TM 5-814-7
charts for determining the peak rate of discharge from
sions from surface impoundments can be minimized by
small watersheds, based on values for surface soil types
increasing impoundment depth and decreasing surface
and antecedent moisture conditions. Basic information
area, and by constructing wind barriers. Removal of
and values are summarized in EPA 60012-79-165 and
volatiles from the waste stream by stream stripping,
detailed in the US Department of Agriculture's
distillation or incineration can also be used, where
engineering field manual.
practical. In all cases, codisposal of reactive and/or
(4) Sedimentation basins are sized based on
incompatible wastes should be avoided.
At land
analysis of settlement time for suspended solids, i.e.,
treatment facilities, volatilization can be mitigated by
sands, silts and clays. Sizing procedures are provided in
injecting volatile substances at least 6 inches below the
TM 5-820-1 through TM 5-820-4. The trapping efficiency
ground surface into moist but friable soils.
of a basin is related to its surface area; the basin's depth
(1) Venting is required at surface impoundments
only provides for sediment storage. The latter document
if gases accumulate beneath a liner and build up
provides an assessment of SCS sizing criteria, and
pressure. Sufficient gas pressure can lift the liner,
demonstrates that constructing basins to control clay-
creating an area where additional gas can accumulate.
sized particles during peak flows may not be practicable
The higher the "gas bubble" rises, the more the
because the basins would need to be ten times larger
membrane stretches and the less the hydrostatic
that those used for control of silts.
pressure is able to restrain the membrane. If this
condition is not controlled by venting, the liner could
rupture or float to the surface of the impoundment.
6-6. Gas control systems
(2) A number of control alternatives are available
at landfills. Choice of the appropriate control system will
a.
Introduction.
Gaseous emissions from
depend on control objectives and involve determination
hazardous waste land disposal facilities-including
of the type of wastes present, the depth of fill, and the
landfills, surface impoundments, and land treatment
subsurface characteristics of the sites and adjacent
sites-generally fall into two categories: (1) methane gas,
areas. In addition, field measurements should be used
produced by the anaerobic decomposition of organic
to determine gas concentrations, positive and negative
wastes, and (2) toxic vapors, produced by the
pressure, and soil permeability.
volatilization of chemical wastes.
Methane gas,
(3) Atmospheric pipe vents, either of the "U" or
explosive in concentrations of 5 to 15 percent by volume
mushroom configuration, can be used in landfills to
in air, is generated mainly in landfills containing organic
control vertical movement of gases; they are most
wastes; waste volatilization can occur at landfills, surface
effective in areas where gases are collecting and
impoundments and land treatment sites.
causing pressure buildup. For example, venting is
(1) There are no specific regulations for control
effective in preventing uplift of the top liner following
of gaseous emissions at hazardous waste facilities. In
closure of a landfill. Forced ventilation, on the other
landfills containing organic wastes, compliance with the
hand, provides an effective means of controlling both
RCRA solid waste criterion for explosive gases is
lateral and vertical migration of gases. Such systems
recommended (40 CFR section 257.3-8). This criterion
usually employ a series of pipe vents or wells installed
stipulates that methane concentrations at the property
within lined landfills and are connected by a manifold to a
boundary not exceed the lower explosive limit (LEL) of 5
motor blower. The effectiveness of vent trenches can be
percent; in facility structures the limit is 25 percent of the
increased by capping the trench with clay or other
LEL, or 1.25 percent methane.
impervious material and employing lateral and riser pipes
(2) EPA regulations do not specifically address
connected by a manifold to a motor blower. The gas to
the effects of hazardous waste land disposal facilities on
be vented or withdrawn from the landfill may require
air quality, due to the limited information on emissions
collection and treatment to control odors and to prevent
from such facilities and the fact that the problem is
discharge of volatile toxics to the atmosphere.
waste-specific.
However, 40 CFR 241.206-2
c. Design considerations and constraints. Pipe
recommends that the need for gas control should be
vents are usually constructed of perforated PVC pipe
assessed; if the need for control measures is warranted,
installed in a gravel pack to prevent clogging and
the location and design elements for vents, barriers or
encourage gas migration to the vent. They should be
related systems should be provided on design plans for
sealed to prevent excess air from entering the system
the facility. A collection system is not required at new
and to prevent methane or volatile toxics from leaking
facilities if the owner/operator can demonstrate that no
out. The key design considerations in installation of pipe
gas will be produced or, if produced, would neither
vents, as part of either atmosphere or forced ventilation
contribute any air pollutant to the atmosphere nor create
systems, are proper placement and spacing.
An
a flammable or explosive environment.
additional consideration for forced ventilation sys-
b. Control techniques. Control techniques for volatile
emissions from surface impoundments and land
treatment sites are largely preventive in nature. Emis-
6-21
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