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Water Treatment Fundamentals Sample Course

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Rapid mixing is the instant and thorough dispersal of coagulants added to raw water stream to start the coagulation process.  Rapid mixing is very fast, with detention time as short as a few seconds, hence it is also called flash mixing.  Typical detention time in the rapid mix chamber could be as short as 10 seconds and may not exceed 5 minutes…

Exercise 4.1

Find the detention time for a 6 ft long, 6 ft wide, and 7 ft deep flash mix operating at 10 mgd flow rate.

V, Flash mix volume = 6 ft × 6 ft × 7 ft = 252 ft3

Q, Flow rate = 10 mgd × 1.547 \(\frac{ft^{3}}{s}\)\(\frac{1}{mgd}\)

= 15.47 \(\frac{ft^3}{s}\)

Detention time = \(\frac{\text{V}}{\text{Q}}\) = \(\frac{\text{252 ft}^{3}}{\text{15.47 ft}^{3}}\)s = 16.3 s



You can compute flash mix detention time by entering values for parameters in red texts below: