5.1 Chlorination Chemistry & Breakpoint Curves

Chlorination Chemistry & Breakpoint Curves

Chlorination is the most widely deployed chemical disinfection process in municipal drinking water and wastewater treatment across California. Developed and refined over more than a century, chlorination provides two distinct public health protections: primary disinfection (inactivating pathogenic bacteria, viruses, and protozoan cysts within the treatment plant contact basin) and secondary disinfection (maintaining a persistent chemical residual throughout the distribution conveyance network to suppress bacterial regrowth and protect against cross-connection contamination).

To manage disinfection processes effectively, certified operators must master the chemical equilibria of aqueous chlorine, the differences among commercial chlorine chemical feedstocks, the kinetics of chlorine reactions with inorganic and organic compounds, and the mechanics of the breakpoint chlorination curve.

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Chemical Delivery Forms of Chlorine

Water and wastewater treatment facilities utilize three primary chemical forms of chlorine, each possessing unique physical properties, available chlorine content, storage requirements, and operational safety protocols.

1. Gaseous Chlorine ($Cl_2$)

• Physical Properties: Elemental chlorine is supplied under pressure as a heavy amber-colored liquid in 150-lb cylinders, 1-ton containers, or railcars. When vaporized, liquid chlorine expands 460 times into a greenish-yellow gas that is approximately 2.5 times heavier than air (specific gravity $\approx 2.49$). Because the gas is denser than ambient air, accidental releases migrate along the floor, collect in low-elevation sumps, and travel downhill.
• Available Chlorine Equivalent: 100% available chlorine by definition.
• Safety & Handling: Chlorine gas is a severe pulmonary irritant and toxic inhalation hazard. Facilities must maintain automatic vacuum-operated chlorinators, sulfur dioxide or caustic scrubbers, emergency repair kits (Kit A for 150-lb cylinders, Kit B for 1-ton containers, Kit C for tank cars), and continuous atmospheric gas detectors calibrated to alarm at 0.5 ppm (warning) and 1.0 to 3.0 ppm (evacuation / emergency scrubber activation).

2. Sodium Hypochlorite ($NaOCl$)

• Physical Properties: Liquid bleach solution containing dissolved sodium hypochlorite in water. Commercial grade typically contains 12.5% trade available chlorine (approximately 125 g/L available $Cl_2$, or ~10.8% available chlorine by weight). It is strongly alkaline, with a pH ranging from 11.0 to 13.0, stabilized with residual sodium hydroxide ($NaOH$) to prevent premature decomposition.
• Decomposition Factors: Sodium hypochlorite solutions are inherently unstable and naturally degrade over time into sodium chloride ($NaCl$), sodium chlorate ($NaClO_3$), and oxygen gas ($O_2$). The decomposition rate accelerates dramatically in the presence of:
  1. Elevated Temperature: Storage temperatures above 20°C (68°F) significantly shorten chemical shelf life; tanks should be stored in temperature-controlled or shaded enclosures.
  2. Ultraviolet Light: Direct sunlight promotes photolytic breakdown; bulk storage requires opaque, UV-stabilized high-density cross-linked polyethylene (HDPE) or fiberglass reinforced plastic (FRP) tanks.
  3. Transition Metal Catalysts: Minute trace concentrations of iron ($Fe$), nickel ($Ni$), copper ($Cu$), or manganese ($Mn$) catalyze rapid decomposition.
• Operational Hazard: Off-gassing of oxygen inside chemical feed suction lines and positive displacement diaphragm pump heads causes gas binding (vapor lock), interrupting chemical dosing unless degassing valves or peristaltic pumps are used.

3. Calcium Hypochlorite ($Ca(OCl)_2$)

• Physical Properties: Solid dry chemical available as white granules, compressed tablets, or briquettes containing approximately 65% available chlorine by weight (trade names include HTH).
• Safety & Storage: Calcium hypochlorite is a potent Class 3 oxidizing agent. It reacts violently and exothermically with organic materials, petroleum products, oils, greases, solvents, and acids, potentially causing spontaneous combustion and toxic gas release. It must be stored in cool, dry, well-ventilated areas away from any combustible materials.
• Operational Impact: When dissolved in water, calcium hypochlorite releases calcium ions ($Ca^{2+}$), which increases the finished water's total calcium hardness. In high-alkalinity waters, this can cause calcium carbonate ($CaCO_3$) scale precipitation on pump impellers, injector nozzles, and carrier piping.

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Aqueous Chlorine Chemistry & pH Dissociation Equilibrium

Regardless of whether chlorine is fed as gas ($Cl_2$), sodium hypochlorite ($NaOCl$), or calcium hypochlorite ($Ca(OCl)_2$), identical chemical disinfecting species are produced once dissolved in water.

Step 1: Hydrolysis of Chlorine

When gaseous chlorine dissolves in water, it undergoes a rapid, virtually instantaneous hydrolysis reaction (completed in less than 1 second) to form hypochlorous acid ($HOCl$), hydrogen ions ($H^+$), and chloride ions ($Cl^-$):

$\text{Cl}_2 + \text{H}_2\text{O} \rightleftharpoons \text{HOCl} + \text{H}^+ + \text{Cl}^- \quad (K_h = 4.5 \times 10^{-4} \text{ at } 25^\circ\text{C})$

Notice that the hydrolysis of chlorine gas produces hydrochloric acid ($H^+ + Cl^-$), which consumes ambient water alkalinity and depresses the pH. For every 1.0 mg/L of chlorine gas ($Cl_2$) added, approximately 0.7 to 1.4 mg/L of alkalinity (as $CaCO_3$) is consumed.

When sodium hypochlorite is added to water, it hydrolyzes without generating free hydrochloric acid:

$\text{NaOCl} + \text{H}_2\text{O} \rightleftharpoons \text{HOCl} + \text{Na}^+ + \text{OH}^-$

This reaction releases hydroxyl ions ($OH^-$), slightly increasing the pH and conserving ambient water alkalinity.

Step 2: Acid-Base Dissociation Equilibrium of Hypochlorous Acid

Hypochlorous acid ($HOCl$) is a weak acid that partially dissociates in water into hydrogen ions ($H^+$) and hypochlorite ions ($OCl^-$) in a reversible, instantaneous equilibrium governed by solution pH and temperature:

$\text{HOCl} \rightleftharpoons \text{H}^+ + \text{OCl}^- \quad (pK_a = 7.54 \text{ at } 25^\circ\text{C})$

The sum of hypochlorous acid and hypochlorite ion concentration is defined as Free Available Chlorine (FAC):

$\text{Free Available Chlorine} = [\text{HOCl}] + [\text{OCl}^-]$

[!IMPORTANT] Germicidal Efficacy: HOCl vs. OCl⁻ Hypochlorous acid ($HOCl$) is 80 to 100 times more potent as a germicide than the hypochlorite ion ($OCl^-$) for inactivating bacteria and viruses.

Why? Bacterial and microbial cell walls carry a net negative surface charge. Hypochlorous acid ($HOCl$) is an electrically neutral molecule ($H\text{-}O\text{-}Cl$), allowing it to readily penetrate the cell's lipid bilayer membrane via passive diffusion to oxidize vital intracellular enzyme systems. In contrast, the hypochlorite ion ($OCl^-$) carries a negative electrical charge and is electrostatically repelled by the cell wall, severely impeding its penetration rate.

Consequently, chlorination is vastly more effective at lower pH levels (pH 6.5 to 7.2) where $HOCl$ predominates than at elevated pH levels (pH > 8.0).

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Chlorine Demand Kinetics & Math

When chlorine is introduced into untreated water, it reacts immediately with various chemical constituents before establishing a stable residual.

The Fundamental Chlorine Balance

$\text{Chlorine Dose (mg/L)} = \text{Chlorine Demand (mg/L)} + \text{Chlorine Residual (mg/L)}$

$\text{Chlorine Demand (mg/L)} = \text{Chlorine Dose (mg/L)} - \text{Chlorine Residual (mg/L)}$

$\text{Total Chlorine Residual} = \text{Free Available Residual} + \text{Combined Available Residual}$

1. Chlorine Dose: The total mass concentration of chlorine chemical added to the water stream (expressed in $\text{mg/L}$ or $\text{ppm}$).
2. Chlorine Demand: The amount of applied chlorine consumed by reactions with:
   • Inorganic Reducing Agents: Ferrous iron ($Fe^{2+}$), manganous manganese ($Mn^{2+}$), hydrogen sulfide ($H_2S$), and nitrite ($NO_2^-$).
   • Organic Matter: Humic and fulvic natural organic matter (NOM), proteins, amino acids, and cellular debris.
   • Ammonia and Nitrogenous Compounds: Free ammonia ($NH_3$) and organic nitrogen.
3. Chlorine Residual: The total concentration of active chlorine compounds remaining in the treated water at the end of a specified contact time (measured via DPD colorimetric or amperometric titration methods).

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Reactions with Ammonia: Inorganic Chloramine Chemistry

When free chlorine is added to water containing natural dissolved ammonia ($NH_3$) or when ammonia is intentionally added in chloramination systems, chlorine reacts in sequential substitution steps to form inorganic chloramines (combined chlorine).

                                  Ammonia Present (NH₃)
                                            │
                                            ▼  + HOCl (pH > 8.0, Cl₂:N < 5:1)
                                 Monochloramine (NH₂Cl)
                                            │
                                            ▼  + HOCl (pH 5.0 - 7.0, Cl₂:N 5:1 to 7.6:1)
                                  Dichloramine (NHCl₂)
                                            │
                                            ▼  + HOCl (pH < 4.5, Cl₂:N > 7.6:1)
                              Trichloramine / Nitrogen Trichloride (NCl₃)

1. Monochloramine ($NH_2Cl$)

$\text{NH}_3 + \text{HOCl} \rightleftharpoons \text{NH}_2\text{Cl} + \text{H}_2\text{O}$

• Formation Conditions: Favored at alkaline pH ($pH > 8.0$) and a chlorine-to-ammonia nitrogen ($Cl_2 : NH_3\text{-}N$) weight ratio of approximately 3:1 to 5:1 (stoichiometric weight ratio is 5.06:1).
• Characteristics: Monochloramine is a relatively stable, weak disinfectant. While slower at killing pathogens than free chlorine (requiring longer contact times), it produces significantly lower concentrations of regulated trihalomethanes (TTHMs) and haloacetic acids (HAA5), does not produce offensive odors, and persists for extended durations in large distribution networks.

2. Dichloramine ($NHCl_2$)

$\text{NH}_2\text{Cl} + \text{HOCl} \rightleftharpoons \text{NHCl}_2 + \text{H}_2\text{O}$

• Formation Conditions: Formed at intermediate pH ($pH\ 5.0 \text{ to } 7.0$) and elevated $Cl_2 : N$ weight ratios between 5:1 and 7.6:1.
• Characteristics: Dichloramine has a strong, pungent, chlorinous odor and taste (often mistaken by consumers as "too much chlorine") and is a respiratory irritant. Operators avoid operating in this regime.

3. Trichloramine / Nitrogen Trichloride ($NCl_3$)

$\text{NHCl}_2 + \text{HOCl} \rightleftharpoons \text{NCl}_3 + \text{H}_2\text{O}$

• Formation Conditions: Formed under highly acidic conditions ($pH < 4.5$) or at very high $Cl_2 : N$ weight ratios exceeding 7.6:1.
• Characteristics: Nitrogen trichloride is an extremely volatile, noxious gas that causes severe eye tearing (lachrymator) and acute respiratory irritation. In indoor swimming pools or enclosed treatment headworks, trichloramine volatilization creates severe operational safety hazards.

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The Breakpoint Chlorination Curve

The breakpoint chlorination curve illustrates the relationship between the applied chlorine dose and the resulting chlorine residual in water containing oxidizable inorganic compounds, organic matter, and ammonia-nitrogen. The curve is divided into four distinct operating zones.

  Residual
  Chlorine
   (mg/L)   │
            │                           Zone 4: Free Chlorine
            │                           (Slope = 1.0)
            │                          / 
            │      Zone 2:            / 
            │     Chloramines        /
            │       Peak            /
            │        /\            /
            │       /  \          /
            │      /    \        /
            │     /      \      /  <── Breakpoint (Lowest Residual)
            │    /        \    /
    0 ──────┴───/──────────\──/───────────────────► Applied Chlorine Dose (mg/L)
             Zone 1       Zone 3
            (Inorganic  (Chloramine
             Demand)    Destruction)

Zone 1: Oxidation of Inorganic Reducing Agents (Initial Demand)

• Chemical Mechanism: Chlorine added in this initial stage is immediately reduced to inactive chloride ions ($Cl^-$) by readily oxidizable inorganic substances: ferrous iron ($Fe^{2+} \rightarrow Fe^{3+}$), manganese ($Mn^{2+} \rightarrow MnO_2$), sulfides ($H_2S + 4Cl_2 + 4H_2O \rightarrow H_2SO_4 + 8HCl$), and nitrites ($NO_2^- \rightarrow NO_3^-$).
• Observed Residual: Zero residual is formed. All applied chlorine is consumed satisfy the immediate inorganic chemical demand.

Zone 2: Formation of Chloramines and Organochlorines

• Chemical Mechanism: Once inorganic demand is satisfied, added chlorine reacts with ammonia and organic amines to form monochloramine ($NH_2Cl$), dichloramine ($NHCl_2$), and chloro-organic complexes.
• Observed Residual: The measured chlorine residual increases proportionally with the applied dose up to a peak. This residual consists almost entirely of Combined Available Chlorine (CAC).
• Peak Point: The peak occurs at a $Cl_2 : NH_3\text{-}N$ weight ratio of approximately 5:1.

Zone 3: Oxidation and Destruction of Chloramines

• Chemical Mechanism: As the chlorine dose is increased past the 5:1 ratio, the excess free chlorine reacts with and oxidizes the previously formed chloramines, converting combined nitrogen into nitrogen gas ($N_2$), nitrous oxide ($N_2O$), and hydrochloric acid:

$2\text{NH}_2\text{Cl} + \text{HOCl} \longrightarrow \text{N}_2\uparrow + 3\text{HCl} + \text{H}_2\text{O}$

• Observed Residual: The measured total chlorine residual decreases sharply despite adding more chlorine chemical. Highly volatile trichloramine ($NCl_3$) and dichloramine intermediates cause strong odors.
• The Breakpoint: The point where the residual curve reaches its absolute lowest level. At the breakpoint, virtually all ammonia-nitrogen and combined chloramines have been oxidized and off-gassed. Stoichiometrically, the breakpoint occurs at a theoretical $Cl_2 : NH_3\text{-}N$ weight ratio of 7.6:1 (in actual water systems containing natural organics, the practical ratio is typically 8:1 to 10:1).

Zone 4: Free Available Chlorine Residual

• Chemical Mechanism: Once the breakpoint is reached and all ammonia is destroyed, any additional chlorine added dissolves as Free Available Chlorine ($HOCl$ and $OCl^-$).
• Observed Residual: The chlorine residual increases in a direct 1:1 linear relationship with the applied dose (the curve rises at a 45-degree angle / slope of 1.0). In this zone, the total residual consists of free available chlorine plus a small baseline of refractory unoxidized organochlorines.

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Practical Operational Considerations

Chloramination vs. Free Chlorination

• Free Chlorination (Operating in Zone 4): Provides powerful, rapid disinfection kinetics; effective against viruses and bacteria; controls biofilms. Disadvantage: reacts with natural organic matter (NOM) to form high levels of regulated disinfection byproducts (TTHMs and HAA5).
• Chloramination (Operating in Zone 2): Minimizes DBP formation; provides a long-lasting, stable residual across large distribution networks. Disadvantage: weaker disinfectant (ineffective against Cryptosporidium oocysts); susceptible to distribution system nitrification if free ammonia is released.

Nitrification in Chloraminated Distribution Networks

If the $Cl_2 : NH_3\text{-}N$ ratio drops below 4:1 (excess free ammonia present), autotrophic ammonia-oxidizing bacteria (Nitrosomonas) and nitrite-oxidizing bacteria (Nitrobacter) proliferate in storage tanks and dead ends:

$\text{NH}_3 + 1.5\text{ O}_2 \xrightarrow{\text{Nitrosomonas}} \text{NO}_2^- + \text{H}^+ + \text{H}_2\text{O}$

$\text{NO}_2^- + 0.5\text{ O}_2 \xrightarrow{\text{Nitrobacter}} \text{NO}_3^-$

Nitrification symptoms include: rapid loss of total chlorine residual, elevated nitrite ($NO_2^-$) and nitrate ($NO_3^-$) levels, drop in dissolved oxygen (DO), and drop in pH. Corrective action involves increasing the chlorine-to-ammonia ratio, deep tank cycling, distribution line flushing, or conducting a periodic "free chlorine burnout" (temporary conversion to Zone 4 free chlorination).