Detention time is expressed in units of time. The most common are seconds, minutes, hours and days.

The simplest way to calculate detention time is to divide the volume of the container by the flow rate into the container. The theoretical detention time of a container is the same as the amount of time it would take to fill the container if it were empty.

The most common volume used are gallons (at times cubic feet may be used).

The time units will be in whatever units are used to express the flow. For instance, if the flow is in gpm, then the detention time will be in minutes. If the flow is in gpd, the detention time will be in days. If your final result is in the wrong units simply convert to the appropriate units.

Example:

The reservoir for the village is 85,000 gallons. The wet well will produce 55 gpm. What is the detention time in the tank in hours?

(85,000 gallons) / (55 gpm) = 1,545 min (but answer need to be in hours)

(1545 min) / (60 min/hr) = 25.8 hrs

**Practice:**

- How long will it take to fill a 50-gallon hypochlorite tank if the flow is 5 gpm?
- Find the detention time in a 45,000 gallon reservoir if the flow rate is 85 gpm.
- If the fuel consumption of a 500 gallon tank to the boiler is 35 gallons per day, how many days will the gallon tank last?
- The sedimentation tank of a water plant contains 5,775 gallons. What is the detention time if the flow is 175 gpm?

**Answers:**

- (50 gal) / (5 gpm) = 10 min
- (45,000 gal) / (85 gal/min) = 529 min

(529 min) / (60 min/hr) = 8.8 hrs - (500 gal) / (35 gal/day) = 14. days
- (5,775 gal) / (175 gpm) = 33 min