Clean wa­ter

Com­ment rendre l’eau po­table ?

Vocable (Anglais) - - Sommaire -

Se­lon l’OMS, 3,6 mil­lions de per­sonnes, soit 7 par mi­nute, meurent chaque an­née du manque de sa­lu­bri­té de l'eau. De­puis le 28 juillet 2010, l'ac­cès à l'eau po­table est re­con­nu comme un droit fon­da­men­tal par l'ONU. Trou­ver une so­lu­tion d’épu­ra­tion fa­cile et peu oné­reuse est de­ve­nu une prio­ri­té. Une équipe de cher­cheurs de Prin­ce­ton pense avoir trou­vé un nou­veau pro­cé­dé ré­vo­lu­tion­naire.

The world’s thirst for clean drin­king wa­ter is vast and gro­wing. It is al­so uns­la­ked, par­ti­cu­lar­ly in poor coun­tries. The World Health Or­ga­ni­sa­tion es­ti­mates that more than 660m people re­ly on what it calls “unim­pro­ved” wa­ter sources. A quar­ter of this is un­trea­ted sur­face wa­ter. Mo­reo­ver, even wa­ter that has un­der­gone at least some treat­ment may not be po­table. Across the pla­net, 1.8bn hu­man beings drink wa­ter conta­mi­na­ted with faeces. All this pol­lu­ted wa­ter spreads di­seases such as cho­le­ra, dys­en­te­ry and ty­phoid. Eve­ry year, more than half a mil­lion people die from wa­ter­borne diar­rhoea alone. As they des­cribe in a pa­per in Na­ture Com­mu­ni­ca­tions, ho­we­ver, Ho­ward Stone of Prin­ce­ton Uni­ver­si­ty and his col­leagues have an idea for a new and cheap way to clean wa­ter up by mixing it with a sub­stance nor­mal­ly re­gar­ded as a pol­lu­tant in its own right—car­bon dioxide.

TECH­NIQUES

2. There are ma­ny exis­ting ways to make wa­ter safe to drink, but each has draw­backs. The first step is usual­ly se­di­men­ta­tion: store the stuff in ponds and let as much of the muck as pos­sible drop out un­der the influence of gra­vi­ty. But that

can­not cleanse wa­ter of mi­nus­cule, buoyant par­ticles, in­clu­ding ma­ny bac­te­ria and vi­ruses, which will not set­tle. These have to be re­mo­ved by a se­cond pro­cess: fil­tra­tion.

3.Fil­te­ring wa­ter may be done through po­rous mem­branes, but that re­quires pres­sure, and thus needs cost­ly pumps. Al­so, the mem­branes foul qui­ck­ly, so re­quire frequent re­pla­ce­ment. Fil­tra­tion through beds of sand needs no mem­branes, but does need che­mi­cals cal­led floc­cu­lants to per­suade pol­lu­tants to coa­gu­late, so that they can be caught by the fil­ter. An al­ter­na­tive, “slow sand” fil­tra­tion, em­ploys the layers of al­gae and bac­te­ria that de­ve­lop on wet sand grains to re­move pol­lu­tants. It thus re­quires fe­wer che­mi­cals. Slow-sand fil­ters must, though, be re­fur­bi­shed re­gu­lar­ly. And both sorts of sand fil­tra­tion miss up to 10% of harm­ful bac­te­ria.

A NEW IDEA

4. Dr Stone’s al­ter­na­tive is to aban­don the idea of fil­tra­tion al­to­ge­ther. Ins­tead, he plans to ap­ply a phe­no­me­non cal­led dif­fu­sio­pho­re­sis to the pro­blem. When CO2 and wa­ter meet at the li­quid’s sur­face they react to make car­bo­nic acid. This is a so­lu­tion of hy­dro­gen ions, which are po­si­ti­ve­ly char­ged, and bi­car­bo­nate ions, which are ne­ga­tive. The new­born ions then dif­fuse away from the sur­face and in­to the main bo­dy of the wa­ter. That creates a gra­dient of io­nic concen­tra­tion per­pen­di­cu­lar to the sur­face. Dr Stone’s in­sight was that, be­cause the gra­vi­ty­re­sis­tant par­ticles which need to be re­mo­ved al­most al­ways have ei­ther po­si­tive or ne­ga­tive sta­tic-elec­tric charges on their sur­faces, their in­ter­ac­tion with an ion gra­dient of this sort, which is it­self com­po­sed of char­ged par­ticles, could be used to move them around.

5.He and his col­leagues the­re­fore crea­ted an ex­pe­ri­men­tal ap­pa­ra­tus through which a chan­nel of wa­ter flo­wed in pa­ral­lel with two chan­nels of gas, one on ei­ther side of it, se­pa­ra­ted from the wa­ter chan­nel by gas-per­meable mem­branes. One of the gas chan­nels car­ried CO2. The other car­ried air. CO2 thus dis­sol­ved in­to the wa­ter on one side of the stream, and out again on the other side, en­te­ring the airs­tream and kee­ping the gra­dient constant.

6.As the team ho­ped, this ar­ran­ge­ment cau­sed sus­pen­ded par­ticles with po­si­tive sur­face charges to concen­trate to­wards the CO2 side of the wa­ter stream, and those with ne­ga­tive sur­face charges to concen­trate to­wards the air side, lea­ving the centre of the stream more or less par­ticle-free. In a wor­king sys­tem it would sim­ply be a ques­tion of split­ting the wa­ter stream in­to three as it left the pro­ces­sor, with the two ou­ter branches being re­cy­cled and the in­ner one tap­ped and pi­ped to consu­mers.

RESULTS

7. Dr Stone’s ap­pa­ra­tus re­mo­ved all but 0.0005% of the tar­get par­ticles. And it used less than a thou­sandth as much ener­gy to do so as mem­brane fil­tra­tion would have re­qui­red. A full-scale ver­sion would not need ad­di­tio­nal che­mi­cals beyond the CO2. And it should, Dr Stone thinks, be ea­sy to main­tain. As to the ne­ces­sa­ry CO2, he ima­gines this would come from po­wer sta­tions and other in­dus­trial pro­cesses, such as ce­ment-ma­king, that pro­duce the gas in large quan­ti­ties as ex­haust. This would res­trict dif­fu­sio­pho­re­tic wa­ter plants to in­dus­trial ci­ties— but, since such ci­ties are huge sources of de­mand, that is hard­ly a pro­blem.

A new and cheap way to clean wa­ter up.

(CHINE NOU­VELLE/SIPA)

Ye­me­ni chil­dren col­lect clean wa­ter from a cha­ri­ty wa­ter pump du­ring a rain as they face clean wa­ter shor­tage in Sa­naa, Ye­men.

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