Hydraulic retention time

The Hydraulic retention time (HRT) also known as Hydraulic residence time or t (tau), is a measure of the average length of time that a soluble compound remains in a constructed bioreactor.

Hydraulic retention time is the volume of the aeration tank divided by the influent flowrate:

$HRT = \frac{Volume\; of\; aeration\; tank}{influent\; flowrate}$

where using SI Units Volume is in [m³] and Influent flowrate is in [m³/h]. HRT is usually expressed in hours (or sometimes days).