Behavior of chromium, nickel, copper, zinc, cadmium, mercury, and lead during the pyrolysis of sewage sludge

Abstract

Raw and digested sewage sludge samples were pyrolyzed in order to investigate the partitioning of selected heavy metals during anoxic thermal treatment. Cr, Ni, Cu, Zn, and Pb were retained quantitatively in the char at temperatures up to 750 °C. Cd compounds were reduced to Cd°, which was volatilized at <I>T</I> > 600 °C. Hg was completely evaporated at the lowest investigated pyrolysis temperature of 350 °C. The metals contained in the char are highly immobile due to the well-buffered neutral to alkaline properties of the char. Thus for sewage sludge, which contains metals with similar chemical and physical properties such as Cr, Ni, Cu, Zn, Cd, and Pb, pyrolysis at 500–600 °C might prove suitable since no metals are released to the atmosphere and a char is produced that is well suited for disposal in an inorganic sanitary landfill.