Generation And Control Of Disinfection By-products Of Chlorine Dioxide Combined With Sodium Hypochlorit

Chlorine dioxide（ClO₂）is considered an excellent alternative to chlorine disinfectants because of its high efficiency,broad-spectrum bactericidal ability and the absence of halogenated disinfection byproducts.However,ClO₂ disinfection generates inorganic by-products chlorite（ClO₂^-）and chlorate（ClO₃^-）,especially ClO₂^-is more likely to exceed the standard,a problem that has received widespread attention in the field of water treatment.In order to control the risk of excessive by-products,chlorine dioxide and sodium hypochlorite（NaClO）combined disinfection has been used in water treatment plants in recent years,but the mechanisms and control methods of combined disinfection by-products（DBPs）are still unclear.In this study,we investigated the by-product generation and factors influencing the combined chlorine dioxide and sodium hypochlorite disinfection process from three aspects:chlorine dioxide preparation method,reaction with natural organic matter（NOM）,dosing method and water treatment process impact,and explored the control methods of combined chlorine dioxide disinfection by-products with actual water samples.The main findings of this paper are as follows:（1）The study of the effect of different preparation methods on by-product generation showed that the by-products ClO₂^-and ClO₃^-introduced by the ClO₂ absorbent solution were much smaller than those of the ClO₂ mixture.The effective chlorine decayed the slowest in the composite absorbent solution,and the inorganic and chlorinated byproducts were lower than those of the high-purity absorbent solution and the NaClO disinfectant,respectively,when disinfection was administered.Taking into account the introduction of byproducts,the decay of effective chlorine and the amount of by-products generated during disinfection,the performance of the compound absorbent solution was optimal.（2）Reaction studies with different types of natural organic matter（NOM）showed that ClO₂,NaClO had the highest reactivity with tannic acid（TA）at the same concentration,followed by humic acid（HA）,bovine serum protein（BSA）,and the lowest reactivity with sodium alginate（SA）;as the concentration increased,ClO₂^-would continue to react with excess HA,while BSA and TA did not react with ClO₂^-reacts.When the concentration of organic matter in water is low,the consumption of ClO₂ in the combined disinfectant is much larger than that of NaClO.As the concentration of organic matter increases,the consumption of NaClO increases and the generation of organic trichloromethane（CHCl₃）begins to increase.The analysis of fluorescence spectra before and after disinfection of natural waters showed that the main precursors of combined disinfection were humic acid and fulvic acid-like substances.（3）The reaction rate of ClO₂^-in combined disinfectants was less affected by temperature than that of NaClO.The ratio of ClO₂^-production decreased with increasing temperature,while the ratio of ClO₃^-and CHCl₃ production increased with increasing temperature.When the p H of the solution increased,the production of ClO₂^-increased gradually,and the production of ClO₃^-decreased and then increased.the production of CHCl₃ was greatest under neutral conditions,and less under acidic and basic conditions.When the combined disinfectant with75%of ClO₂ was applied,the by-products of ClO₂^-disinfection decreased by 68.5%and CHCl₃disinfection decreased by 134.6%compared with the single disinfectant,and the total by-products generated were the lowest.Combined disinfection with ClO₂ first can effectively reduce the generation of the organic by-product CHCl₃ and reduce the total by-products accounted for the standard rate.（4）The conventional water treatment process has limited removal of humic and fulvic acids in water.The disinfection of water samples treated with conventional process reduced the generation of by-products ClO₂^-,ClO₃^-and CHCl₃ by 30%,20%and 26%.The deep water treatment process was effective in removing humic and fulvic acids,and was able to control the generation of disinfection by-products by removing disinfection precursors from the raw water.The ClO₂^-generation of the two water samples treated by the deep process was reduced by more than 55%,and the CHCl₃ generation was reduced by more than 75%.The DBPs generated during the disinfection process were significantly reduced after the deep process for the two actual water samples,with the total DBPs accounting for only 38.0%and 21.3%,respectively.