Impact of Nitrogen Removal in Wastewater Treatment on NDMA Formation at Downstream Drinking-Water Treatment Plants

Abstract

De facto reuse (DFR) occurs when surface waters receiving wastewater effluent are used as source water by a downstream drinking-water treatment plant (DWTP). Wastewater effluent may contain high concentrations of N-nitrosodimethylamine (NDMA) precursors. NDMA is a toxic disinfection byproduct (DBP). Nitrification at wastewater treatment plants (WWTPs) reduces NDMA precursors in this effluent. The present work examines NDMA concentrations in the distribution systems of 31 DWTPs subject to high percentages of DFR. WWTPs contributing to this DFR were categorized by extent of nitrogen treatment: no intentional nitrogen treatment, nitrification, or nitrification followed by denitrification. The fraction of upstream effluent discharges receiving nitrogen treatment was calculated, and the relationship between this fraction and observed NDMA concentrations was assessed. Results indicate that high-DFR DWTPs with large fractions of wastewater undergoing nitrification upstream had NDMA concentrations comparable to a representative national sample of plants. High-DFR plants with little upstream nitrification had significantly higher NDMA detection rates and concentrations, with dependence on the distance between the upstream WWTPs and the downstream DWTP. High DFR low nitrification, high DFR high nitrification, and a national data set of plants had detect rates of 79%, 54%, and 50% and median detects of 8.0 ng/L, 4.2 ng/L, and 3.8 ng/L, respectively, when considering a 150-km cutoff. This suggests that nitrification at WWTPs may have an added benefit of reducing NDMA formation at chloraminating DWTPs located downstream in high-DFR areas.