Chapter 18. Wastewater Story : Science | CBSE Class VII (7th) | Lesson Exercises
1.Question : Fill in the blanks:
Answer :
(a) Cleaning of water is a process of removing pollutants.
(b) Wastewater released by houses is called sewage.
(c) Dried _sludge is used as manure.
(d) Drains get blocked by tealeaves, solid food remains, soft toys,cotton, and sanitary towels .
2. Question : What is sewage? Explain why it is harmful to discharge untreated sewage into rivers or seas.
Answer : 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 carries harmful substances with it. Sewage is a liquid waste which has dissolved and suspended impurities as contaminants or pollutants . If this untreated sewage is dischrged into rivers or seas,It may cause water pollution and soil pollution in which both the surface water and groundwater get polluted. Groundwater is a source of water for wells, tubewells, springs and many rivers. Therefor if it gets polluted, it becomes the most common route for water borne diseases. They include cholera, typhoid, polio, meningitis, hepatitis and dysentery.
3. Question : Why should oils and fats be not released in the drain? Explain.
Answer : 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. Also they may kill microbes that help purify water.Therefor oil and fats should be always discharged after taking due care in the dustbin or if possible in some suitable dumping place
4. Question : Describe the steps involved in getting clarified water from wastewater.
Answer : The steps involved in getting clarified water from 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 through sedimentation .
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 by sedimentation 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.
5. Question : What is sludge? Explain how it is treated.
Answer : This process of removing pollutants from wastewater is commonly known as wastewater treatment” or “Sewage Treatment” . It takes place in several stages. Sludge is one of the By-products of wastewater treatment along with biogas.
During the “Sewage Treatment”, after filterations of floating solid objects through bar screen, the speed of wastewater is decreased to allow sand, grit and pebbles to settledown .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.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.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. Dried sludge is used as manure, returning organic matter and nutrients to the soil.
6. Question : Untreated human excreta is a health hazard. Explain.
Answer : Untreated human excreta is a health hazard because as this will result in poor sanitation. 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. It may cause water pollution and soil pollution. Both the surface water and groundwater get polluted. Groundwater is a source of water for wells, tubewells, springs and many rivers due to this, it becomes the most common carrier for water borne diseases. They include cholera, typhoid, polio, meningitis, hepatitis and dysentery
7. Question : Name two chemicals used to disinfect water.
Answer : In the last stage of “wastewater Treatment”, 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.
8. Question : Explain the function of bar screens in a wastewater treatment plant.
Answer : The function of bar screens in a wastewater treatment plant is to remove coarse solids and other large materials often found in raw wastewater. Removal of these materials is necessary to enhance the operation and maintenance of subsequent treatment units. Preminary treatment operations typically include coarse screening, grit removal and, in some cases, communication of large objects. Wastewater is passed through bar screens. Large objects like rags, sticks, cans, plastic packets, napkins are removed
9. Question : Explain the relationship between sanitation and disease.
Answer : There is a direct relationship between sanitation and disease. Poor sanitation and contaminated drinking water is the cause of a large number of diseases. Under poor sanitation people may resort to defecate in the open, on dry riverbeds, on railway tracks, near fields and many a time directly in water. It may cause water pollution and soil pollution. Both the surface water and groundwater get polluted. Groundwater is a source of water for wells, tubewells, springs and many rivers due to this, it becomes the most common carrier for water borne diseases. They include cholera, typhoid, polio, meningitis, hepatitis and dysentery
10.Question : Outline your role as an active citizen in relation to sanitation.
Answer : We can contribute in maintaining sanitation at public places.
We should not scatter litter anywhere. If there is no dustbin in sight, we should carry the litter home and throw it in the dustbin.
We must realise our responsibility in maintaining the water sources in a healthy state.
We should Adopt good sanitation practices as way of life.
A small initiative as an agent of change on our part will make a great difference
11. Question : Here is a crossword puzzle: Good luck!
Across
3. Liquid waste products
4. Solid waste extracted in sewage treatment
6. A word related to hygiene
8. Waste matter discharged from human body
Down
1. Used water
2. A pipe carrying sewage
5. Micro-organisms which causes cholera
7. A chemical to disinfect water
12. Question : 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
Answer : In progress...
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CBSE Class VII (7th) Science Lesson Exercises | Chapter 18. Wastewater Story
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“By providing clean water and sanitation to the poorest people on the
planet, we can reduce poverty and suffering and ensure education for all
children.”—UNICEF
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An early engineering feat: Indus valley civilisation
One of the ancient civilisations, Harappa and Mohenjodaro had perhaps
the world’s first urban sanitation system. Within the city individual
houses, or groups of houses, obtained water from wells. There was a
separate room for bathing, and wastewater was directed to the covered
drains which lined the major streets. The oldest toilet made of bricks is
about 4500 years old.
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