WATER SOFTENING

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 gt 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
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 --gt Ca(OH)2
• Three basic processes
• Excess lime treatment
• Selective calcium removal
• Split treatment

8
Stoichiometry

• CO2 Ca(OH)2 --gt CaCO3 H2O
• Ca(HCO3)2 Ca(OH)2 --gt 2 CaCO3 2
  H20Mg(HCO3)2 Ca(OH)2 --gt CaCO3 MgCO3
  2H20Mg(CO3) Ca(OH)2 --gt Mg(OH)2
  CaCO3MgS04 Ca(OH)2 --gt Mg(OH)2
  CaS04MgCl2 Ca(OH)2 --gt Mg(OH)2
  CaCl2CaS04 Na2CO3 --gt CaC03 Na2SO4CaCl2
  Na2CO3 --gt CaC03 2NaCl

9
Solubilities

• Ca(OH)2 is very soluble, Mg(OH)2 is 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 --gt 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 lt 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