Dechlorination Agent

In the context of water treatment and wastewater treatment, dechlorination agents are chemicals used to remove or neutralize residual chlorine that remains after the disinfection process.

Background

• In water treatment, chlorine is often used as a disinfectant to kill bacteria, viruses, and harmful microorganisms.

• However, residual free or bound chlorine (chloramine) can cause problems:

  • To the environment: toxic to aquatic organisms.

  • For advanced processes: may interfere with biological treatment units.

  • For water quality: creates undesirable tastes and odors.

Main Functions

Dechlorination Agent functions to:

1. Lowering free/total chlorine levels to safe standards (e.g. before water is discharged into rivers or reused).

2. Protect biological processes (for example in activated sludge or biological membrane installations) so that microbes are not killed by residual chlorine.

3. Reduce the risk of formation of harmful by-products (disinfection by-products, such as trihalomethanes).

Commonly Used Chemical Types

Some chemicals that are often used as dechlorination agents include:

• Sulfur-based compounds

  • Sodium bisulfite (NaHSO₃)

  • Sodium metabisulfite (Na₂S₂O₅)

  • Sodium thiosulfate (Na₂S₂O₃)

• Other reducing agents

  • Ascorbic acid (Vitamin C)

  • Hydrogen peroxide (H₂O₂)

How it works

• The principle is a reduction reaction: the dechlorination agent compound reacts with chlorine/chloramine, reducing it to harmless chloride ions (Cl-).

• Example of reaction with sodium bisulfite:

  Cl2+NaHSO3+H2O→2HCl+NaHSO4Cl₂ + NaHSO₃ + H₂O → 2HCl + NaHSO₄Cl2+NaHSO3+H2O→2HCl+NaHSO4