On-Site Wastewater Treatment and Disposal Systems - Forced ...

Step 8: Select proper pump or siphon.
For a pump system, the total pumping head of the network must
be calculated. This is equal to the elevation difference
between the pump and the distribution lateral inverts, plus
friction loss in the pipe that delivers the wastewater from the
pump to the network at the required rate, plus the desired
pressure to be maintained in the network (the velocity head is
neglected). A pump is then selected that is able to,discharge
the minimum rate (83 gpm) at the calculated pumping head.
For a siphon system, the siphon discharge pipe must be elevated
above the lateral inverts at a distance equal to the friction
losses and velocity head in the pipe that delivers the waste-
water from the siphon to the network at the required rate, plus
the desired pressure to be maintained in the network.
For this example, assume the dosing tank is located 25 ft from
the network inlet, and the difference in elevation between the
pump and the inverts of the distribution laterals is 5 ft.
a. Pump (assume 3-in. diameter delivery pipe)
1. Friction loss in 3-in. pipe at 83 gpm (from Table 7-14)
Friction loss in 25 ft
= 1.38 + 2 (1.73 - 1.38)
= 1.49 ft/loo ft
= (1.49 flYlO ft) x (25 ft)
= 0.4 ft
2. Elevation Head = 5.0 ft
3. Pressure to be maintained = 2.0
Total pumping head = 7.4 ft
Therefore, a pump capable of delivering at least 83 gpm against
7.4 ft of head is required.
b. Siphon (assume 4-in. diameter delivery pipe)
1. Friction loss in 4-in. pipe at83 gpm (from Table 7-14)
= 0.37 + J-j (0.46 - 0.37)
= 0.4 ft/loo ft
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