Wastewater math

Detention Time Calculator

How long wastewater stays in a tank, in hours, from the tank volume in gallons and the flow in MGD.

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The formula

Detention time
Detention Time = Volume ÷ Flow (units must be compatible)
WPI Formula/Conversion Table (2023), Printed page 2 of 10, Detention Time

Worked example

A 60,000 gal tank receives a flow of 0.5 MGD. What is the detention time in hours?

Put the flow in gallons per day to match the tank volume: 0.5 MGD × 1,000,000 gal/MG = 500,000 gpd.
Divide the volume by the flow: 60,000 gal ÷ 500,000 gpd = 0.12 days.
Change days to hours: 0.12 days × 24 hr/day = 2.88 hr.

Answer: 2.88 hr (about 2.9 hr)

Exam traps

Dividing gallons by MGD without converting. 60,000 gal ÷ 0.5 MGD gives 120,000, which is not a time at all.
Stopping at days. Most exam answers want hours, so multiply days by 24 hr/day.
Leaving tank volume in cubic feet. Multiply ft³ by 7.48 gal/ft³ before dividing by a flow in gpd.

Practice

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