NCERT Solutions For Class 7 Science Chapter 18

NCERT Solutions Class 7 Science 8 Wastewater Story

Ncert Solutions For Class 7 Science Chapter 18 PDF Free Download


The NCERT solutions class 7 Science chapter 18 Wastewater Story is crucial for the students of 7th standard. The solutions are provided here to help students understand the chapter in an easy and interesting way.

NCERT solutions for Wastewater Story Class 7 Science by subject experts according to the latest CBSE syllabus. Students must practice the solutions regularly to prepare effectively for their examination.

This chapter specifically deals with the issue of contaminants in the water. Water is an important resource not just for humans but all life on earth. Therefore, contaminating this natural resource has dire consequences. And the main reason water is getting polluted is due to our commercial and industrial activities. All these waste effluents have nowhere else to go but our water bodies. In the long run, this can cause many diseases and health hazards for us.

For instance, in Minamata City, Japan, mercury run off from a factory tainted the waters and the poisoning went undetected for decades. It was only when the people saw their newborns had unusual birth defects that they realized something was wrong. Find out more about wastewater below.

Q1. Fill in the blanks:

  1. Cleaning of water is a process of removing _______.
  2. Wastewater released by houses is called _______.
  3. Dried _______ is used as manure.
  4. Drains get blocked by _______ and _______.


  1. Pollutants
  2. Sewage
  3. Sludge
  4. Chemicals, Kitchen waste


Q2.What is sewage? Explain why it is harmful to discharge untreated sewage into rivers or seas.

Ans. Water containing liquid wastes disposed from domestic household activities, industrial activities and agricultural activities is termed as sewage.

The untreated sewage can harm the rivers and seas as the contaminated water pollutes the whole source of water. As the sewage water may contain harmful substances and disease causing organisms, it is dangerous to release the untreated sewage into the water bodies.


Q3. Why should oils and fats be not released in the drain? Explain.

Ans. Water flow is affected by the blockages produced due to the release of these oils and fats into the streams of water flowing in the drain.They harden the soil in the pipes and results in a block. The holes of the soil in the drains gets clogged by the Fats. Hence, the flow is affected.


Q4. Describe the steps involved in getting clear water from wastewater.


  1. These are the steps involved in the purification of water,
  2. In the first step, all the physical impurities like stones, plastic bags ,cans etc. are removed by passing the water through screens.
  3. Now, water is subjected to sedimentation where impurities are removed by the force of gravity.
  4. Solids settled at the bottom of the sedimentation tank is removed. This is called sludge.
  5. An aerator cleanses the water from other impurities.
  6. Chlorination is done to remove all disease causing bacteria.


Q5. What is Sludge? Explain how it is treated.

Ans. The solid waste collected along with the wastewater during the treatment in the treatment plant is called sludge. It is decomposed in a separate tank by the anaerobic bacteria. The product is called Activated sludge process in which the activated sludge can be used as manure.


Q6. Untreated human excreta is a health hazard. Explain.

Ans. A lot of health hazard may be resulted from untreated human excreta. Besides polluting air, water and soil it could also cause epidemics like Cholera, Meningitis etc,. when it mixes with water.


Q7. Name two chemicals to disinfect water.


  • Chlorine
  • Ozone


Q8. Explain the junction of bar screens in a wastewater treatment plant.

Ans. Physical impurities with considerable size can be removed by Bar screens. The screens are made of meshes which permit water and finer solids to pass through while restricting bigger solids.

Eg:Plastics, Wood chunks, Papers, cans, Food wastes.


Q9 .Explain the relationship between sanitation and disease.

Ans. Proper sanitation should be maintained in order to avoid diseases. They are related to each other. By sanitation, proper disposal of sewage and refuse from household and public places is carried out. Implementation of proper sanitation results in a state where we are free from any diseases. Lack of it may lead to various diseases.


Q10. Outline your role as an active citizen in relation to sanitation.


  • We should take care of our personal sanitation
  • Efforts should be taken to make people aware of the benefits of sanitation.
  • With the help of municipal corporations, all the open drains should be covered.
  • We should remove all the disease causing substances thrown in open.


Q11. Solve this crossword puzzle.




  1. Liquid waste products
  2. Solid waste extracted in sewage treatment
  3. A word related to hygiene
  4. Waste matter discharged from human body


  1. Used water
  2. A pipe carrying sewage
  3. Micro-organisms which cause Cholera
  4. A chemical to disinfect water




12.Study the following statements about ozone:


  1. It is essential for breathing of living organisms.
  2. It is used to disinfect water.
  3. It absorbs ultraviolet rays.
  4. Its proportion in the air is about 3%.


Which of these statement are correct?

I. (1), (2) and (3)

II. (2) and (3)

III. (1) and (4)

IV. All four

Ans. (II) (2) and (3)

Water is a resource that we use for almost all of our activities. And consequently, polluting this resource can adversely affect our health. It is mainly polluted by human activities as a result of commercial or industrial exploits. The waste by products contain many poisonous chemicals that disrupt the marine ecosystem. Also, human excreta poses a major threat and could potentially be a biohazard due to the many harmful microbes it carries. These microbes or chemicals could leech into the aquatic environment tainiting every life form that comes in contact with it.

Similarly, taking the example of Minamata City in Japan again, the mercury runoff had contaminated the marine ecosystem, primarily by fish ingesting the mercury. This mercury accumulated in the fish’s body over a period of time and when these fish were commercially caught and consumed, the mercury had entered the human body. This adversely affected bodily functions and caused a lot of problems for the people and their descendants. Hence, we need to take care of such issues by preventing industrial effluents polluting our most precious resource – water.

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