Treatment Of Typical Micropollutants In Drinking Water By UV/Cl₂ Process And Potential Risks In Water Distribution System

With the development of industrial economy and the increase of population,micropollutants（such as surfactants,pharmaceutical and personal care products,endocrine disrupting chemicals,etc.）have been frequently detected in surface water,groundwater and even drinking water.Although the concentrations of micropollutants are low,they have strong toxicity effects,and are precursors of disinfection byproducts（DBPs）,which affects the safety of water supply.As an advanced oxidation process（AOP）,ultraviolet/chlorine（UV/Cl₂）process has the advantages of high degradation efficiency,simple reaction system and convenient operation.However,the use of UV/Cl₂ to remove micropollutants in drinking water will inevitably create risks of DBPs formation.In addition,when the raw water is rich in bromide,inorganic bromate（Br O₃^-）will be formed after ozone treatment,and then easily reduced to bromide anion（Br^-）by zero-valent iron（ZVI）widely existing on the inner wall in drinking water distribution pipes.When there is residual chlorine in water distribution system,the formation pathway of halogenated DBPs will be affected.On this basis,the study mainly investigated the characteristics of organic micropollutants degradation by UV/Cl₂ process,revealed the contributions of different active species during the degradation process and the formation of DBPs,in order to provide a theoretical basis for the efficient removal of organic micropollutants in drinking water plants.Additionally,the reduction of Br O₃^-by ZVI in chlorination process was studied,and the formation of halogenated DBPs under different conditions was investigated.Combined with the cytotoxicity index（CTI）evaluation of the detected DBPs,the reference was provided for the toxicity risk assessment of bromate and organic DBPs in the process of drinking water transportation and distribution.Firstly,the degradation effects of UV,Cl₂ and UV/Cl₂ on the typical micropollutant,sodium dodecylbenzene sulfonate（SDBS）,were compared.The results showed that the degradation rate of SDBS by UV/Cl₂ process was 3.3 times than the sum of those by UV and Cl₂ alone.Furthermore,the effects of different experimental conditions on the degradation of SDBS by UV/Cl₂ process were investigated.The results showed the degradation of SDBS was strongly p H-dependent.The degradation rate decreased with the increase of p H,and the degradation effect was inhibited to a certain extent by background water matrix components,such as NO₃^-,HCO₃^-and natural organic matter（NOM）.The second-order reaction rate constant of SDBS with hydroxyl radical（HO^*）was determined to be 2.84×10⁹ M^-1 s^-1 by competitive kinetic method.Through using different probe compounds,the main contributors of UV/Cl₂ process to SDBS degradation were found to be UV,HO^*and active chlorine species（RCSs）.Besides,two detectable DBPs,trichloromethane（TCM）and chloral hydrate（CH）were formed during the treatment of SDBS by UV/Cl₂,and total organic chlorine（TOCl）was evaluated.The yields of TCM,CH and TOCl reached 64.0,8.7 and 400μg L^-1 in 30 min,respectively.The TOCl yield was significantly lower than that of Cl₂ alone(3.5 mg L^-1).The combination of UV and Cl₂ is worthy of promotion and application in the field of water treatment.Secondly,the reduction of Br O₃^-by ZVI and the effects of halogenated DBPs formation during chlorination were explored.The presence of ZVI could reduce Br O₃^-to Br^-,which was converted to hypobromic acid（HOBr）during chlorination,leading to the formation of brominated（Br-）DBPs.The DBPs detected during chlorination under different conditions included trihalomethanes（THMs）and haloacetonitriles（HANs）.When the concentration of ZVI increased from 0 to 1.0 g L^-1,the formation of Br-DBPs increased significantly,while the formation of chlorinated（Cl-）DBPs decreased.When p H increased from 5.0 to 9.0,the bromine substitution factors of THMs(BSF_THMs)decreased significantly from 0.61±0.06 to 0.22±0.02.Moreover,the increase of initial Br O₃^-concentration promoted the formation of Br-DBPs and reduced the formation of Cl-DBPs,resulting in an obvious increase of BSF_THMs.With the increase of initial Cl₂ concentration,the concentrations of both THMs and HANs showed an upward trend.Finally,the CTI analysis based on DBPs was carried out.The results showed that the reduction of Br O₃^-by ZVI during chlorination might lead to the increase of cytotoxicity.When the concentration of ZVI was 1.0 g L^-1,the CTI of nine DBPs detected was 23.59×10^-4,which should be paid more attention to in practical application.This work described the technical feasibility of UV/Cl₂ process in drinking water treatment from the aspects of removal of typical micropollutants and evaluation of DBPs formation.Meanwhile,this work explained the risks to water quality in drinking water distribution system from the effects of Br-DBPs formation by ZVI.Generally,it provides a scientific basis for process selection and regulation of DBPs in drinking water plants.