Water Purification
Water purification is the process of changing undrinkable water to drinkable (potable) water.
Water is a very good solvent, it is this one fact which makes it difficult to obtain pure water without treatment. One solute commonly found in water is sodium chloride, or common table salt. Because of the high salt content of seawater, it is generally unfit for human consumption. In the United States, the legal limit for the salt content of municipal water supplies is set at 500 ppm. This is much lower than the 3.5% of salt in seawater or the 0.5% found underground in brackish water in some regions. The removal of salt from these waters is a particular type of water purification called desalination. Water and dissolved salts can be separated by distillation (water is volatile, the salts are not).
On a large scale, distillation is problematical and expensive. A more efficient desalination technique is reverse osmosis where, by applying pressure to the seawater, the water moves across a semi-permeable membrane, leaving the dissolved salt behind. The largest desalination plant in the world is located in Saudi Arabia, and it uses reverse osmosis to produce half of its country's drinking water. In 1992, a similar plant was opened in California that can produce 8 million gallons of drinking water a day.
Water purification occurs naturally when water evaporates from the oceans, leaving the salts and impurities behind. The water vapor then travels through the atmosphere and it eventually returns to the ground in the form of precipitation. When the water returns to the ground, it eventually finds its way into lakes and rivers which may be used to obtain fresh drinking water. In such a process, the water dissolves a number of particles, including sodium, potassium, magnesium, calcium, iron, chloride, sulfate, carbonate, oxygen, nitrogen, carbon dioxide, sewage and other human waste. Before this water can be used, it has to be purified, and in the United States this is usually a five-stage process. The first stage is a coarse filtration to remove large foreign bodies. This is carried out by allowing the water to flow through a mesh screen. The next stage is one of sedimentation. The water is allowed to stand to allow small particles to settle out. This process is aided by calcium oxide to the water followed by aluminum sulfate. A gelatinous mass of aluminum hydroxide is formed by reaction with some of the water. This gelatinous precipitate settles slowly and pulls down particles with it. The water is then filtered through a fine bed of sand and aerated to oxidize dissolved organic compounds. The final stage is sterlization of the water. This is commonly carried out by bubbling chlorine gas through the water, which produces a weak acid responsible for destroying all remaining bacteria. Ozone can also be used in this process in place of chlorine. After this the water is considered drinkable.
Spring water and other bottled waters are purified by porous rocks through which filters out all particles; the water is collected immediately at the source before other particles can dissolve into it. Other natural water purification systems include reed beds, which can concentrate many of the undesirable elements of water into the stems of the plants given sufficient time. Other water purification techniques include the usage of tablets that release chlorine into water, destroying harmful living organisms. Hand-operated reverse osmosis devices are also available and are of particular use to sailors.
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