coagulationflocculationsedimentation

Coagulation, flocculation, and sedimentation are a set of sequential treatment steps used in conventional water and wastewater treatment to remove fine particles that are not easily settled by gravity alone. The process destabilizes colloids, promotes particle aggregation, and allows for gravity-based separation in clarifiers.

Coagulation involves adding chemical coagulants to the water. Common coagulants include aluminum and iron salts, such as aluminum sulfate (alum) and ferric chloride, sometimes supplemented by organic polymers. These chemicals neutralize the surface charges of colloidal particles or create conditions that favor particle collisions, forming microflocs.

Flocculation follows coagulation and uses gentle mixing to encourage collisions between destabilized particles. This step allows microflocs to grow into larger, more easily settled flocs. Polymers or other aids may be added to promote floc formation. The process typically lasts minutes, with mixing intensities lowered compared with rapid mixing used during coagulation.

Sedimentation, or clarification, is the gravity-driven phase where the formed flocs settle to form sludge at the bottom of a basin, while clarified water rises and exits for further treatment. Detention times and basin design influence removal efficiency, with factors such as flow rate, temperature, and sludge blanket depth playing roles.

Design and operation considerations include optimizing coagulant type and dosage (often determined by jar testing and online monitoring), maintaining appropriate pH, and managing residuals and sludge. This trio is frequently followed by filtration and disinfection.

Applications span drinking water treatment, certain industrial processes, and some municipal wastewater systems. Limitations include chemical costs, production of sludge, and potential residuals, requiring proper handling and disposal.