Next comes flocculation . Here, a chemical is added to the water that causes tiny suspended particles to clump together. Usually, these flocs settle out just like large-sized sediment. But if they escape, they are caught by filters farther down the line. To kill unwanted microbes , protozoans, and other living organisms, many municipalities chlorinate the water.
Chlorine is a chemical that kills microorganisms.
In drinking water, the concentrations are low enough that often you can't even taste it. The final stage in most municipal treatments is filtration and aeration. Water is pumped through large tanks filled with fine sand, called rapid sand filters. As the water flows through the spaces between the sand grains, suspended particles and dead microbes get trapped. Sometimes, crushed anthracite coal is used in addition to sand.