Coagulation & Flocculation are unit processes used to remove suspended solids from water or wastewater. Suspended solids present in water vary in terms of source, size and charge. The proper application of these processess depends upon all of these factors. Naturally, the suspended solids, due to their surface charges remain suspended or stabilized. Most of the suspended solids have a negative charge on their surfaces, due to this negative charge the solids mutually repel each other and thus form a layer of suspended particles in the wastewater.
Chemical Coagulation refers to rapid mixing with addition of coagulants such as Ferric Chloride and Aluminium Sulphate to enhance the production of larger particles from smaller particles. The coagulants are used to destablize the suspended particles by neutrilizing charges on their surface. Once the charge is neutrilized the particles can stick together and form microflocs.
The rapid mixing promotes the dispersion of coagulants and enhances the coagulation. Over mixing does not have a negative impact on the coagulation process but incomplete mixing can leave the process incomplete. Typical rapid mixing is carried on for 1 to 3 minutes.
In flocculation, slow mixing is provided to produce visible suspended particles. The time for flocculation ranges from 15 minutes to an hour or more, depending on the type of flocs generated.
The design process of coagulation & flocculation needs to take in to account the mixing velocity of the particles and the amount of energy required to mix the particles.