MSW Management | The Fine-Tuning of Landfill Gas

SUBSCRIBE

COMMENT
ON THIS
ARTICLE

CREATE A LINK
TO THIS ARTICLE
ON YOUR SITE

LFG collection efficiency is improving in Wisconsin.

Michael S. Michels and Gerard M. Hamblin

Wisconsin has 32 landfills that actively receive MSW. Twenty-four of these landfills are required to collect landfill gas (LFG) via active LFG collection and control systems (GCCS). A study was conducted to determine how the LFG collection efficiency at these 24 landfills is changing over time.


	Table 1

The LFG industry has long struggled with calculating the “real” LFG collection efficiency. While this study does not supply the “real” LFG collection efficiency, it does utilize a qualitative approach to track efficiency over time to reveal trends in the data. This is the first study of its type to review LFG collection efficiency trends on a statewide level. This approach can be replicated by other states to determine their statewide LFG collection efficiency trend and to help focus on sectors that need improvement.

Results of this study confirm that Wisconsin’s LFG collection efficiency is improving. For the years 2000 through 2004, the statewide LFG collection efficiency continuously improved from 77.3% to 85%. Further subdivision of the 24 landfills was made into two groups: 1) landfills owned by private industry, and 2) landfills owned by municipalities.

Wisconsin’s 24 landfills accepted over 7.5 million tons of decomposable waste in year 2004. This tonnage data, dating back to 1988, was input into the EPA LandGEM model to estimate the statewide LFG generation. Wisconsin landfills were then contacted to obtain their annual LFG collection rates for the years 2000 through 2004.

This study concluded that: (1) LFG collection efficiency in Wisconsin is improving over time, even as more and more waste is deposited in landfills every year; (2) the 2004 LFG collection efficiency at 24 landfills averaged 85%, and (3) the LFG collection efficiency of the privately owned and municipally owned landfills in Wisconsin is generally the same.

Definition of LFG Efficiency
A rigorous approach to calculating LFG efficiency would be to conduct field test with devices, such as flux chambers, over widespread areas of a landfill and for several seasons. In this study it is not necessary to determine the “real” collection efficiency, but to use a qualitative approach to reveal trends in statewide LFG collection efficiency over time. The general approach used here to determine LFG collection efficiency is to divide the LFG collected by the LFG that is generated. LFG collected is quite simple to determine based on flow meters installed just prior to the control equipment. Estimating LFG generation is much more involved due to the abundance of uncertainties and variables. As stated, field data was not collected for this paper in order to determine LFG generation rates; instead computer modeling was done.

Computer modeling of LFG generation has long been known to be uncertain. In order to minimize modeling uncertainty, all landfills were reviewed as a single facility and an average “k” and “Lo” was used in EPA’s LandGEM model.

This averaging approach normalizes the fact that all landfills in the state have different LFG generation characteristics (such as moisture, pH, particle size, or waste types) and allows a qualitative assessment of LFG collection efficiency trends over time. Since the purpose of this study was to use a qualitative approach, the trends are real but the specific LFG collection efficiencies reported herein should be scrutinized.

Wisconsin Regulatory Requirements
This section summarizes the current rules requiring LFG collection at Wisconsin landfills and Wisconsin Department of Natural Resources (WDNR) draft program to increase LFG collection efficiency. Wisconsin Administrative Code NR 500 requires MSW landfills in the state to install and operate landfill gas collection systems if the design capacity of the landfill is greater than 500,000 cubic yards. The EPA’s new source performance standards (NSPS) and emission guidelines (40 CFR 60.752) require MSW landfills with a design capacity greater than 2.5 million megagrams, and with emission greater than 50 megagrams per year of nonmethane organic compounds to install and operate an LFG collection system.

In year 2003, WDNR adopted an environmental management system (EMS) as apart of ISO 14000 certification. This EMS established goals for the Bureau of Solid Waste that moved it beyond compliance. One goal of the Bureau of Solid Waste EMS was to voluntarily reduce the uncontrolled release of LFG emissions from active landfills (i.e., improve the efficiency of LFG collection systems). The WDNR’s draft guidance indicates more than 85% collection efficiency is desired. The WDNR’s guidance also indicates that the use of the EPA LandGEM model or surface emission monitoring is the desired approach to make this demonstration.

In order to assess the impact of WDNR’s desired LFG collection efficiency on Wisconsin landfills a review of existing LFG collection efficiency was needed. This paper should be considered the first step in assessing existing LFG collection efficiency for landfills located in the State of Wisconsin.

Approach
The approach taken to prepare this paper was:

Tabulate the tonnage of decomposable waste (i.e.: waste that generates LFG), since 1988, that was disposed in Wisconsin landfills;
Estimate the amount of LFG generated using the EPA LandGEM model and the amount of decomposable waste;
Tabulate the amount of LFG actually collected at Wisconsin landfills; and
Calculate and review the historic LFG collection efficiency.

In order to be consistent with the WDNR’s EMS, the only landfills considered in this assessment were the ones that met all of the following criteria:

Landfills that received decomposable MSW (no papermill monofills were considered)
Landfills that accepted waste in 2003 (start of the WDNR’s EMS)
Landfills that were federally or state-required to have an active LFG collection system in the year 2003

Utilizing these criteria, 24 landfills were considered in this assessment. These landfills include nine municipally owned and 15 privately owned landfills.

Waste Tonnage
This section summarizes the amount of decomposable waste that is deposited in Wisconsin landfills. This data is used in subsequent sections of this paper to estimated LFG generation rates. In Wisconsin, solid waste permits require that landfills report waste tonnage deposited by various categories. Waste tonnage information was supplied by WDNR in the form of Microsoft EXCEL spreadsheets. (Note: Some of the WDNR supplied tonnage information may not have included corrections for recycling.) Each MS EXCEL spreadsheet contained a single year of tonnage data for all landfills in the state. The data was sorted to include only the landfills that met the criteria noted above and the data was filtered to include only decomposable waste materials.

Waste materials included as decomposable are: MSW, pulp/papermill, POTW sludge, and “other.” The inclusion of all “other” waste may introduce some error (this error may result in an overprediction of LFG generation and the reporting herein of a lower than actual LFG collection efficiency) because not all parts of that waste are completely decomposable.

Waste types not included in this assessment, because they are believed to not contribute to LFG generation were: ash, foundry waste, fee-exempt waste, industrial waste used for daily cover, shedder fluff, and contaminated soils.

Tonnage data from 1988 to 2004 was considered in this assessment. While the waste from prior to 1988 may also be generating small amounts of LFG flow, it is believed that the amount of LFG contribution from this old waste is insignificant due to its age. In addition, the way waste tonnage was reported prior to 1988 is not as accurate as more recent data due to the changes in waste category definition over time. Therefore, not considering waste older than 1988 in this assessment is not considered to significantly affect the results.


	Table 2
	Table 2 (continued)

A year-by-year summary of the decomposable waste deposited in Wisconsin landfills is provided in Table 2, and a graphic of the waste tonnage is shown in Figure 1. Based on the tonnage data provided by WDNR, nearly 87 million tons of decomposable waste was deposited in these 24 landfills from 1988 thru 2004.

Nearly half of the decomposable waste deposited during this period is deposited in six landfills (i.e., Onyx Emerald Park, Republic Kestrel Hawk, Republic Mallard Ridge, WM Metro, WM Orchard Ridge, and WM Pheasant Run). These six landfills are all located in the southeastern portion of Wisconsin.

During the period of analysis (1988 thru 2004), 21% by weight of the decomposable waste went to municipally owned landfills, 79% went to landfills owned by private companies (WM, Republic, Allied, and Onyx).

Estimates of LFG Generation
This section of the paper predicts how much LFG is generated from all Wisconsin landfills including subsets: 1) privately owned landfills and 2) municipally owned landfills. Determining the amount of LFG generated involved modeling with numerous assumptions. Research has shown that the amount of LFG generated varies depending on many items, but primarily the type/quantity of waste and the moisture content of the waste. Other factors that have less significance but are still important to LFG generation include pH, landfill temperature, and waste particle size.

These estimates were prepared using a computer model, LandGEM, created by the EPA. LandGEM is based on a first-order decomposition rate equation for quantifying emissions from the decomposition of landfilled waste in MSW landfills. The software provides a simple approach to estimating landfill gas emissions. The EPA recommends that LandGEM not be used for determining LFG collection efficiency, however, since WDNR adopted it for its EMS, this is the method used in this paper.

The LandGEM model requires only three inputs, as follows: annual waste tonnage, “k” (the methane generation rate), and “Lo” (methane generation potential). Annual waste deposited from 1988 through year 2004 in Wisconsin was utilized as described in the previous section of this paper. The EPA’s LandGEM model recommends a “k” value of 0.02 per year to 0.07 per year.

Due to the average amount of rainfall received in Wisconsin, as compared to the other 50 States, a “k” value of 0.04 was selected for this paper. EPA’s LandGEM model recommends a “Lo” value of 96 to 170 cubic meters per megagram of waste and indicates the more organic waste in the landfill waste stream results in a higher Lo. A Lo of 100 was selected for this paper.

A k of 0.04 and Lo of 100 are considered by EPA as the AP-42 defaults and EPA believes these are representative of the average landfill in the USA. This author believes that a k of 0.04 and Lo of 100 are reasonable modeling assumptions for reviewing Wisconsin LFG generation, as a whole.

However, this author does not believe this k and Lo values should be used to model individual landfills because the waste moisture and waste types vary too much from landfill to landfill.

LFG Collection Rates
This section summarizes how much LFG has been collected since year 2000. Determining the actual amount of LFG collected is quite simple to obtain from flow meters installed by the landfill owners (assuming that the flow meters are accurate and properly calibrated).


	Table3

LFG Collection for All Landfills—Table 3 tallies the annual amount of LFG collected at all 24 landfills from year 2000 through 2004. The data in Table 3 was obtained via various methods, including flow totalizer, average flow times, or operating hours, each with varying degrees of accuracy. Each landfill manger transmitted data to us via e-mail or personal communication. The results presented in Table 3 have not been corrected to 50% methane, due to the unavailability of this information.

Sauk County Landfill is not included in Table 3, because Sauk County is not required by their WDNR permit to maintain total LFG flow records; therefore, Sauk County could not accurately provide this information. While excluding Sauk County from the tally does introduce some error into the assessment we believe this error is minor because Sauk County Landfill is very small and only accepted 635,170 tons of decomposable waste during the period of assessment (1988 to 2004) or only 0.73% of the total waste collected among all 24 landfills.

For all landfills, combined, the amount of LFG collected has increased every year such that for year 2004 the landfill owners have collected over 13.4 billion cubic feet of LFG.

LFG Collection for Municipally Owned Landfills—Table 3 subtotals the annual amount of LFG collected at the 9 municipally owned landfills from year 2000 thru 2004. For these 9 landfills the amount of LFG collected has increased every year such that for year 2004 the municipal landfill owners have collected over 2.7 billion cubic feet of LFG. For 2003 and 2004, Brown County East Landfill collected the most LFG, of the municipal landfills at 0.63 billion cubic feet in 2004.

LFG Collection for Privately Owned Landfills—Table 3 subtotals the annual amount of LFG collected at the 15 privately owned landfills from 2000 thru 2004.

For these 15 landfills, combined, the amount of LFG collected has increased every year such that for 2004 the private landfill owners have collected over 10.7 billion cubic feet of LFG. WM’s Pheasant Run Landfill collected the most LFG in 2004, at 1.7 billion cubic feet.

LFG Collection Trends
This section and Table 1 summarize the trend in Wisconsin’s LFG collection efficiency from 2000 to the end of 2004.

From 2000 through 2004, for all 24 landfills, the LFG collection efficiency improved by 7.7% (from 77.3% in year 2000 to 85% in 2004).

This 5-year improvement in LFG collection efficiency resulted in 1.2 billion cubic feet of more LFG being collected in year 2004, had no improvement in LFG collection efficiency been made at all. Wisconsin landfill owners should be applauded for this improvement in LFG collection.

For years 2000 through 2004, the LFG collection efficiency (for all sites combined and both the municipal and private subdivisions) was slightly better than EPA’s AP-42 guidance which recommends an average LFG collection efficiency of 75.0%. Again, Wisconsin landfill owners are ahead of the average.

LFG collection efficiency in Wisconsin between the municipally owned and privately owned landfills is nearly the same; with the privately owned landfills having slightly better efficiency.

Findings
The most significant findings of this paper include the following:

Over 7.5 million tons of decomposable waste were deposited in the 24 study landfills during 2004. Approximately 87% of this waste was deposited in 15 privately owned landfills and approximately 13% of this waste was deposited in 9 municipal landfills during 2004.
In 2004, over 6.6 million tons of decomposable waste was deposited in 15 privately owned landfills, and these 15 landfills collected nearly 10.8 billion cubic feet of LFG.
In 2004, nearly 1 million tons of decomposable waste was deposited in Wisconsin municipally owned landfills, and the municipalities collected just over 2.7 billion cubic feet of LFG.
For the period of analysis (1998 through 2004) approximately half the decomposable waste was deposited at six landfills located in the southeast part of the State of Wisconsin.
LFG collection efficiency at Wisconsin’s 24 study landfills has improved nearly 8% since year 2000.
LFG collection efficiency at privately owned and municipally owned landfills has improved since year 2000 (7% and 9.5% respectively).
Comparing the calculated 2004 LFG collection efficiency to WDNR’s EMS voluntary goal of 85% shows that Wisconsin landfills have achieved the goal.

Recommendations
Recommendations for what can be done to refine the findings of this paper are as follows:

Normalize the actual methane concentration in the collected LFG to 50% methane; similar to the normalization that is done with LFG generation rates. Methane concentration data for normalization of the actual LFG flow was not available for this paper. Some of the municipal landfills are not required by permit to totalize the LFG flow. Instead they reported an average flow times the operating time per year and estimated the figure for the annualized LFG flow.
Select a few landfills in the state for further study of the LFG collection efficiency. Instead of using the qualitative approach by modeling the LFG generation rate and calculating the LFG collection efficiency, as done in this paper, measure the flux of LFG escaping collection via: surface emission monitoring, flux boxes, FTIR, and other methods. Select landfills for this study that have a geo cap, geo liners, good history of waste tonnage and type, good history of actual LFG collected via flow totalizer, and measured methane concentrations in their collected LFG.

The WDNR should factor the results of this paper into their EMS as follows.

For year 2004, this study shows that statewide 85% LFG collection efficiency is being achieved.

This is consistent with the EMS goal, therefore only periodic updating of the results by DNR needs to conducted in order to confirm these results over time.

LFG collection efficiency reported herein is likely higher, due to: 1) methane normalization that is not factored into the flow rates; 2) Sauk County’s actual LFG collection rate is not factored into the flow rates; 3) the qualitative nature of modeling can bias gas generation estimates high; and 4) no accounting for capture efficiencies that can occur in landfill covers.

Considering that the LFG collection efficiency continues to rise, current WDNR rules are working and achieving improved environmental protection.

The EMS goal of 85% LFG collection efficiency is being achieved in 2004. Raising the EMS goal to higher levels needs to carefully consider the cost to collect the remaining LFG as compared to the environmental protection that collecting such a small amount of LFG will provide.

As stated above, while some small amounts of LFG may be escaping collection, the LFG may not be emitted to the atmosphere.

Other authors have reported that emissions from landfills can also be controlled by soil covers that oxidize the LFG before it enters the atmosphere, thereby reducing emissions. Oxidation of LFG in the cover soils needs to be considered in the future EMS plans.

Also, surface emissions monitoring performed to meet NSPS requirements consistently show surface concentrations of methane near background levels.

Michael S. Michels is a professional engineer with Cornerstone Environmental Group LLC. Gerard M. Hamblin is a professional engineer with Waste Management of Wisconsin Inc.

MSW - March 2008