EVEN FLOW: Sirkar's recently patented system can deliver about 80 liters of drinking water per 100 liters of seawater.
Image: Courtesy of Journal of Membrane Science
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More than a third of the world already suffers from shortages of potable water—with a rise to 50 percent expected by 2025. Desalination of seawater can help coastal communities can address local shortfalls, although the process is costly, and releasing leftover brine back to the ocean has environmental implications. Now a new system promises to produce more drinkable water with less salty effluent.
Kamalesh Sirkar, a New Jersey Institute of Technology (N.J.I.T.) distinguished professor of chemical engineering, says he has devised a direct-contact membrane distillation (DCMD) system that can efficiently wring drinking water out of up to 20 percent-salt-concentrated brine. (After about 25 percent, salt precipitates out of the solution in the membrane distillation system and could damage the membranes, pumps, lines and other components, Sirkar says.)
Normal seawater has a salt concentration of about 3.5 percent, which means the new system can reprocess the same seawater several times. "More water can be recovered with less residue," Sirkar says.
In Sirkar's system, heated seawater flows across a membrane strung with a series of hollow tubes made of a porous, yet hydrophobic, fiber—meaning only water vapor can be osmotically transferred. Cold distillate water runs through each of the tubes in a direction perpendicular to that of the seawater. The temperature difference between the heated seawater and cold distillate water causes vapor to form on the tubes. This vapor diffuses through the pores and condenses again inside the tubes, joining the flow of cold distillate water. The salt cannot penetrate the tubes and is carried away; with each cycle, more fresh water is drawn off, leaving more highly concentrated brine behind.
Sirkar's recently patented system can deliver about 80 liters of drinking water per 100 liters of seawater, he says. A comparable reverse-osmosis system—which relies on pressure to force seawater through a salt-filtering membrane—would reclaim 41 liters from that same amount of saltwater, according to Sirkar.
Membrane distillation's advantages include its ability to produce drinking water with very low salinity. In addition, seawater can be distilled at a range of temperatures—from 30 to 100 degrees Celsius—reducing the amount of heat typically needed for desalination, an energy savings, Sirkar says. Prolonged use may decrease a typical membrane's efficiency, but Sirkar says his system adds an ultrathin layer of a highly porous silicone–fluoropolymer coating to extend membrane lifetime. Fluoropolymer—a polymer that contains fluorine atoms—has a high resistance to the solvents, acids and bases found in ocean water. As for the environmental impact of desalination, Sirkar says dumping concentrated brine back into the sea creates a "minimal" disturbance to sea life. He adds, "Seawater is a very large volume with enough turbulence to dilute [the brine] very quickly."
That's not to say membrane distillation is without problems. It requires a steady, inexpensive source of heat to prevent the temperatures of the water on either side of the membrane from equalizing, which would impede the vaporization/condensation process. For DCMD to be practical it needs to be easier to use, more cost-effective and able to take advantage of available heat sources, including waste heat produced by places such as shore-based factories and offshore drilling operations, Sirkar says.
Although membranes are getting better, reverse osmosis is more common and has been used in desalination plants since the late 1960s. Recent improvements in reverse-osmosis technology—including more efficient membranes made from carbon nanotubes and energy-recovery devices that boost output while cutting energy consumption and costs—have made it a feasible option for even small communities such as the Sand City, Calif., on the Monterey Peninsula, with a population of less than 350.
Whichever technology is used for desalination, the price tag remains a wild card, dependent on the cost of energy necessary to build and maintain the facility, run the process, and transport seawater in and desalinated water out. A recent study by the WateReuse Association indicates that costs for seawater desalination projects vary widely from about $2 to $12 per 3,785 liters. Smaller capacity units, which produce less than 3.8 million liters daily, are at the higher end of that cost range, in part because they cannot leverage the same economies of scale as larger facilities.
As a result, desalination does not figure prominently in the U.S. Environmental Protection Agency's National Water Program 2012 Strategy: Response to Climate Change report, released in March as a draft for public comment. (pdf) The report notes that "desalination is energy intensive and there may be risks and costs associated with disposing of waste brines from the treatment." Still, the agency does acknowledge that rising sea levels over time may increase saltwater intrusion on coastal freshwater aquifers and notes that desalination is one way to ensure those aquifers remain usable.
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16 Comments
Add CommentI built an experimental desalination system 30 years ago that used a vacuum pump. Water boils at about 20 degrees C under vacuum. It is difficult to maintain a near vacuum & the water vapour needs to be condensed out before it enters the vacuum pump. Another experiment I conducted was to freeze salt water & suck air through it. The salt content was sucked out leaving nearly pure ice. You can conduct this experiment by simply freezing a small block of ice with fruit juice or salt if you wish mixed in it. Once frozen, suck the ice block & spit out the salt or juice & you will quickly have pure ice left. It takes a lot less energy to freeze water than to boil it which was the method most common back then before reverse osmosis systems were developed. I do not know how my experiments further developed would compare to modern osmosis systems as far as energy consumption. The system discussed above looks fairly complicated, having to add heat is a bit of a worry & if you wish to have minimum impact from the residue the last thing you want is more concentrated brine.
Reply | Report Abuse | Link to thisCarlyle: You are a bit naive which is the plague of most of us inventors and creative thinkers. We actually think we can make a difference independently. Wrong, we need money and backing for most of our ideas. As to your idea ,it is cheaper via your method of freezing, but if our sponsers give us money to develop our ideas we have to play a game- who is the most likely winner -GE refrigerator service or Shell that sells power for the heat process of your invention... think about it.
Reply | Report Abuse | Link to thisIf i had it my way, this planet would already be a class 1 planet with zero fossil fuel emmissions.. Who do you think would expend money on such devices? Cheveron? Shell corp, ? Grow up,fella.
Carlyle: You are a bit naive which is the plague of most of us inventors and creative thinkers. We actually think we can make a difference independently. Wrong, we need money and backing for most of our ideas. As to your idea ,it is cheaper via your method of freezing, but if our sponsers give us money to develop our ideas we have to play a game- who is the most likely winner -GE refrigerator service or Shell that sells power for the heat process of your invention... think about it.
Reply | Report Abuse | Link to thisIf i had it my way, this planet would already be a class 1 planet with zero fossil fuel emmissions.. Who do you think would expend money on such devices? Cheveron? Shell corp, ? Grow up,fella.
"A new, more energy-efficient seawater distillation membrane is designed to yield greater amounts of potable water, and less briny discharge". Really?
Reply | Report Abuse | Link to thisStart with seawater of a given salt concentration and separate it into two fractions, potable water and brine. The more potable water (of the same low salinity) you end up with the more concentrated the brine has to be, regardless of the separation technique. The salt doesn't just disappear.
I can certainly imagine more energy-efficient desalination techniques, but the claim that the new technique reduces briny discharge is nonsense. Any day now I expect Scientific American to introduce the first working perpetuum mobile!
The temperatures that they are talking about are not that great and could be atmospheric temperatures in many regions. But if you do need to heat the salt water and cool the non-salted water then the energy balances out!! The system could be quite efficient.
Reply | Report Abuse | Link to thisThere is no solution to the problem of food & portable water shortages and it will get worse. There was a solution: A sustainable Homo sapien population existing in a bio-diverse biosphere. This solution does not exist anymore thanks to religion. The emigrate to another planet group also has it wrong. The best every intelligent humanoid can do is not procreate and do nothing to assist other humans survive or breed. Don't ever give one single cent to charity. Let the stupid species go extinct.
Reply | Report Abuse | Link to thisGood point hb. Maybe they meant to say "less of even more briny discharge".
Reply | Report Abuse | Link to thisI also had a desalination idea, about 45 years ago, that didn't use membranes (in the usual sense), distillation or freezing. It would use electrostatics, treating ions as the charged particles they are, in a relatively non-ionized medium, considerably reducing energy consumption, with possible potential for seawater mining. Some people of course told me it wouldn't work, but I disproved the one specific argument, at least to my own satisfaction.
Along with problems making a working model and convincing people, I eventually concluded that if we solve just one major environmental problem, it will encourage us to continue increasing our human population some more until other problems become worse, as though they wouldn't anyway, so I went on to other projects.
In the process of testing, I also came up with a more efficient way to generate high voltages, compatible with wave action.
If you want to discuss it, please copy thoughts to danrob at efn dot org.
A steady, inexpensive source of heat for this membrane distillation system could be the waste heat from a liquid fluoride thorium reactor.
Reply | Report Abuse | Link to thisI may be showing my lack of knowledge of this process, but why can't we evaporate the remaining water from the brine and then sell the salt? This would solve the brine problem and provide a small amount of additional income. My wife pays a premium for 'sea salt' for cooking. (Maybe that price would come down.)
Reply | Report Abuse | Link to thisWhen this magazine published Earth 3 in 2009 it laid down a challenge for someone to invent a low or zero cost means of desalination. I did this and won the silver award at the Oxford University Venturefest in 2010 with my design. My plant uses only direct heat from the sun.
Reply | Report Abuse | Link to thisIt covers an area the size of two football pitches and also draws heat from the rocky desert floor which is practically infinite. It is made up of stainless steel and borosilicate glass modules each the size of a small car which can be mass produced in tens of thousands to be assembled on site. The plant costs NOTHING TO RUN and the salt is saved for sale as a dry product.It's about time Scientific American ran a story on this. All the information is on SOLAQUA.INFO and I am on the net.
Barry Coots.
I think the article means that, if the separation process is more efficient, there will be a lower volume of brine to discharge, even though that brine's salt concentration is higher.
Reply | Report Abuse | Link to thisBarry Coots, your idea seems interesting (at least to this layman's eye). As I read the article, solar energy and salt extraction at the end of the process came to mind. The clever integration of all the favourable factors is usually critical for new ideas to become practical.
On some sites, pumping and ancillary equipment might also benefit from harvested wind energy.
Thermodynamics would disagree with you. But I would love to get a hold of an air conditioner for my house that does this.
Reply | Report Abuse | Link to thisBarryCoots.. took a look at your website.
Reply | Report Abuse | Link to thisJust one question: Why aren't you rich?
What would be the effect of spinning a quantity of sea water in a centrifuge? Would the outer layers of saltier water leave behind more-pure water for another pass if not for direct consumption? If so, the next consideration would be cost of spinning. Also, what if one were to apply a magnetic field to the centrifuge, would this enhance the movement of the heavier water toward the outer rim? If not, someone please come up with an original idea - the need is already great, and rapidly growing.
Reply | Report Abuse | Link to thisLike several others here I have invented a new concept in desalination. It simplifies the process and saves much of the energy required.
Reply | Report Abuse | Link to thisThe basic concept is to place the membrane deep in the ocean so that the natural pressure is sufficient to push the water through the membrane as long as the fresh water is removed by pumping it to the surface.
The water outside the membrane becomes saltier so heavier so it flows down away from the membrane to be replaced by new salt water.
This means that only the energy needed to lift the fresh water to the surface.
The problems with this technology are 1/ distance to water sufficiently deep (about 200 metres). And 2/ the membrane and pump being relatively inaccessible.
A bonus is how easy it would be to use wave power to operate the pump.
Comment to scientific earthling, this discussion refers to potable water NOT PORTABLE WATER
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