This is the way that many small bits of dirt are helped to stick together to form heavy clumps which then get knocked off balance and fall to the bottom when they reach the sedimentation tank.
Chemicals such as Aluminium sulphate (Alum) are added to water to help the particles meet up and the flocs (clumps of dirt and stuff) to sink to the bottom. This makes the process of SEDIMENTATION faster for the very small floating bits of dirt.