NCERT Class 7 Science Eighteenth Chapter Wastewater Story Exercise Solutions

NCERT Class 7 Science Eighteenth Chapter Wastewater Story Exercise Solutions

Wastewater Story

What is called wastewater?

Ans. Rich in lather, mixed with oil, black– brown water that goes down the drains from sinks, showers, toilets, laundries is dirty. It is called wastewater.

What is called “Sewage Treatment”?

Ans. 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 Clean water put to use Drinking 2018-19 WASTEWATER STORY 221 known as “Sewage Treatment”.

Give some example of Organic impurities?

Ans. –Human faeces, animal waste, oil, urea (urine), pesticides, herbicides, fruit and vegetable waste, etc.

Give some example of Inorganic impurities?

Ans. Nitrates, Phosphates, metals.

Give some example of Nutrients?

Ans. Phosphorus and Nitrogen.

Give some example of Bacteria?

Ans. Such as vibrio cholera which causes cholera and salmonella paratyphi which causes typhoid.

What is sludge?

Ans. Solids like faeces settle at the bottom and are removed with a scraper. This is the sludge.

In which year Government of India has initiated Swachh Bharat mission.

Ans. 2006.

Define sewage.

Ans. Wastewater is generated in homes, industries, agricultural fields and in other human activities. This is called sewage.

Exercise Questions with Answers:

(1) Fill in the blanks:

(a) Cleaning of water is a process of removing ____________.

Ans. Pollutants.

(b) Wastewater released by houses is called ____________.

Ans. Sewage.

(c) Dried ____________ is used as manure.

Ans. Sludge

(d) Drains get blocked by ____________ and ____________.

Ans. Chemicals, Kitchen waste.

(2) What is sewage? Explain why it is harmful to discharge untreated sewage into rivers or seas.

Ans. Wastewater is generated in homes, industries, agricultural fields and in other human activities. This is called sewage.

It is harmful because sewage is a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprophytes and disease causing bacteria and other microbes.

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

Ans. 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.

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

Ans. The steps involved in getting clarified water from wastewater are following:

Step 1. Wastewater is passed through bar screens. Large objects like rags, sticks, cans, plastic packets, napkins are removed.

Step 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.

Step 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 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.

Step 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. Sometimes it may be necessary to disinfect water with chemicals like chlorine and ozone before releasing it into the distribution system.

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

Ans. Solids like faeces settle at the bottom and are removed with a scraper. This is the sludge.

How its treated:

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.

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

Ans. Untreated human excreta is a health hazard. It may cause water pollution and soil pollution.  Groundwater is a source of water for wells, tubewells, springs and many rivers. Thus, it becomes the most common route for water borne diseases. They include cholera, typhoid, polio, meningitis, hepatitis and dysentery.

(7) Name two chemicals used to disinfect water.

Ans. Clorine and Ozone.

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

Ans. Large objects like rags, sticks, cans, plastic packets, napkins are removed through bar screen remover.

(9) Explain the relationship between sanitation and disease.

Ans. Poor sanitation and contaminated drinking water is the cause of a large number of disease. Adopting good sanitation practices should be our way of life.

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

Ans. I. Railway stations, bus depots, airports, hospitals are very busy places. Thousands of people visit them daily. Large amount of waste is generated here. It must be disposed of properly otherwise epidemics could break out.

  1. We should not scatter litter anywhere.

III. If there is no dustbin in sight, we should carry the litter home and throw it in the dustbin.

(12) 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

Ans. (ii) (b) and (c).

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