Knot rejects obtained from pulp screening after sulphite pulping are difficult to dewater, which makes landfilling expensive and burning inefficient. The rejects were found to contain up to 50% cellulose, which is very susceptible to enzymatic hydrolysis to glucose. Knot rejects were hydrolyzed at 20% consistency in a laboratory peg mixer with cellulase enzyme. The thick slurry was liquefied within the first hour of mixing and resulted in a glucose concentration of over 100 g/L after 24 hours of reaction. This solution was fermented by yeast to give an ethanol concentration of over 5%. The laboratory results were confirmed at pilot scale with a mortar mixer (high consistency) or stirred tank reactor (medium consistency) at the 400 L and 6000 L scales, respectively. It was found that washing the knots with ammonia resulted in increased glucose conversion. Enzyme costs could be lowered by separating the enzyme from the hydrolyzed sugars by membrane ultrafiltration and recycling the enzyme to the subsequent batch of substrate. The combination of high-consistency hydrolysis and enzyme recycling minimizes capital investment, energy requirements, and enzyme costs, which are significant factors in the overall economic viability of cellulose conversion to ethanol.