5.5 Chloramination & Nitrification Control

Why California Utilities Chloraminate

The Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules capped total trihalomethanes at 80 µg/L and the five haloacetic acids at 60 µg/L, evaluated as a locational running annual average at each monitoring site. Free chlorine held in a large distribution system for two or three days keeps reacting with natural organic matter and keeps making byproducts. Monochloramine forms dramatically less THM and HAA5, holds a residual for far longer, and penetrates biofilm better. That combination drove most large California systems - Los Angeles, San Diego, the Metropolitan Water District service area, East Bay MUD and many others - to chloraminate.

The tradeoff is that chloramines introduce an entirely new failure mode: nitrification.

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Chloramine Chemistry

Ammonia reacts with hypochlorous acid in three stages, and which species you get depends almost entirely on the chlorine-to-ammonia-nitrogen weight ratio and pH:

$\text{NH}_3 + \text{HOCl} \rightarrow \text{NH}_2\text{Cl (monochloramine)} + \text{H}_2\text{O}$ $\text{NH}_2\text{Cl} + \text{HOCl} \rightarrow \text{NHCl}_2\text{ (dichloramine)} + \text{H}_2\text{O}$ $\text{NHCl}_2 + \text{HOCl} \rightarrow \text{NCl}_3\text{ (trichloramine / nitrogen trichloride)} + \text{H}_2\text{O}$

pH matters. Monochloramine is favored above roughly pH 8. Below pH 7 the equilibrium shifts toward dichloramine and eventually trichloramine. California systems generally chloraminate at pH 8.0 to 8.5, which also supports corrosion control.

[!IMPORTANT] Order of addition. Feed chlorine first, then ammonia after a short free-chlorine contact period, or feed simultaneously with excellent mixing. Feeding ammonia first creates a zone where the ratio is far below 3:1, produces poorly disinfected water, and starts the nitrification cycle at the point of application. Some plants deliberately maintain a brief free chlorine contact time before ammoniation to earn CT credit as free chlorine, then convert to chloramine for the distribution residual.

Disinfection Strength

Chloramines are roughly 20 to 200 times weaker than free chlorine depending on the organism. In practical terms:

• No Cryptosporidium credit at any realistic dose.
• Very limited Giardia credit, requiring enormous CT.
• Some virus credit at long contact time.
• Excellent at penetrating biofilm and holding a residual for days, which is exactly what a large distribution system needs.

The standard California design is therefore primary disinfection with free chlorine, ozone, or UV inside the plant to earn the required log inactivation, then ammoniation to convert to a chloramine residual for the distribution system.

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Nitrification: What Actually Happens

Nitrification is a two-step biological oxidation performed by autotrophic bacteria that colonize pipe walls and stagnant storage:

$\text{NH}_4^+ + 1.5\,\text{O}_2 \xrightarrow{\text{ammonia-oxidizing bacteria}} \text{NO}_2^- + 2\text{H}^+ + \text{H}_2\text{O}$ $\text{NO}_2^- + 0.5\,\text{O}_2 \xrightarrow{\text{nitrite-oxidizing bacteria}} \text{NO}_3^-$

It is a self-accelerating loop. Free ammonia feeds ammonia-oxidizing bacteria. They produce nitrite, which exerts a very strong chloramine demand (roughly 5 mg of chloramine consumed per 1 mg of nitrite as N). Loss of chloramine releases more ammonia, which feeds more bacteria. Meanwhile the reaction releases hydrogen ions, depressing pH and consuming alkalinity, which in a poorly buffered system can destabilize corrosion control and mobilize lead.

Conditions That Trigger It

• Excess free ammonia from running below a 4.5:1 ratio, or from over-dosing ammonia
• Long detention time - oversized or poorly turned-over storage, dead-end mains, low-demand seasons
• Warm water - nitrifiers accelerate sharply above about 15 °C, so California systems see nitrification episodes in late summer and early fall
• Low chloramine residual - below roughly 0.5 mg/L total chlorine, control is lost
• Nutrient-rich biofilm on unlined cast iron

Monitoring and Early Warning

A nitrification action plan is now standard practice for California chloraminating utilities. The parameters, in rough order of how early they warn you:

[!TIP] The pairing that identifies nitrification. Falling total chlorine residual plus rising nitrite at the same site is essentially diagnostic. Falling residual with no nitrite is ordinary chloramine decay - check detention time, temperature, and the feed. Rising nitrite with a stable residual is an early-stage event you can still get ahead of.

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Response Actions, Least to Most Aggressive

1. Fix the ratio at the plant. Verify the chlorine-to-ammonia-nitrogen ratio by calculation and by measurement, and move it toward 4.5:1 or 5:1. Eliminating excess free ammonia removes the nitrifiers' food supply.
2. Increase turnover. Cycle storage tanks through a wider level band, deep-cycle problem reservoirs, and reduce detention time. Stagnation is the enabling condition.
3. Flush. Targeted or unidirectional flushing of dead ends and low-velocity mains removes nitrifying biofilm and stale water. Follow with residual verification.
4. Raise the chloramine residual at the entry point and at booster stations. Adding a chloramine booster at a remote reservoir is often the durable fix for a chronic site.
5. Perform a free chlorine conversion (a "breakpoint burnout"). Stop the ammonia feed and raise the chlorine dose above the breakpoint so free chlorine circulates for a period of weeks. This oxidizes nitrite, kills nitrifiers, and burns off biofilm.

[!WARNING] A free chlorine conversion is a controlled event, not a switch you flip. It temporarily raises THM and HAA5 formation, generates taste and odor complaints, can mobilize corrosion products and cause red water, and must be publicly noticed to two groups who are harmed by the change in disinfectant chemistry: dialysis facilities and owners of fish, reptiles, and aquatic exhibits. Both chloramine and free chlorine must be removed from dialysis water and from aquarium water, but the removal method differs - granular activated carbon handles chloramine much more slowly than free chlorine, and dechlorinators dosed for free chlorine are inadequate for chloramine. Notify hospitals, dialysis centers, aquarium owners, and pet stores in advance of any conversion in either direction.

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Chloramine-Specific Hazards Operators Must Communicate

Operators are the people who take these calls. Knowing that boiling and standing do not remove chloramine while both eventually remove free chlorine is a frequently tested distinction.