Generally, WTE residues can be differentiated into two fractions: The term fly ash refers to the fine particles that are removed from the flue gas. However, usually the fly ash includes also residues from other air pollution control devices, such as scrubbers. Fly ash typically amounts to 10-20% by weight of the total ash.
The rest of the WTE ash is called bottom ash (80-90% by weight). The main chemical components of ash are silica (sand, quartz), CaO, Fe2O3, and Al2O3 for a mass burn WTE plant. Usually the ash has a moisture content of 22-62% by dry weight. The chemical composition of the ash depends strongly on the original MSW feedstock and the combustion process.
Dr. Jurgen Vehlow of Karlsruhe Research and an EEC Research Associate has compiled ash composition data from several studies (see figure below). E.g. Hg can vary from 0.1 to 10 ppm in bottom ash and from 1 to 30 ppm in fly ash (before treatment).