Ozone (O3; CAS 10028-15-6) in water treatment is a strong oxidizing gas generated on site from dry air or oxygen and dissolved into water to disinfect it and oxidize iron, manganese, taste, odor and color compounds. Operators call the process ozonation (ozonisation in older UK texts), and its 3-log Giardia CT of 1.43 mg-min/L at 10 °C (50 °F) is the lowest of the four main disinfectants (EPA disinfection profiling guidance).
Ozone is never stored or delivered: an air-fed generator makes a gas with about 0.5 to 3.0 % O3 by weight, and an oxygen-fed unit reaches about 2 to 4 times that concentration (EPA 832-F-99-063). It is one of the oxidants in this reference of water treatment chemicals, each documented with the same identity, dose and compliance fields.
Three numbers frame most ozone programs: the dose as O3 (0.5 to 1.0 mg per mg of dissolved organic carbon in drinking water, Health Canada), the CT that earns disinfection credit, and the bromate by-product limit of 0.010 mg/L (40 CFR 141.64). Ozone has no NSF/ANSI/CAN 60 chemical listing under its own name, while the oxygen fed to ozone generators is listed.
The identity card below summarizes the attributes that define ozone in water treatment.
| Attribute | Value |
|---|---|
| Name | Ozone |
| Formula and molar mass | O3; 47.998 g/mol |
| CAS number | 10028-15-6 |
| EC number | 233-069-2 |
| Synonyms | ozonation, ozone treatment, ozone disinfection, trioxygen, triatomic oxygen |
| Class | Inorganic oxidant gas, allotrope of oxygen, generated on site |
| Functions | Oxidizing disinfectant; oxidant (Fe/Mn, taste and odor, color, micropollutants); AOP component with H2O2 or UV |
| Appearance | Colorless to blue gas, very pungent odor (NIOSH) |
| Boiling point | -112 °C (-169 °F) (NIOSH) |
| NSF/ANSI/CAN 60 | No chemical listing under "Ozone" (NSF listing search, 2026-10-20) |
| EN standards | EN 1278:2026 (ozone); EN 17971:2024 (in-situ generators) |
| Key by-product limit | Bromate 0.010 mg/L, US MCL (40 CFR 141.64) |
| GHS | No CLP Annex VI entry; notified (ECHA C&L) classification includes H270, H330, H319, H400 |
Every value on this page is checked against primary sources (eCFR, EPA, Health Canada, WHO, NIOSH, ECHA, CEN, AWWA, NSF); see our methodology and fact-checking process.
What Is Ozone (O3) in Water Treatment?#
Ozone in water treatment is a triatomic allotrope of oxygen that a plant makes on site from dry air or oxygen and doses as a gas, because ozone is not stored or shipped (EPA, 1999). The molecule has a molar mass of 47.998 g/mol (PubChem).
What separates air-fed from oxygen-fed ozonation? The feed gas sets the ozone concentration and must be dried to a dew point of -60 °C (-76 °F) or lower. Gaseous ozone becomes explosive at 240 g/m³, so most systems operate at 50 to 200 g/m³ (EPA, 1999). Wetted and gas-side parts use stainless steel and other ozone-resistant materials (EN 17971:2024, clause 8.4).
The table below compares the two forms in which water treatment plants produce ozone.
| Form | O3 content | Condition | Source |
|---|---|---|---|
| Generated from dry air | About 0.5 to 3.0 % O3 by weight | Feed-gas dew point -60 °C (-76 °F) or lower | EPA 832-F-99-063 |
| Generated from oxygen (LOX or on-site oxygen) | About 2 to 4 times the air-fed concentration | Oxygen per ANSI/AWWA B304-21 (US) or EN 12876 (EU) | EPA 832-F-99-063; AWWA; EN 17971:2024 |
| Gas-phase limit (both forms) | Explosive at 240 g/m³; systems run at 50 to 200 g/m³ | Generator outlet and piping | EPA 832-F-99-063 |
How is ozone generated on site from air or oxygen?#
Ozone is generated on site by dielectric barrier discharge (also called silent or corona discharge), in which an electrical discharge splits oxygen and recombines it by the net reaction 3 O2 → 2 O3 (EN 17971:2024). Wastewater generators described by EPA run at 6 to 20 kV AC, and EN 17971:2024 distinguishes air or liquid cooling and negative- or positive-pressure systems. Single generators are large: Veolia states an output above 250 kg/h for its largest Ozonia unit, a supplier figure. Generator types and sizing are covered under ozone generation systems for water treatment.
How Does Ozone Disinfect and Oxidize Water?#
Ozone disinfects and oxidizes water through two parallel pathways: direct attack by molecular ozone on electron-rich bonds, and reaction with hydroxyl (OH) and hydroperoxyl (HO2) radicals formed as ozone decomposes (EPA 832-F-99-063). Microorganisms die by cell-wall oxidation and lysis, nucleic-acid damage and C-N bond breakage, and 40 CFR 141.720 grants ozone Cryptosporidium credit, which free-chlorine CT never earns.
Which pathway earns credit in a contactor? CT credit is calculated from the measured dissolved ozone residual, and adding hydrogen peroxide speeds ozone decay. Health Canada therefore advises against H2O2 addition where a plant needs ozone CT credit. Ozone with H2O2 (peroxone) or with UV forms an advanced oxidation process. WHO lists brominated organics, bromate, iodate, chlorate, aldehydes and carboxylic acids among ozonation by-products.
Where Is Ozone Used? Drinking Water, Wastewater and Cooling Systems#
Ozone is used in 3 systems (drinking water, municipal wastewater and, rarely, cooling towers) for 5 purposes: disinfection, metals oxidation, taste and odor control, color and precursor oxidation, and micropollutant removal. The table below lists each use with its dose basis, feed point and control.
| System and purpose | Dose (as O3) | Feed point | Control | Target | Source |
|---|---|---|---|---|---|
| Drinking water: primary disinfection | O3:DOC 0.5 to 1.0 mg/mg | Pre-, intermediate or post-ozonation contactor | Dissolved ozone residual, CT | Log credit per 40 CFR 141.720; bromate < 0.010 mg/L | Health Canada; 40 CFR 141 |
| Drinking water: Fe and Mn oxidation | 0.87 mg O3 per mg Mn(II) in theory, 2 to 5 times in practice; 0.5 to 2.0 mg/L in 30 to 60 s | Ozone contactor | Filtered Mn | Mn ≤ 0.05 mg/L (US SMCL) | Health Canada (2019) |
| Drinking water: taste and odor control | Site-specific | Ozone followed by biofiltration or GAC | Geosmin, MIB | 94 to 100 % geosmin and 87 to 100 % MIB removal with GAC (one study) | IWA Water Supply; ASU |
| Drinking water: color and DBP precursors | 2 to 5 mg/L for organics | Pre-, intermediate or post-ozonation contactor | Residual about 0.5 mg/L after up to 20 min | Color, precursor reduction | WHO GDWQ |
| Drinking water: arsenic(III) oxidation | Site-specific | Before arsenic removal | As(III) to As(V) | Arsenic MCL compliance | EPA |
| Wastewater: wastewater disinfection | Site-specific by pilot test | After at least secondary treatment | Ozone demand, residual, fecal coliform or E. coli | NPDES permit limits | EPA 832-F-99-063 |
| Wastewater: color removal from wastewater and hydrogen sulfide and odor control | Site-specific | Site-specific | Color, odor | Permit or odor target | EPA 832-F-99-063 |
| Cooling towers | Limited use | n/a | n/a | Life-cycle cost evaluation first | Veolia handbook; DOE FEMP |
Drinking water: primary disinfection and Cryptosporidium credit#
Drinking-water plants use ozone as a primary disinfectant because it earns Giardia, virus and Cryptosporidium credit at a short contact time. Plants dose 0.5 to 1.0 mg O3 per mg of dissolved organic carbon (Health Canada), so water with 4 mg/L DOC needs about 2 to 4 mg/L as O3 (calculated). At 10 °C (50 °F), 1-log Cryptosporidium inactivation requires a CT of 9.9 mg-min/L (40 CFR 141.720).
How does ozone fit the US rules? The Surface Water Treatment Rule requires 3-log Giardia and 4-log virus control, and ozone users add a virus benchmark when profiling (40 CFR 141 Subpart P). Under LT2 (Subpart W), Bin 3 and Bin 4 systems need at least 1-log more treatment, and ozone is a toolbox option. Primary and secondary disinfectant choices are compared under drinking water disinfection.
Drinking water: iron, manganese, taste and odor oxidation#
Ozone oxidizes dissolved manganese at 0.87 mg O3 per mg Mn(II) in theory, and plants apply 2 to 5 times that amount in practice (Health Canada, 2019). Ozone doses of 0.5 to 2.0 mg/L oxidize Mn(II) within 30 to 60 s at pH 5.6 to 7.0. As a worked example, raw water with 0.3 mg/L Mn needs 0.26 mg/L as O3 in theory and about 0.52 to 1.3 mg/L as O3 in practice (calculated). Ozone has the lowest demand per mg Mn(II) of the four chemical oxidants Health Canada compares: 0.87 mg, against 1.3 mg for HOCl, 1.9 mg for KMnO4 and 2.45 mg for ClO2 (dissolved oxygen: 0.29 mg). Oxidant demand and filtration are covered under iron and manganese removal.
Manganese targets differ by jurisdiction: the US SMCL is 0.05 mg/L (40 CFR 143.3, non-enforceable), and Health Canada sets a MAC of 0.12 mg/L and an aesthetic objective of 0.02 mg/L (2019). For taste and odor, ozonation alone removes geosmin and MIB only in part and creates biological instability, so biofiltration follows (Arizona State University study). In one IWA study, ozone plus GAC removed 94 to 100 % of geosmin and 87 to 100 % of MIB.
Why can ozone turn water pink?#
Ozone turns water pink when a high dose re-oxidizes manganese dioxide (MnO2) to permanganate (Health Canada, 2019). The reference point for the Mn(II) oxidation step is an ozone dose of 0.5 to 2.0 mg/L at pH 5.6 to 7.0. Finished-water manganese has a US SMCL of 0.05 mg/L, the same value that NSF/ANSI/CAN 60 sets in its footnote for potassium permanganate products.
Municipal wastewater: disinfection and micropollutant removal#
Municipal wastewater plants apply ozone after at least secondary treatment, in covered deep contact basins, with a contact time of about 10 to 30 minutes and a dose set by pilot testing (EPA 832-F-99-063). Ozonation also raises effluent dissolved oxygen and can remove the need for reaeration. The Indianapolis permit described by EPA set 400 and 200 fecal coliforms per 100 mL as weekly and monthly limits.
When is ozone not economical? EPA rates it uneconomical for wastewater with high suspended solids, BOD, COD or TOC. The same fact sheet reports a combined ozone capacity of 6,380 lb/d (2,894 kg/d) for the Indianapolis Belmont and Southport plants.
In the EU, the recast Urban Wastewater Treatment Directive (EU) 2024/3019, in force since 2025-01-01, adds quaternary treatment for micropollutants, a use of ozonation in municipal wastewater. Ozone and activated carbon are compared under micropollutant removal in wastewater.
Cooling towers: why is ozone rarely used?#
Ozone is rarely used in cooling towers because of its capital cost, as the Veolia Water Technologies Handbook (chapter 27) notes. The US DOE Federal Energy Management Program lists ozonation only as an option after a life-cycle cost evaluation. Oxidizing programs used instead are compared under cooling tower biocides.
How Is the Ozone Dose Calculated and Applied?#
The ozone feed rate equals the dose in mg/L as O3 multiplied by the flow, using lb/day = mg/L x MGD x 8.34 / purity and kg/d = mg/L x m³/d / 1,000, with purity 1 for a dose stated as O3 (Florida DEP operator formulas). The worked example below applies 2.0 mg/L as O3 to a 10 MGD plant.
| Input or result | Metric | US customary |
|---|---|---|
| Dose (input) | 2.0 mg/L as O3 | 2.0 ppm as O3 |
| Flow (input) | 37,854 m³/d | 10 MGD |
| Purity (input) | 1 (dose as O3) | 1 |
| Daily ozone production (calculated) | 75.7 kg/d | 166.8 lb/day |
| Hourly ozone production (calculated) | 3.15 kg/h | 6.95 lb/h |
Worked example, calculated. Dose as O3 with purity 1; transfer efficiency in the contactor is not included.
How do you convert an ozone dose to kg/d and lb/day?#
Multiply the dose in mg/L as O3 by the flow in m³/d and divide by 1,000 to get kg/d, or multiply mg/L by MGD and 8.34 to get lb/day. For 2.0 mg/L at 10 MGD, the formulas give 75.7 kg/d and 166.8 lb/day, or a generator rating of 3.15 kg/h (6.95 lb/h).
For other flows, the dosage and feed rate calculator converts the dose to kg/d and lb/day.
Which feed gas and contactor does ozonation use?#
Ozonation uses either dry air or oxygen as feed gas, and EN 17971:2024 (clause 9.2) sets feed-gas purity requirements. Air-fed systems dry the gas to a dew point of -60 °C (-76 °F) or lower; oxygen-fed systems use liquid oxygen (LOX) or on-site oxygen and produce about 2 to 4 times the air-fed concentration (EPA 832-F-99-063). EPA recommends at least 99.5 % O2 for oxygen used in water treatment, and ANSI/AWWA B304-21 specifies liquid oxygen for ozone generation.
Which contactor positions do plants choose? Drinking-water plants place the contactor at one of 3 points: pre-ozonation, intermediate ozonation after clarification, or post-ozonation. Off-gas from every contactor must be destroyed (EPA). Feed-gas grades are covered under oxygen (LOX) in water treatment.
How Is Ozonation Controlled? Residual, CT and Bromate#
Ozonation is controlled by 5 measurements, which together prove disinfection credit and by-product compliance. The 5 measurements are listed below.
- Dissolved ozone residual: indigo method SM 4500-O3 B (40 CFR 141.74(a)); EN 17971:2024 Annex B also lists DPD and potentiometric methods.
- Contact time: the detention time multiplied by a baffling factor of 0.1, 0.3, 0.5, 0.7 or 1.0 (EPA disinfection profiling guidance).
- CT: residual multiplied by contact time for each contactor cell, at or before the first customer.
- Bromate: 1 sample per month per ozone plant at the entrance to the distribution system (40 CFR 141.132).
- Wastewater: ozone demand, residual and fecal coliform or E. coli against the NPDES permit.
Ozone raises the oxidation-reduction potential (ORP) reading, as other oxidants do, but CT credit rests on the measured residual.
What CT does ozone need for Giardia and Cryptosporidium credit?#
Ozone needs a CT of 1.43 mg-min/L for 3-log Giardia inactivation and 9.9 mg-min/L for 1-log Cryptosporidium inactivation at 10 °C (50 °F). CT is the dissolved ozone residual C in mg/L multiplied by the effective contact time T in minutes, where T equals the detention time multiplied by the baffling factor. Baffling factors and T10 are explained under CT values for disinfection.
The table below lists ozone CT values in mg-min/L by temperature.
| Credit | ≤ 0.5 to 1 °C (33 to 34 °F) | 5 °C (41 °F) | 10 °C (50 °F) | 15 °C (59 °F) | 20 °C (68 °F) | 25 °C (77 °F) | 30 °C (86 °F) |
|---|---|---|---|---|---|---|---|
| 3-log Giardia | 2.9 | 1.90 | 1.43 | 0.95 | 0.72 | 0.48 | n/a |
| 1-log Cryptosporidium | 24 | 16 | 9.9 | n/a | 3.9 | n/a | 1.6 |
| 2-log Cryptosporidium | 48 | 32 | 20 | n/a | 7.8 | n/a | 3.1 |
| 3-log Cryptosporidium | 72 | 47 | 30 | n/a | 12 | n/a | 4.7 |
Giardia values from the EPA disinfection profiling guidance (cold-water row ≤ 1 °C; 40 CFR 141.74 rounds 1.43 to 1.4); Cryptosporidium values from 40 CFR 141.720 (cold-water row ≤ 0.5 °C), which also gives log credit = 0.0397 x 1.09757^T x CT for T in °C.
What contact time does a plant need? At 10 °C (50 °F) and a residual of 0.5 mg/L, 1-log Cryptosporidium credit needs about 20 minutes of effective contact (calculated). Free chlorine needs about 78 times the ozone CT for 3-log Giardia at 10 °C (112 against 1.43 mg-min/L, calculated), and free-chlorine CT is never applied to Cryptosporidium. Log credit at site temperature is computed by the CT value calculator.
How do plants limit bromate formation during ozonation?#
Bromate is an ozonation by-product (WHO), and US plants must keep it below the MCL of 0.010 mg/L (40 CFR 141.64), whose best available technology is control of the ozone treatment process. WHO lists lower ozone dose, shorter contact time, lower residual, a pH of about 6.5 during ozonation and ammonia addition as control options. Bromate health basis and other DBPs are covered under disinfection byproducts.
The table below compares 5 bromate controls and the reductions reported in Health Canada's bromate document and the 2023 critical review by Morrison and colleagues (Eawag).
| Control | Condition | Reported reduction | Source |
|---|---|---|---|
| pH depression | Per pH unit lowered from above 7.5; pH 8 to 6 | 30 to 50 % per unit; 50 to 91 % | Health Canada; Morrison 2023 |
| Ammonia addition | About 200 µg/L NH3; 100 to 900 µg/L NH3-N at pH 8 | About 50 %; 42 to 73 % | Health Canada; Morrison 2023 |
| Chlorine-ammonia process | 0.25 to 1.0 mg/L Cl2 plus 100 to 500 µg/L NH3-N | 75 to 80 %, 68 to 92 % and 86 % in three studies; 44 to 94 % | Health Canada; Morrison 2023 |
| Preformed monochloramine | 1 to 5 mg/L NH2Cl as Cl2 (wastewater) | 68 to 87 % | Morrison 2023 |
| Hydrogen peroxide | Surface water | -130 to +60 % (can increase bromate) | Morrison 2023 |
Hydrogen peroxide is the one control that can raise bromate, and it also removes CT credit, so plants that need disinfection credit rely on pH, ammonia or chlorine-ammonia instead.
What Is Ozone Compatible With? Oxidizable Materials, Peroxide and Polymers#
Ozone is incompatible with all oxidizable materials, organic and inorganic (NIOSH Pocket Guide), so every chemical it meets in a plant is a reactant. The 4 interactions that matter in water treatment are listed below.
- Oxidizable materials: organic and inorganic materials alike react with ozone; gas-side and wetted parts use stainless steel and other ozone-resistant materials (EN 17971:2024, clause 8.4).
- Hydrogen peroxide: accelerates ozone decay and serves as an ozone quench; it removes CT credit (Health Canada).
- Polymers: polymer exposure to ozone is avoided to control NDMA formation (peer-reviewed study indexed in PubMed).
- Chlorine dioxide: ozone applied after chlorine dioxide oxidizes chlorite to chlorate (Irish EPA disinfection manual).
Incompatible pairs for every oxidant are listed under water treatment chemical compatibility.
Which Standards Cover Ozone? EN 1278:2026, EN 17971:2024 and AWWA B304-21#
3 documents cover ozone and its feed gas: EN 1278:2026 for ozone itself, EN 17971:2024 for in-situ generators, and ANSI/AWWA B304-21 for liquid oxygen used to make ozone. AWWA publishes no B-standard for ozone itself. The table below lists each document with its scope and edition.
| Document | Scope | Edition and status | Key point |
|---|---|---|---|
| NSF/ANSI/CAN 60 | Drinking water treatment chemicals, health effects | Listing search 2026-10-20 | No chemical listing under "Ozone"; oxygen feed gas listed |
| EN 1278:2026 | Ozone for water intended for human consumption | BS EN 1278:2026 listed 2026-09-21; replaces EN 1278:1998; see EN standards for drinking water treatment chemicals | Current CEN/TC 164 product standard |
| EN 17971:2024 | In-situ ozone generation devices | Approved by CEN 2024-05-19 | Clause 6 data sheet, clause 9.2 feed gas, Annex B test methods |
| ANSI/AWWA B304-21 | Liquid Oxygen for Ozone Generation for Water, Wastewater, and Reclaimed Water Systems | Current; earlier editions B304-18 and B304-13; see AWWA B-series standards | Feed-gas specification in North America |
| EN 12876 | Oxygen for water intended for human consumption | Current edition per CEN catalogue | Feed-gas specification in Europe |
Is ozone NSF/ANSI/CAN 60 certified?#
No: NSF's listing search returns no NSF/ANSI/CAN 60 chemical listing under "Ozone" (NSF listings, 2026-10-20), so ozone carries no NSF 60 maximum use level. Oxygen for ozone generation is listed: 3 companies, 16 products and 168 product-function rows, with the functions Oxidant and Ozone Reduction and a maximum use of NA (NSF listings, 2026-10-21). The 2020 text of NSF/ANSI/CAN 60 exempts this oxygen from product security requirements, and 49 US states require drinking water treatment chemicals to comply with or be certified to NSF/ANSI/CAN 60 (NSF survey of ASDWA members). Listings and feedstock exemptions are covered under NSF/ANSI/CAN 60 certification.
What Is the Regulatory Status of Ozone in Water Treatment?#
Ozone is regulated mainly through its by-product bromate, through the CT tables that grant disinfection credit and through workplace and process-safety rules for a toxic gas. The figure below shows why regulators credit ozone with the lowest CT of the 4 main disinfectants.
The matrix below lists each instrument that applies to ozone, status 2026-10-22.
| Instrument | Value | Type | Reference |
|---|---|---|---|
| 40 CFR 141.64 and 141.53 | Bromate MCL 0.010 mg/L, MCLG zero; compliance 2002-01-01 (Subpart H systems serving ≥ 10,000) and 2004-01-01 (others) | Enforceable US limit | Disinfection byproducts rules |
| 40 CFR 141.132 | Bromate 1 sample per month per ozone plant; quarterly if the running annual average is ≤ 0.0025 mg/L (Methods 317.0 Rev 2.0, 326.0, 321.8) | US monitoring | eCFR |
| 40 CFR 141.720 and Subpart W (LT2) | Ozone CT tables for Cryptosporidium; Bin 3 and 4 need at least 1-log more | US treatment technique | Surface water treatment rules |
| Directive (EU) 2020/2184 Annex I Part B | Bromate 10 µg/L | EU parametric value | EU Drinking Water Directive 2020/2184 |
| Health Canada (2018) | Bromate MAC 0.01 mg/L as a locational running annual average | Canadian guideline | Health Canada summary table |
| WHO GDWQ | Bromate provisional guideline value 0.01 mg/L | International guidance | WHO guidelines for drinking-water quality |
| FIFRA | Generators sold with pesticidal claims are pesticidal devices (EPA enforcement case, 2020) | US pesticide law | FIFRA registration |
| Regulation (EU) 528/2012 and REACH | Biocidal use of in-situ ozone falls under the BPR; non-biocidal oxidation under REACH (EN 17971:2024) | EU chemicals law | EU Biocidal Products Regulation |
| 29 CFR 1910.119 App. A and 40 CFR 68.130 | OSHA PSM threshold 100 lb (45 kg); ozone is not on the EPA RMP list | US process safety | EPA RMP and OSHA PSM |
| 21 CFR 173.310 | Not listed in 21 CFR 173.310 | US boiler-water additives | eCFR |
| Discharge | No residual-ozone discharge limit found; NPDES bacteria limits apply | US effluent | NPDES permits |
Ozone beside chlorination and corrosion control is shown under drinking water treatment chemicals.
Is the bromate limit the same in the US, EU, Canada and WHO guidance?#
Yes in number, no in type: all four set bromate at 0.010 mg/L (10 µg/L), but the US value is an enforceable MCL (40 CFR 141.64), the EU value a parametric value in Directive (EU) 2020/2184, the Canadian value a MAC (Health Canada, 2018) and the WHO value a provisional guideline. EPA methods quantify bromate with reasonable precision at 0.005 mg/L, and reporting limits down to 0.003 mg/L are achievable (Health Canada).
How Should Ozone Be Handled Safely?#
Ozone is handled safely by never storing it, destroying all contactor off-gas and keeping room air below the OSHA permissible exposure limit of 0.1 ppm (0.2 mg/m³) as an 8-hour time-weighted average. NIOSH sets a ceiling REL of 0.1 ppm and an IDLH of 5 ppm.
Which process-safety rules apply? Ozone is a highly hazardous chemical under OSHA PSM at 100 lb (45 kg) (29 CFR 1910.119 Appendix A), but it is not listed in the EPA RMP table (40 CFR 68.130), so no RMP threshold applies. Personal protective equipment follows the generator maker's SDS.
The table below summarizes the handling values for ozone.
| Item | Value |
|---|---|
| Storage | Not stored; generated on demand |
| Materials | Stainless steel and other ozone-resistant materials (EN 17971:2024, clause 8.4) |
| Incompatibilities | All oxidizable materials, organic and inorganic (NIOSH) |
| OSHA PEL | 0.1 ppm (0.2 mg/m³) TWA |
| NIOSH REL | Ceiling 0.1 ppm (0.2 mg/m³) |
| IDLH | 5 ppm |
| OSHA PSM threshold | 100 lb (45 kg) |
| EPA RMP | Not listed (40 CFR 68.130) |
| Gas-phase explosive limit | 240 g/m³ (systems run at 50 to 200 g/m³) |
What hazard classification applies to ozone?#
Ozone has no harmonised CLP Annex VI entry, and the notified (ECHA C&L) classification applied by the majority of notifiers includes H270 (may cause or intensify fire; oxidizer), H330 (fatal if inhaled), H319 (causes serious eye irritation) and H400 (very toxic to aquatic life), with the signal word Danger and pictograms GHS03, GHS06 and GHS09. This classification is notified, not harmonised. The 4 checks below translate it into plant practice.
- Install an off-gas destruct unit on every contactor.
- Monitor room air against the 0.1 ppm OSHA PEL.
- Check site ozone capacity against the 100 lb (45 kg) OSHA PSM threshold.
- Keep the generator maker's SDS at the generator room.
What Are the Alternatives to Ozone?#
5 oxidants replace or complement ozone: chlorine dioxide, sodium hypochlorite, potassium permanganate, hydrogen peroxide and peracetic acid. The table below compares them with ozone on form, NSF/ANSI/CAN 60 status, CT and the key US limit.
| Oxidant | CAS | Form | NSF/ANSI/CAN 60 status | 3-log Giardia CT at 10 °C (mg-min/L) | Key US limit |
|---|---|---|---|---|---|
| Ozone | 10028-15-6 | Gas generated on site | No listing (2026-10-20) | 1.43 | Bromate MCL 0.010 mg/L |
| Chlorine dioxide | 10049-04-4 | Solution generated on site | Not listed as such; precursors listed (2026-10-20) | 23 | MRDL 0.8 mg/L as ClO2; chlorite MCL 1.0 mg/L |
| Sodium hypochlorite | 7681-52-9 | Bulk liquid, 10 to 15 % active chlorine | 178 companies, 768 products; most common MUL 84 mg/L as product (2026-10-21) | 112 (free chlorine, 1.0 mg/L, pH 7.0) | MRDL 4.0 mg/L as Cl2; TTHM 0.080, HAA5 0.060 mg/L |
| Potassium permanganate | 7722-64-7 | Crystals, ≥ 97 % | MUL 50 mg/L as product for solid grades (2026-10-20) | n/a | Mn SMCL 0.05 mg/L (non-enforceable) |
| Hydrogen peroxide | 7722-84-1 | 35, 50 and 70 % solutions | Product-specific MUL (2026-10-20) | n/a | No MCL |
| Peracetic acid (PAA) | 79-21-0 | 12, 15 and 22 % equilibrium solutions | No listing (2026-10-20) | n/a | Wastewater use |
What are the disadvantages of ozone in water treatment?#
Ozone has 6 disadvantages, each tied to a number that a plant must manage. The 6 disadvantages are listed below.
- Bromate: ozonation forms bromate, capped at 0.010 mg/L with monthly monitoring (40 CFR 141.64, 141.132).
- Capital cost: the ozone system was about 8 % of capital cost in EPA's Indianapolis case, and cost keeps ozone rare in cooling towers.
- Water quality limits: ozone is not economical in wastewater with high suspended solids, BOD, COD or TOC (EPA).
- Biological instability: ozonation alone creates biological instability, so biofiltration or GAC follows.
- Toxic gas: the OSHA PEL is 0.1 ppm, and 100 lb (45 kg) on site triggers OSHA PSM.
- Narrow control window: overdosing re-oxidizes MnO2 to pink permanganate, and peroxide addition removes CT credit.
Ozone vs chlorine dioxide#
Ozone disinfects with a lower CT than chlorine dioxide, needing 1.43 against 23 mg-min/L for 3-log Giardia at 10 °C (EPA). Chlorine dioxide forms chlorite, capped at 1.0 mg/L (40 CFR 141.64), and holds an MRDL of 0.8 mg/L as ClO2, while ozone forms bromate. For manganese, ozone needs 0.87 mg and chlorine dioxide 2.45 mg per mg Mn(II) (Health Canada). Where both are used, ozone applied after chlorine dioxide oxidizes chlorite to chlorate (Irish EPA). Generation and chlorite control are covered under chlorine dioxide.
Ozone vs sodium hypochlorite (chlorine)#
Ozone inactivates Giardia with about 1/78 of the CT that free chlorine needs (1.43 against 112 mg-min/L at 10 °C, pH 7.0, calculated), and it inactivates Cryptosporidium, where free-chlorine CT is never credited. Sodium hypochlorite instead serves as a residual disinfectant in distribution, capped by an MRDL of 4.0 mg/L as Cl2, and forms TTHM and HAA5 (MCLs 0.080 and 0.060 mg/L). Hypochlorite also adds bromate: a mean of 2.5 µg/L (0.10 to 26 µg/L) at the NSF maximum use level, against an allowable 3.3 µg/L per product (Health Canada). Trade percent and maximum use level are covered under sodium hypochlorite.
Ozone Systems, Feed-Gas Suppliers and Sourcing#
Ozone is bought as a generator system plus feed gas, not as a chemical, because it is generated on demand and never stored. Plants specify the generator to EN 17971:2024 and buy liquid oxygen to ANSI/AWWA B304-21. The table below lists the companies that the cited sources name as an ozone generator maker or a liquid oxygen supplier.
| Company | Role |
|---|---|
| Air Liquide | Oxygen and LOX supplier |
| Air Products | Oxygen and LOX supplier |
| Linde | Oxygen and LOX supplier |
| Nippon Sanso Matheson | Oxygen and LOX supplier |
| SYOXSA | Oxygen and LOX supplier |
| Veolia | Ozone generators (Ozonia line) |
Listed alphabetically; no company is a partner of this site and none pays for inclusion.
Producers and distributors by region are listed under water treatment chemical suppliers.
How is an ozone system specified and quoted?#
An ozone system is specified by the technical data that EN 17971:2024 clause 6 requires on the data sheet, so every offer is compared on the same basis. The 5 data items are listed below.
- Nominal ozone output in g/h at the nominal concentration in g/m³n.
- Feed-gas type (air or oxygen) and gas flow in m³n/h.
- Maximum feed-gas dew point in °C.
- Coolant type, flow and temperature (air, water or brine).
- Pressure class: negative-pressure or positive-pressure system.
LOX quotes depend on the standard, NSF/ANSI/CAN 60 listing, annual quantity, freight distance and contract term. Send one specification and request quotes for water treatment chemicals on the same basis.
Where Does Ozone Sit Among Oxidants for Water Treatment?#
Ozone sits at the low-CT end of the oxidant range: an on-site generated gas with the lowest Giardia CT of the main disinfectants, while secondary disinfectants such as chlorine and chloramine carry the residual in distribution. All oxidants are compared under oxidants and oxidizing biocides for water treatment. Ozone with H2O2 or UV is covered under advanced oxidation processes (AOP).
Ozone FAQs#
Is ozone the same as oxygen?#
No: ozone is O3, a triatomic allotrope of oxygen, while oxygen is O2. Ozone generators make ozone from oxygen or dry air, and liquid oxygen for that purpose has its own standard, ANSI/AWWA B304-21.
Does ozone help remove arsenic?#
Yes, as an oxidant: ozone oxidizes arsenic(III) ahead of arsenic removal. EPA lists chlorine, permanganate, ozone and manganese dioxide media as effective oxidants; aeration is not. Oxidation before removal is covered under arsenic removal from drinking water.
Is ozone regulated as a pesticide under FIFRA?#
It depends on the claim: EPA treats ozone generators sold with pesticidal claims as pesticidal devices under FIFRA. EPA's 2020 enforcement case applied this position to generators marketed with such claims.
How harmful is ozone to plant workers?#
Ozone is toxic by inhalation: OSHA limits exposure to 0.1 ppm (0.2 mg/m³) as an 8-hour average, and 5 ppm is immediately dangerous to life or health (NIOSH). NIOSH also sets a 0.1 ppm ceiling, so generator rooms are monitored against these limits and contactor off-gas is destroyed.