Question

Nalgonda technique for defluoridation of water involves the following steps. Arrange them sequentially from first to last step.
A. Addition of two chemicals (lime and alum)
B. Sedimentation
C. Flocculation
D. Filtration
Choose the correct answer from the options given below:

• A. A, B, C, D
• B. A, C, B, D
• C. B, A, D, C
• D. C, B, D, A

Hint:

Remember the standard sequence for water purification by coagulation: 1. Add Chemicals, 2. Mix to form Flocs (Flocculation), 3. Let them Settle (Sedimentation), 4. Separate the clear water (Filtration/Decantation).

Answer 1

The Correct Option is B

Step 1: Introduction to the Nalgonda Technique. Step 1: The Nalgonda technique, an Indian water treatment method, removes excess fluoride via chemical precipitation.
Step 2: Steps of the Chemical Treatment Process.
Step 2:

(A) Chemical Addition: Initially, alum (coagulant) and lime (for alkalinity) are rapidly mixed with the water.
(C) Flocculation: Gentle stirring causes alum to react with fluoride and alkalinity, creating insoluble aluminum hydroxide flocs that trap fluoride ions.
(B) Sedimentation: The water is left undisturbed, allowing heavy flocs to settle.
(D) Filtration: Finally, the clear water is separated, often through filtration, to remove any remaining particles.
Step 3: Correct Operational Sequence.
Step 3: The correct order is A \(\rightarrow\) C \(\rightarrow\) B \(\rightarrow\) D.