Analysis of methane oxidation and dynamics of methanotrophs within a passive methane oxidation barrier

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Proceedings Sardinia 2007, Eleventh International Waste Management and Landfill Symposium
S. Margherita di Pula, Cagliari, Italy; 1 - 5 October 2007

2007 by CISA, Environmental Sanitary Engineering Centre, Italy
ANALYSIS OF METHANE OXIDATION
AND DYNAMICS OF METHANOTROPHS
WITHIN A PASSIVE METHANE
OXIDATION BARRIER
A. CABRAL*, K. ARTEAGA*, D. RANNAUD*, S. AIT-BENICHOU*, M-F
POUËT*, S. ALLAIRE**, L.B. JUGNIA***, C. GREER***
* Department of Civil Engineering, Université de Sherbrooke, Sherbrooke,
Quebec, J1K 2R1, Canada
** Department of Soil Science, Université Laval, Quebec, Quebec, Canada
*** Biotechnology Research Institute, Montreal, Quebec, H4P 2R2 Canada.
SUMMARY: A passive methane oxidation barrier was built on the existing final cover of the St-
Nicéphore MSW landfill, Quebec, Canada. The main goals of this field experiment are to
evaluate CH4 abatement and to study the dynamics of methanotrophs in a Nordic environment.
Preliminary results show that CH4 is being oxidized and that the oxidation zone fluctuates with
changes in environmental parameters, mainly the degree of water saturation. The effects of
atmospheric pressure and temperature are also analyzed. Over the monitoring period, changes in
methanotrophic bacteria counts exhibited different steps and important variations within the CH4
oxidation zone.
1. INTRODUCTION
Methane (CH4) is a potent greenhouse gas. It absorbs infrared radiation more effectively than
CO2 (Crutzen, 1991) and its atmospheric concentration is increasing at a rate of 0.6% per year
(IPCC 2001). This increase has been linked to global climate change. It is now estimated that as
much as 70% of CH4 emissions are due to human activities, and as much as 19% of the increase
can be attributed to landfill emissions (IPCC 2001). Management practices to reduce these
emissions from landfills include gas collection systems; however, even at sites with gas
collection systems, significant amounts of biogas can still escape as fugitive emissions.
It has been shown that CH4 emitted to the atmosphere from landfills can be oxidized by
methanotrophic bacteria in landfill top covers (Gebert and Gröngröft 2006a; Humer and Lechner
2001; etc.). The capacity of this top layer to oxidize CH4 depends on both the physical and
chemical properties of the cover material, on the biogas flow rate and its quality, and on climatic
parameters such as atmospheric pressure and temperature. Therefore, it should be feasible to
develop a mitigation strategy incorporating a combination of engineered and natural
methanotrophic controls, by means of a designed landfill cover that would promote growth of
methane oxidizing bacteria, while performing its other purposes, mainly infiltration control. Such
an approach might provide a complementary strategy for the control of CH4 emissions,
particularly at older sites where flaring or energy recovery is not economically feasible.

Sardinia 2007, Eleventh International Waste Management and Landfill Symposium
This prompted an ongoing multidisciplinary project that jointly considers geotechnical,
hydraulic, physico-chemical, environmental, climatic and microbiological aspects in assessing
the efficiency of landfill cover materials that would act as passive methane oxidation barrier
(PMOB). This paper presents the design of one of three experimental PMOBs built at the St-
Nicéphore landfill, Quebec, Canada, as well as the preliminary results obtained during the
summer and fall of 2006. This includes results of CH4 abatement due to methanotrophic activity,
methanotrophic bacteria counts and an evaluation of the effects of some environmental factors on
CH4 abatement. Other aspects and goals of the project will be addressed in future publications.
2. BACKGROUND ON PMOBs
In addition to fulfilling its role of controlling infiltration, a landfill cover system can be
engineered to attenuate biogas emissions, notably CH4. Several types of PMOB designs have
been proposed: in some cases, additional layers to the cover system have been proposed in order
to be able to count on passive oxidation (e.g. Ameis site, in Austria; Huber-Humer and Lechner
2002). In Germany, Gebert and Gröngröft (2006a) proposed a biofilter consisting mainly of
crushed porous clay covered with a thin layer of top soil. In France, Morcet et al. (2003) reported
experiments conducted at the Montreuil-sur-Barse site, where a PMOB was built consisting of
0.30 m of organic soil on top of a GCL (geotextile clay liner), placed on top of a layer of sand.
Four PMOBs have been built in Australia with a profile of 1.2 m of organic soil (different for
each of the four) on top of 0.5 m of gravel (Dever et al. 2005). As can be observed, there is a
multitude of possible PMOB designs. In addition, there is also great variation in the oxidation
rates obtained (e.g. 10% reported by Czepiel et al. (1996), 70% recorded by Hettiaratchi and
Pokhrel (2003), etc.).
CH4 oxidation within a PMOB is a microbiological process and the choice of substrate used
to support the proliferation of microorganisms responsible for this activity (i.e. methanotrophic
bacteria) is a key factor of expected outcomes. Compost in its pure state would be the preferable
substrate to sustain optimum development of methanotrophic bacteria, due to its high content in
organic matter (Hilger and Humer 2003) and nutrients (nitrogen, phosphorus) that are essential
to biomass biosynthesis. However, compost mixed with a bulking agent would be more effective
since the bulking agent greatly facilitates oxygen migration and helps to limit pore obstruction
by exopolymeric substances (EPS) (Streese and Stegmann 2003). According to Hettiaratchi and
Pokhrel (2003), the minimum percentage of compost that would significantly improve the
growth of methanotrophs, and implicitly the biological oxidation of CH4, would be 50%. This
remains to be confirmed, particularly in the field and in different environments.
In a PMOB, the oxidation rate of CH4 and the growth of methanotrophic bacteria are
contingent upon the concentration of O2 and CH4. The O2/CH4 ratio required is 2:1 (this is
equivalent to 2 L O2 / l L CH4) and the minimum O2 concentration required is in the vicinity of
3% (Czepiel et al. 1996; De Visscher et al. 1999).
The O2/CH4 ratio is greatly influenced by the humidity of the materials. Some laboratory
incubation tests (microcosms) have shown that compost from biosolids (from municipal wastes)
with a water content greater than 45% performed better than a soil with a low water content of
17% (Humer and Lechner 1999; Hilger and Humer 2003). According to Boeckx (1996) and
Figueroa (1993) the optimal water content for CH4 oxidation would be around 50%.
But the parameter that best expresses the relative importance of humidity within a PMOB is
the degree of water saturation, Sr, defined as the ratio between the volume of water and the
volume of voids. This parameter implicitly considers the available space for gases to circulate.
Indeed, despite the fact that a high Sr may favour better oxidation of CH4 (humidity favouring
bacterial growth), when Sr approaches values in the range of 85%, the air phase in the pores risks

Sardinia 2007, Eleventh International Waste Management and Landfill Symposium
becoming occluded (Aachib et al. 2004; Cabral et al. 2004), resulting in a reduced flow of gases,
which reduces the effectiveness of the PMOB.
Generally, passive CH4 oxidation research does not take into account other geotechnical
parameters that provide an idea of the conditions of the porous environment in which
methanotrophs evolve and where biogases and O2 must migrate. For example, it is rare in the
technical literature to find data about the compaction conditions and the water retention capacity
of the materials used in a PMOB. Yet, both affect water and gas flow, therefore the degree of
water saturation of the PMOB.
The performance of a PMOB can also be strongly affected by different environmental and
physico-chemical variables of the environment such as the temperature and the pH of the soil.
Key environmental variables such as precipitation, external temperature and barometric pressure
may also affect gas migration (therefore microbial oxidation of CH4). Precipitation has a direct
influence on the degree of saturation.
Methanotrophic bacteria activity – therefore microbial oxidation of CH4 - is dependent on the
temperature of the soil; most methanotrophic bacteria are mesophilic and grow at temperatures
between 15 and 30°C (Boeckx and Van Cleemput 2000). Their activity is weak at temperatures
lower than 5°C, increases at temperatures close to 15°C and significantly increases above 20°C
(e.g. Boeckx et al. 1999), reaching its optimum at close to 30°C (Nolting et al. 1995).
As for pH, its optimal value for the development of methanotrophic bacteria is between 5.5
and 8.5. This range of values may represent an important advantage, since biosynthesis reactions
related to CH4 oxidation, or the decrease in the concentration of O2, are among the factors that
could contribute to change the pH of the environment (Hilger et al. 2000). Lastly, changes in
barometric pressure may affect O2 flow within the PMOB, especially if the substrate is very
porous (Gebert and Gröngröft 2006b). In cases where the barometric pressure remains low for
extended periods of time, there is greater difficulty for molecular O2 to diffuse into the substrate
beyond 0.4 to 0.5 m; with O2 concentrations falling below the minimum 3% mentioned above,
the efficiency of the PMOB is affected.
3. MATERIALS AND METHODS
Three experimental plots measuring 2.75 m (W) x 9.75 m (L) have been built. The general
design of each of the plots consisted of a layer of substrate that optimizes microbial activity, on
top of a coarse material that ensures uniform distribution of biogas at the base of the substrate
layer. Certain design parameters differ from one plot to the other; in this paper, only details
pertaining to PMOB-1 are presented (Figure 1a). Tensiometers, water content and temperature
probes and gas probes were installed at four separate downgradient points and at different depths
in each profile (Figure 1b). Also, 0.15 m-thick polystyrene panels were placed on the walls of
the plots to thermally insulate the PMOB from the outside environment, thereby preventing
lateral migration of water due to thermal gradients. A weather station has been set up on site.
3.1 Materials constituting the PMOB-1
The plot is fed by biogas produced by the buried waste mass underneath (Figure 1a). A layer of
coarse gravel helps to distribute the biogas uniformly, whereas a second layer of finer gravel
allows for a better transition to the finer substrate layer. The 0.8 m-thick substrate layer consists
of a mixture of sand and compost, in a ratio of 5 volumes of compost for 1 volume of sand
(before sieving). The specific gravity of the substrate is 2290 kg/m³. It was placed in four 0.2 m-
layers and compacted with a vibrating plate to obtain layers with an average density of 838.8
kg/m³ (or 85% of the optimum Proctor standard).

Sardinia 2007, Eleventh International Waste Management and Landfill Symposium
-0.2 m
-0.4 m
-0.6 m
-0.8 m
-0.1 m
-0.25 m
-0.45 m
-0.7 m
-0.2 m
-0.4 m
-0.6 m
-0.8 m
-0.1 m
-0.2 m
-0.3 m
-0.4 m
-0.6 m
-0.82 m
G
T


Gas
Tensiometer Water content Temperature
-0.2 m
-0.4 m
-0.6 m
-0.8 m
-0.1 m
-0.25 m
-0.45 m
-0.7 m
-0.2 m
-0.4 m
-0.6 m
-0.8 m
-0.1 m
-0.2 m
-0.3 m
-0.4 m
-0.6 m
-0.82 m
G
T


Gas
Tensiometer Water content Temperature
2.75 m
Sand / compost
2.45 m
0.80 m
Gravel (6.4 mm net)
Panneau de polystyrène
0.15 m
0.10 m
Wastes
FOAM (insulation)
FOAM (insulation)
Existing cover (mineral soil)
var. Gravel (12.7 mm net)
Figure 1. (a) Lateral cross-section view of PMOB-1; (b) Scheme of instrumentation profile
within the PMOBs
3.2 Monitoring of environmental, physico-chemical and microbiological parameters
Meteorological data, including precipitation and atmospheric pressure, were continuously
recorded by the weather station. Within the PMOB, temperature and water content values were
measured at 4 different reference depths (Figure 1b), with temperature probes (HOBO U12, from
Onset) and water content probes (ECH2O EC-5, from Decagon) connected to dataloggers.
Gas profiles (O2, CO2, and CH4) were established based on weekly analyses conducted in situ.
For this, gas samples collected from the gas probes were analyzed using a portable infrared gas
analyzer (Columbus Instruments).
Soil samples were collected monthly from the uppermost part of the substrate (0-0.1, 0.1-0.2,
0.2-0.3, 0.3-0.4 m) using soil core tubes. The samples were kept at 4ºC until analysis in the
laboratory, which usually was performed within 24-hours. For each sampling campaign, the soil
samples were analyzed for moisture content, organic matter content (by loss-on-ignition at 550ºC
for 2h) and pH (determined in distilled water 1:3 v/v). Methanotroph counting was done by
means of the most probable number method, using soil slurries serially diluted in microliter
plates containing ammonium mineral salts medium. The plates were incubated for 4 weeks at
30ºC in gastight jars containing CH4 in air.
4. RESULTS AND DISCUSSION
4.1 Analysis of oxidation data from the field
To simplify the presentation and discussion of the results, the sand/compost layer was divided in
two 0.4 m-thick layers. The average degree of water saturation throughout the entire monitoring
period was higher in the lower layer, where it approached 90% (Figure 2a). In the upper layer,
the average value fluctuated between 84 and 88%. However, the two curves illustreating the
evolution of the degree of water saturation over time within the layers exhibited similar trends
(Figure 2a).

Sardinia 2007, Eleventh International Waste Management and Landfill Symposium
Figure 2. Evolution of (a) precipitation and degree of saturation in the PMOB-1; (b)
atmospheric pressure (external); and (c) temperature within the PMOB-1
As expected, and as shown in Figure 2a, the degree of saturation was sensitive to variations in
the precipitation level. Indeed, two or three days after rainfall intensities equal or greater than 15
mm/day, peaks in the degree of saturation could be observed. As also expected, the daily
fluctuations of temperature are more perceptible in the upper layer than in the lower layer
(Figure 2c). The atmospheric pressure, the importance of which was previously mentioned,
underwent a few variations of some intensity (for example, -3.5kPa during the day of Sept. 24
and +2.5kPa on Oct. 6/7; Figure 2b).
As shown in Figure 3, the concentrations of CH4 contained in the biogas samples collected at
the base of the substrate layer (0.8 m deep) was around 50% and remained practically constant
from one sampling campaign to another. These percentages are of the same level as those
commonly found in landfill biogas. With the exception of the data collected during the first two
collection campaigns (Figure 3a), where there is a decrease in the CH4 concentration for all the
profile, in most cases the concentrations recorded at the base of the substrate layer decreased
very little or not at all between 0.8 and 0.4 m. This suggests, therefore, that microbial oxidation
of CH4 in this area is low. In fact, the most significant decreases in CH4 concentration are usually
observed between 0.4 m and the surface (particularly near the surface, i.e. from 0.1 to 0.0 m). It
is precisely in the area between 0.4 m and the surface that were observed variations in the total
number of methanotrophic bacteria (Figure 4). Given that O2 and CH4 concentrations at the
surface were obtained from measurements made with dynamic chambers, results from the
surface-most crust (0-0.1 m) should be interpreted with caution, not only for CH4 but also O2.
(a)
(b)
(c)

Sardinia 2007, Eleventh International Waste Management and Landfill Symposium
0 102030405060
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
CH4-20 jul
CH4-2 aug
O2-20 jul
O2-2 aug
Gas concen trati on (%)
0 102030405060
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
CH4-8 aug
CH4-15 aug
O2-8 aug
O2-15 aug
Gas concentration (%)
0 1020304050 60
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
CH4-1 sep
CH4-5 sep
O2-1 sep
O2- 5 sep
Gas concen trati on (%)
0 102030405060
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
CH4-11 sep
CH4-18 sep
O2-11 sep
O2-18 sep
Gas concentration (%)
0 102030405060
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
CH4-25 sep
CH4-2 oct
O2-25 sep
O2-2 oct
Gas co ncentration (%)
0 102030 405060
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
CH4-9 oct
CH4-16 oct
O2-9 oct
O2-16 oct
Gas concentration (%)
* Variations (standard deviations) in other cases were similar to this one, except at 0.2 m in (f))
Figure 3 - CH4 and O2 concentration profiles from Jul. 20 to Oct. 16
The oxygen profiles protrayed in Figure 3 indicate a large drop in O2 concentration starting from
a depth of 0.1 m, where it can be less than the critical value of 3%. Such low concentrations may
explain the low oxidation rates (expressed as decreases in CH4 concentration) observed deep
within the layer (Figure 3). The migration of O2 may also be affected by the presence of other
aerobic microorganisms near the surface.
An increase in the concentration of CH4 from Jul. 20 - Aug. 2 can also be seen in Figure 3a.
This increase follows strong precipitations and a decrease in atmospheric pressure. The
concentration profiles for Aug. 8th and Aug. 15th (which are very similar) are presented in Figure
3b. The latter is a period when the oxidation front moved from 0.2 to 0.4 m and during which the
atmospheric pressures and rainfall remained quite low. Some changes in meteorological
conditions affected the concentration profiles on Sept. 1st and Sept. 5th (Figure 3c). In fact, Sept.
1st corresponds to a time of low precipitation and high atmospheric pressure (which favours the
influx of O2), while 3 days before Sept. 5th, high precipitations were recorded. In addition, the
atmospheric pressure on Sept. 5th is significantly lower than on Sept. 1st (difference of 1.3 kPa).
Consequently, the oxidation front on Sept. 1st is lower (close to 0.3 m) than that on Sept. 5th,
which is above 0.1 m.
(c) (b) (a)
*
(f) (e) (d)

Sardinia 2007, Eleventh International Waste Management and Landfill Symposium
pH
6.7 7.0 7.2 7.5
Soil layer (cm)
0-10
10-20
20-30
30-40
Soil moisture (%)
40 50 60 70 80
0-10
10-20
20-30
30-40
CFU/FW g soil
106107108109
0-10
10-20
20-30
30-40
01 Jun
27 Jun
20 Jul
11 Sep
09 Oct
06 Nov
27 Nov
Organic matter (%)
18 21 24 27 30
Soil layer (cm)
0-10
10-20
20-30
30-40
Figure 4. Temporal evolution of pH, organic matter, soil moisture and colony-forming units
(CFU) of methanotrophs in the profile of PMOB-1
The concentration profiles from the Sept. 11 - 18 (Figure 3d) correspond to a period of a drop
in atmospheric pressure and low rains, both hindering O2 migration. The oxidation front on Sept.
11th is close to 0.3 m. However, with the drop in pressure, this oxidation front is drawn closer to
the surface on Sept. 18th. The concentration profiles for Sept. 25th and Oct. 2nd are presented in
(Figure 3e). Sept. 25th is characterized by little recent precipitation and very low atmospheric
pressure. The latter is notably higher on Oct. 2nd, favouring the influx of O2, which partially
explains the greater drop in CH4 concentration between 0.4 and 0.1 m, compared to the profile
obtained on Sept 25th. However, the difference between the CH4 profiles from Sept. 25th and Oct.
2nd is not as striking as that obtained between the profiles from Sept 1st and Sept. 5th. This is due
to the fact that Oct. 2nd culminates a period of continual precipitation, which caused an increase
in the degree of water saturation (Figure 2a) hindering the downward migration of O2.
The concentration profiles for Oct 9th and Oct. 16th (Figure 3f) correspond to a stable
meteorological period but one where surface temperatures undergo a more striking decrease. The
profiles for these dates are similar to that of Oct. 2nd, with the only difference being a more
marked oxidation at 0.2 m. Considering these first results, conditions favourable to oxidation
seem to be high atmospheric pressure, low precipitation (or sufficiently well spaced), as well as
temperatures greater than 20 degrees.
Soil moisture = grav. water content

Sardinia 2007, Eleventh International Waste Management and Landfill Symposium
4.2 Analysis of physico-chemical parameters and microbial activity
Throughout the monitoring period, the pH of the soil varied very little with time and depth, with
values in the range of 7.0 to 7.3 (Figure 4; Mean ± SD = 7.2 ± 0.1). The growth of
methanotrophic bacteria is generally optimal in this range, which indicates that other factors are
likely at the origin of the fluctuations observed for the activity and abundance of methanotrophic
bacteria.
For all the samples analyzed, the organic material content varied between 20.1 and 27.7%
(Mean ± SD = 23.6 ± 2.2%), while the gravimetric water content (soil moisture in Figure 4)
fluctuated between 41.7 and 76.3% (Mean ± SD = 67.1 ± 8.5%). The values for these two
variables increased very slightly with depth, whereas, with time, the water content fluctuations
recorded were more significant, notably on the surface (0.0-0.1 m) (Figure 4), which
undoubtedly was related to precipitation and air humidity.
A more significant decrease in organic matter near the surface compared to deep layers may
be interpreted as a consequence of its more significant degradation by aerobic microflora. This
seems to uphold since minimum organic material contents in the upper layers of the cover (0.0-
0.1 m) were recorded in July and September, while the maximum oxygen concentration value in
this area was recorded during the month of August (Figure 4).
Counts of methanotrophic bacteria varied from 2.5 x 106 to 3.0 x 108 CFU/ g.f.s. (g of fresh
soil), with the average for all samples analyzed being 5.4 ± 9.0 x 107 CFU/g.f.s. As with the
water content values, the greatest population densities of methanotrophic bacteria were recorded
in the surface layers of the cover soil. On average, the number of methanotrophic bacteria
decreases somewhat with depth: from 1.23 ± 1.32 108 CFU/g.f.s. near the surface (0-0.1 m) it
decreases to 6.68 ± 9.26 x 107 CFU/g.f.s. at the 0.1-0.2 m level; to 2.04 ± 2.79 x 107 CFU/g.f.s.
between 0.2 and 0.3 m and finally reaches a minimum of 7.48 ± 4.79 x 106 CFU/g.f.s. further
down (0.3-0.4 m).
Temporal evolution of the different profiles of abundance in methanotrophic bacteria suggests
three different phases in the development of these micro-organisms. The first, which could be
considered as a latency period following the end of the construction of the PMOBs, consists of
more or less identical abundance profiles during the first three campaigns (June to July).
Thereafter, during the growth phase – which extends from August until the beginning of
November - the number of methanotrophic bacteria seems to increase with time, particularly at
the surface, where it reaches the maximum value observed during this monitoring period. During
the third phase a drop in counts appeared, particularly between 0.1-0.2 m, presumably as a
consequence of the rapid decrease in temperature.
5. CONCLUSIONS
Preliminary results from the first year of monitoring of one of three passive methane oxidation
barriers (PMOB) constructed in a landfill situated in Quebec, Canada, show that CH4 is
effectively being oxidized and that the oxidation front fluctuates with changes in environmental
parameters, most notably the degree of water saturation. The study also revealed the marked
influence of climatic factors, mainly atmospheric pressure and rainfall, on the oxidation
efficiency of the barrier. Over the monitoring period, changes in methanotroph counts exhibited
different steps and important variations within the zone where CH4 oxidation occurred. This last
pattern suggested the role of methanotrophs as a contributing factor which could explain the CH4
abatement observed within the PMOB under study.

Sardinia 2007, Eleventh International Waste Management and Landfill Symposium
ACKNOWLEDGEMENTS
The Authors wish to acknowledge the financial support provided by the National Science and
Engineering Research Council of Canada, Waste Management Canada and Ferti-Val (strategic
grant # GHG 322418-05).
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