Department of Water Resources
A daily compilation of significant news articles and comment
June 6, 2008
4. Water Quality –
Tapping the oceans: Environmental technology: Desalination turns salty water into fresh water. As concern over water’s scarcity grows, can it offer a quick technological fix?
The Economist- 6/5/08
THERE are vast amounts of water on earth. Unfortunately, over 97% of it is too salty for human consumption and only a fraction of the remainder is easily accessible in rivers, lakes or groundwater. Climate change, droughts, growing population and increasing industrial demand are straining the available supplies of fresh water. More than 1 billion people live in areas where water is scarce, according to the United Nations, and that number could increase to 1.8 billion by 2025.
One time-tested but expensive way to produce drinking water is desalination: removing dissolved salts from sea and brackish water. Its appeal is obvious. The world’s oceans, in particular, present a virtually limitless and drought-proof supply of water. “If we could ever competitively—at a cheap rate—get fresh water from salt water,” observed President John Kennedy nearly 50 years ago, “that would be in the long-range interest of humanity, and would really dwarf any other scientific accomplishment.”
According to the latest figures from the International Desalination Association, there are now 13,080 desalination plants in operation around the world. Together they have the capacity to produce up to 55.6m cubic metres of drinkable water a day—a mere 0.5% of global water use. About half of the capacity is in the
Because desalination requires large amounts of energy and can cost several times as much as treating river or groundwater, its use in the past was largely confined to wealthy oil-rich nations, where energy is cheap and water is scarce.
But now things are changing. As more parts of the world face prolonged droughts or water shortages, desalination is on the rise. In
Not everyone is happy about this. Some environmental groups are concerned about the energy the plants will use, and the greenhouse gases they will spew out. A large desalination plant can suck up enough electricity in one year to power more than 30,000 homes.
The good news is that advances in technology and manufacturing have reduced the cost and energy requirements of desalination. And many new plants are being held to strict environmental standards. One recently built plant in
References to removing salt from seawater can be found in stories and legends dating back to ancient times. But the first concerted efforts to produce drinking water from seawater were not until the 16th century, when European explorers on long sea voyages began installing simple desalting equipment on their ships for emergency use. These devices tended to be crude and inefficient, and boiled seawater above a stove or furnace.
An important advance in desalination came from the sugar industry. To produce crystalline sugar, large amounts of fuel were needed to heat the sugar sap and evaporate the water it contained. Around 1850 an American engineer named Norbert Rillieux won several patents for a way to refine sugar more efficiently. His idea became what is known today as multiple-effect distillation, and consists of a cascading system of chambers, each at a lower pressure than the one before. This means the water boils at a lower temperature in each successive chamber. Heat from water vapour in the first chamber can thus be recycled to evaporate water in the next chamber, and so on.
No salt, please
This reduced the energy consumption of sugar refining by up to 80%, says James Birkett of West Neck Strategies, a desalination consultancy based in
A few multi-effect distillation plants were built in the first half of the 20th century, but a flaw in the system hampered its widespread adoption. Mineral deposits tended to build up on heat-exchange surfaces, and this inhibited the transfer of energy. In the 1950s a new type of thermal-desalination process, called multi-stage flash, reduced this problem. In this, seawater is heated under high pressure and then passed through a series of chambers, each at a lower pressure than the one before, causing some of the water to evaporate or “flash” at each step. Concentrated seawater is left at the bottom of the chambers, and freshwater vapour condenses above. Because evaporation does not happen on the heat-exchange surfaces, fewer minerals are deposited.
Countries in the
Research into new ways to remove salt from water picked up in the 1950s. The American government set up the Office of Saline Water to support the search for desalination technology. And scientists at the
Such membranes are common in nature. When there is a salty solution on one side of a semi-permeable membrane (such as a cell wall), and a less salty solution on the other, water diffuses through the membrane from the less concentrated side to the more concentrated side. This process, which tends to equalise the saltiness of the two solutions, is called osmosis. Researchers wondered whether osmosis could be reversed by applying pressure to the more concentrated solution, causing water molecules to diffuse through the membrane and leave behind even more highly concentrated brine.
Initial efforts showed only limited success, producing tiny amounts of fresh water. That changed in 1960, when Sidney Loeb and Srinivasa Sourirajan of UCLA hand-cast their own membranes from cellulose acetate, a polymer used in photographic film. Their new membranes boasted a dramatically improved flux (the rate at which water molecules diffuse through a membrane of a given size) leading, in 1965, to a small “reverse osmosis” plant for desalting brackish water in Coalinga, California.
The energy requirements for thermal desalination do not much depend on the saltiness of the source water, but the energy needed for reverse osmosis is directly related to the concentration of dissolved salts. The saltier the water, the higher the pressure it takes (and hence the more energy you need) to push water through a membrane in order to leave behind the salt. Seawater generally contains 3,337 grams of dissolved solids per litre. To turn it into drinking water, nearly 99% of these salts must be removed. Because brackish water contains less salt than seawater, it is less energy-intensive, and thus less expensive, to process. As a result, reverse osmosis first became established as a way to treat brackish water.
Another important distinction is that reverse osmosis, unlike thermal desalination, calls for extensive pre-treatment of the feed water. Reverse-osmosis plants use filters and chemicals to remove particles that could clog up the membranes, and the membranes must also be washed periodically to reduce scaling and fouling.
Getting better all the time
In the late 1970s John Cadotte of
The energy consumption of such plants has since fallen dramatically, thanks in large part to energy-recovery devices. High-pressure pumps force seawater against a membrane, which is typically arranged in a spiral inside a tube, to increase the surface area exposed to the incoming water and optimise the flux through the membrane.
About half of the water emerges as freshwater on the other side. The remaining liquid, which contains the leftover salts, shoots out of the system at high pressure. If that high-pressure waste stream is run through a turbine or rotor, energy can be recovered and used to pressurise the incoming seawater.
The energy-recovery devices in the 1980s were only about 75% efficient, but newer ones can recover about 96% of the energy from the waste stream. As a result, the energy use for reverse-osmosis seawater desalination has fallen. The
Economies of scale, better membranes and improved energy-recovery have helped to bring down the cost of reverse-osmosis seawater-desalination. Although the cost of desalination plants and their water depends on where they are, as well as the local costs of capital and operations, prices decreased from roughly $1.50 a cubic metre in the early 1990s to around 50 cents in 2003, says Mr Pankratz. As a result, reverse osmosis is preferred for most modern seawater-desalination (though rising energy and commodity prices mean the cost per cubic metre has now risen to around 75 cents). Experts reckon that further gains in energy efficiency, and hence cost reductions, will be increasingly difficult, however. According to a recent report on desalination from
Sometimes, using desalination within water management may be the only way to ensure supply.
To achieve these reductions, researchers want to find better membranes that allow water to pass through more easily and are less likely to get clogged up. Eric Hoek and his colleagues from UCLA, for example, have developed a membrane embedded with tiny particles containing narrow flow channels, producing a significant increase in water flux. The membrane’s smooth surface is also expected to make it harder for bacteria to latch onto. Depending on a plant’s design, the new membranes could reduce total energy consumption by as much as 20%, reckons Dr Hoek. The technology is being commercialised by NanoH2O, a company on UCLA’s campus.
Meanwhile, the possibility of making membranes out of carbon nanotubes, which consist of sheets of carbon atoms rolled up into tubes, has also garnered attention. A study published in the journal Science in 2006 demonstrated unexpectedly high water-flow rates. But insiders think it will be a decade before the idea is ready for commercialisation.
As desalination becomes more widespread, its environmental impacts, including the design of intake and discharge structures, are coming under increased scrutiny.
Some of the damage can be mitigated fairly easily. Reducing the intake velocity enables most fish species and other mobile marine life to swim away from the intake system, though small animals, such as plankton or fish larvae, may still get caught in the intake screens or sucked into the plant.
Measuring the impact
A bigger problem may be the leftover brine, which typically contains twice as much salt as seawater and is discharged back into the ocean. So far little scientific information exists about its long-term effects. In the past, most big seawater-desalination plants were built in places that did not conduct adequate environmental assessments, says Peter Gleick, president of the Pacific Institute, a think-tank based in
Some recent measurements from
A separate problem may be that some metals or chemicals leach into the brine. Thermal-desalination plants are prone to corrosion, and may shed traces of heavy metals, such as copper, into the waste stream. Reverse-osmosis plants, for their part, use chemicals during the pre-treatment and cleaning of the membranes, some of which may end up in the brine. Modern plants, however, remove most of the chemicals from the water before it is discharged. And new approaches to pre-treatment may reduce or eliminate the need for some chemicals.
Based on the limited evidence available to date, it appears that desalination may actually be less environmentally harmful than some other water-supply options, such as diverting large amounts of fresh water from rivers, for example, which can lead to severe reductions in local fish populations. But uncertainties over the environmental impacts of desalination make it hard to draw definite conclusions, the National Research Council concluded. Its report suggested that further research on the environmental impacts of desalination, and how to mitigate them, should be a high priority.
The reverse-osmosis process is increasingly being used not just for desalination, but to recycle wastewater, too. In
As water becomes more scarce, people will want to find several ways to secure their supplies. Many parts of the world also have enormous scope to use water more efficiently, argues Dr Gleick—and that would be cheaper than desalination. But sometimes, making desalination part of the approach to water management may be the only way to ensure a steady supply of drinking water.
In drought-ridden Western Australia, which ordered conservation years ago, the Water Corporation has adopted what it calls “security through diversity”, otherwise known in the industry as the “portfolio” approach. At the moment,
http://www.economist.com/science/tq/displaystory.cfm?story_id=11484059
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