| Biofilm in drinking water distribution system(DWDS)is a potential pathogen reservoir.Recent investigations have found that biofilm can also be employed as precursors to form disinfection byproducts(DBPs).The combination of ultraviolet(UV)and chlorine disinfection has been developed for the control of pathogenic microorganisms and DBPs,which is efficient,safe,and environmentally friendly.Given the sensitivity of biofilm to the disinfection process,it is important to understand how the combination of UV and chlorine disinfection affects the formation of DBPs caused by biofilm.This study sought to examine the changes in DBP formation potential of biofilm(DBPFPB)in simulated continuous-flow DWDS subjected to sequential UV and chlorine disinfection(UV-Cl2)treatments with varying UV and residual chlorine doses.In addition,the key water quality parameters of the biofilm-pipeine water interface,biofilm composition,and microbial community structure were monitored and characterized.The underlying mechanism of the influence of UV-Cl2 on DBPFPB was established based on the relationship between disinfection conditions,biofilm-pipeline water interface microenvironment characteristics,and changes in DBPFPB.The main research contents and conclusions are as follows:(1)The concentration of trihalomethanes(THMs W)and haloacetic acids(HAAs W)in water and the formation potential of trihalomethanes(THMFPB),haloacetic acids(HAAFPB),and the total organic chloride(TOClFPB)produced by biofilm in pipelines cultivated with water disinfected using both Cl2 and UV-Cl2 methods were measured.The results showed that the variations in THMFPB,HAAFPB,and TOClFPB were not significant at the residual chlorine content was 0.7 and 1.4 mg/L,but increased significantly at higher residual chlorine content of 2.1 mg/L.For UV-Cl2 disinfection,a moderate UV irradiation dose(80 mJ/cm~2)can reduce the THMFPB and TOClFPB,while the effect on HAAFPB was minimal.In addition,the statistical study demonstrated that THMs W may be utilized as an indication of DBPFPB.Results indicated that controlling TOClFPB through UV-Cl2 disinfection was most effective at a UV dose of 80 mJ/cm~2 and the residual chlorine dose of 1.4 mg/L.(2)Taking disinfection and DWDS as an integrated consideration,the influence of disinfection conditions on the development of biofilm was analyzed from the aspect of microbial selection pressure(such as residual chlorine,redox potential,and nutritional factors)at the interface between biofilm and pipe network water.The chemical composition,bacterial community structure,and metabolic function of biofilm were characterized.Based on the above studies,the mechanism of disinfection on DBPFPB was proposed.The results indicated that,for UV-Cl2 disinfection,at residual chlorine 0.7 and 1.4 mg/L,the attenuation rate of residual chlorine in pipeline water decreased with increasing UV irradiation dose from 40 mJ/cm~2 to 120 mJ/cm~2,whereas the attenuation kinetics of higher residual chlorine(2.1 mg/L)increased first and then slowed down.That is,the UV dose affected the residual chlorine decay kinetics in DWDS.Compared with chlorine disinfection,UV-Cl2 disinfection had a greater effect on the adjustment of the molecular structure of organic matter and redox potential,etc.In biofilm extracellular polymeric substances,the protein content was higher than polysaccharide content and a high UV dose combined with low residual chlorine could stimulate EPS secretion.Delftia,Stenotrophomonas,Sphingobium,Pseudomonas,and Methylobacterium played a crucial role in protein and polysaccharide biosynthesis.Sphingobium,Pseudomonas,and Methylobacterium were chlorine resistant,while Pseudomonas was UV sensitive.Disinfection conditions affected both the water quality(nitrate nitrogen,phosphate,dissolved organic carbon)and the biofilm characteristics(biological activity,community composition,metabolic function,and EPS components)of the biofilm-pipeline interface. |
