Valorization of Waste Streams from Deinked Pulp Mills through Anaerobic Digestion of Deinking Sludge

Based on the results of this study, a total energy amount of 3,111 TJ/year can be produced from the anaerobic digestion of deinking sludge (DS) arising from German deinked pulp mills, which can then be used to replace up to 5% of the total energy demand for those mills. The DS examined was generated by flotation deinking at the laboratory scale from selected mixes of paper for recycling (PfR). The results from the batch fermentation tests indicated a strong dependence of the methane potential of the DS on the carbohydrates and lignin contents, which in turn are linked to the original PfR quality. The highest methane yield was observed for DS100 (25.8% carbohydrates; 5.1% lignin) with 280.4 mL/g of volatile solids (VS) added, while DS70 (14.2% carbohydrates; 24.9% lignin) showed the lowest methane yield, with 122.1 mL/g(VS). All of the DS samples showed high methane production rates, in the range between 59.4 (DS70) and 118.6 mL/g(VS) d(-1) (DS100), and kinetic constants of 0.66 to 0.79 d(-1). Additionally, no distinguishable lag phases were observed, which strongly indicates the rapid biodegradation of the DS.