Removal of Heavy Metals from Urban Sewage Sludge Using Acidophilic Thiobacillus ferrooxidans

The presence of heavy metals in urban sewage sludge limits its use as a soil modifier. The purpose of this study was to remove heavy metals (Cu, Zn, Cd and Pb) from urban sewage sludge using Acidophilic Thiobacillus ferrooxidans bacteria. In this experimental study, the sludge samples were collected from Shiraz wastewater treatment plant. Bioleaching tests were carried out in 250 ml flasks containing 100 ml fresh sludge, 15 ml Acidophilic Thiobacillus ferrooxidans bacteria and 1-4 g/L Fe+2 at temperature of 28 °C and speed of 180 rpm for 9 consecutive days. Heavy metal removal was monitored by polarograph. The optimum condition for separation of heavy metals in this method was obtained at retention time of 5 days and iron concentration of 2gr/l. Under this condition, 80% of cadmium, 78% of lead, 87% of zinc and 69.9% of copper were removed from sludge. Also, sludge dewatering was increased up to 99%. The residual concentrations of zinc, cadmium, lead and copper in sludge after bioleaching were 783, 1, 18 and 607 mg/kg, respectively, which were within the EPA's standards for agriculture. The results showed that the bioleaching method by Thiobacillus bacterium could be used as a suitable method for removal of heavy metals and increasing sludge dewatering efficiency.