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
(b) Aluminum sulfate added to water containing magnesium
AI2(SO4)3 18 H2O + 3Mg(HCO3) → 3 MgSO4 + 2 Al2(OH)3 + 6 CO2 + 18 H2O
(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