NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story

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Written by Team Trustudies
Updated at 2021-02-11


NCERT solutions for class 7 Science Chapter 18 Wastewater Story Exercise

Q.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 ____________.



NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story


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.

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



NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story


Answer :

Sewage is the wastewater containing both liquid and solid wastes produced by human activities from homes, industries, hospitals, offices, etc.
Sewage contains various contaminants including disease-causing bacteria and other microbes. If untreated sewage is discharged into rivers or seas, then the water in the rivers or seas would also get contaminated. If this contaminated water is used for drinking, then it can cause diseases such as cholera, typhoid, dysentery, etc. which may lead to death. That is why it is harmful to discharge untreated sewage into rivers or seas.

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



NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story


Answer :

Oils and fats harden and block the sewage pipes. In an open drain also, fats block porosity of soil which affects its water filtering efficiency. That is why oils and fats should not be released in the drain.

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



NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story


Answer :

The steps involved in getting clarified water from wastewater are as follows:
(i) Firstly all the physical impurities like stones, plastic bags, cans etc. are to be removed. It is done by passing the water through bar screens.
(ii)Then, the water is filtered through the layers of sand, fine gravel, and medium gravel. Filtration makes the wastewater clean from various types of pollutants. The water is filtered continuously until it becomes clear.
(iii) Solid impurities and faeces etc. are collected from bottom of water. These impurities collected are called sludge.
(iv)Then any disinfectant such as chlorine tablet is added to the filtrate and stirred to obtain completely clear water.

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



NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story


Answer :

Semi-solids such as faeces that settle down during wastewater treatment are called sludge. This sludge is removed using a scraper and then transferred to a tank where it is decomposed by anaerobic bacteria to produce biogas. This biogas is used as a low-cost fuel for heating, cooking, etc. It is also used to produce electricity.

Q.6 Untreated human excreta is a health hazard. Explain.



NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story


Answer :

Untreated human excreta can cause a lot of health related problems. When this contaminated water is used for drinking, it can cause diseases such as cholera, typhoid, hepatitis, dysentery, etc., which may even lead to death.

Q.7 Name two chemicals used to disinfect water.



NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story


Answer :

chlorine and ozone.

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



NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story


Answer :

In a wastewater treatment plant, bar screen removes large solid objects from water. The wastewater is allowed to pass through bar screen so that large solid objects such as rags, napkins, sticks, cans, plastic bags, polythene, etc. present in wastewater can be removed.

Q.9 Explain the relationship between sanitation and disease.



NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story


Answer :

Sanitation and disease are related to each other as lack of sanitation can cause diseases. Sanitation involves proper disposal of sewage and refuse from house and public places. If sanitation is there, no disease will occur, but if sanitation is not there various types of disease will occur and spread. So sanitation should be kept to avoid disease.

Q.10 Outline your role as an active citizen in relation to sanitation.



NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story


Answer :

(i) Ensure that his surroundings (both inside and outside home) are clean.
(ii) Ensure that the sewerage system in his house is properly managed.
(iii) If he notices some leakage or other problem in the sewerage system, he should report it to the municipality.

Q.11.Here is a crossword puzzle. Good luck!

Across:
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 cause cholera
7. A chemical to disinfect water



NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story


Answer :

Across

3. Sewage
4. Sludge
6. Sanitation
8. Excreta
Down
1. Wastewater
2. Sewer
5. Bacteria
7. Ozone

Q.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



NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story


Answer :

(ii) (b) and (c)



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