Page 328 - Water and wastewater engineering

LIME–SODA SOFTENING 7-45

7-24. Determine the lime and soda ash dose, in mg/L as CaCO 3 , to soften the Kool Artesian
( Problem 7-10 ) water to a final hardness of 115 mg/L as CaCO 3 . Assume the water
temperature as pumped from the ground is 10 C. If the price of lime, purchased as
CaO, is $100 per megagram (Mg), and the price of soda ash, purchased as Na 2 CO 3 is
3
$200 per Mg, what is the annual chemical cost of treating 0.500 m /s of this water?
Assume all the alkalinity is bicarbonate, lime is 88% pure, and soda ash is
98% pure.

7-25. Determine the lime and soda ash dose, in mg/L as CaCO 3 , to soften the Village of
Galena’s water to a final hardness of 130.0 mg/L as CaCO 3 . Assume the water
temperature as pumped from the ground is 8.5 C. If the price of lime, purchased as
CaO, is $100 per megagram (Mg), and the price of soda ash, purchased as Na 2 CO 3 is
3
$200 per Mg, what is the annual chemical cost of treating 0.500 m /s of this water?
Assume lime is 93% pure and soda ash is 95% pure.

Village of Galena
Milligrams
Constituent Expressed as per liter

Calcium Ca 2 177.8
Magnesium Mg 2 16.2
Total iron Fe 0.20
Lead a Pb 2 20 a
Sodium Na 4.9
Carbonate alkalinity CaCO 3 0.0
Bicarbonate alkalinity CaCO 3 276.6
Chloride Cl 0.0
2
Sulfate SO 4 276.0
Silica SiO 2 1.2
Total dissolved solids 667
pH units 8.2
Temperature C 8.5
a
Parts per billion.

7-26. Rework Problem 7-21 using caustic soda instead of lime. Soften the water to the
lowest hardness that can be achieved with caustic alone. Assume the caustic is 100%
pure, CO 2 concentration 9.96 mg/L, and that an excess of 40 mg/L of caustic will
be added.
7-27. Design an upflow solids contact basin for a softening plant treating groundwater. The
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plant has a maximum day design flow rate of 50,000 m /d and average winter de-
3
mand of 25,000 m /d. Verify that the design satisfactorily meets the GLUMRB guid-
ance for flocculation and mixing period, detention time, upflow rate (overflow rate),
and weir loading. Use Table 7-4 to select the upflow clarifier(s).
7-28. Design an upflow solids contact basin for a softening plant treating groundwater. The
3
plant has a maximum day design flow rate of 10,000 m /d and average winter demand