EPA 832-F-00-022 - September 2000

1 United StatesEnvironmental ProtectionAgencyOffice of WaterWashington, 832-F-00-022 September 2000 WastewaterTechnology Fact SheetDechlorinationDESCRIPTIOND echlorination is the process of removing residualchlorine from disinfected wastewater prior todischarge into the environment. Sulfur dioxide ismost commonly used for dechlorination and is themajor focus of this fact sheet. Some dechlorinationalternatives include carbon adsorption, sodiummetabisulfite, sodium bisulfite, and hydrogenperoxide. Sodium metabisulfite and sodiumbisulfite are mainly used in small facilities becausethese materials are more difficult to controlcompared to sulfur dioxide.

2 Hydrogen peroxide isnot frequently used because it is dangerous tohandle (WEF, 1996).APPLICABILITYC hlorination has been used widely to disinfectwastewater prior to discharge since passage of the1972 Federal Water Pollution Control Act(WPCA), (Finger et al., 1985). In the first yearsfollowing the WPCA, disinfected wastewater withsignificant levels of residual chlorine was routinelydischarged into the receiving waters. It becameclear, however, that residual chlorine is toxic tomany kinds of aquatic life (see, for example,Mattice and Zittel, 1976, and Brungs, 1973).

3 Moreover, the reaction of chlorine with organicmaterials in the water formed carcinogenictrihalomethanes and organochlorines (WEF andASCE, 1991). As a result, dechlorination wasinstituted to remove residual chlorine fromwastewater prior to discharge into sensitive minimizes the effect of potentiallytoxic disinfection byproducts by removing the freeor total combined chlorine residual remaining afterchlorination. Typically, dechlorination isaccomplished by adding sulfur dioxide or sulfitesalts ( , sodium sulfite, sodium bisulfite, orsodium metabisulfite).

4 Carbon adsorption is also aneffective dechlorination method, but is expensivecompared to other methods. Carbon adsorption isusually implemented when total dechlorination design criteria and monitoringrequirements for a particular region are determinedby the state regulatory agency. Typically, thetreatment plant s National Pollutant DischargeElimination System (NPDES) permit limits effluentchlorine residual and toxicity. Currently, manypermits require very low or non-detect chlorineresiduals, making dechlorination important alternative to dechlorination is toachieve disinfection without the use of means of disinfection, such as ozone orultraviolet disinfection, have also becomeincreasingly prevalent ( EPA, 1986; Blatchley, III, et al.)

5 , 1996).ADVANTAGES AND DISADVANTAGESA dvantages Protects aquatic life from toxic effects ofresidual chlorine. Prevents formation of harmful chlorinatedcompounds in drinking water throughreaction of residual chlorine with water-born organic Chemical dechlorination can be difficult tocontrol when near zero levels of residualchlorine are required. Significant overdosing of sulfite can lead tosulfate formation, suppressed dissolvedoxygen content, and lower pH of thefinished CRITERIAC hemistry of Dechlorination by SulfonationSulfur dioxide (SO2) is a corrosive, nonflammablegas with a characteristic pungent odor.

6 Atatmospheric temperature and pressure, it is acolorless vapor. When compressed and cooled, itforms a colorless liquid. Sulfur dioxide is suppliedas liquefied gas under pressure in 100 or 150 poundcontainers and one-ton cylinders. As an alternativeto sulfur dioxide gas, various dry chemicals areavailable which form sulfur dioxide in include sodium sulfite (Na2SO3), sodiummetabisulfite (Na2S2O5), sodium bisulfite(NaHSO3), a 38 percent aqueous solution of sodiummetabisulfite, and sodium thiosulfate (Na2S2O3),among others (Lind, 1995).When dissolved in water, chlorine hydrolyzes toform hypochlorous acid (HOCl) and hypochloriteions (OClG) which, taken together, are referred to as free chlorine.

7 (Free, uncombined chlorine, Cl2, israrely found in wastewater since the conditions offormation are relatively extreme [Lind, 1995]).Once formed, the free chlorine reacts with naturalorganic matter in water and wastewater to formchlorinated organic compounds. The free chlorinealso combines with ammonia to form mono-, di-,and trichloramines in quantities dependent on theratio of chlorine to ammonia nitrogen (Lind, 1995).When either sulfur dioxide or sulfite salts aredissolved in water, aqueous sulfur compounds inthe +4 oxidation state are produced, often notatedS(IV) (Helz and Nweke, 1995).

8 The S(IV) species,such as the sulfite ion (SO3-2), reacts with both freeand combined forms of chlorine, as illustrated inequations (1) and (2) (WEF, 1996):(1) SO3-2 + HOCl SO4-2 + Cl- + H+(2) SO3-2 + NH2Cl + H20 SO4-2 + Cl- + NH4+Since free chlorine and inorganic chloramines reactrapidly with S(IV) (Helz, 1998), a short contacttime of one to five minutes is considered to besufficient; nevertheless, complete blending at thepoint of application is essential for effectivedechlorination (WEF and ASCE, 1992).Proper dosage is critical to produce a non-detectable chlorine residual.

9 On a mass basis, sulfur dioxide (or parts NaHSO3 or Na2S2O5) is required to dechlorinate partresidual chlorine. In practice, approximately a one-to-one ratio is used (WEF, 1996). Dosing in excessmust be avoided because excess sulfite can reactwith dissolved oxygen (four parts sulfite to one partoxygen) in the wastewater to produce sulfates,potentially leading to reduced dissolved oxygenconcentrations and low pH levels in the finishedeffluent for high levels of overdose (WEF, 1996).Careful process control will help for gaseous sulfonationEquipment required for gaseous sulfonation usingSO2 is similar in design to that used forchlorination, except that the materials are chosenfor their application-specific chemical resistance(WEF, 1996).

10 The four basic components of thesystem include: sufficient gas supply withautomatic switch-over between cylinders; ametering system, usually consisting of a vacuumregulator and a rotameter for feed rate control; oneor more injectors with check valves; and a residualanalyzer to measure and transmit a continuoussignal proportional to the chlorine residual in thesample small concentrations, exposure to SO2 can causeeye and throat irritation. In high concentrations,exposure can produce a suffocating effect caused byirritation to the upper respiratory tract. Therefore,facility design should include features for safestorage, handling, and use of sulfur dioxide.