Impact of Combined Chlorination and Chloramination Conditions on N-Nitrosodimethylamine Formation

Bench-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 chlorine-to-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.