TM 5-813-3/AFM 88-10, Vol 3
reservoirs, sometimes contain dissolved iron and man-
tening is an effective means of removing both iron and
manganese.
ganese. Their presence in solution is associated with
anaerobic conditions near the bottom of the reservoir.
f. Stabilization of iron and manganese. Under some
b. Effects of iron and manganese. Dissolved iron in
circumstances, stabilization of iron and manganese by
application of a polyphosphate compound may be ac-
excess of 1 or 2 mg/L will cause an unpleasant taste,
ceptable. The iron and manganese in the water are
and on standing, the water will develop a cloudy ap-
pearance. Iron concentrations appreciably greater
maintained in a dispersed state through the complet-
than 0.3 mg/L will cause red stains on plumbing fix-
ing action of a polyphosphate compound. Dosages of
about 5 mg/L of sodium hexametaphosphate for each
tures and laundry. Similarly, manganese will cause
mg/L of iron and manganese are reasonably effective
black stains if present to the extent of more than about
however, the total polyphosphate dosage should not
0.05 mg/L. Deposits of iron and manganese can build
up in water distribution systems and periodic "flush-
phate stabilizing compound must be added to the
outs" of these deposits result in objectionable color and
water prior to chlorination. If the chlorine is applied
turbidity at the consumer's tap.
c. Removal by oxidation and filtration. Oxidation
first, it will oxidize the iron and manganese to in-
can be accomplished with dissolved oxygen, added by
soluble forms rendering the stabilizing agent ineffec-
tive. Stabilization of concentrations of iron and man-
aeration, and by the addition of an oxidizing chemical,
such as chlorine, chlorine dioxide, potassium perman-
ganese in excess of approximately 1.0 mg/L is general-
ly not satisfactory. Also, stabilization will not persist
ganate, or ozone. Manganese is more difficult than
iron to oxidize and precipitate. In the absence of man-
if the water is heated because heating coverts poly-
phosphates to orthophosphates which have no stabiliz-
ganese, iron can often be removed with minimum
treatment, consisting of aeration followed by direct fil-
ing power. Stabilization, although helpful, is not a sub-
stitute for iron and manganese removal, and, in gen-
tration. In general, aeration alone will not oxidize
eral, should be viewed as a temporary expedient to be
oxidants, such as chlorine or potassium permanganate,
used pending installation of removal facilities.
are effective at lower pH values. To insure oxidation,
2-12. corrosion and scale control.
precipitation and agglomeration of iron and manga-
"Corrosion" can be defined as the deterioration of
nese and their essentially complete removal, at least
metal by direct chemical or electrochemical reaction
three treatment steps are usually necessary: aeration,
with its environment. "Scale" refers to an accumula-
contact time, and filtration. An aerator containing
tion of solids precipitated out of the water. In water
trays of coke, limestone, etc., as mentioned in para-
graph 2-3c is commonly used. Reaction time is pro-
treatment, corrosion and scale are closely associated.
Both must be considered in connection with the design
vided by a contact or contact-sedimentation basin hav-
ing a detention period of at least 30 minutes. Filtra-
and operation of treatment works. This scale may be
desirable because it can provide a measure of protec-
tion is accomplished by conventional single or multi-
tion against corrosion. However, thick layers of scale
media filters designed for a filtration rate of at least
are detrimental in both hot and cold water systems. It
3.0 gpm per square foot. The aeration step is frequent-
ly supplemented by a chemical oxidant, such as chlo-
is essential to produce a "balanced" water that is
neither highly corrosive nor excessively scale forming.
rine or permanganate. Flocculation is advantageous in
a. Corrosion.
the contact basin, particularly if iron exceeds about 2
mg/L.
(1) The extent and nature of corrosion reactions
d. Removal by ion exchange. The cation exchange
depend upon many factors. Among the most impor-
(sodium zeolite) softening process, under proper condi-
tant are the chemical and physical nature of the water,
tions, is capable of removing limited amounts of dis-
its velocity, pipe metallurgy and pipe coating. In exist-
solved (unoxidized) iron and manganese. For applica-
ing systems, where corrosion is a problem, most of
tion of this process, it is essential that the raw water
these factors, with the exception of the chemical
and wash water contain no dissolved oxygen and that
nature of the water, are not readily susceptible to
the sum of the iron and manganese concentrations not
change. Consequently, for these situations, emphasis
exceed about 0.5 mg/L. The presence of oxygen or
must be placed on adjustment of the water's chemical
higher concentrations of iron and manganese will
quality as the only practical means of corrosion control
cause rapid fouling of the exchange resin with conse-
in an existing system. Controllable factors are princi-
quent loss of removal capacity. If fouling occurs, treat-
pally calcium content, alkalinity and PH. Certain cor-
ment of the resin with sodium bisulfite solution and
rosion inhibitors can also be used, but relatively few
dilute hydrochloric or sulfuric acid will be required to
are suitable for potable water systems.
restore capacity.
(2) Treatment to insure deposition and mainte-
e. Removal by lime-soda softening. Lime-soda so f-
nance of a thin layer of calcium carbonate on the pipe
2-25
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