Removal of heavy metals from natural dried sludge by bioleaching method

Removal of heavy metals is a key issue for utilization of sludge in agriculture. Bioleaching technology was introduced in this study for removal of heavy metals from eleven dried sludge samples which collected from different wastewater treatment plants in Shanxi province. The bioleaching process was incubated for 15 days with Acidithiobacillus thiooxidans as function microbial. The changes of system pH and SO₄^2- were investigated during bioleaching. In addition, the removal efficiency of Cu, Zn, As, Cd, and Cr, and the loss amount of organic matter, total nitrogen, total phosphorus, and total potassium were studied. The results showed that the bioleaching process was slow in original dried sludge system. However, the bioleaching process quickly started with addition of inorganic acid. After acidification, bioleaching process was significantly accelerated as the sulfur was rapidly bio-oxidized, and the removal efficiencies of sludge-borne heavy metals increased with the decrease of pH. After bioleaching, the removal efficiencies of sludge-borne Cu, Zn, As, Cd, and Cr were 23.69%~77.62%, 89.67%~97.80%, 30.24%~84.31%, 18.18%~97.05%, and 28.55%~67.11%, respectively. But the contents of organic matter, total nitrogen, total phosphorus, and total potassium just reduced by 3%, 1%, 44%, and 8%, respectively. The contents of remained nutrients met the limit value given in agricultural application standard (CJ/T309-2009). Removal efficiencies of Cu, Zn, As, Cd, and Cr were significantly and negatively related to the organic matter content of sludge (r=0.935, 0.895, 0.932, 0.516, 0.847, n=11).