Page 319 - Water and wastewater engineering

7-36 WATER AND WASTEWATER ENGINEERING

Many modifications have been made to the basic equation to account for other alkalinity-
contributing species, activity coefficients, and so forth. These are discussed in detail in Schock
(1999). The basic equation will be used for the purpose of illustration of its use in design.

Stabilization Design for Lime-Soda Softened Water
As noted earlier in this chapter, recarbonation is the last step in the softening process. The pur-
pose of the recarbonation step is to “stop” the precipitation reaction by lowering the pH. Carbon
dioxide addition has been the method of choice for lowering the pH. Other chemicals may be
used (for example, H 2 SO 4 ) based on economic and operational considerations. Furthermore, split
treatment of groundwater may bring sufficient CO 2 in the bypassed water to lower the pH with-
out the addition of purchased chemicals.

The objective in stabilization is to achieve an LSI 0. The reactions are given in Equa-
tions 7-27 , 7-28 , 7-29 and 7-30. The equilibrium equations for carbonic acid (7-31 and 7-32)
are used to estimate the concentration of CO 2 that must be added. The estimation of the CO 2
dose is illustrated in the following example.

Example 7-8. Estimate the dose of CO 2 in mg/L to stabilize the water from split treatment
softening. The bypass water has 1.37 meq/L of CO 2 . The estimated constituents and parameters
of interest in the blended water are listed in the table below.
Concentration,
Constituent meq/L as CaCO 3
or parameter or units as shown
2
Ca 1.38
2
Mg 0.80
2
CO 0.50
3
HCO 1.20
3
pH 9.95 units
TDS 320 mg/L
Temperature 10 C

Solution:
a. Calculate the LSI.
2
Converting Ca meq/L to moles /L,
.
(1 38 meq/L )(20 mg/meq ) 4
.
690 110 moles/L,
,
40 000 mg/mole
2
where 20 mg/meq is the equivalent weight of Ca
pCa log (690. 10 4 moles/L 3 16.
)
2
The alkalinity is the sum of CO and HCO . The units are given in meq/L. They must
3 3
be in equiv/L
pAlk log (0 50. 10 3 1 20. 10 3 ) 2 77.