Wastewater math
How long wastewater stays in a tank, from the tank volume and the flow.
The formula
time = volume ÷ flowWPI Formula/Conversion Table (2023), Printed page 2 of 10, Detention Time
volume = flow × timeWPI 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?
Idealized volume is 0.9 MG and flow is 1.2 MGD.
Find theoretical detention time in hours.
MG and MGD describe the same volume scale, and the flow supplies the time basis.
Theoretical detention time = volume ÷ flow.
0.9 MG ÷ 1.2 MGD = 0.75 days.
0.75 day × 24 hr/day = 18 hours.
MG ÷ (MG/day) leaves days; then convert days to hours.
18 hours
Answer: 18 hours
Practice
Practice this math in the Math Center, with hints and worked steps.
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