Influence Of Biofilm On Chlorine Decay And Disinfection Byproducts Trihalomethane Formation In Drinking Water Distribute Systems

With the rapid development of social economy and the acceleration of urbanization,people are paying more and more attention to the safety of drinking water.Since the nutrients in drinking water is low,bacteria that did not inactivated by disinfectants in the watertreatment plant can easily adhere to the surfaces of the water supply pipes because there contain rich nutrients These alive bacteria,dead bacteria and their cell debris are Extracellular polymer(EPS)forms a three-dimensional heterostructure of wall biofilm.On the one hand,biofilm protects the pathogen from the action of disinfectants;on the other hand biofilm can increases the enrichment of pathogenic bacteria.The resistance to disinfectants can be enhanced by the enrichment and adsorption of pathogenic bacteria,but once biofilm shedding,it will increase the risk of water quality associated with desorption of pathogenic bacteria and byproducts of disinfection.Therefore,it is of great significance to guarantee the last mile of water distribution system.In this study,biofilm growth simulators were set up at three different water plants for investigate the effect of the raw water quality and the water treatment process on the growth of biofilm and the generation of trihalomethane,a by-product of disinfection,during chlorine disinfection.Based on the experimental data,the chlorine decay and trihalomethane formation models of the feeding water and biofilm were obtained,and the consumption of chlorine by natural organic matter and biofilm in the water and the contribution to the generation of disinfection byproducts were explored.The main conclusions are as follows:(1)Establishment of HS-GC/MS detection method for four kinds of trihalomethane disinfection byproducts of chloroform,bromodichloromethane,dibromochloromethane,and tribromomethane.The concentration of the four disinfection by-products and instrument response values to be tested showed a good linear relationship.The correlation coefficient R~2 of the four kinds of targets was above 0.99,and the recovery of the method was between 90%and110%.A DNA extraction and q PCR quantitative analysis method was established for the molecular biological analysis of biofilm samples three kinds of pipe network simulation devices(2)Build a pilot-scale PVC pipe loop device for the cultivation of biofilm in three water treatment plants with different raw water and water treatment processes in Xiamen.During the biofilm cultivation,we detected the water quality of the left and right effluent water entering the device for one year to study the effect of water quality on biofilm growth.The study found that the growth of biofilm was directly affected by water quality,and the biofilm grown under different water quality conditions were completely different.Particles and algae in the water are the key factors affecting the growth of biofilm.Treatment conditions in the water plant can potentially affect the formation of biofilm in the water supply network.(3)The feeding water and biofilm play a different role in the process of contact with disinfectants be found.Chlorine decay and trihalomethane generation by feeding water can be fitted by a second-order reaction kinetics model and the fitting correlation coefficient was above 0.9,so the data fitting model is good.After reacting with the disinfectant chlorine in the biofilm of the three water plants,the reaction-diffusion mass transfer model was used to fitting chlorine decay.The results of the Kolmogorov-Smirnov data test showed that there was no significant difference between the actual detected chlorine concentration and the fitted concentration.The data fitting model is good.(4)There was significant difference in the degree of chlorine consumption between the biofilm and the feeding water.The experimental results showed that the residual chlorine with an initial concentration of 4 mg/L reacted with the feeding water and biofilm,respectively.After 48 hours,the residual chlorine residual value of the feeding water from the three water plants averaged 3.36 mg/L.The average residual chlorine in the biofilm pipeline of the water plant was 0.39 mg/L,90%of the residual chlorine was consumed,and 60%was consumed within 1 hour.The results of the study showed that biofilm can quickly and massively consume chlorine in a short period of time,while feeding water consumes chlorine at a slower rate,with a final consumption of only 0.7 mg/L.Therefore,the biofilm in the pipe network consumes most of the free chlorine in the pipeline.(5)The contribution of biofilm and feeding water to the generation of disinfection by-products was significantly different.In the formation of trihalomethanes,the production rate of THMs in the biofilm was slower than that of the feeding water in the early stage of the reaction.Within 6 hours,the amount of THMs generated in the feeding water accounted for 84%of the total amount,while the amount of THMs generated in the biofilm was only 40%of the total;At the later stage of reaction,the amount of THMs produced by feeding water no longer increased,but the amount of THMs generated by biofilm continued to increase.Ultimately,the production of THMs was basically the same,indicating that the potential of biofilm-derived THMs was greater than that of feeding water.