Investigation Of Landfill Gas Diffusionin Soil Cover And Assessment Of Odour Gas Emission

Landfill gas is generated from the biodegradion of the landfilled municipal solid wastes(MSWs).The main components of the landfill gases are greenhouse gases including methane(CH4)and carbon dioxide(CO2).The main odour gases are hydrogen sulfide(H2S)and ammonia(NH3).LFGs also contain a small amount of volatile organic compounds such as alkanes and halogenated compounds.The landfill cover system composed of vegetation,soil impermeable layer and geomembrane is an important barrier to control gas migration.The service environment of it is complex under the combined action of bottom landfill gas pressure,temperature and atmospheric environment.The cover system will be damaged under these complex environmental loads.Therefore,it is necessary to investigate gas transport in the landfill cover and assess the emissions from the landfill cover under these conditions.Laboratory tests were conducted first to investigate the diffusion mechanisms of gas transport in loess.The effects of different water content and density on gas diffusion through loess were considered.Anumerical simulation about gas-water two-phase diffusion model was developed to simulate landfill gas transport in cover soils considering the effect of adsorption of water from vegetation root systems.The mathematical model wassolved by the finite element analysis software COMSOL Multiphysics 5.0.Methane diffusion obtained from laboratory tests was used in this numerical model to study the gas transport between the bare soil cover and the vegetation cover within 0.5m depths.The static chamber technique was used in field experiments to evaluate methane(CH4)and ammonia(NH3)emission fluxes.The results of CH4 emission obtained from field monitoring were compared to those obtainedfrom the California Landfill Methane Inventory Model(CALMIM).The CH4 fluxes were evaluated in different time scales considering the effects ofvegetation covers and gases collections by the CALMIM model.Theobtained results from the GAUSSIAN model and the CALPUFF modelregardingNH3emission were also compared.Finally,comprehensive evaluation was carried out through the results of the laboratory tests,numerical simulations,field experiments and model simulations.The main conclusions are shown as follows:1.With the different water content and density,the orders of magnitude on diffusion coefficient of methane in loess ranges from 10-9cm2/sto 10-6cm2/s.The greater water content of loess,the smaller the diffusion coefficientin soil column when the density is constant.The higher the density,the smaller the diffusion coefficient in the loess when the water content is constant.The diffusion coefficient for the case with water content 5%is 2 orders of magnitudes greater than that of 20%.When the water content is 5%,the diffusion coefficient is the largest.2.The vegetation on top covers has great effect on the LFG transport in soil cover.For the gas concentrations in the soils considering the four kinds of root systems were all lower than those of the bare cover.Soil gases decrease with the increase of soil depth when the amount of rainfall increases.The larger the rainfall,the smaller the effect on the cumulative fluxes.That means it is appropriate to select root length with 30cm and the reduction of the soil gases are more obvious within 0-30cm.3.The results of methane fluxes showed CH4 fluxes ranged from 4.9 g/m2/d to 126.43 g/m2/d.In Jiangcungou landfill,the CALMIM model showed that CH4 fluxes reached maximum during 13:00-18:00 and minimum at 00:00-03:00 in April.Compared to the results of field experiments with the static chambers,the CALMIM Model might overestimate the CH4 emissions.CH4 fluxes in 2015 were at peak in March(88.14 gm-2day-1)and at low in September(25.12 gm-2day-1),respectively.For the vegetation covers,the CH4 fluxes for the cases with 90%coverage was 2.81 times lower than that with no vegetation coverage in September and 3.57 times in November.For the gas recovery systems,CH4 fluxes for the case with 0%coverage is 1.34 times greater than the case with 90%coverage.4.The static chamber was used to monitor the fluxes of ammonia emitted on amunicipal solid waste landfill in Zhejiang.The results show that the maximum andminimum were observed in April 2016(0.775mg/m2/s)and January 2017(0.239mg/m2/s),respectively.The ammonia flux observed in August 2016 can be 4 times greater than that obtained in January 2017 due to the significant difference in temperature.The results of the Calpuff model indicate that the diffusion distance increases with the increase of ammonia source intensity.The maximum concentration of ammonia was 300-600 m away from the working face.The results of the Gaussian model also indicate that ammonia diffusion tends to happen under the slightly stable conditions(e.g.,grade E)because of the lower lateral and vertical dispersion parameters.The ammonia diffusion distance decreases with the increase of wind speed.The results of the impact distance evaluated by Calpuff model is 1.16-1.69 times greater than that obtained by the Gaussian model under the condition of stability grade E.The Gaussian model may underestimate ammonia diffusion distance.5.The results of the laboratory tests,numerical simulations,field experiments and model simulations for odour gas control are useful.Triangular and exponential shapes with root depths are 30cm are recommended when selecting vegetation cover with shallow root systems.It is necessary to sprinkle water for the arid and semi-aridareas(<2mm/d)to maintain a certain soil moisture(>5%).A high gases recovery system and vegetation cover is good for gases reduction,such as the vegetation coverage was 90%,the gas recovery was 50%and the thickness of the cover material(Silty clay)is 115cm(section 4.3.4).Gases emissions were at maximum at 13:00-18:00 in a day because of high temperature,strengthen management is needed at this period.The effect of wind speed on gas diffusion is obvious.Knowing about wind speed in advance when dumping and improving the worker’s covering efficiency if the wind speed is low.