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Water purification is the process of changing undrinkable water to drinkable (potable) water. Water is a very good solvent, it is this one fact that 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 problematic 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 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 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 sedimentation. The water is allowed to stand to allow small particles to settle out. This process is aided by calcium oxide in 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 sterilization 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. Springwater and other bottled waters are purified by porous rocks which filter 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 the water, destroying harmful living organisms. Hand-operated reverse osmosis devices are also available and are of particular use to sailors. after supply problems surfaced in the nineteenth century due to rapid industrialization and the concurrent population growth in major cities across Europe and the eastern United States.

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Suddenly, innumerable factories were in operation, using and polluting water supplies. Additionally, burgeoning cities depended on vastly inadequate sewer systems. Drinking water sources mingled with sewage and industrial waste. Unfortunately, it was not until after 1900 that experts agreed that the germs that caused serious infectious diseases like cholera and typhus flourished in water. Before the turn of the century, people had often boiled water and then filtered it through substances such as gravel, sand, charcoal, and, later, special carbon filters. The ancient Greeks and Romans knew that boiling their drinking water made it safer. Over the centuries, people also used various metals, such as silver and copper, as disinfectants, but these elements did not consistently reduce bacteria count. Only chlorine proved to be an effective germicide.

Consequently, chlorine eventually became the universal choice as a disinfectant. By the mid-nineteenth century, a few water companies had turned to chlorine to disinfect sewage. In the United States and England, some water supplies were treated with chlorine. In 1888, Professor Albert Leeds obtained the first United States patent for water chlorination; his method combined electrolysis and hydrochloric acid. (Electrolysis refers to the use of an electric current to break up a compound by chemical conversion.) It was thought at the time that the electricity was disinfecting the water, but American William Jewell was convinced that chlorine alone could be a disinfectant agent. In 1896, Jewell used chlorine gas for the first time at a testing station in Kentucky.

Filtration is the process of separating material, usually a solid, from a substrate (liquid or gas) by the passage of the substrate through a septum or membrane, which retains most of the materials on or within itself. The septum is called a filter medium, and the equipment assembly that holds the medium and provides space for the accumulated materials is called a filter. Filtration is a physical process; any chemical reaction is inadvertent and normally unwanted. The object of filtration may be to purify the fluid by clarification or recover clean, fluid-free particles, or both.

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