DESALINATION 195
The tropical regions, where the temperature is about 30C
in summer and 22C during winter. The subtropical regions,
where in warm periods the temperature is about 30C, and it
ranges between 10 and 22C in the cold months. The regions,
as in the Mediterranean coasts, where during warm periods
the temperatures are 22 to 33C, and in cold months 10 to
22 C. The cool coastal deserts have in summer temperatures
under 22C and in winter time 10 to 22C. The last desert
regions are the cold places where summer is under 22C and
winter under 10C. The largest single coastal desert of the
third type, with the moderate climatic conditions, is that of
the Mediterranean Sea and covers about 2650 km^2.^1
Coastal deserts have an advantage over the interior
deserts. They are climatically more pleasant, because they are
cooler in summer and warmer in winter. Further, they have
advantages over the interior deserts from the desalination
point of view. Coastal deserts are surrounded by abundant
sea water supply which is in the same level as the desalt-
ing installation, and thus the intake of water can be pumped
with less power consumption than the deep well salty or
brackish waters of the inland deserts. The brine disposal is
also easy, without problems, as it is discharged directly into
the sea, whereas the disposal of brine in inland deserts may
create serious problems. Also, coastal deserts are in favour
over the inland ones concurring the transportation of the
equipment and all other necessary supplies for a desalina-
tion plant. Figure 1 shows a modern desalination plant in
Saudi Arabia.
Some of the most attractive areas and beaches of the
world are almost devoid of water. Not only is this living
space, and space for resort hotels lost, but, in some cases,
profitable resources cannot be exploited. Thus, known min-
erals on Egypt’s Red Sea coast cannot be mined, and fish-
ing industries on South America’s Pacific coast, and other
places around the world, cannot be expanded for lack of
water. These present major losses in the world supplies of
minerals and foods.WATER DEMAND AND USAGEThe water cycle leaves about 9000 km^3 of water worldwide
per year. This amount is enough to provide, with good qual-
ity water, about 20 billion people, but this water is far from
evenly divided, with major shortages in some regions and
abundant quantities in other places.
In a modern urban agglomeration, supply of water may
satisfy domestic, municipal and industrial demand, as well
as agricultural needs. There are no standards of general
acceptance for the quality of water required by each group
of users. Domestic demand includes all water consumed in
housekeeping and gardening. A limit of 500 mg/L (ppm) for
total dissolved solids with a maximum of 250 mg/L for chlo-
ride and sulphate ions, respectively, is recommended by the
World Health Organization (WHO).^2 Nevertheless, there is
a large number of communities, which are still consuming
water containing up to 1000 mg/L total dissolved solids and
sometimes more. Physiological changes may result from theintake of large amounts of the main ions, as well as of some
trace elements.
Municipal requirements, beside the supply of water for
domestic use, include all water needed by offices, public and
commercial establishments, fire-fighting and irrigation of
municipal parks. Although the standards for the latter uses
are not strictly the same, as for drinking water, in practice
all municipal water requirements are identical to drinking
water since it is nearly always supplied by the same piping
system.
A large variety of quality standards is involved in the
use of industrial water, depending on its specific use. They
may vary from high-quality drinking water for food process-
ing to completely demineralized water for specific uses.
Limitations of salt content may be imposed in some cases for
process water. Boiler feed water needs special treatment to
minimize salt content and eliminate dissolved gases. Cooling
water also needs some treatment to meet the process require-
ments. River water and sea water can be used for cooling
purposes and this is the usual practice in plants located on a
river or near the seashore.
About 70% of water withdrawn from the earth goes for
agriculture purposes and the balance, 30%, for various uses,
as household and industrial process water. Overirrigation the
last years, brought salinization of the nearby water resources,
affecting the soil and crop quality, as salts are accumulating
in the soil.
Irrigation water quality, which includes also drinking
water for animals, depends to a large extent on the nature of the
soil, the crops and the climate. The yield and quantity of some
crops can be affected, not only by the total amount of dissolved
solids, but also by the presence of certain specific salts. Thus if
desalinated water is to be used in certain places the make-up of
the product water will be necessary.
The water withdrawn per year and per capita, concern-
ing industry and agriculture is increasing by 8.5%, the main
increase in the developed countries. The USA consumes
2500 m^3 per year per capita, Switzerland 500 and Ghana,
a very poor African country, only 40 m^3.^3
Meanwhile, the majority of fresh water streams are
severely polluted, decreasing the quality water reserves. Self
decontamination is not feasible in many cases and, thus,
treatment methods have to be applied to degrade at least
some of the pollutants in the water. On the other hand, sea
water exist in huge amounts, given free. Although also pol-
luted to some extent, it is a future source of fresh water as
desalination is the future process to produce this valuable
good quality water.SEA WATERThe seas and oceans are great sources of material available
to mankind, though their destiny is very low to be exploited,
but high enough to make the water salty, unsuitable for drink-
ing or processing purposes. Not all the seas around the world
have the same amount of total dissolved solids, the amount
of which range from 20,000 to 50,000 ppm.C004_001_r03.indd 195C004_001_r03.indd 195 11/18/2005 10:18:48 AM11/18/2005 10:18:48 AM