What happens to waste-to-energy incineration ash?

Waste-to-energy plants that incinerate MSW produce two types of ash: fly ash and bottom ash.

Fly ash—or air pollution control residue (APC)—consists of the hazardous and fine particulates removed from a WtE plant’s flue gas, the fumes produced from incineration. Fly ash generally undergoes treatment to reduce its negative environmental impacts, largely in the form of air and water pollution of nearby ecosystems. While there are efforts to recycle and reuse fly ash, it’s commonly sent to hazardous waste landfills.

Bottom ash—or incinerator bottom ash (IBA)—is all ash left over that is not fly ash. It consists mainly of silica, calcium, iron oxide and aluminum oxide. Large magnets can remove some of these materials for recycling and repurposing. For example, construction companies might use bottom ash to make concrete or bulk fill. The rest is sent to landfills.

The ash output generated by WtE plants is significantly smaller than the waste that goes into them. It ranges from 15–25% by weight and 5–15% by volume of the waste precombustion.³