per inch of
fall) per day
(gal per sq. ft)
fall) per day
(gal per sq. ft.)
(Garbage grinders and automatic washing machines not included.)
*Unsuitable for seepage pits if over 30.
Table 1-4. Allowable rate of wastewater application to a soil absorption system.
EXAMPLE: A small military installation has a daily wastewater flow of 5,000 gallons.
The percolation rate is 5 minutes per inch. From table 1-4, the maximum
wastewater application rate is 2.2 gallons per square foot per day. Then
5,000 2.2 = 2,270 square feet of absorption area required. If trenches 2
feet wide are used, 2,270 2 or 1,135 linear feet of absorption trenches
If garbage grinders are used, the requirement must be increased by 20
1,135 + .20(1,135) = 1,362 linear feet.
If automatic washing machines are used, the requirement must be
increased by 40 percent:
1.,135 + .40(1,135) = 1,589 linear feet.
If both garbage grinders and automatic washing machines are used, the
requirement must be increased by 60 percent:
1,135 + .60(1,135) = 1,816 linear feet.
1-15. SEPTIC TANKS
a. Function. Untreated liquid household wastes will quickly clog all but the
most porous gravel formations. A septic tank conditions wastewater so that it may be
more readily percolated into the subsoil of the ground. Thus, the most important
function of a septic tank is to provide protection for the absorptive ability of the subsoil.
(1) Clogging of soil with tank effluent varies directly with the amount of
nonfiltrable residue in the liquids. As wastewater from a building enters a septic tank, its