Revision Notes of Ch 18 Wastewater Story Class 7th Science

Topics in the chapter

  • What is wastewater?
  • Wastewater treatment
  • Sewage
  • Composition of sewage
  • What is sewer?
  • What is sewerage?
  • Sewage treatment
  • Wastewater treatment plant (WWTP)
  • Better housekeeping practices
  • Sanitation
  • Sanitation in public

What is wastewater?

→ That water which gets contaminated after various works like washing, bathing, mopping etc. is called wastewater.

Wastewater treatment

→ It is a process of cleaning of water by removing pollutants before it enters a water body or is reused.


→ It is a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease causing bacteria and other microbes.

Composition of sewage

→ Organic impurities: Animal waste, urea, pesticides, herbicides, fruits and vegetables.

→ Inorganic impurities: Metals, phosphates, nitrates.

→ Bacteria: cholera and typhoid causing bacteria.

→ Nutrients: nitrogen, phosphorus

→ Other microbes: diarrhea, jaundice cause microbes.

What is sewer?

→ Pipe which carries the wastewater is called sewer.

What is sewerage?

→ It is a network of big and small pipes called sewers form the sewerage.

Sewage treatment

(i) Aeration: While continuously stirred air is bubbled through the wastewater.

(ii) Filtration: The aerated water then passes through a deep filter of layered sand, fine gravel and medium gravel.

(iii) Chlorination: chlorine is added to the filtered water until water is clear.

Wastewater treatment plant (WWTP)

→ Waste water is passes through screens to remove large objects

→ Then water goes to a grit and sand removal tank. The speed of incoming waste water is decreased which allow sand, grit and pebbles to settle down.

→ Water then allowed to settle in a large tank in which solids like faeces settle at the bottom and are removed with a scraper.

→ Water is decomposed by anaerobic bacteria in a tank and air is passed. Microbes are settled down at the bottom as a activated sludge and water from top is removed.

Better housekeeping practices

→ Cooking oil and fats should not be thrown down the drain because they can harden and block the pipes.

→ Paints, solvents, insecticides, motor oil, medicines may kill microbes that helps in purification of water.

→ Solid food remains, soft toys, sanitary towels etc should throw in dustbin otherwise these wastes choke the drains. Also they do not allow free flow of oxygen.


→ Poor sanitation cause large number of diseases.

→ Large number of people defecates in the open like dry river beds, on railway tracks and many times directly in water. Because of this both water and soil get polluted.

Sanitation in public

→ Large amount of waste is generated in railway stations, bus depots, and airports. It must be disposed otherwise epidemics could break out

→ We should throw litter in dustbin.

NCERT Solutions of Wastewater Story Class 7
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