CHY 1701

Engineering Chemistry

Module 2-Water Treatment

(Part A)

Dr. M. Akhila Maheswari

1. Phosphate conditioning – For High-Pressure boilers

Scale formation can be prevented by adding sodium

phosphate to the boiler water which reacts with the hardness
producing ions and forms easily removable phosphate salts
of respective ions

3CaCl2 (Boiler water) + 2 Na3PO4 Ca3(PO4)2 + 6 NaCl

Calcium Sodium calcium phosphate
chloride phosphate (non adherent and can be
removed by blow down method)
pH
Na3PO4 (Alkaline)
Optimum pH to precipitate
Ca is pH 9.5 – 10.5 Na2HPO4 (Weakly alkaline)
NaH2PO4 (Acidic)
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 2. Carbonate conditioning

CaSO4 (Boiler water) + Na2CO3 CaCO3 + Na2SO4

Calcium Sodium calcium carbonate
sulfate carbonate
(non adherent loose and
soft ppt and can be
removed by blow down
method)

Caution: Excess Na2CO3 can result in caustic embrittlement

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 3. Calgon conditioning

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

Calgon – sodium hexa meta phosphate

2CaSO4 (Boiler water) + [Na4(PO3) 6]2- [Ca2(PO3) 6]2- + 2Na2SO4

Calcium sulfate Soluble complex ion
of calcium - can be
removed easily

Calgon tablets are used in the

cleaning of washing machine drums

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Softening of hard water –
External treatment

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 External treatment of water – External
Conditioning of water

Softening methods
The process of removing the hardness producing substance
from the water is called softening of water.

Softening of hard water can be done by the following methods

1. Lime soda process
2. Zeolite methods
3. Ion exchange resin method
4. Mixed bed deionizer method

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 1. 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].
Calcium carbonate [CaCO3] and Magnesium hydroxide [Mg(OH)2] so
precipitated, are filtered off.

It is further divided in to two types

i. Cold lime soda process
ii. Hot lime soda process

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 Cold lime soda process

Step 1
1. A calculated quantity of Ca(OH)2 (lime) and Na2CO3 (soda) are mixed
with water at room temperature and added to the hard water.
2. The following reactions takes place depending on the nature of
hardness

Chemical reactions
If it is permanent hardness and due to calcium salt
CaCl2 + Na2CO3 CaCO3 + 2NaCl (soda) …S
slimy suspended precipitate

CaSO4 + Na2CO3 CaCO3 + Na2SO4 (soda) …S

slimy suspended precipitate

If it is permanent hardness and due to Magnesium salt

Mg2+ + Ca(OH)2 Mg(OH)2 + Ca2+ (lime) ….L
slimy suspended precipitate
Chemical reactions contd..

If it is Temporary hardness and due to calcium salt

Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O (lime) …..L
slimy suspended precipitate

If it is due to Magnesium salt

Mg(HCO3)2 + 2Ca(OH)2 2CaCO3 + Mg(OH)2 + 2H2O (2 *lime)….. 2L
slimy suspended precipitates
 Cold lime soda process

Step 2

The precipitates CaCO3 and Mg(OH)2 are very fine and forms sludge like
precipitates in the boiler water and are difficult to remove because it
does not settle easily making it difficult to filter and the removal
process. Finally reduces the efficiency of the boiler.

Therefore, it is essential to add small amount of coagulant (such as

Alum, Aluminium sulfate, sodium aluminate etc) which hydrolyses to
flocculent precipitate of Al(OH)3 which entraps the fine precipitates.

NOTE: Particles finer than 0.1 µm (10-7m) in water remain continuously in motion due to
electrostatic charge (often negative) which causes them to repel each other. Once their
electrostatic charge is neutralized by the use of coagulant chemical, the finer particles start to
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collide and agglomerate (combine together) under the influence of Van der Waals's forces. These
larger and heavier particles are called flocs.
Reactions with coagulants

When coagulants are added flocculation takes place followed

by the formation of flocculants.
NaAlO2 + 2H2O NaOH + Al(OH)3
Coagulant
Flocculent- Gelatinous precipitate
which entraps the fine precipitates
of CaCO3 and Mg(OH)2

Al2(SO4)3 + 3 Ca(HCO3)2 2Al(OH)3 + CaSO4 + CO2

Aluminium Hard water
sulfate sample

The Al(OH)3 formed by the addition of coagulants initiates the

process of flocculation and entraps the fine precipitates and
becomes heavy. The heavier flocs then settles at the bottom and
filtered off easily.
Cold lime-soda process
• 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. and entraps the fine
ppt.

• NaAlO2 + H2O→ NaOH + Al(OH)3

• It provides water with a residual

hardness of 50 to 60 ppm

Al2(SO4)3+3Ca(HCO3)2 →2Al(OH)3+3CaSO4+6CO2
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 Continuous cold lime soda softener

Chemicals Hard water feed

(soda+lime+
coagulant) feed Softened
water
Wood fiber
porous filter

Stirrer
paddles

Sedimented sludge
CaCO3, Mg(OH)2
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 2. Hot lime soda Process

Similar to Cold lime soda process.

Temperature is maintained at 80-110oC.
No need of coagulants.

Advantages of Hot Lime Soda Process

1. The reaction between hardness producing substance and lime soda

proceeds at a faster rate – Softening rate faster.
2. The precipitates and sludges formed are settled at the bottom easily -
No coagulants are required.
3. The dissolved gases such as CO2 escapes and the water becomes free
from dissolved gases.
4. It produces soft water with the residual hardness of 15-30ppm in
contrast to the cold lime soda process which produces soft water with
50-60ppm of residual hardness.
5. Filtration process is simple.

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 Continuous Hot Lime 1. Reaction tank: water, chemicals and
steam are mixed
soda Process
2. Conical sedimentation tank : sludge
Hard water feed settles down
3. Sand filter : complete removal of
sludge from the soft water is ensured
Super heated steam
Chemicals feed (lime
and soda)

Reaction tank
Soft water

Conical sedimentation
tank

Precipitated sludge
(CaCO3, Mg(OH)2 Sludge
outlet

Filtered soft
water

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16
Advantages of Lime soda process
1. It is very economical compared to other methods.
2. Iron and manganese salts are also removed by this process.
3. It increases the pH of the softened water hence corrosion is
minimized also pathogenic bacteria.

Disadvantages of Lime soda process

1. Disposal of large amount of sludge (insoluble precipitates)

poses a problem.
2. This can remove hardness to the extent of 15ppm which is
not good for boilers.

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Rules
1. If Ca(HCO3)2 and Mg(HCO3)2 are considered as ions (Ca2+ + 2HCO3-) and (Mg2+ +
2HCO3-) respectively then the calculation result will be the same based on the
ability of the ions to take up bicarbonate ions
2. If treated water found to contain excess of OH- and CO32- ions these are formed
from excess equivalent each of Ca(OH)2 and Na2CO3 and hence these excess
amounts should be added to the calculation (in temp. hardness and perm.
hardness)
3. When the impurities are given as CaCO3 and MgCO3 present in water it should
be considered as due to bicarbonates of calcium and magnesium respectively
4. Substances like NaCl, KCl, Na2SO4, SiO2, Fe2O3 etc do not contribute to
hardness and therefore, they do not consume any soda or lime and hence if
these present need not be taken in to consideration during calculation.
5. Soda (Na2CO3) neutralizes only permanent hardness

Molecular weight of lime = 74

Molecular weight of soda = 106
Molecular weight of CaCO3 = 100

Therefore, 100 parts by mass of CaCO3 are equivalent to

(i) 74 parts by mass of Ca(OH)2
(ii) 106 parts by mass of Na2CO3 19
Calculation of lime and soda required for the softening of hard water by the
lime soda process

Hardness producing Chemical reaction with lime and soda Need

substance

Permanent Hardness
Ca Salts CaCl2 + Na2CO3 CaCO3 + 2NaCl S

Mg salts MgSO4 + Ca(OH)2 Mg(OH)2 + CaSO4 L+S

CaSO4 + Na2CO3 CaCO3 +Na2SO4

Temp. Hardness
Ca(HCO3)2 Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O L
Mg(HCO3)2 Mg(HCO3)2 + 2Ca(OH)2 2CaCO3 + Mg(OH)2 + 2H2O 2L

Acids
HCl 2H+ + Ca(OH)2 Ca2+ + 2H2O L+S
H2SO4 Ca2+ + Na2CO3 CaCO3 + 2Na+

HCO3- HCO3- + Ca(OH)2 CaCO3 + H2O + CO32- L-S

Fe2+ + Ca(OH)2 Fe(OH)2 + Ca2+

FeSO4 Ca2+ + Na2CO3 CaCO3 + 2Na+ L+S

NaAlO2 NaAlO2 + H2O Al(OH)3 + NaOH

L/2
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Calculation of Lime and Soda requirements to soften water

All in terms of CaCO3 eq

X Volume of water

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Lime Soda Process - Reactions of Lime and Soda

L+S

L-S

2L
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 L

L+S

L+S

23
O

L+S

-L

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 Lime-soda treatment is very slow – super saturated solution – results “after deposition”
- form sludge
- corrosion
Removed: - thorough mixing
- accelerators – bring down the particles of precipitates (activated charcoal)
- Coagulants

Ca(HCO3)2 and Mg(HCO3)2 - calculation results will be the same

Ca2+ + 2HCO3-

If the treated water contains OH- and CO32- due to excess of Ca(OH)2 and Na2CO3
- account to the calculations

NaCl, KCl, Na2SO4, SiO2, Fe2O3 – do not impart any hardness

- do not consume any lime or soda
- do not account for calculating lime or soda requirement
Dissolved Salt/ion Molar Mass MF for eq. CaCO3
Ca(HCO3)2 162 100/162
Mg(HCO3)2 146 100/146
CaSO4 136 100/136
MgSO4 120 100/120
MgCl2 95 100/95
MgCO3 84 100/84
CO2 44 100/44
Ca(NO3)2 164 100/164
Mg(NO3)2 148 100/148
HCO3- 61 100/(61x2) 100/122
OH- 17 100/(17x2) 100/34
CO32- 60 100/60
NaAlO2 82 100/(82x2) 100/164
Al2(SO4)3 342 100/(342/3) 100/114
FeSO4.7H2O 278 100/278
H+ 1 100/(1x2) 100/2
HCl 36.5 100/(36.5x2) 100/73
Zeolite process
Zeolite – Sodium Aluminosilicate

Na+

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 Zeolite (Permutit) method of Softening of water

Zeolite is a Hydrated Sodium Alumino

Silicate (HSAS), capable of exchanging
reversibly its sodium ions for hardness
producing ions in water.

The general chemical structure of zeolite is

given below Na2O.Al2O3.xSiO2.yH2O (x = 2-10
and y = 2-6)

Micro pores of Zeolite Porous Structure of zeolite

❖ Porosity or cavity size of synthetic zeolite structures can be

controlled by varying the Si/Al ratio
❖ Ion-exchange process of zeolite structure is associated with sodium ions
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Zeolite – Sodium Aluminosilicate

Na+

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Why synthetic zeolite is better than natural zeolite for the
softening of water? Ans: Natural zeolites are non-porous
Natural Zeolite

Natrolite

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 Hard water
in

Hard water
spray

Zeolite bed

Gravel

Injector

Softened
NaCl To water
storage sink

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10% NaCl - brine
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36
 Limitations and disadvantages of Zeolite process

1. If the water is turbid ---- the turbidity causing particles clogs the pores
of the Zeolite and making it inactive.
2. The ions such as Mn2+ and Fe2+ forms stable complex Zeolite which can
not be regenerated that easily as both metal ions bind strongly and
irreversibly to the zeolite structure.
3. Any acid present in water (acidic water) should be neutralized with soda
before letting the water to the plant, since acid will hydrolyze SiO2
forming silicic acid.
4. Soft water contains more sodium salts than in lime soda process.
5. It replaces only Ca2+ and Mg2+ with Na+ but leaves all the other ions like
HCO3- and CO32- in the softened water (then it may form NaHCO3 and
Na2CO3 which releases CO2 when the water is boiled and causes
corrosion).
6. It also causes caustic embitterment when sodium carbonate hydrolyses
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to give NaOH.
38
Cation Resin after
exchange Resin treatment

Ion exchange resin

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• Ion-exchange resins are insoluble, cross-linked, long chain
organic polymers with a micro-porous structure and the
functional groups attached to the groups are responsible for
the ion-exchanging properties.

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 ¯H+
SO3¯H+
SO3
RH+

¯H+ SO3¯H+
SO3

Styrene-divinyl benzene copolymers, which on sulphonation or

carboxylation, become capable to exchange their hydrogen ions with
the cations in the water

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 ROH-

Styrene-divinyl benzene or amine-formaldehyde copolymers, which

contain amino or quaternary ammonium or quaternary phophonium or
tertiary sulphonium groups as an integral part of the resin matrix.
These after treatment with dil. NaOH solution capable to exchange their
OH¯ ions with the anions in the water
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Structure of Cation and Anoin exchange resins

Cation exchange resin Anion exchange resin

R = CH3

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H2O

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 Ion exchange purifier or softener
Hard
water

Gravel
Cation exchange Resin Anion exchange Resin bed

Injector
Injector

Acid
solution for Wastages to
regeneratio sink Alkaline solution for
n of resin Wastages to
regeneration of resin
sink
pump

Soft water 45
Hard water in

Soft Water Out

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 Regeneration of cation exchange column

Regeneration of anion exchange column

Advantage

• The process can be used to soften highly acidic or alkaline water

• it produces water of very low hardness (2 ppm)

Disadvantage

• The equipment is costly and more expensive chemical are needed

• If the water contains turbidity then the output of the process is

reduced
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 Softening of water by Mixed Bed deioniser
1. It is a single cylindrical chamber containing a mixture of anion and cation
exchange resins bed
2. When the hard water is passed through this bed slowly the cations and anions
of the hard water comes in to contact with the two kind of resins many number
of times
3. Hence, it is equivalent to passing the hard water many number of times
through a series of cation and anion exchange resins.
4. The soft water from this method contains less than 1ppm of dissolved salts
and hence more suitable for boilers

Hard
water

c a c a Anion
c Mixed bed exchange
Mixed
a deionizer a resin
resin bed
a
c a cc Cation
exchange
resin

Demineralise
d water
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 Regeneration of mixed bed deionizer
1. When the bed (resins) are exhausted or cease to soften the water, the mixed bed is
back washed by forcing the water from the bottom in the upward direction
2. Then the light weight anion exchanger move to the top and forms a upper layer
above the heavier cation exchanger
3. Then the anion exchanger is regenerated by passing caustic soda solution (NaOH)
from the top and then rinsed with pure water
4. The lower cation exchanger bed is then washed with dil.H2SO4 solution and then
rinsed.
5. The two beds are then mixed again by forcing compressed air to mix both and the
resins are now ready for use Low
NaOH
density
resin

c a c a c a c a c aa c a
aa a a a a cRegenerated
c Mixed bed c Exhausted Back washed a
a deionizer a a Mixed bed a
Mixed bed

a a ccccc c
deionizer
a
c a cc c a cc c a c c

Back Compressed
wash High air
water density
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resin
The outgoing water from the mixed-bed contains even less than 1 ppm
of dissolved salts

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