Water- Analysis, Treatments & Industrial Applications

Contents
• Sources of Water

• Impurities of Water

• Water Hardness & Its Units

• Determination of Hardness by EDTA Method

• Alkalinity & Its Determination

• Boiler Troubles

• Softening Methods

• Internal Treatment Methods of Boilers

• Numerical Problems

INTRODUCTION

• For the existence of all living beings (human, animals and plants), water is very
crucial.

• Almost all human activities—domestic, agricultural and industrial, demand use of

water although water is nature’s most wonderful and abundant compound but only
less than 1% of the world’s water resources are available for ready use.

• Hence it is required to use it carefully and economically.

• This chapter deals with the hardness of water and various methods of purification of
water for municipal water supply.

Sources of Water
• The main sources of water are:

• Surface water: It includes flowing water (streams and rivers) and

still water (lakes, ponds and reservoirs).

• Underground water: It includes water from wells and springs.

• Rain water

• Sea water.
Impurities of water
• The impurities present in water may be categorised into following categories:

• Dissolved Impurities

• 2

– Dissolved gases: O2, CO2, H2S etc Inorganic salts:

– Cations: Ca++, Mg++, Na+, K+, Fe++, Al+++ etc.

– Anions: CO3-, Cl-, SO4-, NO3- etc.

– Organic salts.

• Suspended Impurities

– Inorganic: Clay and sand.

– Organic: Oil globules, vegetables, and animal material.

• Colloidal Impurities

- Finally divided clay and silica Al(OH)3 , Fe(OH)3 , organic

waste products, colouring matter, amino acids etc.

• Microscopic Matters

- Bacteria, algae, fungi etc.

SOURCES OF IMPURITIES IN WATER

• Following are the sources of impurities in water:

• Gases (O2, CO2 etc.) are picked up from the atmosphere by rainwater.
Decomposition of plants and animals remains introduce organic
impurities in water.

• Water dissolves impurities when it comes in contact with ground, soil

or rocks.

• Impurities are also introduced in water when it comes in contact with

sewage or industrial waste.

Hardness of Water
“Hardness in water is that characteristics, which prevents the lathering of soap.

Causes of Hardness of Water

Hardness is due to presence of certain salts of Calcium, Magnesium and other heavy metal
ions like Al3+, Fe3+ and Mn2+ in water. It can be explained by the reaction of soap in soft
and hard water.

❖ Reaction of Soap with Soft Water

• When soft water is treated with soap, lather is produced according to the following
reaction:

C17H35COONa + H2O → C17H35COOH + NaOH

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

Sodium stearate Calcium stearate (insoluble)

❖ Reaction of Soap with hard Water

2C17H35COONa + Ca(HCO3)2 → (C17H35COO)2Ca+ NaHCO3

Actually hardness is due to presence of Cl-, SO42-, CO32- and HCO32- of Ca2+, Mg2+ and
other heavy metal ions like Fe++, Al+++ and Mn++

Types of Hardness
It is of following two types:

(i) Temporary Hardness

(ii) Permanent Hardness

Temporary hardness is caused by the presence of dissolved bicarbonates of calcium,

magnesium and other heavy metals and the carbonates of iron and other metals also. Thus,
the main salts responsible for temporary hardness are Ca(HCO3)2 and Mg(HCO3)2 .

Temporary hardness can be largely removed by more boiling of water, when bicarbonates
are decomposed, yielding insoluble carbonates or hydroxides, which are deposited as a
crust at the bottom of vessel.

Boiling

Ca(HCO3)2 → CaCO3↓ + H2O + CO2

Boiling
 Mg(HCO3)2 → Mg(OH)2↓ + 2CO2

Temporary hardness is also known as carbonate hardness or alkaline hardness.

Permanent Hardness: It is due to the presence of dissolved chlorides and sulphates of

calcium, magnesium, iron and other heavy metals. Hence, the salts responsible for
permanent hardness are CaCl2, MgCl2, CaSO4 , MgSO4 , FeSO4 , Al2 (SO4)3 etc. Unlike
temporary hardness, permanent hardness is not destroyed on boiling. It is also known as
non-carbonate or non-alkaline hardness. The difference between total hardness and alkaline
hardness gives the non- alkaline hardness.

DETERMINATION OF HARDNESS OF WATER

• EDTA Method: EDTA method is the most accurate method for determination of
hardness of water. EDTA (Ethylene diamine tetra acetic acid) has the following
structure.

• EDTA acts as a complexing agent. It forms complexes with Ca2+and Mg2+ ions as
well as with many other metals cations, in aqueous solutions.

• In hard water sample, the total hardness can be determine by titrating the Ca 2+ and
Mg2+ present in an aliquot sample with Na2 EDTA solution using NH4Cl-NH4OH
buffer solution of pH-10 and Eriochrome Black-T as the indicator.

• Permanent hardness can be determined by precipitating the temporary hardness by

boiling water for about 30 minutes followed by titration with EDTA solution.

• The difference in the titre values of total hardness & permanent hardness
corresponds to the temporary hardness of the water solution.

• Permanent hardness can be determined by precipitating the temporary hardness by

boiling water for about 30 minutes followed by titration with EDTA solution.

• The difference in the titre values corresponds to the temporary hardness of the water
solution.

Units of Hardness and Their Interrelation

i) Parts per million (ppm): It is defined as the number of parts by weight of calcium
carbonate per million (106) parts by weight of water.

1 ppm = 1 part of CaCO3 equivalent hardness in 106 parts of water.

ii) Milligrams per litre (mg/L): It is defined as the number of milligrams of CaCO3
present in one litre of water.

1mg/L = 1 mg of CaCO3 eq. hardness per L of water

But 1 L of water weighs

= 1kg = 1000 g= 1000x 1000mg = 106 mg

therefore,

1 mg/L = 1 mg of CaCO3 eq. per 106 mg of water

= 1 part of CaCo3 eq. per 106 parts of water= 1ppm

iii) Degree Clarke (oCl ): It is defined as the parts of CaCO3 equivalent hardness per
70,000 parts of water.

iv) Degree French (oFr ): It is defined as the parts of CaCO3 equivalent hardness
per 105 parts of water.

Relationship between various units of hardness

As, 1ppm = 1 part per 106parts of water

1mg/L = 1 mg per 106parts of water

1oFr = 1 part per 105 parts of water

1oCl = 1 part per 70,000 parts of water

Therefore,

106 ppm = 106 mg/L, 105 oFr = 70,000 oCl

Hence, 1 mg/L = 1 ppm = 0.1 oFr = 0.007 oCl

Q. Find the relationship between

1 ppm = ? mg/L = ? 0Fr = ? 0Cl

1 0Cl = ? mg/L = ? 0Fr = ? ppm

 1 0Fr = ? mg/L = ? 1 ppm = ? 0Cl

Boiler Feed Water

Water is mainly used in boilers for the generation of steam for industries and power houses.
For such water, all the impurities lead to operational trouble in boilers are eliminated or
kept within the tolerable limit. Boiler-feed water should correspond with the following
composition:

i) Its hardness should be below 0.2 ppm.

ii) Its caustic alkalinity(due to OH-) should lie in between 0.15 and 0.45ppm.

iii) Its soda alkalinity (due to Na2CO3) should be 0.45-1ppm.

Boiler Troubles: Excess of impurities, if present, in boiler feed water generally cause
the problems such as:

➢ Scale and sludge formation

➢ Corrosion

➢ Carry over (priming and foaming)

➢ Caustic embrittlement.

1, Scale & Sludge Formation

➢ In a boiler, water is continuously evaporated to form steam. This increases the
concentration of the dissolved salts. Finally a stage is reached when the ionic
product of these salts exceeds their solubility product and hence they are thrown as
precipitate.

➢ If the precipitate formed is soft, loose and slimy, these are known as sludges, while
if the precipitate is hard and adhering on the inner wall, it is called as scale.
Scales
Scales are hard deposits firmly sticking to the inner surface of the boiler. They are
difficult to remove, even with the help of hammer and chisel. Scales are major
source of boiler troubles. Scales may be formed inside the boiler due to:

(i) Decomposition of calcium bicarbonate

Ca(HCO3)2 → CaCO3 + H2O + CO2

However, scale composed chiefly of CaCO3 is soft and is the main cause of scale

formation in low pressure boilers. But in high pressure boilers, CaCO3 is soluble.

CaCO3 + H2O → Ca(OH)2 + CO2↑

(ii) Deposition of calcium sulphate

The solubility of CaSO4 in water decreases with increases in temperature.

CaSO4 is soluble in cold water but almost completely insoluble in super-
heated water. This is due to increased ionization at high temperature.
Consequently, CaSO4 gets precipitated as hard scale on the hotter parts of
the boiler. This type of scale causes troubles mainly in high pressure boilers.

(iii) Presence of Silica

Even if a small quantity of SiO2 is present, it may deposit as Calcium

Silicate (CaSiO3) or Magnesium Silicate (MgSiO3). These deposits adhere
very firmly on the inner side of the boiler surface and are very difficult to
remove.
Disadvantages of Scale formation
• Wastage of fuel: Scales have a poor thermal conductivity. So, the rate of heat
transfer from the boiler to the inside water is reduced.

• In order to provide a steady supply of heat to water, excessive or over heating is

done and this causes increase in fuel consumption.

• Decrease in efficiency: Deposition of scale in the valves and condensers of the

boiler, choke them partially. This result is decreases in efficiency of the boiler.

• Danger in explosion: When thick scale crack due to uneven expansion, the water
comes suddenly in contact with over-heated portion and large amount of steam is
formed instantaneously this result in development of sudden high pressure which
may cause explosion of the boiler.

Removal of Scale
Scales are removed by mechanical as well as chemical method.

(i) If the scales are loosely adhering, they can be removed with the help of scraper or
wire brush.

(ii) If the scales are brittle, it can be removed by giving thermal shocks.

(iii) If the scales are loosely adhering, they can also be removed by frequent blow
down operation. (Blow down operation is a partial removal of hard water through
“a tap” at the bottom of the boiler, when extent of hardness in the boiler becomes
alarmingly high.)

(iv) If the scales are adherent and hard, they can be removed by adding chemicals viz.
HCl, EDTA etc.

Sludges
• Sludge is soft, loose and slimy precipitate formed within the boiler.

• These are formed by substances which have greater solubility in hot water than in
cold water. Examples includes Mg(HCO3)2, MgCl2, CaCl2, MgSO4 etc.

• Sludges are formed at comparatively colder portions of the boilers and get collected
at places, where the flow rate is slow; they can be easily removed (scrapped off)
with a wire brush.

• If sludges are formed along with scales, then sludges gets entrapped in the scale and
both get deposited as scale.

Disadvantages of Sludge Formation

 (i) Sludges are poor conductors of heat , so they tend to waste a portion of heat
generated and thus decreases the efficiency of boiler.
(ii) Excessive sludge formation disturbs the working of the boiler. It settles in the
region of poor water circulation such as pipe connections, plug openings,
thereby causing choking of the pipes.

Prevention of sludge formation

(i) By using softened water.
(ii) By frequently 'blow down operation' (i.e. partial removal of concentrated water
through a tap at the bottom of the boiler, when extent of hardness in boiler
becomes alarmingly high.

Carry Over
The phenomenon of carrying of water along with impurities by steam is called “carry over”.
This is mainly due to priming and foaming. Priming and foaming usually occur together.
They are objectionable because:

i) Dissolved salts or suspended solids in boiler water decrease the efficiency of boiler.

ii) Dissolved salts may enter the parts of other machines these by decreasing their life.

Priming: Priming is mainly attributed to the presence of suspended impurities and to some
extent to dissolved impurities in water. Priming is caused by:

i) The presence of considerable quantities of dissolved & suspended impurities.

ii) Steam velocities high enough to carry droplet of water into the steam pipe.

iii) Sudden boiling.

iv) Faulty design of boiler.

Priming can be avoided by

i)Controlling rapid change in steaming velocities.

ii) The proper design of boilers.

iii) Ensuring efficient softening.

iv) Filtration of the boiler - Water carried over to the boiler and blowing off sludge or scale
from time to time.

Foaming: It 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 oil and alkalis in
boiler feed water.
Foaming can be avoided by

i) The addition of anti-foaming agent, which acts by counteracting the reduction in surface
tension .For example, addition of castor oil, neutralizes the surface tension reduction.

ii) By removing the foaming agent from boiler water.

Boiler Corrosion
The decay of boiler material by is environment is known as corrosion. The main causes for
boiler corrosion:

i) Presence of free acids in water.

ii) Acids formed by hydrolysis of certain lubricating oils.

iii) Presence of dissolved oxygen

iv) Presence of dissolved carbon dioxide

v) Presence of Dissolved Inorganic Salts

vi) Formation of galvanic cells

Disadvantage of Corrosion
Corrosion has following disadvantages:

(i) Shortening of boiler life.

(ii) Leakage of joints & bends.

(iii) Increased cost of repairs & Maintenance.

Main factors Responsible for Corrosion in Boilers:

(i) Presence of Dissolved Oxygen:

• Oxygen dissolved in water is mainly responsible for corrosion in boilers.

• Dissolved oxygen in presence of moisture and at high temperature readily attacks

iron of the boiler.
 2Fe + 2 H2O + O2 → 2Fe(OH)2↓

Ferrous hydroxide

4Fe(OH)2↓ + O2 → 2[Fe2O3.2H2O]↓

(Rust)

Removal of Dissolved Oxygen:

(a) Sodium sulphite is suitable for boilers operating at low pressure. The sulphite reacts
with the dissolved oxygen to form sulphate:

2Na2SO3 + O2 → 2Na2SO4 (in low pressure boilers)

This method is very effective for removing oxygen in low pressure boilers cannot be used
in high pressure boilers because in high pressure boilers dissolved salt concentration
increases to produce priming & foaming and sodium sulphite dicomposes & liberates SO2.

(b) Hydrazine (40% aqueous solution) is now extensively used to remove dissolved O2
in high pressure boilers. One of the advantage of hydrazine treatment is that,
combination with O2 does not produce any salt, N2 & H2O are the only reaction
products obtained.

N2H4 + O2 → N2 + 2H2O (in high pressure boilers )

(ii) Presence of Dissolved CO2

• Dissolved carbon dioxide in high pressure boilers gives carbonic acid.

H2O + CO2 → H2CO3

Removal of Dissolved CO2:

• By adding calculated quantity of NH3

NH3 + CO2 + H2O → (NH4)2CO3

(iii) Presence of Dissolved Inorganic Salts:

Some inorganic salts like MgCl2 and CaCl2 are corrosive agents. MgCl2 on
hydrolysis

gives HCl as follows:

MgCl2 + 2 H2O Mg(OH)2↓ +2HCl

• CaCl2 also undergoes hydrolysis but to a lesser extent. The liberated acid reacts with
iron of the boiler in chain like reactions producing HCl again. Thus;
 Fe + 2HCl → FeCl2 + H2

FeCl2 + 2 H2O → Fe(OH)2 ↓ +2HCl

Hence even a small amount of MgCl2 can cause considerable corrosion of the metal. The
amount of HCl is neutralized by the addition of alkali.

(iv) Formation of Galvanic Cells: Corrosion can also occur because of galvanic
cell formation between iron and other metals present in the alloy used in boiler
fittings. This can be prevented by suspending zinc plates which acts as sacrificial
anodes. When two dissimilar metals are electrically connected and exposed to an
electrolyte, the metal higher in electrochemical series undergoes corrosion. This
type of corrosion is called galvanic corrosion. In the above example, zinc higher in
electrochemical series forms the anode and is attacked and gets dissolved, whereas
iron lower in electrochemical series acts as cathode.

Caustic Embrittlement
• It is the phenomenon during which the boiler material becomes brittle due to the
accumulation of caustic substance.

• This type of boiler corrosion is caused by the use of highly alkaline water in the
high pressure boiler. During softening of water by lime soda method, free Na2CO3 is
 usually present in small proportion in the softened water. In high pressure boilers,
Na2CO3 decomposes to give sodium hydroxide and CO2.

Na2CO3 + H2O → 2NaOH +CO2

• This NaOH containing water flows into the small cracks of boiler by capillary
action. Here water evaporates and the dissolved caustic soda attacks the surrounding
area, thereby dissolving iron of boiler as sodium ferroate. This causes embrittlement
of boiler parts, causing even failure of the boiler. The concentrated NaOH dissolves
the protective magnetite layer of the boiler metal. The product of this reaction is
sodium ferroate and sodium ferroite.

4 NaOH + Fe3O4 → 2NaFeO2 + Na2FeO2 + 2H2O

Sod. Ferroite Sod. Ferroate

• Concentrated NaOH then react with freshly exposed base metal to yield sodium
ferroate and hydrogen.

Fe + 2NaOH → Na2FeO2 + H2

• At these areas corrosion takes place in such a manner that inter granular cracks
occur in an irregular fashion. Such a phenomenon is known as caustic
embrittlement.

Prevention of caustic embrittlement

i) During softening process, sodium phosphate is used in place of sodium
carbonate.
ii) Tannin or lignin should be added in boiler water which blocks the hair cracks.
iii) Sodium sulphate should be added in boiler water.

Water Softening
• The process whereby we remove or reduce the hardness of water irrespective of
whether it is temporary or permanent is termed as softening of water. Water
softening is very essential since hard water is unsuitable for domestic as well as
industrial use. Water can be made soft by external as well internal
treatment. External treatment can be done by the following method:
 • Lime soda process

• Zeolite or permutit Method

• Ion-exchange method

Lime soda process

• The basic principle of the process is to chemically convert all the soluble hardness
causing impurities into insoluble precipitate which may be removed by settling and
filtration.

• For this a suspension of milk of lime Ca(OH)2, together with a calculated amount of
sodium carbonate, Na2CO3 (soda) is added in requisite amount.

• Proper mixing of chemicals and water is carried out. Calcium carbonate and
magnesium hydroxide so precipitated are filtered off.

• Lime removes Temporary hardness

Ca(OH)2 + Ca(HCO3)2 → 2CaCO3 ↓ + 2 H2O

(Soluble impurities) (Insoluble ppt.)

Ca(OH)2 + Mg(HCO3)2 → Mg(OH)2 ↓+ 2CaCO3 ↓ + 2 H2O

• Lime removes permanent hardness of Magnesium

Ca(OH)2 + MgSO4 → Mg(OH)2 ↓+ CaSO4

Ca(OH)2 + MgCl2 → Mg(OH)2 ↓+ CaCl2

• Lime removes some mineral acids if present in water

Ca(OH)2 +2HCl → CaCl2 + 2H2O

Ca(OH)2 +H2SO4 → CaSO4 + 2H2O

• Lime removes some gases if present in water

Ca(OH)2 +H2S → CaS↓ + 2H2O

Ca(OH)2 +CO2 → CaCO3↓ + H2O

• Soda removes permanent hardness of Calcium

CaCl2 + Na2CO3 → CaCO3 ↓ + 2NaCl

CaSO4 + Na2CO3 → CaCO3 ↓ + Na2SO4

Lime soda processes are of two types:

i) Cold lime soda process

ii) Hot lime soda process

Cold lime soda process

• At room temperature, the precipitates formed are very fine. They do not settle down
easily and causes difficulty in filtration.

• If small amount of coagulants like alum K2SO4.Al2(SO4)3.24H2O and aluminium

sulphate are added, they hydrolyze to gelatinous precipitate of aluminium hydroxide
which entraps the fine precipitate of CaCO3 and Mg(OH)2.

Al2(SO4)3 + 6H2O → 2Al(OH)3 ↓+ 3H2SO4

NaAlO2 + 2 H2O → Al(OH)3↓ + NaOH

• Thus coagulant helps in the formation of coarse precipitates. Cold lime soda process
provides water containing a residual hardness of 50-60 ppm.

Hot lime soda process

• In this process, water is treated with chemicals at a temperature of 94-1000C. At
high temperature, the reaction proceeds faster and precipitate formed settle down
rapidly and hence no coagulant is needed. Hot lime soda produces water of
comparatively lower residual hardness of 15 to 30 ppm.
Advantages of Lime-soda Process

• It is very economical.

• Treated water is alkaline and hence has less corrosion tendencies.

• It removes not only hardness causing salt but also minerals.

• Due to alkaline nature of treated water, amount of pathogenic bacteria in water is

considerably reduced.

• Iron and manganese are also removed from the water to some extent.

Disadvantages of Lime-soda process

• It requires careful operation and skilled supervision for economical and efficient
softening.

• Sludge disposal is a problem.

• Water softened by this process contains appreciable concentration of soluble salts,

such as sodium sulphate and cannot be used in high pressure boiler.

• This can remove hardness only up to 15 ppm, which is not good for boilers.

Zeolite or permutit Method

• Zeolites are of two types:
(i) Natural zeolite are non porous
e.g. Natrolite Na2O. Al2O3.4SiO2.2H2O
(ii) Synthetic zeolite are porous & possess gel structure. Such zeolites possess
higher
ion exchange capacity than natural zeolites.
• Chemical structure of sodium zeolite may be represented as
Na2O. Al2O3.xSiO2.yH2O
Where x = 2-10 and y = 2-6
• Zeolite is hydrated sodium alumino silicate, capable of exchanging reversibly its
sodium ions for hardness producing ions in water. Zeolites are also known as
permutit.

Process
• For softening of water by zeolite process, hard water is passed at a specified rate
through a bed of zeolite, kept in a cylinder.
• The hardness causing ions (Ca2+, Mg2+ etc) are retained by the zeolite as CaZe and
MgZe respectively, while the outgoing water contains sodium salts. Reactions
taking place during the softening process are:

Na2Ze + Ca(HCO3)2 → CaZe + 2NaHCO3

Na2Ze + Mg(HCO3)2 → MgZe + 2NaHCO3
Na2Ze + CaCl2 → CaZe + 2NaCl
Na2Ze + CaSO4 → CaZe + Na2SO4
Na2Ze + MgCl2 → MgZe + 2NaCl
Na2Ze + MgSO4 → MgZe + Na2SO4

Regeneration

After sometime, the zeolite is completely converted into calcium and magnesium
zeolites and it ceases to soften water, i.e. it gets exhausted. At this stage, the supply
of hard water is stopped and the exhausted zeolite is reclaimed by treating the bed
with a concentrated (10%) brine (NaCl) solution.
CaZe + 2NaCl → Na2Ze + CaCl2
(exhausted zeolite) (brine soln) (reclaimed zeolite) (washings)

MgZe + 2NaCl → Na2Ze + MgCl2

(exhausted zeolite) (brine soln) (reclaimed zeolite) (washings)

Zeolite Exchanger
 Advantages of Zeolite processes
• It removes the hardness almost completely and water of about 10 ppm hardness is
produced.
• The equipment used is compact, occupying a small space.
• No impurities are precipitated, so there is no danger of sludge formation.
• It requires less time, less skills for maintenance.
Disadvantages of Zeolite Process
• The treated water contains more sodium salts than in lime soda process. These
sodium salts create problems in boilers like corrosion, caustic embrittlement etc.
• High turbidity water cannot be treated efficiently by this method, because fine
impurities get deposited on the zeolite bed, thereby creating problem for its
working.

Ion Exchange Process

• Ion exchange is a process by which ions held on a porous, essentially soluble solid
are exchanged for ions in solution that are brought in contact with it.
• Ion exchange resins are insoluble, cross linked, high molecular weight; organic
polymers with a porous structure and the “functional group” attached to the chains
are responsible for the ion-exchange properties.
• There are two types of ion-exchange resins:

\
Cation Exchange Resin
• They are mainly styrene-divinyl benzene co-polymers, which on sulphonation or
carboxylation become capable to exchange their hydrogen ions with the cations in
the water.
• Such resins have acidic functional groups like –SO3H, –COOH etc. group capable
of exchanging the cationic portion of minerals by their hydrogen ions, hence they
are termed as cation exchange resin.
• For example: Ambalite IR-20 and Dowex-50 are commercially available cation
exchange resins.

Anion Exchange Resin

• They are styrene– divinyl benzene or amino formaldehyde copolymers, which
contain basic functional group such as amino as integral part of resin matrix.
• These after treatment with dilute NaOH, become capable to exchange their OH-
anions with anions in water and therefore they are known as anion exchanger.
• For example: Amberlite-400 and Dowex-3 are commercially available anion
exchange resins.

Process
• The hard water is first passed through cation exchange column, where all the cations
like Ca2+, Mg2+ are removed from it, and equivalent amount of H+ ions are released
from this column to water.
2RH+ + Ca2+ → R2Ca2+ + 2H+
2RH+ + Mg2+ → R2Mg2+ + 2H+
• After passing through cation exchange column, the hard water is passed though
anion exchange column, where all the anions like SO42-, Cl-etc. present in water are
removed and equivalent amount of OH- ions are released from this column to water.

R’OH- + Cl- → R’Cl- + OH-

2R’OH- + SO4-2 → R’ SO4-2 + 2OH-

2R’OH- + CO3-2 → R’ CO3-2 + 2OH-

• H+ and OH- ions, released from cation and anion exchange column respectively, get
combined to produce water molecules.

H+ + OH- → H2 O

• Thus the water coming out from the exchanger is free from cations as well as
anions.
• Ion free water is known as de-ionized or de-mineralized water. Thus it is known as
pure as distilled water.

Ion Exchanger
Regeneration
• When capacities of cation and anion exchanger H+ and OH- ions respectively are lost, they
are said to be exhausted.

• The exhausted cation exchange column is regenerated by passing a solution of dilute HCl
or HNO3/H2SO4.

• The regeneration can be represented as:

R2Ca2+ + 2H+ → 2RH+ + Ca2+

R2Mg2+ + 2H+ → 2RH+ + Mg2+

• The exhausted anion exchange column can be regenerated by passing a solution of dilute
NaOH. The regeneration can be represented as:

R’Cl- + OH- → R’OH- + Cl-

R’ SO4-2 + 2OH- → 2R’OH- + SO4-2

R’ CO3-2 + 2OH- → 2R’OH- + CO3-2

Advantages of ion exchange processes

• The process is used to soften highly alkaline or acidic water.

 • It produces water of very low hardness (about 2ppm). So it is very good for treating water
for use in high pressure boilers.

Disadvantages of ion exchange processes

• The equipment is costly and more expensive chemicals are needed.

• If water contains turbidity, then the output of the process is reduced.

• The turbidity must be below 10 ppm. If it is more, it has to be removed first by coagulation
and filteration.

Internal Treatment Methods to Boiler Feed Water

• In this water softening is done by treating raw water inside the boiler. This method is
known as sequestration. Internal treatment methods are followed by ‘blow down
operation’, so that the accumulated sludge is removed. Some of the important internal
treatment methods are:

Colloidal conditioning
• Scale formation can be avoided in low pressure boilers by adding substances like kerosene,
tin, agar-agar etc, which get absorbed over the scale forming precipitates thus giving us
non- sticky and loose deposits, which can be easily removed by blow-down operation.

Carbonate conditioning
• In low pressure boilers, scale formation can be avoided by adding sodium
carbonate to boiler water, when CaSO4 is converted in to calcium carbonate in
equilibrium.

CaSO4 + Na2CO3 → CaCO3 ↓ + Na2SO4

• Consequently, deposition of CaSO4 as scale does not take place and calcium is
precipitated as loose sludge of CaCO3, which can be removed by blow down
operation.

Phosphate Conditioning
• In high pressure boilers, scale formation can be avoided by adding sodium
phosphate, which reacts with scale forming impurity CaSO4 , forming non adherent
and easily removable, soft sludge of calcium and magnesium phosphates, which
can be removed by blow down operation, e.g.,
 3CaSO4 + 2 Na3PO4 → Ca3 (PO4)2 ↓ + 3 Na2SO4

• The main phosphates employed are: NaH2PO4, sodium dihydrogen phosphate

(acidic); (b) Na2HPO4, disodium hydrogen phosphate (weakly alkaline) ; (c) Na3PO4,
trisodium phosphate (alkaline).

Calgon conditioning
• Calgon is the commercial name of sodium hexa-metaphosphate.

• The process involves the addition of sodium hexa-metaphosphate to boiler water

to prevent the scale and sludge formation.

• Calgon converts the scale forming impurity like CaSO4 into soluble complex
compound, which are harmless to the boiler.

Na2[Na4(PO3)6] → 2Na+ + [Na4(PO3)6]2-

Calgon

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

Soluble complex

Alkalinity of Water

Alkalinity is a measure of the ability of water to neutralize the acids. The

alkalinity of water is normally due to the presence of bicarbonates and
hydroxides of sodium, potassium, Ca and Mg. Presence of borates, silicates
phosphates can also contribute to the total alkalinity to some extent.
Depending upon the anion present, the alkalinity can be classified as:

i) Caustic alkalinity (due to OH-and CO32-ions)

iii) Hydroxide only (OH-)
iv) Carbonates only (CO32-)
v) Bicarbonates only (HCO3-)
vii) Carbonates and bicarbonates (CO32-+ HCO3-)

Degree of Hardness
Although hardness of water is never present in the form of the calcium carbonate,
because it is insoluble in water, hardness of water is conveniently expressed in terms
of equivalent amount of CaCO3.

The reason for choosing CaCO3 as the standard for reporting hardness water is the
3
ease in calculation as its molecular weight is exactly 100. Moreover, it is the most
insoluble salt that can be precipitate in water treatment.

EquivalentsofCaCO3=[Amountofhardnessproducingsubstance]×[Chemicalequivalentof
3
CaCO3(=50)]×2 / [Chemical equivalent of hardness producing substance]×2
3

= [Amount of hardness producing substance] × 100/ [Chemical equivalent of

hardness producing substance × 2]
= [Amount of hardness producing substance] × (Multiplication factor) in mg/lit
or ppm.

Multiplication Factor
Weight of MgSO4, Mg(HCO3)2, MgCl2 and CaCl2 actually present, may be
converted in terms of weight of CaCO3 by multiplying 100/120, 100/146, 100/95 and
100/111 respectively. Factors used for such conversion are called multiplication factor. It
can be shown as follows:
Mg(HCO3)2 = CaCO3

146 = 100
Where, we are comparing hardness due to Mg(HCO3)2 in terms of CaCO3 equivalents.
X quantity of Mg(HCO3)2 = X × 100/146 amount of CaCO3 thus the factor 100/146 is
multiplication factor (M.F.) for Mg(HCO3)2.

Multiplication factor =100/ [Chemical equivalent of hardness producing substance × 2]

M.F. for various compounds are shown in Table 1.2.
Table 1.2: Multiplication factors for various compounds

Compounds/Salt/Ions Molar mass Chemical Multiplication factor

equivalent (for conversion into
CaCO3 equivalent )

Ca(HCO3)2 162 81 100/162

Mg(HCO3)2 146 73 100/146

CaCO3 100 50 100/100

MgCO3 84 42 100/84
CaSO4 136 68 100/136

CaCl2 111 55.5 100/111

MgSO4 120 60 100/120

MgCl2 95 47.5 100/95

Mg(NO3)2 148 74 100/148

CO2 44 22 100/44

Ca2+ 40 20 100/40

Mg2+ 24 12 100/24

CO32– 60 30 100/60

H+ 1 1 100/02
–
HCO 61 61 100/2×61 = 100/122
3
OH– 17 17 100/2×17 = 100/34

NaAlO2 82 82 100/64

Al2(SO4)3 342 57 3×100/342 = 100/114 or 100/57

FeSO4.7H2O 278 139 100/278

Ca(NO3)2 164 82 100/164

HCl 36.5 1 100/2×36.5 = 100/73

H2SO4 98 49 100/98