UNIT – II

TECHNOLOGY OF WATER

SOURCES OF WATER:

Water is the main resource for the existence of life on

this planet. Around two- third of human body is made of water.

Surface water and ground water are the two main sources of water .

The Surface water includes river water, lake water ,rain water and

sea water. The rivers and streams are sources of fresh (i.e., not

salty) water. Collecting water from rivers is still a wide spread

practices. Lakes are still bodies of water. They are often replenished

by rain water, river & stream water. The sea water is salty, through

the process of desalination it becomes potable. The sea is essential

for biodiversity. Rain water is now a days contaminated with

pollutants in cities where as in rural areas it is still collected and

used for drinking purpose. Water from wells tends to be very fresh

and clean.

WATER QUALITY PARAMETERS: The physical and chemical quality of water may

affect its acceptability to consumers. The world health organization

had set the water quality parameters for drinking water. Colour,

odour, turbidity,pH, hardness, alkalinity,dissolved oxygen, BOD and

TDS are the important parameters.

a) Turbidity

Turbidity is the measure of lack of water clarity. Turbidity is the result of suspended solids

in water. It can be measured accurately with a turbidimeter. Turbidity is measured in NTUs,

i.e. nephelometric turbidity unit. Turbidity in drinking water should be less than 1 NTU.

Water treatment is required for drinking water when there is excessive turbidity.

b) Hardness

Water is said to be hard due to the presence of various dissolved salts of calcium,

magnesium and iron. Hardness is expressed as ppm of CaCO3. Hardness can be classified

into two types, temporary hardness which can be removed by mere boiling and permanent

hardness which can be removed by certain water treatment process like lime-soda process,

zeolite process etc.

c) PH

It is in general a measure of hydrogen ion concentration of a water sample. The symbol PH

stands for the potential of hydrogen. Water contains both H+ ions and OH

- ions. Pure

distilled water contains equal number of both the ions and therefore considered to be neutral

(PH

= 7). If a water contains more H+ ions than OH

- ions then the water sample is said to be

acidic (PH

less than 7). If the water contains OH- ions in excess than H

+ ions then the water

sample is said to be basic (PH

is more than 7). Rain water has a PH

of almost 7 surface

water have alkaline PH

.

d) Alkalinity

It is the measure of the quantity of alkaline substances in water. Mostly alkalinity in water is

due to the presence of hydroxide, carbonate and bicarbonate ions present in the water

sample. The possible combinations of ions causing alkalinity are

(i) OH- alone

(ii) CO32-

alone

(iii) HCO3- alone

(iv) OH- and CO3

2- together

(v) CO32 and HCO3

- together.

The combination of OH- and HCO3

- ions does not exist as they combine to form CO3

2-

ions. The combination of all the three ions also result in the formation of CO32-

Hence all

the three ions cannot exist together. Alkalinity is measured in ppm or mg/l.

e) BOD

Biological oxygen demand is the measure of oxygen in the water that is required by the

aerobic organisms. BOD is the measure of decrease in oxygen content in milligrams per litre

of a sample of water kept in the dark at a temperature of 20 degree Celsius over a specified

period of time. The difference between initial DO level of the collected water and the final

DO level is due to the consumption of oxygen brought about by bacterial breakdown of

organic material and the oxidation of chemicals in the water during the storage period.

f) TDS

It is a measure of total amount of solid particles dissolved in water. The major dissolved

salts are chlorides, sulphates, bicarbonates and carbonates of Na, K, Ca, Mg, & Fe. High

TDS values make water unsuitable for irrigation and agriculture. It may also cause foaming

and corrosion in boilers. TDS values range from 20 to 2000mg /lit in river water and may be

higher in ground water.

Hardness: Hardness is the property of water which prevents the formation of lather or foam readily with soap. Water is said to be hard due to the presence of various dissolved salts of calcium, magnesium and iron. Hardness is expressed as ppm of .

Hardness can be classified into two types 1) Temporary hardness which can be removed by mere boiling and 2) Permanent hardness which can be removed by certain water treatment process

like lime-soda process, zeolite process etc.

Temporary and permanent hardness

S.No Temporary hardness Permanent hardness

It is due to the presence of It is due to the presence of chlorides

1. bicarbonates of calciumand and sulphates of calcium and

magnesium. magnesium.

It can be removed easily by boiling

2. because bicarbonates are converted It cannot be removed by boiling.

into insoluble carbonates.

Salts which cause temporary

Salts which cause permanent

hardness – calcium chloride,

3. hardness – calcium bicarbonate,

magnesium chloride, calcium magnesium bicarbonate etc.

sulphate etc.

Expression of Hardness

Hardness of water is expressed in terms of equivalent amount of CaCO3. The reason for selecting calcium carbonate as reference is (i) the molecular weight of calcium carbonate is 100 and equivalent weight is 50 which makes the calculations easier.(ii)

CaCO3 is highly insoluble in water. The mathematical form of expression of hardness

ESTIMATION OF HARDNESS (EDTA METHOD)

EDTA is ethylene diamine tetra acetic acid. The structure of EDTA is

Since EDTA is insoluble in water, its sodium salt is used as a complexing agent.

PRINCIPLE: Estimation of hardness by EDTA method is based on the principle that EDTA forms metal complexes with hardness producing metal ions present in water. These complexes are stable when the pH is maintained between 8-10. In order to maintain the pH, buffer solution (ammonium chloride + ammonium hydroxide mixture) is added. The completion of the complexation reaction is indicated by Eriochrome black-T indicator. When this indicator is added to the sample water it forms indicator-metal unstable complexes of wine red colour.

(Unstable complex) Wine red

When this solution is titrated against EDTA, it replaces the indicator from the indicator

complex. When all the hardness causing ions are complexed by EDTA, the indicator is set

free and the end point is the sharp change in colour from wine red to steel blue. The total

hardness is thus determined.

(Stable complex)Steel blue

The temporary hardness is removed by boiling and after the removal of the precipitate by

filtration; the permanent hardness in the filtrate is determined by titration with EDTA as

before.

Total hardness - permanent hardness = temporary hardness.

EXPERIMENTAL PROCEDURE:

TITRATION I: STANDARDISATION OF EDTA A known volume of the standard hard water (Vml) is pipetted out and treated with 10 ml of buffer solution and 2 to 3 drops of EBT indicator .The solution is then titrated against the standard EDTA solution taken in the burette. The end point is the colour change from wine red to steel blue.

Let the volume of EDTA consumed be V1 ml.

TITRATION II: ESTIMATION OF TOTAL HARDNESS A known volume of the sample hard water is pipetted out and treated with 10 ml of buffer solution and 2 to 3 drops of EBT indicator .The solution is then titrated against the standard EDTA solution taken in the burette. The end point is the colour change from wine red to steel blue.

Let the volume of EDTA consumed be V2ml.

TITRATION III: ESTIMATION OF PERMANENT HARDNESS 250 ml of the sample hard water is taken in a beaker. The solution is boiled till the volume is reduced to 50 ml. The residue is filtered and the filtrate is made up to 250ml using distilled water. From this made up solution a known volume is pipetted out and treated with 10 ml of buffer solution and 2 to 3 drops of EBT indicator. The solution is then titrated against the standard EDTA solution taken in the burette. The end point is the colour change from wine red to steel blue.

Let the volume of EDTA consumed be V3 ml.

CALCULATION: STANDARDISATION OF EDTA:

V1 ml of EDTA = V ml of std. hardwater

= V mg of CaCO3 eq. (since 1 ml of std.hardwater = 1 mg. of CaCO3eq Therefore,

1 ml of EDTA = V/V1 mg of CaCO3eq. TOTAL HARDNESS:

V ml of unknown hard water sample consumes V2 ml of EDTA.

i.e., V ml of sample hardwater = V2 ml of EDTA.

= V2 x V/V1 mg of CaCO3eq.

(Since 1 ml of EDTA = V/V1 mg of CaCO3eq.)

Therefore, 1 ml of sample hardwater = mg of CaCO3eq

=V2/V1 mg of CaCO3eq. 1000 ml of sample hard water = V2/V1x 1000 mg of CaCO3eq.

Therefore, TOTAL HARDNESS = V2/V1x 1000 ppm.

PERMANENT HARDNESS: V ml of unknown boiled hard water sample consumes V3 ml of EDTA. i.e.,

V ml of permanent hardwater = V3 ml of EDTA. = V3 x V/V1 mg of CaCO3eq.

(Since 1 ml of EDTA = V/V1 mg of CaCO3eq.)

Therefore, 1 ml of permanent hardwater = mg of CaCO3eq

= V3 /V1 mg of CaCO3eq.

Therefore, 1000 ml of permanent hard water = V3/V1x 1000 mg of CaCO3eq.

Therefore, Permanent Hardness = V3/V1x 1000 ppm.

TEMPORARY HARDNESS:

Total Hardness = Temporary Hardness + Permanent Hardness

Temporary hardness =Total hardness – permanent hardness

x 1000 ppm.

NUMERICALS IN HARDNESS

Calculate the hardness of water containing 146 mg/l of magnesium bicarbonate.

Hardness = 100 ppm

100 ml of standard hard water contains 1 mg of CaCO3 per ml. 20 ml of EDTA

solution was required for titration with EBT as indicator. 100 ml of hard water sample required 30 ml of the same EDTA solution for titration. Calculate the hardness of water sample.

100ml of standard hard water consumes = 20ml of EDTA

Since 1ml of standard hard water contains 1mg of CaCO3

, 1ml of EDTA = 100 / 20 = 5mg / l of CaCO3

100ml of hard water sample consumes 30ml of EDTA

Therefore 100ml of hard water = 30 x 5 = 150 mg / l of CaCO3 equivalent

Therefore 1000ml of hard water = 150 x 1000 / 100 = 1500 ppm

Alkalinity Alkalinity can be estimated by titrimetry method in which water sample is titrated against a

standard acid using phenolphthalein and methyl orange as indicator. The various

neutralization reactions are as follows

I) OH- + H+→H2O

II) CO32- + H+ → HCO3

-

III) HCO3- + H+→ H2O + CO2

The phenolpthalein end point represents the completion of first two reactions and methyl

orange end point represents the completion of all the three reactions. Hence the amount of

acid used after the phenolphthalein end point corresponds to neutralization of complete OH-

ions and neutralization of one half of the carbonate ions and methyl orange end point

corresponds to another half of the carbonate and all the bicarbonates. The total amount of

acid used after the methyl orange end point corresponds to the total amount of hydroxide,

carbonate and bicarbonate ions, i.e. total alkalinity.

The possible combinations of ions causing alkalinity are

i) OH- alone

ii) CO32- alone

iii) HCO3- alone

iv) OH- and CO32- together

v) CO32 and HCO3

- together.

The combination of OH- and HCO3- ions does not exist as they combine to form CO3

2- ions.

The combination all the three ions also result in the formation of CO32- ions. Hence all the

three ions cannot exist together.

TYPES OF ALKALINITY

Depending on the ions present, it is classified as hydroxide, carbonate and

bicarbonate alkalinity.

PROCEDURE:

V ml of water sample is pipetted out into a clean conical flask and 2 or 3 drops of

phenolphthalein indicator is added into the flask. It is then titrated against a standard acid

from the burette. The end point is the disappearance of pink colour. Let the volume of acid

consumed be V1 ml. continue the titration by adding 2 or 3 drops of methyl orange indicator

to the same solution. The end point is the colour change from yellow to pink. Let the

volume of acid consumed be V2 ml.

CALCULATIONS:

Phenolphthalein end point is noted as V1 ml

Methyl orange end point is noted as V2 ml

Phenolphthalein alkalinity in terms of calcium carbonate equivalent

= V1 X Nacid X 50 X 1000

V

Methyl orange alkalinity in terms of calcium carbonate equivalent

= V2 X N acid X 50 X 1000

V

Amount of OH-, CO32-, and HCO3

- in ppm can be calculated as follows:

CASE ALKALINITY

OH- CO3

2- HCO3-

( I ) If P = 0 0 0 M

(II) If P = M / 2 0 2P 0

(III) If P < M / 2 0 2P M-2P

(IV) If P > M / 2 2P-M 2(M-P) 0

(V) If P = M M 0 0

From the above table the type and extent of alkalinity is found out. BOILER FEED WATER

The essential part of boiler operations is the

boiler feed water. In a boiler the water is fed to the steam drum where it is converted

to steam. During this process the dissolved solids may become concentrated and

deposits inside the boiler thereby causing a lot of boiler troubles.

BOILER TROUBLES:

The trouble created to boiler are formation of scales and sludges, priming

and foaming, caustic embrittlement and boiler corrosion.

Sludge and Scale

S.No Sludge scales

1. Sludges are soft and non-adherent Scales are hard deposits which stick very

precipitates firmly to the inner surface of the boiler.

2. Sludges can be removed easily. Scales are very difficult to remove.

3. Sludges can transfer heat to some extent Scales are bad conductors of heat and are

and are less dangerous. more dangerous.

Sludges are formed by substances like Scales are formed by substances like

4. magnesium chloride, calcium chloride calcium sulphate, magnesium hydroxide

etc. etc.

Caustic embrittlement

It is the phenomenon in which the boiler material becomes brittle due to the accumulation of caustic substances. Boiler water usually contains a small portion of

Na2CO3. In high pressure boilers, this hydrolyses to give sodium hydroxide and makes the boiler water more alkaline.

These dilute alkaline boiler water flows into the minute hair cracks and crevices by capillary action. There the water evaporates and the concentration of caustic soda increases progressively. The concentrated alkali present in the crevices, cracks, joints, bents, rivets etc., sets up minute localized electrochemical cells which convert iron in the boiler plate to brittle sodium ferroate.

Priming and Foaming During the production of steam in the boiler, due to rapid boiling some particles of

liquid water are carried along with steam. Steam containing droplets of water is called wet steam. The process of wet steam formation is called as priming.

Priming is due to, i) Very high water level.

ii) Presence of foam on the surface iii) High steam velocity

Priming can be prevented by maintaining low water level in the boiler and by removing oily materials present in the water. The formation of stable bubbles above the surface of water is called as foaming. It is caused by the presence of soluble impurities like alkaline metal salts and oil which form soap. Foaming can be controlled by the action of anti- foaming agents like synthetic poly amides.

Boiler corrosion

Boiler corrosion is the decay of boiler material due to chemical or electrochemical

attack of its environment. The boiler corrosion is due to dissolved oxygen, dissolved

carbondioxide and dissolved acids produced by the hydrolysis of dissolved salts and

dissolved alkali.

1) Corrosion due to dissolved oxygen:

Dissolved oxygen in water is mainly responsible for the corrosion of boiler. The dissolved oxygen in water attacks the boiler material at high temperature.

2Fe + 2H2O + O2 2Fe (OH) 2

4Fe (OH) 2 +O2 + 2H2O 2[Fe2O3. 3H2O] (yellow rust)

Dissolved oxygen can be removed from water by chemical and mechanical methods.

Sodium sulphites and hydrazine are some of the chemicals used for removing dissolved

oxygen.

2Na2SO3 + O2 2Na2SO4

N2H4 + O2 N2 + 2H2O

Dissolved oxygen can also be removed from water by mechanical deaeration. In this process, water is sprayed slowly through the perforated plates fitted inside the tower. Vacuum is applied to this tower and the sides of the tower are also heated. High temperature and low pressure reduce the quantity of dissolved oxygen in the water.

2) Corrosion due to dissolved CO2

Dissolved CO2 in water produces carbonic acid which is corrosive in nature.

CO2 + H2O H2CO3

CO2 can be removed from water by chemical or mechanical means. In the chemical

method CO2 is removed by the addition of a calculated quantity of NH4OH.

2 NH4OH + CO2 (NH4)2CO3 + H2O

Dissolved CO2 along with oxygen can be removed by mechanical deaeration.

3) Corrosion due to dissolved acids:

Acids produced from salts that are dissolved in water during the boiler operations is mainly

responsible for corrosion of boilers. Certain salts like MgCl2, CaCl2 etc. on hydrolysis at

higher temperature produce hydrochloric acid which corrodes the boiler.

MgCl2 + 2H2O Mg (OH) 2 + 2HCl The liberated acid can produce rust in the following way.

Fe +2HCl FeCl2 + H2

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

4Fe (OH)2 +O2 +2H2O 2[Fe2O3.3 H2O] (rust)

Corrosion by acids can be avoided by the neutralization of acids using alkali.

EXTERNAL TREATMENT

The hardness causing salts can be removed before they enter the boiler

system and this is called external treatment.

1. DEMINERALISATION PROCESS

PRINCIPLE:

In this process all the cations and anions except H+ and OH

- ions present in water are

completely removed by ion exchange resins and hence water of zero hardness can be produced. Ion exchange resins are cross linked polymeric molecules. The functional groups attached to these molecules are responsible for the ion exchange reactions. Cation exchange resin contains acidic functional groups eg, sulphonic acid and

carboxylic acid. It is generally represented as RH2. They exchange their H+ ions with

cations present in the hard water. Anion exchange resin contains quaternary ammonium or phosphonium hydroxides as functional groups. It is generally

represented as R’ (OH) 2. They exchange their hydroxyl groups with anions present in the hard wate

PROCESS: The equipment consists of two exchange columns of cation and anion . The natural water is first fed into the cation exchanger column where it exchanges its hydrogen ion with other cations present in the water.

R-H2+ CaSO4 → R-Ca + H2SO4

R-H2+ MgCl2 → R-Mg + 2HCl

R-H2+2NaHCO3 → R-Na2 + 2H2CO3

R-H2+Mg (NO3)2 → R-Mg + 2HNO3

The water coming out of the cation exchange column will be highly acidic and it is

then passed through an anion exchange column. It exchanges hydroxyl ions with

other anions present in the water.

The water obtained is free from all sorts of ions and it is called demineralised water

or de ionized water. Demineralised water is soft water and is free from any type of

ions.

REGENERATION: On prolonged use of ion exchangers, it will lose its exchanging capacity due to the complete removal of hydrogen and hydroxyl ions. In order to regenerate the resins,

dilute solutions of strong acids like HCl or H2SO4 is passed through the cation exchanger and dilute solutions of strong bases like NaOH is passed through the anion exchanger.

ADVANTAGES AND DISADVANTAGES:

(i) Water of zero hardness can be produced by this method. (ii) Highly alkaline or acidic natural water can be treated by this method.

(iii) Turbid water cannot be treated by this method since the active centers

will be blocked by suspended particles. (iv) The ion exchange resins are costly.

Zeolite method of water softening

Zeolites are hydrated sodium alumino silicate. The water of hydration appears in the form of

steam. Zeolites are represented by the formula Na2O.Al2O3.xSiO2.yH2O or Na2Z; x=2-10

and y=2-6. The synthetic form of zeolite is called permutit.

PROCESS:

The hard water containing calcium and magnesium ions, is passed through a zeolite

bed. The sodium ions present in the zeolite is exchanged with calcium and

magnesium ions present in the hard water.

Thus the water obtained is free from hardness causing Ca2+

and Mg2+

ions.

REGENERATION:

Zeolite will be exhausted after repeated use. It can be regenerated by passing 10%

neutral sodium chloride through the zeolite bed where Ca and Mg zeolite will

exchange calcium and magnesium ions with sodium ions.

ADVANTAGES AND DISADVANTAGES OF ZEOLITE PROCESS:

ADVANTAGES:

1. Water of nearly zero hardness is obtained. 2. There is no problem of sludge disposal. 3. The equipment is convenient and compact for use.

DISADVANTAGES:

1. Water treated by this method will have more amounts of sodium salts. The water is

highly alkaline and cannot be used in steam boilers because that will lead to

caustic embrittlement.

2. Highly acidic and alkaline water cannot be treated by this process because zeolite will dissolve in it.

3. Turbid water cannot be treated by this method as the pores of zeolite will be

blocked by the suspended particles.

HOT LIME SODA PROCESS: This process involves the treatment of water using the softening chemicals lime and soda at about 100°C to 150°C temperature.

The softening rate is high as the reactions are carried out at an elevated temperature. Use of coagulants are avoided as the precipitates formed settles easily due to high temperature.

Raw water and chemicals are fed into the central chamber and steam is passed through the inlet to raise the temperature of water. The reaction takes place at a rapid rate and the precipitate settles down at the bottom easily, which will be removed periodically. Softened water is collected through the outlet. The residual hardness of the softened water is 15 – 30 ppm.

COLD LIME- SODA PROCESS

Calculated quantities of lime, soda and

coagulants like Alum, Sodium aluminates are added to the water to be treated into the

inner vertical chamber. Coagulants are added so that the precipitate formed settle

down easily. The vertical chamber is fitted with rotating shaft carrying a number of

paddles. Softening of water take place due to rigorous stirring. The sludges are

removed through an outlet and the treated water are separated after being passed

through wood fiber filters.

COMPARISON OF HOT LIME – SODA PROCESS AND COLD LIME – SODA PROCESS

S.No Cold lime soda process Hot lime soda process

1 Reactions are slow Reactions are fast

2 Filtration is slow Filtration is fast

3 Use of coagulants is essential No coagulants are added

4 Fuel is not required Large amount of fuel is consumed.

5 Time taken for softening will be few Time taken for softening will be few

hours minutes.

Hardness of softened water is 50 - 60 Hardness is 17-34 ppm.

6

ppm.

7 Lime consumed is more Lime consumed is less

SAMPLE PROBLEMS

Calculate the lime( 90% pure) and soda( 95% pure) requirement for treating 1,00,000

litres of water having the following analysis. L3

Ca(HCO3)2 = 162 ppm ; Mg(HCO3)2 = 73 ppm ; MgCl2 = 47.5 ppm ;

CaSO4 = 68 ppm and CaCl2 = 111 ppm

Hardness causing salt Quantity Hardness in mg/l

Ca(HCO3)2 162 162 X 100/162=100ppm

Mg(HCO3)2 73 73 X 100/146=50ppm

MgCl2 47.5 47.5 X 100/95=50ppm

CaSO4 68 68 X 100/136=50ppm

CaCl2 111 111 X 100/111=100ppm

lime requirement =74/100 [temp.Ca + 2 x temp.Mg + per.Mg] =74/100[100 + (2X50) + 50] =74/100[250] = 185ppm

For 1,00,000litres of water Pure lime required = 185 X 1,00,000/1000 X 1000

= 18.5 Kg

Lime provided is only 90% pure

Therefore the impure lime required = 18.5 X 100 /90 = 20.55 kg

ii) Soda requirement = 106/100[ per Ca2+

+ per Mg2+

]

=106/100[(50+100)+50 ] =106/100[200]=212ppm For treating 1,00,000litres of water soda required

=212 X 1,00,000/1000 X1000 =21.2 kg

The soda provided is only 95% pure Therefore the requirement of impure soda

=21.2 X 100/95 = 22.3 kg

INTERNAL CONDITIONING METHODS It involves the removal of scale forming substance, which were not completely removed in the external treatment, by adding chemicals directly into the boiler.

1. COLLOIDAL CONDITIONING:

Scale formation can be avoided by adding chemicals like lignin, agar- agar gel, kerosene etc. to the boiler water during boiler operations. When scale forming compounds get precipitated, these chemicals get coated over them and make them loose and slimy precipitates which settles down and can be removed by blow down operation.

2. CARBONATE CONDITIONING: Scale formation can be avoided by adding calculated quantity of sodium carbonate to boiler water during boiler operations. The scale forming salts are converted into insoluble loose precipitates of respective carbonates which can be removed by blow-down operations. Excess of sodium carbonate added may lead to caustic embrittlement.

CaSO4+ Na2CO3 CaCO3↓ + Na2SO4

(Scale (non scale

Forming salt) forming precipitate)

3. PHOSPHATE CONDITIONING:

Scale formation can be avoided by adding sodium phosphate. It is used in high

pressure boilers. The phosphate reacts with Ca2+

and Mg2+

salts to give soft

sludges of calcium and magnesium phosphates.

3CaSO4 + 2Na3PO4 Ca3 (PO4)2 + 3Na2SO4

Generally 3 types of phosphates are employed

a. Tri sodium phosphate: Na3PO4 (too alkaline) used to treat too acidic water.

b. Disodium hydrogen phosphate: Na2HPO4 (weakly alkaline)- used for

weakly acidic water. c. Sodium dihydrogen phosphate: NaH2PO4 (acidic) - used for alkaline water.

4. CALGON CONDITIONING:

Calgon is sodium hexa meta phosphate Na2 [Na4 (PO3)6]. This substance

interacts with scale forming ions forming a highly soluble complex and thus

prevents the precipitation of scale forming salts.

2CaSO4 + Na2 [Na4 (PO3)6] Na2[Ca2(PO3)6] + 2Na2SO4

The complex Na2 [Ca2 (PO3)6] is soluble in water and there is no problem of

sludge disposal.

DOMESTIC WATER TREATMENT

The water used for domestic purpose should be free from colloidal impurities,

domestic sewage, industrial effluents and disease producing bacteria. Hence domestic

supply of water involves the following stages in the purification processes.

SOURCES OF WATER

SCREENING

SEDIMENTATION

COAGULATION

FILTRATION

STERILISATION (OR) DISINFECTION

1. SCREENING: It is a process of removing the floating materials like leaves, wood

pieces, etc.from water. The raw water is allowed to pass through a screen, having

large number of holes, which retains the floating materials and allows the water to

pass.

2. SEDIMENTATION: It is a process of removing suspended impurities by allowing the water to stand undisturbed for 4 – 8 hours in a big tank. Most of the

suspended particle settles down at the bottom due to forces of gravity, and they are

removed. Sedimentation removes only 75% of the suspended impurities.

3. COAGULATION: Finely divided clay, silica, etc. do not settle down easily and

hence cannot be removed by sedimentation. Such impurities are removed by

coagulation method.

In this method certain chemicals, called coagulants like alum, Al2 (SO4)3 etc. are

added to water. When the Al2(SO4)3 is added to water, it gets hydrolysed to form a

gelatinous precipitate of Al(OH)3. The gelatinous precipitate of Al (OH)3 entraps the

finely divided and colloidal impurities, settles to the bottom and can be removed easily.

4. FILTERATION: It is the process of removing microorganisms and suspended

impurities etc., by passing the water through filter beds containing fine sand, coarse

sand and gravel. A typical sand filter is shown in the figure below.

The sand filter consists of a tank containing a thick top layer of fine sand followed by

a coarse sand, fine gravel and coarse gravel. When the water passes through the

filtering medium, it flows through the various beds slowly. Thus the impurities are

removed. The rate of filteration decreases slowly due to the clogging of impurities in

the pores of the sand bed. When the rate of filteration becomes very slow, the

filteration is stopped and the thick top layer of fine sand is scrapped off and replaced

with clean sand. Bacteria are also partly removed by this process.

5. STERILISATION (OR) DISINFECTION: The process of destroying the

harmful bacteria is known as sterilization or disinfection. The chemicals used for this

purpose are called disinfectants. This process can be carried by the following

methods.

a) By boiling: when water is boiled for 10 – 15 minutes, all the harmful bacteria are killed and the water becomes safe for use.

Disadvantages

i) Boiling alters the taste of drinking water ii) It is impossible to employ it in municipal water-works.

b) By using ozone: ozone is a powerful disinfectant and is readily absorbed by water. Ozone is highly unstable and breaks down to give nascent oxygen.

O3 O2 + [O] The nascent oxygen is a powerful oxidizing agent and kills the bacteria.

Disadvantages i) This process is costly and cannot be used in large scale. ii) Ozone is unstable and cannot be stored for long time.

c) By using ultraviolet radiations: UV rays are produced by passing electric

current through mercury vapour lamp. This is particularly useful for sterilizing

water in swimming pool.

Disadvantages i) It is costly. ii) Turbid water cannot be treated.

d) By chlorination: The process of adding chlorine to water is called chlorination. Chlorination can be done by the following methods.

i) By adding chlorine gas

Chlorine gas can be bubbled in the water as a very good disinfectant.

ii) By adding chloramines

When chlorine and ammonia are mixed in the ratio of 2:1, a compound

chloramine is formed.

Cl2 + NH3 ClNH2 + HCl

Chloramine compounds decompose slowly to give chlorine. It is better disinfectant than chlorine.

iii)By adding bleaching powder

When bleaching powder is added to water, it produces hypochlorous acid (HOCl). It is a powerful germicide.

CaOCl2 + H2O Ca (OH)2 + Cl2

Cl2 + H2O HCl + HOCl HOCl + bacteria bacteria are killed.

BREAK POINT CHLORINATION

It is done for highly polluted water. As chlorine reacts with many of the compounds present

in highly polluted water, the added chlorine is first consumed for different reactions without

providing disinfection. The ammonia present in water reacts with chlorine forming

chloramines. The chlorine will be utilized for the oxidation of other organic compounds.

Only when chlorine demand for all such impurities has been met, remaining free chlorine is

utilized for disinfection. Thus combined residual chlorine decreases to a minimum at which

oxidation of chloramines and other organic compounds complete. This minimum is the

break point. Hence to achieve chlorine as disinfectant, the dosage of chlorine applied should

be more than the break point.

Desalination

Desalination is the process of removal of salt from sea water. It is done by either of the following methods a) Reverse osmosis or

b) Electro – dialysis.

a) REVERSE OSMOSIS

This is a desalination method for converting salt water into potable water. By applying

pressure greater than osmotic pressure, the solvent is made to move from higher

concentration side to lower concentration side through a semi permeable membrane. This is

called Reverse Osmosis.

When two solutions of different concentration are separated by means of a semi permeable

membrane, the solvent molecules migrate from the side of lower concentration to that of the

side of higher concentration due to osmotic pressure and this process is known as osmosis.

If the pressure on the higher concentration solution side is increased more than that of the

osmotic pressure then the flow of solvent is reversed i.e. the solvent molecules migrate from

the side of higher concentration to the side of lower concentration through the semi –

permeable membrane.

The equipment consists of two chambers separated by means of semi permeable membrane

usually made of polymethaacrylate, poly sulphone, cellulose acetate etc. sea water is filled

in the upper chamber and the other chamber is filled with pure water. When high pressure is

applied on the sea water side, the pure water molecules passes through the semi permeable

membrane and migrate to the lower chamber.

b)ELECTRODIALYSIS

The principle of electrodialysis process is that the cations and anions are pulled out from

the sea water under the influence of an electric field using electrodes and ion selective

membranes. Thus when electric current is passed through the saline water, the Na+ ions

moves towards the cathode through cation selective membrane and the Cl- ions moves

towards the anode through the anion selective membrane. Thus the concentration of brine in

alternate chamber is reduced. Desalined water is removed while concentrated brine will be

replaced periodically.

The cation selective membrane is permeable to cations only due to the presence of

negatively charged functional groups suh as RCOO- in the membrane. Similarly anion

selective membrane is permeable only to anions due to the presence of positively charged

functional groups such as R4N+ in the membrane. Usually ion selective membranes are

polystyrene based polymers.