N-Nitrosodimethylamine (NDMA) Formation during Advanced Oxidation Process (AOP) Treatment Following Reverse Osmosis for Potable Water Reuse

A 0.7 ng/L concentration of N-nitrosodimethylamine (NDMA) in drinking water is associated with a 10^(-6) lifetime excess cancer risk. NDMA is a particularly challenging compound for utilities purifying municipal wastewater effluents, where sewage contains higher concentrations of NDMA compared to conventional water sources, and NDMA can form from precursors when chloramines are applied upstream of microfiltration and reverse osmosis (RO) within advanced treatment. In order to reduce NDMA concentrations to below the 10 ng/L California Notification Level, UV-based advanced oxidation processes (AOP), reliant on radical production, such as UV/hydrogen peroxide (UV/H2O2), and, more recently, UV/free chlorine (UV/HOCl) are applied to RO permeates. While UV treatment effectively degrades NDMA, the potential for simultaneous NDMA formation during UV-based AOP treatment to partially offset this degradation has not been evaluated.

The objective of this laboratory-scale study is to evaluate the potential for NDMA formation during UV/hydrogen peroxide (UV/H2O2) and UV/free chlorine (UV/HOCl) AOP treatment. Interest in UV/HOCl processes has recently increased, as radical production from HOCl photolysis can be higher than from H2O2 photolysis resulting in more effective contaminant degradation. However, previous research suggests that the potential for greater NDMA formation during UV/HOCl AOP treatment. In this study, we demonstrate that dark reactions associated with residual chloramines in RO permeates increase NDMA formation during UV treatment. Addition of HOCl to RO permeates for UV/HOCl AOP treatment results in rapid NDMA formation from dark reactions with chloramine and UV-mediated breakpoint chlorination reactions. While these NDMA formation pathways and NDMA photodegradation occur simultaneously, our study shows that NDMA concentrations after UV/HOCl treatment can be double those post UV/H2O2 treatment. A mechanism for formation will be proposed. Given our results, in order to meet NDMA California Notification Levels with UV/HOCl treatment, higher UV fluence is required, and must be considered alongside higher radical production prior to implementation.