Disinfection of water to destroy water-borne pathogens: Which of the following is regularly used as a primary disinfectant in municipal water treatment practice?

Introduction / Context:
Disinfection is the critical step that inactivates pathogenic microorganisms in drinking water. An effective disinfectant must be potent, economical, and able to maintain a residual throughout the distribution system to prevent recontamination.

Given Data / Assumptions:

• Goal: kill bacteria, viruses, and protozoa to safe levels.
• Options include common chemicals with varied uses.
• Municipal scale and routine practice are implied.

Concept / Approach:
Chlorine and its derivatives (sodium hypochlorite, chloramines) are the most widely used disinfectants worldwide due to strong pathogen inactivation, formation of a measurable residual, and relatively low cost. Alkalis adjust pH but are not disinfectants. Benzene hexachloride is a pesticide, not a potable water disinfectant. Alkyl benzene sulphonate (ABS) is a synthetic detergent and surfactant, not used to disinfect municipal water.

Step-by-Step Solution:

Verification / Alternative check:
Alternative disinfectants (ozone, UV) are also used but typically require chlorination or chloramination for residual maintenance in the network.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing water softening/conditioning chemicals with disinfectants or assuming any biocide is appropriate for potable water.

Final Answer:
Chlorine