The feasibility of bioleaching for removal of heavy metals from dewatered sewage sludge using an iron-oxidizing bacterium Acidithiobacillus ferrooxidans was investigated. The influence of seven process parameters including cell adaptation, total amount and particle size of the sludge, initial concentrations of Fe2+ and At ferrooxidans, and addition of inorganic nutrients and sulfur were evaluated in terms of the solubilization of Zn, Cu and Cr. When sludge-adapted cells, addition of inorganic nutrients and lower sludge content were involved, higher yields of metal extraction were obtained. However, higher initial concentrations of At ferrooxidans and Fe2+, fine particle size of the sludge and S addition did not improve the metals' solubilization during an experimental period of 7 days. As a result of a long-term (40 days) bioleaching experiment, 42% of Zn (1300-1648 mg kg(-1)), 39% of Cu (613-774 mg kg(-1)) and 10% of Cr (37-44 mg kg(-1)) in the sludge were leached into the solution. The results indicate that a bioleaching process conducted under operationally optimal conditions can be effectively employed for the removal of heavy metals from sewage sludge before land application. (c) 2005 Society of Chemical Industry.