While in operation only since August 1967, it 
already has achieved a performance ratio of 20 
pounds of product water for every pound of steam 
input, double the performance ratio of the plant 
OSW operated in San Diego in 1962-1964. 
d. Vapor Compression Vapor compression distil- 
lation is very competitive in small portable plants. 
In addition, it has primary application to plant 
systems where water, rather than a combination of 
power and water, is produced. It is also of interest 
for the following reasons: (1) It may be considered 
when only electrical energy is available, and (2) 
unlike the multiple-effect and multistage flash 
distillation processes, no large heat sink is re- 
quired. This can be an advantage at inland sites. 
The largest plant of this type is the OSW Test 
Bed at Roswell, New Mexico, operating on brack- 
ish well water, producing one million gallons per 
day. 
Multiple-stage flash can be combined with 
vapor compression or vertical tube evaporation to 
better utilize thermal energy. Such combinations 
have been termed hybrid processes, and work now 
is under way on analytical investigations of such 
processes for a wide variety of water delivery rates. 
One such study considers an application whereby 
the shaft power from a gas turbine will drive the 
vapor compressor, and waste heat will be used to 
preheat the water in a multiple-stage flash 
system.*! 
3. Crystallization 
The major crystallization process is freezing, 
involving separation of pure water solids (ice) from 
a salt solution. Crystallization also can occur 
through a process whereby a_hydrate-forming 
material combines with water to form a solid. Of 
the two processes, vacuum freezing and secondary 
refrigerant freezing, the former is more advanced. 
The vacuum freezing test bed, which produces 
over 100,000 gallons per day of fresh water at the 
Wrightsville Beach, North Carolina, has been run- 
ning life tests to determine long-term maintenance 
and operating problems and system economics. 
The first pilot plant utilizing the secondary refrig- 
41 Hearings before the Senate Subcommittee on Water 
and Power Resources of the Committee on Interior and 
Insular Affairs, 90th Congress, Second Session on S. 
2912, February 1968, p. 24. 
erant process also is in operation at Wrightsville 
Beach.*? 
Freezing has a unique role to play in brackish 
and polluted water conversion in the salinity range 
of 9,000 to 20,000 ppm, as well as in plant sizes 
from one to 10 million gallons per day. Electro- 
dialysis, although suitable for lower salinities, is 
too expensive in the salinity range of 9,000 to 
20,000 ppm. And, because of the typical higher 
concentration of scale-forming compounds in 
brackish waters, evaporation or distillation proc- 
esses require expensive treatment to remove these 
compounds. Freezing, because of low operating 
temperatures, largely eliminates scale. In addition, 
it requires considerably less energy.* * 
In view of these potentials, a mobile vacuum 
freezing pilot plant now is under construction and 
will be tested on a number of brackish waters to 
determine basic system economics. 
4. Membranes 
Membrane processes involve diffusion through a 
semipermeable membrane. While still in the liquid 
state, salt solution and water are separated. The 
major types are electrodialysis and reverse osmosis. 
The electrodialysis process uses membranes with 
electric current as the driving force. This process 
has reached commercial acceptance in brackish 
water applicable up to 500,000 gpd and salinities 
up to 5,000 ppm. Modifications of this process are 
being studied to reduce capital costs which could 
result in substantial reductions in the cost of 
water. 
In the reverse osmosis process, pressure is the 
driving force. Pressure in excess of the osmotic 
pressure of the saline feed is applied to a special 
membrane and water passes through. 
Both processes appear to be economical for 
desalting brackish waters. However, efforts also are 
being directed toward development of membrane 
processes economical for desalting sea water. 
Presently, the reverse osmosis process is in the 
pilot plant stage and is considered advanced 
technology for ultimate application in converting 
42 Text of presentation to the Marine Engineering and 
Technology Panel by W. F. Savage, OSW, Nov. 16, 1967, 
ps2: 
43 Senate Hearings, May 1965S, testimony by J. W. Pike, 
President of Struthers Scientific & International Corpora- 
tion, pp. 37-38. 
VI-205 
