Wastewater math

Detention Time Formula for Wastewater Operators

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

The formula

Formulas

Find detention time
time = volume ÷ flow
WPI Formula/Conversion Table (2023), Printed page 2 of 10, Detention Time
Find required volume
volume = flow × time
WPI Formula/Conversion Table (2023), Printed page 2 of 10, Detention Time

Worked example

A basin holds 0.9 MG. Flow through the basin is 1.2 MGD. What is the detention time in hours?

What Do We Know?
Idealized volume is 0.9 MG and flow is 1.2 MGD.
What Are We Finding?
Find theoretical detention time in hours.
Are the Units Compatible?
MG and MGD describe the same volume scale, and the flow supplies the time basis.
Choose the Relationship
Theoretical detention time = volume ÷ flow.
Convert the Units
0.75 day × 24 hr/day = 18 hours.
Put In the Values
0.9 MG ÷ 1.2 MGD = 0.75 days.
Check the Units
MG ÷ (MG/day) leaves days; then convert days to hours.
Answer
18 hours

Answer: 18 hours

Practice

Practice this math in the Math Center, with hints and worked steps.

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