Greater Reuse of Industrial W a t e r Seen By 1975 industrial w a t e r requirements will be 1 7 0 % g r e a t e r than in 1950. Fresh water from the ocean and more rain will help, but industrial reuse will b e a major factor RICHARD D . H O A K Me//on institute, Pittsburgh E DAY i s THindustrial
13, Pa.
APPROACHING w h e n m a n y establishments will n o longer enjoy the luxury of using fresh water o n c e a n d t h e n discharging it as waste. T h e reason for this rests u p o n the prediction t h a t b y 1 9 7 5 industrial water r e q u i r e m e n t s will b e 1 7 0 % greater t h a n t h e y were in 1 9 5 0 . I n soime r e g i o n s w a t e r supplies are n o w biirely a d e q u a t e t o s u p p o r t existing industry. It i s t h u s evident t h a t d r a s t i c measures for conserving w a t e r will b e necessary to provide for industrial expansion w h e r e w a t e r is n o t a b u n d a n t . Pure waste of water in industrial plants i s m u c h greater t h a n m a n a g e ment realizes. It is easy t o find s t r e a m s of w a t e r r u n n i n g to the sewer, a p p a r ently b e c a u s e it is too m u c h trouble to sh»ut a valve. I t is c o m m o n t o see many times as m u c h w a t e r b e i n g used than an operation actually r e q u i r e s . If there i s a w a s t e - t r e a t m e n t p l a n t , this practice is d o u b l y wasteful b e c a u s e it increases t r e a t m e n t costs. Loss of water t h r o u g h carelessness c a n r e a d i l y be prevented b y a p r o p e r e d u c a t i o n a l program. A survey of industrial use of w a t e r by The Conservation F o u n d a t i o n disclosed a 36*% increase i n i n t a k e from 1939 t o 1949. T h e survey s h o w e d also t h a t 6 % of the p l a n t s took a b o u t 8 0 % of the total water used b y industry. W^olman h a s p o i n t e d o u t t h a t t h e greatest return p e r unit of e x p e n d i t u r e could p r o b a b l y b e a t t a i n e d b y a d o p t i o n oE conservation practices b y t h e s e very k r g e users of w a t e r . I t is significant, irt this connection, t h a t t h e n e w Fairless Works of U n i t e d States Steel provides for reuse o f some 6 0 % of its process water. The number of locations at w h i c h industry is now reusing w a s t e w a t e r is shown in a U C L A study. Although tine figures should n o t b e r e g a r d e d as precise or c o m p l e t e , t h e y i n d i c a t e t h e extent of c u r r e n t r e c l a m a t i o n practices. It is instructive t h a t two-thirds of t h e instances of industrial r e u s e occur east of the Mississippi River. I t i s almost i n e s c a p a b l e t h a t t h e overriding factor in industrial r e u s e of water will be economics. W h e r e it c a n b e d e m o n s t r a t e d t h a t economies c a n b e realized by reuse, c o m p a n i e s 1278
will generally w e l c o m e t h e i d e a . W h e n a n e w plant is d e s i g n e d , provision c a n often b e m a d e for successive use of w a t e r w h i c h will r e d u c e costs. Unfortunately, this m a y n o t b e the case in older plants w h e r e intricate piping a n d lack of s p a c e result in difficult a n d costly p r o b l e m s . Aside from economics, p r o b a b l y as m a n y p r i m a r y factors will n e e d to b e considered as there a r e special uses for process w a t e r . T h e r e a r e m a n y industrial operations w h e r e w a t e r is in c o n t a c t w i t h t h e p r o d u c t . In s o m e of these instances it m a y be t h a t an increase in t o t a l dissolved solids will limit t h e r e u s e of water; in others a p a r t i c ular ion m a y b e t h e limiting factor. W h e r e concentration of dissolved salts or specific ions limit r e u s e , a m a t e r i a l b a l a n c e will show h o w often fresh w a t e r m a y b e recycled b e f o r e it m u s t b e discarded. Corrosion of e q u i p m e n t is sometimes a p r o b l e m w h e r e w a t e r is recycled. T h e large v o l u m e of b r a c k ish w a t e r u s e d for cooling shows t h a t corrosion c a n b e controlled, especially in n e w p l a n t s d e s i g n e d for its u s e . G r o w t h of slimes in w a t e r systems can b e troublesome, n o t a b l y in p a p e r mills, b u t sliming can b e p r e v e n t e d by chlorination. Tastes a n d odors must b e almost completely e l i m i n a t e d in b e v e r a g e m a n u f a c t u r e a n d food processing, b u t t h e s e industries ordinarily o p e r a t e e q u i p m e n t to p r o v i d e close control of malnavors. Ion exchange is being u s e d to a n increasingly g r e a t e r extent in some industries, e.g., p l a t i n g , to recover values previously w a s t e d a n d to r e claim process w a t e r . W h e n reused w a t e r is finally d i s c a r d e d it usually will h a v e to b e t r e a t e d b e f o r e it can b e wasted. I n some cases i t might b e suitable for irrigating agricultural l a n d , b u t caution must b e exercised in u s i n g this disposal m e t h o d . E x c e p t for nitrogen a n d p h o s p h o r u s , vegetation t a k e s u p relatively insignificant a m o u n t s of the m i n e r a l constituents of irrigation
w a t e r . As a result, u n u s e d salts often a c c u m u l a t e i n t h e soil a n d m a k e it u n suitable for g r o w i n g crops. W h e r e waste w a t e r is p r o c e s s e d in a t r e a t m e n t p l a n t p r i o r to r e u s e , it c a n b e c o n d i t i o n e d to o v e r c o m e a n y d i s a d v a n t a g e s of successive use. T h e F o n t a n a p l a n t of Kaiser Steel p r o v i d e s a n i n s t r u c t i v e e x a m p l e of intensive r e u s e of w a t e r . A n ingenious w a t e r r e c o v e r y system p e r m i t s m a n u f a c t u r e of steel w i t h only 1 4 0 0 gallons of w a t e r p e r t o n as c o m p a r e d w i t h t h e n o r m a l 6 5 , 0 0 0 gallons. U s e by B e t h l e h e m S t e e l of 5 0 million g a l l o n s p e r d a y of t r e a t e d sewage f r o m Baltimore is a well-known i n dustrial r e c l a m a t i o n m e a s u r e . This o p e r a t i o n h a s l e d to other industrial uses of effluents from s e w a g e t r e a t m e n t plants. T h e l a r g e s t s i n g l e i n d u s t r i a l u s e of w a t e r i s for cooling o p e r a t i o n s . In general, t h e larger t h e intake, t h e greater the use of w a t e r for this p u r p o s e . T h e e n o r m o u s v o l u m e of w a t e r t h u s u s e d w o u l d s e e m to p r o v i d e a n obvious p o t e n t i a l for conservation. R e c l a m a t i o n of w o r m w a t e r by circulation t h r o u g h c o o l i n g t o w e r s or spray p o n d s i s widely p r a c t i c e d . Successive use of w a t e r in h e a t e x c h a n g e r s w h e r e progressively h i g h e r w a t e r t e m p e r a t u r e s a r e p e r m i s s i b l e can result in w a t e r e c o n o m y . C o o l i n g w a t e r can b e c o n s e r v e d b y u s i n g closed h e a t exc h a n g e r s i n s t e a d of b a r o m e t r i c cond e n s e r s ; a l t h o u g h closed c o n d e n s e r s r e q u i r e a g r e a t e r flow of w a t e r , it is n o t contaminated and can b e reused. C o o l i n g towers w e r e a v o i d e d a t o n e p l a n t w h e r e w a t e r w a s w i t h d r a w n from a l a k e f e d b y a s t r e a m periodically affected b y d r o u g h t s ; t h e p r o b l e m w a s solved by t a k i n g cooling w a t e r from one e n d of t h e l a k e a n d r e t u r n i n g it to the other. In a n o t h e r case irrigation w a t e r w a s u s e d first for cooling. Steel mills c o m m o n l y p u m p to g a s - w a s h i n g e q u i p m e n t t h e w a t e r u s e d for cooling blast furnaces. Ground water supplies have been augmented by pumping
Number of Instances in Which Specific Measures Have Been Taken to Conserve or Reuse W a f e r by Location Region East of Mississippi R. West of Mississippi R.
Purpose Irrigation 14 234
Industrial 102 58
CHEMICAL
AND
ENGINEERING
Recharge 81 230
NEWS
u^wgmigummtïwmmmMmmmmmm fcsrabjsltmeiiMrepoiting
ZOImilliônlBlriibfêfOTl on
BRACKISH WATER 13413
wmmmmm totmsmsm RECIRCULATED WATER
mmBam
mmm
PublicâWnfei
mmmm
t7MS
imtaKe
•Ki Total Water Intake 2,784 §\
billion gallons
from wells in the center of t h e field during peak demand; during off-peak hours wells around the perimeter of the field are p u m p e d to the interior wells for recharge. These examples illustrate only a few ways to reclaim cooling water. Local conditions will govern possibilities for reusing water economically, b u t management is be coming aware of t h e importance of conservation. How Feasible Is * res h W a t e r f r o m t h e Ocean?
Those who suffer from occasional or continual scarcity of water have thought longingly of the inexhaustible ocean as a source of supply. The difficulty has always been to find a practical way to get rid of the salt. Pure water can be m a d e from ocean or brackish water in several ways, and this is being done where the high cost can b e borne. But where large quantities of relatively pure water are needed, as for irrigation, currently available methods are much too expensive. As described b y C. B. Ellis, president, Atomic Power and Chemical Corp., the schemes that have been proposed for taking the water away from the salt range from the fanciful to the fan tastic. Among the latter is the sug gestion that icebergs b e towed from Antarctica to the California coast. A fanciful idea relates to culturing on a grand scale certain marine organisms which concentrate salts in their cell VOLUME
33, N O . 13
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fluids. By choosing a series of organ isms w h i c h would d o this in water suc cessively less salty and filtering t h e water between stages, it might b e possible t o reclaim water of good quality. This is considered a far fetched scheme today, b u t biological engineering is a n e w field which holds great promise. W e should not forget that research has converted many of the fancies of yesterday into the reali ties of today. The concentration of dissolved solids in ocean water m a y be from about 32,000 to 38,000 p.p.m., t h e normal average being 35,000. Certain lakes and seas w h i c h lack an outlet to t h e ocean may have salt contents ranging above 200,000 p.p.m., for example, 238,000 in the D e a d Sea. T h e ques tion naturally arises, how m u c h of this dissolved matter m u s t be removed to make ocean water useful for most pur poses. While brackish water can be used for some industrial purposes (nearly 1 3 billion gallons was so used in 1 9 5 0 ) , reclaimed water must be nearly of drinking quality to serve most needs. Water for irrigation must also meet certain criteria. L . V. Wilcox of the U . S. Salinity Laboratory lists these as ( a ) total dissolved solids, ( b ) per cent sodium, a n d ( c ) concentration of boron. U n d e r average conditions, total solids should not exceed 1500 p.p.m., though for c e r t a i n tolerant crops this figure may b e much higher. Sodium has an
M A R C H 28,
1955
adverse effect on soil structure. W h e r e total solids are 1400 p.p.m., t h e per cent sodium ( N a χ 1 0 0 / C a -f- Mg -fNa -f- K) should not exceed 4 8 if the water is to b e suitable for irrigating most crops. Boron is essential for normal plant growth, b u t at a concen tration slightly above the o p t i m u m it is very toxic to m a n y plants. Its permis sible concentration ranges from 0.3 to 3.0 p.p.m., depending on c r o p sensi tivity. Since sea water contains about 23 times t h e desirable m i n i m u m of dis solved solids, has a sodium p e r c e n t a g e of 84, a n d contains about 4 p . p . m . of boron, it is clear t h a t considerable puri fication will b e necessary, even for irrigation. ^ T h e cost of accomplishing the de sired degree of purification is a factor of prime importance. W a t e r is t h e cheapest commodity the householder buys. At roughly five cents a ton h e can use it freely a n d not count the cost. But this is not t h e case with farmers. A 1950 irrigation census showed t h a t water was delivered to irrigators at a rate of 3.20 acre-feet p e r acre. T h e average cost of water from t h e Bureau of Reclamation is about $4 per acre-foot. For the normal western farm of 160 acres the annual w a t e r bill would thus be $2048. B u t if the farmer had to pay at New York City rates, his yearly t a b would b e over $33,000! In any feasible process for desalting sea water, the power cost will probably be the dominant factor. A consulting engineer in t h e power generation field has estimated t h a t electric p o w e r can be procured for 3.75 to 4.25 mills per kilowatt-hour u n d e r the following con ditions: The plant would b e located in Texas, close enough to the Gulf coast to avoid any great cost for trans mission of either electric energy or gas for fuel; there would be a continuous load of 800,000 to 1,000,000 kilowatts; the process could tolerate interruption and thus avoid need for reserve power capacity. In t h e current state of devel opment of t h e least expensive saline water conversion processes, produc tion of 1O00 million gallons p e r day of fresh water w o u l d appear t o require energy at a rate of about o n e million kilowatts. T h e cost for p o w e r alone would thus b e nearly 10 cents p e r thousand gallons. It is obvious that practical methods for recovering fresh water from t h e ocean will h a v e to b e very m u c h less costly than a n y process now known. Research is currently u n d e r way on about a dozen different processes for producing fresh water from salines of various compositions. Estimates have been made of t h e cost of recovering water on a large scale b y m o s t of these processes, even though t h e size of ex perimental installations is t o o small to 1279
Big industrial water reclamation job is in operation at American Cyanamid's Sydney mine in Florida. A Dorr Thickener installation is used to clarify 15,000 gal. per minute of phosphate rock tailings water carrying 2 % solids to concentrations of 12 to 1 5 % solids. Overflow is reused in the washing plant—underflow is impounded justify much extrapolation. But, on the assumption that all estimates have been based on sound engineering judgment, they provide a rough means for making economic comparisons. On this basis the two processes which now show the greatest promise are vapor-compression distillation and ion-exchange-mem brane electrolysis. Recovery b y Vapor-Compression Distillation Holds Promise
I n conventional distillation, water is boiled in a vessel and the vapor con densed. Where steam is used t o heat the vessel, a n efficient still will produce nearly a p o u n d of condensate per pound of steam. But t h e vapor leaving the vessel contains heat; this c a n be used to vaporize water in a second ves sel, and the vapor from the second can be used in a third, and so on. In this way as m a n y distillation effects can be built as circumstances demand or eco nomics permit. T h e attractive feature of multiple effect evaporation is that a pound of steam supplied to t h e first effect will yield almost as many p o u n d s of product as t h e r e are effects. But capital and operating costs increase as more effects are used, and economic balance decides the proper number for a particular purpose. Multiple effect evaporation is too expensive for distill ing sea w a t e r except under unusual cir cumstances; estimates range from $1.50 to $3.00 p e r 1000 gal. T h e most re cent use of multiple-effect evaporation for recovery of fresh w a t e r from the ocean in t h e United States appears to 1280
be a n installation u n d e r construction by Pacific Gas and Electric in Cali fornia. T w o 72,000 gallon per d a y triple-effect evaporators will recover 5 0 gallons of water containing 50 p . p . m . of dissolved solids from each 150 gal lons of sea water. T h e recovered w a t e r will then be redistilled to a p r o d u c t containing 1 p.p.m. T h e installation is reported to cost $200,000 and it h a s been estimated that the cost of the final product will b e in the range $1.50 to $4.00 per 1000 gallons. Vapor-compression reduces the cost of distillation by using a different prin ciple. When a vapor, such a s steam, is compressed, its temperature rises. In vapor-compression distillation, once boiling has been started by a n external
source of heat, the steam liberated is compressed and returned to the heating coil in the still-body from w h i c h it finally flows as product. The success of this process is d u e to the fact that w h e n w a t e r boils at one atmosphere pressure, its temperature remains con stant at 212° F., no matter how rapidly h e a t is forced into it. Under these con ditions vaporization of water will re q u i r e 9 7 0 B.t.u.'s p e r pound. By com pressing the steam rising from t h e water enough to raise its temperature only a few degrees, it can be used to vaporize an equivalent amount of water by giv ing up its o w n latent heat of vaporiza tion. I n principle, water can thus b e distilled at a much lower energy input t h a n for multiple-effect evaporation. Hickman regards compression distil lation a s t h e best way to produce pot able water from sea water, f i e is de veloping under contract with the De p a r t m e n t of the Interior, an improve ment in compression distillation which he expects will permit yields in the range 16 t o 50 p o u n d s of w a t e r per 1000 B.t.u. If the upper limit of this range c o u l d b e attained, t h e p o w e r consumption would b e about 50 kilo watt-hour p e r 100O gallons. If p o w e r could b e b o u g h t for 0.4 cents per kilo watt-hour t h e cost would he 20 cents per 10O0 gallons. Scale formation is a serious p r o b lem where water w i t h a high concen tration of dissolved solids is distilled. Such deposits m a y double t h e cost of distillation t h r o u g h lowered heat-trans fer efficiency a n d increased mainte nance expense. At the U. S. A r m y En gineer Research a n d Development L a b oratories it has been found that citric acid c a n b e used to control scale for mation. A procedure has been devised whereby descaling can be accomplished while distillation is in progress, a n d field units c a n be kept in production in definitely. Bliss and Dodge are de veloping mechanical improvements in vapor-compression distillation to in-
Fuel Consumption in S e a W a t e r Distillation Plants
! 1 I 1 ! 1 j 1 1
1
Type of Plant Oil-fired boilers, singleeffect evaporators Oil-fired boilers, doubleeffect evaporators Oil-fired boilers, tripleeffect evaporators Oil-fired boilers, quadrupie-effect evaporators Oil-fired boilers, condensing turbo-generator, motor driven vaporcompression evapora-
tors
1 Diesel driven vapor-com1 pression evaporators
|
Water Produced 1 A m o u n t of Steam or Fuel Fired per 1000 P o w e r t o Evaporators 100 B.t.u., hr. B . t . u . / L b . 1 lb. s t e a m / 0 . 9 lb. water 0.655 12,730 1 lb. s t e a m / 1 . 6 lb. water
7,150
1.150
1 lb. s t e a m / 2 . 2 lb. water
5,220
1.590
1 lb. s t e a m / 3 . 2 lb. water
3,590
2.320
100 kw.-hr./ΙΟΟΟ gal. water
1,050
7.930
770
10.810
76 h p . - h r . / 1 0 0 0 gad.
C H E M I C A L
AND
ENGINEERING
NEWS
crease heat-transier efficiency a n d r e duce scale formation. They have been using a single-tube, forced-circulation evaporator to permit operation at a lower temperature difference without significant c h a n g e in heat-transfer areas. Results of this research h a v e n o t yet been published.
Wringing M o r e W a t e r f r o m the Clouds Will weather control serve to replenish our w a t e r resources in t h e future? Or is this b u t a visionary prospect? To date, data on cloudseeding operations has been too meager to permit statistical treatment, and it cannot be conclusively demonstrated that t h e same amount of rain would have occurred naturally. F. W. Reichelderfer, Chief, U. S. Weather Bureau says that rainmakers assume sufficient moisture is contained in clouds b u t too few ice particles exist to initiate precipitation. Thus they attempt to cause rainfall b y seeding the clouds. Others say too m a n y ice particles inhibit precipita tion and wonder about t h e benefits obtained b y seeding from generators on the ground. Irving P. Krick, President, W a t e r Resources Development Corp., and well known for his cloud-seeding operations, says seeding increases pre cipitation where natural conditions are conducive to storms. Nature seldom releases more than 5% of the moisture in storm winds—efficiency of t h e hydrologie cycle would b e greatly increased if natural fall-out during storms is increased by 5 0 % . Statistical analysis of data on w a t e r content of snow from two areas— Denver, Colo., water-shed where seeding operations were carried out for four-month periods in each of three years, and a similar size area far enough away to b e unaffected b y seeding in target areas—indicates 6 8 % more water a t Denver than could have been expected without seeding. W h e n true weather control is achieved, thunderstorms will b e sup pressed and hurricanes dissipated. Hydroelectric power will cost less; irrigation facilities will improve with resultant productivity, says Krick. This is a n intriguing look into the future—vast research is needed, however, before these potentialities can be attained. Enough rain falls on this country to supply all present and foreseeable needs for water. Runoff from the annual average of 30 inches of rainfall is 8.5 inches, equivalent t o 4.42 χ 101* gallons daily. Withdrawal of fresh water from underground and surface sources for all rural, urban, indus trial, a n d irrigational uses in 1950 was 170.745 χ 10* gallons. Thus t h e runoff was about seven times the withdrawal. T h e water supply situa tion, however, is not as rosy as these figures suggest. The principal problem in w a t e r resources management is t h e fact that rainfall is n o t uniformly distributed in either time or place. T h e 17 western states have learned t o live with a continual -water shortage. T h e humid East has been rather complacent about water because it has usually had a plentiful supply. There are signs, however, that even t h e East will face increasingly serious water resources problems in the future. The obvious way t o extend our water supplies is to use the same water more than once. An inviting source is t h e inexhaustible ocean; extensive research is now directed toward development of practical reclamation processes. Although no method is yet known for making fresh water from the ocean cheaply enough for large uses, research has a habit of converting impossibilities into realities. The expedient of storing excess rainfall would seem to b e a simple way to conserve water, b u t construc tion of large impoundments involves complex technical, economic, and societal problems. Ground water supplies can be augmented b y allowing fresh water to percolate into aquifers, by using treated sewage and industrial water for recharge, and by creating barriers to intrusion of sea water. All these methods are being used or developed.
lon-Transfer-Membrane Electrolysis Practicable for Brackish W a t e r
Ion-exchange resins are widely used to demineralize water where dissolved solids r a n g e u p to several thousand parts per million. Such resins must b e regenerated a t intervals, and t h e amount of regeneration needed i s pro portional to the dissolved salts removed. The costs of régénérant chemicals would b e much too great for deionization of ocean or highly brackish water. An electric current passing through an aqueous solution of salts is carried by t h e ions in the solution. I n this process, cations move toward the negative electrode a n d anions toward t h e positive. Manufacture of ion-exchange resins in thin sheets has led to a n e w use. T h e s e membranes are ion-selective, that is, they permit free passage of ions of o n e sign but are relatively impermeable to ions of the opposite sign. In a n electrolytic cell consisting of a series of compartments alternately separated b y cation- and anion-permeable membranes, pasage of a n electric current will result in increased a n d decreased ion concentrations in alternate compartments. It is possible, i n such a cell, to separate sea w a t e r continuously into two fractions, one relatively pure and t h e other with a much higher concentration of dissolved salts than the original water. Actually, as the purity of t h e product stream increases, the resistance of t h e cell increases and power consumption becomes excessive. A faraday of electric energy (96,500 coulonibs=ampere-seconds) will transfer a gram equivalent of ions. T h e attractive feature of ion-exchange-membrane electrolysis is the fact that the same unit of energy will transfer an equivalent weight of ions times the number of compartment pairs in a cell. This suggests t h a t a small amount of current could b e made t o d o a great deal of work simply b y using a cell with many compartments. But various factors reduce the efficiency of such cells, and t h e r e is an economic limit to the number of compartments in any practical cell. T h e consensus of the moment appears to be that vapor-compression distillation shows the greatest promise for reclaiming sea water, whereas iontransfer-membrane electrolysis is economically superior for brackish water. For some agricultural purposes it is necessary to remove only a small percentage of t h e dissolved solids from VOLUME
3 3, N O . 13
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brackish water; in such instances, iontransfer-membrane electrolysis may b e come economically attractive. Other Demineralizcstion Processes
Energy costs bulk so large in reclaiming fresh water from the ocean that solar heat has b e e n regarded as a natural source of cheap energy. Efficient use of solar heat has attracted t h e imagination of researchers a n d dreamers alike for a long time. Experiments in Massachusetts have shown that about
MARCH
2 8, 1 9 5 5
a quart of water can be produced p e r square foot p e r day under favorable conditions b y a radiation still. T h e evaporator consists of a flat tray containing about a half-inch of water enclosed b y an inclined glass roof. Vapor condenses on t h e glass a n d runs to a collecting trough. Data from the University of California indicate that t h e average yield (January—July) may b e about 0.1 gallon per square foot. Using this figure, about 230,000 effective acres would be needed to provide 1281
1000 million gallons per day. Technical problems associated with use of solar energy in this manner are quite difficult; lack of experience with large installations suitable for irrigation water prevents estimating a minimum overall cost per 1000 gallons. An important consideration is the fact that a successful solar-energy process would avoid depletion of conventional energy sources. Recovery of fresh water from the ocean by freezing is attractive because of the lower theoretical energy requirement of such a process. Since only 80 calories per gram is needed to freeze water as against 540 to vaporize it, the saving in energy would appear to b e considerable. When sea water is frozen, the individual ice crystallites * are quite pure. But if such water is frozen in a large cake the spaces between the ice crystals entrap so much salt that the product is only a little purer than the raw water. This means that an effective freezing process must yield tiny ice crystals from which the entrapped saline water can be separated. Several methods have been developed to do this. The problem of mechanical separation is not as difficult as finding a heat sink to absorb the energy abstracted from the water. The ocean itself should provide a place to discard heat. The temperature of the ocean off the California coast may average about 60° F ; off the Texas coast it would b e at least 10° higher. A process has been designed on a theoretical basis, using the ocean as a heat sink, which indicates the possibility of reclaiming water with a net expenditure of about 26 kilowatt-hours per 1000 gallons. Considerable experimental work would b e needed to establish a practical design, but it seems possible that the economics of a freezing process would be comparable with vapor-compression distillation. Ion-exchange-membrane electrolysis removes all salts from water regardless of species; research is in progress to develop a new kind of membrane demineralization which will separate minerals selectively. Osmosis and solvent extraction are also being investigated as potential processes. The variety of research devoted to demineralization is an encouraging sign that recovery of fresh water from the ocean may soon cease to be an idle dream. The Problem of Replenishment
Water supply would not be a problem if it were possible to store and distribute a relatively small fraction of the runoff from rainfall. Unhappily, any such scheme is fantastically impractical. Actually, we possess an unrivalled fresh water reservoir in t h e Great Lakes. The average flow of t h e Niagara River is 230,000 second-feet, 1282
equivalent t o 0.135 cubic mile per day. In 1950 our daily water use for all rural, municipal, industrial, and irrigational purposes was 0.155 cubic mile; about 52% of this total was used for irrigation. Although considerable water could probably be withdrawn without appreciable effect on lake levels or hydroelectric power, the distribution problem w o u l d b e formidable indeed. Mean lake levels from Superior to Ontario range from 602 to 246 feet above mean sea level. The mean elevations of the 17 western states, where more water is a pressing need, range from 6800 to 1700 feet. The problem of using these lakes to augment water supplies m a y b e illustrated by t h e lack of agreement o n the feasibility of using Lake Erie to supplement the water supply of Columbus, Ohio. Here the distance is a mere 10O miles and the water would have t o be raised only 200 feet. Lake Superior is 225 miles from Fargo, N. D., the nearest of the 17 western states, and t h e hydraulic lift might exceed 1000 feet. Rainfall O f t e n Inadequate To Recharge Aquifers
Withdrawal of ground water faster than recharge can occur has caused difficulties in various localities. In some instances the drawdown has been great enough t o result in subsidence of the surface. This reduces t h e pore space of t h e aquifer and thereby makes the situation even worse. The demand for cool water for air-conditioning in metropolitan areas has been an important factor in local depletion of ground water. A corrective measure is to p u m p the used w a t e r back into the underground supply. In coastal regions excessive drawdown has led to intrusion of brackish water, and withdrawal of ground water in such places is often strictly controlled. Recharge of ground-water reservoirs occurs as rainfall percolates through the soil. The rate of recharge is determined by the geology of a region and the amount of rainfall. The amount of water that can be stored in the ground depends upon the porosity of subsurface formations. Reservoirs, lakes, and farm ponds contribute to ground water storage, and there is an interchange of water between streams and t h e ground-water table, depending upon relative levels. An unknown proportion of water used to irrigate crop land percolates into the ground-water table. Recharge of ground water reservoirs with fresh water has been practiced in some instances b y installing percolation beds. Where there is a heavy draft on ground water, especially in semiarid regions, rainfall is often inadequate to keep aquifers fully charged. This has led
to investigation of the feasibility and economics of using treated domestic sewage and industrial w a s t e s for r e charge. Treated N o r m a l Domestic Sewage for Recharge
Notable research on reclamation of sewage and industrial waste water has been done in California. It is estimated that a million acre-feet o f waste w a t e r could be made available annually for reuse in that state. This represents about 10% of t h e water n o w being distributed. Systems curremtly in use for disposal of sewage and industrial wastes by percolation were valuable sources of information. Supplernenitary data w e r e obtained in t h e laboratory and from test plots. Where waste water i s used for r e charge an important consideration is the effect this practice may have on t h e mineral constitution of ground water. It was found that mineral analyses of ground water near present recharge operations have shown n o appreciable change. In test-plot stixdies, however, there was only a small change in t h e mineral content of water "that percolated through seven feet of s:oil a t rates of 0.5 to 15 feet per day, Where domestic sewage is used to irrigate crop land there is a potential hazard t o health. This study concluded that normal domestic sewage, after treatment b y known processes, can b e considered a water resource that can be used for irrigation, recharge, or industrial purposes. It was also concluded that while transmission of disease through reuse of waste water i s a threat, it presents no fundamental problem b e cause it can b e readily controlled. It was recommended that the study of percolation rates b e continued to provide essential data. T h e need for such research is supported b y t h e fact that it is not yet possible to determine the exact capacity of a soil t o handle a given pollution load, mor to estimate effects on ground water. It is pointed out that community acceptance of r e use of sewage effluents would b e e n hanced if waste treatment plants were designed and operated as reclamation projects. Intrusion of sea water into the water table is a serious proh>lem in coastal areas where drawdown of ground water is excessive. Studies are in progress to determine tiie feasibility of forming underground hydraulic barriers by injection of fresh water. Present data indicate that such barriers may be practical, and thtat sewage effluents may be suitable for the purpose. If they are, it will be possible to save large underground reservoirs from destruction.
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