Municipal Solid Waste (MSW) Landfill: virtual
tour
Passive Gas Collection Systems, Continued
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  Horizontal wells may be appropriate for landfills that need to
recover gas promptly (e.g, landfills with subsurface gas migration
problems), for deep landfills, or for active landfills. Sometimes,
the collection wells vent directly to the atmosphere. Often, the
collection wells convey the gas to treatment or control systems
(e.g., flares).
The efficiency of a passive collection system partly depends on
how well the gas is contained within the landfill. Gas containment
can be controlled and altered by the landfill collection system
design.  Gas can be contained by using liners on the top, sides,
and bottom of the landfill.An impermeable liner (e. g., clay or
geosynthetic membranes) will trap landfill gas and can be used
to create preferred gas migration pathways. For example, installing
an impermeable barrier at the top of a landfill will limit uncontrolled
venting to the atmosphere by causing the gas to vent through collection
wells rather than the cover.
The efficiency of a passive collection system also depends on
environmental conditions, which may or may not be controlled by
the system design. When the pressure in the landfill is inadequate
to push the gas to the venting device or control device, passive
systems fail to remove landfill gas effectively. High barometric
pressure sometimes results in outside air entering the landfill
through passive vents that are not routing gas to control devices.
For these reasons, passive collection systems are not considered
reliable enough for use in areas with a high risk of gas migration,
especially where methane can collect to explosive levels in buildings
and confined spaces.
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