Sulfur dioxide is the most popular chemical method used for dechlorination to date. The reason for the popularity of sulfur dioxide is that it uses existing chlorination equipment and makes extensive training of operators unnecessary.
Sulfur dioxide is a colorless gas with a characteristic pungent odor. SO2 may be cooled and compressed to a liquid. When the gas is compressed to a liquid, a colorless liquid is formed. As with chlorine, when sulfur dioxide is in a closed container the liquid and gases normally are in equilibrium. The pressure within the container bears a definite relation to the container’s ambient temperature. This relationship is very similar to chlorine.
Sulfur dioxide is neither flammable nor explosive in either form, gas or liquid. Dry gaseous sulfur dioxide is not corrosive to most metals; however, in the presence of moisture it forms sulfuric acid (H2SO4) and is extremely corrosive. Due to this corrosive action, similar materials and equipment are used for the storage and application of both sulfur dioxide and chlorine.
The density of sulfur dioxide is very similar to chlorine; so much so, that it is possible to use a chlorine rotameter to measure the flow of sulfur dioxide gas without much difficulty.. When using the chlorine rotameter, multiply the chlorine reading by 0.95 to obtain the pounds per day of sulfur dioxide used.
The chemical reaction of dechlorination results in the conversion of all active positive chlorine ions to the nonactive negative chloride ions.
The reaction of sulfur dioxide with chlorine is as follows:
SO2 + H2O ‡HSO3>†+ H+
HOCL + HSO3>† ‡Cl>†+ SO4 >² + 2H+
SO2 + HOCl + H2O ‡Cl>†+ SO4 >² + 3H+
The formation of sulfuric acid (H2SO4) and hydrochloric acid (HCl) from this reaction is not harmful because of the small amount of acid produced. The pH of the effluent is not changed significantly unless the alkalinity is very low.
Similar reactions are formed with dichloramine and nitrogen trichloride. If some organic materials are present, the reaction rate may change so that an excess of sulfur dioxide may have to be applied. Unwarranted excess sulfur dioxide dosages should be avoided, not only because it is wasteful, but it may result in dissolved oxygen reduction with a corresponding increase in BOD and a drop in the pH in the effluent.
Mixed-Oxidants refer to onsite brine generators, with day tanks, and automated injection systems, which produce a mixed-oxidant solution for wastewater disinfection. The solution leaves a durable chlorine residual which accomplishes the disinfection of the wastewater.
Mixed-Oxidants utilize a electrolytic cell that produces a liquid stream of very aggressive mixed oxidants that are extremely effective in disinfecting water. The electrolytic cell uses sodium chloride (NaCl), water, and electricity to generate the oxidant solution. Dry salt is loaded into a brine tank and combined with water to form a saturated brine solution. This brine is feed to the electrolytic cell. A liquid mixed-oxidant solution is produced at the anode and cathode and collected in a storage tank. The solution is injected into the water stream at a concentration appropriate for treatment objectives. The chlorine residual can be measured using standard DPD test equipment.
As there are no hazardous chemicals used, transported, or stored in the creation of mixed oxidants, they are safer to use than chlorine. Hydrogen gas is produced during the reaction and it should be safely vented from the system. Hazmat training and a Risk Management Plan are not needed.