Vel Tech Rangarajan Dr.

Sagunthala R&D Institute of Science and Technology

UNIT-I : WATER TECHNOLOGY

Introduction:

Water is nature’s most wonderful, abundant, useful compound and is an essential without it one cannot
survive.

It occupies a unique position in industries. Its most important use is as an engineering material in the
steam generation.
Water is also used as coolant in power and chemical plants.

It is also used in other fields such as production of steel, rayon, paper, atomic energy, textiles,
chemicals, ice and for air-conditioning, drinking, bathing, sanitary, washing, irrigation, etc.

Occurrence:

Water is widely distributed in nature. It has been estimated that about 75% matter on earth’s surface
consists of water. The body of human being consists of about 60% of water. Plants, fruits and vegetables
contain 90-95% of water.

Sources of Water:
Different sources of water are:

1. Surface Waters: Rain water (purest form of natural water), River water, Lake Water, Sea water (most
impure form of natural water).

2. Underground Waters: Spring and Well water. Underground waters have high organic impurity.

Hardness of water and its causes :-

Hardness of water is a characteristic property by which water “prevents lathering of soap”.

Causes:

 The hardness of water is due to presence of certain salts (mainly bicarbonates, sulphates and
Chlorides) of Ca, Mg and few other heavy metal salts dissolved in water.

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  A sample of hard water when treated with soap (sodium or potassium salt of higher fatty acid
like oleic, palmitic or stearic) do not produce lather with soap, but on the other hand forms a
white scum or precipitate.

 The precipitate is formed due to insoluble soaps of Ca and Mg.

Chemical reactions:-

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

Sodium Stearate (Sodium soap) hardness Calcium Stearate (Insoluble)

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

Magnesium Stearate (Insoluble)

Thus the water which does not produce lather with soap solution readily is called it as HARD WATER
and water which lathers easily on shaking with soap solution is called it as SOFT WATER.

Types of hardness:

1. Temporary or Carbonate Hardness(CH):-

 It is caused by the presence of dissolved bicarbonates of Ca, Mg, other heavy metals and
carbonate of Iron.

 Temporary hardness is mostly destroyed by mere boiling of water i.e., when bicarbonates
are decomposed, they yield insoluble carbonates or hydroxides, which are deposited as
‘crust’ at the bottom of the vessel.
Chemical reactions:-
Heat
Ca(HCO3)2 → CaCO3↓ + H2O + CO2↑
(Insoluble)
Heat
Mg (HCO3)2 → Mg(OH)2↓ + 2CO2↑
(Insoluble)

2. Permanent or Non Carbonate Hardness(NCH):-

 It is due to presence of chlorides and sulphates of Ca, Mg, Fe and other heavy metals.

 It is not destroyed upon boiling.

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  It can be eliminated by different techniques like, Lime Soda process, Ion exchange
process, Zeolite process, etc.

Total Hardness= Temporary Hardness + Permanent Hardness.

Units to express the hardness:

Equivalents of CaCO3:-

 The concentrations of hardness as well as non-hardness constituting ions are usually expressed
in terms of equivalent amount of CaCO3.

 The choice of CaCO3 in particular is due to its molecular weight 100 (equivalent weight=50)
and it is the most insoluble salt that can be precipitated in water treatment.

 Hardness of the hardness causing salt in terms of CaCO3
The equivalents of CaCO3 =

Mass of hardness causing substance ×chemical equivalent of CaCO3×mol.wt of CaCO3

Chemical equivalent or molecular Wt of hardness causing substance

Mass of hardness causing substance × 50×100
=
Chemical equivalent or molecular Wt of hardness causing substance

Calculation of equivalents of CaCO3:

Multiplication factor
Chemical
Molar for converting into
Salt/ion Equivalent Or
mass equivalents of
Equivalent Weight
CaCO3
Ca(HCO3)2 162 81 100/162
Mg(HCO3)2 146 73 100/146
CaSO4 136 68 100/136
CaCl2 111 55.5 100/111
MgSO4 120 60 100/120
MgCl2 95 47.5 100/95
CaCO3 100 50 100/100
MgCO3 84 42 100/84
CO2 44 22 100/44
Ca(NO3)2 164 82 100/164
Mg(NO3)2 148 74 100/148
HCO3 - 61 61 100/122
OH- 17 17 100/34
CO3 2- 60 30 100/60
NaAlO2 82 82 100/164
Al2(SO4)3 342 57 100/114
FeSO4.7H2O 278 139 100/278
H+ 1 1 100/2
HCl 36.5 1 100/73
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Units of hardness:-
1. Parts Per Million (ppm):- is the parts of CaCO3 equivalent hardness per 106 parts of water i.e., 1ppm=
1 part of CaCO3 equivalent hardness in 106 parts of water.
2. Milligrams Per Litre (mg/L):- number of milligrams of CaCO3 equivalent hardness present per liter of
water.
1mg/L=1mg of CaCO3 equivalent hardness of 1L of water= 1kg=1000g=106mg.
∴1mg/L=1mg of CaCO3 eq per 106 mg of water=1ppm.
3. Clarke’s degree (0Cl):- the no. of grains (1/7000lb) of CaCO3 equivalent hardness per gallon (10lb) of
water or it is parts of CaCO3 equivalent hardness per 70,000 parts of water.
∴1 0Cl= 1 grain of CaCO3 eq hardness per gallon of water
= 1 part of CaCO3 hardness eq per 105 parts of water.
4. Degree French (oFr):- parts of CaCO3 equivalent hardness per 105 parts of
water. ∴ 10 Fr= 1 part of CaCO3 equivalent hardness per 105 parts of
water.

5. Milli-equivalent per liter (meq/L):- is the number of milli-equivalents of hardness present per
liter. 1meq/L= 1meq of CaCO3 per liter of water

= 10-3x 50 g of CaCO3 eq. per liter

= 50 mg of CaCO3 eq. per liter = 50 mg/L of CaCO3 eq. = 50ppm.

Relationship between various units of hardness:

1 ppm = 1 mg/L = 0.1o Fr = 0.07 oCl = 0.02 meq/L

1 oCl = 1.433 oFr = 14.3 ppm = 14.3 mg/L = 0.0286meq/L
1 oFr = 10 ppm = 10 mg/L = 0.7 oCl = 0.2 meq/L

1 meq/L = 50 mg/L = 50 ppm = 5 oFr = 0.35 oCl

Estimation of hardness of water : EDTA method
Estimation of temporary and permanent hardness:-

a) EDTA(Ethylene Di amine Tetra Acetic acid) Method:-

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Principle:

This is a complexometric method. It is in the form of its sodium salt which yields the anion and
this forms complex with Ca+2 and Mg+2 ions.
(Molecular Wt. - 372.24, Equivalent Wt. - 186.14 i.e., M=2N)

In order to determine the equivalence point (i.e., just completion of metal-EDTA complex formation)
indicator Eriochrome Black-T (EBT) an alcoholic solution of blue dye is employed which forms an
unstable wine red complex with Ca+2 and Mg+2 ions. The indicator is effective at about pH 10.
When EBT is added to hard water, buffered to a pH of about 10 (employing NH4OH-NH4Cl buffer), a
wine red unstable complex is formed. Thus,
pH =10
2+
M + EBT [M-EBT] complex
(M+2= Ca+2 or Mg+2) (Unstable wine red)
During the course of titration against EDTA solution, EDTA combines with M+2 (or Ca+2 or Mg+2)
ions from stable complex M-EDTA and releasing free EBT, which instantaneously combines with M+2
ions still present in the solution, thereby wine red color is retained. Thus, titration

[M-EBT] complex + EDTA [M-EDTA] complex + EBT

Wine red (stable complex) (blue)

M+2 + EBT [M-EBT]
(Ca+2 or Mg+2 still present) (blue) wine red complex

When nearly all M+2 (Ca+2 or Mg+2) ions have formed [M-EDTA] complex, then next drop of EDTA
added drop wise displace the EBT indicator from [M-EBT] complex and wine red color changes to blue
color (due to EBT).

Thus at equivalence point,

[M-EBT] complex + EDTA [M-EDTA] complex + EBT

(blue)
Thus, change of wine red to blue color marks the end point of titration.

Steps involved:
1. Preparation of Standard Hard Water: Dissolve 1gm of pure dry CaCO3 in minimum quantity of dil.
HCl and then evaporate the solution to dryness on water bath. Dissolve the residue n distilled water to
make 1L solution. Each 1ml of this solution contains 1mg of CaCO3 hardness.

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2. Standardization of EDTA solution: Rinse and fill the burette with EDTA solution. Pipette out 50ml
of standard hard water in a conical flask. Add 10-15ml of buffer solution and 4 drops of indicator.
Titrate with EDTA solution till wine red color changes to clear blue. Let the volume used be V1ml.

3. Titration of Unknown Hard Water: Titrate 50ml of water sample just in step5. Let the volume used
be V2ml.

4. Titration of Permanent Hardness: Take 250ml of water sample in a large beaker. Boil till the volume
is reduced to about 50ml (all the bicarbonates are decomposed into insoluble CaCO 3+Mg(OH)2).
Filter, wash the precipitate with distilled water collecting filtrate and washings in a 250 ml measuring
flask. Finally make up the volume to 250ml with distilled water. Then titrate 50ml of boiled water
sample just as in step 5. Let the volume used be V3ml.

Calculations:
50 ml of standard hard water = V1 ml of EDTA.
∴ 50 x 1mg of CaCO3 = V1 ml of EDTA
∴ 1ml of EDTA = 50/V1 mg of CaCO3 eq.
50 ml of given hard water = V2 ml of
EDTA.

= mg of CaCO3 eq

∴ 1L (1000ml) of given hard water = mg of CaCO3 eq.

Total Hardness of water = 1000 V2/V1 mg/l
= 1000 V2/V1 ppm.

Now 50ml of boiled water = V3 ml of EDTA.

= mg of CaCO3 eq.
∴ 1L(1000ml) of boiled water = 1000V3/V1 mg of CaCO3 eq
∴ Permanent hardness = 1000V3/V1 ppm.
Temporary Hardness= total hardness- permanent hardness

= - = ppm

Advantages of EDTA method:

This method is preferable because of 1) greater accuracy,

2) Convenience,

3) More rapid procedure.

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Numerical Problems based on hardness of water:-
1. Calculate the temporary and permanent hardness of water sample containing Mg(HCO3)2=
7.3mg/L, Ca(HCO3)2= 16.2mg/L, MgCl2= 9.5mg/L, CaSO4=13.6mg/L).
Solution: conversion into CaCO3 equivalents:

Constituent Multiplication factor CaCO3 equivalent

Mg(HCO3)2= 7.3mg/L 100/146 7.3X100/146= 5mg/L

Ca(HCO3)2= 16.2mg/L 100/162 16.2X100/162=10mg/L
MgCl2= 9.5mg/L 100/95 9.5X100/95= 10mg/L
CaSO4=13.6mg/L 100/136 13.6X100/136= 10mg/L
∴ Temporary hardness of water due to Mg(HCO3)2 and Ca(HCO3)2 =

=5+10=15mg/L or 15ppm.
Permanent hardness due to MgCl2 and CaSO4 = 10+10=20mg/L or 20ppm.

2. Calculate the temporary and total hardness of a water sample containing Mg(HCO3)2= 73mg/L,
Ca(HCO3)2= 162mg/L, MgCl2= 95mg/L, CaSO4=136mg/L.
Solution: calculation of CaCO3 equivalents:

Constituent Multiplication factor CaCO3 equivalent

Mg(HCO3)2= 73mg/L 100/146 73X100/146= 50mg/L

Ca(HCO3)2= 162mg/L 100/162 162X100/162=100mg/L
MgCl2= 95mg/L 100/95 95X100/95= 100mg/L
CaSO4=136mg/L 100/136 136X100/136= 100mg/L
∴ Temporary hardness of water due to Mg(HCO3)2 and Ca(HCO3)2 =

=100 + 50=150mg/L or ppm.

Total hardness of water= 50+100+100+100=350 mg/L or ppm.

3. 50ml of a sample water consumed 15ml of 0.01 EDTA before boiling and 5ml of the same
EDTA after boiling. Calculate the degree of hardness, permanent hardness and temporary
hardness.
Solution: 50ml of water sample = 15ml of 0.01M EDTA
= 15x100 ml of 0.01EDTA=300ml of 0.01M EDTA
50

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 = 2x300ml of 0.01 N EDTA (Molarity of EDTA=2xNormality of EDTA)
= 600 ml or 0.6 L of 0.01 eq. of CaCO3.
= 0.6x0.01x50 gCaCO3 eq.

∴1000ml of boiled water= 5x1000 ml of 0.01M EDTA 50

= 100mL of 0.01M EDTA

= 200mLor 0.2 L of 0.01N EDTA

= 0.2x0.01x50g of CaCO3 eq

= 0.1g or 100mg of CaCO3 eq

Hence permanent hardness = 100mg/L or ppm

∴Temporary hardness= 300-100=200ppm.

4. 0.5g of CaCO3 was dissolved in HCl and the solution made up to 500ml with distilled water.
50ml of the solution required 48ml of EDTA solution for titration. 50ml of hard water sample
required 15ml of EDTA and after boiling and filtering required 10ml of EDTA solution.
Calculate the hardness.
Solution: 500ml of SHW = 0.5g or 500mg CaCO3 eq ∴1ml SHW= 1mg CaCO3 eq

Now 48ml of EDTA solution = 50/48 mg CaCO3 eq

∴1ml of EDTA solution= 50/48 mg CaCO3 eq

Calculation of the total hardness of water:

50ml hard water = 15ml EDTA = 15x50/48 mg of CaCO3 eq ∴1000ml of hard water= 15.625x1000 =
312.5 mg CaCO3 eq 50

Hence total hardness= 312.5mg/L or 312.5 ppm.

5Q. Why is hard water harmful to boilers.

Boiler feed water:

A boiler is a closed vessel in which water under pressure is transformed into steam by the application of
heat.

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Water is mainly used in boilers for the generation of steam( for industries and power houses).

A boiler feed water should correspond to the following composition:

1. Its hardness should be below 0.2 ppm.

2. Its caustic alkalinity (due to OH-) should lie in between 0.15ppm to 0.45ppm.
3. Its soda alkalinity (due to Na2CO3) should lie in between 0.45ppm to

1ppm. Essential requirements of boiler feed water:

1. Hardness causing and scale forming constituents like salts of calcium and magnesium. This is
because scale formation would result in wastage of fuel, lowering of boiler safety, decrease in efficiency
and danger of explosion.

2. Caustic alkali to avoid caustic embrittlement.

3. Dissolved gases like oxygen, carbon dioxide etc. in order to prevent boiler corrosion.

4. Turbidity, oil etc. to reduce the tendency for priming and foaming.
Hence water must be softened before use in boilers.

BOILER TROUBLES:

6Q. What are Scales and sludge’s? How are they formed? What are the disadvantages and what
are the methods of prevention of scale formation?
Scale and sludge formation:

 In boilers, water evaporates continuously and the concentration of the dissolved salts increase
progressively.

 When the dissolved salts concentration increases and reaches saturation point, they are thrown
out of water in the form of precipitates on the inner wall of the boiler.

 If the precipitation takes place in the form of loose and slimy precipitate it is called sludge.

 If the precipitated matter forms a hard adhering crust or coating on the inner walls of the boiler,
it is called scale.

SLUDGE: It is a soft, loose and slimy precipitate formed within the boiler.

Reasons:

It is formed at comparatively colder portions of the boiler and collects where the flow of water rate is
slow or at bends.

Sludge’s are formed by substances which have greater solubility’s in hot water than in cold water.
Eg; MgCO3, MgCl2, CaCl2, MgSO4, etc.

9
 Disadvantages:

Sludge’s are poor conductor of heat which tends to waste a portion of heat generated.

If sludge’s are formed along with scales, then sludge gets entrapped in the latter and both get
deposited as scales.

Sludge formation disturbs the working of the boiler as it settles in the region of poor water
circulation such as pipe connection; plug opening, guage-glass connection thereby causing even
choking of the pipes.

Prevention:
By using well softened water

By frequent “Blow Down” operation i.e., drawing off a portion of the concentrated water.

Sludge Scale

Heat Heat

SCALES: are hard deposits which stick firmly to the inner surfaces of the boiler. These are difficult to
remove.

Formation of Scales:
1. Decomposition of Ca(HCO3)2:-

Ca(HCO3)2 CaCO3↓ + H2O+ CO2

The formation of CaCO3 scale is soft and is the main cause of scale formation in low
pressure boiler. In high pressure boilers, CaCO3 is soluble.
CaCO3+H2O Ca(OH)2 + CO2↑

(Soluble)
2. Deposition of CaSO4:-
The solubility of CaSO4 in water decreases with rise in temperature i.e., CaSO4 is soluble in cold
water but completely insoluble in super-heated water. CaSO4 gets precipitated as hard scale on the
heated portions of the boiler. This is the main cause of scales in high pressure boilers.

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 3. Hydrolysis of Magnesium Salts:-
The Mg salts dissolved undergo hydrolysis at high temperature forming Mg(OH)2 precipitate
which forms a soft type of scale.
MgCl2 + 2H2O → Mg(OH)2↓ + 2HCl↑

(scale)

4 Presence of Silica:-
Silica present in small quantities deposits as calcium silicate (CaSiO3) or MgSiO3. These deposits stick
very firmly and are very difficult to remove. Important source of silica in water is the sand filter.
Disadvantages:

1. Wastage of fuel: Scales have a low thermal conductivity so rate of heat transfer from boiler to
inside water is greatly decreased due to which excessive or over heating is done and this causes
increase in fuel consumption.
The wastage of fuel depends upon the thickness and nature of scale as:

Thickness (mm)) 0.325 0.625 1.25 2.5 12

Wastage of fuel 10% 15% 50% 80% 150%

2. Lowering of boiler safety: the overheating of boiler tube makes boiler softer and weaker and this
causes distortion of boiler tube and makes the boiler unsafe to bear the pressure of steam especially
in high pressure boilers.

3. Decrease in Efficiency: scales may deposit in the valves and condensers of the boilers and choke
them partially. This results in decrease in efficiency of the boiler.

4. Danger of explosion: the thick scales formed if cracked due to uneven expansion the water
comes suddenly in contact with overheated iron plates. This causes infiltration of a large amount of
steam suddenly, so sudden high pressure is developed which causes explosion of boiler.

Removal of Scales:-

If scales are loosely adhering they can be removed with the help of scrapper or wire brush or piece
of wood.

By giving thermal shocks (i.e., heating the boiler and then suddenly cooling with cold water) if they
are brittle.

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 If the scales are hard and adhering, then dissolving them and by adding chemicals. For Eg.
CaCO3 scales are dissolved by using 5-10% of HCl and CaSO4 scales can be dissolved by
adding EDTA with which they form soluble complexes.
By frequent blow-down operation if scales are loosely adhering.
Prevention of scales:

1) External Treatment: includes efficient ‘softening of water’ (i.e. removing hardness producing
constituents of water) which is discussed separately.

2) Internal Treatment: includes softening of water by adding a proper chemical to the boiler
water. It is discussed in softening methods.

7Q. Distinguish between scales and sludges.

S.No Sludge Scale

1. They are soft, loose and slimy precipitate. They form hard deposits.
They form non-adherent deposits and can They stick firmly to the inner surface of the
2.
be easily removed. boiler and are very difficult to remove.

3. They are formed by substances like They are formed by substances like CaSO4,
CaCl2, MgCl2, MgSO4, MgCO3, etc. Mg(OH)2, etc.
They are formed at comparatively colder They are formed at heated portions of the
4.
portions of the boiler. boiler.
They decrease efficiency of boiler but are They decrease efficiency of boiler and chances
5.
less dangerous. of explosion are also there.
They can be removed by blow down They can’t be removed by blow down
6.
operation. operation.

8Q. Write briefly on caustic embrittlement.

Caustic embrittlement:-

 It is a type of boiler corrosion caused by using highly alkaline water in the boiler.

 During softening process by Lime-Soda process, free Na2CO3 is usually present in small
proportion.

 In high pressure boilers, Na2CO3 decomposes to give NaOH and CO2 and this makes the boiler
water ‘caustic’.
Na2CO3 + H2O → 2NaOH + CO2

 The NaOH containing water flows into minute hair cracks always present in inner side of boiler
by capillary action.

 Here water evaporates and the dissolved caustic soda concentration increases progressively.
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  This caustic soda attacks the surrounding area, thereby dissolving iron of the boiler as sodium
ferroate. This causes embrittlement of boiler parts (like bends, joints, etc.,) causing even failure
of the boiler.

 Caustic cracking can be explained by considering the following concentration cell:
- +
Iron at rivets, concentrated Dilute Iron

bends, joints, NaOH solution NaOH at plane

etc. solution surfaces

 The iron surrounded by dil. NaOH becomes the cathodic side; while the iron in contact with con.
NaOH becomes anodic part; which is consequently dissolved or corroded.

Prevention of caustic embrittlement:

Caustic embrittlement can be avoided:
i) By using Na3PO4 as softening reagent instead of Na2CO3.

ii) By adding tannin or lignin to boiler water since these block the hair cracks there by
preventing infiltration of caustic soda solution in these.
iii) By adding Na2SO4 to boiler water. Na2SO4 is added to boiler water so that the ratio

is kept as 1:1:2:1 and 3:1 in boilers working respectively at

pressures up to 10, 20 and 30 atmospheres.
Disadvantages of caustic embrittlement:

The cracking or weakening of the boiler metal causes failure of the

boiler. 9Q. Write a short notes on carry over or priming and foaming.
Carry over or priming and foaming:
Priming:

When a boiler is steaming (i.e., producing steam) rapidly, some particles of the liquid water are carried
along with the steam. This process of ‘wet steam’ formation is known as ‘Priming’.

Causes:
Priming is caused by:
1. The presence of large amount of dissolved salts

2. High steam velocities

3. Sudden boiling

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4. Improper boiler design

5. Sudden increase in steam production rate.

Prevention:
1. By fitting mechanical purifiers
2. Avoiding rapid change in steam rate

3. Maintaining low water levels in boilers

4. Effective softening and filtration of boiler feed water

5. Blow down of the boiler.

Foaming:

The phenomenon of formation of persistent foam or bubbles on the surface of water inside the boiler,
which do not break easily.

Causes:

1. Pure water has very little foaming whereas the water containing dissolved impurities and suspended
matter has a greater tendency to produce foam.

2. The presence of large quantity of suspended impurities and oils lowers the surface tension producing
foam.

Prevention:
1. Adding antifoaming chemicals like castor oil.

2. Besides castor oil, Gallic acid and tannic acids, corn oil, cotton seed oil, sperm oil, bees wax, etc., are
also used as antifoaming agents.
3. Blow down operation of the boiler can prevent foaming.
4. Removing oil form boiler water by adding compounds like sodium aluminate.

Disadvantages of Priming and Foaming:

Priming and Foaming usually occur together. They are objectionable because:-

1. The dissolved salts in boiler water are carried by the wet steam to super heater and turbine blades
where they get deposited and reduce the efficiency.

2. The dissolved salts enter the parts of other machinery where steam is used thereby decreasing the life
of machinery.

3. The actual height of the water column cannot be judged thereby making the maintenance of boiler
pressure becomes difficult.

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10Q. Give a detailed account on boiler corrosion Or Write a note on mechanical deaeration.

Boiler corrosion:

 The decay of boiler material by chemical or electrochemical attack by its environment is

known as “Boiler corrosion”.

 The main reasons for boiler corrosion are:-
Dissolved Oxygen: water contains usually about 8ml of dissolved oxygen per liter at room temperature.

Disadvantages of dissolved oxygen:

In presence of prevailing high temperature, the dissolved Oxygen attacks boiler material as follows:
2Fe+ 2H2O+ O2 2Fe(OH)2↓
4Fe(OH)2↓ + O2 2[Fe2O3.2H2O]↓ (rust)

Removal of dissolved Oxygen:

1) By adding calculated quantity of Sodium sulphite (Na2SO3) or hydrazine(N2H4) or sodium
sulphide (Na2S).

2Na2SO3 + O2 2 Na2SO4

N2H4+ O2 N2+ 2H2O

Na2S + 2O2 Na2SO4

Hydrazine is an ideal internal treatment chemical for the removal of dissolved oxygen.

Azamina 8001-RD (a polyvalent organic compound) has been employed for degassing water in
minimum time.

2) By Mechanical de-aeration i.e., spraying water through a perforated plate fitted in

degasification tower, heated from sides and connected to a vacuum pump as shown in the
figure.

15
 High temperature, low pressure and large exposed surface (provided by perforated plates)
reduce the dissolved oxygen in water.

3) Dissolved Carbon dioxide: has a slow corrosive effect on the materials of boiler plate.
Under the high temperature and pressure the bicarbonates decompose to produce CO2.

Mg(HCO3)2 MgCO3 + H2O + CO2↑

Disadvantages of CO2:

It has slow corrosive effect on boiler plates by producing the carbonic acid.
CO2 + H2O H2CO3

Removal of CO2:

By adding calculated amount of ammonia.

2NH4OH + CO2 → (NH4)2CO3 + H2O

By mechanical deaeration process along with oxygen.

Acids from dissolved salts: water containing dissolved Mg salts liberate acids on hydrolysis.
Ex: MgCl2 + 2H2O → Mg(OH)2 ↓ + 2HCl

Disadvantages of acids:

The acids react with the iron of boiler plate in a chain reaction producing HCl again and
again.
Fe+2HCl → FeCl2 + H2↑

FeCl2+2H2O→ Fe(OH)2↓+2HCl
Presence of even a small amount of MgCl2 will have corrosion of iron to a large extent.

Prevention of acid corrosion:

1) By softening of boiler water to remove MgCl2 from the water.

2) By frequent blow down operation.

3) Addition of inhibitors to form a thin film on the surface of the boiler protecting the metal
from attack. Sodium silicates, sodium phosphates and sodium chromate acts as good
corrosion inhibitors.

11Q. Write short notes on phosphate conditioning Or Write short notes on the following: (a)
Calgon conditioning (b) Carbonate conditioning (c) phosphate conditioning Or Compare
Phosphate conditioning with calgon conditioning.

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Softeninig methods:-

1) Internal treatment
2) External treatment

Water used for industrial purposes should be sufficiently pure. It should therefore be freed from
hardness producing salts before use.

The process of removing hardness producing salts from water is known as ‘Softening’ of water.

1) Internal treatment: (sequesteration)

 In this process, an ion is prohibited to exhibit its original character by complexing or

converting into other more soluble salt by adding appropriate agent.

 An internal treatment is accomplished by adding a proper chemical to the boiler water
either:

 i) To precipitate the scale forming impurities in the form of sludges which can be removed
by blow-down operation or

 ii) To convert them into compounds which will stay in the dissolved form in water, and
thus do not cause harm.

 The important internal conditioning /treatment methods are:

1) Phosphate Conditioning:

In high pressure boilers, scale formation can be avoided by adding sodium phosphate, which reacts with
hardness of water, forming non-adherent and easily removable, soft sludge of Ca and Mg phosphates
which can be removed by blow-down operation.
Eg. 3CaCl2+2Na3PO4 Ca3(PO4)2↓ +6NaCl

The main phosphates employed are

1. NaH2PO4: sodium dihydrogen phosphate(acidic)
2. Na2HPO4: disodium hydrogen phosphate(weakly alkaline)

3. Na3PO4: trisodium phosphate(alkaline)

The choice of salt depends upon the alkalinity of boiler feed water.

2) Carbonate Conditioning:

In low pressure boilers, scale formation can be avoided by adding Sodium carbonate to boiler water,
when CaSO4 is converted to CaCO3 in equilibrium.
CaSO4 + Na2CO3 CaCO3 + Na2SO4

3) Calgon Conditioning:
Involves adding calgon (sodium hexa meta phosphate (NaPO3)6) to boiler water.
17
It prevents the scale and sludge formation by forming soluble complex compound with CaSO4.

Na2[Na4(PO3)6] 2Na+ + [Na4P6O18]-2

(Sodium hexa meta phosphate)

2CaSO4 + [Na4P6O18]-2 → [Ca2P6O18]-2 + 2Na2SO4

(Soluble complex ion)

2) External treatment of water:

14Q. Write short notes on Ion-exchange process. Or What are ion-exchange resins? How will you
purify water by using them? Or What are the advantages of this method over other methods?

Ion exchange/deionization/demineralization:-

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 ion exchanging properties.
Resins containing acidic functional groups (-COOH, -SO3H) are capable of exchanging their cations;
those containing basic functional groups (-NH2=NH-) as HCl are capable of exchanging their anions
with other anions, which comes in their contact.
The ion exchange resins may be classified as :-

1) Cation Exchange Resins (RH+):- are mainly styrene-divinyl benzene co-polymers

which on sulphonation or carboxylation become capable to exchange their H+ ions with cations
in the water.
-
2) Anion Exchange Resins (R’OH ):- are styrene-divinyl benzene and amine
formaldehyde copolymers which contain amino or quaternary ammonium or quaternary
phosphonium or tertiary sulphonium groups as an integral part of resin matrix. These, after
treatment with dil NaOH solution, become capable to exchange their OH- anions with anions in
water.
Process:-

Raw water is first passed through cation exchanger and the removal of cations take place like Ca+2,
Mg+2 etc takes place and equivalent amount of H+ ions are released from this column to water. Thus,
2RH+ + Ca+2 → R2Ca+2 + 2H+

2RH+ +Mg+2 → R2Mg+2 + 2H+

After cation exchange column the hard water is passed through anion exchange column, which
removes all the anions like SO42-, Cl- etc present in the water and equivalent amount of OH- ions are
released from this column to water.

R’OH- + Cl- → R’Cl- + OH-

2 R’OH- + SO42- → R’SO42- + 2OH-

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 2 R’OH- + CO32-→ R’CO32- + 2OH-

H+ and OH- ions (released from cation exchange and anion exchange columns respectively) get
combined to produce water molecule.
H+ + OH- → H2O

Thus, water coming out from the exchanger is free from cations as well as anions. Ion-free water is
known as deionized or demineralized water.

Regeneration:

After the deionization of certain amount of raw water the cation and anion exchangers will be
exhausted.
Regeneration of cation exchanger is carried out by passing dil. HCl or H2SO4 solution into the
bed. R2Ca+2 + 2H+ → 2RH+ + Ca+2 (washing)

The column is washed with deionized water and the washings (Ca+2, Mg+2 etc and Cl- or SO42-) is
passed to sink or drain.
The exhausted anion exchange column is regenerated by passing a solution of dil.
NaOH. R2 SO42- + 2 OH- → 2R’OH- + SO42- (washing)

The column is washed with deionized water and washings containing Na+ and SO42- or Cl- ions is
passed to sink or drain.
The regenerated ion exchange resins are then used again.

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Advantages:
1) Highly acidic or alkaline water samples can be purified by this process.
2) The hardness possessed by the deionised water is 2ppm.

3) The deionised water is most suitable for high pressure boilers.

Disadvantages:

1) The ion exchanging resins are expensive hence the cost of purification is high.

2) Raw water should contain turbidity below 10ppm. Otherwise pores in the resin will be
blocked and output of the process is reduced.

12Q. Write the characteristics of potable water and what are the steps involved in treatment of
municipal water?
Standards required for Potable water:

Potable water is generally obtained from rivers and lakes. Water obtained from these sources are never
pure and as such it is unfit for drinking.

Characteristics:

1. It should be safe.

2. It should be clear and odour less.

3. It should be pleasant in taste.

4. It should be perfectly cool.

5. Its turbidity should not exceed 10 ppm.
6. It should be free from objectional dissolved gases like H2S.

7. It should be free from minerals such as lead, arsenic, chromium, manganese salts.

8. Its alkalinity should not be high.

9. It should be reasonably soft.

10. Its total dissolved solids should be less than 500ppm.

11. It should be free from disease producing microorganisms.

Municipal Water Treatment:

The type of treatment is given to water largely depends upon the quality of raw water and also upon
specified standards.
In general water treatment for municipal supply or domestic use consists of following stages:

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1. Removal of suspended impurities:

a) Screening: the raw water is passed through screens having large no. of holes where floating materials
are retained.

b) Sedimentation: is a process of allowing water to stand undisturbed in big tanks about 5m deep when
most of suspended particles settle down at the bottom due to force of gravity.

c) Coagulation: sedimentation with coagulation is the process of removing fine suspended and colloidal
impurities by addition of coagulants (chemicals) to water before sedimentation.

Coagulant when added to water form insoluble gelatinous, flocculant precipitate which descent through
the water, adsorbs and entangles very fine suspended impurities forming bigger flocs, which settle down
easily.
Eg. Alum [K2SO4. Al2(SO4)3.24H2O], NaAlO2, FeSO4.7H2O.

Al2 (SO4)3+Ca (HCO3)2 2Al (OH)3 +3CaSO4 +6CO2.

NaAlO2 + 2H2O Al (OH)3 + NaOH
The aluminium hydroxide floc causes sedimentation. The sodium hydroxide thus produced, precipitates
magnesium salts such as Mg(OH)2.
MgSO4 + 2 NaOH Mg (OH)2 + Na2SO4

d) Filtration: the process of removing colloidal matter and most of the bacteria, microorganism by
passing water through a bed of fine sand and other proper sized granular materials.

A sand filter consists of a thick top layer of fine sand placed over coarse sand layer and gravels. It is
provided with an inlet for water and an under drain channel at the bottom for exit of filtered water.
Sedimented water entering the sand filter is uniformly distributed over the entire fine sand bed. During
filtration, the sand pores get clogged, due to retention of impurities in the pores.

When the rate of filtration become slow the working of filter is stopped and about 2-3cm of the top fine
sand layer is scrapped off and replaced with clean sand and the filter is put back into use again. The
scrapped sand is washed with water, dried and stored for reuse at the time of next scrapping operation.

2. Removal of microorganisms:

Water used particularly for drinking purpose must be freed from diseases producing bacteria,
microorganism etc from the water and making it safe for use, is called disinfection.
The chemicals or substances which are added to water for killing the bacteria are known as disinfectants.

a) Boiling: water boiled for 10-15 minutes kills all the disease producing bacteria.

b) Adding bleaching powder: 1 kg of bleaching powder per 1000 kilo litres of water is mixed and water
is allowed to stand undisturbed for several hours.

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The chemical action produces hypochlorous acid, a powerful germicide.
CaOCl2+H2O Ca(OH)2+ Cl2
Cl2+H2O HCl + HOCl

Germs + HOCl germs are killed.

The disinfecting action of bleaching powder is due to the chlorine made available by it.

Drawbacks:
Bleaching powder introduces calcium in water which makes it harder.

Bleaching powder deteriorates due to its continuous decomposition during storage.

Excess of bleaching powder gives bad taste and smell to treated water.

c) By chlorination: chlorine (either in gas or in concentrated solution form) produces hypochlorous acid
which is a powerful germicide.
Cl2 + H2O HOCl + HCl

Bacteria + HOCl bacteria are killed.

Liquid Cl2 is most effective when applied to filtered water at such a point where adequate mixing is
done.
Cl2 is the most widely used disinfectant throughout the world.

Mechanism of action: Gleen and Stumpt after long experimentation, reported that the death of
microorganisms, bacteria etc., result from chemical action of hypochlorous acid with the enzymes in the
cells of the organisms, etc. since enzyme is essential for the metabolic processes of the microorganisms,
so death of microorganisms results due to inactivation of enzyme by hypochlorous acid, producing OCl -
which cannot combine with enzymes in the cells of microorganisms.

HOCl H+ + OCl-
This explains the fact that chlorine is found to be more effective disinfectant at lower pH values (below
6.5). This is due to the fact that HOCl is about 80 times more destructive to bacteria than OCl-.
The apparatus used for this purpose is known as chlorinator.
Chlorinator: it is a high tower having a no. of baffle plates. Water and proper quantity of conc. Cl 2
treated water is taken out from the bottom. For filtered water about 0.3-0.5 ppm of Cl2 is sufficient.

Factors affecting efficiency of chlorine:

1. Time of contact: it has been experimentally shown that no. of microorganism destroyed by Cl2 per
unit time is proportion al to the no. of microorganisms remaining alive. Consequently the death rate is
maximum to start with and it goes on decreasing with the time.
2. Temperature of water: the rate of reaction with enzymes increase with temperature. Consequently
death rate of microorganism by Cl2 increases with rise in water temperature.
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3. pH: at lower pH values (5-6.5) a small contact period is required to kill same percentage of organisms.

Advantages of chlorine: It is effective and economical, requires little space for storage, stable, used at
low and high temperatures and an ideal disinfectant.

Disadvantages: Excess of chlorine if added produces a characteristic unpleasant taste and odour.
It causes irritation on mucus membrane.

The quantity of free chlorine in treated water should not exceed 0.1-0.2 ppm.
It is more effective below 6.5 pH and less effective at higher pH values.

13Q. Write a short notes on break point chlorination.

Break point Chlorination:

The process of applying calculated amounts of chlorine to water in order to kill the pathogenic bacteria
is called chlorination.

Chlorine also reacts with water and generates hypochlorous acid, which kills bacteria.
Cl2 +H2O HOCl + HCl

Chlorine is a powerful disinfectant than chloramine and bleaching powder. Calculated amount of
chlorine must be added to water because chlorine after reaction with bacteria and organic impurities or
ammonia, remains in water as residual chlorine which gives bad taste, odour and toxic to human beings.

The amount of chlorine required to kill bacteria and to remove organic matter is called break point
chlorination.

The water sample is treated with chlorine and estimated for the residual chlorine in water and plotted as
a graph as shown below which gives the break point chlorination.

31
From the graph it is clear that the area under represents,
1-2 : chlorine added oxidises reducing impurities of water.

2-3: chlorine added forms chloramine and chloro compounds.

3-4: chlorine added causes destruction of bacteria.

After 4: chlorine is residual chlorine.

So, 4 is the break point for the addition of chlorine to water. This is called break point chlorination.

Advantages of break point chlorination:

1. It removes taste, colour, oxidise completely organic compounds, ammonia and other reducing
impurities.

2. It destroys completely (100%) all disease producing bacteria.

3. It prevents growth of any weeds in water.
4. De chlorination:

The over chlorination is removed by passing the water through a bed of granular carbon and also by
addition of SO2 and sodium thiosulphate.

SO2 +Cl2+2H2O H2SO4+2HCl

Na2S2O3+Cl2+H2O Na2SO4 +2HCl

14Q. What is meant by desalination? Explain the method used for the desalination of brackish
water?
Desalination of brackish water:-
The process of removing common salt (sodium chloride) from the water is known as “Desalination”.

The water containing dissolved salts with a peculiar salty or brackish taste is called “Brackish water”.
Ex. Sea water (contains an average of about 3.5% salts) is an example of brackish water.

It is totally unfit for drinking purpose.

Commonly used methods for desalination of brackish water are:-

1) Electro dialysis
2) Reverse Osmosis

Reverse osmosis:-

When two solutions of unequal concentrations are separated by a semipermeable membrane (which does
not permit selectively the passage of dissolved solute particles i.e., molecules, ions etc) flow of solvent
takes place from dilute to concentrated sides, due to Osmosis.

If a hydrostatic pressure in excess of osmotic pressure is applied on the concentrated side, the solvent
flow reverses i.e., solvent is forced to move from concentrated side to dilute across the membrane. This
is the principle of reverse osmosis.

Thus, in reverse osmosis, pure solvent (water) is separated from its contaminants, rather than removing
contaminants from water. This membrane filtration is also called “Super/ Hyper filtration”.
Method:

In this process, pressure (15-40 kg/cm2) is applied to sea water/impure water (to be treated) to force its
pure water through the semi-permeable; leaving behind the dissolved solids (both ionic and non-ionic).
The principle of reverse osmosis as applied for treating saline/sea water is as follows: given in fig above.

The membrane consists of very thin film of cellulose acetate, affixed to either side of a perforated tube.
Superior membranes recently developed are made of polymethacrylate and polyamide polymers have
come into use.

Advantages:

1) Reverse osmosis process has distinct advantage of removing ionic as well as non-ionic,
colloidal and high molecular weight organic matter.
2) It removes colloidal silica, which is not removed by demineralization.

3) The life time of membrane is quite high, about 2 years.

4) The membrane can be replaced within a few minutes thereby providing nearly
uninterrupted water supply.
5) Low capital cost, simplicity, low operating cost and high reliability.

6) The reverse osmosis is gaining ground at present for converting sea water into drinking
water and for obtaining water for very high pressure boilers.
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