Theory:

As the wastewater includes biological, physical and chemical matter as contaminants, the treatment of wastewater includes biological, physical and chemical processes.
The physical and biological processes involved in the treatment of wastewater are as follows:
 
Separation of large objects from the wastewater:
To remove the large objects like plastic bottles, napkins, rags, cans and sticks, the wastewater is first passed through the bar screens.
 
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Separation of Grit and Sand particles:
 
After the removal of large objects, the wastewater is sent to the grit and sand removal tank. In this tank, the speed of the water is decreased so that the sand, dirt and pebbles to settle down.
 
  
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NCERT: Grade 7 chapter 18
 
Removal of solids:
  • The water is now sent to the tank which is sloped in the centre, known as clarifier.
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  • The slope in this tank allows the solids like faeces to settle at the bottom. This is known as sludge.
  • This sludge is separated from the water with the help of a Scrapper.
  • The skimmer removes certain other floatable substances such as the oil and grease, so that clarified water is obtained. 
  • The sludge is then transferred to a separate tank in which the anaerobic bacteria decompose it to produce biogas.
  • This gas is used as a fuel and to produce electricity.
Removal of suspended sludge:
  • As the clarified water has to be cleaned further, it is moved to the aerator which pumps air so that the aerobic bacteria grows.
 
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Aerator
  • This aerobic bacteria consumes the human wastes, food wastes, soaps and other unwanted matter that are still found in the clarified water.
  • This water is then allowed to settle for several hours so that the waste materials settle down in the tank as activated sludge.
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Settling tank
  • The water is then removed from the top by machines or sand drying beds as the activated sludge is 97% water.
Important!
The dried sludge is used as manure so that the organic matter and nutrients are returned to the soil.
As the treated water contains fewer amounts of impurities and suspended matter, it is dumped into a water body or under the ground and nature cleans it up further.