Page 302 - Water and wastewater engineering

LIME–SODA SOFTENING 7-19

179.0 mg/L as CaCO 3 20 mg/L as CaCO 3 199 mg/L as CaCO 3 . Lime is purchased
and stored as CaO. The amount of 90% pure lime as CaO is
⎛ 28 meq/mg⎞ ⎛ 1 1 ⎞
)
(199 mg/L as CaCO 3 ⎜ ⎝ 50 meq/mg⎠ 090. ⎟ ⎠ 123 8 or 124 mg/L as CaO
.
⎟ ⎜
⎝
where the equivalent weight of CaO 28 meq/mg and the equivalent weight of
CaCO 3 50 meq/mg.
d. The total hardness of the finished water (after recarbonation ) is

Ca 2 Initial 200 mg/L as CaCO 164 mg/L as CCaCO removed
3 3
with bicarbonate
36 mg/L as CaaCO 3
Mg 2 35 mg/L as CaCO 3

Total hardness 36 35 71 mg/L as CaCO 3
e. The changes in the water composition as illustrated by the changes in the bar chart are
shown below.

Bar chart after removal of CO 2 (in mg/L as CaCO 3 ).
0 200 235

Ca 2 Mg 2

HCO SO 2
3 4
0 164 240

Bar chart of the finished water (in mg/L as CaCO 3 ).

0 36 35

Ca 2 Mg 2
SO 2
4
0 76

Comments:
1. Lime neutralization is used because the CO 2 is less than 10 mg/L as CO 2 .

2. The finished water is quite soft and consideration should be given to splitting the flow
to bypass some of the raw water to blend to a higher residual hardness. This would save
capital costs by using smaller tanks and operating costs by reducing chemical usage as
well as the amount of sludge that has to be disposed.