Metal Removal from Sewage Sludge: Bioengineering and Biotechnological Applications

Abstract

Heavy metals such as cadmium, chromium, copper, nickel, lead, zinc, etc., are normally found in varying concentrations in sewage. The metals present in domestic wastewaters come from a number of sources: feces, pharmaceutical products, cosmetics, washing and cleaning chemicals, paints and other surface coatings, etc. Since the source of these metals is non-point in nature, it is extremely difficult to practice source control. As a result, all these metals finally end up at the sewage treatment site. A significant part of these metals are removed during the primary decantation or primary treatment process as part of the primary sludge. During the secondary treatment (generally biological and most often activated sludge) process, a complex between metals and extracellular polymers (produced by the microorganisms used in the treatment) is produced, which results in a biofloc structure. The metals are either adsorbed on the cell surface or are simply entrapped in the bioflocs. These physical and biochemical mechanisms are responsible for the fate of 80–90% of the total metals originally present in the effluent. The metal concentrations observed in the sludge vary with the type of wastewater treatment, the presence of certain industries in the area, the habits of the population covered, the state of the wastewater transport system, etc.