Water Chlorination Chemistry: HOCl, OCl- and pH

Water chlorination is the dosing of chlorine gas or a hypochlorite into water to form hypochlorous acid (HOCl), the disinfecting species, whose share of free chlorine falls from about 78 % at pH 7.0 to about 26 % at pH 8.0 at 25 °C (77 °F). The same 1.0 mg/L dose as Cl2 is therefore a strong or a weak disinfectant depending on pH: how is the split calculated, and what does it mean for dose, residual and compliance?

Chlorine gas, sodium hypochlorite and calcium hypochlorite all end as the same HOCl and hypochlorite ion (OCl-) pair in water. US municipal water and wastewater systems use about 628 million kg of chlorine a year (US EPA chlorine supply-chain profile, 2022 estimate). Chlorine and the hypochlorites form the oxidizing disinfectant family among the water treatment chemicals.

The pKa of 7.54 sets the HOCl share, 3 worked examples turn mg/L as Cl2 into feed rates, and residuals run from 0.2 mg/L in distribution to 20 mg/L in wastewater effluent, under a US MRDL of 4.0 mg/L as Cl2.

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

What Is Water Chlorination?#

Water chlorination is the controlled addition of chlorine (Cl2), sodium hypochlorite (NaOCl) or calcium hypochlorite (Ca(OCl)2) to water to inactivate pathogens, oxidize dissolved contaminants and leave a measurable free chlorine residual. Free available chlorine is the sum of dissolved Cl2, HOCl and OCl-, reported as mg/L as Cl2.

Why is chlorine added to water? Disinfection, residual and oxidation#

Chlorine is added to water for 4 jobs: primary disinfection, a residual that protects the distribution system, oxidation of iron, manganese and other reduced compounds, and microbial control in cooling and membrane pretreatment systems. The 4 jobs are listed below.

• Primary disinfection: 3-log Giardia and 4-log virus objectives under the Surface Water Treatment Rule.
• Distribution residual: not below 0.2 mg/L entering distribution for more than 4 hours (40 CFR 141.72).
• Oxidation: about 1.6 ppm ferrous iron per ppm Cl2.
• Microbial control: 0.5 to 1.0 mg/L free chlorine in cooling towers and RO pretreatment.

In municipal treatment, chlorine is used to disinfect the filtered water and keep a residual to the tap. Disinfection credit, primary and secondary disinfectants and the full program are covered under drinking water disinfection.

Where is chlorine added in the water treatment process?#

Chlorine is added at 3 points in a drinking-water plant: pre-chlorination before treatment, primary chlorination after treatment, and secondary or booster chlorination in the distribution system (WHO, Principles and Practices of Drinking-water Chlorination, 2017). The 3 points, in flow order, are listed below.

1. Pre-chlorination, ahead of treatment.
2. Primary chlorination, after filtration, for disinfection contact time.
3. Booster chlorination, at stations in the distribution system.

Post-chlorination is the primary chlorination step after filtration: it provides contact time and sets the residual leaving the plant. WHO advises avoiding pre-chlorination where possible, because chlorine that meets untreated organic matter raises byproducts. In a conventional plant, chlorination is the last chemical step before the clearwell.

What Happens When Chlorine Is Added to Water? The Chlorination Reactions#

Every chlorine source forms the same disinfecting pair: chlorine gas hydrolyzes to hypochlorous acid and hydrochloric acid, hypochlorites release hypochlorous acid and a base, and hypochlorous acid then splits into hydrogen ion and hypochlorite ion according to pH.

Chlorine gas: Cl2 + H2O → HOCl + HCl#

Cl2 in water treatment is elemental chlorine, a greenish-yellow liquefied gas that hydrolyzes in water to hypochlorous acid and hydrochloric acid (Cl2 + H2O → HOCl + HCl), so it lowers pH and consumes alkalinity. Chlorine of at least 99.5 % w/w arrives in 150-lb (68 kg) cylinders and 1-ton containers and is fed under vacuum through a chlorinator and injector. It dissolves to 14.6 g/L at 0 °C (32 °F).

Cylinder handling, NSF/ANSI/CAN 60 limits and supply are covered under chlorine gas in water treatment.

Hypochlorites: NaOCl + H2O → HOCl + NaOH#

Sodium hypochlorite hydrolyzes in water to hypochlorous acid and sodium hydroxide (NaOCl + H2O → HOCl + NaOH), so a hypochlorite dose raises pH slightly instead of lowering it.

Calcium hypochlorite reacts as Ca(OCl)2 + 2 H2O → 2 HOCl + Ca(OH)2, adding calcium hardness and alkalinity. Trichloroisocyanuric acid tablets release HOCl plus cyanuric acid, which binds part of the free chlorine.

Sodium hypochlorite solution is the liquid chlorine source for disinfection, pre-oxidation and wastewater effluent chlorination. Grades, storage, NSF/ANSI/CAN 60 listings and on-site generation are covered under sodium hypochlorite in water treatment.

Hypochlorous acid dissociation: HOCl ⇌ H+ + OCl-#

Hypochlorous acid (HOCl, CAS 7790-92-3) is a weak acid with a pKa of 7.54 at 25 °C (77 °F), so it is half dissociated into hypochlorite ion at pH 7.54 and mostly undissociated below pH 7 (Morris 1966, cited by Zhang et al. 2023).

Free chlorine is the sum of both forms, measured together as mg/L as Cl2, and pH decides how much of it is HOCl.

Chlorine demand: iron, manganese, ammonia and organics react first#

Chlorine demand is the chlorine consumed by reduced substances before a residual remains: ferrous iron, manganese, ammonia and natural organic matter, each at its own rate and stoichiometry. The 4 demand reactants are listed below.

• Ferrous iron: about 1.6 ppm Fe(II) per ppm Cl2 (Veolia Water Technologies Handbook, chapter 27).
• Manganese: 1.3 mg HOCl per mg Mn(II), effective only above pH 9 or after at least 3 h at pH 7 and 25 °C (Health Canada, 2019).
• Ammonia: forms chloramines before free chlorine remains.
• Organic matter: forms trihalomethanes (THMs) and haloacetic acids (HAAs).

Ammonia drives the breakpoint curve, with about 10 mg/L of chlorine per mg/L of ammonia removed in practice (UK Drinking Water Inspectorate). The curve and the demand test are explained under breakpoint chlorination and chlorine demand.

How pH Controls HOCl and OCl-#

pH controls chlorination because HOCl and OCl- share free chlorine by the acid equilibrium: at 25 °C (77 °F) HOCl is about 92 % of free chlorine at pH 6.5, about 52 % at pH 7.5 and about 10 % at pH 8.5. The DuPont FilmTec chlorination and dechlorination manual measures 90 % and 50 % at those first two points.

Table 1 gives the HOCl and OCl- shares from pH 6.0 to 9.0 at 25 °C, calculated from pKa 7.54 (Morris 1966).

How is the HOCl fraction calculated? (Worked example 1)#

The HOCl fraction of free chlorine equals 1 / (1 + 10^(pH − pKa)), with pKa 7.54 at 25 °C (77 °F). Worked example 1 uses these inputs:

• free chlorine residual: 1.0 mg/L as Cl2
• temperature: 25 °C (77 °F), pKa 7.54
• pH: 7.0 and 8.0

At pH 7.0, 10^(−0.54) = 0.288, so the fraction is 1 / 1.288 = 0.776 and 0.78 mg/L of the residual is HOCl. At pH 8.0, 10^(0.46) = 2.88, so the fraction is 1 / 3.88 = 0.257 and only 0.26 mg/L is HOCl. One more pH unit cuts the HOCl part of the same residual by about two thirds.

Why is HOCl a stronger disinfectant than OCl-?#

HOCl is about 100 times more effective than OCl- as a disinfectant (the same manual), so the HOCl share, not the total residual, sets the killing power of free chlorine. The Veolia Water Technologies Handbook gives "up to 80 times" and places HOCl predominance below pH 7.6.

WHO therefore sets pH below 8.0 for effective chlorination, within an optimum drinking-water pH of 6.5 to 8.5.

How temperature, salinity and bromide shift chlorination chemistry#

Temperature, salinity and bromide shift the HOCl share at a given pH: at pH 7.5 HOCl is about 50 % of free chlorine at 25 °C (77 °F), 62 % at 5 °C (41 °F) and about 30 % at 40,000 mg/L TDS (the same manual). Seawater carries about 65 mg/L of bromide, which converts HOCl to hypobromous acid (HOBr). At pH 8, 72 % of HOCl is dissociated but only about 17 % of HOBr, whose pKa is about 8.7; HOBr is about 83 % of free bromine at pH 8.0.

Activated bromide and BCDMH programs that exploit the higher pKa are covered under bromine water treatment.

How pH changes the CT a chlorine residual must reach#

Higher pH raises the CT that free chlorine needs: for 3-log Giardia at 10 °C (50 °F) and 1.0 mg/L, the CT rises from 79 mg·min/L at pH 6.0 or below to 112 at pH 7.0, 162 at pH 8.0 and 234 at pH 9.0 (US EPA disinfection profiling guidance, Table B-1, from 40 CFR 141.74).

CT is the residual C (mg/L) times the contact time T (min) at or before the first customer (40 CFR 141.2). The 112 value applies at C = 1.0 mg/L; at 0.4 mg/L or less it is 104 mg·min/L. The 4-log virus CT is 6 mg·min/L at 10 °C and pH 6 to 9, and WHO states that free chlorine is not effective against Cryptosporidium.

Baffling factors, log-inactivation arithmetic and every table are on CT values for disinfection.

How to Calculate a Chlorine Dose#

A chlorine dose is calculated in 3 steps: set the residual target, add the chlorine demand of the water, and convert the dose in mg/L as Cl2 into a feed rate using flow, available chlorine and density.

Dose, demand and residual#

Chlorine dose equals chlorine demand plus chlorine residual: a 1.5 mg/L dose that leaves 1.0 mg/L of free residual has met 0.5 mg/L of demand (Michigan Tech operator training, 2023; Florida DEP formula table, 2014).

Chlorine demand is set on a representative sample with ASTM D1291, which fixes the dose, injection point, pH and contact time.

Available chlorine: chlorine gas, sodium hypochlorite and calcium hypochlorite compared#

Available chlorine is the chlorine content of a product expressed as Cl2: liquefied chlorine gas is at least 99.5 % Cl2, calcium hypochlorite granules carry 65 to 70 % available chlorine, and delivered sodium hypochlorite carries 10 to 15 % active chlorine.

Hypochlorite trade % equals grams per liter of available chlorine divided by 10, so 12.5 trade % is 125 g/L, or 11.23 wt % NaOCl at 1.169 g/mL, while a 12.5 wt % solution is 142 g/L, or 14.2 trade % (Olin sodium hypochlorite handbook). Available chlorine equals NaOCl wt % × 0.952. Delivered hypochlorite keeps about 1 month within a 250 to 300 mile (400 to 480 km) truck radius (US EPA), and on-site generation makes 0.5 to 1.0 % m/V (UK DWI).

Table 2 compares the 3 chlorine sources at the feed rate of Worked example 2.

MULs are product-specific and stated as product (NSF listings, 2026-10-21).

US EPA notes that calcium hypochlorite is used most often at very small systems. Tablet and granule feed is covered under calcium hypochlorite in water treatment.

Worked example 2: sodium hypochlorite feed rate in US and metric units#

A 10 MGD (37,854 m³/d) plant dosing 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. Worked example 2 uses these inputs:

• flow: 10 MGD (37,854 m³/d); dose: 2.0 mg/L as Cl2
• product: 12.5 trade % = 125 g/L = 1.04 lb Cl2/gal, density 1.169 g/mL
• formulas: lb/day = mg/L × MGD × 8.34 / purity (Florida DEP, 2014); kg/d = mg/L × m³/d / 1,000

The steps are 2.0 × 10 × 8.34 = 166.8 lb/day and 166.8 / 1.04 = 160 gal/d; in metric, 2.0 × 37,854 / 1,000 = 75.7 kg/d and 75.7 / 0.125 kg/L = 606 L/d.

A dose stated as NaOCl gives a different number: at 2,500 m³/d (0.66 MGD) and 8.5 mg/L as NaOCl, a 12.0 wt % solution at 1.19 kg/L feeds 21.25 kg/d of NaOCl, or 148.8 L/d (39.3 gal/d) (EOCP Math for Operators). That dose is 8.1 mg/L as Cl2 (8.5 × 0.952), so every dose names its basis.

The chemical dosage calculator converts mg/L as Cl2 into kg/d, lb/day, L/h and a pump setting.

Worked example 3: chlorine gas feed rate and alkalinity consumed#

A 1.2 MGD (4,542 m³/d) plant dosing 1.5 mg/L of chlorine gas feeds 15 lb/day (6.8 kg/d) of Cl2 and consumes about 2.1 mg/L of alkalinity as CaCO3. Worked example 3 uses these inputs:

• flow: 1.2 MGD (4,542 m³/d); dose: 1.5 mg/L as Cl2
• purity: 1.0 (at least 99.5 % Cl2)
• alkalinity: 1.4 mg/L as CaCO3 per mg/L Cl2 (Veolia Water Technologies Handbook)

The steps are 1.5 × 1.2 × 8.34 = 15.0 lb/day, 1.5 × 4,542 / 1,000 = 6.8 kg/d and 1.5 × 1.4 = 2.1 mg/L. With 1.0 mg/L of free residual left, demand was 0.5 mg/L. The same handbook does not recommend linking pH control to chlorine gas feed, because the loop risks overchlorination.

How Is Chlorine Measured in Treated Water?#

Chlorine in treated water is measured as free chlorine (HOCl + OCl-), combined chlorine (chloramines) and total chlorine (their sum), all as mg/L as Cl2, by DPD colorimetry, DPD titration or amperometric titration.

The difference between the three readings is explained under chlorine residual in water treatment.

Approved free and total chlorine test methods#

US drinking-water compliance accepts 6 Standard Methods for chlorine residual plus ASTM D1253 (40 CFR 141.74(a)(2)). The accepted methods are listed below, method code first.

• 4500-Cl D and E: amperometric titration, standard and low level.
• 4500-Cl F: DPD ferrous titrimetric (DPD-FAS).
• 4500-Cl G: DPD colorimetric.
• 4500-Cl H and I: syringaldazine (free) and iodometric electrode (total).
• ASTM D1253-14(2021)e1: residual chlorine in water.

DPD kits and the ITS test strip (D99-003) count only where the State approves them, and online analyzers are checked against a grab sample at least every 5 days. NPDES total residual chlorine is analyzed within 15 minutes (40 CFR 136.3); Standard Methods is in its 24th edition (2023).

ORP as a chlorination control signal#

ORP (oxidation-reduction potential) reads the net oxidizing strength of water in mV, so it responds to HOCl but is not specific to chlorine and gives a smaller response at higher pH and higher temperature. The method is ASTM D1498-14(2022)e1.

ORP feed control held effluent total residual chlorine at 0.1 to 0.2 mg/L at the Marsh Creek wastewater plant (US EPA), and RO plants trip at 175 to 200 mV after dechlorination. Probe care and setpoints are covered under oxidation-reduction potential (ORP) in water treatment.

Typical Chlorine Doses and Residuals by System#

Chlorine doses and residuals differ by system: drinking water holds 0.2 to 0.5 mg/L of free chlorine in distribution, wastewater effluent takes 5 to 20 mg/L as Cl2, cooling towers run 0.5 to 1.0 ppm (mg/L) free chlorine, and RO pretreatment holds 0.5 to 1.0 mg/L ahead of dechlorination. Table 3 compares dose or residual, basis, control test and limit for the four systems.

Chlorination in drinking water#

Drinking-water chlorination targets at least 0.5 mg/L of free chlorine after at least 30 minutes of contact at pH below 8.0, then 0.2 to 0.5 mg/L through distribution to the point of delivery (WHO).

In the US, the entry-point residual may not stay below 0.2 mg/L for more than 4 hours, the residual must be detectable in at least 95 % of monthly distribution samples (40 CFR 141.72), and the MRDL caps chlorine at 4.0 mg/L as Cl2 (40 CFR 141.65). NSF/ANSI/CAN 60 lists sodium hypochlorite most commonly at 84 mg/L as product for 12.5 % products and chlorine at 30 mg/L (NSF listings, 2026-10-21).

Turbidity also limits chlorination: above 1 to 2 NTU, efficacy falls and a higher Ct is needed (WHO). NTU limits are explained under turbidity in water.

Chlorination of wastewater effluent#

Wastewater effluent chlorination doses 5 to 20 mg/L as Cl2 into a baffled contact basin with initial mixing in under 1 second, controlled on flow and residual (US EPA fact sheet EPA 832-F-99-062).

The same fact sheet puts dechlorination at about 30 to 50 % of chlorination cost.

EPA aquatic-life criteria for chlorine are 19 µg/L (acute) and 11 µg/L (chronic) in freshwater and 13 and 7.5 µg/L in saltwater (1986). Contact basins, permits and dechlorination are covered under wastewater disinfection and dechlorination.

Chlorination of cooling water#

Cooling-tower chlorination holds a continuous free residual of 0.5 to 1.0 ppm (mg/L) as Cl2 in the hot return water (Cooling Technology Institute Legionella guideline), and its efficiency falls above pH 8 as HOCl gives way to OCl-. The Veolia Water Technologies Handbook gives 0.1 to 0.5 ppm free halogen for open recirculating systems.

The steam electric guideline (40 CFR 423.13) limits once-through cooling at plants of 25 MW or more to 0.20 mg/L total residual chlorine, and tower blowdown to 0.5 mg/L free available chlorine maximum and 0.2 mg/L average, discharged no more than 2 h/day per unit.

Oxidizing and non-oxidizing programs, feed modes and Legionella targets are compared under cooling tower biocides.

Chlorination ahead of RO and membrane systems#

RO pretreatment chlorination holds 0.5 to 1.0 mg/L of free chlorine for 20 to 30 minutes, then removes it before the membranes, because polyamide RO tolerates less than 0.1 mg/L and degrades after about 200 to 1,000 ppm-h (DuPont FilmTec manual).

Sodium metabisulfite removes the residual at 1.34 mg per mg Cl2 in theory and about 3.0 mg in practice, upstream of the cartridge filters, and an ORP interlock at 175 to 200 mV trips the high-pressure pump.

Pretreatment, dechlorination and biofouling control are covered under reverse osmosis and membrane chemicals.

What Changes Chlorination Performance? pH, Temperature, Ammonia and Bromide#

Six factors change chlorination performance: pH, temperature, ammonia, bromide, turbidity and the chlorine source itself, which adds acid (chlorine gas) or base (hypochlorite) to the water. The 6 factors are listed below.

• pH: HOCl falls from 77.6 % at pH 7.0 to 9.9 % at pH 8.5.
• Temperature: 3-log Giardia CT at pH 7.0 and 1.0 mg/L is 210 mg·min/L at 0.5 °C (32.9 °F) against 37 at 25 °C (77 °F).
• Ammonia: converts free chlorine to chloramines.
• Bromide: converts HOCl to HOBr.
• Turbidity: above 1 to 2 NTU, efficacy falls.
• Source chemistry: chlorine gas consumes 1.4 mg/L alkalinity as CaCO3 per mg/L; hypochlorite adds hydroxide.

Caustic soda, lime and acids that set the pH before chlorination are compared under pH adjustment chemicals.

Which Rules Limit Chlorine and Chlorination Byproducts?#

Chlorination is limited in 3 layers: a maximum residual disinfectant level for chlorine, minimum residuals and CT for disinfection, and maximum contaminant levels for the byproducts chlorine forms with organic matter and bromide. Table 4 sets US, WHO and EU values side by side.

US drinking-water rules for chlorine residuals and byproducts#

The US MRDL for chlorine is 4.0 mg/L as Cl2 as a running annual average (40 CFR 141.65, Stage 1 Disinfectants and Disinfection Byproducts Rule), which is not an MCL and is not applied to a single sample.

Monitoring and compliance are set out in the Disinfectants and Disinfection Byproducts Rules.

The byproduct MCLs are 0.080 mg/L for total trihalomethanes and 0.060 mg/L for HAA5, each as a locational running annual average, plus 0.010 mg/L bromate and 1.0 mg/L chlorite (40 CFR 141.64; 71 FR 388, 2006-01-04).

The US has no chlorate MCL, and no revision of the microbial and disinfection byproducts rules had been proposed as of 2026-10-20; EPA anticipates a proposal in summer 2027.

Precursor removal and dose-point changes are covered under disinfection byproducts.

WHO and EU values for chlorine and chlorination byproducts#

WHO sets a health-based guideline value of 5 mg/L for free chlorine, and the EU Drinking Water Directive 2020/2184 limits the chlorination byproducts: total THMs 100 µg/L, HAA5 60 µg/L, bromate 10 µg/L and chlorate 0.25 mg/L (0.70 mg/L where chlorine dioxide is used) from 12 January 2026.

Health Canada considers no maximum acceptable concentration necessary for chlorine (2009).

The guideline values and their basis are listed under WHO Guidelines for Drinking-water Quality.

Product standards for chlorine chemicals: NSF/ANSI/CAN 60, AWWA and EN#

Chlorine chemicals for drinking water are specified by ANSI/AWWA B300-24 (hypochlorites) and B301-24 (liquid chlorine), by EN 901:2024 (sodium hypochlorite) and EN 937:2016 (chlorine), and certified product by product under NSF/ANSI/CAN 60. AWWA B300 requires FIFRA registration for disinfection use.

Maximum use levels are product-specific and stated as product. Under the EU Biocidal Products Regulation, active chlorine from sodium hypochlorite is approved for PT1 to PT5, from calcium hypochlorite for PT2 to PT5 and from chlorine for PT2 and PT5, each from 2019-01-01 to 2028-12-31; drinking-water disinfection is PT5.

How maximum use levels are set and checked is explained under NSF/ANSI/CAN 60.

Common Water Chlorination Mistakes#

Seven chlorination mistakes recur in operator practice, and each one has a number that catches it. The 7 mistakes are listed below.

1. Reading trade % as wt %: 12.5 trade % sodium hypochlorite is 11.23 wt %.
2. Ignoring pH: 1.0 mg/L free chlorine at pH 8.5 is about 0.1 mg/L HOCl.
3. Mixing hypochlorite with acid or an acidic coagulant, which releases chlorine gas.
4. Treating the 4.0 mg/L MRDL as a single-sample limit or an MCL.
5. Applying free-chlorine CT to Cryptosporidium.
6. Storing hypochlorite too long or topping up old solution: WHO advises buying fresh, storing cool and dark and using within about a month.
7. Skipping the grab-sample check of an online analyzer, due at least every 5 days.

Chlorination Calculators and Program Support#

Two calculators apply this chemistry: the dosage calculator turns mg/L as Cl2 into feed rates, and the CT calculator checks contact time against EPA tables.

Enter flow, dose, basis and product strength in the chemical dosage and feed rate calculator.

The dosage inputs are flow in m³/d or MGD, dose with its basis (as Cl2 or as NaOCl), strength in g/L or trade % and density.

The CT inputs are the free residual, the contact time, pH and temperature.

Log inactivation is checked against the EPA tables in the CT value calculator for disinfection.

Is Chlorinated Water Safe, and How Does Chlorination Compare with Other Disinfectants?#

Chlorinated water is safe to drink at the residuals utilities use, which stay below the 4.0 mg/L as Cl2 US MRDL and the 5 mg/L WHO guideline value, and chlorination is one of 5 disinfection options compared below: chlorine, chloramine, chlorine dioxide, ozone and UV.

Scaling, corrosion and coagulation sit beside disinfection among the water treatment chemistry concepts.

Is chlorine in drinking water safe to drink?#

Chlorine in drinking water is safe at normal residuals: WHO's guideline value of 5 mg/L rests on a tolerable daily intake of 150 µg/kg body weight, and NSF/ANSI/CAN 60 caps each certified hypochlorite product at its own maximum use level as product. IARC places hypochlorite in Group 3.

Why does treated water taste or smell of chlorine?#

Treated water tastes of chlorine at about 5 mg/L and smells of it at about 2 mg/L, while sensitive consumers taste it from about 0.3 mg/L (WHO chlorine background document).

What is the difference between chlorination and chloramination?#

Chloramination adds ammonia to chlorine to form monochloramine at a Cl2:NH3-N weight ratio of 4.5:1 to 5:1, giving a longer-lasting but weaker residual: its 3-log Giardia CT at 10 °C (50 °F) is 1,850 mg·min/L against 112 for free chlorine.

Ratios and nitrification control are covered under chloramination.

WHO states that monochloramine is not used as the primary disinfectant, and more than one in five Americans drink chloraminated water (US EPA). Its residual, nitrification and dose arithmetic are on monochloramine in water treatment.

How does chlorination compare with chlorine dioxide, ozone and UV?#

Chlorination leaves a lasting residual in distribution: chlorine dioxide does not provide a residual effect as chlorine would, and UV leaves none, so plants that rely on them add a small chlorine or monochloramine dose (WHO).

The 3-log Giardia CT at 10 °C is 23 mg·min/L for chlorine dioxide and 1.43 for ozone. Chlorine dioxide does not chlorinate organics. The full family of chlorine, bromine, ClO2, ozone and peroxygen oxidants is compared on the oxidants hub.

How is chlorine removed after chlorination?#

Chlorine is removed after chlorination by a reducing agent: sulfur dioxide reacts at 0.90 mg per mg Cl2 in theory, and sodium metabisulfite at 1.34 mg per mg Cl2 (about 3.0 mg in RO practice). Thiosulfates, ascorbates and activated carbon also remove the residual.

Ratios, contact times and overdose effects for every agent are compared under dechlorination chemicals.

Which chemicals are incompatible with chlorine at the plant?#

Hypochlorite releases chlorine gas when it meets any acid or an acidic coagulant such as ferric chloride or alum, and it forms chloramines with ammonia (nitrogen trichloride when acid is also present); hypochlorite and acid pairs caused 56 % of 72 documented unloading accidents between 1953 and 2019.

On 2016-10-21 at MGPI Processing in Atchison, Kansas, a sulfuric acid delivery hose connected to the sodium hypochlorite fill line released a toxic cloud (US Chemical Safety Board). Every dangerous pair is listed under water treatment chemical compatibility and incompatible mixtures.

Which chlorine quantities trigger EPA RMP and OSHA PSM?#

Chlorine triggers EPA RMP at 2,500 lb (1,134 kg) in a process (40 CFR 68.130) and OSHA PSM at 1,500 lb (680 kg) (29 CFR 1910.119 Appendix A). The OSHA PEL is a ceiling of 1 ppm (3 mg/m³), the NIOSH REL a ceiling of 0.5 ppm and the IDLH 10 ppm; the 2026-02-24 RMP proposal is not final.