Class 7 Notes

Class 7 Science Notes Chapter 18 (Wastewater Story) – Science Book

Philoid

Philoid

— 4 min read

Science
Okay, here are the detailed notes for NCERT Class 7 Science Chapter 18: Wastewater Story, focusing on aspects relevant for government exam preparation.

Chapter 18: Wastewater Story - Detailed Notes for Exam Preparation

1. Introduction: Water - Our Lifeline

  • Clean water is a basic human need.
  • Water gets dirty after use (domestic, industrial, agricultural). This used water is called Wastewater.
  • Releasing untreated wastewater into water bodies causes water pollution and spreads water-borne diseases.

2. What is Wastewater?

  • Definition: Water rich in lather, mixed with oil, black-brown water from sinks, showers, toilets, laundries, etc., is wastewater.
  • Sources: Homes, industries, hospitals, offices, agricultural fields, rainwater runoff carrying pollutants.
  • Liquid Waste: Wastewater is essentially liquid waste.
  • Sewage: Wastewater released by homes, industries, hospitals, etc., is commonly called sewage. It is a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprophytic and disease-causing bacteria, and other microbes.

3. Composition of Sewage (Key Components to Remember):

  • Organic Impurities: Human faeces, animal waste, oil, urea (urine), pesticides, herbicides, fruit and vegetable waste, etc.
  • Inorganic Impurities: Nitrates, Phosphates, Metals.
  • Nutrients: Phosphorus and Nitrogen compounds.
  • Bacteria: Such as Vibrio cholerae (causes cholera) and Salmonella paratyphi (causes typhoid).
  • Other Microbes: Such as protozoa which cause dysentery.

4. Water Freshens Up – An Eventful Journey: Sewerage System

  • Sewerage: A network of pipes (small and big) called sewers that carry sewage from homes and other buildings to the point of disposal, i.e., a wastewater treatment plant (WWTP).
  • Sewers: Underground pipes that transport sewage.
  • Manholes: Located at intervals of 50 m to 60 m in the sewerage system, at junctions of sewers, and where there is a change in direction. They allow for inspection and cleaning.

5. Treatment of Polluted Water: Wastewater Treatment Plant (WWTP)

  • Purpose: To remove physical, chemical, and biological matter from wastewater before it is discharged back into the environment or reused.

  • Key Stages of Treatment:

    • (a) Preliminary Treatment (Physical Process):

      • Bar Screens: Wastewater is passed through bar screens. Large objects like rags, sticks, cans, plastic packets, napkins are removed. Purpose: Prevents damage to pumps and equipment later.

    • (b) Primary Treatment (Physical Process):

      • Grit and Sand Removal Tank: Water flows slowly, allowing grit, sand, and pebbles to settle down. Purpose: Reduces abrasion of equipment.
      • Sedimentation Tank (Clarifier): Water is allowed to stand in a large tank with a slope towards the centre. Solids like faeces settle at the bottom, forming primary sludge. Floatable solids like oil and grease are removed with a skimmer. The cleared water is called clarified water. Purpose: Remove settleable solids.

    • (c) Secondary Treatment (Biological Process):

      • Aeration Tank: Clarified water is pumped into an aeration tank where air is pumped into it. This helps aerobic bacteria (bacteria that need oxygen) to grow vigorously. These bacteria consume human waste, food waste, soaps, and other unwanted organic matter remaining in the clarified water. Purpose: Biological removal of dissolved organic matter.
      • Final Sedimentation Tank: After several hours, the suspended microbes settle at the bottom of the tank as activated sludge. The water from the top is removed. Purpose: Separate microbial biomass from treated water.
      • Activated Sludge: About 97% water. Contains the active aerobic bacteria. A part of the activated sludge is recycled back to the aeration tank to provide bacteria for the next batch; the rest is removed.

    • (d) Sludge Treatment:

      • Sludge: Collected solids from primary (primary sludge) and secondary treatment (activated sludge).
      • Digester Tank: Sludge is transferred to a separate tank called a digester. Here, anaerobic bacteria (bacteria that don't need oxygen) break down the organic matter in the sludge.
      • Biogas Production: This anaerobic digestion produces biogas (mainly methane and carbon dioxide), which can be used as fuel or to produce electricity. Key By-product: Energy source.
      • Dried Sludge (Manure): The remaining digested sludge is dried. Dried sludge is used as manure, returning organic matter and nutrients to the soil. Key By-product: Soil conditioner.

    • (e) Tertiary Treatment / Disinfection (Chemical/Physical Process):

      • The treated water has very low levels of organic material and suspended matter.
      • It may be discharged into a sea, river, or into the ground.
      • Sometimes, it needs to be disinfected with chemicals like chlorine or ozone, or using UV light to kill any remaining harmful microbes before release or reuse. Purpose: Ensure water safety.

6. Better Housekeeping Practices (Individual Responsibility)

  • Do Not Dispose: Cooking oils and fats down the drain. They harden and block pipes. Throw them in the dustbin.
  • Do Not Dispose: Chemicals like paints, solvents, insecticides, motor oil, medicines down the drain. They kill microbes that help purify water.
  • Do Not Dispose: Tea leaves, solid food remains, soft toys, cotton, sanitary towels, etc., down the drain. They choke the drains and hamper the free flow of oxygen, interfering with the degradation process. Use a dustbin.

7. Sanitation and Disease

  • Sanitation: Refers to public health conditions related to clean drinking water and adequate treatment and disposal of human excreta and sewage.
  • Poor Sanitation & Contaminated Water: Major cause of diseases.
  • Water-borne Diseases: Cholera, typhoid, polio, meningitis, hepatitis, dysentery. (Remember specific examples).
  • Impact: A large fraction of people in developing countries lack safe sanitation facilities, leading to health crises, especially among children. Open defecation pollutes soil, surface water, and groundwater.

8. Alternative Arrangements for Sewage Disposal

  • In areas without proper sewerage systems, low-cost onsite sanitation systems are encouraged.
  • Examples:
    • Septic Tanks: Suitable for places like hospitals, isolated buildings, clusters of houses. Sewage flows into the tank where anaerobic bacteria decompose solids. Effluent flows out into a soak pit or drain field.
    • Chemical Toilets: Use chemicals to treat waste; require less water. Common in portable settings (construction sites, events, airplanes).
    • Composting Pits / Toilets: Treat human excreta by composting. Turns waste into valuable compost.
    • Vermi-processing Toilets: A design where human excreta is treated by earthworms. It's low-water-use, safe, and converts waste into vermi-cakes (a high-quality soil conditioner).

9. Sanitation at Public Places

  • Maintaining sanitation is crucial in public places like railway stations, bus depots, airports, hospitals, markets.
  • Large amounts of waste are generated daily, requiring proper disposal to prevent epidemics.
  • Cooperation from citizens is essential (not littering, using dustbins).

10. Conclusion: Role of Citizens

  • Everyone has a role in maintaining sanitation.
  • Reduce waste generation.
  • Ensure proper disposal of waste.
  • Prevent clogging of drains.
  • Cooperate with local authorities.
  • Spread awareness about the importance of sanitation.

Key Terms/Concepts for Exams:

  • Wastewater, Sewage, Sewerage, Sewers, Manholes
  • Contaminants (Organic, Inorganic, Nutrients, Microbes)
  • WWTP (Wastewater Treatment Plant)
  • Bar Screen, Grit Chamber, Primary Clarifier, Skimmer
  • Primary Sludge, Clarified Water
  • Aeration, Aerobic Bacteria, Activated Sludge, Anaerobic Bacteria
  • Digester, Biogas, Dried Sludge (Manure)
  • Chlorination, Ozonation, UV Treatment (Disinfection)
  • Sanitation, Water-borne diseases (Cholera, Typhoid, Dysentery etc.)
  • Septic Tanks, Chemical Toilets, Composting Toilets, Vermi-processing Toilets, Vermi-cakes

This detailed breakdown covers the essential points from the chapter relevant for objective and short-answer questions in government exams. Focus on understanding the processes (especially WWTP stages), definitions, key terms, and the link between sanitation and health.

Read more