Composting of two different types of sludge from the recycled paper manufacturing industry was carried out at laboratory scale. Physico-chemical sludge (PCs) from the de-inking process and biological sludge (BS) from the wastewater treatment plant were composted and co-composted with and without addition of a bulking material. Despite its poor initial characteristics (relatively high C/N ratio, low organic content and moisture), PCs showed excellent behaviour in the composting process, reaching and maintaining thermophilic temperatures for more than 7 days at laboratory scale, and therefore complete hygienization. Pilot scale composting of PCs was also studied, and a respiratory quotient of 1.19 was obtained, indicating a full aerobic biological process. Respiration tests showed a complete stabilization of the material, with final values of the static respiration index in the range of 1.1 mg O-2 g TOM-1 h(-1). Composting is proposed as a suitable technology for the effective recycling of this type of sludge from the recycled paper manufacturing industry. (c) 2004 Elsevier Ltd. All rights reserved.