BIOLEACHING OF METALS FROM SEWAGE-SLUDGE BY SULFUR-OXIDIZING BACTERIA

Twenty-three different sewage sludges were studied to adapt indigenous sulfur-oxidizing microflora to solubilize a substantial part of trace metals from the sludges. The results showed that the oxidation of elemental sulfur into sulfuric acid with indigenous sludge thiobacilli causes a sludge acidification, lowering the pH below 2.0. This pH reduction solubilizes trace metals to a level compatible with recommended norms for agricultural use with the following metal solubilization efficiencies: Cd, 82 +/- 13%; Cr, 31 +/- 19%; Cu, 75 +/- 21%; Mn, 92 +/- 7%; Ni, 70 +/- 16%; Pb, 27 +/- 21%; and Zn, 85 +/- 10%. The adaptation of this microflora reduces the trace-metal-leaching period to 1.5 to five days, depending on the sludge used. The growth and metal-leaching capacity of the adapted microflora, which is a mixed culture of thiobacilli, was not significantly influenced by the sludge type (nondigested sludge, aerobically or anaerobically digested sludge) and by sludge total solids content.