Coagulation and flocculation are two of the most critical steps in conventional water treatment. While often mentioned together, they are distinct processes that work in sequence to transform fine, stable particles into settleable aggregates that can be removed by sedimentation and filtration.
Coagulation is the process of destabilizing suspended particles by neutralizing their surface charges. In raw water, particles carry a negative electrical charge that keeps them in suspension — they naturally repel each other. Adding a coagulant (such as ferric chloride or polyaluminum chloride) introduces positively charged metal ions that neutralize these charges, allowing particles to begin aggregating.
Flocculation follows coagulation. During this stage, gentle mixing brings destabilized particles into contact with each other, forming larger aggregates called 'flocs.' Polymer flocculants (such as anionic polyacrylamide) can bridge between particles, creating even larger, denser flocs that settle quickly.
The selection of coagulant type and dose depends on raw water characteristics including turbidity, pH, alkalinity, temperature, and the nature of the contaminants. Jar testing remains the most reliable method for optimizing coagulant selection and dosing for a specific water source.