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 micro‐flora, 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.