BIOLEACHING OF METALS FROM SEWAGE-SLUDGE - MICROORGANISMS AND GROWTH-KINETICS

Microbial leaching is one of the advantageous methods of removing heavy metals from sewage sludge, however, the microbiological aspects of this technology have not been studied. This study presents the characterization of the naturally occurring microorganisms, responsible for the metal leaching activity, in 21 different sewage sludges. The results obtained indicate that the bioleaching of metals is carried out by successive growth of less-acidophilic and acidophilic thiobacilli. Several species of less-acidophilic thiobacilli participate in the sludge acidification, but Thiobacillus thioparus is the most important species. In contrast, Thiobacillus thiooxidans seems to be the only species involved in the acidophilic group of thiobacilli. The growth kinetics of the two groups of thiobaciili was followed in five different sewage sludges. After 5 days of incubation in shake flasks, the pH of the sludge was decreased to about 2.0 and this pH decrease solubilized the toxic metals (Cd: 83-90%; Cr: 19-41%; Cu: 69-92%; Mn: 88-99%; Ni: 77-88%; Pb: 10-54%; Zn: 88-97%). The maximum specific growth rate (mu(max)) for the less-acidophilic thiobacilli varied between 0.079 and 0.104 h-1 and that for the acidophilic thiobacilli varied between 0.067 and 0.079 h-1.