Evaluation of biotechnological processes to obtain ethanol from recycled paper sludge

Finding innovative solutions to manage waste and to expand the production of biofuels are some of the current challenges. Pulp and paper sectors, including recycled paper mills, produce a large amount of solid wastes, such as wastewater sludge containing non-recyclable short fibres. This study aimed to: (1) characterize recycled paper sludge; (2) define conditions for enzymatic hydrolysis and (3) evaluate two processes for ethanol production from the sludge: separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF). The waste showed a very high content of ash (58 %), which negatively affected enzyme action. A commercial enzyme cocktail (Cellic^® CTec2) was used for the saccharification, at 43 °C and 6 % dosage. The highest ethanol yield (55.7 %) with shorter residence time (72 h) resulted from the SSF process. The best performance using the SHF process yielded 45.8 % after 84 h. Despite the intermediate results, sludge from pulp and paper mills could be an attractive raw material due to its low price and not requiring a pretreatment step for delignification. This application could be especially interesting for regions where the generation of paper sludge is high and the ethanol production capacity is still low, such as Asia.