TM 5-813-8
CHAPTER 9
POST-TREATMENT
The Langelier Corrosion Index can be adjusted to a
General. Desalination can and most often does
9-1.
slightly positive value by pH adjustment.
The
produce water so free of dissolved material that
adjustment of the distilled water pH should occur
corrosion and potential health problems are possible.
immediately after reaeration, prior to distribution, as
Electrodialysis reversal is an exception to this general
airborne carbon dioxide may influence the pH.
rule.
Electrodialysis reversal does not remove
d. Residual disinfection.
Residual disinfection
microorganisms.
should be present in the active parts of the distribution
Remineralization and aeration. Pure water is
9-2.
system. A break-point chlorination curve is shown in
a relatively reactive chemical. When air is dissolved in
figure 9-1. Chlorine disinfection of very pure water
extremely pure water, the resulting solution is extremely
leads to a chlorine residual that is free of combined
corrosive. Water without dissolved air tastes flat and
chlorine or chloramines. Chlorination of distilled water
objectionable. Water that contains very little hardness
systems should occur immediately prior to the
appears to be unhealthy for drinking purposes.
distribution system.
Where it is convenient or
Therefore, the designer of a desalination system must
economical, calcium hypochlorite should be used to
consider some posttreatment options, such as
control pH to aid in recalcification and to disinfect the
remineralization and aeration, in order to produce a
distribution system. Disinfection of Army potable water
healthful, noncorrosive, pleasant-tasting water.
supplies is discussed in TB MED 229 and TB MED 576.
Post-treatment of distilled water. The mineral
9-3.
e. Reblending for remineralization. One of the
content of distilled water is much lower than is
most cost-effective ways to remineralize distilled water
considered healthy for drinking purposes. Distilled
is to blend a small stream of pretreated saline feed
potable water should be aerated and remineralized for
water back into the distilled product water. When
health, taste, and to prevent corrosion.
Some
bacterial contamination is sufficiently low and hardness
radionuclides
may
not
be
removed
by
is sufficiently high, then reblending is an acceptable
distillation/condensation, which may require post-
means of distilled water remineralization.
When
treatment by ion exchange, prior to all other post-
reblending is practiced, boiler feed water should be
treatment methods.
removed from the system upstream of the reblend point.
a. Reaeration. Distilled potable waters will be
Saline water used for reblending must have a 30-minute
reaerated before use. Cascade, eductor, or sprayed
chlorine contact time. This chlorinated reblend stream
aeration are acceptable but must be followed by
is an excellent method of providing the total chlorination
disinfection to prevent bacterial contamination.
for the distribution system. As further discussed in
b. Mineralization. Recent studies have indicated
Chapter 10, brine can be used to electrolytically produce
that the hardness of drinking water is important for
hypochlorite ion for disinfection.
human health. The Langelier Corrosion Index was
9-4.
Post-treatment of reverse osmosis product
developed to predict the tendency of water to deposit or
water. Unlike distilled water, reverse osmosis product
dissolve calcium carbonate. Recalcification of distilled
water is not necessarily free of dissolved gases. If
water will be performed in order to reach a positive
degasification is not used as a pretreatment step, the
Langelier Corrosion Index by adding lime. Fluoridation
dissolved gas content of the reverse osmosis product
of Army potable water supplies is discussed in TB MED
stream will be roughly that of the feed stream.
576.
Since divalent and trivalent ions are better eliminated by
c. Corrosion control. Distilled water has a greater
reverse osmosis than are monovalent ions, reverse
corrosion potential once it is aerated.
osmosis product water is usually very soft and has a
Any boiler feed water should bypass reaeration where
hardness level that is unhealthy when used as drinking
possible. Corrosion should be controlled by the addition
water.
of sodium hexametaphosphate or sodium silicate, in
a. Corrosion control. As a consequence of the
conjunction with remineralization, prior to entry into the
dissolved gas content of the reverse osmosis product
distribution system.
water, corrosion control should occur as close as is
convenient to the suck-back tank.
9-1
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