Residual Sulfite

Sulfite can be found in most natural water sources and is one of the major dissolved components of rain.  High concentrations of Sulfite in the water we drink can have a laxative effect when combined with calcium and magnesium, the two most common constituents of hardness.  It can also cause water to have a bitter taste.

Sulfite is widely used for the dechlorination of wastewater in many treatment facilities. Dechlorination minimizes the effect of potentially toxic disinfection by-products by removing the free or total combined chlorine residual remaining after chlorination, which can prove toxic to fish in the receiving stream.

With residual chlorine discharge limits often very close to zero, monitoring residual values to comply with regulations has become very difficult and controlling residuals at values between zero and 10 or 20 parts-per-billion is often not achievable. To meet stringent discharge limits, the Sulfite or bisulfite used for dechlorination is added in slight excess, providing a small Sulfite residual to ensure complete dechlorination.