What is hydraulic retention time (HRT), and how is it calculated?

• A. The time solids spend in the digester
• B. The average time wastewater spends in a tank; HRT = Volume of the tank / Flow rate (hours)
• C. The time water spends in the headworks
• D. A measurement of chemical dosage over time

Answer: (B)

Hydraulic retention time is the average amount of time that wastewater spends inside a treatment tank. It’s determined by how long the water stays in the tank as it flows through the system.

The calculation is straightforward: HRT equals the volume of the tank divided by the flow rate into the tank, with the result in hours. In other words, if a tank holds a certain number of cubic meters and the influent flow is a certain cubic meters per hour, the ratio gives you the time in hours that the water remains in that tank. Dimensional check makes sense: cubic meters divided by cubic meters per hour yields hours.

This is an idealized value that assumes the tank is well-mixed and the flow is steady. In real systems, HRT is an approximation and can vary with changes in flow or tank conditions. It’s different from solids retention time, which concerns how long solids stay in the system, and it’s not about the time spent at headworks or about chemical dosing.