TM 5-813-3/AFM 88-10, Vol 3
Table 2-5 Constant B as a Function of Total Dissolved Solids
(a) One means of chemical control that has been
B
TDS mg/L
reasonably successful is treatment of the water to en-
9.70
0
sure deposition and maintenance of a coating of cal-
9.77
100
cium carbonate. Prediction of the tendency of a water
9.83
200
to precipitate or dissolve a protective coating of cal-
400
9.86
800
9.89
cium carbonate can be based on computation of what is
9.90
1,000
termed the "Langlier Index" (LI). This index is calcu-
Reprinted from Standard Methods, for the Examination of Water
lated as follows:
(eq 2-1)
American Public Health Association.
where:
(c) Examples of the calculation procedure to be
LI = Langlier Index
pH = Actual pH of the water
saturation with calcium carbonate
in the water's alkalinity, calcium content, dissolved
the actual pH, the LI will be positive, indicative of a
greater than the actual pH, the LI will be negative and
this indicates under saturation or a tendency toward
dissolving calcium carbonate, and corrosivity. An LI
value of O indicates exact saturation with calcium
carbonate and no tendency toward deposition or solu-
tion.
(b) The complete equation for the exact calcula-
covering the pH range 6.5 to 9.5, may be used. The
simplified equation is as follows:
where:
(d) The LI is not a quantitative index in the
sense of providing a numerical measure of the amount
A = constant which is a function of water tem-
of calcium carbonate that will be precipitated or dis-
perature
solved. Rather, it merely indicates a tendency in the
B = constant which is a' function of total dissolved
direction of precipitating or dissolving calcium carbon-
solids concentration (TDS, mg/L)
ate. If the water is extremely soft (deficient in calcium
2+
2+
log(Ca ) = logarithm to the base 10 of the Ca
ions) and has a low alkalinity, the water's capacity for
concentration in mg/L
protection will be minimal even though a high pH and
log (Alkalinity) = logarithm to the base 10 of the
a positive LI are consistently maintained. The water
total alkalinity expressed as
should contain at least 50 mg/L of alkalinity and at
CaCo 3, in mg/L.
least 50 mg/L of calcium hardness in order to take ad-
2+
Values of Ca and Alkalinity are obtained from
vantage of calcium carbonate protection. For softened
analytical data.
waters, maintain an LI of about + 1.0, and, in addition,
The values of A and B are obtained from the tables
apply about 0.5 mg/L of polyphosphate to the filtered
2-4 and 2-5.
water in order to prevent excessive deposition in
pumps and mains near the treatment plant.
A
(e) The maintenance of a positive Langlier Index
2,60
32
does not preclude the possibility of corrosion. Condi-
2,50
39
tions may be such that only a partial coating of cal-
2.40
46
2.30
cium carbonate is deposited, resulting in a type of cor-
54
2.20
61
rosion at the uncoated areas, known as "pitting." Pit-
2,10
68
ting corrosion results in loss of metal from relatively
small areas of the pipe rather than informly over the
entire surface. As a consequence, the pipe may fail
*Reprinted from Standard Methods, for The Examination of
fairly quickly because of corrosion penetration of the
Water and Wastewater; 14th ed., by permission. Copyright 01976,
pipe wall.
The American Public Health Association.
2-27
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