Response surface methodology was employed to optimize mixed substrate solid state fermentation for the production of cellulases and xylanase by Aspergillus fumigatus ABK9. Among 11 different parameters, fermentation time (86-88 h), medium pH (6.1-6.2), substrate amount (10.0-10.5 g) and substrate ratio (wheat bran:rice straw) (1.1) had significantly influences on enzyme production. Under these conditions endoglucanase, beta-glucosidase, FPase (filter paper degrading activity) and xylanase activities of 826.2, 255.16, 102.5 and 1130.4 U/g, respectively were obtained. The enzyme cocktail extracted (solid to water ratio of 1:10) from the ferments increased brightness of waste office paper pulp by 82.8% ISO, Ink(D) value by 82.1%, removed chromophores (2.53 OD; A(237) nm) and hydrophobic compounds (1.15 OD; A(465) nm) and also decreased the kappa number to 13.5 from 16.8. (C) 2012 Elsevier Ltd. All rights reserved.