Why more cities are betting on chloramine — and the operational catch that comes with it

For a century, free chlorine has been the go-to for municipal water disinfection because it’s fast and powerful. But it doesn’t linger. In long, warm distribution systems, residuals can fade before water reaches the tap. Monochloramine — a chlorine–ammonia compound known simply as chloramine — lasts longer in pipes and helps many systems meet tough disinfection byproduct (DBP) rules.

After Stage 1 DBP regulations took effect, the EPA estimated that ~30% of U.S. surface-water systems would switch to chloramines, rising toward ~60% under Stage 2 (WWD). By 1998 about 68 million Americans (~22% of the population) received chloraminated water, and major cities including Philadelphia, San Francisco, Tampa Bay, and Washington, D.C. now use chloramines as their secondary disinfectant (CDC).

Residual stability in distribution

EPA guidance is plain: chloramines “are a good choice for secondary disinfection” because the monochloramine residual is “more stable and longer lasting than free chlorine” (EPA). Utilities “have been employing chloramines to maintain a disinfectant residual in their distribution systems for most of the last century,” since it is more persistent in the network (EPA).

In practice, a plant dose that maintains ~1 mg/L (milligrams per liter) as Cl2 can remain >0.5 mg/L at far taps, while free chlorine losses can exceed 50% in a day when water age is high. Reviews confirm chloraminated systems “ensure a relatively higher disinfection stability” than free chlorine (PMC). A field study of four chloraminated systems found monochloramine residual declined only modestly from plant effluent to “maximum residence time” points, while THM/HAA levels stabilized or even fell along the way (PMC) (PMC).

DBP formation and risk profile

Free chlorine reacts rapidly with natural organic matter to form regulated carbonaceous DBPs (notably trihalomethanes, THMs, and haloacetic acids, HAAs). Chloramine, a weaker/slower oxidant, typically yields far fewer of these. EPA documents utilities switching “to minimize the formation of trihalomethanes (THMs) and haloacetic acids (HAAs)” (EPA), noting chloramine “is not as reactive with organics as free chlorine in forming THMs” (EPA).

Quantitatively, studies report 50–90% lower regulated THMs/HAAs under chloramination (conditions and precursors apply) (PMC) (MDPI). Multiple analyses call switching to chloramine “an effective strategy for mitigating DBP formation” (MDPI). Caveat: insufficient chlorine or ammonia dosing can collapse the residual and even increase DBPs or enable nitrification (MDPI).

Chloramination shifts the byproduct mix toward nitrogenous DBPs such as NDMA (N‑nitrosodimethylamine). Typical NDMA levels are very low and are regulated at parts-per-trillion, e.g., 9–10 ng/L in Ontario/California (MDPI). The net effect in practice remains a large drop in regulated THMs/HAAs, and consumers often report less “chlorine” taste/odor even with protective residuals in place (CDC).

Nitrification as the primary risk

Nitrification — the microbial conversion of ammonia to nitrite and then nitrate — is the central operational risk when using chloramine. Because chloramine contains ammonia, nitrifying bacteria can use the residual as a nutrient, especially in warm, low-flow zones. The result can be residual loss and microbial regrowth. “Nitrification is a major challenge in chloraminated drinking water systems, resulting in undesirable loss of disinfectant residual” (PMC).

As chloramine decays or is consumed by biofilms, freed ammonia fuels ammonia‑oxidizers, which can proliferate “to a greater extent” and further deplete disinfectant (PMC). WHO cautions that, in long-retention systems, nitrite formation by organisms in biofilms needs attention where excess ammonia is present (WHO via Studylib). Studies also show ammonia‑oxidizers can adapt to chloramine concentrations as low as ~0.45 mg/L NH2Cl (IWA). Once established, nitrification can accelerate chloramine decay, with nitrite and microbial metabolites consuming disinfectant and locally dropping pH — a corrosion stressor (PMC). Even meter-level nitrite spikes have been recorded during nitrification episodes, triggering flushing or temporary chlorination responses.

On‑site chemistry and monitoring

Chloramines must be made on-site by feeding chlorine and ammonia sequentially at a controlled molar ratio — typically ~4:1 chlorine-to-nitrogen — and keeping formation in the monochloramine range to avoid odorous dichloramine/trichloramine (EPA) (MDPI). Utilities often install ammonia feed systems and re‑train operators; precise ratio control favors the use of accurate chemical dosing equipment such as a dosing pump.

Distribution systems then need frequent nitrite/nitrate and residual monitoring. Many agencies require a formal nitrification action plan with high‑risk site sampling, routine flushing, and periodic “shock” chlorination; the Texas Commission on Environmental Quality and AWWA mandate nitrification monitoring whenever chloramine is used.

Corrosion control and public health

Monochloramine is a weaker oxidant than free chlorine and does not maintain protective oxide films on pipes as aggressively (EPA). EPA and CDC warn that switching to chloramine can raise lead and copper if corrosion control is not re‑optimized (CDC). Washington, D.C., for example, switched in 2000 and had to re‑optimize pH and add buffering or corrosion inhibitors to meet the Lead & Copper Rule. WHO also notes special mitigations for dialysis patients and aquatic pets when chloramine is used (WHO via Studylib).

Costs, complexity, and communications

A chloramine conversion typically adds operational complexity and cost. Ammonia is toxic and corrosive to feed equipment, requiring safety systems; chemical costs and monitoring burdens increase. Utilities often report a “difficult transition,” with a Water Research Foundation survey noting many underestimated the planning required (WaterOnline). Public outreach may be necessary when taste changes or “new” disinfectants enter the conversation.

The trade‑off in one line

Chloramine delivers a longer‑lived residual and far fewer regulated THMs/HAAs than free chlorine — leading to more consistent disinfection at the tap and an easier path to DBP compliance (EPA) (PMC). EPA and WHO both emphasize that long‑lived chloramine residuals typically require smaller initial dosages and generate far fewer THMs/HAAs (EPA) (PMC).

The trade‑offs are operational: getting the chlorine‑to‑ammonia ratio right, maintaining corrosion control, and staying ahead of nitrification — the most significant technical challenge, which can destroy the residual and force emergency flushing or breakpoint chlorination (PMC) (WHO via Studylib). Utilities that invest in nitrification control programs and align residual targets with pipe conditions tend to capture the stability and DBP benefits while keeping the risks in check.

Sources and further reading

Authoritative scientific and regulatory sources support the findings above: EPA, CDC, WHO, and peer‑reviewed literature (CDC) (EPA) (PMC) (EPA) (MDPI) (PMC) (WWD) (WHO via Studylib) (PMC). Case-study extensions include Water Research Foundation work (e.g., Khiari 2016) and AWWA manuals.