Encyclopedia of Environmental Science and Engineering, Volume I and II

DESALINATION 199

Today electrodialysis accounts for 5.7% of plants with

capacities 100 to 4000 m^3 /d and 2.5% for capacities over

4000 m^3 /d.

Freezing processes found no commercial application

though the simplicity of the method. They failed because the

size of produced ice was very small and half of the fresh

water was used to wash out the salt from ice surface, render-

ing the method uneconomical.

Independently from the method or procedure for sea or

brackish water desalination the operation of a desalination

plant includes some general steps to be followed. Figure 2

gives the procedures before and after the main desalina-

tion step.

ENERGY SOURCES

Running a desalination plant many expenses arise, the high-

est of which is energy cost. In a normal chemical plant energy

cost is low, only 1 to 5% and, in some extreme occasion, 10%

of the total operation cost. On the contrary, desalination is a

high energy consuming procedure and the cost of necessary

minimum energy to run the plant is 40% of the total cost.

The main energy sources, depending on the method, are

low pressure stream and electricity, two energy sources easily

available in any industrialized region. Few other energy

sources are given at lower cost or free of charge. These alter-

native energies are suitable for small capacity plants and/or

for remote and arid regions, where fuel and electricity are

not available or the cost of fuel transportation renders its use

uneconomic.

Alternative energy sources include geothermal energy

when and where is available, all kinds of waste heat and

waste heat from nuclear plants.

Renewable energy sources include wind energy, tidal

energy, Ocean Thermal Energy Conversion (OTEC) and,

above all, the abundant solar energy.

Waste heat is available from chemical industry, power

plants and nuclear power plants in large amounts, but in low

heat content. Wind and tidal energy are available in certain

specified regions, transforming the corresponding energy

into electricity. OTEC takes advantage of the temperature

difference between the ocean surface and about the 500 m

depth of the tropical regions. Solar energy is for the time

being the most promising renewable energy. In the earth’s

sunny regions solar radiation is very intensive though also

very spread out, thus the capture of solar energy depends on

large areas.

Although solar, wind and tidal energy are natural forces

given free, the corresponding equipment for transformation

of these energies into a usuable form are yet very expensive

and the yield very low.

DISTILLATION PROCESSES

Aristotle, the ancient Greek philosopher, wrote: “Salt water,

when it turns into vapor, becomes sweet, and the vapor does

not form salt water again when it condenses.” Sailors have

used simple evaporation apparatus to make drinking water

for almost 400 years, and ocean going ships have tradition-

ally used evaporators, often multiple-effect, as an accessory

to steam boilers.

The simplest way to evaporate water is the natural one,

using solar heat. Sun is a free inexhaustible source of energy.

However, this energy has not been captured and stored at its

most concentrated form as yet. The way to use solar energy

for desalination purposes depends on the desalination pro-

cess. The simplest and most common method is the direct

use of the solar energy in specific equipment called “solar

stills” which act simultaneously as converters of solar energy

to heat and as distillers.^8

Indirect use of solar energy, called “solar assisted” or

“solar driven” desalination, captures the solar radiation using

one of the modern procedures which transform the energy

into either heat or electrical power. Horizontal tube, multiple-

effect (HTME), multi-stage-flash (MSF) and thermal vapor

compression (TVC) distillation methods are coupled to the

REMOVAL OF

COARSE

MATERIAL

FEED WATER

PRETREATMENT

SEAWATER

INTAKE

MATERIAL

STORAGE

DESALINATION

INSTALLATION

VAPOR, POWER

CONDENSATE

BRINE

REJECTION

FRESH WATER

FRESH WATER

DISTRIBUTION

FRESH WATER

STORAGE

POST

TREATMENT

POWER

STATION

FIGURE 2 Flow diagram of the main procedures to be followed

in the operation of a desalination plant.

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