2.2 TECHNOLOGY OF WATER
2.2.1 Introduction
Sources of water
Surface water
Underground water
2.2.2 Depletion of underground water
2.2.3 Rain water Harvesting
Water is the most essential compound for all living matter on the
earth. It plays an important role in human living and industries.
The two important sources of water are (1) surface water and (2)
underground water.
The water available on the earth's surface is called as surface water.
Surface water includes rainwater, river water, lake water and seawater.
Underground water includes water present between the rocks in the
earth crust, spring water, well water etc.
The decrease in the quantum of underground water is depletion of
water. Depletion of water is mainly caused by,
1. Modernization, industrialization and population growth
2. Global warming causing excess evaporation of surface water
3. Deforestation
4. Decrease in rainfall caused by seasonal changes and
5. Effluents from the industries spoiling the ground water source.
To meet out this depletion of ground water sources, it is essential to
find alternate plans using water management techniques to recharge the
ground water sources. One of the techniques adopted is rainwater
harvesting.
Rainwater harvesting (RWH) is collection of rainwater for useful
purposes . The methods employed are
1. Roof top harvesting
2. Open space harvesting
36
Roof top harvesting
Open space Harvesting
Types of impurities present in water
2.2.4 Types of water
2.2.5 Hardness of water
Rainwater is directly used for recharging open wells and bore wells
by directing them into it. It can also be stored in sumps or over head tanks
and used directly.
Open spaces around the buildings are used for rainwater harvesting
as follows
1. With percolation/recharge pits
2. Recharge trenches
3. Recharge wells
The recharge method used depends on the soil condition.
There are three types of impurities present in water. They are
(i) Suspended and colloidal impurities.
(ii) Dissolved salts.
(iii) Micro - organisms.
There are two types of water. They are (i) soft water and (ii) hard
water.
(I) Soft water readily gives lather with soap.
(ii) Hard water does not give lather with soap.
There are two types of hardness in water. They are:
(i) Temporary Hardness: (Carbonate hardness)
It is due to the presence of calcium bicarbonate [Ca(HCO ) ] and
magnesium bicarbonate [Mg (HCO ) ]. Temporary Hardness can be
removed by boiling.
(ii) Permanent Hardness: (Non-Carbonate hardness)
It is due to the presence of chloride and sulphate salts of calcium and
magnesium. (CaCl , CaSO , MgCl MgSO ).It cannot be removed by
boiling. Hence it is known as permanent hardness.
3 2
3 2
2 4 2, 4
37
Disadvantages of a hard water sample
2.2.6 Degree of Hardness
Units for measuring hardness
Note
Hard water cannot be used for drinking purpose.
It cannot be used for cooking purposes.
It cannot be used for bathing and washing purposes as it does not
give lather with soap.
Hard water cannot be used in laboratories as it gives unwanted
chemical reactions.
Hard water cannot be used in boilers in steam raising.
It cannot be used in sugar and paper industries.
Hard water cannot be used in textile and leather industries.
.
1.mg/litre of CaCO
2.parts per million of CaCO
Usually ,the hardness of water is expressed in terms of calcium
carbonate equivalents
The formula used to convert the mass of hardness producing salt to
mass of CaCO equivalents is given below
Molecular masses of hardness producing salts are given below.
CaSO 136
MgSO 120
CaCl 111
MgCl 95
Ca(HCO ) 162
Mg(HCO ) 146
CaCO 100
3
3
3
4
4
2
2
3 2
3 2
3
Hardness producing salt Molecular Mass
38
Mass of salt Molecular mass of CaCO3 Calcium carbonate equivalents Molecular mass of salt
Problem - 1
2.2.7 Estimation Of Hardness Of Water - EDTAMethod
PRINCIPLE
PROCEDURE
A water sample contains 48 mg of MgSO per 200ml of water.
Calculate the hardness in terms of CaCO equivalent in mg/litre of CaCO
Mass of MgSO = 48mg
Molecular mass of MgSO = 120
Mass of CaCO present in200 ml of water = 40mg
Therefore, mass of CaCO present in 1000ml of water = 200mg
Hardness of water = 200mg/litre of CaCO
EDTA method is used to determine the hardness of a sample of water.
EDTA refers to Ethylene diamine tetra acetic acid. This method is also
called as Modern method.
This is a volumetric method based on the principle of formation of
complexes. Ethylene diamine tetraacetic acid (E.D.T.A.) forms
colourless complexes with Ca and Mg ions present in water. Similarly
Eriochrome Black-T, another dye, also forms wine red coloured
complexes with Ca and Mg ions. Pure Eriochrome Black-T is blue in
colour. At the pH range of 9 to 10, the Eriochrome complexes are less
stable when compared to E.D.T.A. complexes. Thus when E.D.T.A.
solution is added to Eriochrome-Ca or Mg complexes it displaces pure
Eriochrome to form E.D.T.A-Ca or Mg complexes. Thus at the end
point E.D.T.A. frees the total Eriochrome Black-T to change the colour of
the solution from wine red to steel blue.
Eriochrome-Ca +E.D.T.A.---------> .D.T.A-Ca + Eriochrome Black-T
The burette is filled with the standard E.D.T.A. solution. A 50-ml
pipette is washed with distilled water and rinsed with the sample of hard-
4
3 3.
4
4
3
3
3
2+ 2+
2+ 2+
2+ 2+
2+ 2+
2+ 2+ E
WineRed Steel Blue
39
40mg 120
48 100
Molecular Mass of MgSO
Mass of MgSO Massof CaCO Calcium carbonate equivalents
4
4 3
water. Exactly 50 ml of hard-water is pipetted out into a conical flask and 5
ml of NH Cl - NH OH buffer solution is added. A pinch of Eriochrome
Black-T indicator is added. The colour of the conical flask solution
changes into wine red. The water sample is titrated against the E.D.T.A.
solution taken in the burette. The colour changes from wine red to steel
blue. This is the end point of the titration. The burette reading is noted.
Titrations are repeated until two consecutive values agree. From the
volume of E.D.T.A. the hardness of the sample of water is calculated.
In the estimation of hardness of water, a standard data relating
the mass of CaCO and volume of 0.01M EDTAsolution .is given below
Let,the Volume of water taken =50ml
volume of E.D.T.A. consumed =Vml
1ml of 0.01 M E.D.T.A. 1 mg of CaCO
V ml of 0.01 M E.D.T.A.= V mg of CaCO
V ml of 0.01 M E.D.T.A. 50 ml of Hard water.
Mass of CaCO present in 50 ml of Hard water = V mg
= 20 V mg
A sample of 100 ml of hard water consumes 25 ml of 0.01M EDTA
solution. Calculate the hardness of the sample of water.
25ml of 0.01M EDTAsolution 25 mg of CaCO
25ml of 0.01M EDTAsolution 100 ml of hard water
Mass of CaCO present in 100 ml of hard water = 25 mg
Mass of CaCO present in 1000ml of hard water = 250mg
Hardness of water = 250mg/litre of CaCO
4 4
3
3
3
3
3
3
3
3
Note:
CALCULATION
HARDNESS OF WATER = 20V mg /litre of CaCO
Problem- 1
≡
≡
≡
≡
Mass of CaCO present 3
3
mg 50
V 1000 Mass of CaCO present in1000ml of Hard water 3
1ml of 0.01M EDTA solution 1mg of CaCO ≡ 3
1ml of 0.01M EDTA solution 1mg of CaCO ≡ 3
40
g 100
25 10 10 Mass of CaCO present in 10 g of hard water
-3 6
6
3
To express in ppm
Mass of CaCO present in 100 ml of hard water = 25 mg
Mass of CaCO present in 100g of hard water = 25×10 g
= 250g
Hence, hardness of water = 250 ppm of CaCO
A sample of 100 ml of water consumed 12.5 ml of 0.01 M EDTA
solution. In another titration 100 ml of the same sample, after boiling for
half an hour consumed 8.2 ml of the same EDTA solution. Calculate the
carbonate and non-carbonate hardness of the sample of water.
12.5ml of 0.01M EDTAsolution 12.5 mg of CaCO
12.5ml of 0.01M EDTAsolution 100 ml of hard water
Mass of CaCO present in 100 ml of hard water = 12.5 mg
Mass of CaCO present in 1000ml of hard water = 125mg
Hence, Total hardness of water = 125 mg/litre of CaCO
8.2ml of 0.01M EDTAsolution 8.2 mg of CaCO
8.2ml of 0.01M EDTAsolution 100 ml of hard water
Mass of CaCO present in 100 ml of hard water = 8.2 mg
Mass of CaCO present in 1000ml of hard water = 82 mg
Hence, Non-carbonate hardness of water = 82 mg/litre of CaCO
Therefore, Carbonate Hardness =Total hardness – Non-carbonate
hardness
=(125 – 82) = 43 mg/litre of CaCO
3
3
3
3
3
3
3
3
3
3
3
3
-3
Problem- 2
Total hardness
Non-carbonate Hardness
≡
≡
≡
≡
41
2.2.8 Methods of softening hard water
(1) Ion exchange method
Softening Process
In this method the hard water is first passed through an acidic resin
(RH ) to remove the cations [Ca , Mg ] and then it is passed through a
basic resin [R'(OH) ] to remove the anions. Thus both types of ions are
totally removed.
Acidic resin is represented by RH .
Basic resin is represented by R'(OH)
When the hard water sample is passed through the I-Cylinder
(acidic resin) calcium and magnesium ions are replaced by hydrogen ions
of the acidic resin.
RH + Ca RCa + 2H
RH + Mg ----------> RMg + 2H
When this water is passed through the II-Cylinder (basic resin)
chloride, bicarbonate and sulphate ions are replaced by the hydroxide
ions of the basic resins.
R'(OH) + 2Cl ---------->R'Cl + 2OH¯
R'(OH) + 2HCO ¯---------> R'(HCO ) + 2OH¯
R'(OH) + SO ----------> R'SO + 2OH¯
Thus all the ions responsible for hardness are removed from
water. The H and OH ¯ ions combine together to form water.
H + OH ----------> H O
The quality of water obtained by this method is equivalent to
distilled water.
2
2
2
2.
2
2
2 2
2 3 32
24 4
2
2+ 2+
2+ +
2+ +
2–
+
+ –
---------->
Acidic resin
Acidic resin
42
Diagram
Regeneration ofAcid Resin and Basic Resin
Advantages
(2) Reverse Osmosis Method
Osmosis
:
After a long use, the acidic resin can be regenerated by the addition
of a strong solution of Hydrochloric acid.
RCa + 2HCl -----------> RH +CaCl
The basic resin after a long use, can be regenerated by the addition
of a strong solution of NaOH.
R'Cl + 2OH¯ -----------> R' (OH) + 2Cl ¯
R' (HCO ) + 2OH¯ -----------> R'(OH) + 2HCO ¯
R'SO + 2OH¯ ----------> R'(OH) + SO
1) In this method, both types of hardness are removed.
2) The quality of water obtained is equivalent to distilled water.
3) There is no wastage of water
When a semi-permeable membrane separates two solutions of
different concentrations, solvent molecules move from side to
side until the two concentrations become equal. This
process is called osmosis. The pressure gradient produced due to
osmosis is called osmotic pressure.
2 2
2 2
32 2 3
4 24
2–
dilute
concentrated
43
Hard
water
Acid
resin
Basic
resin
Soft
water
Cation exchanger Anion exchanger
Reverse Osmosis
Method
When a hydrostatic pressure greater than the osmotic pressure is
applied on the concentrated side, solvent molecules move from
concentrated side to the dilute side across the membrane. This is called
reverse osmosis. This principle is used in Reverse Osmosis plants to
soften hard water.
In this method hard water and soft water are taken in two different
chambers separated by a semi permeable membrane.
When a hydrostatic pressure greater than the osmotic pressure is
applied on the hard waterside, the water molecules move from hard
waterside to soft waterside leaving the impurities on the membrane
due to reverse osmosis.
Thus hard water is converted to soft water by Super filtration or hyper
filtration.
The semi permeable membrane is made of polysulphone or cellulose
acetate or polyamide.
Diagram
Hard water
Soft water
Pressure
Piston
Semi-permeable
membrane
44
Advantages
2.2.9 Municipal water supply
Water for Drinking purpose ( Potable water )
1) In this method ionic, non-ionic, colloidal, and organic particles are
removed from water.
2) The semi permeable membrane can be replaced and reused.
3) There is no wastage of water.
Municipal water is mainly used for drinking purposes and for
cleaning, washing and other domestic purposes. The water that is fit for
drinking purposes is called potable water
(1)Characteristics of Potable water
1.It should be colourless, odourless and tasteless.
2.It should be free from turbidity and other suspended Impurities.
3.It should be free from germs and bacteria.
4.It should not contain toxic dissolved impurities.
5. It should be moderately soft.
6. It should not be corrosive to the pipe lines.
7. It should not stain clothes.
(2)Standards of drinking water as recommended by WHO
Parameters WHO standards
pH
BOD
COD
Arsenic
Calcium
Cadmium
Chromium
Ammonia
Copper
Iron
Lead
Mercury
Magnesium
Manganese
Chloride
Cyanide
Nitrate + Nitrite
6.5 - 9.2
6
10
0.05ppm
100ppm
0.01ppm
0.05ppm
0.5ppm
1.5ppm
1.0ppm
0.001ppm
0.1ppm
150ppm
0.5ppm
250ppm
0.05ppm
45ppm
45
(3)Water quality standards in india
The three stages involved in purifying a water sample for drinking
purpose are
1. Sedimentation
2. Filtration
3. Sterilisation
Water from river or lake is taken in the big tank called sedimentation
tank. Here the insoluble matter settles down at the bottom of the tank as
sediments. In this tank the colloidal impurities are converted into
precipitate by adding Alum. The clear water from the top layer is sent to
the next tank, called Filtration tank.
In filtration tank, the suspended impurities and the microorganisms
are removed. In all types of filtration, the filter bed used is constructed as
follows.
Sedimentation
Filtration
46
Parameters Standard
pH
Total Hardness
Turbidity
Chlorides
Cyanide
Fluoride
Nitrate
Sulphate
Manganese
Mercury
Iron
Copper
Cadmiun
Chromium
Lead
Arsenic
Zinc
Magnesium
6.3 - 9.2
600 ppm
25 ppm
1000 ppm
0.05 ppm
1.5 ppm
45 ppm
400 ppm
0.5 ppm
0.001 ppm
1 ppm
1.5 ppm
0.01 ppm
0.05 ppm
0.15 ppm
0.05 ppm
15 ppm
150 ppm
The filter bed consists of a layer of fine sand, followed by a layer of
coarse sand, which is then followed, by a layer of gravel. There is a drain
at the bottom to remove the filtered water. The layer of fine sand acts as
the filtering unit and the other two beds support the fine sand layer.
Generally filtration is done due to the gravitational force. The filtered
water is then taken to the sterilization tank.
In industrial areas where large amount of drinking water is
required in short period, Pressure filters are used in which water is sent
through filter beds using external pressure.)
Sterilization is destroying of bacteria. It is done by Chlorination.
Chlorination is addition of chlorine. Chlorine is added to water in the
pH range of 6.5 to 7. When chlorine is added to water, it forms HCl and
HOCl. The hypochlorous acid enters into the living cells of bacteria and
destroy them.
H O + Cl ---------- >HCl + HOCl
Hypochlorous acid
Other sterilizing agents used are chloramines, bleaching powder
etc. The advantage of using chloramines is that it does not evaporate out
easily and can be carried over to a longer distance along with the water.
Diagram
(Note:
Sterilization
Chlorination
2 2
Drain
Drain
Water
Water from
sedimentation tank
Fire sand
Coarse sand
47
Ultra-violet rays can also be used for sterilizing purpose.
Water is used in boilers, steam engines etc., to raise steam. When a
sample of hard water is used in boiler to prepare steam, the following
problems will occur.
1. Scale formation
2. Corrosion of boiler metal
3. Caustic Embrittlement and
4. Priming and foaming.
When hard water is used in boilers to get steam, the impurities that
are present in the hard water will settle down on the sides of the boiler.
This residue in due course will adhere to the boiler vessel surface in the
form of a sludge or scale. This is called as boiler scale. The following
calcium salts are responsible for the formation of boiler scale.
CaSO , CaCO CaSiO , Ca (OH) Mg (OH) ,etc
1. The salt deposit formed is a poor conductor of heat. Therefore, fuel
is wasted in raising the temperature of the boiler.
2. Due to the increase in the temperature, the plates may melt. This
may lead to explosion of boiler.
3. At higher temperature, more oxygen may be absorbed by the boiler
metal, which causes corrosion of boiler metal.
4. The sudden spalling of the boiler scale exposes the hot metal
suddenly to super-heated steam, which causes corrosion of boiler.
The two types of methods employed to prevent scale formation are,
1. Internal conditioning method
2. External conditioning methods.
1. Internal conditioning methods involve addition of complexing
agents like Calgon to boiler feed water. Another method of internal
conditioning method is Phosphate conditioning. In this method sodium
phosphate is added to boiler feed water which forms non-sticky Calcium
2.2.10 Boiler feed water
(1) Boiler scale formation
Disadvantages of Boiler scale
4 3, 3 2, 2 .
48
and Magnesium Phosphate which can be removed by blow down
operation.
2. In external conditioning methods water is purified either by
Zeolite process or by ion-exchange method before being fed into boilers.
The impurities such as dissolved oxygen, dissolved Carbon di
oxide, mineral acids, dissolved salts of calcium and magnesium, organic
matter etc.are responsible for the corrosion of boilers.
The dissolved matter undergoes hydrolysis and forms acids. The
acid slowly attacks the inner part of the boiler.
The dissolved oxygen attacks iron at high temperature. The CO and
H O form carbonic acid (H CO ), which slowly attacks the metal.
1. By using proper water treatment procedures.
2. By degasification to remove the dissolved gases like oxygen,
CO , etc.,
3. The dissolved CO can be removed by the addition of
limewater.
4. Adding calculated amount of base could neutralize the mineral
acids.
Sometimes cracks appear inside the boiler parts, particularly at the
places, which are under stress. Metal becomes brittle at these places. It is
due to the high concentration of caustic soda (NaOH) and a little amount
of silica in water. This is called as caustic embrittlement.
Caustic soda is formed by the hydrolysis of Na CO .
Na CO + H O ----------> 2NaOH + CO
Removal of Na CO present in water can prevent caustic embrittlement.
This can be done by the following methods.
1. By adding sulphuric acid.
2. By adding CaSO and CaCl to boiler water
3. By adding Na SO
4. By adding trisodium phosphate.
(2) Corrosion of Boiler metal
Prevention of Boiler Corrosion
(3) Caustic Embrittlement:
2
2 23
2
2
2 3
232 2
2 3
4 2
2 4.
49
Foaming is nothing but the formation of foam. Bubbles of water will
enter the surface of water inside the boilers and results in the formation of
foam. Foam comes out of the boiler along with the steam. Hence the
steam becomes wet and the heat content of the steam is reduced
considerably. This type of wet steam spoils the machine parts where it is
used.
The main cause for foaming is the presence of dissolved salts in
water. Hence soft water should be used in boilers to avoid foaming.
Priming is violent and rapid boiling of water inside the boiler. Due to
priming the water particles mix up with the steam when it comes out of the
boiler. Like foaming, priming also reduces the heat content of the steam
and reduces the efficiency of the steam.
Main reasons for Priming
a) Defective design of the boiler.
b) Presence of large quantities of dissolved salts, oily matter, alkaline
and suspended matter.
1. Priming can be controlled by proper design of the boiler
2. By uniformly heating the water in the boiler.
3. By using a better sample of water.
Students have learnt about rain water harvesting, estimation of
hardness, methods of softening and bad effects of hard water in boilers.
