Mechanism of flocculation & coagulation:

• Coagulation-flocculation is a chemical water treatment technique typically applied prior to sedimentation and filtration (e.g. rapid sand filtration) to enhance the ability of a treatment process to remove particles.
• Coagulation and floculation describe the chemical process of contact and adhesion whereby the particles of a dispersion (colloids) form larger-size cluster (flocs or flakes) allowing them to be more easely removed from water (e.g. by settling).
• The coagulation-flocculation processes facilitate the removal of suspended and colloidal particles. It’s used in the first stage of solids-liquids separation: settling, flotation or filtration.

Coagulation mechanism:

• Coagulation is a process used to neutralise charges and form a gelatinous mass to trap (or bridge) particles thus forming a mass large enough to settle or be trapped in the filter.
• Colloidal particles generally carry a negative electrical charge. Their diameter may range between 10-4 to 10-6 mm.
• They are surrounded by an electrical double layer (due to attachment of positively charged ions from the ambient solution) and thus inhibit the close approach of each other.
• They remain finely divided and don't agglomerate. Due to their low specific gravity, they don't settle out. A coagulant (generally positively charged) causes compression of the double layer and thus the neutralization of the electrostatic surface potential of the particles.
• The resulting destabilized particles stick sufficiently together when contact is made. Rapid mixing (a few seconds) is important at this stage to obtain uniform dispersion of the chemical and to increase the opportunity for particle-to-particle contact.
• Subsequent gentle and prolonged (several minutes) mixing cements the still microscopic coagulated particles into larger floes. These floes then are able to aggregate with suspended polluting matter. When increased sufficiently in size and weight, the particles settle to the bottom.

Flocculation mechanism:

• Following coagulation, flocculation, a gentle mixing stage, increases the particle size from submicroscopic microfloc to visible suspended particles.
• The microflocs are brought into contact with each other through the process of slow mixing. Collisions of the microfloc particles cause them to bond to produce larger, visible flocs.
• The floc size continues to build through additional collisions and interaction with inorganic polymers formed by the coagulant or with organic polymers added. Macroflocs are formed.
• High molecular weight polymers, called coagulant aids, may be added during this step to help bridge, bind, and strengthen the floc, add weight, and increase settling rate.
• Once the floc has reached its optimum size and strength, the water is ready for the separation process (sedimentation, floatation or filtration). Design contact times for flocculation range from 15 or 20 minutes to an hour or more.