Supply of Drinking Water in a City
In cities, drinking water is supplied from water works.In water works,the methods like sedimentation,decantation,loading, filtration and chlorination, etc.,are used to move undesirable materials from water.This is described below.
The source of water supply in a city is either a nearby river or a lake (called reservoir).The river water and lake water usually contain suspended solid substances and some germs. So, before this water can be supplied to homes, it must be purified to remove suspended impurities as well as germs. This is done at a place called water works. The water works of a city is usually situated near the bank of a river or a lake.The purification of river water (or lake water) is done in the following steps :
(i) The water from a river (or lake) is pumped by the pumping station into a large reservoir called sedimentation tank (see Figure 65). This water is allowed to stand in the sedimentation tank for some time. During this time, many of the insoluble substances present in water settle down at the bottom of the tank.
(ii) From the sedimentation tank,water is sent to a 'loading tank'.In the loading tank,some alum (phitkari) is added to water. The heavy particles of dissolved alum deposit on the suspended clay particles in water. In this way, the suspended clay particles in water get "loaded' with alum particles, become heavy and settle down at the bottom of the tank. Thus,the process of loading (by using alum) removes the suspended clay particles from water.
(iii) The water is then passed through a filtration tank.The filtration tank has three layers : fine sand layer at the top, coarse sand layer in the middle and gravel (tiny stones) as the bottom layer. The layers of sand and gravel act as filters. When water passes through the layers of sand and gravel, even the small suspended particles and other materials in water get removed.
(iv) The clear water is then passed into a chlorination tank.A little chlorine gas is added to water in this tank. Chlorine is added to water to kill the germs present in it.This is called disinfecting the water (or sterilising the water). The water now becomes fit for drinking (or safe for drinking).
(v) The clean and disinfected water is then pumped by a pumping station into high storage tanks (see Figure 65). From the high storage tank,water is supplied to homes and factories in the entire city through a network of big and small pipes.
Notes
Matter in Our Surroundings - Notes
1. The Case of a Liquid Solute Dissolved in a Liquid Solvent
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2. 11. Metals Usually Have a Silver or Grey Colour
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5. 3. Non-Metals are Bad Conductors of Heat and Electricity.
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9. To Study the Properties of a Colloid
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12. 7. Non-Metals may be Solid, Liquid or Gases at the Room Temperature.
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14. Separation Of Mixture Of a Solid And a Liquids
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16. 1. Separation by a Suitable Solvent
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18. 6. Metals are Usually Strong. They Have High Tensile Strength.
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19. To Separate a Mixture of Alcohol and Water
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21. 6. Non-Metals are Not Strong. They Have Low Tensile Strength.
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22. To Study the Properties of a Compound of Iron and Sulphur
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25. To Study the Properties of a Solution
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28. 3. Metals are Good Conductors of Heat and Electricity.
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29. Separation of Mixture Of Two Solids
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30. 11. Non-Metals Have Many Different Colours.
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32. Chemical Formula for daily use material
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34. To Separate the Salt-Water Mixture (or Salt-Solution)
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44. 7. Metals are Solids at the Room Temperature
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46. 4. Metals are Lustrous (or Shiny), and can be Polished.
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