Chlorine continues to be used throughout industry as a bleaching agent, a disinfectant, and an oxidizer. As a result, it is present in many industrial and municipal wastewaters in concentrations ranging from a few ppb to 1% or more. Industry experience has verified that chlorine residuals, even at very low levels, are toxic to certain fish and other aquatic life. In some areas of the United States an upper limit of 0.1 mg/L chlorine residual already applies to wastewater discharges.

Dechlorination can be accomplished by several means, the most widely used being sulfur dioxide – either as a gas (SO2) or as a salt (e.g., sodium metabisulfite). However, this method has several shortcomings:

• As a corrosive compressed gas, SO2 presents safety and handling problems which require expensive storage and containment facilities
• As a salt, the bisulfite solids must be dissolved into solution before use and the solution often requires heating through the colder months to prevent freezing
• An excess of SO2 must be added to destroy all the available chlorine. Excess SO2 is itself toxic to aquatic life and exerts an oxygen demand on the receiving stream
• The process leaves behind acid sulfate salts which may degrade the effluent for recycling or subsequent processing

When elemental chlorine is dissolved in water, an equilibrium is established between chlorine, hypochlorous acid, and hypochlorite ion (Cl2, HOCl and OCl-, respectively). The relative amount of each present depends primarily on the pH of the system.

The chlorine as HOCl and OCl- is referred to as free available chlorine. This is the form of chlorine typically found in cooling water circuits, industrial bleaching systems, and many chemical processing operations. Nitrogen-containing compounds such as ammonia, amines and proteins are usually present in municipal wastewater. Free available chlorine reacts readily with these materials to form chloramines in which the chlorine is described as combined available chlorine. The available chlorine remaining after disinfection of municipal wastewaters is usually present in the combined form.

As an alternative to sulfur dioxide gas, various dry chemicals are available which form sulfur dioxide in solution.

These include sodium sulfite (Na2 SO3 ), sodium metabisulfite (Na2 S2 O5 ), sodium bisulfite (NaHSO3 ), a 38 percent aqueous solution of sodium metabisulfite, and sodium thiosulfate (Na2 S2 O3 ), among others (Lind, 1995).

When dissolved in water, chlorine hydrolyzes to form hypochlorous acid (HOCl) and hypochlorite ions (OCl-) which, taken together, are referred to as

“free chlorine.” (Free, uncombined chlorine, Cl2 , is rarely found in wastewater since the conditions of formation are relatively extreme [Lind, 1995]).

Once formed, the free chlorine reacts with natural organic matter in water and wastewater to form chlorinated organic compounds. The free chlorine also combines with ammonia to form mono-, di-, and trichloramines in quantities dependent on the ratio of chlorine to ammonia nitrogen (Lind, 1995).

When either sulfur dioxide or sulfite salts are dissolved in water, aqueous sulfur compounds in the +4 oxidation state are produced, often notated

S(IV) (Helz and Nweke, 1995).

The S(IV) species, such as the sulfite ion (SO3 ^-2 ), reacts with both free and combined forms of chlorine, as illustrated in FOLLOWING equations :

Since free chlorine and inorganic chloramines react rapidly with S^+IV (Helz, 1998), a short contact time of one to five minutes is considered to be sufficient; nevertheless, complete blending at the point of application is essential for effective dechlorination (WEF and ASCE, 1992).

Proper dosage is critical to produce a non-detectable chlorine residual.

Dosing in excess must be avoided because excess sulfite can react with dissolved oxygen (four parts sulfite to one part oxygen) in the wastewater to produce sulfates, potentially leading to reduced dissolved oxygen concentrations and low pH levels in the finished effluent for high levels of overdose (WEF, 1996).

Careful process control will help prevent overdosing.

Recommended dosage is 3 ppm DECHLORINATOR-ES per ppm of free chlorine to be removed.

The recommended feeding method is continuous neat or as a dilute solution.

Solution of DECHLORINATOR-ES must be covered with a float, to minimise contact with air.

Piping and mix tanks used for feeding solutions of DECHLORINATOR-ES should be constructed of polyethylene, PVC or 316 SS.