Page 296 - Water and wastewater engineering

LIME–SODA SOFTENING 7-13

1. Add lime CO (to raise pH)
2

Ca 2 Mg 2 2. Add lime HCO (to raise pH)
3
CO 2 3. Add lime Mg 2

2 Mg ) HCO (to remove Ca )
2
2
4. Add soda (Ca
HCO C1 3
3
(a) 5. Consider excess lime
1. Add lime CO 2
Ca 2 Mg 2 2. Add lime HCO
3
CO 2 3. Add lime Mg 2
2
3
HCO C1 4. Add soda (Ca 2 Mg ) HCO
3
(b) 5. Consider excess lime
1. Add lime CO 2

Ca 2 Mg 2 Na 2. Add lime HCO (Need all because need to raise pH)
3
CO 2 3. Add lime Mg 2
4. No soda ash required
HCO 3
(c) 5. Consider excess lime
FIGURE 7-8
2
Cases when Mg concentration is greater than 40 mg/L as CaCO 3 and split treatment is required.
Note that these cases illustrate softening to the practical limits in the first stage of the split-flow
scheme. (Source: Davis and Cornwell, 2008.)
Lime Softening
Also called selective calcium removal, or partial lime softening, this alternative applies to cases
( a ), ( b ), and ( c ) in Figure 7-7 . Only lime is added. The desired control pH is 10.3. This process
removes only carbonate hardness caused by calcium.
Excess Lime Softening
This alternative applies to cases ( b ) and ( c ) in Figure 7-8 . Only lime is added. The desired control
pH is 11.3. This process removes only the carbonate hardness caused by calcium and magnesium.
Lime-Soda Softening
This alternative applies to cases ( a ), ( b ), and ( c ) in Figure 7-8 . Both lime and soda ash are added.
The desired control pH is 11.3. This process removes both carbonate and noncarbonate hardness
caused by calcium and magnesium.

Softening to Practical Limits
One process to achieve a specified magnesium concentration or to achieve a given hardness
is to treat a portion of the water to the practical limits and then blend the treated water with

the raw water to achieve the desired magnesium concentration or hardness. * Stoichiometric
2
2
amounts of lime and soda are added to remove all of the Ca and Mg to the practical limits
2
of softening (that is, 0.60 meq/L or 30 mg/L as CaCO 3 of Ca and 0.20 meq/L or 10 mg/L as
2
CaCO 3 of Mg ).
*Generally, it is not practical to attempt to achieve both a desired magnesium concentration and a specified final hardness with
a single split, and it is not economical to have multiple splits.