Landfill gas emissions

Basics

Landfill gas basics

Landfill gas is a roughly 50/50 mixture of methane and carbon dioxide by volume with trace amounts of many other compounds. It is created through the microbial decomposition of degradable (putrescent) carbon compounds under anaerobic (oxygen free) conditions, and has the potential to cause or contribute to a number of health, environmental and aesthetic problems if not captured and treated before escaping to the atmosphere. These include odors, potential explosion hazards, ground-level ozone formation and global warming.

Consequently, since 1988, Wisconsin's solid waste rules have required that municipal solid waste landfills extract and treat landfill gas. Nearly all of Wisconsin's operating MSW landfills and many of its closed MSW landfills currently extract and burn landfill gas either by “flaring” it (burning in the open air) or utilizing it to generate energy.

Landfill gas extraction systems are required to be installed incrementally as waste is added to the landfill. Generally speaking, extraction systems should be installed in areas of landfills that reach final waste grades. These systems generally consist of vertical or horizontal perforated pipes installed within the waste mass and connected to a blower. (Despite its name, a blower actually applies a vacuum to the perforated pipes and pulls the gas out of the waste.) Upon installation, properly operated landfill gas extraction systems are able to collect the majority of landfill gas produced. However, they cannot prevent the release of all landfill gas.

Landfill gas can also move laterally away from a landfill. To prevent this from happening, for over two decades Wisconsin has required that the base and below-ground sidewalls of landfills be lined with multiple feet of compacted clay. Since 1998, the state has required landfill operators to place a thick plastic membrane over the clay. This landfill liner creates a barrier to the lateral movement of landfill gas and greatly reduces the explosion risk to nearby structures.

A number of larger landfills route gas that is collected to turbines or internal combustion engines to produce electricity. A few others pipe their landfill gas to nearby industries for steam production or compress it for use in vehicles. Several landfills in southeastern Wisconsin have been producing electricity for more than 30 years.

The DNR's Waste and Materials Management Program held a number of listening sessions with stakeholders in the early 2000s in an effort to help guide future policy efforts. A theme heard in those meetings was concern over landfill gas emissions. The general consensus was that the program's past efforts had been primarily directed at groundwater protection, and given the strides made in this area, greater focus can and should now be placed on the air impacts associated with landfills. The stakeholders were interested in reducing air impacts and, if possible, doing so in a non-traditional fashion that might lead to greater environmental results than regulation could achieve alone.

Based on these sessions and on other internal DNR meetings, the Waste and Materials Management Program launched an initiative to reduce estimated uncontrolled or “fugitive” landfill gas emissions.

Known as the “Landfill Gas Initiative,” it involved no new regulations, and partially relies on voluntary actions by landfill owners. A main focus is increasing public access to monitoring results and ensuring better adherence to specific aspects of approved landfill plans that could lead to uncontrolled landfill gas emissions. In addition to the monitoring results, the DNR has also posted estimated landfill gas generation rates for the 36 Wisconsin landfills required to collect it. The “Limitations” tab provides additional information regarding these calculations.

The following link provides much more detailed information on the basics of landfill gas generation, safety and health issues, monitoring and control measures:

• Landfill Gas Primer - An Overview for Environmental Health Professionals [exit DNR]

Measuring landfill gas

Estimating landfill gas generation

Accurately estimating gas generation from landfills is difficult due to many factors, including variations in types and quantities of waste accepted, and variation in landfill design and operational methods. In addition, measuring flow rates of gases consistently in gas collection infrastructure can be challenging, and the accuracy of some readings may be variable.

Even with these limitations, a comparison of estimated landfill gas generation and reported landfill gas extraction can provide some idea of how well landfills are limiting fugitive landfill gas emissions through the use of collection systems. Therefore, the DNR has selected the method presented below to estimate landfill gas generation and evaluate potential landfill gas emissions. To be clear, the DNR's estimates are for landfill gas generation rates only and not emissions rates directly.

To estimate landfill gas generation rates, the DNR has chosen to use a model called Landfill Gas Emissions Model (LandGEM) version 3.02, developed by the U.S. Environmental Protection Agency as a screening tool.

• LandGEM User's Guide [PDF exit DNR] (includes detailed information about the selection of model parameters)

Assumptions used in Wisconsin application of LandGEM

With the selection of the LandGEM model, the DNR has made the following assumptions.

Only the tons of putrescible or decomposable waste accepted by a landfill have been entered into the model because nonputrescible wastes generally contribute very little to landfill gas generation. If you're familiar with the waste categories used in Wisconsin for annual tonnage reporting, the categories 1, 3, 5 and 6 were included for this analysis. The model was originally calibrated by the EPA based on inclusion of all waste tonnages. However, for the purposes of emissions estimation, the EPA now allows use of only putrescible waste tonnages.

• Complete list of DNR waste category codes [PDF]

Two key variables in the model are the methane generation rate constant (k) and the methane generation potential per volume of waste (Lo). k represents the rate of methane generation and Lo represents how much of the waste is subject to decomposition and conversion to methane. The higher the k value, the faster the rate of methane generation; the higher the Lo value, the more the waste type is suited to generate methane.

For estimation of landfill gas generation, the EPA recommends that k be set at a value of 0.04/year and Lo at 100 m3/megagram of waste. These are the values used in applying LandGEM to all municipal solid waste landfills in Wisconsin. (Values of .05/year and 170 m3/megagram respectively are required by the EPA to be used for determining the applicability of federal air rules pertaining to larger landfills. Use of these higher inputs to the model approximately doubles an operating landfill's estimated landfill gas generation.) The DNR will be reviewing choices for k and Lo as time progresses. Other values may subsequently be chosen for specific landfills based on evaluation of actual gas extraction rates and information contained in landfill organic stability plans required under s. NR 514.07(9)(a), Wis. Adm. Code.

Many landfills in Wisconsin have been approved to recirculate leachate. Recirculating leachate by applying it to the surface or within the subsurface of the landfill, generally increases microbial activity and enhances landfill gas generation. For landfills that have been recirculating leachate for at least a few years, allowing for sufficient data, gas generation was estimated twice with the LandGEM model. The second estimate accounts for the effect of leachate recirculation. Gas generation was modeled from the year recirculation started to the present using a value of k = 0.08/year, while Lo remained at 100 cubic meters/megagram of waste.

The only empirical landfill gas extraction rate information currently available for most landfills are instantaneous measurements recorded periodically at various points of the landfill gas extraction and treatment infrastructure, as required by approved operational plans. Locations (points) at which readings are taken include before or after blowers, flares and gas plants. Landfills are required to submit this and other data to the DNR’s Groundwater Environmental Monitoring (GEMS) database annually. To extrapolate to annual rates, each year's measurements are averaged and the average applied to the extraction rate 24 hours per day for the number of days in that year the landfill gas extraction and treatment systems were operational.

(Note: Some years ago at the request of landfill owners, the DNR allowed the option to submit annualized extraction values calculated from measurements not based on GEMS data under the assumption that these alternative measurements would be more accurate. In most of these cases, the data provided was that which facilities are required to submit to the DNR's Air Management Program’s Air Emissions Inventory.)

Because the LandGEM model requires that methane concentrations in landfill gas be equal to 50% by volume, the DNR has adjusted, or "normalized," the landfill gas extraction rate information from landfills to 50% methane as well.

For example, if a landfill reported that it was extracting 500 ft3/minute of landfill gas at a methane concentration of 43% by volume, the DNR would adjust the extraction rate as follows: 500 ft3/min (43/50) = 430 ft3/minute. If the measured methane concentration was, for example, 55% by volume, the adjustment would then be 500 ft3/min (55/50) = 550 ft3/minute.

Limitations

Limitation of the LandGEM model

This landfill gas information is an attempt to provide some transparency and context to the measurement of the control of landfill gas compared to estimates of the volume of landfill gas generated at landfill facilities. There are limitations to this approach both from the generation model used, as well as, the methods used to collect and interpret the data from the landfill gas collection systems.

Landfill gas generation rates are not the same as gas emission rates. Landfill gas generation refers to methane generated by anaerobic microorganisms acting on organic material in the waste of the landfill. Landfill gas emissions refer to, in this case, methane escaping to the atmosphere from the surface of the landfill. Based on research findings, it is clear that at least some of the methane generated in the waste, and perhaps considerable amounts depending on the circumstances, is converted to carbon dioxide by oxidizing bacteria as it migrates through the landfill cover soils before escaping to the atmosphere.

The modeled landfill gas generation curves presented here estimate methane generation in the waste mass and may not represent emissions of methane from the surface of the landfill to the atmosphere, as alluded to above. Landfill gas generation is a biochemical process that is influenced by a number of factors including but not limited to waste composition, moisture content, temperature and pH.

Estimating the effect of recirculation using the LandGEM model on gas generation introduces the potential for additional error and uncertainty. The use of k = 0.08/year and Lo = 100 cubic meters/megagram in the LandGEM model may not represent many sites in Wisconsin very well and may be prone to overestimate gas generation. For this reason, gas generation rates predicted for leachate recirculation could be considered on the high end of the estimation spectrum. Furthermore, for various reasons landfills that were granted approval did not always recirculate leachate consistently. In many instances, leachate may only be recirculated in portions of a landfill’s total site. The gas generation lines accounting for the effect of recirculation for these sites were modeled under the “ideal” assumption that recirculation was applied consistently across the landfill total site and were not meant to reflect the degree to which leachate was actually applied.

The use of modeling to project performance often requires a number of assumptions made for the variable chosen. The use of variables is one of the limitations of models in general as is the selection of constants within the model. The need to make assumptions and the nature of the system being modeled are what make modeling complex. There can be significant variation in waste types, site operations, moisture, temperature, etc. not only on a site-by-site basis, but also over time at the same facility. The value used for k as well as Lo has an impact on the outcome of the model and by running multiple curves under different criteria we can begin to discern the sensitivity of the model to various factors and assumptions.

The data used to generate the graphs presented here is subject to change and revision.

Monitoring data

Modeled landfill gas generation and measured landfill gas collection graphs and tabular data (by landfill type)

Modeled landfill gas generation and measured collection graphs for the 36 individual landfills are accessible from the list below. The landfills are grouped under four categories defined by ownership, size and operational status. The categories are privately owned, large municipally owned, small municipally owned and closed.

The years plotted in the graphs range from 1979 to 2020. The units are million cubic feet (of landfill gas generated)/year, with landfill gas being generally defined as 50% methane by volume.

The graphs were created to examine the potential relationship between modeled landfill gas generation, measured landfill gas collection and waste tonnage received by the landfills over time. An annual waste tonnage graph displaying tonnages (thousands of tons/year) for both total waste tonnage received by the landfill and putrescible tonnage is included with each gas generation and measured collection graph. On the waste tonnage graphs, the green line represents total waste tonnage consisting of waste categories 1–6 and 19–28. The brown line represents the portion of the total that is decomposable waste, categories 1, 3, 5 and 6.

• Complete list of DNR waste category codes [PDF]

On the modeled landfill gas generation and measured extraction graphs, modeled gas generation is represented by the blue line and measured gas collection is represented by the brown line. A few of the landfills do not have gas collection systems and display only modeled gas generation. (Gas extraction is not represented on six of the sites because sufficient data is not yet available or an extraction system was not installed).

Sites having a history of leachate recirculation include an additional line (colored green) displaying gas generation but with the model adjusted to account for the effect of recirculation. See the tab labeled "Measuring landfill gas" for an explanation of how the model was adjusted. The date recirculation was approved is indicated on the graph. For various reasons, landfills that were granted approval did not always recirculate leachate consistently or over the entire landfill area. The gas generation lines accounting for the effect of recirculation in these graphs were modeled under the "ideal" assumption that recirculation was applied consistently across the entire landfill and may not reflect to what degree leachate was actually applied.

For a few landfills, the total gas extracted was measured at two different gas extraction “points” (for example, a blower and a flare). Gas extracted from each point is displayed separately as brown dashed lines in addition to the solid brown line representing the total gas extracted. The point type and/or point ID numbers are shown in the graph legend.

Graphs displaying modeled gas generation and measured extraction as well as waste tonnage for the 36 individual landfills are listed below.

Note: The gas generation curves in these graphs were created using a model subject to variability and may underestimate or overestimate gas generation. Discretion is advised when viewing these graphs. The DNR will periodically update the graphs. The current versions include data through 2020.

Privately owned landfills

Large municipally owned landfills

Small municipally owned landfills

Closed landfills

Summary data

The data used to create the graphs has been aggregated (summed over landfills) and is presented in four tables representing the four categories discussed above, and a fifth table containing data aggregated over all 36 landfills. Annual aggregate values displayed from 1979 to 2020 include Modeled LFG Generation Rate (ft3/yr), Number of Landfills Modelled, LFG Collection Rate (ft3/year), Number of Landfills Collecting LFG, Total Waste (tons) and Decomposable Waste Only (tons).

• PDF version of landfill gas data summary tables [PDF]
• Excel version of landfill gas data summary tables [XLSX]