288 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
The actual energy needed to evaporate fresh water from salt water 
is necessarily much greater than this theoretical requirement, which 
assumes a barely perceptible rate of separation and complete effi- 
ciency in conversion of energy. Murphy [7] has estimated that this 
minimum energy requirement for a practical process may be about 
four times the theoretical, or about 12 kilowatt-hours per thousand 
gallons. Simple distillation, for example, requires 1,000 times as 
much energy, but already distillation systems of today require energy 
expenditures of only about 200 kilowatt-hours per thousand gallons. 
oe 
Fic. 2.—Theoretical energy needed to separate water molecules from salt ions. 
Distillation, known and practiced for long ages, remains to date 
the most advanced (and almost the only) method of applying this 
energy to the work of salt-water separation. While simple in basic 
principle, it is complex and costly in application, and involves the 
use of large and expensive equipment. To discover other methods, 
the Office of Saline Water upon its establishment undertook an ex- 
tensive survey [8] of scientific and technical knowledge. Included 
were various physical, chemical, and electrical actions, as well as 
several modifications of the conventional distillation process designed 
to increase the productivity and reduce the size and cost of the neces- 
sary equipment. 
Some 30 potential conversion processes and pertinent phenomena 
were originally listed from this survey, but they were soon narrowed 
down to 16, which in turn were ultimately reduced to 5 groups of 
processes, namely: (1) thermal and mechanical distillation ; (2) solar 
distillation; (3) membrane processes; (4) freezing; and (5) others, 
the latter including such processes and phenomena as ion exchange, 
solvent extraction, and biological action. 
To date these studies [9], as well as parallel researches abroad, 
have shown that several processes other than distillation are tech- 
nically feasible on a laboratory scale; only one has reached actual 
commercial use, however. This is the recently developed process of 
electrodialysis, in which the ions composing the salt are forced out 
of the saline water through pairs of positive and negative ion-selective 
membranes by the force of an electric current. One other process— 
