Valorisation of Industrial Waste for Second-Generation Ethanol Production: Towards the Scale-Up of High-Solids Paper Sludge Fermentation

The scalability of high-solids paper sludge (PS) fermentation was assessed in a 50 L pilot-scale horizontal, solid-state bioreactor using three industrial strains of S. cerevisiae, two of which were engineered to ferment xylose to ethanol and secrete cellulase that would decrease costly exogenous enzyme supplementation. An ethanol titre of 86.2 g/L was achieved with virgin pulp PS (VP-PS) at a solids loading and enzyme dosage of 40% (w/w) and 10 filter paper units (FPU)/g dry PS, respectively, using Ethanol Red (R). Enzyme dosages could be decreased to 3.5 FPU/g dry PS at 30% (w/w) solids using Cellusec (R) 2.0 and Cellusec (R) 3.3, achieving ethanol yields of 16.5 (81.5% of theoretical maximum) and 15.1 g/100 g dry PS, respectively, due to xylose consumption and cellulase secretion. The study proved an important first step in the potential industrialisation of PS to ethanol, providing suitable operating conditions to reach economically feasible ethanol concentrations > 40 g/L.