in flow, but the demand for electricity (or other products) might
dictate that production be increased at the facility in question or at
another facility altogether; there is usually no guarantee that the
intake flow will be permanently retired. EPA expects flow reductions
due to unit closures could be reasonably included as part of a
facility's impingement mortality and entrainment reductions strategy.
Given the number of variables involved in the decision to retire a unit
and the likelihood of a facility having a unit that is ready to retire
at promulgation of the final rule, unit retirements are not a
nationally available entrainment reduction measure. See Section VIII
for further discussion of how a facility can take credit for flow
reductions attributable to unit closures.
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    \51\ Note that some generating units are retired by the owner
(i.e., the unit is no longer considered sufficiently profitable to
operate) or is rarely dispatched by its independent system operator
for market-driven reasons (i.e., the unit cannot deliver at a
competitive price except during limited peak seasons; see also Sec.
125.94(c)(12)). They may also be mothballed, placed on cold storage,
or maintained in various other states of operational readiness.
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d. Use of Alternate Cooling Water Sources
    While not reducing the overall usage of water at a facility, using
an alternate source of cooling water can reduce impingement and
entrainment if the alternate source substitutes for withdrawals from
surface waters. An example is using ``gray'' water as a source of
cooling water, such as a facility that reaches an agreement with a
nearby wastewater treatment plant to accept the wastewater treatment
plant's effluent as a source of cooling water.\52\ Such alternate
sources are limited by available capacity and consistency of flow.
Increasing competition for these sources of water may make this a more
challenging approach for existing facilities than for new facilities
that are not yet fixed in location. In principle, alternate sources
could be used to fulfill either a fraction or all of a facility's
cooling water demands. In practice, the location of alternate sources,
the costs of moving water from the alternate source to the facility,
and whether the facility uses a once-through or closed-cycle
recirculating system as defined at Sec.  125.92(c) will determine
whether the alternate source can meet all or a portion of the
facility's cooling water needs. All these factors limit the widespread
availability of alternate cooling water sources as an entrainment
reduction measure, however use of alternative sources of cooling water
such as wastewater treatment effluent could be attractive for certain
facilities where the cost of retrofitting or other site-specific
circumstances are favorable.\53\
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    \52\ See, for example, EPA's site visit report for PSEG's Linden
Generating Station (DCN 10-6557), which has a capacity of 1230 MW,
35 percent CUR, and uses 7-8 mgd of gray water as the sole source of
makeup water for its cooling towers.
    \53\ For maps showing which electric generators are near a
source of available reuse water for cooling, see Tidwell, V., J.
Macknick, K. Zemlick, J. Sanchez, and T. Woldeyesus. 2013.
``Transitioning to Zero Freshwater Withdrawal for Thermoelectric
Generation in the United States.'' (submitted). See also the
accompanying presentation given at the American Geophysical Union
Fall 2012 Meeting available at http://www.nrel.gov/docs/fy13osti/57444.pdf.
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e. Water Reuse
    Typically associated with manufacturing facilities, water reuse
(defined as using water for multiple processes) can reduce the volume
of water needed for cooling, process, or other uses. For example, a
facility might withdraw water for non-contact cooling water and then
reuse the heated effluent as part of an industrial process. In effect,
the facility has eliminated the need to withdraw additional water for
the latter

process. EPA has observed significant water reuse at manufacturing
facilities but has not developed national level data for such reuse
because of the range of different manufacturing sectors and the
significant variability in manufacturing processes appropriate for
reuse. For example, during site visits, EPA observed that it may be
difficult to quantify specific water reuse at complex facilities. (See,
for example, the site visit report for ArcelorMittal, a steel mill at
DCN 10-6551.) For additional detail on water usage in specific
industrial sectors, see Chapters 4 and 8 of the TDD.
    Increasingly, electric utilities are adopting water reuse to meet a
portion or all of their cooling water demands. Water reuse can enhance
the reliability of power generation in water-limited environments.
Given the complex use (and reuse) patterns for some facilities and the
lack of reuse at other facilities, water reuse cannot be considered as
a widely available entrainment reduction option.
f. Closed-Cycle Cooling Systems
    Closed-cycle cooling systems allow a facility to transfer its waste
heat to the environment using significantly smaller quantities of water
relative to once-through cooling, and in some cases no water. The main
types of closed-cycle cooling systems are wet cooling, dry cooling,
hybrid cooling, and impoundments. Each is described below.
i. Wet Cooling Systems
    In a wet cooling system, cooling water that has absorbed waste heat
transfers that heat through evaporation of some of the heated water
into the surrounding air and recirculates the now cooled water to
continue the cooling process.\54\ This process enables a facility to
reuse the remaining water, thereby reducing the quantity of water that
must be withdrawn from a waterbody. Because the heat is transferred
through evaporation, the amount of water withdrawn from the water
source is greatly reduced, though not eliminated completely, because
make-up water is required to replace that lost through evaporation and
blowdown.\55\ The two main types of wet cooling systems are natural
draft and mechanical. While wet cooling systems reduce withdrawals
significantly relative to once-through systems, they can increase the
consumptive use of water because they rely on evaporation (which is not
returned to the waterbody) for heat dissipation. When once-through
cooling is used and withdrawals are a significant portion of the source
waterbody, the return of heated water might contribute to greater
evaporation from the waterbody relative to the waterbody's normal
evaporation rate. EPA does not have conclusive data on the relative
magnitude of these effects, but the data do suggest that the relative
difference in evaporation is not so great that it will play a major
role in determining a cooling system type in most watersheds. EPA
examined available information on evaporation losses in DCN 12-6673,
including a comparison to evaporative losses from the downstream
effluent plume of once-through cooling systems. While EPA recognizes
that evaporative losses from closed-cycle systems are greater, EPA's
analysis does not suggest that the difference is substantial enough to
outweigh the significant reduction in adverse environmental impacts to
aquatic organisms. However, the relative loss of water through
evaporation for closed-cycle and once-through systems is site-specific,
depending on the exact design of the systems.
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    \54\ In addition, a smaller portion of the heat is also removed
through direct contact between the warm water and the cooler
surroundings; this is known as sensible heat.
    \55\ Cooling towers must replace water lost to evaporation; this
is referred to as makeup water. Additionally, as water evaporates,
dissolved solids and other materials gradually increase in
concentration in the circulating water and can cause operational
difficulties. To minimize these issues, cooling tower operators
continually discharge a small portion of the circulating flow and
replace it with makeup water; this is referred to as blowdown.
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    There are two common designs for wet cooling systems. A natural
draft cooling tower can be as tall as 500 feet and has a hyperbolic
shape. The height of these towers creates a temperature differential
between the top and bottom of the tower, which creates a natural
chimney effect that transfers heat as heated water contacts rising air.
In contrast, mechanical cooling towers rely on motorized fans to draw
air through the tower and into contact with the heated water.\56\ These
towers are much shorter than natural draft cooling towers (typically 30
to 75 feet tall) and can be built in groups. Mechanical cooling towers
may require more land area than natural draft cooling towers for an
equivalent amount of cooling. Both types of towers require electricity
for pumps, but mechanical draft towers also require electricity to
operate the fans. In both cases, the electricity need of the towers
reduces an electric generating facility's net generating output. Thus,
the monetary and environmental costs of this reduction in energy
efficiency must be considered. These environmental costs include human
health and welfare effects from increased air emissions (from burning
additional fuel to make up for the power that cannot be sold) and the
global climate change effects of increased greenhouse gas output at
fossil-fueled facilities (these costs are now explicitly considered in
the benefit-cost analysis; see Section X below). Both natural draft and
mechanical cooling towers can operate in freshwater or saltwater
environments. Saltwater applications typically require more make-up
water than freshwater applications, making them less efficient in
reducing water withdrawals. Optimized cooling towers can achieve flow
reductions of 97.5 and 94.9 percent or better for freshwater and
saltwater sources, respectively.
---------------------------------------------------------------------------

    \56\ Modular cooling tower units provide an additional cooling
tower alternative. Modular cooling towers resemble mechanical
cooling towers, but are portable, typically rented for short-term
periods and quickly assembled.
---------------------------------------------------------------------------

ii. Dry Cooling Systems
    Dry cooling systems virtually eliminate the need for cooling water
withdrawals.\57\ Unlike wet cooling systems, waste heat in dry cooling
systems is transferred completely through convection and radiation,
rather than evaporation. Direct dry cooling is much like a car
radiator; turbine exhaust steam passes through tubes or fins for
cooling, and the condensate is returned to the boiler to be reheated
into steam to propel the turbine. The system is completely closed to
the atmosphere, and there is no contact between the outside air and the
steam or the resulting condensate. Because of the heavy reliance of dry
cooling on ambient air temperatures and the lower efficiency of heat
transfer through convection and radiation, dry cooling systems are much
larger and therefore more expensive \58\ than wet cooling systems for a
given cooling load. While dry cooling systems are not uncommon in the
U.S. (see DCN 10-6943), they have typically been built at smaller
generating units or in areas where limited water supplies might make

uncertain the availability of either once-through cooling or wet
cooling make-up water, such as the arid southwestern United States. Dry
cooling has not been used for circulating water cooling at nuclear
facilities.
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    \57\ Dry cooling systems blow down some of the circulating water
in the cooling system to prevent the buildup of materials in the
condenser. However, the volume of makeup water is extremely low--a
dry cooling system typically reduces intake flows by 98-99 percent
over a comparable once-through cooling system.
    \58\ The construction and capital costs for dry cooling towers
have been reported as four to 10 times more expensive as wet cooling
towers, and the auxiliary power consumption for dry cooling is
higher than for wet cooling. See DCN 10-6679. EPA recognizes that
costs for dry cooling may have decreased since this document was
written, but costs for dry cooling are still markedly higher than
those for wet cooling. The other challenges associated with dry
cooling remain unchanged.
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iii. Hybrid Cooling Systems
    In certain applications, a facility could choose a hybrid cooling
system design that incorporates elements of both wet and dry cooling.
Typically, the base of the tower functions as a wet cooling system and
the upper portion as a dry cooling system. The most common reason for
this design is to reduce the visible plume of water vapor, which is
accomplished by recapturing some of the water vapor evaporated in the
wet portion of the tower. This design is also usually much shorter than
natural draft wet towers and can also include plume abatement controls.
Another version of the hybrid cooling system also includes both wet and
dry cooling sections, but the dry section functions to directly cool a
portion of the turbine exhaust steam. The benefits of such a tower may
include substantial water savings as well as reduction in power plant
efficiency losses associated with just dry cooling.
iv. Impoundments
    Impoundments are surface waterbodies that serve as both a source of
cooling water and a heat sink. As with cooling towers, impoundments
rely on evaporative cooling to dissipate the waste heat; a facility
withdraws water from one part of the impoundment and then discharges
the heated effluent back to the impoundment, usually in another
location to allow the heated water time to cool. Depending on local
hydrology, impoundments may also require makeup water from another
waterbody. Impoundments can be man-made or natural, and can be offset
from other water bodies or as part of a ``run of the river'' system
(the latter are sometimes referred to as cooling lakes).
2. Exclusion and Collection Technologies
    Over the last several decades, numerous technologies in addition to
specific flow reduction measures such as velocity controls and closed-
cycle cooling have been developed in an effort to minimize impingement
mortality and entrainment associated with cooling water intake systems.
The following section summarizes the most widely used technologies and
the most effective and best-performing technologies, such as screens,
barrier nets, aquatic filter barriers, and collection and return
systems. For additional detailed information on these technologies and
others, also see Chapter 6 of the TDD, ``California's Coastal Power
Plants'' report (DCN 10-6964) or EPRI's ``Fish Protection at Cooling
Water Intake Structures'' report (DCN 10-6813).
a. Screens
    There are several types of screens that offer protection that are
discussed below, including traveling screens and cylindrical wedgewire
screens. Not described in this section are fixed screens that are used
simply for the purpose of debris exclusion but do not offer protection
to fish, larvae, and eggs.
i. Traveling Screens
    Traveling screens are a technology in place as part of most cooling
water intake structures. These screens originally were designed to
prevent debris from entering the cooling water system, but they also
prevent some fish and shellfish from entering the cooling water system.
Traveling screens have been installed in a wide variety of operating
and environmental conditions: salt water, brackish water, freshwater,
and icy water, as well as river, lake and tidal applications. On the
basis of the technical survey, EPA found 93 percent of electric
generators and 73 percent of manufacturers employ traveling water
screens or other intake screens. Many types of traveling water screens
(e.g., through flow, dual flow, center flow) are used. The most common
design in the United States is the through flow system. The screens are
installed behind bar racks (trash racks) but in front of the water
circulation pumps. The screens rotate up and, while out of the water,
debris and impinged organisms are removed from the screen surface by a
high-pressure spray wash. Screen wash cycles are triggered either
manually or by a certain level of head loss across the screen
(indicating clogging). By definition, this technology works by
collecting (i.e., impinging) fish and shellfish on the screen. Ideally,
traveling screens would be used with a fish handling and return system,
as discussed below. The return system should be regularly maintained to
prevent biofouling or other blockages that may affect survival.
ii. Cylindrical Wedgewire Screens
    Unlike traveling screens, cylindrical wedgewire screens are a
passive intake system. Wedgewire screens, also called ``V'' screens or
profile screens, consist of triangular-shaped wires arrayed on a
cylindrical framing system, with long slots between the wires,
lengthwise along the screen. Slot sizes for conventional traveling
screens typically refer to a square opening (\3/8\ inch by \3/8\ inch)
that is punched, molded, or woven into the screen face. Wedgewire
screens are constructed differently, however, with the slot size
referring to the distance between longitudinally adjacent wires. These
screens are designed to have a low through-slot velocity (less than 0.5
fps or 0.15 meter per second) and typically have smaller slot sizes
than a coarse mesh traveling screen. The entire wedgewire structure is
submerged in the source waterbody. (See Chapter 6 of the TDD for an
illustration of these screens.)
    When necessary conditions regarding placement in the waterbody are
met, these screens exploit physical and hydraulic exclusion mechanisms
to achieve consistently high impingement reductions, and as a result,
impingement mortality reductions. Wedgewire screens require an ambient
crossflow current to maximize the sweeping velocity provided by the
waterbody. The screen orientation allows the crossflow to carry
organisms away from the screen allowing them to avoid or escape the
intake. Lower intake velocities also allow fish to escape from the
screen face. Entrainment reductions can also be observed when the
screen slot size is small enough and intake velocity is low enough to
exclude egg and larval life stages.\59\ Limited evidence also suggests
that extremely low intake velocities can allow some egg and larval life
stages to avoid the intake because of hydrodynamic influences of the
crossflow. Therefore, performance is dictated largely by local
conditions that are further dependent on the source waterbody's
biological composition. Costs of wedgewire screens increase
significantly as slot size and design intake velocity decrease because
the cumulative size of the screen (or number of screens) must grow in
order to accommodate the same flow of cooling water. Wedgewire screens
can also employ cleaning and deicing systems such as air-burst sparging
to help maintain open intake structures and low intake velocities.
---------------------------------------------------------------------------

    \59\ Note that this is entrainment exclusion and not necessarily
related to the survival of entrainable organisms.
---------------------------------------------------------------------------

    According to data from the industry questionnaire, EPA's site
visits, and industry documents, dozens of facilities across the United
States employ cylindrical wedgewire screens. However, wedgewire screens
are not feasible for all facilities, particularly where intakes are in
shallow water or have limited shoreline frontage. Also,

wedgewire screens might not be feasible where the size and number of
wedgewire screens would interfere with navigation of vessels. As
described above, locations also need to have an adequate source water
sweeping velocity. Most of the performance data for wedgewire screens
is based on coarse mesh slot sizes with an intake velocity of 0.5 fps.
Because it is extremely difficult to measure impingement and
entrainment reductions in the field, most performance data for
wedgewire screens is based on barge and lab studies.\60\ EPA does not
have data on the performance of fine mesh wedgewire screens on
entrainment survival. Consequently, EPA has considered wedgewire
screens only for impingement mortality. For additional discussion of
the specific design and operation of cylindrical wedgewire screens, see
Chapter 6 of the TDD. The following section discusses the importance of
mesh size to impingement mortality and entrainment reductions.
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    \60\ EPA expects that properly designed wedgewire screens have a
design intake velocity of 0.5 fps, therefore intakes with wedgewire
screens will meet the impingement standard at Sec.  125.94(c)(2) and
there is no need to separately pre-approve this technology as in the
remanded 2004 Phase II rule.
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iii. Screen Mesh Size Considerations
Coarse Mesh
    Coarse mesh traveling screens are the typical traveling screen
fitted on the majority of cooling water intakes. A large number of
facilities have intake screens with \3/8\-inch (9.5 mm) mesh
panels.\61\ This size mesh is common because, as a general rule, the
maximum screen slot size is never larger than one-half of the condenser
tube diameter (the condenser tubing is the narrowest point in the
cooling water system and, as such, is most susceptible to clogging from
debris), and this tubing is typically \3/4\ or \7/8\ inch in diameter.
Mesh of \3/8\-inch (roughly 9.5 mm) size does not prevent entrainment
and without any other precautions can lead to high mortality of
impinged fish. Coarse mesh traveling screens have been in use by both
power plants and manufacturers for more than 75 years and represent the
baseline technology. Similarly, the majority of successful wedgewire
installations are coarse mesh.
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    \61\ In today's rule the EPA recognizes that \1/2\- by \1/4\-
inch mesh is used in some instances and perform comparably to the
\3/8\-inch square mesh.
---------------------------------------------------------------------------

Fine Mesh
    Fine mesh traveling and wedgewire screens are similar to coarse
mesh screens. The only difference is the size of the screen mesh. Fine
mesh traveling screens have been in use since the 1980s. Typically,
facilities have incorporated fine mesh in an effort to reduce
entrainment. The mesh size varies, depending on the organisms to be
protected, but typically range from 0.5 to 5 mm. Data in the record
demonstrate that entrainment typically decreases as mesh size
decreases. Slot sizes larger than 2 mm do not prevent eggs from passing
through the screen. Converting traveling screens from coarse mesh to
fine mesh often requires adding more screens in order to maintain the
same flow, since the open area of a fine mesh screen is less than the
open area of a coarse mesh screen. Adding more screens is one way to
maintain that flow.\62\ EPA estimates that as many as 17 percent of
existing intakes could not be enlarged to accommodate a 2 mm mesh, and
as many as 55 percent of existing intakes could not accommodate a 0.5
mm slot size under conditions of low-intake velocities. For these
reasons, fine mesh screens are available for some locations, but they
are not the best performing technology and are not an available
technology for the industry as a whole for IM&E. For more details, see
Chapter 6 of the TDD.
---------------------------------------------------------------------------

    \62\ A facility could also increase its intake velocity.
---------------------------------------------------------------------------

b. Barrier Nets
    Barrier nets are nets that fully encircle the intake area of water
withdrawal, from the bottom of the water column to the surface, and
prevent fish and shellfish from coming in contact with the intake
structure and screens. According to data from the industry
questionnaire (as of the year 2000), at least a half dozen facilities
employ a barrier net. Typically, barrier nets have large mesh sizes
(e.g., \1/2\-inch or 12.7 mm) \63\ and are designed to prevent
impingement. Because of the large mesh size, they offer no reduction in
entrainment. They are often deployed seasonally, wherever seasonal
migrations create high impingement events or to avoid harsh winter
conditions that jeopardize integrity of the net. Barrier nets also
prevent impingement of shellfish on the intake traveling screen.
Shellfish such as crustaceans can pose a unique issue for traveling
screens; shellfish are not impinged, but they can attach to the
traveling screen surface and are not removed from the traveling screen
by pressure wash sprays. Barrier nets have been shown to be helpful in
this regard.
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    \63\ Barrier net mesh sizes vary, depending on the
configuration, level of debris loading, species to be protected, and
other factors.
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c. Aquatic Filter Barriers
    Aquatic Filter Barriers (AFBs) consist of water-permeable fabric
panels with small pores (less than 20 microns). They are similar to
barrier nets in that they extend throughout the area of water
withdrawal from the bottom of the water column to the surface. AFBs
reduce both impingement mortality and entrainment because they present
a physical barrier to all life stages. The surface area of an AFB is
quite large compared to a traveling screen, allowing for extremely low
water velocities. The low velocity allows non-motile organisms to drift
away. EPA is aware of one power plant that used an AFB but notes that
this facility recently ceased operations.\64\ EPA has updated
performance data for AFB for small flow intakes, but it does not have
enough data to evaluate the technology at large intakes or in all water
bodies. EPA does not consider this technology to be demonstrated and
available as a nationwide BTA candidate.
---------------------------------------------------------------------------

    \64\ This facility ceased operations for reasons unrelated to
any requirements or measures addressing cooling water intake
impingement or entrainment.
---------------------------------------------------------------------------

d. Collection and Return Systems
    Conventional traveling screens were not designed initially with the
intention of protecting fish and aquatic organisms that become impinged
against them. The organisms were often handled in the same manner as
debris on the screens. Marine life can become impinged against the
screens because of high intake velocities that prevent their escape.
Prolonged contact with the screens can suffocate organisms that are
unable to escape or result in descaling injury and latent mortality.
Organisms that survive initial impingement and removal are not always
provided with a specifically designed mechanism to return them to the
waterbody and are often handled in the same way as other screening
debris. Other objects, such as leaves and trash, that are collected on
the screen are typically removed with a high-pressure spray and
deposited in a dumpster or debris return trough for disposal. Exposure
to high pressure sprays and other screening debris can cause
significant injuries that result in latent mortality or increase the
susceptibility to predation or re-impingement. Screens are rotated
periodically on a set time interval or when the pressure differential
between the upstream and downstream faces exceeds a set value.
    Conventional traveling screen systems have been modified to reduce
impingement-related mortalities with

collection and return systems. In its simplest form, these systems are
composed of a return flume or trough with sufficient water volume and
flow to enable impinged organisms to return to the source water. Return
systems should be designed to avoid predation and latent mortality
while organisms are in the flume, maintain an appropriate water depth
in the flume for high survival of the organisms, located at an
appropriate elevation to avoid large drops of the organisms back to the
surface water (or large hydraulic jumps if the end of the return is
below the water's surface), and sited to avoid repeated impingement of
the organisms by the intake structure.
    Some facilities have modified conventional coarse mesh traveling
screen systems to reduce impingement mortality. They did this by
removing fish trapped against the screen and returning them to the
receiving water with as few injuries as possible. The first modified
screens, also known as Ristroph screens, feature capture and release
modifications. In the simplest sense, these screens are fitted with
troughs (also referred to as buckets) containing water that catch the
organisms as the screen rises out of the water and the organisms are
sprayed off of the screen. The return component consists of a mechanism
to remove impinged fish gently from the collection buckets, such as a
low-pressure spray. The buckets empty into a collection trough that
returns fish to a suitable area in the source waterbody. These modified
screens have shown significant reductions in impingement mortality
compared with unmodified screen systems.
    Data from early applications of the Ristroph screen design showed
that while initial survival rates might be high at some installations,
latent mortality rates were higher than anticipated. This indicated
that organisms could sustain significant injuries during the
impingement and return process that were not immediately fatal.
According to a study conducted by Ian Fletcher in the 1990s (see DCN 5-
4387), industry identified several additional critical screen
modifications to address latent mortality. These included redesigning
the collection buckets to minimize turbulence, adding a fish guard
rail/barrier to prevent fish from escaping the collection bucket,
replacing screen panel materials with ``fish-friendly,'' smooth woven
mesh, and using a low-pressure wash to remove fish before any high-
pressure spray to remove debris. The Fletcher analysis also identified
longer impingement duration, insufficient water retention in the
buckets, and exposure to the air and temperature extremes as conditions
that could negatively affect fish survival. Finally, these findings
indicate that modified Ristroph screens must be rotated continually
instead of the periodic rotation schedule common with conventional
screen systems. Performance data for modified traveling screens with
fish-friendly fish return systems, sometimes referred to as post-
Fletcher modifications, show low levels of impingement mortality across
a wide variety of waterbody types and fish species. Additionally,
recently developed screen designs (such as the Passavant Geiger,
Beaudrey WIP, and Hydrolox screens) have also shown promise in reducing
impingement mortality.
    For additional and more detailed discussion of the specific design
and operation of these screen modifications, see Chapter 6 of the TDD.
3. Other Technological Approaches
a. Intake Location and Velocity Caps
    The most common intake location for a cooling water intake
structure is along a shoreline. In some water bodies, however,
shoreline locations are thought to have a potential for greater
environmental impact because the water is withdrawn from the most
biologically productive waters, especially those containing a high
density of organisms in earlier life stages, such as nursery areas.
Some facilities employ an offshore intake to withdraw water from less
biologically productive areas to reduce impingement and entrainment
relative to intakes in more productive shoreline areas. Reduction in
impingement mortality and entrainment due to intake location is highly
site-specific. The greatest potential for reductions is found with far
offshore locations at distances of several hundred feet, not found on
many rivers and streams. Both depth and the offshore location must be
evaluated to determine whether fish densities and species distribution
there are substantially different than those near the shoreline. Two
areas where far offshore locations are commonly used today are the
oceans and Great Lakes.
    EPA found that several offshore intakes are fitted with a velocity
cap.\65\ Velocity caps are a physical structure rising vertically from
the sea bottom and are placed over the top of an intake pipe. Intake
water is withdrawn through openings in the velocity cap so that it
converts the direction of water flow into the pipe from vertical to
horizontal. The velocity cap does not act to reduce the velocity,\66\
but the horizontal flow provides a physiological trigger in fish, which
induces an avoidance response to reduce impingement mortality. The
velocity cap further serves to limit the zone of influence of the
intake to the depth level at which the velocity cap is situated, thus
affecting only the life stages that live at that depth. Velocity caps
are also usually equipped with supports and bar spacing selected to
prevent larger aquatic organisms (e.g., sea turtles or marine mammals)
from entering the intake pipe. Because velocity caps operate under the
principle that the organisms can escape the current, they do not offer
entrainment reductions over and above those achieved by being located
offshore. Reductions in entrainment observed with velocity caps occur
because of the difference in organism densities in far offshore deep
water compared to a surface intake at the shoreline.
---------------------------------------------------------------------------

    \65\ Others can be fitted with a cylindrical wedgewire screen,
or might simply be an open pipe.
    \66\ EPA's data show that velocity caps operate at velocities
above and below the 0.5 fps and can be effective using either
design.
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    Far offshore velocity caps have limited application in oceans and
the Great Lakes, are not available in other water bodies, and are
therefore not available as a candidate for a national BTA. However, the
technology is a demonstrated high performing technology, and is
therefore included as a compliance alternative for those facilities
where the technology is available. For additional and more detailed
discussion of the specific design and operation of offshore intake
locations and velocity caps, see Chapter 6 of the TDD.
b. Reduced Intake Velocity
    Impingement mortality can be reduced greatly by reducing the
through-screen velocity in any screen.\67\ Reducing the rate of flow of
cooling water through the screen (through-screen velocity) to 0.5 fps
or less reduces impingement of most fish because it allows them to
escape the intake current. (See 66 FR 65274 [December 18, 2001] and DCN
2-028A, EPRI's ``Technical Evaluation of the Utility of Intake Approach
Velocity as an Indicator of Potential Adverse

Environmental Impact Under Clean Water Act 316(b).'') As a result, some
facilities have designed and operate their modified traveling screens
or wedgewire screens so as not to exceed a through-screen velocity of
0.5 fps. Swim speed studies demonstrate that for most facilities, an
intake velocity of 0.5 fps or less will result in 96 percent or better
reductions in impingement mortality for most species. EPA notes that
preliminary results from recent studies of fine mesh screens suggest
that at even lower intake velocities such as 0.25 fps, some
hydrodynamic influences may reduce entrainment mortality even more,
because flow dynamics are nonlinear. It is unclear as to whether such
observations hold true when cooling water withdrawals (water volumes)
are large. While higher intake velocities are sufficiently protective
for some species of fish, the higher intake velocities are not
necessarily protective of all life-stages. For example, younger fish
may not be strong swimmers or may have not a developed avoidance
response. Therefore higher intake velocities are not a high performing
technology. As noted previously, low intake velocity has limited
application, and is therefore not available as a candidate BTA
technology. However, the technology is a demonstrated high performing
technology, and is therefore included as a compliance alternative for
those facilities where the technology is available.
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    \67\ Limited lab studies indicate that entrainment also can
decrease as through-screen velocity decreases and that through-
screen velocity can have an effect on entrainment survival rates,
although such data is extremely variable by species (see DCN 10-6802
and DCN 10-6803). In any case, EPA does not consider a reduced
intake velocity as an effective technology for reducing entrainment,
because entrainable organisms generally lack motility.
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D. Technology Basis for Today's Final Rule

    As described above, EPA examined the full range of technologies
that reduce impingement or entrainment or both. From an assessment of
all factors, EPA identified one technology that is best technology
available for minimizing the adverse impacts of impingement mortality
at existing facilities: modified traveling screens with a fish-friendly
fish return. EPA identified no single best technology that is available
for minimizing entrainment at existing facilities for today's final
rule. For new units at existing facilities, EPA identified mechanical
draft wet cooling systems as BTA for both impingement and
entrainment.\68\
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    \68\ Although EPA also identified velocity reduction to 0.5 feet
per second or less as a candidate best performing technology for
impingement mortality, EPA did not promulgate requirements to reduce
intake velocity as BTA because it is not available at all
facilities; however, the final rule does allow facilities to comply
with intake velocity of 0.5 feet per second or less where available.
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    EPA did not identify any single technology or group of technology
controls as the basis for establishing the national BTA standard for
entrainment for existing units. Instead, EPA has established a national
BTA standard for entrainment for existing units that requires
determination of BTA entrainment requirements on a site-specific basis
in a structured permitting setting. The framework for determining
entrainment requirements provides for the consideration at a minimum of
certain specified factors that must be considered in the Director's
determination of the BTA controls.
1. Alternative Impingement Mortality Standards for Existing Units
    After considering all factors identified above, EPA has concluded
that modified traveling screens, such as modified Ristroph screens and
equivalent modified traveling screens with fish-friendly fish returns,
are a best technology available for minimizing impingement
mortality.\69\ These screens use \3/8\ inch, or similar, mesh with
collection buckets designed to minimize turbulence, a fish guard rail/
barrier to prevent fish from escaping the collection bucket; ``fish-
friendly,'' smooth, woven or synthetic mesh; and a low-pressure wash to
remove fish before any high-pressure spray to remove debris. The fish
removal spray must be of lower pressure, and the fish return must be
fish friendly, provide sufficient water and minimize turbulence.
Modified traveling screens generally must be rotated continually to
minimize aquatic exposure to impingement or to the air and thus obtain
the highest reductions in impingement mortality.
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    \69\ EPA also considered recent screen designs (such as the
Passavant Geiger, Beaudrey WIP, and Hydrolox screens) in evaluating
impingement mortality data. In fact, the data set used to calculate
the impingement mortality performance standard at Sec.  125.94(c)(7)
included a study of performance at a facility employing a Passavant
Geiger screen, as well as a facility employing a Beaudrey WIP
screen.
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    Under the seventh option for complying with the BTA impingement
mortality standard in today's final rule, a facility may rely on any
technology it chooses so long as it demonstrates through biological
compliance monitoring that it achieves the required 12 month
impingement mortality performance standard \70\ that EPA calculated
based on the performance of the BTA technology--modified traveling
screens with fish return. As discussed in the TDD (see, for example,
TDD Exhibits 11-1 and 11-3), EPA based the 12 month percent mortality
performance at Sec.  125.94(c)(7) on data from facilities with
traveling screens modified with features to improve the post-
impingement survival of organisms such as smooth mesh, continuous or
near-continuous rotation of the screens, buckets with guard rails, low
pressure sprays for collecting fish, and fish return systems. The
statistical basis for the 12 month impingement mortality performance
standard includes 26 sets of 12 month survival percentages across 17
facilities demonstrating average impingement mortality rates ranging
from 1.6 to 48.8 percent under conditions of 18 to 96 hour holding
times. EPA established the 12 month percent mortality as 24 percent
which is the arithmetic average of the impingement mortality rates from
the 17 facilities. (This is consistent with EPA's proposed rule use of
expected value of the beta distribution which can be calculated as the
arithmetic average.) Note: The 12 month impingement mortality
performance standard means that no more than 24 percent of the impinged
fish may die or alternatively at least 76 percent of the impinged fish
must survive. EPA has occasionally used average annual limitations in
the effluent guidelines program, most recently for the pulp and paper
industry category (40 CFR 430, promulgated in 1998). In these
instances, such as the technology-based BAT, EPA has defined the annual
average limitations to be the average level demonstrated by the
technology. Thus, EPA's approach to calculating the 12 month percent
survival performance standard is consistent with past practice.
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    \70\ In the record, EPA may also refer to this as the 12-month
percent survival performance standard, % SPS, or the IM performance
standard.
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    EPA recognizes that variability in the technology performance
occurs due to changes in seasons, differing intake locations, higher
mortality of certain species, and speciation found in different water
bodies. By using a full 12 months of data, EPA has ensured that the
resulting performance standard reflects the widest range of potential
conditions present in EPA's database. EPA has further incorporated
variability into the 12 month impingement mortality performance
standard by basing it on data from 17 facilities which collectively
performed more than 1,500 sampling events beginning as early as 1977.
EPA notes that seven facilities had mortality rates less than 10
percent which provides evidence that facilities can, and have,
maintained and operated their systems in a manner consistent with the
performance standard. Another four facilities demonstrated impingement
mortality rates significantly greater than the performance standard of
24 percent, however, EPA notes these facilities were

not required to optimize their technology performance as part of their
study, and data collection was not required to achieve a certain level
of performance.\71\ In each study, EPA has identified elements of the
technology operation that a facility could modify to achieve the 12
month percent impingement mortality performance standard. By using the
12 month percent impingement mortality performance standard, EPA has
ensured that the resulting performance standard reflects the widest
range of potential conditions present in EPA's database. In addition to
those studies meeting the criteria for use in the 12 month percent
survival performance standard calculations, there are further studies
in EPA's record that provide additional performance data showing
facilities can, and have, maintained and operated their systems in a
manner consistent with the performance standard. EPA's record includes
approximately 250 total studies related to impingement (see TDD Exhibit
11A-1).
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    \71\ For example, the Indian Point study states ``Because of the
preliminary nature of this study, the effectiveness of the
continuously operating fine mesh traveling screen has not been fully
evaluated. Further studies incorporating controls for survival
testing, regulation of spray wash pressures, collection efficiency
tests, sampling during peak impingement periods for all important
species, and better holding facilities, will provide more conclusive
results.''
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    Despite the overwhelming evidence that the 12 month percent
survival performance standard of 24 percent was consistent with
demonstrated performance for the best technology, EPA considered other
alternatives that might incorporate more variability into a performance
standard. EPA concluded that none of the alternatives were consistent
with the need for facilities to demonstrate ongoing maintenance and
operations over a long period of time, such as a year. Any alternative
would be less stringent and would allow facilities to target long-term
performance at a level that would be less than the optimal performance
demonstrated by facilities with the technology in place. Further, the
12-month average impingement mortality performance standard will
require a facility to actively evaluate performance during the 12 month
period enabling the facility to optimize the technology to improve
performance to counterbalance a result above the standard by one below
the standard. If EPA had included a monthly average standard, it would
have similarly needed to incorporate allowances for exceedances.
Allowing for exceedances would have provided no incentive for improving
operations for such exceedances. Therefore, EPA determined that the 12
month impingement mortality performance standard is sufficient to
ensure performance consistent with best technology available. For this
reason, EPA is not promulgating the monthly average that was included
in the proposal. EPA's decision also is consistent with effluent
guidelines where compliance with the monthly average limitation is not
required for facilities subject to a longer term limitations such as an
annual average limitation (e.g., pulp and paper 40 CFR 430 Subpart B
AOX limitation).
    EPA did not include in the final rule a number of requirements it
had considered at proposal. The proposed rule would have required the
seasonal deployment of barrier nets on estuaries and oceans as one
element of the best technology available for minimizing the impingement
mortality of shellfish. EPA has opted not to include any specific
requirements for shellfish in the final rule, because EPA's review of
the impingement data it used to develop today's final rule impingement
performance standard includes data that incorporate shellfish survival
as part of the performance standard. Further, as previously explained,
the final rule provides for the Director to establish additional
requirements where necessary.
    EPA expects facilities complying with Sec.  125.94(c) of today's
rule by compliance option (7) to track their compliance with the 12
month percent impingement mortality performance standard on an ongoing
basis and to proactively modify their technology or operations when a
trend in the sampling suggests that they might be in danger of
exceeding the 12 month percent impingement mortality performance
standard in the future. The 12 month percent impingement mortality
performance standard requires that impingement mortality not exceed 24
percent, calculated as the sum total number of fish that were impinged
and died within the holding time divided by the sum total number of
fish impinged for a 12-month period. EPA expects the ratio will be
calculated based either on direct sampling counts, or based on both
counts being extrapolated to represent annual counts. Because comments
provided data that expanding the proposed 24 to 48 hour holding time
requirement would generally not affect the observation of mortality due
to impingement, the regulation allows for holding times from 18 to 96
hours.
    As explained in more detail in Section VI.E and G below, the BTA
technology for impingement does not minimize adverse environmental
impacts associated with entrainment.
2. Entrainment Standards for Existing Units
    As discussed below, EPA is not basing BTA for entrainment at
existing units (that is, excluding new units at existing facilities) on
closed-cycle recirculating cooling systems, a highly effective
technology, because this technology is not available nationally and
therefore does not represent BTA. EPA also has not identified any other
effective, available and demonstrated candidate technology (or
combinations of technologies) for entrainment reduction at existing
units that is available nationally. For other entrainment technologies
that might be available on a site-specific basis, see Section VI.E.2
below and Chapter 6 of the TDD. EPA did not select the other flow-
reduction technologies (such as variable-speed drives and seasonal flow
reductions) as the technology basis for entrainment control measures
because these technologies are not uniformly best and are not broadly
available for most facilities. Further, EPA has not identified a basis
for subcategorizing existing units at which flow reduction technologies
are feasible. The effectiveness, availability, and utility to a given
facility of flow reduction or other entrainment reduction methods
depends on site-specific geographical and biological conditions as well
as operations of the facility. For example, this is the reason that EPA
did not select relocation of a shoreline intake to far offshore as a
technology basis for the BTA entrainment standard because this
technology is not widely available for most facilities.
3. Impingement and Entrainment Standards for New Units at Existing
Facilities
    In contrast to existing units, installing a closed-cycle cooling
system at a new unit is far less complex. The technology is also highly
effective, generally achieving greater than 95 percent reductions in IM
and E (mechanical draft (wet) cooling towers achieve flow reductions of
97.5 percent for freshwater and 94.9 percent for saltwater sources, or
by operating the towers at a minimum of 3.0 and 1.5 cycles-of-
concentration, respectively). These reductions in flow and the
concurrent reductions in impingement and entrainment impacts are among
the highest reductions in adverse

environmental impact possible at an intake structure.\72\
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    \72\ Note that these metrics are not explicit requirements for
closed-cycle recirculating systems. They simply represent what EPA
views as examples of characteristics of a properly operated and
maintained closed-cycle recirculating system, as defined at Sec.
125.92(c)(1).
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    As described below, EPA has concluded that new units, in contrast
to existing units, have much greater flexibility in terms of cooling
system design, construction scheduling, and other factors that help
minimize many of the negative aspects associated with closed-cycle
cooling. For a more detailed discussion of this rationale, see below.
    Under the final rule, a new unit at an existing facility, where the
facility that withdraws or will withdraw more than 2 mgd when the new
unit begins operating will have requirements similar to the
requirements of a new facility in Phase I. Under the rule, a new unit
(as defined at Sec.  125.92(u) and described above) is required to have
a flow limited to that which is commensurate with a closed-cycle
recirculating system as it would be applied to the new unit. Today's
final rule also includes an alternative approach (similar to Track II
in Phase I), in which a facility could comply with the new unit
standards by demonstrating that the technologies and operational
measures employed will reduce the level of adverse environmental impact
from any cooling water intake structure used to supply cooling water to
the new unit to a comparable level to that achievable by implementing a
closed-cycle recirculating system as defined at Sec.  125.92(c)(1).
    As discussed above, today's final rule defines a ``new unit'' at an
existing facility as a stand-alone unit the construction of which
commences after the effective date of today's final rule. New unit
includes stand-alone units that are added to a facility for purposes of
the same general industrial operation as the existing facility. This is
in contrast to the definition of new facility, where a new facility
does not include new units that are added to a facility for purposes of
the same general industrial activity (40 CFR 125.83). The provision
``for purposes of the same general industrial operation'' is explicitly
included in today's final rule definition of new unit at an existing
facility for clarity. A new unit may have its own dedicated cooling
water intake structure, or the new unit may use an existing or modified
cooling water intake structure. Any unit at an existing facility that
does not meet the new unit definition in today's rule is subject to the
existing unit provisions.
    EPA is adopting more stringent requirements for new units at
existing facilities because such new units can be designed and
constructed without many of the additional expenses and operational
disadvantages associated with retrofitting an existing unit to closed-
cycle cooling. For example, the incremental downtime that can be
associated with retrofitting to closed-cycle cooling is avoided
altogether at a new unit. In addition, when new units are added, the
condensers can be configured for closed-cycle, reducing energy
requirements (by substantially reducing the turbine backpressure energy
penalty) and associated air emissions.
    The three factors that led EPA to reject closed-cycle cooling as
BTA (described below in Section E) are far less relevant for new units
at existing facilities than for retrofitting existing units. This
section discusses why EPA concluded that each factor is not a
significant concern for new units, and why the record supports EPA's
conclusion that closed-cycle cooling is an available and feasible
technology for new units at existing facilities.
     Land Availability: In contrast to retrofitting the entire
existing facility, the amount of space dedicated to closed-cycle for
the new unit will be limited to the new unit rather than the entire
facility. As a result, space constraints will be much less of an issue.
New units also present the opportunity to design an optimized closed-
cycle recirculating system for the new unit. Retrofitting an existing
facility for the full intake flow of the facility would require a
facility to identify (or possibly obtain) enough space to accommodate
the cooling towers and associated equipment. Furthermore, new units and
their corresponding cooling system can be built in stages rather than
as a facility-wide retrofit, and since the new unit has not yet been
built, there is no energy reliability concern (discussed further
below).
     Air Emissions: EPA expects that emissions are
significantly less of a concern at new units. The condensers will be
optimized for closed-cycle, reducing energy requirements, and high-
efficiency cooling towers can be incorporated into the design of the
new unit, potentially allowing for smaller cooling towers to be
installed. Turbine backpressure and the associated energy penalty can
be substantially reduced in a new unit, but EPA acknowledges new units
will still have auxiliary power consumption for fans. Therefore energy
penalties and air emissions for tower operations can be minimized
(though not eliminated). The emissions effects of requiring closed-
cycle cooling at new units at existing facilities is similar to the
effects of this requirement at new facilities and will not pose an
unacceptable impact. For more information, see Chapters 6, 8, and 10 of
the TDD. Further, the new unit is likely to be more efficient and emit
less pollution than existing units, therefore net emissions are
expected to decrease as new units replace older, less efficient units.
     Remaining Useful Plant Life: This is clearly not an issue
for new units. A new unit has its full useful life remaining and thus
would experience the maximum possible reductions in adverse
environmental impacts throughout that useful life.
    EPA does not expect that the requirements for new units at existing
facilities will be a disincentive for facilities to repower existing
units. The requirements only apply to stand-alone units. Requirements
for entrainment at repowered units will thus be determined by the
Director. EPA notes, however, for facilities that do choose to repower
an existing unit, the costs of employing a closed-cycle cooling system
are not a barrier, as described above. In fact, some facilities may
find closed-cycle cooling to be less costly over the long-term. For
example, in locations with limited water resources such that once-
through cooling of an additional unit is not possible, overall
reliability will be increased by using closed-cycle cooling systems.
    EPA also recognizes that installing closed-cycle cooling systems at
new units is a prevailing trend in industry, regardless of the
regulatory requirements imposed by today's final rule. For example, see
DCN 12-6672 in the record for today's rule, and DCNs 2-009 and 4-4023C
(from the Phase I and Phase II dockets, respectively). These documents
show that, on the basis of responses from facilities to the 316(b)
industry questionnaire, facilities constructed in recent years are
significantly more likely to employ closed-cycle cooling.
    EPA recognizes that at some point in the future, every unit will be
rebuilt, replaced or repowered (or retired). EPA projects that
approximately 227 MW in new generating capacity per year, will be
subject to the new unit provision, reflecting the general industry
trend towards more efficient units. EPA's analysis projects an
equivalent number of new units at manufacturing facilities will be
constructed each year. See the Phase I rule for more information

regarding the affordability and barrier to entry analysis for new
construction.
    EPA notes that the new unit provision is an important element of
the final rule, given the generally long lifespan of equipment at
industrial facilities. For example, generating units at a power plant
are often projected to have a 50-year lifespan. As a result, these
facilities have a slow rate of ``evolution'' in adopting newer
technologies. By requiring closed-cycle cooling in new units, EPA is
ensuring (along with the Phase I rule) that no new once-through cooling
units or facilities will be built.

E. Option Selection

    After considering all factors identified above, EPA has concluded
that it should base the BTA impingement mortality standard for existing
units on the performance of traveling screens (e.g., modified Ristroph
screens and equivalent modified traveling screens with fish-friendly
fish returns)--the ``best technology available'' for minimizing
impingement mortality. While there are a number of technologies that
may perform as well as or better than traveling screens, these
technologies were not feasible or available on a nationwide basis and
thus were not the ``best technology available'' for standard setting
purposes. Moreover, the impingement mortality standard for existing
units provides a number of alternatives, including some of these other
technologies, for compliance with the standard. EPA based the BTA
impingement mortality standard for existing units on the performance of
traveling screens because EPA concluded that this technology is
effective, widely available, feasible,\73\ and does not lead to
unacceptable non-water quality impacts.
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    \73\ As part of the feasibility determination, EPA found that
the costs associated with the IM standards are reasonable for the
industry as a whole.
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    As explained above, EPA has not identified a technology or
combinations of technologies that EPA concluded is ``best technology
available'' for minimizing entrainment at existing units. EPA did not
identify a technology for reducing entrainment that is effective,
widely available, feasible, and does not lead to unacceptable non-water
quality impacts. As such, EPA is unable to identify a nationally
applicable BTA technology on which to base the BTA entrainment
standard.
    While EPA concluded that closed-cycle recirculating systems reduce
entrainment (and impingement mortality) to the greatest extent and are
the most effective performing technology, after careful consideration
of multiple factors, EPA concluded that a closed-cycle recirculating
system is not the ``best technology available'' for existing units
within the meaning of the statute. It is not the best technology
available on a national basis for minimizing adverse environmental
impact and should not form the sole basis for the BTA standard for
entrainment for the reasons explained below.
    EPA also determined that there were no other ``available''
technologies for entrainment whose performance came close to that of
closed-cycle recirculating systems. Further, while reduced intake
velocity was a very effective control for impingement and may also
reduce entrainment of some life stages of fish and shellfish, it does
not significantly reduce entrainment of eggs and non-motile stages of
larvae, and it is not physically available in many locations.
    EPA has broad discretion in what factors it should consider when it
determines the best technology available for minimizing the adverse
environmental impacts of cooling water intake structures. As both the
U.S. Supreme Court and the Second Circuit Court of Appeals have
underscored, section 316(b) is ``sui generis,'' in a class by itself,
unencumbered by ``specified statutory factors,'' Entergy Corp. v.
Riverkeeper, Inc., 556 U.S. 208, 222 (2009); Riverkeeper, Inc. v. EPA,
358 F, 3d 174,187 (2d Cir. 2004). The Second Circuit explicitly
rejected the argument that, because section 316(b) does not mention
costs or other factors, EPA cannot give costs or other factors ``any''
weight in deciding what is the best technology. Riverkeeper, Inc., 358
F.3d at 195. Furthermore, the Second Circuit recognized that EPA may
base its decision on factors other than the effectiveness of a given
technology in reducing impingement and entrainment and that EPA is
entitled to deference in deciding what weight to give to the factors it
considers in its BTA determination. Riverkeeper, Inc., 358 F.3d at 196.
    As noted, costs are one factor EPA may consider in its BTA
determination. Here, while EPA did consider costs, costs were not a
dispositive factor in the decision to reject closed-cycle cooling as
the basis for a uniform national BTA entrainment standard. EPA did not
reject closed-cycle cooling here either because it was not economically
achievable or because the costs of closed-cycle would exceed its
benefits. Instead, EPA rejected closed-cycle cooling as the technology
basis for a uniform national BTA entrainment standard based on three
factors: Land availability, air emissions, and remaining useful plant
life as explained below.
    Central to EPA's evaluation of the availability of closed-cycle as
BTA was EPA's new understanding of the limitations of technologies
other than closed-cycle in reducing entrainment. This presented EPA
with a sharper choice than it had in the Phase II rule. For today's
rulemaking, EPA took a second look at the data it had relied on in the
Phase II rule, particularly in light of new data received since the
Phase II rule. As a result, EPA learned that entrainment exclusion does
not necessarily equate to entrainment survival (76 FR 22185), a key
underpinning to EPA's BTA standards for entrainment in the remanded
Phase II rule.
    For the remanded Phase II rule, EPA had established national BTA
performance standards for entrainment (and impingement) and included a
number of different alternative means to achieve the standards. First,
if a facility demonstrated that it could achieve reductions in flow
associated with closed-cycle cooling, the facility met the BTA
performance standards. Alternatively, a facility could demonstrate that
it met the entrainment performance standards by a combination of
installed technology and operational or other measures (including
restoration measures). See 69 FR 41590 for a description of the final
Phase II rule. Critical to EPA's decision to provide an array of
choices for achieving the national BTA entrainment performance
standards was a key factual conclusion. That conclusion was that a
number of technologies would achieve performance reducing entrainment
that was ``comparable'' to that of closed-cycle cooling. Consequently,
for the Phase II rule, EPA established an entrainment performance
standard of 60 to 90 percent based on data it reviewed for the Phase II
rulemaking. See 69 FR 41598 for information on EPA's rationale for
establishing compliance alternatives as part of the final rule.
    In the Phase II rule, while EPA looked to the performance of
closed-cycle as the benchmark against which it evaluated technologies
for the BTA standards, EPA did not mandate the achievement of flow
reductions that were in all cases equivalent to closed-cycle. Given
that the available data supported the view that there were other much
less expensive technologies that obtained significant reductions in
entrainment, EPA was comfortable with a BTA standard that required
achievement of a level of performance

that was generally comparable though not equivalent to closed-cycle.
    Since the Phase II rulemaking, EPA has received new data and
learned that its understanding of entrainment technology performance
was incomplete. Following the remand of the Phase II rule, EPA
reexamined the data as well as new information on the performance of
various entrainment control technologies it had previously reviewed. As
a result, EPA determined that its conclusion regarding the capability
of these other technologies--a conclusion on which the Agency had based
the Phase II BTA performance standards--was no longer supported by the
data EPA had before it.
    There is a second additional consideration that further required
EPA to focus renewed attention on how widely available closed-cycle
cooling in fact was nationally. The Second Circuit decision in the
Phase II rule removed restoration as a compliance option that EPA could
consider. The decision underscored that restoration measures--one
compliance option included in the Phase II rule--were not an available
tool for complying with any 316(b) standard. However, at the time of
the Phase II promulgation, EPA expected some facilities would use
restoration in lieu of closed-cycle cooling, thus making closed-cycle
or reductions commensurate with closed-cycle feasible (76 FR 41609).
With the court decisions that restoration was not an available tool for
compliance, compliance with a standard based on closed-cycle cooling
alone is less feasible than EPA had expected at the time of the Phase
II promulgation.
    The changed landscape has narrowed markedly EPA's range of options
with respect to the technology basis for today's BTA standards. The gap
between the performance of the most effective entrainment reduction
technologies (closed-cycle) and other less expensive technologies has
widened significantly. EPA's narrowed range of compliance technology
choices required EPA to look even more closely at the feasibility of
closed-cycle cooling and reduced flow. As the Second Circuit has noted,
EPA is clearly entitled to make its choice among alternative BTA
technologies based on more factors other than just a technology's
effectiveness in reducing impingement and entrainment. Riverkeeper,
Inc., 358 F.3d at 196. EPA identified three factors as significant in
its decision to reject closed-cycle cooling as the sole technology
basis for a national BTA entrainment standard. The three factors that
collectively support rejecting closed-cycle cooling systems as a
uniformly applicable BTA for existing facilities (except new units) are
land availability, increased air emissions and remaining useful life.
1. Land Availability and Geographical Constraints Could Be a Factor on
a Local Basis
    While EPA's record indicated that the majority of facilities have
adequate available land to retrofit to closed-cycle cooling, some
facilities have land constraints.\74\ While EPA originally estimated as
many as 23 percent of facilities would not have enough space,\75\ it
observed on site visits that some facilities with a small parcel of
land could still install closed-cycle cooling by using creative
engineering solutions. On the other hand, EPA found that some
facilities with large acreage could not feasibly install cooling towers
because of local zoning or other local concerns. Thus, existing
physical space at the facility was not the only factor contributing to
uncertainty about land availability. Further review has shown that
setback distances to mitigate noise and plume abatement (based on GPS
mapping of residential areas) act as an additional constraint on land
available for retrofitting to closed-cycle, and the cost of acquiring
new land may be prohibitive for some facilities. Consequently, EPA
estimates that 25 percent or more of facilities might have one or more
constraints on land availability that would limit the ability to
retrofit for cooling towers for the entire facility. EPA lacks adequate
support to indicate that land constraints can be accommodated at
existing facilities.
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    \74\ For example, in the case of fossil fuel facilities,
scrubber controls may already have been required to comply with air
rules and standards. This may reduce available land for closed-
cycle.
    \75\ EPRI reported at least 6 percent of sites it evaluated were
deemed ``infeasible'' because no space was available on which to
locate a cooling tower. (DCN 10-6951) While EPA does not have access
to the facility level data, EPRI's report supports EPA's conclusion
that there is significant uncertainty around space constraints for
facilities to install closed-cycle cooling.
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    EPA also attempted to determine criteria based on the data in its
record that would enable it to define a threshold for determining land
availability on a nationwide basis, but was unsuccessful. For example,
one analysis explored a threshold of approximately 160 acres per GW
(gigawatt) below which a facility could not feasibly install cooling
towers. Based on acres and the footprint of the facility and its
surroundings (primarily those sites for which EPA conducted site
visits), EPA found such an approach did not accurately identify which
facilities could feasibly install closed-cycle.
2. Increased Air Emissions Could Be a Factor on a Local Basis
    As previously discussed, retrofitting closed-cycle cooling (without
also repowering) would result in increased air emissions of various
pollutants, including particulates, sulfur dioxide, nitrogen oxides,
mercury, and greenhouse gases, among others.\76\ As a result of
installing closed-cycle cooling structures, fossil-fueled facilities
would need to burn additional fuel, thereby emitting additional PM,
CO2, SO2, NOX, and Hg. Two factors are
responsible: (1) The need to compensate for energy required for
operating cooling towers, and (2) slightly lower generating efficiency
attributed to higher turbine backpressure when the condenser is not
replaced with one optimized for closed-cycle operation when
retrofitting existing units (also referred to as the energy penalty).
While both of these factors contribute to increased air emissions, the
larger contributor to projected increased air emissions is by far the
energy penalty.
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    \76\ EPA recognizes that retrofitting closed-cycle cooling could
be combined with other energy efficiency or pollution control
technologies with the net effect of reducing air emissions; however,
facilities could (and may be required to under other rules) install
such technologies anyway, without converting to closed-cycle
cooling.
---------------------------------------------------------------------------

    The impact of the increased emissions varies according to the local
circumstances. The increased emissions could consist of stack emissions
from increased fuel usage, cooling tower emissions, and plumes of water
vapor. EPA's analysis suggested that the most significant impacts would
be increased PM2.5 emissions, which are associated directly
with an increase in human health effects. EPA notes that cooling plume
abatement and drift elimination technologies exist to address cooling
tower emissions (and EPA included costs for such technologies in its
analysis of Proposal Options 2 and 3). Further, EPA expects most
effects of the particulates from cooling tower emissions would be
limited to the immediate vicinity, confined wholly to the facility
property. (See DCN 10-6954.) Therefore, EPA's primary concern is
increased air emissions associated with additional fuel usage due to
the energy penalty when retrofitting to cooling towers. EPA's review of
emissions data from E-GRID (year 2005) suggests that impacts from these
pollutant discharges could be significant. These include the human
health and welfare and global climate change effects--all associated
with a

variety of pollutants that are emitted from fossil fuel combustion. EPA
is not able to quantify the frequency with which facilities could
experience these local impacts, and therefore has concluded that the
proper forum to address such local impacts fully is in a site-specific
setting.
3. Remaining Useful Plant Life Could Be a Factor on a Facility Basis
    A number of facilities are nearing the end of their useful life.
Considering the long lead time to plan, design, and construct closed-
cycle cooling systems, EPA determined that the Director should have the
latitude to consider the remaining useful plant life in establishing
entrainment mortality requirements for a facility. The remaining useful
plant life, along with other site-specific information, will affect the
entrainment reduction of closed-cycle cooling at a facility. For
example, retrofitting to a closed-cycle system at a facility that is
scheduled to close in three years will result in little entrainment
reduction as compared to retrofitting to closed-cycle at a facility
that will continue to operate for a significantly longer period.
The Decision To Establish a National BTA Standard Requiring Site-
Specific Determination of BTA Entrainment Controls
    Once EPA determined that a ``one-size-fits-all'' approach for
entrainment for existing units is not generally feasible, it is
appropriate to assess the required controls on a site-specific basis.
Therefore, for existing units, EPA decided to adopt as the BTA
entrainment standard an overarching regulatory framework under which
the Director will establish BTA entrainment requirements on a site-
specific basis following prescribed procedures and applying specified
factors for decision-making prescribed in the regulation and as
described below.
    EPA concluded that site-specific proceedings are the appropriate
forum for weighing all relevant considerations in establishing BTA
entrainment requirements. Closed-cycle cooling is indisputably the most
effective technology at reducing entrainment. Closed-cycle reduces
flows by 95 percent and entrainment is similarly highly reduced. But
given that EPA estimates that 25 percent of existing facilities may
face some geographical constraints on retrofitting closed-cycle cooling
and concerns about air emissions and the remaining useful life of a
facility, EPA rejected the option of requiring uniform entrainment
controls based on closed-cycle cooling. Instead, EPA elected to adopt
as the entrainment standard a more flexible process in which, following
consideration of a host of factors, the Director will prescribe 316(b)
entrainment conditions appropriate at a particular site. For additional
discussion on how a site-specific consideration of entrainment control
requirements will be implemented, see Section VIII below.
    EPA has several reasons for adopting the framework approach as the
BTA standard for entrainment. As explained, the record shows that
though closed-cycle cooling is effective, it is neither widely
available nor feasible, and has significant unacceptable non-water
quality impacts. While EPA cannot identify with precision the extent of
these limitations on installing closed-cycle cooling systems
nationwide, the record indicates that the circumstances are neither
isolated nor insignificant. In light of this, EPA decided not to
establish closed-cycle cooling as a presumptive BTA entrainment
standard, pending a site-specific demonstration of the limitations.
Instead, entrainment control requirements will be determined in a site-
specific setting where the opportunity for local input in decision-
making process will be maximized.
    With regard to new units at existing facilities, based on the
performance of properly operated cooling tower operation and the
availability, feasibility and affordability of closed-cycle cooling at
new units, EPA selected closed-cycle recirculating systems based on wet
cooling towers as BTA. For a discussion of how the three factors
(availability, feasibility and affordability) relate to new units, see
Section VI.D.3. Consistent with the Phase I rule for new facilities,
EPA has also included a compliance alternative allowing a facility to
show performance comparable to that of a closed-cycle recirculating
system. The new unit provisions in today's final rule are essentially
the same as the requirements for new facilities under the Phase I rule.

F. Other Options Considered for Today's Final Regulation

    EPA considered several other options for the BTA standards in
developing today's rule, but ultimately rejected them. This section
includes a discussion of these options, as well as some technologies
that EPA considered, but did not include as compliance alternatives to
the impingement mortality standards.
1. Proposal Option 4--Flexible Impingement Mortality Controls Similar
to Final Rule at Existing Facilities With DIF of 50 mgd or More; BPJ
Permits for Impingement Mortality and Entrainment at Existing
Facilities With Design Intake Flow Between 2 mgd and 50 mgd; Site-
Specific Entrainment Standard for Existing Facilities With DIF of 50
mgd or More; and Uniform Impingement Mortality and Entrainment Controls
for All New Units at Existing Facilities Similar to Final Rule
    At proposal, EPA's preferred option was Option 1, which was the
option closest to today's final rule, and the starting point for the
description of the changes to the rule in Section V above. At proposal,
EPA also considered a variant of Option 1, called Option 4, which
changed the impingement mortality requirements for facilities under 50
mgd from the performance standard in Option 1 to BTA as determined by
best professional judgment. In the case of an existing facility below
50 mgd that added a new unit, the flow associated with the new unit
would have been subject to the uniform entrainment requirements based
on closed-cycle cooling. Finally, all existing facilities withdrawing
more than 2 mgd of DIF would have been subject to entrainment
requirements established on a site-specific basis, with the exception
noted above for new units. The option analyzed here, called Proposal
Option 4, is likewise similar to the final rule, but for the
impingement standard based on BPJ for facilities between 2 and 50 mgd.
    EPA ultimately rejected Proposal Option 4 because EPA found that
the technologies on which the impingement mortality performance
standard of today's final rule is based are available, feasible,
demonstrated, and affordable for all regulated facilities on a national
basis. Moreover, EPA's analysis showed that the difference in the total
costs for the two options was nominal. Additionally, EPA notes that
many facilities with a DIF under 50 mgd already use closed-cycle
cooling and would have minimal burden under the final rule. These
facilities would have no difficulty complying with the requirements EPA
is establishing in today's final rule. Proposal Option 4, by not
distinguishing between those facilities under 50 mgd that have already
minimized adverse environmental impacts from those that have not, masks
the actions that would have to be taken by the latter group to comply
with today's final rule. In addition, the flexibilities introduced in
the June 11, 2012 NODA and included

in today's final rule applied to all facilities, rather than taking the
Option 4 approach at proposal of providing for more Director discretion
for only the smaller withdrawing facilities. EPA also concluded that
the data collection activities required under the final rule will be
more protective of threatened and endangered species because they
provide information on a larger number of facilities than Proposal
Option 4 for consideration by the Director in permitting decisions.
Lastly, EPA acknowledges that Proposal Option 4 is more burdensome to
permitting authorities than is the final rule, as it requires more
case-by-case decision making.
2. Proposal Option 2--Flexible Impingement Mortality Controls Similar
to Final Rule at All Existing Facilities That Withdraw Over 2 mgd DIF;
Site-Specific Entrainment Standard for Existing Facilities With DIF at
or Below 125 mgd; Require Flow Reduction Commensurate With Closed-Cycle
Cooling by Facilities Greater Than 125 mgd DIF; and Uniform Impingement
Mortality and Entrainment Controls for All New Units at Existing
Facilities
    As previously explained, EPA assessed a number of different
technologies that reduce impingement mortality and entrainment as the
possible basis for section 316(b) requirements. EPA concluded that
closed-cycle recirculating systems (based on wet cooling towers) are
the most effective technology for reducing impingement mortality and
entrainment. Notwithstanding that conclusion, EPA has decided not to
establish a performance standard for impingement and entrainment based
on closed-cycle recirculating systems for existing facilities.
Furthermore, EPA found that there are no other effective technologies
for entrainment that are available nationally. As described previously,
each of the three factors for rejecting closed-cycle cooling as BTA for
entrainment would also apply in the case of Proposal Option 2, despite
the smaller number of facilities that would be subject to a requirement
to retrofit. The technology basis for entrainment mortality controls
for facilities greater than 125 mgd DIF under this option would have
been wet cooling systems. The constraints discussed above that are
associated with retrofitting a large portion of the universe of
affected facilities, led EPA to conclude that requiring closed-cycle
cooling on a uniform basis scale was not appropriate for a national
regulation.
    EPA notes that it proposed multiple options that included closed-
cycle, and solicited comment on all aspects of closed-cycle cooling.
After fully considering all comments and data, EPA still finds closed-
cycle cooling is not the ``best technology available for minimizing
adverse environmental impact'' required by section 316(b). Because of a
combination of concerns over feasibility/availability, air emissions,
and remaining useful life of the facility, EPA has rejected closed-
cycle recirculating systems as the basis for national impingement and/
or entrainment controls. Nor is EPA able to identify a subcategory for
which these concerns no longer apply. Moreover, the complex interaction
of all of these factors at individual sites does not lend itself to
other regulatory options that would require closed-cycle recirculating
systems with an ``off ramp'' if any of the factors were shown to result
in unacceptable impacts because this would create a presumption for
closed-cycle cooling rather than an equal balancing of all relevant
factors. EPA decided not to establish any presumptive BTA entrainment
outcome. EPA finds the entrainment standards framework in today's final
rule will provide a consistent, more efficient, and more effective
approach than standards with an ``off ramp.''
3. Proposal Option 3--Flexible Impingement Mortality Controls at All
Existing Facilities That Withdraw Over 2 mgd DIF; Require Flow
Reduction Commensurate With Closed-Cycle Cooling at All Existing
Facilities Over 2 mgd DIF
    Proposal Option 3 was, in many ways, the same as requiring closed-
cycle cooling at all existing facilities. As described above, each of
the three factors for rejecting closed-cycle cooling as BTA for
entrainment would apply with equal force for Proposal Option 3. As a
result, EPA has concluded Proposal Option 3, similarly, is not
appropriate as BTA for entrainment.
4. Proposal Option 4 Variant
    EPA also considered a variant of Proposal Option 4. As compared to
Proposal Option 4, this variant did not include flexible alternatives
for complying with the BTA impingement mortality standards (including
pre-approved and streamlined alternatives), but did adopt the 50 mgd
threshold to determine those facilities for which the Director has more
discretion in determining BTA via BPJ. EPA analyzed this option to
directly compare the effects of introducing flexible IM compliance
alternatives at all facilities (as the final rule does) to the effects
of introducing greater Director discretion for a subset of facilities,
via BPJ permitting (as the Proposal Option 4 variant does). The
preferred option at proposal, Option 1, was estimated to be more costly
than Option 4 (Option 1 was estimated to cost $384 annually as compared
with $327 million annually for Option 4). Under the analysis supporting
the final rule the EPA is adopting today, however, today's final rule
is estimated to cost $275 million annually in comparison with an
estimated cost of $284 million annually for the Proposal Option 4
variant. Thus, EPA has concluded that providing flexible alternatives
for compliance with the BTA IM standard at all facilities is both more
effective at reducing costs to society and more readily justified as
best technology available as compared to the approach of introducing
greater Director discretion for only a subset of facilities (below 50
mgd). Hence, EPA rejected the Proposal Option 4 variant, and the
approach of introducing greater Director discretion for only a subset
of facilities (below 50 mgd).
5. Proposal Option 2 Variant
    EPA also considered a variation of Proposal Option 2 that would
have used 125 mgd AIF rather than 125 mgd DIF as the threshold.
However, as described above, EPA rejected Proposal Option 2 and, for
the same reasons, rejected this variant of Option 2.
6. Site-Specific Approach To Addressing Impingement
    Many commenters (primarily from manufacturing facilities) commented
that EPA should adopt a site-specific approach to addressing
impingement mortality, similar to that employed for entrainment. As a
result, EPA also considered an approach that would have established
both impingement mortality and entrainment requirements fully on a
site-specific basis taking into account for the particular facility,
among other factors, those previously described as pertinent to EPA's
316(b) BTA determination. EPA rejected a fully site-specific approach
for impingement controls principally because low-cost technologies for
impingement mortality are available, feasible, demonstrated, and
affordable for facilities nationally. Because technologies are
available, a fully site-specific approach would place an unnecessary
additional burden on state permitting resources. Moreover, the final
impingement mortality standard includes several alternatives that allow
site-specific demonstration that a particular technology performs at a
level representing the best technology available for the site. EPA is
instead promulgating a modified version of the

proposed rule, adding several elements of flexibility (i.e., compliance
alternatives), and thus directly addressing many of the concerns raised
by these commenters.
7. Pre-Approved Technologies
    Many commenters requested that EPA pre-approve technologies that,
once installed, would obviate the need for further regulatory
conditions such as periodic monitoring. This is similar to the approach
taken for cylindrical wedgewire screens in the remanded 2004 Phase II
rule (see 69 FR 41693). EPA has adopted, in significant measure,
commenters' suggestion in the BTA impingement mortality standard in
today's rule by including several pre-approved and several streamlined
compliance alternatives in the form of technologies that may be
approved following a demonstration of required performance, so long as
the facility shows that its alternative technology is operating in a
manner that minimizes adverse environmental impacts. As an option for
achieving the impingement mortality standards, a facility may install
and operate specified impingement controls whose performance is
comparable to or better than the technology EPA concluded was the
``best technology available'' for impingement mortality reductions:

 Closed-cycle recirculating systems, defined at Sec.  125.92(c)
 Existing offshore velocity caps, defined at Sec.  125.92(v)
 Technologies that result in a design intake velocity less than
or equal to 0.5 fps, including most modern cylindrical wedgewire
screens

Although this rule leaves the BTA entrainment determination to the
Director, with the possible BTA decisions ranging from no additional
controls to closed-cycle recirculating systems plus additional controls
as warranted, EPA expects that the Director, in the site-specific
permitting proceeding, will determine that facilities with properly
operated closed-cycle recirculating systems do not require additional
entrainment reduction control measures. Refer to Section E.1 for the
EPA's rationale for selecting these controls.

G. Final Rule BTA Performance Standards

    The rule establishes the following BTA standards for Impingement
Mortality and Entrainment: Impingement Mortality Standards at All
Existing Units at Existing Facilities that withdraw greater than 2 mgd
DIF; an Entrainment Standard that requires site-specific entrainment
controls determined by the Director for Existing Units at Existing
Facilities that withdraw over 2 mgd DIF; BTA standards for impingement
mortality and entrainment for new units at existing facilities. The
previous section described the other options that EPA considered but
ultimately rejected, and the basis for those decisions.
1. Impingement Mortality Controls for Existing Units at Existing
Facilities for the Final Rule
    Today's final rule provides a facility a number of alternatives for
complying with the BTA impingement mortality standard. As discussed
more below, EPA's BTA impingement mortality standard is based on EPA's
conclusion that, on a national basis, modified traveling screens with
fish-friendly return systems are the best performing technology
available for impingement mortality reduction. But EPA is not requiring
compliance with the BTA impingement mortality standards only through
monitoring data that demonstrates achievement of the numeric reduction
in mortality levels that EPA has determined well-operated modified
traveling screen will achieve. Rather, the final rule allows facilities
to comply by employing any of seven alternatives, including monitored
compliance with a numeric impingement mortality performance standard.
    Based on its review of available data and information submitted by
commenters, EPA identified a number of other technologies and
operational measures that could achieve equivalent, or better,
performance to the impingement mortality reductions achieved with
modified traveling screens that may be available for some sites. Thus,
the final rule provides seven alternatives for complying with the BTA
impingement mortality standards. These include three compliance paths
based on pre-approved technologies, and three compliance paths that
offer a streamlined approach to compliance. EPA expects the majority of
facilities will use one of these six options to comply with the BTA
impingement mortality standards (see Exhibit VIII-1 for more
information).
    The following pre-approved technologies will comply with today's
rule and are associated with minimal monitoring and reporting of
operational and/or design parameters. These technologies are (the
numbering reflects the numbering in Sec.  125.94(c)): Operating (1) a
closed-cycle recirculating system; (2) a cooling water intake structure
that EPA or the State NPDES permitting authority determines has a
design maximum through-screen intake velocity of 0.5 feet per second;
or (4) an existing offshore velocity cap. The general intent behind a
compliance path based on a pre-approved technology is to provide a
level of certainty to the regulated entity that they would be deemed
compliant with the relevant rule requirements by designing, installing,
and operating the technology as specified in the regulation. The three
pre-approved compliance alternatives are each based on a particular
technology approach. The permit for each compliance alternative will
necessarily include criteria, design standards, and operational
conditions specific to the pre-approved technology. The compliance
paths based on pre-approved technologies in today's final rule include
simplified permit application requirements (such as reduced or minimal
study), documentation, or reduced monitoring, and will therefore result
in greatly simplified implementation. In today's final rule, there are
no biological compliance monitoring requirements for any of the three
compliance paths based on pre-approved technologies.
    Under the streamlined alternatives, a facility must demonstrate to
the Director that traveling screens or some combination of technology
controls or operational measures represent BTA performance under the
conditions at the site. The three streamlined compliance alternatives
are (the numbering reflects the numbering in Sec.  125.94(c)) operating
(3) a cooling water intake structure that EPA or the State NPDES
permitting authority determines has an actual maximum through-screen
intake velocity of 0.5 feet per second; (5) modified traveling screens
whose demonstrated performance represents the best technology available
for impingement reduction at the site; or (6) a system or combination
of technologies or operational measures whose demonstrated performance
is the best technology available for impingement reduction at the site.
In order to demonstrate BTA performance, a facility will need to
conduct a two-year site-specific study at the same time it conducts its
source water characterization and Entrainment Characterization Study.
This study must demonstrate that its modified traveling screens, or
combination of technology controls and operational measures, have been
adjusted and optimized so as to minimize impingement mortality. If the
Director concludes that the facility has demonstrated optimized
performance for its controls, the facility will have no

subsequent biological monitoring and reporting requirements as compared
to a facility that complies using the impingement mortality performance
standard. If the screens or other measures are not already installed,
the Director may approve postponing the two-year study to be conducted
after the entrainment determination has been made. These three
streamlined compliance alternatives are based on a technology or suite
of technologies and practices with more variable performance, and as
such necessitate some degree of study, in order to optimize technology
performance for the site-specific conditions encountered by a facility.
A streamlined compliance alternative may require some level of
monitoring, but once the optimal performance of the technology has been
identified, conditions included in the permit specifying optimal
operation ensure that the streamlined alternative is similar to or
better than the impingement mortality performance standard. For
example, the streamlined compliance alternatives also do not require
biological compliance monitoring.
    The seventh alternative (at Sec.  125.94(c)(7)) for complying with
the BTA impingement mortality standards requires the owner or operator
to demonstrate compliance with the numeric impingement mortality
performance standard through biological monitoring. Under this
alternative, the owner or operator has the flexibility to choose any
technology, including a new or innovative technology, provided the
compliance monitoring demonstrates the performance standard is
achieved.
    Each of these seven alternatives is further described below. In
addition, further discussion of how each of these alternatives will be
implemented may be found in Section VIII.
a. Closed-Cycle Recirculating Systems
    As described above, in Chapter 6 of the TDD, and in prior
rulemakings, EPA has long recognized the benefits of flow reduction
from closed-cycle recirculating systems for reducing impingement (as
well as entrainment). A facility employing a closed-cycle recirculating
system will typically reduce impingement by more than 95 percent. As a
result, a facility may choose to comply with the BTA impingement
mortality standards in today's final rule by demonstrating that it uses
a properly operated and maintained closed-cycle recirculating system.
    EPA estimates that approximately 18 percent of intake structures
(i.e., those that already have an existing closed-cycle recirculating
system, plus facilities located in California and New York, whose State
regulations are at least as stringent as the final rule) will choose
this alternative.
    EPA does not have the data to determine precisely which
impoundments are serving as part of a closed-cycle recirculating system
as defined at 40 CFR 125.92(c)(2). However, EPA is aware that some
facilities have created their impoundments in a water of the U.S. as
part of their cooling system. EPA does not intend to eliminate the use
of such lawfully created impoundments for their intended purpose, as
doing so could result in a large number of stranded assets. If the
cooling system with the impoundment minimizes the withdrawal of make-up
water for cooling purposes, the Director may determine the cooling
system meets the definition of a closed-cycle recirculating system.
b. Reduced Intake Velocity
    EPA has long recognized the relationship between impingement and
intake velocity. EPA conducted an analysis of fish swim speeds in the
Phase I rule (see 66 FR 65274, December 18, 2001) and concluded that a
design through-screen velocity of 0.5 fps is protective of 96 percent
of motile organisms. However, EPA did not select intake velocity as the
technology basis for the BTA impingement mortality standards. Although
the performance of 0.5 fps intake velocity achieves greater reduction
in impingement mortality than the technology on which the BTA
impingement mortality standards are based, reducing a facility's intake
velocity is not widely available or feasible for all existing
facilities (see Chapter 6 of the TDD).
    EPA is including reductions in intake velocity as an alternative
for complying with the BTA impingement mortality standards through
reduced intake velocity. A facility choosing this alternative must
demonstrate that (1) the through-screen design velocity could not
exceed 0.5 fps or (2) the actual intake velocity does not exceed 0.5
fps.
    EPA estimates that approximately 34 percent of intake structures
will choose this alternative. This estimate includes facilities that
have an existing intake velocity of 0.5 fps or less, plus those
facilities that are projected to install a technology that would reduce
their intake velocity (larger intake, wedgewire screens, or variable
speed pumps).
i. Design Intake Flow Basis
    Consistent with EPA's determination in its earlier 316(b)
regulatory efforts, the final rule allows a facility to comply with the
BTA impingement mortality standards by demonstrating that its intake
has a maximum through-screen design velocity of 0.5 fps. EPA concluded
that facility's operating at this through-screen design velocity will
protect the vast majority of impingeable aquatic organisms. Facilities
choosing to comply with the BTA impingement mortality standards may not
average velocity across multiple intakes at a facility.
ii. Actual Intake Flow Basis
    EPA is also adopting a provision to allow facilities to demonstrate
that the through-screen intake velocity at an intake structure does not
exceed 0.5 fps on the basis of the intake's actual flow. (Again, note
that facilities choosing this compliance alternative may not average
intake velocity across multiple intakes.) In contrast to design flow
above, a facility with an intake having a design through-screen intake
velocity greater than 0.5 fps may be operated at a reduced capacity and
therefore may withdraw cooling water at a velocity less than 0.5 fps.
As long as the actual intake flow is such that the velocity remains at
or below 0.5 fps, the reductions in impingement (and subsequently,
impingement mortality) remain the same as a facility with a maximum
design through-screen intake velocity of 0.5 fps. As described below, a
facility will be required to monitor its intake flow and report this
data to the Director to verify that intake flows do not exceed 0.5 fps.
This approach also permits the Director to allow brief periods where
the intake velocity will exceed 0.5 fps under extreme conditions.
c. Existing Offshore Velocity Caps
    A number of commenters stated that EPA should consider existing
offshore intakes fitted with velocity caps to be pre-approved and
complying with the BTA impingement mortality standards. Locating
submerged intakes in the deeper regions of larger waterbodies
(particularly outside the littoral zone \77\) has the potential to
reduce both impingement and entrainment (I&E), due to the lower
densities of aquatic organisms as compared to a shoreline-based intake.
EPA has identified 11 facilities with offshore velocity caps, and
reviewed a number of studies documenting the performance of these

facilities. These studies show that the impingement reduction
performance of intakes submerged far offshore with velocity caps is
dependent on site-specific conditions. The data show that solely
locating an intake far offshore (i.e., without also employing a
velocity cap) achieves a 60 to 73 percent reduction in impingement, and
therefore does not achieve impingement mortality reduction comparable
to that of well-operated modified traveling screens. Similarly, the
data also show that velocity caps alone achieve a 50 to 97 percent
reduction in impingement, and therefore could result in compliance
performance comparable to or better than modified traveling screens in
some, but not in all cases. However, the combination of an existing
intake located far offshore (i.e., approximately 850 feet, as
identified in the data for Nine Mile Unit 1 and Oswego Unit 5) in
combination with use of a velocity cap will result in performance that
exceeds the 12-month average impingement mortality performance standard
(alternative seven described above).78 79 Because there is
some amount of uncertainty in measuring distances from a shoreline,
including but not limited to due to variations in water levels, storm
swells, or tidal excursions, EPA has set the minimum distance offshore
at 800 feet. As a result, the final rule at Sec.  122.95(c)(4) allows a
facility to comply with the BTA impingement mortality standards with an
existing offshore intake with an existing velocity cap located at least
800 feet offshore, based on the performance data from the 11 identified
facilities.
---------------------------------------------------------------------------

    \77\ The littoral zone extends from the shoreline to roughly the
edge of the continental shelf.
    \78\ An existing facility may also choose to install a new
offshore intake with a velocity cap, but such a facility would not
automatically qualify as meeting the impingement requirements for
the final rule. Such a facility would need to demonstrate equivalent
performance to the impingement mortality performance standard.
    \79\ A velocity cap must also include bar racks or other devices
to exclude large marine organisms (e.g., seals, turtles) from
entering the intake structure.
---------------------------------------------------------------------------

    As noted above, the record shows all existing facilities with a
velocity cap located at least 800 feet offshore will meet or exceed the
12-month average mortality performance standard of Sec.  125.94(c)(7).
EPA does not have data showing velocity caps located at lesser
distances offshore will consistently achieve the impingement mortality
performance standards, but is aware that some facilities may be able to
achieve the impingement mortality standards through a combination of
technologies that includes an offshore location. For example, the
Office of Naval Research states that the littoral zone in ocean
environments generally extends from the shore to 600 ft out in the
water (ONR 2013). SEAMAP data in EPA's record shows installing the
intake to depths where there is a lower concentration of living
organisms (i.e., at least 65 feet) is also expected to decrease
environmental impacts associated with intake operations. Therefore, the
final rule allows facilities with intake structures at significant
distances offshore to demonstrate the performance of their technology
under Sec.  122.95(c)(6), as further discussed below.
    In addition facilities may opt to construct an offshore velocity
cap at new locations. In those circumstances, the facility will need to
demonstrate that theperformance of its velocity caps is the best
technology available for impingement reduction under the alternative
found at Sec.  122.95(c)(6). For more information, see DCN 12-6601.
    EPA estimates that approximately 1 percent of intake structures
(i.e., those with an existing velocity cap meeting the definition at
Sec.  125.92(v) will choose this alternative.
d. Install Modified Traveling Screens
    In the June 11, 2012 NODA, EPA discussed a streamlined compliance
option that would provide facilities with a less burdensome alternative
than the proposed rule. In the final rule, EPA has included an option
at Sec.  125.95(c)(5) for facilities that install traveling screens--
the technology that forms the basis for the numeric IM performance
standards. Under this option, the facility must demonstrate to the
Director that it will install and operate modified traveling screens as
defined at Sec.  125.92(s) that are or will be optimized to minimize IM
mortality at the site. The facility will also be required to submit an
impingement technology performance optimization study (Sec.
122.21(r)(6)) which will include a 2-year optimization study for the
intake technology. The facility will conduct 2 years of monthly
impingement data collection, during which the facility will seek to
optimize the technology performance to minimize impingement mortality.
This study is intended to determine the optimal configuration and
operating conditions of modified traveling screens and the fish
handling and return systems for that intake to be consistently
protective of aquatic organisms. During the course of the study, EPA
expects that a facility will evaluate the interim results and make
changes to the technology or operating conditions as needed to identify
the most appropriate set of operational characteristics to ensure long-
term success. For example, a facility could adjust the spray wash
pressure, adjust the rotating speed of the screens, rotate the screens
more frequently, re-angle the fish sluicing sprays, ensure adequate
water in the return flume, design the fish return to avoid avian and
animal predation on the aquatic organisms, and locate the fish return
in such a way to avoid predation. Once a facility has optimized its
technology performance, the study will identify operational measures
that will serve as observable and enforceable permit conditions. As
evidenced by the data used in determining the performance standard, by
requiring facilities to study the conditions for optimized performance,
many facilities will achieve impingement mortality reductions much
greater than the 12-month average impingement mortality performance
standard without significant additional investment. Biological data
collection beyond this two-year study will not be required. The
facility will simply be required to ensure that it is operating its
technology under the identified conditions for optimized performance.
If the Director concludes that the screens will achieve optimized
performance, the Director will also incorporate operating conditions to
ensure optimized performance as terms of the facility's NPDES permit.
    As discussed in the NODA and Chapter 4 of TDD, EPA's data indicate
that most facilities employ traveling screens.\80\ EPA anticipates
that, as a result, many facilities will view the streamlined screen-
based compliance route as a logical choice for complying with the final
rule. The streamlined option provides an opportunity for a large number
of the affected facilities (i.e., those that do not meet the criteria
for the other compliance technologies) to demonstrate that their
intakes are effectively reducing impingement mortality while
significantly reducing the burden on both facilities and regulatory
agencies. EPA estimates that approximately 30 percent of intake
structures will choose this alternative.\81\
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    \80\ EPA's technical survey found that 93 percent of electric
generators and 73 percent of manufacturers already use screens, the
majority of which are traveling screens.
    \81\ While EPA's data shows 73 to 93 percent of facilities
already use traveling screens, EPA notes that many facilities use
more than one technology. For example, some of these facilities also
have a low intake velocity, an offshore velocity cap, or cooling
towers. EPA expects facilities will choose the IM compliance
alternative corresponding to these pre-approved technologies before
they will choose to comply via optimized performance of their
traveling screens.
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    EPA is aware that some facilities have no technologies installed
and will choose to install modified traveling screens, and further that
some facilities

with traveling screens will choose to either retrofit to modified
traveling screens with fish handling and returns. Obviously, the
impingement technology performance optimization study cannot be
undertaken until the technology is first installed. In this case the
NPDES permit would be issued before the completion of the optimization
study. EPA expects a permit will be issued that includes a schedule for
both the technology installation and the required optimization study.
As discussed earlier, the Director can establish interim measures as
appropriate (40 CFR 125.94(b)).
e. System of Technologies as the BTA for Impingement Mortality
    EPA recognizes that cooling water intake structures have a variety
of configurations and facilities may choose to comply with the final
rule by using more than one of the compliance approaches outlined
above. In the June 11, 2012, NODA, EPA described an approach where
facilities would be able to demonstrate ``credit'' toward meeting the
impingement mortality requirements by reducing the total number of
organisms impinged. EPA also intended for facilities to have the
flexibility to employ any system of technologies or combination of
operational measures to address impingement mortality so long as the
performance of the selected impingement reduction measures represented
the best technology available for the site. The final rule includes an
alternative reflecting these objectives.
    In the broadest sense, facilities have a number of options for
reducing impingement mortality. Some may choose to comply using an
approach where a single technology achieves the level of compliance
necessary. Others may choose an approach of employing multiple
technologies or operational measures, including reducing the number of
organisms that are impinged or susceptible to being impinged. The
following are examples of approaches for which a facility might be able
to take credit for impingement reduction under this alternative:

 Partial closed-cycle cooling
 Variable speed pumps
 Seasonal outages (including standard maintenance outages that
are specifically scheduled to avoid a biologically sensitive period)
 Certain impingement technologies that reduce the number of
organisms exposed to the intake structure (e.g., diversions, louvers,
barrier nets)
 Intake location
 Behavioral technologies (e.g., light or sound barriers) \82\
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    \82\ For example, anadromous clupieds such as alewife, blueback
herring, and American shad have demonstrated avoidance behaviors
when exposed to high frequency sound. Deployments of this technology
at Entergy's FitzPatrick Nuclear Station on Lake Ontario have
resulted in a reduction of over 90 percent in impingement of
alewife. In this case, EPA expects the Director would determine that
impingement requirements regarding alewife have been addressed by
the acoustical deterrent. The Director could disallow such a
technology if it were deemed to have a negative effect on threatened
or endangered species whose habitat includes the facility's intake
location.

    In each case, the technology employed reduces the number of
organisms that potentially are impinged, resulting in a reduction in
the number of organisms actually impinged (i.e., a reduction in the
rate of impingement). By virtue of reducing the actual impingement,
mortality caused by impingement is no longer a consideration--an
organism that is never impinged cannot be killed by the intake
structure. Some technologies work to reduce the intake flow, thereby
reducing the potential organisms exposed to the intake. Others work to
divert organisms away from the screens, either through a physical
exclusion or by being placed in a less biologically productive area.
EPA concluded that it is appropriate to recognize these reductions in
impingement as a step in achieving a BTA impingement mortality
reduction performance at a particular site. As a result, EPA expects
the reduction in impingement will be treated as an equivalent reduction
in impingement mortality, and will therefore be considered by EPA or
the State NPDES permitting authority in evaluating whether the chosen
technologies and operational measures represent BTA performance under
the site's conditions. For example, an intake that operates
infrequently due to the infrequent operation of the electric generating
unit(s) it serves (such as a peaking unit) may use a relatively small
amount of water on an annual basis when compared to the design capacity
of the intake structure. This facility may choose to comply with the
impingement mortality standard at Sec.  125.94(c)(6) by demonstrating
to the Director that the facility operates at an annual intake flow
that is less than or equal to 24 percent of its design intake flow on
an annual basis. This level of flow reduction could achieve a level of
performance equivalent to or better than the impingement mortality
performance standard in Sec.  125.94(c)(7), and therefore could be
considered to be compliant with the requirements of today's final rule.
This demonstration may include design data, several years of past
operating data, and dispatch modeling. These operating conditions would
then be incorporated into the NPDES permit.
    A facility complying under this part, must submit a impingement
technology performance optimization study, which must include the
calculated percent impingement mortality reflecting optimized operation
of the system of technologies, operational measures, and best
management practices and all supporting calculations. Total system
performance is the combination of impingement mortality performance
reflected in all of the following which apply:
     Rate of impingement--The estimated reductions in rate of
impingement must be based on a comparison of the system to a once-
through cooling system with a traveling screen whose point of
withdrawal from the surface water source is located at the shoreline of
the source waterbody. For impoundments that include waters of the
United States, the facility's rate of impingement must be measured at a
location within the cooling water intake system that the Director deems
appropriate.
     Impingement mortality--If the demonstration relies in part
on a credit for reductions in impingement mortality already obtained at
the facility, two years of biological data collection must be provided,
demonstrating the level of impingement mortality the system is capable
of achieving.
     Flow reduction--If the demonstration relies in part on
flow reduction to reduce impingement, the data must include two years
of intake flows, measured daily, as part of the demonstration. This
must include documentation of how the flow reduction results in reduced
impingement.
    The permitting authorities would consider this information shown in
the two-year impingement technology performance optimization study that
must be submitted under this alternative. For example, at facilities
choosing to comply by demonstrating that they are operating below 24
percent of their intake capacity, or that they are peaking units, the
Director should use this study to establish operating conditions that
ensure that the intake continues to operate below 24 percent of its
intake capacity or continues to serve only peaking units and that these
units are not later used as intermediate or baseload units. The
operating conditions and parameters identified in the study will then
be incorporated in the facility's permit conditions. EPA estimates that
approximately 17 percent

of intake structures will choose this alternative.
f. Comply With the Numeric Impingement Mortality Performance Standard
    Facilities complying with the BTA impingement mortality standard by
achieving the numeric performance standard at Sec.  125.94(c)(7) will
perform monthly compliance monitoring to verify that the 12 month
percent impingement mortality resulting from operation of its intake is
below the standard established in today's final rule. (For more details
on complying with the impingement requirements, see Section VIII.) EPA
expects that, save for future technologies or innovations, few
facilities will avail themselves of this option.
2. Entrainment Controls for Existing Units at Existing Facilities
    The BTA entrainment standard for the final rule establishes a
framework under which EPA or the State NPDES permitting authority must
establish site-specific BTA entrainment requirements for each facility
in the scope of today's rule. EPA considered promulgating no further
controls to address entrainment mortality, and to rely instead only on
the BTA impingement mortality controls, which would achieve up to a 34
percent reduction in total AEI. EPA did not select this option as the
basis for national BTA because, in EPA's view, some facilities either
are having a significant impact as a result of entrainment or might be
able to do more to control entrainment at costs that are low relative
to benefits. In addition, EPA's data on entrainment at facilities are
not sufficient to allow the Agency to categorize facilities requiring
no additional controls for entrainment. Thus, the final rule by
requiring prescribed information in the permit application will provide
the Director with adequate information for decision making. Requiring a
structured site-specific analysis of candidate BTA technologies for
entrainment control will allow the Director to determine where it is
appropriate to require such controls. One outcome of the site-specific
analysis could be that the Director would determine that no other
technologies beyond impingement controls are required for BTA
entrainment reductions, either because they are not feasible or because
the social costs of additional control measures are not justified by
the social benefits.
    In the case of site-specific entrainment controls for facilities
withdrawing greater than 125 mgd AIF, the final rule requires
facilities to also develop and submit an Entrainment Characterization
Study and related supporting information, as described in Sec.
122.21(r)(9)-(13) of today's rule, for use by the Director in
establishing site-specific BTA. For facilities above 125 mgd AIF that
also meet the definition of closed-cycle recirculating systems at Sec.
125.92(c), the Director may reduce or waive some or all of this
information.
    EPA considered simply requiring this information of all facilities
above 125 mgd AIF without authorizing Directors to reduce or waive this
information. However, EPA also recognizes that, in some instances,
these same facilities have already minimized adverse environmental
impacts significantly. In such cases, there may be limited value to the
Director requiring a full benefit-cost analysis, or even obtaining the
Entrainment Characterization Study at Sec.  122.21(r)(9).
    EPA also considered not requiring this information of any
facilities above 125 mgd AIF meeting the definition at Sec.  125.92(c).
First, EPA noted that even though these facilities meet the definition
of a closed-cycle recirculating system, they may still withdraw at
least 125 mgd, and in some instances withdraw considerably more than
125 mgd. This is not an insubstantial volume of water withdrawn for
cooling, and in the case of inland waters this withdrawal may comprise
a large proportion of that source waterbody. In addition to withdrawing
large volumes of water, EPA recognizes that some facilities,
particularly those meeting the definition at Sec.  125.92(c)(2),
potentially withdraw water at a rate similar to a once-through facility
not withdrawing from an impoundment, with the potential to cause
adverse environmental impacts similar to those of once-through cooling.
The Director may find the information in Sec.  122.21(r)(9)-(13) to be
useful in determining whether additional controls are warranted. In
these instances, the Director may decide to require the Entrainment
Characterization Study at Sec.  122.21(r)(9) first, in order to
determine if other studies in Sec.  122.21(r)(10) to (13) are also
warranted.
    Facilities at or under the 125 mgd AIF threshold must still provide
certain information under the permit application requirements at Sec.
122.21(r). The Director may require additional information from these
facilities including some or all of the studies at Sec.  122.21(r)(9)-
(13) if there is reasonable concern regarding entrainment impacts at
the facility. Where an owner or operator of a facility intends to
comply with the BTA standards for entrainment using a closed-cycle
recirculating system as defined in Sec.  125.92(c), the Director may
reduce or waive some or all of this information.
    Facilities with a closed-cycle recirculating system as defined at
Sec.  125.92(c)(2) would still submit the studies at Sec.
122.21(r)(9)-(13) if they withdraw greater than 125 mgd AIF, and if the
Director has not waived the requirements. These facilities have cooling
systems that include impoundments of waters of the U.S. where the
impoundment(s) was constructed prior to October 14, 2014 and lawfully
created for the purpose of serving as part of the cooling water system.