NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story

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 ____________.

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.

Answer :

(1) Sewage is the wastewater containing both liquid and solid wastes produced by human activities from homes, industries, hospitals, offices, etc.

(2) Sewage contains various contaminants including disease-causing bacteria and other microbes.

(3) If untreated sewage is discharged into rivers or seas, then the water in the rivers or seas would also get contaminated.

(4) If this contaminated water is used for drinking, then it can cause diseases such as cholera, typhoid, dysentery, etc. which may lead to death.

(5) 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.

Answer :

Oils and fats are not released in the drain because

(1) Oils and fats are not released in the drain because oil is not dissolved in water and it floats on water.

(2) Oils and fats harden and block the sewage pipes.

(3) In an open drain also, fats block the porosity of soil which affects its water filtering efficiency.

(4) It can clog the drains and can affect aquatic life if released into the water body.

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

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 feces etc. are collected from the bottom of the 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.

Answer :

(1) Semi-solids such as faeces that settle down during wastewater treatment are called sludge.

(2) This sludge is removed using a scraper and then transferred to a tank where it is decomposed by anaerobic bacteria to produce biogas.

(3) 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

Answer :

(1) Untreated human excreta can cause a lot of health-related problems.

(2) It consists of various disease-causing microorganisms and pollutants that will contaminate the soil and water resource from where people draw water for drinking and domestic purposes.

(3) When people use water contaminated with human excreta, they get diseases like Cholera, Typhoid, Dysentry, and Hepatitis.

Q.7 Name two chemicals used to disinfect water.

Answer :

Chlorine and Ozone are used to disinfect water.

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

Answer :

(1) In a wastewater treatment plant, a bar screen removes large solid objects from the water.

(2) 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.

Answer :

(1) Sanitation and disease are related to each other as lack of sanitation can cause diseases.

(2) Sanitation involves proper disposal of sewage and refuse from houses and public places.

(3) Poor sanitation leads to the growth of harmful microbes, flies, and mosquitoes which are the source of spreading many diseases like cholera, typhoid, polio, meningitis, hepatitis, and dysentery.

(4) 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.

Answer :

Every citizen has a role to play in maintaining proper sanitization. Following are the things we should follow to play an active role :

(i) Ensure that his surroundings (both inside and outside the 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.

(iv) Separate trash as recycled waste and non-recycled.

(v) Chemicals like paints, insecticides, motor oil, etc. should not be thrown in the drain which may kill useful bacteria.

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

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

Answer :

(ii) (b) and (c)