Effect Of Pre-chlorination Process On Halogenated Disinfection By-products Formation In Drinking Water

In recent years,disinfection has occupied an indispensable position in drinking water treatment and distribution systems,and chlorine disinfection has become one of the most extensive disinfection methods.However,the disinfectant can react with the halogen ions and dissolved organic matter?DOM?to produce halogenated disinfection by-products?X-DBPs?in drinking water,so this reaction process has been studied in depth.This article was mainly discussed about the pre-chlorination that was the starting point of the formation of halogenated disinfection by-products in drinking water,and it could affect the changes of X-DBPs in other treatment processes.So far,the X-DBPs of drinking water has been complex and diverse,and even the molecular composition has been unclear.Therefore,the fourier transform ion cyclotron resonance mass spectrometry?FT-ICR MS?technology was used to characterize the molecular composition of DOM and X-DBPs during the pre-chlorination experiment and the actual drinking water treatment,as well as the toxicity assessment was carried out in the real drinking water.Through pre-chlorination laboratory tests,the molecular characteristics of DOM,Cl-DBPs and Br-DBPs were revealed during the pre-chlorination process.From the perspective of semi-quantitative analysis,the differences in the mass spectra of DOM and X-DBPs were discussed under different experimental conditions.The presence of bromide ion had no significant effect on DOM,which could increase the intensity of Br-DBPs and decrease the intensity of Cl-DBPs.Through Van Krevelen diagram and Kendrick Mass Defect analysis,it was inferred that the possible molecular composition of DOM and X-DBPs,which were mainly composed of lignin substances and molecules with relatively low concentration.And also,the molecular characteristics of obtained molecular formula were speculated by a series of parametric statistics.Formation of X-DBPs with more unsaturated structures might be formed by the DOM with corresponding unsaturated structures.From the perspective of quantitative analysis,the formation of THMs was determined during the pre-chlorination experiment,and the concentration of bromide ion could significantly affect the distribution of THMs.Taking the effluent of actual water treatment processes as the research object,the changes of DOC,UV₂₅₄ and SUVA were analyzed in the effluent of different units,and the overall change trend was gradually decreasing.The transformation of DOM was studied in different water treatment processes.DOM with reducibility and greater than 400 Da was easily removed during the pre-chlorination.In the reaction of pre-ozonation and ozonation,unsaturated DOM would preferentially react with O₃ to produce a certain amount of saturated DOM.Molecules with higher oxidizability were easily removed during the coagulation/sedimentation process.The removal of DOM might be related to the physical adsorption of activated carbon and the biodegradation of microorganisms in the treatment of carbon and sand filter.Taking the effluent of actual water treatment process as the background water quality,the molecular composition and changes of Cl-DBPs were comparatively studied in different processes.A number of 587,514,570,612,473,and 676 Cl-DBPs were detected in pre-chlorination,pre-ozonation,coagulation/sedimentation,ozonation,carbon and sand filter and effluent.The number of Cl-DBPs from high to low was:CHOCl,CHOCl₂,CHOCl₃.The highly unsaturated compounds accounted for more than 80%,while aromatic compounds were basically less than 10%in all Cl-DBPs.Based on thiol reactivity experiments,the EC₅₀ values were obtained in the effluent of various treatment processes,which provided a certain reference for ensuring the water security for residents.