Effect of quantity and composition of waste on the prediction of annual methane potential from landfills

Bioresource Technology 109 (2012) 86–92

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Effect of quantity and composition of waste on the prediction of annual methane potential from landfills Han Sang Cho a,1, Hee Sun Moon b, Jae Young Kim a,⇑ a b

Department of Civil and Environmental Engineering, Seoul National University, Seoul, Republic of Korea School of Earth and Environmental Sciences, Seoul National University, Seoul, Republic of Korea

a r t i c l e Article history: Received 8 October Received in revised Accepted 7 January Available online 15

i n f o

2011 form 6 January 2012 2012 January 2012

Keywords: Waste quantities Waste composition changes LandGEM Annual methane potential

a b s t r a c t A study was conducted to investigate the effect of waste composition change on the methane production in landfills. An empirical equation for the methane potential of the mixed waste is derived based on the methane potential values of individual waste components and the compositional ratio of waste components. A correction factor was introduced in the equation and was determined from the BMP and lysimeter tests. The equation and LandGEM were applied for a full size landfill and the annual methane potential was estimated. Results showed that the changes in quantity of waste affected the annual methane potential from the landfill more than the changes of waste composition. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Waste generated from the residential, industrial, and commercial sectors is eventually disposed of by recycling, landfilling or incineration. In Korea, 24% (by weight) of the generated waste has been placed in landfills (KMOE, 2010). The Korean government recently issued several diversion programs of organic waste, such as banning the disposal of food waste into landfills to reduce the quantity of organic waste in landfills. Consequently, the quantity and composition of waste placed in landfills has been continuously changing since 2000. Also, interest in landfill gas (LFG) recovery has increased as a consequence of the limitations of conventional energy resources and significant worldwide environmental issues. The estimation of annual methane potential from a landfill, depending on the composition of waste (Eleazer et al., 1997; Peer et al., 1993), is very important for the operation of LFG-to-energy plants, as well as for planning clean development mechanism (CDM) projects associated with LFG. In order to predict the annual methane potential from a landfill, various predictive models such as IPCC models (IPCC, 1997, 2006), the Shell Canyon model (Thompson et al., 2009) and LandGEM (USEPA, 2005) have commonly been used. Among these, LandGEM is primarily developed

⇑ Corresponding author. Address: Department of Civil and Environmental Engineer ing, College of Engineering, Seoul National University, 599 Gwanak-ro, Gwanakgu, Seoul 151-744, Republic of Korea. Tel.: +82 2 880 8643; fax: +82 2 873 2684. E-mail addresses: [email protected] (H.S. Cho), [email protected] (J.Y. Kim). 1 Present address: Dongbu Steel Technical Research Laboratories, 103-2, Munji-dong, Yuseong-gu, Daejeon, Republic of Korea. Tel.: +82 42 866 8172; fax: +82 42 866 8199. 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2012.01.026

by USEPA’s researchers to bring most of the large US landfills into the air quality regulatory program (under Clean Air Act amendments) and to extend them for regional emission inventories, which are governed by two main factors, the methane potential (L0) and the decay rate (k) of landfill waste. The L0 of waste can generally be determined via theoretical calculation based on the stoichiometry of the anaerobic degradation of the organic components or actual gas potential data measurements from simulated landfill experiments (e.g. lysimeter experiment) (USEPA, 2005). Between the two approaches, the latter is more desirable and practical because the organics in waste do not completely convert into carbon dioxide and methane within the landfill environment (USEPA, 2005). However, lysimeter experiments are limited since a very long time is required (e.g. a number of years) to collect acceptable data (USEPA, 2005). For this reason, in many studies attempts have been made to obtain the L0 of waste using simple experimental procedures, such as the biochemical methane potential (BMP) assay (Angelidaki et al., 2009; Bogner, 1990; Eleazer et al., 1997; Owens and Chynoweth, 1993). However, this approach also involves certain problems related to obtaining acceptable data for determining the L0. The L0 value obtained via the BMP assay is almost greater than that obtained from a lysimeter experiment (Bogner, 1990), because the waste samples are pre-treated (i.e. size reduction) and digested under optimum conditions (i.e. aqueous phase, no nutrient limitation, neutral pH, etc.). Furthermore, there is still a great deal that is unknown about the reality of LFG recovery at the field scale because it is significantly more complex and highly dependent on management factors as well as waste composition and quantity.

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In this study, a correction factor was suggested to determine the L0 of waste by combining the L0 values of the biodegradable components with the value from a simulated lysimeter experiment. The L0 of each individual major biodegradable waste component was measured via a BMP assay. Also, a lysimeter study to simulate landfill conditions was carried out. Ultimately, a simple empirical equation combining the BMP test with the lysimeter study, which can estimate the L0 of the waste consisted of various waste components, was suggested and verified in this study. Also, the annual methane potential depending on waste compositions and waste quantity at the Sudokwon landfill, Korea was predicted using the LandGEM and L0 from the developed empirical equation in this study. 2. Methods 2.1. Waste sample Fresh waste (45 kg by wet wt.) and digester sludge (5 kg) were directly sampled at the Sudokwon landfill according to the waste standard test method for sampling of waste (KMOE, 2005). The Sudokwon landfill is the largest landfill in Korea and is located in the Gyeonggi-do, Korea. Fresh waste was classified into six different components (i.e. food, paper, textiles, wood, plastics and inorganic). The detailed compositions of each component were as follows: food waste (food fragments, food preparation waste, fish bones and seashells), paper waste (toilet papers and advertisement flyer), textiles waste (frayed underwear), wood waste (ice cream sticks and chopsticks) and plastic waste (several plastic wraps and disposable products). The physical and chemical characteristics of all the classified waste component are shown in Table 1. 2.2. Biochemical methane potential (BMP) assay The L0 of five different components was determined through the BMP assay. All substrates were pre-dried at 100 °C in an oven for 24 h, ground to a maximum size of 2 mm before the BMP assay to reduce uncertainties in the experiments and to compare the L0 for individual components (Cho et al., 1995; Owens and Chynoweth, 1993). Reactors, 250 mL in size, were used for the BMP assay. Each reactor included individual substrates (or cellulose as a reference material) and an inoculated medium (nutrient medium and anaerobic sludge). The nutrient medium that was prepared according to the description of anaerobic basic medium as noted by Angelidaki et al. (2009) was used to minimize the potential of nutrient shortage in biodegradation, and to provide a sufficient buffering capacity. Inoculum was prepared with anaerobic digestion sludge sourced from the anaerobic digester at a municipal sewage treatment plant in Seoul, Korea. The quantity of inoculum was 20% (v/v) of the total inoculated medium volume (Owens and Chynoweth, 1993). A blank reactor, containing inoculated medium only, was also incubated to measure the background methane production from the anaerobic sludge, which was operated until all the other reactors had been dismantled. Before the start of the experiment, each reactor was sufficiently purged with 100% N2 gas, followed by sealing with butyl rubber and an aluminum cap to maintain the anaerobic conditions. Biogas production was measured by shaking the assay bottles vigorously and expelling gas through water-lubricated calibrated glass syringes (10–50 mL depending on gas volume) (Bogner, 1990). All reactors were monitored until there was no measurable methane production. The net methane production was calculated by subtracting the average methane production of the blank reactor. All experiments were carried out in triplicate. All reactors were shaken at 150 rpm in an incubation shaker at 35 °C.

Table 1 Characteristics for five different biodegradable components tested in this study. Moisture (%, w/w)

Food Paper Textiles Wood Sludge

70 8 5 10 60

VS/TS ratio

0.82 0.79 0.92 1.00 0.61

Elemental composition (%, w/w) C

H

O

45.2 36.9 51.6 5.8 22.8

6.5 5.2 5.4 39.9 3.7

27.3 41.8 39.5 0.47 17.6

TMP

643 410 509 487 578

TMP: theoretical methane potential (m3 ton1-VS) calculated using Eq. (1).

2.3. Lysimeter experiment A cylinder lysimeter, made of PVC (30 cm diameter and 120 cm height), was packed with 21 kg (by wet weight) of waste (Fig. 1), of which the composition was determined by the statistical data of the waste landfilled into the Sudokwon landfill (SLSMC, 2009). The waste contained the seven separated components and consisted of 22.0% food (w/w), 17.1% paper, 3.4% textiles, 3.9% wood, 16.5% plastics, 25.5% inorganic and 11.6% sludge. Prior to packing the lysimeter with the waste, 10 cm of gravel was placed at the bottom to retain the waste and stop small particles from leaching out. The initial moisture content of the waste was about 24% (by wet weight). Finally, the waste in the lysimeter was covered with 5 cm of soil and any additional inoculums were not seeded in the lysimeter study to simulate real landfill conditions. Two ports were placed on the top of the lysimeter for the supply of simulated rainwater and the collection of biogas. A PVC reservoir (capacity: about 7 L) was installed at the bottom of the lysimeter to store and drain the leachate. Leachate in a reservoir was completely drained from lysimeter every 2 weeks, and tap water was then supplied to the lysimeter to simulate precipitation. The quantity of the introduced water was equivalent to the average monthly precipitation in Korea, with an assumption of a 35% infiltration ratio (Fig. 1). The volume of biogas generated from the lysimeter was measured using a wet gas meter (Shinagawa, Japan). Landfill inner temperature has been examined in a number of works, which shows that it ranges from 30 °C to 45 °C for approximately 10 years after landfilling (Yoshida and Rowe, 2003). Thus, the lysimeter was operated in a temperature controlled container at 30 °C in the present study. 2.4. Analytical methods The water content, total solid (TS) and volatile solid (VS) of substrates (2540 B for TS and 2540 E for VS) were determined using standard methods (APHA-AWA-WEF, 2005). Elemental compositions (C, H and O) of the individual components were analyzed using an elemental analyzer (CE Instrument, Italy). The composition of the biogas produced (i.e., methane and carbon dioxide) for the BMP assay and lysimeter experiment were analyzed using gas chromatography (Younglin, Korea), equipped with a thermal conductive detector (TCD). The volume of the methane generated for both BMP and the lysimeter study was adjusted to STP (0 °C and 1 atm). The pH and total organic carbon (TOC) of leachate were investigated using a pH meter (Fisher Scientific, UK) and TOC analyzer (Shimadzu, Japan), respectively. 3. Results and discussion 3.1. BMP assay: determination of L0 of five different waste components The L0 of five different components was obtained from the BMP assay. There were no lag-periods in methane production from all

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Fig. 1. Schematic diagrams of the lysimeter experiment.

components during the BMP assays (data not shown). Among these, the L0 of food waste was the highest, at 406 (±62) m3 ton1-VS (100 m3 ton1-waste), but with a relatively high standard deviation compared with other components. This was probably due to the fact that food waste contained many substrates as described earlier. Each substrate has diverse biodegradability (Eleazer et al., 1997). Cho et al. (1995) showed that the L0 of boiled rice, cooked meat, and fresh cabbage was 294, 482, and 277 m3 ton1VS, respectively. The average L0 values of paper, textiles, wood waste and digested sludge were 290 (±2), 216 (±14), 193 (±5) and 92 (±6) m3 ton1-VS (211, 189, 174 and 27 m3 ton1-waste), respectively. The L0 values of paper and wood waste were similar to those determined by Owens and Chynoweth (1993), who mentioned values of 100–369 and 123–209 m3 ton1-VS, respectively. According to Appels et al. (2011), the L0 of sewage sludge was around 590 m3 ton1-organic dry solid, theoretically. However, the L0 of sludge investigated in this study was considerably low because it was secondary digester sludge which has a low ratio VS/TS (see Table 1). It is evident that overall degradation efficiency for organic matter is low in anaerobic digestion, as recently reviewed by Appels et al. (2011). In this regard, the biodegradability of each component tested was evaluated. Here, the biodegradability is the ratio of the L0 obtained from BMP assays to the theoretical methane potentials, which is calculated using Eq. (1) (Buswell and Mueller, 1952) and the elemental composition data (C, H, and O) of the components (Table 1).

Ca Hb Oc þ

    4a  b  2c 4a þ b  2c H2 O ! CH4 4 8   4a  b  2c CO2 þ 8

ð1Þ

The ranking of the biodegradability was in the following order: paper (0.71), food (0.63), wood (0.42), textile (0.40) and sludge (0.16). The biodegradability from the paper waste was the highest. However, it would be expected that the biodegradability for paper waste in the lysimeter would be low due to the larger size of

the tested sample and the restricted mass transfer of the substrate and nutrients in the lysimeter compared with those in the case of the BMP assay. Bogner (1990) mentioned that the use of ground samples accelerated the degradation rate of the organics. Another study revealed that the methane production from tomato waste was increased by 18% with a decrease in the sample size from 20 to 1.3 mm (Hills and Nakano, 1984).

3.2. Lysimeter experiment: assessment of the methane production from waste The L0 of the waste was investigated for 900 days through the lysimeter study. A lag-period which was observed until 50 days (Fig. 2a) might have been due to the microbial adaptation in the initial periods. However, the production of methane dramatically increased between 100 days and 400 days, and then slightly decreased. After 850 days, the production of methane was negligible. The cumulative amount of methane produced from waste in lysimeter was 118 m3 ton1-VS (52 m3 ton1-waste) over a period of 900 days, with a total of 36% methane recovered compared with the theoretical methane potential (325 m3 ton1-VS). Several environmental factors can affect the methane production process of waste under landfill conditions. Of these, the pH and moisture content were the most important factors impacting on the decomposition of waste (Lay et al., 1997). Fig. 2b and 2c shows the variations in the pH and TOC concentrations in the leachate during the decomposition of waste. The initial pH of the leachate was almost neutral (for 2 weeks), but then temporarily decreased to 6 between 20 days and 40 days, which might be due to the accumulation of volatile fatty acids (VFAs). In the digestion of easily biodegradable components such as food waste, VFAs production is fast and high (Fantozzi and Buratti, 2011). As a result, an imbalance between the acidogenesis phase and the methanogenesis phase can sometimes occur. During the same period, the TOC concentration of the leachate was relatively high (1600–2600 mg L1). This could be circumstantial evidence of the accumulation of VFAs produced (Fig. 2b). Therefore, the transient

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Fig. 2. Results from the lysimeter study.

Table 2 Quantity and composition of waste placed in the Sudokwon landfill (between 2001 and 2008) and the estimated L0 of landfilled waste using Eq. (3). Year

Quantity (ton)

2001 2002 2003 2004 2005 2006 2007 2008

6339,924 7342,719 6988,619 6009,428 4844,489 4914,086 5097,648 4807,561

Composition (%, w/w)

L0

Food

Paper

Wood

Rubber

Plastics

Sludge

Textiles

Others

NC

18.2 12.1 9.2 4.7 3.6 2.9 3.0 3.1

12.3 13.1 13.6 16.9 13.1 13.3 13.0 13.0

10.8 7.1 8.6 13.2 16.5 15.8 15.3 15.7

1.3 0.3 0.2 0.3 0.2 0.4 0.4 1.5

13.3 11.7 17.2 18.2 18.0 16.2 12.4 15.3

0.7 1.8 1.5 1.3 2.3 5.1 4.8 4.0

3.0 1.7 2.2 2.3 2.1 2.7 2.2 2.3

2.3 2.1 3.9 4.0 3.3 5.0 5.5 4.5

38.1 50.1 43.7 39.1 41.0 38.6 43.4 40.5

68.6 55.7 57.3 67.8 64.2 64.8 62.4 63.1

NC: non-combustibles (e.g., metals, glasses, soils, bricks, incineration ashes, etc.). Source: SLSMC (2009).

drop in the pH might have resulted in the initial lag-period during the methane production process of waste (Fig. 2a). After 70 days, the pH of the leachate remained between 6.5 and 7.6, with that of the low TOC concentration also continuously maintained. These results suggested that the methane production process of waste was not inhibited by the pH conditions after the short lag-periods in this study. Lay et al. (1998) stated that most methanogenic bacteria function within the pH range between 6.7 and 7.4, and the rate of methane production may decrease if the pH is lower than 6.3 or higher than 7.8. The moisture content of the waste is very important for its decomposition because the moisture state severely affects the circulation of nutrients and mass transfer of degradable substrates to microorganisms; thus, low moisture content can decrease the

biodegradation rate of organic waste. The moisture content of waste should be maintained at a minimum of 20% (w/w) for the degradation of organic materials by microorganisms (Themelis and Ulloa, 2007). In this experiment, the moisture content of the waste might have been maintained above at least 20% because the initial moisture content was 24% (w/w), and the leachate was allowed to continuously flow from the lysimeter throughout the experimental period, with the exception of the first week (Fig. 2d). The moisture content within a landfill is usually less than the field capacity. Field capacity is the amount of water that the waste will absorb and store or retain by capillarity (Christensen et al., 1996). Once the moisture content exceeds the field capacity, the leachate will flow and cause the transfer of medium for the nutrients and bacteria (Gardner, 1993). Actually, the estimated

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Fig. 3. Predictions of the annual methane potential for individual waste landfilled between 2001 and 2008 using LandGEM.

moisture content of waste on the 900th-day (which was calculated by a mass balance on water using the initial water content, the accumulated water quantity used in irrigation and the quantity leached at a given time), was about 21%. 3.3. Empirical equation for estimating the methane potential of waste In this study, an empirical equation was suggested to estimate the L0 of waste in a landfill based on the BMP assay and lysimeter study. This equation was based on two assumptions, which were already introduced from previous other studies (De la Cruz and Barlaz, 2010; Owens and Chynoweth, 1993; IPCC, 2006). The first assumption is that methane is produced from only the biodegradable waste components, and the second is that the L0 of waste (L0,waste) can be calculated from a weighted average of the L0 of each biodegradable component (L0,i), as described in Eq. (2):

L0;waste ¼

n X ðL0;i  ðwt: fractionÞi Þ

ð2Þ

i¼1

In this study, the L0,i was determined from only the BMP study. The L0 measured in the BMP study will likely be higher than that observed in a landfill situation for the reasons explained in the introduction. Therefore, a correction factor (f) was introduced. Finally, Eq. (3) was suggested as an empirical equation for estimating the L0 of all waste placed in a landfill (L0,waste in a landfill). In this study, the f value was determined using Eq. (4):

L0;waste

in a landfill

¼f 

n X

ðL0;i  ðwt: fractionÞi Þ

ð3Þ

i¼1

f ¼

L0;waste in a lysimeter L0;waste

ð4Þ

where the L0,waste in a lysimeter is the L0 of waste observed in the lysimeter study (m3 ton1-waste), and L0,waste is equal to that obtained in Eq. (2) that calculates the estimate of the L0 of the waste investigated in the lysimeter study (m3 ton1-waste). The L0,waste in a lysimeter observed in this lysimeter study was 52 m3 ton1-waste. Also, the L0,waste calculated using the BMP data and Eq. (2) was 74 m3 ton1-waste (L0,waste = (100  0.220) + (211  0.171) + (189  0.034) + (174  0.039) + (27  0.116) = 74 m3 ton1-waste). Consequently, the value of f determined in this study was 0.7. In order to estimate the L0,waste in a landfill of any waste using only Eq. (3), whether Eq. (3) provided the available L0,waste in a landfill first needed to be verified. For this verification, the actual L0 values collected from other long-term lysimeter studies were compared with the estimates calculated using Eq. (3). Bum (2001) and Valencia et al. (2009) operated the lysimeter for 1270 days and 450 days, respectively, to evaluate the L0 of waste with specific compositional ratios of components. The composition of each study was as follows:  Bum (2001): 53.5% (w/w) food, 19.9% paper, 19.2% plastics, 2.7% textiles, 1.0% wood, 0.6% leather and rubber and 3.1% inorganic.  Valencia et al. (2009): 33.2% (w/w) undefined organics (undefined organics were assumed to be food waste in the case of the estimation of L0,waste in a lysimeter), 15.2% paper, 3.7% plastics, 2.1% textiles, 2.5% wood, 0.2% leather and rubber and 43.1% inorganic. These authors also operated lysimeters until the production of methane was negligible. The observed L0 values for both cases were 77 m3-CH4 ton1-waste (Bum, 2001) and 40 m3-CH4 ton1waste (Valencia et al., 2009), respectively. Also, the estimated L0,waste in a landfill for each waste was 72 and 43 m3-CH4 ton1-waste. These results suggested that the use of Eq. (3), with a correction

H.S. Cho et al. / Bioresource Technology 109 (2012) 86–92

factor (f = 0.7), was available to estimate the L0 of various waste with different compositional ratios of components. 3.4. Effect of changes in historical quantity and composition of waste on the prediction of annual methane potential Table 2 shows the variations in the compositions and quantities of all waste placed in the Sudokwon landfill, from 2001 to 2008 (SLSMC, 2009). The ratio of the food waste was significantly decreased due to increased recycling and to the introduction of a waste management policy, which banned the landfilling of food waste in Korea from the mid 2000s. The ratio of wood waste and sludge was increased, while there was no change in the ratio of paper and textiles waste. In this study, the LandGEM (USEPA, 2005) was used to predict the annual methane potential of individual waste landfilled from 2001 to 2008:

Q¼

n X ðMi  k  L0  expðktÞ Þ

91

models, there is still uncertainty in obtaining the L0 of waste. The suggested empirical equation may be an available tool to more readily obtain the L0 of waste. Acknowledgements The authors wish to acknowledge the financial support from the SNU SIR Brain Korea 21 research program and the Brain Korea 21 project through the School of Earth and Environmental Sciences at Seoul National University, funded by the Ministry of Education & Human Resources Development. Also, this study was technically supported by the Engineering Research Institute and Integrated Research Institute of Construction and Environmental Engineering, Seoul National University. References

ð5Þ

i¼1

where Q is the annual methane potential (m3 year1), L0 is the methane potential of waste placed in a landfill (m3 ton1-waste) (here, L0 is equivalent to L0,waste in a landfill), Mi is the quantity of waste placed in the ith section (ton-wet waste), t is the time of waste disposal (year), and k is the decay rate of waste (yr1). The LandGEM has been widely applied to predict the annual landfill methane potentials (Chalvatzaki and Lazaridis, 2010; De La Cruz and Barlaz, 2010; Faour et al., 2007; Paraskaki and Lazaridis, 2005). However, some researchers have noted that LandGEM overestimates annual methane potentials (Lee et al., 2007; Scharff and Jacobs, 2006). Data of the L0 and the k of landfilled waste are required to use the LandGEM. The US EPA offers a binary choice as a default k of either 0.02 yr1 below 635 mm (25 in.) of precipitation or 0.04 yr1 above 635 mm (USEPA, 2004). In this study, 0.04 yr1 was selected because the average annual precipitation in Korea is about 1300 mm. The LandGEM has a disadvantage whereby it cannot allow for changes in the composition of individual waste as a single default L0 was generally used (Scharff and Jacobs, 2006). Thus, in this study, the estimates of L0 of individual waste landfilled from 2001 to 2008 were used, employing Eq. (3) instead of a single default L0. The L0,waste in a landfill of individual waste ranged from 56 to 69 m3 ton1-waste (Table 2), showing that L0,waste in a landfill of individual waste placed in any given year varied; however, there was no significant difference. Fig. 3 shows the annual methane potential for individual waste landfilled between 2001 and 2008 estimated by LandGEM. The trends in the annual methane potential of individual waste landfilled before and after 2005 significantly differed. This might have been because the annual methane potential from a landfill was more influenced by the change in the quantity of waste than the composition in the case of this study, since the L0 values of individual waste were similar (see Table 2). When the quantity of waste was decreased from 7.3  106 (in year 2002) to 4.8  106 (in year 2005) tons, the annual methane potential from the waste was reduced by about 24%. However, after 2005, no significant change in the quantity of the waste was observed and similar trends were shown in the annual methane potential. 4. Conclusion Results showed that the changes in quantity of waste had a greater influence than changes in composition on the prediction of annual methane potential at the Sudokwon landfill. This is because there were no significant differences in the L0 of individual waste landfilled with different compositional ratios of components in the study site. Although many studies have been carried out to predict annual methane potential from landfills by different

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