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What is the goal of Water Treatment
3) Pathogenic Contaminants
from water sources in the most affordable and
expedient manner possible.
Sources of Water
1) Rain Water
2) Surface Water (Rivers and Lakes)
3) Ground Water (Wells and Springs)
4) Sea Water
Uses of Water
Impurities in Water
1) Dissolved Impurities :- carbonates and bicarbonates,
chlorides, magnesium, iron, dissolved gases
2) Suspended Impurities :-
Organic :- oil globules, vegetable and animal
Inorganic :- clay and sand
3) Colloidal Impurities :- finely divided silica and clay,
organic waste products, amino acids
4) Micro organisms :- fungi and bacteria
Methods for removal of impurities
5) Sterilization :-
1) by Chlorine or Bleaching Powder
2) by UV Radiation
3) by Ozone
1) Screening :
Screening is a process of removing floating materials
like wood pieces and leaves from water.
Raw material is allow to pass through a screen having a
large number of holes which removes the small and large
2) Sedimentation :
Sedimentation is the process of removing suspended
impurities by allowing the water to stand undisturbed for
2-6 hours in big tanks.
Due to force of gravity, most of the suspended particles
settled down at the bottom and they are removed.
Sedimentation removes only 70-75% of the suspended
3) Coagulation :
Finely-divided silica, clay etc. do not settle down easily
and hence can not be removed by sedimentation.
Most of these are in colloidal form and are negatively
charged and hence do not coalesce because of mutual
Such impurities are removed by coagulation method
When Al2(SO4)3 is added in water, it hydrolyzes to form a
gelatinous precipitate of Al(OH)3. It entraps impurities and
settle down at the bottom.
4) Filtration :
For removing bacteria, colour, taste, odour, fine
suspended particles, etc and to produce clear water,
filtration is used.
In this process, water is passed through beds of fine
sand, coarse sand and other granular material.
The porous material used is the filtration medium and
the equipment used for filtration is known as filter.
e.g. slow sand filter
5) Sterilization :
1) By Chlorine or Bleaching Powder :- When
bleaching powder is added in the water, HOCl is
produced and it kills the bacteria.
2) By Ultraviolet Radiation :- UV Radiation can kill the
bacteria. It is useful for sterilizing swimming pool
3) By Ozone :- Ozone is readily absorbed by water. It
is highly unstable and decomposes to give nascent
oxygen. It kills the bacteria.
Boiler Feed Water
One of the chief use of water is generation of steam
Essential requirements of Boiler Feed Water :-
It should be free from
- Turbidity, oil, dissolved salts
- Hardness & scale forming constituents
- Dissolved O2 & CO2
- Caustic alkali
If hard Water is directly fed into boiler there arise
many problems such as
- Sludge & Scale Formation
- Caustic Embrittlement
The process of formation of Wet Steam is called
Because of priming efficiency and life of machine
It is caused by
- Very high water level
- Presence of foam on the surface
- High steam velocity
Formation of stable bubbles above the surface of
water is called foaming.
These bubbles are carried over by steam leading to
It is caused by presence of soluble impurities like
alkali metal salts and oil.
Foaming can be controlled by action of anti foaming
agents like synthetic polyamides
1. Sludges are soft and non-adherent
1. Scales are hard deposits which
stick very firmly to the inner
surface of boiler.
2. Sludges can be removed easily. 2. Scales are very difficult to
3. Sludges can transfer heat to
some extent and is less
3. Scales are bad conductors of
heat and are more dangerous.
4. Sludges are formed by
substances like MgCl2 and
4. Scales are formed by
substances like CaSO4 and
Caustic Embrittlement :
It is the phenomenon during which the boiler material
becomes brittle. Na2CO3 present in water breaks up and give
NaOH and water become alkaline.
This alkali boiler water flows into cracks by capillary action.
There the water evaporates and concentration of caustic
soda increases. This leads to failure of boiler.
It can be avoided by using sodium phosphate as a softening
reagent instead of Na2CO3 and adding tannin or lignin to
boiler water which blocks the cracks.
Softening of Water :
Softening can be done in two ways
Zeolite Process :
Zeolite are naturally occurring hydrated sodium
Chemical Formula :- Na2O∙Al2O3∙xSiO2∙yH2O
It is represented by Na2Ze
Sodium ions are replaced by Ca2+ and Mg2+ ions.
When zeolite comes in contact with hard water, it
forms calcium and magnesium zeolites.
1) Water of nearly zero hardness can be obtained.
2) Method is cheap.
3) No sludge formation takes place.
4) Equipments are compact and require small
5) Easy Operation.
6) Process can be made automatic and continues.
1) This method can not be used for acidic and turbid water.
2) This treatment replaces only cations leaving all anions
2- in the soft water.
like HCO3 and Co3
3) This method can not be used for softening of brackish
water because it contains Na+ ions.So, ion exchange
reaction will not take place.
Ion Exchange Process :
Cations like Ca2+ and Mg2+ and anions like Cl- and SO4
can be removed by cation and anion exchange resins.
Resins containing -COOH, -SO3H are capable for
exchanging their H+ ions to cationic portion of minerals. It
is called cation exchanger.
Resins containing –NH2 , NHCH3 are capable for
exchanging the anionic portion of the minerals. It is called
Water is passed through cation and anion
Water coming out side from anion exchanger is
completely free from cations and anions.
It is known as deionised water or demineralised
water and it is pure as distilled water.
Cation exchange resins are regenerated by passing
a dilute solution of HCl through them.
Rca + 2HCl RH2 + CaCl2
Anion exchange resins are generated by passing a
dilute solution of NaOH.
R’Cl2 + 2NaOH R’(OH)2 + 2NaCl
1) Highly acidic or alkaline water can be treated.
2) Water of nearly zero hardness can be obtained.
1) Equipment is costly and expensive chemicals are
2) If water contains turbidity then the output of process is
1) Colloidal Conditioning :
This colloidal Substances get coated over the scale
forming particles and thus the coalescence of this
particles to a compact scale is prevented.
Thus the scale formed remains loose and non-sticky
and can be easily removable.
starch and glue are colloidal conditioning
2) Phosphate Conditioning :
Scale formation can be avoided by adding sodium
phosphate to boiler water.
Phosphate reacts with Ca and Mg salts and formed
easily removable sludge.
3CaSO4 + 2Na3PO4 Ca3(PO4)2 + 3Na2SO4
If water is
- acidic then trisodium phosphate is used
- alkaline then sodium dihydrogen phosphate is used
- neutral then disodium hydrogen phosphate is used
3) Carbonate Conditioning :
Scale formation can be avoided by adding sodium
carbonate to boiler water.
CaSO4 + Na2CO3 CaCO3 + Na2SO4
4) Calgon Conditioning :
It involves the addition of calgon or sodium
hexametaphosphate to boiler water.
This substance reacts with calcium ions and forms highly
soluble complex and thus prevents the precipitation of
scale forming salt.
2CaSO4 Na2[Ca2(PO3) 6]
Na2[Na4(PO3) 6] 2Na2SO4
4) Treatment with sodium aluminate :
When sodium aluminate is treated with boiler water, it
gets hydrolyzed give sodium hydroxide and a gelatinous
precipitate of aluminum hydroxide.
NaAlO2 + 2H2O NaOH + Al(OH)3
2NaOH + MgCl2 Mg(OH)2 + 2NaCl
Al(OH)3 and Mg(OH)2 entrap finely suspended and
colloidal impurities like sand, oil drops and settle to the
bottom and can be removed easily.
Ions present in saline water migrate towards the
oppositely charged electrodes because of applied
Movements of ions takes place through ion selective
Compartments with even number are filled with pure
water and compartments with odd numbers filled with
concentrated brine water when the process finish.
Thus the salinity from the water can be removed.
When two solutions of different concentration are
separated by a semi permeable membrane, flow of
solvent takes from a region of low concentration to
high concentration, until the concentration is equal
both sides. This process is called osmosis and
reverse process is called reverse osmosis.
The driving force in this phenomenon is called
By this method pure water is separated from sea
Membranes used are cellulose, acetate, cellulose