Mechanisms Of N-nitrosamines Formation Of Typical Precursors During Chlorination Of Municipal Wastewater Reuse

Urban sewage is an important freshwater resource in the water cycle system,and its regeneration process has great economic and social value.Since the reclaimed water contains many pathogens such as bacteria and viruses,it must go through a disinfection process to minimize its health risks.Disinfection by-products are an important part of the risk assessment of reused water,especially nitrogen-containing disinfection byproducts,which are much more toxic than traditional carbon-containing disinfection byproducts.As an emerging nitrogen-containing disinfection by-product,N-nitrosamines are generated in the disinfection process of urban sewage reuse,which remains to be studied.There are relatively high levels of(semi-)synthetic organics in urban sewage.After chlorination and disinfection,organics that are more toxic than the main body may be produced,causing a series of environmental problems.In this study,tetracycline antibiotics were used as typical precursors,and simple aliphatic amines(dimethylamine,trimethylamine)were used as model precursors,and the formation potential and reaction mechanism of N-nitrosamines were explored.This article first explores the potential and mechanism of the typical precursors under the ammonia nitrogen system to generate N-nitrosamines during chlorination and disinfection.This study separately explored the potential of N-nitrosamine formation under different initial precursor concentrations,p H conditions,disinfectant activity types,and disinfection time.Under different initial concentrations of precursors,the difference in physical and chemical properties is directly This results in a difference in the potential for N-nitrosamine formation,and the functional groups of different precursors have significant differences in the potential for N-nitrosamine formation.When the ratio of alkaline conditions and chlorine to nitrogen is 2:1,the formation potential of N-nitrosamines is higher,indicating that the unprotonated state and the promotion of N-nitrosamines by dichloramine are more obvious.Through qualitative and semi-quantitative analysis of the intermediate products in the disinfection reaction process,N-nitrosamine basically conforms to the UDMH generation mechanism under the ammonia nitrogen system,and the precursors interact with different types of disinfectants to generate UDMH/UDMH-Cl intermediate products.It further reacts with oxygen or other oxidizing substances to generate N-nitrosamines.Based on the density functional theory(Density Function Theory,DFT),the calculation results of the path of NDMA generated with dimethylamine as the model target show the feasibility of the process in terms of thermodynamics and kinetics.Secondly,we explored the potential mechanism of N-nitrosamine formation in the process of chlorination and disinfection of tetracycline antibiotics in nitros-nitrogen and nitrate-nitrogen in urban sewage.Considering that both nitrite and nitrate are oxygencontaining inorganic nitrogen,in addition to exploring the initial concentration of different precursors,p H conditions,disinfection time,etc.,they also explored the Nnitrosose under different aeration gas conditions.The potential for amine formation,it is found that dissolved oxygen is not the limiting factor in the formation of N-nitrosamines in this process.In this system,the dissociation morphology of the precursor does not have a significant effect on the potential of N-nitrosamine but is related to the electron abundance of the dimethylamine group of the typical precursor.The qualitative analysis of the intermediate products in the disinfection process through the combination of high-performance liquid phase and mass spectrometry technology,and further inferred that the formation of N-nitrosamine in the presence of nitrite/nitrate is significantly different from the UDMH mechanism,so this study proposes the reaction process is based on chlorine-induced nitrosation of reactive nitrogen species to generate Nnitrosamines(HOCl-RNS enhanced mechanism).To further support the hypothesis of the reaction mechanism in the nitrite/nitrate interaction,this study supported the formation mechanism of N-nitrosamine under this system by calculating the reaction process at the same level as the previous method.Finally,based on the above theoretical basis,this study selected two sewage treatment plants with supporting sewage reuse systems in southwest China for field research.The risk of generation of disinfection by-products in the disinfection process of wastewater reuse with different sewage components is explored.Through parallel factor analysis,a multi-component model is established for the sewage components,and the component changes of each sewage treatment unit are explored.Among them,the protein analog component is the main removal object of the biological treatment unit.The wastewater components are further analyzed through techniques such as resin separation and membrane separation to explore the generation risks of disinfection byproducts of each component in actual wastewater.Hydrophilic components and small molecular weight substances are the main contributors to N-nitrosamines.By changing the disinfection conditions and exploring the potential of disinfection by-products,the results show that extending the disinfection time will significantly increase the potential for N-nitrosamine formation.The purpose of this study is to deeply analyze the formation of nitrosamines in the chlorination and disinfection process of typical precursors in recycled water.-Nitrosamine production potential,which provides a theoretical basis for the Nnitrosamine production potential and mechanism of precursors similar in structure to tetracycline antibiotics.Based on ensuring the microbiological safety of the disinfection process,the risk of generation of disinfection by-products in the reused water is evaluated,and a theoretical basis is provided for the improvement of the disinfection process.