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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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 https://www.regulations.gov/ index. Although listed in the index, some information is

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 https://www.regulations.gov.

Written Comments. Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2004-0022, at https://www.regulations.gov/ (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 https://www.regulations.gov/ 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

tions 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

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).

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

g 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.

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: https://rcrapublic.epa.gov/rcra-hwip/trends-and-analysis/details/4.

18 U.S. Environmental Protection Agency. (Last updated Jul. 10, 2025). Biennial Report Management Methods: https://rcrapublic.epa.gov/rcra-hwip/trends-and-analysis/details/3.

d-analysis/details/3.

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

rvening 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

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)

sing 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

another metal HAP standard. Sources may also choose to regulate non-enumerated metal HAP directly as an alternative to the PM standard

re 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

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

, 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

ibed 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.

and Compliance History Online (ECHO) tool ( https://echo.epa.gov ). 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

position 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

l 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

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

ptability 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.

41 Id. at Sept. 14, 1989.

42 54 FR 38061, Sept. 14, 1989.

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

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

ions. 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.

ow 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.

47 Docket ID No. EPA-HQ-OAR-2004-0022, Document ID No. EPA-HQ-OAR-2004-0022-0433. Available at https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous.

We did not have complete data to support unit-specific estimates for all units

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

eed 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

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.

48 For more information about HEM, go to https://www.epa.gov/fera/risk-assessment-and-modeling-human-exposure-model-hem.

a. Dispersion Modeling

tion.

48 For more information about HEM, go to https://www.epa.gov/fera/risk-assessment-and-modeling-human-exposure-model-hem.

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.

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.

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

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 (μg/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 μg/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

e 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 https://www.epa.gov/fera/dose-response-assessment-assessing-health-risks-associated-exposure-hazardous-air-pollutants.

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

ault 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.

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

ct 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

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 https://oehha.ca.gov/air/general-info/oehha-acute-8-hour-and-chronic-reference-exposure-level-rel-summary.

57 National Academy of Sciences, 2001

oduced 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.

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: https://doi.org/10.1080/0960312021000056393 .

64 U.S. Environmental Protection Agency. (Last updated Mar. 21, 2022). Exposure Factors Handbook 2011 Edition (Final Report): https://iris.epa.gov/document/&deid=236252.

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

nd 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

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

mercury), 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

issue 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

66 CAA section 302(h) describes effects on welfare. 42 U.S.C. 7602(h).

or 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

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.

y 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)

nize 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

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.

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

ccur. 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

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

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.

and other factors, as well as uncertainty in being able to accurately estimate the true result.

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

ividual 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

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.

73 70 FR 59433 (Oct. 12, 2005); 80 FR 31473 (June 3, 2015).

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

e 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

T 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.

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)

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 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

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).

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

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.

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).

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

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.

78 Brick and Structural Clay Products Manufacturing NESHAP, 67 FR 47894 (July 22, 2002).

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

ing 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

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).

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

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)

d 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).

AR-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

A 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).

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)

s 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

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

rol 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

a 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).

ubbing solution to form sodium bicarbonate (NaHCO 3 ), sodium chloride (NaCl), and nitrogen (N 2 ), 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

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 ( https://www.epa.gov/stationary-sources-air-pollution/portland-cement-manufacturing-industry-information-collection ). Accessed May 19, 2025.

90 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 13 of 14 existing HWC cement kilns and all new cement kilns would need to install RTO to meet the beyond-the-floor limits. For existing sources, this would result in a 311 tpy reduction of HCN, at approximately a total capital investment of $122 million (2024$) and total annualized costs of $36.3 million (2024$) for a cost-effectiveness of $130,000 (2024$) per ton of HCN reduced

79 FR 36880 (June 30, 2014).

Additional non-air quality health and environmental impacts and energy requirements of RTO must also be considered. Installation of RTO would increase emissions of criteria air pollutants, such as NO X and CO because RTO requires the combustion of additional natural gas for fuel. It would consume an estimated 15,000-16,000 standard cubic feet of natural gas per hour. Based on the foregoing discussions, the EPA is proposing the MACT floor of 56 ppmv HCN, dry basis and corrected to seven percent oxygen, for existing cement kilns and the MACT floor of 1.8 ppmv HCN, dry basis and corrected to seven percent oxygen, for new cement kilns.

The EPA is proposing that compliance with the HCN emission limits for cement kilns 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 would be required to 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.

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 whether the HCN emission limit cement kilns should be subcategorized by kiln type and, if so, how (C-8). The EPA has HCN emission data for two types of HWC cement kilns: a wet process kiln and three preheater/precalciner kilns. There are other types of HWC cement kilns for which EPA does not have HCN emission data, including modified wet process with a preheater/precalciner and dry process without a preheater/precalciner. Without additional data, if we were to subcategorize in response to this proposal, the EPA could set HCN emission limits for wet process kilns separately from preheater/precalciner and other dry process kilns. Using the UPL method to account for variability when determining the level of the MACT floors, the existing and new source MACT floors for wet process HWC cement kilns would be 1.8 ppmv HCN, dry basis and corrected to seven

percent oxygen. 92 Using the UPL method to account for variability when determining the level of the MACT floors, the existing source MACT floor for preheater/precalciner and dry process HWC cement kilns would be 27 ppmv HCN, dry basis and corrected to seven percent oxygen, and the new source MACT floor for preheater/precalciner and dry process HWC cement kilns would be 5.5 ppmv HCN, dry basis and corrected to seven percent oxygen. 93

92 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).

93 Id.

4. Liquid Fuel Boilers

a. Hydrogen Fluoride

For HF emissions from liquid fuel boilers, the EPA is proposing work practice standards with the same multiple compliance options proposed for HF emissions from HWC incinerators. The EPA collected HF emissions data from four HWC liquid fuel boilers in the January 2024 emissions testing request

less than or equal to 250 MMBTU/hr, and capacity greater than 250 MMBTU/hr.

94 42 U.S.C. 7412(d)(1). See also U.S. Sugar 830 F.3d at 593-94 (“[T]he EPA has discretion to differentiate among classes, types, and sizes of sources within a category or subcategory.” (internal citations omitted)).

95 See 40 CFR part 60, subpart Da, Standards of Performance for Electric Utility Steam Generating Units.

For units with a capacity that is less than or equal to 50 MMBTU/hr, the EPA has no data indicating that HCN is emitted because the boiler in this size category had no measurable emissions of HCN. Therefore, we are not proposing HCN emission limits for liquid fuel boilers with capacity less than or equal to 50 MMBTU/hr.

When separate subcategories are established under CAA 112(d)(1), a MACT floor is determined separately for each subcategory. 96 The MACT floor calculation was carried out separately for existing and new liquid fuel boilers in the other two size categories. To the EPA's knowledge, there are fewer than 30 major source liquid fuel boilers with capacity greater than 50 MMBTU/hr but less than or equal to 250 MMBTU/hr. The EPA had HCN emissions data from two of them. Our MACT floor analysis is based on the two sources for which we have data. In determining the level of the MACT floor, we used the UPL method to account for variability in performance, and we calculated the MACT floor at 2.7 ppmv HCN, dry basis and corrected to seven percent oxygen. 97 Based on available data, the EPA estimates that all existing liquid fuel boilers with capacity greater than 50 MMBTU/hr but less than or equal to 250 MMBTU/hr would be able to meet the MACT floor limit with no additional controls. The EPA also calculated a proposed new source limit from the best performing unit using the UPL method at 1.2 ppmv HCN, dry basis and corrected to seven percent oxygen.

96 U.S

proposed new source limit from the best performing unit using the UPL method at 0.57 ppmv HCN, dry basis and corrected to seven percent oxygen. However, the limit calculated by the UPL method is below the 3xRDL value for HCN for liquid fuel boilers (1.1 ppmv), and so the proposed new source limit based on the 3xRDL value is 1.1 ppmv HCN, dry basis and corrected to seven percent oxygen. 99

98 Id.

99 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 rule (Docket ID No. EPA-HQ-OAR-2004-0022).

The EPA evaluated whether BTF emission limits would be appropriate for HCN emissions from all the liquid fuel boiler subcategories except the new source limit for liquid fuel boilers with capacity greater than 250 MMBTU/hr, which is based on the 3xRDL and is already at the EPA's minimum level of the standard. One HWC liquid fuel boiler has a caustic scrubber that we expect to control HCN emissions, so we evaluated whether the incremental emissions reduction achievable with a caustic scrubber would be cost-effective for existing and new liquid fuel boilers with capacity greater than 50 MMBTU/hr. We estimate that a caustic scrubber would achieve approximately 95 percent reduction of HCN from a liquid fuel boiler. A corresponding 95 percent decrease in each UPL MACT floor value would be below the 3xRDL level for HCN emissions from a liquid fuel boiler (1.1 ppmv). Therefore, the BTF emission limits, reflecting the 3xRDL value, would be:

• For existing sources with capacity greater than 50 MMBTU/hr but less than or equal to 250 MMBTU/hr, 1.1 ppmv HCN, dry basis and corrected to seven percent oxygen.

• For new sources with capacity greater than 50 MMBTU/hr but less than or equal to 250 MMBTU/hr, 1.1 ppmv HCN, dry basis and corrected to seven percent oxygen.

• For existing sources with capacity greater than 250 MMBTU/hr, 1.1 ppmv HCN, dry basis and corrected to seven percent oxygen

eview and New Source Performance Standards, 79 FR 36880 (June 30, 2014).

• For existing sources with capacity greater than 50 MMBTU/hr but less than or equal to 250 MMBTU/hr, 2.7 ppmv HCN, dry basis and corrected to seven percent oxygen.

• For new sources with capacity greater than 50 MMBTU/hr but less than or equal to 250 MMBTU/hr, 1.2 ppmv HCN, dry basis and corrected to seven percent oxygen.

• For existing sources with capacity greater than 250 MMBTU/hr, 3.4 ppmv HCN, dry basis and corrected to seven percent oxygen.

• For new sources with capacity greater than 250 MMBTU/hr, 1.1 ppmv HCN, dry basis and corrected to seven percent oxygen.

The EPA is proposing that compliance with the HCN emission limits for all liquid 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 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 would be required to 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.

5. HCl Production Furnaces

a. Hydrogen Fluoride

The EPA is not proposing MACT standards for HF emissions from HCl production furnaces. The EPA surveyed the owners or operators of two HCl production furnaces in the August 2023 questionnaire. Both indicated that they do not burn fluorine-containing materials in their HCl production furnaces

have neither begun operating again nor initiated RCRA closure. 101 These are the only lightweight aggregate kilns in the source category. Because the EPA has no emissions data on which to base decisions about whether or how to regulate HF or HCN emissions from lightweight aggregate kilns, we are not proposing emission standards for HF or HCN emissions from lightweight aggregate kilns at this time. If the existing or new HWC lightweight aggregate kilns begin operating, we expect that we would collect emissions testing data from them and address potential emissions in a subsequent action.

101 See 40 CFR 63.1200(b), 265.351, 266.102(e)(11).

B. What are the results of the risk assessment and analyses?

As described in section III.C., the EPA conducts a risk assessment to estimate the human health and environmental risks posed by HAP emissions from the source category. The following five subsections provide a summary of the results of that risk assessment. Detailed information about the assessment is provided in the document titled Residual Risk Assessment for the Hazardous Waste Combustors Source Category in Support of the 2025 Risk and Technology Review Proposed Rule, which is available in the docket for this proposed rule.

1. Chronic Inhalation Risk Assessment Results

The results of the chronic inhalation cancer risk assessment indicate that, based on estimates of current actual emissions, the MIR posed by emissions from the source category is 9-in-1 million, driven by Ni, Cr(VI) compounds, and As compounds emissions from liquid fueled boilers. The total estimated cancer incidence based on actual emissions is 0.07. Within 50 km of HWC facilities, the population exposed to cancer risk greater than or equal to 1-in-1 million is approximately 540,000 people. The maximum modeled chronic noncancer TOSHI for the source category based on actual emissions is estimated to be 0.3 (for respiratory effects) due to emissions of Ni, HCl, and Co compounds from liquid fuel boilers

s available in the docket for this proposed rule.

4. Environmental Risk Screening Results

As described in section III.A. of this preamble, we conducted a screening assessment for adverse environmental effects for the HWC source category. The environmental screening assessment included the following PB-HAP: As compounds, Cd compounds, PCDD/PCDF, Pb compounds, methylmercury, divalent Hg, and POM. In addition, we conducted an environmental screening assessment for the acid gases HCl and HF.

In the Tier 1 screening analysis for PB-HAP (other than Pb compounds, which were evaluated differently), As

compounds and POM emissions had no exceedances for any ecological benchmark. Cd compounds, PCDD/PCDF, divalent Hg, and methylmercury had Tier 1 screening values above various benchmarks. The maximum Tier 1 screening value was 200 for methylmercury emissions for the surface soil NOAEL avian ground insectivores' benchmark (woodcock). Because there were Tier 1 exceedances, a Tier 2 environmental screening assessment was performed for Cd compounds, PCDD/PCDF, divalent Hg, and methylmercury emissions.

In the Tier 2 screen, Cd compounds and PCDD/PCDF emissions did not exceed any ecological benchmark. The following Tier 2 screening values were exceeded for methylmercury emissions: a screening value of six for the fish-eating birds NOAEL benchmark (specifically for the merganser), a screening value of two for the maximum allowable toxicant level for the merganser, and a maximum screening value of three (a total of eight facilities had screening values from two to three) for avian ground insectivores (woodcock).

The following Tier 2 screening values were exceeded for divalent Hg emissions: a maximum screening value of five for a sediment threshold level (emissions from 10 facilities contributed to this screening value) and a maximum screening value of two for an invertebrate threshold level (a total of five facilities had a screening value of two)

two to three) for avian ground insectivores (woodcock).

Since there were Tier 2 exceedances, we conducted a Tier 3 environmental risk screen. In the Tier 3 environmental risk screen, we looked at aerial photos of the lakes potentially being impacted by Hg emissions. Unnamed “lake” number 139670 is the lake at which the maximum methylmercury screening value of six was modeled for the fish-eating birds NOAEL benchmark (specifically for the merganser). It is also the lake where the maximum divalent Hg screening value of five was modeled for the sediment threshold level. The aerial photos reveal that this “lake” is an open bay off the Gulf of America. As such, it is not a “closed” waterbody, and therefore we do not expect accumulation of Hg concentrations. Therefore, the screening results for “lake” 139670 were removed from the analysis.

Once the screening results for “lake” 139670 were removed, the highest Tier 2 screening values for methylmercury were a screening value of three for a water-column NOAEL benchmark for fish-eating birds (merganser) and a screening value of three for a surface soils NOAEL benchmark for avian ground insectivores (woodcock). The water-column NOAEL benchmark for fish-eating mammals (mink) and the soils NOAEL benchmark for mammalian insectivores (shrew) were not exceeded for methylmercury in Tier 2. In addition, the water-column LOAEL level benchmarks for fish eating birds (merganser) and fish-eating mammals (mink) were not exceeded in Tier 2 for methylmercury.

Once the screening results for “lake” 139670 were removed, the highest Tier 2 screening value for divalent Hg is a screening value of three for a sediment threshold level benchmark

hrew) were not exceeded for methylmercury in Tier 2. In addition, the water-column LOAEL level benchmarks for fish eating birds (merganser) and fish-eating mammals (mink) were not exceeded in Tier 2 for methylmercury.

Once the screening results for “lake” 139670 were removed, the highest Tier 2 screening value for divalent Hg is a screening value of three for a sediment threshold level benchmark. This screening value is the result of emissions from three facilities near one lake (lake 431155), with one facility being the primary contributor (facility 450755720711). The water-column community threshold level benchmark and the surface soil threshold level benchmark for plant communities were not exceeded for divalent Hg in Tier 2.

In summary, Hg emissions from this category resulted in ecological screening values above one (maximum screening value of three) for only some of the most sensitive ecological benchmarks for Hg, while other sensitive benchmarks for Hg were not exceeded. Therefore, we conclude that the ecological impacts of Hg emissions from this category are not widespread and significant.

We did not estimate any exceedances of the secondary Pb NAAQS. The highest annual Pb concentration of 0.004 µg/m 3 is well below the Pb NAAQS (0.15 µg/m 3 in total suspended particles as a three-month average), indicating low potential for environmental risk of concern due to Pb emissions. 103

103 81 FR 71906 (Oct. 18, 2016).

We also conducted an environmental risk screening assessment specifically for acid gases ( i.e., HCl and HF) for the HWC source category. For HCl and HF, the average modeled concentration around each facility ( i.e., the average concentration of all off-site data points in the modeling domain) did not exceed any ecological benchmark. In addition, each individual modeled concentration of HCl and HF ( i.e., each off-site data point in the modeling domain) was below the ecological benchmarks for all facilities

the HWC source category. For HCl and HF, the average modeled concentration around each facility ( i.e., the average concentration of all off-site data points in the modeling domain) did not exceed any ecological benchmark. In addition, each individual modeled concentration of HCl and HF ( i.e., each off-site data point in the modeling domain) was below the ecological benchmarks for all facilities.

Based on the results of the environmental risk screening analysis, we do not expect an adverse environmental effect resulting from HAP emissions from this source category and we are proposing that it is not necessary to set any additional standards, beyond those described above, to prevent, taking into consideration costs, energy, safety, and other relevant factors, an adverse environmental effect. Detailed information about the assessment is provided in the document titled Residual Risk Assessment for the Hazardous Waste Combustors Source Category in Support of the 2025 Risk and Technology Review Proposed Rule, which is available in the docket for this proposed rule.

5. Facility-Wide Risk Results

We conducted an assessment of facility-wide risk as described in section III.C. of this preamble to characterize the source category risk in the context of whole facility risk. We estimated facility-wide risks using the NEI-based data described in section III.C. of this preamble. The maximum lifetime individual cancer risk posed by the 92 facilities modeled based on facility-wide emissions is 200-in-1 million, driven by emissions of ethylene oxide from a different source category (commonly referred to as the Hazardous Organic NESHAP), and the risk review for that source category has already been completed. 104 The total estimated cancer incidence based on facility-wide emission levels is 0.4 excess cancer cases per year

d by the 92 facilities modeled based on facility-wide emissions is 200-in-1 million, driven by emissions of ethylene oxide from a different source category (commonly referred to as the Hazardous Organic NESHAP), and the risk review for that source category has already been completed. 104 The total estimated cancer incidence based on facility-wide emission levels is 0.4 excess cancer cases per year. Within 50 km of HWC facilities, the population exposed to cancer risk greater than 100-in-1 million due to facility-wide emissions is approximately 250 people, and the population exposed to cancer risk greater than or equal to 1-in-1 million is approximately 6.4 million people. The maximum chronic noncancer TOSHI posed by facility-wide emissions is estimated to be three (for respiratory effects) at two different facilities, driven by non-category emissions of chlorine at both. Approximately 170 people are estimated to be exposed to a TOSHI greater than one due to facility-wide emissions.

104 For more information about the Hazardous Organic NESHAP, see https://www.epa.gov/stationary-sources-air-pollution/synthetic-organic-chemical-manufacturing-industry-national.

C. What are our proposed decisions regarding risk acceptability, ample margin of safety, and adverse environmental effect?

1. Risk Acceptability

As noted in section III.A. of this preamble, the EPA weighs a wide range of health risk measures and factors in our risk acceptability determination, including the cancer MIR, the number of persons in various cancer and noncancer risk ranges, cancer incidence, the maximum noncancer TOSHI, the

maximum acute noncancer HQ, and risk estimation uncertainties (54 FR 38044, September 14, 1989).

m noncancer TOSHI, the

maximum acute noncancer HQ, and risk estimation uncertainties (54 FR 38044, September 14, 1989).

The results of the risk assessment indicate that, based on actual emissions, the MIR is 9-in-1 million, driven by emissions of Ni compounds, Cr(VI) compounds, and As compounds. The estimated incidence of cancer due to inhalation exposures is 0.07 excess cancer case per year. No people are estimated to have inhalation cancer risks greater than 100-in-1 million, and the population estimated to be exposed to cancer risks greater than or equal to 1-in-1 million is approximately 540,000. The estimated maximum chronic noncancer TOSHI from inhalation exposure for this source category is 0.07 for respiratory effects. The acute risk screening assessment of reasonable worst-case inhalation impacts indicates a maximum acute HQ of two for the REL for As compounds. In addition, the risk assessment indicates no significant potential for multipathway health effects.

For allowable emissions, the MIR is 100-in-1 million, driven by emissions of Ni compounds, Cr(VI) compounds, and As compounds. The estimated incidence of cancer due to inhalation exposures is 0.8 excess cancer case per year. No people are estimated to have inhalation cancer risks greater than 100-in-1 million, and the population estimated to be exposed to cancer risks greater than or equal to 1-in-1 million is approximately 12.1 million. The estimated maximum chronic noncancer TOSHI from inhalation exposure for this source category is one for respiratory effects. We note that HWC source category actual emissions are much lower than allowable emissions. The allowable emissions are based on the value of the standard and the maximum allowable stack gas flow rate, and they assume that an HWC operates at this maximum capacity for 8,760 hours per year. This is an upper-bound assumption because HWCs cannot operate at their maximum capacity every hour of the year, so the maximum allowable emissions would not be possible

of this preamble, we found that the potential emission reductions were relatively small (0.211 grams of PCDD/PCDF TEQ per year per unit) and we are proposing that the potential control options that we evaluated are not cost-effective (cost-effectiveness of $1.42 million per gram of PCDD/PCDF TEQ reduction).

For the GMCS, emission reductions were estimated to be 13 pounds of Hg per unit per year. The emission reductions would have no impact on the cancer MIR, maximum TOSHI, cancer incidence, or number of people exposed to cancer risk levels of greater than or equal to 1-in-1 million. Similar to the SDDS emission reductions, the GMCS emission reductions could potentially lower the cancer risks estimated in the Multipathway Risk Screening discussed in section IV.B.3. of this preamble. However, also like the SDDS, we are proposing to conclude that the GMCS is not cost-effective, with an estimated annualized cost-effectiveness of $62,000 per pound of Hg reduction (see section IV.D. of this preamble).

Considering the high overall costs of the control options that we evaluated and relatively small emissions reductions, we are proposing to determine that the control technologies are not necessary to provide an ample margin of safety to protect public health. Therefore, based on our weighing of all the relevant factors as presented in the risks analyses for this source category and all of the other information discussed earlier in this section, we propose to conclude that the current standards provide an ample margin of safety to protect public health. We are also requesting comment on whether there are additional control measures for emission sources subject to the HWC standards that are necessary to provide an ample margin of safety to protect public health (C-9).

3. Adverse Environmental Effect

Based on our screening assessment of environmental risk presented in section III.A.4. of this preamble, we did not identify any areas of concern with respect to environmental risk

whether there are additional control measures for emission sources subject to the HWC standards that are necessary to provide an ample margin of safety to protect public health (C-9).

3. Adverse Environmental Effect

Based on our screening assessment of environmental risk presented in section III.A.4. of this preamble, we did not identify any areas of concern with respect to environmental risk. Therefore, we have determined that HAP emissions from the source categories do not result in an adverse environmental effect. Taking into consideration costs, energy, safety, and other relevant factors, we are proposing that it is not necessary to set a more stringent standard to prevent an adverse environmental effect.

D. What are the results and proposed decisions based on our technology review?

As described in section III.B. of this preamble, the EPA's technology review under CAA section 112(d)(6) focused on the identification and evaluation of potential developments in practices, processes, and control technologies that have occurred since the promulgation of

the HWC NESHAP in 2005. We reviewed various sources of information to identify any such developments and found that two new control technologies have been employed in the HWC NESHAP source category on one incinerator since 2005: the SDDS for control of PCDD/PCDF and the GMCS for control of Hg. Detailed information about the technology review can be found in the memorandum titled Clean Air Act Section 112(d)(6) Technology Review for the Hazardous Waste Combustor Source Category, which is available in the docket for this proposed rule (Docket ID No. EPA-HQ-OAR-2004-0022). The EPA is also specifically requesting comment on whether we should consider additional developments not addressed here or in the technical memorandum for emission sources subject to the HWC NESHAP (C-10).

cost information for SDDS installation and operation for HWC units in the U.S. Because the SDDS is similar in principle and design to SCR, we estimated the costs of the SDDS using information from the EPA Control Cost Manual's section on SCR. The EPA estimates a total capital investment cost of $1,776,000 and a total annualized cost of $299,000 per year (2024$) for each unit that installs the SDDS. Using the 95 percent control efficiency demonstrated by the HWC that installed the SDDS and its PCDD/PCDF emissions, we estimate emission reductions of 0.211 grams of PCDD/PCDF TEQ per year for each unit that installs the SDDS. This results in an annualized cost-effectiveness of $1,419,000 per gram of PCDD/PCDF TEQ reduction. Lower control efficiency and lower pre-SDDS PCDD/PCDF emissions (as we expect for HWCs other than the unit that installed the SDDS) would substantially decrease the emission reductions of the SDDS, making it less cost-effective. The EPA has previously considered $300,000 per gram of PCDD/PCDF TEQ reduced (adjusted to 2024$) to not be cost-effective 106 and, in keeping with that prior determination, proposes not to consider $1,419,000 per gram of PCDD/PCDF TEQ reduced to be cost-effective. Due to the high cost and low potential emission reductions of PCDD/PCDF, the EPA proposes not to consider the SDDS a cost-effective technology to further reduce emissions of PCDD/PCDF from sources subject to the HWC NESHAP.

106 See the 2003 proposed and final rules for the National Emission Standards for Hazardous Air Pollutants for Primary Magnesium Refining, 68 FR 2970 (Jan. 22, 2003) and 68 FR 58615 (Oct. 10, 2003).

2. Gore Mercury Control System

ardous Air Pollutants for Primary Magnesium Refining, 68 FR 2970 (Jan. 22, 2003) and 68 FR 58615 (Oct. 10, 2003).

2. Gore Mercury Control System

The GMCS uses a series of modules containing catalysts and sorbents to capture elemental and oxidized Hg and co-control sulfur dioxide emissions. The GMCS has been considered in other EPA rulemakings, including the Phosphoric Acid Manufacturing NESHAP. Specifically, the control efficiency, module capacity, initial costs, and costs of modules from the Phosphoric Acid Manufacturing NESHAP GMCS analysis were used as the basis for the HWC NESHAP estimate. 107

107 85 FR 19412 (Apr. 7, 2020).

The EPA used the average Hg emission rates and stack gas flow rates for HWC incinerators that we developed in the residual risk review to also develop a cost estimate for installing, operating, and maintaining a GMCS at an “average” HWC. The EPA estimates a total capital investment cost of $4,143,000 and a total annualized cost of $804,000 per year (2024$) for each unit that installs a GMCS. Assuming 90 percent control efficiency, we estimate emission reductions of 13 pounds of Hg per unit per year. This results in an annualized cost-effectiveness of $62,000 per pound of Hg reduction. The EPA has previously considered $55,400 per pound of Hg reduced (adjusted to 2024$) to not be cost-effective 108 and, in keeping with that prior determination, proposes not to consider $62,000 per pound of Hg reduced to be cost-effective. Due to the high cost and low potential emission reductions of Hg, the EPA proposes not to consider the GMCS a cost-effective technology to further reduce emissions of Hg from sources subject to the HWC NESHAP.

108 See the 2011 final rule for the National Emission Standards for Hazardous Air Pollutants: Gold Mine Ore Processing and Production Area Source Category, 76 FR 9450 (Feb. 17, 2011)

ically, we are proposing revisions to the SSM provisions of the MACT rule in order to ensure that those provisions are consistent with Sierra Club v. EPA, 551 F.3d 1019 (D.C. Cir. 2008), in which the D.C. Circuit vacated two provisions that the court interpreted as exempting sources from the requirement to comply with otherwise applicable CAA section 112(d) emission standards during periods of SSM.

The EPA is also proposing the following additional changes to the HWC NESHAP:

• Requiring electronic reporting of performance test results, notification of compliance reports, and certain other submissions;

• Allowing states to choose to exempt area sources from the requirement to obtain a title V permit;

• Removing the requirement that CO is kept between the average and maximum reported values during the CfPT;

• Explicitly allowing incorporation by reference of operating parameter limits determined during the CPT into title V permits;

• Clarifying that a relative accuracy test audit (RATA) must be performed within 60 days of every CPT;

• Removing the never-implemented requirement that sources install and operate PM CEMS;

• Removing references that were incorrectly incorporated by reference and have since expired;

• Clarifying the demonstration of compliance timeframe for new standards and removing an outdated demonstration of compliance timeline for the 2005 HWC NESHAP; and

• Other minor editorial corrections.

Our analyses and proposed changes related to these issues are discussed as follows.

1. Emission Standards During Periods of SSM

discussed as follows.

1. Emission Standards During Periods of SSM

In Sierra Club v. EPA, the D.C. Circuit vacated the SSM exemption contained in 40 CFR 63.6(f)(1) and 40 CFR 63.6(h)(1), holding that under CAA section 302(k), emission standards or limitations must be continuous in nature and that the SSM exemption violates the CAA's requirement that some section 112 standards apply continuously. In July 2024, the EPA proposed the removal of the malfunction exemption from the HWC NEHSAP, which, if finalized, would have required the standards for periods of normal operation to apply at all times. 109 We also indicated that we would address standards for periods of startup and shutdown in a future planned rulemaking action. 110 After considering comments received on the proposed removal of the malfunction exemption, the EPA is withdrawing that proposal and instead proposing a different standard for periods of malfunction, as described in this section. The EPA is also proposing standards for periods of startup and shutdown.

109 89 FR 59870.

110 Id.

We are proposing to remove the SSM provision in the HWC NESHAP that appears at 40 CFR 63.1206(b). Consistent with Sierra Club v. EPA, we are proposing standards in this rule that apply at all times, specifically work practice standards that apply for periods of SSM. Although under the current HWC NESHAP emission standards and operating requirements do not apply during periods of SSM, there are other requirements that apply during these periods. Two specific requirements are notable: an approved SSM plan (40 CFR 63.1206(c)(2)) and the AWFCO requirement (40 CFR 63.1206(c)(3)).

Most sources demonstrate compliance with the RCRA requirement to minimize emissions from SSM events by complying with an approved SSM plan during those periods

rements do not apply during periods of SSM, there are other requirements that apply during these periods. Two specific requirements are notable: an approved SSM plan (40 CFR 63.1206(c)(2)) and the AWFCO requirement (40 CFR 63.1206(c)(3)).

Most sources demonstrate compliance with the RCRA requirement to minimize emissions from SSM events by complying with an approved SSM plan during those periods. Under the general provisions of 40 CFR part 63, the SSM plan must describe in detail procedures for operating and maintaining the source during periods of SSM and a program of corrective action for malfunction scenarios that would cause the source to exceed an applicable emission limit. If sources use the SSM plan to comply with RCRA requirements, the SSM plan must include a description of potential causes of malfunctions that may result in significant HAP releases and of actions the source is taking to minimize the frequency and severity of these malfunctions. In addition, when used to demonstrate RCRA compliance, SSM plans must be submitted to the Administrator (or an identified delegate) for approval, and any changes that may significantly increase emissions must also be submitted for approval.

All HWCs are required to have an AWFCO system. Hazardous waste feed to the HWC cannot restart until the event that triggered the AWFCO is resolved, which typically takes no less than one hour. HWCs must comply with the AWFCO system requirements during periods of SSM if they are burning hazardous waste during those periods.

While the D.C. Circuit established that, reading CAA sections 112 and 302(k) together, Congress has required that there must be continuous CAA section 112 standards, the court recognized that in some instances, it may not be feasible to prescribe or enforce an emission standard. 111 For example, the EPA may set different standards for periods of SSM, where feasible

ractice standards for periods of SSM. These work practice standards would be enforceable requirements that minimize emissions of HAP, primarily by preventing emissions. The work practice standards would include the following: (1) a clean fuel requirement for periods of startup and shutdown; (2) a requirement to follow an approved SSM plan during periods of SSM; and (3) the AWFCO system requirement.

The clean fuel requirement for periods of startup and shutdown would limit which supplemental fuels could be burned during those periods to minimize emissions of HAP. For the HWC NESHAP, we are proposing that clean fuels would include one or a combination of natural gas, synthetic natural gas, propane, other Gas 1 fuels, distillate oil, syngas, ultra-low sulfur diesel, kerosene, hydrogen, refinery gas, liquified petroleum gas, and any other fuel authorized in the SSM plan. We are including the option to use any other authorized fuel to allow for cases where another fuel is required due to either combustor design or availability of a facility-produced fuel that is not listed and that has expected combustion emissions similar to those of the listed fuels. An example may include a hazardous waste that is hazardous only because it is flammable and is currently allowed to be burned during startup in accordance with 40 CFR 63.1206(c)(2)(v)(B).

The EPA is proposing that all sources must follow an approved SSM plan during periods of SSM. This is a change from the July 2024 proposal as it relates to malfunctions. This proposal has two differences from the current provisions of the rule. First, all sources, not just sources using the SSM plan for RCRA compliance, would be required to have an approved SSM plan. The EPA expects that most, if not all, sources are already using this option for compliance, so this provision, if finalized, would have minimal impact on most sources. Second, the EPA is proposing to add an explicit requirement that sources must operate according to their SSM plan during periods of SSM

sources using the SSM plan for RCRA compliance, would be required to have an approved SSM plan. The EPA expects that most, if not all, sources are already using this option for compliance, so this provision, if finalized, would have minimal impact on most sources. Second, the EPA is proposing to add an explicit requirement that sources must operate according to their SSM plan during periods of SSM. Based on discussions with regulated parties, the EPA expects that sources are already doing this. This proposal is therefore intended to codify this requirement and, if finalized, would make compliance with the SSM plan an enforceable provision during periods of SSM.

For malfunctions, the EPA is proposing no changes to the current

AWFCO requirements as part of the proposed work practice. 114 The AWFCO requirements minimize emissions during malfunctions that could cause exceedances by requiring swift hazardous waste feed shut off. 115 Because hazardous waste is a primary source of HAP emissions for most HWCs, shutting off hazardous waste feed immediately minimizes emissions while the owner or operator can diagnose and resolve the issue that triggered the AWFCO.

114 80 FR 75178, 75211-14 (Dec. 1, 2015); see also 85 FR 49434, 49441-46 (Aug. 13, 2020).

115 “At the very least, the language [of CAA section 112(d)(3)] permits the EPA to ignore malfunctions in its standard-setting and account for them instead through its regulatory discretion.” U.S. Sugar, 830 F.3d at 608.

Additionally, on September 5, 2025, the D.C. Circuit held in SSM Litigation Group v. EPA, Case No

178, 75211-14 (Dec. 1, 2015); see also 85 FR 49434, 49441-46 (Aug. 13, 2020).

Additionally, on September 5, 2025, the D.C. Circuit held in SSM Litigation Group v. EPA, Case No. 23-1267, that although the EPA lacks authority under the CAA to “create a regulatory `defense' that limits the remedial authority granted by Congress to the federal courts,” a “complete affirmative defense, like the one at issue [in that case], is permissible because it relates to the antecedent question of liability and therefore does not impinge on the judiciary's authority to award `appropriate civil penalties.' ” 116 While this proposal does not involve affirmative defenses, the EPA requests comment on whether and how we should establish regulations within this and other New Source Performance Standards or NESHAPs in response to the D.C. Circuit's SSM Litigation Group decision (C-11). Due to the timing of the D.C. Circuit decision and the Agency's court-ordered deadline, the EPA will address the impacts of the SSM Litigation Group decision in an appropriate future action.

116 Slip Op. at 10-11 (quoting CAA 304(a), 42 U.S.C. 7604(a)).

2. Electronic Reporting

The EPA proposed some provisions for electronic reporting for the HWC NESHAP in July 2024. After considering the comments received on that proposal, the EPA is reproposing the same provisions for electronic reporting in addition to proposing requirements for the use of templates for certain reports. The templates are available in the docket for this proposed rule. The EPA will respond to comments on the July 2024 proposal in the final action for this proposal. There is no need to resubmit any comments that duplicate comments on the July 2024 proposal

ema on the ERT website. The proposed rule would require that the notification of intent to comply, eligibility demonstrations, periodic SSM reports, and compliance progress reports be submitted as PDF uploads in CEDRI.

117 See Document ID No. EPA-HQ-OAR-2004-0022-0646 in Docket ID No. EPA-HQ-OAR-2004-0022.

118 https://www.epa.gov/electronic-reporting-air-emissions/electronic-reporting-tool-ert.

For the NOC and the excess emissions and CMS performance reports and summary reports, the proposed rule would require that owners and operators use the appropriate spreadsheet template to submit information to CEDRI. A draft version of the proposed templates for these reports is included in the docket for this proposed rule. 119 The EPA specifically requests comment on the content, layout, and overall design of the templates (C-12).

119 See Proposed Electronic Reporting Templates for 40 CFR part 63, subpart EEE, available at Docket ID. No. EPA-HQ-OAR-2004-0022.

The electronic submittal of the reports addressed in this proposed rule would increase the usefulness of the data contained in those reports, is in keeping with current trends in data availability and transparency, would further assist in the protection of public health and the environment, would improve compliance by facilitating the ability of regulated facilities to demonstrate compliance with requirements and by facilitating the ability of delegated state, local, Tribal, and territorial air agencies and the EPA to assess and determine compliance, and would ultimately reduce burden on regulated facilities, delegated air agencies, and the EPA. Electronic reporting also eliminates paper-based, manual processes, thereby saving time and resources, simplifying data entry, eliminating redundancies, minimizing data reporting errors, and providing data quickly and accurately to the affected facilities, air agencies, the EPA, and the public

pliance, and would ultimately reduce burden on regulated facilities, delegated air agencies, and the EPA. Electronic reporting also eliminates paper-based, manual processes, thereby saving time and resources, simplifying data entry, eliminating redundancies, minimizing data reporting errors, and providing data quickly and accurately to the affected facilities, air agencies, the EPA, and the public. For more information on the benefits of electronic reporting, see the memorandum Electronic Reporting Requirements for New Source Performance Standards (NSPS) and National Emission Standards for Hazardous Air Pollutants (NESHAP) Rules, referenced earlier in this section.

3. Title V Permits for Area Sources

Under the CAA, sources subject to standards or regulations under sections 111 or 112 generally must obtain a title V operating permit. However, the Administrator has the discretion under CAA section 502(a) to exempt area sources from the requirements of title V if the Administrator finds that “compliance with such requirements is impracticable, infeasible, or unnecessarily burdensome on such categories. . . .” Currently, the HWC NESHAP requires that all sources subject to the rule, both major sources and area sources, obtain and maintain a title V air permit. Title V permits did not replace RCRA permits for HWCs. HWCs are required to obtain and maintain both RCRA and title V permits.

In the 2004 HWC NESHAP proposal, the EPA stated that title V permitting for area sources was not impracticable, infeasible, or unnecessarily burdensome because HWCs were already complying with RCRA permitting requirements, which make no distinction between major and area sources. 120 The EPA did not fully explain why complying with RCRA permitting requirements meant that title V permitting was not impracticable, infeasible, or unnecessarily burdensome

tle V permitting for area sources was not impracticable, infeasible, or unnecessarily burdensome because HWCs were already complying with RCRA permitting requirements, which make no distinction between major and area sources. 120 The EPA did not fully explain why complying with RCRA permitting requirements meant that title V permitting was not impracticable, infeasible, or unnecessarily burdensome. For the reasons provided below, the EPA now proposes to find that the requirement to obtain a title V permit is “unnecessarily burdensome” for HWC area sources and, accordingly, proposes to allow states to exempt HWC area sources from title V permitting requirements. 121 The EPA

seeks comment on this proposal, including any specific reliance interests relevant to the existing requirements for HWC area sources to obtain title V permits (C-13).

120 69 FR 21198, 21325 (Apr. 20, 2004).

121 Unless provided otherwise by statute, an agency may revise or rescind prior actions so long

as it acknowledges the change in position, provides a reasonable explanation for the new position, and considers legitimate reliance interests in the prior position. See FDA v. Wages & White Lion Investments, LLC, 145 S. Ct. 898 (2025); FCC v. Fox Television Stations, Inc., 556 U.S. 502 (2009); Motor Vehicle Mfrs. Ass'n v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29 (1983); Clean Air Council v. Pruitt, 862 F.3d 1, 8 (D.C. Cir. 2017) (“Agencies obviously have broad discretion to reconsider a regulation at any time.”).

The EPA has previously exempted many area sources from title V requirements based on a conclusion that the requirements are “unnecessarily burdensome” under CAA section 502(a)

r Vehicle Mfrs. Ass'n v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29 (1983); Clean Air Council v. Pruitt, 862 F.3d 1, 8 (D.C. Cir. 2017) (“Agencies obviously have broad discretion to reconsider a regulation at any time.”).

The EPA has previously exempted many area sources from title V requirements based on a conclusion that the requirements are “unnecessarily burdensome” under CAA section 502(a). Historically, the EPA has considered four factors in determining whether the “unnecessarily burdensome” criterion is satisfied: (1) whether title V would add any significant compliance requirements to those already required by the NESHAP; (2) whether the area sources subject to a NESHAP possess characteristics that would contribute to title V permitting imposing a significant burden on them and whether this burden could be aggravated by difficulty in obtaining assistance from permitting agencies; (3) whether the costs of title V permitting for the area sources would be justified, taking into consideration any potential gains in compliance; and (4) whether adequate oversight, outreach, and compliance assistance programs by the EPA or a delegated authority could achieve high compliance without relying on title V permitting. 122 The EPA has considered on a case-by-case basis the extent to which one or more of these four factors is present for a given source category and then considered whether, taken together, those factors that are present demonstrate that compliance with title V requirements would be unnecessarily burdensome. 123

122 For further discussion, see the EPA's proposal (70 FR 15250, Mar. 25, 2005) and final rule (70 FR 75320, Dec. 19, 2005) exempting multiple area sources from title V permitting requirements.

123 Id

124 The EPA did not conduct such a comparison as part of the 2004 HWC NESHAP proposal. 69 FR at 21325 (Apr. 20, 2004).

The second factor is whether the area sources subject to a NESHAP possess characteristics that would contribute to title V permitting imposing a significant burden on them, and whether this burden could be aggravated by difficulty in obtaining assistance from permitting agencies. The third factor, which is closely related to the second factor, is whether the costs of title V permitting for area sources subject to a NESHAP would be justified, taking into consideration any potential gains in compliance likely to occur for such sources. The EPA did not discuss either factor as part of the 2004 HWC NESHAP proposal. 125 We estimate that the cost of renewing a title V permit is between $15,000 and $30,000, with costs varying by state and by complexity of the permit, often with additional associated emission fees (the presumptive minimum fee rate was $63.69/ton for both HAP and non-HAP emissions for September 1, 2024, through August 31, 2025). 126 We estimate that the cost of a non-title V state operating permit is approximately half that of a title V permit. Some area sources are small businesses, and the title V permitting cost can represent substantial cost to a small business, making the requirement that area sources have title V permits potentially burdensome on some sources. We propose to find that HWC area sources have characteristics that would contribute to title V permitting imposing a significant burden, and that the costs of title V permitting for these area sources is not justified given the minimal gains in compliance likely to occur for such sources.

125 69 FR at 21325 (Apr. 20, 2004).

126 See https://www.epa.gov/title-v-operating-permits/permit-fees for information about part 70 permit fees and presumptive fee rates

enforcement action; the EPA uses title V as a compliance tool but does not solely rely on it to enforce requirements of NESHAP. Most HWC NESHAP area sources have good compliance history, showing that a title V permit may not be necessary to incentivize compliance for some area sources.

127 The EPA did not discuss this factor as part of the 2004 HWC NESHAP proposal. 69 FR at 21325 (Apr. 20, 2004).

For these reasons, the EPA is proposing to allow states to choose to exempt HWC area sources from the requirement to obtain a title V permit based on the Administrator's determination under CAA section 502(a) that compliance with title V requirements is “unnecessarily burdensome.” If this proposal is finalized, state, local, and Tribal permitting authorities will have the option under 40 CFR 63.1(c)(2)(i) to exclude HWC area sources from the requirement to obtain a title V permit on a source-by-source basis unless the area source is otherwise required by law to obtain a title V permit ( e.g., is an area source of HAP but a major source of criteria pollutants). State, Tribal, and local title V permitting authorities are highly involved in the day-to-day issuance of title V permits and determinations of compliance with the HWC NESHAP. States are well positioned to determine whether the requirement to have a title V permit is burdensome on a small business with a strong record of compliance, or if a title

V permit is a necessary compliance tool for a specific area source. Providing a state with title V permitting authority the option of issuing a title V permit or other non-title V permit decreases unnecessary burden on small entities while ensuring that states have the flexibility to maintain compliance tools.

There are, however, implications of this proposal that are unique to the HWC NESHAP

There are, however, implications of this proposal that are unique to the HWC NESHAP. Because the HWC NESHAP was originally promulgated under joint CAA and RCRA authority, the EPA issued a final rule in 1999 giving sources in authorized states the option to migrate air emissions and related operating requirements established under RCRA regulations from RCRA permits to title V permits in order to consolidate all air requirements for one unit in the same permitting location. 128 All other RCRA related requirements ( e.g., corrective action, general facility standards, material handling, and risk-based emission limits and operating requirements) remain in the RCRA permit. 129 This option was provided pursuant to RCRA section 1006(b), which requires the EPA to demonstrate that the RCRA and CAA provisions are equivalent and that RCRA air emissions requirements could be removed because they were duplicative with HWC NESHAP requirements. 130

128 See 64 FR 52828, 52833-34 (Sept. 30, 1999); see also 70 FR 59516-26 (Oct. 12, 2005).

129 Id.

130 See Chem. Waste Mgmt. v. EPA, 976 F.2d 2, 23, 25 (D.C. Cir. 1992).

The EPA undertook this demonstration most robustly in the 1999 final rule, in which we relied heavily on the federal enforceability of title V permits and concluded that the title V permitting process would provide equivalent opportunities for public participation. 131 The EPA explained that the HWC NESHAP emission limits were generally as protective as

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