Bioethanol Production from Paper Sludge by Subcritical Water Pretreatment and Semi-simultaneous Saccharification and Fermentation

Paper sludge (PS) from paper mills has a significant potential for bioethanol production. In this study, waste-paper-containing PS is used as the raw material for bioethanol production because the annual waste paper utilization rate has increased globally. Although PS does not require delignification, the antiseptics and deinking agents in waste paper-containing PS inhibit enzymatic reactions such as saccharification and fermentation. Their removal is important, but it has not yet been reported. Using subcritical water pretreatment, the selective decomposition of enzyme inhibitors in PS is examined without the generation of other enzyme inhibitors. The optimum pretreatment conditions are identified as 240 °C, 3.3 MPa, 3 min, and pH 4.5. Glucose was obtained in 71% yield from pretreated 5 wt% PS using cellulase, which is 5.5 times higher than that from unpretreated PS. This is because the reactivity of the pretreated PS increases with increasing surface area of the cellulose fibers, and the cellulase inhibitors are decomposed by subcritical water. Next, semi-simultaneous saccharification and fermentation treatments are performed to produce bioethanol from waste-paper-containing PS. The bioethanol yield based on cellulose after 96 h is 68% for PS pretreated with subcritical water, whereas the bioethanol yield is 6% for unpretreated PS. Therefore, subcritical water pretreatment increases the bioethanol yield by 11 times. The proposed method may enable the use of large amounts of PS as ethanol feedstock in future.