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INTRODUCTION
Water purification is the process of removal of contaminants from raw water to
produce water that is fit for human consumption. Water supplied for human
consumption should be free of colour, turbidity, taste, odour and pathogenic bacteria.
It should also be relatively free of hardness. Water being a good solvent dissolves
minerals and pick up contaminants as it passes through the earth. The contaminants
may include bacteria, algae, virus, fungi and man made chemical pollutants. It
becomes in this way a natural carrier for a number of human and animal infections
particularly of the entire group of intestinal tract diseases. Therefore, it is very
important to treat water for removal of these contaminants. The various steps for
treatment of water purification are discussed below:
STEPS FOR TREATMENT OF WATER
1.SCREENINGThe first step in purifying surface water is to remove large debris like sticks,
leaves, trash, fish, etc. by passing raw water through screen with small holes.
Ground water does not need screening before other purification steps. Virtually
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all modern water for urban communities are drawn from surface sources rather
than from the underground.
2.STORAGEWater from rivers may be stored in reservoirs for periods between a few days
and many months to allow natural biological purification to take place. During
storage solid impurities also settle down. Many bacteria fid water an unnatural
habitat and decrease in a storage reservoir through simple devitilisation.
3.COAGULATION AND SEDIMENTATIONNatural storage alone cannot accomplish complete purification of water.
Therefore, artificially controlled sedimentation basins are used for further
removal of suspended impurities. In these the conditions of entrance and exit
are carefully regulated. In order to facilitate settling, artificial stimulation of the
settling is provided through introduction of active chemicals called coagulants.
The primary purpose of coagulants is to provide a nucleus for agglomeration
and suspended particles and increase their specific gravity to cause more rapid
settling. The joining of particles so that they form larger settleable particles is
called flocculants and the larger formed particles are called floc. One of the
commonest coagulants is potash alum. Some others are ferrous sulphate or
chloride or lime, etc.
As the particles settle down in the bottom of the basin, a layer of sludge
is formed on the floor of the tank. The amount of sludge that is generated is
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quite large. It may be up to 3-5% of the total volume of the water that is treated.
The sludge is removed from the bottom of the tank from time to time.
4.FILTRATIONAfter removal of most floc by sedimentation, the water is subjected to filtration
to remove remaining suspended particles and unsettled floc. The most common
type of filter is a rapid sand filter. In this type of filter water moves vertically
trough sand which often has a layer of activated carbon above the sand. The
top layer removes compounds which impart taste and odour to the water. The
space between sand particles is larger than the size of the suspended particles.
Most particles pass trough the surface layer but are trapped I pore spaces or
adhere to sand particles. So, it is not just the top layer of the filter that cleans
the water, but effective filtration leads into the depth of the filter. The filter is
cleaned byback flushing i.e. by passing water through the filter in the
direction opposite to the normal direction.
When sufficient land and space is available, water may be treated
through slow and filters. Slow and filters are constructed using graded layers
of sand with the coarsest at the base and finest at the top. Slow and the filters
rely on biological treatment processes of their action rather than physical
filtration. When a new slow sand filter is brought into use, raw water is
circulated through the filter. Within a few hours, a film of bacteria, fungi and
algae builds on the surface of the sand. It is the layer that removes all the
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impurities. A slow sand filter may remain in service for many weeks or even
months.
SOME OTHER TECHNIQUES
1.BOILINGWater is heated to boiling to inactivate or kill the microorganisms that
normally live in water at room temperature. Near sea level boiling for about
one minute is sufficient. At high altitudes water has to be boiled for 3
minutes. In areas where water is hard boiling decomposes bicarbonates,
resulting in the precipitation of calcium carbonate, magnesium carbonate,
etc. This is the fur that builds up on kettle elements, etc.
2.DISTILLATIONDistillation involves boiling the water to produce water vapour. The vapour
on coming in contact with cold surface condenses as a liquid. Since the
solutes are normally not vaporized, they remain in the boiling solution. This
method provides 99.9% pure water.
3.REVERSE OSMOSISIn this process pressure is applied to impure water to force pure water
through a semi permeable membrane. Reverse osmosis is a very effective
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method of large scale water purification. However, perfect semi permeable
membranes are difficult to make. This treatment removes salts and coloured
compounds from the water but through pre-treatment, high pressures and
careful cleaning is required leading to high running costs.
4.WATER CONDITIONINGThis is a method for reducing the effects of hard water. In this method water
with high concentrations of hardness salts is treated with sodium carbonate
which precipitates out the excess salts through common ion effect.
5.ION EXCHANGEMost common ion exchange systems use a zeolite resin bed to replace
unwanted calcium ion, magnesium ion with sodium ion. This is the common
water softener. Zeolite resin is recharged with sodium chloride solution. A
more rigorous type of ion exchange replaces all cations with H+ ions with all
anions with hydroxide ions. These ion exchange resins are recharged with
hydrochloric acid and sodium hydroxide respectively. The water obtained is
completely free from mineral ions and is called deionized water.
6.ELECTRODE IONIZATIONIn this method water is passed between the oppositely charged electrodes.
Ion selective membranes allow the positive ions to separate from water
towards the negative electrode and negative ions towards the positive
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electrode. Prior to electrode ionization the water is usually subjected to
reverse osmosis so as to remove non-ionic organic contaminants. High
purity deionized water is obtained in this method.
7.PLUMBO-SOLVENCY REDUCTIONPlumbo-Solvency refers to dissolution of lead from any lead pipes through
which water is carried. The problem of plumbo-solvency is more in areas
with naturally acidic waters of low conductivity. The plumbo-solvency can be
reduced by addition of small quantities of phosphate ion and increasing the
Ph. This results in the formation of layer of insoluble lead salts on the inner
surface of the pipes.
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