Coagulation:

In the previous section, we have seen that coarse solids can be effectively removed by sedimentation process as their size enables them to settle them by gravity. Solids which are not coarse (such as fine or colloidal solids) pass on along with the effluents of settling basin. For settlement of such particles, aggregation is essential. The aggregation of such particle requires:

• Particle destabilization to permit attachment when contact occurs, and
• Agglomeration of the established particles.

For this purpose, certain chemicals known as coagulants are added to the water, which on thorough mixing, form floc. The fine colloidal particles present in water get attracted and absorbed on these flocs, forming the bigger size flocs. Coagulation, thus, results in reduction of surface charges of colloidal particles and formation of precipitate.

The coagulation process is practically instantaneous and the initial particles are usually of microscopic in size. These colloidal particles are then gathered to form settleable solids by the process of flocculation. Coagulation utilizes force of electrostatic or inter-ionic nature whereas flocculation occurs by chemical bridging of physical enmeshing mechanisms. For all practical purposes, however, the entire process of addition of chemical (coagulants) and mixing (flocculation) is usually referred to as coagulation. The coagulated water is finally made to pass through the sedimentation tank, where the flocculated particles settle down, and are, thus, removed.