Degradation Of Refractory Organics In SAARB Effluent Of Landfill Leachate By A Fe⁰-O₃/H₂O₂ Advanced Oxidation Process

With the development of the economy,the amount of municipal solid waste generated in China is increasing daily.A large amount of leachate that contains high concentration of organics will be inevitably produced during the treatment of municipal solid waste（e.g.sanitary landfill and incineration）.Semi-aerobic aged refuse bioreactor（SAARB）,as a biological pretreatment method,is mainly filled with aged refuse which is collected from the landfill site with years of operation.The aged refuse has the advantages of large specific surface area,high porosity,good permeability and large cation exchange capacity,and the microorganisms in the aged refuse have good tolerance to the toxic substances in the leachate.Futhermore,SAARB uses the internal and external temperature difference as the power to form aerobic,anoxic and anaerobic zones.Therefore,the organic matter and nitrogen pollutants in the leachate can be efficiently removed by adsorption,biodegradation,assimilation,ammoniation,nitrification and denitrification.However,SAARB only can be a pretreatment method of landfill leachate,its effluent still contains a large amount of bio-refractory organics（e.g.humus substance）,leading to an extremely low biodegradability.Therefore,it is necessary to find further methods to deeply treat the SAARB effluent of landfill leachate.In view of this,Fenton and ozonation which have been applied in the advanced treatment of wastewater were accepted in this study to treat SAARB effluent of landfill leachate.The treatment effects of the above two methods were systematically compared,and we combined Fenton and ozonation in time and space to further degrade the refractory organics in the SAARB effluent of landfill leachate.The main research contents and conclusions are as follows:（1）The difference between the Fenton and the Ozonation process in the treatment of refractory organics in the SAARB effluent of landfill leachate was studied.Results showed that,under the optimal experimental conditions of Fenton and ozonation processes,the removal efficiencies of COD,UV₂₅₄ and CN were44.99%,66.40%,79.52%and 42.33%,60.51%,72.76%respectively.The advantage of Fenton process is that a relatively high removal efficiency of organics can be achieved in the early reaction stage,and the organics degradation in ozonation process is slower than Fenton process but in the late reaction stage ozonation process still showed a relatively high organics removal efficiency.In addition,owing to the selective oxidation of ozone to organics and Fenton mainly unselectively oxidize organics by^·OH,making Fenton process can degrade organics faster and more efficiently than ozonation process.（2）In view of that ozonation and Fenton processes each has its characteristics,ozonation and Fenton processes were conbined in time to sequentially degrade the refactory organics in the SAARB effluent of landfill leachate.Under the condition of ozone dosage of 43.68mg/min and initial pH value of 9,the UV₂₅₄ and CN removal efficiencies of effluent after treatment for 30min were 66.16%and 91.39%,respectively.After the optimal conditions of Fenton treatment,removal efficiencies of UV₂₅₄ and CN increased by 12.22%and 8.32%,respectively.Three-dimensional fluorescence spectroscopy analysis showed that the ozonation process can significantly remove the refractory humus in the SAARB effluent of landfill leachate,and the Fenton process can further improve the removal efficiency of the refractory humus,The final fulvic acid and humic acid removal efficiencies respectively reached87.38%and 81.16%,and some of the refractory organic compounds have been converted to produce tryptophan-like and tyrosine-like substances with simpler molecular structure.The TOC removal efficiency analysis showed that the ozonation process can only achieve the TOC removal efficieny of 5.88%,and the Fenton process can increase the TOC removal efficiency to 33.76%,shows a stronger organic mineralization capacity.Therefore,the combination of ozone and Fenton oxidation in time can effectively degrade the refractory organic matter in SAARB effluent of landfill leachate.（3）Fe²⁺in traditional Fenton process was replaced by Fe⁰ and combined with O₃.A Fe⁰-O₃/H₂O₂ process was constructed to realize the spatial combination of ozone and Fenton processes.The degradation of refractory organics in SAARB effluent of landfill leachate by Fe⁰-O₃/H₂O₂ process and control processes and the reaction mechanism of Fe⁰ in the system were studied.When the initial pH value was 3,the dosage of H₂O₂ was 4.0 mL/L,the dosage of Fe⁰ was 1.0 g/L and the dosage of O₃was 18.92 mg/min,the removal efficiency of UV₂₅₄ in SAARB effluent of landfill leachate after 20 minutes of reaction reached 63.55%,which was higher than that of three control systems,namely,Single-O₃（47.24%）,O₃/H₂O₂（54.74%）and Fe⁰/O₃（53.83%）.The results of three-dimensional fluorescence spectroscopy analysis showed that the removal advantages of Fe⁰-O₃/H₂O₂ process for refractory organic matter in SAARB effluent of landfill leachate were especially shown in the first two minutes of reaction.The peak values of fluorescence in ultraviolet and visible fulvic acid areas decreased by 76.21%and 60.70%,respectively.The removal efficiency of effluent of other control processes after 2 minutes was not more than 25%and the results of ultraviolet-visible spectroscopy analysis also support the conclusion of three-dimensional fluorescence spectrum analysis,and the absorbance of Fe⁰-O₃/H₂O₂process to the SAARB effluent of landfill leachate reached the maximum decreasing degree after 20 minutes of reaction.The results of total iron concentration analysis after reaction and SEM-EDS,XRD and XPS analysis of Fe⁰ before and after reaction showed that Fe⁰ was corroded to form Fe²⁺/Fe³⁺in Fe⁰-O₃/H₂O₂ process,which proves that homogeneous Fenton reaction existed in the process to degrade organics.At the same time,part of Fe⁰ was mainly converted to Fe₂O₃ and Fe₃O₄,which promoted the generation of hydroxyl radicals through heterogeneous catalysis,improving the degradation effect of organics.Generally speaking,Fe⁰-O₃/H₂O₂process realizes the combination of ozonation and Fenton processes in space,and can more effectively degrade the refractory organic matter in the SAARB effluent of landfill leachate.