The aim of this work is the study of recycling urban primary sludge by in-vessel aerobic composting way. Two series of composting trials were carried out in an automated accelerated bioreactor in mixture with agricultural wastes: sugar beet leaves (Cl): straw, sheep manure and sugar beet leaves (C2). Treatments were monitored with regard to physicochemical characteristics, heavy metal amounts and microbiological parameters of the final compost product. The general pattern of the temperature curve was typical for composts of organic waste. The different physicochemical characteristics of the final composts after a retention time in the bioreactor of 30 and 23 days, respectively for C1 and C2 were: pH: 7.3-7.2; C/N: 10.2-12; organic matter: 49.7-58.3%; NH(4)(+)/NO(3)(-): 0.24-0.2. Final compost showed low amounts of heavy metals, relatively high contents of nutrients and significant reduction of pathogens, suggesting the agricultural purposes of urban primary sludge. (C) 2010 Elsevier Ltd. All rights reserved.