Performance And Mechanism Study Of The Degradation Of Refractory Organics In Landfill Leachate By Ozonation Process

With the rapid expansion of urbanization and the improvement of living standards,the amount of municipal solid waste is growing quickly.The conventional biological treatment method is not able to completely remove the refractory organic-matters,such as humic-like substances,etc.In consideration that the division of humic-like substances based on water pH varies a lot in molecular structure.The oxidation efficiency,transformation characteristic and degradation mechanism of ozone process thus differs significantly.In this study,several refractory organic-matters contained in landfill leachate are degraded by ozone process,and the oxidation efficiency,transformation characteristic and degradation mechanism of ozone process are systematically investigated.The main of our work and conclusions are as follows:（1）Firstly,mature and young landfill leachate are used in this study to compare the treatment efficiency in ozone process.Results show that the treatment efficiency on mature and young landfill leachate in ozone process is quite different.The COD,UV₂₅₄and color number（CN）removal effect is 34.72%,58.82%,76.66%（mature）and57.32%m 76.44%,and 84.81%（young）,respectively after ozone treatment for 20 min.Therefore,removal efficiency of dissolved organic matter（DOM）in young landfill leachate is much higher than that in mature landfill leachate under same treatment conditions.It can be explained that the constitution of DOM in young landfill leachate is comparatively simple,the macro-molecular-weight organic substances in mature landfill leachate is more and additionally the molecular structure and humification degree is more complex.However,the molecular structure of DOM in mature landfill leachate can be simplified,thus the biodegradability of wastewater is improved significantly.（2）Concentrated leachate is a typical recalcitrant organic wastewater which is difficult to treat.Coagulation method is used to pre-treatment of concentrated leachate.The treatment efficiency of wastewater which is difficult to coagulate is systematically investigated.Results show that the COD,UV₂₅₄ and CN removal effect is 41.32%,68.82,and 83.50%,respectively,under conditions of ozone dose of 0.5 L/min and reaction time of 30 min.In addition,content of organic substances with molecular weight<1 kDa increases with ozone dose increasing;the remove on organic substances with molecular weight>100 kDa is significant,and biodegradability is greatly improved.UV-Vis shows that the content,humification degree and complex degree both are decreased with increasing ozone dose.Synchronous fluorescence spectrometry shows that protein-and fulvic-like substances in concentrated leachate which is difficult to coagulate are synchronous removed as organic substances in concentrated leachate are being treated by ozone process.Thus,coagulation combined ozone（coagulation-ozone）process is able to remove almost all the refractory organic-matters,favoring sequential biological treatment.（3）Due to that the division of humic-like substances based on water pH varies a lot in molecular structure and solubility,and its constitution is quite complex.Under conditions of initial pH of 8,removal efficiency in ozone process shows the trend as pH11>pH9>pH3>pH5>pH7.FTIR spectra shows that molecular structure is different under different pH.UV-Vis and 3D-EEM show that DOM in wastewater has a lower aromatic degree and a higher molecular weight,especially when the ambient is more acidic and/or more alkaline,thus the remove efficiency increases.（4）Ozone utilization rate show a decrease trend when concentrated leachate is being treated by ozone process with a longer reaction time.However,H₂O₂ can increase the ozone utilization rate by producing hydroxyl radical.The COD,UV₂₅₄,and CN removal effect both increase and then tends to be flat in the latter stage of reaction.The biodegradability of effluent increases to 0.43 owing to the synergetic effect of ozone and H₂O₂.Proportion of molecular weight in regions（>100,50–100,10–50,and 3–10kDa）both significantly decreases,while the proportion of molecular weight in regions（1–3 and<1 kDa）drastically increases.Ozone can gradually degrade DOM in the wastewater,and functional group of humic substance,such as Ar-O and O-H,etc.,is alleviated and/or even eliminated.After adding a certain dose of H₂O₂,remove effect of ozone on benzene-ring compounds is greatly increased.（5）To compare ozone process and Fenton process.Microwave-Fe⁰/H₂O₂ process is set up to the treatment of concentrated leachate.MW-Fe⁰/H₂O₂ process and ozone/hydrogen peroxide process both reveals a great improvement of biodegradability which is higher than 0.3,favoring sequential biological treatment.However,MW-Fe⁰/H₂O₂ process takes a shorter reaction time and shows a better treatment efficiency,while it will produce a large amount of iron-based sludge.Herein,Ec-（ozone-hydrogen peroxide）is 124.18 kWh/m³,while Ec-（MW-Fe⁰/H₂O₂）is 1287.42 kWh/m³,thus energy consumption of ozone/hydrogen peroxide process is much lower than that of Fenton-like process.Overall,the treatment methods including ozone/hydrogen peroxide and MW-Fe⁰/H₂O₂ process can be used to the practical project according to the local circumstances and water quality and quantity.