NCERT Solutions for Class 7th Science
Chapter 18 – WASTEWATER STORY
(Complete Downloadable Chapter Solution PDF file is at the bottom of the page)
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 ____________.
Ans (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.
Ans Sewage is wastewater released by homes, industries, hospitals, offices and other users containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease causing bacteria and other microbes. It also includes rainwater that has run down the street during a storm or heavy rain.
If untreated sewage is discharged into river or seas, it will contaminate these water resources and harm the aquatic life. If this water is used for drinking purpose it can cause various diseases like cholera, typhoid, dysentery etc.
Q.3 Why should oils and fats be not released in the drain? Explain.
Ans Cooking oil and fats should not be thrown down the drain because they can harden and block the pipes. In an open drain the fats clog the soil pores reducing its effectiveness in filtering water. Throw oil and fats in the dustbin.
Q.4 Describe the steps involved in getting clarified water from wastewater.
Ans Treatment of wastewater involves physical, chemical, and biological processes, which remove physical, chemical and biological matter that contaminates the wastewater. The process of treatment involves following steps:
1) Bar screens: The first step in a wastewater treatment plant is passing of waste water through bar screens. They remove the large objects like rags, sticks, cans, plastic packets, napkins.
2) Grit and sand chamber: Water then goes to a grit and sand chamber where sand, grit and pebbles settle down.
3) Sedimentation tank: The water is then allowed to settle in a large tank which is sloped towards the middle. Solids like faeces (called sludge) settle at the bottom and are removed.......
Q.5 What is sludge? Explain how it is treated.
Q.6 Untreated human excreta is a health hazard. Explain.
Q.7 Name two chemicals used to disinfect water.
Q.8 Explain the function of bar screens in a wastewater treatment plant.
Q.9 Explain the relationship between sanitation and disease.
Q.10 Outline your role as an active citizen in relation to sanitation.
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 causes cholera
7. A chemical to disinfect water
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