Page 328 -

11.8 CHAPTER ELEVEN

Using Equation (11.18) for approximate lime feed requirements, we find

CaO (lb/mil gal) = 10.6[CO2 (mg/L)] + 4.7 [alkalinity (mg/L)]
CaO (lb/mil gal) = 10.6(0) + 4.7(180) = 846 lb/mil gal, as 100% pure CaO
Correcting for purity, we get
846
CaO - - 940 lb commercial lime/mil gal
0.9
Adding selected design factor of 10% for minimum feeder size gives
1.1 (940 lb commercial lime/mil gal) = 1,030 lb/mil gal commercial lime
Approximate resulting hardness = 35(approx. minimum solubility of CaCO3)
+ 30(Mg hardness) + 60(remaining calcium NCH) = 125 mg/L as CaCO3t
Approximate remaining alkalinity = 35 mg/L as CaCO3
(approximate minimum solubility of CaCO3)t
Using Equation (11.19) for approximate CO2 feed requirements gives
CO2 (lb/mil gal) = 3.7(40 mg/L finished water bicarbonate alkalinity)

= 148 lb/mil gal
Adding selected design factor of 10% for minimum feeder size gives
1.1(148 lb/mil gal) = 160 lb/mil gal

EXAMPLE 11.2 Reduce the calcium hardness in the finished water, using both lime and
soda ash to reach a target finished water hardness of 100 mg/L.
As shown in Example 11.1, addition of lime at 1,030 lb/mil gal will yield a finished
water with total hardness = 125 mg/L. Removal of a portion of the remaining calcium
NCH will be required to reduce hardness to 100 mg/L.

Noncarbonate hardness in water = 270 - 180 -- 90 mg/L
Noncarbonate hardness to be left in water = 90 - (125 - lO0) = 65 mg/L
Using Equation (l 1.10) to estimate the soda ash feed requirements gives
Soda ash (lb/mil gal) = 8.8(NCH - x) = 8.8(90 - 65) = 220 lb/mil gal

Adding selected design factor of 10% for minimum feeder size gives
1.1 (220 lb/mil gal) = 242 lb/mil gal
Approx. resulting hardness = 35(approx. minimum solubility of CaCO3)
+ 35(remaining calcium NCH) + 30(remaining Mg NCH) -- 100 mg/L as CaCO3t

tNote: Results are approximations based on theoretical equilibrium conditions. These conditions are seldom du-
plicated in practice, and some variations should be expected at the particular plant. Alkalinity in the finished wa-
ter may be increased to 40 mg/L or more by reducing the amount of carbonate hardness removed.