198 DESALINATION
the result of the combined effort of scientists and engineers.
However, it should not be forgotten that desalted water is an
industrial product and its cost can never compete with the
cost of natural fresh water supplies.
The largest desalination plant is Nature. The hydrological
cycle on earth begins by desalination of surface waters. As
the sun’s energy evaporates the water from the oceans and the
land surface waters, the vapors condense again on the earth’s
surface, as desalted water, stored as snow, ice or through the
soil returns to the rivers and seas. This water is the vital liquid
for all creatures on the earth.
The importance of water, as a matter of life, is quoted as
far back as there are records in history. We read in the Old
Testament: “Moses brought the sons of Israel from the Red
Sea and they went into the desert of Shur. They marched three
days in the wilderness and could not find water to drink. And
when they arrived to Merra they could not drink the waters
of Merra, for they were bitter. Therefore, he named this place
‘bitterness.’ And the people murmured against Moses, saying:
What shall we drink? And Moses cried onto the Lord. And
the Lord sweded a wood, which when he had cast into the
waters, the waters were made sweet.”^7 Nobody has guessed
what kind of wood this could be, but is the first known in his-
tory, technical desalination.
The effort of a desalination process is to separate one
of the most common, and most useful and yet most unusual
material—the water from the one of the next common mate-
rial, the salt. Hundreds of processes have been proposed,
based on the various properties of water and its saline solu-
tions. Nevertheless only a few of these methods have reached
such an advanced state of technology to be considered as safe
processes for the commercial conversion of saline waters
into fresh. Distillation processes, reverse osmosis and elec-
trodialysis or in some cases combination of two processes.
The expectations, connected with freezing processes, could
not be met with current freezing technology in large scale
industrial application.
The required separation may be of water from salt, or of
salt from water. Thus the desalination processes can be classi-
fied, according to the operation reference parameter as follow:1: Methods that separate water from salts
1.1: All distillation methods
1.2: Reverse osmosis
1.3: Crystallization (freezing and hydrates)2: Methods that separate the salts from the water
2.1: Electrodialysis
2.2: Ion-exchange
2.3: Piezodialysis
2.4: Osmionic methods3: Methods with phase change
3.1: All distillation methods
3.2: Crystallization4: Methods without phase change
4.1: Reverse osmosis
4.2: ElectrodialysisFrom the energy point of view, the methods are classi-
fied as follow:
5: Methods using heat (thermal methods)
5.1: All distillation methods except mechanical
vapor compression6: Methods using mechanical energy
6.1: Mechanical vapor compression
6.2: Reverse-osmosis7: Methods using electrical energy
7.1: Electrodialysis8: Methods depending on chemical energy
8.1: Ion-exchangeThe methods which found practical applications in large
scale industrial plants are:
Distillation methods: which comprise the following
modifications:1: Multiple-Effect Evaporation ME
2: Multi-Stage-Flash Evaporation MSF
3: Vapor-Compression methods VC
4: Solar distillation method SDDistillation is the most developed process of removing
water from a saline solution. It is applied up to very large
capacities with various types of evaporators and accounts for
about 59.4% of the total world plant capacity.^5
The latent heat of changing phase is an important
factor in the overall process economics, but the degree of
salinity of the raw water is of no importance. Multistage
flash distillation and multi-effect evaporation are reduc-
ing considerably the economic effect of the latent heat of
vaporization.
Reverse osmosis uses mechanical energy, as pres-
sure, to drive the water out of the solution through semi-
permeable membranes. The applied pressure must be higher
than osmotic pressure, its value depending from the salt
content of the brackish or seawater solution. The necessary
counterpressure in reverse osmosis depends greatly upon
the salt content of the raw water and imposes constraints
on membrane life and performance, but also varying energy
consumption according to the salinity of the raw water.
Membrane life is an important cost factor.
Today reverse osmosis plants account, worldwide, for
32.6% for plants having a capacity 100 to 4000 m^3 /d and
19.5% for capacities over 4000 m^3 /d.
Electrodialysis is the most developed process for
eliminating salts from aqueous solutions. The economics
depend closely on the salt content of the raw water, as the
consumption of electric energy is related to the total dis-
solved solids removed from the solution. Electrodialysis
may, therefore, preferably be applied for the purification of
brackish waters. Reversal electrodialysis is a modification,
by which poles are reversing every 20 and which assures
the production of high-quality water and minimizes the
rejection of brine.C004_001_r03.indd 198C004_001_r03.indd 198 11/18/2005 10:18:50 AM11/18/2005 10:18:50 AM