A process for metal removal by thiobacilli was applied to aerobic sludge from three different sources: treated sludge not subjected to phosphate removal, sludge subjected to a FeCl3 phosphate-removal process, and sludge subjected to an alum phosphate-removal process. Solubilization varied, and a number of hypotheses were formulated and tested to explain the results: influence of digestion factors, addition of Percol, metal concentrations of the sludge, and phosphate-removal processes during sludge generation. Sludge digestion and addition of Percol proved not to affect metal solubilization resulting from application of the metal removal process. Solubilized copper concentration was plotted against copper content of the sludge studied, yielding a regression with an r2 of 0.72. For manganese, an r2 of 0.53 was obtained for the same type of regression. The more contaminated the sludge, the higher the element solubilization (%). Copper solubilization correlated with sludge phosphate content (r2 = 0.46), suggesting the phosphate-removal process affected copper solubilization. With manganese, no correlation with phosphate content was established. Under optimum reactor operating conditions, metal content was the main factor in copper and manganese solubilization in the aerobic sludges studied. Key words: heavy metals, biological solubilization, thiobacilli, Thiobacillus ferrooxidans, sewage sludge. [Journal translation]
