The concept of detention time is used in conjunction with many treatment plant processes. “Detention time” refers to the length of time a drop of water or a suspended particle remains in a tank or chamber.
Detention time may also be thought of as the number of minutes or hours required for each tank to fill and overflow. The mental image might be one of the flow from the time water enters the tank until it leaves the tank completely (“plug flow”), as shown in the following figure.
Example
A sedimentation tank has a capacity of 132 m3. If the hourly flow to the clarifier is 47 m3/h, what is the detention time?
Since the flow rate is expressed in hours, the detention time calculated is also in hours:
Detention time = volume of tank/flow rate
=132 m3/47 m3/h
=2.8 h