| Semi-aerobic landfill technology is a popular research field of waste disposal for the simple structure and faster degradation.But most researches are just in laboratory in present.To verify the applicability of semi-aerobic landfill and discuss many important thechnologies of it,three pilot-scale units(10m×10m×5m) are constructed at the Changan landfill site of Chengdu city,the leachate is recirculated in different ways,HDPE and clay are adopted as the cover in the beginning of the research,the degradation rule of leachate quality in different period,the transportation and consumption of the oxygen in the experimental units and the emission rule of landfill gas are discussed.Meanwhile,the influence factors of the contaminant transportation in balanced flow and preferential flow are discussed.Based on the research,technical parameters and theoretical basis are provided for the application of semi-aerobic landfill.The leachate quality of the experimental units are monitored,the results showed that,the leachate concentration of COD dropped from 26172 mg/L,72435 mg/L and 65324 mg/L originally to 1566mg/L,1443mg/L and 2013mg/L finally, and the removal rates are 94.0%,98.0%and 96.9%respectively.The concentration of BOD dropped from 21012mg/L,42800mg/L and 33900mg/L originally to 130mg/L,120mg/L and 140mg/L finally,the removal rates are 99.4%,99.7%and 99.6%respectively.Based on the results,it is obvious that the degradation of organic is effective in the semi-aerobic landfill experimental units.But the degradation of ammonia nitrogen is not good because of the small diameter of leachate collection pipe.The trend of COD is fitted by Excel and Lingo with a result that Lingo is more effective,so the results of predicted by Lingo are more reliable.Under the effect of dispersion,convection,adsorption and degradation,the balanced flow and preferential flow of the landfill contaminant are firstly modelled by FlexPDE.Under the balanced flow condition,with the increasing of the dispersion coefficient and the flow velocity,the contaminant concentration dropped obviously.When the dispersion coefficients are 0.5m~2/d and 0.05m~2/d,the contaminant transportation is negligible in the latter direction.When the flow velocity fell to 0.02m/d,its effect on contaminant transportation is negligible.The contaminant decaying coefficient influenced on the contaminant transportation effectively,when the decaying coefficient is greater,the contaminant dropped faster too,and when the contaminant decaying coefficient fell to 0.0008/d,the effect on contaminant transportation is negligible.From the model of preferential flow,due to the contaminant flow with the faster liquid in the mobile domain,the contaminant concentration changed faster.In the immobile domain,the contaminant concentration is effected by the degradation and first-order mass transfer,so it changed slowly.When the water content of the mobile domain is greater,the contaminant concentration in immobile domain dropped faster.And when the first-order mass transfer coefficient is greater,the contaminant concentration in the immobile domain dropped faster.Meanwhile,the contaminant concentration in the mobile domain increased accordingly.Under the condition of preferential flow,the flow velocity and dispersion coefficient is increasing obviously.When the dispersion coefficient is 5m~2/d,flow velocity is 2m/d,the maximum value of contaminant concentration in the immobile domain decreased 96.7%comparing with the dispersion coefficient is 0.5m~2/d and the flow velocity is 0.2m/d.So,the preferential flow effected the contaminant transportation obviously.The oxygen transportation and consumption in semi-aerobic landfill is tested by the experimental units,and the test data is analyzed by Surfer.Oxygen is transported in the waste mostly by dispersion.The oxygen is imported from the cover is stratifyed in the landfill,and from the upper layer to the under layer,the oxygen concentration is decreasing with the depth.But from the leachate collection pipe, oxygen could be taken in the landfill layer,so the oxygen concentration around the landfill gas collection pipe is increased.In the three experimental units,the average oxygen concentration of the upper layer,middle layer and under layer are 13.8%, 2.1%,0.6%,13.6%,2.3%,0.3%,13.8%,2.4%and 0.4%respectively.The area of waste degradation is partitioned by the oxygen concentration,in the aerobic,anoxic and anaerobic areas,the oxygen concentration is more than 2.4%,more than 0 and less than or equal to 2.4%and equal to 0 respectively.The semi-aerobic landfill gas flux is tested by static chamber method.The results showed that methane flux is closed to 0 mostly in the area around the landfill gas collection pipe,because methane is oxidized almost entirely.With the increasing distance to landfill gas collection pipe,methane flux is enhanced.Because of the difference depth of waste layer,methane and carbon dioxide flux are different.In the area with thicker layer,landfill gas yield is increased,and the flux is increased too.On the side of slopes,atmosphere is taken in deeper layer,so the aerobic area and carbon dioxide flux increased.At the same time,methane flux decreased.In the whole experiment period,methane flux is far smaller than carbon dioxide flux,the ratio of methane flux to carbon dioxide flux is almost more than 688,the maximal ratio is 204940,and the minimal ratio is 47,this is taken place in the second experimental unit in April.From the study,it is obvious that in semi-aerobic landfill,methane flux is very small. |
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