Saturday, September 8, 2012

Class 7 - Science - CH18 - Wastewater Story

Wastewater Story
Potable Water is Precious
(credits: openclipart)
(NCERT Chapter Solutions and Other Q & A)

Q1: Fill in the blanks:


(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 _cooking oil__ and _fats__.

(e) Wastewater is treated in a sewage treatment plant.

(f) Septic tanks, chemical toilets, composting pits are the examples of low cost sewage disposal systems.

Q2: What is sewage? Explain why it is harmful to discharge untreated sewage into rivers or seas.
Waste water discharge kills aquatic life
image credits: Moini

Answer: Sewage is a liquid waste. It consists of waste water from houses, offices, factories, hostpitals etc. It has a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease causing bacteria and other microbes.

If the untreated sewage is discharged into rivers and seas, it will pollute the water resources. It will also harm the aquatic life and spread many diseases like  cholera, typhoid, polio, meningitis, malaria, dengue etc.

Q3: 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.

Oil and fats should be thrown in the dustbin.

Q4: Fill up the mindmap to show different uses of clean water.


Q5: Which day is celebrated as World Water Day?

Answer: March 22nd is celebrated as World Water Day

Q6: What are different types of contaminants or impurities present in sewage?


Q7: Describe the steps involved in getting clarified water from wastewater.

Answer: Treatment of wastewater involves physical, chemical, and biological processes, which remove physical, chemical and biological matter that contaminates the wastewater.

Following steps are involved:
1. Bar Screens: Wastewater is passed through bar screens which remove large objects like rags,
sticks, cans, plastic packets, napkins etc.

2. Grit and Sand Removal: Water then goes through Grit and Sand removal tank, where sand, grit and pebbles settle down.

3. 1st Clarifier Tank: Water is then allowed to settle in a tank where solids like faeces (called sludge) settle at the bottom and are removed with a scraper. A skimmer removes the floatable solids
like oil and grease. Water so cleared is called clarified water. Sludge is used to produce biogas.

4. Aerator: Clarified water is then passed through an aerator tank where air is pumped into the water. It helps aerobic bacteria to grow which decompose organic matter like human waster.

5. 2nd Clarifier Tank: 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 dried activated sludge is used as manure.

6. Chlorination: Water is removed from the top and is stored in a tank. The treated water has low level of organic material and suspended matter. It is discharged into a sea, a river or into the ground. Sometimes it is disinfected through chemicals like Chlorine and ozone and then distributed to towns.

Bird View of  Water Treatment Plant
Q8:  What is sludge? Explain how it is treated.

Answer: Sludge is human waste (like faeces) in waste water left during sewage treatment. Since it is organic waste, it is used to produce bio gas and manure.

From the clarifier tank, sludge is collected by a scrapper. It 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.

After the aerator tank, microbes and human waste settle down and form activated sludge. 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.

Q9: Untreated human excreta is a health hazard. Explain.

Answer: Human excreta is home for many pathogens and disease causing microbe.. If left untreated, it will cause threat to public health. Many water borne diseases like cholera, diarrhea, typhoid,
polio, meningitis, hepatitis and dysentery are caused due to faecal-route. It may cause water pollution and soil pollution. Both the surface water and groundwater get polluted.

Q10: Name two chemicals used to disinfect water.

Answer: Chemicals like chlorine and ozone are commonly used to disinfect water.

Q11: Explain the function of bar screens in a wastewater treatment plant.

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

Q12: Explain the relationship between sanitation and disease.

Answer:  Poor sanitation and contaminated drinking water is the cause of a large number of diseases.
Poor sanitation leads to:
  • growth of harmful microbes, flies and mosquitoes. Hence spread of many diseases like cholera, typhoid, polio, meningitis, hepatitis and dysentery.
  • pollution of soil and water.

Q13: Outline your role as an active citizen in relation to sanitation.

Answer:Wastewater treatment is a costly treatment. It requires costly infrastructure to build and maintain these plants. It is necessary we should be an active citizen and limit the type of waste. A few are listed below:
  1. Open drains be covered.
  2. Should be more considerate about others’ health and keep our surroundings clean.
  3. Should not litter anywhere. 
  4. Adopt good sanitation practices e.g. oil and fats should not be thrown in drains.
  5. Chemicals like paints, insecticides, motor oil etc. should not be thrown in drain which may kill useful bacteria.
  6. Used tealeaves, solid food remains,soft toys, cotton, sanitary towels, etc. should also be thrown in the dustbin
  7. Conserve water.
  8. Make people aware and encourage community practices to keep our environment clean.
  9. Separate trash as recycled waste and non-recycled.

Q14: Here is a crossword puzzle: Good luck!

3. Liquid waste products
4. Solid waste extracted in sewage treatment
6. A word related to hygiene
8. Waste matter discharged from human body
1. Used water
2. A pipe carrying sewage
5. Micro-organisms which causes cholera
7. A chemical to disinfect water


Q15: 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: (b) and (c)

Q16: What are the alternate arrangements for sewage disposal?

Answer: To improve sanitation, low cost onsite sewage disposal systems are being encouraged. These are:
  1. Septic tanks, 
  2. Chemical toilets, 
  3. Composting pits and 
  4. Vermi-processing toilets.

Q17: How sewage is classified based on colour and smell ?

Answer: Based on odour and colour, sewage is classified as:
  1. Sullage water: Waste water released from the kitchen is called sullage water
  2. Foul waste: Waste water released from toilets is termed as foul waste. It is full of germs and microbes and is highly infectious.
  3. Trade waste: Waste from factories and industries. Contains chemicals and toxic in nature.

Q18: What is the function of skimmer in waste water treatment plant?

Answer: The skimmer removes the floatable solids like oil and grease.

Q19: How sludge is decomposed?

Answer: Sludge is decomposed by the action of anaerobic bacteria. During this process biogas is produced which is used as fuel or can be used to produce electricity.

Q20: Why is vermi- compost considered as rich source of nutrients?

Answer:Vermi-compost consist of organic matter which is prepared by the action of earthworms on human or animal waste. It is rich in nutrients and is a good source of manure.

Q21: Why it is suggested to grow eucalyptus trees all along sewage ponds?

Answer: Eucalyptus trees absorb all surplus wastewater rapidly and release pure water vapour into the atmosphere.

Q22: How do the kitchen wastes block the drains?

Answer: Cooking oil and fats harden and block the pipes. In an open drain the fats clog the soil pores
reducing its effectiveness in filtering water. Wastes like used tealeaves, solid food remains, soft toys, cotton, sanitary towels etc. choke the drains. They do not allow free flow of oxygen. This hampers the degradation process.


  1. Hi there! glad to drop by your page and found these very interesting and informative stuff. Thanks for sharing, keep it up!

    - nutrients in wastewater

  2. checkout this blog.. its for helping students....

  3. swathi sri
    do not waste the water and conserve water, because there no water fit to drink because it is polluted by as.

  4. swathi sri
    do not waste the water and conserve water, because there no water fit to drink because it is polluted by as.

  5. We should not defecates in the water or anywhere, we should defecate in the washroom