TM 5-814-5
organics, and wet air oxidation to remove highly
decomposition takes place depleting the oxygen
toxic organics.
present and producing carbon dioxide. In the
e. Final Disposal. Leachate may be (1) dis-
second phase, the percentages of both nitrogen and
charged to a surface water through a National
oxygen are reduced very rapidly, and anaerobic
Pollutant Discharge Elimination System (NPDES)
conditions lead to the production of hydrogen and
permit, (2) discharged to a land application system,
carbon dioxide, the latter reaching its peak during
or (3) recirculated back to the landfill. Recirculation
this phase. Some experts consider the second phase
can accelerate waste stabilization in the land-fill,
to have started when the free oxygen is depleted. In
but does not eliminate the need for ultimate
the third phase, the percentages of carbon dioxide
disposal of the leachate. The exception is arid
regions where the soil and refuse attenuation
and oxygen concentrations are reduced to zero, and
capability eliminate leachate production. Discharge
the percentage of methane increases rapidly to
to a municipal STP eliminates the need for a
reach a relatively constant level. The fourth phase,
separate NPDES permit for the leachate.
which occurs after the landfill has become more
stable, may be termed pseudo steady-state because
3-7. Gas Control.
the percentages of methane, carbon dioxide, and
nitrogen all reach stable values. The time
a. Production. Although gas generated within
dependency of methane production is critical for
some types of landfills may be negligible, most
landfill gas recovery and reuse projects. In most
landfills are expected to generate a significant
cases, over 90 percent of the gas volume produced
quantity of gas. The quality of gas depends mainly
from the decomposition of solid wastes consists of
on the type of solid waste. As with leachate, the
methane and carbon dioxide. The current procedure
quality and quantity of landfill gas both vary with
of sealing off landfills to maintain a dry env-
time. The following discussion on gas quality and
ironment significantly slows these four phases. A
quantity pertains mainly to landfills with municipal
typical analysis of landfill gas is shown in table 3-2.
type wastes, which would be expected at most
(2) Quantity. The quantity of gas generated
installations.
depends on waste volume, waste composition, and
(1) Quality. Landfill gases, specifically methane
time since deposition of waste in the landfill, as
gas, are natural by-products of anaerobic microbial
summarized above. Methane production ranges
activity in the landfill. The anaerobic process
from 0.04-0.24 cubic feet per pound of waste per
requires water and the proper mix of nutrients to
year. Gas production may be increased by adding
maintain optimal conditions. The quality of gas
nutrients, such as sewage sludge or agricultural
varies with time, and may be characterized by four
waste, the removal of bulky metallic goods, and the
distinct phases. In the first phase, which may last
use of less daily and intermediate cover soil.
several weeks under optimum conditions, aerobic
3-11
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