Monitoring Method And Emission Mechanisms Of Landfill Gas In Municipal Solid Waste Landfill

A large amount of landfill gas(LFG)is produced in the degradation process of municipal solid waste.The main components of landfill gas are greenhouse gases including methane and carbon dioxide,and malodorous gases mainly consisting of ammonia and hydrogen sulfide.It also contains a small amount of other harmful volatile organic gases.The escape of landfill gas aggravates the greenhouse effect of the atmosphere.In addition,the health of the residents in and around the landfill site will be greatly influenced by the harmful LFG.In order to control the gas emission during landfill and stabilization process,the daily,temporary and final cover were used to minimize the exposed area,increase the gas collection rate and reduce the release of landfill gas.Due to the difficulties and limitations of on-site monitoring,there is a lack of research on long-term monitoring of landfill gas produced by high kitchen wastes in the world.The mechanisms of the landfill gas migration from the three major sources,such as working area,temporary cover area and final covered area are not well revealed.This leads to frequent air pollution problems induced by LFG emission.In this thesis,a long-term monitoring method for measuring the odorous gas and greenhouse gas concentration and release intensity was proposed,considering the law of degrading process high kitchen waste in China.Based on the multi-component gas migration model,a static chamber measurement error correction method was proposed.The influences of the chamber height,the bottom flux and degree of saturation of soil cover on the test error were analyzed.A long-term monitoring method of methane concentration based on portable laser methane detector combined with GPS was proposed.The emission mechanisms of methane in these two different covering materials were compared.Combined with the release intensity of landfill gas in the landfill working area,the temporary covering area and the final covering area,the Lagrangian particle model based atmospheric diffusion software Calpuff was used to evaluate the odor dispersion of a landfill site in Hangzhou.The following main conclusions have been drawn:1)A multi-component gas migration model was developed for error estimation of the static chamber method.It was found that the key parameter affecting the measurement error of static chamber is the height of the box.For a static chamber with 12 cm high and 20cm in diameter,the measurement error of CH4 and CO2 emission rate could rise to 28.6%and 26.5%,after 20min,with the flux of 6×10-5mol/m2/min,separately.When the chamber height increased to 55cm,the error dropped to 9.1%and 8.6%in the same calculating period,which were 68.1%and 67.5%lower than the previous results.When the flux increases by an order of magnitude,the error in the same calculating time increased by 30.3%and 37.3%,which indicates that the flux has less influence on the measurement error than the size factor.The result shows that the higher degree of saturation of cover soil is helpful to reduce the measurement error.According to these conclusions,the revised static chamber size was set to 55cm in height and 50cm in diameter.The total measurement time is 20min.Combined with the modified test process,the error could be controlled within 10%.2)A one-year field methane monitoring campaign was carred out on the working,temporary and final cover area of a loess-covered landfill.The measured methane emission flux on the working area was 0.004-0.0 1 g/m2/min,which was close to the test results of other sites in China.The emission flux varied greatly on the surface of temporary cover area,ranging from 0.0032-0.796g/m2/min.It indicated that the gas release preferential channel exists on this area.The flux ranged from 6.9×10-6 g/m2/min to 0.019/m2/min on the surface of final cover area.The age of wastes in this area was more than two years.The gas production always last for less than 2 years for the high organic content wastes.The variation of flux was positively correlated with ambient temperature,whereas negatively with humidity,wind speed and air pressure.The results show that the difference of methane concentration on the site can be 5 orders of magnitude.The concentration of 75%monitoring points is lower than the upper limit(500ppm).The concentration variation coefficient of methane is 104.3-502.8%,indicating the high spatial heterogeneity of methane concentration distribution.There was a relatively strong negative correlation between barometric pressure and the exceeding-standard areas,with the correlation coefficient of-0.743(p<0.1),whereas a relatively positive correlation between air humidity and the exceeding-standard areas,with the correlation coefficient of 0.7 8(p<0.1).Ambient pressure interacts with that inside the landfill.The pressure gradient between the surface and the bottom of the cover soil decreases with the increase of ambient pressure.This resulted in the decrease of gas convective flux;The variation of air humidity can approximately reflect the moisture change in the soil.Before the moisture content reaches 30%,the increment of moisture content would promote the methane oxidation that could reduce the methane emission.With the further increase of moisture content,the oxidation process could be inhibited;There was a significant correlation between the air temperature and the soil temperature.The gas emission rate could be controlled by the gas generation rate,which is dependent on the temperature.When the methane transport is dominated by advection in the cracks,the factors that change quickly such as wind velocity play more important roles in gas emissions.The associated near surface pressure fluctuations which induced by the wind could restrain the gas emission.The methane concentration on the final cover is much lower than that on the temporary cover,which is related to the high kitchen content waste in our landfill.More than 90%of the gas generation potentials will be released in the first two years after the waste was landfilled.In the case of membrane damage,the methane concentration in the area covered with HDPE geomembrane is 28.7%higher than that in the area not covered with HDPE geomembrane.These results demonstrate the importance of ensuring the integrity of the geomembrane.A strong linear correction between the methane concentration and emission rate was found on the loess-covered surface.The connection was detected by the simultaneous monitoring of methane concentration and flux on some measurement points.On this basis,methane flux at one location could be estimated approximately by concentration measurement on the corresponding point.3)A three-year long monitoring of methane concentration on the membrane was launched at a landfill site in Hangzhou,where HDPE is the main temporary covering material.The measured methane concentration showed a high spatial variability and positive correction with the ambient temperature.The geomembranes showed huge superiority over the loess in its capability of methane emission control.The median concentration above the geomembrane was 61.7%lower than that on the loess surface.The exceeding-standard rate of methane for loess cover was up to 34.3%,whereas the rate for geomembrane was up to 22.9%.The high methane concentration area could easily form on the locations with geomembrane defects.The maximum detected methane concentration reached 53633ppm on HDPE surface,which exceeded the its explosion limit.Such areas were not detected on the loess cover,but 3 points were detected on the geomembrane.The frequency of defects on HDPE membrane was 2.1/ha,which mainly distributed in the area with the waste age of 6-9 months.24 defects were randomly found on the membrane.The equivalent radius was 0.9 cm-6.1 cm.The variation of hydrogen sulfide rates reaches 3 orders of magnitude and the maximum value is 17.308 mg/m2/min.The correlation analysis showed that the flux of hydrogen sulfide was positively correlated with the defect size and flow rate.The correlation coefficient was 0.447(p<0.1)and 0.798(p<0.1),respectively.The correlation between the flux of methane and the defect size and flux of hydrogen sulfide were only 0.25 and 0.132,respectively.4)The gas transmissivity of Geomembrane/Waste interface on the HDPE geomembrane defect was inversed by a two-dimensional model proposed by Bouazza et al.(2008).The in-situ measured parameters including waste thickness,bottom pressure and permeability coefficient in the unsaturated area were considered in the calculation.The results varied from 7×10-6 m2/s to 2.4×10-4 m2/s.For the same defect,the released amount of hydrogen sulfide is positively correlated with the bottom pressure,the gas released under 1 OkPa were 1.5 times and 3.2 times greater than that under 6.8kPa and 3.2 kPa,respectively.The emission rate decreased with the increase of hole radius because of the higher growth rate of hole size than that of total released amount.The variation of waste permeability coefficient has a more obvious effect on the hydrogen sulfide emission rate at the higher bottom pressure.Compared with the permeability coefficient,the effect of bottom pressure is more obvious.When the bottom pressure and permeability coefficient increased by 1 order of magnitude,the hydrogen sulfide release from the holes increased by 9.4 times and 1.03 times,respectively.5)The results of the long-term monitoring on the working area,temporary cover area and final area of a landfill in Hangzhou showed that the variation of emission rate of H2S on the working area was 0.003 mg/m2/min to 0.98mg/m2/min,and the average flux was positively correlated with the ambient temperature.In the summer of 2017,the flux reached a maximum of 0.2502mg/m2/min throughout the monitoring cycle.The emission amount of H2S on the working area varied between 0.125kg/h and 1.09kg/h.Considering the distribution frequency and radius of the holes on HDPE,the emission amount of H2S on the temporary cover varied between 9.2×10-hand 6.8×10-4kg/h.There is negligible flux of hydrogen sulfide released from the final cover.The contribution rate of the working area and the temporary cover area for hydrogen sulfide release was 90.79%-98.588%,and 0.0008%-0.52%,respectively.The contribution of final cover area was negligible.6)The emission rate of H2S and ammonia were measured from sources on the landfill and analyzed by Calpuff model.H2S is recognized as the leading component which caused the odor pollution on the landfill,whereas the impact of ammonia cannot cross the landfill boundary.Further evaluation showed that wind velocity and gas flux variations are critical factors to affect the odor dispersion.Meanwhile,the relatively low gas collection rate is another important factor.The gas collection rate is only 58%in a landfill in Hangzhou,which indicated that the volume of uncollected gas was more than 1.1 ×108/m3.In order to keep the odorous influence area within landfill site,proper engineering measures should be taken to ensure the emission rate of H2S decreases to the 15%of its original value,such as increase the gas collection rate.It is of great importance to reduce the hydrogen sulfide generation in the landfill gas problem which needs to be solved...