Disinfection choice worldwide: The most widely used and practically ideal disinfectant for public drinking-water supplies is

Introduction / Context:
Disinfection is the final barrier in drinking-water treatment. The disinfectant should be effective, economical, and provide a measurable residual in the distribution system to guard against recontamination.

Given Data / Assumptions:

• Common municipal practice worldwide.
• Focus on primary/secondary disinfection and residual maintenance.

Concept / Approach:

Chlorine is the global workhorse disinfectant. It is effective against many pathogens, relatively inexpensive, and leaves a residual. Alternatives like ozone or UV are powerful but leave no residual; they are often paired with chlorine/chloramines downstream.

Step-by-Step Solution:

Verification / Alternative check:

WHO and many national standards reference chlorine/chloramine residual targets in distribution, confirming routine reliance on chlorine-based disinfectants.

Why Other Options Are Wrong:

• Alum: a coagulant, not a disinfectant.
• Lime: used for pH/softening control, not primary disinfection.
• Nitrogen/ammonia: not disinfectants (ammonia forms chloramines with chlorine but is not used alone for disinfection).

Common Pitfalls:

• Confusing coagulants and disinfectants.
• Assuming ozone/UV are “ideal” without considering lack of residual.

Final Answer:

Chlorine.