NCERT Class 7 Science Chapter 18 Waste Water Story

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NCERT Class 7 Science Chapter 18 Waste Water Story

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Also, you can read the NCERT book online in these sections Solutions by Expert Teachers as per Central Board of Secondary Education (CBSE) Book guidelines. CBSE Class 7 Science Solutions are part of All Subject Solutions. Here we have given NCERT Class 7 Science Chapter 18 Waste Water Story and Textbook for All Chapters, You can practice these here.

Waste Water Story

Chapter: 18


Q. 1. Fill in the blanks: 

(a) Cleaning of water is a process of removing _________.

Ans: Pollutants.

(b) Waste water released by houses is called _________.

Ans: Sewage.

(c) Dried ________ is used as manure.

Ans: Sludge.

(d) Drains get blocked by ________ and _______.

Ans: Solid food remains, sanitary towels.

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

Ans: Sewage is a liquid containing wastes which is disposed by households, industrial and agricultural activities in water. It is harmful to discharge untreated sewage into rivers or seas because it can pollute the whole source of water. Sewage contains harmful substances and disease causing organisms. It is therefore, dangerous to release untreated sewage in water.

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

Ans: Oils and fats should not be released in drains because they harden the soil in the pipes and block them. Fats get clogged in holes of the soil in the drain and block it. It does not allow the water to flow. 

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

Ans: Water is treated physically, chemically and biologically in wastewater treatment plant. 

Following steps are involved in the purification of water:

(i) At first stage all the physical impurities like stones, rags, napkins, plastic bags, cans, packets etc., are removed. It is done by passing the water through bar screens.

(ii) Then water is taken to grit and sand removal tank where impurities are removed by sedimentation.

(iii) Solid impurities and feces etc., are collected from the bottom of the water. These solid impurities collected are called sludge. Water is cleared of floatable solids like oil and grease.

(iv) Clarified water is cleared of other impurities by aerator. All disease causing bacteria are removed by chlorination and water is released in various water bodies. 

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

Ans: Sludge is the collected solid waste from the wastewater during the treatment in water treatment plant. Sludge is decomposed in a separate tank by the anaerobic bacteria. Activated sludge is used as manure.

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

Ans: Untreated excreta can cause a lot of health related problems. It pollutes soil, water and air. The polluted water contains disease causing bacteria, which can spread epidemic like cholera, meningitis etc. 

Q. 7. Name two chemicals used to disinfect water. 

Ans: Chlorine and ozone are the chemicals used to clean the waste water.

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

Ans: Bar screens clear the wastewater of all the physical impurities. Large waste objects like napkins, plastics, can sticks, rags etc., are removed from the wastewater through the bar screens

Q. 9. Explain the relationship between sanitation and disease.

Ans: Sanitation and disease are related to each other. If sanitation is there no disease will occur, but if the sanitation is not there various types of diseases will occur and spread. So sanitation should be kept to avoid diseases. 

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

Ans: As active citizen we should take care of our personal and environmental sanitation. We should make people around us, aware of the benefits of sanitation. We should help the municipal corporation and gram panchayats to cover all the open drains and remove the unhygienic and disease causing substances thrown in open.

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



1. Used water.

2. A pipe carrying sewage.

3. Liquid waste products.

4. Solid waste extracted in sewage treatment. 

5. Micro-organisms which cause cholera.

6. A word related to hygiene.

7. A chemical to disinfect water.

8. Waste matter discharged from human body.


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 about 3%

Which of these statements are correct?

(i) (a), (b) and (c)

(ii) (b) and (c)

(iii) (a) and (d)

(iv) All four

Ans: (ii)-(b) and (c).


Very Short Type Questions Answer 

Q. 1. Name two organic impurities present in sewage.

Ans: Human feces, Animal waste (like Animal dung).

Q. 2. Name two inorganic impurities present in sewage. 

Ans: Nitrates, Phosphates.

Q. 3. Name two nutrients present in sewage. 

Ans: Nitrogen and phosphorus.

Q. 4. What type of microbes present in sewage cause diseases such as cholera and typhoid?

Ans: Bacteria. 

Q. 5. Name two diseases which are caused by bacteria present in sewage. 

Ans: Cholera and typhoid.

Q. 6. Name one disease which is caused by microbes (other than bacteria) present in sewage.

Ans: Dysentery.

Q. 7. Write the full form of WWTP. 

Ans: Waste Water Treatment Plant.

Q. 8. Name two chemicals used to disinfect water.

Ans: Chlorine and ozone.

Q. 9. Name the solid waste extracted during sewage treatment.

Ans: Sludge.

Q. 10. Name two things that can be made from sludge extracted during treatment of sewage.

Ans: Biogas and manure.

Q.11. At a sewage treatment plant, what type of bacteria work: 

(a) in the digester tank.

Ans: Anaerobic bacteria.

(b) in the aeration tank?

Ans: Aerobic bacteria.

Q. 12. What is the role of effluent treatment plants in cities?

Ans: The effluent treatment plants filter out undissolved materials from water.

Q. 13. Name the toilets in which human excreta is treated by earthworm.

Ans: Vermi-processing toilets. 

Q. 14. What is effluent?

Ans: Waste water generated by industries is called effluent.

Q. 15. Name any two low-cost, on side sewage disposal systems for human waste. 

Ans: (i) Septic tanks. 

(ii) Composting pits.

Short Type Questions Answer 

Q.1. Write two properties of pure water. 

Ans: Properties of pure water:

(i) It should not contain any suspended impurity. 

(ii) It should not contain any harmful germs.

Q. 2. How “bar screen” and ‘grit and sand removal tank’ help in clarification of water? 

Ans: When wastewater is passed through bar screens it separates big and large objects like plastics, bags, sticks, can, napkins etc. In grit and sand removal tank other solid impurities like pebbles, sand etc., are removed.

Q. 3. How is sludge treated?

Ans: Sludge is the solid impurities separated from the sewage. It is removed and treated in a separate tank with anaerobic bacteria. During this process biogas is produced which is used to produce electricity. Dried sludge is used as manure.

Q.4. What are the problems arising due to open drains and other unsanitary conditions?

Ans: Open drains and unsanitary conditions produce bad smell. It becomes an idle place of breeding for mosquitoes, flies and other harmful insects. These insects spread many harmful diseases and other health hazards.

Q. 5. Write two uses of sludge.

Ans: (i) It is used as a measure to enrich the soil with humus and nutrients.

(ii) It is used to produce biogas which is used as a fuel. Biogas can also be used to generate electricity.

Q.6. What are the uses of treated water which is obtained from sewage treatment plant? 

Ans: (i) It is used for irrigating fields, lawns and for aquaculture. 

(ii) In Kolkata, the treated water is used for fisheries and growing crops and vegetables.

Q. 7. A sewage treatment plant involves the following steps in the working: Aeration tank, grit and sand removal tank, second sedimentation tank, bar screen, first sedimentation tank.

(a) Arrange all the above steps in the correct order in which they occur in the sewage treatment plant.

Ans: Bar screen, Grit and sand removal tank, First sedimentation tank, Aeration tank, Second sedimentation tank.

(b) Which step gives most of the sludge which is used for obtaining biogas fuel?

Ans: First sedimentation tank.

Q. 8. How the kitchen waste blocks the drains?

Ans: Kitchen waste like oils and fats clogs the pores in soil and reduce the filtration process. It also blocks the pipes by hardening the pipes. Wastes like tea leaves, solid food remains, cotton etc., also choke the pipes and slows down flow of oxygen. This slows down the decomposition process by the aerobic bacteria.

Q. 9. What is meant by: 

(a) Sewer.

Ans: An underground pipe which carries away dirty drainage water and waste matter such as from toilets is called a sewer.

(b) Sewerage?

Ans: The underground network of small and big pipes which carry the wastewater (or sewage) from the point of production to the point of disposal is called sewerage.

Q. 10. Explain the function of bar screen in a wastewater treatment plant.

Ans: The bar screen removes the large rubbish, objects like rags, sticks, cans, polythene bags, napkins, sanitary towels, etc., from the wastewater because they cannot pass through the holes of bar screen. These big pieces of rubbish caught by the bar screen are removed from time to time.

Short Type Questions Answer 

Q. 1. With the help of an example, outline your role as an active citizen in relation to sanitation.

Ans: We can become an active citizen and along with the residents of the area approach the Municipality or Gram Panchayat and insist that the open dirty water drains in our area be covered at the earliest. The covering of open drains will make the environment healthy which, will be free from disease-causing bacteria.

For example, if the sewage of a particular house leaks from the pipes, spills on the road and makes the neighborhood dirty, we should ask the house owner to be more considerate about others health and get the leakage of sewage rectified.

Q. 2. Explain the relation between sanitation and disease. 

Ans: Diseases occur due to lack of clean lines, poor sanitation and contaminated drinking water supplies. In many parts of our country, proper toilet and sewerage facilities are not available to the people. People defecate in the open near fields, or railway tracks and dry river beds. Human excreta lying in the open is a health hazard. As a result, water pollution and soil pollution occur. Water pollution causes diseases like cholera, typhoid, dysentery and hepatitis.

Q. 3. What is a vermi-processing toilet? Name the useful by-product obtained from such toilets.

Ans: In the vermi-processing toilet, the human excreta is treated by earthworms in a pit. The earthworms gradually eat up all the organic matter present in human excreta, decompose it and pass out from their body in the form of warm castings (also called vermi- cakes). The useful by-product obtained from a vermi-processing toilet is vermi-cakes which is kind of high quality manure.

Q. 4. What is potable water? Write its characteristics.

Ans: The water which is fit for drinking by human beings is called potable water.


(i) It is transparent.

(ii) It is smell less and colour less.

(iii) It is germless.

Q. 5. Water pollution is major problem in our country. State three major factors, which lead to water pollution. 

Ans: (i) Industrial waste: Effluent of industries contain large quantities of harmful chemicals and acids that are discharged into water-bodies.

(ii) Fertilisers and pesticides: Fertilisers and pesticides are excessively used in fields to increase production. These are washed by rain water into water-bodies and pollute them.

(iii) Sewage: Organic waste contributed as domestic and commercial sewage are discharged into water-bodies and decrease the quality of water.

Long Type Questions Answer 

Q. 1. What should be done to maintain sanitation and avoid water pollution? 

Ans: (i) Wastes should be disposed off properly and not thrown away anywhere. 

(ii) Proper toilet facilities should be provided to rural people.

(iii) Chemical wastes from the factories should be properly treated before being released into water.

(iv) Sewage from cities should not be discharged directly into rivers. It must be treated and made harmless before release. 

(v) Ensure that there are no leaking sewage pipes.

(vi) Ensure that there are no open drains. Flies and mosquitoes breed in stagnant water in ponds and lakes.

Q. 2. List five ways to control sewage generation from your home.

Ans: (i) Leakage in sewer lines should be checked and repaired regularly.

(ii) Waste water from basins, sinks and washing machines can be used again for watering plants or wiping floors. 

(iii) Never pour house hold products such as cooking oil, fat, butter, plastic, etc., down the drain. These materials can block the pipes and sewage may start overflowing in your home or at public places.

(iv) Never throw paints, solvents, insecticides, automobile oils, and medicines down the drain as they may kill microbes that help to purify water.

(v) Used tea leaves, solid food remains, sanitary towels, toys, etc., should not be thrown in the waste water pipe because these materials may choke the drains.

Q. 3. Write two consequences of malfunctioning of sewage. 

Ans: (i) Rupture or blockage or leakage of sewerage lines.

(ii) Sewerage may overflow on the roads and streets, creating insanitary conditions. 

(iii) The place may stink and become breeding ground for mosquitoes, flies, etc.

(iv) It may upset the ecological balance in lakes and rivers.

(v) Water may become unfit for drinking and other domestic uses. 

Q. 4. What is a septic tank? How does it function?

Ans: Septic tank is a low cost on site sewage disposal system. Septic tanks are suitable where there is no sewerage system for hospitals, isolated buildings or a cluster of four-five houses. Septic tank needs cleaning every four to six months.

Functioning of a septic tank: A septic tank usually consists of a big, covered underground tank made of concrete having an inlet pipe at one end and on outlet pipe at the other end. The toilet seat is connected to the inlet pipe of the septic tank. The human excreta from the toilet seat enters into the septic tank through the inlet pipe. The solid part of excreta keeps on collecting at the bottom of the septic tank in the form of sludge whereas watery waste remains above it. The anaerobic bacteria break down most of the solid organic matter present in human excreta due to which the volume of solid waste is reduced too much. The digested solid waste keeps on depositing at the bottom of septic tank. The watery waste is also cleaned by anaerobic bacteria. The excess water goes out of the septic tank through the outlet pipe and r get absorbed in soil.

Q. 5. What is sludge? How is it used? 

Ans: Solids like feces settle at the bottom and are removed with a scraper. This is the sludge. The sludge is transferred to a separate tank where it is decomposed by the anaerobic bacteria. Air is pumped into the clarified water to help aerobic bacteria to grow. Bacteria consume human waste, food waste, soaps and other unwanted matter still remain in clarified water.

After several hours, the suspended microbes settle at the bottom of the tank as activated sludge. The water is then removed from the tank.

The activated sludge is about 97% water. This water is removed by sand drying beds or machines. Dried sludge is used as manure, returning organic matter and nutrients to the soil.


Q.1. Some of the alternative sewage disposal systems for human waste are given below:

Chemical toilets, composting toilets, vacuum toilets, septic tank toilets, vermi- processing toilets.

(a) Which of these toilets provide high quality manure? 

Ans: Vermi-processing toilets.

(b) Which of these toilets are used in airplanes?

Ans: Vacuum toilets.

(c) Which of these toilets are used at outdoor gatherings like marriages? 

Ans: Chemical toilets.

(d) Which of these toilets are used where water supply is limited or not available at all?

Ans: Composting toilets.

(e) Which of these toilets is most like to contaminate water of a hand pump installed nearby?

Ans: Septic tank toilets.

Q. 2. What are the pollutants present in the city sewage?

Ans: The pollutants present in the city sewage are waste water from homes, industries, hospitals, rundown rain water and many dissolved and suspended impurities.

Q.3. The untreated human excreta is a health hazard. How?

Ans: When it rains, human excreta gets washed down to the rivers, ponds and pollutes the water therein. When polluted water seeps into the ground, it contaminates the ground water. The polluted ground water becomes a source of water borne diseases such as Typhoid, Jaundice, Cholera, Diarrhea, etc.

Q. 4. What problems do we face with open drainage ?

Ans: Open drains easily get blocked and start overflowing. In such cases sewage may collect and stagnate in ponds and lakes. These are perfect places for breeding of flies and mosquitoes. The stagnant water in open drains begins to stink. 

Q.5. What happens when the sewage is passed through bar screens?

Ans: When the sewage is passed through bar screens, large objects such as rags, cans, plastic bags, napkins and sticks are removed.


Q. 1. Observe the following figure and answer the given questions: 

(i) What does this figure show?

(ii) What are the functions of each part of the figure?

Fig. 18. 6.

Ans: (i) This figure shows the supply of drinking water from source to houses. 

(ii) (a) River: It is the source of water.

(ii) What are the functions of each part of the figure?

(b) Pumping station: Pump the water to collect it in reservoir. 

(c) Sedimentation tank: Impurities are settle in the bottom of tank.

(d) Sand and gravel and sand filter: Remove the dirt from the water. 

(e) Chlorinating tank: Chlorine is mixed in water to disinfect the water and to kill the germs.

(f) To overhead tank: Purified water is stored in this tank for supply to the households for drinking.

Q.2. Draw a diagram to show the filtration process.


Fig. 18.7 Filtration process.

Q.3. Draw a diagram to show sewage treatment plant.


Fig. 18. 8. Sewage treatment plant.


1. Fill in the following blanks with suitable words: 

(i) Wastewater released by house is called _________.

Ans: Sewage.

(ii) Cleaning of water is a process of removing ___________.

Ans: Pollutants.

(iii) Dried ________ is used as a manure.

Ans: Sludge.

(iv) Drains get blocked by __________ and __________.

Ans: Solid food remains, sanitary towels

(v) Untreated human excreta is a _______ hazard. 

Ans: Health.

(vi) Used _______ and ______ should not be disposed off in a drain. 

Ans: Oil and ghee.

(vii) During the aeration of clarified water __________ bacteria can be grown.

Ans: Aerobic.

2. State whether the following statements are true or false:

(i) Vacuum toilets are usually used in cinema halls.

Ans: False.

(ii) Waste water cannot be reused. 

Ans: False.

(iii) Wastewater is treated in vermi processing plant.

Ans: False.

(iv) Water pollution is caused by human excreta. 

Ans: True.

(v) The underground network of pipes which carry wastewater from the place of its production to the point of its treatment is called sewerage. 

Ans: True.

(vi) Grit and sand settle down under the influence of gravity.

Ans: True.

(vii) Chlorine is used to sterilize the clean water.

Ans: True.

(viii) All the portable toilets are chemical toilets. 

Ans: True.

(ix) Wastewater is transparent, colorless and odorless. 

Ans: False.

(x) Clean water should contain large quantity of salts.

Ans: False.

3. Match the following:

Column – IColumn – II
1. Sewerage(a) disinfect water.
2. Earthworm(b) remove large objects. 
3. Chlorine or ozone(c) block the pipes
4. Bar screen(d) used in vermi processing toilet 
5. Cooking oil or fats(e) transport system that carries sewage.


Column – IColumn – II
1. Sewerage(e) transport system that carries sewage.
2. Earthworm(d) used in vermi processing toilet 
3. Chlorine or ozone(a) disinfect water.
4. Bar screen(b) remove large objects. 
5. Cooking oil or fats(c) block the pipes

Q. 4. Write one word for the following:

(i) The substances which make water un- fit for human consumption.

Ans: Water pollutants.

(ii) The process of removing pollutants from wastewater. 

Ans: Cleaning of water.

(iii) The underground network of small and big pipes for carrying sewage/waste water.

Ans: Sewerage.

(iv) The set up for the removal of physical, chemical and biological pollutants from waste water. 

Ans: Wastewater treatment plant.

(v) The diseases caused by the polluted/ contaminated ground water.

Ans: Water-borne diseases.

(vi) Discharge of industrial waste water. 

Ans: Effluent.

(vii) The bacteria which function in the presence of oxygen. 

Ans: Aerobic bacteria.

(viii) The bacteria which can work in the absence of oxygen.

Ans: Anaerobic bacteria.

5. Multiple Choice Questions:

(i) Chlorine is used in municipal water treatment plant to:

(a) Change the taste of water 

(b) Prevent evaporation

(c) Kill the harmful microorganisms 

(d) Prevent sedimentation

Ans: (c) Kill the harmful microorganisms.

(ii) The aerobic bacteria can be grown in clarified water by:

(a) Filtration

(b) Sewage

(c) Aeration

(d) Sedimentation

Ans: (c) Aeration.

(iii) Which of the following is not an inorganic impurity present in sewage?

(a) Nitrate

(b) Phosphorus

(c) Urea

(d) Metal salt

Ans: (c) Urea.

(iv) Which of the following organisms produce biogas from sludge in the digester tank of a wastewater treatment plant?

(a) Anaerobic bacteria

(b) Fungi

(c) Aerobic bacteria

(d) Protozoa.

Ans: (a) Anaerobic bacteria.

(v) Which of the following is not a VITA water-borne disease?

(a) Cholera

(b) Tuberculosis

(c) Typhoid

(d) Dysentery

Ans: (b) Tuberculosis.

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