Which chemical is typically used to neutralize residual chlorine in desalinated water prior to discharge?

Sodium bisulfite (NaHSO3) or sodium metabisulfite (Na2S2O5) are typically used to neutralize residual chlorine in desalinated water prior to discharge. Chlorine is often used as a disinfectant in desalination plants to kill or inactivate pathogens. However, residual chlorine can be harmful to aquatic life if discharged into the environment. Sodium bisulfite and sodium metabisulfite are reducing agents that react with chlorine to convert it into chloride, which is much less toxic. The reaction with sodium bisulfite is: NaHSO3 + HOCl -> NaHSO4 + HCl, where HOCl is hypochlorous acid, the active form of chlorine. The reaction with sodium metabisulfite is: Na2S2O5 + 2HOCl + H2O -> 2NaHSO4 + 2HCl. The dosage of sodium bisulfite or sodium metabisulfite is carefully controlled to ensure that the residual chlorine is completely neutralized without adding excessive amounts of the reducing agent, which could also be harmful to the environment. ORP (oxidation-reduction potential) sensors are often used to monitor the dechlorination process and ensure that the chlorine residual is effectively removed. Prior to discharge, it is essential to verify the chlorine level to confirm it meets regulatory requirements.