Paperboard mill sludge (PMS) composed of cellulose, hemicellulose, lignin and ash contents of 36.72 +/- 2.81, 32.91 +/- 1.75, 22.89 +/- 0.56, and 7.48 +/- 0.39%, respectively. Enzymatic hydrolysis process followed by fermentation of native PMS provided an ethanol yield of 0.36 +/- 0.01 g/L which equivalent to net gain energy of -0.84 +/- 0.03 kJ/g(PMS). Choline acetate ionic liquid [Chol[OAc] IL was extensively used as a solvent for PMS to upgrade the performance. Pretreatment with [Cho][OAc] IL/PMS ratio of 10% (w/w) for 1.0 h, at a temperature of 120 degrees C exhibited hemicellulose and lignin removal efficiency of 5.05 +/- 0.52 and 14.71 +/- 1.22%, respectively with 89.19 +/- 5.62% cellulose recovery. This corresponded to net gain energy of 0.60 +/- 0.04 kJ/g(PMS) based on ethanol yield from enzymatic saccharification process which was quite low due to a limited hemicellulose removal and glucose yield of 24.1 +/- 1.4 g/L. [Chol[OAc] IL/PMS ratio of 10% (w/w) supplemented with 1% (v/v) HCl substantially improved the removal efficiency of hemicellulose (36.38 +/- 4.51%), lignin (17.42 +/- 1.19%) and cellulose (82.17 +/- 4.28%) which provided the maximum net energy of 5.36 +/- 0.30 kJ/g(PMS). (C) 2017 Elsevier Ltd. All rights reserved.