Ash Management

The first priority in state-of-the-art ash management is to reduce both the volume and toxicity of the residue left after burning MSW. Removing noncombustibles and material containing toxic substances from the MSW before incineration followed by efficient combustion accomplishes this reduction. The amount of toxic material in ash has been increasing as more effective air pollution control devices capture more pollutants in the fly ash.

State-of-the-art ash management practices are designed to minimize worker and citizen exposure to potentially toxic substances in ash during handling, treatment, and storage, long-term storage, or reuse. Safe ash management has several components:

The bottom ash or residue (noncombustible and partially burned solids left in the incinerator) and fly ash (material captured by emission control devices) is kept separate for rigorous handling of the potentially more toxic fly ash.

The ash is contained while still in the plant. A closed system of conveyors is preferable to handling ash in the open.

The ash is transported wet in leakproof, covered trucks to disposal sites.

The ash is treated to minimize its potential toxic impact. The ash is disposed in ash-only monofills because codis-posal of ash with MSW increases the leachability of the ash when it is exposed to acid.

Fly ash from APC is fine-grained, not unlike soot from fireplaces. For every ton of MSW burned, approximately ]/4 tn becomes some form of ash. Fly ash accounts for about 10 to 15% of the total ash residue; the remaining 85 to 90% is bottom ash.

Operational data from resource recovery incineration facilities throughout the world indicate certain heavy metals, such as lead and cadmium, tend to concentrate in the fly ash, scrubber residue, and small particles (less than 3/8 in) in the bottom ash. Heavy metals, including lead, cadmium, and total soluble salts (including chlorides and sulfates), are potentially leachable components which can impact the environment. Leachable components are those chemical species which dissolve in water and are transported with water. The toxicity characteristic leaching procedure (TCLP) and numerous other techniques exist to estimate the potential environmental impact resulting from ash generation, handling, and disposal.

Two main categories of ash treatment, both recently developed and being improved, are fixation or cementation and vitrification. Both techniques minimize the environmental impact of ash and enable its reuse in situations such as cinderblocks, reefs, and roads. A few incinerators have onsite vitrification facilities.

Another new treatment technology involves washing the toxic materials out of the ash with hot water and then

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