Site Visit Report

	Potomac Generating Station

1400 North Royal Street

Alexandria, Virginia

	December 3, 2007

Background and Objectives

The Environmental Protection Agency (EPA) is in the process of
developing 316(b) cooling water intake structure requirements that
reflect the best technology available (BTA) for minimizing adverse
environmental impact for all existing power plants and manufacturing
facilities. As part of this process, EPA staff is visiting electric
generators and manufacturers to better understand the cooling water
intake structure (CWIS) technologies in use at facilities, including the
site-specific characteristics of each facility and how these affect the
selection and performance of CWIS technologies.  EPA is also visiting
facilities to better understand cooling water use and specific issues or
technologies that can affect 316(b) compliance.  Potomac Generating
Station (Potomac) was selected for a site visit due to its testing of a
new type of intake screen (Geiger screen) and its proximity to the
Washington, D.C. area.

Facility Description

Potomac is owned by the Mirant Corporation and employs approximately 130
people.  The facility is located on 25 acres in Alexandria, Virginia and
is bounded on all sides—the Potomac River (and a National Park Service
walking path), city streets, a railroad, and nearby housing
developments.  The facility began operations in 1949 and withdraws
cooling water from the Potomac River.

Potomac’s NPDES permit is DC0022004.  The permit expired on April 19,
2005.  A draft permit was issued in May 2007 and requires the submittal
of information similar to that required by the now-suspended Phase II
rule’s Comprehensive Demonstration Study (CDS) during the permit term
to provide data for a 316(b) best professional judgment (BPJ)-based
analysis.  The permit also offers three compliance alternatives for the
facility to become compliant.

Electricity Generation and Transmission

	

Potomac has 5 coal-fired generating units which have a combined
generating capacity of 482 MW.  Units 1 and 2 were installed in 1949 and
1950 and are cycling units, generating 88 MW each.  Units 3, 4, and 5
are c baseload units and were installed between 1954 and 1957,
generating 102 MW each.  

PEPCO owns the switchyard at the facility and operates two large
capacity power lines and 16 smaller ones that serve Maryland and the
District of Columbia.  The transmission system in the Washington, D.C.
area is a “spoke and wheel” configuration, where a ring of
transmission lines circles the city with inward “spokes” providing
electricity to the various sections.  Potomac is located near the
interior terminus of one of the spokes, which elevates its importance to
maintaining reliability (i.e., if the transmission lines outside the
spoke from the plant are disrupted, the plant is the only source of
electricity to parts of the District of Columbia).  The facility is
currently operating below its planned capacity, as an emissions study is
being conducted to support Mirant’s application to merge the stacks at
the facility.  The facility hopes to begin construction soon.

Approximately 6% of the power generated by the facility is reclaimed to
operate the plant.  Facility representatives also noted that the
facility is nearing (or may have reached) the capacity of its internal
electrical infrastructure, making the addition of any new processes at
the plant difficult.

Cooling Water Intake Structure

Potomac withdraws water via a shoreline CWIS in a small cove on the
western bank of the Potomac River.  The CWIS has 5 intake bays that
supply cooling and service water to the facility.  All of the generating
units employ once-through cooling, with two pumps per intake (each rated
at 30,000 gpm), and a design delta T of 10-15 degrees at the discharge
point.  No biocides are in use at the facility.  

The total design intake flow (DIF) is 432 million gallons per day (mgd).
 Average intake flows have historically been high (even at or near the
design values), but have decreased in recent years due to a reduction in
the generation at the facility.

The first layer of screening at the CWIS is provided by a bar rack,
which excludes large debris.  The next layer of screening is provided by
Geiger screens.  The facility previously used single-entry, single-exit
traveling screens, but installed Geiger screens on all CWISs in 2004 at
an approximate cost of $3 million.  Potomac selected the Geiger screen
design to reduce the debris carryover experienced by some vertical
traveling screens.  The new screens (mesh size of 3/8”) have virtually
eliminated debris clogging in the condenser.  

In addition, the screen for Unit 1 is equipped with fish buckets, a low
pressure spray wash, and the ability to add a fish return.

Under the previous screen configuration, the design intake velocity for
the CWIS was 1.52 feet per second.

 

A representative from the Electric Power Research Institute (EPRI) added
that it is currently conducting a study of fine mesh Geiger screens at
another location.  

Impingement and Entrainment Information

At the time of the site visit, no information on the effectiveness of
the Geiger screens in reducing impingement or entrainment was publicly
available.  Potomac recently conducted two years of monitoring of the
fish return system for Unit 1 to evaluate its effectiveness in reducing
fish impingement.  Facility representatives stated that impingement
events are historically rare and are generally linked to storms, as
organisms retreat to the shallower waters near the intake.  As such, the
facility did not experience significant impingement problems before
installing the new screens, making analysis of the test results
difficult to interpret.  Nevertheless, the facility estimates they have
enough data for statistically valid results for approximately a half
dozen species that show impingement reductions ranging from 20% to 80%. 
These results were presented at a recent conference of the American
Fisheries Society. 

The facility also considered other technologies for reducing impingement
and entrainment.  Preliminary designs for a Gunderboom estimated a ¾
mile long barrier, which would result in excessive maintenance and
unacceptable interference with existing uses of the river.  Potomac also
considered cylindrical wedgewire screens, but concluded that due to the
relatively shallow depth of the river, a large number of small screens
would be required to meet the facility’s cooling water needs.  These
screens would need to be located in the deeper portion of the river (to
ensure sufficient counterflow currents to remove materials from the
screen surface), which is on the opposite side from the facility,
resulting in long runs of piping and potential conflicts with ongoing
dredging activities in that area.  A representative from EPRI noted that
the preliminary results of a joint test with Alden Laboratories on fine
mesh wedgewire screen use at Potomac was presented at a recent
conference of the American Fisheries Society and that the final results
should be completed by the end of 2008.  EPRI also studied barrier nets
at approximately 15 locations, noting that each facility achieved 80-90%
reductions in impingement, but that problems with debris and hydrilla
would limit the effectiveness of a net at Potomac.

To comply with the now-suspended Phase II rule, the facility stated that
it had intended to demonstrate that the Geiger screens met the
impingement requirements and would have conducted entrainment monitoring
to determine an appropriately-scaled restoration project to meet the
entrainment requirements of the rule.

Cooling Tower Feasibility

Facility representatives stated that cooling towers have not been
extensively considered.  They added that the site is quite small and
nearly all of the existing space is currently in use for critical
activities.  Facility representatives also noted that the facility is
surrounded by land uses that would be problematic for either
constructing towers or for acquiring additional land.  Potomac is also
located approximately 2 miles south of Ronald Reagan Washington National
Airport.

Attachments

Attachment A		List of Attendees

Attachment B		Aerial Photo

Attachment A--List of Attendees

Paul Shriner, EPA

Jan Matuszko, EPA

Josh Hall, EPA

Ron Jordan, EPA

Erik Helm, EPA

Paul Balserak, EPA

Mary Smith, EPA

Richard Witt, EPA

Suzanne Rudzinski, EPA

Julie Hewitt, EPA

Kelly Meadows, Tetra Tech

John Sunda, SAIC

Mike Stumpf, Mirant

Debra Knight, Mirant

Ann Wearmouth, Mirant

Mark Nitz, Mirant

Dave Bailey, EPRI

Also see DCN 10-6512A for an electronic copy of the sign-in sheet.

Attachment B—Aerial Photo

Please see DCN 10-6512B accompanying this document.

 However, due to high suspended sediment loads in the source water, the
facility still regularly shuts down to remove sediment buildup in the
condenser tubes.

 In the current configuration, any fish removed from any of the
facility’s screens would be discharged into the same trough as all
other debris; there is no separate fish return trough for re-routing
impinged organisms.  

 As noted in Section 2.0, the facility is bordered by high-rise
residential buildings to its north, a major city street to its west, and
a rail line to its south.  The National Park Service owns the shoreline,
which operates a frequently used walking path along the river.

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