Wastewater Story Class 7 Science Notes - Chapter 18

According to the CBSE Syllabus 2023-24, this chapter has been renumbered as Chapter 13.



  • Wastewater refers to all effluent from a household, hospitals, commercial organizations and institutions, industries and so on. It is also inclusive of agricultural, horticultural, stormwater and urban runoff, and aquaculture effluent.
  • Effluent is used to refer to the liquid waste or sewage that is discharged into water bodies either from treatment plants or direct sources.


A contaminant is something that contaminates a substance, such as water or food.

Organic Impurities

  • Organic impurities may include substances that pollute abiotic components like water, soil etc. Organic impurities, which pollute water, are collectively called sewage.
  • Organic impurities in sewage include animal waste, urine, oil, vegetable and fruit waste, faeces, pesticides and herbicides.

Inorganic Impurities

Inorganic impurities include phosphates, nitrates and metals. Inorganic impurities generally do not contain any carbon, but there are certain inorganic impurities which contain carbon, like carbon dioxide.


  • Bacteria are a type of microorganism, which are tiny forms of life that can only be seen with a microscope.
  • Harmful bacteria that cause bacterial infections and disease are called pathogenic bacteria.
  • The diseases caused by bacteria are typhoid, cholera, pneumonia etc.

Sewage: Who’s That?


  • Sewage is the wastewater released by hospitals, homes or industrial establishments that is carried away in sewers or drains for dumping or conversion into a form that is not toxic.
  • Sewage is a liquid waste containing a complex mixture of suspended solids, organic and inorganic impurities, nutrients, saprophytic and disease-causing bacteria, and some other microbes.

To know more about Sewage, visit here.


Saprophytic bacteria are the kind of bacteria that feed on dead decaying plants and animals, rotten wood, stagnant water and many other decaying substances, which are rich in organic matter.

To know more about Saprophytes, visit here.

Wastewater Management

Wastewater Management

Wastewater treatment is a process used to remove contaminants and make the water usable.

  • Wastewater Collection
  • Screening
  • Primary Treatment
  • Secondary Treatment
  • Final Treatment


  • Screening is one of the first stages in the process of treating wastewater.
  • In this process, the larger objects are removed from wastewater and then moved into the grit and sand removal tank.

Primary Treatment

  • Wastewater after screening is taken for primary treatment, where all the organic waste is removed.
  • Primary treatment is done by pouring the wastewater into big tanks for the solid matter to settle at the surface of the tanks.


The settled solids, after primary treatment, are called sludge. It is decomposed by bacteria, and the gas emitted is known as biogas, which can be used as fuel or can be used to generate electricity.

Secondary Treatment

  • Water after primary treatment is passed through a tank called ‘aeration lane’’ where the air is tapped into the water to increase the growth of aerobic bacteria.
  • Bacteria break down small particles of sludge that escape after primary treatment.

Activated sludge

After the secondary treatment, the broken down sludge settles down at the base of the huge tank, known as ‘activated sludge’. It contains air in it.

Final treatment

  • The activated sludge is passed through a bed of sand drying machine where the sludge is dried up, and water is filtered out.
  • The water is directed to flow over a wall, wherein it gets filtered through a sand bed to eliminate additional particles, if any.
  • This water that is filtered is then released into the river.

Sewage Systems

Sewage systems

Sewage from each house is collected through the drainage, and the network of pipes called sewers take them to the wastewater treatment plants, from which it’s released into water bodies.

Better Housekeeping Practices

  • Cooking oil and fats should not be thrown down the drain as the fats clog and block the pipes.
  • Used tea leaves, solid food remains, soft toys, cotton, sanitary towels, etc., should not be thrown in drains as they do not allow the free flow of oxygen. This hampers the degradation process.


  • Sanitation generally refers to the provision of facilities and services for the safe disposal of human urine and faeces.
  • Poor sanitation causes a large number of diseases and health hazards.

Vermi-processing Toilet

  • In this process, the waste sewage slurry collected from sewage disposal systems is treated with earthworms, wigglers and tiger worms.
  • They decompose the faecal matter, kitchen waste (organic) and other household organic wastes.
  • It is a very simple, hygienic and low water-consuming process with no odour or flies problem.

Septic Tanks

The septic tank is a buried, water-tight container usually made of concrete or polyethene, in which sewage is collected and allowed to decompose through bacterial activity before draining by means of a soak-away.

To know more about Sewage Management Methods, visit here.

Frequently Asked Questions on CBSE Class 7 Science Notes Chapter 18 Wastewater Story


What is sewage treatment?

Sewage Treatment refers to the process of removing contaminants, micro-organisms and other types of pollutants from wastewater.


What are the bacteria that cause water contamination?

The presence of coliform bacteria, specifically E. coli (a type of coliform bacteria), in drinking water suggests the water may contain pathogens.


What is vermin-processing?

Vermi refers to small worms such as earthworms. Vermi-processing toilets are those toilets in which human excreta is treated by earthworms.


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