Document ID: EPA-HQ-OW-2008-0667-1260
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2011-04-20T04:00Z

Wedgewire Screen Larval Entrainment Reductions at Eddystone Generating
Station 

Apr 2005 – Apr 2006

A recent (20 April 2005 through 28 March 2006) full-scale evaluation of
6.35 mm cylindrical wedgewire screens was performed at the Eddystone
Generating Station located on the Delaware River Estuary in Pennsylvania
(Veritas et al. 2008).  This evaluation supports the applicability of
the wedgewire performance curve to full scale screens of similar design
being considered for IPEC.    The original cooling water intake
structure for Eddystone Units 1 and 2 was located in a shoreline
bulkhead, and was equipped with conventional 9.5 mm (3/8-inch) mesh
traveling screens with no design or operational modifications to reduce
impingement or entrainment.  In the early 1990s, the intake for Units 1
and 2 was modified by removing the traveling screens and installing
sixteen narrow slot cylindrical wedgewire T-type screens (eight per
unit).  Each wedgewire screen is 72-inch in diameter with an effective
length of 144 inches (actual overall length is 240 inches including the
nose cones) and is equipped with 6.35 mm (0.25 inch) wedgewire screening
with the slots oriented radially around each screen and the long axis of
the cylinder aligned parallel to the sweeping flow.  The average
through-screen velocity is approximately 0.4 feet per second (fps). 

Units 3 and 4 at Eddystone were built after Units 1 and 2, and the
cooling water intake structure design included several structural
modifications specifically to reduce impingement and entrainment. Units
3 and 4 employ conventional 9.5 mm (3/8-inch) mesh traveling screens,
with a through-screen velocity of approximately 0.88 fps at design flow
and mean low water level.  

Larvae of tessellated darter (32%), American shad (13%), Cyprinidae
(9%), striped bass (9%), white perch (7%),  Morone spp. (55%), and
American eel (5%) were the predominant fish taxa observed in the
entrainment samples collected during a year-long  entrainment study
performed  from April 2005 through March 2006 at Eddystone Units 1 and
2, with the peak period of entrainment abundance occurring from early
May through mid-July  (Table 2-5 in Veritas et al. 2008).  The 6.35 mm
slot width cylindrical wedgewire screens installed and operated at Units
1 and 2 of Eddystone Generating Station were estimated to reduce total
annual entrainment of fish eggs and larvae combined by 60% from
baseline, with a 95% confidence interval of 48 to 71% (Table 5.2,
Veritas et al. 2008).  The 60% reduction in entrainment estimated for
the 6.35 mm CWW screens installed and operated at Eddystone Units 1 and
2 is a unique and site-specific application of these CWW screens in
combination with the actual flow reductions and other operational
controls unique to that site. 

The entrainment reduction performance of the 6.35 mm CWW screens
installed at Eddystone Units 1 and 2 that is directly applicable to
estimate performance at other facilities of different intake flows and
operational procedures is based on the difference in density of fish
eggs and larvae between simultaneously collected pairs of 2-hour
entrainment samples.  Only 10 eggs were collected throughout the
year-long study (5 American shad, 1 tesselated darter, 1 white perch and
2 striped bass), and 867 larvae were collected, so reduction
calculations for this paired comparison were based solely on larvae
(Table 1).  In 374 pairs of simultaneously collected 2-hour entrainment
samples collected from 20 April 2005 through 5 April 2006 at Eddystone,
the mean density of larvae entrained through the 6.35 mm slot width CWW
screens at the Unit 1 and 2 CWIS was 82% lower than the mean density of
larvae entrained simultaneously through conventional 9.5 mm (3/8-inch)
mesh screens at the adjacent Unit 3 and 4 CWIS (Table 1).  These results
from Eddystone demonstrate that a high percentage of larvae can avoid
entrainment through full-scale cylindrical wedgewire screens operating
under estuarine field conditions with a similar species composition as
IPEC, even at a slot width of 6.35 mm.  

Table 1.  Comparison of the number and density of fish eggs and larvae
(all taxa combined) collected in 374 pairs of simultaneously collected
2-hour entrainment samples taken at the Eddystone Unit 1 and 2 bulkhead
and the Unit 3 and 4 bulkhead during 20 April 2005 through 5 April 2006.

Life Stage	Wedgewire Screens at Unit 1 and 2	Conventional Screens at
Units 3 and 4	

Percent Reduction for WWS

	

n	Mean Density

(n/100 m3)	

n	Mean Density

(n/100 m3)

	Egg	7	0.81	3	0.68	N/A

Larvae	221	0.55	646	3.06	82%

Reference	

Veritas Economic Consulting, Normandeau Associates Inc., and ARCADIS
(Veritas et al.)   December 2008.  Development of annual current and
baseline impingement mortality and entrainment estimates.  Eddystone
Generating Station.  Eddystone, PA.  Prepared for Excelon Generation
Company, LLC