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Coagulation is the agglomeration of colloidal particles to form larger aggregates. This technique purifies wastewater containing suspended colloidal solids, which are usually negatively charged and stabilized by an electrical double layer. Coagulation neutralizes these charges, allowing particles to collide and combine into removable aggregates through precipitation and co-precipitation mechanisms.

Metal coagulants such as aluminum or ferric salts react with bicarbonates in wastewater to yield hydroxides. These hydroxides provide highly charged counter-ions that adsorb onto the suspended solids, neutralizing their negative charge and destabilizing the colloid so particles can aggregate.

Rapid mixing disperses the highly charged coagulant ions throughout the solution and promotes collisions between neutralized colloidal particles. This mixing frequency increases particle coalescence through the hydrophobic effect, enabling suspended solids to combine into larger, removable aggregates that form microflocs and eventually macroflocs.

Microflocs are slightly larger, though still microscopic particles formed when neutralized colloids first coalesce during mixing. Macroflocs develop when microflocs reach optimal size and weight through continued aggregation. Macroflocs are large enough to be removed by sedimentation or filtration to obtain purified water.

Colloidal particles are usually negatively charged and surrounded by an electrical double layer of adsorbed ions. Electrostatic repulsion between these charged double layers prevents particles from colliding with one another, which stabilizes the colloid and keeps particles dispersed in solution.

After macroflocs form, they can be removed from the solution by sedimentation or filtration. Sedimentation allows heavy aggregates to settle to the bottom, while filtration traps them in a filter medium. This separation yields purified water free of the colloidal solids.

Colloidal solids are suspended particles that remain dispersed due to electrostatic repulsion from their electrical double layers. They are often undesirable because they can contain toxins and their small size and charge stability make them resistant to conventional separation methods without chemical treatment like coagulation.