Water purification involves large-scale and small-scale processes. Large-scale purification provides safe drinking water to the public and involves storage, filtration, and disinfection. Storage allows impurities to settle and bacteria to die off over 10-14 days. Filtration removes 98-99% of bacteria using slow sand filters containing a biological layer or rapid sand filters using coagulation, flocculation, sedimentation, and filtration. Disinfection with chlorine kills remaining pathogens, producing clean water for public distribution.

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2.  Water Purification is the process of making water
clean and safe to drink. Normally it is discussed
under two headings:
 1. Purification of water on a large scale.
 2. Purification of water on a small scale.

3.  Large scale purification of water is needed to
provide safe drinking water to public. Normally
this is responsibility of government to provide
continuous safe water supply. Large scale water
purification needs water treatment plant.

4.  Main steps for Large scale purification of
water are as follows-
 1. Storage
 2. Filtration
 3. Disinfection

5.  Storage provides a reserve of water from a
source. Water source may be a river or canal
which provide continuous water supply to the
reservoir. Water is stored in the reservoir till it is
used for next steps of purification

6.  As a result of storage, a very considerable
amount of purification takes place. This is natural
purification. About 90 per cent of the suspended
impurities settle down in 24 hours by gravity.

7.  Some chemical changes also take place during
storage. The aerobic bacteria oxidize the organic
matter present in the water with the help of dissolved
oxygen. By this process the content of free ammonia is
reduced and a rise in nitrates occurs.

8.  On storage the pathogenic organisms gradually die
out. It is found by the research that when river water is
stored the total bacterial count drops by as much as 90
per cent in the first 5-7 days. This is one of the greatest
benefits of storage. The optimum period of storage of
river water is about 10-14 days.

9.  Filtration is the second stage in the purification of
water. It is very important stage where 98-99 per cent
of the bacteria are removed apart from other
impurities. Two types of filters are used for this
process-
 1. Slow sand filters and
 2.Rapid sand filters.

10.  Slow sand filters are also known as biological filters.
the various elements of a slow sand filter are as
follows-
 (1) supernatant water
 (2) a bed of graded sand
 (3) an under-drainage system; and
 (4) a system of filter control valves.

11.  (1) supernatant water- The supernatant water above
the sand bed, whose depth varies from 1 to 1.5 meter,
it provides a constant pressure of water so as to
overcome the resistance of the filter bed and thereby
promote the downward flow of water through the sand
bed.

12.  The supernatant water provides waiting period
of some hours (3 to 12 hours, depending upon
the filtration velocity) for the raw water to
undergo partial purification by sedimentation as
well as filtration.

13.  (2) Sand bed- The most important part of the
filter is the sand bed. it is the main filtration
medium. The thickness of the sand bed is about
1 metre. The sand grains should be preferably
rounded and have an "effective diameter"
between 0.2 and 0.3 mm.

14.  The sand should be clean and free from clay and
organic matter. The sand bed is supported by a
layer of graded gravel of 30-40 cm deep which
prevents the fine sand grains being carried into
the drainage pipes.

16.  Water percolates through the sand bed very slowly. it is
subjected to a number of purification processes -
mechanical straining, sedimentation, adsorption,
oxidation and bacterial action . The designed rate of
filtration of water normally lies between 0.1 and 0.4
m3/hour/per square metre of sand bed surface.

17.  When the filter is newly constructed , it acts only as a
mechanical strainer but soon, the surface of the sand
bed gets covered with a slimy growth known as vital
layer or biological layer. This layer is slimy and
gelatinous and consists of threadlike algae and
numerous forms of life including plankton, diatoms and
bacteria.

18.  The vital layer is very important It removes organic
matter, holds back bacteria and oxidizes ammonia into
nitrates and helps in producing a bacteria-free water.
Until the vital layer is fully formed, the first few days
filtrate is usually considered waste water

19.  (3) Under-drainage system- At the bottom of the filter
bed there is an under-drainage system. It consists of
porous or perforated pipes which acts as outlet system
for the filter. Once the filter bed has been fully formed,
the under-drainage system cannot be seen.

20.  (4) Filter control- The filter is equipped with
certain valves and devices which are
attached to the outlet-pipe system. The
purpose of these devices is to maintain a
constant rate of filtration.

21.  Rapid sand filters are also known as mechanical
filters. It involves following process-
 (1) Coagulation
 (2) Rapid mixing
 (3) Flocculation
 (4) Sedimentation
 (5) Filtration

22.  The raw water is first treated with a chemical
coagulant such as alum to clean the water. The dose
of alum varies from 5 to 40 mg or more per litre,
depending upon the turbidity and colour,
temperature and the pH value of the water.

23.  The alum water is than sent to mixing chamber.
Mixing process uses mechanical rapid mixing
blades to mix raw water with alum completely
and evenly.

24.  This phase involves a slow and gentle stirring of
alum mixed water in flocculation chamber. The
mechanical type of flocculator is the most widely
used. It consists of a number of paddles which
rotate at 2 to 4 rpm.

25.  The coagulated water is now passed into
sedimentation tanks where it is allowed to stay
for periods varying from 2-6 hours. The
flocculent precipitate together with impurities and
bacteria and settle down in the tank.

26.  The precipitate or sludge which settles at
the bottom is removed mechanically from
time to time without disturbing the operation
of the tank. The water now becomes
partially clear.

27.  The partly clear water is now sent to send bed of
rapid sand filter. The depth of the sand bed is
usually about 1 meter supported by graded
gravel. The under-drains at the bottom of the
filter beds collect the filtered water

28. 

29.  Disinfection of clean water is carried out by adding
chlorine to it. Chlorine kills pathogenic bacteria. Apart
from its germicidal effect, chlorine has several other
important functions as it oxidizes iron, manganese and
hydrogen sulphide and it destroys some taste and
odour-producing constituents.

30.  Disinfection of clean water is carried out by adding
chlorine to it. Chlorine kills pathogenic bacteria. Apart
from its germicidal effect, chlorine has several other
important functions as it oxidizes iron, manganese and
hydrogen sulphide and it destroys some taste and
odour-producing constituents.

31.  In this ay large scale purification of water
is achieved. This purified water is ready to
consume. It is sent through water supply
system of water distribution department to
the houses of end users as tap water.

32. By – SURESH KUMAR ( Nursing Tutor )