Hardness of Water for 1st Year B Tech Students

Dr. Vivek Pandey

Rajasthan Technical University Kota

1
 WATER
• Water is an colorless, tasteless, odorless and transparent chemical substance, H2O, which is main constituent of Earth’s
hydrosphere. It exist in solid, liquid and gaseous forms as ice, water and vapor and very useful for the mankind.
H2O(Solid) ⇌ H2O(Liquid) ⇌ H2O (Gas)
• Our earth is blue planet as water covers around 71% of its surface. 97 % of available water is an ocean/sea water which is
almost useless cannot be used for mankind. Only about 2.5 percent of the Earth's water is freshwater and most of that water
(98.8 percent) is in form of ice and groundwater.
• Water is vital for the survival of the all forms of life, and also used for agriculture purpose.
• Water is very use full engineering material it is used as steam for power generation and to run various process industries. It
is also being used as universal solvent in chemical processing units.
• Rain water is supposed to be pure source of water this involves evaporation of water from the earth’s surface during
summers followed by condensation and precipitation as rain.
• Rain water or melted snow flows down the slope over the earth’s surface and due to hydrolysis or hydration reactions water
gets polluted and impurities are added in to the water as dissolved salts and makes the water a hard water.

H2O + CaCO3 → No Reaction

H2O + CaCO3 + CO2 → Ca(HCO3)2
 HARDNESS OF WATER

Hard water is water that has high content of dissolved minerals and is formed when water percolates through deposits
of limestone, chalk, gypsum and other minerals. Hardness in water is imparted by the presence of soluble salts of Ca,
Mg, Fe, Al and Mn in the form of chlorides, sulfates and bicarbonates. However the major contribution of hardness is
by Ca and Mg while Fe, Al and Mn contributes the hardness at a trace level.

Hardness of water may also be defined as the soap consuming capacity of water in giving the lather when water is
mixed with soap. Higher the consumption of soap higher will be the hardness of water. Soap is sodium/potassium salt
of higher fatty acid and when hard water is mixed then it forms insoluble calcium/ magnesium stearate which is useless
white scum having no detergent value.

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

(White Scum)

Hardness of water may have moderate health benefits, but can pose critical problems in its industrial uses where water
hardness may cause costly breakdowns in boilers, cooling towers and other equipment that handles water. Hard water
buildup scale inside water supply pipes, restricting water flow.
 TYPES OF HARDNESS
Hardness of water is categorized as temporary hardness and permanent hardness depending upon the nature salt is dissolved in
water.

TEMPORARY HARDNESS: Hardness of water is temporary when bicarbonates of Ca/Mg/Fe/Al/Mn etc. are soluble in
water and also known as carbonate hardness. This temporary hardness is simply removed by boiling the hard water. When
water boils the soluble bicarbonate decomposes to give the insoluble precipitate of calcium carbonate or magnesium
hydroxide:

Ca(HCO3)2 → CaCO3↓ + CO2↑ + H2O

Mg(HCO3)2 → Mg(OH)2↓ + 2CO2↑

PERMANENT HARDNESS: When chlorides or sulfates of Ca/Mg/Fe/Al/Mn etc. are soluble in water, hardness is called
permanent hardness and also known as non-carbonate hardness. Permanent hardness of water can be removed by using
following water softening methods:
1. Lime Soda softening method
2. Zeolite Softening method
3. Deionization softening method
 UNITS TO EXPRESS HARDNESS OF WATER

The hardness of water is due to the presence of bicarbonates, chlorides and sulfates of calcium and magnesium
and other ions. The hardness of water is expressed in terms of calcium carbonate, CaCO3. The calcium carbonate
is preferably most stable compound among all the hardness causing ions (Ca/Mg/Fe/Al/Mn etc.). Therefore, to
express the hardness we have to convert the concentration of hardness causing salts, equivalent in terms of
CaCO3. Following formula is being used to covert the concentration any hardness causing salt equivalent to
CaCO3:
Concentation of Hardness causing salt
Equivalent as CaCO3 = x 100(M W of CaCO3)
Molecular weight (M W) of hardness causing salt
UNITS FOR HARDNESS:
1. Milligram Per Liter(Mg/Liter): It is the number of mille gram of hardness causing salts, equivalent in
terms of calcium carbonate, present in one liter of water.
2. Parts Per Million(PPM): It is the number of parts of hardness causing salts, equivalent in terms of calcium
carbonate, present in million (106) parts of water.
3. Degree French(oFr): It is the number of parts of hardness causing salts, equivalent in terms of calcium
carbonate, present in 105 parts of water.
4. Degree Clark(oCl): It is the number of grains of hardness causing salts, equivalent in terms of calcium
carbonate, present in one gallon(70000 grains) of water.
1 Mg/Liter = 1 PPM = 0.1 oFr = 0.07 oCl
 DETERMINATION OF HARDNESS OF WATER
Complexometric EDTA Method
The complexometric method is supposed to be a most accurate method of water hardness determination. In this method
ethylene diamine tetra acetic acid (EDTA) and sodium-1-(1- hydroxyl-2-naphthylazo)-6-nitro-2-naphthol-4-sulphonate
also called Eriochrome Black T (EBT) is used. EDTA a strong chelating agent which forms strong stable complexes
with hardness causing Ca2+ and Mg2+ ions present in the water while EBT acts as indicator and forms the weak
complexes with Ca2+ and Mg2+ ions.

Structure of EDTA

Structure of EBT Indicator

The stable complex of EDTA and weak complexes of EBT with hardness causing Ca2+ and Mg2+ ions present in the
water is only formed when pH of the medium is alkaline. Ammonia buffer is therefore used, in this complexometric
titration method, to maintain the pH of the medium at around 10.

Ca2+ (Mg2+) + EDTA → [Ca2+ (Mg2+) EDTA](Stable Complex)

Ca2+ (Mg2+) + EBT → [Ca2+ (Mg2+) EBT] (Weak Complex)

PRINCIPLE: In complexometric determination of hardness, ammonia buffer is first added to a known quantity
of hard water sample to which EBT indicator is added. The weak complex formation takes place as a result wine
red color is developed.
Ca2+ (Mg2+) + EBT → [Ca2+ (Mg2+)EBT] (Weak Complex)
Blue Color Wine Red Color

On further addition of standard EDTA solution from burette the weak complex of hardness causing Ca2+ and
Mg2+ ions and EBT indicator is first braked and a strong complexes between Ca2+ and Mg2+ ions and EDTA is
formed. At the end point all the weak complexes of EBT are converted in to strong complexes of EDTA and
color of the medium changes to blue the color of free EBT indicator.

[Ca2+ (Mg2+)EBT] (Weak Complex) + EDTA → [Ca2+ (Mg2+)EDTA](Stable Complex) + EBT

Wine Red Color Blue Color
REQUIREMENTS: Hard water sample, Standard EDTA solution, Eriochrome Black T indicator, ammonia
buffer, distilled water, burette, pipette, 250 mL conical flask and measuring flask.
PROCEDURE:
• Fill the burette with standard EDTA solution and take known volume of hard water as V1 ml sample into 250 mL
conical flask.
• Add about 5 mL of ammonia buffer and few drops of EBT indicator. Color of the medium becomes wine red because
of the formation of weak complex between EBT and Ca2+ and Mg2+ ions.
• Start adding standard EDTA solution from the burette drop wise in the conical flask till blue color appears as end
point is attained as result of the formation of strong complex of EDTA with Ca 2+ and Mg2+ ions by breaking the weak
complex of EBT.
• Volume of Standard EDTA is noted as V mL.
• Take three readings and finally volume V2 of Standard EDTA is considered as concordant volume for the calculation
purpose.
OBSERVATION TABLE:
Sr Volume of Hard water Volume of standard EDTA use (ml) Actual Volume of
No. taken (ml) Initial Reading Final Reading EDTA used (ml)

1 V1 0.0 V
2 V1 0.0 V V2

3 V1 0.0 V
CALCULATION:
From observation table we know that hardness of V1 volume of hard water is neutralized with V2 ml of standard EDTA
solution having known normality N2.
We know that, N1 × V1 = N2 × V2
Strength of hardness in hard water, N1 = (N2 × V2) / V1
Hardness of hard water in gm/Liter = {(N2 × V2) / V1} × Equivalent Wt. of CaCO3
Hardness as CaCO3 in ppm = {(N2 × V2) / V1} × 50× 1000 ppm or mg/Liter

RESULT:
Hardness of given hard water sample as CaCO3 is …………….. ppm or mg/Liter.
On the basis of the analysis result of the hardness of water sample, water is generally categorized as:

Water Ratings as per the Concentration as Calcium Carbonate

Hardness (mg/L)
Soft 0 to <75
Medium hard 75 to <150
Hard 150 to <300
Very hard 300 and greater
Water Softening -1(Lime-Soda Process) 1st Year B Tech Students
Dr. Vivek Pandey
Rajasthan Technical University Kota
Rawatbhata Road, Akelgargh, Kota-324010

1
 LIME-SODA WATER SOFTENING PROCESS
Water Softening :
Softening of water is a process by which hardness causing soluble salts of calcium, magnesium, iron and aluminum etc.,
present in the water, are removed. Removal of hardness from water is the utmost requirement for the industries to avoid
unwanted breakdowns and also for using water for the domestic purposes. The softening of water is done by following
three main methods:
1. Lime Soda Process
2. Zeolite Process
3. Deionization Process

Lime-Soda Process: The lime soda process is a chemical method of water softening. In this process calcium
hydroxide [Ca(OH)2)] and sodium carbonate (Na2CO3 ), commercially called lime and soda, are used to facilitate the
chemical reactions with the hardness causing metal ions (Ca/Mg/Fe/Al etc.) and convert them into insoluble precipitates
in the form of hydroxides and carbonates. Lime and soda both are added in such a concentration so that they furnishes
sufficient concentrations of hydroxyl (OH)- and carbonate CO3- - ions into the hard water. The hardness causing metal
ions due to the reactions phases out as insoluble precipitates, in the form of hydroxides and carbonates, and are further
filtered out to get soft water.
Role of Lime [Ca(OH)2)] : Calcium hydroxide when added into the hard water it dissociates to give hydroxyl ions.
The quantity of lime is added in such a concentration that it furnishes sufficient hydroxyl ions in water for complete
precipitation of hardness causing metal ions, mineral acids and other unwanted gases which are not required in industry
and are successfully removed.
Ca(OH)2 → Ca++ + 2(OH)-
Lime Hydroxyl ion

1. Removal of soluble salts causing temporary Hardness:

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

Mg(HCO3)2 + 2Ca(OH)2 → Mg(OH)2 (↓) + 2CaCO3 (↓) + 2H2O
2. Removal soluble salts causing permanent hardness:
MgSO4 + Ca(OH)2 → Mg(OH)2 (↓) + CaSO4
FeCl2 + Ca(OH)2 → Fe(OH)2 (↓) + CaCl2

Al2(SO4) 3 + 3Ca(OH)2 → 2Al(OH)3 (↓) + 3CaSO4

NaAlO2 + 2H2O → Al(OH)3 (↓) + NaOH

3. Removal soluble dissolved gases:
CO2 + Ca(OH)2 → CaCO3 (↓) + 2H2O
H2S + Ca(OH)2 → CaS (↓) + 2H2O
4. Removal of free mineral acids:
2H+ + Ca(OH)2 → Ca++ + 2H2O
2HCl + Ca(OH)2 → CaCl2 + 2H2O
H2SO4 + Ca(OH)2 → CaSO4 + 2H2O
5. Removal of bicarbonate ion:
2HCO3- + Ca(OH)2 → CaCO3 (↓) + CO3- - + 2H2O
Role of Soda (Na2 CO3): Sodium carbonate, soda, when added into the hard water it dissociates to give carbonate
ions.
Na2CO3 → 2Na+ + CO3- -
Soda Carbonate ion
The quantity of soda is added in such a concentration in the hard water so that it furnishes sufficient amount carbonate ions
for the complete precipitation of permanent calcium hardness initially present in hard water. The soda is also required for
the precipitation permanent calcium hardness additionally added in to the water due to reaction of lime, as mentioned
above.
CaCl2 + Na2CO3 → CaCO3 (↓) + 2NaCl
CaSO4 + Na2CO3 → CaCO3 (↓) + Na2SO4
Softening Process: In lime-soda softening process sufficient amount lime and soda in the form of suspension is added
into the hard water. The precipitation of hardness causing metal ions takes place as per the chemical reactions of lime and
soda as discussed above. These insoluble precipitates in the form of hydroxides and carbonates are removed from the
softener and we get soft water for industrial use. There are two types of lime-soda softener cold and hot process which
runs at room temperature and at 100oC respectively. The efficiency of the cold process is to reduce the hardness of hard
water up to 50-60 ppm while hot process is more efficient and reduces the hardness up to 15 to 30 ppm.
Based on the chemical reactions in lime-soda softening process the amount of lime and soda can be calculated on the basis
of the analysis report of hard water. The formula used for calculating the amount of lime and soda required for softening
hard water is as follows:
𝟕𝟒
Lime =
𝟏𝟎𝟎
(
X Ca(HCO)3 + 2 Mg(HCO)3 + Mg(Cl2/SO4) + Fe(Cl2/SO4) + Al2(SO4)3 + AlCl3 + CO2 + H2S + HCl+
𝟏𝟎𝟎
H2SO4 + HCO-3 – NaAlO2 ………. All equivalent as CaCO3 ) x V(Volume of Water) x % 𝐏𝐮𝐫𝐢𝐭𝐲 mg

𝟏𝟎𝟔
Soda =
𝟏𝟎𝟎
(
X Ca(Cl2/SO4) + Mg(Cl2/SO4) + Fe(Cl2/SO4) + Al2(SO4)3 + AlCl3 + HCl + H2SO4 - HCO-3 …….. All
𝟏𝟎𝟎
equivalent as CaCO3 ) x V(Volume of Water) x % 𝐏𝐮𝐫𝐢𝐭𝐲 mg
 Batch Type Cold Lime Soda Softener

Continuous Type Cold Lime Soda Softener

 Continuous Type Hot Lime Soda Softener

Comparison of Cold and Hot Lime-Soda process

S.No. Cold Lime-Soda Process Hot Lime-Soda Process

1 Rate of softening is slow at normal temperature Rate of softening is fast at high temperature

2 Reduces the hardness up to 50-60 ppm Reduces the hardness up to 15-30 ppm

3 Runs at normal room temperature and no fuel is required Runs at 100oC temperature and fuel is required
4 More consumption of Lime and soda and Coagulants are required Less consumption of Lime and Soda and coagulants are not required
5 It has low softening capacity It has high softening capacity
 Numerical
Based on
Lime and Soda
Softening Process
 Sr No. Name of Salt Mol. Wt. Multiplication Factor
of Salt (M.F.) = 100/Mol. Wt.
1 Ca(HCO3)2 162 100/162
2 Mg(HCO3)2 146 100/146 Important Points:
3 CaCO3 100 100/100
4 MgCO3 84 100/84 1. If Ca++ and Mg++ are given
Multiplication Factor

then they are treated as

5 Ca++ 40 100/40
permanent calcium and
6 CaCl2 111 100/111 permanent magnesium
7 CaSO4 136 100/136 hardness.

8 Mg++ 24 100/24 2. If CaCO3 and MgCO3 are

9 MgCl2 95 100/95 given then they are treated
as temporary calcium and
10 MgSO4 120 100/120
temporary magnesium
11 CO2 44 100/44 hardness.
12 H 2S 36 100/36
3. If K+ / Na+ / NaCl / KCl /
13 H+ 1 100/(1 x 2) Na2SO4 /K2SO4 / SiO2/
14 HCl 36.5 100/(36.5 x 2) Fe2O3 are given in the
numerical then these salts
15 H2SO4 98 100/98
are not considered and
16 Al2(SO4)3 342 (100 x 3)/342 neglected in calculating the
17 AlCl3 133.5 (100 x 3)/133.5 requirement for lime and
soda.
18 FeSO4.7H2O 278 100/278
19 NaAlO2 82 100/82
20 HCO-3 61 100/(61 x 2)
Numerical of Lime and Soda Softening Process
Water Softening -2 (Zeolite Process) 1st Year B Tech Students
Dr. Vivek Pandey
Rajasthan Technical University Kota
Rawatbhata Road, Akelgargh, Kota-324010

1
 ZEOLITE SOFTENING PROCESS
Zeolite process of water softening, is also called a Permutit Method, it involves the removal hardness causing metal
ions (Ca/Mg/Fe/Al etc.) by the available sodium ions (Na+) of zeolite. This softening process works on the principle
of ion exchange where there is reversible exchange, of loosely bonded sodium ions, present in the Zeolites.
Zeolites are naturally occurring hydrated crystals of sodium alumino silicate and is represented as Na2Z. It has been
considered as good softening material as they are solid, stable and porous in nature so that water can easily
percolates through zeolite crystal. The loosely bonded sodium ions present in the three dimensional framework of
silicates (SiO2)n, are freely available to be replaced by hardness causing metal ions present in hard water.
The efficiency of zeolite softening process is to reduce the hardness of the hard water up to 10 to 15 ppm.

Micro pores
present in Zeolite

Structure of Zeolite
Na2O. Al2O3. xSiO2. yH2O; where x = 2 to 10 and y = 2 to 6
 Softening Process

In zeolite softening process hard water is allowed to pass through the zeolite bed of the softener. The hardness causing
metallic ions are replaced by the loosely bonded sodium ions available in zeolite and the hardness of water is removed. The
softening of water or removal metal ions will continue till loosely bonded sodium ions are available with the zeolite as
shown in the above figure. After running the softener for sufficient period of time, a stage will come when zeolite bed is
fully saturated with hardness causing metal ions and there will be no sodium ions are available for further softening of
water. The reactions taking place during softening of water are as follows:
Na2Z + Ca(HCO3)2 → CaZ + 2NaHCO3
Na2Z + Mg(HCO3)2 → MgZ + 2NaHCO3
Na2Z + MgCl2 → MgZ + 2NaCl
Na2Z + CaSO4 → CaZ + Na2SO4
Fresh Zeolite Saturated Zeolite

In above mentioned reactions Na2Z represents fresh zeolite with available sodium ions while CaZ/MgZ represents
saturated or exhausted zeolite after softening the hard water.
 Regeneration of Zeolite
The exhausted zeolite CaZ/ MgZ after softening the hard water can be regenerated by spraying the NaCl solution through the
injector. When NaCl solution passes through the zeolite the available Ca/Mg ions, of saturated zeolite, are reversibly replaced by
the sodium ions of NaCl. The exhausted/ saturated zeolite can be regenerated again and again and the available sodium ions will
further be used for softening the hard water. The reactions taking place during regeneration of exhausted zeolite are as follows:
CaZ + 2NaCl → Na2Z + CaCl2
MgZ + 2NaCl → Na2Z + MgCl2
Saturated Zeolite Regenerated Zeolite

In the above reactions CaCl2 and MgCl2 released after regenerating the zeolite are removed through the sink valve of the softener
during the regeneration process.

Zeolite Softener
 Limitation of Zeolite Process
1. Hard water containing turbidity or suspended impurities cannot be processed as they can clogs the pores of the zeolite and slowdown
or stop the softening process. Turbidity or suspended impurities are first removed by filtering the hard water.
2. Zeolite softening process is suitable for the removal of Ca and Mg ions. Water containing Fe, Al and Mn ions forms non regenerative
zeolites as a result zeolite gets wasted.
3. The soft water obtained by this process contains several dissolved sodium salts and the soft water is not fit for the boilers use.
4. Hard water containing mineral acids usually destroys the zeolite so they are to be first neutralized first before it's softening.
5. The presence of NaHCO3 in treated soft water causes the problem of priming and foaming when used in boilers. use.

Advantages of Zeolite Process

1. Effective and efficient softening process as treated water has reduced hardness of the order of 10 to 15 ppm.
2. Water softening rate is fast as compared to the lime soda process
3. Zeolite softeners are very compact and its operational cost is also very low as zeolites are easily regenerated again and again using
NaCl solution.
4. There is no problem of sludge in treated soft water as there is no precipitation during softening process.
The formula used to calculate the hardness of water by knowing the amount of NaCl solution used to regenerate the zeolite
for softening V Liters of hard water:
𝒈𝒎
𝑪 (𝑪𝒐𝒏𝒄𝒆𝒏𝒕𝒂𝒕𝒊𝒐𝒏 𝒐𝒇 𝑵𝒂𝑪𝒍 𝒊𝒏 ) 𝐱 𝑽𝒐𝒍𝒖𝒎𝒆 𝑵𝒂𝑪𝒍 𝐱 𝟏𝟎𝟎 𝑴𝑾 𝒐𝒇 𝑪𝒂𝑪𝑶3 𝐱 𝟏𝟎𝟎𝟎
𝒍𝒊𝒕𝒆𝒓
Hardness of water in mg/Liter =
𝟏𝟏𝟕 𝐱 𝑽(𝑽𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝑯𝒂𝒓𝒅 𝑾𝒂𝒕𝒆𝒓)

Comparison of Zeolite and Lime-soda Process

Numerical Based
on Zeolite
Softening Process
Water Softening -3 (Demineralization Process) 1st Year B Tech Students
Dr. Vivek Pandey
Rajasthan Technical University Kota
Rawatbhata Road, Akelgargh, Kota-324010

1
 Demineralization Softening Process
Demineralization(DM) process is the ultimate technology for softening hard water. This is also called Deionization softening process. Water
obtained from DM softener is mineral free, means water without any cationic and anionic impurities. Treated water is called Demineralized
Water in which all the soluble salts of Na/K/Ca/Mg/Al/Fe etc. present in hard water has been totally removed. The efficiency of DM
softening process is best among all the available softening processes and the soft water has hardness up to 0 to 2 ppm only. Demineralized
water finds wide application in generating steam in boilers, electricity and in different process industries and also as coolants.
Demineralization is a physical process which works on the principle of reversible ion exchange where all types of, cationic impurities
(Na+/K+/Ca2+/Mg2+/Fe2+/ Al3+ etc.) and anionic impurities (Cl-/HCO 3-/SO 42-etc.), are replaced by H+ and OH- ions, respectively, available i n
the softening material of DM plant. Softening material used in DM softeners are ion exchange polymeric resins and commonly used resin is
styrene divinylbenzene coploymer which is water insoluble, stable, nonreactive, porous and are of high molecular weight. These water
softening ion exchange resins have acidic H+ and basic OH- functional groups which reversibly replaces all the cationic and anionic
impurities, respectively, present in hard water. Demineralizers gives soft water of exceptionally high purity to be used in industries.

Structure of Styrene Divinylbenzene Copolymer

 Softening Process
DM softener is a two column softener where one is cation exchange used to replace cationic impurities with its acidic H+
function group while other is anion exchange column used to replace anionic impurities with its OH- functional groups. The
working of cation and anion exchange column's can be described as:
Cation Exchange Column
Styrene divinylbenzene copolymer after its carboxylation or sulphonation reactions, becomes cation exchange resin, which
have H+ acidic functional groups used for the reversible replacement of cationic impurities present in hard water. The resin is
represented as RH+ where R represents polymer matrix and H+ is freely available acidic functional group to be used for the
removal cations from the water.
Softening Reactions
The softening reactions taking place when hard water is allowed to pass through
RH+ resin of cation exchange column are as follows:

2RH + + MgCl2 → R2Mg + 2HCl

2RH + + CaSO4 → R2Ca + H2SO4
RH + + NaHCO3 → RNa + H2CO3
Fresh Resin Exhausted Resin
In above reactions RH+ represents fresh Resin while R2Mg/R2Ca/RNa represents
exhausted resin after removal of cations from the hard water while
acids(HCl/H2 SO4/H2CO3) are added into the treated water. The exhausted resin
is further regenerated with H+ acidic functional groups for further replacement Cation Exchange Resin
or removal of cationic impurities of hard water.
 Regeneration of Cation Exchange Resin
The exhausted resin R2Mg/R2Ca/RNa after replacing cations of hard
water, it can be regenerated by spraying the HCl solution through the
injector. When HCl solution passes through the exhausted resin, having
Na+/K+/Ca2+/Mg2+/Fe2+/ Al3+ etc. cations, are reversibly replaced by the
H+ ions of HCl. The exhausted/ saturated resin can be regenerated again
and again and the available H+ acidic functional groups will further be
used for removing cationic impurities from the hard water. The reactions
taking place during regeneration of exhausted cation exchange resin are as
follows:
R2Mg + 2HCl → 2RH+ + MgCl2
R2Ca + 2HCl → 2RH+ + CaCl2
RNa + HCl → RH+ + NaCl
Exhausted Resin Regenerated Resin

In the above reactions CaCl2/MgCl2/NaCl released after regenerating the

exhausted resin are removed through the sink valve of the cation Cation Exchange Column
exchange column of the softener during the regeneration process.
 Anion Exchange Column
Styrene divinylbenzene copolymer having quaternary ammonium (-NR3) group after its reaction with NaOH,
becomes anion exchange resin, which have OH- basic functional groups used for the reversible replacement of
cationic (Cl-/CO3-/SO42-) impurities present as acids in the treated water obtained from cation exchange column.
The resin is represented as ROH- where R represents Polymer matrix and OH- is freely available basic functional
group to be used for the removal anions from the treated of cation exchange column.
Softening Reactions
The softening reactions taking place when treated water of
cation exchange column is allowed to pass through ROH- resin
of anion exchange column, are as follows:
ROH - + HCl → RCl + H2O
2ROH - + H2SO4 → R2SO4 + H2O
2ROH - + H2CO3 → R2CO3 + H2O
Fresh Resin Exhausted Resin
In above reactions ROH- represents fresh Resin while
RCl/R2 SO4/R2CO3 represents exhausted resin after removal of
anions from the treated water and the acids present in the treated
water in cation exchange column is removed and we get only
H2O from the anion exchange column of the softener. The
exhausted resin is further regenerated with OH- basic
functional groups for further replacement or removal of anionic
Anion Exchange Resin
impurities of treated water.
 Regeneration of Anion Exchange Resin

The exhausted resin RCl/R2 SO4/R2CO3 after replacing anions can be

regenerated by spraying the NaOH solution through the injector. When NaOH
solution passes through the exhausted resin, having Cl-/CO3-/SO42- anions are
reversibly replaced by the OH- ions of NaOH. The exhausted/ saturated resin
can be regenerated again and again and the available OH- basic functional
groups will further be used for removing anionic impurities of treated water.
The reactions taking place during regeneration of exhausted anion exchange
resin are as follows:

RCl + NaOH → ROH- + NaCl

R2SO4 + 2NaOH → 2ROH- + NaSO4
R2CO3 + 2NaOH → 2ROH- + Na2CO3
Exhausted Resin Regenerated Resin

In the above reactions NaCl/Na2SO4/Na2CO3 released after regenerating the

exhausted resin are removed through the sink valve of the anion exchange
column the softener during the regeneration process.
Anion Exchange Column
Demineralization Softener
Comparison of Lime-soda Process/Zeolite/Demineralization Processes
Boiler’s Trouble-1(Scale & Sludge Formation)
For 1st Year B Tech Students Of Engineering Chemistry
Prof. Vivek Pandey
Rajasthan Technical University Kota
Rawatbhata Road, Akelgargh, Kota-324010
1. Dimension
2. Material Selection
3. Thickness of Metal
4. Steam Space
5. Blowdown
6. Fire
 Boiler Troubles
Water finds great use in various industries for steam generations in
boilers. A boiler is a closed vessel in which water under pressure is
transformed into steam by the application of heat. Feed water for
boilers containing dissolved salts such as bicarbonates, sulphates,
chlorides of calcium & magnesium, iron, salts, silica, alumina etc. has
more serious effects on smooth functioning of boilers. Improper
supply of boiler feed water causes following boiler troubles :-

• Formation of Scale & Sludge (Formation of Solids)

• Priming & Foaming (Carry Over)
• Caustic Embrittlement
• Corrosion
 Formation of Solids in Boiler
(Scale & Sludge Formation)
In boilers, water in converted into steam which is a continuous process. During the formation of
steam only water molecules of boiler feed water are converted into steam, as a result the total
dissolved solids are precipitated and due to some hydrolysis reactions the solids are formed
within the boiler. These formed solid are called SCALE & SLUDGE deposits.

Reasons for the formation of deposits:-

1) Saturation of Boiler Water:
Boilers are employed for the steam generation and during the formation of steam
evaporation of boiler water takes place. The boiler water is continuously heated to produce
steam as a result there is increase in the concentration of dissolved solids as more and more
water is removed inside the boiler in the form of steam. Due to continuous formation steam
boiler water progressively gets concentrated with dissolved salts and reaches the saturation
point. Finally when the solubility product value these soluble dissolved salts is exceeded the
precipitation of soluble salts takes place and the precipitated insoluble salts are responsible
for the formation of solids in the boilers. The deposition due to this reason is more
pronounced in a high pressure boiler because a greater amount of steam in generated within
a shorter time.
2) Inverted Solubility Trend of Dissolved salt:
In any solution solubility of salt increases with increase in temperature and this is the
natural/normal solubility trend of salts. If the solubility of salt decreases with increase
in temperature they have inverted solubility trend and these types of salt gets
precipitated out in the form of solids at high temperature of solution. In boilers also
boiler water if dissolved salt like CaSO4 is present, the CaSO4 have inverted solubility
trend, therefore at increased boiling temperature inside the boiler water the soluble
CaSO4 is converted into insoluble salt and is responsible for the formation of solids in
boiler.
3) Chemical Reactions inside the Boiler:

A. Decomposition of calcium bicarbonate: The calcium bicarbonate at high

temperature decomposes to calcium carbonate which is insoluble salt and forms the
solids inside the boilers.
Ca(HCO3)2 → CaCO3 ↓ + H2O + CO2

B. Hydrolysis of Magnesium salts: Magnesium salts present in boiler water under goes
hydrolysis reaction at high temperature inside the boiler forming insoluble
precipitate of Mg(OH)2 which leads to solid formation inside the boilers.
MgCl2 + H2O → Mg(OH)2 ↓ + 2HCl

C. Presence of silica: Silica (SiO2) if it is present even in small quantities in boiler

water, they at high temperature easily reacts with calcium and magnesium ions to
form deposits as Calcium silicates (CaSiO3) or Magnesium silicates (MgSiO3) and
forms the solids inside the boilers.
 Formation of Solids/Deposits
Scale
Depending upon the nature of the solids formation inside the boilers they are
categorized as Scales and Sludges. The deposits or solids formed in boilers
which forms tight adherent coatings or getting tightly bonded on the metal
surface inside the boiler are called Scales and the phenomenon of formation
scales is called Scaling. They are formed when the deposits of salts like
CaCO3, Mg(OH)2, CaSO4 , CaSiO3 and MgSiO3 are present in boiler water.

Scale Formation
 Sludge
On the other hand when the deposits or solids which remains in the
suspended form inside the boiler are called Sludges and phenomenon is
called Sludging. Sludges are soft, loosely and slimy solids formed at
comparatively colder portions of the boiler and collected in the area where
flow rate is slow. They are formed when the deposits of salts like MgCO3,
MgCl2, CaCl2 and MgSO4 are present in boiler water.

Sludge Formation
Disadvantages of Sludges:
1. Sludges are poor conductor of heat and results in the wastage of heat and fuel.
2. Sludge may gets entrapped in scale and finally form stable scale.
3. It reduces the steam space in boilers.
4. Excessive sludge formation leads to the settling of sludge in slow circulation areas
such as pipe connections, plug openings leading to the choking of the pipes.

Removal of Sludges:
1. By using boiler feed water as much soft as we can i.e. perfectly softened water.
2. By using soft water which is free from dissolved salts like MgCO3, MgCl2, CaCl2 and
MgSO4 can prevent the sludge formation.
3. By carrying out blow down process in which boiler water concentrated with sludge are
replaced time to time with fresh soft water so that concentration of dissolved salts is
maintained within the prescribed limit in Boiler water.
Disadvantages of Scales:
1) Scales which forms stable layer on the inner metal surface of the boiler are poor
conductor of heat. Formation of scale make boiler metal at higher temperature
causing problem of overheating, cracking & bursting.

T1 Outer Surface > T Outer Surface

2) Scales which are formed inside the boiler may crack allowing penetration of boiler
water to over heated surface which cools the surface and results in excess conversion
of water into steam. This extra excess formation of steam increases the pressure of
steam in side the boiler and due to local overheating of boiler which reduces the
steam bearing capacity of boilers and there may be chance that boiler may burst while
in operation.

3) Due to over heating some portion of boiler gets expanded as strength of boiler metal
gets weakened to bear the pressure of steam and is responsible for differential volume
changes at different portions of boiler as a result it has been difficult to maintain
boiler water level. The formation of scales inside the boilers is not uniform every where
inside the boiler. Due to non-uniform scales formation and problem in maintaining
water level the formation steam affected.

4) Scales are being poor conductor of heat and are responsible for the wastage of fuel. It
has been observed that for 0.325, 1.252 & 2.5 mm thick layer of scales, 10%, 50% &
80% extra fuel is required to get continuous steam.

5) Scale reduces the steam and are responsible for wet steam as a result of priming.
Removal of Scales:
Depending upon the nature of scale they are removed from boiler by different methods:-
1) Soft and loosely adhered scales ae removed by wire brush & scrapers
2) Scales of brittle nature are removed by giving thermal shocks.
3) Strongly bonded and hard scales are removed by dissolving them in suitable chemical
like 5 to 100% HCl solution & EDTA solution.
Prevention of Scales:
Harmful impacts of scaling in smooth functioning of boilers are reduced by proper
conditioning of water which can be done externally and internally. In external treatment
of boiler feed water should be perfectly softened by using efficient softening technique. In
internal treatment some chemicals are added along with boiler water to reduce the
formation scales at the cost of formation of easily removable sludge or at the cost of
formation of extremely soluble salt.
1) Carbonate Conditioning: In low pressure boilers where there is CaCO3 & CaSO4
precipitates are formed as solids in side the boilers. CaSO4 forms very tight and stable
scale in comparison to loose scale of CaCO3. The scale forming CaSO4 are converted
into CaCO3 by adding soda Na2CO3. The Na2CO3 is added along with boiler feed water
in calculated amount which furnishes carbonate CO3- - ions as a result CaCO3 gets
precipitised in preference to CaSO4 as solubility product KCaCO3 < KCaSO4. This
formed CaCO3 are then easily removed by simple blow down process.
CaSO4 + Na2CO3 → CaCO3↓ + Na2SO4
2) Phosphate conditioning:- In high pressure boilers various salt of phosphates are
added along with boiler feed water which converts scale forming salts of Ca & Mg in to
easily removable sludges in form of phosphates. Commonly used phosphates are
Na3PO4 (Tri sodium phosphate), Na2HPO4 (Di sodium hydrogen phosphate) & NaH2PO4
(Mono sodium di hydrogen phosphate). Reactions of these phosphates in boiler water
is as blows:-
2 Na3PO4 + 3 CaCO3 → Ca3 (PO4)2 ↓ + 3 Na2CO3
2 Na3PO4 + 3 CaSO4 → Ca3 (PO4)2 ↓ + 3 Na2SO4
2 Na2HPO4 + 3 CaSO4 → Ca3 (PO4)2 ↓ + 2 Na2SO4 + H2SO4
2 NaH2PO4 + 3 CaSO4 → Ca3 (PO4)2 ↓ + Na2SO4 + 2 H2SO4
3) Colloidal Conditioning: Scale formation in the boilers can be avoided by adding
colloidal conditioning agents which are organic compounds (kerosene, tanin ,agar-
agar and starch etc.), they acts as protective colloids and forms non sticky coating
over the scale forming precipitates as a result they are in the form of loose suspended
precipitates and are easily removed by blow down process.

4) Calgon Conditioning: Calgon in commercial name of sodium hexameta phosphate

(NaPO3)6 or (Na2[Na4P6O18]). Addition of calgon along with boiler feed water prevents
the formation of scales by converting then into highly soluble complexes. Calgon
reacts with the scale forming solids and convert them into highly soluble calcium
hexameta phosphate and thus prevents the precipitation of scale forming salts/solids.

Na2[Na4P6O18] 2 Na+ + [Na4P6O18]2-

2 CaSO4 + 2 Na+ + [Na4P6O18]2-
2 Na2SO4 + 2 Na+ [Ca2P6O18]2-
Soluble Complex
Calcium hexameta phosphate
Calgon, Na2[Na2P6O18)
Comparison of Sludge & Scale
 Boiler’s Trouble-2
(Foaming & Priming/Corrosion/Caustic Embrittlement)
For 1st Year B Tech Students Of Engineering Chemistry
Prof. Vivek Pandey
Rajasthan Technical University Kota
Rawatbhata Road, Akelgargh, Kota-324010
 Carry Over
Steam formed in the boiler may be associated with small droplets of
water. Such steam in called wet steam and small droplets of water
naturally carries some suspended and dissolved impurities present in
boiler water this phenomenon of carrying water by steam along with
the impurities is called Carry Over. Problem of carry over in boiler is
because of

1) Foaming
2) Priming
Foam means the formation of stable bubbles, lather or froth at the water surface in boilers.
Foam consists of a bubble of gas surrounded by a thin liquid film. In boilers foam consists of
a steam bubble surrounded by a liquid film dispersed in boiler water.

In case of pure boiler water, film formation is not possible so there is no foaming in such
case. Generally boiler water is impure containing dissolved solids or suspended matter. The
surface tension of water is changed which is increased by the presence of dissolved salts and
gets decrease by presence of organic matters (oil and grease etc.). This change in surface
tension result in the accumulation or depletion of solids in the interfacial layer as a result
concentration of solid in the interfacial thin liquid film gets increased or decreased as
compared with the concentration in the bulk of water and a problem of Foaming is observed
in boilers. Bubbles or froth actually build up on the surface of the boiler water and pass out
with the steam.
 Reason of Foam Formation in Boiler
Foaming in a boiler may be because of the following:-
1) Presence of too high concentrations of TDS.
2) Presence of organic substances (oil, grease etc.)
Foaming in boiler in not very detrimental if foam bubbles collapse or burst before reaching
the surface of the boiler water. If foam bubbles are said to be stable and carried away
along with the steam in the steam line, super heater, steam turbine or steam engine. It
results some bad effects in these parts.

Prevention of Foaming
Chances of foaming can be overcome by:-
1) Removing total dissolved salts and suspended matter that is
responsible for the formation of foam or bubbles in boilers.
2) Removing sludge forming salts which decrease the viscosity of
the film when TDS is not high and boiler pressure is not high.
3) Use of antifoaming agents like Castor oil.
4) Occasional blow down of boiler water.
5) Placing baffle plates in boiler water portion so that foam gets
Baffle Plate
collapsed and unable to reach in steam space.
 Priming
Priming is carrying over of liquid water from the boiler by generated steam. It is because of
the entrapment of water droplets along with steam. Priming is the main cause of production
of wet steam, although foaming also produces wet steam. But degree of wetness is more in
case of priming.

Reasons for Priming

(1) The presence of a larger amount of stable foam on the surface of the boiler water. Foam
on subsequent bursting result in water droplets.
(2) Carrying too high water level resulting is an insufficient steam space. This minimises the
chance of dropping lock of water droplets into the boiler water.
(3) Steam velocity sufficiently high to carry water droplets into the steam pipe. At high steam
velocity water droplets do not get enough time to settle out of the steam and thereby
never fall back into the boiler water
(4) Sudden and more vigorous boiling brought about by a sudden pressure drop in the
steam line due to rapid increase in steam demand. This results bumping or splashing of
boiler water in the boilers.
 Prevention of Priming
1) Avoid formation of stable foam.
2) Keeping sufficient steam space in boiler so that steam velocity never gets beyond
optimum value. As a result water droplets settle out of the steam.
3) Avoid rapid changes in the steaming rate having smooth control over steam valve by not
opening steam exit value suddenly.
4) Avoid vigorous boiling of boiler water so that bumping /splashing of boiler water can be
controlled.

Impacts of Priming of Foaming

1) Production of wet steam is because of foaming & priming which causes corrosion in all
such parts which are in contact with wet steam.
2) Wet steam gives less heat and reduces the efficiency of steam turbine or steam engine.
3) The super heaters are used in steam pipe line to remove the wetness to increase the
heat content of steam.
4) Deposition of solids of wet steam in the steam line takes place due to evaporation of
water content in super heater. These solids being poor conductor of heat, absorbs heat
of the super heater there by reducing the efficiency of super heater in getting dry steam
over a period of time.
5) The deposition of solids of wet steam causes chocking of the steam line and super heater. These
solids may be carried along with steam to reach turbine and gets deposited which are formed on
evaporation and may gets deposited on the shaft of turbine. As result of solids deposition the
turbine efficiency will decrease and the turbine may also damaged due to dynamic unbalancing
of turbine shafts.

6) Liquid water in the form of small droplets can cause erosion in the machine specially turbine.
 Boiler Corrosion
Corrosion is the destructive conversion of boiler metal into its oxides or salts. Corrosion is
among the serious problem caused due to following reasons.
1) Presence of free acids in boiler water.
2) Dissolved Oxygen in boiler water corrodes boiler metal due to following oxidation reaction
Fe + 3O2 + H2O → Fe(OH)3 (s)

3) Hydrolysis reaction of soluble salts of Ca & Mg in water

CaCl2 + 2H2O → Ca(OH) 2 + 2HCl
MgSO4 + 2H2O → Mg(OH) 2 + H2SO4

These formed acid helps in corrosion by reacting with metal

Fe + 2HCl → FeCl2 + H2
FeCl2 + 2H2O → Fe (OH) 2 + 2HCl

4) Presence of CO2 & SO2 are responsible to make boiler water acidic by the formation of
carbonic and sulphuric acids H2CO3 & H2SO4 which makes metal more susceptive
towards corrosion.
 Remedial Measures
(1) Removal of Dissolved oxygen
(a) Sodium sulphite is used low pressure boilers to remove dissolved oxygen by forming
sodium sulphate
2Na2SO3 + O2 → 2Na2SO4

(b) Hydrazine is used to converts dissolved oxygen in to chemically inert nitrogen to

avoid corrosion.
N2H4 + O2 → N2+ 2H2O
Hydrazine is used in high pressure boilers and its 40% solution in water is quite safe. It
is used in calculated amount as it excess use forms ammonia which bring about
corrosion of some metal alloys.

(2) Removal of dissolved CO2, dissolved oxygen & other gases by mechanical deaeration.

(3) Making boiler water alkaline with addition of requisite amount of NaOH.

(4) Removal of corrosive substance like chlorides & sulphates of Ca & Mg.
 Disadvantages of Boiler Corrosion

1) Boiler corrosion leads to shortening of boiler life because of the decay

or disintegration of boiler metal either due to chemical or
electrochemical reaction with its environment.

2) Corrosion can damage the internal workings of boiler which leads to

pitting and creates localized erosion of boiler tubes.

3) Corrosion that eats boiler metal, affects the working of boilers. Holes
in the boiler metal cause leaks that can cause severe operational
problems which leads to unexpected shut down of boiler to repair the
damage or unexpectedly cause the failure of the system.
 Caustic Embrittlement

Caustic embrittlement is a form of stress corrosion taking place in mild steel in the presence
of alkaline solution at high temperature and at a portion of stress in the boilers. Boiler water,
usually contains a certain proportion of Na2CO3, used in softening process. The formation of
brittle and in crystalline cracks in the boiler shell is due to caustic embrittlement. At high
temperature of boiler water Na2CO3 decomposes to gives NaOH due to which due to which
the boiler water becomes “Caustic Soda”.

Na2CO3 + H2O → 2NaOH + CO2

The H2O evaporates and the concentration of NaOH increases progressively dissolving the
iron of the boiler as sodium ferrate (Na2FeO2).
2NaOH + Fe → Na2FeO2 + H2
NaOH which makes boiler water alkaline and this caustic water flows into the minute hair
line cracks at the portion of stress in boiler by Capillary action. At high temperature of
boilers caustic water in fine cracks evaporates and the concentration of NaOH increases in
the cracks from that of remaining boiler water. This concentrated alkali dissolves iron as
sodium ferroate in cracks at the site of stress. The Na2FeO2 decomposes to give Fe3O4 and
NaOH which again continues the stress corrosion.

3Na2FeO2 + 4H2O → 6NaOH + Fe3O4 + H2

6Na2FeO2 + 6H2O + O2 → 12NaOH + 2Fe3O4
This caustic embrittlement can also explained as a result of electrochemical cell with
different concentration of NaOH and can be expressed as

(-)Anode: ‘Fe’ at bents│Conc.NaOH ║ Dil.NaOH│ ‘Fe’ at plane Surface: Cathode (+)

This causes embrittlement of boiler parts such as bends, joints, reverts etc., due to
which normal working of boiler gets will affected.

Prevention of Caustic Embrittlement

1) By maintaining the pH value of water and neutralization of alkali.
2) By using Sodium Phosphate as softening reagents, in the external treatment of
boilers water.
3) Riveted joints are replaced by welded joints in metallic structures.
4) Caustic embrittlement can also be prevented by adding Tannin or Lignin or
Sodium sulphate which prevents the infiltration of caustic-soda solution
blocking the hair-cracks.
Municipal Water Supply
Prof. Vivek Pandey
Rajasthan Technical University Kota
Rawatbhata Road, Akelgargh, Kota-324010
1
 Municipal Water Supply

Municipal water has been one of the important challenges that is faced by water
technology. Municipal water supply for domestic use, to our houses, should be free
from pathogenic bacteria and should be Clear, Colourless & Pleasant to taste. It
should also be free from excessive dissolved salts, suspended impurities & harmful
micro organism (Organic impurities imparted by animal & vegetable matters).
The municipal corporations of the area process the water to get potable grade
dinking water. The water to be processed should be obtained from such a source
which is least contaminated by animal & vegetable matter as well as industrial
effluents. Water taken from river, lakes & wells are the most common source of
water used by municipalities.

Water taken from the proper source is further processed, as per the following steps,
to get drinking grade municipal water :

(1) Screening (2) Sedimentation (3) Coagulation (4) Filtration (5) Sterilization
Diagram Showing Treatment Process of Municipal Water
 1. SCREENING
Screening is a pre-treatment and is the first treatment water treatment station. This process
essentially involves the removal of large non-biodegradable and floating solids such as rags, papers,
plastics, tins, containers & wood and prevent these to enter into a water treatment plant. These
solids get trapped by inclined screens or bar racks. The spacing between the bars usually is 15 to 40
mm, depending on cleaning patterns. The purpose of water screening is to:
1. Protect the structure downstream against large objects which could create obstructions in some
of the facility's units.
2. Easily separate and remove large matter carried along by the raw water, which might negatively
affect the efficiency of later treatment procedures or make their implementation more difficult.

Screening of Water
 2. SEDIMENTATION
Sedimentation is process of removing suspended types of impurities present in source water using
a large settlement sedimentation tanks of about 5 meter depth. The water is allowed to left in the
settlement tank for a period of few days or even weeks undisturbed as a result most of the
suspended impurities settle down to bottom at the bottom of the tank due to the force of gravity.
The principle involved is to slow down the movement of water so that suspended particles, as an
impurity, which are associated by fast morning water can fall gravitationally to the bottom when
water is kept undisturbed.
The rate of settling of particles at 10 C in still water in called hydraulic settling value of particle
(millimetre/second) the settling rate of particle depends upon following factors:
• Horizontal flow velocity of water
• Size of the particle
• Temperature of water
• Shape of the particle and its specific gravity

Generally efficiency of sedimentation tank to remove suspended impurities is about 70 to 75%

however this method is unsuccessful in removing very fine suspended particles.
 3. COAGULATION
Very fine suspended particles of clay, silica and organic matter having colloidal nature, which are not removed
by sedimentation technique. These fine colloidal particles are generally negatively charged particle and due their
mutual repulsions they are in constant state of motion and then can’t easily settle down under the force of
gravity. In order to remove them chemicals are added which produce oppositely charged ions of that of
negatively charged colloidal particles and bring them to coagulate to increase their size so that they can easily
settle down to the bottom of the tank. Chemicals which in used to aggregate fine colloidal particle to sufficient
size so that it can easily settle down are called coagulants and the process in called COAGULATION. Most
commonly used coagulants are Alums [ K2(SO4.Fe2(SO4). Al2(SO4)3.24H2O], Aluminate [NaAlO2] and Ferrous
Sulphate [FeSO4..7H2O].

These different coagulants are used depending upon the nature of the water.
1. If the water in alkaline one them it in best to use. Alums because H2 SO4 Generated by the dissociations of
alums are neutralized by the alkali nature of water
Al2 (SO4)3+6H2O→2Al(OH)3 +3H2SO4
2. When natural alkalinity is there
Al2(SO4)3+ Ca(HCO3)2→ Al(OH)3 +3CaSO4+6CO2
3. If water have very low alkalinity then it is preferred to add small quantity of Na2Co3
Al2(SO4)3 + Na2CO3+ H2O → 2Al(OH)3 + 3Na2SO4+ 3CO2
4. Removal of particles from Acidic water the use of sodium Aluminate is preferred because of the
dissociation of NaAlO2 in water by the formation NaOH.
NaAlO2 + H2O → NaOH + Al(OH)3
Coagulation method not only removes the fine colloidal particles but also used in the reduction of
bacterial count, organic compounds of water. Due to chemical action of coagulants they releases
oxygen to the water which in used to destroy the bacteria, in breaking up some organic compounds
and also partially remove colour and taste producing organisms present in water. Thus process of
coagulation in very important process in the purification of water.

4. FILTRATION
Filtration in a process of clarification of water by passing the water through a porous
material, which in capable of retaining coarse impurities on its surface and in the pores.
The porous material used in called filter media and the equipment used for filtration in
known as filter. Commonly used filtering material are quartz sand (grain size 0.5 to 1.0
mm) containing not more than 96% SiO2, crushed anthracite and porous clay(piece size
0.8 to 1.5mm).
Water for domestic use may be filtered through large area of finely graded sand beds at a
slow rate of (2gal/sq.ft/hour). The rate of filtration slowly & slowly decreases due to the
entrapment of impurities on and in the pores of the filtering sand which finally becomes
so slow that the had must be cleaned .the top layer of sand up to 2-3 cm is scrapped off
and replaced by clean sand and the filter in put back into use. The scrapped sand in
washed, dried and stored for reuse at the time of next scrapping operation.

Sand Filter used for Filtration of Water

 5. STERLIZATION
Clarification of water by sedimentation, coagulation, filtration removes the suspended impurities and also
reduces the bacteria count to some extent. In order to make potable water it should be totally freed from
pathogenic (disease causing ) bacterias micro-organisms. The process of destroying or killing of the
bacterias & micro-organisms from the water is called Disinfection or Sterilization. The chemicals which
are used for killing the bacterias are called disinfectants. sterilization can be done by following methods:-
(1) Boiling of water :- Boiling of water for 10 to 15 minutes bills all the bacteria and makes water safe for
drinking purpose. This method is very costly and can be used only in individual houses, it is not worthy
to use boiling method in municipal water-works.
(2) Adding Bleaching powder :- In small water works, about 1 kg of bleaching powder per 1000 kilo litres
of water is mixed and water allowed to stand still for several hours. The chemical action produces hypo
chlorous acid (HOCl) which is a powerful disinfectant.
CaOCL2+H2O  Ca(OH) 2+ Cl2
Cl2 + H2O  HOCl + HCl
Germs + HOCl  Germs are killed
The disinfecting action of bleaching powder is due to the chlorine made available by it. However drawback
of this method is that it increases the calcium content in water and makes it hard. Also there is
deterioration in the quality of bleaching powder due to its decomposition during storage as its chlorine
content gradually decreases, so on its use chlorine content of bleaching powder must be checked. The
excess use of bleaching powder imparts bad taste & smell of chlorine into the water.
3. Chlorination :- Chlorine (gas or its concentrated solution) produces by to chlorous acid (HOCl)
when react with treated water.
Cl2 + H2O  HOCl + HCl
Chlorination is widely used for sterilization throughout the world and apparatus used is called
chlorinator. Chlorinator is a high tower, having a number of baffle plates water and a proper
quantity of chlorine solution are introduced from the lop. During the passage of water through
these baffle plates it gets thoroughly mixed and sterilized water is obtained from the outlet at the
bottom of the chlorinator. Filtered water requires 0.3 to 0.5 ppm of chlorine for perfect
sterilization..

Chlorinator used for Chlorination of Water

3. Ozonolysis :- Ozone,O3 , is a powerful disinfectant and is easily absorbed by water.
Ozone being unstable molecule, decomposes easily to give nascent oxygen, [O], which
destroys the bacterias
O3  O2+ [O]
However, this process in relatively expensive but has the advantage of removing bacteria,
colour, odour and taste without leaving any harmful residual effects in the treated water.

Ozonolysis of Water
 BREAK POINT CHLORINATION

Break Point Chlorination is a controlled chlorination process which is used to minimise

the chances of excess use of chlorine because of which unwanted irritating odour & taste
is developed in the sterilized water. In this process sufficient quantity of chlorine is
added to oxidise organic matter and destroy bacterias.

In this process chlorine added in to the water and on the other hand the residual chlorine
in water is idomatrically determined, at a regular interval of time, by titration method. It
has been observed that residual chlorine is less than that of chlorine which was added
initially because of its consumption in oxidising organic matters and bacterias.
Following figure is obtained by plotting a graph between chlorine added and residual
chlorine estimated :
In the figure it has been observed that quantity of residual Chlorine increases with
increasing dose of chlorine giving a straight line ( Point A to B) until at a definite chlorine
does a sudden decrease in the residual chlorine (at point B) is observed. On further
addition of chlorine there is continuous decrease in residual chlorine up to point C. After
point C there is again continuous increase in residual chlorine on further addition of
chlorine dose. This point C is known as break point after which the residual chlorine
increases with the addition of chlorine and is responsible for unwanted irritating odour &
taste in the treated water.