What is desalination? Desalination is the separation process of removing or reducing excess salts and other mineral from substances. 1 The process of desalination may be used on different substances however this paper will focus on the desalination of water. The desalination of water is very important to human and animal life. For centuries humans have used the process of desalination to obtain water safe for consumption. Various methods of desalination have been produced however only some of those methods are widely used today.
Although there are numerous methods of desalination all of them involve three liquid streams: the saline feedwater (seawater or brackish water), low-salinity product water, and very saline concentrate (brine water). 1 The low salinity product water and the saline concentrate are output streams that are produced when the saline feedwater goes through the process of desalination. 1 The desalination of the saline feedwater liquid can be divided into five classifications: distillation, membrane, humidification, freezing, and chemical . The two main desalination processes used today are the desalination by distillation and the desalination by reverse osmosis. Within each method of desalination there are several other processes involved. Distillation processes are the most commonly used methods of desalination as well as some of the oldest methods. 1 In 1869, the British Government built the first steam distillation process on land. 2 There are various distillation processes they include the long-tube vertical distillation, Multistage flash distillation (MSF), Multiple Effect distillation (MED), Vapor Compression distillation (VC), and membrane distillation.
Most of these methods produce freshwater by heating saline water to produce water vapor and then reducing the vapor pressure to allow the water to boil at a lower temperature so more heat will not be used up. All of these methods consume some type of thermal energy or electrical power 4 because energy is needed to produce heat to vaporize the water. However much is done to the designs of the units used in the processes to try to conserve some of that energy. Most of these methods also produce a substance called brine which is basically a concentrated salt solution.
Membrane processes include Electrodialysis, Transport depletion and reverse osmosis What they all have in common is that they all use a membrane as a filter. What is Electrodialysis and Transport depletion? Electrodialysis uses an electric charge to separate the sodium and chloride ions found in salt by passing them through separate filters leaving behind freshwater. 2 However, this process is expensive because more electricity is required for larger amount of salt.
Transport depletion is much like Electrodialysis the only difference is that the membranes screen out the sodium and chloride ions by having one membrane screen for positive ions and the other for negative ions. 2 What is reverse osmosis? The reverse osmosis process is fairly new compared to the other desalination process. In reverse osmosis , feedwater is pressurized and separated from materials by allowing the water to flow through a water-permeable membrane. However this process is difficult because it takes a lot of pressure to make the water molecules diffuse out of the feedwater. The reverse osmosis process has four major steps: (1)Pretreatment, where incoming water is pretreated to make it compatible with the membranes used. (2)
Pressurization, where a pump is used to raise the pressure of the water. (3) Separation, where the water is fed through the membranes to separate them from the excess materials (4) Stabilization, where the water that has been separated is pH adjusted, degassed, and transferred to the distribution system so it can be used as drinking water. The membranes used in the third step consist of a non-woven polyester fiber, a polymer coating, and a thin polyamid microfilm. 3 Humidification processes have nothing in common, they are Solar humidification and Diffusion humidification. In solar humidification, the vapor from the feedwater is evaporated by the sun and condensed into freshwater.
In Diffusion humidification, water is passed through rotating disks, as the disks rotate some of that water sticks to the surface of the disks and is evaporated , then the vapor cools producing freshwater. Freezing processes use the knowledge that when water freezes it forms freshwater ice crystals. There are two freezing processes they are direct freezing process and secondary refrigerant freezing. In the direct freezing process, feedwater is transferred to a vacuum chamber where some of the water vaporizes and some is frozen, then it is separated producing freshwater. In secondary refrigerant freezing, instead of using a vacuum a refrigerant, such as Butane, is used. 2 Chemical processes use chemicals to produce freshwater.
There are two chemical processes they are called the ion-exchange method and the hydrate method. In the ion-exchange method a bed of materials that allows the exchange of ions. Two beds are used to separate the sodium and chloride ions in the salt leaving freshwater behind. In the hydrate method, feedwater is mixed with a hydrating agent such as carbon dioxide. Crystals that have freshwater are formed under the right temperature and pressure they are then separated from the brine, washed, and melted.
1. Source Book of Alternative Technologies for Freshwater Augmentation in Latin America and the Caribbean;IETC Technical Publication Series by UNEP International Environmental Technology Centre, Osaka/Shiga, Japan, 1997. http://www. oas. org/dsd/publications/Unit/oea59e/begin. htm#contents 2. Popkin, Roy. Desalination: Water for the World’s Future. Frederick A. Praeger, Publishers: New York, 1968. 3. Betts, Kellyn. “Desalination, desalination everywhere. ” Environmental Science and Technology: American Chemical Society. 2004, 246A-247A. 4. Levine, Sumner. Desalination and Ocean Technology. Dover Publications, INC: New York, 1968.
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