Chlorine is a toxic substance which makes it ideal for killing harmful pathogens, but it does not stop there. If chlorinated water were released into the wetlands or bay it would also be harmful to fish, birds, wildlife and any other living organisms it contacts. Before water can be released from the contact basin the chlorine must be removed. This dechlorination is accomplished through the use of sulfur dioxide gas.
Before disinfected water leaves the contact basin the chlorine content is removed with sulfur dioxide. The sulfur dioxide gas reacts so rapidly that only a few seconds of treatment is necessary to turn the chlorine compounds into chloride ions. Hence, the dechlorination basin is only a fraction the size of the chlorine contact basin. At the Arcata Wastewater Treatment Plant the decholrination basin is about one and a half meters square.
Near the bottom of the dechlorination basin is a chemical infuser unit that mixes the treated water with sulfur dioxide gas by means of a motor driven propeller. The propeller also creates a vacuum to supply the desired chemical, in this case sulfur dioxide gas. The injection system is also capable of infusing liquid solutions. The Arcata Wastewater Treatment Plant has a sodium bisulfate (NaHSO4) liquid solution standing by that can be used in the event that the gas system fails or needs to be brought offline.
The reactions that occur in the dechlorination are fast and complex.