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Q8:
What is the key difference between primary and secondary sewage treatment?
Solution :
Definition of Sewage Treatment Stages
Sewage treatment is a multi-stage process designed to eliminate contaminants from wastewater before its release into the environment. The distinction between primary and secondary treatment lies in the fundamental mechanisms—physical separation versus biological degradation.
| Feature | Primary Treatment | Secondary Treatment |
|---|---|---|
| Mechanism | Physical separation (Sedimentation/Filtration) | Biological degradation (Microbial activity) |
| Key Agents | Screens, grit chambers, settling tanks | Aerobic bacteria, fungi (flocs) |
| Target Contaminants | Large debris, grit, and suspended solids | Dissolved organic matter (BOD reduction) |
| End Product | Primary sludge | Activated sludge and treated effluent |
Step 1: Analysis of Primary Treatment
Primary treatment is essentially a mechanical process [Per the Principles of Sedimentation]. It involves two distinct steps:
- Filtration: Sequential removal of floating debris using screens.
- Sedimentation: The sewage is placed in settling tanks where grit (sand, pebbles) and larger suspended solids settle by gravity to form primary sludge. The liquid supernatant, known as the effluent, is then passed to the secondary stage.
Step 2: Analysis of Secondary Treatment (Biological Treatment)
Secondary treatment is a biochemical process that utilizes heterotrophic microbes to purify the effluent. This stage is crucial for reducing the Biochemical Oxygen Demand (BOD) [Per the Biological Oxygen Demand Theory].
- Aeration: The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it.
- Microbial Action: This environment promotes the growth of aerobic microbes into flocs (masses of bacteria associated with fungal filaments). These microbes consume the organic matter, significantly lowering the BOD of the water.
- Settling: The mixture is then passed into a settling tank where the bacterial 'flocs' are allowed to sediment. This sediment is termed activated sludge.
Step 3: Justification of the Biological Difference
The core difference is the metabolic capacity of the system. Primary treatment only removes solids that can settle mechanically; it does nothing to address the dissolved organic pollutants which exert high oxygen demand upon the aquatic ecosystems into which they might be discharged. Secondary treatment uses the respiration of aerobic bacteria to convert these dissolved pollutants into biomass (flocs) and stable mineral byproducts, thereby restoring the ecological balance of the water [By the Principle of Microbial Mineralization].
Final Solution: The key difference is that primary sewage treatment relies on physical processes (sedimentation and filtration) to remove large solid waste, while secondary sewage treatment relies on biological processes (aerobic microbial metabolism in flocs) to break down dissolved organic matter and significantly reduce the Biochemical Oxygen Demand (BOD) of the wastewater.
More Questions from Class 12 Biology Microbes in Human Welfare EXERCISES
- Q1: Bacteria cannot be seen with the naked eyes, but these can be seen with the help of a microscope. If you have to carry a sample from your home to your biology laboratory to demonstrate the presence of microbes with the help of a microscope, which sample would you carry and why?
- Q1: Bacteria cannot be seen with the naked eyes, but these can be seen with the help of a microscope. If you have to carry a sample from your home to your biology laboratory to demonstrate the presence of microbes with the help of a microscope, which sample would you carry and why?
- Q2: Give examples to prove that microbes release gases during metabolism.
- Q2: Give examples to prove that microbes release gases during metabolism.
- Q3: In which food would you find lactic acid bacteria? Mention some of their useful applications.
- Q3: In which food would you find lactic acid bacteria? Mention some of their useful applications.
- Q4: Name some traditional Indian foods made of wheat, rice and Bengal gram (or their products) which involve use of microbes.
- Q4: Name some traditional Indian foods made of wheat, rice and Bengal gram (or their products) which involve use of microbes.
- Q5: In which way have microbes played a major role in controlling diseases caused by harmful bacteria?
- Q5: In which way have microbes played a major role in controlling diseases caused by harmful bacteria?
- Q6: Name any two species of fungus, which are used in the production of the antibiotics.
- Q6: Name any two species of fungus, which are used in the production of the antibiotics.
- Q7: What is sewage? In which way can sewage be harmful to us?
- Q7: What is sewage? In which way can sewage be harmful to us?
- Q8: What is the key difference between primary and secondary sewage treatment?
- Q9: Do you think microbes can also be used as source of energy? If yes, how?
- Q9: Do you think microbes can also be used as source of energy? If yes, how?
- Q10: Microbes can be used to decrease the use of chemical fertilisers and pesticides. Explain how this can be accomplished.
- Q10: Microbes can be used to decrease the use of chemical fertilisers and pesticides. Explain how this can be accomplished.
- Q11: Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled \(A\), \(B\) and \(C\); but the laboratory attendant did not note which was which. The BOD values of the three samples \(A\), \(B\) and \(C\) were recorded as 20mg/L, 8mg/L and 400mg/L, respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean?
- Q11: Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled (A), (B) and (C); but the laboratory attendant did not note which was which. The BOD values of the three samples (A), (B) and (C) were recorded as (20mg/L), (8mg/L) and (400mg/L), respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean?
- Q12: Find out the name of the microbes from which Cyclosporin \(A\) (an immunosuppressive drug) and Statins (blood cholesterol lowering agents) are obtained.
- Q12: Find out the name of the microbes from which Cyclosporin (A) (an immunosuppressive drug) and Statins (blood cholesterol lowering agents) are obtained.
- Q13(a): Find out the role of microbes in the following and discuss it with your teacher. (a) Single cell protein (SCP)
- Q13(a): Find out the role of microbes in the following and discuss it with your teacher. (a) Single cell protein (SCP)
- Q13(b): Find out the role of microbes in the following and discuss it with your teacher. (b) Soil
- Q13(b): Find out the role of microbes in the following and discuss it with your teacher. (b) Soil
- Q14: Arrange the following in the decreasing order (most important first) of their importance, for the welfare of human society. Give reasons for your answer. Biogas, Citric acid, Penicillin and Curd
- Q14: Arrange the following in the decreasing order (most important first) of their importance, for the welfare of human society. Give reasons for your answer. Biogas, Citric acid, Penicillin and Curd
- Q15: How do biofertilisers enrich the fertility of the soil?
- Q15: How do biofertilisers enrich the fertility of the soil?
CBSE Solutions for Class 12 Biology Microbes in Human Welfare
Chapters in CBSE - Class 12 Biology
- Reproduction in Organisms
- Human Reproduction
- Reproductive Health
- Principles of Inheritance and Variation
- Molecular Basis of Inheritance
- Evolution
- Human Health and Disease
- Strategies for Enhancement in Food Production
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
- Environmental Issues
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