Module V
Water Technology and Sewage Water Treatment
About 75% earth’s crust is water. Water is essential for living beings as well as for industries. It is used as an engineering material, as a coolant in power plants and also used for power generation purpose. But only 1% of earth’s water r...
, Module V
Water Technology and Sewage Water Treatment
About 75% earth’s crust is water. Water is essential for living beings as well as for
industries. It is used as an engineering material, as a coolant in power plants and also used for
power generation purpose. But only 1% of earth’s water resources are available for ready use.
Hence it is urgently required to treat the water most carefully and economically. Such type of all
treatments comes under the heading water technology.
Hardness of water
The property of water which prevents lather formation with soap solution is called hardness of
water. It is due to the presence of dissolved salts of Ca, Mg and some other heavy metals. On the
basis of hardness, water is of two types;
1. Soft water
2. Hard water
Soft water:
Water which can form ready and permanent lather with soap solution is called soft water.
Hard water:
Water which cannot form ready and permanent lather with soap solution is called hard water.
Soap is sodium or potassium salts of higher fatty acid which when treated with hard water
produces insoluble Ca-Soap and Mg-Soap (white scum).
2C17H35COONa + CaCl2 → (C17H35COO)2Ca +2NaCl
(Soap) (Ca- Soap)
2C17H35COONa + MgSO4 → (C17H35COO)2Mg +Na2SO4
Hardness is of two types.
1. Temporary (Carbonate) hardness
2. Permanent (Non-carbonate) hardness
Temporary (Carbonate) hardness:
Hardness which will remain for a shorter period and can be easily removed by boiling is called
temporary (Carbonate) hardness. It is due to the presence of bicarbonates of Ca & Mg. On
boiling, soluble bicarbonate changes to insoluble carbonates and hydroxides.
Ca(HCO3)2 → CaCO3 + H2O + CO2
Mg(HCO3)2 → Mg(OH)2 + 2CO2
Removal of temporary hardness
1. Boiling
On boiling, soluble bicarbonate changes to insoluble carbonates and hydroxides.
Ca(HCO3)2 → CaCO3 + H2O + CO2
Mg(HCO3)2 → Mg(OH)2 + 2CO2
2. Clark’s process
In this method, calculated quantity of lime is added to convert it as insoluble carbonate.
Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O
Mg(HCO3)2 + Ca(OH)2 → MgCO3 +CaCO3 + H2O
Permanent (Non-carbonate) hardness:
Hardness due to the presence of chlorides and sulphates of Ca, Mg, Al, Fe, etc. are called
permanent hardness.
Removal of Permanent (Non-carbonate) hardness:
1. Lime soda process
2. Zeolite process
3. Ion exchange process
Sree Buddha College of Engineering, Pattoor Page 1
, Degree of hardness
Hardness is expressed in terms of degree of hardness. Degree of hardness is expressed in
terms of CaCO3 equivalent hardness. The concentration of hardness producing ions and non-
hardness producing ions are expressed in terms of equivalent amount of CaCO3 and is called
CaCO3 equivalent hardness. The choice of CaCO3 in particular due to,
1. Molecular weight is 100.
2. Equivalent weight is 50.
3. It is the most insoluble precipitate in most of the water treatment processes.
CaCO3 equivalent hardness =
Where HPS is hardness producing substance.
The same formula can be modified by replacing the equivalent weight by molecular weight.
CaCO3 equivalent hardness =
Let X be the mass of HPS, then CaCO3 equivalent hardness =
Units of hardness
The important units of hardness are
1. Ppm
2. Mg/L
3. Degree French (°F)
4. Degree Clark (°Cl)
5. Equivalents per million (epm)
Ppm:
It is the number of parts of CaCO3 equivalent hardness present per million parts of water.
1ppm = 𝟏𝟎𝟔 ppm =1
1𝟏𝟎𝟔 ppm =
Mg/L:
It is the number of milligrams of CaCO3 equivalent hardness present per litre of water.
1mg/L = = 1ppm 𝟏𝟎𝟔 mg/L = 1
Degree French:
It is the number of parts of CaCO3 equivalent hardness present per partsof water.
1°F = 𝟏𝟎𝟓 𝑭 = 1
Degree Clark:
It is the number of parts of CaCO3 equivalent hardness present per 70000 parts of water.
1°Cl =
Equivalents per million:
It is the number of equivalents of CaCO3 equivalent hardness present per million parts of
water.
𝟏𝟎𝟔
1 epm = epm = 1
𝟓𝟎
Inter conversion of various units of hardness 70000°Cl = 1
Ppm = mg/L = =70000°Cl = epm = 1
1ppm = 1mg/L = 0.1°F = 0.07°Cl =0.02epm
Sree Buddha College of Engineering, Pattoor Page 2
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