Hydraulic Detention Time


The term detention time, or hydraulic detention time (HDT), refers to the average length of time (theoretical time) a drop of water, wastewa-ter, or suspended particles remains in a tank or channel. It is calculated by dividing the water/wastewater in the tank by the flow rate through the tank. The units of flow rate used in the calculation are dependent on whether the detention time is to be calculated in seconds, minutes, hours, or days. Detention time is used in conjunction with various treatment processes, including sedimentation and coagulation-flocculation.

Generally, in practice, detention time is associated with the amount of time required for a tank to empty. The range of detention time varies with the process; for example, in a tank used for sedimentation, detention time is commonly measured in minutes. The calculation methods used to determine detention time are illustrated in the following sections.


The general hydraulic detention time calculation is:

Hydraulic Detention Time (HDT) = Tank Volume (6.1)

Flow Rate

This general formula is then modified based on the information provided or available and the normal range of detention times for the unit being evaluated:

Hydraulic Detention Time (days) = (6.2)

Flow (gpd)

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