Effects Of Oxidation-Reduction Potential Change Of Solution On The Formation Potential Of Disinfection By-products During Preoxidation

Adding a certain amount of preoxidant to raw water can not only alleviate the operation problems caused by microbial breeding,but also effectively control the smell of micro-polluted water,so the technology is widely used in conventional drinking water treatment plants.During the preoxidation process,the oxidant may react with the natural organic matter or trace organic matter existing in water,changing the structure and physicochemical properties of these substances,and then affecting the formation potential of the disinfection by-products?DBPs?of subsequent treatment and disinfection.Generally,it is believed that the stronger the oxidation ability of the preoxidant,the easier it is to oxidize the organic matter,it will have a greater impact on the subsequent DBPs formation potential.However,previous studies have used the standard re oxidation-reduction dox potential?ORP?of oxidants to explore their oxidation ability.In real water samples,the ORP of these oxidants may be quite different from the standard ORP,so their oxidation ability is also quite different.However,the relationship between the actual oxidation capacity of oxidants and formation potential of DBPs has not been reported.Firstly,the effects of solution pH value?4,7 and 10?,oxidant type?KMnO₄,ClO₂,Cl2 and NH₂Cl?and dosage on ORP value were systematically studied.Then,taking the raw water from inlet of a drinking water plant as a research object,the effects of ORP values in preoxidation stages on organic matter and the formation potential of DBPs?Trihalomethanes?THMs?,haloacetic acid?HAAs?,haloacetonitrile?HANs?,haloketones?HKs?,and trichloronitromethane?TCNM??in the process of chlorination and chloramination disinfection were explored.The results of this study are as follows:?1?Through the determination and analysis of ORP values of four oxidants?KMnO₄,ClO₂,Cl₂ and NH₂Cl?under different pH values and dosages,it was found that the ORP values of these oxidants were all decreasing with the increase of pH value of solutions,and the ORP values of these oxidants showed a trend of increasing rapidly at first and remaining basically unchanged with the increase of dosages.Among them,the pH value of the solution has little effect on ClO₂,while the effect on the other three oxidants is greater;when the dosage of oxidant is 0?0.5 mg/L,the ORP value of solution increases rapidly,while when the concentration is greater than 0.5 mg/L,the ORP value increases slowly.At the same pH value and dosage,the ORP values of the four oxidants are as follows:ClO₂>Cl₂>KMnO₄>NH₂Cl.?2?When using different pre-oxidants to oxidize the raw water,according to the EEMs?region I?EM<330nm,EX<250nm?:tyrosine protein organics;II region?330<EM<380nm,EX<250nm?:tryptophan-type protein organics;III region III?380<EM<550nm,EX<250nm?:fulvic acid organics;IV region?EM<380nm,EX>250nm?:microbial metabolites;V region?EM>380nm,EX>250nm?:humic acid organics?integral calculation of the five EEM regions,it can be seen that increasing the ORP value of solution?adding preoxidant?can reduce the total fluorescence intensity of NOM to different degrees,but there is no obvious rule between the change of ORP value and the removal rate of organic matter in different regions.?3?Increasing the solution ORP value in the preoxidation stage will significantly affect the types and concentrations of DBPs during the subsequent disinfection process,and the addition of different prexidants under the same water quality conditions will also have a greater impact on the DBPs formation potential;the formation potential of THMs decreases with the increase of the ORP value of the pre-oxidant,regardless of the type of preoxidant;ClO₂ can reduce the HAAs formation potential,while the other three preoxidants will cause the HAAs formation potential to increase or remain unchanged;ClO2 and Cl2 cause the formation potential of TCP decreased slightly,while the remaining two pre-oxidants had little effect on it;all four types of pre-oxidants could reduce the HANs formation potential to varying degrees,but for specific pre-oxidants,the solution pH or ORP has little effect on it.The choice of post-chlorinating agent has a significant effect on the DBPs formation potential.Compared with chlorine disinfection,the formation potential of all DBPs decreases significantly when using chloramine disinfection.This may be related to that the chloramine ORP value is lower than that of chlorine under neutral reaction conditions?pH=7?.?4?According to the experimental results,when chlorine is used as a disinfectant,ClO₂ as a pre-oxidant could better reduce the formation potential of C-DBPs,and KMnO₄ as a pre-oxidant could better reduce the formation of N-DBPs.When chloramine is used as a disinfectant,chlorine as a pre-oxidant can significantly increase the formation potential of C-DBPs by 65.5%,while ClO₂ and NH₂Cl had little influence on the formation potential of DBPs,and KMnO₄ as a pre-oxidant could reduce the formation potential of N-DBPs by 43.4%.Therefore,the combination of ClO₂/KMnO₄ preoxidation+chlorine and KMnO₄ preoxidation+chloramine can reduce the formation potential of DBPs.