Which chemical is used to neutralize residual chlorine in treated water?

• A. sodium chloride
• B. hydrogen peroxide
• C. sodium bicarbonate
• D. sodium thiosulfate

Answer: (D)

Dechlorination of treated water is the step of removing residual chlorine before discharge to protect aquatic life and prevent corrosion. Sodium thiosulfate fits this role because it acts as a reducing agent that neutralizes chlorine species, turning them into chloride ions and other non-oxidizing byproducts. The reaction happens quickly and reliably, leaving water with minimal residual chlorine for safe release and compatibility with downstream processes. The other options don’t remove chlorine: sodium chloride adds salt but doesn’t dechlorinate; sodium bicarbonate buffers pH without eliminating chlorine; hydrogen peroxide can react with chlorine but is not the standard dechlorinating agent used in most treatment setups.