Diurnal patterns of N-nitrosodimethylamine and formaldehyde behaviors in different seasons in surface water influenced by effluent from sewage treatment plants

N-nitrosodimethylamine (NDMA) and formaldehyde (FAH) are probable carcinogenic disinfection by-products and have been found to occur in areas of the Yodo River basin that are influenced by treated wastewater. The diurnal patterns of them were examined with water samples collected over 4 years in different seasons at five outlets of four sewage treatment plants (STPs), seven main stream sites, and five tributary sites in the basin. Based on mass flux calculations, STPs were shown to be the main sources of the downstream for NDMA and FAH loads in the study area. Moreover, results revealed that photo degradable NDMA and non-photo degradable FAH showed different fates during the river transportation. For NDMA, in addition to direct photolysis, water temperature was identified as an important factor in NDMA attenuation in surface waters. NDMA attenuated significantly in the daytime and even during summer nights but persisted during winter nights, while attenuation of FAH was found to mainly be influenced by water temperature, and persisted during winter. Their behaviors were compared in an artificial river channel and whole study area, clearly demonstrating the importance of monitoring them in the water environment during periods of low UV intensity and water temperature, especially winter nights.