Figure 4. Dosing Tank
The pumping system is submerged in a watertight dosing tank as shown in Figure 4. The dosing tank should be sized to accommodate the daily volume of wastewater plus two additional days of storage. See Box 3 for guidance on dosing tank selection. Note the smaller tank volumes required for a system that includes a stabilization pond. The retention and storage of water in the pond compensates for the smaller dosing tank capacity.
Centrifugal pumps are typically used for residential irrigation systems. Only use pumps that have been specifically designed for use with septic tank effluent or pond water for irrigation of treated wastewater. Examine pump curves to determine an appropriate pump. Total dynamic head (TDH) and flow rate (GPM) are used to make the selection. The total dynamic head is the sum of the total friction losses and the operating pressure of the sprinkler heads, while the flow rate is the output in gallons per minute for all of the sprinkler heads in the largest zone in the system. Avoid selecting too large a pump, with the GPM versus TDH well below the pump 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 most efficient operation.
| Minimum Dosing Tank Size | ||
|---|---|---|
| (in gallons) | ||
| Treatment System | ||
| Bedrooms | Bioreactor | Stabilization Pond |
| 1 | 500 | 500 |
| 2 | 1000 | 500 |
| 3 | 1500 | 500 |
| 4 | 2000 | 1000 |
| 5 | 2000 | 1000 |
Note: Two tanks in series connected at the bottom will work in areas where it is difficult to set a large tank.
Determine Total Dynamic Head from step 8.
Total friction loss X 2.34 ft -> 10.88 psi X 2.34 ft/psi = 25.46 ft of head.
Calculate the Total Flow Rate for the largest zone from step 6.
2.4 + 2.4 + 2.4 = 7.2 gpm
Using the pump curve determine the size pump needed for the irrigation system.
Select pump A since the desired gpm and TDH are just below the curve and within the middle two-thirds of the curve.
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