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Important Questions: CBSE Class 7 Science Chapter 13 Wastewater Story

Chapter 13 Wastewater Story in CBSE Class 7 Science covers sewage sources, composition, treatment plant operations, sanitation and disease linkages, and better housekeeping practices. Questions from this chapter regularly appear in CBSE exams worth 3–5 marks. This page organizes 18 real exam-style questions by marks and provides model answers using exact NCERT terminology, so you can revise effectively and score full marks.

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Key takeaways

  • ✓Chapter 13 Wastewater Story typically carries 3–5 marks in the CBSE Class 7 Science final exam.
  • ✓Sewage composition, treatment plant processes, and sanitation practices are high-weightage topics.
  • ✓1-mark MCQs test definitions of sludge, sewage, clarifier, and aerator; 2-mark questions ask for process steps.
  • ✓3-mark questions demand explanation of wastewater treatment stages with diagrams or flow charts.
  • ✓5-mark case-based questions link sanitation, diseases, and alternative systems like on-site sanitation.
  • ✓Common mistakes include confusing sludge with sewage, skipping NCERT diagrams, and incomplete answers on biogas generation.
  • ✓Practicing NCERT in-text and end-chapter questions first builds a strong foundation for board-style questions.

Chapter Overview and Marks Weightage in CBSE Exams

Wastewater Story is part of the Natural Resources unit in CBSE Class 7 Science. The chapter introduces students to sewage (liquid waste from homes, industries, hospitals), its treatment in wastewater treatment plants, and the importance of sanitation in preventing waterborne diseases. CBSE typically allots 3–5 marks from this chapter in the final exam. Questions appear as MCQs (1 mark), very short answers (2 marks), short answers (3 marks), and case-based or long-answer questions (5 marks). Since the 2024 syllabus emphasizes application and real-world linkage, expect one case study linking sanitation to diseases or alternative systems like biogas from sludge. Diagrams of clarifiers, aerators, and sedimentation tanks are frequently asked for 2–3 marks, so labelling practice is crucial.
  • Sewage composition — organic waste, nutrients, microbes, suspended solids.
  • Treatment plants — bar screens, grit chambers, sedimentation, aeration, clarifier, sludge digestion.
  • Sanitation and disease — open drains, lack of toilets, cholera, typhoid, dysentery linkage.
  • Better housekeeping — reducing cooking oil disposal, avoiding chemicals in drains, composting.

1-Mark Questions (MCQ and Very Short Answer)

CBSE Class 7 Science MCQs from Wastewater Story test your recall of definitions, terminologies, and NCERT facts. These questions demand one-word or one-sentence answers. Always use NCERT keywords. For instance, 'sludge' is the solid waste that settles at the bottom during sedimentation — never call it 'dirt' or 'mud'. Similarly, 'clarifier' is the tank where activated sludge is separated from treated water, not a 'filter'. Below are five representative 1-mark questions with concise model answers. Revise NCERT in-text bold terms before attempting these. In the 2025 board exam, expect 1–2 MCQs or fill-in-the-blanks from this chapter. Keep answers precise; examiners award zero for vague or incorrect technical terms.
  • Q1. What is sewage? A: Sewage is wastewater released by homes, industries, hospitals, offices and agricultural fields, containing organic waste, nutrients, microbes and suspended solids.
  • Q2. Name the process that removes floating debris from wastewater. A: Bar screens or screening removes large floating debris like plastics, rags, and sticks.
  • Q3. Which microorganisms help decompose organic matter in sewage? A: Aerobic bacteria decompose organic matter in the presence of oxygen during the aeration step.
  • Q4. What is sludge? A: Sludge is the solid waste that settles at the bottom of sedimentation tanks during wastewater treatment.
  • Q5. True or False: Cooking oil and fats should be poured down the kitchen sink. A: False. Oil and fats should be collected in bins or composted; they clog pipes and harm treatment plants.

2-Mark Questions (Short Answer)

2-mark questions in CBSE Class 7 Science usually ask for two distinct points, a brief explanation, or a comparison. For Wastewater Story, examiners commonly test the steps of sewage treatment, differences between terms, or the role of a specific component. Write two clear sentences or two bulleted points. Avoid long paragraphs — you will run out of time and space. Use NCERT diagrams if the question asks to 'draw and label' (for example, a bar screen or aerator). Below are four high-probability 2-mark questions. Practice writing answers in 30–40 words each. In the 2024 board paper, one 2-mark question appeared on the function of an aerator. Always begin with a definition or context sentence, then give the second point as an example or consequence.
  • Q6. Why is aeration done in wastewater treatment? A: (i) Aeration supplies oxygen to aerobic bacteria. (ii) These bacteria rapidly decompose organic waste, reducing pollution load before water is released or reused.
  • Q7. Differentiate between sewage and sludge. A: Sewage is liquid wastewater containing dissolved and suspended impurities. Sludge is the solid waste that settles at the bottom during sedimentation in treatment plants.
  • Q8. What happens to the sludge after it is removed from the clarifier? A: Sludge is transferred to a digester tank where anaerobic bacteria decompose it, producing biogas (methane). The remaining dried sludge is used as manure.
  • Q9. List two better housekeeping practices to reduce wastewater generation. A: (i) Do not pour cooking oil or fats down the drain. (ii) Use minimum chemicals and detergents; prefer eco-friendly cleaners.

3-Mark Questions (Descriptive Answer)

3-mark questions demand structured explanations with three clear points or a step-wise process. CBSE examiners look for NCERT sequence and correct terminology. For Wastewater Story, popular 3-mark topics include the complete sewage treatment process, the role of microorganisms, sanitation and disease linkage, and alternative sanitation systems. Always number your points (i), (ii), (iii) or use subheadings like 'Physical treatment', 'Biological treatment', 'Chemical disinfection'. Diagrams fetch you 1–1.5 marks if labelled correctly, so include a flow chart if time permits. Below are three representative 3-mark questions with model answers. Write 60–80 words per answer. In the 2023 Delhi board paper, a 3-mark question on 'steps in a wastewater treatment plant' was asked. Students who skipped the clarifier or sludge digestion step lost 1 mark.
  • Q10. Describe the main steps involved in wastewater treatment. A: (i) Physical removal: Bar screens remove floating debris; grit and sand settle in chambers. (ii) Sedimentation: Wastewater is kept in large tanks where suspended solids settle as sludge. (iii) Biological treatment: Air is pumped into water (aeration) so aerobic bacteria decompose organic waste. (iv) Clarification: Treated water is separated from activated sludge in a clarifier. (v) Disinfection: Chlorine or UV light kills remaining pathogens before release.
  • Q11. Explain how untreated sewage causes waterborne diseases. A: (i) Untreated sewage contains disease-causing microorganisms (pathogens) like bacteria and viruses. (ii) When sewage mixes with drinking water sources, people consuming that water contract cholera, typhoid, dysentery and hepatitis. (iii) Open drains and poor sanitation in slums and rural areas increase the spread of these diseases, especially among children.
  • Q12. What is the role of anaerobic bacteria in sewage treatment? A: (i) Anaerobic bacteria work in the absence of oxygen inside the sludge digester. (ii) They decompose organic matter in sludge, producing biogas (mainly methane and carbon dioxide). (iii) Biogas is used as fuel for heating or electricity; dried sludge residue is used as manure, making the process eco-friendly.

5-Mark Questions and Case-Based Questions

5-mark questions test deep understanding, real-world application, and the ability to connect multiple concepts. CBSE Class 7 Science papers since 2022 include one case study or source-based question worth 4–5 marks. For Wastewater Story, cases revolve around sanitation campaigns (Swachh Bharat Abhiyan), on-site sewage systems (septic tanks), biogas plants in villages, or the water cycle and sewage linkage. Read the case paragraph carefully, underline keywords, and answer sub-questions in the exact sequence asked. Allocate 1 mark per sub-part. Write 100–120 words total. Below are two full 5-mark questions with model answers. Practice these under timed conditions. In the 2024 All India paper, a case on a village installing a biogas plant from community toilets was asked; students who mentioned methane production and manure use scored full marks.
  • Q13. (Case Study) A municipality installed a new wastewater treatment plant. The plant has bar screens, a grit chamber, two large sedimentation tanks, an aeration tank, a clarifier, and a sludge digester that produces biogas. (a) Why are bar screens placed at the entry? (b) What settles in the sedimentation tank? (c) Why is air pumped in the aeration tank? (d) How is biogas produced? (e) Suggest one use of dried sludge. A: (a) Bar screens remove large floating debris like plastics, rags, and sticks to prevent clogging. (b) Suspended solid impurities settle as sludge at the bottom of the sedimentation tank. (c) Air is pumped to supply oxygen for aerobic bacteria, which decompose organic waste rapidly. (d) Anaerobic bacteria in the digester break down sludge in the absence of oxygen, producing biogas (methane and CO₂). (e) Dried sludge is rich in nutrients and is used as manure in agriculture.
  • Q14. Explain the importance of sanitation and proper sewage disposal in preventing diseases. Describe two alternative systems for areas without underground sewers. A: Importance: (i) Proper sanitation ensures human waste does not contaminate water sources. (ii) Untreated sewage carries pathogens causing cholera, typhoid, dysentery, and hepatitis. (iii) Open defecation and overflowing drains spread these diseases, especially in crowded slums. Alternative systems: (i) Septic tanks: On-site system where solid waste settles and is decomposed by bacteria; liquid overflows to a soak pit. (ii) Composting toilets: Waste is composted with sawdust or ash, producing safe manure without water. Both reduce dependency on centralized sewage networks and are suitable for rural or hilly areas.

How CBSE Frames Questions from This Chapter

CBSE question setters follow NCERT textbook structure closely. Most questions directly paraphrase NCERT in-text 'Think and Answer' boxes or end-chapter exercises. For Wastewater Story, the 2023–2025 papers show a pattern: one MCQ on definitions (sewage, sludge, clarifier), one 2-mark diagram-based question (label a sedimentation tank or aerator), one 3-mark process question (steps of treatment or role of bacteria), and one 5-mark application or case study linking sanitation to health or describing alternative systems. Questions often use real-world contexts — a village, a city, a school — to test if students can apply NCERT knowledge. Examiners value NCERT terminology: always write 'aerobic bacteria' not 'oxygen bacteria', 'clarifier' not 'final tank', 'activated sludge' not 'leftover mud'. Diagrams must match NCERT figures; hand-drawn sketches are acceptable if neat and labelled. Marks are split: 1 mark for diagram, 2 marks for labels and explanation. Practice past years' board papers and NCERT exemplar problems to recognize these patterns.
  • Direct NCERT lifts: MCQs and VSAs often copy in-text 'Do You Know' or activity boxes verbatim.
  • Diagram-based: 'Draw and label a bar screen' or 'Show the flow of wastewater in a treatment plant'.
  • Application: 'Why should we not throw medicines down the toilet?' tests understanding of chemical effects on bacteria.
  • Case studies: A paragraph on Swachh Bharat or a village biogas plant, followed by 4–5 sub-questions (1 mark each).

Common Mistakes Students Make and How to Avoid Them

Every year, CBSE Class 7 Science answer scripts lose marks on Wastewater Story due to avoidable errors. The biggest mistake is confusing 'sewage' (liquid wastewater) with 'sludge' (settled solid waste). Write definitions clearly. Second, students skip labelling in diagrams or mislabel parts — for example, calling the grit chamber a 'sand filter' or the clarifier a 'settling tank'. Use exact NCERT terms. Third, when explaining the treatment process, many skip the sludge digestion and biogas step, costing 1–2 marks. Always mention anaerobic bacteria and biogas in any 3-mark or 5-mark answer on treatment. Fourth, answers on sanitation and disease lack specific disease names; write 'cholera, typhoid, dysentery' not 'stomach diseases'. Fifth, students write long paragraphs without numbering points, making it hard for examiners to award sectional marks. Use (i), (ii), (iii) format. Sixth, copying answers from guidebooks instead of NCERT leads to terminology mismatches. Stick to NCERT textbook language. Finally, many ignore the 'better housekeeping practices' section, yet CBSE loves asking 'Suggest two ways to reduce wastewater at home' for 2 marks. Revise pages 151–152 of NCERT thoroughly.
  • Confusing sewage with sludge — learn definitions by heart.
  • Skipping biogas production in treatment answers — always mention anaerobic bacteria and methane.
  • Mislabelling diagrams — match NCERT figure 13.2 (wastewater treatment plant) exactly.
  • Writing vague disease names — specify cholera, typhoid, dysentery, hepatitis.
  • Ignoring better housekeeping practices — easy 2-mark questions, don't leave them.
  • Using guidebook language instead of NCERT terms — examiners cross-check with NCERT answer key.

Additional Practice Questions (Mixed Marks)

Here are three more exam-style questions to round out your revision. These mirror the variety seen in CBSE board papers and school pre-boards. Attempt each under timed conditions, then compare your answer with the model. Notice how model answers use numbered points, NCERT terms, and concise sentences. For MCQs, eliminate obviously wrong options first. For descriptive answers, plan your points in rough before writing. If a question says 'Explain with an example', always give a real-world or NCERT activity example. If it says 'Draw and label', spend 2 minutes on a neat diagram. These questions cover topics not yet addressed above: vermicomposting toilets, water cycle linkage, and chemical disposal. Together with the 14 questions already provided, you now have 18 important questions spanning every mark type and topic in Chapter 13.
  • Q15. What is a septic tank and where is it used? (2 marks) A: A septic tank is an on-site sewage disposal system where solid waste settles and is decomposed by bacteria. It is used in areas without underground sewage systems, such as rural homes and small colonies.
  • Q16. Why should we not dispose of medicines, paints, and insecticides in the drain? (3 marks) A: (i) These chemicals kill the bacteria used in wastewater treatment plants, disrupting the decomposition process. (ii) They pollute rivers and groundwater when untreated sewage is discharged. (iii) They harm aquatic life and can enter the food chain, affecting human health.
  • Q17. (MCQ) Which gas is produced during anaerobic digestion of sludge? (a) Oxygen (b) Nitrogen (c) Methane (d) Hydrogen A: (c) Methane (along with carbon dioxide, forming biogas).

How CBSETUTOR.ai Helps You Master Wastewater Story Questions

Memorizing 18 questions is a start, but real mastery comes from understanding the logic behind each answer and practicing variations. CBSETUTOR.ai gives your child a 24×7 AI tutor that explains every NCERT concept in Wastewater Story using simple language and visual aids. Students can upload a photo of any question — from school worksheets, sample papers, or past boards — and receive step-by-step solutions within seconds. The platform covers all CBSE classes 6–12 at a flat ₹999 per month, with a 3-day free trial so you can explore Chapter 13 question banks, video explanations, and diagram practice tools risk-free. Parents in cities like Bengaluru and Delhi report that daily 15-minute AI-tutor sessions improve diagram accuracy and answer structuring, leading to 3–5 extra marks in Science. The AI tutor highlights common mistakes in real time, ensuring your child never confuses sludge with sewage or skips the biogas step again. Try the free trial today and watch confidence grow before the board exam.

Revision Strategy for Chapter 13 Before the Exam

Start revision three weeks before the CBSE Class 7 Science exam. In week one, read NCERT pages 147–154 twice, underline bold terms, and copy all diagrams (bar screen, sedimentation tank, clarifier, flow chart) into your notebook. In week two, solve all NCERT in-text and end-chapter questions without looking at answers, then self-correct using the textbook. Mark difficult questions. In week three, attempt the 18 questions on this page under timed conditions — 1 mark in 1 minute, 2 marks in 2–3 minutes, 3 marks in 4 minutes, 5 marks in 7–8 minutes. Write answers on paper, not just mentally. Compare with model answers and note gaps in terminology or skipped points. On the day before the exam, revise only definitions (sewage, sludge, clarifier, aerator, sanitation), disease names, and the treatment flow chart. Do not start new guidebooks. Sleep well; Wastewater Story is scoring if you stick to NCERT language. Keep a one-page summary: definitions, treatment steps (5 points), diseases (4 names), better housekeeping (3 practices), alternative systems (2 types). Revise this summary in the 15-minute reading time before the exam starts.
  • Week 1: NCERT reading, diagram copying, term highlighting.
  • Week 2: Solve all NCERT in-text and end-chapter questions; self-correct.
  • Week 3: Attempt these 18 important questions in timed mode; compare answers.
  • Day before exam: Revise one-page summary of definitions, steps, diseases, practices.
  • During exam: Spend reading time labelling diagrams mentally; write numbered points for descriptive answers.

Frequently asked questions

How many marks does Chapter 13 Wastewater Story carry in CBSE Class 7 Science final exam?+
Typically 3–5 marks. Expect one MCQ (1 mark), one short answer on treatment steps or diagrams (2 marks), and one descriptive or case-based question linking sanitation and health (3–5 marks).
What is the difference between sewage and sludge?+
Sewage is liquid wastewater containing dissolved and suspended impurities from homes and industries. Sludge is the solid waste that settles at the bottom of sedimentation tanks during treatment.
Do I need to draw diagrams for Wastewater Story questions?+
Yes. CBSE often asks 'Draw and label a wastewater treatment plant' or 'Show bar screen and sedimentation tank' for 2–3 marks. Practice NCERT Figure 13.2 and label all parts neatly.
Which diseases are caused by untreated sewage and poor sanitation?+
Cholera, typhoid, dysentery, and hepatitis are the four diseases mentioned in NCERT. Always write specific names in answers; 'waterborne diseases' alone fetches partial marks.
What is the role of aerobic bacteria in wastewater treatment?+
Aerobic bacteria decompose organic waste in the presence of oxygen during the aeration step. Air is pumped into the tank to supply oxygen, enabling rapid breakdown of pollutants.
How is biogas produced from sewage sludge?+
Sludge is transferred to a digester tank where anaerobic bacteria decompose it in the absence of oxygen, producing biogas (mainly methane and carbon dioxide). Biogas is used as fuel.
What are two better housekeeping practices to reduce wastewater pollution?+
Do not pour cooking oil or fats down the drain; collect them separately. Use eco-friendly, minimal detergents and cleaners instead of harsh chemicals that harm treatment bacteria.
What is a septic tank and where is it used?+
A septic tank is an on-site sewage system where solid waste settles and bacteria decompose it. Liquid overflows to a soak pit. It is used in rural areas and places without underground sewers.
Can I score full marks by learning only these 18 questions?+
These 18 questions cover all high-probability patterns and NCERT topics. However, always solve NCERT in-text and end-chapter questions first, then use this page for focused revision and exam practice.
How does CBSETUTOR.ai help with Chapter 13 Wastewater Story preparation?+
CBSETUTOR.ai provides a 24×7 AI tutor that answers doubts, solves uploaded question photos step-by-step, and offers diagram practice. Flat ₹999/month for classes 6–12, with a 3-day free trial to explore Chapter 13 resources.

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