FRESH WATER FOR ARID LANDS—JENKINS 293 
about 20 gallons to several hundred gallons per day of fresh water. 
The large stills would cover many acres. They would be most useful 
where other conversion methods are more costly or impracticable and 
where solar intensities are high and land costs low. Both types have 
attractive possibilities for areas where the stills can be constructed 
with local materials and labor. Much developmental work is neces- 
sary to reduce the unit cost on a per-square-foot basis. 
It is anticipated that the solar distillation center in Florida will 
result in engineering designs and specifications for practical small 
units and will point the way for future solar distillation plants of 
much larger capacity. 
MEMBRANE PROCESSES 
Desalinization processes utilizing membranes have been developed 
during the past few years to the point where several are known to be 
technically feasible and one appears to be economically feasible for 
the treatment of brackish waters under certain conditions [26]. 
Specifically, the membrane processes showing promise for the puri- 
fication of brackish waters consist of (1) electrodialysis where an 
electromotive force is applied to a cell consisting of ion-selective 
membranes; (2) “osmionic” where the concentration gradient be- 
tween the solutions supplies the potential to drive ions through 
ion-selective membranes; and (8) reverse osmosis where sufficient 
pressure is applied to the solution to force water through an ion- 
restraining membrane into the fresh-water side. The practicality of 
using biological membranes through the use of algae is now being 
investigated by the Office of Saline Water [27]. 
One of the limiting factors in the use of these processes has been 
the membranes themselves. Consequently, during the past few years 
considerable research has been conducted by organizations in several 
countries aimed at improving the characteristics of those membranes. 
As a result, greatly improved ion-selective membranes have been de- 
veloped. Such membranes are now available commercially at a cost 
per unit area of approximately one-fifth of what it was formerly. 
If the cost of membranes and equipment can be further reduced, 
some authorities believe that the use of ion-selective membranes for 
the demineralization of sea water might become economically feasible. 
Electrodialysis—An electrodialysis cell consists of alternate cation 
and anion permeable membranes (pl. 4, fig. 2). Upon the applica- 
tion of an electromotive force the positive ions, such as sodium, pass 
through the cation permeable membranes, whereas the negative ions, 
such as chloride, move in the opposite direction and pass through the 
anion permeable membranes. Thus, the water passing between alter- 
nate membrane pairs is depleted of salt, while that passing through 
the intervening pairs is enriched. 
