Detention time calculator explained
Detention time (also called hydraulic residence time) describes how long liquid stays in a basin or clarifier. This calculator multiplies the plan area {base} by liquid height {height} to get volume {volume}, then divides by inflow {flow} to report detention time {dt}. Keeping these relationships visible makes it easier to understand how geometry and flow changes impact process performance.
Use it to size equalization tanks, confirm clarifier residence time during peak wet-weather events, or explain treatment-plant kinetics to operators.
How the conversion works
For rectangular or cylindrical basins with uniform depth:
where is surface area, is liquid height, is influent flow rate, and is detention time. If you already know the desired detention time, invert the relationship to solve for the volume or depth needed.
Units and conversions
| Quantity | Units | Notes |
|---|---|---|
| Area | m, ft | Use the wetted plan view. |
| Height | m, ft | Measure effective liquid depth, not wall height. |
| Volume | m, ft | Calculated as . |
| Flow | m/s, m/h, MGD | Keep volume and flow in consistent units. |
| Detention time | s, min, h | Convert seconds to hours by dividing by 3600. |
Worked examples
- Existing equalization basin
, , so . Flow .
The basin provides 45 minutes of buffering at that flow; lower flows stretch detention proportionally.
- Designing a clarifier depth
Target (10800 s) at . Required volume . With available area :
Increase area or split the flow between tanks if that depth is impractical.
Tips and pitfalls
- Always use peak wet-weather flow for worst-case detention time checks.
- Subtract sludge blanket depth or freeboard from so the calculation reflects actual water volume.
- For circular clarifiers, compute before using the calculator.
- Combine detention time with mixing energy checks; long detention with stagnant corners can still hurt performance.