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Survival of Fish Impinged on a Rotary Disk Screen
DANIEL L. BIGBEE,* RONALD G. KING, AND KENT M. DIXON
EA Engineering, Science, and Technology, Inc., 221 Sun Valley Boulevard, Suite D,
Lincoln, Nebraska 68528, USA
DOUGLAS A. DIXON
Electric Power Research Institute, 3420 Hillview Avenue, Palo Alto, California 94304, USA
ELGIN S. PERRY
2000 Kings Landing Road, Huntingtown, Maryland 20639, USA
Abstract. —An impingement survival study was conducted to determine 48-h survival of fish impinged on a
modified rotary disk screen equipped with fish protection features. The rotary disk screen was installed for a
technology evaluation at the cooling-water intake structure of the North Omaha Station located on theMissouri River in Omaha, Nebraska. Hatchery-raised fish and native fish collected from the Missouri River
were released in batches into the rotary disk screen bay and collected with a system that was constructed to
recover fish from the screen’s vacuum system. That system was designed to remove fish from the rotary disk
screen and return them to the river. Screen performance was assessed in April and August, representing spring
and summer environmental conditions. The 48-h survival rates of hatchery-raised fathead minnow
Pimephales promelas, channel catfish Ictalurus punctatus, and bluegills Lepomis macrochirus and a group
of mixed native species approached 100%. Survival rates were not statistically different between test groups
and controls, indicating that impingement did not contribute to the observed mortality. High survival rates of
impinged fish removed from the screen indicated that the rotary disk screen would reduce impingement losses
at the North Omaha Station, where losses due to impingement on the existing vertical traveling screens are
assumed by the U.S. Environmental Protection Agency to approach 100% because the screens lack fish
protection features. Our study results suggest that the rotary disk screen tested could be considered analternative technology under section 316(b) of the Clean Water Act, which requires power plants to install the
best technology available to reduce impingement. Use of hatchery-reared fish and native fish collected from
the river assured that an adequate number of fish were tested to provide statistically reliable results and
allowed the use of controls to account for mortality due to handling stresses experienced by test fish.
The use of surface waters for cooling water at
conventional power plants results in the impingement
of aquatic organisms at cooling water intake structures
that are screened to limit the size of particles passing
through condenser systems. The primary objective of
our study was to evaluate survival of impinged fish
removed by a rotary disk screen installed for evaluation
at Omaha Public Power District’s North Omaha Station
(NOS; Figures 1, 2) as an alternative technology to the
conventional vertical traveling screens used at the
facility. The Clean Water Act (CWA; CWA 1972)
requires that ‘‘the location, design, construction, and
capacity of cooling water intake structures reflect the
best technology available [BTA] for minimizing
adverse environmental impact.’’ Our study was planned
and implemented based on the U.S. Environmental
Protection Agency’s (EPA) Phase II Rule (U.S. Office
of the Federal Register 2004) and preceded the U.S.
Second Circuit Court of Appeals remanding of several
provisions on 25 January 2007 and the EPA’s
subsequent suspension of the rule in July 2007 (U.S.
Office of the Federal Register 2007). Until a new rule
is promulgated, states and EPA regions have reverted
to administration of section 316(b) on a ‘‘best
professional judgment ’’ basis. Under the Phase II Rule,
the EPA required reducing impingement losses by 80–
95% relative to a baseline. The baseline at NOS would
represent 100% mortality of the fish impinged on the
conventional vertical traveling screens used at the
facility because the cooling water intake structure does
not provide fish protection features.
A modified rotary disk screen manufactured by
Beaudrey USA was selected for evaluation by the
Omaha Public Power District as a potential engineering
option for meeting the section 316(b) BTA require-
ments. The modified rotary disk screen provides fish
protection and was expected to alleviate maintenance
issues associated with debris carryover, bearing wear,
* Corresponding author: dbigbee@eaest.com
Received April 17, 2009; accepted August 13, 2010Published online December 13, 2010
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North American Journal of Fisheries Management 30:1420–1433, 2010Ó Copyright by the American Fisheries Society 2010DOI: 10.1577/M09-059.1
[Article]
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and plugged spray wash nozzles associated with the
existing vertical traveling screens, which meet EPA’s
definition of a baseline intake screening system that
does not provide fish protection features (e.g., fishbuckets, low-pressure screen washes, or a fish return
system). The existing vertical traveling screens at NOS
have 9.5-mm mesh and are equipped with pressure
spray washes (4.9–7.0 kg/cm2
) to remove accumulated
debris and impinged fish. These screens are typically
operated intermittently, based on pressure differentials
across the screens.
The modified rotary disk screen can be installed in
intake bays without the need for civil works, does not
require a pressurized screen wash system, and
eliminates debris carryover associated with verticaltraveling screens. Excessive maintenance of the
existing spray wash at the NOS was necessary because
the sediment load in the Missouri River consists
primarily of sand, which causes excessive wear to
spray pumps and spray nozzles. In addition, the
modified rotary disk screen has only one bearing at
the center of the wheel; this was expected to eliminate
wear of the pin joints connecting the vertical travelingscreens, which are pinned at the corners of each screen
panel. These pin joints wear due to sand in the source
water and eventually causes the chain to lose tension
leading to additional wear to other screen parts,
including the upper and lower sprockets on which the
screens are rotated.
The traditional rotary disk screen consists of a flat
disk (wheel) covered with screening material that
rotates on a horizontal axis perpendicular to the water
flow. As water flows through the submerged portion of
the disk, impinged organisms and debris are retainedon the screen. That material is removed by a spray
wash system when the disk rotates above the water
line. The modified rotary disk screen installed at NOS
consists of one rotating wheel that rotates within a
FIGURE 1.—Location of the North Omaha Station, where a rotary disk screen was installed for a technology evaluation at the
cooling water intake structure on the Missouri River.
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frame at 2 revolutions/min and provides a fish
protection system. The modified rotary disk screen
system consists of a 2.438-m rotating screen wheel and
backwash pump (Figures 3, 4). The screen openings on
the modified rotary disk screen are 6.1 mm
2
, comparedwith 9.5-mm screen openings for the existing vertical
traveling screens (i.e., 36% larger). A hydraulic pump
and drive motor rotate the screen, which is divided into
pie-shaped sections by plates on the inlet side of the
screen. Impinged fish and debris are vacuumed from
the plates as they rotate under a stationary scoop
mounted over one section of the rotary disk screen. A
Hidrostal screw-centrifugal backwash pump used by
fish hatcheries to transfer live fish is used to vacuum
the screen. The backwash pump impeller is designed to
separate the water into pockets that provide a safe place
for the fish as they move through the pump. The
backwash pump causes a backwash flow that removes
fish, sand, and debris off the screen wheel. During
operation, the disk rotates at a constant 2 revolutions/
min that limits retention time on the screen to 30 s or
less, depending on position of the impinged object on
the screen. The angular velocity of the screen is 0.21
rad/s, so at the outside diameter the screen would be
moving at about 0.256 m/s. The backwash pump has a
variable speed drive that maintains a constant flow rate
through the pump in response to river stage and allows
the pump to remain at peak operating efficiency.
North Omaha Station is a coal-fired generating
station located in Omaha, Nebraska, on the bank of the
Missouri River at river kilometer 1,006 (Figure 1). It
has five generating units with a combined capacity of
663 MW that utilizes a once-through water cooling
system. Cooling water is withdrawn from the MissouriRiver through three onshore cooling water intake
structures. The study was performed at intake 3, which
has three circulating water pumps that provide
condenser cooling water for unit 5 (Figure 2). There
are two sets of trash racks, inlet bays, sluice gates, and
vertical traveling screens for each circulating water
pump. One of the six upstream vertical traveling
screens at intake 3 was replaced with a modified rotary
disk screen to evaluate its effectiveness (Figure 2).
Impingement survival studies have been conducted
since the mid-1970s in response to requirements of
section 316(b). Although study results have varied
widely, both among species and among the screen
systems tested, they have shown species-dependent
impingement survival rates of 70–80% or higher at
facilities with adequate screen design and operation
(EPRI 2003). That report reviewed 71 studies at 35
power plants with survival data for angled, dual-flow,
and single-flow traveling screens, most of which
evaluated modifications to conventional screen designs
(e.g., vertical traveling screens with the addition of
Ristroph-type modifications) or operational changes
(e.g., modifying screen wash operation from intermit-
tent to continuous) to improve impingement survival.
FIGURE 2.—Schematic layout of intake 3 at the North Omaha Station, showing the locations of the rotary screen, collection
tank, acclimation tanks, and control and test tanks.
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These studies generally showed that screen wash
frequency, screen travel time, screen modifications
for separating fish, and debris handling were important
factors influencing survival rates. Survival increased
with decreased time between screen washes, continu-
ous screen rotation providing the highest survival rates
(King et al. 1978; Tatham et al. 1978). Several studies
have shown that survival rates increase at higher screen
speeds because the faster screen travel time, the less
time impinged organism are retained on the screens
(Beak 1988).
Studies of the physical modifications to the rotary
disk screen evaluated to protect fish have not been
previously conducted. Results from field, laboratory,
prototype, and full-scale pilot studies of other screening
systems have shown that survival of impinged fish
depends on the species and site-specific factors (EPRI
2003), making it difficult to estimate survival of
untested screen designs. The primary objective of the
survival study at the NOS was to evaluate survival of
impinged fish removed by the rotary disk screen and its
ability to reduce impingement losses relative to the
existing vertical traveling screens, which do not
provide fish protection. The use of both hatchery-
raised and wild, native fish allowed separation of
handling and holding effects from impingement effectson survival through the use of controls, an approach
that also yielded adequate sample sizes amenable to
valid statistical analyses of the test results.
Methods
The study consisted of three components: a pilot
study, two impingement survival tests, and a collection
efficiency study. The pilot study was conducted to (1)
develop a method for introducing fish into the rotary
disk screen bay that would minimize handling stress,
(2) determine safe procedures for personnel while
performing study tasks, and (3) determine the recovery
rate of fish so that the number of released fish could be
estimated to yield statistically valid results. Impinge-
FIGURE 3.—Front view of the rotary screen installed in
intake 3 at the North Omaha Station, showing the hydraulic
pump, rotating screen wheel, and stationary scoop. FIGURE 4.—Side view of the rotary screen installed in intake
3 at the North Omaha Station, showing the hydraulic pump,
drive motor, rotating screen wheel, and backwash pump.
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ment survival tests were conducted during the weeks of
28 April and 11 August 2008 to represent spring and
summer conditions when impinged fish are most
stressed due to warm water temperatures and lower
dissolved oxygen (DO). The collection efficiency studywas conducted to evaluate differences in recovery rates
observed during the pilot study and impingement
survival tests.
Source of test fish.—Our original intent was to use
wild, resident fish impinged during scheduled im-
pingement monitoring events as the test fish for the
impingement survival study; however, an impingement
monitoring study from April 2007 to July 2008 (D. L.
Bigbee, unpublished) reported impingement rates of
about 1.0 fish/h on the rotary disk screen, which were
too low to provide a statistically adequate number of fish for testing of survival following impingement.
Those low rates necessitated the use of hatchery-raised
and wild, native fish collected from the Missouri River
rather than using fish impinged during normal screen
operations. That approach provided statistically ade-
quate sample sizes and increased the ratio of
information to effort. We selected three hatchery-raised
species native to the Missouri River and readily
available from a local hatchery for testing: fathead
minnow Pimephales promelas, channel catfish Ictalu-
rus punctatus, and bluegills Lepomis macrochirus.These species were selected because they were
impinged on the rotary disk screen during the
impingement monitoring study (Bigbee, unpublished).
The selected species were purchased from a licensed
Nebraska hatchery and transported to NOS in an
aerated tank. Upon delivery of fish, Missouri River
source water was slowly mixed with hatchery water
until the water temperature equalized. Hatchery fish
were acclimated in three 750-L aerated tanks with a 4-h
exchange rate for 48 h prior to introduction to the
rotary disk screen bay. Only fish exhibiting normal
swimming behavior and free of external abnormalities
(e.g., signs of infection, wounds, or loss of scales) at
the end of the 48-h acclimation period were tested.
For the August tests, wild, native fish were collected
along the shoreline of the Missouri River near NOS via
seining (6.1 m, 4.8-mm mesh) in shallow areas with
firm substrate and low water velocity. Fish were
transferred from the seine to a holding tank and
subsequently transferred to an acclimation tank at
intake 3 near the rotary disk screen bay (Figure 2).
Native fish were not segregated by species prior to
testing to minimize handling stress and were observed
for 24 h prior to introduction to the rotary disk screen
bay. Only fish exhibiting normal swimming behavior
and free of external abnormalities (e.g., signs of
infection, wounds, or loss of scales) were introduced
into the rotary disk screen bay.
Pilot study.—Bluegills and channel catfish were
introduced as separate 500-fish batches into the rotary
disk screen bay during the pilot study by lowering thefish into the screen bay. A tether was used to invert the
buckets so that fish were released below the water
surface. The rotary disk screen was shut down at 30-
min, 60-min, and 120-min following release of each
species in order to remove recovered fish from the
collection tank. Released bluegills averaged 52 mm
total length (TL) and channel catfish averaged 76 mm
TL. Recovery results from the pilot study were used to
estimate the number of test fish to release for the
impingement survival tests with the goal of recovering
50 fish for each survival test.
Impingement survival tests.—The impingement sur-
vival study consisted of 48-h tests conducted in April
and August 2008. Fish that were stunned and failed to
recover upon recovery from the collection tank were
considered dead, as were damaged fish. This assump-
tion potentially underestimated survival rates; however,
only 7.0% of 1,134 test fish were dead or died during
the impingement survival tests, indicating few fish
were stunned or damaged. Only minor damage was
observed on fish classified as alive, which was
consistent with results from the NOS impingement
monitoring study classifying that 2.4% of the fishremoved from the rotary disk screen were damaged
(Bigbee, unpublished). Acclimation of both hatchery-
raised and wild, native fish captured for this study
reduced the likelihood of diseased or injured fish
influencing survival through the selection of healthy
fish.
Collection efficiency study.—Tests using live and
freshly killed fish were conducted to evaluate the fate
of fish released into the rotary disk screen bay that were
not recovered. Those tests compared recovery rates
between live fish that could actively avoid the rotarydisk screen and freshly killed fish that were passively
impinged. The same hatchery-raised species used
during the pilot study and impingement survival tests
(fathead minnow, bluegills, and channel catfish) were
used for the collection efficiency study. Fresh-killed
fish received five different fin clips (anal fin, dorsal fin,
left pectoral, right pectoral, and the bottom portion of
the tail fin) that allowed tracking of the released
batches. One group of live fish with anal fin clips was
also introduced so that comparisons could be made
between percent recovery of freshly killed and live fish.Marked fish were introduced to the rotary disk screen
bay at 15-min intervals and were collected from the
screen and processed after each 15-min interval. Live
test fish were enumerated on-site, and dead fish were
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taken to the laboratory for processing. The collection
efficiency test was terminated 105 min after fish were
first released.
Collection and test tanks.—A collection tank (2.443
1.98 m, 0.67 m deep) was constructed to receive fishremoved from the rotary disk screen during the pilot
study, survival tests, and collection efficiency study.
The collection tank contained a screen in front of a weir
upstream from the outlet. The weir served to reduce
velocity through the screen to less than 0.15 m/s.
Screen openings in front of the weir matched the 6.1-
mm2 mesh of the rotary disk screen. The collection tank
had a bypass outlet with a removable screen and gate.
Test tanks for bluegills, fathead minnow, and native
fish were 80 L, whereas 120-L tanks were used for
channel catfish (only used for the impingement
survival tests). All test tanks were continuously
supplied with source water from the Missouri River
and monitored during each event to assure that
environmental conditions were adequate to support
the species used in the tests and to minimize external
factors that could affect survival. Water temperature
and dissolved oxygen (DO) was monitored throughout
each 48-h test. Test date and time, species, the number
of fish placed in each tank were recorded. Water
temperature, DO, and flow were recorded at the
beginning of each test. Observation times, water
quality readings, and flow to the tanks were recordedevery 8 h, along with general observations of the
aeration system. Dead fish (those with no opercular
movement and no response to stimuli) were removed at
8-h intervals.
The test tanks included two control groups and four
test groups for each species. The test tanks, which were
set near the collection tank (Figure 2), held approxi-
mately 50 fish, with a goal of limiting fish biomass to
less than 5 g/L of source water to prevent overcrowd-
ing and to ensure adequate DO levels. The test tanks
were conditioned and water temperatures stabilized byregulating source water flow at an exchange rate of at
least one volume every 4 h. Each test tank had an air
stone as a DO source.
Introduction of fish.—Hatchery-raised fish for the
pilot study, two impingement survival tests, and a
collection efficiency study were introduced to the
rotary disk screen bay one species at a time after
grouping the test species fish in a 19-L bucket. The
bucket was attached to a rope and lowered through an
open floor grate. A tether attached to the bottom of the
bucket was used to invert the bucket so that fish werereleased below the water surface. Native fish collected
from the Missouri River for the August impingement
survival test were similarly introduced, but as a batch
of mixed species in order to reduce handling stresses
that would have occurred if the fish had been separated
by species. Impingement rates at the NOS were low
(Bigbee, unpublished), and we assumed that only
released fish were recovered. That assumption was
supported by the fact that only the three hatchery-raisedspecies were collected during each impingement
survival test.
Upon initiation of each test, the rotary disk screen
was shut off and the gate to the bypass outlet was
removed to allow the collection tank to drain. Once the
collection tank was drained, a net was used to remove
fish and debris that had accumulated in the collection
tank. The gate was replaced and the rotary disk screen
restarted. A net was placed in front of the inlet for 2
min, allowing the screen to make four revolutions so
that fish or debris that may have accumulated on the
screen when it was shut down could be removed and
discarded. The collection tank was monitored to
prevent debris from accumulating on the collection
tank screen.
Removal of fish from collection tank .—Fish recov-
ered during the pilot study, survival tests, and
collection efficiency study were removed from the
collection tank with a net that was swept through the
collection tank. The net was kept submerged while fish
were removed with a dip cup. After multiple net
sweeps yielded no fish, screens were placed in front of
both the inlet and bypass outlet and the rotary diskscreen was shut off. The gate to the bypass outlet was
then removed to lower the water level in the collection
tank to approximately 10 cm, at which point remaining
fish were recovered. A dip-cup was used to keep the
fish submerged at all times to reduce stress while
transferring fish from the collection tank to the test
tanks (Figure 2).
Sample processing.—Control and test tanks for the
impingement survival tests were observed approxi-
mately every 4 h during the 48-h impingement survival
tests. During each observation, dead specimens wereremoved, identified, weighed, and measured. At the
conclusion of the 48-h tests, a small submersible pump
was used to withdraw water from the test tanks. When
approximately 4 L of water remained in the test tanks,
all fish were transferred to a smaller bucket for a final
count. The number of surviving fish was then recorded.
Twenty specimens of each species were weighed and
measured (TL) to obtain an average size of the test
population. Counts and batch weights of the remaining
test fish were recorded for the subsequent counts.
Native species that could not be easily identified werepreserved for identification in the laboratory.
Controls.—Fish used as controls for the impinge-
ment survival tests were subjected to the same handling
procedures used for the impinged fish except for
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introduction into the screen bay and subsequent
collection from the rotary disk screen. Control fish
were removed from the acclimation tanks and placed in
19-L buckets covered with a fine-mesh screen. Control
fish buckets were then submerged in the screen bay,inverted, and retrieved without releasing the fish. The
control buckets were then submerged in the collection
tank and emptied. Control fish were subsequently
retrieved from the collection tank and transferred to
covered control tanks where they were held for the 48-
h test period. Each control group consisted of
approximately 50 fish. The native controls consisted
of 112 fish composed of six species.
Tests.—Test groups of each species and the native
fish group for the impingement survival tests were
populated with approximately 50 fish/test (range, 49–
78) when recovery yielded sufficient numbers of
impinged fish. Survival of 229 channel catfish, 199
bluegills, and 208 fathead minnow in four batches were
tested during the April tests. The average total lengths
of fish used in the April tests (96 mm for channel
catfish, 76 mm for bluegills, and 56 mm for fathead
minnow) were representative of the size of fish
collected during impingement monitoring at the NOS
(Bigbee, unpublished).
Test fish for the August survival tests included both
hatchery-raised fish and wild, native fish collected from
the Missouri River. Fathead minnow obtained from thehatchery exhibited poor survival during the August
acclimation period and, therefore, were not tested. The
August survival tests of channel catfish, bluegills, and
the native species group were conducted with 37–59
fish/test, except one of the three bluegill tests used just
a single bluegill. The average total lengths of the fish
tested in August (72 mm for channel catfish, 57 mm for
bluegills, and 45 mm for the native fish) were
representative of the size of fish collected during
impingement monitoring at the NOS (Bigbee, unpub-
lished).Test conditions.—The pilot study, impingement
survival tests, and collection efficiency study were
conducted with the cooling water pumps fully
operational at river levels that ranged from 294.7 to
295.9 m above mean sea level. The top of the rotary
disk screen is at 295.0 m. The rotary disk screen was
fully submerged during the pilot study (295.8 m) and
the April (295.7 m) and August (295.9 m) impinge-
ment survival tests, whereas river level was lower
(294.7 m) during the collection efficiency study (water
surface was 0.3 m below the top of the rotary diskscreen). Water temperatures were lowest during the
collection efficiency study (4.58C) compared with
13.58C for the pilot study, 14.08C for the April survival
tests, and 27.08C for the August survival tests. The DO
concentrations in the source water averaged 12.8 mg/L
during the April survival tests (range, 10.4–11.7 mg/L)
and 7.9 mg/L for the August tests (7.4–8.2 mg/L).
Statistical analysis.—Survival data were analyzed
using logistic regression (McCullagh and Nelder 1989)as a function of the treatment (impinged versus control)
and the date of the survival tests. Logistic regression is
a statistical technique that makes the output of a
generalized linear regression model suitable for
modeling survival probabilities. The logistic regression
model used in this study was
logit ð pÞ ¼ logeð p=1 À pÞ¼ b0 þ b1 3 treatment þ b2 3 date;
p ¼ survival probability;
treatment ¼ coding variable for treatment (0 ¼control, 1 ¼ impinged);
date ¼ coding variable for date of test;
b0,b1,b2¼ model coefficients.
Since survival probabilities follow a binomial
distribution, the estimates for the model coefficients
and their associated standard errors were obtained
using a goodness-of-fit test called quasi-likelihood
estimation that allows for overdispersion (i.e., covari-
ance) among survival trials relative to the binomial
distribution (McCullagh and Nelder 1989).Pairwise comparisons among levels of treatment
factors were computed based on asymptotic z-scores of
the model coefficients and their associated estimated
covariances. All analyses were performed using the R
statistical programming language (R Development
Core Team 2007).
Results
Pilot Study
During the pilot study, 433 of the 500 bluegills
(86.6%) were collected within 30 min after they werereleased into the rotary screen bay, and 12 additional
bluegills were recovered after a total of 60 min (89.0%
accumulative); no additional bluegills were recovered
60–120 min after release. For channel catfish, 285 were
recovered within 30 min (57.0%), 12 within 60 min
(59.4%), and 6 additional channel catfish were
collected after 120 min (60.6%); no additional channel
catfish were recovered 120 min after release.
Impingement Survival Tests
Recovery rates (i.e., the number of tested fishcollected after treatment and then held for 48-h survival
observation) for the April survival tests were 49.8% for
bluegills, 23.1% for fathead minnow, and 20.6% for
channel catfish, yielding a sample size of at least 49
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fish per species for each test (Table 1). For the August
tests, 19.7% of the channel catfish, 9.6% of the
bluegills, and 15.9% of the native fish that were
released were recovered, yielding at least 37 fish per species for each test except for two bluegill tests (Table
2). Tested fish were recovered within 60–120 min of
release into the rotary disk screen bay for both the April
and August tests.
Impinged fish tested in April exhibited 48-h strong
survival rates similar to control fish: 99% for bluegills
versus 100.0% for controls, 96.1% for channel catfish
versus 90.0% for the controls, and 79.3% for fathead
minnow versus 81.0% for controls (Table 1). Bluegills
had the highest survival rates, and fathead minnow had
the lowest survival rates.
Impinged fish tested in August exhibited 48-h
survival rates similar to or better than control fish:
99.5% for channel catfish versus 100.0% for controls,
91.5% for bluegills versus 89.0% for the controls, and
84.6% for the native group versus 82.1% for the
controls (Table 2). Channel catfish experienced the
highest 48-h survival rate, and the native fish group had
the lowest survival rate. Of the native species tested,
emerald shiner Notropis atherinoides accounted for
96.3% (26 of 27 fish) of the test fish that did not
survive the 48-h test (Table 3). The overall survival rate
of impinged native fish (84.6%) tested in August was
similar to the survival of impinged fathead minnow
(79.3%) in April.
Collection Efficiency Tests
Results from the pilot study and impingement
survival tests raised questions regarding the fate of
unrecovered fish introduced into the rotary disk screen
bay, although recovery rates were generally adequate to
meet the targeted sample sizes for the survival tests.
Tests using both live and freshly killed fish were
conducted to evaluate collection efficiency. We
assumed that freshly killed fish, which were unable
to avoid the rotary disk screen, would have higher
recovery rates than live fish that presumably could
avoid impingement on the rotary disk screen.
A total of 250 channel catfish, 194 fathead minnow,
and 91 bluegills (all freshly killed) were introduced in
groups of 15–50 fish at 15-min time intervals over a
90-min period for the collection efficiency test. The
average total lengths of the released fathead minnow
(55 mm), bluegills (73 mm), and channel catfish (53
mm) were representative of the fish collected during
impingement monitoring at the NOS (Bigbee, unpub-
lished). All of the recovered fathead minnow and
bluegills were collected within 30 min of release, and
all except one channel catfish were collected 45 min
following release. The recovery rate for bluegills
(95.6%) was the highest of the three species: 87
recovered of 91 introduced. The recovery rates for
channel catfish (93.6%) and fathead minnow (94.3%)
were only slightly lower than the recovery rate for
bluegills.
TABLE 1.—Percent of hatchery-reared channel catfish, bluegills, and fathead minnow in control and test tanks that were alive
48 h after being impinged on a rotary disk screen installed in intake 3 at the North Omaha Station during April 2008.
Species
Test type
and tank
Sample
size
Number
alive at 48 h
Survival
at 48 h (%)
Channel catfish Control 1 50 41 82.0Control 2 50 49 98.0Total 100 90 90.0
Test 3 50 49 98.0
Test 4 51 50 98.0Test 5 78 74 94.9
Test 6 50 47 94.0
Total 229 220 96.1Bluegills Control 1 51 51 100.0
Control 2 50 50 100.0Total 101 101 100.0
Test 3 50 48 96.0Test 4 50 50 100.0
Test 5 49 49 100.0
Test 6 50 50 100.0Total 199 197 99.0
Fathead minnow Control 1 50 42 84.0Control 2 50 39 78.0
Total 100 81 81.0
Test 3 53 28 52.8Test 4 51 42 82.4
Test 5 51 47 92.2Test 6 53 48 90.6
Total 208 165 79.3
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Discussion
The impingement survival study conducted at the
NOS showed that fish impinged, removed, and
recovered from the rotary disk screen exhibited
survival rates that were not statistically different from
those observed for the controls, suggesting that
impingement on the rotary disk screen did not
contribute to the observed mortality. Study results
were consistent with field (EPRI 2003) and pilot-scale
(EPRI 2006; EPRI 2007) impingement survival studies
of other screen configurations or modifications de-
signed to improve impingement survival. Survival rates
of the species we tested were generally higher thandocumented in field studies (Table 5). The average
survival rate for channel catfish for the only extended
survival study (96 h) reviewed by EPRI (2003) was
93.0%, compared with the average 48-h survival rates
of 96.1% and 99.5% we observed, rates that wouldapproach 100.0% if adjusted for control survival rates.
Although our 48-h rates may not be directly compa-
rable to the 96-h rates of EPRI (2003), delayed
(‘‘extended’’) survival has been traditionally studied
at 24-h, 48-h, or 96-h intervals. The EPRI (2003) report
shows that latent effects of impingement appears to be
greatest between 24 and 48 h after impingement and
levels off rapidly after 48 h.
Ictalurids as a group had an average survival rate of
74.3% for the reviewed field studies (Table 5). None of
the 33 records for ictalurid survival tests were controladjusted, including the 96-h survival rate for channel
catfish. Although survival rates for our study were not
TABLE 4.—Summary of point estimates and 95% confidence intervals for survival of impinged and control fish in 48-h
postimpingement survival tests of fish removed from a rotary disk screen installed in intake 3 at the North Omaha Station, 2008.
Species Treatment
Survival
Estimate Lower bound Upper bound
Channel catfish Control 0.984 0.920 0.997
Impinged 0.990 0.950 0.998
Bluegill Control 0.911 0.810 0.961
Impinged 0.971 0.917 0.990Fathead minnow Control 0.780 0.213 0.979
Impinged 0.795 0.509 0.935Native (wild) fish Control 0.821 0.630 0.925
Impinged 0.850 0.728 0.923
TABLE 3.—Percent of native fish in control and test tanks that were alive 48 h after being impinged on a rotary disk screen
installed in intake 3 at the North Omaha Station during August 2008. The percent of total population is the number impinged
divided by the number of all species impinged.
Species
Number
impinged
Number
alive at 48 h
Survival
at 48 h (%)
Percent of total
population
Controls
Emerald shiner Notropis atherinoides 86 66 76.7 76.8Red shiner Cyprinella lutrensis 19 19 100.0 17.0
River shiner Notropis blennius 4 4 100.0 3.5Temperate bass Morone spp. 1 1 100.0 0.9
Gizzard shad Dorosoma cepedianum 1 1 100.0 0.9
White crappie Pomoxis annularis 1 1 100.0 0.9All species combined 112 92 82.1
Tests
Emerald shiner 115 89 77.4 65.7River shiner 27 27 100.0 15.4
Silver chub Macrhybopsis storeiana 10 10 100.0 5.7
Buffaloes Ictiobus spp. 6 6 100.0 3.4White bass Morone chrysops 5 5 100.0 2.9
Red shiner 4 4 100.0 2.3
Channel catfish Ictalurus punctatus 2 1 50.0 1.1Plains minnow Hybognathus placitus 2 2 100.0 1.1
Gizzard shad 1 1 100.0 0.6
Goldeye Hiodon alosoides 1 1 100.0 0.6Orangespotted sunfish Lepomis humilis 1 1 100.0 0.6
Sand shiner Notropis stramineus 1 1 100.0 0.6
All species combined 175 148 84.6
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control-adjusted, statistical analyses suggest that the
observed mortality of channel catfish impinged on the
rotary disk screen was not the result of impingement.
Survival of bluegills from seven extended (24-h and96-h) survival studies ranged from 79% to 100% and
averaged 99.3%; as a family, centrarchids had an
average extended survival rate of 71.1% (Table 5). The
bluegills and centrarchid tests included in the review,
similar to ictalurids, were not control adjusted.
The EPRI (2003) review did not include tests for
fathead minnow, which was the primary cyprinid tested
during the current study, but did include results for
emerald shiners, which had an average 24-h survival
rate of 94.2% (Table 5), compared with the 77.4%
survival rate of emerald shiners in the native fish testsfor the current study. However, the results from our
study indicated that impingement on the rotary disk
screen did not contribute to the observed mortality
because there was no statistical difference between
control and test groups.
Survival rates of fish removed from the rotary disk
screen were compared with results from another 2-year
pilot-scale study of a Geiger multi-disk screen
(hereafter, Geiger screen; another modified screen
system with fish protection features), where fish
impingement rates and subsequent survival were tested.The Geiger screen is composed of circulating sickle-
shaped mesh panels connected to a revolving chain
installed across an intake (EPRI 2007). Prior to its
installation at the Potomac River Generating Station,
the Geiger screen had only been used for debris
handling at the D.C. Cook Nuclear Station on Lake
Michigan (Peltier 2004). Ten Geiger screens were
installed at the Potomac River facility to improve
removal of debris and aquatic weed loads. One of the
Geiger screens was provided with fish protection
features so that impingement survival could beevaluated. Fish protection features included a fish
bucket for carrying submersed fish to a return trough, a
low-pressure through-screen wash, and external wash
to transfer impinged fish to the fish bucket. The screen
panels were drilled plastic with 9.5-mm openings,
which were larger than the 6.1-mm2
screen openings
on the rotary disk screen we evaluated.
The 2-year study of the modified Geiger screen
demonstrated the difficulty in obtaining reliablesamples for survival tests when relying on ambient
impingement rates. Most species were impinged at low
rates that limited the number of fish available for
survival testing. Only 4 of 20 impinged species were
collected in numbers greater than 100 during the 2-year
study. Bluegills and channel catfish were collected in
sufficient numbers during the Geiger screen study to
evaluate impingement survival. Bluegills exhibited
annual survival rates of 94% and 95%, and channel
catfish had annual survival rates of 50.0% and 100.0%.
Because almost all fish were collected during and
immediately following major runoff events, reduced
water quality during these events may have compro-
mised survival results during the study (EPRI 2007).
Survival rates on rotary disk screen tested during our
study were higher than survival rates of fish impinged
on the Geiger screen.
We also compared the survival rates from our study
to those from a recent laboratory evaluation of
modified Ristroph vertical traveling screens (hereafter,
Ristroph screens) because the study also used hatchery
fish. Ristroph screens are conventional vertical travel-
ing screens equipped with fish buckets attached to thescreens. They have low-pressure spray washes to
remove fish, separate debris and fish returns, and a
continuous screen operation. The 48-h survival of
fathead minnow, channel catfish, and bluegills had
survival rates of at least 95% at three different approach
velocities following removal from the Ristroph screens
(EPRI 2006). As in our study, test fish for the Ristroph
screen were obtained from hatcheries and therefore
may have been in better health than wild fish from the
source water, based on a recent impingement study that
incorporated a health assessment of both impinged fishand fish collected from the source water (Baker 2007;
Knight 2008). That study showed that the majority of
impinged fish were in poorer health than nonimpinged
fish. Based on those results and the assumption that
site-specific environmental factors (e.g., water quality,
water temperature, and turbulence) could reduce
impingement survival, the use of hatchery fish may
overestimate survival. However, a comparison of
results from our study with results from field studies
indicate that the survival estimates achieved using
hatchery-raised fish are within the range of survivalobserved in the field and may more accurately estimate
the ability of the rotary disk screen to return fish to the
Missouri River.
Monitoring of DO and water temperature during the
TABLE 5.—Summary of extended (24-h to 96-h) post-
impingement survival rates (%) from historical field studies
conducted at other facilities for those fish taxa that were
impinged on the rotary disk screen installed in intake 3 at the
North Omaha Station. Source: Appendix B of EPRI (2003).
Taxon Average Minimum Maximum SD
Ictaluridae 74.3 20.0 100.0 10.8
Channel catfish 93.0Centrarchidae 71.1 0.00 100.0 35.4
Bluegill 99.3 79.0 100.0 8.2
Cyprinidae 67.2 0.00 100.0 34.2Emerald shiner 94.1 7.7 97.5 40.5
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survival tests showed that environmental conditions in
the control and test tanks were stable and probably did
not affect 48-h survival rates in either the control or test
groups. Although variations in river levels and water
temperatures observed during the study potentiallyinfluenced recovery of released fish, they did not
explain the lower recovery rates during both the April
and August impingement survival tests compared with
recovery rates during the pilot study.
The survival rates documented in our study may also
be due to the continuous screen rotation that reduces
the retention time of fish on the rotary disk screen and
the use of a vacuum to remove impinged fish from the
screen. Retention time on the rotary disk screen is 30 s
or less, depending on position of the impinged object
on the screen. In comparison, retention time on vertical
traveling screens can range from a few seconds to
several minutes, assuming continuous screen operation
and depending on where on the screen fish are
impinged. Retention time for the Ristroph laboratory
study was about 40 s, based on a rotation speed of 2.4
m/s and an assumption that fish were impinged at the
deepest point of the screen (EPRI 2006). Retention
time was not provided for the Geiger screen study.
The screens evaluated for the Ristroph and Geiger
screening systems were operated continuously, as in
our study, whereas conventional vertical traveling
screens are typically operated intermittently based onpressure differentials across the screen, which increases
the retention time of impinged fish on the screens. Fish
impinged on the rotary disk screen remain immersed in
water prior to removal from the screen so they are not
exposed to air, an obvious source of stress, as is the use
of spray washes, especially high pressure washes
typical of the conventional traveling screens currently
at the NOS.
Survival tests of fish removed from the rotary disk
screen demonstrated that introduction of fish in front of
the screen was an effective alternative to relying onimpingement rates to provide adequate sample sizes for
survival tests, especially when impingement rates are
low and unpredictable, as they were at the NOS. It
allowed for scheduling tests and reduced the number of
visits that would have been required if the tests were
based solely on ‘‘ambient ’’ impingement rates. The
pilot study provided a reliable method for introducing
fish to the screen bay that yielded an adequate number
of fish for valid statistical analyses of the test results.
Percent recovery of introduced fish for the April and
August survival tests was lower than that observed inboth the pilot study and collection efficiency tests. All
fish used in the pilot study, survival tests, and
collection efficiency study were similar in size and
the same procedures were used to introduce test fish
into the rotary disk screen bay. The lowest recovery
rate occurred during the August tests when swimming
performance should have improved due to water
temperatures that were 13.18C warmer than during
the April tests.Fish could have avoided impingement on the rotary
disk screen, based on the calculated approach velocities
that range from 0.06 m/s at a high-river stage (304 m)
to 0.46 m/s at a low-river stage (294 m). The calculated
approach velocity at the normal river stage (296 m) is
0.16 m/s, which is close to the 0.15 m/s through-screen
velocity recommended by the EPA in its Phase II
316(b) rule (U.S. Office of the Federal Register 2004).
The calculated approach velocities are the average
water velocity found at 5–8 cm in front of the screen
taken in the same direction as the general flow (EPRI
2000). They are representative of the area between the
screen and intake bay inlet and are the velocities
experienced by fish as they swim near the screen,
which lower than through-screen velocities (i.e., the
velocity of water as it passes through the screen; EPRI
2000). Through-screen velocities would be experienced
only by fish when they are at the face of the screen and
are probably not as important a factor as approach
velocity, but may affect whether impinged fish can
remove themselves from the screen once impinged
(EPRI 2000). A review of swimming capabilities of
freshwater fish indicate fish can avoid impingement at approach velocities of 0.15–0.30 m/s (EPRI 2000),
particularly when exposed for short periods (120 min),
such as during our tests. Approach velocities during the
pilot study and both impingement survival tests ranged
from 0.179 to 0.191 m/s. Higher approach velocities
(0.265 m/s) occurred during the collection efficiency
study, when river level was 1.2–1.3 m lower than
during the pilot study and survival tests.
The high recovery rate of dead fish during the
collection efficiency study indicates that unrecovered
live fish probably sought refuge in the screen bay awayfrom the rotary disk screen or moved out of the screen
bay into the Missouri River. The small mesh (6.1-mm2
screen openings) of the rotary disk screen should have
retained all test fish; thus, it is not likely that fish were
extruded through the screen and its installation
prevented fish from passing between the screen and
bay walls without being removed by the screen. In
addition, the bay was blocked to prevent fish from
finding refuge in an adjacent intake bay.
Factors contributing to differences in recovery rates
of released fish during the pilot study, impingement survival tests, and collection efficiency study are
unclear. Only live fish were released during the pilot
study and survival tests, whereas both live and freshly
killed fish were released during the collection efficien-
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cy study. As shown by the collection efficiency study,
recovery rates of freshly killed fish approached 100%,
as anticipated, and recovery was better than for live
fish, which could avoid the rotary disk screen. All fish
were released consistently during each test, followingthe same procedures by the same personnel. The
release depths relative to the top of the rotary disk
screen (295.0 m) for the April and August survival tests
varied by about 0.1 m, and fish were released at depths
that were 0.6–0.9 m above the top of the rotary disk
screen. Although fish were released closer to the top of
the rotary disk screen during the April tests (approx-
imately 0.6 m), the resultant recovery was lower than
for the pilot study. Recovery rates in August were even
lower than in both the pilot study and April survival
tests when fish were released about 0.9 m above the top
of the rotary disk screen.
Water temperatures during the tests were also
examined to evaluate whether differences among tests
could have affected swimming performance, possibly
accounting for the apparent avoidance of the rotary
disk screen in April and August compared with the
pilot study. Water temperatures were similar during the
pilot study (13.58C) and April (14.08C) survival tests,
much higher during the August tests (27.08C), and
much lower during the collection efficiency tests
(4.58C). Differences in recovery between the pilot
study and the April survival tests were probably not due to water temperature, which varied by 0.58C. More
fish may have avoided the rotary disk screen because
of the warmer temperature in August and because fish
were released further above the top of the rotary disk
screen than during the pilot study and the April
survival tests. Cooler water temperatures during the
collection efficiency study probably reduced swim-
ming performance and thereby contributed to the
generally higher recovery rates of live fish compared
with recoveries during the pilot study and survival
tests. Additionally, the river level was about 0.4 mbelow the top of the rotary disk screen during the
efficiency study, indicating higher approach velocities
than during the pilot study and survival tests, albeit the
volume of cooling water pumped through the screen
was the same during all three studies.
The impingement survival studies were not per-
formed during the winter season since impingement
rates are either low (which is the case on the Missouri
River) or if high, have been found to be primarily
associated with cold-stressed or moribund clupeids
(EPRI 2000).We also examined the size of the fish tested to see
whether recovery rates could be related to swimming
performance. However, even though mean total lengths
were greater for channel catfish (97 mm) and bluegills
(76 mm) in April than during the pilot study (76 and 52
mm, respectively), it is not likely that those size
differences (20–24 mm) would account for recoveries
that were 35–39% lower, especially because they were
released closer to the rotary disk screen than during thepilot study. The mean length of fish released during the
August survival tests were 21–24 mm shorter than in
April and were similar to the mean lengths of test fish
used in the pilot study. However, the August fish were
released about 0.9 m above the top of the rotary disk
screen and the low recovery rates suggest fish were
able to avoid impingement.
The collection efficiency study results support the
hypothesis that unrecovered fish avoided impingement,
based on the 94–96% recovery rates of the freshly
killed fish, but those results were not particularly useful
for explaining the lower recovery during the survival
tests. The relatively good recovery of live fish during
the collection efficiency study was probably due to the
cold water (4.58C) and low river levels that were about
0.4 m below the top of the rotary disk screen. The
recovery rate for live channel catfish during the
collection efficiency study (82.0%) was much better
than for the other tests and their smaller mean total
length (53 mm) may have contributed to that recovery
rate. In contrast, the recovery rate of live bluegills
(70.0%) was lower during the collection efficiency
study than during the pilot test; those fish were larger (73 mm) than those in the pilot study (52 mm), which
could have contributed to the different recoveries.
Overall, the results suggest that, when introducing fish
in front of screens to examine impingement survival,
the release point should be adjusted with water level so
that fish are released at a consistent depth relative to the
center of the screen; this would assure good recovery of
test fish.
The use of hatchery-raised fish and wild, native fish
collected from the source water for the survivability
tests allowed clear interpretation of the results becausetargeted sample sizes for the survival tests and controls
were met and allowed valid statistical analyses. Study
results demonstrated that the absence of controls in
survivability tests can overestimate mortality or
underestimate survival. Use of controls allowed
evaluation of factors that could overestimate impinge-
ment mortality, including handling mortality, and
thereby provided accounting for prior moribund fish
and interactions of sampling stress and holding times
(EPRI 2005). Controls allowed for an adequate
evaluation of the screen’s ability to collect and returnfish to the river. Impinged wild fish probably reflect
site-specific environmental stress factors that poten-
tially increase impingement mortality; however, a
comparison of these pilot-scale results with observed
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field results indicate that the nominal survival rates
observed in the rotary disk screen pilot-scale study is
within the range of results from field studies that were
not adjusted for control mortality. The lack of statistical
differences between impingement and control survivalrates indicate that fish removed from the rotary disk
screen were unaffected by impingement.
Acknowledgments
The authors acknowledge the support provided by
Omaha Public Power District (OPPD), the corporate
sponsors and their technical staff, as well as personnel
at the North Omaha Station that provided site access
and security. They include Ron Stohlmann, Karen
Belek, Russ Baker, Igor Cherko, Nate Staroscik, and
Eric Pohl. We also acknowledge the technical and
managerial support provided by Greg Seegert of EA
Engineering, Science, and Technology, Inc. Field and
office staff that supported this effort included Mitch
Wallman, Jamie Suing, and Ben Carlson. We also
acknowledge the support provided by reviewers of the
draft paper including Dave Michaud (We Energy), Bob
Reider (DTE Energy), Casey Knight (Alabama Power),
Ron Stohlmann (OPPD), Ray Tuttle and Jon Black
(Alden Research Laboratory).
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