1. WATER SOFTENING removal of hardness How is Softening done?...
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 mg/l as CaCO3 range considered excessive
Even > 150 mg/l may result in consumer objection
mg/l as CaCO3 is considered a moderate amount
high soap consumption
scaling in heating vessels and pipes

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

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 H20 Mg(HCO3)2 + Ca(OH)2 --> CaCO3 + MgCO3 + 2H20 Mg(CO3) + Ca(OH)2 --> Mg(OH)2 + CaCO3 MgS04 + Ca(OH)2 --> Mg(OH)2 + CaS04 MgCl2 + Ca(OH)2 --> Mg(OH)2 + CaCl2 CaS04 + Na2CO3 --> CaC03 + Na2SO4 CaCl2 + Na2CO3 --> 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 --> CaCO3 + H2O
CO2 must be considered because it consumes lime

11. Effectiveness
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
2 0
1 1
0 1