The document discusses water treatment and the importance of water. It provides information on water sources and common impurities. Standards for drinking water parameters according to BIS and WHO are listed. Hardness of water is defined and types are described. Methods for determining hardness including complexometric titration are outlined. Issues caused by hard water in industries and domestic use are explained. Boiler troubles from hard water like scaling, corrosion and carryover are discussed along with their causes and prevention methods.

1. WATER AND ITS TREATMENT
Dr. E. Laxminarayana
Associate Head (Chemistry Section)
Associate Dean R&D

2. Introduction
• Water is essential for life. In fact, life is originated from water
• Without Water, there is no Life
• It is the most abundant and essential Natural Resource
• Even though 70% Water is available on earth crust
there is a shortage of Water for every activity
• It is estimated that the hydrosphere contains about 1360 million
cubic km ( 1.3 x 1018 m3) of Water.
• Of this, about 97% is in the oceans and inland seas, which is not
suitable for human consumption because of its high salt content
• Of the remaining 3%, 2% is locked in the glaciers and polar ice caps
and only 1% is available as fresh Water in rivers, lakes, streams,
reservoirs and ground water, which is suitable for human
consumption.

3. Specification for Water
Different uses of Water demand, different
specifications
1. Domestic Use
2. Agricultural Use
3. Industrial Use
4. Safe Drinking Water

4. STANDARDS FOR SAFE DRINKING WATER
(Maximum Permissible Limit)
Parameter BIS Standards
(in ppm except pH)
WHO Standards
(in ppm except pH)
pH 6.5-8.5 6.5-8.5
Total Solids 500 -1000 500
Dissolved Oxygen 5.0-7.0 6.0-7.0
Calcium 100 100
Magnesium 75 75
Total Hardness 200-250 200
Iron 0.1-0.3 0.1
Manganese 0.05 0.05
Copper 0.05 0.05
Zinc 5.0 5.0
Arsenic 0.01 0.01
Chromium 0.05 0.05
Lead 0.01 0.01
Sulphate 200 150

5. Parameter BIS Standards
(in ppm except pH)
WHO Standards
(in ppm except pH)
Chloride 200 150
Fluoride 1.0 - 1.5 1.0
Nitrate 20 20
Nitrite 1.0 0.5
Ammonia 0.5 0.5
Phenols 0.001 0.001
Polycyclic Aromatics 0.002 0.002
Bacteria 1 coliform / 100 ml 1 coliform / 100 ml
Drinking Water Standards for Fluoride ions Prescribed by Various Authorities
S. No. Authority Permissible
Limit (mg/l) (ppm)
1 WHO (For Indian Context) 1.50
2 WHO (For International Standard) 0.50
3 BIS (ISO – 10500) 1.0-1.5
4 ICMR 1.0-2.0
5 US Public Health 0.7-1.2

6. Sources of Water
• Surface water
• Underground Water
• Rain Water
Common Impurities of Water
• Dissolved Impurities
• Colloidal Impurities
• Suspended Impurities
Water Quality Standards
1. Physical Characteristics
Color, Taste and Odor, Temperature and Electrical Conductivity
2. Chemical Characteristics
pH, Hardness, Total Dissolved Solids (TDS), Total Suspended Solids (TSS). Total Solids (TS)
Dissolved Oxygen (DO), BOD, COD, Fluorides, Chlorides, Sulphates, Nitrates
3. Biological Characteristics: Water should be free from all types of bacteria, viruses, protozoa, algae

7. Hardness of Water
It is defined as the characteristic of water that prevents lathering of soap
Causes: Hardness of water is due to the presence of certain soluble salts of Ca and Mg in
water.
A sample of water when treated with soap (sodium salts of oleic, palmitic or stearic acid)
does not form lather or foam but forms a white precipitate instead. This is because of the
formation of insoluble salts of Ca and Mg.
2C17H35COONa + CaCl2 → (C17H35COONa)2Ca (ppt) + 2 NaCl
2C17H35COONa + MgSO4 → (C17H35COONa)2Mg (ppt) + Na2SO4

8. Types of Hardness
1. Temporary Hardness:
(a) It is also called carbonate hardness or alkaline hardness. It is due to the presence of
bicarbonate, carbonate and hydroxide and can be determined by titration with HCl using
methyl orange as an indicator.
(b) It can be removed by boiling of water
Ca(HCO3)2 → CaCO3 + H2O + CO2
Mg(HCO3)2 → Mg(OH)2 + H2O + 2CO2
2. Permanent Hardness:
(a) It is also called non-carbonate or non-alkaline hardness
(b) It is due to the presence of dissolved chlorides, sulphates and nitrates of calcium,
magnesium and other heavy metals.
3. It can not be removed by simple boiling but can be removed by lime-soda process, ion-
exchange process or zeolite process

9. Degree of Hardness
Hardness is always calculated in terms of equivalents of CaCO3, based on
(i) Molecular weight is 100 and equivalent weight is 50
(ii) It is the most insoluble salt that can be precipitated in water treatment
Calculation of Equivalents of CaCO3
Equivalents of CaCO3 = Mass of hardness producing
substance x 50 /
Chemical equivalent of hardness
producing substance

11. Units of hardness and their interrelationship
i) Parts per million (ppm): It is defined as the number of parts by weight of CaCO3 equivalent
hardness present in million ( 106) parts by weight of water
(ii) Milligrams per litre (mg/l) It is defined as the number of milligrams of CaCO3 equivalent
hardness present per litre of water.
1 mg/l = 1 mg of CaCO3 equivalent hardness in one litre of water
Since weight of 1 litre of water = 1 Kg = 1000 g = 1000 x 1000 = 106 mg
1 mg / l = 1 mg of CaCO3 per 106 mg of water
= 1 part of CaCO3 per 106 parts of water
= 1 ppm

12. (iii) Degree Clarke ( 0Cl): it is defined as the number of grains (1/7000 lb) of CaCO3 equivalent
hardness per gallon ( 10 lb) of water.
1 0Clarke = 1 grain of CaCO3 equivalent hardness per gallon of water
1 grain = 1 / 7000 lb (pounds) and 1 gallon = 10 lb
1 grain / gallon = 1/7000 lb/10lb
= 1 : 70, 000
1 0Cl can also be defined as the number of parts of CaCO3 equivalent hardness per 70, 000
parts of water
(iv) Degree French ( 0Fr): It is defined as the number of parts of CaCO3 equivalent hardness
per 105 parts of water.
1 0Fr = 1 part of CaCO3 equivalent hardness in 105 parts of water
Hence, 1 ppm = 1 mg/l = 0.07 0Cl = 0.1 0Fr

13. Impurity Ca(HCO3)2 Mg(HCO3)2 CaSO4 MgSO4
Quantity mg/L 4 6 8 10
M.wt 162 146 136 120
 A sample of water on analysis is found to contain the following impurities.
Calculate the temporary, permanent and total hardness of water in ppm,
o
Fr and
o
Cl.
Solution:
Constituent mg/L 100/M.wt CaCO3 equivalent
Ca(HCO3)2 4 100/162 0.617 X 4 = 2.47 mg/L
Mg(HCO3)2 6 100/146 0.684 X 6 = 4.11 mg/L
CaSO4 8 100/136 0.735 X 8 = 5.58 mg/L
MgSO4 10 100/120 0.833 X 10 = 8.33 mg/L

14. Determination of temporary Hardness:-
THS is due to presence of bicarbonates of Ca and Mg
THS = 2.47+ 4.11 =6.58 mg/L (or) ppm
As 1mg/L = 1ppm = 0.1 oFr = 0.07 oCl
Temporary hardness = 6.58 mg/L (or) ppm
= 6.58 x 0.1 =0.658 oFr
= 6.58 x0.07= 0.46 oCl
Determination of permanent hardness:-
PHS is due to presence of CaSO4 and MgSO4
Permanent hardness = 5.88 + 8.33 = 14.21 mg/L
= 14.21ppm
= 14.21 x 0.1 = 1.421 oFr
= 14.21x 0.07 = 0.995oCl

15. Total hardness = Temporary Hardness + permanent
Hardness
=6.58 + 14.21 = 20.79 mg/L
= 20.79 ppm
= 20.79 x 0.1 = 2.079 oFr
= 20.79 x 0.07 = 1.4553 oCl

16. Determination of Hardness of Water by Complexometric Method
 The ethylene diamine tetraacetic acid (EDTA) forms stable soluble complex with Ca2+
and Mg2+ ions in the pH range 8-10
 EDTA is chelating ligand and it acts as a hexadentate ligand coordinating through two
nitrogen atoms of amine group and four oxygen atoms of carboxylic acid with Ca2+
and Mg2+ ions at a pH of 8-10
 The structure of metal-EDTA complex is octahedral
 Even though EDTA contains 8 oxygen atoms and two nitrogen atoms, it can not use its
four oxygen atoms of CO group for coordination (Due to steric effect)
 To maintain the pH 8-10, ammonical buffer is added
 The indicator added is Eriochrome black-T, is known to be a metal ion indicator

17.  The indicator acts a tridentate ligand coordinating through nitrogen atom and two
oxygen atoms of OH groups
 The indicator also form complexes with metal ions at a pH of 8-10. However, indicator
metal complex is less stable when compared to EDTA-Metal complex
M2+ + EBT → [M (EBT)2] ( M = Ca or Mg)
Wine red color
[M (EBT)2] + EDTA → [M (EDTA)] + 2 EBT
(blue color)
 The color change is from wine red to blue
 This is the best method to determine the hardness of water
 We can determine temporary, permanent and total hardness of water

18. Ethylenediaminetetraacetic acid
Metal-EDTA Complex
Eriochrome Black T indicator

19. Disadvantages of Hard Water
1) For industries, if hard water is used the slats present in water causes problems in
sugar industry, paper industry, textile industry, dyeing and pharmaceutical industries
2) Steam generation in boilers: The use of hard water in boilers causes problems like scale
and sludge formation , boiler corrosion, caustic embrittlement, priming and foaming.
3) In Domestic Use:
4) Drinking: Hard water can have bad effect on our appetite and digestive system. Some
times it produces calcium oxalate that causes different urinary problems.
5) Cooking: The boiling point of hard water increases because of the presence of various
salts. This causes wastage of time and fuel.
6) Bathing and Washing: As hard water does not form lather with soap, lot of soap is
wasted

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

21. Boilers are employed in industries for generation of steam.
When raw water is directly feed into the boilers various
physical changes and chemical reactions are induced in the
boiler with the action of the heat.
The ultimate result being the problem such as
1. Carry over,
2. Boiler corrosion,
3. Caustic embrittlement and
4. Sludge and Scale formation.

22. 1.Carry over:
The process of carrying of water with
impurities by steam is called carry over.
a) PRIMING: When boiler is steaming rapidly,
some droplets of liquid water carried along
with the steam. This process of wet steam
formation is called priming.
 It is caused by the presence of large
amount of dissolved salts,
 Improper boiler design,
 High steam velocities and sudden boiling.

23. Priming can be avoided
 By fitting mechanical steam
purifiers,
 Avoiding rapid change in
steaming rate,
 Maintaining low water levels
in boilers,
 Efficient softening and
filtration of the boiler-feed
water.

24. b) FOAMING:
It is the production of persistent
foam, bubbles in boilers, which
do not break easily.
It is due to presence of oils and
alkalis in boiler water.
 Oils, alkalis react to form
soaps which lower the surface
tension of water and increase
the foaming.

25. Foaming can be avoided by
Addition of antifoaming agents like castor oil.
By removing foaming agent like oil and alkalis in
boiler water.
The oils can be removed by adding compounds like
sodium aluminates and aluminum sulphate.
Disadvantages of priming and foaming:-
 The actual height of the water in boiler is not
judged,
 Wastage of heat,
 Efficiency f the boiler is lowered and
 corrosion of the machineries.

26. 2.Boiler Corrosion:
The decay (or) disintegration of boiler material either
due to chemical (or) electrochemical reaction with its
environment is called boiler corrosion.
Disadvantages of boiler corrosion are:
1.Shortening of boiler life,
2.Leakages of boiler life,
3.Increased cost of repairs.
Main sources for boiler corrosion are
A.Dissolved oxygen
B.Dissolved carbon dioxide
C.Acids from dissolved salts

27. A. Dissolved oxygen: Water usually contains about 8ml of
dissolved oxygen per lit. at RT.
It attacks boilers at high temperature.
 4Fe +2H2O + O → 4Fe [OH] 2↓
 4Fe [OH] 2↓ + O2 → 2[Fe2O32H2O] ↓
(Ferrous hydroxide) (Rust)
Removal of dissolved oxygen:
By adding sodium sulphite , hydrazine and sodium sulphide
2Na2SO3 + O2  2 Na2SO4
N2H4 + O2  N2 + 2H2O
Na2S + 2O2  Na2SO4

28. B. Disolved carbon dioxide: The disolved CO2 in water
produces carbonic acid, which has slow corrosive effect.
This is also produced inside the boiler, if water contains
bicarbonates.
H2O + CO2 → H2CO3
Mg (HCO3)2  MgCO3 + H2CO3
Removal of dissolved CO2:
a. By adding ammonia:
NH4OH + CO2 (NH4)2CO3 + H2O
b. By mechanical de-aeration:
Spraying water over preheated plates stacked in a
tower.

30. C. Acids from dissolved salts:
Mineral acids are generated by the hydrolysis of disolved acidic
salts.
The liberated acid reacts with Fe of boiler to form Fe(OH)2
subsequently get converted into rust .
Small amount of MgCl2 will cause corrosion of Fe to a large
extent.
MgCl2 + 2H2O  Mg(OH)2 + 2HCl
2HCl + Fe  FeCl2 + H2
FeCl2 + 2H2O  Fe(OH)2 + 2HCl
Fe(OH)2 + O2  Fe2O3 .2H2O

31. Prevention of Acid corrosion :
 HCl can be removed by adding alkalis
 By addition of inhibitors
 By frequent blow down operation
 By softening of boiler water .

32. 3. Caustic Embrittlement:
 Caustic embrittlement is a phenomenon in which the boiler material
becomes brittle due to accumulation of caustic substances.
 The boilers feed water contains carbonates and bi –carbonates of
alkali metals.
 Na2CO3 used for softening of the water decomposes to give NaOH
and CO2.
Na2CO3 + H20 → 2NaOH + CO2
 The NaOH concentration increases by evaporation and attacks the
surround area there by Fe dissolves as sodium ferrate.
 As per electrochemical theory highly concentrated parts of
boilers(covered by NaOH) will become cathode and other parts will
become anode.
 Corrosion occurs at anode
 This causes embrittlement of boiler parts causing even failure of the
boiler.

33.  By using sodium phosphate as softening reagent
instead of Na2CO3 for boiler water.
 By adding Na2SO4 to boiler water which blocks the
hair cracks in the boiler.
 By adding tannin or lignin to boiler water which
blocks the hair cracks in the boiler there by
preventing formation of concentration cells.

34. 4. SLUDGE & SCALE FORMATION
 In boilers, water evaporates continuously and concentration of
dissolved salts increases. When it reaches saturation point they
will be precipitated on the inner walls of the boilers.
 If the precipitation takes place in the form of loose and slimy, it
is called sludge.
 If the precipitation takes place in the form of hard, adhering on
the inner walls of the boilers, it is called scale.

35. Sludges Scales
1. Sludges are soft loose and slimy
precipitate.
2. They are non-adherent deposits
and can be easily removed.
3. Formed by substances like CaCl2 ,
MgCl2 ,MgSO4, MgCO3 etc.
4. Formed at comparatively colder
portion of the boiler.
5. They decrease the efficiency of
boiler but are less dangerous.
6. Can be removed by blow-down
operation.
1. Scales are hard deposits.
2. They sticks very firmly to the inner
surface of boiler and are very
difficult to remove.
3. Formed by substance like CaSO4,
Mg(OH)2 etc.
4. Formed generally at heated
portions of the boiler.
5. Decrease the efficiency of boiler
and chances of explosions are
also there.
6. Cannot be removed by blow-down
operation.

36. a) SLUDGE:
 It is formed at colder portions of the boilers.
 It is formed by substances, which have greater solubility
in hot water than cold water(MgCO3, MgCl2, MgSO4,
CaCl2).
 It can easily scrapped off with a wire brush.
Disadvantages:
 Sludges are poor conductors of heat.
 Disturbs the working of boilers.
Prevention of Sludge Formation:
 By using well-softened water.
 By frequently blow-down operation(nothing but the
removal concentrated water through an out let at the
bottom of the boiler).

37. b) Scales
Scales may be formed inside the boiler due to:
A).Decomposition of Ca (HCO3)2:-
Ca (HCO3)2 → CaCO3↓ +H2O +CO2↑
CaCO3 + H2O → Ca(OH)2 + CO2↑
B).Deposits of CaSO4:-
The solubility of CaSO4 decreases with increasing the temperature. So it is soluble
in cold water and insoluble in hot water. Hence it get precipitated in hotter region.
C). Hydrolysis of MgCl2:-
MgCl2 + 2H2O → Mg(OH)2 ↓ + 2HCl
D).Presence of silica:-
Even if small quantity of SiO2 is present, it may deposits as CaSiO3 (or) MgSiO3.
These deposits adhere very firmly on inner side of boiler surface.

38.  In low pressure boilers CaCO3 causes scale formation, but high pressure
boilers CaCO3 is soluble
CaCO3 + H2O → Ca(OH)2 + CO2↑
Disadvantages of scale formation:
 Wastage of fuel,
 Lowering of boiler safety,
 Decrease in efficiency,
Danger of explosion
Removal of scales: removed by chemical and mechanical method.
 If scales are loosely sticking, removed with help of wire brush.
 By frequently blow-down operation.
 If the scales are brittle, it can be removed by giving thermal shocks.
 If the scales are hard, they can be removed with chemicals.

39. Treatment of boiler Feed water:-
Water used in boiler should be free from impurities.
Otherwise cause operational troubles.
The removal of hardness is done by following methods.
I. Internal Treatment of water.
II.External Treatment of water

40. I. Internal Treatment
 In this method, raw water is treated inside the boiler.
Addition of suitable chemicals to reduce scale and sludge
formation and to convert the scale forming chemicals into
sludge which can be removed by blow down process.
Some of the internal treatment methods used for the removal
of scale formation in boilers are.
A) Phosphate conditioning
B) Carbonate conditioning
C) Calgon conditioning

41.  It is generally applicable to high pressure boilers.
 Scale formation is avoided by adding sodium phosphate, which reacts with
magnesium and calcium salts forming non-sticky and easily removable
sludge of calcium and magnesium phosphate and easily removed by blow
down operation.
3MCl2 + 2Na3PO4 → M3( PO4)2↓ + 6NaCl
[Where M = Ca2+, Mg2+]
3MSO4 + 2Na3PO4 → M3 ( PO4)2↓ + 6NaSO4

42.  In low pressure boilers, scale formation can be avoided by
adding sodium carbonate to boiler water.
 When salts like CaSO4 can be converted into CaCO3. CaCO3
can be removed by blow down process.
CaSO4 + Na2CO3 → CaCO3↓ + Na2SO4↓

43.  It involves in adding Sodium hexa meta phosphate (calgon)
to boiler water to prevent the scale and sludge formation.
 Calgon converts the scale forming impurity like CaSO4
into soluble complex compound.
Na2 [Na4 (PO3)6] ↔ 2Na+ + [Na4 (PO3)6]2-
Calgon
2CaSO4 + [Na4 (PO3)6]2- → [Ca2 (PO3)6]2- + 2Na2SO4
Insoluble soluble insoluble

44. External Treatment Methods
Ion-Exchange Process-Deionization or Demineralization of Water :
 Ion-exchange process is defined as the reversible exchange of ions in the structure of
an ion exchanger to ions in solution that is brought in contact with it
 The resin used for the purpose are called ion exchange resins. They are porous,
insoluble, cross linked, long chain organic polymers capable of exchanging ions
 Two types of resins are employed for the softening of water
(i) Cation exchange resin and
(ii) Anion exchange resin

45. Cation exchange resin:
 They are materials capable of exchanging a cation in their structure to the cation in
solution.
 For softening of water, the resins used should be capable of exchanging H+ ions in
their structure to other cations in solution.
 Commonly used resins are styrene divinyl benzene copolymers, which on
sulphonation or carboxylation become capable of exchanging hydrogen ion with
the cations in water
 They are represented as R-H+ (R- = Insoluble polymer matrix and H+ is the
exchangeable ion)
 Amberlite IR-120 and Dowex 50 are examples of commercially available cation
exchange resins

46. Anion exchange Resins:
 These are materials capable of exchanging an anion (OH-) in their structure to
anion in solution
 Anion exchangers employed for water softening are styrene divinyl benzene or
amine formaldehyde copolymers which contain basic functional group such as
amino or quaternary ammonium or quaternary phosphonium or tertiary
sulphonium groups as an integral part of the resin matrix.
 They are represented as R+OH- (R+ = Insoluble polymer matrix and OH- is the
exchangeable ion)
 Amberlite 400 and Dowex-3 are examples commercially available anion exchange
resins

48. Process:
 The hard water is first passed through a column containing cation exchange resin.
 All cations are removed and equivalent amount of H+ ions are released from this column to water.
 The water coming out of this chamber has low pH
 The exchange reactions are
2RH + Ca2+ → R2Ca + 2H+
2RH + Mg2+ → R2Mg + 2H+
 Then the water is passed into second column containing anion exchange resin.
 All the anions are removed and an equivalent amount of OH- ions are released.
ROH + Cl- → RCl + OH-
2ROH + SO4
2- → R2SO4 + 2OH-
2ROH + CO3
2- → R2CO3 + 2OH-
The H+ ions released from cation exchange column and OH- ions released from anion exchange column
combine to produce water molecule. The water coming out of the exchanger is free from all
cations and anions. The ion free water is known as deionized water or demineralized water
H+ + OH- → H2O

49. Regeneration:
 After some time the resin loses all their H+ and OH- ions and then their capacity to
exchange ions is lost.
 In such a condition the resins are said to exhausted
 The exhausted cation exchange resin is regenerated by passing a dilute solution of
HCl
R2Ca + 2HCl → 2RH + CaCl2
R2Mg + 2HCl → 2RH + MgCl2
 The anion exchange resin is regenerated by passing a dilute solution of NaOH and
then washing with distilled water
R2SO4 + 2NaOH → 2ROH + Na2SO4
RCl + 2NaOH → ROH + NaCl
The salts are removed by washing with distilled water

51. Advantages:
 The process produces water of very low residual hardness (about 2 ppm)
 The process can be used to soften highly acidic or alkaline water
 It removes all cations and anions
Disadvantages:
 The process is expensive, both the equipment and resin are costly
 Turbid water decreases the efficiency of the process

52. Potable Water
The common requirements for potable water is
 Should be clear, colorless, free from solids
 Should be odouless
 It should be soft
 It should be abundant and cheap
 Organism and disease causing bacteria should not be present
 The dissolved oxygen should be 5-7 ppm
Water Treatment For Domestic Use Employs the Following Steps:
1. Screening
2. Sedimentation
3. Filtration
4. Disinfection

53. Screening:
 It is used to remove suspended impurities by passing water through screens having
large number of holes when the floating matters are retained by them.
 Screens are vertical bars with perforations
 Water is passed through them where the suspended and floating impurities are
removed.

54. Sedimentation:
 It is the process of removing fine, suspended colloidal impurities by allowing water to
stand undisturbed in big tank about 5 m deep.
 Most of the suspended particles settle down at the bottom, due to the force of gravity
 The clear supernant water is then drawn from the tank with the help of pumps
 The retention period in a sedimentation tank ranges from 2-6 hrs.
 Fine suspended particles like mud particles and colloidal matter present in the water can
not settle by plain sedimentation

55. Sedimentation with Coagulation:
 It is the process of removing fine suspended and colloidal impurities by addition of
requisite amount of chemicals (coagulants) to water before sedimentation
 Coagulants, when added to water, forms an insoluble gelatinous, flocculent
precipitate, which traps very fine suspended impurities forming bigger flocs, which
settle down.
 The coagulants mainly used are alum, ferrous sulphate, sodium aluminate, etc.
Al2 (SO4)3 + 3Ca(HCO3)2 → 2Al(OH)3 + 3CaSO4 + 6CO2
NaAlO2 + 2H2O → Al(OH)3 + NaOH
MgSO4 + 2NaOH → Mg(OH)2 + Na2SO4
FeSO4 + Mg(HCO3)2 → Fe(OH)2 + MgSO4 + 2CO2
4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3

56. Filtration:
 It is the process of separating colloidal and suspended impurities from water by
passing it through a porous bed made of fine sand and other proper sized granular
materials
 Filtration is carried out in a sand filter. It consists of a top thick layer of fine sand,
placed over coarse sand layer followed by gravel
 Water comes from the top. The suspended particles present in water are of bigger
size than the voids in the sand layer are retained there itself and water becomes
free of them
 The sand layer may get choked after sometime and then it is to be replaced with
clean sand for further action
Advantages: Removes all suspended particles, colloidal impurities and organic matter

58. Disinfection:
 It is the process of removing pathogenic bacteria, viruses and protozoa from water
 Several methods are there:
(i) By boiling
(ii) By adding Bleaching powder
(iii) By direct Chlorination
(iv) By adding chloramines
(v) Ozonization
(vi) By adding KMnO4

59. Direct Chlorination:
 Chlorine (either gas or in concentrated solution form) produces
hypochlorous acid which is a powerful germicide
Cl2 + H2O → HCl + HOCl
HOCl → H+ + OCl-
Germs + OCl- → Germs are killed
Advantages:
 Economical, require little space for storage
 Stable does not deteriorate on storage Does not introduce calcium
Disadvantages:
 Excess chlorine produces unpleasant taste and odor
 Excess produces irritation on mucous membrane

60. It involves in addition of sufficient amount of chlorine to oxidize
a) Organic matter
b)Reducing substances
c)Free ammonia
The dosage of applied chlorine and the free Chlorine can be depicted
graphically in which appearance of following four stages occurs:

62. The graph below shows what happens when chlorine (either chlorine
gas or a hypochlorite) is added to water.
 First (between points 1 and 2), the water reacts with reducing
compounds in the water, such as hydrogen sulfide. These compounds
use up the chlorine, producing no chlorine residual.
 Next, between points 2 and 3, the chlorine reacts with organics and
ammonia naturally found in the water. Some combined chlorine
residual is formed - chloramines.
 Note that if chloramines were to be used as the disinfecting agent,
more ammonia would be added to the water to react with the
chlorine. The process would be stopped at point 3.
 Using chloramines as the disinfecting agent results in little tri halo
methane production but causes taste and odor problems since
chloramines typically give a "swimming pool" odor to water.

63.  Finally, the water reaches the breakpoint, shown at point 4. The
breakpoint is the point at which the chlorine demand has been totally
satisfied - the chlorine has reacted with all reducing agents, organics,
and ammonia in the water. When more chlorine is added past the
breakpoint, the chlorine reacts with water and forms hypo chlorous acid
in direct proportion to the amount of chlorine added. This process,
known as breakpoint chlorination, is the most common form of
chlorination, in which enough chlorine is added to the water to bring it
past the breakpoint and to create some free chlorine residual.

65. Ozonization:
 Ozone is produced by passing silent electric discharge through cold and dry oxygen
3O2 → 2O3
 Ozone is highly unstable and breaks down liberating nascent oxygen
O3 → O2 + O (Nascent oxygen)
 Nascent oxygen is a powerful oxidizing agent and kills all the bacteria as well as oxidizes the organic
matter present in water
 The contact period is about 10-15 minutes and dose is 2-3 ppm
Advantages:
 Ozone sterilizes, bleaches, decolorizes and deodorizes water.
 Excess of ozone in water is not harmful as it decomposes to give oxygen
 Ozone improves the taste of water. Highly potable water is thus sterilized with ozone
Disadvantages:
 Ozonization is a very expensive process. Ozone is also a corrosive agent. It corrodes stainless steel,
cast iron, copper and rubber

66. Desalination of Brackish Water:
 Water is a good solvent. It contains dissolved solids in it. Depending upon the amount of dissolved
solids water may be grouped as follows
(i) Fresh water (having < 1000 ppm of dissolved solids)
(ii) Brackish water (having > 1000 ppm but < 35000 ppm of dissolved solids
(iii) Sea water (having > 35000 ppm of dissolved solids)
 Water containing high concentration of salts in it is called brackish water
 Due to the presence of large quantity of salts, it is salty in taste and is unfit for domestic and industrial
purpose
 This water can be put to good use by removing the salts contents from it.
 The process used for the removal of salts from water is called desalination
 Various techniques employed for desalination are
• Reverse osmosis
• Electro dialysis
• Ultra filtration
• Flash evaporation

67. Reverse osmosis:
 Osmosis is defined as the process of spontaneous flow of solvent through a semi
permeable membrane from a dilute solution to a concentrated solution ( From lower
concentration to higher concentration)
 The diffusion of solvent particles takes place on account of hydrostatic pressure
called osmotic pressure, which drives solvent molecules in search of equilibrium
 If an external pressure equal to osmotic pressure is applied on the concentrated
solution the process of osmosis will stop and if the external pressure exceeds osmotic
pressure, solvent will be squeezed out of the concentrated solution. This process is
called reverse osmosis

69.  In reverse osmosis, pure water is separated from its contaminants rather than
removing contaminants from water
 This process is also known as super filtration or hyper filtration
 The semi permeable membranes are usually made up of cellulose acetate,
polymethacrylate or polyamide polymers.
Advantages:
This process removes lead, calcium, magnesium, sodium, potasium, aluminium,
chloride, nitrate, fluoride, sulphate, boron, most microorganisms and organic chemicals
from water.
Disadvantages:
It requires large volume of water and it produces huge volume of waste water, it
is difficult to manage the waste water.

70.  Electrodialysis
 Dialysis is a process in which diffusion of smaller particles takes
place through a semi-permeable membrane.
 By this process, crystalloids are removed from colloids. The
process has been successfully applied for the purification of sea
water. Sea water is called brackish water (salty water). It has
3.5% salt. Dialysis removes salt from sea water (brine) through a
membrane.
 Principle:
In electro dialysis, ion-selective membrane is used, which
permits the passage of only one kind of ions having specific
charge, i.e., cation selective membrane allows the passage of
cations only but not anions and vice versa.

71. Line diagram of electrodialysis