Page 345 - Materials Chemistry, Second Edition

CAT3525_C10.qxd 1/31/2005 12:00 PM Page 316
316 Waste Management Practices: Municipal, Hazardous, and Industrial
Slowly decomposable
C H O N 9H O → 11CH 9CO NH 3
2
4
10
20
30
2
g/mol 436 162 176 396 17
Note. The equations do not balance exactly due to rounding.
Calculate the volume of CH and CO that can be generated from the different waste fractions.
4
2
Rapidly decomposable
3
CH [(912)(23.6 kg)]/[(1684.1)(0.718 kg/m )] 17.8 m 3 at STP
4
3
CO [(1056)(23.6 kg)] / [(1684.1)(1.98 kg/m )] 7.5 m 3 at STP
2
3
3
(density of CH 0.718 kg/m ; density of CO 1.98 kg/m )
4
2
Slowly decomposable
3
CH [(176)(3.81 kg)] / [(436.4)(0.718 kg/m )] 2.14 m 3
4
3
CO [(396)(3.81 kg)] / [(436.4)(1.98 kg/m )] 1.75 m 3
2
Determine the total theoretical quantity of gas generated per unit weight of organic matter.

Rapidly decomposable

3
3
volume/kg (17.83 m 7.5 m )/23.6 kg 1.07 kg/m 3
Slowly decomposable
3
3
volume/kg (2.14 m 1.75 m )/3.81 kg 1.02 kg/m 3
(Adapted from Tchobanoglous et al., 1993)

10.4.14.7 Control of Explosive Gases
Landfill gas emissions contribute to local smog and cause unpleasant odors and trigger complaints
from neighbors. Methane is the primary concern in evaluating landfill gas generation because it is
highly combustible. Methane accumulation in structures near a landfill may result in fire and explo-
sions. Methane hazards can be prevented through monitoring of landfill gas and corrective action.
Although methane is lighter than air and carbon dioxide is heavier, these gases tend to remain
mixed. They migrate as a function of the density of the mixture and other gradients such as tem-
perature and partial pressure (U.S. EPA, 1994; Tchobanoglous et al., 1993). In an ideal situation,
landfill gas would simply diffuse to the surface of the unit and disperse into the atmosphere.
Unfortunately, there are a number of circumstances that will force landfill gas to migrate laterally
rather than vertically. Landfill gas will migrate through the path of least resistance. The direction of
migration is controlled in part by the permeability of the soil and fill material. This is especially rel-
evant in pre-RCRA landfills, which may lack a complete subsurface liner. Coarse, porous soils such
as sand and gravel adjacent to the landfill will promote greater lateral transport of gases than would
fine-grained soils. If the unit is closed, landfill gas will migrate laterally if the final cover is dense
or impermeable and if the side slopes do not contain a gas barrier. Additionally, with an increase in