Aluminum Sulfate (Alum).

water and the minerals present, which is combine with the coagulant, are the principal

determinants as to which chemical is used. The following chemicals are some of the

more commonly used coagulants:

a. Aluminum Sulfate (Alum). Aluminum sulfate (AI2 (SO4) 3), commonly known

as alum or fiIter alum, is one of the most commonly used coagulants in military water

treatment. It is available commercially in solid form or as a solution called "Iiquid alum."

Aluminum sulfate reacts with naturally occurring alkalinity in water to form aluminum

hydroxide, a white precipitate that forms a floc and settles readily.

(1) Two chemical reactions showing the use of aluminum sulfate as a

coagulant are given below.

(a) Aluminum sulfate added to water containing calcium bicarbonate.

AI2(SO4)3 ·.18 H2O + 3 Ca(HCO3) → 3 CaSO4 + 2 Al(OH)3 + 6 CO2 + 18 H2O

aluminum

calcium

aluminum carbon

water

sulfate

bicarbonate

sulfate

hydroxide dioxide

(b) Aluminum sulfate added to water containing magnesium

bicarbonate.

AI2(SO4)3 · 18 H2O + 3Mg(HCO3) → 3 MgSO4 + 2 Al2(OH)3 + 6 CO2 + 18 H2O

aluminum

magnesium

aluminum

carbon

water

sulfate

bicarbonate

sulfate

hydroxide

dioxide

(2) Aluminum sulfate has an optimum pH range of 4.0 to 7.0, which is

slightly acid. When treating water within this pH range, the usual practice is to add soda

ash (see c, below) to provide sufficient alkalinity for precipitation of all the aluminum as

aluminum hydroxide. Aluminum sulfate is desirable in that it is inexpensive; it is easy to

store, transport, and apply; and it reduces the temporary (carbonate) hardness in the

water. Disadvantages of aluminum sulfate are that it produces carbon dioxide (carbon

dioxide is corrosive to pipes and other metals) and that the calcium or magnesium

sulfate, which is, produced causes permanent hardness.

b. Ferric Chloride. Ferric chloride (FeCI3) is available in liquid, crystal, or

anhydrous form. Optimum results are obtained with ferric chloride under both acid (pH

3.5 to 6.5) and alkaline (pH above 8.5) conditions. The floc formed by ferric chloride is

heavier than that formed by alum; therefore, it settles more rapidly. It also settles more

rapidly in cold water than does alum floc. Ferric chloride is very corrosive, both

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