NCERT Class VII Science Solutions: Chapter 18-Wastewater Story Part 2 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 2:

What is sewage? Explain why it is harmful to discharge untreated sewage into rivers or seas.


Sewage is a liquid waste which has water as its largest component along with various types of impurities like waste water from houses, offices, factories, hospitals etc. , is called sewage. It has a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease causing bacteria and other microbes. Discharging untreated sewage into rivers or seas will pollute the water resources. Contaminated water is dangerous for aquatic plants and animals. It also spread many diseases like cholera, typhoid, polio, meningitis, malaria, dengue etc.

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Question 3:

Why should oils and fats be not released in the drain? Explain.


Oil and fats can clog the drains reducing its effectiveness in filtering water. In open drains, they block the pores in the soil and thus hamper the filtration by soil. Hence, oil and fats should not be released in the drain.

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Question 4:

Describe the steps involved in getting clarified water from wastewater.


Treatment of wastewater involves physical, chemical, and biological processes, which remove physical, chemical and biological matter that contaminates the wastewater.

Following steps are involved:

  • Wastewater is passed through bar screens which remove large objects like rags, sticks, cans, plastic packets, napkins etc.
  • Water then goes through Grit and Sand removal tank, where sand, grit and pebbles settle down.
  • Water is then allowed to settle in a tank where solids like faces (called sludge) settle at the bottom and are removed with a scraper. A skimmer removes the floatable solids like oil and grease. Water so cleared is called clarified water. Sludge is used to produce biogas.
  • Clarified water is then passed through an aerator tank where air is pumped into the water. It helps aerobic bacteria to grow which decompose organic matter like human waste.
  • After several hours, the suspended microbes settle at the bottom of the tank as activated sludge. The water is then removed from the top. The dried activated sludge is used as manure.
  • Water is removed from the top and is stored in a tank. The treated water has low level of organic material and suspended matter. It is discharged into a sea, a river or into the ground. Sometimes it is disinfected through chemicals like Chlorine and ozone and then distributed to towns.
As Showing in Images is a Result for Clarified Water from Wa …

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