Removal of the chlorine residual before discharge with a sulfur reducing agent. The dose follows the reaction with chlorine (as Cl₂), times a practical excess:
| Agent | Reaction | mg per mg Cl₂ |
|---|---|---|
| Sodium bisulfite | NaHSO₃ + Cl₂ + H₂O → NaHSO₄ + 2 HCl | 1.47 |
| Sodium metabisulfite | Na₂S₂O₅ + 2 Cl₂ + 3 H₂O → 2 NaHSO₄ + 4 HCl | 1.34 |
| Sodium sulfite | Na₂SO₃ + Cl₂ + H₂O → Na₂SO₄ + 2 HCl | 1.78 |
Dr=resteqfexc(CR,e−CR,s)
Results include the product consumption at its strength and density, the dosing pump flow at peak and the storage volume. An excess beyond what the residual needs consumes dissolved oxygen in the receiving water and lowers the pH.
METCALF & EDDY. Wastewater Engineering: Treatment and Resource Recovery. 5th ed. McGraw-Hill, 2014. Chapter 12 (disinfection).
U.S. EPA. Design Manual: Municipal Wastewater Disinfection. EPA/625/1-86/021, 1986.