Sewage Ejector Basin Cycle Calculator
Enter the basin geometry, pump-down level separation, pump flow, and design inflow to check whether the basin gives enough active volume. The result shows the exact worst-case starts per hour and the float spacing needed for the entered limits.
Required pump-on/off separation (in)20.42
- Existing active basin volume (gal)
- 23.5
- Pump capacity margin (gpm)
- 20
- Runtime at maximum inflow (min)
- 1.18
- Fill time at maximum inflow (min)
- 2.35
- Worst-case inflow for cycling (gpm)
- 10
- Maximum starts per hour (starts/hr)
- 17.02
- Required active volume (gal)
- 40
- Cycling assessment
- Fails: the entered pump-on/off separation does not provide the required active volume.
Required active volume checks
| Check | Result | Limit |
|---|---|---|
| Starts per hour | 17.02 starts/hr | 10 starts/hr max |
| Runtime near zero inflow | 0.78 min | 1.0 min minimum |
| Starts-limit volume | 40.00 gal | Included in required volume |
| Runtime-limit volume | 30.00 gal | Included in required volume |
Assumes steady inflow, constant pump delivery at actual TDH, one fixed-speed pump, and a constant-area basin between float levels.
Round-basin inputs use inside diameter; rectangular-basin inputs use inside length and width.
How to use this calculator
- Choose round or rectangular basin geometry for the section between the pump-off and pump-on levels.
- Enter the inside basin dimensions and the vertical pump-on to pump-off separation.
- Use pump flow from the pump curve at the installed total dynamic head.
- Enter the expected maximum inflow, permitted starts per hour, and desired minimum runtime.
- Compare the existing active volume with the required volume and separation.
How the basin cycle is calculated
The calculator treats the working part of the basin as a constant-area shape. For a round basin, cross-sectional area is A = pi(D / 12)^2 / 4. For a rectangular basin, it is A = (L / 12)(W / 12). The existing active volume between the pump-on and pump-off levels is V = A(h / 12)(7.48052), with volume in U.S. gallons.
During a cycle, inflow fills the basin while the pump is off, then the pump removes water faster than it enters while the pump is on. At inflow Qi, fill time is tf = V / Qi and pump runtime is tp = V / (Qp - Qi). Starts per hour are N = 60 / (tf + tp), which reduces to N = 60Qi(Qp - Qi) / (VQp).
That equation does not always peak at the maximum design inflow. It peaks at half the pump capacity, so the worst cycling point inside the entered inflow range is Qw = min(Qm, Qp / 2). The active volume required by the starts-per-hour limit is Vs = 60Qw(Qp - Qw) / (SQp).
The runtime check uses the conservative no-inflow pump-down case, because that is the shortest pump run for a fixed active volume. The volume needed for runtime is Vr = Qp x tr. The required active volume is Vreq = max(Vs, Vr), and the required pump-on/off separation is hreq = 12Vreq / (7.48052A).
What changes the result most
Basin diameter or plan area has a large effect because every inch of float separation holds more gallons in a larger basin. Pump flow also matters in two ways: it improves pump-down margin against the maximum inflow, but a higher-capacity pump can require more active volume to satisfy a minimum runtime. The most severe cycling rate occurs near half of pump capacity, not necessarily at the largest inflow.
What this calculator leaves out
This is an active-sump-volume and cycling check only. It does not calculate total dynamic head, discharge pipe losses, solids handling, emergency storage, alarm elevation, freeboard, venting, pump submergence, basin structural limits, duplex controls, or code-required details. It also assumes steady inflow and constant pump delivery, so a tapered, benched, irregular, or partially obstructed basin needs a project-specific volume check.
Worked example
For a 24 in round basin with 12 in between pump-on and pump-off, the area is pi(24 / 12)^2 / 4 = 3.1416 ft2. Active volume is 3.1416 x 1 ft x 7.48052 = 23.50 gal.
With a 30 gpm pump, 10 gpm maximum inflow, 10 permitted starts per hour, and a 1.0 min minimum runtime, the worst cycling inflow is 10 gpm because it is below half the pump flow. The exact cycling equation gives about 17.02 starts per hour, so the starts limit requires 40.00 gal. The runtime limit requires 30 x 1.0 = 30.00 gal, so the required active volume is 40.00 gal and the required separation is about 20.42 in.
Common questions
Why does the highest cycling rate occur at half the pump capacity?
The starts-per-hour equation contains the product of inflow and net pump-down rate. That product is largest when the two rates are equal, which happens when inflow is half of pump flow. If the entered maximum inflow is lower than that, the worst point is the maximum inflow instead.
How do I find pump flow at total dynamic head?
Use the selected pump curve and the installation total dynamic head, including static lift and friction loss in the discharge piping. The free-flow rating is not appropriate because a pump delivers less flow as head increases. If the curve does not cover the installed condition, choose another pump or ask the manufacturer.
What is active or drawdown volume in an ejector basin?
Active volume is the gallons between the pump-on level and the pump-off level. It is the volume removed during a normal pump cycle. It is not the total basin volume below the lid or the emergency storage above the pump-on elevation.
How far apart should pump-on and pump-off floats be?
The spacing must provide enough active volume to stay within the pump manufacturer starts-per-hour limit and any minimum runtime requirement. This calculator reverses the volume equation to report that required separation for a round or rectangular basin. The result still has to fit the usable basin height and float switch travel.
What happens when sewage inflow exceeds pump capacity?
The basin cannot pump down under that design condition because water enters as fast as, or faster than, the pump removes it. More active volume may delay an overflow, but it does not fix the capacity problem. The pump, discharge system, or inflow assumption has to be changed.