Determining Detention Time

Water operators must be able to determine the DT (detention time) of the water they are treating to make sure the process is working correctly. To solve for this, operators use the formula DT= Volume (gals) ÷ Flow (gpm).

What is the detention time in hours for a basin that measures 35 ft. by 115 ft., has a water depth of 12 ft. and is flowing at a rate of 700 gpm?

Before we start to work the problem, look and see what they are asking, if there is a formula and in what units the answers will be. For this question, we want to know DT, and yes, there is a formula. The answer needs to be in hours.

First, we write out the formula and plug in what we know.

DT= Volume (gals) ÷ flow (gpm)

DT= ? ÷ 700 gpm

Now we need to solve for the volume in gallons in that tank. We can use the volume formula for a rectangle, which is vol (cu/ft) = L ft x W ft x H ft. Now you notice this formula will give you volume in cubic ft. To get gallons, we need to convert cubic feet to gallons. We do this by taking the cu/ft number and multiplying it by 7.48.

DT= ((115 x 35 x 12)7.48) ÷700

DT= (48,300 x 7.48) ÷ 700

DT= 361,284 gal ÷ 700 gpm

DT= 516 minutes

Convert minutes to hours by dividing by 60.

DT= 516 minutes ÷ 60 minutes

DT = 8.6 hrs

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