Prolonged bioleaching period was required to remove heavy metals from anaerobically digested sewage sludge in the presence of low molecular weight organic acids. The purpose of the present study was therefore to enhance metal solubilization efficiencies through introducing organic acid-degrading microorganisms into this artificial bioleaching system. An acetic and propionic acid-degrading yeast Blastoschizomyces capitatus Y5 was successfully isolated from a local Yuen Long sewage sludge and it could achieve optimum growth in synthetic liquid media containing 2,000 mg l-1 acetic acid or 1,000 mg l-1 propionic acid. When it was inoculated simultaneously with Acidithiobacillus ferrooxidans ANYL-1 into anaerobically digested sewage sludge, which contained 648 mg l-1 acetic acid and 731 mg l-1 propionic acid, both acids were completely decomposed within 24 hours. As a result, ferrous iron oxidation was greatly improved, resulting in enhanced metal solubilization. Compared with the 8, 10 and 12 days required for maximum solubilization of Zn, Cu and Cr for the control sludge, the bioleaching period was significantly shortened to 4, 5 and 8 days respectively for sludge receiving co-inoculation.

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