Fly ash and pollutants are collected in a flue gas cleanup stage. Heavier ash, inerts, and metals are collected and recycled if appropriate. Outputs may include power, steam, fly ash (via treatment with
pollution control measures), and bottom ash or other post-combustion materials (e.g. metals).
Typically, the composition by weight of output ash is approximately three quarters bottom ash and
one quarter fly ash. Collective residual ash typically comprises approximately five to 15 percent by
volume of input MSW processed, or roughly 15 to 25 percent by weight of input MSW processed.