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Detention TimeIntermediate 24 min read

Detention Time: Process Control Basics

Covers unit-consistency challenges, circular tank volume, partial-fill basins, and rearranging the detention time formula to solve for required volume — with four fully worked process-control examples.

The formula t=V/Qt = V/Q looks simple, but most detention-time errors on the job and on exams don't come from the formula itself — they come from mismatched units, circular tanks calculated like rectangles, basins that aren't actually full, and not knowing how to flip the formula around when you're the one designing the detention time instead of just measuring it. This guide builds the process-control skill set: getting units right every time, handling real basin geometry, and sizing basins to hit a target.
Unit Consistency: The #1 Source of Errors
Every detention-time problem is really two problems: get the volume and the flow into compatible units, then divide. Keep these conversions on hand:
  • 1 ft3=7.48 gal1\ \text{ft}^3 = 7.48\ \text{gal}
  • 1 MGD=1,000,000 gpd=41,666.7 gph=694.4 gpm1\ \text{MGD} = 1{,}000{,}000\ \text{gpd} = 41{,}666.7\ \text{gph} = 694.4\ \text{gpm}
  • 1 hr=60 min1\ \text{hr} = 60\ \text{min}; $1\ \text{day} = 1{,}44

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