Coagulation & Flocculation Mixing

Coagulation & Flocculation Mixing – in wastewater there is particulate matter which must be removed before final treatment and clarification. Colloids are minute particles and will take a long time to settle naturally however these particles are negatively charged. The process, which takes part after the PH adjustment stage, is called coagulation in which chemicals like ferric sulphate are used to speed up the settling rate.

Flocculation Process

At this stage, the flocculation treatment mixers mix another chemical, a polymer, is added which helps keep the macroflocs together. This chemical helps make the bridge between the microflocs stronger when forming the macroflocs.

A Flocculator is a 2 or 3 stage treatment tank with separate mixers a flash mixer, a flocculation mixer and a coagulation mixer. At each stage there are various sizes of flocs, as the size of the flocs increase, the power on the mixers is decreased, to prevent shear, and not damage the flocs.  Taking each flocculator stage, various impeller configurations are used.

1st – Flash Mixing

Essentially its microflocs developing into macroflocs. For this stage, the mixer required is a radial or pitched blade impeller, with a higher mixer RPM, and higher mixer power.   At this stage, the more particle collisions, the faster the flocculation process, therefore the higher mixing energy results in more collisions per second and helps a more rapid flocculation process.

2nd – Flocculation Mixing

This is where medium-sized macroflocs become larger and the mixing energy has to be reduced.  As the flocs grow in size they become more fragile.  The mixer in this section of the 3 stage tank generally would have a pitch blade impeller, reduced medium RPM and medium mixer power.

3rd – Coagulation mixing

The 3rd section of the tank is where the flocs continue to become larger macroflocs and become ready for the next section of the process which occurs in a sedimentation tank.  In this area, the mixer requires low RPM, low power and flow to help maintain the flocs and improve settling for the next stage.

The configuration of the mixers at various stages in the flocculator requires various flocculation mixers with various impellers and drives. has configured many wastewater mixers and 3 stage treatment for such applications.  More details are available here on our Coagulation & Flocculation Mixing or call us today on 01778 562810.


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