onto particle surfaces. It can also contain residues from reagents, such as lime and activated carbon, themselves with condensed or absorbed contaminants. Fly ash is collected in hoppers beneath the APCDs.

Scrubber water is a slurry that results from the operation of wet scrubbers and contains salts, excess caustic or lime, and contaminants (particles and condensed organic vapors) scrubbed from the flue gas.

In addition, there are various other waste streams that may be generated by the incinerator. For example, waste-to-energy plants produce blow-down water from the heat recovery boilers; some municipal solid-waste incinerators recover small quantities of condensed metals (e.g., lead alloys) from parts of their flue gas system. The initial sorting of municipal-solid waste produces a stream of large items unsuitable for burning (such as whole refrigerators, gas stoves, and auto batteries).

In 1995, the International Ash Working Group reviewed the available scientific data and developed a treatise on municipal solid-waste incinerator-residue characterization, disposal, treatment, and use (IAWG 1995). It found that the different temperature regimes in a municipal solid-waste incineration facility impart different characteristics to the residues collected from the various operational steps in a facility. Its report concluded that the development of management strategies for municipal solid-waste incinerator residues requires knowledge of the intrinsic properties of the material, including the physical, chemical, and leaching properties.

Cement kilns burning hazardous waste are in a class by themselves. All cement kilns are major sources of particulate emissions and are regulated as such by EPA and the states. Kiln-exhaust gases contain large amounts of entrained particulate matter known as cement-kiln dust, a large fraction of which is collected in APCDs. The kiln dust so collected is generally recycled to the kiln feed. Under the current BIF regulation, residue generated primarily by the combustion of fossil fuels may be exempted as RCRA hazardous waste provided that the facility operator can demonstrate that such wastes are no different from normal process residues or that any change caused by the combustion of hazardous waste as supplemental material in the fuel will not cause harm to human health or the environment. Cement-kiln dust is in that category.

Ash Handling

Two concerns of on-site ash management at incineration facilities are the safety of workers and the possibility that fugitive ash will escape into the environment during handling or removal of the ash for disposal. Both concerns require that the ash be contained at all times both inside and outside the facility, as described above. In the facility, water is used to quench the ash, simultaneously reducing dust generation and minimizing the possibility of ash-dust inhalation or ingestion by workers. In modern systems, a closed system of con-



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