|
Water, water everywhere but not a drop to drink
(New Scientist Via Thomson Dialog NewsEdge) IN THE Grimms' fairy tale, Rumpelstiltskin could spin straw into gold. But for parched regions around the world, turning saltwater into fresh would be a more valuable talent.
One-third of the world's population is living without sufficient water and of the 1400 million cubic metres of water on the planet, 98 per cent is undrinkable seawater. Worldwide, underground aquifers are being sucked dry.
The problem is likely only to worsen as the global population grows, so countries are looking to desalination to turn saltwater into drinking water (New Scientist
, 10 July 2004, p 22).
The US built its first large-scale desalination plant in Florida in 2003, and plans are afoot for another, as well as over a dozen new plants in California and three in Texas. Desalination is also being considered for landlocked states such as New Mexico, Nevada and Utah, where a decade of drought has caused surface water to run dry, while saltwater lies untouched in underground reserves. Elsewhere, countries including Spain, Israel and China have already built plants.
However, the existing technology has serious drawbacks. Desalination plants not only consume large amounts of energy to produce freshwater, they also generate a concentrated brine waste, which has to be disposed of. In the UK Ken Livingstone, the mayor of London, has blocked plans to build a desalination plant for the city, claiming the energy demands would be too high. The proposal is now the subject of a public inquiry.
On the horizon are technological breakthroughs that could significantly reduce the energy consumed by the plants and the waste they produce, making desalination plants viable all over the world. "If scientists can solve these two big questions, every community along the coast will go for it," says Ricardo de la Cruz, co-founder of the International Desalination Institute, a research organisation based in Monterey, California.
A US task force is putting the final touches to a technology road map for Congress, recommending the most promising areas of desalination research. Chief among these are new membrane technologies.
Conventional desalination is relatively simple most modern plants use reverse osmosis, in which membranes act like super-fine coffee filters, allowing water to pass through while screening out the salt. Since gravity alone is not enough to push water through the membranes, hydraulic pumps force it through under enormous pressure. This requires a lot of energy, typically around 4 kilowatt-hours per thousand litres of freshwater produced. As a result, if a typical US household were to get all its water from desalination, overall electricity usage would increase by 20 to 25 per cent.
Even in areas of the US where drinking water is particularly expensive and desalinated water relatively cheap, desalinated water still costs around 25 per cent more. New membrane technologies are expected to reduce this cost significantly. Olgica Bakajin and her colleagues at Lawrence Livermore National Laboratory, California, for example, have created a membrane filled with carbon nanotubes between 1.4 and 2 nanometres in diameter around six times the diameter of a single water molecule.
Unlike the pores in a conventional membrane, which are randomly orientated, vary in size and are spaced relatively far apart, the nanotubes are smooth and uniformly spaced. This reduces resistance and turbulence, allowing the water to flow through more smoothly. The nanotubes are also tightly packed together, increasing the number of pores per square centimetre and allowing more water to flow through. As a result, water flows through the nanotubes 10 to 100 times faster than existing membranes, reducing energy costs.
Other researchers believe they can build better membranes by taking their cue from nature. Cell membranes allow water to flow in or out by osmosis with almost no resistance, while blocking any other molecules or ions. Water passes through the cell membrane via channels called aquaporins. These channels are so small that water molecules travel through in single file, while larger molecules cannot enter. They are also extremely efficient: up to one billion water molecules can pass through a single aquaporin channel each second.
Now a number of research teams are hoping to adapt this process for reverse osmosis by developing desalination membranes containing artificial aquaporins. Danish company AQUAporin is using natural aquaporins extracted from plants to build a membrane, which they hope to launch in 2009.
Since aquaporins allow only pure water to pass through, they hope the membrane might also be able to treat water contaminated with pollutants such as the gasoline additive methyl tert-butyl ether, or gender-bending endocrine disrupters.
Progress is also being made in reducing the waste brine produced by desalination plants. Plants desalinating water from the sea can pump the brine back into the ocean, but inland plants have real problems disposing of the waste, as in some areas injecting it into the ground would contaminate groundwater.
The waste brine can be dried, leaving a mixture of salts which are usually dumped in a landfill site, adding to the overall cost. Researchers are developing ways of separating out these salts, so they can be sold off individually.
Ultimately, the best desalination plant is likely to be one that runs cheaply off its own renewable energy source, such as an onshore or offshore wind turbine or wave power generator. This could be particularly beneficial in the developing world, where water shortages are at their most severe and electricity supplies are unreliable.
Larry Flowers at the National Renewable Energy Laboratory in Golden, Colorado, has found that wind-powered desalination already makes economic sense in the rural south-west of the US where there are many areas with large underground deposits of brackish water, high wind speeds and poor access to the electricity grid. Efforts are also under way to develop small, stand-alone systems that can be used in the developing world.
As membranes improve and less energy is needed to push the seawater through, it will become more practical to desalinate water with renewable energy, says Patrick Brady, a geochemist at Sandia National Laboratories. "It actually is fairly realistic."
Banish the biofilmsJulie Rehmeyer The surface of a desalination membrane makes a cosy place for bacteria to live, complete with a steady supply of food. But bacteria don't make great house guests, as they clog up the pores, ruining the membrane.
Without a way of removing the bacteria, "you might have a fantastic filtration membrane, but it will go to pot very quickly", says Snezna Rogelj of the New Mexico Institute of Mining and Technology in Socorro.
The problem isn't the bacteria themselves, but the structures they build. Different kinds of bacteria come together to form a community, creating a "biofilm" on surfaces. Within these communities, the bacteria communicate with one another and share nutrients. They also secrete proteins and DNA to form an incredibly tough exoskeleton that gums up the pores in membranes.
While these biofilms can be removed with bleach, this ruins the membranes. So researchers are searching for new techniques to break down the biofilms or better still stop them forming in the first place.
Rogelj is hijacking tricks the bacteria use themselves when food gets low. The bacteria stop cooperating with each other and start releasing enzymes that chew up the exoskeleton, freeing them to move to richer feeding territory. Rogelj is attempting to capture these enzymes and use them to remove the biofilms.
One problem is that the chemistry of biofilms varies substantially from one to the next, making it difficult to find enzymes that work consistently. So she is also looking at ways to stop the biofilms from forming in the first place, by filtering the bacteria out before they get to the membrane using a sand-like substance called zeolite which is coated with detergent.
Another approach would be to attach detergent directly to the membrane, or to release a concentrated amount of detergent into the water before it reaches the membrane. The relatively large detergent molecules could then be filtered out.
[ Back To TMCnet.com's Homepage ]
|
Find Solutions for Enterprises, SMBs & Service Providers at the INTERNET TELEPHONY Conference and EXPO West, September 16-18, 2008. Los Angeles, California.
