Conversion of Industrial Paper Sludge to Ethanol: Fractionation of Sludge and Its Impact

Paper sludge is an attractive biomass source for the conversion to ethanol due to its low cost and the lack of severe pretreatment required. Four sludges from pulp and paper operations including both virgin kraft (VK) and recycled and deinking (RD) paper mills were analyzed. A fractionation process using a laboratory screen was utilized to produce a fiber-rich stream for enzymatic hydrolysis. This process removed 82-98 % of the ash with fiber yields from 39 to 69 %. Even though sludges in both non-fractionated and fractionated scenarios were pH-adjusted, total sugar conversion was still improved by 12-27 % by fractionation with 4.5 times less acid required for pH adjustment. Fermentation of the fractionated sludges showed very high ethanol yields. Acid insoluble clay adsorbs 3-5 mg enzyme per gram of clay depending on enzyme dosage. Acid soluble CaCO3 adsorbs about half of the enzyme compared to clay. Fractionation efficiency was also evaluated by testing different size mesh screen openings (100 to 500 mesh). The 400-mesh screen presented the best fiber yield, ash removal and ash fractionation ratio for both VK and RD sludges. The ash-rich streams have a lower C/N ratio than the original sludge which improves its suitability as soil amendment.