MIL-HDBK-1005/16
6.3 Septic Tank Systems. Septic tanks, with appropriate
effluent disposal systems, are acceptable as a treatment system
for isolated buildings or for single-unit residential buildings
when permitted by regulatory authority and when alternative
treatment is not practical. When soil and drainage
characteristics are well documented for a particular site, septic
tank treatment may be permanently feasible.
Because the septic tank treatment system is a
biological process, and because it usually discharges directly
above shallow groundwater, it is particularly important that
toxic or hazardous chemicals are not discharged to it.
Discharging industrial wastewater to septic tanks violates the
underground injection provisions of the Safe Drinking Water Act
(SDWA). In addition, grease and nonbiodegradable products should
not be discharged into these systems since these products can
clog system components.
For this reason, use of garbage grinders in domestic
and light commercial facilities serviced by septic tank systems
should be discouraged. For these users, food and other kitchen
wastes should be disposed of entirely through the garbage
collection system. Designers are cautioned to evaluate the
sources of waste carefully before designing a septic tank system,
and to advise upstream sewer users of appropriate discharges and
source control to prevent improper releases to septic tank
systems.
6.3.1
Size. Septic tanks perform settling and digestion
functions and are effective in treating from 1 to 300 population
equivalents of waste. (A population equivalent is considered to
be approximately 100 to 120 [380 to 450L] gallons per capita per
day of domestic strength wastewater.) Generally, septic tanks
should be used only for 1 to 25 population equivalents, except
when septic tanks are the most economical solution for larger
populations within the above range. Minimum size will be at
least a 500-gallon (1,900-L) capacity. In designing tanks, the
length-to-width ratio should be between 2:1 and 3:1, and the
liquid depth should be between 4 and 6 feet (1.2 and 1.8 m).
(Refer to Figure 11 and see Military Standard Drawings
No. 26-20-01 and 26-20-02 for details of construction.)
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