Key Finding: Nitrosamines—a class of potent carcinogenic disinfection byproducts (DBPs)—have been detected in treated drinking waters across North America at concentrations that vary significantly by disinfection method and source water quality. N-nitrosodimethylamine (NDMA) is the most commonly detected nitrosamine and forms primarily when chloramine-based disinfection reacts with certain precursor compounds in source water.

Krasner, S. W.; Roback, S.; (…); Bukhari, Z.

2020 | AWWA Water Science

Eight N-nitrosamines were measured at 37 water plants in the United States and Canada. Five tobacco-specific nitrosamines (TSNAs) were measured in selected waters. N-Nitrosodimethylamine (NDMA) was preferentially formed in chloraminated systems (maximum detention time: median 4.4ng/L). A small amount was detected in some chlorinated systems (90th percentile <2.0 ng/L). After ozone (before chloramines), NDMA was sometimes detected (90th percentile 2.9 ng/L), suggesting that the ozone did not react with precursors to form NDMA. The chloramine plants that temporarily switched to chlorine typically produced less NDMA (Plant 29 reduced NDMA formation, on average, from 34 to 4 ng). More NDMA was produced during spring runoff, when there were elevated levels of ammonia and NDMA precursors in the source water. More NDMA was formed when there were higher levels of poly (diallyldimethylammonium chloride) (polyDADMAC) used. N-Nitrosomorpholene was found to be a contaminant and not a disinfection byproduct; it did not increase during chloramination. TSNAs were produced during spring runoff; source water ammonia impacted the chlor (am) ine chemistry. © 2020 American Water Works Association

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Source: Water Feed

Nitrosamines in North American Drinking Water: Occurrence, Formation, and Treatment

Nitrosamines are a group of N-nitroso compounds formed as unintended byproducts during drinking water disinfection. Unlike trihalomethanes (THMs) and haloacetic acids (HAAs), nitrosamines are not currently regulated under the U.S. EPA Stage 1 or Stage 2 Disinfectants and Disinfection Byproducts Rules, yet they are significantly more carcinogenic on a per-mass basis. The California Department of Public Health (CDPH) has established a notification level of 10 ng/L for NDMA—the most prevalent nitrosamine—while Health Canada has proposed a maximum acceptable concentration of 40 ng/L.

Research across North American water systems has identified monochloramine as the primary driver of NDMA formation. Utilities that switched from free chlorine to chloramine disinfection to comply with THM/HAA regulations paradoxically increased NDMA formation in many cases. Key precursor compounds include dimethylamine (DMA)-containing pharmaceuticals (e.g., ranitidine), tertiary amines from wastewater effluent, and certain ion exchange resins used in water softening processes. Source waters with high wastewater influence tend to carry elevated precursor loads.

From a treatment standpoint, UV photolysis at doses of 500–1,000 mJ/cm² has demonstrated effective destruction of NDMA (>90% removal) and is considered the most practical technology for nitrosamine control at full-scale water treatment plants. However, UV at typical disinfection doses (40 mJ/cm²) provides minimal NDMA reduction. Biological activated carbon (BAC) filtration, when positioned after ozonation, can also biodegrade NDMA precursors before disinfection occurs. Reverse osmosis achieves modest NDMA rejection (approximately 50–70%) due to the compound's low molecular weight and relatively high polarity, making it less amenable to membrane exclusion than many other DBPs.

Industrial and municipal water system operators should assess their disinfection strategies, particularly if transitioning to or already using chloramine. Conducting precursor analysis and NDMA occurrence monitoring before and after treatment process changes allows operators to quantify risk and justify investment in UV or advanced oxidation systems. Maintaining detailed records of source water quality and disinfectant residuals is essential for compliance and risk management.