18-16: Production of Lactic Acid from the Mixture of Softwood Pre-hydrolysate and Paper Mill Sludge by Simultaneous Saccharification and Fermentation

Paper mill sludge is a solid waste material composed of pulp residues and ash generated from pulping and paper making process. The carbohydrate portion of the sludge from a Kraft mill has chemical and physical characteristics similar to those of commercial-grade wood pulp. Because of its high carbohydrate content and well-dispersed structure, the sludge can be biologically converted to value-added products without pretreatment. In bioconversion of solid feedstock such as paper mill sludge, a certain amount of water must be added to attain fluidity. In this study, hemicellulose pre-hydrolysate, in place of water, was added to the sludge to increase the concentration of the final product. Pre-hydrolysate was obtained by hot-water treatment of pine wood in which the total sugar concentration reached 4 wt. %. The mixture was subjected to Simultaneous Saccharificiaton and Fermentation (SSF) using enzymes (cellulase and pectinase) and Lactobacillus delbrueckii (ATCC-7830) or Lactobacillus pentosus (ATCC-8041). During the SSF of the mixture, calcium carbonate existing in the paper sludge acted as a buffer keeping the pH to near optimum. In the SSF, L. delbrueckii converted only hexoses (glucose and mannose) into lactic acid, whereas L. pentosus converted hexose and xylose into lactic acid. The overall product yield on the basis of total carbohydrate content of the initial feed ranged 80 -90% of the theoretical maximum. Use of the mixture of pre-hydrolysate and pulp mill sludges as the fermentation feed also increased the product concentration to 60 g of lactate/L.