9-13: Production of lactic acid from the slurry of paper mill sludge and softwood pre-hydrolysate

Kraft pulp mills generate 100-300 tons of sludge/day as solid waste. Because of the high carbohydrate content and well-dispersed structure, the sludges can be biologically converted to value-added products without pretreatment. Lactic acid in the US is produced entirely from corn, of which the non-food demand has rapidly risen causing price hikes. In this study, sludges obtained from a Kraft paper mill was evaluated as an alternative feedstock for lactic acid. For the conversion scheme, we employed simultaneous saccharification and fermentation (SSF), applying cellulase enzyme and L. delbrueckii and L. pentosus. Paper mill sludges also contain high amount of ash in the form of calcium carbonate which acts as a built-in neutralizing reagent in the SSF. In repeated experiments, the yields of lactic acid from paper mill sludge were found to be in the range of 85-95% (theoretical maximum) for both microorganisms. Most of the hemicellulose in the pulp feed is lost during the pulping process, which can also be used as a supplementary feedstock. In the second part of this study, a portion of hemicellulose sugars from softwood chips was selectively recovered by hot-water treatment. The pre-hydrolysate containing hemicellulose sugars (mannose, xylose, and oligomers) were detoxified by overliming and charcoal treatment. The mixture of the sludge and the prehydrolysate was put through the SSF process in which an extra enzyme (pectinase) was added to hydrolyze the oligomers.  Bioconversion of the slurry was proven to increase the total sugar and lactic acid concentration significantly above that of the sludge-alone operation.