Water found in nature is never 100 percent pure. There are always substances dissolved in the water. Some of these substances, as well as bacteria and particles of dirt, must be removed before the water is fit to drink.
- Coarse filter: A screen prevents large items such as leaves from entering the water treatment plant.
- Aeration: During aeration, air may be bubbled into the water or water may be sprayed into the air. Aeration removes substances from water, such as iron compounds, that give water and unpleasant taste.
- Forming a colloid: Alum (KAl(SO4)2 • 12H2O) is added to the mixing tank. Alum causes small particles in the water to form a type of colloid called a gel. Most of the bacteria in the water are trapped in the gel.
- Sedimentation: In the sedimentation tank, large lumps slowly settle to the bottom of the tank where they can be removed.
- Sand and gravel filter: Next, the water trickles through sand and gravel filter beds, which trap the remaining suspended particles. Because the filter beds can become clogged, they are washed every 24 hours.
- Carbon filter: The water is sometimes passed through a carbon (C) filter. This filter removes tiny amounts of dissolved impurities, and improves the water's taste and color.
- Additives: The fluorine (F) compound prevents tooth decay and the chlorine (Cl) kills bacteria. The water softener removes some magnesium (Mg) and calcium (Ca) compounds.
- Storage: Finally, the water is held in storage tanks before passing into the public water supply.
About half of the drinking water in the United States comes from natural underground sources. Although this groundwater is filtered as is passes through rocks and sand, it sometimes contains high levels of dissolved minerals or chemical pollutants. So groundwater is purified using a modified version of surface water filtration.