Hydraulic Aspects of the Design of a Passive Methane ...

GeoFlorida 2010: Advances in Analysis, Modeling & Design (GSP 199) © 2010 ASCE

Hydraulic Aspects of the Design of a Passive Methane Oxidation Biocover A. M. Abdolahzadeh1, A. Cabral1, J. Lafond2 and S. Allaire2

Downloaded from ascelibrary.org by UNIVERSITE LAVAL on 01/08/15. Copyright ASCE. For personal use only; all rights reserved.

1

Geoenvironmental Group, Department of Civil Engineering, Université de Sherbrooke, Sherbrooke, Quebec, J1K 2R1, CA; +1 819 821 8061; Corresponding author: email: [email protected]

2

Horticultural Research Center, Université Laval, Quebec, Quebec, G1K 7P4, Canada ABSTRACT Passive methane oxidation biocovers (PMOB) have been recently proposed as a viable option for migration residual emissions. The efficiency of a PMOB depends on the degree of saturation, which controls the migration of molecular O2; a necessary element in the CH4 oxidation process. The evolution of the degree of saturation Sr is regulated by the unsaturated flow of moisture across the PMOB, which can only be describe with the water retention curve (WRC) and hydraulic conductivity function (k-fct) of the material. This paper discusses the reliability of laboratory-obtained WRCs as input parameter in the prediction of the hydraulic behavior of PMOBs in the field. Three WRC were determined based on field data and in the laboratory using a tension plate. The field data was obtained from an experimental PMOB constructed at St-Nicéphore landfill, Quebec, Canada, where tensiometers and water content probes were installed. INRODUCTION 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 landfilling of wastes (IPCC 2001). Management practices to reduce these emissions from landfills include gas collection systems; however, these systems have limited efficiencies (Humer and Lechner 1999; USEPA 2002; Morcet et al. 2003), letting significant amounts of biogas escape as fugitive emissions. Recent research efforts have been directed towards the development of a mitigation strategy for landfill gas emissions whereby an engineered cover would act as infiltration control, in addition to promoting the growth of methane oxidizing bacteria (Huber-Humer 2004; Huber-Humer et al. 2008). One such system is a passive methane oxidation biocover (PMOB), within which methanotrophic bacteria must find favourable conditions to oxidize CH4 into CO2, in the presence of O2. Several types of PMOB designs have been proposed (e.g. Ameis site, in Austria; Huber-Humer and Lechner 2002; Montreuil-sur-Barse site, France; Morcet et al. 2003). The efficiency of a PMOB depends on the degree of saturation (Sr), which controls the migration of molecular O2; a necessary element in the CH4 oxidation process. And the evolution of Sr depends on the unsaturated flow of moisture across the PMOB (Cabral et al. 2007). This behaviour can only be described once the water retention curve (WRC) of the material is known. A complete PMOB design includes concerns about the geotechnical behaviour of the materials and about their physical and chemical composition,

GeoFlorida 2010

2916



which ultimately affects gas flow, microbial growth and activity, therefore CH4 oxidation. The goal of this paper is to discuss about another concern in the design of an efficient PMOB: the hydraulic aspect. More specifically, one aspect of the study of unsaturated flow in sloped PMOBs, namely the reliability of laboratoryobtained WRCs. In order to tackle this question, three WRC were determined based on field data (suction and volumetric water content) and in the laboratory. The latter was obtained using a tension plate, whereas field data was obtained from an experimental PMOB constructed at St- Nicéphore landfill, Quebec, Canada. BACKGROUND Influence of humidity in PMOBs In a PMOB, the CH4 oxidation rate and the growth of methanotrophic bacteria are contingent upon the concentration of O2 and CH4 (Czepiel et al. 1996; De Visscher 1999), and the O2/CH4 ratio is greatly influenced by the humidity of the medium. Gas permeability decreases with increasing water content. The highest CH4 turnover rate in an Austrian experiment was achieved under the same ambient conditions in biowaste composts, with moisture contents between 40 to 80% of the maximum water holding capacity (Humer and Lechner 2001). The parameter that best expresses the relative importance of humidity within a PMOB is the degree of water saturation, Sr, This parameter implicitly considers the available space for gases to circulate. Despite the fact that a high Sr may favour better oxidation of CH4 (humidity favouring bacterial growth), when its value approaches 85%, the air phase in the pores start to become occluded (Brooks and Corey 1966; Nagaraj et al. 2006), resulting in a drastic reduction in gas migration, thereby diminishing the effectiveness of the PMOB. This is an important concern in the design of sloped PMOBs. The performance of a PMOB can also be strongly affected by different physico-chemical and environmental variables of the environment, such as the pH of the soil (e.g. Hütsch et al. 1994; Bender and Conrad 1995), temperature (e.g. Boeckx and Van Cleemput 1996; Börjesson et al. 2004) and barometric pressure (Gebert and Gröngröft 2006). Design parameter-Water Retention Curve (WRC) The basic PMOB is composed of a gas distribution layer composed of very coarse materials whose main function is to uniformly distribute gas (e.g. HuberHumer and Lechner 2002; Wawra and Holfelder 2003; Berger 2005; HuberHumer et al. 2008) and a substrate layer, where CH4 oxidation occurs. The latter may be composed of different types of materials, from coarse, purely mineral material (crushed porous clay; Gebert and Gröngröft 2006) to pure compost (Humer and Lechner 1999), and to mixtures of materials, such as the sand/compost employed in this study (Cabral et al. 2007; Cabral et al. 2009). In most cases, the substrate layer has smaller pores than the Gas Distribution Layer (GDL). The stacking of the materials with different pore-size distribution, thus different hydraulic properties, leads to the formation of a capillary break at the interface. The schematic water retention curves (WRC) of a GDL and substrate are presented in Figure 1(a), where the capillary block layer (CBL) represents the GDL and the moisture retaining layer (MRL) represents the substrate. These curves dictate the hydraulic behaviour of the PMOB. Due to the high

GeoFlorida 2010

2917

2918

compressibility of some substrates, it is preferred to present the WRC in the form of suction vs. Sr, rather than suction vs. volumetric water content. Using the WRC and the k-fct of the materials (the latter are not presented), and for a given infiltration rate (the value of which is part of the design), it is possible to obtain the Sr profiles shown schematically in Figure 1(b). In this figure, it is presented the worst case scenario possible, i.e. the case where the Sr at the base is equal to 100% (suction is zero). This would happen in the unlikely case where the leachate level would touch the base of the cover. It is clear from the figure that there is a gap in Sr values at the interface. As the MRL becomes more saturated as water is drained down through the MRL (substrate), gas flow becomes more difficult and the probability of preferential biogas flow increases. Such a situation must be avoided by design. The first important step of the design was the choice of materials, which required their respective WRC, and, ultimately, their k-fct (Parent and Cabral 2005). u l av e l b iss o P M eht nihti w S

(a)

CBL MRL

L BC LRM

Possible values of Sr in the MRL

ψ

xam

Suction (L) Ψbot_MRL= Ψmax_CBL Ψ max

oitcuS

S

tob

S

te grat

S

po t

ψ = LRM_tobψ

LBC_xam

( ) tu ra tSoibotn % D e g re e oSftopS aStarget

Height above base of CBL (L)

Sr (%)

qq

(b)

Height above base of CBL [L]



MRL

(

Sbot: Sr at the bottom of the MRL Straget: Provided Sr Stop: Sr at the top of the MRL

CBL Stop Starget Sbot Stop Softarget Sbot S Degree saturation (%) r (%)

Figure 1. (a) Schematic water retention curves, (b) Schematic degree of saturation profiles for a PMOB (adapted from (Cabral et al. 2007)

MATERIALS AND METHODS Experimental PMOB The profile adopted for the experimental PMOB discussed in this paper constructed is presented in Figure 2. This cell included a 0.80 m thick layer of

GeoFlorida 2010


2919

2.45 m

0.1 m of Gravel (1/4 net)

S

G

T

Polystyrene foam FOAM (insulation)

0.8 m of Sand/compost

ψ

Panneau de polystyrène Polystyrene foam FOAM (insulation)

θ

0.15 m

Legend: θ: Water content probe

Ψ: Suction probe T: Temperature probe G: Gas probe S: Sampling from depth of 0–0.2,

1.9 m of Gravel (1/2 net)

Silty clay (existing cover)

0.15 m

Existing cover (mineral soil)


substrate underlain by a 0.10 m thick transitional layer consisting of 6.4-mm gravel and a 2.0- m thick gas distribution layer (GDL) consisting of 12.7-mm gravel. This plot was fed directly by biogas coming from a 3-year old buried waste mass (as of 2009, it is 6 years old). Tensiometers (Irrometer Company), water content capacitance sensors (ECH2O EC-5, from Decagon Devices) and temperature probes (HOBO U12, from Onset) were installed at four separate downgradient points and at different depths in each profile (Figure 2) and connected to dataloggers. Other instruments were installed.

0.2–0.4, 0.4–0.6 and 0.6-0.8 m

Wastes

Gas flow

Gas flow

Not to scale

Figure 2. Lateral cross-section view of PMOB-1 and its instrumentation Section A

Section B

Section C

0.8 m Sand/compost

Gas flow

Profile 1

Gas flow

Profile 2

Gas flow

Profile 3

Profile 4

Figure 3. Scheme representing instrumentation profiles and sampling sections within the PMOB-1

GeoFlorida 2010



Field-based WRC: Non direct method In order to determine the WRC of a given material, one needs suction values and water contents (or degree of saturation). Suction values were obtained from tensiometers installed at each of the profiles indicated in Figure 3, where four tensiometers were installed at the depths of 0–0.2, 0.2–0.4, 0.4–0.6 and 0.60.8 m and were connected to a data logger. In order to calculate the Sr associated with these suction values, the gravimetric water contents of sand/compost samples were obtained. Sampling took place in sections A (upstream), B (middle) and C (downstream) (Figure 3) at depths of 0–0.2, 0.2–0.4, 0.4–0.6 and 0.6-0.8 m. A PVC tube (0.10 m (D) x 0.076 m (H)) was used. Sr values were calculated for all the periods during which tensiometer data was monitored. This calculation required volumetric water content data and the porosity (0.63), with the latter being calculated based on the dry unit weight (8.4 kN m-3) and the Gs (2.24) of the substrate. Sr values were associated with suction values from the closest tensiometer in the profile. For example, for a sample taken from section B at depth of 0.4 m, the tensiometer installed at profile 2 at depth of 0.4 m was considered. Field-based WRC: Direct method In this method the suction values were also directly obtained from the four tensiometers installed at depth of 0–0.2, 0.2–0.4, 0.4–0.6 and 0.6-0.8 m at each profile. At each profile, four water content probes were installed at depths of 0– 0.2, 0.2–0.4, 0.4–0.6 and 0.6-0.8 m. The degrees of saturation were calculated using water content data recorded with a data acquisition system, plus the dry unit weight and the Gs. Laboratory-obtained WRC The water retention curve was obtained in the laboratory using a tension table for small suction potentials (0 to 10 kPa), and a pressure plate for suctions greater than 10 kPa. Only the drainage curve was retained in this study. Intact and repacked cores have been used to perform the WRCs of the biocover soil. Intact cores indicate the importance of the soil structure while repacked cores indicates the impact of particle size (Klute 1986). Six intact cores of the sand-compost biocover (17.8% organic matter content by weight) have been collected from the first 0.10 m using PVC tubes. Six cores of the sandcompost-gravel biocover were hand packed at the unit weight observed in the field (14.56 kN m-3). The cores were saturated for 24 hours by capillary rise and then weighted and placed on tension tables. A stepwise method was followed to increment suction up to 10 KPa. In parallel, cores were installed on pressure plates and submitted to suctions from 33 KPa to 300 KPa. Equilibrium was obtained between each measurement. The samples were then taken to the oven for water content measurements (gravimetric and then volumetric) at each suction level, which allowed for the calculation of the degrees of saturation. RESULTS AND DISCUSSION The WRCs obtained using all the above described methods are presented in Figure 4. The results show that the WRCs obtained using field data agree relatively well with those obtained in the laboratory. Due to some defective water content probes, some curves do not match as well. The results show that well

GeoFlorida 2010

2920


2921

known methods to obtain the WRC in the laboratory can be reliable tools for designing a PMOB; at least as far as unsaturated flow is concerned. Of particular concern is the repeatability of laboratory tests involving soils with significant amounts of coarse material. Lab-Tension table (test #3) Lab-Tension table (test #4)

100

lab-Tension table (test #38)

Degree of saturation (%)

90

Lab-Tension table (test #36) Lab-Tension tabe (test#10) Field-Sampling (Section A)

80

Field-Sampling (Section B) Field- Sampling (Section C)

70

Field-Water content probes (Section A) Field-Water content probes (Section B)

60

Field-Water content probes (Section C)

50 40 30 0,1

1

10

100

1000

Suction (KPa) Figure 4 - WRC obtained from three methods 1,0E-05

Test l#3

1,0E-06

Hydraulic conductivity (m/s) '


Lab-Tension table (test#32)

Test #4

1,0E-07

Test #38

1,0E-08

Test #32

1,0E-09

Test #36

1,0E-10 1,0E-11 1,0E-12 1,0E-13 1,0E-14 0

1

10

100

1000

Suction (kPa) Figure 5. Sand/compost fitting K-function by van Genuchten (1980) model based on the laboratory obtained-WRC

GeoFlorida 2010



For modelling purposes, the hydraulic conductivity functions (k-fct) are required. Figure 5 shows the k-fct of sand/compost fitted by van Genuchten (1980) model. It can be observed that the results reported are quite similar, which was expected, since the k-fct is intimately related to the WRC. More importantly, it can be expected that the hydraulic behaviour of the material can be described with relatively good precision by a series of lab tests. CONCLUSIONS The efficiency of a PMOB depends on the Sr, which controls the migration of molecular O2. And the evolution of the degree of saturation depends on the unsaturated flow of moisture across the PMOB. This paper compared WRCs determined in the laboratory and using two methods that utilized field data. The curves obtained in all cases indicate that WRC obtained by means of the laboratory investigations are reliable and can be used to design a PMOB. However, as usual, judgement is necessary. ACKNOWLEDGEMENTS The Authors wish to acknowledge the financial support provided by the National Science and Eng. Research Council of Canada (Strategic grant # GHG 322418-05), Waste Management Canada and the BIOCAP Foundation. The second Author also wishes to thank the Rotary Foundation and CAPES/Brazil. REFERENCES Bender, M. and Conrad, R. (1995). "Effect of CH4, concentrations and soil conditions on the induction of CH4 oxidation activity." Soil Biol. Bioch. 27: 1517-1527. Berger, J., Fornés, L.V., Ott, C., Jager, J., Wawra, B., Zanke, U. (2005). "Methane oxidation in a landfill cover with capillary barrier." Waste Management 25: 369-373. Boeckx, P. and Van Cleemput, O. (1996). "Methane oxidation in a neutral landfill cover soil: influence of moisture content, temperature, and nitrogenturnover." Journal of environmental quality 25(1): 178-183. Börjesson, G., Sundh, I. and Svensson, B. (2004). "Microbial oxidation of CH4 at different temperatures in landfill cover soils." Fems Microbiology Ecology 48(3): 305-312. Brooks, R. H. and Corey, A. T. (1966). "Poperties of Porous Media Afecting Fluid Fow." Journal Irrig. and Drainage Div., ASCE(June): 61-88. Cabral, A. R., Arteaga, K., Rannaud, D., Aït-Benichou, S., Pouët, M. F., Allaire, S., Jugnia, L.-B. and Greer, C. W. (2007). Analysis of methane oxidation and dynamics of methanotrophs within a passive methane oxidation barrier. 11th International Waste Management and Landfill Symposium, Sta. M. di Pula, Italy. Cabral, A. R., Capanema, M. A., Gebert, J., Moreira, J. F. and Jugnia, L. B. (2009). "Quantifying microbial methane oxidation efficiencies in two experimental landfill biocovers using stable isotopes." Water, Air, and Soil Pollution: Accepted July 2009. Cabral, A. R., Parent, S.-É. and El Ghabi, B. (2007). Hydraulic aspects of the design of a passive methane oxidation barrier. 2nd BOKU Waste Conf., Vienna. Czepiel, P., Mosher, B., Crill, P. and Harriss, R. (1996). "Quantifying the effect of

GeoFlorida 2010

2922



oxidation on landfill methane emissions." Journal of Geophysical Research 101(11): 16721-16729. De Visscher, A., Thomas, D., Boeckx, P., and Van Cleemput, O. (1999). "Methane oxidation in simulated landfill cover soil environments." Environmental science and technology 33: 1854-1859. Gebert, J. and Gröngröft, A. (2006). "Passive landfill gas emission - Influence of atmospheric pressure and implications for the operation of methaneoxidising biofilters." Waste Management 26(3): 245-251. Gebert, J. and Gröngröft, A. (2006). "Performance of a passively vented fieldscale biofilter for the microbial oxidation of landfill methane." Waste Management 26(4): 399-407. Huber-Humer, M. (2004). "International research into landfill gas emissions and mitigation strategies - IWWG working group "CLEAR"." Waste Management 24: 425-427. Huber-Humer, M., Gebert, J. and Hilger, H. (2008). "Biotic systems to mitigate landfill methane emissions." Waste Management & Research 26(1): 33-46. Huber-Humer, M. and Lechner, P. (2002). Proper bio-covers to enhance methane oxidation - findings from a two year field trial. 25th annual Landfill Gas Symposium., Monterey, CA, SWANA (publ. GR-LG-00325). Humer, M. and Lechner, P. (1999). "Alternative approach to the elimination of greenhouse gases from old landfills." Waste Management Research(17): 443-452. Humer, M. and Lechner, P. (1999). Methane oxidation in compost cover layers in landfills. 7th International Waste Management and Landfill Symposium, Sta Margarita di Pula, Italy. Humer, M. and Lechner, P. (2001). Microorganisms against the Greenhouse Effect as Suitable Cover Layers for the Elimination of Methane Emissions from Landfills. 6th Annual Landfill Symposium, San Diego, CA Solid Waste Association of North America (SWANA). Hütsch, B. W., Webster, C. P. and Powlson, D. S. (1994). "Methane oxidation in soil as affected by land-use, soil-pH and N-fertilization." Soil Biol. Biochem. 26: 1613–1622. IPCC (2001). Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. J. T. Houghton, Y. Ding, D. J. Griggs and M. Noguer. Cambridge, IPCC: 881, http://www.grida.no/climate/ipcc_tar/wg1/index.htm. Klute, A. (1986). Water Retention: Laboratory Methods. Methods of Soil Analysis, Part 1. Physical and Mineralogical Methods A. Klute. Madison, WI, American Society of Agronomy - Soil Science Society of America: 635-662. Morcet, M., Aran, C., Bogner, J., Chanton, J., Spokas, K. and Hebe, I. (2003). Methane mass balance: a review of field results from three french landfill case studies. Ninth International Waste Management and Landfill Symposium, Sta Margarita di Pula, Italy, CISA. Nagaraj, T. S., Lutenegger, A. J., Pandian, N. S. and Manoj, M. (2006). "Rapid estimation of compaction parameters for field control." Geotechnical Testing Journal 29(6): 497-506. Parent, S.-É. and Cabral, A. R. (2005). Material selection for the design of

GeoFlorida 2010

2923



inclined covers with capillary barrier effect. Geo-Frontiers 2005, Austin, TX. USEPA (2002). Solid Waste Management And Greenhouse Gases: A Life-Cycle Assessment of Emissions and Sinks, 2nd edition. http://www.epa.gov/mswclimate/greengas.pdf, United States Environmetal Protection Agency: 136, http://www.epa.gov/mswclimate/greengas.pdf. Wawra, B. and Holfelder, T. (2003). Development of a landfill cover with capillary barrier for methane oxidation - the capillary barrier as gas distribution layer. 9th Int. Waste Mgmt and Landfill Symp., Italy.

GeoFlorida 2010

2924