METALS REMOVAL FROM SEWAGE-SLUDGE BY INDIGENOUS IRON-OXIDIZING BACTERIA

Indigenous sewage sludge iron-oxidizing bacteria was acclimated in twenty-three different sludges to remove toxic metals from sewage sludge. The acclimation was carried out at pH 4.0 and 21 +/- 1-degree-C. The required microflora was adapted in two to three successive transfers. The results showed that the oxidation of added Fe2+ in the form of FeSO4.7H2O decreased the sludge pH to 2.5 and increased oxido-reduction potential (ORP) to 500 mV This pH reduction solubilized toxic metals to a level compatible with the recommended norms for agricultural use of the sludge. The adaptation of the microflora reduced the metal leaching period from 11-28 days to 2-10 days depending upon the sludge used. The metal leaching capacity of the adapted iron-oxidizing bacteria was not significantly influenced by the sludge type (non-digested sludge or aerobically or anaerobically digested sludge). Two strains of T. ferrooxidans were isolated from sewage sludge, and their activity was compared with the standard strains of T ferrooxidans (from American Type Culture Collection). The use of indigenous iron-oxidizing bacteria resulted in the following metal solubilization efficiency: Cd: 74 +/- 13%, Cr: 7 +/- 8%, Cu: 60 +/- 22%, Mn: 89 +/- 8%, Ni: 63 +/- 19%, Pb: 9 +/- 8%, and Zn: 82 +/- 12%.