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Temporal and spatial variation of greenhouse gas emissions from a limited-controlled landfill site
Environment International 127 (2019) 387–394
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Temporal and spatial variation of greenhouse gas emissions from a limitedcontrolled landfill site Chengliang Zhanga,b, Yan Guoa,b, Xiaojun Wanga, Shaohua Chena, a b
T
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Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C LE I N FO
A B S T R A C T
Handling Editor: Yong-Guan Zhu
Landfilling biodegradable waste is an important source of global greenhouse gas (GHG) emissions. Among the several types of landfill, limited-controlled landfill is a common method used to dispose of domestic solid waste, especially in developing countries. However, information about GHG emissions from limited-controlled landfill sites has rarely been reported. In this study, the GHG emissions from a typical limited-controlled landfill site were investigated under a regular period for one year. The number and positions of static chambers were arranged according to the guidance on Monitoring Landfill Gas Surface Emissions by the UK Environment Agency to obtain representative data from the heterogeneous surface of the landfill. Inverse distance weighting (IDW) was applied to evaluate and visualise the GHG emissions from the whole landfill surface based on the measurements of distributed static chambers. As an important GHG source of the landfill site, the emissions from the landfill leachate treatment plant were also measured. The results revealed that CH4 and N2O emission fluxes from the landfill area were 1324.73 ± 2005.17 mg C m−2 d−1 and 2.16 ± 2.33 mg N m−2 d−1, respectively, and the fluxes from the leachate treatment plants were 23.92 ± 29.20 mg C m−2 d−1 and 16.40 ± 16.89 mg N m−2 d−1, respectively. CH4 and N2O releases preferred to present spatial heterogeneity, while temporal heterogeneity was expected to exist in CH4 and CO2 emissions. The annual GHG emissions from the limited-controlled landfill was calculated to be 1.078 Gg CO2-eq yr−1, which was the least among all types of landfill sites. In addition, the GHG emission factor was 0.042 t CO2-eq t−1 waste yr−1 which could not be ignored compared to the sanitary landfills. Therefore, it is advisable to give more attention and determine a potential solution for reducing GHG emissions from limited-controlled landfill sites.
Keywords: Greenhouse gas emission Methane Nitrous oxide Limited-controlled landfill Landfill leachate Emission factor
1. Introduction Approximately 5% of anthropogenic greenhouse gas (GHG) emissions are derived from solid waste disposal worldwide (Solomon et al., 2013). In China, almost 1.5% (111.81 Tg CO2-eq yr−1) of the total anthropogenic GHG originated from waste treatment in 2005 (Department of Climate Change, 2013). Landfill is a common land use type, and the GHG emissions from which have received much public attention (Stocker et al., 2014). For instance, previous study reported that emission fluxes ranged from 0.9 to 433 mg CH4 m−2 h−1, 2.7 to 1200 μg N2O m−2 h−1, and 12.3 to 964.4 mg CO2 m−2 h−1 from a sanitary landfill at Perungudi in Chennai, a mega-city in India (Jha et al., 2008). In comparison, scholars have given less concern to GHG emissions from limited-controlled landfills. It has been found that average CH4, CO2, and N2O in emission ranged from < 0.04 to 1800, 4.9 to 1800, and < 0.0001 to 0.35 mL m−2 min−1, respectively, from a
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dumping landfill site with waste ages of approximately 0.5 year (Nagamori et al., 2013). Moreover, studies on the factors affecting their release and control techniques also received wide concern (Börjesson and Bo, 1997; Bogner et al., 2011; Mcbain et al., 2005; Rinne et al., 2005; Zhang et al., 2009). As a developing country, China is yet to reach the capacity of developed countries to manage municipal solid waste (MSW). Landfill is currently and will continue to be the main method of MSW treatment long into the future (Hua and Zhao, 2004). Although China's Minister of Housing and Urban-Rural Development released a comprehensive technical standard for municipal solid waste landfills, standardized landfill practices are not well understood and limited-controlled landfill sites are often poorly managed in small and medium-sized cities, unlike metropolitan sanitary landfill sites (Zhang et al., 2010). The limitedcontrolled landfills in this study means that a kind of landfill located in rural areas far from the big cities. And most of limited-controlled
Corresponding author. E-mail address: [email protected] (S. Chen).
https://doi.org/10.1016/j.envint.2019.03.052 Received 19 November 2018; Received in revised form 22 March 2019; Accepted 22 March 2019 0160-4120/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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five leachate treatment plants in South China, landfill leachate treatment could be a significant potential source of N2O emission, with the N2O flux and dissolved N2O of 58.8 ng mL−1 h−1 and 1309 ng mL−1, respectively (Lin et al., 2008). In addition, the cumulative GHG emissions from fresh leachate storage ponds, fresh leachate treatment systems, and aged leachate treatment systems were measured as 19.10, 10.62, and 3.63 Gg CO2-eq yr−1, respectively (Wang et al., 2014). Due to the shortage of treatment facilities, dissolved GHGs in the leachate discharged from limited-controlled landfills may be higher than that from sanitary landfills. Therefore, this is a nonnegligible potential source of GHGs. To complement the information of GHG emissions from limitedcontrolled landfill sites that are located in most small and medium-sized cities in China, a typical limited-controlled landfill site depositing MSW was selected in this study, and its GHG emissions were measured throughout one year. The sampling points were arranged following advice from the UK Environment Agency, allowing for a more detailed profiling of GHG release. The results from the analysis of the static chamber measurements utilising geospatial techniques (IDW) were used to estimate the flux from the whole landfill surface. Moreover, comprehensive GHG emissions from leachate storage ponds and fullscale treatment systems were investigated to provide basic data for assessing GHG emissions from the whole limited-controlled landfill site. Furthermore, the annual cumulative GHG emissions from the landfill area and leachate treatment system were evaluated to provide a scientific basis for estimating GHG emissions from limited-controlled landfills.
landfills receive solid waste from both urban areas and the countryside. Unclassified livestock carcasses, crop stalks and even building materials are dumped in these places frequently. Therefore, the composition of refuse from limited-controlled landfill differs and has more extensive heterogeneity than the waste from sanitary landfill sites. Some management of sanitary landfills are adopted in controlled landfill, such as impermeable layers, landfill gas (LFG) recovery system and leachate collection lines and leachate treatment installation. But there are still some flaws in equipment management and operation in limited-controlled landfills, and cannot meet the environmental standards due to shortages of capital investment or maintenance. Prior to 2016, there were almost 1248 limited-controlled landfill sites in China, which received 5.91 × 107 t waste annually (Department of Housing and UrbanRural Development, 2017). These limited-controlled landfill sites are an important source of GHG emissions, however, there is little information about this. Accurate sampling and analysis methods play a significant role in quantifying GHG emission fluxes from the whole landfill surface. The static chamber method is used for measuring GHG emission fluxes in many landfill sites due to its rapidity and suitability for remote regions. The reliability and accuracy of this method have been verified by many studies (Levy et al., 2011). However, measuring the emissions of the whole landfill surface by the static chamber method can be problematic, as it is measured by limited discontinuous surface chambers. The spatial heterogeneity of landfill waste would cause great fluctuations in the measured values (Spokas et al., 2003). Therefore, designing a feasible spatial sampling plan for a landfill area is extremely important for undertaking spatial structure analysis, and is of considerable economic interest (Wang and Qi, 1998). In this study, the number and locations of sampling points were planned based on monitoring advice from the UK Environment Agency (Rosevear, 2004). This method could increase the representativity of the sampling points by dividing the landfill surface into grids that can provide better estimation than random or cellular stratified sampling schemes (Levy et al., 2011). In addition, geospatial techniques (inverse distance weighting, IDW) were utilised to calculate the total GHG emissions of the landfill area through the distributed static chamber results and visualise the spatial heterogeneity of GHG emissions from the landfill area. IDW is an interpolation technique that interpolates values based on measurement values at nearby locations, weighted only by the distance from the interpolation location. This technique does not assume any type of spatial relationship, except the basic assumption that nearby points are more closely related to one another than the more distant points (Maguire et al., 1991). This method is used to describe a range of statistical techniques for determining the relationship between spatially distributed values, resulting in the estimation of the property at unsampled locations, and can also interpret the fluctuations in data with respect to spatial and temporal variation (Spokas et al., 2003). The landfill area is not the sole source of GHG emissions from the landfill site. GHG emissions from landfill leachate treatment plants are also of concern, which has been noted in the literature. In a study on
2. Materials and methods 2.1. Nanjing landfill site Nanjing Landfill Site (24°33′45″N, 117°22′18″E), a typical limitedcontrolled landfill, was selected as the object of investigation. It is located in the rural area of Nanjing County, Fujian Province. The annual average temperature is approximately 21.5 °C (−0.5–38.9 °C). The landfill area is shown in Fig. 1(a). The designed total storage capacity is 1,009,600 m3, and the landfill occupies approximately 15 ha. It has been receiving household waste from towns and rural areas in Nanjing County since 2008, with an average daily load of 70 tons. The amount of waste received during the measurement period was 25,460 tons. The site has an impermeable HDPE membrane layer, landfill leachate collection system, and a treatment plant, but no landfill gas collection system. The leachate treatment plant (Fig. 1(b)) within the landfill site covered an area of 8000 m2, which can treat approximately 150 m3 leachate per day. The leachate flows into the storage pond (SP), and is then drawn into the oxidation ditch (OD). The total area of the SP is almost 1400 m2 and provides a capacity of 3500 m3. As the storage pond is not covered, the leachate quality is greatly affected by rain dilution. The main treatment process unit is an oxidation-ditch, in
Fig. 1. Bird's-eye view of Nanjing Landfill Site. (a) Landfill area. (b) Leachate treatment plant. 388
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Fig. 2. Layout of the sampling points in the Nanjing Landfill area.
used to measure the GHG emissions from each unit of the leachate treatment plants (Wang et al., 2014). Both types of chambers were 38 L. During a sampling campaign, five 60-mL air samples were withdrawn into syringes through the sampling port with a gas-tight, three-way valve at 10-min intervals for a period of 40 min. Meanwhile, the moisture content (Water-Determination Apparatus, MP-406, Corp. ZYYD Beijing) and temperature (Digital Thermometer; JM624, Corp. JingMing Tientsin) were monitored at a depth of 6 cm below the surface of sampling points and the inside chamber.
which a surface aerator, namely a brush rotor, is used to alternately circulate the mixed liquor into the aerobic and anoxic zones. Chemical oxygen demand (COD) degradation, nitrogen, and phosphorus removal all occur in this unit. The areas of the OD and SST (secondary sedimentation tank) are 300 and 8.6m2, respectively, and the effluent is discharged through the secondary sedimentation tank. 2.2. Samples collection and analyses 2.2.1. Sampling points arrangement According to the “Guidance on monitoring landfill gas surface emissions” from the UK Environment Agency (Rosevear, 2004), the number of sample sites and their interval distance were calculated by Eqs. (1) and (2).
N = 6 + 0.15 A
(1)
D=
(2)
A/N
2.2.3. Analyses methods N2O, CH4, and CO2 fluxes were measured based on previous study (Towprayoon et al., 2005). The GHG emission fluxes were calculated from either the nonlinear or linear fitting of the change in gas concentration over time, depending on the regression coefficient previously discussed (Hutchinson and Livingston, 1993). Typically, the IPCC does not include CO2 in modeled landfill GHG inventories because anaerobic degradation of biomass is biogenic (Lemke et al., 2007). But CO2 emission is an important indicator of decomposition rate of organic matter and non-ignorable component of GHG inventory from landfill (Wang et al., 2017a). Therefore, the CO2 flux was also measured for the integrity of data in this study. The correlation between environmental factors and the of N2O, CH4, and CO2 emission fluxes were analysed by Pearson correlation analysis. If p > 0.05, the correlation was regarded as insignificant, while the correlation was treated as significant at p < 0.05. The average value and the deviation of this study were calculated by the arithmetic mean and standard deviation (SD), respectively. SPSS IBM 17 (Corp. IBM, USA) was used for statistical analysis. ArcGIS10.1 (Corp. Esri, USA) was used in this study to provide a spatial interpolation method (IDW) for illustrating the spatial patterns of GHG emissions, as well as visualise the impact of environmental factors on the GHG concentrations in the landfill area. We defined the landfill block with the former two ranges of GHG emissions as a “hotspot”. Subsequently, ArcGIS was used to separately determine the contribution of the GHG fluxes of “hotspots” to the total. Chromatograph (GC, Agilent 7890A, Palo Alto, CA) was used in GHG analysis. An electron captured detector (ECD) and a thermal conductivity detector (TCD) were equipped for monitoring N2O and CH4 + CO2. The oven temperature, ECD temperature and TCD temperature were set at 50, 340, and 250 °C, respectively (Wang et al.,
where N is the number of sample sites, D is the interval distance between adjacent sites, and A is the area of the landfill area. The using area of Nanjing Landfill is 11,474 m2. The calculated number of sampling points was 37, and their layout is shown in Fig. 2. The static chamber was placed at the centre of the grid. The distance between two adjacent chambers was 18 m. So, the GHG emission of each grid was calculated by multiplying the GHG flux of the grid and the corresponding grid area, and the total emission of the whole landfill was the sum of the above. For the leachate treatment system, three sampling rounds (RD 1–3) were conducted, during which both gas (about 3*250 mL at each point) and water samples (about 3*50 mL at each point) were collected. The leachate treatment system and sampling sections are illustrated in Fig. 3. 2.2.2. Sampling action The GHG emissions were measured every two months from April 2015 to March 2016 for over one-year period, so six sampling rounds were conducted. Long-term and intensive sampling rounds have supplied reliable data for GHG emissions (Wang et al., 2017a). The static chamber method followed the manual protocol of GHG emissions from the landfill surface (Bogner et al., 1999), and a floating chamber was 389
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Fig. 3. Sample point distribution in the leachate treatment system.
Fig. 4. (a) CH4, (b) N2O, and (c) CO2 emission fluxes from Nanjing Landfill for a one-year study period.
microbial metabolic activity and increased the GHG transfer rate inside the waste mass. These resulted in an increase of the GHG emission (R. Henneberger et al., 2015). As for in winter, the low air temperature led to the break-up of soil aggregates and low solubility of CH4 and CO2, as a result, more internal cumulative CH4 and CO2 escaped from the cracks of soil aggregates. In summary, dramatic temporal variations of GHG emission were related to the seasonal changes of environmental factors. And many environmental factors and operational parameters have been investigated to understand the mechanisms of GHG emissions, especially those related to anaerobic fermentation, nitrification, denitrification, and methane oxidation (Guo et al., 2013). The parameters affecting these biological processes, such as the moisture content, temperature, pH, and oxygen concentrations of the landfill, are key factors regulating GHG emissions (Cabral et al., 2010; Spokas and Bogner, 2011). As illustrated in Table 1, there were significant correlations between CH4 and CO2 fluxes and cover soil temperature in the landfill site (p < 0.01). The significant correlation with R = 0.245 at p < 0.01 between CH4 release and temperature was also reported in sanitary landfill (Mcbain et al., 2005). The cover soil moisture significantly affected CO2 emissions (p < 0.05). However, there was an insignificant relationship between CH4 fluxes and the cover soil moisture. This differed from the results that soil temperature and moisture were negatively and positively correlated with the CH4 emission, respectively, in an old landfill over different time-scales (Rachor et al., 2013). The influence of environmental factors on the release of GHGs varied dramatically among different landfills. For example, it was reported that CH4 and N2O emissions from a MSW landfill increased during short-term precipitation, and strong correlations existed between CH4 and N2O emissions and waste moisture (p < 0.05) (Zhang et al., 2013). N2O emissions were remarkable. In this study, N2O fluxes did not correlate with any measured environmental factors. Previous monitoring results also presented no correlation between N2O emissions and soil temperature or moisture (Rinne et al., 2005). They speculated that the emission of N2O may be related with the age of the deposited waste and the characteristic differences in the covering soil. Soil nutrients (dissolved organic matters, dissolved organic N, and total organic C), C:N ratio, and the abundance and activity of nitrifiers and denitrifiers could determine the flux of N2O (Long et al., 2018). But some
Table 1 Correlation analysis of the relationship between GHG fluxes in the landfill area and environmental factors.
N2O CH4 CO2 ⁎⁎ ⁎
N2O
CH4
CO2
Cover soil temperature
Cover soil moisture
1 0.000 0.067
0.000 1 0.805⁎⁎
0.067 0.805⁎⁎ 1
0.131 0.304⁎⁎ 0.432⁎⁎
−0.047 0.147 0.153⁎
Correlation is significant at the 0.01 level (2-tailed). Correlation is significant at the 0.05 level (2-tailed).
2017b). Each gas sample was assayed in triplicate. 3. Results and discussion 3.1. Temporal variation of GHG emissions from landfill area As shown in Fig. 4, CH4, N2O, and CO2 emissions ranged from 303.65 to 1435.36 mg C m−2 h−1, −4.18 to 0.85 mg N m−2 h−1, and 798.89 to 2106.09 mg C m−2 h−1, respectively, during the monitoring period. The N2O fluxes between each sampling time show no significant differences (p = 0.073), while CH4 and CO2 (p = 0.026 and p = 0.014) were statistically different. The GHG emissions in this study were similar to those of a landfill without a LFG recovery system, and the correspond emissions ranged from 4.7 to 796 mg C m−2 h−1, 0.09 to 2.21 mg N m−2 h−1, and 131 to 811 mg C m−2 h−1, respectively (Zhang et al., 2013). However, GHG emission fluxes from a landfill in north-eastern Illinois were almost negligible compared to ours due to the high level of engineered gas control, such as a pumped gas recovery system with vertical wells (Bogner et al., 1999). This indicates that ineffective management results in high GHG emissions. However, all the data they obtained were based on short-term observation. The long observation period in this study might provide more accurate results. There was no raining day during the sampling period. The magnitude of CH4 and CO2 emissions varied with seasonal variation, presenting high values in the summer (06/08/2015,08/12/2015) and winter (01/06/2016), and low values in the spring and autumn, while there were no obvious differences in the variations of N2O emissions (Fig. 4b). In summer, the rise in temperature probably enhanced 390
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Fig. 5. Contour plots of the GHG emission fluxes of the landfill area.
As shown in Fig. 5, the “hotspots” of CH4 and N2O were only partial overlapped. It may take years for solid waste to completely stabilise and release large amounts of CH4 during the decomposition of its biodegradable components. Therefore, the hotspot regions of CH4 did not vary dramatically. The hotspot areas of CO2 almost covered the same zones as the CH4 hotspot. However, the N2O hotspots were more dependent on the movement of active dumping areas (Fig. 5). The presentation of contour maps was consistent with the correlation analysis results (Table 1). Namely, there was a significant correlation between CH4 and CO2 fluxes, which was also reported by other studies (Abushammala et al., 2012; Chen et al., 2008). The hotspots of GHG release were dispersed throughout the landfill area and accounted for a large proportion of the total GHG emissions. Therefore, there is a great risk when accounting the GHG emissions of a landfill site through as the results measured at a random distribution of sampling points.
researchers also found that environment factors, such as temperatures and oxygen content inside the landfill site, would be the potential factors influencing N2O production (Nag et al., 2016b), which had been proved by batch experiments (Nag et al., 2016a). Therefore, high N2O emissions would be the joint combinations of high waste moisture content, available N, and restricted aeration in the fresh refuse (Bogner et al., 2011). In this study, N2O emissions may be correlated with the nitrogen content of the waste, rather than the environmental factors of the covering soil. It would be discussed in more detail in the next section. 3.2. Spatial variation of GHG emissions from landfill area Due to the spatial heterogeneity of waste in the landfill, there would be noticeable discrepancies between the GHG emissions of different sampling points. Fig. 5 shows the contoured results of the IDW analysis of the CH4, N2O, and CO2 fluxes from the whole landfill surface. The contour plots of the GHG emissions showed that the “hotspots” of N2O emissions in this study were entirely overlapped with the active dumping areas (Fig. 5). Lower N2O emissions were observed in the regions filled with aged waste and a sandy soil cover. It has been found that the N2O emissions from MSW landfill could be minimised by covering the waste with infertile sandy soil owing to its low content of organic matter and nitrogen compounds (Pinjing et al., 2008; Zhang et al., 2008). As shown in Table 2, the N2O emissions from the “hotspots” (approximately 5% of total landfill area) accounted for approximately 25% of the total N2O emissions at each sampling time. This was further an evidence demonstrating that higher N2O emissions were more related with the age of the deposited waste than the covering soil (Rinne et al., 2005), and fresh waste was speculated to be an source of intense N2O release. Similar to N2O, the release of CH4 from “hotspots” (2.72–13.02% of the total area) significantly affected the total release magnitude (7.48–39.41% of the total CH4 emission) (Table 2). The spatial variability in CH4 fluxes was more dramatic throughout the landfill surface, with a CV ranging from 114% (08/12/2015) to 248% (01/06/2016). There was a large gap between the highest and lowest flux values, spanning up to five orders of magnitude.
3.3. GHG emissions from the leachate treatment system The leachate treatment system was an important source of GHG generation (Monni et al., 2007), particularly for N2O. The nitrification and denitrification reactions inside the waste mass were very limited (Barton and Atwater, 2002), therefore, N2O was mainly generated from the disposal process of leachate treatment. Fig. 6 shows the GHG emissions from the different units of the leachate treatment process. The emissions of CH4, N2O, and CO2 in the SP and STT were positive, but negligible compared with those in the OD. Low N2O fluxes (−0.01–3.61 mg N m−2 h−1) were detected at the storage pond, which were consistent with our other measurements in Dongbu and Dongfu landfill in Xiamen, China (Wang et al., 2014). CH4 and CO2 fluxes varied considerably from the storage pond, ranging from 3.24 to 51.61 mg C m−2 h−1 and from 31.03 to 179.89 mg C m−2 h−1, respectively, indicating that the dissolved organic matters may continue to anaerobically decay in the storage ponds (Czepiel et al., 1995). The N2O production ranged from 8.14 to 171.98 mg N m−2 h−1 with an average of 54.92 mg N m−2 h−1 from the OD-O. The N2O emissions from the OD was almost 10-to 90-fold higher than those from the other units. However, the changes of CH4 and CO2 did not present the similar 391
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Table 2 Proportion of hotspot flux to the total flux of the landfill area. Sampling date
Average flux of whole landfill area
Flux of the hotspot (mg C/N m−2 h−1)
Proportion of hotspot area to total area (%)
Proportion of hotspot flux to total flux
(mg C/N m−2 h−1)
04/02/2015 06/08/2015 08/12/2015 10/18/2015 01/06/2016 03/01/2016
(%)
CH4
N2O
CO2
CH4
N2O
CO2
CH4
N2O
CO2
CH4
N2O
CO2
482.80 1046.59 761.21 321.46 1324.73 564.58
4.75 2.15 1.76 1.15 0.87 1.2
1469.29 1916,13 1923.98 809.78 2495.91 997.45
1536.42–4305.31 2572.220–6625.06 1712.38–4475.74 572.54–2484.32 5778.61–16,956.03 2844.37–9149.34
12.63–35.70 10.88–31.16 4.72–12.04 2.69–7.20 3.60–10.95 3.06–7.86
2923.27–6360.02 3143.84–7641.89 2603.87–6383.59 1296.34–2909.89 6338.57–20,458.60 1862.42–5901.81
5.24 2.23 4.87 5.88 3.35 6.84
3.03 11.15 3.69 13.02 3.54 2.72
11.45 15.82 4.07 14.94 5.34 10.12
21.58 18.98 18.79 20.02 23.68 25.14
28.26 30.59 7.48 39.41 25.13 22.37
13.89 4.93 8.30 12.42 16.10 18.64
system were unified in terms of CO2-equivalent, as CH4 and N2O have a GWP 21 and 298 times that of CO2 over a 100-year horizon (IPCC, 2007), respectively (Table 4). Meanwhile CO2 emission was also accounted in this table for data integrity and consistency. It should be noted that the compare between our results and model estimates based on the IPCC inventory method should not be conducted because the IPCC only considered CH4. The GHG emission proportions of the landfill area and leachate treatment system were 94.01% and 5.99%, respectively. CH4 was the major contributor to the total emissions. This result differed from those of our previous studies, in which N2O accounted for up to 96.47% of the warming effect of all emitted GHG in the leachate system (Wang et al., 2014). The annual GHG emissions from the limited-controlled landfill were 1.078 Gg CO2-eq yr−1. Limited-controlled landfills might be an insignificant GHG emission source compared to large-scale landfills. However, based on China's nationwide data, there were nearly 1248 limited-controlled landfills that accepted 5.91 × 107 t municipal annually waste up to 2016 (Department of Housing and Urban-Rural Development, 2017). Therefore, according to our measurements, approximately 1334 Gg CO2-eq GHG was released from these landfill sites per year. Furthermore, based on the corresponding waste treating capacity, the total GHG emission could be translated to 0.042 t CO2-eq t−1 waste yr−1. As for sanitary landfills of South Africa, this value was in the range of 145 to 1016 kg CO2-eq t−1 waste (Elena and Cristina, 2013). This indicated that the GHG emission factor of limited-controlled landfill was less than some sanitary landfills. In contrast, the GHG emissions from some sanitary landfills were at low levels because of the installation of GHG recovery system and good management. Up to about 1000 kg CO2-eq t−1 waste for the open dump, 300 kg CO2eq t−1 waste for conventional landfilling of mixed waste and 70 kg CO2eq t−1 waste for low-organic‑carbon waste landfills in EU were reported (Manfredi et al., 2009). In the light of this, limited-controlled landfills would be a considerable GHG release source. Therefore, GHG emissions from limited-controlled landfills should be paid more attention when we determine the GHG emissions of waste sector. In order to obtain accurate GHG emissions data, typical limited-controlled landfills should
pattern as N2O, and the emissions differed at each sampling time. The GHG emission fluxes from this system were much lower than those from the leachate treatment plants of the sanitary landfill sites. The N2O emissions were 105 to 106 mg N m−2 h−1 from treatment systems in sanitary landfills, and the total annual GHG emission in CO2 equivalent was 10.62 Gg CO2-eq yr−1 when the system was applied to treat fresh leachate (Wang et al., 2014). This difference is due to many reasons. First, the absence of a stormwater draining channel and cover membrane of landfill area led to the interfusion of the leachate and stormwater before they flowed into SP, resulting in lower concentrations of COD and NH4+-N in the leachate. Therefore, the relatively low concentration of pollutant led to low GHG production from this leachate treatment system. Second, the open-air state of the storage pond resulted in non-strict anaerobic conditions. The anaerobic zones highly contributed to CH4 emissions, which has been demonstrated in several studies (Ahn et al., 2010; Daelman et al., 2013; Rajagopal and Béline, 2011; Wang et al., 2014). Third, the oxide ditch was operated unsteadily, resulting in low biodegradation efficiency. Due to the inactive nitrification and denitrification (Table 3) in the oxidation ditch, the N2O potential production pathways were inhibited and resulted in poor N2O emissions. The average leachate characteristics of each treatment unit are shown in Table 3. The management and maintenance of the leachate treatment facilities were improper. The leachate treatment was not functional for nearly the whole month of January 2016, therefore, we could not obtain a leachate sample. Abnormal conditions, such as mechanical failure or pipe blockage, also occurred several times during our sampling period. Therefore, high TN-content leachate was discharged. This remains a concern as N2O would be produced by the microorganisms in the leachate receptor through nitrification and denitrification, and the system would be a potential GHG release source of the limited-landfill site.
3.4. GHG emissions from Nanjing landfill The GHG emissions of the landfill area and leachate treatment
Fig. 6. Fluxes of (a) CH4, (b) N2O and (c) CO2 in different treatment units. 392
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Table 3 Characteristics of leachate (Unit: mg L−1). NO2−-N
NH4+-N SP OD-O OD-A SST Effluent
893.2 780.3 634.2 554.8 523.1
± ± ± ± ±
511.0 521.1 612.5 541.7 531.3
NO3−-N
74.9 ± 45.9 224.2 ± 102.3 228.8 ± 101.8 214.5 ± 97.8 292.8 ± 83.1
197.8 262.4 271.6 241.0 216.6
± ± ± ± ±
112.3 192.0 200.3 162.4 130.4
TN 1202.0 1184.8 1089.6 1081.2 1058.7
± ± ± ± ±
COD 842.4 827.5 862.1 669.8 743.2
1486.3 ± 1068.2 1289.3 ± 908.5 1280.9 ± 948.5 1300.8 ± 924.6 1196.4 ± 777.8
Table 4 Annual GHG emissions from Nanjing Landfill Site.
Landfill area Landfill leachate plant
Unit SP OD SST Total
Proportion of total emissions (%)
CH4 (Gg CO2-eq yr−1)
N2O (Gg CO2-eq yr−1)
CO2 (Gg CO2-eq yr−1)
Total (Gg CO2-eq yr−1)
Proportion of total emissions (%)
0.832 CH4 (T CO2-eq yr−1) 7.000 0.390 0.002 7.392 77.90
0.028 N2O (T CO2-eq yr−1) 4.258 42.851 0.045 47.154 7.00
0.153 CO2 (T CO2-eq yr−1) 5.124 4.768 0.077 9.969 15.10
1.013 Total (T CO2-eq yr−1) 16.382 48.010 0.124 64.516
94.01
be selected to measure the emission flux from areas with different economic development levels and climatic situations over a long period. Furthermore, more funding should be provided to improve the management level of such landfills, so as to introduce the standardized operation to reduce GHG emissions and degrade pollutants. For example, the landfill area and leachate storage pond should be covered by a HDPE membrane or other materials, and a rainwater collection and discharge system need to be constructed to reduce leachate production. In-site leachate treatment system should be well operated and maintained to ensure the effluent meet the discharge standard.
5.99 100
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4. Conclusion In this study, intense GHG emissions were observed in the landfill area, especially from the “hotspot” zones. The results revealed that the “hotspot” regions were discrete blocks, while significantly affected the magnitude of total GHG emissions. Moreover, the release of N2O was concentrated in the area of newly dumped waste, while CH4 and CO2 release varied with environmental factors. The leachate treatment system of Nanjing landfill was not a significant GHG source owing to insufficient nitrification-denitrification activities. Taking landfill reservoir and leachate treatment system into account, the annual GHG emissions was 1.078 Gg CO2-eq yr−1, which was much lower than that of sanitary landfills in large cities. However, in consideration of the large number of limited-controlled landfills, the total GHG emissions from limited-controlled landfills should not be neglected. It was estimated that approximately 1334 Gg CO2-eq GHG was released from this type of landfill. The estimation of GHG emissions from limited-controlled landfill sites and their contribution to GHG inventories are indispensable, which may help to improve GHG guidelines for landfill management in the vast rural area of China. However, the emissions from limited-control landfills should be further investigated, as the climate and waste composition vary in different places, and more typical limited-controlled landfills should be taken as case studies. Acknowledgement This research was financially supported by the National Natural Science Foundation of China (Grant No. 41475130, 51708536), and the ‘Strategic Priority Research Program-Climate Change: Carbon Budget and Relevant Issues’ of the Chinese Academy of Sciences (Grant No. XDA05020602). 393
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Levy, P.E., Gray, A., Leeson, S.R., Gaiawyn, J., Kelly, M.P.C., Cooper, M.D.A., Dinsmore, K.J., Jones, S.K., Sheppard, L.J., 2011. Quantification of uncertainty in trace gas fluxes measured by the static chamber method. Eur. J. Soil Sci. 62, 811–821. Lin, L., Lan, C.Y., Huang, L.N., Chan, G.Y., 2008. Anthropogenic N2O production from landfill leachate treatment. J. Environ. Manag. 87, 341–349. Long, X.E., Ying, H., Chi, H., Li, Y., Ahmad, N., Yao, H., 2018. Nitrous oxide flux, ammonia oxidizer and denitrifier abundance and activity across three different landfill cover soils in Ningbo, China. J. Clean. Prod. 170. Maguire, D.J., Goodchild, M.F., Rhind, D.W., 1991. Geographical Information Systems: Principles and Applications. Volume 2: Applications. Wiley. Manfredi, S., Tonini, D., Christensen, T.H., Scharff, H., 2009. Landfilling of waste: accounting of greenhouse gases and global warming contributions. Waste Manag. Res. 27, 825–836. Mcbain, M.C., Warland, J.S., Mcbride, R.A., Wagnerriddle, C., 2005. Micrometeorological measurements of N2O and CH4 emissions from a municipal solid waste landfill. Waste Manag. Res. 23, 409–419. Monni, S., Pipatti, R., Lehtilä, A., Savolainen, I., Syri, S., 2007. Global Climate Change Mitigation Scenarios for Solid Waste Management. Vtt Technical Research Centre of Finland. Nag, M., Shimaoka, T., Komiya, T., 2016a. Nitrous oxide production during nitrification from organic solid waste under temperature and oxygen conditions. Environ. Technol. 37, 2890–2897. Nag, M., Shimaoka, T., Nakayama, H., Komiya, T., Chai, X., 2016b. Field study of nitrous oxide production with in situ aeration in a closed landfill site. J. Air Waste Manage. Assoc. 66, 280–287 (Taylor & Fran. Nagamori, M., Koide, T., Wijewardane, N.K., Watanabe, Y., Isobe, Y., Mowjood, M.I.M., Ishigaki, T., Kawamoto, K., 2013. Flux Measurements of Greenhouse Gases from an Abandoned Open Dumping Site of Solid Waste in Sri Lanka. Pinjing, H., Miao, C., Houhu, Z., Liming, S., 2008. Effects of leachate irrigation and cover soil type on N2O emission from municipal solid waste landfill. Chin. J. Appl. Ecol. 19, 1591–1596. Rachor, I.M., Gebert, J., Gröngröft, A., Pfeiffer, E.M., 2013. Variability of methane emissions from an old landfill over different time-scales. Eur. J. Soil Sci. 64, 16–26. Rajagopal, R., Béline, F., 2011. Nitrogen removal via nitrite pathway and the related nitrous oxide emission during piggery wastewater treatment. Bioresour. Technol. 102, 4042–4046. Rinne, J., Pihlatie, M., Lohila, A., Thum, T., Aurela, M., Tuovinen, J.P., Laurila, T., Vesala, T., 2005. Nitrous oxide emissions from a municipal landfill. Environ. Sci. Technol. 39,
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Temporal and spatial variation of greenhouse gas emissions from a limitedcontrolled landfill site Chengliang Zhanga,b, Yan Guoa,b, Xiaojun Wanga, Shaohua Chena, a b
T
⁎
Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C LE I N FO
A B S T R A C T
Handling Editor: Yong-Guan Zhu
Landfilling biodegradable waste is an important source of global greenhouse gas (GHG) emissions. Among the several types of landfill, limited-controlled landfill is a common method used to dispose of domestic solid waste, especially in developing countries. However, information about GHG emissions from limited-controlled landfill sites has rarely been reported. In this study, the GHG emissions from a typical limited-controlled landfill site were investigated under a regular period for one year. The number and positions of static chambers were arranged according to the guidance on Monitoring Landfill Gas Surface Emissions by the UK Environment Agency to obtain representative data from the heterogeneous surface of the landfill. Inverse distance weighting (IDW) was applied to evaluate and visualise the GHG emissions from the whole landfill surface based on the measurements of distributed static chambers. As an important GHG source of the landfill site, the emissions from the landfill leachate treatment plant were also measured. The results revealed that CH4 and N2O emission fluxes from the landfill area were 1324.73 ± 2005.17 mg C m−2 d−1 and 2.16 ± 2.33 mg N m−2 d−1, respectively, and the fluxes from the leachate treatment plants were 23.92 ± 29.20 mg C m−2 d−1 and 16.40 ± 16.89 mg N m−2 d−1, respectively. CH4 and N2O releases preferred to present spatial heterogeneity, while temporal heterogeneity was expected to exist in CH4 and CO2 emissions. The annual GHG emissions from the limited-controlled landfill was calculated to be 1.078 Gg CO2-eq yr−1, which was the least among all types of landfill sites. In addition, the GHG emission factor was 0.042 t CO2-eq t−1 waste yr−1 which could not be ignored compared to the sanitary landfills. Therefore, it is advisable to give more attention and determine a potential solution for reducing GHG emissions from limited-controlled landfill sites.
Keywords: Greenhouse gas emission Methane Nitrous oxide Limited-controlled landfill Landfill leachate Emission factor
1. Introduction Approximately 5% of anthropogenic greenhouse gas (GHG) emissions are derived from solid waste disposal worldwide (Solomon et al., 2013). In China, almost 1.5% (111.81 Tg CO2-eq yr−1) of the total anthropogenic GHG originated from waste treatment in 2005 (Department of Climate Change, 2013). Landfill is a common land use type, and the GHG emissions from which have received much public attention (Stocker et al., 2014). For instance, previous study reported that emission fluxes ranged from 0.9 to 433 mg CH4 m−2 h−1, 2.7 to 1200 μg N2O m−2 h−1, and 12.3 to 964.4 mg CO2 m−2 h−1 from a sanitary landfill at Perungudi in Chennai, a mega-city in India (Jha et al., 2008). In comparison, scholars have given less concern to GHG emissions from limited-controlled landfills. It has been found that average CH4, CO2, and N2O in emission ranged from < 0.04 to 1800, 4.9 to 1800, and < 0.0001 to 0.35 mL m−2 min−1, respectively, from a
⁎
dumping landfill site with waste ages of approximately 0.5 year (Nagamori et al., 2013). Moreover, studies on the factors affecting their release and control techniques also received wide concern (Börjesson and Bo, 1997; Bogner et al., 2011; Mcbain et al., 2005; Rinne et al., 2005; Zhang et al., 2009). As a developing country, China is yet to reach the capacity of developed countries to manage municipal solid waste (MSW). Landfill is currently and will continue to be the main method of MSW treatment long into the future (Hua and Zhao, 2004). Although China's Minister of Housing and Urban-Rural Development released a comprehensive technical standard for municipal solid waste landfills, standardized landfill practices are not well understood and limited-controlled landfill sites are often poorly managed in small and medium-sized cities, unlike metropolitan sanitary landfill sites (Zhang et al., 2010). The limitedcontrolled landfills in this study means that a kind of landfill located in rural areas far from the big cities. And most of limited-controlled
Corresponding author. E-mail address: [email protected] (S. Chen).
https://doi.org/10.1016/j.envint.2019.03.052 Received 19 November 2018; Received in revised form 22 March 2019; Accepted 22 March 2019 0160-4120/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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five leachate treatment plants in South China, landfill leachate treatment could be a significant potential source of N2O emission, with the N2O flux and dissolved N2O of 58.8 ng mL−1 h−1 and 1309 ng mL−1, respectively (Lin et al., 2008). In addition, the cumulative GHG emissions from fresh leachate storage ponds, fresh leachate treatment systems, and aged leachate treatment systems were measured as 19.10, 10.62, and 3.63 Gg CO2-eq yr−1, respectively (Wang et al., 2014). Due to the shortage of treatment facilities, dissolved GHGs in the leachate discharged from limited-controlled landfills may be higher than that from sanitary landfills. Therefore, this is a nonnegligible potential source of GHGs. To complement the information of GHG emissions from limitedcontrolled landfill sites that are located in most small and medium-sized cities in China, a typical limited-controlled landfill site depositing MSW was selected in this study, and its GHG emissions were measured throughout one year. The sampling points were arranged following advice from the UK Environment Agency, allowing for a more detailed profiling of GHG release. The results from the analysis of the static chamber measurements utilising geospatial techniques (IDW) were used to estimate the flux from the whole landfill surface. Moreover, comprehensive GHG emissions from leachate storage ponds and fullscale treatment systems were investigated to provide basic data for assessing GHG emissions from the whole limited-controlled landfill site. Furthermore, the annual cumulative GHG emissions from the landfill area and leachate treatment system were evaluated to provide a scientific basis for estimating GHG emissions from limited-controlled landfills.
landfills receive solid waste from both urban areas and the countryside. Unclassified livestock carcasses, crop stalks and even building materials are dumped in these places frequently. Therefore, the composition of refuse from limited-controlled landfill differs and has more extensive heterogeneity than the waste from sanitary landfill sites. Some management of sanitary landfills are adopted in controlled landfill, such as impermeable layers, landfill gas (LFG) recovery system and leachate collection lines and leachate treatment installation. But there are still some flaws in equipment management and operation in limited-controlled landfills, and cannot meet the environmental standards due to shortages of capital investment or maintenance. Prior to 2016, there were almost 1248 limited-controlled landfill sites in China, which received 5.91 × 107 t waste annually (Department of Housing and UrbanRural Development, 2017). These limited-controlled landfill sites are an important source of GHG emissions, however, there is little information about this. Accurate sampling and analysis methods play a significant role in quantifying GHG emission fluxes from the whole landfill surface. The static chamber method is used for measuring GHG emission fluxes in many landfill sites due to its rapidity and suitability for remote regions. The reliability and accuracy of this method have been verified by many studies (Levy et al., 2011). However, measuring the emissions of the whole landfill surface by the static chamber method can be problematic, as it is measured by limited discontinuous surface chambers. The spatial heterogeneity of landfill waste would cause great fluctuations in the measured values (Spokas et al., 2003). Therefore, designing a feasible spatial sampling plan for a landfill area is extremely important for undertaking spatial structure analysis, and is of considerable economic interest (Wang and Qi, 1998). In this study, the number and locations of sampling points were planned based on monitoring advice from the UK Environment Agency (Rosevear, 2004). This method could increase the representativity of the sampling points by dividing the landfill surface into grids that can provide better estimation than random or cellular stratified sampling schemes (Levy et al., 2011). In addition, geospatial techniques (inverse distance weighting, IDW) were utilised to calculate the total GHG emissions of the landfill area through the distributed static chamber results and visualise the spatial heterogeneity of GHG emissions from the landfill area. IDW is an interpolation technique that interpolates values based on measurement values at nearby locations, weighted only by the distance from the interpolation location. This technique does not assume any type of spatial relationship, except the basic assumption that nearby points are more closely related to one another than the more distant points (Maguire et al., 1991). This method is used to describe a range of statistical techniques for determining the relationship between spatially distributed values, resulting in the estimation of the property at unsampled locations, and can also interpret the fluctuations in data with respect to spatial and temporal variation (Spokas et al., 2003). The landfill area is not the sole source of GHG emissions from the landfill site. GHG emissions from landfill leachate treatment plants are also of concern, which has been noted in the literature. In a study on
2. Materials and methods 2.1. Nanjing landfill site Nanjing Landfill Site (24°33′45″N, 117°22′18″E), a typical limitedcontrolled landfill, was selected as the object of investigation. It is located in the rural area of Nanjing County, Fujian Province. The annual average temperature is approximately 21.5 °C (−0.5–38.9 °C). The landfill area is shown in Fig. 1(a). The designed total storage capacity is 1,009,600 m3, and the landfill occupies approximately 15 ha. It has been receiving household waste from towns and rural areas in Nanjing County since 2008, with an average daily load of 70 tons. The amount of waste received during the measurement period was 25,460 tons. The site has an impermeable HDPE membrane layer, landfill leachate collection system, and a treatment plant, but no landfill gas collection system. The leachate treatment plant (Fig. 1(b)) within the landfill site covered an area of 8000 m2, which can treat approximately 150 m3 leachate per day. The leachate flows into the storage pond (SP), and is then drawn into the oxidation ditch (OD). The total area of the SP is almost 1400 m2 and provides a capacity of 3500 m3. As the storage pond is not covered, the leachate quality is greatly affected by rain dilution. The main treatment process unit is an oxidation-ditch, in
Fig. 1. Bird's-eye view of Nanjing Landfill Site. (a) Landfill area. (b) Leachate treatment plant. 388
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Fig. 2. Layout of the sampling points in the Nanjing Landfill area.
used to measure the GHG emissions from each unit of the leachate treatment plants (Wang et al., 2014). Both types of chambers were 38 L. During a sampling campaign, five 60-mL air samples were withdrawn into syringes through the sampling port with a gas-tight, three-way valve at 10-min intervals for a period of 40 min. Meanwhile, the moisture content (Water-Determination Apparatus, MP-406, Corp. ZYYD Beijing) and temperature (Digital Thermometer; JM624, Corp. JingMing Tientsin) were monitored at a depth of 6 cm below the surface of sampling points and the inside chamber.
which a surface aerator, namely a brush rotor, is used to alternately circulate the mixed liquor into the aerobic and anoxic zones. Chemical oxygen demand (COD) degradation, nitrogen, and phosphorus removal all occur in this unit. The areas of the OD and SST (secondary sedimentation tank) are 300 and 8.6m2, respectively, and the effluent is discharged through the secondary sedimentation tank. 2.2. Samples collection and analyses 2.2.1. Sampling points arrangement According to the “Guidance on monitoring landfill gas surface emissions” from the UK Environment Agency (Rosevear, 2004), the number of sample sites and their interval distance were calculated by Eqs. (1) and (2).
N = 6 + 0.15 A
(1)
D=
(2)
A/N
2.2.3. Analyses methods N2O, CH4, and CO2 fluxes were measured based on previous study (Towprayoon et al., 2005). The GHG emission fluxes were calculated from either the nonlinear or linear fitting of the change in gas concentration over time, depending on the regression coefficient previously discussed (Hutchinson and Livingston, 1993). Typically, the IPCC does not include CO2 in modeled landfill GHG inventories because anaerobic degradation of biomass is biogenic (Lemke et al., 2007). But CO2 emission is an important indicator of decomposition rate of organic matter and non-ignorable component of GHG inventory from landfill (Wang et al., 2017a). Therefore, the CO2 flux was also measured for the integrity of data in this study. The correlation between environmental factors and the of N2O, CH4, and CO2 emission fluxes were analysed by Pearson correlation analysis. If p > 0.05, the correlation was regarded as insignificant, while the correlation was treated as significant at p < 0.05. The average value and the deviation of this study were calculated by the arithmetic mean and standard deviation (SD), respectively. SPSS IBM 17 (Corp. IBM, USA) was used for statistical analysis. ArcGIS10.1 (Corp. Esri, USA) was used in this study to provide a spatial interpolation method (IDW) for illustrating the spatial patterns of GHG emissions, as well as visualise the impact of environmental factors on the GHG concentrations in the landfill area. We defined the landfill block with the former two ranges of GHG emissions as a “hotspot”. Subsequently, ArcGIS was used to separately determine the contribution of the GHG fluxes of “hotspots” to the total. Chromatograph (GC, Agilent 7890A, Palo Alto, CA) was used in GHG analysis. An electron captured detector (ECD) and a thermal conductivity detector (TCD) were equipped for monitoring N2O and CH4 + CO2. The oven temperature, ECD temperature and TCD temperature were set at 50, 340, and 250 °C, respectively (Wang et al.,
where N is the number of sample sites, D is the interval distance between adjacent sites, and A is the area of the landfill area. The using area of Nanjing Landfill is 11,474 m2. The calculated number of sampling points was 37, and their layout is shown in Fig. 2. The static chamber was placed at the centre of the grid. The distance between two adjacent chambers was 18 m. So, the GHG emission of each grid was calculated by multiplying the GHG flux of the grid and the corresponding grid area, and the total emission of the whole landfill was the sum of the above. For the leachate treatment system, three sampling rounds (RD 1–3) were conducted, during which both gas (about 3*250 mL at each point) and water samples (about 3*50 mL at each point) were collected. The leachate treatment system and sampling sections are illustrated in Fig. 3. 2.2.2. Sampling action The GHG emissions were measured every two months from April 2015 to March 2016 for over one-year period, so six sampling rounds were conducted. Long-term and intensive sampling rounds have supplied reliable data for GHG emissions (Wang et al., 2017a). The static chamber method followed the manual protocol of GHG emissions from the landfill surface (Bogner et al., 1999), and a floating chamber was 389
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Fig. 3. Sample point distribution in the leachate treatment system.
Fig. 4. (a) CH4, (b) N2O, and (c) CO2 emission fluxes from Nanjing Landfill for a one-year study period.
microbial metabolic activity and increased the GHG transfer rate inside the waste mass. These resulted in an increase of the GHG emission (R. Henneberger et al., 2015). As for in winter, the low air temperature led to the break-up of soil aggregates and low solubility of CH4 and CO2, as a result, more internal cumulative CH4 and CO2 escaped from the cracks of soil aggregates. In summary, dramatic temporal variations of GHG emission were related to the seasonal changes of environmental factors. And many environmental factors and operational parameters have been investigated to understand the mechanisms of GHG emissions, especially those related to anaerobic fermentation, nitrification, denitrification, and methane oxidation (Guo et al., 2013). The parameters affecting these biological processes, such as the moisture content, temperature, pH, and oxygen concentrations of the landfill, are key factors regulating GHG emissions (Cabral et al., 2010; Spokas and Bogner, 2011). As illustrated in Table 1, there were significant correlations between CH4 and CO2 fluxes and cover soil temperature in the landfill site (p < 0.01). The significant correlation with R = 0.245 at p < 0.01 between CH4 release and temperature was also reported in sanitary landfill (Mcbain et al., 2005). The cover soil moisture significantly affected CO2 emissions (p < 0.05). However, there was an insignificant relationship between CH4 fluxes and the cover soil moisture. This differed from the results that soil temperature and moisture were negatively and positively correlated with the CH4 emission, respectively, in an old landfill over different time-scales (Rachor et al., 2013). The influence of environmental factors on the release of GHGs varied dramatically among different landfills. For example, it was reported that CH4 and N2O emissions from a MSW landfill increased during short-term precipitation, and strong correlations existed between CH4 and N2O emissions and waste moisture (p < 0.05) (Zhang et al., 2013). N2O emissions were remarkable. In this study, N2O fluxes did not correlate with any measured environmental factors. Previous monitoring results also presented no correlation between N2O emissions and soil temperature or moisture (Rinne et al., 2005). They speculated that the emission of N2O may be related with the age of the deposited waste and the characteristic differences in the covering soil. Soil nutrients (dissolved organic matters, dissolved organic N, and total organic C), C:N ratio, and the abundance and activity of nitrifiers and denitrifiers could determine the flux of N2O (Long et al., 2018). But some
Table 1 Correlation analysis of the relationship between GHG fluxes in the landfill area and environmental factors.
N2O CH4 CO2 ⁎⁎ ⁎
N2O
CH4
CO2
Cover soil temperature
Cover soil moisture
1 0.000 0.067
0.000 1 0.805⁎⁎
0.067 0.805⁎⁎ 1
0.131 0.304⁎⁎ 0.432⁎⁎
−0.047 0.147 0.153⁎
Correlation is significant at the 0.01 level (2-tailed). Correlation is significant at the 0.05 level (2-tailed).
2017b). Each gas sample was assayed in triplicate. 3. Results and discussion 3.1. Temporal variation of GHG emissions from landfill area As shown in Fig. 4, CH4, N2O, and CO2 emissions ranged from 303.65 to 1435.36 mg C m−2 h−1, −4.18 to 0.85 mg N m−2 h−1, and 798.89 to 2106.09 mg C m−2 h−1, respectively, during the monitoring period. The N2O fluxes between each sampling time show no significant differences (p = 0.073), while CH4 and CO2 (p = 0.026 and p = 0.014) were statistically different. The GHG emissions in this study were similar to those of a landfill without a LFG recovery system, and the correspond emissions ranged from 4.7 to 796 mg C m−2 h−1, 0.09 to 2.21 mg N m−2 h−1, and 131 to 811 mg C m−2 h−1, respectively (Zhang et al., 2013). However, GHG emission fluxes from a landfill in north-eastern Illinois were almost negligible compared to ours due to the high level of engineered gas control, such as a pumped gas recovery system with vertical wells (Bogner et al., 1999). This indicates that ineffective management results in high GHG emissions. However, all the data they obtained were based on short-term observation. The long observation period in this study might provide more accurate results. There was no raining day during the sampling period. The magnitude of CH4 and CO2 emissions varied with seasonal variation, presenting high values in the summer (06/08/2015,08/12/2015) and winter (01/06/2016), and low values in the spring and autumn, while there were no obvious differences in the variations of N2O emissions (Fig. 4b). In summer, the rise in temperature probably enhanced 390
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Fig. 5. Contour plots of the GHG emission fluxes of the landfill area.
As shown in Fig. 5, the “hotspots” of CH4 and N2O were only partial overlapped. It may take years for solid waste to completely stabilise and release large amounts of CH4 during the decomposition of its biodegradable components. Therefore, the hotspot regions of CH4 did not vary dramatically. The hotspot areas of CO2 almost covered the same zones as the CH4 hotspot. However, the N2O hotspots were more dependent on the movement of active dumping areas (Fig. 5). The presentation of contour maps was consistent with the correlation analysis results (Table 1). Namely, there was a significant correlation between CH4 and CO2 fluxes, which was also reported by other studies (Abushammala et al., 2012; Chen et al., 2008). The hotspots of GHG release were dispersed throughout the landfill area and accounted for a large proportion of the total GHG emissions. Therefore, there is a great risk when accounting the GHG emissions of a landfill site through as the results measured at a random distribution of sampling points.
researchers also found that environment factors, such as temperatures and oxygen content inside the landfill site, would be the potential factors influencing N2O production (Nag et al., 2016b), which had been proved by batch experiments (Nag et al., 2016a). Therefore, high N2O emissions would be the joint combinations of high waste moisture content, available N, and restricted aeration in the fresh refuse (Bogner et al., 2011). In this study, N2O emissions may be correlated with the nitrogen content of the waste, rather than the environmental factors of the covering soil. It would be discussed in more detail in the next section. 3.2. Spatial variation of GHG emissions from landfill area Due to the spatial heterogeneity of waste in the landfill, there would be noticeable discrepancies between the GHG emissions of different sampling points. Fig. 5 shows the contoured results of the IDW analysis of the CH4, N2O, and CO2 fluxes from the whole landfill surface. The contour plots of the GHG emissions showed that the “hotspots” of N2O emissions in this study were entirely overlapped with the active dumping areas (Fig. 5). Lower N2O emissions were observed in the regions filled with aged waste and a sandy soil cover. It has been found that the N2O emissions from MSW landfill could be minimised by covering the waste with infertile sandy soil owing to its low content of organic matter and nitrogen compounds (Pinjing et al., 2008; Zhang et al., 2008). As shown in Table 2, the N2O emissions from the “hotspots” (approximately 5% of total landfill area) accounted for approximately 25% of the total N2O emissions at each sampling time. This was further an evidence demonstrating that higher N2O emissions were more related with the age of the deposited waste than the covering soil (Rinne et al., 2005), and fresh waste was speculated to be an source of intense N2O release. Similar to N2O, the release of CH4 from “hotspots” (2.72–13.02% of the total area) significantly affected the total release magnitude (7.48–39.41% of the total CH4 emission) (Table 2). The spatial variability in CH4 fluxes was more dramatic throughout the landfill surface, with a CV ranging from 114% (08/12/2015) to 248% (01/06/2016). There was a large gap between the highest and lowest flux values, spanning up to five orders of magnitude.
3.3. GHG emissions from the leachate treatment system The leachate treatment system was an important source of GHG generation (Monni et al., 2007), particularly for N2O. The nitrification and denitrification reactions inside the waste mass were very limited (Barton and Atwater, 2002), therefore, N2O was mainly generated from the disposal process of leachate treatment. Fig. 6 shows the GHG emissions from the different units of the leachate treatment process. The emissions of CH4, N2O, and CO2 in the SP and STT were positive, but negligible compared with those in the OD. Low N2O fluxes (−0.01–3.61 mg N m−2 h−1) were detected at the storage pond, which were consistent with our other measurements in Dongbu and Dongfu landfill in Xiamen, China (Wang et al., 2014). CH4 and CO2 fluxes varied considerably from the storage pond, ranging from 3.24 to 51.61 mg C m−2 h−1 and from 31.03 to 179.89 mg C m−2 h−1, respectively, indicating that the dissolved organic matters may continue to anaerobically decay in the storage ponds (Czepiel et al., 1995). The N2O production ranged from 8.14 to 171.98 mg N m−2 h−1 with an average of 54.92 mg N m−2 h−1 from the OD-O. The N2O emissions from the OD was almost 10-to 90-fold higher than those from the other units. However, the changes of CH4 and CO2 did not present the similar 391
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Table 2 Proportion of hotspot flux to the total flux of the landfill area. Sampling date
Average flux of whole landfill area
Flux of the hotspot (mg C/N m−2 h−1)
Proportion of hotspot area to total area (%)
Proportion of hotspot flux to total flux
(mg C/N m−2 h−1)
04/02/2015 06/08/2015 08/12/2015 10/18/2015 01/06/2016 03/01/2016
(%)
CH4
N2O
CO2
CH4
N2O
CO2
CH4
N2O
CO2
CH4
N2O
CO2
482.80 1046.59 761.21 321.46 1324.73 564.58
4.75 2.15 1.76 1.15 0.87 1.2
1469.29 1916,13 1923.98 809.78 2495.91 997.45
1536.42–4305.31 2572.220–6625.06 1712.38–4475.74 572.54–2484.32 5778.61–16,956.03 2844.37–9149.34
12.63–35.70 10.88–31.16 4.72–12.04 2.69–7.20 3.60–10.95 3.06–7.86
2923.27–6360.02 3143.84–7641.89 2603.87–6383.59 1296.34–2909.89 6338.57–20,458.60 1862.42–5901.81
5.24 2.23 4.87 5.88 3.35 6.84
3.03 11.15 3.69 13.02 3.54 2.72
11.45 15.82 4.07 14.94 5.34 10.12
21.58 18.98 18.79 20.02 23.68 25.14
28.26 30.59 7.48 39.41 25.13 22.37
13.89 4.93 8.30 12.42 16.10 18.64
system were unified in terms of CO2-equivalent, as CH4 and N2O have a GWP 21 and 298 times that of CO2 over a 100-year horizon (IPCC, 2007), respectively (Table 4). Meanwhile CO2 emission was also accounted in this table for data integrity and consistency. It should be noted that the compare between our results and model estimates based on the IPCC inventory method should not be conducted because the IPCC only considered CH4. The GHG emission proportions of the landfill area and leachate treatment system were 94.01% and 5.99%, respectively. CH4 was the major contributor to the total emissions. This result differed from those of our previous studies, in which N2O accounted for up to 96.47% of the warming effect of all emitted GHG in the leachate system (Wang et al., 2014). The annual GHG emissions from the limited-controlled landfill were 1.078 Gg CO2-eq yr−1. Limited-controlled landfills might be an insignificant GHG emission source compared to large-scale landfills. However, based on China's nationwide data, there were nearly 1248 limited-controlled landfills that accepted 5.91 × 107 t municipal annually waste up to 2016 (Department of Housing and Urban-Rural Development, 2017). Therefore, according to our measurements, approximately 1334 Gg CO2-eq GHG was released from these landfill sites per year. Furthermore, based on the corresponding waste treating capacity, the total GHG emission could be translated to 0.042 t CO2-eq t−1 waste yr−1. As for sanitary landfills of South Africa, this value was in the range of 145 to 1016 kg CO2-eq t−1 waste (Elena and Cristina, 2013). This indicated that the GHG emission factor of limited-controlled landfill was less than some sanitary landfills. In contrast, the GHG emissions from some sanitary landfills were at low levels because of the installation of GHG recovery system and good management. Up to about 1000 kg CO2-eq t−1 waste for the open dump, 300 kg CO2eq t−1 waste for conventional landfilling of mixed waste and 70 kg CO2eq t−1 waste for low-organic‑carbon waste landfills in EU were reported (Manfredi et al., 2009). In the light of this, limited-controlled landfills would be a considerable GHG release source. Therefore, GHG emissions from limited-controlled landfills should be paid more attention when we determine the GHG emissions of waste sector. In order to obtain accurate GHG emissions data, typical limited-controlled landfills should
pattern as N2O, and the emissions differed at each sampling time. The GHG emission fluxes from this system were much lower than those from the leachate treatment plants of the sanitary landfill sites. The N2O emissions were 105 to 106 mg N m−2 h−1 from treatment systems in sanitary landfills, and the total annual GHG emission in CO2 equivalent was 10.62 Gg CO2-eq yr−1 when the system was applied to treat fresh leachate (Wang et al., 2014). This difference is due to many reasons. First, the absence of a stormwater draining channel and cover membrane of landfill area led to the interfusion of the leachate and stormwater before they flowed into SP, resulting in lower concentrations of COD and NH4+-N in the leachate. Therefore, the relatively low concentration of pollutant led to low GHG production from this leachate treatment system. Second, the open-air state of the storage pond resulted in non-strict anaerobic conditions. The anaerobic zones highly contributed to CH4 emissions, which has been demonstrated in several studies (Ahn et al., 2010; Daelman et al., 2013; Rajagopal and Béline, 2011; Wang et al., 2014). Third, the oxide ditch was operated unsteadily, resulting in low biodegradation efficiency. Due to the inactive nitrification and denitrification (Table 3) in the oxidation ditch, the N2O potential production pathways were inhibited and resulted in poor N2O emissions. The average leachate characteristics of each treatment unit are shown in Table 3. The management and maintenance of the leachate treatment facilities were improper. The leachate treatment was not functional for nearly the whole month of January 2016, therefore, we could not obtain a leachate sample. Abnormal conditions, such as mechanical failure or pipe blockage, also occurred several times during our sampling period. Therefore, high TN-content leachate was discharged. This remains a concern as N2O would be produced by the microorganisms in the leachate receptor through nitrification and denitrification, and the system would be a potential GHG release source of the limited-landfill site.
3.4. GHG emissions from Nanjing landfill The GHG emissions of the landfill area and leachate treatment
Fig. 6. Fluxes of (a) CH4, (b) N2O and (c) CO2 in different treatment units. 392
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Table 3 Characteristics of leachate (Unit: mg L−1). NO2−-N
NH4+-N SP OD-O OD-A SST Effluent
893.2 780.3 634.2 554.8 523.1
± ± ± ± ±
511.0 521.1 612.5 541.7 531.3
NO3−-N
74.9 ± 45.9 224.2 ± 102.3 228.8 ± 101.8 214.5 ± 97.8 292.8 ± 83.1
197.8 262.4 271.6 241.0 216.6
± ± ± ± ±
112.3 192.0 200.3 162.4 130.4
TN 1202.0 1184.8 1089.6 1081.2 1058.7
± ± ± ± ±
COD 842.4 827.5 862.1 669.8 743.2
1486.3 ± 1068.2 1289.3 ± 908.5 1280.9 ± 948.5 1300.8 ± 924.6 1196.4 ± 777.8
Table 4 Annual GHG emissions from Nanjing Landfill Site.
Landfill area Landfill leachate plant
Unit SP OD SST Total
Proportion of total emissions (%)
CH4 (Gg CO2-eq yr−1)
N2O (Gg CO2-eq yr−1)
CO2 (Gg CO2-eq yr−1)
Total (Gg CO2-eq yr−1)
Proportion of total emissions (%)
0.832 CH4 (T CO2-eq yr−1) 7.000 0.390 0.002 7.392 77.90
0.028 N2O (T CO2-eq yr−1) 4.258 42.851 0.045 47.154 7.00
0.153 CO2 (T CO2-eq yr−1) 5.124 4.768 0.077 9.969 15.10
1.013 Total (T CO2-eq yr−1) 16.382 48.010 0.124 64.516
94.01
be selected to measure the emission flux from areas with different economic development levels and climatic situations over a long period. Furthermore, more funding should be provided to improve the management level of such landfills, so as to introduce the standardized operation to reduce GHG emissions and degrade pollutants. For example, the landfill area and leachate storage pond should be covered by a HDPE membrane or other materials, and a rainwater collection and discharge system need to be constructed to reduce leachate production. In-site leachate treatment system should be well operated and maintained to ensure the effluent meet the discharge standard.
5.99 100
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4. Conclusion In this study, intense GHG emissions were observed in the landfill area, especially from the “hotspot” zones. The results revealed that the “hotspot” regions were discrete blocks, while significantly affected the magnitude of total GHG emissions. Moreover, the release of N2O was concentrated in the area of newly dumped waste, while CH4 and CO2 release varied with environmental factors. The leachate treatment system of Nanjing landfill was not a significant GHG source owing to insufficient nitrification-denitrification activities. Taking landfill reservoir and leachate treatment system into account, the annual GHG emissions was 1.078 Gg CO2-eq yr−1, which was much lower than that of sanitary landfills in large cities. However, in consideration of the large number of limited-controlled landfills, the total GHG emissions from limited-controlled landfills should not be neglected. It was estimated that approximately 1334 Gg CO2-eq GHG was released from this type of landfill. The estimation of GHG emissions from limited-controlled landfill sites and their contribution to GHG inventories are indispensable, which may help to improve GHG guidelines for landfill management in the vast rural area of China. However, the emissions from limited-control landfills should be further investigated, as the climate and waste composition vary in different places, and more typical limited-controlled landfills should be taken as case studies. Acknowledgement This research was financially supported by the National Natural Science Foundation of China (Grant No. 41475130, 51708536), and the ‘Strategic Priority Research Program-Climate Change: Carbon Budget and Relevant Issues’ of the Chinese Academy of Sciences (Grant No. XDA05020602). 393
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