Colorado Adopts New State Regulations For Methane Emissions From Landfills

Key Takeaways

What Happened? On December 18, 2025, the Colorado Air Quality Control Commission (the Commission) adopted the Air Pollution Control Division's (the Division) proposed Regulation 31, establishing state regulations to reduce methane emissions from municipal solid waste (MSW) landfills. Colorado's new regulations go beyond existing federal regulations by requiring more landfills to install gas collection and control systems (GCCS), implement stronger monitoring requirements, and phase out open flares.

Who is Affected? Owners and operators of active, inactive, and closed MSW landfills in Colorado, as well as third parties that own or operate MSW landfill gas treatment or combustion equipment. The new regulations are expected to cover up to 32 of the state's 82 MSW landfills, including both open and closed sites.

Next Steps? Landfill owners, operators, and landfill gas project developers should start taking the following measures to ensure compliance with applicable deadlines:

• Confirm whether their facilities fall within the scope of Regulation 31 and whether any exemptions apply.
• Calculate waste-in-place, and, for landfills with more than 450,000 short tons of waste-in-place, methane generation rates. These initial reports are due June 30, 2026.
• Plan for open flare replacement, enclosed flare installation, and potential use of biofilters at closed landfills, including timely submission of any required permit applications. Applicants should plan sufficient time for the Division to process and approve related permit applications.
• Develop or update monitoring programs, including surface, leak, and remote methane monitoring, as well as cover integrity programs.
• Review contracts with third-party landfill gas purchasers to clarify compliance responsibilities.

Background

The Commission designed Regulation 31 to reduce methane emissions from MSW landfills to help Colorado meet statutory greenhouse gas reduction goals established in Colorado Revised Statutes § 25-7-102(2)(g). Regulation 31 is part of a recent trend of states tightening landfill air regulation, and has some similarities to rules adopted in recent years by California, Maryland, Michigan, Oregon, and Washington.

Scope and Applicability

Regulation 31 applies to MSW landfills that have received solid waste after November 8, 1987, excluding those that satisfy certain limited exemption conditions. Landfills that meet or exceed specific waste-in-place and methane emissions thresholds must implement requirements intended to reduce methane emissions. The Commission anticipates that approximately 17 landfills will be required to install a GCCS.

Additionally, any third-party entity that purchases or receives landfill gas from a covered MSW landfill and operates stationary equipment to treat or combust that gas is responsible for complying with the requirements that apply to its equipment.

Municipal or county-owned MSW landfills with less than 8 million tons of waste-in-place as of December 31, 2025, are allowed three additional years to comply.

Reporting and Emissions Control Requirements

• Landfills subject to Regulation 31 must submit an initial Waste-in-Place report by June 30, 2026. Those reporting less than 450,000 tons must track waste monthly and submit annual reports to the Division by March 31 of each following year.
• Landfills with at least 450,000 tons of waste-in-place must also submit a methane generation rate report by June 30, 2026.
• Landfills that generate at least 664 metric tons of methane per year are subject to additional compliance requirements. Depending on the circumstances at each site, these landfills may be required to install a GCCS, calculate the average methane concentration of their landfill gas, and begin and/or expand their quarterly surface emissions monitoring.
• Closed landfills with emission combustion devices, such as flares, must install biofilters when those devices are removed, to ensure ongoing methane control.
• Owners and operators required to install or upgrade their GCCS must submit new or revised Design Plans to reflect these changes. Regulation 31 sets out more detailed requirements for the Design Plan's contents than the federal rules, and generally requires more rapid installation and operation of a compliant GCCS than the federal rules.

Open Flare Phase-Out

Regulation 31 phases out the use of open flares at MSW landfills. Operating an open flare at an MSW landfill is prohibited as of January 1, 2029, unless the owner or operator demonstrates that one of several specified conditions applies and receives written approval from the Division. Limited exceptions include when landfill gas flow rate and methane concentration are insufficient to support operation of an enclosed flare; use of an open flare during repair or maintenance of the GCCS; and use of an open flare as a backup or secondary control device to a treatment system or non-open-flare control device.

Surface, Leak, and Remote Methane Monitoring

Surface Monitoring: Regulation 31 requires more granular surface methane monitoring. Routine surface monitoring must follow a 25-foot spacing pattern. If a landfill does not record any exceedances of the specified emission limits during four consecutive quarterly monitoring periods, it may expand the pattern to 100-foot spacing for subsequent monitoring, subject to certain exceptions. Regulation 31 also requires "integrated" surface emissions monitoring – where average emissions are calculated across defined grids on the landfill's surface – in addition to instantaneous monitoring. For integrated monitoring, Regulation 31 establishes an emissions limit of 25 ppm for each defined grid area. Landfills may rely on EPA-approved monitoring methods, such as drone technology under OTM-51.

GCCS Leak Inspection: Owners and operators of GCCS systems must conduct quarterly leak monitoring of all GCCS components operating under positive pressure, leachate collection risers, and knock-out drums. Leak monitoring must begin when a newly required GCCS starts operating, or no later than April 1, 2027, for systems installed before January 1, 2026.

Remote Methane Monitoring: Regulation 31 also establishes procedures under which owners and operators may be notified of remote methane monitoring detections identified by the Division or third parties. All remote detections must use Division-approved technology or technology that has obtained third-party certification under EPA's Methane Super Emitter Program for remote monitoring.The Division will notify landfill owners or operators electronically of remote methane detections within seven days of receiving monitoring data.Owners or operators must investigate a remote methane detection within five calendar days of receiving the Division's notice.They must report the investigation results and any corrective actions to the Division within 15 calendar days of receiving the notification, using a Division-approved form.

Cover Requirements and Integrity Programs

For active MSW landfills with a GCCS, Regulation 31 sets new cover standards. For new intermediate cover installed on or after July 1, 2026, that will remain in place longer than 180 days, owners or operators must use a biocover or a cover that incorporates a biocover, subject to approval as an alternative cover by the Hazardous Materials and Waste Management Division, or apply at least 18 inches of earthen or other suitable material as intermediate cover, subject to approval, and comply with related cover integrity requirements. Landfills may not use petroleum-contaminated soils or soils containing volatile organic compounds for intermediate or final cover. Additionally, Regulation 31 elaborates on the requirements for monthly cover integrity monitoring, specifying that the monitoring must include protocols to identify and address exposed waste, leachate breakouts, and erosion gullies.

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