The water we drink must be drawn from natural freshwater sources, such as lakes, rivers and underground aquifers. However, the water that comes from these sources is not always fit for drinking - - it may be contaminated with disease-causing microorganisms, harmful chemicals and other unwanted contaminants. Therefore, before piping water through to your home's faucet, municipal water suppliers put the water through a treatment process, which is designed to make the water safe for drinking, cooking, bathing and other uses.

Water purification plants start by adding coagulants (such as alum or lime) to water. These additives make the unwanted particles in the water cling together and form clumps. The water is then shaken, which causes the clumps to collect into larger flocs; the water is left to stand for 24 hours to allow the flocs and other sediment to settle to the bottom of the container. After the particles have settled, the water is ready to be filtered. It is then disinfected with chlorine and aerated to remove radon and similar contaminants.

If you are unsure about the cleanliness of a water source, you can purify water yourself in a number of ways. There are two primary goals in water purification: One is the removal of sediment and particles, and the other is the removal of microorganisms. Just letting the water sit can allow some of the sediment in it to sink, and the clean water can then be poured off. You can also filter water through layers of clean cloth to remove sediment and grit. Boiling water for at least a minute kills most microorganisms. Water can also be cleaned with a few drops of iodine (5 drops per gallon), with chlorine tablets (available at camping stores) or with chlorine bleach. Typically, about 1/8 of a teaspoon of chlorine bleach per gallon of water is enough to do the trick, but you might need more if the water is murky. After adding disinfectant chemicals, let the water stand at least 30 minutes before drinking.