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Flash Mixing in Water Treatment

The water treatment process truly begins with a very brief turn in a flash mixing chamber.

After screening out debris and testing raw water, chemicals that encourage coagulation are added to the water stream. The mixture is agitated quickly and thoroughly in a process called flash mixing. The chemicals introduced into the water stream will attract any very fine particles, such as silt, that will not readily settle or filter out and make them clump together. These larger, heavier formations are called floc, which are much easier to remove from the water.

http://www.dynamixinc.com/flash-mixing-rapid-mixer

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Note added at 56 mins (2017-07-08 13:20:18 GMT)
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After coagulant chemicals are introduced, the water is mixed quickly and forcefully by the flash mixer so that the chemicals are evenly distributed throughout the water. This step is very important to create the conditions for efficient, effective water treatment. Flash mixing must last at least 30 seconds, or else the chemicals will not be properly distributed, but it typically lasts for less than 60 seconds. When water is flash-mixed for a longer period, the mixer blades will tend to chop or shear the aggregating material back into small particles. Coagulation actual begins during flash mixing as the coagulants neutralize the electrical charge of the fine particles. This stops the repulsion of like-charged particles and allows the particles to begin bonding and forming larger clumps.
http://filtronics.com/flocculation.html