UNIT - I WATER TREATMENT PROCESS - Vidyarthiplus

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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.,



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.



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



(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



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.



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


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



Sodium stearate Calcium chloride Calcium stearate


2C17H35COONa + MgSO4 (C17H35COO)2 Mg ↓ + Na2SO4

Sodium stearate Magnesium sulphate Magnesium stearate


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



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)



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)



_________________________________________ × 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 =




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



Amount equivalent to CaCO3 =




We know that, the molecular weight of CaSO4 = 136


= 150 mgs/lit

Result


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


= 125 mgs/lit

Result


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


= 500 mgs/lit

Result




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



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.



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



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.


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.


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



= 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








25 ml of EDTA = 2.5 X 25


Therefore 50 ml of hard water contains = 62.5 mg of CaCO3 eq.


Total hardness = 1250 ppm


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


Permanent hardness = 900 ppm

C.Temporary hardness

Temporary hardness = Total hardness – Permanent hardness

= 1250 – 900

= 350 ppm

Result

Temporary hardness = 350 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








8 ml of EDTA = 2.0833 X 8


Therefore 25 ml of hard water contains = 16.6664 mg of CaCO3




After boiling EDTA consumed = 6 ml

1 ml of EDTA = 2.0833 mg of CaCO3 equivalent

6 ml of EDTA = 2.0833 X 6


25 ml of sample hard water = 12.4998 mg of CaCO3 equivalent


1000 ml of sample hard water = 12.4998/25 X1000



C.Temporary hardness

Temporary hardness = Total hardness – Permanent hardness

= 666.656 – 500

= 166.656 ppm

Result

Temporary hardness = 166.656 ppm



Example: 4




Solution


17 ml of EDTA = 50 ml of std. hard water






12 ml of EDTA = 2.9412 X 12





Result


Example: 5




Solution


22 ml of EDTA = 100 ml of std. hard water




18 ml of EDTA = 4.5454 X 18





Result


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



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.



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



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



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,



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.



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 ↑



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



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



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.



(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.



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



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



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



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.



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)



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



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



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



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)?



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.



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.



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



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



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



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.

Documents

UNIT - I WATER TREATMENT PROCESS - Vidyarthiplus