Impact of Combined Chlorination and Chloramination Conditions onN‐Nitrosodimethylamine Formation

Abstract

SwitzerlandBench-scale chloramination under uniform formation conditions was used to examine N-nitrosodimethylamine (NDMA) formation in settled and (bio)filtered drinking water and treated wastewater. In this study, water temperature, pH, postchloramination time, and chlorineto-nitrogen (Cl 2 /N) weight ratio were varied to investigate NDMA formation in various water types. Wastewater and certain polymers were investigated as sources of NDMA precursors. Nitrified biofilters were found to be another precursor source. NDMA formation in nitrified biofilter effluent and polymer-impacted water was the highest at Cl 2 /N of 3-5; NDMA formation was the highest at Cl 2 /N of 5-7 in wastewater-impacted waters. Other tests at Cl 2 /N of 4.75 evaluated the impact of pH and temperature, with a 3 min prechlorination, which can result in some precursor abatement. At pH 7, NDMA formation increased with increasing temperature, whereas at pH 8 and 9, low temperature sometimes yielded higher NDMA formation because of lower precursor abatement during prechlorination and, thus, higher NDMA formation during postchloramination.

Occurrence of nitrosamines and their precursors in North American drinking waters

“…This suggests that, once NDMA formed by a certain point in time in the distribution system, its formation then tended to plateau out. For example, in developing uniform formation conditions for bench-scale testing of NDMA, NDMA tended to plateau out after 3 days (Krasner, Lee, et al, 2018a). Note that some of the DS/max were somewhat lower than the DS/ ave.…”

“…N‐ Nitrosamine samples were collected at the plant influent to see if N‐ nitrosamines were present from upstream contamination, at the plant effluent, and in the distribution system at average and maximum detention times, where N‐ nitrosamine formation was typically slow (e.g., at a conventional drinking water plant with ozone, NDMA formation at 25 °C at pH 8 at 0.04, 1, 3, and 7 days was 2.0, 6.8, 8.5, and 10 ng/L, respectively [Krasner, Lee, Mitch, & von Gunten, 2018a]). FP tests were conducted at many plants to determine precursor loadings in source and settled waters (to determine watershed‐ and polymer‐derived precursors, respectively) and before and after unit processes that may destroy or remove precursors (preoxidation, RBF, PAC, GAC).…”

“…However, the water was cold, slowing the reactions between HOCl and the precursors. HOCl can destroy (deactivate) NDMA precursors (Krasner, Lee, et al, 2018a). Then, when ammonia was added to convert the HOCl residual to chloramines, this Another way to examine the data is as follows.…”

Production of N-Nitrosodimethylamine Precursors by Biofilters Is Highly Dynamic and Affected by Filter Media Type and Backwashing Conditions

“…Both the SDS and UFC chloramination protocols approach typical chloramination conditions; the SDS protocol mimics the conditions expected at a particular facility, while the UFC protocol approximates average chloramination conditions across utilities. 25 Regarding differences expected between the protocols, the ∼3.4 mg min/L free chlorine contact time employed in the SDS protocol is expected to result in only slight deactivation of NDMA precursors 26 while the higher pH should promote NDMA formation during chloramination. 1 Temperature is another important factor.…”

“…Previous research on a nitrified biofilter effluent indicated that more NDMA formed at pH 9 and 5 °C than at pH 8 and 25 °C under SDS conditions. 25 While the lower temperature should reduce NDMA formation during chloramination, this effect is outweighed by the reduction in NDMA precursor deactivation during the preceding free chlorine contact time. Even though there may be differences in NDMA formation between the SDS and UFC protocols employed in this work, we drew comparisons only between water samples treated by a common protocol.…”