Oxidants and Oxidizing Biocides for Water Treatment: 17 Types, from Chlorine and Bromine to ClO2, Ozone and Peroxygens

Oxidants and oxidizing biocides for water treatment are 17 chemicals in 7 classes, from sodium hypochlorite and chlorine dioxide to ozone, peracetic acid and permanganate, that kill microorganisms or oxidize iron, manganese, sulfide and organic matter by taking electrons from them. Their doses run from 0.07 mg/L (ppm) of chlorine dioxide in drinking water to 20 mg/L of chlorine in wastewater effluent, so which oxidant fits which system?

The classes are chlorine and hypochlorites, chloramines, chlorine dioxide and its precursors, bromine biocides, ozone and oxygen, peroxygens and permanganates. Also called oxidising biocides, they form one of the 16 families of water treatment chemicals.

Chlorine leads by volume. The tables give doses with their basis, CT, NSF/ANSI/CAN 60 maximum use levels, AWWA and EN editions, and the US MRDLs of 4.0 mg/L as Cl2 and 0.8 mg/L as ClO2.

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

What Is an Oxidizing Biocide?#

An oxidizing biocide is a chemical, such as chlorine, chlorine dioxide, bromine, ozone or peracetic acid, that kills bacteria, algae and biofilm by oxidizing cell walls and cell components, and that leaves a measurable residual while it is still active. Permanganates and hydrogen peroxide are dosed mainly to oxidize contaminants, not to disinfect.

Non-oxidizing biocides kill through specific reactions and need longer contact, typically at least 4 hours, with two biocides of different kill mechanisms alternated (UK HSE, HSG274, 2024). Both groups are compared on biocides for water treatment.

What do oxidants do in water treatment?#

Oxidants do 6 jobs in water treatment: primary disinfection, a protective residual in distribution or cooling water, oxidation of iron and manganese, oxidation of sulfide and odor, destruction of taste, odor and color compounds, and biofouling control ahead of membranes and heat exchangers.

• Disinfection: Giardia and virus credit; Cryptosporidium only with ozone or chlorine dioxide.
• Residual: at least 0.2 mg/L at a US entry point.
• Iron and manganese: 1.9 mg of permanganate per mg of Mn(II).
• Sulfide: about 1.0 mg of hydrogen peroxide per mg of H2S (calc.).
• Taste and odor: ozone with GAC removes 94 to 100 % of geosmin.
• Biofouling: 0.5 to 1.0 mg/L free chlorine through RO pretreatment.

Where are oxidants dosed in a treatment system?#

Oxidants are dosed at 4 points: at the intake or rapid mix as a pre-oxidant, at the contact tank for primary disinfection, at the entry point and booster stations for the residual, and in cooling, wastewater and membrane systems ahead of the equipment they protect. Permanganate goes in at the head of the plant, ammonia follows chlorine after the clearwell, and wastewater chlorine needs mixing in under 1 second (US EPA, 1999).

How Do Oxidants Work?#

Oxidants work by electron transfer: they take electrons from cell walls, enzymes and nucleic acids, which kills microorganisms, and from reduced species such as Fe(II), Mn(II) and sulfide, which converts them into solids or harmless ions.

Electron transfer: how oxidants kill and convert#

Each oxidant class transfers electrons its own way: chlorine dioxide takes 1 electron and becomes chlorite, hydrogen peroxide takes 2, ozone attacks directly and through hydroxyl radicals, and permanganate is reduced to solid manganese dioxide that filters out. Why does chlorine dioxide form no trihalomethanes? It does not chlorinate organic matter (WHO). Permanganate follows 3 Mn2+ + 2 MnO4- + 4 OH- to 5 MnO2 + 2 H2O.

Why pH controls chlorine and bromine#

pH decides how much free chlorine works: hypochlorous acid, about 100 times more effective than hypochlorite ion, makes up about 50 % of free chlorine at pH 7.5 but only about 10 % at pH 8.5, while hypobromous acid still makes up 83 % at pH 8.0. At 25 °C (77 °F) the HOCl pKa is 7.54 and the HOBr pKa about 8.7:

• pH 6.5: about 90 % HOCl
• pH 7.5: about 50 % HOCl, 94 % HOBr
• pH 8.5: about 10 % HOCl (calc.), 57 % HOBr

Seawater carries about 65 mg/L of bromide, which turns dosed chlorine into HOBr. The equilibrium is worked through on water chlorination chemistry.

Oxidant demand, residual and breakpoint#

An oxidant dose equals demand plus residual: if water consumes 0.5 mg/L of a 1.5 mg/L chlorine dose, 1.0 mg/L remains as free residual.

Ammonia raises demand: breakpoint takes 7.6 mg of Cl2 per mg of NH3-N in theory and 8 to 10 mg in practice (Health Canada, 2020).

The four zones of the curve are explained under breakpoint chlorination and chlorine demand.

17 Types of Oxidants and Oxidizing Biocides in 7 Classes#

The 17 oxidants used in water treatment fall into 7 classes: 4 chlorine and hypochlorite chemicals, 2 chloramine chemicals, 3 chlorine dioxide chemicals, 2 bromine biocides, ozone and its oxygen feed gas, 2 peroxygens and 2 permanganates. Table 1 compares the classes; advanced oxidation is a process row, not counted.

1. Chlorine and hypochlorites (4)#

Chlorine and hypochlorites are the chlorination chemicals that release hypochlorous acid in water; this class has 4 chemicals: sodium hypochlorite, chlorine gas, calcium hypochlorite and TCCA.

Sodium hypochlorite#

Sodium hypochlorite (NaOCl) is the most widely handled chlorination chemical, delivered as 10 to 15 % active chlorine solution or generated on site at about 0.8 % from salt brine.

Chlorine gas#

Chlorine gas (Cl2) is liquefied elemental chlorine, at least 99.5 % pure, shipped in 150-lb (68 kg) cylinders and 1-ton containers; it lowers pH because it forms hydrochloric acid.

Calcium hypochlorite#

Calcium hypochlorite (Ca(OCl)2) is a solid chlorine source with 65 to 70 % available chlorine, sold as granules or tablets and used most often at very small systems.

Trichloroisocyanuric acid (TCCA)#

Trichloroisocyanuric acid (TCCA) is a slow-release chlorine donor in tablet form that hydrolyzes to hypochlorous acid and cyanuric acid, which binds part of the free chlorine.

2. Chloramines (2)#

Chloramines are combined-chlorine disinfectants formed from chlorine and ammonia; this class has 2 chemicals: monochloramine and its ammonia source ammonium sulfate.

Monochloramine#

Monochloramine (NH2Cl) is the secondary disinfectant formed when ammonia meets free chlorine, about 200 times less effective than free chlorine but far more persistent in distribution.

Ammonium sulfate#

Ammonium sulfate ((NH4)2SO4) is the ammonia source most often fed as a 40 % liquid for chloramination, at 4.72 mg of salt per mg of NH3-N (calc.).

3. Chlorine dioxide and its precursors (3)#

Chlorine dioxide is generated on site from sodium chlorite or sodium chlorate, so this class has 3 chemicals: chlorine dioxide and its 2 precursors.

Chlorine dioxide#

Chlorine dioxide (ClO2) is a gas generated on site and dosed at 0.07 to 2.0 mg/L (ppm) as ClO2 in drinking water, where up to 70 % of the dose returns as chlorite.

Sodium chlorite#

Sodium chlorite (NaClO2) is the usual chlorine dioxide precursor, sold as 25 and 31 % solutions and fed only to a generator, where 1.34 lb of NaClO2 and 0.5 lb of chlorine yield 1 lb of ClO2.

Sodium chlorate#

Sodium chlorate (NaClO3) is the chlorine dioxide precursor of larger plants, reduced by hydrogen peroxide in strong acid inside the generator and never dosed into water itself.

4. Bromine biocides (2)#

Bromine biocides release hypobromous acid, which stays active at higher pH than chlorine; this class has 2 chemicals: sodium bromide and BCDMH. Stabilized bromine liquids and bromamines belong to the bromine node of this family.

Sodium bromide#

Sodium bromide (NaBr) is a bromine precursor, fed as a 40 % solution and activated by hypochlorite at 1.45 mg of NaBr per mg/L of chlorine (calc.).

BCDMH#

BCDMH (bromochlorodimethylhydantoin) is a solid bromine and chlorine donor, 98 % active, dissolved in a bypass erosion feeder for cooling towers and closed loops.

5. Ozone and oxygen (2)#

Ozone is generated on site from dry air or oxygen, so this class has 2 chemicals: ozone and the liquid oxygen that feeds the generator.

Ozone#

Ozone (O3) is generated on site at 0.5 to 3.0 % by weight from dry air, or 2 to 4 times higher from oxygen, and earns 3-log Cryptosporidium credit at a CT of 30 mg·min/L at 10 °C (50 °F).

Oxygen (LOX)#

Liquid oxygen (O2, LOX) is the feed gas of oxygen-fed ozone generators and of aeration, delivered by cryogenic trailer at a recommended purity of at least 99.5 %.

6. Peroxygens (2)#

Peroxygens are oxygen-oxygen bonded oxidants that leave no halogenated byproducts; this class has 2 chemicals: hydrogen peroxide and peracetic acid.

Hydrogen peroxide#

Hydrogen peroxide (H2O2) is a peroxygen sold at 35, 50 and 70 %, dosed to oxidize sulfide, to quench chlorine and as the radical source of advanced oxidation.

Peracetic acid (PAA)#

Peracetic acid (PAA) is an equilibrium mix of peracetic acid, hydrogen peroxide and acetic acid, dosed at 2 to 7 mg/L to disinfect secondary and tertiary effluent without chlorinated byproducts.

7. Permanganates (2)#

Permanganates are manganese(VII) salts that oxidize iron, manganese and taste and odor compounds rather than disinfect; this class has 2 chemicals: potassium and sodium permanganate.

Potassium permanganate#

Potassium permanganate (KMnO4) is a purple crystalline oxidant, at least 97 % pure, dosed at 0.3 to 5 mg/L (ppm) ahead of filters to oxidize iron and manganese.

Sodium permanganate#

Sodium permanganate (NaMnO4) is the liquid permanganate, sold at 20 and 40 %, which avoids dissolving crystals and their dust.

Which Oxidant for Which System? Selection by Application and Water Quality#

Oxidant selection depends on 4 factors: the disinfection credit or residual the system needs, the water's pH, ammonia and bromide, the contaminant to oxidize, and the byproducts the system must keep under its limits.

Drinking water: primary and secondary disinfection#

Drinking water systems pair a strong primary disinfectant with a persistent secondary one: free chlorine, chlorine dioxide or ozone earns the Giardia and virus credit, and free chlorine or chloramine holds at least 0.2 mg/L at the entry point. US rules also require a detectable residual in at least 95 % of distribution samples (40 CFR 141.72).

Why switch to chloramine? It persists longer in distribution, but about two thirds of medium and large chloraminating systems reported nitrification in a 1996 survey.

How each disinfectant forms or avoids THMs, HAAs, chlorite and bromate is covered under disinfection byproducts: formation and control.

By pH, ammonia and bromide#

Water chemistry narrows the choice: above pH 7.5 to 8 bromine outperforms chlorine, chlorine dioxide keeps working from pH 6 to 9 without reacting with ammonia, and bromide-rich water makes ozone a bromate risk. Health Canada's literature summary reports that lowering pH can cut bromate formation by 30 to 50 % per pH unit, and that ammonia can cut it by about 50 % at 200 µg/L.

By target: iron, manganese, sulfide, taste and odor#

Oxidant demand per mg of dissolved manganese ranges from 0.29 mg for oxygen to 2.45 mg for chlorine dioxide (Health Canada, 2019), but chlorine oxidizes manganese effectively only above pH 9.

• Oxygen: 0.29 mg
• Ozone: 0.87 mg
• Hypochlorous acid: 1.3 mg
• Potassium permanganate: 1.9 mg
• Chlorine dioxide: 2.45 mg

Permanganate also needs 0.94 mg per mg of Fe(II).

By application: cooling, wastewater and membrane systems#

Each industrial system has a default oxidant: hypochlorite with or without bromide in cooling towers, chlorine or peracetic acid for wastewater effluent, hydrogen peroxide or permanganate for sulfide, and chlorine ahead of membranes only when it is removed first. Table 2 matches 10 applications with their oxidants and one key number.

Cooling residuals follow Cooling Technology Institute guidance, remediation the CDC Legionella toolkit. Every cooling program is compared on cooling tower water treatment.

Oxidant Dosage Ranges by Chemical#

Oxidant doses range from 0.07 mg/L (ppm) of chlorine dioxide in drinking water to 20 mg/L of chlorine in wastewater effluent, and each range carries its basis: as Cl2, as ClO2, as O3, as active peroxide or as product. Table 3 lists doses per oxidant and system (mg/L = ppm).

Potable doses are capped by the product MUL (Table 5); cooling and wastewater doses follow residual targets and permits.

How is an oxidant dose expressed: as Cl2, as product or as active?#

Chlorine-family doses are stated as Cl2: 12.5 trade % hypochlorite holds 125 g/L of available chlorine, about 11.2 % by weight, so a dose as product is never the same number as a dose as Cl2. A MUL of 84 mg/L as product for 12.5 % hypochlorite equals about 9 mg/L as Cl2 (calc., mass basis). Bromine read by DPD as Cl2 times 2.25 gives Br2.

How is an oxidant dose turned into a feed rate?#

Worked example: 10 MGD (37,854 m³/d) dosed at 2.0 mg/L as Cl2 needs 166.8 lb/day (75.7 kg/d) of available chlorine, which is about 160 gal/d (606 L/d) of 12.5 trade % sodium hypochlorite at 1.04 lb Cl2/gal. The inputs are flow 10 MGD, dose 2.0 mg/L as Cl2 and the factor 8.34; assuming 12.5 wt % (1.18 lb/gal) instead sets the pump about 12 % low. The chemical dosage calculator converts mg/L into kg/d, lb/day and pump settings.

How Are Oxidant Residuals Controlled and Tested?#

Oxidant residuals are controlled in 3 layers: residual tests, CT calculations that prove disinfection credit, and ORP signals that pace feed in cooling and wastewater systems.

Free, combined and total chlorine residual tests#

Total chlorine equals free plus combined chlorine, and US systems measure it by amperometric titration or DPD methods listed in 40 CFR 141.74, all expressed as Cl2. Combined halogen above about 0.5 ppm can give a phantom free reading (supplier figure).

Analyzer checks are covered under chlorine residual in water treatment.

CT and disinfection credit#

CT, residual concentration times contact time, sets disinfection credit: at 10 °C (50 °F), 3-log Giardia inactivation needs 112 mg·min/L of free chlorine at pH 7.0 but 1,850 of chloramines, 23 of chlorine dioxide and 1.43 of ozone. Table 4 gives CT in mg·min/L at 10 °C (50 °F).

Sources: US EPA Disinfection Profiling and Benchmarking Guidance (2022); 40 CFR 141.74 and 141.720.

Free chlorine CT rises from 79 at pH 6 to 234 at pH 9. Chlorine earns no Cryptosporidium credit; see CT values for disinfection.

Contact time is counted from the dosing point to the first customer.

A CT value calculator applies these tables to a flow and temperature.

ORP control in cooling and wastewater systems#

ORP paces oxidant feed by millivolts: a positive reading means oxidizing conditions, but the same residual reads lower at higher pH or temperature, so setpoints are tied to grab residual tests.

In one US EPA wastewater case, ORP control held total residual chlorine at 0.1 to 0.2 mg/L.

Setpoints are covered under oxidation-reduction potential (ORP).

Dechlorination before discharge and membranes#

Residual oxidant is removed wherever it would harm a receiving water or a membrane: once-through cooling at steam electric plants of 25 MW or more may discharge at most 0.20 mg/L total residual chlorine (40 CFR 423.13), and polyamide RO tolerates less than 0.1 mg/L free chlorine.

EPA's 1986 freshwater criteria for chlorine are 19 µg/L acute and 11 µg/L chronic.

Bisulfite, metabisulfite and thiosulfate ratios are compared on dechlorination chemicals.

Which Standards and Regulations Apply to Oxidants?#

Oxidants are governed in 4 layers: NSF/ANSI/CAN 60 certification, AWWA and EN product standards, limits on residual disinfectant and byproducts, and biocide registration under FIFRA and the EU Biocidal Products Regulation. No oxidant is listed in 21 CFR 173.310.

NSF/ANSI/CAN 60 maximum use levels for oxidants#

NSF/ANSI/CAN 60 maximum use levels for oxidants are product-specific and stated as product, from 3 mg/L for some BCDMH products to 176 mg/L for most 20 % sodium permanganate, with 84 mg/L the most common value for 12.5 % sodium hypochlorite (NSF listings, 2026-10-20). Table 5 lists values in mg/L as product.

MULs count the total dose at all application points; ozone, chlorine dioxide, monochloramine, peracetic acid and sodium bromide have no listing under their own name. MUL rules are explained on NSF/ANSI/CAN 60.

AWWA and EN product standards for oxidants#

Eight AWWA B-standards and 17 EN standards specify oxidant purity, from ANSI/AWWA B300-24 for hypochlorites to EN 1278:2026 for ozone and EN 17971:2024 for in-situ ozone generators. Table 6 maps each oxidant to its edition (AWWA standards list and CEN/TC 164 catalogue, 2026-10-20).

TCCA, sodium chlorate, sodium bromide, BCDMH and peracetic acid have no AWWA or drinking-water EN standard.

ANSI/AWWA B303-24 also covers sodium chlorite for wastewater and reclaimed water; scopes are listed under AWWA B-series standards.

EN 901:2024 replaced EN 901:2013 for sodium hypochlorite, and EN 1278:2026 replaced EN 1278:1998 for ozone.

EN 12672 caps arsenic at 20 mg/kg and mercury at 10 mg/kg in potassium permanganate.

Impurity limits by product are set out in EN standards for drinking water treatment chemicals.

Residual disinfectant and byproduct limits#

US drinking water limits the disinfectant by MRDLs, 4.0 mg/L as Cl2 for chlorine and chloramines and 0.8 mg/L as ClO2 as running annual averages (40 CFR 141.65), and its byproducts by MCLs, including chlorite at 1.0 mg/L and bromate at 0.010 mg/L (40 CFR 141.64).

• United States: TTHM 0.080 and HAA5 0.060 mg/L; no chlorate MCL.
• European Union: chlorate and chlorite 0.25 mg/L each, 0.70 mg/L where chlorine dioxide is used, bromate 10 µg/L, from 2026-01-12.
• Health Canada: chlorate and chlorite 1 mg/L (2008), bromate 0.01 mg/L (2018), no chloramine MAC (2020).
• WHO (2022 consolidation): chlorite 0.7 mg/L provisional, monochloramine 3 mg/L, no chlorine dioxide value.

EPA had proposed no revision as of 2026-10-20; perchlorate is a proposal only (91 FR 398, 2026-01-06). See the Disinfectants and Disinfection Byproducts Rules.

Chlorite is tested daily at the entrance and monthly in distribution (40 CFR 141.132).

The higher EU value of 0.70 mg/L applies only where chlorine dioxide or another chlorate-generating disinfection method is used.

Member-state rules are set out under the EU Drinking Water Directive 2020/2184.

Biocide registration: FIFRA and the EU Biocidal Products Regulation#

Oxidizing biocides sold to disinfect water are registered pesticides: in the US every hypochlorite product for disinfection needs FIFRA registration, and in the EU active chlorine from sodium hypochlorite is approved for product types 1 to 5 until 2028-12-31 ((EU) 2017/1273).

Calcium hypochlorite holds PT2 to PT5 ((EU) 2017/1274) and chlorine PT2 and PT5 ((EU) 2017/1275); drinking-water disinfection is PT5.

Hydrogen peroxide PT1 to PT5 runs to 2029-07-31 ((EU) 2026/2131). See the EU Biocidal Products Regulation.

Chlorine dioxide and sodium chlorite share EPA reregistration Case 4023 (2006).

ANSI/AWWA B300-24 states in its scope that hypochlorite sold for disinfection must be a FIFRA-registered product.

Labels are covered under FIFRA registration of water treatment biocides.

Complete List of Oxidants: All 17 Chemicals in One Table#

The complete list of oxidants below gives all 17 chemicals with their CAS number, class, function, NSF/ANSI/CAN 60 status and product standards.

Every other chemical is filterable by CAS, function and NSF status in the water treatment chemical database.

Who Supplies Oxidants and Disinfection Programs?#

Chlorine is the bulk oxidant of US water treatment: about 628 million kg a year goes to municipal water, and the February 2021 winter storm cut about 28 % of US chlor-alkali capacity (EPA chlorine supply-chain profile).

Hypochlorite keeps about 1 month and travels 250 to 300 miles (400 to 480 km) by truck; producers and distributors are profiled under water treatment chemical suppliers.

Chlorine dioxide and ozone are generated on site, so buyers purchase precursors, oxygen and generators.

A program provider adds residual testing and feed control to supply; see water treatment companies.

Are Oxidants Safe to Store and Handle at the Plant?#

Oxidants are among the most hazardous chemicals at a water plant: sodium hypochlorite carries the harmonised CLP Annex VI classification EUH031, H314, H318, H400 and H410 (index 017-011-00-1), and OSHA caps chlorine at a ceiling of 1 ppm (3 mg/m³) in workplace air. For other products the supplier SDS governs.

Which chemicals must never be mixed with oxidants?#

Hypochlorite must never meet an acid, ferric chloride or alum, because the mixture releases chlorine gas; hypochlorite with ferric chloride caused 13 % of 72 documented chemical-unloading accidents between 1953 and 2019. Chlorite with acid releases chlorine dioxide.

The full matrix is under chemical compatibility and incompatible mixtures.

Which oxidants fall under EPA RMP and OSHA PSM?#

Chlorine triggers EPA RMP at 2,500 lb (1,134 kg) and OSHA PSM at 1,500 lb (680 kg), chlorine dioxide triggers both at 1,000 lb (454 kg), and ozone falls under PSM at 100 lb (45 kg) but not RMP; sodium and calcium hypochlorite fall under neither. The 2026-02-24 RMP proposal is not final.

Coverage tests are under EPA RMP and OSHA PSM for water treatment chemicals.

Does sodium hypochlorite lose strength in storage?#

Yes: 15 % sodium hypochlorite loses half its strength in about 100 days, 10 % in about 220 days and 5 % in about 790 days, and stored hypochlorite forms chlorate and perchlorate (WHO, citing AWWA M20).

What is the most common disinfectant in water treatment?#

Chlorine, as gas or hypochlorite, is the most widely used disinfectant: about 628 million kg a year reaches US municipal water, two thirds of it for wastewater, and more than one in five Americans drink chloraminated water.

What is the difference between oxidizing and non-oxidizing biocides?#

Oxidizing biocides kill fast and non-selectively by electron transfer and are controlled as a residual, while non-oxidizing biocides kill through specific reactions over hours and are dosed as slugs.

Glutaraldehyde, isothiazolinone, DBNPA and the other classes are compared on non-oxidizing biocides.