Compared
Detention time vs. contact time
Detention time and contact time both measure how long water stays somewhere, but for different reasons. Detention time is how long water sits in any tank or basin, volume divided by flow. Contact time is how long a disinfectant stays with the water to inactivate organisms, the heart of disinfection.
Last reviewed 2026-08-04
| Detention time | Contact time | |
|---|---|---|
| What it times | Water sitting in a tank or basin | A disinfectant working on the water |
| How | Volume divided by flow | How long the water holds a disinfectant residual |
| Purpose | Process control and sizing | Making sure disinfection works |
| Where | Clarifiers, basins, tanks | Clearwell, contact chamber |
The bottom line
Detention time is how long water sits in a tank; contact time is how long a disinfectant gets to work on it.
Common questions
- Are they the same?
- No. Detention time is general holding time; contact time is specifically for disinfection.
- How do you find detention time?
- Divide the volume by the flow, keeping the units matched.
- Why does contact time matter?
- A disinfectant needs enough time with the water to inactivate organisms.