NCERT Solutions Class 7 Science Chapter 13 Wastewater Story

Exercises

Fill in the blanks:

1. (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 oils and fats.

Short Answer Questions:

2. What is sewage? Explain why it is harmful to discharge untreated sewage into rivers or seas.
   • Sewage is wastewater released from homes, industries, hospitals, and other places. It contains organic and inorganic impurities, nutrients, and harmful microbes.
   • Discharging untreated sewage into rivers or seas pollutes water, harms aquatic life, spreads diseases like cholera and dysentery, and causes environmental degradation.
3. Why should oils and fats not be released in the drain? Explain.
   • Oils and fats can harden and clog pipes, leading to blockages in drains. In open drains, they reduce the soil’s ability to filter water, which affects the degradation process and pollutes the environment.
4. Describe the steps involved in getting clarified water from wastewater.
   • Steps to clarify water:
     1. Bar Screening: Removes large objects like plastic, rags, and cans.
     2. Grit and Sand Removal: Allows sand, grit, and pebbles to settle.
     3. Sedimentation: Solids like feces settle at the bottom as sludge.
     4. Aeration: Air is pumped to promote bacterial growth, which decomposes organic matter.
     5. Filtration: Water is passed through sand and gravel layers.
     6. Disinfection: Chlorine or ozone is added to kill harmful microbes.
5. What is sludge? Explain how it is treated.
   • Sludge is the solid waste that settles at the bottom of the sedimentation tank during wastewater treatment.
   • Treatment of sludge: It is decomposed by anaerobic bacteria, producing biogas, which can be used as fuel or to generate electricity. The dried sludge is used as manure.
6. Untreated human excreta is a health hazard. Explain.
   • Untreated human excreta can contaminate water and soil, spreading diseases like cholera, typhoid, hepatitis, and dysentery. It pollutes drinking water sources and harms the environment.
7. Name two chemicals used to disinfect water.
   • Chlorine and ozone.
8. Explain the function of bar screens in a wastewater treatment plant.
   • Bar screens are used to remove large objects like rags, sticks, plastic, and cans from wastewater to prevent clogging and protect further treatment processes.
9. Explain the relationship between sanitation and disease.
   • Poor sanitation leads to the contamination of water and the environment, spreading waterborne diseases like cholera, typhoid, and dysentery. Proper sanitation prevents disease outbreaks and promotes public health.
10. Outline your role as an active citizen in relation to sanitation.
    • As an active citizen:
      • Ensure proper disposal of household waste.
      • Avoid throwing oils, fats, and solid waste into drains.
      • Promote awareness about sanitation in the community.
      • Report unhygienic practices to local authorities.
      • Participate in cleanliness drives and support government initiatives like “Swachh Bharat.”

Crossword Puzzle:

11. Complete the crossword puzzle using the hints provided:

Across:
3. Sewage (Liquid waste products)
4. Sludge (Solid waste extracted in sewage treatment)
6. Sanitation (A word related to hygiene)
8. Excreta (Waste matter discharged from the human body)

Down:

1. Wastewater (Used water)
2. Sewer (A pipe carrying sewage)
3. Bacteria (Microorganisms that cause cholera)
4. Chlorine (A chemical to disinfect water)

Multiple Choice Question:

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 is about 3%.

Which of these statements are correct?

• Answer: (ii) (b) and (c).

Extended Learning — Activities and Projects

1. Construct your own crossword puzzle:
   (Activity to engage creativity; students create a crossword using chapter keywords.)
2. Then and now: Report on sewage systems in earlier times and now:
   • Collect information about traditional and modern sewage disposal systems. Highlight the progress and remaining challenges.
3. Visit a sewage treatment plant:
   (Students record observations, such as the plant’s location, treatment capacity, processes used, and the final treated water’s safety.)

General Understanding

1. What is wastewater?
   • Wastewater is water that has been used and becomes dirty. It includes water from sinks, toilets, laundries, and rainwater mixed with harmful substances.
2. Why is clean water important?
   • Clean water is essential for drinking, cooking, sanitation, and maintaining good health. Lack of clean water leads to diseases and deaths.
3. What is sewage?
   • Sewage is wastewater released from homes, industries, hospitals, and other places. It contains organic and inorganic impurities, nutrients, and microbes.
4. What is sewage treatment?
   • Sewage treatment is the process of removing pollutants from wastewater before releasing it into water bodies or reusing it.
5. What are the main sources of sewage?
   • Homes, industries, hospitals, offices, stormwater runoff, and agricultural activities.

Components of Sewage

6. List two organic impurities found in sewage.
   • Human feces and vegetable waste.
7. List two inorganic impurities in sewage.
   • Nitrates and phosphates.
8. What harmful microbes are present in sewage?
   • Bacteria like Vibrio cholera (causing cholera) and Salmonella paratyphi (causing typhoid).
9. What are saprophytes in sewage?
   • Saprophytes are microorganisms that feed on decaying organic matter in wastewater.
10. What is the role of nutrients like nitrogen and phosphorus in sewage?
    • These nutrients can cause excessive algal growth in water bodies, leading to eutrophication.

Processes in Wastewater Treatment

11. What happens in the bar screen stage of treatment?
    • Large objects like rags, sticks, and plastic are removed.
12. What is the function of the grit and sand removal tank?
    • It slows down wastewater to allow sand, grit, and pebbles to settle at the bottom.
13. What is sludge?
    • Sludge is the solid waste, including feces, that settles at the bottom of the treatment tank.
14. What are anaerobic bacteria?
    • These bacteria decompose sludge without oxygen, producing biogas.
15. What is biogas, and how is it used?
    • Biogas is a mixture of gases produced during anaerobic decomposition. It is used as fuel or for electricity production.

Alternative and On-Site Solutions

16. What is a septic tank?
    • A septic tank is an on-site sewage disposal system where wastewater is partially treated before being released into the ground.
17. What is a vermi-processing toilet?
    • A toilet where human excreta is decomposed by earthworms into vermicompost.
18. How do eucalyptus trees help in sewage ponds?
    • They absorb excess wastewater and release pure water vapor into the atmosphere.
19. What is the role of a biogas plant in sewage management?
    • It processes human excreta into biogas, which can be used as an energy source.
20. Why are composting pits considered eco-friendly?
    • They convert human and kitchen waste into compost, reducing environmental pollution.

Impact of Poor Sanitation

21. What diseases are caused by poor sanitation?
    • Cholera, typhoid, hepatitis, dysentery, and polio.
22. What is the effect of untreated human excreta on water sources?
    • It pollutes surface and groundwater, making it unsafe for drinking and irrigation.
23. How does open defecation affect the environment?
    • It leads to soil and water pollution and provides breeding grounds for disease-causing organisms.
24. Why is sanitation important in public places?
    • Poor sanitation can lead to the spread of epidemics in crowded places like fairs, bus stops, and railway stations.
25. What is the government’s role in improving sanitation?
    • Implementing standards for waste disposal, providing sewerage systems, and running campaigns like “Swachh Bharat.”

Better Housekeeping Practices

26. Why shouldn’t cooking oil and fats be thrown down the drain?
    • They can harden and block pipes, reducing soil filtration effectiveness in open drains.
27. What types of waste should not be thrown into sinks?
    • Tealeaves, solid food, cotton, and sanitary towels as they choke drains and hinder oxygen flow.
28. How can citizens improve sanitation in their neighborhoods?
    • By ensuring proper waste disposal, covering open drains, and reporting unhygienic practices.
29. What are the benefits of chlorinating treated water?
    • Chlorine disinfects water by killing harmful bacteria and microbes.
30. How can individuals contribute to better sanitation?
    • By adopting responsible waste disposal habits, reducing pollutants, and spreading awareness about sanitation.