Journal of Envi ro nmental Prote c tion, 2011, 2, 1310-1316
doi:10.4236/jep.2011.210151 Published Online December 2011 (http://www.SciRP.org/journal/jep)
Effect of Solid Wastes Composition and
Confinement Time on Methane Production
in a Dump
Carlos González1, Otoniel Buenrostro1*, Liliana Marquez1, Consuelo Hernández2, Edgar Moreno3,
Fabián Robles4
1Agronomics and Forestry Research Institute, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Mexico; 2Technological
Institute of Toluca, Toluca, Mexico; 3Technological Institute of Morelia, Morelia, Mexico; 4Interdisciplinary Professional Unit of
Biotechnology, Instituto Politécnico Nacional, Mexico City, Mexico.
Received September 21st, 2011; revised October 23rd, 2011; accepted November 25th, 2011.
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 escapes 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 measures of biogas emissions collected in the final disposal
site located at Morelia (Mexico), along with an evaluation of the organic fraction and confinement time participation
on biogas production. Biogas emission data were taken with a portable analyzer from 49 ventila tion pip e s for 52 weeks.
For the composition and degradability analysis of solid wastes, the required samples have been collected from 16 sites.
The results sho w a heterogeneou s composition of solid wa stes: 38 separate components are present, from those, 19 be-
long to organic categories and 28 of total comp onents accounts fo r almost 99% of the waste. The mean biogas concen-
tration detected was: 45.5% CH4, 32.4% CO2, 3.1% O2, and 18.9% balance gas (i.e., N2, CO or H2S). The ANOVA
procedure clearly corroborated the influence of composition, biodegradability and time of co nfinement of solid wastes
on the production of methane, despite the deficiencies in the final soil layer cover in these sites.
Keywords: Mexico, Biodegradability, Emissio ns, Biogas, Methane
1. Introduction
Intergovernmental agencies worldwide are making at-
tempts to create awareness about the contribution to glo-
bal warming of landfill gas emissions from decomposing
municipal solid wastes (MSW), recommending the use of
biogas emissions as a source of alternative energy instead
of their release to the atmosphere [1]. The amount of land-
fill gas generated largely depends on climatic conditions,
geography, waste characteristics and other local factors
[2]. Several researchers have identified the factors influ-
encing the degradation of MSW and have assessed their
individual effects on the methane production. These fac-
tors include presence or absence of oxygen and hydrogen
[3-5]; temperature [6,7]; MSW confinement time [8];
MSW field capacity and hydraulic retention time [9-11];
compaction and compressibility of MSW [12]; pH [13,14];
type of material used as final cover layer and the codispo-
sition of wastes from the construction and demolition in-
dustry [15,16]; humidity content and water flow [17,18]
and; the use of inoculants such as biosolids and compost,
and leachate recycling [19-26]. Although the aforemen-
tioned factors are interdependent, [27] identified pH and
humidity content as being most critical, whereas [28] em-
phasized humidity and nutrient contents as the main fac-
tors affecting the stabilization of MSW. However, 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 materi-
als is often incomplete [29].
There is clearly a need to extend the knowledge on
biogas emissions in MSW disposal sites in order to opti-

Effect of Solid Wastes Composition and Confinement Time on Methane Production in a Dump1311
mize measures for landfill gas capture and to minimize
emissions to the environment [30], but such information
is still very limited in developing countries, and when
available, comparisons and the determination of precise
data is difficult. The available information derives from
short-term studies made in final disposal sites that have
heterogeneous composition, different climatic conditions,
while management and measuring techniques vary widely
[2,31-33]. Table 1 shows the results of some studies of
biogas composition.
In the present research the composition of landfill gas
emission was analyzed at the final disposition site in the
city of Morelia (Mexico), the scope was to determine the
effect of MSW composition in the production of biogas.
The specific objectives were: 1) To determine the physi-
cochemical characteristics of confined MSW, and 2) to
study the amount and composition of the biogas generated
in the studied site.
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 18.7˚C, intermediate humidity, a summer rainfall re-
gime of 700 - 1000 mm and an average of 5 mm of win-
ter precipitation. The site has an extension of 17 ha, an
irregular topography, an approximate slope of 15˚; dis-
posal cells have a maximum depth of 10.1 m and the
estimated amount of deposited MSW is of 3,859,642 t
[34]. At the time of its closure, 49 venting pipes were
installed at a 3 m depth.
2.2. Sampling and Characterization of Solid
Waste Composition
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 boreholes were perforated at a
depth of 3 m and from each one approximately 4 kg of
refuse were collected in transparent plastic bags. The
Table 1. C ompo sition of landfill gas (L FG) in differe nt worl d regions (% volume).
LFG composition S D W
Landfill location Year CH4 CO2 O2 Balance
P. de Montaña, Mexico 2000-2001 46 34.6 2.6 16.86 16 S/D Horizontal
Nakhonpatho, 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
Site of Study 2010-2011 45.5 32.4 3.1 18.9 7 3 Vertical
Sources: [2,31,32]. S (Sampling period (months)); D (Depth (meters)); W (Wells).
N
Figure 1. Location of the Morelia, M ichoacán dump.

Effect of Solid Wastes Composition and Confinement Time on Methane Production in a Dump 1312
numbers of boreholes and venting pipes in each quadrant
are shown in Table 2.
2.3. Physical and Chemical Analyses of Solid
Waste Samples
Physical and chemical analyses of samples were con-
ducted in triplicate. Samples were grinded to homogenize
the components to one inch fractions. These analyses in-
cluded: characterization of waste components (NOM-AA-
22-1985) [35], temperature (NMX-AA-25-1984) [36], pH
(NMX-AA-25-1984) [36], moisture content (NMX-AA-
016-1984) [37], total solids (TS) (APHA, 1998), volatile
solids (VS) [38], ashes (NMX-AA-018-1984) [39] and me-
thane emission (using a GEM-2000 plus (Landtec) portable
analyzer).
2.4. Measurement of Biogas Emissions
Measurement of biogas emissions were made in the 49
venting pipes using a GEM-2000 plus (Landtec) portable
analyzer. Measurements for methane (CH4), carbon dio-
xide (CO2), oxygen (O2) and hydrogen sulfide (H2S) were
periodically made during 52 weeks.
Measurements were made during morning time (be-
tween 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. Data Statistical Analysis
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 ana-
lysis. Descriptive statistics and three series of one-way
analysis of variance (ANOVA) were performed to estab-
lish the influence between the production of gas and the
surveyed variables. Those variables showing a statistically
significant difference were analyzed by means of the
Tukey test.
3. Results and Discussion
3.1. Composition of Solid Wastes Samples
The composition of solid waste included 38 components,
Table 2. Number of wells and venting pipes in each quad-
rant of the study site.
Quadrant Boreholes Ventilation pipes
I 5 15
II 4 11
III 3 11
IV 4 12
as described in the Mexican norm. Table 3 shows a high
heterogeneity of results among quadrants. It is likely that
these differences result from the different factors such as:
a seasonal variation in the consumption and the modifica-
tion in consumption patterns due to economic and demo-
graphic challenges experienced in the country [40,41].
Table 3 also shows a variety of the more abundant com-
ponents of the 38 categories, of which, food waste/Un-
identifiable fraction was dominant; it is noteworthy that
the organic fraction content is inversely proportional to
the confinement period of the solid residues, showing the
influence in the organic fraction content of time of dis-
position of solid wastes in the site. Oldest quadrants have
a lower content of organic waste, as can be seen in the
quadrant I to the more recent (quadrant IV).
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, pa-
per, cloth, shoes, hair, coating and wax) averaged 63%, a
proportion that is relatively high, which reflects that al-
though the city of Morelia has experienced economic cha-
llenges these have not been widespread for all the po-
pulation, and also that a rural life style consumer pattern
prevails.
Also, 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
Table 3. Composition of solid waste in the study site (%,
w/w, fresh basis).
Quadrant
Category I IIIIIIV
Food Waste/Unidentifiable fraction* 31.6 36.366.460.3
Paper and Cardboard 7.1 10.24.43.9
Yard Waste 3.3 2.34.71.5
Wood 2.2 1.40.02.4
Cloth 1.3 1.54.12.7
Other Organics** 1.2 1.50.40.6
Diapers 4.6 5.31.42.6
Metals 3.2 3.70.10.1
Glass and Ceramics 4.9 2.52.23.6
Plastics 16.8 21.29.512.2
Stones 19.7 13.26.88.0
Other Inorganic 4.3 0.30.02.1
*Organic components difficult to identify due to the stage of degradation;
**Organic components such as cotton, leather, bone, wood, etc; ***Inorganic
components, such as synthetic fibers, cellophane, aluminum foil, electrical
material, cigarette butts, modeling clay, and shoes, etc.

Effect of Solid Wastes Composition and Confinement Time on Methane Production in a Dump1313
and ferrous materials have been diminishing their amounts
with respect to previously measurements in the solid waste
stream [40].
3.2. Results of Physical and Chemical Analyses
of Solid Waste Samples
An increment in temperature was observed related to the
depth of the layer and to the season (results no shown).
Nonetheless all the studies boreholes presented mesophilic
temperatures (35˚C to 40˚C) (Table 4) during the sam-
pling period and methane production was detected in all
ventilation pipes in the site, which is in agreement with the
report of [42] that a mesophilic range of temperature is a
critical factor for the optimal degradation of the organic
fraction of the wastes.
Moisture values ranged between 29% - 38% for all
boreholes, commonly this value is 25% - 60% [42], and
is linked to its composition. However, obtained data did
not show a clear difference in the moisture content be-
tween the quadrants within the 3 m depth at which solid
wastes samples were taken.
Solid waste pH values were detected in the range of 8 -
8.4 (Table 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
because the acidic/basic conditions influence the metha-
nogenic phase [19].
The results of TS showed values of between 62% -
68%, the highest being concentrated in stratum III, which
is related to the lower content of humidity in that level.
Volatile solid values ranged from 33% - 56% (Table
4), an ample range that reflects the level of degradation of
MSW among quadrants. Higher values of volatile solids
and of the organic fraction in quadrant IV are showing a
relation with the stage of degradation of the organic con-
tents 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 [43],
volatile solids are a reliable parameter to indicate the
degradation of the organic materials throughout time, and
Table 4. Phys i cal and chemical characteristics of waste sam-
ples.
Quadrant
Parameters I II IIIIV
Physical Characteristics
Temperature (˚C) 35.7 28.5 28.336.9
pH 8.4 8.4 8.38.0
Moisture Content (%) 34.2 35.0 37.131.7
Chemical Characteristics
Total Solids (% w/w) 65.8 65.0 62.968.2
Volatile Solids (% TS) 33.5 47.3 53.555.7
Ash (% TS) 66.5 52.7 46.544.3
consequently, an indicator of the potential for methane
production.
3.3. Measurement of Biogas Emissions
The biogas generated in the studied site contained an ave-
rage of 45.5% CH4, 32.4% CO2, 3.1% O2, 18.9% balance
gas (i.e., N2, H2S, CO) and emissions differed between
quadrants (Ta ble 5). Figure 2 shows the average methane
emissions in the four quadrants during the sampling pe-
riod.
The observed seasonal variation in methane generation
is shown in Figure 2, a fluctuation that reinforces the effe-
cts of corresponding changes in temperature and preci-
pitation. An increase in methane emission was observed
during the rainy season (July-October, weeks 5 to 21)
and a decrease in such activity was registered during the
dry season (November-June, weeks 23 to 52), 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 rea-
ched its maximum level.
The average generation of methane observed in the stu-
dy site (45.5%) was below according with the values re-
ported by [33], but some boreholes had values that fall
within the range reported in other studies (58% - 64%)
[2,31,32]. The aforementioned studies also report the in-
fluence on methane generation of climate, solid waste
composition and type of cover. Most landfills commonly
used in developing countries lack infrastructure for cap-
turing methane; nevertheless, being able to identify the
level of biogas production is relevant in order to assess
both its environmental impact and the economic feasi-
bility of building sanitary landfills having methanecap-
ture systems.
3.4. Statistical Data Analysis
The ANOVA test revealed a statistically significant dif-
ference (P < 0.05) among quadrants in temperature, pH,
Table 5. Composition of the biogas in the site of study (av-
erage per quadrant % volume).
Composition of Biogas (% volume)
QCH
4SD CO2SD O2 SD B SD
I41,315,127,710,3 4,4 4,9 26,521,0
II49,4 8,4 35,4 5,3 2,2 2,9 12,710,6
III48,5 5,9 35,4 4,5 2,7 2,0 13,4 8,4
IV44,5 16,6 32,89,2 2,5 3,3 20,2 22,9
Q (Quadrant); SD (Standard Deviation); B (Balance).

Effect of Solid Wastes Composition and Confinement Time on Methane Production in a Dump 1314
Figure 2. Methane emissions i n the site studied (average per quadrant % in volume).
ashes, VS, organic and inorganic fractions and methane
production (Table 6).
of a statistically significant difference between sampled
quadrants.
The results of the Tukey test for temperature showed a
statistically significant difference (P < 0.05) between qua-
drants 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 indicated
such a difference (P < 0.05). Ashes did not show a differ-
ence 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 di-
fferences but a statistically significant difference was
observed between quadrants I and IV (P < 0.05). The or-
ganic and inorganic fractions were similar between qua-
drants I and II as between quadrants III and IV, a sta-
tistically significant difference existing between both grou-
pings (P < 0.05). Concentration of methane showed a sta-
tistically significant difference (P < 0.05) of quadrant III
with respect to the other three quadrants.
4. Conclusions
1) Statistical analyses reveal heterogeneity of solid waste
composition within the studied site, differences which are
due to differences in confinement chronology.
2) 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.
3) 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.
4) Differential methane generation between quadrants
and the statistical analyses corroborated the effect in bio-
gas production of solid wastes composition (organic ma-
tter and its degradability) and of the time elapsed since
their confinement.
Our study showed that the differences in methane
production were related to the different stages of degrada-
tion of solid wastes, although the effect of residue compo-
sition 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
5) The way in which variables were analyzed over time
allowed for corroboration of differences in methane gene-
ration, not only between different sites but also within the
same site.
Table 6. Result s of ANOVA for the studied variables.
6) Studies like the present are relevant to increase the
information regarding biogas production in final disposal
sites that have no control over the variables known to have
an effect on methane generation.
Source No. parametersDF Sum of SquaresF Ratio
Temperature 3 3 1331.6439 35.5321
pH 3 3 10.817776 6.5915
Humidity 3 3 244.122 1.5909
Ashes 3 3 5125.5327 4.7511
TS 3 3 245.23178 1.6077
VS 3 3 5125.5327 4.7511
Organic fraction 3 3 2647.4535 4.4391
Inorganic fraction 3 3 2647.4535 4.4391
Methane 3 3 2018.2282 28.3443
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