agua potable del mar.pdf

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SCIENFEstablished 1845 .ERIC March, 1957 Vlume 96 Number

Fresh Water from Slthe thirst of (lzan w preses ard up t urally

(alable spply f rs wat. It s easy eng t esalt

sea atr, bt t mst be e ceply abuatl a s

In the face f ur increasing ctrl

ver nature, it is irnic that we havesteadily been sing gru wth re

gard t ne f mankins mst vtalneeswter. Over much f the civiliedwrld, water shrtage is a grae ngrwing prble

T be sue, water has always been amajr cncern f man. The children fsael ecvered thei fath in G oly

when ses smote the rck an pr-uce wter. Egypt se an fell withthe w f the Nile and eve tay isacing its hpes fr the future n pansto evel the resurces f tht greatriver. Few things have mre pwerfullyinuence the curse f the humanrace thn the perennial search fr freshwater

But n tas wrld the need fr wa-ter has becme acute in many areas. nsome ard cunies the per capita cn-smtion f water, thanks t imprvedsanitatio, has suenly risen fm tw

o thee uts per da t 0 r 0 more. Unerevelope cuntries seekingto rase ther stanar f living industraliztion an irigtin nd them-selves with huge new needs fr waterslies. Een ur wn waterfavredoutry s beginnng t be cncerne,with may commntes alreay facingshrtages. The rblem is widespread,ot cnne t lcalities such as therghtstriken Suthwest. Since 900the U. S. hs ncreased its cnsumptinf water almst sevenfld. By 97 urwater requieent will have nearly du-

bled again. We shall then e using aut7 per cent f the ttal supply f natural

y Dv k

fresh water i ur rives lakes, pringsn wells. Many areas will he reachethe imit f their lcal resources. The remaning 7 per cent f the ttal suppl,argely strea water, will probbl bephibitie expensive t collect, streand distribute to the plces whee it iseee.

Thee e two mjr steps we cte. The st, most imprtt for

the es imeitely hea, is t reuce our ais wste of water. Amoothe things, we can educe the olltio f our strems, ecoer se wteb pricatio, cature oowters dmanae the inustril use of wter mecaefully. Sme inustries have lreyshw what cn be accomlshe alonthis line. Fr instance, the Kser steelmill t Fntana, Calif, hs rece thecnsumptin f water i mnufactigsteel frm the average f 65,000 gallospe tn t nly 00 galns per ton, yrecirculating the water t uses.

But in the last analysis we must alsoincrease the water suply itself. Theeis ne wa this can be achieve n a very

large scale: y cnverting salt water.There is plenty f water n the oceans,ad mans ingenuity is certainl equal tothe task f cverting sea water ther saine waters int fresh water at areasonabe cst.

ive years ag the . S. Cngress,recgniing the gravity f the situatin,passed a Saine Water Cnversin Actwhich authzed a prgram f researchand evelpment nder the administra-

tin f the epartment f the nterr acrdinated cmpaign f investigatin is

nw under way in the . S., with coeratin frm rad. Goemet ecies, privte inustries other stittins re engage in more th soef lbratry investigtios few ofwhich have prgresse to ilotlanttest. The sltwter cnerso metosuer iestigtion inclue ume ofwellknwn rocesses an several om-letel ew ieas.

Fresh wter is cmmnly dene swte cotin less th ,000 parts

er illo of sslve salts. Bt hofresh the wte nees to e epens nthe se to be mae f it. Dring wte,acoig to the U. S. Pulic HealthServie stas, shoul have morethan ,000 n referably less tha 500rts e million. In eeral siity fwte for agrcultural tio sholbe o more tha 200 ts er millio,the llowable cncettio deeigo the secic salts it cotis For some

idstil oses, suh s ooin shi, ene sea wter will o othe other han, in highese oilersit my be necessar t hve lmost ewater cntaining nt re tha two orthree pats per milli f salt. Thus theecnomic fesibilt of sewter coversion epens n the use t whi thewater is t be put: fr some osesthe cost my be esole, for othesnot. For the uice of the researchrogrm a sey of the varous iustries wter eqiemets is uretlneeed.

The salinity f the wters avaiable frcnversi varies grety. The cens re

37

1957 SCIENTIFIC AMERICAN, INC


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0

0 0 0 00

0 0 0

000

0 1

0 0 0 0 0 0 0 O 0 .=-- .---I __--

r- - -

" " I

0 0 " \

o 0 ( 0 o

0 o 0 " 0 -- 1

-- - -

--=,

--

THERETCA ENERGY needed to epara ater moeue

from at on a be auated from experment hon here

the ame preure and temperatre more ater moeue o

nto he vapor phae from freh ater left) than from at ater

(center). By ompren the vapor from the at ater o he am

denty or vapor preure a that of the freh ater vapor, t an be

jut made to ondene to freh ater right) The enery needed

equa to the enery that bnd the ater moeue to at on

irly uniform, verin bout 35,000rts per milion of issolve slt. But in

the Persin Gulf it is nerly 40,000 rtser million in hespeke By bout5,000; in the Blti Se only ,000. Anyter less slty thn the oens but ithmore thn 1,000 rts per million iscle brkish

ommon slt, soim hlorie, -ounts for most of he sltiness of seter. Hoever, se ter ontinsmll mounts of mny other slts

rI ,'. ::TAM BIL

j

some 44 issolve elements in ll [seetable 2

Wht is require to eslt te hesi fcts re simple enouh A sltissolve in ter is seprte into ions- in the se of soim horie thepositively hre soium ion n thenetively hre hlorine ion heions re boun to ter moleules bytheir eletri hres. The problem,then, is to pull the ter moleules nthe ions pt

We n mke lultion of homuh enery this takes. onsier to

sele sks, one prtly le ith pureter the other ith se ter. At roomtemperture, sy, ertin mount ofthe ter in eh sk evportes intothe unlle prt of the vessel, n thisestblishes ertin equilibium vporpressre. The vpor pressure in the ontiner of se ter is loer thn in thesk of pure ter, beuse its termoleules, bein boun to slt ions, o

W

TSTAGE ST ue h aten heat reeaed by ondena

ton of ater vapor at eah tae o eaporate the brne on n

from the next tae The rt tae left) provde a ondeer for

the team from a trboenerator The team colored dots) on

deed to ater by he ooer at aer ray) on n throh

the evaporator tube. The at ater heated by the ondenato

of the team, eparate no ater vapor colored dots) and drop

et of brne black dots) n the team het above the evaporator

38



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. ' ; ' : :

'. , '.< ::: : .' ::.WORK

FESH

ENERGY needed to evaporate freh ater rom at

aer i neceariy much reater than the heoretica opposite

ae) which aume a bare pereptibe rae of evaporaion and

per ent ecieny n onverin of enery Smpe tiation

(left) may require , ime a m eey omeio tiation right i more ecient oreion o the vaor raie

i emperature; the uerheate vaor i pipe throuh he boier

where t ondene yiedn it atent heat to evaporate more brine

evaprate as easily Nw the exraergy needed t separate water mle-ules frm the ins can easily be meas-ured: it is jst equal t the energy weave t spply t cmpress the vaprrm the sewater bttle s that itsressre is the same as that f the vaprin the bttle f pre water see digrms oposte pge J Fr sea water f aver-e saltiness this eergy amnts t 28iwatt hrs per 000 gallns

ut this is merely te minimum

amunt needed t tip the scae f vaprpressre eiibrim s that evapraicn prceedt a barely perceptiberate T raise the rate f evaprati ta usef leve reires a great dea mreenergy. Frthermre there are fndmenta limitatis n the eciency fcnversin f eergy t sef wrk iany prcess r machine; cnseqenty cnsiderae prt f the energy we feeint the machine is unavidably wastedn ractice t separate water rm salt

by evapratin in a simple still akes000 times the amnt f energy givenbve as the theretica thermynamicreqiremetthat is abt 2800 kiwatt hrs f eergy (in he fr fheat) er 000 gallns f water Btthere are f crse mch mre ecietrcees tan simple sigestage dis-iati It is estimated that se f thercesses nw nder stdy may redcethe energy reqiremet t abut fr rve times the thermdynamic minimm

BIN

Here the brine roet are opped y the onia bale pae and

a to the bottom of the team het, drann out at ef The water

apor ruhe out at the to an o the riht nto he eond taer ge p a aora te a aer own n from

the hrd tae and ondene to freh water Becaue the tempera

ture of he at water at eah tae i ower the boiin point mut be

brouht ower; th acomihe by reucn the preure at eachae a ndcated by nreae ize of team het fr om eft o riht

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EXPT T dened y ara Teke are

hon on rootop at e York nverty. n unt at et ater

vapor rn rom haow poo o ea ater ondene on under

de o nned ga or pat roo. nt at rht are "opn

t, h ea ate oed trough a ak k tted at

rght ane to un' ray th onderae nreae n output.

r abut 10 t 1 kilwatt hurs per1000 gallns

Let us see what evices we ca em-ply t enhance the eciency f istilla-tin The simple still evaprates water tsteam at atmspheric pressure But nwif we cmpress the steam t a fewpuns per square inc abve atmspheric pressure the temperature f thesteam will rise slightly an we can usethis ae heat t evaprate mre wa-

ter. n ther wrs we have increasethe yiel f istille water withut feeing mre heat t the system merelypening a little energy t rive a me-chanical cmpressr This methcalle cmpressin istillatin re-uces the ttal energy reuirement frm2,800 kilwatt hurs t abut 00 kil-watt hurs per 1,000 gallns f watere iea is mre than 100 years l itws st patente by a renchmanPierre Peetan in 1840. The U S armeservices use it extensively uring WrldWar fr upplying wter t trp in

areas lacking reay fresh water

0

n the past three years interest i cm-pressin istillatin ha been heighteneby an exciting new system t was e-vise by Keneth C Hickman a cl-labratr in the gvernmental researchprgram n essence what Hickman hasae is a simple evice fr increasingphenmenally the rate f heat transfert the water: namely spreaing it utin thin lm The salient feature f hisdevice is a rtating rum sape sme

thing like a chil's musical tp [see - pe Salt water at a tem-perture f 12 egrees ahrenheit isspraye n the insie surface f therum The cetrifugal frce f therum's rtatin spreas the water verthis surface as a very thin turbulentlm Sme f the water evaprates theunevaprate brine is cnstantly rawn thrugh a scp The water vaprleaves the rum ia a pipe where a blw-er cmprees it slightly raising it tempaue The warme apr then circu-ate t th utide ufa f the um

ther it ndnse ad ie up its latent

heat the rum shell transmits this heatt the m f water n its insie surfacespeeing evapratin The cnensevapr is cllecte a istille water

Thi system is recmmene nt nlyby its sipliciy an lw pwer reuiement but als by anther great avan-tage the lw perating temperatures(1 t 10 egrees istillatiprcesses using much higher tempera-tures the seawater salts are epsite

n metal surfaces as scale Scale frmtin which mpees the transfer f heatt the water is the greatest single enemyf erts t bring wn the cst f isstillatin n Hickman's apparatus littlescale frms because f the lw peratingtemperatures he main imitatin f thertary cmpressin still is that uch astill bviusly must be limite i size

here are ther highly prmising attack n the prblem f imprving

the eciency f isillatin ne f themt peful i the multistae still n

this system the latent heat reease by



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the condensation of th evaporated watat ch stage is used fo the next stageprovidig a chain ect or example nhe rst stag sa water is evaporated toseam at atmosphric pessure or hgher;the stam passs to coils in a seconevaporator condnses there and s col-lect as distlld wate in condensing releass ts latent hat to evaporate sea

ater in that conainer and so onhrough a nubr of stages [see di-gm pges 8 nd 9 Vacuumumps kep each succssv evaporatornder lowe prssure so that ts waterboils at a lowr tmprature Such a sys-tem could opeate on the xhast(waste) stam fm an lctricitygen-eratn plant The main poblm s toprevent the frmation of scale and themethod is promising enough to justifyhe cosderable resarch being con-ductd on that aspect If scale can beiminatd a 20stage still may be fasi-be and t might prodce fresh water atbewen 30 and 0 cents per 1000 gal-lons

Thr is a comparatvely nw distlla-ion method whch uses a sdden reduc-tion of essure istead o heat to evapo-rae water At a givn temperatur theaont of water vapor that ar can holdepends on the ar pressre If salt watris ed into a closed chamber in whichthe pressre is lower han outside parto h water will "ash to steam Thsmethod is being used extensively in

mtstag sysems. In rench Westica egneers are attempting to d-veo a as evapoato whch wll op-eae o the temerare dieences be-een upe and oer levels of theocean sing te colde atr to chil andcondense the vapor from the ash ham-ber The U S Department o the Int-io and e University of Californiahav been oring on slar systes

Varios oter dsilation proceseshave been stdied includin distllationat "supercriical teperares and presses I is not possibe to discss here

a the distillaion idas tha are ndestdy But even ths brie descrtion ohe ork n ogress mst make cleartha h pospects fo distllng salt waterto resh at a redc cost are brht

The sn which showers us wth a vastbndance o energy free of charge

is esonsible fo our natural suplyo fresh water by ts evaporaion ofthe seas Is there any way to harnesssoa enery to provide s with more?e ingnios solar stills have benroosed

e smlest fr f solar stll s a a

t BN

. '

AR T o type under deveopment by Department o the Interior i diarammed

Water vapor evaporatn rom at water condee on nderide o a or patic pate

and drain nto tter on either ide o nit p diagm. Incmin ea water middlerm ow nto rt unit and retr thrh econd tet pipe o reh water andrne are mmered in ncomin ea water i order to preheat it by heatexchane eect

rawn a bottom how the pan o a are intaation whch mht cover many acr

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7/9

IONEXCHANGE SEPARATION of fresh water from sal employs

membranes which are alernaely permeale o he sodum or

chlorine on and impermeable o he oher. B applyg an elecurren across he sysem (top diagram), e sdum ons are

raced oward oe end of he sysem and he chlorine ons oward

he oher The ions are hus conenaed in alernae ells, leavn

desaled waer in he ells eween bottom diagram). he rnecan hen e drawn o va one ppe and desaled waer va anohe

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FRSHAT

BRINE OUT

ROTATINGDRUM ILL incororates rincile of comression

distillaton see diagram on pge 39). Salt water s jetted againsthe hot nner surface o the rum and sread ot in thin lm by

4

CMPD M

: '

L WATER

cenrfuga force. Waer vaor (colored dots) s scked ot and

copreed by blower t rgh and then codene on ouer srfaceof dru Waste brine s scoope nto dran a ef nde du



9/9

e erveg ones s desaled ee di on e 43].

s roess, alled elerodalyss, ise deeed eea e S e Neerla ga ae o S ra ease oe ee er eremen oeso l eoomal romsg for -ve ea aer oers good ros-

e o esalg ras aesGeorge W Mry, a e -

ersy o Oklaoma, as roose sge eleral arge of os rm a re sead o an ele rre as e drvg er o s osg e memrae. Resear o s eg doe a e SoerReear Ie rmgam

oer romsg memrae meods ased o e enomeo of osms every sde f emsry a sa slo s dvded rom essa oe a osmo memrae, mermeale o sa o oaer aer asses rog e memae no e more aly ol edg o eqaze e saly n o sdesf e memae s roess ae reversed y ag o he more oneraed oon a meanal ressrereaer han e osmo resre ag he aer amos o 350ods er sqae eee reshaer and sea aer) a s y, ereverse osmoss ores aer roghe memrae o o e a soo

le e memrae os a e sals Cares E. Red o e versy Florda as demosaed a memraes made o eose aeae0 95 er e o e sal a e reoved from sea aer oe ass

o oer roeses oerromse are eg deeled. Oe earao o om aer freezg e Carrer Coao, er otract e Deame of e Ie, s og resear a verave omao of reezg adevarao ad smlar develomes

ave ee reored y Iae a Ygosava e oer omg roess searao ae olg orga sove o e sas

We are already overg sal wae re r ome roses I

ve 5 yeas e sod e ale ove eaoale s or a mhlager me se dsralses rs. ed ma l d nooe ma solos o e oemd ll eg maarg aer a me deve ad o a saleagg om smll eold sl olrge mal ad nsral la

MULI-ROTOR SI me of ei roin rm of ye sown on oosie e

ec ei fee in imeer is ner eeomen Ber Mnufcturin Comny inCmbrie Ms Rory rm ciees i re of e rnfer n ts i eien_

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