Page 321 - Water and wastewater engineering

7-38 WATER AND WASTEWATER ENGINEERING

and

3 moles/L

[HCO ] . 145 10 144 10 3 mooles/L
.
3
. 1 0094
and
2 3 5
(
)
[CO ] 0 .0094 1 .44 10 moles/L . 1 35 10 moles/L
3
c. The dose of CO 2 is estimated assuming that CO 2 H 2 CO 3 . The reaction is
2
CO HCO 2 HCO
3 2 3 3

Because one mole of CO 2 produces two moles of HCO , the dose of CO 2 to convert
3
carbonate to bicarbonate is
⎛ 1 35 10 5 moles/L ⎞
.
3
⎜ ⎟ ( 44 10 mg/mole) 0.30 mg/L of CO 2
3
⎝ 2 ⎠
The total concentration of bicarbonate after the conversion is
1 35 10 5 moles/L 1 44 10 3 moles/L 1 45 10 3 moles/L
.
.
.
Comments:
1. The small addition of CO 2 is the result of blending the raw water with the treated water. The
raw water CO 2 converted the hydroxyl ion to carbonate. Otherwise, the high pH required to
remove the magnesium (>11.3) would have resulted in a higher CO 2 requirement.
2. The fact that the water is “stable” does not mean that it is noncorrosive.
2
3. To estimate the concentrations from split treatment, assume Ca 30 mg/L as CaCO 3
2
and Mg 10 mg/L as CaCO 3 in the discharge from first stage of softening because
the water has been softened to the practical solubility limits. Associated with this as-

sumption are the related quantities of CO 2 and OH , that is CO 2 30 mg/L as CaCO 3

and OH 10 mg/L as CaCO 3 .

The recarbonation basin should provide (GLUMRB, 2003):

• A detention time of 20 minutes.
• Two compartments with a diffuser depth not less than 2.5 m.
• One compartment (the mixing compartment) should have a detention time 3 minutes.

The practice of on-site generation of CO 2 is discouraged. Bulk pressurized or liquified CO 2
is commonly available and often used because it eliminates operation and maintenance problems
associated with on-site generation by combustion.
Approximately 50 to 75 percent of the applied CO 2 goes into solution. The room housing the
recarbonation basin must be ventilated to prevent the accumulation of the 25 to 50 percent of the