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Class 7 science Wastewater stories NCERT Solution

Chapter 18: Wastewater stories

N C E R T Textbook Questions Solved

Fill in the blanks:

a. Cleaning of water is the process of removing————-.

b. Wastewater released by houses is called————-.

c. Dried __________ is used as manure.

d. Drains get blocked by————- and—————.


a. pollutants.

b. sewage.

c. sludge (d)

d. chemicals, kitchen waste

2. What is sewage? Explain why it is harmful to discharge untreated sewage into rivers or seas.

Answer: The wastewater which is disposed of by households, industrial and agricultural activities is called sewage. It is harmful to discharge untreated sewage into rivers or seas. It pollutes the whole sources of water. Sewage contains harmful substances and disease-causing organisms and therefore it is dangerous to release untreated sewage in the water.

3. Why should oils and fats be not released in the drain? Explain.

Answer: Oils and fats disposed of in drains harden the soil in the pipes and block them. Fats get clogged in holes of the soil in the drain and block it. It hinders the flow of water.

4. Describe the steps involved in getting clarified water from wastewater.

Answer: Following steps are involved in the purification of water:

i. Firstly all the physical impurities like stones, plastic bags, cans etc. are removed by passing the water through bar screens.

ii. This water is taken to grit and sand removal tank. Here impurities are removed by sedimentation.

iii. Solid impurities and faeces etc. are collected at the bottom of the water. These impurities get collected as sludge.

iv. The clarified water is cleaned of other impurities by aerating it. Afterwards, all disease-causing bacteria are removed by chlorination.

5. What is sludge? Explain how it is treated.

Answer: Sludge is the solid waste collected at the bottom of the tank during the treatment of wastewater in the water treatment plant. It is decomposed in a separate tank by the anaerobic bacteria. Activated sludge is used as manure.

6. Untreated human excreta are a health hazard. Explain.

Answer: Untreated human excreta causes many health hazards. It pollutes water, air and soil. The polluted water contains disease-causing

causing bacteria which spreads epidemics like cholera, meningitis etc.

7. Name two chemicals used to disinfect water.

Answer: Chlorine and ozone are used to purify water.

8. Explain the Junction of bar screens in a wastewater treatment plant.

Answer: Bar screens clear the wastewater of all the large-sized impurities. E.g. napkins, plastics, cans etc. are removed from the wastewater through the bar screens.

9. Explain the relationship between sanitation and disease.

Answer: Sanitation and disease are closely related. Sanitation involves proper disposal of sewage from the house and public places. Sanitation checks the spreading of diseases.

10. Outline your role as an active citizen in relation to sanitation. Answer: As an active citizen, we should take care of personal and environmental sanitation. We should make our neighbours and people aware of the benefits of sanitation. We should ask municipal corporations to cover all the open drains and clean disease-causing substances thrown in open.