As far as ascertaining the optimum aeration mode for the AALM is concerned, different aeration modes were tried in Lys. A and B. The duration that each aeration mode was executed in Lys. A and B ranged from 273 days to 464 days. The different influences of aeration modes on degradation of contaminants can be convincingly illustrated by Stage
I. However, the influences of other aeration modes on landfill stabilization cannot be fully observed due to the properties of the solid waste that was already different at the beginning of Stage II. In order to better reveal how different aeration modes affect the degradation of contaminants in landfilled solid waste, a one–dimensional landfill model was established and used to estimate the effects of the different aeration modes.
Fig. 5-2 shows a comparison of the results of the numerical simulation for Lys. B with experimental data for the organic carbon and nitrogen in leachate from different depths. As shown in the figure, the one–dimensional landfill model can basically simulate contaminant degradation trends under various aeration modes. This calibrated model was used to estimate the influence of various aeration modes on contaminant degradation.
5.4.2 Simulation on the influences of aeration modes on discharged leachate quality In this section, a total of 7 cases, including the control group, which were top, middle, and bottom (0.5, 1.5, and 2.5 m) with low (0.5 L/ min) and high (1.0 L/ min) air injection rates were simulated for over 1000 days. Multiple air injection depths were not considered due to the difficulty of their implementation in a real landfill. Moreover, the influence of variations of precipitation on contaminant degradation was not considered either, due to its significant influence under some cases, as mentioned before. Instead,
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0 500 1000
1500 TOC
TOC (mg/L)
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Sim.
1.75 m 3.00 m
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Exp.
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NH4+-N
Elapsed time (days)
Nitrogen (mg/L)
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NO2--N
Elapsed time (days)
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NO3 --N
Elapsed time (days)
Fig. 5-2. Comparison of numerical simulation and experimental data.
average daily precipitation of 2.4 mm/day was used during the simulations. Furthermore, the influence of temperatures on contaminant degradation was also estimated during the simulations. The temperature profile adopted was exactly the same as with the ambient temperature (as shown in Fig. 2-3 of Chapter 2).
Fig. 5-3 shows the results of the simulation of the contaminant degradation under various aeration modes. Similar to the experimental results, bottom aeration was the most effective aeration mode for organic carbon degradation. On the other hand, middle and top aeration brought moderate and lesser influences on organic carbon, respectively.
This was attributed to anaerobic conditions that still existed near the leachate collection pipe. This drawback will be aggravated especially under mesothermal conditions (as shown in the TOC evolution figure). Precipitation appears to have a lesser influence (wash–out effect) on organic carbon degradation since the degradation trends were similar to the experimental results obtained under varying precipitation conditions. This is probably due to the hydrophobic property of the organic matter.
The influence of aeration modes on nitrogen evolution was similar to the experimental results. Bottom aeration was the most effective aeration mode for nitrogen removal considering the long term effects. However, unexpectedly, sequenced nitrification and denitrification was not observed under the case of bottom aeration with a low air injection rate. It is probably the real situation when this aeration mode was employed at the beginning of experiment (organic carbon is abundant). Therefore, further experiments are required to verify this suspicion. It is also important to note that the air injection rate appeared to have lesser influence on contaminant degradation compared with the air injection depth. This might be due to the low diffusion rate of oxygen in the waste matrix. Therefore, it is hard for oxygen to reach the area below the air injection pipe. For area where air can penetrate, oxygen supplementation appeared to be sufficient for aerobic activity even under the low air injection rate (0.5 L/min) condition. However, the situations might be different if larger diffusion rate of oxygen was adopted. Regardless, high pressure aeration has a positive effect on landfill stabilization (Ritzkowski and Stegmann, 2012).
5.4.3 Simulation on the influences of aeration modes on carbon mass balance and green house gases emission
Fig. 5-4 shows the numerical simulation results on the integrated influences of aeration modes on carbon mass balance and green house gases (GHGs) emission for 5 years. As shown in Fig. 5-4, totally around 10% of solid organic matter was decomposed under no air injection condition, in which 3.2% was reduced through leachate discharged, while
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TOC
TOC concentratioin (mg/L)
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T-N concentration (mg/L)
T-N
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NH4+-N
Elapsed time (days)
Concentratioin (mg/L)
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Control
Top(0.5 L/min) Top (1.0 L/min) Mid.(0.5 L/min) Mid.(1.0 L/min) Bot. (0.5 L/min) Bot.(1.0 L/min)
NO2 --N
Elapsed time (days)
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NO3--N
Elapsed time (days)
Fig. 5-3. Simulation of the degradation of contaminants under various aeration modes.
6.8% through gasification. Top aeration with low or high air flow rate brought less influence on organic matter decomposition. The case where air was injected at the bottom yielded the maximum organic matter decomposition (around 84%), which was approximately 1.5% higher than middle aeration. According to the numerical simulation results, most of the organic matter was decomposed through gasification. There was 1.5% of organic matter reduced through leachate discharged which was only half of it found in the control group.
According to the pollutions degradation pathways used for the model construction, gasified organic matter was emitted as CO2 and CH4 forms. The gasified nitrogen was emitted as N2 and N2O forms. In this study, global warming potential (GWP) of CH4
and N2O were 23 and 293 times greater, respectively, than a unit of CO2. Fig. 5-4 shows the CO2 equivalent GWP associated with air injection. In the light of the simulation results, although the least amount of organic carbon gasified in control group, the largest GWP was found under this landfill scenario. It is also important to be aware that CH4
contributed large fraction to GWP in the cases of top and middle aeration, and its contribution was diminished as the air injection depth becomes deeper. As shown in the figure, the contribution of N2O to GWP was limited under all cases. It is attributed to low N2O yield rate employed in the model. The specific N2O yield rate of the solid waste will be investigated in the future experiments. To sum up, aeration at the deepest layer near the leachate collection pipe tends to inhibit anaerobic fermentation, while
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emitted gas Discharged leachate Solid waste
Amount of carbon (%)
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N2O(296 times) CH
4(23 times) CO
2
GHGs emission (kg-CO 2 eq./t-dry)
_
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8.3
18.9 23.7
Fig. 5-4. Influences of aeration modes on carbon mass balance and GHGs emission.
promotes aerobic decomposition to the maximum extent, and therefore it may be the most suitable aeration mode for the control of GHGs emission from landfill.