Class 7 Science Notes Chapter 18 (Chapter 18) – Examplar Problems Book

Alright class, let's focus on Chapter 18, 'Wastewater Story', from your Science Exemplar book. This chapter is crucial not just for understanding our environment but also frequently appears in various government examinations. Pay close attention as we break down the key concepts.

Chapter 18: Wastewater Story - Detailed Notes for Exam Preparation

1. What is Wastewater?

• Wastewater is essentially used water. It includes water discharged from homes (domestic), industries (industrial), hospitals, offices, and other sources.
• It also includes rainwater that runs off rooftops, roads, and other surfaces, picking up various pollutants (stormwater).
• Wastewater generated in homes and public buildings is commonly called Sewage.

2. Composition of Sewage:

• Sewage is a complex mixture, mostly water (about 99.9%), containing dissolved and suspended impurities.
• These impurities are called contaminants.
• Major Components:
  • Organic Impurities: Human faeces, animal waste, urine, oil, urea, pesticides, herbicides, fruit and vegetable waste.
  • Inorganic Impurities: Nitrates, phosphates, metals (like arsenic, lead).
  • Nutrients: Phosphorus and Nitrogen compounds.
  • Microbes: Bacteria (like Vibrio cholerae causing cholera, Salmonella typhi causing typhoid), protozoa (causing dysentery), viruses, and other disease-causing microorganisms (pathogens).
  • Suspended Solids: Sand, silt, grit, cloth fibres, paper.

3. Harmful Effects of Untreated Sewage:

• Water Pollution: Contaminates surface water (rivers, lakes) and groundwater, making it unfit for drinking, bathing, or agriculture.
• Spread of Diseases: Pathogens in sewage cause water-borne diseases like cholera, typhoid, polio, meningitis, hepatitis, and dysentery.
• Oxygen Depletion (Eutrophication): Organic waste and nutrients (nitrates, phosphates) promote excessive growth of algae (algal bloom). When these algae die, bacteria decompose them, consuming large amounts of dissolved oxygen in the water. This lack of oxygen kills fish and other aquatic life.

4. Sewerage System:

• A network of pipes designed to transport sewage from its point of origin (homes, buildings) to a treatment plant.
• Components:
  • Sewers: Underground pipes carrying sewage. Made of cast iron, concrete, PVC etc.
  • Manholes: Access points located at intervals (every 50-60 m) in the sewerage system, at junctions, and where direction changes. Used for inspection and cleaning.

5. Wastewater Treatment Plant (WWTP):

• A facility where wastewater is treated to remove pollutants before it is discharged back into the environment (like a river or sea) or reused.
• Treatment involves physical, chemical, and biological processes.
• Stages of Treatment:
  • a) Preliminary Treatment:
    • Bar Screens: Wastewater is passed through large screens to remove large objects like rags, sticks, cans, plastic packets.
  • b) Grit and Sand Removal:
    • Water flows through tanks where speed is reduced, allowing heavy solids like grit, sand, and pebbles to settle down.
  • c) Primary Treatment:
    • Water is allowed to stand in large sedimentation tanks.
    • Solid faecal matter and other heavier organic solids settle at the bottom, forming primary sludge.
    • Lighter materials like oil and grease float on the surface and are removed using a skimmer.
    • The partially cleaned water is called clarified water.
  • d) Secondary Treatment (Biological Treatment):
    • Aeration: Clarified water is pumped into an aeration tank. Air is bubbled through the water to promote the growth of aerobic bacteria. These bacteria consume the remaining dissolved organic matter. This mixture of microbes and waste is called activated sludge.
    • Sedimentation: The mixture is then transferred to another sedimentation tank where the activated sludge (containing bacteria) settles down. A part of this activated sludge is recycled back to the aeration tank to maintain the bacterial population.
  • e) Tertiary Treatment (Optional/Advanced):
    • Disinfection: The treated water may still contain harmful microbes. Chemicals like chlorine or ozone, or UV (Ultraviolet) radiation are used to kill these pathogens before discharge.

6. Sludge Treatment and Disposal:

• The sludge collected from primary and secondary treatment needs further processing.
• Sludge Digestion: Sludge is transferred to a separate tank called a digester. Here, anaerobic bacteria (bacteria that work without oxygen) break down the organic matter in the sludge.
• Biogas Production: This anaerobic digestion produces biogas (a mixture primarily of methane and carbon dioxide). Biogas can be used as fuel or to generate electricity.
• Drying: The digested sludge is dried. Dried sludge can be used as manure or soil conditioner, returning organic matter and nutrients to the soil.

7. Better Housekeeping Practices (Preventing Clogging and Pollution):

• Do Not dispose of cooking oils and fats down the drain. They harden and block pipes. Collect them in a container and dispose of them in the dustbin.
• Do Not throw chemicals like paints, solvents, insecticides, medicines down the drain. They kill microbes that help purify water.
• Do Not throw solid waste like tea leaves, food remains, cotton, sanitary towels, plastic bags, etc., into drains. They choke the drains and hamper the free flow of oxygen, affecting the degradation process. Use a dustbin.

8. Alternative Sewage Disposal Systems:

• In areas without a proper sewerage network (rural areas, isolated buildings):
  • Septic Tanks: Suitable for individual houses or small clusters. Sewage flows into a tank where solids settle and are decomposed by anaerobic bacteria. The liquid effluent flows out into a soak pit or drain field. Requires periodic emptying.
  • Chemical Toilets: Use chemicals to treat waste; often portable.
  • Composting Pits/Toilets: Treat human excreta by composting.
  • Vermi-processing Toilets: Use earthworms to treat human excreta, converting it into valuable vermi-compost. Low water usage.

9. Sanitation and Disease:

• Poor sanitation and contaminated drinking water are major causes of diseases globally.
• Lack of proper sewage disposal leads to contamination of water sources, flies breeding on excreta, and spread of infections.
• Maintaining public hygiene and proper sanitation is crucial for community health.

10. Role of an Active Citizen:

• Be conscious of water usage and waste generation.
• Follow good housekeeping practices.
• Report leaking pipes or overflowing drains to the concerned authorities immediately.
• Ensure septic tanks are maintained properly.
• Promote awareness about sanitation in the community.

Now, let's test your understanding with some Multiple Choice Questions (MCQs). Choose the best option for each.

Multiple Choice Questions (MCQs)

1. Which of the following is NOT typically considered an organic impurity in sewage?
   (a) Human faeces
   (b) Urea
   (c) Nitrates
   (d) Fruit waste

2. The process of removing large objects like rags and sticks from wastewater at a WWTP is done using:
   (a) Grit chambers
   (b) Bar screens
   (c) Skimmers
   (d) Aeration tanks

3. Aerobic bacteria are primarily used during which stage of wastewater treatment?
   (a) Preliminary treatment
   (b) Primary treatment
   (c) Secondary treatment
   (d) Sludge digestion

4. Biogas produced during sludge digestion mainly consists of:
   (a) Oxygen and Nitrogen
   (b) Methane and Carbon dioxide
   (c) Hydrogen and Oxygen
   (d) Chlorine and Methane

5. Which of the following diseases is NOT commonly spread through contaminated water?
   (a) Cholera
   (b) Typhoid
   (c) Tuberculosis
   (d) Dysentery

6. The solid waste that settles down during primary treatment of wastewater is called:
   (a) Activated sludge
   (b) Primary sludge
   (c) Grit
   (d) Scum

7. Which of the following should NOT be disposed of down the kitchen sink?
   (a) Water used for washing vegetables
   (b) Leftover tea
   (c) Cooking oil and fats
   (d) Fruit juice

8. An alternative sewage disposal system suitable for areas without sewerage, which uses earthworms, is:
   (a) Septic tank
   (b) Chemical toilet
   (c) Vermi-processing toilet
   (d) Composting pit

9. The function of adding air to the clarified water during secondary treatment is to:
   (a) Help solids settle faster
   (b) Kill harmful bacteria using oxygen
   (c) Promote the growth of aerobic bacteria to consume waste
   (d) Remove oil and grease

10. Anaerobic bacteria are important in WWTPs for:
    (a) Removing large objects
    (b) Digesting sludge and producing biogas
    (c) Killing pathogens using chlorine
    (d) Aerating the clarified water

Answer Key:

1. (c) Nitrates (These are inorganic impurities)
2. (b) Bar screens
3. (c) Secondary treatment (in the aeration tank)
4. (b) Methane and Carbon dioxide
5. (c) Tuberculosis (It's primarily an airborne disease)
6. (b) Primary sludge
7. (c) Cooking oil and fats
8. (c) Vermi-processing toilet
9. (c) Promote the growth of aerobic bacteria to consume waste
10. (b) Digesting sludge and producing biogas

Study these notes thoroughly. Understanding the process of wastewater treatment and the importance of sanitation is vital. Let me know if any part needs further clarification.