NCERT Exemplar Problems Class 7 Science – Wastewater Story

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
Answer:
(d) Water Goming out of a laundry is wastewater.

Question 2:
Sewage is mainly a
(a) liquid waste
(b) solid waste
(c) gaseous waste
(d) mixture of solid and gas
Answer:
(a) Sewage is mainly a liquid waste. The waste water and faeces from homes and other buildings which is carried away in sewers, is called sewage.

Question 3:
Which of the following is/are products of wastewater treatment?
(a) Biogas
(b) Sludge
(c) Both biogas and sludge
(d) Aerator
Answer:
(c) Biogas and sludge both are the 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
Answer:
(d) Open drain system is a breeding place for flies, mosquitoes and organisms which cause diseases.

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
Answer:
(c) Asthma is not a water-borne disease. It is caused by polluted air.

Question 6:
Pick from the following one chemical which is used to disinfect water.
(a) Chlorine
(b) Washing soda
(c) Silica
(d) Coal
Answer:
(a) Chlorine is the chemical used as water disinfectant.

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
Answer:
(b) The system of a network of pipes used for taking away wastewater from homes or public buildings to the treatment plant is known as sewerage.

Question 8:
Which of the following is a part of inorganic impurities of the sewage?
(a) Pesticides
(b) Urea
(c) Phosphates
(d) Vegetable waste
Answer:
(c) Phosphate is an inorganic impurity of the sewage.

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
Answer:
(d) In a filtration plant, water is filtered using layers of sand, fine gravel and medium gravel.

Question 10:
Which of the following is not a source of wastewater?
(a) Sewers
(b) Homes
(c) Industries
(d) Hospitals
Answer:
(a) Sewers are not considered as sources of wastewater. Homes, industries and hospitals are considered as a source of wastewater.

Very Short Answer Type Questions

Question 11:
Why open drains is a concern?
Answer:
Open drain is a big concern for the society because they create unhygienic conditions and flies, mosquitoes and other insects breed can spread a number of diseases.

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

1. Sewage is a solid waste which causes water pollution and soil pollution.
2. Used water is wastewater.
3. Wastewater could be reused.
4. Where underground sewerage systems and refuse disposal systems are not available, the high cost onsite sanitation system can be adopted.

Answer:
(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 onsite sanitation system can be adopted.

Question 13:
How are open drains harmful for human health?
Answer:
Open drain system creates the most unhygienic and unsanitary conditions in its neighbourhood. Open dirty water drains are breeding places for flies, mosquitoes and other disease causing organism.

Short Answer Type Questions

Question 14:
Name two inorganic impurities present in sewage.
Answer:
Inorganic impurities present in sewage are nitrates and phosphates.

Question 15:
Animal waste, oil and urea are some of the organic impurities present in sewage. Name two more organic impurities present in sewage.
Answer:
Fruits and vegetable wastes, pesticides and herbicides are organic impurities present in sewage other than animal wastes, oil and urea.

Question 16:
Name two alternative arrangements for sewage disposal where there is no sewerage system.
Answer:
The two alternative arrangements for sewage disposal, where there is no sewerage are as below:
(i) Septic tanks
(ii) Composting pits

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.
Answer:
No, one must always put the waste in a nearby dustbin or carry it home and dispose it in dustbins there.
Waste, not properly disposed may enter into the drains and choke them, it also makes public places dirty and unhygienic.

Question 18:
Why should we not throw
(a) used tea leaves into sink?
(b) cooking oil and fats down the drain?
Answer:
(a) We should not throw used tea leaves into sink because it may choke the drain-pipe of the sink and do not allow the free fall of waste water through them.
(b) We should not throw cooking oil and fats down the drain because cooking oil and fats down the drain as it can harden and block the drainage pipes. And in an open drain, the fats clog the soil pores reducing its effectiveness in filtering water.

Question 19:
Match the Column I with Column II.

Answer:
The correct matching is as given:
(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 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.
Answer:
The correct sequence of wastewater treatment in treatment plant is:
(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, i.e. sludge, sewage and 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.
Answer:
(a) The settled solids that are removed in wastewater treatment with a scraper is sludge.
(b) Water from kitchen which is used for washing dishes is wastewater.
(c) Wastewater released from homes, industries, hospitals and other public buildings is 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)?
Answer:
A mixture of sewage (x) in water contains suspended solids, organic impurities, inorganic impurities like nitrates, phosphates and metals (a), nutrients like phosphorus and nitrogen (b), disease causing bacteria and other microbes.

Long Answer Type Questions

Question 23:
What are the different types of inorganic and organic impurities generally present in sewage?
Answer:
(a) The different types of inorganic and organic impurities present in sewage are as below:
(b) Inorganic impurities – nitrates, phosphates and metals.
(c) 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?
Answer:
The terms like ‘sewage’, ’sewers’ and ‘sewerage’ are interlinked with each other because sewage is a mixture of wastewater coming out of homes and other places.
Sewers are pipes which carry sewage and sewerage is a network of sewage carrying pipes.

Question 25:
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 most common route for… borne diseases like …, dysentery, etc.
Answer:
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.
Groundwater is the source for wells, tubewells and handpumps. Thus, it be’comes 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.
Answer:
The various steps are:
(a) Wastewater is passed through bar screens. Large objects like rags, sticks, cans, plastic packets, napkins are removed.
(b) 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.
(c) The water is then allowed to settle in 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.
(d) 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.
Answer:
We can minimise waste and pollutants entering the water and create less wastewater by taking following few steps at home:
(i) By not throwing used tea leaves, solid food remains, etc. in the drain. We should throw it in the dustbin.
(ii) By not throwing chemicals like medicines, paints, insecticides, etc. in the drain as they increase the pollution load of the sewage.