Heavy metals concentrated in sewage sludge during a wastewater-treatment process can be leached out by using sulphur-oxidizing bacteria. The growth and associated sulphate production by the less-acidophilic and acidophilic Thiobacilli lower the sludge pH, resulting in solubilization of metals present in the sludge. This series of events, which forms part of the bioleaching operation, is expressed mathematically. The process of metal solubilization, as a function of pH, solids concentration, metal concentration, time, and sulphate concentration, is presented as a trained neural net. A sensitivity analysis of the process shows that (i) higher initial concentrations of bacteria sewed to reduce the time; (ii) an initial sludge pH of 7.0 is optimal in terms of process time; (iii) a higher solids concentration increased the rate of metal solubilization; and (iv) a higher temperature resulted in faster leaching at temperatures below 40 degrees C.