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Effective Lime/Soda Ash Water Softening By Douglas Rittmann, Ph.D., P.E. Water/Wastewater Consultant www.dougrittmann.com.

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Presentation on theme: "Effective Lime/Soda Ash Water Softening By Douglas Rittmann, Ph.D., P.E. Water/Wastewater Consultant www.dougrittmann.com."— Presentation transcript:

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

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

3 II. What is Hardness? C. Carbonate Hardness as CaCO 3 = T. Alkalinity as CaCO 3 D. Non-Carbonate Hardness = T. Hardness – T. Alkalinity Water Softening a. Removed by Boiling b. Removed by Lime a. Unaffected by boiling b. Removed by Soda Ash B. T. Hardness, mg/L, as CaCO 3 = (Ca X 2.5) + (Mg X 4.12) (MW=100) (40 X 2.5 = 100) (24.3 X 4.12 = 100) (MW=100) (40 X 2.5 = 100) (24.3 X 4.12 = 100) 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

4 Water Softening Methods of Softening III. Methods of Softening A. Lime-Soda Ash Chemistry 1. 1 st Stage Treatment (Lime only) * CO 2 + Ca(OH) 2 CaCO 3 + H 2 O * CO 2 + Ca(OH) 2 CaCO 3 + H 2 O * Ca + 2HCO 3 + Ca(OH) 2 2CaCO 3 + 2H 2 O(pH ) * Mg + 2HCO 3 + Ca(OH) 2 CaCO 3 + Mg + CO 3 + 2H 2 O(pH >10.8) a. Carbon Dioxide Removal (< 8.3 pH) b. Carbonate Hardness Removal c. Magnesium Hardness Removal (>pH 10.8) * Mg + CO 3 + Ca(OH) 2 CaCO 3 + Mg(OH) 2 * Mg + SO 4 + Ca(OH) 2 Ca + SO 4 + Mg(OH) 2 * Ca + SO 4 + Na 2 CO3 Na 2 SO 4 + CaCO nd Stage Treatment(Soda Ash) 2. 2 nd Stage Treatment (Soda Ash) * Ca + Cl 2 + Na 2 CO 3 CaCO 3 + 2NaCl

5 Analyses Water #1 Water #2 Water #3 Total Hardness Calcium Hardness Mg Hardness Total Alkalinity Interpretations Water #1 Water #2 Water #3 Calcium Alkalinity Mg. Alkalinity None50100 Sodium Alkalinity NoneNone50 Ca N.C. Hardness 50NoneNone Mg. N.C. Hardness 10050none 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. calcium hardness but less than total hardness. C. Sodium alkalinity = total alkalinity – total hardness IV. Chemical Analyses Interpretations D. NCH = Total Hardness – Total Alkalinity ( If Mg Alkalinity present then no Ca NCH)

6 Water Softening 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 Mg++ = 21 mg/L Alkalinity = 170 mg/L as CaCO3 Alkalinity = 170 mg/L as CaCO3 Carbon Dioxide = 6 mg/L Carbon Dioxide = 6 mg/L Lime Requirement: Carbon Dioxide = (6) (56) / (44) = 8 Alkalinity = (170) (56) / (100) = 95 Alkalinity = (170) (56) / (100) = 95 Mg ++ = (21) (56) / (24.3) = 48 Mg ++ = (21) (56) / (24.3) = 48 Excess Lime = = 35 Excess Lime = = 35 Total CaO required = 273 mg/L Total CaO required = 273 mg/L Soda Ash Requirement: NCH = 280 – 170 = 110 mg/L Soda Ash (Na2CO3) = (110) (106) / (100) = 117 mg/L Soda Ash (Na2CO3) = (110) (106) / (100) = 117 mg/L A. Theoretical Solubility of Ca & Mg: Mg(OH)2 = 9 mg/L Solubility Mg(OH)2 = 9 mg/L Solubility CaCO3 = 17 mg/L Solubility CaCO3 = 17 mg/L Solubility Total = ~ 26 mg/L Solubility Total = ~ 26 mg/L Solubility B. Practical Minimum Total Hardness = 50 to 80 mg/L V. Theoretical versus Practical

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