WATER TREATMENT – MODULE 2
SOFTENING OF WATER
The hardness producing salts can be removed from water by the following
methods:
1) External treatment
=> The external treatment of water is carried out before it is fed into the boiler.
This treatment prevents boiler troubles.
=> It can be done by the following methods:
(i) Lime-Soda process
-> It is a process in which Lime (Ca(OH)2) and Soda (Na2CO3) are added to the hard
water to convert the soluble calcium and magnesium salts to insoluble
compounds by a chemical reaction. The CaCO3 and Mg(OH2) so precipitated are
filtered off and removed easily.
-> It is further divided into two types:
1. Cold Lime Soda-process
⦁ In this process a calculated quantity of Lime (Ca(OH )2) and Soda (Na2CO3) are
added to hard water at room temperature. The following reactions takes place:
For permanent hardness due to calcium salt:
Ca2+ + Na2CO3 -------> CaCO3 ↓ + 2Na+
Soda Slimy suspended precipitate
For permanent hardness due to magnesium salt:
Mg2+ + Ca(OH)2 --------> Mg(OH)2 ↓ + Ca2+
Lime Slimy suspended precipitate
Ca2+ + Na2CO3 --------> CaCO3 ↓ + 2Na+
Soda Slimy suspended precipitate
For temporary hardness due to calcium salt:
, Ca(HCO3)2 + Ca(OH)2 --------> 2CaCO3 ↓ + 2H2O
Lime Slimy suspended precipitate
For temporary hardness due to magnesium salt:
Mg(HCO)3 + 2Ca(OH)2 --------> 2CaCO3 ↓ + Mg(OH)2 + 2H2O
Lime Slimy suspended precipitate
⦁ In this method, the precipitates formed are finely divided and form sludge in
the boiler water, so they do not settle easily and cannot be filtered easily. This
reduces efficiency of the boiler. Therefore, we add small amount of coagulants
such as alum, aluminium sulphate, sodium aluminate etc.
⦁ When coagulants are added flocculation takes place followed by the formation
of flocculants.
NaAlO2 + 2H2O --------> NaOH + Al(OH)3 ↓
Coagulant (Sodium Aluminate) Flocculant
Al2 (SO4)3 + 3Ca(HCO3)2 ---------> 2Al(OH)3 ↓ + CaSO4 + CO2
Aluminuim sulphate Flocculent
⦁ Flocculent, on formation, entraps the fine precipitates which make the
precipitates heavy. The heavier flocs then settle at the bottom and filtered off
easily.
⦁ Process:
, a. Raw water containing the dissolved salts of calcium and magnesium is fed
through the water inlet.
b. Water fills the inner cylindrical vessel completely. A hopper with a connecting
tube is attached to the inner cylinder containing the raw water.
c. Requisite quantities of both soda ash and lime as desired in equivalents are
fed through the hopper into the inner cylinder.
d. A stirrer attached to a motor swirls the raw water continuously along with the
soda ash and lime. As a result of chemical interaction, carbonates of the metal
ions are precipitated as a fine powder that settles down very slowly.
e. A coagulant like NaAlO2, aluminium sulphate, sodium aluminate is introduced
to grow the size of the carbonates of calcium and magnesium particles and to
settle quickly at the bottom, as sludge that can be easily removed.
f. The soft water moves to the outer cylinder and passes through a filter that
filters out unwanted materials. Finally, soft water can be withdrawn through
the outlet at the top of the outer cylinder.
2. Hot Lime-Soda process
• In this process a calculated quantity of Lime (Ca(OH)2) and Soda (Na2CO3)
are mixed with hot water at a temperature range of 80° C to 150°C.
• The rate of precipitation reactions can be greatly accelerated by increasing
the temperature, because, this not only increases the rate of the ionic
reactions themselves, but also the rate at which particles of measurable
size are formed.
• Process:
SOFTENING OF WATER
The hardness producing salts can be removed from water by the following
methods:
1) External treatment
=> The external treatment of water is carried out before it is fed into the boiler.
This treatment prevents boiler troubles.
=> It can be done by the following methods:
(i) Lime-Soda process
-> It is a process in which Lime (Ca(OH)2) and Soda (Na2CO3) are added to the hard
water to convert the soluble calcium and magnesium salts to insoluble
compounds by a chemical reaction. The CaCO3 and Mg(OH2) so precipitated are
filtered off and removed easily.
-> It is further divided into two types:
1. Cold Lime Soda-process
⦁ In this process a calculated quantity of Lime (Ca(OH )2) and Soda (Na2CO3) are
added to hard water at room temperature. The following reactions takes place:
For permanent hardness due to calcium salt:
Ca2+ + Na2CO3 -------> CaCO3 ↓ + 2Na+
Soda Slimy suspended precipitate
For permanent hardness due to magnesium salt:
Mg2+ + Ca(OH)2 --------> Mg(OH)2 ↓ + Ca2+
Lime Slimy suspended precipitate
Ca2+ + Na2CO3 --------> CaCO3 ↓ + 2Na+
Soda Slimy suspended precipitate
For temporary hardness due to calcium salt:
, Ca(HCO3)2 + Ca(OH)2 --------> 2CaCO3 ↓ + 2H2O
Lime Slimy suspended precipitate
For temporary hardness due to magnesium salt:
Mg(HCO)3 + 2Ca(OH)2 --------> 2CaCO3 ↓ + Mg(OH)2 + 2H2O
Lime Slimy suspended precipitate
⦁ In this method, the precipitates formed are finely divided and form sludge in
the boiler water, so they do not settle easily and cannot be filtered easily. This
reduces efficiency of the boiler. Therefore, we add small amount of coagulants
such as alum, aluminium sulphate, sodium aluminate etc.
⦁ When coagulants are added flocculation takes place followed by the formation
of flocculants.
NaAlO2 + 2H2O --------> NaOH + Al(OH)3 ↓
Coagulant (Sodium Aluminate) Flocculant
Al2 (SO4)3 + 3Ca(HCO3)2 ---------> 2Al(OH)3 ↓ + CaSO4 + CO2
Aluminuim sulphate Flocculent
⦁ Flocculent, on formation, entraps the fine precipitates which make the
precipitates heavy. The heavier flocs then settle at the bottom and filtered off
easily.
⦁ Process:
, a. Raw water containing the dissolved salts of calcium and magnesium is fed
through the water inlet.
b. Water fills the inner cylindrical vessel completely. A hopper with a connecting
tube is attached to the inner cylinder containing the raw water.
c. Requisite quantities of both soda ash and lime as desired in equivalents are
fed through the hopper into the inner cylinder.
d. A stirrer attached to a motor swirls the raw water continuously along with the
soda ash and lime. As a result of chemical interaction, carbonates of the metal
ions are precipitated as a fine powder that settles down very slowly.
e. A coagulant like NaAlO2, aluminium sulphate, sodium aluminate is introduced
to grow the size of the carbonates of calcium and magnesium particles and to
settle quickly at the bottom, as sludge that can be easily removed.
f. The soft water moves to the outer cylinder and passes through a filter that
filters out unwanted materials. Finally, soft water can be withdrawn through
the outlet at the top of the outer cylinder.
2. Hot Lime-Soda process
• In this process a calculated quantity of Lime (Ca(OH)2) and Soda (Na2CO3)
are mixed with hot water at a temperature range of 80° C to 150°C.
• The rate of precipitation reactions can be greatly accelerated by increasing
the temperature, because, this not only increases the rate of the ionic
reactions themselves, but also the rate at which particles of measurable
size are formed.
• Process:
