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LIME SOFTENING 11.11

Correcting for purity gives

CaO = 1,899 = 2,110 lb commercial lime/mil gal
0.9
Adding selected design factor of 10% for minimum feeder size gives

1.1(2,110 lb commercial lime/mil gal) = 2,320 lb/mil gal commercial lime
Approx. resulting hardness = 35(approx. minimum solubility Ca) + 10(approx.
minimum solubility of Mg as CaCO3) + 81(NCH) = 126 mg/L as CaCO3~

Approx. remaining alkalinity = 45 mg/L as CaCO3
(approx. minimum solubility of Ca plus Mg as CaCo3)~

Using Equations (11.19) and (11.21) for approximate CO2 feed requirements gives
CO2 (lb/mil gal) = 3.7(40 mg/L bicarbonate alkalinity)
+ 3.7(65 mg/L excess OH alkalinity)
= 389 lb/mil gal
Adding selected design factor of 10% for minimum feeder size gives
1.1(389 lb/mil gal) = 428 lb/mil gal CO2

EXAMPLE 11.4 Reduce the noncarbonate hardness for the water shown in Example 11.3
enough to reach a target finished water hardness of 100 mg/L.
The required lime dosage is calculated as in Example 11.3.
Noncarbonate hardness to be removed = 126 - 100 = 26 mg/L
Noncarbonate hardness to be left in water = 81 - 26 = 55 mg/L
Using Equation (11.10) to estimate soda ash feed requirements gives

Soda ash (lb/mil gal) = 8.8(NCH - x) = 8.8(81 - 55) = 229 lb/mil gal
Adding selected design factor of 10% for minimum feeder size yields
1.1(229 lb/mil gal) = 252 lb/mil gal
Approx. resulting hardness = 35(approx. minimum solubility Ca)
+ 10(approx. minimum solubility of Mg as CaCO3) + 55(remaining NCH)
= 100 mg/L as CaCO3~

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,