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. Desalination 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 specialized, expensive infrastructure, making it very costly compared to the use of fresh water from rivers or groundwater.
However, along with recycled water this is one of the only non-rainfall dependent water sources particularly relevant to countries like Australia which traditionally have relied on rainfall in dams to provide their drinking water supplies.
The world's largest desalination plant is the Jebel Ali Desalination Plant 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 metres of water per year. By comparison the largest desalination plant in the United States is located in Tampa Bay, Florida and operated by Tampa Bay Water, which began desalinating 34.7 million cubic meters of water per year in December 2007.The Tampa Bay plant runs at around 12% the output of the Jebel Ali Desalination Plants. A January 17, 2008, article in the Wall Street Journal states, "World-wide, 13,080 desalination plants produce more than 12 billion gallons of water a day, according to the International Desalination Association.
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 vapor pressure equals the ambient pressure and vapor pressure increases with temperature. Thus, because of the reduced temperature, energy is saved. A leading distillation method is multi-stage flash distillation accounting for 85% of production worldwide in 2004.
Reverse osmosis desalination plant in Barcelona, Spain
The principal competing processes use membranes to desalinate, principally applying reverse osmosis technology. Membrane processes use semi-permeable membranes and pressure to separate salts from water. Reverse osmosis plant 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.
Considerations and criticism
Cogeneration is the process of using excess heat from power production to accomplish another task. For desalination, cogeneration is the production of potable water from seawater or brackish groundwater in an integrated, or "dual-purpose", facility in which a power plant is used as the source of energy for the desalination process. The facility’s energy production may be dedicated entirely to the production of potable water (a stand-alone facility), or excess energy may be produced and incorporated into the energy grid (a true cogeneration facility). There are various forms of cogeneration, and theoretically any form of energy production could be used. However, the majority of current and planned cogeneration desalination plants use either fossil fuels or nuclear power as their source of energy. Most plants are located in the Middle East or North Africa, due to their petroleum resources and subsidies. The advantage of dual-purpose facilities is that they can be more efficient in energy consumption, thus making desalination a more viable option for drinking water in areas of scarce water resources.
Shevchenko BN350, a nuclear-heated desalination unit
In a December 26, 2007, opinion column in the The Atlanta Journal-Constitution, Nolan Hertel, a professor of nuclear and radiological engineering at Georgia Tech, wrote, "... nuclear reactors can be used ... to produce large amounts of potable water. The process is already in use in a number of places around the world, from India to Japan and Russia. Eight nuclear reactors coupled to desalination plants are operating in Japan alone ... nuclear desalination plants could be a source of large amounts of potable water transported by pipelines hundreds of miles inland..."
Additionally, the current trend in dual-purpose facilities is hybrid configurations, in which the permeate from an RO desalination component is mixed with distillate from thermal desalination. Basically, two or more desalination processes are combined along with power production. Such facilities have already been implemented in Saudi Arabia at Jeddah and Yanbu.
A typical aircraft carrier in the U.S. military uses nuclear power to desalinate 400,000 US gallons (1,500,000 l; 330,000 imp gal) of water per day.
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.
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." and "Indeed, one needs to lift the water by 2,000 metres (6,600 ft), or transport it over more than 1,600 kilometres (990 mi) to get transport costs equal to the desalination costs. Thus, it may be more economical to transport fresh water from somewhere else than to desalinate it. In places far from the sea, like New Delhi, or in high places, like Mexico City, high transport costs would add to the high desalination costs. Desalinated water is also expensive in places that are both somewhat far from the sea and somewhat high, such as Riyadh and Harare. In many places, the dominant cost is desalination, not transport; the process would therefore be relatively less expensive in places like Beijing, Bangkok, Zaragoza, Phoenix, and, of course, coastal cities like Tripoli.After being desalinated at Jubail, Saudi Arabia, water is pumped 200 miles (320 km) inland through a pipeline to the capital city of Riyadh.For cities on the coast, desalination is being increasingly viewed as an untapped and unlimited water source.
Desalination makes sense only after less expensive options are exhausted, including recycling water and fixing broken infrastructure. Water is reused in Las Vegas NV, Fountain Valley CA, Fairfax VA, El Paso TX and Scottsdale AZ. Compared to desalinated sea water, recycling requires 50% less energy due to the significantly lower salt content and produces new water at 30% less cost to the consumer, without the damage to marine life and ecosystems common to desalination plants.
Israel is now desalinating water at a cost of US$0.53 per cubic meter.Singapore is desalinating water for US$0.49 per cubic meter. 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 stormwater harvesting infrastructure. Studies have shown that the desalination option is more cost-effective than large-scale recycled water for drinking, and more cost-effective in Sydney than the vastly expensive option of mandatory installation of rainwater tanks or stormwater harvesting infrastructure. The city of Perth has been successfully operating a reverse osmosis seawater desalination plant since 2006, and the Western Australian government have announced that a second plant will be built to serve the city's needs. A desalination plant is now operating in Australia's largest city of Sydney,and the Wonthaggi desalination plant under construction in Wonthaggi, Victoria.
The Perth desalination plant is powered partially by renewable energy from the Emu Downs Wind Farm. A wind farm at Bungendore in NSW has been purpose-built to generate enough renewable energy to offset the energy use of the Sydney plant,mitigating concerns about harmful greenhouse gas emissions, 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 Queensland state government also purchased renewable energy certificates on behalf of its Gold Coast plant which will see the plant offset its carbon emissions for the initial 18 to 20 months of operations, bringing its environmental footprint down, in line with the other major plants that will be operating around the same time, in Perth and Sydney.
In December 2007, the South Australian government announced that it would build a seawater desalination plant for the city of Adelaide, Australia, located at Port Stanvac. The desalination plant is to be funded by raising water rates to achieve full cost recovery.An online, unscientific poll showed that nearly 60% of votes cast were in favor of raising water rates to pay for desalination.
A January 17, 2008, article in the Wall Street Journal states, "In November, Connecticut-based Poseidon Resources Corp. won a key regulatory approval to build the US$300 million water-desalination plant in Carlsbad, north of San Diego. The facility would produce 50,000,000 US gallons (190,000,000 l; 42,000,000 imp gal) of drinking water per day, enough to supply about 100,000 homes ... Improved technology has cut the cost of desalination in half in the past decade, making it more competitive ... Poseidon plans to sell the water for about US $950 per acre-foot [1,200 cubic metres (42,000 cu ft)]. That compares with an average US$700 an acre-foot [1200 m³] that local agencies now pay for water." $1,000 per acre-foot works out to $3.06 for 1,000 gallons, or $.81 for 1 cubic meter, which is the unit of water measurement that residential water users are accustomed to being billed in.
While this regulatory hurdle was met, Poseidon Resources is not able to break ground until the final approval of a mitigation project for the damage done to marine life through the intake pipe, as is required by California law. Poseidon Resources has made progress in Carlsbad, CA, despite its unsuccessful attempt to complete construction of Tampa Bay Desal, a desalination plant in Tampa Bay, FL, in 2001. The Board of Directors of Tampa Bay Water were forced to buy Tampa Bay Desal from Poseidon Resources in 2001 to prevent a third failure of the project. Tampa Bay Water faced five years of engineering problems and operation at 20% capacity due to marine life and growth captured and stuck to reverse osmosis filters prior to fully utilizing this facility in 2007.
According to a May 9, 2008, article in Forbes, a San Leandro, California, company called Energy Recovery Inc. has been desalinating water for US $0.46 per cubic meter.
According to a June 5, 2008, article in the Globe and Mail, a Jordanian-born chemical engineering doctoral student at the University of Ottawa, named Mohammed Rasool Qtaisha, has invented a new desalination technology that is alleged to be between 600% and 700% more water output per square meter of membrane than current technology. According to the article, General Electric is looking into similar technology, and the U.S. National Science Foundation announced a grant to the University of Michigan to study it as well. Because the patents were still being worked out, the article was very vague about the details of this alleged technology.
While desalinating 1,000 US gallons (3,800 l; 830 imp gal) of water can cost as much as $3, the same amount of bottled water costs $7,945.
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 90%, of the life in the ocean; 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.
To limit the environmental impact of returning the brine to the ocean, it can be diluted with another stream of water entering the ocean, such as the outfall of a waste water treatment plant or power plant. While seawater power plant cooling water outfalls are not freshwater like waste water treatment plant outfalls, the salinity of the brine will still 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. Another method to reduce the increase in salinity is to spread the brine via a diffuser to mix in a mixing zone 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 method can be used in combination with the joining of the brine with power plant or waste water plant outfalls.
There are methods of desalination, particularly in combination with open pond evaporation (solar desalination), that do not discharge brine back into the ocean at all.
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.
The UAE, Qatar, Bahrain, Saudi Arabia, Kuwait and Iran have 120 desalination plants between them. These plants flush nearly 24 tons of chlorine, 65 tons of algae-harming antiscalants used to descale pipes, and around 300 kg of copper into the Persian Gulf every day.
Because the brine is denser than the surrounding sea water due to the 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 the dispersal of the concentrated seawater, such that it will be indistinguishable from normal seawater between 50 and 75 metres (160 and 246 ft) from the outlet points. Sydney is fortunate to have typical oceanographic conditions off the coast that allow for such rapid dilution of the concentrated byproduct, thereby minimizing harm to the environment.
In Perth, Australia, in 2007, the Kwinana Desalination Plant was opened. The water is sucked in from the ocean at only 0.1 metres per second (0.33 ft/s), which is slow enough to let fish escape. The plant provides nearly 140,000 cubic metres (4,900,000 cu ft) of clean water per day.This is the same at Queensland's Gold Coast Desalination Plant and Sydney's Desalination Plant.