Occurrence, fate, and transport of N-nitrosamines and precursors in sewage treatment plants and receiving rivers in a highly urbanized basin

Abstract

N-nitrosamines (NAs), highly carcinogenic disinfection by-products, were frequently detected in raw sewage, sewage treatment plants (STPs), and receiving rivers. This study investigated five NAs, including N-nitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA), N-nitrosomorpholine (NMOR), N-nitrosodi-n-butylamine (NDBA), and N-nitrosopiperidine (NPIP), and their formation potentials (FPs) in a highly urbanized basin. The results showed that total NAs and their FPs ranged from 101 to 141 ng/L and 72.6 to 203 ng/L in the influent, implying that NAs and their FPs in the raw sewage might be affected by the sewage type, especially for NDMA (up to 103 ng/L) influenced by industrial wastewater. NDMA FP was positively correlated with NH₄⁺, TN, and TOC, while NDMA, NDEA, and NDEA FP were strongly associated with heavy metals, especially Hg, implying factories using Hg as potential sources. The biological treatment effectively removed NAs in STPs, but NMOR showed the weakest biological removal. In addition, the removal efficiency of NDMA was related to the type of biological treatment in the following order: Modified anaerobic-anoxic-oxic-membrane-bioreactor (Modified AAO-MBR) (81.2%) > AAO (60.1%) > Oxidation ditch (53.3%) > UNITANK (46.5%) > Modified AAO (25.8%). After treatment, total NAs (mainly NDMA and NMOR) in the effluent still ranged from 7.09 to 31.8 ng/L. In the receiving rivers, although NMOR was mainly photodegraded, Patescibacteria discharged from STPs was the first time to be identified as a potential contributor for NMOR. NDMA was primarily degraded through photodegradation and biodegradation, NDMA FP was probably biodegraded, with Proteobacteria probably contributing to the biodegradation of NDMA and NDMA FP.