Study On Washing-recovery Of Disordered Semi-aerobic Aged Refuse Bioreactor For Landfill Leachate Treatment

The landfill leachate produced in the process of harmless disposal of MSW is always a difficulty of the treatment.Due to the great existence of organic matters,ammonia nitrogen,inorganic salt,and heavy metals,the discharge of treated landfill leachate meeting the related criteria is the research hotspot.Semi-aerobic aged refuse bioreactor（SAARB）is a cost-effective and effective technique for landfill leachate treatment which utilizes aged refuse as the filter and is based on the concept of“treat waste by waste”.At present,many landfills have been closed or are close to closing,the landfill leachate shares many common features with mature leachate.Comparing to young leachate,mature leachate is characterized by high salinity,high ammonia nitrogen content,and imbalanced carbon-to-nitrogen ratio,etc.Long-term treatment/recirculation of mature leachate inevitably decreases treatment capacity and results SAARB into a disorder state.Therefore,study on the recovery of disordered SAARB after the long-term treatment or recharge high salinity and low C/N ratio leachate should be pay special attention.In addition,washing-recovery strategy featuring in wide application and cost-effective has been the research hotspot,while the application of this strategy in recovery of disordered SAARB is not comprehensively investigated.This study first applied a mixture leachate with different ratio of mature leachate and young leachate for SAARB acclimation to restore its processing capacity;Second,different operation conditions,e.g.,long practice period,high salinity,and low C/N ratio,were introduced aiming to make SAARB disorder;Third,domestic sewage,ozonation-treated mature leachate,and young leachate were used to washing-recovery strategy for disordered SAARB.This paper reveals the variation characteristics of leachate quality and microorganisms,and the results of this study can provide a feasible strategy for leachate treatment by SAARB.In acclimation stage,the removal efficiencies of color number（CN）and absorbance at254 nm(UV₂₅₄)significantly increased with higher ratio of mature leachate in the mixture leachate,and chemical oxygen demand（COD）removal efficiency stables ca.80%during the process.Three-dimensional excitation-emission（3D-EEM）indicated that micro-and macro-molecular weight aromatic substances can be efficiently removed.In addition,removal efficiency of ammonia nitrogen was higher than 90%during this process.Concentrations of NO₂^--N,NO₃^--N,and TN continued to increase,and emission flux of N₂O also increased with prolonging operation time.After acclimation,diversity of microbial community in SAARB increased,but the evenness decreased.Functional species,e.g.,organics degrader and ammonia oxidation species,can be significantly accumulated in acclimation process,including Actinomadura,Nitrosospira,Nitrosomonas,and Nitrosopumilus,etc.In long treatment period,concentrations of CN,UV₂₅₄,and COD of mature leachate（denoted as 1#）and young leachate（dented as 4#）increased with operation time,and CO₂emission flux continuously decreased.The performance reduction of 1#and 4#follow the order COD>UV₂₅₄>CN,while the removal efficiencies of CN,UV₂₅₄,and COD by 1#were overall higher than that of 4#.3D-EEM indicated that removal efficiency of humic-like substances and fulvic-like substances gradually decreased in both 1#and 4#.Nitrogen pollutants accumulated in 1#and 4#in the long treatment period.In washing-recovery process,the washing effluent can take the accumulated organic matters,nitrogen pollutants,and inorganic salts from 1#and 4#,therefore improving the removal efficiency for humic-and fulvic-like substances.Although microbial diversity of 1#and 3#in disorder-washing-recovery period was different,the washing-recovery process can effectively increase the richness,diversity,and evenness of microbial community and recover the treatment ability of SAARB.In high salinity operation condition,removal efficiencies of CN,UV₂₅₄,and COD by SAARB gradually decreased with increasing salinity.In addition,it was observed that CO₂emission flux decreased and nitrogen pollutants as well as salinity continuously increased in this period.3D-EEM showed that high salinity inhibited the removal of humic-and fulvic-like substances by SAARB.In washing-recovery process,domestic sewage as the feed influent can wash the accumulated organic matters,nitrogen pollutants,and inorganic salts out of SAARB.3D-EEM indicated that the removal efficiency of humic-and fulvic-like substances increased with longer washing-recovery period.Salinity will cause a negative effect on microbial richness,diversity,and evenness,and the stability of microorganisms will be decreased.Washing-recovery process can increase the richness,diversity and evenness of microbial community of SAARB and the stability of microbial community was improved.Moreover,functional species will be accumulated in SAARB by washing-recovery process,and thus the treatment capacity of SAARB was improved.In low C/N ratio condition,removal efficiencies of CN,UV₂₅₄,and COD by SAARB decreased with lowering C/N ratio,the CO₂ emission flux decreased,and the concentration of chloride and sulfate ions continuously increased.In addition,nitrogen pollutants concentration in leachate increased with higher salinity.3D-EEM indicated that lowering C/N ratio depressed the removal efficiency of humic-and fulvic-like substances.In washing-recovery process,domestic sewage,likewise,can wash the accumulated organic matters,nitrogen pollutants,and inorganic salts out of the disordered SAARB.Therefore,the removal of humic-and fulvic-like substances by recovered SAARB greatly increased with longer washing-recovery time.Low C/N ratio will lead to the decrease of richness,diversity,and evenness of microbial community of SAARB.The composition of microbials of disordered SAARB can be greatly improved by washing-recovery process,the key functional species,e.g.,organics degrader and nitrogen metabolic species,can be accumulated and dominant in the community.Therefore,the treatment capacity of SAARB can be greatly improved by washing-recovery process.