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Depending upon the magnitude of treatment required, proper unit operations are selected and arranged in the proper sequential order for the purpose of modifying the quality of raw water to meet the desired standards. Indian Standards for drinking water are given in the table below.
Indian Standards for drinking water
Parameter |
Desirable-Tolerable |
If no alternative source available, limit extended upto |
Physical
|
Turbidity (NTU unit) |
< 10 |
25 |
Colour (Hazen scale) |
< 10 |
50 |
Taste and Odour |
Un-objectionable |
Un-objectionable |
Chemical
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pH |
7.0-8.5 |
6.5-9.2 |
Total Dissolved Solids mg/l |
500-1500 |
3000 |
Total Hardness mg/l (as CaCO3) |
200-300 |
600 |
Chlorides mg/l (as Cl) |
200-250 |
1000 |
Sulphates mg/l (as SO4) |
150-200 |
400 |
Fluorides mg/l (as F ) |
0.6-1.2 |
1.5 |
Nitrates mg/l (as NO3) |
45 |
45 |
Calcium mg/l (as Ca) |
75 |
200 |
Iron mg/l (as Fe ) |
0.1-0.3 |
1.0 |
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The typical functions of each unit operations are given in the following table:
Functions of Water Treatment Units
Unit treatment |
Function (removal) |
Aeration, chemicals use |
Colour, Odour, Taste |
Screening |
Floating matter |
Chemical methods |
Iron, Manganese, etc. |
Softening |
Hardness |
Sedimentation |
Suspended matter |
Coagulation |
Suspended matter, a part of colloidal matter and bacteria |
Filtration |
Remaining colloidal dissolved matter, bacteria |
Disinfection |
Pathogenic bacteria, Organic matter and Reducing substances |
The types of treatment required for different sources are given in the following table:
Source |
Treatment required |
1. Ground water and spring water fairly free from contamination |
No treatment or Chlorination |
2. Ground water with chemicals, minerals and gases |
Aeration, coagulation (if necessary), filtration and disinfection |
3. Lakes, surface water reservoirs with less amount of pollution |
Disinfection |
4. Other surface waters such as rivers, canals and impounded reservoirs with a considerable amount of pollution |
Complete treatment |
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