Measuring Monochloramine
Without Reagents
For utilities using chloramine disinfection, accurately measuring monochloramine has required either expensive colorimetric reagent systems ($29,000–$33,000 in equipment alone) or accepting that DPD measurements cannot distinguish between chloramine species. A new sensor approach changes both — measuring free chlorine and monochloramine simultaneously, in-pipe, without reagents or a waste stream, and enabling a new process control method.
Why Utilities Use Chloramine
Monochloramine is used as a secondary disinfectant by hundreds of U.S. water utilities, particularly those with large distribution systems. Its stability and reduced byproduct formation make it preferable to free chlorine for maintaining disinfectant residuals at the tap.
Longer-Lasting Disinfection
Monochloramine is more stable than free chlorine and remains effective over longer distances in distribution systems — making it ideal for large systems with extended pipe runs.
Fewer Disinfection Byproducts
Monochloramine produces significantly fewer regulated disinfection byproducts (DBPs) than free chlorine, helping utilities meet EPA standards for trihalomethanes and haloacetic acids.
Better Taste & Odor
Water treated with monochloramine generally has less of the "chlorine" taste and smell compared to free chlorine treatment, resulting in fewer consumer complaints.
Chloramine Chemistry: Species & Formation
Chloramines form when chlorine reacts with ammonia. The species that forms depends on the Cl₂:NH₃ weight ratio, pH, and temperature. Getting this right is critical — the wrong ratio produces dichloramine and trichloramine, which cause taste and odor problems and are less effective disinfectants.
Monochloramine (NH₂Cl) — Target Species
- check_circleForms at Cl₂:NH₃ ratio of 4.5:1 to 5:1
- check_circleProvides longer-lasting disinfection than free chlorine
- check_circleFewer disinfection byproducts
- check_circleStable above pH 8.0 — strongly favored in alkaline conditions
Dichloramine & Trichloramine — Avoid These
- cancelForm when Cl₂:NH₃ ratio exceeds 5:1
- cancelCause "swimming pool" taste and odor
- cancelLess effective disinfectants
- cancelMore prevalent at lower pH — minimized above pH 8.0
The pH Rule of Thumb
Above pH 8.0 there will be virtually zero dichloramine. At pH 7.5 there may be a 0.01–0.02 mg/L residual at 1.5 ppm total chloramine. Maintaining pH above 8.0 strongly favors monochloramine formation and stability.
Why Total Chlorine Measurement Isn't Enough
The most common monitoring approach — DPD total and DPD free measurements — cannot provide enough information about the chloramination process. When using a total chlorine sensor or DPD measurement, an operator cannot know whether a 3.0 mg/L result is monochloramine, a mixture of species, or free chlorine.
Same total chlorine reading — completely different process conditions
| Total Chlorine | Free Chlorine | Monochloramine | Dichloramine | Condition |
|---|---|---|---|---|
| 3.0 mg/L | 0 | 3.0 | 0 | ✅ Ideal — all monochloramine |
| 3.0 mg/L | 1.0 | 2.0 | 0 | ⚠️ Free chlorine present |
| 3.0 mg/L | 0 | 2.0 | 1.0 | ❌ Dichloramine forming |
| 3.0 mg/L | 0 | 2.0 | 0 | ⚠️ Unknown species present |
Without knowing the individual species, operators must overfeed ammonia to ensure no free chlorine remains — which risks excess free ammonia and nitrification. There is simply no way to know where you are on the breakpoint curve using total chlorine alone.
Operational Challenges of Chloramination
Nitrification Risk
As chloramine decays it releases ammonia. Bacteria oxidize ammonia to nitrite and nitrate, which accelerates chloramine decay and can lead to significant disinfectant residual loss, HPC violations, and coliform issues.
Dichloramine Formation
If the chlorine-to-ammonia ratio exceeds 5:1, dichloramine forms — causing "swimming pool" taste and odor complaints and reducing disinfection effectiveness.
Excess Ammonia
Feeding too much ammonia leads to free ammonia in the distribution system, which promotes nitrification. Operators currently have no easy way to know where they are on the breakpoint curve.
DPD Measurement Limitations
The conventional DPD method cannot distinguish between chloramine species. A 3.0 mg/L reading could be all monochloramine, a mix of species, or free chlorine — the operator simply cannot tell.
Cost Comparison: Monitoring Methods
| Method | Equipment | Reagents/yr | Waste Stream | Update Time |
|---|---|---|---|---|
| Monochloramine & Ammonia Reagent Feed Analyzers | $29,000–$33,000 | $3,000/yr | 70,000–150,000 gal/yr | 4.5–20 min |
| Free and Total Reagent Systems | $15,000 | $1,400/yr | 140,000 gal/yr | < 1 min |
| Halogen MP-TOTAL (Free + Monochloramine) | $10,700 | $0 | 0 gallons | < 2 min |
A New Control Method for Chloramination
Because the Halogen MP-TOTAL measures free chlorine without monochloramine interference, it enables a fundamentally new approach to process control. Instead of ensuring a small free ammonia residual is present (which requires overfeeding ammonia), operators can now maintain a very small free chlorine residual of 0.04–0.08 ppm.
The New Control Logic
A free chlorine residual of 0.04–0.08 ppm confirms you are at the top of the breakpoint curve with no excess ammonia. By monitoring both free chlorine and monochloramine simultaneously, the operator can adjust ammonia feed to maintain this precise operating point — minimizing excess ammonia in treated water and eliminating nitrification risk. At pH above 9.0, dichloramine will not form, so a small free chlorine residual optimizes the entire process.
The Halogen MP-TOTAL updates its measurement every two minutes — more than twice as fast as online reagent systems that update every 4.5 minutes, enabling more precise process control. All other amperometric and DPD measurements list monochloramine as an interferent. The Halogen MP-TOTAL is the only sensor that can be used in this control scheme.