Heavy metals accumulated in sewage sludge during wastewater treatment can be leached our by (i) acid addition (acidification process), (ii) iron oxidation using Thiobacillus ferrooxidans or (iii) sulphur oxidation using Thiobacillus thioparus and Thiobacillus thiooxidans. An algorithm was developed for estimating the cost involved in leaching out metals from sewage sludges using these three processes. Cast estimations showed that the sulphur oxidation process is the most attractive at low plant capacities and high solids concentrations. Acid addition was more attractive at high plant capacities. A combined metal leaching and sludge digestion process compared well with aerobic digestion followed by acid addition process in terms of cost per tonne of dry raw sludge treated. Sludge solids concentrations below 20 g litre(-1) were found to increase the treatment costs appreciably, for all three processes. Aeration rate was found to play a significant role in the cost of metal leaching by both bioleaching processes.