2. Hardness
Hardness of water is that characteristic which prevents the
lathering of soap.
Hardness of water is due the presence of Ca2+ , Mg2+ ions.
2 C17H35COONa + Ca2+ → (C17H35COO)2Ca
Soap hard water ppt
The degree of hardness is measured in Parts Per Million(ppm).
1ppm=1mg/liter
Eg: 20ppm means 20 mg /liter.
4. Temporary hardness
Temporary hardness is a type of hardness
caused by the presence of carbonates and
bicarbonates of Ca+2 and Mg+2
“Temporary" hardness can be reduced either
by boiling the water, or by softening process.
5. Permanent hardness
Permanent hardness is caused by sulfates and
chlorides of Ca+2 and Mg+2
Permanent hardness cannot be removed by
boiling.
Because calcium sulfate and/or magnesium
sulfates in the water, not precipitate out as the
temperature increases.
Ions causing permanent hardness of water can be
removed using softening methods like ion
exchange column
6. Determination of hardness of water
by EDTA Method
Principle
M+2 + EBT M-EBT (Wine red colour)
M+2 + EDTA M-EDTA ( colourless)
M-EBT + EDTA M-EDTA+ EBT (Blue colour)
Colour change : Wine red to Blue
7. Procedure:
PART A: Standardization of EDTA.
PART B: Determination of total hardness.
PART C: Determination of Permanent
hardness.
Temporary hardness=total-permanent
hardness..
8. PART A-Standardisation of EDTA
50ml of standard hard water (CaCO3 solution
1mg/ml)+10 ml buffer +EBT indicator—Conical flask
Titrated with EDTA from Burette
Let the volume of EDTA consumed is V1 ml.
50 ml of CaCO3 solution = V1 ml of EDTA
50 mg of CaCO3 = V1 ml of EDTA
I ml of EDTA = 50/ V1 mg of CaCO3 equivalent
50…… V1
X-------1ml
X=50*1/VI=50/V1
9. PART B: Total hardness
50ml of hard water +10 ml buffer +EBT indicator
Titrated with EDTA.
Let the volume of EDTA consumed is V2 ml.
50 ml of hard water= V2 ml of EDTA
= V2 (50/ V1 )mg of CaCO3 eq
=50(V2 / V1 ) mg of CaCO3 eq
1000 ml of hard water = 1000(V2 / V1 ) mg of CaCO3
eq
= 1000(V2 / V1 ) ppm
10. PART C-PERMANENT
HARDNESS
50ml of BOILED hard water +10 ml buffer +EBT indicator
Titrated with EDTA.
Let the volume of EDTA consumed is V3ml.
50 ml of BOILED hard water = V3 ml. of EDTA
= V3 (50/ V1 )mg of CaCO3 eq
= 50(V3 / V1 ) mg of CaCO3 eq
1000 ml of hard water = 1000(V3 / V1 ) mg of CaCO3 eq
= 1000(V3 / V1 ) ppm
13. Boilers
A boiler is a device used to generate steam in
industries.
The steam from the boiler is used in various
processes including reactors, food processing
, sanitation and other types of heat transfer
equipment.
Proper treatment of boiler feed water is an
important for operating and maintaining a
boiler.
14. Boiler Troubles
1.Scale and sludge
formation
2.Priming and Foaming
3.Caustic Embrittlement
4.Boiler corrosion
15.
16. Scale and sludge
In a boiler, water is continuously evaporated to form
steam .This increases the concentration of dissolved
salts in the water
Finally a stage is reached when their concentration
reach a saturation point , dissolved salts are thrown out
as precipitates on the inner walls of the boiler.
If the precipitates formed are soft loose and slimy,
these are known as sludges .
If the precipitate is hard and adhering on the inner
walls, it is called as scale.
17. SLUDGE
Sludge is a soft, loose and slimy precipitate
formed within the boiler.
Sludges are formed by substances which
have greater solubilities in hot water than
in cold water, e.g., MgCO3 , MgCl2, CaCl2 ,
MgSO4 ; etc.
They can be easily removed with a wire
brush
18. Disadvantages of sludge
formation
1.Wastage of Fuel- Sludges are poor conductors of
heat, so the rate of heat transfer from boiler to inside
water is reduced. In order to provide a steady supply
of heat to water, overheating is done and this
causes increase in fuel consumption.
2. Decrease in efficiency :Excessive sludge
formation disturbs the working of the boiler. It settles
in the regions of poor water circulation such as pipe
connection, plug opening, thereby blocking the
pipes.
19. Prevention of sludge formation
By using softened water.
By frequently “blow-down operation', (i.e.,
partial removal of concentrated water through
a tap at the bottom of boiler)
20. SCALES
Scales are hard deposits firmly sticking to the
inner surfaces of the boiler.
They are difficult to remove, even with the
help of hammer and are the main source of
boiler troubles.
21. Scales may be formed inside the
boiler due to:
i) Decomposition of calcium bicarbonate in low-pressure
boilers.
Ca(HCO3)2 CaCO3 + H2O + CO2
But in high pressure boilers, CaCO3 is soluble due to the
formation of Ca(OH)2
(ii) Hydrolysis of magnesium salts: Dissolved magnesium
salts get hydrolyzed forming magnesium hydroxide
precipitate.
MgCl2 + 2H2O Mg(OH)2+2HCl
22. (iii) Decomposition of calcium sulphate : The
solubility of CaSO4 in water decreases with increase in
temperature;
CaSO4 is soluble in cold water, but almost completely
insoluble in superheated water. Consequently, CaSO4
gets precipitated as hard scale on the hotter parts, of
the boiler.
(iv) Presence of silica: Even if a small quantity of SiO2
'is present, it may deposit as calcium silicate
(CaSiO3)"and/ or magnesium silicate (MgSiO3).
23. Disadvantages of Scale formation
(i)Wastage of fuel : Scales have a poor thermal
conductivity so the rate of heat transfer from boiler to
inside water is greatly reduced. In order to provide a
steady supply of heat to water, excessive or
overheating is done and this causes increase in fuel
consumption.
(ii) Lowering of boiler safety : Due to scale formation,
over-heating of boiler is done in order to maintain a‘
steady supply of steam. It makes the boiler material
softer and Weaker. This makes the boiler unsafe.
24. (iii) Decrease in efficiency : Deposition of scales in the
valves and condensers of the boiler, choke them
partially. This results in decrease in efficiency of the
boiler.
(iv) Danger of explosion : When thick scales crack due
to uneven expansion, the water comes suddenly in
contact with over-heated portion and large amount of
steam is formed instantaneously. This results in
development of sudden high-pressure which may cause
explosion of the boiler.
25. Removal of Scales
1)It can be removed by giving thermal shocks
(i .e. heating the boiler and then suddenly
cooling with cold water),
2) Hard scales can also be removed by
dissolving them by adding chemicals
e.g., CaCO3 scales can be dissolved by
using 5 - 10% HCl .
Calcium sulphate scales can be removed
by adding EDTA, since the Ca - EDTA
complex is highly soluble in water
26. Prevention of scales
By using softened water.
By frequently “blow-down operation', (i.e.,
partial removal of concentrated water through
a tap at the bottom of boiler)
27. Priming and foaming
Priming
When steam is produced rapidly in the boilers,
some droplets of liquid water are also carried
along with the steam. This process of 'wet steam'
formation is called priming.
Priming is caused by :
(i) the presence of of dissolved solids
(ii) High Steam velocities
(iii) Sudden increase in temperature.
(iv) Faulty design of boiler.
28. Priming can be avoided by
by taking soft water
controlling rapid change in steaming velocities.
the proper design of boilers
maintaining low water levels in boilers
By fitting steam filters.
29. Foaming
Foaming is the formation of small but persistent
foam or bubbles at the water surface in boilers,
which do not break easily.
Foaming is caused by the presence of an oil in
boiler-feed water.
Traces of oils are generally introduced in boiler feed
water through the lubricating materials used for
pumps etc.
Actually oils react to form soaps which greatly
lowers the surface tension of water and thus
increase the foaming tendency of the liquid.
30. Foaming can be avoided by
(i) By the addition of anti-foaming agents
(ii) By the removal of foaming agent (oil) from
boiler water.
Oils can be removed by the addition of sodium
aluminate which are hydrolysed to form aluminium
hydroxide floc which entrap oil drops.
The flocs of Al(OH)3 containing oil droplets are
removed by filtration.
31. Disadvantages of priming and
foaming
The dissolved salts in water are carried by
wet steam to other machinery parts, get
deposited as scale when water evaporates.
This reduces their efficiency.
Actual height of the water level cannot be
judged properly, there by making the
maintenance of the boiler pressure becomes
difficult.
32. DESALINATION of Brackish
water
Brackish water-The water containing
dissolved salts with a peculiar salty taste.
Desalination-The process of removing
salt from water .
Methods used are: (1) Reverse osmosis
(2) Electro dialysis
33. 1.Reverse osmosis method
Osmosis: When two solutions of unequal
concentrations are separated by a semipermeable
membrane t he solvent molecules (water) move
spontaneously from low concentrated solution to
high concentrated solution.
The transfer of solvent molecules will continue until equilibrium is
attained
34. Reverse osmosis
When pressure greater than osmotic pressure
is applied, solvent molecules (water) move
through a semipermeable membrane from
HIGH concentrated solution to LOW
concentrated solution.
36. Process:
RO unit consists of a chamber fitted with a semi
permeable.
In this process, pressure (15-40 kg/cm2) is applied to
the salt water side to force its pure solvent molecules
out (water) through the semi permeable membrane .
Therefore , pure water is separated from its
contaminants rather than removing contaminants from
the water.
Membranes used-cellulose acetate, polymethacrylate
and polyamide polymers
37. Advantages
1.Both ionic and non ionic, colloidal and high
molecular weight organic matter even
microorganisms can be removed.
2. The process does not add any other
chemical to water. It separates the dissolved
substances from the incoming water.
38. Disadvantages
The primary disadvantage is the amount of water wasted
by the process. Expensive process with slow output
Hard water can shorten the life span of the RO
membrane. A water softener might be necessary to keep
the membrane working at its best.
39. Electro dialysis
It is a method in which ions (of the salts) are
removed from salt water by passing direct
current, using two electrodes and two rigid
plastic membranes.
41. Process
When DC current is passed through saline water, the
positive ions (cations) start moving towards negative
electrode and negative ions (anions) start moving
towards positive electrode through the membrane.
As a result concentration of salt decreases in the middle
compartment, while it increases in the side
compartments.
42. Desalinated water is collected from central
compartment .
The process is repeated by taking fresh brackish in the
side compartment .
Anion and cation selective membranes
To increase the efficiency of the process cation and
anion selective membrane pairs are used in
electrodialysis process.
43.
44. Anion–selective membranes (AMs) and Cation-selective
membranes (CMs) are placed between an Anode (+)
and a Cathode (-).
When the electric field is applied, anions will move
towards the Anode and cations will move towards the
Cathode.
Anion–selective membrane permit only anions and
stops cations.
Cation–selective membrane permit only cations and
stops anions.
Therefore, water in one compartment of the cell is
deprived of salts and while the salt concentration in
adjacent compartment is increased.
Finally, we get alternative streams of pure water and
concentrated salt water.
45. Advantages
1.Electro dialysis removes only ionised species, it is
particularly suitable for separating non-ionised from
ionised components.
2.Compact unit.
46. Disadvantages
1.Only ions are removed. Organic matter,
colloids and SiO2 are not removed by Electro
Dialysis unit.
2.Feed Water pre-treatment is necessary to
prevent destroying of Electro Dialysis unit.
47. What is softening
Hardness in water is due to the presence of
ions Ca2+ and Mg2+ .
The process of removing hardness causing
ions is called softening.
Softening methods-1)Zeolite Method
2)Ion exchange Method
3) Lime soda method
48. Zeolite Method
Zeolite Na2O.Al2O3.x SiO2.y H2O = Na2Ze
Where x=2-10 and y=2-6
Types of Zeolites-Natural or synthetic
Natural zeolite-Non porous
Synthetic Zeolite-Porous – prepared by heating
(Clay+ faldspar+sodium carbonate).
Removes cations in the water when hard water is
allowed to flow through the zeolite bed.
50. Process
In zeolite process, hard water is percolated
(moves slowly through the pores of zeolite
bed) at a specified rate.
The hardness causing ions in the water (Ca2+
,Mg2+ ,etc.) are retained by the zeolite as
CaZe and MgZe while the outgoing
water contains sodium salts.
52. Regeneration of Zeolite
After some time, the zeolite is completely
converted into calcium zeolite and
magnesium zeolite and it ceases to soften the
water.
By injecting concentrated NaCl solution
through Zeolite Bed, it can be regenerated.
53. LIMITATIONS OF ZEOLITE
METHOD
1)Highly acidic water is not suitable
because it destroys the zeolite bed
2)Water must be free from ions like Mn+2,
Fe+2 otherwise they produce corresponding
zeolites which cannot be regenerated easily.
3)The water should not be TURBID,
otherwise the turbidity will clog the pores of
zeolite.
54. ADVANTAGES OF ZEOLITE
METHOD:
1)Water of about less than 15 ppm can be
obtained.
2)Water obtained is clear and pure.
3)The equipment is compact and hence require
less space.
4)Requires less skill in maintenance as well as
operation.
55. Ion exchange method (or)
Demineralisation Process
Ion exchanger is a insoluble, three dimensional,
cross linked organic polymer.
The functional groups attached to the polymer are
responsible for ion exchanging properties.
56. Two types of Ion exchangers
(1) Cation exchanger-Polymer carrying -SO3
-H+
groups-Capable of exchanging H+ ion with other
cations.
Cation exchanger is represented by R H+
(2)Anion exchanger-Polymer carrying -N(CH3)3
+OH-
groups-Capable of exchanging OH- ions with other
anions.
Anion Exchanger is represented by R-OH-
57.
58.
59. Cation exchanger
Hard water are passed first through cation
exchange resin .
The process replaces the hard ions with H+ ions
of the resin
2R H+ Ca+2 R Ca+2 2 H+
2R H+ Mg+2 R Mg+2 2 H+
The hardness causing ions(Ca+2 , Mg+2 ,etc.) are
retained by the cation exchanger as
R Ca+2 and R Mg+2 while the outgoing
water contains H+ ion.
60. Now the water is passed through anion exchange
resin .
The process replaces the anions with OH- ions of
the resin
R OH- Cl- R Cl- OH-
R OH- HCO3
- R HCO3
- OH-
All the anions(Cl-, HCO3
- etc.) are retained by
the anion exchanger as RX- while the outgoing
water contains OH- ions.
Anion exchanger
61. H+ from cation exchanger and OH- ion from
anion exchanger will combine
H+ OH- H20
i.e Water is free from all the ions
62. Regeneration of exchanger
Regeneration of anion exchanger
R Cl- + OH- R OH- + Cl-
R HCO3
- + OH- R OH- + HCO3
-
Regeneration of cation exchanger
R Ca+2 + 2H+ 2 R H+ + Ca+2
R Mg+2 + 2H+ 2R H+ + Mg+2
63. Advantages of ion exchange
1.The process can be used to soften highly
acidic or highly basic water.
2.It produces water of low hardness (2 ppm)
64. Limitations of ion exchange
1.The equipment is costly.
2. If Water contains turbidity, efficiency of the
process is reduced.
