In mound systems, the pump delivers septic tank effluent to the mound situated at a higher elevation. The pump also pressurizes the lateral system to provide uniform distribution in the dosing application. Pumps appropriate for septic tank effluent, called effluent pumps, are designed to operate in the corrosive environment of a sewage system. Effluent pumps can also handle a small amount of solid material without damage.
The pump size is selected based on the system flow rate in gallons per minute (GPM) and the total dynamic head (TDH). The total dynamic head is determined by adding together:
Using pump performance curves, select the pump that best matches the required flow rate at the operating head. Using the pump performance curve, determine if the pump will produce the flow rate at the required head. Do not choose the undersized pump. Pumps can be oversized but will be more costly.
Follow steps 8 through 13 to determine the pump size. They are presented here along with an example.
The needed pump capacity in gallons per minute is the system flow rate (determined when sizing the main). The system flow rate is the sum of the flows out of all the holes in the laterals. The pump size is selected based on the system flow rate in gallons per minute (GPM) and the total dynamic head (TDH)
Design Example
1) System flow rate (GPM)
2) Total dynamic head (TDH) = Static lift + Main pipe loss + Network loss
Selected Parameter
(18) System flow rate: 88.4 gpm
Establish the relative positions of the mound and the dosing tank, both vertically and horizontally, as shown in Figure 11. Determine the elevation difference between the pump outlet and the laterals. This is called the static lift. Minimize 90° angle fittings. Choose where possible 2 × 45° angles to gradually move the wastewater from the pump to the laterals. 90° angle fittings make it difficult to clear the pipe if a clog occurs.
Design Example
Static lift (Elevation difference between the pump outlet and the laterals) = 10 ft
Figure 11. Elevation difference from dosing tank to mound.
Selected Parameter
(21) Static lift: 10 ft
Calculate the head losses in the main pipe and fittings. Determine the total length of the main pipe and the types and number of fittings, as illustrated in Figure 12. Add to the length of the main the equivalent lengths of pipe for each fitting, as presented in Table 3. Multiply the equivalent length of pipe by the head loss per 100 feet of pipe from Table 2. Then divide by 100 to get the head loss in the main pipe and fittings.
Design Example Total equivalent length of pipe = Main pipe length + Sum of equivalent length of pipe for fitting (from Table 3 at 4 inch main diameter)
Main pipe length = 5 ft + 15 ft + 15 ft + 5 ft = 40 ft
Equivalent length of pipe for 4 – 90° Std. Elbow = 4 elbow × 14 ft/elbow = 56 ft
Equivalent length of pipe for 2 – 45° Std. Elbow = 2 elbows × 8 ft/elbow = 16 ft
Quick disconnection coupling = 5.0 ft
Equivalent length of pipe for tee = 22 ft
Total equivalent length of pipe = 40 ft + 56 ft + 16 ft + 5 ft + 22 ft = 139 ft
Head loss in 100 ft of 4 inch pipe at 90 gpm = 0.46 ft (from Table 2, page 19)
Head loss in 139 ft of 4 inch pipe = (0.46 ft × 139 ft) / 100 ft = 0.64 ft
Table 3. Head losses through plastic fittings in terms of equivalent lengths of plastic pipe (after Clemons, 1991).
| Nominal Size Fittings and Pipe—inches | ||||||
|---|---|---|---|---|---|---|
| Type of Fitting | 1 ¼ | 1 ½ | 2 | 2 ½ | 3 | 4 |
| Equivalent Lengths of Pipe—feet | ||||||
| 90° Std. Elbow | 7.0 | 8.0 | 9.0 | 10.0 | 12.0 | 14.0 |
| 45° Std. Elbow | 3.0 | 3.0 | 4.0 | 4.0 | 6.0 | 8.0 |
| Std. Tee | 7.0 | 9.0 | 11.0 | 14.0 | 17.0 | 22.0 |
| Check Valve | 11.0 | 13.0 | 17.0 | 21.0 | 26.0 | 33.0 |
| Coupling or Quick Disconnect | 1.0 | 1.0 | 2.0 | 3.0 | 4.0 | 5.0 |
| Gate Valve | 0.9 | 1.1 | 1.4 | 1.7 | 2.0 | 2.3 |
Figure 12. Main length and fittings from dosing tank to mound.
Selected Parameter
(22) Main pipe head loss: 0.64 ft
Determine the network losses by multiplying the desired head at the end of the laterals (from step 5) by 1.3 (multiplier related to the friction loss in the manifold and laterals which assumes that the laterals and manifold are sized correctly).
Design Example
Network loss = Head at end of laterals × 1.3 = 4 ft × 1.3 = 5.2 ft
Selected Parameter
(23) Network loss: 5.2 ft
Determine total dynamic head (TDH) by adding together the elevation difference (from step 9), the head loss in the main pipe and fittings (from step 10), and the network head losses (from step 11).
Design Example
Total dynamic head (TDH) = Static head + Main pipe loss + Network loss = 10 ft + 0.64 ft + 5.2 ft = 15.84 ft
Selected Parameter
(24) Total dynamic head (TDH): 15.84 ft
Determine pump curves to help select an appropriate pump. Three examples of pump curves are presented in Figure 13. Each pump will have a rating curve, which compares pump capacity (in GPM) to the head (in feet) provided by the manufacturer. Select a pump that will provide sufficient head for the capacity needed. Avoid selecting too large a pump, with the GPM versus TDH far below the curve. Large pumps are more expensive. The required TDH for the GPM should be on or just below the pump curve, within the middle two-thirds of the curve, for the most efficient operation.
Design Example
Pump (1) is undersized and Pump (3) is oversized, while Pump (2) is large enough to provide sufficient head at the necessary flow rate. Select Pump (2)
Figure 13. Example pump curves for three different pumps.