MIL-HDBK-1138
Section 3:
SEPTIC TANKS
3.
3.1
Description. Septic tank systems are used to
treat and dispose of wastewater from individual homes and
buildings where it is not feasible to provide a community
wastewater collection and treatment system. All components of
the septic tank system are underground. Figure 2 depicts a
standard two-compartment septic tank system.
3.1.1
System Components. The individual home or
building discharges through a pipeline to the septic tank, which
in turn discharges to the effluent leaching system. Where
groundwater levels are high, the elevation may be insufficient
for gravity flow and pumps may be required on either side of the
septic tank. Pumps located before the septic tank require
diligent maintenance because they directly receive untreated
wastewater. Refer to manufacturer's recommendations for
maintenance of these pumps.
3.1.1.1
Tank. The septic tank is an underground,
watertight concrete or fiberglass receptacle that typically has a
liquid depth of at least 42 inches (1 m). Inlets and outlets are
located at opposite ends of the tank. The inlets generally have
invert elevations 1 to 3 inches (3 to 8 cm) above the liquid
level of the tank. The outlets generally have a vented tee so
that the intake to the outlet device is below the liquid level.
A system may have two or more septic tanks placed in a series.
3.1.1.2
Effluent Leaching Systems. Wastewater discharges
through a pipe from the septic tank to the underground effluent
leaching system. The effluent leaching system consists of a
distribution box or header pipe and a drain field. The drain
field is a system of open-jointed or perforated piping that
allows the wastewater effluent to be distributed gradually into
the soil.
Typically, all portions of the effluent leaching system
are installed below the elevation of undisturbed native soil. A
mound system is used in locations where the depth is insufficient
for the effluent leaching system because of high groundwater
levels, insufficient permeable soil, or other conditions. For
mound systems, all or part of the leaching system is located
above the elevation of undisturbed native soil. Mound systems
require a pump to deliver the effluent to the elevated leaching
system. Electric controls outside the dosing chamber and a power
supply may be required to operate the pump.
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