Unit-I Water Technology

Dr. K. Thirunavukkarasu
Department of Chemistry
 Introduction
70% of our body contains water, which regulates the all the processes and also
maintain the body temperature by sweating and evaporation.
Water act as a universal solvent and widely used in laboratories, irrigation, steam
generator, industrial purpose.
80% of the earth surface is surrounded with water.
97% of the total resources are not useful for domestic, agricultural and industry.
0.7% is ready to use.
From an engineer’s point of view- water is required in boilers for production of
steam, which act as a source of energy and a coolant in many power and chemical
plants and many other industries.
 To develop appropriate and cheap technologies to treat ocean water is known as
water technology or water treatment.
Sources of water
 Types of Impurities
Types of impurities

Dissolved Suspended Collodial Microorganism

Inorganic Organic
Inorganic Gases Silica
Clay Bacteria
Fungi
Virus

Ca2+ NO2 Vegetables

Sand
Mg2+ CO2 Clay Leaves
Na+ SO2 dead materials
CO32-
SO42-
Cl-
 Hard water and Hardness
Depending on salts presents in water and reaction with soap, water is categorized
into hard water and soft water.
Due to presence of chlorides, sulphates and bicarbonates salts of calcium and
magnesium, water does not produce lather (foam) and forms white scum.
Soap is the sodium or potassium salt of fatty acids like stearic acid.

2 C17H35COONa + MgCl2 (C17H35COO)2Mg + 2 NaCl

Sodium stearate Salt White scum

Water which can produces lather with soap easily is called soft water and doesn't
produce lather is called hard water.
 Types of Hardness
Depending on salts present in the water, hardness is of two types
Temporary or Carbonate Hardness: Dissolved bicarbonates of calcium and
magnesium.
By boiling water, temporary hardness can be removed.

Heat
Ca(HCO3)2 CaCO3 + H2O + CO2

soluble in water Insoluble in water

Heat
Mg(HCO3)2 MgCO3 + H2O + CO2

soluble in water Insoluble in water

 Types of Hardness
Permanent or Non-carbonate Hardness: Chlorides and sulphates of calcium and
magnesium. It cannot removed by boiling and special methods are followed.

Lime soda process:

CaCl2 + Na2CO3 CaCO3 + 2 NaCl

Zeolite process:
(-Micro porous, aluminosilicate minerals, used as catalyst and adsorbents)

MgSO4 + Na2Ze MgZe + Na2SO4

Total hardness is sum of the temporary and permanent hardness

 Expression of Hardness
Hardness of water is expressed in terms of equivalents of calcium carbonate.
 Reason for choosing CaCO3:
CaCO3 is a insoluble salt: Its molecular weight is 100 and equivalent weight is 50.

The amount of dissolved salts present in water is first converted into calcium
carbonate equivalent by using the formula;

 If the weight of CaSO4 is 43 mg/lit, then the weight equivalent to CaCO3 is

 Units of Hardness
Parts per million (ppm): Number of parts of calcium carbonate equivalent hardness
present per 106 parts of water.
1 ppm = 1 part of CaCO3 equivalent hardness per 106 parts of water
 Milligrams per liter (mg/lit): Number of milligrams of calcium carbonate equivalent
hardness present per liter of water.
1 mg/L = 1 mg of CaCO3 equivalent hardness per 106 mL of water
Degree French ( 0Fr): Number of parts of calcium carbonate equivalent hardness
present per 105 parts of water.
1 0Fr = 1 part of CaCO3 equivalent hardness per 105 parts of water
 Degree Clarkes ( 0Cl): Number of parts of calcium carbonate equivalent hardness
present per 70000 parts of water.
1 0Cl = 1 part of CaCO3 equivalent hardness per 70000 parts of water

1 ppm = 1 mg/L = 0.1 0Fr = 0.07 0Cl

 Determination of Hardness ( EDTA method)
Hardness of water can be determined by complexometric titration using ethylene
diamine tetra acetic acid (EDTA).
 It is the most important and more accurate method to determine the hardness of
water.
EDTA has limited solubility in water, hence, disodium salt of EDTA is used which is
soluble in water.
 Determination of Hardness ( EDTA method)
Principle:
EDTA can form complex with salts (Ca2+ and Mg2+) which are present in hard water.
 These ions forms unstable wine red complex with Erichrome Black T (EBT) indicator
in ammonical buffer solution.
When it is titrated with EDTA solution, the metals ions give a stable deep blue colour
(M-EDTA) complex and releases the free indicator.
EDTA
2+
M + EBT M-EBT M-EDTA + EBT

( Ca2+ or Mg2+) Unstable Stable Steel blue

Wine red

O
OH HO O O
C M C O
NaO3S N N
H2C N N CH2
OOCH2C
CH2COO
EBT
O2N CH2
CH2
 Determination of Hardness ( EDTA method)
Preparation of solutions:
Standard hard water: Dissolve 1 g of pure and dry CaCO3 in minimum

quantity of dil. HCl and evaporate. Redissolve in 1000 mL of distilled water.

EDTA solution: Dissolve 4 g disodium EDTA in 1000 mL of distilled water.

EBT indicator: Dissolve 0.5 g EBT in 100 mL of methanol.

Buffer solution: Dissolve 67.5 g NH4Cl in 570 mL of NH4OH and make up to

1000 mL distilled water.

 Determination of Hardness ( EDTA method)
Procedure:
Step 1: Standardization of EDTA solution: Pipette out 50 mL of

standard hard water (SHW) in a 250 mL conical flask. Add 10- 15 mL buffer solution and
two drops of EBT indicator. Titrate the flask solution against EDTA solution from the
burette until the colour changes from wine red to steel blue. Repeat the procedure to
get concordant readings. Let the volume of EDTA be consumed as V1 mL.

Step 2: Determination of total hardness: Titrate 50 ml of

unknown water sample with EDTA solution by addition of 10- 15 mL buffer solution
and two drops of EBT indicator till the colour changes from wine red to steel blue. Let
the volume of EDTA be consumed as V2 mL.
 Determination of Hardness ( EDTA method)
Procedure:
Step 3: Determination of permanent hardness: Take 250 mL

of water sample in a 500 mL beaker and boil gently for half an hour. Cool, filter, and
wash the precipitate with distilled water, collecting filtrate and washing in a 250 mL
measuring flask, and make up to the mark. Now titrate 50 ml of unknown water
sample with EDTA solution by addition of 10- 15 mL buffer solution and two drops of
EBT indicator till the colour changes from wine red to steel blue. Let the volume of
EDTA be consumed as V3mL.
 Determination of Hardness ( EDTA method)
Calculation:
Step 1: Standardization of EDTA solution:
Volume of SHW taken for titration = 50 mL
Concordant volume of EDTA used = V1 mL
V1 mL of EDTA = 50 mL of SHW = 50 X 1 mg of CaCO3
1 mL of EDTA = 50/V1 mg of CaCO3

Step 2: Determination of total hardness:

Volume of unknown water sample taken for titration = 50 mL
Concordant volume of EDTA used = V2 mL
50 mL of unknown water sample = V2 mL of EDTA = V2 X 50/V1 mg of CaCO3
1 mL of unknown water sample = V2 /50 X 50/V1 mg of CaCO3
1000 mL of unknown water sample = V2 /V1 X 1000 mg /L
 Determination of Hardness ( EDTA method)
Calculation:
Step 3: Determination of permanent hardness:
Volume of hard water sample taken after boiling and filtering for titration = 50 mL
Concordant volume of EDTA used = V3mL
50 mL of unknown water sample = V3mL of EDTA = V3 X 50/V1 mg of CaCO3
1 mL of unknown water sample = V3 /50 X 50/V1 mg of CaCO3
1000 mL of unknown water sample = V3 /V1 X 1000 mg/L
Permanent hardness = V3 /V1 X 1000 ppm
Temporary hardness = Total hardness – permanent hardness
= V2 /V1 X 1000 ppm - V3 /V1 X 1000 ppm
= (V2-V3/V1 ) 1000 ppm
 Boiler feed water
The water feed into the boiler for the production of steam is called boiler feed water.
Requirements: Water should be
 Clear, clean, colorless and odourless.
Free from hardness, suspended particles and pathogenic bacteria.
 Free from harmful dissolved solids like arsenic, manganese, chromium, lead etc.
Free from harmful gases like H2S, SO2, etc.
TDS should be less than 500 ppm.
pH should be about 7-8.
Turbidity should be less than 10 ppm.
Hardness should be about 300 ppm.
Alkalinity should not exceed 600 ppm.
Fluoride should be less than 3 ppm
Chloride and sulphate should be less than 250 ppm.
 Disadvantages of using hard water in boilers
Scale and sludge formation: In boilers, when water is vaporized to steam gradually the
concentration of dissolved salts increases. When the concentration of salts reaches
their saturation, they are thrown out in the form of precipitates.
Sludge:
 Soft, loose, and slimy precipitate.
Due to the presence of MgCO3, MgCl2, CaCl2, MgSO4, etc.
Disadvantages:
They are poor conductor of heat, so they tend to waste a portion of heat generated.
Decrease the efficiency of the boiler.
 Sludges settle in the areas of poor water circulation such as joints, bends, etc.
therefore chocking of pipes takes place.
Removal of sludge:
 Using soft water; Scrapping of sludge with hard brush.
Blow-down operation – replacement of concentrated water with fresh water.
 Disadvantages of using hard water in boilers
Scale:
 Hard, adhering and sticky deposit on the inner walls of boilers.
Due to the presence of Ca(OH)2, CaCO3, Mg(OH)2, CaSO4, CaSiO3, MgSiO3, etc.
Disadvantages:
They are poor conductor of heat, so they tend to waste a portion of heat generated.
Decrease the efficiency of the boiler.
 Due to high pressure, the scales crack and water suddenly comes in contact with
overheated iron plates and may cause explosion.
 Super heating of boiler makes the boiler material softer and weaker, which causes
distortion of boiler tube.
Removal of sludge:
 Scrapping of scale with wire brush and blow-down operation
Brittle scale can be removed by giving thermal shocks to the boilers.
 Adding chemicals ( Acids, EDTA, etc)
Scale and sludge formation
 Disadvantages of using hard water in boilers
Priming and Foaming: During the production of steam in the boilers, due to rapid
boiling some droplets of liquid water are carried along with the steam. Steam
containing water droplet is called wet steam. Carrying of suspended and dissolved
solids along with wet steam is called carryover.

Priming:
 Priming is carrying of small droplets of water ( wet steam) along with steam.
Causes of priming:
Improper design of the boiler.
Very high water level in the boiler.
 High steam generation velocities.
 Sudden increase in steam production rate.
 Presence of large amount of finely divided particles and dissolved solids.
 Disadvantages of using hard water in boilers
Priming:
Precaution/ Prevention:
 Fitting of mechanical steam purifier.
Maintaining low water level in the boiler.
 Avoiding rapid change in steam generation.
 Using soft and filtered water.
 Removal of scale and sludge frequently.
Foaming:
 Foaming is the formation of stable bubbles in the boilers.
Causes of priming: Due to the presence of
 oil and grease in water; Finely divided sludge in water;
Precaution/ Prevention:
 By adding anti-foaming agents like castor oil or polyamides.
Using soft and filtered water.
 Disadvantages of using hard water in boilers
Boiler corrosion: It is the disintegration or decay of boiler material either due to
chemical or electrochemical reaction with its environment.
Factors causing the boiler corrosion:
 Formation of rust with dissolved oxygen: Dissolved oxygen present in the water
attacks the boiler material and forms rust.

2 Fe + 2H2O + O2 2 Fe(OH)2

4 Fe(OH)2 + O2 2[Fe2O3. 2H2O]

Rust
Due to dissolved CO2: Dissolved CO2 present in water forms weak carbonic acid.

CO2 + H2O H2CO3

H2CO3 + Fe FeCO3 + H2
 Disadvantages of using hard water in boilers
Boiler corrosion:
Due to formation of acids from dissolved salts: MgCl2 and CaCl2, etc which is present
in the water can produce HCl and can corrode the boilers.

MgCl2 + 2H2O Mg(OH)2 + 2 HCl

CaCl2 + 2H2O Ca(OH)2 + 2 HCl

 The liberated HCl reacts with boiler material in chain-like reaction.

MgCl2 + 2H2O Mg(OH)2 + 2 HCl

Fe + 2 HCl FeCl2 + H2

FeCl2 + 2 H2O Fe(OH)2 + 2 HCl

 Disadvantages of using hard water in boilers
Boiler corrosion:
Prevention:
 By removal of dissolved oxygen (DO):
(i) Preheating: As solubility of gases decreases with increases in temperature, at
approximate 65 0C, complete DO is removed.
(ii) Chemical treatment: The DO is removed through the addition of sodium sulphite
(Na2SO3 )or sodium sulphide (Na2S) or hydrazine (N2H4)

2 Na2SO3 + O2 2 Na2SO4

Na2S + 2 O2 Na2SO4

N2H4 + O2 N2 + 2H2O

Hydrazine is found to be ideal for removing DO because the products are water and N2
 Disadvantages of using hard water in boilers
Boiler corrosion:
Prevention:
 By removal of dissolved oxygen (DO):
(iii) Mechanical deaeration: Water
passes through the perforated plates and
undergoes deaeration at high temp. and
low pressure - dissolved oxygen and
carbon dioxide escapes. The solubility of
a gas in water is directly proportional to
pressure and inversely proportional to
temperature, hence the water gets
deaeration.
 Disadvantages of using hard water in boilers
Boiler corrosion:
Prevention:
 By removal of dissolved CO2:
(i) Preheating: As solubility of gases decreases with increases in temperature
(ii) Chemical treatment: The CO2 is removed through the addition of ammonia.

(iii) Mechanical deaeration: It removes DO as well as CO2

Addition of alkali: Corrosion by acids may be prevented by adding some alkalies

from outside so that product acid may be neutralized.

 By using soft water: Soft water for steam generation

 Disadvantages of using hard water in boilers
Caustic Embrittlement/Intercrystalline cracking: It is special type of boiler corrosion
caused by the use of highly alkaline water. During the softening of water by lime soda
process, usually small amount of free Na2CO3 is present. In high-pressure boilers,
sodium carbonate decomposes and gives sodium hydroxide and this makes the boiler
caustic.
The concentration of NaOH is increased by evaporation of water, and attacks the boiler
material by giving sodium ferroate, which decomposes and forms rust.

Na2CO3 + H2O 2 NaOH + CO2

Fe + 2 NaOH Na2FeO2 + H2

Na2FeO2 + 4 H2O 6 NaOH + Fe3O4 + H2

 Disadvantages of using hard water in boilers
Caustic Embrittlement/Intercrystalline cracking:
Prevention:
 By using sodium phosphate as a softening agents instead of sodium carbonate.
By adding chemicals like lignin and tannin to boiler water because they block the hair
cracking inside the boiler.
By adding sodium sulphate to boiler water, which blocks the minute cracks thereby
preventing the entry of sodium hydroxide solution.
 Softening/Conditioning of water
Process of removing hardness producing salts from water is known as softening of
water or conditioning of water.

Internal treatment/conditioning:
 Appropriate chemicals are added to the water to precipitate scale forming
impurities in the form of sludge which can be removed by blow-down operation.
(a) Carbonate conditioning: In low-pressure boilers, scale formation can be reduced by
adding sodium carbonate (Na2CO3) to boiler to prevent the precipitation of scale
forming calcium sulphate (CaSO4).

CaSO4 + Na2CO3 CaCO3 + Na2SO4

When calcium sulphate is converted into calcium carbonate by the addition of

sodium carbonate, CaCO3 acts as a loose sludge, which can be removed by blow-
down operation.
 Softening/Conditioning of water
Internal treatment/conditioning:
(b) Phosphate conditioning: Conversion of scale-forming Ca & Mg salts into the most
soluble calcium phosphate and magnesium phosphate, which form easily
removable non-adherent soft sludge, which can be removed by blow-down
operation.

3 CaCl2 + 2 Na3PO4 Ca3(PO4)2 + 6 NaCl

3 MgCl2 + 2 Na3PO4 Mg3(PO4)2 + 6 NaCl

Three sodium orthophosphates may be used depending upon the alkalinity of the
boiler feed water
(i) In acidic medium- Sodium dihydrogen phosphate (NaH2PO4)
(ii) In weakly acidic medium – Disodium hydrogen phosphate (Na2HPO4)
(iii) In alkaline medium – Trisodium phosphate (Na3PO4)
 Softening/Conditioning of water
Internal treatment/conditioning:
(c) Calgon conditioning: Addition of calgon [Sodium hexameta phosphate – (NaPO3)6] to
boiler water converts calcium salts into soluble complex compound thereby
preventing scale or sludge formation.

Na2[Na4(PO3)6] 2 Na+ + [Na4(PO3)6]2-

Calgon

2 CaSO4 + [Na4(PO3)6]2- 2 Na2SO4 + [Ca2(PO3)6]2-

soluble complex ion

 Softening/Conditioning of water
External treatment/conditioning:
 Hard water causes number of harmful effects when used for domestic, industrial,
and boiler purposes. So we have to remove or reduce hardness-causing impurities
present in water before using it for any purpose.
(a) Lime soda process: Treatment of water sample with calculated quantities of lime
[Ca(OH)2] and soda [Na2CO3], which react with calcium and magnesium salts to
form insoluble precipitates as CaCO3 and Mg(OH)2.
(i) Functions of lime:
Ca(HCO3)2 + Ca(OH)2 2 CaCO3 + 2H2O

MgSO4 + Ca(OH)2 Mg(OH)2 + CaSO4

(ii) Functions of soda:

CaCl2 + Na2CO3 CaCO3 + 2NaCl

CaSO4 + Na2CO3 CaCO3 + Na2SO4

 Softening/Conditioning of water
External treatment/conditioning:
(b) Zeolite or permutit process: Zeolite is known as permutit, i.e. boiling stone.
Zeolites are hydrated alumino silicate minerals or sodium aluminium orthosilicate
and it is represented as Na2O.Al2O3.xSiO2.yH2O (x = 2-10, y = 2-6) [Na2Z].
Process: The raw water is percolate through a bed of zeolite (Na2Z), which is packed in
a vertical cylindrical tank. The zeolite bed retains the Ca2+and Mg2+ ions from hard
water by exchanging with Na+ ions thereby the out-flowing water contains sodium
salts.

CaSO4 + Na2Z CaZ + Na2SO4

CaCl2 + Na2Z CaZ + 2 NaCl

Ca(HCO3)2 + Na2Z CaZ + 2 NaHCO3

 Softening/Conditioning of water
External treatment/conditioning:
(c) Demineralization process/ Ion exchange process: The cations and anions present in
the water and which can produce hardness are removed by ion-exchange resins.
Resins are long, cross linked organic polymers with a porous structure. There are
two types of ion-exchange resins namely cation exchange resins and anion
exchange resins.
Cation exchange resins: These resins have acidic functional groups such as –COOH, -
SO3H, etc., which are capable of exchanging the cation by their hydrogen ions.
Hence, they are called cation exchangers. They can be represented as R-H, where R
is the insoluble polymeric part.

Ex: styrene-divinylbenzene copolymers

 Softening/Conditioning of water
External treatment/conditioning:
Anion exchange resins: These resins have basic functional groups such as amine,
substituted amine, quaternary ammonium groups, etc., which are capable of
exchanging the anion. Hence, they are called anion exchangers. They can be
represented as R’-OH, where R is the insoluble polymeric part.
Ex: Styrene-divinylbenzene copolymer containing basic groups
 Softening/Conditioning of water
External treatment/conditioning:
Process: The hard water is first passed through cation-exchange resin chamber, which
removes all the cations (e.g. Ca2+ and Mg2+) from it, and equal amount of H+ ions
are released from its column to water.

Then the hard water is now pumped through anion-exchange resin chamber, which
removes all the anions (e.g. Cl- and SO42-) from it, and equal amount of OH- ions are
released from this resin bed to water.

H+ and OH- ions are combined to produce water, Thus, the treated water is completely
free from anions and cations, so it is known as demineralized or deionized water.
 Softening/Conditioning of water
External treatment/conditioning:
Regeneration of Resins: After some time of usage of cation and anion exchange resins
will exhaust, and it need to be regenerate.
(i) Regeneration of cation-exchange resins: By addition of dil. HCl or H2SO4
R2Ca + 2 H+ 2 R-H + Ca2+
(ii) Regeneration of anion-exchange resins: By addition of dil. NaOH

R2SO4 + 2 OH- 2 R'-OH + SO42-

Advantages: (i) Produces water of very low hardness ( 2 ppm)

(ii) Highly acidic or alkaline water also can be softened.
Disadvantages: (i) The equipment is very costly.
(ii) Expensive chemicals are required.
(iii) Turbid water decreases the efficiency of resins.
 Desalination
Removal of dissolved salts (eg. NaCl) from water. Sea water contains more than 35,000
ppm dissolved solids while brackish water contains 1000-3500 ppm.

(a)Reverse osmosis: Reverse osmosis is a process by which a solvent such as water

is purified from the solutes by passing through a semipermeable membrane. It is
exactly opposite of osmosis and hence it is known as reverse osmosis.
Osmosis: When two different concentrated solutions are separated by a
semipermeable membranes, solvent (water) flows from the region of low
concentration to higher concentration. The excess pressure applied on the
concentrated solution side to prevent osmosis is called osmotic pressure.
This natural osmosis process can be reversed by applying pressure on the concentrated
side, the solvent will flow in the reverse direction. 100-150 micron thick membrane
made from cellulose acetate or polymeric membranes having pores in the range of
0.0001 – 0.001 μm in diameter; it allows only water to pass through it and not the
salt.
 Desalination
(a)Reverse osmosis:
 Desalination
(a)Reverse osmosis:
Process: High pressure is applied to the sea water or brackish water, which is to be
treated. The semipermeable membrane allows only the solvent (water) to pass
through it. Thus dissolved ionic and non-ionic solvents are left behind. Generally,
polymethacrylate and polyamide polymer membranes are used for the process.

Advantages:
 It removes ionic as well as non-ionic salts present in saline water.
 It is economical, compact, and very simple.
 It is effective in removing colloidal matters also.
 It is suitable for converting sea water into drinking water,
 It has long life and easy to replace the membranes.
 The water obtained from the process can be used in high-pressure boilers.
(b) Electrodialysis: Desalination
Principle: The ions are pulled out of the salt water by using DC current using
electrodes and thin cation and anion selective membranes. The cation selective
membranes are consisting of sulphated polystyrene, which allows only cations and
rejects anions. However, anion selective membranes are consisting of polystyrene
with quaternary ammonia, which allows only anions. Multiple membranes
alternatively allow cation or anions to flow through.
Process: The process is carried out in a special type of the cell called electrodialysis cell. It
consists of two electrodes and ion selective membranes which are permeable to
either cation or anion. The anode is placed near anion- selective membrane while the
cation placed near cation - selective membrane. The anion selective membrane has
positively charged functional groups such as R4N+ and therefore allows negatively
charged ions only to pass through them. Similarly, cation selective membrane has
negatively charged functional groups such as RCOO- and therefore allows positively
charged ions.
 Desalination
(b) Electrodialysis:
(b) Electrodialysis: Desalination
Process: Saline water is passed from the top of the cell and it passes between membrane
pair. When an EMF is applied across two electrodes the cations (Na+) present in salt
water move towards cathode through cation selective membrane and anions (Cl-)
present in salt water move towards anode through anion selective membrane. As a
result, the concentration of ions in alternative compartments 2,4,6 etc. decreases,
while it increases in the alternate compartment 1,3,5 etc. Thus water in the even
number compartments becomes pure and it is collected from bottom of the cell.
Similarly, water in the odd number compartment becomes rich in the saline water.

Advantages:
 It is economical
 It is convenient and may be applied at room temperature
 It is most compact in size and requires electricity for operation.
 Domestic water treatment
Water should be free from colloidal impurities, domestic sewage, industrial effluent
and disease producing bacteria. The various stage in the treatment of water for
drinking can be indicated in the following steps.
 Domestic water treatment
Screening: The raw water is allowed to pass through screens having large number of
holes, where floating impurities like rags, paper, leaves, etc., held by them, and
water is passed through the holes.

Aeration: The process of mixing water with air is called aeration. The main purpose
of aeration is
-to promote taste & adour to water
-to increase oxygen content in water for giving it freshness
-to remove CO2, H2S and other volatile matter causing bad taste and adour in water
-to remove Fe and Mn salts as their insoluble precipitate.

Sedimentation: Sedimentation is a process for retention of water for certain

period in a deep tank. Most of the suspended particles are settle down due to the
force of gravity. This process takes two to eight hours and removes 70 -75% of
suspended impurities.
 Domestic water treatment
Coagulation: Coagulation is the process by which the fine, suspended , and
colloidal impurities are removed from the water by the addition of suitable
chemicals (coagulants). The finely divided suspended inorganic matters don’t settle
down so easily, so these particles are converted into larger ones.

The commonly used coagulants are alum (K2SO4 Al2(SO4)3.24H2O), ferrous sulphate
(FeSO4.7H2O), sodium aluminate (NaAlO2) etc. These coagulants react with alkaline
salts and form a thick gelatinous precipitate known as Flock. Flock had the
property to attract finely suspended particles and form big flock, which settles
down rapidly. This process is called flocculation.

Al2(SO4)3 + 3 Ca(HCO3)2 2 Al(OH)3 + 3 CaSO4 + 6 CO2

 Domestic water treatment
Filtration: Almost all suspended impurities are removed through filtration
process. During filtration, all types of insoluble colloidal impurities are also
removed by passing water through gravity sand filter.

It consists of three layers. The upper layer

consists of fine sand and the middle layer
consists of coarse sand and the bottom layer
consists of gravels. Sedimented water enters
the sand filter from the top and finely
suspended particles & bacteria are retained by
the top layer. The rate of filtration slow after
some time due to clogging of pores of the top
sand layer. Therefore, the portion of the top fine
sand layer is scrapped off and replaced by a
new sand layer.
 Domestic water treatment
Sterilization: Sterilization of water means complete destruction of all living
microorganisms ( bacteria and virus etc.) present in water. Chemicals used for
sterilization are known as sterilizers or disinfectants.
(i) Boiling method: Water is boiled for about 20-30 minutes, and all the harmful
disease – causing bacteria and gems are destroyed. But this method is useful only
for household purposes because this is very much expensive for municipal supply
of water.
(ii) Chlorination method: Process of addition of chlorine to water in order to kill
pathogens are called chlorination.
(a) Chlorine gas: Used directly as gas. It reacts with water to form hypochlorous
acid and nascent oxygen, which are powerful germicides.
 Domestic water treatment
Sterilization:
(b) Bleaching powder (CaOCl2): Bleaching powder is a strong oxidizing agent and
having 30% available chlorine. When water is treated with bleaching powder,
hypochlorous acid is formed. It releases nascent oxygen which deactivates the
enzymes of micro-organisms.
CaOCl2 + H2 O Ca(OH)2 + Cl2

Cl2 + H2O HCl + HOCl

HOCl HCl + [O]

Germs + [O] Deactivate the enzyme

Stop metabolic activities

of microorganisms

Kill
 Domestic water treatment
Sterilization:
(c) Chloramines (NH2Cl): By mixing of chlorine and ammonia in 2:1 ratio,
chloroamine is formed.

When water is treated with chloroamine , hypochlorous acid is formed. It releases

nascent oxygen which deactivates the enzymes of micro-organisms.

NH2Cl + H2O NH3 + HOCl

HOCl HCl + [O]

Germs + [O] Deactivate the enzyme

Kill
 Domestic water treatment
Sterilization:
(iii) Ozonolysis method: Ozone is a highly unstable and excellent disinfectant. It
breaks down and gives nascent oxygen which kills bacteria and germs in the water.

It removes colour and adour from water. Improves the taste of the water. Excess dose
of ozone is not harmful, because it releases O2 on decomposition.
It is costly and cannot stored for longtime.

(iv) UV-rays method: When water is exposed to UV-rays from electric mercury lamp,
most of the pathogenic bacteria are destroyed. This method is widely used for the
disinfection of swimming pool water.
It does not require any chemicals. It does not produce any odour in water.
It is expensive, so it is not widely used on a large scale.
 Break-point chlorination
Chlorination of water is done carefully in a controlled manner with the dip or break is
called break-point chlorination. The point at which free residual chlorine begins to
appear is called break-point chlorination. With this method not only living organisms
but also organic impurities and free NH3 present in water are destroyed.

Added chlorine consumed for different reactions such as

(i) Oxidation of reducing substance
(ii) Chlorination of organic substance
(iii) Oxidation of ammonia and disinfection of bacteria

Break-point chlorination shows four stages of sterilization.

Stage 1: Initially, with the lower dosage of Cl2, there is no free residual chlorine since all
the added Cl2 gets consumed in complete oxidation of reducing substances present
in water.
Break-point chlorination
 Break-point chlorination
Stage 2: As the amount of Cl2 dose is increased, the amount of residual Cl2 also shows
steady increase. This stage corresponds to the formation of chloro-organic and
chloramines compounds without undergoing oxidation.
Stage 3: As more amount of Cl2 is applied, the amount of free residual chlorine also
decreases, due to oxidation of chloro-organic and chloramines. When the oxidation
destruction is complete, it reaches a minima.
Stage 4: After minima, the added Cl2 is not used in any reaction. Thus, the residual
chlorine keeps on increasing in direct proportion to added Cl2.

The point ‘C’ is called break-point. It is a point at which free residual chlorine begins to
appear. Thus, break-point chlorination helps in eliminating disagreeable odour and
bad taste in water.
All The Best
All The Best