selective passage of water, with rejection of salts 

 and other pollutants through the membrane under 

 an applied pressure in excess of the osmotic 

 pressure of the saline feed. The second major 

 membrane process, electrodialysis, involves the 

 selective passage of ionic salt species through 

 alternating pairs of cation and anion selective 

 membranes under the influence of an electrical 

 potential. This process may be economical for 

 brackish waters and a major research effort is to 

 develop new and improved membranes which are 

 more resistant to fouling by various pollutants. 



Newer ion exchange processes, such as Sul-by 

 Sol, Sirotherm, and Kurnin, developed by private 

 industry, may be competitive with reverse osmosis 

 and electrodialysis. 



Crystallization processes are also being inves- 

 tigated. The vacuum freezing-vapor compression 

 method appears especially promising, and pilot 

 plants utilizing both seawater and brackish water 

 are in operation. Investigations are also continuing 

 on the order of melting inversion process. Hydrate 

 and secondary refrigerant processes have been 

 found to be uneconomic and efforts to develop 

 them have recently been curtailed. 



On Jan. 1, 1967, there were 153 seawater-based 

 plants of all types in operation, of which 28, with 

 a desalting capacity of 15.1 mgd, were in the 

 United States.'' At the end of 1967 a total of 625 

 desalting plants with a combined capacity of 222.2 

 mgd were in operation or under construction 

 worldwide (Table 1). It is not known how many of 

 these are seawater-based plants. 



Five operating plants in the United States have 

 capacities of one mgd or greater; the largest, 

 placed in operation in 1967, is in Key West, 

 Florida and has a capacity of 2.6 mgd. Three of 

 these five plants are OSW test bed plants. About 

 95 per cent of all plants now in operation use the 

 multiple stage flash distillation process. 



A strong market is seen for small scale desalting 

 plants (less than 10 mgd) over the next decade* 

 for the smaller metropolitan areas. Demand, over 

 the near term at least, will probably be mainly for 

 industrial and metropolitan uses. 



Table 1 

 DISTRIBUTION OF DESALTING PLANTS 

 AND PLANT CAPACITY IN OPERATION OR 

 UNDER CONSTRUCTION, BY GEOGRAPHI- 

 CAL SUBDIVISION AS OF DEC. 31, 1967 



Information supplied by W. F. Mcllhenny, Dow 

 Qiemical Corp., February 1968, adapted from the 1966 

 Saline Water Conversion Report by the Office of Saline 

 Water, Appendix E. 



Conversation with J. A. Hunter, Office of Saline 

 Water, Department of the Interior, 1968. 



Source: Office of Saline Water, Department of the Inter- 

 ior, June 1968. 



According to the Office of Saline Water,^ 43 

 new desalting construction starts were made, 

 worldwide, in 1967. Of these, four will have a 

 desalting capacity between 5 and 10 mgd, and nine 

 will have a capacity between 1 and 5 mgd. 



During the period 1960-1965, worldwide de- 

 salting capacity showed an average annual growth 

 rate of 12 per cent. Tentative OSW projections 

 indicate an annual growth rate of 24 per cent 

 during the decade 1966-1975, and 32 per cent 

 during 1976-1980. Capacity is projected to grow 

 from 222 mgd on Jan. 1, 1968 to 1.0 bgd by the 

 end of 1975, and to 4.0 bgd by the end of 1980. 



iV. UNIFORM COSTS COMPARISONS 



In 1952, when only a few small land-based 

 desalting plants were in existence, the cost of 



Written communication, July 1968. 



VII-227 



