310 ENVIRONMENTALENGZNEERING
The effectiveness of each method is highly waste-specific, and tests must be conducted
on a site-by-site basis before a reliable leachate treatment system can be designed. All
methods produce waste sludge that must reach ultimate disposal.
A secondary safeguard system consists of another barrier contoured to provide a
backup leachate collection system. In the event of failure of the primary system, the
secondary collection system conveys the leachate to a pumping station, which in turn
relays the wastewater to the surface for treatment.
A final safeguard system is also advisable. This system consists of a series of dis-
charge wells up-gradient and down-gradient to monitor groundwater quality in the area,
and to control leachate plumes if the primary and secondary systems fail. Up-gradient
wells act to define the background levels of selected chemicals in the groundwater and
to serve as a basis for comparing the concentrations of these chemicals in the discharge
from the down-gradient wells. This system thus provides an alarm mechanism if the
primary and secondary systems fail.
If methane generation is possible in a hazardous waste landfill, a gas collection
system must be designed into the landfill. Sufficient vent points must be allowed so
that the methane generated may be burned off continuously.
Operation. As waste containers are brought to a landfill site for burial, specific
precautions should be taken to ensure the protection of public health, worker safety, and
the environment. Wastes should be segregated by physical and chemical characteristics,
and buried in the same cells of the landfill. Three-dimensional mapping of the site is
useful for future mining of these cells for recovery purposes. Observation wells with
continuous monitoring should be maintained, and regular core soil samples should be
taken around the perimeter of the site to verify the integrity of the liner materials.
Site Closure. Once a site is closed and does not accept any more waste, the
operation and maintenance of the site must continue. The impervious cap on top of
the landfill must be inspected and maintained to minimize infiltration. Surface water
runoff must be managed, collected, and possibly treated. Continuous monitoring of
surface water, groundwater, soil, and air quality is necessary, as ballooning and rupture
of the cover material may OCCUT if gases produced or released from the waste rise to
the surface. Waste inventories and burial maps must be maintained for future land use
and waste reclamation. A major component of postclosure management is maintaining
limited access to the area.
CONCLUSIONHazardous waste is a relatively new concern of environmental engineers. For years,
the necessary by-products of an industrialized society were piled “out back” on land
that had little value. As time passed and the rains came and went, the migration of
harmful chemicals moved hazardous waste to the front page of the newspaper and
into the classroom. Engineers employed in all public and private sectors must now
face head-on the processing, transportation, and disposal of these wastes. Hazardous
waste must be appropriately addressed at the “front end” of the generation process,
either by maximizing resource recovery or by detoxification at the site of generation.