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UNIT - I
WATER TREATMENT PROCESS
“Nothing on earth can function without water”.
-Thiruvalluvar.
“Water is the driver of life on earth”.
-Leo Nardo Davinci.
Introduction
The Molecular Formula H2O
The Molecular Mass 18
Structure
O
H H
Water is nature’s gift of life to us. It is the most widely distributed
compound, which exists naturally as liquid, solid and gas.
It covers about 80% of the earth’s surface; about 70% of the human body is
water.
The water content in the human body accounts for more than half of its total
weight.
It is the most important compound for the existence of human beings,
animals and plants. All plants, animals (Human being 70%) and fruits
(Water melon 99%, tomato 95%) contain water.
In 1781, Cavendish prepared water by the combustion of hydrogen in air.
Later Lavoisier proved that water is a compound of hydrogen and oxygen.
Composition of Water
One molecule of water contains atleast two atoms of hydrogen and one atom
of oxygen. There fore, the atomic ratio of hydrogen and oxygen is 2:1.
The atomic weights of hydrogen and oxygen are 1 and 16.
The gravimetric composition (by weight) of water is 1:8 and the volumetric
composition of water is 2:1.
Volumetric composition of water
The ratio by volume of hydrogen and oxygen present in water is called
volumetric composition of water.
Its value is 2:1. It was established by Davy in 1800 AD. It is determined by
using Hopeman’s voltameter.
Water has greater applications in industries such as textiles, chemicals,
paper, pharmaceuticals, food processing, leather etc.,
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Water is mainly used in power generation.
It is also used as a coolant in atomic reactors, as well as in chemical plants.
It is also largely used in irrigation for agricultural purpose and fire fighting.
Now – a – days, the quantity of water is gradually deteriorating due to
pollution. So, engineers need to have a wide knowledge about the quality of
water, the problems posed by hard water in industries and its treatment
processes.
Sources of water
Water is essential for the survival of all living organisms. About 80% of the
earth’s surface is occupied by water.
The main sources of water are,
1. Surface water.
2. Underground water.
Surface water Surface runoff precipitation that does not infiltrate the ground or return to
the atmosphere or return to the evaporation (including transpiration). This runoff
flows into streams, lakes, wetlands, estuaries and reserviours.
It can be further classified into four major sources.
1. Rain water
2. River water.
3. Lake water.
4. Sea water.
Rain water
It is the purest form of water. It is made impure by polluted atmosphere, like
CO2, SO2, and NO2 etc.,
River water
River water starts from spring water and fed by rain water. Chlorides,
sulphates, bicarbonates of Na, Ca, Mg and Fe are some of the major mineral salts
present in river water.
Lake water
Lake water has constant chemical composition. It usually contains fewer
amounts of dissolved minerals and a high quantity of organic matter.
Sea water
This is the most impure form of natural water. It contains NaCl (2.6%),
sulphates of Na, bicarbonates of K, Mg and Ca and bromides of K, Mg and a
number of other compounds.
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Underground water
A part of rain water which falls on earth surface percolates into the earth and
continues its journey till it meets a hard rock where it may be stored or come in the
form of spring.
Properties of water
Physical properties
(i) It exists in three states ie., solid, liquid and gas. The solid form of water is
known as ice. It exists only below 00c. It exists as a liquid between 0 and
1000c and as gas (steam) above 1000c. Hence the boiling point of water is
1000c (373 K) and freezing point is 00c (273 K).
(ii) Pure water is a transparent, colourless, odourless and tasteless liquid.
(iii) It is a good solvent for the ionic compounds and it dissolves almost all
substances (solid, liquid or gas). Hence, it is known as universal solvent.
(iv) Pure water is a bad conductor of electricity, but acidified water is an
electrolyte.
(v) The density of water is maximum at 40c, which is equal to 1000 kg m-3.
Chemical properties
(i) Heating Process
(ii) Action with metals
(iii) Action with non – metals
(iv) Action with metallic oxides
(v) Action with non – metallic oxides
(vi) Action with carbides, phosphides and nitrides.
(i). Heating Process
At very high temperature, water is decomposed into hydrogen and oxygen.
20000c
2H2O ∆ 2H2 + O2 ↑
Water Hydrogen + oxygen
(ii). Action with metals
(a). Cold water reacts with metals like sodium, potassium and calcium to
form hydrogen and their respective hydroxides.
2Na + 2H2O 2Na(OH) + H2 ↑
Sodium Sodium hydroxide
2K + 2H2O 2K(OH) + H2 ↑
Potassium Potassium hydroxide
Ca + 2H2O Ca(OH) 2 + H2 ↑
Calcium Calcium hydroxide
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(b). Metals like magnesium and zinc react with hot water or steam to form
hydrogen and their respective oxides.
Mg + H2O MgO + H2 ↑
Magnesium Magnesium oxide
Zn + H2O ZnO + H2 ↑
Zinc Zinc oxide
(iii). Action with non – metals
(a). Carbon reacts with water at red hot condition to produce water gas (a
mixture of carbon monoxide and hydrogen).
1273 K
C + H2O ∆ CO + H2 ↑
Carbon Carbon monoxide
(b). Chloride reacts with water in the presence of sun light to form oxygen
and hydrochloric acid.
2Cl2 + 2H2O 4HCl + O2 ↑
Chlorine Hydrochloric acid
(iv). Action with metallic oxides
Water reacts with metallic oxides like sodium oxide and potassium oxide to
form their respective hydroxides.
Na2O + H2O 2Na(OH)
Sodium oxide Sodium hydroxide
K2O + H2O 2K(OH)
Potassium oxide Potassium hydroxide
(v). Action with non – metallic oxides
Non – metallic oxides like carbon dioxide, sulphur dioxide and sulphur tri
oxide react with water to form their respective acids.
CO2 + H2O H2CO3
Carbon dioxide Carbonic acid
SO2 + H2O H2SO3
Sulphur dioxide Sulphurous acid
SO3 + H2O H2SO4
Sulphur trioxide Sulphuric acid
(vi). Action with carbides, phosphides and nitrides
Water decomposes carbides, phosphides and nitrides to produce methane,
phosphine and ammonia respectively.
Al4C3 + 12H2O 4Al(OH)3 + 3CH4
Aluminium carbide Aluminium hydroxide + Methane
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Ca3P2 + 6H2O 3Ca(OH)2 + 2PH3
Calcium phosphide Calcium hydroxide + Phosphine
Ca3N2 + 6H2O 3Ca(OH)2 + 2NH3
Calcium nitride Calcium hydroxide + Ammonia
Types of impurities
The impurities present in water may be broadly classified into four types.
They are,
1. Dissolved impurities
2. Suspended impurities
3. Colloidal impurities and
4. Micro organisms
Dissolved impurities
The dissolved impurities are mainly the carbonates, bi-carbonates,
Chlorides and sulphates of Ca, Mg, Fe, Na and K. The dissolved impurities also
include dissolved gases like O2, CO2, etc.,
The presence of these salts imports hardness to water.
Suspended impurities
There are two types of suspended impurities. They may either be;
1. Inorganic suspended impurities: Clay and sand
2. Organic suspended impurities: Oil globules, Vegetable and animal
matter.
The inorganic and organic suspended impurities impart turbidity, colour and
odour to water.
Colloidal impurities
Finely divided silica and clay, organic waste products, complex protein
amino acids, etc.,
Micro Organisms
They include algae, fungi and bacteria.
Water Treatment
Among the sources of water (surface and underground water) are normally
used for domestic and industrial purposes. Such water must be free from
undesirable impurities. The process of removing all types of impurities from water
and making it fit for domestic or industrial purposes is called water treatment or
water technology.
Hardness of water
Define – Hardness
Hardness is the property present in water which prevents lathering of soap.
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Classification of water
Water from different sources, differ in taste and odour. This difference is
due to the presence of dissolved salts and minerals. Based on the quality, water can
be classified into two types.
They are,
1. Hard water.
2. Soft water.
Define - Hard water
“Water which does not give much foam lather with soap solution is called
hard water”. On the other hand, it forms a white scum or precipitate.
The hardness of water is due to the presence of soluble bicarbonates,
chlorides and sulphates of ‘Ca’ and ‘Mg’.
2C17H35COONa + CaCl2 (C17H35COO)2 Ca ↓ + 2NaCl
Sodium stearate Calcium chloride Calcium stearate
(Sodium soap) (Hardness causing salt in water)
2C17H35COONa + MgSO4 (C17H35COO)2 Mg ↓ + Na2SO4
Sodium stearate Magnesium sulphate Magnesium stearate
(Sodium soap) (Hardness causing salt in water)
Define - Soft water
“Water which gives good foam lather with soap solution is called soft
water”. This is due to the absence of ‘Ca’ and ‘Mg’ salts.
Give the Reason for Hardness?
Reason for Hardness
The hardness of water is due to the presence of bicarbonates (HCO3-),
carbonates (CO32-), chlorides (Cl-) and sulphates (SO4
2-) of calcium or magnesium
or both.
How to identify Hardness?
Hardness of water can be identified into two ways.
1. Reaction with soap solution.
2. Reaction with EBT indicator.
Reaction with soap solution
When, the water is treated with soap solution, if it does not give much foam
lather. This water is called hard water.
2C17H35COONa + CaCl2 (C17H35COO)2 Ca ↓ + 2NaCl
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Sodium stearate Calcium chloride Calcium stearate
(Sodium soap) (Hardness causing salt in water)
2C17H35COONa + MgSO4 (C17H35COO)2 Mg ↓ + Na2SO4
Sodium stearate Magnesium sulphate Magnesium stearate
(Sodium soap) (Hardness causing salt in water)
Reaction with EBT indicator
When the water is added two to three drops of EBT indicator, if it gives
wine red colour, the water is hard water.
Classification of Hardness of water
How is Hardness in water classified? Give Example?
On the basis of dissolved ions, hardness of water can be classified into two
types. They are,
1. Temporary hardness (or) Carbonate hardness (or) Alkaline hardness.
2. Permanent hardness (or) Non-carbonate hardness (or) Non-Alkaline
hardness.
Temporary hardness (or) Carbonate hardness (or) Alkaline hardness
If bicarbonates of Ca and Mg are present in water, such hardness is called
carbonate hardness or temporary hardness or alkaline hardness. It can be easily
removed by boiling the water and Clark’s process.
Removal of Temporary hardness
Temporary hardness of water may be removed by the following methods.
1. Boiling
2. Clark’s process
(i). Boiling
When the temporary hard water is heated strongly, the following reactions
take place.
Ca(HCO3)2 CaCO3 ↓+ H2O + CO2 Calcium bicarbonate Calcium carbonate
Mg(HCO3)2 MgCO3 ↓+ H2O + CO2 Magnesium bicarbonate Magnesium carbonate
i.e, Calcium bicarbonate and magnesium bicarbonate are decomposed into
calcium and magnesium carbonate. These salts are insoluble in water and settle at
the bottom of the vessel. It can be removed by filtration. The filtrate obtained, is
soft water.
(ii). Clark’s process
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In this process a calculated quantity of slacked lime (calcium hydroxide) is
added to temporary hardness of water. This converts the soluble bicarbonates into
insoluble carbonates which are removed by filtration. Filtered water is thus free
from calcium and magnesium bicarbonates and is soft.
Ca(HCO3)2 + Ca(OH) 2 CaCO3 ↓+ 2H2O
Calcium bicarbonate Calcium hydroxide Calcium carbonate
Mg(HCO3)2 + Ca(OH)2 MgCO3 ↓+ CaCO3 ↓+ 2H2O
Magnesium bicarbonate Calcium hydroxide Magnesium carbonate
Permanent hardness (or) Non - carbonate hardness (or) Non - alkaline
hardness
If chlorides and sulphates of Ca and Mg are present in water, such hardness
is called permanent hardness or non – carbonate hardness or non – alkaline
hardness. It cannot be removed by boiling the water, because permanent hardness
producing salts do not decompose on heating.
But it can be removed by the following methods.
1. Lime – soda (washing soda – sodium carbonate) process
2. Calgon process (Internal conditioning method)
3. Zeolite or Permutit (External conditioning method) process
4. Ion – exchange or Demineralisation or Deionisation process
5. Reverse osmosis method
Removal of Permanent hardness
Lime – soda process
When washing soda is added to hard water, the chlorides and sulphates of
calcium and magnesium are converted into their respective carbonates.
CaCl2 + Na2 CO3 CaCO3 ↓+ 2NaCl
Calcium chloride Sod. Carbonate Calcium carbonate
Calgon process
‘Calgon’ is the commercial name of sodium hexameta phosphate. It means
‘Calcium gone’. When Calgon is added to hard water, the magnesium and calcium
salts present in it are converted into soluble complex salts and soft water is
produced. As these salts are soluble in water, filtration is not required.
2CaSO4 + Na2 [Na4(PO3) 6] Na2 [Ca2 (PO3)6] + 2Na2SO4
Units of hardness
The following four common units are used in hardness measurements.
1. Milligrams per litre (mg/l)
2. Parts per million (ppm)
3. Degree Clark’s ( 0Cl)
4. Degree French ( 0Fr)
5. Milliequivalent per litre (Meq/l)
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Milligrams per litre
It is defined as the number of milligrams of CaCO3 present in one litre of
water.
Parts per million
It is defined as the number of parts by weight of CaCO3 present in million
(106) partsof water.
Degree Clark’s
It is defined as the number of parts of CaCO3 equivalent hardness per 70,000
parts of water.
Degree French
It is defined as the number of parts of CaCO3 equivalent hardness per
1, 00,000 (105) parts of water.
Milliequivalent per litre
It is defined as the number of Milliequivalents of hardness present per litre.
Relation between the hardness units
1 mg/lit = 1 ppm = 0.07 0Cl = 0.10F = 0.02 Meq/l
Disadvantages of using hard water
1. Hard water when used for drinking affects the digestive system and leads to
formation of kidney stones (Calcium oxalate).
2. Hard water is not suitable for laboratory analysis because the hardness
producing ions (Ca2+ and Mg2+) interface in various reactions.
3. When hard water is used for cooking, more fuel and time consumption are
required. Because of the presence of salts Ca and Mg, this increases the
boiling point of water.
4. When hard water is used for steam production, the boiler affected by the
problems like Scale – Sludge formation priming and foaming and caustic
embrittlement.
5. When hard water is used for concrete making, the hydration of the cement
and the strength of the concrete are affected.
Expression of hardness in terms of equivalents of CaCO3
The concentrations of hardness producing salts are usually expressed in
terms of an equivalent amount of CaCO3. The reason for choosing CaCO3 as the standard for calculating hardness of
water is because,
Its molecular weight is exactly 100 and equivalent weight is 50, which
makes the calculations easier.
It is the most insoluble salt, thus can be easily precipitated in water
treatment processes. Amount equivalent to CaCO3 =
Weight of hardness producing salt or ions (cations)
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_________________________________________ × Molecular Weight of CaCO3
Molecular Weight of hardness producing salt or ions
Molecular weight of some hardness producing salts
Hardness producing
salt & Cation
Molecular
weight
Hardness producing
salt & Cation
Molecular
weight
Ca(HCO3) 2 162 Mg(HCO3) 2 146
CaCl2 111 MgCl2 95
CaSO4 136 MgSO4 120
CaCO3 100 MgCO3 84
Ca2+ 40 Mg2+ 24
Ca(NO3) 2 164 Mg(NO3) 2 148
Problems based on hardness in terms of Calcium Carbonate equivalents
Example: 1
A water sample contains 120 mgs of MgSO4 per litre. Calculate the hardness
in terms of CaCO3 equivalents.
Solution
The amount of MgSO4 = 120 mgs/lit
Amount equivalent to CaCO3 =
Weight of hardness producing salt or ions (cations)
_________________________________________ × Molecular Weight of CaCO3
Molecular Weight of hardness producing salt or ions
We know that, the molecular weight of MgSO4 = 120
Amount equivalent to CaCO3 = 120/120 X 100
= 100 mgs/lit
Result
Therefore amount equivalent to CaCO3 = 100 mgs/lit
Example: 2
A water sample contains 204 mgs of CaSO4 per litre. Calculate the hardness
in terms of CaCO3 equivalents.
Solution
The amount of CaSO4 = 204 mgs/lit
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Amount equivalent to CaCO3 =
Weight of hardness producing salt or ions (cations)
_________________________________________ × Molecular Weight of CaCO3
Molecular Weight of hardness producing salt or ions
We know that, the molecular weight of CaSO4 = 136
Amount equivalent to CaCO3 = 204/136 X 100
= 150 mgs/lit
Result
Therefore amount equivalent to CaCO3 = 150 mgs/lit
Example: 3
If a sample of water contains 50 mgs of Ca2+ ions per litre. Calculate its
hardness in terms of CaCO3 equivalent.
Solution
The amount of Ca2+ = 50 mgs/lit
We know that, the molecular weight of Calcium = 40
Amount equivalent to CaCO3 = 50/40 X 100
= 125 mgs/lit
Result
Therefore amount equivalent to CaCO3 = 125 mgs/lit
Example: 4
What is the hardness of a solution containing 0.585 grams of NaCl and 0.6
grams of MgSO4 per litre?
Solution
1. The amount of NaCl = 0.585 grms/lit
= 0.585 X 1000
= 585 mgs/lit
2. The amount of MgSO4 = 0.6 grms/lit
= 0.6 X 1000
= 600 mgs/lit
NaCl does not contribute to hardness. So it is ignored.
We know that, the molecular weight of MgSO4 = 120
Amount equivalent to CaCO3 = 600/120 X 100
= 500 mgs/lit
Result
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Therefore amount equivalent to CaCO3 = 500 mgs/lit
Example: 5
A sample of water is found to contain the following analytical data in
mgs/lit.
(i) Mg(HCO3)2 = 14.6 mgs/lit
(ii) MgCl2 = 9.5 mgs/lit
(iii) MgSO4 = 6.0 mgs/lit and
(iv) Ca(HCO3) 2 = 16.2 mgs/lit
Calculate the temporary and permanent hardness of the sample of water
(Atomic weight of Ca, Mg, Cl, C, S, O and H are 40, 24, 35.5,12, 32, 16, and 1
respectively).
Solution
Name of the hardness
producing salt
Amount
in mgs/lit
Molecular
weight
Amount equivalent
to CaCO3
Mg(HCO3)2 14.6 146 = 14.6/146 X100
= 10 mgs/lit
MgCl2 9.5 95 = 9.5/95 X 100
= 10 mgs/lit
MgSO4 6.0 120 = 6.0/120 X 100
= 5 mgs/lit
Ca(HCO3) 2 16.2 162 = 16.2/162 X 100
= 10 mgs/lit
Temporary hardness producing salts = Mg(HCO3)2 + Ca(HCO3) 2
= 10 + 10
= 20 mgs/lit
Permanent hardness producing salts = MgCl2 + MgSO4
= 10 + 5
= 15 mgs/lit
Result
Temporary hardness = 20 mgs/lit
Permanent hardness = 15 mgs/lit
Estimation of Hardness
The estimation of hardness of water is very essential for its use in boilers for
steam generation, as well as for industries uses.
Methods of estimation of hard water
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There are mainly three basic methods of estimation of hard water. They are,
1. EDTA Method
2. Alkalinity Method
3. O.Hehner’s Method
(i) Determination of Temporary hardness
(ii) Determination of permanent hardness
EDTA Method (or) Complexometric Method
EDTA
Ethylene Diamine Tetra Acetic acid
Structure of EDTA
HOOCH2C CH2COOH
N ─ CH2 ─ CH2 ─ N
HOOCH2C CH2COOH
Structure of disodium salt of EDTA
HOOCH2C CH2COONa
N ─ CH2 ─ CH2 ─ N
NaOOCH2C CH2COOH
‘EDTA’ is insoluble in water; its disodium salt is used as a complexing
agent. This method is also known as “ Versenate” Method.
Principle
The amount of hardness causing ion (Ca2+ and Mg 2+) can be estimated by
titrating the water sample against EDTA using ‘Eriochrome Black –T’ (EBT)
indicator at a pH range of 8 -10.
Before starting the titration to the hard water, ammonia buffer and EBT are
added which
Ca2+ pH 8 -10 Ca
+ EBT EBT
Mg2+ Mg
Unstable complex with
wine red coloured
When this solution is titrated against EDTA, it replaces the indicator from an
unstable complex and form stable EDTA complex.
After the titration, all the hardness causing ions are complexed by EDTA,
the indicator is set free.
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Therefore the colour of the free indicator is steel blue. Thus the end point is
the change of colour from wine red to steel blue.
Ca Ca
EBT + EDTA EDTA + EBT
Mg Mg
Unstable complex with Stable complex with steel blue coloured
Wine red coloured (alkaline medium)
Preparation of solutions
1. Preparation of EDTA solution
Dissolve ‘4g’ of pure disodium salt of EDTA crystals in 1 litre of
distilled water.
Preparation of standard hard water
Dissolve 1g of pure CaCO3 + dil HCl Residue
(small quantity) upto dryness
Dissolve the residue in one litre of distilled water. “1 ml” of this solution
contains “1 mg” of CaCO3.
2. Preparation of Buffer solution
Add 67.5g of NH4CL to 570ml of concentrated NH3 solution and dilute he
solution with one litre of distilled water.
3. Preparation of EBT Indicator
Dissolve 0.5g of EBT in 100ml alcohol.
“Structure of EBT”
Procedure
In this method, three titrations are carried out to estimate the total,
permanent and temporary hardness.
Standardization of EDTA solution
The burette is filled with EDTA solution. 50ml of standard hard water is
pipetted out into a clean conical flask. Add 10-15ml of buffer solution and few
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drops of EBT indicator. The wine red solution present in the conical flask is
titrated against the burette EDTA solution till the wine red colour changes to steel
blue colour.
Let the volume of EDTA consumed be V1 ml.
Estimation of total hardness
Pipette out 50ml of sample hard water into the conical flask add the
ammonia buffer (NH4OH + NH4Cl) and EBT indicator and titrate it against the
same EDTA burette solution to get the end point.
Let the volume of EDTA consumed be V2 ml.
Estimation of permanent hardness
The water sample of 250ml is taken in a beaker and evaporates nearly to
50ml. the temporary hard salts settle down. Filter and wash thoroughly and make
up the solution again to 250ml. Pipette out 50ml of the made-up solution in to a
clean conical flask and titrate it against the (EDTA) burette solution to get the end
point.
Let the volume of EDTA consumed be V3 ml.
Calculations
A.Total hardness
(i) V1 ml of EDTA is consumed by 50ml of standard hard water.
V1 ml of EDTA = 50 mg of CaCO3
Therefore 1ml of EDTA = 50/V1 mg of CaCO3
(ii) V2 ml of EDTA is consumed by 50ml of sample hard water.
1ml of EDTA = 50/V1 mg of CaCO3
V2 ml of EDTA = 50/V1 X V2 mg of CaCO3
Therefore 50ml sample hardwater contain = 50/V1 X V2 mg of CaCO3
Therefore 1000 ml sample hardwater = 50/V1 X V2/50X1000mg/l
= V2 /V1 X1000mg/l
Therefore total hardness = V2 / V1 X 1000mg/l of CaCO3 (ppm)
B.Permanent hardness
50ml of sample hard water after removing temporary hardness consumes V3
ml EDTA.
1ml of EDTA = 50/V1 mg of CaCO3
Therefore V3 ml of EDTA = 50/V1 X V3 mg of CaCO3
50ml of sample hard water
(after boiling) contain = 50/V1 X V3 mg of CaCO3
Therefore 1000ml of sample hard water = 50/V1 X V3/50 X 1000mg/l
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= V3 /V1 X 1000mg/l
Therefore permanent hardness = V3 /V1 X 1000mg/l of CaCO3 (ppm)
C.Temporary Hardness
Temporary Hardness = Total hardness – permanent hardness
= (V2 /V1 X 1000) – (V3 /V1 X 1000)
= 1000 [(V2/V1) – (V3/V1)]
= 1000 X (V2–V3/V1) ppm
Problems based on EDTA method
Example: 1
50 ml of a standard hardwater containing 1 mg of pure CaCO3 per ml
consumed 24 ml of EDTA. 50ml of sample of hard water consumed 16 ml of
EDTA. Calculate the total hardness in ppm.
Solution
We know that, 1ml of std. hard water = 1mg of CaCO3
24ml of EDTA = 50 ml of std. hard water
Therefore 1ml of EDTA = 50/24 ml of std. hard water
= 50/24 mg of CaCO3 equivalent
Therefore 1ml of EDTA = 2.0833 mg of CaCO3 equivalent
16 ml of EDTA = 2.0833 X 16
= 33.3328 mg of CaCO3 equivalent
Therefore 50 ml of hard water contains =33.3328 mg of CaCO3
Therefore 1000ml of hard water contains = 33.3328/50 X1000
Total hardness = 666.656 ppm
Result
Therefore total hardness = 666.656 ppm.
Example: 2
(i) 50 ml of standard hard water containing 1mg of pure CaCO3 per
ml consumed 20 ml EDTA.
(ii) 50 ml of sample consumed 25 ml of EDTA solution.
(iii) 50 ml of water sample after boiling and filtering consumed 18
ml of EDTA. Calculate the temporary, permanent and total hardness.
Solution
A.Total hardness
We know that, 1ml of std. hard water = 1mg of CaCO3
20ml of EDTA = 50 ml of std. hard water
Therefore 1ml of EDTA = 50/20 ml of std. hard water
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= 50/20 mg of CaCO3 equivalent
Therefore 1ml of EDTA = 2.5 mg of CaCO3 equivalent
25 ml of EDTA = 2.5 X 25
= 62.5 mg of CaCO3 equivalent
Therefore 50 ml of hard water contains = 62.5 mg of CaCO3 eq.
Therefore 1000ml of hard water contains = 62.5/50 X1000
Total hardness = 1250 ppm
B.Permanent hardness
After boiling EDTA consumed = 18 ml
1 ml of EDTA = 2.5 mg of CaCO3 equivalent
18 ml of EDTA = 2.5 X 18
= 45 mg of CaCO3 equivalent
50 ml of sample hard water = 45 mg of CaCO3 equivalent
(after boiling) contains
1000 ml of sample hard water = 45/50 X1000
(after boiling) contains
Permanent hardness = 900 ppm
C.Temporary hardness
Temporary hardness = Total hardness – Permanent hardness
= 1250 – 900
= 350 ppm
Result
Temporary hardness = 350 ppm
Permanent hardness = 900 ppm
Total hardness = 1250 ppm
Example: 3
(i) 25ml of standard hard water consumes 12 ml of standard EDTA solution.
(ii) 25 ml of sample hard water consumes 8 ml of standard EDTAsolution.
(iii) After boiling the sample 25ml of the boiled and cooled hard water
consumes 6 ml of standard EDTA solution. Calculate the total, temporary and
permanent hardness.
Solution
A.Total hardness
We know that, 1ml of std. hard water = 1mg of CaCO3
12ml of EDTA = 25 ml of std. hard water
Therefore 1ml of EDTA = 25/12 ml of std. hard water
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= 25/12 mg of CaCO3 equivalent
Therefore 1ml of EDTA = 2.0833 mg of CaCO3 equivalent
8 ml of EDTA = 2.0833 X 8
= 16.6664 mg of CaCO3 equivalent
Therefore 25 ml of hard water contains = 16.6664 mg of CaCO3
Therefore 1000ml of hard water contains = 16.6664/25 X1000
Total hardness = 666.656 ppm
B.Permanent hardness
After boiling EDTA consumed = 6 ml
1 ml of EDTA = 2.0833 mg of CaCO3 equivalent
6 ml of EDTA = 2.0833 X 6
= 12.4998 mg of CaCO3 equivalent
25 ml of sample hard water = 12.4998 mg of CaCO3 equivalent
(after boiling) contains
1000 ml of sample hard water = 12.4998/25 X1000
(after boiling) contains
Permanent hardness = 500 ppm
C.Temporary hardness
Temporary hardness = Total hardness – Permanent hardness
= 666.656 – 500
= 166.656 ppm
Result
Temporary hardness = 166.656 ppm
Permanent hardness = 500 ppm
Total hardness = 666.656 ppm
Example: 4
50 ml of a standard hardwater containing 1 mg of pure CaCO3 per ml
consumed 17 ml of EDTA. 50ml of sample of hard water consumed 12 ml of
EDTA. Calculate the total hardness in ppm.
Solution
We know that, 1ml of std. hard water = 1mg of CaCO3
17 ml of EDTA = 50 ml of std. hard water
Therefore 1ml of EDTA = 50/17 ml of std. hard water
= 50/17 mg of CaCO3 equivalent
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Therefore 1ml of EDTA = 2.9412 mg of CaCO3 equivalent
12 ml of EDTA = 2.9412 X 12
= 35.2944 mg of CaCO3 equivalent
Therefore 50 ml of hard water contains =35.2944 mg of CaCO3
Therefore 1000ml of hard water contains = 35.2944/50 X1000
Total hardness = 705.888 ppm
Result
Therefore total hardness = 705.888 ppm.
Example: 5
100 ml of a standard hardwater containing 1 mg of pure CaCO3 per ml
consumed 22 ml of EDTA. 100ml of sample of hard water consumed 18 ml of
EDTA. Calculate the total hardness in ppm.
Solution
We know that, 1ml of std. hard water = 1mg of CaCO3
22 ml of EDTA = 100 ml of std. hard water
Therefore 1ml of EDTA = 100/22 ml of std. hard water
= 100/22 mg of CaCO3 equivalent
Therefore 1ml of EDTA = 4.5454 mg of CaCO3 equivalent
18 ml of EDTA = 4.5454 X 18
= 81.8172 mg of CaCO3 equivalent
Therefore 100 ml of hard water contains =81.8172 mg of CaCO3
Therefore 1000ml of hard water contains = 81.8172/100 X1000
Total hardness = 818.172 ppm
Result
Therefore total hardness = 818.172 ppm.
Uses of EDTA
1. It is used to measure the total hardness of water.
2. EDTA is used in volumetric and gravimetric analysis of metal (Ca2+, Mg2+)
ions.
3. The formation of scale (CaSO4) in the boilers can be prevented by EDTA
solution.
4. Fruits, fruit juices and food stuffs are preserved by the addition of EDTA.
BOILER FEED WATER
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In industry, one of the main uses of water is generation of steam by boilers.
Water used in boilers for steam production is known as boiler feed water.
Boiler feed water should be free from dissolved salts (MgCl2, CaCl2), gases
(O2, CO2), suspended impurities (silica and oil) etc.,
Essential requirements of boiler feed water
Boiler feed water should be free from,
Hardness producing ions like Ca2+ and Mg2+ to avoid scale and sludge
formation.
Turbidity, oil and non – scaling dissolved salts to produce priming and
foaming.
Caustic alkali (NaoH) to remove caustic embrittlement and
Dissolved oxygen and CO2 in order to prevent corrosion in the boiler.
Disadvantages of using hard water in boilers (or) Boiler Troubles
Hard water when used in boiler, it leads to the following troubles.
1. Sludge and scale formation
2. Priming and foaming
3. Caustic embrittlement
4. Boiler corrosion
Sludges and scale formation:
Due to continuous evaporation of water in boilers, the concentration of
dissolved salts increases gradually and get deposited as precipitates on the inner
walls and bottom of the boiler. This precipitate is known as sludge or scale.
Sludge
If the precipitate is a soft, loose and slimy it is called sludge.
Scale
If the precipitate forms hard and adherent coating on the inner walls of the
boiler, is called scale.
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Main reasons for the boiler scale or sludge formation
The solubility product of the salt must be exceeded by the product of the
concentration of the constituent ions.
The solubility of the salt decreases with rise of temperature.
Salts responsible for Sludge formation
Salts like calcium chloride (CaCl2),
Magnesium carbonate (MgCO3),
Magnesium Chloride (MgCl2) and
Magnesium sulphate (MgSO4)
Removal of Sludge
Sludge formation can be removed by,
Frequent ‘blow down operation’
Using soft water and
Scrapping off with a wire brush.
Blow Down Operation - Definition
“Removing the bottom portion of salt concentrated water of the boiler is
known as blow down operation”.
Salts responsible for scale formation
Salts like calcium carbonate (CaCO3)
Calcium sulphate (CaSO4)
Calcium Silicate (CaSiO3) and
Magnesium hydroxide (Mg (OH)2).
Dangers of scale formation
Wastage of fuel
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Decrease in efficiency and
Danger of explosion of boiler.
Removal of Scales
Scales can be removed by applying thermal shocks.
Using Scrapers, Wire brush etc., and
Using certain chemicals.
For example, using 5-10%HCl, CaCO3 Scales can be removed and using EDTA
Solution CaSO4 scales can be removed.
PRIMING AND FOAMING
Priming
During the production of steam in the boiler, due to rapid boiling some
particles of liquid water are carried along with steam. Steam containing droplets of
water is called wet steam. The process of wet steam formation is called Priming.
Reasons for priming
Priming is due to
Some dissolved salts
High steam velocity and very high water level in the boiler
Improper design of boiler and
Sudden boiling of water etc.,
Removal of Priming
Priming can be removed by,
Controlling the velocity of steam
Maintaining medium water level
Removing oily materials present in water
Good boiler design and
Using treated water.
Foaming
The formation of stable bubbles above the surface of water is called
foaming.
Reasons for priming
Foaming is due to
The presence of oil, grease and
The presence of finely divided particles.
Removal of Foaming
Foaming can be removed by
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Adding certain anti – foaming chemicals like cotton seed oil, castor oil and
synthetic polyamides etc.,
Adding coagulants like Sodium aluminates and aluminium hydroxide etc.,
Caustic Embrittlement
Caustic Embrittlement means intercrystalline cracking of boiler metal. It is a
type of boiler corrosion, caused by using highly alkaline water in the boiler. Boiler
water usually contains small amounts of NaHCO3 and Na2CO3. In high pressure
boilers, Na2CO3 undergoes hydrolysis to produce NaOH.
∆
i) 2Na HCO3 Na2CO3 + H2O + CO2
ii) Na2CO3 + H2O 2NaOH + CO2
The NaOH thus formed flows into the minute hair cracks that are usually
present in inner side of the boiler by capillary action. As water evaporates,
its concentration increases and dissolve the iron of boiler forming Sodium
ferroate.
Fe + 2NaOH Na2FeO2 + H2
This type of electrochemical corrosion occurs when concentration of NaOH
is above 100 ppm.
This causes embrittlement of boiler parts particularly stressed parts like
bends, joints, rivets etc., causing even failure of the boiler.
Removal of Caustic embrittlement
Caustic embrittlement can be avoided by
Neutralizing the alkali with a very small quantity of acid.
Adding trisodium phosphate as softening agent for water.
Adding tannin or lignin which also blocks hair cracks.
BOILER CORROSION
Corrosion
Any process of destruction or loss of a solid metallic material, through an
unwanted chemical or electrochemical attack by its environment on the surface of
the metal is called corrosion.
Boiler corrosion
Boiler corrosion is decay of boiler material by a chemical or electro
chemical attack by its environment.
Main reasons of boiler corrosion
Boiler corrosion is mainly due to the presence of,
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Dissolved Oxygen
Dissolved Carbon-di-oxide
Acid produced by the hydrolysis of dissolved salts like MgCl2.
I. Dissolved Oxygen
Water usually contains about 8 ppm of dissolved oxygen per liter at room
temperature. The dissolved oxygen in water attacks the boiler material at high
temperatures.
2Fe + 2H2O + O2 2Fe (OH)2 ↓
Ferrous Hydroxide
4Fe (OH) 2 + O2 2[Fe2 O3.2H2O]↓
Rust
Removal of dissolved Oxygen
Dissolved Oxygen can be removed from water by two methods. They are,
1. Chemical Method
2. Mechanical Method
(i). Chemical Method
Sodium sulphite, sodium sulphide and Hydrazine are some of the chemicals
used for removing oxygen.
2Na2SO3 + O2 2Na2SO4
Na2S + 2O2 Na2SO4
N2H4 + O2 N2+2H2O
‘Hydrazine’ is an ideal internal treatment chemical for the removal of
dissolved oxygen. It results with oxygen, forming nitrogen and water. Nitrogen is
harmless.
(ii). Mechanical Method
De-aeration
Dissolved oxygen can be removed from the water by mechanical de-
aeration. In this process, water is allowed to fall slowly on the perforated plates
fitted inside the tower. The sides of the tower are heated and a vacuum pump is
also attached to it. The high temperature and low pressure produced inside the
tower considerably reduce the oxygen content of the water.
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II. Dissolved Carbon-di-oxide
a) Dissolved CO2 in water produces carbonic acid (H2CO3) which is corrosive
in nature.
CO2 + H2 O H2CO3
b) CO2 gas is also produced from decomposition of bicarbonate salts usually
present in water.
Mg (HCO3)2 MgCO3 ↓ + H2O + CO2 ↑
Removal of carbon-di-oxide
CO2 can be removed from water by two methods. They are
1. Chemical Method.
2. Mechanical Method.
Chemical method
CO2 can be removed from water by adding NH4OH (Calculated quantity)
into water.
2NH4OH + CO2 (NH4)2 CO3 + H2O
Ammonium Hydroxide Ammonium Carbonate
Mechanical Method
Dissolved Carbon-di-oxide along with oxygen can be removed by
mechanical de-aeration method.
III. Acid produced by the Hydrolysis of dissolved salts
Acids produced from salts dissolved in water are also mainly responsible for
the corrosion of boilers. Certain salts like MgCl2, CaCl2 etc., on hydrolysis at
higher temperature produce hydrochloric acid which corrodes the boiler.
Mg Cl2 +2H2O Mg (OH)2 ↓ + 2HCl
The liberated acid reacts with boiler (iron) producing rust.
Fe + 2HCl FeCl2 + H2 ↑
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FeCl2 + 2H2O Fe (OH)2 ↓ + HCl
4Fe (OH) 2 +O2 2[Fe2 O3.2H2O] ↓
Rust
Removal of Acids
Corrosion by acid (HCl) can be avoided by the addition of alkali to the
boiler water.
HCl + NaOH NaCl + H2O
Softening of water (or) Treatment of boiler feed water
Water used for industrial purposes (steam generation) should be pure, that is
it should be free from hardness, scale forming substances and corrosive agents like
dissolved oxygen, dissolved carbon dioxide, etc.,
Softening of water
The process of removing hardness producing substances from water is
known as softening of water.
Softening of water can be done in two methods. They are
I. External treatment (or) External conditioning method
II. Internal treatment (or) Internal conditioning method
External treatment of boiler feed water
In external treatment process, the hardness producing salts are removed
before feeding the water into the boiler. The external treatment process can be
done by any one of the following methods.
1. Zeolite (or) Permutit process
2. Ion exchange (or) Deionization (or) demineralization process
3. Soda lime process
The above methods of softening are not only used for boiler feed water but
also for domestic and industrial use.
Zeolite process
Zeolite
Zeolite is hydrated sodium alluminium silicate. Its chemical formula is
Na2O.Al2O3
.XSiO2.YH2O (where X = 2 – 10 and Y = 2 – 6)
It is represented as Na2Ze, which is capable of exchanging reversibly its Na
ions for hardness producing ions in water. It is also known as Permutits.
Classification
They are classified into two types
(a). Natural zeolites
(b). Synthetic zeolites
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Natural zeolites
Natural zeolites are derived from green sand. They are non – porous
zeolites.
Example
Netrolite (Na2O.Al2O3
.4SiO2.2H2O)
Synthetic zeolites
Syntetis zeolites are porous and gelly structure. It is prepared by heating
together china clay, feldspar and soda ash. These zeolites are higher exchange
capacity per unit weight than natural zeolites.
Process
(i) In this process the hard water is allowed to perculate through a bed of
sodium zeolite (Na2Ze)
(ii) The hardness causing ions (Ca2+ and Mg2+) in hard water is replaced by
loosely held sodium ions in zeolite bed.
(iii) The outgoing soft water contains sodium ions.
Reaction
Na2Ze + Ca(HCO3)2 CaZe + 2NaHCO3
Na2Ze + Mg(HCO3)2 MgZe + 2NaHCO3
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Na2Ze + CaCl2 CaZe + 2NaCl
Na2Ze + MgCl2 MgZe + 2NaCl
Na2Ze + CaSO4 CaZe + Na2SO4
Na2Ze + MgSO4 MgZe + Na2SO4
Regeneration
(iv) After the softening process, the zeolite is completely converted into
calcium and magnesium zeolites and it gets exhausted.
(v) At this stage the hard water supply is stopped and the exhausted bed is
regenerated by treating with a concentrated (10%) NaCl (brine) solution.
CaZe + 2NaCl Na2Ze + CaCl2
MgZe + 2NaCl Na2Ze + MgCl2
Exhausted zeolite regenerated zeolite
Advantages of Zeolite process
(i) It reduces hardness upto 5 ppm.
(ii) The equipment is quite compact.
(iii) It requires less time for softening.
(iv) It requires less skill for maintenance and operation.
(v) No impurities are precipitated, so there is know danger of sludge
formation.
(vi) This method is very cheap because the regenerated permutit can be used
again.
Disadvantages of zeolite process
(i) Highly turbid water can not be treated by this method.
(ii) This process removes only the cations (Ca2+ and Mg2+).
(iii) All the acidic ions like HCO3-, CO3
2-, Cl- and SO42-, etc., are not treated
by this method, which can cause corrosion.
(iv) Acidic water can not be treated because it decomposes the structure of
zeolite.
(v) Brackish water can not be treated by this method.
Demineralization (or) Deionization (or) Ion exchange process
In this process almost all the ions (both anions (Cl-, SO42-) and cations (Ca2+,
Mg2+)) present in hard water are removed. This process is also called
demineralization process.
In the demineralization process, the ions present in water are removed by ion
exchangers. Ion exchange resins are insoluble; cross - linked, long chain organic
polymers with a micro porous structure, and the “functional groups” attached to the
chains are responsible for the ion exchanging properties. They are of two types.
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(i). Cation exchangers.
(ii). Anion exchangers.
Cation exchangers
Materials capable of exchanging cations are called cation exchangers. Cation
exchanger resins containing acidic groups (-COOH,-SO3H) are capable of
exchanging their H+ ions with other cations (Ca2+, Mg2+) of hard water.
Cation exchange resin is represented as RH2 (or) RH.
Anion exchangers
Materials capable of exchanging anions are called anion exchangers. Anion
exchanger resins containing basic groups (-NH2,-OH) are capable of exchanging
their OH- ions with the other anions of hard water.
Anion exchange resin is represented as R1 (OH) 2 (or) R1OH.
Process
Water is passed through a tank having cation exchanger which absorbs all
the cations present in water.
RH2 + CaCl2 RCa + 2HCl
RH2 + MgSO4 RMg + H2SO4
The cation free water is now passed through another tank having anion
exchanger which absorbs all the anions present in water.
R1 (OH) 2 + 2HCl R1Cl2 + 2H2O
R1 (OH) 2 + H2SO4 R1SO4 + 2H2O
The water coming out of the anion exchanger is completely free from
cations and anions responsible for hardness. It is known as deionized water (or)
deminaralized water. It is as pure as distilled water.
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Regeneration
Cation exchange resins are regenerated by passing a dilute solution of HCl
through them.
RCa + 2HCl RH2 + CaCl2
Resin
Similarly, the anion exchange resins are regenerated by passing a dilute
solution of NaOH through them.
R1Cl2 + 2NaOH R1 (OH)2 + 2NaCl
Resin
Advantages of ion exchange process
(i) Highly acidic (or) alkaline water can be treated by this
process.
(ii) This produces water of very low hardness nearly 2 ppm.
Disadvantages of ion exchange process
(i) The equipment is costly and more expensive chemicals are
needed.
(ii) If water contains turbidity, then the output of the process is
reduced. The turbidity must be below 10 ppm.
Internal treatment of boiler feed water (or) Boiler compounds
Internal treatment (or) internal conditioning refers to the treatment of water
in the boiler itself. This treatment consists of adding chemicals directly to the water
in the boiler for removing dangerous scale forming salts which were not
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completely removed by the external treatment for water softening. These chemicals
convert the scale forming substances into insoluble precipitates (or) soluble
complexes. These chemicals are also called boiler compounds.
Some important internal conditioning methods are
(i) Phosphate conditioning
(ii) Carbonate conditioning
(iii) Calgon conditioning
(iv) Colloidal conditioning
Phosphate conditioning
Sodium phosphate is added to avoid scale formation in high pressure boilers.
The phosphate reacts with calcium and magnesium salts in the boiler water,
forming easily removable soft sludge of calcium and magnesium phosphates.
3CaSO4 + 2Na3PO4 Ca3 (PO4)2 + 3Na2SO4
Calcium Trisodium
sulphate phosphate
The main phosphates employed are,
1. Trisodium phosphate: - Na3PO4
2. Disodium hydrogen phosphate: - Na2HPO4
3. Sodium dihydrogn phosphate: - NaH2PO4
However, the choice of salts depends upon the nature of the water to be
treated.
Nature of water Salts used
Acid water Na3PO4
Neutral water Na2HPO4
Alkaline water NaH2PO4
Carbonate conditioning
Sodium carbonate is added to avoid scale formation in low pressure boilers.
The scale forming salt (CaSO4) is converted into calcium carbonate, which can be
removed easily.
CaSO4 + Na2CO3 CaCO3 + Na2SO4
Calgon process
‘Calgon’ is the commercial name of sodium hexameta phosphate (Na2 [Na4
(PO3) 6] ). It means ‘Calcium gone’. When Calgon is added to hard water, the
magnesium and calcium salts present in it are converted into soluble complex salts
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and soft water is produced. As these salts are soluble in water, filtration is not
required.
2CaSO4 + Na2 [Na4(PO3) 6] Na2 [Ca2 (PO3)6] + 2Na2SO4
Colloidal conditioning
In low pressure boiler, scale formation can be avoided by adding organic
substances like tannin, agar – agar, kerosene, starch, glue etc., These substances
get coated over the scale forming materials there by yielding non – sticky deposits,
which can be removed easily.
Desalination
The process of removal of extra common salt (NaCl) from the water is
known as desalination.
Depending upon the quantity of dissolved salts the water is graded as,
1. Fresh water
It contains less than (<) 1000 ppm of dissolved salts
2. Brackish water
It contains 1000 – 35,000 ppm of dissolved salts
3. Sea water
It contains greater (>) than 35,000 ppm of dissolved salts.
Brackish water
Water containing dissolved salts with a peculiar salty (or) brackish taste is
called brackish water. It is totally unfit for drinking purposes. Sea water and
brackish water can be made available as drinking water through desalination
process.
Desalination is carried out by the following methods
Reverse osmosis
Electro – dialysis
Freezing method
Distillation method
Reverse osmosis
Osmosis
When two solutions of different concentrations are separated by a semi
permeable membrane, flow of solvent takes place from the region of low
concentration to high concentration until the concentration is equal on both the
sides. This process is called osmosis. The driving force in this phenomenon is
called osmotic pressure.
Principle of reverse osmosis
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If a hydrostatic pressure in excess of osmotic pressure is applied on the
higher concentration side, the solvent flow reverse. That is solvent is forced to
move from higher concentration to lower concentration. This is the principle of
reverse osmosis.
Using this method pure water is separated from sea water. This process is
also known as super – filtration (or) hyper – filtration.
Process
In this method, pressure (15 – 40 kg/cm2) is applied to the sea water to force
its pure water out through the semi permeable membrane leaving behind the
dissolved salts. Earlier, cellulose acetate membrane was used for this purpose.
Now – a – days a number of synthetic semi permeable membranes such as poly
amide, poly sulphones, etc., are used.
Advantages
(i) It removes ionic, non – ionic, colloidal and high molecular weight
organic maters.
(ii) It also removes colloidal silica, which is not removed by
demineralization process.
(iii) The process is cheap, simple and does not require skilled labour.
(iv) The maintenance cost depends on the replacement of the semi –
permeable membrane, usually once in three years.
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Treatment of domestic water (or)
Purification of water for drinking purpose
Drinking water
Water which is safe to drink and fit for human consumption is called
drinking water. It is otherwise called potable water or municipal water.
What are the essential requirements of drinking water?
Essential requirements of drinking water
(i) It should be sparking clear and odourless.
(ii) It should be pleasant in taste.
(iii) It should be perfectly cool.
(iv) Its turbidity should not exceed 10ppm.
(v) It should be free from dissolved gases like H2S, CO2, NH3, etc.,
(vi) It should be free from minerals like lead (Pb), Arsenic (As), Chromium
(Cr) and manganese (Mn) salts.
(vii) It should be free from disease producing micro - organism.
(viii) It’s TDS (Total Dissolved Solids) is less than 500 ppm.
(ix) PH of the drinking water should be 6.5 – 8.5.
What are the various stages in the treatment of water for domestic supply
with block diagram?
Block Diagram
Source of water
The main sources of water is,
(i) Surface water
(ii) Underground water
These untreated waters are called raw water.
Source of water (Raw
water)
(Raw water)
Sterilization and
disinfection
(Raw water)
Screening
(Raw water)
Filtration
(Raw water)
Aeration
(Raw water)
Sedimentation and
coagulation
Storage and
distribution
(Raw water)
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Screening
The raw water is passed through screens having large number of
small holes, where floating matters like wood pieces, leaves etc., are
removed.
Aeration
“The process of mixing water with air is known as aeration”.
The main purpose of aeration is,
(i) Increase the content of oxygen in water and makes it fresh and
promotes taste.
(ii) Remove unwanted gases like H2S, CO2 and other volatile substances.
(iii) Salts of iron and manganese are also removed.
Sedimentation
It is the process of removing suspended impurities by allowing the water to
stand undisturbed for 2-5 hours in a big sedimentation tanks about 5 m deep. Most
of the suspended particles are settle down at the bottom due to forces of gravity
and they are removed. Sedimentation process removes only 75% of the suspended
impurities.
Coagulation
“In sedimentation process all the impurities cannot be removed. So certain
chemicals are added to fasten the sedimentation and the process is called
coagulation.”
Alum [Al2(SO4)3] and sodium aluminate (NaAlO2) are widely used in water
treatment plants. These are called coagulants.
Al2(SO4)3 + 3Ca(HCO3)2 2Al (OH)3 +3CaSO4 + 6CO2↑
Alum calcium bicarbonate Aluminium hydroxide
(Flocculant. precipitate)
NaAlO2 + 2H2O Al(OH)3 + NaOH
Sodium aluminate Aluminium hydroxide
(Gelatinous precipitate)
The gelatinous precipitate of Aluminium hydroxide settles to the bottom and
can be removed by filtration method.
*Salts of iron [(FeSO4, FeCl3)] are also used as coagulant.
FeSO4 + Mg(HCO3)2 Fe(OH)2 ↓ + MgCO3 +CO2 +SO3
Ferrous sulphate Ferrous hydroxide
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4Fe (OH) 2 ↓ + O2 + 2H2O 4Fe(OH)3 ↓
Ferric hydroxide
(Heavy floc)
Fe(OH)3 is in the form of heavy floc, which causes quick sedimentation.
Filtration
It is the process of removing colloidal matter and most of the bacteria,
micro-organisms etc, by passing water through a bed of fine sand and other proper-
sized granular materials.
Generally filtration is carried out by using sand filter.
Sterilization (or) Disinfection
The complete removal of harmful bacteria is known as sterilization. The
chemicals (or) substances used for this purpose are called disinfectants.
This process can be carried out by the following methods.
(a) Boiling method.
(b) Ozonation (By using ozone).
(c) UV Radiation method (By using UV Radiations)
(d) Chlorination method.
By adding chlorine gas (Cl2).
By adding chloramines (ClNH2).
By adding bleaching powder (CaOCl2).
Break point chlorination (or) free residual chlorination.
Boiling method
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Just boiling the water 100oC for 10 to 15 minutes, all the disease producing
bacteria are killed and water becomes safe for use.
Ozonation
Ozone (gas) is an excellent disinfectant. Ozone is produced by passing silent
electric discharge through cold and dry oxygen.
3O2 (Oxygen ) 2O3 (Ozone)
Ozone (O3) is highly unstable and decomposes to give molecular and
nascent oxygen [O].
O3 O2 + [O]
Ozone Nascent Oxygen
The nascent oxygen is highly powerful oxidizing agent and kills all the
bacteria’s and germs. It also oxidizes the organic matter present in the water.
Advantages
1. Ozone not only removes bacteria’s but also removes colour,
unpleasant taste and bad odour.
2. If present excess in water, it is not harmful, because it is unstable
and decomposes to oxygen.
Dis advantages
1. This method is expensive and cannot be employed for
municipal water works.
UV Radiation method
UV rays are produced by passing electric current through mercury vapour
lamp. This is particularly used for sterilizing swimming pool water. This process is
highly expensive.
Advantages
1. It effectively kills the majority of bacteria, viruses and other
harmful micro organisms.
2. It does not introduce any chemicals to the water and produces no bi
– products.
3. It does not alter the taste, PH or other properties of the water.
Dis advantages
1. This method requires electrical connection
2. Prefilteration is a must for effective disinfection.
Chlorination Method
The process of adding chlorine to water is called chlorination. Chlorination
can be done by the following methods.
By adding chlorine gas
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Chlorine (gas or liquid form) produce hypochlorous acid (powerful
germicide) with filtered water.
Cl2 + H2O HOCl + HCl.
Hypochlorous acid
Bacteria / germs+ HOCl Bacteria / germs are killed.
By adding chloramines (ClNH2)
When chlorine and ammonia are mixed in the ratio 2:1 compound
chloramines is formed.
Cl2 + NH3 ClNH2 + HCl
Chloramine
ClNH2 + H2O HOCl + NH3
Hypochlorous acid
Chloramine is a better disinfectant than chlorine and it gives good taste to
treated water.
By adding bleaching powder (CaOCl2)
When bleaching powder is added to water, it produces hypochlorous acid,
which is a powerful germicide.
CaOCl2 + H2O Ca(OH)2 + Cl2
Cl2 + H2O HOCl + HCl
Hypochlorous acid
HOCl + Bacteria / Germs Bacteria/ Germs are destroyed
Hypochlorous acid
Advantages
1. This method is very effective and economical.
2. Storage requires only little space.
3. It can be used at both high and low temperatures.
4. It does not produce any salt impurities in the treated water.
Disadvantages
1. Excess chlorine when added, imparts unpleasant taste and bad
odour.
2. It is effective at lower PH (below 6.5) and less effective at higher
PH (>6.5).
Break point chlorination or Free residual chlorination
What is break point chlorination (BPC)?
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It involves in addition of sufficient amount of chlorine to water in order to
oxidize organic matter, reducing substances and free ammonia; leaving behind
mainly chlorine for disinfecting disease producing bacteria.
Explanation
This involves in addition of sufficient amount of chlorine to oxidize;
(a) Organic matter
(b) Reducing substances and
(c) Free ammonia in raw water, leaving behind mainly free
chlorine, which possesses disinfecting against pathogenic bacteria.
When chlorine is added, it first kills the bacteria; further addition will appear
as residual chlorine. After a certain point, the residual chlorine suddenly decreases
with the evolution of bad smell and objectionable taste. That is, the chlorine being
used for oxidizing the organic impurities or ammonia.
After sometime, there is sudden increase in residual chlorine indicating that
oxidation is over. The addition of chlorine at the dip or break is called “break-
point” chlorination. This indicates the point at which free residual chlorine begins
to appear.
BREAK – POINT CHLORINATION CURVE
Advantages
1. It oxidizes organic compounds reducing substances and free
ammonia.
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2. It removes unwanted colour from water, bad odour and taste.
Disadvantages
1. Over – chlorination after BPC may lead to unpleasant taste and
odour in water.
Alkalinity (Acidic capacity)
Alkalinity is generally known as basicity of water. It is defined as the
measure of the ability of water to neutralize acids.
It is expressed in terms of CaCO3 equivalent of the hydrogen ions
neutralized.
Alkalinity of water is due to the presence of the following,
(a) Carbonate (CO32-), bicarbonate (HCO3
-) and hydroxides
(OH-) of Na, K, Ca, and Mg.
(b) Salts of weak acids and strong bases as
i. Borates, silicates and phosphates.
ii. Salts of acetic, propionic and hydro sulphuric acids.
(c) Salts of organic acids like humic acid.
Types of alkalinity
Depending on the type of anion present in water, alkalinity is classified into
three types. They are
(i) Hydroxide alkalinity due to (OH-) ions
(ii) Carbonate alkalinity due to (CO32-) ions
(iii) Bicarbonate alkalinity due to (HCO3-) ions.
Principle
Alkalinity in water is due to the presence of hydroxide, carbonate and
bicarbonate. There are five alkalinity conditions are possible in water. They are,
(i) OH- (Hydroxide) alkalinity only
(ii) CO32- (Carbonate) alkalinity only
(iii) HCO3- (Bicarbonate) alkalinity only
(iv) Combination of OH- and CO32- alkalinity
(v) Combination of CO32- and HCO3
- alkalinity.
The OH- and HCO3- ions cannot exist together in water because they will
react together and forms H2O and CO32-.
Alkalinity may be estimated by
(a) Potentiometric method
(b) Using PH meter and
(c) Titrimetry using different indicators.
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The various type and amount of alkalinities can be easily estimated by
titrating with standard acid using different indicators successively.
Determination of alkalinity is based on the following reactions
(i) [OH-] + [H+] H2O
(ii) [CO32-] + [H+] HCO3
-
(iii) [HCO3-] + [H+] H2CO3 H2O + CO2
Titration of water sample against standard acid up to phenolphthalein end
point, indicates, the completion of reactions (i) & (ii). This amount of acid used
thus corresponds to OH- plus one half of the normal carbonate (CO32-) present. On
the other hand titration with methyl orange as indicator indicates the completion of
all the three reactions.
From the two titre values the different alkalinities are calculated.
When, P = 0 → Bicarbonate alkalinity
P = M → Hydroxide alkalinity
P = ½ M → Carbonate alkalinity
P > ½ M → Hydroxide and carbonate alkalinity
P < ½ M → Carbonate and bicarbonate alkalinity
Estimation of Phenolphthalein alkalinity
About 20 ml of sample water is pipetted out into a clean conical flask and
one drop of phenolphthalein indicator is added. The pink colour solution is titrated
against the acid solution taken in burette. The end point is the disappearance of
pink colour. From the volume of acid required phenolphthalein alkalinity (P) is
estimated.
When P > ½ M then,
Volume of HCl required for [OH-] alkalinity = 2 [P] – [M]
= 2 x ……. – ……..
= ………. ml
Calculation of OH- alkalinity
Volume of HCl (V1) = …….. ml
Strength of HCl (N1) = 0.01 N
Volume of sample water (V2) = 20.0 ml
Strength of sample water (OH- alkalinity) (N2) = ? N
According to volumetric principle
V1N1 = V2N2
V1N1 / V2 = N2
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Strength of sample water (OH- alkalinity) (N2) = (..... X 0.01) / 20
(N2) = ………. N
Alkalinity due to OH- ion = Strength of water sample x Equivalent weight
of CaCO3 x 1000
= ………. x 50 x 1000
= …….. ppm
Estimation of methyl orange alkalinity
To the above solution, one drop of methyl orange is added. The straw yellow
solution is titrated against the acid solution taken in burette. The end point is the
colour change from straw yellow to red orange.
When P > ½ M then,
Volume of HCl required for [CO3 2-] alkalinity = 2 [M] – 2 [P]
= 2 x …… – 2 x…….
= ………. ml
Calculation of CO3 2- alkalinity
Volume of HCl (V1) = …….. ml
Strength of HCl (N1) = 0.01 N
Volume of sample water (V2) = 20.0 ml
Strength of sample water (CO3 2- alkalinity) (N2) = ? N
According to volumetric principle
V1N1 = V2N2
V1N1 / V2 = N2
Strength of sample water (CO3 2- alkalinity) (N2) = (.. X 0.01) / 20
(N2) = ………. N
Alkalinity due to CO3 2- ion = Strength of water sample x Equivalent weight
of CaCO3 x 1000
= ………. x 50 x 1000
= …….. ppm
Amount of total alkalinity in water sample = OH- alkalinity + CO3 2- alkalinity
Result
The given water sample contains,
(i) OH- alkalinity = --------- ppm
(ii) CO3 2- alkalinity = --------- ppm
(iii) Total alkalinity = --------- ppm
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Different alkalinities and titre value
Results of Phenolphthalein
end point and
phenolphthalein, methyl
orange end point
Hydroxide
alkalinity
(OH-)
Carbonate
alkalinity
(CO32-)
Bicarbonate
alkalinity
(HCO3-)
Nature of
alkalinity
present in
water
P = 0
0
0
M
Only HCO3-
ions
P = M
P
0
0
Only OH-ions
P = ½ M 0 2P 0 Only CO32-
ions
P > ½ M 2 [P] – [M]
2 [M – P] 0 OH- and CO32-
ions
P < ½ M 0 2P [M – 2P] CO32- and
HCO3- ions
Conclusion
1. When P = 0, both OH- and CO32- ions are absent and alkalinity
is due to HCO3- ions only.
2. When P = M, only OH- ion is present alkalinity due to OH-.
3. When P = ½ M, only CO32- is present, half of CO3
2-
neutralization reaction takes place with Phenolphthalein
indicator.
That is [CO32-] + [H+] HCO3
-
Complete carbonate neutralization reaction occurs when methyl
orange indicator is used.
[CO32-] + [H+] HCO3
-
[HCO3-] + [H+] H2CO3 H2O + CO2
Thus alkalinity due to CO32-
4. When P > ½ M besides CO32-, OH- ions are also
present. Now half of CO32- equal to [M – P].
So alkalinity due to CO32- = 2 [M – P]
alkalinity due to OH- = M – 2 [M – P]
= M – 2 M + 2P
= [2P – M]
5. When P < ½ M besides CO32-, HCO3
- ions are also
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Present.
Now alkalinity due to CO32- = 2P
alkalinity due to HCO3- = [M – 2P]
Significance
1. Water with high alkalinity is undesirable as far as consumers are
concerned, because highly alkaline waters are usually unpalatable.
2. A minimum amount of alkalinity (30 mg/l of CaCO3) is necessary for
effective coagulation.
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