• Stages in Municipal Rapid Sand Water Purification:

• Storage / Sedimentation (B): Raw water is stored to allow coarse suspended solids to settle.
• Coagulation and Flocculation (A): Chemical coagulants (alum) are added with rapid mixing to aggregate fine colloidal impurities into settleable flocs.
• Filtration (C): Water passes through a rapid gravity sand and gravel filter bed to remove remaining particulate matter.
• Chlorination (D): Terminal chemical disinfection (gaseous chlorine/bleaching powder) to destroy pathogenic microorganisms and provide residual protection.
  • Sequence:

• nalysis:
• ((B), (A), (C), (D)): Correct.

Correct option: (3) (B), (A), (C), (D)

• Neutralizing Residual Chlorine: Sodium Thiosulphate ($\text{Na}_2\text{S}_2\text{O}_3$, typically added at $0.1\text{ ml}$ of a $10\%$ solution per $100\text{ ml}$ sample bottle) is the standard reducing chemical used to instantly neutralize residual free and combined chlorine in treated drinking water.
• Microbiological Rationale: Chlorine neutralization is critical when collecting water samples for bacteriological examination (e.g., coliform counts) to prevent continued bactericidal action during transit to the laboratory, which would cause false-negative culture results.

Correct option: (4) Sodium Thiosulphate