Home NCERT Solutions NCERT Exemplar CBSE Sample Papers NCERT Books Class 12 Class 10

Chapter 18 Wastewater Story

  • Water is essential for the survival of life.
  • Used water is called as waste water.
  • Waste water comes from home usually like from sink, showers, toilets etc.
  • The process of removing impurities or pollutants from water is called as cleaning of water.
  • Wastewater treatment is commonly called as sewage treatment.
  • Sewage is a mixture of solid and liquid waste which comes from home, industries, hospitals etc.
  • Different types of impurities present in water like organic, inorganic, nutrients, bacteria and other microbes.
  • Sewer is the network of big and small pipes, farming the sewerage.
  • The process by which air is mixed with a liquid. Aeration is done during sewage water treatment to provide growth of aerobic bacteria as they consume waste materials like humans waste, food waste etc. is called as aeration.
  • The bacteria which need oxygen to survive are known as aerobic bacteria.
  • Anaerobic bacteria are those who don’t need oxygen to survive.
  • Biogas is produced when sludge is decomposed by the anaerobic bacteria.
  • A polluting or poisonous substance that makes something impure.
  • Keeping our surrounding clean known as sanitation.
  • During waste water treatment the solid waste which settle down in the tank known as sludge.

Download pdf of NCERT Solutions for Class Science Chapter 18 Wastewater Story

Download pdf of NCERT Examplar with Solutions for Class Science Chapter 18 Wastewater Story

Exercise 1 ( Page No. : 229 )

  • Q1

    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 ____________.


    (a) Contaminants
    Cleaning of water is a process of eliminating pollutants before it enters a water or aquatic body or is reused. This wastewater treatment process is commonly known as sewage treatment. It takes place in various stages.

    (b) Sewage
    Sewage is a waste. Most of the dirty water having dissolved and suspended impurities or contaminants in it. The impurities present in sewage are known as contaminants or pollutants.

    (c) Dung
    Cow dung, which is dark brown in colour is mainly used as manure. If the dung is not recycled into the soil by species like earthworms and dung beetles, cow dung can dry out remain on the pasture, creating an area of grazing land which is unpalatable to animals.

    (d) Plastic and sludge
    The waste water is passed into a sedimentation tank and allowed to stand there for a while. Most of the solid organic matter settles down on the bottom of the slope of sedimentation tank in the form of a sludge; the solid part of sewage is called sludge.


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


    Sewage: The waste water and faeces (human excretion) from homes and other buildings which is carried into the sewers is called sewage. Sewage includes household wastewater from kitchen sink, washing machine, washbasin, bathroom shower, bath tub and faeces from the toilet. Sewage is also produced and released by shops, offices, schools, hospitals, factories, industries and all other places where humans work or live. Sewage also includes rainwater that has run down the streets and roads during rains carrying harmful substances with it which also spreads various diseases along it, and flows into sewer pipes. Sometimes, sewage contains harmful and poisonous chemicals that are being discharged by factories and industries. Most of the dirty water contains dissolved and suspended impurities in it which are known as contaminants or pollutants.


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


    Waste cooking oils and fats should not be released in the kitchen sink to go down the drain because cooking oils and facts can block the drainage pipes or system. On the other hand, in an open drains, the fats blocks the soil pores and reduces its effectiveness in filtering water. The waste oils and fats should always be thrown in the dustbin. The oil travelling into waterways can cause water’s oxygen level to drop which can also affects the wildlife living in it. Cooking oils are commonly liquid at room temperature but can get solid at the low temperature if the oils contains fat such as coconut oil, palm oil and palm kernel oil. The oil extracted from plants sources are olives, soybean oil, palm oil, musturd oil, peanut oil and other vegetables oil etc.


    Describe the steps involved in getting clarified water from wastewater.


    We need water for all our as usual activities. Each and every process is in connection to the water that we use. Now a days, we waste water too much. Water conservation is not being done properly. So water is clarified from the wastewater (also known as sewage) generated by various activities like washing, laundries and etc. 

    Clarified water from wastewater:
     Wastewater is collected and passed through bar screens where plastic bags, sticks, cans, napkins etc. are removed. This water then enters a grit and a sand removal tank.
     The speed of the water is decreased so that the sand particles get settled at the bottom of the tank. After this, the water is allowed to settle in a large tank which is tilted towards the centre.
     Solid wastes are settled and removal by the scraper and which is known as clarified water. Air is pumped in this water to help aerobic bacteria to grow.
     The bacteria consume the wastes by microbial action. These microbes settle down after several hours and the water at the top is removed.
     This water is discharged into rivers and oceans or is put to other uses. Nature cleans it up further. Sometimes it may be necessary to disinfect water with chemicals like chlorine and ozone before releasing it into the distribution system.


    What is sludge? Explain how it is treated.


    The waste water (or sewage) is then passed into a sedimentation tank and allowed to stand there for a while. Most of the solid organic matter settles down on the slopping bottom of the sedimentation tank in the form of a sludge. Thus, the solid part of sewage is called sludge.

    The sludge is taken out from the bottom of first sedimentation tank and put into a large, closed tank called digester tank. In the digester tank, many types of anaerobic bacteria decompose the organic matter present in sludge to produce biogas (also called sewage gas). The anaerobic bacteria do not need oxygen of air to carry out the process of decomposition. That is why the digester tank is a closed tank. The biogas produced here can be used as a fuel directly or it can be used to generate electricity. The digested sludge left after the removal of biogas is taken out, dried and used as a manure. The wastewater treatment gives us two useful products: biogas and sludge. The biogas is used as a fuel whereas sludge (or rather ‘digested sludge’) is used as a manure. The use of dried sludge as a manure returns the nutrients to the soil.


    Untreated human excreta is a health hazard. Explain.


    Majority of people in our country still do not contains sewage facilities for the safe disposal of human excreta. Due to this, a very large number of our people defecate in the open in fields, near railway tracks, on dry river beds and many a time directly in water (in ponds, lakes and rivers etc.) The untreated human excreta is a health hazards because it can cause a large number of diseases and also cause soil pollution and water pollution which can lead to various diseases. Since, the human excreta is carried along by rainwater, both in lakes and rivers as well as groundwater get polluted with it. The river water and groundwater are the sources for performing various purposes for many people.

    The drinking water contaminated with untreated human excreta becomes the most common route for the spreading the diseases.


    Name two chemicals used to disinfect water.


    It may be necessary to disinfect the treated water with chemicals like chlorine or ozone before releasing it into the distribution system. The purpose of disinfection in the treatment of wastewater is to eliminate completely the harmful micro- organisms in the water to be discharged back into the environment. The treated wastewater can also be passed through layers of sand (called sand filters). Sand filtration removes most of the remaining suspended matter from the treated wastewater. It has been suggested that we should plant Eucalyptus trees all along the sewage ponds. This is because eucalyptus trees absorb all the surplus
    wastewater from the sewage ponds rapidly and release pure water vapour into the atmosphere (through transpiration).


    Explain the function of bar screens in a wastewater treatment plant.


    A place where wastewater from houses and other building like industrial area is brought for processing is called wastewater treatment plant. Waste-water treatment plant can be written as WWTP. The wastewater treatment plant is also called sewage treatment plant. In a wastewater treatment plant, bar screen removes large solid components from water. The wastewater is allowed to pass through bar screen so that large solid components like rags, napkins, sticks, cans, plastic bags, polythene etc. present in wastewater can then be removed. These bar screens can then be used as a primary filtration method in the waste water treatment plants.

    The bar screen is a kind of mechanical filter and is used in the waste water treatment plant to remove the bigger and larger impurities or contaminants that leads to pollution like plastics or used cups, etc. After removal of larger impurities, it becomes easy to treat the water to get rid of the smaller impurities or impurities mixed in water with the help of chemical treatment.


    Explain the relationship between sanitation and disease.


    A large number of diseases are caused due to poor sanitation and contaminated drinking water. Poor sanitation leads to the growth of harmful microbes, flies and mosquitoes which are the source of spreading many diseases like cholera, typhoid, polio, meningitis, hepatitis and dysentery. So it’s better to have good sanitation and proper hygiene. Sanitation refers to public health conditions related to clean drinking water and adequate treatment and disposal of human wastes and sewage. Preventing human contact with faeces is part of sanitation, as is hand washing with soap.

    Relationship between Sanitation and Diseases:
    1. In our country, a large number of people even today do not have sewage facilities and thus defecate in open fields, railway, tracks, etc. The untreated human excreta thus pollute soil and water sources including the underground water. When this contaminated water is used for drinking, it can cause diseases such as cholera, typhoid, hepatitis, dysentery, etc. which may even lead to death. Therefore, the lack of sanitation resulting in the drinking of contaminated water can cause health problems.
    2. If sanitation is there, no disease will occur; but if sanitation is not there various types of diseases will occur and spread. So sanitation should be kept to avoid diseases.


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


    The open drains system creates the most unhygienic and unsanitary conditions in its surroundings. Open dirty water drains are breeding places for flies, mosquitoes and other disease causing organisms and bacterias. We can become an active citizen and approach the municipality or we can say Gram panchayat and insist that the open dirty water drains in our area be covered as soon as possible. The covering of open drains will make the environment healthy which will be free from disease causing organisms. If the sewage of a particular house leaks from the pipes, spills on the road and makes the neighbourhood dirty, we should ask the house owner to be more considerate about other’s health and get the leakage of sewage rectified.


    Here is a crossword puzzle: Good luck!


    3. Liquid waste products
    4. Solid waste extracted in sewage treatment
    6. A word related to hygiene
    8. Waste matter discharged from human body

    1. Used water
    2. A pipe carrying sewage
    5. Micro-organisms which causes cholera
    7. A chemical to disinfect water


    3. Liquid waste product- Sewage
    4. Solid waste extracted in sewage treatment- Sludge
    6. A word related to hygiene- Sanitation
    8. Waste matter discharged from human body- Excreta

    Down:                                                                                                                                                                                                                                                  1. Used water- waste water
    2. A pipe carrying sewage- Sewer
    5. Microorganisms which causes cholera- Bacteria
    7. A chemical to disinfect water- Ozone


    Study the following statements about ozone:
    (a) It is essential for breathing of living organisms.
    (b) It is used to disinfect water.
    (c) It absorbs ultraviolet rays.
    (d) Its proportion in air is about 3%.

    Which of these statements are correct?
    (i) (a), (b) and (c)
    (ii) (b) and (c)
    (iii) (a) and (d)
    (iv) All four


    (ii); (b) and (c); It is used to disinfect water and it absorbs ultraviolet rays; this statement is correct.

    Ozone is a powerful oxidising agent which, when dissolved in water, produces a broad spectrum biocide that destroys all bacteria, viruses etc. Ozone water treatment is being used commercially for the treatment of potable water. It absorbs ultraviolet rays or radiations, present in the stratosphere, ozone is created primarily by ultraviolet radiation. Ozone is extremely valuable since it absorbs a range of ultraviolet energy which is very harmful for the earth and our body; and by absorbing these radiations it protects from the damage that could happen to our earth. 

Popular Questions of Class 7 Science

Recently Viewed Questions of Class 7 Science