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Evaluating the biochemical methane potential (BMP) of low-organic waste at Danish landfills
Waste Management 34 (2014) 2251–2259
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Evaluating the biochemical methane potential (BMP) of low-organic waste at Danish landfills Zishen Mou ⇑, Charlotte Scheutz, Peter Kjeldsen Department of Environmental Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark
a r t i c l e
i n f o
Article history: Received 4 March 2014 Accepted 25 June 2014 Available online 5 August 2014 Keywords: Biochemical methane potential (BMP) Low-organic waste Landfill gas (LFG) generation model Degradable organic carbon content (DOCC) Greenhouse gas (GHG) emission
a b s t r a c t The biochemical methane potential (BMP) is an essential parameter when using first order decay (FOD) landfill gas (LFG) generation models to estimate methane (CH4) generation from landfills. Different categories of waste (mixed, shredder and sludge waste) with a low-organic content and temporarily stored combustible waste were sampled from four Danish landfills. The waste was characterized in terms of physical characteristics (TS, VS, TC and TOC) and the BMP was analyzed in batch tests. The experiment was set up in triplicate, including blank and control tests. Waste samples were incubated at 55 °C for more than 60 days, with continuous monitoring of the cumulative CH4 generation. Results showed that samples of mixed waste and shredder waste had similar BMP results, which was in the range of 5.4–9.1 kg CH4/ton waste (wet weight) on average. As a calculated consequence, their degradable organic carbon content (DOCC) was in the range of 0.44–0.70% of total weight (wet waste). Numeric values of both parameters were much lower than values of traditional municipal solid waste (MSW), as well as default numeric values in current FOD models. The sludge waste and temporarily stored combustible waste showed BMP values of 51.8–69.6 and 106.6–117.3 kg CH4/ton waste on average, respectively, and DOCC values of 3.84–5.12% and 7.96–8.74% of total weight. The same category of waste from different Danish landfills did not show significant variation. This research studied the BMP of Danish low-organic waste for the first time, which is important and valuable for using current FOD LFG generation models to estimate realistic CH4 emissions from modern landfills receiving low-organic waste. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Methane (CH4) is one of the most important greenhouse gases (GHG), with a 100-year global warming potential 28 times that of carbon dioxide (CO2) (the digital value of 28 does not include CO2 from CH4 oxidation or climate-carbon feedbacks) (IPCC, 2013). In 2011, landfills contributed to 4907 and 3052 Gg CH4, accounting for 17.5% and 19.6% of anthropogenic CH4 emissions in the US and Europe, respectively (EEA, 2013; US EPA, 2013). European landfills with a total disposal capacity greater than 25,000 tons must report their annual CH4 emission to the Protocol on Pollutants Release and Transfer Registers (PRTR) (CEC, 2006). Due to difficulties in precisely monitoring whole site’s CH4 emissions (Scheutz et al., 2011b), first order decay (FOD) landfill gas (LFG) generation models are currently widely used to estimate
⇑ Corresponding author. Address: Department of Environmental Engineering, Technical University of Denmark, Bygningstorvet – Building 115, Miljøvej 113, 2800 Kgs. Lyngby, Denmark. Tel.: +45 4525 1498; fax: +45 4593 2850. E-mail address: [email protected] (Z. Mou). http://dx.doi.org/10.1016/j.wasman.2014.06.025 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
CH4 emissions from landfills, and the biochemical CH4 potential (BMP) is an essential parameter in this procedure (Blaha et al., 1991; Bogner and Matthews, 2003; Cho et al., 2012; Thompson et al., 2008). For example in the US EPA LandGEM version of the FOD model, the parameter L0 (m3 or kg CH4/ton waste, wet weight) is used to describe the CH4 generation potential (US EPA, 2005). In the IPCC model, the degradable organic carbon content (DOCC) (%, kg C/kg waste, wet weight) is used instead of BMP (IPCC, 2006). DOCC is one of the main parameters affecting the CH4 emissions from landfills, and is estimated based on the waste composition, and varies for different waste fractions (IPCC, 2006). Eq. (1) was defined by the IPCC guidelines for transformation between BMP and DOCC (IPCC, 2006).
BMP ¼ DOCC F 16=12 103
ð1Þ
where F is the volume fraction of CH4 in generated LFG, 16 and 12 are the molar masses of CH4 and carbon, 103 is the factor between kg and ton. Different models have various numeric values for both parameters, as listed in Table 1. Two of the models listed in Table 1, the E-PRTR (Fr) and LandGEM models, are single-phase models. Thus, they have no specific sub-categories for each kind of waste
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Table 1 Default BMP values (kg CH4/ton waste, wet weight) and DOCC (%, kg C/kg waste, wet weight) for the various waste categories that were used in different models. FOD models
BMP (DOCC)a values Industrial waste
MSW
E-PRTR (Fr)b LandGEMc
28 (4) Not specified
55 (8) 122 or 72 (18 or 11)
IPCC (waste fractions)d
Sludge Industrial
33 (5) 67 (10)
Food Garden Paper Textiles
100 133 267 160
Afvalzorg (waste fractions)e
Soil C&D Shredder Sludge Cleansing
3 (2) 11 (3) 13 (4) 25 (6) 19 (4)
Household Bulky mixed Garden Commercial RDF
91 80 47 56 63
(15) (20) (40) (24)
(18) (19) (9) (13) (18)
Country based
Latest version
French U.S.
2003 2005
International
2011
Holland
2014
a
Digits given parenthetically present DOCC values of corresponding waste categories. The E-PRTR (Fr) model describes CH4 generation rates of 4.8, 2.4, 1.3 and 0.6 kg CH4 per ton household waste per year at 0, 5, 10 and 20 years after landfilling, respectively. For moderately decomposable waste such as nonhazardous industrial waste, the values are 50% of household waste for the corresponding landfilling period (ADEME, 2003; Oonk, 2010). c The default value of 122 was based on requirements for U.S. landfills as specified in the Clean Air Act. The default value of 72 was based on results of an inventory by the US EPA (Oonk, 2010; US EPA, 1990). d Ranges of default values for MSW were calculated based on default DOCC values of various MSW fractions, as shown in the following rows (IPCC, 2011, 2006). e Ranges of default values were calculated based on default organic carbon content and fractions of wet waste (Afvalzorg, 2014). b
(ADEME, 2003; Oonk et al., 1994; US EPA, 1990). The other two models, the IPCC and Afvalzorg models, are multi-phase models, which operate with a number of more detailed waste categories (Mata-Alvarez et al., 2011; Scharff and Jacobs, 2006; Thompson et al., 2009). As listed in Table 1, the IPCC model divides waste into industrial waste (incl. sludge) and municipal solid waste (MSW), and MSW includes food, garden, paper, and textiles. The Afvalzorg model, which was developed by a Dutch waste management company (Jacobs and Scharff, 2001), holds datasets for different waste categories as listed in Table 1. Default values were determined based on the IPCC model and field measurements in three Dutch landfills disposing low-organic waste (Afvalzorg, 2014). Instead of organic fractions of traditional MSW such as paper and food waste, the Afvalzorg model provides relative low-organic waste fractions such as bulky mixed waste (incombustible), construction and demolition (C&D) waste, soil (contaminated with oil and other residues), shredder (shredded pieces of abandoned vehicles or machines), cleansing (residues from street cleansing) waste and refuse-derived fuel (RDF). Above all, the shredder and bulky mixed waste are the major components of deposited waste categories at current Danish landfills. As EU Council legislated, landfilling is limited to inert materials that are not biodegradable or combustible (containing a significant fraction of organic matter) since 1997 (EU, 1999). Consequently more and more EU member states (e.g. the Netherlands as of 1996, Denmark as of 1997, and Germany as of 2005) have banned the landfilling of organic waste (Manfredi et al., 2010). Researchers have paid more attention to the biological stability and degradability of organic waste (Cossu and Raga, 2007; De la Cruz et al., 2013). Danish landfills currently only receive low-organic and non-combustible waste permanently, such as bulky mixed waste (Scheutz et al., 2011a), shredder waste, low-organic dewatered sludge, etc. Some landfills receive combustible waste for temporary storage (up till several months), which is later sent to incineration plants for electricity and heat generation (Manfredi et al., 2010). Even if the majority of MSW is incinerated, previous studies have shown significant gas emission from Danish landfills (Scheutz et al., 2011a, 2007; Scheutz and Kjeldsen, 2007). In this situation, LFG generation modeling is still necessary when reporting CH4 generation from landfills. Due to the implementation of the European Waste Directive (EU, 2008), the composition of disposed waste at
landfills in other European countries is expected to change, as organic waste is expected to be recycled or incinerated. As an essential parameter, BMP values for different types of organic waste are widely studied and reported in the literature, and applied in the fields of renewable energy (Alzate et al., 2012), anaerobic digestion (Elbeshbishy et al., 2012) and especially LFG generation (Kim and Townsend, 2012). BMP assays for organic waste have also been developed and improved over the past decade (Angelidaki et al., 2009; Hansen et al., 2004; Li et al., 2013; Owens and Chynoweth, 1993; Raposo et al., 2011). Table 2 lists BMP determinations in anaerobic incubation batch tests, in which solid waste was usually co-digested with an inoculum (such as sewage sludge or cattle manure) at intermediate (35 °C) or high temperature (55 °C). The volume in each experimental reactor varied between 100 and 2000 ml and the organic load (concentration of organic material in inoculum) varied from 0.25 to 4 g/L based on the mass of volatile solids (VS), total solids (TS), organic carbon (OC), or chemical oxygen demand (COD). The unit of BMP is expressed as Nm3 CH4/ton VS or kg CH4/ton waste (wet weight). The relative standard deviation was determined via duplicate or triplicate parallel tests. The CH4 recovery (%) was calculated as the ratio between the actual and theoretical CH4 generation from the substrate. From Tables 1 and 2, it can be seen that BMP values vary significantly among different waste categories, as well as in models. To estimate CH4 generation from Danish landfills, which receive low-organic waste, realistic BMP values are highly important. However, very little is known about BMP values for non-combustible waste or low-organic waste. Furthermore, most FOD models were developed based on traditional waste fractions such as food waste and MSW, which have much higher DOCC, as well as BMP according to Eq. (1). Even with the Afvalzorg model, previous research has shown its default BMP values could cause overestimations when applied to Danish landfills (Scheutz et al., 2011a, 2007; Scheutz and Kjeldsen, 2007). Therefore, current models are highly uncertain for Danish landfills, especially when default BMP values were based on tests from traditional waste fractions. In this research, three categories of low-organic waste (bulky mixed, shredder and sludge waste from waste water treatment plant), which are all major components of permanently deposited waste, and temporarily stored combustible waste were sampled
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Z. Mou et al. / Waste Management 34 (2014) 2251–2259 Table 2 BMP determined by anaerobic co-digestion experiments using different types of organic waste as substrate. Waste fractions
Organic fractions of MSWb Food wastec Paper and cardboardd Sludge wastee Fresh samples from landfillsf
Substrate
BMP 3
TOC (% wet waste)
VS (% wet waste)
(Nm CH4/ ton VS)
(kg CH4/ ton waste)a
8.1–18.4 7.3–14.6 83.2–95.5 0.8–2.6 12.8–68.2
14.5–33.6 22.9–28.0 84.2–97.4 1.4–4.1 25.1–54.8
336–495 467–675 192–277 143–379 127–284
14.8–44.5 61–106 114–189 2.1–3.7 54–91
Relative Standard deviation (± %)
CH4 recovery (%)
4–5 1–3 12–17 2–6 5–26
28–37 57–89 20–30 15–22 43–60
a
BMP results were based on wet weight when presented as kg CH4/ton waste. Waste fractions came from Nordic and British MSW streams that did not contain food/paper waste (Davidsson et al., 2007; Hansen et al., 2004; Jansen et al., 2004; Jørgensen, 2009; Zhang et al., 2012). c Waste fractions came from the food industry, university canteens and restaurants in Ireland and southern Europe (Browne and Murphy, 2013; Cabbai et al., 2013; Cavaleiro et al., 2013; Rincon et al., 2013). d Waste fractions included office paper, newspaper, magazines and cardboard from Chinese and French waste collection stations. Samples were pretreated before running the BMP assay (Pommier et al., 2010; Yuan et al., 2012). e Sludge waste came from waste water treatment plants. Pretreatment including mechanical dewatering was performed before running the BMP assay (Astals et al., 2013; Seng et al., 2010; Tomei et al., 2009; Yan et al., 2013). f Waste fractions were fresh samples from South Korean, Finnish and American landfills. Results for paper, food and sludge waste were excluded (Jeon et al., 2007; Jokela et al., 2005; Kim and Townsend, 2012). b
from four Danish landfills. The objectives were to determine the physical characteristics and BMP values of low-organic waste via batch experiments. DOCC values of low-organic waste that are representative of the current and future disposed waste at modern landfills are presented for the first time. The outcomes are also valuable for improving current FOD models to more precisely estimate LFG generation from low-organic waste disposal sites in Denmark, as well as other countries, which have or will have similar landfilling strategies. 2. Materials and methods 2.1. Waste samples from Danish landfills 2.1.1. Description of landfills and waste categories Waste samples were collected from four Danish landfills. All four landfills were still in service and currently receive only low-organic waste for permanent disposal. Two of the four sites are currently running gas collection system in the area of old landfill cells which disposed mixed household waste in 1990s. Table 3 provides an overview of the four landfills in terms of location, start year, total landfill area, storage capacity, and amounts of disposed waste and temporarily stored combustible waste in 2010. Two sites, AV Miljø and Audebo, have special cells for combustible waste such as paper, plastic, and wood. The combustible waste is stored temporarily before being sent to incineration plants in the winter for energy recovery. The deposited waste categories are similar at all four sites, including shredder waste, dewatered sludge, bulky mixed waste, soil and C&D waste. Only one landfill, Audebo, does not receive any shredder waste. Table 4 shows waste amounts for the major waste categories disposed in 2010. 2.1.2. Sampling and preparation of waste Samples of shredder, sludge, bulky mixed and combustible waste were collected from four landfills. The whole process was approached by using a method developed by Jansen et al. (2004). For each sample, approximate 300 kg of wet waste was collected as raw material, either from same-day-deposited waste piles or waste transporting trucks when they arrived at the site. When sampling from waste piles, shovels that were long enough to reach the bottom were used to avoid missing bottom layers (Gy, 1996). When sampling sludge and combustible waste, the top or cover
layer was removed to avoid unrepresentativeness (Laine-Ylijoki et al., 2009). The combustible waste was covered daily with wood pieces and soil at AV Miljø landfill, or packed in large plastic bags at Audebo landfill by the landfill operators. Therefore, samples of combustible waste were quite dry when deposited at the landfills. In total, 13 waste samples were obtained over a period of approximately 24 days in October 2011. The waste was collected in plastic bags (100 L, 0.07 mm in thickness) and placed in sealed steel drums (115 L) for transportation. All waste samples were kept at 4 °C in the dark before treatment. Within 7 days after sampling, mass and size reduction of the samples was approached. Each waste sample was spread out on a large plastic sheet placed on the floor and was mixed manually. Then using a grid method (Laine-Ylijoki et al., 2009), about 20% of the sample was retained and then shredded in a cutter mill machine (SM 2000, Retsch). About 5–6 kg of samples for analysis of TS and VS were taken. Approximately 20% of the machine´s outcome was collected using the same method and then dried at 80 °C until a constant weight was achieved. The dry sample was afterwards milled in a small hammer mill (Macsa 300, Eriez) with a 2 mm screen. The material was thoroughly mixed for BMP assay and the finely grained sample was smashed into powder manually with a mortar and pestle for analyzing total carbon (TC) and total organic carbon (TOC). 2.2. Physical and chemical tests of waste samples Waste samples were characterized in terms of moisture content, TS, VS, ash, TC and TOC following standard procedure. The pH of sludge waste leachate was tested and according to the landfill manager’s information, there was no acid substrate among other waste fractions. The moisture content was measured by oven-drying 2–4 kg of sample at 105 °C for at least 24 h until a constant weight was achieved. Ash from each sample was obtained by oven drying the dry samples at 550 °C for at least 2 h until no further weight loss was obtained. All experiments were run in triplicate. Dry powder from each sample was used to test TC and TOC using a LECO Induction Furnace CS-200 oven. When testing TOC, 2 ml of sulfurous acid (5% H2SO3 solution) was added to approximately 0.5 g of powder to remove inorganic carbon. Each experiment was run in triplicate and the systemic error was ±2%. All results of TS, VS, TC and TOC are presented as % (mass fraction of wet waste, similarly hereinafter) in this research.
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Table 3 Overview of four Danish landfills and the total amount of disposed and combustible waste in 2010. Landfill
Location
Start year
Total area (ha)
Capacity (Mm3)
Disposed waste amount-2010 (t)a
Combustible waste amount-2010 (t)b
AV Miljø Reno Djurs Odense Audebo
Southwest of Copenhagen Northeast in Jutland Northeast in Fyn Northwest in Zealand
1989 1981 1994 1988
34 72 110 24
2 4 10 7
80,334 64,692 145,142 19,644
32,179 – – 49,782
a
This column shows the amount of permanently disposed waste in 2010 at each landfill. Combustible waste for temporary storage is not included. This column shows the amounts of temporarily stored combustible waste in 2010 at AV Miljø and Audebo. The Reno Djurs and Odense landfills do not receive combustible waste. b
Table 4 Amounts of waste (t) in different categories disposed of at four Danish landfills in 2010.
a
Landfill
Shredder
Sludge
Bulky mixed
C&D
Soil and sand
Other inert
AV Miljø Reno Djurs Odense Audeboa
43,279 24,506 73,027 –
10,054 2680 1886 640
6414 8359 6854 2341
12,853 6469 15,966 982
4017 1941 29,028 59
3717 20,737 18,381 15,622
Audebo does not receive any shredder waste.
2.3. Batch tests for determining BMP 2.3.1. Set up and monitoring The experiment was carried out in terms of triplicate batch tests following the method developed by Hansen et al. (2004). Table 5 provides an overview of the different series set-up in terms of waste sample, substrate mass, inoculum volume, organic load (weight of TOC in substrate per unit volume of inoculum, g OC/L) and the substrate to inoculum ratio (S/I, g VS of waste material added as substrate per g VS added as inoculum). In Series A, three waste samples were tested. Higher BMP values were observed for combustible, shredder and sludge waste when setting the organic load at 2 g OC/L in comparison to 4 g OC/L. In these batch tests, the reactors were 2 L reactors containing 1 L of inoculum. CH4 generation curves, calculation of BMP, CH4 recovery and DOCC are presented in the result section, as well as for series B and C. For comparison, CH4 generation curves for these three waste samples when organic load was set at 4 g OC/L are also presented. Insignificant differences and acceptable variance were observed when using a 1 L glass bottle with 0.5 L inoculum. Therefore in series B, for combustible, shredder and sludge waste from other landfills, organic load was set at 2 g OC/L and the inoculum volume was 0.5 L. Reducing the reactor size also conserved the inoculum so that all samples could be incubated with the same inoculum. When using bulky mixed waste samples as substrate, inhibition was observed for set-up of 1 g OC/L during the initial experimental research and BMP was under the detection limit (Scheutz et al., 2007). Therefore, in series C, the organic load was set at 0.5 g OC/ L and the inoculum volume was kept as 0.5 L. The reactors were placed on a scale when setting them up and were flushed for over 5 min with N2 before sealed. All reactors were placed in the incubator at 55 ± 1 °C for over nine weeks using gas chromatograph (Shimadzu GC 14A); and the CH4 concentration in the headspace was measured once or twice per week. Thermophilically (55 °C) digested material from a full-scale plant, located at Vegger (Nibe, Denmark), was used as inoculum in the experiments. The Vegger biogas plant co-digests mainly cattle manure (80%) together with different wastes from the food industry (20%). The TS and VS values of the inoculum were 3.01 ± 0.55% and 2.06 ± 0.71%, respectively.
2.3.2. Blank and control experiments Blank and control experiments with the same set up (starting and running date, inoculum and reactor volume) were also
performed in triplicate in each series. Blank experiments, which contained only water and inoculum, were used to measure CH4 generation originating from the inoculum alone, and to indicate the detection limit of BMP in this method. Control experiments, which also contained standard substrate, were used to test the quality of the inoculum (i.e. to address the variation among triplicates) and to indicate if the incubation method for determining BMP was functioning as expected. In this method, Avicel (Fluka, Sigma–Aldrich, Vallensbæk Strand, Denmark), which is a microcrystalline cellulose powder, was used as standard substrate in the control experiments. The TOC value of Avicel was 44.44% according to its molecular formula (C6H10O5)n. In all series, the concentration of substrate in control experiments was 1 g/L as showed in Table 5. The monitoring of gas samples from blank and control experiments were the same as stated above for all batch tests. The cumulative CH4 generated (the average value of triplicates) from the substrate alone (waste sample) was obtained by subtraction of the CH4 generated in the blank and transformed into a function of incubation time. With each measurement, the actual temperature and atmospheric pressure were recorded. Thus, all measures of gas volume could be converted to standard temperature and pressure (STP, 0 °C and 1 atm) conditions according to the Ideal Gas Law Equation. BMP results are presented as kg CH4/ton waste (wet weight, similarly hereinafter) according to default parameters in FOD models. By using TS (TOC) values, BMP results can be easily converted VS or dry matter (TOC or COD) based. The uncertainty in the blanks, i.e. the triplicate tests with inoculum but no substrate, determined the detection limit of the BMP measurement. The average CH4 generated in the blank tests was subtracted from the average of all batch tests when calculating the BMP value of the corresponding substrate. In this research the detection limit was given as the limit above, from which one could quantify the BMP result (the difference between a sample and a blank test) with a probability of 95%. The detection limit (ml) was calculated by Eq. (2) (Hansen et al., 2004; Lavagnini and Magno, 2007; Johnson et al., 2011; US EPA, 1993).
Detection limit ¼ 1:943 s 2
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1=n1 þ 1=n2 Þ
ð2Þ
where 1.943 was the 95% confidence level in the t-distribution with 6 degrees of freedom (Johnson et al., 2011); s was the standard deviation of the blank tests (ml); n1 and n2 were the numbers of replicates used to measure the samples and blanks (in this research they were both 3).
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Previous research on the BMP method indicated that an average value of triplicate control tests should be in the range of 315– 439 ml CH4/g VS when using Avicel as a standard substrate (Hansen et al., 2004). If the average value was outside this range, it might suggest the inoculum, although evenly distributed in the reactors, was inhibited or for other reasons producing too little CH4. 2.3.3. Calculation of CH4 recovery and DOCC The CH4 recovery (%) was defined as the ratio between the actual and theoretical CH4 generation, and was calculated using Eq. (3).
CH4 recovery ¼ ðBMP 103 Þ=ðTOC F 16=12Þ 100%
ð3Þ
where BMP is determined via batch tests and presented as kg CH4/ton waste, TOC is presented as % (kg OC/kg waste), F is the volume fraction of CH4 in generated gas, 16 and 12 are the molar masses of CH4 and carbon. The default value of F in the IPCC model is 50% with 5% of uncertainty range (IPCC, 2006). Most waste in landfills generates a gas with approximately 50% CH4 (Cavaleiro et al., 2013; Elfadel et al., 1996; Jeon et al., 2007). Only material including substantial amounts of fat or oil can generate gas with substantially more than 50% CH4. Therefore in the research, F with a value of 50% was used for calculation in Eq. (3). As stated before, DOCC is used in some FOD instead of BMP when estimating LFG generation. To adapt models such as the IPCC version to Danish landfills, the DOCC (%) of low-organic waste can be calculated by Eq. (4).
DOCC ¼ CH4 recovery TOC
ð4Þ
where DOCC is presented as % (kg C/kg waste, wet weight). Mathematically, it can be seen from Eqs. (3) and (4) that the ratio between the BMP and DOCC values for any waste material should be a constant when F (the volume fraction of CH4 in LFG) was determined. 3. Results and discussion 3.1. TS, VS, TC and TOC of waste samples Table 6 shows the average results for TS, VS, TC and TOC of all the waste samples. The pH value of sludge waste leachate was in the range of 7.3–7.6. All values were obtained from triplicate experiments with a relative error less than ±5%. In general, little variation was observed within the same category of waste sampled at different landfills. Sludge had the lowest TS content, which was
about 20%, while other samples such as shredder waste had a relatively high TS content that was above 80%. Combustible waste had the highest VS content, which was about 70%, as well as the highest ratio of VS/TS (over 80%). Bulky mixed waste samples also contained a relatively high fraction of TS (over 80%), but a significantly lower fraction of VS, which was only 7–8%. Shredder waste samples contained equally high fractions of TS as combustible and bulky mixed waste, which were over 85% and over 4 times of sludge waste. However, shredder and sludge waste samples contained similar fractions of TC and TOC, which were in the range of 9–13%. Combustible and bulky mixed waste samples contained the highest and lowest carbon fractions, which were 29–32% and 4–5% respectively. Traditional landfill waste such as food or kitchen waste, sludge waste and garden waste (Bolan et al., 2013; De la Cruz et al., 2013; Jones et al., 2013; Yang et al., 2013) contains a significant fraction of organic matter. From Table 2, it can be seen that MSW and landfill samples had a TOC content varying between 8.1–68.2%, which is significantly higher than bulky mixed waste (3–4%, Table 6) at Danish landfills. In this case an overestimation is unavoidable if traditional FOD models are applied to predict LFG generation from Danish landfills. If the TOC and VS contents of sludge waste are compared, Table 6 shows much higher value. This is because the deposited sludge at Danish landfills was dehydrated and had a much lower moisture content (80–82%, calculated from TS results). Traditional sludge waste deposited at landfills has a moisture level of 90–95% (Astals et al., 2013; Kim and Townsend, 2012; Seng et al., 2010). The shredder and sludge waste showed similar TC and TOC values, but different VS values because of their lower moisture contents and various characteristics of the dry fractions. The shredder waste was consisting mainly of metals, plastic, rubber, wood and foam. The main components of combustible waste at Danish landfills include paper, wood, plastic, and cardboard. The TOC value is lower than traditional paper waste (Table 2) because most paper waste is recycled and reused in Denmark (Manfredi et al., 2010). Theoretically, the DOCC value of waste fractions must be lower than the TOC value because some OC such as fossil carbon is nondegradable and results in carbon storage at landfills (De la Cruz et al., 2013; Law et al., 2013). As shown in Table 1 (digits given parenthetically), the default DOCC values in the Afvalzorg and IPCC model, for instance, are 15–40% and 10–20% for MSW waste, respectively. Single phase models use default DOCC values of 8% and 11–18% in the E-PRTR (Fr) and LandGEM models. However, the TC and TOC values of bulky mixed waste at Danish landfills (in Table 6) are only 3–5% as tested, which was even lower. There-
Table 5 Substrate and inoculum set-up of batch tests for determining the BMP of waste. Series
Waste category
Landfill
Substrate mass (g)a
Inoculum volume (L)
Organic load (g OC/L inoculum)
Substrate to inoculum ratio (S/I, g VS/g VS)
A
Combustible Shredder Sludge
Audebo Reno Djurs AV Miljø
7.6 (15.2) 19.2 (38.4) 20.6 (41.2)
1.0
2 (4)a
4–6 (7–12)
B
Combustible Shredder
AV Miljø AV Miljø Odense Reno Djurs Odense Audebo
3.6 9.6 9.4 11.7 11.0 8.9
0.5
2
4–6
Sludge
a b
C
Bulky mixed
AV Miljø Reno Djurs Odense Audebo
7.5 8.4 6.7 7.1
0.5
0.5
15–20
A B/C
Avicel
Controlb
1 0.5
1 0.5
0.44
21
Digits given parenthetically present the substrate mass when organic load was 4 g OC/L and the inoculum volume is 1.0 L in Series A. Avicel was used as substrate in control experiments. In all series, the concentration of material in inoculum was 1 g/L.
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Table 6 Waste characterization in terms of TS, VS, TC and TOC (%, kg/kg waste, wet weight).
Combustible
AV Miljø Audebo
84 86
Shredder
AV Miljø Reno Djurs Odense
Sludge
Bulky mixed
Avicel
TC
TOC
70 69
32 29
28 26
87 86 90
30 31 29
12 12 13
10 10 11
AV Miljø Reno Djurs Odense Audebo
20 18 19 21
17 15 17 19
11 9 10 12
10 9 9 11
AV Miljø Reno Djurs Odense Audebo
82 81 80 84
7 7 8 8
4 4 5 4
3 3 4 4
100
100
44
44
Control
a
TS
VS
a
Avicel was used as substrate in control experiments. Digits present theoretical values based on chemical formula (C6H10O5)n.
fore these default DOCC values in FOD models were obviously not suitable for Danish LFG generation estimation.
Cumulave CH4 generatoin (ml)
Landfill
3000
Control
Combusble - Audebo 1
Combusble - Audebo 2
(a)
2400 1800 1200 600 0
0
10
Blank
Ccumulave CH4 generatoin (ml)
Waste
Blank
20
Control
30
40
Shredder - Reno Djurs 1
50
60
70
Shredder - Reno Djurs 2
(b)
2500 2000 1500 1000 500 0
0
10
20
30
40
50
60
70
3.2. BMP batch tests Cumulave CH4 generatoin (ml)
Blank
3.2.1. Cumulative CH4 generation curves Fig. 1–3 show cumulative CH4 generation curves changing with time in all series. All results are the average of triplicates. The standard deviations are shown in Table 7. The results of blank and control experiments with a 1 L inoculum are shown in Fig. 1, and those with a 0.5 L inoculum are shown in both Fig. 2 and 3. In general, curves within the same waste category showed similar trends in CH4 accumulation over time. In all series, CH4 generation from the blank experiments was lower than those from control and other incubation experiments with waste. Fig. 1 shows the results from Series A, which incubating different waste fractions with two organic load settings. From Fig. 1(a) and (b) it can be seen that for combustible and shredder waste, normalized CH4 generation was unaffected by the concentration of waste material. With twice the mass of substrate, higher CH4 generation is reasonable for combustible – Audebo 2 and shredder – Reno Djurs 2. If normalized by mass (g) of substrate, CH4 generation (after subtracting CH4 generated from the blank test) based on wet weight was 74.6 and 73.6 ml CH4/g combustible waste, and 5.1 and 5.0 ml CH4/g shredder waste, respectively. Therefore similar BMP values as well as CH4 recovery and DOCC were obtained when setting the organic load at 2 and 4 g OC/L. Both substrate and inoculum had good internal homogeneity. As showed in Fig. 1(c), sludge – AV Miljø 1 and 2 resulted in 46.3 and 33.40 ml CH4/g waste, respectively. The difference might be due to the inhibition during the anaerobic degradation process. A larger amount of substrate might lead to accumulation of ammonia and thus lower CH4 generation (Cavaleiro et al., 2013; MataAlvarez et al., 2011). Ammonia is produced by the biological degradation of the nitrogenous matter present in the waste material (Chen et al., 2008). Intermediate compounds during hydrolysis of proteins, phospholipids, nitrogenous lipids and nucleic acids are composed of amino acids, which could inhibit the growth of methanogens (Kayhanian, 1999). Since lower organic load for combustible, sludge and shredder waste showed higher BMP values, it was set as 2 g OC/L in Series B. For bulky mixed waste samples, former research also observed inhibition during 1 g OC/L incubation experiments, (Scheutz et al., 2007). Therefore in Series C, BMP was determined based on S/I = 0.5 g OC/L batch tests. Fig. 2(a–c) show the results from Series B, incubating the same waste categories as in Fig. 1(a–c), correspondingly. Fig. 3 shows
3500
Control
Sludge - AV Miljø 1
Sludge - AV Miljø 2
(c)
2800 2100 1400 700 0
0
10
20
30
40
50
60
70
Incubaon me (days) Fig. 1. Series A of BMP batch tests: organic load = 2 and 4 g OC/L for (a) combustible waste – Audebo; (b) shredder waste – Reno Djurs; (c) sludge waste – AV Milø.
cumulative CH4 generation from bulky mixed waste samples. Curves of combustible and sludge waste in Fig. 2(a) and (c) showed a period of rapid increase in the first two weeks. Combustible waste from AV Miljø, as shown in Fig. 2(a), resulted in 1440 ml of total CH4 generated over 60 days of incubation. However, on the 12th and 15th day after starting the batch tests, average cumulative CH4 generation reached 863 and 951 ml, respectively. Therefore, 2/3 of the potential CH4 was generated in the first 14 days from samples of combustible waste, and only 1/3 was produced in the remaining 45 days. Similar results were also observed for sludge waste samples, as shown in Fig. 2(c). The cumulative CH4 generation reached 1180–1330 ml on the 15th day after starting incubation. Incubation of sludge waste resulted in 1647–1825 ml CH4 by the end of 60 days. More than 70% of the potential CH4 was generated in first 14 days. This might be caused by co-digestion of cattle manure and sludge, which could accelerate the hydrolysis process of the organic waste due to the synergetic effects and improved nutrients for methanogens growth (Borowksi and Weatherley, 2013; Zhang et al., 2014). On the other hand, shredder and bulky mixed waste samples have different CH4 generation trends, which were relatively stable at the beginning. If the curves in Figs. 2(b) and 3 are considered as linear trends in the first 55 days, rates of daily increasing of 17–18 ml CH4 can be fitted. The average cumulative CH4 generated by shredder and bulky mixed waste resulted in 932–969 and 902–922 ml CH4, respectively.
Z. Mou et al. / Waste Management 34 (2014) 2251–2259
Cumulave CH4 generatoin (ml)
Blank
Ccumulave CH4 generatoin (ml)
Combusble - AV Miljø
1600
(a) 1200
800
400
0
0
10
Blank
20
30
Control
40
50
Shredder - AV Miljø
60
70
Shredder - Odense
1200
(b) 800
400
0
0
Blank
Cumulave CH4 generatoin (ml)
Control
10
Control
20
30
40
Sludge - Audebo
50
60
Sludge - Odense
70
Sludge - Reno Djurs
triplicates exceeded the average of a set of blank triplicates by more than 47 ml CH4 in Series A, or 32 ml CH4 in series B and C, then significant CH4 generation from the samples was obtained and it could be quantified with a confidence of at least 95%. In this research, all batch tests showed higher CH4 generation than the limits of detection. In Series A, the mean and standard deviation of triplicate control tests were 2195 and 64.5 ml CH4, respectively. In Series B and C, the mean and standard deviation of triplicate control tests were 1026 and 21.1 ml CH4, respectively. By subtracting the CH4 generated from the blank experiments, the BMP of Avicel was 256.7 and 253.5 kg CH4/ton Avicel, or 359 and 355 ml CH4/g VS, in Series A and B/C, respectively. The standard deviation was 13 ml CH4/g VS for all series. The results were entirely in the expected range (315–439 ml CH4/g VS). Similar results (379 ml CH4/g VS generated from Avicel substrate) have been reported by Hansen et al. (2004). As calculated by Eqs. (3) and (4), the CH4 recovery was 85–87% and the DOCC was approximately 38%, as showed in Table 7. It was in good agreement with literatures (Raposo et al., 2011). From Figs. 1–3 it can be seen that the CH4 generation curves of the blank and control experiments had same trend as other waste samples. No significant or unexpected changes were observed during the whole CH4 generation process. Therefore, the performance of the inoculum in all series of batch tests was acceptable.
2000
(c) 1600 1200 800 400 0
0
10
20
30
40
50
60
70
Incubaon me (days) Fig. 2. Series B of BMP batch tests: organic load = 2 g OC/L for (a) combustible waste – AV Milø; (b) shredder waste – AV Milø and Odense; (c) sludge waste – Reno Djurs, Odense and Audebo.
Blank
Cumulave CH4 generatoin (ml)
2257
Control
AV Miljø
Reno Djurs
Odense
Audebo
1200 1000 800 600 400 200 0
0
10
20
30
40
50
60
70
80
Incubaon me (days) Fig. 3. Series C of BMP batch tests: organic load = 0.5 g OC/L for bulky mixed waste – four landfills.
3.2.2. Blank and control In Series A, blank and control experiments were performed with a 1 L inoculum. The mean and standard deviation of triplicate blanks were 1836 and 14.9 ml CH4, respectively. In Series B and C, blank and control experiments were performed with a 0.5 L inoculum. The mean and standard deviation of triplicate blanks were 849 and 10.2 ml CH4, respectively. Using two standard deviation values and Eq. (2), the detection limits in Series A and B/C were 47 and 32 ml, respectively. If the average of a set of sample
3.2.3. Calculation of BMP, CH4 recovery and DOCC Table 7 provides an overview of experimental and calculated results in terms of cumulative and theoretical CH4 generation, BMP, standard deviation of triplicates, average CH4 recovery and DOCC for different waste samples. In general, little variation can be seen within the same category of waste sampled from different landfills. The same conclusion can also be applied to other physical and chemical test results based on Table 6. The highest BMP and DOCC values were observed in combustible waste, 106.6–117.3 kg CH4/ton waste and 7.96–8.74%, respectively. The CH4 recovery was 30.6–31.2%, indicating that less than one third of the organic carbon in the waste samples was released as gases. Because paper and wood are the main combustible waste, the degradation process would be considerable slow. In comparison to the default DOCC values in the IPCC model (Table 1) and other tested results of BMP (Table 2), it can be concluded that traditional model and waste characters are unsuitable to apply to the Danish scenario. Sludge waste showed lower BMP values (51.8–69.6 kg CH4/ton waste) in comparison to combustible waste, but the highest CH4 recovery (42.7–52.2%) among all waste fractions. This is reasonable because active microorganisms and easily degradable organic matter are likely to remain in the sludge waste (Astals et al., 2013; Tomei et al., 2009; Yan et al., 2013). Unlike the combustible waste, the BMP values of Danish landfill sludge waste are actually higher than the default values (Table 1) in the IPCC and Afvalzorg model, as well as tested results from references (Table 2). This is due to the dehydration process, which would enhance the VS and TOC content as well as the BMP values. The shredder and bulky mixed waste samples showed similar BMP and DOCC results, in the range of 5.1–9.1 kg CH4/ton waste and 0.44–0.70%, respectively. These waste categories are special and quite different from traditional landfill MSW. They have much lower BMP and DOCC values than all model default values in Table 1, as well as the organic fractions of MSW and fresh samples from landfills in Table 2. In addition, CH4 recovery from bulky mixed waste samples was 2–3 times higher than from shredder waste. From Fig. 3 it can be seen that the incubation time for bulky mixed waste was also longer than the others, which indicated that
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Table 7 Overview of batch test results in terms of cumulative and theoretical CH4 generation, calculated BMP, CH4 recovery and DOCC values.
a b c
Waste category
Landfill
Cumulative CH4 generation (ml) a
Theoretical CH4 generation (ml)
BMP (kg CH4/ton waste)
CH4 recovery (%)
Combustible
AV Miljøb Audeboc
1440 (24) 2403 (62)
1897 1853
117.3 106.6
31.2 30.6
8.74 7.96
Shredder
AV Miljøb Reno Djursc Odenseb
932 (18) 1934 (51) 969 (21)
1867 1873 1868
6.2 7.3 9.1
4.4 5.2 6.4
0.44 0.52 0.70
Sludge
AV Miljøc Reno Djursb Odenseb Audebob
2790 1825 1647 1716
1941 1868 1868 1864
63.7 59.6 51.8 69.6
49.1 52.2 42.7 46.5
4.91 4.70 3.84 5.12
Bulky mixedb
AV Miljø Reno Djurs Odense Audebo
922 918 911 902
467 468 466 466
7.0 5.9 6.6 5.4
15.7 14.8 13.3 11.3
0.47 0.44 0.53 0.45
Avicel (control)
Series B and Cb Series Ac
1026 (21) 2195 (65)
(71) (37) (32) (34)
(10) (12) (11) (10)
207 415
253 256
85.5 86.5
DOCC (%, kg C/kg waste)
37.9 38.4
Digits given parenthetically present standard deviation (ml) obtained from triplicate batch experiments. All digital results for gas volume have been converted to STP. Gas generation from 0.5 L inoculum (blank tests) in series B and C was 849 (10) ml. Gas generation from 1 L inoculum (blank tests) in Series A was 1836 (15) ml.
LFG generated from bulky mixed waste would be a slow and longterm process. 4. Conclusions and perspectives In this research, four categories of waste from Danish landfills were sampled and their characteristics (TS, VS, TC and TOC) were analyzed. BMP and DOCC values of low-organic waste were determined and are presented for the first time. In comparison to Tables 1 and 2, Danish landfill waste shows significantly different characteristics. Shredder and bulky mixed waste had significantly lower BMP and DOCC values than traditional MSW, as well as default values in widely used FOD models nowadays. Since they are major components at Danish landfills (Table 4), it can be concluded that using current FOD models with default values, or referencing experimental values from traditional waste fractions to estimate or evaluate LFG generation from Danish landfills would result in significant overestimation. Because FOD models are still widely used to estimate LFG generation, numeric values for relevant parameters need to be revised as realistically as possible. E.g. if applying lab determined BMP values to the Afvalzorg model when estimating LFG generation from shredder and bulky mixed waste at Danish landfills, results would be 60–90% less than original model outputs. Besides of indicating a realistic characterization of low-organic waste, achievements in this research are valuable for revising current FOD models to estimate LFG generation from modern landfills. Models would be more reliable if applying site-specific values for key parameters. Accurate estimation would give landfill managers better ideas for reporting to the PRTR protocol. It could be also referred for making decisions, e.g. choosing proper technologies for landfill operation and aftercare aiming at mitigating GHG emission or CH4 reduction. Nevertheless, other parameters such as decay rates (k values) for different low-organic waste still need to be studied for precise estimation. More relevant research such as long term anaerobic degradation of modern landfills needs to be studied. Acknowledgements This research was supported by the Danish landfill network DepoNet. The authors would like to thank the four Danish landfills (AV Miljø represented by Jonas Nedenskov and Per Palle
Wellendorph, Reno Djurs represented by Henrik Rolsted and Peter Lindequist Madsen, Odense Renovation represented by Jan Thrane and Finn Andersen, and Danish waste management company Kara/ Noveren I/S, which is responsible for Audebo Renovation, represented by Finn Kjær and Lotte Fjelsted) and the DHI Water and Environment (represented by Ole Hjelmar and Jette Bjerre Hansen) for providing waste data and help with waste sampling.
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Further reading Mønster, G.J., Samuelsson, J., Kjeldsen, P., Scheutz, C., in press. Quantification of methane emission from 15 Danish landfills using mobile tracer dispersion method. Waste Manage. Mou, Z.S., Scheutz, C., Kjeldsen, P., in press. Evaluating the methane generation rate constant (k value) of low-organic waste at Danish landfills. Waste Manage.
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Evaluating the biochemical methane potential (BMP) of low-organic waste at Danish landfills Zishen Mou ⇑, Charlotte Scheutz, Peter Kjeldsen Department of Environmental Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark
a r t i c l e
i n f o
Article history: Received 4 March 2014 Accepted 25 June 2014 Available online 5 August 2014 Keywords: Biochemical methane potential (BMP) Low-organic waste Landfill gas (LFG) generation model Degradable organic carbon content (DOCC) Greenhouse gas (GHG) emission
a b s t r a c t The biochemical methane potential (BMP) is an essential parameter when using first order decay (FOD) landfill gas (LFG) generation models to estimate methane (CH4) generation from landfills. Different categories of waste (mixed, shredder and sludge waste) with a low-organic content and temporarily stored combustible waste were sampled from four Danish landfills. The waste was characterized in terms of physical characteristics (TS, VS, TC and TOC) and the BMP was analyzed in batch tests. The experiment was set up in triplicate, including blank and control tests. Waste samples were incubated at 55 °C for more than 60 days, with continuous monitoring of the cumulative CH4 generation. Results showed that samples of mixed waste and shredder waste had similar BMP results, which was in the range of 5.4–9.1 kg CH4/ton waste (wet weight) on average. As a calculated consequence, their degradable organic carbon content (DOCC) was in the range of 0.44–0.70% of total weight (wet waste). Numeric values of both parameters were much lower than values of traditional municipal solid waste (MSW), as well as default numeric values in current FOD models. The sludge waste and temporarily stored combustible waste showed BMP values of 51.8–69.6 and 106.6–117.3 kg CH4/ton waste on average, respectively, and DOCC values of 3.84–5.12% and 7.96–8.74% of total weight. The same category of waste from different Danish landfills did not show significant variation. This research studied the BMP of Danish low-organic waste for the first time, which is important and valuable for using current FOD LFG generation models to estimate realistic CH4 emissions from modern landfills receiving low-organic waste. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Methane (CH4) is one of the most important greenhouse gases (GHG), with a 100-year global warming potential 28 times that of carbon dioxide (CO2) (the digital value of 28 does not include CO2 from CH4 oxidation or climate-carbon feedbacks) (IPCC, 2013). In 2011, landfills contributed to 4907 and 3052 Gg CH4, accounting for 17.5% and 19.6% of anthropogenic CH4 emissions in the US and Europe, respectively (EEA, 2013; US EPA, 2013). European landfills with a total disposal capacity greater than 25,000 tons must report their annual CH4 emission to the Protocol on Pollutants Release and Transfer Registers (PRTR) (CEC, 2006). Due to difficulties in precisely monitoring whole site’s CH4 emissions (Scheutz et al., 2011b), first order decay (FOD) landfill gas (LFG) generation models are currently widely used to estimate
⇑ Corresponding author. Address: Department of Environmental Engineering, Technical University of Denmark, Bygningstorvet – Building 115, Miljøvej 113, 2800 Kgs. Lyngby, Denmark. Tel.: +45 4525 1498; fax: +45 4593 2850. E-mail address: [email protected] (Z. Mou). http://dx.doi.org/10.1016/j.wasman.2014.06.025 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
CH4 emissions from landfills, and the biochemical CH4 potential (BMP) is an essential parameter in this procedure (Blaha et al., 1991; Bogner and Matthews, 2003; Cho et al., 2012; Thompson et al., 2008). For example in the US EPA LandGEM version of the FOD model, the parameter L0 (m3 or kg CH4/ton waste, wet weight) is used to describe the CH4 generation potential (US EPA, 2005). In the IPCC model, the degradable organic carbon content (DOCC) (%, kg C/kg waste, wet weight) is used instead of BMP (IPCC, 2006). DOCC is one of the main parameters affecting the CH4 emissions from landfills, and is estimated based on the waste composition, and varies for different waste fractions (IPCC, 2006). Eq. (1) was defined by the IPCC guidelines for transformation between BMP and DOCC (IPCC, 2006).
BMP ¼ DOCC F 16=12 103
ð1Þ
where F is the volume fraction of CH4 in generated LFG, 16 and 12 are the molar masses of CH4 and carbon, 103 is the factor between kg and ton. Different models have various numeric values for both parameters, as listed in Table 1. Two of the models listed in Table 1, the E-PRTR (Fr) and LandGEM models, are single-phase models. Thus, they have no specific sub-categories for each kind of waste
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Table 1 Default BMP values (kg CH4/ton waste, wet weight) and DOCC (%, kg C/kg waste, wet weight) for the various waste categories that were used in different models. FOD models
BMP (DOCC)a values Industrial waste
MSW
E-PRTR (Fr)b LandGEMc
28 (4) Not specified
55 (8) 122 or 72 (18 or 11)
IPCC (waste fractions)d
Sludge Industrial
33 (5) 67 (10)
Food Garden Paper Textiles
100 133 267 160
Afvalzorg (waste fractions)e
Soil C&D Shredder Sludge Cleansing
3 (2) 11 (3) 13 (4) 25 (6) 19 (4)
Household Bulky mixed Garden Commercial RDF
91 80 47 56 63
(15) (20) (40) (24)
(18) (19) (9) (13) (18)
Country based
Latest version
French U.S.
2003 2005
International
2011
Holland
2014
a
Digits given parenthetically present DOCC values of corresponding waste categories. The E-PRTR (Fr) model describes CH4 generation rates of 4.8, 2.4, 1.3 and 0.6 kg CH4 per ton household waste per year at 0, 5, 10 and 20 years after landfilling, respectively. For moderately decomposable waste such as nonhazardous industrial waste, the values are 50% of household waste for the corresponding landfilling period (ADEME, 2003; Oonk, 2010). c The default value of 122 was based on requirements for U.S. landfills as specified in the Clean Air Act. The default value of 72 was based on results of an inventory by the US EPA (Oonk, 2010; US EPA, 1990). d Ranges of default values for MSW were calculated based on default DOCC values of various MSW fractions, as shown in the following rows (IPCC, 2011, 2006). e Ranges of default values were calculated based on default organic carbon content and fractions of wet waste (Afvalzorg, 2014). b
(ADEME, 2003; Oonk et al., 1994; US EPA, 1990). The other two models, the IPCC and Afvalzorg models, are multi-phase models, which operate with a number of more detailed waste categories (Mata-Alvarez et al., 2011; Scharff and Jacobs, 2006; Thompson et al., 2009). As listed in Table 1, the IPCC model divides waste into industrial waste (incl. sludge) and municipal solid waste (MSW), and MSW includes food, garden, paper, and textiles. The Afvalzorg model, which was developed by a Dutch waste management company (Jacobs and Scharff, 2001), holds datasets for different waste categories as listed in Table 1. Default values were determined based on the IPCC model and field measurements in three Dutch landfills disposing low-organic waste (Afvalzorg, 2014). Instead of organic fractions of traditional MSW such as paper and food waste, the Afvalzorg model provides relative low-organic waste fractions such as bulky mixed waste (incombustible), construction and demolition (C&D) waste, soil (contaminated with oil and other residues), shredder (shredded pieces of abandoned vehicles or machines), cleansing (residues from street cleansing) waste and refuse-derived fuel (RDF). Above all, the shredder and bulky mixed waste are the major components of deposited waste categories at current Danish landfills. As EU Council legislated, landfilling is limited to inert materials that are not biodegradable or combustible (containing a significant fraction of organic matter) since 1997 (EU, 1999). Consequently more and more EU member states (e.g. the Netherlands as of 1996, Denmark as of 1997, and Germany as of 2005) have banned the landfilling of organic waste (Manfredi et al., 2010). Researchers have paid more attention to the biological stability and degradability of organic waste (Cossu and Raga, 2007; De la Cruz et al., 2013). Danish landfills currently only receive low-organic and non-combustible waste permanently, such as bulky mixed waste (Scheutz et al., 2011a), shredder waste, low-organic dewatered sludge, etc. Some landfills receive combustible waste for temporary storage (up till several months), which is later sent to incineration plants for electricity and heat generation (Manfredi et al., 2010). Even if the majority of MSW is incinerated, previous studies have shown significant gas emission from Danish landfills (Scheutz et al., 2011a, 2007; Scheutz and Kjeldsen, 2007). In this situation, LFG generation modeling is still necessary when reporting CH4 generation from landfills. Due to the implementation of the European Waste Directive (EU, 2008), the composition of disposed waste at
landfills in other European countries is expected to change, as organic waste is expected to be recycled or incinerated. As an essential parameter, BMP values for different types of organic waste are widely studied and reported in the literature, and applied in the fields of renewable energy (Alzate et al., 2012), anaerobic digestion (Elbeshbishy et al., 2012) and especially LFG generation (Kim and Townsend, 2012). BMP assays for organic waste have also been developed and improved over the past decade (Angelidaki et al., 2009; Hansen et al., 2004; Li et al., 2013; Owens and Chynoweth, 1993; Raposo et al., 2011). Table 2 lists BMP determinations in anaerobic incubation batch tests, in which solid waste was usually co-digested with an inoculum (such as sewage sludge or cattle manure) at intermediate (35 °C) or high temperature (55 °C). The volume in each experimental reactor varied between 100 and 2000 ml and the organic load (concentration of organic material in inoculum) varied from 0.25 to 4 g/L based on the mass of volatile solids (VS), total solids (TS), organic carbon (OC), or chemical oxygen demand (COD). The unit of BMP is expressed as Nm3 CH4/ton VS or kg CH4/ton waste (wet weight). The relative standard deviation was determined via duplicate or triplicate parallel tests. The CH4 recovery (%) was calculated as the ratio between the actual and theoretical CH4 generation from the substrate. From Tables 1 and 2, it can be seen that BMP values vary significantly among different waste categories, as well as in models. To estimate CH4 generation from Danish landfills, which receive low-organic waste, realistic BMP values are highly important. However, very little is known about BMP values for non-combustible waste or low-organic waste. Furthermore, most FOD models were developed based on traditional waste fractions such as food waste and MSW, which have much higher DOCC, as well as BMP according to Eq. (1). Even with the Afvalzorg model, previous research has shown its default BMP values could cause overestimations when applied to Danish landfills (Scheutz et al., 2011a, 2007; Scheutz and Kjeldsen, 2007). Therefore, current models are highly uncertain for Danish landfills, especially when default BMP values were based on tests from traditional waste fractions. In this research, three categories of low-organic waste (bulky mixed, shredder and sludge waste from waste water treatment plant), which are all major components of permanently deposited waste, and temporarily stored combustible waste were sampled
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Z. Mou et al. / Waste Management 34 (2014) 2251–2259 Table 2 BMP determined by anaerobic co-digestion experiments using different types of organic waste as substrate. Waste fractions
Organic fractions of MSWb Food wastec Paper and cardboardd Sludge wastee Fresh samples from landfillsf
Substrate
BMP 3
TOC (% wet waste)
VS (% wet waste)
(Nm CH4/ ton VS)
(kg CH4/ ton waste)a
8.1–18.4 7.3–14.6 83.2–95.5 0.8–2.6 12.8–68.2
14.5–33.6 22.9–28.0 84.2–97.4 1.4–4.1 25.1–54.8
336–495 467–675 192–277 143–379 127–284
14.8–44.5 61–106 114–189 2.1–3.7 54–91
Relative Standard deviation (± %)
CH4 recovery (%)
4–5 1–3 12–17 2–6 5–26
28–37 57–89 20–30 15–22 43–60
a
BMP results were based on wet weight when presented as kg CH4/ton waste. Waste fractions came from Nordic and British MSW streams that did not contain food/paper waste (Davidsson et al., 2007; Hansen et al., 2004; Jansen et al., 2004; Jørgensen, 2009; Zhang et al., 2012). c Waste fractions came from the food industry, university canteens and restaurants in Ireland and southern Europe (Browne and Murphy, 2013; Cabbai et al., 2013; Cavaleiro et al., 2013; Rincon et al., 2013). d Waste fractions included office paper, newspaper, magazines and cardboard from Chinese and French waste collection stations. Samples were pretreated before running the BMP assay (Pommier et al., 2010; Yuan et al., 2012). e Sludge waste came from waste water treatment plants. Pretreatment including mechanical dewatering was performed before running the BMP assay (Astals et al., 2013; Seng et al., 2010; Tomei et al., 2009; Yan et al., 2013). f Waste fractions were fresh samples from South Korean, Finnish and American landfills. Results for paper, food and sludge waste were excluded (Jeon et al., 2007; Jokela et al., 2005; Kim and Townsend, 2012). b
from four Danish landfills. The objectives were to determine the physical characteristics and BMP values of low-organic waste via batch experiments. DOCC values of low-organic waste that are representative of the current and future disposed waste at modern landfills are presented for the first time. The outcomes are also valuable for improving current FOD models to more precisely estimate LFG generation from low-organic waste disposal sites in Denmark, as well as other countries, which have or will have similar landfilling strategies. 2. Materials and methods 2.1. Waste samples from Danish landfills 2.1.1. Description of landfills and waste categories Waste samples were collected from four Danish landfills. All four landfills were still in service and currently receive only low-organic waste for permanent disposal. Two of the four sites are currently running gas collection system in the area of old landfill cells which disposed mixed household waste in 1990s. Table 3 provides an overview of the four landfills in terms of location, start year, total landfill area, storage capacity, and amounts of disposed waste and temporarily stored combustible waste in 2010. Two sites, AV Miljø and Audebo, have special cells for combustible waste such as paper, plastic, and wood. The combustible waste is stored temporarily before being sent to incineration plants in the winter for energy recovery. The deposited waste categories are similar at all four sites, including shredder waste, dewatered sludge, bulky mixed waste, soil and C&D waste. Only one landfill, Audebo, does not receive any shredder waste. Table 4 shows waste amounts for the major waste categories disposed in 2010. 2.1.2. Sampling and preparation of waste Samples of shredder, sludge, bulky mixed and combustible waste were collected from four landfills. The whole process was approached by using a method developed by Jansen et al. (2004). For each sample, approximate 300 kg of wet waste was collected as raw material, either from same-day-deposited waste piles or waste transporting trucks when they arrived at the site. When sampling from waste piles, shovels that were long enough to reach the bottom were used to avoid missing bottom layers (Gy, 1996). When sampling sludge and combustible waste, the top or cover
layer was removed to avoid unrepresentativeness (Laine-Ylijoki et al., 2009). The combustible waste was covered daily with wood pieces and soil at AV Miljø landfill, or packed in large plastic bags at Audebo landfill by the landfill operators. Therefore, samples of combustible waste were quite dry when deposited at the landfills. In total, 13 waste samples were obtained over a period of approximately 24 days in October 2011. The waste was collected in plastic bags (100 L, 0.07 mm in thickness) and placed in sealed steel drums (115 L) for transportation. All waste samples were kept at 4 °C in the dark before treatment. Within 7 days after sampling, mass and size reduction of the samples was approached. Each waste sample was spread out on a large plastic sheet placed on the floor and was mixed manually. Then using a grid method (Laine-Ylijoki et al., 2009), about 20% of the sample was retained and then shredded in a cutter mill machine (SM 2000, Retsch). About 5–6 kg of samples for analysis of TS and VS were taken. Approximately 20% of the machine´s outcome was collected using the same method and then dried at 80 °C until a constant weight was achieved. The dry sample was afterwards milled in a small hammer mill (Macsa 300, Eriez) with a 2 mm screen. The material was thoroughly mixed for BMP assay and the finely grained sample was smashed into powder manually with a mortar and pestle for analyzing total carbon (TC) and total organic carbon (TOC). 2.2. Physical and chemical tests of waste samples Waste samples were characterized in terms of moisture content, TS, VS, ash, TC and TOC following standard procedure. The pH of sludge waste leachate was tested and according to the landfill manager’s information, there was no acid substrate among other waste fractions. The moisture content was measured by oven-drying 2–4 kg of sample at 105 °C for at least 24 h until a constant weight was achieved. Ash from each sample was obtained by oven drying the dry samples at 550 °C for at least 2 h until no further weight loss was obtained. All experiments were run in triplicate. Dry powder from each sample was used to test TC and TOC using a LECO Induction Furnace CS-200 oven. When testing TOC, 2 ml of sulfurous acid (5% H2SO3 solution) was added to approximately 0.5 g of powder to remove inorganic carbon. Each experiment was run in triplicate and the systemic error was ±2%. All results of TS, VS, TC and TOC are presented as % (mass fraction of wet waste, similarly hereinafter) in this research.
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Table 3 Overview of four Danish landfills and the total amount of disposed and combustible waste in 2010. Landfill
Location
Start year
Total area (ha)
Capacity (Mm3)
Disposed waste amount-2010 (t)a
Combustible waste amount-2010 (t)b
AV Miljø Reno Djurs Odense Audebo
Southwest of Copenhagen Northeast in Jutland Northeast in Fyn Northwest in Zealand
1989 1981 1994 1988
34 72 110 24
2 4 10 7
80,334 64,692 145,142 19,644
32,179 – – 49,782
a
This column shows the amount of permanently disposed waste in 2010 at each landfill. Combustible waste for temporary storage is not included. This column shows the amounts of temporarily stored combustible waste in 2010 at AV Miljø and Audebo. The Reno Djurs and Odense landfills do not receive combustible waste. b
Table 4 Amounts of waste (t) in different categories disposed of at four Danish landfills in 2010.
a
Landfill
Shredder
Sludge
Bulky mixed
C&D
Soil and sand
Other inert
AV Miljø Reno Djurs Odense Audeboa
43,279 24,506 73,027 –
10,054 2680 1886 640
6414 8359 6854 2341
12,853 6469 15,966 982
4017 1941 29,028 59
3717 20,737 18,381 15,622
Audebo does not receive any shredder waste.
2.3. Batch tests for determining BMP 2.3.1. Set up and monitoring The experiment was carried out in terms of triplicate batch tests following the method developed by Hansen et al. (2004). Table 5 provides an overview of the different series set-up in terms of waste sample, substrate mass, inoculum volume, organic load (weight of TOC in substrate per unit volume of inoculum, g OC/L) and the substrate to inoculum ratio (S/I, g VS of waste material added as substrate per g VS added as inoculum). In Series A, three waste samples were tested. Higher BMP values were observed for combustible, shredder and sludge waste when setting the organic load at 2 g OC/L in comparison to 4 g OC/L. In these batch tests, the reactors were 2 L reactors containing 1 L of inoculum. CH4 generation curves, calculation of BMP, CH4 recovery and DOCC are presented in the result section, as well as for series B and C. For comparison, CH4 generation curves for these three waste samples when organic load was set at 4 g OC/L are also presented. Insignificant differences and acceptable variance were observed when using a 1 L glass bottle with 0.5 L inoculum. Therefore in series B, for combustible, shredder and sludge waste from other landfills, organic load was set at 2 g OC/L and the inoculum volume was 0.5 L. Reducing the reactor size also conserved the inoculum so that all samples could be incubated with the same inoculum. When using bulky mixed waste samples as substrate, inhibition was observed for set-up of 1 g OC/L during the initial experimental research and BMP was under the detection limit (Scheutz et al., 2007). Therefore, in series C, the organic load was set at 0.5 g OC/ L and the inoculum volume was kept as 0.5 L. The reactors were placed on a scale when setting them up and were flushed for over 5 min with N2 before sealed. All reactors were placed in the incubator at 55 ± 1 °C for over nine weeks using gas chromatograph (Shimadzu GC 14A); and the CH4 concentration in the headspace was measured once or twice per week. Thermophilically (55 °C) digested material from a full-scale plant, located at Vegger (Nibe, Denmark), was used as inoculum in the experiments. The Vegger biogas plant co-digests mainly cattle manure (80%) together with different wastes from the food industry (20%). The TS and VS values of the inoculum were 3.01 ± 0.55% and 2.06 ± 0.71%, respectively.
2.3.2. Blank and control experiments Blank and control experiments with the same set up (starting and running date, inoculum and reactor volume) were also
performed in triplicate in each series. Blank experiments, which contained only water and inoculum, were used to measure CH4 generation originating from the inoculum alone, and to indicate the detection limit of BMP in this method. Control experiments, which also contained standard substrate, were used to test the quality of the inoculum (i.e. to address the variation among triplicates) and to indicate if the incubation method for determining BMP was functioning as expected. In this method, Avicel (Fluka, Sigma–Aldrich, Vallensbæk Strand, Denmark), which is a microcrystalline cellulose powder, was used as standard substrate in the control experiments. The TOC value of Avicel was 44.44% according to its molecular formula (C6H10O5)n. In all series, the concentration of substrate in control experiments was 1 g/L as showed in Table 5. The monitoring of gas samples from blank and control experiments were the same as stated above for all batch tests. The cumulative CH4 generated (the average value of triplicates) from the substrate alone (waste sample) was obtained by subtraction of the CH4 generated in the blank and transformed into a function of incubation time. With each measurement, the actual temperature and atmospheric pressure were recorded. Thus, all measures of gas volume could be converted to standard temperature and pressure (STP, 0 °C and 1 atm) conditions according to the Ideal Gas Law Equation. BMP results are presented as kg CH4/ton waste (wet weight, similarly hereinafter) according to default parameters in FOD models. By using TS (TOC) values, BMP results can be easily converted VS or dry matter (TOC or COD) based. The uncertainty in the blanks, i.e. the triplicate tests with inoculum but no substrate, determined the detection limit of the BMP measurement. The average CH4 generated in the blank tests was subtracted from the average of all batch tests when calculating the BMP value of the corresponding substrate. In this research the detection limit was given as the limit above, from which one could quantify the BMP result (the difference between a sample and a blank test) with a probability of 95%. The detection limit (ml) was calculated by Eq. (2) (Hansen et al., 2004; Lavagnini and Magno, 2007; Johnson et al., 2011; US EPA, 1993).
Detection limit ¼ 1:943 s 2
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1=n1 þ 1=n2 Þ
ð2Þ
where 1.943 was the 95% confidence level in the t-distribution with 6 degrees of freedom (Johnson et al., 2011); s was the standard deviation of the blank tests (ml); n1 and n2 were the numbers of replicates used to measure the samples and blanks (in this research they were both 3).
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Previous research on the BMP method indicated that an average value of triplicate control tests should be in the range of 315– 439 ml CH4/g VS when using Avicel as a standard substrate (Hansen et al., 2004). If the average value was outside this range, it might suggest the inoculum, although evenly distributed in the reactors, was inhibited or for other reasons producing too little CH4. 2.3.3. Calculation of CH4 recovery and DOCC The CH4 recovery (%) was defined as the ratio between the actual and theoretical CH4 generation, and was calculated using Eq. (3).
CH4 recovery ¼ ðBMP 103 Þ=ðTOC F 16=12Þ 100%
ð3Þ
where BMP is determined via batch tests and presented as kg CH4/ton waste, TOC is presented as % (kg OC/kg waste), F is the volume fraction of CH4 in generated gas, 16 and 12 are the molar masses of CH4 and carbon. The default value of F in the IPCC model is 50% with 5% of uncertainty range (IPCC, 2006). Most waste in landfills generates a gas with approximately 50% CH4 (Cavaleiro et al., 2013; Elfadel et al., 1996; Jeon et al., 2007). Only material including substantial amounts of fat or oil can generate gas with substantially more than 50% CH4. Therefore in the research, F with a value of 50% was used for calculation in Eq. (3). As stated before, DOCC is used in some FOD instead of BMP when estimating LFG generation. To adapt models such as the IPCC version to Danish landfills, the DOCC (%) of low-organic waste can be calculated by Eq. (4).
DOCC ¼ CH4 recovery TOC
ð4Þ
where DOCC is presented as % (kg C/kg waste, wet weight). Mathematically, it can be seen from Eqs. (3) and (4) that the ratio between the BMP and DOCC values for any waste material should be a constant when F (the volume fraction of CH4 in LFG) was determined. 3. Results and discussion 3.1. TS, VS, TC and TOC of waste samples Table 6 shows the average results for TS, VS, TC and TOC of all the waste samples. The pH value of sludge waste leachate was in the range of 7.3–7.6. All values were obtained from triplicate experiments with a relative error less than ±5%. In general, little variation was observed within the same category of waste sampled at different landfills. Sludge had the lowest TS content, which was
about 20%, while other samples such as shredder waste had a relatively high TS content that was above 80%. Combustible waste had the highest VS content, which was about 70%, as well as the highest ratio of VS/TS (over 80%). Bulky mixed waste samples also contained a relatively high fraction of TS (over 80%), but a significantly lower fraction of VS, which was only 7–8%. Shredder waste samples contained equally high fractions of TS as combustible and bulky mixed waste, which were over 85% and over 4 times of sludge waste. However, shredder and sludge waste samples contained similar fractions of TC and TOC, which were in the range of 9–13%. Combustible and bulky mixed waste samples contained the highest and lowest carbon fractions, which were 29–32% and 4–5% respectively. Traditional landfill waste such as food or kitchen waste, sludge waste and garden waste (Bolan et al., 2013; De la Cruz et al., 2013; Jones et al., 2013; Yang et al., 2013) contains a significant fraction of organic matter. From Table 2, it can be seen that MSW and landfill samples had a TOC content varying between 8.1–68.2%, which is significantly higher than bulky mixed waste (3–4%, Table 6) at Danish landfills. In this case an overestimation is unavoidable if traditional FOD models are applied to predict LFG generation from Danish landfills. If the TOC and VS contents of sludge waste are compared, Table 6 shows much higher value. This is because the deposited sludge at Danish landfills was dehydrated and had a much lower moisture content (80–82%, calculated from TS results). Traditional sludge waste deposited at landfills has a moisture level of 90–95% (Astals et al., 2013; Kim and Townsend, 2012; Seng et al., 2010). The shredder and sludge waste showed similar TC and TOC values, but different VS values because of their lower moisture contents and various characteristics of the dry fractions. The shredder waste was consisting mainly of metals, plastic, rubber, wood and foam. The main components of combustible waste at Danish landfills include paper, wood, plastic, and cardboard. The TOC value is lower than traditional paper waste (Table 2) because most paper waste is recycled and reused in Denmark (Manfredi et al., 2010). Theoretically, the DOCC value of waste fractions must be lower than the TOC value because some OC such as fossil carbon is nondegradable and results in carbon storage at landfills (De la Cruz et al., 2013; Law et al., 2013). As shown in Table 1 (digits given parenthetically), the default DOCC values in the Afvalzorg and IPCC model, for instance, are 15–40% and 10–20% for MSW waste, respectively. Single phase models use default DOCC values of 8% and 11–18% in the E-PRTR (Fr) and LandGEM models. However, the TC and TOC values of bulky mixed waste at Danish landfills (in Table 6) are only 3–5% as tested, which was even lower. There-
Table 5 Substrate and inoculum set-up of batch tests for determining the BMP of waste. Series
Waste category
Landfill
Substrate mass (g)a
Inoculum volume (L)
Organic load (g OC/L inoculum)
Substrate to inoculum ratio (S/I, g VS/g VS)
A
Combustible Shredder Sludge
Audebo Reno Djurs AV Miljø
7.6 (15.2) 19.2 (38.4) 20.6 (41.2)
1.0
2 (4)a
4–6 (7–12)
B
Combustible Shredder
AV Miljø AV Miljø Odense Reno Djurs Odense Audebo
3.6 9.6 9.4 11.7 11.0 8.9
0.5
2
4–6
Sludge
a b
C
Bulky mixed
AV Miljø Reno Djurs Odense Audebo
7.5 8.4 6.7 7.1
0.5
0.5
15–20
A B/C
Avicel
Controlb
1 0.5
1 0.5
0.44
21
Digits given parenthetically present the substrate mass when organic load was 4 g OC/L and the inoculum volume is 1.0 L in Series A. Avicel was used as substrate in control experiments. In all series, the concentration of material in inoculum was 1 g/L.
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Table 6 Waste characterization in terms of TS, VS, TC and TOC (%, kg/kg waste, wet weight).
Combustible
AV Miljø Audebo
84 86
Shredder
AV Miljø Reno Djurs Odense
Sludge
Bulky mixed
Avicel
TC
TOC
70 69
32 29
28 26
87 86 90
30 31 29
12 12 13
10 10 11
AV Miljø Reno Djurs Odense Audebo
20 18 19 21
17 15 17 19
11 9 10 12
10 9 9 11
AV Miljø Reno Djurs Odense Audebo
82 81 80 84
7 7 8 8
4 4 5 4
3 3 4 4
100
100
44
44
Control
a
TS
VS
a
Avicel was used as substrate in control experiments. Digits present theoretical values based on chemical formula (C6H10O5)n.
fore these default DOCC values in FOD models were obviously not suitable for Danish LFG generation estimation.
Cumulave CH4 generatoin (ml)
Landfill
3000
Control
Combusble - Audebo 1
Combusble - Audebo 2
(a)
2400 1800 1200 600 0
0
10
Blank
Ccumulave CH4 generatoin (ml)
Waste
Blank
20
Control
30
40
Shredder - Reno Djurs 1
50
60
70
Shredder - Reno Djurs 2
(b)
2500 2000 1500 1000 500 0
0
10
20
30
40
50
60
70
3.2. BMP batch tests Cumulave CH4 generatoin (ml)
Blank
3.2.1. Cumulative CH4 generation curves Fig. 1–3 show cumulative CH4 generation curves changing with time in all series. All results are the average of triplicates. The standard deviations are shown in Table 7. The results of blank and control experiments with a 1 L inoculum are shown in Fig. 1, and those with a 0.5 L inoculum are shown in both Fig. 2 and 3. In general, curves within the same waste category showed similar trends in CH4 accumulation over time. In all series, CH4 generation from the blank experiments was lower than those from control and other incubation experiments with waste. Fig. 1 shows the results from Series A, which incubating different waste fractions with two organic load settings. From Fig. 1(a) and (b) it can be seen that for combustible and shredder waste, normalized CH4 generation was unaffected by the concentration of waste material. With twice the mass of substrate, higher CH4 generation is reasonable for combustible – Audebo 2 and shredder – Reno Djurs 2. If normalized by mass (g) of substrate, CH4 generation (after subtracting CH4 generated from the blank test) based on wet weight was 74.6 and 73.6 ml CH4/g combustible waste, and 5.1 and 5.0 ml CH4/g shredder waste, respectively. Therefore similar BMP values as well as CH4 recovery and DOCC were obtained when setting the organic load at 2 and 4 g OC/L. Both substrate and inoculum had good internal homogeneity. As showed in Fig. 1(c), sludge – AV Miljø 1 and 2 resulted in 46.3 and 33.40 ml CH4/g waste, respectively. The difference might be due to the inhibition during the anaerobic degradation process. A larger amount of substrate might lead to accumulation of ammonia and thus lower CH4 generation (Cavaleiro et al., 2013; MataAlvarez et al., 2011). Ammonia is produced by the biological degradation of the nitrogenous matter present in the waste material (Chen et al., 2008). Intermediate compounds during hydrolysis of proteins, phospholipids, nitrogenous lipids and nucleic acids are composed of amino acids, which could inhibit the growth of methanogens (Kayhanian, 1999). Since lower organic load for combustible, sludge and shredder waste showed higher BMP values, it was set as 2 g OC/L in Series B. For bulky mixed waste samples, former research also observed inhibition during 1 g OC/L incubation experiments, (Scheutz et al., 2007). Therefore in Series C, BMP was determined based on S/I = 0.5 g OC/L batch tests. Fig. 2(a–c) show the results from Series B, incubating the same waste categories as in Fig. 1(a–c), correspondingly. Fig. 3 shows
3500
Control
Sludge - AV Miljø 1
Sludge - AV Miljø 2
(c)
2800 2100 1400 700 0
0
10
20
30
40
50
60
70
Incubaon me (days) Fig. 1. Series A of BMP batch tests: organic load = 2 and 4 g OC/L for (a) combustible waste – Audebo; (b) shredder waste – Reno Djurs; (c) sludge waste – AV Milø.
cumulative CH4 generation from bulky mixed waste samples. Curves of combustible and sludge waste in Fig. 2(a) and (c) showed a period of rapid increase in the first two weeks. Combustible waste from AV Miljø, as shown in Fig. 2(a), resulted in 1440 ml of total CH4 generated over 60 days of incubation. However, on the 12th and 15th day after starting the batch tests, average cumulative CH4 generation reached 863 and 951 ml, respectively. Therefore, 2/3 of the potential CH4 was generated in the first 14 days from samples of combustible waste, and only 1/3 was produced in the remaining 45 days. Similar results were also observed for sludge waste samples, as shown in Fig. 2(c). The cumulative CH4 generation reached 1180–1330 ml on the 15th day after starting incubation. Incubation of sludge waste resulted in 1647–1825 ml CH4 by the end of 60 days. More than 70% of the potential CH4 was generated in first 14 days. This might be caused by co-digestion of cattle manure and sludge, which could accelerate the hydrolysis process of the organic waste due to the synergetic effects and improved nutrients for methanogens growth (Borowksi and Weatherley, 2013; Zhang et al., 2014). On the other hand, shredder and bulky mixed waste samples have different CH4 generation trends, which were relatively stable at the beginning. If the curves in Figs. 2(b) and 3 are considered as linear trends in the first 55 days, rates of daily increasing of 17–18 ml CH4 can be fitted. The average cumulative CH4 generated by shredder and bulky mixed waste resulted in 932–969 and 902–922 ml CH4, respectively.
Z. Mou et al. / Waste Management 34 (2014) 2251–2259
Cumulave CH4 generatoin (ml)
Blank
Ccumulave CH4 generatoin (ml)
Combusble - AV Miljø
1600
(a) 1200
800
400
0
0
10
Blank
20
30
Control
40
50
Shredder - AV Miljø
60
70
Shredder - Odense
1200
(b) 800
400
0
0
Blank
Cumulave CH4 generatoin (ml)
Control
10
Control
20
30
40
Sludge - Audebo
50
60
Sludge - Odense
70
Sludge - Reno Djurs
triplicates exceeded the average of a set of blank triplicates by more than 47 ml CH4 in Series A, or 32 ml CH4 in series B and C, then significant CH4 generation from the samples was obtained and it could be quantified with a confidence of at least 95%. In this research, all batch tests showed higher CH4 generation than the limits of detection. In Series A, the mean and standard deviation of triplicate control tests were 2195 and 64.5 ml CH4, respectively. In Series B and C, the mean and standard deviation of triplicate control tests were 1026 and 21.1 ml CH4, respectively. By subtracting the CH4 generated from the blank experiments, the BMP of Avicel was 256.7 and 253.5 kg CH4/ton Avicel, or 359 and 355 ml CH4/g VS, in Series A and B/C, respectively. The standard deviation was 13 ml CH4/g VS for all series. The results were entirely in the expected range (315–439 ml CH4/g VS). Similar results (379 ml CH4/g VS generated from Avicel substrate) have been reported by Hansen et al. (2004). As calculated by Eqs. (3) and (4), the CH4 recovery was 85–87% and the DOCC was approximately 38%, as showed in Table 7. It was in good agreement with literatures (Raposo et al., 2011). From Figs. 1–3 it can be seen that the CH4 generation curves of the blank and control experiments had same trend as other waste samples. No significant or unexpected changes were observed during the whole CH4 generation process. Therefore, the performance of the inoculum in all series of batch tests was acceptable.
2000
(c) 1600 1200 800 400 0
0
10
20
30
40
50
60
70
Incubaon me (days) Fig. 2. Series B of BMP batch tests: organic load = 2 g OC/L for (a) combustible waste – AV Milø; (b) shredder waste – AV Milø and Odense; (c) sludge waste – Reno Djurs, Odense and Audebo.
Blank
Cumulave CH4 generatoin (ml)
2257
Control
AV Miljø
Reno Djurs
Odense
Audebo
1200 1000 800 600 400 200 0
0
10
20
30
40
50
60
70
80
Incubaon me (days) Fig. 3. Series C of BMP batch tests: organic load = 0.5 g OC/L for bulky mixed waste – four landfills.
3.2.2. Blank and control In Series A, blank and control experiments were performed with a 1 L inoculum. The mean and standard deviation of triplicate blanks were 1836 and 14.9 ml CH4, respectively. In Series B and C, blank and control experiments were performed with a 0.5 L inoculum. The mean and standard deviation of triplicate blanks were 849 and 10.2 ml CH4, respectively. Using two standard deviation values and Eq. (2), the detection limits in Series A and B/C were 47 and 32 ml, respectively. If the average of a set of sample
3.2.3. Calculation of BMP, CH4 recovery and DOCC Table 7 provides an overview of experimental and calculated results in terms of cumulative and theoretical CH4 generation, BMP, standard deviation of triplicates, average CH4 recovery and DOCC for different waste samples. In general, little variation can be seen within the same category of waste sampled from different landfills. The same conclusion can also be applied to other physical and chemical test results based on Table 6. The highest BMP and DOCC values were observed in combustible waste, 106.6–117.3 kg CH4/ton waste and 7.96–8.74%, respectively. The CH4 recovery was 30.6–31.2%, indicating that less than one third of the organic carbon in the waste samples was released as gases. Because paper and wood are the main combustible waste, the degradation process would be considerable slow. In comparison to the default DOCC values in the IPCC model (Table 1) and other tested results of BMP (Table 2), it can be concluded that traditional model and waste characters are unsuitable to apply to the Danish scenario. Sludge waste showed lower BMP values (51.8–69.6 kg CH4/ton waste) in comparison to combustible waste, but the highest CH4 recovery (42.7–52.2%) among all waste fractions. This is reasonable because active microorganisms and easily degradable organic matter are likely to remain in the sludge waste (Astals et al., 2013; Tomei et al., 2009; Yan et al., 2013). Unlike the combustible waste, the BMP values of Danish landfill sludge waste are actually higher than the default values (Table 1) in the IPCC and Afvalzorg model, as well as tested results from references (Table 2). This is due to the dehydration process, which would enhance the VS and TOC content as well as the BMP values. The shredder and bulky mixed waste samples showed similar BMP and DOCC results, in the range of 5.1–9.1 kg CH4/ton waste and 0.44–0.70%, respectively. These waste categories are special and quite different from traditional landfill MSW. They have much lower BMP and DOCC values than all model default values in Table 1, as well as the organic fractions of MSW and fresh samples from landfills in Table 2. In addition, CH4 recovery from bulky mixed waste samples was 2–3 times higher than from shredder waste. From Fig. 3 it can be seen that the incubation time for bulky mixed waste was also longer than the others, which indicated that
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Z. Mou et al. / Waste Management 34 (2014) 2251–2259
Table 7 Overview of batch test results in terms of cumulative and theoretical CH4 generation, calculated BMP, CH4 recovery and DOCC values.
a b c
Waste category
Landfill
Cumulative CH4 generation (ml) a
Theoretical CH4 generation (ml)
BMP (kg CH4/ton waste)
CH4 recovery (%)
Combustible
AV Miljøb Audeboc
1440 (24) 2403 (62)
1897 1853
117.3 106.6
31.2 30.6
8.74 7.96
Shredder
AV Miljøb Reno Djursc Odenseb
932 (18) 1934 (51) 969 (21)
1867 1873 1868
6.2 7.3 9.1
4.4 5.2 6.4
0.44 0.52 0.70
Sludge
AV Miljøc Reno Djursb Odenseb Audebob
2790 1825 1647 1716
1941 1868 1868 1864
63.7 59.6 51.8 69.6
49.1 52.2 42.7 46.5
4.91 4.70 3.84 5.12
Bulky mixedb
AV Miljø Reno Djurs Odense Audebo
922 918 911 902
467 468 466 466
7.0 5.9 6.6 5.4
15.7 14.8 13.3 11.3
0.47 0.44 0.53 0.45
Avicel (control)
Series B and Cb Series Ac
1026 (21) 2195 (65)
(71) (37) (32) (34)
(10) (12) (11) (10)
207 415
253 256
85.5 86.5
DOCC (%, kg C/kg waste)
37.9 38.4
Digits given parenthetically present standard deviation (ml) obtained from triplicate batch experiments. All digital results for gas volume have been converted to STP. Gas generation from 0.5 L inoculum (blank tests) in series B and C was 849 (10) ml. Gas generation from 1 L inoculum (blank tests) in Series A was 1836 (15) ml.
LFG generated from bulky mixed waste would be a slow and longterm process. 4. Conclusions and perspectives In this research, four categories of waste from Danish landfills were sampled and their characteristics (TS, VS, TC and TOC) were analyzed. BMP and DOCC values of low-organic waste were determined and are presented for the first time. In comparison to Tables 1 and 2, Danish landfill waste shows significantly different characteristics. Shredder and bulky mixed waste had significantly lower BMP and DOCC values than traditional MSW, as well as default values in widely used FOD models nowadays. Since they are major components at Danish landfills (Table 4), it can be concluded that using current FOD models with default values, or referencing experimental values from traditional waste fractions to estimate or evaluate LFG generation from Danish landfills would result in significant overestimation. Because FOD models are still widely used to estimate LFG generation, numeric values for relevant parameters need to be revised as realistically as possible. E.g. if applying lab determined BMP values to the Afvalzorg model when estimating LFG generation from shredder and bulky mixed waste at Danish landfills, results would be 60–90% less than original model outputs. Besides of indicating a realistic characterization of low-organic waste, achievements in this research are valuable for revising current FOD models to estimate LFG generation from modern landfills. Models would be more reliable if applying site-specific values for key parameters. Accurate estimation would give landfill managers better ideas for reporting to the PRTR protocol. It could be also referred for making decisions, e.g. choosing proper technologies for landfill operation and aftercare aiming at mitigating GHG emission or CH4 reduction. Nevertheless, other parameters such as decay rates (k values) for different low-organic waste still need to be studied for precise estimation. More relevant research such as long term anaerobic degradation of modern landfills needs to be studied. Acknowledgements This research was supported by the Danish landfill network DepoNet. The authors would like to thank the four Danish landfills (AV Miljø represented by Jonas Nedenskov and Per Palle
Wellendorph, Reno Djurs represented by Henrik Rolsted and Peter Lindequist Madsen, Odense Renovation represented by Jan Thrane and Finn Andersen, and Danish waste management company Kara/ Noveren I/S, which is responsible for Audebo Renovation, represented by Finn Kjær and Lotte Fjelsted) and the DHI Water and Environment (represented by Ole Hjelmar and Jette Bjerre Hansen) for providing waste data and help with waste sampling.
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Z. Mou et al. / Waste Management 34 (2014) 2251–2259 Chen, Y., Cheng, J.J., Creamer, K.S., 2008. Inhibition of anaerobic digestion process: a review. Bioresource Technol. 99, 4044–4064. Cho, H.S., Moon, H.S., Kim, J.Y., 2012. Effect of quantity and composition of waste on the estimated of annual methane potential from landfills. Bioresource Technology 109, 86–92. Cossu, R., Raga, R., 2007. Test methods for assessing the biological stability of biodegradable waste. Waste Manage. 28, 381–388. Davidsson, Å., Gruvberger, C., Christensen, T.H., Hansen, T.L., Jansen, J.C., 2007. Methane yield in source-sorted organic fraction of municipal solid waste. Waste Manage. 27, 406–414. De la Cruz, F.B., Chanton, J.P., Barlaz, M.A., 2013. Measurement of carbon storage in landfills from the biogenic carbon content of excavated waste samples. Waste Manage. 33, 2001–2005. EEA, 2013. Annual European Union Greenhouse Gas Inventory 1990–2011 and Inventory Report. European Environment Agency, Technical Report No. 8/2013, May 2013.
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Mata-Alvarez, J., Dosta, J., Macé, S., Astals, S., 2011. Codigestion of solid wastes: a review of its uses and perspectives including modeling. Crit. Rev. Biotechnol. 2, 99–111. Oonk, H., 2010. Literature Review: Methane from Landfills. Sustainable Landfill Foundation.
Further reading Mønster, G.J., Samuelsson, J., Kjeldsen, P., Scheutz, C., in press. Quantification of methane emission from 15 Danish landfills using mobile tracer dispersion method. Waste Manage. Mou, Z.S., Scheutz, C., Kjeldsen, P., in press. Evaluating the methane generation rate constant (k value) of low-organic waste at Danish landfills. Waste Manage.