NCERT Solutions for Class 7 Science Chapter 18 - Wastewater Story

Answer:

Water coming out of a laundry is waste water because it is rich in lather.

Hence, the correct answer is option D.

Answer:

Sewage is mainly a liquid waste. Water constitutes the majority of sewage.

Hence, the correct answer is option A.

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The biogas is produced by decomposition of sludge by the anaerobic bacteria during wastewater treatment. The dried sludge at the end of the process is used as manure, returning organic matter and nutrients to the soil.

Hence, the correct answer is option C.

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Most unhygienic and unsanitary conditions prevail over the open drain system. It is a breeding place for flies, mosquitoes and other organisms which cause diseases.

Hence, the correct answer is option D.

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Asthma is a respiratory disease caused due to air pollution.

Hence, the correct answer is option C.

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Chlorine is used to disinfect treated water before releasing it into the distribution system.

Hence, the correct answer is option A.

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Sewerage is the system of a network of big and small pipes, called sewers, used for taking away wastewater from homes or public buildings to the treatment plant.

Hence, the correct answer is option B.

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Phosphates, nitrates and metals are part of inorganic impurities of the sewage.

Hence, the correct answers is option C.

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In a filtration plant, water is filtered using layers of sand, fine gravel and finally medium gravel.

Hence, the correct answer is option D.

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Sewers are a network of big and small pipes under the ground to carry waste water. They are not the source of waste water.

Hence, the correct answer is option A.

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Open drains produce offensive smell. They are the sites where most unhygienic and unsanitary conditions prevail. As a result, flies, mosquitoes and other insects breed in them giving rise to various water borne diseases. Therefore, open drains are a concern.

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(a) Sewage is a solid waste which causes water pollution and soil pollution. False
Corrected statement: Sewage is a liquid waste which causes water pollution and soil pollution.

(b) Used water is wastewater. True

(c) Wastewater could be reused. True

(d) Where underground sewerage systems and refuse disposal systems are not available, the high cost on-site sanitation system can be adopted. False
Corrected statement: Where underground sewerage systems and refuse disposal systems are not available, the low cost on-site sanitation system can be adopted.

Answer:

Open drains produce offensive smell. They are the sites where most unhygienic and unsanitary conditions prevail. As a result, flies, mosquitoes and other insects breed in them giving rise to various water borne diseases such as cholera, typhoid, polio, meningitis, hepatitis and dysentery.

Answer:

Nitrates and phosphates are two inorganic impurities present in sewage.

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Herbicides and vegetable waste are the other two organic impurities present in sewage.

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Two alternative arrangements for sewage disposal where there is no sewerage system are as follows:

• Low cost onsite sewage disposal systems such as septic tanks and composting pits.
• On-site human waste disposal technology in which excreta from the toilet seats flow through covered drains into a biogas plant.

Answer:

No, it is not a proper waste disposal method. One should always throw the waste in dustbin and should not scatter litter anywhere. One must behave like a responsible citizen and dispose off the waste properly to prevent any disease which may spread due to accumulation of waste in open.

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(a) Used tea leaves choke the drains and do not allow free flow of oxygen. This affects the degradation process. So, we should not thrown them in sinks.

(b) Cooking oil and fats can harden and block the pipes. The effectiveness of an open drain in filtering water can get reduced because the fats clog the drain's soil pores. So, they should not be thrown down the drain.

Answer:

Answer:

The correct sequence of the processes involved in a wastewater 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.

 

Answer:

Sludge: The settled solids that are removed in wastewater treatment with a scraper.

Sewage: Wastewater released from homes, industries, hospitals and other public buildings.

Wastewater: Water from kitchen used for washing dishes.

Answer:

(x) is sewage

(a) are nitrates, phosphates and metals

(b) are nitrogen and phosphorus

Answer:

The different types of inorganic impurities generally present in sewage are nitrates, phosphates and metals while organic impurities are human faeces, animal waste, oil, urea (urine), pesticides, herbicides, fruit and vegetable waste, etc.

Answer:

Sewage is wastewater released by homes, industries, hospitals, offices, rainwater that has run down the street during a storm or heavy rain. Sewers are a network of small and big pipes under the ground carrying wastewater from homes and public buildings. Sewerage is the overall transport system of sewers that carries sewage from the points of being produced, i.e. homes or public buildings to the point of disposal, i.e. treatment plant

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 ground water get polluted. Ground water is the source for wells, tubewells and handpumps. Thus it becomes the most common route for water borne diseases like cholera, dysentery, etc.

Answer:

In wastewater treatment plant, wastewater is treated through physical, chemical, and biological processes to remove all the impurities. The various steps involved in cleaning wastewater are described below:

• Firstly,  large objects such as sticks, plastic packets, cans and napkins are removed from wastewater by passing it through bar screens.
• Then, water is sent to a grit and sand removal tank where the speed of the incoming wastewater is decreased to allow sand, grit and pebbles to settle down.
• Subsequently, 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 known as sludge.
• Floatable solids such as oil and grease are removed with a skimmer. The cleared water so obtained is called clarified water.
• The sludge is transferred to a separate tank where it is decomposed by the anaerobic bacteria to produce biogas.
• Air is then pumped into the clarified water to help in the growth of aerobic bacteria which consume human waste, food waste, soaps and other unwanted substances still remaining in clarified water.
• After several hours, the suspended microorganisms settle at the bottom of the tank as activated sludge (about 97% water).
• The water is then removed from the top by sand drying beds or machines.
• The treated water, so obtained, has a very low level of organic material and suspended matter and it is discharged into a river , a sea or into the ground. Sometimes it is disinfected with chlorine and ozone before releasing it into the distribution system.

Answer:

The ways to minimise waste and pollutants at home are as follows:

• Cooking oil and fats can harden and block the pipes. The effectives of an open drain in filtering water gets reduced because the fats clog the soil pores of the drain. So, they should not be thrown down the drain. 
• Chemicals such as paints, insecticides, medicines may kill microorganisms that help in purifying water. Therefore, they should also not be thrown down the drain.
• Used tealeaves, soft toys, cotton, leftover food and sanitary towels should be thrown in the dustbin because these wastes choke the drains and do not allow free flow of oxygen which affects the degradation process.