A 1, 000 års tørke rammer Vesten:Kunne afsaltning være en løsning?

USA og mange andre dele af verden raser under følgerne af alvorlig tørke. En mulig løsning er afsaltning af havvand, men er det en sølvkugle?

Det vestlige USA oplever i øjeblikket, hvad en palæoklimatolog kaldte "potentielt den værste tørke i 1, 200 år." Regionen har tidligere haft mange tørkeperioder, inklusive "megatørke", der varer årtier, men klimaændringer gør tørre år tørrere og våde år vådere. Højere temperaturer opvarmer jorden og luften hurtigere, og den øgede fordampning tørrer jorden og mindsker mængden af ​​nedbør, der når reservoirer. Opvarmning fører også til mindre af den snepakke, der er nødvendig for at genopbygge floder, vandløb, reservoirer og fugtig jord om foråret og sommeren.

Omkring 44 procent af USA oplever en vis grad af tørke med næsten 10 procent i "ekstraordinær tørke." Naturbrande raser i øjeblikket i 13 stater, forværres af de varme og tørre forhold. Der har været hidtil usete vandafskæringer til Colorado-floden - som giver vand til syv stater - og nedlukninger af vandkraftværker. Akvifererne i byer, der er afhængige af brøndvand, er udtømt, tvinger beboerne til at køre i vand. Normalt, landbruget forbruger over 90 procent af vandet i mange vestlige stater, men tørken har fået udbyttet til at falde; nogle landmænd har reduceret deres areal eller ændret afgrøder til mindre vandintensive, mens andre sandsynligvis vil gå konkurs. Landbrugere er nødt til at sælge ud af dele af deres besætninger. Men selvom de lokale kæmper med disse vanskeligheder, flere mennesker flytter til området.

Mellem 1950 og 2010, sydvestens vækstrate var dobbelt så stor som resten af ​​landet. Den amerikanske befolkning forventes at fortsætte med at vokse frem til 2040, med mere end halvdelen af ​​den vækst i områder, der har oplevet alvorlig tørke i de sidste ti år. Mange mennesker fortsætter med at flytte til et område, der forventes at blive endnu tørrere i de kommende år, ligesom den seneste IPCC-rapport forudsiger, at klimaforandringerne vil intensivere tørken i disse regioner.

Alle andre kontinenter i verden oplever også alvorlig tørke, undtagen Antarktis. Og FN har advaret om, at 130 flere lande kan stå over for tørke inden 2100, hvis vi ikke gør noget for at bremse klimaændringerne. Men allerede i 2025, to tredjedele af verdens befolkning kan stå over for vandmangel, ifølge World Wildlife Fund. Dette kan resultere i konflikter, politisk ustabilitet, og fordrivelse af millioner af mennesker.

Manglen på ferskvand kan også gøre det sværere at dekarbonisere samfundet - noget vi skal gøre for at forhindre katastrofale klimaændringer - fordi nogle strategier til at gøre dette kan yderligere stresse vandressourcerne. Grøn brint, betragtes som nøglen til at eliminere emissioner fra luftfart, Forsendelse, lastbilkørsel, og tung industri, fremstilles ved elektrolyse, som spalter vand til brint og ilt. Imidlertid, processen kræver store mængder renset vand. Et skøn er, at ni tons af det er nødvendigt for at producere et ton brint, men faktisk kræver behandlingsprocessen, der bruges til at rense vandet, dobbelt så meget urent vand. Med andre ord, Der skal virkelig 18 tons vand til for at producere et ton grøn brint. Atomenergi, set af IPCC som et vigtigt værktøj til at nå vores klimamål, afhænger også af ferskvand til afkøling, men efterhånden som vandmangel stiger, atomkraftværker kan blive tvunget til at reducere deres kapacitet eller lukke ned.

Hvor der er vand

Mens det meste af vores planet er dækket af vand, kun tre procent af det er ferskvand, og kun en tredjedel af det er tilgængeligt for mennesker, da resten er frosset i gletsjere eller er utilgængeligt dybt under jorden. I mellemtiden global opvarmning fortsætter med at smelte flere gletschere hvert år og øge fordampningen, mindske vores ferskvandsressourcer.

Som følge af vandmangel, nogle dele af verden har vendt sig til afsaltning til drikkevand. Afsaltning (afsaltning) involverer fjernelse af salt og mineraler fra saltvand, normalt havvand. Denne proces sker naturligt, når solen opvarmer havet - frisk vand fordamper fra overfladen og falder derefter som regn. Tørre regioner som Mellemøsten og Nordafrika har længe været afhængige af afsalingsteknologi til deres ferskvand. I dag har over 120 lande afsalningsanlæg, hvor Saudi-Arabien producerer mere ferskvand gennem afsaltning end nogen anden nation. USA har også en række afsalningsanlæg med de største på den vestlige halvkugle i Carlsbad, CA. En ny 1,4 milliarder dollar afsalningsfabrik i Huntington Beach, CA vil sandsynligvis snart blive godkendt.

Afsaltning nærmer sig

Desal udføres normalt på en af ​​to måder. Termisk destillation involverer kogende havvand, som producerer damp, der efterlader saltet og mineralerne. Dampen opsamles derefter og kondenseres gennem afkøling for at producere rent vand. Den anden metode er membranfiltrering, som skubber havvand gennem membraner, der fanger saltet og mineralerne på den ene side og slipper rent vand igennem.

Før 1980'erne, 84 procent af desal brugte den termiske destillationsmetode. I dag, omkring 70 procent af verdens afsaltning sker med en membranfiltreringsmetode kaldet omvendt osmose, fordi det er den billigste og mest effektive metode. I naturlig osmose, molekyler bevæger sig spontant gennem en membran fra en opløsning med mindre opløste stoffer til en mere koncentreret opløsning, udligne de to sider. Men i omvendt osmose, saltere vand bevæger sig gennem en membran til en mindre salt opløsning. Fordi dette virker mod naturlig osmose, omvendt osmose kræver højt tryk for at skubbe vand gennem de semipermeable membraner. Det resulterende ferskvand steriliseres derefter, normalt med ultraviolet lys.

Bekymringer om afsaltning

Selvom desal kan være den eneste løsning for nogle regioner, det er dyrt, bruger meget energi og har skadelige miljøpåvirkninger.

"Afsaltning af havvand er en af ​​de dyreste måder at få vand på, " sagde Ngai Yin Yip, assisterende professor i jord- og miljøteknik ved Columbia University. "Det hænger bare sammen med, at det ikke er let at få salt ud af vandet. Men vi skal have vand - der er bare ingen erstatning for vand. Så det kan være dyrt. Men det faktum, at vi ikke kan overleve uden vand betyder, at det er en nødvendig omkostning."

Storskala afsalningsanlæg er meget dyre at bygge, og anlæggene bruger meget energi. Termiske destillationsanlæg kræver energi til at koge vand til damp og elektricitet til at drive pumper. Omvendt osmose kræver ikke energi til at generere varme, men er afhængig af energi til elektriciteten til at drive sine højtrykspumper. Ud over, tilsmudsning af membraner med mindre opløselige salte, kemikalier, og mikroorganismer kan påvirke deres permeabilitet og reducere produktiviteten, øger vedligeholdelses- og driftsomkostningerne.

Ifølge Yip, den mest økonomiske måde at gøre afsaltning på er at målrette mod kilder til vand, der indeholder mindre salt, såsom grundvand. "Jo mindre salt der er, jo mindre arbejde skal du gøre for at tage det ud, " sagde han. "Så fra et rent økonomisk perspektiv, grundvand ville være mere økonomisk end havvand." Afsaltning af grundvand kan udføres bæredygtigt på steder, hvor det er rigeligt. Men hvor det er faldende, opsugning af grundvand kan føre til jordsynkning, eller i kystområder, til saltvandsindtrængning af grundvandsmagasinet. Hvis der ikke er grundvand tilgængeligt, Yip føler omvendt osmose af havvand er den bedste teknologi at bruge.

Mange mellemøstlige planter, imidlertid, bruge ældre termiske anlæg, der kører på fossile brændstoffer. Som resultat, afsaltningsanlæg er i øjeblikket ansvarlige for at udlede 76 millioner tons CO ₂ hvert år. Da efterspørgslen efter salg forventes at stige, De globale emissioner relateret til afsaltning kan nå op på 400 millioner tons CO ₂ om året i 2050.

Desal har også indvirkning på havmiljøet på grund af mængden af ​​saltlage, det producerer. For hver enhed rent vand, der produceres, omkring 1,5 enheder koncentreret saltlage - dobbelt så salt som havvand og forurenet med kobber og klor, der bruges til at forbehandle vandet for at forhindre det i at forurene membranerne - resulterer. Globalt, hver dag udledes over 155 millioner tons saltlage tilbage i havet. Hvis saltlage frigives i et roligt område af havet, det synker til bunds, hvor det kan true livet i havet. En undersøgelse fra 2019 af Carlsbad-afsaltningsanlægget nær San Diego, der fortynder sin saltlage, før den frigives, fandt, at der ikke var nogen direkte indvirkning på livet i havet, imidlertid, saltniveauerne overskred de tilladte grænser, og saltlagefanen strakte sig længere ud til kysten end tilladt.

Forbedring af afsaltning

Forskere over hele verden forsøger at løse desals udfordringer. Her er et par eksempler på nogle af deres løsninger.

Vedvarende energi

NEOM er en futuristisk smart bystat på 500 milliarder dollar, der bygges i det nordvestlige Saudi-Arabien langs kysten af ​​Det Røde Hav. For at skaffe vand til de anslåede en million fremtidige beboere, it will construct an innovative solar desal system comprising a dome of glass and steel 25 meters high over a cauldron of water. Seawater is piped through a glass enclosed aqueduct and heated by the sun as it travels into the dome. der, parabolic mirrors concentrate solar radiation onto the dome, superheating the seawater. As it evaporates, highly pressurized steam is released and condenses as fresh water, which is piped to reservoirs and irrigation systems. The system is completely carbon neutral and theoretically reduces the amount of brine waste produced. NEOM, expected to be completed in 2025, claims it will produce 30, 000 cubic meters of fresh water per hour at 34 cents per cubic meter.

The U.S. Army and the University of Rochester researchers have developed a simple and efficient method of desalinating water also dependent on the sun's energy. Using a laser treatment, they created a "super-wicking" aluminum panel with a grooved black surface that makes it super absorbent, enabling it to pull water up the panel from a water source. The black material, heated by the sun, evaporates the water, a process made more efficient because of its super-wicking nature. The water is then collected, leaving contaminants behind on the panel, which is easy to clean. It can be reconfigured and also be angled to face the sun, absorbing maximum sunlight, and because it is moveable, could easily be used by military troops in the field. Larger panels would potentially enable the process to be scaled up.

European companies are developing the Floating WINDdesal in the Middle East, a seawater desal plant powered almost entirely by wind energy. The floating semi-submersible plant is being built in three sizes, with the largest expected to be able to produce enough water for 500, 000 people. The plants can be moved by sea, making them easy to mobilize for emergencies and can be deployed in deeper water where brine disposal would have less impact on marine life. Because they float, they will not be affected by rising sea levels.

Membranes

Membrane research is focused on increasing membrane permeability which would reduce the amount of pressure needed, reducing the fouling that occurs, and making membranes more resilient to high pressure.

A discovery by scientists at the University of Texas, Penn State and DuPont could improve the flow of water through membranes and increase their efficiency, which would mean that reverse osmosis would not require as much pressure. Using an electron microscope technique, the researchers discovered that the densely packed polymers that make up even the thinnest membranes could slow the water flow. The most permeable membranes are those that are more uniformly dense at the nanoscale, and not necessarily the thinnest. The discovery could help makers of membranes improve their performance.

Reverse osmosis desal is hindered when microorganisms grow on the membrane surface, slowing the flow of water. Some coatings that have been used to prevent this "biofouling" of membranes are hard to remove, so they result in more energy use as well as more chemicals released into the sea. King Abdullah University of Science and Technology (KAUST) researchers created a nontoxic coating that adheres to the membrane and can be removed with a flush of high-saline solution.

Desal without membranes

Columbia University engineers led by Yip, developed a method called temperature swing solvent extraction (TSSE) that doesn't use membranes at all to desalinate. The efficient, scalable, and low-cost technique uses a solvent whose water solubility—the amount of a chemical substance that can dissolve in water—changes according to temperature.

At low temperatures, the solvent mixed with salt water draws in water molecules but not salt. After all the water is sucked into the solvent, the salts form crystals that can easily be removed. The solvent and its absorbed water are then heated to a moderate temperature, enabling the solvent to release the water, which forms a separate layer below. The water can then be collected. Yip explained that the process is designed to deal with very salty water, which reverse osmosis cannot handle. For eksempel, the water that comes up during oil and gas extraction can be five to seven times saltier than regular seawater. The textile industry also produces very salty water because of the solutions it uses to dye cloth. According to Yip, TSSE is not the best way to obtain drinking water, but it could help replenish our water resources for other needs.

Brine

Brine impacts can be lessened by how much brine is discharged and how the desal process is carried out.

Stanford University researchers have developed a device that can turn brine into useful chemicals.Through an electrochemical process, it splits the brine into positively charged sodium and negatively charged chlorine ions. These can then be combined with other elements to form sodium hydroxide, brint, and hydrochloric acid. Sodium hydroxide can be used to pretreat seawater going into the desal plant to minimize fouling of the membranes. It is also involved in the manufacture of soap, papir, detergents, explosives and aluminum. Hydrochloric acid is useful for cleaning desal plants, producing batteries, and processing leather; it is also used as a food additive and is a source of hydrogen. Turning brine components into chemicals that have other purposes would decrease brine waste and its environmental damage, as well as improve the economic viability of desalination.

Diluting brine can also lessen its impacts. "You take more seawater, and you premix it [with the brine] in an engineered reactor, " said Yip. "Now the salinity of that mix is not two times saltier than seawater. It's still saltier than seawater, but it's lower. And instead of discharging it at one point, you discharge it at several points with diffusers. These are engineering approaches to try to minimize the impacts of brine, " forklarede han.

Other solutions for the drought

Despite improvements in desal's environmental and economic profile, imidlertid, it is still an expensive solution to water scarcity. This is especially so given that most water in the U.S. is used for agriculture, taking showers, and flushing toilets. Newsha Ajami, the director of urban water policy at Stanford, said "I disagree with using tons of resources to clean the water up just to flush it down the toilet."

Water recycling

Paulina Concha Laurrari, a senior staff associate at the Columbia Water Center, said "Water reuse definitely has to be an important part of the solution. Our wastewater can get treated, either to potable standards, like it's been done in other parts of the world and even in California, or to a different standard that can be used for agriculture or other things."

Recycling the approximately 50 million tons of municipal wastewater that is discharged daily around the U.S. into the ocean or an estuary could supply 6 percent of the nation's total water use. Recycled water can be used for irrigation, watering lawns, parks and golf courses, for industrial use and for replenishing aquifers. The House of Representatives is considering a bill that would direct the Secretary of the Interior to establish a program to fund water recycling projects and build water recycling facilities in 17 western states through 2027.

The technology to recycle water has been around for 50 years. Wastewater treatment facilities add microbes to wastewater to consume the organic matter. Membranes then are used to filter out bacteria and viruses, and the filtered water is treated with ultraviolet light to kill any remaining microbes. The water can be used for agriculture or industry, or it can be pumped into an aquifer for storage. When it is needed for drinking water, it can be pumped out and repurified. If the water is for human consumption, some minerals are added back in to make it more drinkable.

Waste not

Every year in the U.S., approximately 9 billion tons of drinking water are lost due to leaking faucets, pipes and water mains, and defective meters. President Biden's $1.2 billion infrastructure plan includes substantial sums for upgrading clean drinking water and wastewater infrastructure.

I USA, 42 billion tons of untreated stormwater enter the sewage system and waterways and ultimately the ocean each year. This means that the rainwater that could soak into the ground to replenish groundwater supplies is lost. Green infrastructure, such as green roofs, rain gardens, træer, and rain barrels, would reduce some of this water waste.

Sensible water use

It's also important to figure out how to put the water that's available to the best use in a particular area. "For eksempel, having a better planning strategy of what is the best use for water, like what to plant where, " said Laurrari. "Instead of using it, sige, for alfalfa, how do we use it for higher value crops? Or even tell farmers, "I will pay you not to use this water' and the state can have it to replenish our aquifers or to source cities or something else."

Determining the most reasonable and economical uses for water would help everyone understand and appreciate its true value. "In some of these places where they're having droughts, there are still people who are watering their lawns, and happily paying the fine, " said Yip. "So really, there's a mismatch between what is happening and what the reality is. We need to adjust our activities such that we are not putting that kind of a human-imposed strain on the water supply. We need to be thinking about how we make drastic wholesale changes to the way we organize our activities that actually make sense."

Israel's example

Israel is located in one of the driest regions of the world and has few natural water resources, imidlertid, it is considered "the best in the world in water efficiency" according to Global Water Intelligence, an international water industry publisher.

Israeli children are taught about water conservation beginning in preschool, and adults are reminded not to waste water in television ads. Low-flow showerheads and faucets are mandatory, and Israeli toilets usually have two different flushing options for urine and bowel movements. The country adopted drip irrigation, which uses half the water than does traditional irrigation while producing more yield. Israel also resolutely attends to small leaks in pipes before they become large. Ud over, 75 percent of its wastewater is recycled, more than that of any other country. And because Israelis pay for their water themselves, they are careful about how much they use and readily adopt water-saving technology. Som resultat, it's estimated that the average Israeli consumes half as much water each day as the average American.

Israel began desalination in the 1960s. Today it has five desal plants with two more on the way and will soon get 90 percent of its water from desal.

While Israel has invested a lot of money in desal, it has also made huge investments in water awareness and water efficiency. These other measures enabled the country to delay building desal plants and build them more economically and smaller than they would otherwise have needed to be because the citizens were already conserving water.

101 things you can do

Here are 100 ways to conserve water.

And one more. "Become more actively involved with the decisions that government makes in terms of investments of infrastructure, " said Laurrari, "Because yes, you can conserve water at home, but what is really going to matter is what's done at the larger scale by politicians. So having a more active role, knowing where your water comes from, and what your local issues are is important."

Denne historie er genudgivet med tilladelse fra Earth Institute, Columbia University http://blogs.ei.columbia.edu.