These NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.
Wastewater Story NCERT Solutions for Class 7 Science Chapter 18
Class 7 Science Chapter 18 Wastewater Story Textbook Exercise Questions and Answers
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 ……………….
d. oils, fats
What is sewage? Explain why it is harmful to discharge untreated sewage into rivers or seas.
Water containing liquid wastes disposed from domestic household activities, industrial activities and agricultural activities is termed as sewage.
The untreated sewage can harm the rivers and seas as the contaminated water pollutes the whole source of water. As the sewage water may contain harmful substances and disease causing organisms, it is dangerous to release the untreated sewage into the water bodies.
Why should oils and fats be not released in the drain? Explain.
Oils and fats can block the drains. In open drains, they block the pores in the soil and thus hamper the filtration by soil. Hence, oils and fats should not be released in the drains.
Describe the steps involved in getting clarified water from wastewater.
The following steps are involved in clarifying wastewater:
- In the first step, all the physical impurities like stones, plastic bags, cans, etc., are removed by passing the water through bar screens.
- Now, water is subjected to sedimentation where impurities are removed by the force of gravity.
- Solids settled at the bottom of the sedimentation tank are removed. This is called sludge.
- An aerator cleanses the water from other impurities.
- Chlorination is done to remove all disease causing bacteria.
What is sludge? Explain how it is treated.
The solid waste which settles at the bottom of the sedimentation tank is called sludge. The sludge is further treated by the process of aeration. This helps in growth of bacteria which eat away the human waste. Activated sludge is used as manure.
Untreated human excreta is a health hazard. Explain.
Untreated human excreta contains many microbes which can cause deadly diseases like cholera, diarrhoea, jaundice and typhoid. The germs of these diseases can easily reach human population through insects and through contaminated food and water. Thus, untreated human excreta is a health hazard.
Name two chemicals used to disinfect water.
Bleaching powder and chlorine tablets.
Explain the function of bar screens in a wastewater treatment plant.
Bar screens are large filters which are placed in the form of vertical screens. Large solid wastes like sticks, pebbles, sand, cans, plastic bottles, plastic bags, etc., are filtered out when the wastewater moves through the bar screens.
Explain the relationship between sanitation and disease.
Proper sanitation should be maintained in order to avoid diseases. They are related to each other. By sanitation, proper disposal of sewage and refuse from household and public places is carried out. Implementation of proper sanitation results in a state where we are free from any diseases. Lack of it may lead to various diseases.
Outline your role as an active citizen in relation to sanitation.
As an active citizen, we should take care of our personal environmental sanitation. For this, we should follow these steps:
- Throwing garbage at designated places and in garbage bins.
- Preventing clogging of drains by not releasing oils and fats and solid wastes into the drains.
- Using a dustbin to throw trash at public places.
- By not spitting at public places.
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 humans
1. Used water
2. A pipe carrying sewage
5. Micro-organisms which cause cholera
7. A chemical to disinfect water
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)
NCERT Extended Learning Activities and Projects
Then and now: Talk to your grandparents and other elderly people in the neighbourhood. Find out the sewage disposal systems available to them. You can also write letters to people living in far off places to get more information. Prepare a brief report on the information you collected.
Do it yourself
Visit a sewage treatment plant.
It could be as exciting and enriching as a visit to a zoo, a museum or a park. To guide your observation here are a few suggestions.
Record in your notepad:
Name of the official at the plant ………….. Guide/
a. The location of the sewage plant.
b. Treatment capacity.
c. The purpose of screening as the initial process.
d. How is air bubbled through the aeration tank?
e. How safe is the water at the end of the treatment? How is it tested?
f. Where is the water discharged after treatment?
g. What happens to the plant during heavy rains?
h. Is biogas consumed within the plant or sold to the other consumers?
i. What happens to treated sludge?
j. Is there any special effort to protect nearby houses from the plant?
k. Other observations.
Do it yourself with the help of your teacher.
Objective: To locate an open drain near your home, school or on the road side and inspect water flowing through it.
Procedure: Visit the nearby drain and observe it carefully.
|Type of sewage
|Point of origin
|Substances which contaminate
|Fruits and vegetable waste, oil
|Human feces, urine
|Industrial and commercial organisations
|Nitrates, phosphates of metals
Conclusion: The open drain water is dirty and has a foul smell. Many organic impurities are found there such as human faeces, animal wastes, fruit and vegetable wastes, etc.
Wastewater Treatment Plant (WWTP) or Sewage Treatment Plant: These are large plants where wastewater is cleaned before being sent to the nearest water bodies or being reused. The sewage treatment involves physical, chemical and biological processes to remove impurities from the wastewater. This is done in the following steps:
i. Wastewater is passed through bar screens to remove large objects such as rags, cans, plastic packets, polythene, etc.
ii. Sand and Grit removal tank: The speed of incoming wastewater in this tank is decreased to allow sand, grit, pebbles, etc., to sediment and settle down.
iii. The water then goes into a large tank sloping in the middle where solids, like faeces, settle at the bottom and are removed with a scraper. This solid is called the sludge.
A skimmer removes the floatable solids like oil and grease. Water so cleared is called the clarified water.
iv. Anaerobic bacteria: Bacteria which can grow and multiply in absence of free oxygen are called anaerobic bacteria. The sludge is transferred to a separate tank for decomposition by anaerobic bacteria.
The gas produced by the decomposition of sludge by the anaerobic bacteria is known as biogas. The biogas produced during decomposition can be used as fuel or used to produce electricity.
v. Role of aerobic bacteria: The clarified water also needs to be cleaned further. Hence, it is moved to an aerator that pumps air into it. This allows aerobic bacteria to grow in this water. Bacteria which require free oxygen for their growth and multiplication are called aerobic bacteria. The aerobic bacteria consume organic wastes, soaps, food waste and other elements that remain in the clarified water.
vi. Activated sludge: After several hours of decomposition in the tank, the microbes settle down at the bottom as activated sludge. It contains about 97% water. The water is then removed from the top by sand drying beds or machines. The treated water with very low amount of suspended matter is discharged into water bodies where it is further cleaned by nature. Water can also be disinfected with chemicals like chlorine UV radiations and ozone before releasing it into the distribution system.
Objective: To treat a sample of polluted water and make it clean.
Materials Required: Funnel, a glass jar, filter paper, funnel stand, soil, grass pieces, orange peels, detergent, ink, strainer and beaker.
- Fill a glass jar three-fourths with tap water. Add a little of soil, some dirty organic matter such as grass pieces and orange peels, a small amount of detergent and a few drops of ink to it. Cap the glass jar, shake it well and keep it in the sun for at least two days.
- Shake the contents of the glass jar well and pour the content over a strainer kept over a beaker. The large pieces of grass and orange peels will not pass through the holes of strainer and hence get removed. The remaining part of the polluted water will collect in the beaker kept below the strainer.
- Allow the polluted water to stand in the beaker for some time. The solid fine soil present in the polluted water will settle down at the bottom of the beaker.
- Transfer the polluted water from this beaker into another beaker by decantation. Pass air into this polluted water for several hours by using an aerator.
- When the aeration is complete, we will find that the foul smell from polluted water has almost disappeared. Filter the aerated water through the layer of sand, fine gravel and medium gravel arranged on a filter paper in a funnel. Collect the clean water as a filtrate in a beaker kept below the funnel.
- Add a small piece of chlorine tablet to the clean water collected in the beaker. Mix well until the water is clear and colourless. This is the clean and clear safe water obtained by the treatment of polluted water.
|Raw sample (before treatment)
Conclusion: With this treatment, we can clean polluted water. This treated water does not give foul smell and it is safe to use.
Sanitation: Sanitation refers to the health and hygienic conditions of people related to the disposal of sewage, human excreta and provision of clean drinking water. Poor sanitation and polluted drinking water can lead to many diseases. Hence, a sanitation system aims to provide a clean environment for us so that we can stay away from diseases. Improper sanitation can give rise to different diseases that arise from contaminated water such as typhoid, dysentery, hepatitis, polio, cholera and meningitis. The human excreta is a health hazard and can lead to soil and water pollution. It can pollute the surface water and groundwater. Hence, people should never defecate in open areas.
Alternative Arrangement for Sewage Disposal:
i. In the absence of a sewerage system, arrangements for onsite sewage disposal can be made. For example, septic tanks are built in which human excreta are collected. In due course of time, the human excreta gets decomposed into compost.
ii. Composting pits can be made to dump waste and to make manure from them. Sewage can be collected into biogas plants to produce useful biogas.
Chemical toilets are new discoveries. They do not require much water for the disposal of human excreta and are environment friendly. Such toilets are ideal for the trains.
Sanitation at Public Places: There are numerous places in our country which are very busy such as airports, railway stations and bus depots. Many times, large gatherings such as fairs and festivals are also organised where a large number of people visit. This results in the generation of large amounts of waste and if the waste is not treated properly, it can lead to several diseases and even an epidemic. Hence, we should all become aware and contribute to maintaining sanitation not only in our homes but also at public places. If we adopt certain practices at public places we can help in maintaining their sanitation, for instance:
- We should not throw garbage in public places and always use the dustbins.
- We should not spit around in a public place.
- We should wash our hands thoroughly before eating anything at a public place.
Class 7 Science Chapter 18 Wastewater Story Additional Important Questions and Answers
Very Short Answer Type Questions
Which day is celebrated as World Water Day?
What is sewerage?
It is like a transport system that carries sewage from the point of being produced to the point of disposal, i.e., treatment plant.
What are the harmful effects of drinking contaminated water?
Contaminated drinking water is the cause of a large number of diseases such as cholera, typhoid, polio, meningitis, hepatitis and dysentery.
What is cleaning of water?
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”.
Why chemicals should not be thrown down the drain?
Chemicals should not be thrown down the drain because chemicals like paints, solvents, insecticides, motor oil and medicines may kill microbes that help purify water.
Name two nutrients present in the sewage?
Nitrogen and phosphorus.
Name the solid waste extracted during sewage treatment.
Name any two things that can be made from the sludge extracted during treatment of sewage.
Biogas and manure.
Write the examples of on site sewage disposal systems.
Septic tanks, chemical toilets, composting pits, etc.
Name the toilet in which human excreta is treated by earthworms.
What is the use of dried sludge?
Dried sludge is used as manure, thus returning organic matter and nutrients to the soil.
How can we improve the sanitation?
To improve sanitation, low cost onsite sewage disposal systems must be encouraged.
Why are ozone and chlorine used in water purification?
Ozone and chlorine are used to kill the bacteria, etc., present in the clarified water.
How are floatable solids removed from the wastewater?
A skimmer is used to remove floatable impurities.
Which microorganism decomposes the sludge?
Anaerobic bacteria decompose the sludge.
Short Answer Type Questions
Define community hygiene? How can it be effectively maintained?
Community hygiene means maintenance, protection and improvement of health of the whole community. It can be maintained by:
- Provision of clean and safe drinking water.
- Maintaining green open space.
- Maintaining proper sanitation.
- Maintaining clean air.
What are the problems arising due to open drains and other unsanitary conditions?
Open drains and unsanitary conditions produce bad smell. It becomes an idle place of breeding for mosquitoes, flies and other harmful insects. These insects spread many harmful diseases and other health hazards.
How kitchen waste blocks the drain?
Cooking oils that drain from kitchen into the pipes, hardens and blocks the pipes. The oil in open drains clogs the soil pores thereby preventing the filtration process and its effectiveness. It further hampers the degradation process.
How bar screen and grit and sand removal tank help in clarification of water?
When wastewater is passed through bar screens, it separates big and large objects like rags, sticks, cans, plastic packets, napkins, etc. In grit and sand removal tank, other solid impurities like pebbles, sand, etc., are removed.
What is vermi-processing toilet?
Vermi-processing toilet is a new toilet system developed in India. In this system, human excreta is treated by earthworms for safe processing of human waste. The operation of this toilet is very simple and hygienic. It has the potential to reduce freshwater use and improve sanitation levels.
What is the need to chlorinate any water supply? Name two other methods which help to do the same.
For making water safe for drinking, chlorination can be done. In this process, chlorine tablets, bleaching powder or alum is added to water to prevent it from diseases. Filtration or sedimentation can also be done in order to make the water safe for drinking.
Why is air pumped into the clarified water?
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 the clarified water.
Why should eucalyptus trees be planted along sewage ponds?
It has been suggested that we should plant eucalyptus trees all along the sewage ponds. These trees absorb all surplus wastewater rapidly and release pure water vapour into the atmosphere.
Why are bacteria important in sewage treatment?
Bacteria are important in sewage treatment because, they consume human waste, food waste, soaps and other unwanted matter still remaining in clarified water. Also, they help in the decomposition of solid sludge to make useful things like biogas.
Suggest some alternative arrangements for sewage disposal.
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 organisations 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.
Long Answer Type Questions
What are the different types of inorganic and organic impurities generally present in sewage?
Wastewater is composed primarily of natural organic substances which are by-products of human, animal and plant processes. The primary inorganic elements in domestic wastewater are nitrogen, phosphorous, ammonia and carbon. These elements are released with the growth of bacteria in standing water. The bacteria consume the oxygen present in water and as a result, living organisms in the water start to die.
We know that sewage is a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease causing bacteria and other microbes.
|Human faeces, animal waste oil, urea (urine)
|Nitrates, phosphates, metals
|Phosphorus and nitrogen
|Such as which cause cholera and typhoid
|Such as which cause dysentery
List some ways to minimise or eliminate waste and pollutants at their source.
One of me ways to minimise or eliminate waste and pollutants at their source is to see what you are releasing down the drain. Other ways may include:
- 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 and medicines may kill the microbes that help purify water. So we should not throw them into the drain.
- Used tea-leaves, solid food remains, soft toys, cotton, sanitary towels, etc., should also be thrown in the dustbin as these wastes choke the drains. They do not allow free flow of oxygen which hampers the degradation process.
Write some consequences of the malfunctioning of sewage.
- Rupture or, blockage or leakage of sewerage lines.
- Sewerage may overflow on the roads and streets, creating poor sanitary conditions.
- It may upset the ecological balance in lakes and rivers.
- Water may become unfit for drinking and other domestic uses.
- The place may stink and become breeding ground for mosquitoes, flies, etc.
What is a septic tank? How does it function?
Septic tank is a low cost on-site sewage disposal system. Septic tanks are suitable where there is no sewerage system for hospitals, isolated buildings or a cluster of four to five houses. Septic tank needs cleaning every four to six months.
Septic tanks serve three functions:
a. Removal of solids: As sewage enters the septic tank, its rate of flow is reduced so that the larger solids sink to the bottom and soaps, grease and smaller solids rise to the surface. These solids are retained in the tank and the clarified effluent with suspended and dissolved solids is discharged.
b. Bacterial Action: The solids and the liquids in the tank are partially decomposed by bacteria and other natural processes. These bacteria are called anaerobic because they thrive in the absence of free oxygen. This decomposition of sewage under anaerobic conditions is termed “septic,” hence the name of the system (and the cause of the odour).
c. Sludge and scum storage: Sludge is the accumulation of solids at the bottom of the tank, while scum is a partially submerged mat of floating solids that may form at or near the surface. Space must exist in the tank to store these residues during the intervals between pumping. Otherwise, the sludge and scum will eventually be scoured from the tank and will clog the leaching field and receiving soil.
The final Stage of Disposal:
The treated effluent from the septic tank is discharged to the leach field where it percolates through suitable “septic stone” and finally into the subsoil for further purification.
Observe the diagram shown below and answer the following questions.
a. What does it depict?
b. Which layers are used in the funnel?
a. It depicts purification of polluted water by filtration.
b. Layers of sand, silt and gravel are used in the funnel.
Identify the diagram shown below.
Sewage treatment plant