Aquaculture Magazine
Mixed-Cell Raceway: combining the best characteristics of tanks
and raceways
James M. Ebeling
Environmental Engineer
Carla F. Welsh
Research Associate
Conservation Fund Freshwater Institute
1098 Turner Rd., Shepherdstown, WV 25443, USA
j.ebeling@freshwaterinstitute.org
Michael B. Timmons
Professor
Cornell University
Biological and Environmental Engineering Department
302 Riley-Robb Hall Ithaca, NY 14853
Raceways have been used for years for the production of salmonids and other species by federal
and state agencies for stocking purposes and commercial growers. Where there are large
groundwater resources, such as the Thousand Springs area in Idaho, raceways are the most
common production tank design, being used to grow the majority of rainbow trout produced in
the USA. Two significant advantages of raceways is their better utilization of floor space and
easier handling and sorting of fish as compared to traditional, circular tanks. Their primary
disadvantages are the large volume of water required, high turnover rates, and limited selfcleaning ability. Even using lower exchange rates and lower velocities, their use has been
severely limited due to the unavailability of large quantities of high quality water, increased
concern about their environmental impacts on receiving waters, and the difficulty in treating
large flows from effluent discharge. One solution to these problems is to convert the raceways
into a series of counter-rotating mixed-cells that act as hydraulically separated round tanks, thus
providing both the advantages of a raceway and the improved solids removal capability of
circular tanks. This would allow the raceways to be managed either as partial reuse systems or
as intensive recirculation systems.
A prototype raceway was constructed to better understand the operations, management, and
hydraulics of a large, mixed-cell raceway using a research greenhouse at the Conservation Funds
Freshwater Institute. The basic design concept was to operate the raceway as a series of
square/octagonal tanks (cells), each having a center drain for continuous removal of solids and
sludge. Each cell received water from a vertical pipe section (downleg) extending to the tank
floor and located in the corners of each cell through which water is pumped and discharged
through several orifices (jets) to establish rotary circulation in the cells. The rotational velocity
in the cells can then be controlled by the design of the orifice discharges, either by increasing the
flow rate, the discharge velocity, or the total number of orifices. This was then combined with
the concept of the ‘Cornell double drain system’, where 10% to 20% of the total flow into a tank
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Aquaculture Magazine
is removed from a center bottom drain and 80 to 90% of the flow is removed from the side drain.
Settable wastes and sludge are then removed from the center drains and collected in a settling
sump.
Design and Construction
The experimental raceway measured 16.3 m x 5.44 m x 1.22 m and was constructed of structural
lumber with a HDPE liner. The floor of the raceway was covered with 5 cm of fine sand and
graded to provide a slight slope to the three center drains. The walls and floor were insulated
with 2.54 cm foam insulation board to minimize heat loss through the sidewalls. The tank was
lined with a 20 ml high density cross laminated polyethylene (HDPE) raceway liner from
Permalon, Reef Industries, Inc. A 15 cm drainline with three discharge drains (tee fittings)
centered on each of the three cells was buried along the longitudinal axis of the tank. The three
drains discharged into a 1.83 m x 1.83 m x 1.83 m fiberglass sump tank. The sump tank had
both a standpipe for water level control and a drainline to empty the system. The sump tank was
intended to fulfill several roles, including solids management by acting as a solids settling basin,
maintaining desired water level by using a standpipe, and as a harvesting basin by draining the
production tank through a screened harvesting cage.
Eight 0.75 kW pumps were installed along the outside walls on platforms and discharged into a
10 cm schedule 40 PVC manifold that encircled the raceway. Two of the pumps were located at
the sump collection tank, while the remaining six were placed equal distant along the length of
the tank outside walls. The suction lines with check valves were located approximately one third
below the water level in the tanks. The pump discharges were connected to the manifold with a
5cm flexible hose with a bronze gate valve to control flow and a clear plastic manometer to
measure downleg back pressure. Four 5cm PVC vertical manifolds with seven orifices were
placed in the four corners of the tank and four 8 cm PVC vertical manifolds were located along
the sidewalls, dividing the tank into three equal cells. Two of the 8 cm vertical manifolds had 14
orifices; seven discharging along each edge of the wall in opposite directions and the other two
vertical manifolds had 7 orifices each discharging perpendicular to the walls. The orifice
opening were constructed by welding a threaded bushing 3.18 cm by 2.54 cm, which allowed a
2.54 cm threaded plug to be inserted. This allowed orifice sizes to be easily modified or
plugging of unused orifice openings for research trials.
Flow Measurement
A SonTek Argonaut Acoustic Doppler Velocimeter from Yellow Springs Instruments was used
to measure speed and direction within the hydraulically separated cells. The SonTek velocity
meter is a single-point, 3D Doppler current meter designed for shallow water flow monitoring.
Water velocity is measured via the Doppler shift in frequency of sound from a moving object, in
this case small particulate matter in water current. The SonTek probe assembly was mounted on
an aluminum beam supported above the width of the raceway, which allowed the probe to be
moved across the tank width.
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Aquaculture Magazine
Hydraulic Characteristics
The color contour plot for cell #3 (end cell farthest from sump) shows the velocity profile at
intervals of 5 cm/sec using downleg discharge orifices of 15 mm. The discharge orifice pressure
head was approximately 0.22 m. Research has shown that water velocities greater than 15 to 30
cm/sec (0.5 to 1.0 fps) are required to drive settable solids to the center of a tank for quick
removal by the center drain. The outside perimeter of the cell exhibits these high scouring
velocities and significantly lower velocities are seen near the center of the cell. The contour plot
also shows the four discharge plumes at each corner. During a research production trial for
marine shrimp, excellent solids removal was seen even at significantly lower cell velocities.
Summary and Conclusions
In order to better understand the hydraulics of a large, mixed-cell raceway, a prototype raceway
was constructed in a research greenhouse at the Conservation Fund Freshwater Institute. Studies
have been conducted on the velocity profiles for several different orifice discharge diameters and
flow rates through the system. Results show excellent bottom velocities for scouring solids and
transporting them towards the center drains in each cell. Further work will focus on determining
the relationship between the orifice discharge velocities and the water rotational velocity, solids
removal efficiency of the system, and management studies on how best to use the sump tank for
solids collection and as a solids settling basin. In short, mixed-cell raceways show excellent
potential for either retrofitting existing raceways or as a design for a new production system.
This work was supported by the United States Department of Agriculture, Agricultural Research
Service under Cooperative Agreement number 59-1930-1-130 and Magnolia Shrimp, LLC,
Atlanta, Georgia.
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Aquaculture Magazine
Sludge
Disposal
Pump
Pump
Pump
Sludge
Tank
Sump &
Settling
Basin
6” drain
line
Harvest by
screen capture
Pump
Pump
Pump
4” Manifold
Schematic of Mixed-cell Production Tank showing the three counter rotating cells and settling
and harvest sump.
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Aquaculture Magazine
15
10
2.5 25
15 20
20
15
10
40
35
30
35
30
25
25
2.0
15
5
10
1.5
10
20
15
20
1510
35 30
25
20
10
5
35
20
20
5
15
25
20
15
20
1.0
25
10
10
20
25
20
0.5 15
20
0.0
20
10
15
25
15
5
5
35
10
-0.5
-1.0
20
10
10
5
20
3540
10
5
10
15
35
30
25 20
-1.5 40
25 30
5
15
15
15
25
20
15
20 25 25
10
-2.0
20
15
15
20
-2.5
20 25
15
10
-2.5
-2.0
-1.5
25
30
15
20
25
-1.0
35
-0.5
0.0
0.5
1.0
25
30
1.5
20
20
10
5
2.0
2.5
40
Cell #3: velocity contour plot, 5 cm/sec intervals
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Overview of Mixed-cell Production Tank showing construction, pump manifold and pumps, and
vertical manifolds.
Mixed-cell raceway showing two vertical manifolds, pumps, and heat exchanger.
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