Sodium thiosulfate, dechlorination

Sodium bicarbonate is generally added to increase alkalinity and muriatic acid (HCl) or sodium bisulfate (NaHSO ) to reduce it. In general, with acidic sanitizers such as chlorine gas or trichloroisocyanuric acid, ideal total alkalinity should be in the 100—120 ppm range, whereas, with alkaline products such as calcium, lithium, or sodium hypochlorite, a lower ideal total alkalinity of 80—100 ppm is recommended (14). Alkalinity is deterrnined by titration with standard sulfuric acid using a mixed bromcresol green—methyl red indicator after dechlorination of the sample with thiosulfate. Dechlorination with thiosulfate causes higher readings due to formation of hydroxyl ion (32) ... 

Water from the wastewater treatment plants of paper mills, power plants, etc., contains high chlorine residues in aqueous media, which causes environmental concern. Several methods have been used for dechlorination, including granular activated carbon, hydrogen peroxide, sodium thiosulfate, ammonia, sodium sulfite, and metabisulfite. In addition, ferrous sulfate hep-tahydrate has also been proposed for the removal of chlorine residues. 

Sodium thiosulfate is a colorless, transparent monoclinic crystal widely used by utilities for dechlorination. The reactions of sodium thiosulfate with free and combined chlorine varies with solution pH (13,15). Reaction of sodium thiosulfate with chlorine yields the following ... 

The amount of thiosulfate required is independent of the form of chlorine (free or combined chlorine) present. However, the amount of thiosulfate required for dechlorination may vary with solution pH (15). Approximately 2.23 parts of thiosulfate are required to neutralize one part of chlorine at pH 6.5 and nearly 1.6 parts of sodium thiosulfate is sufficient to neutralize one part of chlorine at pH 9.0. 

Bureau of Water Works, Portland, OR Combined chlorine Sodinm bisnlfite Sodium sulfite Sodium thiosulfate Calcium thiosulfate Ascorbic acid Sodium ascorbate 1% solution Stoichiometric concentrations needed for dechlorination 1.1 8.0 Surface water 300... 

When no dechlorination chemical was added, the dissolved oxygen concentration of the released water decreased from an initial concentration of 11 by less than 0.3 mg/L after traveling 450 ft in one test. When stoichiometric amounts of dechlorination chemicals were added, the DO decreased by 1.18,0.3,0.55, and 0.5 mg/L in the presence of sodium metabisulfite, sodium sulfite, sodium thiosulfate, and calcium thiosulfate, respectively. When twice the stoichiometric amounts of dechlorination chemicals were added, the dissolved oxygen concentration decreased hy 1, 0.9, 0.9, and 0.7 mg/L, respectively, in the presence of these chemicals. With the addition of stoichiometric concentrations of ascorbic acid and sodium ascorbate, the DO of the water increased by 0.3 mg/L, after a travel of 450 ft. When twice the stoichiometric concentrations of these chemicals were used, the DO decreased hy 0.2 mg/L. 

In summary, results indicated that sodium metabisulfite had a greater impact (1.0-1.18 mg/L depletion) on the DO concentrations of the water tested. Sodium sulfite, sodium thiosulfate, and calcium thiosulfate decreased the DO concentration hy 0.3-0.9 mg/L, depending on the amount of dechlorination chemical used. Ascorbic acid and sodium ascorbate had the least impact on the DO of the water tested. 

When no dechlorination chemical was added, the chlorine concentration decreased from 1.05 to 0.95 mg/L after 1000 feet (Fig. 2). This indicated that only a small amount (0.1 mg/L) of the chloramines dissipated through chlorine demand of paved surfaces. Sodium bisulfite, sodium sulfite, ascorbic acid, and sodium ascorbate neutralized all detectable chlorine to below 0.1 mg/L within 2 ft downstream of the mixing hose (approx 2 s). Sodium thiosulfate neutralized more than 80% of the chlorine within 2 ft. However, chlorine concentrations decreased below 0.1 mg/L (the discharge limit in most states) after about 500 ft (elapsed time 3 min, 2 s). Calcium thiosulfate neutralized 60% of the chlorine within 2 ft and neutralized 90% of the chlorine after 1000 ft (elapsed time 7 min, 10s). 

Dechlorination Using Ascorbic Acid Powder/Sodium Thiosulfate Crystals... 

The field studies indicated that all of the dechlorination chemicals were effective in neutralizing free and combined chlorine to below 0.1 mg/L. In most cases the stoichiometric amount of dechlorination chemicals removed more than 90% of the chlorine. However, the reaction rates and the water quality impacts varied with the type, amount, and form of the chemicals used. In general, the rates of dechlorination using sodium/calcium thiosulfate were slower than those using the other chemicals. However, studies by others indicated that dechlorination of wastewater samples using sodium thiosulfate was more rapid that using ascorbic acid (24). 

When used in powder or crystal form, dechlorination chemicals (ascorbic acid and sodium thiosulfate) dissolved rapidly causing water-quality concerns, although physical methods (tablets) have been developed since to slow down dissolution rates. Sodium sulfite, when used in tablet form, was very effective in dose control. One tablet was sufficient to dechlorinate 2 mg/L of chloraminated water to below 0.1 mg/L for 45 min when water was released at 100 gpm. Finally, these field tests also indicated that the flow rates of chlorinated waters can significantly impact the efficiency of dechlorination operations. 

Neutralization. Many health codes regulate bather entry to specified concentrations of hypochlorous acid in the water. This varies, but in the US bathers are generally not allowed in the water if chlorine levels are above 3.0 ppm. This is referred to as the re-entry level . After superchlorination it may be necessary to partially dechlorinate the water to re-entry levels. This is accomplished by the addition of a chlorine neutralizer (reducing agent), such as sodium sulfite, sodium thiosulfate or hydrogen peroxide. 

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. 

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