The Prospects for Advanced Processing of Paper Sludge Using Enzymes, Microalgae and Yeast

This article presents the information on the valorization of waste generated during the production of tissue paper. The possibility of bioconversion of the polysaccharide part of paper sludge into simple sugars has been evaluated. The options for processing the paper sludge before enzymic hydrolysis to achieve the maximum yield of monosaccharides have been considered. Pretreatment with acids has been found to be a key step before the biocatalytic cleavage of waste polysaccharides. An additional yield of enzymic hydrolysis products after pretreatment with acids has been obtained by pre-extraction of the paper sludge with spirit or acetone. It has been established that the most intense enzymic hydrolysis of readily available fractions of the paper sludge takes place in the first 10–12 hours. Further, the process slows down, probably due to the action of the remaining components of the fillers, as well as an increase in the proportion of the difficult-to-hydrolyze polysaccharide part. In all cases, the proportion of absolutely dry non-hydrolyzed residue has been about 43±2 % of the dry matter of the paper sludge. The main products of enzymic hydrolysis have been glucose and xylose. The resulting sugars have been used for mixotrophic cultivation of the Tetradesmus obliquus and Chlorella vulgaris algae. Yeast strains have been selected for the conversion of monosaccharides from the paper sludge. In a series of experiments on non-sterile yeast cultivation, the Candida utilis PAL D and Debaryomyces hansenii SWING R cultures have turned out to be the most productive (within 2.10±0.14 g of air-dry yeast weight/dm3 per 24 hours). The degree of conversion of hydrolysate sugars has been 70±2 %. Most of the remaining sugars (about 80 %) have been represented by xylose. Complete utilization of the sugars has taken place on the 2nd day when having added an extra nitrogen source to the medium. On the other hand, the spent nutrient medium after yeast separation has been suitable for mixotrophic cultivation of microalgae. It has been revealed that the economic costs of pre-treatment of the paper sludge with nitric acid can be leveled by using the resulting salts as a nitrogen source for cultivating yeast. In doing so, the yield of yeast biomass increases by almost 2 times.