HAZARDOUS WASTE TREATMENT - Environmentally Conscious Materials Handling - page 191

potential of all types of liners, landfill leachate collection systems have multiple
leachate collection zones to protect groundwater from contamination. The
secondary leachate collection zone—also known as leak detection system—
controls the leachate that may pass through the primary LCS (LaGrega
et al. 2001).
Leachate contains many of the same constituents that are typical contaminants
of concern when considering disposal of any waste stream, especially very high
COD and ammonium concentrations. Unique to leachate, however, are xenobiotic
organic compounds (XOCs) and heavy metals. XOCs and the heavy metals are
hazardous and can be toxic, corrosive, flammable, reactive, and carcinogenic,
among other hazards, and can also be bioaccumulative and persistent.
Given the levels of ammonium present in leachate, it is also necessary to
deal with it on a hazardous level, as it is the primary pollutant of groundwater
from landfills (Slack et al. 2005). This is a problem in all types of landfills,
not specifically those designed for hazardous waste storage and disposal. The
highly reactive environment of the leachate permits a wide range of chemical and
biological transformation of the waste with the XOCs and heavy metals. These
transformations in the leachate can lead to the formation of toxic byproducts
from relatively harmless organic substances.
In the primarily anaerobic conditions found in capped landfills or in the lower
layers of actively operating landfills, there are methanogenic conditions through
redox environments. The heavy metal content of leachate reduces from acid phase
to methanogenic phase. Depending on the contaminant, little or no degradation
may occur in the treatment conditions of the landfill, with the toxic hazardous
waste percolating down into the leachate and thereby requiring treatment. These
transformations and degradation of the wastes can cause those wastes disposed of
in the landfill to become even more toxic than they were at the time of disposal
(Slack et al. 2005).
The design of the landfill cover system is an important consideration. A landfill
cover system controls the infiltration through the top of the landfill and reduces
recharge of precipitation into waste or contaminated soil; it prevents direct contact
with waste or contaminated soil and also prevents fugitive emissions. Landfill
cover also allows the site to be returned to some beneficial use and makes the
site aesthetically acceptable to nearby residents.
Construction of a landfill results in large increases in off-site flows and sedi-
ments. Therefore, runoff from non-landfilled areas is diverted off site. The off-site
flow is controlled by constructing swales and stormwater recharge ponds. Runoff
from exposed excavation area is either directed to siltation basins and discharged
off site or directed to localized holding sumps and sampled for contamination.
Landfill gas is generated during the natural process of bacterial decomposi-
tion of organic material contained in the wastes placed in landfills. By volume,
landfill gas is about 50 percent methane and 50 percent carbon dioxide and
water vapor, and also contains small amounts of nitrogen, oxygen, and hydrogen,