Bioleaching of heavy metals from anaerobically digested sewage sludge
Pathak, A.; Dastidar, M.G.; Sreekrishnan, T.R.
Journal of Environmental Science and Health. Part A Toxic/Hazardous Substances and Environmental Engineering 43(4): 402-411
ISSN/ISBN: 1093-4529 PMID: 18273746 DOI: 10.1080/10934520701795624
The effects of sulfur concentration, initial pH of the sludge and sludge solid content on metal bioleaching were examined using anaerobically digested sewage sludge procured from a typical sewage treatment plant in Delhi, the capital city of India. Experiments on effect of sulfur concentration were carried out using 0-4 g L(-1) of elemental sulfur to optimize the concentration of elemental sulfur for efficient bioleaching. For the type of sludge (20 g L(-1) solid content) used in the present study, 2 g L(-1) of elemental sulfur was found sufficient in metal bioleaching in the following order: Zn 86%, Cu 71.5%, Mn 70%, Ni 58.3% and Cr 43.8%. Changes in pH, sulfate concentration and oxidation-reduction potential (ORP) as a function of time were experimentally monitored. A rapid change in the above parameters took place in 4-6 days followed by a slow change until the 10th day. The change in pH with time was observed to vary at different initial pH values (pH 7 to pH 3) of the sludge; however, there was not much difference in the final pH achieved and final metal solubilization which ranged from: Cu 83.6-94.2%, Ni 27.7-29.8%, Zn 89-94.8%, Mn 67.5-79% and Cr 34.1-44.1% The results of the present studies strongly indicate that using 2 g L(-1) elemental sulfur, indigenous sulfur oxidizing microorganisms can bring down pH to a value needed for significant metal solubilization. Also, bioleaching can be carried out successfully over a wide range of initial pH values of the sludge. Further, at higher sludge solid concentration than 20 g L(-1), lower metal solubilization was achieved due to the buffering capacity of the sludge.