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Science of Water : Concepts and Applications

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<strong>Water</strong> Treatment Calculations 339<br />

Example 10.25<br />

Problem:<br />

A fl occulation basin is 50 ft long, 22 ft wide, <strong>and</strong> contains water to a depth <strong>of</strong> 11 ft 6 in. How many<br />

gallons <strong>of</strong> water are there in the tank?<br />

Solution:<br />

First convert the 6 in. portion <strong>of</strong> the depth measurement into feet:<br />

Then calculate basin volume:<br />

6 in.<br />

05 . ft<br />

12 in./ft<br />

Volume, ft length, ft)(width, ft)(depth, ft)(7.48 gal ft 3<br />

( )<br />

3<br />

( 50 ft)(22 ft)(11.5 ft)(7.48 galft) 94,622 gal<br />

Detention Time<br />

Because coagulation reactions are rapid, detention time for fl ash mixers is measured in seconds,<br />

whereas the detention time for fl occulation basins is generally between 5 <strong>and</strong> 30 min. The equation<br />

used to calculate detention time is shown below.<br />

Example 10.26<br />

Detention time, min <br />

volume <strong>of</strong> tank, gal<br />

flow rate, gpm<br />

Problem:<br />

The fl ow to a fl occulation basin 50 ft long, 12 ft wide, <strong>and</strong> 10 ft deep is 2100 gpm. What is the detention<br />

time in the tank, in minutes?<br />

Solution:<br />

3<br />

Tank volume, gal (50 ft)(12 ft)(10 ft)(7.48 gal/ft )<br />

44,880 gal<br />

Detention time, min <br />

volume <strong>of</strong> tank, gal<br />

flow rate, gpm<br />

44,880<br />

gal<br />

<br />

2100 gpm<br />

<br />

21.4 min<br />

(10.24)

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