Desalination (or desalinization or desalinisation) refers to any of several processes that remove excess salt and other minerals from water. The term desalination may also be used in a general sense, to refer to the removal of salts and minerals from a mixture, as in soil desalination, but this article focuses on water desalination.
Water is desalinated to obtain freshwater suitable for animal consumption or irrigation, or, if almost all of the salt is removed, for human consumption. Sometimes the process produces table salt as a by-product. It is used on many ships and submarines. Most of the modern interest in desalination is focused on developing cost-effective ways of providing freshwater for human use in regions where the availability of water is limited.
Large-scale desalination typically requires large amounts of energy as well as specialized, expensive infrastructure, making it very costly compared to the use of freshwater from rivers or groundwater. Thus, desalination is a viable technology in affluent regions close to coastlines, but it is currently not an option for poverty-stricken areas or places that are at high altitudes or far inland. In addition, the wastewater from desalination plants can adversely affect the local marine ecosystem unless care is taken to ensure that the temperature and salinity of the wastewater are not too different from the temperature and salinity of the ocean.
The large energy reserves of many Middle Eastern countries, along with their relative water scarcity, have led to extensive construction of desalination plants in this region. Saudi Arabia's desalination plants account for about 24 percent of total world capacity. The world's largest desalination plant is the Jebel Ali Desalination Plant (Phase 2) in the United Arab Emirates. It is a dual-purpose facility that uses multi-stage flash distillation and is capable of producing 300 million cubic meters of water per year.
Desalination may be done by any of a number of different technologies, as listed below.
As of July 2004, the two leading methods of desalination were reverse osmosis (47.2 percent of installed capacity worldwide) and multi-stage flash distillation (36.5 percent).
The traditional process used for desalination has involved vacuum distillation. In this method, water is boiled at below atmospheric pressure, and thus at a much lower temperature than normal. Because the temperature is reduced, energy is saved.
During the last decade, membrane processes have grown rapidly, and most new facilities use reverse osmosis technology. These processes use semi-permeable membranes and pressure to separate salts from water. Membrane systems typically use less energy than thermal distillation, leading 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.
Forward osmosis employs a passive membrane filter that is hydrophilic and slowly permeable to water, and blocks a portion of the solutes. Water is driven across the membrane by osmotic pressure created by food-grade concentrate on the clean side of the membrane. Forward osmosis systems are passive in that they require no energy input. They are used for emergency desalination purposes in seawater and floodwater settings.
Under some circumstances, it may be possible to use energy more efficiently. As heat is produced during distillation processes, it is possible to design a desalination plant that also reuses the heat generated to produce electricity. For example, in the Middle East and North Africa, it has become fairly common for dual-purpose facilities to produce both electricity and water. The main advantage is that a combined facility consumes less fuel than would be needed by two separate facilities.
A number of factors determine the capital and operating costs for desalination: capacity and type of facility, location, feed water, labor, energy, financing and concentrate disposal. Desalination stills now control pressure, temperature and brine concentrations to optimize the water extraction efficiency. Nuclear-powered desalination might be economical on a large scale, and there is a pilot plant in the former USSR.
Critics point to the high costs of desalination technologies, especially for poverty-stricken developing countries, the difficulty in transporting or piping massive amounts of desalinated seawater throughout the interiors of large countries, and the byproduct of concentrated seawater, which some environmentalists have claimed "is a major cause of marine pollution when dumped back into the oceans at high temperatures."
It should be noted that the reverse osmosis technology used for desalination typically does not produce this "hot water" as a by-product. Additionally, depending on the prevailing currents of receiving waters, the seawater concentrate by-product can be diluted and dispersed to background levels within relatively short distances of the ocean outlet.
While noting that costs are falling, and generally positive about the technology for affluent areas that are proximate to oceans, one study argues that "Desalinated water may be a solution for some water-stress regions, but not for places that are poor, deep in the interior of a continent, or at high elevation. Unfortunately, that includes some of the places with biggest water problems." It further says, "… desalinated water is only expensive in places far from the sea, like New Delhi, or in high places, like Mexico City. Desalinated water is also expensive in places that are both somewhat far from the sea and somewhat high, such as Riyadh and Harare. In other places, the dominant cost is desalination, not transport. This leads to relatively low costs in places like Beijing, Bangkok, Zaragoza, Phoenix, and, of course, coastal cities like Tripoli." For cities on the coast, desalination is being increasingly viewed as an untapped and unlimited water resource.
Many large coastal cities in developed countries are considering the feasibility of seawater desalination, due to its cost effectiveness compared with other water supply options, which can include mandatory installation of rainwater tanks or storm water harvesting infrastructure. Studies have shown that desalination is among the most cost-effective options for boosting water supply in major Australian state capitals. The city of Perth has been successfully operating a reverse osmosis seawater desalination plant since 2006, and the West Australian government has announced that a second plant will be built to service the city's needs. A desalination plant is to be built in Australia's largest city, Sydney, and in Wonthaggi, Victoria, in the near future.
The Perth desalination plant is powered partially by renewable energy from the Emu Downs Wind Farm. The Sydney plant will be powered entirely from renewable sources, thereby eliminating harmful greenhouse gas emissions to the environment, a common argument used against seawater desalination due to the energy requirements of the technology.
The purchase or production of renewable energy to power desalination plants naturally adds to the capital and/or operating costs of desalination. However, recent experience in Perth and Sydney indicates that the additional cost is acceptable to communities, as a city may then augment its water supply without doing environmental harm to the atmosphere. The Gold Coast desalination plant will be powered entirely from fossil fuels, at a time when the coal-fired power stations have significantly reduced capacity due to the drought. At a rate of over 4 kWh per cubic meter of production, this will be the most expensive source of water in Australia.
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. The initial plans of many proposed ocean desalination plants relied on these intakes, despite their huge potential 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 90 percent of the life force of the ocean—that is, the plankton, fish eggs, and fish larvae. There are alternatives, including beach wells, that eliminate this concern, but require more energy and higher costs while limiting output. Other environmental concerns include air pollution and greenhouse gas emissions from the power plants that provide electricity and/or thermal energy to the desalination plants.
Regardless of the method used, there is always a highly concentrated waste product, consisting of everything that was separated from the newly generated freshwater. This is sometimes referred to as brine, which is also a common term for the by-product of recycled water schemes that is often disposed of in the ocean. These concentrates are classified by the United States Environmental Protection Agency (EPA) as industrial wastes. With coastal facilities, it may be possible to return the concentrate to the sea without harm if it does not exceed the normal ocean salinity gradients to which osmoregulators are accustomed. Reverse osmosis, for instance, may require the disposal of wastewater with a salinity twice that of normal seawater. The benthic community cannot accommodate such an extreme change in salinity, and many filter-feeding animals would be destroyed when the water is returned to the ocean. This presents an increasing problem further inland, where one needs to avoid ruining existing freshwater supplies such as ponds, rivers and aquifers. As such, proper disposal of concentrate needs to be investigated during the design phases.
To limit the environmental impact of returning the brine to the ocean, one approach is to dilute the brine with another stream of water entering the ocean, such as the outfall of a wastewater treatment plant or power plant. In this manner, the salinity of the brine can be reduced. If the power plant is medium- to large-sized, and the desalination plant is not enormous, the flow of the power plant's cooling water is likely to be at least several times larger than that of the desalination plant.
An alternative approach is to spread the brine over a very large area, so that there is only a slight increase in salinity. For example, once the pipeline containing the brine reaches the sea floor, it can split off into many branches, each one releasing the brine gradually along its length through small holes. This approach can be used together with the combining of brine with power plant or wastewater plant outfalls.
The concentrated seawater has the potential to harm ecosystems, especially marine environments, in regions with low turbidity and high evaporation that already have elevated salinity. Examples of such locations are the Persian Gulf, the Red Sea, and, in particular, coral lagoons of atolls and other tropical islands around the world. Because the brine is denser than the surrounding seawater due to higher solute concentration, discharge into water bodies means that the ecosystems on the bed of the water body are most at risk because the brine sinks and remains there long enough to damage the ecosystems. Careful re-introduction can minimize this problem. For example, for the desalination plant and ocean outlet structures to be built in Sydney from late 2007, the water authority states that the ocean outlets will be placed in locations at the seabed that will maximize dispersal of the concentrated seawater, such that it will be indistinguishable from normal seawater between 50 and 75 meters from the outlet points. Sydney is fortunate to have typical oceanographic conditions off the coast that allow for such rapid dilution of the concentrated by-product, thereby minimizing harm to the environment.
In Perth, Australia, a wind-powered desalination plant was opened in 2007. The water is sucked in from the ocean at only 0.1 meter per second, which is slow enough to let fish escape. The plant provides nearly 40 million gallons of clean water per day.
Increased water conservation and water use efficiency remain the most cost-effective priority for supplying water. While comparing ocean water desalination to wastewater reclamation for drinking water shows desalination as the first option, using reclamation for irrigation and industrial use provides multiple benefits. Urban runoff and storm water capture also provide multiple benefits in treating, restoring and recharging groundwater.
In the past, many novel desalination techniques have been researched, with varying degrees of success. Some are still on the drawing board, 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 maximizing energy efficiency and cost effectiveness, one may consider the Passarell Process.
Other approaches involve the use of geothermal energy. An example would be the work being done by the San Diego State University CITI International Consortium for Advanced Technologies and Security. 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 these water resources have long been under severe stress.
Recent research in the United States indicates that nanotube 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.
All links retrieved August 15, 2013.
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