Title: Reclamation of Water From Sea Water
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Title: Reclamation of Water From Sea Water
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Language: English
Publisher: Fla. Engineering and Industrial Experiment Station
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Spatial Coverage: North America -- United States of America -- Florida
 Notes
Abstract: Richard Hamann's Collection - Reclamation of Water From Sea Water
General Note: Box 12, Folder 1 ( Materials and Reports on Florida's Water Resources - 1945 - 1957 ), Item 32
Funding: Digitized by the Legal Technology Institute in the Levin College of Law at the University of Florida.
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Reclamation of Water from Sea Water

by
DAVID B. SMITH*


The oceans hold most of the earth's waters. For
each gallon of fresh water on the earth's surface it is
estimated that there are 2690 gallons of salt water in
the oceans and seas.' Almost without exception this
large portion of the waters of the globe is unfit for
municipal, agricultural, or industrial uses, the notable
exception being the use of brackish waters for cooling
purposes. Within recent years, however, much prog-
ress has been made in a program of research and de-
velopment on methods of obtaining fresh water from
the sea. The purpose of this paper is to present a
brief survey of the current status of work in this
important field.

Quality Criteria Demands of Water Users
Sea water is objectionable in potable water sup-
plies because of its taste and its physiological effects.
The "U. S. Public Health Service Drinking Water
Standards, 1946" specifies that ordinarily total solids
should not exceed 500 p.p.m., but does concede that
a total solids content of 1000 p.p.m. is permissible in
drinking water. Supplies containing 2500 p.p.m. are
often considered satisfactory in arid regions, and 4000
p.p.m. are found in a few drinking waters in the south-
western United States.2 Sea water purified only to
5000 p.p.m. is somewhat toxic and its continued use
is inimical to health. Waters of 2500 to 3000 p.p.m.
dissolved salts can be used by persons acclimated to
them, but special consideration must be given to
eliminating even low concentrations of certain toxic
components such as barium and boron.3
Water quality requirements for industrial uses are
highly variable depending upon the process for which
water is to be used. Most industries are content to
draw upon the local municipal supply which can
usually be modified to fit the needs of the process.
Where industrial limitations are more severe than
those of the municipality, the limitation is usually
placed on some particular ion and not on the total
concentration of dissolved solids.3 Nevertheless an
industrial plant which is supplied with a water of low
dissolved solids content is more advantageously located
than a similar plant which must use water of higher
solids content.
Although the standards for irrigation waters have

*Professor of Civil Engineering, College of Engineering, Uni-
versity of Florida.


not been determined precisely, they are usually judged
by three criteria: (a-Btor I concentration; (b) ration
of Na+ content to, tthe sum of the Na+, Mg++,
Ca++, and K+; and () totaldissolved salts. No more
than 3 p.p.m. baoralas boron is permissible for even
the more tolerant cs.Via_ ce boron in any form is
toxic to plants. SetAi~, too much sodium adversely
influences the permeability of soils, an effect which
is counteracted by calciuto magnesium and potassium.
The highest permissible,percentage ratio of sodium
to the sum of the four cations listed is 60 per cent. The
third limitation placed on irrigation waters is the dis-
solved solids content, which, if too high, results in
-deposition of salts in the ground. In regions of low
rainfall the salt build-up in the soil may reach levels
sufficient to retard or prevent growth of vegetation.
The Department of Agriculture suggests limits of
500 to 1400 p.p.m. dissolved solids for waters of "per-
missible" grade.2 *
It may be concluded that any process designed to
reduce the salt content of sea water for general use
by municipalities, industries, or agriculturists must
have as a maximum limit 1000 pip.m. concentration
of dissolved solids. Any toxic elements present in
the raw water must also be limited in the final prod-
uct where human consumption or agricultural usage
is involved. In general, the 1000 p.p.m. limit on
total content is the criterion upon which deminerali-
zation techniques and cost estimates are based.

Properties of Sea Water
The salinity of sea water is extremely constant at
about 3.4 to 3.7 per cent solids throughout the world
with an average salt content of 3.46 per cent by
weight. In addition, the ratios between the major
constituents of sea water have been found to be
constant regardless of the location of the sampling
point. Brackish waters derived from sea water also
exhibit this characteristic since dilution modifies only
the total dissolved solids content. Table 1 lists average
values for the important ions and molecules found in
sea water.
The properties of pure water are unique in com-
parison with those of other liquids, and the presence
of dissolved salts of the level found in sea water does
not alter most of these qualities significantly. A brief
discussion of some of the peculiar characteristics of
water will help to explain the difficulties and economies
involved in the several demineralization processes.


---. ___~____~__cC~*C-c~"-L--- ir"-









Heat capacity. The heat capacity of water is the
highest of all known solids and liquids except am-
monia. The costs of some methods, such as distillation
and freezing, are increased by this property since
large amounts of energy must be added to or removed
from the water in the separatory process.
Latent heat of fusion. The latent heat of fusion of
water is the highest of any known liquid except that of
ammonia. This factor must be considered in attempts
to obtain fresh water from sea water by freezing.
Latent heat of evaporation. The latent heat of
evaporation of water is the highest of all known liquids.
This property obviously increases the cost of dis-
tillation and evaporation methods of desalting sea
water by virtue of the large energy requirements which
must be met.
Surface tension. The surface tension of water is the
highest of all known liquids, a property which reduces
the effectiveness of processes involving evaporation.
Dissolving power. In general, water dissolves more'
substances and in greater quantities than any other
known liquid. Consequently, the quantity and quality
of dissolved solids in sea water is greater than would
be the case if any other liquid were involved. If sea
water of 3.5 per cent salinity is to be reduced to a
1000 p.p.m. level, it is necessary to remove 280 pounds
of dissolved solids from each 1000 gallons treated.
Dielectric constant. Pure water has the highest
dielectric constant of all known liquids. The polarity
of water molecules creates strong attraction between
the molecules and the salt ions, a factor which results
in a number of water molecules becoming attracted to
and clustered about each ion. At 3.5 per cent salinity
there are about 100 water molecules for each pair of
sodium and chloride ions. Few of the water molecules
are not included in the region of influence exerted
by the electric charge of the ions. Any separatory pro-
cess will require the input of energy in an amount
sufficient to overcome the mutual attraction between
the ions and molecules. An irreducible minimum of
TABLE I
Major Constituents of Sea Water (from The Oceans by Sverdrup,
Johnson & Fleming, Prentice-Hall, 1942)
Constituent Per cent by weight
CI- 1 8980


so4- -
HCO,-
Br-
F-
Na
Mg-
Ca+
K+
Sr+
H ,BO3


Total


0.2649
0.0140
0.0065
0.0001
1.0556
0.1272
0.0400
0.0380
0.0013
0.0026
3.4482


2.6 kilowatt-hours of energy is required to produce
1000 gallons of fresh water from sea water of 3.5 per
cent dissolved solids content. All processes utilizing
sieves or membranes must maintain a high efficiency
level in order to be economically feasible. For ex-
ample, at an energy cost of one cent per kilowatt-hour
the theoretical energy costs for desalting one acre-foot
of sea water is $8.50. In actual practice low process
efficiencies cause this cost item to be much higher.

Desalting processes and their costs
Although there are many factors which increase the
costs of sea-water demineralization, several processes
are known which are theoretically feasible. At the
present stage of development, however, the process
efficiencies are quite low and cannot compare in cost
with conventional methods of obtaining fresh water.
For comparative purposes, the average water rates
in the United States for untreated and undistributed
municipal water range from $50 to $75 per acre-foot
with a maximum figure of $125 per acre-foot.4 The
prevailing charges for industrial water vary greatly
and in some instances exceed the costs to municipalities
by several times the $125 figure, depending upon the
location, operation, and the quantity and quality of
the particular industry. Average charges, however,
are approximately equal to those for municipal sup-
plies. For irrigation water the maximum charge on
record is $40, but the cost to most farmers is between
$1.50 and $6.00 per acre-foot.4, 5 In comparison with
these costs the present status of methods to desalt sea
water has been summed up by the United States De-
partment of the Interior -which states ". any sig-
nificant reduction in the present minimum cost of
demineralized water (about $400 per acre-foot for
sea water by vapor compression distillation) must be
viewed as substantial progress".5
The development of practicable low-cost means of
producing fresh water from sea or other saline waters
was implemented by an act of the U. S. Congress in
1952. The act appropriated $2 million prorated over
a five-year period to be used in a research and develop-
ment program. At the end of 1953 fourteen contracts
had been signed by the United States Department of
the Interior (the administering agency), and forty-one
contracts were in effect with other governmental
agencies. In addition, much headway has been made
by private industries which must conduct operations
in areas where fresh water is a rare and precious com-
modity. For example, the Gulf and Anglo-Arabian Oil
Companies own and operate six triple-effect evapo-
rators converting sea water to fresh in Kuwait on the
Persian Gulf. The units have a total capacity output
of 720,000 gallons a day.










In recent years much progress has been made
towards developing desalting processes that will com-
pete with the usual methods of collecting and proces-
sing fresh water. The following paragraphs review
the status of the more promising methods. Cost com-
parisons which appear in this paper are based on the
total cost of producing, but got distributing, one acre-
foot of fresh water of the desired salinity content. A
summary of the best current cost observations or esti-
mates appears in Table 2.
It should be pointed out that as in every industrial
process the cost of fresh water production is based on
five factors. They are: (a) fixed charges, (b) operating
labor costs, (c) plant maintenance costs, (d) power
costs, and (e) raw material costs. In general, only one
or two of these factors control the overall costs of a
particular desalting process. For example, the com-
petitive position of the solar evaporation process is


TABLE 2
Cost of Producing Fresh Water from Sea


almost wholly dependent upon the cost of plant con-
struction, whereas the energy requirement is a major
cost factor in the electric membrane process.
Electric membrane process. The electric-membrane
process for demineralizing sea water is one in which
the principles of selective membranes and electrolysis
have been combined. It is a development made pos-
sible only by recent advances in knowledge of ion
exchange resins possessing the property of selective
ion permeability. Basically the system consists of an
electrolytic cell in which the space between electrodes
is divided by a large number of membranes. The
anion-permeable and the cation-permeable membranes
are placed alternately so that under the influence of
an electric current ions are removed from alternate
compartments and transferred to adjacent compart-
ments. Since ions can migrate electrically no farther,
the salt content in every second compartment is in-


Water by Various Methods


5. Total Dissolved
"b solids (p.p.m.)


Plant used to Raw Finished Total Cost*
Method of Production obtain cost data Authority ". Water Water ($/acre-ft.)

Collection only New York, N. Y.
Municipal system Kikers 1000 None
required No Change $ 34
Lime-soda ash Gainesville, Fla.
Municipal system Kiker9 5.0 1.0 250 130 59
Electric membrane Pilot plant Judas 0.0024 0.3 10,000 350 40
Electric membrane Laboratory scale plant Eliassenlo 0.275 0.3 35,000 500 430
Electric membrane Laboratory scale plant 6.....------ 75 -.. 35,000 .... 410-490
Electric membrane Estimated Held2 1000 0.5 35,000 1000 105
Vapor-compression distillation Estimated Aultmanll 1.0 0.5 35,000 0 410
Vapor-compression distillation Plant scale ............- 75 35,000 0 410-490
Vapor-compression distillation Laboratory scale plant .--- ..-- 6 75 35,000 0 196-245
High temperature and
pressure distillation Laboratory scale plant --..--... 6 75 --.. 35,000 0 98-260
Solar evaporation -- .------ 6 75 None
required 35,000 0 650
Solar evaporation at latitude
of Southern Arizona Laboratory scale plant U.S.D.I.4 -- None
required 35,000 0 980
Multiple-effect evaporation by
steam from solar radiation in
latitude of southern California Estimated Landry5 0.24 1.0 35,000 0 900-1080
Triple-effect evaporation Kuwait on the
Persian Gulf U.S.D.I.5 0.75 (natural
gas) 35,000 0 490-650**
Evaporation by thermal
differential Laboratory scale plant ............... -75 -. 35,000 0 230-260

*Costs of distribution not included.
**Costs based on estimates of an equivalent plant in the U. S.


r









creased up to 100 per cent depending upon the degree
to which the electrolytic process is carried. The en-
riched brine is replaced with sea water containing a
lower salt concentration with the flow from alternate
cell compartments consisting of water of reduced salt
content and of brine. The electric energy required to
remove a given quantity of salts depends upon the num-
ber of compartments, the resistance of the electrolysis
cell, and the hydraulic flow rate. Several laboratory and
pilot plant models have been built and operated by
Ionics, Inc. The cost of producing fresh water from
sea water by this process has been estimated at $105
to $490 per acre-foot (see Table 2).
Vapor-compression distillation. The vapor-com-
pression distillation process operates on the heat-
pump principle in which the latent heat of vaporiza-
tion is continuously re-used. In the operation of a
vapor-compression distillation unit sea water is first
vaporized at atmospheric pressure. The vapor is then
compressed to raise its pressure and temperature after
which it is returned to the heating side of the evapora-
tor tubes to vaporize more brine. The water vapor
is allowed to condense in the evaporator tubes. In so
doing substantially all of the latent heat of evaporation
of the compressed steam is transferred through the
tubes to the boiling brine where it evaporates more
water from the brine. The fresh water condensate is
then withdrawn. Various estimates have placed the
cost of producing one acre-foot of fresh water by this
method at $196 to $490. About 40 per cent of the
total cost is for fuel. Units of up to 4800 gallons per
day capacity are manufactured by the Badger Manu-
facturing Co. of Cambridge, Mass.5
High temperature and pressure distillation. Under
conditions of critical temperature and pressure (ca.
7000F and 3,200 p.s.i.) there is no distinction between
the liquid and vapor phases of water. Under these
conditions salt water can be distilled into fresh water
with greater efficiency. Although laboratory-scale
equipment has indicated that costs of producing one
acre-foot of fresh water by this method are extremely
low (see Table 2), scaling up the unit's size could
reveal difficulties in the design of plant scale equip-
ment to use the high temperatures and pressures.6
Multiple-effect evaporation. The process of vapori-
zation applied to demineralization methods consists
of evaporating a portion of the water from the brine
after which the vapor is condensed to produce fresh
water. The energy requirements are almost inde-
pendent of the salt content of the brine. When single-
stage distillation is practiced a considerable portion of
the applied energy is lost in the condensation chamber,
but the process efficiency can be increased if several
stages of evaporation are used. In multiple-effect


evaporation the vapor from the first stage is condensed
in heating compartments of a second unit where steam
is being produced from the brine at a temperature
and pressure less than those of the first stage. In
turn the vapor produced in the second stage is used
as a heat source for the third stage, and so on. This
method has been under extensive commercial de-
velopment for years, and at present almost all fresh
water produced from sea and brackish waters is by
one of the evaporation methods. Sea water evaporators
have been in service aboard ships for some time, and
land-based installations for producing potable water
from sea water are located in Kuwait, Bermuda, Aruba,
Curacao, Johnson Islands, and elsewhere.5 The cost
of producing one acre-foot of potable water from sea
water by thermal evaporation is estimated to be be-
tween $490 to $650.
Evaporation by thermal differential. The tempera-
ture difference method depends upon the availability
of a source of warm saline water and another of colder
water. Where these conditions are found it is possible
to evaporate the warmer water under reduced pressure
and to use the colder water for condensing the vapor.
The energy requirements are then reduced to that
required to maintain the lower pressure and to pump
the saline waters. A plant utilizing this principle can
be used as a source of steam for electric power genera-
tion, and the power may then be utilized to drive the
pressure reducing units. At present a plant of 10,000
kilowatts and 150,000 gallons of fresh water per day
capacity is under construction at Abidjan in French
West Africa.7 The estimated production cost of $230
to $260 per acre-foot of fresh water is attractive, but
the process appears to be limited to special locations
where waters with at least 160 F difference in tempera-
ture are available.
Solar evaporation. Single-stage evaporation is the
most obvious and simplest use of solar energy in ob-
taining fresh water from the sea. Investigations to date
have centered around methods of evaporating sea water
in the solar energy collector itself, and the cost of pro-
ducing one acre-foot of fresh water by this method
has been estimated at $650 to $980. Since there is no
energy cost, the expenditures depend almost entirely
upon maintenance and capital costs.
There is at least one variation from the single-
stage evaporation unit. Some study has been devoted
to the feasibility of producing distilled water from
brines by multiple-effect evaporation using steam gen-
erated in solar collectors of the parabolic cylindrical
type.5 The estimated cost of production is of the same
order of magnitude as that of the single-stage process.
Other methods. Some attention has also been
given to other methods which are theoretically capable









of producing satisfactory water from almost any saline
water. Among these may be listed chemical precipi-
tation, osmotic membranes, freezing, and ion-exchange.
In the present state of development none of these
methods can compete economically with the methods
described above in the demineralization of water.
Chemical precipitation has been accomplished on
a laboratory scale,4 but the great amount of both
chemicals required and solids precipitated make the
process very expensive.
Osmotic membranes have been tested on a labora-
tory scale with some success at the University of
Florida, but cost estimates for this process are not yet
available. The practical application of this method
will depend in large part upon the development of
membranes which will permit the passage of water
molecules but not the salt ions when sufficient pressure
is applied to the brine in contact with the membrane.
In the present state of development the ion-ex-
change process is not sufficiently advanced to be com-
petitive in the sea water demineralization program.
Where saline waters of less than 2,500 p.p.m. dissolved
solids are being desalted, however, the ion-exchange
process has marked advantages which in many instances
make it the method of choice. At higher levels of
salinity the fresh water requirements for regeneration
and rinse increase rapidly, and in treatment of sea
water practically all the product of the ion-exchange
system is consumed in these operations.3
In summary the electric membrane system and
the vapor-compression distillation method are leading
in the race to develop practicable means of reclaiming
fresh water from the sea. It is not implied, however,
that other methods should be excluded from con-
sideration. Work on any of the processes mentioned
in this paper may result in developments which will
reduce the fresh water production costs to a point
comparable with the more conventional systems.

Sanitary Engineering Considerations
There are at least three major sanitary engineering
considerations in the design and operation of any de-
mineralization plant. The first is that of salt water
pre-treatment. While sea water in the open ocean is
practically free of interfering substances, it is to be
expected that the normal source of saline water will
be from inshore or ground waters or even from salt
lakes. Such waters are likely to contain turbidity,
suspended and colloidal solids as well as many micro-
organisms. In this case a conventional water treatment
plant may be required preceding the demineralization
unit.
Secondly, the bacteriological and physical quality
of desalted waters must be reduced to a level compat-


ible with the ultimate usage. Irrigation waters rarely
require bacteriological control or removal of color
and turbidity, but such is not the case with potable
waters or waters used in many industrial processes. In
normal operation any of the evaporation or distilla-
tion methods will produce water free of contamination.
The fresh water, however, is subject to contamination
in two ways-by carry-over into the condenser and by
leakage of feed water into the condenser and heat-
exchangers. On the other hand effluents from electric
membrane and ion-exchange units will retain much
of the original contaminants and would be subject to
the usual water treatment processes now in practice.
It is also unlikely that any desalting process will re-
move taste and odor-producing substances. If they
are present in the salt water then either pre-treatment
of the salt water or treatment of the fresh water
effluent would be needed for potable supplies. In any
event, bacteriological and corrosion control should
be maintained on drinking water supplies.
The third consideration is a wastes disposal prob-
lem. Any process for desalting saline water will pro-
duce concentrated brine which may vary between
10 and 50 per cent of the product volume. If the
operation is located near the ocean, the wastes can
be disposed of quite easily by discharging the brine
into tidal waters. At inland locations the problem may
be quite serious since large quantities of brine will
have handled in such a manner that the surrounding
areas will not be damaged.
For this reason it is necessary that the sanitary
engineering profession keep abreast of developments
in the program to reclaim fresh water from the sea.

REFERENCES
1. Sverdrup, H. U., Johnson, M. W., and Fleming, R. H., The
Oceans, Prentice-Hall, Inc., 1946.
2. Ellis, C. B., et al, Fresh Water from the Ocean, Ronald Press
Co., 1954, p. 202.
3. Moore, E. W., "The Desalting of Saline Waters: A Review of
the Present Status," Journal New England Water Works
Association, 65, 319 (1951).
4. U. S. Dept. of the Interior, A Preliminary Discussion of a
Research Program on Demineralization of Saline Waters,
Oct. 1952.
5. Second Annual Report of the Secretary of the Interior on
Saline Water Conversion, January, 1954.
6. Anon., "Seawater Desalted Six Ways," Engineering News-
Record, 152, 18, 25 (May 6, 1954).
7. Anon., Engineering News Record, 150, 38 (March 12, 1953).
8. J. E. Kiker, Jr., unpublished report on Yonkers Water Supply,
May 8, 1947.
9. J. E. Kiker, Jr., unpublished correspondence.
10. Eliassen, R., "Reclamation of Saline Waters by Electro-dialysis
Shows Promise," Civil Engineer, 44, 366 (1954).
11. Aultman, W. W., "Desalting Sea Water for Domestic Use,"
Journal Am. Water Works Assn., 42, 786 (1950).


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