Desalting refers to a water treatment process that removes salts from water. It is also called desalination or desalinization, but it means the same thing. Desalting can be done in a number of ways, but the result is always the same: fresh water is produced from brackish (somewhat salty) or seawater. Desalting technologies can be used for a number of applications, but the purpose of this report is to discuss the use of desalting to produce potable water from saline water for domestic or municipal purposes. Throughout history, people have continually tried to treat salty water so that it could be used for drinking and agriculture.
Of all the globes water, 94 percent is salt water from the oceans and 6 percent is fresh. Of the latter, about 27 percent is in glaciers and 72 percent is underground. While this salt water is important for transportation and fisheries, it is too salty to sustain human life or farming. Desalting techniques have increased the range of water resources available for use by a community. Until recently, only water with a dissolved solid (salt) content generally below about 1,000 milligrams per liter (mg/L) was considered acceptable for a community water supply.
This limitation sometimes restricted the size and location of communities around the world and often led to hardship for many who could not afford to live near a ready supply of fresh water. The application of desalting technologies over the past 50 years has changed this in many places. Villages, cities, and industries have now developed or grown in many of the arid and water-short areas of the world where sea or brackish waters are available and have been treated with desalting techniques.
This change has been very noticeable in parts of the arid Middle East, North Africa, and some of the islands of the Caribbean, where the lack of fresh water severely limited development. Now, modern cities and major industries have developed in some of those areas thanks to the availability of fresh water produced by desalting brackish water and seawater. About half of the world s desalted water is produced with heat to distill fresh water from seawater.
The distillation process mimics the natural water cycle in that salt ater is heated, producing water vapor that is in turn condensed to form fresh water. In a laboratory or industrial plant, water is heated to the boiling point to produce the maximum amount of water vapor. The process that accounts for the most desalting capacity for seawater is multi-stage flash distillation, commonly referred to as the Multi-Stage Flash process or MSF. In the MSF process, seawater is heated in a vessel called the brine heater. This is generally done by condensing steam on a bank of tubes that carry seawater, which passes through the vessel or brine-heater.
This heated seawater then flows into another vessel, called a stage; where the ambient pressure is lower, causing the water to immediately boil. The sudden introduction of the heated water into the chamber causes it to boil rapidly, almost exploding or flashing into steam. Generally, only a small percentage of this water is converted to steam (water vapor), depending on the pressure maintained in this stage, since boiling will continue only until the water cools (furnishing the heat of vaporization) to the boiling point.
This process of lowering ambient pressure continues for many stages depending on the size of the plant. Distillation is not the only means of saltwater purification. Another process includes membranes. In nature, membranes play an important role in the separation of salts, including both the process of dialysis and osmosis, which occur in the body. Membranes are used in two commercially important desalting processes: Electrodialysis and reverse osmosis. Each process uses the ability of the membranes to differentiate and selectively separate salts and water.
However, membranes are used differently in each of these processes. Electrodialysis is a voltage driven process and uses an electrical potential to move salts selectively through a membrane, leaving fresh water behind as product water. Reverse osmosis is a pressure-driven process, with the pressure used for separation by allowing fresh water to move through a membrane, leaving the salts behind. Scientists have explored both of these concepts since the turn of the century, but their commercialization for desalting water for municipal purposes has occurred in only the last 30 to 40 years.
Electrodialysis was commercially introduced in the early 1960s, about 10 years before reverse osmosis. The development of electrodialysis provided a cost-effective way to desalt brackish water and spurred considerable interest in the whole field of using desalting technologies for producing potable water for municipal use. Electrodialysis depends on the following general principles: Most salts dissolved in water are ionic, being positively (cationic) or negatively (anionic) charged. These ions migrate toward the electrodes with an opposite electric charge.
Membranes can be constructed to permit selective passage of either anions or cations. Another means of purification is the freezing of the water. Extensive work was done in the 1950s and 1960s to develop freezing desalination. During the process of freezing, dissolved salts are naturally excluded during the initial formation of ice crystals. Cooling saline water to form ice crystals under controlled conditions can desalinate seawater. Before the entire mass of water has been frozen, the mixture is usually washed and rinsed to remove the salts in the remaining water from adhering to the ice crystals.
The ice is then melted to produce fresh water. Theoretically, freezing has some advantages over distillation, which was the predominant desalting process at the time the freezing process was developed. These advantages include a lower theoretical energy requirement for single stage operation, a reduced potential for corrosion, and few scaling or precipitation problems. The disadvantage is that it involves handling ice and water mixtures that are mechanically complex to move and process. There are several different processes that use freezing to desalt seawater, and a few plants have been built over the past 50 years.
However, the process has not been a commercial success in the production of fresh water for municipal purposes. At this stage, freeze-desalting technology probably has better application in the treatment of industrial wastes than in the production of municipal drinking water. Along with membrane and freezing purification comes solar humidification. The use of direct solar energy for desalting saline water has been investigated and used for some time. During World War II, considerable work went into designing small solar stills for use on life rafts.
This work continued after the war, with a variety of devices being made and tested. These devices generally imitate a part of the natural hydrologic cycle in that the suns rays heat the saline water so that the production of water vapor (humidification) increases. The water vapor is then condensed on a cool surface, and the condensate collected as fresh water product. An example of this type of process is the greenhouse solar still, in which the saline water is heated in a basin on the floor, and the water vapor condenses on the sloping glass roof that covers the basin.
By any means, salt water purification has a definite economic impact. Since desalination facilities exist in over 100 countries around the world, specifying exact costs for desalting is not appropriate. What can be said with certainty is that the capital and operating costs for desalination have tended to decrease over the years. At the same time desalting costs have been decreasing, the cost of obtaining and treating water from conventional sources has tended to increase because of the increased levels of treatment being required in various countries to meet more stringent water quality standards.
This rise in cost for conventionally treated water also is the result of an increased demand for water, leading to the need to develop more expensive conventional supplies, since the readily obtainable water sources have already been used. Many factors enter into the capital and operating costs for desalination: capacity and type of plants, plant location, feed water, labor, energy, financing, concentrate disposal, and plant reliability. In general, the cost of desalted seawater is about 3 to 5 times the cost of desalting brackish water from the same size plant.
During the past decade in a number of areas of the USA, the economic cost of desalting brackish water has become less than the alternative of transferring large amounts of conventionally treated water by long-distance pipeline. In 1999 in the USA, the total production costs, including capital recovery, for brackish water systems with capacities of 4,000 to 40,000m3/d typically ranges from $0. 25 to $0. 60/m3 ($1. 00 to $2. 40/1000 gallons). In many recent privatized seawater-desalting plants ranging from 4,000 to 100,000 m3/d, the total cost of water estimated at $3 to $0. 75 m3 ($12 to $. 0/1000 gallons).
These amounts give some idea of the range of costs involved, but the site-and country-specific factors will affect the actual costs. In any country or region, the economics of using desalination is not just the number of dollars, pesos, or dinars per cubic meter, but also the cost of desalted water versus the other alternatives. In many water-short areas, the cost of alternative sources of water is already very high and often above the cost of desalting. Any economic evaluation of the total cost of water delivered to a customer must include all the costs involved.
This includes the costs for environmental protection (such as brine or concentrate disposal), distribution and losses in the storage and distribution system. Desalination technology has been extensively developed over the past 50 years to the point where it is routinely considered and reliably used to produce fresh water from saline sources. This has effectively made the use of saline waters for water resource development possible. The cost for desalination can be significant because of its intensive use of energy.
However, in many areas of the world, the cost to desalinate saline water is less than other alternatives that may exist or may be considered for the future. Desalinated water is used as a main source of municipal supply in many areas of the Caribbean, Mediterranean, and Middle East. Desalting is also being used or considered for many coastal urban areas in the USA, Asia, and other areas and where it is proving more economical than available conventional sources. The use of desalination technologies, especially for softening mildly brackish waters is rapidly increasing in the USA. There is no best method of desalination.
Generally, distillation and reverse osmosis are used for seawater desalting, while reverse osmosis and electrodialysis are used to desalt brackish water. However, the selection of a process should depend on a careful study of site conditions and the application at hand. Local circumstances will always play a significant role in determining the most appropriate process for an area. The best desalination system should be more than economically reasonable in the study stage. It should work when it is installed and continue to work and deliver suitable amounts of fresh water at the expected quantity, quality, and cost for the life of a project.
