TM 5-813-3/AFM 88-10, Vol 3
and the feed equipment should be designed to mini-
b. Fluoridation chemicals. Chemicals most fre-
mize the possibility of free flow (flooding) of chemical
quently used for fluoridation are: Sodium silicofluo-
through the feeder. Normally, the feed machine's
supply hopper should hold no more than 100 to 200
--
pounds of chemical. Large extension hoppers holding `
of chemical will depend principally on delivered cost
much greater quantities of dry fluoride chemical in-
and availability.
crease the danger of flooding and overfeeding and are
(1) Sodium fluoride. This chemical is commercial-
not recommended for most installations. Solutions of
ly available as a white crystalline powder having a
sodium silicofluoride are acidic and corrosion-resistant
purity of 95 to 98 percent. (Sometimes it is artificially
dissolvers and solution piping must be provided where
colored nile blue.) Volubility is approximately 4 per-
this chemical is employed. If fluosilicic acid is used, it
cent at 770 F. The pH of a saturated solution is 6.6.
can be applied by means of a small metering pump into
The 100 percent pure material contains 45.25 percent
an open channel or a pressure pipe. Storage tanks,
fluoride. It is available in 100-pound bags, 125 to 400
feeders, and piping for fluosilicic acid must be made of
pound drums, and bulk.
corrosion-resistant material. The acid is slightly vola-
(2) Sodium silicofluoride. This compound is com-
tile and the feed system should be enclosed. If not en-
mercially available as a white powder with a purity of
closed, special exhaust ventilation should be provided
98 to 99 percent. Volubility is only about 0.76 percent
to protect personnel from fluoride fumes.
at 770 F. The pH of a saturated solution is 3.5. The
100 percent material contains 60.7 percent fluoride. It
e. Fluoride removal. Fluoride removal can be accom-
is available in 100 pound bags, 125 to 400 pound
plished by passage of the water through beds of acti-
drums, and bulk.
vated alumina, bone char, or tricalcium phosphate.
(3) Fluosilicic acid. This chemical is commercially
When the capacity of the bed to remove fluoride is ex-
available as a liquid containing 22 to 30 percent by
hausted, it can be regenerated by treatment with a
weight of fluosilicic acid. It is sold in 13 gallon car-
caustic soda solution followed by rinsing and acid neu-
boys, 55 gallon drums, and in bulk. The 100 percent
tralization of the residual caustic soda. Other methods
pure acid contains 79.2 percent fluoride. The pH of a 1
of fluoride removal include electrodialysis, reverse os-
percent solution is 1.2, and the use of fluosilicic acid as
mosis and ion exchange. Some fluoride reduction can
a fluoridation agent in a water of low alkalinity will
be obtained by water softening using excess lime treat-
significantly reduce the pH of the water. It should not
ment. Fluoride reduction by this method is associated
be used for fluoride adjustment of waters of this type
with magnesium precipitation and the extent of fluo-
unless pH adjustment is also provided.
ride removal is a function of the amount of magnesium
c. Point of application. It is essential that all water
precipitated from the water. All removal processes
pass the point of injection of the fluoridation chemical
produce liquid wastes and suitable provision must be
and that the flow rate past this point be known with
made for their disposal. Guidance as to the fluoride re-
reasonable accuracy. At a water treatment plant, the
moval process to be employed can be obtained from
preferred application point is usually the combined ef-
laboratory studies of process effectiveness and fluo-
fluent of all filters. The fluoride chemical can be fed at
ride removal capacity, using samples of the water that
an earlier stage of treatment, for example, the com-
is to be treated.
bined filter influent, but part of the fluoride applied
will be removed by the filtration process. Coagulation
2-9. Taste and odor control.
and lime softening will also remove a small amount of
Most taste and odors in surface water are caused by
the applied fluoride. A larger dose is required to offset
low concentrations of organic substances derived from
treatment process losses. If ground water is the supply
decomposing vegetation, microscopic organisms, sew-
source, the fluoride chemical should be injected into
age and industrial waste pollution, etc. Treatment for
the discharge pipe of the well pump. Where the supply
taste and odor removal involves destruction of the
is from several wells, each pumping independently to
odorous substance by chemical oxidation or its re-
the distribution system, it will be necessary to provide
moval by aeration or adsorption or activated carbon.
an injection point at each well. If flow past the injec-
a. Chemical oxidation. Chemical oxidizing agents
tion point is variable, automatic equipment that will
which have been found effective and which can be
feed fluoride chemical at a rate proportional to flow is
used in the treatment of potable water are chlorine,
a requirement.
chlorine dioxide, potassium permanganate, and ozone.
d. Fluoride feeders. Volumetric or gravimetric dry
No single chemical is completely effective under all
feeders equipped with dissolvers are suitable for
operating conditions.
sodium fluoride or sodium silicofluoride. Feeders
b. Aeration. Aeration is helpful in eliminating odor
should be equipped with weighing devices that will ac-
caused by hydrogen sulfide, but is ineffective in signif-
curately measure the weight of chemical fed each day
2-18
Previous Page
