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Detention time is the volume of the tank divided by the flow through it, with both in matching units: gallons divided by gallons per minute gives minutes. Find the volume in cubic feet, convert to gallons with 7.48 gal/ft³, divide by the flow in gallons per minute (gpm), then divide by 60 if the answer is wanted in hours.
Detention time (min) = Volume (gal) ÷ Flow (gal/min)
| Term | Meaning | Units |
|---|---|---|
| Volume | length × width × depth for a rectangular basin, converted at 7.48 gal/ft³ | gal |
| Flow | flow through the basin | gal/min |
| Detention time | theoretical time a parcel of water spends in the basin | min (÷ 60 for hours) |
The drinking water sheet gives detention time in minutes from gallons and gpm and in hours from cubic feet and gallons per day; the wastewater sheets give it in hours. Official sheets: Exam Conversion Sheet (revised 4/30/2026) (State Water Resources Control Board, Drinking Water Operator Certification Program; Water Treatment and Water Distribution exams); Equivalents and Formulas Sheet, Version 1 (State Water Resources Control Board, Wastewater Operator Certification Program; Wastewater Treatment Plant Operator exams); Equivalents and Formulas Sheet, Version 2 (State Water Resources Control Board, Wastewater Operator Certification Program; Wastewater Treatment Plant Operator exams).
Detention time · Original practice item, not a real exam question.
A rectangular sedimentation basin is 90 ft long, 35 ft wide and 14 ft deep, and it receives 2,100 gpm. What is the detention time in minutes and in hours?
90 ft × 35 ft × 14 ft = 44,100 ft³44,100 ft³ × 7.48 gal/ft³ = 329,868 gal329,868 gal ÷ 2,100 gal/min = 157.1 min157.1 min ÷ 60 min/hr = 2.62 hrAnswer: 157.1 minutes, or 2.62 hours
No. Detention time assumes every parcel of water stays the full average time. Disinfection credit uses T₁₀, the time by which 10 percent of the water has passed through, which is the detention time multiplied by a baffling factor less than 1.
Put the volume in million gallons and multiply by 24. For the basin above, 2,100 gpm is 3.024 MGD and the volume is 0.3299 million gallons (MG), so 0.3299 ÷ 3.024 × 24 = 2.62 hours, the same answer.
The free study app has a step-by-step lesson on this calculation, with fraction-tile unit cancellation and graded practice problems.
Open the detention time lessonFormulas follow the official sheets linked above, which are the reference at the exam. The worked example and the mistakes are original practice material written for this page; they are not real exam questions, which are confidential. Written by the site's author.
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