Partitioning of heavy metals in sewage sludge incineration

The behaviour of seven metals (Cd, Cr, Cu, Mn, Ni, Ph, Zn) during sewage sl udge incineration was studied in two pilot plant tests performed with a rot ary drum furnace at 900 degrees C. A first group of metals (Cr, Cu, Mn, Ni) show similar concentrations in bottom and fly ashes. In contrast, lead, ca dmium and zinc concentrations in fly ashes are one or two order of magnitud e higher than those in bottom ash. A thermodynamic model was used to predic t metal behaviour assuming perfect mixing and equilibrium conditions in the combustion chamber. The model may predict in a satisfactory way the behavi our of Cr, Cu, Mn, Ni (" non volatile" metals) and of Cd and Pb ("volatile" metals), with the exception of Zn, that is not predicted to volatilize in the combustion chamber, in contrast with the clear zinc enrichment in fly a sh. The use of the thermodynamic model was proved to be very useful in unde rstanding and predicting metal behaviour during sludge incineration.