Dechlorination in water treatment: Which chemical is most commonly applied for removing excess chlorine after disinfection?

Introduction / Context:
Dechlorination removes free chlorine residual when it could be problematic (e.g., before discharge to receiving waters or prior to processes sensitive to chlorine). Several reducing agents can neutralize chlorine; selection depends on scale, handling, and residual impacts.

Given Data / Assumptions:

• Conventional municipal facility practices.
• Liquid or gas chemical feed systems available.

Concept / Approach:

Sulphur dioxide (SO2) is widely used at plant scale due to controllable feed, rapid reaction, and cost. Alternatives include sodium sulphite and sodium thiosulphate, commonly used in laboratory or small-scale applications. Sodium bisulphate is an acidulant, not a standard dechlorinating agent, thus “all the above” is not correct in this list.

Step-by-Step Solution:

Verification / Alternative check:

Standard operating procedures and design manuals cite SO2 as the predominant dechlorination agent at wastewater and water plants; sodium sulphite/thiosulphate are often used for sampling/dechlorinating small volumes.

Why Other Options Are Wrong:

• Sodium thiosulphate / sodium sulphite: Effective but typically used for small-scale or sampling; not the most commonly applied at plant-scale.
• Sodium bisulphate: Primarily lowers pH; not a dechlorination standard.
• All the above: Incorrect because one listed chemical is not appropriate for dechlorination.

Common Pitfalls:

• Over-dosing SO2 causing sulphite carryover and oxygen depletion in receiving waters.
• Failing to interlock dosing with chlorine feed to avoid simultaneous overdosing.

Final Answer:

sulphur-dioxide.