MIL-HDBK-1005/16
Onsite Wastewater Treatment and Disposal Systems, for complete
details and leach field special design information. Minimum
depth of trench will be 18 inches (46 cm), with 12 inches (30 cm)
of backfill. Invert slopes will be 0.3 percent when dosing tanks
are used and 0.5 percent when not used. Soil absorption systems
will be 100 feet (30 m) from water supply wells, 50 feet (15 m)
from streams, 10 feet (3 m) from any dwelling or property lines.
Soil testing is a required prerequisite for any subsurface
disposal of waste. EPA 625/1-80-012 Chapter 3 specifies soil
testing methodologies, including the standard Falling Head
Percolation Test procedure used to estimate local percolation
rates. Local and state regulations should also be consulted
because they often provide meaningful guidance for the soil types
in specific geographical areas.
6.3.3.2
Leaching Wells. Leaching wells can be used for septic
tank effluent disposal where subsoil is porous. Although
absorption beds are generally preferred, site characteristics and
cost considerations may encourage the use of a leaching well.
Wells are constructed with masonry blocks or stone with lateral
openings and gravel outside to prevent sand from entering the
well. If more than one well is required, space the wells at
intervals with at least twice the diameter of a well as distance
between well hole sides.
The percolation area is the area on the side and bottom
of the hole for the leaching well. The bottom of a leaching well
should be 4 feet (1.2 m) above seasonal high water. See
Figures 17 and 18 and EPA 625/1-80-012.c.
6.3.3.3
Subsurface Sand Filters. Septic tank effluent can also
be applied to subsurface sand filters. Subsurface explorations
are always necessary. Clogging and installation costs are
significant disadvantages. Where recirculatory sand filters are
used, dose rate may range between 3 and 5 gallons per day per
square foot (gpd/sq ft) (0.12 and 0.2 cubic meters per day per
square meter [cu m/d/sq m]). Consult EPA 625/1-80-012;
Intermittent Sand Filtration for Upgrading Waste Stabilization
Pond Effluents, Harris et al., 1977; and Intermittent Sand Filter
Design and Performance: An Update, Royayne et al., 1982, for
appropriate procedures for site evaluation and design parameters.
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