because of the chlorine ion present and because carbon dioxide is formed in the
coagulation process. The chemical reaction that occurs when ferric chloride is added to
water containing calcium bicarbonate is as follows:
3 Ca(HCO3)2 → 2 Fe(OH)3 + 3 CaCI2 + 6 CO2
2 FeCI3
+
ferric
calcium
ferric
calcium
carbon
chloride
bicarbonate
hydroxide
chloride
dioxide
c. Coagulant Aids. These materials do not produce coagulation when they
alone are added to water, but they improve the results when they are used with
coagulants. The two coagulant aids commonly used in military water treatment are
(1) Soda ash (sodium carbonate). Soda ash (Na2CO3) is used with alum to
supply the necessary alkalinity for precipitation of the aluminum. It does not increase
the hardness, but it does increase the corrosiveness somewhat. The chemical reaction
is as follows:
AI2(SO4)3 18 H2 + 3 Na2CO3 → 3 Na2SO4 + 2 AI(OH)3 + 3 CO2 + 15 H2O
Aluminum
sodium
sodium
aluminum
carbon
water
sulfate
carbonate
sulfate
hydroxide
dioxide
(2) Limestone (calcium carbonate). Crushed or powdered limestone
(CaCO3) is used with ferric chloride in the same manner as soda ash and three
aluminum sulfate. The reaction is as follows:
3 CaCO3 + 3 H20 → 2 Fe(OH)3 + 3 CaCI2
2 FeCI3 +
+
3 CO2
ferric
calcium
water
ferric
calcium
carbon
chloride
carbonate
hydroxide
chloride
dioxide
1-12. CHEMICAL APPLICATION
a. Feeding. The amounts of chemicals added to the water must be carefully
controlled to insure uniform treatment. Many types of chemical feeders are available,
but dry feeders (see figure 1-2) are used most often because of their greater accuracy
and simpler operation. They can be adjusted to feed automatically by connecting a flow
measuring device, which proportions chemical feed to rate of water flow. The dry
chemical is fed at the proper rate to a small tank or mixing chamber where it is
suspended in water. The resulting slurry is added to the water to be treated.
MD0160
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