With the help of diagram explain the purification system
Answers
Purification of water on a large scale!
Water is purified on a large scale in establishments that are known as waterworks. Usually, the following steps are taken to purify water.
1. Sedimentation:Water pumped from the source (river, lake or dam) is allowed to stand in large tanks, called settling tanks, for a few hours. Mud, sand and other suspended particles settle down. The addition of a small quantity of potash alum (K2SO4 . A12(SO4)3 . 24H2O) helps the suspended particles to settle down quickly. The solid that settles at the bottom of the tank is called sediment. That is why these tanks are also called sedimentation tanks. The water over the sediment layer is more or less clean.
2. Filtration:The water from the sedimentation tank is filtered through layers of sand and gravel. Filtration is also done through activated charcoal, which affords better-quality water. Activated charcoal is a special type of charcoal made by heating ordinary charcoal in vacuum. This charcoal holds the impurities strongly.
3. Chlorination:The filtered water is then treated with chlorine gas, which kills germs. You can see for yourself that alum helps the suspended particles of water to settle down quickly. Take two similar glasses containing equal volumes of muddy water and label them A and B. (You can prepare muddy water by mixing a teaspoonful of mud from your garden with the water in the glass.)
Crush a small crystal of alum (which you can obtain from a store) between the folds of a paper and add it to A. Stir the water in the two glasses and allow them to stand side by side. Within a short while, you will find that the mud has settled down with clear water above it in glass A, but the water is still not clear in glass B.
Purification of water at home:It is advisable that water meant for drinking be purified at home also.
This is done by the following processes.
1. Boiling and filtering:Water is boiled (preferably for 20 minutes) to kill the microorganisms and allowed to stand for some time. Some solid may also settle down. The clear water is decanted off and filtered through ceramic candles. Ceramic candles have fine pores which allow only the water to pass through, but not the suspended particles.
2. Radiating water with ultraviolet rays:Ultraviolet lamps radiate high-energy rays which kill germs. Nowadays, domestic water purifiers are available, which purify water in three stages. Water from the tap enters the purifier, where it gets filtered first through a candle and then through activated charcoal (carbon). The filtered water is finally irradiated by ultraviolet radiation to render it free from germs. This water is safe to drink.
3. Using bleaching powder:In rural and backward areas, water is lifted in buckets from wells. Well water gets contaminated, especially during the rains, and is generally disinfected by mixing some bleaching powder. The bleaching powder slowly reacts with water to chlorine, which kills the germs.
Even after treating it with bleaching powder, the water should be boiled to make it germ-free. Domestic water tanks—overhead or underground—are also disinfected by bleaching powder at regular intervals, say of a fortnight or a month.
Bottled water:Bottled potable water is prepared by a technique called reverse osmosis (RO), about which you will learn in higher classes. Domestic water purifiers using RO are also available now, and are becoming popular. Disinfecting water using chlorine gives it an unacceptable taste.
Hence, ozone is often used in place of chlorine. Ozone kills the microorganisms and gets converted into oxygen. The unreacted ozone also decomposes to give oxygen, which adds to the dissolved oxygen.