recycled water may actually increase fugitive emissions because HF
resides primarily in residual and make-up waters used to transport the
gypsum slurry to the gypsum dewatering stack. One commenter contended
that determining hot or dry periods is too subjective; therefore, it
would be difficult to know when the control technique would apply.
Another commenter illustrated the uncertainty of wetting of the active
gypsum dewatering stack as a fugitive HF emissions control technique by
identifying two studies with contradicting conclusions (one concluded
that most HF is emitted from aqueous surfaces and trends with solar
radiation, and the other study concluded that drying gypsum is a major
source of ambient fluoride emissions from gypsum storage areas).
    One commenter challenged the EPA's lack of evidence on the
effectiveness of applying slaked lime to gypsum dewatering stacks as a
fugitive HF emissions control technique, and claimed that it would not
be feasible, referring to rain as threat to eliminate the potential for
effectiveness. On the contrary, another commenter described how they
apply a lime solution on top of reachable drying gypsum stack areas,
and that the reaction of fluoride with slaked lime does result in the
``tie-up'' of volatile F, although they are not aware of any studies
that have measured or quantified reductions.
    In addition, commenters also claimed that enormous costs would be
associated with the fugitive HF emissions control technique requiring
facilities to apply soil caps and vegetation to all side slopes of the
active gypsum dewatering stack up to 50 feet below the stack top. Some
of these commenters mentioned that there are state rules that require
soil caps and side vegetation on side slopes for erosion/water impact
control, but not for the purpose of fugitive HF emissions control.
    Furthermore, commenters requested that the closure of a gypsum
dewatering stack not be considered a fugitive HF emissions control
technique. One commenter contended that the EPA should allow the final
cover on a closed stack to consist of a synthetic liner, as this would
achieve the same purpose as a vegetative liner and may be more
appropriate in some instances. Another commenter explained that some
states and the EPA have closure requirements under Resource
Conservation and Recovery Act (RCRA), including, for example,
requirements for long term care practices (beyond 20-50 years); shaping
and configuration of gypsum dewatering stacks; site security. They
suggested that due to these detailed requirements, it would be best to
defer to stack closure requirements within other regulations and not
have NESHAP requirements that involve or require stack closure.
    Finally, commenters requested that if the EPA proceeds with a final
rule that includes work practices for reducing fugitive HF emissions
from gypsum dewatering stacks or cooling ponds, the work practices
should include a flexibility mechanism for facilities to use additional
practices not codified during this rulemaking. One commenter asserted
that work practice standards that might commonly be practicable for
other industries are not universally practicable (or legally
permissible) throughout the phosphoric acid and phosphate fertilizer
industries, and some practices might be appropriate for some
facilities, but not others (depending on location, climate, etc.).
    Response. We are adopting the proposed work practice standard that
requires owners or operators to prepare, and operate in accordance with
a gypsum dewatering stack and cooling pond management plan; however,
based on analysis of public comments, we are making some changes to the
specific control measures that we proposed as options in the plan for
controlling fugitive HF emissions. In the final rule, the Agency is
using the terminology ``control measures'' in lieu of the proposed
terminology ``control techniques'' because it more accurately describes
the list of options in the rule and avoids confusion with other CAA
programs. We are finalizing standards that will reduce HAP emissions
from gypsum dewatering stacks and cooling ponds because, as explained
in the preamble to the proposed rule, the 1999 Phosphoric Acid
Manufacturing NESHAP (i.e., NESHAP subpart AA) did not regulate
fugitive HF emissions from gypsum dewatering stacks or cooling ponds.
As explained in the preamble to the proposed rule, we are adopting a
work practice standard instead of numeric emission limits because it is
``not feasible to prescribe or enforce an emission standard'' for these
emissions because they ``cannot be emitted through a conveyance
designed and constructed to emit or capture such pollutant'' (see CAA
section 112(h)(2)(A)) as the several hundred acres average size of
these emission sources makes conveyance impractical. The size of these
emission sources also makes it difficult to quantify the emission
reductions that any control measure employed will achieve. However, in
the paragraphs below, we explain how each control measure is feasible
and effective in reducing fugitive HF emissions. We also provide
details on the changes we have made to the gypsum dewatering stack and
cooling pond management plan since proposal. Even after these changes,
the measures are consistent with CAA section 112(d) controls and
reflect a level of performance analogous to a MACT floor.
    We noted in the preamble to the proposed rule that we believe that
it is most effective for sources to determine the best practices that
are to be incorporated into their site-specific gypsum dewatering stack
and cooling pond management plan. We also stated in the preamble to the
proposed rule that sources would be required to incorporate control
measures from the list of options being proposed, and we solicited
comment on the proposed site-specific gypsum dewatering stack and
cooling pond management plan. In addition, we made considerable effort
\5\ before and after proposal in identifying a list of control measure
options that encompass enough variety that at least one control measure
option is feasible for at least one of each facility's existing gypsum
dewatering stacks and/or cooling ponds. In fact, we are not aware

of any facility that does not use a rim ditch (cell) building
technique. Therefore, we disagree with commenters that the options we
have listed for the gypsum dewatering stack and cooling pond management
plan are not technically feasible.
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    \5\ See the following documents which are all available in
Docket ID No. EPA-HQ-OAR-2012-0522): ``USEPA Meeting with The
Fertilizer Institute, July 24, 2013''; ``TFI meeting with USEPA to
discuss RTR for Phosphoric Acid and Phosphate Fertilizer NESHAPs,
September 11, 2014''; ``EPA Meeting Minutes for TFI Discussion March
12, 2015''; ``Summary of Potential Costs for Implementing Phosphate
NESHAPs/Recommendations for Phosphogypsum Stack Work Practices, May
5, 2015''; ``Notes from Meeting with Florida DEP Regarding Gypsum
Dewatering Stack and Cooling Pond Management Plan, March 4, 2015'';
and ``Site Visits to Mosaic Plant City and Mosaic New Wales, March
4, 2015.''
---------------------------------------------------------------------------

    Additionally, personnel from the Florida Department of
Environmental Protection (DEP) had concerns regarding how the plan
would be implemented, as well as how a facility would show compliance
with the control measure it chooses (see ``Notes from Meeting with
Florida DEP Regarding Gypsum Dewatering Stack and Cooling Pond
Management Plan, March 4, 2015,'' which is available in Docket ID No.
EPA-HQ-OAR-2012-0522). Therefore, in an effort to improve compliance
demonstration with a facility's site-specific gypsum dewatering stack
and cooling pond management plan, we are including a condition in the
final NESHAP subpart AA rule that requires facilities to submit their
plan for approval to the Administrator. Facilities will be required to
provide details on how they plan to implement and show compliance with
the control measure(s) that they choose. The Administrator will approve
or disapprove the facility's site-specific gypsum dewatering stack and
cooling pond management plan within 90 days after it is received. There
may be a benefit to facilities and permitting authorities for the
gypsum dewatering stack and cooling pond management plan and the title
V major modification application to be submitted and reviewed at the
same time. To change any of the information submitted in the plan, the
facility must submit a revised plan 60 days before the change is to be
implemented in order to allow time for review and approval by the
Administrator before the change is implemented.
    We are not including an option in the NESHAP subpart AA final rule,
as commenters requested, that would provide a flexibility mechanism for
facilities to use additional practices not codified during this
rulemaking. This type of flexibility does not provide regulatory
certainty that is needed for both industry and the EPA.
    Although some commenters opposed using a submerged discharge pipe
(with necessary siphon breaks to a level below the surface of the pond)
as a fugitive HF emissions control measure, we believe submerging a
discharge pipe can be appropriate and effective for reducing emissions
from process water discharges into a cooling pond, although some
facilities may not choose this option. Moreover, we agree with
commenters that submerging siphon breaks could impede the ability of
these devices to prevent backflow; therefore, we are removing this
requirement from the final rule. On a recent site visit (see ``Site
Visits to Mosaic Plant City and Mosaic New Wales, March 4, 2015,''
which is available in Docket ID No. EPA-HQ-OAR-2012-0522), we noted
strong vapor odors coming from splash operations occurring at a non-
submerged pipe that was discharging process water into a cooling pond.
According to AP-42, Chapter 5.2--Transportation and Marketing of
Petroleum Liquids (01/95), significant turbulence and vapor/liquid
contact that occur during splash discharge operations will result in
higher levels of vapor generation and emissions loss compared to using
a submerged discharge operation. Liquid turbulence is controlled
significantly during submerged discharge operations, resulting in much
lower vapor generation than encountered during splash discharge
operations. We believe this demonstrates that submerging the pipe is an
effective technique for mitigating HF emissions, and we are therefore
retaining this option for cooling ponds.
    However, we are removing the option of submerging a discharge pipe
that is associated with the gypsum pond because it is not a feasible
option due to high solids volume in the slurry. (A gypsum pond, also
called a settling pond, often is located in the middle of a gypsum
disposal pile and receives waste gypsum slurry.) Based on information
received from industry after the public comment period ended for the
proposal (see Docket ID No. EPA-HQ-OAR-2012-0522-0048), it is much more
likely for this particular pipe to become clogged, creating
backpressure on pump seals. Submerging the discharge pipe under water
in the gypsum pond creates a potential restriction against the
discharging slurry that could get worse as solids build up around and
against the end of the pipe. The discharge pipe for the gypsum pond is
also routinely moved, which complicates submersing it.
    As we stated earlier in our response, we are not aware of any
facility that uses a gypsum dewatering stack building technique that is
different from rim ditch (cell) building. With regard to commenters'
assertions that the EPA did not provide data or explain the link
between minimizing the gypsum dewatering stack surface area and
reducing fugitive HF emissions, we believe that using the rim ditch
technique over the lifespan of a gypsum dewatering stack will reduce
the surface area of the gypsum pond and thereby reduce fugitive HF
emissions. Fugitive HF emissions are calculated using an emission
factor that is directly related to the total acreage from the gypsum
dewatering stack, which includes the pond surface area (tons HF per
acre per year); therefore, minimizing the pond surface area would
minimize HF emissions. The rim ditch (cell) building technique is
mainly used for gypsum dewatering stack stability since inner and outer
dikes are used to create a rim ditch that provides better protection
against overflow of the gypsum pond. However, as rim ditches are filled
with slurry, the gypsum pond area will gradually decrease within each
cell, thereby shrinking the amount of surface area of the pond that is
exposed to the atmosphere (reducing the amount of fugitive HF
emissions). An alternative to the rim ditch technique is to simply
discharge gypsum slurry into the gypsum pond. With this technique,
there is no inner dike to control slurry flow and the pond surface area
would not be reduced as quickly or consistently. This increased surface
area would allow greater potential for fugitive HF emissions due to the
larger amount of surface water exposed to the atmosphere. We are
revising this control measure option in the NESHAP subpart AA final
rule to clarify that owners or operators must minimize the surface area
of the gypsum pond associated with the active gypsum dewatering stack
(and not the surface area of the active gypsum dewatering stack as we
had proposed) by using a rim ditch (cell) building technique or other
building technique. This clarification also addresses industry's
suggestion to reword the control measure in response to a meeting that
occurred after the public comment period closed (see ``EPA Meeting
Minutes for TFI Discussion March 12, 2015,'' and ``Summary of Potential
Costs for Implementing Phosphate NESHAPs/Recommendations for
Phosphogypsum Stack Work Practices, May 5, 2015,'' which are both
available in Docket ID No. EPA-HQ-OAR-2012-0522). Moreover, in this
same correspondence that occurred after the public comment period
closed, industry provided a suggestion for the definition of ``rim
ditch.'' We agree with industry's suggested definition; however, we
believe the definition more appropriately covers the meaning of ``rim
ditch (cell) building technique'' and not just ``rim ditch.'' We are
including this definition in the final rule for ``rim ditch (cell)
building technique'' in an effort to clarify what we mean by this
control measure. The

final rule defines ``rim ditch (cell) building technique'' as a gypsum
dewatering stack construction technique that utilizes inner and outer
dikes to direct gypsum slurry flow around the perimeter of the stack
before directing the flow and allowing settling of finer materials into
the settling compartment. For the purpose of this definition, the rim
ditch (cell) building technique includes the compartment startup phase
when gypsum is deposited directly into the settling compartment in
preparation for ditch construction, as well as the step-in or terminal
phases when most solids must be directed to the settling compartment
prior to stack closure. Decant return ditches are not rim ditches.
    Based on commenters' objection to wetting active gypsum dewatering
stacks as a fugitive HF emissions control measure, and additional
discussion with industry (see ``EPA Meeting Minutes for Simplot
Discussion April 1, 2015,'' which is available in Docket ID No. EPA-HQ-
OAR-2012-0522), we determined that the proposed rule was not clear on
how this control measure would be used. This control measure is not
applied to the side slopes of the gypsum dewatering stacks, and instead
is used on certain gypsum areas within cells of a gypsum dewatering
stack. According to one facility located in arid climate (see ``EPA
Meeting Minutes for Simplot Discussion April 1, 2015,'' which is
available in Docket ID No. EPA-HQ-OAR-2012-0522), these areas may be
more susceptible to drying out in warmer months due to higher surface
temperatures of the gypsum dewatering stack; therefore, a system of
weirs can be used to help direct gypsum pond water (not fresh water) to
these areas to keep them wet. We agree with the commenter who pointed
out that that applying water to a gypsum stack may actually increase
fugitive emissions because HF resides primarily in the water used to
transport the gypsum slurry to the gypsum dewatering stack. We realize
that this option might increase the surface area of the gypsum pond
water which conflicts with our understanding that minimizing surface
area of the gypsum pond will minimize HF emissions. Therefore, we are
not adopting this proposed control measure in the NESHAP subpart AA
final rule.
    In response to a commenter's assertion that there is lack of
evidence of the effectiveness of applying slaked lime to gypsum
dewatering stacks as a fugitive HF emissions control measure, we
received information after the public comment period ended (see Docket
ID No. EPA-HQ-OAR-2012-0522-0048) that at least one facility uses this
technique to help meet its state ambient air standard for F. This
commenter stated that, based on data from their site-specific ambient
air monitoring, they apply a lime solution to their gypsum dewatering
stack areas during periods where they are close to violating their 30-
day state ambient air standard for F, measured as HF, in order to stay
below the standard. Slaked lime can precipitate fluorides from gypsum
dewatering stacks and cooling ponds, thus reducing the availability of
fluorides in solution that could then be released into the air during
evaporation. This is an example of the type of detail that the
Administrator may require be included in the facility's site-specific
plan (in addition to how compliance would be demonstrated) before it
could be approved. We have clarified in the final rule that if this
control measure is chosen, then the plan must include the method used
to determine the specific locations slaked lime is applied. The plan
must also include the methods used to determine the quantity of, and
when to apply, slaked lime (e.g., slaked lime may be applied to achieve
a state ambient air standard for F, measured as HF).
    With respect to the measure involving application of soil caps and
vegetation to side slopes of a gypsum dewatering stack, on recent site,
visits personnel from Mosaic and the Florida DEP had concerns that this
control measure was too specific in that it could be difficult for
facilities to demonstrate compliance with the ``50 feet below the stack
top'' requirement as well as the requirement to apply soil caps and
vegetation to all side slopes (see ``Site Visits to Mosaic Plant City
and Mosaic New Wales, March 4, 2015,'' and ``Notes from Meeting with
Florida DEP Regarding Gypsum Dewatering Stack and Cooling Pond
Management Plan, March 4, 2015,'' which are available in Docket ID No.
EPA-HQ-OAR-2012-0522). We recognize that applying soil caps and
vegetation to side slopes of a gypsum dewatering stack is an ongoing
process that continuously changes over time based on facility-specific
operations. Therefore, we have revised this control measure option in
the NESHAP subpart AA final rule to acknowledge that this technique
will only be applied to portions of the side slopes that are no longer
active on a gypsum dewatering stack instead of all side slopes up to 50
feet below the top of the gypsum dewatering stack. We also have revised
this option to allow the use of a synthetic cover in lieu of soil caps
and vegetation. Furthermore, we expect that if a facility chose to use
this specific control measure in their plan, the Administrator may
require details on schedule, and how the portion of side slopes that
received soil caps and vegetation, or a synthetic cover, is determined
(in addition to how compliance would be demonstrated), before the plan
could be approved. Therefore, we have clarified in the final rule that
the plan must include the method used to determine the specific
locations of soil caps and vegetation, or synthetic cover, and specify
the acreage and locations where soil caps and vegetation, or synthetic
cover, is applied. The plan must also include a schedule describing
when soil caps and vegetation, or synthetic cover, is to be applied.
    Additionally, we believe that this control measure creates a
barrier on the surface of the gypsum dewatering stack side slopes that
reduces HF emissions; therefore, we disagree with commenters' assertion
that applying soil caps and vegetation may not be an effective option
for fugitive HF emissions control. The Florida DEP has used this
control measure as part of its overall management of fluorides from
gypsum dewatering stacks; and Wyoming has approved this control measure
in a facility's title V permit as an optional method for reducing
fugitive fluoride emissions. We also disagree with a request \6\ to
reword this control measure to require a gypsum dewatering stack
construction and operation plan because the commenter did not provide
any justification on how this activity reduces fugitive HF emissions
from gypsum dewatering stacks.
---------------------------------------------------------------------------

    \6\ See ``EPA Meeting Minutes for TFI Discussion March 12,
2015,'' and ``Summary of Potential Costs for Implementing Phosphate
NESHAPs/Recommendations for Phosphogypsum Stack Work Practices, May
5, 2015,'' which are both available in Docket ID No. EPA-HQ-OAR-
2012-0522.
---------------------------------------------------------------------------

    We disagree with commenters' requests to exclude closure from the
list of measures for controlling fugitive HF emissions from gypsum
dewatering stacks. We believe that closing a gypsum dewatering stack is
one of the best solutions for reducing fugitive HF emissions because it
permanently reduces the emissions from the greatest contributing
source. However, we are revising this control measure option in the
NESHAP subpart AA final rule to allow a facility to design its own
closure requirement plan, provided that the closure requirements, at a
minimum, contain: (1) A specific trigger mechanism for when owners or
operators must begin the closure process on the gypsum dewatering
stack, and (2) a requirement to install a final cover. As with all
gypsum dewatering stack and

cooling pond management plans, this closure requirement must be
submitted to the Agency for approval. Although we are not identifying a
specific trigger mechanism in the final rule, one example of a trigger
mechanism is a facility-specified length of time where the gypsum
dewatering stack is inactive and no longer receives gypsum (i.e., once
the gypsum dewatering stack stops receiving gypsum for a period of
time, the facility must begin closing it). Also, we are clarifying that
a final cover means the materials used to cover the top and sides of a
gypsum dewatering stack upon closure. This addresses commenters request
that the EPA should allow the final cover on a closed stack to consist
of a synthetic liner. Finally, in light of our decision to revise the
control measure option for closing a gypsum dewatering stack, we are
also removing the definition of a ``closed gypsum dewatering stack''
from the NESHAP subpart AA final rule. Since the revised language
relies on a specific trigger mechanism for when owners or operators
must begin the closure process on the gypsum dewatering stack, the
definition of a ``closed gypsum dewatering stack'' is no longer
necessary in the final rule. Because we are removing the definition of
a ``closed gypsum dewatering stack'' from the final rule, we are
revising the definition of an ``active gypsum dewatering stack.'' In
the NESHAP subpart AA final rule, an ``active gypsum dewatering stack''
means a gypsum dewatering stack that is currently receiving gypsum,
received gypsum within the last year, or is part of the facility's
water management system. A gypsum dewatering stack that is considered
closed by a state authority is not considered an active gypsum
dewatering stack.
    As we have stated before, the final list of NESHAP subpart AA
control measures is exhaustive enough that a facility has a number of
options for selecting a control measure that would be feasible for
their particular operations. We assume that facilities would choose the
lowest cost option, and that all facilities are using at least one of
the control measure options already (e.g., we are not aware of any
facilities that do not use a rim ditch (cell) building technique).
Therefore, we disagree with the commenters' claim that enormous costs
would be incurred if they were required to apply soil caps and
vegetation to all side slopes of the active gypsum dewatering stack up
to 50 feet below the stack top. We are not requiring that facilities
implement this control measure since this specific control technique is
not a requirement, but instead an option for how a facility may
demonstrate compliance with the work practice standards for fugitive HF
emissions from the gypsum management system.
    iii. Requirement to Use At Least Two of the Fugitive HF Emissions
Control Measures--Comment. One commenter requested that the EPA
eliminate the ``dual practice'' approach for new sources. Two
commenters declared that the requirement to implement ``at least two of
the control techniques'' listed for ``each regulated gypsum dewatering
stack and cooling pond'' is not possible without a broader list that
includes at least two practices for cooling ponds. Additionally, with
regard to closing an active gypsum dewatering stack as a control
technique option, the commenter contended that giving an owner of a new
gypsum dewatering stack the option of closing it in tandem with a
mandatory second control technique is ``nonsensical'' because the ``new
stack would immediately have to be closed to implement the practice.''
Another commenter wanted clarification as to whether the lateral
expansion of an existing gypsum dewatering stack is considered a new
stack, and thus would trigger the proposed work practice standards
related to the size of active gypsum dewatering stacks and production
ratio. The commenter also sought clarification as to whether at least
two of the control techniques be used in the gypsum dewatering stack
and cooling pond management plan for controlling fugitive HF emissions
would be required.
    Response. We agree with the commenter that the proposed requirement
for new gypsum dewatering stacks and cooling ponds to implement ``at
least two of the control techniques'' listed for ``each'' regulated
``gypsum dewatering stack and cooling pond,'' would make compliance for
cooling ponds impossible for new sources without a broader list with at
least two control measures for cooling ponds. In the final rule, the
Agency is using the terminology ``control measures'' in lieu of the
proposed terminology ``control techniques'' because it more accurately
describes the list of options in the rule and avoids confusion with
other CAA programs. As stated in a previous response, in an effort to
clarify the specific emission source that we are regulating in the
final rule (NESHAP subpart AA), we have included a new term, ``gypsum
dewatering stack system,'' (see sections V.C.3.b.i of this preamble for
further details) in the final rule. This revision also clarifies our
original intent that the two control measure options that a facility
selects can be for any combination of gypsum dewatering stacks and/or
cooling ponds in the gypsum dewatering stack system. For example, if a
facility operates a cooling pond considered a new source, the facility
may choose to not implement the control measure option requiring a
submerged discharge pipe for the new cooling pond, and instead
implement two control measures at one or more gypsum dewatering stacks
no matter whether they be considered a new or existing source.
Furthermore, we have revised the control measure option for closing a
gypsum dewatering stack (see section V.C.3.b.ii of this preamble for
further details). Because of this change to the NESHAP subpart AA final
rule, there is no longer a requirement to immediately close the active
gypsum dewatering stack in tandem with a mandatory second control
measure option.
    Lastly, the Agency has revised the definitions in the NESHAP
subpart AA final rule for when a gypsum dewatering stack is considered
``new'' or ``existing'' in order to address whether a lateral expansion
of an existing gypsum dewatering stack is considered a new gypsum
dewatering stack. The revised definitions in the final rule also deal
with a concern one commenter raised during the comment period about
triggering the proposed regulation for a ``new'' source each time they
rotate the functionality of their three gypsum dewatering stack sites
at their facility (this topic was also discussed after the comment
period closed, see ``USEPA Meeting Minutes for PCS Aurora Discussion
(2.2.2015),'' which is available in Docket ID No. EPA-HQ-OAR-2012-
0522). We are revising the NESHAP subpart AA final rule such that a
gypsum dewatering stack or cooling pond is considered ``new'' if it
meets two criteria: (1) It was constructed or reconstructed after
August 19, 2015, and (2) it was required to obtain a permit by a state
authority for the construction or reconstruction. Some lateral
expansions may build beyond a facility's existing permitted capacity
(and design dimensions of the gypsum dewatering stack); therefore,
these lateral expansions would be considered ``new'' in the final rule
because the facility would be required to obtain (or revise) their
existing permitted capacity (and design dimensions). Because of this
change in the NESHAP subpart AA final rule, we are also revising the
criteria for when a gypsum dewatering stack or cooling pond is
considered ``existing.'' Specifically, a gypsum dewatering stack or
cooling pond is

considered ``existing'' if it meets one of two criteria: (1) It was
constructed or reconstructed on or before August 19, 2015, or (2) it
was constructed or reconstructed after August 19, 2015 and it was not
required to obtain a permit by a state authority for the construction
or reconstruction.
    iv. Fugitive HF Emissions Control Measure Considerations for
Cooling Ponds--Comment. One commenter referenced a 1978 EPA document:
``Evaluation of Emissions and Control Techniques for Reducing Fluoride
Emissions from Gypsum Ponds in the Phosphoric Acid Industry'' and
questioned why the EPA proposed work practice standards focused solely
on gypsum dewatering stacks, while the EPA has in the past studied and
documented more work practices for controls of cooling pond emissions,
which are not discussed as alternatives to the proposed rule. Another
commenter requested that if EPA keeps cooling ponds as part of the
gypsum dewatering stack and cooling pond management plan, then EPA
should provide more than one work practice that could be implemented at
a cooling pond. They suggested that EPA add a control measure option
(for cooling ponds) that would require developing a plan to optimize
the size of cooling ponds to address fugitive HF emissions (as
appropriate based on the conditions at the facility).
    In addition, another commenter suggested additional control measure
options for reducing fugitive HF emissions from cooling ponds. This
commenter suggested EPA include an option to develop and implement a
plan for dredging cooling ponds which helps maintain cooling capacity,
and, therefore, can reduce fugitive emissions by reducing the vapor
pressure of fluoride in the pond water. This commenter also suggested
EPA include an option to implement a system for the recovery of
fluoride for water that is directed to cooling ponds. The commenter
pointed out that one of its facilities has the capability to recover
fluoride as hydrofluorosilicic acid during the phosphoric acid
evaporation process. The commenter stated that this recovery process is
operated as needed to meet the market demand for hydrofluorosilicic
acid. Finally, the commenter suggested EPA include an option to
implement a system for the removal of fluoride for water that is
directed to cooling ponds (for example, by adding lime to increase the
pH).
    Response. We are aware of the 1978 EPA document, ``Evaluation of
Emissions and Control Techniques for Reducing Fluoride Emissions from
Gypsum Ponds in the Phosphoric Acid Industry,'' and the six potential
control techniques it examines for reducing fluoride emissions from
gypsum ponds. These six potential control techniques include: (1) Use
of the ``Kidde'' process; (2) use of the ``Swift'' process; (3) use of
lime to raise pH; (4) dry conveyance of gypsum, (5) pretreatment of ore
by calcining; and (6) changing the entire phosphoric acid production
process to a ``hemi/dehydrate'' process. The 1978 EPA document
clarifies that the first four of these potential control techniques
could also reduce fluoride emissions from cooling ponds. The ``Swift,''
``Kidde,'' and ``hemi/dehydrate'' processes each use byproduct fluoride
in the WPPA to produce hydrofluorosilicic acid (an acid generally used
in fluoridation of drinking water, but also has other industry uses) or
ammonium silicofluoride. We are aware of at least two facilities that
are equipped and capable of making hydrofluorosilicic acid; however, it
is not clear which process they use, nor is it clear if either facility
is actively making hydrofluorosilicic acid. However, facilities have
expressed that production of hydrofluorosilicic acid for the primary
purpose of controlling HF emissions is not practical. Facilities that
produce hydrofluorosilicic acid seek to sell the product for use in
water fluoridation.\7\ In fact, one commenter stated that their
recovery process is operated as needed to meet the market demand for
hydrofluorosilicic acid. Facilities would not produce this product in
the absence of a market demand, as the hydrofluorosilicic acid would be
another waste stream that would need to be disposed of. Therefore, we
do not believe this to be a reasonable control technique option for
fugitive HF emissions from these sources.
---------------------------------------------------------------------------

    \7\ http://www.cdc.gov/fluoridation/factsheets/engineering/wfadditives.htm.
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    We have determined that using lime (or any other caustic substance)
to raise the pH of liquid discharged into the cooling pond could be a
feasible control measure option for reducing fluoride emissions from
cooling ponds; therefore, we are including this option in the NESHAP
subpart AA final rule. The control measure option simultaneously raises
the pH of the cooling pond water and lowers the concentration of
soluble F, and, thus reducing the concentration of fluoride (including
HF) that could be potentially evaporated into the atmosphere. Based on
information provided in the 1978 EPA document, a greater than 90
percent emission reduction in fluoride can be achieved by raising the
pond water from pH 1.4 to pH 3.9. In the final rule, if this control
measure is chosen, then the plan must include: the method used to raise
the pH of the liquid discharged into the cooling pond, the target pH
value (of the liquid discharged into the cooling pond) expected to be
achieved by using the method, and the analyses used to determine and
support the raise in pH. Moreover, this control measure is similar to
an option that industry suggested in response to a meeting that
occurred after the public comment period closed (see ``EPA Meeting
Minutes for TFI Discussion March 12, 2015,'' and ``Summary of Potential
Costs for Implementing Phosphate NESHAPs/Recommendations for
Phosphogypsum Stack Work Practices, May 5, 2015,'' which are both
available in Docket ID No. EPA-HQ-OAR-2012-0522). Industry suggested
including an option that would require providing inputs to the gypsum
dewatering stack system to react with and precipitate fluoride
compounds to insoluble forms.
    With regard to the remaining potential control techniques
identified in the 1978 EPA document (i.e., dry conveyance of gypsum and
pretreatment of ore by calcining), we have determined that these
control techniques are not likely to be used by industry because
significant process changes would be required. Furthermore, with regard
to pretreatment of ore by calcining, the 1978 EPA document states that
off-gases from pretreating ore would still need to be scrubbed to
remove F, and the scrubbing liquid from this process would likely be
disposed of in a cooling pond (which would defeat the purpose of this
technique). Therefore, we are not finalizing the NESHAP subpart AA
final rule to include these two control measure options for controlling
fugitive HF emissions from cooling ponds.
    Lastly, we agree with a commenter's request to add a control
measure option (for cooling ponds) that would require developing a plan
to optimize the size of cooling ponds to address fugitive HF emissions
(as appropriate based on the conditions at the facility); therefore, we
are including this option in the NESHAP subpart AA final rule. However,
in order for a facility to be able to use this control measure option,
its cooling pond evaluation must result in a reduction in overall
cooling pond surface area. Fugitive HF emissions are calculated using
an emission factor that is directly related to gypsum dewatering stack
and pond surface area (tons HF per acre per year); therefore,
minimizing

the surface area of the cooling pond(s) would minimize HF emissions. On
a recent site visit (see ``Site Visits to Mosaic Plant City and Mosaic
New Wales, March 4, 2015,'' which is available in Docket ID No. EPA-HQ-
OAR-2012-0522), we noticed that one company evaluated whether a
reduction in the size of its cooling ponds could still support
additional water due to rainfall and plant process water needs.
However, the result of these evaluations did not lead to a change in
size of its cooling ponds, and thus did not lead to a reduction in
fugitive HF emissions from the cooling ponds. In the final rule, if
this control measure is chosen, then the facility-specific evaluation
plan must be certified by an independent licensed professional engineer
or similarly qualified individual, and include the method used to
reduce the total cooling pond footprint, the analyses used to determine
and support the reduction in the total cooling pond surface area, and
the amount of total cooling pond surface area that was reduced due to
the facility-specific evaluation plan. Furthermore, we agree with the
commenter who stated dredging cooling ponds is a good practice for
maintaining cooling capacity. With regard to the commenter's request to
include this activity (i.e., dredging cooling ponds) as a specific
control measure option,\8\ we determined that this activity could be
considered in the cooling pond evaluation; however, the evaluation
would still need to lead to a change in size of the surface area of the
cooling pond for it to qualify as a control measure in the final rule.
---------------------------------------------------------------------------

    \8\ Industry also suggested this control measure as an option to
reducing fugitive HF emissions from cooling ponds in response to a
meeting that occurred after the public comment period closed (see
``EPA Meeting Minutes for TFI Discussion March 12, 2015,'' and
``Summary of Potential Costs for Implementing Phosphate NESHAPs/
Recommendations for Phosphogypsum Stack Work Practices, May 5,
2015,'' which are both available in Docket ID No. EPA-HQ-OAR-2012-
0522).
---------------------------------------------------------------------------

    We also evaluated an additional control measure option suggested by
industry in response to a meeting that occurred after the public
comment period closed (see ``EPA Meeting Minutes for TFI Discussion
March 12, 2015,'' and ``Summary of Potential Costs for Implementing
Phosphate NESHAPs/Recommendations for Phosphogypsum Stack Work
Practices, May 5, 2015,'' which are both available in Docket ID No.
EPA-HQ-OAR-2012-0522). Industry suggested including the option to
``operate the cooling pond systems to adjust the active cooling surface
area to address weather conditions, seasonal cooling needs and
associated production changes. Cooling circuit adjustments may be
accomplished through utilization of either fixed or floating flow
diversion devices or by changing flows such that some of the heated
water is diverted away from portions of the ponded area.'' However, we
are not including this option in the final rule because it is not clear
how the option reduces fugitive HF emissions from cooling ponds.
    v. Excluding Cooling Ponds from Management Plan--Comment. One
commenter requests that the EPA revise the regulatory language in
proposed 40 CFR 63.602 (d) through (f) that refers to each ``gypsum
dewatering stack and cooling pond'' to instead refer only to each
``gypsum dewatering stack.'' The commenter stated that the regulatory
direction seems to encompass ponds that are not part of a ``gypsum
dewatering stack.'' Another commenter claimed the rule implies that
control measure options apply to cooling ponds distinctly from gypsum
dewatering stacks. An additional commenter alleged that work practice
standards should not apply to cooling ponds that are physically
separate from gypsum stacks. This commenter pointed out that only one
practice (submerging the discharge pipe) relates to cooling ponds, and
because of the requirement to implement at least one practice for each
``gypsum dewatering stack and cooling pond,'' then cooling ponds that
fall within the proposed definition of a gypsum dewatering stack
seemingly could choose to submerge the discharge pipe at the pond, or
they could implement other techniques from the list.
    Response. The NESHAP subpart AA final rule clarifies that the
gypsum dewatering stack and cooling pond management plan is intended to
cover both gypsum dewatering stacks and cooling ponds. In response to a
previous comment, we have included a new term ``gypsum dewatering stack
system,'' revised the definition of ``gypsum dewatering stack'' to
exclude cooling ponds, and have retained the proposed definition of
``cooling pond'' in the final rule (see section V.C.3.b.i of this
preamble for further details).
4. What is the rationale for our final approach pursuant to CAA
sections 112(d)(2), 112(d)(3), and 112(h)?
    For the reasons provided above and in the preamble for the proposed
rule, we are finalizing our proposal to eliminate the use of PM as a
surrogate for Hg and are adding Hg and total fluoride emission limits
for phosphate rock calciners to the NESHAP subpart AA final rule.
    For the reasons provided above, we are making the revisions,
clarifications, and corrections noted in section V.C.2 in the NESHAP
subpart AA final rule.

D. NSPS Review for the Phosphoric Acid Manufacturing Source Category

    The NSPS review focused on the emission limitations that have been
adequately demonstrated to be achieved in practice, taking into account
the cost of achieving such reduction and any non-air quality health and
environmental impact and energy requirements. Determining the BSER that
has been adequately demonstrated and the emission limitations achieved
in practice necessarily involves consideration of emission reduction
methods in use at existing phosphoric acid manufacturing plants. To
determine the BSER, the EPA performed an extensive review of several
recent sources of information, including a thorough search of the RACT/
BACT/LAER Clearinghouse (RBLC), section 114 data received from
industry, and other relevant sources.
    Our review considered the emission limitations that are currently
achieved in practice, and found that more stringent standards are not
achievable for this source category. When evaluating the emissions from
various process lines, we observed differences in emissions levels, but
did not identify any patterns in emission reductions based on control
technology configuration. More information concerning our NSPS review
can be found in the memorandum, ``CAA Section 111(b)(1)(B) and
112(d)(6) Reviews for the Phosphoric Acid Manufacturing and Phosphate
Fertilizer Production Source Categories.'' Though some of the sources
are emitting at levels well below the current NSPS, other sources are
not. We evaluated emissions based on control technologies and practices
used by facilities, and found that the same technologies and practices
yielded different results for different facilities. Therefore, we
determined that we cannot conclude that new and modified sources would
be able to achieve a more stringent NSPS. As explained in the proposed
rule, all Phosphoric Acid Manufacturing NSPS (under subpart T and
subpart U) emission sources, and the control technologies that would be
employed, are the same as those for the NESHAP regulating phosphoric
acid plants, such that we reached the same conclusion that there are no
identified developments in technology or practices that results in
cost-effective emission

reductions strategies. Therefore, we are finalizing our determination
that revisions to NSPS subpart T and subpart U standards are not
appropriate pursuant to CAA section 111(b)(1)(B).

E. Startup, Shutdown, and Malfunction Provisions for the Phosphoric
Acid Manufacturing Source Category

1. What SSM provisions did we propose for the Phosphoric Acid
Manufacturing source category?
    In its 2008 decision in Sierra Club v. EPA, 551 F.3d 1019 (D.C.
Cir. 2008), the U.S. Court of Appeals for the District of Columbia
Circuit vacated portions of two provisions in the EPA's CAA section 112
regulations governing the emissions of HAP during periods of SSM.
Specifically, the Court vacated the SSM exemption contained in 40 CFR
63.6(f)(1) and 40 CFR 63.6(h)(1), holding that under section 302(k) of
the CAA, emissions standards or limitations must be continuous in
nature and that the SSM exemption violates the CAA's requirement that
some CAA section 112 standards apply continuously.
    We proposed to eliminate the SSM exemption in NESHAP subpart AA.
Consistent with Sierra Club v. EPA, the EPA proposed standards in this
rule that apply at all times. We also proposed to revise appendix A of
subpart AA (the General Provisions Applicability Table) in several
respects as is explained in more detail below. For example, we proposed
to eliminate the incorporation of the General Provisions' requirement
that the source develop an SSM plan. We also proposed to eliminate and
revise certain recordkeeping and reporting related to the SSM exemption
as described in detail in the proposed rule and summarized again here.
    In proposing the standards in this rule, the EPA took into account
startup and shutdown periods and, for the reasons explained below,
proposed work practice standards for periods of startup and shutdown in
lieu of numeric emission limits. CAA section 112(h)(1) states that the
Administrator may promulgate a design, equipment or operational work
practice standard in those cases where, in the judgment of the
Administrator, it is not feasible to prescribe or enforce an emission
standard. CAA section 112(h)(2)(B) further defines the term ``not
feasible'' in this context to apply when ``the application of
measurement technology to a particular class of sources is not
practicable due to technological and economic limitations.''
    Startup and shutdown periods at phosphoric acid manufacturing
facilities generally only last between 30 minutes and 6 hours. Because
of the variability and the relatively short duration, compared to the
time needed to conduct a performance test, which typically requires a
full working day, the EPA has determined that it is not feasible to
prescribe a numeric emission standard for these periods. Furthermore,
according to information provided by industry, it is possible that the
feed rate (i.e., equivalent P2O5 feed, or rock
feed) can be zero during startup and shutdown periods. During these
periods, it is not feasible to consistently enforce the emission
standards that are expressed in terms of lb of pollutant/ton of feed.
    Although we requested information on emissions and the operation of
control devices during startup and shutdown periods in the CAA section
114 survey issued to the Phosphoric Acid Manufacturing source category,
we did not receive any emissions data collected during a startup and
shutdown period (nor did we receive data during public comment of the
proposed rule), and we do not expect that these data exist. However,
based on the information for control device operation received in the
survey, we concluded that the control devices could be operated
normally during periods of startup or shutdown. Also, we believe that
the emissions generated during startup and shutdown periods are lower
than during steady-state conditions because the amount of feed
materials introduced to the process during those periods is lower
compared to normal operations. Therefore, if the emission control
devices are operated during startup and shutdown, then HAP emissions
will be the same or lower than during steady-state operating
conditions.
    Consequently, we proposed a work practice standard rather than an
emissions limit for periods of startup or shutdown. We proposed that
control devices used on the various process lines in this source
category are effective at achieving desired emission reductions
immediately upon startup; therefore, during startup and shutdown
periods, we proposed that sources begin operation of any control
device(s) in the production unit prior to introducing any feed into the
production unit. We also proposed that sources must continue operation
of the control device(s) through the shutdown period until all feed
material has been processed through the production unit.
    Periods of startup, normal operations and shutdown are all
predictable and routine aspects of a source's operations. Malfunctions,
in contrast, are neither predictable nor routine. Instead, they are, by
definition, sudden, infrequent and not reasonably preventable failures
of emissions control, process or monitoring equipment (40 CFR 63.2)
(definition of malfunction). The EPA interprets CAA section 112 as not
requiring emissions that occur during periods of malfunction to be
factored into development of CAA section 112 standards. Under CAA
section 112, emission standards for new sources must be no less
stringent than the level ``achieved'' by the best controlled similar
source and for existing sources generally must be no less stringent
than the average emission limitation ``achieved'' by the best
performing 12 percent of sources in the category. There is nothing in
CAA section 112 that directs the EPA to consider malfunctions in
determining the level ``achieved'' by the best performing sources when
setting emission standards. As the U.S. Court of Appeals for the
District of Columbia Circuit has recognized, the phrase ``average
emissions limitation achieved by the best performing 12 percent of''
sources ``says nothing about how the performance of the best units is
to be calculated.'' Nat'l Ass'n of Clean Water Agencies v. EPA, 734
F.3d 1115, 1141 (D.C. Cir. 2013). While the EPA accounts for
variability in setting emission standards, nothing in CAA section 112
requires the Agency to consider malfunctions as part of that analysis.
A malfunction should not be treated in the same manner as the type of
variation in performance that occurs during routine operations of a
source. A malfunction is a failure of the source to perform in a
``normal or usual manner'' and no statutory language compels EPA to
consider such events in setting CAA section 112 standards.
    Further, accounting for malfunctions in setting emission standards
would be difficult, if not impossible, given the myriad different types
of malfunctions that can occur across all sources in the category and
given the difficulties associated with predicting or accounting for the
frequency, degree and duration of various malfunctions that might
occur. As such, the performance of units that are malfunctioning is not
``reasonably'' foreseeable. See, e.g., Sierra Club v. EPA, 167 F.3d
658, 662 (D.C. Cir. 1999) (``The EPA typically has wide latitude in
determining the extent of data-gathering necessary to solve a problem.
We generally defer to an agency's decision to proceed on the basis of
imperfect scientific information, rather than to `invest the resources
to conduct the perfect study.' ''). See also Weyerhaeuser v. Costle,
590 F.2d 1011,

1058 (D.C. Cir. 1978) (``In the nature of things, no general limit,
individual permit, or even any upset provision can anticipate all upset
situations. After a certain point, the transgression of regulatory
limits caused by `uncontrollable acts of third parties,' such as
strikes, sabotage, operator intoxication or insanity, and a variety of
other eventualities, must be a matter for the administrative exercise
of case-by-case enforcement discretion, not for specification in
advance by regulation.''). In addition, emissions during a malfunction
event can be significantly higher than emissions at any other time of
source operation. For example, if an air pollution control device with
99 percent removal goes off-line as a result of a malfunction (as might
happen if, for example, the bags in a baghouse catch fire) and the
emission unit is a steady-state type unit that would take days to
shutdown, the source would go from 99 percent control to zero control
until the control device was repaired. The source's emissions during
the malfunction would be 100 times higher than during normal
operations, and the emissions over a 4-day malfunction period would
exceed the annual emissions of the source during normal operations. As
this example illustrates, accounting for malfunctions could lead to
standards that are not reflective of (and significantly less stringent
than) levels that are achieved by a well-performing non-malfunctioning
source. It is reasonable to interpret CAA section 112 to avoid such a
result. The EPA's approach to malfunctions is consistent with CAA
section 112 and is a reasonable interpretation of the statute.
    In the event that a source fails to comply with the applicable CAA
section 112(d) standards as a result of a malfunction event, the EPA
would determine an appropriate response based on, among other things,
the good faith efforts of the source to minimize emissions during
malfunction periods, including preventative and corrective actions, as
well as root cause analyses to ascertain and rectify excess emissions.
The EPA would also consider whether the source's failure to comply with
the CAA section 112(d) standard was, in fact, sudden, infrequent, not
reasonably preventable and was not instead caused in part by poor
maintenance or careless operation 40 CFR 63.2 (definition of
malfunction).
    If the EPA determines in a particular case that an enforcement
action against a source for violation of an emission standard is
warranted, the source can raise any and all defenses in that
enforcement action, and the federal district court will determine what,
if any, relief is appropriate. The same is true for citizen enforcement
actions. Similarly, the presiding officer in an administrative
proceeding can consider any defense raised and determine whether
administrative penalties are appropriate.
    In summary, the EPA interpretation of the CAA and, in particular,
CAA section 112 is reasonable and encourages practices that will avoid
malfunctions. Administrative and judicial procedures for addressing
exceedances of the standards fully recognize that violations may occur
despite good faith efforts to comply and can accommodate those
situations.
    To address the U.S. Court of Appeals for the District of Columbia
Circuit vacatur of portions of the EPA's CAA section 112 regulations
governing the emissions of HAP during periods of SSM, Sierra Club v.
EPA, 551 F.3d 1019 (D.C. Cir. 2008), we proposed to revise and add
certain provisions to the Phosphoric Acid Manufacturing rule. As
described in detail below, we proposed to revise the General Provisions
table (appendix A) to change several references related to requirements
that apply during periods of SSM. We also proposed to add other
provisions to the Phosphoric Acid Manufacturing rule as described
below.
    a. 40 CFR 63.608(b) General Duty. We proposed to revise the entry
for 40 CFR 63.6(e)(1)(i) and (e)(1)(ii) in the General Provisions table
(appendix A) by changing the ``yes'' in column three to ``no.'' Section
63.6(e)(1)(i) describes the general duty to minimize emissions. Some of
the language in that section is no longer necessary or appropriate in
light of the elimination of the SSM exemption. We proposed instead to
add general duty regulatory text at 40 CFR 63.608(b) that reflects the
general duty to minimize emissions while eliminating the reference to
periods covered by an SSM exemption. The current language in 40 CFR
63.6(e)(1)(i) characterizes what the general duty entails during
periods of SSM. With the elimination of the SSM exemption, there is no
need to differentiate between normal operations, startup and shutdown
and malfunction events in describing the general duty. Therefore, the
language the EPA proposed does not include that language from 40 CFR
63.6(e)(1). We also proposed to revise the entry for 40 CFR
63.6(e)(1)(ii) in the General Provisions table (appendix A) by changing
the ``yes'' in column three to ``no.'' Section 63.6(e)(1)(ii) imposes
requirements that are not necessary with the elimination of the SSM
exemption or are redundant of the general duty requirement being added
at 40 CFR 63.608(b).
    b. SSM Plan. We proposed to revise the entry for 40 CFR 63.6(e)(3)
in the General Provisions table (appendix A) by changing the ``yes'' in
column three to ``no.'' Generally, these paragraphs require development
of an SSM plan and specify SSM recordkeeping and reporting requirements
related to the SSM plan. As noted, the EPA proposed to remove the SSM
exemptions. Therefore, affected units will be subject to an emission
standard during such events. The applicability of a standard during
such events will ensure that sources have ample incentive to plan for
and achieve compliance and thus the SSM plan requirements are no longer
necessary.
    c. Compliance with Standards. We proposed to revise the entry for
40 CFR 63.6(f) in the General Provisions table (appendix A) by changing
the ``yes'' in column three to ``no.'' The current language of 40 CFR
63.6(f)(1) exempts sources from non-opacity standards during periods of
SSM. As discussed above, the Court in Sierra Club v. EPA vacated the
exemptions contained in this provision and held that the CAA requires
that some CAA section 112 standard apply continuously. Consistent with
Sierra Club v. EPA, the EPA proposed to revise standards in this rule
to apply at all times.
    d. 40 CFR 63.606 Performance Testing. We proposed to revise the
entry for 40 CFR 63.7(e)(1) in the General Provisions table (appendix
A) by changing the ``yes'' in column three to ``no.'' Section
63.7(e)(1) describes performance testing requirements. The EPA instead
proposed to add a performance testing requirement at 40 CFR 63.606(d).
The performance testing requirements that were proposed differ from the
General Provisions performance testing provisions in several respects.
The proposed regulatory text does not allow testing during startup,
shutdown, or malfunction. The proposed regulatory text does not include
the language in 40 CFR 63.7(e)(1) that restated the SSM exemption and
language that precluded startup and shutdown periods from being
considered ``representative'' for purposes of performance testing.
Furthermore, as in 40 CFR 63.7(e)(1), performance tests conducted under
this subpart should not be conducted during malfunctions because
conditions during malfunctions are often not representative of
operating conditions.
    e. Monitoring. We proposed to revise the entry for 40 CFR
63.8(c)(1)(i) and (iii) in the General Provisions table by changing the
``yes'' in column three to

``no.'' The cross-references to the general duty and SSM plan
requirements in those subparagraphs are not necessary in light of other
requirements of 40 CFR 63.8 that require good air pollution control
practices (40 CFR 63.8(c)(1)) and that set out the requirements of a
quality control program for monitoring equipment (40 CFR 63.8(d)).
    We proposed to revise the entry for 40 CFR 63.8(d)(3) in the
General Provisions table (appendix A) by changing the ``yes'' in column
three to ``no.'' The final sentence in 40 CFR 63.8(d)(3) refers to the
General Provisions' SSM plan requirement, which is no longer
applicable. The EPA proposed to add to the rule at 40 CFR 63.608(c)(3)
text that is identical to 40 CFR 63.8(d)(3), except that the final
sentence is replaced with the following sentence: ``You must include
the program of corrective action required under Sec.  63.8(d)(2) in the
plan.''
    f. 40 CFR 63.607 Recordkeeping. We proposed to revise the entry for
40 CFR 63.10(b)(2)(i) in the General Provisions table (appendix A) by
changing the ``yes'' in column three to ``no.'' Section 63.10(b)(2)(i)
describes the recordkeeping requirements during startup and shutdown.
These recording provisions are no longer necessary because the EPA
proposed that recordkeeping and reporting applicable to normal
operations will apply to startup and shutdown. In the absence of
special provisions applicable to startup and shutdown, such as a
startup and shutdown plan, there is no reason to retain additional
recordkeeping for startup and shutdown periods.
    We proposed to revise the entry for 40 CFR 63.10(b)(2)(ii) in the
General Provisions table (appendix A) by changing the ``yes'' in column
three to ``no.'' Section 63.10(b)(2)(ii) describes the recordkeeping
requirements during a malfunction. The EPA proposed to add such
requirements to 40 CFR 63.607(b). The regulatory text we proposed to
add differs from the General Provisions it is replacing in that the
General Provisions requires the creation and retention of a record of
the occurrence and duration of each malfunction of process, air
pollution control and monitoring equipment. The EPA proposed that this
requirement apply to any failure to meet an applicable standard and
that the source record the date, time and duration of the failure
rather than the ``occurrence.'' The EPA also proposed to add to 40 CFR
63.607(b) a requirement that sources keep records that include a list
of the affected source or equipment and actions taken to minimize
emissions, an estimate of the volume of each regulated pollutant
emitted over the applicable standard and a description of the method
used to estimate the emissions. Examples of such methods would include
product-loss calculations, mass balance calculations, measurements when
available or engineering judgment based on known process parameters.
The EPA proposed requiring that sources keep records of this
information to ensure that there is adequate information to allow the
EPA to determine the severity of any failure to meet a standard, and to
provide data that may document how the source met the general duty to
minimize emissions when the source has failed to meet an applicable
standard.
    We proposed to revise the entry for 40 CFR 63.10(b)(2)(iv) in the
General Provisions table (appendix A) by changing the ``yes'' in column
three to ``no.'' When applicable, the provision requires sources to
record actions taken during SSM events when actions were inconsistent
with their SSM plan. The requirement is no longer appropriate because
SSM plans will no longer be required. The requirement previously
applicable under 40 CFR 63.10(b)(2)(iv)(B) to record actions to
minimize emissions and record corrective actions is now applicable by
reference to 40 CFR 63.607.
    We proposed to revise the entry for 40 CFR 63.10(b)(2)(v) in the
General Provisions table (appendix A) by changing the ``yes'' in column
three to ``no.'' When applicable, the provision requires sources to
record actions taken during SSM events to show that actions taken were
consistent with their SSM plan. The requirement is no longer
appropriate because SSM plans will no longer be required.
    We proposed to revise the entry for 40 CFR 63.10(c)(15) in the
General Provisions table (appendix A) by changing the ``yes'' in column
three to ``no.'' The EPA proposed that 40 CFR 63.10(c)(15) no longer
apply. When applicable, the provision allows an owner or operator to
use the affected source's SSM plan or records kept to satisfy the
recordkeeping requirements of the SSM plan, specified in 40 CFR
63.6(e), to also satisfy the requirements of 40 CFR 63.10(c)(10)
through (12). The EPA proposed to eliminate this requirement because
SSM plans would no longer be required, and, therefore, 40 CFR
63.10(c)(15) no longer serves any useful purpose for affected units.
    g. 40 CFR 63.607 Reporting. We proposed to revise the entry for 40
CFR 63.10(d)(5) in the General Provisions table (appendix A) by
changing the ``yes'' in column three to ``no.'' Section 63.10(d)(5)
describes the reporting requirements for startups, shutdowns and
malfunctions. To replace the General Provisions reporting requirement,
the EPA proposed to add reporting requirements to 40 CFR 63.607. The
replacement language differs from the General Provisions requirement in
that it eliminates periodic SSM reports as a stand-alone report. We
proposed language that requires sources that fail to meet an applicable
standard at any time to report the information concerning such events
in the excess emission report already required under this rule. We
proposed that the report must contain the number, date, time, duration
and the cause of such events (including unknown cause, if applicable),
a list of the affected source or equipment, an estimate of the volume
of each regulated pollutant emitted over any emission limit, and a
description of the method used to estimate the emissions (e.g.,
product-loss calculations, mass balance calculations, direct
measurements or engineering judgment based on known process
parameters). The EPA proposed this requirement to ensure that adequate
information is available to determine compliance, to allow the EPA to
determine the severity of the failure to meet an applicable standard,
and to provide data that may document how the source met the general
duty to minimize emissions during a failure to meet an applicable
standard.
    The proposed rule eliminates the cross reference to 40 CFR
63.10(d)(5)(i) that contains the description of the previously-required
SSM report format and submittal schedule from this section. We proposed
that these specifications would no longer be necessary because the
events will be reported in otherwise required reports with similar
format and submittal requirements. We proposed that owners or operators
no longer be required to determine whether actions taken to correct a
malfunction are consistent with an SSM plan because the plans would no
longer be required.
    We proposed to revise the entry for 40 CFR 63.10(d)(5)(ii) in the
General Provisions table (appendix A) by changing the ``yes'' in column
three to ``no.'' Section 63.10(d)(5)(ii) describes an immediate report
for SSM when a source failed to meet an applicable standard but did not
follow the SSM plan. We proposed that we would no longer require owners
and operators to report when actions taken during a startup, shutdown,
or malfunction were not consistent with an SSM plan because the plans
would no longer be required.

2. How did the SSM provisions change for the Phosphoric Acid
Manufacturing source category?
    We are finalizing the proposed work practice standards for periods
of startup and shutdown; however, in consideration of comments received
during the public comment period for the proposed rulemaking (as
discussed in sections V.E.3.a and V.E.3.b of this preamble), we are
making changes to this work practice in order to clarify the standard
applies in lieu of numeric emission limits and to clarify how
compliance with the standard is demonstrated. Additionally, as
discussed in section V.E.3.c of this preamble, we added a definition of
``startup'' and ``shutdown'' in the final rule to specify when startup
begins and ends, and when shutdown begins and ends.
3. What key comments did we receive on the SSM provisions, and what are
our responses?
    We received comments regarding the proposed revisions to remove the
SSM exemptions for the Phosphoric Acid Manufacturing source category,
and the proposed work practice standards for periods of startup and
shutdown. The following is a summary of some of the comments specific
to the proposed work practice standards and our response to those
comments. Other comments and our specific responses to those comments
can be found in the Comment Summary and Response document available in
the docket for this action (EPA-HQ-OAR-2012-0522).
    a. Work Practice Standard In Place Of Emission Limits--Comment. One
commenter argued that the EPA should specify that the proposed work
practices for plant startup and shutdown periods apply ``in lieu of''
any other emission standards, and that such periods should not be
counted for testing, monitoring, or operating parameter requirements.
The commenter noted that the proposed rule at 40 CFR 63.602(h) requires
the use of work practices ``to demonstrate compliance with any emission
limits'' during periods of startup and shutdown. The commenter agrees
with the EPA's conclusion that it is not feasible to apply numeric
limits to startup and shutdown because certain variables required to
calculate emissions would be zero during such periods. The commenter
also agreed with the EPA that existing emission control devices would
still be effective during periods of startup or shutdown, if activated.
However, the commenter recommended that the rule should clarify that
startup and shutdown events should not be required to comply with the
monitoring and operating parameter requirements because startup and
shutdown events generally are not representative of operating
conditions for other compliance purposes, such as emissions testing.
Instead, the commenter, as well as a second commenter, recommended
that, because the startup and shutdown periods are not representative,
the rule should only require that (1) all emission control devices be
kept active, and (2) owners and operators follow the general duty to
control emissions, and owners and operators should not be required to
monitor operating parameters during startup and shutdown periods.
    The commenter argued that the approach in the proposed rule at 40
CFR 63.602(h) to require the use of work practices ``to demonstrate
compliance with any emission limits'' during periods of startup and
shutdown is ``directly inconsistent'' with the approach that the EPA
has applied to other source categories, where such practices clearly
were prescribed ``in lieu of'' numeric emission limits that would
otherwise apply. (The commenter cites, for example, 78 FR 10015,
February 12, 2013.) According to the commenter, the EPA made it clear
in other industries' rules that such work practice standards apply ``in
place of'' or ``in lieu of'' numeric standards, including with respect
to monitoring and recordkeeping requirements. (See id. at 10013 and
10015.) The commenter argues that according to the preamble language
cited for those other industries, ``there will no longer be a numeric
emission standard applicable during startup and shutdown,'' and the EPA
recognizes that ``the recordkeeping requirement must change to reflect
the content of the work practice standard''(Id. at 10014).
    Therefore, the commenter recommended that the EPA should clearly
explain that work practices are not applied to ``demonstrate
compliance'' with numeric limits under subpart AA, which the EPA
acknowledges are ``not feasible'' for startup and shutdown periods,
and, instead, the work practices should be written to apply ``in lieu
of'' the numeric limits during those periods. The commenter argues that
without this clarification, it will appear that both the numeric
standards and the work practice standards would apply during startup
and shutdown. The commenter suggests that this can be corrected in the
rule by using the ``in lieu of'' language used for other industries.
    Response. The commenter is correct that our intention at proposal
was that the numeric emission limits would not apply during periods of
startup and shutdown, but that facilities would comply with the work
practice instead. We did not intend for the work practice to be a
method to demonstrate compliance with the emission limit. We are
replacing the phrasing ``to demonstrate compliance'' with ``in lieu
of'' as this language is more consistent with our original intent.
Accordingly, in the final rule, 40 CFR 63.602(f) specifies that the
emission limits of 40 CFR 63.602(a) do not apply during periods of
startup and shutdown. Instead, owners and operators must follow the
work practice specified in 40 CFR 63.602(f). See section V.E.3.b of
this preamble for our response to commenters' argument that owners and
operators should not be required to monitor operating parameters during
startup and shutdown periods.
    b. Applicability Of Operating Limits--Comment. Two commenters
recommended that the EPA amend the rule to make clear that the work
practice standards for startup and shutdown also apply in lieu of the
parametric monitoring requirements set forth in subpart AA and make
explicit that parametric operating requirements do not apply during
times of startup and shutdown.
    One commenter argued that when the EPA established the flow rate
and pressure drop parametric monitoring requirements in its 1999 final
rule, the EPA concluded that requiring continuous monitoring of these
parameters ``help[ed] assure continuous compliance with the emission
limit'' (64 FR 31365, June 10, 1999). The commenter also asserted that
the rules specify that ``[t]he emission limitations and operating
parameter requirements of this subpart do not apply during periods of
startup, shutdown, or malfunction . . .'' (40 CFR 63.600(e)). The
commenter argued that this was a reasonable action because the
operating parameter ranges are established during annual performance
tests, and these tests cannot be performed during startup and shutdown
conditions.
    The commenter suggested that in the proposed rule, the EPA exempted
compliance with the emission limits during startup and shutdown
periods, imposed work practice standards in lieu thereof, and retained
the prohibition on conducting a performance test during periods of
startup or shutdown (79 FR 66570 (proposed 40 CFR 63.606(d)). The
commenter suggested that the proposed rule is silent on the
applicability of the parametric monitoring requirements during startup
and shutdown. The commenter asserted that because the parametric
monitoring provisions

provide an inference of compliance with the emission limits (64 FR
31365, June 10, 1999), and these emission limits do not apply during
startup and shutdown, the commenter concluded that the parametric
monitoring provisions similarly should not apply during startups and
shutdowns.
    The commenters pointed to two recent EPA NESHAP rulemakings to
support their conclusion. First, the commenters argued that in its
industrial, commercial and institutional boilers and process heaters
NESHAP reconsideration proposal (hereinafter, the ``Boiler NESHAP''),
the EPA, responding to a comment soliciting clarification ``that the
operating limits and opacity limits do not apply during periods of
startup and shutdown,'' stated that with the finalization of work
practice standards, ``EPA agrees that the requested clarification is
what was intended in the final rule'' (76 FR 80598 and 80615, December
23, 2011). The commenters asserted that to this end, in its response to
the reconsideration, the EPA made clear that affected sources must
comply with ``all applicable emissions and operating limits at all
times the unit is operating except for periods that meet the
definitions of startup and shutdown in this subpart, during which times
you must comply with these work practices'' (78 FR 7138 and 7142,
January 31, 2013). The commenters noted that in the Boiler NESHAP
regulations, the EPA required the implementation of work practice
standards in lieu of compliance with the operating parameter
requirements during startup and shutdown by (1) Excluding periods of
startup and shutdown from the averaging period (Id. at 7187, 40 CFR
63.7575, the definition of a 30-day rolling average'' excludes ``hours
during startup and shutdown''), and (2) expressly stating that the
``standards'' (the emission limits and operating requirements) do not
apply during periods of startup or shutdown. (Id. at 7163, 40 CFR
63.7500(f), titled ``What emission limitations, work practice
standards, and operating limits must I meet?'' applies ``at all times
the affected unit is operating, except during periods of startup and
shutdown during which time you must comply only with Table 3 of this
subpart.'')
    Second, the commenters argued that in its Portland Cement NESHAP,
the EPA specified an operating limit for kilns, identified as a
temperature limit established during a performance test, and that the
temperature limit applied at all times the raw mill is operating,
``except during periods of startup and shutdown'' (78 FR 10039,
February 12, 2013, 40 CFR 63.1346(a)(1)). Further, for the continuous
monitoring requirements, including operating limits, the Portland
Cement NESHAP required operating of the monitoring system at all times
the affected source is operating, ``[e]xcept for periods of startup and
shutdown'' (Id. at 10041, 40 CFR 63.1348(b)(1)(ii)).
    The commenters argued that given the EPA's conclusion in the
proposed rule that the emission limits should not apply during startup
and shutdown, and because the parametric monitoring requirements are
established during a performance test (which cannot be performed during
a startup or a shutdown) and used to infer compliance with the emission
limits, the EPA should make clear in the final rule that the operating
parameters requirements do not apply during startup or shutdown. The
commenter recommended that the EPA should make this explicit: (1) In
the operating and monitoring requirement section of subpart AA
(proposed 40 CFR 63.605), and (2) by defining the averaging period
(currently daily) as excluding periods of startup and shutdown
(proposed 40 CFR part 63, subpart AA, Table 4.) As an alternative, the
commenters recommended that if the EPA continues to require compliance
with the parametric monitoring requirements during startup and shutdown
periods, then the EPA should adopt a longer averaging period, from
daily to 30 days, to allow for the effects of startups and shutdowns to
be reduced by a longer period of steady-state operations. The commenter
noted that the Boiler NESHAP has a 30-day averaging period for pressure
drop and liquid flow rate, and excludes periods of startup and shutdown
from the averaging period (40 CFR 63.7575, definition of ``30-day
rolling average'' and 40 CFR part 63, subpart DDDDD, Table 4.) The
commenter stated that a 30-day averaging period would be substantially
more stringent than the Boiler NESHAP approach since it would include
periods of startup and shutdown, while at the same time avoid
misleading ``exceedances'' caused by the inclusion of periods of
startup and shutdown compared to daily average parametric limits.
    Response. We disagree with the commenters about the applicability
of the operating limits. Based on these comments, we have clarified in
the final rule at 40 CFR 63.602(f) that to comply with the work
practice during periods of startup and shutdown, facilities must
monitor the operating parameters specified in Table 3 to subpart AA and
comply with the operating limits specified in Table 4 of subpart AA.
The purpose of the work practice is to ensure that the air pollution
control equipment that is used to comply with the emission limit during
normal operations is operated during periods of startup and shutdown.
Monitoring of control device operating parameters is necessary to
demonstrate compliance with the work practice. We have concluded that
it is reasonable for the control device at phosphoric acid processes to
meet the same operating limits during startup and shutdown that apply
during normal operation, and that it is not necessary to specify
different averaging times for periods of startup and shutdown. Meeting
the operating limits of Table 4 of subpart AA will ensure that owners
and operators meet the General Duty requirement to operate and maintain
the affected source and associated air pollution control equipment in a
manner consistent with safety and good air pollution control practices
for minimizing emissions.
    The analogies that the commenters made to the Industrial Boiler
NESHAP and the Portland Cement NESHAP are not relevant to this
rulemaking. In each rulemaking, we consider the feasibility of applying
standards during startup and shutdown based on relevant process
considerations for each source category, the pollutants regulated, and
control devices on which the rule is based. In developing this rule, we
obtained information on the operation of control devices during startup
and shutdown periods in the CAA section 114 survey issued to the
phosphoric acid manufacturing industry. Based on survey results, we
concluded that for this source category, control devices (i.e.,
absorbers and WESP) could be operated during periods of startup and
shutdown. We found no indication that process operations during startup
and shutdown would interfere with the ability to operate the relevant
control devices according to good engineering practice. Moreover, the
commenters provided no technical justification as to why a different
operating limit is needed during startup and shutdown.
    Regarding the comparison to the Industrial Boiler NESHAP, the
operation of boilers and their associated control devices are different
than phosphoric acid plants. While boiler control devices do not have
to comply with specific operating limits during startup or shutdown,
they must meet a work practice that includes firing clean fuels,
operating relevant control devices (e.g., absorbers) as expeditiously
as possible, and monitoring the applicable operating parameters (e.g.,
flow rate) to demonstrate that the control devices are being operated
properly. The EPA

currently is reconsidering the control requirements for industrial
boilers during startup and shutdown (80 FR 3090, January 21, 2015). In
the proposed action on reconsideration, we pointed out that some of the
control devices used for boilers cannot be operated during the full
duration of startup and shutdown because of safety concerns and the
possibility of control equipment degradation due to fouling and
corrosion. The control devices used for phosphoric acid production do
not pose these same risks. Likewise, the fact the Portland Cement
NESHAP does not require monitoring of kiln temperature during startup
and shutdown is not relevant. The Portland Cement NESHAP requires
maintaining a kiln temperature as part of the MACT operating limit. The
operating limit for the Portland Cement NESHAP does not apply during
startup and shutdown because it is not physically possible to maintain
a constant temperature during startup and shutdown of a kiln. In
contrast, the feasibility of operating the control devices used to
control HAP emissions from phosphoric acid manufacturing is not limited
by specific process operating conditions. Therefore, it is feasible to
operate the devices during startup and shutdown, and we have determined
that it is reasonable to do so considering cost, nonair health and
environmental impacts, and energy requirements.
    c. Definition Of Startup And Shutdown--Comment. Several commenters
argued that the EPA's proposed work practice standard for periods of
startup and shutdown failed to account for how equipment in the
phosphoric acid industry works. In order to comply with the proposed
startup and shutdown requirements, the operator must begin operation of
any control device(s) being used at the affected source prior to
introducing any feed into the affected source and continue operation of
the control device(s) through the shutdown period until all feed
material has been processed through the affected source. The commenters
noted that it is not feasible to process all feed material from a
process prior to shutting down most equipment at a facility. For
example, the phosphoric acid reactors and beds in the calciners may not
be able to process all the feed material in them prior to shutdown and
there would always still be feed material left in the equipment even
after it is shutdown. The same would be true for nearly all process
units in the industry. The commenters requested that the EPA revise 40
CFR 63.602(h) to require compliance with the work practice standard
only up to the point in time when no more feed or in-process materials
are being introduced into the production unit.
    Two commenters agreed with other commenters that it is not feasible
to base the conclusion of a ``shutdown'' on the point at which all feed
has ``been processed.'' Instead, they suggested that the EPA should
clarify the work practice standard of keeping all emission control
equipment active during shutdowns. The commenters reported that
facilities in the industry consider the commencement of ``shutdown'' as
the moment at which the plant ceases adding feed to the affected
process, rather than basing shutdown on when all feed materials have
been processed through the process. The commenters recommended that the
EPA should define ``shutdown'' to begin when the facility ceases adding
feed to an affected process line, and to conclude when the affected
process line equipment is deactivated, even though some feed or
residues may still be present within particular parts of the process.
    One of the commenters also noted that it is common practice to have
short-term shutdown of process inputs for temporary maintenance work
(including work on emission control equipment) where the entire system
is not emptied. In these cases, feed of phosphoric acid and ammonia to
the process is suspended as is flow from the reactor to the granulator.
The commenter argued that because the source of fluoride to the system
has ceased and dust generating material flows are suspended, there
should be no significant source of emissions to control, and it is not
necessary to require the use of control devices until all feed material
has been processed. Instead, the commenter recommended that an affected
entity should be allowed to turn off control devices when reactor and
granulator feeds have been stopped, unless the system is being emptied,
in which case control devices should be required as long as the
material handling system is in operation.
    Response. We agree with the commenters that the rule needs to have
a more precise definition of startup and shutdown that more clearly and
reasonably establishes the times when the work practice applies and
when the emission limits apply. Accordingly, we added a definition of
``startup'' and ``shutdown'' in the definitions section of the final
rule to specify when startup begins and ends, and when shutdown begins
and ends.
    Based on additional information provided by industry (see ``Email
Correspondence Received After Comment Period re Startup Shutdown (May
5, 2015),'' which is available in Docket ID No. EPA-HQ-OAR-2012-0522),
we are including a definition of startup in the final rule. The final
rule defines startup as commencing when any feed material is first
introduced into an affected source and ends when feed material is fully
loaded into the affected source. Regarding shutdown, we agree with the
commenters that it is not feasible to process all feed material from a
process prior to shutting down most equipment at a facility. Such
requirement would imply that the control device must be operated after
the shutdown ends. The final rule defines shutdown as commencing when
the facility ceases adding feed to an affected source and ends when the
affected source is deactivated, regardless of whether feed material is
present in the affected source. This definition will address concerns
about temporary shutdowns as well as shutdowns of longer duration.
    In addition, the final rule at 40 CFR 63.602(f) specifies that any
control device used at the affected source must be operated during the
entire period of startup and shutdown, and must meet the operating
limits in Table 4 of the final rule.
4. What is the rationale for our final decisions for the SSM
provisions?
    For the reasons provided above and in the preamble for the proposed
rule, we are finalizing the proposed revisions to the General
Provisions table (appendix A of NESHAP subpart AA) to change several
references related to requirements that apply during periods of SSM.
For these same reasons, we are also finalizing the addition of the
following proposed provisions to NESHAP subpart AA: (1) Work practice
standards for periods of startup and shutdown in lieu of numeric
emission limits; (2) the general duty to minimize emissions at all
times; (3) performance testing conditions requirements; (4) site-
specific monitoring plan requirements; and (5) malfunction
recordkeeping and reporting requirements.

F. Other Changes Made to the Phosphoric Acid Manufacturing NESHAP and
NSPS

1. What other changes did we propose for the Phosphoric Acid
Manufacturing NESHAP and NSPS?
    a. Clarifications to Applicability and Certain Definitions--i.
NESHAP Subpart AA. As stated in the preamble to the proposed rule, to
ensure the emission standards reflect inclusion of HAP emissions from
all sources in the source category, we proposed to amend the

definitions of WPPA process line, SPA process line, and PPA process
line to include relevant emission points, including clarifiers and
defluorination systems at WPPA process lines, and oxidation reactors at
SPA production lines. We also proposed removing text from the
applicability section that is duplicative of the revised definitions.
    We also proposed revising the term ``gypsum stack'' to ``gypsum
dewatering stack'' in order to help clarify the meaning of this
fugitive emission source, and to alleviate any potential misconception
that the ``stack'' is a point source. Other changes we proposed
included the addition of definitions for ``cooling pond,'' ``phosphoric
acid defluorination process,'' ``process line,'' and ``raffinate
stream.''
    ii. NSPS Subpart T. As stated in the preamble to the proposed rule,
to ensure the emission standards we proposed reflected inclusion of
total fluoride emissions from all sources in the defined source
category, we proposed to amend the definition of WPPA plant to include
relevant emission points, including clarifiers and defluorination
systems. We also proposed to remove text from the applicability section
that is duplicative of the revised definitions.
    iii. NSPS Subpart U. To ensure the emission standards we proposed
reflected inclusion of total fluoride emissions from all sources in the
defined source category, we proposed to amend the definition of SPA
plant to include relevant emission points, including oxidation
reactors. We also proposed to remove text from the applicability
section that is duplicative of the revised definitions.
    b. Testing, Monitoring, Recordkeeping and Reporting --i. NESHAP
Subpart AA. As stated in the preamble to the proposed rule, to provide
flexibility, we proposed several monitoring options, including pressure
and temperature measurements, as alternatives to monitoring of absorber
differential pressure. We also proposed monitoring the absorber inlet
gas flow rate along with the influent absorber liquid flow rate (and
determining liquid-to-gas ratio) in lieu of monitoring only the
absorber inlet liquid flow rate.
    In addition, we proposed removing the requirement that facilities
may not implement new operating parameter ranges until the
Administrator has approved them, or 30 days have passed since
submission of the performance test results. We proposed that facilities
must immediately comply with new operating ranges when they are
developed and submitted; and new operating ranges must be established
using the most recent performance test conducted by a facility, which
allows for changes in control device operation to be appropriately
reflected.
    As stated in the preamble to the proposed rule, we modified the
language for the conditions under which testing must be conducted to
require that testing be conducted at ``maximum representative operating
conditions'' for the process.\9\
---------------------------------------------------------------------------

    \9\ Based on the EPA memorandum, ``Issuance of the Clean Air Act
National Stack Testing Guidance,'' dated April 27, 2009.
---------------------------------------------------------------------------

    In keeping with the general provisions for CMS (including CEMS and
continuous parameter monitoring system (CPMS)), we proposed the
addition of a site-specific monitoring plan and calibration
requirements for CMS. Provisions were also proposed that included
electronic reporting of stack test data. We also proposed modifying the
format of NESHAP subpart AA to reference tables for emissions limits
and monitoring requirements.
    Finally, we proposed HF standards in NESHAP subpart AA by
translating the current total fluoride limits (lb total F/ton
P2O5 feed) into HF limits (lb HF/ton
P2O5 feed). To comply with HF standards, we
proposed that facilities use EPA Method 320.
    ii. NSPS Subpart T. We proposed new monitoring and recordkeeping
requirements for any WPPA plant that commences construction,
modification, or reconstruction after November 7, 2014 to ensure
continuous compliance with the standard. As stated in the preamble to
the proposed rule, to ensure that the process scrubbing system is
properly maintained over time; ensure continuous compliance with
standards; and improve data accessibility, we proposed the owner or
operator establish an allowable range for the pressure drop through the
process scrubbing system. We also proposed that the owner or operator
keep records of the daily average pressure drop through the process
scrubbing system, and keep records of deviations.
    For consistency with terminology used in the associated NESHAP
subpart AA, we proposed changing the term ``process scrubbing system''
to ``absorber'' in NSPS subpart T.
    iii. NSPS Subpart U. We proposed new monitoring and recordkeeping
requirements for any SPA plant that commences construction,
modification or reconstruction after November 7, 2014 to ensure
continuous compliance with the standard. As stated in the preamble to
the proposed rule, to ensure that the process scrubbing system is
properly maintained over time; ensure continuous compliance with
standards; and improve data accessibility, we proposed the owner or
operator establish an allowable range for the pressure drop through the
process scrubbing system. We also proposed that the owner or operator
keep records of the daily average pressure drop through the process
scrubbing system, and keep records of deviations.
    For consistency with terminology used in the associated NESHAP
subpart AA, we proposed changing the term ``process scrubbing system''
to ``absorber'' in NSPS subpart U.
2. How did the provisions regarding these other proposed changes to the
Phosphoric Acid Manufacturing NESHAP and NSPS change since proposal?
    a. Clarifications to Applicability and Certain Definitions--i.
NESHAP Subpart AA. In consideration of comments received during the
public comment period for the proposed rulemaking, we are adopting the
proposed clarifications for oxidation reactors as discussed in section
V.F.3.a.i of this preamble; however, we are also revising the
definition of oxidation reactor in the final rule to clarify that
oxidizing agents may include: Nitric acid, ammonium nitrate, or
potassium permanganate. Also, in consideration of comments received
(see section V.F.3.a.ii of this preamble for details), we are not
adopting the proposed clarifications for defluorination systems and
clarifiers.
    We have not made any change to the proposed revision to rename
``gypsum stack'' to ``gypsum dewatering stack.'' We have also not made
any changes to the proposed definitions for ``cooling pond'' and
``raffinate stream''; however, we are removing the proposed definitions
for ``phosphoric acid defluorination process'' and ``process line'' for
reasons discussed in sections V.F.3.a.ii and V.F.3.a.iii of this
preamble, respectively.
    Finally, we are removing the proposed language ``includes, but is
not limited to'' in the definitions of WPPA, SPA, and PPA process lines
for reasons discussed in section V.F.3.a.iv of this preamble.
    ii. NSPS Subpart T. In consideration of comments received (see
section V.F.3.a.ii of this preamble for details), we are not adopting
the proposed clarifications for defluorination systems and clarifiers.
We are also removing the proposed language ``includes, but is not
limited to'' in the definitions of WPPA plant for reasons discussed in
section V.F.3.a.iv of this preamble.

    iii. NSPS Subpart U. In consideration of comments received during
the public comment period for the proposed rulemaking, we are adopting
the proposed clarifications for oxidation reactors as discussed in
section V.F.3.a.i of this preamble; however, we are also revising the
proposed definition of oxidation reactor in the final rule to clarify
that oxidizing agents may include: Nitric acid, ammonium nitrate, or
potassium permanganate. We are also removing the proposed language
``includes, but is not limited to'' in the definitions of SPA plant for
reasons discussed in section V.F.3.a.iv of this preamble.
    b. Testing, Monitoring, Recordkeeping and Reporting--i. NESHAP
Subpart AA. We have not made any changes in our proposed determination
that pressure drop is not an appropriate monitoring parameter for
absorbers that are designed to operate with pressure drops of 5 inches
of water column or less. However, in consideration of comments received
during the public comment period for the proposed rulemaking, we are
not adopting the proposed options to monitor: (1) The temperature at
the wet scrubber gas stream outlet and pressure at the liquid inlet of
the absorber, or (2) the temperature at the scrubber gas stream outlet
and scrubber gas stream inlet. Instead, we have revised Table 3 of
NESHAP subpart AA to require liquid-to-gas ratio monitoring for low-
energy absorbers, and influent liquid flow and pressure drop monitoring
for high-energy absorbers; and we are keeping liquid-to-gas ratio
monitoring as an option for high-energy absorbers in the final rule.
(See section V.F.3.b.i and V.F.3.b.ii of this preamble for details.)
    In addition to these revisions, we are making corrections at 40 CFR
63.607(a) to clarify the procedures for establishing a new operating
limit based on the most recent performance test. We are also revising
the requirements at 40 CFR 63.605(d)(1)(ii)(B) of the final rule to
remove the requirement that facilities must request and obtain approval
of the Administrator for changing operating limits. (See section
V.F.3.b.iii and V.F.3.b.iv of this preamble for details.)
    Also, for reasons discussed in the in the Comment Summary and
Response document available in the docket, we are revising the annual
testing schedule in the final rule at 40 CFR 63.606(b), and the
terminology for ``maximum representative operating conditions'' in the
final rule at 40 CFR 63.606(d).
    We are not making any changes to the proposed addition of a site-
specific monitoring plan and calibration requirements for CMS. We are
also keeping the proposed term ``absorber'' in lieu of ``scrubber,'' as
well as the proposed format of NESHAP subpart AA to reference tables
for emissions limits and monitoring requirements.
    Lastly, we are retaining the current total fluoride limits and not
adopting the proposed HF standards and associated EPA Method 320
testing in NESHAP subpart AA (see section V.F.3.c of this preamble for
details).
    ii. NSPS Subpart T. We are not making changes to the proposed
monitoring and recordkeeping requirements for any WPPA plant that
commences construction, modification or reconstruction after August 19,
2015 to ensure continuous compliance with the standard. We are also
keeping the proposed term ``absorber'' in lieu of ``process scrubbing
system.''
    iii. NSPS Subpart U. We are not making changes to the proposed
monitoring and recordkeeping requirements for any SPA plant that
commences construction, modification, or reconstruction after August
19, 2015 to ensure continuous compliance with the standard. We are also
keeping the proposed term ``absorber'' in lieu of ``process scrubbing
system.''
3. What key comments did we receive on the other changes to the
Phosphoric Acid Manufacturing NESHAP and NSPS, and what are our
responses?
    Several comments were received regarding the proposed
clarifications to applicability and certain definitions, revisions to
testing, monitoring, recordkeeping and reporting, translation of total
fluoride to HF emission limits, and revisions to other provisions for
the Phosphoric Acid Manufacturing source category. The following is a
summary of significant comments and our response to those comments.
Other comments received and our responses to those comments can be
found in the Comment Summary and Response document available in the
docket for this action (EPA-HQ-OAR-2012-0522).
    a. Applicability Clarifications and Certain Definitions--i.
Oxidation Reactors--Comment. Several commenters remarked that the
proposed definition of SPA process line to include oxidation reactors
is problematic and goes beyond clarification. These commenters
requested that the EPA develop more specific language or provide a
clear technical basis under the CAA because any equipment that was not
expressly included in EPA's MACT floor calculations should not be
included in the affected source definition.
    Commenters mentioned that the EPA's memorandum ``Applicability
Clarifications to the Phosphoric Acid Manufacturing Source Category,''
which is available in the docket for this action, captured four
facilities, but it was not clear whether the PCS Aurora facility was
included in the count. These commenters stated that the oxidation step
at this facility is carried out in agitated tanks that do not have any
emissions control, and the emissions from the oxidation step are not
included in their annual performance testing (when demonstrating
compliance with the current total fluoride limits). The commenters said
that it was not clear whether this oxidation step involves an
``oxidation reactor'' as proposed; and, if it does, the commenters
argued that the EPA has not considered additional costs imposed by
including ``any equipment that uses an oxidizing agent to treat
phosphoric acid'' within the scope of the NESHAP at 40 CFR part 63,
subpart AA.
    Response. We are adopting the proposed SPA process line definition
in NESHAP subpart AA, and the proposed SPA plant definition in NSPS
subpart U, to include oxidation reactors. Based on information in
process flow diagrams provided by facilities, we initially believed
that oxidation reactors were part of the SPA process lines that would
have been considered in the original MACT analysis, and, thus subject
to the existing limits. In response to comments that stated the
opposite was true, we searched historical data, specifically the 1996
memorandum ``National Emission Standards for Hazardous Air Pollutants
from Phosphoric Acid Manufacturing and Phosphate Fertilizers
Production; Proposed Rules--Draft Technical Support Document and
Additional Technical Information'' (1996 TSD). The 1996 TSD lists, in
Attachment 2, the test data for SPA process lines that were assembled
for the MACT floor analysis (the 1996 TSD is item II-B-20 in Docket A-
94-02). Based on this review as well as a facility construction air
permit, we determined that oxidation reactor emissions from at least
one facility, PCS White Springs (see the emission point ``Occidental,
Suwanee Rv., FL-G'' in the 1996 TSD), were included with this assembled
SPA test dataset. It is possible that three other facilities (see the
emission points ``J.R. Simplot, Pocatello, ID'' for the Simplot Don-
Pocatello facility, ``Nu-West, Soda Springs, ID'' for the Agrium Nu-
West facility, and ``Texasgulf, Aurora, NC'' for the PCS Aurora
facility in the 1996 TSD) with oxidation reactors were also included in
this original dataset since we know today that these facilities have
oxidation reactors; however, it is unclear whether the oxidation
reactors

at these facilities were operating when the dataset was assembled.
Nevertheless, based on the emission point ``Occidental, Suwanee Rv.,
FL-G,'' SPA process lines that incorporate an oxidation reactor were
included as part of the SPA emissions dataset that was evaluated in
order to conduct the MACT floor analysis.
    In addition, the EPA's technology review revealed that SPA process
lines at four different facilities include an oxidation reactor to
remove organic impurities from the acid. We determined that one of
these facilities (Simplot Don-Pocatello) already ducts their oxidation
reactor emissions through their SPA process line wet scrubber, and is
achieving compliance with the SPA total fluoride emission limit. For
two of these facilities (PCS White Springs and Agrium Nu-West), we
determined that when their oxidation reactor emissions are combined
with the rest of their SPA process line emissions, the facilities are
in compliance with the total fluoride emission limit. Therefore, for
these three facilities it would not be necessary to upgrade existing
control systems, or to install a control system, in order to comply
with the rule.
    With regard to the oxidation reactor at the fourth facility (PCS
Aurora), the Agency has determined that this process (i.e., an
oxidation step carried out in agitated tanks) does qualify as an
oxidation reactor. Based on information that we received from industry
after the public comment period ended for the proposal (see docket item
EPA-HQ-OAR-2012-0522-0051), potassium permanganate is used in the PCS
Aurora oxidation step. This oxidizing agent was one of three
specifically cited in our memorandum ``Applicability Clarifications to
the Phosphoric Acid Manufacturing Source Category,'' which is available
in the docket for this action, so based on the data available, this
oxidation step should be included as part of the SPA process line
emissions when determining compliance with the SPA total fluoride
emission limit. Furthermore, based on this same information that we
received from industry after the public comment period ended for the
proposal, PCS Aurora may need to install a new absorber in order to
control its oxidation process emissions due to logistical complications
and concerns about inadequate capacity of other existing absorbers at
their SPA units. PCS Aurora estimated the absorber (venturi scrubber)
would incur capital costs of approximately $270,500, based on prior
absorber purchases for its facility. We estimated annual costs of
approximately $95,000. The costs associated with this change are
discussed further in the memorandum ``Control Costs and Emissions
Reductions for Phosphoric Acid and Phosphate Fertilizer Production
Source Categories--Final Rule,'' which is available in Docket ID No.
EPA-HQ-OAR-2012-0522.
    The definition of oxidation reactor in the final rule for NESHAP
subpart AA has been revised to clarify that oxidizing agents may
include: Nitric acid, ammonium nitrate, or potassium permanganate. The
words ``or step'' has also been added to the definition of oxidation
reactor, for instances when a facility may not typically identify their
oxidation process as occurring in a reactor. The definition now states
that ``oxidation reactor means any equipment or step that uses an
oxidizing agent (e.g., nitric acid, ammonium nitrate, or potassium
permanganate) to treat SPA.'' Similarly, the definition of ``SPA
plant'' in the final rule for NSPS subpart U has also been revised to
reflect these changes.
    ii. Defluorination and Clarifiers--Comment. Many commenters opposed
the proposed expanded definition of ``wet-process phosphoric acid
line'' to include ``clarifiers'' and ``defluorination processes.''
These commenters stated that the proposed revisions have the potential
to pull in several ``defluorination processes'' and ``clarifiers'' that
are not subject to the current rule (e.g., animal feed phosphate
production operations that have traditionally been outside the scope of
this subpart). These commenters argued that any unit operation that
conducts evaporation or concentrates phosphoric acid will have the
effect of defluorinating to some extent. One of these commenters stated
that they have a desulfation process at one of their facilities that
reduces F; the commenter also said that this facility's WPPA process
line has several filter product tanks, evaporator feed tanks, and
evaporator product tanks that could potentially be deemed clarifiers,
and thus be pulled into the proposed rule. Another of these commenters
argued that it is not logical to include clarifier and defluorination
systems in the definition because they operate independently of process
lines, and are often operated when feed is not put into process lines
(and so are not a process line manufacturing phosphoric acid by
reacting phosphate rock and acid). This commenter added that clarifiers
often operate more like tanks than process equipment and are not
routinely emptied; and emissions from clarifiers are not a function of
phosphate feed material to the reactor. The commenter stated that the
addition of clarifiers will require significant facility modifications
to accommodate emissions testing because although some clarifiers are
evacuated to WPPA scrubbers, others are not; and even though some
clarifiers have independent evacuation and scrubbing systems, other
clarifiers have no evacuation and scrubbing systems. Another commenter
also stated that one of their facilities contains clarifiers that are
not source tested or vented to a wet scrubber. This commenter stated
that it was not possible for one of their facilities to determine
whether they meet the proposed standard for a WPPA process line that
includes defluorination processes because their defluorination units
are not only integrated with their WPPA process, but also with
processes that do not meet the definition of WPPA lines. A commenter
added that defluorination processes and clarifiers are often subject to
separate emissions control requirements in their title V permits.
    Two commenters stated that since the original rule was adopted, the
definition of ``wet-process phosphoric line'' has not been interpreted
to extend or apply to clarifiers or defluorination processes. One of
these commenters claimed that the only rationale the EPA provides is
that the rules were ``initially intended'' to cover these sources, but
argued that neither the original proposal, nor the original final rule
mentioned the term ``clarifier'' or ``defluorination process.'' The
commenters requested that the EPA conduct CAA section 112(d)(2) or
112(d)(3) analyses for these new affected units. If the EPA conducts
these analyses, and decides to expand the definition of ``wet-process
phosphoric acid line'' to include ``clarifiers'' and ``defluorination
processes,'' a commenter suggested that the definition exclude units
that partially clarify or defluorinate an in-process stream
incidentally.
    Response. Based on information in process flow diagrams provided by
facilities, we initially believed that clarifiers and defluorination
systems were part of the WPPA process lines that would have been
considered in the original MACT analysis, and, thus, subject to the
existing limits. However, the EPA agrees that clarifiers and
defluorination systems should not be included in the WPPA process line
definition of NESHAP subpart AA, based on the new information
available. We also agree that clarifiers and defluorination systems
should not be included in the WPPA plant definition of NSPS subpart T.
    In the proposed rules, the EPA was specifically referring to
defluorination

processes that use diatomaceous earth and are included as part of the
WPPA process line; however, commenters explained that this type of
process is used solely in animal feed production. Because
defluorination processes that use diatomaceous earth are not related to
phosphoric acid manufacturing, as we first surmised, it is not
appropriate to include defluorination processes in the WPPA process
line definition.
    In response to comments regarding the inclusion of clarifiers in
the WPPA process line definition, we searched historical data.
Specifically, we reviewed the 1996 memorandum ``National Emission
Standards for Hazardous Air Pollutants from Phosphoric Acid
Manufacturing and Phosphate Fertilizers Production; Proposed Rules--
Draft Technical Support Document and Additional Technical Information''
(1996 TSD) to determine if clarifier emissions were included in the
MACT floor evaluation for WPPA process lines (the 1996 TSD is item II-
B-20 in Docket A-94-02). The 1996 TSD lists, in Attachment 2, the WPPA
test data that were assembled for the MACT floor analysis. Based on
this review, we were not able to confirm that clarifiers were included
as part of the WPPA emissions dataset that was evaluated in order to
conduct the MACT floor analysis; therefore, we are not including
clarifiers in the WPPA process line definition. Similarly, we are not
including clarifiers in the WPPA plant definition of NSPS subpart T.
    iii. Generic Process Line Definition--Comment. One commenter stated
that the EPA has introduced ambiguity and vagueness with its definition
of a generic ``process line'' that includes ``all equipment associated
with the production of any grade or purity of a phosphoric acid product
including emission control equipment.'' The commenter asserted that
under this expansive definition, every hypothetical fugitive emission
source would have to be accounted for in determining compliance. The
commenter explained that the EPA has not collected emission data from
``all equipment'' nor provided guidance on estimating emissions for
such sources in order to allow entities with process lines to
demonstrate compliance. The commenter stressed the ``process line''
definition, as it currently stands, could include a wash plant that
prepares phosphate ore or product storage tanks due to these sources
being considered ``associated'' with production and thus subject to the
proposed NESHAP.
    Response. The Agency agrees with the commenter that it is not
necessary to include the generic ``process line'' definition, and has
removed it from the NESHAP subpart AA final rule. This definition did
not provide additional clarity to facilities, and it was not our intent
to include emissions from ``all equipment'' that is ``associated'' with
phosphoric acid production for compliance determinations. Specific
definitions are provided for WPPA process line, SPA process line, and
PPA process line and, therefore, enough specificity is already provided
in the rule.
    iv. ``Includes, but is Not Limited to''--Comment. A commenter
remarked that incorporating the language ``includes, but is not limited
to'' in the definitions of WPPA, SPA, and PPA process lines is overly
broad and creates ambiguity. They stated that industry should have
certainty as to the applicability and scope of the rule, but the
language ``includes, but is not limited to'' creates uncertainty as to
where the affected equipment begins and ends for purposes of
demonstrating compliance.
    Response. We agree that this language creates overly broad process
line definitions and can lead to regulatory uncertainty for affected
sources. Therefore, we are not finalizing the language ``includes, but
is not limited to'' in the definitions of WPPA, SPA, and PPA process
lines of NESHAP subpart AA. Similarly, we are not finalizing the
language ``includes, but is not limited to'' in the definitions of WPPA
plant and SPA plant of NSPS subpart T and NSPS subpart U, respectively.
    b. Testing, Monitoring, Recordkeeping and Reporting--i. Pressure
Drop Across Absorber--Comment. Several commenters requested the EPA
delete the requirement that pressure drop across an absorber must be
greater than 5 inches of water in order to use the option of measuring
pressure drop as an operating parameter. These commenters contended
that the EPA has not articulated any basis for the requirement. These
commenters provided data demonstrating that units operate in compliance
with the emission standards when the pressure drop across an absorber
is less than 5 inches of water. One of these commenters expressed
safety concerns associated with operating scrubbers at higher range
pressure drop settings, citing one of its facilities that experienced
the entrainment of moisture within the absorbing tower when operating
at pressure drops in excess of 8 inches of water, and another that
experienced the buildup of excessive fumes on the digester floor when
operating the digester scrubber as high as 6 inches of water.
    Response. The Agency maintains its determination that pressure drop
is not an appropriate monitoring parameter for absorbers that do not
use the energy from the inlet gas to increase contact between the gas
and liquid in the absorber (see ``Use of Pressure Drop as an Operating
Parameter,'' which is available in Docket ID No. EPA-HQ-OAR-2012-0522).
Therefore, we are not revising this proposed amendment.
    High-energy (i.e., high pressure drop) absorbers, such as venturi
scrubbers, are designed to use the energy in the inlet gas to atomize
the liquid stream entering the absorber which increases the contact
between the liquid droplets and gas. For these types of absorbers,
pressure drop is an appropriate monitoring parameter because changes in
pressure drop values indicate that either liquid droplets are not being
formed effectively inside the absorber (falling pressure drop), or that
the absorber is fouled (increasing pressured drop). Pressure drop is
not an appropriate monitoring parameter for low-energy absorbers (i.e.,
absorbers that are designed to operate with pressure drops of 5 inches
of water column or less) because pressure drop is not integral to the
mechanism used in the absorber to mix the scrubbing liquid and inlet
gas. Furthermore, in a meeting that occurred after the public comment
period closed (see ``EPA Meeting Minutes for TFI Discussion March 12,
2015,'' which is available in Docket ID No. EPA-HQ-OAR-2012-0522),
industry stated that there is no correlation between pressure drop and
absorber performance.
    With regard to the safety concerns raised by one commenter when
operating low-energy absorbers at high pressure drop settings, the
proposed rule (NESHAP subpart AA) did not require low-energy absorbers
(i.e., absorbers that are designed to operate with pressure drops of 5
inches of water column or less) to operate at pressure drops greater
than 5 inches of water column. Instead, the proposed rule required a
different parameter to be monitored for these types of absorbers.
Nevertheless, based on other comments received, we are not adopting the
proposed monitoring for low-energy absorbers, and have revised the
final rule (NESHAP subpart AA) to require liquid-to-gas ratio
monitoring for low-energy absorbers in lieu of monitoring influent
liquid flow and pressure drop through the absorber (see section
V.F.3.b.ii of this preamble for further details).
    ii. Absorber Monitoring Options--Comment. Several commenters called
attention to the options of either measuring: (1) The temperature at
the

wet scrubber gas stream outlet and pressure at the liquid inlet of the
absorber, or (2) the temperature at the scrubber gas stream outlet and
scrubber gas stream inlet. One of these commenters said that they do
not believe monitoring gas temperature in locations of large ambient
temperature ranges would provide accurate monitoring of the absorbers
performance. The commenter argued that temperature and pressure probes
would be very susceptible to scaling issues. In addition, this
commenter contended that liquid inlet pressure does not provide any
additional monitoring of the absorber performance, since the inlet
liquid flow rate is already measured and monitored. Another commenter
contended that the EPA has not provided any data or analysis to show
that there is a correlation between temperature and emissions; the
commenter stated that they were not aware of any data suggesting a
relationship between exit temperature and emissions, or that monitoring
temperature difference across an absorber would be effective. One of
these commenters argued that they were not in a position to evaluate
the difficulties associated with performing the associated monitoring
and establishing the requisite operating ranges.
    Response. Absorber outlet gas temperature is often used to indicate
a change in operation for absorbers that are used to control thermal
processes. Because this source category uses the wet process in lieu of
a thermal process to produce phosphoric acid, the Agency agrees with
the commenters that temperature is not an appropriate monitoring
parameter for absorbers used in this source category, and has removed
these monitoring options from Table 3 of the final rule (NESHAP subpart
AA). However, in light of this comment, the Agency has revised Table 3
of NESHAP subpart AA to require liquid-to-gas ratio monitoring for low-
energy absorbers (i.e., absorbers that are designed to operate with
pressure drops of 5 inches of water column or less) in lieu of
monitoring influent liquid flow and pressure drop through the absorber.
(See section V.F.3.b.i of this preamble for further details of why we
are not allowing pressure drop monitoring for low-energy absorbers.)
Although liquid flow to the absorber is the most critical parameter for
monitoring absorption systems, monitoring the inlet gas flow rate along
with the influent liquid flow rate (and determining liquid-to-gas
ratio) provides better indication of whether enough water is present to
provide adequate scrubbing for the amount of gas flowing through the
system. Furthermore, the Agency has revised Table 3 of NESHAP subpart
AA to require influent liquid flow and pressure drop monitoring for
high-energy (i.e., high pressure drop) absorbers, such as venturi
scrubbers; and we are keeping liquid-to-gas ratio monitoring as an
option for high-energy absorbers in the final rule. Rather than
calculating one minimum flow rate at maximum operating conditions that
must be continuously adhered to, this alternative provision (i.e.,
liquid-to-gas ratio monitoring for high-energy absorbers) allows a
facility to optimize the liquid flow for varying gas flow rates. By
using a liquid-to-gas ratio, sources may save resources by reducing the
liquid rate with reductions in gas flow due to periods of lower
production rates.
    The Agency believes the cost to implement these finalized
monitoring requirements is minimal for facilities. For low-energy
absorbers, we are allowing the gas stream to be measured by either
measuring the gas stream flow at the absorber inlet or using the design
blower capacity, with appropriate adjustments for pressure drop.
Therefore, facilities would not need to purchase new equipment to
measure gas flow at the inlet of the absorber since they may choose to
use design blower capacity. Furthermore, we are not requiring any new
monitoring for high-energy absorbers; therefore, these facilities are
already equipped to monitor as required in the final rule.
    iii. Operating Range Established From a Previous Test--Comment. One
commenter stated that 40 CFR 63.607(a) is somewhat ambiguous, tending
to suggest that affected facilities would be immediately required to
implement new equipment operating ranges following a source test, even
if operating conditions from previous source tests demonstrated
compliance with fluoride emission standards. The commenter argued that
there is no reason that a new performance test at a new operating range
should invalidate a previous performance test at a different operating
range.
    Response. The Agency has clarified in the final rule at 40 CFR
63.607(a) that during the most recent performance test, if owners or
operators demonstrate compliance with the emission limit while
operating their control device outside the previously established
operating limit, then limits must be established. Owners or operators
must establish a new operating limit based on that most recent
performance test and notify the Administrator that the operating limit
changed based on data collected during the most recent performance
test. Public comments on the 1999 rule stated that the equipment and
control devices in these source categories are subject to harsh
conditions that cause corrosion and scaling of the process components.
Accordingly, the performance of the emissions controls will vary over