Valorization of Residual Streams from Pulp and Paper Mills: Pretreatment and Bioconversion of Primary Sludge to Bioethanol

Primary sludge is a lignocellulosic residue from pulp and paper mills consisting of cellulosic fibers and ash. However, the high ash content (35%, mainly CaCO3) and pH value affect the enzymatic hydrolysis of cellulosic fibers. Several pretreatments were used to reduce the CaCO3 content and to adjust the pH. Enzymatic hydrolysis of primary sludge was enhanced when it was pretreated with HCl or spent acid (another residual stream of the same plant). Cellulosic fibers were converted to monomeric sugars by Cellic CTec(2) cellulase with a dosage of 35 FPU g(CH)(-1) for a carbohydrate concentration of 46 g L-1. This conversion was enhanced from 20% to 88% for primary sludge pretreated with HCl and to 72% for samples pretreated with spent acid. The fermentation of 27 g L-1 of available sugars with Pichia stipitis led to ethanol concentrations of up to 10.5 g L-1 with a yield of 0.39 g(EtOH) g(sug)(-1).