Incineration is often mistakenly referred to as a waste disposal technology, when in fact it is a waste treatment technology. This is because incineration, like other treatment technologies, produces residues that themselves require treatment and/or disposal, most often in a landfill.

Ash - or, in the case of pyrolysis, slag - is the residue from incineration produced in the greatest quantity. Both ash and slag are defined as hazardous wastes under international law. Other significant residues include scrubber water and filter cake (the solids from scrubber water treatment), both of which are usually heavily contaminated with toxics.

There are two basic types of incinerator ash: bottom ash and fly ash. Bottom ash, also known as clinker, is the residue from the furnace itself, while fly ash is the fine particles trapped by the air pollution control equipment. Bottom ash makes up about 90 percent of the total ash produced and has been shown to contain significant concentrations of heavy metals, organohalogens, and other chemical pollutants. However, the fly ash, although much smaller in volume, is generally far more hazardous. If there is no air pollution control equipment, or it is not functioning, many of the hazardous byproducts will be released into the air instead of being trapped in the fly ash. This reveals a central conundrum of incineration: the cleaner the air emissions, the more hazardous the ash.

Source of information

Global Anti Incineration Alliance