1. Effective Lime/Soda Ash Water SofteningBy
Douglas Rittmann, Ph.D., P.E.
Water/Wastewater Consultant

2. Water Softening I. Introduction A. Reasons to Soften
1. Reduce Soap Consumption
2. Improve Aesthetics of Water
3. Hot Water Heaters last longer
B. Reasons not to Soften
1. Expensive Process
2. May be less healthy
3. Competes with health related costs

3. Water Softening II. What is Hardness? A. Hardness Classifications
a. Soft Water = 0 to 70 mg/L
b. Moderate Hardness = 71 to 150 mg/L
c. Hard Water = > 150 mg/L
B. T. Hardness, mg/L, as CaCO3 = (Ca X 2.5) + (Mg X 4.12)
(MW=100) (40 X 2.5 = 100) (24.3 X 4.12 = 100)
C. Carbonate Hardness as CaCO3 = T. Alkalinity as CaCO3
a. Removed by Boiling
b. Removed by Lime
D. Non-Carbonate Hardness = T. Hardness – T. Alkalinity
a. Unaffected by boiling
b. Removed by Soda Ash

4. Water Softening III. Methods of Softening A. Lime-Soda Ash Chemistry
1. 1st Stage Treatment (Lime only)
a. Carbon Dioxide Removal (< 8.3 pH)
* CO2 + Ca(OH) CaCO3 + H2O
b. Carbonate Hardness Removal
* Ca + 2HCO3 + Ca(OH) CaCO3 + 2H2O(pH )
* Mg + 2HCO3 + Ca(OH) CaCO3 + Mg + CO3 + 2H2O(pH >10.8)
c. Magnesium Hardness Removal (>pH 10.8)
* Mg + CO3 + Ca(OH) CaCO3 + Mg(OH)2
* Mg + SO4 + Ca(OH) Ca + SO4 + Mg(OH)2
2. 2nd Stage Treatment (Soda Ash)
* Ca + SO4 + Na2CO Na2SO4 + CaCO3
* Ca + Cl2 + Na2CO CaCO3 + 2NaCl

5. IV. Chemical Analyses Interpretations
A. Calcium Alkalinity = Ca Hardness or T. Alkalinity whichever is smaller
B1. Magnesium Alkalinity = Mg. Hardness if T. Alkalinity > or = than total hardness
B2. Magnesium Alkalinity = Total Alkalinity – calcium hardness if total alkalinity is > than
calcium hardness but less than total hardness.
C. Sodium alkalinity = total alkalinity – total hardness
D. NCH = Total Hardness – Total Alkalinity ( If Mg Alkalinity present then no Ca NCH)
Analyses
Water #1
Water #2
Water #3
Total Hardness
300
Calcium Hardness
200
Mg Hardness
100
Total Alkalinity
150
250
350
Interpretations
Water #1
Water #2
Water #3
Calcium Alkalinity
150
200
Mg. Alkalinity
None 50
100
Sodium Alkalinity
Ca N.C. Hardness
Mg. N.C. Hardness
none

6. Water Softening V. Theoretical versus Practical
A. Theoretical Solubility of Ca & Mg:
Mg(OH)2 = 9 mg/L Solubility
CaCO3 = 17 mg/L Solubility
Total = ~ 26 mg/L Solubility
B. Practical Minimum Total Hardness = 50 to 80 mg/L
Example: Calculate the hydrated lime (100%), soda ash, and carbon dioxide requirement to
Reduce the hardness of a water with the following analysis to about 50 to 80 mg/L by the excess
Lime-soda ash process.
Analyses: Total Hardness = 280 mg/L as CaCO3
Mg = 21 mg/L
Alkalinity = 170 mg/L as CaCO3
Carbon Dioxide = 6 mg/L
Lime Requirement: Carbon Dioxide = (6) (56) / (44) = 8
Alkalinity = (170) (56) / (100) = 95
Mg = (21) (56) / (24.3) = 48
Excess Lime = = 35
Total CaO required = 273 mg/L
Soda Ash Requirement: NCH = 280 – 170 = 110 mg/L
Soda Ash (Na2CO3) = (110) (106) / (100) = 117 mg/L

7. Questions???