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The Sanitary Landfill 313

TABLE 10.12
Landfill Gas Composition Over the First 48 months
after the Closure of a Landfill Cell
Average (% by vol.)
Time since N 2 CO 2 CH 4
Closure (months)
0–3 5.2 88 5
3–6 3.8 76 21
6–12 0.4 65 29
12–18 1.1 52 40
18–24 0.4 53 47
24–30 0.2 52 48
30–36 1.3 46 51
36–42 0.9 50 47
42–48 0.4 51 48
Source: Tchobanoglous, T. et al., Integrated Solid Waste Management:
Engineering Principles and Management Issues. McGraw-Hill,
New York, 1993. Data reproduced with kind permission of the
McGraw-Hill Companies, Inc.

be estimated using Equation 10.8, assuming the complete conversion of the biodegradable organic
waste into CO and CH :
2
4
C H O N (4a b 2c 3d)/4H O → (4a b 2c 3d)/8CH 4
d
c
2
a
b
(4a b 2c 3d)/8CO dNH 3 (10.8)
2
The organic component of MSW can be divided into two categories: (1) those materials that
will decompose rapidly (3 months to 5 years) and (2) those materials that will decompose slowly
(up to 50 years or more) (Vesilind et al., 2002). The rapidly and slowly decomposable components
of the organic fraction of MSW are listed in Table 10.11.
10.4.14.6 Predicting Gas Production
It is essential for landfill operators to estimate the volume of gas produced from an operating or
closed landfill. Similarly, the composition of the gas (e.g., methane, moisture, and sulfur content)
is important to energy users. Engineers use mathematical models to predict landfill gas generation.
Models are designed based on population data, per capita waste generation, waste composition,
waste moisture content, and expected gas yield per unit dry weight of waste. Mathematical models
are also used to model gas recovery systems including layout, equipment type, operation parame-
ters, and failure simulation (Vesilind et al., 2002). The following parameters must be known if gas
production is to be accurately estimated: gas yield per unit weight of waste, lag time prior to gas
production, shape of the gas production curve, and duration of gas production.
3
3
In theory the biological decomposition of 1 ton of MSW produces 442 m (15,600 ft ) of land-
3
3
fill gas containing 55% CH and a heat value of 19,730 kJ/m (530 Btu/ft ). Only a portion of the
4
waste converts into CH due to the presence of inaccessible waste and non-biodegradable fractions;
4
3
3
therefore, the actual average methane yield is closer to 100 m /MT (3,900 ft /ton) of MSW.
Significant variation in gas production data has been noted at landfills across the United States due
to differences in climate, waste types, and landfill management. The methane generation usually is