Chapter 13:
Wastewater Story

Class 7 Science | CBSE Curriculum

Learn about sewage, wastewater treatment, sewerage, sludge, biogas, sanitation, waterborne diseases, septic tanks, vermi-processing toilets explained with more than 100 practice questions.

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Time:
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Quick Revision: Wastewater Story

  • Wastewater/Sewage: Used water from homes, industries, hospitals, offices. Contains organic/inorganic impurities, nutrients, pathogens.
  • Sewerage: Network of pipes (sewers) that transport sewage to treatment plants. Manholes every 50-60 m for access.
  • Wastewater Treatment Steps:
    • Bar screens: Remove large objects (rags, sticks, plastic).
    • Grit and sand removal: Sand, pebbles settle due to reduced flow.
    • Clarifier (sedimentation): Solids (sludge) settle; oil/grease (scum) skimmed.
    • Aeration tank: Air pumped; aerobic bacteria consume organic waste.
    • Final clarifier: Activated sludge settles; treated water discharged.
    • Disinfection: Chlorine/ozone kills remaining pathogens.
  • Sludge treatment: Anaerobic bacteria digest sludge → produces biogas (fuel). Dried sludge used as manure.
  • Better housekeeping: Don't throw oils, fats, chemicals, tealeaves, sanitary waste down drains.
  • Sanitation & disease: Poor sanitation causes waterborne diseases: cholera, typhoid, polio, hepatitis, dysentery.
  • Onsite systems: Septic tanks, vermi-processing toilets (using earthworms), chemical toilets, composting pits.
  • Swachh Bharat Mission (2016): Government initiative to eliminate open defecation and improve sanitation.
  • Historical fact: Indus Valley civilisation (4500 years ago) had advanced sanitation with brick toilets and covered drains.
  • UN Water for Life Decade (2005-2015): Aimed to halve the number of people without access to safe drinking water.

Chapter Summary:

Wastewater Story addresses the critical issue of used water management. Wastewater (sewage) is generated from homes, industries, hospitals, and offices, containing organic/inorganic impurities, nutrients, and disease-causing pathogens. Sewerage systems—networks of pipes (sewers) with manholes every 50-60 metres—transport sewage to treatment plants.

Wastewater treatment involves physical, chemical, and biological processes: bar screens remove large objects; grit tanks settle sand; clarifiers allow sludge to settle and scum to be skimmed; aeration tanks use aerobic bacteria to consume organic waste; final clarifiers separate activated sludge. Sludge is digested by anaerobic bacteria to produce biogas (used as fuel), then dried and used as manure. Treated water is disinfected with chlorine or ozone before discharge.

Better housekeeping practices include not disposing of oils, fats, chemicals, or solid waste down drains. Poor sanitation causes waterborne diseases like cholera and typhoid. Onsite systems (septic tanks, vermi-processing toilets) serve areas without sewerage. The Swachh Bharat Mission (2016) aims to eliminate open defecation. The Indus Valley civilisation had advanced sanitation 4500 years ago. The UN 'Water for Life' decade (2005-2015) highlighted global water access challenges. Individual responsibility and collective action are essential for maintaining sanitation and public health.