BACTERIAL LEACHING OF METALS FROM SEWAGE-SLUDGE BY INDIGENOUS IRON-OXIDIZING BACTERIA

Bioleaching of metals can be achieved in sewage sludge using Thiobacil lus ferrooxidans, which obtains its energy requirements from the oxida tion of added ferrous iron. The purpose of this study was to verify th e presence of indigenous T. ferrooxidans and to evaluate their adaptiv e capacity and leaching potential. Nineteen sludges (primary, secondar y, aerobically and anaerobically digested, oxidation pond) were tested and all of them contained indigenous iron-oxidizing bacteria. The acc limation of these organisms by successive transfers allowed a rise of sludge redox potential over 450 mV and a decrease of sludge pH between 3.8 and 2.2 over a 10-day incubation period. The metal solubilization efficiencies were Cd: 55-98%, Cr: 0-32%, Cu: 39-94%, Mn.- 71-98%, Ni: 37-98%, Pb: 0-31% and Zn: 66-98%, were reached with these indigenous strains. The results obtained show that the metal bioleaching may be e asily realized by direct acclimation of sludge microflora.