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Water is desalinated in order to convert salt water to fresh water so it is suitable for human consumption or irrigation. Sometimes the process produces table salt as a by-product. It is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on developing cost-effective ways of providing fresh water for human use in regions where the availability of fresh water is, or is becoming, limited.
Large-scale desalination typically uses extremely large amounts of energy as well as specialised, expensive infrastructure, making it very costly compared to the use of fresh water from rivers or groundwater. The large energy reserves of many Middle Eastern countries, along with their relative water scarcity, have led to extensive construction of desalination in this region. By mid-2007, Middle Eastern desalination accounted for close to 75% of total world capacity.
As of July 2004, the leading method is multi-stage flash distillation (85% of production worldwide). The traditional process used in these operations is vacuum distillation—essentially the boiling of water at less than atmospheric pressure and thus a much lower temperature than normal. This is because the boiling of a liquid occurs when the vapour pressure equals the ambient pressure and vapour pressure increases with temperature. Thus, because of the reduced temperature, energy is saved.
In the last decade, membrane processes have developed very quickly, and most new facilities use reverse osmosis technology. Membrane processes use semi-permeable membranes and pressure to separate salts from water. Membrane systems typically use less energy than thermal distillation, which has led to a reduction in overall desalination costs over the past decade. Desalination remains energy intensive, however, and future costs will continue to depend on the price of both energy and desalination technology.
One of the main environmental considerations of ocean water desalination plants is the impact of the open ocean water intakes, especially when co-located with power plants. Many proposed ocean desalination plants' initial plans relied on these intakes despite perpetuating ongoing impacts on marine life. In the United States, due to a recent court ruling under the Clean Water Act, these intakes are no longer viable without reducing mortality, by ninety percent, of the life in the ocean; the plankton, fish eggs and fish larvae.
In the past, many novel desalination techniques have been researched with varying degrees of success. Some, such as forward osmosis, are still on the drawing board now while others have attracted research funding. For example, to offset the energy requirements of desalination, the U.S. government is working to develop practical solar desalination.
As an example of newer theoretical approaches for desalination, focusing specifically on maximising energy efficiency and cost effectiveness, the Passarell Process (http://www.waterdesalination.com/theory.htm) may be considered.
Other approaches involve the use of geothermal energy. From an environmental and economic point of view, in most locations geothermal desalination can be preferable to using fossil groundwater or surface water for human needs, as in many regions the available surface and groundwater resources already have long been under severe stress.
Recent research in the U.S. indicates that nano-tube membranes may prove to be extremely effective for water filtration and may produce a viable water desalination process that would require substantially less energy than reverse osmosis.
On June 23, 2008, it was reported that Siemens Water Technologies had developed a new technology, based on applying electric field on sea-water, that desalinates one cubic meter of water while using only 1.5 kWh of energy, which, according to the report, is one half the energy that other processes use.
Fresh water can also be produced by freezing sea-water, as happens naturally in the polar regions, and is known as freeze-thaw desalination.
According to MSNBC, a report by Lux Research estimated that the worldwide desalinated water supply will triple between 2008 and 2020.
Low Temperature Thermal Desalination (LTTD) uses low pressures inside chambers created by vacuum pumps and the principle that water boils at low pressures, even at ambient temperature. To cool the water vapours, cold sea water located 600 metres below the sea level is pumped through coils to condense the water vapours and then collect the pure water into storage tanks. The temperature of ocean water declines with an increase in depth, the water on the surface of sea water is hot and water below 600 metres is much cooler. It is also possible to use the LTTD process for power plants where huge amounts of warm water are discharged continuously from the plant.
The technology was developed by India's National Institute of Ocean Technology (NIOT) and the world's first LTTD plant was opened in the Lakshadweep islands in 2005. The plant has a 100,000 litres/day capacity.
A low-cost water desalination system developed by New Mexico State University engineers can convert saltwater to pure drinking water on a round-the-clock basis – and its energy needs are so low it could be powered by the waste heat of an air conditioning system.
A prototype built on the NMSU campus in Las Cruces can produce enough pure water continuously to supply a four-person household, said Nirmala Khandan, an environmental engineering professor in NMSU’s Department of Civil Engineering.
This research project, funded by the NMSU-based New Mexico Water Resources Research Institute, explores the feasibility of using low-grade heat – such as solar energy or waste heat from a process such as refrigeration or air conditioning – to run a desalination process.
Khandan said the project builds on a process, first developed by researchers in Florida, that makes distillation of saline water possible at relatively low temperatures – 45 to 50 degrees Celsius (113 to 122 degrees Fahrenheit) rather than the 60 to 100 C (140 to 212 F) required by most distillation processes.
The system utilises the natural effects of gravity and atmospheric pressure to create a vacuum in which water can evaporate and condense at near-ambient temperatures. Two 30-foot vertical tubes – one rising from a tank of saline water and the other from a tank of pure water – are connected by a horizontal tube. The barometric pressure of the tall water columns creates a vacuum in the head-space.
At normal temperatures, Khandan said, evaporation from the pure-water side will travel to the saline side and condense as the system seeks equilibrium. “That’s nature,” he said. “We want it to go the other way.”
Raising the temperature of the water in the head-space over the saline column slightly more than that of the freshwater column causes the flow to go in the other direction, so that pure, distilled water collects on one side and the brine concentrate is left behind in a separate container. A temperature increase of only 10 to 15 degrees is needed, Khandan said. “That’s the trick of this vacuum,” he said. “We don’t have to boil the water like normal distillation, so you can use low-grade heat like solar energy or waste heat from a diesel engine or some other source of waste heat.”
Potentially a desalination system using this method could be coupled to a home’s refrigerated air conditioning system, Khandan said. “When you air condition a house, you are pumping the heat outside the house, and the heat is wasted into the atmosphere,” he said. “We want to capture that heat and use it to power this desalination system.”
The 30-foot-tall NMSU prototype is powered by a solar panel. Khandan and his research assistant, civil engineering doctoral student Veera Gnaneswar Gude, have modified the process originally developed by Florida researchers to incorporate a thermal energy storage device that allows the system to operate around-the-clock, using stored energy at night.
Desalinisation or desalination refers to any of several processes that remove excess salt and other minerals from water. More generally, desalination may also refer to the removal of salts and minerals, as in soil desalination.