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. Because the coal grains carry a charge on their surface, they act like tiny magnets attracting other charged contaminants. Aeration is exactly what it sounds like - adding air to water. When sprayed through large, fountainlike devices, water not only gains a great deal of oxygen, but any volatile compounds that may have been in the water escape into the atmosphere.