N-Nitrosodimethylamine Formation during UV/Hydrogen Peroxide and UV/Chlorine Advanced Oxidation Process Treatment...

Chloramines applied to control microfiltration and reverse osmosis (RO) membrane biofouling in potable reuse trains form the potent carcinogen, N-nitrosodimethylamine (NDMA). In addition to degrading other contaminants, UV-based advanced oxidation processes (AOPs) strive to degrade NDMA by direct photolysis. The UV/chlorine AOP is gaining attention because of its potential to degrade other contaminants at lower UV fluence than the UV/hydrogen peroxide AOP, although previous pilot studies have observed that the UV/chlorine AOP was less effective for NDMA control. Using dimethylamine (DMA) as a model precursor and secondary municipal wastewater effluent, this study evaluated NDMA formation during the AOP treatment via two pathways. First, NDMA formation by UV treatment of monochloramine (NH₂Cl) and chlorinated DMA (Cl-DMA) passing through RO membranes was maximized at 350 mJ/cm² UV fluence, declining at higher fluence, where NDMA photolysis outweighed NDMA formation. Second, this study demonstrated that chlorine addition to the chloramine-containing RO permeate during the UV/chlorine AOP treatment initiated rapid NDMA formation by dark breakpoint reactions associated with reactive intermediates from the hydrolysis of dichloramine. At pH 5.7, this formation was maximized at a chlorine/ammonia molar ratio of 3 (out of 0-10), conditions typical for UV/chlorine AOPs. At 700 mJ/cm² UV fluence, which is applicable to current practice, NDMA photolysis degraded a portion of the NDMA formed by breakpoint reactions. Lowering UV fluence to ∼350 mJ/cm² when switching to the UV/chlorine AOP exacerbates effluent NDMA concentrations because of concurrent NDMA formation via the UV/NH₂Cl/Cl-DMA and breakpoint chlorination pathways. Fluence >700 mJ/cm² or chlorine doses greater than the 3:1 chlorine/ammonia molar ratios under consideration for the UV/HOCl AOP treatment are needed to achieve NDMA control.