Wizards of the water cycle

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s p e c i a l r e p o r t n w a t e r v s. e n e r g yThe company calculated that the recycledwater’s exceeding purity saves it morethan half a million dollars a year, in partby cutting out steps in its internal waterpurificationprocess.Singapore also started priming thepublic. The prime minister drank a bottleof NEWater at a national festival, andthe crowd cheered. The subtext wasclear—patriotic Singaporeans drinkwastewater. But the rest of Singaporewas slower to follow. A parody of a popularnationalist song, “Count on Me,Singapore,” cheerily urged residents to“Drink Our Pee, Singapore.”T he queasy reac t ions of someSingaporeans didn’t deter the utility,which built four more wastewater treatmentplants and is about to increase itsNEWater production to 555 megalitersa day. By the end of this year the plantswill treat enough sewage to cover nearlya third of Singapore’s water needs.spectrum.ieee.orgFor now, only a tiny fraction of thedrinking supply is NEWater. About40 ML are blended in with the city-state’spotable water each day, equivalent to2 percent of consumption. The utility alsobuilt a seawater desalination plant thatproduces 136 ML of water each day, or a bitless than one-tenth of Singapore’s supply.The city-state has all the water it needs.N at u r e ’ s h y d r o l o g i c a l c y c l e r e l i e son sunlight to evaporate water, leavingbehind salt and other impuritiesand returning freshwater to Earth inthe form of rain and snow. But it can becapricious: Most freshwater evaporatesfrom the oceans, where it rises throughthe atmosphere and then cools to formclouds. Winds push around the clouds,driving a few of them over land, wherethey deposit a mere tenth of the world’sprecipitation, according to the U.S.Geological Survey. Singapore decided tocut the atmosphere out of the transactionand to replace evaporation with speediermechanisms—membranes.Here’s how it works: First, a treatmentplant gathers up the city’s used water andseparates out the large, easily removableimpurities. About 60 percent of the wateris released into the sea. The rest gets sentto a NEWater factory. The water enterscontaminated with bacteria, viruses, andall sorts of carbon-based particles andemerges cleaner than what flows fromjust about any tap in the world.The main tactic of the water trade isto simply push water through tiny holes—the smaller the holes, the fewer the undesirablemolecules that can sneak through.The art is to do so without sending theelectricity bill skyrocketing or cloggingup the tiny holes with grime. Here, thefirst step is to force the water through amembrane that blocks particles of up to0.2 micrometers in size, catching mostbacteria and protozoa. The membranelooks like a cylinder filled with skinny,hollow tubes. The stream flows into theporous straws. Water molecules pushthrough the pores and collect outsidethe membranes, while the larger particlescontinue traveling inside the tubes,to be disposed of separately.The water still needs to be strippedof any viruses that might be adrift in theflow. For this the partially treated streamneeds a reverse-osmosis membrane. Inone configuration, paperlike sheets ofmembrane more than a meter long aresandwiched between sheets called spacers.The stack of membranes and spacersis wound up into a cylinder, like a thickroll of wrapping paper. The core of thecylinder is an empty channel where theclean water collects.As the stream is pushed into one endof the roll, the impurities slide along thespacers and never penetrate the polymermembrane. The water molecules, however,escape through the 0.0001-µm poresand slip into the central channel.These two steps remove just abouteverything, but in a third and final step,mercury lamps generate ultraviolet-lightrays that penetrate the water. The radiationscrambles the genetic material of anybacteria and other microorganisms thatjune 2010 • IEEE Spectrum • NA 59
3. t i n y t e s t b e d s n s i n g a p o r e n d e s a l i n a t i o n n m a l t aglacier or how much rain falls duringmonsoon season. Suddenly, Singapore iswater rich. The water that falls from thesky may have once washed the streets ofParis or filled Cleopatra’s bathtub. Nowit’s augmented with some of the waterused by Harry Seah, Asit Biswas, andeverybody else in Singapore. The utilityisn’t waiting for nature to turn its usedwater fresh again—it’s letting technologydo the job.might have slipped through, destroyingtheir ability to reproduce. Now the wateris ready for the tap.t h e S i g N i F i c a N c e o F N e Wat e r i S M u c hgreater than its literal contribution indrops or buckets or liters per person.“NEWater is key to our whole strategy,”Seah says.Seah is a soft-spoken, dapper manwith a face that collapses frequentlyinto a brassy laugh. He’s been with thePublic Utilities Board for almost twodecades, and he’s seen the NEWaterproject through to maturity. “The realbeauty of NEWater is its multiplyingeffect,” he explains. What he means isthat if the utility recycles 50 percent ofits wastewater, Singapore can squeezeone more drop out of every two it desalinatesor collects from the sky. “If I canachieve 100 percent recycling, I wouldn’teven need rain,” Seah says.That’s critical, given how few optionsSingapore has. Half of the island hasalready been converted into rainwatercatchment areas, and three new reservoirswill soon bring it to two-thirds. Theother source of freshwater is Singapore’sone desalination plant, which consumesso much energy it can cover only a fractionof the overall demand.Consider the details. To remove saltfrom seawater, Seah says the treatmentplant must apply a pressure of about7 megapascals. To remove impuritiest h e m a r i n ab a r r age t u r n Sa b ay i n t o ar e S e r voi r[at le f t ]p r o t e c t e d f r o mt h e S e a [ r ight ] .from wastewater, he requires less than1 MPa. That translates directly into theenergy cost for the whole plant—morethan 4 kilowatt-hours per thousand litersof water for seawater desalination, versus0.7 kWh for NEWater. “It’s a no-brainer,”says Seah, and his eyes crinkle into alaugh. “Win, win, win!” he says.Asit Biswas, an international waterpolicyexper t who splits his timebetween the National University ofSingapore and the Third World Centerfor Water Management, in Mexico, seesSingapore as one of the few places wherethe water authorities agree with his owncontroversial position—that water scarcitysimply does not exist. What doesexist, just about everywhere, is badwater management.The problem, he says, is that peopletend to assume that water is like oil orany other fossil fuel and that it behavesaccording to the same basic economicprinciples. “But water is not oil,” hesays. “Once we use oil, it breaks downinto various components. You can’tput it back together.” Water, despiteits tendency to evaporate and trickleout of reach, doesn’t change its molecularstructure. “There is no limit tohow many times water can be reused,”Biswas points out.That simple fact radically changes theway that water is counted. What’s availableto a city or country is not just a matterof how much ice sits in a HimalayanN o t e V e r y o N e c a N B e c a J o l e d i N t odrinking recycled urine. In Queensland,Australia, and in San Diego, public opinionhas thwarted local utilities’ water-recyclingambitions. But at least three sitesin the United States—Scottsdale, Ariz.;Orange County, Calif.; and northernVirginia—purify wastewater and blend itinto their drinking water.In fact, most places reuse water—justwithout meaning to do so. According toPeter Gleick’s The World’s Water 2002–2003, each drop in the Colorado Riveris used 17 times. Similarly, residents ofLondon drink water that was dischargedfrom numerous wastewater treatmentplants upstream on the Thames.In Singapore’s case, the deliberatereuse of water has played a prominentrole in the country’s development.Seetharam Kallidaikurichi is the directorof the Institute of Water Policy at theNational University of Singapore, and heargues that Singapore’s water strategyhas been the foundation of the country’seconomic success in the last 45 years.“The evidence is beginning to emerge,”he says, “that it’s not water among manyother things but water as central, the keyto unlocking the vicious cycles of deprivationand poverty into economic successand a good quality of life.”But prioritizing water means using aconsiderable amount of energy to makeit, clean it, and move it. The question ishow to optimize the juggle of resources.Because Singapore must import some290 billion cubic feet of natural gas to runits power plants, it can be said that thecountry has merely substituted one formof dependence for another.The consequences of that dependenceare starting to hit home. Darren60 nA • ieee spectrum • june 2010 spectrum.ieee.org
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Wizards of the water cycle

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