Formation Mechanisms And Control Of Disinfection By-products During Mixed Residual Chlorine Process In Water Treatment Plant

Residual chlorine is an indicator to ensure the biological safety of water quality in the urban water supply system.Unlike the commonly believed single available chlorine（free chlorine or monochloramine（NH₂Cl））,the total residual chlorine in water is actually composed of free chlorine,inorganic chloramines and organic chloramines.The mixed residual chlorine would transform into each other continuously,resulting in faster total chlorine decay and more highly toxic disinfection by-products（DBPs）formation.The mixed residual chlorine and their derivative risks are important water quality issues of concern in this field.This study focused on the formation mechanisms and control of disinfection by-products during mixed residual chlorine process in water treatment plant.The following contents will be mainly elaborated:（1）to monitor the distribution of mixed residual chlorine in water supply system and risks of derivative DBP formation;（2）to establish an efficient preparation method of NH₂Cl;（3）to investigate the formation and control of I-THMs during breakpoint chlorination of iodide-containing water;（4）to analyze the transformation mechanisms of mixed residual chlorine and formation and control of DBP formation in a UV-activated mixed residual chlorine system.The results obtained in this study will provide a theoretical basis and technical support for water quality risk control during the disinfection process.Based on the results of residual chlorine and DBP formation from several drinking water supply system in different seasons in Shanghai,the residual chlorine in water supply system was a common situation.For the water treatment by using chloramination process,the proportion of inorganic and organic chloramines was up to8%in finished water,and increased to 18%in pipe network.Besides,the consumption rate of disinfectants could reach 48%in a mixed chlorine system,which was 23%and46%higher than those of chlorine and chloramine alone system,respectively.Moreover,more I-THM formation was observed in a mixed chlorine system,which was 13 and 2times than those of chlor（am）ine alone system,while little difference was observed in the formation of C/N-DBPs.Therefore,considering the fast decay of disinfectants and the derivative I-THMs formation in a mixed chlorine system,more attention should be paid to this issues.In order to avoid the water quality problems caused by mixed residual chlorine system,we developed an efficient preparation method of NH₂Cl for drinking water treatment plants（DWTPs）.Meanwhile,DBP formation in this process was also evaluated.The results showed that inorganic nitrogen sources,mass ratios of Cl₂/N and p H values played vital role in the formation of NH₂Cl compared to other factors（such as G values,temperature,reaction time,and chlorine dosing rate）.DBPs formation increased with the increasing mass ratios of Cl₂/N,temperature,reaction time,and chlorine dosing rate,while decreased with the increasing G values.In sum,the optimal result of chloramine disinfection can be achieved at Cl₂/N 5:1,reaction time 2h,25℃,G 300 s^-1 with（NH₄）₂SO₄ as the nitrogen source and low chlorine dosing rate at neutral or alkaline p H conditions.In this way,the proportion of NH₂Cl can be up to 90%in the finished water.This study also systemically investigated the I-THM formation during breakpoint chlorination of iodide-containing waters.The results indicated that the formation of I-THMs varied in different zones of the breakpoint curve.I-THM concentration increased with the increasing chlorine dosage within breakpoint value and then dropped remarkably beyond it.Moreover,CHI₃ and CHCl I₂ were the dominant compounds in the pre-breakpoint zone,while CHCl₂I was the major one in the post-breakpoint zone.I-THM concentration increased with the increasing I^-concentration,NOM concentration and p H values,but decreased with increasing SUVA₂₅₄ value.For all the above-mentioned factors,IIF showed decreasing trend with increasing chlorine dosage during the breakpoint chlorination,especially in the post-breakpoint zone.The decomposition mechanisms of chlorine species in a mixed residual chlorine system in the presence UV irradiation were further investigated in this study.The results demonstrated that the mass ratios of free chlorine to NH₂Cl and UV fluence had vital influence on the decay of disinfectants in a mixed residual chlorine system.UV irradiation could significantly accelerate the chlorine consumption rate.Besides,the chlorine consumption rate was the fastest as the mass ratio of free chlorine to NH₂Cl at4:1,and the consumption rate could reach 99%at 60 min.However,NO₂^-and NO₃^-concentrations could be easily improved in a UV-activated mixed residual chlorine system,especially as the mass ratio of free chlorine to NH₂Cl at 1:1.It should be noted that OH·was significant to the consumption of free chlorine and NH₂Cl compared to other radicals（such as Cl·and Cl O·）.Meanwhile,the removal efficiency of UV₂₅₄（94%）was much higher than that of DOC（24%）in a UV-activated mixed residual chlorine system.The formation of C/N-DBPs in a mixed residual chlorine system and the control mechanisms in the presence of UV irradiation were further systematically investigated.The results indicated that the C/N-DBP formation was the highest in a chlorine alone system,which was about 4 and 11 times than those of mixed residual chlorine and chloramine systems,respectively.Besides,C/N-DBP formation obviously decreased as the mass ratios of HOCl to NH₂Cl varied from 5:0 to 0:5.C/N-DBP formation in HA-containing water was the highest,followed by those in FA-and AOM-containing water,respectively.C/N-DBP formation increased significantly as the p H values changed from 6.0 to 8.0.The findings also demonstrated that C/N-DBP formation could be effectively suppressed in a UV-activated mixed residual chlorine system in alkaline conditions.Moreover,in the presence of Br^-,better control of the CTI and GTI values were achieved with UV irradiation compared to that during mixed residual chlorine process without UV irradiation.The formation of I-THMs in a mixed residual chlorine system as well as the control method by UV irradiation was also investigated.The results indicated that I-THM formation decreased in the order of mixed chlorination/chloramination>chloramination>>chlorination,which agreed well with the toxicity evaluation using CHO cells.Conversely,TOX concentration followed by the trend of chlorination>>chloramination≈mixed chlorination/chloramination.Influencing factors of p H values and Br^-/I^-on I-THM formation were also systematically investigated in a mixed residual chlorine system.Enhanced I-THM formation was observed with increased p H values（6.0 to 8.0）and Br^-/I^-molar ratios（1:1 to 10:1）.Besides,I-THM formation can be efficiently suppressed during UV irradiation in the mixed residual chlorine system.