Desalination of Seawater0 pages

Application Data Sheet

Water and Wastewater Industry

ADS 4950-15/rev.C
January 2011

Desalination of sea water
BACKGROUND

SCALING AND pH CONTROL

Desalination is the name given to processes that
remove salt from water. Although 70% of the world’s
surface is covered with water, almost all of this water
cannot be used without some removal of salt. Areas
with large, growing populations and little fresh water
resources are increasingly turning to desalination for
their water needs. Many industrial plants and electrical
utilities are located by the ocean so that they can be
assured of a large, dependable, fixed-cost water supply.
Sea water, however, is so corrosive that it is generally
more cost effective to remove the salt than to replace
piping or install pipe of more resistant materials.

There are two separate scaling problems that must
be dealt with in a sea water evaporator. The first
includes both calcium carbonate (limestone) and
magnesium hydroxide scale. These are formed
under higher pH conditions when bicarbonate ions
can form carbonate ions and when more hydroxyl
(OH-) ions are present. Carbonate and hydroxyl ions
react with the calcium and magnesium normally
present in seawater (Figure 1).

In a typical desalination process, raw seawater is
filtered to remove solids and then combined with acid
in a treatment tank. The treated seawater is heated in
an evaporator, with its resulting vapor collected and
recondensed in the condenser (see Figure 1). The
condenser product is fresh water, and the salt from
the seawater is removed as brine. One of the problems in this process is scale formation on the hot
surfaces of the evaporation equipment. Careful pH
control can minimize some of the scaling problems.

CaCO3 and Mg(OH)2 scaling can be minimized by
lowering the pH of the feedwater under 5.7, thus
preventing the formation of appreciable amounts
of scale. This is commonly achieved by adding citric
acid, ferric chloride, or sulfuric acid. However, too
much acid will result in corrosion. A proper pH
balance can be maintained using the Rosemount
Analytical Model 1056 dual input analyzer and the
Model 3900 pH/ORP sensor.
The second scale problem appears when the feed
solution becomes saturated with calcium sulfate.

DUAL INPUT ANALYZER
(pH & CONDUCTIVITY)

FIGURE 1. Seawater Desalination

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