Removal and process of heavy metals in sludge by microbiological leaching for agricultural use

The high concentration of heavy metals in wastewater sludge has limited the application of the sludge on agricultural land. Under the circumstances of high Cu content in the sludge from wastewater treatment plants in Tianjin City, the microbiological leaching experiments using ferrous sulfate and sulfur powder as the culture respectively, were performed. The results showed that Cu could be removed more efficiently using ferrous sulfate as the culture than that using sulfur powder. Using the ferrous sulfate as the culture, the effects of the dose of ferrous sulfate, sludge-inoculation ratio, pH and temperature on the removal of heavy metals were investigated. The removal efficiencies of Cu, Pb, Cr, Ni, Zn and Cd were 89%, 38%, 61%, 76%, 72% and 57%, respectively, and the corresponding residual contents in sludge were 116 mg/kg, 62.5 mg/kg, 98 mg/kg, 22 mg/kg, 174 mg/kg and 1.1 mg/kg, respectively, with 10 g/L of dose of ferrous sulfate, 20% of sludge-inoculation ratio, 28℃ of temperature, and four days of biological leaching time in the batch experiments. In addition, it indicated that the activity of Thiobacillus Ferrooxidans increased and the biological leaching time shortened when the temperature rose from 7℃ to 28℃. And the activity of Thiobacillus Ferrooxidans at 28℃ and 35℃ had little significance. The contents of heavy metals in sludge treated by biological leaching using ferrous sulfate as the substrate can meet Control Standards for Pollutants in Sludges for Agricultural Use (GB4284-1984).