Desalination

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, as in soil desalination, 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
 * 2) Multi-stage flash distillation (MSF)
 * 3) Multiple-effect evaporator (MED|ME)
 * 4) Vapor-compression evaporation (VC)
 * 5) Evaporation/condensation
 * 6) Membrane processes
 * 7) Electrodialysis reversal (EDR)
 * 8) Reverse osmosis (RO)
 * 9) Nanofiltration (NF)
 * 10) Forward osmosis (FO)
 * 11) Membrane distillation (MD)
 * 12) Freezing
 * 13) Geothermal desalination
 * 14) Solar humidification (HDH, MEH)
 * 15) Methane hydrate crystallisation
 * 16) 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%).

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.

In the last decade, membrane processes have grown very fast, 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.

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.

Co-generation
Cogeneration is the process of using excess heat from power production to accomplish another task. In the sense of 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.

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. The advantage to hybrid configurations is that two qualities Such facilities have already been implemented in Saudi Arabia at Jeddah and Yambu-Medina.

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.

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"

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." 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. Singapore is desalinizing water for 49 cents 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 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 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 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. 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 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. 

Desalination compared to other water supply options
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.

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. 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.