HRT Hydraulic retention time (residence time) also t (tau)

HRT Hydraulic retention time (residence time) also referred to as t (tau)

The Hydraulic retention time (HRT) or t (tau) is a measure of the average length of time that a soluable compound remains in a constructed bioreactor.

The volume  of the aeration tank divided by the influent flowrate is τ (tau), the hyraulic retention time.

source: Metcalf and Eddy page 592