flocculation

Flocculation is a process used in water and wastewater treatment and in various industrial applications to promote the aggregation of fine suspended particles into larger clusters, or flocs, that can settle, float, or be filtered more easily. It follows coagulation, which destabilizes particles, and relies on gentle mixing to encourage collisions and attachment between particles and polymers or multivalent ions.

The mechanisms of flocculation include charge neutralization, where positively charged coagulants reduce electrostatic repulsion between negatively charged particles; polymer bridging, where long-chain flocculants attach to multiple particles to form networks; and sweep flocculation, where the formation of metal hydroxide precipitates enmeshes particles. The result is a sedimentable or filterable floc that enhances clarification and solids removal.

Operationally, flocculation involves a rapid mixing phase to distribute coagulants, followed by a slower mixing phase that promotes floc growth. Dosing of reagents, pH control, temperature, and mixing intensity all influence performance. Common coagulants include aluminum and ferric salts, while flocculants are often polymers such as polyacrylamides. In some systems, natural flocculation or lime softening can play a role.

Applications span drinking water treatment, municipal and industrial wastewater treatment, mining effluents, pulp and paper processing, and various manufacturing streams. Performance is evaluated by changes in turbidity, particle size distribution, settling rate, and filter feasibility. Environmental considerations include residual coagulants or polymers, sludge generation, and the fate of removed contaminants; operations aim to minimize chemical use while achieving target clarity and solids removal.