Environmental Engineering Reference
In-Depth Information
may tear it. Acids in the leachate may eventually breach the liner. Even-
tually all pipes clog and all buried pumps fail. Landfi lls are not the best
method of garbage disposal.
Three companies own more than two-thirds of all landfi lls in the
United States. Landfi lls are valuable because it gets harder every year to
open a new one because of public opposition; another reason is the
expense of fi nding a location and building it to federal standards. It takes
an average of ten years to open a new landfi ll.
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It is much cheaper to
expand an existing one.
Landfi ll Gas
When the organic portion of garbage decomposes in the absence of
oxygen, as in a sealed and impermeable landfi ll, gas is produced that
is typically composed of 35 to 55 percent methane, 25 to 45 percent
carbon dioxide, and up to 10 percent water vapor.
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But some landfi lls
produce gas that is 85 percent methane, a greenhouse gas twenty times
more potent than carbon dioxide, so keeping it from dispersing in the
atmosphere is environmentally signifi cant. Gas generation from a landfi ll
can continue at stable rates for many decades and, when purifi ed, the
methane (natural gas) can be a valuable energy source. Municipal solid
waste landfi lls are the largest source of human-related methane emis-
sions in the United States, accounting for about 34 percent of these
emissions.
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Landfi ll gas is extracted from landfi lls using a vacuum system and a
series of wells drilled into the waste pile. This system directs the collected
gas to a central point where it can be cleaned of hydrogen sulfi de, volatile
organic compounds, and carbon dioxide that landfi ll gas is likely to
contain. It can then generate electricity, replace fossil fuels, fuel green-
house operations, or be upgraded to pipeline quality gas. In mid-2008,
469 projects were functioning in the United States, and according to the
EPA, 520 landfi lls exist that could economically support a project (fi gure
4.2).
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Those operating in 2008 were supplying 11 billion kilowatt-hours
of electricity per year (enough for about 870,000 households) and 77
billion cubic feet of gas (enough to heat 534,000 homes), and they pre-
vented emissions from 182 million barrels of oil (U.S. consumption is
about 8 billion barrels per year) or the greenhouse gas emissions from
14.3 million cars (there are 251 million cars in the United States).
Landfi ll gas has safety and economic benefi ts as well. Whether or not
the gas is captured, landfi lls must be vented to prevent accumulations of
methane because the gas is explosive at concentrations of greater than 5
