Case Study:

In many Indian cities like Delhi and Jaipur, municipal corporations draw raw water from nearby rivers and lakes. This water often contains mud, sand, dissolved salts, microorganisms, and other impurities. Before it reaches households, it passes through a water purification plant.

At a local treatment plant, 50,000 litres of river water are treated every hour. The process begins with sedimentation tanks where heavier impurities settle at the bottom due to gravity. Alum is added to speed up the settling process by forming larger particles (flocs). The water then passes through sand and gravel filters to remove smaller suspended particles.

After filtration, chlorine is added in controlled amounts (about 0.5 mg per litre) to kill harmful bacteria. Engineers regularly test the turbidity (cloudiness) of water before and after treatment. Initially, the turbidity is measured at 120 NTU (Nephelometric Turbidity Units), and after treatment, it reduces to 5 NTU.

During a maintenance check, one filter bed becomes partially blocked. As a result, turbidity after filtration rises to 25 NTU. The plant supervisor must decide whether to continue supply or temporarily stop the process for cleaning.

This situation highlights the importance of physical separation methods, gravity, filtration principles, and safe water standards in everyday life.

Questions:

Section A – MCQs

1. The primary reason heavier impurities settle in sedimentation tanks is due to:
A. High temperature
B. Gravitational force
C. Magnetic force
D. Air pressure

2. If turbidity after filtration increases from 5 NTU to 25 NTU, it most likely indicates:
A. Excess chlorine
B. Filter malfunction
C. Increase in dissolved salts
D. Increase in temperature

3. Adding alum helps mainly by:
A. Killing bacteria
B. Increasing water temperature
C. Forming larger particles for faster settling
D. Adding oxygen to water

4. If chlorine is added at 0.5 mg per litre, how much chlorine is required for 10,000 litres of water?
A. 5 mg
B. 50 mg
C. 500 mg
D. 5000 mg

Section B – Short Answer Questions

1. Why is sedimentation considered a physical method of separation? Explain with reference to the plant process.

2. The turbidity reduces from 120 NTU to 5 NTU after treatment. What does this indicate about the efficiency of the plant?

3. Why should the supervisor stop the supply if turbidity remains at 25 NTU? Give two reasons.

Section C – Long Answer Question

1. Suppose the filtration unit completely stops working, but chlorination continues.
(a) Will the water be safe to drink? Explain scientifically.
(b) Suggest two corrective measures the plant should take immediately.
(c) Explain how this situation shows the importance of multiple purification stages.

Answer Key:

Section A – MCQs

1. B
2. B
3. C
4. D
Calculation:
0.5 mg × 10,000 litres = 5000 mg

Section B - Short Answers

1. Sedimentation is a physical method because no chemical composition changes occur. Heavier particles settle due to gravity without forming new substances.
2. It indicates high efficiency. The plant successfully removed most suspended particles, making water clearer and safer.

3. (Any two)
a) High turbidity indicates suspended impurities.
b) It may contain harmful microorganisms.
c) It does not meet safe water standards.

Section C - Long Answer

(a) No. Chlorination kills bacteria but does not remove suspended impurities. Without filtration, water may still contain harmful particles.

(b) 1. Immediately clean or replace the filter bed.
      2. Temporarily stop supply until turbidity returns to safe limits.

(c) Multiple stages ensure complete purification. Sedimentation removes heavy particles, filtration removes fine particles, and chlorination kills microbes. Each stage has a specific role; skipping one compromises safety.