desalinate

 

process of removing soluble salts from water to render it suitable for drinking, irrigation, or industrial uses. The principal methods used for desalination include distillation (or evaporation), electrodialysis, freezing, ion exchange, and reverse osmosis.

In distillation saltwater is heated in one container to make the water evaporate, leaving the salt behind. The desalinated vapor is then condensed to form water in a separate container. Although long known, distillation has found limited application in water supply because of the fuel costs involved in converting saltwater to vapor. Representative of the early attempts in this direction were the solar distillation methods employed (c.49 B.C.) by the legions of Julius Caesar for using water from the Mediterranean. Modern technological advances led to the development of more efficient distillation units using solar energy; however, since these units have small capacities, their utility is restricted.

Distillation plants having high capacities and using combustible fuels employ various devices to conserve heat. In the most common system a vacuum is applied to reduce the boiling point of the water, or a spray or thin film of water is exposed to high heat, causing flash evaporation; the water is flashed repeatedly, yielding fresh distilled water. This multistage flash distillation method is used in more than 2,000 desalination plants, including one in Saudi Arabia that produces 250 million gallons of freshwater per day.

Another method of desalination is by electrodialysis. When salt dissolves in water, it splits up into charged particles called ions. Placed in a container with a negative electrode at one end and a positive electrode at the other, the ions are filtered by the membranes as they are attracted toward the electrodes; they become trapped between semipermeable membranes, leaving outside the membranes a supply of desalinated water that can be tapped. The first large installation using this process began operating in South Africa in 1958, but its electrical demands make it impractical except where such energy is abundant.

By far the most promising approach is the reverse osmosis process, in which pressure is applied to saltwater to force it through a special membrane. Only pure water passes, leaving concentrated seawater behind. Where multistage flash distillation costs about $4 per 1,000 gallons, reverse osmosis costs about half that amount. This process is used by a plant in the Tampa Bay area, Florida, that produces 25 million gallons of drinking water a day. Another type uses an empty hollow sphere of semipermeable material that is lowered into the sea. The water flowing into the sphere is fresh, since the salt is excluded by the membrane that covers the entire sphere and is its guard.

One final approach is under development in Hawaii, where different layers of seawater display a large temperature differential. Here an Ocean Thermal Energy Conversion plant is being built which will use steam produced by the flash method to produce energy, then condense the steam into freshwater. Three such plants could produce a hundred megawatts of power, as well as supply 30% of Hawaii's water needs.

For emergency use, i.e., in lifeboats, various systems are available in addition to solar or fuel-heated distillation devices. One device made of flexible plastic is worn around the waist of the user to employ body heat for evaporation.

Desalination, desalinization, or desalinisation refer to any of several processes that remove excess salt and other minerals from water. Desalination may also refer to the removal of salts and minerals more generally,^[1] as in soil desalination,^[2][3] but the focus of this article is on water desalination.

Water is desalinated in order to obtain fresh water 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 fresh water 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 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. Saudi Arabia's desalination plants account for about 24% 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.

Methods

1. Distillation
   1. Multi-stage flash distillation (MSF)
   2. Multiple-effect evaporator (MED|ME)
   3. Vapor-compression evaporation (VC)
   4. Evaporation/condensation
2. Membrane processes
   1. Electrodialysis reversal (EDR)
   2. Reverse osmosis (RO)
   3. Nanofiltration (NF)
   4. Forward osmosis (FO)
   5. Membrane distillation (MD)
3. Freezing
4. Geothermal desalination
5. Solar humidification (HDH, MEH)
6. Methane hydrate crystallisation
7. High grade water recycling

As of July 2004, the two leading methods were reverse osmosis (47.2% of installed capacity world-wide) and multi-stage flash (36.5%).^{[4][not in citation given]}

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. Due to the reduced temperature, energy is saved.^{[citation needed]}

In the last decade, membrane processes have grown very fast, and most new facilities use reverse osmosis technology.^{[citation needed]} Membrane processes use semi-permeable membranes and pressure to separate salts from water.^{[citation needed]} 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.^{[citation needed]}

Forward osmosis employs a passive membrane filter that is hydrophilic and slowly permeable to water, and blocks a portion of the solutes.^{[citation needed]} Water is driven across the membrane by osmotic pressure created by food grade concentrate on the clean side of the membrane.^{[citation needed]} Forward osmosis systems are passive in that they require no energy input.^{[citation needed]} They are used for emergency desalination purposes in seawater and floodwater settings.^{[citation needed]}

Considerations and criticism

Co-generation

There are circumstances in which it may be possible to use energy more efficiently. With cogeneration this occurs as energy quality drops from a high level (typically in the form of electricity) toward lower levels (typically in the form of heat). Distillation processes, in particular, can be designed to take advantage of co-generation by recovering and reusing heat. 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 can consume less fuel than would be needed by two separate facilities.

Economics

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.^[5]

Critics point to the high costs of desalination technologies, especially for poor third world countries, the impracticability and cost of 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"^[6]

It should be noted that typically the reverse osmosis technology that is used to desalinate water does not produce this "hot water" as a byproduct. Additionally, depending on the prevailing currents of receiving waters, the seawater concentrate byproduct 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." and "Indeed, one needs to lift the water by 2000 m, or transport it over more than 1600 km to get transport costs equal to the desalination costs. Thus, 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."^[7] For cities on the coast, desalination is being increasingly viewed as an untapped and unlimited water storage.

Israel is now desalinizing water at a cost of 53 cents per cubic meter.^[8] Singapore is desalinizing water for 49 cents per cubic meter.^[9] 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 desalination is among the most cost-effective options for boosting water supply in major Australian state capitals.^{[citation needed]} The city of Perth has been successfully^{[citation needed]} 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 Wonthaggi, Victoria in the near future.^[10]

The Perth desalination plant is powered partially by renewable energy from the Emu Downs Wind Farm^[11]. The Sydney plant will be powered entirely from renewable sources^[12], 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 and 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 to produce this will be the most expensive source of water in Australia.

Environmental

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 huge 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 lifeforce of the ocean; the plankton, fish eggs and fish larvae.^[13] There are alternatives including beach wells that eliminate this concern, but require more energy and higher costs while limiting output.^[14] 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 removed from the created fresh water. This is sometimes referred to as brine, which is also a common term for the byproduct of recycled water schemes that is often disposed of in the ocean. These concentrates are classified by the United States Environmental Protection Agency as industrial wastes. With coastal facilities, it may be possible to return it to the sea without harm if this concentrate 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 fresh water 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, it can be diluted with another stream of water entering the ocean, such as the outfall of a wastewater treatment plant or power plant. While seawater power plant cooling water outfalls are not freshwater like wastewater 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 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 method can be used in combination with the joining of the 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 more dense 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 maximise the dispersal of the concentrated seawater, such that it will be indistinguishable from normal seawater between 50 metres and 75 metres 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 minimising harm to the environment.

In Perth, Australia, in 2007, a wind powered desalination plant was opened. 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. [2]

Desalination compared to other water supply options

Increased water conservation and water use efficiency remain the most cost effective priority for supplying water .^[15] 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.^[16] Urban runoff and storm water capture also provide multiple benefits in treating, restoring and recharging groundwater.^[17]

Experimental techniques and other developments

In the past many novel desalination techniques have been researched with varying degrees of success. Some 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 maximizing energy efficiency and cost effectiveness, we may consider the Passarell Process.

Other approaches involve the use of geothermal energy. An example would be the work being done by SDSU CITI International Consortium for Advanced Technologies and Security.^[18] 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 US 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.^[19]

See also

• Soil salination
• Kwinana Desalination Plant
• Solar Powered Desalination Unit
• Point Paterson Desalination Plant

References

1. ^ "Desalination" (definition), The American Heritage Science Dictionary, Houghton Mifflin Company, via dictionary.com. Retrieved on 2007-08-19.
2. ^ "Australia Aids China In Water Management Project." People's Daily Online, 2001-08-03, via english.people.com.cn. Retrieved on 2007-08-19.
3. ^ Takashi, Kume, Amaya Takao, and Mitsuno Tooru. "The Effect of Soil Desalinization in the Hetao Irrigation District, Inner Mongolia, China." Transactions of the Japanese Society of Irrigation, Drainage and Reclamation Engineering, No. 223, pp. 133-139, 2003, abstract via sciencelinks.jp. Retrieved on 2007-08-19.
4. ^ Source: 2004 IDA Worldwide Desalting Plants Inventory Report No 18; published by Wangnick Consulting
5. ^ "Nuclear Desalination: UIC Nuclear Issues Briefing Paper #74," Uranium Information Centre Ltd., Melbourne, Australia, October 2006. Retrieved on 2007-08-20.
6. ^ Barlow, Maude, and Tony Clarke, "Who Owns Water?" The Nation, 2002-09-02, via thenation.com. Retrieved on 2007-08-20.
7. ^ Zhoua, Yuan, and Richard S.J. Tolb. "Evaluating the costs of desalination and water transport." (Working paper). Via a Hamburg University website. 2004-12-09. Retrieved on 2007-08-20.
8. ^ Sitbon, Shirli. "French-run water plant launched in Israel," European Jewish Press, via ejpress.org, 2005-12-28. Retrieved on 2007-08-20.
9. ^ "Black & Veatch-Designed Desalination Plant Wins Global Water Distinction," (Press release). Black & Veatch Ltd., via edie.net, 2006-05-04. Retrieved on 2007-08-20.
10. ^ "Sydney desalination plant to double in size," ABC News (Australian Broadcasting Corporation), via abc.net.au, 2007-06-25. Retrieved on 2007-08-20.
11. ^ Australia Turns to Desalination by Michael Sullivan. Morning Edition, National Public Radio, June 18, 2007
12. ^ http://www.sydneywater.com.au/EnsuringTheFuture/Desalination/pdf/Desalinationfactsheet_Poweredgreenenergy.pdf
13. ^ http://www.desalresponsegroup.org/files/RiverkeepervEPA1-25-07_decision.pdf
14. ^ http://www.pacinst.org/reports/desalination/desalination_report.pdf
15. ^ Gleick, Peter H., Dana Haasz, Christine Henges-Jeck, Veena Srinivasan, Gary Wolff, Katherine Kao Cushing, and Amardip Mann. (November 2003.) "Waste not, want not: The potential for urban water conservation in California." (Website). Pacific Institute. Retrieved on 2007-09-20.
16. ^ Cooley, Heather, Peter H. Gleick, and Gary Wolff. (June 2006.) "Desalination, With a Grain of Salt – A California Perspective." (Website). Pacific Institute. Retrieved on 2007-09-20.
17. ^ Gleick, Peter H., Heather Cooley, David Groves. (September 2005.) "California water 2030: An efficient future." (Website). Pacific Institute. Retrieved on 2007-09-20.
18. ^ [1]
19. ^ Lawrence Livermore National Laboratory Public Affairs (2006-5-18). Nanotube membranes offer possibility of cheaper desalination. Press release. Retrieved on 2007-9-7.

External links

• Making Water, WWF report on desalination
• The Desal Response Group
• Middle East Desalination Research Centre
• Encyclopedia of Desalination and water and Water Resources
• Global Water Intelligence - monthly newsletter featuring 'Desal Project Tracker'
• Water Desalination Report - weekly newsletter on international desal
• International Desalination Association
• Desalination & Water Reuse - Official magazine of the International Desalination Association
• European Desalination Society
• IAEA - Nuclear Desalination
• DME - German Desalination Society
• Geothermal Desalination
• SDSU (San Diego State University) Center for Information Technology and Infrastructure (CITI) International Consortium of Advanced Technologies and Security
• "Desalination Journal" and Desalination Directory of the European Desalination Society
• Desalination by humidification and dehumidification of air: state of the art
• Zonnewater - optimized solar thermal desalination (distillation)
• SOLAR TOWER Project - Clean Electricity Generation for Desalination.
• Solar Desalination using the MEH-Method
• Article: Water issues prompt new look at desalination
• Desalination bibliography Library of Congress
• Water-Technology
• Cheap Drinking Water from the Ocean - Carbon nanotube-based membranes will dramatically cut the cost of desalination
• BBC2 Newsnight Film: Israel build the world's largest desalination plant
• Water Desalination Free pdf book
• Tampa Bay Water Seawater Desalination Reverse Osmosis Deslaniation Plant on Tampa Bay
• Solar thermal-driven desalination plants based on membrane distillation
• Encyclopedia of Water Sciences, Engineering and Technology Resources
• Report on other byproducts of desalination plants

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