Chlorination sequencing in water treatment: Which process is paired correctly as the one that de-chlorination follows in the super-dosing approach to achieve breakpoint control and then remove excess residual?

Introduction / Context:
Utilities sometimes apply a high chlorine dose to achieve breakpoint chlorination, oxidize demanding substances, and ensure disinfection. Because this can leave an excessive residual, a subsequent step removes the excess for taste/odour control and compliance.

Given Data / Assumptions:

• Conventional surface-water treatment context.
• Use of chemical reduction (e.g., sulphur dioxide, sodium bisulphite) for de-chlorination.
• Goal is an acceptable final chlorine residual.

Concept / Approach:

In “super-chlorination followed by de-chlorination,” the plant purposely overdoses chlorine, then removes the excess with a reducing agent. This sequence ensures strong initial oxidation/disinfection while delivering palatable water downstream.

Step-by-Step Solution:

Verification / Alternative check:

Standard design texts describe breakpoint chlorination methods and subsequent residual trimming with sulphur dioxide or similar agents.

Why Other Options Are Wrong:

• Pre-/post-chlorination describe locations in the train, not the super-dose pair.
• “Double-chlorination” is not the standard term for the super-dose then quenching sequence.

Common Pitfalls:

• Reversing the order—de-chlorination cannot precede the super dose.
• Confusing breakpoint chlorination with continuous low-dose residual control.

Final Answer:

Super-chlorination.