Performance Of Biological Nitrification And Electrochemical Oxidation Pretreatment Of Landfill Leachate

Landfill leachate is a complicated organic wastewater generated in the sanitary landfilling process,which contains high concentrations of ammonia and non-degradable organic matter.It is considered to be one of the most refractory wastewaters.When landfill leachate was treated with A/O（nitrification and denitrification）process,the treatment process often face challenges associated with poor adaptability and low denitrification efficiency.In this study,the actual landfill leachate was selected as the treatment object.First,the biological nitrification performance of landfill leachate treatment was evaluated by the partial nitrification reactor operation.Then,based on the three-dimensional electrode electrochemical oxidation technology,the electrochemical oxidation pretreatment performance of landfill leachate was analyzed.Further,combined the removal of TOC and the spectral analysis of dissolved organic matter（DOM）,the degradation pathways of DOM were revealed during the three-dimensional electrode electrochemical oxidation process.Main conclusions are presented as follows:（1）The rapid start-up performance of the partial nitrification process was studied.To start up the partial nitrification reactors,four methods were applied,including controlling high concentration ammonia nitrogen,adding free ammonia（FA）,hydroxylamine or benzethonium chloride（BZC）.The comparative analysis of the partial nitrification performance with the four methods indicated that the addition of BZC effectively achieved the rapid start-up of the partial nitrification reactor.The nitrite accumulation rate reached 87%in a short period（24 days）.The relative abundance of ammonia-oxidizing bacteria Nitrosomonas increased by14.1%,and the relative abundance of nitrite-oxidizing bacteria Nitrospira was extremely low（close to 0%）.（2）The partial nitrification performance and microbial community structure were analyzed when the landfill leachate was directly treated by the partial nitrification reactor.The long-term operation performance showed that nitrogen removal was always kept at a low level.Specifically,the NH₄⁺-N removal rate was lower than 39%and the NO₂^--N accumulation rate was close to 0.Additionally,the TOC removal rate was maintained at 25%.The microbial community structure analysis and metagenomic analysis showed that the relative abundance of Nitrosospira decreased by 15.8%,and the relative abundances of denitrifying Rhodanobacter,Hyphomicrobium and Bradyrhizobium increased with the operation of the reactor.The relative abundance of Rhodanobacter increased by 27%.The abundance of amo,hao and nxr genes involved in the nitrification process decreased by 55.8%,39.4%and 79.3%,respectively.The abundance of nar,nir,nor and nos genes involved in the denitrification process increased by 1062.5%,714.3%,787.5%and 2.3%,respectively.Therefore,when landfill leachate was directly treated by the partial nitrification reactor,the nitrifying bacteria were inhibited to a certain extent,but the denitrification performance was enhanced.（3）The performance of activated carbon three-dimensional electrode for landfill leachate treatment was studied,and the degradation rules of DOM were revealed.When the voltage was 6V and the activated carbon content was 80 g/L,the activated carbon three-dimensional electrode achieved optimal performance for landfill leachate treatment.Results indicated that the TOC removal rate reached 85%and NH₄⁺-N removal rate reached 69%.During the electrochemical oxidation process,the fluorescence excitation-emission matrix-parallel factor（EEM-PARAFAC）analysis results showed that the removal rates of humic acid-like and fulvic acid-like components were up to 78%and 77%.The two-dimensional correlation spectroscopy（2DCOS）results showed that the electrochemical oxidation degradation order of different components and structures in landfill leachate followed:fulvic acid-like component/conjugated double bond>humic acid-like component/aromatic structure.（4）The ammonia removal mechanisms during electrochemical oxidation of landfill leachate and effects of landfill leachate pretreatment by electrochemical oxidation on nitrifying sludge were discussed.During the electrochemical oxidation process with the three-dimensional electrode,Cl^-was oxidized to Cl₂,and then reacted with water to produce HClO.HClO could oxidize NH₄⁺to N₂ finally,which was the main pathway of ammonia nitrogen removal.Batch test results indicated that the TOC removal rate and NH₄⁺-N removal rate were lower than 5%.The relative abundance of ammonia-oxidizing Nitrosospira and Nitrosomonas decreased by 27.6%and 5.2%,respectively.28 figures,4 tables,andreferences,and 108 references.