Speciation of heavy metals in non-volatile solids of sewage sludge

The aim of this study was to investigate if the combustion of sewage sludge at 600 degrees C (removal of organic matter) can influence the formation of immobile chemical forms of heavy metals (Zn, Cu, Ni, Pb, Cd, and Cr) in ash. Dried sewage sludge samples were collected at two different mechanical-biological wastewater treatment plants. For sewage sludge and residue on ignition (ash) heavy metal speciation analysis according to BCR procedure was conducted. It was demonstrated that metals found in sludge are mainly bound to organic/sulfide fraction. Zinc was present in significant amounts in hydrated iron and manganese oxide fractions, nickel and cadmium in exchangeable-carbonate fraction, and lead in residue fraction. In the ash obtained after calcination at the temperature of 600 degrees C, mainly metal enrichment of nearly insoluble compound fraction (residual fraction) occurred. The content of lead in this fraction was 93-96%. The residual fraction is considered to be chemically stable and biologically inactive. For other metals: zinc, copper, nickel, cadmium, and chromium, their content in the residual fraction was 48-62; 50-68; 69-92; 44-48; and 91-97%, respectively. Metal accumulation in this non-mobile fraction is advantageous with respect to soil-water environment protection.