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When thinking about garbage and its ultimate destination, most people think
of a "dump." Traditional open dumps are quite rare today; instead
trash is deposited in modern sanitary landfills. The reason for this evolution
is that simply dumping garbage in a large pile created several problems. Aside
from being unsightly and foul smelling, dumps attracted insects, gulls, rats,
and other rodents. These animal "vectors" are harmful to the health of
the people living nearby because they can carry disease. Uncontrolled fires,
either set or spontaneously combusting, plagued open dumps. The most serious
problem resulted from rain percolating through the garbage and carrying harmful
bacteria and hazardous chemicals from dumps into groundwater and nearby lakes or
streams. This polluted runoff is called leachate.
As a result of these problems, open dumps were banned by the U.S.
Environmental Protection Agency (EPA) in 1979, and have been replaced by
sanitary landfills. The sanitary landfill concept developed in Great Britain
during the 1920s. The procedure entails alternating layers of compacted garbage
with cover material. This can be soil, compost, or any other approved material.
Garbage is dumped and then compacted by special bulldozers aptly called
compactors. At the end of each day when all the garbage has been dumped and
flattened, bulldozers cover the fresh layer of garbage with at least six inches
of cover material. This process slows decay, prevents exposure to health
hazards, and reduces odor problems. Burying each day's waste eliminates the
problems historically associated with dumps.
The Resource Conservation and Recovery Act of 1992 requires that all
landfills operating be "lined" and equipped with leachate collection
systems. A typical liner is composed of layers of clay, gravel, plastic and
synthetic material to prevent leachate from escaping. Lined landfills are also
fitted with pipes to collect and drain the leachate. Collected leachate is
treated and discharged, or can be recirculated through the landfill. Some tests
indicate that recirculating leachate may speed the decomposition process, but
this practice is not widely used. In 1992 Only 11% of existing landfills were
lined with leachate-collection systems.
In addition to leachate formation, decomposition is another dynamic occurring
in a landfill. Inside a landfill, innumerable microorganisms are hard at work.
These microbes may be fungi or microscopic bacteria. Microbes feeding on the
organic matter in the landfill transform it into smaller and smaller particles.
This rotting or decaying process is called decomposition. Decomposition occurs
very slowly in a landfill because there is very little air and moisture (and no
sunlight) in the compacted layers of garbage. The lack of these elements means
that even biodegradable materials will take many years to break down.
The odor sometimes present at a landfill is caused by the gases created by
microorganisms as they break down organic material. Landfill Gas (LFG) is
created as a result of the decomposition that is occurring. Left uncontrolled,
LFG will migrate out of a landfill and into the atmosphere, creating odor,
safety, and environmental problems. LFG is composed of methane (50-55%), carbon
dioxide (45-50%), and trace amounts of other gases such as oxygen, nitrogen, and
hydrogen sulfide.
The methane component of LFG is of particular concern and must be controlled
by the operators of the landfill for several reasons. Although the gas itself is
odorless, it suspends other gasses with unpleasant odors (hydrogen sulfide), is
highly flammable and will explode if trapped. In addition, the methane in a
cubic foot of LFG is believed to be 20 to 30 times as powerful a contributor to
global warming as an equivalent amount of carbon dioxide.
There are different systems employed by modern sanitary landfills to vent or
collect methane. Some landfills use permeable gravel trenches; others combine a
system of pipes with gravel trenches; and others use a pumping system of a
connected network of pipes or wells to draw gas out of the landfill. These
systems are operated only in portions of landfills that have been closed
temporarily or permanently, although they can be installed as the landfill is
built and later connected.
When LFG is captured it can be burned off in a process called
"flaring," or recovered to generate power. The Johnston Landfill has
the largest methane gas recovery/power generating system in the northeast.
Recovering methane for power generation involves drawing LFG out of the Johnston
landfill from over 100 methane wells by a huge vacuum called an "extraction
blower." The gas is then cleaned and dried, and the methane component
separated from the other gases. The purified methane gas then flows through
eight 12-cylinder engines, which work much like a turbocharged car engines.
Outside air is filtered in and combined with the gas to produce an energy-rich
mix. The engines power a huge generator which creates electricity. In 1992, only
123 landfills in the country actually collect methane to recover energy. Rhode
Island's Johnston landfill gas-to-energy facility generates 13.6 megawatts of
electricity, a portion of which is used for internal plant needs. The remaining
12.3 megawatts is sold to Narragansett Electric and provides power for
approximately 18,000 homes annually.
Eventually a landfill reaches its maximum capacity, and cannot accept any
more garbage. At this point the landfill must be closed. It is
"capped" with a layer of clay and a six-foot layer of earth. Closed
landfills have become parks, playing fields, golf courses, and even
amphitheaters and ski slopes. The land is not suitable for building homes
however, as it shifts and settles slightly as the waste slowly decomposes. Sixty
percent of the nation's waste was landfilled in 1994, and it remains the primary
method of municipal solid waste (MSW) management in the United States today. It
will continue to be needed for disposing of non-recyclable or non-combustible
materials, as well as residual waste from recycling and incineration.
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