What process involves the gathering of fine particles in water by gentle mixing after the addition of coagulant chemicals?

The process that involves the gathering of fine particles in water through gentle mixing after the addition of coagulant chemicals is flocculation. During flocculation, coagulants such as aluminum sulfate or ferric chloride are introduced to the water, which neutralizes the charge of the suspended particles. Once the coagulants are added, gentle agitation helps to promote the formation of larger aggregates or "flocs" from the smaller particles.

This step is critical in the drinking water treatment process because it increases the size of the particles, making them easier to remove during subsequent treatment stages such as sedimentation and filtration. Flocculation typically occurs in a designated tank called a flocculator, where the conditions are controlled to encourage optimal particle agglomeration without overwhelming turbulence that might disrupt the forming floc.

In contrast, sedimentation involves the settling of these formed flocs under the influence of gravity but does not involve mixing with coagulants. Filtration is the process of passing water through media to remove particulates and does not involve the aggregation of particles. Centrifugation uses centrifugal force to separate components, which is not typically employed in conventional water treatment for aggregating fine particles.