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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
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
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
formation of hardness

Precipitation

Topsoil

Subsoil

CO2 + H2O  H2CO3

CaCO3(s) + H2CO3 Ca(HCO3)2

MgCO3(s) + H2CO3  Mg(HCO3)2

Limestone

Formation of Hardness
hardness
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
hardness5
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
hardness units
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)
lime soda ash softening
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
stoichiometry
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

solubilities
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

removal by precipitation
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

effectiveness
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
stoichiometry table
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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