HAZARDOUS WASTE TREATMENT - Environmentally Conscious Materials Handling - page 189

wastes with lower heating values. It also works well for wastes with smaller
solid size and generates less ash than rotary kiln incinerators.
Multiple hearth incinerators have limited use in hazardous waste treatment
because the temperatures cannot be raised high enough to destroy the hazardous
constituents due to equipment limitations. They are more commonly used for
sewage sludge treatment. Waste is fed from the top. As it combusts, residues are
raked to the center, where they fall on progressively lower hearths until only ash
remains. These are usually larger unit processes. They are used for wastes with
higher solids content or larger solids size and generates higher ash residue.

5 LAND DISPOSAL SYSTEMS

One of the options for ultimately disposing of hazardous wastes is landfilling.
Landfills are relatively inexpensive when compared to other treatment and dis-
posal options, but they do have some negatives. U.S. federal regulations (RCRA)
developed in the late 1980s require separate landfill disposal of municipal (house-
hold) solid waste and hazardous waste. RCRA Subtitle D regulates municipal
solid waste (MSW) landfills, and RCRA Subtitle C is used for hazardous waste
landfills. Before RCRA, most landfills were dumps. Many closed dumps still
exist, and a majority of these dumps continue to pollute groundwater and surface
waters.
Hazardous wastes that are disposed of in a landfill in many cases still exist in
toxic form, but are suitably contained for landfill disposal. However, the toxicity
will remain and has the potential to leak/leach into surrounding soils and ground-
water. For this reason, the landfill liners and the leachate collection and treatment
systems are viewed to be the two most important factors when designing a new
landfill.
In weighing the options for hazardous waste disposal, landfills are often deter-
mined to be the best option, based primarily on disposal costs. A major portion
of hazardous wastes are disposed of in this method. According to Visvanathan
(1996), landfilling accounts for 79 percent of the hazardous waste disposal in the
United Kingdom, storing 2.7 million tons per year of waste. A similar percentage
is likely in the United States.
The most important landfill design considerations when designing a hazardous
waste landfill include site selection, properly installing clay and synthetic liners to
limit leachate percolating into the groundwater, the final cap to a closed landfill,
and the leachate and gas collection and treatment systems.
Wastes have to be contained or stabilized prior to disposal in a landfill site. In
order for the wastes to be stored in the landfill, they must be contained in sealed
drums or other containers or other stabilization to limit the waste's migration
from its landfill cell into other areas of the landfill, where there may be leachate
infiltration into the groundwater. Leachate is the liquid that forms at the bottom
of the landfill, resulting from percolation, precipitation, uncontrolled runoff, and