ABSTRACT
Nitrous oxide (N2O) has gained considerable attention as a contributor to global warming and depilation of stratospheric ozone layer. Landfill is one of the high emitters of greenhouse gas such as methane and N2O during the biodegradation of solid waste. Landfill aeration has been attracted increasing attention worldwide for fast, controlled and sustainable conversion of landfills into a biological stabilized condition, however landfill aeration impel N2O emission with ammonia removal. N2O originates from the biodegradation, or the combustion of nitrogen-containing solid waste during the microbial process of nitrification and denitrification. During these two processes, formation of N2O as a by-product from nitrification, or as an intermediate product of denitrification. In this study, air was injected into a closed landfill site and investigated the major N2O production factors and correlations established between them. The in-situ aeration experiment was carried out by three sets of gas collection pipes along with temperature probes were installed at three different distances of one, two and three meter away from the aeration point; named points A-C, respectively. Each set of pipes consisted of three different pipes at three different depths of 0.0, 0.75 and 1.5 m from the bottom of the cover soil. Landfill gases composition was monitored weekly and gas samples were collected for analysis of nitrous oxide concentrations. It was evaluated that temperatures within the range of 30–40°C with high oxygen content led to higher generation of nitrous oxide with high aeration rate. Lower O2 content can infuse N2O production during nitrification and high O2 inhibit denitrification which would affect N2O production. The findings provide insights concerning the production potentials of N2O in an aerated landfill that may help to minimize with appropriate control of the operational parameters and biological reactions of N turnover.
Implications: Investigation of nitrous oxide production potential during in situ aeration in an old landfill site revealed that increased temperatures and oxygen content inside the landfill site are potential factors for nitrous oxide production. Temperatures within the range of optimum nitrification process (30–40°C) induce nitrous oxide formation with high oxygen concentration as a by-product of nitrogen turnover. Decrease of oxygen content during nitrification leads increase of nitrous oxide production, while temperatures above 40°C with moderate and/or low oxygen content inhibit nitrous oxide generation.
Introduction
Nitrous oxide (N2O) is a trace gas that has gained considerable attention, as it makes an important contribution to global warming and participates in the depletion of stratospheric ozone layer. Therefore, even a very low amount of N2O is undesirable. Around the world, landfilling continues as a primary method to manage municipal solid waste (MSW), which is one of the high emitters of greenhouse gases such as CH4 and N2O during the biodegradation of solid waste. In recent years, landfill aeration with or without leachate recirculation has attracted increasing attention worldwide for fast, controlled, and sustainable conversion of landfills into a biological stabilized condition. In a traditional anaerobic landfill ammonia is a long-term pollutant; it can be dramatically removed in an aerobic landfill since the introduced air can create a favorable environment for simultaneous nitrification and denitrification (Price, Barlaz, and Hater, Citation2003; Prantl et al., Citation2006; Ritzkowski, Heyer, and Stegmann, Citation2006; Berge, Reinhart, and Batarseh, Citation2007). N2O originates from the biodegradation or combustion of nitrogen (N)-containing solid waste during microbial nitrification and denitrification. During these two processes, formation of N2O is as a by-product from nitrification, or as an intermediate product of denitrification (Wrage et al., Citation2001). However, the fraction of N2O produced in each of these reactions varies considerably (Firestone and Davidson, Citation1989; Bleakley and Tiedje, Citation1982). Nitrification is carried out by autotrophs in aerobic conditions, whereas denitrification is accomplished by heterotrophs in anoxic conditions. Therefore, landfilling and landfill leachate treatment may make a huge contribution to the release of N2O into the atmosphere.
Recently, N2O emission from aerobic landfill started to draw much attention from the world since landfill aeration not only induced N removal, but can also impel N2O emission. Powell et al. (Citation2006) inferred that during field-scale closed-landfill aeration, N2O production was promoted, on the basis of the observation that increasing gas flow did not lead to the reduction of N2O concentration. In a lab-scale landfill column experiment, He et al. (Citation2011) observed that N2O production was positively correlated with the prolonged aerobic time, and negatively related with the C/N ratio in the recycled leachate.
N2O emissions have been investigated intensively in various ecosystems, such as agricultural fields, wetlands, forests, and grasslands (Chen et al., Citation2000; Kiese and Butterbach-Bahl, Citation2002; Ghosh, Majumdar, and Jain, Citation2003; Erisman et al., Citation2008), whereas there have been few investigations of N2O emissions in municipal solid waste (MSW) landfills. However, fluxes of N2O from MSW landfills are comparatively higher than those from other systems such as agricultural and forest soils (Rinne et al., Citation2005). Some studies have examined the effects of cover soils and leachate surface irrigation on N2O emissions from MSW landfill to determine several biochemical reactions that may responsible for N2O emission (Zhang, He, and Shao, Citation2008; Zhang, He, and Shao, Citation2010). However, to our knowledge no study has investigated the production potential of N2O inside the aerated MSW landfill, where landfill aeration technology may induce N2O production in presence of oxygen.
The main purpose of this study was to investigate the production potential of N2O inside the aerated landfill. In this study, air was injected into a closed landfill site and the major N2O generation factors were analyzed with landfill gases. Correlative analysis of temperature in the landfill cell has been conducted to understand the effects on N2O production as well. The findings provide insights concerning the production potential of N2O in an aerated landfill that may help to minimize greenhouse gas emissions with appropriate control of operational parameters and biological reactions of N turnover.
Materials and methods
Characteristics of landfill waste
About 50.44% of the waste deposited in the Chongming landfill site, China, was kitchen/organic waste in original composition and about 15 and 12% was plastic and paper, respectively. Before starting the in situ aeration experiment, kitchen/organic waste was about 20% (wt) and 19 and 10% were measured as plastic and paper, respectively. The deposited landfill solid waste was collected from the experimental site after removing the cover soil, and the composition was measured on a weight basis.
Site description
Chongming MSW landfill started in operation in 2006 and closed in 2013. The landfill site is located northwest of Chongming Island at 31°27’00’’ N, 121°09’30’’ E and 3.5–4.5 m above sea level. In this area, the mean annual precipitation is 1003.7 mm and the mean annual temperature is 15.3°C (Baidu Encyclopedia, Citation2015). The total landfill site has an area of 3.3 ha, where 1,250,000 tons (fresh matter) of untreated municipal and commercial waste were deposited. The depth of disposed solid waste was between 3 and 8 m and covered with a thin layer of soil material and a compost/soil mixture. The area selected for the in-situ aeration project has a surface of approximately 125 m2 (25 m × 5 m) and was situated at the center of the Chongming landfill site.
Experimental setup
The outline of the experiment to observe and verify the in situ aeration effect on N2O generation in the closed landfill is shown in . The aeration system consisted of an air compressor (0.8 MPa and capacity 42 L/min, BEBICON, Hitachi), gas regulator, flowmeter, air hoses, and air injection pipe. The air injection pipe was installed at the experimental site, and the air injection point, which is the tip of the air injection pipe, was located at 3.8 m depth from the landfill surface. First, cover soil was removed from the experimental site, and then the landfilled solid waste was dug out and the aeration pipe and three sets of gas collection pipes along with temperature probes were installed. Three sets of gas collection pipes were installed at three different distances of 1.0, 2.0, and 3.0 m away from the point where the aeration pipe was installed, points A, B, and C, respectively. After installing all pipes, landfilled waste and the covered soil were replaced. Each set of gas collection pipes consisted of three pipes different in length, and their tips were set at three different depths of 0.0, 0.75, and 1.5 m from the bottom of the cover soil, respectively. Names of monitoring points different in depth are given as the depth from cover soil bottom in this study. The cover soil thickness was 2.3 m from the surface. In each gas monitoring pipe, concentrations of O2, CO2, and CH4 were monitored every week using a portable landfill gas analyzer (GA 5000). Temperature was measured at each monitoring depth by thermocouples connected to the portable data logger (TDS-150 and FSW-10, Tokyo Sokki, Kenkyujo Co., Ltd., Japan) at 15-min intervals. During monitoring of landfill gas (LFG), water came out from 1.5 m monitoring depth, as the water table was very close to the monitoring depth of 1.5 m. Therefore, gas samples could be collected and monitored only from 0.0 and 0.75 m depths. Airflow rate was controlled at 1.0 L/min until 24 days, and from 24 days the airflow rate was maintained at 5.0 L/min. The in situ aeration experiment lasted for 107 days.
Figure 1. (a) Outline of the in situ aeration experiment. (b) Measurement method and parts. (c) Aeration system.
Analytical methods
Initial solid waste characteristics were measured by the Japanese Leaching Test (JLT-46, a method to test the conformity of a material to the soil environmental standard and also the environmental safety of a material that as solid waste is recycled in Japan) for analyzing pH, Electric conductivity (EC), total organic carbon (TOC), ammonium-N (NH4+-N), nitrite-N (NO2- -N), and nitrate-N (NOx- -N). pH and EC were measured by Horiba pH and EC meters. Loss on ignition (LOI) was measured to estimate the organic carbon content in the solid phase using a muffle furnace to heat the dried sample (at 105°C for 24 hr) to 550°C for 4 hr. The total organic carbon and total N contents of the sampled leachate were measured using a TOC analyzer with total nitrogen measuring unit (TNM-1) (Shimadzu, Japan). Ion chromatography (DX-120, Dionex, Japan) was carried out to detect the concentrations of NH4+-N, NO2–N, and NO3–N ions in the collected leachate. Compositions of O2, CO2, and CH4 were measured using a portable gas analyzer (Geotech GA-5000, United Kingdom). The N2O concentrations in the gas samples were analyzed using a gas chromatograph (GC-Agilent 7820, USA) equipped with an electron capture detector (ECD). The GC was equipped with a precolumn (Porapak Q80/100 mesh, 1 m × 2 mm) and a main column (Porapak Q80/100 mesh, 3 m × 2 mm). A 1-mL sample loop and a 10-port valve were used to inject the gas samples. The temperatures of the columns and the ECD were 100 and 300°C, respectively. The carrier gas used was a mixture of 91% argon and 9% CH4. N2O was quantified using the standard curves generated from certified standard gases (National Institute of Metrology, PR China).
Results and discussion
Preliminary landfill characterization
shows the physicochemical characteristics of collected solid waste before commencing aeration in the experimental site. At 24 days, a leachate sample was collected from 2 m away from the aeration point at 1.5 m depth and the chemical characteristics of the leachate were analyzed, as shown in . It is thought that the leachate percolation driven by the gravity tends to higher TOC and NH4+-N in landfill cells. In addition, under aerobic conditions the positive redox potential affects the mobility of organic compounds, and other reactions are also affected, such as solubility and sorption properties of organic contaminants (Rich, Gronow, and Voulvoulis, Citation2008).The initial increase was due to increased mobilization of organic and inorganic nitrogen compounds, as well as release of leachate already containing NH4-N and T-N in the pore spaces of waste. Formation of NOx–N indicated that N conversion had also commenced at 2 m away from aeration point.
Table 1. Solid waste characteristics before started the experiment.
Table 2. Leachate characteristics after 24 days of the in situ aeration.
Effects of aeration on temperature and the landfill gas composition
Generally, landfill aeration leads to temperature in a range of 35–50°C due to intensive aerobic conversion processes (Heyer et al., Citation2005). shows how the temperature development in the in situ aeration landfill site with LFG composition changes with time.
Figure 2. (a) Temperature development. (b) LFG compositions with time by in situ aeration.
Before starting the air supply, the temperature was about 24°C; after aeration started at a rate of 1 L/min, temperature had no significant change, although there was a slight increase at 1.5 m depth near the aeration point. The airflow rate was increased at 5 L/min from 24 days and the temperature profiles rapidly started to increase, especially for points A and B, and there was a gradually increasing trend during the entire aeration period, as shown in .
In situ aeration of waste leads to a significant change in gas composition and production when compared with the anaerobic condition. The presence of oxygen accelerates the decomposition of organic matter in waste, and distribution of carbon load during aerobic decomposition is concentrated on the creation of gaseous products. represents the O2, CO2, CH4, and N2O concentration changes with depth and time during the in situ aeration of the experimental landfill site. Before commencing aeration (i.e., when no O2 was available), CH4 contents were 60–71% (v/v) and CO2 contents around 35% (v/v) at all gas wells. It was observed that during aeration at 1 L/min, CO2 and CH4 were very high due to the low O2 concentration, although a slight decrease occurred immediately after starting the aeration. This result suggests that the aeration rate was not sufficient to enhance the microbial activity, and the high organic carbon and ammonium concentrations probably made nitrification and dentrification very weak in this stage. When aeration rate was increase to 5 L/min there was a dramatic decrease in CO2 and CH4 throughout the aeration periods at points A and B, with O2 concentration within the range of 10–20% (v/v). After starting aeration, CH4 concentration was reduced by about 85 % within 3 months, in accordance with the study of a pilot cell in Live Oak Landfill (USA) by Read, Hudgins, and Phillips (Citation2001). It was also noticed that the CH4 concentration reduced from 60 to 15% at point A at 0.75 m depth, and from 60 to 9% at 0 m depth, whereas the ratio of CO2 to CH4 rose from 0.55 to 0.88 at 0 m depth. Aeration enhances the carbon conversion mainly as CO2, which complies with this study’s results at deeper depth (0.75 m) at points A and B. Heyer et al. (2005a) found that in an aerated landfill, CH4 concentration rapidly decreased (3–15%) and CO2 is within the range of 10–20%, which is consistent with results of the present study. In light of , at points A and B, the rapid decrease in CH4 and increase CO2 levels with O2 consumption evidently shows the influence of the aeration measures. The rapid decrease of methane occurred by the in situ aeration, which inhibits the activity of methanogen and accelerates the activity of methane-oxidizing bacteria simultaneously through increase of oxygen content. At point C, CO2 and CH4 contents decreased during 24–40 days for O2 concentration in the range of 5–15% (v/v), while the concentrations of both CO2 and CH4 were found higher at the end, approximately 35 and 60% (v/v), respectively, under relatively lower percentages of oxygen (below 5%).
In contrast, during the high aeration rate (5 L/min), observed N2O generation ranged from undetectable to 338 and 310 ppm at points A and B, respectively, whereas at point C, N2O generation was very low (up to 5 ppm) or below detection limit and O2 concentration was observed below 5% (v/v). It can be inferred that the existing and adequate oxygen caused by in situ aeration is the dominant factor on N2O production. However, in situ aeration provided inadequate O2 to inhibit nitrification and promote the decomposition of immediate product, NH2OH in nitrification, and N2O production (Davidson, Citation1992).
After increasing the airflow rate, N2O production began to increase; this phenomenon can be explained by the reaction rates of ammonia oxidation and nitrite oxidation studied by Tadahiro et al. (2014) on wastewater treatment. It was evaluated that due to high airflow rate, higher N2O was produced because of relatively high ammonia oxidation rate in the beginning, which caused NO2-N accumulation. On the other hand, with low airflow rate both NO2-N and N2O concentrations were low, probably because nitrite oxidation was relatively high. Accumulation of NO2–N had good correlation with N2O production observed by He et al. (Citation2001) in aerated composting of food waste during nitrification. Therefore, it is hypothesized that with high aeration rate, accumulation of nitrite is one of the potential factors for N2O production. Moreover, observed NOx–N () in this study is possibly owing to the beginning of nitrification even at low airflow rate. It is assumed that a sufficient O2 concentration due to high aeration rate prompted NO2–N accumulation, which was also observed in a previous study (Ritzkowski, Heyer, and Stegmann, Citation2006) in an aerated landfill and in a laboratory-scale bioreactor study by Nag, Shimaoka, and Komiya (Citation2015).
In , at high aeration rate, a very sharp increase of N2O was found at point A (at both depths), and it was often observed when O2 concentration was above 15% (v/v). Increasing oxygen concentration from 5 to 20% (v/v) led to increased N2O production at points A and B revealed that the nitrification process is the dominant pathway of N2O formation. It has been suggested that N2O concentration increases with decreasing O2 concentration during nitrification; the results of the present study from 73 days, when O2 concentration started to decrease, were in accordance with this suggestion. Robinson, Barr, and Last (Citation1992) mentioned that the accumulation of nitrite and nitrate occurs when nitrification becomes stronger and denitrification become weaker as a result of decreased organic carbon. Hence N2O production did not accord with increased nitrite and nitrate concentrations as a key influencing factor studied by Sun et al. (Citation2013). However, at point C, N2O generation was detected at a very low level when O2 concentration was below 5% (v/v). It is noticeable that at point C, although O2 concentration ranged between 0 and 15% at the initial period of high aeration from day 24 to 40, N2O generation was undetectable. This might happen for the incomplete nitrification or denitrification. Weakening of both nitrification and denitrification is thought to change the trend of N2O production (Sun et al., Citation2013). These observations suggest that N2O reduction is possible by controlling the rates of each nitrification by changing airflow rate. High production of N2O inside the aerated landfill occurred for O2 concentrations above 15% v/v in this study might show development of a strong nitrification process, which stimulated N2O formation as a by-product. He et al. (Citation2011) mentioned that prolonged aerobic conditions can result in strong production of N2O.
Correlative analysis among landfill gases
In order to find out the in situ aeration effects on greenhouse gases (GHGs; CO2, CH4, and N2O) and volume ratio of CO2 and CH4, a Pearson correlation analysis based on the collected sampling data of points A and B was conducted.
From , it can be found that both CH4 and CO2 had a significant (p < 0.01) negative correlation with O2 concentration. Usually, abundant O2 content can inhibit CH4 production and improve CO2 production through methane oxidation. Oxygen distribution in a waste layer is a multistage process influenced by concentration diffusion in the pore space in the waste layer, aerobic metabolism of organic carbon, and the nitrification process. But N2O formation occurs during nitrification and denitrification processes. The negative but not close correlation between CH4 and N2O indicated that CH4 and N2O are formed through different biological processes. On the other hand, O2 and N2O had a significant positive correlation at point A (0 m depth, p < 0.01, and 0.75 m depth, p < 0.05). At point B, a positive correlation was observed but was not very close. This phenomenon indicates that higher O2 content induces formation of higher N2O. Therefore, during the inhibition of CH4 production by in situ aeration, acceleration of N2O production is not inevitable. Optimization of O2 distribution is a key in attenuation of N2O and CH4 production synchronously.
Table 3. Correlation among landfill gases: Pearson’s correlation coefficients (R value) and p value.
Correlative analysis of temperature and landfill gas composition
At points A and B, oxygen concentration was mostly in the range of 15 to 20% (v/v), and therefore aerobic microbial activity accelerated the organic matter decomposition, leading to a release of energy and increased temperature. Temperature at 0 and 0.75 m depth for point A reached up to 50°C and 40°C, respectively,and at point B ranged within 30–40°C. The raised temperature in the waste body was an unequivocal indication of release thermal energy as a result of an intensive aerobic conversion process with reduction of CH4 and increase of the concentration of CO2, simultaneously. Temperatures at point C remained below 30°C throughout the experimental period as oxygen concentration was observed in a range between 0 to 5% (v/v), except the initial period of high aeration rate, which indicates that release of energy from accelerated decomposition of organic matter was limited by insufficient O2 content.
It was evaluated that during the experimental period, N2O was below detection limit at low aeration rate as temperature ranged within 25 to 30°C. Increased aeration rate (5 L/min) stimulated N2O production with increased temperature up to 50°C. These results reveal that temperature had a strong influence on N2O production with high aeration rate, as temperature increases due to energy release from aerobic decomposition of organic matter. Increasing temperatures are the major factor determining N dynamics and the moisture losses during aeration. Temperatures above 40°C are intensifying both hydrolysis and ammonification processes (Scheffer and Schachtschabel, Citation2009) while at the same time inhibiting nitrification (Robinson, Olufsen, and Last, Citation2015). Data from the in situ aeration in this study showed a significant (p < 0.01) positive correlation between temperature and N2O generation at points A and B within the range of 30–50°C, as shown in .
Table 4. Correlation between temperature and landfill gases: Pearson’s coefficients (R value) and p value.
From , it was evaluated that temperature has a significant (p < 0.01) negative correlation with CO2 and CH4, indicating that optimum aerobic condition has a notable impact on GHGs reduction in landfill cell. At points A and B, N2O concentrations were higher for deeper than shallower depths (0.75 and 0.0 m, respectively) (). When temperature increased to 30°C, N2O generation started to increase, with gradual increase at temperatures up to 40°C; N2O production was increased very sharply for both points A and B at both depths (0.0 and 0.75 m), while for point A at 0.0 m depth, when temperatures rose above 40°C, N2O production showed a decreasing trend. The generation of N2O showed a decreasing trend when temperature started to decrease. However, at point C, N2O concentration was much lower, with the highest content of 5 ppm for temperature within the range of 25–30°C. Oxygen concentration was observed in a range between 0 and 5% (v/v) except for the initial period of high aeration rate; this indicates that release of energy from accelerated decomposition of organic matter was limited by inadequate O2 content. The data of N2O concentrations showed that temperature was clearly a potential factor in N2O formation. N2O production with temperature in this in situ aeration study revealed that temperature controlled the nitrification process up to 40°C, in accordance with results of Robinson, Olufsen, and Last (Citation2015), and N2O was mainly generated as a by-product of nitrification during aeration for the in situ landfill site.
Figure 3. Evaluation of greenhouse gases by in situ aeration.
Evaluation of greenhouse gases by in situ aeration
Results of greenhouse gases (GHGs) had shown the obvious reduction of CH4 and production of N2O observed simultaneously. Therefore, the total effect on GHGs under in situ aeration is evaluated. At minimum aeration rate of 1 L/min, the total average GHGs concentrations in CO2 equivalent were 4.1 × 106, 4.9 × 106, and 5.4 × 106 ppm at sampling points A, B, and C, respectively, where N2O was not detected. On the other hand, at high aeration rate of 5 L/min, total average concentrations of GHGs reduced to 9.3 × 105, 1.9 × 106, and 3.3 × 106 ppm (CO2 equivalent) at sampling points A, B, and C, respectively, including N2O concentration as shown in (2.2 × 104, 1.6 × 104, and 3.4 × 102 ppm CO2 equivalent at points A, B, and C, respectively). It has been evaluated that net total CO2 equivalent concentration reductions at points A, B, and C were 77, 61, and 38%, respectively, at increased aeration rate from 1 L/min to 5 L/min. The N2O production impelled by in situ aeration does not offset the greenhouse effect reduction caused by CH4 reduction. Therefore, in situ aeration in MSW landfill is an effective way to minimize greenhouse effects.
Conclusions
In situ aeration in the MSW landfill accelerated the decomposition of organic matter and therefore released energy that increased temperature and posed a positive impact on N2O production. The N2O production varied with the temperature range of 30–50°C and O2 concentration within a range of 5–15% (v/v). Low O2 content can lead to N2O production during nitrification and high O2 inhibits denitrification, which would affect N2O production. Therefore, optimization of O2 concentration inside the MSW landfill is a key factor to control N2O production. It was hypothesized that high airflow rate is a potential parameter of N2O production because of nitrite accumulation. Nitrification is found to be the dominant pathway of N2O production during in situ aeration. Results of this study should aid in understanding the N and temperature dynamics of N2O production. N2O flux measurement in an aerated landfill site, with consideration of cover soil materials and adjustment of temperature by a controlled addition of water, and with intermittent aeration, would be beneficial in future studies. The findings provide insights concerning the production potential of N2O in an aerated landfill that may help to minimize with appropriate control of the operational parameters and biological reactions of N turnover, although in situ aeration helps to minimize the total greenhouse gas emissions.
Acknowledgments
The authors acknowledge Wu Boran for his assistance in sample analyses and Dr. Wu Chuanfu and Kazutoshi Manabe for their assistance during installation of the experiment in the landfill site.
Additional information
Notes on contributors
Mitali Nag
Mitali Nag is a Ph.D. student in the Department of Urban and Environmental Engineering, Graduate School of Engineering, Kyushu University, Japan.
Takayuki Shimaoka
Takayuki Shimaoka is a professor in the Department of Urban and Environmental Engineering, Faculty of Engineering, Kyushu University, Japan.
Hirofumi Nakayama
Hirofumi Nakayama is an associate professor in the Department of Urban and Environmental Engineering, Faculty of Engineering, Kyushu University, Japan.
Teppei Komiya
Teppei Komiya is an assistant professor in the Department of Urban and Environmental Engineering, Faculty of Engineering, Kyushu University, Japan.
Chai Xiaoli
Chai Xiaoli is a professor at the School of Environmental Science and Engineering, Tongji University, Shanghai, China
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