Bio-tarp alternative daily cover prototypes for methane oxidation atop open landfill cells

Waste Management 31 (2011) 1065–1073

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Bio-tarp alternative daily cover prototypes for methane oxidation atop open landfill cells Bryn L. Adams a, Fabien Besnard b, Jean Bogner c, Helene Hilger b,⇑ a

Department of Biology, University of North Carolina at Charlotte, USA Department of Civil and Environmental Engineering, University of North Carolina at Charlotte, USA c Landfills +, Inc., Wheaton, IL, USA b

a r t i c l e

i n f o

Article history: Received 31 May 2010 Accepted 4 January 2011 Available online 26 February 2011

a b s t r a c t Final landfill covers are highly engineered to prevent methane release into the atmosphere. However, methane production begins soon after waste placement and is an unaddressed source of emissions. The methane oxidation capacity of methanotrophs embedded in a ‘‘bio-tarp’’ was investigated as a means to mitigate methane release from open landfill cells. The bio-tarp would also serve as an alternative daily cover during routine landfill operation. Evaluations of nine synthetic geotextiles identified two that would likely be suitable bio-tarp components. Pilot tarp prototypes were tested in continuous flow systems simulating landfill gas conditions. Multilayered bio-tarp prototypes consisting of alternating layers of the two geotextiles were found to remove 16% of the methane flowing through the bio-tarp. The addition of landfill cover soil, compost, or shale amendments to the bio-tarp increased the methane removal up to 32%. With evidence of methane removal in a laboratory bioreactor, prototypes were evaluated at a local landfill using flux chambers installed atop intermediate cover at a landfill. The multilayered bio-tarp and amended bio-tarp configurations were all found to decrease landfill methane flux; however, the performance efficacy of bio-tarps was not significantly different from controls without methanotrophs. Because highly variable methane fluxes at the field site likely confounded the test results, repeat field testing is recommended under more controlled flux conditions. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Landfill gas emissions contribute an estimated 23.8–38.1 Tg of methane (CH4) to the atmosphere annually (IPCC, 2007) and are considered a target for reducing this potent greenhouse gas. To avoid CH4 emissions, the EU Landfill Directive (Council of the European Union, 1999) has severely restricted the quantity of biodegradable organic waste that can be landfilled, while other countries require or promote highly engineered landfill covers and gas collection systems for energy capture (US Code, 1976; EPA, 2010). However, filling an active landfill cell may take weeks or even months, during which time no CH4 collection occurs. In the US, regulations require that buried waste at the active site be covered. This ‘‘daily cover’’ is typically 15 cm of soil, although alternative daily cover materials (ADC) may be used. After a cell is completed, it is covered with a 46 cm soil layer that constitutes ‘‘intermediate’’ cover until final capping occurs. Some CH4 is likely produced after waste placement but before final capping, and escapes through daily and intermediate cover materials. Although there are few reports of

⇑ Corresponding author. E-mail address: [email protected] (H. Hilger). 0956-053X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2011.01.003

open cell monitoring; emissions from one site in France ranged from 100 to 200 g CH4 m 2 d 1 (Bogner, personal communication), and at a site in California, where gas extraction occurred during filling, emissions averaged 0.053 ± 0.03 g m 2 d 1 (Bogner et al., accepted for publication), suggesting that such emissions can vary greatly depending on site conditions. One emerging and promising method of mitigating fugitive landfill CH4 emissions is the use of bio-based systems such as biofilters (Gebert et al., 2003), biocovers (Huber-Humer, 2004), and biowindows (Scheutz et al., 2006). All these systems rely on methanotrophic bacteria for CH4 removal. Methanotrophs are a diverse group of bacteria that use CH4 as their sole carbon and energy source, which is derived from the oxidation of CH4 to CO2 and H2O in the presence of O2 (Hanson and Hanson, 1996). Methanotrophs are abundant in ecosystems where CH4 and O2 coincide, such as wetlands, rice paddies and landfill cover soil (Bachelet and Nueue, 1993; Chen et al., 2003; Jones and Nedwell, 1993; Stralis-Pavese et al., 2004; Svenning et al., 2003; Wise et al., 1999). In addition to CH4 consumption, these organisms can also co-metabolize some non-CH4 hydrocarbons (Aziz et al., 1999; Linder et al., 2000; Scheutz et al., 2003, 2008; Schütz et al., 1989). Studies have shown that biotic treatment systems (biocovers, biofilters, and biowindows) are viable tools for mitigating landfill

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CH4 emissions from closed landfills (Barlaz et al., 2004; Fredenslund et al., 2007, 2008; Gebert and Gröngröft, 2006; Huber-Humer, 2004, 2008; Humer and Lechner, 2001a; Kjeldsen et al., 2007). To date, no treatment has been proposed to mitigate CH4 emitted from open landfill cells. Therefore, we conceived of a ‘‘bio-tarp’’ that could function as a bio-based ADC. One popular type of ACD is a tarp that can be placed on the landfill cell surface each evening and removed each morning. There are several commercially available systems, and their appeal lies in their ability to conserve landfill volume (‘‘airspace’’) that would otherwise be filled with 15 cm of daily soil cover. If a tarp-like matrix could be embedded with active methanotrophic bacteria to consume CH4 during nighttime placement, it would offer a means to reduce CH4 emissions from open cells. The purpose of this research was to develop the bio-tarp concept by evaluating potential materials for supporting the methanotrophs and serving as an ADC. Several candidate materials were tested in laboratory reactors and in field tests for their capacity to support methanotrophs and consume CH4. Critical features of the bio-tarp design included providing sufficient gas-filled porosity (for landfill gas influx from below and atmospheric O2 influx from above); retention time; moisture; and temperature conditions to achieve critical rates of methanotrophy. Because significant gas pressures do not develop in new cells with daily cover only (Kim and Benson, 2004; Moldrup et al., 2000, 2003, 2004), bio-tarp operation would rely primarily on diffusive landfill gas fluxes, and ‘‘edge sealing’’ issues were considered less important in design than the structure and overall porosity of the bio-tarp.

absorption capacity and water retention capacity of each geotextile was measured. Triplicate square pieces (58 cm2) of each geotextile were weighed under the following conditions: dry; water-saturated for 10 min, then drained; and water saturated for 10 min, then physically squeezed. 2.3. Methane oxidation capacity To compare CH4 uptake by methanotrophs immobilized on the different candidate bio-tarp geotextiles, triplicate 7.6 cm squares of each material were tested in batch experiments. A culture was prepared by inoculating 500 mL of NMS with a mixed methanotroph stock in a 1 L air tight bottle. The cells were grown for 18 h as described in Section 2.1. Each test sample was washed and sterilized prior to inoculation with 10 mL of the 18 h mixed methanotroph culture (approx.108 cfu mL 1, as determined by plate counts). Preliminary tests showed that a 10 mL volume was completely absorbed by the swatches after 3–5 min. The headspace of gas-tight jars was adjusted to 8% methane-in-air (19% O2) and the bottles were incubated at 20 °C for 24 h. Methane concentrations in the bottles at the start and end of the incubation were measured, and observed O2 consumption and CH4 depletion measurements were calculated. Two readings were taken at each time point. Furthermore, for each geotextile, three independent replicates were prepared and monitored simultaneously. This allowed three CH4 uptake (mg d 1) measurements to be calculated for each material tested and allowed a one-way ANOVA to be used to compare the geotextile samples to one another for significant differences.

2. Methods 2.4. Laboratory landfill simulation reactors

2.1. Methanotrophic bacteria A mixed culture of methanotrophic bacteria was developed from enriched landfill cover soil (Adams et al., 2006) and maintained in liquid Whittenbury’s Nitrate Mineral Salts (NMS) medium (Whittenbury and Phillips, 1970) at 22 °C with shaking under a 10% methane-in-air headspace. Microarray analysis of the culture revealed it contained methanotrophs belonging to the Methylobacter, Methylosinus, and Methylocystis genera (Adams, 2009). 2.2. Geotextiles Nine geotextiles manufactured by TenCate Geosynthetics were tested for their ability to support methanotrophs and CH4 oxidation. The samples differed in thickness, fiber density, water affinity, and chemical composition (Table 1). Some were made specifically for this project, while others are existing commercial products. Water holding capacity was thought to be important for successful colonization (Adams, 2009), as this likely allows cells to be adsorbed as moisture is absorbed. For this reason, both the water

In order to efficiently oxidize landfill CH4, immobilized bio-tarp methanotrophs must remove each CH4 molecule during the time that molecule passes through the support matrix. A set of continuous flow laboratory reactors was designed to simulate landfill conditions. The reactors were configured to route a synthetic landfill gas mix through a gravel dispersion layer and then to the bottom cross-section of a bio-tarp prototype affixed in the chamber (Fig. 1A). The gas flowed upward through the tarp and then exited the chamber. Compressed air was introduced near the top of the chamber above the prototype; to simulate an atmospheric O2 source as would be experienced under field conditions. The synthetic landfill gas was a mixture of equal parts ultra pure CH4 and dry bone CO2. A 0.5 mL min 1 flow of each gas was metered through a flow controller and then into a mixing chamber (45.7 cm long, 5.4 cm dia. glass wool-filled PVC tubing) before entering the reactor chamber. Medical grade air was also metered through a flow controller to deliver it at 5 mL min 1 to the reactor. The flow of CH4 and CO2 constituted 8.33% of the total flow into the chamber, and the air supply provided more O2 than CH4, to ensure that CH4 oxidation could occur. At room temperature (22 °C), the

Table 1 TenCate geosynthetics geotextile types and properties.

A B C D E F G H I

Geotextile

Thickness (cm)

Color

Characteristics

20 osy wettable PP 160N 20 osy wettable PP 3 denier 6 osy wettable PP 3 denier FR 60 160N + 6 osy phosphate 30 osy PP S1600 IR 26

0.81 ± 0.04 0.30 ± 0.06 0.97 ± 0.01 0.46 ± 0.04 0.36 ± 0.05 0.61 ± 0.05 1.27 ± 0.01 0.50 ± 0.01 0.70 ± 0.01

White Black White White White White and Black White Grey Black

High water holding capacity Common nonwoven geotextile; polypropylene fibers Version of 20 osy wettable PP with a lighter thread Version of the 160N with a lighter thread Polyphosphate-based additive, and. phosphate released when wetted. Composite of two geotextiles Version of the 20 osy wettable PP but thicker Needle-punched nonwoven; polypropylene fibers; inert High water retention. One side fused, creating a skin-like surface on one side

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Fig. 1. Schematic of laboratory continuous flow chamber bioreactor. Panel A shows the overall design with dimensional measurements and key features are noted by letters. (A) open chamber outlet (B) commercial furnace filter for gas mixing (C) gravel layer for gas distribution (D) air inlet (E) landfill gas (LFG) inlet (F) bio-tarp platform. Panel B shows the bio-tarp secured inside the removable frame. This frame is then secured to the bio-tarp platform (labeled as F in Panel A).

flux through the tarp was 23.2 g m 2 d 1, based on maximum measured open cell landfill CH4 emission rates of 100–200 g m 2 d 1 (Bogner, personal communication) and preliminary experiments at various flow rates. The chambers were constructed of acrylic plastic cylinders, and 3.2 mm dia. stainless steel tubing was used for gas transport. Swagelok fittings were used for tubing connections and to penetrate the reactor walls. Commercial furnace filter fabric was sized to snugly cover the cross-sectional area of a reactor near the far end of the chamber to ensure complete mixing of the exit flow. Bio-tarp prototypes were sandwiched between two Plexiglas plates. Each swatch was secured with bolts located around the perimeter of the plates. The secured prototype was then placed in the chamber and sealed to an internal platform with wing nuts and a silicone sealant at the time of testing (Fig. 1B). This ensured that gases passed through the bio-tarp and prevented short-circuiting. 2.5. Bio-tarp design and preparation for testing Prototype bio-tarps were constructed based on the results of the batch studies (Section 3.1.2). Preliminary trials showed that multiple tarp layers were needed for measurable CH4 uptake activity to occur. Prototype bio-tarps consisted of a four layer composite

prepared by alternating layers of 16.5  25.4 cm pieces of washed and sterilized IR26 (Sample H) and S1600 (Sample I) geotextiles. The IR26 pieces were placed fused side up in the second and fourth (top) layers to yield a configuration with a fused top surface. A culture was prepared by inoculating 1 L of NMS from a mixed methanotroph stock in a 2 L gas-tight bottle. The cells were grown for 18 h as described in Section 2.1, after which the entire 1 L was centrifuged in 50 mL aliquots and the pellets resuspended in 50 mL fresh NMS. The aliquots were combined in a sterile beaker to provide a 1 L fresh mixed methanotroph culture (approx. 108 cells mL 1, as determined by plate counts) for application to the bio-tarp. The geotextile layers were soaked in the 1 L culture for at least 15 min, after which they were allowed to drain for 15 min, and no discernable liquid volume was collected. Four prototype configurations were tested: one unamended and three with various amendments inserted between the second and third layers of material. These amendments were (i) a 0.33 cm layer of intermediate landfill cover soil; (ii) a 0.33 cm layer of finished yard waste compost; or (iii) a 0.33 cm layer of shale fines sieved to produce a 2.00– 4.76 mm particle size fraction. The soil and compost were each pre-incubated in gas-tight jars under 50% methane-in-air headspace for 2 d before addition to the bio-tarp. The washed and

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sterilized shale was first pre-incubated in excess solution of an overnight mixed methanotroph population (prepared as described above) for 30 min before insertion in the tarp matrix. Negative control multilayered tarps were prepared without amendments, and an additional shale control was tested where NMS-only soaked shale was included. Duplicate gas samples were collected through a septa located in the wall of the gas pre-mixing chamber (for influent concentrations) and in tubing downstream of the reactor outlet (for effluent concentrations). Inlet and outlet flows were measured with an ADM 2000 volumetric flow meter that fed flow data to a computer where software allowed measurement of the flow at 2 s intervals. The flows were recorded, averaged over 7 min, and used with the inlet and outlet gas concentrations to calculate the bio-tarp CH4 removal efficiencies. 2.6. Field tests of bio-tarp prototypes 2.6.1. Flux chambers The flux chamber (Bogner et al., 1997) base was constructed of 6.35 mm thick stainless steel Fig. 2. A stainless steel ring was welded to the inside circumference of the cylinder to accommodated a removable chamber cover. The covers were stainless steel hemisphere (13 L volume, 3.2 mm thick) that were inverted, set into the channel on the base, and then sealed with water and four spring clamps. The top of the cover contained a gas-tight septum for gas sampling by syringe. Prototype bio-tarps were placed atop the inside ring at the bottom of each base. Another stainless steel ring was set on top of the tarp to secure it and prevent gas short circuiting. Field trials were conducted at the Allied Waste landfill in Cabarrus County, NC at a section where intermediate soil cover overtopped 1 year old municipal waste. The intermediate cover was composed of about 30 cm of clay topped with 30 cm of top soil. Six flux chambers were installed at random locations within a 6  6 m area. The base of each chamber was set firmly into the ground, with about 4 cm of the base depth below grade. Additional soil was placed around the perimeter of the chamber and packed down tightly to seal the interface between the base and the surface. The site soil temperature and atmospheric pressure were

documented each time samples were collected from the flux chambers. Preliminary landfill flux measurements were taken with six flux chambers installed at each of three different intermediate cover depths, (i) at the intermediate cover surface; (ii) about 20 cm below the intermediate cover surface; and (iii) atop bare refuse, approximately 60 cm below the intermediate cover surface, to determine which depth yielded CH4 emission rates most similar to those measured over open landfill cells and used in laboratory bioreactor trials. The flux chambers were sealed and CH4 samples collected to determine the flux at each of the three different depths. Fifty milliliter gas samples were collected in 3 or 5 min intervals for 15 min and analyzed. The field testing protocol was designed to assess the flux at each chamber site with and without the bio-tarp in place. First samples were collected in the absence of any bio-tarp by collecting 50 mL gas samples in 3 or 5 min intervals for 15 min. A bio-tarp prototype was then secured inside the flux chamber, sealed, and samples collected again within 10–15 min of the baseline reading. Finally, the flux chamber top was removed and the bio-tarp was left inside the chamber overnight and exposed to landfill gases. The following morning, the tarps were removed for a short period before testing began to prevent biased readings from any possible accumulations beneath the tarps from the overnight incubation. Then the tarp was replaced, and data for flux measurements were collected. 2.6.2. Flux calculations The CH4 concentrations of measured field gas samples were plotted over time and linear regression was performed. If the regression coefficient exceeded 0.9, the best-fit line was considered acceptable, and the slope of the line was used to calculate the CH4 mass flow rate in ppm min 1. Methane flux was calculated as described by Rolston (1986). 2.7. Field bio-tarp prototype configuration and preparation Bio-tarp prototypes for field tests were similar in composition to the laboratory composites, except that they were configured as 40.6 cm dia. circles. Washed and sterilized geotextiles were prepared as multi-layer units of TenCate IR26 (Sample H) and S1600

Fig. 2. Flux chamber schematic drawing.

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B.L. Adams et al. / Waste Management 31 (2011) 1065–1073 Table 2 Water absorbency and water retention in geotextiles.

a b

Sample

Geotextile

Thickness (cm)

Water absorbed (g cm

A B C D E F G H I

20 osy wettable PP 160N 20 osy wettable PP 3 denier 6 osy wettable PP 3 denier FR 60 160N + 6 osy phosphate 30 osy PP S1600 IR 26

0.81 0.28 0.97 0.41 0.41 0.64 1.27 0.5 0.7

0.803 0.237 0.3 0.571 0.456 0.529 0.522 0.748 0.808

3

)

Water retained (g cm

3

)

0.378 0.176 0.068 0.211 0.139 0.203 0.26 0.456 0.403

Abs. ranka

Ret. rankb

99.3 29.3 37.1 70.7 56.4 65.5 64.6 92.5 100

82.8 38.7 15 46.4 30.5 44.6 56.9 100 88.5

Percent of highest water absorbing Sample (I). Percent of highest water retaining Sample (H).

(Sample G) geotextiles. One prototype was tested without any further amendments, while three contained amendments prepared as described for the laboratory prototypes. The bio-tarps were inoculated with a fresh mixed methanotroph culture. This culture was prepared by inoculating 2 L of NMS from a mixed methanotroph stock in a 2 L gas-tight spinner flask. The cells were grown for 18 h as described in Section 2.1, after which the entire 2 L was centrifuged in 50 mL aliquots and the pellets resuspended in 50 mL fresh NMS. The aliquots were combined in a sterile tub to provide a 2 L fresh mixed methanotroph culture (approx. 108 cells mL 1, as determined by plate counts), in which the geotextiles were soaked to saturation for at least 10 min. Afterwards, they were allowed to drain for 20 min, and no discernable liquid volume was collected. Control bio-tarps were four layer composites prepared with either sterile NMS or DI water. 2.8. Gas chromatography A gas chromatograph (GC) with a thermal conductivity detector (TCD) was used for all gas monitoring (CH4, CO2, O2, and N2) of laboratory culture and bench-scale experiments. For field studies, a GC equipped with both a TCD and flame ionization detector (FID) was used because it could simultaneously detect high and low ranges of CH4. 3. Results 3.1. Batch experiments 3.1.1. Water absorption and retention capacity The geotextiles differed in thickness, fiber density, water affinity, and chemical composition. The thickness varied from 0.28 to 1.27 cm, but thickness did not correlate with higher water absorption or water retention capacities (Table 2). For example, Sample A was 0.81 cm thick and absorbed 0.803 g cm 3, while Sample C was 0.97 cm thick and only held 0.300 g cm 3. Similarly, when wrung dry, Sample A retained 47% of the water absorbed, while Sample C retained only 23%. When the absorption and retention capacities of each sample were calculated as percentages of the most absorbent geotextile (Sample I) and best water retaining geotextile (H), it was evident that while Samples A, H, and I clustered in the 92–100 range and the 80–100 range on absorption and retention, respectively; there was a marked drop in relative performance of other samples relative to these three.

Fig. 3. Comparison of CH4 uptake by methanotrophic cells immobilized on various geotextile materials, as described in Table 1. A significantly higher CH4 uptake as determined by one-way ANOVA (p < 0.05) is indicated by the small letter above the bar. The various letters represent the geotextiles described in Table 1. Error bars represent the standard deviation of three replicate samples.

Although all three geotextiles were among the thicker matrices (Table 2), thickness did not directly correlate with high CH4 oxidation activity. For example, the CH4 uptake level of Sample H (S1600), one of the thinner materials tested, was significantly higher than all other geotextiles. Furthermore, Sample C was a thicker geotextile but showed lower uptake (521 mg d 1). Final O2 concentrations were typically 10–14% and were always above concentrations that would affect CH4 oxidation (Czepiel et al., 1996; Gebert et al., 2003). Therefore, O2 limitation likely had no affect on lower CH4 uptake observations. These data suggest that thickness alone was not the determining factor for high CH4 oxidation rates. Geotextiles H and I were selected for subsequent continuous flow tests because they had high observed CH4 uptake levels and high water retention capacity, factors judged to be critical for good performance under varying weather conditions. Also, geotextile I, (IR 26) had a fused side, which was deemed a potentially valuable characteristic. If the fused surface was placed face-up, gas flow would be slowed resulting in longer retention times in the biotarp. 3.2. Laboratory chamber experiments

3.1.2. Methane oxidation capacity Samples G, H, and I showed the highest CH4 uptake activity, oxidizing 764, 999 and 726 mg m 2 d 1, respectively (Fig. 3). Their performance was found to be statistically more robust (p < 0.05; one-way ANOVA) than the other geotextiles tested.

The four layer bio-tarps were prepared using S1600 (Sample H) and IR26 (Sample I) matrices because of their good performance in water absorption and retention tests (Section 3.1.1), and their high CH4 uptake levels relative to other candidate geotextiles

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(Section 3.1.2). The overall average CH4 uptake of two independent evaluations of a four-layered bio-tarp was 16% (equivalent mass rate of about 3.7 mg CH4 m 2 d 1). Methane consumption was higher as the trials began, but decline after 4 d, reaching only 3% uptake on Day 9 (Fig. 4A). Control chambers, containing tarps with NMS only showed no CH4 oxidation. The addition of amendments to the four layer bio-tarps proved beneficial. Three independent trials of bio-tarps containing enriched landfill soil (Fig. 4B) shows an overall average CH4 uptake of 26% (equivalent mass rate of about 6.0 mg CH4 m 2 d 1), 1.6fold higher than the four-layered prototype. Unlike the unamended bio-tarp prototype, CH4 uptake was sustained over the 9 d trial. The use of an enriched compost (Fig. 4C) amendment yielded results similar to the enriched landfill soil. The overall average CH4 removal of two independent trials was 27% (equivalent mass rate of about 6.2 mg CH4 m 2 d 1). The addition of shale (Fig. 4D) to the four-layered bio-tarp CH4 uptake increased twofold relative to the unamended tarp. The overall average CH4 removal for three independent evaluations of a shale amended bio-tarp was 32% (equivalent mass rate of about 7.4 mg CH4 m 2 d 1). Methane consumption began at 50%, however, unlike the landfill soil and compost amendments, this level of consumption was not sustained and declined to 28% by Day 8. During each continuous flow chamber trial, condensation was evident on the continuous flow chamber walls that contained the bio-tarp, but not on the control chamber walls. This is noteworthy because water is a product and indicator of CH4 oxidation. The removal of CH4 and O2 with an accompanying production of CO2 and condensed water were not observed in any negative control chambers. Therefore, the CH4 removal detected was not the result of CH4 adsorption or dissolution but rather the result of biotic activity by the geotextile methanotrophs. Unfortunately, the high water accumulation in the non-control chambers precluded use of CH4:CO2 ratios, since some of the CO2 produced likely dissolved in the moisture. Also, the landfill soil and compost amendment likely contained heterotroph populations that were actively producing CO2 from the metabolism of organic materials in the soil or compost.

At the conclusion of some of the continuous flow chamber trials, portions of the bio-tarp were incubated in batch with a 10% CH4 in air headspace to confirm that methanotrophic activity was present (data not shown). This measurable CH4 uptake observed suggests that although CH4 consumption generally declined over the trial period, methanotrophic cells remained viable and active. 3.3. Field experiments A survey of CH4 flux rates at the study site was conducted with six chambers at each of three depths: (i) atop bare refuse; (ii) about 20 cm below the intermediate cover surface; and (iii) at the intermediate cover surface. Methane fluxes over bare refuse varied widely for each chamber and among the chambers. For example, the flux measured for one chamber at one location varied from 1830 to 4400 g CH4 m 2 d 1 during one four week monitoring interval when eight different flux measurements were performed. The overall chamber means ranged from 408 to 5103 g CH4 m 2 d 1 (Table 3). CH4 flux rates directly atop the waste layer were higher than the target range corresponding to those used in the laboratory and those previously reported in the field. However, fluxes atop the intermediate cover soil, (8–33 g CH4 m 2 d 1) were judged to be too low. For this reason, the chambers were ultimately set at 20 cm into the intermediate cover (fluxes ranged from 22 to 145 g CH4 m 2 d 1) for all bio-tarp tests, as this range was found to be most similar to laboratory and field tests. While the addition of landfill cover soil, compost, and shale to the bio-tarp prototype yielded increased CH4 removal in laboratory landfill simulation reactors, similar bio-tarp efficacy was not evident in the field. There was no statistically significant difference between CH4 removal in a negative control tarp (NMS only, no methanotrophs) and a four-layered unamended bio-tarp. Methane flux tests using tarps amended with enriched landfill soil (Fig. 5A), compost (Fig. 5B) and shale (Fig. 5C) similarly showed no significant differences between biotically active amended, unamended or negative control tarps. Temperature and atmospheric pressure readings were collected during each trail were analyzed, but no correlations with CH4 flux were found.

Fig. 4. Methane uptake by four-layered biotarp prototypes. Error bars represent the standard deviation of duplicate samples. (A) four-layered biotarp prototype with no amendments (B) four-layered biotarp prototype with landfill soil amendment (C) four-layered biotarp prototype with compost amendment (D) four-layered biotarp prototype with shale amendment.

B.L. Adams et al. / Waste Management 31 (2011) 1065–1073 Table 3 Mean CH4 flux (g m

a

2

d

1

) at various depths on intermediate landfill cover.

Chamber

Bare refuse fluxa

20 cm deep fluxa

Intermediate cover fluxa

1 2 3 4 5 6

1076.46 ± 159.13 2723.04 ± 213.05 5103.76 ± 893.00 2311.88 ± 360.00 3902.29 ± 685.41 408.56 ± 142.89

123.78 ± 40.29 209.99 ± 114.81 165.7 ± 45.77 59.96 ± 55.89 22.01 ± 32.38 165.94 ± 39.22

8.99 ± 5.86 33.5 ± 21.49 6.38 ± 5.49 32.93 ± 38.63 12.09 ± 8.50 8.68 ± 12.13

Average flux of 6–8 independent measurements ± the standard devation (SD).

Fig. 5. Methane flux reduction by bio-tarp prototypes amended with (A) enriched landfill cover soil (n = 2), (B) enriched compost (n = 2), or (C) shale (n = 3). Negative control tarps contain NMS only (no methanotrophs) and unamended bio-tarps consist of methanotrophs only (no amendments). Error bars represent the standard deviation (SD) of replicate samples.

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4. Discussion Although the absolute amount of daily CH4 uptake in the laboratory chambers was low (approx. 4 g m 2 d 1), the results suggested that the bio-tarp concept was worthy of further investigation. Both the increase in methanotrophs present and retention time available in the four layer tarp configuration likely contributed to the performance observed. Although a two layer tarp may have some advantages, the tarp depth offered for a given surface area did not provide sufficient retention time for marked CH4 uptake to occur. A tarp that was four layers thick had a greater depth for the same surface area (and therefore greater volume) so that there was greater retention time for each CH4 molecule before it exited the bio-tarp matrix. The decline in CH4 uptake over time in the chamber (Fig. 4) may have been due to changes in the growth rates of the cells (i.e. rapid growth initially when nutrients were most plentiful) or accumulation of exopolymer that reduced the rate of O2 diffusion to the cells (Hilger et al., 2000). Despite the general decline in CH4 uptake over time, variability between samples seemed to decrease over the test duration. This trend may have been due to the fact that the inlet gases were equilibrating in the earliest days of testing as the reactor filled. Amendments were added to the geotextile tarp to assess the overall contribution to enhanced CH4 uptake by materials previously shown to be good supports for methanotrophs. Rather than increase the tarp thickness with another geotextile layer, such materials were incorporated into the tarp construction. The amendments may have acted as an additional gas distribution layer, which has been shown to be an important design consideration in biofilters (Huber-Humer, 2004). They may also function as methanotrophic host matrices (Huber-Humer et al., 2008) and generally enhance CH4 oxidation (Abichou et al., 2006a; Barlaz et al., 2004; Gebert et al., 2003; Humer and Lechner, 2001b; Mor et al., 2006; Stern et al., 2006; Streese and Stegmann, 2003; Zeiss, 2006). As natural materials, the amendment surfaces may have been preferable to those offered by synthetic geotextile fibers. It is well known that soil microbes, including methanotrophs, can require trace micronutrients or by-products from other microbes to flourish (Jones, 1997). Methanotrophs have been found with a wide array of other organisms, including annelids, fungi, and heterotrophic bacteria (Chiemchaisri et al., 2001; Hery et al., 2007; Watzinger et al., 2008), in landfill cover soil and biocover. The compost and soil amendments may also have provided missing unknown micronutrients or trace constituents not present in the NMS support matrix. In sum, the effects of each amendment represented the composite influence of additional methanotrophs plus other possible nutrients or surface property factors that could contribute to the baseline performance of a four layer geotextile. There was no attempt to calibrate the number or growth stage of the methanotrophs in each amendment (although the number of methanotrophs in the NMS used to soak the shale amendments was known). Therefore, the statistical comparisons between treatments reflect performance differences between the treatments as configured, but they do not isolate the specific factor that contributed to the differences. While such factors were not identified in this study, a related study using methanotroph specific probes and exopolymeric-specific stains showed that cells were dispersed throughout the geotextile matrix (Adams, 2009). Although the number of cells present could not be determined with this method, microscopic examination of stained geotextile sections showed that there was room for further cell densification. The addition of amendments to the bio-tarp did increase CH4 removal, with removal rates of 6.0 to 7.4 g m 2 d 1. Although these oxidation rates are lower than the reported 100–200 g CH4 m 2 d 1 emission rates from open landfill cells in France (Bogner, personal

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communication), they are higher than emission rates of 0.053 g m 2 d 1 reported for open landfill cells in California with an active gas extraction system (Bogner et al., accepted for publication). Further design modifications and optimization of the biotarp prototype would be needed to increase the CH4 removal rate to levels suitable for open landfill emission rates. Despite evidence of CH4 removal in the laboratory, field experiments failed to replicate the results from the laboratory simulations. The testing protocol was modified in a variety of ways to test several hypotheses that could explain the inability of the bio-tarp to outperform control tarps in the field. The physical integrity of the flux chambers was examined to ensure that there was no short-circuiting at the chamber base or cover seal. Several pre-conditioning steps were tested to increase the initial tarp moisture or refresh it at the time of flux measurement. An acclimatization period was also provided to accustom newly applied methanotrophs to CH4 for 24 h before placement in the field. Similarly, tarps were placed in the chambers with the lids loosely affixed for 24 h before measurements began. Other pilot trials were undertaken where the tarps were sprayed with additional nutrients just before testing, and more repeated measures were collected in an attempt to overcome the high variability among flux readings. The large spatial and temporal CH4 flux variations observed here (Table 3) are consistent with reports from other sites (Abichou et al., 2006a,b; Boeckx et al., 1996; Bogner and Spokas, 1993; Bogner et al., 1997; Borjesson et al., 2000; Czepiel et al., 1996). These variations, however, made collection of analyzable data difficult. Care was taken to only collect data when the soil was dry and to ensure that the chambers were sealed properly. Despite careful control and operation of the test systems, CH4 fluxes taken just 10–15 min apart showed a high degree of temporal variability. In the end, it was judged that it was not possible to collect enough sequential flux data under constant conditions to overcome the variability between readings. This does not imply that a bio-tarp could not respond to widely varying fluxes; it simply means that that more controlled flux conditions than were available to us were needed to discern statistically significant differences between biotarps and controls. No attempt was made to test a bio-tarp with more than four layers of geotextile because it was judged that any further thickness would be functionally infeasible. Interestingly, the field tests showed that there was high potential for the NMS-wetted fourlayer controls to reduce CH4 emissions. While a dry assemblage of the composite geotextiles offered little resistance to CH4 fluxes at the test sites (data not shown), the wetted control tarps reduced emissions by an average of 55–80%. This was likely a physical phenomenon wherein the CH4 dissolved in the water layer, which slowed its passage through the pore spaces.

5. Conclusion The purpose of this research was to test the feasibility of using a methanotroph embedded bio-tarp designed to serve as an ADC and mitigate CH4 emitted from an open landfill cell. Composite prototype bio-tarps comprised of multiple layers of water absorbent geotextiles with and without other amendments were tested under laboratory and field conditions. While there was clear CH4 oxidation activity exhibited by bio-tarps tested in the laboratory trials, no statistically significant activity was observed in the field trials. Because the field site was characterized by highly variable fluxes at a given chamber and also between chambers, the testing paradigm proved to be unsuitable for generating conclusive data about the field performance of the bio-tarps. Different testing conditions where consistent fluxes are present or artificially created

and maintained outdoors will be necessary to properly assess bio-tarp performance against controls. Nevertheless, these studies introduce the concept of bio-tarps as a potential means to mitigate open cell CH4 emissions and suggest various avenues for further investigation. Future research should address: identification or development of new bio-tarp materials that might be thicker but lightweight and that might include highly durable natural materials; exploration of the stimulatory factors offered by the amendments tested here and perhaps exploitation of synergistic effects among them; methods to further densify the cells applied to the bio-tarp; more controlled flux testing conditions; and perhaps some accommodation for timed nutrient release to stimulate methanotroph growth and activity. This work also suggests that there is a need for more thorough documentation of CH4 emissions from open landfill cells, through soil, and ADC. Aknowledgments This work was supported by the US Department of Energy (grant number: DOE Award Number: DE-FC26-05NT42433). The authors would like to thank Mr. Gary Abernathy for the fabrication of the continuous flow chambers. References Abichou, T., Chanton, J., Powelson, D., Fleiger, J., Escoriaza, S., Lei, Y., Stern, J., 2006a. Methane flux and oxidation at two types of intermediate landfill covers. Waste Manag. 26, 1305–1312. Abichou, T., Powelson, D., Chanton, J., Escoriaza, S., Stern, J., 2006b. Characterization of methane flux and oxidation at a solid waste landfill. J. Environ. Eng. 132, 220– 228. Adams, B.L., 2009. The Development of a Methanotrophic Alternative Daily Cover To Reduce Early Methane Emissions. University of North Carolina at Charlotte, Charlotte. Adams, B.L., Hamm, J., Oliver, J.D., Hilger, H., 2006. Assessment of immobilization techniques for methanotrophic bacteria in a biotarp. In: Intercontinental Landfill Research Symposium. Gällivare, Sweden. Aziz, C.E., Georgiou, G., Speitel Jr., G.E., 1999. Cometabolism of chlorinated solvents and binary chlorinated solvent mixtures using M. trichosporium OB3b PP358. Biotechnol. Bioeng. 65, 100–107. Bachelet, D., Nueue, H.U., 1993. Methane emissions from wetland rice areas of Asia. Chemosphere 26, 219–237. Barlaz, M.A., Green, R.B., Chanton, J.P., Goldsmith, C.D., Hater, G.R., 2004. Evaluation of a biologically active cover for mitigation of landfill gas emissions. Environ. Sci. Technol. 38, 4891–4899. Boeckx, P., van Cleemput, O., Villaralvo, I., 1996. Methane emission from a landfill and the methane oxidising capacity of its covering soil. Soil Biol. Biochem. 28, 1397–1405. Bogner, J., Spokas, K.A., 1993. Landfill CH4: rates, fates, and role in global carbon cycle. Chemosphere. 26, 369–386. Bogner, J.E., Spokas, K.A., Burton, E.A., 1997. Kinetics of methane oxidation in landfill cover soil: temporal variations, a whole-landfill oxidation experiment, and modeling of net CH4 emissions. Environ. Sci. Technol. 31, 2504–2514. Bogner, J.E., Spokas, K.A., Chanton, J.P., 2011. Seasonal greenhouse gas emissions from engineered landfills: daily, intermediate, and final California cover soils. J. Environ. Qual., accepted for publication. Borjesson, G., Danielsson, A., Svensson, B.H., 2000. Methane fluxes from a Swedish landfill determined by geostatistical treatment of static chamber measurements. Environ. Sci. Technol. 34, 4044–4050. Chen, A.-C., Ueda, K., Sekiguchi, Y., Ohashi, A., Harada, H., 2003. Molecular detection and direct enumeration of methanogenic Archaea and methanotrophic Bacteria in domestic solid waste landfill soils. Biotechnol. Lett. 25, 1563–1569. Chiemchaisri, W., Wu, J.S., Visvanathan, C., 2001. Methanotrophic production of extracellular polysaccharide in landfill cover soils. Water Sci. Technol. 4, 151– 159. Council of the European Union, 1999. COUNCIL Directive, 1999/31/EC of 26 April 1999 on the landfill of waste. Council of the European Union, ed. Czepiel, P.M., Mosher, B., Crill, P.M., Harriss, R.C., 1996. Quantiyfing the effect of oxidation on landfill methane emissions. J. Geophys. Res. 101, 16721–16729. Fredenslund, A.M., Kjeldsen, P., Scheutz, C., Lemming, G., 2007. Biocover – reduction of greenhouse gas emissions from landfills by use of engineered bio-covers. Kalmar ECO-Tech, Kalmar, Sweden. Fredenslund, A.M., Kjeldsen, P., Scheutz, C., Pederersen, G.B., 2008. Design and performance of a full scale biocover to reduce greenhouse gas emissions from Faske landfill in Denmark. In: International Landfill Research Symposium, Copper Mountain, CO.

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