Transformation Of DOM Molecules During Simultaneous Denitrification And Methanogenesis For Fresh Waste Leachate With High Organic And Nitrogen Contaminants

Waste leachate with high nitrogen and organic contaminants is a typical secondary pollutant derived from municipal solid waste management system in China.As waste source classification is carried out,treatment requirement of such wastewater is going increasing.Simultaneous denitrification and methanogenesis with activated sludge system（SDM-AS）can treat fresh waste leachate as well with the concise processes.SDM-AS can sufficiently use the carbon source in the raw leachate for denitrification,and it can be robust to clogging.However,it is still a bottleneck to cope with the remnant organics in the effluent for biological treatment technologies.Additionally,dissolved organic matters（DOM）are the key components that affect bio-treatment effluent of fresh leachate and fouling of membrane system.Due to the complexity of leachate DOM and the leachate treatment system,ultra-performance liquid chromatography coupled with Orbitrap mass spectrometry（UPLC Orbitrap MS）was applied to acquire the molecular information of leachate DOM.Furthermore,a data-mining framework was established to disclose the DOM transformation during the wastewater treatment processes.These methodologies facilitate to clarifying the chemodiversity of leachate DOM,and molecular transformation of leachate DOM during the aging of landfill.The DOM transformation was clarified in a continuous SDM-AS reactor in terms of thermodynamics.Aerobic bio-derived DOM may be degraded in the SDM unit.It further was confirmed by quantitive calculation in the sequencing batches of DOM degradation processes in terms of dynamics.The main results were as following:（1）Based on the formulae inferenced from molecular information,molecular indexes derived from formulae can reflect the part of structural information of DOM.The DOM classification based on van Krevelen diagram was summerized.According to the number of samples,several samples collected from a wastewater treatment process would be recommended to apply a generalized classification mode based on 3-set Venn diagram,which was firstly proposed to distinguish the native and derived DOM.Whereas,correlation analysis is suitable a time and spatial series of samples.Such correlation analysis can be expanded by machine learning.（2）The present study acquired the molecular information for DOM in 22 waste leachate samples using UPLC Orbitrap MS.These molecular characteristics of raw leachate were determined to compensate for the evolution of leachate with waste age.Unique mass peaks occupied more than 20%of the detected mass peaks in each leachate,implying that the molecular information for DOM could be the fingerprint of waste landfills and storage pits.The double-bond equivalent increased and the H/C decreased with waste age.We further found that 57 precursor ion peaks and artificial matter（confirmed as N-butylbenzenesulfonamide）were significantly correlated with waste age by multiple test and posteriori non-target screening.（3）Organic loading rate and nitrogen loading rate of SDM reactor reached up to6.70 kg COD/（m³·d）and 0.19 kg NO₃^--N/（m³·d）after 440-days operation,respectively.Chemical oxygen demand（COD）of raw fresh leachate from waste-to-energy plant was reduced to 854±120 mg/L from 63000±470 mg/L,corresponding the COD removal rate of 98.6%.Total nitrogen（TN）decreased from 2500±647 mg/L to 404±75 mg/L,corresponding the TN removal rate of 83.7%.BOD5 decreased to 2 mg/L.440-days steady operation confirmed the satisfying performance of SDM-AS processes for the treatment of waste leachate with high nitrogen and organic contaminants.（4）The generalized classification mode based on 3-set Venn diagram was conducted to differentiate 7 kinds of native and bio-derived DOM in the SDM-AS reactor.Persistence of these DOM were quantificationally determined.Bio-derived DOM contributed the 76-84%of effluent DOM which is predominated by DOM category derived from AS unit.SDM unit can remove 25-32%of bio-derived DOM by the recirculation of NO₃^-/NO₂^-but produce bio-derived DOM occupying 36-52%of SDM digestate in terms of abundance.A hyperbolical and dual relationship can theoretically explain the bio-derived DOM production and removal behaviour in SDM and AS units at the molecular level.Enhancing NO₃^-/NO₂^-circulation ratio may facilitate the removal bio-derived DOM with diminishing marginal benefit.（5）Inert native DOM accounting for 16-24%effluent DOM and partly confirmed as anthropogenic micropollutants by posteriori non-target screening,e.g.phosphate flame retardants,anticorrosive,paint,adhesive and industrial agents,were at levels of5-200μg/L.SDM-AS processes hardly can remove and degrade such compounds.The subsequent treatment processes should put the remnant anthropogenic micropollutants in concern from the perspective of environmental health.（6）The anaerobic degradation of aerobically stabilized DOM was investigated with DOM substrate isolated through electrodialysis.15%of aerobic bio-derived DOM can be degraded in terms of TOC.Nearly 30%of DOM mass features were completely diminished.Majority were transformed to new intermediate metabolites,which were reactively labile but treatability refractory compounds.10%of mass features with high abundance were persistent during the degradation process.Reactivity continuum existed among these mass features based on machine learning technologies.Weighted average degradation rate of the mass features was-0.0035～-0.0027 d^-1,further confirming degradation of aerobic bio-derived DOM at the molecular level.（7）Network between DOM mass features and amplicon sequence variants can pick up the active microbes.Six kinds of genus were identified to be responsible for refractory DOM degradation.They were Lysobacter,Parvibaculum,SWB02,unclassified family of PHOS-HE36 and Caldilineaceae,and classified order of SBR1031.It indicated that methanogenesis of labile compounds and anaerobic degradation of aerobic bio-derived DOM could be the independently parallel reactions in the SDM processes.