Water treatment for municipality water supply
Water treatment
Removal of suspended impurities (4 Disinfection or sterilisation
stages) Addition of bleaching
1. screening powder
2. Plain sedimentation chlorination
3. Sedimentation by
coagulation ozonisation
4. filtration
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Water treatment for drinking purpose
The water which is obtained from river, lakes, ponds, dams etc..
May contain suspended impurities and pathogenic microorganisms.
For the drinking and domestic purpose, the water should be treated
in two stages.
1. Removal of suspended impurities
2. Disinfection or sterilisation
Removal of suspended impurities
The suspended impurities are removed by four stages completely.
i. Screening
ii. Sedimentation
iii. Sedimentation by coagulation
iv. Filtration
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Water treatment for drinking purpose
Removal of suspended impurities
Screening
Water is sent through the screen and arresting the floating material in the
water.
Large size floating matters arrested in the screening.
Plain sedimentation
In this process screened water is stored and stand for few hours to few days.
Due to gravitational force of suspended particles, most of the particles settle
down at bottom, and the water is send for coagulation sedimentation
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Water treatment for drinking purpose
Removal of suspended impurities
sedimentation by coagulation
Plain sediment water may contains dissolved salt. These salts are
removed by adding coagulants like alums, sodium aluminate,
ferrous sulphate etc..
By adding alums, suspended salts got coagulation with alums and
form flock like substance which settle down at bottom.
K2SO4.Al2(SO4)3.24 H2O K2SO4 + Al2(SO4)3 + 24 H2O
Al2(SO4)3 + 3 Ca(HCO3)2 pH>8 2 Al(OH)3 + 3 CaSO4 + 6 CO2
coagulant Flocculent
precipitate
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Water treatment for drinking purpose
Removal of suspended impurities
sedimentation by coagulation
Sodium aluminate acts as coagulant at pH less than 7 for
salts which are dissolved in water
NaAlO2 + 2 H2O pH<7 Al(OH)3 + NaOH
2 NaOH + MgSO4 Mg(OH)2 + Na2SO4
coagulant Flocculent
precipitate
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Water treatment for drinking purpose
Removal of suspended impurities
sedimentation by coagulation
Ferrous Sulphate also works as coagulant at pH > 8.5
FeSO4 + Mg(HCO3)2 Fe(OH)2 + MgCO3 + CO2 + H2SO4
coagulant
2 Fe(OH)2 +O2 + 2 H2O 4 Fe(OH)3 2Fe2O3 + 6H2O
Flocculent heavy Floc.
precipitate precipitate
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Water treatment for drinking purpose
Removal of suspended impurities
Filtration
Suspended sediments, remaining suspended particles are
filtered by sending the water through the filtration bed. The
filtered water is send for sterilization tower.
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Water treatment for drinking purpose
Sterilization or Disinfection
The killing of pathogenic bacteria, viruses, other microorganisms present in
the water is called sterilization or disinfection of water. This can be done in
various methods.
By adding bleaching powder
By adding bleaching powder to the water, bleaching releases free chlorine,
which produce free nascent oxygen through hypochlorous acid. Nascent
oxygen kills all the pathogenic germs.
H2O + CaOCl2 Ca(OH)2 + Cl2
Cl2 + H2O HCl + HOCl
HOCl HCl +(O)
Pathogenic germs + (O) Dead gems or Deactivated germs
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Water treatment for drinking purpose
Sterilization or Disinfection
The killing of pathogenic bacteria, viruses, other microorganisms present in
the water is called sterilization or disinfection of water. This can be done in
various methods.
By adding bleaching powder
Advantages
✓ Cheapest and easiest method
Disadvantages
✓ Excess of bleaching gives bad taste and bad smell to the water.
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Water treatment for drinking purpose
Sterilization or Disinfection
By Chlorination
The tower which is used for chlorination is called chlorinator.
Chlorinator contains baffle plates which are useful for proper mixing.
Chlorine solution and water are sent from the top of the chlorinator and
collected at bottom. Here chlorine liberates nascent oxygen through
hypochlorous acid, which act as powerful germicide.
Cl2 + H2O HCl + HOCl
HOCl HCl + (O) nascent oxygen
Pathogenic germs + (O) Dead germs or Deactivated germs
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Water treatment for drinking purpose
Sterilization or Disinfection
By Chlorination
Advantages
✓ Storage requires less space.
✓ Effective and economical.
✓ Ideal disinfectant.
Disadvantages
✓ Excess chlorine causes unpleasant taste and odour.
✓ More effective at pH < 6.5 and less effective at
high pH.
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Ozonisation
Ozone is a powerful and an excellent disinfectant, which is produced by ozoniser.
3 O2 silent electric discharge 2 O3
Oxygen Ozone
Ozone is highly unstable and give nascent oxygen on breaking.
O3 O2 + (O) nascent Oxygen
Pathogenic germs + (O) Dead gems or Deactivated germs.
Advantages
Oxidises other reducing substances such as organic or inorganic matter.
Disadvantages
Method is costly and which is not useful for large scale.
Storage of ozone is difficult due to its unstableness.
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Water treatment for drinking purpose
Break point chlorination or Free residual chlorine
Break point chlorination involves in addition of sufficient amount of chlorine
to oxidise (a) Organic matter, (b) reducing substances in raw water, (c) free
ammonia in raw water, leaving behind free chlorine, which posses
disinfecting action against pathogenic germs.
When increasing the dosage of chlorine to water which contains organic
matter or reducing substances or free ammonia, gradually some changes
takes place which shown in the following figure.
4-Sep-23 13
Dr. K. Chandra Mohan M.Sc.,
4-Sep-23 14 Ph.D.,
Assistant Professor,
Department of Chemistry,
drkchandramohan@mits.ac.in
Break point chlorination or Free residual chlorine
From the graph, initially the applied chlorine is used to destroy reducible
substances present in water and there is no free residual chlorine (Section 1)
As the amount of applied chlorine increases, the amount of combined residual
chlorine also increases. This is due to the formation of chloramine and other
chlorocompounds. (Section 2)
On further increase of chlorine dose, the combined residual chlorine decreases
i.e.. monochloramines destroyed and converted to dichloramines(Section 3)
Dichloramines give bad taste and odour to water due to volatile in nature. Due to
this combined chlorine reaches to minimum point, which is known as break point
chlorination.
After the break point of chlorination, free residual chlorine is on increase in dosage
of chlorine(Section 4).
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Break point chlorination or Free residual
chlorine
Advantages
Before reaching the break point of chlorination
✓ Oxidising the reducing substances, organic matter and fee ammonia
completely.
✓ Removes colour, taste and odour from the water.
After reaching breakpoint destroy the all pathogenic bacteria and
other germs.
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Brackish water
The water which contains dissolved salts with salty taste is
called brackish water.
The water which contains about 3.5% of salts with brackish taste
is comes under brackish water. The brackish water is unfit for
drinking. Ex. Sea water, ocean water etc..
Water attains brackishness due to presence of dissolved sodium
chloride and small amounts of other salts. The removal of those
salts from the water is known as desalination of water or
demineralisation of water.
Reverse osmosis and electro dialysis are commonly used methods
for the desalination of water.
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Electrodialysis
Softening of water by Reverse Osmosis
The diffusion of solvent through a semi
permeable membrane from pure solvent to
solution or dilute solution to concentrated
solution
The external pressure applied to the solution in order to
stop the osmosis of pure solvent into solution or dilute
solution into concentrated solution separated
by a semipermeable membrane is called osmotic
pressure.
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Reverse Osmosis
force water through membrane
removes many contaminants
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Softening of water by Reverse Osmosis
Advantages
✓ Removes ionic, non-ionic, colloidal and high molecular
weight organic matter.
✓ Microscopic parasites (including viruses) are usually
removed by properly functioning RO units.
✓ Low maintenance cost with high reliability.
Disadvantages
✓ Damaged membranes are not easily detected, so it is
hard to tell if the system is functioning normally and
safely.
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Uses:
1. For eliminating certain certain hazardous compounds
related to industrial wastes, chemicals and pesticides
2. For removing unpleasant tastes and odour caused by
decaying organic matter, dissolved gases, and residual
chlorine
Limitations:
1. Does not treat microorganism
2. Water has to be chlorinated before passing through an
activated carbon filter so as to prevent bacterial growth
on the filter.
Candle Filtration
The Candle Filter consists of three
major components:
• The vessel
• The filtering elements
• The cake discharge mechanism
• Candle Filters are very well suited for
handling flammable, toxic and
corrosive materials.
Candle Filtration
• Advantages
• Excellent cake discharge.
• Adapts readily to slurry thickening.
• Minimum floor space.
• Mechanically simple since there are no complex sealing
glands or bearings.
• Disadvantages
• High headroom is required for dismantling the filtering
elements.
• The emptying of the vessel in between cake filtration, washing
and drying requires close monitoring of the pressure inside
the vessel to ensure that the cake holds on to the candles.
