Colorado Finalizes Landfill Methane Reduction Rule

On December 18, 2025, the Colorado Air Quality Control Commission (AQCC) approved the Colorado Department of Public Health and Environment (CDPHE), Air Pollution Control Division’s (Division) proposed rule establishing requirements for methane emissions reductions from municipal solid waste landfills (MWSL) (Regulation 31). This rule is a result of over a year of work by a technical working group, along with public participation, and is part of Colorado’s Greenhouse Gas Pollution Reduction Roadmap 2.0.

What Landfills are Subject to Regulation 31?

The rule applies to active MSWL that received solid waste after November 8, 1987 and closed landfills that have a gas collection and control system (GCCS) that closed before November 8, 1987. MSWLs that are owned by a municipal or county government and have less than eight million short tons of waste-in-place as of December 31, 2025 will be allowed an additional three years to comply with date-based requirements. Additionally, third-party entities that purchase or receive landfill gas from an MSWL and operate equipment for the treatment of combustion of gas from an MSWL subject to this regulation will have to meet certain requirements.

MSWLs that meet the following criteria are exempt from the requirements of Regulation 31:

• Located on tribal lands,
• Receive only construction and demolition debris, inert material, or non-decomposable solid waste,
• Do not have a GCCS installed, closed before October 31, 1993, and have less than 450,000 short tons of waste-in-place or a design capacity less than 2,7500,000 short tons and 3,260,000 cubic yards, or
• Are regulated under the Resource Conservation and Recovery Act (RCRA) Subtitle C, or the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA).

Reporting Requirements

Owners or operators of active, inactive, or closed MSWL must submit an initial waste-in-place report to the Division by June 30, 2026, and annually by March 31 each year after the initial report. Active landfills with less than 450,000 short tons waste-in-place must track and record total waste-in-place on a monthly basis and submit annual waste-in-place reports. If the MSWL becomes inactive with a waste-in-place of less than 450,000 short tons, the owner or operator must submit the waste-in-place report before March 31 of the following year and will not be required to submit further reports. If the MSWL meets or exceeds 450,000 short tons, the owner or operator must report to the Division within 90 days and meet the methane generation calculation requirements described below.

MSWL owners or operators that meet or exceed 450,000 short tons of waste-in-place on or before December 31, 2025 must calculate the calendar year 2025 methane generation rate based on waste-in-place at the end of the calendar year and submit the methane generation rate to the Division no later than June 30, 2026 and annually by March 31 thereafter. For MSWL that meet or exceed 450,000 short tons of waste-in-place after December 31, 2025, methane generation rates must be calculated and reported for the waste-in-place at the end of the preceding calendar year. The timing of the report submittal is dependent on the quarter in which the threshold was met or exceeded.

Gas Collection and Control Systems

MSWL that have a calculated methane generation rate greater than or equal to 664 metric tons may be subject to additional compliance requirements depending on landfill status including:

• Methane concentration evaluations,
• Quarterly surface emissions monitoring, and
• Installation of a GCCS.

Landfills that require the installation or upgrade of a GCCS must submit a Design Plan for Division approval. The Design Plan must meet all the requirements specified in Regulation 31 and be prepared and certified by a Colorado Professional Engineer. Active MSWL that are required to install a GCCS must install and operate the system within 18 months of the submission deadline of the Design Plan or request an extension in writing to the Division for approval.

When an enclosed flare is installed to control methane emissions from the landfill, the flare must meet additional requirements, including achieving a 99% methane destruction efficiency. Compliance demonstrations must be performed by conducting an initial performance test within 180 calendar days of startup of the system and annually thereafter. There are many other compliance requirements applicable to enclosed flares such as combustion temperature operating range, gas flow measurement, and visual inspections.

For systems utilizing an open-tip flare, the device must be equipped with a device to detect the continuous presence of a pilot light or flame among other requirements. Perhaps more importantly, open tip flares at MSWL will be prohibited on and after January 1, 2029, unless certain conditions are met and an application for continued use of the open flare is approved by the Division.

Methane Monitoring and Leak Detection

Quarterly surface emissions monitoring must be performed utilizing one of the technologies listed in Regulation 31. Owners or operators may also use an alternative technology approved by the Division or the United States Environmental Protection Agency (U.S. EPA), allowing for proven and emerging technologies to be employed. Readings of greater than 200 parts per million by volume (ppmv) or parts per million meter (ppm-m), depending on monitoring method, must be recorded, while readings greater than 500 ppm or ppm-m must be addressed, including initiating corrective actions.

Additional quarterly leak inspections are required on the GCCS, utilizing Division approved technologies. Component leaks greater than 500 ppm or ppm-m must be tagged, recorded, and repaired. First attempts at repair must be made no later than five calendar days after discovery with completion and re-monitoring no later than 15 calendar days after discovery, unless parts aren’t available.

Other Operational Requirements

Regulation 31 prescribes the use of biocover or biocover techniques for landfill cover. MSWL with greater than or equal to 450,000 short tons of waste-in-place must implement a cover integrity monitoring program and implement repairs or maintenance monthly no later than July 1, 2026 if the landfill had at least 450,000 short tons of waste-in-place by December 31, 2025.

Landfills subject to Regulation 31 must maintain prescribed records for five years and comply with detailed reporting requirements outlined in the rule.

Next Steps

Initial compliance requirements are quickly approaching. MSWL owners and operators should confirm whether your landfill is subject to Regulation 31 or if any exemptions apply, perform waste-in-place calculations, and prepare to submit initial reports which are due on June 30, 2026. Based on applicability and the specifics of your landfill, you must develop monitoring plans and programs following the requirements in the rule, and if currently using an open-tip flare as part of a GCCS, plan for replacement with enclosed flares before January 1, 2029.

ALL4 has an experienced team that can work with your landfill to determine applicability, develop compliant monitoring plans, support flare conversions, and develop continuous flare monitoring programs to meet the new requirements. If you have any questions or need help evaluating compliance strategies for Colorado Regulation 31, reach out to Brian Taylor at btaylor@all4inc.com to discuss how ALL4 can help.