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A mixed culture of two fast-growing bacterial strains for heavy-metal solubilization of municipal sewage sludge has been developed. Strain VA-7 decreases the initial sludge pH (7-8.5) to a value between 4.0 and 4.5. Then, strain VA-4 begins growing and further reduces the pH to values below 2.0. The rapid decrease of sludge pH by a mixed culture through sulfur oxidation into sulfuric acid solubilizes the toxic metals (Cd 83-96%, Cr 16-54%, Cu 85-87%, Mn 91-94%, Ni 78-79%, Pb 28-46%, Zn 82-96%) to levels recommended for intensive use of residual sludge in agriculture. A study of the physiological and metabolic characteristics of these strains revealed that isolate VA-7 is a strain of Thiobacillus thioparus (ATCC 55127), while isolate VA-4 corresponds to a Thiobacillus thiooxidans (ATCC 55128). These bacterial strains possess distinctive physiological characteristics that allow them to easily grow and solubilize heavy metals in municipal sludge.
Blais et al. (Sun,) studied this question.