MIL-HDBK-1005/16
6.3.2
Detention Time. Detention time depends largely on the
method of effluent disposal. When effluent is disposed of in
subsurface absorption fields or leaching pits, 24 hours of
detention time based on average flows is required. The septic
tank should be sized to provide the required detention (below the
operating liquid level) for the design daily flow plus an
additional 25 percent capacity for solids storage. If secondary
treatment such as a subsurface sand filter or an oxidation pond
is provided, this period can be reduced to 18 hours. Open sand
filter treatment can further reduce detention time to 10 to
12 hours.
6.3.3
Effluent Disposal. Absorption field and leaching well
disposal should normally be limited to small facilities (less
than 50 population equivalents). If the total population is over
50, then more than one entirely separate field or well would be
acceptable. For 10 or more population equivalents, discharge of
effluent will be through dosing tanks, which periodically
discharge effluent quantities near 80 percent of the absorption
system capacity.
6.3.3.1
Subsurface Absorption. Subsurface absorption can be
used in conjunction with septic tank treatment when soil
conditions permit. Percolation tests should be performed as
required by the U.S. Public Health Service, and the groundwater
table at the highest known or anticipated level should not reach
any higher than 2 feet (0.6 m) below the invert of the lowest
distribution line.
Absorption fields normally consist of open-joint or
perforated distribution pipe laid in trenches 1 to 5 feet (0.3 to
1.5 m) deep and 1 to 3 feet (0.3 to 0.9 m) wide. The bottoms of
the trenches are filled with a minimum of 6 inches (15 cm) of
3/4- to 2-1/2-inch rock or gravel (Figure 16). The perforated
distribution pipe is laid on top of this rock, and the open
joints between pipe lengths are covered to prevent clogging.
More rock is placed carefully over the pipe network, and then a
semipermeable membrane is used over the rock layer to prevent
fine, grained backfill from clogging the drainage zone.
Distribution pipe may be spaced as close as 2 feet (0.6 m) if the
rock beneath is deep, the subsoil porous, and distance to bedrock
greater than 4 feet (1.2 m).
Generally, distribution pipelines are 3 to 6 feet
(0.9 to 1.8 m) apart laterally and are no longer than 100 feet
(30 m). Consult EPA 625/1-80-012, Process Design Manual for
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