Grim building

Water scarcity is a frequently discussed probable impact of climate change. As glaciers and snowcaps diminish, less fresh water will accumulate in the mountains during the winter; that increases both flooding (during wet seasons) and drought. Higher temperatures also increase water usage for everything from irrigation to cooling industrial processes. Given the extent to which the world’s aquifers are already depleted (see: Ogallala Aquifer), relatively few additional natural sources exist.

The big alternative to natural sources is the desalination of seawater. This is done in one of two ways: using multistage flash distillation or reverse osmosis. About 1,700 flash distillation plants exist in the Middle East already, processing 5.5 billion gallons of seawater per day (72% of the global total). These plants use superheated steam, a by-product of fossil fuel combustion, to pressurize and heat a series of vessels. As salt water flows into each successively lower pressure vessel, it flash boils. Condensers higher in the vessel cause the fresh water to precipitate out from the hot pressurized air solution. This is a simple process, but an energy intensive one.

Reverse osmosis, by contrast, uses a combination of high pressure pumps and specialized membranes to desalinate water. Essentially, the pressure drives fresh water through the membranes more quickly than the accompanying salts. As such, it is progressively less saline with each membrane crossing. In this process, there are both relatively high energy requirements (for high pressure pumping) and the costs associated with building and maintaining the membranes. Because it can be done at different scales, portable reverse osmosis facilities are the preferred option for combat operations or disaster relief.

Unfortunately, both processes are highly energy intensive. Particularly when that energy is being generated in greenhouse gas intensive ways, this is hardly a sustainable solution. Part of the solution is probably to sharply reduce or eliminate water subsidies – especially for industry and agriculture. More transparent pricing should help ensure that the whole business of desalination is only undertaken in situations where the need for water justifies all the expenses incurred.

Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford. Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here.

24 thoughts on “Desalination”

  1. “Nature has had the benefit of a pretty long R&D period,” says Mr Pawlyn, who hopes to take the exploitation of natural designs to a completely new level. His firm has borrowed a trick used by fog-basking beetles and in the nostrils of camels for a novel desalination plant.

    When Grimshaw was given the brief to regenerate the Santa Catalina Isthmus, a narrow stretch of land on the coast of Las Palmas in Grand Canaria, the firm came up with the concept of a 3km (1.9 mile) promenade with a theatre and botanic garden as its focus. But rather than creating yet another drain on local resources the architects wanted the structure to be cooled and irrigated by natural means. “We decided to put forward a scheme that showed how the island could move towards self-sufficiency in water and energy, without relying on fossil fuels,” says Mr Pawlyn.

    This meant finding a way to turn seawater into clean drinking water without expending too much energy. Fog-basking beetles, which are found in Namibia, have an ingeniously simple way of doing this. They hide underground during the day so that when they come out at night, their dark backs are relatively cool compared with the ambient night air. As moisture-laden breezes roll in from the Atlantic, the water in the air condenses on the beetles’ backs (just as a cold bottle of beer left on a table causes water in the air to condense on its surface). The beetles simply have to tilt their bodies to make the water trickle into their mouths. A similar trick is also used by camels to prevent them losing moisture as they exhale. Moisture secreted through the nostrils evaporates as the camel breathes in, cooling the nostrils in the process. When the camel breathes out, moisture within the air then condenses on the nostrils.

    Inspired by this, Mr Pawlyn and his colleagues have designed their theatre around the same principles. A series of tall, vertical evaporation “gills” are positioned so that they face towards the sea and the incoming coastal breeze. Warm seawater, taken from close to the surface, is pumped so that it trickles down these units. As the breeze blows through the gills some of the seawater evaporates, leaving salt behind. The clean, moist air then continues its journey until it encounters a series of vertical condensing pipes. These are kept cold by pumping deep-sea water, from 1,000 metres below the surface, through them. As the moist, warm air hits the pipes the water condenses and trickles down to be collected.

    “You get a very powerful desalination effect,” says Mr Pawlyn. This system is able to supply enough water for the 70,000-square-metre complex. A traditional flash-distillation desalination plant consumes between five and 12 kilowatt hours (kWh) of energy per cubic metre of water. The biomimetic approach, however, requires just 1.6 kWh per cubic metre. And since the water pumps will be mostly powered by a wind turbine, driven by the same prevailing winds that provide the plant’s airflow, the overall energy consumption of the site is reduced even further. In the process, the same system can also help to cool neighbouring buildings, says Mr Pawlyn.


  2. KANSAS CITY, Missouri (Reuters) – The U.S. craze for ethanol could severely strain an already ailing aquifer in key farm states, increasing demand for scarce water supplies by more than 2 billion gallons a year, according to a report issued Thursday by the nonprofit group Environmental Defense.

    The environmental group’s report focused on the Ogallala aquifer, an 800-mile-long underground pool that stretches from Texas to South Dakota. The Ogallala feeds one-fifth of all the irrigated land in the United States, and is critical to farmers growing corn, cotton, wheat, soybeans and other crops.

    “The Ogallala Aquifer is a microcosm of the challenges we’ll face in America as we develop renewable fuels,” said Martha Roberts, co-author of the report and a fellow at Environmental Defense. “Nine new ethanol plants are already planned for some of the most water-depleted areas of the Ogallala Aquifer, even though those areas are vulnerable to erosion and the entire region’s water resources are stretched thin.”


  3. Case history
    Tapping the oceans

    Jun 5th 2008
    From The Economist print edition
    Environmental technology: Desalination turns salty water into fresh water. As concern over water’s scarcity grows, can it offer a quick technological fix?

  4. 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 33-37 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.

  5. The Perth plant, which uses technology from Energy Recovery, a firm based in California, consumes only 3.7kWh to produce one cubic metre of drinking water, according to Gary Crisp, who helped to oversee the plant’s design for the Water Corporation, a local utility.

  6. Your article on wave power caused me to question the obvious. I wonder if anyone has proposed marrying the Limpet wave-power-to-electricity concept to a reverse osmosis plant.

    The numbers you quoted seem to make such a concept interesting: the Limpet can apparently produce about 300kw; the high efficiency reverse osmosis plant being built in Perth consumes about 3.7kwh to produce a cubic meter of pure water.

    Folding these numbers together I come up with a rough estimate that such a combination plant could purify enough ocean water to supply nearly 400 homes a day. Furthermore it would be energy independent and carbon neutral.

  7. Parched Perth embarks on water rescue

    By Phil Mercer
    BBC News, Perth, Western Australia

    Authorities in Western Australia say they can show the world how to conquer a water crisis that had threatened to decimate the state capital, Perth, amid a long-standing drought and declining rainfall.

  8. Desalination – Energy Down the Drain

    Posted by Nate Hagens on March 2, 2009 – 9:47am

    The next worst idea to turning tar sands into synthetic crude is turning ocean water into municipal drinking water. Sounds great until you zoom in on the environmental costs and energetic consequences. It may be technically feasible, but in the end it is unsustainable and will be just one more stranded asset.

    In 2003, I was one of two elected officials invited to serve on the California Desalination Task Force. The task force was the result of Assembly Bill 2717 (Hertzberg), authorizing the Department of Water Resources to study desalination facilities and “report on potential opportunities and impediments…”

  9. Cheaper desalination
    Current thinking

    Oct 29th 2009
    From The Economist print edition
    A fresh way to take the salt out of seawater

    Existing desalination plants work in one of two ways. Some distil seawater by heating it up to evaporate part of it. They then condense the vapour—a process that requires electricity. The other plants use reverse osmosis. This employs high-pressure pumps to force the water from brine through a membrane that is impermeable to salt. That, too, needs electricity. Even the best reverse-osmosis plants require 3.7 kilowatt hours (kWh) of energy to produce 1,000 litres of drinking water.

    Mr Sparrow and Mr Zoshi, by contrast, reckon they can produce that much fresh water with less than 1 kWh of electricity, and no other paid-for source of power is needed. Their process is fuelled by concentration gradients of salinity between different vessels of brine. These different salinities are brought about by evaporation.

  10. One notable thing about desalination is that it creates a kind of floor price for water, at least for those near the sea.

    If you can get 1,000L of fresh water for about $0.15 worth of energy, it is hard to imagine people in Vancouver or San Francisco going thirsty, no matter how bad droughts become in the continental interior.

  11. Example 1: the cost of desalinating sea water. [This method of making it stick came from Jim Gill, Chancellor of Curtin University, via Sam Wylie.] In SEWTHA (p 93), I report that desalination has an energy cost of 8 kWh per m3. A nice way to make this number more meaningful is to work out what temperature rise you would get if the same energy were put directly into heat in the same volume of water. The answer is ((8 kWh) / (1000 litres)) / (4.2 ((kJ / C) / litre)) = 7 degrees C.

    This result brings home that if the desalinated water is going to be used for a shower or for cooking, the energy cost of the desalination is fairly tiny compared to the energy that will be used later in the water’s lifecycle.

  12. Water technology
    Striking the stone
    Israeli firms offer technology to slake the world’s thirst

    MOSES parted the waters. Strauss aims merely to separate the waters from their yucky impurities. On May 18th in Shanghai Israel’s second-largest food and drinks firm will launch a high-tech purifier that not only filters water but also heats it to exactly the right temperature for making tea. Strauss has forged a joint venture with China’s Haier Group, the world’s biggest maker of white goods, to distribute it.

    China is the perfect first market for such an appliance, says Rami Ronen, the boss of Strauss’s water arm. Chinese people drink a lot of tea, and their taps emit a lot of undrinkable gunge. At 4,490 yuan ($692) a pop the device is not cheap, but Chinese wallets are increasingly capacious.

    Israel wants to become the Silicon Valley of water technology. The conditions are ripe: the country has plenty of scientists, an entrepreneurial culture and a desperate shortage of fresh water. Netafim, one of the world’s biggest “blue-tech” firms, got its start on a kibbutz in the Negev desert. Intrigued by an unusually large tree, an agronomist discovered that a cracked pipe fed droplets directly to its roots. After much experimentation the firm was founded in 1965 to sell what has become known as drip irrigation. Today it boasts annual sales of over $600m and a global workforce of 2,800.

  13. Yet with around 7,000 people per square kilometre, Singapore is the third most densely populated country in the world. Its land mass is not large enough to supply the thirst of its 5m inhabitants.

    One answer is to desalinate seawater. That, though, is expensive, so the Singaporean government is keen to find cheaper ways of doing it. And, in collaboration with Siemens, a German engineering conglomerate, it may have done so, for Siemens says its demonstration electrochemical desalination plant on the island can transform seawater into drinking water using less than half the energy required by the most efficient previous method.

    To make seawater fit for human consumption its salt content of approximately 3.5% must be cut to 0.5% or less. Existing desalination plants do this in one of two ways. Some employ distillation, which needs about 10 kilowatt-hours (kWh) of energy per cubic metre of seawater processed. The energy is used to heat the brine, partially evaporating it, and to condense the resulting water vapour. Other plants employ reverse osmosis. This uses special membranes which act as molecular sieves by passing water molecules while holding back the ions, such as sodium and chloride, that make water salty. Generating the pressure needed to do this sieving consumes about 4kWh per cubic metre of water. The Siemens system, by contrast, consumes only 1.8kWh per cubic metre, and the firm hopes to get that down to 1.5kWh.

    It works using a process called electrodialysis, in which the seawater is pumped into a series of channels walled by membranes that have slightly different properties from those used in reverse osmosis. Instead of passing water molecules, these membranes pass ions. Moreover, the membranes employed in electrodialysis are of two types. One passes positively charged ions. The other passes negatively charged ones. The two types alternate, so that each channel has one wall of each type. Two electrodes flanking the system of channels then create a voltage that pulls positively charged ions such as sodium in one direction and negatively charged ions such as chloride in the other.

  14. Monitor
    Drops to drink

    Desalination: A technique called electrodialysis may provide a cheaper way to freshen seawater for human consumption

    SINGAPORE’S average annual rainfall is more than double that of notoriously soggy Britain, so the casual observer might be surprised to learn that the place has a shortage of drinking water. Yet with around 7,000 people per square kilometre, Singapore is the third most densely populated country in the world. Its land mass is not large enough to supply its 5m inhabitants with water.

    One answer is to desalinate seawater. That, though, is expensive, so the Singaporean government is keen to find cheaper ways of doing it. And, in collaboration with Siemens, a German engineering conglomerate, it may have done so, for Siemens says its demonstration electrochemical desalination plant on the island can turn seawater into drinking water using less than half the energy required by the most efficient previous method.

  15. The very idea of consuming reprocessed human, animal and industrial waste can turn people’s stomachs. But it happens more than most realise. Even municipalities that do not pump waste-water back into aquifers or reservoirs, often draw their drinking supply from rivers that contain the treated effluent from communities upstream.

    A survey done in 1980 for the Environment Protection Agency (EPA), which looked at two dozen water authorities that took their drinking water from big rivers, found this unplanned use of waste-water (known as “de facto reuse”) accounted for 10% or more of the flow when the rivers were low. Given the increase in population, de facto reuse has increased substantially over the past 30 years, says a recent report on the reuse of municipal waste-water by the National Research Council (NRC) in Washington, DC.

    Along the Trinity River in Texas, for instance, water now being drawn off by places downstream of Dallas and Fort Worth consists of roughly 50% effluent. In summer months, when the natural flow of the river dwindles to a trickle, drinking water piped to Houston consists almost entirely of processed effluent.

    The main problem is not changes in the weather (though global warming hardly helps), but population growth. The American population has doubled, to over 300m, since the middle of last century—and is expected to increase by a further 50%, to 450m, over the next half century. Meanwhile, households as a whole have been consuming water at an even faster rate, thanks to the housing boom and the widespread use of flushed toilets, dish washers, washing machines, swimming pools and garden sprinklers.

  16. Israelis once obsessed over the level of their largest natural reservoir, the Sea of Galilee. As The Economist went to press, it was just 11cm above its “red line”, the point at which Israel stops pumping water to avoid ecological damage. Yet this no longer causes public concern, for most of Israel’s water is artificially produced. About a third comes from desalination plants that are among the world’s most advanced. Farmers rely on reclaimed water for irrigation. Israel recycles 86% of its wastewater, the highest level anywhere; Spain, the next best, reuses around 20%.

  17. That breakdown of where desalination is used hints at two reasons it is not a panacea. One is geography. If the sea is the feedstock it will be too costly to transport desalinated water long distances inland—to western China, for example. Secondly, even for coastal regions, desalination is very expensive, which explains why two-thirds of existing facilities are located in high-income countries. The expense comes partly in the capital cost of the plants. Sorek required a total investment of about $400m. In Israel the desalination industry marks a departure from one of the cardinal principles of its water-management policies—that all water is a public good. From the moment a raindrop leaves a cloud it is the state’s property. However, four out of the five desalination plants are privately owned.

    The second big reason for the expense is the energy they use—typically between one-half and two-thirds of the cost of desalinated water. Israel has managed to achieve relatively good energy efficiency, partly through the use of innovative membranes. The price of Sorek’s water is $0.50-0.55 a cubic metre, down from $0.78 for water from the first Israeli plant built on the public-private model at Ashkelon, which opened in 2005.

    The unu paper concentrated on a third drawback to desalination: what happens to the salty sludge (known as brine) left behind by the pristine, desalinated water. At Sorek, as is typical, it is taken out by a pipe and discharged nearly 2km out at sea. Around the world, desalination plants produce nearly 50% more brine (141.5m cubic metres a day) than freshwater.

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