Wastewater Story, Class 7 Solutions of NCERT (CBSE) Science Textbook Exercise


Class 7 Ncert Solutions
CBSE Board Class 7 Science Textbook Chapter Exercise Answers
Wastewater Story
Solutions of NCERT Textbook Exercise (Important Questions Only)

Q.1: Fill in the blanks:
a)     Cleaning of water is a process of removing _____.
b)     Wastewater released by houses is called ______.
c)      Dried _______ is used as manure.
d)     Drains get blocked by ______ and ______.

Solution: (a) pollutants (b) sewage (c) sludge (d) solid food remains, sanitary towels

Q.2: What is sewage? Explain why it is harmful to discharge untreated sewage into rivers or seas.

Solution: Sewage is wastewater released by homes, industries, hospitals, offices, other users and also the rain water that run down the street during the storm or heavy rain. Sewage is a liquid waste. It is harmful to discharge untreated sewage into rivers or seas because sewage is a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease causing bacteria and other microbes. 

Q.3: Why should oils and fats not be released in the drain. Explain.

Solution: Oils and fats should not be thrown down the drain. They can harden and block the pipes. In an open drain the fats clog the soil pores reducing its effectiveness in filtering water.

Q.4: Describe the steps involved in getting clarified water from wastewater.

Solution: The various steps involved for water clarification (water treatment) can be sequenced as follows:
  • Step 1 - Wastewater is passed through bar screens. Large objects like rags, napkins, sticks, Solution, plastic packets are removed. 
  • Step 2 - 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.
  • Step 3 - The water is then allowed to settle in a large tank which is sloped towards the middle. Solids like faces settle at the bottom and removed with scraper. And a skimmer removes the floatable solids like oil and grease. Water so, cleared is called ‘Clarified water’.
  • Step 4 - Air is pumped into clarified water to help aerobic bacteria to grow. Bacteria consume human waste, food waste, soap and other unwanted matter still remaining in clarified water.
After several hours, the suspended microbes settle at the bottom of the tank as activated sludge. The water is then removed from the top.    

Q.5: What is sludge? Explain how it is treated.

Solution: Solids like faces settle at the bottom of the tank in the process of water treatment and are removed with a scrapper, which is called sludge. The sludge is transferred to a separate tank where it is decomposed by the anaerobic bacteria. The biogas produced in the process can be used as fuel or can be used to produce electricity. And the dried sludge is used as manure.

Q.6: Untreated human excreta are a health hazard. Explain.

Solution: Human excreta may cause water pollution and soil pollution. Both surface water and ground water get polluted. Ground water is a source of water for wells, tube-wells, springs and many rivers. Thus it becomes the most common route for water borne diseases. They include cholera, typhoid, polio, meningitis, hepatitis and dysentery.     

Q.7: Name two chemicals used to disinfect water.  

Solution:  Chlorine and ozone.

Q.8: Explain the function of a bar screen in a wastewater treatment plant.


Solution: When wastewater is passed through bar screens, large objects like rags, sticks, Solution, plastic packets, napkins that are present in the wastewater are removed.

Q.10: Outline your role as an active citizen in relation to sanitation.

Solution: Waste generation is a natural human activity. But we can limit the type and quantity of wastes produced. Open drains are common in our localities causing unhygienic and unsanitary conditions. As an active citizen we can approach the municipality or the gram panchayat. Insist that the open drains are covered. There are no dust bins in the localities. Dust bins should be kept at some distances for the disposal of house wastes (litter).   

View sample questions and their solutions on 
CBSE (NCERT) 7th Science Wastewater Story

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