Use of a biologically active cover to reduce landfill methane emissions and enhance methane oxidation

Abstract

Biologically-active landfill cover soils (biocovers) that serve to minimize CH₄ emissions by optimizing CH₄ oxidation were investigated at a landfill in Florida, USA. The biocover consisted of 50 cm pre-composted yard or garden waste placed over a 10–15 cm gas distribution layer (crushed glass) over a 40–100 cm interim cover. The biocover cells reduced CH₄ emissions by a factor of 10 and doubled the percentage of CH₄ oxidation relative to control cells. The thickness and moisture-holding capacity of the biocover resulted in increased retention times for transported CH₄. This increased retention of CH₄ in the biocover resulted in a higher fraction oxidized. Overall rates between the two covers were similar, about 2 g CH₄ m⁻² d⁻¹, but because CH₄ entered the biocover from below at a slower rate relative to the soil cover, a higher percentage was oxidized. In part, methane oxidation controlled the net flux of CH₄ to the atmosphere. The biocover cells became more effective than the control sites in oxidizing CH₄ 3 months after their initial placement: the mean percent oxidation for the biocover cells was 41% compared to 14% for the control cells (p < 0.001). Following the initial 3 months, we also observed 29 (27%) negative CH₄ fluxes and 27 (25%) zero fluxes in the biocover cells but only 6 (6%) negative fluxes and 22 (21%) zero fluxes for the control cells. Negative fluxes indicate uptake of atmospheric CH₄. If the zero and negative fluxes are assumed to represent 100% oxidation, then the mean percent oxidation for the biocover and control cells, respectively, for the same period would increase to 64% and 30%.

Introduction

Atmospheric CH₄ has more than doubled in concentration over the last 150 years (Schlesinger, 1997, Dlugokencky et al., 2003). Methane is the third most important greenhouse gas after water vapor and carbon dioxide, accounting for approximately 20% of positive forcing relative to CO₂ (Hansen et al., 1998). Over a period of 100 years, the global warming potential, or GWP, of CH₄ is 23 times that of an equal mass of CO₂ (IPCC, 2001).

Landfills are estimated to account for about 35% of anthropogenic CH₄ emissions in the United States and 5–10% of global CH₄ emissions to the atmosphere (Stern and Kaufmann, 1996, EIA, 2000, USEPA, 2000, IPCC, 2001, Czepiel et al., 2003). Landfills represent a large CH₄ source with a potential for mitigation through management practices. The difference between global atmospheric sources and sinks of CH₄ is less than 6% of the total CH₄ production. Therefore, even a small reduction in anthropogenic CH₄ emissions would be significant (Dlugokencky et al., 1994, Dlugokencky et al., 1998, Etheridge et al., 1998, Dlugokencky et al., 2003). In addition, the relatively short atmospheric lifetime for CH₄ (7–10 yr) means that the beneficial effects of management schemes to reduce emissions could be observed in a relatively short period of time.

Over the last half century, solid waste disposal in developed countries has been largely transformed from open dumping and burning practices to sanitary landfilling, consisting of engineered burial of waste with use of cover materials and management of leachate and gas. As a result, decomposition of solid waste proceeds anaerobically with the microbial generation of large quantities of CH₄ by methanogenic microorganisms. Methane emissions from landfills are in part controlled by the rate of oxidation as CH₄ is transported through the aerobic soil cover materials on top of the landfill. Oxidation of CH₄ is achieved by aerobic methanotrophic microorganisms that consume CH₄ and oxidize it to CO₂. As CH₄ migrates through aerobic soil layers, the residual CH₄ become increasingly enriched in ¹³CH₄ due to the preferential consumption of ¹²CH₄ (Barker and Fritz, 1981, Coleman et al., 1981, Happell et al., 1994, Tyler et al., 1994, Happell et al., 1995, Liptay et al., 1998). In this study, we exploited this fractionation to calculate the % oxidation (Chanton et al., 1999).

Methane oxidation is controlled by several factors, including soil temperature, moisture, and texture, as well as pH and nutrient content (Kightley et al., 1995, Boeckx et al., 1996, Chanton and Liptay, 2000, Borjesson et al., 2001). Previous studies have shown seasonal variation in CH₄ oxidation, which is greater during warmer months (Chanton and Liptay, 2000, Borjesson et al., 2001). Oxidation is also higher in organic-rich soils than in clay (Chanton and Liptay, 2000). In addition, there appears to be an optimum soil moisture for CH₄ oxidation, 10–20% (w/w) at temperatures from 25 ° C to 30 °C (Whalen et al., 1990, Boeckx et al., 1996). One incubation study of composted municipal solid waste used as landfill cover showed a high percentage oxidation at a soil moisture content of 45% (w/w) (Hilger and Humer, 2003).

Soil composition is also an important parameter, as soil texture and grain size affect oxygen diffusion into landfill cover soils. Coarser grained soils and porous mulch have been found to be superior to finer grained soils and clays. Methane oxidation in landfills can be enhanced by the emplacement of a biologically active compost or mulch cover (Humer and Lechner, 1999, Hilger and Humer, 2003, Barlaz et al., 2004).

This study examines CH₄ emission and oxidation in landfill soils with and without a “biocover,” a biologically active layer of mulch placed over a gas dispersion layer on top of an existing interim cover soil. The purpose of this biocover is to optimize the environment for methanotrophic bacteria. The biocover must be sufficiently permeable for oxygen transport but also have good moisture-holding capacity. The depth of oxygen penetration controls the depth and thickness of the zone of CH₄ oxidation. Moreover, at greater depth, oxidation can typically proceed under more stable moisture and temperature regimes (Hilger and Humer, 2003). In an Austrian landfill, the pioneering work of Humer and Lechner, 1999, Huber-Humer, 2004 showed that a 1 m layer of sewage sludge composted with woodchips overlying a 0.3 m gas dispersion layer can mitigate CH₄ emissions of several hundred g m⁻² d⁻¹. Positive results were also seen from a biocover consisting of 1 m yard waste mulch underlain by 0.15 m tire chips and 0.15 m clay placed at the Outer Loop landfill in Louisville, Kentucky, USA (Barlaz et al., 2004).

At the Leon County landfill (Florida, USA), the site of this study, it was previously shown that just 15 cm of mulch (composted yard waste and woodchips) overlaying a clay cover significantly increased CH₄ oxidation (Chanton and Liptay, 2000). We hypothesized that a relatively thin (∼50 cm) biocover consisting of the same material would, likewise, be more effective in oxidizing CH₄ than untreated landfill soils. Here we present the results of flux and oxidation field measurements for biocover and control cells for one annual cycle beginning in March 2004 and ending May 2005.