HOLD THE SALT: This desalination plant in Tampa, Fla., is the biggest reverse-osmosis facility in the U.S.
Image: Tampa Bay Water
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Even with all of the water in Earth's oceans, we satisfy less than half a percent of human water needs with desalinated water.* We currently use on the order of 960 cubic miles (4,000 cubic kilometers) of freshwater a year, and overall there's enough water to go around. There is increasing regional scarcity, though.
So why don't we desalinate more to alleviate shortages and growing water conflicts?
The problem is that the desalination of water requires a lot of energy. Salt dissolves very easily in water, forming strong chemical bonds, and those bonds are difficult to break. Energy and the technology to desalinate water are both expensive, and this means that desalinating water can be pretty costly.
It's hard to put an exact dollar figure on desalination—this number varies wildly from place to place, based on labor and energy costs, land prices, financial agreements, and even the salt content of the water. It can cost from just under $1 to well over $2 to produce one cubic meter (264 gallons) of desalted water from the ocean. That's about as much as two people in the U.S. typically go through in a day at home.
But switch the source to a river or an aquifer, and the cost of a cubic meter of water can plummet to 10 to 20 cents, and farmers often pay far less.
That means it's still almost always cheaper to use local freshwater than to desalinate seawater. This price gap, however, is closing. For example, meeting growing demand by finding a new source of water or by building a new dam in a place like California could cost up to 60 cents per cubic meter of water.
And sometimes these traditional means of “harvesting” water are no longer available. As such, this cost figure is expected to continue to rise, which is why California is now seriously considering desalination and why the city of Tampa, Fla., decided to build the biggest desalination plant in the U.S.
The International Desalination Association says that as of 2007 there were about 13,000 desalination plants operating around the world. They pumped out approximately 14.7 billion gallons (55.6 billion liters) of drinkable freshwater a day. A lot of these plants are in countries like Saudi Arabia, where energy from oil is cheap but water is scarce.
So how is energy used to separate salt from water?
There are two basic methods for breaking the bonds in saltwater: thermal distillation and membrane separation. Thermal distillation involves heat: Boiling water turns it into vapor—leaving the salt behind—that is collected and condensed back into water by cooling it down.
The most common type of membrane separation is called reverse osmosis. Seawater is forced through a semipermeable membrane that separates salt from water. Because the technology typically requires less energy than thermal distillation, most new plants, like Tampa's, now use reverse osmosis.
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35 Comments
Add CommentWhat an embarrassing error in the first sentence. I'm sure that the intended meaning was that less than .5% of freshwater used is the result of desalination (i.e. on the order of 20 cubic kilometers a year, which matches figures given later), but what it actually says is that less than .5% of the water in the oceans (i.e. less than 6,500,000 cubic kilometers) is desalinated for use.
Reply | Report Abuse | Link to thisAuthor response: hthall is exactly right -- an embarrassing error on my part - mea culpa. I should have caught it in editing! What this should say (of course) is that of all the water humans currently use, less than half a percent of it comes from desalination. Thank you hthall, and we'll try to post a corrected copy!
Reply | Report Abuse | Link to thisPeter Gleick
I was shocked when I read that too ( in the RSS version ) but please don't be embarrassed: You made a small mistake, admitted it when it was pointed out and tried to get it corrected.
Reply | Report Abuse | Link to thisI wish all authors - on the web, and elsewhere - were as good about this sort of thing !
This is where I see solar and wind energy being an enormous help. They are inconsistent energy supplies but that is fine in something like a desalination plant. In some ways, if you think about it, it would be an energy storage system.
Reply | Report Abuse | Link to thisShane makes an excellent point. What could be more environmentally friendly than a wave-powered generator far out at sea pumping fresh water back to shore and disposing of the highly saline waste back where it will be heavily diluted and dispersed by the surrounding seawater, away from shorelines where it would damage aquatic life? (Answers on a postcard to...)
Reply | Report Abuse | Link to thisIn the mid 1950's, a French Scientist named Henri Coanda had a solar desalination tower working in a tidal pool environment . No extra energy was required. The story of Coanda's experiments was written up in Popular Mechanics magazine, in the 1961-1962 time frame.
Reply | Report Abuse | Link to thisThe French government had a monopoly on salt. Coanda insisted he was making fresh water; the government insisted he was making salt, because it was piling up under his tower. They shut him down, in typical government shortsightedness.
The last time I looked, the Coanda papers were still available on line.
This has now been corrected. Thanks to the sharp-eyed SciAm readers!
Reply | Report Abuse | Link to thisPeter Gleick
all these methods of making water from sea water etc is the same as continuing to give aspirin to someone with pain. On is trying to cure a sympton not the cause which as the author of the article referred to i.e. the world population is increasing. Cure that and no more problem. Very difficult of course politically.
Reply | Report Abuse | Link to thisIt's great, it's technology. Fresh water from seawater! Very different of the salty tearfully blahs about restrictions of water consumption. Articles like that bring hope to our minds.
Reply | Report Abuse | Link to thisA great, relavant article. Using fossil fuels for energy source to desalinate sea water seems like substituting another problem for the first. Solar, wind, and geothermal energy are other options, but the condensation process will require heat rejection into the already warming ocean. Reverse osmosis is clearly the preferred method using currently available technology.
Reply | Report Abuse | Link to thisAs for the brine, it seems the earth and sun can be used as a massive retort by pumping the brine into the Dead Sea, Salt Lake, Salton Sea, or salt flat, and let the sun further evaporate to solid salt. The solid salt itself might be a source of sodium, magnesium, or calcium like aluminum is from bauxite. Sorry, I'm not a chemist, but chemical engineers can develop processing plants if there is enough incentive.
We can get hot air by recycling the heat so we can get drinkable water at low prices.
Reply | Report Abuse | Link to thisIt has been found that the heat is recyclable like any other object. When the heat is recycled an additional energy emerges. Thermal Energy Recycling system (TERS) will be the energy of the future. Open your minds and we will be able to replace fuels.
Thousands of letters I have sent commenting how to get hot air by recycling the heat. But the Scientific community is in the worst paradigmatic paralysis of all times. The Galileos time has not ended yet. The founders of thermodynamics are responsible for the problem of pollution, global warming and dependence on fuels now.
I am inviting all of the governments of the world and institutions, to have a demonstration with a hot air generator prototype in Cali, Colombia. It uses 5.3 Kw/hour and produces 12 Kw/hour. It produces 1050 cubic meter of hot air/hour up to 73 �C. If some components of the prototype are improved we will be able to get the same volume of air up to 250 �C with the same 5.3 kw/hour. The prototype uses a blower; (0.35 psi and the heat produced by the electric motor is evacuated). The prototype does not use fuels or electric resistor to heat the air.
More information would be sent if required.
Regards.
Ra�l Caicedo Astudillo
Why is there no discussion of the use of Nuclear Energy to produce fresh water? A nuclear power plant set up on a coastal location would produce both electrical power AND fresh water, and return water with only a small increase in salinity back to the ocean. To me, this would be a win-win situation.
Reply | Report Abuse | Link to thisTry this:
Reply | Report Abuse | Link to this1) Expose ocean water to radiation from a nuclear reactor
2) Allow the evaporated water to cool and precipitate out as drops and ice crystals.
3) Store the ice crystals at high elevations to keep them cool. Store the drops in cheap containers made of rocks and dirt.
I know it works, we've done it for millions of years on a scale that dwarfs the desalination plants. Yet it's still not enough to satisfy the demand from the burgeoning population.
I believe that perhaps, freshwater must be used only to drink. It is necesary to develop technologies in order to use salt water to wash or industrial uses. So the "drink water" is lowered about two galon by person (including water directly used in food preparation).
Reply | Report Abuse | Link to thisTwo cavemen, Og and Erk
Reply | Report Abuse | Link to thisOg: Lets use fire
Erk: But won't that put carbon dioxide in the air?
Og: Yeah, but just a little bit, we can't hurt anything.
Erk: Okay, lets also desalinate water
Og: But won't that increase ocean salinity?
Erk: Yeah, but just a little bit, we can't hurt anything.
that is why god invented war
Reply | Report Abuse | Link to thisI guess I don't understand why it has to be so expensive. If an African family built two 4x8 rectangular frames that can seal together a black and a clear plastic sheet at the edges in direct sunlight, dig a 4x8 trench for it to sit it and fill it with saltwater, fill the space with hydrogen (far superior H2O carrier than air), then a large plastic trough suspended between the two sheets should provide enough water out of there for their needs and it will cost them about $20 in materials (though they'd need access to a community salt-water irrigation system). Of course they'd probably need new Visqueen every year (maybe $1 worth). The hydrogen could be trickle fed by a little 20W PV panel - that would be the big one-time expense ($100) but should last for 20-30 years.
Reply | Report Abuse | Link to thisTotal amortized cost = $3 to $5 per year per family. That's a far cry from the $1/M^3water quoted in this article
We can get hot air by recycling the heat so we can get drinkable water at low prices.
Reply | Report Abuse | Link to thisIt has been found that the heat is recyclable like any other object. When the heat is recycled an additional energy emerges. The Thermal Energy Recycling system (TERS) will be the energy of the future. Open your minds and we will be able to replace fuels.
Thousands of letters I have sent commenting how to get hot air by recycling the heat, but the Scientific community is in the worst paradigmatic paralysis of all times. The Galileo’s time has not ended yet. The founders of thermodynamics are responsible for the problem of pollution, global warming and dependence on fuels now.
I am inviting the governments, institutions and foundations to send the experts in order to have a demonstration with a TERS hot air generator prototype in Cali, Colombia. It uses 5.3 Kw/hour and produces 12 Kw/hour. It produces 1050 cubic meter of hot air/hour up to 73 ºC. If some components of the prototype are improved we will be able to get the same volume of air up to 250 ºC with the same 5.3 kw/hour. The prototype uses a blower; (0.35 psi and the heat produced by the electric motor is evacuated). The prototype does not use fuels or electric resistors to heat the air.
More information would be sent if required.
Regards.
Raúl Caicedo Astudillo
Since the early 1920s, all the new human problems are due to overpopulation. Efforts should be made in the future to reverse this trend. Remember, in 1960 the world population was less than half what it is today.
Reply | Report Abuse | Link to thisWith today's technological capabilities and people's awareness and with three billion people on earth, life could become more interesting to live. I forsee future generations more conscious of their social choices.
If the problem is how to get rid of the salt, why not add it to the discharged water? It'll bring the salinity back to the same level as it came in.
Reply | Report Abuse | Link to thishey I'm just a student, so forgive me if I'm mistaken, but can't the production of brine be turned into a positive? The main products of the electrolysis of brine are chlorine and hydrogen, both of which are industrially valuable. The caustic liquor which remains can then be boiled down to leave 50% sodium hydroxide solution and 1% sodium chloride solution... I thought that that option would have been apparent from the beginning?
Reply | Report Abuse | Link to thisI suppose the main question which remains is where to obtain the energy for the electrolysis of brine, but i mean, if desalination plants are normally near the shore ( please correct me if I'm wrong) then couldn't this allow for the utilization of tidal/wind power, both for the desalination process and for the electrolysis?
Maybe I missed an important point, so any feedback would be appreciated.
As always, we are blinded by high tech solutions. SEAWATER algae are perfect natural desalinating plants. And it is easy to extract fresh water from some larger, fleshy ones. Farmers could easily grow algae on barren land and use them to fertilise and water fertile parts of their property. And the story does not end here: http://www.abc.net.au/news/stories/2008/07/01/2290426.htm
Reply | Report Abuse | Link to thisKind regards,
Just thinking; all those endless expanses of salt, the Med was totally gone once, all that remained was salt. Salt deserts everywhere geologists look, and now consider that freshwater is coming to an end and what would our world look like if we have to consume sea water and dump the salt...............would it look like it does now, with surprisingly large salt pans and salt domes and salt expanses.............geologically unexplainable enormous expanses of evaporated salt. Leaves the question of where all the water went, doesn't it?
Reply | Report Abuse | Link to thisI have recently read Trevor Norton's book, Diving to get out of the Rain, and it is so well written, I enjoyed it.
Reply | Report Abuse | Link to thisHe addresses many different issues and speaks to the SEAWATER ALGAE usages for many things but, as always, the single commodity is ALWAYS integrated in the community of living things, where the success of one depends on the success of all, and as we find in fish farming, an effort to single one thing out from many entails unintended consequences.
I am, of course, old. Age makes for skepticism and justifiably so. Just a student also.
maybe what the author intended to say is that the freshwater available in the ground is less than .5% of the water in oceans.
Reply | Report Abuse | Link to thisWhat about red tide? Can this desalinization method also get rid of germs?
Reply | Report Abuse | Link to thisI remember a few years ago I read an article about research into nodes that absorb freshwater out of the sea, which would not require the amount of energy and costs of reverse osmosis. Has any progress come of this research?
Reply | Report Abuse | Link to thisI remember a few years ago I read an article about research into nodes absorbing fresh water from the sea which would not require the energy or the costs that reverse osmosis does. Has any progress been made on this research?
Reply | Report Abuse | Link to thisCouldn't the sea salt just be used in place of table salt, salt blocks for live stock, etc, rather than mining it?
Reply | Report Abuse | Link to thisThe following idea sounds too good to be true, but I cannot find the fault in it. Perhaps someone else can.
Reply | Report Abuse | Link to thisWhy can't reverse osmosis be accomplished by using the ambient water pressure of the ocean at great depths? All you have to do would be to take a long pipe with an osmotic filter on the end and lower it deep into the water to a depth where the pressure would force pure water through the filter into the pipe. I've seen calculations that demonstrate that this is not all that deep and certainly within our technological capabilities. Eventually, the pipe would fill up with pure water, thus killing the pressure differential that drives the osmosis. This problem would be fixed by simply pumping the fresh water out of the pipe. I don't understand why this idea cannot be implemented on a commercial scale. It eliminates the huge power requirments to force water through the filter (some power would be required to pump out the fresh water, but no more than a regular well). It also eliminates the problem of residual brine. What am I missing?
Middle-west agriculture is based on irrigation by pumping the Ogalla aquifer that will run dry in 30 years time. I read here that farmers pay less than 10 cents a cubic meter for water. It is surely high time to make agriculture pay the true replacemant cost of (desalinated) water if US farming is to survive into the second half of this century. The alternative doesn't bear thinking about...
Reply | Report Abuse | Link to thisMidwest agriculture depends on irrigation by pumping the Ogallala aquifer which will run dry in 25 years time. I read here that farmers only pay 10 cents per m3 for water. Surely they should pay the replacement cost of water, (via desalination), if the US is to prepare for agriculture in 30 years time. The alternative will be mass starvation both in America and the World.
Reply | Report Abuse | Link to thisDesalination is expensive way. It is good if the energy can be convert or fully utilized as global warming and animal extinction is the hot issue curently.
Reply | Report Abuse | Link to thisDesalinazation is very costly method. It is great oif the energy can be converted into others use as global warming and animal extinction is the hot issue.
Reply | Report Abuse | Link to thisThere is a multi-billion dollar international business opportunity here for the right engineer-entreprenur. We have two overlapping disruptive technologies that could solve this problem: a) HHO as an alternative to Oil/Gas; the electrolysis of water to separate the hydrogen and oxygen into a combustable gas form, which functions as an alternative to gasoline and runs in any gasoline-based engine without modification. the downside being the same, electrolysis costs a lot. but the benefits being that the byproduct of the Browns Gas combustion is water again. so it can be made into a closed-loop system. There are already cars and houses running on this technology as open-source efforts to perfect it. In order to make electrolysis effective, salt is often added as an electrolyte. See the connection? b) Sea Water Desalinization as an alternative to Fresh Water. Instead of taking a centralized on-grid approach, if you filled gas tanks with sea water then there are potentially two uses out of a single separation event: fuel for machines, and fresh drinking water for humans. Water is the most abundant resource on the Earth. Additionally, it does not disappear; we consume it, but through perspiration, evaporation, and natural planetary cycles it is a closed-loop system. The water you began with is the water you end with. It is life-giving. This approach is not only scalable, it is sustainable, and it threatens the livelihood of the centralized energy capitalists who seek to enslave the Earth and all its inhabitants. Research it.
Reply | Report Abuse | Link to thisAlso check out the architectural approaches to desalinization; buildings which utilize wind, angles, evaporation, and the sun to capture massive amounts of moisture from the sea breeze; with zero energy input they produce fresh water.