COVID-19 impacts on characterization of N-nitrosamines and their precursors during transport in sewer systems

During the COVID-19 outbreak, N-nitrosodimethylamine (NDMA) and N-nitrosomorpholine (NMOR) and their specific precursors (N,N-dimethylformamide [DMF] for NDMA and morpholine [MOR] for NMOR) were widely detected in sewerage systems of an urban area, in which chlorine-containing disinfectants were discharged with effluent of hospitals, etc. However, little is known about the effect of chlorine influx on formation and distribution of NDMA and NMOR in sewer systems in a sudden major public health event. We investigated the spatiotemporal patterns of NDMA, NMOR, DMF and MOR in influents of sewage treatment plants (STPs), as well as its upstream sewer sites during the COVID-19 pandemic. During the pandemic, there was a significant decrease of industry-related NMOR and DMF, however, with an increase of concentration (up to 243 ng/L) and detection frequency for NDMA in influents of the biggest STP in Kyoto Prefecture. Moreover, it was found that NDMA reached a maximum of 187 ng/L with 57 % detection frequency, while NMOR reached a maximum of 101 ng/L with 51 % detection frequency in the sewer systems connecting to all the STPs for service area during the pandemic. Especially, during the pandemic, concentration (median value) of NDMA increased from 40.9 ng/L with 42 % detection frequency in 2020 to 72.5 ng/L with 77 % detection frequency in 2021, which was coincident with the change of infected population. In addition, this research clearly exhibited the possibility that unintentional chlorination and nitrosation of precursors formed NDMA and NMOR in sewer systems influenced by COVID-19 pandemic. The NDMA formation was ranked according to increased concentration (median value) as follows: addition of ClO^- (669 ng/L) > addition of NO₂^- (138 ng/L) > without addition (34.3 ng/L), while additional ClO^- and NO₂^- did not significantly increase NMOR formation probably caused by low existence of NMOR precursors (e.g., MOR) in raw sewage. Therefore, it is necessary to make an urgent attention on environmental issues caused by high-dose chlorine-containing disinfectants residue, because increased byproducts induced by disinfectants in raw sewage caused higher risk during the future pandemic by unexpected pollution from insufficiently treated sewage (e.g. combined sewer overflow and primary effluent bypass discharge) to receiving water bodies.