Page 308 - Water and wastewater engineering

LIME–SODA SOFTENING 7-25

removed. Removal of the calcium equivalent to the bicarbonate will leave 40 mg/L as
CaCO 3 of calcium hardness plus the 40 mg/L as CaCO 3 of magnesium hardness for a
total of 80 mg/L as CaCO 3 . The additions are as follows.

Lime
Constituent mg/L as CaCO3 Lime meq/L
12.5 0.25
CO 2

HCO 198.0 3.96
3
210.5 4.21

Addition of lime equal to CO 2 and HCO (even in the second stage) is necessary to
3
achieve the control pH of 10.3.
g. Excluding the extra lime to drive the reaction, the total chemical additions are in propor-
tion to the flows:
(
.
Lime 0 628 313 6) 0 372 210 5) 2753 or 275 mg/L as CaCO
.
.
.
.
(
3
Soda 0 628 1306 0 372 0 0) 82 mg/L as CaCO

(
.
)
(
.
.
.
3
h. Because the magnesium concentration is greater than 40 mg/L as CaCO 3 , the rule-of-
thumb addition of extra lime to the first stage is 40 mg/L as CaCO 3 .
Comment. In this case the final hardness is at the low end of the acceptable range. Because the
capital cost of installing a second stage is quite high, other alternative process schemes should be
considered. For example, treating more water in the first stage would result in a total hardness in
the acceptable range without the need for a second stage. The resulting water would have a mag-
nesium concentration lower than the design goal of 40 mg/L as CaCO 3 . An economic analysis
would have to be conducted because the capital cost would be less but the chemical and operating
costs including sludge disposal might be higher.
Other Estimating Methods
The method of estimating dosages used here is a traditional technique that provides a direct
link to the chemical reactions and, with the bar charts, provides a means of illustrating
the chemical processes. An alternative to the stoichiometric approach is the solution of the
simultaneous equilibria equations to estimate the dosage. A series of diagrams called the
Caldwell-Lawrence diagrams have been developed to solve these equations graphically.
Examples of their use may be found in AWWA (1978), Merrill (1978), Benefield et al.
(1982), and Benefield and Morgan (1999).
The American Water Works Association has computer software for working with the
Caldwell-Lawrence diagrams. It is called The Rothberg, Tamburini, and Winsor Model for
Corrosion Control and Process Chemistry.
Use of Caustic Soda
Caustic soda (NaOH) is an alternative to the use of lime for softening. It has the advantages of
decreased sludge production, reduction in dust generation, and the option of simpler storage and
feed systems because it is purchased as a liquid. There are several disadvantages in using caustic