How do cities purify drinking water and where does the water come from?

Water from lakes, rivers, and reservoirs isn’t usually clean enough for our drinking glasses, so before it gets to our tap, it goes through a water treatment plant.

First the water runs through large filters that rid it of large objects like sticks, leaves, dead animals, trash, and so on. From there, the plant injects alum—a salt that gets sticky when stirred with water. It attracts grit, sand, and dirt.

Clumps of these substances, called floc, fall to the bottom of the tank, where they’re removed.

The cleaner water is forced through more and finer filters as well as through activated charcoal, which further absorbs small particles.

Finally, the water is spritzed with a small amount of chlorine, —to kill viruses, bacteria, and certain harmful amoebas—, and fluoride to strengthen teeth.

After this, the water’s sent flowing underground toward your water taps.

About Karen Hill

Karen Hill is a freelance writer, editor, and columnist. Born in New York, her work has appeared in the Examiner, Yahoo News, Buzzfeed, among others.

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