Heavy metals in sewage sludge and their behaviour in incineration

Fundamental aspects of the behaviour of a number of heavy metals such as As, Cd, Cr, Cu, Hg, Mn, Ni, Sb, Sn, and Zn in sewage sludge combustion in a fluidized-bed unit are investigated. Main sources of uncertainty in computation of thermodynamic equilibria are the homogeneity of the combustion system, the state of the condensed phases, and the kinetics limitations. Nevertheless, the calculations are able to reveal general trends. Practically all Hg and most Cd and Pb volatilize, as observed in practice. The amounts of the volatilized Zn and Cu depend on the reduction conditions and on the amount of present Cl. As and Sb also volatilize under the prevailing oxidation conditions. The presence of sulfur stabilizes many of the metals at low temperatures but has little influence above 800 C. The residual particulate matter (ash) from incineration becomes progressively enriched in more volatile metals (Cd and Pb) with decreasing particle size and in the order bottom ash < cyclone ash < water scrubber particles < stack particles. The less volatile metals (Cu and Ni) are not similarly enriched. The heavy metals in the sludge are often completely retained in the output ash. The leachability of heavy metals from the ash is much lower than that from the digested sludge.