NCERT Class VII Science Solutions: Chapter 18-Wastewater Story Part 4 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 21:

Three statements are provided here which define the terms

(a) Sludge (b) sewage and (c) wastewater. Pick out the correct definition for each of these terms.

  1. The settled solids that are removed in wastewater treatment with a scraper.
  2. Water from kitchen used for washing dishes.
  3. Wastewater released from homes, industries, hospitals and other public buildings.


  1. Sludge
  2. Wastewater
  3. sewage

Question 22:

A mixture (x) in water contains suspended solids, organic impurities, inorganic impurities (a) , nutrients (b) , disease causing bacteria and other microbes. Give names for (x) , (a) and (b) ?


(X) Sewage

(A) Nitrates, phosphates and metal

(B) Phosphorus and nitrogen


Question 23:

What are the different types of inorganic and organic impurities generally present in sewage?


  1. Inorganic impurities – nitrates, phosphates and metals
  2. Organic impurities – fruit and vegetable wastes, oil, urea, human faeces, animal waste, pesticides and herbicides
Difference between Inorganic and Organic Impurities

Question 24:

The terms sewage, sewers and sewerage are interlinked with each other. Can you explain, how?



  1. Sewage – mixture of wastewater coming out of homes and other places.
  2. Sewers – pipes which carry sewage.
  3. Sewerage – network of sewage carrying pipes.

Question 25:

Fill in the blanks in the following statements using words given in the box.

Air, hand pumps, cholera, water, large, ground

A very ________ number of our people defecate in the open. It may cause ________pollution and soil pollution. Both the surface water and ________ water get polluted. ________water is the source for wells, tube wells and. Thus it becomes the most common route for ________ borne diseases like ________, dysentery, etc.


A very large number of our people defecate in the open. It may cause water pollution and soil pollution. Both the surface water and ground water get polluted. Hand pump water is the source for wells, tube wells and. Thus it becomes the most common route for water borne diseases like cholera, dysentery, etc.

Question 26:

Describe various steps of cleaning wastewater in a wastewater treatment plant.


The various steps are:

  • Wastewater is passed through bar screens. Large objects like rags, sticks, cans, plastic packets, napkins are removed.
  • Water then goes to a grit and sand removal tank. The speed of the incoming wastewater is decreased to allow sand, grit and pebbles to settle down.
  • The water is then allowed to a settle ion a large tank which is sloped towards the middle. Solids like faeces settle at the bottom and are removed with a scraper. This is the A skimmer removes the floatable solids like oil and grease. Water so cleared is called clarified water.
  • Air is pumped into the clarified water to help aerobic bacteria to grow. Bacteria consume human waste, food waste, soaps and other unwanted matter still remaining in clarified water.

Question 27:

Think and suggest some ways to minimise waste and pollutants at their source, taking your home as an example.


Some points are given here, you can add more.

  1. Do not throw used tea leaves, solid food remains, etc. in the drain. Throw them in the dustbin.
  2. Chemicals like medicines, paints, insecticides, etc. should not be thrown in the drain, as they increase the pollution load of the sewage.

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