Hydraulic Detention Time

t = \frac{V}{Q}

Hydraulic residence (detention) time is tank volume divided by the volumetric flow rate. Use consistent units (e.g. m³ and m³/d → days). It is the mean time a fluid element spends in an ideal plug-flow or completely-mixed basin and is the θ in reactor design.

$t$ (h or d): Detention time
$V$ (m³): Basin volume
$Q$ (m³/s or m³/d): Flow rate

Step-by-step solved example

V = 3600 m³, Q = 0.10 m³/s. Find t in hours.

t = V/Q. Hydraulic residence time in a tank.

1. Seconds
t = V/Q = 3600/0.10 = 36,000 s.
2. Hours
36,000/3600 = 10.0 h.
$t = 10.0 h$

Answer: t = 10.0 h

10 practice questions

0/10 correct

1.V = 800 m³, Q = 200 m³/h. t =

2.If Q doubles at fixed V, detention time

3.θ = 8 h, Q = 6000 m³/d. V =

4.Longer primary-clarifier detention generally

5.CMFR mean residence time equals V/Q even though

6.A 3 m deep tank, surface 20 m × 8 m, Q = 5000 m³/d. t =

7.Contact time in a chlorine contact basin is a form of

8.Units of t when V is m³ and Q is m³/s are

9.Short-circuiting makes the effective detention time

10.For the same V, a PFR and a CMFR have