Abhishek Pandey-12th Sci

8/3/2019 Abhishek Pandey-12th Sci

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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.

Abhishek Pandey-12th Sci

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