Water that is safe to drink is called potable water, or drinking water, in contrast to safe water, which can be used for bathing or cleaning. In the United States, the Environmental Protection Agency sets maximum levels for the 90 most commonly occurring contaminants. If something happens to your water supply, your supplier has to contact you to let you know what precautions you should take.

Photodisc/Getty Images
Homes line a polluted canal in Manila, Philippines.

Water treatment requires six basic steps.

In coagulation, coagulants like lime and alum are added to the water, which causes particulates to clump together.
Next, the water is shaken into larger clumps, called flocs.
The sedimentation process requires that the water stand for 24 hours, which allows the clumps to settle to the bottom.
The water is then filtered, disinfected (usually with chlorine) and aerated.
Aeration helps to remove certain contaminants like radon.

Water Survival Techniques
If your water becomes contaminated and you don't have bottled water, you can purify it in a few different ways. If it is cloudy, first filter it through clean cloths or allow it to settle and then pour off the clear water. Then, you can boil the water for one minute to kill most disease-causing organisms.

You can also add one-eighth of a teaspoon of household chlorine bleach per gallon of water (or follow directions on the label). You should double the amount if the water is discolored or murky. Stir and let it stand for 30 minutes. Chlorine bleach tablets are sold at camping supply stores to purify water for drinking. You can also use five drops of iodine per gallon to disinfect water.

Store boiled or disinfected water in clean, covered containers. If the boiled water tastes too flat or the chlorine taste is too strong, pour it from one container into another.

In the next section, we'll take a closer look at exactly how water circulates in animal and plant cells.