298 



Economic Aspects 



and elsewhere, has focused attention on 

 schemes of converting sea water into fresh 

 water. Because of the gravity of the situa- 

 tion here and elsewhere, the California 

 Legislature in 1951 appropriated funds for 

 the University of California to undertake 

 research in conversion, and the U. S. Con- 

 gress in 1952 passed a Sahne Water Conver- 

 sion Act authorizing a program of research 

 and development administered by the Office 

 of Saline Water in the Department of the 

 Interior. More than a score of laboratory 

 investigations are underway and several pilot 

 plants are now being operated. Very good 

 outlines of the various methods being inves- 

 tigated are given by Howe (1952) and Jenkins 

 (1957, 1958). 



The simplest conversion method is that of 

 using solar energy to evaporate sea water 

 under glass or plastic covers, but the effi- 

 ciency is low, only about a quarter of a gal- 

 lon per square foot per day. If all the fresh 

 water required within Los Angeles County 

 alone, about 300,000,000 gallons per day, 

 were produced in this way, an area of about 

 2 square miles (5 sq km) of solar stills would 

 be needed. The cost of the land alone pre- 

 vents the use of solar stills for development 

 of urban water supplies. Obviously a more 

 concentrated source of energy than sunlight 

 is needed. Energy as heat, electricity, and 

 pressure has been applied in a variety of 

 ways. Multieffect stills also use the latent 

 heat released by condensation of water vapor 

 from each stage to evaporate the brine from 

 the next stage. Reductions in pressure at 

 each successive stage reduce the boiling 

 point in accordance with the lower tempera- 

 ture of the brine at each stage. In vapor- 

 compression stills improved efficiency results 

 from compression of the water vapor to raise 

 its temperature and the transfer of heat from 

 the vapor to the brine to aid in evaporation. 

 A great improvement over either type of still, 

 especially for small supplies of water, is the 

 spinning top in which a thin film of sea water 

 sprayed on the inside surface partly evapo- 

 rates. This vapor is drawn off", is compressed 

 to raise its temperature, and is condensed on 

 the outside surface, thus yielding heat that 

 is transmitted through the thin metal of the 



spinning top to evaporate the film of sea 

 water on the inside surface. Another 

 method operates on temperature diff'erences 

 existing between the ocean surface and 

 depth, using the colder water from depth to 

 condense vapor from the warmer surface 

 water or vapor from a flash chamber. This 

 method has the advantage of producing 

 power as well as fresh water, but it has a low 

 yield for the capital outlay. 



Ion exchange with zeolites and various 

 resins was used in war-time survival kits for 

 life rafts. In principle, the unwanted ions 

 are exchanged for less olfensive ones; for 

 permanent installations the unwanted ions 

 are washed off the exchange medium which 

 can then be reused. Thus the method is 

 similar to that of the family water softener, 

 but it is expensive and useful for making 

 only small quantities of fresh water. A more 

 complex ion exchange method, electrodialy- 

 sis, makes use of selective membranes, 

 whereby membranes permeable to cations 

 alternate with ones permeable to anions in 

 subdividing a long tank into compartments. 

 When an electric current is passed from one 

 end of the tank to the other, alternate com- 

 partments become fresh, having yielded their 

 salts to the intervening compartments. By 

 another method, reverse osmosis, water is 

 forced through a semipermeable membrane 

 which screens out the salts. Freezing has 

 long been known by oceanographers to pur- 

 ify sea water because sea ice when melted is 

 relatively fresh. As shown by Thompson 

 and Nelson (1954), increased efficiency of 

 converting salt into fresh water can be ob- 

 tained by freezing from the top of a water 

 column and by discarding the first 70 per 

 cent of the meltwater. Because the latent 

 heat of fusion is less than the latent heat of 

 evaporation, freezing when properly apphed 

 may become cheaper than distillation as a 

 source of fresh water. All these and other 

 methods are being intensively tested, some 

 in southern California. 



Only the multieffect distillation and 

 vapor compression distillation methods have 

 reached a commercial level, but both are too 

 expensive at their present stages of develop- 

 ment (Howe, 1958). The costs of distillation 



