Dechlorination chemicals are 6 reducing agents in 3 classes, from sodium bisulfite and sulfur dioxide to the thiosulfates and vitamin C, that convert chlorine residual into chloride before water reaches a stream, a membrane or a sensitive process. Their dose ratios run from 0.90 mg of sulfur dioxide to about 2.8 mg of sodium ascorbate per mg of chlorine, so which agent fits which system?
The 6 agents are 3 sulfur(IV) compounds (sodium bisulfite, sodium metabisulfite and sulfur dioxide), 2 thiosulfates (sodium and calcium thiosulfate) and 1 ascorbate group, also spelled sulphite, thiosulphate and sulphur dioxide. They form the eleventh of the 16 families of water treatment chemicals.
Effluent permits, polyamide membranes (below 0.1 mg/L free chlorine) and flushed mains set the choice, and dechlorination adds about 30 to 50 % to the cost of wastewater chlorination (US EPA 832-F-99-062). NSF/ANSI/CAN 60 caps drinking-water use at product-specific maximum use levels, with residual sulfite held to 0.1 mg/L.
Every value on this page is checked against primary sources (NSF listings, eCFR, AWWA, CEN, US EPA, membrane and resin manufacturers' manuals); see our methodology and fact-checking process.
What Is Dechlorination in Water Treatment?#
Dechlorination in water treatment is the practice of partially or totally removing the chlorine residual from treated water, usually by adding a reducing agent such as sulfur dioxide, sodium bisulfite or sodium thiosulfate (AWWA M20, Water Chlorination/Chloramination Practices and Principles).
Free available chlorine is the sum of dissolved Cl2, hypochlorous acid (HOCl) and hypochlorite (OCl-), combined chlorine is the chloramines, and total residual chlorine (TRC) is both, expressed as Cl2. How HOCl and OCl- shift with pH is explained under water chlorination chemistry.
Why is chlorine removed from treated water?#
Chlorine is removed for 3 reasons: residual chlorine in effluent is toxic to aquatic life, polyamide reverse osmosis membranes degrade under free chlorine, and ion-exchange resins lose life when chlorine reaches them. The 3 reasons carry these numbers.
- Aquatic toxicity: the US EPA freshwater acute criterion for chlorine is 19 µg/L (1986).
- Membrane damage: polyamide RO elements tolerate less than 0.1 mg/L (ppm) of free chlorine.
- Resin life: a gel cation resin lasts about 10 years below 0.2 ppm of chlorine and about 6 years at 1 ppm (DuPont AmberLite Water Conditioning Manual).
Each reason sets the point where the dechlorination chemical is dosed.
Where is a dechlorination chemical dosed?#
A dechlorination chemical is dosed at the end of the chlorine contact time: after the contact tank at a wastewater plant, upstream of the cartridge filters ahead of an RO train, and at the outlet of a flushed main or hydrant in the field. RO intakes typically carry 0.5 to 1.0 mg/L free chlorine through pretreatment for 20 to 30 minutes first. Field crews feed by venturi, gravity carboy, metering pump or diffuser.
How Do Dechlorination Chemicals Work?#
Dechlorination chemicals work by reduction: the agent donates electrons to hypochlorous acid or chloramine, chlorine leaves as chloride, and the agent leaves oxidized, for example sulfite as sulfate. Sulfur(IV) agents react fastest, thiosulfates slowest.
Reducing free chlorine and chloramine#
Sulfur(IV) agents reduce chlorine in one fast step: each bisulfite ion converts one hypochlorous acid molecule into chloride and leaves as sulfate (HSO3- + HOCl → SO4 2- + Cl- + 2 H+), and the same chemistry reduces monochloramine. Why does metabisulfite behave like bisulfite? It hydrolyzes to 2 bisulfite ions (Na2S2O5 + H2O → 2 NaHSO3), so 1 kg of sodium metabisulfite equals 1.095 kg of sodium bisulfite.
Thiosulfate products depend on pH (sulfate, sulfur or tetrathionate), and sodium thiosulfate is the slowest common agent (Basu and De Souza, Journal of Water Supply, 2011). Activated carbon reacts as C + 2 Cl2 + 2 H2O → 4 HCl + CO2, and ascorbic acid is not effective for monochloramine.
Why do sulfite agents also remove oxygen and alkalinity?#
Sulfite agents remove oxygen and alkalinity because the same S(IV) that reduces chlorine also reacts with dissolved oxygen and releases acid: sulfur dioxide consumes about 2.8 mg of alkalinity as CaCO3 per mg of chlorine reduced (AWWA M20). Sodium bisulfite consumes 1.41 mg as CaCO3 per mg Cl2 and 6.5 mg per mg of dissolved oxygen, and sodium sulfite about 8 parts per part of oxygen. AWWA M20 ranks the agents by oxygen scavenging as follows.
- Strong: sulfur dioxide, sodium bisulfite and sodium metabisulfite.
- Moderate: sodium sulfite.
- Weak: sodium and calcium thiosulfate.
- None reported: the ascorbates (hydrogen peroxide adds oxygen).
In steam systems the same sulfite chemistry protects boilers, and its feedwater doses are compared on oxygen scavengers for boiler water.
6 Types of Dechlorination Chemicals in 3 Classes#
The 6 dechlorination chemicals used in water treatment fall into 3 classes: 3 sulfur(IV) compounds (sodium bisulfite, sodium metabisulfite and sulfur dioxide), 2 thiosulfates (sodium and calcium thiosulfate) and 1 ascorbate group (ascorbic acid and sodium ascorbate). Table 1 compares the 3 classes by members, active species, oxygen scavenging, 1 % solution pH and typical system.
| Class | Members on this site | Active species | Oxygen scavenging (AWWA M20) | 1 % solution pH (AWWA M20) | Typical system |
|---|---|---|---|---|---|
| Sulfur(IV) compounds | sodium bisulfite, sodium metabisulfite, sulfur dioxide | HSO3- / SO3 2- | strong | SBS 4.3; SMBS 4.3 | wastewater effluent, RO feed |
| Thiosulfates | sodium thiosulfate, calcium thiosulfate | S2O3 2- | weak | 7.0; 6.5 to 7.0 | water-main and field discharges, wastewater |
| Ascorbates | ascorbic acid (with sodium ascorbate) | ascorbate | none reported | sodium ascorbate 7.0 | field discharges of mains, hydrants and tanks |
1. Sulfur(IV) compounds (3)#
Sulfur(IV) dechlorination agents are sulfur dioxide and the sulfite salts that form bisulfite in water; this class has 3 chemicals: sodium bisulfite, sodium metabisulfite and sulfur dioxide, all strong oxygen scavengers.
Sodium bisulfite (SBS)#
Sodium bisulfite (NaHSO3) is the liquid workhorse of dechlorination: it is sold as a solution of about 40 % (520 g/L, EN 12120:2022) or 38 %, with a relative density of about 1.34, and it crystallizes near 2 °C (36 °F). As supplied it has a pH of 4.5 to 5.0. Drinking-water plants dose it up to the product MUL, keeping residual sulfite at or below 0.1 mg/L.
Sodium metabisulfite (SMBS)#
Sodium metabisulfite (Na2S2O5) is the dry form of bisulfite chemistry: a white powder that operators dissolve on site to 2 to 30 % solutions, which keep 1 week at 10 %, 1 month at 20 % and 6 months at 30 % (DuPont FilmTec Reverse Osmosis Technical Manual). RO plants use food-grade SMBS that is not cobalt-activated.
Sulfur dioxide (SO2)#
Sulfur dioxide (SO2) is the gas-phase dechlorination agent of large wastewater plants, fed from 100 % liquefied-gas cylinders or 907 kg (1-ton) containers through a vacuum sulfonator, a process called sulfonation. It boils at -10 °C (14 °F) and has the lowest theoretical ratio, 0.90 mg per mg Cl2.
2. Thiosulfates (2)#
Thiosulfates are S2O3 2- salts that reduce chlorine with weak oxygen scavenging and a near-neutral pH (7.0 and 6.5 to 7.0 as 1 % solutions); this class has 2 chemicals: sodium thiosulfate and calcium thiosulfate.
Sodium thiosulfate#
Sodium thiosulfate (Na2S2O3) is the slowest common dechlorination agent and the classic choice for field discharges: a 30 % solution stays at strength for 2 to 3 days after mixing (AWWA M20). UK practice uses 10 g of pentahydrate crystals per 1,000 L (264 gal) at 5 mg/L free chlorine (Water UK TGN 14).
Calcium thiosulfate#
Calcium thiosulfate (CaS2O3) is a near-neutral 20 to 30 % solution that neither scavenges oxygen nor releases sulfur dioxide, and it needs nearly 5 minutes to neutralize chlorine completely at a stoichiometric dose (AWWA M20). In spent membrane-cleaning solutions it showed the shortest half-life and reaction time of the reducing chemicals tested (Ko, 2016).
3. Ascorbates (1)#
Ascorbates are vitamin C compounds used for field dechlorination of mains, hydrants and tanks; this class has 1 entity, ascorbic acid, which also covers its neutral salt sodium ascorbate.
Ascorbic acid and sodium ascorbate#
Ascorbic acid (C6H8O6) and sodium ascorbate are the vitamin C dechlorination agents for field use, dosed at about 2.5 and 2.8 mg per mg of chlorine (AWWA M20), with sodium ascorbate the pH-neutral form. The US Forest Service rule of thumb is 1 g per 100 gal (379 L) per 1 mg/L of chlorine; solutions degrade within days.
Other dechlorination methods: sodium sulfite, hydrogen peroxide, activated carbon and holding#
Four other methods remove chlorine without a dechlorination-family chemical: sodium sulfite, hydrogen peroxide, activated carbon and holding the water until chlorine dissipates. The 4 methods differ as follows.
- Sodium sulfite: 1.78 g per g Cl2 in theory (DuPont AmberLite manual) and 1.96 in the field (AWWA M20).
- Hydrogen peroxide: 0.48 mg per mg Cl2 (AWWA M20); it adds oxygen.
- Activated carbon: oxidized to CO2; certified under NSF/ANSI/CAN 61 as process media, not NSF/ANSI/CAN 60.
- Holding: 0.5 to 2.0 mg/L of chlorine dissipates in several hours to a few days; chloramines take 3 to 4 times longer.
Which Dechlorination Chemical for Which System?#
Dechlorination chemical selection depends on 4 factors: the system, the chlorine species, the allowed effect on oxygen and pH, and the feed form (gas, liquid, powder or field kit), and the discharge or membrane limit decides the target residual. Plants should pick the agent whose side effect the receiving system tolerates.
Municipal wastewater effluent#
Municipal wastewater plants dechlorinate with sulfur dioxide or sodium bisulfite after the chlorine contact tank, because both react fast and their gas or liquid feed tracks large flows, and they then check dissolved oxygen in the effluent. Typical wastewater chlorine doses are 5 to 20 mg/L (US EPA 832-F-99-062), and sulfur dioxide enters through 316 stainless steel diffusers. Contact-tank design and the full effluent program sit on the wastewater disinfection and dechlorination page.
Reverse osmosis and ion-exchange feed#
RO systems dechlorinate with sodium metabisulfite or sodium bisulfite upstream of the cartridge filters, because polyamide membranes tolerate less than 0.1 mg/L of free chlorine and degrade after about 200 to 1,000 ppm-h of exposure (DuPont FilmTec Chlorination and Dechlorination Manual). The SMBS enters through a static mixer ahead of the 5 µm cartridge filter and the high-pressure pump.
Resins allow about 0.1, 0.2 and 0.6 mg/L as Cl2 for gel, high-crosslink gel and macroporous types at 15 to 20 °C (59 to 68 °F). Antiscalants, cleaners and the other RO chemicals are compared under reverse osmosis and membrane chemicals.
Water mains and field discharges#
Field dechlorination of flushed mains, hydrants and tanks relies on sodium thiosulfate and the ascorbates, because the 2025 revision notice for AWWA C655-25 listed sulfur dioxide, sodium sulfite, sodium bisulfite and sodium metabisulfite for removal from the field chemical list. The same notice (2025-02-18) added a required Disinfection and Dechlorination Plan. Crews target a non-detect residual or the state limit, 0.1 mg/L in several states (AWWA M20).
By application: drinking water, wastewater, membranes and cooling water#
Each application has a default dechlorination chemical: sodium bisulfite or SO2 for wastewater effluent, SMBS or SBS for RO feed, thiosulfate or ascorbate for water-main discharges and a sulfite salt within its NSF limit for drinking water. Table 2 lists the default agents, the reason and the key number per application.
| Application | Default agents | Why | Key number |
|---|---|---|---|
| drinking water treatment chemicals | SBS, SMBS, calcium thiosulfate, ascorbates within product MULs | plant or storage dechlorination | residual sulfite ≤ 0.1 mg/L (NSF listing footnote) |
| wastewater disinfection and dechlorination | SO2, SBS, SMBS | effluent TRC permit | chlorine dose 5 to 20 mg/L; dechlorination adds about 30 to 50 % to chlorination cost |
| RO pretreatment chemistry | SMBS, SBS | polyamide protection | < 0.1 mg/L free chlorine; ORP trip 175 to 200 mV |
| RO membrane preservation | SMBS | storage solution | 1 to 1.5 % SMBS; replace at pH 3 or lower |
| water main disinfection and field dechlorination | sodium thiosulfate, ascorbates, calcium thiosulfate | flushing and hydrant discharges | AWWA C655-25; 2025 revision notice |
| cooling tower water treatment | SBS (blowdown or once-through return) | 40 CFR 423.13 limits | FAC 0.2 mg/L average, 0.5 mg/L maximum |
| ion exchange resins for water treatment | sodium sulfite or SO2 ahead of the resin | resin life | 1.78 g Na2SO3 or 0.91 g SO2 per g Cl2 |
Effluent dechlorination is one step of a larger discharge program, and every effluent program is compared on wastewater treatment chemicals.
Dechlorination Dose Ratios: mg of Chemical per mg of Chlorine#
Dechlorination dose ratios run from 0.90 mg of sulfur dioxide to 1.78 mg of sodium sulfite per mg of chlorine in theory, while AWWA M20 field values at pH 7.0 sit about 10 % higher for the sulfur(IV) agents, from 0.99 mg for SO2 to 1.96 mg for sodium sulfite. Theoretical ratios come from molar masses (64.06 g of SO2 per 70.90 g of Cl2), field ratios from AWWA M20 Table A-2.
Membrane plants dose above both: sodium metabisulfite at 3.0 mg per mg Cl2 in practice (DuPont FilmTec) and sodium bisulfite at a design range of 1.8 to 3.0 (Hydranautics TAB-111). Table 3 compares the ratios and side effects.
| Agent | Theoretical | Source | Field, pH 7.0 (AWWA M20 Table A-2) | RO practice | Side effect per mg Cl2 |
|---|---|---|---|---|---|
| Sodium bisulfite (as NaHSO3) | 1.47 | Hydranautics TAB-111 | 1.61 | 1.8 to 3.0 design | 1.41 mg alkalinity as CaCO3 (calculated); strong O2 scavenger |
| Sodium metabisulfite (as Na2S2O5) | 1.34 | DuPont FilmTec | 1.47 | 3.0 | strong O2 scavenger |
| Sulfur dioxide (as SO2) | 0.90 | molar masses 64.06 / 70.90 | 0.99 | n/a | about 2.8 mg alkalinity as CaCO3 (AWWA M20); strong O2 scavenger |
| Sodium thiosulfate (as Na2S2O3) | not stated | n/a | 1.9 | n/a | weak O2 scavenger |
| Calcium thiosulfate | not stated | n/a | not stated | n/a | no O2 scavenging, no SO2 release |
| Ascorbic acid / sodium ascorbate | n/a | n/a | about 2.5 / about 2.8 | n/a | ascorbic acid lowers pH in low-alkalinity water; raises TOC |
| Related: sodium sulfite (as Na2SO3) | 1.78 | DuPont AmberLite manual | 1.96 | n/a | moderate O2 scavenger |
| Related: hydrogen peroxide (as H2O2) | n/a | n/a | 0.48 | n/a | adds oxygen |
All ratios are for the pure chemical, never as product; divide by the product strength to get the product dose.
How is a dose ratio turned into a product feed rate?#
A dose ratio becomes a feed rate in 3 steps: multiply the chlorine to remove by the ratio, divide by the product strength, then multiply by flow, so removing 1.0 mg/L of chlorine from 10,000 m³/d (2.64 MGD) with 38 % sodium bisulfite at the field ratio of 1.61 takes 42.4 kg/d (93.4 lb/day) of product. Table 4 is the worked example with every input stated.
| Input or step | Metric | US customary |
|---|---|---|
| Flow | 10,000 m³/d | 2.64 MGD |
| Chlorine to remove | 1.0 mg/L as Cl2 | 1.0 ppm as Cl2 |
| Ratio (AWWA M20 field, pH 7.0) | 1.61 mg NaHSO3 per mg Cl2 | same |
| Product | 38 % SBS, density 1.34 kg/L | 38 % SBS, SG 1.34 |
| Active feed | 16.1 kg/d as NaHSO3 | 35.5 lb/day |
| Product feed | 42.4 kg/d | 93.4 lb/day |
| Product volume | 31.6 L/d | 8.35 gal/d |
Worked example arithmetic: 1.61 x 1.0 mg/L x 10,000 m³/d / 1,000 = 16.1 kg/d; / 0.38 = 42.4 kg/d; / 1.34 kg/L = 31.6 L/d. In US units, 1.61 x 2.642 MGD x 8.34 = 35.5 lb/day, and / 0.38 = 93.4 lb/day, or 4.24 mg of product per mg Cl2. The chemical dosage calculator converts mg/L into kg/d, lb/day and pump settings.
What does a dechlorination overdose do?#
A dechlorination overdose causes 4 problems: it lowers dissolved oxygen, lowers pH, adds sulfate or organic carbon, and with thiosulfates feeds Thiobacillus in the receiving water. The 4 effects come from different agents.
- Lower dissolved oxygen and pH: excess sulfur(IV) agents (US EPA 832-F-00-022).
- Sulfate: every sulfite agent ends as sulfate, whose US secondary standard is 250 mg/L (40 CFR 143.3, non-enforceable).
- Organic carbon: ascorbates raise TOC.
- Thiobacillus: excess thiosulfate in continuous discharges (AWWA M20); overdosed calcium thiosulfate also forms milky solids.
How Is Dechlorination Controlled and Tested?#
Dechlorination is controlled in 3 layers: total residual chlorine sets and proves the dose, ORP guards RO membranes in real time, and feed-forward or feedback loops adjust the pump to flow and residual.
Total residual chlorine measurement#
Total residual chlorine after dechlorination is measured by amperometric titration or DPD (Standard Methods 4500-Cl D, E, F or G under 40 CFR 136.3), and each sample is analyzed within 15 minutes of collection. Residual sulfite is measured by 4500-SO3 2- B, also within 15 minutes.
US EPA Region 1 uses 20 µg/L as the TRC minimum level for compliance. Free, combined and total chlorine are compared under chlorine residual in water treatment.
ORP control ahead of RO membranes#
ORP control protects RO membranes by tripping the high-pressure pump when the reading downstream of the dechlorination mixer rises past the threshold: 175 to 200 mV for DuPont FilmTec elements, while Hydranautics warns above 300 mV and shuts down at 350 mV. No single setpoint fits every membrane.
Setpoints by system are covered under oxidation-reduction potential (ORP).
Feed-forward and feedback dose control#
Feed-forward control sets the dechlorination dose from flow times the chlorine residual measured before the dosing point, and US EPA reserves it for plants that must reach a zero residual without a biased analyzer (EPA 832-F-00-022). Feedback control trims the dose from a downstream residual, and the same fact sheet calls ORP suitability for effluent control disputed.
Which Standards and Regulations Apply to Dechlorination Chemicals?#
Dechlorination chemicals are governed in 4 layers: NSF/ANSI/CAN 60 certification for drinking water, AWWA and EN product standards, discharge limits on residual chlorine, and food-steam and process-safety rules for specific agents.
NSF/ANSI/CAN 60 maximum use levels for dechlorination chemicals#
NSF/ANSI/CAN 60 maximum use levels for dechlorination chemicals are product-specific and stated as product, from 8 to 160 mg/L for sodium bisulfite, most commonly 46 mg/L, to 10 mg/L for sulfur dioxide (NSF listings, 2026-10-20). The standard covers dechlorinators as chemicals added but not intended to remain (NSF/ANSI/CAN 60-2020). Table 5 gives the MULs per agent.
| Agent | Most common MUL, mg/L as product | Range, mg/L as product | Listing note |
|---|---|---|---|
| Sodium bisulfite | 46, then 50 | 8 to 160 (160 = 10 % dilute product) | residual sulfite ≤ 0.1 mg/L (100 ppb) |
| Sodium metabisulfite | 15 | 10 to 50 | residual sulfite ≤ 0.1 mg/L |
| Sulfur dioxide | 10 | 10 | function Dechlorination |
| Sodium thiosulfate | 50 (30 % solutions) | 50 | no "Sodium Thiosulfate" category; listed as Miscellaneous Treatment Chemical |
| Calcium thiosulfate | 50 (all listed products) | 50 | use only to the extent necessary to ensure chlorine removal |
| Ascorbic acid / sodium ascorbate | 12 | 12 | function Dechlorination |
MULs count the total dose across all application points, and the 0.1 mg/L sulfite footnote is a recommendation, not an EPA MCL. How MULs are set is explained on NSF/ANSI/CAN 60.
AWWA and EN standards for dechlorination chemicals#
Two AWWA B-standards and four EN standards specify dechlorination chemicals, ANSI/AWWA B601-25 for sodium metabisulfite and B512-23 for sulfur dioxide, while AWWA C655-25 governs field dechlorination as a practice standard. Table 6 lists editions and 21 CFR 173.310 status (AWWA standards list read 2026-10-20; eCFR as of 2026-09-01).
| Agent | AWWA | EN | 21 CFR 173.310 |
|---|---|---|---|
| Sodium bisulfite | no B-standard; listed for removal from C655 in the 2025 revision notice | EN 12120:2022 | not listed |
| Sodium metabisulfite | B601-25; listed for removal from C655 in the 2025 revision notice | EN 12121:2022 | listed |
| Sulfur dioxide | B512-23; listed for removal from C655 in the 2025 revision notice | none stated | not listed |
| Sodium thiosulfate | C655-25 field use; no B-standard | EN 12125:2022 | not listed |
| Calcium thiosulfate | none | none found | not listed |
| Ascorbic acid / sodium ascorbate | C655-25 field use; not among the 4 listed for removal | none | not listed |
| Sodium sulfite (related) | listed for removal from C655 in the 2025 revision notice | EN 12124:2022 | listed (neutral or alkaline) |
Every AWWA edition is tracked under AWWA B-series standards.
AWWA C651-23 covers disinfecting water mains, the step before field dechlorination.
EN 12120:2022 superseded the 2012 edition.
The EN titles keep British spelling ("sodium hydrogen sulphite") and sit in the CEN/TC 164 series under EN standards for drinking water treatment chemicals.
Discharge limits for residual chlorine#
Residual chlorine limits in discharge permits follow the US EPA aquatic-life criteria of 19 µg/L acute and 11 µg/L chronic for freshwater (1986), so many effluents must dechlorinate to below the analytical minimum level. Saltwater criteria are 13 and 7.5 µg/L. Steam electric plants meet 40 CFR 423.13: 0.2 mg/L average free available chlorine in cooling tower blowdown and 0.20 mg/L TRC maximum for once-through units of 25 MW or more.
Drinking-water residuals sit far higher, with an MRDL of 4.0 mg/L as Cl2 (40 CFR 141.65). How a TRC limit enters a permit is covered under NPDES permits and chemical discharges.
Food-contact steam listing and process safety#
Sulfur dioxide is the only dechlorination chemical with process-safety thresholds: 5,000 lb (2,268 kg) anhydrous under EPA RMP (40 CFR 68.130) and 1,000 lb (454 kg) liquid under OSHA PSM (29 CFR 1910.119 Appendix A), and of the 6 agents only sodium metabisulfite is listed in 21 CFR 173.310. The other 5 are not listed in 21 CFR 173.310 (eCFR as of 2026-09-01).
The full additive list sits under 21 CFR 173.310.
Sodium bisulfite has no threshold in either rule.
Sulfur dioxide installations require gas detectors (AWWA M20).
All thresholds are listed under EPA RMP and OSHA PSM.
Complete List of Dechlorination Chemicals: All 6 in One Table#
The complete list of dechlorination chemicals below gives all 6 agents with their CAS number, class, function, systems, NSF/ANSI/CAN 60 maximum use level, AWWA and EN standard and 21 CFR 173.310 status.
| # | Dechlorination chemical | CAS | Class | Function | Systems | NSF 60 most common MUL (as product, NSF listings, 2026-10-20) | AWWA / EN | 21 CFR 173.310 |
|---|---|---|---|---|---|---|---|---|
| 1 | sodium bisulfite | 7631-90-5 | sulfur(IV) | dechlorination; oxygen scavenging; membrane preservation | wastewater effluent; RO feed; drinking water; cooling discharge | 46 mg/L (range 8 to 160) | none / EN 12120:2022 | not listed |
| 2 | sodium metabisulfite (SMBS) | 7681-57-4 | sulfur(IV) | dechlorination; oxygen scavenging; RO biostatic and preservation | RO feed; wastewater effluent; drinking water | 15 mg/L (range 10 to 50) | B601-25 / EN 12121:2022 | listed |
| 3 | sulfur dioxide | 7446-09-5 | sulfur(IV), gas | dechlorination (sulfonation); oxygen scavenging | large wastewater plants | 10 mg/L | B512-23 / none stated | not listed |
| 4 | sodium thiosulfate | 7772-98-7 (anhydrous); also 10102-17-7 (pentahydrate) | thiosulfate | dechlorination; ozone residual quenching | water mains and field discharges; wastewater | 50 mg/L (30 % solution, Miscellaneous Treatment Chemical) | C655-25 field use / EN 12125:2022 | not listed |
| 5 | calcium thiosulfate | 10124-41-1 | thiosulfate | dechlorination; ozone reduction; hypobromite neutralization | field discharges; drinking water; wastewater; spent membrane-cleaning solutions | 50 mg/L | none / none found | not listed |
| 6 | ascorbic acid (vitamin C) dechlorination | 50-81-7 (ascorbic acid); 134-03-2 (sodium ascorbate) | ascorbate | field dechlorination | mains, hydrants and tanks | 12 mg/L | C655-25 field use / none | not listed |
Three related agents from other families also dechlorinate; Table 8 lists them outside the count of 6.
| Agent | Family | Ratio, mg per mg Cl2 | Note |
|---|---|---|---|
| sodium sulfite | oxygen scavengers | 1.78 theoretical (DuPont AmberLite manual); 1.96 field (AWWA M20) | EN 12124:2022; listed in 21 CFR 173.310 (neutral or alkaline) |
| hydrogen peroxide | oxidants | 0.48 (AWWA M20) | adds oxygen; no AWWA B-standard |
| activated carbon in water treatment | media | reaction C + 2 Cl2 + 2 H2O → 4 HCl + CO2 | certified under NSF/ANSI/CAN 61, not 60 |
Every other chemical is filterable by CAS, function and NSF status in the water treatment chemical database.
Who Supplies Dechlorination Chemicals and Programs?#
Dechlorination chemicals reach plants by 3 supply routes: bulk sodium bisulfite solution of 38 to 40 %, dry sodium metabisulfite or thiosulfate crystals, and liquefied sulfur dioxide in 45 or 68 kg (100 or 150 lb) cylinders and 907 kg (1-ton) containers.
Producers, regional distributors and repackagers serve these routes, compared under water treatment chemical suppliers.
Price depends on concentration and basis, volume and container, freight distance, contract term and NSF/ANSI/CAN 60 certification. Public procurement records show what buyers paid at a given date and place; they include delivery and local terms and are not a current price. The Appomattox River Water Authority and South Central Wastewater Authority bid tabulation (Virginia, US, bids opened 2025-01-09) shows 4 bids for 38 % sodium bisulfite from USD 358.60 to 459.50 per wet ton as product.
Buyers choose a program provider that sizes the dose, control loop and permit reporting, or a supplier that delivers to an NSF/ANSI/CAN 60, AWWA or EN specification; comparing providers is covered under water treatment companies.
Are Dechlorination Chemicals Safe in Drinking Water and at the Plant?#
Certified dechlorination chemicals meet the NSF/ANSI/CAN 60 health-effects requirements at or below each product's maximum use level, with residual sulfite in finished water capped at 0.1 mg/L by the listing footnote, while at the plant sulfur dioxide is the main hazard: its harmonised CLP Annex VI classification (index 016-011-00-9) includes H331, H370 and H314. NIOSH sets a REL of 2 ppm TWA and OSHA a PEL of 5 ppm. The harmonised classifications of sodium bisulfite and sodium metabisulfite (index 016-063-00-2) include H302, the latter also H318.
Which chemical is used most for dechlorination?#
Sulfur compounds are the standard dechlorination chemicals: AWWA M20 lists sulfur dioxide, sodium bisulfite, sodium metabisulfite, sodium sulfite and sodium thiosulfate as the common agents, with calcium thiosulfate, the ascorbates, hydrogen peroxide and activated carbon as alternatives.
Does dechlorination remove chloramine?#
Yes, sulfite agents such as sodium bisulfite reduce monochloramine as well as free chlorine, but ascorbic acid is not effective for monochloramine (Basu and De Souza, 2011).
How utilities form and hold that residual is covered under monochloramine.
Does sodium bisulfite remove chlorine dioxide and chlorite?#
No: in a toxicity study sodium bisulfite did not remove chlorine dioxide or chlorite toxicity to Daphnia magna even at 10 times the stoichiometric dose, although it removed chlorine toxicity.
How fast do dechlorination chemicals react?#
Sulfite agents and ascorbic acid react within seconds, and Hydranautics allows at least 20 seconds for sodium bisulfite ahead of RO, while sodium thiosulfate is slower and calcium thiosulfate needs nearly 5 minutes at a stoichiometric dose.
Which chemicals must never be mixed with dechlorination chemicals?#
Acids and oxidizers must never meet dechlorination chemicals in storage: an acid releases sulfur dioxide gas from bisulfite or metabisulfite, and bisulfite is incompatible with hypochlorite and other oxidizers, so the products are stored apart. Thiosulfate with acid releases SO2 and H2S. Every pair is listed under water treatment chemical compatibility.