Treatment Performance And Reaction Mechanisms Of A Combination Process Of Semi-Aerobic Aged Refuse Biofilter And O3-Based Advanced Oxidation Process Treating Landfill Leachate

At present,practical treatment of landfill leachate mainly adopts a combination process of pre-treatment,biological treatment,and advanced treatment.Due to the vast fluctuation of leachate quality,the complexity of organic composition,and the severe humification degree of landfill leachate,the treatment efficiency of biological process and advanced treatment process is greatly affected and negatively impacted.Here presents the obstacles that urgently need to be addressed,including(1)low treatment performance of humic substances with macro molecular weight in landfill leachate by biological process as well as the unclear understanding of microbial response characteristics when treating different types of leachates,(2)little available literatures in relation to the effects of humification degree and anions concentration on the performance of advanced process and the degradation,and(3)an in-depth understanding of organics degradation and transformation properties during the biological process and advanced treatment process,considering that refractory organics in landfill leachate are the mixed complex organics.Summing from above,this thesis proposed a combination process of semi-aerobic aged refuse biofilter(SAARB)and O₃-based advanced oxidation process(AOPs)for treating landfill leachate.The aims of this study include(1)treatment performance of different leachates,differing from the leachate age,by SAARB and the microbial response along and within the process,(2)investigation on the degradation characteristics of refractory organics in SAARB effluent arising from treating leachate(denoted as SAARB leachate)during ozonation process,(3)degradation and transformation of refractory organics in SAARB leachate by Fe⁰-enhanced O₃/H₂O₂ process(Fe⁰-O₃/H₂O₂),and(4)mechanisms of SAARB combining O₃-AOP treating landfill leachate.The main conclusions are shown as follows:(1)SAARB showed a stable performance for organic removal from mature leachate(ML),young leachate(YL),and mixed leachate(MYL)with a COD removal ranging from67%to 94%.The treatment efficiency followed by the order of YL>MYL>ML.In addition,nitrogen removal from three leachates by SAARB was also significant.The nitrogen concentration in each effluent was at the same level,and the nitrogen concentration of YL effluent was the lowest.After treating YL,MYL,and ML respectively,microbial abundance and diversity of AR changed.The abundance and diversity order were observed as YL#>MYL#>ML#.Microbial abundance and diversity were mainly affected by the relative content of refractory organics in leachates,which showed a negative relationship.In addition,results showed that,at phylum level,Proteobacteria was dominant in all four samples(AR,ML#,MYL#,and YL#),Doinococcus-Thermus was easily affected by water quality(positively related).While Chloroflexi showed a opposite trend with Doinococcus-Thermus.At genus level,microbials disperse evenly in AR sample.Thauera and other bacteria were dominant in YL#,by which aerobic denitrification is carried out consuming easily degradable organics.While Thruepera and Iodidimonas and other bacteria which can adapt to extreme environments are the dominant species in ML#.(2)The removal efficiencies of COD,UV₂₅₄,and CN from SAARB leachate by ozonation at 25 min were 39.24%,63.2%,and 87.81%,respectively.And the removal efficiencies of COD,UV₂₅₄,and CN from MBR leachate by ozonation at 25 min were 29.3%,62.14%,and 88.14%,respectively.The overall treatment efficiency of SAARB leachate featuring lower humification degree was better than that of MBR leachate.It was worthying noted that discoloration of both SAARB and MBR leachate after ozonation was observed.It would be mainly attributed to the reaction of organics with residual ozone,chemical bonds formation between organics via shared electron pairs,and recombination of functional groups,thus affecting leachate apparent color.The presence of regular anions had different impacts on humic substances removal.CO₃^2-decreased the removal efficiency from 74.69%to51.77%,and the reaction kinetic constant k decreased from 0.036 to 0.026 min^-1.HCO₃^-decreased the removal efficiency to 4.26%,and the reaction kinetic constant k changed less.SO₄^2-increased the removal from 64.55%to 76.09%,and the reaction kinetic constant k decreased from 0.036 to 0.026 min^-1.Cl^-decreased the removal from 81.26%to 64.55%,but the reaction kinetic constant k changed less.NH₄⁺-N and NO₂^-decreased the removal,and k decreased from 0.036 and 0.030 to 0.018 and 0.005 min^-1.NO₃^-had less effect on the promotion or inhibition of ozonation.The interference of anions was mainly attributed to their reaction with ozone or hydroxyl radicals,which in turn reduced the probability of organic degradation reactions.(3)Comparison experiments were carried out between single processes(Fe⁰,O₃,and H₂O₂),binary processes(Fe⁰/H₂O₂,Fe⁰/O₃,and O₃/H₂O₂)and Fe⁰ enhanced O₃-AOP(Fe⁰-O₃/H₂O₂).Results showed that Fe⁰ can greatly enhance the treatment capability of O₃/H₂O₂ process,and the CN,UV₂₅₄,and TOC removal efficiencies were 98.82%,84.30%,and 66.38%,respectively.In Fe⁰-O₃/H₂O₂ process,higher dosages of O₃ and Fe⁰ favored organics removal,H₂O₂ can promote the organics removal within certain dosage range,however,too much H₂O₂ will inhibit the removal results.In addition,the best treatment performance was observed under acidic condition.Three-dimensional excitation and emission matrix showed that SAARB leachate mainly compromise two fluorescent substances which are fulvic-like substances in ultraviolet region(Ex/Em=235-255nm/410-450 nm)and fulvic-like substances in visible light region(Ex/Em=310-360nm/370-450 nm).Fluorescent substances can be greatly destroyed by Fe⁰-O₃/H₂O₂ process at early stage of reaction,therefore reducing the condense degree.Analysis of Fe⁰ morphology and element valance changes showed that micro Fe⁰ particles used in this study remain high reactivity during the reaction.The contribution of Fe⁰ in the enhanced O₃-AOPs includes homogenous Fenton,heterogenous Fenton,and coagulation-flocculation effects.(4)Electron spray ionization(ESI)coupled Fourier Transform Ion Cyclotron Resonance(FT-ICR MS)was applied to investigate the DOM characteristics of different leachates and the transformation characteristics in O₃ and Fe⁰-O₃/H₂O₂ process.Results showed that DOM composition in ML was complex,and DOM content was significantly reduced after SAARB treatment.However,SAARB leachates mainly compromised high O unsaturated substances which are relative more resistant to bio-degrade.In addition,DOMs with different O numbers showed overall higher unsaturated degree than that in ML.Furthermore,carbon number of DOM in SAARB leachate showed a positive relationship with DBE.Being treated by O₃ and Fe⁰-O₃/H₂O₂ process,the oxidation degree of DOM overall increased and the unsaturated degree decreased.Owing to the stronger oxidation ability of Fe⁰-O₃/H₂O₂ process,reduction in the abundance of condensed aromatics and aromatics were greater.In addition,S-containing DOM showed the most significant increase in oxidation degree and decrement in the relative content.According to the functional group changes,it was found that the main reaction mechanism of the O₃ process included ring open and decarboxylation,while Fe⁰-O₃/H₂O₂ process was more drastic for the ring opening and decarboxylation of DOM.