there was awareness of potential health hazards associated with soil contamination, land dumping was often seen as the most practical way to dispose of chemical wastes.

Concern over contamination of air, surface water, and groundwater from uncontrolled land-disposal sites provoked the emergence of tougher regulations for land disposal. Those provided incentives for industry to use a wide variety of traditional and advanced technologies for managing hazardous wastes. The regulations also require that many wastes be treated by incineration or other methods to reduce organic content to specified levels before the wastes can be disposed of in a secured landfill.

EPA estimates that regulations forbidding land disposal of any hazardous waste that contains liquid will substantially increase the quantity of hazardous waste directed to incinerators, boilers, and furnaces. Although industrial growth is also likely to increase hazardous-waste generation, increasing emphasis on waste minimization and recycling is likely to exert pressure to reduce such generation.

Many kinds of hazardous waste are fed to incinerators, boilers, and industrial furnaces essentially as received. These wastes are often difficult to handle because of their consistency or hazardous nature, so minimal handling is preferred. Where feasible, however, pretreatment operations are desirable to facilitate homogenization of the waste and continuous feeding to the combustor. Common pretreatment operations for liquid wastes are blending and solids filtration; for solids, screening and size reduction (crushing or shredding); and for wastes in containers, liquid-phase decanting and shredding to allow continuous auger feeding.

Several types of industrial furnace systems are used to incinerate hazardous waste to recover energy or material. The major ones are cement kilns, lightweight-aggregate kilns, halogen-acid furnaces, and metal-recovery and smelting furnaces.

Cement is produced by feeding raw materials into a rotary kiln and burning them with fuel under controlled-temperature conditions. Suitable hazardous waste is used as an auxiliary or replacement fuel. Lightweight aggregate is produced much like cement, in a kiln configured and fueled much like a cement kiln using feed stocks that include special clays, pumice, scoria, shale, and slate. It is used to make insulation and monostructural and lightweight concrete. Halogen-acid furnaces are typically modified firetube boilers that process secondary waste streams containing 20-70% chlorine or bromine. The combustion gases are “scrubbed” with water to produce a halogen-acid product.

Cement kilns have been used to burn hazardous waste since 1972, when PCBs were combusted in Ontario, Canada. Since then, the use of waste-fueled kilns has become widespread in the United States, Belgium, and Switzerland. An intended benefit of combustion of waste as fuel in kilns is the recovery of energy from the waste and the consequent conservation of nonrenewable fossil fuels. Moreover, there is a strong economic incentive in that the kiln operators

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