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WATER CHEMISTRY
We never know the worth of water till the well is dry
English Proverb
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The availability of water, both in quantity and quality, is
one of the prime factor in deciding the growth of townsand cities as well as industries. For chemical industries,
the available water must be as near as possible to the
factory site and should also be soft. Otherwise the
manufacturing cost will increase.
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A phase diagram is a convenient way of representing the phases
of a substance as a function of temperature and pressure. The
phase diagrams are the primary visualizing tools in material
sciences because they help to predict and interpret changes of a
composition of a material from phase to phase as a function of
temperature and pressure.
The phase diagram consists of following important headings:
i. Phase Curves
ii. Phase Areas
iii. Triple Point
iv. Critical Point
WATER PHASE DIAGRAM
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i. Phase Curves
The phase diagram contains three curves, each indicating theequilibrium between two phases.
Fusion/ Melting/ Freezing curve i,e Phase boundary line betweenSolid and Liquid phases show Melting Point/ Freezing Point.
Boiling curve/ vapour pressure curve of water i.e Phase boundaryline between Liquid and Gaseous phases show Boiling Point/liquefying point/ vapour pressure of water at that temperature.
Sublimation curve or vapour pressure curve of ice i.e phaseboundary line between Solid and Gaseous phases showSublimation Point/ Solidifying point/ vapour pressure of ice at thattemperature.
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ii. Phase AreaThere are three phase areas each indicating range of set ofconditions in terms of temperature and pressure at which thatmaterial is capable of stable existance in that particular single-phasestate.
Solid phase area is the area bounded by sublimation and fusioncurves.
Liquid phase area is the area bounded by fusion and boiling curves.
Gaseous phase area is the area bounded by boiling and sublimationcurves.
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iii. Triple Point
The point in the phase diagram, where all three lines meet
together, a unique combination of temperature and pressure
exist, where all three phases are in dynamic equilibrium together
i.e solid is melting & sublimating, liquid is boiling & freezing and
gas is liquefying and solidifying. This point is called Triple Point.
For water this point is at 0.00075oC and 4.58 mm pressure.
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iv. Critical Point:
The point above which it is impossible to condense a gas into liquid,no matter how much pressure is applied. Temperature and pressurefunctions correspond to this point are called critical temperature andcritical pressure respectively.
So, Critical temperatureis the temperature required to vaporize a
liquid at its critical pressure. Beyond critical temperature it is notpossible to distinguish between liquid and gaseous phases.
CT of water is 374oC, NH3 132 degree, O2119 degree & CO2 is
31.2 degree.
Critical pressureis the pressure required to liquefy a gas at its
critical temperature.
CP of water is 217.7 atm, NH3 111.5 atm, O2 49.7 atm and CO2 is 73
atm.
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SPECIAFICATION OF WATER FOR
INDUSTRIAL PURPOSE
S.
No
Purpose specification Remarks
1. Boilers Hardness of water
should be zero
Untreated water may cause corrosion
of boiler and scale formed Prevents
the efficient heart transfer.
2. cooking Soft and free from
dissolved salts
Hard water produces unpleasant taste
and it also requires more fuels.
3. coolant Should be non-
corrosive and non-
scale forming
Corrosiveness cause damage to
cooling lines & radiators while scale
decrease cooling efficiency
4. Beaverage Should be free from
alkalinity
Alkaline water neutralizes the fruit
acids which destroys or modified the
taste.
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5. Dairies Water should be
colourless, tasteless,
odourless and free
from pathogens.
6. Sugarindustry
Soft water In presence of hard waterdeliquescent sugar is formed which
causes problem in crystallization.
7. Textile
industry
Water should be soft,
free from turbidity,
organic matter, colour,
iron and manganese.
Turbidity in hard water causes
uneven dying; hard water causes
precipitation of basic dyes and
decreases the solubility of acidic
dyes. Organic matter imparts foul
smell.
8. laundry Water should be soft
and free from colour,
iron and manganese
Hard water consumes more soaps
and detergents. Salts of iron and
manganese impart undesirablecolour to the fabric.
9. Paper
mills
Should not contain
excess of lime and
magnesia and should
be free from iron salts.
It will destroy quality of paper pulp.
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PARAMETERS OF WATER QUALITY:
1. Hardness:
Hardness is the property of water which prevents
the lather formation with soap.
Hardness in water is due to the presence of cations of
calcium, magnesium and other heavy metals. Other
metal ions like Al3+
, Mn3+
, Fe2+
etc. also react with soapbut their contribution to hardness is very less because
these are present only in traces in natural water.
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Soap action
Soaps are generally sodium and potassium salts of higher fatty acidslike oleic acid, stearic acid, palmitic acid. When a sample of hard
water is treated with soap it does not produce lather rather it forms
insoluble white scum or precipitate which do not possess any
cleansing action. This is because the Ca2+ and Mg2+ ions in water
form insoluble salt of these fatty acids.
2 C17H35COONa + Ca2+ (C17H35COO)2Ca + 2Na+Soap from Insoluble
(Sodium stearate) hard water Calcium stearate
2 C17H35COONa + Mg2+ (C17H35COO)2Mg + 2Na+Soap from Insoluble
(Sodium stearate) hard water Magnesium stearate
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Lather is not produced until the cations causing the precipitation of soap are
completely removed and hence the need for a large quantity of soap to
produce lather with hard water. Therefore hardness may be defined as
soap consuming/wasting capacity of water. The present days detergent
used in laundry work, contains a long hydrocarbon alkene of the typeCnH2n where n is between 12 and 20. The alkene is first sulphonated with
oleum and then converted into sodium salt. This sodium salt is the
detergent.
C15H30O + H2SO4 C15H31SO4H
C15H31SO4H + NaOH C15H31SO4Na + H2O
They are better than soap as they are not affected by hardness in water.
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Types of Hardness
A. On the basis of sustain abilityhardness is of two types
i) Temporary hardness.
ii) Permanent hardness.
B. On the basis of alkaline characterhardness is of two types
i) Alkaline hardness
ii) Non-alkaline hardness
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(i) Temporary hardness:
Temporary hardness of water is due to the presence ofbicarbonates of
calcium, magnesium and other heavy metals and carbonate of iron.
Temporary hardness is removed by mere boiling of water, which
converts bicarbonates into insoluble carbonates or hydroxides which
are deposited as crust at the bottom of the vessel. It needs only
physical treatment.
Ca(HCO3)2 CaCO3 + H2O + CO2Calcium Bicarbonate Calcium carbonate (Insoluble)
Mg(HCO3)2 Mg(OH)2 + 2CO2Magnesium bicarbonate Magnesium hydroxide (Insoluble)
T
T
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ii) Permanent hardness:
Permanent hardness is due to the presence ofchlorides and
sulphates of calcium magnesium, iron and other heavy metals.
Permanent hardness cant be removed by boiling and need
chemical treatment.These are removed by special chemical methods like
Lime-soda process
Zeolite process Ion exchange process
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i) Alkaline hardness
Alkaline hardness is defined as the hardness due to Alkaline
anions as bicarbonates, carbonates and hydroxides of the
hardness producing cations.
OH- + H+ H2O
CO3-2 + H+ HCO3-
HCO3- + H+ H2O + CO2
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ii) Non-alkaline hardness
Non-alkaline hardness is due to non-alkaline anions like Cl- and SO4
2- ofhardness producing cations. The non alkaline hardness is obtained by
substracting the alkaline hardness from the total hardness.
Ca(HCO3)2
Temporary Mg(HCO3)2 Alkaline
Hardness FeCO3 Hardness
Ca(OH)2
Total CaCl2
Hardness MgCl2
FeCl2
FeCl3
Permanent CaSO4 Non-Alkaline
Hardness MgSO4 HardnessFeSO4
Fe2(SO4)3
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Units of Hardness:
Hardness usually measured in terms of CaCO3 due to its round
figure molecular weight i.e 100 a.m.u and its abudance in hardwater after treatment.
a) Parts Per Million (ppm):
Number of parts by weight of CaCO3 equivalent hardness present
per million (106) parts by weight of H2O.Hence
X ppm = X part CaCO3 equivalent hardness in 106 parts of H2O
b) Milligrams Per Liter:
Number of milligrams of CaCO3 equivalent hardness present perliter of H2O is known as mg/liter hardness.
Hence
X mg/liter = X mg CaCO3 equivalent hardness per liter of H2O
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d = m/v = 1 gm/cm3 for waterso m = v for water
1 Kg H2O = 1 L of H2O
103 x 103 mg = 1 Kg H2O
106 mg = 1 L of H2O
X mg/liter = X mg of CaCO3 equivalent per 106 mg of water
X mg/liter = X parts of CaCO3 equivalent per 106 parts of water
X mg/liter = X ppm
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c) Degree French:
Number of parts by weight of CaCO3 equivalent hardness presentper lakh (105) parts of water is called degree French. It is denotedby Fr.
X oFr = X Parts of CaCO3 equivalent hardness per 105 parts of H2O
d)Degree Clarks:
Number of parts by weight of CaCO3 equivalent hardness presentper 70,000 parts of water
or It may also be defined as
the number of grams of CaCO3 equivalent hardness present pergallon or in 10 lb of water.
It is denoted by oCl.
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Relation Between Various Units
Xppm = Xparts CaCO3 equi hardness per106 parts of H2O
Xmg/L = Xppm
XoFr = Xparts CaCO3 equi hardnessper105 parts of H2O
XoCl = Xparts CaCO3 equi hardnessper 70,000 parts of H2O
From above equations the inter-relation of various units can be writtenas
106ppm = 106mg/L = 105oFr= 70,000 oCl
1ppm = 1mg/L = 0.1 oFr= 0.07 oCl
1 oCl = 14.3 ppm (approximately) = 1.43oFr(approximately)
1oFr= 10ppm = 0.7 oCl
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Water hardnessscale
Parts per Gallon Parts per
Million(ppm)
Classification
Less than 1.0 Less than 17.1 soft
1.0 - 3.5 17.1 60.0 Slightly hard
3.5 - 7.0 60.0 120.0 Moderately hard
7.0 - 10.5 120.0 180.0 Hard
More than 10.5 More than 180.0 Very hard
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2. Alkalinity:
Alkalinity of water may be defined as its capacity to neutralize acids.
The alkalinity of water may be attributed to the presence of
OH- ions
CO3 -2 ions
HCO3 - ions
The determination is based on the following reactions
(i) OH- + H+ H2O P
(ii) CO3-2 + H+ HCO3- M
(iii) HCO3- + H+ H2O + CO2
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The titration of water sample against a standard acid up to
phenolphthalein end point (P) marks the completion of reactions (i)
and (ii) only. The amount of acid used upto phenolphthalein endpoint thus corresponds to neutralization of whole OH- and one half
of the normal carbonate present. While the titration of water sample
against the standard acid upto methyl orange end point (M) marks
the completion of reactions (i), (ii) and (iii). Therefore the additional
acid used after phenolphthalein end point corresponds to one half ofnormal carbonate and all the bicarbonate present. The total amount
of acid used represents the total alkalinity (due to OH-, CO3-2 and
HCO3-).
Therefore, P = OH- + CO3-2
M = OH- + CO3-2 + HCO3-
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The possible combination of ions causing alkalinity in water are:
OH- only
CO3-2 only
HCO3- only
OH- and CO3-2 only
CO3-2 and HCO3- only
The possibility of OH- and HCO3- together is not possible since
they combine together to form CO3-2 and H2O
OH- + HCO3- CO3-2+ H2O
Similarly, all the three (OH-, CO3-2 and HCO3-) can not exist together.
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RELATIONSHIP B/W P & M ALKALINITY
P = OH- + CO32- , M = OH- + CO32- + HCO3-
Case-I : When only OH- is present in water
P = OH- + CO32- , M = OH- + CO32- + HCO3-
P = OH- , M = OH-
P = M
Case-II : When only CO32- is present in waterP = OH- + CO32- , M = OH- + CO32- + HCO3-
P = CO32- , M = CO32-
P = M
M = 2P
Case-III : When only HCO3- is present in waterP = OH- + CO32- , M = OH- + CO32- + HCO3-
P = 0 , M = HCO3-
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Case-IV : When only OH- & CO32- are present in water
(i).
P = OH- + CO32- , M = OH- + CO32- + HCO3-
P = OH- + CO32- , M = OH- + CO32-
P = OH- + OH- + CO32-P = OH- + (OH- + CO32-)
P = OH- + M
P = M + OH-
P > M
(ii).P = M + OH-
P = (M + OH-)
2P = M + OH-
OH- = 2PM
(iii).
P = OH- + CO32- , M = OH- + CO32-M = OH- + CO32- + CO32-
M = P + CO32-
M - P = CO32-
2(MP) = CO32-
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Case-V : When only CO32- & HCO3- are present in water
P = OH- + CO32- , M = OH- + CO32- + HCO3-
P = CO32- , M = CO32- + HCO3-
2P = CO32- , M = CO32- + HCO3-
M = 2P + HCO3-
2P = M - HCO3-
P = M HCO3-
P < M
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Water absorbs more heat for a given temperature rises than any
other common inorganic substance and expands 1600 times as it
evaporates to form steam at atmospheric pressure. The steam is
capable of carrying large quantities of heat.
If hard water is directly fed into the boilers it may lead to the
following problems:
(I) Scale and sludge formation.
(II) Boiler corrosion.
(III) Caustic embrittlement.
(IV) Priming and foaming.
BOILER TROUBLES BY FEEDING WATER
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(I) Scale and Sludge
In boilers steam is generated continuously by the evaporation of
water. As the water evaporates continuously, the concentration of
dissolved salts increases, finally the solution becomes saturated.
The point at which ionic product exceeds the solubility product, they
are thrown out as precipitates.
A) Scale:Scales are the hard deposits firmly sticking on the
inner wall of the boiler and cant be removed easily by
scrapping.
B) Sludge: If the precipitate formed is soft, loose and floats in
boiler water it is called sludge.
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Why scale formation takes place?
Scaling is mainly due to the presence ofcalcium and magnesium
salts (carbonates or sulphates), which are less soluble in hot than
cold water, or due to presence oftoo high concentration of silica
in relation to the alkalinity of the water in the boiler. They may be
formed in boilers due to following reasons:
Hydrolysis of magnesium slats:
MgCl2 + 2H2O Mg(OH)2 + 2HClscale
Decomposition of calcium bicarbonate:
Ca(HCO3)2 CaCO3 + CO2 + H2Oscale
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Calcium carbonate scale is soft and can be easily removed by wire brush. Inhigh pressure boilers CaCO3 is soluble because it undergoes hydrolysis to
form calcium hydroxide and carbon dioxide.
CaCO3 + H2O Ca(OH)2 + CO2
On continuous heating CaSO4present in hard water also gets precipitated ashard scale. (A sulphate deposit is much harder and denser than a carbonate
deposit because the crystals are smaller and cemented together more
tightly). A sulphate deposit is brittle, does not pulverize easily, and does not
show any effervescence when dropped into acid. If silica is present in small
amount in water it may form calcium silicate (CaSiO3) and magnesium
silicate (MgSiO3) scales which adhere very firmly to the inner walls of the
boiler. Scales are generally removed by chemical reactions.
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A high silica deposit is very hard, resembling porcelain. The crystals of silica
are extremely small, forming a very dense and impervious scale. This scale
is extremely brittle and very difficult to pulverize. It is not soluble in
hydrochloric acid and is usually very light colored.
Iron deposits, either due to corrosion or iron contamination in the water, are
very dark coloured. Iron deposits in boilers are most often magnetic. Theyare soluble in hot acid giving a dark brown colored solution.
Iron oxide scales consist of ferric and ferrous compounds such as iron
silicates, ferrous phosphate Fe3(PO4)2, sodium ferrophosphate (NaFePO4)
and iron oxides (Fe2O3, Fe3O4).
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Removal of scale formation:
Brittle scales can be removed by giving thermal shocks, i.e, boileris heated and then suddenly cooled with cold water.
Hard and adherent scales can be removed by adding chemicals.The added chemical dissolves the scale and hence removes it,
e.g.,(i) Silicate and calcium sulphates scales are removed
by dissolving them with EDTA solution.
(ii) CaCO3 scale can be dissolved by using 10% HCl solution.
Loose and adherent scale can be removed with the help ofscrapper and frequently blow down operation.
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Prevention of scale formation:
Scale formation can be prevented by- External treatment
- Internal treatment
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External treatment:
In this process water is externally treated before feeding it into the
boiler and hence the name external treatment.
The various external treatment methods are: Lime soda process.
Zeolite process.
Ion-exchange process.
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Internal treatment:The treatment of raw water inside the boiler is known as
internal treatment.
This is also known as internal conditioning or sequestration.
In internal treatment suitable chemicals are added to the boiler
water either
To precipitate the scale forming impurities in the form of Sludges
which can be removed by blow down operation or
To convert them into compounds which will stay in dissolved form inwater.
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Internal treatment methods
(i) Phosphate conditioning:
In phosphate conditioning scale formation is prevented by addition of sodium
phosphate which reacts with magnesium and calcium salts giving non-adherent
and soft sludge of magnesium and calcium phosphate respectively.
This process is effective in between the pH range 9.5 to 10.5.
3MgCl2+ 2Na3PO4 Mg3(PO4)2 + 6NaCl3CaCl2 + 2Na3PO4 Ca3(PO4)2 + 6NaCl
The phosphates employed are NaH2PO4, Na2HPO4 and Na3PO4.
The precipitate formed is then removed by blow down operation.
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(ii) Calgon conditioning:
Calgon is sodium hexametaphosphate Na2[Na4(PO3)6].
It is extensively used in internal treatment and prevents the scale and
sludge formation by converting scale forming impurity like CaSO4 to
highly soluble complex.
Na2[Na4(PO3)6] Na2[Na4P6O18]
2CaSO4 + Na2[Na4P6O18] Na2[Ca2P6O18] + 2Na2SO4Soluble complex
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Why sludge formation takes place?
Salts like MgCl2, MgCO3, CaCl2 & MgSO4 having greater solubility in
hot water than in cold water are responsible for the formation of
Sludges and hence are generally formed at comparatively colder
parts/portions of the boilers. They get collected at places where the
rate of flow is slow. Sludge can easily be removed with a wire brush.
They may lead to chocking of pipes. Sludges are usually sparingly
soluble compound while scales are highly insoluble compounds.
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Disadvantages of sludge formation:
Sludges are bad conductor of heat and hence a portion of heat
generated is wasted which decreases the efficiency of boiler.
Excessive sludge may cause chocking of pipe in the region
where there is less water circulation.
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Prevention of sludge formation
Formation of Sludge can be prevented by
Frequently blow down operation
Using soft water
Coagulation and dispersion are two general approaches
When the total amount of sludge is high (as the result of high feed-water hardness) it is better to coagulate the sludge to form largeflocculent particles.
When the amount of sludge is not high (low feed water hardness) itis preferable to use a higher percentage of phosphates in thetreatment.
The materials used for conditioning sludge include various organicmaterials of the tannin, lignin or alginate classes.
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(II) Boiler corrosion
Loss of boiler body material or its useful properties by chemical
or electrochemical interaction with its environment is known as
boiler corrosion.
Corrosion may occur in the feed-water system as a result of
low pH water
presence of dissolved oxygen and carbon dioxide.
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Dissolved carbon dioxide:
CO2 gas dissolved in water, forms carbonic acid which has a corrosive
effect on boiler material like any other acid. CO2 is also produced in waterby the decomposition of bicarbonates.
Mg(HCO3)2 MgCO3 + CO2 + H2OCa(HCO3)2 CaCO3 + CO2 + H2OCO2 + H2O H2CO3
Removal:
Carbon dioxide can be removed
(a) By addition of calculated amount of ammonium hydroxide (NH4OH)
CO2 + 2NH4OH (NH4)2CO3 + H2O
(b) By mechanical de-aeration along with oxygen (sonication).
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(ii) Dissolved oxygen:
Dissolved oxygen is the most important factor causing boiler corrosion.Water usually contains about 8 ppm dissolved oxygen at room temperature.
Dissolved oxygen present in water attacks the boiler material according to
the reactions:
Fe Fe2
+ + 2e-] x 2 at anodeO2 + 2H2O + 4e- 4OH- at cathode
2Fe + O2 + 2H2O 2Fe2+ + 4OH-
2Fe(OH)2
4Fe(OH)2 + 2H2O + O2 4Fe(OH)3 Fe2O3.XH2O (Rust)
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Removal:
(a) O2 is removed by adding oxygen scavengers like Na2SO3,hydrazine etc.
2Na2SO3 + O2 2Na2SO4
Sodium Sulfite Sodium Sulfate
N2H4 + O2 N2 + 2H2O
(b) it is also removed by mechanical de-aeration.
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(III). Caustic Embrittlement
Caustic embrittlement is a type of boiler corrosion due to which
boiler material becomes brittle in presence of high conc of
caustic and static tensile stress (thermal stress).
It is characterized by the formation of inter-granular cracks on theboiler metal particularly at places of high stress like bends, joints,
riveted seams etc.
During softening by lime-soda process, residual sodium carbonate is
usually present in softened water. Under the condition of highpressure in boiler sodium carbonate decomposes to form sodium
hydroxide and CO2.
Na2CO3 + H2O 2NaOH + CO2
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Prevention:
Caustic embrittlement can be prevented by:
Using sodium phosphate as softening agent instead of sodium
carbonate (soda) in external treatment.
By adding sodium sulphate or sodium phosphate to boiler water
which clogs the hair cracks opening, reducing the chance ofcaustic embrittlement.
Tannin or lignin addition to boiler water which blocks the hair
cracks preventing the infilteration of caustic soda. All these
substances fill hairline cracks in boiler to avoid caustic
embrittlement.
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(IV) Priming and Foaming
As steam rises from the surface of boiling water in boiler it may be
associated with small droplets of water. Steam containing liquid water is
called wet steam.
The process of wet steam formation is known as priming.
Priming is the carryover of varying amounts of droplets of water in the steam
(foam and mist). Which lowers the energy efficiency of the steam and leads
to the deposit of salt crystals on the super heaters and in the turbines.
Steam-carried solids produce turbine blade deposits. These conditions often
lead to super heater tube failures as well. Priming is related to the viscosity
of the water and its tendency to foam. These properties are governed by
alkalinity, the presence of certain organic substances and by total salinity or
TDS. The degree of priming also depends on the design of the boiler and its
steaming rate.
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Priming is mainly attributed to the presence of
Suspended impurities and to some extent to dissolve some
impurities in water.
Sudden boiling (bumping). High steam velocity.
Faulty boiler design.
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Priming can be minimized by:
Proper designing (i.e dimensions).
Maintaining low water levels.
Controlled rate of steam velocity.
Efficient softening.
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Foaming:
The formation of persistent bubbles in boiler which do not break
easily is known as foaming.
Foaming is generally caused by the presence of oils and alkalis in
water. Clay or organic matterin raw water, oil and grease in condensed
make up water and finely divided particles of sludge may also cause
foaming. Foaming also increase priming.
Foaming can be minimized by
Removal of foaming agent like oil, grease from boiler water.
Addition of castor oil and antifoaming chemicals like 2-Octanol,sulfonated oils, Organic phosphate, silicon fluids etc.
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The process of removing or reducing the hardness (temporary
or permanent) from water is known as softening of water.
The important method employed for the softening of water are:
Zeolite process
lime-Soda process
Ion exchange process
All these three are external treatments.
Water Softening Methods
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1. Zeolite Process or permutit Process
Zeolites are hydrated sodium alumino silicates capable ofexchanging its sodium ions reversibly with the hardness
producing cations in water.
The formula of sodium zeolite is Na2OAl2O3.xSiO2.yH2O Where x = 2
to 10 and y = 2 to 6. They are also known as permutit.
Zeolite are of two types:
Natural Zeolites Synthetic Zeolites
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(i) Natural Zeolites:
They are amorphous and non-porous in nature. They are derived from
green sands by washing, heating and treating with NaOH. The exchange
value of green sand is 7000 to 9000 gm of hardness per m3 of zeolite. e.g.Natrolite Na2O.Al2O3.4SiO2.2H2O
(ii) Synthetic Zeolites:
They are porous and gel structured synthetic zeolites are prepared by
heating together solutions of Sodium silicate, aluminium sulphate and sodium aluminate.
China clay, feldspar and soda ash.
The most common artificial zeolite is the permutit. The permutit is white
in colour and its chemical formula is 2SiO2
Al2
O3
.Na2
O. The exchangevalue of permutit is 35000 to 41000 gm of hardness per m3 of
zeolite.
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A.Principle:
Zeolites can be simply represented as Na2Z where Z represents insolubleradical frame work. They hold sodium ion loosely. When hard water is
passed through a bed of Zeolite, the hardness causing ions are retained by
zeolite as CaZ and MgZ. Therefore water becomes free from the main
hardness producing cations but gets more concentrated with respect to
sodium salts and eventually zeolite gets exhausted.
Ca(HCO3)2 + Na2Z CaZ + 2NaHCO3
Mg(HCO3)2 + Na2Z MgZ + 2NaHCO3
MgCl2 + Na2Z MgZ + 2NaCl
CaCl2+ Na2Z CaZ + 2NaCl
CaSO4 + Na2Z CaZ + Na2SO4
MgSO4 + Na2Z MgZ + Na2SO4
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B. Regeneration
During softening, Zeolites exchange its sodium ions with magnesium
and calcium ions and after some time they are completely converted
into calcium and magnesium zeolites and the zeolite bed cease to
soften water, i.e gets exhausted
The process by which the exhausted zeolite is reclaimed bytreating it with 10% brine solution is known as regeneration
CaZ + 2NaCl Na2Z + CaCl2
MgZ + 2NaCl Na2Z + MgCl2exhausted Brine reclaimed (Washings)
Zeolie solution zeolite
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Process:
Hard water is percolated (filtered/passthrough) at a definite rate through
the bed of zeolite housed in a cylindrical unit. The hardness causing
Ca+2 and Mg+2 ions are retained by zeolite as CaZ and MgZ
respectively. The outgoing water contains sodium salts. After some
time the bed gets exhausted.
At this stage the supply of hard water is stopped and regeneration is
carried out. Thus softening by Zeolite involves alternate cycles of
softening run and regeneration. The regeneration step comprises of (1)
backwashing (2) brining (3) rinsing before reuse. The soft water thus
obtained has hardness less than 30 ppm.
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Advantages of zeolite process softener
Hardness is almost completely removed and water of about 10 ppmhardness is produced.
It automatically adjust itself to the water of different hardness.
The equipment used is compact and occupies less space.
It required less time for softening.
Less skill is required for maintenance as well as operation.
There is no danger of sludge formation because impurities are not
precipitated.
Disadvantages of zeolite process
Only cations (Ca+2, Mg+2) are replaced by sodium ion sand not the
acidic ions.
Treated water contains more sodium salts than in lime-soda
process.
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Limitations:
If water is turbid, it will cause the clogging of pores of zeolite bed, therebymaking it inactive.
Acid radicals are not removed by this process. Such waters used in boilers
causes highly alkaline condition which leads to
Caustic embrittlement
Boiler corrosion.
NaHCO3 NaOH + CO2
CO2 + H2O H2CO3
(Carbonic acid)
If large quantities of Fe+2 and Mn+2 are present in water the zeolite bed is
converted into iron and manganese zeolites which cant be regenerated.
Zeolite process gives good results only with cold water in terms of hardness
removal.
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2. Lime-soda processPrinciple:
Lime-soda process is based upon the precipitation of solublecalcium and Magnesium salts by addition of calculated amount
of lime and soda.
Function of lime:The lime used in this process may be quick lime or
hydrated lime. Calcium ion is precipitated as calcium carbonate
(CaCO3) and magnesium as magnesium hydroxide [Mg(OH)2] which
then filtered off.
At room temperature precipitates formed are very fine and
they do not settle down easily, causing difficulty in filtration. Hence a
small amount ofcoagulant like alum, aluminium sulphate or sodium
aluminate are added for the coarsening of precipitates.
The following reactions take place during this process:
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The following reactions take place during this process:
1) Lime removes any free acid present:
2HCl + Ca(OH)2 CaCl2+ 2H2O
H2SO4 + Ca(OH)2 CaSO4 + 2H2O
2) Lime precipitates iron and aluminium salts as hydroxides:
Al2(SO4)3+ 3Ca(OH)2 2Al(OH)3 + 3CaSO4FeSO4+ Ca(OH)2 Fe(OH)2 + CaSO4
2Fe(OH)2 + H2O + O2 2Fe(OH)33) Lime dissolves carbon dioxide as calcium carbonate:
CO2+ Ca(OH)2 CaCO3 + H2O4) Lime precipitates bicarbonate of calcium as calcium carbonate:
Ca(HCO3)2 + Ca(OH)2 2CaCO3+ 2H2O5) Lime precipitates magnesium salts as hydroxides:
Mg(HCO3)2 + 2Ca(OH)2 2CaCO3 + Mg(OH)2 + 2H2OMgCl2(or MgSO4) + Ca(OH)2 Mg(OH)2 + CaCl2(orCaSO4)
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6)Lime converts bicarbonate ions (like NaHCO3, KHCO3, etc)
into carbonates:
2NaHCO3 + Ca(OH)2 Na2CO3
+ CaCO3
+ 2H2O
7) Soda coverts all soluble calcium permanent hardness:
Na2CO3+ CaCl2 CaCO3 + 2NaClNa2CO3 + CaSO4 CaCO3 + Na2SO4
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Function of soda:The added ingredient soda ash reacts with calcium permanent hardness
including calcium hardness introduced during the reaction of lime with HCl(equations in point vii above).
Bicarbonate ions (NaHCO3, KHCO3), if present in the hard water, produced
carbonate ions (as Na2CO3 or K2CO3) during their reaction with lime and this
may be imagined to be equivalent to production of Na2CO3. Hence, the
amount of carbonate ions thus produced from bicarbonate ions are to be
subtracted from the total requirement of soda for softening.
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Lime-soda process are of two types:
Cold lime-soda process.
Hot lime-soda process.
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Advantages of Lime-soda process:
Economical.
cost effective.
Lesser amount of coagulants are required.
Iron and manganese could be removed. Minerals are also removed
Reduce corrosion tendency.
The alkaline nature of water reduces the amount of pathogenic
bacteria.
Suitable for turbid and acidic water.
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Disadvantages of Lime-soda process:
Disposal of large amount of sludge is a problem.
Skilled supervision and careful operation are required for efficient
and economical softening.
3 I E h d i i ti
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3. Ion Exchange or de-ionization or
Mineralization process.
The process used for removal of all dissolved salts from water is referred to
as deionization
Deionization requires the flow of water through two ion exchange materials in
order to effect the removal of all salt content. The terms demineralization and
deionization are used somewhat interchangeably by the industry. While the
term demineralization is generally better understood, deionization is especially
apt. The word soft water means it does not have hardness producing Ca2+ and
Mg2+ ions but it may contain other ions like Na+, Cl-, K+ etc. Alternatively,
demineralized water does not have any ion including hardness producing ones.
Thus, it should be noted that every soft water is not de-mineralized water;
whereas every demineralized water is soft water
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Polished/Intrinsic water
The extremely pure water used for washing in the manufacturing of
delicate electronic equipments like TV tubes, calculators, watches,
transistors etc is known as polished water or intrinsic water which is
usually obtained by passing water through several ion exchange
resin columns.
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Ion exchange resins
They are insoluble cross-linked long chain organic polymers with a
micro porous structure. The functional groups attached to the
chains are responsible for the ion exchanging properties.
Resins containing carboxylic (-COOH) or sulphonic acid (-SO3H)
functional groups are able to replace their H+ ions with other
cations, which comes in their contact whereas those containing
basic amino (-NH2OH) or substituted amino (quaternary ammonium
salts) functional group are able to replace their anions with other
anions, which comes in their contact.
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i) Cation exchange resins:Cations exchange resins are mainly styrene-divinyl benzene
copolymers, which on sulphonation or caboxylation become capable to
exchange their hydrogen ions with the cations in the water. For
example,AmberliteIR 120, Dowex-50, Nalcite-HCR. These can be
represented as R-H+. Their exchange reactions with cations (for
example, Ca2+, Mg2+) are shown below:
2R-H+ + Ca2+ R22-Ca2+ + 2H+ 2R-H+ + Mg2+ R22-Mg2+ + 2H+
CLASSIFICATION OF RESINS
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CH2 CH CH2 CH CH2 CH CH2 CH2 CH2
SO3-H
+ SO3-H
+SO3
-H
+SO3
-H
+
M+ M
+
M++M
+ M+
M+
M
+
M++
M++
M++
CH2 CH CH2 CH CH2 CH2 CH2 CH2 CH2
SO3-M
+ SO3-M
+ SO3-
-O3S
M++
M+
M+
M+
M+
M++
M++
M++
H+
H+
H+
H+
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ii) Anion exchange resins:
They are styrene-divinyl benzene or amine formaldehyde copolymers,
which contain amino or quaternary ammonium or quaternary phosphine
or tertiary sulphonium groups as an integral part of the resin medium.
After treatment with dilute NaOH solution the above groups are able to
exchange their OH- anions with the anions in the water. For example,
Amberlite-400, Dowex-3, and Zeolite-F. These can be represented as
R+OH-, their exchange reactions with anions (for example, SO42-, Cl-,
CO32-, etc,) are shown as below:
R+OH- + Cl- R-Cl- + OH- 2R+OH- + SO42- R22+SO42- + 2OH-
2R+OH- + CO32- R22+CO32-+ 2OH-
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CH2 CH CH2 CH CH2 CH CH2 CH CH2
CH2 CH CH2 CH CH2 CH CH2 CH CH2
NMe2+OH
-NMe2
+OH
- NMe2+OH
-
NMe2+OH
-
NMe2
+
NMe2
+
X
-
NMe2
+
X
- +
Me2NX2 -
X2 -
X2 -
X2 -
X2 -
X2 -
X2 -
X2 -
X-
X-
X- X
- X-
X-
X- X
-
X-
X-
OH-
OH-
OH- OH
-
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The process:
The raw water is passed first through the cation exchange material to
remove the Ca2+
and Mg2+
ions just as in the normal softening process.
The metallic ions in the water get attached to the ion exchange material,
which releases its hydrogen ions on a chemically equivalent basis.
At this point the deionization process is just half complete. While thepositive metallic ions have been removed, the water now contains positive
hydrogen ion and the anions originally in the raw water.
The partially treated water then is passed through a second unit containing
an anion exchange material normally consists of replaceable hydroxyl
anions and fixed irreplaceable cations. Now the negative ions in solution(the anions) are absorbed into the anion exchange material and hydroxyl
anions are released in their place. All that emerges from such two-unit
system is ion-free water.
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Regeneration:
The ion exchange capability of these resins towards the exchange of
ions from the water is based upon their ion exchange potential. These
resins are said to be exhausted when ion exchange potential is
lost/decreased. These exhausted Cation exchange resins can be
regenerated by passing a solution of dil.HCl or dil. H2SO4 through the
Cation exchange column.
The renewal is represented as:
R22-Ca2+ + 2H+ Ca2+ + 2R-H+
The column is washed with de-ionized water and washing (contains
Ca2+
Mg2+
and Cl-
or SO42-
ions) is passed to a sink or drain. Theexhausted anion exchange column can be regenerated by passing a
solution of dil. NaOH, the regeneration can be represented as:
R22+SO42- + 2OH- 2R+OH- + SO42-
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Advantages of the ion exchange process:
Highly efficient process, >99.9% removal of desired ions.
Predictable performance.
The process can be used to soften highly acidic or alkaline
water. Produces water of very low hardness (say 2ppm).
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Disadvantages of the ion exchange process:
The equipment is costly and more expensive chemicals are
needed.
If water contains turbidity, then the output of the process is
reduced.
The turbidity must be below 10 ppm. If it is more, it has to be removed first by co adulation and
filtration.
This water softening does not remove bacteria, sand,
pesticides, and many other organic and inorganic compounds.
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Water is indisputably the most essential resource of the earth has to
offer to the human race. As already explained oceans contribute about
97% of the total water on earth. The ocean (sea) water contains about
2.5% salt and it has a peculiar salty (brackish) taste and is also known
as brackish water(If the water contains 1000 to 35,000 mg/lit of
dissolve salts, it is called brackish water).This brackish water is totally
unfit for drinking purposes.
The process of removing common salt (NaCl) from sea (brackish)
water is called desalination
SALT WATER DESALINATION METHODS
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The most commonly used methods for desalination of water are:
Electrodialysis,
Reverse osmosis
Distillation.
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1) Electro-dialysis
It is a process by which oppositely charged ions (Na+ and Cl- ) are pulledout of the salt water by passing direct current using electrodes and ion selective
natural or synthetic membranes. When current is passed sodium ions start
moving towards cathode whereas chloride ions move towards anode through
the ion selective membranes [which allows only one kind of ion with specific
charge to pass through it, for example, As a result of movement of Na+
and Cl- ions, the concentration of brine decreases in the central compartment.
Desalinated (pure) water in the central compartment is removed from time to
time and replaced with fresh seawater and the process continues.
The advantage of electro-dialysis is that the cost of the plant installation is
economical.
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2) Reverse Osmosis
When a semipermeable membrane separates two solutions of differentconcentrations, the flow of solvent takes place from dilute solution to
concentrated solution and the process is called osmosis. However, on
application of a hydrostatic pressure in excess of osmotic pressure, the solvent
flow can be reversed, that is, the flow of solvent takes place from concentrated
solution to dilute solution and the process is called as reverse osmosis. So, in
Reverse Osmosis (RO) process, the pure solvent (water) is separated from its
contaminants (in case of brackish water contaminant is NaCl). In this process
the pressure (20-100 atm ) is applied to seawater (higher conc.) to force
pure water (lower conc.) out through semipermeable membrane. The mostcommonly used polymeric RO membranes are made from cellulose acetate or
Polymethamethaacrylate or polyamides.
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The advantages of reverse osmosis include:
It significantly reduces salt, most other inorganicmaterial present in the water, and some organiccompounds.
It usually removes microscopic parasites (including
viruses), but any defect in the membrane would allowthese organisms to flow undetected into the filteredwater.
The process is less expensive to operate andmaintain.
Th di d t f i i l d
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The disadvantages of reverse osmosis include:
The process makes only a few gallons of treated water a day for
drinking or cooking. Wastage of water is more. 2-4 gallons of waste water are flushed
down the drain for each gallon of filtered water produced.
Some pesticides, solvents and other volatile organic chemicals are notcompletely removed by reverse osmosis systems.
The efficiency of RO membrane is dependent on many factors [thecontaminant concentrations, chemical properties of the contaminants,the membrane type and condition, and operating conditions (like pH,water temperature, and water pressure)] in reducing the amount ofcontaminant in the water.
These systems require periodic maintenance. The pre and post filtersand the reverse osmosis membranes must be changed according tothe manufacturers recommendation, and the storage tank must becleaned periodically.
Damaged membranes are not easily detected, so it is hard to tell if thesystem is functioning normally and safely.
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Potable Water
Water which is safe and palatable for drinking
purposes is known as potable water
Characteristics of potable ater
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Characteristics of potable water
It must possess the following characteristics:
It should be colourless, odourless and sparkling clear.
Potable water should be perfectly cool.
It should have pleasant taste.
It should be free from objectionable gases (CO2
, NH3
,H2
S etc)and minerals such as Pb, Mn, Cr, As salts.
pH should be about 8.0.
It should be reasonably soft.
Turbidity should be less than 10 ppm. Free chlorine should beless than 0.1-0.2 ppm. Dissolved solid less than 500 ppm is
desirable. It should be free from pathogens. Water obtained from most of
the natural sources is generally impure and is thus not fit fordrinking purposes.
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Water treatment plant:
There are two types of water treatment for potability
1) POE water treatment2) POU water treatment
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1. POE water treatment
Schematic diagram of a POE watertreatment plant
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In point of Entry (POE) water treatment, the following steps are taken
for purification of the water before domestic supply:
(i) Screens:
The untreated water is passed through screens having large number of
holes of different sizes to retain the floating material in the water.
(ii) Pre-chlorination:
Pre-chlorination is utilized mainly in situations where the inflow is taken
from a surface waste source such as a river, lake, or reservoir.
(iii) Sedimentations with coagulation and flocculation:
Sedimentation is a process in which the water is allowed to stand
undisturbed in big tanks for 2-6 hours. During sedimentation most of
the suspended particles settle down at the bottom of the tank due toforce of gravity. Coagulants are added before sedimentation to remove
impurities like fine clay particles or colloidal matter.
The various chemical reactions involved in the coagulation/flocculation
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The various chemical reactions involved in the coagulation/flocculation
are given below:
a) Alum [K2SO4.Al2(SO4)3.24H2O] is the most widely used coagulant in water
treatment. Alum reacts with the natural alkalinity present in the water asfollows:
Al2(SO4)3+ Ca(HCO3)2 2Al(OH)3 + 3CaSO4 + 6CO2(coagulant) (Flocculant ppt)
b) Sodium Aluminate (NaAlO2) is used as coagulant when the water
undergoing treatment have pH less than 7 (no alkalinity). It gives best
results in the pH range of 5.5-7.0.
NaAlO2+ 2H2O Al(OH)3 + NaOH
The sodium hydroxide formed reacts with magnesium salts present in
the water to form magnesium hydroxide flocculants as given below.
MgCl2(orMgSO4) + 2NaOH Mg(OH)2 + 2NaCl(orNa2SO4)
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c) Ferrous sulphate [FeSO4.7H2O] is commonly used as
flocculant above pH 8.5. The chemical reactions involved aregiven below
FeSO4 + Mg(HCO3)2+ H2O Fe(OH)2 + MgCO3 + CO2+ H2SO4(coagulant) (Flocculant ppt.)
4Fe(OH)2 + O2 + 2H2O 4Fe(OH)3(coagulant) (heavy Flocculant ppt.)
Fe(OH)3 is a heavy flocculant which quickly settles down during
sedimentation process.
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(iv) Filtration:
Slow filtration through a fine sand filter to remove any remaining small
particles, microorganisms, bacteria etc.
(v) Aeration:
of water to oxidize small quantities of organic material to carbon dioxide
and water.
The BOD (biological oxygen demand)value is the amount of oxygen
required, with the assistance of bacteria, to oxidize the oxidizable organic
materials present in 1 liter of water to carbon dioxide and water.
Therefore, BOD is a measure of the degree of pollution of water and is
expressed in mg of oxygen /L.
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The COD value (chemical oxygen demand)indicates how muchoxygen, measured in mg/L is required to oxidize most of the
organic material in 1 liter of dirty water by oxidizing agent.
This test method oxidized more organic material than the BOD test
and therefore the COD value is always higher than the BOD value.
Despite this, measurement of the COD value is oftenpreferredbecause the results can be obtained more quickly.
(vi) Disinfection:
Disinfection with either ozone or chlorine to render the remaining
bacteria harmless and to reduce the growth of algae in waterpipes. 3-6 mg of chlorine per liter are added to the water with which
it reacts to form hypochlorous acid, which is a more efficient
disinfectant than the hypochlorous anion.
A Chl i ti
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A. ChlorinationWater for drinking purposes, the potable water, is treated to make it
acceptable and free from harmful bacteria. This is most often
accomplished by adding chlorine to the water, which forms the strong
oxidizing hypochlorous acid, HOCl.
Cl2 + H2O HOCl + H+ + Cl-
HOCl H++ OCl-
Depending on pH and products formed, the effective residual
concentration of chlorine [free available chlorine (FAC)], HOCl or OCl-
(hypochlorite ion) is 0.1 0.2 mg/l. Higher concentrations of chlorine (or
products of chlorination) tend to give water a definite taste. The
hypochlorous acid produced by the reaction of chlorine with water isresponsible for the death of microorganisms and bacteria, etc. present
in water.
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The advantages of using chlorine as disinfectant are:
Chlorine provides a strong residual in the distribution system.
Chlorine can be easily converted to chloramines, which also
provide a strong residual and do not produce by-products.
Chlorine is easy to apply.
Chlorine is relatively inexpensive.
Chlorine is effective even at low concentrations.
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The disadvantages of using chlorine are:
When chlorine reacts with organic material its concentration is
reduced and trihalomethanes (THMs) are formed which are
carcinogenic.
Chlorine provides poor cryptosporidium and Giardia control. Bothof these disease cause gastrointestinal problems, which in severe
instances can cause death.
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B. Ozonation
A mixture of ozone and air can be bubbled through water to oxidizeimpurities. Ozone is toxic, and can be detected by its odour at about
0.01 ppm whereas its total limit value (TLV) is 0.1 ppm. Unlike chlorine,
ozone does not produce known carcinogens as a byproduct of its water
treatment and, therefore, is gaining increased use for domestic as wellas industrial water supplies. Chlorination must follow ozonation in
public water supply because ozone decomposes rapidly, and chlorine
residual may be carried throughout a distribution system. For domestic
water purifiers, ozone is often combined with activated carbon filtration
to achieve a more complete water treatment.
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The advantages of using ozone include:
Ozone is primarily a disinfectant that effectively kills biological
contaminants.
It oxidizes and precipitates iron, sulphur, and manganese which
can be filtered out from solution.
It oxidizes and breaks down many organic chemicals including
many that cause odour and taste problems.
Ozonation produces no taste or odour in the water.
Since ozone is made of oxygen and reverts to pure oxygen, it
vanishes without trace once it has been used.
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The disadvantages of using ozone include:
Ozone treatment can produce undesirable by-products that can
be harmful to health (for example, formaldehyde and bromate).
The process requires electricity and hence, it cannot be used in
an emergency.
Ozone is not effective in removing dissolved minerals and salts.
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2. POU water treatment:
Point of use (POU) methods treat water at the point where it is used
frequently (at the kitchen sink). Only the water that is actually used for
drinking, cooking, beverage preparation, etc. is treated. This has the
advantage of economy only a few hundred gallons of water need to be
treated per year instead of many thousands if all of the water enteringthe home were to be treated.
POU generally include following three processes
A) Filtration by filter
B) Adsorption by activated carbon
C) UV irradiation by UV lamp
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( ) Filt ti b filt
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(a) Filtration by filterFiber filters:
These filters contain cellulose, rayon or some other material spun into a mesh
with small pores. Suspended sediment (or turbidity) is removed as water
pressure forces water through tightly wrapped fibers. Some small organic
particles that cause disagreeable odour and taste may also be removed.
The finer the filter, the more particles are trapped and the more often the filtermust be changed. Fiber filters are often used as pre-filters to reduce the
suspended contaminants that could clog carbon or RO filters. The main
drawback of fiber filters is that these can not remove contaminants that are
dissolved in the water, like chlorine, lead, mercury, trihalomethanes or other
organic compounds.
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(b) Adsorption by activated carbon
Granular activated carbon (GAC):
In this types of filter, water flows through a bed of
activated carbon granules which trap some particulate
matter and remove organic contaminants causing
undesirable taste and odour.
(c) Ultra violet light irradiation:
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(c) Ultra-violet light irradiation:
Water passes through a clear chamber where it isexposed to Ultra-violet (UV) light. UV light effectively
destroys bacteria and viruses. However, how well the UV
system work depends on the energy dose that the
organism absorbs. If the energy dose is not high enough,the organisms genetic material may only be damaged
rather than disrupted.
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The advantages of using UV include:
(i)it does not produce any toxic or significant nontoxic by productsduring the process.
(iii) it leaves no smell or taste in the treated water.
(iv) It requires very little contact time (seconds versus minutes for
chemical disinfection).
(v) It improves the taste of water because microorganisms aredestroyed.
(vi) Many pathogenic microorganisms are killed or rendered inactive by
UV radiations.
(vii) It does not affect minerals in water.
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The disadvantages of using UV include:
(i) UV radiation is not suitable for water with high level of suspended solids,
turbidity, colour, or soluble organic matter. These materials can react with
UV radiation, and reduce disinfection performance. Turbidity makes it
difficult for radiation to penetrate water and pathogens can be shadowed,
protecting them from the light.
(ii) UV light is not effective against many non-living contaminant, lead,
asbestos, many organic chemicals, chlorine, etc.
(iii) It requires electricity to operate so can not be used in an emergencysituation when the power is out.