systems through the formation of scale. Materials that
General. Before raw water is desalinated, the
precipitate easily to form scale are double-charged
undesirable materials will be removed or reduced to
positive ions, such as calcium and magnesium salts. As
acceptable levels. Such materials to be removed
water volume is reduced, the concentration of all ions
include solids, immiscible liquids, and sparingly soluble
and materials is increased. When the concentration
salts. Accumulations of these materials on desalination
reaches saturation, scale is formed. With alkaline earth
process surfaces greatly reduce efficiencies. Without
sulfates and carbonates, the saturation concentration is
adequate pretreatment, desalination facilities are
reduced by increasing temperature. Ion exchange (see
destined for reduced lifetimes, shortened periods of
Chap. 8) as well as chemical treatment followed by
operation, and high maintenance.
filtration can reduce the levels of sparingly soluble salts
limitations for various desalination processes are
before desalination processes.
However, such a
summarized in table 5-1. An overview of typical
treatment system may not be cost effective, economic
pretreatment options is shown in figure 5-1.
evaluation is needed before proceeding with detailed
Physical and chemical treatment
design. The concentration factor limits for calcium
processes will be used to remove solid materials. The
sulfates in sea water are shown in figure 5-2.
level of solids removal will depend on the desalination
Chemical treatment. Many materials, e.g.,
process to be used.
calcium salts, magnesium salts, oil and grease, total
(1) Settleable solids. Settleable solids are
suspended solids and some organic contaminates, that
easily removed from water by gravity settling or
are harmful to desalination processes can be reduced to
acceptable levels by chemical treatment. Chemical
treatment may involve any of the following: coagulation,
(2) Non-settleable solids.
either with or without clarification; pH adjustment, which
solids are removed by chemical treatment. Chemical
may influence fluoride removal; or scale inhibition.
treatment may be designed to remove not only non-
When required, degasification and secondary filtration
settleable solids but to precipitate and remove scale-
follow chemical treatment.
a. Coagulation. Non-settleable solids and some
b. Immiscible liquids. Emulsified oil and
suspended materials do not precipitate because of
grease are the principle sources of immiscible liquid
electrical charges on the surface of the particles. If the
fouling in desalination facilities. Use gravity settling and
charges on the particles can be reduced, the particles
chemical treatment processes followed by a filtration
may precipitate. Chemicals that lower surface charges
system to maintain immiscible liquid concentrations
are lime, alum, ferric salts, and polyelectrolytes.
within acceptable levels. To measure low levels of oil
Evaluate each chemical used in pretreatment to
and grease, a sophisticated concentration procedure
determine its effect on the successive steps in the
must be performed.
desalination process. For example, calcium present in
lime can cause an increase in scale formation in
common organic materials, tannins and humic acids,
can be removed from water by chemcial treatment.
ferric salts can cause excessive fouling in membrane
Some organic materials will carry across a
desalination systems; alum is usually the best coagulant
for desalination systems. Do not use pretreatment
Pesticides and industrial organic chemicals may be
chemicals without prior operational experience and an
difficult to remove by distillation/condensation. Reverse
exhaustive actual application review.
osmosis will usually remove the majority of the large
settleable solids and the suspended solids in conjunction
pesticide molecules. If during pretreatment tests a non-
with any added coagulants will be removed by either
ionized organic contaminate is not reduced to
standard clarification techniques or direct filtration.
acceptable levels, then a full treatability investigation
will be performed to ensure that potable water standards
As physical and
d. Sparingly soluble salts.
chemical conditions change during desalination, some
soluble materials become insoluble (precipitate). This