Dr.

Harish Shinde Chapter- Water 1

Chapter 1

WATER

 INTRODUCTION:

Water plays a vital part in our daily life. It is mostly available in large excess amount
on earth surface. The 70% surface of earth is covered by water and ice. About 97% of
available water is in oceans and it is not directly useful for our daily use. A major part of the
remaining water is trapped in south and north ice poles of earth. The part of water which is
just remaining after all this is available for domestic, industrial, agricultural etc. Water is a
basic material needed by almost all industries. In industries water is useful for the following
purposes.

1. Cooling,
2. As a solvent,
3. Steam generation in boilers,
4. In industries like paper, textile, sugar, chemical, ice manufacturing for all routine
operations.

Instead of industrial applications, water is also essential for domestic applications like
drinking, cooking, bathing, washing etc.

 PHYSICAL AND CHEMICAL CHARACTERISTICS OF WATER:

The chemical formula of water is H2O. The pure water consists of two parts of hydrogen and
one part of oxygen by volume or one part of hydrogen and eight parts of oxygen by weight.
Water is a very good solvent; it dissolves almost all substances comes in contact. This is why
Natural water is not pure.

Physical characteristics: The physical characteristics of water are as follows,

1. Pure water is colourless but if it is associated with some dissolved salts and
colloidal impurities then it produces yellow or brownish colour.

2. Pure water is odourless but if it mixes with organic matter and gases like hydrogen
sulphide, then it produces odour. Chlorination of water also produces odour to the water.

3. Pure water is tasteless but when it mixes with sufficient oxygen and salts then it
gives pleasant taste.

4. Pure water is clear and transparent but when it mixes with suspended and colloidal
impurities then it becomes turbid. Turbid water is not suitable for industrial and domestic
use.

5. Pure water freezes at 0 0C but if it has impurities then it freezes for less than 0 0C.

DYPCEPT
 Dr. Harish Shinde Chapter- Water 2

6. The density of pure water is 1.0 gm/ml at 4 0C and 0.989 gm/ml at 25 0C.

7. Boiling point of pure water is 100 0C at 1 atmospheric pressure but if it contains

impurities boiling point elevates.

8. Pure water shows neutral test to the litmus but if it contains some acidic or alkaline
impurities then its pH value becomes less than 7 or greater than 7. For drinking and industrial
use pH of water should be around 7– 8.

9. Latent heat of water is 587 cal/mole; therefore, it is most suitable for using as a
coolant.

10. Water is the covalent compound and it contains polar O – H bonds, therefore it
acts as a very good solvent.

11. Pure water is bad conductor of electricity but when it contains dissolved salts it
becomes conductor of electricity.

12. Heat capacity of water is 1 cal/gm/0C and its refractive index is 1.33.

13. Surface tension of water is 72.8 dynes/cm and its viscosity is 1 centipoise at 20
0
C.

Chemical characteristics: The water when flows over the surface of earth it gets
contaminated with following chemical changes,

1. Stability: Chemically water is highly stable and does not decompose below 700 0C.

2. Action on metals: The metals and alloys react at slow or fast rate with water and it
forms metal oxides or hydroxides. For example, Na, K, Ca, Mg reacts much faster with
water.

3. Hydration: The mineral salts like anhydrite (CaSO4) and olivine (Mg2SiO4)
undergoes hydration and forms products of increased volume. Because of hydration rocks
undergoes disintegration.

CaSO4 + 2H2O CaSO4 .2H2O (Gypsum)

Mg2SiO4 + xH2O Mg2SiO4.xH2O (Serpentine)

4. Action of dissolved oxygen: Dissolved oxygen brings about oxidation and

hydration reactions.

2Fe3O4 3Fe2O3 3Fe2O3.2H2O

Magnetite Hematite Limonite

DYPCEPT
 Dr. Harish Shinde Chapter- Water 3

2FeS2 + 2H2O + 7O2 2FeSO4 + 2H2SO4

5. Action of dissolved carbon dioxide: It converts insoluble carbonates of Ca, Mg and

Fe into soluble carbonates.

CaCO3 + CO2 + H2O Ca(HCO3)2

MgCO3 + CO2 + H2O Mg(HCO3)2

6. Water as a catalyst: Water acts as an effective catalyst in many reactions. For this
purpose, just few drops of moisture are sufficient.

7. Ionization solvent: As the water is polar solvent it helps in ionization of acids,

bases and salts.

8. Reactivity: Water reacts with hydrides, phosphides, nitrides and carbides to liberate
H2, PH3, NH3 and acetylene respectively.

 Sources of water:
The natural sources of water are generally classified as,

A) Surface water: The surface water is flowing water and still water, e.g. rain water,
river water, lake water, seawater, etc.

i) Rain water: Rainwater is the purest form of natural water. It is obtained

from the evaporation of seawater and other surface water sources. The water vapours are
drifted to high altitude by the wind. They condense at this level and converts into water
droplets. The droplets then lead to the formation of clouds. When they aggregate sufficiently
they fall down in the form rain. The rainwater when approaches towards earth through an air,
it mixes with CO2, SO2, NO2, NH3, etc. and some suspended organic and inorganic particles.

ii) River water: This water is obtained from rain and springs. Rain water
when flows over the surface of earth it dissolves the mineral salts like chlorides, sulphides
and bicarbonates of sodium, calcium, magnesium and iron and finally collects in river. River
water also contains some organic matter derived from decomposition of plants and animals,
small sand and soil particles.

iii) Lake water: Lake water has a constant chemical composition. It contains
lesser amount of dissolved minerals but high percentage of organic matter.

iv) Seawater: Seawater is the most impure form of water. The seawater
continuously undergoes evaporation by which the concentration of dissolved salts goes on
increasing. It contains about 3.5% of dissolved salts and other suspended matter. Out of
which 2.6% is sodium chloride. This water is not suitable for industrial and domestic
consumption.

DYPCEPT
 Dr. Harish Shinde Chapter- Water 4

B) Underground water: Some part of the rainwater, which percolates into the earth,
is called as underground water. It consists of the large amount of dissolved mineral salts. It
comes out in the form of springs. The spring water is clearest but it is rich of dissolved salts.

 IMPURITIES IN NATURAL WATER:

The common impurities present in the natural water are of the following types,

1. Suspended impurities: Natural water contains suspended impurities like solid

particles lighter than water with size greater than 10-4 cm, which floats on the surface and
suspended inside the water. For example, leaves and branches of trees, pieces of wood,
blades of grasses and pieces of other organic matter. Water may contain the suspended
impurities like clay, mud, bacteria, algae and industrial wastes. They cause turbidity,
diseases, colour, odour and turbidity to the water. Some of the solid particles settle down by
the gravitational force but others cannot. So these impurities can be removed by
sedimentation or by filtration.

2. Colloidal impurities: If the size of the suspended particle lies between 10-5 -10-7
then they are called as colloidal particles. The examples of colloidal particles are finely
divided clay particles, soil, silica, hydroxides of iron and aluminium, organic particles etc.
Colloidal particles cause the turbidity to the water and they cannot settle down with
gravitational force effect. They can be removed by coagulation process.

3. Dissolved impurities: There are two types of dissolved impurities present in the
natural water, minerals and gases. They cause the following changes in water,

Table Dissolved impurities in water

Dissolved impurity Causes
1. Calcium and Magnesium bicarbonates Alkalinity and hardness
and carbonates.
2. Calcium and Magnesium chloride and Hardness
sulphate.
3. Sodium bicarbonate, Calcium and Alkalinity and softening
Magnesium Chloride.
4. Sodium fluoride and chloride Molted enamel of teeth and taste
5. Metals and oxides Colour, poisoning, taste, hardness
(iron oxide), corrosiveness
6. Oxygen, carbon dioxide, hydrogen Acidity, corrosion of metals, smell
sulphide, ammonia, etc. etc.

The dissolved impurities in water can be removed by chemical treatment like soda
lime, permutit, ion exchange etc.

DYPCEPT
 Dr. Harish Shinde Chapter- Water 5

4. Biological impurities: Biological impurities are of two types, suspended and

dissolved. For example, they include bacteria and other disease-producing microorganisms,
vegetables and dead animals etc. These impurities cause colour, taste, acidity, diseases and
pollution to the water. They can be removed by filtration or by sterilization.

The chemical formula of water is H2O. The pure water consists of two parts of
hydrogen and one part of oxygen by volume or one part of hydrogen and eight parts of
oxygen by weight. Water is a very good solvent; it dissolves almost all substances comes in
contact. This is why Natural water is not pure.

 Water Quality Parameters

1) Total solids
Inpurities present in a water sample, whether suspended or dissolved that have a negligible
vapour pressure at 105°C may be termed as solids". Causes Water accumulates solid
impurities from surrounding surfaces,

vegetation, animal decay. Such solids are both organic and inorganic nature. Inorganic solids
mainly contribute as "Impurities". Total solids is the amount of residue left in the vessel after
evaporation of a sample and subsequent drying in an oven at a specified temperature.

Solids present in water are of three types:

(1) Suspended solids (iii) Settleable solids

(ii) Fixed and volatile solids

Significance of measurement of Total solids It gives an idea of the extent of dissolved

inorganic salts. It is reported in mg/l. Presence of such solid matter makes water dirty, affects
colour and odour of water and thus such contaminated water is unacceptable for any

(ii) Acidity Definition

Acidity can be defined as, "Capacity to combine with hydroxyl ions to form a molecule of
water." Or excess H+ ions
Causes

The water becomes acidic due to dissolution of CO, or SO, or SO, as,

CO2+H2O — HCO3

Carbon di-oxide Carbonic acid

DYPCEPT
 Dr. Harish Shinde Chapter- Water 6

SO2+H2O--------H2SO3
Sulphur di-oxide Sulphurous acid

Significance of measurement of acidity

Acidic waters are more corrosive, and therefore acidity has to be controlled.

The extent of acidity of water governs the quantities of lime and soda to be added during L.S.
Method.

In phosphate conditioning method to control boiler corrosion, the

choice of phosphate depends upon the extent of acidity of water. Methyl orange acidity of
water is more significant in water pollution control.

3. Alkalinity
By alkalinity of water we mean the total content of those substances in water that causes an
increased concentration of OH ions upon dissociation or due to hydrolysis.

Causes

Alkalinity of natural water is generally due to the presence in them of

HCO,, SiO, HSiO, and some times CO,. In addition to the above the

alkalinity of boiler water is also due to the presence of PO, OH ions.

With respect to the constituents causing alkalinity in natural waters

following situations may arise.

6) OH only co, only

(iii) HCO, only

(iv) OH and CO

(v) CO, and HCO3

4. Chloride
Chloride content in water may be either in the form of Cl, or as salt such as CaCl, MgCl, or
NaCl.

DYPCEPT
 Dr. Harish Shinde Chapter- Water 7

Causes

Chloride ions are present in water samples usually as CaCl, MgCl, NaCl. It enters water from
polluting materials, trade wastes, from the sea, by solvent action of water on salts present in
soil.

Significance Its determination is necessary as,

Chloride content above 250 ppm imparts unacceptable taste to drinking water.

Type of desalting apparatus has to be chosen accordingly. For treating industrial wastes
before discharging them into natura bodies of water.

5. Dissolved oxygen:
The dissolved oxygen is the main corrosion causing impurity in boiler fee water. Normal
water usually contains about 8 ppm, dissolved oxygen at room temperature. At high
temperature dissolved oxygen attacks on boiler material and converts it into ferrous
hydroxide and then into ferric oxide rust.

4Fe + 2H2O + O2 4Fe(OH)2

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

6. HARDNESS OF WATER:

The natural water is classified in two types,

1. Hard water: Water, which does not produce good leather with soap, is called as
hard water.

2. Soft water: Water, which produces good leather with soap, is called as soft water.

The hardness of water is a characteristic by which it prevents the formation of good

leather with the soap. The causes of hardness are the dissolution of mineral salts to the water.

1. Causes of hardness of water: Rainwater when passes through an air, it absorbs

carbon dioxide from it and forms carbonic acid.

H2O + CO2 H2CO3

This acid dissolves in water to make it acidic. This acidic water when flows over the
surface of rocks or soil, then carbonic acid reacts with insoluble carbonates, to form soluble
bicarbonates.

H2CO3 + CaCO3 Ca (HCO3) 2

DYPCEPT
 Dr. Harish Shinde Chapter- Water 8

The soil also contains the chlorides and sulphates of calcium and magnesium. The
bicarbonates along with these the chlorides and sulphates on dissolution makes the water
hard.

Hard and Soft Water:

(Soap test): To distinguish hard water from soft water soap test is applied. Soap is the
mixture of sodium and potassium salts of higher fatty acids. When hard water is treated with
soap, dissolved salts from water reacts with soap to form insoluble precipitates called as
‘curd’ or ‘scum’.

2C17H35COONa + CaCl2 (C17H35COO)2 Ca + 2NaCl

Sodium stearate Salt from. ‘ Scum’ or ‘Curd’
(Soap) Water

Soft water does not produce ‘Scum’ or ‘Curd’ with soap.

Types of hardness:

The hardness of water has two types.

a) Temporary hardness: It is also known as carbonate hardness because it is

obtained due to the soluble bicarbonates of calcium and magnesium. This type of hardness
can be easily removed by mere boiling of water. Boiling decomposes the bicarbonates into
insoluble carbonates and carbon dioxide. The insoluble carbonates are removed on filtration
and carbon dioxide escapes out.

Ca(HCO3)2 CaCO3 + H2O + CO2 

Calcium bicarbonate Calcium carbonate
Soluble Insoluble

Mg(HCO3)2 MgCO3 + H2O + CO2 

Magnesium bicarbonate Magnesium carbonate

b) Permanent hardness: It is also known as non-carbonate hardness. It is

obtained due to the chlorides and sulphates of calcium and magnesium. It cannot be removed
on easy boiling but it can be removed by using certain chemical methods.

 Measurement of Hardness of Water

Equivalents of Calcium carbonate: The total hardness of water is estimated by using

standard EDTA or soap solution method. The amount of hardness causing substances can be
expressed in terms of equivalents of calcium carbonate. The molecular weight of calcium

DYPCEPT
 Dr. Harish Shinde Chapter- Water 9

carbonate is 100 and its equivalent weight is 50 then the equivalents of calcium carbonate is
defined as,

Mass of hardness producing substance x 50

Equivalents of CaCO3 
Chemical equivalentof hardness producing substance
or

Mass of hardness producing substance x 100

Equivalents of CaCO3 
Molecular weight of hardness producing substance

Therefore, when 162 parts by mass of calcium bicarbonate reacts with the same
amount of soap as 100 parts by mass of calcium carbonate. Therefore the mass of calcium
bicarbonate is multiplied by a factor 100/162 or 0.616 to give mass in terms of calcium
carbonate.
Table Carbonate conversion table
Dissolved Molecular Chemical To convert to To convert
Impurity mass Equivalent into CaCO3 CaCO3 to
HCO3 61 61 100/61=0.820 61/100=0.610
CO3 60 30 100/60=1.660 60/100=0.600
CO2 44 22 100/44=2.272 44/100=0.440
OH 17 17 100/17=2.940 17/100=0.170
Cl 35.5 35.5 100/35.5=1.41 35.5/100=0.355
CaCO3 100 50 100/100=1.000 100/100=1.000
Ca(HCO3)2 162 81 100/162=0.616 162/100=1.620
CaCl2 111 55.5 100/111=0.902 111/100=1.110
CaSO4 136 68 100/136=0.735 136/100=1.360
Mg(HCO3)2 146 73 100/146=0.684 146/100=1.460
MgCl2 95 47.5 100/95=1.050 95/100=0.950
MgSO4 120 60 100/120=0.831 120/100=1.200
Mg(NO3)2 148 74 100/148=0.674 148/100=1.480
NaCl 58.5 58.5 100/58.5=0.85 58.5/100=5.850
NaOH 40 40 100/40=1.250 40/100=0.400
Na2SO4 142 71 100/142=0.705 142/100=1.420
MgCO3 84 42 100/84=1.190 84/100=0.840

 Units of hardness: The most common units of hardness are as follows,

1. Parts per million (ppm): It is the parts of calcium carbonate equivalent hardness
per 106 parts of water.

1ppm = 1 part of calcium carbonate equivalent hardness in 106 parts of water,

1ppm = 1 milligram per liter,

DYPCEPT
 Dr. Harish Shinde Chapter- Water 10

One liter of water weighs 1000 gram = 1000 x 1000 mg =106 mg

2. mg/litre
1 mg/liter = 1 mg of calcium carbonate equivalent per 106 mg of water
1 mg/liter = 1ppm

3. Clarke’s degree (0Cl): It is the number of grains of CaCO3 equivalent hardness

per gallon (10 lb) of water or it is parts of CaCO3 equivalent hardness per 70,000
parts of water.

10 Clarke = 1 grain of calcium carbonate equivalent hardness per gallon of water,

10 Clarke = 1 part of calcium carbonate equivalent hardness per 70,000 parts of water,

4. Degree French (0Fr): It is the parts of CaCO3 equivalent hardness per 105 parts of
water.

10 Fr = 1 part of calcium carbonate equivalent per 105 parts of water,

Relation between various units of hardness:

1 ppm =1 mg/liter = 0.10 Fr = 0.070 Cl
1 mg/liter = 1 ppm = 0.10 Fr = 0.070 Cl
10 Cl = 14.3 ppm = 1.430 Fr = 14.3 mg/liter
10 Fr = 10 ppm =10 mg/liter = 0.70 Cl

 EFFECT OF HARD WATER INDUSTRIAL APPLICATIONS:

Disadvantage in steam generation in boilers: In all industries lot of water is

required for carrying out various operations. Form this the most important use of water is the
steam generation in boilers. The water used in boilers, for steam generation, is known as
boiler feed water. If hard water is used for steam generation in boilers then salts like
sulphates, bi-carbonates and chlorides of calcium and magnesium affects the boilers. The use
of hard water causes corrosion, caustic embrittlement, priming and foaming and scale and
sludge formation in boilers.

1. Scale and sludge formation in boilers:

a) Scale and sludge formation: In boilers, water is boiled continuously for the steam
generation. Due to this continuous boiling the concentration of dissolved salts from hard
water increases and water becomes saturated. At this stage salts get precipitated and deposits
at inner side of boilers in the form of
scales and sludges. Water

Boiler

Sludges Scales
DYPCEPT
Figure Scales and sludges in boiler
 Dr. Harish Shinde Chapter- Water 11

Scales: Scales are the hard deposits, which sticks very firmly to the inner surface of
the boilers. Scales are so hard and adherent to the boiler surface in such a way that we cannot
remove them easily. They can be removed with the help of hammer and chisel only. Scales
are bad conductors of heat and more troublesome. Generally, scales are formed in the hot area
of boiler. Scales are of carbonate, sulphate, silicate, phosphate and oxide type.

Sludges: Sludges are the soft, loose and slimy deposits formed within the boiler. They
can be easily removed with the help of only wire brushes. Sludges are formed in
comparatively cold area of boilers and collects in area where the flow of water is slow or
bends. Sludges are formed from the substances, which have greater solubility in hot water,
e.g. magnesium carbonate, magnesium chloride, calcium chloride, magnesium sulphate etc.

b) Causes of Scale and Sludge formation: There are three main sources of scale
formation,

1. Decomposition of calcium bicarbonate: When hard water is boiled for

steam generation calcium bicarbonate undergoes decomposition to calcium carbonate and
carbon dioxide.

Ca(HCO3)2 CaCO3 + H2O + CO2

The insoluble calcium carbonate deposits in the form of scales and sludge’s. This type
of scale and sludge formation is observed in low-pressure boilers.

2. Decreased solubility of calcium sulphate: The solubility of all salts in water

increases with rise in temperature but solubility of calcium sulphate decreases with increase
in temperature. In other words calcium sulphate is soluble in cold water and insoluble in
boiling water. Thus, when hard water is boiled for steam generation, calcium sulphate
precipitates and deposits in the form of scales. This type of scale formation is observed in
high-pressure boilers.

3. Hydrolysis of magnesium salts: The dissolved magnesium salts from water

undergoes hydrolysis at high temperature in boiler. The product obtained is the magnesium
hydroxide. Magnesium hydroxide deposits inside the boiler in the form of sludges.
Mg (HCO3)2 MgCO3 + H2O + CO2
MgCO3 + H2O Mg(OH)2 + CO2

c) Disadvantages of scale formation: The following are the disadvantages of scale and
sludge formation,

1. Wastage of fuel: Scales are bad conductors of heat. They decrease the rate
transfer of heat from boiler to the inside water. Thus, in order to get continuous supply of
steam, it is necessary to heat the boiler excessively. This overheating causes the increase in
fuel consumption and there is wastage of fuels. The wastage of fuels depends upon thickness
of scales, e.g.

DYPCEPT
 Dr. Harish Shinde Chapter- Water 12

Thickness of scales in mm 0.325 0.625 1.25 2.5

Wastage of fuels in % 10% 15% 20% 50%

2. Lowering of boiler safety: Due to scale formation, overheating of boiler is

required to get continuous supply of steam. The overheating of the boiler makes the boiler
material soft and weak. This causes the distortion and sometimes at high temperature it
oxidizes making the boiler unsafe.

3. Danger of explosion: The co-efficient of thermal expansion of boiler and

scales is different. This causes the development of cracks in scale layer. Due to these cracks,
water suddenly enters into them and comes in contact with hot boiler material. The red-hot
boiler suddenly converts the water into steam and high pressure is developed. This sudden
high pressure may cause the explosion of boiler.

4. Decrease in efficiency: Scales and sludges may deposits in values and pipes
of boiler and choke them partially. This results in decrease in efficiency of boilers.

d) Removal of scales: The scales can be removed from by using following methods,

1. Loosely adhered scales are removed with the help of scraper or a piece of
wood or wire brush.
2. If the scales are brittle they scales can also be removed by giving thermal
shocks i.e. heating the boiler and then suddenly cooling with cold water.
3. Some times, the scales are very hard and adherent, then it is required to
dissolve them by using some chemicals. The scales of calcium carbonate scales can be
dissolved by using 5-10% HCl. Calcium sulphate scales can be dissolved by adding ethylene
diamine tetra acetic acid (EDTA) with which they form soluble complexes.
4. If the scales are loosely adhered then they can be removed by frequent
blow-down operation.

e) Prevention of scale formation: The above methods of scale removal are

cumbersome, expensive and time-consuming. Thus, it is better to prevent the boiler scale
formation. The scale formation can be prevented or at least minimized by giving certain
chemical treatment to boiler water. Sometimes the external treatment is given to water,
before supplying it to the boiler. On the other hand, internal treatment is given to water by
treating it inside the boiler during evaporation process.

1. External treatment: It includes the softening of water i.e. to remove hardness

producing salts from water by lime-soda process or zeolite process or ion exchange process.

DYPCEPT
 Dr. Harish Shinde Chapter- Water 13

2. Internal treatment: An internal treatment includes the addition of chemicals to the

boiler water either to precipitate the scale forming impurities in the form of sludge or to
convert them into compounds. The precipitates are then removed by blow-down operation or
they will stay in dissolved form in water, which are not harmful. The internal treatment of
water is carried out as follows,

i) Colloidal conditioning: In low-pressure boilers, scale formation is avoided by

adding organic substances like kerosene, tannin, agar-agar gel, etc. These substances get
deposited over the scale forming precipitates and yields the non-sticky and loose deposits like
sludges. The loose sludges are then removed by frequent blow-down operation.

ii) Phosphate conditioning: In high-pressure boilers, scale formation is avoided by

adding sodium phosphates like sodium dihydrogen phosphate [NaH2PO4], disodium
phosphate [Na2HPO4], trisodium phosphate [Na3PO4), etc. These phosphates react with
calcium and magnesium salts in boiler water and forms non-adherent and easily removable
soft sludges of calcium and magnesium phosphates. The loose phosphates are then removed
by blow-down operation.

3MgSO4 + 2Na3PO4 Mg3(PO4)2 + 3Na2SO4

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

iii) Carbonate conditioning: In low-pressure boilers, scale formation can be avoided

by adding sodium carbonate to boiler water. Due to this the scale forming salts like CaSO4
are converted into calcium carbonate in equilibrium.

CaSO4 + NaCO3 CaCO3 + Na2SO4

This prevents the deposition of calcium sulphate as scale and it gets precipitated as
loose sludge of calcium carbonate, which is then removed by blow-down operation.

iv) Calgon conditioning: It involves the addition of calgon or sodium hexameta

phosphate [Na(PO4)6] to boiler water. It prevents the scale and sludge formation by forming
soluble complex compound with calcium sulphate.

2CaSO4 + Na2[Na4 (PO4)6] Na2[Ca2(PO4)6] + 2Na3SO4

v) Electrical conditioning: Sealed glass bulbs containing mercury are connected to the
battery and then allowed to float in boiler water. When water boils, mercury bulbs emit the
electrical discharges and prevent scale-forming particles to adhere or stick to the boiler in the
form of scale.

vi) Radioactive conditioning: Sometimes, the tablets containing radioactive salts are
placed inside the boiler water at a few points. The tablets emits the energy radiations prevents
scale formation.

DYPCEPT
 Dr. Harish Shinde Chapter- Water 14

2. Caustic embrittlement: Caustic embrittlement is similar with boiler corrosion. It

takes place by highly alkaline water in the boiler. Caustic embrittlement is generally,
observed in high pressures boilers. In a lime-soda process water is treated with lime and soda,
which forms the sodium carbonate. This sodium carbonate is soluble in water and it remains
in water. If this water is directly used as boiler feed water in high-pressure boilers then
Na2CO3 decomposes to give sodium hydroxide and carbon dioxide.

Na2CO3 + H2O 2NaOH + CO2

The formation of sodium hydroxide or caustic soda makes the boiler-water alkaline.
This alkaline water enters into the small cracks on inner side of the boiler by capillary action.
The red-hot boiler evaporates the water and increases the concentration of caustic soda
progressively. The caustic soda attacks on the surrounding area and dissolves the boiler
material (iron) in the form of sodium ferroate. This causes embrittlement of boiler parts,
particularly stressed parts like bends, joints rivets etc. causing even failure of the boiler.

Caustic embrittlement can be avoided by using following treatment,

(i) Caustic embrittlement can be avoided by using sodium phosphate as softening

reagent instead of sodium carbonate,
(ii) Tannin or lignin is added to boiler water, since they blocks the hair-cracks of
boiler and avoids the infiltration of caustic soda solution in cracks,
(iii) Sodium sulphate is added to the boiler water, which also blocks the hair-cracks
on boiler material and prevents the infiltration of caustic soda and
(iv) The pH of the boiler feed water is adjusted carefully at about 8-9.

3. Boiler corrosion: Boiler corrosion is the most dangerous problem, created by the
use of unsuitable water for steam generation in boilers. The boiler material gets attacked by a
chemical or electro-chemical action of surrounding water and causes the corrosion. The
boiler corrosion takes place due to the following reasons,

i) Dissolved oxygen: The dissolved oxygen is the main corrosion causing impurity in
boiler fee water. Normal water usually contains about 8 ppm, dissolved oxygen at room
temperature. At high temperature dissolved oxygen attacks on boiler material and converts it
into ferrous hydroxide and then into ferric oxide rust.

4Fe + 2H2O + O2 4Fe(OH)2

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

To prevent the boiler corrosion, it is required to remove the free oxygen from boiler
feed water. Calculated amount of hydrazine or sodium sulphite or sodium sulphide is added
to the water. Dissolved oxygen can also be removed by mechanical de-aeration of water i.e.
by spraying. Water is allowed to flow through perforated plates fitted inside the tower and the
tower is heated from sides and connected to vacuum pump. At higher temperature, low

DYPCEPT
 Dr. Harish Shinde Chapter- Water 15

pressure and large exposing surface area, the concentration of dissolved oxygen from water
gets reduced. Raw water

To avoid the excess time

required for de-aeration some
To vacuum
polyvalent organic compounds are water
added for the removal of free oxygen,
e.g. Azamina 8001-RD.
Steam
ii) Dissolved carbon dioxide: jacket
Water obtained from any natural
source contains the dissolved carbon Perforated
dioxide, which reacts with water to plates
form carbonic acid.
De-aerated water
CO2 + H2O H2CO3
Figure De-aeration of
Carbonic acid attacks slowly water
on boiler material and causes its
corrosion. Another source of carbon dioxide is decomposition of bicarbonates. If boiler feed
water contains bicarbonates of calcium and magnesium, they decompose at higher
temperature into insoluble carbonates and the carbon dioxide.

Ca(HCO3)2 CaCO3 + H2O + CO2

Mg(HCO3)2 MgCO3 + H2O + CO2

To avoid the boiler corrosion by carbon dioxide, it required to remove it from boiler
feed water. Carbon dioxide is removed by adding calculated amount of ammonia or
ammonium hydroxide. Mechanical de-aeration of water removes the dissolved oxygen as
well as carbon dioxide.

iii) Dissolved acids: If the boiler feed water contains dissolved magnesium salts, they
create the acids on hydrolysis. The liberated acids react with iron material of the boiler in a
chain reaction to produce HCl again and again.

MgCl2 + 2H2O Mg(OH)2 + 2HCl

Fe + 2HCl FeCl2 + H2
FeCl2 + 2H2O Fe(OH)2 + 2HCl

Consequently, the presences of even a small amount of MgCl2 will cause-corrosion of

the boiler metal to a large extent. This type of boiler corrosion can be avoided by adding
alkalies from outside to neutralize the acids.

DYPCEPT
 Dr. Harish Shinde Chapter- Water 16

4. Priming and foaming: When a boiler is continuously heated for steam generation,
some particles or the liquid water are carried along with the steam. This causes the “wet
steam” formation in boiler and it is called as priming. The extent of priming is depending
upon the presence large amount of dissolved solids, high steam velocities, sudden boiling,
improper boiler design and sudden increase in steam production rate.
Foaming is the production of persistent foam or bubbles in boilers, which do not
break easily. Foaming is observed when the boiler feed water contains the substances like
oils, which reduces the surface tension of water. Priming and foaming, usually, occur
together. If there is priming and foaming, the dissolved salts in boiler water are carried away
by the wet steam to super-heater and turbine blades, where they gets deposited as water
evaporates. These deposits reduce the efficiency of turbine blades. The dissolved salts may
enter in the parts of machinery, which decreases their life. Actual height of the water column
cannot be judged in presence of priming and foaming.

Priming can be avoided by fitting mechanical steam purifiers, avoiding rapid change
in steaming rate, maintaining low water levels in boilers and efficient softening and filtration
of the boiler-feed water. Foaming can be avoided by adding anti-foaming agents like castor
oil, removing oil from boiler water by adding compounds like sodium aluminate.

REMOVAL OF HARDNESS OF WATER:

The water used for industrial purposes must be free from calcium, magnesium and
iron salts. The process of removing the hardness producing salts from hard water is known as
softening of water. The principle used in softening of hard water is that to convert soluble
salts into insoluble one and then to remove them by filtration. The following methods are
used for water softening for industrial purposes. The methods of removal of hardness depend
upon the type of hardness, i.e. temporary or permanent.

A) Removal of temporary hardness: Temporary hardness of water can be removed

by using following two methods,

1. Boiling: Temporary hardness of water can be removed by mere boiling of water.

The bicarbonates of calcium and magnesium decompose into insoluble carbonates and carbon
dioxide.

Ca (HCO3)2 CaCO3 + H2O + CO2

Mg (HCO3)2 MgCO3 + H2O + CO2

The insoluble carbonates settles down and carbon dioxide escapes out. The upper
layer of soft water is taken out. This method is not useful on large scale, but it is useful for
domestic treatment of water.

2. Clark’s method: The temporary hard water containing excess of bicarbonates can
be softened by Clark’s process. In this method the raw water is treated with calculated

DYPCEPT
 Dr. Harish Shinde Chapter- Water 17

amount of slacked lime [Ca(OH)2]. Soluble bicarbonates react with slacked lime and get
converted unto insoluble carbonates, which are then removed by filtration.

Ca (HCO3)2 + Ca (OH)2 2CaCO3 + 2H2O

Mg (HCO3)2 + 2Ca(OH)2 2CaCO3 + Mg (OH)2 + 2H2O

If two much lime is added during treatment, excess lime will remain in water and the
hardness will retained.

B) Removal of permanent hardness: Permanent hardness of water can be removed

by using following methods,

1. Soda ash process: Sodium carbonate or soda ash is useful to remove the temporary
as well as permanent hardness of water. In this process the calculated amount of soda ash is
added to the raw water. Soda ash reacts with bicarbonates, chlorides and sulphates of calcium
and magnesium and converts them into respective insoluble carbonates. The insoluble
carbonates are then removed by filtration and soft water is separated.

CaCl2 + Na2CO3 CaCO3 + 2NaCl

CaSO4 + Na2CO3 CaCO3 + Na2SO4
Mg (HCO3)2 + Na2CO3 MgCO3 + 2NaHCO3

2. Lime-Soda process: In this process hard water is treated with calculated amount of
slaked lime, Ca(OH)2 and soda ash, Na2CO3. The chemicals and water are thoroughly mixed
with stirrers. The treatment of water with these chemicals converts the soluble salts of
calcium and magnesium into insoluble compounds. The following reactions take place,

a) The lime removes calcium and magnesium bicarbonates i.e. temporary hardness.

Ca (HCO3)2 + Ca (OH)2 2CaCO3 + 2H2O

Mg (HCO3)2 + 2Ca(OH)2 2CaCO3 + Mg (OH)2 + 2H2O

b) The soda ash removes calcium chloride and sulphate permanent hardness.

CaCl2 + Na2CO3 CaCO3 + 2NaCl

CaSO4 + Na2CO3 CaCO3 + Na2SO4

c) The soda ash and lime removes magnesium chloride and sulphate permanent
hardness.
MgCl2 + Na2CO3 MgCO3 + 2NaCl
MgSO4 + Na2CO3 MgCO3 + Na2SO4
MgCO3 + Ca(OH)2 Mg(OH)2 + CaCO3

Thus, lime is require by temporary as well as permanent harness, while soda ash is
require by both calcium and magnesium permanent hardness. The Lime soda process is

DYPCEPT
 Dr. Harish Shinde Chapter- Water 18

carried out in cold or in hot water. If it is carried out in cold water then it is called as ‘cold-
lime soda process.’ and if it is carried out in hot water then it is called as ‘Hot lime soda
process.’

A) Cold lime soda process: It is carried out either in batch or continuously.

a) Batch
Raw water Chemicals process: In this process water
is softened in batches or at
intervals. This process is
Soft carried out in a large
water rectangular tank provided with
a mechanical stirrer. A
calculated amount of chemicals
Sludge
(lime + soda) are added to the
outlet
water in the tank and mixed
Figure Batch cold lime-soda process well with a mechanical stirrer.
After stirring small amount of
coagulant like alum is added
and the mixture is kept undisturbed for some time. Alum reacts with bicarbonate of calcium
or magnesium to form a gelatinous precipitate of aluminium hydroxide called as ‘flock’. The
flock helps to settle the precipitates down completely and the soft water is taken out. The
precipitated impurities i.e. sludges are then removed from sludge outlet provided at the
bottom of tank. The water is then pumped out and allowed to pass through a filter consisting
coal different layers of sand. Filtration removes the remaining sludges and a clear soft water
is obtained. The time require to complete the process is around 2-3 hours. After softening of
one batch of water next batch is taken for treatment.

b) Continuous process: In this process water is continuously softened and the

wastage of time is prevented. The process
is carried out in two concentric cylindrical Stirrer
tanks. The inner cylinder is provided with
a stirrer and two inlets one for raw water Raw
Water Chemicals
and the other for chemicals. The outer
cylinder is provided with a fiber filter and
outlets for soft water and sludges.
Soft Water
The raw water and calculated Filter bed
amount of chemicals are introduced to the
inner chamber. This mixture is then
stirred vigorously by using mechanical
Sludge
stirrer, thereby chemical reactions will
complete. The soft water and the
insoluble sludges are then comes in outer Sludge outlet
cylinder. The sludges settles at the bottom
Figure Continuous lime-soda process

DYPCEPT
 Dr. Harish Shinde Chapter- Water 19

and soft water moves in upward direction. The water then passes through a fiber filter, which
removes remaining sludge particles and clear soft water will come out from the top outlet.
Sludges are taken out from the bottom through sludge outlet. The water obtained from this
process may have 50 to 60 ppm hardness.

B) Hot Lime-Soda process: In this process hard water is treated with chemicals (lime
and soda ash) at a temperature about 80 to 150 0C. This process consists of a reaction tank in
which raw water; chemicals and steam are thoroughly mixed to complete all chemical
reactions within considerably short time. The sludges are allowed to settle or they can be
removed by filtration.

Hot lime soda process is carried out either in batch or continuous. Batch process is
similar with that of used in cold process except that the water is heated by using heating coils.
The temperature of water is kept around 150 0C, so that reactions will complete rapidly. The
water is then filtered to separate insoluble sludges.

Raw water The continuous hot lime

soda process is carried out in two
concentric cylindrical chambers.
Steam
The upper end of inner chamber is
Chemicals funnel shaped and it is provided
Soft water with three inlets, for raw water,
chemicals and steam. The outer
chamber is big with large cross
sectional area at the top. This helps
in forcing the water in outward
direction. The raw water and
chemicals are added in inner
chamber and steam is allowed to
Sludge outlet pass in water. Temperature of
water increases and the chemicals
Figure Hot lime-soda process
will convert soluble salts into
insoluble sludges. Sludges then settle at the bottom of outer chamber and clear soft water is
taken out from the soft water outlet. The hardness of water obtained from this process is
about 25 ppm.

Advantages of hot lime-soda process: Since the process is carried out at a boiling
point temperature of water, it has some advantages over a cold process.

i) Rate of reactions is fast, completes within 15 minutes whereas cold lime soda
process takes several hours.
ii) Water softening capacity is more than cold lime soda process.
iii) The settling of sludges is faster than cold lime soda process and no coagulant is
needed.
iv) The dissolved gases like CO2 will escape out at high temperature.

DYPCEPT
 Dr. Harish Shinde Chapter- Water 20

v) The rate of filtration is more than cold process.

vi) The water obtained from this process has just 15 to 30 ppm hardness.

3. Zeolite or Permutit Process: This is the modern method of softening of water.

Zeolites are the complex hydrated silicates of
aluminium and sodium. Their chemical 10% Brine
solution
formula is Na2O, Al2O3, 2SiO2, 6H2O but they
can be represented in short as Na2Z, or Na2P Raw water
where Z stands for zeolite and P stands for
permutit. In zeolites sodium ions are loosely
held and they can be easily exchanged with Zeolite
++ ++
cations like Ca and Mg ions. This process
consists of a cylindrical tank in which zeolite
bed is placed. The upper part has two inlets Washings Soft water
one for raw water and other for 10% brine
(NaCl) solution. The bottom of the tank is Figure Zeolite process
provided with outlets for soft water and
washings.
When hard water is allowed to percolate through zeolite bed then Ca++ and Mg++ ions
from salts in water exchanges with sodium ions from zeolite.

CaCl2 + Na2Z CaZ + 2NaCl

CaSO4 + Na2Z CaZ + Na2SO4
MgSO4 + Na2Z MgZ + Na2SO4
MgCl2 + Na2Z MgZ + 2NaCl
Mg(HCO3)2 + Na2Z MgZ + 2NaHCO3

After sometime all the sodium zeolite gets exhausted into calcium and magnesium
zeolite. Due to this water will not be softened further. Therefore, the addition of raw is
stopped and exhausted calcium and magnesium zeolites are treated with 10% brine (NaCl)
solution. This process of converting calcium and magnesium zeolite back into sodium zeolite
is called as ‘regeneration’.

CaZ + 2NaCl Na2Z + CaCl2

MgZ + 2NaCl Na2Z + MgCl2

The regenerated washings are taken out from the outlet provided at the bottom of
tank. The regenerated zeolite used for further softening of water. The zeolite process has
following limitations,

i) This process is not applicable to turbid water because the turbidity clogs the pores
of zeolite and makes it inactive.
ii) If hard water contains coloured ions like Fe++ and Mn3+ then zeolite process is not
applicable because, once the Fe and Mn zeolites are formed it is difficult to regenerate them.

DYPCEPT
 Dr. Harish Shinde Chapter- Water 21

iii) If hard water contains mineral acids, then they destroy the zeolite, therefore it is
require to neutralize the water before passing through the zeolite.

Advantages of zeolite process:

i) It removes the hardness almost completely up to 10ppm.
ii) The equipment used is compact and requires small space.
iii) There is no danger of sludge formation.
iv) The process automatically adjusts itself to different harnesses.
v) It requires less time for softening.
vi) It is very clean process.

Disadvantages of zeolite process:

i) Water obtained from this process contains more quantity of sodium salts.
ii) This method removes only positive metal ions and not the negative ions.

4. Ion exchange process: In this process ion exchange resins are used for softening of
hard water. Ion exchange process is also known as ‘Demineralization’ or ‘Deionisation’.
Resins are the insoluble, synthetic or natural polymers having ionizable functional groups and
called as ion exchangers. There are two types of ion exchangers,

i) Cation exchange resins: These resins are capable to exchange cations (Ca++ and
Mg++) from water with the hydrogen ions. They are generally represented as RH2, where R
stands for resin.
ii) Anion exchange resins: These resins are capable to exchange anions (Cl- and SO4- -,
-
HCO3 ) from water with the hydroxyl ions. They are generally represented as R(OH)2.

Thus, if hard water is allowed to pass through resins then it removes the soluble salts
of calcium and magnesium.
Raw water To vacuum

Steam
jacket

Dil. HCl
Degasifier
Cation Exchange Dil. Soft water
Resin NaOH
Anion Exchange
Resin
Acid washings Alkali washings

Cation Exchanger Anion Exchanger

Figure Demineralization of water

DYPCEPT
 Dr. Harish Shinde Chapter- Water 22

The ion exchange process consists of three cylindrical tanks. In first cylinder cation
exchange resin is placed and it is called as ‘cation exchanger’. In second cylinder anion
exchange resin is placed and it is called as ‘anion exchanger’. The third cylinder is the
degasifier.

The hard water is allowed to pass first through the cation exchanger and then through
anion exchanger. Cation exchange resin removes the cations like Ca++ and Mg++.

Ca++ +H2R CaR + 2H+

Mg++ + H2R MgR + 2H+

The remaining anions like of sulphate, chloride and bicarbonate are then removed by
anion exchange resin.

2Cl- + R(OH)2 RCl2 + 2OH-

SO4-- + R(OH)2 RSO4 + 2OH-
HCO3- + R(OH)2 RHCO3 + 2OH-

The resulting H+ and OH- ions combines to form the water free from all cations and
anions and it is called as deionized or demineralised water.

2H+ + OH- H2O

After removal of cations and anions water is allowed to pass through a ‘degasifier’.
The degasifier is connected to a vacuum pump and heated by supplying steam. At high
temperature and under pressure the dissolved gases like CO2 and O2 escapes out. Thus, at this
stage water becomes almost soft. The hardness of water obtained from ion exchange process
is about 0-2 ppm hardness.

Regeneration: After a certain time cation and anion resins get exhausted by the
exchange of H+ and OH- ions. These exhausted resins are then required to regenerate by
passing dilute HCl and dilute NaOH through the cation exchanger the anion exchanger
respectively. The washings of dilute HCl and dilute NaOH are allowed to drain from the
outlet at bottom of cylinders. Regenerated resins are then further used for water softening.

CaR + 2HCl RH2 + CaCl2

MgR + 2HCl RH2 + MgCl2
RCl2 + 2NaOH R (OH)2 + 2NaCl
RSO4 + 2NaOH R(OH)2 + Na2SO4.

Ion exchange process is useful to remove the hardness of acidic as well as alkaline
water. The water treated with this process has very low hardness, therefore useful to threat
the high-pressure boiler feed water. The equipment and chemicals required for this process
are somewhat costly and it cannot be used directly for treatment of turbid water.

DYPCEPT
 Dr. Harish Shinde Chapter- Water 23

PURIFICATION OF DRINKING WATER:

In our domestic routine water is mainly used for drinking purposes. The water, which
is suitable for drinking, is called as ‘potable water.’ The essential characteristics of drinking
water are as,

1. It should be obtained from the source free from pollution.

2. It should be obtained from naturally purified source or from municipal water
supply.
3. It should be sparking clear, colourless, and odourless.
4. It should have a pleasant taste.
5. It should be free from disease producing bacteria and microorganisms.
6. It should be free from objectionable dissolved gases like hydrogen sulphide.
7. It should be soft or free from objectionable minerals such as lead, arsenic,
chromium and manganese salts.
8. Its turbidity should not be more than 10ppm.
9. It should be free from sanitary defect and health hazards.
10. Its carbonate alkalinity should not be more than 150ppm and its pH should be
around 8.
11. Its total dissolved solid should be less than 500 ppm.
12. It should be perfectly cool.
13. Water should be protected from artificial treatment.

Natural water obtained from the sources like rivers, canals etc. do not provide all
these requirements of drinking water. Thus, to make it suitable for domestic consumption its
domestic treatment is carried out as follows,

1. Screening: It is the process of removing floating impurities like lives and branches
of trees, blades of grasses, papers etc. from natural water. In this process raw water is allowed
to pass through a screen having large number of pores, which removes the floating matter.

2. Sedimentation: It is the process of removing suspended impurities from water. In

this process water is allowed to stand in a large tank, without any disturbance, so that
suspended matter settles down at the bottom due to force of gravity. The time required for
sedimentation is about 8-10 hours.

It can be carried out in continuous flow type of tank in which the flow of water is
adjusted at slow rate for incoming and outgoing water. The water flows in horizontal
direction at uniform speed. Due to the effect of gravitational force, suspended particles like
mud, fine sand etc. having specific gravity more than one get settled down at the bottom of
tank. They can be removed periodically. The clear supernatant water from the top comes out
which is free from suspended solids. About 75% of suspended matter gets removed in
sedimentation process.

DYPCEPT
 Dr. Harish Shinde Chapter- Water 24

3. Coagulation: Sometimes coagulation is carried out along with sedimentation. This

process is used to remove colloidal impurities, i.e. solid particles having specific gravity less
than one. The colloidal particles are having very small size (10-4 to 10-7 cm) and carries either
positive or negative charge. Due to the similar charge they repel each other and do not
aggregate. Therefore, they do not settle during sedimentation. In coagulation process some
chemicals called as coagulants are added to remove colloidal particles. The examples of well
known coagulants are alum (K2SO4, Al2(SO4)3, 24H2O), ferrous sulphate [FeSO4.7H2O],
sodium aluminate [NaAlO2], etc.

The mixture of alum 5-10% solution and water is thoroughly mixed by rotating the
deflector plate as shown in figure The coagulant reacts with bicarbonates from water and
produces white gelatinous precipates called as ‘flock’.

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

‘Flock’
FeSO4 + Mg(HCO3)2 Fe(OH)2 + MgCO3 + H2O + CO2

NaAlO2 + 2H2O Al(OH)3 + NaOH

Chemicals

Inlet for
Outlet
water

Deflector
plate

Figure Coagulation process

The flock when passes downward through the water, collects all the colloidal particles
so that its gravity goes on increasing and finally it settles down. On the other hand Al+++ ions
with positive charge, neutralizes the negative charge of colloidal particles. After
neutralization particles combines with each other to form bigger particles, which settles down
easily at the bottom. The addition of coagulant to the water also removes colour, odour and
improves the taste.

DYPCEPT
 Dr. Harish Shinde Chapter- Water 25

4. Filtration: Water obtained after sedimentation and coagulation is still containing

small amount of colloidal and bacterial impurities. Filtration is used to remove these
impurities. In filtration water is allowed to pass through a bed of proper sized materials.
There are two types of filters used for filtration.

a) Gravity sand filter: It consists of a large rectangular concrete tank. At the

bottom of tank there are channels of bricks to collect filtered water. Over the channels there
are three layers made of gravels (30cm) course sand (20cm) and fine sand (50cm).
Sedimented water is allowed to percolate from the top layer. When water percolates through
the sand, it removes all suspended, colloidal particles and bacteria.

The rate of filtration of

Fine sand gravity sand filter becomes slow
Coarse sand after 24 hours because of clogging of
Gravels pores of top sand layer. At that time
it is required to replace the top
Filtered
portion of upper layer with new sand.
Water
These types of filters are used in
Channels Sedimented water municipal water supply.
Figure Gravity sand filter
b) Pressure filter: To
increase the rate of filtration pressure
filter is used. It consists of a cylindrical vertical steel tank containing three layers of filtering
media such as pebbles (10-35 mm size), coarse sand (5-7mm size) and Fine sand (1-2-mm
size). The raw water from
Raw Wash
sedimentation is mixed with small water
water
amount of alum and forced under
pressure from the top of tank as Layer of Deflector
Al(OH)3 plate
shown in figure 4.8.
Fine
Coarse
Alum forms a slimy layer of sand
sand
Al(OH)3 on the fine sand. When
Pebbles
water percolates through Al(OH)3 Compressed
and different layers of sand, which air
removes the colloidal and bacterial
impurities. Filtered water is Wash water Filtered
collected from the bottom. inlet water

Figure Pressure Filter

Pressure filter gets clogged
after its use for some time. Therefore
it is required to clean it. Wash water is forced from the bottom for this purpose and the
backwash water is allowed to go. Pressure filters are used on small scale and they are
economical.

DYPCEPT
 Dr. Harish Shinde Chapter- Water 26

5. Sterilization: Water obtained after treatment in sedimentation, coagulation and

filtration, still contains small amount of pathogenic bacteria and microorganisms. Therefore,
it is required to remove them from water. The process of destroying bacteria and
microorganisms from water is known as sterilization of water. The substances used for
sterilization are called as ‘sterilizers’. The different methods used for sterilization of water
are simple boiling, chlorination, ozonolysis, aeration and ultraviolet rays.

a) Boiling: Sterilization of water is carried out by simple method like boiling. Boiling
kills all the disease producing bacteria and microorganisms. This method is useful only for
household purposes, because on large scale it becomes more expensive.

b) Chlorination: Sterilization of water to destroy bacteria and microorganisms by

using chlorine gas is called as disinfection by chlorine. It is carried out by using following
methods,

i) By using chlorine gas (Cl2): Chlorine gas or chlorine water is used directly for
sterilization of water in city water supply. Chlorine reacts with water to form hypochlorous
acid, which on further decomposition gives HCl and nascent oxygen. Both the hypochlorous
acid and nascent oxygen are powerful germicides and they kills the germs and bacteria.

Cl2 + H2O HOCl + HCl

HOCl HCl + (O) nascent oxygen

The apparatus used for carrying out the chlorination is called as chlorinator. It consists
of a cylindrical tower provided with number of baffle plates. The filtered water and required
quantity of chlorinated water are introduced from the top of tank. When water passes over the
baffle plates it mixes
thoroughly with chlorine. The Filtered Chlorine
treated water is then taken out water solution
from the bottom of tank. or

For filtered water about

0.3 to 0.5ppm chlorine is
sufficient. If chlorine is added
in excess, then it gives bad
odour and taste to the water. Baffle plate
Therefore, water should not
contain more than 0.1-0.2 ppm
of free chlorine.

Chlorination is most Sterilized

water
effective and economical
method for the disinfection of
water. The effect of Figure Chlorination
chlorination is permanent and it

DYPCEPT
 Dr. Harish Shinde Chapter- Water 27

cannot be deteriorate on keeping. Chlorination is useful at low as well as high temperature.

Water obtained from chlorination is free from salts.

ii) By adding bleaching powder (CaOCl2): Bleaching powder is soluble in water and it
is a good sterilizer. In practice on small scale one kg of bleaching powder is sufficient for one
thousand liters of water. Water is mixed with required quantity of bleaching powder and
allowed to stand for several hours. It reacts with water and produces Cl2 and nascent oxygen.
Nascent oxygen and hypochlorous acid oxidizes the bacteria and microorganisms.

CaOCl2 + H2O Ca(OH)2 + Cl2

Cl2 + H2O HOCl + HCl
HOCl HCl + (O) nascent oxygen.

The excess of bleaching powder produces bad taste, disagreeable odour to the water
and increases the hardness of water.

iii) By using chloramine (ClNH2): If chlorine and ammonia are mixed in the ratio of
2:1 by volume then a compound called chloramine is formed. Now a day’s chloramine has
taken the place of chlorine because ammonia removes the bad smell of chlorine and produces
a good taste to water. Chloramine when added to the water, it reacts with water and gives
nascent oxygen. Nascent oxygen thus produced then disinfects the water by destroying the
bacteria and microorganisms.

Cl2 + NH3 ClNH2 + HCl

‘Chloramine’
ClNH2 + H2O HOCl + NH3
HOCl HCl + [O] nascent oxygen

c) Ozonisation: Sometimes the water is treated with ozonized oxygen and it is very
effective method of sterilization of water. The ozonisation plant consists of gravels bed
through which water is allowed to percolate with slow rate and in wide area. At the same
time ozone gas is allowed to bubble through the water. Ozone (O3) gas is unstable and it
decomposes easily into oxygen and nascent oxygen (O).

O3 O2 + (O) nascent oxygen.

Impure Waste Gases

water

Gravel Bed

Curved
partition with
Ozone
perforation
Gas

Sterilized
water
DYPCEPT

Figure Ozonisation
 Dr. Harish Shinde Chapter- Water 28

The nascent oxygen thus produced is more effective for killing all germs and bacteria
from water. Ozone is also acting as bleaching, de-colourizing and de-odourzing agent. It
improves the taste of water and if it becomes excess, it will not be harmful. This method is
quite expensive and hence not useful on large scale in municipal water supply.

QUESTIONS
Q.1 Enlist different impurities present in natural water and suggest methods for minimizing it.
Or
Enlist different impurities present in natural water? How are they removed?

Or
What are the different impurities present in natural water?
Q.2 Write note on Total Dissolved Solids (TDS)
Q.3 What is acidity of water? Explain in details.
Q.4 Write short note on ‘Alkalinity of water sample’.
Q.5 What is chloride content in water? Explain in detail.
Write note on chloride content in water.
Q.6 What is hardness of water? Explain in detail.
Q.7 Explain in brief ill effects of hard water when used in boilers.
Q.8 What is scale and sludge formation? Explain disadvantages and prevention of scale
formation.
Q.9 What are causes & disadvantages of scale and sludge formation?
Q.10 Distinguish between scale & sludge.
Q.11 Explain the principle of ion exchange process for removal of hardness of water.
Explain ion exchange process for treatment of hard water.
Q.12 Explain reverse osmosis technique for the treatment of hard water.
Write note on Reverse Osmosis.
Q. 13 Describe dissolved oxygen of water.

DYPCEPT