Page 343 - Materials Chemistry, Second Edition

CAT3525_C10.qxd 1/31/2005 12:00 PM Page 314
314 Waste Management Practices: Municipal, Hazardous, and Industrial
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between 0.06 and 0.12 m /kg (1 to 2 ft /lb) of waste on a dry basis over 10 to 40 years. Gas yields
based on waste generation have been predicted using assumptions such as:
● 50% of the organic material placed in the landfill will actually decompose
● 50% of the landfill gas generated is recoverable
● 50% of landfills are operating within a favorable pH range

Once the expected yield is determined, a model is utilized to show the pattern of gas produc-
tion over time. The U.S. EPA has published a model called The Landfill Gas Emissions Model
(LandGEM) based on the following equation (U.S. EPA, 1998):
n
T
Q 2kL M e –kt i (10.9)
i
0
j 1
where Q is the total gas emission rate from a landfill (volume and time), n the total time period of
T
–1
waste placement, k the landfill gas emission constant (time ), L the methane generation potential
0
(volume/mass of waste), t the age of the ith section of waste (time) and M the mass of wet waste,
i
i
placed at time i.
In this model, the gas generation rate is based on a first-order decomposition model, which uses
two parameters: L , the potential methane generation capacity of the waste and k, the methane gen-
o
eration decay rate. The methane generation rate is assumed to be maximal upon MSW placement
in the landfill. This model allows the user to enter L and k values using test data and landfill-spe-
o
cific parameters or use default L and k values derived from research data (U.S. EPA, 1998).
o
The amount of MSW in the landfill is calculated for this model using site-specific characteristics
entered by the user, such as the years the landfill has been in operation, the amount of MSW in place
in the landfill, and landfill capacity. Emission rates are estimated for CH ,CO , nonmethane organic
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compounds (NMOCs), and air pollutants expected to be emitted from landfills based on test data com-
piled in the U.S. EPA compilation of air pollutant emission factors, AP-42 (U.S. EPA, 1997a).
EXAMPLE 10.4
A landfill cell receives about 225,000 metric tons of MSW per year. Calculate the gas production
for the first year, given a landfill gas emission constant of 0.0335 year 1 and a methane generation
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potential of 175 m /metric ton.
SOLUTION

For the first year,
Q 2 (0.0335) (175) (225,000) (e 0.0335(1) ) 2,551,067 m 3
T
Note. In the second year, this same cell will produce less total gas; however, the new layer for the
second year will produce gas, and the yields of the two cells will be combined to calculate the total
gas production for the second year, and so on.
The lag period prior to CH generation may range from a few weeks to a few years, depending
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on landfill conditions. The duration of gas production is also influenced by environmental condi-
tions within the landfill.
An example utilizing the full LandGEM model appears in the Appendix to this chapter.

EXAMPLE 10.5

Using data from the table below, estimate the chemical composition and the amount of gas that can
be derived from the organic constituents in MSW. Determine the chemical composition and the