What Is Coagulation and Flocculation? How Mixers Affect This Process

Coagulation & Flocculation is a mixing process in wastewater to particulate matter which must be removed before further treatment and clarification.  These particles (colloids) are negatively charged and being very minute take a very long time to settle.  To help increase the time for these particles to settle to a few minutes the coagulation process has to be applied early in the water treatment stage.   

Generally, the coagulation stage happens after the Ph adjustment stage, where the water is adjusted to be more alkaline so that additives like aluminium sulphate or ferric sulphate (coagulants) can be effective for coagulation and flocculation process.

Coagulation Process

The objective is to create microfloc from particulate matter and agglomerate the particles so that it can settle.   The particulate matter is non-settleable and generally carries a negative charge.  To neutralize negatively charged particle we look at adding a coagulant that is positively charged.   With the addition of this, within a few seconds, micro particles are created of as the negative charged particles bond with the positive charged.  Once the particles are neutralized the become microflocs and become bigger and form macro flocs that can settle to the bottom.  At this stage the mixer power is usually high as flash mixing is essential to get the microflocs to floc together to create larger macroflocs which settle quicker.

Flocculation Process

At this stage,  a polymer is added, which helps keep the macro flocs together.  Essentially it helps make the bridge between the micro flocs stronger when forming the macroflocs.

A flocculator is 3 or more basins with separate mixers and 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.


1st Basin – Microflocs developing into Macroflocs

  • Radial or Pitched Blade Impeller
  • Higher RPM and HP
  • Higher Mixer Power

At this stage, the more particle collisions the faster the flocculation process, thus the higher mixing energy results in more collisions per second and helps a more rapid flocculation process.

2nd Basin – Medium-sized Macroflocs

  • Pitch Blade Impeller
  • Medium RPM and HP
  • Medium Mixer Power

At this stage, the flocs become larger and the mixing energy is to be reduced, as the flocs are more fragile.

3rd Basin – Larger Macroflocs

  • Blade Impeller
  • Low RPM and HP
  • Low Mixer Power

At the last basin where the flocs are biggest is where the flocs are ready to move to the sedimentation basin.  The low mixer power and flow helps maintain the flocs and improve settling for the next stage.

Enduramaxx manufactures a range of 2 stages and 3 stage flocculation tanks here which can be customised to your requirements.  Need more help on what is coagulation and flocculation?, – Please get In touch.

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