TM 5-814-3/AFM 88-11, Volume III
(9) Calculate approximate void volume: Laterals and manifold.
Lateral volume (cu in) = A length
= (r)2 (No. of laterals length (ft) 12)
= 3,393 cu in.
= (1)2(125)(12)
Manifold volume
= 4,710 cu in.
Total void volume = 8,103 cu in = 35 gallons.
(10) Dosing volume/pump selection: To be certain that all laterals are quickly and completely
filled upon each dosing, dosing volume is estimated at 10 times void volume, or 350 gallons. Controls,
pump size and septic tanks should be chosen based upon conventional engineering practices and local
codes. The design should include a high pressure cutoff to prevent over--pressurization of the system, a
pump chamber with appropriate control, and a pump pedestal to protect the pump from settled solids.
Use figure C-10 to check flow rate against mound absorption area. Dosing volume should be about 1/5th
of the septic tank capacity. In this case, the septic tank is about 2,000 gallons. Since each dose will be
350 gallons, the tank will always remain 80 percent full, allowing settlement of solids and pumping (or
siphoning) of relatively clear effluent to the mound. The pump must be sized to deliver 2 ft of head at the
distal end of laterals after due consideration of:
(a) elevation difference between pump and lateral invert;
In this case, a pump might supply 2.5 ft of head at the supply end to overcome friction losses. A pump
capable of delivering 2.5 ft of head at 120 gals/mm would pump for about 3 min to deliver the 350-gal
dose (fig C-10). After installation, pressure, timing and dose rate will be adjusted prior to final burial of
the piping. Each dose must fill all laterals to about 2 ft head to assure even distribution. However,
pressure should not be too great as pipes may burst.
C-45
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