Desalination

Desalinationis a process that removes mineral components fromsaline water.More generally, desalination is the removal of salts and minerals from a substance.One example issoil desalination.This is important for agriculture. It is possible to desalinate saltwater, especiallysea water,to produce water for human consumption or irrigation. The by-product of the desalination process isbrine.Many seagoing ships andsubmarinesuse desalination. Modern interest in desalination mostly focuses on cost-effective provision offresh waterfor human use. Along with recycledwastewater,it is one of the fewwater resourcesindependent of rainfall.

Reverse osmosis desalination plant in Barcelona, Spain

Due to its energy consumption, desalinating sea water is generally more costly than fresh water fromsurface waterorgroundwater,water recyclingandwater conservation;however, these alternatives are not always available and depletion of reserves is a critical problem worldwide.Desalination processes are using either thermal methods (in the case ofdistillation) or membrane-based methods (e.g. in the case ofreverse osmosis).

An estimate in 2018 found that "18,426 desalination plants are in operation in over 150 countries. They produce 87 million cubic meters of clean water each day and supply over 300 million people."The energy intensity has improved: It is now about 3 kWh/m³(in 2018), down by a factor of 10 from 20–30 kWh/m³in 1970.Nevertheless, desalination represented about 25% of the energy consumed by thewater sectorin 2016.

History

See also:Distillation § Desalination by distillation,andDistilled water § History

Ancient Greek philosopherAristotleobserved in his workMeteorologythat "salt water, when it turns into vapour, becomes sweet and the vapour does not form salt water again when it condenses," and that a fine wax vessel would hold potable water after being submerged long enough in seawater, having acted as a membrane to filter the salt.

At the same time the desalination of seawater was recorded in China. Both theClassic of Mountains and Water Seasin thePeriod of the Warring Statesand theTheory of the Same Yearin theEastern Han Dynastymentioned that people found that the bamboo mats used for steaming rice would form a thin outer layer after long use. The as-formed thin film hadadsorptionandion exchangefunctions, which could adsorb salt.

Numerous examples of experimentation in desalination appeared throughout Antiquity and theMiddle Ages,but desalination became feasible on a large scale only in the modern era.A good example of this experimentation comes fromLeonardo da Vinci(Florence, 1452), who realized that distilled water could be made cheaply in large quantities by adapting astillto a cookstove.During the Middle Ages elsewhere in Central Europe, work continued on distillation refinements, although not necessarily directed towards desalination.

The first major land-based desalination plant may have been installed under emergency conditions on an island off the coast ofTunisiain 1560.It is believed that a garrison of 700 Spanish soldiers was besieged by the Turkish army and that, during the siege, the captain in charge fabricated astillcapable of producing 40 barrels of fresh water per day, though details of the device have not been reported.

Before theIndustrial Revolution,desalination was primarily of concern to oceangoing ships, which otherwise needed to keep on board supplies of fresh water. SirRichard Hawkins(1562–1622), who made extensive travels in theSouth Seas,reported that he had been able to supply his men with fresh water by means of shipboard distillation.Additionally, during the early 1600s, several prominent figures of the era such asFrancis BaconandWalter Raleighpublished reports on desalination.These reports and others,set the climate for the first patent dispute concerning desalination apparatus. The two first patents regarding water desalination were approved in 1675 and 1683 (patents No.184and No. 226,published byWilliam Walcotand Robert Fitzgerald (and others), respectively). Nevertheless, neither of the two inventions entered service as a consequence of scale-up difficulties.No significant improvements to the basic seawater distillation process were made during the 150 years from the mid-1600s until 1800.

When the frigateProtectorwas sold to Denmark in the 1780s (as the shipHussaren) its still was studied and recorded in great detail.In the United States,Thomas Jeffersoncatalogued heat-based methods going back to the 1500s, and formulated practical advice that was publicized to all U.S. ships on the reverse side of sailing clearance permits.

Beginning about 1800, things started changing as a consequence of the appearance of thesteam engineand the so-calledage of steam.Knowledge of the thermodynamics of steam processesand the need for a pure water source for its use in boilersgenerated a positive effect regarding distilling systems. Additionally, the spread ofEuropean colonialisminduced a need for freshwater in remote parts of the world, thus creating the appropriate climate for water desalination.

In parallel with the development and improvement of systems using steam (multiple-effect evaporators), these type of devices quickly demonstrated their desalination potential.In 1852,Alphonse René le Mire de Normandywas issued a British patent for a vertical tube seawater distilling unit that, thanks to its simplicity of design and ease of construction, gained popularity for shipboard use.Land-based units did not significantly appear until the latter half of the nineteenth century.In the 1860s, the US Army purchased three Normandy evaporators, each rated at 7000 gallons/day and installed them on the islands ofKey WestandDry Tortugas.Another land-based plant was installed atSuakinduring the 1880s that provided freshwater to the British troops there. It consisted of six-effect distillers with a capacity of 350 tons/day.

After World War II, many technologies were developed or improved such as Multi Effect Flash desalination (MEF) and Multi Stage Flash desalination (MSF). Another notable technology is freeze-thaw desalination.Freeze-thaw desalination, (cryo-desalination or FD), excludes dissolved minerals from saline water through crystallization.

The Office of Saline Water was created in theUnited States Department of the Interiorin 1955 in accordance with the Saline Water Conversion Act of 1952.This act was motivated by a water shortage in California and inland western United States. The Department of the Interior allocated resources including research grants, expert personnel, patent data, and land for experiments to further advancements.

The results of these efforts included the construction of over 200 electrodialysis and distillation plants globally,reverse osmosis(RO) research, and international cooperation (for example, the First International Water Desalination Symposium and Exposition in 1965).The Office of Saline Water merged into the Office of Water Resources Research in 1974.

The first industrial desalination plant in the United States opened inFreeport, Texasin 1961 after a decade of regional drought.

By the late 1960s and the early 1970s, RO started to show promising results to replace traditional thermal desalination units. Research took place at state universities in California, at theDow Chemical CompanyandDuPont.Many studies focus on ways to optimize desalination systems.The first commercial RO plant, the Coalinga desalination plant, was inaugurated in California in 1965 forbrackish water.Dr. Sidney Loeb,in conjunction with staff atUCLA,designed a large pilot plant to gather data on RO, but was successful enough to provide freshwater to the residents of Coalinga. This was a milestone in desalination technology, as it proved the feasibility of RO and its advantages compared to existing technologies (efficiency, no phase change required, ambient temperature operation, scalability, and ease of standardization).A few years later, in 1975, the firstsea waterreverse osmosis desalination plant came into operation.

As of 2000, more than 2000 plants were operated. The largest are in Saudi Arabia, Israel, and UAE and the biggest plant with a volume of 1,401,000 m3/d is in Saudi Arabia (Ras Al Khair).

As of 2021 22,000 plants were in operationIn 2024 the Catalan government installed a floating offshore plant near the port of Barcelona and purchased 12 mobile desalination units for the northern region of the Costa Brava to combat the severe drought.

In 2012, cost averaged $0.75 per cubic meter. By 2022, that had declined (before inflation) to $0.41. Desalinated supplies are growing at a 10%+ compound rate, doubling in abundance every seven years.

Applications

External audio
"Making the Deserts Bloom: Harnessing nature to deliver us from drought",Distillations Podcast and transcript, Episode 239, March 19, 2019,Science History Institute

Schematic of amultistage flash desalinator
A – steam in B – seawater in C – potable water out
D – brine out (waste) E –condensateout F – heat exchange G – condensation collection (desalinated water)
H – brine heater
Thepressure vesselacts as acountercurrent heat exchanger.Avacuum pumplowers the pressure in the vessel to facilitate the evaporation of the heated seawater (brine) which enters the vessel from the right side (darker shades indicate lower temperature). The steam condenses on the pipes on top of the vessel in which the fresh sea water moves from the left to the right.

There are now about 21,000 desalination plants in operation around the globe. The biggest ones are in theUnited Arab Emirates,Saudi Arabia,and Israel. The world's largest desalination plant is located inSaudi Arabia(Ras Al-Khair Power and Desalination Plant) with a capacity of 1,401,000 cubic meters per day.

Desalination is currently expensive compared to most alternative sources of water, and only a very small fraction of total human use is satisfied by desalination.It is usually only economically practical for high-valued uses (such as household and industrial uses) inaridareas. However, there is growth in desalination for agricultural use and highly populated areas such as Singaporeor California.The most extensive use is in thePersian Gulf.

While noting costs are falling, and generally positive about the technology for affluent areas in proximity to oceans, a 2005 study argued, "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 the 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, 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, likeMexico City,transport costs could match desalination costs. Desalinated water is also expensive in places that are both somewhat far from the sea and somewhat high, such asRiyadhandHarare.By contrast in other locations transport costs are much less, such as Beijing,Bangkok,Zaragoza,Phoenix,and, of course, coastal cities likeTripoli.After desalination atJubail,Saudi Arabia, water is pumped 320 km inland toRiyadh.For coastal cities, desalination is increasingly viewed as a competitive choice.

In 2023, Israel was using desalination to replenish theSea of Galilee's water supply.

Not everyone is convinced that desalination is or will be economically viable or environmentally sustainable for the foreseeable future.Debbie Cookwrote in 2011 that desalination plants can be energy intensive and costly. Therefore, water-stressed regions might do better to focus on conservation or other water supply solutions than invest in desalination plants.

Technologies

Desalination is an artificial process by which saline water (generallysea water) is converted to fresh water. The most common desalination processes aredistillationandreverse osmosis.

There are several methods.Each has advantages and disadvantages but all are useful. The methods can be divided into membrane-based (e.g.,reverse osmosis) and thermal-based (e.g.,multistage flash distillation) methods.The traditional process of desalination isdistillation(i.e., boiling and re-condensationofseawaterto leave salt and impurities behind).

There are currently two technologies with a large majority of the world's desalination capacity:multi-stage flash distillationandreverse osmosis.

Distillation

Solar distillation

Solar distillationmimics the natural water cycle, in which the sun heats sea water enough for evaporation to occur.After evaporation, the water vapor is condensed onto a cool surface.There are two types of solar desalination. The first type uses photovoltaic cells to convert solar energy to electrical energy to power desalination. The second type converts solar energy to heat, and is known as solar thermal powered desalination.

Natural evaporation

Water can evaporate through several other physical effects besidessolar irradiation.These effects have been included in a multidisciplinary desalination methodology in theIBTS Greenhouse.The IBTS is an industrial desalination (power)plant on one side and a greenhouse operating with the natural water cycle (scaled down 1:10) on the other side. The various processes of evaporation and condensation are hosted in low-tech utilities, partly underground and the architectural shape of the building itself. This integrated biotectural system is most suitable for large scaledesert greeningas it has a km²footprint for the water distillation and the same for landscape transformation in desert greening, respectively the regeneration of natural fresh water cycles.

Vacuum distillation

Invacuum distillationatmospheric pressure is reduced, thus lowering the temperature required to evaporate the water. Liquids boil when thevapor pressureequals the ambient pressure and vapor pressure increases with temperature. Effectively, liquids boil at a lower temperature, when the ambient atmospheric pressure is less than usual atmospheric pressure. Thus, because of the reduced pressure, low-temperature "waste" heat from electrical power generation or industrial processes can be employed.

Multi-stage flash distillation

Water is evaporated and separated from sea water throughmulti-stage flash distillation,which is a series offlash evaporations.Each subsequent flash process uses energy released from the condensation of the water vapor from the previous step.

Multiple-effect distillation

Multiple-effect distillation(MED) works through a series of steps called "effects".Incoming water is sprayed onto pipes which are then heated to generate steam. The steam is then used to heat the next batch of incoming sea water.To increase efficiency, the steam used to heat the sea water can be taken from nearby power plants.Although this method is the most thermodynamically efficient among methods powered by heat,a few limitations exist such as a max temperature and max number of effects.

Vapor-compression distillation

Vapor-compression evaporationinvolves using either a mechanical compressor or a jet stream to compress the vapor present above the liquid.The compressed vapor is then used to provide the heat needed for the evaporation of the rest of the sea water.Since this system only requires power, it is more cost effective if kept at a small scale.

Wave-powered desalination

Wave powered desalination systems generally convert mechanical wave motion directly to hydraulic power for reverse osmosis.Such systems aim to maximize efficiency and reduce costs by avoiding conversion to electricity, minimizing excess pressurization above the osmotic pressure, and innovating on hydraulic and wave power components. One such example isCETO,awave powertechnology that desalinates seawater using submerged buoys.Wave-powered desalination plants began operating onGarden Islandin Western Australia in 2013and inPerthin 2015.

Membrane distillation

Membrane distillationuses a temperature difference across a membrane to evaporate vapor from a brine solution and condense pure water on the colder side.The design of the membrane can have a significant effect on efficiency and durability. A study found that a membrane created via co-axialelectrospinningofPVDF-HFPandsilica aerogelwas able to filter 99.99% of salt after continuous 30-day usage.

Osmosis

Reverse osmosis

Schematic representation of a typical desalination plant usingreverse osmosis.Hybrid desalination plants usingliquid nitrogen freeze thawin conjunction with reverse osmosis have been found to improve efficiency.

The leading process for desalination in terms of installed capacity and yearly growth isreverse osmosis(RO).The RO membrane processes use semipermeable membranes and applied pressure (on the membrane feed side) to preferentially induce water permeation through the membrane while rejecting salts.Reverse osmosis plantmembrane systems typically use less energy than thermal desalination processes.Energy cost in desalination processes varies considerably depending on water salinity, plant size and process type. At present the cost of seawater desalination, for example, is higher than traditional water sources, but it is expected that costs will continue to decrease with technology improvements that include, but are not limited to, improved efficiency,reduction in plant footprint, improvements to plant operation and optimization, more effective feed pretreatment, and lower cost energy sources.

Reverse osmosis uses a thin-film composite membrane, which comprises an ultra-thin, aromatic polyamide thin-film. This polyamide film gives the membrane its transport properties, whereas the remainder of the thin-film composite membrane provides mechanical support. The polyamide film is a dense, void-free polymer with a high surface area, allowing for its high water permeability.A recent study has found that the water permeability is primarily governed by the internal nanoscale mass distribution of the polyamide active layer.

The reverse osmosis process requires maintenance. Various factors interfere with efficiency: ionic contamination (calcium, magnesium etc.);dissolved organic carbon(DOC); bacteria; viruses;colloidsand insoluble particulates;biofoulingandscaling.In extreme cases, the RO membranes are destroyed. To mitigate damage, various pretreatment stages are introduced. Anti-scaling inhibitors include acids and other agents such as the organic polymerspolyacrylamideandpolymaleic acid,phosphonatesandpolyphosphates.Inhibitors for fouling arebiocides(as oxidants against bacteria and viruses), such as chlorine, ozone, sodium or calcium hypochlorite. At regular intervals, depending on the membrane contamination; fluctuating seawater conditions; or when prompted by monitoring processes, the membranes need to be cleaned, known as emergency or shock-flushing. Flushing is done with inhibitors in a fresh water solution and the system must go offline. This procedure is environmentally risky, since contaminated water is diverted into the ocean without treatment. Sensitivemarine habitatscan be irreversibly damaged.

Off-gridsolar-powered desalination unitsuse solar energy to fill a buffer tank on a hill with seawater.The reverse osmosis process receives its pressurized seawater feed in non-sunlight hours by gravity, resulting in sustainable drinking water production without the need for fossil fuels, an electricity grid or batteries.Nano-tubes are also used for the same function (i.e., Reverse Osmosis).

Forward osmosis

Forward osmosisuses a semi-permeable membrane to effect separation of water from dissolved solutes. The driving force for this separation is an osmotic pressure gradient, such as a "draw" solution of high concentration.

Freeze–thaw

Freeze–thaw desalination (or freezing desalination) uses freezing to remove fresh water from salt water. Salt water is sprayed during freezing conditions into a pad where an ice-pile builds up. When seasonal conditions warm, naturally desalinated melt water is recovered. This technique relies on extended periods of natural sub-freezing conditions.

A different freeze–thaw method, not weather dependent and invented byAlexander Zarchin,freezes seawater in a vacuum. Under vacuum conditions the ice, desalinated, is melted and diverted for collection and the salt is collected.

Electrodialysis

Electrodialysisuses electric potential to move the salts through pairs of charged membranes, which trap salt in alternating channels.Several variances of electrodialysis exist such as conventionalelectrodialysis,electrodialysis reversal.

Electrodialysis can simultaneously remove salt andcarbonic acidfrom seawater.Preliminary estimates suggest that the cost of suchcarbon removalcan be paid for in large part if not entirely from the sale of the desalinated water produced as a byproduct.

Microbial desalination

Main article:Microbial desalination cell

Microbial desalination cells are biologicalelectrochemicalsystems that implements the use of electro-active bacteria to power desalination of waterin situ,resourcing the natural anode and cathode gradient of the electro-active bacteria and thus creating an internalsupercapacitor.

Design aspects

Energy consumption

The desalination process's energy consumption depends on the water's salinity.Brackish waterdesalination requires less energy thanseawaterdesalination.

The energy intensity of seawater desalination has improved: It is now about 3 kWh/m³(in 2018), down by a factor of 10 from 20-30 kWh/m³in 1970.This is similar to the energy consumption of other freshwater supplies transported over large distances,but much higher than local freshwater suppliesthat use 0.2 kWh/m³or less.

A minimum energy consumption for seawater desalination of around 1 kWh/m³has been determined,excluding prefiltering and intake/outfall pumping. Under 2 kWh/m³has been achieved withreverse osmosismembrane technology, leaving limited scope for further energy reductions as thereverse osmosisenergy consumption in the1970swas 16 kWh/m³.

Supplying all US domestic water by desalination would increase domesticenergy consumptionby around 10%, about the amount of energy used by domestic refrigerators.Domestic consumption is a relatively small fraction of the total water usage.

Energy consumption of seawater desalination methods (kWh/m³)

Desalination Method   ⇨	Multi-stage Flash "MSF"	Multi-Effect Distillation "MED"	Mechanical Vapor Compression "MVC"	Reverse Osmosis "RO"
Energy ⇩
Electrical energy	4–6	1.5–2.5	7–12	3–5.5
Thermal energy	50–110	60–110	none	none
Electrical equivalent of thermal energy	9.5–19.5	5–8.5	none	none
Total equivalent electrical energy	13.5–25.5	6.5–11	7–12	3–5.5

Note: "Electrical equivalent" refers to the amount of electrical energy that could be generated using a given quantity of thermal energy and an appropriate turbine generator. These calculations do not include the energy required to construct or refurbish items consumed.

Given the energy-intensive nature of desalination and the associated economic and environmental costs, desalination is generally considered a last resort afterwater conservation.But this is changing as prices continue to fall.

Cogeneration

Cogenerationis generating excess heat and electricity generation from a single process. Cogeneration can provide usable heat for desalination in an integrated, or "dual-purpose", facility where a power plant provides the energy for desalination. Alternatively, the facility's energy production may be dedicated to the production of potable water (a stand-alone facility), or excess energy may be produced and incorporated into the energy grid. Cogeneration takes various forms, and theoretically any form of energy production could be used. However, the majority of current and planned cogeneration desalination plants use eitherfossil fuelsornuclear poweras their source of energy. Most plants are located in the Middle East or North Africa, which use their petroleum resources to offset limited water resources. The advantage of dual-purpose facilities is they can be more efficient in energy consumption, thus making desalination more viable.

TheShevchenko BN-350,a former nuclear-heated desalination unit in Kazakhstan

The current trend in dual-purpose facilities is hybrid configurations, in which the permeate from reverse osmosis desalination is mixed with distillate from thermal desalination. Basically, two or more desalination processes are combined along with power production. Such facilities have been implemented in Saudi Arabia atJeddahandYanbu.

A typicalsupercarrierin the US military is capable of using nuclear power to desalinate 1,500,000 L (330,000 imp gal; 400,000 US gal) of water per day.

Alternatives to desalination

Increasedwater conservationand efficiency remain the most cost-effective approaches in areas with a large potential to improve the efficiency of water use practices.Wastewater reclamation provides multiple benefits over desalination of saline water,although it typically uses desalination membranes.Urban runoffand storm water capture also provide benefits in treating, restoring and recharging groundwater.

A proposed alternative to desalination in the American Southwest is the commercial importation of bulk water from water-rich areas either byoil tankersconverted to water carriers, or pipelines. The idea is politically unpopular in Canada, where governments imposed trade barriers to bulk water exports as a result of aNorth American Free Trade Agreement(NAFTA) claim.

TheCalifornia Department of Water Resourcesand theCalifornia State Water Resources Control Boardsubmitted a report to the state legislature recommending that urban water suppliers achieve an indoor water use efficiency standard of 55 US gallons (210 litres) per capita per day by 2023, declining to 47 US gallons (180 litres) per day by 2025, and 42 US gallons (160 litres) by 2030 and beyond.

Costs

Factors that determine the costs for desalination include capacity and type of facility, location, feed water, labor, energy, financing and concentrate disposal. Costs of desalinating sea water (infrastructure, energy, and maintenance) are generally higher than fresh water from rivers orgroundwater,water recycling,andwater conservation,but alternatives are not always available. Desalination costs in 2013 ranged from US$0.45 to US$1.00/m³.More than half of the cost comes directly from energy cost, and since energy prices are very volatile, actual costs can vary substantially.

The cost of untreated fresh water in the developing world can reach US$5/cubic metre.

Cost Comparison of Desalination Methods

Method	Cost (US$/liter)
Passive solar (30.42% energy efficient)	0.034
Passive solar (improved single-slope, India)	0.024
Passive solar (improved double slope, India)	0.007
Multi Stage Flash (MSF)	< 0.001
Reverse Osmosis (Concentrated solar power)	0.0008
Reverse Osmosis (Photovoltaic power)	0.000825

Average water consumption and cost of supply by sea water desalination at US$1 per cubic metre (±50%)

Area	Consumption Litre/person/day	Desalinated Water Cost US$/person/day
US	0378	00.38
Europe	0189	00.19
Africa	0057	00.06
UN recommended minimum	0049	00.05

Desalinationstillscontrol pressure, temperature and brine concentrations to optimize efficiency.Nuclear-powereddesalination might be economical on a large scale.

In 2014, the Israeli facilities of Hadera, Palmahim, Ashkelon, and Sorek were desalinizing water for less than US$0.40 per cubic meter.As of 2006, Singapore was desalinating water for US$0.49 per cubic meter.

Environmental concerns

Intake

In the United States, cooling water intake structures are regulated by theEnvironmental Protection Agency(EPA). These structures can have the same impacts on the environment as desalination facility intakes. According to EPA, water intake structures cause adverse environmental impact by sucking fish and shellfish or their eggs into an industrial system. There, the organisms may be killed or injured by heat, physical stress, or chemicals. Larger organisms may be killed or injured when they become trapped against screens at the front of an intake structure.Alternative intake types that mitigate these impacts include beach wells, but they require more energy and higher costs.

TheKwinana Desalination Plantopened in the Australian city ofPerth,in 2007. Water there and atQueensland'sGold Coast Desalination PlantandSydney'sKurnell Desalination Plantis withdrawn at 0.1 m/s (0.33 ft/s), which is slow enough to let fish escape. The plant provides nearly 140,000 m³(4,900,000 cu ft) of clean water per day.

Outflow

Desalination processes produce large quantities ofbrine,possibly at above ambient temperature, and contain residues of pretreatment and cleaning chemicals, their reaction byproducts and heavy metals due to corrosion (especially in thermal-based plants).Chemical pretreatment and cleaning are a necessity in most desalination plants, which typically includes prevention of biofouling, scaling, foaming and corrosion in thermal plants, and of biofouling, suspended solids and scale deposits in membrane 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 awastewater treatmentor power plant. With medium to large power plant and desalination plants, the power plant's cooling water flow is likely to be several times larger than that of the desalination plant, reducing the salinity of the combination. Another method to dilute the brine is to mix it via a diffuser in a mi xing zone. For example, once a pipeline containing the brine reaches the sea floor, it can split into many branches, each releasing brine gradually through small holes along its length. Mi xing can be combined with power plant or wastewater plant dilution. Furthermore, zero liquid discharge systems can be adopted to treat brine before disposal.

Another possibility is making the desalination plant movable, thus avoiding that the brine builds up into a single location (as it keeps being produced by the desalination plant). Some such movable (ship-connected) desalination plants have been constructed.

Brine is denser than seawater and therefore sinks to the ocean bottom and can damage the ecosystem. Brine plumes have been seen to diminish over time to a diluted concentration, to where there was little to no effect on the surrounding environment. However studies have shown the dilution can be misleading due to the depth at which it occurred. If the dilution was observed during the summer season, there is possibility that there could have been a seasonal thermocline event that could have prevented the concentrated brine to sink to sea floor. This has the potential to not disrupt the sea floor ecosystem and instead the waters above it. Brine dispersal from the desalination plants has been seen to travel several kilometers away, meaning that it has the potential to cause harm to ecosystems far away from the plants. Careful reintroduction with appropriate measures and environmental studies can minimize this problem.

Energy Use

The energy demand for desalination in the Middle East, driven by severewater scarcity,is expected to double by 2030. Currently, this process primarily usesfossil fuels,comprising over 95% of its energy source. In 2023, desalination consumed nearly half of the residential sector's energy in the region.

Other issues

Due to the nature of the process, there is a need to place the plants on approximately 25 acres of land on or near the shoreline.In the case of a plant built inland, pipes have to be laid into the ground to allow for easy intake and outtake.However, once the pipes are laid into the ground, they have a possibility of leaking into and contaminating nearby aquifers.Aside from environmental risks, the noise generated by certain types of desalination plants can be loud.

Health aspects

Iodine deficiency

Desalination removes iodine from water and could increase the risk ofiodine deficiencydisorders. Israeli researchers claimed a possible link between seawater desalination and iodine deficiency,finding iodine deficits among adults exposed to iodine-poor waterconcurrently with an increasing proportion of their area's drinking water from seawater reverse osmosis (SWRO).They later found probable iodine deficiency disorders in a population reliant on desalinated seawater. A possible link of heavy desalinated water use and national iodine deficiency was suggested by Israeli researchers.They found a high burden of iodine deficiency in the general population of Israel: 62% of school-age children and 85% of pregnant women fall below the WHO's adequacy range.They also pointed out the national reliance on iodine-depleted desalinated water, the absence of a universal salt iodization program and reports of increased use of thyroid medication in Israel as a possible reasons that the population's iodine intake is low.In the year that the survey was conducted, the amount of water produced from the desalination plants constitutes about 50% of the quantity of fresh water supplied for all needs and about 80% of the water supplied for domestic and industrial needs in Israel.

Experimental techniques

Other desalination techniques include:

Waste heat

Thermally-driven desalination technologies are frequently suggested for use with low-temperaturewaste heatsources, as the low temperatures are not useful forprocess heatneeded in many industrial processes, but ideal for the lower temperatures needed for desalination.In fact, such pairing with waste heat can even improve electrical process: Diesel generatorscommonly provide electricity in remote areas. About 40–50% of the energy output is low-grade heat that leaves the engine via the exhaust. Connecting a thermal desalination technology such asmembrane distillationsystem to the diesel engine exhaust repurposes this low-grade heat for desalination. The system actively cools thediesel generator,improving its efficiency and increasing its electricity output. This results in an energy-neutral desalination solution. An example plant was commissioned by Dutch companyAquaverin March 2014 forGulhi,Maldives.

Low-temperature thermal

Originally stemming fromocean thermal energy conversionresearch,low-temperature thermal desalination(LTTD) takes advantage of water boiling at low pressure, even atambient temperature.The system uses pumps to create a low-pressure, low-temperature environment in which water boils at a temperature gradient of 8–10 °C (14–18 °F) between two volumes of water. Cool ocean water is supplied from depths of up to 600 m (2,000 ft). This water is pumped through coils to condense the water vapor. The resulting condensate is purified water. LTTD may take advantage of the temperature gradient available at power plants, where large quantities of warm wastewater are discharged from the plant, reducing the energy input needed to create a temperature gradient.

Experiments were conducted in the US and Japan to test the approach. In Japan, a spray-flash evaporation system was tested by Saga University.In Hawaii, the National Energy Laboratory tested an open-cycle OTEC plant with fresh water and power production using a temperature difference of 20 °C (36 °F) between surface water and water at a depth of around 500 m (1,600 ft). LTTD was studied by India's National Institute of Ocean Technology (NIOT) in 2004. Their first LTTD plant opened in 2005 at Kavaratti in theLakshadweepislands. The plant's capacity is 100,000 L (22,000 imp gal; 26,000 US gal)/day, at a capital cost of INR 50 million (€922,000). The plant uses deep water at a temperature of 10 to 12 °C (50 to 54 °F).In 2007, NIOT opened an experimental, floating LTTD plant off the coast ofChennai,with a capacity of 1,000,000 L (220,000 imp gal; 260,000 US gal)/day. A smaller plant was established in 2009 at the North Chennai Thermal Power Station to prove the LTTD application where power plant cooling water is available.

Thermoionic process

In October 2009, Saltworks Technologies announced a process that uses solar or other thermal heat to drive anioniccurrent that removes allsodiumandchlorineions from the water using ion-exchange membranes.

Evaporation and condensation for crops

TheSeawater greenhouseuses natural evaporation and condensation processes inside agreenhousepowered by solar energy to grow crops in arid coastal land.

Ion concentration polarisation (ICP)

In 2022, using a technique that used multiple stages of ionconcentration polarisationfollowed by a single stage ofelectrodialysis,researchers fromMITmanage to create a filterless portable desalination unit, capable of removing both dissolved salts andsuspended solids.Designed for use by non-experts in remote areas ornatural disasters,as well as on military operations, the prototype is the size of a suitcase, measuring 42 × 33.5 × 19 cm³and weighing 9.25 kg.The process is fully automated, notifying the user when the water is safe to drink, and can be controlled by a single button or smartphone app. As it does not require a high pressure pump the process is highly energy efficient, consuming only 20 watt-hours per liter of drinking water produced, making it capable of being powered by common portablesolar panels.Using a filterless design at low pressures or replaceable filters significantly reduces maintenance requirements, while the device itself is self cleaning.However, the device is limited to producing 0.33 liters of drinking water per minute.There are also concerns that fouling will impact the long-term reliability, especially in water with highturbidity.The researchers are working to increase the efficiency and production rate with the intent to commercialise the product in the future, however a significant limitation is the reliance on expensive materials in the current design.

Other approaches

Adsorption-based desalination (AD) relies on the moisture absorption properties of certain materials such as Silica Gel.

Forward osmosis

One process was commercialized by Modern Water PLC usingforward osmosis,with a number of plants reported to be in operation.

Hydrogel based desalination

Scheme of the desalination machine: the desalination box of volume$V_{box}$contains a gel of volume$V_{gel}$which is separated by a sieve from the outer solution volume$V_{out}=V_{box}-V_{gel}$.The box is connected to two big tanks with high and low salinity by two taps which can be opened and closed as desired. The chain of buckets expresses the fresh water consumption followed by refilling the low-salinity reservoir by salt water.

The idea of the method is in the fact that when the hydrogel is put into contact with aqueous salt solution, it swells absorbing a solution with the ion composition different from the original one. This solution can be easily squeezed out from the gel by means of sieve or microfiltration membrane. The compression of the gel in closed system lead to change in salt concentration, whereas the compression in open system, while the gel is exchanging ions with bulk, lead to the change in the number of ions. The consequence of the compression and swelling in open and closed system conditions mimics the reverse Carnot Cycle of refrigerator machine. The only difference is that instead of heat this cycle transfers salt ions from the bulk of low salinity to a bulk of high salinity. Similarly to the Carnot cycle this cycle is fully reversible, so can in principle work with an ideal thermodynamic efficiency. Because the method is free from the use of osmotic membranes it can compete with reverse osmosis method. In addition, unlike the reverse osmosis, the approach is not sensitive to the quality of feed water and its seasonal changes, and allows the production of water of any desired concentration.

Small-scale solar

The United States, France and the United Arab Emirates are working to develop practicalsolar desalination.AquaDania's WaterStillar has been installed at Dahab, Egypt, and in Playa del Carmen, Mexico. In this approach, asolar thermal collectormeasuring two square metres can distill from 40 to 60 litres per day from any local water source – five times more than conventional stills. It eliminates the need for plasticPETbottles or energy-consuming water transport.In Central California, a startup company WaterFX is developing a solar-powered method of desalination that can enable the use of local water, including runoff water that can be treated and used again. Salty groundwater in the region would be treated to become freshwater, and in areas near the ocean, seawater could be treated.

Passarell

The Passarell process uses reduced atmospheric pressure rather than heat to drive evaporative desalination. The pure water vapor generated by distillation is then compressed and condensed using an advanced compressor. The compression process improves distillation efficiency by creating the reduced pressure in the evaporation chamber. The compressorcentrifugesthe pure water vapor after it is drawn through a demister (removing residual impurities) causing it to compress against tubes in the collection chamber. The compression of the vapor increases its temperature. The heat is transferred to the input water falling in the tubes, vaporizing the water in the tubes. Water vapor condenses on the outside of the tubes as product water. By combining several physical processes, Passarell enables most of the system's energy to be recycled through its evaporation, demisting, vapor compression, condensation, and water movement processes.

Geothermal

Geothermal energy can drive desalination. In most locations,geothermal desalinationbeats using scarce groundwater or surface water, environmentally and economically.

Nanotechnology

Nanotube membranesof higher permeability than current generation of membranes may lead to eventual reduction in the footprint of RO desalination plants. It has also been suggested that the use of such membranes will lead to reduction in the energy needed for desalination.

Hermetic, sulphonatednano-composite membranes have shown to be capable of removing various contaminants to the parts per billion level, and have little or no susceptibility to high salt concentration levels.

Biomimesis

Biomimeticmembranesare another approach.

Electrochemical

In 2008, Siemens Water Technologies announced technology that applied electric fields to desalinate one cubic meter of water while using only a purported 1.5 kWh of energy. If accurate, this process would consume one-half the energy of other processes.As of 2012 a demonstration plant was operating in Singapore.Researchers at the University of Texas at Austin and the University of Marburg are developing more efficient methods of electrochemically mediated seawater desalination.

Electrokinetic shocks

A process employing electrokinetic shock waves can be used to accomplish membraneless desalination at ambient temperature and pressure.In this process, anions and cations in salt water are exchanged for carbonate anions and calcium cations, respectively using electrokinetic shockwaves. Calcium and carbonate ions react to formcalcium carbonate,which precipitates, leaving fresh water. The theoreticalenergy efficiencyof this method is on par withelectrodialysisandreverse osmosis.

Temperature swing solvent extraction

Temperature Swing Solvent Extraction (TSSE) uses a solvent instead of a membrane or high temperatures.

Solvent extractionis a common technique inchemical engineering.It can be activated by low-grade heat (less than 70 °C (158 °F), which may not require active heating. In a study, TSSE removed up to 98.4 percent of the salt in brine.A solvent whose solubility varies with temperature is added to saltwater. At room temperature the solvent draws water molecules away from the salt. The water-laden solvent is then heated, causing the solvent to release the now salt-free water.

It can desalinate extremely salty brine up to seven times as salty as the ocean. For comparison, the current methods can only handle brine twice as salty.

Wave energy

A small-scale offshore system uses wave energy to desalinate 30–50 m³/day. The system operates with no external power, and is constructed of recycled plastic bottles.

Plants

Trade Arabia claims Saudi Arabia to be producing 7.9 million cubic meters of desalinated water daily, or 22% of world total as of 2021 yearend.

Main article:Desalination by country

• Perthbegan operating a reverse osmosis seawater desalination plant in 2006.The Perth desalination plant is powered partially by renewable energy from theEmu Downs Wind Farm.
• A desalination plant now operates inSydney,and theWonthaggi desalination plantwas under construction inWonthaggi, Victoria.A wind farm atBungendoreinNew South Waleswas purpose-built to generate enoughrenewable energyto offset the Sydney plant's energy use,mitigating concerns about harmfulgreenhouse gas emissions.
• A January 17, 2008, article inThe Wall Street Journalstated, "In November, Connecticut-based Poseidon Resources Corp. won a key regulatory approval to build the $300 million water-desalination plantinCarlsbad,north ofSan Diego.The facility would produce 190,000 cubic metres of drinking water per day, enough to supply about 100,000 homes.As of June 2012, the cost for the desalinated water had risen to $2,329 per acre-foot.Each $1,000 per acre-foot works out to $3.06 for 1,000 gallons, or $0.81 per cubic meter.

As new technological innovations continue to reduce the capital cost of desalination, more countries are building desalination plants as a small element in addressing theirwater scarcityproblems.

• Israel desalinizes water for a cost of 53 cents per cubic meter
• Singapore desalinizes water for 49 cents per cubic meterand also treats sewage withreverse osmosisfor industrial and potable use (NEWater).
• China and India, the world's two most populous countries, are turning to desalination to provide a small part of their water needs
• In 2007 Pakistan announced plans to use desalination
• All Australian capital cities (exceptCanberra,Darwin, Northern TerritoryandHobart) are either in the process of building desalination plants, or are already using them. In late 2011,Melbournewill begin using Australia's largest desalination plant, theWonthaggi desalination plantto raise low reservoir levels.
• In 2007Bermudasigned a contract to purchase a desalination plant
• Before 2015, the largest desalination plant in the United States was atTampa Bay,Florida, which began desalinizing 25 million gallons (95000 m³) of water per day in December 2007.In the United States, the cost of desalination is $3.06 for 1,000 gallons, or 81 cents per cubic meter.In the United States, California,Arizona,Texas,and Florida use desalination for a very small part of their water supply.Since 2015, theClaude "Bud" Lewis Carlsbad Desalination Planthas been producing 50 million gallons of drinking water daily.
• After being desalinized atJubail,Saudi Arabia,water is pumped 200 miles (320 km) inland though a pipeline to the capital city ofRiyadh.

As of 2008, "World-wide, 13,080 desalination plants produce more than 12 billion gallons of water a day, according to the International Desalination Association."An estimate in 2009 found that the worldwide desalinated water supply will triple between 2008 and 2020.

One of the world's largest desalination hubs is theJebel AliPower Generation and Water Production Complex in theUnited Arab Emirates.It is a site featuring multiple plants using different desalination technologies and is capable of producing 2.2 million cubic meters of water per day.

A typicalaircraft carrierin the U.S. military uses nuclear power to desalinize 400,000 US gallons (1,500,000 L) of water per day.

In nature

Mangrove leaf with salt crystals

Evaporation of water over the oceans in thewater cycleis a natural desalination process.

The formation ofsea iceproduces ice with little salt, much lower than in seawater.

Seabirds distill seawater usingcountercurrent exchangein aglandwith arete mirabile.The glandsecretes highly concentrated brinestored near the nostrils above the beak. The bird then "sneezes" the brine out. As freshwater is not usually available in their environments, some seabirds, such aspelicans,petrels,albatrosses,gullsandterns,possess this gland, which allows them to drink the salty water from their environments while they are far from land.

Mangrovetrees grow in seawater; they secrete salt by trapping it in parts of the root, which are then eaten by animals (usually crabs). Additional salt is removed by storing it in leaves that fall off. Some types of mangroves have glands on their leaves, which work in a similar way to the seabird desalination gland. Salt is extracted to the leaf exterior as smallcrystals,which then fall off the leaf.

Willowtrees andreedsabsorb salt and other contaminants, effectively desalinating the water. This is used in artificialconstructed wetlands,for treatingsewage.

Society and culture

Despite the issues associated with desalination processes, public support for its development can be very high.One survey of a Southern California community saw 71.9% of all respondents being in support of desalination plant development in their community.In many cases, high freshwater scarcity corresponds to higher public support for desalination development whereas areas with low water scarcity tend to have less public support for its development.

See also

• Metal–organic framework
• Atmospheric water generator
• Dewvaporation
• Flexible barge
• Peak water
• Pumpable ice technology
• Soil desalination model
• Soil salinity
• Soil salinity and groundwater model

External links

• International Desalination Association
• European Desalination Society
• Working principles in desalination systems
• Classification of Desalination Technologies (CDT)
• SOLAR TOWER Project – Clean Electricity Generation for Desalination.
• Desalination bibliography Library of Congress
• Encyclopedia of Desalination and water and Water Resources