[Federal Register: June 11, 2008 (Volume 73, Number 113)]
[Proposed Rules]
[Page 33257-33282]
[DOCID:fr11jn08-25]

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Part V

Environmental Protection Agency

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40 CFR Part 63

National Emission Standards for Hazardous Air Pollutants: Mercury
Emissions from Mercury Cell Chlor-Alkali Plants; Proposed Rule

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2002-0017; FRL-8576-3]
RIN 2060-AN99

National Emission Standards for Hazardous Air Pollutants: Mercury
Emissions from Mercury Cell Chlor-Alkali Plants

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: This action proposes amendments to the national emission
standards for hazardous air pollutants (NESHAP) for mercury emissions
from mercury cell chlor-alkali plants. This NESHAP (hereafter called
the ``2003 Mercury Cell MACT'') limited mercury air emissions from
these plants. Following promulgation of the 2003 Mercury Cell Maximum
Achievable Control Technology (MACT) NESHAP, EPA received a petition to
reconsider several aspects of the rule from the Natural Resources
Defense Council (NRDC). NRDC also filed a petition for judicial review
of the rule in the U.S. Court of Appeals for the DC Circuit. By a
letter dated April 8, 2004, EPA granted NRDC's petition for
reconsideration, and on July 20, 2004, the Court placed the petition
for judicial review in abeyance pending EPA's action on
reconsideration. This action is EPA's proposed response to NRDC's
petition for reconsideration.
    We are not proposing any amendments to the control and monitoring
requirements for stack emissions of mercury established by the 2003
Mercury Cell MACT. This proposed rule would amend the requirements for
cell room fugitive mercury emissions to require work practice standards
for the cell rooms and to require instrumental monitoring of cell room
fugitive mercury emissions. This proposed rule would also amend aspects
of these work practice standards and would correct errors and
inconsistencies in the 2003 Mercury Cell MACT that have been brought to
our attention.

DATES: Comments. Comments must be received on or before August 11,
2008.
    Public Hearing. If anyone contacts EPA by June 23, 2008 requesting
to speak at a public hearing, a hearing will be held on July 11, 2008.

ADDRESSES: You may submit comments, identified by Docket ID No. EPA-HQ-
OAR-2002-0017, by any of the following methods:
     Federal eRulemaking Portal: http://www.regulations.gov:
Follow the instructions for submitting comments.
     Agency Web Site: http://www.epa.gov/oar/docket.html.
Follow the instructions for submitting comments on the EPA Air and
Radiation Docket Web site.
     E-mail<EMAIL_ADDRESS>Include Docket ID No. EPA-
HQ-OAR-2002-0017 in the subject line of the message.
     Fax:<PHONE_NUMBER>.
     Mail: National Emission Standards for Hazardous Air
Pollutants for Mercury Cell Chlor-alkali Plants Docket, Environmental
Protection Agency, EPA Docket Center (EPA/DC), Air and Radiation
Docket, Mail Code 2822T, 1200 Pennsylvania Ave., NW., Washington, DC
20460. Please include a total of two copies.
     Hand Delivery: EPA Docket Center, Public Reading Room, EPA
West, Room 3334, 1301 Constitution Ave., NW., Washington, DC 20460.
Such deliveries are only accepted during the Docket's normal hours of
operation, and special arrangements should be made for deliveries of
boxed information.
    Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2002-0017. EPA's policy is that all comments received will be included
in the public docket without change 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 www.regulations.gov
or e-mail. The 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
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.
    Docket: All documents in the docket are listed in the
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, is not placed on the Internet and will be
publicly available only in hard copy form. Publicly available docket
materials are available either electronically through
www.regulations.gov or in hard copy at the National Emission Standards
for Hazardous Air Pollutants for Mercury Cell Chlor-alkali Plants
Docket, 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 Air Docket is<PHONE_NUMBER>.

FOR FURTHER INFORMATION CONTACT: Dr. Donna Lee Jones, Sector Policies
and Programs Division, Office of Air Quality Planning and Standards
(D243-02), Environmental Protection Agency, Research Triangle Park,
North Carolina 27711, telephone number:<PHONE_NUMBER>; fax number:
(919) 541-3207; e-mail address<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:

I. General Information

A. Does this action apply to me?

    The regulated categories and entities potentially affected by this
proposed action include:

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             Category                   NAICS code \1\                 Examples of regulated entities
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Industry..........................  325181...............  Alkalis and Chlorine Manufacturing.
Federal government................  .....................  Not affected.

State/local/tribal government.....  .....................  Not affected.
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\1\ North American Industry Classification System.

    This table is not intended to be exhaustive, but rather provides a
guide for readers regarding entities likely to be affected by this
action. To determine whether your facility would be regulated by this
action, you should examine the applicability criteria in 40 CFR 63.7682
of subpart IIIII, National Emission Standards for Hazardous Air
Pollutants (NESHAP): Mercury Emissions from Mercury Cell Chlor-Alkali
(hereafter called the ``2003 Mercury Cell MACT''). If you have any
questions regarding the applicability of this action to a particular
entity, consult either the air permitting authority for the entity or
your EPA regional representative as listed in 40 CFR 63.13 of subpart A
(General Provisions).

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

    Do not submit information containing confidential business
information (CBI) to EPA through www.regulations.gov or e-mail. Send or
deliver information identified as CBI only to the following address:
Roberto Morales, OAQPS Document Control Officer (C404-02),
Environmental Protection Agency, Office of Air Quality Planning and
Standards, Research Triangle Park, North Carolina 27711, Attention
Docket ID EPA-HQ-OAR-2002-0017. 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 so marked will not be disclosed except in accordance with
procedures set forth in 40 CFR part 2.

C. Where can I get a copy of this document?

    In addition to being available in the docket, an electronic copy of
this proposed action will also be available on the Worldwide Web (WWW)
through the Technology Transfer Network (TTN). Following signature, a
copy of this proposed action will be posted on the TTN's policy and
guidance page for newly proposed or promulgated rules at the following
address: http://www.epa.gov/ttn/oarpg/. The TTN provides information
and technology exchange in various areas of air pollution control.

D. When would a public hearing occur?

    If anyone contacts EPA requesting to speak at a public hearing
concerning the proposed amendments by June 23, 2008, we will hold a
public hearing on July 11, 2008. If you are interested in attending the
public hearing, contact Ms. Pamela Garrett at<PHONE_NUMBER> to verify
that a hearing will be held. If a public hearing is held, it will be
held at 10 a.m. at the EPA's Environmental Research Center Auditorium,
Research Triangle Park, NC, or an alternate site nearby.

E. How is this document organized?

    The supplementary information in this preamble is organized as
follows:

I. General Information
    A. Does this action apply to me?
    B. What should I consider as I prepare my comments to EPA?
    C. Where can I get a copy of this document?
    D. When would a public hearing occur?
    E. How is this document organized?
II. Background Information
    A. Reconsideration Overview
    B. Industry Description
    C. Regulatory Background
    D. Details of the Petition for Reconsideration
III. Summary of EPA's Reconsideration and Proposed Amendments
    A. What were the issues that EPA reconsidered, and what are
EPA's proposed responses?
    B. What amendments are EPA proposing?
    C. What are the impacts of these proposed rule amendments?
IV. 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 (Energy Effects)
    I. National Technology Transfer Advancement Act
    J. Executive Order 12898: Federal Actions to Address
Environmental Justice in Minority Populations and Low-Income
Populations

II. Background Information

A. Reconsideration Overview

    On December 19, 2003, EPA promulgated the National Emission
Standards for Hazardous Air Pollutants for Mercury Emissions from
Mercury Chlor-alkali Plants (40 CFR part 63, subpart IIIII, 68 FR
70904), hereafter called the ``2003 Mercury Cell MACT.'' This rule for
mercury cell chlor-alkali plants implemented section 112(d) of the
Clean Air Act (CAA), which required all categories and subcategories of
major sources listed under section 112(c) to meet hazardous air
pollutant emission standards reflecting the application of the maximum
achievable control technology (MACT). Mercury cell chlor-alkali plants
are a subcategory of the chlorine production source category listed
under the authority of section 112(c)(1) of the CAA. In addition,
mercury cell chlor-alkali plants were listed as an area source category
under section 112(c)(3) and (k)(3)(B) of the CAA. The 2003 Mercury Cell
MACT satisfied our requirement to issue 112(d) regulations under each
of these listings (for mercury).
    The 2003 Mercury Cell MACT contained numerical emission limitations
for the point sources of mercury emissions at mercury cell chlor-alkali
plants. It also required that the plants either install mercury
monitoring systems on the point source vents or that they test each
vent manually at least once per week. The compliance date for the 2003
Mercury Cell MACT was December 19, 2006.
    The 2003 Mercury Cell MACT also contained a set of work practice
standards to address fugitive mercury emissions from the cell rooms. We
determined that these procedures represented the MACT for the industry,
and were considerably more stringent than the 40 CFR part 61 subpart E
NESHAP requirements for control of mercury emissions (hereafter called
the ``part 61 Mercury NESHAP'') that were applicable to this industry
prior to the 2003 Mercury Cell MACT. An alternative compliance option
was included in the 2003 Mercury Cell MACT that required mercury

monitoring systems to be installed in the cell rooms with mandatory
problem correction when a site-specific mercury concentration action
level is exceeded. As of December 19, 2006, the compliance date for the
2003 Mercury Cell MACT, all facilities but one have chosen this
alternative compliance option.
    On February 17, 2004, the Natural Resources Defense Council (NRDC)
submitted to EPA an administrative petition asking us to reconsider
several aspects of the 2003 Mercury Cell MACT under Clean Air Act
section 307(d)(7)(B). On the same day, NRDC and the Sierra Club filed a
petition for judicial review of the 2003 Mercury Cell MACT in the U.S.
Court of Appeals for the DC Circuit (Civ. No. 04-1048). The focus of
many of the issues raised in the petition for reconsideration was EPA's
treatment of the fugitive cell room emissions in the 2003 Mercury Cell
MACT. Specifically, NRDC asked EPA to reconsider (1) the decision to
develop a set of work practice requirements under Clean Air Act section
112(h) in lieu of a numeric emission limitation for cell rooms; (2) the
decision to make the promulgated work practices optional for sources
that choose to undertake continuous monitoring; (3) the decision to not
require existing facilities to convert to a mercury-free chlorine
manufacturing process; (4) the elimination of the previously applicable
part 61 rule's 2,300 grams/day plant-wide emission limitation; and (5)
the decision to create a subcategory of mercury cell chlor-alkali
plants within the chlorine production category.
    By a letter dated April 8, 2004, Jeffrey Holmstead, then-EPA
Assistant Administrator for Air and Radiation, notified the NRDC that
EPA had granted NRDC's petition for reconsideration of the 2003 Mercury
Cell MACT. On July 20, 2004, the Court granted EPA's motion to hold the
case in abeyance pending EPA's action on reconsideration of the 2003
Mercury Cell MACT. Today's notice is EPA's proposed response to NRDC's
petition for reconsideration.

B. Industry Description

    There currently are five operating mercury cell chlor-alkali plants
in the U.S., with one of these plants planning to convert to non-
mercury technology by 2012. These five plants are in Augusta, Georgia;
Ashtabula, Ohio; Charleston, Tennessee; New Martinsville, West
Virginia; and Port Edwards, Wisconsin. The Port Edwards, Wisconsin
facility is the one that is expected to convert to non-mercury
technology.
    Mercury cell chlor-alkali plants produce chlorine and caustic soda
(sodium hydroxide) or caustic potash (potassium hydroxide) in an
electrolytic reaction using mercury. A mercury cell plant typically has
many individual cells housed in one or more cell buildings. Mercury
cells are electrically connected together in series.
    At a mercury cell chlor-alkali plant, mercury is emitted from point
sources (i.e., stacks) and fugitive sources. Mercury also leaves the
plant in wastewater and solid wastes. There are three primary point
sources of mercury emissions at mercury cell plants: The end-box
ventilation system vent, the by-product hydrogen system vent, and the
mercury thermal recovery unit vents. Every mercury cell plant has a
hydrogen by-product stream, and most have an end-box ventilation
system. However, not all of the plants have thermal mercury recovery
units. Of the five plants currently operating, all five facilities have
end-box ventilation systems and two have thermal mercury recovery
units.
    In addition to the stack emissions, there are fugitive mercury
emissions at these plants. The majority of fugitive mercury emissions
occur from sources inside the cell room such as leaks from cells,
decomposers, hydrogen piping, and other equipment. Fugitive mercury
emissions also occur during maintenance activities such as cell or
decomposer openings, mercury pump change-outs, and end-box seal
replacements, etc. All of this equipment and activities are located in
the cell room, so these fugitive mercury emissions would be emitted via
the cell room ventilation system.
    There are potential fugitive air emission sources outside of the
cell room. These potential outside sources include leaks of mercury-
contaminated brine in the brine treatment area, the wastewater system,
and the handling and storage of mercury contaminated wastes.

C. Regulatory Background

    The part 61 Mercury NESHAP, which applied to all mercury cell
chlor-alkali chlorine production plants prior to the 2003 Mercury Cell
MACT, contained a numerical emission limit for mercury of 2,300 grams
per day (g/day) for the entire plant. Point sources were limited to
1,000 g/day of mercury. If plants conducted a series of detailed
design, maintenance, and housekeeping procedures, they were permitted
under the part 61 rule to assume that fugitive mercury emissions from
the cell room were 1,300 g/day, without having to demonstrate as such.
All the mercury cell plants complied with the part 61 Mercury NESHAP
using these assumptions rather than testing and determining actual
fugitive cell room mercury emissions. Therefore, the extent of actual
plant-wide and cell room emissions that occurred under the part 61 rule
could not be precisely determined.
    In the 2003 Mercury Cell MACT rulemaking, pursuant to Clean Air Act
section 112(d)(2) and (3), the regulatory analyses for the stack
control requirements were based on the practices and controls of the
lowest emitting plants out of the eleven facilities operating at the
time of the MACT analyses. Existing mercury cell chlor-alkali
production facilities with end-box ventilation systems were required by
the 2003 Mercury Cell MACT to limit the aggregate mercury emissions
from all by-product hydrogen streams and end-box ventilation system
vents to not exceed 0.076 grams (g) mercury (Hg) per megagram (Mg)
chlorine (Cl2) for any consecutive 52-week period. Existing
mercury cell chlor-alkali production facilities without end-box
ventilation systems were required to limit the mercury emissions from
all by-product hydrogen streams to not exceed 0.033 g Hg/Mg
Cl2 for any consecutive 52-week period.
    The 2003 Mercury Cell MACT contained a set of work practice
standards to address and mitigate fugitive mercury releases at mercury
cell chlor-alkali plants. The MACT analysis for the requirements to
reduce fugitive mercury emissions was based on the best practices of
the eleven facilities operating at the time of the July 2002 proposal
for the Mercury Cell MACT (see 67 FR 44672, July 3, 2002). These work
practice provisions included specific equipment standards such as the
requirement that end boxes either be closed (that is, equipped with
fixed covers), or that end box headspaces be routed to a ventilation
system (40 CFR 63.8192, ``What work practice standards must I meet?'',
and Tables 1 through 4 to subpart IIIII of part 63). Other examples
include requirements that piping in liquid mercury service have smooth
interiors, that cell room floors be free of cracks and spalling (i.e.,
fragmentation by chipping) and coated with a material that resists
mercury absorption, and that containers used to store liquid mercury
have tight-fitting lids (Table 1 to subpart IIIII of part 63). The work
practice standards also included operational requirements. Examples of
these include requirements to allow electrolyzers and decomposers to
cool before opening, to keep liquid mercury in end boxes and mercury

pumps covered by an aqueous liquid at a temperature below its boiling
point at all times, to maintain end box access port stoppers in good
sealing condition, and to rinse all parts removed from the decomposer
for maintenance prior to transport to another work area (Table 1 to
subpart IIIII of part 63).
    A cornerstone of the work practice standards was the inspection
program for equipment problems, leaking equipment, liquid mercury
accumulations and spills, and cracks or spalling in floors and pillars
and beams. Specifically, the 2003 Mercury Cell MACT required that
visual inspections be conducted twice each day to detect equipment
problems, such as end box access port stoppers not securely in place,
liquid mercury in open containers not covered by an aqueous liquid, or
leaking vent hoses (Table 2 to subpart IIIII of part 63). If a problem
was found during an inspection, the owner or operator was required to
take immediate action to correct the problem. Monthly inspections for
cracking or spalling in cell room floors were also required as well as
semiannual inspections for cracks and spalling on pillars and beams.
Any cracks or spalling found were required to be corrected within 1
month. Visual inspections for liquid mercury spills or accumulations
were also required twice per day. If a liquid mercury spill or
accumulation was identified during an inspection, the owner or operator
was required to initiate cleanup of the liquid mercury within 1 hour of
its detection (Table 3 to subpart IIIII of part 63). In addition to
cleanup, the 2003 Mercury Cell MACT required inspection of the
equipment in the area of the spill or accumulation to identify the
source of the liquid mercury. If the source was found, the owner or
operator was required to repair the leaking equipment as discussed
below. If the source was not found, the owner or operator was required
to reinspect the area every 6 hours until the source was identified or
until no additional liquid mercury was found at that location.
Inspections of specific equipment for liquid mercury leaks were
required once per day. If leaking equipment was identified, the 2003
Mercury Cell MACT required that any dripping mercury be contained and
covered by an aqueous liquid, and that a first attempt to repair
leaking equipment be made within 1 hour of the time it is identified.
Leaking equipment was required to be repaired within 4 hours of the
time it is identified, although there are provisions for delaying
repair of leaking equipment for up to 48 hours (Table 3 to subpart
IIIII of part 63) under certain conditions.
    Inspections for hydrogen gas leaks were required twice per day. For
a hydrogen leak at any location upstream of a hydrogen header, a first
attempt at repair was required within 1 hour of detection of the
leaking equipment, and the leaking equipment was required to be
repaired within 4 hours (with provisions for delay of repair if the
leaking equipment was isolated). For a hydrogen leak downstream of the
hydrogen header but upstream of the final control device, a first
attempt at repair was required within 4 hours, and complete repair
required within 24 hours (with delay provisions if the header is
isolated) (Table 3 to subpart IIIII of part 63).
    The work practice standards in the 2003 Mercury Cell MACT required
that facilities institute a floor level mercury vapor measurement
program (See Sec.  63.8192, ``What work practice standards must I
meet?'', specifically paragraph (d)). Under this program, mercury vapor
levels are periodically measured and compared to an action level of
0.05 mg/m3. The 2003 Mercury Cell MACT specified the actions to be
taken when the action level is exceeded. If the action level was
exceeded during any floor-level mercury vapor measurement evaluation,
facilities were required to take specific actions to identify and
correct the problem (Sec.  63.8192(d)(1) through (4)).
    As an alternative to the full set of work practice standards
(including the floor-level monitoring program), the 2003 Mercury Cell
MACT included a compliance option to institute a cell room monitoring
program (See Sec.  63.8192, ``What work practice standards must I
meet?'', specifically paragraph (g)). In this program, owners and
operators continuously monitor the mercury concentrations in the upper
portion of each cell room and take corrective actions as soon as
practicable when a site-specific mercury vapor level is detected. The
cell room monitoring program was not designed to be a continuous
emissions monitoring system inasmuch as the results would be used only
to determine relative changes in mercury vapor levels rather than
compliance with a cell room emission or operating limit (68 FR 70922).
    As part of the cell room monitoring program, the owner or operator
was required to establish an action level for each cell room based on
preliminary monitoring to determine normal baseline conditions (See
Sec.  63.8192, ``What work practice standards must I meet?'',
specifically paragraph (g)(2)). Once the action level(s) was
established, continuous monitoring of the cell room was required during
all periods of operation. If the action level was exceeded at anytime,
actions to identify and correct the source of elevated mercury vapor
were required to be initiated as soon as possible. If the elevated
mercury vapor level was due to a maintenance activity, the owner or
operator was required to ensure that all work practices related to that
maintenance activity were followed. If a maintenance activity was not
the cause, inspections and other actions were needed to identify and
correct the cause of the elevated mercury vapor level. Owners and
operators utilizing this cell room monitoring program option were
required to develop site-specific cell room monitoring plans describing
their monitoring system and quality assurance/quality control
procedures that were to be used in their monitoring program (Table 5 to
subpart IIIII of part 63).
    The 2003 Mercury Cell MACT established the requirement for owners
and operators to routinely wash surfaces throughout the plant where
liquid mercury could accumulate (See Sec.  63.8192, ``What work
practice standards must I meet?'', specifically paragraph (e)). Owners
and operators were required to prepare and follow a written washdown
plan detailing how and how often certain areas specified in the 2003
Mercury Cell MACT were to be washed down to remove any accumulations of
liquid mercury (Table 7 to subpart IIIII of part 63).
    For new or reconstructed mercury cell chlor-alkali production
facilities, the 2003 Mercury Cell MACT prohibited mercury emissions.
    Several mercury cell plants have closed or converted to membrane
cells since the promulgation of the 2003 Mercury Cell MACT. When these
situations have occurred at plants with on-site thermal mercury
recovery units, it has been common for these units to continue to
operate to assist in the treatment of wastes associated with the
shutdown/conversion. Under the applicability of the 2003 Mercury Cell
MACT, these units are no longer an affected source after the chlorine
production facility ceased operating. Although these mercury recovery
units were required to continue to use controls as per their state
permits, these proposed amendments would require any mercury recovery
unit to continue to comply with the requirements of the Mercury Cell
MACT for such units even after closure or conversion of the chlorine
production facility, as long as

the mercury recovery unit continues to operate to recover mercury.

D. Details of the Petition for Reconsideration

    On February 17, 2004, under section 307(d)(7)(B) of the Clean Air
Act, the NRDC submitted to EPA an administrative petition asking us to
reconsider the 2003 Mercury Cell MACT. NRDC and the Sierra Club also
filed a petition for judicial review of the rule in the U.S. Court of
Appeals for the DC Circuit (NRDC v. Sierra Club v. EPA, Civ. No. 04-
1048). Underlying many of the issues raised in the petition for
reconsideration was the uncertainty associated with the fugitive
emission estimates used by EPA in the rulemaking. In particular, the
NRDC had concerns over the inability of mercury cell plants to account
for all the mercury added to their processes to replace mercury that
leaves in products or wastes or leaves via air emissions. NRDC, along
with a number of other concerned parties who submitted comments on the
July 2002 proposed rule, believed that the majority of this ``missing''
or unaccounted mercury must be lost through fugitive emissions. They
also contended that recognition of this asserted fact would cause EPA
to change many of the decisions that had been made in developing and
promulgating the 2003 Mercury Cell MACT. Specifically, NRDC raised the
following five issues in its petition:

    (1) EPA refused to establish a numeric emission standard for the
cell room, choosing instead to develop a set of work practices
designed to minimize emissions. NRDC argued that under Clean Air Act
section 112(h) EPA is permitted to substitute work practices for
emission limits only upon a finding that ``it is not feasible * * *
to prescribe or enforce an emission standard.''
    (2) EPA's 2003 Mercury Cell MACT unreasonably backtracked from
the work practices the Agency proposed. As part of the regulatory
effort, EPA had surveyed the work practices used by facilities in
the industry and concluded that the housekeeping activities that
sources followed to comply with the part 61 Mercury NESHAP
represented the MACT floor. The EPA then required these detailed
housekeeping practices that were based upon the best levels of
activity in the industry. But despite the results of its survey and
findings, EPA made the work practices optional in the 2003 Mercury
cell MACT, allowing facilities to choose not to do the housekeeping
activities and to instead perform continuous monitoring. EPA then
stated that ``a comprehensive continuous cell room monitoring
program should be sufficient to reduce fugitive mercury emissions
from the cell room without imposing the overlapping requirements of
the detailed work practices.''
    (3) EPA failed to consider non-mercury technology as a beyond-
the-floor MACT control measure for existing sources even though
eliminating the mercury cell process would totally eradicate mercury
emissions and also would be cost-effective, based on NRDC's
expectations of the amount of fugitive mercury emissions from
subject sources.
    (4) EPA eliminated a 2,300 g/day limit on plant-wide mercury
emissions that existed under the part 61 Mercury NESHAP. NRDC stated
that doing so violated the CAA because the law generally prohibits
the new emission standards under section 112 from weakening more
stringent existing requirements.
    (5) EPA inappropriately decided to create a subcategory of
mercury cell plants within the chlorine production category.

    In a letter dated April 8, 2004, EPA generally granted NRDC's
petition for reconsideration, and indicated we would respond in detail
in a subsequent rulemaking action. In addition, in meetings between EPA
staff and NRDC representatives, EPA agreed to address the uncertainty
of EPA's fugitive mercury emissions from this industry. The Court
stayed the litigation while the Agency addressed the uncertainty
issues, conducted additional testing, and reconsidered the rulemaking.

III. Summary of EPA's Reconsideration and Proposed Amendments

    In this section, we describe actions that we undertook in support
of the proposed reconsideration of the rule, especially as related to
the issues raised by NRDC in its petition for reconsideration. We
present our proposed conclusions and decisions in response to NRDC's
petition, and we summarize the rule amendments that we are proposing in
today's action, along with our estimate of the impacts of these
amendments.
    These proposed amendments would be applicable to affected
facilities when the final rule amendments are published, with proposed
compliance periods of 60 days for facilities that have complied with
the 2003 Mercury Cell MACT by selecting the continuous cell room
monitoring option of that rule, and 2 years for facilities that have
complied with the 2003 Mercury Cell MACT by selecting the work practice
option. Mercury recovery units at sites where mercury cells are closed
or converted after the date that the final rule amendments are
published would be required to comply with the requirements of the
final amendments as long as they are in operation.

A. What were the issues that EPA reconsidered, and what are EPA's
proposed responses?

    As discussed above in section (II)(D), NRDC's petition listed five
specific issues. Our reconsideration of each of these issues is
addressed below. First, however, we also present a discussion of
another issue that we believe relates to much of NRDC's petition: The
magnitude of the fugitive mercury emissions from mercury cell chlor-
alkali plants.
1. Magnitude of Fugitive Mercury Emissions from Mercury Cell Chlor-
alkali Plants
    It has been difficult to quantify fugitive mercury emissions from
mercury cell chlor-alkali plants. During most of the time when the 2003
Mercury Cell MACT was being developed, we were aware of fewer than five
mercury emissions studies conducted over the last 30 or more years in
the U.S. and Europe that measured fugitive emissions from mercury cell
plants. Two of these studies were conducted by EPA in the early 1970's
and formed the basis for the assumption of 1,300 g/day mercury cell
room emissions of the part 61 Mercury NESHAP. During the development of
the 2003 Mercury Cell MACT, EPA conducted a study at Olin Corporation's
mercury cell plant in Augusta, Georgia (hereafter called ``Olin
Georgia''), that provided an additional estimate of fugitive mercury
emissions.
    In the time period since mercury cell chlor-alkali plants were
required to comply with the part 61 Mercury NESHAP, which was
promulgated in April of 1973, we are not aware of any facility that
conducted testing to demonstrate compliance with the cell room emission
limitation of the part 61 Mercury NESHAP. Instead, all facilities
carried out the set of approved design, maintenance, and housekeeping
practices and assumed fugitive mercury emissions of 1,300 g/day, as was
permitted by the part 61 NESHAP.
    The sensitivity and concern over the actual levels of fugitive
mercury emissions from the cell rooms was exacerbated by the inability
of the industry to fully account for all the mercury that was added to
the cells. In the preamble to the final 2003 Mercury Cell MACT (68 FR
70920), we stated the following: ``Even with this decrease in
consumption, significant mercury remains unaccounted for by the
industry. The mercury releases reported to the air, water, and solid
wastes in the 2000 Toxics Release Inventory (TRI) totaled around 14
tons. This leaves approximately 65 tons of consumed mercury that is not
accounted for in the year 2000.'' While industry representatives
provided explanations for this discrepancy, they could not fully
substantiate their theories.

    Although we acknowledged the uncertainty in the accounting of all
the mercury, we stated in the 2003 Mercury Cell MACT that no evidence
has ever been provided to indicate that the unaccounted mercury is
emitted to the atmosphere via fugitive emissions from the cell room or
otherwise. In its petition for reconsideration and in other
correspondence, NRDC cites information that it believes supports a
conclusion that the unaccounted mercury is emitted from the cell room.
However, NRDC did not address studies that have been conducted to
measure fugitive mercury emissions from mercury cell plants that rebut
that conclusion.
    Historically, the highest daily emission rate reported for any cell
room has been approximately 2,700 g/day for a plant operating in 1971,
which was before the part 61 Mercury NESHAP was in effect. More recent
studies show fugitive mercury emissions considerably lower than the
1,300 g/day assumption in the part 61 Mercury NESHAP. For example, a
study in 1998 at the Holtrachem facility in Orrington, Maine, estimated
a fugitive mercury emission rate between 85 and 304 g/day. A study in
Sweden in 2001 estimated a daily fugitive emission rate of 252 g/day.
While NRDC cites various peripheral aspects of the EPA study in 2000
study at Olin's Georgia mercury cell plant, NRDC does not discuss a
primary conclusion of the test: That the facility was estimated to have
an average fugitive mercury emission rate of 472 g/day.
    While we were confident that the fugitive emissions from cell rooms
were not at the very high levels estimated by NRDC (at several tons per
year (tpy) per plant), we recognized that the body of fugitive mercury
emissions data could be improved. Therefore, as part of our
reconsideration of the 2003 Mercury Cell MACT, we collected additional
information on fugitive mercury emissions from mercury cell chlor-
alkali plants. The primary purpose of this effort was to address
whether the fugitive emissions from a mercury cell chlor-alkali plant
are on the order of magnitude of the historical assumption of 1,300 g/
day, corresponding to 0.5 tons per year (tpy) per plant, or on the
order of magnitude of the unaccounted for mercury in 2000, which would
correspond to 3 to 5 tpy per plant, or at some other level.
    In planning our information gathering efforts for this test
program, we recognized that all of the previous studies were relatively
short term. Fugitive mercury emissions from a mercury cell plant occur
for numerous reasons, with significant emission sources likely being
leaking or malfunctioning equipment and maintenance activities that
expose mercury normally enclosed in process equipment to the
atmosphere. One noteworthy NRDC criticism of the Olin Georgia study was
that no major ``invasive'' maintenance activities were performed during
the testing. Therefore, in designing our new study, we collected data
over a number of months during a wide range of operating conditions and
during times when all major types of maintenance activities were
conducted.
    Consequently, as part of the reconsideration efforts for the 2003
Mercury Cell MACT, EPA sponsored a test program to address the issue of
the magnitude of the fugitive mercury emissions at mercury cell chlor-
alkali plants. We visited five mercury cell chlor-alkali plants to
identify and evaluate the technical, logistical, and/or safety issues
associated with the measurement of fugitive emissions from the mercury
cell rooms as part of a test program. The result of these efforts was
that we sponsored two emissions testing programs: One at the Olin
mercury cell chlor-alkali plant in Charleston, Tennessee (hereafter
called ``Olin Tennessee''), to estimate mercury emissions from one of
its three cell rooms; and the other at the Occidental Chemical mercury
cell chlor-alkali plant in Muscle Shoals, Alabama (hereafter called
``Occidental Alabama''), to estimate their total site mercury
emissions. These testing programs are discussed in detail later in this
notice.
    In addition to these emissions measurements, we also collected
mercury emissions data from the continuous mercury monitoring system
installed at three mercury cell plants: The Occidental facility in
Delaware City, Delaware (hereafter called ``Occidental Delaware'');
Occidental Alabama; and Olin Tennessee, which was also a site for the
EPA emissions measurement tests. We also performed validation studies
of the air flow measurement systems and mercury monitors at these three
facilities.
    In addition, we compared maintenance logs and mercury emissions
data to establish the correlation, if any, between maintenance
activities and mercury emissions using data from Occidental's
facilities. And finally, we addressed the issue of significant sources
of fugitive mercury emissions from outside the cell room from the data
acquired at the EPA-sponsored total site emissions tests at Occidental
Alabama.
    The descriptions of the emissions testing and data gathering
efforts are summarized below along with our estimates of fugitive
mercury emissions derived from these studies. The full emissions test
reports, two memoranda that summarize the test reports, validation
reports, and summaries of the mercury monitoring system emissions data
analyses can be found in the docket to this proposed rule (EPA-HQ-OAR-
2004-0017), and were previously provided to NRDC and industry
representatives.
a. Description of EPA-Sponsored Mercury Emissions Tests at Two
Facilities
    Olin--Charleston, Tennessee. This test was performed over a six-
week period from August to October 2006 using a long-path ultraviolet
differential optical absorption spectrometer (UV-DOAS) to continuously
measure the mercury concentration in the ventilator and an optical
scintillometer (anemometer) to measure the velocity. Emission estimates
were reported for each 24-hour period. The test report can be found in
the docket, item number EPA-HQ-OAR-2002-0017-0056.3.
    The Olin Tennessee facility has three cell rooms installed adjacent
to one another. The E510 cellroom (startup in 1962) is a simple
rectangular design with two rows of cells. The E812 cell room (startup
in 1968) is also a simple rectangular design with two rows of cells. In
1974, Olin added a third cell room with additional E812 cells just
south of the existing E812 cell room. A central control area was
installed between the E510 and E812 cell rooms. In addition, an
elevator and computer equipment area was installed between the two
original plants. The area between the original E812 cells and the E812
10-cell Expansion is fully open. Each of the three cell rooms has a
full length, natural draft ventilator mounted on the roof. Fans have
been installed at the cell floor level around the perimeter of the E510
and E812 cell rooms to enhance cool air flow in key work areas. In
addition, high velocity fans were installed near the central control
area to aid air movement in ``dead zones'' created by the control area
walls. There are no exhaust fans in any of the cell rooms.
    Logistical and cost considerations resulted in the E510 cell room
being selected for the EPA test. Continuously measuring the mercury
emissions from more than one ventilator simultaneously was not
practical, based on the limited availability of equipment and the
complexities related to the operation of a number of highly
sophisticated

measurement devices. The small size of the E812 Expansion cell room
excluded it from consideration, and the complicated flow patterns
between the E812 and E812 Expansion rooms would have made it very
difficult to account for all the associated uncertainties using only
one monitor. The configuration of the E510 cell room, the relatively
straightforward air flow pattern, and the structure of the ventilator
(which allowed easy access and a clear path for the beams) made it the
obvious choice for the test program to optimize our ability to obtain
the most reliable data.
    Occidental--Muscle Shoals, Alabama. This test was conducted over 53
days, from September 21, 2006, through November 12, 2006, to measure
total site mercury emissions. For this study, the ``total site''
included emissions via the cell room ventilation system, the stacks/
point sources (thermal mercury recovery unit vent, hydrogen byproduct
vent, end-box ventilation vent), and any fugitives that occurred
outside of the cell room in adjacent process areas. The measurement
approach used a Vertical Radial Plume Mapping (VRPM) measurement
configuration employing three open-path UV-DOAS instruments for
elemental mercury concentration measurements, in conjunction with
multipoint ground level mercury measurements with a Lumex mercury
analyzer. The total site mercury emissions were estimated using these
concentration measurements and meteorological data (e.g., wind speed,
wind direction).
    The measurement systems operated on a 24 hour, 7 day per week basis
for the 53-day campaign. The 3-beam VRPM configuration used to estimate
elemental mercury emissions from the facility was located at a fixed
position and fixed orientation on site for the duration of the project.
Calculations of mercury flux through the VRPM plane were conducted only
when specific data quality indicators involving wind speed, wind
direction, path averaged concentration ratios and instrument operation
were met. During the 53-day emissions test program, VRPM mercury flux
values were able to be calculated for 23 days. Data were reported as
daily (24 hour) emission values that were extrapolated from rolling 20-
minute averages calculated every four minutes. A total of 1,170 mercury
emission flux estimates were produced during the 23 days. The test
report can be found in the docket, item number EPA-HQ-OAR-2002-0017-
0056.5.
    The cell room at the now closed Occidental Alabama plant was a
rectangular building measuring 260 feet by 357 feet. The cell room
consisted of two rows of cells broken into four sections. The cell room
took up half of a larger building, with a wall separating the cell room
from the other half of the building that was used for equipment
storage. The peak of the roof was over the wall separating the cell
room from the other side of the building. The ventilation for the cell
room consisted of both induced and forced draft fans. There were 43
forced-draft fans positioned on the side wall of the building pushing
air towards the center of the building. There were two rows of induced-
draft fans on the roof of the cell building. One row, containing 33
fans, was directly over the center of the two rows of cells. The other
row, which contained 32 fans, was at the peak of the roof. The result
was that the building was constantly under a slightly negative
pressure.
b. EPA Validations of Mercury Monitoring Systems in Cell Rooms of
Mercury Chlor-Alkali Plants
    During the time we were planning the testing programs to estimate
fugitive mercury emissions via an EPA-sponsored test program, the
mercury cell chlor-alkali industry was undertaking its own long-term
mercury emissions estimation efforts. Two Occidental mercury cell
plants (Delaware and Alabama) installed mercury monitoring systems in
their cell rooms in 2005, and the Olin Tennessee facility installed a
mercury monitoring system in 2006. The plants used these systems to
identify and correct mercury emission episodes in accordance with the
alternative cell room monitoring program of the 2003 Mercury Cell MACT.
Specifically, the facilities monitored physical and chemical parameters
in the cell room, such as air flow and mercury concentration, that
allowed the continuous estimation of the relative mass of mercury
emissions leaving the cell room. Since these plants had already
installed and were currently running their mercury monitoring systems,
we included the collection and evaluation of data from these systems in
our data gathering program. The overall goal of our validation program
was to provide a qualitative assessment of the mercury monitoring
systems at these three facilities.
    There were three specific objectives of the EPA validation studies.
The first objective was to verify that facility data processing and
archiving were being performed correctly. This was accomplished through
comparison of facility data with independently calculated values for
elemental mercury mass emission rates. These independent calculations
utilized the same equations and raw input data as the company data
systems. The second objective was to establish a confidence level for
the accuracy of the measured elemental mercury concentrations. To
accomplish this, a systems assessment was performed using calibration
standards to challenge the mercury analyzer with a known concentration
of mercury and to compare the analysis results with the certified
concentration of the calibration standard. The goal of this assessment
was an evaluation of short-term operation of the elemental mercury
analyzer and effectiveness of routine maintenance and calibration
activities that may impact long-term operation of the instrument. The
third objective was to establish a confidence level associated with the
flow determinations. Since each cell room has a unique ventilation
system, this flow determination validation was done somewhat
differently for each mercury monitoring system.
    The following are descriptions of the mercury monitoring system at
each faculty and the results of the corresponding validation studies.
The final reports for the validation program at the two Occidental
facilities can be found in the docket to this rule (see docket items
EPA-HQ-OAR-2002-0017-0057 and 0017-0058). The validation tests
performed at Olin's Tennessee facility are included within the
emissions test report described above (see docket item number EPA-HQ-
OAR-2002-0017-0056.3).
    Occidental--Delaware City, Delaware. Validation tests were
performed by EPA at Occidental's now closed facility in Delaware the
weeks of August 22, 2005, and September 9, 2005. The cell room at the
Delaware City Plant was a rectangular building measuring 352 feet by
140 feet. The cell room consisted of two independent circuits, and each
circuit was broken into two sections, resulting in four quadrants. The
air flow in the cell room was via natural convection; there were no
fans to provide either induced or forced draft air flow. During the
summer months, approximately 40 percent of the sides on the lengthwise
span were removed to improve ventilation. There were two rows of roof
ventilators. Each ventilator was in two discrete sections for a total
of four sections (corresponding to the four quadrants of the cell
room).
    The mercury monitoring system at the Occidental Delaware facility
was a Mercury Monitoring System Model MMS-16 analyzer manufactured by
Mercury Instruments GmbH Analytical Instruments in Germany. It collects
samples from 16 points and analyzes

them for elemental mercury using a Model VM-3000 ultraviolet absorption
analyzer. The mercury monitoring system takes one sample per minute,
meaning that a sample is taken from each point once every 16 minutes.
The sampling sequence is established so that a sample is taken from
each quadrant once every four minutes. The flow rate for the building
is estimated using a convective air flow model. The inputs to this
model are atmospheric and ridge vent temperatures (which are
continuously monitored), intake and discharge areas, and stack height.
    The validation of the Occidental Delaware mercury monitoring system
confirmed the accuracy of the data collection, calculation, and
archiving system. With regard to the data quality of the mercury
analyzer, mercury calibration accuracy results for the Delaware City
instrument were 20 percent and 10 percent for the mid- and high-range
calibration standards, respectively. Specifically, the analyzer
reported a concentration of 8 micrograms per cubic meter ([mu]g/
m3) for the 10 [mu]g/m3 standard and a
concentration of 45 [mu]g/m3 for the 50 [mu]g/m3
standard. These results, along with the line integrity test results,
suggest that the high range calibration of this instrument was offset
in a negative direction.
    A qualitative assessment of the accuracy of the Delaware City
facility's approach to flow estimation was made with independent, on-
site, flow measurements using a vane anemometer at the roof vents.
These measurements, covering multiple sampling points, were averaged
and compared to the average air flow determined using the convective
flow model equations used to estimate the flow. This evaluation showed
that the difference between the anemometer and convective flow model
methods was 29 percent, with the convective flow model reporting a
higher value than the anemometer tests.
    Occidental--Muscle Shoals, Alabama. Validation tests were performed
by EPA at Occidental Alabama the week of September 12, 2005. The
mercury monitoring system at this facility was a Mercury Monitoring
System Model MMS-16 analyzer manufactured by Mercury Instruments GmbH
Analytical Instruments in Germany. The elemental mercury concentration
is measured using a Model VM-3000 ultraviolet absorption analyzer. The
mercury monitoring system collects samples from 65 points (at the inlet
to each induced draft fan) and combines them in groups of three or four
to provide a representative profile of the cell room in a 20 point
sample array. The mercury monitoring system takes one sample per
minute, meaning that a sample is taken from each point once every 20
minutes. We previously described the cell room at Occidental Alabama,
above.
    To estimate the flow rate from the cell room, Occidental tested
each fan to determine the flow rate at standard conditions and to
correct the actual flow rate based on continuous monitoring of
temperature, pressure, and humidity. The assessment of the accuracy of
the Muscle Shoals facility's flow estimation procedure was made with
independent, on-site, flow measurements at each of the 65 fan outlets.
The total flow through all 65 fans was measured at five points within
the fan exhaust area using an anemometer. The exhaust flow from each
fan was determined by averaging these five flow values. Total flow from
the cell room was determined by subsequently summing the flow from each
fan during the test period. The difference between the anemometer and
fan flow model methods was slightly more than 7 percent, with the
exhaust fan model reporting a higher value than the anemometer
validation tests.
    The validation of the Occidental Alabama continuous mercury
monitoring system confirmed the accuracy of the data collection,
calculation, and archiving system of the facility. The mercury
calibration accuracy results for the Muscle Shoals facility instruments
were 4.0 percent and 0.2 percent, for the mid- and high-range
calibration standards, respectively. These results indicate that the
Muscle Shoals mercury analyzer was in good operating condition with no
apparent calibration problems at the time of the validation test.
    Olin--Charleston, Tennessee. Validation tests were performed by EPA
at the Olin Tennessee facility during the month of September 2006. We
previously described the cell rooms at the Olin Tennessee plant, above.
This facility has two separate mercury monitoring systems: One for the
E510 cell room and one for the E812/E812 Expansion rooms. These mercury
monitoring systems are Mercury Monitoring System Model MMS-16 analyzers
manufactured by Mercury Instruments GmbH Analytical Instruments in
Germany. The mercury monitoring system collect samples from individual
points and analyze them for elemental mercury using a Model VM-3000
ultraviolet absorption analyzer. In each of the cell rooms, there are
five sampling points evenly spaced along the ventilators. In addition
to the sample points in the ventilators (five for the E510 system and
ten for the E812/812 Expansion system), each mercury monitoring system
has one sample point dedicated to continuously measuring mercury for
point sources subject to the 2003 Mercury Cell MACT, and one point used
for calibration. Each point is sampled for one minute and the
concentration is held and used in calculating the overall cell room
average concentration until the point is sampled in the next cycle.
Hourly and daily rolling averages are then calculated and stored. The
flow rates for the cell rooms are estimated separately using a
convective air flow model. The inputs to this model are atmospheric and
ridge vent temperatures (which are continuously monitored), intake and
discharge areas, discharge height, and fans on/off operation.
    The mercury calibration accuracy results for the instrument in the
E510 cell room were approximately 8 percent and 19 percent for the mid
and high range calibration standards, respectively. For the E812/812
Expansion System, the results were approximately 5 percent and 20
percent for the mid and high range calibration standards, respectively.
Both analyzers indicated higher concentrations than the certified
calibration standards provided by the manufacturer.
    Manual flow measurements were made in each of the cell room roof
vents using a vane anemometer. These manual flow measurements were not
compared directly with flow rates estimate by Olin's convective flow
model. The accuracy of the facility's model was assessed in a two-step
process. The manual measurements for the E510 cell room were first
compared with the air flow measurements estimated using the optical
anemometer in the EPA test, and then compared with the estimates from
the Olin flow model. The accuracy determination between the optical
flow monitor and the manual flow measurements was slightly lower than
10 percent. The flow rate estimated using the Olin flow model was
approximately 5 percent higher than the flow rate measured by the
optical flow monitor over the entire testing period.
c. Analyses of Cell Room Maintenance Logs and Mercury Emissions Data
    Occidental also provided detailed maintenance records for the April
through November 2005 (Delaware) and August 2005 through January 2006
(Alabama) time periods in addition to their emissions data. They also
provided production data and details of ``alarm events'' for this
period, where an alarm event was a situation in which the monitoring
system recorded a mercury concentration above established action
levels. When such an alarm occurred,

Occidental personnel were dispatched to the area of the cell room where
the elevated concentration was detected to identify the specific cause
and to take corrective actions. We performed an analysis of the effect
of maintenance activities, alarm events, production levels, and ambient
conditions on daily fugitive mercury emission levels. While we
recognize that maintenance activities and alarm events can result in
short-term spikes in emissions, our analyses of the data did not show
any correlation between daily fugitive mercury emissions and these
events. The only factor that showed any correlation, albeit weak, to
daily emissions was the ambient temperature. The report of these
analyses can be found in the docket.
d. No Significant Fugitive Sources of Mercury From Outside the Cell
Room
    In addition to obtaining total site emission estimates at
Occidental Alabama, we attempted to ascertain whether fugitive sources
outside of the cell room were contributors of measurable emissions by
performing a material balance on the contributors to the total site
emissions and solving for the outside fugitive component.
    The ``total site'' mercury emissions for this study included
emissions via the cell room ventilation system, the stacks/point
sources (thermal mercury recovery unit vent, hydrogen by-product vent,
end-box ventilation vent), and any fugitives that occurred outside of
the cell room in adjacent process areas. From a material balance
analysis of these data, we concluded that fugitive sources outside the
cell room do not contribute measurable mercury emissions when compared
to fugitive emissions from the cell room (see docket items EPA-HQ-OAR-
2002-0017-0056.5 and 0017-0056.6).
e. New EPA Fugitive Mercury Emission Estimates for Cell Rooms
    We used eight separate fugitive mercury emission data sets from
three different mercury cell chlor-alkali plants in 2005 and 2006 to
produce a new estimate of fugitive mercury emissions from cell rooms.
The time periods of data collection range from 6 weeks to over 30
weeks, all of which provided an opportunity to include a complete range
of maintenance activities and operating conditions. Two of the data
sets were generated via EPA-sponsored test programs and the others were
collected from cell room mercury monitoring systems that were validated
by EPA. Summaries of the data sets can be found in the docket.
    The daily mercury emission rates extrapolated from these data sets
ranged from around 20 to 1,300 g/day per facility. The average daily
emission rates ranged from around 420 g/day to just under 500 g/day per
facility, with the mean of these average values being slightly less
than 450 g/day per facility.
    The purpose of this effort was to address whether the fugitive
emissions from a mercury cell chlor-alkali plant are on the order of
magnitude of the historical assumption of 1,300 g/day (or 0.5 tpy per
plant) or on the order of magnitude of the unaccounted for mercury in
2000 (3 to 5 tpy per plant, which equates to around 10,000 g/day). The
information we obtained shows that fugitive emissions are on the order
of magnitude of the historical assumption of 1,300 g/day. There was no
evidence obtained during any of the studies that indicated that
fugitive mercury emissions were at levels higher than 1,300 g/day. In
addition, all of the studies that produced these data were of
sufficient duration to encompass all types of maintenance activities,
including the major ``invasive'' procedures that were not conducted
during the earlier test at the Olin Georgia facility. The length of
these studies was also sufficient to include emissions from a variety
of process upsets, such as: Liquid mercury spills, leaking cells, and
other process equipment, and other process upsets (see docket items
EPA-HQ-OAR-2002-0017-0021 and 0017-0029).
    The results of the almost one million dollar study of fugitive
emissions from mercury cell chlor-alkali plants sponsored by EPA
enables us to conclude that the levels of fugitive emissions for
mercury chlor-alkali plants are much closer to the assumed emissions in
the part 61 Mercury NESHAP, of 1,300 g/day/plant (around 0.5 tons/yr/
plant) than the levels assumed by NRDC (3 to 5 tons/yr/plant). The
results of this study suggest that the emissions are routinely less
than half of the 1,300 g/day level, with overall fugitive emissions
from the five operating facilities estimated at less than 1 ton per
year of mercury.
f. Conclusions on the Use of Mercury Monitoring Systems as a Work
Practice Tool
    In the data we obtained or examined, we saw discrepancies between
the measured concentrations and the calibrated standards, and
differences between the flow rates estimated by the cell room systems
and those estimated by anemometers (manual or optical), as summarized
above. The differences for the measurement of the mercury concentration
were as high as 20 percent, and the differences in the measurements for
the flow rates were as high as 29 percent. Such differences lead us to
conclude that these systems would not be suitable to accurately
demonstrate compliance with a numeric standard, because of the
potential for errors in compliance determinations due to uncertainties
in the measurement techniques. However, since the goal of this effort
was to assess the order of magnitude of fugitive mercury emissions from
the cell room, we concluded that data from these systems were
appropriate for that purpose since the differences were well within an
order of magnitude.
    Our observations at these three plants during the validation
programs resulted in recognition of the ability of the mercury
monitoring system to be used as a work practice tool to reduce fugitive
emissions in the cell room. When the 2003 Mercury Cell MACT was
promulgated, we thought that the mercury monitoring system could help
identify problems before significant emission events occurred. However,
at that time no mercury cell plant in the United States had installed
such technology so there was no opportunity to assess their
effectiveness. Now, with data from the three plants described above, we
can conclusively say that the mercury monitoring systems aid in the
identification and correction of fugitive emission problems and help
plants refine their standard operating procedures and work practices to
further reduce emissions. Therefore, we believe that the use of such
systems as a tool to determine the effectiveness of work practices has
been demonstrated. We estimate that the cost of installing a system in
a cell room is about $120,000, which equates to a total annual cost
(including annualized capital cost and operation and maintenance costs)
of slightly over $25,000 per year. We believe that in the long term
these systems will result in continued decreases in fugitive mercury
emissions as plants will be able to identify emission-reducing
improvements in their processes and practices. Therefore, we are
proposing to require all mercury cell chlor-alkali plants to install
cell room mercury monitoring systems and to develop a cell room
monitoring plan.
g. Estimate of the Efficiency of the Cell Room Monitoring Program To
Reduce Fugitive Emissions
    In the 2003 Mercury Cell MACT, we noted our inability at that time
to quantify the emission effects of adopting the cell room work
practices, a point also noted by NRDC in its petition for

reconsideration. However, we are now able to better estimate the
emissions reductions achieved by the cell room monitoring program and
work practices for these amendments using the results of the test
programs and other information gathering efforts, as described above.
    We estimated that baseline mercury emissions prior to the 2003
Mercury Cell MACT were 1,300 g/day per facility (68 FR 70923). This
equated to nationwide pre-MACT baseline fugitive emissions of 4.7 tpy.
The test program data suggest that on average, the fugitive mercury
emissions from a single facility are approximately 450 g/day, which
equates to nationwide emissions of 0.9 tpy. Therefore, we estimate that
the combination of the work practices promulgated in the 2003 Mercury
Cell MACT combined with cell room monitoring reduces fugitive mercury
emissions from a single facility by over 65 percent from the pre-MACT
levels. On a nationwide basis, we estimate that fugitive mercury
emissions have been reduced by approximately 86 percent, including
plant closures.
    The point source emissions (from hydrogen vents, end-box
ventilation systems, and mercury recovery units) from the five mercury
cell plants expected to be in operation after these amendments are
finalized are around 0.4 tons/yr total. Therefore, our estimate of the
nationwide total mercury emissions from all emission sources (point and
fugitive) at these plants is around 1.3 tons/yr.
2. Elimination of Uncertainty Regarding the ``Missing'' Mercury
    Mercury is not consumed in the mercury cell chlor-alkali plant
process. Therefore, in theory, the amount of mercury that is added to
the process should be equal to the amount of mercury that leaves the
process in either air, water, or waste pathways. In other words, the
mercury going into the system should approximately equal the mercury
leaving the system, where the ``system'' is the entire plant.
Historically, the industry has had a difficult time closing this
mercury balance, as the amount of mercury added has exceeded the amount
measured in the wastes, wastewater, products, and air leaving the
plant. This difference has been referred to as the ``missing'' or
unaccounted mercury. The primary basis for NRDC's estimates of fugitive
mercury emissions from mercury cell chlor-alkali plants was the 65 tons
of mercury that could not be fully accounted for by the industry at
that time in their plant-wide inventories (in 2000).
    The EPA emissions testing and data gathering efforts discussed
above did not independently resolve the unaccounted mercury issue.
However, since promulgation of the 2003 Mercury Cell MACT, the level of
mercury that is unaccounted for by the industry has diminished
drastically. The industry reported a total of 7 tons of unaccounted for
mercury in 2004, and 3 tons in 2005,\a\ with the estimate for 2006 even
lower.
---------------------------------------------------------------------------

    \a\ ``NINTH ANNUAL REPORT TO EPA for the Year 2005, May 15,
2006.'' http://www.epa.gov/region5/air/mercury/9thcl2report.pdf.
---------------------------------------------------------------------------

    This reduction in the unaccounted mercury is likely due to
increased efforts by the affected industry to inventory and track
mercury in their plants, rather than to large reductions in mercury
being released to the air, water, or in wastes. During our visits to
mercury cell plants since promulgation of the 2003 Mercury Cell MACT,
we have developed a fuller understanding of the components of a plant-
wide mercury balance.
    One of the most significant improvements in estimating this balance
has been in the estimation of the amount of mercury in the cells. Most
plants now utilize a radioactive tracer method to estimate the mercury
inventory in the cells. Previously, some plants did not use scientific
methods to conduct an inventory of the mercury in the cells. The
radioactive tracer method is accurate to around 1 percent. So, for a
mercury cell plant that has about 300 tons of mercury in the cells,
this error could cause the mercury balance to be inaccurate by about 3
tons. For plants that did not conduct a scientific inventory, their
errors could result in significantly greater variability in the mercury
inventory estimates for the mercury cells. If each of 10 plants had
only factors of two errors in the accuracy of their mercury cell
measurements, the effect could be 60 or more tons of unaccounted
mercury for the cells alone.
    Another area where significant improvement in the mercury balances
has occurred is in estimating the amount of liquid mercury present in
pipes and other process equipment. As plants perform maintenance on
process equipment, they have measured the amount of mercury recovered
and have developed accumulation factors that are now incorporated into
the mercury balances procedures.
    The 3 tons of unaccounted mercury reported in 2005 for the eight
plants then in operation is, on average, approximately 750 pounds (lb)
per plant. Significantly contributing to this number are the
uncertainties in the various measurement techniques used to develop the
inventory. While the affected industry must continue to strive to
account for every pound of mercury that enters their processes, the
degree of uncertainty regarding the unaccounted mercury has been
substantially reduced since the time of promulgation of the 2003
Mercury Cell MACT.
3. Emission Limitation for Cell Room
    Two of the issues raised by NRDC in its petition for
reconsideration are related to their objection that the 2003 Mercury
Cell MACT did not include a numeric emission standard for fugitive
emissions from the cell room. First, NRDC states that EPA failed to
adequately justify that a numeric emission limitation was not feasible
per the criteria prescribed in section 112(h) of the Clean Air Act
(CAA). These criteria govern EPA's decisions to require a work practice
standard (or a design, equipment, or operational standard) in lieu of a
numerical standard under section 112. The CAA section 112(h)(1)
provides that the EPA can prescribe, consistent with sections 112(d) or
(f), a work practice if in the judgment of the Administrator it is not
feasible to prescribe or enforce an emission standard. The CAA section
112(h)(2) then defines the phrase ``not feasible to prescribe or
enforce an emission standard'' to mean either ``(A) a hazardous air
pollutant or pollutants cannot be emitted through a conveyance designed
and constructed to emit or capture such pollutant, or that any
requirement for, or use of, such a conveyance would be inconsistent
with any Federal, State or local law, or (B) the application of
measurement methodology to a particular class of sources is not
practicable due to technological and economic limitations.'' NRDC
argued that EPA did not provide sufficient rationale that a numeric
limit for the cell room is infeasible in order to support a work
practice standard in lieu of a numeric standard. Rather, NRDC referred
to the EPA test program at Olin's Georgia plant in 2000 as evidence
that the technology is available to monitor the cell room. Second, NRDC
states that EPA illegally eliminated the 2,300 g/day limit on plant-
wide mercury emissions that existed under the part 61 Mercury NESHAP.
    Both of NRDC's objections regard the 2003 Mercury Cell MACT's
addressing of emissions from the cell rooms only through maintenance
activities. NRDC noted in their petition that while EPA stated that we
expected these maintenance activities would minimize

mercury emissions, we did not quantify the effect adopting these
practices would have on the emissions.
    In setting the work practice standards in the form of maintenance
activities in the 2003 Mercury Cell MACT, we referred to section 112(h)
of the CAA to provide clarification on how EPA must determine the
feasibility of prescribing or enforcing an emission standard. NRDC
claims that EPA failed to provide adequate justification that any of
the section 112(h)(2) conditions were met, and therefore that we did
not validly conclude that the establishment or enforcement of a numeric
emission limitation is infeasible.
    We continue to maintain that it is not feasible to prescribe or
enforce an emission limitation for fugitive emissions from the cell
room. We also maintain that fugitive emissions from mercury cells and
associated equipment is a clear example of the type of situation to be
addressed by the provisions of section 112(h). The various points
leading to our opinion on the feasibility of establishing an emission
standard, as well as our response to the claim that we inappropriately
removed a previously existing standard, are discussed below.
a. Mercury Emissions From Mercury Cells and Associated Equipment Cannot
Be Emitted Through a Conveyance Designed and Constructed To Emit or
Capture Mercury
    In its petition, NRDC discusses the ``cell room'' as if the room
itself is the source of mercury emissions. This perception
oversimplifies the actual situation. There are numerous potential
sources of fugitive mercury emissions associated with mercury cells,
ranging from the cells and decomposers to the hydrogen processing
system to hundreds of pumps, valves, and connectors in the process
piping. On average, cell rooms contain around 60 mercury cells, each
with a decomposer. Fugitive mercury emissions primarily occur when the
cells and the other process equipment develop leaks.
    EPA has a long history of demonstrating that ``equipment leaks'' in
the chemical industry are justifiably regulated by design, equipment,
work practice, and operational standards in accordance with section
112(h). One of the best examples of EPA's regulation of equipment leaks
is the Hazardous Organic NESHAP, or HON (40 CFR part 63, subpart H),
which regulates equipment leaks from the synthetic organic chemical
manufacturing industry through only work practices 57 FR at 62666
(December 31, 1992). A few examples of many other MACT standards that
use similar work practice programs to address equipment leaks include
the Gasoline Distribution MACT (40 CFR part 63, subpart R) 59 FR at
5868 (February 8, 1994); the Generic MACT which covers numerous source
categories (40 CFR part 63, subparts TT and UU) 63 FR at 55197 (October
14, 1998); and the Miscellaneous Coatings MACT (40 CFR part 63, subpart
HHHHH) 67 FR at 16168 (April 4, 2002).
    However, design, equipment, work practice, and operational
standards are not unique to organic HAP emissions. Other examples
include the MACT for Hydrogen Fluoride, which is covered under the
Generic MACT cited above and the Coke Ovens Pushing, Quenching, and
Battery Stacks MACT (40 CFR part 63, subpart CCCCC) 66 FR at 35338
(July 3, 2001).
    We do not believe that the cell room building can be considered as
a conveyance designed and constructed to emit or capture mercury. The
primary purpose of the cell room building is not to capture mercury
emissions, but rather, to protect the process equipment from the
weather and other potentially damaging elements. Similarly, the primary
purpose of the ventilation systems in the cell room is to remove the
heat generated in the electrolytic process, and not to remove the
mercury. As noted earlier, there are numerous sources of fugitive
emission sources in the cell room, ranging from the large cells and
decomposers to individual valves. In order to effectively emit and
capture mercury emissions from these sources, separate enclosed
conveyance systems would need to be designed and constructed for
individual potential emission sources or for groups of potential
emission sources. Even if construction of such enclosures was
physically possible, it would severely limit access to process
equipment, thus hindering plant personnel from performing maintenance.
This could, in effect, result in increased fugitive emissions.
    Therefore, due to the nature of the sources of fugitive emissions
from mercury cells and associated equipment, we conclude that these
emissions cannot be emitted through a conveyance designed and
constructed to emit or capture mercury.
b. The Application of Measurement Methodology to Fugitive Emission
Sources From Mercury Cells and Associated Processes in Cell Rooms for
Compliance Purposes is not Practicable due to Technological and
Economic Limitations
    In the 2003 Mercury Cell MACT, we stated that our reason for
establishing work practices instead of numeric emission limits was
based on factors associated with the practicality and feasibility of
setting a limit against which compliance realistically can be measured
and enforced. EPA cited three reasons for our conclusion in the 2003
Mercury Cell MACT:

    (1) Mercury emission monitors have not been used in the past to
monitor fugitive emissions at mercury cell chlor-alkali facilities
for compliance demonstrations;
    (2) Variability in the number and location of exhaust vents at
these facilities affects the amount and potential variability of air
moved through the cell rooms, thus affecting calculations of
fugitive mass emission rates; and
    (3) Variability of the cell room roof configurations within the
industry affects the feasibility of using continuous mercury
monitoring systems at each facility.

    While NRDC did not directly refute these statements, it provided
three specific points to support its view that emissions from cell
rooms could be feasibly measured from a technological perspective: (1)
Although EPA envisioned that chlor-alkali plants could install cell
room mercury vapor monitoring to comply with the 2003 Mercury Cell
MACT, EPA did not show why this monitoring could not also
quantitatively measure mercury emissions from the cell room for a
standard; (2) since all of the operating plants already conduct basic
monitoring of the cell room in keeping with Occupational Safety and
Health Administration (OSHA) standards for worker exposure to mercury,
EPA should also be able to require testing for its own standards; and
(3) EPA ignored and failed to take advantage of a substantial EPA
monitoring initiative at the Olin Georgia mercury cell plant, launched
in 2000, which demonstrated that a measurement program needed to
support an emission limit can be feasibly applied to the cell room.
According to NRDC, the mercury vapor monitoring program required by the
2003 Mercury cell MACT and the monitoring programs conducted by mercury
cell plants to comply with OSHA standards are proof that a numeric
standard is technically feasible.
    We know that the two types of monitoring cited by NRDC can be used
reliably to identify leaks and thereby reduce fugitive mercury
emissions. The floor-level monitoring program of the 2003 Mercury Cell
MACT, which is used to identify potential mercury leaks and other
problems that could result in increased fugitive mercury emissions, is
similar to the use of Method 21 to identify leaking equipment in
volatile organic chemical service.

    Method 21 requires that a portable instrument be used to detect
volatile organic compound (VOC) leaks from individual sources such as
pumps, valves, etc. This instrument, often called a ``sniffer,''
measures the VOC concentration. Concentrations above specified levels
that are defined to constitute a leak result in a requirement for
corrective action to repair the leak. Though Method 21 is an extremely
useful method for identifying leaking equipment, it could not and has
not ever been required to demonstrate compliance with a numerical
emission standard. In fact, section 2.1 of Method 21 specifically
states ``This method is intended to locate and classify leaks only, and
is not to be used as a direct measure of mass emission rate from
individual sources.''
    The OSHA worker safety program requires plants to measure mercury
concentrations in areas where workers could be exposed to mercury
vapor. According to OSHA standards, employee exposure to airborne
mercury compounds may not exceed an 8-hour time-weighted average limit
of 1 mg/10 M3 (0.1 mg/M3). Mercury cell plants
typically comply with this standard by periodically measuring the
mercury concentration at selected points throughout the cell room at
the floor level. If concentrations approach the exposure limit, workers
are required to wear respirators to lessen their exposure in areas
where the high concentrations were identified. However, these
measurements of employee exposure to mercury vapor do not represent the
mercury concentration from the entire cell room and cannot be linked to
continuous compliance with a numeric standard.
    The EPA test at Olin's Georgia facility in 2000 not only provided
insights into monitoring techniques that could be implemented at
mercury cell plants to help reduce fugitive emissions, it also helped
answer some of the questions regarding the magnitude of fugitive
mercury emissions at mercury cell plants. This knowledge and experience
were a key aspect of our conclusions that a cell room monitoring
program could be an effective means of reducing fugitive emissions. The
success of this test program also played a large role in moving the
industry forward to develop and implement cell room monitoring programs
that are proving to be valuable in minimizing potential mercury
emission events in a manner not previously possible.
    However, the Olin Georgia test program was not used to demonstrate
the ability of the Olin Georgia plant, or any other facility, to comply
with a numeric emission standard. In the conclusions of the test report
from the Olin Georgia tests, it was stated that ``roof vent
instrumentation may be a useful tool for process monitoring in some
facilities to identify problems in the operation of the cells that may
require corrective action.'' In the report for the Olin Georgia study,
it is further noted that cell room conditions changed rapidly, which
affected their emissions measurements; therefore, mercury emission data
collection worked best when it was taken over a short period of time.
It was also stated in the Olin Georgia report that the mercury
concentrations in the roof vent were not homogeneously stratified and
the concentration of mercury was not consistent along the length of the
ventilator.
    We do not agree with NRDC that the success of the Olin Georgia
tests can be extrapolated to the mercury chlor-alkali industry's
ability to quantitatively measure fugitive emissions from all mercury
cell rooms for the purposes of an emission standard. We provide
additional information on this subject, below.
    Olin Georgia Cell Room Configuration--The Olin Georgia cell
building is a single structure that is approximately 200 feet long and
100 feet wide. The peak of the building is around 50 feet tall, and
there is a single ventilator that runs the entire length of the
building at the peak. The building has two stories, with the bottom
floor open to the atmosphere on three sides. The second floor, which
contains the mercury cells and decomposers, has wall panels that can be
opened or closed depending on ambient conditions. Ventilation occurs
via natural convection. Therefore, in periods when ambient temperatures
are higher and the sides are opened, the flow rate through the building
increases significantly.
    In EPA's Olin Georgia study, the mercury concentration was measured
by a UV-DOAS, and an optical scintillometer (anemometer) was used to
measure the air flow rate from the cell room. A single beam from each
of these instruments was shot along the path of the ventilator slightly
above the ``throat'' of the ventilator. A preliminary hypothesis might
be that concentration and flow measurements taken along this exit point
could provide a ``reasonable representation'' of the emissions from the
cell building. However, a ``reasonable representation'' to obtain an
estimate of mercury emissions for monitoring purposes is not equivalent
to an ``exact measurement'' for the purpose of demonstrating compliance
with a numeric emission standard. There were several aspects of the
Olin Georgia study that prevent us from considering the measurement
methodologies used in this study as methods to determine compliance,
not the least of which is the potential adverse effect of high
electromagnetic field on air flow measurement made with the current
state-of art instrument operation. These include the variability of air
flow due to the bottom floor being open to the atmosphere on three
sides, and the second floor, which contains the mercury cells and
decomposers, having wall panels that are open or closed depending on
ambient conditions, with the ventilation occurring via natural
convection, hence the inherent variability.
    Cell Room Configurations of Three Other Facilities in the
Industry--Prior to the Olin Georgia tests, EPA and the industry's trade
organization, the Chlorine Institute, worked together to examine the
facilities in the industry to be able to select a mercury cell chlor-
alkali plant that would provide the best opportunity for a testing
program to be successful. Olin's Georgia, plant was a clear choice for
this program, given the configuration of the cell room and the
ventilation system. The cell rooms at many of the other operating
mercury cell plants, however, were not nearly as conducive to accurate
measurement of flow and concentration.
    As the first example, Olin Tennessee has three cell rooms adjacent
to one another in one cell building. At this facility, the bottom floor
is largely open on all sides. Two of the cell rooms are simple
rectangular designs with an enclosed space for control equipment
between them. One of these cell rooms has wall panels that can be
removed on three sides. The second of these cell rooms has removable
panels on the ends, but is fully open to the third cell room on the
side opposite the control equipment. The third cell room has another
industrial process sharing the building at one end, and has removable
panels on two of the walls. Each of the three cell rooms has a full
length, natural draft ventilator mounted on the roof. Although the room
ventilation is designed to allow the hot air to naturally flow out to
the cool outside environment (convective), fans have been installed at
the cell floor level around the perimeter of the first two cell rooms
to move the cool air to flow in and around key work areas. In addition,
high velocity fans were installed near the central control equipment
space to aid air movement. There is also cross-mixing of air flow
between the three cell rooms. Although we used one of the cell rooms
for our 2006 monitoring study,

described in detail above, we rejected the other two rooms based on the
same analysis that we used to choose the E510 room. The inability to
accurately estimate air flow in two of these three cell rooms would be
a barrier to quantitatively estimating a flow rate and in turn an
emission rate for compliance purposes.
    As another example, the cell room building at the Pioneer mercury
cell chlor-alkali plant in St. Gabriel, Louisiana, has a rectangular
shape, with the bottom floor basically open on all sides. The roof over
the upper floor where the mercury cells are housed is double-pitched to
produce two bays, with a full-length vent along each roof ridge that
allows convective air flow out of the cell building. In addition, there
are induced draft fans in each bay along the narrow (end) wall of the
cell room to pull air out of the room. Therefore, the ventilation is a
combination of convection and induced draft in a number of directions.
    A third example is the ventilation for the cell room at ERCO's
mercury cell chlor-alkali plant in Port Edwards, Wisconsin, which
consists of three different types of vents on the cell room roof. Two
natural convection ridge ventilators are located at the two roof peaks
of the building. Each ridge is equipped with dampers. Six exhaust fans
are located on the cell room roof on either side of the roof gutter
running down the center of the building. The round opening for these
exhaust fans is approximately six feet in diameter. Eight rectangular
natural convection ventilators are also located on the roof, on either
side of the roof gutter running down the center of the building,
between the ridge ventilators and the exhaust fans. The windows and
doors to the cell room are opened or closed as needed to control the
temperature in the cell room. In the summertime nearly all the doors
and windows are open, and in the wintertime they are nearly all shut.
In addition, there are two adjoining buildings with openings to the
cell room.
    From the above descriptions of cell rooms at Olin Georgia and three
other facilities in the industry, the single UV-DOAS and optical
anemometer system employed in the roof vents at the Olin Georgia plant
would not be sufficient to quantitatively measure mercury emissions
from this facility or any other cell room for compliance with a
standard. Specifically, with the natural drafts, numerous ridge
ventilators and other discharge points from these cell rooms, it would
not be feasible to configure a system using multiple instruments to
accurately measure the concentration and flow rate of the exhaust
streams over all operating time periods to comply with an emission
standard. The detailed cell room design information and test results
described above for facilities in this industry supports our conclusion
in the 2003 Mercury Cell MACT that it is not technologically feasible
to accurately measure the mercury emissions from mercury cell rooms
throughout the industry in a manner sufficient for compliance with an
emission standard.
    Estimating Building Replacement Costs--While this does not relate
to identification of the MACT floor and, as discussed below, we do not
believe it is practical to impose such a requirement as a beyond-floor
requirement, for the purposes of this proposed rule we explored a
scenario where all facilities would tear down their existing cell room
structures and replace them with a design equivalent to Olin Georgia's.
We chose this facility since it was used to provide short-term cell
room mercury emission estimates that have been generally accepted as a
good representation of the magnitude of facility cell room emissions
during the tests, and was cited as an example by NRDC in its petition.
    We estimate that the cost for such construction efforts could be in
the range of $10 to $20 million per facility. Documentation of this
analysis can be found in the docket. We conclude that this is not an
economically feasible option. We also do not believe that an industry-
wide construction effort of this type to be practical, given that we do
not expect any difference in the emission reduction that would be
achieved by a numeric standard as opposed to combination of a cell room
monitoring program and work practices that would be required if we
promulgated today's proposed amendments. Details of our cost estimate
can be found in the docket.
c. Part 61 Mercury NESHAP Allowed Facilities to Assume Cell Room
Emissions of 1,300 g/day and did not Require Compliance with an
Emission Standard
    With regard to the second objection raised by NRDC relating to the
lack of a numeric standard (i.e., that EPA illegally eliminated the
numeric emission limit for the cellroom in the part 61 Mercury NESHAP),
NRDC stated that this long-existing regulation included a numeric
emission standard that applied plant wide, which included the cell
room. NRDC also stated in its petition that one alternative for
demonstrating compliance with a standard such as that in the part 61
Mercury NESHAP is an EPA-approved emission test method, such as EPA
Method 101 (part 61, Appendix B).
    The part 61 Mercury NESHAP contained a plant-wide mercury emission
limitation of 2,300 g/day, which included a 1,000 g/day limit for stack
sources of mercury (end-box ventilation system and hydrogen vents).
However, there was no other limit specified as such in the rule. The
stack limit at 1,000 g/day and the total facility limit of 2,300 g/day
effectively resulted in a 1,300 g/day default limit for fugitive
mercury sources from the cell room by subtraction, but no such separate
limit for fugitive emissions existed in the rule.
    The part 61 Mercury NESHAP further required compliance tests using
Methods 101 and 102 for the point sources. While the part 61 Mercury
NESHAP did include testing provisions for cell room ventilation systems
using Method 101, that rule also allowed sources to alternatively
demonstrate compliance with the rule by using approved design,
maintenance, and housekeeping practices. In this case, the part 61
Mercury NESHAP allowed facilities to assume that their cell room
emissions were 1,300 g/day, without actually requiring them to
demonstrate achievement of this level of emissions.
    The part 61 Mercury NESHAP applied to mercury cell chlor-alkali
plants for more than 30 years. During that time, we are not aware of a
single facility that has demonstrated compliance with the rule by
conducting a test of a cell room ventilation system and showing that
fugitive emissions were in fact no higher than 1,300 g/day. This fact
further supports our conclusions regarding the infeasibility of
applying measurement methodology to fugitive emissions from the cell
rooms for purposes of demonstrating compliance with a numeric limit.
    Prior to the 2003 Mercury Cell MACT, all of the mercury cell chlor-
alkali industry instituted the design, maintenance, and housekeeping
practices in the part 61 Mercury NESHAP and used the default 1,300 g/
day emissions assumption for fugitive mercury emissions from the cell
room. For all practical purposes, the establishment of more detailed
and more stringent MACT-level work practices in the 2003 Mercury Cell
MACT was an improvement of the requirements used to comply with the
part 61 Mercury NESHAP. This is evident in the findings of our testing
and information gathering efforts discussed earlier, which showed cell
room emission levels consistently lower than 1,300 g/day. As also
discussed

previously, the average fugitive emission rate measured during the
testing and other information gathering efforts was around 450 g/day.
In 2006, the average reported mercury emissions from point sources
averaged around 200 g/day, meaning that the overall plant average
emission rate is on the order of around 650 g/day. A 2,300 g/day
emission limit would not be representative of the average fugitive
emissions level achieved by the best performing sources. In fact, a
2,300 g/day limit represents a level of emissions that is likely three
or four times as high as the average emissions of the worst performing
source. Accordingly, in our view the combination of the point source
limits and work practice requirements in the 2003 Mercury Cell MACT is
more stringent than the 2,300 g/day emission limitation in the part 61
Mercury NESHAP. Further, we believe the amendments proposed today
further strengthen the fugitive emissions reduction program beyond both
the part 61 NESHAP and the 2003 Mercury Cell MACT.
d. Conclusion Regarding the Lack of Emission Limitation for Cell Room
    In conclusion, consistent with CAA section 112(h), we believe that
we have established in the discussions above that it is not feasible to
prescribe or enforce an emission standard in this case. There are two
independent bases for this conclusion. First, consistent with CAA
section 112(h)(2)(A), we have concluded that fugitive mercury emissions
from a mercury cell chlor-alkali plant cannot be emitted through a
conveyance designed and constructed to emit or capture such pollutant.
Second, consistent with CAA section 112(h)(2)(B), we have established
that the application of measurement technology to mercury cell rooms is
not practicable due to technological and economic limitations. Finally,
we believe that the plant-wide emission limit from the part 61 Mercury
NESHAP was a standard to which no mercury cell facility had ever
demonstrated compliance by way of emissions testing, is not an
enforceable standard today, and, more importantly, does not reflect the
MACT level of emissions control required under CAA section
112(d)(3)(B). Therefore, we did not unlawfully remove any actual
requirement of the part 61 Mercury NESHAP. Instead, the 2003 Mercury
Cell MACT adopted a set of MACT-level work practice requirements under
section 112(h) that are more stringent in terms of controlling fugitive
mercury emissions than was allowed in the part 61 NESHAP.
    We believe that the enhanced work practices and operational
standards of today's proposed rule would be a more reasonable and
effective method in reducing fugitive mercury emissions than inaccurate
attempts to meet a numeric emissions limit. The 60 percent reduction in
mercury emissions obtained by comparing the assumed part 61 Mercury
NESHAP emission levels for the cell rooms to the measured post-2003
Mercury Cell MACT emissions levels, as noted above, have shown that
work practices alone are effective. The work practices that would be
required in today's proposed amendments would allow sources to spend
their time and efforts identifying and correcting problems rather than
attempting to perform testing to determine compliance with an emissions
limit which would not provide representative data. The detailed
documentation of the work practices during the setting of the action
level we are proposing in today's rule would also ensure that the
lowest emissions levels are maintained through the year. For these
reasons, the effectiveness of today's proposed amendments is not
compromised by the absence of a numeric emission limit for fugitive
emissions from the cell room.
4. Combining the Monitoring Program with Work Practices
    Section 63.8192 of the 2003 Mercury Cell MACT, ``What work
practices standards must I meet?'', allows facilities to institute a
cell room monitoring program to continuously monitor the mercury vapor
concentration in the upper portion of each cell room as an alternative
to work practice standards. One of the objections raised by NRDC was
that this provision backtracked from the Agency's proposed work
practice standards. NRDC pointed out that in the 2003 Mercury Cell
MACT, EPA concluded that the housekeeping activities that facilities in
the industry follow to comply with the part 61 mercury NESHAP
represented the MACT floor and that requiring practices based upon the
most detailed activities in the industry (i.e., ``beyond-the-floor''
practices) was justified. But NRDC was concerned because the work
practices in the 2003 Mercury Cell MACT were optional if facilities
chose to do continuous monitoring and, therefore, this option would
allow sources to avoid conducting activities that represent the MACT
floor. NRDC argued that this was a violation of section 112(d)(3) of
the CAA, which requires all facilities to meet the MACT floor.
    We believe that facilities should continue to perform housekeeping
activities when the action level for the cell room monitoring program
is established. The facilities that have chosen to implement the cell
room monitoring program have continued to perform the housekeeping
activities. Since we know that there is benefit to doing both the
monitoring and the work practices, we are proposing to amend the 2003
Mercury Cell MACT to require both a cell room monitoring program and
work practice standards. This should remove the basis for NRDC's
objection to the 2003 Mercury Cell MACT having made the work practice
requirements optional. Because it is our intention that the primary
focus of the facility should be towards finding and correcting leaks
quickly, which directly results in emission reductions, and we believe
the level of recordkeeping for the routine work practices in the 2003
Mercury Cell MACT detracts from the work practice efforts, we are
reducing the burden of paperwork for the work practices, except during
the setting of the action level. Therefore, the amendments proposed
today would reduce the day-to-day recordkeeping provisions associated
with the work practices and would instead include a requirement for
weekly ``checklists'' certifying that the work practices are being
performed.
    The proposed amendments would add the requirements for detailed
records of work practices during the semi-annual period of 14 to 30
days when the action level is established. Because we are proposing to
require both work practice measures and a cell room monitoring program,
we believe that a reduction in day-to-day recordkeeping will not
diminish the effectiveness of the cell room fugitive emission reduction
program.
    As part of the proposed amendments, we would eliminate the floor-
level monitoring program required in the 2003 Mercury Cell MACT for
facilities that chose the work practice option since it would be
redundant and a less effective alternative to the cell room monitoring
program. The cell room monitoring program accomplishes the same
purpose, except that it requires continuous monitoring of the mercury
concentration. In addition to its continuous nature, the monitoring is
also required to be conducted in the upper portion of the cell room
building. The floor-level program primarily identifies only leaking
equipment at the floor level. By monitoring all the process equipment,
the cell room monitoring program would detect elevated concentrations
from any equipment in the cell room.

5. Other Monitoring Amendments
    In addition to proposing to require all facilities to develop and
implement a cell room monitoring program, we are proposing to amend
some of the requirements of the existing cell room monitoring program
as well as correcting errors from the 2003 Mercury Cell MACT. These
proposed monitoring amendments are described below.
a. Establishment of the cell-room monitoring action level
    The cell-room monitoring action level of the 2003 Mercury Cell MACT
was a concentration that set in motion a series of required procedures
to identify and correct problems that could result in increased
fugitive mercury emissions. To establish the action level, the 2003
Mercury Cell MACT required that the owner or operator collect cell room
concentration data for the first 30 days following the compliance date
and establish an action level at the 75th percentile of the data. As
mercury cell chlor-alkali plants installed and began to operate these
continuous mercury monitoring systems, we became aware of several
aspects of these provisions that could be improved. First, we believe
that the 75th percentile is not the appropriate level for the action
level. When the action level is exceeded, the 2003 Mercury Cell MACT
required that owners and operators take significant actions to identify
and correct the situation causing the increased mercury concentration.
Establishing the level at the 75th percentile resulted in the action
level being exceeded approximately 25 percent of the time. We would
prefer that plant resources be expended when there is a real problem
that can impact mercury emissions (e.g., a leak in hydrogen piping, a
seal failure on a decomposer, etc.), rather than to constantly
investigate and document action level exceedences caused by normal
process variations. Therefore, we are proposing that the action level
be established at the 90th percentile of the data set. Since this level
would be established during the performance and documentation of the
work practices, we believe that an action level at 90 percent would be
sufficient to ensure proper equipment operation.
    We also have come to realize that ambient conditions (temperature,
humidity, etc.), and the seasonal reconfiguration of the cell rooms can
have a significant impact on the cell room concentration. Therefore, we
are proposing that the facilities re-establish their action level at
least once every six months. Due to the increased frequency of action
level determinations and the work practice documentation, we are
reducing the minimum amount of time that plants must collect data to 14
days, although time periods up to 30 days can be used.
b. Weekly Certification of Work Practice Inspections
    Sources that elected to comply with the work practice standards in
the 2003 Mercury Cell MACT were required to keep detailed records of
each inspection. Sources that elected to comply with the cell room
monitoring program were required to keep detailed records of actions
taken whenever an action level is exceeded. We believe that if sources
are required to comply with both the work practice provisions and the
cell room monitoring program provisions, these levels of recordkeeping
are not necessary. Therefore, we are proposing to eliminate the
requirements for detailed records associated with the work practice
inspections and instead we are proposing to require a weekly
certification that all the required work practices are being conducted.
We believe that it is still important that the facilities keep records
of instances where elevated mercury concentrations are measured, along
with records of the associated causes and corrective actions.
Therefore, we are proposing to maintain the detailed recordkeeping
requirements during the 14 to 30 days of setting the action level of
the cell room monitors.
c. Miscellaneous Measurement Amendments
    Detection limit for mercury emission monitor analyzers. Paragraph
(a)(2) of Sec.  63.8242, ``What are the installation, operation, and
maintenance requirements for my continuous monitoring systems?,''
requires that mercury continuous emission monitor analyzers have a
detector with the capability to detect a mercury concentration at or
below 0.5 times the mercury concentration level measured during the
performance test. Since promulgation of the 2003 Mercury Cell MACT, we
determined that setting the analyzer detection capability in reference
to the concentration level during the performance test could be
problematic. We realized that a concentration of 0.5 times the mercury
concentration could, in cases of low mercury concentrations, be
infeasible for the monitoring devices on the market. Information
available to us at this time shows that 0.1 [mu]g/m3 is the detection
limit of commonly commercially available analyzers. We believe that
analyzers with detection limits at this level are more than sufficient
to determine compliance with the emission limitations in the 2003
Mercury Cell MACT. Therefore, we are proposing to revise this paragraph
to require a detector with the capability to detect a mercury
concentration at or below 0.5 times the mercury concentration measured
during the test, or 0.1 [mu]g/m3, whichever is greater.
    Averaging period for mercury recovery unit compliance. The 2003
Mercury Cell MACT is inconsistent as to whether the rule requires a
daily average or an hourly average to determine continuous compliance
with the emissions standard for mercury recovery units found at Sec.
63.8190(a)(3) of Sec.  63.8190 ``What emission limitations must I
meet?''. Paragraph (b) of Sec.  63.8243, ``What equations and
procedures must I use to demonstrate continuous compliance?'', clearly
indicates that this averaging period is daily: ``You must calculate the
daily average mercury concentration using Equation 2 * * *'' However,
paragraph (b) of Sec.  63.8246, ``How do I demonstrate continuous
compliance with the emission limitations and work practice
standards?'', states that for each mercury thermal recovery unit vent,
``you must demonstrate continuous compliance with the applicable
emission limit specified in Sec.  63.8190(a)(3) by maintaining the
outlet mercury hourly-average concentration no higher than the
applicable limit.''
    It was our intention for compliance to be based on a daily average,
as detailed below, and the inclusion of ``hourly'' in paragraph (b) of
Sec.  63.8246, ``How do I demonstrate continuous compliance with the
emission limitations and work practice standards?'', was a drafting
error. Therefore, we are proposing to correct this error by replacing
``hourly'' in Sec.  63.8246(b) with ``daily.'' In the proposal Federal
Register notice for the 2003 Mercury Cell MACT (67 FR 44678, July 3,
2002), we clearly stated our intention when we summarized the
requirements as follows:

    ``To continuously comply with the emission limit for each by-
product hydrogen stream, end-box ventilation system vent, and
mercury thermal recovery unit, we are proposing that each owner and
operator would continuously monitor outlet elemental mercury
concentration and compare the daily average results with a mercury
concentration operating limit for the vent * * * .''
    ``Continuous compliance would be demonstrated by collecting
outlet elemental mercury concentration data using a continuous
mercury vapor monitor, calculating daily averages, and documenting
that the calculated daily average values are no higher than
established operating limits. Each daily average vent elemental
mercury concentration greater than the established

operating limit would be considered a deviation.

6. Creation of the Mercury Cell Chlor-Alkali Subcategory
    As stated in the preamble to the final 2003 Mercury Cell MACT (68
FR 70905), we divided the chlorine production source category into two
subcategories: (1) Mercury cell chlor-alkali plants and (2) chlorine
production plants that do not rely upon mercury cells for chlorine
production. In December 2003 (68 FR 70949), we issued our final
decision to delete the subcategory of the chlorine production source
category for chlorine production plants that do not utilize mercury
cells to produce chlorine and caustic. This action was made under our
authority in CAA section 112(c)(9)(B)(ii), and was not challenged in a
petition for judicial review. Nor did anyone ask us to reconsider that
action pursuant to CAA section 307(d)(7)(B). The objection raised by
NRDC in its petition for reconsideration of the 2003 Mercury Cell MACT
was that by subcategorizing mercury cell chlor-alkali plants, the worst
industry performers are insulated from controls that could otherwise be
driven by sources with no mercury emissions at all (i.e., the non-
mercury chlorine producers), resulting in standards inconsistent with
what NRDC believes is the MACT floor. According to NRDC, if the MACT
floor for mercury emissions was determined for the chlorine production
source category as a whole, the best-performing 12 percent of sources
in the category would be mercury-free. NRDC stated that well over half
of the chlorine production industry as a whole uses either membrane or
diaphragm cell technology. Therefore, NRDC asserted that EPA is
compelled by section 112(d)(3)(A) of the CAA to require sources to
convert to a non-mercury process as MACT.
    We have a long history of using subcategorization to appropriately
differentiate between types of emissions and/or types of operations
when analyzing whether air pollution control technology is feasible for
groups of sources. As we stated in the preamble to the Initial List of
Categories of Sources under section 112(c)(1) of the CAA Amendments of
1990, we have the authority to distinguish among classes, types, and
sizes of sources in establishing emission standards (57 FR 31576, July
16, 1992). Subcategories, or subsets of similar emission sources within
a source category, may be defined if technical differences in emissions
characteristics, processes, control device applicability, or
opportunities for pollution prevention exist within the source
category. This policy is supported by section 112(d)(1), the
legislative history, our prior rulemakings, and judicial precedent.
    EPA's broad authority to establish categories and subcategories of
industry sources is firmly established, and has been recognized as
entitled to substantial deference by the U.S. Court of Appeals for the
D.C. Circuit and by the U.S. Supreme Court. See, e.g., Davis County
Solid Waste Mgmt v. EPA, 101 F.3d 1395, 1405 (DC Cir. 1996) (EPA has
``substantial discretion to create categories of sources for which
standards must be promulgated''); see also Lignite Energy Council v.
EPA, 198 F.3d 930, 933 (DC Cir. 1999) (upholding EPA's refusal to
subdivide a category and noting that the Court was ``[m]indful of the
high degree of deference we must show to EPA's scientific judgment'' on
this question); Chemical Mfrs. Ass'n v. EPA, 470 U.S. 116, 131 (1985)
(``the means used by EPA to define subcategories'' under the Clean
Water Act ``are particularly persuasive cases for deference to the
Agency's interpretation'').
    Under CAA section 112, that authority is subject only to the
consideration that, ``to the greatest extent practicable,'' categories
and subcategories be established ``consistent with'' the source
categories that EPA had established under other CAA programs (i.e., CAA
section 111's ``new source performance standards'' (NSPS) and the
``prevention of significant deterioration'' (PSD) program). 42 U.S.C.
7412(c)(1). Having identified these general touchstones, however,
Congress stated that ``Nothing in the preceding sentence limits the
Administrator's authority to establish subcategories under this
section, as appropriate.'' 42 U.S.C. 7412(c)(1). Further, CAA section
112(d)(1) provides that EPA ``may distinguish among classes, types, and
sizes of sources within a category or subcategory.'' 42 U.S.C.
7412(d)(1). The legislative history confirms Congress' intent to give
EPA broad discretion, noting that the CAA ``provides discretionary
authority to the Administrator to list categories or subcategories
under section 112(c),'' and that ``it is vital to utilize
subcategorization to prevent the cost-ineffective application of * * *
MACT.'' Statement of Rep. Bliley, Oct. 26, 1990, 1 Legis. Hist. at
1225-26.
    Traditionally, EPA has established CAA section 112 subcategories
for regulation based upon ``factors such as process operations (type of
process, raw materials, chemistry/formulation data, associated
equipment, and final products); emission characteristics (amount and
type of HAP); control device applicability; and opportunities for
pollution prevention.'' 64 FR 56493, 56494 (Oct. 20, 1999). These
factors relate to the appropriate application and achievement of
emission standards.
    When EPA has declined to establish subcategories for CAA section
112 standards, we have done so because subcategorization would not
affect the achievability of the standards, due to a lack of
differences, for example, between sources' sizes or designs. (See, for
example, 64 FR 52828, 52859 in regard to declining to subcategorize
hazardous waste incinerators because it would not result in standards
that are more achievable.) On the other hand, where differences in
design and operation between types of sources in a category clearly do
affect the achievability of standards, EPA has reasonably
subcategorized. As the DC Cir. has observed, ``one legitimate basis for
creating additional subcategories must be the interest in keeping the
relation between `achieved' and `achievable' in accord with common
sense and the reasonable meaning of the statute.'' Sierra Club v. EPA,
479 F.3d 875, 885 (DC Cir. 2007)(Williams, concurring)(remanding and
vacating NESHAP for brick and ceramic kilns on other grounds).
    One example of EPA's reasonable subcategorization that presented
issues very similar to those raised in the chlorine production industry
was in the NESHAP for primary copper smelters, 67 FR 40478 (June 12,
2002). There, the existing source MACT determination focused only on
the emissions levels achieved by primary copper smelters using the
relatively older batch copper converter process, while the more state
of the art continuous flash converter process, due to its unique design
and operation, achieved significantly more stringent levels, especially
in terms of controlling process fugitive emissions. 67 FR at 40488.
Commenters argued that EPA should have included the flash converter
smelters in the existing source MACT analysis, but we concluded that
batch converters and continuous flash converters were so distinct that
it was necessary to place them in separate subcategories and to apply
the rule's requirements only to the batch converter smelters. 67 FR at
40489. However, we did identify the continuous flash converter smelter
as the ``best controlled similar source,'' and thereby required that
level of performance as new source MACT and prohibited construction of
new batch converter smelters. 67 FR at 40489. While this issue was not

challenged in the subsequent litigation of the rule, it should be noted
that the Court was fully aware of EPA's differentiation and remarked
upon it without criticism. Sierra Club v. EPA, 353 F.3d 976, 981 (DC
Cir. 2004) (``The rulemaking only concerned those primary copper
smelters that use `batch copper converters'''). We maintain that the
creation of the mercury cell chlor-alkali chlorine production
subcategory was warranted, was consistent with our prior practice (and,
in particular, with the differentiated approach we took for primary
copper smelters), and add the following in support of our conclusion.
    With regard to differences in emission characteristics, the HAP
emitted by mercury cell chlor-alkali processes and non-mercury cell
chlor alkali processes are different, due to the fundamental
differences in production processes and materials used at the two types
of plants. While chlorine and hydrogen chloride are emitted by all
chlor-alkali processes, mercury emissions are unique to the mercury
cell subcategory. There are no mercury emissions from chlor-alkali
plants that utilize electrolytic cells other than mercury cells, simply
because those plants do not use or depend upon mercury as a material in
their production processes. Therefore, it is not realistic to think of
those plants as ``controlling'' mercury emissions levels, or of having
any level of performance in ``limiting'' mercury emissions. It would
likewise be unrealistic to base a MACT level of mercury emissions
performance on such sources, where no mercury emissions at all are even
possible and no actual control measures are, in fact, taken to limit
mercury emissions. Rather, within the chlorine production source
category, these plants represent a different process type, which does
not provide information to assess the best levels of emissions control
performance at source types where mercury emissions in fact occur.
    Second, while chlorine and caustic are produced in all chlor-alkali
processes via an electrolytic reaction, the processes are significantly
different, apart from the basic difference in one subcategory using
mercury and the other not using it. In addition, there are differences
in the products, particularly the caustic products. The basic reaction
that occurs in any chlor-alkali process is the electrolysis of brine,
which contains sodium (or potassium) chloride in water, to form
chlorine, hydrogen, and sodium (or potassium) hydroxide. However, the
manner in which this reaction occurs and associated equipment (i.e.,
the ``cells'') is vastly different.
    In diaphragm cells, a diaphragm separates the electrolytic cell
into an anode compartment and a cathode compartment. Chlorine is formed
in the anode compartment, and hydrogen and sodium (potassium) hydroxide
are produced in the cathode compartment. Membrane cells have the same
basic design, except that the compartments are separated by a membrane
instead of a diaphragm. The primary difference is that the membrane
only allows migration of sodium ions from the anode compartment to the
cathode compartment, which results in a purer raw hydroxide product.
While cell models differ, typical diaphragm cells are around 10 feet
wide and 8 feet long. Membrane cells are of comparable size to
diaphragm cells.
    Mercury cells are considerably different from diaphragm and
membrane cells. First, the reaction occurs in two distinct operations
in two separate vessels. The electrolytic cell, which is typically
around 50 feet long and 5 feet wide, produces chlorine gas. A separate
decomposer, which is typically a cylindrical vessel around 5 feet tall
and 3 feet in diameter, produces hydrogen gas and sodium (or potassium)
hydroxide. The cell and decomposer are linked at the two ends by an
inlet endbox and an outlet endbox.
    While the basic products are the same between mercury cell and non-
mercury cell processes, there are distinct differences in the quality
of the products produced. The products from mercury cell processes
include a concentrated (50 percent) hydroxide and very pure hydrogen
and chlorine. In contrast, diaphragm cells produce very low
concentration and impure hydroxide solutions that require expensive
multi-stage evaporators to strengthen the solution, and the chlorine
produced in membrane cells typically has a high oxygen content.
    Therefore, we believe that there are significant differences in
mercury cell and non-mercury cell processes. While there may be common
aspects of auxiliary processes (e.g., chlorine liquefaction), the most
basic aspect of chlor-alkali facilities (i.e., the electrolytic cells
that produce the chlorine, hydrogen, and caustic) are dissimilar.
    Finally, a comparison of mercury controls or pollution prevention
opportunities between mercury cell processes and non-mercury cell
processes is not possible since the non-mercury cell processes do not
emit any mercury. We do not believe that it would be reasonable to
impose the multi-million dollar conversion of a mercury cell process to
a non-mercury cell process as either a control device application or a
pollution prevention procedure for this industry. In conclusion, we
continue to maintain that non-mercury chlor-alkali chlorine production
processes are separate processes from mercury cell chlor-alkali
chlorine production and, specifically, are not methods of controlling
mercury emissions.
7. Consideration of Non-Mercury Chlor-Alkali Technology as a Beyond-
The-Floor Control Requirement
    Section 112(d)(3) of the Clean Air Act establishes the minimum
requirements (i.e., the ``floor'') for MACT rules. Section 112(d)(2)
requires us to consider alternatives that are more stringent than the
MACT floor (i.e., ``beyond-the-floor'' options). In beyond-the-floor
controls, we are required to consider the impacts that might result
from imposing such controls, including cost, non-air quality health and
environmental impacts, and energy requirements. In developing the 2003
Mercury Cell MACT, we considered beyond-the-floor alternatives for
every emission source. In fact, each numerical emission limit for point
sources, along with the work practices for fugitive sources, represents
a beyond-the-floor level of control. In addition, mercury emissions
from new mercury cell chlor-alkali production facilities were
prohibited, as we identified as the ``best controlled similar source''
a non-mercury chlorine production facility, even though such a source
is not in the same subcategory as existing mercury cell chlor-alkali
facilities. This approach is similar to how we differentiated between
batch converter primary copper smelters (which comprised the existing
source subcategory) and continuous flash converter smelters (which were
not in the regulated subcategory, but drove the new source floor) in
the primary copper smelters MACT rulemaking, discussed above. See 67 FR
40478, 40488-89 (June 12, 2002).
    In its petition NRDC argued that the 2003 Mercury Cell MACT does
nothing to limit the use of mercury cell technology by existing chlor-
alkali plants, and that the Agency ignored a known technique for
reducing mercury emissions from this industry, namely, conversion to
non-mercury processes. According to NRDC, requiring the industry to
convert to a non-mercury process is cost-justified and would provide
significant non-air quality benefits. In support of its argument, NRDC
pointed to EPA's determination at proposal that a cost effectiveness of
$9,000 per pound was warranted for the beyond-the-floor control level
for