U.S. EPA Releases Significant Update to AP-42 Section 2.4 for Solid Waste Landfills

In May 2025, the United States Environmental Protection Agency (U.S. EPA) released a significant update to AP-42 Section 2.4 for Municipal Solid Waste Landfills and the associated LandGEM model for estimating air emissions from solid waste landfills. AP-42 Section 2.4 was last updated in November 1998 (with a draft update in 2008 that was not finalized). The 2025 updates reflect advancements in data collection, regulatory frameworks, and a continued focus on refining methane emissions calculation methods. This article provides an overview and comparison of the key differences between the new and previous versions.

1) Methane Generation Rate Equation:

1998 Version: Used a first-order kinetic equation with specific “k” (methane generation rate) and “L₀” (potential methane generation capacity) values derived from older datasets for solid waste landfills in the U.S. EPA database.

May 2025 Update: Removes the original first-order kinetic equation and replaces it with the Mandatory Greenhouse Gas Reporting Program (GHGRP) equation HH-1 from 40 CFR 98.343(a)(1) (Subpart HH). This update is designed to ensure consistent methane generation values across various U.S. EPA programs, particularly aligning AP-42 with the GHGRP, which is a mandatory reporting program for large sources of greenhouse gas (GHG) emissions. This is not merely a technical adjustment but a strategic policy choice by U.S. EPA to standardize methane generation calculations across its different regulatory programs. This harmonization streamlines compliance for facilities that must report emissions under both Federal air permitting programs that require use of AP-42 Section 2.4 procedures and the GHGRP, reducing the potential for discrepancies in emissions estimates and administrative burden, thereby fostering a more consistent and efficient national approach to landfill emissions quantification. This change also implicitly elevates the importance of GHGRR methodologies for all landfill emissions calculations.

While the equation itself is updated, the New Source Performance Standards (NSPS) under 40 CFR Part 60 and Emissions Guidelines (EG) under 40 CFR Part 62 still reference the older k and L₀ values from the previous AP-42 section, which may cause some confusion when preparing calculations for regulatory applicability analyses. ALL4 is not aware of a planned effort by U.S. EPA to update these rules in alignment with Subpart HH as of the date of this article.

Additionally, Equation 3 of AP-42 Section 2.4 was updated to allow for the incorporation of a site-specific fraction of methane volume in landfill gas. This change provides greater flexibility and accuracy when site-specific data is available, moving towards more tailored emissions estimates. The default average collection efficiency of 75% has been replaced with a directive for the reader to refer to Table HH-3 of Subpart HH for Landfill Gas Collection Efficiencies. This change emphasizes reliance on specific regulatory tables for gas collection and control systems (GCCS) efficiency calculations, ensuring consistency with other Federal requirements.

2) Non-Methane Organic Compound (NMOC) and Volatile Organic Compound (VOC) Default Concentrations:

1998 Version: Default NMOC/VOC values were based on older, smaller datasets (e.g., 18 landfills for no/unknown co-disposal sites). A 39% ratio of VOC to NMOC was used to adjust calculations for regulatory applicability purposes.

May 2025 Update: Updated default NMOC concentrations for co-disposal sites (prior to 1992) use 2,400 parts per million by volume (ppmv). NMOC concentrations for non-co-disposal sites opened before 1992 use 600 ppmv (previously 595 ppmv) while NMOC concentrations for sites opened during and after 1992 use 550 ppmv.

The May 2025 update clarifies that NMOC emissions are a good surrogate for VOC emissions, with less than a 50 ppmv difference, and removes the previous reference to a 39% ratio of VOC to NMOC. This reflects a more accurate understanding of the relationship between these compounds in landfill gas. U.S. EPA reviewed existing test reports for these updates, while addressing previous critiques regarding the data used for the original AP-42 section. The May 2025 update incorporates a larger and more complete dataset which includes 242 complete test reports from 116 landfills used for the 2019 Residual Risk Assessment conducted for proposed revisions of the 40 CFR part 63, Subpart AAAA National Emissions Standards for Hazardous Air Pollutants (NESHAP) for Municipal Solid Waste Landfills¹.

3) Emissions Factors for Combustion Sources:

1998 Version: Provides less-detailed or specific emissions factors for various combustion devices such as flare control devices and landfill gas-fired engines.

May 2025 Update: Adds six new emissions factors and revises five existing emissions factors for landfill gas flares, engines, and fugitive emissions sources. This update also combines emissions factors for pollutants from open flares and enclosed combustors and provides additional NMOC emissions factors for engines at various loads. Emissions factors for nitrogen oxides (NO_X) and carbon monoxide (CO) from these sources are also generally lower than the factors in the 1998 version.

Notably, the May 2025 Update provides new emissions factors with a “Highly Representative” emissions factor rating for enclosed flares, including for NO_X, NMOC, and CO. This is a new inclusion and a significant change for enclosed flare emissions factors, which had previous been given only a “C” rating in the 1998 Version.

4) Organization and Structure:

The May 2025 update also includes minor organizational changes, such as adding Section 2.4.5 for Source Classification Codes for municipal landfills and updated references to more recent federal regulations like NSPS/EG and 40 CFR Part 63 NESHAPs.

See the table below for a side-by-side comparison of the major updates to AP-42 Section 2.4:

 Updates to U.S. EPA LandGEM Model

The Landfill Gas Emissions Model (LandGEM) is designed to calculate emissions rates for total landfill gas, methane, carbon dioxide, NMOC, and hazardous air pollutants (HAP) from MSW landfills. In general, both AP-42 Section 2.4 and LandGEM work together to provide the emissions factors and methods and a tool that implements a model based on those principles. Both the May 2025 update to AP-42 Chapter 2.4 and LandGEM itself, as of its most recent publicly available version (LandGEM Version 3.1, 2024, with a User Guide from December 2024), have replaced the previous first-order kinetic equation with equation HH-1 from Subpart HH.

Here are the key aspects of the LandGEM Version 3.1 updates:

1. Core Functionality: LandGEM continues to be a macro-enabled Microsoft Excel file that allows users to calculate emissions using either site-specific data or default parameters.
2. Model Parameters: LandGEM relies on key parameters like the methane generation rate (k), potential methane generation capacity (L₀​), assumed NMOC concentrations, and methane content of landfill gas.
3. Equation Differences: LandGEM Version 3.02 (which predates 3.1) introduced a revised first-order decomposition rate equation that integrates emissions over a 0.1-year time increment, improving accuracy, especially for higher k This resulted in slightly lower emissions for typical k values compared to earlier versions. Version 3.1 continues with this improved calculation method.
4. Default Parameters: LandGEM continues to offer two sets of default parameters:

1. 1. Regulatory defaults: Based on federal Clean Air Act (CAA) regulations for MSW landfills, used for determining regulatory applicability.
   2. Inventory defaults: Based on AP-42 Section 2.4 emissions factors, used for emissions inventories and air permits.

5. Alignment with AP-42 Section 2.4: LandGEM now uses a first-order decay model based on equation HH-1 from Subpart HH, and its default parameters are intended to align with AP-42 Section 2.4. The updated default NMOC concentrations from the May 2025 update to AP-42 Section 2.4 are expected to be reflected in the latest LandGEM versions, or at least be the recommended inputs for users.

Ongoing Development/Considerations:

U.S. EPA is exploring a transition from an NMOC emissions threshold to a methane emissions threshold for requiring GCCS in future solid waste landfill regulations. This could influence how LandGEM is used for regulatory compliance in the future. There is ongoing discussion and research into improving methane emissions quantification, including the use of advanced monitoring technologies (like aerial monitoring) which may, in the long term, influence how models like LandGEM are refined or supplemented. However, this effort to refine both the regulatory applicability methodology and supporting tools such as AP-42 and LandGEM may be limited or halted altogether if U.S. EPA’s pending proposal to eliminate most subparts in 40 CFR Part 98 is adopted by the Federal government. This pending decision would separate both AP-42 Section 2.4 and LandGEM from their recently established basis in the GHGRP equation HH-1 from Subpart HH. This decoupling could create uncertainty over the appropriate emissions quantification methodology between regulatory agencies and affected facilities and would nullify U.S. EPA’s effort to streamline compliance for facilities that must report emissions under both Federal air permitting programs that require use of AP-42 Section 2.4 procedures and the GHGRP.

In summary, AP-42 Section 2.4 has made a direct shift to the GHGRP equation for methane generation, and LandGEM, as a modeling tool, has been updated to reflect the new default parameters and emissions factors provided in AP-42 Section 2.4. The key takeaway is that the latest LandGEM version incorporates the application of the newly updated values from AP-42, particularly for NMOC concentrations and potentially other parameters influencing emissions calculations. As the May 2025 update to AP-42 Section 2.4 and the LandGEM model are more widely utilized, it is possible that users will encounter other bugs, issues, or have more suggestions for improvements. If you have any questions about these updates or encounter an issue, feel free to reach out to Daniel Brese at dbrese@all4inc.com or your ALL4 project manager.

¹Residual Risk Assessment for the Municipal Solid Waste Landfills Source Category in Support of the Risk and Technology Review 2019 Proposed Rule. EPA’s Office of Air Quality Planning and Standards Office of Air and Radiation May 2019.