The amount of sludge generated from wastewater treatment plants has been increasing year-by-year as the sewerage service area expanded. The composting process is considered to be one of the best management methods to stabilize different organic wastes. The main goal of this investigation was to find the optimal ratio of the quantity of the organic waste to the quantity of the sewage sludge. The process of composting was carried out in the Horstmann bioreactor (pilot scale) for 2-3 weeks and then in a compost pile for 6-12 weeks. Seven experiments were performed, with different ratios of the quantity of organic wastes to the quantity of sewage sludge. The results enabled an estimation of the quality of compost and its usefulness and also the usefulness of the bioreactor to be deduced. (C) 2003 Published by Elsevier Science Ltd.