This aim of this study was to optimize the conditions for efficient saccharification of paper sludge (PS) considering PS cellulose concentration, buffer capacity, and cellulase activity from Acremonium cellulolyticus. PS is a cellulosic biomass, the residue of paper making industry, composed of 24.5% cellulose, 10.5% clay, and 65% water. Clay present in PS did not show any inhibitory effect on the saccharification process but influenced the buffer pH. Maleate buffer in the pH range 5.2-5.6 was suitable for the saccharification of PS cellulose. The maximum reducing sugar concentration that could be obtained from 75.6 kg m (3) of PS cellulose was predicted to be 37.8 kg m (3). This was experimentally confirmed to be 38.4 kg m(-3) under the optimal saccharification conditions at pH 5.2 (1.06 kmol m(-3) Maleate buffer) using Cellulase 20,000 FPU m (3). Almost 100% saccharification yield (SY) was obtained at a low PS cellulose concentration of 15 kg m (3), but this value decreased when the cellulose content increased. When PS was hydrolyzed with commercial cellulase GC220, the SY and glucose content were 54% and 60% of SY, respectively. The corresponding values with Cellulosin T2 were 63% and 75% of SY, respectively. A. cellulolyticus yielded SY and glucose content values almost 10% higher than those obtained with other cellulases. (C) 2010 Elsevier Ltd. All rights reserved.