Water Chemistry 01

7/28/2019 Water Chemistry 01

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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.

Water Chemistry 01

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Transcript of Water Chemistry 01