By-products Formation Control In Algae-containing Water Treated With Chlorine Dioxide

Eutrophication of lakes and reservoirs is an increasingly prominent problem of water environment in China.Algae blooms has many adverse effects on conventional water purification processes.Chlorine dioxide（ClO₂）has been widely concerned in drinking water treatment because of its high efficiency,broad spectrum and no organic by-products（DBPs）.However,the generation of inorganic by-products such as chlorite（ClO₂^-）and chlorate（ClO₃^-）are harmful to human health,which has become a potential problem when applying ClO₂ for drinking water treatment.Some researches on the utilization of ClO₂ for the removal of algae have been carried out,but mostly focused on the effects of the amount of dosing of ClO₂,the reaction time on the efficiency of algae removal,instead of studying on the formation of by-products.In this research,the effect of algae on the formation of ClO₂ inorganic by-products was investigated by batch test.On this basis,the process parameters of ClO₂ pre-oxidation/coagulation sedimentation were optimized.Besides,ClO₂ combined process was applied to treat different algae containing reservoirs water,which provided technical references for the control of by-products in water treatment plants.The formation regularity of ClO₂^-during the preo-xidation process for algal water was determined.The experimental results showed that the amount of ClO₂^-increased with the increase of ClO₂ dosage,the initial concentration of algae,pH and NOM concentration.Fe²⁺and Mn²⁺promoted the production of ClO₂^-,while NH₄⁺had no effect on that.In addition,the amount of ClO₂^-generated during the process of removing Cyanophyta was significantly higher than that of Chlorophyta.In addition,the ratio of Cl O₂^-converted from algae in raw water taken for about 1/3 of total ClO₂^-generation when the dosage of ClO₂ was 2.0 mg/L.In the research,we compare the effects on the conventional process of algae-containing reservoir water when applying ClO₂,KMnO₄ and NaClO for pre-oxidation.The results showed that applying ClO₂ for pre-oxidation had the best effect on the comprehensive treatment of algae-containing water.When the concentration of chlorophyll-a[ρ（Chla）]was 50μg/L,we conducted 10 min of pre-oxidation with 0.5 mg/L ClO₂,followed by the coagulation sedimentation with 10mg/L PAC,and the experimental results showed that the removal efficiency of algae,turbidity,UV₂₅₄54 and permanganate index were improved by about 30%,15%,10%,and 10%,respectively.The formation of ClO₂^-was significantly correlated with the dosage of ClO₂ used in the process of treating reservoir water.In order to control the by-product concentration within the national standard limit,the highest ClO₂ dosage during pre-oxidation should be less than 1.0 mg/L.Based on the results obtained from the study of enhancements on coagulation sedimentation with pre-oxidation of ClO₂ for reservoir water,the experiment of the whole process（pre-oxidation,coagulation,sedimentation,filtration and disinfection）were carried out for different algae-containing reservoir water.The results demonstrated that,for reservoir water with low algae concentration[ρ（Chla）=18.14μg/L],conventional ClO₂ disinfection process was feasible.For reservoir water with medium algae concentration[ρ（Chla）=51.62μg/L],there was no risk of exceeding the inorganic by-products standard when applying 0.5 mg/L ClO₂ for pre-oxidation and 0.6 mg/L ClO₂ for disinfection.For reservoir water with high algae concentration[ρ（Chla）=124.16μg/L],the ClO₂^-concentration was below 0.6 mg/L and DBPs concentrations were lower than 60%of the limit value when ClO₂-NaClO or ClO₂-KMnO₄ combination process was applied.