NCERT Solutions for Class 7 Science Chapter 18 : Wastewater Story


Here is a crossword puzzle. Good luck!


3. Liquid waste products

4. Solid waste extracted in sewage treatment

6. A word related to hygiene

8. Waste matter discharged from human body


1. Used water

2. A pipe carrying sewage

5. Micro-organism which causes cholera

7. A chemical to disinfect water



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 (b)

(iv) All four


The correct option is option (ii).


Name two chemicals used to disinfect water.


Two chemicals used to disinfect water are chlorine and ozone.


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


Our role as an active citizen should be in maintaining sanitation in neighbourhood and at public places. We can limit the type of waste and quantity of waste produced. For this every citizen should follow these steps.

  • Do not drop litter. Carry the litter to home if any dustbin is not found nearby.
  • Contribute and coordinate with government plans.
  • If drains are not covered, report it to municipality.
  • If the sewage of any particular house makes the neighbourhood dirty, request them to be more considerable about others health.


Explain the relationship between sanitation and disease.


Lack of sanitation may cause diseases. Large fraction of people defaecate in open, on dry river boards, railway tracks and many times directly in water. It results in pollution of water and many diseases. Diseases like typhoid, meningitis, hepatitis, dysentery, etc. are caused due to poor sanitation.


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


In wastewater treatment plant bar screens are used to separate large objects like rags, plastic packets, napkins, etc. by passing water through them.


What is sludge? Explain how it is treated.


Semi-solid wastes such as faeces that settle down at the bottom during wastewater treatment are called sludge. Sludge is removed with a scraper. The sludge is then transferred to a separate tank where it is decomposed by the anaerobic bacteria. In this process biogas is produced.

The biogas produced can be used as a fuel or can be used to produce electricity.


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


Oils and fats should not be released in the drain because they can harden and block the pipes. In an open drain the fats clog the soil pores which affect its water filtering efficiency. Therefore, oils and fats should be thrown in dustbins.


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 _________.


(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.


Untreated human excreta is a health hazard. Explain.


Untreated human excreta is a health hazard because it pollutes soil and water. Both the surface water and ground water gets polluted. Due to contamination of ground water, various water bodies such as wells, tube wells, ponds, etc. also get contaminated as ground water is the source of water for them. When water from these contaminated sources is used for domestic purposes, it may cause several water borne diseases such as cholera, dysentery, meningitis, etc.


Describe the steps involved in getting clarified water from wastewater.


Steps involved in getting clarified water from wastewater are as follows:

  1. Wastewater is passed through bar screens. Large objects like rags, plastic packets and napkins are separated by passing water through bar screens.
  2. Water is then passed through grit and sand removal tank. Here, water is passed at a slow speed to allow pebbles and grit to settle down.
  3. Water is then allowed to settle in a large tank which is sloped towards the middle. Here, solid wastes like faeces settle down at the bottom. These solid wastes are called sludge. Sludge is removed with a scraper. A skimmer removes the floating wastes like oil and grease. Water is then passed through water clarifier. Water cleared by this process is called clarified water.


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


Sewage is wastewater having dissolved and suspended impurities released from homes, industries, hospitals, offices and other users.

It is harmful to discharge untreated sewage into rivers or seas because:

  • Untreated sewage contaminates seas and rivers.
  • Sewage contains various contaminants including disease causing bacteria and other microorganism. Disease causing bacteria and other microorganism may cause dysentery, cholera, typhoid, etc.
  • Pesticide, herbicides present in the sewage may cause various disorders in living beings.

Therefore, it is harmful to discharge untreated sewage into rivers or seas.

Benefits of NCERT Solutions

Following are some of the benefits of using NCERT Solutions by Extramarks.

1. The NCERT solutions for class 7 are prepared keeping in mind the necessary rules and format issued by the CBSE board thus providing students much-needed preview of the exam pattern of questions and preview of the exam that they are going to face during the final exams.

2. The students can be sure about the quality of the solutions as they are committed by highly experienced and expert teachers. These teachers have years of experience and they make sure that there is no mistake committed when preparing the solutions.

3. The NCERT 7 solutions are provided for the questions that are most likely to appear in the exam thus improving the efficiency of the students by helping them prepare for the important parts of the syllabus.

Tips & Strategies for Class 7 Exam Preparation

1. Start your revision early so you can revise well and grasp all the difficult concepts.

2. Organize your study time by making a timetable as it will help you manage your time accordingly.

3. Take mock tests and solve sample papers as they will help you learn time management during the exam and make you familiar with the exam pattern.

4. Take regular breaks, look after yourself by sleeping properly and eating well.

5. Know your exam well, explore the mock tests, and practice sample papers as much as you can.

Why Choose Extramarks CBSE Class 7 Study Materials?

1. The solutions have been compiled keeping in mind the purpose of scoring high marks in exams thus they have everything that the students need to prepare for exams.
2. The NCERT solutions for class 7 are designed in a clear and simple language complete with diagrams so that the students understand the subject well.
3. The NCERT 7 solutions are accurate thus improving their reliability.
4. The solutions are designed by expert and experienced teachers that help in removing all doubts out of the solutions and improving the credibility of the answers.

Frequently Asked Questions

Is class 7 Math NCERT book sufficient for prepare for the exams?

Yes, the NCERT book has been designed keeping in mind the pattern of paper that will be presented in the exam to students. The NCERT book is sufficient to study for the exam as it has everything related to the exam and contains all information out of which the questions will be asked in the exam. But if you want to prepare a little bit extra, we suggest that you go through the NCERT class 7 solutions.

When to refer to NCERT solutions for class 7th?

When you are studying the NCERT book for math and other subjects for your class 7 exams, refer to the NCERT class 7 solutions simultaneously so you can see where you are going wrong and what are you getting right in your exercises’ attempt.

Why are the NCERT solutions important for Class 7th?

The class 7th is very crucial as it is among the major middle school classes during which the child’s base is formed in math and science and even in other subjects like English. Thus, it is students must study smartly in this class and ace their exams while getting all the help they need from the NCERT 7 solutions given by Extramarks.

Can I rely on the NCERT solutions for class 7 Math at Extramarks to perform well in the annual exam?

Yes, absolutely. The NCERT solutions provided by Extramarks for class 7 are eloquent and elaborate at best and are thus useful for students studying for the final exams of class 7. The students will have many questions and doubts regarding the NCERT solutions for class 7 and this is why the solutions have been elaborated well to help students understand the answers to exercises.

Do you provide NCERT solutions for Class 7 Math for free?

Yes, the NCERT solutions for Math and other subjects for class 7 are available for free from Extramarks.

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