Page 307 - Water and wastewater engineering

7-24 WATER AND WASTEWATER ENGINEERING

Constituent mg/L EW EW CaCO 3 /EW ion mg/L as CaCO 3 meq/L
Alkalinity 198.0 3.96

Cl 67.8 35.5 1.41 95.6 1.91
2
SO 73 48.0 1.04 76 1.52
4
b. Bar chart of raw water in meq/L.
0.25 0 4.76 6.56 7.68

Ca 2 Mg 2 Na
CO 2
HCO Cl SO 2
3 4
0.25 0 3.96 5.87 7.39

This is similar to the case shown in Figure 7-8a . Split treatment must be used to achieve
a magnesium concentration goal of 40 mg/L as CaCO 3 .
c. In the first stage the water is softened to the practical solubility limits; lime and soda
must be added as shown below.

Addition Lime, Lime, Soda, Soda,
equal to: mg/L as CaCO 3 meq/L mg/L as CaCO 3 meq/L
12.5 0.25
CO 2

HCO 198.0 3.96
3
2
Ca minus HCO 40 0.80
3
2
Mg 90.6 1.80 90.6 1.80
Total 301.1 6.01 130.6 2.60
d. The split is calculated in terms of mg/L as CaCO 3 :
40 10
X 0 372.
90 6 10
.
The fraction of water passing through the first stage is then 1 0.372 0.628.
e. The total hardness of the water after passing through the first stage is the sum of the
practical solubility limits, that is, 30 10 40 mg/L as CaCO 3 . Because the total hard-
ness in the raw water is 238 90.6 328.6 mg/L as CaCO 3 , the mixture of the treated
and bypass water has a hardness of:

.
)
.
(0 372 )(328 6 mg/L as CaCO (0 628 )(40 mg/LLas CaCO )
.
3 3
147 4 mg/L as CaCO
.
3
This is above the specified finished water criteria range of 80–120 mg/L as CaCO 3 , so
further treatment is required.
f. Because the split is designed to yield 40 mg/L as CaCO 3 of magnesium, no further
magnesium is removed. To achieve the desired total hardness more calcium must be