What is hydraulic detention time?

Hydraulic detention time (HDT or HRT) is the theoretical time a unit volume of water spends inside a tank or basin. It equals tank volume divided by the volumetric flow rate. Longer detention times allow more treatment to occur.

What is the typical detention time for a sedimentation basin?

Primary clarifiers in wastewater treatment typically have detention times of 1–3 hours. Secondary clarifiers range from 1.5–4 hours. Coagulation–flocculation basins are usually 20–60 minutes.

How does detention time affect treatment efficiency?

Longer detention time generally allows more settling of suspended solids, more contact time for disinfection chemicals, and more complete biological reactions. However, overly long times can cause anaerobic conditions, odors, or increased footprint cost.

What units should I use?

Any consistent units work — the calculator handles all conversions internally. Common combinations are cubic meters with m³/hour, million liters with MLD, or US gallons with MGD.

Is detention time the same as hydraulic retention time?

Yes. Hydraulic detention time (HDT), hydraulic retention time (HRT), and nominal retention time (NRT) all refer to the same quantity: V / Q. In practice, short-circuiting and dead zones mean actual residence time is shorter than the theoretical value.

Detention Time Calculator

Name: Detention Time Calculator
Availability: InStock
Author: Nham Vu

Enter tank volume and influent flow rate to compute hydraulic detention (retention) time in hours, minutes, or days.

Tank / Basin Parameters

Tank Volume

Influent Flow Rate

Detention Time Results

Enter values and click Calculate

Typical Detention Times by Process

Treatment Process	Typical HRT	Notes
Rapid mix / flash mix	10 – 60 s	Coagulant addition
Flocculation basin	20 – 40 min	Gentle mixing for floc growth
Primary sedimentation	1 – 3 h	Municipal wastewater
Secondary clarifier	1.5 – 4 h	After biological treatment
Activated sludge aeration	4 – 8 h	Conventional process
Chlorine contact tank	15 – 30 min	CT value compliance
Stabilization lagoon	5 – 30 days	Facultative ponds

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Summary

Enter tank volume and influent flow rate to compute hydraulic detention (retention) time in hours, minutes, or days.

How it works

Enter the tank or basin volume and select the volume unit (liters, gallons, cubic meters, etc.).
Enter the influent flow rate and select the flow-rate unit (L/s, m³/h, MGD, GPM, etc.).
The calculator converts both values to a common base (liters and liters/second) and divides volume by flow rate.
Detention time is displayed in seconds, minutes, hours, and days simultaneously.
Use the swap button to reverse the calculation — enter detention time to back-calculate required volume or flow rate.

Use cases

Size a sedimentation basin for a municipal water treatment plant.
Verify detention time of an existing clarifier meets regulatory minimums.
Design a contact tank for disinfection (chlorine or UV contact time).
Calculate retention time for an equalization basin in industrial wastewater treatment.
Check aeration tank HRT for biological treatment (activated sludge).
Determine mixing time for chemical dosing reactors.
Evaluate lagoon or pond detention time for stormwater management.
Quick field check when commissioning or troubleshooting a treatment system.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu