Water purification is the removal of contaminants from untreated water to produce drinking water that is pure enough for human consumption. Substances that are removed during the process of drinking water treatment include bacteria, algae, viruses, fungi, and man-made chemical pollutants. Many contaminants, such as man-made chemicals and heavy metals, can be dangerous—but depending on the quality desired, some are removed to improve the water's smell, taste, and appearance. There really is no such thing as pure water. As the universal solvent, the moment that purified water is exposed to the environment it interacts, even with carbon dioxide in the air. Water purification therefore is a process describing the treatments employed to meet the objectives of the user.
A small amount of disinfectant is usually intentionally left in the water at the end of the treatment process to reduce the risk of re-contamination in the distribution system.
Many environmental and cost considerations affect the location and design of water purification plants. It is not possible to tell whether water is safe to drink just by looking at it. Simple procedures such as boiling or the use of a household charcoal filter are not sufficient for treating water from an unknown source. Even natural spring water—considered safe for all practical purposes in the past must now be tested before determining what kind of treatment is needed.