Water treatment disinfection: Across municipal water plants worldwide, which method remains the most widely used primary disinfectant for inactivating pathogens?

Introduction / Context:
Disinfection is a critical barrier in drinking water treatment, preventing waterborne diseases by inactivating bacteria, viruses, and protozoa. Among several technologies—chlorination, chloramination, ozone, and UV irradiation—one has historically dominated due to effectiveness, residual protection, and cost. This question asks which method is most widely used globally.

Given Data / Assumptions:

• Focus is on primary disinfection at municipal scale.
• Consider factors such as efficacy, residual maintenance in distribution, and infrastructure cost.
• Many systems also use secondary disinfectant residuals.

Concept / Approach:
Chlorine (as gas, hypochlorite, or generated on-site) is widely used because it is effective against a broad spectrum of microbes and leaves a measurable residual that persists through the distribution network, guarding against recontamination. UV is highly effective for primary inactivation, especially of protozoa, but provides no residual and requires post-disinfection residual maintenance. Cation exchange and coagulation are treatment processes for ions and particles, not disinfectants.

Step-by-Step Solution:
Identify which option is a disinfectant with residual capability: chlorine.Exclude unit processes that are not disinfectants: cation exchange, coagulation.Note UV's strong role but non-residual limitation; global adoption remains highest for chlorine.

Verification / Alternative check:
Surveys of utilities show chlorination as the most common primary and/or secondary disinfection method, especially in small- to medium-sized systems and developing regions.

Why Other Options Are Wrong:
UV irradiation: Effective but lacks residual; often combined with downstream chlorination.Cation exchanger: Ion removal, not disinfection.Coagulation: Particle/organic removal step; may aid pathogen removal but does not inactivate by itself.

Common Pitfalls:

• Assuming the newest technology is the most widely used; legacy systems and residual requirements matter.
• Confusing removal (coagulation/filtration) with inactivation (disinfection).

Final Answer:
Chlorine