Study Materials

NCERT Solutions for Class 7th Science

 

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Chapter 18. Wastewater Story

NCERT Exercise

 

 

 

Exercise


1. 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 ____________.

Answer:

(a) Contaminants  (b) Sewage  (c) Sludge  (d) plastic, sludge.

2. 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 streets during a storm or heavy rain.

The water that washes off roads and rooftops carries harmful substance with it. Sewage is a dissolved and suspended impurities which are called contminnts.

That is why, it is harmful to discharge untreated sewage into rivers to sea. 

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

Answer:

Oil does not mix with water. Being lighter, oil floats on water. This prevents mixing of water and air. This reduses oxygen supply for aqutic plants and animals.

Alongwith, in an open drain the fats clog the soil pores reducing its effectiveness in filtering water.   

4. Describe the steps involved in getting clarified water from wastewater.

Answer:

Treatment of wastewater involves physical, chemical, biological process, which remove physical, chemical, biological matter that contaminates the water.

Physical: 

(i) Waste water is passed through bar screens. Large objects like rags, cans, plastic, packets, napkins are removed.

(ii) Water 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.

Biological:

(i)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 removed with scaper. This is the sludge. 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 anearobic bacteria.

(ii) Air pumped into the clerified water to help aerobic bacteria to grow.

Bacteria consume human waste, food waste, soap 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.

Chemical: 

Something it may be necessary to disinfected water with the chemicals like chlorine and ozone before releasing it into the distribution system.    

5. What is sludge? Explain how it is treated.

Answer:

During physical treatment, solids like feaces settle at the bottom of the the large tank. This is called sludge.

Treatment of sludge:

(i) Sludge are first removed from the bottom using a sraper.

(ii) Sludge is then transferred to a seprate tank. Where it is decomposed by the anearobic bacteria.

The biogas prodused in the process can be used as fuel.

6. Untreated human excreta is a health hazard. Explain.

Answer:

Untreated human excreta is a health hazard. it may cause water pollution and soil pollution. Both the surface water and groundwater is a source of water for wells, tubewells, springs and many rivers. Thus, it becomes the most common route for water-botne diseases. They include cholera, typhoid, polio, meningits, hepatitis and dysentery.  

7. Name two chemicals used to disinfect water.

Answer:

Ozone, chlorine.

8. Explain the function of bar screens in a wastewater treatment plant.

Answer:

Wastewater is passed through bar screens to remove large objects like rags, sticks, cans, plastic packets, napkins.

9. Explain the relationship between sanitation and disease.

Answer:

Untreated human excreta is a health hazard. Improper sanitation may cause water pollution and soil pollution. Both the surface water and groundwater get polluted. Groundwater is a souce of water for wells, tube wells, springs and many rivers. Thus, it becomes the most common route for water- borne disease. They include cholera, typhoid, polio meningitis, hepatits and dysentery.

That is why, proper sanitation is must to avoid some of the deadliest disease.  

10. Outline your role as an active citizen in relation to sanitation.

Answer:

We all have a role to play in keeping our enviorment clean and healthy. We must realise our responsiblty in maintaning the water source in a healthy state. Adopting good sanitation practices should be our way of life. As an agent of chance our individual intiative will make a great difference. We can influence others with our energy, ideas and optimies. A lot can be done if people work together. There is great power in collective action.

11. 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:

(ii) (b) and (c)

 

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