Dechlorination

Sulfur dioxide gas is the most commonly used dechlorination agent in large municipal treatment plants. It reacts with both free chlorine and combined chlorine (chloramines) on a near-stoichiometric basis — approximately 0.9 mg SO₂ per mg of chlorine residual. SO₂ systems use gas feed equipment similar to chlorination systems and provide precise, rapid dechlorination. However, SO₂ is a toxic gas that requires careful handling, safety equipment, and regulatory compliance.

Sodium bisulfite is the most widely used liquid dechlorination agent, particularly in small to medium-sized facilities. It is safer to handle than SO₂ gas and is typically available as a 38–40% solution. The practical dose rate is approximately 1.5 mg of sodium bisulfite per mg of chlorine residual, with many operators dosing at 3 mg/mg to ensure complete dechlorination with a safety margin. Sodium bisulfite is injected using chemical metering pumps — making Milton Roy's portfolio of electromagnetic, hydraulic diaphragm, and peristaltic pumps an ideal fit for these applications.

Sodium metabisulfite dissolves in water to form sodium bisulfite, and is used interchangeably in many facilities. It offers a slightly higher concentration of active reducing agent per unit weight, which can reduce storage and handling requirements.

For applications requiring food-grade or NSF-certified dechlorination agents — particularly in potable water systems — ascorbic acid and sodium ascorbate provide effective chlorine neutralization without adding sulfite residuals to the treated water. These agents are also used in laboratory and pharmaceutical water treatment applications.