Sulfur dioxide dechlorination

Effluents from sewage treatment plants are not allowed to contain residual chlorine in excess of tolerable values as determined by water quality standards. For example, in discharges to trout streams, the residual chlorine should not exceed 0.02 mg/L. Thus, chlorinated effluents should be dechlorinated. Sulfur dioxide, sodium sulfite, sodium metabisullite, and activated carbon have been used for dechlorination. Because sulfur dioxide, sodium sulfite, and sodium metabisulfite contain sulfur, we will call them sulfur dechlorinating agents. Dechlorination is an oxidation-reduction reaction. The chemical reactions involved in dechlorination are discussed next. 

Chemicals required for dechlorination Sulfur dioxide (SO2) and sulfite salts are the most common chemicals used. Sodium metabisulfite (Na2S205) can also be used, but is much less common. In fact, any reducing agent can be considered, depending on cost and availability. 

Disinfection, 8 605-672. See also Disinfection processes antimicrobial nanoemulsion technology, 8 630-631 bromine, 8 621-626 bromine chloride, 8 626-628 chlorination, 8 610-615 chlorine dioxide, 8 617-619 dechlorination with sulfur dioxide, 8 615-617... 

Example 17.17 A total flow of 25,000 mVd is to be dechlorinated after disinfection nsing chlorine. Sulfur dioxide is to be used for dechlorination. If the total residnal chlorine (TRC) is 0.5 mg/L, how many kilograms per day of snifur dioxide are needed ... 

Calculate the residual sulfur dioxide in a dechlorination process that will result in a decrease of pH to 5.0 in a receiving stream assuming the original pH is 7.5. 

Another consideration in breakpoint chlorination is dechlorination to remove the chlorine residual from the final effluents before it is discharged. Very often, dechlorination using sulfur dioxide or activated carbon may be needed when the breakpoint chlorination process is used. A new dechlorination technology has been introduced in another chapter of this handbook series (2). UV dechlorination is recommended by Wang (45). 

Dechlorination is the process of converting highly reactive chlorine from these waters into less reactive chloride ions prior to disposal into receiving streams. Various chemical and nonchemical techniques are currently used for disposal of chlorinated waters by water and wastewater agencies. For example, wastewater treatment plants use sulfur dioxide gas or sodium metabisulfate to dechlorinate treated effluent prior to release into receiving streams. Many water utilities often use passive, non-chemical methods such as discharge to sanitary sewers for disposal of chlorinated waters. Impurities such as organics, iron, and sulfide in the sanitary sewer exert a chlorine demand and neutralize chlorine in the released water. 

Technology for large-scale application of chemical reduction is well developed. The reduction of residual chlorine in a chlorination or superchlorination process system is termed dechlorination, which is the most common process in municipal water and wastewater treatment. The reduction of chromium waste by sulfur dioxide is another classic process and is in use by numerous plants employing chromium compounds in operations such as electroplating. 

Since about 1970, much attention has been focused on me toxic effects of chlorinated effluents. Bom free chlorine and chloramine residuals are toxic to fish and omer aqnatic organisms. Dechlorination involves the addition of sulfur dioxide (or equivalent redncing agent) to me water or wastewater. Please discuss me following ... 

Process performance Available chlorine residuals can be reduced to essentially zero by sulfur dioxide dechlorination. Dechlorination process, if control properly, generates no residuals. 

Process reliability Sulfur dioxide addition for dechlorination purposes is reasonably reliable from a mechanical standpoint. The greatest problems are experienced with analytical control which may lower the process reliability. 

Process modifications and alternatives Metabisulfite, bisulfite, or sulfite salts can also be used. Automatic or manually fed systems can also be used. If chlorine is used at the site, sulfur dioxide is preferred, since identical equipment can be used for the addition of both chemicals. Alternative dechlorination systems include activated carbon, and ponds (sunlight and aeration). UV is also an effective dechlorination technology (60,61). 

To eliminate residual free chlorine from hquid, granular activated carbon adsorption or chemical reduction (with reducing agents, such as sulfur dioxide, sodium bisulfite, and sodium metabisulfite) are the most common processes for dechlorination. Ultraviolet (UV) irradiation process is gaining wider acceptance as a dechlorination process (30,45,46, 60,61). 

If the final chlorine-disinfected effluent is to be discharged to a sensitive watercourse, dechlorination may be necessary to avoid problems from the toxicity of the residual chlorine. After a 20- to 30-min holding time for completion of the disinfection to take place, the treated effluent is dechlori-nated by adding a reducing agent such as sulfur dioxide to convert chlorine to chloride, followed by pH neutralization with lime (Eqs. 5.20 and 5.21). 

Since a slight excess of chlorine is usually employed to obtain cyanide destruction, the waste stream may still be quite toxic from the residual chlorine present. Dechlorination using sulfur dioxide can be used to reduce any residual chlorine to chloride, which removes the toxicity before discharge (Eq. 5.20). 

Following the chlorine chemical contact for disinfection, described in the previous section, residual chlorine must be removed using a dechlorination agent such as sulfur dioxide (Fig. 8). This quenching of the chlorine (or chloramine) by a strong reductant provides a potential for the chemical reduction of transformation products in the wastewater. Minimal research has addressed these reductive reactions for transformation products of synthetic organic chemicals. 

The water that evaporates from the dechlorinated brine is condensed in a cooler. The condensate, which may be chemically dechlorinated, is returned to the brine circulation system if necessary to maintain the volume of the brine circuit. If necessary, the remaining chlorine content can be further reduced by blowing with compressed air, by a second vacuum treatment, by treatment with activated carbon (63], or by chemical treatment with hydrogen sulfite, thiosulfate, sulfur dioxide, or sodium hydrogensulfide. 

Sulfites occur naturally in very small concentrations in the earth and in the atmosphere, and they are a natural ingredient in certain vegetables, foods and beverages as a result of endogenous fermentation by the yeasts in beer and wine (Taylor et al., 1986). Sulfites also occur naturally in the human body (Taylor et al., 1986 Adams, 1997 Food and Drug Administration [FDA], 2000), and they can be found in greater concentrations in natural waters or wastewaters as a result of industrial pollution, and in treatment plant effluents dechlorinated with sulfur dioxide, or as a consequence of volcanic activity, etc. 

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