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Rule2026-11047

National Emission Standards for Hazardous Air Pollutants From Hazardous Waste Combustors: Residual Risk and Technology Review

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Published

June 3, 2026

Effective

June 3, 2026

Issuing agencies

Environmental Protection Agency

Abstract

The U.S. Environmental Protection Agency (EPA) is finalizing the residual risk and technology review (RTR) conducted for the National Emission Standards for Hazardous Air Pollutants (NESHAP) from Hazardous Waste Combustors (HWC). Specifically, the EPA is finalizing that risks due to emissions of hazardous air pollutants (HAP) from this source category are adequately addressed by the existing standards; that the NESHAP provides an ample margin of safety to protect public health; and that no developments in practices, processes, or control technologies necessitate revision of the standards. In addition, the EPA is promulgating emission standards for hydrogen fluoride (HF) and hydrogen cyanide (HCN) emissions from major source HWC incinerators, cement kilns, solid fuel boilers, and liquid fuel boilers under Clean Air Act (CAA) sections 112(d)(2) and (3) and 112(h). These final amendments also include work practice standards under CAA section 112(h) for periods of startup, shutdown, and malfunction (SSM); new electronic reporting provisions and requirements; provisions allowing States to choose to exempt area source HWCs from certain permitting requirements; and certain typographical and technical corrections and clarifications.

Full Text

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[Federal Register Volume 91, Number 106 (Wednesday, June 3, 2026)]
[Rules and Regulations]
[Pages 33484-33572]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2026-11047]

[[Page 33483]]

Vol. 91

Wednesday,

No. 106

June 3, 2026

Part IV

Environmental Protection Agency

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40 CFR Part 63

National Emission Standards for Hazardous Air Pollutants From Hazardous
Waste Combustors: Residual Risk and Technology Review; Final Rule

Federal Register / Vol. 91, No. 106 / Wednesday, June 3, 2026 / Rules
and Regulations

[[Page 33484]]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2004-0022; FRL-10654-02-OAR]
RIN 2060-AV96

National Emission Standards for Hazardous Air Pollutants From
Hazardous Waste Combustors: Residual Risk and Technology Review

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: The U.S. Environmental Protection Agency (EPA) is finalizing
the residual risk and technology review (RTR) conducted for the
National Emission Standards for Hazardous Air Pollutants (NESHAP) from
Hazardous Waste Combustors (HWC). Specifically, the EPA is finalizing
that risks due to emissions of hazardous air pollutants (HAP) from this
source category are adequately addressed by the existing standards;
that the NESHAP provides an ample margin of safety to protect public
health; and that no developments in practices, processes, or control
technologies necessitate revision of the standards. In addition, the
EPA is promulgating emission standards for hydrogen fluoride (HF) and
hydrogen cyanide (HCN) emissions from major source HWC incinerators,
cement kilns, solid fuel boilers, and liquid fuel boilers under Clean
Air Act (CAA) sections 112(d)(2) and (3) and 112(h). These final
amendments also include work practice standards under CAA section
112(h) for periods of startup, shutdown, and malfunction (SSM); new
electronic reporting provisions and requirements; provisions allowing
States to choose to exempt area source HWCs from certain permitting
requirements; and certain typographical and technical corrections and
clarifications.

DATES: This final rule is effective on June 3, 2026. The incorporation
by reference of certain material listed in the rule was approved by the
Director of the Federal Register as of September 8, 2020.

ADDRESSES: The EPA established a docket for this action under Docket ID
No. EPA-HQ-OAR-2004-0022. All documents in the docket are listed in
<a href="https://www.regulations.gov/">https://www.regulations.gov/</a>. Although listed, some information is not
publicly available, e.g., Confidential Business Information or other
information whose disclosure is restricted by statute. The EPA does not
place certain other material, such as copyrighted material, on the
internet; this material is publicly available only as Portable Document
Format (PDF) versions and accessible only on EPA computers in the
docket office reading room. The public cannot download certain
databases and physical items from the docket but may request these
items by contacting the docket office by telephone at (202) 566-1744.
The docket office has 10 business days to respond to such requests.
Except for these items, publicly available docket materials are
available electronically at <a href="https://www.regulations.gov">https://www.regulations.gov</a> or on EPA
computers in the docket office reading room at the EPA Docket Center,
WJC West Building, Room Number 3334, 1301 Constitution Ave. NW,
Washington, DC. The Public Reading Room hours of operation are 8:30
a.m. to 4:30 p.m. Eastern Time (ET), Monday through Friday. The
telephone number for the Public Reading Room is (202) 566-1744, and the
telephone number for the EPA Docket Center is (202) 566-1742.

FOR FURTHER INFORMATION CONTACT: For information about this final rule,
contact U.S. EPA, Attn: Rachel Smoak, Mail Drop: Natural Resources
Division (E143-02), 109 T.W. Alexander Drive, P.O. Box 12055, RTP,
North Carolina 27711; telephone number: (919) 541-0253; and email
address: <a href="/cdn-cgi/l/email-protection#3e4d53515f55104c5f5d565b527e5b4e5f10595148"><span class="__cf_email__" data-cfemail="a4d7c9cbc5cf8ad6c5c7ccc1c8e4c1d4c58ac3cbd2">[email&#160;protected]</span></a>. Individuals who are deaf or hard of
hearing, as well as individuals who have speech or communication
disabilities, may use a telecommunications relay service. To learn more
about how to make an accessible telephone call to any of the telephone
numbers shown in this preamble, please visit the web page \1\ for the
relay service of the Federal Communications Commission, and a list of
relay services is available on their directory page.\2\
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\1\ See <a href="https://www.fcc.gov/trs">https://www.fcc.gov/trs</a>.
\2\ See <a href="https://www.fcc.gov/general/trs-state-and-territories">https://www.fcc.gov/general/trs-state-and-territories</a>.

SUPPLEMENTARY INFORMATION:
Preamble acronyms and abbreviations. Throughout this preamble, the
use of ``we,'' ``us,'' or ``our'' refers to the EPA. We use multiple
acronyms and terms in this preamble. While this list may not be
exhaustive, to ease the reading of this preamble and for reference
purposes, the EPA defines the following terms and acronyms here:

APCD air pollution control device
AWFCO automatic waste feed cutoff
CAA Clean Air Act
CDX Central Data Exchange
CEDRI Compliance and Emissions Data Reporting Interface
CEMS continuous emission monitoring system(s)
CISWI commercial and industrial solid waste incinerator
CFR Code of Federal Regulations
CfPT confirmatory performance test
CMAS chemical manufacturing area sources
CMS continuous monitoring system(s)
COMS continuous opacity monitoring system(s)
CPT comprehensive performance test
DRE destruction and removal efficiency
[deg]F degrees Fahrenheit
EPA Environmental Protection Agency
ERT Electronic Reporting Tool
GACT generally available control technology
GMCS GORE Mercury Control System
HAP hazardous air pollutants(s)
HBEL health-based emission limit
HCl hydrochloric acid
HCN hydrogen cyanide
HF hydrogen fluoride
Hg mercury
HQ hazard quotient
HWC hazardous waste combustor
ICR Information Collection Request
MACT maximum achievable control technology
MMBTU/hr million British thermal units per hour
MTEC maximum theoretical emissions concentration
NESHAP national emission standards for hazardous air pollutants
NSPS new source performance standards
NOC Notification of Compliance
NTTAA National Technology Transfer and Advancement Act
OMB Office of Management and Budget
OPL operating parameter limit
PAH polycyclic aromatic hydrocarbons
PCB polychlorinated biphenyls
PCDD/PCDF polychlorinated dibenzo-p-dioxins and polychlorinated
dibenzofurans
PM particulate matter
POM polycyclic organic matter
ppmv parts per million by volume
RATA relative accuracy test audit
RCRA Resource Conservation and Recovery Act
RFA Regulatory Flexibility Act
RTR risk and technology review
SDDS Shell Dioxin Destruction System
SSM startup, shutdown, and malfunction
TEQ toxic equivalency quotient
THC total hydrocarbons
TOSHI target organ-specific hazard index
UMRA Unfunded Mandates Reform Act
UPL upper prediction limit

Table of Contents

I. General Information
A. Executive Summary
B. Does this action apply to me?
C. What is the statutory authority for this final action?
D. Where can I get a copy of this document and other related
information?
E. Judicial Review and Administrative Reconsideration
F. Severability
II. Background
A. What is the HWC NESHAP source category and how does the HWC

[[Page 33485]]

NESHAP regulate HAP emissions from the source category?
B. What changes did we propose for the HWC NESHAP source
category in our November 10, 2025, proposal?
III. What is included in this final rule?
A. What are the final rule amendments based on the risk review
for the HWC NESHAP source category?
B. What are the final rule amendments based on the technology
review for the HWC NESHAP source category?
C. What are the final rule amendments pursuant to CAA sections
112(d)(2) and (3) and 112(h) for the HWC NESHAP source category?
D. What are the final rule amendments addressing emissions
during periods of startup, shutdown, and malfunction?
E. What other changes have been made to the NESHAP?
F. What are the effective and compliance dates of the standards?
IV. What is the rationale for our final decisions and amendments for
the HWC NESHAP source category?
A. Residual Risk Review for the HWC NESHAP Source Category
B. Technology Review for the HWC NESHAP Source Category
C. Amendments Pursuant to CAA Sections 112(d)(2) and (3) and
112(h) for the HWC NESHAP Source Category
D. Changes to Provisions for Periods of Startup, Shutdown, and
Malfunction
E. Other Amendments to the HWC NESHAP
V. Summary of Cost, Environmental, and Economic Impacts and
Additional Analyses Conducted
A. What are the affected facilities?
B. What are the air quality impacts?
C. What are the cost impacts?
D. What are the economic impacts?
E. What are the benefits?
VI. Statutory and Executive Order Reviews
A. Executive Orders 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
B. Executive Order 14192: Unleashing Prosperity Through
Deregulation
C. Paperwork Reduction Act (PRA)
D. Regulatory Flexibility Act (RFA)
E. Unfunded Mandates Reform Act (UMRA)
F. Executive Order 13132: Federalism
G. Executive Order 13175: Consultation and Coordination with
Indian Tribal Governments
H. Executive Order 13045: Protection of Children From
Environmental Health Risks and Safety Risks
I. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use
J. National Technology Transfer and Advancement Act (NTTAA)
K. Congressional Review Act (CRA)

I. General Information

A. Executive Summary

The EPA promulgated the current HWC NESHAP to address emissions of
HAP from hazardous waste burning incinerators, cement kilns,
lightweight aggregate kilns, solid fuel-fired boilers, liquid fuel-
fired boilers, and HCl production furnaces under CAA section 112 in
2005.\3\ This followed the vacatur \4\ of the 1999 standards \5\ and a
period of regulation under a 2002 interim final rule \6\ while the EPA
developed the updated HWC NESHAP. In response to multiple petitions for
reconsideration, the EPA sought and received a full voluntary remand of
the rule in 2009 to reexamine the HWC NESHAP.\7\
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\3\ 70 FR 59402 (Oct. 12, 2005).
\4\ Cement Kiln Recycling Coal. v. EPA, 255 F.3d 855, 872 (D.C.
Cir. 2001).
\5\ 64 FR 52828 (Sept. 30, 1999).
\6\ 67 FR 6792 (Feb. 13, 2002).
\7\ Sierra Club v. EPA, Docket No. 05-1441 (consolidated with
Docket Nos. 05-1442, 05-1443, 05-1445, 05-1449) (D.C. Cir. 2008).
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CAA section 112(f)(2) requires the EPA to review the standards
initially promulgated for this source category within eight years to
identify and address residual risk to human health and the environment.
CAA section 112(d)(6) also requires the EPA to review and revise the
standards ``as necessary'' at least every eight years to address
developments in practices, processes, and control technologies.
The EPA conducted the required RTR and proposed those results in
2025.\8\ In the same action, the EPA withdrew a previous proposal
regarding emission standards during periods of malfunction \9\ and
proposed new standards for HF and HCN for HWCs with demonstrated
emissions of those HAP, which had not previously been regulated by the
HWC NESHAP; revisions to the SSM provisions; electronic reporting
provisions; provisions regarding area source HWC title V permitting
requirements; and other technical, typographical, and clarifying
corrections.\10\
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\8\ 90 FR 50814 (Nov. 10, 2025).
\9\ 89 FR 59867 (July 24, 2024).
\10\ 90 FR 50814 (Nov. 10, 2025).
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In this final rule, the EPA is finalizing the results of the RTR,
determining that the risks due to HAP emissions from the HWC NESHAP are
adequately addressed by the existing standards, that the HWC NESHAP
provides an ample margin of safety to protect public health, and that
there are no developments in practices, processes, or control
technologies that warrant revisions to the standards pursuant to the
technology review. The EPA is finalizing a new work practice standard
for HF for HWC incinerators, numerical emission limits for HCN for HWC
cement kilns, numerical emission limits for HF and HCN for HWC solid
fuel boilers, and a work practice standard for HF and numerical
emission limits for HCN for HWC liquid fuel boilers. The EPA is also
finalizing new work practice standards for periods of SSM; electronic
reporting provisions; provisions that title V air permitting
authorities may choose to exempt area sources not otherwise subject to
title V air permitting requirements on a case-by-case basis; and other
technical, typographical, and clarifying corrections to the HWC NESHAP.
Following consideration of comments and evaluation of additional
information received on the proposed rule, the EPA is revising our
assessment of the emission limits for HF and HCN for HWC cement kilns.
The EPA received additional data on the emission of HF from cement
kilns demonstrating that the HF emissions the EPA relied upon to
propose a work practice standard for HF were artifacts of the
measurement technique, not data demonstrating HF emissions from HWC
cement kilns. The EPA does not currently have any credible data
demonstrating that HWC cement kilns measurably emit HF. Accordingly,
the EPA is not finalizing any emission standard for HF emissions from
HWC cement kilns. The EPA is also revising the HCN emission limit for
new source HWC cement kilns based on information about the best similar
source for new cement kilns. The EPA has also made other minor
revisions in response to comments.
The EPA estimates that this final rule will result in present value
costs of $2.4 million at a three percent discount rate and $1.8 million
at a seven percent discount rate over the 2027 to 2041 time frame, with
equivalent annualized values of $200,000 per year for both discount
rates (in 2024 dollars). Averaged over the first three years, the EPA
does not expect any affected entity to incur an annual cost of more
than 0.16 percent of their revenues, and the Agency expects nine
affected parent entities to have cost savings associated with this
final rule.

B. Does this action apply to me?

Regulated entities. Table 1 of this preamble presents categories
and entities that this action potentially regulates.

[[Page 33486]]

[GRAPHIC] [TIFF OMITTED] TR03JN26.072

Table 1 of this preamble, although not exhaustive, provides a guide
for readers regarding entities that this final action likely affects
for the source categories listed. To determine if this action affects
your facility, you should examine the applicability criteria in title
40 of the Code of Federal Regulations (CFR), part 63, subpart EEE. If
you have any questions regarding the applicability of any aspect of
this NESHAP, please contact the appropriate person listed in the
preceding FOR FURTHER INFORMATION CONTACT section of this preamble. To
our knowledge, State, local, and Tribal government entities do not own
or operate sources that would be affected by this action.

C. What is the statutory authority for this final action?

CAA sections 112, as amended, and 502(a) provide the statutory
authority for this action.\11\ CAA section 112 establishes a multi-
stage regulatory process to develop standards for emission of HAP from
stationary sources. Generally, the first stage involves the EPA
establishing technology-based standards that reflect the maximum
achievable control technology (MACT) or an appropriate alternative.\12\
The second stage involves evaluating those standards within eight years
pursuant to CAA section 112(f)(2) to determine whether additional
standards are needed to address remaining risk associated with HAP
emissions.\13\ This second stage is commonly referred to as the
``residual risk review.'' In addition to the residual risk review, CAA
section 112(d)(6) also requires the EPA to review the standards every
eight years and ``revise as necessary'' taking into account
``developments in practices, processes, and control technologies.''
\14\ This review is commonly referred to as the ``technology review.''
The following discussion identifies the most relevant statutory
sections and briefly explains the contours of the methodology used to
implement these statutory requirements.
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\11\ 42 U.S.C. 7412, 7601(a), 7661a(a).
\12\ Id. 7412(d)(1)-(4).
\13\ Id. 7412(f)(2).
\14\ Id. 7412(d)(6).
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In the first stage of the CAA section 112 standard-setting process,
the EPA promulgates technology-based standards under CAA section 112(d)
for categories of sources identified as emitting one or more of the HAP
listed in CAA section 112(b). Sources of HAP emissions are either major
sources or area sources, and CAA section 112 establishes different
requirements for major source standards and area source standards. The
HWC NESHAP regulates both major and area sources, but only the
requirements for establishing emissions limitations for major sources
are relevant to the present rulemaking. ``Major sources'' are those
that emit or have the potential to emit 10 tons per year or more of a
single HAP or 25 tons per year or more of any combination of HAP.\15\
All other sources are ``area sources.'' \16\ For major sources, CAA
section 112(d)(2) provides that the technology-based NESHAP must
reflect the maximum degree of reduction in emissions of HAP achievable
(after considering cost, energy requirements, and non-air quality
health and environmental impacts). These standards are commonly
referred to as MACT standards. CAA section 112(d)(3) also establishes a
minimum control level for MACT standards, known as the MACT ``floor,''
which is based on emission control achieved in practice by the best
performing sources. For new sources, the MACT floor cannot be less
stringent than the emission control achieved in practice by the best-
controlled similar source. The MACT standards for existing sources can
be less stringent than floors for new sources, but they cannot be less
stringent than the average emission limitation achieved by the best-
performing 12 percent of existing sources in the category or
subcategory (or the best-performing five sources for categories or
subcategories with fewer than 30 sources). The EPA also considers
control options that are more stringent than the floor and may
establish standards more stringent than the floor, based on the
consideration of the cost of achieving the emissions reductions, any
non-air quality health and environmental impacts, and energy
requirements.\17\ Standards that are more stringent than the floor are
commonly referred to as ``beyond-the-floor'' standards.\18\
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\15\ 42 U.S.C. 7412(a)(1).
\16\ Id. 7412(a)(2).
\17\ Id. 7412(d)(2).
\18\ Nat'l Lime Ass'n v. EPA, 233 F.3d 625, 634 (D.C. Cir. 2000)
(``Once the Agency sets statutory floors, it then determines,
considering cost and the other factors listed in section 7412(d)(2),
whether stricter standards are `achievable.' The Agency calls such
stricter requirements `beyond-the-floor' standards.'').
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In certain instances, as provided in CAA section 112(h), the EPA
may set work practice standards in lieu of numerical emission
standards. Under CAA section 112(h), the EPA may adopt a work practice
standard in lieu of a numerical emission standard if it is ``not
feasible in the judgment of the Administrator to prescribe or enforce
an emission standard for control of a hazardous air pollutant.'' \19\
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\19\ 42 U.S.C. 7412(h)(1); Sierra Club v. EPA, 479 F.3d 875,
883-84 (D.C. Cir. 2007). The EPA may ``adopt[ ] a method to account
for measurement imprecision that has a rational basis in the
correlation between increased emission values and increased testing
precision.'' Nat'l Ass'n of Clean Water Agencies (NACWA) v. EPA, 734
F.3d 1115, 1154-55 (D.C. Cir. 2013).
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The next stage in standard-setting focuses on identifying and
addressing any remaining (i.e., ``residual'') risk within eight years
pursuant to CAA

[[Page 33487]]

section 112(f)(2). The approach incorporated into the CAA and used by
the EPA to evaluate residual risk and develop standards under CAA
section 112(f)(2) is also a two-step approach. In the first step, the
EPA determines whether risks are adequately addressed by existing
standards. This determination ``considers all health information,
including risk estimation uncertainty, and includes a presumptive limit
on maximum individual lifetime [cancer] risk (MIR) of approximately 1
in 10 thousand.'' \20\ If risks are unacceptable, the EPA determines
the emission standards necessary to reduce risk to an acceptable level
without considering costs. In the second step of the approach, the EPA
considers whether the emission standards provide an ample margin of
safety to protect public health ``in consideration of all health
information, including the number of persons at risk levels higher than
approximately 1 in 1 million, as well as other relevant factors,
including costs and economic impacts, technological feasibility, and
other factors relevant to each particular decision.'' \21\ The EPA
promulgates emission standards necessary to provide an ample margin of
safety to protect public health or determine that the standards being
reviewed provide an ample margin of safety without any revisions. After
conducting the ample margin of safety analysis, the EPA considers
whether a more stringent standard is necessary to prevent an adverse
environmental effect, taking into consideration costs, energy, safety,
and other relevant factors.
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\20\ 54 FR 38045 (Sept. 14, 1989). Although defined as ``maximum
individual risk,'' MIR refers only to cancer risk. MIR, one metric
for assessing cancer risk, is the estimated risk if an individual
were exposed to the maximum level of a pollutant for a lifetime.
\21\ Id.
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CAA section 112(d)(6) separately requires the EPA to review MACT
standards promulgated under CAA section 112 and revise them ``as
necessary (taking into account developments in practices, processes,
and control technologies)'' no less frequently than every eight years.
In conducting this review, the EPA is not required to recalculate the
MACT floors that were established in earlier rulemakings.\22\ In
Louisiana Environmental Action Network (LEAN) v. EPA, the D.C. Circuit
held that the EPA must address previously unregulated HAP known to be
emitted from a major source category as part of its periodic review
under CAA section 112(d)(6).\23\
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\22\ See Ass'n of Battery Recyclers, Inc. v. EPA, 716 F.3d 667
(D.C. Cir. 2013); NRDC v. EPA, 529 F.3d 1077, 1084 (D.C. Cir. 2008).
\23\ 955 F.3d 1088 (D.C. Cir. 2020).
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CAA section 112(d)(6) and relevant case law provide the EPA with
flexibility to consider additional relevant factors other than those
enumerated in CAA section 112(d)(6) when deciding whether revisions to
existing standards are ``necessary.'' The D.C. Circuit has held that
the CAA section 112(d)(6) requirement to periodically review and revise
CAA section 112 emission standards ``as necessary'' is not limited to
the consideration of ``developments in practices, processes and control
technologies.'' \24\ Rather, ``the operative standard is `revise as
necessary,' with the parenthetical pointing to a non-exhaustive list of
considerations.'' \25\ The Supreme Court also emphasized in Michigan v.
EPA that unless the statute provides otherwise, broad terms such as
``necessary'' direct the relevant agency to consider all relevant
factors, including cost.\26\ That decision is particularly relevant
here because the Court was interpreting a provision of CAA section 112
that instructs the Administrator to determine whether it is
``appropriate and necessary'' to regulate HAP emissions from electric
utility steam generating units.\27\ Thus, under relevant case law, when
the EPA is deciding whether it is ``necessary'' to revise standards
pursuant to CAA section 112(d)(6), the Agency considers the costs of
any developments in practices, processes, and control technologies.
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\24\ LEAN, 955 F.3d at 1097.
\25\ Id.; see also Nat'l Ass'n for Surface Finishing v. EPA, 795
F.3d 1, 11 (D.C. Cir. 2015); Ass'n of Battery Recyclers, 716 F.3d at
673-74.
\26\ 576 U.S. 743, 752-53 (2015).
\27\ See id. (interpreting 42 U.S.C. 7412(n)(1)(A)).
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The EPA is also required to specify relevant test methods, best
practices, procedures, or protocols and recordkeeping requirements for
standards promulgated under CAA section 112. Finally, CAA section
502(d)(l) requires each State to develop and submit to the EPA an
operating permit program to meet the requirements of title V of the CAA
and the EPA's implementing regulations at 40 CFR part 70 (``title V'').
Major stationary sources of air pollution and certain other non-major
sources are required to apply for and operate in accordance with title
V operating permits that include emission limitations and other
conditions as necessary to assure compliance with applicable
requirements of the CAA, including the requirements of the applicable
implementation plan.

D. Where can I get a copy of this document and other related
information?

In addition to the docket, an electronic copy of this final action
is available on the internet. A brief summary of this rule is available
at <a href="https://www.regulations.gov">https://www.regulations.gov</a>, Docket ID No. EPA-HQ-OAR-2004-0022.
Following signature by the EPA Administrator, the EPA will post a copy
of this rule at: <a href="https://www.epa.gov/hazardous-waste-combustors-national-emission-standards-hazardous">https://www.epa.gov/hazardous-waste-combustors-national-emission-standards-hazardous</a>. Following publication in the
Federal Register, the EPA will post the Federal Register version and
key technical documents at this same website.
Additional information is available on the RTR website at <a href="https://www.epa.gov/stationary-sources-air-pollution/risk-and-technology-review-national-emissions-standards-hazardous">https://www.epa.gov/stationary-sources-air-pollution/risk-and-technology-review-national-emissions-standards-hazardous</a>. This information
includes an overview of the RTR program and links to project websites
for the RTR source categories.

E. Judicial Review and Administrative Reconsideration

Under CAA section 307(b)(1), judicial review of this final action
is available only by filing a petition for review in the United States
Court of Appeals for the District of Columbia Circuit by August 3,
2026. CAA section 307(b)(2) prohibits a party from challenging this
final rule separately in any civil or criminal proceedings brought by
the EPA for enforcement.
CAA section 307(d)(7)(B) further provides that only an objection to
a rule or procedure that was raised with reasonable specificity during
the period for public comment (including any public hearing) may be
raised during judicial review. This section also requires the EPA to
reconsider the rule if the person raising an objection can demonstrate
to the Administrator that it was impracticable to raise such objection
within the period for public comment or if the grounds for such
objection arose after the period for public comment (but within the
time specified for judicial review) and if such objection is of central
relevance to the outcome of the rule. Any person seeking to make such a
demonstration should submit a Petition for Reconsideration to the
Office of the Administrator, U.S. EPA, Room 3000, WJC South Building,
1200 Pennsylvania Ave. NW, Washington, DC 20460, with a copy to both
the person(s) listed in the preceding FOR FURTHER INFORMATION CONTACT
section, and the Associate General Counsel for the Air and

[[Page 33488]]

Radiation Law Office, Office of General Counsel (Mail Code 2344A), U.S.
EPA, 1200 Pennsylvania Ave. NW, Washington, DC 20460.

F. Severability

This final rule contains several discrete components, which the EPA
views as severable as a practical matter--i.e., they are functionally
independent and will operate in practice independently of the other
components. These discrete components are detailed in sections III.A
through III.E of this preamble and the technical memoranda available in
the docket.\28\ For example, the outcome of the risk review, the
outcome of the technology review, the final emission limits for HF and
HCN for each type of HWC, and the final work practice standards for
periods of SSM generally function independently of one another and
would not be impacted if a reviewing court were to vacate one or more
of the other final provisions. In addition, as this final rule revises
an existing NESHAP, the EPA notes that if a reviewing court were to
vacate one or more of the standards finalized here, the affected
standards will revert to those present in the 2005 HWC NESHAP as
revised in 2005, 2006, and 2008.\29\
---------------------------------------------------------------------------

\28\ See Docket ID. No. EPA-HQ-OAR-2004-0022.
\29\ See 70 FR 59402 (Oct. 12, 2005); 70 FR 75042 (Dec. 19,
2005); 71 FR 62388 (Oct. 25, 2006); 73 FR 18970 (Apr. 8, 2008); 73
FR 64068 (Oct. 28, 2008).
---------------------------------------------------------------------------

II. Background

A. What is the HWC NESHAP source category and how does the HWC NESHAP
regulate HAP emissions from the source category?

HWCs are units that combust hazardous waste and they can be located
at many types of facilities including those listed in table 1 of this
preamble. The source category covered by the HWC NESHAP currently
includes approximately 160 HWCs located at approximately 90 facilities
in the United States. HWCs are incinerators, cement kilns, lightweight
aggregate kilns, boilers, or HCl production furnaces that combust
hazardous waste for waste reduction, thermal energy recovery, and/or
production of a product. Hazardous waste is defined under the Resource
Conservation and Recovery Act (RCRA), which establishes a comprehensive
regulatory structure overseeing the treatment, storage, and disposal of
hazardous waste.\30\ In 2023, approximately 32.2 million tons of
hazardous waste were generated in the United States, all of which must
be treated or disposed of in a manner that protects human health and
the environment.\31\ Hazardous waste incineration provided that manner
of disposal for approximately 1.1 million tons of that hazardous waste,
and energy recovery in units like hazardous waste burning boilers
accounted for an additional 1.4 million tons.\32\
---------------------------------------------------------------------------

\30\ 42 U.S.C. 6901-6992k.
\31\ U.S. Environmental Protection Agency. (Last updated Dec.
30, 2024). Biennial Report Summary: <a href="https://rcrapublic.epa.gov/rcra-hwip/trends-and-analysis/details/4">https://rcrapublic.epa.gov/rcra-hwip/trends-and-analysis/details/4</a>.
\32\ U.S. Environmental Protection Agency. (Last updated July
10, 2025). Biennial Report Management Methods: <a href="https://rcrapublic.epa.gov/rcra-hwip/trends-and-analysis/details/3">https://rcrapublic.epa.gov/rcra-hwip/trends-and-analysis/details/3</a>.
---------------------------------------------------------------------------

The key HAP that the HWC NESHAP regulates include polychlorinated
dibenzodioxins and furans (PCDD/PCDF); mercury (Hg); cadmium and lead
as semi-volatile metals; arsenic, beryllium, and chromium as low-
volatile metals; antimony, cobalt, manganese, nickel, and selenium as
non-enumerated metal HAP; HCl and chlorine gas; and other hydrocarbon
HAP, including polychlorinated biphenyls (PCB) and polycyclic aromatic
hydrocarbons (PAH). The HWC NESHAP also includes several other emission
limits used as surrogate standards to regulate emissions of other HAP
such as a carbon monoxide (CO) or total hydrocarbon (THC) limit
associated with demonstrating good combustion practices, a destruction
and removal efficiency (DRE) standard also for demonstrating good
combustion practices, and a particulate matter (PM) emission limit for
some subcategories.\33\
---------------------------------------------------------------------------

\33\ For more information about the HWC NESHAP, see the notice
of proposed rulemaking associated with this final action at 90 FR
50814 (Nov. 10, 2025).
---------------------------------------------------------------------------

The EPA originally promulgated the HWC NESHAP, codified at 40 CFR
part 63, subpart EEE, in 1999.\34\ It regulated incinerators, cement
kilns, and lightweight aggregate kilns that burned hazardous waste.
These standards were vacated in 2001 \35\ and replaced with interim
standards in 2002.\36\ The EPA promulgated replacement standards for
hazardous waste incinerators, cement kilns, and lightweight aggregate
kilns and first-time standards for hazardous waste solid fuel boilers,
liquid fuel boilers, and HCl production furnaces in 2005.\37\ In
response to multiple petitions for reconsideration, the EPA sought and
received a voluntary remand of the rule in 2009 to reexamine the HWC
NESHAP.\38\
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\34\ 64 FR 52828 (Sept. 30, 1999).
\35\ Cement Kiln Recycling Coal., 255 F.3d at 872.
\36\ 67 FR 6792 (Feb. 13, 2002).
\37\ 70 FR 59402 (Oct. 12, 2005).
\38\ Sierra Club v. EPA, Docket No. 05-1441 (consolidated with
Docket Nos. 05-1442, 05-1443, 05-1445, 05-1449) (D.C. Cir.).
---------------------------------------------------------------------------

In October 2022, Earthjustice filed an action in the U.S. District
Court for the District of Columbia to compel the EPA to review and
revise the HWC NESHAP under CAA sections 112(d)(6) and (f)(2) (i.e.,
complete the RTR). In December 2024, the district court issued an order
requiring that the EPA sign the final RTR rule for this source category
by December 31, 2025.\39\ In response to the EPA's request for an
extension of time, the district court subsequently extended the
deadline from December 31, 2025, to May 29, 2026.\40\ The EPA is
finalizing this action in response to the court order.\41\
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\39\ Order, Blue Ridge Envtl. Def. League v. Regan, 22-cv-3134
(APM), at 4 (D.D.C. Dec. 12, 2024).
\40\ Order, Blue Ridge Envtl. Def. League v. Regan, 22-cv-3134
(APM), at 3-4 (D.D.C. Dec. 23, 2025).
\41\ In finalizing standards for previously unregulated HAP as
part of this rulemaking, the EPA is exercising its authority under
CAA section 112(d)(6) to revise the standards as necessary. Nothing
in this final rule should be taken as endorsing the language in the
district court's order discussing the EPA's obligations under CAA
section 112 or the LEAN decision. The D.C. Circuit has exclusive
jurisdiction to review final action taken by the EPA to promulgate
standards pursuant to CAA section 112, including, when appropriate,
to interpret relevant statutory provisions, see 42 U.S.C.
7607(b)(1), while district courts have jurisdiction, subject to
standing, venue, and other requirements, to order the performance of
``any act or duty under [the CAA] which is not discretionary with
the Administrator,'' see id. 7604(a). In context, the district
court's order was addressing the parties' arguments about the
appropriate remedy, that is, the appropriate timeline for completing
the RTR, for plaintiffs' claim that the EPA had failed to act by the
statutory deadline.
---------------------------------------------------------------------------

B. What changes did we propose for the HWC NESHAP source category in
our November 10, 2025, proposal?

On November 10, 2025, the EPA published a proposed rule in the
Federal Register for the HWC NESHAP, 40 CFR part 63, subpart EEE, that
discussed the results of the RTR and proposed actions reflecting those
results. Specifically, the EPA proposed: that the results of the risk
review demonstrated that no revisions to the existing standards were
required to address residual risk; that no revisions to the existing
standards were necessary based on developments in practices, processes,
or control technologies under the technology review; numeric emission
limits for HF and HCN for major source HWC solid fuel boilers; a work
practice standard for HF for major source HWC incinerators; a work
practice standard for HF and numeric emission limits for HCN for major
source HWC cement kilns; a work practice standard for HF and numeric
emission limits for HCN for major source liquid fuel boilers; work
practice standards for periods of SSM; electronic reporting provisions;
allowing title V air

[[Page 33489]]

permitting authorities to choose to exempt area sources from the
requirement to obtain a title V permit if such area sources are not
otherwise subject to title V air permitting requirements; and other
items and technical corrections.\42\
---------------------------------------------------------------------------

\42\ For more information about what was proposed, see the
notice of proposed rulemaking associated with this final rule at 90
FR 50814 (Nov. 10, 2025).
---------------------------------------------------------------------------

III. What is included in this final rule?

This action finalizes the EPA's determinations pursuant to the RTR
provisions of CAA section 112 for the HWC NESHAP source category. This
action also finalizes other changes to the NESHAP, including emission
limits and work practice standards for HF and HCN from major source HWC
incinerators, cement kilns, solid fuel boilers, and liquid fuel
boilers; work practice standards for periods of SSM; new electronic
reporting provisions and requirements; provisions allowing title V air
permitting authorities to choose to exempt area source HWCs from the
requirement to have a title V air permit if such area sources are not
otherwise subject to title V air permitting requirements; and minor
corrections and clarifications to a number of other rule provisions.
This action also reflects several changes to the November 2025 proposal
in consideration of comments received during the public comment period,
as described in section IV of this preamble.

A. What are the final rule amendments based on the risk review for the
HWC NESHAP source category?

The EPA is finalizing, as proposed, that pursuant to CAA section
112(f), risks from this source category are adequately addressed by the
existing standards and therefore acceptable, the existing standards
provide an ample margin of safety to protect public health, and more
stringent standards are not necessary to prevent an adverse
environmental effect. The EPA is not promulgating any additional
control requirements pursuant to CAA section 112(f)(2) but instead
reaffirming the existing standards.\43\
---------------------------------------------------------------------------

\43\ The D.C. Circuit upheld this approach to CAA section
112(f)(2) in NRDC: ``If EPA determines that the existing technology-
based standards provide an `ample margin of safety,' then the Agency
is free to readopt those standards during the residual risk
rulemaking.'' 529 F.3d at 1083.
---------------------------------------------------------------------------

Sections IV.A.2 through IV.A.4 of this preamble provide a more in-
depth analysis of the decision to finalize that the risks from the
source category are acceptable and provide an ample margin of safety
pursuant to CAA section 112(f).

B. What are the final rule amendments based on the technology review
for the HWC NESHAP source category?

The EPA is finalizing, as proposed, that pursuant to CAA section
112(d)(6), there are no developments in practices, processes, and
control technologies that warrant revisions to the MACT standards for
this source category. Sections IV.B.2 through IV.B.4 of this preamble
provide a more in-depth analysis of the decision not to revise the
existing MACT standards under CAA section 112(d)(6).

C. What are the final rule amendments pursuant to CAA sections
112(d)(2) and (3) and 112(h) for the HWC NESHAP source category?

Consistent with our authority to review and revise the HWC NESHAP
and with the proposed rule,\44\ the EPA is finalizing regulatory
provisions to address previously unregulated HAP emissions within the
source category in this rule. Based on a review of available
information, the EPA is finalizing the following pursuant to CAA
sections 112(d)(2), (d)(3), and (h)(1): \45\
---------------------------------------------------------------------------

\44\ See 90 FR 50814 (Nov. 10, 2025).
\45\ See LEAN, 955 F.3d at 1091-99.
---------------------------------------------------------------------------

<bullet> Numeric emission limits for HF and HCN for major source
HWC solid fuel boilers.
<bullet> Work practice standard for HF for major source HWC
incinerators.
<bullet> Numeric emission limit for HCN for major source HWC cement
kilns.
<bullet> Work practice standard for HF for all major source HWC
liquid fuel boilers and numeric emission limits for HCN for some major
source HWC liquid fuel boilers.
The EPA presents the results and final decisions based on the
analyses performed pursuant to CAA sections 112(d)(2), (d)(3), and
(h)(1) below, with separate discussion for each subcategory and HAP.
All emission standards discussed here regulate only HWCs at facilities
that are major sources of HAP. Sections IV.C.2 through IV.C.4 of this
preamble provide a more in-depth discussion of the HF and HCN emission
limitations.
1. Solid Fuel Boilers
a. Hydrogen Fluoride
The EPA is promulgating standards for HF emissions from major
source HWC solid fuel boilers pursuant to CAA sections 112(d)(2) and
(3). The EPA is promulgating a limit at the MACT floor of 6.2 parts per
million by volume (ppmv) HF, dry basis and corrected to seven percent
oxygen, for both existing and new solid fuel boilers.\46\ The EPA is
finalizing these standards as proposed.
---------------------------------------------------------------------------

\46\ New source standards apply to major source HWC solid fuel
boilers for which construction or reconstruction commences after
November 10, 2025.
---------------------------------------------------------------------------

The EPA is finalizing as proposed that existing sources must comply
with the HF emission limits for solid fuel boilers within three years
after June 3, 2026, and must conduct an initial compliance test
demonstrating compliance no later than six months after the compliance
date using EPA Methods 26A or 320. For affected facilities that
commence construction or reconstruction after November 10, 2025, owners
or operators must comply with all requirements of the subpart,
including the HF emission limits, no later than the effective date of
the final rule or upon startup, whichever is later, and must
demonstrate compliance no later than six months after the compliance
date using the same methods. All sources must subsequently demonstrate
compliance once every five years during the comprehensive performance
test (CPT).
b. Hydrogen Cyanide
The EPA is promulgating standards for HCN emissions from major
source HWC solid fuel boilers pursuant to CAA sections 112(d)(2) and
(3). The EPA is promulgating a limit at the MACT floor of 5.0 ppmv HCN,
dry basis and corrected to seven percent oxygen, for both existing and
new solid fuel boilers.\47\ The EPA is finalizing these standards as
proposed.
---------------------------------------------------------------------------

\47\ New source standards apply to major source HWC solid fuel
boilers for which construction or reconstruction commences after
November 10, 2025.
---------------------------------------------------------------------------

The EPA is finalizing as proposed that existing sources must comply
with the HCN emission limits for solid fuel boilers within three years
after June 3, 2026, and must conduct an initial compliance test
demonstrating compliance no later than six months after the compliance
date using EPA Method 320 or, if there are entrained water droplets in
the flue gas, an alternative test method submitted and approved by the
Administrator.\48\ For affected facilities that commence construction
or reconstruction after November 10, 2025, owners or operators must
comply with all requirements of the subpart, including the HCN emission
limits, no later than the effective date of the final rule or upon
startup, whichever is later, and must demonstrate compliance no later
than six months after the compliance date

[[Page 33490]]

using the same methods. Sources must subsequently demonstrate
compliance once every five years during the CPT.
---------------------------------------------------------------------------

\48\ 40 CFR 63.7(f).
---------------------------------------------------------------------------

2. Incinerators
a. Hydrogen Fluoride
The EPA is promulgating a work practice standard with multiple
compliance options for HF emissions from major source HWC incinerators
pursuant to CAA section 112(h) because it is not feasible to prescribe
or enforce a standard of performance.\49\ The same work practice
standard is applicable to both new and existing sources.\50\ In
response to comments, the EPA is finalizing the requirement that one
automatic waste feed cutoff (AWFCO)-interlocked operating parameter
limit (OPL) other than chlorine feed rate, rather than two, is
appropriate for complying with the work practice standard.
---------------------------------------------------------------------------

\49\ See CAA section 112(h)(2) for more information about the
circumstances under which prescribing or enforcing a standard of
performance is not reasonable and the notice of proposed rulemaking
associated with this final action for discussion of why it is not
feasible in this circumstance (90 FR 50814 (Nov. 10, 2025)).
\50\ New source standards apply to major source HWC incinerators
for which construction or reconstruction commences after November
10, 2025.
---------------------------------------------------------------------------

The work practice standard requires a source to comply with their
choice of one of three options. The options of the work practice
standard are as follows:
Option 1: If a source actively controls HCl emissions and the
source has at least one AWFCO-interlocked OPL other than chlorine feed
rate to control HCl, then the source may comply with the HCl and
chlorine gas OPL or limits and indicate in the CPT report and
Notification of Compliance (NOC) that they are demonstrating compliance
with the HF work practice standard by complying with the HCl and
chlorine gas OPL requirements.
Option 2: If a facility does not feed any material with detectable
levels of fluorine to the source, then the source may certify in the
CPT report that no fluorine is fed and indicate in the CPT report and
NOC that they are demonstrating compliance with the HF work practice
standard through the certification.
Option 3: If a facility feeds fluorine to a source and the source
has no active HCl control with at least one AWFCO-interlocked OPL other
than chlorine feed rate to control HCl emissions (Option 1), then the
facility must monitor and record the total fluorine fed to the unit as
a 12-hour rolling average. If at any point the feed rate suggests that
HF emissions may exceed the solid fuel boiler existing source emission
limit for HF (6.2 ppmv HF as calculated according to the HWC NESHAP's
maximum theoretical emissions concentration (MTEC) procedure), then the
source must complete a one-time HF emissions test during the next CPT
at the maximum recorded fluorine feed rate and include the test results
in the CPT report. The source must include the comparison of the HF
MTEC to the solid fuel boiler existing source emission limit for HF in
the CPT plan.
The EPA is finalizing as proposed that existing sources must comply
with the HF work practice standard for incinerators within three years
after June 3, 2026, and must demonstrate compliance through a
certification, test plan, or initial compliance test no later than six
months after the compliance date. Emission testing for HF must use EPA
Methods 26A or 320. For affected facilities that commence construction
or reconstruction after November 10, 2025, owners or operators must
comply with all requirements of the subpart, including the HF work
practice standard, no later than the effective date of the final rule
or upon startup, whichever is later, and must demonstrate compliance no
later than six months after the compliance date. Sources must
subsequently demonstrate compliance once every five years during the
CPT.
b. Hydrogen Cyanide
The EPA did not propose and is not finalizing standards for HCN
emissions from major source HWC incinerators because the EPA does not
have credible data indicating that major source HWC incinerators
measurably emit HCN.\51\
---------------------------------------------------------------------------

\51\ 90 FR 50814 (Nov. 10, 2025).
---------------------------------------------------------------------------

3. Cement Kilns
a. Hydrogen Fluoride
As explained in sections IV.C.2 through IV.C.4 of this preamble,
the EPA is not finalizing standards for HF emissions from major source
HWC cement kilns because the Agency does not have credible data
indicating that HWC cement kilns measurably emit HF.
b. Hydrogen Cyanide
The EPA is promulgating standards for HCN emissions from major
source HWC cement kilns pursuant to CAA sections 112(d)(2) and (3). For
existing sources, the EPA is finalizing as proposed a limit at the MACT
floor of 56 ppmv HCN, dry basis and corrected to seven percent oxygen.
As explained in sections IV.C.2 through IV.C.4 of this preamble, for
new sources the EPA is finalizing for new sources a limit at the
revised MACT floor of 5.5 ppmv HCN, dry basis and corrected to seven
percent oxygen.\52\
---------------------------------------------------------------------------

\52\ New source standards apply to major source HWC cement kilns
for which construction or reconstruction commences after November
10, 2025.
---------------------------------------------------------------------------

The EPA is finalizing as proposed that existing sources must comply
with the HCN emission limits for cement kilns within three years after
June 3, 2026, and must demonstrate compliance through an initial
compliance test no later than six months after the compliance date
using EPA Method 320 or, if there are entrained water droplets in the
flue gas, an alternative test method submitted and approved by the
Administrator.\53\ For affected facilities that commence construction
or reconstruction after June 3, 2026, owners or operators must comply
with all requirements of the subpart, including the HCN emission
limits, no later than the effective date of the final rule or upon
startup, whichever is later, and must demonstrate compliance no later
than six months after the compliance date using the same methods.
Sources must subsequently demonstrate compliance once every five years
during the CPT using EPA Method 320 or, if there are entrained water
droplets in the flue gas, an alternative test method submitted and
approved by the Administrator.\54\
---------------------------------------------------------------------------

\53\ 40 CFR 63.7(f).
\54\ Id.
---------------------------------------------------------------------------

4. Liquid Fuel Boilers
a. Hydrogen Fluoride
The EPA is promulgating a work practice standard with multiple
compliance options for HF emissions from major source HWC liquid fuel
boilers pursuant to CAA section 112(h) because it is not feasible to
prescribe or enforce a standard of performance.\55\ This is the same
work practice standard with multiple compliance options described in
section III.C.2.a of this preamble for incinerators. The same work
practice standard is applicable to both new and existing sources.\56\
The EPA is finalizing these standards as proposed.
---------------------------------------------------------------------------

\55\ See CAA section 112(h)(2) for more information about the
circumstances under which prescribing or enforcing a standard of
performance is not reasonable and the notice of proposed rulemaking
associated with this final action for discussion of why it is not
feasible in this circumstance (90 FR 50814 (Nov. 10, 2025)).
\56\ New source standards apply to major source HWC liquid fuel
boilers for which construction or reconstruction commences after
November 10, 2025.
---------------------------------------------------------------------------

The EPA is finalizing as proposed that existing sources must comply
with the HF work practice standard for liquid

[[Page 33491]]

fuel boilers within three years after the publication of the final rule
and that demonstration through a certification, test plan, or initial
compliance test would occur no later than six months after the
compliance date. For affected facilities that commence construction or
reconstruction after June 3, 2026, owners or operators must comply with
all requirements of the subpart, including the HF work practice
standard, no later than the effective date of the final rule or upon
startup, whichever is later, and must demonstrate compliance no later
than six months after the compliance date. Sources must subsequently
demonstrate compliance once every five years during the CPT. Emission
testing for HF must use EPA Methods 26A or 320.
b. Hydrogen Cyanide
The EPA is promulgating standards for HCN emissions from two
subcategories of major source HWC liquid fuel boilers pursuant to CAA
sections 112(d)(2) and (3). The EPA is finalizing as proposed the
subcategorization of liquid fuel boilers by size, under CAA section
112(d)(1), for the purposes of the HCN emission standard.\57\ The size
categories are as follows: capacity less than or equal to 50 million
british thermal units per hour (MMBTU/hr), capacity greater than 50
MMBTU/hr but less than or equal to 250 MMBTU/hr, and capacity greater
than 250 MMBTU/hr.
---------------------------------------------------------------------------

\57\ 42 U.S.C. 7412(d)(1); see also U.S. Sugar Corp. v. EPA 830
F.3d 579, 593-94 (D.C. Cir. 2016) (``[T]he EPA has discretion to
differentiate among classes, types, and sizes of sources within a
category or subcategory.'' (internal citations omitted)).
---------------------------------------------------------------------------

The EPA did not propose and is not finalizing standards for HCN
emissions from major source HWC liquid fuel boilers with a capacity
that is less than or equal to 50 MMBTU/hr because the Agency does not
have credible data indicating that major source HWC incinerators
measurably emit HCN.
For existing major source HWC liquid fuel boilers with capacity
greater than 50 MMBTU/hr but less than or equal to 250 MMBTU/hr, the
EPA is finalizing as proposed a limit at the MACT floor of 2.7 ppmv
HCN, dry basis and corrected to seven percent oxygen. For new major
source HWC liquid fuel boilers with capacity greater than 50 MMBTU/hr
but less than or equal to 250 MMBTU/hr, the EPA is finalizing as
proposed a limit at the MACT floor of 1.2 ppmv HCN, dry basis and
corrected to seven percent oxygen.\58\
---------------------------------------------------------------------------

\58\ New source standards apply to major source HWC liquid fuel
boilers for which construction or reconstruction commences after
November 10, 2025.
---------------------------------------------------------------------------

For existing major source HWC liquid fuel boilers with capacity
greater than 250 MMBTU/hr, the EPA is finalizing as proposed a limit at
the MACT floor of 3.4 ppmv HCN, dry basis and corrected to seven
percent oxygen. For new major source HWC liquid fuel boilers with
capacity greater than 250 MMBTU/hr, the EPA is finalizing as proposed a
limit at the MACT floor of 1.1 ppmv HCN, dry basis and corrected to
seven percent oxygen.
The EPA is finalizing as proposed that existing sources must comply
with the HCN emission limits for liquid fuel boilers within three years
after June 3, 2026, and must demonstrate compliance through an initial
compliance test no later than six months after the compliance date
using EPA Method 320 or, if there are entrained water droplets in the
flue gas, an alternative test method submitted and approved by the
Administrator.\59\ For affected facilities that commence construction
or reconstruction after June 3, 2026, owners or operators must comply
with all requirements of the subpart, including the HCN emission
limits, no later than the effective date of the final rule or upon
startup, whichever is later, and must demonstrate compliance no later
than six months after the compliance date using the same methods.
Sources must subsequently demonstrate compliance once every five years
during the CPT using EPA Method 320 or, if there are entrained water
droplets in the flue gas, an alternative test method submitted and
approved by the Administrator.\60\
---------------------------------------------------------------------------

\59\ 40 CFR 63.7(f).
\60\ Id.
---------------------------------------------------------------------------

5. HCl Production Furnaces
a. Hydrogen Fluoride
The EPA did not propose and is not finalizing standards for HF
emissions from major source HWC HCl production furnaces because the
Agency does not have credible data indicating that major source HWC HCl
production furnaces measurably emit HF.\61\
---------------------------------------------------------------------------

\61\ 90 FR 50814 (Nov. 10, 2025).
---------------------------------------------------------------------------

b. Hydrogen Cyanide
The EPA did not propose and is not finalizing standards for HCN
emissions from major source HWC HCl production furnaces because the
Agency does not have credible data indicating that major source HWC HCl
production furnaces measurably emit HCN.\62\
---------------------------------------------------------------------------

\62\ Id.
---------------------------------------------------------------------------

6. Lightweight Aggregate Kilns
The EPA did not propose and is not finalizing standards for HF or
HCN emissions from lightweight aggregate kilns because the EPA has no
credible emissions data on which to base decisions about whether or how
to regulate such emissions.\63\ If an HWC lightweight aggregate kiln
begins operating after the promulgation of this final rule, the EPA
expects to collect emissions testing data from them and address
potential emissions in a subsequent action.
---------------------------------------------------------------------------

\63\ Id.
---------------------------------------------------------------------------

D. What are the final rule amendments addressing emissions during
periods of startup, shutdown, and malfunction?

Consistent with Sierra Club v. EPA \64\ and the proposed rule,\65\
the EPA is finalizing revisions to the SSM provisions of the NESHAP to
ensure that HWCs meet MACT standards at all times when controlling HAP
emissions. The EPA is promulgating work practice standards for periods
of SSM. These work practice standards include the following: (1) a
clean fuel requirement for periods of startup and shutdown; (2) a
requirement to follow an approved SSM plan during periods of SSM; and
(3) the AWFCO system requirement. The EPA is also finalizing as
proposed various other changes to modify recordkeeping and reporting
requirements as a result of the SSM provisions.
---------------------------------------------------------------------------

\64\ 551 F.3d 1019 (D.C. Cir. 2008).
\65\ 90 FR 50814 (Nov. 10, 2025).
---------------------------------------------------------------------------

The EPA is finalizing a work practice standard for periods of
startup and shutdown. This work practice standard is the combination of
firing prescribed supplemental clean fuels during periods of startup
and shutdown and operating in accordance with an approved SSM plan
during periods of startup and shutdown. The EPA is also finalizing a
work practice standard for periods of malfunction. This work practice
standard is the combination of AWFCO system requirements and operating
in accordance with an approved SSM plan during periods of malfunction.
Because the SSM plan also includes a description of potential causes of
malfunctions that may result in significant releases of HAP and actions
the source takes to minimize the frequency and severity of those
malfunctions, the source must also comply with any requirements to
minimize the frequency and severity of malfunctions prescribed in the
SSM plan for periods of normal operation.
The EPA is also finalizing that owners or operators must submit SSM
plans to the Administrator for approval within 180 days of the
effective date of this rule or upon initial startup, whichever is

[[Page 33492]]

later, and that sources must begin complying with the SSM plan
immediately upon submittal. While the EPA expects SSM plans to be
reviewed within no more than 90 days of submittal, commenters pointed
out, and the Agency agrees, that owners or operators of HWCs can only
control the submittal date, not the approval date of an SSM plan, and
so the EPA considers that it is more appropriate to tie the compliance
timeline to the action that HWC owners or operators can control.\66\ In
response to comments, the EPA is also finalizing that if changes are
made to the SSM plan as a result of the approval process, sources must
begin complying with the revised SSM plan upon notification of
approval.
---------------------------------------------------------------------------

\66\ See 40 CFR 63.1206(c)(2)(ii)(B) for information about SSM
plan review.
---------------------------------------------------------------------------

As discussed in sections IV.D.2 through IV.D.4 of this preamble,
the EPA is finalizing revised definitions of startup, shutdown, and
supplemental fuel in response to comments received on the proposal.\67\
Commenters requested, and the EPA is finalizing, separate definitions
of startup and shutdown for different types of HWCs and is clarifying
that transitioning from operating under the requirements of an
otherwise applicable requirement constitutes a period of startup.\68\
The definitions of startup are:
---------------------------------------------------------------------------

\67\ See the revised 40 CFR 63.1206(c)(10) and (c)(11).
\68\ See 40 CFR 63.1206(b)(1)(ii) for information about
operating according to an otherwise applicable requirement.
---------------------------------------------------------------------------

<bullet> For incinerators, startup begins with the firing of
supplemental fuel in the combustion chamber or with transitioning from
a period of shutdown. All air pollution control devices (APCDs) must be
in operation as expeditiously as possible and prior to the introduction
of any waste material that is not supplemental fuel into the HWC.
Startup ends once the system has stabilized but no later than 15
minutes after either hazardous waste that is not fed in accordance with
the AWFCO requirements when burning hazardous waste during startup and
shutdown or any waste material that is not supplemental fuel is fed
into the HWC, whichever occurs first.\69\
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\69\ See 40 CFR 63.1206(c)(2)(v)(B) for the AWFCO requirements
when burning hazardous waste during startup and shutdown.
---------------------------------------------------------------------------

<bullet> For cement kilns and lightweight aggregate kilns, startup
begins when a kiln either begins firing supplemental fuel or
transitions from a period of shutdown. All APCDs must be in operation
as expeditiously as possible and prior to the introduction of kiln feed
or any waste material that is not supplemental fuel into the kiln.
Startup ends 120 minutes after the continuous introduction of kiln
feed, when the feed rate exceeds 60 percent of the kiln design
limitation rate, or 15 minutes after hazardous waste that is not fed in
accordance with the AWFCO requirements when burning hazardous waste
during startup and shutdown is fed into the HWC, whichever occurs
first.\70\ Cement kilns may fire traditional fuels as defined in 40 CFR
241.2 once the HWC achieves 1200 degrees Fahrenheit ([deg]F) measured
at a location that best represents, as practicable, the bulk gas
temperature in the combustion zone and all APCDs are operational.
---------------------------------------------------------------------------

\70\ See 40 CFR 63.1206(c)(2)(v)(B) for the AWFCO requirements
when burning hazardous waste during startup and shutdown.
---------------------------------------------------------------------------

<bullet> For solid fuel boilers and liquid fuel boilers, startup
begins with either the first-ever firing of supplemental fuel in a
boiler for the purpose of supplying useful thermal energy (such as heat
or steam) for heating, cooling, or process purposes, or producing
electricity or the firing of fuel in a boiler for any purpose after a
shutdown event. All APCDs must be in operation as expeditiously as
possible and prior to the introduction of any waste material that is
not supplemental fuel into the boiler. Startup ends at the earliest of
the following: four hours after when the boiler supplies useful thermal
energy (such as heat or steam) for heating, cooling, or process
purposes; the boiler produces electricity; or 15 minutes after either
hazardous waste that is not fed in accordance with the AWFCO
requirements when burning hazardous waste during startup and shutdown
or any waste material that is not supplemental fuel is fed into the
boiler.\71\
---------------------------------------------------------------------------

\71\ See 40 CFR 63.1206(c)(2)(v)(B) for the AWFCO requirements
when burning hazardous waste during startup and shutdown.
---------------------------------------------------------------------------

<bullet> For HCl production furnaces, startup begins when the HCl
production furnace either begins firing supplemental fuel or
transitions from a period of shutdown. All APCDs must be in operation
as expeditiously as possible and prior to the introduction of any waste
material that is not supplemental fuel into the HCl production furnace.
Startup ends either 120 minutes after the continuous introduction of
materials intended to produce HCl to the HCl production furnace or 15
minutes after either hazardous waste that is not fed in accordance with
the AWFCO requirements when burning hazardous waste during startup and
shutdown or any waste material that is not supplemental fuel is fed
into the HCl production furnace, whichever is earlier.\72\
---------------------------------------------------------------------------

\72\ See 40 CFR 63.1206(c)(2)(v)(B) for the AWFCO requirements
when burning hazardous waste during startup and shutdown.
---------------------------------------------------------------------------

<bullet> Notwithstanding the previous definitions, transitioning
from an otherwise applicable standard initiates a period of startup
lasting no more than 15 minutes in duration.
The definitions of shutdown are:
<bullet> For incinerators, shutdown begins when hazardous waste
feed to the combustion chamber has been cut off for a period of time
not less than the hazardous waste residence time and the feed of non-
hazardous waste materials to the combustion chamber is cut off.
Shutdown ends when fire is extinguished in the combustion chamber, the
incinerator enters another mode of operation, or when a startup is
initiated.
<bullet> For cement kilns and lightweight aggregate kilns, shutdown
begins when hazardous waste feed to the combustion chamber has been cut
off for a period of time not less than the hazardous waste residence
time and kiln feed is halted. Shutdown ends when continuous kiln
rotation ceases, the kiln enters another mode of operation, or when a
startup is initiated.
<bullet> For solid fuel boilers and liquid fuel boilers, shutdown
begins when the boiler no longer supplies useful thermal energy (such
as heat or steam) for heating, cooling, or process purposes and/or
generates electricity or when no fuel is being fed to the boiler,
whichever is earlier, and when hazardous waste feed to the combustion
chamber has been cut off for a period of time not less than the
hazardous waste residence time. Shutdown ends when the boiler no longer
supplies useful thermal energy (such as steam or heat) for heating,
cooling, or process purposes and/or generates electricity, and no fuel
is being combusted in the boiler, the boiler enters another mode of
operation, or when startup is initiated.
<bullet> For HCl production furnaces, shutdown begins when
hazardous waste feed to the combustion chamber has been cut off for a
period of time not less than the hazardous waste residence time and raw
material feed to the HCl production furnace is halted. Shutdown ends
when the HCl production furnace flame is extinguished, the HCl
production furnace enters another mode of operation, or when a startup
is initiated.
The EPA defines supplemental fuel as one or a combination of the
following fuels: natural gas, synthetic natural gas,

[[Page 33493]]

propane, other gas 1 fuels, distillate oil, syngas, ultra-low sulfur
diesel, kerosene, hydrogen, refinery gas, liquified petroleum gas, and
any other fuel approved in the SSM plan. For solid fuel boilers, the
definition of supplemental fuel includes coal. The EPA also defines
other gas 1 fuels as gaseous fuel that is not natural gas, refinery
gas, or a hazardous waste and does not exceed a maximum Hg
concentration of 40 micrograms per cubic meter of gas. The SSM plan
must specify the basis for determining that any gas qualifies as other
gas 1 fuel.
The EPA is also finalizing as proposed changes in recordkeeping
requirements that are associated with periods of SSM. In accordance
with this final action, sources must retain in the operating record the
start date, start time, and duration (hours) of each startup, shutdown,
or malfunction of affected process, air pollution control, and
monitoring equipment and whether the source followed the SSM plan. For
periods of SSM when the SSM plan was not followed, sources must record
and retain a list of the affected sources or equipment; actions taken
to minimize emissions in accordance with 40 CFR 63.6(e)(1)(i) and 40
CFR 63.8(c)(1)(i); any corrective actions taken to return the affected
unit to its normal or usual manner of operation; whether the failure
occurred during a period of SSM; an estimate of the quantity of each
regulated pollutant emitted; and a description of the method used to
estimate the emissions.
The EPA is finalizing as proposed changes to the HWC NESHAP General
Provisions table related to periods of SSM by revising the
applicability of 40 CFR 63.10. The following general provisions remain
applicable to the HWC NESHAP: 40 CFR 63.10(a), (b)(1), (b)(2)(iii),
(b)(2)(v)-(xiv), (b)(3), (c), (d)(1), (d)(3), (d)(4), (e), and (f). The
following general provisions are no longer applicable to the HWC
NESHAP: 40 CFR 63.10(b)(2)(i) and (ii), (b)(2)(iv), and (d)(2).
Sections IV.D.2 through IV.D.4 of this preamble provide a more in-
depth analysis of the EPA's decisions regarding work practice standards
for periods of SSM.

E. What other changes have been made to the NESHAP?

The EPA is promulgating other changes to the HWC NESHAP. Regarding
electronic reporting, the EPA is finalizing as proposed the requirement
to electronically report performance test results, NOC reports, and
certain other submissions. Regarding technical corrections, the EPA is
finalizing, as proposed, the removal of the requirement that CO is kept
between the average and maximum reported values during the confirmatory
performance test (CfPT) and the never-implemented requirement that
sources install and operate PM continuous emission monitoring systems
(PM CEMS). Regarding clarifications, the EPA is finalizing the
following issues as proposed: explicitly allowing incorporation by
reference of OPLs determined during the CPT in air permits, removing
references that were incorrectly incorporated by reference and have
since expired, clarifying the demonstration of compliance timeframe for
new standards by removing an outdated demonstration of compliance
timeline for the 2005 HWC NESHAP, and other minor editorial
corrections. The EPA is also clarifying that a relative accuracy test
audit (RATA) must be performed within 180 days before every CPT. The
EPA is also finalizing that title V air permitting authorities
including but not limited to States may choose to exempt area sources
not otherwise subject to title V air permitting requirements from the
requirement to obtain a title V permit on a case-by-case basis. Section
IV.E of this preamble provides a more in-depth analysis of the EPA's
decisions regarding these revisions.
EPA Method 0023A is referenced in the amendatory text of this
document and was previously approved for Sec. 63.1208.

F. What are the effective and compliance dates of the standards?

The revisions to the HWC NESHAP standards promulgated in this
action are effective on June 3, 2026. The EPA is finalizing the
timeline for compliance with the HF and HCN limits as proposed.
Existing sources must comply with the HF and HCN emission limits as
applicable by June 3, 2029, which is three years after publication of
this final rule. For existing sources, CAA section 112(i) provides that
the compliance date for standards promulgated under CAA section 112(d)
shall be as expeditious as practicable, but no later than three years
after the effective date of the standard.\73\ As the EPA explained in
the proposal, and as some commenters agreed, owners and operators need
at least three years to implement the requirements that the EPA is
finalizing under CAA section 112(d)(2), (3), and (6).\74\ For example,
sources that complete performance testing to demonstrate compliance
with the HF or HCN emission limits must determine if they need to make
any modifications to comply with the limits, implement any changes,
submit a performance test plan at least one year before testing
commences, get the performance test plan approved, and schedule and
conduct a performance test to demonstrate compliance. Owners or
operators may also require modifications to their title V, other air,
or RCRA permits if they modify operations of their HWC to comply with
the HF or HCN emission limits. As provided in CAA section 112(i) and 5
U.S.C. 801(3), all new affected sources, which are for the purposes of
the HF and HCN standards sources constructed or reconstructed after
November 10, 2025, must comply with all requirements under CAA sections
112(d)(2), (3), (6), and 112(h) immediately upon the effective date,
which is June 3, 2026, or upon startup, whichever is later. Both new
and existing sources must commence performance testing to demonstrate
compliance with the HF and HCN emission limits no later than six months
after their respective compliance dates. The EPA provided additional
rationale for these compliance dates in the preamble to the proposed
rule.\75\ The EPA is clarifying in this final rule that the
Administrator may grant an extension of the compliance timeline, as
appropriate.\76\
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\73\ U.S. Sugar Corp. v. EPA, 113 F.4th 984, 995 (D.C. Cir.
2024) (CAA section 112(i)(3)(A) ``permits EPA to establish a delayed
`compliance date' for any existing-source emission standard, which
may fall up to 3 years after the effective date of such standard'');
see also Ass'n of Battery Recyclers, 716 F.3d at 672 (``Section
112(i)(3)'s three-year maximum compliance period applies generally
to any emission standard . . . promulgated under [section 112].'').
\74\ See chapter 1 of U.S. EPA, Summary of Public Comments and
Responses for the National Emission Standards for Hazardous Air
Pollutants from Hazardous Waste Combustors, available in the docket
for this rulemaking.
\75\ 90 FR 50814, 50851 (Nov. 10, 2025).
\76\ See 40 CFR 63.6(i) and 40 CFR 63.1213 for more information
about an extension of the compliance timeline.
---------------------------------------------------------------------------

The EPA is promulgating new electronic reporting requirements for
all sources, and the Agency is finalizing all electronic reporting
compliance dates as proposed. Owners and operators must submit
notifications of intent to comply, eligibility demonstrations, periodic
SSM reports, and compliance progress reports electronically through a
PDF upload in Compliance and Emissions Data Reporting Interface (CEDRI)
beginning 60 days from the effective date of the final rule, which is
August 3, 2026. As the EPA is not changing the contents and structure
of these reports, but only the manner of submission, the Agency
believes that 60 days is sufficient time for facilities to enroll in

[[Page 33494]]

CEDRI if not already enrolled and to submit these reports
electronically. Owners and operators must submit performance tests and
performance evaluations including RATA electronically using the
Electronic Reporting Tool (ERT) beginning 90 days after the effective
date of the final rule, which is September 1, 2026. In the EPA's
experience, since the ERT has been available for use for over a decade
and stack testing firms are well acquainted with its use, 90 days is
sufficient time to begin electronic reporting using the ERT. Owners and
operators must submit the NOC and the excess emissions and continuous
monitoring system (CMS) performance reports and summary reports
electronically in CEDRI using a spreadsheet template beginning one year
from the effective date of the final rule, which is June 3, 2027, or
one year from the date the EPA makes the template available on the
CEDRI homepage, whichever is later. The EPA believes that one year is
necessary to ensure that facilities can become acquainted with the
spreadsheet template and begin entering data into the new format.
The EPA is promulgating new work practice standards for periods of
SSM, which are described in sections III.D and IV.D of this preamble.
In response to comments, the EPA is finalizing that all facilities must
submit the SSM plan to the Administrator for approval either postmarked
within 180 days of June 3, 2026, or upon initial startup, whichever is
later. For clarity, November 30, 2026, is 180 days after the date of
this rule's effective date. The EPA anticipates that most facilities
are already operating in accordance with their SSM plan. Based on the
EPA's experience and comments received from affected entities, the 180-
day compliance period is a reasonable timeframe for facilities to
review their SSM plans, revise them, if required, based on the contents
of this action, and submit them for approval. All facilities already
implement the AWFCO system requirement of the SSM work practice
standards and so HWC owners and operators do not require additional
time for compliance.
The EPA is finalizing as proposed that all other revisions to the
HWC NESHAP would become applicable on the effective date of the final
rule, which is June 3, 2026, or as otherwise indicated in the
regulatory text. These revisions are technical corrections,
clarifications, and deregulatory actions that do not require
demonstrations of compliance or immediate action on the part of
regulated entities.

IV. What is the rationale for our final decisions and amendments for
the HWC NESHAP source category?

For each issue, this section provides a description of what the EPA
proposed and what the Agency is finalizing, the Agency's rationale for
the final decisions and amendments, and a summary of key comments and
responses. For all comments not discussed in this preamble, comment
summaries and the EPA's responses are in the document titled Summary of
Public Comments and Responses for the National Emission Standards for
Hazardous Air Pollutants from Hazardous Waste Combustors (``Comment
Response Document''), which is in the docket for this rulemaking.\77\
---------------------------------------------------------------------------

\77\ See Docket ID. No. EPA-HQ-OAR-2004-0022.
---------------------------------------------------------------------------

A. Residual Risk Review for the HWC NESHAP Source Category

1. What did we propose pursuant to CAA section 112(f) for the HWC
NESHAP source category?
On November 10, 2025, the EPA proposed that risks posed by HAP
emissions from HWCs are adequately address by existing standards and
acceptable, that the current NESHAP provides an ample margin of safety
to protect public health, and that additional standards are not
necessary to prevent an adverse environmental effect.\78\ The EPA's
residual risk review found that the estimated cancer risks were below
the presumptive limit of acceptability and that the noncancer risk
results indicates a minimal likelihood of adverse noncancer health
effects due to HAP emissions from HWCs. The EPA based the proposed
decision on ample margin of safety on a weighing of relevant factors,
including the risk posed by HWCs, the costs and effectiveness of
additional controls to further reduce risk, and uncertainties and
conservative assumptions in the emission rates used in estimating risk.
---------------------------------------------------------------------------

\78\ 90 FR 50814 (Nov. 10, 2025).
---------------------------------------------------------------------------

In the proposed review, the EPA evaluated risk based on estimates
of current actual HAP emissions from HWCs, maximum allowable HAP
emissions from HWCs, and facility-wide risk based on reported actual
HAP emissions to characterize the risk from HWCs in the context of
whole facility risk. The EPA modeled several metrics for risk due to
HAP inhalation in the proposed review, including cancer risk to the
individual most exposed, total cancer incidence, population within 50
kilometers of an HWC facility exposed to cancer risk greater than or
equal to one in one million, modeled chronic noncancer target organ-
specific hazard index (TOSHI), and maximum modeled acute noncancer
hazard quotient (HQ). The EPA also evaluated in the proposed review
multipathway human health risks including the cancer risk for the
highest exposed individual, the maximum chronic noncancer HQ for
persistent and bioaccumulative HAP, and if the Agency expected an
adverse environmental effect as a result of HAP emissions from HWCs
based on the results of an environmental risk screening analysis. Table
2 of this preamble presents a summary of the results of the proposed
risk analysis. The EPA estimates that no people will be at an increased
risk of cancer greater than or equal to 100-in-1 million based on
source category actual or allowable emissions.

[[Page 33495]]

[GRAPHIC] [TIFF OMITTED] TR03JN26.073

The EPA also considered the uncertainty associated with the
proposed risk assessment. Some of the major uncertainties in the
proposed risk assessment were associated with the RTR emissions
dataset, dispersion modeling, inhalation exposure estimates, and dose-
response relationships.\79\ Considering all of the health risk
information, including the uncertainties, the EPA proposed to conclude
that the risks for this source category under the current NESHAP
provisions are acceptable.
---------------------------------------------------------------------------

\79\ See the preamble to the proposed rule at 90 FR 50830 (Nov.
10, 2025), section III.C.8, and the memorandum Residual Risk
Assessment for the Hazardous Waste Combustor Source Category in
Support of the Risk and Technology Review 2025 Proposed Rule, which
is available in the docket for this rulemaking.
---------------------------------------------------------------------------

As part of the ample margin of safety analysis performed for the
proposal, the EPA evaluated additional potential technologies for
controlling emissions to further reduce risk from HWCs, taking into
consideration costs and economic impacts of controls, technological
feasibility, uncertainties, and other relevant factors.
The EPA evaluated the installation of a Shell Dioxin Destruction
System (SDDS) for control of PCDD/PCDF in the proposal. The EPA
estimated in the proposal that emissions reductions of PCDD/PCDF due to
SDDS installation would have no impact on the cancer maximum individual
risk or the maximum noncancer TOSHI, a minimal impact on the cancer
incidence, and little impact on the number of people exposed to cancer
risks greater than or equal to one, but could potentially lower the
cancer risks that the EPA estimated in the multipathway risk screening.
The EPA did not propose revisions based on this control scenario
pursuant to CAA section 112(f) because of the relatively high capital
and annualized costs compared to relatively low reductions in cancer
risks and emissions reductions.
The EPA similarly evaluated the installation of a GORE Mercury
Control System (GMCS) for control of Hg in the proposal. The EPA
estimated in the proposal that the emission reductions due to GMCS
installation would have no impact on the cancer maximum individual
risk, maximum TOSHI, cancer incidence, or number of people exposed to
cancer risk levels of greater than or equal to one in one million but
could potentially lower the cancer risks estimated in the multipathway
risk screening. Like the SDDS, the EPA did not propose revisions based
on this control scenario pursuant to CAA section 112(f) because of the
relatively high capital and annualized costs compared to relatively low
reductions in cancer risk and emissions reductions.
Based on the EPA's weighing of all the relevant factors presented
in the risk analyses for the HWC NESHAP and all of the other
information discussed earlier in this section, the Agency proposed to
conclude that the current standards provide an ample margin of safety
to protect public health. Therefore, the EPA did not propose any
changes to the HWC NESHAP in order to provide an ample margin of safety
to protect public health or to prevent an adverse environmental effect.
2. How did the risk review for the HWC NESHAP source category change
since proposal?
The EPA did not make any changes to the risk review in this final
rule and is finalizing the residual risk assessment as proposed.
3. What key comments did we receive on the risk review, and what are
our responses?
The EPA received comments in support of and against the proposed
risk

[[Page 33496]]

review. Commenters generally discussed the statutory requirements for
the residual risk review, the methodology used to conduct the residual
risk review, and the EPA's conclusions concerning the residual risk
review. This section provides a summary of and response to key comments
received regarding the statutory requirements for the residual risk
review. Comment summaries and the EPA's responses for additional issues
raised regarding the residual risk review for the HWC NESHAP are in the
Comment Response Document, which is available in the docket for this
rulemaking.
Comment: Two commenters asserted that the D.C. Circuit previously
affirmed the EPA's interpretation of CAA section 112(f)(2) in NRDC v.
EPA \80\ under the second step of the Chevron framework and that, per
Loper Bright, the Agency's interpretations must now reflect the best
reading of the statute.\81\ The commenters stated that the proposed
rule does not reflect the best reading of the statute because CAA
section 112(f)(2)(A) requires the EPA to promulgate risk-based
standards to reduce the maximum individual lifetime cancer risk to less
than one in one million, which they assert the Agency has not done. The
commenters further argued that the EPA's interpretation of CAA section
112(f)(2)(B), which they characterized as a savings provision, makes
CAA section 112(f)(2)(A) meaningless because the maximum individual
cancer risk for the HWC source category would remain above one in one
million. Additionally, the EPA's interpretation allows for readopting
standards that have ``already been promulgated,'' which is contrary to
language contained in CAA section 112(f)(2)(A). One commenter further
asserted that since the EPA has not promulgated any standards under CAA
section 112(f) for this source category, it is in violation of the best
reading of the statute.
---------------------------------------------------------------------------

\80\ 529 F.3d 1077.
\81\ Chevron U.S.A., Inc. v. Natural Res. Def. Council, 467 U.S.
837 (1984); Loper Bright Enters. v. Raimondo, 603 U.S. 369 (2024).
---------------------------------------------------------------------------

Response: The EPA disagrees with the commenters' assertion that the
best reading of CAA section 112(f)(2) requires the EPA to promulgate
risk-based standards to reduce the maximum individual lifetime cancer
risk to less than one in one million. As described in the preamble for
the notice of proposed rulemaking,\82\ CAA section 112(f)(2)(B)
expressly preserves the EPA's use of the two-step approach for
developing standards to address any residual risk and the Agency's
interpretation of ``ample margin of safety'' developed in the National
Emissions Standards for Hazardous Air Pollutants: Benzene Emissions
from Maleic Anhydride Plants, Ethylbenzene/Styrene Plants, Benzene
Storage Vessels, Benzene Equipment Leaks, and Coke By-Product Recovery
Plants (``Benzene NESHAP'').\83\ Specifically, CAA section 112(f)(2)(B)
states that nothing in CAA section 112(f)(2)(A) or in any other
provision of CAA section 112 shall be construed as affecting or
applying to the Administrator's interpretation of CAA section 112 as
set forth in the Benzene NESHAP final rule.\84\ The commenters cite
language in CAA section 112(f)(2)(A), which is subject to the plain
language of CAA section 112(f)(2)(B). The best interpretation of CAA
section 112(f)(2)(B) indicates that the Administrator's interpretation
of a residual risk review as set forth in the Benzene NESHAP should be
used for the residual risk review conducted by the EPA for the HWC
source category. The EPA does not agree that CAA section 112(f)(2)(A)
is best read as compelling the Agency to promulgate standards pursuant
to subsection (f)(2) where, as here, the residual risk is acceptable
and there is an ample margin of safety.
---------------------------------------------------------------------------

\82\ See 90 FR 50814 sections II.A, III.A, III.C, IV.B, and IV.C
(Nov. 10, 2025).
\83\ See 54 FR 38044 (Sept. 14, 1989).
\84\ 42 U.S.C. 7412(f)(2)(B).
---------------------------------------------------------------------------

The EPA further disagrees that the D.C. Circuit precedents
rejecting commenters' argument are no longer valid under Loper Bright.
The D.C. Circuit has rejected the commenters' alternative reading of
CAA section 112(f)(2) on multiple occasions, and the court's reasoning
makes clear that the EPA's longstanding position is at least the better
reading of the statute. In any event, the Supreme Court noted in Loper
Bright that it did ``not call into question prior cases that relied on
the Chevron framework.'' \85\
---------------------------------------------------------------------------

\85\ 603 U.S. at 412.
---------------------------------------------------------------------------

The EPA also disagrees with the commenters' assertion that the
Agency is not promulgating standards pursuant to CAA section 112(f)(2).
As described in the preamble to the proposed rule, the EPA completed
the residual risk review pursuant to CAA section 112(f)(2), proposed to
determine that the risks are acceptable with an ample margin of safety
under the statutory authority of CAA section 112(f)(2), and is
finalizing that determination under the same authority. As previously
discussed, CAA section 112(f)(2)(B) dictates that the Administrator may
continue to use the interpretation set forth in the Benzene NESHAP for
the residual risk review conducted pursuant to CAA section 112(f)(2).
In the Benzene NESHAP, the Administrator determined that existing
levels of control provided an ample margin of safety for ethylbenzene/
styrene process vents and benzene equipment leaks and so the
Administrator did not promulgate a new standard for those processes,
instead drawing a conclusion that additional standards were not
warranted for those processes.\86\ The conclusion that no new standards
were warranted and so no new standards were promulgated in the Benzene
NESHAP combined with CAA section 112(f)(2)(B) leads the EPA to
determine that the best plain language reading of CAA section 112(f)(2)
allows for the Administrator to affirm or readopt existing standards
for the HWC source category under CAA section 112(f)(2).\87\
---------------------------------------------------------------------------

\86\ See 54 FR 38044 (Sept. 14, 1989).
\87\ NRDC, 529 F.3d at 1083 (``If EPA determines that the
existing technology-based standards provide an `ample margin of
safety,' then the Agency is free to readopt those standards during
the residual risk rulemaking.'').
---------------------------------------------------------------------------

4. What is the rationale for our final approach and final decisions for
the risk review?
The EPA's residual risk review focused on whether risks due to HAP
emissions from HWCs are acceptable and if the standards provide an
ample margin of safety to protect public health. The EPA considered
multiple measures of health risk, including cancer risks and incidence
rates, the presence of non-cancer health effects due to chronic or
acute exposure, the potential for an adverse environmental effect, and
the uncertainties of the risk assessment. Based on consideration of all
of the health risk information, factors, results, and uncertainties
discussed in section IV.A.1 of this preamble and in the proposal, the
EPA concludes that the risks due to HAP emissions from the HWC NESHAP
source category are acceptable.\88\ Furthermore, based on the analyses
described in the proposal and elsewhere in this preamble, including the
evaluation of the costs and effectiveness of potential controls to
reduce emissions and risks, the EPA concludes that the HWC NESHAP
provides an ample margin of safety to protect public health. Finally,
based on our evaluation of environmental risks, the EPA concludes that
more stringent standards are not necessary to prevent an adverse
environmental effect. Therefore, the EPA is not promulgating any
additional control requirements pursuant to CAA section 112(f)(2) but

[[Page 33497]]

instead reaffirming the existing standards.\89\
---------------------------------------------------------------------------

\88\ 90 FR 50814 (Nov. 10, 2025).
\89\ The D.C. Circuit upheld this approach to CAA section
112(f)(2) in NRDC: ``If EPA determines that the existing technology-
based standards provide an `ample margin of safety,' then the Agency
is free to readopt those standards during the residual risk
rulemaking.'' 529 F.3d at 1083.
---------------------------------------------------------------------------

B. Technology Review for the HWC NESHAP Source Category

1. What did we propose pursuant to CAA section 112(d)(6) for the HWC
NESHAP source category?
The EPA's technology review under CAA section 112(d)(6) focused on
the identification and evaluation of potential developments in
practices, processes, and control technologies that have occurred since
the promulgation of the HWC NESHAP in 2005. The EPA reviewed various
sources of information to identify any such developments and found that
since 2005, two new control technologies have been employed in the HWC
NESHAP source category on one incinerator, the SDDS for control of
PCDD/PCDF and the GMCS for control of Hg. Based on an evaluation of the
capital cost, annualized cost, potential emission reductions, and HAP
cost effectiveness, the EPA proposed not to consider either the SDDS or
the GMCS a cost-effective technology to further reduce HAP emissions
from sources subject to the HWC NESHAP.
In summary, the EPA did not identify any additional relevant cost-
effective developments in technologies, practices, or processes since
promulgation of the HWC NESHAP in 2005 to further reduce HAP emissions.
Therefore, the EPA did not propose any changes to the MACT standards in
this action as a result of our technology review under CAA section
112(d)(6).\90\
---------------------------------------------------------------------------

\90\ See section IV.D of the proposed rule (90 FR 50814 (Nov.
10, 2025)) and the memorandum Clean Air Act Section 112(d)(6)
Technology Review for the Hazardous Waste Combustor Source Category,
which is available in the docket for this rulemaking, for a more
detailed discussion of the EPA's findings.
---------------------------------------------------------------------------

2. How did the technology review for the HWC NESHAP source category
change since proposal?
The EPA did not make any changes to the technology review in this
final rule and is finalizing the technology review as proposed.\91\ The
EPA provided one additional scenario to estimate the cost-effectiveness
of the SDDS at the average rate of HWC PCDD/PCDF emissions, which is
available in the document titled Clean Air Act Section 112(d)(6)
Technology Review for the Hazardous Waste Combustor Source Category in
the docket for this rulemaking.
---------------------------------------------------------------------------

\91\ 90 FR 50814 (Nov. 10, 2025).
---------------------------------------------------------------------------

3. What key comments did we receive on the technology review, and what
are our responses?
The EPA received comments in support of and against the proposed
technology review results. Commenters generally discussed the criteria
used to evaluate potential developments, the developments evaluated,
and the scope of the technology review. This section provides a summary
of and response to key comments received regarding the EPA's evaluation
criteria for the technology review for the HWC NESHAP. Comment
summaries and the EPA's responses for additional issues raised
regarding the technology review for the HWC NESHAP are in the Comment
Response Document, which is available in the docket for this
rulemaking.
Comment: A commenter disagreed with both the proposed technology
review's calculation of cost-effectiveness for new HAP control
technologies and the conclusion that there would be high cost and low
potential emissions reductions for the identified pollution control
technologies. The commenter was concerned about the social and economic
harm caused by exposure to toxic chemicals on human health, including
families dealing with radiation and chemotherapy treatments, impacts on
child development, reproductive harm, and funeral costs. The commenter
asserted that ``petrochemical polluters'' can easily absorb the costs
from more stringent standards.
Response: The EPA disagrees with the commenter's assertion that the
Agency's calculations of emissions reduction, cost, and cost-
effectiveness in the technology review were inappropriate. As described
in the preamble for the proposed rule, the EPA based its technology
review decisions on the criteria described in CAA section 112(d)(6),
that is, to review and revise the standards as necessary taking into
account developments in practices, processes, and control
technologies.\92\ The EPA's decision on whether it is ``necessary'' to
revise the HWC NESHAP emission standards was based on cost, cost-
effectiveness, technical feasibility, energy implications, non-air
environmental impacts, and emission reductions associated with the
potential application of each development.\93\
---------------------------------------------------------------------------

\92\ See 90 FR 50814 (Nov. 10, 2025) section III.B for more
information about what factors the EPA considers in the technology
review.
\93\ See 90 FR 50814 (Nov. 10, 2025) section IV.D and Document
ID No. EPA-HQ-OAR-2004-0022-0720 entitled Clean Air Act Section
112(d)(6) Technology Review for the Hazardous Waste Combustor Source
Category for more information about the analyses conducted for the
technology review.
---------------------------------------------------------------------------

The EPA estimated that the SDDS could reduce emissions of PCDD/PCDF
by between 7.66 and 211 milligrams of PCDD/PCDF toxic equivalency
quotient (TEQ) per year per unit, with the average reduction on the low
end of the range.\94\ Because the EPA does not expect an adverse
environmental effect resulting from HAP emissions from HWCs, as
described in the environmental risk screening results, the EPA also
does not expect a positive non-air environmental effect from these
modest emission reductions of PCDD/PCDF.\95\ The EPA also estimated a
total capital investment cost of $1,776,000 and a total annualized cost
of $299,000 per year (2024$) per unit. This results in an annualized
cost-effectiveness of between $1.4 million and $39 million (2024$) per
gram of PCDD/PCDF TEQ emission.\96\ Given the modest emission
reductions, lack of non-air environmental effects, high cost of
installation, and lack of cost-effectiveness, the EPA is determining
that the SDDS does not constitute a development that makes it necessary
to revise the HWC NESHAP emission limits.
---------------------------------------------------------------------------

\94\ See the document entitled Clean Air Act Section 112(d)(6)
Technology Review for the Hazardous Waste Combustor Source Category
Final Rule, which is available in the docket for this rulemaking.
\95\ See 90 FR 50814 (Nov. 10, 2025) section IV.B.4 for
discussion of ample margin of safety.
\96\ See the document entitled Clean Air Act Section 112(d)(6)
Technology Review for the Hazardous Waste Combustor Source Category
Final Rule, which is available in the docket for this rulemaking.
---------------------------------------------------------------------------

The EPA estimated that the GMCS could reduce emissions of Hg by
approximately 13 pounds of Hg per year per unit.\97\ Because the EPA
does not expect an adverse environmental effect resulting from HAP
emissions from HWCs, as described in the environmental risk screening
results, the EPA also does not expect a positive non-air environmental
effect from these modest emission reductions of Hg.\98\ The EPA also
estimated a total capital investment cost of $4,143,000 and a total
annualized cost of $804,000 per year (2024$) per unit. This results in
an annualized cost-effectiveness of $62,000

[[Page 33498]]

(2024$) per pound of Hg emission.\99\ Given the modest emission
reductions, lack of non-air environmental effects, high cost of
installation, and lack of cost-effectiveness, the EPA is determining
that the GMCS does not constitute a development that makes it necessary
to revise the HWC NESHAP emission limits.
---------------------------------------------------------------------------

\97\ Id.
\98\ See 90 FR 50814 (Nov. 10, 2025) section IV.B.4 for
discussion of ample margin of safety.
\99\ See the document entitled Clean Air Act Section 112(d)(6)
Technology Review for the Hazardous Waste Combustor Source Category
Final Rule, which is available in the docket for this rulemaking.
---------------------------------------------------------------------------

4. What is the rationale for our final approach for the technology
review?
The EPA's technology review focused on the identification and
evaluation of developments in practices, processes, and control
technologies that have occurred since the EPA promulgated the 2005 HWC
NESHAP. In the proposal, the EPA identified two potential developments
but proposed that they did not necessitate a change in the HWC NESHAP
standards. During the public comment period, the EPA received several
comments on the Agency's proposed determinations for the technology
review. The comments and the EPA's specific responses and rationale for
the Agency's final decisions are in section IV.B.3 of this preamble and
in the Comment Response Document, which is in the docket for this
rulemaking.
No information presented by commenters has led the EPA to change
the Agency's proposed determination under CAA section 112(d)(6). For
the reasons explained in the proposed rule and in this final rule
preamble, the EPA is finalizing that there are no developments in
practices, processes, or control technologies to further reduce HAP
emissions that warrant revisions to the standards.\100\ Consequently,
the EPA is not promulgating any new or revised standards in this action
for the HWC NESHAP as a result of our technology review under CAA
section 112(d)(6).
---------------------------------------------------------------------------

\100\ For more information, see the preamble to the proposed
rule, 90 FR 50814 (Nov. 10, 2025), and section IV.B.1 of this
preamble.
---------------------------------------------------------------------------

C. Amendments Pursuant to CAA Sections 112(d)(2) and (3) and 112(h) for
the HWC NESHAP Source Category

1. What did we propose pursuant to CAA sections 112(d)(2) and (3) and
112(h) for the HWC NESHAP source category?
On November 10, 2025, the EPA proposed emission standards for HF
and HCN pursuant to CAA sections 112(d)(2), (d)(3), and (h)(1) for
major source HWC solid fuel boilers, incinerators, cement kilns, and
liquid fuel boilers. For existing and new major source solid fuel
boilers, the EPA proposed a MACT floor emission limit of 6.2 ppmv HF,
dry basis and corrected to seven percent oxygen. The EPA proposed that
existing sources must comply with the HF emission limit within three
years after publication of the final rule, and that new sources must
comply with the HF emission limits no later than the effective date of
the final rule or upon startup, whichever is later. For both existing
and new sources, the EPA proposed that owners or operators must
demonstrate compliance with this limit through an initial compliance
test using EPA Methods 26A or 320 occuring no later than six months
after the applicable compliance date.\101\ The EPA also proposed that
subsequent performance testing would be required once every five years
during the CPT using the same methods.
---------------------------------------------------------------------------

\101\ New sources are affected facilities that commence
construction or reconstruction after November 10, 2025.
---------------------------------------------------------------------------

For existing and new major source HWC incinerators, cement kilns,
and liquid fuel boilers, the EPA proposed a work practice standard for
HF emissions with multiple compliance options. The EPA proposed that a
source would have a choice of complying with only one of the three
following options:
<bullet> Option 1: If a source actively controls HCl emissions and
the source has at least two AWFCO-interlocked OPLs other than chlorine
feed rate to control HCl, then the facility must comply with the HCl
and chlorine gas OPLs and indicate in the CPT report and NOC that
compliance is demonstrated by complying with the HCl and chlorine gas
OPLs.
<bullet> Option 2: If a facility does not feed any material with
detectable levels of fluorine to the source, then the facility must
certify in the CPT report that no fluorine is fed and indicate in the
CPT report and NOC that compliance is demonstrated through the
certification.
<bullet> Option 3: If a facility feeds fluorine to a source and the
source has no active HCl control with at least two AWFCO-interlocked
OPLs other than chlorine feed rate to control HCl emissions, then the
facility must monitor and record the total fluorine fed to the unit as
a 12-hour rolling average. If at any point the feed rate suggests that
HF emissions may exceed the solid fuel boiler existing source emission
limit for HF (as calculated according to the HWC NESHAP's MTEC
procedure), then the source would complete a one-time HF emissions test
using EPA Methods 26A or 320 during the next CPT at the maximum
recorded fluorine feed rate and include the test results in the CPT
report. The demonstration that HF MTEC does not exceed the solid fuel
boiler existing source emission limit for HF would be included in the
CPT plan.
The EPA proposed that existing sources must comply with the work
practice standard for HF within three years after publication of the
final rule, and that new sources must comply with the work practice
standard for HF no later than the effective date of the final rule or
upon startup, whichever is later. For both existing and new sources,
owners or operators must demonstrate compliance with this work practice
through a certification, test plan, or initial compliance test occuring
no later than six months after the applicable compliance date. The EPA
also proposed to require subsequent demonstration of compliance once
every five years during the CPT.
The EPA also proposed emission standards for HCN pursuant to CAA
sections 112(d)(2) and (d)(3) for major source HWC solid fuel boilers,
cement kilns, liquid fuel boilers with capacity greater than 50 MMBTU/
hr but less than or equal to 250 MMBTU/hr, and liquid fuel boilers with
capacity greater than 250 MMBTU/hr. The EPA did not propose emission
standards for HCN for major source HWC incinerators, liquid fuel
boilers with capacity less than or equal to 50 MMBTU/hr, HCl production
furnaces, or lightweight aggregate kilns.
For existing and new major source HWC solid fuel boilers, the EPA
proposed a MACT floor emission limit of 5.0 ppmv HCN. For existing
major source HWC cement kilns, the EPA proposed a MACT floor emission
limit of 56 ppmv HCN. For new major source HWC cement kilns, the EPA
proposed a MACT floor emission limit of 1.8 ppmv HCN. For existing
major source HWC liquid fuel boilers with capacity greater than 50
MMBTU/hr but less than or equal to 250 MMBTU/hr, the EPA proposed a
MACT floor emission limit of 2.7 ppmv. For new major source HWC liquid
fuel boilers with capacity greater than 50 MMBTU/hr but less than or
equal to 250 MMBTU/hr, the EPA proposed a MACT floor emission limit of
1.2 ppmv HCN. For existing major source HWC liquid fuel boilers with
capacity greater than 250 MMBTU/hr, the EPA proposed a MACT floor
emission limit of 3.4 ppmv HCN. For new major source HWC liquid fuel
boilers with capacity greater than 250 MMBTU/hr, the EPA proposed a
MACT floor emission limit of 1.1 ppmv HCN. All proposed emission limits
are on a dry basis and corrected to seven percent oxygen.

[[Page 33499]]

For all subcategories with proposed limits, the EPA proposed that
existing sources must comply with the applicable HCN emission limit
within three years after publication of the final rule, and that new
sources must comply with the HCN emission limits no later than the
effective date of the final rule or upon startup, whichever is later.
For both existing and new sources, the EPA proposed that owners or
operators must demonstrate compliance with this limit through an
initial compliance test occuring no later than six months after the
applicable compliance date using EPA Method 320 or, if there are
entrained water droplets in the flue gas, an alternative test method
submitted and approved by the Administrator.\102\ The EPA also proposed
that subsequent performance testing would be required once every five
years during the CPT using the same methods.\103\
---------------------------------------------------------------------------

\102\ See 40 CFR 63.7(f) for information about alternative test
methods. New sources are affected facilities that commence
construction or reconstruction after November 10, 2025.
\103\ Section IV.A of the preamble to the proposed rule contains
a complete discussion of the EPA's proposed requirements for HWC
sources under CAA sections 112(d)(2), (3) and 112(h). 90 FR 50814
(Nov. 10, 2025).
---------------------------------------------------------------------------

2. How did the revisions pursuant to CAA sections 112(d)(2) and (3) and
112(h) for the HWC NESHAP source category change since proposal?
The EPA is finalizing as proposed the numeric emission limits for
HF and HCN for solid fuel boilers and the numeric emission limits for
HCN for liquid fuel boilers with capacity greater than 50 MMBTU/hr but
less than or equal to 250 MMBTU/hr and liquid fuel boilers with
capacity greater than 250 MMBTU/hr.\104\ The EPA is finalizing the
proposed work practice standard for HF for incinerators and liquid fuel
boilers with one minor revision in response to comment. Specifically,
sources must have one AWFCO-interlocked OPL other than chlorine feed
rate to control HCl to use Option 1 of the work practice standard. For
cement kilns, in response to information submitted by commenters, the
EPA is finalizing a revised numeric emission limit for HCN. Further,
the EPA is not finalizing the work practice standard for HF for cement
kilns. In addition, the EPA is clarifying that the Administrator may
extend the timeline for compliance with these standards.\105\
---------------------------------------------------------------------------

\104\ 90 FR 50814 (Nov. 10, 2025).
\105\ See CAA section 112(i)(3)(B) and (i)(5), 40 CFR 63.6(i),
and 40 CFR 63.1213.
---------------------------------------------------------------------------

3. What key comments did we receive on the proposed revisions pursuant
to CAA sections 112(d)(2) and (3) and 112(h), and what are our
responses?
This section provides summaries of and responses to key comments
received regarding the EPA's proposed emission standards for HF and
HCN. The comments are organized as: (1) the EPA's beyond-the-floor
evaluation methodology, (2) HF work practice standard, (3) HF emissions
from cement kilns, (4) HCN emission limits for cement kilns, and (5)
health-based emission limits (HBELs). Comment summaries and the EPA's
responses for additional issues raised regarding the revisions pursuant
to CAA sections 112(d)(2) and (3) and 112(h) for the HWC NESHAP are in
the Comment Response Document, which is available in the docket for
this rulemaking.
a. The EPA's Beyond-the-Floor Evaluation Methodology
Comment: One commenter stated that the EPA did not establish
emission limits or evaluate potential beyond-the-floor options for HF
for incinerators, cement kilns, liquid fuel boilers, HCl production
furnaces or lightweight aggregate kilns, which according to the
commenter encompasses 157 out of 164 sources in the HWC category. The
commenter also argued that the EPA did not evaluate beyond-the-floor
controls for HCN emissions at incinerators, HCl production furnaces,
and lightweight aggregate kilns, which according to the commenter
encompasses 81 out of 164 sources in the HWC category. The commenter
noted that there is a ``self-created lack of data,'' but that the EPA
can still determine whether there are additional controls that should
be implemented, and that the EPA must consider what beyond-the-floor
controls are available for these sources.
Response: The EPA did not propose and, as such, is not finalizing
numeric emission limits for HF emissions from HWC incinerators, cement
kilns, lightweight aggregate kilns, liquid fuel boilers, and HCl
production furnaces and for HCN emissions from incinerators,
lightweight aggregate kilns, and HCl production furnaces.\106\ Thus,
the commenter's count of sources appears to be a combination of
subcategories the EPA did not propose to establish emission limits or
evaluate beyond-the-floor options for because of no demonstrated
emissions of HF or HCN and subcategories that the EPA proposed to
regulate through a work practice standard. The EPA disagrees that the
Agency must consider beyond-the-floor standards where we are not
setting emission limits.\107\
---------------------------------------------------------------------------

\106\ 90 FR 50841 (Nov. 10, 2025).
\107\ See NACWA, 734 F.3d at 1157 (noting that ``Congress gave
EPA broad discretion in considering whether to go beyond-the-
floor'').
---------------------------------------------------------------------------

The EPA collected emissions data regarding emissions of HCN from
HWC incinerators and HCl production furnaces. The emissions data
indicated in all test runs for both subcategories that HCN is not
emitted from HWC incinerators and HCl production furnaces.\108\ Under
CAA section 112(d)(1), the EPA is required to set emissions standards
for major source categories and subcategories that emit HAP.\109\ Under
the EPA's historic approach to standard setting, the first step
requires the establishment of the MACT standard developed under CAA
section 112(d)(3). In the second step, which is after establishing the
MACT standard, the EPA then determines whether to set more stringent
standards that control emissions beyond-the-floor considering criteria
and methods contained in CAA section 112(d)(2) (i.e., cost, non-air
quality health and environmental impacts, and energy
requirements).\110\ The EPA has no obligation or statutory authority to
establish emissions limitations for HAP that are not emitted from a
source category and as such the EPA is also under no obligation to
undertake a beyond-the-floor analysis under CAA section 112(d)(2).
---------------------------------------------------------------------------

\108\ For discussion about the EPA's emissions data collection
for HCN from these sources, see section 3.2.2 of the Comment
Response Document, which is available in the docket for this
rulemaking.
\109\ 42 U.S.C. 7412(d)(1).
\110\ 42 U.S.C. 7412(d)(2)-(3); Nat'l Lime Ass'n, 233 F.3d at
634 (``Once the Agency sets statutory floors, it then determines,
considering cost and the other factors listed in section 7412(d)(2),
whether stricter standards are `achievable.' The Agency calls such
stricter requirements `beyond-the-floor' standards.'').
---------------------------------------------------------------------------

As previously explained, the EPA made a reasonable attempt to
collect emissions data where the Agency had reason to believe that a
type of HWC could emit a given HAP. At proposal, the EPA did not have
any credible emissions data regarding HF emissions from lightweight
aggregate kilns and HCl production furnaces and HCN emissions from
lightweight aggregate kilns.\111\ Therefore, the EPA did not propose
MACT standards for HF emissions from HCl production furnaces.
Additionally, the EPA did not propose MACT standards for HF and

[[Page 33500]]

HCN emissions for lightweight aggregate kilns.\112\ As such, the EPA
did not conduct a beyond-the-floor analysis pursuant to CAA section
112(d)(2) for HF emissions from lightweight aggregate kilns or HCl
production furnaces or HCN emissions from lightweight aggregate kilns.
---------------------------------------------------------------------------

\111\ For discussion about the EPA's emissions data collection
for HCl production furnaces and lightweight aggregate kilns, see
sections 3.2.3 and 3.2.4, respectively, of the Comment Response
Document, which is available in the docket for this rulemaking.
\112\ 90 FR 50841 (Nov. 10, 2025).
---------------------------------------------------------------------------

The EPA is finalizing work practice standards for HF emissions from
HWC incinerators and liquid fuel boilers. The EPA requested and
analyzed data on HF emissions \113\ and practices used to control HF
emissions from incinerators and liquid fuel boilers.\114\ The EPA
generally considers a work practice standard to be justified if a
significant majority (e.g., more than 55 percent of test runs) of
emissions data available indicate that emissions are so low that they
cannot be reliably measured (i.e., emissions are below detection
limit). Emissions testing data showed that 94 percent of HF
measurements from HWC incinerators and 75 percent of HF measurements
from HWC liquid fuel boilers were below the detection limit.\115\ The
EPA identified the best performing incinerators and liquid fuel boilers
as those where all HF measurements were below the detection limit.
Sources with non-detectable HF emissions reported that they used the
following methods that control HF emissions: not feeding fluorinated
organics, various control devices that control HCl emissions, and
tracking the feedrate of fluorine.\116\ Based on this data, the EPA
determined that the best performing HWC incinerators and liquid fuel
boilers control HF emissions through one or a combination of not
feeding fluorinated organics, actively controlling HCl emissions, and
tracking the feedrate of fluorine, which forms the basis of the EPA's
work practice standard for HF emissions.
---------------------------------------------------------------------------

\113\ See Document ID No. EPA-HQ-OAR-2004-0022-0724.
\114\ See Document ID No. EPA-HQ-OAR-2004-0022-0651.
\115\ See Document ID No. EPA-HQ-OAR-2004-0022-0724.
\116\ See Document ID No. EPA-HQ-OAR-2004-0022-0651, responses
to questions 39, 48, and 69. Note that no liquid fuel boilers
reported tracking the feedrate of fluorine.
---------------------------------------------------------------------------

The EPA also considered additional measures that would be more
stringent than those reported by HWCs. The EPA proposed expanding the
scope of the feed restrictions from not feeding fluorinated organics to
not feeding fluorine-containing materials. Chemically, if no fluorine
enters an HWC, then the HWC cannot produce HF; this control cannot be
improved upon. As explained in the preamble to the proposed rule, APCDs
that control HCl are equally or more effective at controlling HF than
HCl. As these HWCs already control HF emissions, there is no need for
further control requirements. The information collection indicated that
some HWCs are monitoring fluorine content in their combustor feed; it
did not indicate that HWCs routinely report on the amount of fluorine
fed to the HWC or perform emissions testing for HF. The EPA proposed
and is finalizing the requirement that HWCs tracking fluorine input
would need to calculate a theoretical HF output and if that theoretical
output exceeds the numerical emission limit for existing solid fuel
boilers, perform a one-time HF emission test at their next CPT. This
one-time test performs at least three important functions. First, it
provides data for the EPA to consider in a future technology review if
HF emissions are more common or higher than the current data
suggests.\117\ Second, it provides State regulators with information
they can use to determine if State emission limits for HF are
warranted. Third, it provides the public with transparent information
about HWC emissions of HF. There was no single best performer from this
pool of sources with all HF measurements below detection limit, so the
EPA proposed and is finalizing the same work practice standard for
existing and new sources.
---------------------------------------------------------------------------

\117\ See, e.g., Sierra Club v. EPA, 884 F.3d 1185, 1203 (D.C.
Cir. 2018) (``[D]ata EPA gathers while these rules are in effect
should inform . . . future refinements of these rules when they are
periodically reviewed.'').
---------------------------------------------------------------------------

b. HF Work Practice Standard
Comment: Four commenters supported the work practice standards for
HF emissions. One commenter suggested modifying Option 1 to require one
AWFCO-interlocked operating parameter instead of two.\118\ According to
the commenter, a single AWFCO-interlocked operating parameter would cut
off waste feed as appropriate. The commenter gave the example of a
hazardous waste incinerator that uses HCl CEMS interlocked with the
AWFCO system as the control and concluded that a failure of any OPL or
CEM would trigger an AWFCO. The commenter also questioned the EPA's
proposed numerical emission limit for solid fuel boilers and suggested
the same work practice options for solid fuel boilers because the
fundamental principles for the three options are also valid for solid
fuel boilers. Finally, the commenter reinforced their conclusions by
explaining the technical aspects of HCl and HF control in HWCs.
---------------------------------------------------------------------------

\118\ See proposed 40 CFR 63.1209(s)(1)(i) in Document ID No.
EPA-HQ-OAR-2004-0022-0708.
---------------------------------------------------------------------------

One commenter pointed out that the AWFCO system requirements in the
proposed work practice standards are already in use by many hazardous
waste incinerators.
One commenter suggested that the EPA should allow owners or
operators to demonstrate compliance for Option 1 or 2 with a
notification to the Administrator (e.g., NOC Status Report) without
necessarily including it in the CPT report.
Response: The EPA acknowledges the commenter's support for the work
practice standard for HF emissions from incinerators and liquid fuel
boilers and the many HWCs incinerators that are already in compliance.
In response to comment, the EPA is modifying the work practice standard
option to actively control HCl emissions by finalizing that the HWC
must have at least one AWFCO-interlocked OPL other than chlorine feed
rate and then comply with the HCl and chlorine gas OPLs. The EPA agrees
that complying with one interlocked parameter other than chlorine
feedrate meets the objective of an OPL controlling chlorine emissions;
controlling only chlorine feed rate does not control HF emissions.
The EPA disagrees with the commenter that solid fuel boilers should
be able to use the work practice standard for HF, given that the
emissions test data from solid fuel boilers demonstrated measurable HF
emissions. CAA section 112(h)(1) authorizes the EPA to promulgate a
work practice standard where it is not feasible to prescribe and
enforce a standard.\119\ Because it is feasible to prescribe and
enforce a numerical standard for HF emissions from solid fuel boilers,
the EPA is doing so.
---------------------------------------------------------------------------

\119\ 42 U.S.C. 7412(h)(1).
---------------------------------------------------------------------------

HWCs must report the option they select to comply with the work
practice standard for HF in both the NOC and the CPT report to reduce
confusion and for completeness. The NOC is a streamlined summary of the
outcomes of a CPT, which is in a more readable format. The CPT report
contains all the information needed to determine how a source is
complying with the HWC NESHAP, and so the EPA believes that it is
reasonable to include this compliance information in the CPT report.
Given the minimal burden associated with stating how an HWC incinerator
or liquid fuel boiler is complying with the HF work practice standard,
the EPA is finalizing as proposed the requirement to state the

[[Page 33501]]

method of compliance in both the CPT report and NOC.
c. HF Emissions From Cement Kilns
Comment: One commenter asserted that cement kilns do not emit HF
and provided information to show that HF from cement kiln stack tests
during testing for the EPA's emissions testing request was invalid data
because of calibration gas contamination. The commenter challenged the
EPA's conclusion at proposal that 71 percent of HF data were below the
detection limit for cement kilns, asserting that this 71 percent
included invalid data and, without the invalid data, 100 percent of the
remaining HF data were below the detection limit. The commenter
referenced previous communications with the EPA warning against using
HF reference gas and explaining why HF contaminates the performance
tests. The commenter also cited a previous emissions testing request
for the Portland Cement NESHAP to support these conclusions.
The commenter identified the following issues with the EPA's
proposed approach for HF emissions from cement kilns:

<bullet> The inability to identify facilities with HCl controls.
<bullet> Non-compliance with an HCl OPL would lead to an automatic
non-compliance for HF even though cement kilns do not emit HF.
<bullet> Incinerators can certify that there is no fluorine fed,
but there might be a detectable amount of HF in the raw materials
including fluorine found in limestone.
<bullet> Comparing MTEC against the solid fuel boiler numeric HF
limit is inappropriate for cement kilns because they are not boilers.
<bullet> Fluorine monitoring would be needlessly burdensome because
it would require updates to feedstream analysis, data acquisition and
handling systems to calculate, and procuring new lab equipment and
training staff on new lab methods.
Response: After reviewing the information provided by the
commenter, the EPA agrees that HF was not measurably present in cement
kiln emissions and further agrees that the long period of time
necessary for HF to purge from the sample system when there is no
measurable HF in the sample stream caused the few values that were
above detection limit. The EPA disagrees with the commenter that it is
always inappropriate to calibrate using HF, as the ``sticky'' nature of
HF makes it more important to demonstrate that the sampling system can
accurately quantify HF in the gas stream. This is a not an otherwise
unknown situation with certain gases in stack sampling and is analogous
to SO<INF>2</INF> CEMS calibrations on low emitting SO<INF>2</INF>
sources, where purging SO<INF>2</INF> from a system post calibration
requires an atypically long time compared to other typical CEMS gases.
The EPA is not promulgating any final HF work practice for cement kilns
as HWC cement kilns do not emit measurable quantities of HF.
d. HCN Emission Limits for Cement Kilns
Comment: One commenter challenged the EPA's proposed HCN emission
limit for new cement kilns. The commenter highlighted that the limit
was based on a single best performing HWC cement kiln for HCN, which is
a long wet-process kiln with no APCD for HCN. The commenter attested
that this type of kiln has the worst emission profile for criteria
pollutants. The commenter cautioned that there are no feasible controls
to reduce HCN and explained why caustic scrubbers and RTOs are not
feasible control measures for cement kilns. The commenter concluded
that new and modified sources would be unable to simultaneously meet
the proposed HCN new source limit and the criteria pollutant standards,
even with pollution controls, and that existing hazardous waste cement
kilns could not modernize and would have to close. The commenter
recommended requiring sources to develop site-specific HBELs if
necessary, in the future.
Response: In response to comment, the EPA is finalizing an HCN
emission limit for new HWC cement kilns of 5.5 ppmv, dry basis at seven
percent oxygen. The EPA agrees with the commenter that the Agency
calculated the proposed HCN limit for new HWC cement kilns, 1.8 ppmv,
dry basis at seven percent oxygen, based on data from a long wet-
process kiln. In a follow-up conversation with representatives of all
companies that own or operate HWC cement kilns, the EPA learned about
the low likelihood of future construction of long wet process cement
kilns based on industry clarification that included long wet-process
kiln emitting more criteria pollutants than preheater/precalciner
cement kilns, along with energy inefficiencies of long wet-process
kilns in comparison to other kiln types.\120\ The EPA further confirmed
that the HWC wet process kilns owned by Heidelberg Materials in
Logansport, IN were built between 1961 and 1965 \121\ while the HWC wet
process kilns owned by Amrize in Paulding, OH were constructed in the
1950s.\122\ To the EPA's knowledge, no other long wet-process HWC
cement kilns have been constructed since 1965. Additionally, companies
are even replacing wet-process kilns with dry process kilns.\123\
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\120\ See the memorandum entitled Meeting and Communication
Record Between the U.S. EPA and Representatives of the Cement Kiln
Recycling Coalition, which is available in the docket for this
rulemaking.
\121\ See Document ID No. EPA-HQ-OAR-2004-0022-0695, attachments
8 and 11.
\122\ See <a href="https://edocpub.epa.ohio.gov/publicportal/ViewDocument.aspx?docid=928108">https://edocpub.epa.ohio.gov/publicportal/ViewDocument.aspx?docid=928108</a>.
\123\ See Document ID No. EPA-HQ-OAR-2004-0022-0650.
---------------------------------------------------------------------------

CAA section 112(d)(3) requires ``that the maximum degree of
reduction in emissions that is deemed achievable for new sources in a
category or subcategory shall not be less stringent than the emission
control that is achieved in practice by the best controlled similar
source, as determined by the Administrator.'' \124\ Based on comments
received at proposal, the EPA reasonably believes that owners or
operators of HWC cement kilns will not construct new long wet-process
kilns in the future. Accordingly, the EPA does not view a long wet-
process kiln as an appropriate ``best controlled similar source'' for
new HWC cement kilns.\125\ Instead, the EPA views the best performing
dry cement kiln as the ``best controlled similar source'' for new HWC
cement kilns.\126\ The EPA has the ``discretion to determine what
metric to use in defining the `best' source, so long as it is
reasonable.'' \127\ Additionally, the D.C. Circuit ``has generally
acknowledged that EPA may exercise discretion and utilize its expertise
when calculating emission standards for categories of sources'' under
CAA sections 112(d)(2) and (d)(3).\128\ Accordingly, the MACT floor
emission limit for HCN emissions from new HWC cement kilns is 5.5 ppmv,
dry basis at seven percent oxygen. It is also worth noting that the EPA
solicited comment on this limit as a new source HCN limit for a
potential subcategory of dry cement kilns in the preamble to the
proposed rule.\129\
---------------------------------------------------------------------------

\124\ 42 U.S.C. 7412(d)(3).
\125\ Cement Kiln Recycling Coal., 255 F.3d at 871(explaining
that ``[f]loors need not be perfect mirrors of the best performers'
emissions'').
\126\ See Ne. Md. Waste Disposal Auth. v. EPA, 358 F.3d 936, 945
(D.C. Cir. 2004) (``The word `similar' [in CAA section 129(a)(2)]
may reasonably be read as referring to a unit that is in the same
subcategory.'').
\127\ Sierra Club, 895 F.3d at 15.
\128\ U.S. Sugar Corp., 113 F.4th at 999.
\129\ 90 FR 50814 (Nov. 10, 2025).
---------------------------------------------------------------------------

At proposal, the EPA evaluated whether the incremental emissions
reduction achievable with RTO would be cost-effective. The EPA
estimated

[[Page 33502]]

that RTO would achieve approximately 95 percent reduction of HCN. This
may be an overestimation of effectiveness given the relatively high HCN
emissions from one Portland cement kiln with RTO installed.\130\ A 95
percent reduction from the UPL MACT floor due to RTO results in an
emission limit of 0.275 ppmv, which is below three times the RDL value
for HCN for cement kilns (1.1 ppmv, dry basis at seven percent oxygen).
Therefore, the evaluated beyond-the-floor-limit for new sources is 1.1
ppmv, dry basis at seven percent oxygen. This is the same beyond-the-
floor new source limit that the EPA evaluated in the preamble to the
proposed rule at section IV.A.3.b with the same associated costs and
benefits.\131\ For the reasons explained in the preamble to the
proposed rule, the EPA does not consider that installation and
operation of RTO for the beyond-the-floor control of HCN emissions from
HWC cement kilns is cost-effective for new HWC cement kilns and would
have additional non-air quality health and environmental impacts and
energy requirements.\132\ Therefore, the EPA is finalizing a new source
limit for HCN emissions from HWC cement kilns as 5.5 ppmv, dry basis at
seven percent oxygen.
---------------------------------------------------------------------------

\130\ U.S. Environmental Protection Agency. (Last updated Mar.
30, 2026). Portland Cement Manufacturing Industry: Information
Collection Request Data: <a href="https://www.epa.gov/stationary-sources-air-pollution/portland-cement-manufacturing-industry-information-collection">https://www.epa.gov/stationary-sources-air-pollution/portland-cement-manufacturing-industry-information-collection</a>).
\131\ 90 FR 50814 (Nov. 10, 2025).
\132\ 90 FR 50837-39. For additional discussion about RTO cost-
effectiveness, see section 3.6 of the Comment Response Document,
which is available in the docket for this rulemaking.
---------------------------------------------------------------------------

e. Health-Based Emission Limits
Comment: Several commenters supported adopting HBELs for HF and HCN
under CAA section 112(d)(4). Some of these commenters suggested that
the EPA should adopt the site-specific risk assessment approach
currently used for the alternative HCl HBEL.\133\ Two commenters
pointed out that emissions of HF and HCN from sites electing to
implement an HBEL will necessarily be at a level that provides an ample
margin of safety. Similarly, another commenter argued that using HBELs
provides a flexible alternative that allows the EPA to ensure standards
remain protective of public health with an ample margin of safety
without unnecessarily locking future HBELs into a particular form of
limit or method for establishing them.
---------------------------------------------------------------------------

\133\ 40 CFR 63.1215.
---------------------------------------------------------------------------

Many commenters argued that CAA section 112(d)(4) allows the EPA to
set risk-based standards in lieu of the technology-based standards. The
commenters said that rather than relying on the MACT floor approach in
CAA section 112(d)(2) and (3) and (h), which can overregulate by
emphasizing technological performance instead of actual risk reduction,
Congress intentionally allows the EPA, in CAA section 112(d)(4), to
avoid unnecessary over-regulation by tailoring the stringency of
emission limits for a given threshold HAP to the specific health risks
posed by that pollutant in a particular source category. A commenter
elaborated that given that CAA section 112(d)(4) is under the same
subsection as 112(d)(1), the EPA's authority to utilize a health
threshold approach is in addition to its authority to promulgate an
emissions standard; therefore, the EPA may elect to establish a health
threshold with an ample margin of safety under CAA section 112(d)(4) or
adopt a MACT standard as set forth under CAA section 112(d)(2). The
commenter pointed out that in Sierra Club v. EPA, the Court recognized
that the ``EPA may use a health threshold rather than MACT standards
for `pollutants for which a health threshold has been established.'
Such a health-based standard must include an `ample margin of safety.'
'' \134\ The commenter said that according to the EPA, an HBEL does not
need to be as stringent as a MACT standard.\135\ The commenter added
that the D.C. Circuit Court has confirmed the EPA's understanding that
HBELs can be less stringent than MACT standards.\136\
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\134\ See Sierra Club v. EPA, 895 F.3d 1, 8 (D.C. Cir. 2018);
see also U.S. Sugar Corp., 830 F.3d at 624 (recognizing the EPA's
authority to set HBEL); Desert Citizens Against Pollution v. EPA,
699 F.3d 524, 525 (D.C. Cir. 2012) (determining that seven
bioaccumulative hazardous air pollutants listed under CAA section
112(c)(6) are subject to standards under subsections 112(d)(2) or
(d)(4)).
\135\ 63 FR 18754, 18765 (Apr. 15, 1998) (explaining that an
HBEL may be less stringent than a MACT under CAA section 112(d)(4)
in the NESHAP for Chemical Recovery Combustion Sources at Kraft,
Soda, Sulfite, and Stand-Alone Semichemical Pulp Mills); 69 FR
55218, 55241 (Sept. 13, 2004); 70 FR 59402, 59479 (Oct. 12, 2005).
\136\ See U.S. Sugar Corp., 830 F.3d at 623-24 (recognizing the
EPA's authority to establish HBELs and finding that the EPA ``may
consider adopting alternative health-based emission standards--which
are more lenient--for pollutants with an established health
threshold'').
---------------------------------------------------------------------------

Commenters in favor of HBELs for HF and HCN provided the following
additional arguments to reinforce their support:
<bullet> The EPA has often tried to set MACT floors using limited
data, resulting in standards that are overly burdensome and technically
infeasible.
<bullet> Neither HF or HCN has been determined to be a non-
threshold pollutant by the EPA or any other similar public health
agency.
<bullet> Citing the 2024 supplemental proposed rule for the lime
manufacturing NESHAP, completion of the one-time CAA section 112(f)
residual risk review lays a solid foundation for a subsequent
HBEL.\137\
---------------------------------------------------------------------------

\137\ 89 FR 9088, 9093 (Feb. 9, 2024) (``Because the hazards
associated with HCl were acceptable with an ample margin of safety
in the 2020 RTR, it is possible to contemplate setting an HBEL for
this rule.'').
---------------------------------------------------------------------------

<bullet> Risk from small levels of HCN emissions is presumably even
lower than 9-in-1 million even at the highest risk site, which is far
under the EPA's 100-in-1-million residual risk threshold.
<bullet> The EPA's precedent on HBELs in the HWC NESHAP, Lime
NESHAP, and the Brick and Structural Clay Products Manufacturing and
Clay Ceramic Manufacturing NESHAPs demonstrate the flexibility that the
EPA can employ to ensure that HBELs are implemented in a manner
appropriate for each applicable industry.
On the contrary, a commenter said that adopting HBELs for HF and
HCN under CAA section 112(d)(4) would be unlawful and arbitrary and
capricious. The commenter argued that: (1) the CAA does not authorize
the EPA to set health-based emission limitations under section
112(d)(4) that are less stringent than the floor limits required under
CAA section 112(d)(2) and (3); (2) the EPA may only set health-based
limits with substantial evidence that the regulated pollutant(s) is not
carcinogenic; and (3) the EPA may, in all cases, only set health-based
limits if it has substantial evidence of an established threshold for
the regulated HAP.
The commenter elaborated by citing the language of CAA section
112(d)(4) that allows the Administrator to ``consider'' health
threshold levels ``when establishing emission standards under this
subsection.'' The commenter says that this language indicates that
Congress wanted CAA section 112(d)(4) to provide for more stringent
standards. The commenter said that CAA section 112(d)(2) provides that
standards ``promulgated under this subsection'' must meet the
stringency requirements in section 112(d)(2) and (3). The commenter
concluded that because CAA section 112(d)(2) requires the ``maximum''
reductions that are achievable ``including a prohibition on such
emissions, where achievable,'' reading CAA section 112(d)(4) as
authorizing more stringent standards is consistent with section
112(d)(2). The commenter added that the EPA cannot rely on Senator
Durenberger's floor

[[Page 33503]]

statement for authority to set limits that are less stringent than CAA
section 112(d)(2) and (3) require.\138\
---------------------------------------------------------------------------

\138\ National Emission Standards for Hazardous Air Pollutants
from Chemical Recovery Combustion Sources, 63 FR 18754, 18765 (Apr.
15, 1998) (quoting 1 Comm. on Env't & Pub. Works, 103d Cong., A
Legislative History of the Clean Air Act Amendments of 1990, at 876
(1993) (statement of Sen. Durenberger)).
---------------------------------------------------------------------------

The commenter also asserted that the EPA cannot show with
substantial evidence that HF or HCN are not carcinogens; therefore, the
EPA cannot set a health-based emission threshold for these pollutants.
The commenter said that for HCN, the EPA has found that it does not
have adequate evidence to determine whether it is carcinogenic.\139\
The commenter pointed out that the EPA has stated that there are no
studies on the carcinogenicity of HCN.\140\ The commenter said that for
HF, the EPA has stated that ``[s]tudies investigating the carcinogenic
potential of hydrogen fluoride are unreliable due to confounding
factors and because of lack of breakdown by type of fluoride
exposure.'' \141\
---------------------------------------------------------------------------

\139\ U.S. EPA, Hydrogen Cyanide and Cyanide Salts, <a href="https://iris.epa.gov/ChemicalLanding/&substance_nmbr=60">https://iris.epa.gov/ChemicalLanding/&substance_nmbr=60</a> (last updated Sep.
28, 2010).
\140\ U.S. EPA, Cyanide Compounds (``Cyanide Fact Sheet'') 2
(2000), <a href="https://www.epa.gov/sites/default/files/2016-09/documents/cyanide-compounds.pdf">https://www.epa.gov/sites/default/files/2016-09/documents/cyanide-compounds.pdf</a>.
\141\ U.S. EPA, Hydrogen Fluoride (Hydrofluoric Acid) (``HF Fact
Sheet'') 3 (2016), <a href="https://www.epa.gov/sites/default/files/2016-10/documents/hydrogen-fluoride.pdf">https://www.epa.gov/sites/default/files/2016-10/documents/hydrogen-fluoride.pdf</a>.
---------------------------------------------------------------------------

Finally, the commenter said that even if the EPA could set an HBEL
for carcinogens under CAA section 112(d)(4), it could not establish a
threshold for HF and HCN because: (1) the EPA has insufficient evidence
to assess whether either pollutant is carcinogenic, so it cannot
establish any cancer-related threshold; (2) for HCN, no inhalation
studies on reproductive/developmental effects exist, and the EPA's
reference concentration is rated ``low confidence,'' \142\ which the
D.C. Circuit has already ruled cannot support an HBEL; \143\ and (3)
for HF, the EPA has no reference concentration or dose, and no human
inhalation data on reproductive/developmental effects, so no
``established'' threshold exists.\144\
---------------------------------------------------------------------------

\142\ U.S. EPA, Cyanide Compounds (``Cyanide Fact Sheet'') 2
(2000), <a href="https://www.epa.gov/sites/default/files/2016-09/documents/cyanide-compounds.pdf">https://www.epa.gov/sites/default/files/2016-09/documents/cyanide-compounds.pdf</a>.
\143\ Sierra Club, 895 F.3d at 12.
\144\ U.S. EPA, Hydrogen Fluoride (Hydrofluoric Acid) (``HF Fact
Sheet'') 3 (2016), <a href="https://www.epa.gov/sites/default/files/2016-10/documents/hydrogen-fluoride.pdf">https://www.epa.gov/sites/default/files/2016-10/documents/hydrogen-fluoride.pdf</a>.
---------------------------------------------------------------------------

Response: The EPA acknowledges commenter support and opposition
regarding an HBEL for HF or HCN in the HWC NESHAP. The EPA finds that
additional time is needed to evaluate the existing body of evidence
regarding toxicology and ecological impacts of HCN and HF in order to
determine whether there is an existing threshold for HCN or HF.\145\
For this reason, the EPA is not promulgating an HBEL for HCN or HF at
this time.
---------------------------------------------------------------------------

\145\ See generally Sierra Club v. EPA, 895 F.3d at 9-13.
---------------------------------------------------------------------------

4. What is the rationale for our final approach and final decisions for
the revisions pursuant to CAA sections 112(d)(2) and (3) and 112(h) for
the HWC NESHAP source category?
The EPA evaluated the comments on the proposed emission and work
practice standards for HF and HCN. Consistent with the order issued by
the DC District Court addressing our obligations to review and revise
the HWC NESHAP \146\ and the proposal,\147\ the EPA is finalizing
actions to address HF and HCN emissions from HWCs in this final rule
pursuant to CAA sections 112(d)(2) and (3) and 112(h). The EPA notes
that if the Agency had set the HF and HCN limits pursuant to CAA
section 112(d)(6) as compared to CAA sections 112(d)(2) and (3), the
Agency would not have any cost or impact differences because the
estimated costs are for testing, recordkeeping, and reporting under all
provisions. Additionally, for HF and HCN emission limits in all
subcategories if the EPA had set the standard when promulgating the HWC
NESHAP in 2005, the Agency would have set substantially similar
standards because the best performing sources have not modified their
operations for the purpose of reducing HF or HCN emissions between 2005
and 2026.
---------------------------------------------------------------------------

\146\ Order, Blue Ridge Envtl. Def. League v. Regan, 22-cv-3134
(APM) (D.D.C. Dec. 12, 2024).
\147\ See 90 FR 50814 (Nov. 10, 2025).
---------------------------------------------------------------------------

Therefore, the EPA is finalizing the proposed emission limits for
HF from solid fuel boilers, HCN from solid fuel boilers, and HCN from
liquid fuel boilers. The EPA is also finalizing emission limits for HCN
from cement kilns and a work practice standard for HF emissions from
incinerators and liquid fuel boilers. More information and rationale
concerning the amendments that the EPA is finalizing pursuant to CAA
section 112(d)(2) and (3) and 112(h) are in the preamble to the
proposed rule, section IV.C.3 of this preamble and in the Comment
Response Document, which is in the docket for this rulemaking.

D. Changes to Provisions for Periods of Startup, Shutdown, and
Malfunction

1. What SSM provisions did we propose for the HWC NESHAP source
category?
On November 10, 2025, the EPA proposed revisions to the SSM
provisions of the HWC NESHAP consistent with Sierra Club v. EPA, to
ensure that HWCs meet MACT standards at all times when controlling HAP
emissions.\148\ The EPA proposed to remove the exemption from emission
standards and operating requirements during periods of SSM and to add
new work practice standards that apply for periods of SSM.\149\
---------------------------------------------------------------------------

\148\ 551 F.3d 1019 (D.C. Cir. 2008) (vacating the SSM exemption
contained in 40 CFR 63.6(f)(1) and 40 CFR 63.6(h)(1), and holding
that under CAA sections 112 and 302(k), emission standards or
limitations must be continuous in nature).
\149\ See 40 CFR 63.1206(b)(1)(i) for the SSM exemption.
---------------------------------------------------------------------------

For periods of startup and shutdown, the EPA proposed a clean fuel
requirement that would limit which supplemental fuels could be burned
during those periods to minimize emissions of HAP. For all periods of
SSM, the EPA also proposed that all sources must have an approved SSM
plan and also proposed adding an explicit requirement that sources must
operate according to their approved SSM plan during periods of SSM. For
periods of malfunction, the EPA also proposed the current AWFCO
requirements as part of the work practice standards.\150\
---------------------------------------------------------------------------

\150\ 80 FR 75178, 75211-14 (Dec. 1, 2015); see also 85 FR
49434, 49441-46 (Aug. 13, 2020).
---------------------------------------------------------------------------

At proposal, the EPA explained that the HWC NESHAP did not involve
issues relating to affirmative defenses like the one at issue in SSM
Litigation Group v. EPA, but nevertheless requested comment on whether
and how the Agency should establish regulations within this and other
New Source Performance Standards (NSPS) or NESHAPs in response to the
court decision.\151\ The EPA further explained that the Agency intended
to address the impacts of the SSM Litigation Group decision in an
appropriate future action given the court-ordered deadline for the HWC
NESHAP and the timing of the decision.\152\ The EPA received comments
in support of a future Agency action to add affirmative defenses in
NSPS and NESHAPs through a separate rulemaking. However, these
supportive

[[Page 33504]]

comments did not provide a suggested specific timeframe for the EPA to
promulgate such a rule. Conversely, the EPA also received comments
urging the Agency not to engage in rulemaking for purposes of creating
any affirmative defense or exemption. A summary of these comments is
available in the Comment Response Document, which is in the docket for
this rulemaking.
---------------------------------------------------------------------------

\151\ SSM Litig. Grp. v. EPA, 150 F.4th 593, 599 (D.C. Cir.
2025) (holding that a ``complete affirmative defense . . . is
permissible because it relates to the antecedent question of
liability and therefore does not impinge on the judiciary's
authority to award `appropriate civil penalties'' ').
\152\ 90 FR 50814, 50847 (Nov. 10, 2025).
---------------------------------------------------------------------------

2. How did the SSM provisions for the HWC NESHAP source category change
since proposal?
The EPA is finalizing work practice standards for periods of SSM
that are generally consistent with the Agency's proposal. Additionally,
in response to comments received on the proposed SSM provisions, the
EPA is making the following revisions to the proposal: (1) adding
distinct definitions of startup and shutdown for different types of
HWCs, (2) requiring that all APCDs must be in operation as
expeditiously as possible and before any waste material that is not
supplemental fuel is introduced into the HWC, (3) clarifying that
transitioning from a mode of operation representing an otherwise
applicable standard triggers a period of startup, (4) for solid fuel
boilers only, including coal in the definition of supplemental fuel,
(5) allowing cement kilns to burn traditional fuels during periods of
startup after the combustion chamber reaches 1200 [deg]F and the HWC is
operating all APCDs, (6) adding a definition for ``other gas 1 fuel,''
(7) requiring that the SSM plan be submitted, not approved, within 180
days of the effective date of this final rule, (8) requiring HWCs to
comply with their SSM plans upon submittal and any changes to their SSM
plans as a result of the approval process upon notification of
approval, and (9) clarifying text regarding AWFCOs during periods of
SSM.
3. What key comments did we receive on SSM provisions for the HWC
NESHAP source category?
This section provides summaries of and responses to key comments
received regarding the EPA's proposed work practice standards for
periods of SSM. The comments are organized as follows: (1) legal
sufficiency of SSM work practice standards, (2) definitions of startup
and shutdown, (3) definition of supplemental fuel, (4) SSM plan
approval, and (5) AWFCO during periods of SSM. Comment summaries and
the EPA's responses for additional issues raised regarding the
revisions pursuant to CAA sections 112(d)(2) and (3) and 112(h) for the
HWC NESHAP are in the Comment Response Document, which is available in
the docket for this rulemaking.
a. Legal Sufficiency of SSM Work Practice Standards
Comment: Several commenters supported the EPA's partial withdrawal
of the July 2024 proposal to apply numeric emission standards during
malfunction events. Some of these commenters said that the EPA's
proposed work practice standards for periods of SSM align with waste
combustor design and pollution control systems that improve safety and
reduce emissions, while also matching established industry best
practices for operating during SSM conditions. A commenter said that
SSM events are brief, typically lasting under three hours, which is
shorter than the minimum test duration required by the EPA, and
represent only temporary operating conditions for HWCs. Another
commenter said that the EPA's proposed work practice standards for
periods of SSM are consistent with the design of waste combustors and
associated pollution control systems, which are based on best available
control technology and allow for the use of Emergency Safety Vents
(ESVs). The commenter pointed out that the ESV feature is integral to
the design of a combustor and intended to prevent a catastrophic
failure of pollution control equipment and possible danger to plant
personnel and combustor equipment in the event of an unplanned shutdown
or malfunction. Similarly, another commenter noted that enforceable SSM
standards improve safety and environmental accountability. The
commenter requested that the EPA spell out exactly what compliance
should look like during SSM events and to acknowledge that actions
taken to protect workers, equipment, and nearby communities should not
create unintended liability or discourage responsible emergency
decision-making.
Another commenter warned that imposing numerical emission limits
uniformly during SSM events: (1) disregards the physical limitations of
combustion systems and control equipment, (2) does not necessarily
reduce emissions, and (3) may instead encourage unsafe or unstable
operating practices. The commenter provided the following rationale in
support of work practice standards for periods of SSM in lieu of
emission limits:
<bullet> Properly designed work practice requirements focus on
operator actions, system controls, and procedural safeguards that
influence emissions during transitional events. They provide clearer
compliance expectations and more meaningful environmental protection
than after-the-fact numerical violations.
<bullet> Penalizing facilities for malfunctions even when they
follow approved procedures could discourage transparency and timely
fixes, undermining the cooperative approach needed for effective
environmental regulation.
<bullet> Overly prescriptive SSM requirements may discourage
facilities from performing necessary maintenance or upgrades,
increasing long-term environmental and safety risks.
<bullet> Safety-driven operational choices sometimes need to take
precedence over strict emissions control, and regulations should allow
for that rather than impose conflicting obligations.
<bullet> SSM requirements should not become enforcement traps;
compliance should hinge on following approved procedures in good faith,
not on hindsight divorced from real operating conditions.
<bullet> HWCs vary widely in design and operation, so a uniform SSM
rule can impose irrelevant or counterproductive requirements, making
flexibility essential to ensure both practicality and environmental
protection.
Some commenters said the use of work practice standards for periods
of SSM is supported by statute and the courts. A commenter said that
CAA section 302(k) explicitly provides that ``any design, equipment,
work practice or operational standard promulgated under [the CAA]'' may
constitute a valid ``emission limitation.'' The commenter added that in
Sierra Club v. EPA, the D.C. Circuit relied on Kamp v. Hernandez, where
the court specifically held ``the requirement of regulation on a
continuous basis does not necessarily imply that the source always be
subject to precisely the same limitation.'' The commenter said that
while CAA section 302(k) may confer a ``requirement of regulation on a
continuous basis,'' this does not necessarily equate to continuous
numeric limits.\153\ The commenter concluded that numeric limits during
malfunction periods are not required; work practice standards are an
acceptable alternative that maintain continuous compliance with CAA
section 112.
---------------------------------------------------------------------------

\153\ Kamp v. Hernandez, 752 F.2d 1444, 1452 (9th Cir. 1985).
---------------------------------------------------------------------------

Another commenter echoed that while CAA section 112 requires
emission limitations to apply continuously, the broad definition of the
term ``emission limitation'' in CAA section 302(k) includes in
pertinent part ``. . . any requirement relating to the

[[Page 33505]]

operation or maintenance of a source to assure continuous emission
reduction, and any design, equipment, work practice or operational
standard promulgated under this chapter.'' The commenter added that the
D.C. Circuit found in Sierra Club v. EPA that under certain conditions
it is not feasible to accurately measure emissions or enforce a numeric
standard, and that in such instances the EPA may establish work
practice standards as authorized in CAA section 112(h).\154\
---------------------------------------------------------------------------

\154\ 551 F.3d 1019.
---------------------------------------------------------------------------

Another commenter added that Sierra Club v. EPA did not address SSM
provisions under any specific NESHAP, including the HWC NESHAP, so the
decision has no bearing on whether final HWC NESHAP rule's SSM
provisions violate the requirement for continuous CAA section 112
standards.\155\ The commenter said that the Court noted that the EPA
had not tried to justify the general duty provision as a CAA section
112(h) work practice, which is allowed when numerical limits are not
feasible. The commenter stated that this signaled the Court's
acceptance that work practices are permissible so long as they ensure
continuous compliance with CAA section 112. The commenter argued that
the EPA's proposed work practice standards for periods of SSM meet the
requirements of the CAA that emission limits must apply at all times
and provided the following rationale in support of their position:
---------------------------------------------------------------------------

\155\ Id.
---------------------------------------------------------------------------

<bullet> The commenter pointed to the joint initial brief filed by
industry petitioners on August 16, 2000, which challenged the EPA's SSM
provisions in the 1999 final HWC NESHAP Rule which stated that the
standards and operating parameter limits apply during periods of SSM
except when hazardous waste was not being combusted. At that time,
industry argued that the provisions were non-achievable because all
technologies fail on occasion and the EPA provided no recourse for
those occurrences, thereby conflicting with (sic) CAA section
112(b)(3).
<bullet> Citing Cement Kiln Recycling Coalition v. EPA, the
commenter stated that the Court cast doubt on the EPA's approach to SSM
but declined to rule on the industry's challenge to SSM provisions
while also noting that industry challengers might have valid concerns
over the Agency disallowing exemptions during SSM, and uncontrollable
emergency valve events.\156\
---------------------------------------------------------------------------

\156\ 255 F.3d 855.
---------------------------------------------------------------------------

The EPA agreed with commenters ``who state that sources must be
exempt from technology-based emission standards and operating limits
during SSM events'' in the preamble to the 2005 final HWC NESHAP
rule.\157\
---------------------------------------------------------------------------

\157\ 70 FR 59402, 59494 (Oct. 12, 2005).
---------------------------------------------------------------------------

The commenter stated that the AWFCO requirement already minimizes
emissions when malfunctions occur; and this has been a requirement
since the first HWC NESHAP regulations were promulgated in 1999.\158\
The ``hopelessly generic'' problem discussed in U.S. Sugar Corp. v. EPA
does not apply to the AWFCO requirement.\159\
---------------------------------------------------------------------------

\158\ See 40 CFR 63.1206(c)(3) for information about the AWFCO
requirement.
\159\ U.S. Sugar Corp., 831 F.3d at 608-09.
---------------------------------------------------------------------------

Some commenters stated that the combined requirements of a SSM plan
and AWFCO system ensure continuous compliance and function like a work
practice by limiting emissions through a non-numerical control. These
commenters further noted that the combined requirements of a SSM plan
and AWFCO system restricts emissions at all times and, therefore, align
with the continuous compliance principles in Sierra Club.
On the other hand, other commenters opposed the EPA's proposal to
not apply numeric emission standards during SSM events. A commenter
argued that the EPA's proposed work practice standards for periods of
SSM do not ensure continuous compliance. The commenter said that Sierra
Club requires emissions standards or limits to apply continuously
during SSM events. The commenter contended that requiring an SSM plan
just allows polluters to follow their own plans, which may or may not
be meaningful. The commenter concluded that having a procedure on paper
is no substitute for requiring facilities to meet CAA section 112-
compliant emission standards at all times. Another commenter argued
that the proposed work practice standards for periods of SSM would
greatly increase pollution exposure for already burdened communities,
cannot plausibly protect public health, and are especially alarming
given the extraordinarily high number of SSM events (i.e., 20,000 to
30,000 based off compliance documents) reported in the last five years
at some incinerators.
Response: The EPA acknowledges comments in support of work practice
standards during periods of SSM.
The EPA disagrees with the comment that the work practice standards
for periods of SSM do not constitute emission limits that apply
continuously.\160\ The EPA may set work practice standards when ``the
application of measurement methodology to a particular class of sources
is not practicable due to technological or economic limitations.''
\161\ According to the D.C. Circuit, ``[w]ork practice standards can be
thought of as a statutory Plan B; EPA may resort to them only when
using numeric limits is `not feasible.' . . . The statute defines when
EPA may conclude that numeric limits are infeasible, including--as
relevant here--when `the application of measurement methodology to a
particular class of sources is not practicable due to technological or
economic limitations.'' \162\ Thus, the ``EPA's authority to resort to
a work practice standard does not depend on its determining that
numerically gauging emissions would be impractical throughout the
entire startup period for every single source to which a work practice
applies; the Act requires only that EPA determine that it is
impractical to measure emissions for the `particular class of sources'
at issue.'' \163\
---------------------------------------------------------------------------

\160\ U.S. Sugar Corp., 830 F.3d at 666-67 (``A work-practice
standard that requires facilities to minimize the time their boilers
spend in startup or shutdown thus seems calculated to maximally
reduce emissions during those periods.'').
\161\ 42 U.S.C. 7412(h)(2)(B).
\162\ Sierra Club, 884 F.3d at 1190 (upholding work practice
standards for periods of startup and shutdown that included
requirements to initiate startup and shutdown with clean fuels, to
start certain pollution control devices ``as expeditiously as
possible,'' and to develop and follow an SSM plan).
\163\ Id. at 1201 (quoting 42 U.S.C. 7412(h)(2)(B)).
---------------------------------------------------------------------------

As the EPA explained at proposal, the combination of a clean fuel
requirement during periods of startup and shutdown, a requirement to
develop and follow an approved SSM plan during periods of SSM, and the
AWFCO system requirement constitutes a work practice standard for
periods of malfunction.\164\ Here, specifically, the EPA is
promulgating work practice standards for periods of SSM because it is
often not feasible to accurately measure emissions of HWCs during
periods of SSM. Periods of SSM are transitory and often unstable for
HWCs. The isokinetic sampling required in the primary means of
compliance demonstration during stack testing cannot be met during
unstable periods of operation and, therefore, it is not technically
feasible for operators to conduct the emissions testing necessary to
demonstrate compliance with numeric emission limits during those
periods.

[[Page 33506]]

Accordingly, the EPA does not have any data on which to base numeric
emission limits for periods of SSM. In addition, many OPLs required
under the HWC NESHAP cannot be met during startup and shutdown,
including minimum combustion temperature. Further, some APCDs cannot
operate during the full duration of startup and shutdown. One example
is that stack gas cannot be directed to a baghouse until the
temperature surpasses the dew point.\165\
---------------------------------------------------------------------------

\164\ See the preamble to the proposed rule, 90 FR 50814 (Nov.
10, 2025), section IV.E.1, for more information about the proposed
work practice standards for periods of SSM.
\165\ Sierra Club, 884 F.3d at 1204 (recognizing ``technological
limitations on the use of control devices during the volatile
conditions that characterize startup'').
---------------------------------------------------------------------------

The EPA also disagrees that approved SSM plans may or may not be
meaningful. In addition to the elements of the SSM plan described in
general provisions of 40 CFR part 63, the SSM plan must include a
description of potential causes of malfunctions that may result in
significant HAP releases and of actions the source is taking to
minimize the frequency and severity of these malfunctions.\166\ This
final rule requires that all owners or operators of HWCs submit their
SSM plans for approval by the Administrator, which serves to ensure
that SSM plans are appropriate to minimize both the occurrence of
unusual combustion events like malfunctions and emissions during
periods of SSM. Any changes that may significantly increase emissions
must also be submitted for approval. Further, it bears note that the
D.C. Circuit cast doubt on our removal of the SSM exemption as far back
as 2001 in Cement Kiln Recycling Coalition v. EPA.\167\
---------------------------------------------------------------------------

\166\ Id. (upholding work practice standards for periods of
startup and shutdown that included requirements to develop and
follow an approved SSM plan).
\167\ 255 F.3d 872 (declining request to remand 1999 standards
to the EPA and instead vacating standards in their entirety because
``industry petitioners may be correct that EPA should have exempted
HWCs from regulatory limits during periods of SSM, permitting
sources to return to compliance by following the steps of a SSM plan
filed with the Agency. We have similar doubts about EPA's decision
to require sources to comply with standards even during openings of
emergency safety valves caused by events beyond the sources'
control.'').
---------------------------------------------------------------------------

Owners or operators of HWCs must meet the requirements of the HWC
NESHAP at all times, including the work practice standard for periods
of SSM during such periods.
The EPA disagrees that the work practice standards for periods of
SSM will increase emissions of HAP and are not protective of public
health. Given that emission limits under the HWC NESHAP historically
have not applied during periods of SSM, the EPA fails to see how an
enforceable work practice standard could lead to increased emissions of
HAP, and the commenter has not provided support for that statement. The
commenter also provides no support for the statement that the work
practice standards for periods of SSM cannot plausibly protect public
health and did not provide the alleged compliance documents showing
``20,000 to 30,000 SSM events'' reported in the last five years at some
incinerators. The EPA reiterates that the definition of ``malfunction''
governing the HWC NESHAP requires that a malfunction event must be
sudden, infrequent, and not reasonably preventable; failures that are
caused in part by poor maintenance or careless operation are not
malfunctions.\168\
---------------------------------------------------------------------------

\168\ See 40 CFR 63.2 definition of ``Malfunction.''
---------------------------------------------------------------------------

Comment: A commenter argued that the alternative fuel requirement
is not sufficient to comply with Sierra Club. The commenter asserted
that the EPA's claim that the clean fuel requirement will ``minimize''
emissions says nothing about how much emissions will be reduced and
whether such reductions reflect the emissions of the best performers.
The commenter said that the EPA failed to demonstrate that the SSM plan
and clean fuel requirements together represent both: (1) the ``average
emission limitation achieved'' by the best performing sources, and (2)
the maximum reduction in emissions ``achievable'' considering cost and
other relevant factors.
Response: The EPA disagrees with the commenter's assumption that
the alternative fuel requirement is the only requirement under the work
practice standard for periods of startup and shutdown. Rather, the work
practice standard for periods of startup and shutdown is a combination
of the alternative fuel requirement and following an approved SSM plan.
As the EPA stated in the proposed rule preamble, the Agency cannot
feasibly measure accurate HAP emissions during periods of startup and
shutdown because the transitory and unstable emissions during periods
of startup and shutdown mean that sources cannot achieve the isokinetic
sampling required for the primary means of compliance demonstration
during stack testing.\169\ Because the EPA cannot feasibly measure HAP
emissions for HWCs during periods of startup and shutdown, the EPA also
cannot quantitatively measure emissions reductions achieved by the work
practice standard or numerically define best performing sources.\170\
To determine the best performers for periods of startup and shutdown,
the EPA requested and analyzed information on how sources minimized
emissions during these periods.\171\ Sources generally indicated that
they were complying with the requirements already in the HWC NESHAP to
minimize emissions during periods of startup and shutdown (e.g.,
following their SSM plan, operator training, following startup and
shutdown procedures). Some sources indicated that in addition to
complying with the requirements already in the HWC NESHAP, they burned
only clean fuel during periods of startup and shutdown.
---------------------------------------------------------------------------

\169\ See 90 FR 50814, 50846 (Nov. 10, 2025).
\170\ Sierra Club, 884 F.3d at 1201 (CAA section 112(h)(2)(B)
``requires only that EPA determine that it is impractical to measure
emissions for the `particular class of sources' at issue'').
\171\ See Document ID No. EPA-HQ-OAR-2004-0022-0651.
---------------------------------------------------------------------------

Based on this data, the EPA determined that the best performing
HWCs both burned only clean fuels during periods of startup and
shutdown and complied with their SSM plan during such periods. The HWC
NESHAP already requires operator training and certification, with
annual review or refresher training.\172\ Both the initial and annual
training must include operation of the combustor, including proper
startup and shutdown procedures; operation of air pollution control
equipment; and actions to correct malfunctions or conditions that may
lead to malfunctions. This training requirement ensures that operators
know and can follow the HWC's SSM plan, augmenting the SSM plan portion
of the work practice standards for periods of SSM. Further, the
definitions of startup and shutdown described in section IV.D.3.b of
this preamble constitute an implicit emissions limitation by ensuring
that periods of startup and shutdown are ``not needlessly drawn out,''
thereby minimizing emissions that are not constrained by the numeric
emissions limitations of periods of normal operations.\173\
---------------------------------------------------------------------------

\172\ See 40 CFR 63.1206(c)(6).
\173\ Sierra Club, 884 F.3d at 1204.
---------------------------------------------------------------------------

While the information collection generally did not indicate if the
SSM plans were approved, the EPA reasoned that having an approved SSM
plan would better ensure that the contents of the SSM plan were
reasonable and protective of human health and the environment.
Accordingly, the EPA proposed a work practice standard of burning only
clean, supplemental fuels during periods of startup and shutdown and
requiring HWCs to follow an

[[Page 33507]]

approved SSM plan during those periods. Combined they ``are materially
more precise and demanding than the general duty standard . . .
disapproved in Sierra Club.'' \174\ The EPA notes that the D.C. Circuit
has previously upheld very similar work practice standards for
industrial boilers consisting of using clean fuels, following a startup
and shutdown plan, engaging APCDs as expeditiously as possible, and
limiting the amount of time spent in startup and shutdown as consistent
with CAA section 112's MACT approach.\175\
---------------------------------------------------------------------------

\174\ Id.
\175\ Id. (upholding work practice standards for periods of
startup and shutdown that included requirements to initiate startup
with clean fuels, develop and follow an approved SSM plan and to
start certain pollution control devices ``as expeditiously as
possible'').
---------------------------------------------------------------------------

The commenter neither suggested that another work practice standard
would better reduce emissions during periods of startup and shutdown
nor provided additional work practices that the EPA could incorporate
into the proposed work practice standard to minimize HAP emissions.
Comment: Several commenters supported the EPA's proposal to require
an SSM plan. However, other commenters argued that the EPA's proposed
SSM plan is legally inadequate because it functions like the ``general
duty'' standard that the D.C. Circuit struck down in Sierra Club v.
EPA. The commenter asserted that an SSM plan merely requires facilities
to minimize emissions and fix malfunctions promptly, which are
obligations the Court already rejected as insufficient substitutes for
real, enforceable emission limits. The commenter concluded that the EPA
must impose actual CAA section 112-compliant emission standards that
apply during SSM periods rather than relying on SSM plans.
Response: The EPA acknowledges commenters' support for requiring an
SSM plan.
The EPA disagrees with commenters who equate the requirement for an
approved SSM plan as part of work practice standards for periods of SSM
to the ``general duty'' standard that the D.C. Circuit struck down in
Sierra Club v. EPA. In Sierra Club v. EPA, the Court's decision
centered around the requirement that there must be continuous section
112-compliant standards and the EPA's general duty regulatory
provisions that exempt sources from emission limitation under CAA
section 112(d), or an alternate standard under CAA section 112(h)
during periods of SSM. The Court ruled that ``[b]ecause the general
duty is the only standard that applies during SSM events--and
accordingly no section 112 standard governs these events--the SSM
exemption violates the CAA's requirement that some section 112 standard
apply continuously.'' \176\ The Court did not rule that requirements to
minimize emissions, fix malfunctions immediately, and have an SSM plan
are inappropriate, but that as promulgated, the general duty standard
was not a CAA section 112(d) or 112(h) standard. Moreover, since then
the D.C. Circuit has rejected a somewhat similar argument to the one
made by the commenter finding instead that work practice standards that
include the use of clean fuels at start up and shut down, starting
certain APCDs as expeditiously as possible, and the requirement to
develop and follow an SSM plan to be ``meaningful constraint[s].''
\177\
---------------------------------------------------------------------------

\176\ Sierra Club, 551 F.3d at 1028.
\177\ Sierra Club, 884 F.3d at 1203 (upholding the ``EPA's
conclusion that its work practice standard has constraining effect
that a general-duty standard lacks''); see also See U.S. Sugar
Corp., 830 F.3d at 663.
---------------------------------------------------------------------------

As previously explained, it is not feasible to measure emissions
during periods of SSM and so work practice standards under CAA section
112(h) are appropriate.\178\ The EPA has also explained that the clean
fuel requirements during periods of startup and shutdown, AWFCO
requirement, and the requirement to follow an approved SSM plan
constitute work practice standards for periods of SSM consistent with
requiring the maximum degree of emissions reductions based on the best
performing sources for which the Administrator has data.\179\
Promulgating these requirements as a CAA section 112(h) work practice
standard makes them CAA section 112-compliant, enforceable emission
standards that apply during periods of SSM.
---------------------------------------------------------------------------

\178\ See 90 FR 50814, 50846 (Nov. 10, 2025) and section
IV.D.3.a of this preamble.
\179\ See section IV.D.3.a of this preamble.
---------------------------------------------------------------------------

Comment: Commenters said that combined with an SSM plan, the use of
an AWFCO system ensures that corrective actions may be taken to
minimize emissions during malfunction periods, during which time waste
will not be fed to the combustor. A commenter said that the AWFCO
system immediately (or within a minute), automatically cuts off the
hazardous waste feed to the HWC when an AWFCO event occurs. The
commenter pointed out that where an AWFCO is triggered, operators must
continue to send combustion gases to the air pollution control system
while hazardous waste remains in the combustion chamber.
On the other hand, a commenter argued that the use of an AWFCO
system combined with an SSM plan does not meet the requirement for CAA
section 112-complaint standards under the Sierra Club decision. The
commenter said that the EPA failed to demonstrate that the SSM plan and
AWFCO system requirements together represent both (1) the ``average
emission limitation achieved'' by the best performing sources, and (2)
the maximum reduction in emissions ``achievable'' considering cost and
other relevant factors.
Response: The EPA acknowledges commenters' support. Similar to how
the EPA developed the work practice standard for periods of startup and
shutdown, the Agency requested and analyzed information on how sources
minimized emissions during periods of malfunction.\180\ Sources
generally indicated that they were complying with the requirements
already in the HWC NESHAP to minimize emissions during periods of
malfunction (e.g., following their SSM plan, operator training,
complying with the AWFCO requirements). Sources did not indicate
additional methods for minimizing emissions during periods of
malfunction. Based on this data, the EPA determined that the best
performing HWCs both complied with the AWFCO provisions and complied
with their SSM plan during periods of malfunction. While the
information collection generally did not indicate if the SSM plans were
approved, the EPA reasoned that having an approved SSM plan would
better ensure that the contents of the SSM plan were reasonable and
protective of human health and the environment.
---------------------------------------------------------------------------

\180\ See Document ID No. EPA-HQ-OAR-2004-0022-0651.
---------------------------------------------------------------------------

All HWCs are also required to operate an AWFCO system, which is a
system that immediately (or within one minute in some circumstances),
automatically cuts off the hazardous waste feed to the HWC when an OPL
or other certain monitoring condition indicated in the HWC NESHAP is
exceeded or any component of the AWFCO system fails. These monitoring
conditions are set to ensure that HWCs always comply with the emission
limits of the HWC NESHAP. To avoid any potential exceedances, many HWC
owners or operators set their AWFCO system to trigger when a monitored
parameter approaches the monitoring condition limit instead of when the
parameter exceeds it. During an AWFCO, as one

[[Page 33508]]

commenter noted, owners or operators must continue to send combustion
gases to the air pollution control system while hazardous waste remains
in the combustion chamber of the HWC. Hazardous waste feed to the HWC
cannot restart until the monitoring conditions are within the specified
limits, which typically takes no less than one hour. The AWFCO system
must generally be tested at least weekly. The AWFCO system triggering
does not necessarily indicate that the HWC is malfunctioning, but a
malfunction that may lead to excess HAP emissions will trigger the
AWFCO system.\181\ The AWFCO requirements minimize emissions during
malfunctions that could cause exceedances by requiring swift hazardous
waste feed shut off. Because hazardous waste is a primary source of HAP
emissions for most HWCs, shutting off hazardous waste feed immediately
minimizes emissions while the owner or operator can diagnose and
resolve the issue that triggered the AWFCO. Finally, it bears note that
the D.C. Circuit cast doubt on our removal of the SSM exemption as far
back as 2001 in Cement Kiln Recycling Coalition v. EPA.\182\
Accordingly, the EPA proposed a work practice standard of the AWFCO
provision and requiring HWCs to follow an approved SSM plan during
periods of malfunction. The commenter neither suggested that another
work practice standard would better reduce emissions during periods of
malfunction nor provided additional work practices that the EPA could
incorporate into the proposed work practice standard to minimize HAP
emissions.
---------------------------------------------------------------------------

\181\ For the HWC NESHAP, malfunction is defined in 40 CFR part
63.2 as ``any sudden, infrequent, and not reasonably preventable
failure of air pollution control and monitoring equipment, process
equipment, or a process to operate in a normal or usual manner which
causes, or has the potential to cause, the emission limitations in
an applicable standard to be exceeded. Failures that are caused in
part by poor maintenance or careless operation are not
malfunctions.''
\182\ 255 F.3d 872 (vacating standards in the entirety instead
of remanding to the EPA).
---------------------------------------------------------------------------

b. Definitions of Startup and Shutdown
Comment: A commenter said that although the EPA's proposed SSM
approach is ``lawful'' and ``reasonable,'' it cannot be finalized until
the EPA creates startup and shutdown regulations that fit how HWC
cement kilns operate. The commenter argued that the proposed work
practices are designed for incinerators and fail to account for the
fact that cement kilns exist to produce Portland cement. The commenter
argued that the SSM requirements must incorporate concepts reflecting
that both raw materials and fuel are introduced during startup and
shutdown, building on the framework in the Portland Cement NESHAP.\183\
The commenter said that the EPA's proposed ``shutdown'' regulation is
unworkable because it prohibits a facility--once it initiates
shutdown--from going back to startup without first firing supplemental
fuel. The commenter contended that this is inconsistent with operating
procedures, and a short interruption in hazardous waste combustion
should not lead to this result.
---------------------------------------------------------------------------

\183\ 40 CFR 63.1341.
---------------------------------------------------------------------------

The commenter requested the EPA adopt the following definition for
``startup'':
Startup means the time from when a shutdown hazardous waste burning
cement kiln begins firing supplemental fuel. Startup ends at the
earlier of either 120 minutes after the continuous introduction of kiln
feed or 15 minutes after hazardous waste is continuously fired into the
hazardous waste burning cement kiln.
The commenter also requested the EPA finalize the following
requirements:
<bullet> During startup, the hazardous waste burning cement kiln
shall not start the flow of hazardous waste fuels to the hazardous
waste burning cement kiln until the applicable operating parameters and
emission levels are within the limits specified in the NOC,\184\ unless
the owner or operator does so in accordance with the AWFCO requirements
when burning hazardous waste during startup and shutdown.\185\
---------------------------------------------------------------------------

\184\ See 40 CFR 63.1207(j) and 40 CFR 63.1210(d).
\185\ 40 CFR 63.1206(c)(2)(v)(B).
---------------------------------------------------------------------------

<bullet> The owner or operator must operate in accordance with the
SSM plan during periods of shutdown.
The commenter explained that their suggested rule text makes it
clear that a source would not have to keep burning supplemental fuel
during a later startup; and instead would allow the source to switch
back to traditional or hazardous-waste fuel sooner once the hazardous
waste burning cement kiln reaches the necessary operating conditions or
emission levels. The commenter added that their suggested rule text
would allow HWC cement kilns the ability to restart hazardous waste
fuel firing after shutdown begins by complying with the requirements
for restarting waste feed after an AWFCO.\186\
---------------------------------------------------------------------------

\186\ 40 CFR 63.1206(c)(3)(iii).
---------------------------------------------------------------------------

Another commenter also suggested that there is considerable
variation in construction and operation of HWCs between and within HWC
subcategories, which makes it difficult to develop definitions of
startup and shutdown that fit all sources. The commenter offered the
example of a solid fuel boiler that may start up on supplemental fuel,
begin supplying useful thermal energy to a steam header, then begin
combusting non-hazardous waste, and stated that the solid fuel boiler
could operate in this way (i.e., normal operation) for an extended
period of time before it needs to manage hazardous waste. The commenter
pointed out that under the proposed definition of startup, this would
mean that the solid fuel boiler was operating in a ``startup'' period
until it introduces hazardous waste. The commenter suggested that the
EPA should define either ``startup'' and ``shutdown'' on a site-
specific basis in approved SSM plans or develop different startup and
shutdown definitions for incinerators, cement kilns, boilers, and HCl
production furnaces.
Response: The EPA acknowledges the commenter's general support for
work practice standards for periods of SSM.
In response to comments on the proposed SSM provisions that
highlighted the differences in startup and shutdown for the HWC
subcategories, the EPA is finalizing separate definitions of startup
for incinerators, cement kilns and lightweight aggregate kilns, solid
fuel and liquid fuel boilers, and HCl production furnaces. As suggested
by a commenter, the EPA has utilized the frameworks developed in
corresponding rules for similar sources that do not burn hazardous
waste: the Portland Cement NESHAP \187\ for cement kilns, lightweight
aggregate kilns, and HCl production furnaces and the major-source
industrial boilers NESHAP \188\ for solid fuel boilers and liquid fuel
boilers. The new definitions for startup are as follows: \189\
---------------------------------------------------------------------------

\187\ See 40 CFR 63.1341.
\188\ See 40 CFR 63.7575.
\189\ The EPA is codifying these definitions in 40 CFR
63.1206(c)(10).
---------------------------------------------------------------------------

<bullet> For incinerators, startup begins with the firing of
supplemental fuel in the combustion chamber or with transitioning from
a period of shutdown. All APCDs must be in operation as expeditiously
as possible and prior to the introduction of any waste material that is
not supplemental fuel into the HWC. Startup ends once the system has
stabilized but no later than 15 minutes after either hazardous waste
that is not fed in accordance with the AWFCO requirements when burning
hazardous waste during startup and shutdown or any waste material that
is

[[Page 33509]]

not supplemental fuel is fed into the HWC, whichever occurs first.\190\
---------------------------------------------------------------------------

\190\ See 40 CFR 63.1206(c)(2)(v)(B) for the AWFCO requirements
when burning hazardous waste during startup or shutdown.
---------------------------------------------------------------------------

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