Pulp and paper industry primary sludge being largely composed of lignocellulosic fibres, it could be used as carbon source by bacteria having cellulolytic capability. The aim of this study was to evaluate the use of cellulose contained in this type of sludge for Clostridium thermocellum to produce ethanol, hydrogen and cellulases. In an ATCC 1191 medium containing 5 kg m(-3) dry primary sludge from recycled paper mill, batch culture reached stationary phase after 2 days. All of the available cellulose was hydrolysed after 60 h of incubation, with a final pH of 5.83. Metabolites produced after 60 h of fermentation were acetate (8.50 mol m(-3)), ethanol (11.30 mol m(-3)), lactate (8.75 mol m(-3)), formate (0.27 mol m(-3)), hydrogen (11.20 mol m(-3)) and carbon dioxide (18.41 mol m(-3)). Cellulase activity was detected in the supernatant after 36 h, with a maximal activity of 0.25 U cm(-3) at 72 h. Pulp and paper primary sludge appeared to be a readily usable substrate for C. thermocellum at this concentration, yielding both potential biofuels (hydrogen and ethanol) as well as active cellulases. (C) 2014 Elsevier Ltd. All rights reserved.