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Effect of solid wastes composition and confinement time on methane production in a
landfill
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Key words: Mexico; biodegradability; emissions; biogas; methane
Carlos GONZÁLEZ1, Otoniel BUENROSTRO1,*, Liliana MARQUEZ1, Fabián ROBLES2,
Consuelo HERNÁNDEZ3, Edgar MORENO4
1
Agricultural and Forestry Research Institute. Universidad Michoacana de San Nicolás de
Hidalgo. Posta Veterinaria Km. 1.5 Morelia-Zinápecuaro. CP. 58880. Morelia, Michoacán,
México.
2
Unidad Profesional Interdisciplinaria de Biotecnología. Instituto Politécnico Nacional. Av.
Acueducto s/n, C.P. 07340. México D.F., México.
3
Technological Institute of Toluca. Av. Tecnológico S/N, Frac. Ex-Rancho La Virgen.
Metepec, Estado de México, México.
4
Technological Institute of Morelia. Av. Tecnológico 1500, C.P. 58120, Morelia, Michoacán,
México.
Abstract: In developing countries, illegal dump structures or even some landfills do not
include methane collecting systems, even if local environmental laws exist. In this condition,
the greenhouse gas leaks to the atmosphere uncontrolled and practical solutions to tackle this
problem are not obvious. To make a solution approachable, first-hand reliable data from
dump emissions are required as starting point. The methane production is not homogeneous
throughout the dump, therefore to estimate its global methane emissions, various
representative gas monitoring sites distributed along the dump becomes necessary.
This research work presents the estimation of biogas emissions from measurements collected
in the dump located at Morelia, Michoacán, Mexico alongside an evaluation of the organic
fraction and confinement time participation on biogas production. Biogas emission data have
been taken with a portable analyzer from 49 ventilation pipes for 52 weeks. For the
composition and degradability analysis of solid wastes, the required samples have been
collected from 16 sites. The results show a heterogeneous composition of solid wastes: 38
separate components are present, from those, 19 belong to organic categories and 28 accounts
for almost 99% of the waste. The mean biogas concentration matrix is: 45.5% CH4, 32.4%
CO2, 3.1% O2, and 18.9% balance gas (i.e., N2, CO or H2S). The ANOVA procedure clearly
corroborates the influence of composition, biodegradability and time of confinement of solid
wastes on the production of methane, despite the deficiencies in the final soil layer cover in
these sites.
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1. Introduction
Intergovernmental agencies worldwide are making attempts to create awareness about
the contribution to global warming of landfill gas emissions from decomposing municipal
solid wastes (MSW) in landfills, recommending the use of biogas emissions as a source of
alternative energy instead of their release to the atmosphere (IPCC, 2007). The amount of
landfill gas generated largely depends on climatic conditions, geography, waste
characteristics and other local factors (Chiemchaisri et al; 2007). Several researchers have
identified the factors influencing the degradation of MSW and have assessed their individual
effects on the methane production. These factors include presence or absence of oxygen and
hydrogen (Borglin et al., 2004; Fielding et al., 1988; Wilshusen et al., 2004); temperature
(Kalyuzhnyi et al., 2003; Rajasekaran et al., 1986); MSW confinement time (Wang et al.,
1994); MSW field capacity and hydraulic retention time (Munasinghe, 1996; Orta de
Velázquez et al., 2003; Zeiss and Major, 1993); compaction and compressibility of MSW
(Hossain et al., 2003); pH (Bergman et al., 2000; Bernard and Merlin, 1995); type of material
used as final cover layer and the codisposition of wastes from the construction and demolition
industry (Márquez and Watson, 2003; Méndez et al; 2002); humidity content and water flow
(Klink and Ham, 1982; Korfiatis et al., 1984) and; the use of inoculants such as biosolids and
compost, along with the leachate recycling (Francois et al., 2007; Swati et al., 2005; Ellis et
al., 2005; Sanphoti et al., 2006; Chugh et al., 1998; Reinhart, 1996; Sinan et al., 2007; Sponza
et al., 2004).
Although the above-mentioned factors are interdependent, Barlaz (1996) identified pH
and humidity content as being most critical, whereas Reinhart et al. (2002) emphasized
humidity and nutrient contents as the main factors affecting the stabilization of MSW.
However Generally, only the solid wastes containing cellulose are degraded in a landfill, such
as food and yard wastes and two thirds of the paper and cardboard; the degradation of other
materials is often incomplete (Gendebien et al; 1992). CARLOS Y OBD, CUAL ES LA
IDEA ORIGINAL AQUI? NO LOGRO CAPTAR EL FLUJO O LOGICA
Although the need to extend the knowledge on biogas emissions in MSW disposal sites
in order to optimize measures for landfill gas capture and for minimizing emissions to the
environment has globally been emphasized (Scharff and Jacobs 2006), such information is
still very limited in developing countries, when available, comparisons and the determination
or precise data is difficult: the available information derives from short-term studies made in
MSW final disposal sites having variable composition, climatic conditions and management
and measuring techniques vary widely . Table 1 shows the results of some studies of biogas
composition. LA INCLUSION DE ESTA TABLA ES DESCONCERTANTE, DADO QUE
LA PREMISA DEL PARRAFO ES DEJAR CLARO QUE EN MEXICO (1) LOS
ESTUIDOS SON POCOS Y (2) SON POCO CONFIABLES, POR LO TANTO NO
ALCANZO A VER LA RELACION. ALGUIEN POR FAVOR PUEDE INCLUIR UN
PARRAFO ADECUADO
Y/O LA TABLA QUE SI DEBA IR? CON BUENA
REDACCION EN ESPAÑOL Y YO LA PASO AL INGLES, GUARDEN UN ARCHVO
APARTE CON LAS INCIIALES DE QUIEN ESTE ATENDIENDO ESTO, POR FAVOR.
OTONIEL: O QUIZAS EN LA VERSION EN ESPAÑOL SI HACE SENTIDO LAS
IDEAS?
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Table 1 Composition of landfill gas (LFG) in different world regions CARLOS/OBD
CUALES SON LAS UNIDADES DE LA COMPOSIICON DEL GAS??(% volumen)
LFG composition
Sampling
Depth
period
Landfill location
Year
CH4 CO2 O2 Balance (months)
(m)
Wells
P. de Montaña, Mexico
2000-2001 46
34.6 2.6
16.86
16
S/D
Horizontal
Nakhonpathom,Thailand
2003
64.1 31.1
1
3.82
8
3
Vertical
Samutprakan, Thailand
2004
50.6 38.9 1.1
8.5
3
50
Horizontal
Louisville. KY, USA
2001-2005 58
41
0.4
1
48
18.5
Horizontal
Site of Study
2010-2011 45.5 32.4 3.1
18.9
7
3
Vertical
92
Sources: Hernández et al., 2006; Chiemchaisri et al., 2007; Wang Yao et al., 2004; Tolaymat et al., 2010.
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In the present research the composition of landfill gas emission was analyzed in the final
disposition site in the city of Morelia, Michoacán, Mexico, aiming to determining the effect
of MSW composition in the production of biogas. The specific objectives were: 1)
Determining the physicochemical characteristics of confined MSW, and 2) Knowing the
amount and composition of the biogas generated in the studied site.
Carlos (o alguien) Por favor, aunque sea en español reescribe este parrafo a que no
incluya tan seco los Obje especificos, ponlos en format de un parrafo de articulo.
LA SECCION COMPLETA ME DEJA ALGUNAS DUDAS, POR EJEMPLO, SI SE
ESTABLECE
QUE
LAS
VARIABLES
MAS
IMPRTANTES
SON
HUMEDAD/PH/NUTRIENTES COMO LAS MAS IMPORTANTES O INFLUYENTES…
arioENTONCES COMO SE JUSTIFICA EL ESTUDIO POR COMPOSICION?
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2. Materials and methods
2.1. Description of the studied site and sampling of solid waste
The present study was conducted in final disposal site that was closed for operation four
years ago, after 20 years of activity. The dump is located 15 km west of Morelia, the capital
city of the state of Michoacán, in Mexico (19° 41’ 40’’ N, 101° 20’ 54’’ W), at an elevation of
2075 m.a.s.l. (Figure 1). The predominant climate in the area is of the temperate subtype
Cwa, with a mean annual temperature of 18.7ºC, intermediate humidity, a summer rainfall
regime of 700-1000 mm and an average of 5 mm of winter precipitation. The site has an
extension of 17 ha, an irregular topography, an approximate slope of 15o; disposal cells have
a maximum depth of 10.1 m and the estimated amount of deposited MSW is of 3,859,642 t
(Israde et al; 2005). At the time of its closure, 49 venting pipes were installed at a 3 m depth.
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2.2. Sampling and characterization of solid waste composition
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The study area was divided into four quadrants according to the confinement period of
MSW, the oldest being quadrant I and the more recent, quadrant IV. Using a simple random
design, 16 wells were perforated at a depth of 3 m and from each one approximately 4 kg of
Figure 1. (HERE)
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waste were collected in transparent plastic bags. The numbers of perforated wells and venting
pipes in each quadrant are shown in Table 2.
Table 2. Number of wells and venting pipes in each quadrant of the study site
Quadrant
Wells
Ventilation pipes
I
5
15
II
4
11
III
3
11
IV
4
12
2.3. Physical and chemical analyses of solid waste samples
Physical and chemical analyses of samples were conducted in triplicate. Samples were
grinded to homogenize the components to one inch fractions AQUI POR FAVOR
OBD/CARLOS, PUEDEN ACLARAR ESTE PASO? CONOZCO EL MOLINO Y NO HAY
MANERA DE QUE SE HAYA MOLIDO A UN TAMAÑO DE 2.5 CM,. This CUAL?
analysis included: characterization of waste components, temperature, pH, moisture content,
total solids (TS), volatile solids (VS) and methane emission.?? EN QUE UNIDADES?
2.3.1. Physical analysis
2.3.1.1. Solid waste characterization
One kilogram of solid waste was hand-sorted and individual components were weighed,
Te envio la version que se tradujo. Unos de los principales errores, y lo reconozco. Es que el
artículo se escribió en inglés muy malo y español . Tambien, estoy detectando otro garrafal.
La version traducida tiene las tablas 3 y 4 erroneas¡¡. La discusión del artículo está bien, con
base en las tablas que van en el archivo 14. Solo están cambiadas¡. Porqué?, aun no lo sé que
pasó.
saludosaccording with the Mexican official standard norm (NOM-AA-22-1985)
(SECOFI, 1985).
2.3.1.2. Temperature
The temperature of the sampling sites was measured at a depth of 3 m with a digital
thermometer (Taylor Precision Products, model 9878) for about 20 seconds to ensure the
stability of the measurement.
2.3.1.3. Potential of hydrogen (pH)
The pH in solution of the solid waste was measured in accordance with the Mexican
standard norm (NMX-AA-25-1984) (SECOFI, 1984a) using a potentiometer (model pc-18,
Felisa) that was previously calibrated to pH = 7.0 using the buffer solution provided by the
manufacturer of the equipment.
2.3.2. Chemical analysis
2.3.2.1. Moisture content
The moisture content was measured in accordance with the Mexican standard norm
(NMX-AA-016-1984) (SECOFI, 1984b). 15 g samples of solid waste were placed in
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aluminum capsules that were previously washed with distilled water and dried. The capsules
containing the samples were heated in an oven at 105°C for 12 hours. Afterwards, the
capsules were left to cool in a dryer for 40 minutes and weighed until reaching a constant
weight. Finally, the initial weight was subtracted from the final weight to obtain the sample's
net weight.
Calculations were made with the following equation:
%H= PH − PS ∗ 100
Where:
PH
% H = percentage humidity,
PH= weight in grams of wet sample (before drying),
PS= weight in grams of dry sample.
Equation 1
2.3.2.2. Total solids (TS)
The analysis of total solids was made in accordance with the methodology set out in APHA
(1998). The calculations were made with the following equation:
Where:
% TS = percentage of total solids,
PH = weight in grams of wet sample,
PS = weight in grams of dry sample.
ST=
PS
∗ 100
PH
Equation 2
2.3.2.3. Volatile solids (VS)
Analysis of volatile solids was made in accordance with the methodology described in
APHA (1998). Data obtained from the analysis were processed using the following equation:
SV= PS − PC ∗ 100
PS
Where:
% SV = percentage of volatile solids,
PS= weight in grams of dry sample,
PC= weight in grams of burned sample.
Equation 3
2.3.2.4. Ashes
The ash content was measured in accordance with the Mexican standard norm (NMXAA-018-1984) (SECOFI, 1984c). The samples obtained from the moisture content were
placed in capsules and put inside a muffle for one hour at 550°C. The calculations were made
with the following equation:
Where:
% C = percentage of ashes,
PA=weight of ashes,
PS= weight in grams of dry sample.
%C= PA∗ 100
PS
Equation 4
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2.4. Measurement of biogas emissions
Measurement of biogas emission was made in the 49 venting pipes using a GEM-2000
plus (Landtec) portable analyzer. Measurements for methane (CH4), carbon dioxide (CO2),
oxygen (O2) and hydrogen sulfide (H2S) were made during 52 weeks.
Measurements were made during morning time (between 9:00 AM and 13:00 PM) to
assure less interference from external factors such as changes in humidity and temperature,
placing both plastic connectors of the analyzer inside venting pipes.
2.5. Statistical analysis of data
Field and laboratory data from the physical and chemical analyses of composition of
residues and of emission of methane in the dump were captured in the program Excel
(Microsoft Office XP). The JMP program (Version: 6.0. SAS Inc. Institute, 2005) was used
for the statistical analysis. Descriptive statistics and three series of one-way analysis of
variance (ANOVA) were performed to establish the influence between the production of gas
and the surveyed variables. Those variables showing a statistically significant difference were
analyzed with the Tukey test.
3. Results and discussion
3.1. Composition of solid wastes samples
The composition of solid wastes was highly heterogeneous including 38 components,
which could have been due to the variation trough time of MSW composition. Table 3 shows
the more relevant components, of which Food Waste/Unidentifiable fraction was dominant; it
is noteworthy that the organic fraction content is inversely proportional to the confinement
period of the solid residues, clearly showing the influence in the organic fraction content of
time of disposition of solid wastes in the site. Older wastes have a higher degree of
degradation, as can be seen in the organic fraction contents from the oldest (quadrant I) to the
more recent (quadrant IV) residues.
Of the total solid waste composition in the site of study, the organic fraction (fine
residue, food and yard wastes, hard plant fiber, bone, wood, feces, cardboard, leather, paper,
cloth, shoes, hair, coating and wax) averaged 63%, a proportion that is relatively high, which
reflects that although the city of Morelia has experienced economic challenges these have not
been widespread for all the population, and also that a rural life style consumer pattern
prevails.
Also noteworthy is that, despite the implementation in Morelia of MSW separation
programs, some recyclable components such as cardboard, rigid and film plastics, and glass,
continue to be elevated in the solid waste stream. It was also observed that only those
materials having high economic value and demand, such as aluminum, polyethylene
terephthalate and ferrous materials, have been diminishing their amounts in the solid waste
stream.
Table 3. Composition of solid waste in the study site (% fresh weight)
Parameters
Waste Composition (% w/w)
I
II
Quadrant
III
IV
6
Food Waste/ Unidentifiable fraction
Paper and Cardboard
Yard Waste
Wood
Leather, Bone and Rubber
Cloth
Other Organics
Diapers
Metals
Glass and Ceramics
Plastics
Stones
Other Inorganic
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53.3
4.7
4.4
2.3
0
2.8
1.2
5.7
0.1
3.5
13.1
7.2
1.6
40.3
9.8
0.5
1.3
0
4.5
0.3
1.6
4.2
5.6
22.8
9.2
0
68.6
2.5
2.1
1.9
0
0.9
0.8
3.8
0.8
1.6
9.7
7.3
0
25.5
9.0
3.4
0.0
0.7
1.6
0.9
4.0
2.9
3.6
14.4
27.9
6.1
3.2. Results of physical and chemical analyses of solid waste samples
An increment in temperature was observed that depended on the depth of the layer and
on the season of the year (results no shown). Nonetheless, only quadrants I and IV presented
mesophilic temperatures (35 to 40ºC) (Table 4) and all the ventilation pipes in the site
registered an optimum production of methane, which is in agreement with the report of
Yilmaz et al. (2003) that a mesophilic range of temperature is a critical factor for the optimal
degradation of the organic fraction of the wastes.
Table 4. Physical and Chemical characteristics of buried waste samples (% wet weight).
Quadrant
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Parameters
Physical Characteristics
Temperature (°C)
pH
Chemical Characteristics
Moisture Content (% w/w)
Total Solids (% w/w)
I
II
III
IV
26.8
8.6
27
7.7
33.1
7.8
35.6
7.7
35.1
64.9
31.9
66.3
29.6
68.6
32.5
65.5
Volatile Solids (% TS)
Ash (% TS)
49.1
50.9
53.0
45.8
44.5
54.2
36.3
62.3
The high contents of water in solid wastes found in the present study are explained by
their high organic content. Humidity values ranged between 29% and 35% for all wells. A
water content of wastes of 25% to 60% is common, and is linked to its composition.
However, data did not show a clear difference in the moisture content between the quadrants.
This data indicates no variation in the water content within the 3 m depth at which solid
wastes samples were taken. According to El-Fadel et al. (1996a, 1996b), humidity is an
important factor that contributes to production of biogas in a landfill because it is necessary
for microbial activity. Yilmaz et al. (2003) state that humidity is one of the most important
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factors in biogas generation in sanitary landfills because water is necessary for chemical and
biological reactions to take place within the solid residue matrix, and because it stimulates
microbial activity by providing a better contact between substrates and microorganisms. A
water content of 65% to 70% in the solid wastes is required to allow for an accelerated
anaerobic degradation (Reinhart et al; 2002). Besides, water is necessary for the transport of
nutrients and microorganisms between the layers of waste.
The solid waste's pH values were in the range of 8 to 8.4, which is indeed a suitable
range for the optimal degradation of solid wastes; pH is an important parameter in the
optimum decomposition of solid wastes (Francois et al; 2007).
The results of TS show values of between 62% and 68%, the highest such values being
concentrated in stratum III, which is related to the lower content of humidity in that level.
Volatile solid values went from 33% to 56% (Table 4), an ample range that reflects the
type of organic content and the level of degradation of MSW among quadrants. Higher values
of volatile solids and of the organic fraction in quadrant IV are showing a directly
proportional relation of both the degradability of the organic contents and the confinement
time of the solid wastes. The volatile solid content has been used to express the total organic
matter present in solid wastes. According to Mehta et al. (2002), volatile solids are a
trustworthy parameter to indicate the degradation of the organic materials throughout time,
and consequently, a good indicator of the potential for methane production.
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Figure 2. (HERE).
3.3. Measurement of biogas emissions
The biogas generated in the studied site contained 45.5% CH4, 32.4% CO2, 3.1% O2,
18.9% balance gas (i.e., N2, H2S CO). Gas emissions differed between quadrants during the
sampling period. Figure 2 shows the average methane emissions in the four quadrants.
The observed seasonal variation in methane generation can be seen in Figure 2, a
fluctuation that reinforces the effects of corresponding changes in temperature and
precipitation. An increase in methane emission was observed during the rainy season (JulyOctober) and a decrease in such activity was registered during the dry season (April-June),
the latter due to low water availability. The lower level of methane generation observed in
quadrants I and IV suggests the effect of the time of solid wastes confinement and, hence,
their degradation stage. In the case of quadrant I, which is the oldest deposit, it was expected
that after 20 years a large proportion of the organic fraction had already been degraded. In
contrast, in quadrant IV which is the most recent deposit, the methanogenic phase has not yet
reached its maximum level.
The average generation of methane observed in the study site (45.5%) was below the
values reported by Tolaymat et al. (2005), but some wells had values that fall within the
range reported in other studies (58-64%) (Wang Yao et al., 2005; Chiemchaisri et al., 2007;
Hernández Cano et al., 2006). These latter studies also report the influence on methane
generation of climate, solid waste composition and type of cover. The soil landfills
commonly used in developing countries are in general devoid of infrastructure appropriate for
capturing methane; nevertheless, knowing the level of production of this gas is relevant in
order to assess both its environmental impact and the economic feasibility of building
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sanitary landfills having methane-capture systems.
3.4. Statistical data analysis
The ANOVA revealed a statistically significant difference (P < 0.05) between quadrants in
temperature, pH, ashes, VS, organic and inorganic fractions and methane production (Table
5).
Table 5. Results of ANOVA for the studied variables.
Source
Temperature
pH
Humidity
Ashes
TS
VS
Organic fraction
Inorganic fraction
Methane
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328
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330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
No. parameters
3
3
3
3
3
3
3
3
3
DF
3
3
3
3
3
3
3
3
3
Sum of Squares
1331.6439
10.817776
244.122
5125.5327
245.23178
5125.5327
2647.4535
2647.4535
2018.2282
F Ratio
35.5321
6.5915
1.5909
4.7511
1.6077
4.7511
4.4391
4.4391
28.3443
The results of the Tukey test for temperature showed a statistically significant difference
(P < 0.05) between quadrants I and IV with respect to quadrants II and III. The pH did not
show a statistically significant difference between quadrants II and IV, while quadrants I and
III did show such a difference (P < 0.05). Ashes did not show a difference between quadrants
II and III, while quadrants I and IV did show a statistically significant difference (P < 0.05).
In the case of VS, quadrants II and III did not show differences but a statistically significant
difference was observed between quadrants I and IV (P < 0.05). The organic and inorganic
fractions were similar between quadrants I and II as between quadrants III and IV, a
statistically significant difference existing between both groupings (P < 0.05). Concentration
of methane showed a statistically significant difference (P < 0.05) of quadrant III with respect
to the other three quadrants.
Our study showed that the differences in methane production were related to the different
stages of degradation of solid wastes, although the effect of residue composition is also to be
considered together with the fact that the dump operated without an efficient cover of solid
wastes. The statistical analyses corroborate the existence of a statistically significant
difference between sampled quadrants.
4. Conclusions
Statistical analyses reveal heterogeneity of solid waste composition within the studied
site, differences which are attributable to different confinement times.
A larger amount of organic fraction was found in more recent quadrants, likewise, the
ANOVA and the Tukey test showed a statistically significant difference between quadrants
corroborating the effect of time of confinement of solid wastes, despite the deficiencies in
their covering.
The contents of VS observed in the solid wastes together with the statistical analyses of
the four studied quadrants confirm existing differences in organic matter content
(degradability) and in degradation stages of the latter.
Differential methane generation between quadrants and the statistical analyses herein
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made corroborate the effect in biogas production of solid wastes composition (organic matter
and its degradability) and of the time elapsed since their confinement.
The way in which variables were analyzed over time allowed for corroboration of
differences in methane generation, not only between different sites but also within the same
site.
Studies like the present are relevant for increasing the information regarding biogas
production in dumps, MSW disposal sites having no control over the variables known to have
an effect on methane generation.
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5. References
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