Chemical and Biological Leaching Methods to Remove Heavy Metals from Sewage Sludge: A Review

Abstract

The removal of heavy metals (HMs) in sewage sludge (SS) is important since sludge is often disposed or applied on farmland to enhance soil fertility. The present study reviewed two conceptual approaches (chemical and biological leaching) of removing HMs present in SS. In the chemical leaching method, traditional acid treatment together with novel methods such as aeration, complexation and sequential extraction procedure have been reviewed extensively. Certain factors influence the removal of HMs in SS. These factors include; pH, leaching agent, redox potential, and contact time. Nitric acid (HNO3), hydrochloric acid (HCl), sulphuric acid (H2SO4), phosphoric acid (H3PO4), ethylenediamine tetraacetic acid (EDTA), as well as Thiobacillus thiooxidans and Thiobacillus ferrooxidans are the most widely studied leaching agents and substrates involved in the chemical and bioleaching processes. However, the bioleaching process has been proposed as a safe, efficient, economical, environmental friendly method to remove HMs in SS due to its simplicity, high yield of metal extraction, low acid consumption, and low sludge solids concentration. Nevertheless, the present review has noted that most researchers are of the opinion that more studies are needed in the bioleaching method in order to enhance its commercial attraction.

Application of biological method for removing selected heavy metals from sewage sludge

“…Metals can be removed from sewage sludge by the method of chemical leaching with inorganic and organic acids (Veeken and, Hamelers, 1999;Marchioretto et al, 2002;Ukiwe et al, 2008), ion exchange, adsorption or chemical extraction (Zagury et al, 1999;Babel et al, 2006;Lee et al, 2006;Chen et al, 2005, Karwowska et al, 2014. As a result of these operations, an environmentally safe material is obtained and, at the same time, metals are recovered.…”