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

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

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

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)2 CaCO3 + H2O b. Carbonate Hardness Removal * Ca + 2HCO3 + Ca(OH)2 2CaCO3 + 2H2O(pH 8.3-9.4) * Mg + 2HCO3 + Ca(OH)2 CaCO3 + Mg + CO3 + 2H2O(pH >10.8) c. Magnesium Hardness Removal (>pH 10.8) * Mg + CO3 + Ca(OH)2 CaCO3 + Mg(OH)2 * Mg + SO4 + Ca(OH)2 Ca + SO4 + Mg(OH)2 2. 2nd Stage Treatment (Soda Ash) * Ca + SO4 + Na2CO3 Na2SO4 + CaCO3 * Ca + Cl2 + Na2CO3 CaCO3 + 2NaCl

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

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

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