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STD 12 - 8. microbes in human welfare — Biology STD 12 Science — Question

CBSE BoardEnglish MediumSTD 12 ScienceBiologySTD 12 - 8. microbes in human welfare1 Mark
MCQ
The domestic sewage in large cities
  • A
    has a high $BOD$ as it contains both aerobic and anaerobic bacteria
  • ✓
    is processed by aerobic and then anaerobic bacteria in the secondary treatment in Sewage Treatment Plants $(STPs)$
  • C
    when treated in $STPs$ does not really require the aeration step as the sewage contains adequate oxygen
  • D
    has very high amount of suspended solids and dissolved salts.
✓

Answer

Correct option: B.
is processed by aerobic and then anaerobic bacteria in the secondary treatment in Sewage Treatment Plants $(STPs)$
b
(b) : Sewage water can be purified by passing it through sewage treatment plants with the action of heterotrophic microorganisms. There are three stagesof this treatment -primary, secondary and tertiary. Primary treatment removes floating and suspended solids from sewage through two processes of filtration and sedimentation. First floating matter is removed through sequential filtration. The filtrate is kept in large open settling tanks where grit settles down. The sediment is called primary sludge while the supernatant is called effluent. The primary sludge traps a lot of microbes and debris. In secondary treatment, the primary effluent is taken to aeration tanks. A large number of aerobic heterotrophic microbes grow in the aeration tank. They form flocs. Flocs are masses of bacteria held together by slime and fungal filaments to form mesh like structures. The microbes digest a lot of organic matter, converting it into microbial biomass and releasing a lot of minerals. As the $BOD$ of the waste matter is reduced to $10-­15\%$ of raw sewage, it is passed into settling tank. Thus secondary treatment is more or less biological. The sediment of settling tank is called activated sludge. The remaining is passed into a large tank called anaerobic sludge digester. It is designed for continuous operation. The aerobic microbes present in the sludge get killed. Anaerobic microbes digest the organic mass as well as aerobic microbes of the sludge. They are of two types, nonmethanogenic and methanogenic. Methanogenic bacteria produce a mixture of gases containing methane, $H_2S$ and $CO_2$.

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