Page 300 - Water and wastewater engineering

LIME–SODA SOFTENING 7-17

c. With p K a 1 6.35, solve Equation 7-31 for [H 2 CO 3 ].

[H ][HCO 3 ]
[HCO 3
]
2
K 1 a
6 35
1 10 . . 10 7 moles/L at 25 C
4 47
K a
224
(. 10 8 moles/L))(620. 10 3 moles/L )
]
[ HCO 3
2
447 10 7 moles/L
.

3 11 10 4 moles/L
.
d. Assume that all the CO 2 in water forms carbonic acid. Thus, the estimated CO 2 concen-
tration is
[CO 3 11 10 4 moles/L
]
.
2
In other units for comparison and calculation:
)
7
)
(
CO ( 3 11 10 4 moles/L 44 10 3 mg/mole 13.7 mg/L as CO
.
2 2
and
⎛ 50 mg/meq ⎞
CO ( 13 7 . mg/L as CO ) 3114 or 311 . mg/L as CaCO
.
2 ⎜
2 ⎜ ⎝ 22 mg/meq)⎠ ⎟ 3
The equivalent weight of CO 2 is taken as 22 because it effectively behaves as carbonic acid
(H 2 CO 3 ) and thus n 2.
Comment. This CO 2 concentration is high enough to warrant consideration of air stripping to
remove it.
Selective Calcium Removal
When the magnesium concentration is less than 40 mg/L as CaCO 3 , lime softening (also
called partial lime softening ) can produce the desired final hardness. The alternative dos-
ing schemes are dependent on the amount of carbonate alkalinity as shown in Figure 7-7 .
In each instance CO 2 removal is shown by lime neutralization. This assumes that this is the
economic alternative. In addition, it should be noted that lime must be added to the stoichio-
metric equivalent of the bicarbonate present regardless of the concentration of calcium. If the
bicarbonate is not neutralized, the pH objective of 10.3 required to precipitate the calcium
will not be achieved.
Example 7-4 illustrates one case of those shown in Figure 7-7 , using both mg/L as CaCO 3
and milliequivalents/L (meq/L) as units of measure.

Example 7-4. Prepare a bar chart for Sweetwater’s water analysis given below and determine
the chemical dosage required for selective calcium removal. Estimate the dosage of quicklime
(CaO) that needs to be added if the purity of lime is 90%.