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Additional Article Content

• Table One
• Table Two
• Table Three
• Table Four
• Table Five

Landfill gas (LFG) is formed as a natural byproduct of the decomposition of wastes in landfills. To recover its energy value and minimize its pollutant emissions, many landfill managers have installed LFG recovery and utilization systems. 

At past SWANA LFG Symposia, numerous papers have been presented that measure the LFG collection efficiency, which is defined in this report as the amount of LFG (or methane) that is collected in the LFG recovery system, as compared to the amount generated. Collection efficiencies of over 90% have been reported. 

This article presents highlights of a report resulting from an investigation into the topic of gas collection efficiencies of LFG recovery systems. The report study was conducted by staff with the SWANA Applied Research Foundation (ARF), with input and guidance provided by the ARF LFG Project Sponsors. (The ARF was founded in 2001 with the purpose of conducting collectively defined and funded applied research on pressing solid waste issues. It is funded by local governments and other organizations that contribute a “penny per ton” of waste managed on an annual basis. For more information on the SWANA Applied Research Foundation, please contact Jeremy O’Brien, director of applied research, SWANA, 301-585-2898.)

LFG is generated by the biological decomposition of the organic fraction of landfilled solid waste. The primary constituents of LFG are methane and carbon dioxide. However, LFG also contains a number of trace constituents, including hydrogen sulfide, water vapor, ammonia, and a variety of volatile organic compounds (VOCs). 

LFG is typically extracted with wells drilled into the completed areas of a landfill. Drilled wells are generally limited to completed fill areas because wellhead facilities, valves, and monitoring ports are incompatible with active filling. To a limited extent, other types of vertical collectors have been raised in active fill areas as new lifts are constructed and eventually interconnected. Because of the time required to attain final fill grade, horizontal collectors (sometimes referred to as trenches) may be installed as an interim measure.

Microbial gas generation is highly sensitive to a number of factors, such as moisture, temperature, oxygen, and the refuse waste degradability. For these and other reasons, including the size and depth of the landfill, it is difficult to determine the LFG generation rate through the use of generic computer models. Despite these difficulties, landfill managers have historically used LFG generation rates predicted by computer models in estimating the collection efficiencies of their LFG recovery systems. 

In 2002, the EPA commissioned one of its contractors—Eastern Research Group Inc.—to conduct a review of available data and industry information regarding LFG collection efficiencies. The internal memorandum that resulted from this task (draft memorandum from Chad Leatherwood of Eastern Research Group Inc. to Brian Guzzone and Meg Victor of the US Environmental Protection Agency, dated October 24, 2002) summarized the data and information available at that time regarding the efficiency of LFG collection systems. The memorandum notes that the Agency’s AP-42 emission factor document states that: “Reported collection efficiencies range from 60% to 85%, with an average of 75% most commonly used.” (AP-42 emission factors are estimates of emission rates that are developed by the EPA in compliance with Section 130 of the Clean Air Act. These factors serve as fundamental tools in the development of national, regional, state, and local emissions).

Important points contained in the memorandum include the following:

• Overall, there are minimal data on LFG collection efficiency. Industry contacts cited the difficulty in documenting uncontrolled LFG emissions as the primary reason.
• Documenting uncontrolled LFG emissions is problematic because: 1) Emissions can migrate horizontally as well as vertically and can be released from almost anywhere on the surface of the landfill cell; 2) LFG generation rates are highly variable due to the heterogeneity of MSW and variations in rainfall and landfill temperature.
• As a result, LFG emission levels are site specific and vary over time as well as space across the landfill surface. Therefore, the collection of samples, which are representative of the entire landfill over a long period of time, is extremely difficult.
• Finally, LFG systems historically have been satisfied with capturing a majority of the LFG generated. As a result, LFG system owners and/or operators have not been particularly interested in expending additional efforts in trying to determine actual LFG emissions.

The memorandum included actual LFG collection efficiency estimates from Pacific Energy for three California landfills. These estimates were 85%, 90%, and 95%, and were the only numerical estimates identified by ERG for individual landfills in its research.

Literature Review
Of the numerous publications reviewed for this project, three directly addressed the issue of LFG collection system recovery efficiencies and provided quantitative estimates of LFG recovery efficiencies that were significantly higher than the 75% average recovery efficiency stated in AP-42. These three publications—two of which were published in 2006 and the third in 2005—are summarized below. 

A paper titled, “Methane Mass Balance at Three Landfill Sites: What is the Efficiency of Capture by Gas Collection Systems?” (Spokas, K. et. al., Waste Management, Vol. 26, Issue 5, 2006, pp. 516–525) presents the findings resulting from in-depth field investigations of the generation and fate of methane gas (CH₄) in seven landfill cells at three French landfills. The project presented in the paper was referred to as the METAN program, established in France to study methane mass balances in actual landfill settings. Financial support for METAN was provided by the French environment agency, Agence de l’Environnement et de la Maitrise de l’Energie (ADEME) and Veolia Environment.

As noted in the paper, once generated within a landfill, LFG methane can take one of five pathways. It can:

• migrate horizontally;
• be emitted from the landfill surface;
• be oxidized by bacteria in the soil within the landfill or at the surface of the landfill;
• be collected in the LFG collection system; or
• be temporarily stored within the landfill.

The corresponding “mass balance” equation that accounts for these variables is:

CH₄ generated = CH₄ emitted + CH₄oxidized +
CH₄ recovered + CH₄ migrated + CH₄ stored in the landfill

The project investigators attempted to account for the LFG generated at each landfill cell by measuring or estimating the quantity of methane that was diverted through each of these pathways. For each landfill cell, the measured values for each of the parameters except the storage parameter were inserted into the mass balance equation. The value of the storage parameter (i.e., methane storage in the landfill) was then set to the value needed to balance the equation. That calculated value was then compared to the maximum value that was estimated for methane storage. If the calculated value was less than the maximum value, then the measured values for each of the parameters in the methane mass balance equation were assumed to be reasonable.

The field studies at each of the three landfills were each conducted over a two-week period. Two study periods were conducted at one of the landfills to account for seasonal variations. The results of the field studies are summarized in Table 2. The following points are noteworthy:

• The methane recovery rate (i.e., the amount of methane generated that was collected in the LFG collection system for recovery) ranged from 84% to 98% and averaged 91%.
• The percentage of methane generated that was emitted to the atmosphere averaged 3.7%.
• On average, only 0.7% of the methane generated either migrated laterally or was oxidized in the landfill cover soil.
• As much as 15% of the methane generated was temporarily stored in the landfill.

As can be seen in Table 2, the landfill cell that had a geosynthetic clay liner/cover performed poorly with respect to methane recovery—recovering only 41% of the methane generated.

I

Palos Verdes Landfill

Another important finding is that an additional landfill mechanism—namely, the oxidation of methane by bacteria in the landfill cover soils—further reduces the amount of methane emitted to the atmosphere through the landfill surface. As a result of these two mechanisms—namely, LFG recovery systems in landfills capped with geomembrane and/or clay liners, and methane oxidation in cover soils, the amount of methane emitted to the atmosphere was determined to be less than 4% of the amount generated.

Another paper, “Measuring Landfill Gas Collection Efficiency Using Surface Methane Concentrations,” presented in 2006 by R. Huitric and D. Kong at the SWANA 2006 Landfill Gas Symposium in Saint Petersburg, Florida, presents an innovative approach developed by the Los Angeles Sanitation Districts for estimating the efficiency of LFG collection systems using surface methane concentration data. The Districts are a confederation of 25 independent special districts serving the water pollution control and solid waste management needs of more than 5 million people in Los Angeles County, CA. The Districts manage a comprehensive solid waste management system that includes three active sanitary landfills which dispose of approximately 16,000 tons per day, three closed landfills, three material recovery/transfer facilities, three landfill gas-to-energy facilities, two recycle centers, a refuse-to-energy facility, and participation in a second refuse-to-energy facility.

The study was conducted based on data collected in FY2001 at the Palos Verdes landfill (PVLF), a 291-acre landfill that was closed in 1980 after receiving 23.6 million tons of waste (see Figure 1). The PVLF, which is owned and was operated by the Districts, has a 5-foot-thick clay cover and an active gas-collection system.

Efficiency is defined by the study authors as “the ratio of collected-to-generated gas during the period of collection using a well-operated gas recovery system that fully extends throughout the landfill.”

In the 1980s, the Districts developed an integrated surface methane (ISM) monitoring methodology to determine surface methane emissions. This methodology was later adopted by the Southern California Air Quality Management District as one of its Rule 1150.1 requirements.

In this methodology, the landfill surface is divided into 50,000-square-foot grids (e.g., 200 feet by 250 feet). Each grid is monitored within 3 inches of the landfill surface in a looped fashion with readings taken every four seconds (approximately every seven feet) using an automatic methane reader and data logger. 

The surface emissions are monitored on a quarterly basis when wind conditions are below 5 miles per hour. To avoid higher winds speeds, monitoring generally starts early each day and terminates by noon. It takes a technician 12 days to walk the landfill and take the required measurements. In this way, precise and voluminous data (50,000 measurements per quarterly monitoring period) are collected. The data are then statistically analyzed and are corrected for background methane concentrations.

To correlate the measured emission levels to LFG emission generation rates, the Districts utilized the EPA’s Industrial Source Complex (ISC) model, one of the most often-used air quality models simulating air-dispersion mechanisms to study air-quality impacts.

For an “area” source such as a landfill, the ISC model demonstrates that the emission rate and the resulting emission levels are directly related in a linear fashion.

Using the model, the Districts estimated the surface methane concentration reduction resulting from LFG collection.

The LFG collection efficiency was calculated by the following equation:

E = ISM_r ÷ ISM_r + ISM_e

Where:
E = LFG collection efficiency
ISM_r = integrated surface methane recovered
ISM_e = integrated surface methane emitted

In this equation, the “surface methane recovered” term (ISMr) includes the methane collected through the landfill’s LFG recovery system as well as the methane oxidized in the landfill’s cover soils. Therefore, the LFG collection efficiency is defined to include LFG that is collected as well as LFG that is oxidized in the cover soils as compared with the total LFG that is collected, oxidized, and/or emitted to the atmosphere.

It should be noted that a key assumption in the Districts’ approach is that the LFG stored in the landfill is relatively constant over time.

Other assumptions are that the horizontal gas migration rate is negligible and that emission-monitoring data collected during the morning hours are representative of the daily emissions. (The Districts assumes that any migrating gases are intercepted and are collected in the LFG collection system; gas migration from the landfill, therefore, is negligible. This assumption appears reasonable in light of the Spokas study reviewed in this report.)

The results of the ISC model, which can be run in an “urban” or “rural” mode, are presented in Table 3 (these modes adjust for the relative amounts of dispersion associated with urban or rural settings). The collection efficiency estimated by the model was 93.2% (urban) and 96.4% (rural). 

In summary, based on coupling of surface emissions data measurements with the ISC model, the Districts found that the efficiency of the LFG collection and cover-soil oxidation systems at the PVLF is 93% to 96%.  Based on this project, the study authors concluded that:

“The commonly assumed default collection efficiency value of 75% is dated and does not reflect modern conditions for NSPS-regulated landfills and other landfills operated for emission control purposes.”

In the fall 2006, the Districts conducted a follow-up study that compared the results of the above approach—referred to as the Integrated Surface Methane/Industrial Source Complex Model (ISM/ISC) method—with the conventional Static Flux Chamber Emissions method (Huitric, R., Kong, D., Scales, L., Maguin, S., and Sullivan, P. “Field Comparison of Landfill Gas Collection Efficiency Measurements.” Proceedings—30th Annual Landfill Gas Symposium, March 5, 2007, Monterey, and CA. Silver Spring, MD: SWANA, March 2007).

The flux chambers consisted of square (1 meter²) stainless steel boxes that were placed at 10 representative locations on the surface of the landfill. The chambers include a pressure and temperature probes as well as mixing fans to ensure that the gas samples—taken via a syringe at five-minute intervals—were representative of the contents of the chamber. The flux chambers evenly divided between locations with average and peak surface methane levels as measured by the ISM method. This technique addressed the concern for spatial representation that arises when placing flux chambers on a landfill.

The flux chambers found no significant methane accumulation, indicating that LFG collection efficiencies of essentially 100%. These results correlated well with the results of the ISM/ISC method, which was also utilized and which showed LFG collection efficiencies of over 99.2% in 2006. 

The Districts attributed the increase in observed LFG collection efficiencies from 2002 (about 95%) to 2006 (over 99%) to improvements in the LFG collection system made during the intervening time.

“Baro-Pneumatic Estimation of Landfill Gas Generation Rates at Four Operating Landfills” (Bentley, H, Smith, S., and Schrauf, T. Proceedings, SWANA’s 28th Annual Landfill Gas Symposium. Silver Spring, MD: SWANA, March 2005. See also: www.hgcinc.com) describes the baro-pneumatic method and presents the results obtained at four MSW landfills located in the southeastern United States. 

The baro-pneumatic method attempts to directly measure the LFG generation rate through the correlation of the changes in gas pressures within the landfill with changes in barometric pressure. In this way, the effects of atmospheric pressure—shown to have a significant impact on measured emission and collection rates—can be normalized.

By developing actual, field-measured LFG generation rates and comparing them with known LFG recovery rates, the LFG collection system efficiency can be accurately determined—especially if a sufficient number of sampling locations are constructed and field measurements are taken seasonally and annually.

This approach requires the installation of several small-diameter monitoring probes in landfill refuse for multiple depths at locations evenly distributed over the landfill surface. Typically, 10 locations and 20 to 30 total monitoring points, placed at various depths in the landfill, are employed.

Pressures are measured at 10-minute intervals to obtain a continuous record of atmospheric and subsurface landfill pressures over a three- to four-day period—enough to observe the effects of six to eight diurnal atmospheric pressure peaks. Establishing the relationships between the pressure variations at the landfill surface and the monitoring probe pressure responses enables the estimation of landfill permeabilities and LFG generation rates. Short-duration (i.e., four hours) gas extraction tests are also conducted to obtain independent measurements of gas porosity and horizontal gas permeability.  

The pressures at the implanted probes respond to atmospheric pressure waves that have been imposed on the entire landfill surface and traversed a significant volume of the landfill. The large scale of the measurement tends to reduce the uncertainties normally associated with smaller-scale flow and pressure measurements in heterogeneous landfill materials. 

A numerical gas-flow model of the landfill is then constructed and calibrated by varying gas permeabilities to match the measured atmospheric and subsurface pressures. The observed landfill pressure gradients and the gas permeabilities obtained from the calibration process are incor¬porated into Darcy’s Law to determine LFG generation rates. A simple form of Darcy’s Law applied to LFG generation is:

LFGgen = –k_gA(∆P/∆z)

where:
LFGgen   is the landfill gas generation rate,
kg is the gas permeability,
A is the cross-sectional area between the measuring points,
∆P/∆z is the pressure gradient,
∆P is the pressure difference between the atmosphere and landfill monitoring point, and
∆z is the depth of the monitoring point below landfill surface. 

The baro-pneumatic method is based on the assumption that LFG generation rates are essentially constant over the three- to four-day monitoring period. 

The LFG generation rates obtained at various locations in the landfills are then used in conjunction with the characteristics of the waste disposed at those locations to construct and calibrate first-order LFG-generation models.

The decay equations for each of the landfills were thus provided with site-specific values of methane potential (L_o) and a first-order rate constant (k). Importantly, this approach requires no additional measurements to account for gas diverted through other pathways (e.g., surface emissions, methanotrophic consumption, gas migration, or gas storage) to determine the LFG generation rate. 

This approach was used to estimate LFG generation rates at four landfills in the southeastern United States.  LFG collection efficiency was determined for the one landfill studied—the North Shelby Landfill in Millington, Tennessee—that was equipped with an LFG collection system. The landfill’s LFG collection efficiency was obtained by dividing the measured gas collection rate by the LFG generation rate obtained from the baro-pneumatic method.

The North Shelby Landfill’s total LFG generation rate and LFG collection rate, reported in standard cubic feet per minute (scfm), and the percentage LFG collection efficiency are presented in Table 4. The calibrated first-order decay model for the North Shelby Landfill was found to have a methane potential L_o of 3,300 cubic feet per ton (103 cubic meters per metric ton) and a rate constant (k) of 0.078 year-1. By way of comparison, the use of the AP-42 default parameters, L_o = 100 cubic meters per metric ton and k = 0.04, resulted in a LFG generation rate of 2,853 scfm—16% less than the amount actually collected.  

Conclusions
The three field studies reviewed in this report provided quantitative estimates of the LFG collection efficiencies at five closed, capped landfills. As shown in Table 5, the average LFG collection efficiency for these five landfills is over 90%.  This average LFG collection system efficiency—which is based on recent field investigations—is significantly higher than the average LFG collection efficiency of 75% that is reported in the EPA AP-42 documentation.

Based on the data and findings presented in the project report, SWANA recommended that the EPA consider revising its AP-42 factors to reflect the higher LFG collection efficiencies that can be expected from MSW landfills that have traditional Subtitle D landfill cover systems and active LFG collection systems.

For more information regarding this or other research conducted by SWANA’s Applied Research Foundation, contact Jeremy O’Brien at jobrien@swana.org.