The residual waste treated in Waste-to-Energy plants is burned under controlled conditions, reducing the volume of the waste by about 90%.

In a Waste-to-Energy plant the bottom ash is collected at the end of the furnace’s grate. It consists of non-combustible materials, and is the residual part from the incineration of waste. Even if the waste was sorted before hand, there are metals found in it. Both ferrous metals and non-ferrous metals can be taken out of the bottom ash and recycled. After the metals are taken out, the rest is ashes, like gravely sand. This is stockpiled before being used in road construction or as a covering layer on landfill sites.

The residues from the flue gas cleaning system amount only to 3-4 % of the mass of the waste entering the plant. These residues are collected after the filtration process and must be carefully stored to ensure no escape of the material into the local environment. The material is then transported in sealed containers to landfill sites.