Partitioning of heavy metals in sewage sludge incineration

The behaviour of seven metals (Cd, Cr, Cu, Mn, Ni, Ph, Zn) during sewage sludge incineration was studied in two pilot plant tests performed with a rotary 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, cadmium and zinc concentrations in fly ashes are one or two order of magnitude higher than those in bottom ash. A thermodynamic model was used to predict metal behaviour assuming perfect mixing and equilibrium conditions in the combustion chamber. The model may predict in a satisfactory way the behaviour 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 ash. The use of the thermodynamic model was proved to be very useful in understanding and predicting metal behaviour during sludge incineration.