Wastewater Story — Class 7 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 7 Science chapter "Wastewater Story" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 7 Science chapter "Wastewater Story" — 8 important questions with detailed answers for CBSE board exam pre…
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NCERT Solutions for Class 7 Science — Wastewater Story. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Wastewater Story — All 8 Questions Solved
Q1. What is wastewater? What are its sources?
Wastewater: water from households, industries, and agriculture containing dissolved and suspended impurities and pollutants. Sources:
1. Household: bathrooms, kitchens, laundry (sewage)
2. Industrial: factories, mills, chemical plants
3. Agricultural: runoff from fields containing fertilizers, pesticides, soil
4. Commercial: restaurants, hotels, hospitals
5. Stormwater: rainwater mixed with dust and pollutants
6. Groundwater: mineral-rich water from deep sources
Contains harmful pathogens, chemicals, nutrients causing water pollution.
Q2. What is sewage? Distinguish between sewage and wastewater.
Sewage: wastewater from houses (toilets, sinks, bathrooms) containing feces, urine, and household waste. More concentrated in pollutants.
Wastewater: broader term; includes sewage plus water from industries, agriculture, and other sources.
Both contain pathogens and pollutants. Sewage is most dangerous due to fecal matter containing disease-causing bacteria (E. coli, Salmonella), viruses, parasites. Must be treated before disposal to prevent waterborne diseases and environmental contamination.
Q3. Describe the main steps in wastewater treatment.
Wastewater treatment process:
1. Screening: remove large objects (sticks, rags, plastic)
2. Sedimentation: solid particles settle (primary treatment), sludge collected
3. Aeration: pump air into wastewater, promote bacterial decomposition
4. Secondary settling: remaining suspended particles settle
5. Biological treatment: aerobic bacteria decompose organic matter
6. Filtration: sand/gravel filters remove remaining particles
7. Disinfection: chlorination/UV light kills pathogens
8. Final treatment: polish water quality
Treated water can be reused for irrigation or safely discharged into water bodies.
Q4. What is the difference between primary and secondary treatment?
Primary treatment:
1. Screening: remove large objects
2. Grit removal: separate sand, gravel
3. Sedimentation: heavy solids settle (sludge removed)
4. Purpose: remove 50-60% suspended solids
Secondary treatment:
1. Aeration: introduce oxygen
2. Biological decomposition: microorganisms (bacteria) break down organic matter
3. Settling: remaining solids precipitate
4. Purpose: remove 80-95% organic matter, reduce BOD (biological oxygen demand)
Secondary treatment requires longer time but produces cleaner effluent suitable for discharge.
Q5. What is activated sludge? How does it work in treatment?
Activated sludge: mixture of flocculent microorganisms (bacteria, protozoa, fungi) concentrated in wastewater treatment. Formation: primary sludge is aerated to activate microbial growth.
How it works:
1. Microorganisms decompose dissolved organic matter in aeration tank
2. Sludge settles in secondary clarifier
3. Some sludge returned to aeration tank (maintains microbial population)
4. Excess sludge removed (waste sludge)
5. Clarified effluent flows out for disinfection
Advantage: efficient removal of organic pollutants and harmful microorganisms.
Q6. What is biosolids or sludge? What is its disposal?
Biosolids (sludge): residual solid matter separated during treatment; contains microorganisms, undecomposed organic matter, mineral nutrients. High in nitrogen and phosphorus.
Disposal methods:
1. Landfill: dump in designated sites
2. Composting: decompose aerobically to form compost for agriculture
3. Land application: spread on agricultural fields as fertilizer (nutrient-rich)
4. Incineration: burn at high temperature (reduce volume, kill pathogens)
5. Drying beds: solar drying reduces moisture and volume
Composting and land application preferred as they recycle nutrients sustainably.
Q7. How can wastewater be recycled and reused?
Wastewater recycling and reuse (water conservation):
1. Agricultural irrigation: treated wastewater for crops, reduces freshwater demand
2. Industrial use: cooling systems, manufacturing processes need not use potable water
3. Toilet flushing: grey water (from sinks, baths) for non-potable use in homes
4. Landscape irrigation: parks, gardens
5. Toilet cistern filling: water harvesting in buildings
Advantages: conserves freshwater, reduces pollution, reduces treatment cost, sustainable water use.
Must ensure adequate treatment level for intended use to prevent health hazards.
Q8. What are the harmful effects of untreated wastewater? How can they be prevented?
Harmful effects of untreated wastewater:
1. Waterborne diseases: cholera, typhoid, dysentery (from pathogens)
2. Water pollution: eutrophication from excess nutrients
3. Aquatic ecosystem damage: toxic chemicals harm fish and plants
4. Soil contamination: groundwater pollution from seepage
5. Bad odor and aesthetics: unpleasant appearance
Prevention:
1. Mandatory wastewater treatment before discharge
2. Proper sewage system infrastructure
3. Regular monitoring of water quality
4. Industrial pollution control regulations
5. Public awareness and education
6. Septic tanks in areas without sewage network
7. Individual water treatment systems in homes
Goal: safe disposal and water resource protection.
Wastewater Story — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. What is the other name for dirty water that contains dissolved and suspended impurities?
- A. Potable water
- B. Wastewater
- C. Rainwater
- D. Groundwater
Answer: B. Wastewater — Wastewater is the used water released from homes, industries, agriculture, and other human activities, containing various impurities.
MCQ 2. Which of the following is NOT a component of raw sewage?
- A. Organic impurities
- B. Inorganic impurities
- C. Biogas
- D. Bacteria
Answer: C. Biogas — Biogas is a product obtained from the anaerobic digestion of sludge during wastewater treatment, not a direct component of raw sewage.
MCQ 3. What is the first step in a wastewater treatment plant (WWTP) to remove large objects like rags, sticks, and plastic bags?
- A. Sedimentation
- B. Aeration
- C. Bar Screens
- D. Chlorination
Answer: C. Bar Screens — Bar screens are the initial physical treatment where large debris is filtered out from the incoming wastewater to prevent damage to machinery and blockages.
MCQ 4. Consuming water contaminated with untreated sewage can lead to which of the following diseases?
- A. Scurvy
- B. Cholera
- C. Rickets
- D. Night blindness
Answer: B. Cholera — Cholera is a severe diarrheal disease caused by bacteria typically transmitted through contaminated water or food, often due to sewage pollution.
MCQ 5. During wastewater treatment, what is the term for the solid waste that settles down at the bottom of the primary sedimentation tank, and what is the floating material called?
- A. Scum, Sludge
- B. Sludge, Scum
- C. Grit, Sludge
- D. Scum, Grit
Answer: B. Sludge, Scum — In the sedimentation tank, the heavy solid impurities that settle at the bottom are called sludge, while lighter floating impurities like oil and grease are called scum.
MCQ 6. What is the primary purpose of pumping air into the aeration tank during wastewater treatment?
- A. To kill all bacteria
- B. To make the water colder
- C. To help aerobic bacteria grow and consume organic waste
- D. To increase the water's density
Answer: C. To help aerobic bacteria grow and consume organic waste — Air is pumped into the aeration tank to provide oxygen for aerobic bacteria, which then multiply and decompose the organic matter in the wastewater.
MCQ 7. Which of these is a major source of domestic wastewater?
- A. Rainwater runoff
- B. Industrial discharge
- C. Water from toilets and kitchens
- D. Agricultural runoff
Answer: C. Water from toilets and kitchens — Domestic wastewater primarily originates from household activities such as flushing toilets, washing dishes, bathing, and laundry.
MCQ 8. What useful product can be obtained from the digestion of sludge by anaerobic bacteria?
- A. Pure oxygen
- B. Biogas
- C. Chlorine
- D. Water for drinking
Answer: B. Biogas — Anaerobic bacteria digest the organic matter in sludge, producing biogas (a mixture mainly of methane and carbon dioxide), which can be used as a fuel.
MCQ 9. What is 'activated sludge' and what is its role in wastewater treatment?
- A. It's the treated water ready for release.
- B. It's the primary sludge used as fertilizer.
- C. It's bacteria-rich sludge returned to the aeration tank to speed up decomposition.
- D. It's the scum removed from the sedimentation tank.
Answer: C. It's bacteria-rich sludge returned to the aeration tank to speed up decomposition. — Activated sludge is a mixture of aerobic bacteria and other microorganisms developed in the aeration tank. A portion of it is recycled back to the aeration tank to inoculate incoming wastewater and enhance the decomposition process.
MCQ 10. Why is open defecation considered a serious health hazard?
- A. It pollutes the air with pleasant smells.
- B. It attracts tourists.
- C. It contaminates soil and water sources, spreading diseases.
- D. It makes the landscape more natural.
Answer: C. It contaminates soil and water sources, spreading diseases. — Open defecation directly contaminates the environment, especially soil and water sources, leading to the spread of waterborne and other infectious diseases.
MCQ 11. Which chemical is commonly used to disinfect treated wastewater before releasing it into water bodies?
- A. Sulphuric acid
- B. Chlorine
- C. Sodium bicarbonate
- D. Methane
Answer: B. Chlorine — Chlorine is a powerful disinfectant often used in the final stage of wastewater treatment to kill any remaining harmful microorganisms, making the water safe for discharge.
MCQ 12. How do septic tanks primarily treat wastewater in areas without a municipal sewage system?
- A. By chemical oxidation using strong acids.
- B. By physical screening and then direct discharge.
- C. By allowing solids to settle and anaerobic bacteria to decompose organic matter.
- D. By continuously pumping air to promote aerobic digestion.
Answer: C. By allowing solids to settle and anaerobic bacteria to decompose organic matter. — Septic tanks are underground chambers where wastewater flows, allowing solids to settle and be digested by anaerobic bacteria, while the liquid effluent flows out into a drainage field.
MCQ 13. The chapter 'Wastewater Story' primarily focuses on:
- A. The history of water on Earth
- B. Different types of clean drinking water
- C. The management and treatment of used water
- D. The water cycle in nature
Answer: C. The management and treatment of used water — The chapter 'Wastewater Story' specifically deals with understanding what wastewater is, how it's generated, why it needs treatment, and the processes involved in treating it.
MCQ 14. Which of the following is a good wastewater management practice at home?
- A. Pouring cooking oil and fats down the drain
- B. Flushing expired medicines down the toilet
- C. Sweeping solid food waste into the sink
- D. Disposing of used tea leaves and coffee grounds in the dustbin
Answer: D. Disposing of used tea leaves and coffee grounds in the dustbin — Disposing of solid food waste like tea leaves and coffee grounds in the dustbin prevents them from clogging drains and interfering with the wastewater treatment process.
MCQ 15. In the aeration tank, what are 'flocs' and why are they important?
- A. They are chemical pollutants that need to be removed.
- B. They are large objects that bypass the bar screens.
- C. They are clumps of bacteria and fungi that consume organic waste.
- D. They are solidified fats and greases that float on the surface.
Answer: C. They are clumps of bacteria and fungi that consume organic waste. — Flocs are aggregates of aerobic bacteria and fungi that grow in the aeration tank. These flocs effectively trap and consume organic matter from the wastewater, biologically purifying it.
MCQ 16. After digestion, the treated sludge can be used as:
- A. An antiseptic
- B. A raw material for plastic
- C. Manure for fields
- D. A component of biogas only
Answer: C. Manure for fields — Digested sludge, rich in organic matter and nutrients, can be dried and used as a valuable fertilizer or manure for agricultural fields.
MCQ 17. In places where sewage systems are not available, what is a common alternative for human waste disposal that promotes hygiene?
- A. Discharging directly into rivers
- B. Using chemical toilets
- C. Storing waste in open pits
- D. Burning waste in the open
Answer: B. Using chemical toilets — Chemical toilets are a viable alternative for waste disposal, especially in remote or temporary locations, as they use chemicals to decompose waste and control odor, ensuring better hygiene than open dumping.
MCQ 18. The grit and sand removal tank in a WWTP is designed to:
- A. Remove large floating debris like plastic bags.
- B. Promote the growth of aerobic bacteria.
- C. Allow sand, grit, and pebbles to settle at a slower flow rate.
- D. Disinfect the water using chemicals.
Answer: C. Allow sand, grit, and pebbles to settle at a slower flow rate. — After bar screens, wastewater flows into the grit and sand removal tank, where the flow velocity is reduced to allow heavier particles like grit, sand, and pebbles to settle down without allowing lighter organic solids to settle.
MCQ 19. Which international day is observed to raise awareness about the importance of sanitation and ending the global sanitation crisis?
- A. World Water Day
- B. World Environment Day
- C. World Toilet Day
- D. Earth Day
Answer: C. World Toilet Day — World Toilet Day, observed on November 19th, aims to inspire action to tackle the global sanitation crisis and achieve Sustainable Development Goal 6 (water and sanitation for all).
MCQ 20. What is produced when aerobic bacteria decompose organic matter in wastewater in the presence of oxygen?
- A. Biogas and methane
- B. Carbon dioxide, water, and energy
- C. Hydrogen sulfide and ammonia
- D. Heavy metals and acids
Answer: B. Carbon dioxide, water, and energy — Aerobic decomposition is a process where microorganisms use oxygen to break down organic substances into simpler, stable end products like carbon dioxide, water, and energy.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.