Saturday , May 21 2022

NCERT 7th Class (CBSE) Science: Wastewater Management

Question: What is waste water? How it is produced? Schematically represent the process of waste water treatment.

Answer: The used water that comes into drains from bathroom, toilets and kitchen. The process is:

  1. The waste water sewage reaches through domestic sewerage’s.
  2. Bar screens are used to remove large substances like cans rags and plastic bottles.
  3. After the clarification the  known as activated sludge.
  4. Waste water is allowed into a tank which is sloped in the middle. Solid waste settles in the middle which is scraped is called sludge.
  5. Floating solids are skimmed and the water is called clarified water.
  6. Water from the activated sludge is removed by allowing it to evaporate from sand beds.
  7. After the clarification the microbes are allowed to settle at the bottom which is called activated sludge.
  8. Waste from the activated sludge is removed by allowing it to evaporate from the sand beds.
  9. In tertiary treatment process chlorine is added to kill germs.

Question: Explain the contaminants with examples. Write a short note on the sources of contamination.

Answer: Waste water or sewage is produced due to human activities and natural cause like rain. The main contribution to sewage is from activities like bathing, sanitary needs, cooking, washing, utensils, clothes and farming. This sewage water contains detergents, toxic chemicals, food wastes, blood, human animal tissues etc. These kinds of materials present in the waste water is called contaminants.

Question: How is sludge disposed?

Answer: Some methods to dispose sludge are:

  1. Sludge can be incinerated in incinerator tanks to convert it into ashes, which can be used in construction purpose or to supplement the nutrients in the soil.
  2. De-watering the sludge can be done in two different ways by dry sludge machine or by sand bed, where the water evaporates and percolates into sand. The left over solid waste is used in the manufacture of fertilizers.
  3. The gas produced by sludge and aerobic bacteria is known as bio gas. It is used for lighting purpose and cooking purpose.

Question: Draw a septic tank and explain how it works.

Answer: In this system, sewage from houses flows through drains and into the underground tanks made of cement and stones. The solid waste in the sewage settles down at the bottom. Anaerobic bacteria in the tank decompose the solid waste. Waste water then moves out of perforated pipes into soil and percolates into the soil. It then get gets filtered through the pores of the soil and joins the ground water.


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