Biology Class 12 NCERT Solutions: Chapter 10 Microbes in Human Welfare Part 3

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Q.7 What is sewage? In which way can sewage be harmful to us?


  • Sewage is the municipal waste matter that is carried away in sewers and drains. It includes both liquid and solid wastes, rich in organic matter and microbes.

  • Many of these microbes are pathogenic and can cause several water- borne diseases. Sewage water is a major cause of polluting drinking water.

  • Hence, it is essential that sewage water is properly collected, treated, and disposed.

Image of Sewage Pollution

Image of Sewage Pollution

Q.8 What is the key difference between primary and secondary sewage treatment?


Table of Primary V/S Secondary Sewage Treatment
Table of Primary V/S Secondary Sewage Treatment

Primary sewage treatment

Secondary sewage treatment


It is a mechanical process involving the removal of coarse solid materials.


It is a biological process involving the action of microbes.


It is inexpensive and relatively less complicated.


It is a very expensive and complicated process.

Q.9 Do you think microbes can also be used as source of energy? If yes, how?


  • Yes, microbes can be used as a source of energy. Bacteria such as Methane bacterium is used for the generation of gobar gas or biogas.

  • The generation of biogas is an anaerobic process in a biogas plant, which consists of a concrete tank (10−15 feet deep) with sufficient outlets and inlets.

  • The dung is mixed with water to form the slurry and thrown into the tank. The digester of the tank is filled with numerous anaerobic methane – producing bacteria, which produce biogas from the slurry.

  • Biogas can be removed through the pipe which is then used as a source of energy, while the spent slurry is removed from the outlet and is used as a fertilizer.

Q.10 Microbes can be used to decrease the use of chemical fertilizers and pesticides. Explain how this can be accomplished.


  • Microbes play an important role in organic farming, which is done without the use of chemical fertilizers and pesticides. Bio-fertilizers are living organisms which help increase the fertility of soil.

  • It involves the selection of beneficial micro-organisms that help in improving plant growth through the supply of plant nutrients. Bio-fertilizers are introduced in seeds, roots, or soil to mobilize the availability of nutrients.

  • Thus, they are extremely beneficial in enriching the soil with organic nutrients. Many species of bacteria and cyanobacteria have the ability to fix free atmospheric nitrogen.

  • Rhizobium is a symbiotic bacteria found in the root nodules of leguminous plants. Azospirillium and Azotobocter are free living nitrogen-fixing bacteria, whereas Anabaena, Nostoc and Oscillitoria are examples of nitrogen -fixing cyanobacteria.

  • Bio-fertilizers are cost effective and eco-friendly. Microbes can also act as bio-pesticides to control insect pests in plants.

  • An example of bio-pesticides is Bacillus thuringiensis, which produces a toxin that kills the insect pests.

  • Dried bacterial spores are mixed in water and sprayed in agricultural fields. When larvae of insects feed on crops, these bacterial spores enter the gut of the larvae and release toxins, thereby it.

  • Similarly, Trichoderma are free living fungi. They live in the roots of higher plants and protect them from various pathogens.

  • Baculoviruses is another bio-pesticide that is used as a biological control agent against insects and other arthropods.

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