BIOLEACHING OF METALS FROM SEWAGE-SLUDGE - EFFECTS OF TEMPERATURE

The biological leaching of heavy metals from sewage sludge with ferrous iron and elemental sulfur as substrate was investigated in shake flask experiments. The tests were performed at six different temperatures in the range of 7-42-degrees-C. The temperature effects on the kinetics of metal solubilization with the two bioleaching processes (ferrous iron and sulfur) followed first-order kinetics for cadmium, copper, nickel and zinc. The activation energies calculated from the rate constants varied between 30 and 64 kJ/mol, whereas the temperature coefficients (Q10) in the 7-28-degrees-C range, were evaluated as 1.6 and 2.5. The temperature effects on bacterial growth (less-acidophilic and acidophilic thiobacilli) were measured in the bioleaching process with sulfur. The growth rate constants of the two types of thiobacilli yielded a linear Arrhenius plot, an activation energy of 42 kJ/mol and a temperature coefficient (Q10) of 1.8 for the 7-28-degrees-C temperature interval. These results suggested that the growth of leaching bacteria was the rate-limiting step for metal solubilization from the sludge, and that the metal removal from sludge by biological methods was strongly influenced by the temperature of the bioreaction. However, the metal leaching tests carried out in this study, showed that the two biological processes can be used for sludge decontamination at very low temperatures (7-degrees-C)