[Federal Register: February 6, 2007 (Volume 72, Number 24)]
[Proposed Rules]
[Page 5509-5550]
[DOCID:fr06fe07-20]

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Part II

Environmental Protection Agency

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40 CFR Part 60

Standards of Performance for New Stationary Sources and Emission
Guidelines for Existing Sources: Hospital/Medical/Infectious Waste
Incinerators; Proposed Rule

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 60

[EPA-HQ-OAR-2006-0534; FRL-8274-9]
RIN 2060-A004

Standards of Performance for New Stationary Sources and Emission
Guidelines for Existing Sources: Hospital/Medical/Infectious Waste
Incinerators

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: On September 15, 1997, EPA adopted new source performance
standards (NSPS) and emission guidelines for hospital/medical/
infectious waste incinerators (HMIWI). The NSPS and emission guidelines
were established under sections 111 and 129 of the Clean Air Act (CAA).
On November 14, 1997, the Sierra Club and the Natural Resources Defense
Council (Sierra Club) filed suit in the U.S. Court of Appeals for the
District of Columbia Circuit (the Court) challenging EPA's methodology
for adopting the regulations. On March 2, 1999, the Court issued its
opinion. The Court remanded the rule to EPA for further explanation of
the Agency's reasoning in determining the minimum regulatory ``floors''
for new and existing HMIWI. The Court did not vacate the regulations,
so the NSPS and emission guidelines remained in effect during the
remand and were fully implemented by September 2002. This action
provides EPA's proposed response to the questions raised in the Court's
remand.
    Section 129(a)(5) of the CAA requires EPA to review and, if
appropriate, revise the NSPS and emission guidelines every 5 years. In
this action, EPA also is proposing our response to this 5-year review,
which would revise the emission limits in the NSPS and emission
guidelines to reflect the levels of performance actually achieved by
the emission controls installed to meet the emission limits set forth
in the September 15, 1997, NSPS and emission guidelines.

DATES: Comments. Comments must be received on or before April 9, 2007.
Under the Paperwork Reduction Act, comments on the information
collection provisions must be received by the Office of Management and
Budget (OMB) on or before March 8, 2007. Because of the need to resolve
the issues raised in this action in a timely manner, EPA will not grant
requests for extensions beyond these dates.
    Public Hearing. If anyone contacts EPA by February 26, 2007
requesting to speak at a public hearing, EPA will hold a public hearing
on March 8, 2007. If you are interested in attending the public
hearing, contact Ms. Pamela Garrett at<PHONE_NUMBER> to verify that a
hearing will be held.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-
OAR-2006-0534, by one of the following methods:
    http://www.regulations.gov: Follow the on-line instructions for submitting

comments.
    E-mail: Send your comments via electronic mail to
<EMAIL_ADDRESS>Attention Docket ID No. EPA-HQ-OAR-2006-0534.

    Facsimile: Fax your comments to<PHONE_NUMBER>, Attention Docket ID
No. EPA-HQ-OAR-2006-0534.
    Mail: Send your comments to: EPA Docket Center (EPA/DC),
Environmental Protection Agency, Mailcode 6102T, 1200 Pennsylvania
Ave., NW., Washington, DC 20460, Attention Docket ID No. EPA-HQ-OAR-
2006-0534.
    Hand Delivery: Deliver your comments to: EPA Docket Center (EPA/
DC), EPA West Building, Room 3334, 1301 Constitution Ave., NW.,
Washington, DC 20460, Attention Docket ID No. EPA-HQ-OAR-2006-0534.
Such deliveries are accepted only during the normal hours of operation
(8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal
holidays), and special arrangements should be made for deliveries of
boxed information.
    Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2006-0534. The EPA's policy is that all comments received will be
included in the public docket and may be made available online at
http://www.regulations.gov, including any personal information provided,

unless the comment includes information claimed to be Confidential
Business Information (CBI) or other information whose disclosure is
restricted by statute. Do not submit information that you consider to
be CBI or otherwise protected through http://www.regulations.gov or e-mail.

The http://www.regulations.gov Web site is an ``anonymous access'' system,

which means EPA will not know your identity or contact information
unless you provide it in the body of your comment. If you send an e-
mail comment directly to EPA without going through http://www.regulations.gov,

your e-mail address will be automatically captured and included as part
of the comment that is placed in the public docket and made available
on the Internet. If you submit an electronic comment, EPA recommends
that you include your name and other contact information in the body of
your comment and with any disk or CD-ROM you submit. If EPA cannot read
your comment due to technical difficulties and cannot contact you for
clarification, EPA may not be able to consider your comment. Electronic
files should avoid the use of special characters, any form of
encryption, and be free of any defects or viruses.
    Public Hearing: If a public hearing is held, it will be held at
EPA's Campus located at 109 T.W. Alexander Drive in Research Triangle
Park, NC, or an alternate site nearby. Persons interested in presenting
oral testimony must contact Ms. Pamela Garrett at<PHONE_NUMBER> at
least 2 days in advance of the hearing.
    Docket: EPA has established a docket for this action under Docket
ID No. EPA-HQ-OAR-2006-0534 and Legacy Docket ID No. A-91-61. All
documents in the docket are listed in the http://www.regulations.gov index.

Although listed in the index, some information is not publicly
available, e.g., CBI or other information whose disclosure is
restricted by statute. Certain other material, such as copyrighted
material, will be publicly available only in hard copy form. Publicly
available docket materials are available either electronically at
http://www.regulations.gov or in hard copy at the EPA Docket Center EPA/DC,

EPA West, Room 3334, 1301 Constitution Ave., NW., Washington, DC. The
Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through
Friday, excluding legal holidays. The telephone number for the Public
Reading Room is<PHONE_NUMBER>, and the telephone number for the EPA
Docket Center is<PHONE_NUMBER>.

FOR FURTHER INFORMATION CONTACT: Ms. Mary Johnson, Energy Strategies
Group, Sector Policies and Programs Division (D243-01), Environmental
Protection Agency, Research Triangle Park, North Carolina 27711;
telephone number:<PHONE_NUMBER>; fax number:<PHONE_NUMBER>; e-mail
address<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION: Organization of This Document. The following
outline is provided to aid in locating information in this preamble.
I. General Information
    A. Does the proposed action apply to me?
    B. What should I consider as I prepare my comments?
II. Background
III. Summary
    A. Litigation and Proposed Remand Response

    B. Proposed Amendments (CAA Section 129(a)(5) 5-Year Review)
IV. Rationale
    A. Rationale for the Proposed Response to the Remand
    B. Rationale for the Proposed Amendments (CAA Section 129(a)(5)
5-Year Review)
V. Impacts of the Proposed Action for Existing Units
    A. What are the primary air impacts?
    B. What are the water and solid waste impacts?
    C. What are the energy impacts?
    D. What are the secondary air impacts?
    E. What are the cost and economic impacts?
VI. Impacts of the Proposed Action for New Units
VII. Relationship of the Proposed Action to Section 112(c)(6) of the
CAA
VIII. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review
    B. Paperwork Reduction Act
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination with
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children from
Environmental Health and Safety Risks
    H. Executive Order 13211: Actions That Significantly Affect
Energy Supply, Distribution or Use
    I. National Technology Transfer Advancement Act

I. General Information

A. Does the proposed action apply to me?

    Regulated Entities. Categories and entities potentially affected by
the proposed action are those which operate HMIWI. The NSPS and
emission guidelines for HMIWI affect the following categories of
sources:

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                                                                                Examples of
                   Category                                     NAICS Code                       potentially
                                                                                regulated entities
----------------------------------------------------------------------------------------------------------------
Industry.....................................  622110, 622310, 325411, 325412, 562213,       Private hospitals,
                                                611310.                                       other health care
                                                                                facilities,
                                                                                commercial
                                                                                research
                                                                                laboratories,
                                                                                commercial waste
                                                                                disposal
                                                                                companies, private
                                                                                universities.
Federal Government...........................  622110, 541710, 928110......................  Federal hospitals,
                                                                                other health care
                                                                                facilities, public
                                                                                health service,
                                                                                armed services.
State/local/Tribal Government................  622110, 562213, 611310......................  State/local
                                                                                hospitals, other
                                                                                health care
                                                                                facilities, State/
                                                                                local waste
                                                                                disposal services,
                                                                                State
                                                                                universities.
----------------------------------------------------------------------------------------------------------------

    This table is not intended to be exhaustive, but rather provides a
guide for readers regarding entities likely to be affected by the
proposed action. To determine whether your facility would be affected
by the proposed action, you should examine the applicability criteria
in 40 CFR 60.50c of subpart Ec and 40 CFR 60.32e of subpart Ce. If you
have any questions regarding the applicability of the proposed action
to a particular entity, contact the person listed in the preceding FOR
FURTHER INFORMATION CONTACT section.

B. What should I consider as I prepare my comments?

1. Submitting CBI
    Do not submit information that you consider to be CBI
electronically through http://www.regulations.gov or e-mail. Send or deliver

information identified as CBI to only the following address: Ms. Mary
Johnson, c/o OAQPS Document Control Officer (Room C404-02), U.S. EPA,
Research Triangle Park, NC 27711, Attention Docket ID No. EPA-HQ-OAR-
2006-0534. Clearly mark the part or all of the information that you
claim to be CBI. For CBI information in a disk or CD ROM that you mail
to EPA, mark the outside of the disk or CD ROM as CBI and then identify
electronically within the disk or CD ROM the specific information that
is claimed as CBI. In addition to one complete version of the comment
that includes information claimed as CBI, a copy of the comment that
does not contain the information claimed as CBI must be submitted for
inclusion in the public docket. Information marked as CBI will not be
disclosed except in accordance with procedures set forth in 40 CFR part
2.
    If you have any questions about CBI or the procedures for claiming
CBI, please consult the person identified in the FOR FURTHER
INFORMATION CONTACT section.
2. Tips for Preparing Your Comments
    When submitting comments, remember to:
    a. Identify the rulemaking by docket number and other identifying
information (subject heading, Federal Register date and page number).
    b. Follow directions. The EPA may ask you to respond to specific
questions or organize comments by referencing a Code of Federal
Regulations (CFR) part or section number.
    c. Explain why you agree or disagree; suggest alternatives and
substitute language for your requested changes.
    d. Describe any assumptions and provide any technical information
and/or data that you used.
    e. If you estimate potential costs or burdens, explain how you
arrived at your estimate in sufficient detail to allow for it to be
reproduced.
    f. Provide specific examples to illustrate your concerns, and
suggest alternatives.
    g. Explain your views as clearly as possible, avoiding the use of
profanity or personal threats.
    h. Make sure to submit your comments by the comment period deadline
identified in the preceding section titled DATES.
3. Docket
    The docket number for the proposed action regarding the HMIWI NSPS
(40 CFR part 60, subpart Ec) and emission guidelines (40 CFR part 60,
subpart Ce) is Docket ID No. EPA-HQ-OAR-2006-0534.
4. Worldwide Web (WWW)
    In addition to being available in the docket, an electronic copy of
this proposed action is available on the WWW through the Technology
Transfer Network Web site (TTN Web). Following signature, EPA posted a
copy of the proposed action on the TTN's policy and guidance page for
newly proposed or promulgated rules at http://www.epa.gov/ttn/oarpg.

The TTN provides information and technology exchange in various areas
of air pollution control.

II. Background

    Section 129 of the CAA, entitled ``Solid Waste Combustion,''
requires EPA to develop and adopt NSPS and emission guidelines for
solid waste incineration units pursuant to CAA sections 111 and 129.
Sections 111(b) and 129(a) of the CAA (NSPS program) address emissions
from new HMIWI units, and CAA sections 111(d) and 129(b) (emission
guidelines program) address emissions from existing HMIWI units. The
NSPS are directly enforceable Federal regulations. The emission
guidelines are not directly enforceable but, rather, are implemented by
State air pollution control agencies through sections 111(d)/129 State
plans.
    An HMIWI is defined as any device used to burn hospital waste or
medical/

infectious waste. Hospital waste means discards generated at a
hospital, and medical/infectious waste means any waste generated in the
diagnosis, treatment, or immunization of human beings or animals, in
research pertaining thereto, or in the production or testing of
biologicals (e.g., vaccines, cultures, blood or blood products, human
pathological waste, sharps). Hospital/medical/infectious waste does not
include household waste, hazardous waste, or human and animal remains
not generated as medical waste. An HMIWI typically is a small, dual-
chamber incinerator that burns about 800 pounds per hour (lb/hr) of
waste. Smaller units burn as little as 13 lb/hr while larger units burn
as much as 3,700 lb/hr.
    Incineration of hospital/medical/infectious waste causes the
release of a wide array of air pollutants, some of which exist in the
waste feed material and are released unchanged during combustion, and
some of which are generated as a result of the combustion process
itself. These pollutants include particulate matter (PM); heavy metals,
including lead (Pb), cadmium (Cd), and mercury (Hg); toxic organics,
including chlorinated dibenzo-p-dioxins/dibenzofurans (CDD/CDF); carbon
monoxide (CO); nitrogen oxides (NOX); and acid gases,
including hydrogen chloride (HCl) and sulfur dioxide (SO2).
In addition to the use of good combustion control practices, HMIWI
units are typically controlled by wet scrubbers or dry sorbent
injection fabric filters (dry scrubbers).
    Combustion control includes the proper design, construction,
operation, and maintenance of HMIWI to destroy or prevent the formation
of air pollutants prior to their release to the atmosphere. Test data
indicate that as secondary chamber residence time and temperature
increase, emissions decrease. Combustion control is most effective in
reducing CDD/CDF, PM, and CO emissions. The 0.25-second combustion
level includes a minimum secondary chamber temperature of 1700 [deg]F
and a 0.25-second secondary chamber residence time. These combustion
conditions are typical of older HMIWI. The 1-second combustion level
includes a minimum secondary chamber temperature of 1700 [deg]F and
residence time of 1 second. These combustion conditions are typical of
newer HMIWI. Compared to 0.25-second combustion, 1-second combustion
will achieve substantial reductions in CDD/CDF and CO emissions, and
will provide some control of PM, but will not reduce emissions of acid
gases (HCl and SO2), NOX, or metals (Pb, Cd, and
Hg). The 2-second combustion level includes a minimum secondary chamber
temperature of 1800 [deg]F and residence time of 2 seconds. These
combustion conditions will provide additional control of CDD/CDF, CO,
and PM, but will not reduce emissions of acid gases (HCl and
SO2), NOX, or metals (Pb, Cd, and Hg). The 2-
second combustion conditions are considered to be the best level of
combustion control (i.e., good combustion) that is applied to HMIWI.
Wet scrubbers and dry scrubbers provide control of PM, CDD/CDF, HCl,
and metals, but do not influence CO, SO2 (at the low
concentrations emitted by HMIWI units), or NOX; in fact,
there are no technologies currently used by HMIWI that will
consistently reduce SO2 or NOX emissions. (See
Legacy Docket ID No. A-91-61, item II-A-111; 60 FR 10669, 10671-10677;
and 61 FR 31742-31743.)
    On September 15, 1997, EPA adopted NSPS (40 CFR part 60, subpart
Ec) and emission guidelines (40 CFR part 60, subpart Ce) for entities
which operate HMIWI. The NSPS and emission guidelines are designed to
reduce air pollution emitted from new and existing HMIWI, including
HCl, CO, Pb, Cd, Hg, PM, CDD/CDF (total, or 2,3,7,8-tetrachlorinated
dibenzo-p-dioxin toxic equivalent (TEQ)), NOX, SO2, and
opacity. The NSPS apply to HMIWI for which construction began after
June 20, 1996, or for which modification began after March 16, 1998.
The NSPS became effective on March 16, 1998, and its requirements apply
as of that date or at start-up of a HMIWI unit, whichever is later. The
emission guidelines apply to HMIWI for which construction began on or
before June 20, 1996, and required compliance by September 2002.
    CAA section 129 requires EPA to establish technology-based emission
standards that reflect levels of control EPA determines are achievable
for new and existing units, after considering costs, non-air quality
health and environmental impacts, and energy requirements associated
with the implementation of the standards.
    In setting forth the methodology EPA must use to establish the
technology-based performance standards and emissions guidelines, CAA
section 129(a)(2) provides that standards ``applicable to solid waste
incineration units promulgated under section 111 and this section shall
reflect the maximum degree of reduction in emissions of [certain listed
air pollutants] that the Administrator, taking into consideration the
cost of achieving such emission reduction, and any non-air quality
health and environmental impacts and energy requirements, determines is
achievable for new and existing units in each category.'' This level of
control is referred to as a maximum achievable control technology, or
MACT standard.
    In promulgating a MACT standard, EPA must first calculate the
minimum stringency levels for new and existing solid waste incineration
units in a category, generally based on levels of emissions control
achieved or required to be achieved by the subject units. The minimum
level of stringency is called the MACT floor, and CAA section 129(a)(2)
provides that the ``degree of reduction in emissions that is deemed
achievable for new units in a category shall not be less stringent than
the emissions control that is achieved in practice by the best
controlled similar unit, as determined by the Administrator. Emissions
standards for existing units in a category may be less stringent than
standards for new units in the same category but shall not be less
stringent than the average emissions limitation achieved by the best
performing 12 percent of units in the category.''
    The minimum stringency requirements form the first and least
stringent regulatory option EPA must consider in the determination of
MACT for a source category. EPA must also determine whether to control
emissions ``beyond the floor,'' after considering the costs, non-air
quality health and environmental impacts, and energy requirements of
such more stringent control. These are the two steps EPA took in the
1997 HMIWI rulemaking. Finally, every 5 years after adopting a MACT
standard under section 129, CAA section 129(a)(5) requires EPA to
review and, if appropriate, revise the incinerator standards. In
addition to responding to the Court's remand in Sierra Club v. EPA, 167
F.3d 658 (D.C. Cir. 1999), this proposed action includes our first set
of proposed revisions to the HMIWI standards, also known as the 5-year
review.

III. Summary

A. Litigation and Proposed Remand Response

1. What was EPA's general methodology for determining MACT?
    The methodology used to determine MACT is similar for source
categories under sections 112 and 129 of the CAA. However, because each
source category is unique and the data available to determine the
performance capabilities of technology can vary from one source
category to another, the basic methodology must be adapted to fit the

source category in question. As the Court pointed out in the HMIWI
litigation, it ``generally defer[s] to an agency's decision to proceed
on the basis of imperfect scientific information, rather than to
`invest the resources to conduct the perfect study.' '' Sierra Club v.
EPA, 167 F.3d at 662.
    In general, all MACT analyses involve an assessment of the air
pollution control systems or technologies used by the better performing
units in a source category. The technology assessment can be based
solely on actual emissions data, on knowledge of the air pollution
control in place in combination with actual emissions data, or on State
regulatory requirements, which give an indication of the actual
performance of the regulated units. For each source category, the
assessment of the technology involves a review of actual emissions data
with an appropriate accounting for emissions variability. Where there
is more than one method or technology to control emissions, the
analysis results in a series of potential regulations (called
regulatory options), one of which is selected as MACT.
    The first regulatory option considered by EPA must be at least as
stringent as the CAA's minimum stringency requirements. However, MACT
is not necessarily the least stringent regulatory option. EPA must
examine more stringent regulatory options to determine MACT. Unlike the
minimum stringency requirements, EPA must consider various impacts of
the more stringent regulatory options in determining MACT. Only if the
more stringent regulatory options are considered to have unreasonable
impacts does EPA select the first ``floor-based'' regulatory option as
MACT.
    As stated earlier, the CAA requires that MACT for new sources be no
less stringent than the emissions control achieved in practice by the
best controlled similar unit. After EPA's assessment of technology, EPA
determines the best control currently in use for a given pollutant and
establishes one potential regulatory option at the emission level
achievable by that control. More stringent potential regulatory options
might reflect controls used on other sources that could be applied to
the source category in question.
    For existing sources, the CAA requires that MACT be no less
stringent than the average emissions limitation achieved by the best
performing 12 percent of units in a source category. EPA must determine
some measure of the average emissions limitation achieved by the best
performing 12 percent of units to form the least stringent regulatory
option. Sometimes, a direct calculation of the actual emissions values
from the best performing 12 percent of sources provides the basis for
this regulatory option. More often, EPA determines the technology used
by the average source in the best performing 12 percent of sources and
establishes the floor based on the technology assessment for that
average source. More stringent regulatory options reflect other
technologies capable of achieving better performance.
2. What was EPA's methodology in the 1997 HMIWI rulemaking?
    On February 27, 1995, EPA published a notice of proposed rulemaking
regarding emissions standards for HMIWI units (60 FR 10654). The
proposal was the result of several years of reviewing available
information. During the public comment period for the proposal, EPA
received over 700 letters, some of which contained new information or
indicated that the commenters were in the process of gathering more
information for EPA to consider. The new information led EPA to
consider the need for numerous changes to the proposed rule, and on
June 20, 1996, the Agency published a re-proposal (61 FR 31736).
Following an additional public comment period, EPA published the final
rule on September 15, 1997 (62 FR 48348).
    During the data-gathering phase of developing the 1995 proposal,
EPA found it difficult to obtain an accurate count of the thousands of
HMIWI units nationwide, or to find HMIWI units with add-on air
pollution control systems in place. A few HMIWI units with combustion
control were tested to assess performance of combustion control in
reducing emissions. One unit with a wet scrubber, and a few units with
dry scrubbing systems were tested to determine performance capabilities
of add-on controls. (See 61 FR 31738.)
    Altogether, data were available from only 7 out of the estimated
then-operating 3,700 existing HMIWI units (60 FR 10674). Because EPA
was under a court-ordered deadline to propose and adopt standards for
HMIWI that did not provide sufficient time to collect more actual
emissions data (see consent decree entered in Sierra Club v. EPA, Nos.
CV-92-2093 and CV-93-0284 (E.D.N.Y.)), EPA proceeded to develop the
regulations with the existing data, as described below. However, EPA
specifically requested comment on EPA's MACT determinations and on
EPA's conclusions about the performance capabilities of air pollution
control technologies on HMIWI in light of the relatively small database
(60 FR 10686).
    a. EPA's Methodology for New HMIWI. In determining the least
stringent regulatory option allowed by the CAA for new HMIWI, EPA first
examined the data available for various air pollution control
technologies applied to HMIWI to determine the performance capabilities
of the technologies (i.e., the achievable emission limitations) (60 FR
10671-73, 61 FR 31741-43). To determine the performance capabilities,
EPA grouped all of the test data by control technology and established
the numerical value for the achievable emission limitations somewhat
higher than the highest test data point for each particular control
technology. (See Legacy Docket ID No. A-91-61, items IV-B-46, 47, 48,
and 49.) Following the determination of performance capability, EPA
identified the best control technology for each air pollutant for each
subcategory of HMIWI, and established the numerical values for the
least stringent regulatory option at the achievable emission limitation
associated with that particular control technology. (See 60 FR 10673;
Legacy Docket ID No. A-91-61, item IV-B-38; 61 FR 31745-46.) Other,
more stringent, regulatory options were developed reflecting the actual
performance of other, more effective, control technologies (61 FR
31766-68).
    As stated in the 1996 re-proposal, the least stringent regulatory
option for new large HMIWI units (units with maximum waste burning
capacity of more than 500 lb/hr) was based on good combustion (i.e., 2-
second combustion level) and a combination of two control technologies,
high-efficiency wet scrubbers and dry injection/fabric filter dry
scrubbers with carbon (61 FR 31746). New medium units (units with
maximum waste burning capacity of more than 200 lb/hr but less than or
equal to 500 lb/hr) would need to use good combustion and a combination
of two control technologies, high-efficiency wet scrubbers and dry
injection/fabric filter dry scrubbers without carbon, to meet the least
stringent regulatory option. Id. New small units (units with maximum
waste burning capacity of less than or equal to 200 lb/hr) would need
to use good combustion and a moderate-efficiency wet scrubber to meet
the least stringent regulatory option. Id.
    In EPA's final standards promulgated in 1997, EPA selected an
overall more stringent regulatory option for new HMIWI (62 FR 48365).
The final standards were based on emission limits achievable with good
combustion and a

moderate-efficiency wet scrubber for new small HMIWI, and good
combustion and a combined dry/wet control system with carbon for new
medium and large HMIWI. Id. These standards reflected the MACT floor
emissions levels for new small and large HMIWI, but were more stringent
than the MACT floor for new medium HMIWI. Id. EPA estimated that the
standards would reduce emissions from these units of HCl by up to 98
percent, PM and Pb by up to 92 percent, Cd by up to 91 percent, CDD/CDF
by up to 87 percent, Hg by up to 74 percent, and CO, SO2,
and NOX by up to 52 percent (62 FR 48366).
    b. EPA's Methodology for Existing HMIWI. For existing units, EPA
did not have sufficient emissions data to fully characterize the actual
emissions performance of the best performing 12 percent of existing
HMIWI, and, based exclusively on such data, EPA did not have a clear
indication of the technology used by the best 12 percent of units. As a
result, EPA used emission limits included in State regulations and
State-issued permits (hereinafter referred to as regulatory limits) as
surrogate information to determine emissions limitations achieved by
the best performing 12 percent of units in each subcategory (60 FR
10674). EPA believed this information could be expected to reliably
reflect levels of performance achieved on a continuous basis by better-
controlled units that must meet these limits or risk violating
enforceable requirements. EPA assumed that all HMIWI were achieving
their regulatory limits (60 FR 10674). Where there were regulatory
limits for more than 12 percent of units in a subcategory, the
regulatory limits were ranked from the most stringent to least
stringent, and the average of the regulatory limits for the top 12
percent of units in the subcategory was calculated. Id.; 61 FR 31744-
45. Where the number of units subject to specific emissions limitations
did not comprise 12 percent of the population in a subcategory, EPA
assumed those units with regulatory limits were the best performing
units, and the remaining units in the top 12 percent were assigned an
emission value associated with ``combustion control.'' (See 60 FR
10674; 61 FR 31745; Legacy Docket ID No. A-91-61, item IV-B-24 at 2.)
In previous Federal Register notices regarding HMIWI (60 FR 10654, 61
FR 31736, and 62 FR 48348), this level of control was referred to as
``uncontrolled,'' which is misleading because sources with combustion
control emit lesser amounts of CDD/CDF, CO, and PM. In the latter
situation described above, the average of the regulatory limits plus
enough combustion-controlled emission values to account for 12 percent
of units in the subcategory was calculated. (See Legacy Docket ID No.
A-91-61, item IV-B-24 at 2-4.)
    After calculating the averages of regulatory limits and combustion-
controlled emission values, EPA examined the resulting calculated
values to determine what level of air pollution control would be needed
to meet the calculated average values. (See 60 FR 10675-78; 61 FR
31755-56.) For many pollutants, the calculated averages presented no
clear indication of the type of air pollution control used by the best
performing units. However, the calculated values for three key
pollutants, PM, CO, and HCl, did provide a good indication of the type
of air pollution control used on the best performing 12 percent of
units. The level of air pollution control associated with the
calculated average values for PM, CO, and HCl formed the technical
basis of the least stringent regulatory option considered by EPA (61 FR
31756, Table 13). The emission limitations assigned to each pollutant
reflected the actual performance of the technology on which they were
based. Finally, EPA developed a series of regulatory options based on
progressively more stringent technologies and assigned emission
limitations to each regulatory option based on the actual performance
capabilities of the technologies (61 FR 31757, Table 14).
    As stated in the 1996 re-proposal, large existing units would need
to use good combustion and a high-efficiency wet scrubber to meet the
least stringent regulatory option, while medium existing units would
need to use good combustion and a moderate-efficiency wet scrubber,
although dry scrubbers could also be used with good combustion at large
and medium existing units (61 FR 31745). EPA further stated that its
inclination was to establish emission limitations for large and medium
existing units based on regulatory options representing the MACT floors
(61 FR 31778). Small existing units would need only to use good
combustion practices to meet the regulatory option representing the
MACT floor (61 FR 31745). With respect to small existing units, EPA
stated that it had no inclination with regard to which regulatory
option should be used to establish emission limitations and requested
comment on requiring use of good combustion and a low-efficiency wet
scrubber (61 FR 31778-79).
    In EPA's final standards promulgated in 1997, EPA selected an
overall more stringent regulatory option for existing HMIWI (62 FR
48371). The final standards were based on emission limits achievable
with good combustion and a low-efficiency wet scrubber for most
existing small HMIWI, good combustion and a moderate-efficiency wet
scrubber for existing medium HMIWI, and good combustion and a high-
efficiency wet scrubber for existing large HMIWI (62 FR 48371). The
final standards allow small HMIWI that meet certain rural criteria to
meet emissions limits achievable with good combustion alone. Id. These
standards reflected the MACT floor emissions levels for existing small
HMIWI meeting rural criteria, medium HMIWI, and large HMIWI, but were
more stringent than the MACT floor for most existing small HMIWI (i.e.,
non-rural) (62 FR 48371-72). The final standards for existing medium
and large HMIWI were structured so that either a dry scrubber or a wet
scrubber could be used to achieve the emission limits. EPA estimated
that the final emission guidelines would reduce emissions of CDD/CDF by
up to 97 percent, Hg by up to 95 percent, PM by up to 92 percent, Pb by
up to 87 percent, Cd by up to 84 percent, CO by up to 82 percent, HCl
by up to 98 percent, and SO2 and NOX by up to 30
percent (62 FR 48372). Table 1 of this preamble summarizes the emission
limits for the NSPS and emission guidelines promulgated in 1997.

                                Table 1.--Summary of Promulgated Emission Limits
----------------------------------------------------------------------------------------------------------------
                                                        Limit for existing
        Pollutant (units)             Unit Size \1\          HMIWI \2\             Limit for new HMIWI \2\
----------------------------------------------------------------------------------------------------------------
HCl (parts per million by volume  L, M, S.............  100 or 93%          15 or 99% reduction.
 (ppmv)).                                                reduction.
                                  SR..................  3,100.............  N/A.\3\
CO (ppmv).......................  L, M, S.............  40................  40
                                  SR..................  40................  N/A.
Pb (milligrams per dry standard   L, M................  1.2 or 70%          0.07 or 98% reduction.\3\
 cubic meter (mg/dscm)).                                 reduction.

                                  S...................  1.2 or 70%          1.2 or 70% reduction.
                                                         reduction.
                                  SR..................  10................  N/A.
Cd (mg/dscm)....................  L, M................  0.16 or 65%         0.04 or 90% reduction.
                                                         reduction.
                                  S...................  0.16 or 65%         0.16 or 65% reduction.
                                                         reduction.
                                  SR..................  4.................  N/A.
Hg (mg/dscm)....................  L, M, S.............  0.55 or 85%         0.55 or 85% reduction.
                                                         reduction.
                                  SR..................  7.5...............  N/A.
PM (grains per dry standard       L...................  0.015.............  0.015
 cubic foot (gr/dscf)).
                                  M...................  0.03..............  0.015
                                  S...................  0.05..............  0.03.
                                  SR..................  0.086.............  N/A.
CDD/CDF, total (nanograms per     L, M................  125...............  25
 dry standard cubic meter (ng/
 dscm)).
                                  S...................  125...............  125
                                  SR..................  800...............  N/A.
CDD/CDF, TEQ (ng/dscm)..........  L, M................  2.3...............  0.6
                                  S...................  2.3...............  2.3
                                  SR..................  15................  N/A.
NOX (ppmv)......................  L, M, S.............  250...............  250
                                  SR..................  250...............  N/A.
SO2 (ppmv)......................  L, M, S.............  55................  55
                                  SR..................  55................  N/A.
----------------------------------------------------------------------------------------------------------------
\1\ L = Large; M = Medium; S = Small; SR = Small Rural
\2\ All emission limits are measured at 7 percent oxygen.
\3\ Not applicable.

    c. Compliance by HMIWI. At the time of promulgation (September
1997), EPA estimated that there were approximately 2,400 HMIWI
operating in the United States. Those units combusted approximately 830
thousand tons of hospital/medical/infectious waste annually. Of those
existing HMIWI, about 48 percent were small units, 29 percent were
medium units, and 20 percent were large units. About 3 percent of the
HMIWI were commercial units. EPA projected that no new small or medium
HMIWI would be constructed, and that up to 60 new large units and 10
new commercial units would be constructed.
    After shutdown of approximately 97 percent of the 2,400 HMIWI that
were operating in 1997, there are currently 72 existing HMIWI at 67
facilities. Additionally, only 4 new HMIWI at 3 facilities began
operation following the 1997 rulemaking. These 76 existing and new
units are estimated to combust approximately 165 thousand tons of waste
annually. Of the 72 existing HMIWI subject to the emission guidelines,
44 are large units, 20 are medium units, and 8 are small units (6 of
which meet the rural criteria). Twenty-one percent of the existing
HMIWI are commercially owned. Of the four new HMIWI, three are large
units, and one is a medium unit. Two of the new units are county-owned
but accept waste from other sources, similar to commercial units. The
actual emissions reductions achieved as a result of implementation of
the standards exceeded the 1997 projections for all nine of the
regulated pollutants. A comparison of the estimated pollutant
reductions versus the actual reductions is presented in Table 2 of this
preamble.

  Table 2.--Comparison of Estimated Pollutant Reductions Versus Actual
                          Pollutant Reductions
------------------------------------------------------------------------
                                       Estimated
                                       emissions       Actual emissions
            Pollutant                 reduction,      reduction, percent
                                        percent               \1\
------------------------------------------------------------------------
HCl.............................  98................                99.2
CO..............................  75 to 82..........                98.1
Pb..............................  80 to 87..........                98.7
Cd..............................  75 to 84..........                99.0
Hg..............................  93 to 95..........                99.0
PM..............................  88 to 92..........                98.1
CDD/CDF, total..................  96 to 97..........                99.5
CDD/CDF, TEQ....................  95 to 97..........                99.6
NOX.............................  0 to 30...........                70.6
SO2.............................  0 to 30...........                92.6
------------------------------------------------------------------------
\1\ Reflects the effect of unit shutdowns as well as the effect of
  compliance with the promulgated standards.

3. What was the Sierra Club's challenge?
    On November 14, 1997, the Sierra Club and the Natural Resources
Defense Council (Sierra Club) filed suit in the U.S. Court of Appeals
for the District of Columbia Circuit (the Court). The Sierra Club
claimed that EPA had violated CAA section 129 by setting emission
standards for HMIWI under CAA sections 129 and 111 that are less
stringent than the statutory minimum stringency required by section
129(a)(2); that EPA had violated section 129 by not including mandatory
pollution prevention or waste minimization requirements in the HMIWI
standards; and that EPA had not adequately considered the non-air
quality health and environmental impacts of the standards. For new
units, the Sierra

Club argued that to satisfy the statutory phrase ``best controlled
similar unit'' in CAA section 129(a)(2), EPA should have identified the
single best performing unit in each subcategory and based the MACT
floor for that subcategory on that particular unit's performance,
rather than consider the performance of other units using the same
technology. The Sierra Club also argued that EPA erroneously based the
new unit floors on the emissions of the worst performing unit using a
particular technology. Regarding existing units, the Sierra Club
claimed that the plain meaning of CAA section 129(a)(2)'s words,
``average emissions limitation achieved by the best performing 12
percent of units,'' precludes the use of regulatory data, and claimed
that the legislative history of section 129(a)(2) reflects
congressional intent to prohibit EPA from relying on regulatory data.
Moreover, the Sierra Club claimed that, for HMIWI, using regulatory
data was impossible because such data existed for fewer than 12 percent
of units, and because doing so would impermissibly import an
achievability requirement into the unit floor determination. Finally,
the Sierra Club argued that EPA failed to require HMIWI units to
undertake programs to reduce the Hg and chlorinated plastic in their
waste streams, in violation of CAA section 129(a)(3), and that EPA
failed to consider the fact that CDD/CDF and Hg from HMIWI can
contaminate water, sediment, and soil, and can bioaccumulate in food,
in violation of the CAA's requirement that EPA consider non-air quality
impacts of setting HMIWI emissions standards.
4. What was the Court's ruling?
    On March 2, 1999, the Court issued its opinion in Sierra Club v.
EPA, 167 F.3d 658 (D.C. Cir. 1999). While the Court rejected the Sierra
Club's claims regarding pollution prevention and non-air quality
impacts, and rejected the Sierra Club's statutory arguments under CAA
section 129, the Court remanded the rule to EPA for further explanation
regarding how EPA derived the MACT floors for new and existing HMIWI
units. Furthermore, the Court did not vacate the regulations, stating
that ``[i]t is possible that EPA may be able to explain [EPA's basis
for the standards]'' in response to the concerns raised by the Court.
Id., at 664. The regulations remain in effect during the remand.
    a. The Court's Ruling on New Units. In response to the Sierra
Club's claims regarding EPA's treatment of new units, the Court opined
that ``EPA would be justified in setting the floors at a level that is
a reasonable estimate of the performance of the `best controlled
similar unit' under the worst reasonably foreseeable circumstances [* *
*]. It is reasonable to suppose that if an emissions standard is as
stringent as `the emissions control that is achieved in practice' by a
particular unit, then that particular unit will not violate the
standard. This only results if `achieved in practice' is interpreted to
mean `achieved under the worst foreseeable circumstances.' In National
Lime Ass'n v. EPA, 627 F.2d 416, 431 n. 46 (D.C. Cir. 1980), we said
that where a statute requires that a standard be `achievable,' it must
be achievable `under most adverse circumstances which can reasonably be
expected to recur.' The same principle should apply when a standard is
to be derived from the operating characteristics of a particular
unit.'' Sierra Club v. EPA, 167 F.3d at 665. Thus, the Court refused to
embrace the Sierra Club's interpretation of CAA section 129(a)(2) as
requiring EPA to base the MACT floor on only the lowest emissions data
points observed (i.e., the level achieved by the best performing unit
for each pollutant).
    Relating to the Sierra Club's claim that EPA erred in considering
the emissions of units other than the best controlled unit, the Court
refused to rule that EPA's approach was unlawful, and posited that
``[p]erhaps considering all units with the same technology is
justifiable because the best way to predict the worst reasonably
foreseeable performance of the best unit with the available data is to
look at other units' performance. Or perhaps EPA reasonably considered
all units with the same technology equally `well-controlled,' so that
each unit with the best technology is a `best-controlled unit' even if
such units vary widely in performance.'' Sierra Club v. EPA, 167 F.3d
at 665.
    However, the Court concluded that the possible rationale for this
treatment of new units was not presented in the rulemaking record with
enough clarity for the Court to determine that EPA's ``path may
reasonably be discerned.'' Id. Moreover, the Court ruled that EPA had
``not explained why the phrase best controlled similar unit encompasses
all units using the same technology as the unit with the best observed
performance, rather than just that unit itself[. * * * W]e do not know
what interpretation the agency chose, and thus cannot evaluate its
choice.'' Sierra Club v. EPA, 167 F.3d at 665. The Court further
directed EPA to provide additional explanation regarding how the Agency
had calculated the upper bound of the best-controlled unit's
performance through rounding. Id.
    b. The Court's Ruling on Existing Units. With respect to existing
units, the Court first rejected the Sierra Club's ``claim that EPA's
decision to base the floors on regulatory data fails the first step of
the Chevron test. None of the Sierra Club's arguments establish that
Congress has `directly addressed' and rejected the use of regulatory
data.'' Id., at 661. After noting that the Sierra Club's statutory
objections to EPA's methodology appeared to be premised on ``the
counterintuitive proposition that an `achieved' level may not be
`achievable,' or, as Sierra Club puts it, may be better than `EPA's
notions about what is achievable,' '' id. at 662, the Court rejected
the Sierra Club's statutory objections to using regulatory data and
uncontrolled (i.e., combustion-controlled) emissions values. In other
words, the Court implicitly embraced EPA's view, under the principle of
National Lime, that the MACT floor is premised on the fundamental
concept that it be ``achievable,'' and should not be set at a level
that happens to be reflected by the lowest observed data point without
consideration of variability in operating conditions. Then, after
analyzing and rejecting the Sierra Club's arguments that the plain
language of the CAA and its legislative history forbid EPA's
methodology, the Court further ruled that it found ``nothing inherently
impermissible about construing the statute to permit the use of
regulatory data--if they allow EPA to make a reasonable estimate of the
performance of the top 12 percent of units. Indeed, the Sierra Club
conceded at oral argument that `a reasonable sample' may be used `to
find out what the best 12 percent are doing.' Oral Arg. Tr. at 11. To
be sure, the Sierra Club did not concede that permit data may be used.
But neither has it provided any basis for believing that state and
local limitations are such weak indicators of performance that using
them is necessarily an impossible stretch of the statutory terms. [* *
*] We therefore reject the Sierra Club's argument that the CAA forbids
the use of permit and regulatory data, and hold that the use of such
information is permissible as long as it allows a reasonable inference
as to the performance of the top 12 percent of units. Similarly, as
long as there is a reasonable basis for believing that some of the best
performing 12 percent of units are uncontrolled [i.e., combustion
controlled], EPA may include data points giving a reasonable
representation of the performance of those units in its averaging.''
Sierra Club v. EPA, 167 F.3d at 662, 663. Thus, the Court rejected all
of the Sierra Club's

arguments that the CAA prohibits EPA from basing MACT floor
determinations on permit or regulatory data, or on uncontrolled (i.e.,
combustion-controlled) emissions values.
    However, in addressing the manner in which EPA had specifically
relied upon such data in the HMIWI rulemaking, the Court concluded that
``[a]lthough EPA said that it believed the combination of regulatory
and uncontrolled [i.e., combustion-controlled] data gave an accurate
picture of the relevant [HMIWI]s` performance, it never adequately said
why it believes this. [* * *] First, EPA has said nothing about the
possibility that [HMIWI]s might be substantially overachieving the
permit limits. If this were the case, the permit limits would be of
little value in estimating the top 12 percent of [HMIWI]s' performance.
[* * *] Second, EPA never gave any reason for its apparent belief that
[HMIWI]s that were not subject to permit requirements did not deploy
emission controls of any sort. Unless there is some finding to this
effect, it is difficult to see the rationality in using `uncontrolled'
[i.e., combustion-controlled] data for the units that were not subject
to regulatory requirements.'' Id., at 663-664. The Court further
questioned the rationality of EPA using the highest of its test run
data in cases where the regulatory data did not alone comprise the
necessary 12 percent. Id., at 664.
    c. Subsequent Court Rulings Relevant to the Remand. Following the
Court's remand of the HMIWI MACT floors in Sierra Club v. EPA, the
Court issued a series of rulings in other cases addressing MACT rules
that bear on EPA's proposed response regarding HMIWI. The first of
these was Nat'l Lime Ass'n v. EPA, 233 F.3d 625 (D.C. Cir. 2000) (``NLA
II''), which involved challenges to EPA's MACT standards under CAA
section 112(d) for portland cement manufacturing facilities. In that
case, the Sierra Club argued that EPA should have based its estimate of
the top performing 12 percent of sources on actual emissions data, in
order to ``reasonably estimate'' such performance. But the Court
determined that EPA's approach of selecting ``the median [performing]
plant out of the best twelve percent of the plants for which it had
information and set[ting] the * * * floor at the level of the worst
performing plant in its databases using th[e same] technology [as the
median plant]'' had not been shown by the Sierra Club to reflect a not
reasonable estimate. NLA II, 233 F.3d at 633.
    In addition, the Court partially clarified its position regarding
EPA's approach of accounting for emissions performance variability by
setting floors at a level that reasonably estimates ``the performance
of the `best controlled similar unit' under the worst reasonably
foreseeable circumstances.'' Sierra Club, 167 F.3d at 665. In NLA II,
the Court stressed that EPA should not simply set floors at levels
reflecting the worst foreseeable circumstances faced by any worst
performing unit in a given source category, and that while considering
all units with the same technology may be justifiable because the best
way to predict the worst reasonably foreseeable performance of the best
unit with available data is to look at other units' performance, such
an approach would satisfy the CAA ``if pollution control technology
were the only factor determining emission levels of that HAP.'' NLA II,
233 F.3d at 633.
    In Cement Kiln Recycling Coalition v. EPA, 255 F.3d 855 (D.C. Cir.
2001) (``CKRC''), the Court again refined its view on when it is
appropriate for EPA to base MACT floors on the performance of air
pollution control technology. In that case, the Sierra Club challenged
EPA's MACT standards for hazardous waste combustors (HWC), and argued
that factors other than MACT technology influenced the emissions
performance of the best performing sources.
    The Court agreed that since EPA's record evidence in the HWC
rulemaking showed that factors besides MACT controls significantly
influenced HWC emission rates, ''emissions of the worst-performing MACT
source may not reflect what the best-performers actually achieve.''
CKRC, 255 F.3d at 864. EPA had claimed that MACT floors must be
achievable by all sources using MACT technology, and that to account
for the best-performing sources'' operational variability we had to
base floors on the worst performers'' emissions. But the Court stressed
that ``whether variability in the MACT control accurately estimates
variability associated with the best performing sources depends on
whether factors other than MACT control contribute to emissions[,]''
id., and that ``the relevant question here is not whether control
technologies experience variability at all, but whether the variability
experienced by the best-performing sources can be estimated by relying
on emissions data from the worst-performing sources using the MACT
control.'' Id., at 865.
    In the specific case of the HWC rule, the Court concluded that,
since record evidence showed that non-MACT factors influenced emissions
performance, EPA could not base floors simply on the worst-performing
MACT sources' emissions. Id., at 866. However, the Court also
reiterated that ``[i]f in the case of a particular source category or
HAP, the Agency can demonstrate with substantial evidence--not mere
assertions--that MACT technology significantly controls emissions, or
that factors other than the control have a negligible effect, the MACT
approach could be a reasonable means of satisfying the statute's
requirements.'' Id.
5. Are revisions to the emission limits being proposed in response to
the remand?
    Yes, the proposed response to the remand would revise some of the
emission limits in both the NSPS and emission guidelines. Relative to
the NSPS, the emission limits for CO, Pb, Cd, Hg, PM, and CDD/CDF would
be revised. Relative to the emission guidelines, the emission limits
for HCl, Pb, Cd, and CDD/CDF would be revised. EPA believes that the
revised emission limits being proposed as a result of its response to
the remand can be achieved with the same emission control technology
currently used by HMIWI. The proposed emission limits for the NSPS and
emission guidelines necessary to respond to the Court's remand are
summarized in Table 3 of this preamble. Note that in several cases,
further amendments to the emission limits are being proposed as a
result of our 5-year review under CAA section 129(a)(5). Those proposed
amendments are discussed in the following section of this preamble.

                     Table 3.--Summary of Proposed Emission Limits in Response to the Remand
----------------------------------------------------------------------------------------------------------------
                                                          Proposed remand
       Pollutant  (units)             Unit size \1\          limit for       Proposed remand limit for new HMIWI
                                                        existing HMIWI \2\                   \2\
----------------------------------------------------------------------------------------------------------------
HCl (ppmv)......................  L, M, S.............  78 or 93%           15\3\ or 99% reduction \3\.
                                                         reduction \3\.
                                  SR..................  3,100 \3\.........  N/A \4\.
CO (ppmv).......................  L, M, S.............  40 \3\............  32
                                  SR..................  40 \3\............  N/A \4\.

Pb (mg/dscm)....................  L, M................  0.78 or 71%         0.060 or 98% reduction \3\.
                                                         reduction.
                                  S...................  0.78 or 71%         0.78 or 71% reduction.
                                                         reduction.
                                  SR..................  8.9...............  N/A \4\.
Cd (mg/dscm)....................  L, M................  0.11 or 66%         0.030 or 93% reduction.
                                                         reduction \3\.
                                  S...................  0.11 or 66%         0.11 or 66% reduction \3\.
                                                         reduction \3\.
                                  SR..................  4 \3\.............  N/A \4\.
Hg (mg/dscm)....................  L, M................  0.55 \3\ or 87%     0.45 or 87% reduction.
                                                         reduction.
                                  S...................  0.55 \3\ or 87%     0.47 or 87% reduction.
                                                         reduction.
                                  SR..................  6.6...............  N/A \4\.
PM (gr/dscf)....................  L...................  0.015 \3\.........  0.009
                                  M...................  0.030 \3\.........  0.009
                                  S...................  0.050 \3\.........  0.018
                                  SR..................  0.086 \3\.........  N/A \4\.
CDD/CDF, total (ng/dscm)........  L, M................  115...............  20
                                  S...................  115...............  111
                                  SR..................  800 \3\...........  N/A \4\.
CDD/CDF, TEQ (ng/dscm)..........  L, M................  2.2...............  0.53
                                  S...................  2.2...............  2.1
                                  SR..................  15 \3\............  N/A \4\.
NOX (ppmv)......................  L, M, S.............  250 \3\...........  225
                                  SR..................  250 \3\...........  N/A \4\
SO2 (ppmv)......................  L, M, S.............  55 \3\............  46
                                  SR..................  55 \3\............  N/A \4\.
----------------------------------------------------------------------------------------------------------------
\1\ L = Large; M = Medium; S = Small; SR = Small Rural
\2\ All emission limits are measured at 7 percent oxygen.
\3\ No change proposed.
\4\ Not applicable.

B. Proposed Amendments (CAA Section 129(a)(5) 5-Year Review)

    Section 129(a)(5) of the CAA requires EPA to conduct a review of
the NSPS and emissions guidelines at 5 year intervals and, if
appropriate, revise the NSPS and emission guidelines pursuant to the
requirements under sections 111 and 129 of the CAA. In conducting such
reviews, EPA attempts to assess the performance of and variability
associated with the installed emissions control equipment (and
developments in practices, processes and control technologies) and to
revise as necessary and appropriate the NSPS and emission guidelines.
In these reviews, EPA takes into account the currently installed
equipment and its performance and operational variability. As
appropriate, we also consider new technologies that have been
demonstrated to reliably control emissions from the source category. In
setting numerical emission limits from single, ``snap shot'' stack test
data, EPA must exercise technical judgment to ensure the achievability
of such limits over the course of anticipated operating conditions. EPA
has completed the 5-year review, and the proposed amendments discussed
below reflect the changes that EPA has determined are appropriate in
addition to the amendments that are necessary to respond to the Court's
remand. These proposed amendments do not reflect adoption of new
control technologies or processes, but do reflect more efficient
practices in operation of the control technologies that sources used in
order to meet the 1997 MACT standards.
    Following year 2002 compliance with the emission guidelines, EPA
gathered information on the performance levels actually being achieved
by HMIWI that were operating under the guidelines. After implementation
of the guidelines in 1997, approximately 94 percent of HMIWI shut down,
and 3 percent demonstrated eligibility for exemptions from the HMIWI
regulation. Those HMIWI that remained in operation either continued
operation with their existing configuration or were retrofitted with
add-on air pollution control devices in order to meet the standards.
The retrofits were completed on time, and the controls installed to
meet the required emission limitations were highly effective in
reducing emissions of all of the CAA section 129 pollutants emitted by
HMIWI. For those HMIWI, relative to a 1995 baseline, the emission
guidelines reduced organic emissions (CDD/CDF) by about 90 percent,
metals emissions (Pb, Cd, and Hg) by more than 80 percent, and acid gas
emissions (HCl and SO2) by more than 70 percent. Including
shutdowns and exemptions, nationwide HMIWI emissions of organics,
metals, and acid gases each decreased by about 99 percent or more
relative to a 1995 baseline. It should be noted that the original HMIWI
emission limits were based primarily on permit information and other
regulatory requirements, and not on actual performance or stack test
data. To this end, it was highly uncertain at promulgation what the
precise performance efficiency and day-to-day operational variability
associated with the promulgated regulatory requirements would yield.
Thus, the 2002 compliance test information provided the first
quantitative assessment of the performance of the installed control
equipment's ability to attain the NSPS and emissions guideline limits.
    The goal of the current technology review is to assess the
performance efficiency of the installed equipment and to ensure that
the emission limits reflect the performance of the technologies
required by the MACT standards. In addition, the review addresses
whether new technologies and processes and improvements in practices
have been demonstrated at sources subject to the emissions limitations.
EPA's intent for future technology reviews is to include similar
analyses that also assess risk along with new technologies. For the
current review, while new technologies have not yet been demonstrated
to reliably control emissions more efficiently at reasonable cost at
HMIWI units than those used to meet MACT, improvements in operational
practices

do support some additional revision of the standards, in order to
better reflect the best operation of the MACT controls.
    These proposed amendments would revise the NSPS and emission
guidelines, in some cases beyond the point needed to respond to the
Court's remand, based on the performance levels currently being
achieved by HMIWI. The revisions discussed in the following text apply
to both the NSPS and the emission guidelines, unless otherwise
specified.
1. Are revisions to the emission limits being proposed?
    Yes, the proposed amendments would revise the emission limits in
both the NSPS and emission guidelines. EPA's technology review
demonstrates that the proposed emission limits can be achieved with the
same emission control technology currently used by HMIWI. The proposed
emission limits for the NSPS and emission guidelines are summarized in
Tables 4 and 5 of this preamble.

                    Table 4.--Summary of Proposed 5-Year Review Emission Limits for New HMIWI
----------------------------------------------------------------------------------------------------------------
          Pollutant (units)                    Unit Size \1\                      Proposed Limit \2\
----------------------------------------------------------------------------------------------------------------
HCl (ppmv)...........................  L, M, S......................  15 \3\ or 99% reduction \3\.
CO (ppmv)............................  L, M, S......................  25
Pb (mg/dscm).........................  L, M.........................  0.060 or 99% reduction.
                                       S............................  0.64 or 71% reduction.
Cd (mg/dscm).........................  L, M.........................  0.0050 or 99% reduction.
                                       S............................  0.060 or 74% reduction.
Hg (mg/dscm).........................  L, M.........................  0.19 or 96% reduction.
                                       S............................  0.33 or 96% reduction.
PM (gr/dscf).........................  L, M.........................  0.0090.
                                       S............................  0.018.
CDD/CDF, total (ng/dscm).............  L, M.........................  16
                                       S............................  111
CDD/CDF, TEQ (ng/dscm)...............  L, M.........................  0.21
                                       S............................  2.0
NOX (ppmv)...........................  L, M, S......................  212
SO2 (ppmv)...........................  L, M.........................  21
                                       S............................  28
----------------------------------------------------------------------------------------------------------------
\1\ L = Large; M = Medium; S = Small
\2\ All emission limits are measured at 7 percent oxygen.
\3\ No change proposed.

                 Table 5.--Summary of Proposed 5-Year Review Emission Limits for Existing HMIWI
----------------------------------------------------------------------------------------------------------------
          Pollutant (units)                    Unit Size \1\                      Proposed Limit \2\
----------------------------------------------------------------------------------------------------------------
HCl (ppm)............................  L, M, S......................  51 or 94% reduction.
                                       SR...........................  398
CO (ppm).............................  All..........................  25
Pb (mg/dscm).........................  L, M, S......................  0.64 or 71% reduction.
                                       SR...........................  0.60
Cd (mg/dscm).........................  L, M, S......................  0.060 or 74% reduction.
                                       SR...........................  0.050
Hg (mg/dscm).........................  L, M, S......................  0.33 or 96% reduction.
                                       SR...........................  0.25\3\
PM (gr/dscf).........................  L............................  0.015
                                       M............................  0.030 \3\
                                       S............................  0.030
                                       SR...........................  0.030
CDD/CDF, total (ng/dscm).............  L, M, S......................  115
                                       SR...........................  800 \3\
CDD/CDF, TEQ (ng/dscm)...............  L, M, S......................  2.0
                                       SR...........................  15 \3\
NOX (ppmv)...........................  All..........................  212
SO2 (ppmv)...........................  All..........................  28
----------------------------------------------------------------------------------------------------------------
\1\ L = Large; M = Medium; S = Small; SR = Small Rural
\2\ All emission limits are measured at 7 percent oxygen.
\3\ No change proposed.

    As indicated by Table 5 of this preamble, the proposed emission
limits for Pb, Cd, and Hg for existing small rural HMIWI are more
stringent than those being proposed for existing large, medium, and
small HMIWI. We believe that this better emissions performance by
existing small rural HMIWI is a result of the waste stream of a small
rural hospital not including certain materials that are in the waste
stream of a non-rural hospital and that cause relatively higher Pb, Cd
and Hg emissions.
2. Are other amendments being proposed?
    The proposed amendments would also make the following changes based
on information received during implementation of the HMIWI NSPS and
emission guidelines and would

apply equally to the NSPS and emission guidelines, unless otherwise
specified.
    a. Performance Testing and Monitoring Amendments. The proposed
amendments would allow sources to use the results of previous emissions
tests to demonstrate compliance with the revised emission limits as
long as the sources certify that the previous test results are
representative of current operations. Only those sources whose previous
emissions tests do not demonstrate compliance with one or more revised
emission limits would be required to conduct another emissions test for
those pollutants (note that sources are already required to test for
HCl, CO, and PM on an annual basis). The proposed amendments would
require, for existing HMIWI, annual inspections of scrubbers and fabric
filters, and a one-time Method 22 visible emissions test of the ash
handling operations to be conducted during the next compliance test.
For new HMIWI, the proposed amendments would require CO continuous
emissions monitoring systems (CEMS), bag leak detection systems for
fabric-filter controlled units, annual inspections of scrubbers and
fabric filters, and Method 22 visible emissions testing of the ash
handling operations to be conducted during each compliance test. For
existing HMIWI, use of CO CEMS would be an approved alternative, and
specific language with requirements for CO CEMS is included in the
proposed amendments. For new and existing HMIWI, use of PM, HCl, multi-
metals, and Hg CEMS, and semi-continuous dioxin monitoring (continuous
sampling with periodic sample analysis) also are approved alternatives,
and specific language for these alternatives is included in the
proposed amendments.
    b. Other Amendments. The proposed amendments would revise the
definition of ``Minimum secondary chamber temperature'' to read
``Minimum secondary chamber temperature means 90 percent of the highest
3-hour average secondary chamber temperature (taken, at a minimum, once
every minute) measured during the most recent performance test
demonstrating compliance with the PM, CO, and dioxin/furan emission
limits.''
    The proposed amendments would require sources to submit, along with
each test report, a description of how operating parameters are
established during the initial performance test and subsequent
performance tests.
3. Is an implementation schedule being proposed?
    Yes; under the proposed amendments to the emission guidelines, and
consistent with CAA section 129, revised State plans containing the
revised emission limits and other requirements in the proposed
amendments would be due within 1 year after promulgation of the
amendments. That is, revised State plans would have to be submitted to
EPA 1 year after the date on which EPA promulgates revised standards.
    The proposed amendments to the emission guidelines then would allow
HMIWI units up to 3 years from the date of approval of a State plan,
but not later than 5 years after promulgation of the revised standards,
to demonstrate compliance with the amended standards. Consistent with
CAA section 129, EPA expects States to require compliance as
expeditiously as practicable. HMIWI units have already installed the
emission control equipment necessary to meet the proposed revised
limits, and EPA, therefore, anticipates that most State plans will
include compliance dates sooner than 5 years following promulgation of
the amendments. In most cases, the only changes necessary are to review
the revisions and adjust the emission monitoring and reporting
accordingly.
    In revising the emission limits in a State plan, a State has two
options. First, it could include both the current and the new emission
limits in its revised State plan, which allows a phased approach in
applying the new limits. That is, the State plan would make it clear
that the current emission limits remain in force and apply until the
date the new emission limits are effective (as defined in the State
plan). States whose HMIWI units do not find it necessary to improve
their performance in order to meet the new emission limits may want to
consider a second approach where the State would insert the new
emission limits in place of the current emission limits, follow
procedures in 40 CFR part 60, subpart B, and submit a revised State
plan to EPA for approval. If the revised State plan contains only the
new emission limits (i.e., the current emission limits are not
retained), then the new emission limits must become effective
immediately since the current limits would be removed from the State
plan.
4. Has EPA changed the applicability date of the 1997 NSPS?
    No; however, HMIWI may be treated differently under the amended
standards than they were under the 1997 standards in terms of whether
they are ``existing'' or ``new'' sources, and there will be new dates
defining what are ``new'' sources and imposing compliance deadlines
regarding any amended standards. The applicability date for the NSPS
units, with respect to the standards as promulgated in 1997, remains
June 20, 1996; however, units for which construction is commenced after
the date of this proposal, or modification is commenced on or after the
date 6 months after promulgation of the amended standards, would be
subject to more stringent NSPS emission limits than units for which
construction or modification was completed prior to those dates. Under
the proposed amendments, units that commenced construction after June
20, 1996, and on or before February 6, 2007, or that are modified
before the date 6 months after the date of promulgation of any revised
final standards, would continue to be or would become subject to the
NSPS emission limits that were promulgated in 1997 and that remain in
the 40 CFR part 60, subpart Ec NSPS, except where the revised emission
guidelines would be more stringent. In that case, HMIWI that are NSPS
units under the 1997 rule would also need to comply with the revised
emission guidelines for existing sources, by the applicable compliance
date for such existing sources. Similarly, emission guidelines units
under the 1997 rule would need to meet the revised emission guidelines
by the applicable compliance date for the revised guidelines. HMIWI
that commence construction after February 6, 2007 or that are modified
6 months or more after the date of promulgation of any revised
standards would have to meet the revised NSPS emission limits being
added to the subpart Ec NSPS and any remaining NSPS limits from the
1997 rule, as applicable, within 6 months after the promulgation date
of the amendments or upon startup, whichever is later.

IV. Rationale

A. Rationale for the Proposed Response to the Remand

    This action responds to the Court's remand by (1) further
explaining the reasoning processes by which EPA determined the MACT
floors and the MACT standards for new and existing HMIWI for the
portions of those processes that are being retained under our remand
response, and (2) explaining revisions to the processes, the MACT
floors, and the MACT standards for new and existing HMIWI that result
from our response to the remand.
1. New HMIWI
    The Court raised three issues with regard to EPA's treatment of the
MACT floor for new units and the achievable

emission limitations. First, the Court asked EPA to explain why the
floor was based on the highest emissions levels of the ``worst-
performing'' unit employing the MACT technology rather than on the
lowest observed emissions levels of the best performing unit using the
MACT technology. (See Sierra Club v. EPA, 167 F.3d at 665.) Second, the
Court requested further explanation of why EPA considered multiple
units employing the MACT technology, rather than identify the single
best-performing unit and basing the floor on that particular unit's
performance with that technology. Id. Third, the Court requested
further explanation of EPA's procedure for determining the achievable
emission limitation from the available data, where EPA selected a
numerical value somewhat higher than the highest observed data point.
The Court stated that EPA's procedure ``[m]ay be justifiable as a means
of reasonably estimating the upper bound of the best-controlled unit's
performance, but in the absence of agency explanation of both the
decision to increase the levels and the choice of method for
determining the increases, we are in no position to decide.'' Id.
    As discussed in detail below, for the first two issues, the Court
described potential rationale for EPA's method. However, because the
Court concluded that this rationale was not adequately presented in the
rulemaking record, the Court asked for further clarification by EPA. In
subsequent cases the Court further addressed these potential
rationales, and discussed under what circumstances they would and would
not be persuasive. In fact, the Court's potential rationale for EPA's
method reflects the principles used by EPA in determining the MACT
floor for new units and the achievable emission limitations for this
source category, and is the method that has been used by EPA throughout
most of the Agency's 30-year history in developing achievable
technology-based emission limitations for source categories in cases
where the application of control technology has been the only means by
which sources have limited emissions, and the variability of technology
performance is a critical factor in determining an emission
limitation's achievability. (See, e.g., American Iron and Steel Inst.
v. EPA, 115 F.3d 979, 1000 (D.C. Cir. 1997); BP Exploration & Oil,
Inc., v. EPA, 66 F.3d. 784, 794 (6th Cir. 1995); NRDC v. EPA, 790 F.2d
289, 299 (3d Cir. 1986); National Ass'n of Metal Finishers v. EPA, 719
F.2d 624, 659 (3d Cir. 1983); rev'd on other grounds sub nom, Chemical
Mfrs. Ass'n v. NRDC, 470 U.S. 116 (1985); American Petroleum Inst. v.
EPA, 661 F.2d 340, 347 n. 23 (5th Cir. 1981); Bunker Hill Co. v. EPA,
572 F.2d 1286, 1302 (9th Cir. 1977); Marathon Oil Co. v. EPA, 564 F.2d
1253, 1266-67 (9th Cir. 1977); FMC v. Train, 639 F.2d 973, 985-86 (4th
Cir. 1976).) As discussed elsewhere in this preamble, in CKRC the Court
stressed that where record evidence suggests that factors other than
application of control technology influence emissions, EPA will not be
able to demonstrate ``that floors based on the worst-performing MACT
sources' emissions represent `a reasonable estimate of the performance
of the [best-performing] units.' '' CKRC, 255 F.3d at 866, quoting
Sierra Club, 167 F.3d at 662. However, the Court reiterated that where
EPA's record demonstrates that MACT technology significantly controls
emissions, or that factors other than the control have a negligible
effect, the approach of accounting for variability by basing the floor
on the highest emissions resulting from a source using MACT technology
``could be a reasonable means of satisfying the statute's
requirements.'' CKRC, at 866.
    a. Applicability of National Lime to CAA Section 129. CAA section
129(a)(3) states that ``[s]tandards under section 111 and this section
applicable to solid waste incineration units shall be based on methods
and technologies for removal or destruction of pollutants before,
during, or after combustion [* * *].'' This language requires that such
a standard be based on the degree of reduction in air pollutant
emissions that can be achieved through application of a particular
method of pollution control, and any other factors that record evidence
shows significantly affect emissions performance. Much like the
language in CAA sections 111 and 129 governing the HMIWI standards,
Congress has used similar language in other statutes to direct adoption
of technology-based standards. (See, e.g., CAA section 169(3) defining
``best available control technology''; Clean Water Act section
301(b)(2)(A), for ``best available technology economically achievable''
or ``BAT'' standards; Clean Water Act section 304(b)(1) for ``best
practicable technology'' or ``BPT'' standards.)
    As the Court has stated, ``[t]echnology-based provisions [in the
CAA] require EPA to promulgate standards only after finding that the
requisite technology exists or may be feasibly developed. Absolute
standards, on the other hand, require compliance with statutorily
prescribed standards and time tables, irrespective of present
technologies.'' (See NRDC v. Reilly, 983 F.2d 259, 268 (D.C. Cir. 1993)
(holding that elimination of feasibility requirements and specification
of particular control systems indicated that congressional amendment of
CAA section 202(a)(6) resulted in an ``absolute'' standard).) MACT
standards under CAA sections 111 and 129 are ``technology-based,''
rather than ``absolute'' standards. The legislative history to the 1990
CAA Amendments clearly shows that Congress intended the MACT standards
to be technology-based. (See I A Legislative History, at 863 (Senator
Durenberger referring to ``the MACT technology-based standards'' in
debates on the bill reported by the Conference Committee); id., at 1128
(Senator Dole explaining that changes made to CAA section 129 in the
Conference Committee ``make the technology test more closely
approximate the role of the NSPS''); S. Rep. No. 101-228, at 133-134
(1989) (referring to CAA section 112 MACT standards as ``technology-
based standards'' and noting that technology-based effluent standards
under the Clean Water Act served as a model for the new MACT
standards).)
    CAA section 129 does not specify a type of control technology for
HMIWI, but instead requires EPA to develop floor levels already
achieved in practice by one or more units, and then issue standards
that EPA determines are ``achievable'' for units in that source
category. As the Court stated in National Lime Ass'n v. EPA (627 F.2d
416, 431 n. 46 (D.C. Cir. 1980)) (``NLA I''), and restated in Sierra
Club, ``where a statute requires a standard to be achievable, it must
be achievable `under most adverse circumstances which can reasonably be
expected to recur.' '' (See Sierra Club, 167 F.3d at 665.) In other
words, ``EPA would be justified in setting floors at a level that is a
reasonable estimate of the performance of the `best controlled similar
unit'' under the worst reasonably foreseeable circumstances[.]'' Id.
This concept of ``worst reasonably foreseeable circumstances'' is
fundamental in developing achievable technology-based emission
limitations, since, once the standard is in force, sources will be
expected to comply with it at all times by relying on the technology
that formed the basis for EPA's determination that the promulgated
emissions limitation is achievable. As the Court stated in Sierra Club,
`[i]t is reasonable to suppose that if an emissions standard is as
stringent as `the emissions control that is achieved in practice' by a
particular unit, then that particular unit will not violate the
standard. This only results if `achieved in practice' is interpreted to

mean `achieved under the worst foreseeable circumstances.' '' Id.
    EPA agrees with the Court that, in order to satisfy the
requirements of NLA I, ``[t]he same principle should apply when a
standard is to be derived from the operating characteristics of a
particular unit[,]'' as is the case under CAA section 129(a)(2). Id.
CAA section 129(a)(2) requires that the new unit MACT floor be ``not
less stringent than the emissions control that is achieved in practice
by the best controlled similar unit, as determined by the
Administrator.'' It would have been unreasonable for EPA to base the
MACT floors solely on the lowest levels of emissions observed without
an assessment of whether those observed levels could be met on a
continuous basis, and the CAA and its legislative history provide no
support in deviating from the general practice EPA has followed in the
wake of NLA I. In a report on H.R. 3030, the House Committee on Energy
and Commerce explained that ``MACT is not intended to require unsafe
control measures, or to drive sources to the brink of shutdown.'' (See
H.R. Rep. No. 101-490, pt. 1, at 328 (1990).) This view is consistent
with NLA I, which involved challenges to standards EPA promulgated
under section 111 of the CAA and is particularly applicable to the
HMIWI rulemaking under CAA section 129, since this rule has its basis
in authority in both section 129 and section 111. (See CAA section
129(a)(1)(A) and (C).)
    Moreover, interpreting CAA section 129 as subject to the principles
of NLA I appropriately notes the critical distinction between a level
of emissions that has been continuously achieved through performance
using control technology, and one that has been observed at a single
point in time. A level that has been continuously achieved is capable
of being met under most conditions which can reasonably be expected to
recur because variability in operating conditions is taken into
account. Such a level best effectuates Congress' intent because it
ensures that the MACT floor will result in reduced emissions without
forcing sources to shut down. A lowest observed emission level,
however, is not representative of a unit's performance under most
conditions which can reasonably be expected, and may be impossible to
achieve on a regular, let alone continuous, basis. While an observed
lowest emissions level may be appropriate for use in determining
whether a source is in compliance with an emission standard that must
be continuously met, it is not an appropriate level upon which to base
the minimum stringency level of such a standard.
    In addition, Congress' use of the phrases ``as determined by the
Administrator'' and ``achieved in practice'' in CAA section 129(a)(2)
in the directive to establish MACT floors shows that Congress expected
EPA to consider variability in operating conditions and other relevant
factors in the Agency's determinations. The term ``practice'' is
defined as ``[r]epeated or customary action; habitual performance; a
succession of acts of a similar kind; custom; usage.'' (See Black's Law
Dictionary 1172 (6th ed. 1990).) Thus, achieved in ``practice'' means
achieved on a repeated, customary, or habitual basis. Under the
statutory mandate that the level ``achieved in practice'' be
``determined by the Administrator,'' EPA must exercise its judgment,
based on an evaluation of the relevant factors and available data, to
determine the level of emissions control that can be customarily
achieved using the relied-upon technology under variable conditions.
Merely locating the lowest emissions data point and setting the MACT
standard at that level would not constitute a considered
``determination by the Administrator'' as to what has been ``achieved
in practice.'' (See, e.g., Senate Debate on Conference Report, 10-26-
90, reprinted in I A Legislative History of CAA Amendments of 1990,
103d Cong., 1st Sess. at 1128-1129 (Comm. Print 1993) (exchange between
Senators Dole and Durenberger confirming that the phrase ``achieved in
practice'' accounts for the distinction between research-type pollution
control systems and systems that are ``economically viable for
widespread use,'' and stressing that MACT floors should rely upon
technologies that can ``stand the rigors of day to day operations'').)
    Ultimately, NLA I is controlling because the case addressed how
standards must be set in the face of variable operating conditions, and
involved one of the same provisions of the CAA, section 111, under
which the HMIWI rule was promulgated. NLA I held that EPA is required
to use data that is representative of emissions that could be achieved
in the industry as a whole. (See 627 F.2d at 433.) In developing the
standards at issue in that case, EPA relied upon tests of the emissions
from particular units to determine the level of emissions control that
was achievable across the entire industry. The Court directed EPA to
identify ``variable conditions that may contribute substantially to the
amount of emissions, or otherwise affect the efficiency of the
emissions control systems.'' Id. The Court then stated that ``where
test results are relied upon, it should involve the selection or use of
test results in a manner which provides some assurance of the
achievability of the standard for the industry as a whole, given the
range of variable factors found relevant to the standards'
achievability.'' Id. This does not mean that EPA must test every plant,
but it does mean that ``due consideration must be given to the possible
impact on emissions of recognized variations in operations and some
rationale offered for the achievability of the promulgated standards
given the tests conducted and the relevant variables identified.'' Id.,
at 434. Thus, applying NLA I to the HMIWI rule adopted under CAA
sections 111 and 129, it is really a misnomer to characterize EPA as
basing the MACT floor on the emissions of the ``worst performing'' unit
using the technology in question, since that unit's level of emissions
necessarily more closely represents the level ``achieved in practice''
by the given technology than would the lowest emissions level observed
at a source using that ``best'' technology.
    b. Variability Between Facilities or Units. In remanding the NSPS
at issue in NLA I, the Court noted that its decisions under CAA section
111 ``evince a concern that variables be accounted for, that the
representativeness of test conditions be [sic] ascertained, that the
validity of tests be assured and the statistical significance of
results be determined.'' (See NLA I, 627 F.2d at 452-53.) (See, also,
Portland Cement Ass'n v. Ruckelshaus, 486 F.2d 375, 396 (D.C. Cir.
1973), cert. denied, 417 U.S. 921 (1974).) When floors and standards
are developed based on emissions data, EPA accounts for several types
of variability to avoid adopting unachievable standards. The first type
of variability is that concerning operational distinctions between
facilities or units. As the Sierra Club Court stated in reviewing the
HMIWI rule, ``[p]erhaps considering all units with the same technology
is justifiable because the best way to predict the worst reasonably
foreseeable performance of the best unit with the available data is to
look at other units' performance. Or perhaps EPA reasonably considered
all units with the same technology equally `well-controlled,' so that
each unit with the best technology is a `best-controlled unit' even if
such units vary widely in performance.'' (See 167 F.3d at 665.) These
are two ways of saying essentially the same thing, and these concepts
have

been used by EPA throughout most of the Agency's history in determining
achievable technology-based emission limitations, in cases where
application of control technology significantly controls emissions and
no record evidence indicates that factors other than the control have
more than a negligible effect. Examining multiple units using the same
technology gives the best picture of the performance capability of that
particular technology, since it provides EPA with a more complete set
of data by which to evaluate what levels of emissions control a
technology can achieve as it is applied to varying sources. Such an
analysis is necessary especially when adopting standards that all
sources in a category will have to be able to meet by using the
identified technology. Since MACT floors and standards are generally
expressed as numerical emissions limits, it is necessary to account for
this variability in order to adopt a regulation that is `achievable' by
the industry as a whole.'' (See NLA I, 627 F.2d at 437.)
    Section 129(a)(2) of the CAA requires that EPA determine the
emissions control achieved by the ``best controlled similar unit'' when
establishing the MACT floors for new units. A solid waste incineration
``unit'' is defined as ``a distinct operating unit of any facility
which combusts any solid waste material'' (CAA section 129(g)(1)). To
achieve the best level of pollution control, that unit will utilize a
particular method of pollution control (and possibly use other means
that affect its emissions performance). The emissions control achieved
by that method (and by any additional means) is the emissions control
achieved by the ``best controlled similar unit.'' Thus, the MACT floor
for new units is based on the ``emissions control'' that is attained by
the specific method of pollution control and any other means used to
limit emissions at the best similar unit, rather than merely on the
emissions measured at a particular unit.
    In this way, by basing the MACT floor on the capability of a
particular method of pollution control used at ``similar'' ``best''
``units,'' instead of on the emissions measured at a single unit, EPA
ensures that the floors would not only be achievable by the single best
performing unit, but are also achievable by other units using the same
technology and/or emissions limiting means as the best similar unit,
and that it is reasonable to require the best similar unit and all
future new units to meet this floor on a continuous basis. In contrast,
identifying the ``emissions control'' of the ``best controlled similar
unit'' as being a single data point from a single source provides
merely a snapshot of emissions performance that may not be replicable
by either that single source or by other sources using the same control
technology, and, therefore, does not provide a basis for enforceably
requiring all sources to perform to that level.
    Thus, the most reasonable way to interpret the statutory phrase
``best controlled similar unit'' in CAA section 129 is as encompassing
all units using the same technology and emissions limiting means as the
single unit with the best observed performance, rather than just that
single best performing unit itself. A contrary interpretation would
seem to directly conflict with the Court's directive in NLA I, and is
not compelled by the Court rulings in Sierra Club, NLA II, and CKRC.
Applying this approach to evaluating ``best technologies'' at ``best
controlled similar units,'' where different design characteristics are
identified (e.g., low-efficiency versus moderate-efficiency versus
high-efficiency wet scrubbers), the data are grouped such that each
data set reflects the performance of an ``identical'' control device,
providing the best indication of the true performance of each control
device and enabling the Agency to adopt a numerical standard that can
be met with the subject technology at all units employing this
technology, and can be enforced. Again, where the record evidence
indicates that the only means of control of emissions at units is
application of control technology, and there is no record evidence
showing that other means of emissions limitation significantly affect
emissions performance, basing the MACT floor on this approach is fully
consistent with the Court's rulings in the MACT cases.
    c. Variability Between and Within Tests at Facilities. Another type
of variability that EPA accounts for in order to ensure the
achievability of technology-based standards that rely upon application
of pollution controls concerns operational distinctions between and
within tests at the same unit. Regarding ``between-test variability,''
even where conditions appear to be the same when two or more tests are
conducted, variations in emissions are often caused by different
settings for emissions testing equipment and differences in sample
handling. Varying results may also be caused by use of different field
teams to conduct the testing, or different laboratories to analyze the
results. All these variations are typical.
    An achievable standard needs to account for these differences
between tests, in order for ``a uniform standard [to] be capable of
being met under most adverse conditions which can reasonably be
expected to recur[.]'' (See NLA I, 627 F.2d at 431, n. 46.) (See also
Portland Cement Ass'n, 486 F.2d at 396 (noting industry point that ``a
single test offered a weak basis'' for inferring that plants could meet
the standards).) Without accounting for variation among different
emissions tests, it can be determined with a significant degree of
statistical confidence that even a single unit will not be able to meet
the standard over a reasonable period of time, when one can expect
adverse conditions to recur. The Courts have recognized this basic
principle in reviewing technology-based effluent standards under the
Clean Water Act. As the U.S. Court of Appeals for the 5th Circuit
stressed regarding ``best practicable technology'' or ``BPT'' standards
under section 304(b)(1) of the Clean Water Act, ``[t]he same plant
using the same treatment method to remove the same toxic does not
always achieve the same result. Tests conducted one day may show a
different concentration of the same toxic than are shown by the same
test on the next day. This variability may be due to the inherent
inaccuracy of analytical testing, i.e., `analytical variability,' or to
routine fluctuations in a plant's treatment performance.'' (See
Chemical Mf'rs Ass'n v. EPA, 870 F.2d 177, 228 (5th Cir. 1989).) (See
also American Petroleum Institute v. EPA, 540 F.2d 1023, 1035-36 (10th
Cir. 1976) (``Even in the best treatment systems, changes occur in
ability to treat wastes. [* * *] [V]ariability factors present[] a
practical effort to accommodate for variations in plant operations'');
FMC Corp. v. Train, 539 F.2d 973, 985 (4th Cir. 1976) (variability
factors account for ``the fact that even in the best treatment systems
changes continually occur in the treatability of wastes'').)
    The same types of differences leading to between-test variability
also cause variations in results between various runs comprising a
single test, or ``within-test variability.'' A single test at a unit
usually includes at least three separate test runs. (See 40 CFR
63.7(e)(3) (for MACT standards under section 112 of the CAA), and 40
CFR 60.8(f) (for NSPS under CAA section 111).) (See also Portland
Cement Ass'n, 486 F.2d at 397 (noting differences in conditions among
several test runs).)
    d. Application of NLA I, Sierra Club, NLA II, and CKRC Principles
in HMIWI Rulemaking. Based on the record for the 1997 rulemaking, the
best way to determine the worst reasonably foreseeable circumstances
for the

particular technologies used to control emissions at HMIWI was to first
examine the highest data point actually observed from HMIWI equipped
with each particular technology. If an emission value has been observed
and there is no reason to believe it represents poor performance (i.e.,
there is nothing that can be done to prevent its recurring), it is
likely to occur again in the future and, therefore, reflects a
foreseeable circumstance. It is incorrect to characterize the highest
data point as the ``worst performance'' of the best performing unit, or
to characterize one control device's performance as ``better'' than
another's based solely on the results of a single emission test. This
is because such focuses relate to essentially random single data
occurrences, rather than to estimating what a particular technology can
be expected to continuously achieve. Rather, each data point, whether
from one unit or from several identical units using the same
technology, should be viewed as a snapshot of the actual performance of
the technology in use. Along with an understanding of the factors
affecting the performance of the technology, each of these snapshots
gives information about the normal, and unavoidable, variation in
emissions that would be expected to recur over time when using the
identified technology. Conversely, when there is evidence that an
emission data point reflects poor performance (design, operation, or
both), such a data point should not be considered in determining the
achievable emission limitation associated with the technology.
    Furthermore, a distinction must be made between an emission level
that has been ``observed'' and an emission limitation that can be
continuously ``achieved.'' The purpose of the MACT program is to compel
sources to replicate emission reduction strategies used by the best-
performing sources. Thus, MACT floors are based on the control
strategies used by the best-performing sources to reduce emissions, not
based on a snapshot level of emissions from sources without regard to
whether this level reflects application of any replicable emission
control strategies. CAA section 129(a)(2) does not direct EPA to assess
relative emission ``levels'' in determining MACT floors; it directs EPA
to assess the degree of emissions ``control'' or ``reduction'' or
``limitation'' ``achieved'' by the best-controlled or best-performing
sources. The plain meaning of these words implies that a source is
utilizing some method or technique to reduce emissions that is within a
source operator's power to adopt. The reference to a ``degree of
reduction'' supports the view that the words ``control'' and
``limitation'' appearing in section 129(a)(2) require a source to have
reduced emissions from uncontrolled levels through some control
technique. See NLA II, 233 F.3d at 631-32 (rejecting position that EPA
is required to set new source floors at the lowest recorded emission
level for which it has data and to set existing source floors at the
average of the lowest 12 percent or recorded emission level data
points).
    The Court has recognized that EPA may consider variability in
estimating the degree of emission reduction achieved by best-performing
sources and in setting MACT floors. See Mossville Envt'l Action Now v.
EPA, 370 F.3d 1232, 1241-42 (D.C. Cir 2004) (holding EPA may consider
emission variability in estimating performance achieved by best-
performing sources and may set floor at level that best-performing
source can expect to meet ``every day and under all operating
conditions''). Since an emission limitation must be complied with at
all times, for it to be achievable it must be set at a level that will
not force sources to violate it when operating conditions are not ideal
and higher emissions levels might be observed. For example, a car which
has been observed to consume 0.02 gallons of gasoline in a one-mile
downhill stretch of highway cannot be said to have ``achieved'' a
minimum 50 miles per gallon fuel efficiency rate when that same car is
later certain to consume 0.04 gallons of gasoline in a one-mile uphill
stretch of highway (25 miles per gallon). Rather, the minimum fuel
efficiency of the car will be that which the car can meet in adverse
circumstances, the uphill stretch. So it is with emissions limitations,
which cannot reasonably be set at levels which would force sources to
operate in violation even when properly employing the control
technology upon which the standards are based.
    The emission data used to develop the emission limitations in the
HMIWI regulations reflect properly designed and operated air pollution
control technology on properly designed and operated HMIWI, and
emission data that reflected poor operation of the HMIWI unit or the
air pollution control technology were excluded. (See Legacy Docket ID
No. A-91-61, items II-A-111 and IV-B-14.) The incinerators selected by
EPA for testing represented a range of incinerator designs and air
pollution control systems in use on this source category. (See Legacy
Docket ID No. A-91-61, item IV-B-46.) The incinerators and air
pollution controls were inspected thoroughly, and maintenance was
performed where necessary to ensure that the incinerators and pollution
controls were functioning properly. (See Legacy Docket ID No. A-91-61,
items II-A-93, II-A-94, and II-A-85.) During testing, most test runs
were conducted under representative conditions to minimize emissions.
(See Legacy Docket ID No. A-91-61, items II-A-111, IV-B-46, and IV-B-
47.) However, some test runs were purposely conducted under conditions
that would represent poor operation (e.g., overcharging waste to the
incinerator) to determine the effect of improper operation on
emissions. (See Legacy Docket ID No. A-91-61, items II-A-111 and IV-B-
46.) These test runs demonstrated that improper operation results in
higher emissions. (See Legacy Docket ID No. A-91-61, items II-A-111,
IV-B-46, and II-A-81.) Of course, the test runs reflecting poor
operation were not used in developing the achievable emission
limitations. Id. It is important to note that such poor operation is
precluded by the good combustion requirements and the parametric
monitoring requirements in the 1997 final rule. In addition to data
gathered by EPA directly, vendors of air pollution control systems
submitted test reports to EPA. (See Legacy Docket ID No. A-91-61, items
II-I-230 through 237, II-I-243 and 244, II-I-248, IV-B-48 and 49, IV-J-
11, IV-J-15 and 16, IV-J-20, IV-J-24, IV-J-27, IV-J-29 through 31, IV-
J-33 and 34, IV-J-39 and 40, and IV-J-47.) The test reports were
submitted primarily by wet scrubber vendors to demonstrate to EPA that
wet scrubbers could achieve lower emissions than EPA had concluded from
the EPA-collected data. (EPA had conducted testing on only one wet
scrubber system.) (See 61 FR 31742; Legacy Docket ID No. A-91-61, item
IV-B-48.) The test reports and the data collected by EPA reflect the
best performance of the air pollution controls that can reasonably be
expected when continuously applied on HMIWI.
    MACT and other technology-based standards are necessarily derived
from short-term emissions test data, but such data are not
representative of the range of operating conditions that facilities
face on a day-to-day basis. In statistical terms, each test produces a
limited data sample, not a complete enumeration of the available data
for performance of the unit over a long period of time. (See Natrella,
Experimental Statistics, National Bureau of Standards Handbook 91,
chapter 1 (revised ed., 1966).) EPA, therefore, often needs to adjust
the

short-term data to account for these varying conditions, so facilities
properly employing optimal controls can remain in compliance with the
standards on a continuous basis.
    With the relatively small data sets EPA had to work with in the
1997 HMIWI rulemaking, it is possible that EPA has not recorded the
highest emissions levels that would occur under the worst reasonably
foreseeable circumstances. As the Court noted, it would ``generally
defer to an agency's decision to proceed on the basis of imperfect
scientific information, rather than to `invest the resources to conduct
the perfect study.' '' (See Sierra Club, 167 F.3d at 662.) ``[S]ince
EPA had data on only one percent of about 3,000 [HMIWI], the data
gathering costs of any non-sampling method may well have been
daunting.'' Id., at 663. In fact, the ``perfect study'' cannot be
conducted, regardless of the resources expended to conduct it. Every
study ends with some uncertainty in the results. There is no
``cookbook'' methodology for determining achievable emission
limitations from data. In every case, but especially in cases where
data are limited as with the 1997 HMIWI rulemaking, EPA must make
judgments about what constitutes the worst reasonably foreseeable
circumstance and put those judgments out for public comment. In the
case of the HMIWI rulemaking, the ``high'' data points simply reflected
the normal, and unavoidable, variation in emissions that would be
expected to recur over time when properly using the best control
technologies and strategies we determined were being used at HMIWI
units. In fact, while the highest observed value is a ``foreseeable
circumstance,'' it may not reflect the worst reasonably foreseeable
circumstance. In determining the 1997 final MACT standards, EPA chose
to account for the ``worst reasonably foreseeable circumstance'' by
adding 10 percent to the highest observed emissions levels in the data,
and then rounding up those figures. Upon review of this approach in
responding to the Court's remand, we have determined that although the
highest observed data point may not reflect the ``worst reasonably
foreseeable circumstance,'' we do not have information to support
accounting for the ``worst reasonably foreseeable circumstance'' by
adding 10 percent to the highest observed emissions levels, and then
rounding up those figures. We, therefore, propose to base revised MACT
standards for new HMIWI units on the highest observed data points
associated with employed control strategies.
    In the CKRC case, the Court left open the possibility that the
approach of basing floors on the ``worst-performing MACT sources''
emissions represent `a reasonable estimate of the performance of the
[best-performing] units,' '' CKRC at 866, quoting Sierra Club at 662,
provided that ``in the case of a particular source category or HAP, the
Agency can demonstrate with substantial evidence--not mere assertions--
that MACT technology significantly controls emissions, or that factors
other than the control have a negligible effect[.] CKRC at 866, citing
NLA II at 633. The Court in Sierra Club essentially already found this
to be the situation for the HMIWI rulemaking, and it was, therefore,
appropriate for EPA to base its MACT floor review in the 1997 rule
strictly on the emissions reductions achieved by use of control
technologies. The Sierra Club had claimed that EPA wrongly failed to
require HMIWI units to undertake programs to reduce the Hg and
chlorinated plastics in HMIWI waste streams. Sierra Club, at 666. While
the petitioner raised this objection in its challenge to the
promulgated standards, rather than its objection to the floor
methodology, the Court's response to the Sierra Club's claim shows that
in the case of the 1997 HMIWI rulemaking, EPA appropriately focused on
the control technologies used at HMIWI units, and that, therefore,
under the CKRC ruling it was appropriate, in this instance, to base
floors on the highest emissions levels achieved by units employing the
MACT technologies.
    The Court observed that ``EPA does not deny that the waste stream
reductions the Sierra Club calls for would reduce pollution. The less
mercury in, the less mercury out, and the less chlorinated plastic in,
the less HCl out. But the EPA has consistently argued in its response
to comments and here that it does not have evidence that allows
quantification of the relevant output reduction. For mercury, the only
quantitative evidence before EPA was that a pollution prevention
program aimed at mercury could reduce mercury emissions from very high
levels to typical levels. See RTC at 7-14 to 7-15. For chlorinated
plastics, there was no quantitative evidence before the agency. See RTC
at 7-16, 7-18. The Sierra Club does not contest the adequacy of EPA's
data-gathering with respect to these measures.'' Id. (Note that the
emission guidelines and NSPS require HMIWI to prepare a waste
management plan under Sec. Sec.  60.35e and 60.55c that would segregate
from the health care waste stream certain solid waste components
contributing to toxic emissions from the incinerator (62 FR 48380,
48387).)
    e. Development of the Proposed Revised Emission Limits. While we
are proposing to respond to the Court's remand regarding new units by
basing floors and standards on the same control technologies that
formed the basis for the 1997 standards, in some cases it is necessary
to adjust the emission limits in order to correct for the concerns
regarding our 1997 methodology that the Court raised. As at
promulgation of the 1997 rule, EPA examined the data available for
various air pollution control technologies applied to HMIWI to
determine the performance capabilities of the technologies; identified
the best control technology for each air pollutant for each subcategory
of HMIWI (i.e., MACT floor); considered control technologies more
stringent than the MACT floor; made a determination regarding the
achievable emissions levels from using control technologies upon which
the emission standards would be based; and then established numerical
emission limits achievable with those technologies. The proposed
revised standards are based on the same technologies upon which the
1997 final standards were based--good combustion and a moderate-
efficiency wet scrubber for new small HMIWI, and good combustion and a
combined dry/wet control system with carbon for new medium and large
HMIWI--and reflect the MACT floor emissions levels for new small and
large HMIWI, but are more stringent than the MACT floor for new medium
HMIWI. The rationale for these determinations regarding identification
of MACT can be found at 62 FR 48365.
    As explained earlier in this preamble, we are proposing emission
limits for each air pollutant for each subcategory of new HMIWI based
on the highest observed data points associated with the control
technologies upon which the emission standards are based, since we
identified the ``best controlled similar unit'' as one using the
relevant control technologies for each subcategory of new units. The
proposed percent reduction limits for HCl, Pb, Cd, and Hg were
established based on average combustion-controlled emissions estimates
and highest observed data points associated with the control
technologies upon which the emission standards for each of these
pollutants for each subcategory are based. This is the same approach
used at the time of promulgation with two exceptions--the proposed
percent reduction limits do not include the addition of 10 percent

to the highest observed emissions levels, nor does it include the
rounding up of those figures. A summary of the control technologies
upon which the proposed standards for new HMIWI are based, the highest
observed data points associated with those control technologies, and
the proposed emission limits for new HMIWI in response to the remand
are presented in Table 6 of this preamble. Note that MACT for
NOX and SO2 are ``combustion control,'' although
combustion control results in no emission reductions for those
pollutants because NOX emissions are not reduced by
combustion control, and NOX add-on controls have not been
demonstrated on HMIWI; and SO2 emissions are not reduced by
combustion control, and acid gas controls are not effective in reducing
SO2 emissions from HMIWI at the low SO2 levels
associated with HMIWI.

                                                   Table 6.--Summary of Remand Response for New HMIWI
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Highest
          Pollutant  (units)                  Unit Size \1\                  MACT             observed data           Proposed emission limit \2\
                                                                                point \2\
--------------------------------------------------------------------------------------------------------------------------------------------------------
HCl (ppmv)............................  L, M, S..................  Wet scrubber............            9.3    15 \3\ or 99% reduction \3\.
CO (ppmv).............................  L, M, S..................  Good combustion.........           32      32.
Pb (mg/dscm)..........................  L, M.....................  Dry scrubber w/carbon...            0.06   0.060 or 98% reduction \3\.
                                        S........................  Wet scrubber............            1.1    0.78 \4\ or 71% reduction.
Cd (mg/dscm)..........................  L, M.....................  Dry scrubber w/carbon...            0.03   0.030 or 93% reduction.
                                        S........................  Wet scrubber............            0.14   0.11 \4\ or 66% reduction \3\.
Hg (mg/dscm)..........................  L, M.....................  Dry scrubber w/carbon...            0.45   0.45 or 87% reduction.
                                        S........................  Wet scrubber............            0.47   0.47 or 87% reduction.
PM (gr/dscf)..........................  L, M.....................  Dry scrubber w/carbon...            0.009  0.0090.
                                        S........................  Moderate-efficiency wet             0.018  0.018.
                                                                    scrubber.
CDD/CDF, total (ng/dscm)..............  L, M.....................  Dry scrubber w/carbon...           20      20.
                                        S........................  Wet scrubber............          111      111.
CDD/CDF, TEQ (ng/dscm)................  L, M.....................  Dry scrubber w/carbon...            0.53   0.53.
                                        S........................  Wet scrubber............            2.1    2.1.
NOX (ppmv)............................  L, M, S..................  Combustion Control \5\..          225      225.
SO2 (ppmv)............................  L, M, S..................  Combustion Control \5\..           46      46.
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ L = Large; M = Medium; S = Small.
\2\ All values are measured at 7 percent oxygen.
\3\ No change proposed.
\4\ Remand standards for existing small non-rural HMIWI are proposed.
\5\ Combustion control results in no emissions reduction.

    Note that no change is proposed for the emission limit for HCl for
new large, medium, and small HMIWI. In this situation, the highest
observed emission point (i.e., 9.3 ppmv) is not used as a basis for the
proposed emission limits. Public comments concerning use of EPA Method
26A when testing for HCl emissions at sources with wet scrubbers were
submitted with respect to the recently promulgated standards for other
solid waste incineration units (70 FR 74870, December 16, 2005). The
commenter asserted that EPA Method 26A is not adequate for
demonstrating compliance with an HCl standard below 20 ppmv when
sampling sources with wet scrubbers. Although EPA did not concede that
there is an outright problem, we acknowledged that a tester may need to
take certain precautions to ensure that there is no bias when sampling
streams with low HCl concentrations in certain environments and
promulgated an HCl emission limit of 15 ppmv (versus the proposed limit
of 3.7 ppmv). Method 26A also notes that there is a possible measurable
negative bias below 20 ppmv HCl perhaps due to reaction with small
amounts of moisture in the probe and filter (40 CFR part 60, appendix
A). Accordingly, because many of the wet-scrubber controlled HMIWI used
Method 26A to measure HCl emissions below 20 ppmv and did not take
precautions to ensure no negative bias, in this action we are proposing
to retain the emission limit of 15 ppmv and also are including
provisions that require sources to condition the filter before testing,
and use a cyclone and post test purge if water droplets may be present.
In the cases of Pb and Cd for new small HMIWI, using the highest
observed data points would result in emission limits less stringent
(i.e., higher) than the proposed emission limits for existing small
non-rural HMIWI. Because the existing source analysis provides limits
that can be achieved by existing HMIWI, there is no reason to believe
that new HMIWI could not also meet the more stringent limits. This
unanticipated result may be due to the small amount of Pb and Cd
emissions data available for wet scrubbers at promulgation. Regardless,
we are proposing emission limits for Pb and Cd for new small HMIWI that
are the same as those proposed for existing small non-rural HMIWI.
2. Existing Units
    The Court raised three specific concerns regarding EPA's approach
for existing units in concluding that EPA had not adequately explained
why the combination of regulatory and uncontrolled (i.e., combustion-
controlled) data provided a ``reasonable estimate'' of HMIWI
performance: ``First, EPA has said nothing about the possibility that
[HMIWI] might be substantially overachieving the [regulatory] limits.
[Footnote:] Although the agency conceded in its response to comments
that `actual emission data routinely fall below the State permit
emission limits,' [* * *] the context makes reasonably clear that the
EPA was referring to data on `actual emissions' during tests; EPA
implied that `these levels are not routinely achieved in practice.' [*
* *] [End Footnote] If this were the case, the permit limits would be
of little value in estimating the top 12 percent of [HMIWI]s'
performance'' (167 F.3d at 663, and at n. 3). According to the Court,
``[d]ata in the record suggest that the regulatory limits are in fact
much higher than emissions that units achieve in practice.'' Id., at
663.
    ``Second, EPA never gave any reason for its apparent belief that
[HMIWI]s that were not subject to [regulatory limits] did not employ
emission controls of any sort. Unless there is some finding to this
effect, it is difficult to see the rationality in using `uncontrolled'
data for the units that

were not subject to regulatory requirements'' (167 F.3d at 664). The
Court pointed out that ``[d]ata submitted by the American Hospital
Association [AHA] in 1995 indicate that over 55% of [HMIWI]s in each
category were controlled by wet scrubbers.'' Id., footnote omitted. As
a result, the Court found it ``difficult to see how it was rational to
include any uncontrolled [i.e., combustion-controlled] units in the top
12 percent, at least with respect to pollutants that wet scrubbing
controls.'' Id.
    Third, the Court held that ``assuming the regulatory data was a