FACTORS AFFECTING TOXIC METALS REMOVAL FROM DIGESTED SEWAGE-SLUDGE BY ENRICHED SULFUR-OXIDIZING MICROORGANISMS

Enriched sulphur-oxidizing microorganisms were applied to remove metals from anaerobically digested sewage sludge in laboratory reactors. The pH drop and rise in oxidation-reduction potential (ORP) of the sludge by microorganisms were mainly responsible for the metal solubilization. There was a significant effect of the sulphur amount per unit volume of sludge, the initial pH of the sludge, the inoculum size and the sludge solids concentration on the pH, the sulphate production rate, the ORP, the maximum metal solubilization rate, and the time required to achieve the acceptable level of Cu for sludge land application. The enriched microorganisms were able to grow and reduce the pH at a wide range of initial sludge pH values. Metals (Cu, Zn) were solubilized efficiently as a result of decreased pH.