Feasible process development and techno economic evaluation of paper sludge to bioethanol conversion: South African paper mills scenario
Paper sludge samples collected from recycling mills exhibited high ash content in the range of 54.59% 65.50% and glucose concentrations between 21.97% and 31.11% Washing the sludge reduced the total ash content to between 10.7% and 19.31% and increased the concentration of glucose, xylose and lignin. Samples were screened for ethanol production and fed batch simultaneous saccharification and fermentation (SSF) was optimised for the washed samples that resulted in highest and lowest ethanol concentrations. Maximum ethanol concentrations of 57.31 g/L and 47.72 g/L (94.07% and 85.34% of the maximum theoretical yield, respectively) was predicted for high and low fermentative potential samples, respectively, and was experimentally achieved with 1% deviation. A generic set of process conditions were established for the conversion of high ash containing paper sludge to ethanol. Techno economic analysis based on three different revenue scenarios, together with Monte Carlo analysis revealed 95% probability of achieving IRR values in excess of 25% at a paper sludge feed rate of 15 t/d. Feed rates of 30 t/d and 50 t/d exhibited a cumulative probability of 100% This study presents the technical feasibility and economic viability of paper mills expansion towards bioethanol production from paper sludge.