Control of nitrosamines during non-potable and de facto wastewater reuse with medium pressure ultraviolet light and preformed monochloramine

Abstract

Water utilities practicing chlorination of poorly nitrified wastewater effluents for non-potable reuse, or of ammonia-containing, wastewater-impacted source waters during de facto reuse may form N-nitrosodimethylamine (NDMA) by reactions between wastewater-derived NDMA precursors and inorganic chloramines formed in situ when chlorine reacts with ammonia. Treatment with moderate doses (186 mJ cm⁻² germicidal fluence) of UV from medium pressure mercury lamps (MPUV) upstream of chloramination minimized NDMA formation by destroying ambient NDMA (i.e., the NDMA occurring in wastewater) by >70% and deactivating chloramine-reactive NDMA precursors by 46%. UV from low pressure mercury lamps (LPUV) was less effective for NDMA precursor deactivation. Ozonation does not destroy ambient NDMA and formed significant levels of NDMA from ozone-reactive precursors. For waters with low ammonia concentrations, breakpoint chlorination deactivated NDMA precursors, but does not destroy ambient NDMA, and formed an array of regulated and unregulated halogenated byproducts. Even over a 3 d contact time, application of preformed monochloramine reduced NDMA formation by 39% by minimizing the exposure to inorganic dichloramine, the inorganic chloramine precursor to NDMA. However, the benefit of preformed monochloramine was restricted to waters with low to moderate ammonia concentrations (<20 mg L⁻¹ as N), because dichloramine formation during in situ chloramination is less important for high ammonia waters. Overall, the combination of MPUV with preformed monochloramine reduced final NDMA concentrations by 73–93% relative to in situ chloramination, a common current practice for non-potable reuse.