Page 349 - Materials Chemistry, Second Edition

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320 Waste Management Practices: Municipal, Hazardous, and Industrial

TABLE 10.13
Typical Concentrations of Trace Compounds Found in Landfill
Gas at 66 MSWLFs, California
Compound Mean ppb, v
Concentration Maximum
Acetone 6,838 240,000
Benzene 2,057 39,000
Chlorobenzene 82 1,640
Chloroform 245 12,000
1,1-Dichloroethane 2,801 36,000
Dichloromethane 25,694 620,000
1,1-Dichloroethene 130 4,000
Diethylene chloride 2,835 20,000
trans-1,2-Dichloroethane 36 850
Ethylene dichloride 59 2,100
Ethyl benzene 7,334 87,500
Methyl ethyl ketone 3,092 130,000
1,1,1-Trichloroethane 615 14,500
Trichloroethane 2,079 32,000
Toluene 34,907 280,000
1,1,2,2-Tetrachloroethane 246 16,000
Tetrachloroethylene 5,244 180,000
Vinyl chloride 3,508 32,000
Styrenes 1,517 87,000
Vinyl acetate 5,663 240,000
Xylenes 2,651 38,000
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.

10.4.16.1 Passive Systems
Passive gas control systems rely on natural pressure and convection to vent landfill gas to the atmos-
phere. Passive systems involve ‘high-permeability’ or ‘low-permeability’ techniques. High-permeabil-
ity systems incorporate pathways such as trenches, vent wells, or perforated vent pipes surrounded by
coarse material to vent landfill gas to the surface. Low-permeability systems block lateral migration via
barriers such as synthetic membranes and clayey soils. Passive systems may be incorporated into a land-
fill design or may be installed later for corrective purposes. They may also be installed within a landfill
unit along the perimeter, or between the landfill and the facility property boundary (U.S. EPA, 1985).
At the time of landfill closure, a passive system may be incorporated into the final cover sys-
tem. This may consist of perforated collection pipes and high-permeability soils located directly
below the impermeable cover. Passive vent systems may be connected to header pipes located along
the perimeter of the landfill unit. Figure 10.16 illustrates two passive systems.
Some practical problems have been associated with passive systems. For example, snow and
soil may accumulate in vent pipes thus preventing gas migration and venting. Biological clogging
of pipes and soil pores is also common in passive systems.

10.4.16.2 Active Systems

Active gas control systems employ some mechanical means to remove landfill gas and consist of
either positive pressure (air injection) or negative pressure (extraction) systems. Negative pressure