On Aug. 29, 2016, the EPA finalized a rulemaking updating the standards of performance and emission guidelines for Municipal Solid Waste (MSW) landfills. Under CAA Section 111(d), ADEQ developed a State Plan that updated ADEQ’s rules with the current emission guidelines for MSW landfills under ADEQ’s jurisdiction through incorporation by reference of 40 Code of Federal Regulations (C.F.R.) Part 60, Subpart Cf. The State Plan for implementing MSW landfill emission guidelines for Arizona was submitted to the EPA on July 24, 2018 | View Plan >

The EPA finalized its approval of ADEQ's State Plan on July 28, 2020 | View EPA Approval >

In 1997, the EPA finalized a rulemaking promulgating emission guidelines and compliance times for Hospital/Medical/Infections Waste Incinerators (HMIWI), that was codified at 40 C.F.R. Part 60, Subpart Ce. 62 FR 48379 (Sept. 15, 1997). On Nov. 16, 1999, ADEQ submitted a negative declaration letter to the EPA asserting that the State of Arizona contained no HMIWIs subject to the EPA’s final rule and therefore a State Plan would not be required. On June 22, 2000, the EPA approved ADEQ’s negative declaration referenced in 65 FR 38740 | View EPA approval 65 FR 38740 (June 22, 2000) >

On June 30, 2025, the EPA finalized its emissions guideline pursuant to CAA Section 111(d) that established new emission limits for Other Solid Waste Incinerators (OSWI) units. The purpose of EPA’s rule is to control emissions of ten different pollutants from OSWI units.. There are two categories of incinerators that are governed by this rule: 

1. The first category includes Very Small Municipal Waste Combustion (VSMWC) units, which are municipal waste combustion units that have the capacity to combust less than 35 tons per day of municipal solid waste or refuse-derived fuel. This category also includes air curtain incinerators that burn less than 35 tons per day of municipal solid waste. This category also includes air curtain incinerators located at institutional facilities burning any amount of institutional waste generated on-site. 
2. The second category includes Institutional Waste Incineration (IWI) units, which are any combustion units that combusts institutional waste and is a distinct operating unit of the institutional facility that generated the waste. This category also includes air curtain incinerators that burn less than 35 tons per day and are limited to burning specific materials collected from the general public and/or from residential, commercial, institutional, and industrial sources. These materials include wood waste, clean lumber, yard waste, or any combination of these materials. This category also includes air curtain incinerators located at institutional facilities that burn these same materials when they are generated on-site.

Under CAA Section 111(d) and the EPA’s final rule, Arizona is required to submit a State Plan addressing OSWI units no later than June 30, 2026. If there are no existing OSWI units in the state, the state may submit a negative declaration letter to the EPA in place of a State Plan | View Final Rule 90 FR 27910 (June 30, 2025) >

ADEQ will accept comments on the proposed negative declaration letter beginning April 3, 2026, through May 6, 2026, and at the public hearing on May 6, 2026. For more information on the public comment period and public hearing, and to view the proposed negative declaration letter, visit ADEQ’s OSWI public notice webpage | View Public Notice and Negative Declaration Letter >

On March 10, 2026, the EPA finalized its emissions guidelines pursuant to CAA Section 111(d) that established new emission limits for Large Municipal Waste Combustors (LMWC). The purpose of EPA’s rule is to control emissions of 11 different pollutants from LMWCs. The rule applies to municipal waste combustors that have the capacity to combust greater than 250 tons per day of municipal solid waste.

Under CAA Section 111(d) and the EPA’s final rule, Arizona is required to submit a State Plan addressing LMWCs no later than March 10, 2027. If there are no existing LMWCs in the state, the state may submit a negative letter to the EPA in place of a State Plan | View Final Rule 91 FR 11802 (March 10, 2026) >

On Mar. 8, 2024, the EPA issued its final emission guidelines for regulating methane emissions from existing oil and natural gas operations, which includes, but is not limited to, well sites, compressor stations, storage vessel/tanks batteries, and other operations related specifically to existing crude oil and natural gas operations. Federal Rule: 40 C.F.R. Part 60, Subpart OOOOc | View Guidelines 89 FR 17140 (Mar. 8, 2024) >

On Mar. 12, 2025, the EPA announced that it would be reconsidering the regulations proposed for the oil and natural gas sector | View Reconsideration Notice > 

Following reconsideration, the EPA published an interim final rule on July 31, 2025, that extended the State Plan submission deadline by 18 months to Jan. 22, 2027 | View Final Rule 90 FR 35966 (July 31, 2025) >  

For more information about the corresponding ADEQ rulemaking related to this State Plan and opportunities for involvement, please visit ADEQ’s active rulemaking webpage | Visit Rulemaking Page >

On May 9, 2024, the EPA published final carbon pollution standards for power plants that set carbon dioxide (CO₂) emission guidelines for existing coal-, oil- and gas-fired steam generating units. In this action, the EPA also repealed the Affordable Clean Energy (ACE) Rule. Federal Rule: 40 C.F.R. Part 60, Subpart UUUUb | View Final Rule 89 FR 40047 (May 9, 2024) >

Under this rule, States have until May 11, 2026, to submit State Plans addressing the EPA’s emission guidelines for these sources.

ADEQ held a public information session regarding the Power Sector State Plan on March 20, 2025 | View Presentation >

On June 17, 2025, the EPA proposed to repeal all greenhouse gas (GHG) emissions standards for fossil fuel-fired power plants. The EPA also proposed that the CAA requires the EPA to make a finding that GHG emissions from fossil fuel-fired power plants contribute significantly to dangerous air pollution. The EPA proposed to make a finding that GHG emissions from fossil fuel-fired power plants do not contribute significantly to dangerous air pollution. As an alternative, the EPA proposed to repeal a narrower set of requirements that includes the emission guidelines for existing fossil fuel-fired steam generating units, the Carbon Capture and Sequestion/Storage (CCS)-based standards for coal-fired steam generating units undertaking a large modification, and the CCS-based standards for new base load stationary combustion turbines | View Proposed Rule 90 FR 25752 (June 17, 2025) >

Before the existing emissions guidelines for Fossil Fuel-Fired Power Plants were finalized by the EPA, ADEQ worked on two prior State Plans for fossil fuel-fired power plants under CAA Section 111(d): the Clean Power Plan and the Affordable Clean Energy Rule.

The Clean Power Plan (CPP)

On Aug. 3, 2015, the EPA announced the Clean Power Plan (CPP), a rule that required states to develop and implement a plan to reduce CO_­2 emissions from existing fossil fuel-fired power plants. The CPP was part of a comprehensive federal program to mitigate climate change | View Final Rule 80 FR 64662 (Oct. 23, 2015) >

The CPP required states to adopt standards to meet CO­₂ emission reduction goals established by the EPA. States had until Sept. 6, 2016, to transmit a final plan or an initial submittal to the EPA. If the EPA approved the initial submittal, the deadline for submitting a final plan would have been extended to Sept. 6, 2018.

The proposed CPP would have required Arizona to achieve a 52 percent reduction in the CO₂ emissions rate for affected power plants and to achieve about 90 percent of that reduction by 2020. The reduction for Arizona was much more stringent than the reductions proposed for our neighbors in the West and was among the most severe imposed on any state. As a result, at least in part, of extensive comments and technical data submitted by ADEQ and Arizona stakeholders, the EPA lowered the CO_­2 emission rate to 34 percent, and established a much more gradual timeline for achieving that reduction.

In response to the EPA’s CPP, ADEQ’s Air Quality Division initiated the State Plan development process. However, on Oct. 16, 2017, the EPA proposed to repeal the CPP. As a result, Arizona’s State Plan for the CPP was never submitted to the EPA | View Proposed Rule 82 FR 48035 (Oct. 16, 2017) >  

The Clean Power Plan’s Replacement: Affordable Clean Energy (ACE) Rule

On Dec. 28, 2017, the EPA published an Advanced Notice of Proposed Rulemaking, seeking information from the public and stakeholders regarding certain components of the CPP. Comments were due to the EPA by Feb. 26, 2018 | View ADEQ Comments > | View Proposed Rule 82 FR 61507 (Dec. 28, 2017) >

On June 19, 2019, the EPA issued the final Affordable Clean Energy (ACE) rule. In this same rulemaking, the EPA also rescinded the CPP and established new regulations for CAA Section 111(d) State Plans | View Final Rule 84 FR 32520 (July 8, 2019) >

On Nov. 8, 2019, ADEQ held an initial stakeholder meeting to discuss the ACE Rule planning processes. 

On April 23, 2020, ADEQ and representatives from the affected coal-fired electric generation units commenced a technical working group. This group met on a monthly basis to address technical issues regarding the Heat Rate Improvement (HRI) analysis of the EPA’s identified Best System of Emissions Reduction (BSER) as mandated by the ACE Rule. 

On Dec. 19, 2021, the Federal D.C. Circuit Court of Appeals struck down the ACE rule and remanded it to the EPA. 

On June 30, 2022, the U.S. Supreme Court held that the EPA lacked authority under CAA Section 111(d) to set a cap on GHG emissions from power plants based on generation shifting. Following the Supreme Court’s decision, the EPA began development of a new rule for greenhouse gas standards and guidelines for fossil fuel-fired power plants. 

The ACE Rule was repealed on May 9, 2024, and Arizona’s State Plan for the ACE Rule was never submitted to the EPA.