Water softening removes hardness primarily caused by calcium and magnesium ions through precipitation or ion exchange processes. Hardness above 150 mg/L as CaCO3 can cause issues like increased soap consumption and scaling. Hardness is formed as carbon dioxide interacts with limestone and other minerals in soil and groundwater. There are two types of hardness - carbonate (temporary) and non-carbonate (permanent) - which are removed through different softening methods. Lime-soda ash softening uses the addition of lime and soda ash to precipitate calcium and magnesium out of solution, leaving softer water. This process can typically reduce hardness to 80-100 mg/L as CaCO3.

1. 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. 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. 3
Formation of Hardness
CO2 + H2O → H2CO3
Subsoil
Limestone CaCO3(s) + H2CO3 → Ca(HCO3)2
MgCO3(s) + H2CO3 → Mg(HCO3)2
Precipitation
Topsoil

4. 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
-
, CO3
2-
» CH = TH or Total alkalinity, whichever is less

5. 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
-
,
SO4
2-
» NCH = TH - CH
» If Alkalinity ≥ Total hardness, then NCH = 0

6. 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. 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. 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. 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 ?...
5.8 mg/l as Mg(OH)2
29 mg/meq
58 mg/mmol

10. 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. 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. 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