WATER TREATMENT
BY BAHADIR KESKİN
YTU 2012
HARD WATER
Hard water does not produce good lather or form with soap and consume more soap.
The hard water contains bicarbonates chloride and sulphates of calcium & magnesium.
The hard water when treated with soap i.e. sodium stearate, then no lather will be
formed because sodium stearate of soap reacts with salts of calcium and magnesium giving
insoluble Ca & Mg stearate.
2C17H35 COONa + CaCl2 — 2C17H35 COOCa + 2NaCl
sodium
calcium
stearate
stearate
2C17H35 COONa + MgCl2 — 2C17H35 COOMg + 2NaCl
sodium
magnesium
stearate
stearate
SOFT WATER
The soft water when treated with soap produces more lather and consume less soap
and this is due to the absence of dissolved salts of Ca & Mg in water.
C17H35 COONa + H2O — NaOH + 2C17H35 COOH
TEMPORARY HARDNESS :
There are two types of hardness.
(1) TEMPORARY HARDNESS
It is due to the presence of only bicarbonate of calcium & magnesium. This type of
hardness can be removed by boiling the water.
Ca(HCO3)2
boil
CaCO3  + CO2 + H2O
insoluble
Mg(HCO3)2
boil
MgCO3  + CO2 + H2O
insoluble
(2) PERMANENT HARDNESS
This type of hardness is caused by the presence of chloride and sulphates of calcium&
magnesium. It can not be removed by boiling the water.
Salts producing hardness of water
❖ Temporary Hardness :
Calcium bicarbonate
Ca(HCO3)2
Mol.wt: 162gm/mole
Magnesium bicarbonate
Mg(HCO3)2
Mol.wt: 146gm/mole
❖ Permanent Hardness :
Calcium chloride
CaCl2
Mol.wt -111gm/mole
Magnesium chloride
MgCl2
Mol.wt - 95gm/mole
Calcium sulphate
CaSO4
Mol.wt-136gm/mole
Magnesium sulphate
MgSO4
Mol.wt -120gm/mole
UNITS OF HARDNESS :
(1)
DEGREE CLARK
If in a 7.0 x 104 parts (by weight) of water contains one part (by weight) of CaCO3 or
equivalent salts of Ca & Mg, then the hardness is said to be one degree clark.
(2)
DEGREE FRENCH
If in a 1 x 105 parts (by wight) of water contain one part (by weight) of CaCO3 or
equivalent salts of Ca & Mg, then the hardness is said to be one degree French.
(3)
PPM
If in a 106 parts (by weight) of water contain one part (by weight) of CaCO3 or
equivalent salts of Ca & Mg, then the hardness is said to be one ppm.
Relationship
0.070 degree clark = 0.10 degree french = 1 ppm
Hardness of solub le salt equivalent to CaCO3
=
=
W x Mol. wt. of CaCO3
Mol. wt.of soluble salt
W x 100
Mol.wt.of solublesalt
Note : The hardness is measured in terms of CaCO3 because CaCO3 is less soluble in water
and as the molecular weight of calcium carbonate is 100 gm/mole, it makes easy in
calculation.
Example - 1
A water sample contains following dissolved salt.
Ca(HCO3)2
- 8.1mg/lit.
Mg(HCO3)2
- 29.2mg/lit.
CaCl2 - 11.1mg/lit.
MgSO4 - 6.0mg/lit.
Find out temporary hardness, permanent hardness & total hardness.
Hardness of soluble salt equivalent to CaCO3 = W x lOO /Mol. wt. of soluble salt
Hardness due to Ca(HCO3)2 = 8.1 X 100/ 162 = 5mg/lit
Hardness due to Mg(HCO3)2= 29.2 X 100 / 146 = 20mg/lit
Hardness due to CaCl2 = 11.1 X 100 / 111 = 10mg/lit
Hardness due to MgSO4 = 6.0 X 100 / 120= 5mg/lit
.'. Temparary Hardness = Hardness due
+
to Ca(HCO3)2
Hardness due
to Mg(HCO3)2
= 5 + 20 = 25 mg / lit.
.'.
Permanant Hardness = Hardness due
+
to CaCl2
Hardness due
to MgSO4
= 10 + 5 = 15 mg / lit.
.'. Totalhardness = Temporary hardness + Permanant hardness
= 25 + 15 = 40 mg / lit.
To remove hardness from water, three method are used on a large scale.
(1) Soda-lime method
(2) Permutit process
(3) Ion-exchange process
PERMUTIT PROCESS
Permutit is also known as Zeolite. They are capable of exchanging ions reversibly. The
chemical formula for permutit is Na2O, Al2O3SiO2 6H2O. In short it is written as Na2-P or
Na2-Z.
For softening of water by this method, hard water is percolated at a specified rate through a
bed of zeolite kept in a cylinder. The hardness causing ions (Ca++ & Mg++) are retained by the
permutit as Ca-P & Mg-P. While the out going water contains sodium salts.
Permutit undergo following chemical reaction with temporary hardness forming salt.
Ca(HCO3)2 + Na2-P  Ca-P + 2NaHCO3
Mg(HCO3)2 + Na2-P  Mg-P + 2NaHCO3 permutit undergo following chemical
reaction with permanent hardness forming salt.
CaCl2 + Na2-P  Ca-P + 2NaCl
MgCl2 + Na2-P  Mg-P + 2NaCl
CaSO4 + Na2-P  Ca-P + Na2SO4
MgSO4 + Na2-P  Mg-P + Na2SO4
When permutit is completely converted into calcium & magnesium permutit & it
ceases to soften the water. It gets exhausted. At this stage, the supply of hard water is stopped
& the exhausted permutit is regenerated by treating the bed with concentrated 10% NaCl
solution.
Ca-P + 2NaCl 
Na2-P + CaCl2
Mg-P + 2NaCl  Na2-P + MgCl2
The washing with water is led to remove CaCl2 & MgCl2 & thus obtained regenerated
permutit can be again used.
ION-EXCHANGE PROCESS :
This is most modern method for softening hard water. By using this method almost
all salts can be removed completely from hard water and the water obtained is as good as
distill water.
In this process two types of resins are used i.e. cation exchange resin & anion
exchange resin, cation exchange resin contains (-COOH, -SO3H) function groups and are
capable of exchanging their H+ ions with cations. While anion exchange resin contains
(-NH2, -OH) functional group and are capable of exchanging OH- ions with anions.
In the process two columns, one consist of cation exchange resin & another consist of
anion exchange resin are used.
The hard water is first allowed to pass through a column containing cation exchange
resins. Which remove all the cations like Ca+2, Mg+2 etc. and release H+ ions. Reaction in
first column takes place as under.
R-H2 + CaCl2
 Ca-R + 2HCl R-H2 +
MgCl2  Mg-R -- 2HCl
R-H2 + CaSO4  Ca-R -- H2SO4
R-H2 + MgSO4  Mg-R + H2SO4
The anions like chloride & sulphates are converted into acid like HCl & H2SO4.
Which is passed through another column containing anion exchanger i.e. R-(OH)2 resin where
the following reaction takes place.
R-(OH)2 + 2HCl  R-O2 + 2H2O
R-(OH)2 + H2SO4  R-SO4 + 2H2O
Water thus obtained is free from all cations and anions & is called soft water or distill
water.
When both resins get fully exhausted then they are regenerated. The acidic resin is
regenerated by passing dilute acid solution (in first column). While basic resin is regenerated
by passing dilute NaOH solution (in second column), followed by washing with water.
Ca-R + 2HCl  R-H2 + CaCl2
in first

column
Mg-R + 2HCl
R-H2 + MgCl2
R-O2 + 2NaOH  R-(OH)2 + 2NaCl
R-SO4 + 2NaOH
 R-(OH)2 + 2Na2SO4
in second
column
The regenerated acidic and basic resins are again reused for softening of hard water.
EFFECT OF HARD WATER IN BOILER
The hard water when used for boiler, causes the following effects in boiler.
1. scale or sludge formations
2. priming and foaming
3. caustic embrittlement
4. corrosion.
(1) SCALE OR SLUDGE FORMATION :
The main causes of scale formation are
(1) On boiling water, the bicarbonates are converted into insoluble carbonate & gets
precipitated to form scale.
Ca(HCO3)2
boil
CaCO3  + CO  + H2O
Mg(HCO3)2
boil
MgCO3  + CO2  + H2O
(2) Due to evaporation, the concentration of soluble matter is increased & finally
precipitated.
(3) Colloidal matter such as silica etc., due to high temperature and concentration are
precipitated in to boiler.
Disadvantages of scale or sludge formation :
(1)
(2)
(3)
(4)
Sludges are poor conductor of heat, so they tend to waste a portion of heat generated.
Excessive sludge formation disturbs the working of boiler.
It causes choking of pipes.
Due to scale formation, over heating of boiler is done in order to maintain a constant
supply of steam. The overheating of the boiler tube makes the material
softer & weaker & this causes distortion of boiler tube & also makes the boiler unsafe to
bear the pressure of the steam.
Prevention of sludge formation
(1)
(2)
(3)
(4)
(5)
(6)
By using soft water
By frequently blow-down operation.
By removing the scale with the help of brush.
By giving thermal shocks. (i.e. heating the boiler & then suddenly cooling with cold
water)
CaCO3 scale can be removed by passing 5-10% HCl while CaSO4 scale can be removed
by passing EDTA solution.
By the addition of sodium phosphate (Na3PO4) or disodium hydrogen phosphate
(Na2HPO4), which forms a non-adherent & easily removable soft sludge of calcium &
magnesium phosphate.
3CaCl2 + 2Na3PO4  Ca3(PO4)2 + 6NaCl
(7)
By the addition of calgon [Na2{Na4(PO3)6}] (sodium hexa meta phosphate) sludge or
scale formation can be prevented.
(2) PRIMING & FOAMING
(A) PRIMING
When a boiler is producing steam rapidly some particles of the water liquid carried
along with the steam. This process of 'Wet Steam' formation is called priming.
Causes of Priming
(1)
(2)
(3)
(4)
(5)
the presence of large amount of dissolved solids
high steam velocities.
sudden boiling
improper boiler design
sudden increase in steam-production rate.
Disadvantages of Priming
(1)
(2)
(3)
Dissolved salt in boiler water are carried out by the wet steam to turbine blades which
reduces their efficiency.
Dissolved salts may enter the parts of other machinery may decrease the life of the
machinery.
Actual height of the water column cannot be judge properly, Thereby making the
maintenance of the boiler pressure becomes difficult.
Prevention of Priming
(1) By improving boiler design.
(2) By fitting mechanical steam purifiers.
(3) By maintaining low water level in boilers
(4) By using soft water.
(5) By decreasing the amount of dissolved salts.
(B) FOAMING :
It is the production of foam or bubbles in boiler which do not break easily. Causes of
Foaming :
It is due to the presence of oily substances in water.
(1) Low level of water in boiler.
(2) The presence of dissolved salts in water.
(3) Sudden increase in steam production rate.
Disadvantages of foaming :
(1) Actual height of the water column cannot be judge.
(2) Dissolved salts in water carried by the wet steam may damage turbine blads or
machinery parts.
(3) Boiler pressure cannot be maintained.
Prevention of Foaming :
(1) By the addition of anti-foaming chemicals like castor oil, Gallic acid, tennic acid
etc.
(2) removing oil from boiler water by adding compounds like sodium aluminate.
(3) CAUSTIC EMBRITTLEMEN :
:
It is a type of boiler corrosion caused by using highly alkaline water in the boiler. The
NaOH containing water flows into the minute hair-cracks, bends, and joints rivets causes the
failure of the boiler by corrosion.
Caustic embrittlement can be avoided
(i) By using sodium phosphate as softening agent instead of sodium
carbonate.
(ii) by adding tannin or lignin to boiler water
(iii) by adding sodium sulphate to boiler water.
(iv) By adjusting the pH of boiler water to 8.0 - 8.5.
(4) CORROSION :
Corrosion of boiler takes place by chemical or electro chemical reaction.
(i) Due to dissolved oxygen :
Water usually contains about 8ml. of dissolved oxygen per liter at room
temperature. Dissolved oxygen in water in the presence of prevailing high
temperature, attack the boiler materials.
2Fe + 2H2O + O2  2Fe(OH) 2 
4Fe(OH)2  + O2

2(Fe2O3 • 2H2O) 
(ii) Due to dissolved carbon dioxide
The produced carbon dioxide reacts with water to give carbonic acid
CO2 + H2O  H2CO3
which has slow corrosive effect.
(iii) Due to acids from dissolved salt
Water containing dissolved magnesium salts liberates acid on hydrolysis
CaCl2 + 2H2O

Ca(OH)2 + 2HCl
MgCl2 + 2H2O

Mg(OH)2 + 2HCl
CaSO4 + 2H2O

Ca(OH)2 + H2SO4
MgSO4 + 2H2O 
Mg(OH)2 + H2SO4
the liberaled acid produces corrosion effect.
Prevention :
(1) Corrosion can be prevented by adding alkali to neutralize acidity & anti-oxidant to
remove oxygen
(2) By keeping pH value 8 to 9.
(3) Oxygen is removed from boiler feed-water by adding Na2SO3.
(4) Oxygen can also removed by treating it with hydrazine hydrate NH2-NH2
NH2-NH2 + O2  2N2 + 2H2O
PURIFICATION OF DRINKING WATER BY SCREENING,
SEDIMENTATION, CO-AGULATION & FILTERATION.
Water used for domestic purposes must be free from germs & bacteria. It should be
free from objectionable dissolved gases like H2S & dissolved salts like, lead arsenic &
manganese salts. Generally for domestic supply, surface water is used & it is contaminated
with large number of impurities such as, organic matter, suspended impurities etc. therefore to
make it safe for drinking purpose, following treatment processes are employed.
(1) Screening :
Screening is the process of removing floating materials from water. Raw water is
allowed to pass through a screen having a large number of perforations which removes the
large & small floating matter.
(2) Sedimentation :
It is the process of removing insoluble impurities by allowing the water to stay
undisturbed for some time. Water is allowed to settle in big tanks or reservoirs for a number
of hours. The suspended materials, other than living organisms which float due to the gases
they produce, settle down at the bottom due to the force of gravity. The process of
sedimentation is generally carried out in continuous flow type tanks in which water flows
continuously in horizontal, radial or vertical directions at uniform rate.
Horizontal flow tank consists of a rectangular tank with channel type inlet & outlet
extending throughout the width of the tank. It is provided with buffle walls to
reduce the velocity of the incoming water. The sedimented sludge is taken out through the
outlet provided at the bottom.
(3) Co-agulation :
Co-agulation is the process of removing fine sized particles from water with addition
of certain chemicals known as co-agulants.
Actually the fine sized particles present in water either do not settle down at all or
take a long time. In order to facilitate quick settling of these particles, some chemical agents
known as co-agulants are used. The commonly used co-agulants are the salts of iron &
aluminium. e.g. alum [ K2SO4 Al2(SO4)3 • 24H2O ], aluminium sulphate Al2(SO4)3 18H2O,
ferrous sulphate FeSO4 • 7H2O, ferric chloride FeCl3 etc. These salts react with carbonate &
bicarbonate radicals present in water & form co-agulable precipitates (flocks) of hydroxide of
these metals. The precipitated hydroxide absorbs the suspended impurities, bacteria & other
micro-organisms & causes them to settle down.
(4) Filtration :
When sludge etc. after co-agulation has settled down, Alteration is carried out by
means of filters. Filtration is the process of removing insoluble, colloidal & bacterial
impurities from water by means of filters. The filters used in water filtration consist of several
layers of sand particles of different size.
Gravity send filter consists of a large rectangular take made of concrete. The filtering
medium in it consists of 3 feet of find send below which 1 feet thick coarser sand which
inturn is supported by 8 inches thick bed of graded gravel. Below the graded gravels are drain
through which filtered water goes out.
As the water percolates ( due to gravity ) through the fine sand bed, most of the
objectionable materials are absorbed by it & clear water collects in the under drain channel,
form where it is drawn out.
Although most of bacteria & micro-organisms present in water are removed by
co-agulation followed by settling & filtering through sand beds, yet water contains a small
percentage of pathogenic bacteria. These bacteria must be completely removed particularly
from water supplied by municipality for drinking water purposes. Otherwise water borne
diseases may be produced. The process of destroying these pathogenic bacterial &
micro-organisms is known as sterilization or disinfection.
Chlorine is a commonly used sterilizer in municipal water supply. It can be used
directly as a gas or as chlorine water. Its germicidal action is based on its reaction with water.
It reacts with water to form hypochlorous acid & nascent oxygen both of which are powerful
germicides.
Cl2 + H2O  HOCl + HCl
HOCl 
HCl + [O]
However, excess of chlorine should be avoided because it produces characteristic
unpleasant taste & odour & irritating effect on mucous membrane. For filtered water about
0.3-0.5 ppm of chlorine is sufficient. The treated water should not contain more than 0.1-0.2
ppm of free chlorine.
(ii) Bleaching powder :
Bleaching powder is a good sterilizer for small water works. In practice about 100
liters of water is mixed well with about 1kg of the powder & the resulting solution allowed to
stand for several hours. Hypochlorous acid & nascent oxygen produced by action of water on
bleaching powder are powerful germicides.
CaOCl2 + H2O
—
Ca(OH)2 + Cl2
Cl2 + H2O
—
HOCl +
HOCl
—
HCl + [O]
HCl
Bleaching powder should be used only in calculated amount because excess of it will
give a bad taste & disagreeable odour. While lesser amount of it will not sterilize the water
completely.
Question asked in T.E.B. exam
Q.1
Write the chemical reactions involved in permutit process.
Q.2
A sample of hard water gave on analysis the following results. Calculate the
temporary and permanent hardness in ppm.
Ca(HCO3)2 = 81 mg/l
CaCl2 = 11.1 mg/l
Mg(HCO3)2 = 73 mg/l
MgCl2 = 47.5 mg/l
MgSO4
= 6.0 mg/l
CaSO4 = 6.8 mg/l
Q.3
List the effects of hard water when used in boiler. Explain scale and studge formation
and its prevention.
Q.4
What is Hard water ?
Q.5
What is French degree ?
Q.6
Total hardness of a water sample is 250 ppm. Calculate the hardness of that water
sample in degree clark and degree French.
Q.7
What is co-agulation ?
Q.8
How to prevent foaming in boiler ?
Q.9
Explain zeolite method for softening of water with figure.
Q.10
Explain sedimentation with co-agulant.
Q.11
Explain chlorination.
Q.12 Write bad effects of scale formation in boiler
Q.13
A sample of water on analysis gives the following results. Calculate total hardness of
the given sample of water.
Mg(HCO3)2
- 7.3 mg/l
CaSO4
- 13.6 mg/l
MgCl2
- 19.0 mg/l
Q.14
Write only the names and formulas of the salts which produce the hardness of water
Q.15
A sample of water on analysis give the following results. Calculate the total
hardness of the given sample of water and convert it in to degree French.
Mg(HCO3)2 = 14.6 mg/lit.
MgCl2
= 19.0 mg /lit.
CaSO4
= 34.0 mg/lit.
Q.16 What is hard water ? Give the types of hardness.
Q.17
A sample of hard water gave on analysis the following results. Calculate the
temporary and permanent hardness of water in ppm.
Ca(HCO3)2 = 8.1 mg/l
MgSO4 = 6.0 mg/l
Mg(HCO3)2 = 29.2 mg/l
MgCl2 = 47.5 mg/l
CaCl2
= 11.1 mg/l
CaSO4 = 6.8 mg/l
Q.18 Define caustic Embrittlement and explain how it can be prevented.
Q.19
Write short note on Demineratization of water. (Ion-exchange process)