NCERT Solutions for Class 7th: Ch 18 Wastewater Story Science
Page No: 228
1. Fill in the blanks:
(a) Cleaning of water is a process of removing ____________.
(b) Wastewater released by houses is called ____________.
(c) Dried ____________ is used as manure.
(d) Drains get blocked by ____________ and ____________.
Answer
(a) Cleaning of water is a process of removing pollutants.
(b) Wastewater released by houses is called sewage.
(c) Dried sludge is used as manure.
(d) Drains get blocked by cooking oil and fats.
2. What is sewage? Explain why it is harmful to discharge untreated sewage into rivers or seas.
Answer
Sewage is a liquid waste released by homes, industries, hospitals, offices and other users which also includes rainwater that has run down the street during a storm or heavy rain.
Sewage water contains harmful substances with it which washes off roads and rooftops. It is a complex
mixture containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease causing bacteria and other microbes. So, when sewage discharge untreated into rivers or seas, it will be dangerous for aquatic plants and animals.
3. Why should oils and fats be not released in the drain? Explain.
Answer
Oil and fats should not be released in the drain because they can harden and block the pipes. In an
open drain the fats block the soil pores reducing its effectiveness in filtering water.
4. Describe the steps involved in getting clarified water from wastewater.
Answer
Steps involved in getting clarified water from wastewater:
(i) Wastewater is shaken well and left to stand in the sun for some days.
(ii) After that, it is stir several times using an aerator or mechanical stirrer or a mixer which remove bad odour from water.
(iii) The stirred product is then filtered and sediment to remove insoluble waste.
(iv) Chlorination is used to kill the germs in filtered water. Now, the wastewater is fit for drinking purpose.
5. What is sludge? Explain how it is treated.
Answer
Solids waste like faeces settle at the bottom of tank during sedimentation is called sludge.
Sludge is removed with a scraper and then transferred to a separate tank where it is decomposed by the anaerobic bacteria to produce biogas.
6. Untreated human excreta is a health hazard. Explain.
Answer
Untreated human excreta is a health hazard as it can cause water pollution and soil pollution. Both the surface water and groundwater get polluted. Thus, it becomes the most common route for water borne diseases which include cholera, typhoid, polio, meningitis, hepatitis and dysentery. It is also a big threat to sanitation.
7. Name two chemicals used to disinfect water.
Answer
Two chemicals used to disinfect water are chlorine and ozone.
8. Explain the function of bar screens in a wastewater treatment plant.
Answer
Bar screen in a wastewater treatment plant is used to remove the large objects like rags, sticks, cans, plastic packets and napkins.
9. Explain the relationship between sanitation and disease.
Answer
Sanitation and disease are closely related as lack of sanitation leads to unhygienic condition and ultimately disease. Proper sanitation is necessary to avoid certain kinds of disease. Where there is sanitation, there is no disease and vice versa.
10. Outline your role as an active citizen in relation to sanitation.
Answer
Role as an active citizen in relation to sanitation:
(i) Contribute in maintaining sanitation at public places and should not scatter litter anywhere.
(ii) Throwing garbage or waste at its desired place.
(iii) Ensure that my surrounding and neighbourhood is clean and help the municipality in maintaining proper sanity.
(iv) Preventing clogging of drains by not releasing oil and fat and solid waste into the drain.
(v) Immediately inform the municipality about any sewage leakage and for the maintenance of dustbins.
11. Here is a crossword puzzle: Good luck!
3. Liquid waste products
4. Solid waste extracted in sewage treatment
6. A word related to hygiene
8. Waste matter discharged from human body
Down
1. Used water
2. A pipe carrying sewage
5. Micro-organisms which causes cholera
7. A chemical to disinfect water
Answer
3. Liquid waste products → Sewage
4. Solid waste extracted in sewage treatment → Sludge
6. A word related to hygiene → Sanitation
8. Waste matter discharged from human body → Excreta
Down
1. Used water → Wastewater
2. A pipe carrying sewage → Sewer
5. Micro-organisms which causes cholera → Bacteria
7. A chemical to disinfect water → Ozone
Page No: 229
12. Study the following statements about ozone:
(a) It is essential for breathing of living organisms.
(b) It is used to disinfect water.
(c) It absorbs ultraviolet rays.
(d) Its proportion in air is about 3%.
Which of these statements are correct?
(i) (a), (b) and (c)
(ii) (b) and (c)
(iii) (a) and (d)
(iv) All four
► (ii) (b) and (c)
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