The long game in tap water: why many utilities trade chlorine’s punch for chloramine’s staying power

How long does your disinfectant last after it leaves the plant? In bench-scale decay tests, a typical free chlorine dose of 2.4 mg/L (as Cl₂; milligrams per liter) fell below the detection limit (~0.05 mg/L) in about 7 days, while the same 2.4 mg/L monochloramine dose took ~11 days to hit that mark (MDPI Water).

Full-scale data tell a similar story: at plant entry, total chlorine (free+combined) was ~4.7 mg/L (as Cl₂) and “codechlorine” dropped to ~1.7 mg/L at maximum residence time, whereas the monochloramine residual went from ~3.4 to ~1.1 mg/L (PMC). Put simply, “chloramines are more stable than free chlorine” (PMC).

That stability is precisely why many utilities switch. The CDC notes chloramine “can keep killing germs in water pipes longer than chlorine,” which “gets used up quickly” (CDC). In hot or low‑flow networks, chloramine can maintain a 0.2–0.5 mg/L residual at taps much more reliably than chlorine. By contrast, free chlorine often falls to near‑zero between the plant and distal mains without booster dosing.

Residual stability in distribution

Modeled networks show monochloramine can maintain ≥0.2–0.3 mg/L at far points without booster stations, whereas free chlorine often requires continuous re‑chlorination or flushing to avoid “residual deficits.” In one simulation, converting a network from chlorine to chloramine cut the number of required flushing (blowoff) devices by half (MDPI Water).

In practice, utilities dose 2–4 mg/L total chlorine (as Cl₂) at the plant to ensure ≥0.2–0.5 mg/L at taps; studies have observed utilities finding 0.2–0.7 mg/L in customer taps (ResearchGate).

Primary vs. secondary disinfection performance

There’s a trade-off: monochloramine is a weaker oxidant. For 99.9% pathogen inactivation, reported CT (concentration × time; mg·min/L) values are on the order of 10,000 mg·min/L for chloramine versus <150 mg·min/L for free chlorine (MDPI Water). That’s why chloramine is rarely used for primary disinfection. Utilities commonly apply chlorine (or UV/ozone) to kill microbes, then convert to chloramine for distribution (MDPI Water; PMC). Where ultraviolet is part of the barrier strategy, plants deploy ultraviolet systems to deliver non-chemical inactivation upstream of a long‑lasting chloramine residual.

Disinfection byproducts and regulations

Regulated chlorination byproducts — trihalomethanes (THMs) and haloacetic acids (HAAs) — form mainly when free chlorine reacts with natural organic matter (NOM). Chloramination greatly reduces formation of these regulated DBPs. After the U.S. Stage 1 and 2 DBP rules (1998–2006), many utilities switched to chloramine “to reduce the formation of regulated DBPs” (PMC; MDPI Water). The CDC likewise notes that chloramine “makes fewer disinfection byproducts than chlorine” (CDC).

In engineered systems, >65–100% of THM/HAA formation may occur during treatment, with any ongoing formation in distribution much lower under chloramination. In practice, systems using chloramine almost always measure markedly lower THM and HAA levels — often by an order of magnitude — than comparable free‑chlorine systems (to meet MCLs of 80 µg/L THM4 and 60 µg/L HAA5).

Chloramination, however, can generate different byproducts: nitrogenous DBPs such as N‑nitrosodimethylamine (NDMA) and haloacetonitriles. These typically occur at much lower concentrations than THMs but are highly toxic (MDPI Water). Utilities monitor them (for example, California’s NDMA action level ~10 ng/L). In summary: chloramine reduces THM/HAA formation (since it is less reactive with NOM) (PMC), while requiring vigilance for nitrogen‑DBPs.

Conversion to chloramine and nitrification risk

Converting from free chlorine to chloramine brings operational hurdles — chiefly nitrification, the biological oxidation of chloramine’s ammonia to nitrite/nitrate by microbes (e.g., Nitrosomonas, Nitrobacter). As ammonia is stripped from NH₂Cl (monochloramine), the disinfectant is destroyed. A key indicator is a sudden drop in total chlorine residual accompanied by nitrite emergence (Missouri DNR PDF).

Missouri DNR guidance notes nitrification causes “loss of disinfection capabilities… most noticeably a drop in total chlorine residual” (Missouri DNR PDF). It also causes pH/alkalinity to fall and heterotrophic bacteria counts to spike (e.g., >500 CFU/mL) (Missouri DNR PDF; PMC). Once nitrification begins, a chloramine level >2 mg/L can fall to below detection “in a short time” (PMC). Incomplete nitrification can also yield toxic nitrite (and eventually nitrate) build‑up.

Risk is greatest under warm water (25–30 °C), high pH (7.5–8.5), and stagnation (dead‑ends, tanks) (Missouri DNR PDF). It is especially a concern in tropical climates (such as Indonesia’s). Utilities converting to chloramine must implement rigorous monitoring of ammonia, nitrite, and residual chlorine throughout the network (Missouri DNR PDF; PMC).

Many large systems build in preventive measures: cycling to free chlorine on a scheduled basis (“chlorine burn”) to knock back nitrifiers. Missouri guidance notes some systems perform an annual free‑chlorine conversion (weeks‑long) — dosing ≥0.5 mg/L to ≥4 mg/L chlorine and flushing — until residuals stabilize, then resume chloramination (Missouri DNR PDF; Missouri DNR PDF). Such conversions often cause temporary taste/odor issues and customers are urged to flush taps (Missouri DNR PDF).

Chemistry and control considerations

Chloramine’s different chemistry can alter corrosion control. Because it is less oxidative, it can increase lead/copper solubility if the distribution system chemistry shifts; utilities must adjust corrosion inhibitors (e.g., orthophosphate) and pH when changing disinfectants (CDC).

Chloramination also requires ammonia feed infrastructure and tight Cl₂:NH₃ control — typically ~4:1 by mass — to ensure stable NH₂Cl formation without excess NH₃. Plants lean on precise feed equipment; accurate chemical dosing is central to this control, which is where a well‑specified dosing pump becomes part of the operational backbone.

Operational trade-offs, quantified

Overall, chloramine provides a much more persistent residual and helps meet THM regulations (PMC; PMC), but it is a weaker disinfectant and brings risks of nitrification and secondary chemistry changes. In one modeled network study, switching to chloramine reduced the volume of water needed for flushing and the number of node‑bleeds, reflecting its stability (MDPI Water).

Conversely, maintaining chloramine requires ongoing attention to microbiological control. Through careful design and monitoring, many large utilities successfully manage chloraminated systems, but achieving that in practice requires investment in training, monitoring, and operational procedures. Indonesian studies (Azwaruddin et al. 2025) note that current PDAM practice targets ~0.2–0.7 mg/L chlorine residual (ResearchGate).

Sources and further reading

Abulikemu, G., Mistry, J. H., Wahman, D. G., Alexander, M. T., Kennicutt, A. R., Bollman, J. D., & Pressman, J. G. (2022). Investigation of Chloramines, Disinfection Byproducts, and Nitrification in Chloraminated Drinking Water Distribution Systems. Journal of Environmental Engineering (New York) 149(1):1–12. DOI:10.1061/(ASCE)EE.1943-7870.0002062 (PMC).

Pfaller, S., King, D., Mistry, J., Alexander, M., Abulikemu, G., Pressman, J., Wahman, D., & Donohue, M. (2021). Chloramine Concentrations within Distribution Systems and Their Effect on Heterotrophic Bacteria, Mycobacterial Species, and Disinfection Byproducts. Water Research 205:117689. DOI:10.1016/j.watres.2021.117689 (PMC; PMC).

Bertone M., Buonanno, G., Natale, P., & Santonastaso, G. (2022). Comparison of Techniques for Maintaining Adequate Disinfectant Residuals in a Full-Scale Water Distribution Network. Water 14(7):1029. DOI:10.3390/w14071029 (MDPI Water; MDPI Water).

Centers for Disease Control and Prevention (CDC). (2024 Feb 14). About Water Disinfection with Chlorine and Chloramine. https://www.cdc.gov/drinking-water/about/about-water-disinfection-with-chlorine-and-chloramine.html (CDC; CDC).

Missouri Department of Natural Resources. Nitrification and Temporary Conversion From Chloramine to Free Chlorine (PUB 2646). PDF: dnr.mo.gov (Residual drop/nitrite; Risk conditions; Annual conversion; Duration note; Customer flushing).

Hossain, S., Chow, C. W. K., Cook, D., Sawade, E., & Hewa, G. A. (2022). Review of Nitrification Monitoring and Control Strategies in Drinking Water System. International Journal of Environmental Research and Public Health 19(7):4003. DOI:10.3390/ijerph19074003 (PMC; PMC).

Azwaruddin, A., Abdullah, T., & Wahyuningsih, S. (2025). Study of Chlorine Use in Water Treatment at the Giri Menang Regional Drinking Water. Jurnal Pijar MIPA 20(3):462–467. DOI:10.29303/jpm.v20i3.8885 (ResearchGate).