The Characteristics And Mechanisms Of Greenhouse Gas Emissions From Simulated Bioreator Landfills

With the rapid development of economy and the vigorous advancement of urbanization in china,the production of municipal solid waste?MSW?has been increasing.At present,more than 60%of MSW are disposed by landfills,but landfills are one of the major anthropogenic sources of greenhouse gases in China.Therefore,it is particularly necessary to reduce greenhouse gas emissions from landfills.The bioreactor landfill with leachate recirculation as the main measure has the advantages of accelerating the speed and efficiency to degradate and transform MSW,effectively reducing the volume of leachate and the concentration of leachate pollutants,realizing the rapid stabilization of landfills,etc.However,after the leachate recirculation,the carbon-nitrogen matrix and water content distribution of the landfill system will change,then,the greenhouse gas emission characteristics will also have a great change.Studies of CH₄ or N₂O emissions from anaerobic or semi-aerobic bioreactor landfills have also shown that semi-aerobic landfills can reduce CH₄ emissions but have an effect of increasing N₂O and CO₂ emissions.Until now,studies on the total emission characteristics of three types of greenhouse gases,including CH₄,N₂O and CO₂,have been rare in different types of bioreactor landfills.Especially the study of total emission characteristics,influencing factors,and microbiological mechanisms for semi-aerobic bioreactor landfills greenhouse gases under different conditions is rarely involved.Therefore,the characteristics and mechanism of total greenhouse gas emissions from bioreactor landfills are researched by laboratory simulation tests,high-throughput sequencing technologies,mathematical statistics,and comprehensive analysis.The research results can provide a theoretical basis for the control of greenhouse gases emitted from landfills,and it also contributes to the deepening and expansion of landfill technology,which has important practical value.Through the construction of anaerobic,semi-aerobic and aerobic bioreactor landfill simulated columns?ANBL,SABL,and ABL?,the leachate quality and greenhouse gas?CH₄,N₂O and CO₂?emissions were monitored based on static-chamber gas chromatography.The results found that,compared with ANBL,the stabilization process of SABL and ABL was faster and the denitrification effect was better,the removal rates of COD_Cr in leachate were46.99%,97.69%and 97.64%,respectively,and the removal rates of TN in leachate were9.42%,88.01%and 70.25%,respectively.And there were mainly 3 substances in the leachate DOM,which are high-excitation-wavelength tryptophan,ultraviolet fulvic acid and visible fulvic acid.The total greenhouse gas emissions?calculated by CO₂ equivalents?of SABL,ABL,and ANBL are 15630.07,12113.82,and 6051.92g,respectively.The minimum greenhouse gas emission of ANBL was due to the fact that it has not reached the stage of methanogenesis.The contribution ratio of CH₄,CO₂,and N₂O to total greenhouse gas emissions was 40.75%⁶7.56,23.64%⁵0.71%and 1.68%⁸.79%,respectively,so it can be inferred that total greenhouse gas emissions from ANBL will increase significantly when it reached the methanogenesis stage.SABL was a suitable technology for greenhouse gas emission reduction which considering the stabilization process,denitrification efficiency,economy,and total greenhouse gas emissions.The SABL simulation columns with different diameters of vent-pipe?25,50,75mm??SABL1#,SABL2#?5#?,SABL3#?and recirculation frequencies?1 time in 5 days,1 time in3 days,1 time in 1 day??SABL4#?SABL5#?2#?and SABL6#)were constructed.The results showed that the increase in the diameter of vent-pipe was beneficial to the stabilization process of SABL.Methane was the main source of greenhouse gas emissions from SABLs?the ratio was 71.42%⁷9.17%?,followed by CO₂?the ratio was 14.06%²1.90%?,and the contribution of N₂O was the smallest?the ratio was 6.77%¹2.60%?.In addition,after the landfill operation for 465d,the N₂O emission was SABL1#>SABL6#>SABL2#?5#?>SABL4#>SABL3#,the CH₄ emission was SABL6#>SABL2#?5#?>SABL1#>SABL4#>SABL3#,the CO₂ emission was SABL6#>SABL4#>SABL3#>SABL1#>SABL2#?5#?,and the total greenhouse gas emission was SABL6#>SABL2#?5#?>SABL1#>SABL4#>SABL3#,whose values were 175517.27,144969.20,141719.76,129283.78,and 122766.91g,respectively.Correlation analysis was conducted by the greenhouse gas emission flux and the leachate indexes of SABL1#⁶#,the result showed that the factors affecting the greenhouse gas emissions are different at different landfilling stage.In the first and second stages?landfill 0-86d?,the impact factors of N₂O emission flux from SABL1#³#were landfill temperature and NH₄⁺-N concentration,and the N₂O emission flux from SABL2#and SABL3#were positively related to the NO₃^--N concentration.The impact factor of N₂O emission flux from SABL4#⁶#was the NO₃^--N concentration,and the N₂O emission flux from SABL4#and SABL5#were positively related to the NH₄⁺-N concentration.But there was no significant correlation between N₂O emission flux and COD_Cr,TN concentration and CO₂ emission flux from SABLs.In the third stage?landfill 87-234d?,the impact factor of N₂O emission flux from SABL1#³#was the NH₄⁺-N concentration,and SABL3#can reduce the N₂O emission by regulating the concentration of COD_Cr,VFA,TN,NH₄⁺-N and NO₃^--N in leachate.In the fourth stage?landfill 235-465d?,the N₂O emission fluxes of SABL1#⁶#were very small,and the correlation with the leachate index was low.In addition,the CH₄ emission fluxes of SABL1#⁶#were positively correlated with the CO₂emission flux at the 0.01 level,and the CO₂ emission flux was positively correlated with pH and temperature.Moreover,the CH₄ and CO₂ emission fluxes of SABL1#⁶#were negatively correlated with the concentration of COD_Cr and VFA in leachate.High-throughput sequencing technology was used to dissect the community structure and relative abundance of waste samples from the initial stable stage of SABL1#⁶#.It was found that SABL samples all had high microbial diversity,and the phylums were similar?32common phylums?.SABL could produce a unique phylum when the diameter of vent-pipe changed,but this phenomenon did not occur when the recirculation frequency changed.In addition,the dominant phylums of SABL1#⁶#were Proteobacterica and Bacteroidetes.The relative abundances of Proteobacteria were SABL3#?50.25%?>SABL2#?5#??37.56%?>SABL4#?36.86%?>SABL6#?36.02%?>SABL1#?27.44%?.The relative abundance of Bacteroidetes were SABL1#?37.03%?>SABL3#?25.64%?>SABL4#?24.35%?>SABL6#?22.15%?>SABL2#?5#??16.31%?.Moreover,Firmicutes was also one of the main bacteriaes.The dominant genuses of SABL3#and 6#samples were both the methylomicrobium species in?-Proteobacteria?the relative abundances were 13.61%and12.74%,respectively?,which belonged to type I methane oxidizing bacteria,that can facilitates the methane reduction.The dominant genus of SABL1#sample was the vadinBC27-wastewater-sludge-group genus?the relative abundance was 12.90%?,which function was degrade macromolecular organisms,and indicating SABL1#degraded slowly and its greenhouse gas emission time is long.However,there was no obvious dominant genus in SABL2#?5#?and SABL4#samples.