flocs

Flocs are aggregates of dissolved and suspended particles that form during coagulation and flocculation in water, wastewater, and other suspensions. They typically range from several micrometers to a few millimeters in size and settle more readily than individual particles, aiding clarification and filtration.

Formation and mechanisms: Flocs arise when destabilized particles collide and stick together, often after the addition of coagulants that reduce surface charge. There are several mechanisms that promote aggregation: charge neutralization, where repulsive forces are diminished; bridging, in which long polymer chains connect multiple particles; and electrostatic patching, where positively charged areas on particle surfaces attract oppositely charged particles. The resulting flocs can be loose or dense and often have a fractal, porous structure that traps water and fine solids.

Factors and properties: Floc size, strength, and settling behavior depend on water chemistry, polymer type and dosage, mixing intensity and duration, temperature, pH, and ionic strength. Flocs are commonly characterized by their settling velocity, turbidity reduction, zeta potential, and particle-size distribution. They are typically fragile and susceptible to breakage under high shear, but can strengthen with aging and appropriate conditions.

Applications: Flocculation is used in drinking-water and wastewater treatment to remove suspended solids and natural organic matter, enabling sedimentation and filtration. It also facilitates solid-liquid separation in mining and mineral processing, papermaking, and municipal sludge treatment. In natural systems, flocs form in rivers and lakes from organic matter, clays, and microorganisms, influencing transport and sedimentation.

Materials: Coagulants (eg, alum, ferric salts) destabilize particles, while flocculants (polymers, including polyacrylamides) promote bridging. Selection depends on water chemistry and treatment goals.