Page 329 - Water and wastewater engineering

7-46 WATER AND WASTEWATER ENGINEERING

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of 5,000 m /d. The primary softening clarifier is to be followed by a secondary clari-
fier that is to be used as settling tank for coagulation of the unsettled precipitate from
the first tank. Verify that the design satisfactorily meets the GLUMRB guidance for
side water depth, flocculation and mixing period, detention time, upflow rate (over-
flow rate), and weir loading. Use Table 7-4 to select the upflow clarifier(s).
7-29. Estimate the dose of CO 2 in mg/L to stabilize the water from split treatment soften-
ing. The estimated constituents and parameters of interest in the blended water are
listed in the table below.
Concentration,
Constituent mg/L as CaCO 3
or parameter or units as shown
Ca 2 63.0
Mg 2 35.0
2
CO 84.7
3
HCO 128.4
3
pH 10.1 units
TDS 240 mg/L
Temperature 4 C

7-30. Estimate the dose of CO 2 in mg/L to stabilize the water from split treatment soften-
ing. The estimated constituents and parameters of interest in the blended water are
listed in the table below.
Concentration,
Constituent mg/L as CaCO 3
or parameter or units as shown
2
Ca 53.65
2
Mg 40.0
2
CO 25.2
3
HCO 53.2
3
pH 9.89 units
TDS 560 mg/L
Temperature 12 C
7-11 DISCUSSION QUESTIONS
7-1. Is the Lime Ridge water (Problem 7-19) a likely candidate for air stripping to remove
CO 2 before lime-soda softening? Explain why or why not.
7-2. Explain why many lime-soda softening utilities have raised their target hardness from
between 75 and 120 mg/L as CaCO 3 to a target hardness between 120 and 150 mg/L
as CaCO 3 .
7-3. A water that contains only carbonate hardness can be softened with NaOH alone.
True or false?