The sulfite mill as a sugar-flexible future biorefinery

The production of single- and mixed-sugar streams and their conversion to bioproducts were studied, using sulfite pulping streams as feedstocks. Sulfite pulp, sludge, and spent sulfite liquor are concurrently generated alongside of bleached pulp, and the pulping process renders pretreatment of solid streams unnecessary. Streams were converted separately; however, due to their low production volume, solid and liquid streams were also combined as a means to increase the quantity of starting feedstock. Spent sulfite liquor, comprising mostly monomeric hexose and pentose sugars, was directly fermented to ethanol and xylitol with Candida guilliermondii. Single-sugar streams were generated through hydrolysis of pulp and sludge in water, followed by fermentation to ethanol with Saccharomyces cerevisiae. Mixed-sugar streams were generated through both separate hydrolysis and fermentation and simultaneous saccharification and fermentation of pulp and sludge in spent sulfite liquor using S. cerevisiae. The best single-sugar source was obtained by hydrolysis of pulp in water, which produced 78.8 g/L of glucose after 96 h. The glucose concentration from hydrolysis of sludge in water was lower (33.5 g/L). Both of these streams were easily converted to ethanol, with yields of 77.8% and 76.2%, respectively. Hydrolyzability of solids was the limiting factor in separate hydrolysis and fermentation conversion of pulp and sludge in water, but hydrolyzability of sludge was not affected when mixed with spent sulfite liquor.