Environmental Engineering Reference
In-Depth Information
the migration of landfill gases: diffusion (concentration), pressure, and permeability. Gases
can travel off-site and into neighboring buildings, posing indoor air quality threats such as
odors or exposures to inhalation hazards, or even fire and explosion. For these reasons gas
control is needed.
There are three types of systems used to control landfill gas migration (Tchobanoglous
and O'Leary 1994; McBean et al 1995; Pescod 1991-93; Waste Management Paper 26B,
1995; Waste Management Paper 27, 1994): passive venting; physical barriers; pumping
extraction systems.
Passive venting
Passive systems use existing variations in landfill pressure and gas concentrations to vent
landfill gas into the atmosphere or a control system. Passive venting systems are only
recommended for old sites in the late stages of gas generation where gas generation rates are
low, or where inert wastes are landfilled and similarly low or negligible rates of gas
generation are found. The passive venting pit consists of a highly permeable vent of gravel
material encased in a geotextile fabric to prevent ingress of fine material and reduction of
permeability. The gases flow up the highly permeable layer, and vent passively into the
atmosphere through a permeable capping layer of sand and granular soil or crushed stone.
The vent may also be constructed of granular material but with a central perforated plastic
pipe, the pipe venting directly to the atmosphere. Construction of the passive venting system
may be as emplacement of the waste proceeds or afterwards by drilling or excavation into the
mass of waste. Typically the vents are placed at intervals of between 20 and 50 m and to
depths ranging from 50% to 90% of the waste thickness. If groundwater is encountered within
the waste, wells end at the groundwater table. Other designs of passive gas venting systems
include trenches, which are excavated into or at the boundary of the waste. The trench is lined
at the outer edge with a low-permeability barrier and the trench is filled with high-
permeability gravel, or perforated pipes are used to vent the migrating gas to the surface
(Waste Management Paper 26B, 1995; Waste Management Paper 27, 1994).
Physical barriers
Physical barriers use low-permeability barriers of, for example, flexible polymeric
geomembranes, bentonite cement or clay, to contain and restrict the gas migration. Whilst
these barriers might form part of a leachate containment system, they are less effective in
containing gas. Coefficients of permeability for gas containment are required to be lower than
10−9 m/s. Efficiencies of barriers are improved if they are combined with a means of
removing the gas by either passive venting or pumped extraction.
Pumping extraction systems
Pumping extraction systems pump the gas out of the landfill. The gas migrates to gas pits
or wells, within the waste, which consist of highly permeable gravel, stones or rubble with a
central perforated plastic pipe. The gases pass through the high-permeability vent to a plain
imperforated pipe which draws the gases through to the pump. Leachate vapor may also be
pumped out with the gas, which has high moisture content, and therefore a leachate
condensation trap is required. Figure 9 shows a typical pumping extraction well. The gas
pumped to the surface is either flared by self-sustaining combustion or the use of a support
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