Class 7 Science Chapter 18 Important Questions

Class 7 Science Chapter 18 Important Questions Answers of Wastewater Story for academic session 2021-2022 free to use based on latest NCERT Books. Class VII Science Extra Questions are important to clear the doubt related to chapter 18.

All the answers are brief and easy to understand for a student of standard 7 in CBSE and State board.

Class 7 Science Chapter 18 Important Questions for 2021-2022

Class: 7Science
Chapter: 18Waste Water Story
Contents:Extra Questions with Answers

Class 7 Science Chapter 18 Important Questions Set – 1

What do you mean by “wastewater”?

Clean water is a basic necessity of our life. We use clean water every-day for drinking, bathing, washing clothes, cooking food, washing utensils, and flushing toilets etc. When we use water for various purposes which make the water dirty. The water which is rich in lather, mixed with oil, black– brown water that goes down the drains from sinks, showers, toilets, laundries used water called wastewater.

Why did the United Nations proclaim the period 2005–2015 as the International Decade for action on “Water for life”?

Clean water is a basic need of human being. The used water should not be wasted. We must clean it up by removing pollutants. Clean water that is fit for use is unfortunately not available to all. It has been reported that more than one billion of our fellow human beings have no access to safe drinking water. This accounts for a large number of water-related diseases and even deaths. Women and girls walk for several kilometers to collect clean water.
The increasing scarcity of fresh- water due to population growth, pollution, industrial development, mismanagement and other factors. Realizing the urgency of the situation on the World Water Day, on 22 March 2005, the General Assembly of the United Nations proclaimed the period 2005–2015 as the International Decade for action on “Water for life”. All efforts made during this decade aim to reduce by half the number of people who do not have access to safe drinking water.

What do you mean by “water cleaning”?

Cleaning of water is a process of removing pollutants before it enters a water body or is reused. This process of wastewater treatment is commonly known as “sewage treatment”. It takes place in several stages.

With the help of an activity, how would you study the sewage route in your home, school or building?
Sewage is a liquid waste. Sewage is wastewater released by homes, industries, hospitals, offices and other users. We should study the sewage route by the followings:

    • i) Make a line diagram of the sewage route.
    • ii) Walk down the street or survey the campus to find the number of manholes.
    • iii) Follow an open drain and find out where it ends and which living organisms are found in and around it.




What do you mean by sewage, sewers and sewerage?

Sewage is a liquid waste. Sewage is wastewater released by homes, industries, hospitals, offices and other users. It also includes rainwater that has run down the street during a storm or heavy rain. The water that washes off roads and rooftops carry harmful substances with it.
In a home or a public building generally one set of pipes brings clean water and another set of pipes takes away wastewater, called sewers, forming the sewerage.
Sewerage is like a transport system that carries sewage from the point of being produced to the point of disposal, i.e., treatment plant. In other words, sewerage refers to the physical facilities or provision (e.g., pipes, lift stations, and treatment and disposal facilities) through which sewage flows.

Class 7 Science Chapter 18 Important Questions Set – 2

Write down the components which are present in wastewater or sewage.

We know that sewage is a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease-causing bacteria and other microbes.
i) Organic impurities as – Human faeces, animal waste, oil, urea (urine), pesticides, herbicides, fruit and vegetable waste, etc.
ii) Inorganic impurities as – Nitrates, Phosphates, metals.
iii) Nutrients as – Phosphorus and Nitrogen.
iv) Bacteria – Such as which cause cholera and typhoid.
v) Other microbes – Such as which cause dysentery.

What is sludge? Explain, how it is treated.

The waste water passes into a sedimentation tank, in which organic material settle down and are drawn for disposal. This organic matter is then converted to methane, carbon dioxide and a humus like material by a process called digestion. This digested sludge is placed on sand beds for air drying. Dried sludge can be used as a fertilizer.

Describe the various steps involved in waste water treatment.

Treatment of wastewater involves physical, chemical, and biological processes, which remove physical, chemical and biological matter that contaminates the wastewater.
1. Wastewater is passed through bar screens. Large objects like rags, sticks, cans, plastic packets, napkins are removed.
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.
3. 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 sludge. A skimmer removes the floatable solids like oil and grease. Water so cleared is called clarified water.
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.
4. 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.
After several hours, the suspended microbes settle at the bottom of the tank as activated sludge. The water is then removed from the top.
The activated sludge is about 97% water. The water is removed by sand drying beds or machines. Dried sludge is used as manure, returning organic matter and nutrients to the soil. The treated water has a very low level of organic material and suspended matter. It is discharged into a sea, a river or into the ground. 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.

Describe various house-keeping practices for efficient working of sewage system.
    • One of the ways to minimize or eliminate waste and pollutants at their source is to see what you are releasing down the drain.
    • Cooking oil 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. Throw oil and fats in the dustbin.
    • Chemicals like paints, solvents, insecticides, motor oil, medicines may kill microbes that help purify water. So do not throw them down the drain.
    • Used tea-leaves, solid food remains, soft toys, cotton, sanitary towels, etc. should also be thrown in the dustbin. These wastes choke the drains. They do not allow free flow of oxygen. This hampers the degradation process.




With the help of an activity, discuss the process of water treatment plant.

    • Fill a large glass jar 3/4 full of water. Add some dirty organic matter such as grass pieces or orange peels, a small amount of detergent, and a few drops of an ink or any color.
    • Cap the jar, shake it well and let the mixture stand in the sun for two days. After two days, shake the mixture and pour a small sample into test tube. Label this test tube “Before treatment; Sample 1”.
    • Use an aerator from an aquarium to bubble air through the sample in the glass jar. Allow several hours for aeration; leave the aerator attached overnight. If you do not have an aerator, use a mechanical stirrer or a mixer. You may have to stir it several times. The next day when aeration is complete, pour another sample into a second test tube. Label it as “After aeration; Sample 2”.
    • Fold a piece of filter paper to form a cone. Wet the paper with tap water and then insert the cone in a funnel. Mount the funnel on a support. Place layers of sand, fine gravel and finally medium gravel in the funnel. (An actual filtration plant does not use filter paper, but the sand filter is several meters deep).
    • Pour the remaining aerated liquid through the filter into the beakers. Do not allow the liquid to spill over the filter. If the filtered liquid is not clear, filter it a few times till you get clear water. Pour a sample of the filtered water into a third test tube labelled “Filtered; Sample 3”.
    • Pour another sample of the filtered water into a fourth test tube. Add a small piece of a chlorine tablet. Mix well until the water is clear. Label the test tube “Chlorinated; Sample 4”. Observe carefully the samples in all the test tubes. Do not taste!

Class 7 Science Chapter 18 Important Questions Set – 3

Are you familiar with the word “vacuum toilet”? Discuss about it.

Yes, the aeroplanes have a special type of toilet called “vacuum toilets”. So, the human excreta or sewage in an aeroplane is disposed of through vacuum toilets. When we flush the toilet in an aeroplane, then a valve opens in a pipeline which has vacuum inside it. The vacuum in the pipeline sucks the human excreta from the toilet seat with a great force making a loud noise and carries it into a tank on board the aeroplane. When the aeroplane lands at an airport, then the tank containing all the human excreta is emptied into a sewer line into the ground through connecting pipes.

What do you mean by “chemical toilet”?

A chemical toilet uses a chemically treated reservoir located directly below the toilet seat. The chemicals reduce the odor or foul smell coming from the human waste and carry out partial disinfection of the human waste. Chemical toilets have a limited storage capacity for human waste, so their reservoirs need to be emptied into a sewer line after a certain time. All the portable toilets are chemical toilets. They can be shifted from one place to another. Chemical toilets are used at construction site and at a large outdoor gathering such as music, festivals and marriage etc.

Write short notes on low-cost onsite sewage disposal systems.

To improve sanitation, low-cost onsite sewage disposal systems are being encouraged. Examples are septic tanks, chemical toilets, composting pits. Septic tanks are suitable for places where there is no sewerage system, for hospitals, isolated buildings or a cluster of 4 to 5 houses. Some organizations offer hygienic on-site human waste disposal technology. These toilets do not require scavenging. Excreta from the toilet seats flow through covered drains into a biogas plant. The biogas produced is used as a source of energy.

What is your role as an active citizen towards waste generation? Discuss.

Waste generation is a natural part of human activity. But you can limit the type of waste and quantity of waste produced. Often, you have been repelled by offensive smell. The sight of open drains is disgusting. The situation worsens in the rainy season when the drains start overflowing. You have to wade through the mud pools on the roads. Most unhygienic and unsanitary conditions prevail. Flies, mosquitoes and other insects breed in it.
You can be an enlightened citizen and approach the municipality or the gram panchayat. Insist that the open drains be covered. If the sewage of any particular house makes the neighborhood dirty, you should request them to be more considerate about others’ health.




How can we contribute in maintain sanitation at public places?
In our country fairs are organized periodically. A large number of people participate in them. In the same way railway stations, bus depots, airports, hospitals are very busy places. Thousands of people visit them daily. Large amount of waste is generated here. It must be disposed of properly otherwise epidemics could break out. We can contribute in maintaining sanitation at public places by observing some simple practices such as:

    1. We should never urinate on the road side.
    2. After eating a banana, we should not throw banana peels on the road side.
    3. We should not spit in public places.
    4. We should not scatter rubbish such as paper, food waste, packets etc., in public places. The rubbish should be put in the dustbins provided for this purpose. If, dustbin is not available at a public place, the rubbish should be carried home by us and thrown in the dustbin.

Class 7 Science Chapter 18 Important Questions Set – 4

Explain the relationship between sanitation and disease.

Poor sanitation and contaminated drinking water is the cause of a large number of diseases. A vast number of our people are still without sewerage facilities. A very large fraction of our people defecates in the open, on dry riverbeds, on railway tracks, near fields and many a time directly in water. Untreated human excreta is a health hazard. It may cause water pollution and soil pollution. Both the surface water and groundwater get polluted. Groundwater 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.

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

Oil and fats can clog the drinks reducing its effectiveness in filtering water. In open drains, they block the pores in the soil and thus hamper the filtration by soil. Hence, oil and fats should not be released in the drain.

“We should plant eucalyptus trees all along sewage ponds”. Why?

It has been suggested that we should plant eucalyptus trees all along sewage ponds. These trees absorb all surplus wastewater rapidly and release pure water vapor into the atmosphere.

Discuss, how sewage is harmful to people and the environment.
The sewage or wastewater released by houses and other buildings contains a larger number of harmful substances and disease-causing microorganisms. If this untreated sewage is discharged into rivers, lakes or ground, it will contaminate our fresh water sources (like rivers, lakes and groundwater). Drinking of water contaminated with sewage will spread diseases in people which can even lead to death.
The discharge of untreated sewage into rivers and sea, can also kill fish and other aquatic animals. Since, it is very harmful, untreated sewage should not be allowed to run into any water source or underground. Sewage water should be treated suitably or processed before releasing it back to the environment. So that, it reduces the pollutants or harmful substances present in it. And a low level of contaminants that it may not remain a health hazard for anyone.




Write any three benefits of vermi processing toilet?

A design of a toilet in which humans’ excreta is treated by earthworms has been tested in India, known as vermi processing toilet.

    • i) It has been found to be a novel, low water-use toilet for safe processing of human waste.
    • ii) The operation of the toilet is very simple and hygienic.
    • iii) The human excreta is completely converted to vermi cakes — a resource much needed for soil.

Class 7 Science Chapter 18 Important Questions Set – 5

Why is untreated human excreta a health hazard?

Untreated human excreta contain many pathogens and disease-causing microbes like cholera, diarrhea, typhoid, polio, meningitis, hepatitis etc. The germs of these diseases can easily reach human population through air, water or insects and through contaminated food and water. Thus, untreated human excreta are a health hazard.

What you have learnt from this chapter? Discuss.

We all have a role to play in keeping our environment clean and healthy. We must realise our responsibility in maintaining the water sources in a healthy state. Adopting good sanitation practices should be our way of life. As an agent of change our individual initiative will make a great difference. Influence others with our energy, ideas and optimism. A lot can be done if people work together. There is great power in collective action.

Name two water disinfectants used during treatment of waste water.

The two water disinfectants used during treatment of waste water are chlorine and ozone.

“Water in a river is clean naturally”. Do you agree with this statement? Explain it.
Yes, river water is clean naturally by a process that is similar to the process used in wastewater treatment plant. As muddy water flows through grass or weeds on its way to a stream, mud and solid particles get filtered out. At the bottom of a lake or stream, micro-organisms bring chemical changes in the water. The natural filtration process removes pollution from the groundwater throughout the process making it clean and fit for drinking.

Think and suggest some ways to minimize waste and pollutants at their source, taking your home as an example.
We can minimize 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.