Low fraction of methane in landfill gas emissions in an industrial waste landfill containing incineration ash and gypsum board waste under anaerobic conditions

Abstract

The behaviour of carbon dioxide (CO₂) and methane (CH₄) emissions at the surface and below the soil cover in an industrial waste landfill under anaerobic operating conditions was evaluated for six years. This landfill contained gypsum board waste and incineration ash – a practice currently allowed because of a change in Japanese regulations. The CO₂ and CH₄ fluxes decreased throughout the six years of the survey. Almost all of the survey points exhibited fractions of CH₄ in landfill gas emissions of <0.5 (mean values: 0.0–0.1 [surface], 0.0–0.3 [subsurface]) under anaerobic conditions. In addition, a relatively high first-order reaction rate constant for the landfill gas emissions (0.3 year⁻¹) was observed. The landfill leachate showed a relatively high sulphate ion (SO₄ ^2–) concentration, although other environmental conditions, such as the pH, oxidation–reduction potential and ammonium concentration, were not at levels that could have inhibited CH₄ production. These findings suggest that the low fractions could have been related to the lower amounts of CH₄ generation caused by competition between methanogens and sulphate-reducing bacteria (SRB). Therefore, SRB could play a major role in the degradation of organic carbon in the landfill.

Keywords

Industrial waste landfill landfill gas flux gypsum board waste anaerobic conditions sulphate-reducing bacteria

Introduction

Emissions of landfill gas, of which the main components are the greenhouse gases carbon dioxide (CO₂) and methane (CH₄), are a global concern (Allen et al., 2019; Di Bella et al., 2011; Zhang et al., 2008). Evaluations of gas emissions are important for assessing the state of stabilisation in landfills. The behaviour of landfill gas emissions can be affected by various factors, such as waste composition, structure, landfill management, climate and geological conditions, as well as soil cover characteristics (Ishigaki et al., 2005; Kim et al., 2010; Krause et al., 2016a; Omar and Rohani, 2015; Zhang et al., 2008). It is important to understand the behaviour of landfill gas under various conditions at landfill sites, which will lead to appropriate operation and maintenance procedures.

Many studies have examined the composition of landfill gas from gas venting pipes and monitoring wells installed in landfill sites in Japan (Kim et al., 2010; Matsuto et al., 2015; Nagamori et al., 2008; Takuwa et al., 2009; Yanase et al., 2011; Yoshida, 2009). In contrast, there have been few reports on the evaluation of landfill gas fluxes from the landfill surface in Japan (Ishigaki et al., 2005), even though this knowledge is also important to the understanding of landfill gas behaviour.

Currently, the majority of landfill sites in Japan are being operated as semi-aerobic landfills (66% of municipal waste landfills and 63% of landfills for industrial waste; Ministry of the Environment, Japan, 2012; 2018; Matsuto et al., 2015). Operation of the industrial waste landfill in this study is as a controlled landfill; however, this is not a common case in Japan. The industrial waste landfill is covered by sandy soil and is operated under anaerobic conditions due to the high inner water level. This site contains incineration ash and gypsum board waste. There have been few investigations of landfills with these particular characteristics. Tanaka and Kamura (2017) studied the dissolution characteristics of inorganic ions and the balance of the ions in this landfill. The results showed that the sulphate ion (SO₄ ^2–) concentration in the landfill leachate in a recently established section was higher than that in the old section. This may be related to a change in Japanese regulations (Ministry of the Environment, Japan, No. 060601001), which stipulated that gypsum board waste was no longer to be considered as an inert waste but is to be disposed of in controlled landfills with waste such as incineration ash and sludges. This change in regulations should affect the microbial reactions and transformation of substances in the landfills, and gas generation is one important process that could be impacted. It is known that CH₄ generation is competitively inhibited by sulphate-reducing bacteria (SRB), which utilise SO₄ ^2- as an electron acceptor (Kim et al., 1997). Hydrogen sulphide (H₂ S) is generated by sulphate-reducing reactions while CH₄ generation is suppressed. Since H₂ S generation should be avoided in landfill management practices from the viewpoint of occupational and environmental safety, a thorough understanding of the gas generation behaviour under the conditions of management for a particular landfill site is essential.

In this study, we investigated the behaviour of CO₂ and CH₄ fluxes in an industrial landfill site for six years. Specifically, we evaluated the CH₄ fractions in landfill gas emissions and the first-order reaction rate constant of landfill gas emissions. From the results, the factors affecting gas emissions from this landfill are discussed.

Materials and methods

Site description

The industrial waste landfill examined in the study is located in an area facing the coast and has weather characteristics in the zone of the Sea of Japan, where there is relatively high precipitation (normal value: 2238 mm year⁻¹) compared to the average value for Japan. The distance from the sea is approximately 500 m. This landfill uses a type of excavation that excavates flat ground and lays a liner sheet. The landfill has received waste since 1982 and consists of six sections (I–VI). Each section is completely independent and no landfill waste comes into contact with waste in other sections. The total area is approximately 47,000 m². The depth of the landfill layer is 3.5 m in section I and 6 m in the other sections (II–VI). Section V (in operation from 2001 to 2009) was investigated in this study. This site received mainly incineration ash (55.8%), sludge (38.8%), gypsum board waste (4.6%) and a small amount of other waste types (0.8%) (% w/w). The area and volume were 11,790 m² and 52,650 m³, respectively. Sandy soil was used as a final cover.

Landfill waste was disposed of into the section that previously stored approximately 2 m of rainwater, due to the prevention of breakage of the liner sheet by external water pressure from the groundwater because of the high groundwater level around the landfill. Even after the section was closed, the high level of retained water was maintained and the majority of the waste layer was submerged. Leachate collection pipes were laid at the bottom of all sections, and landfill leachate was introduced into the leachate collection basin through the pipe. A submerged pump was installed in the basin and leachate was introduced into the leachate treatment plant under the control of a water level sensor and timer. Since the pipes are separate for each landfill section, landfill leachate could be collected from each. In Japan, the majority of the landfill sites are operated as semi-aerobic landfill systems, which rapidly discharge leachate and passively aerate through the leachate collection pipes without using mechanical equipment to promote stabilisation of landfill leachate and waste (Chong et al., 2005; Matsufuji and Tachifuji, 2007; Matsuto et al., 2015; Tachifuji and Hirata, 2009). Matsuto et al. (2015) explained that “The key concept of a semi-aerobic landfill is the connection of a leachate collection pipe with gas vents that directly connect to the atmosphere” (p.204). Operation of the landfill in this study is quite different from semi-aerobic landfills.

A boring survey was conducted on 13 November 2009. Core samples were collected at 0.5-m intervals from 0–5 m depths. The depth of the sandy soil cover was confirmed as approximately 0.75 m from the surface. To install solar panels at the landfill surface, a 10–15 cm depth from the surface of the sandy cover was replaced and a mat foundation was installed in a part of the landfill site. Construction work was carried out from November 2013 to September 2014.

The level of retained water in the monitoring well was mostly higher than −0.75 m from the ground surface (Figure 1), indicating that the whole waste layer had been submerged for the majority of the time, as described above. The temporary decline of the level in 2013 was caused by the promotion of retained water drainage due to the construction work.

Landfill leachate quality

To reveal the mechanisms of the behaviour of gas generation, the quality of leachate in section V was investigated. We collected landfill leachate samples from section V several times a year. The pH, chemical oxygen demand (COD_Mn) and oxidation–reduction potential (ORP) in the samples were measured from 2005 to 2016. Ammonium and SO₄ ^2- concentrations in the samples were analysed from 2005 to 2013. The pH and ORP were measured with a pH meter (F-23, Horiba, Kyoto) and an ORP meter (PHL-20, DKK-TOA, Tokyo), respectively. The COD_Mn was analysed according to the Japanese Industrial Standard (JIS K0102, 2003). Ammonium was determined using an ammonium electrode (5002A, Horiba, Kyoto). The SO₄ ^2- concentration was analysed using ion chromatography (Dionex DX-500, Thermo Fisher Scientific, Tokyo).

Gas emissions

The field survey campaign was conducted for six years (2011–2016), and the pertinent collected information is shown in Table 1. Gas fluxes of CO₂ and CH₄ from the landfill surface were measured with a closed static chamber according to the method described by Ishigaki et al. (2005). To check H₂ S gas generation in the waste layer, a hole (ϕ6 mm, depth 50–80 cm) was made in the layer, then a stainless-steel pipe (ϕ6 mm, length 1 m) with a tube attached to the upper portion was inserted into the hole and sealed for between several hours and half a day, following which the H₂ S gas concentration in the accumulated gas was measured. Japan’s Industrial Safety and Health Act specifies a control concentration of 1 ppm for H₂ S in work environments; however, at landfills the action of SRB converts sulphur into H₂ S gas, leading to the detection of gas concentrations from dozens to tens of thousands of ppm (Kikuchi et al., 2001; Takuwa et al., 2009). Since herein we did not examine the atmosphere but the gas concentration in the waste layer to assess the state of sulphur gas conversion, a 1 ppm concentration threshold of H₂ S gas was used. Portable gas analysers (GA2000Plus and GA5000) capable of detecting H₂ S gas at 1 ppm or higher were used as the field survey instruments. Gas fluxes of CO₂ and CH₄ below the sandy cover were measured at the hole described above using the chamber method.

Table 1.

Information on the field survey.

Years	2011	2012	2013	2014	2015	2016
Survey date	27–29 Sep.	5–6 Sep.	15–16 May	29–30 Oct.	20–22 Oct.	24–26 Oct.
Number of measured points of surface gas flux	30	30	37	44	43	37
Number of measured points of subsurface gas flux	18	5	2	7	19	13

The degree of dissociation of H₂ S in water was estimated by the acid dissociation constant (pK _a = 7.02) (The Chemical Society of Japan, 1993) at pH 8.

The detection ratio of the gas flux of CO₂ and CH₄ was calculated using the following equation

Detection ratio of the gas flux (%) = \frac{Number of points where gas flux detected}{Number of measured points} \times 100

The landfill gas flux was defined as follows

{Landfill gas flux (L d}^{−1} m^{−2}) = {CO}_{2} {flux (L d}^{−1} m^{−2} {) + CH}_{4} {flux (L d}^{−1} m^{−2})

The detection limit of the gas flux of CO₂ and CH₄ was 0.017 L d⁻¹ m⁻².

The geospatial distribution of landfill gas flux was estimated using the kriging method. Kriging is a method used for the prediction of data at unobserved points using the spatial correlation at neighbouring observation points, which has been applied to estimate the spatial distribution of landfill gas emissions (Franzidis et al., 2008; Pokryszka et al., 1995; Spokas et al., 2003). Landfill gas emissions from the whole landfill were estimated by geospatial analysis of the landfill gas flux. Surfer (Golden Software, Golden, Colorado) was used for the geospatial analysis and drawing of the contour map.

In IPCC (2016), the fraction of CH₄ was described as the fraction of CH₄ in generated landfill gas (F). Generally, the composition of gas emitted from landfills is mostly composed of CO₂ and CH₄ gases (Krause, 2018; Thermelis and Ulloa, 2007). In this study, the fraction of CH₄ in landfill gas emissions was determined using the following equation

{Fraction of CH}_{4} in landfill gas emissions = \frac{{CH}_{4} flux}{{CO}_{2} {flux + CH}_{4} flux}

The time-course of landfill gas emissions from the whole landfill was fitted to a first-order decay model

C_{t} = C_{0} exp (- k t)

where C_t represents the landfill gas emissions from the whole landfill at time t (m³), C ₀ represents the landfill gas emissions from the whole landfill initially (m³), k is the first-order reaction rate constant (year⁻¹) and t is the time (year). The k value was calculated using landfill gas emissions from the whole landfill evaluated over six years.

Homogenisation test

Homogenisation tests were conducted to understand the carbon behaviour and metal abundance in the landfill site. Boring core samples from depths of 175–200, 250–275, 350–375 and 450–475 cm were used for these tests. First, 10 g (wet weight) of each boring core sample was added to 500 mL of ultrapure water and homogenised for 1 min using an homogeniser (ULTRA-TURRAX T25, IKA Japan, Osaka). The mixture was then centrifuged (20,500 rpm, 5 min) and the supernatant was used for organic carbon and metal analyses. Organic carbon concentrations were measured using a total organic carbon (TOC) analyser (TOC-VCSH, Shimadzu, Kyoto). For metal analyses, nitric acid was added to the supernatant to a concentration of 1%. Then, metal concentrations were determined using inductively coupled plasma-mass spectrometry (ICP-MS; ELAN DRC-e, Perkin Elmer, Japan, Kanagawa).

Ratio of discharge of organic carbon from the landfill

To assess the ratio of the discharge of organic carbon from the landfill, orgC_r and orgC_i were estimated by the amount of organic carbon in the boring core samples, landfill leachate and landfill gas emissions.

Here, orgC_r is the organic carbon remaining in the landfill site in 2016 (mol) and orgC_i is the organic carbon in the landfill site initially (mol)

o r g C_{r} = o r g C_{b} - o r g C_{l} - o r g C_{g}

where orgC_b is the organic carbon in the landfill site at the time of the boring survey (mol), orgC_l is the emission of organic carbon from the landfill site through landfill leachate after the boring survey to 2016 (mol) and orgC_g is the emission of organic carbon from this landfill site through gas from after the boring survey to 2016 (mol)

o r g C_{b} = C B \times W \times 10^{9}

where CB is the concentration of organic carbon in the boring core sample (mol g⁻¹), which is obtained by homogenisation tests (CB = 6.3 × 10⁻⁴ mol g⁻¹) and W is the mass of the waste deposited (Gg) (W = 62 Gg), as shown in the following equation

o r g C_{l} = \sum_{i = 1}^{n} C L \times I

Here, CL is the mean annual concentration of organic carbon in the landfill leachate (mol L⁻¹) and I is the amount of infiltration (l year⁻¹)

o r g C_{g} = \sum_{i = 1}^{n} G_{w h o l e}

Here, G_whole is the landfill gas emissions from the whole landfill (mol year⁻¹).

To estimate the organic carbon in the landfill site at the initial time (mol), orgC_i , we assumed that organic carbon remaining in the landfill will be decreased according to a first-order decay model.

orgC_i was calculated as shown in the following equation

o r g C_{i} = o r g C_{b} \times e x p (k_{B} t_{1})

k_{B} = \frac{1}{t_{2}} \times ln \frac{o r g C_{b}}{o r g C_{r}}

In Equation (9), k_B is the first-order reaction rate constant of organic carbon (year⁻¹), t ₁ is the duration from the start of landfilling to the boring survey and t ₂ is the duration from the boring survey to 2016.

The ratio of discharge of organic carbon from the landfill (R_orgC ) was estimated using the following equation

R_{o r g} C (%) = \frac{o r g C_{i} - o r g C_{r}}{o r g C_{i}} \times 100

Results

Evaluation of the landfill leachate in section V indicated a highly reductive environment in this landfill (ORP: −370 to −17 mV [−231 ± 92 mV, mean ± SD] and pH: 7.1–8.4 [8.0 ± 0.3, mean ± SD]). This environment and the whole waste layer had been submerged for the majority of the time (Figure 1), indicating anaerobic conditions. The high SO₄ ^2- concentrations (400–1600 mg l⁻¹ [1140 ± 282 mg l⁻¹, mean ± SD]) may have originated from the gypsum board waste in this section. Low organic matter concentrations (COD_Mn: 12–54 mg l⁻¹ [25 ± 11 mg l⁻¹, mean ± SD]) and relatively high ammonium nitrogen concentrations (36–190 mg l⁻¹ [119 ± 38 mg l⁻¹, mean ± SD]) were also observed.

The time-course of the detection ratios of fluxes of CO₂ and CH₄ at the surface and below the sandy cover are shown in Figure 2. The detection ratios of the CO₂ flux at the surface remained high (>92%) throughout the six years of the surveys. Detection ratios of the CH₄ flux amounted to 33% in 2011 and declined to 3% in 2016; a surface CH₄ flux was not detected in 2015. The trend in the detection ratios of landfill gas flux below the cover was similar to that at the surface. A CO₂ flux below the cover was detected at all the measured points throughout the six years, whereas that of the CH₄ flux was 56% in 2011 and declined to 15% in 2016. Figure 3 shows the distribution of CO₂ and CH₄ fluxes from the landfill surface. The findings showed that there was spatial heterogeneity at the surface in terms of the landfill gas flux, and that hot-spots existed at some points where the flux was particularly high. The physically and chemically heterogeneous nature of waste materials in the landfill likely contributed to the flux distributions. The locations of hot-spots in the CH₄ flux exhibited similar patterns over the six years (Figure 3(b)), whereas those of the CO₂ flux shifted (Figure 3(a)). The mean and range of the CO₂ flux in 2011 were 7.7 and 2.9–34 L m⁻² d⁻¹, respectively, while the mean and range of the CO₂ flux in 2016 were 1.6 and 0.28–6.1 L m⁻² d⁻¹, respectively (Figure 4). The mean and range of the CH₄ flux in 2011 were 1.9 and −1.3–33 L m⁻² d⁻¹, respectively. Few points exhibited a relatively high CH₄ flux (>10 L d⁻¹ m⁻²) in 2011 or 2013. However, the CH₄ flux decreased to <0.64 L m⁻² d⁻¹ in 2014 and was not detected at any points in 2015. In 2016, the surface CH₄ flux was only detected at one out of 37 points with a low flux of 0.2 L m⁻² d⁻¹. On the other hand, H₂ S gas in the waste layer was detected at only two points out of 145 measuring points throughout the investigation (i.e., 1 ppm in 2011, 4 ppm in 2013).

Landfill gas emissions at the surface from the whole landfill decreased linearly, except for during 2014, throughout the six years (Figure 5). In 2014, landfill gas emissions from the whole landfill were markedly decreased; however, emissions returned to exhibiting a similar decreasing trend in the following year (2015). The first-order reaction rate constant of the landfill gas emission (k) in the landfill site was 0.3 year⁻¹.

The estimation of R_orgC indicated that 33% of the organic carbon in this landfill site had been discharged through landfill gas and leachate by 2016. In addition, the majority of the organic carbon was discharged as gas (99%) rather than as leachate (1%), which aligns with observations noted in previous reports (IPCC, 2006).

Figure 6 presents the fraction of CH₄ in landfill gas emissions at the surface and below the sandy cover as obtained through the surveys. The fraction at the surface exhibited values of <0.5 at almost all points (i.e., 205 of 209 points) regardless of the landfill gas flux. Only four points showed values >0.5 for the fractions at the surface, amounting to 0.6 and 0.7 in 2011 and 0.6 and 0.8 in 2013. The fraction below the sandy cover was also <0.5 at almost all points (57 of 62 points). Only five points exhibited values >0.5 for the fraction below the sandy cover, and these values amounted to 0.6, 0.6 and 0.8 in 2011, 0.5 in 2013 and 0.8 in 2015. The mean values of the CH₄ fraction at the surface and below the sandy cover were in the ranges of 0.0–0.1 and 0.0–0.3, respectively, throughout the field surveys.

Discussion

The field investigation of surface fluxes of CO₂ and CH₄ showed that the detection ratio of the CO₂ flux remained high (92%), whereas that of the CH₄ flux decreased to 3% throughout the six years of surveys (Figure 2). On the other hand, the mean and range of the surface gas flux indicated that both CO₂ and CH₄ decreased over the six years (Figure 4). The range of the CH₄ flux in this study was −1.3 to 33 L m⁻² d⁻¹ (−0.8 to 21 g CH₄ m⁻² d⁻¹). Bogner et al. (1997) summarised CH₄ emissions from landfills and reported the range in the CH₄ flux from −0.06 to 4560 g CH₄ m⁻² d⁻¹, and Ishigaki et al. (2005) reported that the CH₄ flux in Japan was −1.5 to 384 g CH₄ m⁻² d⁻¹. The range of the CH₄ flux in this study was within that described in previous reports (Di Bella et al., 2011; Bogner et al., 1997; Ishigaki et al., 2005; Zhang et al., 2008). Landfill gas emissions at the surface from the whole landfill decreased linearly, except for in 2014, over the six years of analysis (Figure 5). Temporary decreases of landfill gas emissions from the whole landfill were observed in 2014, which could be ascribed to the installation works for the mat foundation, such as replacement of the sandy cover. After a period of time for the pores of the new sandy cover to become filled with landfill gas, the release of landfill gas to the atmosphere would commence again. The temporary decrease may also have been related to the dissolution of landfill gas into the water contained in the new sandy cover. The landfill gas emissions from the whole landfill returned to a similar trend of decrease in the following year (2015). Thus, this survey revealed that there was a constant decrease in the whole landfill gas emissions in addition to the decline of the surface gas fluxes of CO₂ and CH₄ throughout the investigation.

The estimation of R_orgC indicated that 33% of the organic carbon in this landfill site had been mainly discharged as landfill gas by 2016. It is known that some organic carbon in wastes, such as paper and wood, degrades very slowly or can remain under anaerobic conditions. Generally, approximately half of the organic carbon in landfills is considered to be degraded under anaerobic conditions (IPCC, 2006). The fraction of degradable organic carbon that anaerobically degrades (DOC_f) is affected by many factors, such as temperature and waste composition (Krause et al., 2016a; Wang et al., 2015). Krause (2018) noted that the DOC_f was reported to range from 50% to 83% and that higher and lower values were experimentally determined for a variety of waste components, such as wood (0–50%) and food waste (50–75%). Thus, the R_orgC of this study (33%) suggests that the decomposition and gasification of organic carbon by microbial reactions had occurred in this landfill site.

Almost all of the survey points at both the surface and below the sandy cover exhibited fractions of CH₄ in landfill gas emissions that were <0.5 throughout the surveys (Figure 6). The mean values of the fractions at the surface and below the sandy cover were 0.0–0.1 and 0.0–0.3, respectively. IPCC (2006) described that the default value of F is 0.5 and that the range is 0.5–0.55 in anaerobic landfills. Krause (2018) reviewed previous studies on F; F = 0.5 is almost universally applied and is appropriate for modelling anaerobic decomposition. Values of F in field surveys were generally >0.5 (Burattiet et al., 2015; Dong et al., 2013; Krause, 2018; Powelson et al., 2006). The fractions in this study were less than those in other reports.

A negative CH₄ flux was observed at three points in 2011, indicating that CH₄ oxidation does occur at this landfill site. However, CH₄ oxidation should not be considered as the major reason for the low fractions, because low fractions of CH₄ below the sandy cover were also observed (Figure 6(b)). It has been reported that CH₄ generation is affected by some environmental conditions, including ammonium, oxygen and sulphate concentrations, which are factors related to bacterial reactions (Krause et al., 2016b). Since the waste layer had been submerged for the majority of the investigation period (Figure 1) and the ORP value was −231 ± 92 mV, the majority of the waste body in the landfill would have been under anaerobic conditions. The ammoniacal nitrogen concentration (119 ± 38 mg L⁻¹) and pH (8.0 ± 0.3) in the leachate were not at levels that could have strongly inhibited CH₄ formation (Chen et al., 2008; Clark and Speece, 1971). This landfill contained approximately 4.6% w/w gypsum board waste and the landfill leachate exhibited a relatively high SO₄ ^2- concentration (1140 ± 282 mg L⁻¹). It is well known that inhibition of CH₄ generation can occur in the presence of sulphate because of competition between methanogens and SRB for substrates. The SRB convert organic carbon to CO₂, which may have led to the decrease in the fraction of CH₄ in landfill gas emissions. Choi and Rim (1991) reported that SRB predominated when COD/SO₄ ^2- was <1.7, and in this landfill leachate the value was 0.02. Decreases in the fraction of CH₄ under landfill conditions were observed in other reports (Calabro et al., 2010; Kim et al., 1997). These results suggest that the low fraction may have been the result of competition between methanogens and SRB. The SRB may have become a major contributor to the degradation of organic carbon in this landfill site at the later stage.

Calculation of the k value in this landfill site resulted in a value of 0.3 year⁻¹. The IPCC (2006) has set a default k value of 0.1 within the range of 0.06–0.1 year⁻¹ in temperate wet climate zones. The k value in this study was higher than those values. It has been reported that the degradation rate of organic carbon under sulphate-reducing conditions is higher than that in methanogenic conditions (Kim et al., 1997). The relatively high first-order reaction rate in this study could have resulted from the microbial activity of sulphate reduction rather than from methanogenesis.

Although active sulphate-reducing reactions in this landfill were suggested by the findings, as discussed above, H₂ S gas concentration in the waste layer was less than 1 ppm at almost all of the measuring points during the investigation. At landfills where sulphate-reducing reactions are active and sulphur converts into a gas, this leads to the detection of gas concentrations from dozens to tens of thousands of ppm (Kikuchi et al., 2001; Takuwa et al., 2009). In contrast, in our study, the maximum H₂ S gas concentration in the waste layer was found to be 4 ppm, and levels of 1 ppm or more were observed at only two of the 145 observation points. This could be associated with the pH conditions and sulphide formation at that landfill. The dissociation degree of H₂ S in water is dependent on the pH. The mean pH in the leachate at this site was 8.0 ± 0.3. It was estimated that approximately 90% of the sulphur was present as HS^- and 10% was present as H₂S_(aq) at pH 8. Homogenisation test results for the boring core samples showed that the waste in this landfill contained several metals, such as Fe (7.7 × 10⁻ ³ g/wet-g), Ni (2.9 × 10⁻ ³ g/wet-g), Al (7.0 × 10⁻ ³ g/wet-g), Mn (4.1 × 10⁻ ⁴ g/wet-g), Zn (8.1 × 10⁻ ⁴ g/wet-g), Cu (6.3 × 10⁻ ⁴ g/wet-g), Pb (2.2 × 10⁻ ⁴ g/wet-g) and Ca (9.9 × 10⁻ ³ g/wet-g), which are well known for their ability to form sulphides. HS^- produced by SRB would soon generate metal sulphides in this landfill site. In the case of gaseous H₂ S, it is adsorbed to the soil and sand, although H₂S_(aq) is partially converted to H₂S_(g). Metal immobilisation by metal sulphide formation under landfill conditions was well known (Calabro et al., 2010; Du et al., 2014; Erses and Onay, 2003). The coexistence both of a high pH and metals could have led to the control of H₂ S gas generation at this site.

Conclusion

This study investigated the behaviour of gas emissions of CO₂ and CH₄ in an industrial waste landfill that contains incineration ash and gypsum board waste together under anaerobic conditions. Almost all of the survey points showed that the CH₄ fractions in landfill gas emissions were <0.5. The mean values of the fraction at the surface and below the sandy cover were 0.0–0.1 and 0.0–0.3, respectively. Environmental conditions, such as the pH, ORP, temperature and ammonium concentration, at this landfill site were not at levels that could cause strong inhibition of CH₄ formation. The landfill leachate showed a relatively high SO₄ ^2- concentration (1140 ± 282 mg L⁻¹) and a low COD_Mn/SO₄ ^2- (0.02). These results suggest that the low CH₄ fraction may have been the result of lower CH₄ generation caused by competition between methanogens and SRB. A relatively high first-order reaction rate constant for the landfill gas emissions (0.3 year⁻¹) was also observed. The SRB could be playing a major role in organic carbon degradation in this landfill. Hydrogen sulphide gas was undetected at almost all of the measuring points during the surveys, which could have been caused by the formation of metal sulphides at the higher pH and in the presence of sufficient amounts of metals. The results obtained in this study provide new knowledge on the behaviour of the emissions of landfill gas.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors received no financial support for the research, authorship and/or publication of this article.

ORCID iD

Yuka Ogata

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