Water treatment—what is the principal purpose of using chloramines? In a municipal or industrial water chemical treatment plant, the use of chloramines primarily ensures which outcome?

Introduction / Context:
Chloramines (formed by combining chlorine with ammonia) are widely used in drinking-water treatment as an alternative or complement to free chlorine. They provide a persistent disinfectant residual in distribution systems and can reduce formation of regulated disinfection by-products compared to free chlorine alone.

Given Data / Assumptions:

• Treated water already has solids and hardness largely controlled by standard processes.
• The goal is safe distribution over long networks with minimized by-products.
• Chloramines act primarily as secondary disinfectants.

Concept / Approach:
Disinfection is the primary mission of chloramination—especially maintaining a durable residual that survives long residence times. While chloramines can modestly influence taste/odour, they are not principally used for weed control or hardness removal; those require different unit processes (e.g., algaecides, lime-soda softening, ion exchange).

Step-by-Step Solution:

Verification / Alternative check:
Operational guidance from utilities emphasizes chloramination for residual maintenance and DBP control, validating disinfection as the primary purpose.

Why Other Options Are Wrong:

• Taste & odour control: not the main reason, often managed by activated carbon or oxidation strategies.
• Weed control: involves reservoir management, algaecides, or mixing—chloramines are not used for this.
• Permanent hardness removal: requires softening; disinfectants do not remove hardness ions.

Common Pitfalls:
Assuming all chlorine-based chemicals remove hardness or solve taste issues; their primary role is microbiological safety.

Final Answer:
Disinfection (with a long-lasting residual)