Water Pollution - Causes, Effects, and Control - UNSOLVED PRACTICE SET

Q7. Differentiate between BOD and COD (Chemical Oxygen Demand). Which one gives a more complete picture of water pollution?

Q8. What is biomagnification? Explain with reference to DDT or mercury in an aquatic food chain.

Q9. Why does the Ganga river remain polluted despite the Namami Gange programme? Mention two persistent challenges.

Q10. What is the difference between point source and non-point source water pollution? Give one example of each.

Q11. Your village well water has turned yellowish and smells of rotten eggs. What could be the cause? Name the chemical responsible for the smell.

Q12. Why is chlorination used for disinfecting drinking water? What are the disadvantages of excessive chlorination?

Q13. Discuss water pollution in detail:

(a) Major sources — domestic sewage, industrial effluents, agricultural runoff, thermal pollution

(b) Types of water pollutants — organic, inorganic, biological, radioactive, thermal

(c) Parameters for measuring water quality — pH, BOD, COD, dissolved oxygen, turbidity, heavy metals

(d) Effects on human health, aquatic ecosystems, and agriculture

(e) The specific case of arsenic contamination in groundwater of West Bengal and Bangladesh

Q14. Explain the methods for controlling water pollution:

(a) Primary treatment — screening, sedimentation, coagulation

(b) Secondary treatment — biological treatment (activated sludge process, trickling filters)

(c) Tertiary treatment — nutrient removal, disinfection, advanced oxidation

(d) Natural methods — constructed wetlands, phytoremediation

(e) The role of STPs (Sewage Treatment Plants) in Indian cities and their current status

Q15. Water pollution is a crisis in India affecting health, economy, and culture. Discuss:

(a) Why 70% of India's surface water is unfit for drinking and the health burden of waterborne diseases

(b) The crisis of groundwater depletion and contamination with fluoride, arsenic, and nitrates

(c) The Namami Gange Mission — objectives, achievements, and remaining challenges

(d) Community-led water conservation models in Rajasthan (Johads, Baolis) and their relevance today

Q16. A river water sample has the following characteristics:

BOD = 80 mg/L

Dissolved oxygen (DO) = 4 mg/L

pH = 6.5

Temperature = 32°C

(a) Classify the pollution level of this river based on BOD (clean: <5 mg/L, moderately polluted: 5-20 mg/L, heavily polluted: >20 mg/L).

(b) Calculate the oxygen deficit if the saturation DO at 32°C is 7.5 mg/L.

(c) If a sewage treatment plant reduces BOD by 90%, calculate the BOD of the treated effluent and assess if it meets the Indian standard of <30 mg/L for discharge.

Q17. A factory discharges effluent containing 50 mg/L of chromium (Cr) into a river with a flow rate of 10 m³/s.

(a) Calculate the mass of chromium discharged per day.

(b) If the permissible limit for Cr in drinking water is 0.05 mg/L, calculate the dilution factor needed to make the river water safe.

(c) If the river flow decreases to 2 m³/s during summer, calculate the new Cr concentration and assess the risk.

Q18. The Yamuna River in Delhi receives 3,600 million litres per day (MLD) of sewage, of which only 50% is treated.

(a) Calculate the volume of untreated sewage entering the Yamuna daily.

(b) If the BOD of untreated sewage is 300 mg/L and treated sewage is 30 mg/L, calculate the total BOD load entering the river.

(c) If the river has a minimum flow of 100 m³/s during summer, calculate the BOD concentration in the river after mixing and compare with the standard of <3 mg/L for bathing quality.