The design of the water treatment system depends on the quality of the raw water source. Larger particles are easier to remove by sedimentation.
Larger particles are easier to remove by filtration.
Small particles, or colloids, are usually negatively charged. Since they have like charges, they are repulsed by each other and are very stable. A chemical coagulant with a positive charge and high valence is mixed into the water. The coagulant acts to prevent repulsion and destabilize the particles. Mixing the water will tend to increase the chances of contact between the particles.