The aim of this work was to study the effect of ratio of substrate dosage to solid content (Sd/SC) on sewage sludge bioleaching. The inocula - indigenous sulfur-oxidizing bacteria were enriched and cultured from the fresh activated sludge to a wastewater treatment plant. The results showed that Sd/SC significantly influenced the sludge bioleaching process. With increase in Sd/SC the sludge bioleaching was enhanced, which was represented by the acceleration of sludge acidification, oxidizing environment formation, and substrate (sulfur) utilization. Higher Sd/SC was more efficient to solubilize the heavy metals and total phosphorus (TP) than lower Sd/SC, while total nitrogen (TN) release was not influenced by Sd/ SC. Zinc and copper were efficiently bioleached because of sludge acidification and sludge oxidation, but lead was bioleached with a low efficiency because of the formation of low soluble PbSO(4) precipitates. After bioleaching the biotoxicity of sewage sludge greatly reduced. (c) 2008 Elsevier Ltd. All rights reserved.