Hydraulic Retention time (HRT) is the theoretical time a drop of water remains in a clarifier.  It’s calculated as clarifier volume divided by effluent flow rate. Clarifier HRT is typically 3 to 6 hours, but the actual HRT is often shorter due to factors like uneven weirs, poor inlet conditions, and density currents, which can reduce clarifier efficiency and lead to higher effluent TSS concentrations.