Class 7 Science Chapter 13 Question Answers - Wastewater story

Q1. Open drain is a big concern now-a-days. Justify.
Ans: Open drain is a big concern for the society now-a-days because they create unhygienic conditions. It provides favourable conditions to flies, mosquitoes and other insects to breed and spread a number of diseases.

Q2 Sewage also contains some nutrients Comment.
Ans: Sewage is a complex mixture which contains suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease causing bacteria and other microbes.
The nutrients present in sewage are phosphorus and nitrogen.

Q3. Recall and enlist some better house keeping practices.
Ans: We must minimise and manage waste at our houses before its disposal in the following manner:

• 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.
• Used tealeaves, 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.
• The chemicals like paints, solvents, insecticides, medicines and motor oils should 1 State the role of screen bars in wastewater treatments, not be thrown in drains as they kill helpful 2 Chlorination makes water fit for human consumption. How? microbes which digest the organic waste.

Q4. With reference to the treatment of polluted water at home by aeration, filtration, chlorination processes answer the following questions.
(a) What changes did we observe in the appearance of liquid after aeration? Did aeration change the odour?
(b) What is removed by the sand filter?
(c) Does chlorine change the colour of treated water?
(d) Do chlorine have an odour?
Ans:
(a) Aerated water contains some suspended impurities and the foul odour of the polluted water disappears completely after aeration.
(b) Sand filter removes tiny suspended impurities.
(c) Chlorine makes the water clear and colourless.
(d) Yes, chlorine have a peculiar odour which is not worse than wastewater.

Q5. Explain, how sewage is disposed of in an aeroplane.
Ans: Aeroplanes have their closed waste sewage tanks in them, which suck the wastewater and collect it in their tanks.
Once the aeroplane lands on the ground, the crew disposed of the sewage properly into airport sewage facility.

Q6. Suggest two alternative arrangements for sewage disposal where there is no sewerage system.
Ans: The two alternative arrangements for sewage disposal, where there is no sewerage system are as below

• Septic tanks
• Composting pits.

Q7. Describe onsite disposal systems.
Ans: Some organisations offer hygienic onsite 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.

Q8. Can we contribute in sanitation at public places? Explain.
Ans: Yes, we all have a role to play in keeping our environment clean and healthy. All of us 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.

Q9. Explain the different types of inorganic and organic impurities generally present in sewage.
Ans: It is wastewater released by homes, industries agricultural fields and other human activities. It also includes rainwater that has run down the street during a storm or heavy rain and it is a liquid waste. Most of its water has dissolved and suspended impurities called contaminants.
Composition of Sewage
The following components make the sewage:

• The organic impurities present in sewage are human faeces, animal wastes (like animal dung), urea (as urine), oil, fruits and vegetable wastes, pesticides, herbicides, etc.
• The inorganic impurities present in sewage are nitrates, phosphates and metals.
• The nutrients present in sewage are nitrogen and phosphorus.
• The bacteria present in sewage include those bacteria which cause water-borne diseases such as cholera and typhoid.
• The other microbes present in sewage are Protozoa which cause a water-borne disease called dysentery.

Q10. Discuss vermi-processing toilets. Name the useful product obtained from such toilets.
Ans: In the vermi-processing toilet, the human excreta is treated by earthworms in a pit. The earthworms gradually eat up all the organic matter present in human excreta, decompose it and pass out from their body in the form of warm casting (also called vermi cakes).
The useful product obtained from a vermi-processing toilet is vermi cakes which is kind of high quality manure.

Q11. Describe septic tank. How does it function?
Ans: Low cost outside sewage disposal system have been developed to take care of places where there is no sewage system, e.g. rural areas, isolated buildings. These are described below:
(i) Septic tanks: Septic tank is a low cost onsite sewage disposal system. Septic tanks are suitable where there is no sewerage made. These tanks need cleaning every four to six months.

• A septic tank usually consists of a big, covered underground tank made of concrete having an inlet pipe at one end and on outlet pipe at the other end. The toilet seat is connected to the inlet pipe of the septic tank. The human excreta from the toilet seat enters into the septic tank through the inlet pipe. The solid part of excreta keeps on collecting at the bottom of the septic tank in the form of a sludge whereas watery waste remains above it.
• The anaerobic bacteria breakdown most of the solid organic matter present in human excreta due to which the volume of solid waste is reduced too much. The digested solid waste keeps on depositing at the bottom of septic tank. The watery waste is also cleaned by anaerobic bacteria. The excess water goes out of the septic tank through the outlet pipe and get absorbed in soil.

(ii) Composting pits: These are self sustained human waste disposal units which is not connected to a sewer line or a septic tank. A composting toilet breaks down and dehydrate human waste to compost.
(iii) Chemical toilets: These toilets have limited storage capacity for human waste and need to be emptied periodically. These are the toilets which use chemically treated reservoir located just below the toilet seats. The chemicals reduce the foul smell coming out of human excrete and carry out partial disinfection of human waste.

Q12. Think and suggest some ways to minimise waste and pollutants at their source, taking your home as an example.
Ans: We can minimise waste and pollutants entering the water and create less wastewater by taking following few steps at home:

• By not throwing used tea leaves, solid food remains, etc., in the drain. We should throw it in the dustbin.
• By not throwing chemicals like medicines, paints, insecticides, etc., in the drain as they increase the pollution load of the sewage.

Q13. What is meant by sewerage?
Ans: Sewerage is a network of big and small pipes called sewage placed under the ground. It can be considered as a transport system that carries wastewater from the point where it is produced to the point of disposal.

Q14. What is clarified water. What can it be used for?
Ans: Clarified water is the water we get from bar screening, sedimentation, aeration and skimming of the wastewater. It can be used for watering plants, flushing toilets and cleaning public places.

Q15. What should be done to wastewater?
Ans: Wastewater should not be wasted. It must be cleaned up by removing pollutants.

Q16. How does water get polluted?
Ans: Water is used for numerous purpose in homes, industries, agriculture, etc. It gets polluted when it is used for dying, bathing, flushing toilets, etc. During these activities; many unwanted chemicals, organic and inorganic substances get mixed with water. This pollutes water.

Q17. Define potable water. Write any two characteristics of potable water.
Ans: Water that is fit for drinking is called potable water. Its characteristics are following:

• It is odourless and colourless.
• It is transparent.
• It is free from germs and harmful chemicals.

Q18. List some sources of water pollution.
Ans:

• Discharge of sewage
• Agricultural wastes
• Domestic wastes
• Industrial wastes

Q19. How do bar screen, and grit and sand removal tank help in cleaning of water?
Ans: Bar screen removes large objects like rags, cans, plastic packets, etc., from sewage. In grit and sand removal tank heavy solid impurities like pebbles, etc., are removed by sedimentation process.

Q20. Write one word for the following:
(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.
Ans:
(a) sludge
(b) wastewater
(c) sewage

Q21. What is function of a septic tank?
Ans: A septic tank consists of a big covered underground tank made of concrete. It also consists of an inlet pipe at one end and an outlet pipe at the other end. The toilet seat is connected to the inlet pipe. The human excreta enters the tank through the inlet pipe, the solid portion of the excreta gets collected at the bottom of the tank in the form of sludge whereas the liquid waste gets collected above it.
The anaerobic bacteria break down most of the solid wastes owing to which the volume of solid reduces to a great extent. The liquid waste is also decomposed by the bacteria. Bacteria deposit the digested solid at the bottom of the septic tank. The excess water goes out of the septic tank through the outlet pipe and is absorbed by the soil.

Q22. List various steps involved in waste water treatment.
Ans: Treatment of wastewater involves physical, chemical, and biological processes, which remove physical, chemical and biological matter that contaminates the wastewater.

• First wastewater is passed through bar screens. Large objects like rags, sticks, cans, plastic packets, napkins are removed.
• Water than 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.
• 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 settled solid is called 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.
• 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 contains about 97% water which 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 now, has a very low level of organic material and suspended matter. It is discharged into a sea, or 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.

Q23. What is vermi-processing toilet? Explain.
Ans: Vermi-processing toilet is a type of toilet in which the human excreta is processed by earthworms. This has been tested in India. It has proved to be a low-water use, nobel method for disposal of human ex¬creta. Toilet is operated in a very simple and hygienic way. The human excreta is converted into vermi- cakes. Hence it becomes a resource much needed for the soil.

Q24. How can we minimise the discharge of pollutants at their source?
Ans: To minimise or eliminate waste and pollutants at their source we must see what we 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. We should throw oil and fats in the dustbin.
• Chemicals like paints, solvents, insecticides, motor oil, medicines may kill microbes that help purify water. So we should 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.

Q25. Name various components of sewage.
Ans: Sewage is a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease-causing bacteria and other microbes.

• Organic impurities: Human faeces, animal waste, oil, urea (urine), pesticides, herbicides, fruit and vegetable waste, etc.
• Inorganic impurities: Nitrates, phosphates, metals.
• Nutrients: Phosphorus and nitrogen.
• Bacteria: Such as which causes cholera, typhoid, dysentery, etc.