Methods of purification of water:

Various methods which may be adopted for purifying public water supplies are:
(i) Screening, (ii) Plain sedimentation, (iii) Sedimentation aided with coagulation, (iv) Filtration, (v)
Disinfection, (vi) Aeration, (vii) Softening, (viii) Other miscellaneous treatments.

These units are described briefly below:

1. Screening:
 Generally provided in front of the pumps or the intake works.
 Coarse screens - Parallel iron rods placed vertically at about 2 to 10 cm centre to centre
Fine screens - Openings are less than 1 cm wide.
 Velocity through the screens should not be more than 0.8 to 1 m per second.

2. Plain sedimentation:
 Most of the suspended impurities present in water to have a specific gravity greater than that of
water. As soon as the turbulence of flow is retarded by offering storage to the water, these
impurities tend to settle down at the bottom of the tank which is used for such storage. This
process is called sedimentation. The tank used for this storage is called sedimentation tank.
 It is easy to guess that very small sized particle will settle very slowly and vice versa. Settling
velocity of various particles are governed by Stokes law.
 Sedimentation Tanks:
(a) Generally made of reinforced concrete and maybe rectangular or circular in plan.
Rectangular tanks are generally preferred.
(b) A plain sedimentation tank under normal conditions may remove as much as 70% of the
suspended impurities present in water.

(c) The parameter Q/BL is called overflow rate or overflow velocity or surface loading which
is the main factor governing the efficiency of sedimentation tanks. Smaller particles will also
settle down, if this overflow rate is reduced.
(d) Normal values of overflow rates are 12 to 18 m /day/m (plain sedimentation) and 24 to 30
3 2

m /day/m of plan area (for sedimentation with coagulants).

3 2

(e) The average theoretical time required for the water to flow through the sedimentation tank
is called detention period. It is obtained by dividing the volume of the tank by the rate of flow
through the tank.
(f) The detention time usually ranges between 4 to 8 hours for plain sedimentation and from to
2 to 4 hours when coagulants are used.

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 (g) The materials which settle down at the bottom of the sedimentation tank are called sludge.
Sludge should be removed from the sedimentation tank either by using mechanical equipment
or periodically by any other means.

3. Sedimentation aided with coagulation:

 Very fine suspended mud particles cannot settle down in plain sedimentation. For this, certain
chemical compounds called coagulants are added to the water, which on thorough mixing form
a gelatinous precipitate called ‘floc’. Very fine colloidal particles present in water get attracted
and absorbed in these flocs and forms bigger sized flocculated particles.
 The coagulated water is finally made to pass through the sedimentation tank, where the
flocculated particles settle down and are removed.
 The use of coagulant is generally necessary for raw water containing turbidites greater than 30
to 50 mg/l.
 The chemicals which are used as coagulants are most effective when water is slightly alkaline.
 The following chemicals are generally used as coagulants:

(a) Alum:
 Alum is the common name of Aluminium Sulphate i.e. Al (SO ) , 18H O.
2 4 3 2

 When added to raw water, it reacts with the bicarbonate alkalinity present in the water and form
a gelatinous precipitate (floc) of aluminium hydroxide.This floc attract other fine particles and
suspended matters.
 Addition of alum to water impart permanent hardness to it in the form of calcium sulphate. The
carbon dioxide gas which is evolved causes corrosiveness.
 The dose of alum may vary from 5 mg per litre to 85 mg per litre for highly turbid waters. The
average normal dose is about 17 mg per litre.
 It is a very effective coagulant. It helps in reducing taste and colour in addition to removing its
turbidity. However, it is very difficult to dewater the sludge formed. Also the effective pH range
for its use is small, i.e. 6.5 to 8.5.

(b) Copperas:
 It is the common name of ferrous sulphate i.e. FeSO , 7H O.
4 2

 Generally added to raw water in conjunction with lime.

 Ferric hydroxide i.e. Fe(OH) forms the floc.
3

 Used for water that are not coloured.

 Functions effectively when pH range is 8.5 and above.
 It’s cheaper than alum and the dosage is almost as same as that of alum.

(c) Chlorinated copperas:

 When chlorine is added to the solution of copperas then ferric sulphate and ferric chloride
forms. This combination of ferric sulphate and ferric chloride i.e Fe (SO ) and FeCl , is called
2 4 3 3

chlorinated copperas.
 This is a valuable coagulant for removing colours, especially where the raw water has a very
low pH value.
 Ferric sulphate is effective in the pH range of 4 to 7 and above 9. On the other hand, ferric
chloride is effective in the pH range of 3.5 to 6.5 and above 8.5. The combination is therefore
effective over a wide pH range.

(d) Sodium Aluminate i.e Na Al O :

2 2 4

 This chemical when dissolved and mixed with water results in the formation of calcium or
magnesium aluminate.
 It is very useful for waters which do not have the natural desired alkalinity.
 This chemical further reduces the temporary as well as permanent hardness present in raw
supplies, rather than increasing the same as is done by alum. That’s why it is effectively used
for treating boiler feed waters.

Page| 2 Prepared by Mr. R. Sinha

 The coagulation sedimentation plant commonly called as clariflocculator, contains following
four units:

(i) Feeding device: The chemical coagulant is first of all fed into the raw water through
this feeding device. If the chemical is added in dry form, then it’s called ‘dry feeding and if
added in solution form then the feeding is called ‘wet feeding’.
(ii) Mixing device: The mix of water and chemical is thoroughly mixed and agitated in the
mixing basin. The mixing basins can be fitted with baffle walls, or they can be equipped with
mechanical devices. The detention period in this thanks is normally kept between 20 to 50
minutes.
(iii) Flocculation tank or Flocculator: The floc which is formed as a result of the chemical
reaction taking place in the mixing basin, is then allowed to consolidate in the flocculation tank.
This consolidation is achieved by a relatively slow and gentle stirring of the water in rectangular
tanks fitted with paddles. The paddles rotate at 2 to 3 r.p.m and permit build up and
agglomeration of the floc.
(iv) Settling tank or sedimentation tank: The flocculated water is finally passed into the
sedimentation tank where those flocculated particles settle down and be removed. These tanks
are same as those used for plain sedimentation; but they have a lower value of detention period
(say 2 to 4 hrs). Also a little higher value of surface loading varying between 1000 to 1250
litre/hr/m of plan area may be permitted. The settled mass below the tank is called ‘sludge’, is
2

periodically removed and safely disposed of.

 The dose of the coagulant is determined by jar test.

 In this test, the water is placed in a number of jars each having capacity of 1 litre. Different
amounts of coagulants are added in each jar and the driving unit is started. Floc formation in
each is noted. The amount of coagulant in the jar which produces a good floc with the least
amount of coagulant, indicates the optimum dosage.
 The complete process of coagulation-sedimentation may help in removing turbidites up to as
low as 10-20 mg/lit and reduce B-coli index by as much as 70%.

4. Filtration:
 Even though screening and sedimentation removes a large percentage of the suspended solids
and organic matter present in raw supplies, the resultant water will not be pure and may content
some very fine suspended particles and bacteria. To remove or to reduce the remaining
impurities still further the water is filtered through the beds of fine granular material such as
sand. This process is known as filtration.

 Two types of filters are commonly used for treating municipal water supplies
a. Slow sand gravity filters and
b. Rapid sand gravity filters
A third type of rapid sand filter works under pressure and is known as pressure filter.

 Slow sand gravity filters are useful in the sense that they can remove much larger percentage
of impurities and bacteria from the water. However slow sand filters have a very low rate of
filtration and require large areas and thus are costly.

 With the advancement of modern disinfection techniques, slow sand filters are becoming
obsolete these days. The water from the coagulation sedimentation plant is directly fed into the
rapid gravity filters and the result and supplies are disinfected for complete killing of germs
and colour removal.

Theory of filtration

 The filters purify water through four different processes, as explained below:

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 1. Mechanical straining- The suspended particles present in the water which are of bigger size
than the size of the voids get arrested in them.

2. Flocculation and sedimentation- The void spaces act like tiny coagulation sedimentation tanks.
The colloidal matter arrested in these voids is a gelatinous mass and therefore attract other finer
particles. Thus, even particles having smaller size than that of the voids are trapped within the voids.

3. Biological metabolism- Various microorganisms and bacteria are generally present in the voids
of the filter. These organisms utilise such organic impurities for food and convert them into harmless
compounds by the process of biological metabolism. The harmless compound so formed generally form
a layer on the top which is called dirty skin. This layer further helps in absorbing and straining out the
impurities.

4. Electrolytic changes- It has been observed that sand grains of the filter media and the
impurities in water, carry electrical charges of opposite nature and when come in contact they neutralize
each other thereby changing the character of water and make it purer. After a certain time the electrical
charges of sand grains get exhausted and have to be restored by cleaning the filter.

Filter materials
 Generally sand is used as filter media for all type of filters. Sometimes, anthrafilt (a material
made from anthracite coal stone) is used as filter media.

Slow sand filters

 It normally utilises effluents from the plain sedimentation tanks.
 The filtering sand and other media are kept in an open ended watertight RCC rectangular tank,
the depth of which is 2.5 to 3.5 m. Its plan area is huge and varies from 100 to 2000 m or so.
2

 The filtering media of sand layer is about 90 to 110 cm in depth.

 D of the sand varies from 0.2 to 0.4 mm.
10

 (D /D ) of the sand varies from 1.8 to 2.5 or 3.0.

60 10

 The sand is supported by base material of 30 to 75 cm thick gravels of different grades.

 The water entering the slow sand filters should not be treated by coagulants.
 These filters are cleaned by scrapping and removing 1.5 to 3 cm of top sand layer.
 Cleaning is repeated at an interval of 1 to 3 months, until the sand depth is reduced to 40 cm
depth. Then new sand is added to the filter.
 Rate of filtration obtained from the slow sand filters are very low and usually ranges between
100 to 200 litres/hr/m of filter area.
2

 These filters can remove bacteria upto 98 to 99 percent or more along with odours and tastes of
organic impurities (like algae or plankton). However, they are less efficient in removing colours
of raw waters and can remove turbidity only upto 50 mg/l or so.

Page| 4 Prepared by Mr. R. Sinha

Rapid sand filters
 It normally utilises effluents from the coagulation sedimentation tanks.
 Like slow filters, the filtering sand and other media are also kept in an open ended watertight
RCC rectangular tank, the depth of which is 2.5 to 3.5 m. Its plan area however is less and
varies from 10 to 80 m or so for each unit. Minimum 2 units are used for any plant.
2

 These filters employ coarse sand with D value 0.35 to 0.55 mm.
10

 (D /D ) of the sand varies from 1.2 to 1.8.

60 10

 These filters are cleaned by passing the wash water upward at a high rate at every 24 to 48 hrs.
However, this rate shouldn't be too high as to cause the washing of smallest filter sand particles
with it.

 These filters remove only 80 to 90% of bacterial load and thus disinfection of the effluent water
is a must. However, they are highly efficient in removing colours.

Pressure filters
 They are like small rapid gravity filters placed in closed steel vessels through which water is
passed under pressure.
 The raw coagulated water is neither flocculated nor sedimented before it enters the filter. The
flocculation takes place inside the pressure filter itself.
 Generally alum is used in these filters.
 Requires slightly more frequent cleaning than rapid gravity filters.
 Filtration rate is as high as 6000 to 15000 lit/hr/m . 2

Prepared by Mr. R. Sinha Page| 5

  They are less efficient than rapid gravity filters in removing bacteria and turbidity and hence
generally not used for public supplies. However, they are preferred for treating smaller
quantities of relatively clearer water.

5. Disinfection:
 Water obtained from filters contain harmful disease producing bacterium in it which are killed
using certain chemicals, to make the water safe for drinking. These chemicals are called
disinfectants and the process is called disinfection.
 The aim of disinfection is not only to eliminate the existing bacteria from the water at the plant
but to also ensure their killing even afterwards in the distribution system.
 Chlorine has been found to be the best and the most ideal disinfectant satisfying the above
mentioned criterion. Hence chlorination is the major method of disinfection adopted almost
worldwide.
 Following are some minor methods of disinfection:

(i) Boiling of water: Only possible for individual household and not for the whole public
supply system. Only kills existing germs but cannot take care of future contaminations.

(ii) Treatment with excess lime: It can be very effective in removing 100% bacterial load
from the water. However it simultaneously raises the pH of the water to about 9.5 or so. Hence
before public supply, this excess lime should be removed from the water by using methods like
recarbonation. Also,it cannot protect the water from future contaminations.

(iii) Treatment with ozone: A very good disinfectant as it removes the organic matter,
bacterium, colour, bad taste, odour and additionally adds taste to the water. However, it is very
costly and does not ensure safety against future contaminations.

(iv) Treatment with iodine and bromine: Generally, not used for large scale public supplies
but can be used for small water supplies for private plants, swimming pools etc.

(v) Treatment with ultraviolet rays: These rays are highly effective in killing the bacteria,
but the water to be treated should be less turbid and low in colour.

(vi) Treatment with potassium permanganate: It is generally used in villages for

disinfecting well waters. It removes 98% bacteria from the water but mainly it’s famous for
100% removal of organism causing cholera. However, it’s of little use against other disease
producing bacteria.

Chlorination

 When chlorine is added to water it forms hypochlorous acid (i.e. HOCl) which have an
immediate and disastrous effect on microorganisms.
Cl2 + H2O → HOCl + HCl
Page| 6 Prepared by Mr. R. Sinha
 However, HOCl is formed only when the pH of the water slightly less than 7.
 If NH is also present in water then it reacts with chlorine and forms chloramines (i.e. NH Cl,
3 2

NHCl and NCl ). This chlorine combined with ammonia is called combined chlorine, which is
2 3

much less effective than free chlorine regarding disinfection.

 When chlorine is added to water, it first of all reacts with inorganic impurities and forms
chlorides which have no disinfecting power. Excess chlorine after this point is consumed by
ammonia to form Chloramines. The total chlorine thus consumed is called chlorine demand.
Excess chlorine after the chlorine demand is satisfied will appear as free chlorine.
 The free chlorine will kill the pathogens instantaneously whereas the combined chlorine
provides long term germicidal effect.
 The dose of chlorination is decided in such a way so that the free chlorine residual is about 0.2
mg/l after a contact period of 10 minutes.
 Chlorine can be applied in the following forms:

(i) As free chlorine (in the form of liquid chlorine or chlorine gas): Liquid chlorine can be
stored easily and it’s also very cheap. It’s a very powerful disinfectant and may remain in water
as residual for good enough time. Also no sludge is formed in its application.

(ii) Bleaching powder i.e. CaOCl : When freshly made it contains almost 30% of available
2

chlorine, but, being unstable it loses its chlorine content very fast due to exposure to the air.
They are not used for water supplies now a day.

(iii) Hypochlorites: Calcium and sodium hypochlorites when dissolved in water form
hypochlorite ions which may further combine with the hydrogen ions present in water so as to
form hypochlorous acid. This process is called hypochlorination. They are also not used in
modern days because they raise the pH of water and they contain very less amount of chlorine.

(iv) Chloramines: As said earlier, they are formed by reaction between ammonia and
chlorine. They are very stable and can remain in water for quite a long time unlike chlorine
which is unstable and disappears from the system after some time. Hence they provide a greater
safeguard against future pollution.

(v) Chlorine dioxide gas: ClO is a very powerful disinfectant (2.5 times stronger than Cl )
2 2

and its action remains unaffected even in highly alkaline water having pH 6 to 10. But it is very
unstable and costly and thus not suitable for public water supplies.

Types of chlorination:

(i) Plain chlorination: This term is used to indicate that only the chlorine treatment and
no other treatment has been given to the raw water. The quantity of chlorine required for plain
chlorination is about 0.5 milligram per litre or more.

(ii) Pre-chlorination: It is the process of applying chlorine to the water before filtration or
rather before sedimentation coagulation process. the dose of chlorine should be such that about
0.1 to 0.5 mg/l of residual chlorine comes to the filter plant. The normal doses required are as
high as 5 to 10 mg/l. Pre chlorination is always followed by post chlorination.

(iii) Post chlorination: It is the normal standard process of applying chlorine in the end
when all other treatments have been completed.It is normally adapted after filtration and before
the water enters the distribution system. the dosage of chlorine should be such as to leave a
residual chlorine of about 0.1 to 0.2 mg/lit, after a contact period of about 20 minutes.

(iv) Double chlorination: This term is used to indicate that the water has been chlorinated
twice. Basically this is the same as those of prechlorination.

Prepared by Mr. R. Sinha Page| 7

 (v) Breakpoint chlorination: It represents a dose of chlorination, beyond which any further
addition of chlorine will appear as a free residual chlorine. The graph below shows what
happens when chlorine (either chlorine gas or a hypochlorite) is added to water. First (between
points 1 and 2), the water reacts with reducing compounds in the water, such as hydrogen
sulfide. These compounds use up the chlorine, producing no chlorine residual. Next, between
points 2 and 3, the chlorine reacts with organics and ammonia naturally found in the water.
Between points 3 and 4, the organic matter present in the water gets oxidised and thus residual
chlorine content suddenly falls down. Finally, the water reaches the breakpoint, shown at point
4.

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

(vi) Super chlorination: This indicates the addition of excessive amount of chlorine (5 to
15 mg/l) to the water. It is required in some special places of highly polluted water, or during
epidemics of waterborne diseases.The dosage should be such as to give about 1 to 2 mg/l of
residue be on the breakpoint.

(vii) Dechlorination: When super chlorination has been practiced, additional chlorine
should be removed from water before supplying it to the public. This process is called
dechlorination. Dechlorination may be carried out by adding certain chemicals like sulphur
dioxide gas, activated carbon, sodium thiosulfate, sodium metabisulphate, sodium sulphite,
sodium bisulfite etc. to the water or by simply aerating the water.

6. Aeration:
 In this process water is brought in intimate contact with air, so as to absorb oxygen and to
remove carbon dioxide gas. It also helps in removing H S gas, iron and manganese to a certain
2

extent from water.

 It can be done by the following ways:
(a) By using spray nozzles through which water is sprinkled in the air, and gets broken into
droplets.
(b) By permitting the water to trickle over cascades
Page| 8 Prepared by Mr. R. Sinha
 (c) By blowing compressed air through the water so as to thoroughly mix it with water
(d) By allowing the water to trickle down the base of coke supported over perforated
bottom trays.
 Aeration should however be used only to a limited extent, because too much of absorbed
oxygen will make the water corrosive and may necessitate deaeration process.

7. Fluoridation and defluoridation:

 Fluoride concentration of less than 1 mg/l in drinking water is harmful from health point of
view. So when the water is deficient in fluoride, extra additional fluoride dose may be added to
the treated water. This process of adding fluoride compounds is called fluoridation.
 Sodium fluoride is the most widely used compound for this purpose which is generally fed
under pressure in a solution form.
 Similarly, excess fluoride is also harmful for health. The technique of removal of fluoride from
the water is known as the defluoridation.
 Defluoridation is done by using activated alumina or bone char. Water is percolated through
the insoluble granular beds of such materials which absorb fluoride from the water.
 In rural areas of India, ground water containing excess fluoride is treated by Nalgonda
technique. This technique uses alum for removing fluoride. Water is mixed with adequate
amount of lime or sodium carbonate. Alum solution is then added, and the water is stirred
slowly for about 10 minutes and allowed to settle for nearly 1 hour. The precipitated sludge is
discarded, and the clear supernatant containing permissible amount of fluoride is withdrawn for
use.

8. Water Softening:
 The reduction or removal of hardness from water is known as water softening. For industrial
supplies the softening is more important.
 Water hardness are of two types, i.e. temporary hardness and permanent hardness. Temporary
or carbonate hardness is caused by carbonates and bicarbonates of calcium and magnesium
whereas the permanent or non-carbonate hardness is caused by sulphates, chlorides and nitrates
of calcium and magnesium.
 Temporary hardness can be removed either by boiling or by adding lime to the water. However,
it should be mentioned here that boiling cannot satisfactorily remove the temporary hardness
caused by magnesium.
 Permanent hardness can be removed mainly by lime soda process, base exchange process
(generally called Zeolite process) and demineralization process.
 Lime soda process: In the lime soda process, lime i.e. Ca(OH)2 and soda Ash i.e. Na2CO3 are
added to the hard water. It reacts with the calcium and magnesium salts so as to form insoluble
precipitates of calcium carbonate and magnesium hydroxide. These precipitates can be
sedimented out in a sedimentation tank. However, in this method a large quantity of sludge is
formed and the total operation needs careful supervision. Also incrustation of pipe walls of the
distribution system happens in this method if the water is not properly re-carbonated.
 Zeolite process: Zeolite are the natural or synthetic cation or base exchange hydrated silicates
of sodium and aluminium. During the softening operation the calcium and magnesium
carbonate bicarbonate or sulphate which causes the hardness in water is replaced by the
complex zeolite radical of calcium and Magnesium.
 The following chemical reactions show the exchange process, where X represents zeolite, the
complex zeolite exchange radical.
Removal of carbonate hardness:
Ca(HCO3)2 + Na2X ------> CaX + 2NaHCO3
Mg(HCO3)2 + Na2X ------> MgX + 2NaHCO3

Removal of non-carbonate hardness:

CaSO4 + Na2X ----- > CaX + Na2SO4
CaCl2 + Na2X ------> CaX + CaCl2
Prepared by Mr. R. Sinha Page| 9
 MgSO4 + Na2X ------> MgX + Na2SO4
MgCl2 + Na2X ------> MgX + 2NaC1
 A zeolite softener resembles a sand filter in which the filtering medium is a zeolite rather than
sand. The biggest advantage of the zeolite process is that water of zero hardness can be obtained
and no sludge is formed. However, it is not suitable for treating highly turbid waters.
 Demineralisation process: A complete removal of minerals can be carried out first by passing
the water through a bed of cation exchange resin and then through a bed of anion exchange
resin. This treated water is as pure as distilled water and is very suitable for industrial uses.

P a g e | 10 Prepared by Mr. R. Sinha