NCERT Exemplar for Class 7 Science Solutions Chapter 18 Wastewater Story

Multiple Choice Questions

1. Which of the following is waste water?

(a) Water trickling from a damaged tap

(b) Water coming out of a shower

(c) Water flowing in a river

(d) Water coming out of a laundry

Ans: The correct option is (d) water coming out of a laundry. Discharged water during household activities or in industries is considered to be waste water.

2. Sewage is mainly a

(a) Liquid waste

(b) Solid waste

(c) Gaseous waste

(d) Mixture of solid and gas

Ans: The correct option is (a) liquid waste. Sewage is discharged water from various sources and contains impurities.

3. Which of the following is/are products of wastewater treatment?

(a) Biogas

(b) Sludge

(c) Both biogas and sludge

(d) Aerator

Ans: The correct option is (c) Both biogas and sludge. Biogas and sludge go through wastewater treatment to remove the impurities from them.

4. Open drain system is a breeding place for which of the following-

(a) Flies

(b) Mosquitoes

(c) Organisms which cause diseases

(d) All of these

Ans: The correct option is (d) All of these.

5. Water polluted by various human activities causes a number of water borne diseases. Which of the following is not a water borne disease?

(a) Cholera

(b) Typhoid

(c) Asthma

(d) Dysentery

Ans: The correct option is (c) Asthma. Asthma is a respiratory disorder in which a patient is allergic to allergens present in air like pollen grains or dust.

6. Pick from the following one chemical used to disinfect water.

(a) Chlorine

(b) Washing soda

(c) Silica

(d) Coal

Ans: The correct option is (a) Chlorine. Chlorine destroys harmful microorganisms in water, such as bacteria and viruses.

7. The system of a network of pipes used for taking away wastewater from homes or public buildings to the treatment plant is known as

(a) Sewers

(b) Sewerage

(c) Transport system

(d) Treatment plant

Ans: The correct option is (b) sewerage. Sewerage is a system that is used in buildings to discharge wastewater into a sewage system. It is a system of pipes known as sewers that are linked together.

8. Which of the following is a part of inorganic impurities of the sewage?

(a) Pesticides

(b) Urea

(c) Phosphates

(d) Vegetable waste

Ans: The correct option is (c) phosphates. Phosphate or phosphorus is a metallic impurity in the sewage.

9. In a filtration plant water is filtered using layers of

(a) Sand and clay

(b) Clay and fine gravel

(c) Sand and fine gravel

(d) Sand, fine gravel and medium gravel

Ans: The correct option is (d) sand, fine gravel and medium gravel. In wastewater treatment, discharged or waste water is made to enter a grit and sand removal tank, where the speed of wastewater is reduced to allow sand, grit and pebbles to settle down.

10. Which of the following is not a source of wastewater?

(a) Sewers

(b) Homes

(c) Industries

(d) Hospitals

Ans: The correct option is sewers. Sewers are the pipes via which waste water runs into the sewage, whereas wastewater is collected in the sewage from households, businesses, and hospitals.

Very Short Answer Questions

11. Why are open drains a concern?

Ans: Mosquitoes and flies mostly breed in dirty water or areas and in open water. Open drains are filthy and contain garbage, making them ideal breeding grounds for mosquitoes and flies.

12. State whether the following statements are “True or False”. In case a statement is false, write the correct statement.

(a) Sewage is a solid waste which causes water pollution and soil pollution

Ans: The given statement is False. Sewage is a liquid waste, not a solid garbage, and it pollutes the water and soil.

(b) Used water is wastewater

Ans: The given statement is true.

(c) Wastewater could be reused

Ans: The given statement is true.

(d) Where underground sewerage systems and refuse disposal systems are not available, the high cost on-site sanitation system can be adopted

Ans: The given statement is False. The low cost on-site sanitation systems can be adopted when the underground sewerage systems and refuse disposal systems are unavailable.

13. How are open drains harmful for human health?

Ans: Open drains harmful for human health

i. Open drains have a foul smell and by that they cause air pollution.

ii. Mosquitoes and flies mostly breed in dirty water or areas and in open water.

iii. Humans are infected with water-borne diseases such as cholera, typhoid, and diarrhoea, as open drains infect clean water.

Short Answer Questions

14. Name two inorganic impurities present in sewage.

Ans: Inorganic impurities present in sewage are

i. Nitrate

ii. Phosphate

15. Animal waste, oil and urea are some of the organic impurities present in sewage. Name two more organic impurities present in sewage.

Ans: Organic impurities present in sewage are

i. Vegetable and fruit peels waste

ii. Pesticides

16. Name two alternative arrangements for sewage disposal where there is no sewerage system.

Ans: Where there is no sewerage system, there are two alternatives for sewage disposal-

i. The low cost on-site sanitation systems can be adopted like a septic tank.

ii. Composting toilets- It is a dry toilet. Excreta, which contains no water or odour, is decomposed aerobically in this procedure.

17. A man travelling in a train threw an empty packet of food on the platform. Do you think this is a proper waste disposal method? Elaborate.

Ans: No, this is not an acceptable way of trash disposal. He should have tossed the packet in the nonbiodegradable waste bin because food packets are typically made of nonbiodegradable plastic.

18. Why should we not throw-

(a) Used tea leaves in the sink?

Ans: On throwing used tea leaves into the sink would not allow free flow of oxygen and would cause the degradation process as it chokes the drain.

(b) Cooking oil and fats down the drain?

Ans: When fats and cooking oils cool, they become rigid. So, by throwing these down the drain would block the drain pipe. And in open drains these would hamper the pores of the soil and deduct the efficiency of soil in filtering water.

19. Match the items of Column I with the items of Column II with reference to sewage.

Ans: 

20. Given below is a jumbled sequence of the processes involved in a wastewater treatment plant. Arrange them in their correct sequence.

(a) Sludge is scraped out and skimmer removes the floating grease.

(b) Water is made to settle in a large tank with a slope in the middle.

(c) Large objects like plastic bags are removed by passing wastewater through bar screens.

(d) Sand, grit and pebbles are made to settle by decreasing the speed of incoming wastewater.

(e) Wastewater enters a grit and sand removal tank.

Ans: The correct sequence is-

1-(c) Large objects like plastic bags are removed by passing wastewater through bar screens.

2-(e) Wastewater enters a grit and sand removal tank.

3-(d) Sand, grit and pebbles are made to settle by decreasing the speed of incoming wastewater.

4-(b) Water is made to settle in a large tank with a slope in the middle.

5-(a) Sludge is scraped out and skimmer removes the floating grease.

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.

(a) The settled solids that are removed in wastewater treatment with a scraper.

(b) Water from the kitchen used for washing dishes.

(c) Wastewater released from homes, industries, hospitals and other public buildings.

Ans: Correct definitions of given terms are-

(a) Sludge- The settled solids that are removed in wastewater treatment with a scraper.

(b) Sewage- Wastewater released from homes, industries, hospitals and other public buildings.

(c) Wastewater- Water from the kitchen used for washing dishes.

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)?

Ans: The names are-

x is wastewater or sewage.

(a) is nitrogen or phosphorus

(b) is Vibrio comma or Salmonella typhi.

Long Answer Questions

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

Ans: In water, the most common types of pollutants are organic and inorganic. The organic are biodegradable products such as human faeces, oil, and animal waste like excreta and urea from the urine, pesticides. Metals, phosphate, nitrate, and other non-biodegradable materials are examples of inorganic products.

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

Ans: Because sewage is a mixture of wastewater coming out of homes and other places, phrases like 'sewage, ‘sewers,' and 'sewerage' are interlinked with each other. Suspended solids, organic and inorganic pollutants, nutrients, saprotrophic and disease-causing bacteria, and other microbes make up this complicated mixture. Sewerage is similar to a transportation system that transports sewage from the place of production to the point of disposal, such as a treatment plant. Sewers are a network of large and small pipes.

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

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, tubewells and

_________. Thus it becomes the most common route for _________ borne diseases like _________, dysentery, etc.

Ans: A very large number of our people defecate in the open. It may cause water

pollution and soil pollution. Both the surface water and groundwater get polluted.

Ground water is the source for wells, tube-wells and hand-pumps. Thus it becomes the

most common route for water borne diseases like cholera, dysentery, etc.

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

Ans: The steps performed for cleaning the wastewater before it is released into the water bodies is known as wastewater treatment plant. The steps are-

i. A bar screen is placed through which the wastewater is passed to eradicate impurities from it like sticks, rags, cans, plastic packets, etc.

ii. After that, a grit is added after the bar screen, through which the filtered water is passed for additional filtering of sand, grit, and stones.

iii. Now, the water is passed to a large tank which is sloppy towards the middle and impurities like faeces settle down and later are removed with a scraper. Sludge is the byproduct of the wastewater treatment process.

iv. The sludge is further treated in a bio-gas plant for the production of bio-gas. The sludge had oils and fats remaining in it which are removed by skimmer.

v. Clear water is the treated product that has been pumped with air to prevent the growth of aerobic bacteria. Soap, human faeces, and any other undesired material are consumed by the air. It is kept undisturbed for several hours for the microbes to settle down at the bottom of the tank as activated sludge. The water is removed from the top.

vi. The water is dumped into a sea, river, or the ground once it has been properly treated.

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

Ans: We can take following measures to minimize pollutants and waste water at their sources

i. Plastic bags should be outlawed or used sparingly. Cotton or jute bags can be used instead.

ii. Fruit peels, tea leaves, and vegetables, as well as their peels, should all be composted in the kitchen garden.

iii. Reuse old newspaper, polythene bags, jars.

iv. Biodegradable and non-biodegradable waste should be thrown in separate dustbins.

v. Follow the 3R rule that is reduce, recycle and reuse to have a healthy environment.

About Class 7 Science Chapter 18

The chapter throws light on the wastewater management system in our country. We in our daily activities make a lot of dirty water. The chapter tells us how this water is treated to make it useable again and how we should become active citizens and maintain healthy hygiene in our surroundings.