What phenomenon occurs when particulate impurities in water settle after coagulation?

The correct phenomenon that occurs when particulate impurities in water settle after coagulation is flocculation. This process follows coagulation, where coagulants are added to water to neutralize the charge of suspended particles, allowing them to clump together into larger aggregates known as "flocs." During flocculation, gentle mixing encourages these flocs to grow in size by colliding and joining together.

Once the flocs reach a sufficient size and density, they eventually settle out of the water due to gravity, which is the next step in the water treatment process. Flocculation is crucial because it significantly aids in reducing turbidity and removing contaminants from water, ensuring cleaner water for consumption.

Understanding flocculation is essential for water treatment operators, as it is a critical step towards achieving effective purification of drinking water. This step should not be confused with other terms such as hydration, which refers to the process of incorporating water into a substance, or filtration, which is a physical separation method. Separation, while it might suggest a removal process, does not specifically describe the interaction and settling process that occurs during flocculation.