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Sources of Water
1
...
It is the purest form of natural water
...
and other solids
...
Sea water
...
5%
dissolved salts of which about 2
...
Other salts present include
sulfates, bicarbonates, bromides of sodium, potassium, magnesium etc
...
River water
...
River water while flowing through the land collects lots of
organic matters from falling trees and nearby habitats and also other
soluble and suspended matters from the lands, soils etc
...
Lake water
...

5
...
The rainwater and other surface water percolate
down through the soil and rocks and get filtered and finally collected on
rocky surface or again come out as spring
...


Hardness of Water
Hardness of water is the characteristic of preventing lather formation of
water with soap
...

This hard water on treatment with soap which is stearic or palmitic acid
salts of sodium or potassium causes white precipitate formation of
calcium or magnesium stearate or palmitate
...
When the hardness causing ions are removed
as insoluble soaps, water becomes soft and forms lather
...
Since bicarbonates readily get precipitated on boiling
the water, the temporary hardness

(ii) Permanent hardness is due to the presence of chlorides and sulfates of
Ca, Mg, Fe, etc
...


Units of Hardness
Both temporary and permanent hardnesses are expressed in ppm as
CaCO3
...
wt
...

Equivalent of CaCO3

2

Units of Hardness
Hardness is principally expressed in ppm unit
...


Disadvantage of hardwater
• In domestic use
– Washing: Hard water, when used for washing purposes, does
not lather freely with soap
...
Similar problem exists in
bathing
...
Consequently more
fuel is and time are required for cooking
...

The possibility of forming calcium oxalate crystals in urinary
tracks is increased (Kidney stones)
...

Precipitate of calcium and magnesium soaps adhere to the fabrics
...

– Sugar industry : Water containing sulphates, nitrates, alkali
carbonates, etc
...
Moreover, the sugar so produced may be
deliquescent
– Concrete making : Water containing chlorides and sulphates, if used
for concrete making, affects the hydration of the cement and the final
strength of the hardened concrete
...


•

Sludge can be easily scrapped off with the wire brush
...
g
...


Disadvantage of sludge formation
•

Sludges are poor conductor of heat, so they tend to waste a portion of heat
...
It settle at the
bends thereby causing blocking

Prevention of sludge formation
By using well softened water and by frequently blow down operation

Scales
Decomposition of calcium bicarbonate
•

Scales are the main source of boiler troubles
...


Scale

•

But in high-pressure boilers, CaCO3 is soluble
...


•

The solubility of CaSO4 is 3,200 ppm at 15° and it reduces to
C
55 ppm at 230°C and 27 ppm at 320°
C

•

CaSO4 gets precipitated as hard scale on the heated portion of the
boiler
...


Hydrolysis of magnesium salts
Dissolved magnesium salts undergo hydrolysis forming magnesium
hydroxide precipitate which forms a soft type of scale
MgCl2 + 2H2O

Mg(OH)2 + 2HCl

Presence of Silica
presence of silica in small quantities deposits as calcium silicate
(CaSiO3) or magnesium silicate (MgSiO3)
...
325

0
...
25

2
...

– Calcium carbonate scales can dissolved by using 5-10% HCl
...


•

Frequent blow down operation

7

Ethylene diamine tetra acetic acid (EDTA)

EDTA Complex with Manganese

8

EDTA Complex with Calcium

Prevention of scale formation
• External Treatment
– Removing hardness-producing constituents of water
(will be discussed later)

• Internal Treatment
–
–
–
–
–

Colloidal Conditioning
Phosphate conditioning
Carbonate conditioning
Calgon conditioning
Treatment with sodium aluminate

9

Priming
During rapid steam production, some liquid water drops are
also carried along with the steam
...

Priming occurs due to
(i) Presence of large amount of dissolved solids,
(ii) High steam velocities,
(iii)Sudden boiling and
(iv)Sudden increase in steam production
...


Foaming
Foaming is the production of bubbles and foams which do
not break easily
...

Foaming can be reduced by
(i) removing oil from boiler-feed water and
(ii) adding anti-foaming agents
...


•

Actual height of the water column cannot be judged due to
foaming hence creating difficulty in the maintenance
...
Its called priming
...
It is due to the presence of oil
...
)
Sodium Ferrate
+
Iron at
stressed
parts

__

Conc
...

NaOH
solution

Iron at
plane
surfaces

11

Caustic embrittlement can be avoided
•

By using sodium phosphate as softening reagent instead of sodium
carbonate

•

By adding tannin or lignin to boilers water, since it blocks the haircracks, thereby preventing infiltration of caustic soda

•

By adding sodium sulphate to boiler water – It also blocks the haircracks and preventing infiltration of caustic soda
...

Fe + 2H2O + O2
Fe(OH)2 + O2
(Ferrous hydroxide)

Fe(OH)2
2[Fe2O3
...
High temperature, low pressure and large
exposed surface reduces dissolved oxygen in water

Dissolved Carbon dioxide
H2CO3 is carbonic acid which has corrosive effect on the boiler material
Removal of CO2
By adding calculated quantity of ammonia
2NH4OH + CO2

(NH4)2CO3 + H2O

Acids from Dissolved Salts
Water containing dissolved magnesium salts liberate acids on hydrolysis
MgCl2 + 2H2O

Mg(OH)2

+

2HCl

The liberated acid reacts with iron of the boiler in chain-like reactions
producing HCl again and again
...

Fe + 2HCl

FeCl2 + H2

FeCl2 + 2H2O

Fe(OH)2 + 2HCl

13

Softening methods
• The process of removing the hardness producing substance
from the water is called softening of water
• In Industry three main methods are employed for softening of
water
– Lime soda process
– Zeolite process
– Ion exchange process

Lime-Soda process
Soluble calcium and magnesium salts in water are chemically converted into
insoluble compounds by adding calculated amount of lime [Ca(OH)2] and
Soda [Na2CO3]
...


Lime Soda Process - Reactions of Lime and Soda

14

15

•

Occurring at room temperature

•

precipitate formed are finely
divided hence do not settle down
easily

•

It is essential to add small amount
of coagulant (alum, sodium
aluminate)

•

Coagulant hydrolyse to form
gelatinous ppt
...


•

NaAlO2 + H2O

•

It provides water with a residual
hardness of 50 to 60 ppm

NaOH + Al(OH)3

Hot lime-soda process
•

Occurring at 80 to 150 °C close to the boiling poi nt of the solution

•

Reaction proceed faster

•

The precipitate and sludge formed settle down rapidly so no
coagulant needed

•

Viscosity of the softened water is lower, so filtration of water
becomes much easier

•

Produce water contain the residual hardness of 15 to 30 ppm

16

17

Advantage of Lime-Soda process
•
•
•

It is very economical
This process increases the pH value of the treated-water, thereby
corrosion of the distribution pipes is reduced
Besides the removal of the hardness, the quantity of minerals in
water reduced

Disadvantage of Lime-Soda process
•
•

Disposal of large amounts of sludge poses a problem
This can remove hardness only up to 15 ppm, which is not good for
boilers

18

Aluminosilicate

Zeolite and permutit process

Natural Zeolite

Natrolite

19

Artificial Zeolite

China clay

20

21

Ion-exchange

•

Ion-exchange resins are soluble, cross-linked, long chain organic
polymers with a microporous structure and the functional groups
attached to the groups are responsible for the ion-exchanging
properties
...
These after
treatment with dil
...


•

By adding bleaching powder
– About 1kg of bleaching powder per 1000 kiloliters of water is mixed
– Produces hypochlorous acid (powerful germicide)
CaOCl2 + H2O
Cl2 + H2O
Germs + HOCl

Ca(OH)2 + Cl2
HCl + HOCl
Germs are killed

Disadvantage
Introduces Calcium in water, thereby making it more hard
excess of it gives a bad taste and smell to treated water

• Chlorination
– Chlorine either gas or in concentrated solution form produces
hypochlorous acid, which is a powerful germicide
Cl2 + H2O
Germs + HOCl

HCl + HOCl
Germs are killed

• Factors affecting efficiency of chlorine
– Number of micro-organism destroyed by chlorine per unit time is
proportional to the number of micro-organism alive
...
5, less contact time is enough

•

Advantage
– (i) Effective and economical (ii) used low as well as high temp (iii) most
ideal

•

Disadvantage
– (i) should not exceed 0
...
2 ppm (ii) less effective in higher pH values
(iii) excess chlorine produces unpleasant taste and odour
...

The addition of chlorine at the dip or break is called break-point chlorination

• Chloramine (ClNH2)
Cl2 + NH3

ClNH2 + HCl
(Chloroamine)

ClNH2 + H2O

HOCl + NH3
(Disinfectant)

• Disinfection by ozone
3O2

2O3

O3

O2 + [O]
Nascent oxygen

29

Desalination of Brackish water

30

Reverse Osmosis
Super filtration
15-40 kg cm2

Advantages
Removes colloidal silica
Long life
Can be replaced within
few minutes

31

32

Estimation of permanent hardness of water
sample using standard EDTA
• Take 100 ml of hard water sample in a 250 ml beaker
...
Cool and filter the
solution into a 100 ml standard flask
...

– Let the volume of EDTA consumed be V3 ml

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