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Chris,

Would the salt in sodium metabisulfate harm the plants if it gets into the soil?

Here is some more info on chlorine neutralization using chemicals.

Chemical Dechlorination

              Whenever it is not possible to dispose of chlorinated waters safely by non-chemical methods, chlorine may be neutralized using chemicals.  Several solid, liquid and gaseous dechlorination chemicals are commercially available and are widely used by water and wastewater utilities.  This section describes the reactions of various chemicals with free and combined chlorine, related water quality, health & safety issues, ease of use, cost and other issues related to the application of the chemicals commonly used for dechlorination.

Sulfur Dioxide (SO₂)

            Sulfur dioxide is a colorless gas with a suffocating pungent odor.  It is widely used in water and wastewater treatment plants for dechlorinating backwash water and wastewater disinfected with chlorine.  Sulfur dioxide reacts instantaneously with free chlorine according to the following stoichiometry (1):

SO₂  +             H₂O +    HOCl                          ®  SO₄^-2  + Cl^-  +  3H⁺

^{Sulfur dioxide                   Hypochlorous acid                           Sulfate}         

In the field, nearly 1.1 parts of SO₂ are required to neutralize 1 part of chlorine (2).  SO₂ is an oxygen scavenger.  It can deplete dissolved oxygen in the discharge water and receiving stream.  SO₂ can also reduce pH of water significantly.  Approximately 2.8 mg of alkalinity as CaCO₃ is consumed per milligram of chlorine reduced.

Sulfur dioxide is a toxic chemical subject to reporting requirements of the Superfund Amendments and Reauthorization Act (SARA).  It has a National Fire Protection Association (NFPA) rating of 2, 0 and 0 for health, fire and reactivity, respectively.  (Hazard ratings range from 0 to 4, with 0 indicating no hazard and 4 indicating extremely hazardous).  It is an extremely irritating gas.  While it is suitable for use in facilities such as treatment plants and pumping stations, it is not best suited for field applications.

Sodium Thiosulfate (Na₂S₂O₃)

Sodium thiosulfate is a colorless, transparent monoclinic crystal widely used by municipalities for dechlorination.  It undergoes multiple reactions with free and combined chlorine, depending on solution pH (1,3).  Reaction with chlorine yields the following:

Na₂S₂O₃ +       4HOCl +   H₂O            ® 2NaHSO₄  +           4HCl

^{Sodium thiosulfate         Hypochlorous acid                             Sodium bisulfate                 Hydrochloric acid}

Na₂S₂O₃    +   HOCl               ® Na₂SO₄      + S   +              HCl

^{Sodium thiosulfate         Hypochlorous acid                     Sodium sulfate            Hydrochloric acid}

2Na₂S₂O₃     +             HOCl               ®  Na₂S₄O₆    +  NaCl             +    NaOH

^{Sodium thiosulfate       Hypochlorous acid                 Sodium tetrathionate  Sodium chloride   Sodium hydroxide}

 The amount of thiosulfate required for dechlorination may vary with solution pH (3).   Sodium thiosulfate is a reducing agent.  However, it scavenges less oxygen than sodium sulfite, bisulfite or metabisulfite.

Sodium thiosulfate is a skin, eye, nose and throat irritant.  It has a NFPA Rating of 1,0, 0 for health, fire and reactivity, respectively. An EPA toxicity study indicated that sodium thiosulfate is not very toxic to aquatic species.  Sodium thiosulfate may react slowly with chlorine under some conditions, and requires more time for dechlorination than most dechlorination chemicals (4).  Over-dechlorination with sodium thiosulfate may encourage thiobacillus and some other bacterial growth in receiving streams, particularly during low flow conditions.

Sodium Sulfite (Na₂SO₃)

Sodium sulfite is another dechlorinating agent widely used by utilities.  It is available in powder/crystalline and tablet form.  It undergoes the following reaction with free chlorine (3):

Na₂SO₃  +    HOCl      ®Na₂SO₄  +  HCl

^{Sodium sulfite     Hypochlorous acid         Sodium sulfate                 Hydrochloric acid}

On a weight-to-weight basis, approximately 1.775 parts of sodium sulfite are required to remove one part of chlorine (5).  Sodium sulfite is a reducing agent and is reported to scavenge more oxygen than sodium thiosulfate. 

The major advantage of using sodium sulfite is that, it is available in tablet form.  Many utilities find the tablets easier to store, handle and apply as compared to solutions or powders.   In addition, dechlorination tablets are very effective for dechlorinating constant, low flow rate chlorinated releases.

Sodium Bisulfite (NaHSO₃)

Sodium bisulfite is available as a white powder, granule or clear liquid solution.  It is highly soluble in water.  Currently, many industries and wastewater utilities use sodium bisulfite solution for dechlorination.  It undergoes the following reactions with free chlorine:

NaHSO₃     +      HOCl             ®  NaHSO₄   +   HCl

^{Sodium Bisulfite          Hypochlorous acid                 Sodium bisulfate       Hydrochloric acid}

On a weight-to-weight basis, approximately 1.45 parts of sodium bisulfite are required to dechlorinate 1 part of chlorine.  Sodium bisulfite is a good oxygen scavenger.  Sodium bisulfite can cause skin, eye and respiratory tract irritation.  It is harmful if swallowed or inhaled.  Sodium bisulfite may crystallize at room temperatures.  It is highly viscous and sometimes difficult to handle.  Sodium bisulfite is highly corrosive and caution must be exercised in safely handling this chemical.  It has a NFPA rating of 1,0,1.

Sodium Metabisulfite (Na₂S₂O₅)

Sodium metabisulfite is available as crystal, powder or solution.  It reacts with chlorine as follows. 

Na₂S₂O₅    +                2HOCl + H₂O              ®2NaHSO₄   +   2HCl

^{Sodium metabisulfite    Hypochlorous acid          Sodium bisulfate     Hydrochloric acid}

Scavenging properties of sodium metabisulfite are comparable to that of sodium bisulfite.  On a weight-to-weight basis, approximately 1.34 parts of sodium metabisulfite are required to remove 1 part of free chlorine.  Sodium metabisulfite is an eye, throat, skin and lung irritant.  The hazard ratings for sodium metabisulfite are 3,0,1.

Calcium Thiosulfate (CaS₂O₃)

Calcium thiosulfate is a clear crystalline substance, with little color, a faintly sulfurous odor and near neutral pH.  It reacts with free as well as combined chlorine.  Calcium thiosulfate undergoes the following reactions with free chlorine (6). 

CaS₂O₃     +      4HOCl        + H₂O  ®    CaSO₄ +     4HCl     +       H₂SO₄   

^{Calcium thiosulfate     Hypochlorous acid                       Calcium sulfate    Hydrochloric acid       Sulfuric acid}    

Approximately 0.99 and 0.45 mg of calcium thiosulfate is required to neutralize one mg of residual chlorine at pH 7.35 and 11, respectively.  Calcium thiosulfate is not toxic to aquatic species.  The 96-hour LC₅₀ for fathead minnows is greater than 750 mg/L.  The NFPA hazard rating of calcium thiosulfate is 0,0,0.

One concern with using calcium thiosulfate is that dechlorination reactions using stoichiometric concentrations require nearly five minutes for complete neutralization (6).  Over-dosing of calcium thiosulfate may produce milky-colored suspended solids, causing turbidity violations.  Also, excess thiosulfate release may promote thiobacillus bacterial growth. 

Ascorbic Acid (Vitamin C)

Ascorbic acid has recently been used by several water utilities for dechlorination.  The reactions with chlorine are shown below:

C₅H₅O₅CH₂OH  +  HOCl      ® C₅H₃O₅CH₂OH + HCl  +  H₂O

^{Ascorbic acid                Hypochlorous acid      Dehydroascorbic acid        Hydrochloric acid}

Approximately 2.5 parts of ascorbic acid are required to neutralize 1 part of chlorine.  Ascorbic acid is not reported to scavenge DO.  Since ascorbic acid is weakly acidic, the pH of water may decrease slightly in low alkaline waters.  Caution must be exercised to prevent accidental inhalation or contact with the skin, eyes or lungs. 

Sodium Ascorbate

In addition to ascorbic acid, sodium ascorbate is also currently under evaluation by several utilities for dechlorination.  Sodium ascorbate undergoes the following reaction with free chlorine. 

C₅H₅O₅CH₂ONa  +  HOCl      ®  C₅H₃O₅CH₂OH   +   NaCl    +    H₂O

^{Sodium ascorbate                Hypochlorous acid         Dehydroascorbic acid        Sodium chloride}

Approximately 2.80 parts of sodium ascorbate are required to neutralize one part of chlorine.  Sodium ascorbate is not reported to impact the DO or pH of the receiving streams.  The pH of sodium ascorbate is approximately 7.0.  The chemical is very stable with a shelf life of at least one year in a dry state, if kept in a cool, dark place.  However, once in solution, the chemical degrades within a day or two. 

Dechlorination Chemical Summary

Currently, sodium bisulfite, sodium sulfite and sodium thiosulfate are most frequently used by water utilities for dechlorination.  The choice of a particular dechlorination chemical is dictated by site-specific issues such as the nature of water release, strength of chlorine, volume of water release, and distance from receiving waters.  Sodium bisulfite is used by some utilities due to its lower cost and higher rate of dechlorination.  Sodium sulfite tablets are chosen by utilities due to ease of storage and handling, and its ease of use for dechlorinating constant, low flow rate releases.  Sodium thiosulfate is used for dechlorination since it is less hazardous and consumes less oxygen than sodium bisulfite and sodium sulfite.  Ascorbic acid and sodium ascorbate are used because they do not impact DO concentrations. 

Knowledge on dechlorination efficiencies of various chemicals is incomplete.  A comprehensive study evaluating all the chemicals for various chlorinated water release scenarios is not currently available. 

Field Methods for Residual Chlorine Measurement

            Several methods such as water quality test strips, swimming pool test kits and orthotolidine indicator kits can be used to measure TRC in the field.  Many of these methods lack sensitivity required for ensuring regulatory compliance. 

A colorimetric kit supplied by Hach company is widely used to monitor dechlorination in the field.  The kit can measure free or combined chlorine residuals at concentrations of 0 to 4.5 mg/L with a detection limit of 0.1 mg/L.  In this method, a pre-measured amount of reagent is added to the water sample, mixed well, and the sample analyzed for chlorine concentration.  A liquid crystal detector indicates the chlorine concentration in solution based on the intensity of the color formed.