Stolling

Coagulation is the agglomeration of colloidal particles to form larger aggregates.

This technique is applied to purify wastewater containing suspended colloidal solids, which are usually negatively charged and surrounded by an electrical double layer of adsorbed ions.

Electrostatic repulsion prevents the particles from colliding, stabilizing the colloid.

When metal coagulants, such as aluminium or ferric salts, are added, they react with the bicarbonates in wastewater to yield the respective hydroxides.

These provide highly charged counter-ions that can be adsorbed on the suspended solids, neutralizing their charge.

Rapid mixing promotes collisions between the suspended solids, which coalesce.

Now, the particles come together and form slightly larger, though microscopic, particles called microflocs.

When microflocs reach the optimum size and become macroflocs, they can be removed by sedimentation or filtration to obtain purified water.