1. Introduction
In December 2015, the 21st Conference of the Parties to the United Nations (UN) Framework Convention on Climate Change adopted the Paris Agreement. Since 2021, governments have been adopting a new climate change system to replace the Kyoto Protocol. The Paris Agreement, which emphasizes the contribution of both developed and developing countries to the reduction of greenhouse gases (GHGs), has been the most important environmental agreement of the international community since the adoption of the UN Framework Convention on Climate Change in 1992. In this pursuit, South Korea aims to reduce GHG emissions by 24.4% from 2017 levels and reduce emissions forecasts (business as usual) by 37% in consideration of emissions growth by 2030. GHGs should be reduced across all sectors to achieve this goal.
Typical GHG-emitting facilities in the waste sector include sewage and wastewater treatment, incineration, and solid waste landfill facilities. According to the GHG emission figures of the waste sector in Korea, landfills account for 7.8 million tons CO2eq. of GHG emissions, which constitutes 45.8% of total waste sector emissions and the highest in the waste sector, followed by incineration (7.1 million tons CO2eq.) and sewage and wastewater treatment (1.7 million tons CO2eq.). However, the amount of methane (CH4) recovery for landfill gas (LFG) energy, which was 2.1 million tons CO2eq. as of 2018, is gradually decreasing [1]. The amount of LFG generated in existing LFG energy facilities is gradually reducing due to the diversion of organic matter away from landfills [2]. Moreover, the LFG recovery amount is gradually declining because the use of LFG energy is suspended due to economic problems surrounding LFG energy projects [3].
Landfill aeration is an alternative to LFG energy projects that reduces long-term GHG emissions and shortens the required aftercare period of landfills [4], particularly in cases where energy recovery is no longer economically viable. Moreover, with the development of the concept of sustainable landfilling, landfill aeration has become essential. Microbial decomposition in anaerobic landfills can take decades or even centuries. Thus, landfill aeration is conducted by injecting air into landfilled municipal solid waste (MSW) to create an aerobic environment inside the landfill. Landfill aeration effectively accelerates waste degradation, improves leachate quality, reduces GHG emissions, and shortens aftercare [5–7]. Aerobic landfills have lower costs than anaerobic landfills because of their reduced leachate treatment and aftercare management costs [7, 8]. Aerobic bioreactor landfills are 13% less costly than traditional anaerobic landfills because of their airspace recovery and decreased leachate treatment costs [8]. Nonetheless, the cost reduction of landfill aeration is mainly expected in the medium and long terms [7].
However, despite these benefits, landfill aeration may cause economic problems during operation due to the power consumption of blowers [9]. High aeration rates benefit landfills by enhancing waste degradation, but aeration rates cannot be increased excessively because of aeration costs [10]. Long-term air injection into a landfill incurs high operational costs. In addition, secondary carbon dioxide (CO2) emissions caused by the required energy production of blowers for air compression must be considered. Thus, to save energy and reduce operating costs, researchers have shifted their focus to semi-aerobic landfills [9]. The use of a landfill as a semi-aerobic landfill should be considered from the initial design and construction, including the proper size of leachate collection pipes. Converting an anaerobic landfill in operation into a semi-aerobic landfill is difficult. Moreover, a landfill designed and constructed as a semi-aerobic landfill may become an anaerobic landfill over time because its leachate collection pipes may be full in the summer and rainy seasons [11].
Thus, the application of landfill aeration can be further expanded to GHG emission mitigation measures in existing MSW landfills by solving the economic problem caused by the power consumption of landfill aeration. For instance, intermittent aeration can be implemented as an alternative to continuous aeration to reduce power consumption costs [12]. Intermittent aeration reduces energy costs by approximately 75% compared with continuous aeration [13].
This paper presents the application of an energy-saving aerobic landfill concept consisting of a blower and an air amplifier. The proposed air amplifier is an aerodynamic device that induces large amounts of airflow using a small quantity of compressed air. The field applicability of the air amplifier is studied by measuring its air amplification, energy consumption, and GHG reduction efficiencies through field experiments.
2. Materials and Methods
2.1. Landfill Site and Aeration Concept
Our investigation was conducted at the closed S landfill site (landfill area of 44,900 m2 and landfill capacity of 671,000 m3) in Gyeonggi-do. The landfill accepted MSW between 1997 and 2008.
Air was supposed to be injected using a blower (maximum capacity of 40 m3/min), and the developed air amplifier was supposed to change the internal conditions of the waste landfill from anaerobic to aerobic. However, air injection wells could not be installed at the top of the landfill because of landfill capping. Thus, the existing leachate collection and drainage pipes were used for aeration. The diameter of the leachate collection and drainage pipes was 300 mm, and an aeration tube was inserted into the drainage pipe for aeration. The air amplifier was installed between the blower and aeration tube. The air volume injected through the air amplifier was approximately 5 m3/min. The extracted gas was collected using a gas collection system, conveyed into a biofilter system, and released to the atmosphere. A schematic description of the installation of the in-situ aeration system is given in Fig. 1.
2.2. Air Amplifier Principle
In this study, the air volume from the blower was amplified using the proposed air amplifier to reduce the power consumption of air injection into the landfill. The principle of the air amplifier is explained in Fig. 2. The air injected into the air amplifier using the blower is released through a gap inside the amplifier as the pressure and velocity of the air increase according to the Bernoulli principle. The fast-flowing air released through the gap flows along the inner wall of the air amplifier caused by viscosity according to the Coanda effect (adhesion of fluids to a surface), and the internal core of the air amplifier lowers the air pressure. At this time, the pressure difference between the inside and the outside of the air amplifier causes the surrounding air to flow into the air amplifier and combine with the air injected using the blower to create a larger flow. Consequently, the air volume increases without consuming additional energy.
The air amplifier was installed at the experimental landfill site to evaluate the efficiencies of air amplification and energy saving. The blower capacity was set to 3 m3/min, and the airflow rates with and without the air amplifier were measured. After that, the blower capacity was set such that the aeration volume was similar to the airflow rate during the operation of the air amplifier. Then, the energy-saving effect was assessed by measuring the electric power consumption for one week after. The power consumption was measured using a meter.
2.3. LFG Composition and CH4 Emission Measurement
The change in LFG composition (oxygen [O2], CH4, and CO2) was analyzed before gas flowed into the biofilter system using a portable LFG analyzer (GA5000, Geotechnical Instruments). The surface CH4 emissions from the landfill site before and after aeration were measured using Eqs. (1) and (2), respectively. The measurement was conducted using the flux chamber (chamber diameter of 0.2 m and chamber height of 1 m) and a laser CH4 detector (Tokyo Gas Engineering) [14]. The chamber was fixed to the ground by placing soil around the outer rim to prevent any disturbance from ambient air.
where M is the path-integrated CH4 concentration (ppm·m) and X is the distance (m).
The CH4 concentration over time was measured, and the CH4 surface emission flux was calculated using Eq. (2) with the CH4 concentration gradient over time. The CH4 concentration gradient over time was calculated through linear regression analysis, and the data used for regression analysis were based on a correlation coefficient of 0.8 or higher [15].
where Q is the CH4 emission flux (mg/m2/min), V is the chamber volume (m3), A is the chamber area (m2), and dC/dt is the change in the headspace CH4 concentration with time.
The number of surface emission measurement points, which was calculated using Eq. (3) [15], was set to 40. The 40 sites were divided into constant grid intervals (30 m), and the measurement points were selected (Fig. S1).
where n is the number of field measurements and Z is the size of the investigated area (m2).
A flow meter was installed at the front end of the biofilter system to measure the LFG gas flow rate and CH4 concentration. The CH4 emission from the experimental landfill site during air injection was evaluated by calculating the CH4 amount emitted through the gas collection system and combining the result with the surface emission amount.
3. Results and Discussion
3.1. Evaluation of Aeration Volume and Economic Feasibility of Air Amplifier
Table 1 shows the air volumes with and without the air amplifier. When the blower capacity was 3 m3/min, the air volume from the aeration pipe was 1.47 m3/min before the installation of the air amplifier, and the loss rate was approximately 51%. After the installation of the air amplifier, the average air volume was 5.11 m3/min, which showed an improvement in the air amplification efficiency of approximately 3.5 times.
The energy consumption values of the blower with and without the air amplifier were examined and compared. The blower capacity was set to 10 m3/min when the air amplifier was not applied, considering the abovementioned enhancement in the air amplification efficiency of approximately 3.5 times. When the air amplifier was applied, the blower capacity of the air amplifier was set to 3 m3/min. The blower was operated for one week under each condition. Without the air amplifier, the airflow rate was 4.13 m3/min on average, approximately 1 m3/min less than that when the airflow was applied (5.11 m3/min; 3 m3/min blower capacity). The power consumption values were 588 and 42 kWh at the blower capacities of 10 and 3 m3/min, respectively (Table 2). Therefore, the energy-saving efficiency was expected to increase further, as the airflow rate increased by approximately 1 m3/min when the air amplifier was used. In conclusion, the use of the air amplifier will reduce energy consumption by at least 90% compared with existing in-situ aeration in aerobic landfills.
3.2. GHG Reduction Efficiency
Fig. 3 presents the LFG compositions and O2 utilization rates with and without aeration. The CH4 concentration before aeration averaged 53%, whereas the CO2 concentration averaged 26%. However, when the volume of the air injected through the air amplifier was approximately 5 m3/min, the CH4 concentration declined to less than 5% within 10 days and did not increase, maintaining the O2 concentration at a constant level (Fig. 3(a)). Thus, aeration using the air amplifier was efficient despite the pressure inside the landfill. In addition, if the existing leachate collection and drainage pipes are not full, aeration using the leachate collection and drainage pipes should be more economical than air injection using air injection wells.
Intermittent aeration can be an efficient approach to energy saving. Thus, the LFG concentration was evaluated after the pump stopped. The CH4 concentration increased from 0.5% to 14.7% in about six days after aeration stopped, whereas the O2 concentration decreased from 12.8% to 2.3%. When the pump was stopped, the O2 concentration was less than 2%, which may have been due to air intrusion into the side slopes. Air intrusion can occur through cracks on side slopes, where landfill capping is not easily compacted. CH4 emissions depend on changes in barometric pressure. An increase (decrease) in barometric pressure suppresses (enhances) such emissions. This effect, called barometric pumping, can be overcome by the pressure difference between the landfill and the atmosphere through the side slopes, resulting in air ingress. Thus, in this case, if intermittent aeration is to be applied, aeration and suspension should be conducted daily or regular times to maintain the aerobic conditions of the landfill. Nonetheless, further research is needed to determine whether the increase in CH4 concentration is related to the aeration volume and duration.
After aeration stopped, the CH4 concentration gradually increased, and it took about 30 days to recover to its level under anaerobic conditions. This suggested the existence of a partial anaerobic environment inside the landfill; the injected air was not evenly distributed because of the inhomogeneous characteristics of the landfill and the moisture between waste pores [16, 17]. The continuous detection of CH4 concentrations (0.5%–3%) during the aeration period supported this inference.
The CH4 concentration reduced during aeration due to gas dilution by high aeration, inhibition of CH4 production due to the conversion of the environment into an aerobic one, and biological oxidation of CH4 [18]. In this study, the proportions of the individual causes of CH4 reduction could not be determined. However, when air was injected again after the first aeration phase, the tendency of CH4 concentration reduction was similar to that during the initial aeration, but the CO2 concentration differed from that during the first aeration phase. When the CH4 concentration decreased to under 5% in the first aeration, the CO2 concentration was 6%–7.6%. During the second aeration phase, the CO2 concentration (7.8%–11.8%) was slightly higher than that during the first aeration. Therefore, unlike in the first aeration, where air-induced gas dilution was the main factor, the CO2 concentration slightly increased in the second aeration because CH4 oxidation or organic matter decomposition was achieved. In general, the concentration of CO2 is approximately 15% under sufficient aerobic decomposition [16]. The use of O2 in start-up aeration results in CH4 oxidation rather than organic matter decomposition [19].
Fig. 3(b) shows the O2 utilization rate during the experiment. During the aeration phases, it remained stable at 50%–60%. Previous studies showed an initially high O2 utilization rate of approximately 30%–60% in old landfills [20, 21]. In cases where O2 utilization rates are low during aeration, the aerobic conditions of a landfill body may endure for some time after air injection is switched off, making continuous aeration unnecessary. However, in this study, the O2 utilization rate was high during air injection and extremely decreased immediately after aeration stopped. Thus, in this case, intermittent aeration is unsuitable.
The CH4 emissions from the experimental landfill site were evaluated after the experiment. The CH4 emissions were 1,945 CH4 kg/d before aeration. After aeration, the CH4 reduction efficiency was 90.3%, indicating CH4 emissions of 188 CH4 kg/d. This study achieved a CH4 reduction efficiency similar to those in previous studies (77%–95%) [20].
The aeration volume in this study was compared with those of different aerobic landfills. The experimental landfill had a capacity of 671,000 m3. Assuming a landfill density of 1 ton/m3, the landfill volume was 671,000 tons. Assuming a moisture content of 25%, the dry landfill volume was 503,250 tons, and the aeration volume through the air amplifier was 7,200 m3/d (based on the 5 m3/min air volume). Therefore, the aeration volume per kg-DM (dry matter) was 0.014 L/d. According to previous studies, and the aeration volume per kg-DM is 0.08–2.9 L/d [20, 22–23]. Therefore, the aeration volume in this study was significantly lower than previous findings (Fig. S2). A possible explanation for this is that the experimental landfill in this study had a large capacity and that the experiment was conducted approximately 12 years since its closure. Nonetheless, the appropriate aeration volume according to landfill characteristics should be continuously assessed to clarify the cause of this difference.
3.3. CH4/CO2 Ratio
The CH4/CO2 ratio indicates the proportion of the anaerobic condition to the aerobic condition in a landfill body. Fig. 4 shows the CH4/CO2 ratio according to the O2 concentration. The CH4/CO2 ratio before aeration was more than 1, which is typical under anaerobic conditions [24]. After aeration, the CH4/CO2 ratio was 0.12 (0.06–0.25) on average and decreased with an increase in the O2 concentration. According to Yang et al. [25], the CH4/CO2 ratios under anaerobic and semi-aerobic conditions are 2.43 and 0.64, respectively. Ma et al. [26] found CH4/CO2 ratios of 1.64 and 0.44–0.68 under anaerobic and semi-aerobic conditions, respectively; however, this experiment was a laboratory-scale one, so these CH4/CO2 ratios might be lower than field results. Powell et al. [18] found that the CH4/CO2 ratio under aerobic conditions is 0.44 on average. According to Ban et al. [19], the range of CH4/CO2 ratios under these conditions is 0.2–0.6. Therefore, aeration using air amplifiers efficiently reduces the CH4 concentration of LFG.
3.4. Percentage of Anaerobic Activity
CH4 and CO2 concentrations can be used to estimate the fraction of waste that is degraded anaerobically at any point in time. Anaerobic degradation can be assumed to produce LFG with a 50/50% ratio of CH4/CO2. In this case, the aerobic and anaerobic degradation equations for carbon-containing compounds are as follows:
Based on the stoichiometry of these two reactions, the percentage of waste degraded anaerobically, P, is estimated as follows [27]:
where CCH4 and CCO2 are the measured concentrations (% v/v) of CH4 and CO2, respectively.
The value of P can be computed assuming that waste degradation produces different LFG compositions. In this study, the concentrations of CH4 and CO2 in LFG before aeration were 53.6% and 26.3%, respectively. A value of the CH4+CO2% in LFG of under 100% suggests air intrusion into the LFG. However, because the O2 concentration was very low in this study, the generation of CO2 by O2 was not considered. All carbon decomposed under anaerobic conditions was assumed to be converted into CH4 and CO2. This study also did not consider carbon discharge via leachate. Because carbon discharge via leachate in landfills is generally an order of magnitude lower than that of LFG [28, 29]. Assuming a CH4+CO2% in LFG of 100%, the CH4 and CO2 concentrations were approximately 65% and 35%, respectively. Thus, in this study, the anaerobic degradation reaction was expressed as follows:
With the use of Eq. (7) and the same expression for aerobic degradation in Eq. (5), the percentage of anaerobic activity, P, was calculated as follows:
During the aeration phases, the P values ranged from 15% to 20%, as shown in Fig. 5. Therefore, approximately 80% of biodegradable organic carbon was decomposed aerobically, and the remaining 20% was decomposed anaerobically. Interestingly, under aerobic conditions, where O2 concentrations exceeded 10%, P was seldom below 10%. Thus, even with sufficient O2 supply to the landfill, anaerobic zones persisted, where a portion of the waste degraded anaerobically. Raga and Cossu [23] obtained similar results; some anaerobic zones within the landfill body were not affected by air injection. According to Yazdani et al. [27], even at O2 concentrations above 15%, anaerobic zones persisted in the studied landfill. This could cause the transport of air through preferential flow paths, resulting in the simultaneous presence of aerobic and anaerobic zones in aeration landfills. The value of P exceeded 100% because it was calculated by assuming that waste degradation produced different LFG compositions. If the assumption regarding the LFG composition changes, the P value will also change.
In addition, the P value increased immediately after air injection stopped, suggesting that the maintenance of the aerobic conditions in the landfill body stopped immediately after air injection stopped. In this case, intermittent aeration is unsuitable, so the utilization of the air amplifier will result in efficiency.
Categorizing landfills is important for GHG inventories. Thus, this study proposes classifying anaerobic, semi-aerobic, and aerobic landfills according to their CH4/CO2 ratios. Fig. 6 presents landfill types according to their anaerobic activities and CH4/CO2 ratios. Because the degree of reduction of anaerobic microbial activity due to air penetration should be considered for semi-aerobic landfills, Intergovernmental Panel on Climate Change guidelines suggest an anaerobic activity of 50%–70% for semi-aerobic landfills [30]. Thus, if the anaerobic activity is 50%–70%, then the CH4/CO2 ratio is 0.56–0.90; this range can then be categorized to a semi-aerobic landfill. Jeong et al. [11] proposed the use of the CH4/CO2 ratio as an indicator for evaluating semi-aerobic landfills and argued that a landfill with CH4/CO2 > 1 is difficult to classify as a semi-aerobic landfill. However, the structural aspects of landfills, including the leachate levels in leachate collection pipes, should be investigated to classify semi-aerobic landfills. In addition, the CH4/CO2 ratio of LFG in semi-aerobic landfills should be further studied to validate the range presented in this study. Nevertheless, using the CH4/CO2 ratio criterion along with site investigation will help identify semi-aerobic landfills.
4. Conclusions
In this study, the power consumption of landfill aeration was reduced using an air amplifier to solve the economic problem of aerobic landfilling. Field experiment results demonstrated that the use of the air amplifier could reduce energy consumption by at least 90% and exhibited an air amplification effect of approximately 3.5 times. In addition, evaluation findings of the gas composition, CH4/CO2 ratio, and CH4 emissions indicated that the conditions inside the landfill were sufficiently converted into aerobic conditions and the GHG reduction efficiency was high.
Even after air injection is stopped, intermittent aeration will be possible if the aerobic conditions within the landfill are maintained for a certain period. However, if the aerobic conditions in the landfill are not maintained after air injection is stopped, continuous aeration will be required; in this case, aeration using the air amplifier will be sufficiently effective in reducing GHGs and power consumption.
Aeration using existing leachate collection and drainage pipes may be more economical than air injection using air injection wells. However, approximately 20% of the organic carbon was decomposed anaerobically because of the transport of air through preferential flow.
In this study, the CH4/CO2 ratio range of 0.56–0.90 was presented as a criterion for classifying landfills as semi-aerobic. However, the CH4/CO2 ratio in LFG in semi-aerobic landfills should be further explored to verify the proposed range.
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