Aquacultural Engineering 22 (2000) 3 – 16
The design and operational characteristics of
the CP&L
/
EPRI fish barn: a demonstration of
recirculating aquaculture technology
Thomas M. Losordo
a,*, Alexander O. Hobbs
b,
Dennis P. DeLong
aaDepartment of Zoology,North Carolina State Uni6ersity,Campus Box 7646,Raleigh, NC27695, USA
bDepartment of Mechanical Engineering,North Carolina State Uni6ersity,Campus Box 7910, Raleigh,NC27695, USA
Abstract
The Carolina Power & Light Company, in conjunction with the Electric Power Research Institute of Palo Alto, California has developed a commercial fish production demonstration utilizing water reuse technology developed at the North Carolina State University Fish Barn. The fish production system is housed in a 39.6 m long by 9.75 m wide barn structure located on the campus of North Carolina State University in Raleigh, NC. Fish production activities began in the spring of 1998. The facility is designed to produce 45 tonnes of fish annually, with the first crop being tilapia. The project is being operated as a public demonstration of this technology, with biological, engineering and economic data being collected by research and extension personnel at North Carolina State University. This paper outlines the design of the recirculating system technology used to recycle water through the main fish production tanks. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Recirculating system; Biological filter media; Copper – cadmium reduction
www.elsevier.nl/locate/aqua-online
* Corresponding author. Tel.: +1-919-5157587; fax:+1-919-5155110. E-mail address:[email protected] (T.M. Losordo)
1. Introduction
The North Carolina Fish Barn project at North Carolina State University (NCSU) has been active in the evaluation and development of recirculating technology for the intensive production of fish since 1989. In 1993, with funding from AGA Gas, Inc. (Cleveland, OH), AGA AB (Stockholm, Sweden), AquaOp-tima AS (Trondheim, Norway), and the Energy Division of the North Carolina Department of Commerce (Raleigh, NC), a ‘third generation’ Fish Barn recirculat-ing system was designed and tested. The Fish Barn system incorporated a tank and particle trap technology referred to as ECO-TANK and ECO-TRAP, respectively. The ECO-TANK and ECO-TRAP technology are described by Skybakmoen (1989) and more recently by Timmons et al. (1998) and are products of AquaOptima AS. The North Carolina Fish Barn system is described by Twarowska et al. (1997) and is referred to as the ECOFISH™/NCSU system. The system consisted of a tank with a circular flow pattern, a particle trap, a sludge collector, a drum screen filter, a trickling biofilter (gravity fed, referred to as a BioSump), a downflow oxygen contactor, and a vertical manifold water inlet in the culture tank.
In 1994, Carolina Power & Light Company (CP&L) became a corporate sponsor of work ongoing at the NC Fish Barn. In 1996, NC State and CP&L proposed the development of the large-scale recirculating fish production demonstration system to be funded by the Electric Power Research Institute (Palo Alto, CA). The facility was designed by NCSU and CP&L personnel based upon the ECOFISH™/NCSU system. The overall layout of the facility is described in Hobbs et al. (1997). The fish production system consists of a 5.1 m3 quarantine tank, a 13.3 m3 secondary quarantine or nursery tank, and four 60 m3 growout tanks. The tank systems are housed in a 39.5 m long×9.75 m wide agricultural barn structure, with much of the treatment equipment contained in two 3 m×6 m shed type ‘mechanical’ rooms. Referred to as The CP&L/EPRI Fish Barn, the facility is designed to produce approx. 45 metric tonnes (mt) of tilapia fish annually based on 1 g fingerlings growing to market size of 580 g in approx. 210 days. This paper details the design and operational characteristics of the main growout tank system and associated water treatment components.
2. Water treatment system design and layout
5 T.M.Losordo et al./Aquacultural Engineering22 (2000) 3 – 16
Water exits from the two culture tanks (A) through the particle traps (B) to the sludge collectors (C) and a common standpipe well (D). The water then passes through a drum screen filter (E) to the biofilter (F). The water is returned to the culture tanks by centrifugal pumps (G) via downflow oxygen contactors (H), which add pure oxygen to the flow stream. The water re-enters the culture tanks through
Fig. 2. Elevation view of tank system, particle trap, sludge collector, standpipe well and drum screen filter (after Hobbs et al. (1997)).
two vertical manifolds (I) per tank. Water from the main treatment flow stream is diverted at a rate of 230 liters per minute (l min−1) through two foam fractionators (J). System piping cross connections provide operational flexibility and heating capabilities via a heat pump (5). Table 1 provides a detailed description of the equipment used in the main growout system.
A typical growout tank system layout is shown in ‘elevation’ view in Fig. 2. This diagram shows the fiberglass tank (A) with the particle trap (B) set in the concrete tank foundation (floor). Using the tank water level as a reference elevation, water flows from the particle trap through two separate pipes to the sludge collector (C) and the standpipe well (D) where the flows are rejoined. The flow proceeds through the drum screen filter (E) towards the BioSump, which is not shown on this diagram. Gravity flow is used as much as possible to carry water through the treatment processes. While this can be viewed as an energy savings feature, the greater advantage is the improvement in solids removal. In general, solids are larger and more easily removed by gravity settling and/or mechanical screening processes before they are subjected to the shearing forces of a centrifugal water pump impeller.
Settleable solids are removed rapidly within the culture tank by the particle trap which is shown in detail in Fig. 3 (ECO-TRAP 300; AquaOptima AS, Trondheim, Norway). Studies have shown that fecal solids and uneaten feed are removed from the tank within minutes of settling to the flat bottom of the culture tank (Twarowska et al., 1997). The ECO-TRAP has two outlets in which water flows from the culture tank. Settleable solids are captured by the particle trap as they slide beneath a plate located in the tank center just above and parallel to the tank bottom. The uneaten feed and fecal solids are collected in a bowl within the particle trap and are removed via a 30 l min−1 flow stream designated B in Fig. 3.
7
Specifications of components in one 2-tank growout system in the CP&L/EPRI fish barn
Supplier/manufacturer Quantity
Component function Description Model
N/A Glass Boat Works, PO Box 674, Exmore, 60 m3 at 6.4 m dia.×1.98 m deep,
fiber-Hydrotech w/40 micron screen 802 Water Management Tech., PO Box 66125, Drum screen filter (E) 1
Baton Rouge, La. 70896 USA N/A
Corrugated steel pipe, 2.44 m diameter× Contech Construction Products, Inc., PO Box 2
Trickling filter BioSump
800, Middletown, OH 45042, USA
(F) 5.66 m deep, concrete bottom
BioBlok 200 EXPO-NET Danmark A/S, Georg Jensens 15.4 m3 BioBlok, plastic media, 55×55×55 cm
Biofilter media
Vej 5, DK-9800 Hjørring, Denmark blocks, 200 m2m−3
4 Sweetwater, 2 hp, 8.9 full load amps at 230 PS 6 Aquatic Eco-Systems, Inc., 1767 Benbow Centrifugal pumps (G)
VAC, 8.07 running amps at 230 VAC Court, Apopka, FL 32703 USA (1.856 kW)
Aquatic Eco-Systems, Inc. 4 Oxygen Saturator fabricated from welded OY 110
Downflow oxygen
contac-tors (H) polyethylene pipe
AquaOptima AS
Vertical manifold (I) 4 ECO-FLOW, PVC 110
2 Top Fathom, PVC venturi driven TF12
Foam fractionator (J) Aquanetics Systems, Inc., 5252 Lovelock, San
Diego, CA 92110, USA Aquatic Eco-Systems, Inc. S-45
1
Regenerative blower (1) Sweetwater, 1.5 hp, 10.4 full load amps at 230 VAC, 8.87 running amps at 230 VAC (2.04 kW)
1 Dayton, High Pressure, Direct Drive Model 6K481B W.W. Grainger, Inc., 4820 Signett Dr., High volume blower (3)
Blower, Stock No. 7C483, 2.7 full load Raleigh, NC 27604 USA amps at 230 VAC, 2.77 running amps at
230 VAC (0.637 kW)
Model GT026 FHP Manufacturing, Inc., 601 Northwest Florida heat pump, 10.25 kW heating, 10.5
1 Heat pump water heater
to the adjacent standpipe well. Flow stream A shown in Fig. 3 carries suspended solids through the elevated strainer of the particle trap (B) at a design rate of 800 l min−1 per tank.
Fig. 3. The ECO-TRAP particle trap showing high solids/low flow stream B and high flow/low solids stream A, (after Hobbs et al. (1997)).
9 T.M.Losordo et al./Aquacultural Engineering22 (2000) 3 – 16
Fig. 5. Tank, sludge collector, drum screen filter and BioSump biofilter layout (after Hobbs et al. (1997)).
The settleable solids and suspended solids flow streams from each tank come together in the standpipe well (D) where the flows from both tanks combine and are carried to a drum screen filter (E) (Hydrotech, Water Management Technologies, Baton Rouge, LA) at a combined rate of 1660 l min−1
. At this point, all solids larger than the size of the screen on the drum screen filter (40 microns) are removed by the screen and then by the intermittent high-pressure rinse spray to a waste stream. The filtered water leaves the drum screen filter (E) and exits through the discharge pipe which then divides the stream in two, flowing to the two 2.44 m diameter BioSumps (F) shown in Fig. 5.
The water is distributed over the top of the BioSump biofilter media with a single rotating distribution nozzle (Balanced AquaSystems, 1051 Swanston Drive, Sacra-mento, CA). The water falls through 1.65 m of plastic biofilter media (BioBlok 200, EXPO-NET) which has a specific surface area of 200 m2 m−3. The ammonia is converted to nitrate at a (design) rate of approx. 0.45 g TAN m−2 day−1 by the bacteria attached to the media. Carbon dioxide is removed from the downward cascading water stream with a counter-current air flow generated by a 0.62 kW ‘high-volume blower’ (Dayton, Model 6K481B, W.W. Grainger, Inc.) which pro-vides a total of 6.8 m3
min−1
EnviroQuip International, Cincinnati, OH) provides aeration in each BioSump with a supply of 1.4 m3min−1of air. A 2.0 kW regenerative blower (Sweetwater Model S-45, Aquatic Eco-Systems, Inc.) provides the air to the aeration arrays in both BioSumps at a combined rate of 2.8 m3 min−1at 76 cm H
2O pressure, to liberate carbon dioxide and add dissolved oxygen. Water is then pumped from the bottom of each BioSump with two-2 kW centrifugal pumps (G) (Sweetwater, Aquatic Eco-Systems, Apopka, FL) at a rate of 415 l min−1each through oxygen injection components to each tank (Fig. 6). For each tank, two downflow oxygen contactors (H) (Oxygen Saturator Model OY110, Aquatic Eco-Systems, Apopka, FL) are used. Water flows into the top of the downflow oxygen contactors, is mixed with gaseous oxygen, and exits the bottom in a pressurized (0.5 – 1.0 bar) flow stream for delivery to the culture tank. The oxygenated water re-enters the culture tank through two vertical manifolds (I) (ECO-FLOW Model 110, AquaOptima AS, Trondheim, Norway) that allow for even distribution of the water from top to bottom in the tank water column.
3. Water treatment system design and operational characteristics
The following is a description of the operational characteristics of the growout tank’s water treatment system described above.
11 T.M.Losordo et al./Aquacultural Engineering22 (2000) 3 – 16
Table 2
Design and operational characteristics of the CP&L/EPRI water treatment system
Average
mea-Recycle flow rate per unit of feed added 16.6 KW kg−1per
Pumping energy per unit of feed added 0.082 0.085 day
Aeration/degassing energy per unit of feed KW kg−1per 0.030 0.031 day
added
g TAN/m−2per
Biofilter nitrification 0.45 0.43
day
3.1.Design characteristics
The water treatment system for the two-tank growout system is designed to remove or neutralize the waste created by approximately 100 kg of fish feed per day (38% protein by weight). The system was designed to have the capacity to treat, renovate and recycle water back to the culture tanks at a rate of 1660 l min−1. Much of the system component sizing is dictated by the daily feed amounts required to grow the specified biomass of fish in each culture tank (Losordo and Hobbs, 2000). Table 2 lists some important sizing and design characteristics per unit of feed input to the growout system.
In general, the system was designed to use approx. 5 – 10% of the system volume per day in replacement water. The new water replaces that which is lost to discharge at the drum screen filter, the sludge collectors, draining and refilling tanks during harvesting, and evaporation. In the growout system, with a tank volume of 120 m3 and a BioSump water volume of 9.35 m3, a daily replacement volume of 7.5% is approximately 9.7 m3of water. With an estimated feed rate of 100 kg per day, the design rate for replacement water based on feed rate is 97 l kg−1of feed per day (9,700 l/100 kg feed per day) (Table 2).
Thus the design rate listed in Table 2 for energy used for recycled water pumping per unit of feed per day is estimated at 0.082 kW per kg of feed per day (8.2 kW/100 kg feed day).
Similarly, the energy used in aeration and degassing can be estimated by summing the total FLA for the regenerative blower (10.4 amps) and high volume blower (2.7 amps) and multiplying by 230 volts. Therefore, the design rate listed in Table 2 for energy used for aeration and degassing was 0.030 kW kg−1feed per day (3.01 kW/100 kg per day).
Previous studies at the North Carolina Fish Barn have indicated that an appropriate design nitrification rate (Table 2) for the type of trickling filter which was used in this study is 0.45 g TAN m−2
per day (Twarowska et al., 1997).
3.2.Preliminary operational characteristics
Tank 1 of the two-tank system was stocked on 29 June 1998 with 6752 – 140.5 g (average weight) tilapia fingerlings (Oreochromis niloticus). Tank 2 of the same system was stocked on 5 August 1998 with 9653 – 95.1 g tilapia fingerlings. The biofilters were allowed to populate naturally with nitrifying bacteria. Tank 1 harvests began on 5 October 1998 and were completed by 10 November 1998. A total of 3813.5 kg was harvested from this tank. On 13 November 1998 Tank 1 was restocked with 4668 – 364 g tilapia (1/2 the population from Tank 2 weighing 1699.2 kg). As noted above, the Tank 2 population remained stable until it was divided and half was transferred into Tank 1.
As one can readily see, the populations and biomass in this two-tank system fluctuated considerably during this study period. This is typical of tank systems that share water treatment components and of those that are harvested numerous times to satisfy local markets. Maximum biomass for the two-tank system occurred around late October or early November with a total estimated fish biomass of 6860 kg (57.2 kg m−3). The feed rates listed in the figures fluctuated according to the system biomass and fish appetite.
Although the following data do not represent steady-state conditions, they do provide a preliminary evaluation of the water treatment systems’ capabilities. Water samples were immediately transported to the Water Quality/Waste Management Laboratory at North Carolina State University. Samples were analyzed by auto-mated analysis (Technicon, Auto-analyzer Model II) for total ammonia nitrogen (TAN) by the salicylate method, nitrite – nitrogen (NO2– N) by the cadmium reduction method, and nitrite – nitrogen plus nitrate – nitrogen (NO2−N+NO3− N) by the copper – cadmium reduction method (EPA, 1979). Filterable suspended solids (FSS) were analyzed according to Standard Methods (APHA, 1989).
13 T.M.Losordo et al./Aquacultural Engineering22 (2000) 3 – 16
The actual volume of new water used within the growout system was recorded daily and averaged 12.3 m3 per day over the 12-week sampling period. With an average daily feed rate of 86.9 kg per day, the actual rate for replacement water based on the feed input rate (Table 2) was 141 l kg−1of feed per day (12300 l/86.9 kg feed per day). Similarly, the actual recycle flow rate for the 12 week sample period was estimated at 18.3 l min−1of recycle flow per kg of feed per day (1590 l min−1/86.9 kg feed per day).
The water quality and associated feed rate data for the two-tank growout system for the 12 week sampling period can be found in graphical form in Figs. 7 and 8. The TAN and NO2– N concentrations for 22 September 1998 and 6 October 1998 were extremely high. Prior to these dates the system was operating with one centrifugal pump per culture tank (415 l min−1
recirculation flow rate per tank). With the second pump operating, the flow rate approximately doubled, however the biofilter distribution nozzle was not adjusted properly. For a period of time, the water from the system was overshooting the biofilter media and much of the water was running down the sides of the filter reactor. The situation was corrected on or around 22 September 1998 and, as can be seen by the data in Fig. 7, the TAN and NO2– N declined to more appropriate levels. TAN concentration ranged from 0.87 to 1.83 mg l−1with an average of 1.34 mg l−1over the period between 14 October and 8 December 1998. The pH within the system ranged from 6.9 to 7.4. Nitrite – nitrogen concentration over the same period varied from 0.83 to 3.98 mg l−1with an average concentration of 1.83 mg l−1. While these NO
2– N concentra-tions were higher than desirable levels, they are typical of production systems not
Fig. 8. Measured nitrate-nitrogen and filterable suspended solids (FSS) concentration in the two-tank growout system during a 12 week period.
yet in steady-state conditions (e.g. mature biological filters). Chloride concentration of the system water was maintained in excess of 100 mg l−1
to ‘protect’ the fish from the harmful effects of nitrite toxicity.
Data in Fig. 8 show a characteristically high nitrate-nitrogen concentration that is expected within a recirculating system (that replaces 10% or less of the system volume per day) operating without an active denitrification system. The filterable suspended solids concentration within the tanks averaged just over 32 mg l−1. While 20 mg l−1 or lower is desirable, this level was acceptable given the fact that the foam fractionation system had not been put into operation during this initial test period.
Areal nitrification rates for the tricking filters in the system are shown graphed versus growout tank TAN concentration in Fig. 9.
These nitrification rates were estimated by multiplying the average flow rate (l per day) to the filter by the difference between the biofilter inflow and outflow TAN concentration (g l−1), then dividing by the total area (m2) of the biological filter media ((l per day×g l−1
) m−2 ).
The data indicate that for tank effluent TAN concentrations of between 1 and 1.5 mg l−1
15 T.M.Losordo et al./Aquacultural Engineering22 (2000) 3 – 16
Fig. 9. Areal nitrification rate for the trickling filters servicing the two-tank growout system.
4. Conclusions
The CP&L/EPRI Fish Barn project at North Carolina State University provides a unique opportunity to collect and analyze data from a recirculating fish produc-tion system at the commercial scale in a university setting. This manuscript provides a detailed review of the main growout system technology within the facility and an examination of preliminary data collected at the facility.
The data from operation of the system indicate that the actual operational characteristics of the system approached the design goals of the two tank growout system. In most cases (Table 2), variances from the design rates were caused by reduced amounts of feed input to the system. The reduced average feed rate was caused by fluctuations in the system biomass as a result of multiple harvests during this study. Increased new water usage occurred early in the study as a result of startup activities and the multiple harvests. Subsequent water use has approached or equaled the design rate.
Acknowledgements
References
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