BACTERIAL LEACHING OF TOXIC METALS FROM MUNICIPAL SLUDGE - INFLUENCE OF SLUDGE CHARACTERISTICS

The presence of high concentrations of heavy metals in wastewater sludge is one of the most important factors that limit the application of sludge on agricultural land. In the last few years, the microbiological process of heavy metals removal from wastewater sludge has been studied in a batch mode. In the present study, the effects of metals, solids concentration, and initial pH on indigenous sulfur-oxidizing thiobacilli growth and metal solubilization were examined. The results obtained indicate that metals removal is possible even in high sludge solids concentration (70 g/L). The rate of sulfuric acid production through the oxidation of added elemental sulfur is increased by the increased sludge solids concentration. The initial pH of the sludge (7.0-3.0) does not affect the capacity of metal leaching. The high concentration of metallic ions (Cu2+ and Zn2+) adjusted in the sludge did not greatly affect the microbiological activity of metal-leaching bacteria. The leaching tests conducted with metal sulfides as substrate (chalcocite, galena, pyrite, sphalerite) showed that metals solubilization from sulfides occurred by an indirect mechanism (production of acid followed by metal solubilization due to the action of the acid produced). No evidence of direct oxidation of metal sulfides was observed.