TM 5-814-5
(2) Passive Venting. Passive venting systems
ing, and finally collecting for beneficial energy
are installed where gas generation is low and off-
reuse. Most decisions regarding landfill gas disposal
site migration of gas is not expected. Essentially
will depend on local regulations.
passive venting is suitable for small municipal
3-8. Runoff Control.
landfills (less than 50,000 cubic yards) and for most
nonmunicipal, containment type landfills. A typical
a. General. Control of storm water runoff at a
system may consist of a series of isolated gas vents
landfill disposal facility is necessary to minimize the
that only penetrate as far as the top layer of landfill
potential of environmental damage to ground and
waste. No design procedure is available to calculate
surface waters. Direct surface water contamination
the number of vents required, but one vent per
can result when solid waste and other dissolved or
10,000 cubic yards of waste may be sufficient. The
suspended contaminants are picked up and carried
more stringent requirements for landfill liners which
by storm water runoff that comes into contact with
exist today will help prevent gas movement away
the working face of the landfill. Uncontrolled
from the landfill site. Some techniques, such as
surface water runoff can also increase leachate
gravel filled trenches and perforated pipes, not only
production, thereby increasing the potential for
help direct the flow of leachate, but assist in the
ground-water contamination. The resulting
passive venting of landfill gases. A passive gas vent
unwanted gas generation may also increase the
is shown in figure 3-5. A typical gas venting layer
potential for explosions.
is composed of sand or fine aggregates. If fine
b. Criteria. The USEPA requires a storm water
aggregate is used, a layer of sand must be placed
control system to prevent surface water discharge
between it and the flexible membrane liner.
into the working portion of the landfill during a
(3) Active Venting. An active venting system
peak storm discharge, defined as a 25-year storm.
consists of a series of deep extraction wells con-
Surface water runoff control should be accom-
nected by header pipe to a mechanical blower, that
plished at a landfill disposal facility in accordance
either delivers the gas to a combustion boiler for
with the following:
energy reuse, transports it to an on-site waste
(1) Landfill disposal facilities should be located
burner, or simply releases it to the atmosphere. An
and designed so that the potential for surface
active system is more effective for controlling gas
drainage from adjacent areas onto the landfill is
movement than a passive system, but the layout and
minimal. Control is accomplished by constructing
design of an active system is much more detailed.
diversion structures to prevent surface water runoff
An active system will require as much engineering
from entering the working portion of the facility.
and design effort as other piping systems that
(2) Landfill disposal facilities should be
contain or transport potentially hazardous
equipped with suitable channeling devices, such as
materials.
ditches, berms or dikes, to divert surface water
(4) Gas Release. Whether landfill gases can be
runoff from areas contiguous to the landfill.
released to the atmosphere either before or after
(3) Precipitation that falls on a landfill will
burning depends on the following:
either infiltrate into the soil, run off the site, or be
(a) Chemical constituents of landfill gases.
reduced by evapotranspiration or direct evapora-
Hazardous air contaminants such as vinyl-chloride
tion. To control leachate generation the final cover
or benzene may be present in landfill gases, and
on the landfill should inhibit moisture penetration
combustion may produce other harmful chemical
and limit surface erosion. This can be done by
by-products. Air quality regulations may not permit
sloping the final side grades at a maximum of 30
the release of these chemicals.
percent to enhance runoff.
(b) Landfill location. If the landfill is located
(4) Well-compacted, fine-grained soils should
near or within a community or neighborhood, then
be used for the final cover to promote surface water
gas collection and disposal techniques may be
runoff by minimizing infiltration.
necessary to minimize the nuisance of odors, and
(5) Ground cover and plant growth should be
the explosive potential of the gas.
included to aid in erosion control and to help
c. Gas Disposal No one plan for gas disposal
dissipate moisture in the soil.
will be suitable for all landfill situations. Each
(6) Runoff which does not come into contact
design will require different design decisions rela-
with the working portion of the landfill, and thus is
tive to acceptable landfill wastes, leachate control
not contaminated, should be dispersed overland to
and treatment, liner design, and gas production
reduce the flow rate and suspended solids load.
expectations. The primary options for gas disposal
Other sedimentation control measures such as
include: venting to the atmosphere, collecting for
retaining ponds may be equally effective.
transport and disposal off-site, collecting and burn-
3-13
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