Water treatment is the process of cleaning the water and making sure it is safe for people to use, whether for drinking, washing, bathing or cooking. Almost every large city in the world treats its drinking water in some way.  Drinking water treatment plants select the processes necessary for their type of source water, as water can require different treatment based on where it comes from and the type of environment it flows through. Therefore, only the steps that are necessary for the particular type of supply are utilized.

Adapted from Pollution Probe's Drinking Water Primer