FREE FROM SALT: Special membranes help desalinize 25 million gallons of water a day at the Tampa Bay Seawater Desalination Plant in Florida.
Image: COURTESY OF DOW
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Almost three quarters of Earth's surface is covered with water, but most of it is too salty to drink. And the 2.5 percent that is freshwater is locked up either in soil, remote snowpacks and glaciers or in deep aquifers. That leaves less than 1 percent of all freshwater for humans and animals to drink and for farmers to use to raise crops—and that remnant is shrinking as rising global temperatures trigger more droughts. The upshot: it's becoming increasingly difficult to slake the world's thirst as the population grows and water supplies dwindle. Analysts at the investment bank Goldman Sachs estimate that worldwide water use doubles every 20 years.
So the search for new water sources is on. One proved candidate is desalinization—technologies that extract the salt from brine drawn from the oceans or saline aquifers to create potable water. But the historically high price of desalinization has largely kept it at bay, a situation that's changing as technology improves and growing demand squeezes freshwater supplies .
"The two main desalinization techniques are distillation and reverse osmosis, or RO," says Menachem Elimelech, an environmental engineer at Yale University. "Distillation, in which the raw water is evaporated and then condensed as freshwater, is energy-intensive, so it's mainly used in the Middle East where oil is abundant." Thermal salt-removing processes require high temperatures so they tend to be expensive (more than $1 per cubic meter of freshwater), but the use of rejected "waste" heat from other industrial or power plant operations for co-generation can cut energy expenditure.
More commonly, however, desalinization plants rely on RO, which is based on high-tech polymer membranes that are permeable to water, but reject the passage of dissolved salts, Elimelich says. When a saline solution sits on one side of a semipermeable membrane and a less salty solution is on the other, he explains, water diffuses through the membrane from the less concentrated to the more concentrated side. Scientists call this phenomenon osmosis, which tends to equalize the salinity of the two solutions.
In the 1950s and '60s researchers realized that they could reverse the process by applying pressure to the more concentrated solution, causing water molecules there to traverse the membrane, leaving behind a condensed brine. To counter the osmotic pressure that arises between the solutions and force water back through the membrane, desalinization plants must utilize high pressures of 7,000 to 8,300 kilopascals (71 to 86.5 kilogram-force per square centimeter or 1,000 to 1,200 pounds per square inch), he notes.
Common RO membranes are thin-film composites that combine a mechanically robust support layer made of microporous polysulfone with a micron-thick polyamide "filter" layer through which water molecules can pass but nothing else. The latter substance is "a second cousin to DuPont's Kevlar—the super-strong aramid polymer fiber used in lightweight body armor," says Bill Mickols, senior research scientist at Dow Water Solutions (DWS) of Edina, Minn., the biggest supplier of such products. RO membranes have matured during the past two decades, he says, with marked improvements in water permeability, salt-rejection capability, operating life (now as long as three to five years) and cost.
These advances, in combination with energy-recovery devices that take pressure from the concentrated brine stream and transfer most of it to the incoming water flow, have made desalinization more affordable. Current RO facilities desalinize seawater for 68 to 90 cents per cubic meter. The average delivery price of municipal water in the U.S. is around 60 cents a cubic meter, according to the American Water Works Association.
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26 Comments
Add CommentWhat to do with the brine created by desalination?
Reply | Report Abuse | Link to thisThere's a place on our planet that needs brine to function, and it's a globally important issue. The ocean conveyor depends upon the glaciers and icebergs near Greenland to extract freshwater from the North Atlantic, as the waters freeze. The result is that the area around this freezing process becomes more salty, and starts the "push" of the conveyor as this salty heavier water sinks.
As the waters around Greenland become warmer, there's a fear that this will not cause the water in the area to become saltier (due to the glaciers/icebergs not absorbing freshwater out of the ocean), and thus there wont be anything to cause the water in that area to sink. This could disrupt/interrupt the conveyor, which many speculate would be catastrophic to the global climate.
And it all depends on saltier water that is heavy and sinks, happening right in a specific location. So, if we measure the right salinity level in that region, and we have lots of briny/extra-salty water on hand, when the salinity level drops into a region that is outside of our safe zone ... we could add our man-made brine (the byproduct of desalination) to the area, giving the ocean conveyor a boost.
Desalination, membrane wastewater reclamation, and conservation are the future of water management.
Reply | Report Abuse | Link to thisFrom village-scale plants on remote shores to irrigation-scale plants in the deserts, we'll see a willingness and capability, brought on by necessity, to supply our potable and irrigation grade water needs from these sources. The cost of desalination is already competitive with most new sources, and combined with aggressive conservation measures to keep the volumes down, it will continue to look better and better. Many uses of potable water can be switched to non-potable, including flushing toilets, industrial process, and cooling water. Irrigation techniques can conserve a high percentage of that used today.
Our water problems are a matter of delivering the right quality of water, to the right place, at the right time. We can do that, but only if we recognize that water is now ridiculously cheap.
One way to drastically increase the amount of water available is to reduce its use. I have long advocated composting toilets because they use no water, do not require pluming and provide the owner with valuable composting material to use instead of industrial fertilizer.
Reply | Report Abuse | Link to thisThe average household uses 50% of their water flushing their toilets! With this simple change, you can drastically reduce the amount of water you use. Additionally, there is plenty else you can do to reduce your water usage.
Now, as for desalination. I would say that distillation is the best bet for the simple reason that it can be used in a cogeneration system using waste heat from producing power as the heat source. This treats waste the way it should be treated, as another resource. Rain water capture also isn't a bad idea.
Why don't they use brine to get cement from co2?
Reply | Report Abuse | Link to thisConsider this option: develop desalination/hydrogen plants in areas like Death Valley and the Afar Depression in Africa where saltwater from the ocean can be gravity fed to treatment/production facilities that operate on the local geothermal (if available), solar heat and photovoltaic resources. Both freshwater could be produced along with hydrogen, both of which could be shipped back to the coast for distribution through port facilities or pipelines, as appropriate.
Reply | Report Abuse | Link to thisHow about Green Sahara - water makes miracles in Chad basin at modest costs.
Reply | Report Abuse | Link to thisSahara has been during last 10000 years mostly covered by vegetation.
Green Sahara holds vast amount of carbon: about 10 miljon km2 forests, lakes, swamps, ... If there is on average 100 kg of biomass per m2 , then there about 2000 000 m * 5000 000 m* 0.1 ton/m2 or 1 trillion tons of carbon in the Green Sahara. This about the same as the carbon now in the atmosphere?
But how to get enough water there at reasonable costs?
By damming Oubangui- river and letting 1000 m3/sec of river water flow to Lake Chad, costs are few hundred million bugs .
See:
http://en.wikipedia.org/wiki/Lake_Chad
http://lakechad.iwlearn.org/
ABUJA DECLARATION ON LAKE CHAD BASIN 2007:
"....positive impact of the proposed Oubangui-Lake Chad Water Transfer project and the fact that the project will serve as an opportunity to rebuild the ecosystem, rehabilitate and replenish the lake in a manner that will increase and improve the level of irrigation activities thus boosting agricultural production and reforestation...."
http://lakechadparlcomm.org/index.php?option=com_content&task=view&id=33&Itemid=48
Kai, a geologist
i think instead of doing all such stuff about desalination and all every person on earth must change their attitude ......save fresh water instead of convertivg saltwater to fresh water thereby disturbing the salinity levels of the seas ...instead every person must bother to turn off the water taps and showers after every use ....
Reply | Report Abuse | Link to this"Distillation, in which the raw water is evaporated and then condensed as freshwater, is energy-intensive, so it's mainly used in the Middle East where oil is abundant." Thermal salt-removing processes require high temperatures so they tend to be expensive (more than $1 per cubic meter of freshwater), but the use of rejected "waste" heat from other industrial or power plant operations for co-generation can cut energy expenditure.
Reply | Report Abuse | Link to thisSo, why aren't people building solar energy powered plants to distill the water? It should be remarkably cheap: you just needs lots of mirrors focused upon heat exchangers onto which you concentrate the sunlight.
While a hurricane makes fresh water fast, the cheaply part is irrelevant. It costs a hurricane zero dollars to produce all that water. If we can create such speedy recover cheaply is another question altogether.
Reply | Report Abuse | Link to thisWe drank RO water when I was in Iraq. They reprocessed waste water, not salt water, but it is the same process and I can vouch for the effectiveness. The RO water is purer than anything we drink at home.
Reply | Report Abuse | Link to thisThis seems like a perfect solution to water problems, but I am reminded that yesterday's solution is today's problem and we have to assume that today's soltultion will be tomorrow's problem. These developments could avoid the coming water crisis. The next solution is up to the next generation. Life is a lot like a giant chain letter. It has worked so far, and each generation gets to blame the one before and claim that their problems are worse - all the while living a better life.
Nobody has mentioned the negative impacts of desalinization such as:
Reply | Report Abuse | Link to this- releasing water has double the salt content back in to the ocean and the effects this will have on ocean health
- the large amount of small ocean creatures that are brought in along with the water
- finally - if humans are given unlimited quantities of water - we will likely use it with no regard to the effects this will have on marine life. We must learn to live within our bounds, live sustainably and conserve water rather then look to the ocean to satisfy our ever growing population and water use.
Right. Because, of course, we all know that a drop of water flushed down a toilet disappears from the planet forever. After all, it's been flushed, right?
Reply | Report Abuse | Link to thisOh, for crying out loud. Are you people for real? Daily evaporation from the ocean is going to pull out more fresh water than the entire human race could possibly hope to do.
Reply | Report Abuse | Link to this"Chlorine is added to sterilize the water," he says, "but operators usually need to dechlorinate it afterwards to protect the membrane from chemical degradation."
Reply | Report Abuse | Link to thisWhy are'nt they ionizing the salted water (like in modern swiming pool) instead of adding chlorine?
One of the concepts being studied in Florida is the process of brine injection wells to place the resulting brine below the aquifer level. I believe the theory is that brine, being much heavier than fresh water, or sea water, would remain at the level it is injected. Time will tell whether this technique proves viable.
Reply | Report Abuse | Link to thishttp://www.springerlink.com/content/a80372n841pm41k0/
As of March 17, 2009, the Tampa Bay seawater desal plant has yet to prove itself commercially dependable and economically viable. Read the story here [ http://tinyurl.com/ceesoq ]. Tampa Bay's government controlled regional water monopoly - Tampa Bay Water - has thus far made no public comment on the actual cost their desalinated water. The idea of manufacturing water in a state with the largest acquifier in the U.S. and the second highest annual rainfall in the continental United States is absolutely criminal.
Reply | Report Abuse | Link to this@roseroserose: I totally agree! While desalination may seem to fix our freshwater shortage problem, it is simply a fix; not a sustainable solution. A careful analysis of what we do with the waste (in this case brine) is a key aspect to any sustainable project; somehow omitted from this article. Bad job SciAm! The brine not only concentrates the salts found in the sea water, but also all the other toxins we have been spewing into the ocean. As a mental exercise, think of the dirtiest sea water you can imagine and then concentrate the dirty/toxic aspects to 5 times its original concentration and then sending that back out to sea. Would you like to swim in that? Do you think the marine life want to?
Reply | Report Abuse | Link to thisAs for the other so-called solutions involving injecting it underground and others, they are equivalent to sweeping the dust under the rug. Or for a more real comparison, shipping our trash out of sight and out of mind until our world turns to that in Wall-E.
Conservation is more sustainable AND more economical in the long run with no waste to deal with. I mean really, does you poo or plants really need potable freshwater?
Heres an idea from a science enthusiast (not a scientist !!).
Reply | Report Abuse | Link to thisIf RO takes a lot of pressure to operate, could it be done on the ocean floor where the pressure is already available at no cost?
Basically, but a container with a RO membrane exposed to sea water. On the inside of the container, would be a pump to move the fresh water to land.
Would this pump use the same or more energy than the conventional system?
Also, to clean the membrane a few jets could propel the water against it, perhaps UV lights could remove algae.
Im sure there is many obstacles to make a viable system, but I hope someone can take this simple idea and make it real!
Am I missing some thing?
Reply | Report Abuse | Link to thisWhy are the using energy (oil) intensive plants in a desert country?
Why aren't they using dome evaporators?
Glass (or plexy) domes or even giant inflatables. Sunlight is free (and intense in these areas).
The article cites the rise in construction of desalination facilities and states "The trend worries many local environmental groups, such as California's Surfrider Foundation...". For those curious about our concerns, see this article from our website:
Reply | Report Abuse | Link to thishttp://www.surfrider.org/a-z/desal.php
This is really effective information
Reply | Report Abuse | Link to thisThis is really effective information
Reply | Report Abuse | Link to thisPressure at the 'Challenger Deep', bottom of pacific ocean is 6,248 psi. So we can probably operate RO by drawing out fresh water to the surface in very large amounts because density of pure water is 1000kg/m3 but density of salt water is 1027 kg/m3.
Reply | Report Abuse | Link to thisWater tax is the solution
Reply | Report Abuse | Link to thisWhy not offer a billion dollar reward to the entity that can find a cost effective way to desalinate sea water? We could ask the Bill Gates Foundation, the governments of the g-20 nations, the U.N. and Greenpeace to provide financial backing.
Reply | Report Abuse | Link to thisThere is a method for creating large, inexpensive, high strength concrete spheres capable of withstanding pressures greater than 1,000 psi and higher. These concrete spheres are made from manufactured concrete block. These spheres can be sunk to depth (over 3,000 feet) where a portal with a RO membrane performs desalination. Then the sphere is hoisted to the surface and emptied (the only real work required) and the process is repeated. This method comes very close to the thermodynamic limit of energy required for desalination. I have written about this several times on my blog. If anyone has any interest or questions, please contact me. If you look at my other blog entries you will get a better idea, and there are pictures of this masonry system being used in other applications.
Reply | Report Abuse | Link to thishttp://masonrydesign.blogspot.com/2011/09/energy-budget-for-desalination.html