HRT = V / Q
Hydraulic Retention Time (HRT) is a critical design parameter in water and wastewater treatment. HRT = V/Q where V is the reactor volume and Q is the volumetric flow rate. It represents the average time water stays in a treatment unit. Design HRT values: primary clarifiers 1.5-2.5 hours, activated sludge aeration 4-8 hours, anaerobic digesters 15-30 days, constructed wetlands 5-14 days, UV disinfection 10-30 seconds. HRT must be long enough for the treatment process to complete but short enough to be economically feasible. Increasing HRT improves treatment efficiency but requires larger tanks. In biological systems, HRT also affects the food-to-microorganism ratio (F/M) and sludge age, which determine treatment performance.