Application Of Fluidized-bed Fenton Technology In Industrial Wastewater Treatment

Fenton technology has been commonly applied in the pre-treatment or advanced treatment of industrial wastewater as a mature,effective and low-cost process,However,the large amount of iron sludge production in the reaction has limited its widely application.The fluidized-bed Fenton technology,enhancing the oxidation and reducing the production of iron sludge via adding carriers to Fenton system in a fluidized-bed reactor,has attracted lots attention of the researchers in many countries.However,the crystallization mechanism isn't clear enough to explain the efficiency decrease in continuous crystallization.Besides,effects of different factors on crystallization,such as temperature and properties of different carriers,have not been studied systematically.And the study of wastewater treatment with fluidized-bed Fenton technology is not available.According to the limitation of the current researches on fluidized-bed Fenton technology,this study focused on the relationship between the characteristics and the crystallization efficiencies with different carriers to comprehensively elaborate the crystallization mechanism.This paper also studied efficiencies of crystallization and wastewater treatment under different water qualities.On the basis of these researches,fluidized-bed Fenton process was applied in the wastewater treatment and was successfully finished the engineering application in the advanced treatment of papermaking wastewater.The conclusions of this paper are shown as follows.(1)The increase of temperature was proved to be effective to the crystallization in the fluidized-bed Fenton.Properties of the carrier had no influences on the iron crystallization efficiency,though it did affect the crystallization kinetics.As a result,both increasing surface area and decreasing the surface charge of the carrier could both increase the crystallization kinetics.Besides the agglomeration of FeOOH particles over the surface of the carrier based on the theory of "metastable region",the electrostatic attraction between the carrier and the FeOOH particles was also involved in the crystallization mechanism.Hence,the covered carrier inhibited iron crystallization since the electrostatic repulsion between the FeOOH particles on the carrier surface and in the solution.Although carriers with porous structure have stronger combination with the FeOOH crystal,the broken of the covered carrier speeds crystallization by increasing the total carrier surface area.Taken together,construction sand was chosen to be the carrier applied in fluidized-bed Fenton process.(2)Organic compounds or the intermediates of the oxidation would inhibit the iron crystallization via complexing with Fe3+,and the order of inhibition degree was:nitrobenzene<aniline<benzoic acid<phenol.The mineralization of organic compounds was very important for high crystallization efficiency.The influence of different inorganic anions was in order of H2PO4->NO3->Cl->SO42-.It was proved that SO42-had little of effect on iron crystallization.The effects of Cl-and H2PO4-came from the formation of complexes with Fe3+,and H2PO4-was removed with Fe3+together as a complex with the ratio of 1:1.The effect of NO3-came from the formation of HNO3 under acidic condition.Compared to the raw wastewater with the same COD concentration,the treatment efficiency of the biological treated sewage effluent was much higher.The crystallization efficiency was depended on the Fe3+saturation.The remaining organic compounds might decrease the Fe3+ saturation and consequently inhibit the iron crystallization.However,the increase of pH increases the Fe3+saturation if the remaining organic compounds kept stable.As a result,the crystallization efficiency was enhanced.(3)Fluidized-bed Fenton process achieved a better treatment efficiency than conventional Fenton in both pre-treatment and advanced treatment of wastewater.In the pre-treatment of wastewater via fluidized-bed Fenton process,the COD removal efficiency was 24.9%,and the iron reduction rate was 37.5%with the B/C of the wastewater improved from 0.112 to 0.304.The process of reduction followed with oxidation,was proved not to be capable of improving the COD degradation and B/C improvement efficiency effectively.Besides,the iron crystallization was significantly inhibited since the nitrobenzene structure was converted to aniline structure.In the advanced treatment of wastewater by fluidized-bed Fenton process,the COD removal efficiency was kept over 70%and iron reduction rate was kept over 65%.Compared with the conventional Fenton process,about 66%ferrous sulfate could be saved in fluidized-bed Fenton process to catalyze enough hydrogen peroxide.Besides,the rate constant of pseudo second order kinetics equation in fluidized-bed Fenton process was higher than that of conventional Fenton process.The COD degradation in both homogeneous and heterogeneous catalysis could be described by pseudo second order kinetics equation,but the rate constant of homogeneous catalysis was higher than that of heterogeneous catalysis.And increasing pH might lead to the increase of rate constant.The heterogeneous catalysis was considered to be the combination of homogeneous and heterogeneous process because of the iron released from the covered carrier,which may partly reform new iron hydroxide oxide agglomerated onto the solid surface.The latest fluidized-bed Fenton reactor has been successfully applied in the advanced treatment of papermaking wastewater with the capacity of 24000 t/d,and the investment cost of 216.2 yuan per ton.To ensure the effluent quality meeting the standard,the operation cost was 1.38 yuan per ton.