water and its treatment

1. UNIT-IV
WATER AND ITS TREATMENT

2. HARDNESS OF WATER
Hardness is the property of water which prevents lather formation with soap. This is due to the
presence of certain salts of Ca and Mg. These dissolved Ca and Mg ions do not produce lather
but form insoluble white ppt.
2C17H35COONa + CaCl2→(C17H35COO)2Ca + 2NaCl
Other metal ions like Fe3+, Al3+, Mn2+ also contribute to hardness but these ions are present in
natural water only in traces. However the hardness of a sample of water is usually taken as a
measure of its Ca2+, Mg2+ ions content. Water can be classified as:
HARD WATER
Thus water which does not produce a ready and permanent lather with soap solution is called
hard water. Eg. Sea water, River water etc.
SOFT WATER
Water and which produces a ready and permanent lather with soap solution is called soft water.
Eg. Distilled water, Rain water etc.

3. TYPES OF HARDNESS
1. Temporary Hardness It is due to the presence of dissolved bi-carbonates of Ca, Mg and
other heavy metals. Presence of bicarbonates causes alkalinity to water. Temporary
hardness can be removed by simple boiling of water.
2. Permanent Hardness It is due to the presence of dissolved chlorides and sulphates of Ca,
Mg and other heavy metals it can not be removed by simple boiling, chemical treatment
is required.
DISADVANTAGES OF USING HARD WATER
A. In domestic purpose
i. Washing and bathing When hard water is used it does not lather freely with soap. Only
after all the Ca and Mg ions are removed from water as insoluble salts, lather will be
produced. So a lot of soap will be wasted.
ii. Drinking and Cooking Hard water causes bad effect on the digestive system. The
possibility of formation of calcium oxalate stones in the urinary tracts is also increased.
More fact and time are required since the boiling point of water is increased due to the
presence of dissolved salts.

4. B. In industrial purposes
1. Textile Industry Consider quantities of soap are wasted during the washing of cloth.
Moreover the insoluble salts of Ca and Mg may adhere to the fabric.
2. 2. Paper industry Ca2+, Mg2+ and Fe2+ ions may react to the chemical used and affect the
smoothness and colour of paper.
3. Sugar Industry Dissolved salts cause problems in the Crystallisation of sugar.
4. Dyeing Industry The dissolved salts may react with the functional groups present in dyes
causing precipitation of dye or changing the colour.
5. Alcoholic distillaries Alkaline water tends to utilize the fruit acids and changes the taste of
beverage.
6. Dairies and Pharmaceuticals These industries require highly soft water as salt present in hard
water may react with chemicals to form undesirable products.
C. In steam generation boilers
Most of the water used for industrial purpose is in the form of steam. Boilers are used for steam
generation. If hard water is used in boilers, the following boiler troubles may arise.

5. SCALE AND SLUDGE FORMATION IN BOILERS
As water evaporates continuously in boilers, the concentration of dissolved salts increases. When
their concentration exceeds the saturation point they get precipitated on the inner walls of the
boilers. If the ppt formed is soft, loose and non-adherent it is called sludge. If the ppt formed is
hard, thick and strongly adhering on the inner walls of the boiler, it is called scale.
Sludges are formed by substances like MgCl2, MgCO3, MgSO4 and CaCl2 which have greater
solubilities in hot water than in cold water. Sludges are generally formed at the colder parts of the
boiler. They can be easily removed by scrapping off with a wire brush. Scales are the firmly
sticking hard substances which cannot be easily removed. Scales are the main source of boiler
troubles.
Disadvantages of sludge formation
1. Heat loss because of poor conductance of sludge.
2. Disturbs the proper working of boiler.
3. Deposits on the colder parts of the boiler like bend, joint etc.
Disadvantages of scale formation
1. Waste of fuel
2. Lowering of boiler safety
3. Decrease in efficiency
4. Boiler explosion

6. CAUSTIC EMBRITTLEMENT
Caustic embrittlement is a type of boiler corrosions caused by using highly alkaline
water in the boiler and also due to stress. Water softened by lime soda process may
contain NaOH which is formed by the hydrolysis of Na2CO3.
Na2CO3 + H2O→2NaOH + CO2
The NaOH containing water flows through small pores in the stressed areas like bends,
joints hair-cracks etc. When water evaporates the concentration of NaOH increases and
it corrodes the surrounding area by forming sodium ferrate. This causes embrittlement
of boiler parts particularly stressed parts causing even failure of the boiler.

7. WATER SOFTENING
The process whereby we remove or reduce the hardness of water is termed as
softening of water. It is very essential process since hard water is unsuitable for
domestic as well as industrial use. The hardness causing salts can be removed from
water by following two ways.
a. Internal treatment
The internal treatment of boiler feed water refers to the conditioning of water in the
boiler itself by the addition of chemicals. The following conditioning methods are
used in the internal treatment. Colloidal, phosphate, calgon and carbonate
conditioning
b. External treatment
The external treatment of water is carried out before its entry in to the boiler. This
treatment prevents boiler problems. It can be done by lime soda process, zeolite or
ion exchange process.

8. i. Lime – Soda process
The basic principle behind this method is the chemical conversion of all the
soluble hardness causing impurities in to insoluble precipitates which may be
removed by settling and filtration. This method consisted of simultaneous
addition of calculated quantities of both lime and soda in to hard water
followed by removal of the precipitates. Both temporary and permanent
hardness can be removed by this process.
Ca(OH)2+CO2→CaCO3↓+H2O
Ca(OH)2+Ca(HCO3)2→2CaCO3↓+2H2O
Ca(OH)2+Mg(HCO3)2→MgCO3+CaCO3↓+2H2O
Ca(OH)2+MgCO3→CaCO3↓+Mg(OH)2↓
Ca(OH)2+MgSO4→CaSO4+Mg(OH)2↓
Na2CO3 + CaSO4 → Na2SO4 + CaCO3↓

9. II. Zeolite Process
Zeolite are hydrated sodium alumino siticates Na2Al2 Si2O8. xH2O. They work as
water softness by replacing the calcium and magnesium ions in water with the
sodium ions in zeolite. In this process hard water is allowed to pass through a bed
of zeolite at a specified rate. Then the sodium ions present in the zeolite bed
continuously replace the calcium and magnesium ions present in water and hence
the water becomes soft.
Na2Ze + Ca(HCO3)2 → CaZe + 2NaHCO3
Na2Ze + Mg(HCO3)2→ MgZe + 2NaHCO3
When the zeolite bed becomes exhausted it requires regeneration. This is achieved
by passing 10% NaCl solution through it.
CaZe+2NaCl→Na2Ze+CaCl2

11. III. Ion Exchange Method
Ion exchange resins are insoluble cross linked long chain organic polymers having a
microporous structure where acidic or basic functional groups attached to the chain are
responsible for the ion exchange capacity. Cation exchange resins contain acidic
functional groups like –COOH, -SO3H etc. while onion exchange resins contain basic
functional groups like –OH, -NH2 etc. The hard water is allowed to pass through a
cation exchange column to remove all the cations like Ca2+, Mg2+ etc.
2RH+ + Ca2+ → R2Ca2+ + 2H+
2RH+ + Mg2+ → R2Mg2+ + 2H+
Afterwards the water is allowed to pass through an anion exchange resin column to
remove anions like SO4
2-, Cl- etc.
ROH- + Cl- → RCl- + OH-
The H+ and OH- ions so produced from the cation and anion exchange resins combine
to become water
H+ + OH- → H2O
Water thus coming out of the ion exchanger will be free from both cations and anions
and hence called demineralised water.

14. INTERNAL TREATMENTS OR INTERNAL CONDITIONING
In this method appropriate chemicals are added to water in the boiler when the
hardness producing ions are converted in to either more soluble salts or insoluble
ppts. The following are the important internal treatment methods done inside the
boiler.
1. Colloidal Conditioning Certain organic substance like tannin is added to the
boiler water they get absorbed over scale forming substances thus preventing
scale deposition.
2. Phosphate conditioning In high pressure boilers, scale formation is prevented by
adding sodium phosphate to boiler feed water. This reacts with Ca2+ and Mg2+
ions in water forming non adherent and easily removable soft sludge of Ca and
Mg phosphate. The commonly used phosphate are NaH2PO4, Na2HPO4 and
Na3PO4.
2Na3PO4 + 3CaCl2 → Ca3(PO4)2 + 6NaCl

15. The choice of phosphate depends up on alkalinity of boiler feed water. For eg. Ca can
not be precipitated below PH 9.5. Hence Na3PO4 is selected to remove Ca2+ ions.
3. Carbonate Conditioning
In low pressure boilers, scale formation can be prevented by adding Na2CO3 to boiler
feed water. Ca salts are precipitated as loose sludge of CaCO3 which can be removed by
blow down operation.
4. Calgon Conditioning
Addition of sodium hexameta phosphate called calgon prevents scale formation by
converting Ca2+ and Mg2+ ions in to soluble complexes.
5. Aluminium conditioning
Boiler water is treated with sodium meta aluminate whenNaOH and Al(OH)3 are
formed. NaOH precipitates Mg salt as Mg(OH)2. Both Al(OH)3 and Mg(OH)2 ppts
formed entraps all the scale forming materials. These loose ppt formed can be removed
by blow down operation.
NaAlO2 + 2H2O → NaOH + Al(OH)3
2NaOH + MgCl2 → Mg(OH)2 + 2Nacl

16. SAVE WATER
Thank You