The Formation Characteristics Of Bromate Disinfection Byproducts In Hybrid Ozonation-coagulation System

Hybrid ozonation-coagulation（HOC）as an advanced treatment process of sewage treatment plants,has a high removal efficiency of dissolved organic matter in secondary effluent,but there are ozone oxidation and hydroxyl radical oxidation in the HOC system,which makes this system has a high risk of bromate（BrO₃^-）disinfection by-products.This paper studied the formation characteristics of BrO₃^-in HOC system under different conditions,clarified the optimal process conditions of HOC process,revealed the contribution rate of BrO₃^-formation pathway,and explored the inhibition method of BrO₃^-,in order to ensure the safety of HOC process effluent quality,a certain theoretical basis is provided.The removal efficiency of the dissolved organic matter in the secondary effluent by the HOC process can reach about 40%,which is about 20%higher than that of the single ozone oxidation process,but the generation of BrO₃^-disinfection by-products in this system is higher than that of the ozone oxidation process about 50%,resulting in increased acute toxicity of effluent.Based on this,the paper analyzed the effects of ozone dosage,p H value,coagulant type,coagulant dosage and organic matter on the formation of BrO₃^-in the HOC process.The results show that the increase of p H and the increase of ozone dosage will promote the formation of BrO₃^-,and the BrO₃^-produced by using aluminum chloride as coagulant in the HOC process is slightly higher than that when aluminum sulfate is used as the coagulant.The addition amount has no obvious effect on the formation of BrO₃^-,and the organic matter has obvious inhibitory effect on the formation of BrO₃^-.In this study,by measuring the concentration changes of Br^-,BrO₃^-and the intermediate product HOBr/OBr^-during the reaction process,the generation pathway of BrO₃^-in the HOC process was further clarified.The results show that in the early stage of the reaction,organic matter can react with the intermediate product HOBr/OBr^-,which inhibits the formation of BrO₃^-,while in the middle and later stages of the reaction,with the increase of the removal rate of organic matter,the formation rate of BrO₃^-is accelerated.Based on the above research results,it was determined that the optimal reaction conditions for the HOC process to treat the secondary effluent were p H=7,the dosage of ozone was 1.0mg O₃·mg TOC^-1,and the coagulant was aluminum chloride(15 mg Al·L^-1),and the reaction time was 8 min.Under this condition,the removal rate of organic matter could be kept above 30%,and the production of BrO₃^-was less than 10μg/L,which met the standard for drinking water（GB 5749-2006）.The generation pathways of BrO₃^-are divided into direct oxidation by ozone and indirect oxidation by hydroxyl radicals（·OH）.The contribution rate was calculated by fitting and·OH generation rate analysis.The results show that the contribution rate of ozone oxidation in the HOC process is above 70%,and as the reaction progresses,it increases rapidly and then remains stable,oxidation is the main way of BrO₃^-generation in HOC system.Based on this,the studies the formation of BrO₃^-when H₂O₂（H₂O₂:O₃=0.5,1,1.5 and 2）is added to the HOC process under different ozone dosage.The results show that when H₂O₂:O₃>1,it has inhibitory effect on BrO₃^-,and the greater the dosage,the more obvious the effect,the maximum inhibition rate reaches 60%,but the inhibitory effect of H₂O₂ will be worse when the dosage of ozone increases.The concentration changes of Br^-,BrO₃^-and HOBr/OBr^-in the aluminum chloride HOC system were compared before and after the addition of H₂O₂,and the inhibition mechanism of H₂O₂ on the formation of BrO₃^-in the HOC process was further clarified.The results show that H₂O₂ reduces the consumption rate of Br^-and reduces the contribution rate of ozone oxidation to the formation of BrO₃^-by inhibiting the formation of the intermediate product HOBr/OBr^-,thus achieving the effect of inhibiting the formation of BrO₃^-.