What is the first step in water treatment?

The first step in water treatment is called Coagulation or Flocculation. During this step, liquid aluminum sulfate (alum) and/or polymer is added to untreated (raw) water. When mixed with the water, this causes the tiny particles of dirt in the water to stick together or coagulate. Further mixing causes these tiny particles to stick to each other and form larger, heavier particles called flocs.