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WATER SOFTENING

WATER SOFTENING. removal of hardness Hardness is?... How is Softening done?. primarily Ca, Mg, plus Fe, Mn, St, Al. Precipitation of Ca and Mg, or Ion exchange of Ca / Mg with ion such as Na. Why bother?. Hardness in 300-500 mg/l as CaCO 3 range considered excessive

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WATER SOFTENING

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  1. WATER SOFTENING • removal of hardness • Hardness is?... • How is Softening done?... • primarily Ca, Mg, plus Fe, Mn, St, Al Precipitation of Ca and Mg, or Ion exchange of Ca / Mg with ion such as Na

  2. Why bother? • Hardness in 300-500 mg/l as CaCO3 range considered excessive • Even > 150 mg/l may result in consumer objection • 60-120 mg/l as CaCO3 is considered a moderate amount • high soap consumption • scaling in heating vessels and pipes

  3. Precipitation Topsoil Subsoil CO2 + H2O  H2CO3 CaCO3(s) + H2CO3 Ca(HCO3)2 MgCO3(s) + H2CO3  Mg(HCO3)2 Limestone Formation of Hardness

  4. Hardness • Carbonate Hardness • Often called "temporary hardness" because heating the water will remove it. When the water is heated, the insoluble carbonates will precipitate and tend to form bottom deposits in water heaters. • Ca2+, Mg2+ associated with HCO3-, CO32- • CH = TH or Total alkalinity, whichever is less

  5. Hardness • Non-Carbonate Hardness • Called permanent hardness because it is not removed when the water is heated. It is much more expensive to remove non-carbonate hardness than carbonate hardness. • Ca2+, Mg2+ associated with other ions, Cl-, NO3-, SO42- • NCH = TH - CH • If Alkalinity  Total hardness, then NCH = 0

  6. Hardness Units • milligrams per liter (mg/L) as calcium carbonate (most common) • parts per million (ppm) as calcium carbonate • grains per gallon of hardness (to convert from grains per gallon to mg/L, multiply by 17.1) • equivalents/liter (eq/L)

  7. LIME - SODA ASH SOFTENING • Addition of lime, Ca(OH)2, & soda ash, Na2CO3 causes precipitation of Ca, Mg • Lime often added as CaO, quick lime • CaO + H20 --> Ca(OH)2 • Three basic processes • Excess lime treatment • Selective calcium removal • Split treatment

  8. Stoichiometry CO2 + Ca(OH)2 --> CaCO3 + H2O Ca(HCO3)2 + Ca(OH)2 --> 2 CaCO3 + 2 H20Mg(HCO3)2 + Ca(OH)2 --> CaCO3 + MgCO3 + 2H20Mg(CO3) + Ca(OH)2 --> Mg(OH)2+ CaCO3MgS04 + Ca(OH)2 --> Mg(OH)2 + CaS04MgCl2 + Ca(OH)2 --> Mg(OH)2 + CaCl2CaS04 + Na2CO3 --> CaC03+ Na2SO4CaCl2 + Na2CO3 --> CaC03+ 2NaCl

  9. Solubilities • Ca(OH)2 is very soluble, Mg(OH)2is not • MgCO3 is very soluble, CaCO3 is not • CaCO3 and Mg(OH)2 are relatively insoluble • CaCO3: ~ 30 mg/l as CaCO3 • 0.6 meq/l • Mg(OH)2: ~ 10 mg/l as CaCO3 • 0.2 meq/l • MW is ?... • EW is?... • mg/l is ?... 58 mg/mmol 29 mg/meq 5.8 mg/l as Mg(OH)2

  10. Removal by precipitation • Is complete removal possible?... No, lime-soda ash softening cannot remove all hardness What about CO2? • CO2 + Ca(OH)2 --> CaCO3 + H2O CO2 must be considered because it consumes lime

  11. Effectiveness • 80-100 mg/l as CaCO3 is usually considered acceptable result of lime-soda ash softening, • as long as Mg is < 40 mg/l as CaCO3 • any more causes scaling in heating vessels

  12. Stoichiometry Table • meq of lime and soda ash to remove a meq of X initially present X Lime Soda Ash CO2 Ca(HCO3)2 Mg(HCO3)2 MgCO3 MgSO4 CaSO4 1 0 1 0 2 0 1 0 1 1 0 1

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