Application Of Combined Ultraviolet Disinfection On The Removal Of Chlorine-resistant Bacteria Isolated From Water Supply Pipe Network

The problem of drinking water biosafety has attracted much attention.The emergence of chlorine-resistant bacteria has brought great challenges to the water treatment industry.In this study,The chlorine-resistant Bacillus subtilis isolated from in pipe network was used as the test organism,and the sterilization effect and disinfection kinetics characteristics of four single disinfection methods including chlorine,ultraviolet,chlorine dioxide and potassium Potassium peroxymonosulfate（PMPS）were studied.On this basis,the combined disinfection process based on ultraviolet was selected to investigate the inactivation effect of combined disinfection on the tested organisms,and the influence of the combination sequence of different disinfectants and ultraviolet disinfection on the inactivation efficiency of chlorine-resistant bacteria was discusssed.So as to screen out the optimal process for inactivation of chlorine-resistant Bacillus subtilis in the water supply system.The parameters under the optimal process combination were optimized by response surface analysis.The main research results are as follows:The results showed that the inactivation efficiency Cl₂,UV,Cl O₂,and PMPS was low.At room temperature,the inactivation sufficiency of Cl₂disinfection with the concentration of1mg·L^-1on Bacillus subtills was only only 0.34 lg when it was inactivated for 240 min.Under the same effective chlorine condition with Cl O₂(0.38 mg·L^-1)was disinfection for 120 min,and the inactivation rate was 0.35 lg.When the concentration of Cl O₂ was 2.4 mg·L^-1,and the inactivation rate of Bacillus subtilis reached 3.4 lg after disinfection for 120 minutes.Under the same effective chlorine conditions,the inactivation rate of Cl₂ disinfection(6mg·L^-1)was 4.26 lg.In other words when the effective chlorine was the same and the concentration of the disinfectant increases,and the]inactivation effect of Cl₂disinfection is greater than that of Cl O₂ disinfection.When the Cl₂ concentration was 3.0 mg·L^-1 and 6.0mg·L^-1,the inactivation rate of Bacillus subtilis reached 99.9%,the CT value was 157 mg·min L^-1 and 120 mg·min L^-1,respectively.When the inactivation rate of Cl O₂disinfection reached 99.9%,the CT value was 100 mg·min L^-1.The standard UV dose drinking water is 40mJ·cm^-2.In this study,when the UV radiation intensity was 10 mJ·cm^-2 and 40 mJ·cm^-2,the inactivation rates were 0.18 lg and 0.98 lg,respectively.When the UV radiation intensity was160 mJ·cm^-2,the inactivation rate of Bacillus subtilis was 4.1 lg.The inactivation rate of Bacillus subtilis at different concentrations of PMPS was relatively low.In this study the concentration of PMPS was 25 mg·L^-1,50 mg·L^-1,75 mg·L^-1,and the inactivation rate of Bacillus subtilis was about 0.23 lg,and the inactivation efficiency was not obvious.The disinfection kinetics of UV,Cl₂,and Cl O₂were analyzed by Chick model,Chick-Watson model,Hom model,and first-order kinetic model.The optimal kinetic model of Cl₂ disinfection was the Hom model,and the best disinfection fitting model was obtained based to data analysis:=0.036C^0.805T^0.746.indicating that the survival curve of Bacillus subtilis was curved rather than linear in the process of chlorine disinfection,and both the concentration of disinfectant and the inactivation time in the process of disinfection had an impact on the inactivation rate.The optimal kinetic model of Cl O₂ disinfection was the Chick model,and the disinfection fitting model:=0.03CT,indicating that the first-order reaction can be used to express the inactivation rate of chlorine dioxide against Bacillus subtilis.The UV inactivation of Bacillus subtilis does not conform to the simple first-order kinetic equation,and the fitting equation was:N_t=N₀*exp（-0.51It）.The inactivation coefficient k=0.51 was higher than the recommended value k=0.067～0.38 cm²/（W·s）in the US EPA design manual.Indicating that Bacillus subtilis had strong resistant to UV disinfection.The inactivation rate of potassium bisulphate monophosphate was low and the change was not obvious,so it could not be expressed by a kinetic model.In this study,three disinfectants of Cl₂,Cl O₂,PMPS and UV rays were combined to disinfect,and the inactivation rate of Bacillus subtilis was discussed.The results showed that the combined sequence of disinfectants had a great influence on the inactivation effect.UV disinfection enhances the inactivation efficiency of chlorine-resistant bacteria.The possible reason is that the combined process of disinfectant generates hydroxyl free radicals,which has a certain synergistic effect on the inactivation of chlorine-resistant bacteria.Cl₂ concentration4 mg·L^-1,UV intensity 80 mJ·cm^-2,time 60 min,pre-chlorine disinfection,followed by UV disinfection（Cl₂-UV）combined disinfection process,the inactivation rate was 4.56 lg,which was higher than the single disinfection and inactivation under the same conditions.The sum of efficiencies was 1.4 lg higher,and the synergy efficiency was obvious.The inactivation rate of Bacillus subtilis by the combined disinfection process of UV disinfection（UV-Cl₂）and chlorine disinfection（UV-Cl₂）was 3.3 lg,the inactivation rate was low,and the synergy efficiency was not obvious.Under the same effective chlorine conditions,the Cl O₂-UV combined disinfection process has a Cl O₂ concentration of 1.6 mg·L^-1,an UV intensity of 80mJ·cm^-2,and a 60 min inactivation rate of 3.8 lg,which is lower than the Cl₂-UV combined disinfection process for chlorine-resistant bacteria The combined process has a certain synergistic efficiency only when the Cl O₂ concentration was 1.6 mg·L^-1,and the combined process has a certain synergistic efficiency,and the UV disinfection pretreatment was carried out first,and then the Cl O₂ disinfection（UV-Cl O₂）combined disinfection process could affect the spores of subtilis.The inactivation rate of bacilli was 2.15 lg,the inactivation rate was low,and the synergy efficiency was not obvious.Under the maximum concentration experimental conditions of the Cl₂-PMPS combined disinfection process(Cl₂ concentration 4 mg·L^-1,ultraviolet intensity 80 mJ·cm^-2,time 60 min),the Cl₂-PMPS combined disinfection process had an inactivation rate of Bacillus subtilis 2.32 lg,the inactivation rate of UV-PMPS combined disinfection process was only 1.4 g under the same conditions,which does not have synergistic effect.Based on the above results,it could be seen that the inactivation rate of Bacillus subtilis was much greater than that of the first UV pretreatment followed by the disinfectant disinfection.And the synergistic effect of Cl₂-UV combined disinfection process was the most obvious.This study to pipe the water separating the chlorine resistance of bacillus subtilis as an example,the resistance of bacteria in drinking water disinfectant and disinfectant inactivated effect of combination process,pick out the Cl₂-UV for the chlorine resistance of Bacillus subtilis optimal disinfection process,the results for the chlorine resistance of bacteria control and the improvement of drinking water biological stability provides technical reference.