Solid waste landfills need to have final covers to 1) reduce the infiltration of rainfall into the waste mass and 2) reduce surface greenhouse gas emissions. Most regulations require that such final covers include hydraulic barriers, such as compacted clays with or without geomembrane. Significant research has been undertaken to allow the use of evapotranspiration-based covers (often termed: Evapotranspiration (ET) Cover, Water Balance Covers, or Phyto Covers) as an alternative to the barrier concept covers. ET covers are designed so that they have the capacity to store water by the soil and also have plants or vegetation to remove the stored water. In ET covers, plant roots can enhance the aeration of soil by creating secondary macropores which improve the diffusion of oxygen into soil. Therefore, biological methane oxidation (a natural process in landfill soils) can be improved considerably by the soil structuring processes of vegetation, along with the increase of organic biomass in the soil associated with plant roots. This paper summarizes a study to investigate the capacity of an ET cover to reduce surface greenhouse gas emissions when implemented on a solid waste landfill. This study consisted of using a numerical model to estimate methane emission and oxidation through an ET cover under average climatic conditions in Bennignton, Nebraska, USA. Different simulations were performed using different methane loading flux (5 to 200 gm -2·d -1) as the bottom boundary. For all simulations, surface emissions were the lowest during the growing season and during warmer days of the year. Percent oxidation is the highest during the growing season and during warmer days. The lowest modeled surface emissions were always obtained during the growing season. Finally, correlations between percent oxidation and methane loading into simulated ET covers were proposed to estimate methane emissions and methane oxidation in ET covers.
Final covers are used to reduce the quantity of water that infiltrates into waste deposits at landfills. Reducing the volume of infiltrating water reduces the amount of leachate that is generated and the risk of groundwater contamination. Generally, the applicable rules and regulations require that landfill covers employ resistive principles, i.e., layers having low saturated hydraulic conductivity such as compacted clay barriers, geosynthetic clay liners with or without a geomembrane. This design philosophy is often referred to as “raincoat”, barrier, or “umbrella” approach. Barrier type covers, such as compacted clay covers, have been shown to loose their impermeable qualities over time because of the influence of climate variations on the integrity of the soil layer [
Evapotranspiration (ET) covers are earthen final covers that control the percolation of infiltrating rainwater into the waste by balancing the water storage capacity of finer textured soils and the ability of plants to extract the water stored. They are designed to transmit equal or less percolation than the conventional resistive covers built with low hydraulic conductivity materials. Evapotranspiration (ET) covers (also referred to as: water balance covers, “store and release” covers, phyto-covers) are designed to have a capacity to store water by the soil and also to have the capacity to remove the stored water. Therefore, ET covers are essentially well-de- signed-vegetated soil landfill covers. The soil layer (s) of an ET cover stores moisture during rain events, and the sun and plants remove the water from the soil by evaporation and evapotranspiration, returning the moisture to the atmosphere.
Under landfill setting, final covers are also exposed to landfill gases from the waste mass below. Landfill gas is composed of 50% by volume of methane. Landfill gas is transported by diffusive and convective flux mechanisms from the anaerobic zone toward the atmosphere. The microbiological process of oxidizing CH4 to CO2 by methanotrophs is called microbial CH4 oxidation. Several previous studies on landfill CH4 oxidation in soil cover layers have demonstrated the ability of methane oxidation as a mechanism to reduce CH4 emissions from landfill surfaces [
In ET cover context, methane oxidation is impacted by vegetation, water content and soil temperature. The soil water content and the soil temperature in the soil profile govern gas transport and therefore methane oxidation in ET covers. Water content in the ET cover soil profile is a very important factor affecting CH4 oxidation in landfill cover soils. First, the optimum environment for CH4-oxidizing bacteria (methanotrophic) is obtained within a specific water content range. Second, water content affects the penetration of O2 into cover soils and O2 is the main reactor for CH4 oxidation. As water content increases in soil, O2 diffusion into the soil is hindered. Third, water content affects the air-filled porosity of soil thus influences gas transport. As water fills up voided pores within the cover soil, it blocks the flow of gas upward. [
The potential superior capability of ET covers to reduce fugitive release of methane to the atmosphere is an additional characteristic that should be considered to encourage the use of the types of covers. The basis of ET cover design is the predictive capabilities of the water balance modeling and the ability to quantify the maximum water infiltration from the atmosphere into the waste mass below the ET cover. ET covers provide a unique opportunity to introduce a modeling approach to methane oxidation in landfill covers since the modeling approach is already undertaken during the water balance assessment of the design. The objective of this study is to use a similar modeling methodology for methane oxidation to extend the water balance modeling-based design associated with ET covers to estimate gas transport and methane oxidation in and across an ET cover. The modeling approach consists of modeling gas transport using our existing gas transport and oxidation model. Our model accounts for dynamic parameters associated with the change in water content and temperature caused by change in climatic conditions at the upper boundary of the ET cover. The model also incorporates the changing temperature and water content to methanotrophic activity in the soil. The numerical model (FSU Model) is developed by our research team at Florida State University and combines water and heat flow with a gas transport and oxidation [
An ET cover was constructed and monitored as a demonstration project in Bennington, Nebraska. Bennington, NE, receives760 mm of rain per year. Snowfall is 740 mm. The number of days with any measurable precipitation is 92. On average, there are 216 sunny days per year in Bennington, NE. The July high temperature is around 30˚C. The January low is −11˚C. The project was intended to demonstrate that an ET cover as constructed in the demonstration project is functionally equivalent to the regulatory default cover based on water balance performance. The cover demonstration profile was constructed and instrumented in 2008. The instrumentation was activated during December 2008. The demonstration cover surface was then seeded in 2009. Surface vegetation and underlying rooting have been established. The soil moisture measurement equipment utilized on this project includes water content sensors and water potential sensors (soil suction). There were two sets of probes installed in instrumentation nests.
A numerical model (FSU Model) was developed by our research team at Florida State University that combined water and heat flow with a gas transport and oxidation model [
The new Land SEM model comprises four major modules, bundled via a graphical user interface:
A climate module which generates daily minimum and maximum air temperature along with daily rainfall based on the landfill site geographical location.
A soil property generation module which generates soil properties, e.g. saturated hydraulic conductivity, porosity, and methane oxidation capacity based on a built-in database of soil property data from the literature. The user can also input measured soil properties, if available.
A soil water content and temperature simulation module which uses the landfill site location, soil texture information and the climate generation data developed in the first two steps and then predicts the daily soil moisture and temperature at any depth of the soil cover profile for an average climatic year.
A core computational module, based on [
The climate simulator is a module adopted from the models Global Temp SIM and Global Rain [
The continuity equation describing the reactive transport of gas component i in landfill cover soils can be written [
where
time on right hand side represents the transport process while the second term accounts for the soil gas reaction (oxidation for methane).
The flux
where
The diffusion coefficient
where
where
The gas flow velocity is due to advection and is assumed to follow Darcy’s law.
where
where R is the universal gas constant (8.314
The reaction rate of CH4 is given
where
tors among which soil temperature and moisture content are more significant. Temperature, moisture, and scaling correction factors were considered for the kinetic parameters in Equation (7) as described in [
The simulated ET cover was divided into five30-cm-thick layers with different soil properties. The top layer was assumed to be 30 cm thick and represented by the 15 cm deep soil water and soil suction sensors. The second layer was also assumed to be 30 cm thick and be represented by the sensors buried at 45 cm, etc. The top 60 cm ET cover soil was assumed to have undergone several freeze-thaw and desiccation cycles. The long-term saturated hydraulic conductivity of the top 30-cm-thick layer was assigned to be 1 × 10−4 cm/s. The second layer was a signed to have a saturated hydraulic conductivity of 1 × 10−5 cm/s. The third layer was assumed to have a 10 times lower saturated hydraulic conductivity. The hydraulic conductivity of the lowest layer was measured from an undisturbed sample as 2.7 × 10−7 cm/s. The hydraulic conductivity of the second layer from the bottom was assumed to be 5 times higher than the bottom layer. The porosity of the soil profile was assumed as follow: from top to down 0.45, 0.42, 0.40, 0.38, and 0.38. The long term unsaturated soil properties are shown in
One of the major inputs to the gas transport model is the maximum methane oxidation capacity (a lab measured quantity Vmax) of the soil. This quantity represents the maximum capacity of bacteria in the soil to oxidize methane at room temperature. Based on our experience with measuring Vmax on landfill cover soils and for the purpose of this document, different simulations of two different values of Vmax (150, 300 nmol∙s−1∙g−1 dry soil) were performed. Several simulations were performed using different rates of methane loading into the ET cover. For each Vmax, several simulations were performed using methane loading varying from 2 to 200 gm−2∙d−1. The landfill gas entering the bottom of soil profile was partitioned equally to CH4 and CO2 v/v.
The output of the gas transport model consists of daily surface emissions of methane and daily percent oxidized for each simulation performed with each methane loading flux (MLF) during the modelled average one year period in Bennignton, NE. The daily simulations was repeated for different MLFs (2, 5, 10, 20, 50, 100, 200 gm−2∙d−1) and with each Vmax.
during colder periods of the year. The higher percent oxidation and the lowest surface emissions occur during the growing season and during the spring and summer seasons indicating the effects of soil temperature and water contents on surface emissions and therefore methane oxidation.
The data presented in
The shape of the graph in
The same simulation results were used to calculate the average percent year methane oxidation, and plotted versus MLF for each Vmax in
One way to estimate uncontrolled emissions or fugitive emissions from the surface of landfills is to use the AP- 42 protocol proposed by the United States Environmental Protection Agency (USEPA, 1997). AP-42, when applied to landfill methane surface emissions, can be written as follows:
where CMP is the controlled mass emissions of methane (kg/year), UMP is the methane generation from the waste mass in kg/year (estimated by LANDGEM or any other allowed model),
One application of this model is to be used in conjunction of AP-42 guidelines but with a modeled methane oxidation,
sions, along with quantifying the improvement of performance (emissions and oxidation) of ET covers. Such protocol can help decision makers quantify additional methane oxidation capabilities of ET covers. The modeled percent oxidation for each Vmax was expressed as a power function of methane loading flux (MLF) as shown in
TarekAbichou,TarekKormi,ChengWang,HaykelMelaouhia,TerryJohnson,StephenDwyer, (2015) Use of Evapotranspiration (ET) Landfill Covers to Reduce Methane Emissions from Municipal Solid Waste Landfills. Journal of Water Resource and Protection,07,1087-1097. doi: 10.4236/jwarp.2015.713089