NCERT Exemplar Class 7 Science Chapter 18 Wastewater Story

NCERT Exemplar Class 7 Science Chapter 18 Wastewater Story are part of NCERT Exemplar Class 7 Science. Here we have given NCERT Exemplar Class 7 Science Solutions Chapter 18 Wastewater Story.

NCERT Exemplar Class 7 Science Solutions Chapter 18 Wastewater Story

Multiple Choice Questions

Question 1.
Which of the following is wastewater?
(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.
Solution:
(d) : Water discharged during daily chores-like washing clothes, flushing toilet, washing dishes, bathing etc., becomes dirty. This dirty water is called wastewater. Water trickling from a damaged tap, water coming out of a shower and water flowing in a river are not dirty and hence it is not a wastewater.

Question 2.
Sewage is mainly a
(a) liquid waste
(b) solid waste
(c) gaseous waste
(d) mixture of solid and gas.
Solution:
(a) Sewage is a wastewater that includes domestic or industrial liquid wastes. It contains various dissolved and suspended impurities.

Question 3.
Which of the following is/are products of wastewater treatment?
(a) Biogas
(b) Sludge
(c) Both biogas and sludge
(d) Aerator
Solution:
(c) The sewage is treated to destroy the germs and reduce the contaminants. In wastewater treatment, wastewater is passed through sedimentation tank, in which organic materials settle down and are removed with scraper. This is called sludge. In separate tank, sludge is acted upon by anaerobic bacteria which produces methane, carbon dioxide, and biogas by process known as digestion. Therefore, both sludge and biogas are products of wastewater treatment.

Question 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
Solution:
(d) In some places there are open drains to dispose off the sewage. These drains can easily get blocked by plastic bags, and can overflow. Therefore, the problems of smell, breeding of mosquitoes, flies and germs are encountered in open drains.

Question 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) Dysentry
Solution:
(c) Asthma is an air borne respiratory disease. It is caused due to an allergy to dust, pollen grains or any other allergen present in air.

Question 6.
Pick from the following one chemical used to disinfect water.
(a) Chlorine
(b) Washing soda
(c) Silica
(d) Coal
Solution:
(a) During wastewater treatment, disinfectant like chlorine is added to the water to kill harmful microorganisms.

Question 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.
Solution:
(b) Sewage goes through a system of pipes, called sewers, which are linked to each other to form a network called sewerage system.

Question 8.
Which of the following is a part of inorganic impurities of the sewage?
(a) Pesticides
(b) Urea
(c) Phosphates
(d) Vegetable waste
Solution:
(c) Various metals and phosphates are inorganic impurities of sewage, while pesticides, urea and vegetable wastes are organic impurities.

Question 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.
Solution:
(d) In wastewater treatment, waste¬water is allowed to enter a grit and sand removal tank. Here, the speed of wastewater is reduced in order to allow sand, grit and pebbles to settle down.

Question 10.
Which of the following is not a source of waste water?
(a) Sewers
(b) Homes
(c) Industries
(d) Hospitals
Solution:
(a) Sewers are the pipes through which wastewater generated from homes, industries and hospitals flows. They are not the source of wastewater.

Very Short Answer Type Questions

Question 11.
Why are open drains a concern?
Solution:
Drains that are not covered may get blocked with solid wastes like plastic bags, rags and gravel. Such drains are flooded with dirty water that stagnates and are polluted emitting foul smell and provides breeding grounds to various microbes. Mosquitoes lay eggs and breed causing the spread of diseases like malaria and dengue. Sewage wastes also carry germs that cause serious waterborne diseases like cholera, typhoid, dysentery and jaundice.

Question 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.
(b) Used water is wastewater.
(c) Wastewater could be reused.
(d) Where underground sewerage systems and refuse disposal systems are not available, the high-cost on-site sanitation system can be adopted.
Solution:
(a) False – Sewage is a liquid waste which causes water pollution and soil pollution.
(b) True
(c) True
(d) False – Where underground sewerage systems and refuse disposal systems are not available, the low cost on site sanitation systems can be adopted.

Question 13.
How are open drains harmful for human health?
Solution:
Refer to answer 11.

Short Answer Type Questions

Question 14.
Name two inorganic impurities present in sewage.
Solution:
Two inorganic impurities present in sewage are phosphate and nitrate.

Question 15.
Animal waste, oil and urea are some of the organic impurities present in sewage. Name two more organic impurities present in sewage.
Solution:
Pesticides and fruits and vegetable wastes are organic impurities present in sewage.

Question 16.
Name two alternative arrangements for sewage disposal where there is no sewerage system.
Solution:
Two alternative arrangements for sewage disposal where there is no sewerage system are:
(i) Septic tanks: In septic tank, the sewage is allowed to flow into a tank in which anaerobic bacteria decompose the waste. The septic tank acts as sedimentation-cum-digestion tank. Anaerobic (in the absence of air) digestion of the settled sludge occurs in its bottom zone, and the remaining liquid has to undergo treatment in a soak pit/filter bed. This can be done in areas where the water table is very low and the rainwater from the surface also does not interfere with their functioning. Septic tanks are suitable for places where there is no sewerage system, for hospitals, isolated buildings or a cluster of 4 to 5 houses.
(ii) Composting toilets: The composting toilet is a dry toilet that uses a predominantly aerobic processing system that treats excreta, typically with no water or small volumes of flush water, by aerobic decomposition.

Question 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.
Solution:
Throwing an empty packet of food on the platform is not a proper waste disposal method. One must always put the waste in a nearby dustbin or carry it until a proper place to dispose it off is found. Waste, if not disposed properly would litter the floor and may go into the drains and choke them, it make public places dirty and unhygienic, and would spread many diseases.

Question 18.
Why should we not throw
(a) used tea leaves into sink?
(b) cooking oil and fats down the drain?
Solution:
(a) Used tea leaves should not be thrown in kitchen sinks as they clog drains and prevent free flow of oxygen that interferes with the decomposition process. They should be thrown in dustbins.
(b) Cooking oil and fats should not be disposed off down the drains as these get harden and block pipes. They reduce the flow of water and results in stagnant water, which provides breeding ground for mosquitoes and flies and spreads infectious diseases.

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

Solution:
(a) (ii)
(b) (iv)
(c) (i)
(d) (iii)

Question 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.
Solution:
The correct sequence of processes involved in wastewater treatment plant are as follows:
(c) Large objects like plastic bags are removed by passing wastewater through bar screens,
(e) Wastewater enters a grit and sand removal tank.
(d) Sand, grit and pebbles are made to settle by decreasing the speed of incoming wastewater.
(b) Water is made to settle in a large tank with a slope in the middle.
(a) Sludge is scraped out and skimmer removes the floating grease.

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.
(a) The settled solids that are removed in wastewater treatment with a scraper.
(b) Water from kitchen used for washing dishes.
(c) Wastewater released from homes, industries, hospitals and other public buildings.
Solution:
(a) The settled solids that are removed in wastewater treatment with a scraper – sludge
(b) Water from kitchen used for washing dishes – wastewater
(c) Wastewater released from homes, industries, hospitals and other public buildings – 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)?
Solution:
(x) – sewage
(a) – nitrates, phosphates and metals
(b) – phosphorus and nitrogen

Long Answer Type Questions

Question 23.
What are the different types of inorganic and organic impurities generally present in sewage?
Solution:
Wastewater is generated in homes, industries, hospitals, hotels, offices and at other places. Domestic wastewater contains urine,’ faeces, detergent and dirt released after washing clothes and utensils, and food wastes. Industrial wastewater contains poisonous chemicals. Wastewater from hospitals may contain several types of disease-causing germs, and dangerous radioactive materials (material that emit harmful rays). Rainwater that has run down the land also forms wastewater as it contains fertilisers, pesticides from agricultural fields. All this wastewater forms sewage. Sewage contains dissolved and suspended impurities known as contaminants. Various inorganic and organic impurities present in sewage are as follows:
(i) Inorganic impurities – nitrates, phosphates and metals
(ii) Organic impurities – fruit and vegetable wastes, oil, urea, human faeces, animal waste, pesticides and herbicides.

Question 24.
The terms sewage, sewers and sewerage are interlinked with each other. Can you explain, how?
Solution:
Sewage is wastewater that flows down the drains of houses, industries, hospitals, offices, fields, etc. Rainwater that runs down the streets during storm or heavy rain also adds to the sewage. This is because rainwater falls on rooftops and roads and carries harmful substances with it. The dissolved and suspended solid impurities present in sewage are called contaminants. They include organic and inorganic impurities, nutrients and disease-causing bacteria and other microbes. Sewage goes through a system of pipes, called sewers. Sewer is an underground pipe for carrying off drainage water and waste matter. Sewers are linked to each other to form a network called sewerage system.

Question 25.
Fill in the blanks in the following statements using words given in the box.
[ air, handpumps, 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.
Solution:
large, water, ground, ground, hand-pumps, water, cholera

Question 26.
Describe various steps of cleaning wastewater in a wastewater treatment plant.
Solution:
Wastewater treatment is the process of removing contaminants from wastewater to get clean, drinking water. It involves removal of physical, chemical and biological contaminants through series of steps like filtration, sedimentation and microbial degradation. The various steps involved in this method are:
(i) Preliminary treatment: First of all, wastewater is passed through bar screens which remove large solid materials like plastics, cans, rags. Wastewater is then passed through grit and sand removal tank, where the speed of wastewater is reduced to allow sand, silt and gravel to settle down.
(ii) Primary treatment: Wastewater is then made to pass through sedimentation tanks where organic matter like faeces settle down at the bottom. This is removed with the help of a scraper. Floatable solids like oil and grease are removed with skimmer. The water so obtained is clarified water.
(iii) Secondary treatment: This is a biological process, where the organic matter is broken down by the action of natural decomposers like bacteria to release biogas, which can be used as fuel or to produce electricity. This is referred to as activated sludge system. Bacteria, protozoans and worms feed on the organic matter, which tends to form clumps of humus-like material that settle at the bottom.
(iv) Final cleansing and disinfection: Water is removed from the top. Compressed air is passed through it and disinfectants such as chlorine and ozone gas are added to kill the microorganisms. This treated water is finally released into natural water bodies like lakes and rivers.

Question 27.
Think and suggest some ways to minimise waste and pollutants at their source, taking your home as an example.
Solution:
A lot of the waste that we generate at home is thrown into the sink or flushed down the toilet. This adds to the sewage. Sewage is generally treated before being disposed off in a water body. The same water body may be the source of water supply to the town. A few simple steps at home can minimize the strain imposed on the water supply system in cleaning the water for our use.
(i) Food, especially Indian food contains lot of fats and oils. The leftover food items should not be thrown down the drain because they can choke the pipes when they harden. These when thrown in open drains block the soil pores, thereby decreasing its effectiveness to filter water. Therefore, such waste should be thrown in dustbins only.
(ii) Solid wastes such as used tea leaves, food remains, plastic bags, soft toys, cotton and sanitary towels should not be thrown down the drains. Such wastes block the drains. They reduce free flow of oxygen and thus, slow down the process of degradation.
(iii) Various chemical products such as medicines, motor oils, paints, grease, machine oils and insecticides may kill water purifying microorganisms and hamper the degradation process. These should not be thrown down the drain.

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