The nature of the ash or slag emerging from the grate of a power-from-waste plant will depend on both the type of waste being burnt and the combustion conditions. While its primary constituents will be solid, incombustible mineral material from the wastes, this residue will be contaminated with traces of a variety of metals. These traces may be in a toxic or a harmless form.

By careful control of the temperature in the furnace, it is possible to incorporate the metals into the mineral content of the ash and render them effectively harmless. This is a process called sintering. The effectiveness of the sintering process in rendering toxic metals harmless will be determined by measuring the amounts capable of being leached out by water. The ash may also contain some toxic organic compounds such as dioxins. Furnace conditions can minimise these too since a sufficiently high temperature will normally destroy such compounds. The effectiveness of this will again be determined by a leaching test.

If the ash or slag is too toxic it will have to be buried in a landfill. Modern facilities aim to render it sufficiently stable and benign that it can be used for road building or for similar purposes. When they succeed, only a residual 1% of the original waste needs to be buried.

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