Purification of Water EVS Grade 4 | Orchids International School
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Concept: Purification of Water

Why should we purify water?

Water is a vital natural resource. However, raw water contains different types of solid and liquid impurities. These impurities may be present either in a dissolved or undissolved state.

Since, drinking such contaminated water may lead to various ailments, water must be purified before it is consumed.


Some diseases caused by drinking impure water:

  • Dysentery
  • Typhoid
  • Cholera
  • Diarrhoea

Different methods of purification of water:

  1. Boiling and evaporation:
    • In this method, water is boiled, which kills harmful microbes at that high temperature.
    • To separate soluble solids like sugar or salt, water is evaporated first.
    • Then water is extracted separately by condensation.
    • The solid residues are left behind in the boiling pot.
  2. Filtration:
    • Filtration is used to separate insoluble solid impurities like mud and sand from water.
    • Filtration can be performed by using filter paper, muslin cloth or strainer.
    • The filter paper is folded and placed on a funnel.
    • The impure water is then passed through it.
    • The filter paper has tiny pores through which water passes but all the solid impurities are left behind.
    • The purified water is collected for further use.

  3. Sedimentation and decantation:
    • This method is used to separate clay, mud and sand from the water.
    • First, raw water is collected in a container and left undisturbed for a while. It is called sedimentation.
    • All impurities settle at the bottom of the container, called sediment.
    • Clean water is seen at the top of the sediment, which is carefully collected in a vessel.
    • It is called decantation.

  4. Distillation:
    • This method is used to separate water from liquid impurities such as oil or alcohol.
    • In this method, water is first heated up to its boiling point.
    • When water starts to evaporate, it produces vapour leaving behind all kinds of impurities.
    • The water vapour is collected in a condenser which condenses the vapour back into the water.
    • Distilled water obtained by this method is one of the purest forms of water.

  5. Chlorination:
    • Chlorination is a cheap and effective way of purifying water.
    • Chlorine tablets are added to water before drinking as they can effectively kill germs and disease-causing microorganisms from tap water and groundwater.

What is a water treatment plant?

  • A water treatment plant purifies water on a large scale and distributes water to various urban localities for further use.
  • These plants also treat industrial wastewater before it is dumped into nearby water bodies, thereby minimising water pollution.
  • These water treatment plants have several types of machinery and equipment to purify water.

Steps involved in wastewater treatment:

  1. Filtration:
  2. The wastewater is passed through a screen. This process removes big objects like bottles, rags, sticks, plastic bags, cans, etc.

  3. Removal of grit and sand:
  4. The wastewater is gradually passed through the grit and sand removal tank. As a result, grit, sand, and pebbles settle at the bottom.

  5. Sedimentation:
  6. The wastewater is then sent to the sedimentation tanks. Solid impurities settle at the bottom, whereas floating impurities float on the surface. A scraper removes the solid impurities from the bottom of the tank. A skimmer removes the floating impurities.

  7. Aeration:
  8. Air is passed through the obtained water, which encourages the growth of bacteria. This step is carried out because bacteria decompose food waste and other unwanted materials in the water. After several hours, the bacteria settle at the bottom, whereas clear water is collected.

  9. Disinfection:
  10. Water purified through aeration is unsuitable for drinking water. It needs to be disinfected by treating it with chlorine or passing UV rays through the water.


New Words

Grit: Small, loose particles of sand and soil.

Scraper: A device to scrape out dirt or unwanted matter.

Skimmer: A flat perforated scoop-type device used for skimming, i.e., removing impurities from the water.

Did You Know?

  • Chlorine tablets are often used to disinfect the water in swimming pools.
  • Spring water is the healthiest to drink as it is enriched with minerals and essential compounds that offer a wide variety of health benefits.
  • The impurity collected after the water is passed through the sedimentation tank is called sludge. It is used to produce biogas or manure.
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