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Desalination is a separation process used to reduce the dissolved salt content of saline water to a usable level. All desalination processes involve three liquid streams: the saline feedwater brackish water or seawater , low-salinity product water, and very saline concentrate brine or reject water.
The saline feedwater is drawn from oceanic or underground sources. It is separated by the desalination process into the two output streams: the low-salinity product water and very saline concentrate streams.
The use of desalination overcomes the paradox faced by many coastal communities, that of having access to a practically inexhaustible supply of saline water but having no way to use it. Although some substances dissolved in water, such as calcium carbonate, can be removed by chemical treatment, other common constituents, like sodium chloride, require more technically sophisticated methods, collectively known as desalination.
In the past, the difficulty and expense of removing various dissolved salts from water made saline waters an impractical source of potable water. However, starting in the s, desalination began to appear to be economically practical for ordinary use, under certain circumstances.
A by-product of desalination is brine. Technical Description There are two types of membrane process used for desalination: reverse osmosis RO and electrodialysis ED. The latter is not generally used in Latin America and the Caribbean. In the RO process, water from a pressurized saline solution is separated from the dissolved salts by flowing through a water-permeable membrane.
The permeate the liquid flowing through the membrane is encouraged to flow through the membrane by the pressure differential created between the pressurized feedwater and the product water, which is at near-atmospheric pressure.
The remaining feedwater continues through the pressurized side of the reactor as brine. No heating or phase change takes place. The major energy requirement is for the initial pressurization of the feedwater. For brackish water desalination the operating pressures range from to psi, and for seawater desalination from to 1 psi. In practice, the feedwater is pumped into a closed container, against the membrane, to pressurize it. As the product water passes through the membrane, the remaining feedwater and brine solution becomes more and more concentrated.
To reduce the concentration of dissolved salts remaining, a portion of this concentrated feedwater-brine solution is withdrawn from the container. Without this discharge, the concentration of dissolved salts in the feedwater would continue to increase, requiring ever-increasing energy inputs to overcome the naturally increased osmotic pressure.
Figure 16 illustrates the basic components of a reverse osmosis system. Pretreatment: The incoming feedwater is pretreated to be compatible with the membranes by removing suspended solids, adjusting the pH, and adding a threshold inhibitor to control scaling caused by constituents such as calcium sulphate. Pressurization: The pump raises the pressure of the pretreated feedwater to an operating pressure appropriate for the membrane and the salinity of the feedwater.
Separation: The permeable membranes inhibit the passage of dissolved salts while permitting the desalinated product water to pass through.
Applying feedwater to the membrane assembly results in a freshwater product stream and a concentrated brine reject stream. Because no membrane is perfect in its rejection of dissolved salts, a small percentage of salt passes through the membrane and remains in the product water. Reverse osmosis membranes come in a variety of configurations.
Two of the most popular are spiral wound and hollow fine fiber membranes see Figure They are generally made of cellulose acetate, aromatic polyamides, or, nowadays, thin film polymer composites. Both types are used for brackish water and seawater desalination, although the specific membrane and the construction of the pressure vessel vary according to the different operating pressures used for the two types of feedwater. Stabilization: The product water from the membrane assembly usually requires pH adjustment and degasification before being transferred to the distribution system for use as drinking water.
The product passes through an aeration column in which the pH is elevated from a value of approximately 5 to a value close to 7. In many cases, this water is discharged to a storage cistern for later use. Source: O. Buros, et. During the last 15 years, this capacity has continued to increase as a result of cost reductions and technological advances. RO-desalinated water has been used as potable water and for industrial and agricultural purposes. Potable Water Use : RO technology is currently being used in Argentina and the northeast region of Brazil to desalinate groundwater.
New membranes are being designed to operate at higher pressures 7 to 8. Agricultural Use : Greenhouse and hydroponic farmers are beginning to use reverse osmosis to desalinate and purify irrigation water for greenhouse use the RO product water tends to be lower in bacteria and nematodes, which also helps to control plant diseases. Reverse osmosis technology has been used for this type of application by a farmer in the State of Florida, U. In some Caribbean islands like Antigua, the Bahamas, and the British Virgin Islands see case study in Part C, Chapter 5 , reverse osmosis technology has been used to provide public water supplies with moderate success.
During the eighteen-month period between January and June , the Antigua plant produced between 6. In addition, the major resort hotels and a bottling company have desalination plants. On Tortola, there are about 4 water connections serving a population of 13 year-round residents and approximately visitors annually.
In , the government water utility bought million liters of desalinated water for distribution on Tortola. On Virgin Gorda, there are two seawater desalination plants.
Both have open seawater intakes extending about m offshore. These plants serve a population of 2 year-round residents and a visitor population of 49 , annually. There are connections to the public water system on Virgin Gorda. In , the government water utility purchased 80 million liters of water for distribution on Virgin Gorda.
In South America, particularly in the rural areas of Argentina, Brazil, and northern Chile, reverse osmosis desalination has been used on a smaller scale. Fewer plants have had long-term operational problems. Assuming that a properly designed and constructed unit is installed, the major operational elements associated with the use of RO technology will be the day-to-day monitoring of the system and a systematic program of preventive maintenance.
Preventive maintenance includes instrument calibration, pump adjustment, chemical feed inspection and adjustment, leak detection and repair, and structural repair of the system on a planned schedule. The main operational concern related to the use of reverse osmosis units is fouling.
Fouling is caused when membrane pores are clogged by salts or obstructed by suspended particulates. It limits the amount of water that can be treated before cleaning is required. Membrane fouling can be corrected by backwashing or cleaning about every 4 months , and by replacement of the cartridge filter elements about every 8 weeks. The lifetime of a membrane in Argentina has been reported to be 2 to 3 years, although, in the literature, higher lifespans have been reported.
Operation, maintenance, and monitoring of RO plants require trained engineering staff. Level of Involvement The cost and scale of RO plants are so large that only public water supply companies with a large number of consumers, and industries or resort hotels, have considered this technology as an option.
Small RO plants have been built in rural areas where there is no other water supply option. In some cases, such as the British Virgin Islands, the government provides the land and tax and customs exemptions, pays for the bulk water received, and monitors the product quality.
The government also distributes the water and in some cases provides assistance for the operation of the plants. Costs The most significant costs associated with reverse osmosis plants, aside from the capital cost, are the costs of electricity, membrane replacement, and labor. All desalination techniques are energy-intensive relative to conventional technologies.
Table 5 presents generalized capital and operation and maintenance costs for a 5 mgd reverse osmosis desalination in the United States. The variation in these costs reflects site-specific factors such as plant capacity and the salt content of the feedwater. The International Desalination Association IDA has designed a Seawater Desalting Costs Software Program to provide the mathematical tools necessary to estimate comparative capital and total costs for each of the seawater desalination processes.
Table 5 U. Today's state-of-the-art technology uses thin film composite membranes in place of the older cellulose acetate and polyamide membranes. The composite membranes work over a wider range of pH, at higher temperatures, and within broader chemical limits, enabling them to withstand more operational abuse and conditions more commonly found in most industrial applications. In general, the recovery efficiency of RO desalination plants increases with time as long as there is no fouling of the membrane.
Suitability This technology is suitable for use in regions where seawater or brackish groundwater is readily available. Cultural Acceptability RO technologies are perceived to be expensive and complex, a perception that restricts them to high-value coastal areas and limited use in areas with saline groundwater that lack access to more conventional technologies. At this time, use of RO technologies is not widespread.
Johns, Antigua. Fax E-mail: cehi isis. Chuquicamata Fono, Calama, Chile. William T. Angamos , Casilla de Correo , Antofagasta, Chile. E-mail: respejo socompa. E-mail: solon omega. Joseph E. Bibliography Birkett, J. New York, United Nations, pp. Buros, O. Englewood, N. Hadwen ed. Childs, W. Dodero, E. Eisenberg, Talbert N. Joe Middlebrooks. Furukawa, D.
Gibbs, Robert, Gomez, Evencio G.
Desalination is a separation process used to reduce the dissolved salt content of saline water to a usable level. All desalination processes involve three liquid streams: the saline feedwater brackish water or seawater , low-salinity product water, and very saline concentrate brine or reject water. The saline feedwater is drawn from oceanic or underground sources. It is separated by the desalination process into the two output streams: the low-salinity product water and very saline concentrate streams. The use of desalination overcomes the paradox faced by many coastal communities, that of having access to a practically inexhaustible supply of saline water but having no way to use it. Although some substances dissolved in water, such as calcium carbonate, can be removed by chemical treatment, other common constituents, like sodium chloride, require more technically sophisticated methods, collectively known as desalination.
Jurnal Imbibisi Tya (1)
Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated. Show full item record Recommend this item. Jurnal Hayati, Juni , Vol. Naiola, Beth Paul. Lack of oae of the common resources such as water or excessive concentration of a specific ion such as sodium will cause plants suffer from drought and salinity stresses.