Study On The Formation Mechanism Of Halogenated By-products During UV/PS And UV/NaClO Treatment Of Bromine-containing Reclaimed Water

When the new crown pneumonia epidemic is approaching,most sewage treatment plants adopt the method of increasing the dosage of chlorine-containing disinfectants to improve the killing effect of pathogenic microorganisms,which will lead to an increase in the production of disinfection by-products(DBPs).In this study,UV/persulfate process(UV/PS)and UV/sodium hypochlorite process(UV/NaClO)were used to remove DBPs precursors in reclaimed water.The analysis included oxidant concentration,UV radiation intensity,reaction time,initial p H value,Br^-concentration,transition metal ions and other factors on the treatment effect and the formation mechanism of DBPs in the oxidation process.UV/PS was used to remove typical organics(sulfamethoxazole)in reclaimed water,and the formation of DBPs was studied.The main test results are as follows:(1)The increase of Br^-concentration promoted the formation of brominated DBPs(Br-DBPs)during UV/PS and UV/NaClO oxidation.With the increase of Br^-concentration in solution,the amount of brominated trihalomethanes(Br-THMs)generated during UV/PS oxidation increased.The increase of Br^-concentration promoted the formation of bromine active substances in the UV/PS system,and the degradation effect of UV/PS oxidation on haloacetic acids(HAAs)precursors in reclaimed water did not change with the increase of Br^-concentration.Similarly,Br-THMs were preferentially generated during UV/NaClO oxidation of reclaimed water when the Br^-concentration in the solution increased.Compared with the unoxidized state,the amount of trichloromethane(TCM)generated after UV/NaClO oxidation increased at different levels.Low concentration of Br^-may increase the oxidized the formation potential of haloacetic acids(HAAs FP)relative to the raw water.(2)The amount of DBPs generated during UV/PS oxidation of reclaimed water is not affected by factors such as oxidant concentration,UV radiation intensity,reaction time,etc.The generated dichloroacetic acid(DCAA)is more stable than TCM in the oxidation system.Acidic conditions can promote the formation of Br-DBPs,which in turn increases the types and concentrations of trihalomethanes(THMs)and HAAs during the oxidation process.UV/PS/Co²⁺oxidation has an inhibitory effect on the formation of trichloroacetic acid(TCAA),while UV/PS/Fe²⁺oxidation promotes the formation of chloral hydrate(CH)and dibromoacetonitrile(DBAN).The pure water distribution experiment was designed to degrade SMX by UV/PS oxidation.When the Br^-concentration was 500?g/L and the Cl^-concentration was 300 mg/L,no THMs,HAAs and haloacetonitriles(HANs)were detected during the UV/PS oxidation of SMX.(3)Compared with UV/PS oxidation,the DBPs generated in the UV/NaClO oxidation process of reclaimed water were various and the total amount was large,the generation of Br-THMs and brominated haloacetic acids(Br-HAAs)increased significantly,and HANs were only generated when NaClO oxidation alone.The increase of reaction time promoted the generation of various THMs.The generation of TCM,TCAA,bromochloroacetic acid(BCAA)and other chlorinated DBPs(Cl-DBPs)showed an increasing trend with the increase of NaClO concentration.Lower UV radiation intensity also increased the generation of Cl-DBPs.Similar to UV/PS oxidation,alkaline conditions significantly inhibited the formation of HAAs.(4)The removal effect of UV/PS oxidation on DBPs precursors in reclaimed water showed an overall increasing trend with the increase of oxidant concentration,UV radiation intensity and reaction time.The mineralization effect of UV/PS/Cu²⁺oxidation on organic matter in reclaimed water is higher than that of UV/PS.After oxidation,the formation potential of THMs(THMs FP)is reduced by 30.72%compared with UV/PS oxidation.UV/PS/Cu²⁺oxidation had the best removal effect on HANs precursors,and only 0.66?g/L of bromochloroacetonitrile(BCAN)was generated.As the main precursor of THMs,HAAs and HANs,SMX can not only effectively reduce the formation potential of DBPs(DBPsFP),but also reduce the generation of DBPs by UV/PS oxidation,and the degradation products are no longer dichloroacetonitrile(DCAN)precursors.(5)The degradation effect of UV/NaClO oxidation on HANs precursors is not affected by UV radiation intensity,initial p H value and Br^-concentration,and UV oxidation alone and shorter reaction time will increase the amount of TCAN.Low concentration of NaClO can increase THMs FP and HAAs FP relative to raw water,and increasing the concentration of NaClO can effectively remove Br-DBPs precursors such as bromochloromethane(BDCM),BCAA,and DBAN.