Breakpoint Chlorination and Chlorine Demand: The 4-Zone Curve, 7.6:1 Ratio and Worked Examples

Breakpoint chlorination is the chlorine dose at which every reducing compound and ammonia-derived chloramine in the water has been oxidized, so that each further mg/L of chlorine appears as free chlorine residual; with ammonia present, it falls at a theoretical 7.6 mg/L of chlorine per mg/L of ammonia-nitrogen. Practical plants need 8 to 10 mg/L as Cl2 per mg/L NH3-N, so how much chlorine does a given water demand before a free residual forms?

The dose passes through 4 zones: A (reducing compounds), B (chloramine formation), C (chloramine destruction) and D (free residual). US municipal water uses about 628 million kg of chlorine a year, 67 % of it in wastewater and 33 % in drinking water (US EPA Chlorine Supply Chain Profile, 2022 estimate). Chlorine and the hypochlorites belong to the oxidant family of water treatment chemicals.

Below: the zones, the 5:1, 7.6:1 and 8:1 to 10:1 ratios, 3 worked doses (up to 51.7 lb/day for a 1 MGD well), the demand test, targets by system and common mistakes.

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

What Is Breakpoint Chlorination?#

Breakpoint chlorination is dosing chlorine past the point where chlorine demand and combined chlorine are destroyed, so the residual changes from chloramines to free chlorine (HOCl and OCl-). The breakpoint lies at a theoretical 7.6:1 and an actual 8:1 to 10:1 weight ratio of Cl2 to NH3-N (Health Canada, Chloramines guideline technical document, 2020). Below the breakpoint the residual is combined chlorine; at the breakpoint it is at its minimum; above it, residual rises one for one with dose.

What is chlorine demand?#

Chlorine demand is the part of a chlorine dose consumed by reactions with substances in the water during a stated contact time: demand equals dose minus residual, so a 1.5 mg/L dose that leaves 1.0 mg/L of free chlorine has a 0.5 mg/L demand (Michigan Tech training, 2023).

One part per million (ppm) of chlorine equals 1 mg/L only in dilute water with a density of about 1. Demand depends on contact time, pH and temperature, which is why the demand test states them.

Why does disinfection need a free residual past the breakpoint?#

Disinfection needs a free residual because free chlorine reaches a Giardia credit with far less contact: a 3-log Giardia credit at 10 °C (50 °F) needs a CT of 112 min.mg/L with free chlorine at 1.0 mg/L and pH 7.0, against 1,850 min.mg/L with chloramines (40 CFR 141.74). The chloramine CT for a 1-log Giardia credit is about 18 times higher (US EPA disinfection profiling and benchmarking guidance, 2022).

WHO defines effective chlorination as at least 0.5 mg/L of free chlorine after at least 30 minutes of contact at pH below 8.0, a Ct of 15 min.mg/L, and at least 0.2 mg/L at the point of delivery. Breakpoint chlorination also oxidizes ammonia nitrogen, which makes it a route to ammonia removal (worked example 3).

The calculation of log inactivation is covered under CT values for disinfection.

The Breakpoint Chlorination Curve: 4 Zones#

The breakpoint chlorination curve plots chlorine residual against chlorine dose and has 4 zones: A, where reducing compounds consume chlorine; B, where chloramines form; C, where chloramines are destroyed; and D, where free residual rises after the breakpoint.

The 4 zones of the breakpoint curve are compared below by dose range, chlorine species and residual type.

Ratios by weight, chlorine as Cl2, ammonia as NH3-N (Health Canada, 2020).

Zone A: iron, manganese, sulfide and other reducing compounds#

Zone A is the immediate demand: reduced iron, manganese and sulfide consume chlorine before any measurable residual forms, at about 0.63 mg of chlorine per mg of Fe(II), 1.3 mg per mg of Mn(II) and 2.22 mg per mg of sulfide oxidized to elemental sulfur.

Why does the practical dose exceed these ratios? Health Canada (manganese technical document, 2019) puts the practical HOCl dose for Mn(II) at about 4 times the stoichiometric value, and sewer and wastewater sulfide control uses 10 to 15 lb of Cl2 per lb of sulfide (kg per kg). The reducing compounds of zone A are compared below by theoretical and practical chlorine demand.

Zone B: chlorine and ammonia form monochloramine#

In zone B, chlorine reacts with ammonia to form monochloramine (NH3 + HOCl → NH2Cl + H2O), and total residual rises with dose up to a weight ratio of about 5:1 Cl2 : NH3-N, which is 1:1 on a molar basis. Chloramination plants design at 4.5:1 to 5:1, never above 5:1, and stay in zone B on purpose (Health Canada, 2020).

The chemistry, dose and limits of the combined disinfectant itself are on monochloramine in water treatment.

Zone C: chloramines are destroyed up to the breakpoint#

In zone C, chlorine added beyond 5:1 converts monochloramine to dichloramine and oxidizes the chloramines, so total residual falls as dose rises until the breakpoint at a theoretical 7.6:1 Cl2 : NH3-N. Dichloramine is favored between 5:1 and 7.6:1 and at pH 4.0 to 6.0; nitrogen trichloride forms below pH 4.4 or near 15:1 (Health Canada, 2020).

Zone C is the odor zone: dichloramine has taste and odor thresholds of 0.13 and 0.15 mg/L and nitrogen trichloride an odor threshold of 0.02 mg/L, against 0.48 and 0.65 mg/L for monochloramine (Health Canada, 2020).

Zone D: free chlorine residual after the breakpoint#

Zone D begins at the breakpoint: the combined residual is at its minimum, and every further mg/L of chlorine adds free chlorine residual that is available for disinfection. Free available chlorine is the sum of Cl2, HOCl and OCl-; combined chlorine is the chloramines; total residual chlorine is free plus combined, all as Cl2.

Breakpoint Chemistry: Reactions and Chlorine to Ammonia Ratios#

Breakpoint chemistry rests on 3 reactions and 4 weight ratios: chlorine hydrolysis, hypochlorite hydrolysis and monochloramine formation, and the ratios 5:1, 7.6:1, 8:1 to 10:1 and about 15:1 Cl2 : NH3-N.

Chlorination reactions behind the breakpoint#

Three reactions set the start of the breakpoint curve, listed below with their effect on pH.

• Chlorine gas hydrolysis: Cl2 + H2O → HOCl + HCl, which consumes alkalinity and lowers pH.
• Hypochlorite hydrolysis: NaOCl + H2O → HOCl + NaOH, which raises pH slightly; for calcium hypochlorite, Ca(OCl)2 + 2 H2O → 2 HOCl + Ca(OH)2, which adds calcium hardness and alkalinity.
• Monochloramine formation: NH3 + HOCl → NH2Cl + H2O.

Hypochlorous acid dissociates to hypochlorite ion with a pKa of 7.54 at 25 °C (77 °F) (Morris, 1966), so about half of the free chlorine is HOCl at pH 7.5. The speciation and its pH dependence are explained on water chlorination chemistry.

Why is the breakpoint 7.6:1 in theory and 8:1 to 10:1 in practice?#

The theoretical breakpoint is 7.6 mg of chlorine per mg of ammonia-nitrogen, while plants observe 8:1 to 10:1, because zone A demand and other reactions in the water consume chlorine on top of the ammonia stoichiometry (Health Canada, 2020).

What dose removes ammonia in the field? The UK Drinking Water Inspectorate gives roughly 10 mg/L of chlorine per mg/L of ammonia removed as a practical figure and calls it site-specific. No single ratio is a constant; the demand test fixes it.

How do pH, chlorine gas and hypochlorite shift the curve?#

pH sets which chloramine forms: monochloramine forms between pH 6.5 and 9.0 and fastest at 8.3, dichloramine is favored at pH 4.0 to 6.0, and nitrogen trichloride forms below pH 4.4. The chlorine source moves pH in one of 3 directions, listed below.

• Chlorine gas lowers pH and consumes alkalinity.
• Sodium hypochlorite raises pH slightly.
• Calcium hypochlorite adds calcium hardness and alkalinity.

The strength, storage and dosing of sodium hypochlorite in water treatment are covered on its own page.

How to Calculate the Breakpoint Chlorine Dose: 3 Worked Examples#

The breakpoint chlorine dose equals zone A demand plus 8 to 10 mg/L as Cl2 per mg/L of ammonia-nitrogen plus the free residual target, and a chlorine demand test on the actual water confirms it. The 3 worked examples are summarized below with their inputs and results.

Worked example 1: chlorine demand from a bench test#

Worked example: a water dosed at 1.5 mg/L as Cl2 that holds 1.0 mg/L of free chlorine after the contact time has a chlorine demand of 0.5 mg/L. The inputs are a 1.5 mg/L (ppm) dose as Cl2, a 1.0 mg/L free residual and the plant's own contact time.

Demand = dose − residual = 1.5 − 1.0 = 0.5 mg/L as Cl2. The identity holds only when the residual is free chlorine (zone D); a mostly combined total residual means the dose sits in zone B or C.

Worked example 2: breakpoint dose and hypochlorite feed rate for groundwater#

Worked example: groundwater at 1 MGD (3,785 m³/d) with 0.5 mg/L NH3-N and 0.3 mg/L ferrous iron needs 5.2 to 6.2 mg/L as Cl2 to pass the breakpoint and hold 1.0 mg/L free chlorine, or up to 51.7 lb/day (23.5 kg/d) of chlorine. The source is 12.5 trade % sodium hypochlorite (125 g/L available chlorine, 1.04 lb Cl2 per gallon at 1.169 g/mL, Olin hypochlorite handbook); feed in lb/day = dose (mg/L) x flow (MGD) x 8.34 / purity. The 4 steps are listed below.

1. Calculate iron demand: 0.3 / 1.6 = 0.19 mg/L as Cl2.
2. Calculate ammonia to the breakpoint: 0.5 x 7.6 = 3.8 mg/L in theory; 0.5 x 8 to 0.5 x 10 = 4.0 to 5.0 mg/L in practice.
3. Add the residual target: 0.19 + 4.0 to 5.0 + 1.0 = 5.2 to 6.2 mg/L as Cl2.
4. Convert to feed at 6.2 mg/L: 6.2 x 1 x 8.34 = 51.7 lb/day (23.5 kg/d) as Cl2, or 51.7 / 1.04 = 49.7 gal/d (188 L/d) of 12.5 trade % hypochlorite.

The estimate excludes organic demand, so a chlorine demand test on the well water sets the final dose.

The chemical dosage and feed rate calculator converts any breakpoint dose in mg/L into kg/d, lb/day and pump settings.

Worked example 3: breakpoint ammonia removal in wastewater effluent#

Worked example: removing 15 mg/L of ammonia-nitrogen from 10,000 m³/d (2.642 MGD) of effluent by breakpoint takes 120 to 150 mg/L as Cl2, about 1,500 kg/d (3,305 lb/day) of chlorine, against 5 to 20 mg/L for disinfection alone. The 15 mg/L input is a treated-effluent value; domestic raw wastewater carries 20 to 70 mg/L of total nitrogen, 60 to 70 % of it ammonia-N.

The theoretical dose is 15 x 7.6 = 114 mg/L; the practical dose is 15 x 8 to 15 x 10 = 120 to 150 mg/L as Cl2. At 150 mg/L, the feed is 150 g/m³ x 10,000 m³/d = 1,500 kg/d, or 150 x 2.642 x 8.34 = about 3,305 lb/day.

The residual left after the breakpoint must be removed before discharge, and the acid formed by chlorine gas lowers pH, so pH control runs with the feed. Biological nitrification consumes 7.14 mg of alkalinity as CaCO3 per mg of NH3-N oxidized (Minnesota Pollution Control Agency).

Biological and chemical routes are compared under chemical support for nitrogen removal.

How Is a Chlorine Demand Test Done?#

A chlorine demand test doses a series of samples of the actual water with known chlorine amounts, holds them for the plant's contact time and measures free and total residual, so the breakpoint and demand can be read from the results (ASTM D1291).

Chlorine demand test procedure#

The chlorine demand test runs in 6 steps, from a representative sample to a plotted residual curve; they are listed below.

1. Collect a representative sample at the planned injection point.
2. Measure pH, temperature and ammonia as NH3-N.
3. Dose a series of samples with increasing chlorine as Cl2, bracketing 8 to 10 times the NH3-N value.
4. Hold each sample for the plant's contact time at the plant's pH.
5. Measure free and total chlorine within 15 minutes of sampling.
6. Plot residual against dose and read the minimum (the breakpoint) and the demand at the target residual.

ASTM D1291 is the standard practice for this test; its result sets dose, injection point, pH and contact time (DuPont FilmTec chlorination and dechlorination manual). The 15-minute limit is the holding time for total residual chlorine in 40 CFR 136.3.

The bench setup of jars and a dose series is the same as in the jar test procedure, with chlorine instead of a coagulant.

Measuring free and total chlorine residual along the curve#

Free and total chlorine along the curve are measured by the US-approved methods in 40 CFR 141.74(a)(2): amperometric titration (SM 4500-Cl D and E), DPD-FAS titration (F), DPD colorimetry (G), syringaldazine (H, free only) and the iodometric electrode (I, total), with DPD test kits only where the State approves them.

Combined chlorine is the difference between total and free chlorine, all as Cl2. The definitions and test choices are explained under chlorine residual in water treatment.

Typical Breakpoint Doses and Residual Targets by System#

Breakpoint doses and residual targets differ by system: drinking water aims for a free residual of at least 0.2 mg/L at the entry point, wastewater doses 5 to 20 mg/L for disinfection, and cooling water holds 0.5 to 1.0 mg/L of free chlorine. The 5 systems are compared below by purpose, residual target and the role of the breakpoint.

Drinking water: free chlorine disinfection and NSF/ANSI/CAN 60 dose caps#

Drinking water past the breakpoint must keep a free residual of at least 0.2 mg/L at the entry point, never lower for more than 4 hours, and a detectable residual in at least 95 % of distribution samples (40 CFR 141.72).

NSF/ANSI/CAN 60 caps the dose a breakpoint program can feed. Maximum use levels are product-specific and stated as product: 30 mg/L in 102 of 106 chlorine product-function rows, most commonly 84 mg/L for sodium hypochlorite (the typical 12.5 % products) and most commonly 15 mg/L for calcium hypochlorite (NSF listings, 2026-10-21). How maximum use levels are set is explained on NSF/ANSI/CAN 60.

Chloraminated systems: nitrification and breakpoint chlorination#

Chloraminated systems use breakpoint chlorination as a corrective action when nitrification starts, with a nitrite action level of 0.05 mg/L plus a falling residual as one trigger example (US EPA Nitrification white paper, 2002). About two thirds of medium and large US chloraminating systems experience nitrification (AWWA survey, 1996), and nitrite runs at 0.005 to 0.5 mg/L NO2-N during episodes, usually 0.015 to 0.1 mg/L.

Prevention keeps about 2 mg/L as Cl2 leaving the plant and more than 1.5 mg/L at all monitoring points (Health Canada, 2020). Ratio control and nitrification action plans are covered under chloramination.

Municipal wastewater: disinfection, dechlorination and ammonia#

Municipal wastewater is chlorinated at 5 to 20 mg/L as Cl2 for disinfection, far below the 8 to 10 mg/L per mg/L NH3-N that breakpoint needs, so effluents that carry ammonia are disinfected below the breakpoint, with combined chlorine.

The residual is removed before discharge at a theoretical 0.90 mg/L of SO2 per mg/L of Cl2, and compliance rests on total residual chlorine analyzed within 15 minutes. In one US EPA case at the Marsh Creek plant, ORP feed control held total residual chlorine at 0.1 to 0.2 mg/L. Bisulfite, metabisulfite and thiosulfate ratios are compared on dechlorination chemicals.

Cooling water and RO pretreatment#

Cooling towers hold 0.5 to 1.0 mg/L of free chlorine continuously in the hot return water (Cooling Technology Institute), and steam electric plants may discharge at most 0.5 mg/L free available chlorine in tower blowdown, 0.2 mg/L on average (40 CFR 423.13). RO pretreatment carries 0.5 to 1.0 mg/L of free chlorine for 20 to 30 minutes and removes it before polyamide membranes, which tolerate less than 0.1 mg/L (DuPont FilmTec manual).

What Changes Chlorine Demand and the Breakpoint?#

Six factors move chlorine demand and the breakpoint dose; they are listed below in the order the curve meets them.

• Ammonia-nitrogen in the source, each mg/L adding 8 to 10 mg/L as Cl2.
• Reduced iron, manganese and sulfide (Table 2).
• Ammonia sources dosed on purpose, such as ammonium sulfate and aqua ammonia for chloramination.
• pH, which decides between monochloramine, dichloramine and nitrogen trichloride.
• The chlorine source: chlorine gas lowers pH, hypochlorites raise it.
• Contact time, which the demand test fixes for each water.

Other oxidants meet the same reducing compounds: Health Canada gives 2.45 mg of chlorine dioxide, 0.87 mg of ozone and 1.9 mg of permanganate per mg of Mn(II). Chlorine and these oxidants are compared under oxidants and oxidizing biocides for water treatment.

Common Breakpoint Chlorination Mistakes#

Five mistakes recur in breakpoint calculations and operation; they are listed below.

• A ratio written without its basis: it should always read by weight, Cl2 : NH3-N.
• A total residual read as free residual in zones B and C, where most of it is combined chlorine.
• Hypochlorite trade % treated as weight %: 12.5 trade % is 125 g/L, about 11.2 wt % NaOCl.
• A chloramination design above 5:1, which pushes the dose into zone C.
• A breakpoint dose in drinking water that exceeds the product's NSF/ANSI/CAN 60 maximum use level.

Is Breakpoint Chlorination Safe? Byproducts, Odors and Residual Limits#

Breakpoint chlorination carries 4 known risks: disinfection byproducts from free chlorine, chloramine odors in zone C, the 4.0 mg/L as Cl2 maximum residual disinfectant level and the hazards of mixing hypochlorite with ammonia or acids.

Does breakpoint chlorination form disinfection byproducts?#

Free chlorine forms trihalomethanes and haloacetic acids with natural organic matter, regulated in the US at 0.080 mg/L TTHM and 0.060 mg/L HAA5 as locational running annual averages (40 CFR 141.64). WHO lists the operational measures: optimize the chlorine dose and dose point, avoid pre-chlorination where possible and reduce water age. Formation and control are covered under disinfection byproducts.

What causes the chlorinous odor near the breakpoint?#

The odor near the breakpoint comes mostly from dichloramine and nitrogen trichloride, whose odor thresholds of 0.15 and 0.02 mg/L lie far below that of monochloramine at 0.65 mg/L (Health Canada, 2020). Free chlorine itself has odor and taste thresholds of 2 and 5 mg/L, with some taste at 0.3 mg/L (WHO).

What is the maximum disinfectant residual in drinking water?#

The maximum residual disinfectant level is 4.0 mg/L as Cl2 for chlorine and for chloramines, as a running annual average, not a single-sample limit (40 CFR 141.65). Chlorine or chloramine may exceed it as necessary for line breaks, storm runoff, source contamination or cross-connection events (40 CFR 141.2). MRDL compliance is set out in the Disinfectants and Disinfection Byproducts Rules.

What happens when chloraminated and free-chlorinated water mix?#

Blending chloraminated water with free-chlorinated water causes an uncontrolled breakpoint at the point where the two waters meet, with loss of residual, odors and byproduct formation.

Which chemicals are incompatible with hypochlorite at a breakpoint feed?#

Hypochlorite must never meet ammonia or ammonium salts in concentrated form, because they form chloramines and, with acid, nitrogen trichloride, which decomposed explosively in a documented drain incident; acidic coagulants such as ferric chloride or alum release chlorine gas from hypochlorite.

The full matrix of incompatible mixtures is on the compatibility page.