Detention time is expressed in hours.

• A. minutes
• B. hours
• C. days
• D. seconds

Answer: (B)

Detention time refers to the average time water spends inside a treatment unit. It’s calculated from the unit’s volume and the flow rate, and since plant flows are usually measured in volume per hour, the resulting time is naturally expressed in hours. Using minutes or seconds would imply much shorter times than most treatment steps require, while days would suggest an unrealistically long delay for ordinary unit processes. For example, a basin with 20,000 gallons of capacity and an inflow of 2,000 gallons per hour has a detention time of 10 hours, which fits common design expectations. While some specific processes like disinfection contact may discuss shorter times in minutes, the standard way to express detention time for a unit is in hours.