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Q1:
What are the various constituents of domestic sewage? Discuss the effects of sewage discharge on a river.
Solution :
Section 1: Constituents of Domestic Sewage
Domestic sewage, often referred to as municipal wastewater, comprises the liquid waste discharged from residential areas and commercial establishments. It consists of the following primary constituents:
- Suspended Solids: Includes sand, silt, and clay (inorganic) as well as fecal matter, paper, cloth, and bits of food (organic).
- Colloidal Materials: Fecal matter, bacteria, cloth, and paper fibers that are finely divided and remain in suspension [Per Stokes' Law regarding sedimentation].
- Dissolved Materials: Nutrients such as nitrates ($NO_3^-$), phosphates ($PO_4^{3-}$), calcium ($Ca^{2+}$), sodium ($Na^+$), and potassium ($K^+$).
- Biological Contaminants: Pathogenic microorganisms including bacteria (e.g., Escherichia coli, Vibrio cholerae), viruses, and protozoan cysts which pose significant health risks.
- Biodegradable Organic Matter: Complex carbon-based compounds that serve as substrate for decomposer microorganisms.
Section 2: Effects of Sewage Discharge on River Ecosystems
The discharge of untreated domestic sewage into a river triggers a cascade of ecological transformations, primarily driven by the metabolic activity of microorganisms.
Step 3: Analytical Breakdown of Biochemical Oxygen Demand (BOD)
When organic waste enters the water, it acts as a food source for aerobic decomposers. These microorganisms consume high amounts of dissolved oxygen ($DO$) to break down the organic load [Per the concept of biological oxidation].
| Parameter | Effect of Sewage Discharge | Ecological Consequence |
|---|---|---|
| BOD (Biochemical Oxygen Demand) | Increases sharply | High microbial respiration rate |
| Dissolved Oxygen ($DO$) | Decreases rapidly | Asphyxiation of aquatic fauna |
Step 4: Long-term Ecological Consequences
- Fish Mortality: As $DO$ levels drop below the threshold required for aerobic life, mass mortality of fish and other gill-breathing organisms occurs.
- Eutrophication: The influx of inorganic nutrients (Nitrogen and Phosphorus) stimulates excessive algal growth, known as an 'algal bloom'. This leads to the deterioration of water quality and the production of toxins [Per the principle of nutrient enrichment].
- Anaerobic Decomposition: Once oxygen is entirely depleted, anaerobic bacteria thrive, producing foul-smelling gases like hydrogen sulfide ($H_2S$) and methane ($CH_4$), rendering the riverbed biologically "dead" or septic.
Final Solution: Domestic sewage consists of suspended, colloidal, and dissolved solids, nutrients, and pathogens. Its discharge into rivers significantly increases the Biochemical Oxygen Demand (BOD), causing a sharp decline in Dissolved Oxygen (DO), which leads to the death of aquatic life, the acceleration of eutrophication, and the promotion of anaerobic conditions that degrade the riverine ecosystem.
More Questions from Class 12 Biology Environmental Issues EXERCISES
- Q1: What are the various constituents of domestic sewage? Discuss the effects of sewage discharge on a river.
- Q2: List all the wastes that you generate, at home, school or during your trips to other places. Could you very easily reduce the generation of these wastes? Which would be difficult or rather impossible to reduce?
- Q2: List all the wastes that you generate, at home, school or during your trips to other places. Could you very easily reduce the generation of these wastes? Which would be difficult or rather impossible to reduce?
- Q3: Discuss the causes and effects of global warming. What measures need to be taken to control global warming?
- Q3: Discuss the causes and effects of global warming. What measures need to be taken to control global warming?
- Q4: Match the items given in column A and B:
- Q4: Match the items given in column A and B: Column A Column B (a) Catalytic converter (i) Particulate matter (b) Electrostatic precipitator (ii) Carbon monoxide and nitrogen oxides (c) Earmuffs (iii) High noise level (d) Landfills (iv) Solid wastes
- Q5(a): Write critical notes on the following: (a) Eutrophication
- Q5(a): Write critical notes on the following: (a) Eutrophication
- Q5(b): Write critical notes on the following: (b) Biological magnification
- Q5(b): Write critical notes on the following: (b) Biological magnification
- Q5(c): Write critical notes on the following: (c) Groundwater depletion and ways for its replenishment
- Q5(c): Write critical notes on the following: (c) Groundwater depletion and ways for its replenishment
- Q6: Why does ozone hole form over Antarctica? How will enhanced ultraviolet radiation affect us?
- Q6: Why does ozone hole form over Antarctica? How will enhanced ultraviolet radiation affect us?
- Q7: Discuss the role of women and communities in protection and conservation of forests.
- Q7: Discuss the role of women and communities in protection and conservation of forests.
- Q8: What measures, as an individual, would you take to reduce environmental pollution?
- Q8: What measures, as an individual, would you take to reduce environmental pollution?
- Q9(a): Discuss briefly the following: (a) Radioactive wastes
- Q9(a): Discuss briefly the following: (a) Radioactive wastes
- Q9(b): Discuss briefly the following: (b) Defunct ships and e-wastes
- Q9(b): Discuss briefly the following: (b) Defunct ships and e-wastes
- Q9(c): Discuss briefly the following: (c) Municipal solid wastes
- Q9(c): Discuss briefly the following: (c) Municipal solid wastes
- Q10: What initiatives were taken for reducing vehicular air pollution in Delhi? Has air quality improved in Delhi?
- Q10: What initiatives were taken for reducing vehicular air pollution in Delhi? Has air quality improved in Delhi?
- Q11(a): Discuss briefly the following: (a) Greenhouse gases
- Q11(a): Discuss briefly the following: (a) Greenhouse gases
- Q11(b): Discuss briefly the following: (b) Catalytic converter
- Q11(b): Discuss briefly the following: (b) Catalytic converter
- Q11(c): Discuss briefly the following: (c) Ultraviolet B
- Q11(c): Discuss briefly the following: (c) Ultraviolet B
CBSE Solutions for Class 12 Biology Environmental Issues
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
- Microbes in Human Welfare
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
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