Formation and Control of N-Nitrosodimethylamine (NDMA) in Wastewater Reclaimed for Indirect Potable Reuse

Abstract

A large variety of disinfection by-products (DBPs) are formed during water treatment processes using chlorination. Disinfection with chloramines is often used to significantly reduce the formation of regulated DBPs. However, chloramination favours the formation of N-nitrosamines, which are emerging by-products of health concern. Of all the nitrosamines, N-nitrosodimethylamine (NDMA) has been most commonly detected in drinking water and wastewater. Nitrosamines can also be produced using different disinfectants, including chlorine dioxide and ozone. Wastewater often contains considerable concentrations of NDMA and NDMA precursors. Therefore, the reuse of treated wastewater for drinking water augmentation might pose a risk due to the presence of nitrosamines. To date, the NDMA formation mechanisms through the use of different oxidants are not fully understood and all precursors have not yet been identified making the NDMA control challenging during water treatment processes. The main goals of the present study were: i) to investigate the formation and control of NDMA by disinfection and advanced wastewater treatment processes, including chloramination, ozonation, reverse osmosis (RO) and advanced oxidation processes (i.e. UV/hydrogen peroxide (H2O2)); ii) to identify source of NDMA precursors in wastewater; iii) to determine the role of inorganic water constituents on NDMA formation and the possible formation mechanisms. Full scale experiments and lab scale tests were performed using wastewater collected at treatment plants placed in the South Western United States. Selected experiments were carried out using synthetic water containing NDMA precursors and inorganic constituents (i.e. dimethylamine (DMA), dimethylformamide (DMF), ammonia, bromide). Results of this study showed that ozonation of wastewater often produced elevated NDMA, whereas chloramination or post chloramination has always resulted in far lower amounts of NDMA under realistic dosages and contact times. RO removes the vast majority of NDMA precursors but it has low effectiveness on NDMA rejection. During UV treatment, the presence of H2O2 can reduce NDMA photolysis due to absorption/screening of UV light. Furthermore, UV or UV/H2O2 treatment can enhance NDMA (re)formation during post-chloramination. Likely, it happens because more reactive by-products are formed. Bench scale ultraviolet experiments confirmed an enhanced NDMA formation during chloramination after UV/H2O2 treatment of DMF. Centrates sent back to the head of the treatment plant were identified as the main source of NDMA precursors in the investigated wastewater. The easily degradable structure of Mannich polymer used for sludge treatment was the principal cause of precursors occurrence. Ammonia and bromide presence in water was found to improve the NDMA yield by ozonation from DMA. Ozonation of ammonia and secondary amines forms hydroxylamine (HA); whereas brominated oxidants are formed in the presence of bromide. This study proved that when these oxidant species are present, the NDMA formation from DMA is significantly enhanced. Finally, NDMA formation during ozonation of DMF occurred only in the presence of ammonia and it was attributable to the oxidation of DMF by hydroxyl radicals. Overall, wastewater characterized by elevated ammonia and NDMA precursors load can induce high and rapid NDMA formation by ozone. Therefore, complete biological nitrification may represent an effective treatment method for reducing NDMA formation potential. During wastewaters treatments, RO and UV are very effective to remove NDMA precursors and degrade NDMA, respectively. Nevertheless, UV/H2O2 treatment may enhance the NDMA formation during post-chloramination. Thus, UV dose and the use of H2O2 should be validated considering potential NDMA reformations. The use of Mannich polymer in sludge treatment should be avoided due to its easily degradable structure that can release elevated amount of NDMA precursors.