The time required to fill a tank at a given flow is measured by which term?

• A. Surface overflow rate
• B. Solids retention time
• C. Hydraulic detention time
• D. Weir overflow rate

Answer: (C)

The time required for liquid to fill and reside in a tank at a given flow is the hydraulic detention time. This is the liquid residence time in the tank and is calculated by dividing the tank’s volume by the flow rate (HDT = Volume / Flow). It tells you how long the water stays in the tank, which influences how well treatment processes can occur before the water moves on. For example, a 5000-gallon tank filled at 1000 gallons per hour would have an HDT of 5 hours, meaning it takes about 5 hours for the tank to fill at that flow. Surface overflow rate and weir overflow rate describe how fast water can move over surfaces or through a weir, not how long the tank takes to fill. Solids retention time, on the other hand, concerns how long solids stay in the system, not the liquid’s residence time in the tank.