Water & Wastewater
Detention Time Calculator
How long water stays in a tank or basin at a given flow, in hours and minutes, with the volume turnover per day.
Calculated reference results
- Detention time
- 3.33 h
- Detention time
- 3.333hours
- Detention time
- 0.1389days
- Turnovers per day
- 7.20
- Flow
- 0.7200MGD
- Septicity risk at this detention
- Moderate
200.0 minutes
A rough guide for raw wastewater in a wet well or holding structure. Temperature and the strength of the wastewater both matter.
The arithmetic
- Detention = 100,000 gal / 500.00 gpm = 200.0 minutes
- = 3.333 hours
What this does not account for
- This is theoretical detention time. Short-circuiting means the actual time water spends in a basin is often much shorter, which is why baffling and inlet design matter for a clarifier or a contact chamber.
- Where contact time is a regulatory requirement, the calculation uses a baffling factor and the peak hourly flow, not a simple volume divided by average flow. Follow the method your regulator requires.
- In a wet well, long detention means septicity, odour, and hydrogen sulfide that corrodes concrete and metal downstream.
How this is calculated
Detention time is the volume of a tank divided by the flow through it. A 100,000 gallon basin passing 500 gallons per minute has a detention time of 200 minutes, or about 3.3 hours. It is the basic measure behind contact time, clarifier performance, and wet well septicity.
Formulas
detention time = volume / flow
Assumptions and limitations built into this calculator
- Theoretical detention with ideal plug flow. Does not account for short-circuiting or dead zones.
- Does not apply a baffling factor.
Frequently asked questions
- Is detention time the same as contact time?
- No. Regulatory contact time for disinfection applies a baffling factor to account for short-circuiting and uses peak flow, so it is shorter than the theoretical detention time.
Direct contact
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