After screening out debris and testing the raw water, water treatment really begins at the flash mix chamber. Here, chemicals are added to the water, primarily to aid in coagulation and flocculation. In the flash mixer, the water is agitated violently for a short period of time before being released into the flocculation basin.

The duration of mixing in the flash mix chamber is carefully controlled and is usually between thirty seconds and one minute. If the water is mixed for less than thirty seconds, then the chemicals will not be properly mixed into the water. But if the water is mixed for more than sixty seconds, then the blades will shear the newly forming floc back into smaller particles. When determining the length of time that water must spend in the flash mix chamber, flow rates must be calculated. The volume of the flash mix chamber and the amount of flow determine the contact time.