Chlorine Dioxide in Water Treatment: Uses, Dosage, Specifications and Safety

Chlorine dioxide (ClO2; CAS 10049-04-4) is a yellow-green oxidizing gas that water plants generate on site as a dilute solution and dose as an oxidant and primary disinfectant. Drinking water doses run from 0.07 to 2.0 mg/L (ppm) as ClO2, under a US maximum residual disinfectant level (MRDL) of 0.8 mg/L as ClO2 (40 CFR 141.65) and a chlorite MCL of 1.0 mg/L (40 CFR 141.64).

Chlorite sets the real ceiling: up to 70 % of each dose becomes chlorite, so dose ceilings fall between about 0.7 and 1.4 mg/L as ClO2 depending on the jurisdiction. Chlorine dioxide is one of the oxidants in this reference of water treatment chemicals, each documented with the same identity, dose and compliance fields.

The identity card below summarizes the attributes that define chlorine dioxide in water treatment.

Every value on this page is checked against primary sources (eCFR, EPA, WHO, Health Canada, EUR-Lex, NSF listings, AWWA, CEN); see our methodology and fact-checking process.

What Is Chlorine Dioxide (ClO2)?#

Chlorine dioxide is a free-radical oxidizing gas (ClO2, 67.45 g/mol, also called chlorine(IV) oxide) that is generated where it is used because it cannot be shipped or stored safely. The gas is yellow-green, its solution yellow, and it boils at 11 °C (52 °F) (WHO background document).

Why does a plant never keep a tank of it? The gas becomes explosive at 10 % or more in air (Irish EPA Disinfection Manual), and the Veolia water handbook reports a decomposition wave above 14 % with a spark. It is also about 700 times more volatile than hypochlorous acid (same handbook). The table below lists the physical properties that matter for dosing and handling.

"Chlorine dioxide gas treatment" means this same gas dissolved into water.

Is chlorine dioxide sold as a solution or a gas?#

Chlorine dioxide is sold as neither: plants buy precursor chemicals and generate a dilute ClO2 solution on demand. US EPA disinfection profiling guidance describes a typical stock of about 500 mg/L, Xylem's supplier FAQ gives 1 to 3 g/L for its generators, and the Irish EPA manual reports vacuum generators producing up to 40 g/L. One precursor grade is a 31 % Active solution with at least 30.5 % NaClO2, which plants order as sodium chlorite on its own specification.

What Does Chlorine Dioxide Do in Water Treatment?#

Chlorine dioxide works in 5 roles, each based on one-electron oxidation that adds no chlorine to the target compound. The 5 roles are listed below.

• Pre-oxidant: raw or settled water ahead of filtration.
• Primary disinfectant: Giardia and virus inactivation at low CT.
• Oxidizing biocide: building water and cooling systems; ClO2 penetrates biofilm (UK HSE HSG274 Part 1).
• Taste and odor control: oxidation of odor compounds.
• Iron and manganese oxidation: Mn(II) oxidation above pH 7.

In neutral water each ClO2 molecule accepts one electron and becomes chlorite (ClO2-); under strongly acidic conditions it can accept 5 electrons (WHO background document). Chlorine dioxide does not react with ammonia, and its efficacy stays largely unchanged between pH 6 and 9.

Why does chlorine dioxide form chlorite instead of trihalomethanes?#

Chlorine dioxide forms chlorite because it oxidizes by electron transfer, not by chlorine substitution, so its by-products are chlorite, chlorate and chloride rather than chlorinated organics (WHO Guidelines for Drinking-water Quality). Up to 70 % of the applied dose is reduced to chlorite, while chlorate typically stays below 20 % of the dose. High-purity ClO2 forms no trihalomethanes (THM) or haloacetic acids (HAA), and it does not form bromate. Chlorate still matters: the WHO background document attributes up to 35 % of chlorate in distribution to generator type and tuning.

Where Is Chlorine Dioxide Used? Drinking Water, Building Water, Cooling and Wastewater#

Chlorine dioxide is used in 4 water systems: drinking water, building water, cooling water and municipal wastewater. The table below compares the dose, feed point and control test in each system.

Drinking water: pre-oxidation and primary disinfection#

Drinking water plants dose 0.07 to 2.0 mg/L (ppm) as ClO2, either at raw or settled water for pre-oxidation or after filtration for primary disinfection (WHO; Irish EPA). The residual leaving the plant is typically a few tenths of a mg/L. The US Stage 1 Disinfectants and Disinfection Byproducts Rule also covers transient non-community systems that use chlorine dioxide (40 CFR 141.130).

Because ClO2 gives no lasting residual (WHO), plants pair it with chlorine or chloramine for distribution, a sequence described in the drinking water disinfection overview. Under the LT2 rule, chlorine dioxide is one of the toolbox options that Bin 3 and Bin 4 systems can use for additional Cryptosporidium treatment (40 CFR 141.720).

What CT values does chlorine dioxide need for Giardia, viruses and Cryptosporidium?#

Chlorine dioxide needs a CT of 23 mg·min/L for 3-log Giardia inactivation at 10 °C (50 °F), against 830 mg·min/L for 3-log Cryptosporidium at the same temperature (40 CFR 141.74 and 141.720). The table below lists the regulatory CT values by water temperature.

CT in mg·min/L, pH 6 to 9. At 10 °C the Irish EPA manual adds 4.2 and 25.1 for 2-log and 4-log viruses and 138 for 0.5-log Cryptosporidium; 40 CFR 141.720 gives Cryptosporidium log credit = 0.001506 x 1.09116^T x CT. Other temperatures: CT value calculator.

At 10 °C a CT of 23 mg·min/L earns 3-log Giardia credit but stays below the 138 needed for 0.5-log Cryptosporidium; the method is explained under CT values for disinfection.

Iron, manganese and taste and odor oxidation#

Chlorine dioxide oxidizes manganese at 2.45 mg ClO2 per mg Mn(II) above pH 7, against 1.9 mg for potassium permanganate, 1.3 mg for hypochlorous acid and 0.87 mg for ozone (Health Canada manganese guideline document, 2019). A raw water with 0.1 mg/L Mn(II) therefore needs about 0.25 mg/L as ClO2 for manganese alone (calculated). Where ClO2 is also the primary disinfectant, Health Canada caps the feed at 1.2 mg/L; oxidant choice is compared under iron and manganese removal. The same oxidation serves taste and odor control.

Legionella control in building water systems#

Building water programs dose 0.1 to 0.5 mg/L total ClO2 at the tap, and some programs target 0.3 mg/L or more at distal taps, with daily ClO2 and chlorite tests (US EPA, Technologies for Legionella Control in Premise Plumbing Systems, 2016). Hot-water loops are the weak point: one study in the same EPA review found about 88 % of the residual lost in hot-water recirculation, and chlorite above 0.7 mg/L was reported in hot water. Program design, monitoring and water management plans are covered under Legionella control in cooling towers and water systems.

Cooling towers and wastewater#

Chlorine dioxide is a primary microbial control option in cooling systems with high pH, ammonia contamination or persistent slime, according to OxyChem's chlorine dioxide guide (a supplier source). UK HSE guidance (HSG274 Part 1) notes that above pH 8 a chlorine residual needs a 3 to 4 times increase while ClO2 stays effective, and describes ClO2 as a volatile, niche biocide dosed by microbial results. Program choices are compared under cooling tower biocides.

In municipal wastewater, chlorine dioxide disinfects effluent and oxidizes phenolics, simple cyanides and sulfides, and the same guide reports hydrogen sulfide oxidation between pH 5 and 9. Sewer and plant odor programs sit with hydrogen sulfide and odor control chemicals.

How Is the Chlorine Dioxide Dose Calculated and Fed?#

The daily chlorine dioxide demand equals the dose as ClO2 times the flow, and each precursor demand equals that ClO2 mass times the reaction ratio divided by the precursor assay. In metric units, kg/d = mg/L x m³/d / 1,000; in US units, lb/day = mg/L x MGD x 8.34 / purity (Florida DEP operator formulas). The worked example below applies these formulas to the chlorine-chlorite route.

The ratios are theoretical (OxyChem chlorine dioxide drinking water guide), and the 30.5 % assay is the minimum of the same maker's sodium chlorite grade, so real feed rates rise with generator yield below 100 %. Other flows and doses run through the dosage and feed rate calculator.

Which precursor chemicals generate chlorine dioxide?#

Chlorine dioxide is generated by 4 chemical routes (chlorine-chlorite, acid-chlorite, chlorite with hypochlorite and acid, and chlorate with hydrogen peroxide and acid) plus electrochemical units. The table below compares the routes by reaction, yield and typical use.

Packaged generators reach up to 10 kg/h (22 lb/h) at 90 to 95 % efficiency (Irish EPA). Reactor designs, yields and feed controls are covered under chlorine dioxide generators.

What limits the maximum chlorine dioxide dose?#

Chlorite limits the maximum dose: at 70 % conversion, about 1.4 mg/L as ClO2 keeps chlorite under the 1.0 mg/L US MCL (Irish EPA). The 3 dose ceilings in use are listed below, from the highest to the lowest.

1. About 1.4 mg/L as ClO2 to keep chlorite below the US MCL of 1.0 mg/L (40 CFR 141.64) at 70 % conversion.
2. 1.2 mg/L as ClO2, the maximum feed dose in the Health Canada guideline (2008).
3. About 0.7 mg/L as ClO2 under UK Regulation 31, which limits ClO2 plus chlorite plus chlorate entering supply to 0.5 mg/L combined.

Plants that need more oxidant remove chlorite with ferrous iron, sulfite or thiosulfate, granular activated carbon, anion exchange or 10 to 20 mg/L of powdered activated carbon; chlorate cannot be removed at low cost (Irish EPA). At typical final-disinfection doses, chlorite normally stays below 0.2 mg/L (WHO chemical fact sheets).

How Are Chlorine Dioxide and Chlorite Residuals Tested?#

Chlorine dioxide is measured by DPD (Standard Methods 4500-ClO2 D), amperometric titration (4500-ClO2 E) or EPA Method 327.0 Rev 1.1, the methods approved in 40 CFR 141.131. The 3 residuals a ClO2 plant tests are listed below.

• Chlorine dioxide: daily at the entrance to distribution against the 0.8 mg/L MRDL; building programs also test daily.
• Chlorite: daily at the entry point by amperometric 4500-ClO2 E or EPA 327.0 Rev 1.1; monthly distribution samples by ion chromatography (EPA 300.0, 300.1, 317.0 Rev 2.0, 326.0 or ASTM D6581-00).
• Chlorate: no US MCL exists; EU systems test it as a parametric value under Directive (EU) 2020/2184.

Plants that pair chlorine dioxide with chlorine or chloramine for distribution track those residuals as well; free and total readings are explained under chlorine residual.

How is chlorite monitored under the US disinfection byproducts rule?#

Chlorite is monitored daily at the entrance to the distribution system and monthly as a 3-sample set in distribution (40 CFR 141.132(b)(2)). The 3 steps of the routine program are listed below.

1. Sample chlorite every day at the entrance to the distribution system.
2. Take a monthly 3-sample set (first customer, average and maximum residence time); compliance is the set average.
3. Collect 3 distribution samples the next day whenever a daily entry sample exceeds the 1.0 mg/L MCL.

Distribution monitoring can drop to quarterly after 1 year; entry monitoring cannot. For the ClO2 MRDL, a daily entry sample above 0.8 mg/L plus 1 or more next-day distribution samples above it is an acute violation, and 2 consecutive daily entry exceedances are a non-acute violation (40 CFR 141.133). Unlike chlorine, ClO2 has no short-term MRDL exception for line breaks (40 CFR 141.2). The full monitoring framework sits under the disinfection byproducts rules.

What Is Chlorine Dioxide Compatible With? Ammonia, Membranes, Acids and Light#

Chlorine dioxide coexists with ammonia, but it is incompatible with organic materials, heat, light and polyamide membranes. The 5 compatibility groups are listed below.

• Ammonia: compatible; ClO2 does not react with ammonia, so it forms no chloramines.
• Organic materials: incompatible, as are heat, phosphorus, potassium hydroxide, sulfur, mercury and carbon monoxide (NIOSH).
• Light and UV: ClO2 is unstable in light, and UV forms chlorate (WHO).
• Polyamide membranes: ClO2 is not recommended on polyamide RO and NF elements (DuPont FilmTec Reverse Osmosis Technical Manual); membrane-safe options are compared under reverse osmosis and membrane chemicals.
• Acids with chlorite: sodium chlorite plus acid rapidly releases spontaneously explosive chlorine dioxide (HSDB 733).

Plants keep acid and chlorite precursor lines and tanks separate for this reason. The segregation rules for every precursor pair are listed under chemical compatibility.

Which Standards Cover Chlorine Dioxide? NSF/ANSI/CAN 60, AWWA B303 and EN 12671#

No standard covers chlorine dioxide as a delivered product; certification and specifications attach to the precursors and the generator. The table below lists the documents a specification cites.

Is chlorine dioxide NSF/ANSI/CAN 60 certified?#

Chlorine dioxide is certified through its precursors: plants specify sodium chlorite or sodium chlorate certified to NSF/ANSI/CAN 60, and the generator to NSF/ANSI/CAN 61. Sodium chlorite listings cover 41 companies, 144 products and 283 product-function rows, with maximum use levels most often 22 mg/L as product (78 rows, typical of 31 % solutions) and 28 mg/L (47 rows, typical of 25 % solutions) (NSF listings, 2026-10-21). Sodium chlorate listings cover 3 companies, 8 products and 8 product-function rows at 8, 10 or 18 mg/L as product (NSF listings, 2026-10-21).

The sodium chlorate footnote requires finished-water monitoring of chlorine residuals, ClO2, chlorate, chloramine and disinfection by-products. Drinking water treatment chemicals must comply with or be certified to the standard in 49 US states (NSF survey of ASDWA members). How maximum use levels and footnotes work is explained under NSF/ANSI/CAN 60 certification.

What Is the Regulatory Status of Chlorine Dioxide in Drinking Water?#

Chlorine dioxide is regulated twice in US drinking water: as a disinfectant with an MRDL of 0.8 mg/L as ClO2 (40 CFR 141.65) and through its by-product chlorite, with an MCL of 1.0 mg/L (40 CFR 141.64). Both values date from the Stage 1 Disinfectants and Disinfection Byproducts Rule (63 FR 69390, 1998-12-16), with compliance dates of 2002-01-01 and 2004-01-01 depending on system type. Other jurisdictions control chlorine dioxide through chlorite and chlorate instead.

Chlorite and chlorate limits by jurisdiction; formation and control are covered under disinfection byproducts.

The table below lists every instrument that governs chlorine dioxide use (status 2026-11-02).

The same compliance picture beside coagulation, corrosion control and fluoridation appears under drinking water treatment chemicals.

Is chlorine dioxide safe in drinking water?#

Chlorine dioxide is permitted in US drinking water up to an MRDL of 0.8 mg/L as ClO2, the same value as its MRDLG, with its by-product chlorite capped at 1.0 mg/L. The WHO sets no guideline value for chlorine dioxide itself and controls it through provisional values of 0.7 mg/L for chlorite and chlorate. In the EU, chlorite and chlorate are each limited to 0.70 mg/L where chlorine dioxide is used.

How Should Chlorine Dioxide Be Handled Safely?#

Chlorine dioxide is generated on demand in sealed systems and never stored, because the gas becomes explosive at 10 % or more in air (Irish EPA). The precursors carry the storage risk instead: sodium chlorite tanks are FRP with UV protection, HDPE kept out of sunlight, or titanium, and carbon steel or stainless steel piping is not recommended (OxyChem sodium chlorite handbook).

EPA RMP (40 CFR 68.130) and OSHA PSM (29 CFR 1910.119 Appendix A) both set a threshold quantity of 1,000 lb (454 kg) for chlorine dioxide, and on-demand generation normally keeps the on-site quantity far below both; duties above a threshold are explained under EPA RMP and OSHA PSM. The table below summarizes the handling facts.

Which hazard classification and exposure limits apply to chlorine dioxide?#

Chlorine dioxide carries the harmonised CLP Annex VI classification H270, H330, H314 and H400 with signal word Danger (index 017-026-00-3), and a US OSHA PEL of 0.1 ppm (0.3 mg/m³) as an 8-hour TWA. The 4 hazard and limit items are listed below.

• CLP Annex VI, gas (index 017-026-00-3): H270, H330 (fatal if inhaled), H314, H400; Danger, 5 pictograms.
• CLP Annex VI, solution entry (index 017-026-01-0): H301, H314, H400.
• Workplace air: OSHA PEL TWA 0.1 ppm (0.3 mg/m³); NIOSH REL TWA 0.1 ppm, short-term 0.3 ppm (0.9 mg/m³); IDLH 5 ppm (NIOSH Pocket Guide).
• Transport: the only 49 CFR 172.101 entry is NA9191, chlorine dioxide hydrate, frozen, class 5.1, packing group II; transport codes for the precursors are on their own SDS.

What Are the Alternatives to Chlorine Dioxide?#

The 4 main alternatives to chlorine dioxide are sodium hypochlorite, ozone, potassium permanganate and monochloramine, and they differ in by-products, CT and residual. The table below compares them on the attributes that drive selection.

Chlorine dioxide vs chlorine and sodium hypochlorite#

Chlorine dioxide is a different molecule from chlorine and from bleach (sodium hypochlorite, NaOCl, CAS 7681-52-9): it oxidizes without chlorinating, so it forms chlorite instead of trihalomethanes. The difference shows in the limits: the US MRDL is 4.0 mg/L as Cl2 for chlorine but 0.8 mg/L as ClO2 for chlorine dioxide (40 CFR 141.65), and 3-log Giardia at 10 °C needs a CT of 112 mg·min/L with free chlorine against 23 with ClO2. Chlorine, by contrast, holds a lasting distribution residual. Strength units, MUL and storage of bleach are covered under sodium hypochlorite.

Chlorine dioxide vs ozone#

Ozone needs the lower CT, 1.43 mg·min/L against 23 for 3-log Giardia at 10 °C, and 30 against 830 for 3-log Cryptosporidium (40 CFR 141.74 and 141.720), but it leaves no residual. Chlorine dioxide forms no bromate, which is the main ozone by-product in bromide-bearing water. Using both in sequence has a catch: ozone dosed after chlorine dioxide oxidizes the chlorite to chlorate (WHO). Generation, bromate control and CT for the alternative are covered under ozone.

Chlorine Dioxide Generators, Precursor Suppliers and Sourcing#

Chlorine dioxide is sourced as 2 purchases: a generator system and a precursor supply. Sodium chlorite is on the US EPA list of 36 chemicals with a water treatment supply-chain profile, while chlorine dioxide itself is not on that list. Precursor distributors are listed among water treatment chemical suppliers. The table below lists companies named as makers of chlorine dioxide systems or precursors.

Listed alphabetically; no company is a partner of this site and none pays for inclusion.

How is chlorine dioxide bought and quoted?#

Chlorine dioxide is not bought by the drum: plants buy sodium chlorite or sodium chlorate by grade and package, and a generator sized in kg/h (lb/h). The 31 % Active sodium chlorite grade ships by railcar, tank truck, 330 gal tote or 55 gal drum, and products sold for pesticidal ClO2 generation carry an EPA registration number, for example EPA Reg. No. 5382-45. A precursor quote depends on 6 factors: strength and assay basis, NSF/ANSI/CAN 60 listing, package and annual volume, freight distance, contract term, and documents (certificate of analysis, SDS, FIFRA registration). Same-basis offers come from the form to request quotes for water treatment chemicals.

Where Does Chlorine Dioxide Sit Among Oxidants for Water Treatment?#

Chlorine dioxide is one of 17 oxidants documented on this site and the only one generated as a dissolved gas from a chlorite or chlorate precursor. The full family is compared under oxidants and oxidizing biocides for water treatment. Non-oxidizing options for cooling and membrane systems appear under biocides for water treatment.

Chlorine dioxide FAQs#

Does chlorine dioxide react with water?#

No, chlorine dioxide dissolves in water without hydrolysis, unlike chlorine, which forms hypochlorous acid. It stays dissolved as ClO2 gas, about 700 times more volatile than hypochlorous acid.

Does chlorine dioxide leave a residual in the distribution system?#

Chlorine dioxide leaves a residual of a few tenths of a mg/L at the plant, but the WHO notes it does not provide a lasting residual effect the way chlorine does.

Plants therefore add chlorine or chloramine for distribution; how those residuals persist is explained under water chlorination chemistry.

Can chlorine dioxide be stored?#

No, chlorine dioxide is not stored; it is generated on demand as a dilute solution of about 500 mg/L to 40 g/L. Only the precursors are kept in tanks.