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Proposed Rule2025-19815

National Emission Standards for Hazardous Air Pollutants From Hazardous Waste Combustors: Residual Risk and Technology Review; Withdrawal of Proposed Revisions to Standards for Periods of Malfunction

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Published

November 10, 2025

Issuing agencies

Environmental Protection Agency

Abstract

This proposal presents the results of the U.S. Environmental Protection Agency's (EPA) residual risk and technology review for the National Emission Standards for Hazardous Air Pollutants (NESHAP) from Hazardous Waste Combustors (HWC) as required under the Clean Air Act (CAA). In this action, the EPA is proposing to establish emission limits and work practice standards for hydrogen fluoride and hydrogen cyanide emissions from HWC incinerators, cement kilns, solid fuel boilers, and liquid fuel boilers; eliminate the startup, shutdown, and malfunction (SSM) exemption; add a work practice standard for periods of SSM; add electronic reporting procedures and requirements; allow states to choose to exempt area sources from certain permitting requirements; and other clarifications and corrections. In response to comments received on certain aspects of the July 24, 2024, proposed revisions for periods of malfunction, the EPA is withdrawing that proposed rule and instead proposing different provisions to address periods of SSM.

Full Text

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[Federal Register Volume 90, Number 215 (Monday, November 10, 2025)]
[Proposed Rules]
[Pages 50814-50855]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2025-19815]

[[Page 50813]]

Vol. 90

Monday,

No. 215

November 10, 2025

Part III

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; Withdrawal of
Proposed Revisions to Standards for Periods of Malfunction Proposed
Rule

Federal Register / Vol. 90, No. 215 / Monday, November 10, 2025 /
Proposed Rules

[[Page 50814]]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2004-0022; FRL-10654-01-OAR]
RIN 2060-AV96

National Emission Standards for Hazardous Air Pollutants From
Hazardous Waste Combustors: Residual Risk and Technology Review;
Withdrawal of Proposed Revisions to Standards for Periods of
Malfunction

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule and withdrawal of proposed rule.

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SUMMARY: This proposal presents the results of the U.S. Environmental
Protection Agency's (EPA) residual risk and technology review for the
National Emission Standards for Hazardous Air Pollutants (NESHAP) from
Hazardous Waste Combustors (HWC) as required under the Clean Air Act
(CAA). In this action, the EPA is proposing to establish emission
limits and work practice standards for hydrogen fluoride and hydrogen
cyanide emissions from HWC incinerators, cement kilns, solid fuel
boilers, and liquid fuel boilers; eliminate the startup, shutdown, and
malfunction (SSM) exemption; add a work practice standard for periods
of SSM; add electronic reporting procedures and requirements; allow
states to choose to exempt area sources from certain permitting
requirements; and other clarifications and corrections. In response to
comments received on certain aspects of the July 24, 2024, proposed
revisions for periods of malfunction, the EPA is withdrawing that
proposed rule and instead proposing different provisions to address
periods of SSM.

DATES:
Comments. Comments must be received on or before December 26, 2025.
As of November 10, 2025, the proposed rule published on July 24, 2024,
at 89 FR 59867, is withdrawn. Under the Paperwork Reduction Act (PRA),
comments on the information collection provisions are best assured of
consideration if the Office of Management and Budget (OMB) receives a
copy of your comments on or before December 10, 2025.
Public hearing: If anyone contacts us requesting a public hearing
on or before November 15, 2025, we will hold a virtual public hearing.
See SUPPLEMENTARY INFORMATION for information on requesting and
registering for a public hearing.

ADDRESSES: You may send comments, identified by Docket ID No. EPA-HQ-
OAR-2004-0022, by any of the following methods:
<bullet> Federal eRulemaking Portal: <a href="https://www.regulations.gov/">https://www.regulations.gov/</a>
(our preferred method). Follow the online instructions for submitting
comments.
<bullet> Email: <a href="/cdn-cgi/l/email-protection#fc9dd19d9298d18ed198939f979988bc998c9dd29b938a">[email&#160;protected]</a>. Include Docket ID No. EPA-
HQ-OAR-2004-0022 in the subject line of the message.
<bullet> Mail: U.S. Environmental Protection Agency, EPA Docket
Center, Docket ID No. EPA-HQ-OAR-2004-0022, Mail Code 28221T, 1200
Pennsylvania Avenue NW, Washington, DC 20460.
<bullet> Hand/Courier Delivery: EPA Docket Center, WJC West
Building, Room 3334, 1301 Constitution Avenue NW, Washington, DC 20004.
The Docket Center's hours of operation are 8:30 a.m.-4:30 p.m., Monday-
Friday (except Federal holidays).
Instructions: All submissions received must include the Docket ID
No. for this rulemaking. Comments received may be posted without change
to <a href="https://www.regulations.gov/">https://www.regulations.gov/</a>, including any personal information
provided. For detailed instructions on sending comments and additional
information on the rulemaking process, see the SUPPLEMENTARY
INFORMATION section of this document.

FOR FURTHER INFORMATION CONTACT: For information about this proposed
rule, contact U.S. EPA, Attn: Rachel Smoak, Mail Drop: 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#1a6977757b7134687b79727f765a7f6a7b347d756c">[email&#160;protected]</a>. For specific information regarding the risk
modeling methodology, contact U.S. EPA, Attn: Matt Woody, Ph.D., Mail
Drop: C539-02, 109 T.W. Alexander Drive, P.O. Box 12055, RTP, North
Carolina 27711; telephone number: (919) 541-1535; and email address:
<a href="/cdn-cgi/l/email-protection#c9bea6a6adb0e7a4a8bdbd89acb9a8e7aea6bf">[email&#160;protected]</a>.

SUPPLEMENTARY INFORMATION: Participation in virtual public hearing. To
request a virtual public hearing, contact the public hearing team at
(888) 372-8699 or by email at <a href="/cdn-cgi/l/email-protection#693a39392d191c0b05000a010c081b00070e290c1908470e061f">[email&#160;protected]</a>. If requested,
the hearing will be held via virtual platform on November 25, 2025. The
EPA may close a session 15 minutes after the last pre-registered
speaker has testified if there are no additional speakers. The EPA will
announce further details at <a href="https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous">https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous</a>.
The EPA will begin pre-registering speakers for the hearing no
later than one business day after a request has been received. To
register to speak at the virtual hearing, please use the online
registration form available at <a href="https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous">https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous</a> or contact the public hearing team at (888) 372-8699 or by
email at <a href="/cdn-cgi/l/email-protection#3566656571454057595c565d5054475c5b52755045541b525a43">[email&#160;protected]</a>. The last day to pre-register to
speak at the hearing will be November 22, 2025. Prior to the hearing,
the EPA will post a general agenda that will list pre-registered
speakers at: <a href="https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous">https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous</a>.
The EPA will make every effort to follow the schedule as closely as
possible on the day of the hearing; however, please plan for the
hearings to run either ahead of schedule or behind schedule. Each
commenter will have four minutes to provide oral testimony. The EPA
encourages commenters to submit the text of your oral testimony as
written comments to the rulemaking docket. The EPA may ask clarifying
questions during the oral presentations but will not respond to the
presentations at that time. Written statements and supporting
information submitted during the comment period will be considered with
the same weight as oral testimony and supporting information presented
at the public hearing.
Please note that any updates made to any aspect of the hearing will
be posted online at <a href="https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous">https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous</a>. While the EPA expects the hearing to go forward as set forth
above, please monitor this website or contact the public hearing team
at (888) 372-8699 or by email at <a href="/cdn-cgi/l/email-protection#a1f2f1f1e5d1d4c3cdc8c2c9c4c0d3c8cfc6e1c4d1c08fc6ced7">[email&#160;protected]</a> to determine
if there are any updates. The EPA does not intend to publish a document
in the Federal Register (FR) announcing updates.
If you require special accommodation such as audio description,
please pre-register for the hearing with the public hearing team and
describe your needs by November 17, 2025. The EPA may not be able to
arrange accommodations without advanced notice.
Docket. The EPA has established a docket for this proposed rule
under Docket ID No. EPA-HQ-OAR-2004-0022. All documents in the docket
are listed in the <a href="https://www.regulations.gov/">https://www.regulations.gov/</a> index. Although listed
in the index, some information is

[[Page 50815]]

not publicly available, e.g., Confidential Business Information (CBI)
or other information whose disclosure is restricted by statute. Certain
other material, such as copyrighted material, is not placed on the
internet and will be publicly available only as Portable Document
Format (PDF) versions that can only be accessed on the EPA computers in
the docket office reading room. Certain databases and physical items
cannot be downloaded from the docket but may be requested by contacting
the docket office at 202-566-1744. The docket office has up to 10
business days to respond to these requests. With the exception of such
material, publicly available docket materials are available
electronically at <a href="https://www.regulations.gov">https://www.regulations.gov</a>.
Written Comments. Submit your comments, identified by Docket ID No.
EPA-HQ-OAR-2004-0022, at <a href="https://www.regulations.gov/">https://www.regulations.gov/</a> (our preferred
method), or the other methods identified in the ADDRESSES section. Once
submitted, comments cannot be edited or removed from the docket. The
EPA may publish any comment received to its public docket. Do not
submit to the EPA's docket at <a href="https://www.regulations.gov/">https://www.regulations.gov/</a> any
information that you consider to be CBI or other information whose
disclosure is restricted by statute. This type of information should be
submitted as discussed in the Submitting CBI section of this document.
The EPA is soliciting comment on numerous aspects of the proposed
rule. The EPA has indexed each comment solicitation with a unique
identifier (e.g., ``C-1,'' ``C-2,'' ``C-3'' . . .) to provide a
consistent framework for effective and efficient provision of comments.
Accordingly, we ask that commenters include the corresponding
identifier when providing comments relevant to that comment
solicitation. We ask that commenters include the identifier either in a
heading or within the text of each comment (e.g., ``In response to C-1,
. . .'') to make clear which comment solicitation is being addressed.
We emphasize that we are not limiting comment to these identified areas
and encourage provision of any other comments relevant to this proposed
action.
Multimedia submissions (audio, video, etc.) must be accompanied by
a written comment. The written comment is considered the official
comment and should include discussion of all points you wish to make.
The EPA will generally not consider comments or comment contents
located outside of the primary submission (i.e., on the Web, cloud, or
other file sharing system). Please visit <a href="https://www.epa.gov/dockets/commenting-epa-docket">https://www.epa.gov/dockets/commenting-epa-docket</a> for additional submission methods; the full EPA
public comment policy; information about CBI or multimedia submissions;
and general guidance on making effective comments.
The <a href="https://www.regulations.gov/">https://www.regulations.gov/</a> website allows you to submit your
comment anonymously, which means the EPA will not know your identity or
contact information unless you provide it in the body of your comment.
If you send an email comment directly to the EPA without going through
<a href="https://www.regulations.gov/">https://www.regulations.gov/</a>, your email address will be automatically
captured and included as part of the comment that is placed in the
public docket and made available on the internet. If you submit an
electronic comment, the EPA recommends that you include your name and
other contact information in the body of your comment and with any
digital storage media you submit. If the EPA cannot read your comment
due to technical difficulties and cannot contact you for clarification,
the EPA may not be able to consider your comment. Electronic files
should not include special characters or any form of encryption and
should be free of any defects or viruses.
Submitting CBI. Do not submit information containing CBI to the EPA
through <a href="https://www.regulations.gov/">https://www.regulations.gov/</a>. Clearly mark the part or all of
the information that you claim to be CBI. For CBI information on any
digital storage media that you mail to the EPA, note the docket ID,
mark the outside of the digital storage media as CBI, and identify
electronically within the digital storage media the specific
information that is claimed as CBI. In addition to one complete version
of the comments that includes information claimed as CBI, you must
submit a copy of the comments that does not contain the information
claimed as CBI directly to the public docket through the procedures
outlined in Written Comments above. If you submit any digital storage
media that does not contain CBI, mark the outside of the digital
storage media clearly that it does not contain CBI and note the docket
ID. Information not marked as CBI will be included in the public docket
and the EPA's electronic public docket without prior notice.
Information marked as CBI will not be disclosed except in accordance
with procedures set forth in 40 Code of Federal Regulations (CFR) part
2.
Our preferred method to receive CBI is for it to be transmitted
electronically using email attachments, File Transfer Protocol (FTP),
or other online file sharing services (e.g., Dropbox, OneDrive, Google
Drive). Electronic submissions must be transmitted directly to the
OAQPS CBI Office at the email address <a href="/cdn-cgi/l/email-protection#9ef1ffefeeedc1fdfcf7defbeeffb0f9f1e8">[email&#160;protected]</a>, and as
described above, should include clear CBI markings and note the docket
ID. If assistance is needed with submitting large electronic files that
exceed the file size limit for email attachments, and if you do not
have your own file sharing service, please email <a href="/cdn-cgi/l/email-protection#96f9f7e7e6e5c9f5f4ffd6f3e6f7b8f1f9e0">[email&#160;protected]</a> to
request a file transfer link. If sending CBI information through the
postal service, please send it to the following address: OAQPS Document
Control Officer (C404-02), OAQPS, U.S. Environmental Protection Agency,
109 T.W. Alexander Drive P.O. Box 12055 RTP, North Carolina 27711,
Attention Docket ID No. EPA-HQ-OAR-2004-0022. The mailed CBI material
should be double wrapped and clearly marked. Any CBI markings should
not show through the outer envelope.
Preamble acronyms and abbreviations. Throughout this preamble the
use of ``we,'' ``us,'' or ``our'' is intended to refer 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:

AEGL acute exposure guideline level
AERMOD air dispersion model used by the HEM model
APCD air pollution control device
AWFCO automatic waste feed cutoff
CAA Clean Air Act
CalEPA California EPA
CBI Confidential Business Information
CEDRI Compliance and Emissions Data Reporting Interface
CEMS continuous emission monitoring system
CFR Code of Federal Regulations
CfPT confirmatory performance test
CMS continuous monitoring system
CPT comprehensive performance test
DRE destruction and removal efficiency
EPA Environmental Protection Agency
ERPG emergency response planning guideline
ERT Electronic Reporting Tool
HAP hazardous air pollutant(s)
HBEL health-based emission limit
HCl hydrochloric acid
HCN hydrogen cyanide
HEM Human Exposure Model
HF hydrogen fluoride
HI hazard index
HQ hazard quotient
HWC hazardous waste combustor
ICR information collection request
IRIS Integrated Risk Information System
km kilometer
LOAEL lowest-observed-adverse-effect level
MACT maximum achievable control technology
mg/kg-day milligrams per kilogram per day

[[Page 50816]]

mg/m3 milligrams per cubic meter
MIR maximum individual risk
NAAQS National Ambient Air Quality Standards
NAICS North American Industry Classification System
NESHAP national emission standards for hazardous air pollutants
NOAEL no-observed-adverse-effect level
NRC National Research Council
NTTAA National Technology Transfer and Advancement Act
OAQPS Office of Air Quality Planning and Standards
OECA Office of Enforcement and Compliance Assurance
OMB Office of Management and Budget
PAH polycyclic aromatic hydrocarbons
PB-HAP hazardous air pollutants known to be persistent and
bioaccumulative in the environment
PCDD/PCDF polychlorinated dibenzo-p-dioxins and polychlorinated
dibenzofurans
PM particulate matter
POM polycyclic organic matter
ppm parts per million
REL reference exposure level
RFA Regulatory Flexibility Act
RfC reference concentration
RfD reference dose
RTR residual risk and technology review
SAB Science Advisory Board
SBA Small Business Administration
SSM startup, shutdown, and malfunction
TEQ toxic equivalency quotient
TOSHI target organ-specific hazard index
tpy tons per year
TRIM.FaTE Total Risk Integrated Methodology.Fate, Transport, and
Ecological Exposure model
UF uncertainty factor
[micro]g/m3 micrograms per cubic meter
UMRA Unfunded Mandates Reform Act
UPL upper prediction limit
URE unit risk estimate
VCS voluntary consensus standards

Table of Contents

I. General Information
A. Does this action apply to me?
B. Where can I get a copy of this document and other related
information?
II. Background
A. What is the statutory authority for this proposed action?
B. What is this source category and how does the current NESHAP
regulate its HAP emissions?
C. What data collection activities were conducted to support
this action?
D. What other relevant background information and data are
available?
III. Analytical Procedures and Decision-Making
A. How do we consider risk in our decision-making?
B. How do we perform the technology review?
C. How do we estimate post-MACT risk posed by the source
category?
IV. Analytical Results and Proposed Decisions
A. What actions are we proposing pursuant to CAA sections
112(d)(2) and 112(d)(3)?
B. What are the results of the risk assessment and analyses?
C. What are our proposed decisions regarding risk acceptability,
ample margin of safety, and adverse environmental effect?
D. What are the results and proposed decisions based on our
technology review?
E. What other actions are we proposing?
F. What compliance dates are we proposing?
V. Summary of Cost, Environmental, and Economic Impacts
A. What are the affected sources?
B. What are the air quality impacts?
C. What are the cost impacts?
D. What are the economic impacts?
E. What are the benefits?
F. What analysis of children's environmental health did we
conduct?
VI. Request for Comments
VII. Submitting Data Corrections
VIII. Statutory and Executive Order Reviews
A. Executive Order 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 Mandate 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)

I. General Information

A. Does this action apply to me?

Table 1 of this preamble lists the NESHAP and associated regulated
industrial source categories that are the subject of this proposal.
Table 1 is not intended to be exhaustive but rather provides a guide
for readers regarding the entities that this proposed action is likely
to affect. The proposed standards, if finalized, would be directly
applicable to the affected sources. State, local, and Tribal government
entities do not own or operate sources that would be affected by this
proposed action. The hazardous waste combustor (HWC) source category,
which is the subject of this proposal, is regulated under 40 CFR part
63, subpart EEE, the National Emission Standards for Hazardous Air
Pollutants from Hazardous Waste Combustors (HWC NESHAP). The HWC NESHAP
includes hazardous waste combusting sources from five initial source
categories: Hazardous Waste Incineration, Portland Cement
Manufacturing, Clay Products Manufacturing (including lightweight
aggregate kilns), Industrial Boilers, and Hydrochloric Acid (HCl)
Production.
Hazardous waste combusting sources from five initial source
categories are regulated as HWCs under 40 CFR part 63, subpart EEE, the
HWC NESHAP. As defined in the Initial List of Categories of Sources
Under Section 112(c)(1) of the Clean Air Act Amendments of 1990 (57 FR
31576, July 16, 1992) and Documentation for Developing the Initial
Source Category List, Final Report (EPA-450/3-91-030, July 1992), the
``Hazardous Waste Incineration'' source category includes any source
that incinerates hazardous waste in ``any furnace, or other device,
used in the process of burning waste for the primary purpose of
reducing the volume of the waste by removing combustible matter.'' The
``Portland Cement Manufacturing'' source category includes ``any
facility engaged in manufacturing Portland cement by either the wet or
dry process.'' The ``Clay Products Manufacturing'' source category
includes lightweight aggregate kilns and is defined as ``any facility
engaged in manufacturing of clay products such as brick, vitrified clay
pipe, structural clay tile, and clay refractories.'' The ``Industrial
Boilers'' source category includes ``boilers used in manufacturing,
processing, mining, and refining or any other industry to provide
steam, hot water, and/or electricity.'' In 2004, the Industrial Boilers
source category was combined with the Institutional/Commercial Boilers
and the Process Heaters source categories into the Industrial/
Commercial/Institutional Boilers and Process Heaters source
category.\1\ The ``Hydrochloric Acid Production'' source category
includes ``any facility engaged in the production of hydrochloric
acid.''
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\1\ 70 FR 37819 (June 30, 2005).

[[Page 50817]]

Table 1--NESHAP and Source Categories Affected by This Proposed Action
------------------------------------------------------------------------
Source category NESHAP NAICS code \1\
------------------------------------------------------------------------
Petroleum and coal products 40 CFR part 63, 3241
manufacturing. subpart EEE.
Chemical manufacturing........ 40 CFR part 63, 325
subpart EEE.
Cement and concrete product 40 CFR part 63, 3273
manufacturing. subpart EEE.
Other nonmetallic mineral 40 CFR part 63, 3279
product manufacturing. subpart EEE.
Hazardous waste treatment and 40 CFR part 63, 562211
disposal. subpart EEE.
Remediation and other waste 40 CFR part 63, 5629
management services. subpart EEE.
------------------------------------------------------------------------
\1\ North American Industry Classification System.

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

In addition to being available in the docket, an electronic copy of
this action is available on the internet. In accordance with 5 U.S.
Code (U.S.C.) 553(b)(4), a brief summary of this rule may be found at
<a href="https://www.regulations.gov">https://www.regulations.gov</a>, Docket ID No. EPA-HQ-OAR-2004-0022.
Following signature by the Administrator, the EPA will post a copy of
this proposed action at <a href="https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous">https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous</a>. Following publication in the Federal Register, the EPA will
post the Federal Register version of the proposal and key technical
documents at this same web page. Information on the overall residual
risk and technology review (RTR) program is available 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>.
A memorandum showing the rule edits that would be necessary to
incorporate the changes to 40 CFR part 63, subpart EEE, proposed in
this action is available in the docket (Docket ID No. EPA-HQ-OAR-2004-
0022). Following signature by the Administrator, the EPA also will post
a copy of this document to <a href="https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous">https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous</a>.

II. Background

A. What is the statutory authority for this proposed action?

The statutory authority for this proposed action is provided by CAA
sections 112, 301(a), and 502(a) (42 U.S.C. 7412, 7601(a), 7661a(a)).
CAA section 112 establishes a two-stage regulatory process to develop
standards for emissions of hazardous air pollutants (HAP) from
stationary sources. Generally, the first stage involves establishing
technology-based standards that reflect the maximum achievable control
technology (MACT) or an appropriate alternative.\2\ The second stage
involves evaluating those standards within eight years to determine
whether additional standards are needed to address any remaining risk
associated with HAP emissions.\3\ This second stage is commonly
referred to as the ``residual risk review.'' In addition to the
residual risk review, CAA section 112 also requires the EPA to review
the standards every eight years and ``revise as necessary'' taking into
account ``developments in practices, processes, or control
technologies.'' \4\ This review is commonly referred to as the
``technology review.'' When the two reviews are combined into a single
rulemaking, it is commonly referred to as the ``risk and technology
review'' (RTR). The discussion that follows identifies the most
relevant statutory sections and briefly explains the contours of the
methodology used to implement these statutory requirements.
---------------------------------------------------------------------------

\2\ 42 U.S.C. 7412(d)(1)-(4).
\3\ Id. 7412(f)(2).
\4\ Id. 7412(d)(6).
---------------------------------------------------------------------------

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 and area source standards. ``Major sources'' are
those that emit or have the potential to emit 10 tons per year (tpy) or
more of a single HAP or 25 tpy or more of any combination of HAP.\5\
All other sources are ``area sources.'' \6\ For major sources, CAA
section 112(d)(2) provides that the technology-based NESHAP must
reflect the maximum degree of emission reductions 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,''
based on emission controls achieved in practice by the best performing
sources. 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.'' \7\
CAA section 112(h)(2)(A) defines this phrase as applying in any
situation where a HAP ``cannot be emitted through a conveyance designed
and constructed to emit or capture such pollutant, or that any
requirement for, or use of such a conveyance would be inconsistent with
any Federal, State or local law.'' \8\ This phrase is further defined
in CAA section 112(h)(2)(B) as applying where ``the Administrator
determines that the application of measurement methodology to a
particular class of sources is not practicable due to technological and
economic limitations.'' \9\ The EPA has long considered situations
where the majority of the measurements are below the detection limit as
being a situation where measurement is not ``technologically
practicable'' within the meaning of CAA section 112(h)(2)(B).
Additionally, unreliable measurements raise issues of practicability,
feasibility and enforceability. The application of measurement
methodology in this situation would also not be ``practicable due to .
. . economic limitation'' within the meaning of CAA section
112(h)(2)(B) because it would just result in cost expended to produce
analytically

[[Page 50818]]

suspect measurements. The EPA also considers control options that are
more stringent than the floor.\10\ Standards more stringent than the
floor are commonly referred to as ``beyond-the-floor'' standards. For
area sources, CAA section 112(d)(5) allows the EPA to set standards
based on generally available control technologies or management
practices (GACT standards) in lieu of MACT standards.
---------------------------------------------------------------------------

\5\ 42 U.S.C. 7412(a)(1).
\6\ Id. 7412(a)(2).
\7\ Id. 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 v. EPA, 734 F.3d 1115, 1154-55
(D.C. Cir. 2013).
\8\ 42 U.S.C. 7412(h)(2)(A).
\9\ 42 U.S.C. 7412(h)(2)(B).
\10\ Id. 7412(d)(2).
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For categories of major sources and any area source categories
subject to MACT standards, the second stage focuses on identifying and
addressing any remaining (i.e., ``residual'') risk within eight years
pursuant to CAA section 112(f). Specifically, CAA section 112(f)(2)
requires the EPA to determine not later than eight years after
establishment of the MACT standards whether promulgation of additional
standards is needed to provide an ample margin of safety to protect
public health or to prevent an adverse environmental effect. CAA
section 112(d)(5) provides that this residual risk review is not
required for categories of area sources subject to GACT standards. 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'').\11\ The EPA notified Congress in
the Residual Risk Report that the Agency intended to use the Benzene
NESHAP approach in making CAA section 112(f) residual risk
determinations (EPA-453/R-99-001, p. ES-11). The EPA subsequently
adopted this approach in its residual risk determinations, and the
United States Court of Appeals for the District of Columbia Circuit
upheld the EPA's interpretation that CAA section 112(f)(2) incorporates
the approach established in the Benzene NESHAP. See NRDC v. EPA, 529
F.3d 1077, 1083 (D.C. Cir. 2008).
---------------------------------------------------------------------------

\11\ 54 FR 38044, Sept. 14, 1989.
---------------------------------------------------------------------------

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 acceptable. 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''.\12\ If risks are
unacceptable, the EPA must determine 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''.\13\ The EPA must promulgate 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, we consider whether a more stringent standard is
necessary to prevent, taking into consideration costs, energy, safety,
and other relevant factors, an adverse environmental effect.
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\12\ 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.
\13\ Id.
---------------------------------------------------------------------------

CAA section 112(d)(6) separately requires the EPA to review
standards promulgated under CAA section 112 and revise them ``as
necessary (taking into account developments in practices, processes,
and control technologies)'' no less often than every eight years. In
conducting this review, which we call the ``technology review,'' the
EPA is not required to recalculate the MACT floors that were
established during earlier rulemakings.\14\ The EPA may consider cost
in deciding whether to revise the standards pursuant to CAA section
112(d)(6).\15\
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\14\ Ass'n of Battery Recyclers, Inc. v. EPA, 716 F.3d 667, 673-
674 (D.C. Cir. 2013); NRDC, 529 F.3d at 1084.
\15\ 42 U.S.C. 7412(d)(2), (6); Ass'n of Battery Recyclers, 617
F.3d at 673-74.
---------------------------------------------------------------------------

CAA sections 112(c)(3) and (k) require the EPA to identify and list
the area source categories that represent 90 percent of the emissions
of the 30 urban air toxics associated with area sources and subject
them to standards under the CAA. CAA section 112(k)(3), which cross-
references CAA section 112(c)(3), requires the EPA to identify a list
of at least 30 air toxics that pose the greatest potential health
threat in urban areas (the ``urban'' HAP). Taken together, these
requirements are known as the Urban Air Toxics Strategy. These are the
HAP that present the greatest threat to public health in the largest
number of urban areas (CAA section 112(k)(3)(B)(i)). CAA sections
112(k)(3)(B)(ii) and 112(c)(3) also require the EPA to ``assure that
sources accounting for 90 percent or more of the 30 identified
hazardous air pollutants are subject to standards.''
In Louisiana Environmental Action Network (LEAN) v. EPA, the D.C.
Circuit held that the EPA must address missing MACT standards for
listed HAP known to be emitted from a major source category as part of
its periodic review of MACT standards under CAA section 112(d)(6). 955
F.3d 1088 (D.C. Cir. 2020). 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, and establish
standards for any previously unregulated HAP in the final RTR. Order,
Blue Ridge Envtl. Def. League v. Regan, 22-cv-3134 (APM), at 4 (D.D.C.
Dec. 12, 2024). The EPA is proposing this action in response to that
court order.
Further, under CAA section 301(a) ``[t]he Administrator is
authorized to prescribe such regulations as are necessary to carry out
his functions.'' 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.

B. What is this source category and how does the current NESHAP
regulate its HAP emissions?

HWCs are incinerators, cement kilns, lightweight aggregate kilns,
boilers, or HCl production furnaces that combust

[[Page 50819]]

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 safe treatment, storage, and
disposal of hazardous waste.\16\ HWCs act as a disposal method for
hazardous waste but can also provide other benefits to their owners and
operators. The primary purpose of HWC incinerators is the destruction
or volume reduction of hazardous waste. The primary purpose of HWC
cement kilns is the production of cement using hazardous waste as a
fuel to reduce the need for non-waste energy inputs. Similarly, HWC
lightweight aggregate kilns use hazardous waste to provide energy for
producing lightweight aggregate. HWC boilers produce thermal energy
(often used in the form of steam), with hazardous waste often replacing
the need for some non-waste fuel. An HWC HCl production furnace
produces HCl, often using hazardous waste as a chlorine source.
---------------------------------------------------------------------------

\16\ 42 U.S.C. 6901-6992k.
---------------------------------------------------------------------------

HWCs may either burn only hazardous waste produced onsite or by the
owner, which is referred to as a ``captive'' HWC, or burn hazardous
waste produced offsite or by someone other than the owner, which is
referred to as a ``commercial'' HWC. Facilities with captive HWCs
typically use their HWC as a waste management strategy. The most common
captive HWCs are solid fuel boilers, liquid fuel boilers, HCl
production furnaces, and some incinerators. Facilities with commercial
HWCs typically use their HWC for revenue generation. The most common
commercial HWCs are cement kilns, lightweight aggregate kilns, and
incinerators. The main line of business for some commercial HWC
incinerators is waste management. There are approximately 160 HWCs
located at approximately 90 facilities in the United States. 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 ways
that protect human health and the environment.\17\ Hazardous waste
incineration provided that 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.\18\
---------------------------------------------------------------------------

\17\ 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>.
\18\ U.S. Environmental Protection Agency. (Last updated Jul.
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>.
---------------------------------------------------------------------------

HWCs are regulated under both the CAA and RCRA. Under the CAA, all
unit types are regulated under the HWC NESHAP. Prior to demonstrating
compliance with the HWC NESHAP, incinerators were primarily regulated
by 40 CFR part 264, subpart O, and cement kilns, lightweight aggregate
kilns, boilers, and HCl production furnaces were primarily regulated by
40 CFR part 266, subpart H. For most sources, air emission standards
and associated operating requirements are no longer contained in their
RCRA permits. Sources continue to hold RCRA permits for activities
related to hazardous waste management, including general facility
standards, manifest requirements, closure, financial responsibility,
and any risk-based emission limit and associated operating conditions
deemed necessary to protect human health and the environment.
The HWC NESHAP, which was originally promulgated in 1999, regulated
hazardous waste incinerators, cement kilns, and lightweight aggregate
kilns.\19\ These standards were vacated in 2001 \20\ and replaced with
interim standards in 2002.\21\ 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.\22\ Subsequently, the EPA received four petitions for
reconsideration of the final rule. In 2006, the EPA granted
reconsideration for eight issues raised by the petitions \23\ and in
2007 reopened the 2005 rule \24\ (the ``Solicitation of Comment on
Legal Analysis'') to consider comments relating to an intervening
decision by the D.C. Circuit.\25\ The EPA took final action on the
eight reconsideration issues, responded to comments on the Solicitation
of Comment on Legal Analysis, and made technical corrections in
2008.\26\ In response to a petition for reconsideration of the 2008
final rule, the EPA sought and received a full voluntary remand of the
rule in 2009 to reexamine the HWC NESHAP in totality.\27\
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\19\ 64 FR 52828 (September 30, 1999).
\20\ Cement Kiln Recycling Coal. v. EPA, 255 F.3d 855, 872 (D.C.
Cir. 2001).
\21\ 67 FR 6792 (Feb. 13, 2002).
\22\ 70 FR 59402 (Oct. 12, 2005).
\23\ 71 FR 14665 (Mar. 23, 2006); 71 FR 52624 (Sept. 6, 2006).
\24\ 72 FR 54875 (Sept. 27, 2007).
\25\ Sierra Club v. EPA, 479 F.3d 875 (D.C. Cir. 2007).
\26\ 73 FR 64068 (Oct. 28, 2008).
\27\ Sierra Club v. EPA, Docket No. 05-1441 (consolidated with
Docket Nos. 05-1442, 05-1443, 05-1445, 05-1449) (D.C. Cir.).
---------------------------------------------------------------------------

In July 2024, the EPA issued a notice of proposed rulemaking for
the HWC NESHAP regarding emission standards during periods of
malfunction, electronic reporting provisions, emergency safety vent
provisions, and other minor technical corrections.\28\ The EPA is
withdrawing certain aspects of that proposal in this document for the
reasons explained in section IV.E. of this preamble. The EPA is instead
proposing different requirements and soliciting comments on certain
topics from the 2024 proposal that include emission standards during
periods of malfunction and electronic reporting provisions in this
notice of proposed rulemaking. The EPA will respond to other comments
on aspects of the July 2024 proposal that are not withdrawn in the
final action for this proposal.
---------------------------------------------------------------------------

\28\ 89 FR 59867 (Jul. 24, 2024).
---------------------------------------------------------------------------

The key pollutants that the HWC NESHAP regulates include
polychlorinated dibenzodioxins and furans (PCDD/PCDF); mercury (Hg);
cadmium (Cd) and lead (Pb) as semi-volatile metals (SVM); arsenic (As),
beryllium (Be), and chromium (Cr) as low-volatile metals (LVM);
antimony (Sb), cobalt (Co), manganese (Mn), nickel (Ni), and selenium
(Se) as non-enumerated metal HAP; HCl and chlorine gas; and other
hydrocarbon HAP, including polychlorinated biphenyls (PCBs) and
polycyclic aromatic hydrocarbons (PAHs). The HWC NESHAP also includes
several other emission limits 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 in some subcategories.
The HWC NESHAP regulates HAP through a combination of numeric
emission limits and surrogate standards, where compliance with one
emission standard demonstrates compliance with the standard for another
HAP. For example, emissions of non-PCDD/PCDF organic HAP, including
PCBs and PAHs, are regulated by the combination of the DRE standard and
either the CO or THC standard, as chosen by the source. Another example
of a surrogate is the PM standard, which primarily regulates emissions
of non-enumerated metal HAP. These metals are not regulated by

[[Page 50820]]

another metal HAP standard. Sources may also choose to regulate non-
enumerated metal HAP directly as an alternative to the PM standard. An
alternative health-based emission limit (HBEL) based on a site-specific
risk assessment is also available for HCl and chlorine gas.\29\
---------------------------------------------------------------------------

\29\ For more information on the alternative HBEL for HCl and
chlorine gas, see 70 FR 59413-25 (Oct. 12, 2005); see also 69 FR
21298-305-06 (Apr. 20, 2004).
---------------------------------------------------------------------------

The HWC NESHAP regulates HAP emissions from HWCs at major and area
sources, as defined by CAA sections 112(a)(1) and (2). The HWC NESHAP
also requires both major and area sources to obtain a title V air
permit. Major and area sources are subject to the same standards for
HWC incinerators, cement kilns, and lightweight aggregate kilns. Area
source HWC boilers and HCl production furnaces are only subject to the
same emission standards as major sources for Hg, PCDD/PCDF, and non-
PCDD/PCDF organic HAP. RCRA standards for Cd and Pb, Cr, HCl and
chlorine gas, and PM under 40 CFR part 266, subpart H, apply to area
source HWC boilers and HCl production furnaces unless an area source
elects to comply with the HWC NESHAP major source standards in lieu of
the RCRA standards. Area sources otherwise have the same requirements
as major sources, including recordkeeping, reporting, operator
training, the startup, shutdown, and malfunction (SSM) plan, and the
automatic waste feed cutoff (AWFCO) system.\30\
---------------------------------------------------------------------------

\30\ See 70 FR 59432 (Oct. 12, 2005) for further discussion on
the similarities and differences between major and area source
standards.
---------------------------------------------------------------------------

Periods of SSM are addressed under both the RCRA rules and the HWC
NESHAP. The requirements of both rules apply simultaneously to HWCs.
Under the RCRA rules, sources may choose to retain or revise certain
RCRA permit conditions that are specific to periods of SSM, including a
requirement not to feed most types of hazardous waste during periods of
SSM. Alternatively, sources may choose to remove RCRA operating permit
conditions specific to periods of SSM if an SSM plan has been developed
under the HWC NESHAP and approved by the Administrator.\31\ Most
sources have chosen to remove conditions specific to periods of SSM
from their RCRA permits because they have approved SSM plans under the
HWC NESHAP.
---------------------------------------------------------------------------

\31\ See 40 CFR part 270, subpart I, for the integration of RCRA
and CAA standards during periods of SSM.
---------------------------------------------------------------------------

The emission standards and operating requirements of the HWC NESHAP
currently do not apply during periods of SSM.\32\ However, there are
two requirements relating to periods of SSM that apply to all HWCs.
Specifically, all HWCs must develop an SSM plan and operate an AWFCO
system. All HWCs are subject to the SSM plan provisions of the 40 CFR
part 63, subpart A general provisions, including the requirements to
develop an SSM plan and update it as necessary. Under the general
provisions, if actions taken by the owner or operator cause the source
to exceed any applicable emission limitation and are consistent with
the procedures specified in the SSM plan, then the owner or operator
must keep records and confirm in their reporting that their actions
were consistent with the SSM plan. If actions taken by the owner or
operator cause the source to exceed any applicable emission limitation
and are not consistent with the procedures specified in the SSM plan,
then the owner or operator must also record and report those actions.
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\32\ In July 2024, the EPA proposed to remove the malfunction
exemption from the HWC NESHAP, which, if finalized, would have
required standards for periods of normal operation to apply at all
times (89 FR 59870). After considering the comments received on that
proposal, the EPA is withdrawing the proposed removal of the
malfunction exemption and is instead proposing different
requirements for periods of malfunction, as described in section
IV.E of this preamble.
---------------------------------------------------------------------------

If an HWC uses their CAA SSM plan to comply with RCRA requirements,
then the SSM plan must be submitted to the Administrator for review and
approval and the SSM plan must include a description of potential
causes of malfunctions and actions the source is taking to minimize the
frequency and severity of those malfunctions. The Administrator must
also approve any changes to the SSM plan if the changes may
significantly increase emissions of HAP.
All HWCs are also required to operate an AWFCO system, which is a
system that immediately (or within one minute in some circumstances)
and automatically cuts off the hazardous waste feed to the HWC when an
operating parameter limit (OPL) established per the HWC NESHAP is
exceeded, an emission standard monitored by a continuous emission
monitoring system (CEMS) is met or exceeded, the allowable combustion
chamber pressure is exceeded, the span value of any continuous
monitoring system (CMS) detector except a CEMS is met or exceeded, a
CMS monitoring an emission level or an OPL established per the HWC
NESHAP malfunctions, or any component of the AWFCO system fails. During
an AWFCO, 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 OPLs and emission levels are within the specified
limits, which typically takes no less than one hour. The AWFCO system
must generally be tested at least weekly.
HWCs must comply with the described AWFCO system requirements
during malfunctions, although an exceedance of an OPL or emission
standard interlocked with the AWFCO system is not a violation of the
HWC NESHAP if the corrective measures prescribed in the SSM plan are
correctly followed.
Additionally, HWCs must comply with AWFCO requirements during
periods of startup and shutdown if they burn hazardous waste during
those periods. An exceedance of an OPL or emission standard interlocked
with the AWFCO system is not a violation of the HWC NESHAP if the
corrective measures prescribed in the SSM plan are correctly followed.
If owners or operators of HWCs feed hazardous waste during periods of
startup or shutdown, they must include waste feed restrictions and
other appropriate operating conditions and limits in the SSM plan and
interlock those OPLs with the AWFCO system. Under the RCRA incinerator
(40 CFR part 264, subpart O) and boiler and industrial furnaces (BIF;
40 CFR part 266, subpart H) requirements, hazardous waste may be fed
into an HWC during startup and shutdown if all OPLs are being met. This
is typically the case shortly before startup ends and shortly after
shutdown begins. In addition, certain types of hazardous waste may be
fed during startup and shutdown under certain stipulations, regardless
of whether OPLs for periods of normal operation are being met. One
example is a waste that is only considered hazardous because it is
ignitable and easily burned; an owner or operator might feed this waste
into the combustor during startup and use the energy released by its
combustion to raise the HWC's temperature to the allowable range for
periods of normal operation. Most HWCs do not combust hazardous waste
during startup and shutdown. In cases where an HWC does so, we expect
that the HWC NESHAP's SSM plans closely mirror RCRA's restrictions on
hazardous waste feed during periods of startup and shutdown.

C. What data collection activities were conducted to support this
action?

The EPA conducted multiple data collection activities to support
this action, including collecting HWC facility permits and emissions
testing

[[Page 50821]]

information available through state and local authorities, a two-phased
request for information under CAA section 114, and site visits to HWC
facilities.\33\
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\33\ In its 1999 report to Congress on how the agency planned to
address residual risks, the EPA stated that ``source and emissions
data can be derived from broad-scale emissions inventories, specific
data collection efforts with particular industries, or information
from regional, State, or local air toxics agencies.'' Residual Risk
Report to Congress, EPA-453/R-99-001 at 13 (March 1999) (emphasis
added).
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To develop the facility list, we began with a partial facility list
provided by the EPA Office of Land and Emergency Management (OLEM),
which administers the RCRA rules for HWCs. We also gathered a list of
all facilities listed as subject to the HWC NESHAP in the EPA Office of
Enforcement and Compliance Assurance's (OECA) Enforcement and
Compliance History Online (ECHO) tool (<a href="https://echo.epa.gov">https://echo.epa.gov</a>). We
reviewed and cross-referenced these lists and confirmed that facilities
were subject to the HWC NESHAP by gathering title V air permits from
the websites of state and local governments or agencies, where
available. The resulting facility list is available in the docket for
this proposed rule (Docket ID No. EPA-HQ-OAR-2004-0022).
The EPA also collected emissions test reports for the comprehensive
performance test (CPT) and confirmatory performance test (CfPT)
required by the HWC NESHAP. These test reports were gathered from the
websites of state and local governments or agencies where available,
from the EPA regional offices, and from some industry stakeholders who
voluntarily provided courtesy copies. The CPT and CfPT reports provide
unit- and site-specific emissions information for the HAP regulated by
the HWC NESHAP and are the basis for emissions of the currently
regulated HAP in the risk modeling for the HWC source category. The
collected emissions test reports used to develop emissions for the HWC
source category are available in the docket for this proposed rule
(Docket ID No. EPA-HQ-OAR-2004-0022).
In August 2023 and January 2024, the EPA issued requests to collect
information from HWC facilities owned and operated by nine entities
(i.e., corporations) pursuant to CAA section 114. These facilities were
chosen to represent the six HWC subcategories and commercial and
captive units. The August 2023 request was a questionnaire designed to
collect comprehensive information about process equipment, control
technologies, emissions, composition of the hazardous waste feed to the
unit, periods of SSM, and other aspects of facility operations.
Companies submitted responses (and follow-up responses) in November
2023. A copy of the questionnaire and the information not claimed as
CBI by respondents is available in the docket for this proposed
rule.\34\
---------------------------------------------------------------------------

\34\ Docket ID No. EPA-HQ-OAR-2004-0022, Document ID No. EPA-HQ-
OAR-2004-0022-0651.
---------------------------------------------------------------------------

Following the review of the August 2023 questionnaire information,
the EPA issued an emissions testing request to the same nine entities
in January 2024 to obtain emissions information about targeted
pollutants and to characterize emissions of any HAP not currently
regulated by the HWC NESHAP. Companies submitted responsive emissions
testing results (and follow-up responses) between September 2024 and
November 2024. The EPA did not receive emissions testing results from
one company that temporarily ceased operation of their HWCs between
January 2024 and September 2024. The January 2024 request generally
required emissions testing of PCBs, PAHs, hydrogen fluoride (HF),
hydrogen cyanide (HCN), hydrogen bromide (HBr), THC, and supporting
measurements like oxygen and moisture, though the requests were
tailored to the specific survey responses of each recipient. Notably,
all HCl production furnaces indicated in their survey responses that
they do not feed any fluorine to their units because it would
contaminate their HCl product with HF, and so HCl production furnaces
were not required to test for HF. The EPA has used the collected
information to identify and quantify emissions of HAP not measured
during a CPT or CfPT, fill data gaps, and estimate the public health,
environmental, and cost impacts associated with the regulatory options
considered in this proposed action. A copy of the emissions testing
request and the information not claimed as CBI by respondents is
available in the docket for this proposed rule (Docket ID No. EPA-HQ-
OAR-2004-0022).
The EPA also conducted two site visits to HWC facilities in 2023.
The primary goals of these site visits were to learn about the day-to-
day operations of incinerators, solid fuel boilers, and cement kilns.
Reports documenting these site visits are available in the docket for
this proposed rule.\35\
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\35\ Docket ID No. EPA-HQ-OAR-2004-0022, Document ID Nos. EPA-
HQ-OAR-2004-0022-0649 and EPA-HQ-OAR-2004-0022-0650.
---------------------------------------------------------------------------

D. What other relevant background information and data are available?

The EPA used emissions and supporting data from the National
Emissions Inventory (NEI) based on emissions year 2022, supporting data
from the database developed for the 2005 HWC NESHAP Final Rule,
available RCRA trial or risk burn data, and CPT and CfPT stack test
data from as many sources as possible to develop model file inputs for
the residual risk assessment of sources subject to the HWC NESHAP.
The NEI is a database that contains information about sources that
emit criteria air pollutants, their precursors, and HAP. The NEI
contains data necessary for conducting risk modeling, including annual
HAP emissions estimates from individual emissions sources at facilities
and the related emissions release parameters. The database includes
estimates of annual air pollutant emissions from point, nonpoint, and
mobile sources in the 50 states, the District of Columbia, Puerto Rico,
and the U.S. Virgin Islands. The EPA collects this information and
releases a full, updated version of the NEI database every three years.
The 2022 emissions data is not a full, triennial NEI; instead, the 2020
NEI was taken as a basis and more recent 2022 data was incorporated. In
cases where we had emissions and release point parameters for the same
HWC from multiple data sources (e.g., the NEI, CPT, and August 2023
questionnaire), we prioritized information in the following order:
August 2023 questionnaire, CPT or CfPT data, then NEI data. Additional
information on the development of the modeling file can be found in the
docket for this proposed rule (Docket ID No. EPA-HQ-OAR-2004-0022).
To identify control technologies in use and determine whether there
have been developments in practices, processes, or control technologies
to consider under the technology review, the EPA collected information
from the August 2023 questionnaire, January 2024 emissions testing
request, CPT and CfPT reports, consent decrees involving HWCs regulated
by the HWC NESHAP, and the Reasonably Available Control Technology
(RACT)/Best Available Control Technology (BACT)/Lowest Achievable
Emission Rate (LAER) Clearinghouse (RBLC). The EPA established the RBLC
to provide a central database of air pollution technology information
(including technologies required in source-specific permits) to promote
the sharing of information among permitting agencies

[[Page 50822]]

and to ais in identifying future control technology options that might
apply to numerous sources within a category or only on a source-by-
source basis.\36\ The EPA also reviewed subsequent CAA regulatory
actions for other source categories to determine whether there have
been other developments in practices, processes, or control
technologies that may also be applicable to the HWC NESHAP source
category. Additional information about the technology review can be
found in the docket for this proposed rule (Docket ID No. EPA-HQ-OAR-
2004-0022).
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\36\ U.S. Environmental Protection Agency. (Last updated Sept.
29, 2025). RACT/BACT/LAER Clearinghouse (RBLC) Basic Information:
<a href="https://www.epa.gov/catc/ractbactlaer-clearinghouse-rblc-basic-information">https://www.epa.gov/catc/ractbactlaer-clearinghouse-rblc-basic-information</a>.
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III. Analytical Procedures and Decision-Making

In this section, we describe the analyses performed to support the
proposed decisions for the RTR and other issues addressed in this
proposal.

A. How do we consider risk in our decision-making?

As discussed in section II.A. of this preamble and in the Benzene
NESHAP, in evaluating and developing standards under CAA section
112(f)(2), we apply a two-step approach to determine whether or not
risks are acceptable and to determine if the standards provide an ample
margin of safety to protect public health. As explained in the Benzene
NESHAP, ``the first step judgment on acceptability cannot be reduced to
any single factor'' and, thus, ``[t]he Administrator believes that the
acceptability of risk under section 112 is best judged on the basis of
a broad set of health risk measures and information.'' \37\ Similarly,
with regard to the ample margin of safety determination, ``the Agency
again considers all of the health risk and other health information
considered in the first step'' (id.). ``Beyond that information,
additional factors relating to the appropriate level of control will
also be considered, including cost and economic impacts of controls,
technological feasibility, uncertainties, and any other relevant
factors'' (id.).
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\37\ 54 FR 38046, Sept. 14, 1989.
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The Benzene NESHAP approach provides flexibility regarding factors
the EPA may consider in making determinations and how the EPA may weigh
those factors for each source category.\38\ The EPA conducts a risk
assessment that provides estimates of the MIR posed by emissions of HAP
that are carcinogens from each source in the source category, the
hazard index (HI) for chronic exposures to HAP with the potential to
cause noncancer health effects, and the hazard quotient (HQ) for acute
exposures to HAP with the potential to cause noncancer health
effects.\39\ The assessment also provides estimates of the distribution
of cancer risk within the exposed populations, cancer incidence, and an
evaluation of the potential for an adverse environmental effect. The
scope of the EPA's risk analysis is consistent with the explanation in
the EPA's response to comments on our policy under the Benzene NESHAP:
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\38\ See NRDC, 529 F.3d at 1082-1084.
\39\ The MIR is defined as the cancer risk associated with a
lifetime of exposure at the highest concentration of HAP where
people are likely to live. The HQ is the ratio of the potential HAP
exposure concentration to the noncancer dose-response value; the HI
is the sum of HQs for HAP that affect the same target organ or organ
system.

The policy chosen by the Administrator permits consideration of
multiple measures of health risk. Not only can the MIR figure be
considered, but also incidence, the presence of non-cancer health
effects, and the uncertainties of the risk estimates. In this way,
the effect on the most exposed individuals can be reviewed as well
as the impact on the general public. These factors can then be
weighed in each individual case. This approach complies with the
Vinyl Chloride mandate that the Administrator ascertain an
acceptable level of risk to the public by employing his expertise to
assess available data. It also complies with the Congressional
intent behind the CAA, which did not exclude the use of any
particular measure of public health risk from the EPA's
consideration with respect to CAA section 112 regulations, and
thereby implicitly permits consideration of any and all measures of
health risk which the Administrator, in his judgment, believes are
appropriate to determining what will ``protect the public health.''
\40\
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\40\ 54 FR 38057, Sept. 14, 1989.

Thus, the level of the MIR is only one factor to be weighed in
determining acceptability of risk. As the EPA explained in the Benzene
NESHAP: ``[A]n MIR of approximately one in 10 thousand should
ordinarily be the upper end of the range of acceptability. As risks
increase above this benchmark, they become presumptively less
acceptable under [CAA] section 112 and would be weighed with the other
health risk measures and information in making an overall judgment on
acceptability. Or, the Agency may find, in a particular case, that a
risk that includes an MIR less than the presumptively acceptable level
is unacceptable in the light of other health risk factors.'' \41\ In
other words, risks that include an MIR above 100-in-1 million (1-in-10
thousand) may be determined to be acceptable, and risks with an MIR
below that level may be determined to be unacceptable, depending on the
available health information. Similarly, with regard to the ample
margin of safety analysis, the EPA stated in the Benzene NESHAP that:
``EPA believes the relative weight of the many factors that can be
considered in selecting an ample margin of safety can only be
determined for each specific source category. This occurs mainly
because technological and economic factors (along with the health-
related factors) vary from source category to source category.'' \42\
We also consider the uncertainties associated with the various risk
analyses, as discussed later in this preamble, in our determinations of
acceptability and ample margin of safety.
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\41\ Id. at Sept. 14, 1989.
\42\ 54 FR 38061, Sept. 14, 1989.
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The EPA notes that, as a matter of longstanding practice, we do not
attempt to quantify the HAP risk that may be associated with emissions
from other facilities that do not include the source category under
review, mobile source emissions, natural source emissions, persistent
environmental pollution, or atmospheric transformation in the vicinity
of the sources in the category. The EPA understands the potential
importance of considering an individual's total exposure to HAP in
addition to considering exposure to HAP emissions from the source
category and facility. We recognize that such consideration may be
particularly important when assessing noncancer risk, where pollutant-
specific exposure health reference levels (e.g., reference
concentrations (RfCs)) are based on the assumption that thresholds
exist for adverse health effects. For example, the EPA recognizes that,
although exposures attributable to emissions from a source category or
facility alone may not indicate the potential for increased risk of
adverse noncancer health effects in a population, the exposures
resulting from emissions from the facility in combination with
emissions from all of the other sources (e.g., other facilities) to
which an individual is exposed may be sufficient to result in an
increased risk of adverse noncancer health effects. In May 2010, the
Science Advisory Board (SAB) advised the EPA ``that RTR assessments
will be most useful to decision makers and communities if results are
presented in the broader context of aggregate and cumulative risks,
including background

[[Page 50823]]

concentrations and contributions from other sources in the area.'' \43\
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\43\ Recommendations of the SAB Risk and Technology Review
Methods Panel are provided in their report, which is available at:
<a href="https://www.epa.gov/sites/default/files/2021-02/documents/epa-sab-10-007-unsigned.pdf">https://www.epa.gov/sites/default/files/2021-02/documents/epa-sab-10-007-unsigned.pdf</a>.
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In response to the SAB recommendations, the EPA incorporates
cumulative risk analyses into its RTR risk assessments. The Agency (1)
conducts facility-wide assessments, which include source category
emission points, as well as other emission points within the
facilities; (2) combines exposures from multiple sources in the same
category that could affect the same individuals; and (3) for some
persistent and bioaccumulative pollutants, analyzes the ingestion route
of exposure. In addition, the RTR risk assessments consider aggregate
cancer risk from all carcinogens and aggregated noncancer HQs for all
noncarcinogens affecting the same target organ or target organ system.
Although we are interested in placing source category and facility-
wide HAP risk in the context of total HAP risk from all sources
combined in the vicinity of each source, we note there are
uncertainties of doing so. Estimates of total HAP risk from emission
sources other than those that we have studied in depth during this RTR
review would have greater associated uncertainties than the source
category or facility-wide estimates. We further note that CAA section
112(f)(2) does not require or authorize the EPA to promulgate standards
based on cumulative assessments of a person's total exposure to HAP
from all sources.

B. How do we perform the technology review?

The EPA's technology review primarily focuses on the identification
and evaluation of developments in practices, processes, and control
technologies that have occurred since the MACT standards were
promulgated. Where we identify such developments, we analyze their
technical feasibility, estimated costs, energy implications, and non-
air environmental impacts. We also consider the emission reductions
associated with the potential application of each development. This
analysis informs our decision whether it is ``necessary'' to revise the
emission standards. In addition, we consider the appropriateness of
applying controls to new sources versus retrofitting existing sources.
For this exercise, we consider any of the following to be a
``development'' :\44\
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\44\ Nat'l Ass'n for Surface Finishing v. EPA, 795 F.3d 1, 11
(D.C. Cir. 2015) (upholding EPA's interpretation of what is
considered ``developments'' under CAA section 112(d)(6) and
deferring to EPA's methodology and balancing decisions for a
technology review under the Skidmore standard of review).
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<bullet> Any add-on control technology or other equipment that was
not identified and considered during development of the original MACT
standards;
<bullet> Any improvements in add-on control technology or other
equipment (that were identified and considered during development of
the original MACT standards) that could result in additional emissions
reduction;
<bullet> Any work practice or operational procedure that was not
identified or considered during development of the original MACT
standards;
<bullet> Any process change or pollution prevention alternative
that could be broadly applied to the industry and that was not
identified or considered during development of the original MACT
standards; and
<bullet> Any significant changes in the cost (including cost
effectiveness) of applying controls (including controls the EPA
considered during the development of the original MACT standards).
In addition to reviewing the practices, processes, and control
technologies that were considered at the time we originally developed
the NESHAP, we review a variety of data sources in our investigation of
potential practices, processes, or controls. Pursuant to the D.C.
Circuit's decision in LEAN, we also review available data to determine
if there are any unregulated emissions of HAP within the source
category and evaluate this data for use in developing new emission
standards. The LEAN decision requires the EPA to address regulatory
gaps when reviewing MACT standards, such as missing standards for
listed air toxics known to be emitted from a major source category. See
sections II.C. and II.D of this preamble for information on the
specific data sources that were reviewed as part of the technology
review.

C. How do we estimate post-MACT risk posed by the source category?

In this section, the EPA provides a description of the types of
analyses that we generally perform during the risk assessment process.
In some cases, we do not perform a specific analysis because it is not
relevant. For example, in the absence of emissions of HAP known to be
persistent and bioaccumulative in the environment (PB-HAP), we would
not perform a multipathway exposure assessment. Where we do not perform
an analysis, we state that we do not and provide the reason. While we
present all of our risk assessment methods, we only present risk
assessment results for the analyses actually conducted (see section
IV.B. of this preamble).
The EPA conducts a risk assessment that provides estimates of the
MIR for cancer posed by the HAP emissions from each source in the
source category, the HI for chronic exposures to HAP with the potential
to cause noncancer health effects, and the HQ for acute exposures to
HAP with the potential to cause noncancer health effects. The
assessment also provides estimates of the distribution of cancer risk
within the exposed populations, cancer incidence, and an evaluation of
the potential for an adverse environmental effect, taking into
consideration the factors set out in CAA section 112(f). The following
eight subsections describe how we estimated emissions and conducted the
risk assessment. The docket for this proposed rule contains the
following document which provides more information on the risk
assessment inputs and models: Residual Risk Assessment for the
Hazardous Waste Combustor Source Category in Support of the 2025 Risk
and Technology Review Proposed Rule. The methods used to assess risk
(as described in the eight primary steps below) are consistent with
those described by the EPA in the document reviewed by a panel of the
EPA's SAB in 2009; \45\ and described in the SAB review report issued
in 2010.\46\ They are also consistent with the key recommendations
contained in that report.
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\45\ U.S. Environmental Protection Agency. (Last updated Mar.
20, 2012). Risk and Technology Review (RTR) Risk Assessment
Methodologies: For Review by the EPA's Science Advisory Board--Case
Studies--MACT I Petroleum Refining Sources and Portland Cement
Manufacturing (EPA-452/R-09-006): <a href="https://cfpub.epa.gov/si/si_public_record_report.cfm?LAB=OAQPS&dirEntryID=238928">https://cfpub.epa.gov/si/si_public_record_report.cfm?LAB=OAQPS&dirEntryID=238928</a>.
\46\ U.S. Environmental Protection Agency. (Last updated May 7,
2010). Review of EPA's draft entitled, ``Risk and Technology Review
(RTR) Risk Assessment Methodologies: For Review by the EPA's Science
Advisory Board--Case Studies--MACT I Petroleum Refining Sources and
Portland Cement Manufacturing'': <a href="https://www.epa.gov/sites/default/files/2021-02/documents/epa-sab-10-007-unsigned.pdf">https://www.epa.gov/sites/default/files/2021-02/documents/epa-sab-10-007-unsigned.pdf</a>.
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1. How did we estimate actual emissions and identify the emissions
release characteristics?
The EPA used the actual emissions and emissions release
characteristics from the NEI based on emissions year 2022 for each HWC
facility to create the initial risk modeling input file. For each NEI
record, the EPA reviewed the standard classification code, emission
unit description, and process description to classify each record as

[[Page 50824]]

either belonging to the source category (i.e., the record represents
emissions from an HWC) or not belonging to the source category. Source
category emissions of HAP are stack emissions only because the source
category is specific to the HWC unit. We included both stack and
fugitive emissions in non-category records. We removed duplicate
emission records for an HWC. For example, some facilities listed
emissions from the HWC when the unit was and was not combusting
hazardous waste separately. To consolidate these records, we removed
the records identified as periods when hazardous waste was not being
combusted. We then cross-referenced each source category record against
the facility list and added units in the facility list that could not
be identified in the NEI records. We identified emission release
characteristics from emissions testing information, as available.
Emissions test data or values derived from emissions test data
replaced or augmented NEI data for HWC emissions of PCDD/PCDF, HAP
metals, HCl, chlorine gas, PCBs, PAHs, HF, and HCN. When we had CPT
data or data responsive to the CAA section 114 emissions testing
request for a unit, we used that data to estimate emissions. Most of
the data were concentration data, and we calculated emissions on a tpy
basis using the stack gas flow rate and by assuming that units operate
8,760 hours (hr) per year (i.e., continuous operation). Some of the
data were in a thermal concentration format (like pounds (lb) per
million british thermal units (MMBTU)), and we used the thermal
hazardous waste feedrate (MMBTU/hr) to calculate emissions. When we had
units with data from multiple CPTs, test conditions, or runs, we used
the mean to estimate annual emissions (tpy). For cement kilns with in-
line raw mills, we calculated weighted averages to account for the
typical time spent with the raw mill on (85 percent) and off (15
percent).
In accordance with the HWC NESHAP, owners and operators conduct
CPTs at worst-case test conditions. Companies use multiple strategies
to ensure that CPTs are conducted at worst-case conditions, including
operating at worst-case operating parameter limits (e.g., low
combustion chamber temperature, high hazardous waste feed rate, high
stack gas velocity) and intentionally adding extra HAP, HAP surrogates,
or HAP precursors to the feed of the HWC to account for potential
variability in the HWC feed. This means that emissions estimates based
on CPT data are conservative, worst-case estimates. We expect that
actual annual HAP emissions are lower than the estimates based on CPT
data.
The EPA used CfPT data to help account for the conservative, worst-
case estimates, where available. Unlike CPTs, CfPTs are conducted at
normal operating conditions, and so we expect operating parameters to
better reflect average operations of HWCs. CfPTs are only conducted for
incinerators, cement kilns, lightweight aggregate kilns, and some
liquid fuel boilers. Only PCDD/PCDF is measured when CfPTs are
conducted. To scale the estimates produced by the CPT data to better
resemble normal operating conditions, we developed ratios between the
average stack gas flow rate during CPTs and CfPTs (``CPT adjustment
factor'') and used them to adjust the CPT emissions estimates, with a
maximum value of one.
The HWC NESHAP regulates metal HAP in groups: Hg; Cd and Pb are
regulated as SVM; As, Be, and Cr are regulated as LVM (except for
liquid fuel boilers, where the LVM standard regulates Cr only); and Sb,
Co, Mn, Ni, and Se are typically regulated by PM surrogate. Because the
HWC NESHAP regulates metals in groups, metals are often reported by
group in CPT results; however, the residual risk review requires the
emissions of each metal to be determined separately. Cr also required
further speciation because Cr species vary widely in both
physiochemical properties and toxicity. To separate these results, the
EPA developed speciation factors for Cr, SVM, LVM, and the other metals
(regulated using PM as a surrogate) by unit type using the database
developed for the 2005 HWC NESHAP final rule.\47\ Generally, we
developed the speciation factors by calculating the ratio of the
emission of the chemical species in question to the emission of the
total group. We averaged speciation factors by unit type, and only the
unit type averages were used to account for variability in chemical
speciation. We also used standard Hg speciation factors to calculate
emissions for Hg species with different physiochemical and toxicity
properties. For liquid fuel boilers, the only regulated LVM is Cr, so
speciation of Cr from LVM was not required. We did not speciate liquid
fuel boiler emissions of As and Be from PM; instead, we collected
separate emissions testing data (from trial or risk burns, or to
demonstrate compliance with state or RCRA limits) for As and Be and
used the data to estimate emissions.
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\47\ Docket ID No. EPA-HQ-OAR-2004-0022, Document ID No. EPA-HQ-
OAR-2004-0022-0433. Available at <a href="https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous">https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous</a>.
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We did not have complete data to support unit-specific estimates
for all units. The EPA made several informed assumptions to help fill
the gaps. First, the EPA calculated average CPT adjustment factors for
each unit type and applied those factors for units without a unit-
specific CPT adjustment factor. Two HWC subcategories, solid fuel
boilers and HCl production furnaces, are required to conduct CPTs but
not CfPTs. We assumed that the unit type average CPT adjustment factor
for these units was the average of the four other unit subcategory
average CPT adjustment factors. Second, some companies have units that
have been deemed identical units (``sister units'') for the purpose of
demonstrating compliance with the HWC NESHAP. Typically, the companies
perform emissions testing on one HWC of a set of sister units, and they
attribute the results to the sister units. When an HWC did not have
emissions data, but a sister unit did, we attributed those emissions to
all sister unit HWCs without unit-specific data. Third, we calculated
average emissions for each unit subcategory. We assumed that the
emissions for an HWC without emissions data for a specific HAP were the
average emissions of the unit subcategory. We applied this assumption
widely for PCBs, PAHs, HF, and HCN because most of the emissions data
represented a limited number of HWCs in the January 2024 emissions
testing request. No emissions data were submitted in response to the
January 2024 emissions testing request for lightweight aggregate kilns;
instead, we combined the maximum concentration of each pollutant
measured in the January 2024 emissions testing request and the average
stack gas flow rate for lightweight aggregate kilns to estimate
conservative emissions of these HAP from lightweight aggregate kilns.
Fourth, for units with HAP emissions estimated by the HWC subcategory
average emission, we substituted the estimated allowable emissions for
the HWC subcategory average emission if the HWC subcategory average
emission exceeded the estimated allowable emissions (calculated as
described in section III.C.2. of this preamble). This affected HWCs
with a known flow rate but no known concentration, and it allowed us to
better estimate actual emissions from HWCs with smaller-than-average
flow rates since the average emissions from the HWC subcategory are
based on units with higher flow rates.

[[Page 50825]]

Additional information on the development of the modeling file for
the HWC NESHAP source category, including the development of the actual
emissions and emissions release characteristics, can be found in the
docket for this proposed rule (Docket ID No. EPA-HQ-OAR-2004-0022).
2. How did we estimate MACT-allowable emissions?
The available emissions data in the RTR emissions dataset include
estimates of the mass of HAP emitted during a specified annual time
period. These ``actual'' emission levels are often lower than the
emission levels allowed under the requirements of the current MACT
standards. The emissions allowed under the MACT standards are referred
to as the ``MACT-allowable'' emissions. We discussed the consideration
of both MACT-allowable and actual emissions in the final Coke Oven
Batteries RTR (70 FR 19992, 19998-99, Apr. 15, 2005) and in the
proposed and final Hazardous Organic NESHAP RTR (71 FR 34421, 34428,
June 14, 2006; 71 FR 76603, 76609, Dec. 21, 2006). In those actions, we
noted that assessing the risk at the MACT-allowable level is inherently
reasonable since that risk reflects the maximum level facilities could
emit and still comply with NESHAP. We also explained that it is
reasonable to consider actual emissions, where such data are available,
in both steps of the risk analysis, in accordance with the Benzene
NESHAP approach (54 FR 38044).
The current HWC NESHAP specifies numeric emission limits for PCDD/
PCDF, HAP metals (directly, as groups, or through a surrogate), HCl,
and chlorine gas for existing and new HWCs. These limits were used as
the basis for calculating the MACT-allowable emissions. CPT stack gas
flow rates and thermal hazardous waste feed rates were identified for
each HWC, where possible. For HWCs without stack gas flow rate or
thermal feed rate data, average rates were calculated for each type of
HWC and used to fill gaps. These rates were combined with the emission
limits and the assumption of 8,760 hours of operation per year (i.e.,
24 hours per day, 365 days per year) to produce the upper bound of
MACT-allowable emissions. In the case where a standard has two formats
(i.e., both a mass and thermal concentration basis), we took the
greater of the two as the MACT-allowable emission. We then speciated
the MACT-allowable emissions following the same procedures as the
actual emissions, except that we speciated As and Be MACT-allowable
emissions for liquid fuel boilers were speciated from PM. For PAHs,
PCBs, HCN, and HF, we estimated the unit type allowable emissions from
the average actual emissions for each unit type.
For the HWC source category, actual emissions tend to be lower than
allowable emissions, in some cases much lower. This shows that HWCs are
generally performing better than they are required to by the HWC
NESHAP. We generally expect that actual emissions will be lower than
allowable emissions because HWCs must demonstrate that their emissions
are consistently below the emission limits, which practically means
that they operate in such a manner as to be far enough below the
emission limit that slight variations in the combustor operation would
not cause them to exceed the emission limit. We use the full value of
the emission limit to calculate allowable emissions. Another
contributing factor in some cases could be that additional non-HWC
NESHAP emission limits are established for HWCs under RCRA. Most HWCs
have completed site-specific risk assessments using RCRA methodology
and under specific provisions of RCRA. Some HWCs may have additional
emission restrictions under RCRA based on those results. The
methodologies in the RCRA site-specific risk assessments and this CAA
residual risk review are not comparable, and one should not be used in
lieu of the other. Additional information on the development of the
modeling file for the HWC NESHAP source category, including the
estimation of MACT-allowable emissions, can be found in the docket for
this proposed rule (Docket ID No. EPA-HQ-OAR-2004-0022).
3. How do we conduct dispersion modeling, determine inhalation
exposures, and estimate individual and population inhalation risk?
Both long- and short-term inhalation exposure concentrations and
health risk from the source category addressed in this proposal were
estimated using the Human Exposure Model (HEM).\48\ The HEM performs
three primary risk assessment activities: (1) conducting dispersion
modeling to estimate the concentrations of HAP in ambient air; (2)
estimating long- and short-term inhalation exposures to individuals
residing within 50 kilometers (km) of the modeled sources; and (3)
estimating individual and population-level inhalation risk using the
exposure estimates and quantitative dose-response information.
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\48\ For more information about HEM, go to <a href="https://www.epa.gov/fera/risk-assessment-and-modeling-human-exposure-model-hem">https://www.epa.gov/fera/risk-assessment-and-modeling-human-exposure-model-hem</a>.
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a. Dispersion Modeling
The air dispersion model AERMOD (American Meteorological Society/
EPA Regulatory Model dispersion modeling system), used by the HEM
model, is one of the EPA's preferred models for assessing air pollutant
concentrations from industrial facilities.\49\ To perform the
dispersion modeling and to develop the preliminary risk estimates, HEM
draws on three data libraries. The first is a library of meteorological
data, which is used for dispersion calculations. This library includes
one year (2019) of hourly surface and upper air observations from over
800 meteorological stations, selected to provide coverage of the United
States and Puerto Rico. A second library of United States Census Bureau
census block \50\ internal point locations and populations provides the
basis of human exposure calculations (U.S. Census, 2020). In addition,
for each census block, the census library includes the elevation and
controlling hill height, which are also used in dispersion
calculations. A third library of pollutant-specific dose-response
values is used to estimate health risk. These are discussed below.
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\49\ U.S. EPA. Revision to the Guideline on Air Quality Models:
Adoption of a Preferred General Purpose (Flat and Complex Terrain)
Dispersion Model and Other Revisions (70 FR 68218, Nov. 9, 2005).
\50\ A census block is the smallest geographic area for which
census statistics are tabulated.
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b. Risk From Chronic Exposure to HAP
In developing the risk assessment for chronic exposures, we use the
estimated annual average ambient air concentrations of each HAP emitted
by each source in the source category. The HAP air concentrations at
each nearby census block centroid located within 50 km of the facility
are a surrogate for the chronic inhalation exposure concentration for
all the people who reside in that census block. A distance of 50 km is
consistent with both the analysis supporting the 1989 Benzene NESHAP
(54 FR 38044) and the limitations of Gaussian dispersion models,
including AERMOD.
For each facility, we calculate the MIR as the cancer risk
associated with health protective assumptions, such as a continuous
lifetime (24 hours per day, seven days per week, 52 weeks per year, 70
years) exposure to the maximum annual average concentration at the
centroid of each inhabited census block. This is meant to provide an
upper bound estimate of cancer risks as people

[[Page 50826]]

are unlikely to be in the same location for 70 years. We calculate
individual cancer risk by multiplying the estimated lifetime exposure
to the ambient concentration of each HAP (in micrograms per cubic meter
([mu]g/m\3\)) by its unit risk estimate (URE). The URE is an upper-
bound estimate of an individual's incremental risk of contracting
cancer over a lifetime of exposure to a concentration of 1 [mu]g/m\3\
of air. For residual risk assessments, we currently use UREs from the
EPA's Integrated Risk Information System (IRIS) when they are
available. For carcinogenic pollutants without IRIS values, we look to
other reputable sources of cancer dose-response values, often using
California EPA (CalEPA) UREs, where available. In cases where new,
scientifically credible dose-response values have been developed in a
manner consistent with EPA guidelines and have undergone a peer review
process similar to that used by the EPA, we may use such dose-response
values in place of, or in addition to, other values, if appropriate.
The pollutant-specific dose-response values used to estimate health
risk are available at <a href="https://www.epa.gov/fera/dose-response-assessment-assessing-health-risks-associated-exposure-hazardous-air-pollutants">https://www.epa.gov/fera/dose-response-assessment-assessing-health-risks-associated-exposure-hazardous-air-pollutants</a>.
To estimate individual lifetime cancer risks associated with
exposure to HAP emissions from each facility in the source category, we
sum the risks for each of the carcinogenic HAP \51\ emitted by the
modeled facility. We estimate cancer risk at every census block within
50 km of every facility in the source category. The MIR is the highest
individual lifetime cancer risk estimated for any of those census
blocks. In addition to calculating the MIR, we estimate the
distribution of individual cancer risks for the source category by
summing the number of individuals within 50 km of the sources whose
estimated risk falls within a specified risk range. We also estimate
annual cancer incidence by multiplying the estimated lifetime cancer
risk at each census block by the number of people residing in that
block, summing results for all of the census blocks, and then dividing
this result by a 70-year lifetime.
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\51\ The EPA's 2005 Guidelines for Carcinogen Risk Assessment
identifies five recommended standard hazard descriptors:
``Carcinogenic to Humans,'' ``Likely to Be Carcinogenic to Humans,''
``Suggestive Evidence of Carcinogenic Potential,'' ``Inadequate
Information to Assess Carcinogenic Potential,'' and ``Not Likely to
Be Carcinogenic to Humans.'' The first three are treated as
carcinogenic and coincide with the terms ``known carcinogen,
probable carcinogen, and possible carcinogen,'' respectively, which
are the terms advocated in the EPA's Guidelines for Carcinogen Risk
Assessment, published in 1986 (51 FR 33992, Sept. 24, 1986). In
August 2000, the EPA published a document entitled Supplemental
Guidance for Conducting Health Risk Assessment of Chemical Mixtures
(EPA/630/R-00/002) as a supplement to the 1986 document. Copies of
both documents can be obtained from <a href="https://cfpub.epa.gov/ncea/risk/recordisplay.cfm?deid=20533&CFID=70315376&CFTOKEN=71597944">https://cfpub.epa.gov/ncea/risk/recordisplay.cfm?deid=20533&CFID=70315376&CFTOKEN=71597944</a>. Summing
the risk of these individual compounds to obtain the cumulative
cancer risk is an approach that was recommended by the EPA's SAB in
their 2002 peer review of the EPA's National Air Toxics Assessment
(NATA) titled NATA--Evaluating the National-scale Air Toxics
Assessment 1996 Data--an SAB Advisory, available at <a href="https://archive.epa.gov/airtoxics/nata/web/html/sabrev.html">https://archive.epa.gov/airtoxics/nata/web/html/sabrev.html</a>.
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To assess the risk of noncancer health effects from chronic
exposure to HAP, we calculate either an HQ or a target organ-specific
hazard index (TOSHI). We calculate an HQ when a single noncancer HAP is
emitted. Where more than one noncancer HAP is emitted, we sum the HQ
for each of the HAP that affects a common target organ or target organ
system to obtain a TOSHI. The HQ is the estimated exposure divided by
the chronic noncancer dose-response value, which is a value selected
from one of several sources. The preferred chronic noncancer dose-
response value is the EPA RfC, defined as ``an estimate (with
uncertainty spanning perhaps an order of magnitude) of a continuous
inhalation exposure to the human population (including sensitive
subgroups) that is likely to be without an appreciable risk of
deleterious effects during a lifetime.'' \52\ In cases where an RfC
from the EPA's IRIS is not available or where the EPA determines that
using a value other than the RfC is appropriate, the chronic noncancer
dose-response value can be a value from the following prioritized
sources, which define their dose-response values similarly to the EPA:
(1) the Agency for Toxic Substances and Disease Registry (ATSDR)
Minimum Risk Level (<a href="https://www.atsdr.cdc.gov/minimal-risk-levels/about/index.html">https://www.atsdr.cdc.gov/minimal-risk-levels/about/index.html</a>); (2) the CalEPA Chronic Reference Exposure Level
(REL) (<a href="https://oehha.ca.gov/air/general-info/oehha-acute-8-hour-and-chronic-reference-exposure-level-rel-summary">https://oehha.ca.gov/air/general-info/oehha-acute-8-hour-and-chronic-reference-exposure-level-rel-summary</a>); or (3) as noted above, a
scientifically credible dose-response value that has been developed in
a manner consistent with EPA guidelines and has undergone a peer review
process similar to that used by the EPA. The pollutant-specific dose-
response values used to estimate health risks are available at <a href="https://www.epa.gov/fera/dose-response-assessment-assessing-health-risks-associated-exposure-hazardous-air-pollutants">https://www.epa.gov/fera/dose-response-assessment-assessing-health-risks-associated-exposure-hazardous-air-pollutants</a>.
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\52\ U.S. Environmental Protection Agency. (Last updated May 2,
2025). IRIS Glossary: <a href="https://www.epa.gov/iris/iris-glossary">https://www.epa.gov/iris/iris-glossary</a>.
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c. Risk From Acute Exposure to HAP That May Cause Health Effects Other
Than Cancer
For each HAP for which appropriate acute inhalation dose-response
values are available, the EPA also assesses the potential health risks
due to acute exposure. For these assessments, the EPA makes health
protective assumptions about emission rates, meteorology, and exposure
location. As part of our efforts to continually improve our
methodologies to evaluate the risks that HAP emitted from categories of
industrial sources pose to human health and the environment,\53\ we
revised our treatment of meteorological data to use reasonable worst-
case air dispersion conditions in our acute risk screening assessments
instead of worst-case air dispersion conditions. This revised treatment
of meteorological data and the supporting rationale are described in
more detail in Residual Risk Assessment for the Hazardous Waste
Combustor Source Category in Support of the 2025 Risk and Technology
Review Proposed Rule and in appendix 5 of the report: Technical Support
Document for Acute Risk Screening Assessment, which are available in
the docket for this proposed rule. This revised approach has been used
in this proposed rule and in all other RTR rulemakings proposed on or
after June 3, 2019.\54\
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\53\ See, e.g., U.S. EPA. Screening Methodologies to Support
Risk and Technology Reviews (RTR): A Case Study Analysis (Report,
Sept. 2018). <a href="https://cfpub.epa.gov/si/si_public_record_report.cfm?Lab=OAQPS&dirEntryID=307074">https://cfpub.epa.gov/si/si_public_record_report.cfm?Lab=OAQPS&dirEntryID=307074</a>.
\54\ See for example, 85 FR 40740, (July 7, 2020) (Organic
Liquids Distribution RTR); 85 FR 40386, (July 6, 2020) (Ethylene
Production RTR); 85 FR 15608, (March 18, 2020) (Solvent Extraction
for Vegetable Oil Production RTR).
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To assess the potential acute risk to the maximally exposed
individual, we use the peak hourly emission rate for each emission
point,\55\ reasonable worst-case air dispersion conditions (i.e., 99th
percentile), and the point of highest off-site exposure. Specifically,
we assume that peak emissions from the source category and reasonable
worst-

[[Page 50827]]

case air dispersion conditions co-occur and that a person is present at
the point of maximum exposure.
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\55\ In the absence of hourly emission data, we develop
estimates of maximum hourly emission rates by multiplying the
average actual annual emission rates by a factor (either a category-
specific factor or a default factor of 10) to account for
variability. We used the default factor of 10 for this risk
assessment because we did not have hourly emissions data. This is
documented in Residual Risk Assessment for the Hazardous Waste
Combustor Source Category in Support of the 2025 Risk and Technology
Review Proposed Rule and in appendix 5 of the report: Technical
Support Document for Acute Risk Screening Assessment. Both are
available in the docket for this rulemaking.
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To characterize the potential health risks associated with
estimated acute inhalation exposures to a HAP, we generally use
multiple acute dose-response values, including acute RELs, acute
exposure guideline levels (AEGLs), and emergency response planning
guidelines (ERPG) for 1-hour exposure durations, if available, to
calculate acute HQs. The acute HQ is calculated by dividing the
estimated acute exposure concentration by the acute dose-response
value. For each HAP for which acute dose-response values are available,
the EPA calculates acute HQs.
An acute REL is defined as ``the concentration level at or below
which no adverse health effects are anticipated for a specified
exposure duration.'' \56\ Acute RELs are based on the most sensitive,
relevant, adverse health effect reported in the peer-reviewed medical
and toxicological literature. They are designed to protect the most
sensitive individuals in the population through the inclusion of
margins of safety. Because margins of safety are incorporated to
address data gaps and uncertainties, exceeding the REL does not
automatically indicate an adverse health impact. AEGLs represent
threshold exposure limits for the general public and are applicable to
emergency exposures ranging from 10 minutes to eight hours.\57\ They
are guideline levels for ``once-in-a-lifetime, short-term exposures to
airborne concentrations of acutely toxic, high-priority chemicals.''
\58\ The AEGL-1 is specifically defined as ``the airborne concentration
(expressed as ppm (parts per million) or mg/m\3\ (milligrams per cubic
meter)) of a substance above which it is predicted that the general
population, including susceptible individuals, could experience notable
discomfort, irritation, or certain asymptomatic nonsensory effects.
However, the effects are not disabling and are transient and reversible
upon cessation of exposure.'' The document also notes that ``Airborne
concentrations below AEGL-1 represent exposure levels that can produce
mild and progressively increasing but transient and nondisabling odor,
taste, and sensory irritation or certain asymptomatic, nonsensory
effects.'' \59\ AEGL-2 are defined as ``the airborne concentration
(expressed as parts per million or milligrams per cubic meter) of a
substance above which it is predicted that the general population,
including susceptible individuals, could experience irreversible or
other serious, long-lasting adverse health effects or an impaired
ability to escape.'' \60\
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\56\ CalEPA issues acute RELs as part of its Air Toxics Hot
Spots Program, and the 1-hour and 8-hour values are documented in
Air Toxics Hot Spots Program Risk Assessment Guidelines, Part I, The
Determination of Acute Reference Exposure Levels for Airborne
Toxicants, which is available at <a href="https://oehha.ca.gov/air/general-info/oehha-acute-8-hour-and-chronic-reference-exposure-level-rel-summary">https://oehha.ca.gov/air/general-info/oehha-acute-8-hour-and-chronic-reference-exposure-level-rel-summary</a>.
\57\ National Academy of Sciences, 2001. Standing Operating
Procedures for Developing Acute Exposure Levels for Hazardous
Chemicals, at 2. Available at <a href="https://www.epa.gov/sites/production/files/2015-09/documents/sop_final_standing_operating_procedures_2001.pdf">https://www.epa.gov/sites/production/files/2015-09/documents/sop_final_standing_operating_procedures_2001.pdf</a>. Note that the
National Advisory Committee for Acute Exposure Guideline Levels for
Hazardous Substances ended in October 2011, but the AEGL program
continues to operate at the EPA and works with the National
Academies to publish final AEGLs (<a href="https://www.epa.gov/aegl">https://www.epa.gov/aegl</a>).
\58\ Id. at 21.
\59\ Id.
\60\ Id.
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ERPGs are developed by the American Industrial Hygiene Association
(AIHA) for emergency planning and are intended to be health-based
guideline concentrations for single exposures to chemicals. The ERPG-1
is the maximum airborne concentration established by AIHA below which
it is believed that nearly all individuals could be exposed for up to
one hour without experiencing other than mild transient adverse health
effects or without perceiving a clearly defined, objectionable odor.
Similarly, the ERPG-2 is the maximum airborne concentration established
by AIHA below which it is believed that nearly all individuals could be
exposed for up to one hour without experiencing or developing
irreversible or other serious health effects or symptoms which could
impair an individual's ability to take protective action.
An acute REL for 1-hour exposure durations is typically lower than
its corresponding AEGL-1 and ERPG-1. Even though their definitions are
slightly different, AEGL-1s are often the same as the corresponding
ERPG-1s, and AEGL-2s are often equal to ERPG-2s. The maximum HQs from
our acute inhalation screening risk assessment typically result when we
use the acute REL for a HAP. In cases where the maximum acute HQ
exceeds 1, we also report the HQ based on the next highest acute dose-
response value (usually the AEGL-1 and/or the ERPG-1). In our acute
inhalation screening risk assessment, acute impacts are deemed
negligible for HAP for which acute HQs are less than or equal to 1, and
no further analysis is performed for these HAP. In cases where an acute
HQ from the screening step is greater than 1, we assess the site-
specific data to ensure that the acute HQ is at an off-site location.
For this source category, the data refinements employed consisted of
reviewing satellite imagery of the locations of the maximum acute HQ
values to determine if the maximum was off facility property. For any
maximum value that was determined to be on facility property, the next
highest value that was off facility property was used. These
refinements are discussed more fully in the Residual Risk Assessment
for the Hazardous Waste Combustor Source Category in Support of the
2025 Risk and Technology Review Proposed Rule, which is available in
the docket for this proposed rule.
4. How do we conduct the multipathway exposure and risk screening
assessment?
The EPA conducts a tiered screening assessment examining the
potential for significant human health risks due to exposures via
routes other than inhalation (i.e., ingestion). We first determine
whether any sources in the source category emit any HAP known to be
persistent and bioaccumulative in the environment, as identified in the
EPA's Air Toxics Risk Assessment Library.\61\
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\61\ See volume 1, appendix D, at <a href="https://www.epa.gov/fera/risk-assessment-and-modeling-air-toxics-risk-assessment-reference-library">https://www.epa.gov/fera/risk-assessment-and-modeling-air-toxics-risk-assessment-reference-library</a>.
---------------------------------------------------------------------------

For the HWC source category, we identified PB-HAP emissions of As
compounds, Cd compounds, PCDD/PCDF, Pb, polycyclic organic matter
(POM), and Hg, so we proceeded to the next step of the evaluation.
Except for Pb, the human health risk screening assessment for PB-HAP
consists of three progressive tiers. In a Tier 1 screening assessment,
we determine whether the magnitude of the facility-specific emissions
of PB-HAP warrants further evaluation to characterize human health risk
through ingestion exposure. To facilitate this step, we evaluate
emissions against previously developed screening threshold emission
rates for several PB-HAP that are based on a hypothetical upper-end
screening exposure scenario developed for use in conjunction with the
EPA's Total Risk Integrated Methodology.Fate, Transport, and Ecological
Exposure (TRIM.FaTE) model. The PB-HAP with screening threshold
emission rates are As compounds, Cd compounds, PCDD/PCDF, Hg compounds,
and POM. Based on the EPA estimates of toxicity and bioaccumulation
potential, these pollutants represent a conservative list for inclusion
in multipathway risk assessments for RTR rules.\62\ In this assessment,
we compare the facility-

[[Page 50828]]

specific emission rates of these PB-HAP to the screening threshold
emission rates for each PB-HAP to assess the potential for significant
human health risks via the ingestion pathway. We call this application
of the TRIM.FaTE model the Tier 1 screening assessment. The ratio of a
facility's actual emission rate to the Tier 1 screening threshold
emission rate is a ``screening value.''
---------------------------------------------------------------------------

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

We derive the Tier 1 screening threshold emission rates for these
PB-HAP (other than Pb compounds) to correspond to a maximum excess
lifetime cancer risk of 1-in-1 million (i.e., for As compounds, PCDD/
PCDF, and POM) or, for HAP that cause noncancer health effects (i.e.,
Cd compounds and Hg compounds), a maximum HQ of one. If the emission
rate of any one PB-HAP or combination of carcinogenic PB-HAP in the
Tier 1 screening assessment exceeds the Tier 1 screening threshold
emission rate for any facility (i.e., the screening value is greater
than 1), we conduct a second screening assessment, which we call the
Tier 2 screening assessment. The Tier 2 screening assessment separates
the Tier 1 combined fisher and farmer exposure scenario into fisher,
farmer, and gardener scenarios that retain upper-bound ingestion rates.
In the Tier 2 screening assessment, the location of each facility
that exceeds a Tier 1 screening threshold emission rate is used to
refine the assumptions associated with the Tier 1 fisher and farmer
exposure scenarios at that facility. A key assumption in the Tier 1
screening assessment is that a lake and/or farm is located near the
facility. As part of the Tier 2 screening assessment, we use a U.S.
Geological Survey (USGS) database to identify actual waterbodies within
50 km of each facility and assume the fisher only consumes fish from
lakes within that 50 km zone. We also examine the differences between
local meteorology near the facility and the meteorology used in the
Tier 1 screening assessment. We then adjust the previously developed
Tier 1 screening threshold emission rates for each PB-HAP for each
facility based on an understanding of how exposure concentrations
estimated for the screening scenario change with the use of local
meteorology and the USGS lakes database.
In the Tier 2 farmer scenario, we maintain an assumption that the
farm is located within 0.5 km of the facility and that the farmer
consumes meat, eggs, dairy, vegetables, and fruit produced near the
facility. We may further refine the Tier 2 screening analysis by
assessing a gardener scenario to characterize a range of exposures,
with the gardener scenario being more plausible in RTR evaluations.
Under the gardener scenario, we assume the gardener consumes home-
produced eggs, vegetables, and fruit products at the same ingestion
rate as the farmer. The Tier 2 screen continues to rely on the high-end
food intake assumptions that were applied in Tier 1 for local fish
(adult female angler at 99th percentile fish consumption) \63\ and
locally grown or raised foods (90th percentile consumption of locally
grown or raised foods for the farmer and gardener scenarios).\64\ If
PB-HAP emission rates do not result in a Tier 2 screening value greater
than 1, we consider those PB-HAP emissions to pose risks below a level
of concern. If the PB-HAP emission rates for a facility exceed the Tier
2 screening threshold emission rates, we may conduct a Tier 3 screening
assessment.
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\63\ Burger, J. (2002). Daily consumption of wild fish and game:
Exposures of high end recreationists. International Journal of
Environmental Health Research, 12, 343-354: <a href="https://doi.org/10.1080/0960312021000056393">https://doi.org/10.1080/0960312021000056393</a>.
\64\ U.S. Environmental Protection Agency. (Last updated Mar.
21, 2022). Exposure Factors Handbook 2011 Edition (Final Report):
<a href="https://iris.epa.gov/document/&deid=236252">https://iris.epa.gov/document/&deid=236252</a>.
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There are several analyses that can be included in a Tier 3
screening assessment, depending upon the extent of refinement
warranted, including validating that the lakes are fishable, locating
residential/garden locations for urban and/or rural settings,
considering plume-rise to estimate emissions lost above the mixing
layer, and considering hourly effects of meteorology and plume-rise on
chemical fate and transport (a time-series analysis). If necessary, the
EPA may further refine the screening assessment through a site-specific
assessment.
In evaluating the potential multipathway risk from emissions of Pb
compounds, rather than developing a screening threshold emission rate,
it is our longstanding practice to compare maximum estimated chronic
inhalation exposure concentrations to the level of the current National
Ambient Air Quality Standard (NAAQS) for Pb.\65\ Values below the level
of the primary (health-based) Pb NAAQS are considered to have a low
potential for multipathway risk.
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\65\ In doing so, the EPA notes that the legal standard for a
primary NAAQS--that a standard is requisite to protect public health
and provide an adequate margin of safety (CAA section 109(b))--
differs from the CAA section 112(f) standard (requiring, among other
things, that the standard provide an ``ample margin of safety to
protect public health''). However, the primary lead NAAQS is a
reasonable measure of determining risk acceptability (i.e., the
first step of the Benzene NESHAP analysis) since it is designed to
protect the most susceptible group in the human population--
children, including children living near major lead emitting
sources. 73 FR 67002, (Oct. 18, 2006); 73 FR 67000; 73 FR 67005. In
addition, applying the level of the primary lead NAAQS at the risk
acceptability step is conservative, since that primary lead NAAQS
reflects an adequate margin of safety.
---------------------------------------------------------------------------

For further information on the multipathway assessment approach,
see the 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 proposed rule.
5. How do we assess risks considering emissions control options?
In addition to assessing baseline inhalation risks and screening
for potential multipathway risks, we also estimate risks considering
the potential emission reductions that would be achieved by the control
options under consideration. In these cases, the expected emission
reductions are applied to the specific HAP and emission points in the
RTR emissions dataset to develop corresponding estimates of risk and
incremental risk reductions.
6. How do we conduct the environmental risk screening assessment?
a. Adverse Environmental Effect, Environmental HAP, and Ecological
Benchmarks
The EPA conducts a screening assessment to examine the potential
for an adverse environmental effect as required under CAA section
112(f)(2)(A). This section authorizes the Agency to adopt more
stringent standards than MACT standards, if necessary, ``to prevent,
taking into consideration costs, energy, safety, and other relevant
factors, an adverse environmental effect.'' CAA section 112(a)(7)
defines ``adverse environmental effect'' as ``any significant and
widespread adverse effect, which may reasonably be anticipated, to
wildlife, aquatic life, or other natural resources, including adverse
impacts on populations of endangered or threatened species or
significant degradation of environmental quality over broad areas.''
In conducting the screening assessment during the risk review,
under CAA section 112(f)(2)(A), it is the EPA's long-standing practice
to focus on eight HAP, which are referred to as ``environmental HAP'':
six PB-HAP and two acid gases. The PB-HAP included in the screening
assessment are As

[[Page 50829]]

compounds, Cd compounds, PCDD/PCDF, POM, Hg (both inorganic and
methylmercury), and Pb compounds. The acid gases included in the
screening assessment are HCl and HF.
HAP that persist and bioaccumulate are of particular environmental
concern because they accumulate in the soil, sediment, and water. The
acid gases, HCl and HF, are included due to their well-documented
potential to cause direct damage to terrestrial plants. In the
environmental risk screening assessment, we evaluate the following four
exposure media: terrestrial soils, surface water bodies (includes
water-column and benthic sediments), fish consumed by wildlife, and
air. Within these four exposure media, we evaluate nine ecological
assessment endpoints, which are defined by the ecological entity and
its attributes. For PB-HAP other than Pb, both community-level and
population-level endpoints are included. For acid gases, the ecological
assessment evaluated is terrestrial plant communities.
An ecological benchmark represents a concentration of HAP that has
been linked to a particular environmental effect level. For each
environmental HAP, we identified the available ecological benchmarks
for each assessment endpoint. We identified, where possible, ecological
benchmarks at the following effect levels: probable effect levels,
lowest-observed-adverse-effect level (LOAEL), and no-observed-adverse-
effect level (NOAEL). In cases where multiple effect levels were
available for a particular PB-HAP and assessment endpoint, we use all
of the available effect levels to help us determine whether ecological
risks exist and, if so, whether the risks could be considered
significant and widespread.
For further information on how the environmental risk screening
assessment was conducted, including a discussion of the risk metrics
used, how the environmental HAP were identified, and how the ecological
benchmarks were selected, see appendix 9 of the 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 proposed rule.
b. Environmental Risk Screening Methodology
For the environmental risk screening assessment, the EPA first
determined whether any facilities in the HWC source category emitted
any of the environmental HAP. For the HWC source category, we
identified emissions of As compounds, Cd compounds, PCDD/PCDF, Pb, POM,
Hg, HCl, and HF. Because one or more of the environmental HAP
evaluated--As compounds, Cd compounds, PCDD/PCDF, Pb, POM, Hg, HCl, and
HF--are emitted by at least one facility in the source category, we
proceeded to the second step of the evaluation.
c. PB-HAP Methodology
The environmental screening assessment includes six PB-HAP--As
compounds, Cd compounds, PCDD/PCDF, POM, Hg (both inorganic and
methylmercury), and Pb compounds. With the exception of Pb, the
environmental risk screening assessment for PB-HAP consists of three
tiers. The first tier of the environmental risk screening assessment
uses the same health-protective conceptual model that is used for the
Tier 1 human health screening assessment. TRIM.FaTE model simulations
were used to calculate Tier 1 screening threshold emission rates. The
screening threshold emission rates represent the emission rate in tpy
that results in media concentrations at the facility that equal the
relevant ecological benchmark. To assess emissions from each facility
in the category, the reported emission rate for each PB-HAP was
compared to the Tier 1 screening threshold emission rate for that PB-
HAP for each assessment endpoint and effect level. If emissions from a
facility do not exceed the Tier 1 screening threshold emission rate,
the facility ``passes'' the screening assessment and therefore is not
evaluated further under the screening approach. If emissions from a
facility exceed the Tier 1 screening threshold emission rate, we
evaluate the facility further in Tier 2.
In Tier 2 of the environmental screening assessment, the screening
threshold emission rates are adjusted to account for local meteorology
and the actual location of lakes in the vicinity of facilities that did
not pass the Tier 1 screening assessment. For soils, we evaluate the
average soil concentration for all soil parcels within a 7.5-km radius
for each facility and for each PB-HAP. For the water, sediment, and
fish tissue concentrations, the highest value for each facility for
each pollutant is used. If emission concentrations from a facility do
not exceed the Tier 2 screening threshold emission rate, the facility
``passes'' the screening assessment and typically is not evaluated
further. If emissions from a facility exceed the Tier 2 screening
threshold emission rate, we evaluate the facility further in Tier 3.
As in the multipathway human health risk assessment, in Tier 3 of
the environmental screening assessment, we examine the suitability of
the lakes around the facilities to support life and remove those that
are not suitable (e.g., lakes that have been filled in or are
industrial ponds), adjust emissions for plume-rise, and conduct hour-
by-hour time-series assessments. If these Tier 3 adjustments to the
screening threshold emission rates still indicate the potential for an
adverse environmental effect (i.e., the facility emission rate exceeds
the screening threshold emission rate), we may elect to conduct a more
refined assessment using more site-specific information. If, after
additional refinement, the facility emission rate still exceeds the
screening threshold emission rate, the facility may have the potential
to cause an adverse environmental effect.
To evaluate the potential for an adverse environmental effect from
Pb, we compared the average modeled air concentrations (from HEM) of Pb
around each facility in the source category to the level of the
secondary Pb NAAQS. The secondary Pb NAAQS is a reasonable means of
evaluating environmental risk because it is set to provide substantial
protection against adverse welfare effects, which can include ``effects
on soils, water, crops, vegetation, man-made materials, animals,
wildlife, weather, visibility and climate, damage to and deterioration
of property, and hazards to transportation, as well as effects on
economic values and on personal comfort and well-being.'' \66\
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\66\ CAA section 302(h) describes effects on welfare. 42 U.S.C.
7602(h).
---------------------------------------------------------------------------

d. Acid Gas Environmental Risk Methodology
The environmental screening assessment for acid gases evaluates the
potential phytotoxicity and reduced productivity of plants due to
chronic exposure to HF and HCl. The environmental risk screening
methodology for acid gases is a single-tier screening assessment that
compares modeled ambient air concentrations (from AERMOD) to the
ecological benchmarks for each acid gas. To identify a potential
adverse environmental effect (as defined in CAA section 112(a)(7)) from
emissions of HF and HCl, we evaluate the following metrics: the size of
the modeled area around each facility that exceeds the ecological
benchmark for each acid gas, in acres and square kilometers; the
percentage of the modeled area around each facility that exceeds the
ecological benchmark for each acid gas; and the area-weighted average
screening value

[[Page 50830]]

around each facility (calculated by dividing the area-weighted average
concentration over the 50-km modeling domain by the ecological
benchmark for each acid gas). For further information on the
environmental screening assessment approach, see appendix 9 of the
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 proposed rule.
7. How do we conduct facility-wide assessments?
To put the source category risks in context, we typically examine
the risks from the entire facility, where the facility includes all
HAP-emitting operations within a contiguous area and under common
control. In other words, we examine not only the HAP emissions from the
source category emission points of interest, but also emissions of HAP
from all other emission sources at the facility for which we have data.
For the HWC source category, we conducted the facility-wide assessment
using a dataset compiled from the NEI based on emissions year 2022. The
source category records of that NEI dataset were removed, evaluated,
and updated as described in section II.C. of this preamble. Once we
completed the quality assurance review, the dataset was placed back
with the remaining records from the NEI for that facility. The
facility-wide file was then used to analyze risks due to the inhalation
of HAP that are emitted facility-wide for the populations residing
within 50 km of each facility, consistent with the methods used for the
source category analysis described above. For these facility-wide risk
analyses, the modeled source category risks were compared to the
facility-wide risks to determine the portion of the facility-wide risks
that could be attributed to the source category addressed in this
proposal. We also specifically examined the facility that was
associated with the highest estimate of risk and determined the
percentage of that risk attributable to the source category of
interest. The Residual Risk Assessment for the Hazardous Waste
Combustor Source Category in Support of the Risk and Technology Review
2025 Proposed Rule, available through the docket for this proposed
rule, provides the methodology and results of the facility-wide
analyses, including all facility-wide risks and the percentage of
source category contribution to facility-wide risks.
8. How do we consider uncertainties in risk assessment?
Uncertainty and the potential for bias are inherent in all risk
assessments, including those performed for this proposal. Although
uncertainty exists, we believe that our approach, which used health
protective tools and assumptions, ensures that our decisions are health
and environmentally protective. A brief discussion of the uncertainties
in the RTR emissions dataset, dispersion modeling, inhalation exposure
estimates, and dose-response relationships follows. Also included are
those uncertainties specific to our acute screening assessments,
multipathway screening assessments, and environmental risk screening
assessments. A more thorough discussion of these uncertainties is
included in the 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 proposed
rule. If a multipathway site-specific assessment was performed for this
source category, a full discussion of the uncertainties associated with
that assessment can be found in appendix 11 of that document, Site-
Specific Human Health Multipathway Residual Risk Assessment Report.
a. Uncertainties in the RTR Emissions Dataset
Although the development of the RTR emissions dataset involved
quality assurance/quality control processes, the accuracy of emissions
values will vary depending on the source of the data, the degree to
which data are incomplete or missing, the degree to which assumptions
made to complete the datasets are accurate, errors in emission
estimates, and other factors. The emission estimates considered in this
analysis generally are emissions during worst-case scenario performance
tests corrected based on a stack gas flow rate or hazardous waste
thermal concentration feed rate more typical of normal operations.
Results were averaged across multiple years, where available, and
emissions averages across HWC unit subcategories were used when
specific emissions data was not available. The estimates of peak hourly
emission rates for the acute effects screening assessment were based on
an emission adjustment factor applied to the average annual hourly
emission rates, which are intended to account for emission fluctuations
due to normal facility operations.
b. Uncertainties in Dispersion Modeling
We recognize that there is uncertainty in ambient concentration
estimates associated with any model, including AERMOD. In using a model
to estimate ambient pollutant concentrations, the user chooses certain
options to apply. For RTR assessments, we select some model options
that have the potential to overestimate ambient air concentrations
(e.g., not including plume depletion or pollutant transformation). We
select other model options that have the potential to underestimate
ambient impacts (e.g., not including building downwash). Other options
that we select have the potential to either underestimate or
overestimate ambient levels (e.g., meteorology and receptor locations).
On average, considering the directional nature of the uncertainties
commonly present in ambient concentrations estimated by dispersion
models, the approach we apply in the RTR assessments should yield
unbiased estimates of ambient HAP concentrations. We also note that the
selection of meteorology dataset locations could have an impact on the
risk estimates. As we continue to update and expand our library of
meteorological station data used in our risk assessments, we expect to
reduce this variability.
c. Uncertainties in Inhalation Exposure Assessment
Although we make every effort to identify all of the relevant
facilities and emission points, as well as to develop accurate
estimates of the annual emission rates for all relevant HAP, the
uncertainties in our emission inventory are likely the highest-
contributing factors of the uncertainties in the exposure assessment.
Some uncertainties in our exposure assessment include human mobility,
using the centroid of each census block, assuming lifetime exposure,
and assuming only outdoor exposures. For most of these factors, there
is neither an underestimate nor overestimate when looking at the MIR or
the incidence, but the shape of the distribution of risks may be
affected. With respect to outdoor exposures, actual exposures may not
be as high if people spend time indoors, especially for very reactive
pollutants or larger particles. For all factors, we reduce uncertainty
when possible. For example, with respect to census block centroids, we
analyze large blocks using aerial imagery and adjust locations of the
block centroids to better represent the population in the blocks. We
also add additional receptor locations where

[[Page 50831]]

the population of a block is not well-represented by a single location.
d. Uncertainties in Dose-Response Relationships
There are uncertainties inherent in the development of the dose-
response values used in our risk assessments for cancer effects from
chronic exposures and noncancer effects from both chronic and acute
exposures. Some uncertainties are generally expressed quantitatively,
and others are generally expressed qualitatively. We note, as a preface
to this discussion, a point on dose-response uncertainty that is stated
in the EPA's 2005 Guidelines for Carcinogen Risk Assessment; namely
that ``the primary goal of EPA actions is protection of human health;
accordingly, as an Agency policy, risk assessment procedures, including
default options that are used in the absence of scientific data to the
contrary, should be health protective'' (the EPA's 2005 Guidelines for
Carcinogen Risk Assessment, at 1-7). This is the approach followed here
as summarized in the next paragraphs.
Cancer UREs used in our risk assessments are those that have been
developed to generally provide an upper-bound estimate of risk.\67\
That is, they represent a ``plausible upper limit to the true value of
a quantity'' (although this is usually not a true statistical
confidence limit). In some circumstances, the true risk could be as low
as zero; however, in other circumstances the risk could be greater.\68\
Chronic noncancer RfC and reference dose (RfD) values represent chronic
exposure levels that are intended to be health-protective levels. To
derive dose-response values that are intended to be ``without
appreciable risk,'' the methodology relies upon an uncertainty factor
(UF) approach,\69\ which considers uncertainty, variability, and gaps
in the available data. The UFs are applied to derive dose-response
values that are intended to protect against appreciable risk of
deleterious effects.
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\67\ U.S. Environmental Protection Agency. (Last updated Jun.
22, 2022). Integrated Risk Information System (IRIS) Glossary:
<a href="https://sor.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?details=&vocabName=IRIS%20Glossary&filterTerm=unit%20risk&checkedAcronym=false&checkedTerm=false&hasDefinitions=false&filterTerm=unit%20risk&filterMatchCriteria=Contains">https://sor.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?details=&vocabName=IRIS%20Glossary&filterTerm=unit%20risk&checkedAcronym=false&checkedTerm=false&hasDefinitions=false&filterTerm=unit%20risk&filterMatchCriteria=Contains</a>.
\68\ An exception to this is the URE for benzene, which is
considered to cover a range of values, each end of which is
considered to be equally plausible, and which is based on maximum
likelihood estimates.
\69\ See A Review of the Reference Dose and Reference
Concentration Processes, U.S. EPA, December 2002, and Methods for
Derivation of Inhalation Reference Concentrations and Application of
Inhalation Dosimetry, U.S. EPA, 1994.
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Many of the UFs used to account for variability and uncertainty in
the development of acute dose-response values are quite similar to
those developed for chronic durations. Additional adjustments are often
applied to account for uncertainty in extrapolation from observations
at one exposure duration (e.g., four hours) to derive an acute dose-
response value at another exposure duration (e.g., one hour). Not all
acute dose-response values are developed for the same purpose, and care
must be taken when interpreting the results of an acute assessment of
human health effects relative to the dose-response value or values
being exceeded. Where relevant to the estimated exposures, the lack of
acute dose-response values at different levels of severity should be
factored into the risk characterization as potential uncertainties.
Uncertainty also exists in the selection of ecological benchmarks
for the environmental risk screening assessment. We established a
hierarchy of preferred benchmark sources to allow selection of
benchmarks for each environmental HAP at each ecological assessment
endpoint. We searched for benchmarks for three effect levels (i.e., no-
effects level, threshold-effect level, and probable-effect level), but
not all combinations of ecological assessment/environmental HAP had
benchmarks for all three effect levels. Where multiple effect levels
were available for a particular HAP and assessment endpoint, we used
all of the available effect levels to help us determine whether risk
exists and whether the risk could be considered significant and
widespread.
Although we make every effort to identify appropriate human health
effect dose-response values for all pollutants emitted by the sources
in this risk assessment, some HAP emitted by this source category lack
dose-response assessments. Accordingly, these pollutants cannot be
included in the quantitative risk assessment, which could result in
quantitative estimates understating HAP risk. To help to alleviate this
potential underestimate, where we conclude similarity with a HAP for
which a dose-response value is available, we use that value as a
surrogate for the assessment of the HAP for which no value is
available. To the extent use of surrogates indicates appreciable risk,
we may identify a need to increase priority for an IRIS assessment for
that substance. We additionally note that, generally speaking, HAP of
greatest concern due to environmental exposures and hazard are those
for which dose-response assessments have been performed, reducing the
likelihood of understating risk. Further, HAP not included in the
quantitative assessment are assessed qualitatively and considered in
the risk characterization that informs the risk management decisions,
including consideration of HAP reductions achieved by various control
options.
For a group of compounds that are unspeciated (e.g., glycol
ethers), we conservatively use the most protective dose-response value
of an individual compound in that group to estimate risk. Similarly,
for an individual compound in a group (e.g., ethylene glycol diethyl
ether) that does not have a specified dose-response value, we also
apply the most protective dose-response value from the other compounds
in the group to estimate risk.
e. Uncertainties in Acute Inhalation Screening Assessments
In addition to the uncertainties highlighted in section III.C.8. of
this preamble, there are several factors specific to the acute exposure
assessment that the EPA conducts as part of the risk review under CAA
section 112(f). The accuracy of an acute inhalation exposure assessment
depends on the simultaneous occurrence of independent factors that may
vary greatly, such as hourly emission rates, meteorology, and the
presence of a person. In the acute screening assessment that we conduct
under the RTR program, we assume that peak emissions from the source
category and reasonable worst-case air dispersion conditions (i.e.,
99th percentile) co-occur. We then include the additional assumption
that a person is located at this point at the same time. Together,
these assumptions represent a reasonable worst-case actual exposure
scenario. In most cases, it is unlikely that a person would be located
at the point of maximum exposure during the time when peak emissions
and reasonable worst-case air dispersion conditions occur
simultaneously.
f. Uncertainties in the Multipathway and Environmental Risk Screening
Assessments
For each source category, we generally rely on site-specific levels
of PB-HAP or environmental HAP emissions to determine whether a refined
assessment of the impacts from multipathway exposures is necessary or
whether it is necessary to perform an environmental screening
assessment. This determination is based on the

[[Page 50832]]

results of a three-tiered screening assessment that relies on the
outputs from models--TRIM.FaTE and AERMOD--that estimate environmental
pollutant concentrations and human exposures for five PB-HAP (PCDD/
PCDF, POM, Hg, Cd compounds, and As compounds) and two acid gases (HF
and HCl). For Pb, we use AERMOD to determine ambient air
concentrations, which are then compared to the secondary Pb NAAQS. Two
important types of uncertainty associated with the use of these models
in RTR risk assessments and inherent to any assessment that relies on
environmental modeling are model uncertainty and input uncertainty.\70\
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\70\ In the context of this discussion, the term ``uncertainty''
as it pertains to exposure and risk encompasses both variability in
the range of expected inputs and screening results due to existing
spatial, temporal, and other factors, as well as uncertainty in
being able to accurately estimate the true result.
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Model uncertainty concerns whether the model adequately represents
the actual processes (e.g., movement and accumulation) that might occur
in the environment. For example, if the model adequately describes the
movement of a pollutant through the soil. This type of uncertainty is
difficult to quantify. However, based on feedback received from
previous EPA SAB reviews and other reviews, we are confident that the
models used in the screening assessments are appropriate and state-of-
the-art for the multipathway and environmental screening risk
assessments conducted in support of RTRs.
Input uncertainty is concerned with how accurately the models have
been configured and parameterized for the assessment at hand. For Tier
1 of the multipathway and environmental screening assessments, we
configured the models to avoid underestimating exposure and risk. This
was accomplished by selecting upper-end values from nationally
representative datasets for the more influential parameters in the
environmental model, including selection and spatial configuration of
the area of interest, lake location and size, meteorology, surface
water, soil characteristics, and structure of the aquatic food web. We
also assume an ingestion exposure scenario and values for human
exposure factors that represent reasonable maximum exposures.
In Tier 2 of the multipathway and environmental screening
assessments, we refine the model inputs to account for meteorological
patterns in the vicinity of the facility versus using upper-end
national values, and we identify the actual location of lakes near the
facility rather than the default lake location that we apply in Tier 1.
By refining the screening approach in Tier 2 to account for local
geographical and meteorological data, we decrease the likelihood that
concentrations in environmental media are overestimated, thereby
increasing the usefulness of the screening assessment. In Tier 3 of the
screening assessments, we refine the model inputs again to account for
hour-by-hour plume-rise and the height of the mixing layer. We can also
use those hour-by-hour meteorological data in a TRIM.FaTE run using the
screening configuration corresponding to the lake location. These
refinements produce a more accurate estimate of chemical concentrations
in the media of interest, thereby reducing the uncertainty with those
estimates. The assumptions and the associated uncertainties regarding
the selected ingestion exposure scenario are the same for all three
tiers.
For the environmental screening assessment for acid gases, we
employ a single-tiered approach. We use the modeled air concentrations
and compare those with ecological benchmarks.
For all tiers of the multipathway and environmental screening
assessments, our approach to addressing model input uncertainty is
generally cautious. We choose model inputs from the upper-end of the
range of possible values for the influential parameters used in the
models, and we assume that the exposed individual exhibits ingestion
behavior that would lead to a high total exposure. This approach
reduces the likelihood of not identifying high risks for adverse
impacts.
Despite the uncertainties, when individual pollutants or facilities
do not exceed screening threshold emission rates (i.e., ``passes''), we
are confident that the potential for adverse multipathway impacts on
human health is very low. On the other hand, when individual pollutants
or facilities do exceed screening threshold emission rates, it does not
mean that impacts are significant, only that we cannot rule out that
possibility and that a refined assessment for the site might be
necessary to obtain a more accurate risk characterization for the
source category.
The EPA evaluates the following HAP in the multipathway and/or
environmental risk screening assessments, where applicable: As
compounds, Cd compounds, PCDD/PCDF, Pb, Hg (both inorganic and
methylmercury), POM, HCl, and HF. These HAP represent pollutants that
can cause adverse impacts either through direct exposure to HAP in the
air or through exposure to HAP that are deposited from the air onto
soils and surface waters and then through the environment into the food
web. These HAP represent those for which we can conduct a meaningful
multipathway or environmental screening risk assessment. For other HAP
not included in our screening assessments, the model has not been
parameterized such that it can be used for that purpose. In some cases,
depending on the HAP, we may not have appropriate multipathway models
that allow us to predict the concentration of that pollutant. The EPA
acknowledges that other HAP beyond these that we are evaluating may
have the potential to cause adverse effects and, therefore, the EPA may
evaluate other relevant HAP in the future, as modeling science and
resources allow.

IV. Analytical Results and Proposed Decisions

A. What actions are we proposing pursuant to CAA sections 112(d)(2) and
112(d)(3)?

In this proposal, we are proposing actions to address unregulated
HAP pursuant to the D.C. Circuit's decision in LEAN. The D.C. Circuit
has held that the EPA is required to address any previously unregulated
HAP emissions as part of its periodic review of MACT standards under
CAA section 112(d)(6). Based on a review of available information
pursuant to the LEAN decision, we are proposing the following pursuant
to CAA sections 112(d)(2), (d)(3), and (h)(1): \71\
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\71\ See LEAN, 955 F.3d at 1091-99.
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<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> Work practice standard for HF and numeric emission limit
for HCN for major source HWC cement kilns.
<bullet> Work practice standard for HF and numeric emission limits
for HCN for major source liquid fuel boilers.
The results and proposed decisions based on the analyses performed
pursuant to CAA sections 112(d)(2) and (3) are presented below, with
separate discussion for each subcategory and HAP.
Consistent with the EPA's longstanding position, we do not believe
that we are required to regulate emissions of HF or HCN from area
sources in the HWC NESHAP because the EPA did not identify either HF or
HCN as urban HAP pursuant to CAA sections 112(k)(3)(B) and 112(c)(3)
\72\

[[Page 50833]]

and because CAA section 112(c)(6) does not identify either HF or HCN as
a pollutant of specific concern. Neither HF nor HCN is an urban HAP or
a pollutant of specific concern, therefore, we are not proposing any
emission limits for HF or HCN for area sources. All emission standards
discussed here are only for HWCs at facilities that are major sources
of HAP.
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\72\ 64 FR 38706, 38715, (July 19, 1999); see, e.g., Proposed
Gas-Fired Melting Furnaces Located at Wool Fiberglass Manufacturing
Area Sources NESHAP, 78 FR 22370, 22375-76, (Apr. 15, 2013); 80 FR
45280, 45319-20, (July 29, 2015) (finalized as proposed).
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As previously noted, the D.C. Circuit has held that the EPA must
address any previously unregulated HAP known to be emitted from major
sources as part of its periodic review of MACT standards under CAA
section 112(d)(6). LEAN, 955 F.3d at 1091-99. The order issued by the
D.C. District Court addressing our obligations to review and revise the
HWC NESHAP also requires the EPA to establish standards for any
previously unregulated HAP in this rulemaking. Order, Blue Ridge Envtl.
Def. League v. Regan, 22-cv-3134 (APM) (D.D.C. Dec. 12, 2024). During
the technology review, the EPA identified HF and HCN as unregulated HAP
through permit review and emissions testing. We also collected
information on PCBs and PAHs in the emission testing request. Although
we mistakenly identified PCBs as an unregulated HAP to the D.C.
District Court, we more recently conducted a careful analysis of the
HWC NESHAP's rule record, which revealed that the EPA already
promulgated MACT standards for PCBs and PAHs through the combination of
the DRE and CO or THC standards as a surrogate for non-PCDD/PCDF
organic HAP.\73\ Because PCBs are already regulated through surrogacy,
no additional emission standards are required. Therefore, we are not
proposing additional standards regulating PCB emissions.
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\73\ 70 FR 59433 (Oct. 12, 2005); 80 FR 31473 (June 3, 2015).
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To address the missing HF and HCN standards, we are proposing
emission limits for HF and HCN under CAA sections 112(d)(2), (d)(3),
and (h)(2) as described in this section. While the proposed emission
limits for these HAP were calculated under CAA sections 112(d)(3) and
(h)(2), we are soliciting comment on setting the HF and HCN standards
pursuant to CAA section 112(d)(6) rather than setting the HF and HCN
standards exclusively pursuant to CAA section 112(d)(2), (d)(3), and
(h)(2) (C-1). Although the D.C. Circuit held in LEAN that the EPA is
required to address previously unregulated HAP from major sources
during a CAA section 112(d)(6) technology review, it is not entirely
clear how that process functions under the statutory text. The
difference in the approach would be that we would not be constrained to
any minimum stringency level and would, therefore, not conduct a
beyond-the-floor analysis. We would not anticipate any cost or impact
differences associated with setting the HF and HCN limits pursuant to
CAA section 112(d)(6) as compared to CAA sections 112(d)(2) and (3).
The estimated costs would be for testing, recordkeeping, and reporting.
It bears noting that the standards under review were first promulgated
in 2005 and our review found an overall reduction of emissions from
this source category that could likely be attributed to concerted
efforts of sources since promulgation. We are also soliciting comments,
data, and other information regarding the analyses for our proposed
MACT floor standards, the beyond-the-floor options, and our
determinations (C-2).
1. Solid Fuel Boilers
a. Hydrogen Fluoride
The EPA is proposing MACT standards for HF emissions from solid
fuel boilers. As further explained below, the EPA is also soliciting
comment on establishing an HBEL under CAA section 112(d)(4) for HF
emissions from solid fuel boilers.
Under the D.C. Circuit's decision in LEAN, the EPA must set
emission limits for major sources with known unregulated HAP emissions
as part of its periodic review of MACT standards under CAA section
112(d)(6). These standards can take at least three forms: technology-
based standards that reflect the maximum reductions of HAP achievable
(after considering cost, energy requirements, and non-air health and
environmental impacts) and are commonly referred to as MACT standards;
an HBEL for HAP with an established health threshold; or a work
practice standard when another standard is not feasible to prescribe or
enforce. Because the EPA did not have previous HF emissions data, the
EPA collected HF emissions data from one HWC solid fuel boiler in the
January 2024 emissions testing request, and this boiler had detected
emissions of HF in all emissions test runs. Based on that emissions
test data, the EPA considers that a numerical emission standard is
feasible to prescribe and enforce for emissions of HF from solid fuel
boilers.
We are proposing MACT emission limits for HF emissions from solid
fuel boilers. CAA section 112(d)(3)(B) provides that MACT shall not be
less stringent than ``the average emission limitation achieved by the
best performing 5 sources (for which the Administrator has or could
reasonably obtain emissions information) in the category or subcategory
for categories or subcategories with fewer than 30 sources.'' Because
we have HF emissions data for only one of the seven solid fuel boilers,
the proposed MACT floor is based on the HF data for this unit. In
determining the level of the MACT floor, we used the Upper Prediction
Limit (UPL) method to account for variability in solid fuel boiler
performance and calculated the MACT floor at 6.2 parts per million by
volume (ppmv) HF, dry basis and corrected to seven percent oxygen.\74\
Based on available data, the EPA estimates that all solid fuel boilers
would be able to meet the MACT floor limit with no additional controls.
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\74\ MACT Floor and Beyond-the-Floor Analysis for Hazardous
Waste Combustors, which is available in the docket for this proposed
rulemaking (Docket ID No. EPA-HQ-OAR-2004-0022); The UPL
``reflect[s] a reasonable estimate of the emissions achieved in
practice by the best-performing sources.'' U.S. Sugar Corp. v. EPA,
830 F.3d 579, 639 (D.C. Cir. 2016) (alteration in original).
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For new sources, CAA section 112(d)(3) provides that the MACT 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.'' Because we only have HF emissions data from one solid
fuel boiler, the proposed MACT floor limit for new sources is the same
as the MACT floor limit for existing sources: 6.2 ppmv HF, dry basis
and corrected to seven percent oxygen.
When establishing an emission standard pursuant to CAA section 112,
the EPA also determines whether to control emissions ``beyond-the-
floor'' (BTF) after considering the costs, non-air quality health and
environmental impacts, and energy requirements of such more stringent
control.\75\ Further, CAA section 112 does not prescribe a methodology
for the Agency's costs analysis. Therefore, where cost is a
consideration for standard setting under CAA section 112(d)(2), we have
historically used cost-effectiveness (cost/ton-reduced) in supporting
analyses.\76\ The EPA solicits comment

[[Page 50834]]

on whether strategies other than cost per ton of pollutant reduced for
considering cost when evaluating beyond-the-floor standards would be
more appropriate (C-3).
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\75\ 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.'').
\76\ See NRDC v. EPA, 749 F.3d 1055, 1060 (D.C. Cir. 2014)
(upholding the EPA's consideration of cost-effectiveness as a
component of the CAA section 112(d)(2) cost analysis); see also
Ass'n of Battery Recyclers, 716 F.3d at 673-74 (the EPA may rely on
cost-effectiveness in CAA section 112(d)(6) decision-making); Nat'l
Ass'n of Clean Water Agencies v. EPA, 734 F.3d 1115, 1156-57 (D.C.
Cir. 2013) (the EPA may rely on cost-effectiveness in setting BTF
standards under CAA section 129(a)(2)); Husqvarna AB v. EPA, 254
F.3d 195, 200 (D.C. Cir. 2001) (``because section 213 does not
mandate a specific method of cost analysis, we find reasonable the
EPA's choice to consider costs on the per ton of emissions removed
basis'').
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The EPA evaluated whether a BTF emission limit would be appropriate
for HF emissions from solid fuel boilers. One HWC liquid fuel boiler
has a caustic packed bed scrubber (caustic scrubber) with sodium
hydroxide (NaOH) added to the scrubbing liquid which can control HF
emissions. While no HWC solid fuel boilers have a caustic scrubber,
based on substantial similarities in design and operations of both
types of boilers, we expect that a caustic scrubber would be a
technically feasible option for HWC solid fuel boilers. Therefore, we
evaluated whether the incremental emissions reduction achievable with a
caustic scrubber would be cost-effective. A caustic scrubber would also
offer some co-control of HCl and HCN emissions. We estimate that a
caustic scrubber would achieve approximately 95 percent reduction of HF
from the solid fuel boiler. A corresponding 95 percent reduction in the
HF MACT floor would result in a standard that is below three times the
representative detection level (3xRDL) of the method.\77\ The EPA uses
3xRDL as its minimum standard to account for variability in the test
method measurements and ensure that compliance with a standard can be
reliably measured. Therefore, the BTF emission limit would be 0.60 ppmv
HF, dry basis and corrected to seven percent oxygen, which reflects the
3xRDL for HF emissions from solid fuel boilers.
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\77\ See the memorandum Representative Detection Limit (RDL) for
Hydrogen Fluoride for Hazardous Waste Combustion Sources, which is
available in the docket for this proposed rulemaking (Docket ID No.
EPA-HQ-OAR-2004-0022).
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The EPA estimates that all solid fuel boilers would need to install
caustic scrubbers to meet the BTF level. This would result in an
industry-wide 16.35 tpy reduction of HF (i.e., 95 percent reduction of
emissions), at approximately a total capital investment of $14.3
million (2024$) and total annualized costs of $4.46 million (2024$) for
a cost-effectiveness of $273,000 (2024$) per ton of HF reduced. The
installation of a caustic scrubber at a single new source would achieve
a 2.3 tpy reduction of HF, at approximately a total capital investment
of $2.04 million (2024$) and total annualized costs of $637,000 (2024$)
for a cost effectiveness of $272,000 (2024$) per ton of HF reduced. If
other acid gases are present, then the amount of caustic required would
increase from the amount we estimated, and there would be corresponding
annual cost increases. The EPA has previously considered $68,000 per
ton of HF reduced (adjusted to 2024$) to not be cost-effective \78\
and, in keeping with that prior consideration, proposes not to consider
either $273,000 or $272,000 per ton of HF reduced to be cost-effective.
A caustic scrubber would also produce additional wastewater that would
need to be treated onsite or removed from the site for treatment or
disposal. Additional energy is required both to operate the scrubber
and to treat or otherwise dispose of wastewater. After considering both
the MACT floor and BTF options for existing and new sources, the EPA
proposes to conclude that the installation of a caustic scrubber as a
BTF option is not warranted considering the cost, non-air quality
health and environmental impacts, and energy requirements for either
existing or new solid fuel boilers. Therefore, the EPA is proposing the
MACT floor of 6.2 ppmv HF, dry basis and corrected to seven percent
oxygen, for both existing and new solid fuel boilers.
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\78\ Brick and Structural Clay Products Manufacturing NESHAP, 67
FR 47894 (July 22, 2002).
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The EPA is proposing that compliance with the HF emission limits
for solid fuel boilers would be required within three years after the
publication of the final rule and that demonstration through an initial
compliance test would occur no later than six months after the
compliance date. This would be followed by subsequent demonstration of
compliance once every five years during the CPT 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.
The EPA is also soliciting comment on whether an HBEL for HF
emissions from solid fuel boilers should be established (C-4). For HAP
with an established health threshold, CAA section 112(d)(4) allows the
EPA to consider such health thresholds when establishing emission
standards under CAA section 112(d). CAA section 112(d)(4) states,
``[w]ith respect to pollutants for which a health threshold has been
established, the Administrator may consider such threshold level, with
an ample margin of safety, when establishing emission standards under
this subsection.'' \79\ In other words, for HAP with a health
threshold, such as HCl, the EPA may promulgate standards under a
different process from that otherwise specified in CAA sections
112(d)(2) and (3). This kind of standard is commonly referred to as an
HBEL. It also bears noting that the EPA previously established an
alternative HBEL for HCl in the HWC NESHAP that was based on a site-
specific risk assessment or, more conservatively, values based on
general release parameters.\80\ More recently, the EPA solicited
comment on establishing an HBEL for HCl in the supplemental proposal
for the Lime Manufacturing Plants NESHAP.\81\ For solid fuel boilers,
the EPA is soliciting comment on whether an HBEL for HF should be
established (C-4) and, if so, whether that should be a single HBEL,
like the one for HCl in the Lime Manufacturing Plants NESHAP, or an
alternative HBEL based on the existing framework in the HWC NESHAP for
HCl (C-5).
---------------------------------------------------------------------------

\79\ 42 U.S.C. 7412(d)(4). See also U.S. Sugar, 830 F.3d at 624
(``This provision thus allows, but does not require, the EPA to
adopt a standard more lenient than the MACT floor, subject to two
critical restrictions: the Agency must determine (1) that there is
an established health threshold, and (2) that the established
threshold would provide `an ample margin of safety.' '').
\80\ See the 2005 HWC NESHAP final rule (70 FR 59432, Oct. 12,
2005) and its technical support documents for more discussion on the
current alternative health-based emission limit for HCl, which is
available in the docket for this proposed rulemaking (Docket ID No.
EPA-HQ-OAR-2004-0022).
\81\ 89 FR 9088 (Feb. 9, 2024).
---------------------------------------------------------------------------

b. Hydrogen Cyanide
The EPA is proposing MACT standards for HCN emissions from solid
fuel boilers. As further explained below, the EPA is also soliciting
comment on establishing an HBEL under CAA section 112(d)(4) for HCN
emissions from solid fuel boilers (C-4).
The EPA collected HCN emissions data from one HWC solid fuel boiler
in the January 2024 emissions testing request, and this boiler had
detected emissions of HCN in all emissions test runs. The EPA is
proposing MACT emission limits for HCN emissions from solid fuel
boilers. CAA section 112(d)(3)(B) provides that MACT shall not be less
stringent than ``the average emission limitation achieved by the best
performing 5 sources (for which the

[[Page 50835]]

Administrator has or could reasonably obtain emissions information) in
the category or subcategory for categories or subcategories with fewer
than 30 sources.'' Because we have HCN emissions data for only one of
the seven HWC solid fuel boilers, the proposed MACT floor is based on
the HCN emissions data from this one unit. In determining the level of
the MACT floor, the UPL method was used to account for variability in
solid fuel boiler performance, and the MACT floor was calculated at 5.0
ppmv HCN, dry basis and corrected to seven percent oxygen.\82\ Based on
available data, the EPA estimates that all solid fuel boilers would be
able to meet the MACT floor limit with no additional controls. For new
sources, CAA section 112(d)(3) provides that the MACT 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.'' Because we only have HCN emissions data from one solid
fuel boiler, the proposed MACT floor limit for new sources is the same
as the MACT floor limit for existing sources: 5.0 ppmv HCN, dry basis
and corrected to seven percent oxygen.
---------------------------------------------------------------------------

\82\ MACT Floor and Beyond-the-Floor Analysis for Hazardous
Waste Combustors, which is available in the docket for this proposed
rulemaking (Docket ID No. EPA-HQ-OAR-2004-0022).
---------------------------------------------------------------------------

When establishing an emission standard pursuant to CAA section 112,
the EPA must also determine whether to control emissions BTF after
considering the costs, non-air quality health and environmental
impacts, and energy requirements of such more stringent control. The
EPA evaluated whether BTF emission limits would be appropriate for HCN
emissions from solid fuel boilers. No HWC solid fuel boiler has a
control device that the EPA expects to control HCN emissions.
Furthermore, no HWC has a control device specifically designated as
controlling HCN emissions. Good combustion practices like high
combustion temperature, thorough mixing, sufficient residence time,
excess oxygen, and control of flue gas temperature can prevent
emissions of HCN from HWCs by encouraging the oxidation of HCN in the
combustion zone and preventing its formation after the flue gas exits
the combustion chamber. Many HWCs have incorporated secondary
combustion chambers or afterburners that may serve to promote good
combustion and control HCN emissions, especially if they are followed
by a quench. One HWC, a liquid fuel boiler, combusts HCN and uses it as
the primary organic hazardous constituent (POHC) in its DRE
demonstration. This unit demonstrated at least 99.99 percent DRE and
had low HCN emissions. The unit does not have any air pollution control
devices (APCDs) that control HCN emissions and instead relies on good
combustion practices and operational parameters appropriate for
limiting HCN emissions.
Several HWCs have control devices for other HAP that we expect to
co-control HCN emissions, including caustic scrubbers with NaOH added
to the scrubbing liquid. One HWC liquid fuel boiler has a caustic
scrubber and based on substantial similarities in design and operations
of both types of boilers we expect that a caustic scrubber would be a
technically feasible option for HWC solid fuel boilers, so we evaluated
whether the incremental emissions reduction achievable with a caustic
scrubber would be cost-effective. We estimate that a caustic scrubber
would achieve approximately 95 percent reduction of HCN from one solid
fuel boiler. A corresponding 95 percent reduction in the MACT floor
would result in a standard below the 3xRDL value for HCN for solid fuel
boilers (1.1 ppmv).\83\ Therefore, the BTF emission limit would be 1.1
ppmv HCN, dry basis and corrected to seven percent oxygen, which
reflects the 3xRDL for HCN emissions from solid fuel boilers.
---------------------------------------------------------------------------

\83\ See the memorandum Representative Detection Level for
Hydrogen Cyanide for Cement Kilns and Hazardous Waste Combustors,
which is available in the docket for this proposed rulemaking
(Docket ID No. EPA-HQ-OAR-2004-0022).
---------------------------------------------------------------------------

The EPA estimates that all solid fuel boilers would need to install
caustic scrubbers to meet the BTF limit. This would result in an
industry-wide 27.4 tpy reduction of HCN (i.e., 95 percent reduction of
emissions), at a total capital investment of $14.3 million (2024$) and
total annualized costs of $4.46 (2024$) for a cost-effectiveness of
$163,000 (2024$) per ton of HCN reduced. The installation of a caustic
scrubber at a single new source would achieve a 3.9 tpy reduction of
HF, at approximately a total capital investment of $2.04 million
(2024$) and total annualized costs of $637,000 (2024$) for a cost
effectiveness of $162,000 (2024$) per ton of HCN reduced. A caustic
scrubber would also offer some co-control of HCl and HF. If other acid
gases are present, then the amount of caustic required would increase
from the amount estimated, and there will be corresponding annual cost
increases. The EPA has previously considered $15,900 per ton of HCN
reduced (adjusted to 2024$) to not be cost-effective \84\ and, in
keeping with that prior consideration, proposed not to consider either
$163,000 or $162,000 per ton of HCN reduced to be cost-effective. A
caustic scrubber would also produce additional wastewater that would
need to be treated onsite or removed from the site for treatment or
disposal. Additional energy is required both to operate the scrubber
and to treat or otherwise dispose of wastewater. After considering both
the MACT floor and BTF options for existing and new sources, the EPA
proposes to conclude that the installation of a caustic scrubber as a
BTF option is not warranted considering the cost, non-air quality
health and environmental impacts, and energy requirements for either
existing or new solid fuel boilers. Therefore, the EPA is proposing the
MACT floor of 5.0 ppmv HCN, dry basis and corrected to seven percent
oxygen for both existing and new solid fuel boilers.
---------------------------------------------------------------------------

\84\ Petroleum Refinery Sector Risk and Technology Review and
New Source Performance Standards, 79 FR 36880 (June 30, 2014).
---------------------------------------------------------------------------

The EPA is proposing that compliance with the HCN emission limits
for solid fuel boilers would be required within three years after the
publication of the final rule and that demonstration through an initial
compliance test would occur no later than six months after the
compliance date. This would be followed by subsequent demonstration of
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
according to 40 CFR 63.7(f). 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.
The EPA solicits comment on establishing an HBEL under CAA section
112(d)(4) for HCN (C-4). The EPA also solicits comment on whether a
single HBEL under CAA section 112(d)(4) for HCN should be established,
like that discussed in the supplemental proposal for HCl in the Lime
Manufacturing Plants NESHAP (89 FR 9088; Feb. 9, 2024), or whether an
alternative HBEL for HCN based on the framework already in the HWC
NESHAP for HCl (40 CFR 63.1215) would be more appropriate (C-5).

[[Page 50836]]

2. Incinerators
a. Hydrogen Fluoride
The EPA is proposing a work practice standard with multiple
proposed compliance options for HF emissions from HWC incinerators. The
EPA collected HF emissions data from seven HWC incinerators in the
January 2024 emissions testing request. We did not require an eighth
incinerator to test for HF emissions because it had reported in the
August 2023 questionnaire that it did not burn fluorinated waste. CAA
section 112(h)(1) authorizes the Administrator to promulgate ``a
design, equipment, work practice, or operational standard, or
combination thereof'' if, in his judgment, ``it is not feasible to
prescribe or enforce a standard of performance.'' CAA section 112(h)(2)
provides the circumstances under which prescribing or enforcing a
standard of performance is ``not feasible,'' such as when the pollutant
cannot be emitted through a conveyance designed to emit or capture the
pollutant, or when there is no practicable measurement methodology for
the particular class of sources. Further, ``application of measurement
methodology'' is more than just taking a measurement. The measurement
must also have some reasonable relation to what the source is emitting
(i.e., the measurement must yield a meaningful value). 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).\85\ In the case of HWC incinerators, we found that 94 percent
of the HF data was below the detection limit, and so we find it
appropriate to propose a work practice standard.
---------------------------------------------------------------------------

\85\ See the memorandum titled Determination of ``non detect''
from EPA Method 29 (multi-metals) and EPA Method 23 (dioxin/furan)
test data when evaluating the setting of MACT floors versus
establishing work practice standards (Johnson, 2014), which is
available in the docket for this proposed rulemaking (Docket ID No.
EPA-HQ-OAR-2004-0022). (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, 734 F.3d at 1155).
---------------------------------------------------------------------------

The EPA is proposing a work practice standard for HF emissions from
HWC incinerators with multiple compliance options. We are proposing
that a source would only comply with one of the three options. The
options of the proposed work practice standard are as follows:
<bullet> Option 1: If a source actively controls HCl emissions and
the source has at least two AWFCO-interlocked operating parameter
limits other than chlorine feed rate to control HCl, then comply with
the HCl and chlorine gas operating parameter limits and indicate in the
CPT report and notice of compliance that compliance is demonstrated by
complying with the HCl and chlorine gas operating parameter limits.
<bullet> Option 2: If a facility does not feed any material with
detectable levels of fluorine to the source, then certify in the CPT
report that no fluorine is fed and indicate in the CPT report and
notice of compliance 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
operating parameter limits 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 maximum theoretical emissions concentration (MTEC)
procedure), then 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 demonstration that HF MTEC does not
exceed the solid fuel boiler existing source emission limit for HF
would be included in the CPT plan.
Compliance with this work practice standard will minimize emissions
of HF from HWC incinerators. For the Option 1 work practice, all
utilized controls of HCl emissions except chlorine feed rate control
also control HF, as both are acid gases with similar chemistry in
APCDs; these APCDs are equally or more effective at controlling HF than
HCl. Because HCl, and by extension HF, is already controlled, no
further control requirements are necessary. For the Option 2 work
practice, if no fluorine is fed to an HWC, then HF will not be emitted
from the HWC. While most commercial HWCs accept some hazardous waste
containing fluorine, the results of the August 2023 questionnaire
indicate that some captive HWCs do not feed fluorine, and there is no
reason to expect HF emissions. The EPA anticipates that most HWC
incinerators will fall into the Option 1 or Option 2 work practices. By
our estimate, approximately 70 percent of HWC incinerators have an APCD
that controls HCl, and approximately 33 percent of captive incinerators
do not feed fluorine.
Any HWC incinerators that cannot meet the Option 1 or Option 2 work
practices would be required to comply with Option 3 by monitoring the
fluorine fed to the unit and completing an HF emissions test if
significant amounts of fluorine are ever fed. The feed monitoring
requirement is similar to the monitoring requirements of other HAP
precursors (like metals or chlorine). The Option 3 work practice is
designed to operate as a backstop and to provide the EPA with emissions
data to use in a future CAA section 112(d)(6) technology review if HF
emissions are more significant than our current data indicate.
The EPA is soliciting comment on whether this proposed work
practice standard is appropriate for the control of HF emissions and
whether additional work practice options should be added (C-6).
The EPA is proposing that compliance with the HF work practice
standard for incinerators would be required 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. This would be followed
by subsequent demonstration of compliance once every five years during
the CPT. 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 would be required to 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.
b. Hydrogen Cyanide
The EPA is not proposing MACT standards for HCN emissions from HWC
incinerators. The EPA collected HCN emissions data from eight HWC
incinerators in the January 2024 emissions testing request. HCN was not
measured in any test run. Because the EPA emissions data indicates that
HCN is not measurably emitted from HWC incinerators, the EPA is not
proposing any emission standard for HCN from HWC incinerators.
3. Cement Kilns
a. Hydrogen Fluoride
For HF emissions from cement kilns, the EPA is proposing work
practice

[[Page 50837]]

standards with the same multiple compliance options proposed for HF
emissions from HWC incinerators. The EPA collected HF emissions data
from four HWC cement kilns in the January 2024 emissions testing
request. We found that 71 percent of the HF data was below the
detection limit, and so the Administrator finds it appropriate to
propose a work practice standard for emissions of HF from major source
cement kilns under CAA section 112(h)(1).
The EPA is proposing the same multi-option work practice standard
for cement kilns as described in section IV.A. of this preamble for
incinerators. Approximately 15 percent of HWC cement kilns have
integrated APCDs with AWFCO-interlocked operating parameter limits that
control HCl emissions, and these kilns would fall into the Option 1
work practice. To our knowledge, all cement kilns burn at least some
fluorine-containing material, and so we do not expect that any would
fall into the Option 2 work practice. Any cement kilns that cannot meet
the Option 1 or Option 2 work practices would follow the Option 3
monitoring work practice.
While only 15 percent of cement kilns have APCDs that control HCl
emissions, the EPA views the cement production process as offering some
degree of inherent control of HCl (and thus HF). When hot effluent gas
flows out of a cement kiln, it is not immediately directed to the air
pollution control train like it may be for other types of HWC. Instead,
the effluent gas is used to preheat raw materials before they enter the
kiln, which serves as a form of energy recovery. Raw materials that are
fed to a cement kiln contain large amounts of alkaline materials
including calcium carbonate, which is used in dry scrubbing APCDs for
control of acid gases because it reacts readily with HCl and HF. The
effluent gas continues to contact alkaline cement kiln dust throughout
the process until the dust collection APCD, which is often the final
control device for an HWC cement kiln. For ``inherent'' control of HCl
from cement kilns to qualify as an Option 1 work practice, there must
be operating parameter limits related to the inherent control
interlocked with the AWFCO system. The EPA is soliciting comment on
which operating parameter limits (e.g., maximum stack gas flow rate)
may be appropriate parameterization for cement kiln's inherent control
of HCl and thus HF (C-7).
The EPA is proposing that compliance with the HF work practice
standard for HWC cement kilns would be required 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. This would be followed
by subsequent demonstration of compliance once every five years during
the CPT. 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 would be required to 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.
b. Hydrogen Cyanide
The EPA is proposing MACT standards for HCN emissions from HWC
cement kilns. As further explained below, the EPA is also soliciting
comment on whether the HCN standards for cement kilns should be
subcategorized by kiln type and, if so, how (C-8).
The EPA collected HCN emissions data from four HWC cement kilns in
the CAA section 114 emissions testing request, and HCN was detected in
all emissions test runs. The EPA is proposing MACT emission limits for
HCN emissions from cement kilns. CAA section 112(d)(3)(B) provides that
MACT shall not be less stringent than ``the average emission limitation
achieved by the best performing 5 sources (for which the Administrator
has or could reasonably obtain emissions information) in the category
or subcategory for categories or subcategories with fewer than 30
sources.'' Because we have HCN emissions data from only four HWC cement
kilns, the proposed MACT floor is based on the HCN emissions data from
those four units. Many HWC cement kilns have in-line raw mills that
operate approximately 85 percent of the time when the kiln is in
operation. Whether the raw mill is in operation can affect the HAP
emissions profile of the cement kiln. In the January 2024 emissions
testing request, the EPA requested that data be collected both while
the raw mill was on and off if the kiln had an in-line raw mill. When
the raw mill is running, a portion of the kiln exhaust is recycled back
to the raw mill to heat raw materials fed to the kiln, resulting in a
different emission profile at the stack. When the raw mill is not
running, typically for maintenance, the kiln's exhaust is routed
directly to the APCDs and stack. The raw mill off data were used to
develop a correction factor for HCN emissions. Specifically, the
average HCN emission concentration when the raw mill was off was
calculated for each HWC cement kiln with a raw mill. Then, the raw mill
off average was used with the raw mill on data for each test run to
calculate a raw mill-corrected HCN emission concentration as a weighted
mean assuming that the raw mill is on 85 percent of the time and off 15
percent of the time. This allows us to correct for any differences in
emission profile depending on the operational status of the raw mill
while maintaining the variability displayed in the raw mill on test
runs. In determining the level of the MACT floor, the UPL method was
used to account for variability in cement kiln performance, and the
MACT floor was calculated at 56 ppmv HCN, dry basis and corrected to
seven percent oxygen.\86\ Based on available data, the EPA estimates
that all existing cement kilns would be able to meet the MACT floor
limit with no additional controls.
---------------------------------------------------------------------------

\86\ MACT Floor and Beyond-the-Floor Analysis for Hazardous
Waste Combustors, which is available in the docket for this proposed
rulemaking (Docket ID No. EPA-HQ-OAR-2004-0022).
---------------------------------------------------------------------------

For new sources, CAA section 112(d)(3) provides that the MACT 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.'' The cement kiln with the best controlled emissions is
a wet process kiln. The EPA calculated a proposed new source limit from
the unit with the best controlled emissions using the UPL method, and
this limit was calculated at 1.8 ppmv HCN, dry basis and corrected to
seven percent oxygen.\87\
---------------------------------------------------------------------------

\87\ Id.
---------------------------------------------------------------------------

[[Page 50838]]

may be a potential control option for other subcategories of HWC, it is
not a demonstrated control strategy for HWC cement kilns. Caustic
scrubbers remove HCN by reacting it with NaOH to produce sodium cyanide
(NaCN) and water. In some applications, sodium hypochlorite (NaClO) is
also added to the scrubbing solution to form sodium bicarbonate
(NaHCO<INF>3</INF>), sodium chloride (NaCl), and nitrogen
(N<INF>2</INF>), which are often more favored reaction products. As
implied by their name, caustic scrubbers operate at a basic pH.
However, wet scrubbers employed by the cement kiln industry for acid
gas control by necessity operate at an acidic pH to avoid precipitation
and fouling of scrubber components and pumps. The product of these wet
scrubbers is synthetic gypsum, which can be used in the cement
production process. Caustic scrubbers could not replace wet scrubbers
for multiple reasons, including that elevated levels of sodium would
interfere with the cement production process. Instead, caustic
scrubbers would have to be added after the final component of the
cement kiln's current air pollution control system, likely followed by
a demister to prevent interference with stack CEMS. The EPA has no
evidence that this APCD configuration has been demonstrated on any
cement kiln.
---------------------------------------------------------------------------

\88\ See the email from the Cement Kiln Recycling Coalition in
the docket for this proposed rulemaking (Docket ID No. EPA-HQ-OAR-
2004-0022).
---------------------------------------------------------------------------

The EPA considers regenerative thermal oxidizers (RTO) to be a
technically feasible option for control of HCN emissions, but RTO have
an additional energy requirement due to use of natural gas. While no
HWC cement kilns have RTO installed, two Portland cement kilns do. The
EPA has considered, and continues to consider, combustion as a viable
control technology for HCN. HWCs are, by nature, combustors. However,
the data show that HCN is emitted from cement kilns. This is because
gas that exits cement kilns can stay in the post-combustion system at
elevated temperatures for relatively long times. These conditions
create an environment for the potential formation of certain HAP (e.g.,
PCDD/PCDF) after the gas leaves the combustion zone of the kiln but
before it exits to the atmosphere. Therefore, the EPA evaluated whether
the incremental emissions reduction achievable with RTO would be cost-
effective. We estimated 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.\89\ Assuming a 95 percent reduction from the UPL MACT
floor due to RTO, the BTF emission limit for existing sources would be
2.8 ppmv HCN, dry basis and corrected to seven percent oxygen. A
corresponding 95 percent reduction in the new source MACT floor would
result in a standard below the 3xRDL value for HCN for cement kilns
(1.1 ppmv).\90\ Therefore the evaluated beyond-the-floor levels are 2.8
ppmv HCN for existing sources and 1.1 ppmv HCN for new sources, both on
a dry basis and corrected to seven percent oxygen.
---------------------------------------------------------------------------

\89\ See ``Section 114 Facility Responses'' for the Portland
Cement NESHAP (<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>).
Accessed May 19, 2025.
\90\ See the memoran

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