SRAC Publication No. 451 September 1998 VI PR Revised Recirculating Aquaculture Tank Production Systems An Overview of Critical Considerations Thomas M. Losordo1, Michael P. Masser2 and James Rakocy3 Traditional aquaculture produc- ally use tanks for aquaculture pro- Critical production tion in ponds requires large quan- duction, substantially less land is tities of water. Approximately 1 required. considerations million gallons of water per acre All aquaculture production sys- Aquatic crop production in tanks are required to fill a pond and an tems must provide a suitable and raceways where the environ- equivalent volume is required to environment to promote the ment is controlled through water compensate for evaporation and growth of the aquatic crop. treatment and recirculation has seepage during the year. Critical environmental parameters been studied for decades. Assuming an annual pond yield include the concentrations of dis- Although these technologies have of 5,000 pounds of fish per acre, solved oxygen, un-ionized ammo- been costly, claims of impressive approximately 100 gallons of nia-nitrogen, nitrite-nitrogen, and yields with year-round production water are required per pound of carbon dioxide in the water of the in locations close to major markets fish production. In many areas of culture system. Nitrate concentra- and with extremely little water the United States, traditional tion, pH, and alkalinity levels usage have attracted the interest aquaculture in ponds is not possi- within the system are also impor- of prospective aquaculturists. In ble because of limited water sup- tant. To produce fish in a cost- recent years, a variety of produc- plies or an absence of suitable effective manner, aquaculture pro- tion facilities that use recirculating land for pond construction. duction systems must maintain technology have been built. Recirculating aquaculture produc- Results have been mixed. While good water quality during peri- tion systems may offer an alterna- there have been some notable ods of rapid fish growth. To tive to pond aquaculture technolo- large-scale business failures in this ensure such growth, fish are fed gy. Through water treatment and sector, numerous small- to medi- high-protein pelleted diets at rates reuse, recirculating systems use a um-scale efforts continue produc- ranging from 1.5 to 15 percent of fraction of the water required by tion. their body weight per day ponds to produce similar yields. depending upon their size and Prospective aquaculturists and species (15 percent for juveniles, Because recirculating systems usu- investors need to be aware of the 1.5 percent for market size). basic technical and economic risks 1Department of Zoology, North Carolina involved in this type of aquacul- Feeding rate, feed composition, State University, North Carolina ture production technology. This fish metabolic rate and the quanti- 2Department of Fisheries and Allied fact sheet and others in this series ty of wasted feed affect tank water Aquaculture, Auburn University, are designed to provide basic quality. As pelleted feeds are Alabama information on recirculating aqua- introduced to the fish, they are 3University of the Virgin Islands, either consumed or left to decom- culture technology. Agricultural Experiment Station, U.S. pose within the system. The by- Virgin Islands products of fish metabolism body weight per day, then 37.5 cost-effective water treatment sys- include carbon dioxide, ammo- pounds of feed would produce tem components. All recirculating nia-nitrogen, and fecal solids. If approximately 1.1 pounds of production systems remove waste uneaten feeds and metabolic by- ammonia-nitrogen per day. solids, oxidize ammonia and products are left within the cul- (Approximately 3 percent of the nitrite-nitrogen, remove carbon ture system, they will generate feed becomes ammonia-nitrogen.) dioxide, and aerate or oxygenate additional carbon dioxide and Additionally, if the ammonia- the water before returning it to ammonia-nitrogen, reduce the nitrogen concentration in the tank the fish tank (see Fig. 1). More oxygen content of the water, and is to be maintained at 1.0 mg/l, intensive systems or systems cul- have a direct detrimental impact then a mass balance calculation on turing sensitive species may on the health of the cultured ammonia-nitrogen indicates that require additional treatment product. the required flow rate of new processes such as fine solids In aquaculture ponds, proper water through the tank would be removal, dissolved organics environmental conditions are approximately 5,600 gallons per removal, or some form of disinfec- maintained by balancing the hour (93 gpm) to maintain the tion. inputs of feed with the assimila- specified ammonia-nitrogen con- tive capacity of the pond. The centration. Even at this high flow Waste solids constraints pondÕs natural biological produc- rate, the system also would require aeration to supplement Pelleted feeds used in aquaculture tivity (algae, higher plants, zoo- production consist of protein, car- plankton and bacteria) serves as a the oxygen added by the new water. bohydrates, fat, minerals and biological filter that processes the water. The portion not assimilat- wastes. As pond production ed by the fish is excreted as a intensifies and feed rates increase, Recirculating systems highly organic waste (fecal solids). supplemental and/or emergency design When broken down by bacteria aeration are required. At higher within the system, fecal solids and rates of feeding, water must be Recirculating production technol- ogy is most often used in tank uneaten feed will consume dis- exchanged to maintain good solved oxygen and generate water quality. The carrying capac- systems because sufficient water is not available on site to ÒwashÓ ammonia-nitrogen. For this rea- ity of ponds with supplemental son, waste solids should be aeration is generally considered fish wastes out of production tanks in a flow-through configura- removed from the system as to be 5,000 to 7,000 pounds of fish quickly as possible. Waste solids per acre (0.005 to 0.007 pound of tion or production system that uses water only once. In most can be classified into four cate- fish per gallon of pond water). gories: settleable, suspended, cases, a flow-through requirement The carrying capacity of tank sys- of nearly 100 gallons per minute floatable and dissolved solids. In tems must be high to provide for to maintain one production tank recirculating systems, the first two cost-effective fish production would severely limit production are of primary concern. Dissolved because of the higher initial capi- capacity. By recirculating tank organic solids can become a prob- tal costs of tanks compared to water through a water treatment lem in systems with very little earthen ponds. Because of this system that ÒremovesÓ ammonia water exchange. expense and the limited capacity and other waste products, the Settleable solids control: of the ÒnaturalÓ biological filtra- same effect is achieved as with the Settleable solids are generally the tion of a tank, the producer must flow-through configuration. The easiest of the four categories to rely upon the flow of water efficiency with which the treat- deal with and should be removed through the tanks to wash out the ment system ÒremovesÓ ammonia from the tank and filtration com- waste by-products. Additionally, from the system, the ammonia ponents as rapidly as possible. the oxygen concentration within production rate, and the desired Settleable solids are those that will the tank must be maintained concentration of ammonia-nitro- generally settle out of the water through continuous aeration, gen within the tank determine the within 1 hour under still condi- either with atmospheric oxygen recirculating flow rate from the tions. Settleable solids can be (air) or pure gaseous oxygen. tank to the treatment unit. Using removed as they accumulate on The rate of water exchange the example outlined above, if a the tank bottom through proper required to maintain good water treatment system removes 50 per- placement of drains, or they can quality in tanks is best described cent of the ammonia-nitrogen in be kept in suspension with contin- using an example. Assume that a the water on a single pass, then uous agitation and removed with 5,000-gallon production tank is to the flow rate from the tank would a sedimentation tank (clarifier), be maintained at a culture density need to be twice the flow required mechanical filter (granular or of 0.5 pound of fish per gallon of if fresh water were used to flush screen), or swirl separator. The tank volume. If the 2,500 pounds the tank (93 gpm/0.5 = 186 gpm). sedimentation and swirl separator of fish are fed a 32% protein feed A key to successful recirculating processes can be enhanced by at a rate of 1.5 percent of their production systems is the use of adding steep incline tubes (tube Fine & Dissolved Carbon Dioxide Solids Removal Removal Foam fractionation Air stone diffuser Packed column Fish Culture Tank Round, Octagonal, Rectangular or Aeration or D-ended Disinfection Oxygenation Ultraviolet light Air stone diffuser Ozone contact Packed column Down-flow contactor Low head oxygenator U-tube Waste Solids Removal Biological Filtration (Nitrification) Sedimentation Swirl separators Fluidized bed filters Screen filters Mixed bed filters Bead filters Trickling filters Double drain Rotating bio contactor Figure 1. Required unit processes and some typical components used in recirculating aquaculture production systems. settlers) in the sedimentation tank devices see SRAC 453, that creates foam at the top to reduce flow turbulence and Recirculating Aquaculture Tank air/water interface. As the bub- promote uniform flow distribu- Production Systems: A Review of bles rise through the water col- tion. Component Options. umn, solid particles attach to the bubblesÕ surfaces, forming the Suspended solids control: From Fine and dissolved solids foam at the top of the column. an aquacultural engineering point control: Fine suspended solids The foam build-up is then chan- of view, the difference between (< 30 micrometers) have been nelled out of the fractionation unit suspended solids and settleable shown to contribute more than to a waste collection tank. Solids solids is a practical one. 50 percent of the total suspended concentration in the waste tank Suspended solids will not settle to solids in a recirculating system. can be five times higher than that the bottom of the fish culture tank Fine suspended solids increase the of the culture tank. Although the and cannot be removed easily in oxygen demand of the system and efficiency of foam fractionation is conventional settling basins. cause gill irritation and damage in subject to the chemical properties Suspended solids are not always finfish. Dissolved organic solids of the water, the process generally dealt with adequately in a recircu- (protein) can contribute signifi- can be used to significantly reduce lating production system. If not cantly to the oxygen demand of water turbidity and oxygen removed, suspended solids can the total system. demand of the culture system. significantly limit the amount of Fine and dissolved solids cannot fish that can be grown in the sys- be easily or economically Nitrogen constraints tem and can irritate the gills of removed by sedimentation or fish. The most popular treatment Total ammonia-nitrogen (TAN), mechanical filtration technology. method for removing suspended consisting of un-ionized ammonia Foam fractionation (also referred solids generally involves some (NH3) and ionized ammonia to as protein skimming) is suc- form of mechanical filtration. The (NH4+), is a by-product of protein cessful in removing these solids two types of mechanical filtration metabolism. TAN is excreted from from recirculating tank systems. most commonly used are screen the gills of fish as they assimilate Foam fractionation, as employed filtration and granular media fil- feed and is produced when bacte- in aquaculture, is a process of tration (sand or pelleted media). ria decompose organic waste introducing air bubbles at the bot- For more information on these solids within the system. The un- tom of a closed column of water ionized form of ammonia-nitro- gen is extremely toxic to most concentrations should not exceed ner in which it comes into contact fish. The fraction of TAN in the 10 mg/l for long periods of time with wastewater define the water un-ionized form is dependent and in most cases should remain treatment characteristics of the upon the pH and temperature of below 1 mg/l. biological filtration unit. The most the water. At a pH of 7.0, most of Nitrates are not generally of great common configurations for bio- the TAN is in the ionized form, concern to the aquaculturist. logical filters include rotating bio- while at a pH of 8.75 up to 30 per- Studies have shown that aquatic logical contactors (RBC), fixed cent of TAN is in the un-ionized species can tolerate extremely film reactors, expandable media form. While the lethal concentra- high levels (> 200 mg/l) of filters, and mixed bed reactors. tion of ammonia-nitrogen for nitrate-nitrogen in production sys- For more information on biologi- many species has been estab- tems. Nitrate-nitrogen concentra- cal filters and components see lished, the sub-lethal effects of tions do not generally reach such SRAC 453, Recirculating ammonia-nitrogen have not been high levels in recirculating sys- Aquaculture Tank Production well defined. Reduction in growth tems. Nitrate-nitrogen is either Systems: A Review of Component rates may be the most important flushed from a system during sys- Options. sub-lethal effect. In general, the tem maintenance operations (such concentration of un-ionized as settled solids removal or filter pH and alkalinity constraints ammonia-nitrogen in tanks should backwashing), or denitrification not exceed 0.05 mg/l. The measure of the hydrogen ion occurs within a treatment system (H+) concentration, or pH, in Nitrite-nitrogen (NO2- ) is a prod- component such as a settling tank. water indicates the degree to uct of the oxidation of ammonia- Denitrification occurs when anaer- which water is either acidic or nitrogen. Nitrifying bacteria obic bacteria metabolize nitrate- basic. The pH of water affects (Nitrosomonas) in the production nitrogen to produce nitrogen gas many other water quality parame- system utilize ammonia-nitrogen that is released to the atmosphere ters and the rates of many biologi- as an energy source for growth during the aeration process. For cal and chemical processes. Thus, and produce nitrite-nitrogen as a more information on the effects of pH is considered an important by-product. These bacteria are the water quality on fish production, parameter to be monitored and basis for biological filtration. The see SRAC 452, Recirculating controlled in recirculating aqua- nitrifying bacteria grow on the Aquaculture Tank Production culture systems. Alkalinity is a surface of the biofilter substrate Systems: Management of measure of the waterÕs capacity to although all tank production sys- Recirculating Systems. neutralize acidity (hydrogen ions). tem components will have nitrify- Ammonia and nitrite-nitrogen Bicarbonate (HCO3-) and carbon- ing bacteria present to some control: Controlling the concen- ate (CO3-) are the predominant extent. While nitrite-nitrogen is tration of un-ionized ammonia- bases or sources of alkalinity in not as toxic as ammonia-nitrogen, nitrogen (NH3) in the culture tank most waters. Highly alkaline it is harmful to aquatic species is a primary objective of recircu- waters are more strongly buffered and must be controlled within the lating treatment system design. against pH change than less alka- tank. Ammonia-nitrogen must be line waters. Nitrite-nitrogen binds with hemo- ÒremovedÓ from the culture tank Nitrification is an acid-producing globin to produce methemoglo- at a rate equal to the rate of pro- process. As ammonia-nitrogen is bin. Methemoglobin is not capable duction to maintain a safe concen- transformed to nitrate-nitrogen by of binding and transporting oxy- tration. While there are a number nitrifying bacteria, hydrogen ions gen and the affected fish become of technologies available for are produced. As hydrogen ions starved for oxygen. The toxicity of removing ammonia-nitrogen from combine with bases such as nitrite-nitrogen is species specific. water, biological filtration is the hydroxide (OH-), carbonate and However, a common practice for most widely used. In biological bicarbonate, alkalinity is con- reducing nitrite-nitrogen toxicity filtration (also referred to as biofil- sumed and the pH decreases. is to increase the chloride concen- tration), there is a substrate with a Levels of pH below 4.5 are dan- tration of the culture water. Main- large surface area where nitrifying gerous to fish; a pH below 7.0 will taining a chloride to nitrite-nitro- bacteria can attach and grow. As reduce the activity of nitrifying gen ratio of 10:1 generally will previously noted, ammonia and bacteria. If the source water for a protect against methemoglobin nitrite-nitrogen in the recycle recirculating system is low in build-up and nitrite-nitrogen toxi- stream are oxidized to nitrite and alkalinity, then pH and alkalinity city. Fortunately, Nitrobacter bacte- nitrate-nitrogen by Nitrosomonas should be monitored and alkalini- ria, which also are present in most and Nitrobacter bacteria, respec- ty must be maintained with addi- biological filters, utilize nitrite- tively. Gravel, sand, plastic beads, tions of bases. Some bases com- nitrogen as an energy source and plastic rings, plastic tubes, and monly used include hydrated lime produce nitrate as a by-product. plastic plates are common biofil- [Ca(OH)2] quick lime (CaO), and In a recirculating system with a tration substrates. The configura- sodium bicarbonate (NaHCO3). mature biofilter, nitrite-nitrogen tion of the substrate and the man- Dissolved gas constraints 20 mg/l to maintain good grow- are an effective and simple means ing conditions. of aerating water that is already in Although ammonia-nitrogen The build-up of dissolved nitro- a flow-stream. In a PCA, water build-up can severely limit a recir- gen gas is rarely a problem in low in oxygen is introduced into a culating systemÕs carrying capaci- warm water aquaculture systems. small tower filled with plastic ty, maintaining adequate dis- However, caution is advised medium. A perforated plate or solved oxygen (DO) concentra- when pressurized aeration or oxy- spray nozzle evenly distributes tions in the culture tank and filter genation systems are used the incoming water over the system also is of critical impor- because atmospheric nitrogen can medium. The packed column is tance. In most cases, a systemÕs become supersaturated in water if operated under non-flooded con- ability to add dissolved oxygen to air is entrained into the pressur- ditions so that air exchange water will become the first limit- ized flow stream. Aquatic organ- through the tower is maintained. ing factor in a systemÕs fish carry- isms subjected to elevated concen- If the PCA is to be used for carbon ing capacity. To maintain ade- trations of dissolved nitrogen gas dioxide stripping, a low pressure quate DO levels in the culture can develop Ògas bubblesÓ in air blower will be required to pro- tank, oxygen must be added to their circulatory systems and die. vide a large quantity of air flow the tank at a rate equal to that of through the packed medium. the rate of consumption by fish Maintaining adequate dissolved and bacteria. The consumption oxygen levels and minimizing A number of recirculating system rate of dissolved oxygen in a recir- carbon dioxide concentrations in designs use air-lift pumps (verti- culating system is difficult to cal- the culture tank cannot be over- cal pipes with air injection) to culate, yet an estimate is essential looked in recirculating system recycle water through treatment for proper system design. The design. In a typical intensively processes and back to the culture overall rate of oxygen consump- loaded recirculating system, aera- tank. Air lifts agitate the water tion for a system is the sum of the tion or oxygenation system failure with air bubbles in the process respiration rate of the fish, the can lead to a total loss of the fish and remove CO2 and add dis- oxygen demand of bacteria break- crop in 1/2 hour or less. solved oxygen. ing down organic wastes and Pure Oxygen Injection: In inten- Aeration and Degassing: The uneaten food (also referred to as sive production systems, the rate addition of atmospheric oxygen Biochemical Oxygen Demand or of oxygen consumption by the to water or the release of excess BOD), and the oxygen demand of fish and bacteria may exceed the carbon dioxide from water can be nitrifying bacteria in the filter. capabilities of typical aeration accomplished in recirculating sys- The amount of oxygen required equipment to diffuse atmospheric tems through a variety of devices by the system is largely dictated oxygen into the water. In these such as air diffusers, surface agi- by the length of time waste solids cases, pure gaseous oxygen diffu- tators, and pressurized or non- remain within the system and the sion is used to increase the rate of pressurized packed columns. biofilter configuration. In systems oxygen addition and allow for a System aeration is commonly car- with non-submerged biofilters, higher oxygen utilization rate. ried out in the culture tanks, where solids are removed quickly, The saturation concentration of although this is not a particularly as little as 0.3 pound of oxygen atmospheric oxygen in water good place to add dissolved oxy- can be consumed for every pound rarely exceeds 8.75 mg/l in warm gen. This is because the oxygen of feed added. In systems with water applications (> 20o C). transfer efficiency of aerators submerged biological filters, When pure oxygen is used with drops as the concentration of dis- where solids are retained within gas diffusion systems, the satura- solved oxygen increases to near the system between backwashings tion concentration of oxygen in saturation levels in the tank of solid-removing filters, as much water is increased nearly five fold water. Because saturated condi- as 0.75 pound of oxygen will be to 43 mg/l at standard atmos- tions are desirable in the culture consumed for every pound of pheric pressure. This condition tank, aeration in this location is feed added. allows for more rapid transfer of extremely inefficient. Carbon dioxide (CO2) is a by- oxygen into water even when the In recirculating systems, a better ambient tank dissolved oxygen product of fish and bacterial respi- place to aerate and degas water is concentration is maintained close ration and it can accumulate with- in the recycled flow-stream just to atmospheric saturation (> 7 in recirculating systems. Elevated prior to re-entry into the culture mg/l). carbon dioxide concentrations in tank. At this location, in systems the water are not highly toxic to A measure of success in using using submerged biological filtra- fish when sufficient dissolved pure oxygen in aquaculture is the tion, the concentration of dis- oxygen is present. However, for oxygen absorption efficiency of solved oxygen should be at its most species, free carbon dioxide the injection or diffusion equip- lowest and carbon dioxide con- concentrations in the culture tank ment. The absorption efficiency is centration will be at its highest. should be maintained at less than defined as the ratio of the weight Packed column aerators (PCAs) of oxygen absorbed by the water live or on ice to local niche mar- nomic evaluation. Construction to the weight of oxygen applied kets. These high-priced markets costs of pond production systems through the diffusion or injection appear to be necessary for finan- in the Southeast are approximate- equipment. Properly designed cial success due to the high cost of ly 90 cents per pound of annual oxygen diffusion devices can pro- fish production in recirculating production. Recirculating sys- duce an oxygen absorption effi- systems. In fact, the variable costs tems, on the other hand, cost ciency of more than 90 percent. (feed, fingerling, electricity and between $1 and $4 per pound of However, as with tank aeration labor) of producing fish in recir- annual production. A $1 increase (with air), the culture tank is not culating systems is not much dif- in investment cost per pound of the best location for oxygen diffu- ferent than other production annual production can add more sion with common Òair stoneÓ methods. Where pond culture than 10 cents per pound to the diffusers. Because of the short methods require a great deal of production cost of fish. contact time of bubbles rising electricity (at least 1 kW per acre Given these conditions, producers through a shallow (< 6 feet) water of pond) for aeration during the using recirculating technology column in tanks, air stone dif- summer months, recirculating generally do not attempt to com- fusers have oxygen absorption systems have a steady electrical pete in the same markets as pond efficiencies of not greater than 40 load over the entire year. While it producers. However, in specialty percent. Efficient oxygen injection may appear that recirculating sys- high-value niche markets, such as systems are designed to maxi- tems require more labor in system gourmet foods, tropical or orna- mize both the oxygen/water con- upkeep and maintenance than mental fish, or year-round supply tact area and time. This can be ponds, when the long hours of of fresh product, recirculating sys- achieved through the use of a nightly labor for checking oxygen tem products are finding a place. counter-current contact column, a in ponds and moving emergency The key to niche market success is closed packed-column contact aerators and harvest effort are to identify the market size and unit, a u-tube column or a down- considered, the difference is mini- meet commitments before market flow bubble contactor. For more mal. Recirculating systems can expansion. In most cases, niche information on aeration and oxy- actually have an advantage in markets will limit the size of the genation equipment see SRAC reducing feed costs. Tank produc- production units. 453, Recirculating Aquaculture Tank tion systems generally yield better Production Systems: Component feed conversion ratios than pond Before investing in recirculating Options. systems. systems technology, the prospec- tive aquaculturist should visit a Why, then, are production costs Other production commercial system and learn as generally higher for recirculating much about the technology as considerations systems? The answer usually can possible. As in all aquaculture be found when comparing the There have not been many well- enterprises, the decision to begin capital cost of these systems. documented successes in large- production and the size of the scale fish production in recirculat- Comparing the investment costs production unit one chooses ing systems. Most reports of suc- of recirculating systems with should be based on the market. cessful production have been other production methods is criti- from producers who supply fish cal in making an informed eco- The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045 from the United States Department of Agriculture, Cooperative States Research, Education, and Extension Service. SRAC Publication No. 452 March 1999 Revision Recirculating Aquaculture Tank Production Systems Management of Recirculating Systems Michael P. Masser1, James Rakocy2 and Thomas M. Losordo3 Recirculating systems for holding Production Systems: Component bacteria and algae, which prolifer- and growing fish have been used Options. ate in response to high levels of by fisheries researchers for more Recirculating systems are mechan- nutrients and organic matter. This than three decades. Attempts to ically sophisticated and biological- can cause increases or decreases in advance these systems to com- ly complex. Component failures, tank water levels, reduce aeration mercial scale food fish production poor water quality, stress, dis- efficiency, and reduce biofilter effi- have increased dramatically in the eases, and off-flavor are common ciency. Flow rate reduction can be last decade. The renewed interest problems in poorly managed avoided or mitigated by using in recirculating systems is due to recirculating systems. oversized pipe diameters and con- their perceived advantages, Management of these systems figuring system components to including: greatly reduced land takes education, expertise and shorten piping distances. The and water requirements; a high dedication. fouling of pipes leaving tanks (by degree of environmental control gravity flow) is easily observed allowing year-round growth at Recirculating systems are biologi- because of the accompanying rise optimum rates; and the feasibility cally intense. Fish are usually in tank water level. If flow rates of locating in close proximity to reared intensively (0.5 pound/gal- gradually decline, then pipes prime markets. lon or greater) for recirculating must be cleaned. A sponge, clean- systems to be cost effective. As an ing pad or brush attached to a Unfortunately, many commercial analogy, a 20-gallon home aquari- systems, to date, have failed plumber’s snake works well for um, which is a miniature recircu- scouring pipes. Air diffusers because of poor design, inferior lating system, would have to management, or flawed econom- should be cleaned periodically by maintain at least 10 pounds of fish soaking them in muriatic acid ics. This publication will address to reach this same level of intensi- the problems of managing a recir- (available at plumbing suppliers). ty. This should be a sobering culating aquaculture system so thought to anyone contemplating Flow blockage and water level that those contemplating invest- the management of an intensive fluctuations also can result from ment can make informed deci- recirculating system. the clogging of screens used to sions. For information on theory retain fish in the rearing tanks. and design of recirculating sys- Screen mesh should be the largest tems refer to SRAC Publication System operation size that will retain the fish (usu- No. 451, Recirculating Aquaculture To provide a suitable environment ally 3/4 to 1 inch). The screened Tank Production Systems: An for intensive fish production, area around pipes should be Overview of Critical Considerations, recirculating systems must main- much larger than the pipe diame- and SRAC Publication No. 453, tain uniform flow rates (water and ter, because a few dead fish can Recirculating Aquaculture Tank air/oxygen), fixed water levels, easily block a pipe. Screens can be and uninterrupted operation. made into long cylinders or boxes that attach to pipes and have a 1Auburn University; The main cause of flow reduction large surface area to prevent 2University of the Virgin Islands; is the constriction of pipes and air blockage. Screens should be tight- 3North Carolina State University diffusers by the growth of fungi, ly secured to the pipe so that they Biological filters (biofilters) can particulates are too small to be cannot be dislodged during feed- fail because of senescence, chemi- removed by conventional particu- ing, cleaning and harvesting oper- cal treatment (e. g., disease treat- late filters and cause or compli- ations. ment), or anoxia. It takes weeks to cate many other system problems. An essential component of recir- months to establish or colonize a culating systems is a backup biofilter. The bacteria that colonize Water quality management power source (see SRAC a biofilter grow, age and die. These bacteria are susceptible to In recirculating systems, good Publication No. 453). Electrical water quality must be maintained power failures may not be com- changes in water quality (low dis- solved oxygen [DO], low alkalini- for maximum fish growth and for mon, but it only takes a brief optimum effectiveness of bacteria power failure to cause a cata- ty, low or high pH, high CO2, etc.), chemical treatments, and in the biofilter (Fig. 1). Water qual- strophic fish loss. For example, if ity factors that must be monitored a power failure occurred in a oxygen depletions. Biological fil- ters do not take rapid change and/or controlled include temper- warmwater system (84o F) at sat- ature, dissolved oxygen, carbon urated oxygen concentrations well! dioxide, pH, ammonia, nitrite and containing 1/2-pound fish at a solids. Other water quality factors density of 1/4 pound of fish per Particulates that should be considered are gallon of water, it will take only Particulate removal is one of the alkalinity, nitrate and chloride. 16 minutes for the oxygen con- most complicated problems in centration to decrease to 3 ppm, a recirculating systems. Particulates Temperature stressful level for fish. The same come from uneaten feed and from system containing 1-pound fish at undigested wastes. It has been Temperature must be maintained a density of 1 pound of fish per estimated that more than 60 per- within the range for optimum gallon would plunge to this cent of feed placed into the sys- growth of the cultured species. At stressful oxygen concentration in tem ends up as particulates. Quick optimum temperatures fish grow less than 6 minutes. These scenar- and efficient removal of particu- quickly, convert feed efficiently, ios should give the prospective lates can significantly reduce the and are relatively resistant to manager a sobering feeling for biological demand placed on the many diseases. Biofilter efficiency how important backup power is biofilter, improve biofilter efficien- also is affected by temperature but to the integrity of a recirculating cy, reduce the overall size of the is not generally a problem in system. biofilter required, and lower the warmwater systems. Temperature Certain components of backup oxygen demand on the system. can be regulated with electrical systems need to be automatic. An Particulate filters should be immersion heaters, gas or electric automatic transfer switch should cleaned frequently and main- heating units, heat exchangers, start the backup generator in case tained at peak efficiency. Many chillers, or heat pumps. Tempera- personnel are not present. Auto- matic phone alarm systems are inexpensive and are essential in CLOSED RECIRCULATING SYSTEM alerting key personnel to power N2 DENITRIFICATION failures or water level fluctua- NO2 tions. Some phone alarm systems NO3 NITRIFICATION allow in-dialing so that managers ION BALANCE can phone in and check on the status of the system. Other com- H+ GAS STRIPPING ponent failures can also lead to disastrous results in a very short CO2 ALKALINITY time. Therefore, systems should ADDITION be designed with essential backup TAN BOD BOD REDUCTION components that come on auto- SOLIDS BACTERIA matically or can be turned on DISSOLVED quickly with just a flip of a REFRACTORY MANAGEMENT switch. Finally, one of the sim- O2 plest backups is a tank of pure AERATION oxygen connected with a solenoid INERT valve that opens automatically SOLIDS during power failures. This oxy- RFM 6/6/90 SOLIDS REMOVAL gen-solenoid system can provide Figure 1. Diagram of fish wastes and their effects on bacterial and chemical sufficient dissolved oxygen to interactions in a recirculating system. keep the fish alive during power Courtesy of Ronald F. Malone, Department of Civil Engineering, Louisiana State University, from failures. Louisiana Aquaculture 1992, “Design of Recirculating Systems for Intensive Tilapia Culture,” Douglas G. Drennan and Ronald F. Malone. ture can be manipulated to reduce Water said to be “saturated” with sent). Lethargic behavior and a stress during handling and to con- oxygen contains the maximum sharply reduced appetite are com- trol certain diseases (e.g., Ich and amount of oxygen that will dis- mon symptoms of carbon dioxide ESC). solve in it at a given temperature, stress. salinity and pressure (Table 1). Carbon dioxide can accumulate in Dissolved oxygen Pure oxygen systems can be incor- recirculating systems unless it is porated into recirculating systems. physically or chemically removed. Continuously supplying adequate These inject oxygen into a con- amounts of dissolved oxygen to Carbon dioxide usually is fined stream of water, creating removed from the water by fish and the bacteria/biofilter in supersaturated conditions (see the recirculating system is essen- packed column aerators or other SRAC Publication No. 453). aeration devices (see SRAC tial to its proper operation. Dissolved oxygen (DO) concentra- Supersaturated water, with DO Publication No. 453). tions should be maintained above concentrations several times high- 60 percent of saturation or above 5 er than saturation, is mixed into pH ppm for optimum fish growth in the rearing tank water to maintain DO concentrations near satura- Fish generally can tolerate a pH most warmwater systems. It is range from 6 to 9.5, although a also important to maintain DO tion. The supersaturated water should be introduced into the rapid pH change of two units or concentrations in the biofilter for more is harmful, especially to fry. maximum ammonia and nitrite rearing tank near the bottom and be rapidly mixed throughout the Biofilter bacteria which are impor- removal. Nitrifying bacteria tant in decomposing waste prod- become inefficient at DO concen- tank by currents generated from the water pumping equipment. ucts are not efficient over a wide trations below 2 ppm. pH range. The optimum pH range Proper mixing of the supersaturat- Aeration systems must operate ed water into the tank is critical. for biofilter bacteria is 7 to 8. continuously to support the high Dissolved oxygen will escape into The pH tends to decline in recir- demand for oxygen by the fish the air if the supersaturated water culating systems as bacterial nitri- and microorganisms in the sys- is agitated too vigorously. If the fication produces acids and con- tem. As fish approach harvest size water is mixed too slowly, zones sumes alkalinity, and as carbon and feeding rates (pounds/sys- of supersaturation can cause gas dioxide is generated by the fish tem) are near their maximum lev- bubble disease. In gas bubble dis- and microorganisms. Carbon els, oxygen demand may exceed ease, gases come out of solution dioxide reacts with water to form the capacity of the aeration system inside the fish and form bubbles carbonic acid, which drives the to maintain DO concentrations in the blood. These bubbles can pH downward. Below a pH of 6, above 5 ppm. Fish show signs of result in death. Fry are particular- the nitrifying bacteria are inhibit- oxygen stress by gathering at the ly sensitive to supersaturation. ed and do not remove toxic nitro- surface and swimming into the gen wastes. current produced by the aeration Carbon dioxide device (e. g., agitator, air lift, etc.) Optimum pH range generally is where DO concentrations are Carbon dioxide is produced by maintained in recirculating sys- higher. If this occurs, a supple- respiration of fish and bacteria in tems by adding alkaline buffers. mental aeration system should be the system. Fish begin to stress at The most commonly used buffers used or the feeding rate must be carbon dioxide concentrations are sodium bicarbonate and calci- reduced. above 20 ppm because it interferes um carbonate, but calcium with oxygen uptake. Like oxygen hydroxide, calcium oxide, and Periods of heavy feeding may be sodium hydroxide have been sustained by multiple or continu- stress, fish under CO2 stress come to the surface and congregate used. Calcium carbonate may dis- ous feedings of the daily ration solve too slowly to neutralize a over a 15- to 20-hour period rather around aeration devices (if pre- rapid accumulation of acid. than in two or three discrete meals. As fish digest food, their respiration rate increases dramati- Table 1. Oxygen saturation levels in fresh water at sea level cally, causing a rapid decrease in atmospheric pressure. DO concentrations. Feeding small Temperature DO Temperature DO amounts continuously with auto- oC oF oC oF matic or demand feeders allows mg/L (ppm) mg/L (ppm) DO to decline gradually without 10 50.0 10.92 24 75.2 8.25 reaching critical levels. During 12 53.6 10.43 26 78.8 7.99 periods of heavy feeding, DO 14 57.2 9.98 28 82.4 7.75 should be monitored closely, par- ticularly before and after feedings. 16 60.8 9.56 30 86.0 7.53 Recirculating systems require con- 18 64.4 9.18 32 89.6 7.32 stant monitoring to ensure they 20 68.0 8.84 34 93.2 7.13 are functioning properly. 22 71.6 8.53 36 96.8 6.95 Calcium hydroxide, calcium oxide Table 2. Percentage of total ammonia in the un-ionized form at and sodium hydroxide dissolve differing pH values and temperatures. quickly but are very caustic; these compounds should not be added Temperature (oC) to the rearing tank because they pH 16 18 20 22 24 26 28 30 32 may harm the fish by creating zones of very high pH. The pH of 7.0 0.30 0.34 0.40 0.46 0.52 0.60 0.70 0.81 0.95 the system should be monitored 7.2 0.47 0.54 0.63 0.72 0.82 0.95 1.10 1.27 1.50 daily and adjusted as necessary to maintain optimum levels. Usually, 7.4 0.74 0.86 0.99 1.14 1.30 1.50 1.73 2.00 2.36 the addition of sodium bicarbon- 7.6 1.17 1.35 1.56 1.79 2.05 2.35 2.72 3.13 3.69 ate at a rate of 17 to 20 percent of 7.8 1.84 2.12 2.45 2.80 3.21 3.68 4.24 4.88 5.72 the daily feeding rate is sufficient 8.0 2.88 3.32 3.83 4.37 4.99 5.71 6.55 7.52 8.77 to maintain pH and alkalinity within the desired range (Fig. 2). 8.2 4.49 5.16 5.94 6.76 7.68 8.75 10.00 11.41 13.22 For example, if a tank is being fed 8.4 6.93 7.94 9.09 10.30 11.65 13.20 14.98 16.96 19.46 10 pounds of feed per day then 8.6 10.56 12.03 13.68 15.40 17.28 19.42 21.83 24.45 27.68 approximately 2 pounds of bicar- 8.8 15.76 17.82 20.08 22.38 24.88 27.64 30.68 33.90 37.76 bonate would be added daily to adjust pH and alkalinity levels. 9.0 22.87 25.57 28.47 31.37 34.42 37.71 41.23 44.84 49.02 Alkalinity, the acid neutralizing 9.2 31.97 35.25 38.69 42.01 45.41 48.96 52.65 56.30 60.38 capacity of the water, should be 9.4 42.68 46.32 50.00 53.45 56.86 60.33 63.79 67.12 70.72 maintained at 50 to 100 mg as cal- 9.6 54.14 57.77 61.31 64.54 67.63 70.67 73.63 76.39 79.29 cium carbonate/L or higher, as 9.8 65.17 68.43 71.53 74.25 76.81 79.25 81.57 83.68 85.85 should hardness. Generally, the addition of alkaline buffers used 10.0 74.78 77.46 79.92 82.05 84.00 85.82 87.52 89.05 90.58 to adjust pH will provide ade- 10.2 82.45 84.48 86.32 87.87 89.27 90.56 91.75 92.80 93.84 quate alkalinity, and if the buffers also contain calcium, they add to daily. If total ammonia concentra- tissue damage in several species hardness. For a more detailed dis- tions start to increase, the biofilter of warmwater fish. However, cussion of alkalinity and hardness may not be working properly or tilapia tolerate high un-ionized consult a water quality text. the feeding rate/ammonia nitro- ammonia concentrations and sel- dom display toxic effects in well- gen production is higher than the Nitrogen wastes design capacity of the biofilter. buffered recirculating systems. Ammonia is the principal nitroge- Ammonia should be monitored nous waste released by fish and is mainly excreted across the gills as ammonia gas. Ammonia is a 8.5 Discontinue byproduct from the digestion of supplemental aeration protein. An estimated 2.2 pounds of ammonia nitrogen are pro- 8.0 duced from each 100 pounds of feed fed. Bacteria in the biofilter Optimum Add convert ammonia to nitrite and 7.5 sodium Reduce daily nitrite to nitrate, a process called bicarbonate bicarbonate nitrification. Both ammonia and addition nitrite are toxic to fish and are, 7.0 therefore, major management Increase problems in recirculating systems aeration (Fig. 2). 6.5 Add Ammonia in water exists as two sodium compounds: ionized (NH4+) and bicarbonate un-ionized (NH3) ammonia. Un- & aerate 6.0 ionized ammonia is extremely 0 100 200 300 400 500 toxic to fish. The amount of un- ionized ammonia present depends Alkalinity, mg/L as CaCO3 on pH and temperature of the water (Table 2). Un-ionized Figure 2. The pH management diagram, a graphical solution of the ionization constant ammonia nitrogen concentrations equation for carbonic acid at 25oC. as low as 0.02-0.07 ppm have been Courtesy of Ronald F. Malone, Department of Civil Engineering, Louisiana State University, from Master’s Thesis of Peter A. Allain, 1988, “Ion Shifts and pH Management in High Density Shedding shown to slow growth and cause Systems for Blue Crabs (Callinectes sapidus) and Red Swamp Crawfish (Procambarus clarkii),” Louisiana State University. maintain a pH of 7.5. After the Table 3. Nutrient solution for pre-activation of biofilter. activation period the nutrient Nutrient Concentration (ppm) solution is discarded. Dibasic ammonium phosphate, (NH4)2HPO4 40 Many fish can die during this Dibasic sodium phosphate, Na2HPO4 40 period of biofilter activation. Managers have a tendency to Sea salts “solids” 40 overfeed, which leads to the gen- Sea salts “liquids” 0.5 eration of more ammonia than the Calcium carbonate, CaCO3 250 biofilter can initially handle. At first, ammonia concentrations increase sharply and fish stop Biofilters consist of actively grow- tioning properly. Subdividing or feeding and are seen swimming ing bacteria attached to some sur- compartmentalizing biofilters into the current produced by the face(s). Biofilters can fail if the reduces the likelihood of a com- aeration device. Deaths will soon bacteria die or are inhibited by plete failure and gives the manag- occur unless immediate action is natural aging, toxicity from chem- er the option of “seeding” active taken. At the first sign of high icals (e. g., disease treatment), lack biofilter sludge from one tank or ammonia, feeding should be of oxygen, low pH, or other fac- system to another. stopped. If pH is near 7 the fish tors. Biofilters are designed so that may not show signs of stress Activating a new biofilter (i. e., because little of the ammonia is in aging cells slough off to create developing a healthy population space for active new bacterial the un-ionized form. of nitrifying bacteria capable of growth. However, there can be sit- removing the ammonia and As nitrifying bacteria, known as uations (e. g., cleaning too vigor- nitrite produced at normal feed- Nitrosomonas, become established ously) where all the bacteria are ing rates) requires a least 1 in the biofilter, they quickly con- removed. If chemical additions month. During this activation vert the ammonia into nitrite. This cause biofilter failure, the water in period, the normal stocking and conversion takes place about 2 the system should be exchanged. feeding rates should be greatly weeks into the activation period The biofilter would then have to reduced. Prior to stocking it is and will proceed even if feeding be re-activated (taking 3 or 4 advantageous, but not absolutely has stopped. Once again, fish will weeks) and the pH adjusted to necessary, to pre-activate the seek relief near aeration and mor- optimum levels. biofilters. Pre-activation is accom- talities will occur soon unless During disruptions in biofilter plished by seeding the filter(s) steps are taken. Nitrite concentra- performance, the feeding rate with nitrifying bacteria (available tions decline when a second group should be reduced considerably commercially) and providing a of nitrifying bacteria, known as or feeding should be stopped. synthetic growth medium for a Nitrobacter, become established. Feeding, even after a complete period of 2 weeks. The growth These problems can be avoided if water exchange, can cause ammo- medium contains a source of time is taken to activate the biofil- nia nitrogen or nitrite nitrogen ammonia nitrogen (10 to 20 ters slowly. concentrations (Fig. 3) to rise to mg/l), trace elements and a buffer Nitrite concentrations also should stressful levels in a matter of (Table 3). The buffer (sodium be checked daily. The degree of hours if the biofilter is not func- bicarbonate) should be added to toxicity to nitrite varies with species. Scaled species of fish are generally more tolerant of high nitrite concentrations than species 24 System Ammonia - N such as catfish, which are very Nitrite - N sensitive to nitrite. Nitrite nitrogen Concentration, mg/L 21 as low as 0.5 ppm is stressful to 18 catfish, while concentrations of as nitrogen 15 less than 5 ppm appear to cause little stress to tilapia. Nitrite toxici- 12 ty causes a disease called “brown 9 blood,” which describes the blood color that results when normal 6 blood hemoglobin comes in con- 3 tact with nitrite and forms a com- pound called methemoglobin. 0 Methemoglobin does not transport oxygen properly, and fish react as Figure 3. Typical ammonia and nitrite curves showing time delays in establishing if they are under oxygen stress. bacteria in biofilters. Fish suffering nitrite toxicity come Courtesy of Ronald F. Malone, Department of Civil Engineering, Louisiana State University, from to the surface as in oxygen stress, Master’s Thesis of Don P. Manthe, 1982, “Water Quality of Submerged Biological Rock Filters for Closed Recirculating Blue Crab Shedding Systems,” Louisiana State University. sharply reduce their feeding, and are lethargic. Nitrite toxicity can a low level of suspended solids take 3 to 4 weeks. Table 4 summa- be reduced or blocked by chloride may serve a beneficial role within rizes general water quality ions. Usually 6 to 10 parts of chlo- the system as long as they do not requirements of recirculating sys- ride protect fish from 1 part irritate the fishes’ gills. tems. nitrite nitrogen. Increasing con- If organic solids build up to high centrations of nitrite are a sign levels in the system, they will Water exchange that the biofilter is not working stimulate the growth of microor- properly or the biofilter is not Most recirculating systems are ganisms that produce off-flavor designed to replace 5 to 10 per- large enough to handle the compounds. The concentration of amount of waste being produced. cent of the system volume each solids at which off-flavor com- day with new water. This amount As with ammonia buildup, check pounds develop is not known, pH, alkalinity and dissolved oxy- of exchange prevents the build-up but the system water should of nitrates and soluble organic gen in the biofilter. Reduce feed- never be allowed to develop a ing and be prepared to flush the matter that would eventually foul or fecal smell. If offensive cause problems. In some situa- system with fresh water or add odors develop, increase the water salt (NaCl) if toxic concentrations tions, sufficient water may not be exchange rate, reduce feeding, available for these high exchange develop. increase solids removal, and/or rates. A complete water exchange Nitrate, the end product of nitrifi- enlarge biofilters. should be done after each produc- cation, is relatively nontoxic tion cycle to reduce the build-up except at very high concentra- Chloride of nitrate and dissolved organics. tions (over 300 ppm). Usually nitrate does not build up to these Adding salt (NaCl) to the system For emergency situations it is rec- concentrations if some daily is beneficial not only for the chlo- ommended that the system have exchange (5 to 10 percent) with ride ions, which block nitrite toxi- an auxiliary water reservoir equal fresh water is part of the manage- city, but also because sodium and to one complete water exchange ment routine. Also, in many recir- chloride ions relieve osmotic (flush). The reservoir should be culating systems some denitrifica- stress. Osmotic stress is caused by maintained at the proper temper- tion seems to occur within the the loss of ions from the fishes’ ature and water quality. system that keeps nitrate concen- body fluids (usually through the trations below toxic levels. gills). Osmotic stress accompanies Fish production handling and other forms of Denitrification is the bacteria- stress (e. g., poor water quality). management mediated transformation of nitrate to nitrogen gas, which A salt concentration of 0.02 to 0.2 percent will relieve osmotic stress. Stocking escapes into the atmosphere. This concentration of salt is bene- Fish management starts before the ficial to most species of fish and fish are introduced into the recir- Solids invertebrates. It should be noted culating system. Fingerlings Solid waste, or particulate matter, that rapidly adding salt to a recir- should be purchased from a rep- consists mainly of feces and culating system can decrease utable producer who practices uneaten feed. It is extremely biofilter efficiency. The biofilter genetic selection, knows how to important to remove solids from will slowly adjust to the addition carefully handle and transport the system as quickly as possible. of salt but this adjustment can fish, and does not have a history If solids are allowed to remain in the system, their decomposition will consume oxygen and pro- Table 4. Recommended water quality requirements of recirculating duce additional ammonia and systems. other toxic gases (e. g., hydrogen Component Recommended value or range sulfide). Solids are removed by filtration or settling (SRAC Temperature optimum range for species cultured - less Publication No. 453). A consider- than 5o F as a rapid change able amount of highly malodor- Dissolved oxygen 60% or more of saturation, usually 5 ppm ous sludge is produced by recir- or more for warmwater fish and greater than culating systems, and it must be disposed of in an environmental- 2 ppm in biofilter effluent ly sound manner (e. g., applied to Carbon dioxide less than 20 ppm agricultural land or composted). pH 7.0 to 8.0 Very small (colloidal) solids Total alkalinity 50 to 100 ppm or more as CaCO3 remain suspended in the water. Total hardness 50 to 100 ppm or more as CaCO3 Although the decay of this mater- ial consumes oxygen and pro- Un-ionized ammonia-N less than 0.05 ppm duces some additional ammonia, Nitrite-N less than 0.5 ppm it also serves as attachment sites Salt 0.02 to 0.2 % for nitrifying bacteria. Therefore, of disease problems in his/her growth and near the system maxi- mum—the highest feeding rates at Table 5. Estimated food con- hatchery. Starting with poor quali- ty or diseased fingerlings almost which acceptable water quality sumption by size of a ensures failure. conditions can be maintained. typical warmwater fish. Fish should be checked for para- When more feed is required, fish Average Body weight sites and diseases before being stocks should be split and moved weight per fish consumed introduced into the system. New to new tanks. This would gradual- (lbs.) (g) (%) fish may need to be quarantined ly reduce the stocking rate over the production cycle. 0.02 9 5.0 from fish already in the system so that diseases will not be intro- Another approach is to divide the 0.04 18 4.0 duced. A few fish should be rearing tank(s) into compartments 0.06 27 3.3 checked for parasites and diseases with different size groups of fish 0.25 113 3.0 by a certified fish diagnostician. in each compartment. In this approach, the optimum feeding 0.50 227 2.75 Once diseases are introduced into a recirculating system they are rate for all the compartments is 0.75 340 2.5 generally hard to control, and consistently near the biofilter’s 1.0 454 2.2 treatment may disrupt the biofil- maximum performance. As one 1.5 681 1.8 ter. group of fish is harvested, finger- Fish are usually hauled in cool lings are immediately stocked into water. As they come into the sys- the vacant compartment or tank. Table 6 approximates a feeding tem they usually have to be tem- Compartment size within a tank schedule for a warmwater fish pered or gradually acclimated to may be adjusted as fish grow, by (e.g., tilapia) stocked into an 84o F the system temperature and pH. using movable screens. recirculating system as fry and Fish can generally take a 5o F harvested at a weight of 1 pound change without much problem. Feeding after 250 feeding days. Feed con- Temperature changes of more Knowing how much to feed fish version is estimated at 1.5: 1, or than 5o F should be done at about without overfeeding is a problem 1.5 pounds of feed to obtain 1 1o F every 20 to 30 minutes. Stress in any type of fish production. pound of gain. can be reduced if the system is Feeding rates are usually based on Tables 5 and 6 are estimates and cooled to the temperature of the fish size. Small fish consume a should be used only as guidelines hauling water and then slowly higher percent of their body which can change with differing increased over a period of several weight per day than do larger fish species and temperatures. hours to days. (Table 5). Most fish being grown Growth and feed conversion are Recirculating systems must oper- for food will be stocked as finger- estimated by weighing a sample ate near maximum production lings. Fingerlings consume 3 to 4 of fish from each tank and then (i. e., maximum risk) capacity at percent of their body weight per calculating the feed conversion all times to be economical. It is not day until they reach 1/4 to 1/2 ratios and new feeding rates from cost effective to operate pumps pound, then consume 2 to 3 per- this sample. For example, 1,000 and aeration devices when the cent of their body weight until fish in a tank have been consum- system is stocked with fingerlings being harvested at 1 to 2 pounds. ing 10 pounds of feed a day for at only one-tenth of the system’s A rule-of-thumb for pond culture the last 10 days (100 pounds carrying capacity. Therefore, fin- is to feed all the fish will consume total). The fish were sampled 10 gerlings should be stocked at very in 5 to 10 minutes. Unfortunately, days earlier and weighed an aver- high rates, in the range of 30 fish this method can easily lead to age of 0.33 pounds or an estimat- per cubic foot. Feeding rates overfeeding. Overfeeding wastes ed total of 330 pounds. should be optimum for rapid feed, degrades water quality, and can overload the biofilter. Table 6. Recommended stocking and feeding rates for different size groups of tilapia in tanks, and estimated growth rates. Stocking rate Weight (g) Growth rate Growth period Feeding rate (number/ft3) Initial Final (g/day) (days) (%) 225 0.02 0.5-1 - 30 20 - 15 90 0.5-1 5 - 30 15 - 10 45 5 20 0.5 30 10 - 7 28 20 50 1.0 30 7 - 4 14 50 100 1.5 30 4 - 3.5 5.5 100 250 2.5 30 3.5 - 1.5 3 250 450 3.0 70 1.5 - 1.0 A new sample of 25 fish is collect- minutes. Multiple feedings at the (depending on the type and sever- ed from the tank and weighed. same location in a tank can ity of off-flavor). If fish remain in The 25 fish weigh 10 pounds or an increase dominance because a few the purging tanks for an extended average of 0.4 pounds per fish. If fish jealously guard the area and period, their feeding rate may this is a representative sample, do not let other fish feed. In this need to be reduced, or off-flavor then 1,000 fish should weigh 400 situation, use feeders that distrib- may develop within the purging pounds. Therefore, the change in ute feed widely across the tank. system. total fish weight for this tank is Fish can be fed by hand, with See SRAC Publication No. 431, 400 minus 330, or 70 pounds. The demand feeders, or by automatic Testing Flavor Quality of Preharvest fish were fed 100 pounds of feed feeders, but stationary demand Channel Catfish, for detailed infor- in the last 10 days and gained 70 and belt type feeders tend to mation on off-flavor. pounds in weight. Feed conver- encourage dominance. Whichever sion then is equal to 1.43 to 1 (i.e., method is used, be careful to 100 ÷ 70). In other words, the fish evenly distribute feed and not to Stress and disease control gained 1 pound of weight for each overfeed. The key to fish management is 1.43 pounds of feed fed. The daily Always purchase high quality stress management. Fish can be feeding rate should now be feed from a reputable company. stressed by changes in tempera- increased to adjust for growth of Keep feed fresh by storing it in a ture and water quality, by han- the fish. cool, dry place. Never use feed dling (including seining and haul- To calculate the new feeding rate, that is past 60 days of the manu- ing), by nutritional deficiencies, multiply the estimated total fish facture date. Never feed moldy, and by exposure to parasites and weight (400 pounds) by the esti- discolored or clumped feed. diseases. Stress increases the sus- mated percent body weight of Molds on feed may produce afla- ceptibility of fish to disease, which feed consumption for a 0.4-pound toxins, which can stress or kill can lead to catastrophic fish losses fish (from Table 5). Table 5 sug- fish. Feed quality deteriorates if not detected and treated quick- gests that the percent body weight with time, particularly when ly. To reduce stress fish must be consumed per day should be stored in warm, damp conditions. handled gently, kept under proper between 2.75 and 3 percent. If 3 A disease known as “no blood” is water quality conditions, and pro- percent is used, then 400 times associated with feed that is defi- tected from exposure to poor 0.03 is 12.0. Thus, the new feeding cient in certain vitamins. In a case water quality and diseases. Even rate should be 12 pounds of feed of “no blood,” the fish appear sound and light can stress fish. per day for the next 10 days, for a pale with white gills and blood Unexpected sounds or sudden total of 120 pounds. Using this appears clear, not red. Another flashes of light often trigger an sampling technique the manager nutritional disease known as “bro- escape response in fish. In a tank, can accurately track growth and ken back syndrome” is caused by this escape response may send feed conversion, and base other a vitamin C deficiency. The only fish into the side of the tank, caus- management decisions on these management practice for “no ing injury. Fish are generally sen- factors. blood” disease and “broken back sitive to light exposure, particular- syndrome” is to discard the feed ly if it is sudden or intense. For Feeding skills being used and purchase a differ- this reason many recirculating ent batch or brand of feed. systems have minimal lighting Feeding is the best opportunity to around the fish tanks. observe overall vitality of the fish. Fines, crumbled feed particles, are A poor feeding response should not generally consumed by the Diseases be an immediate alarm to the fish but add to the waste load of manager. Check all aspects of the the system, increasing the burden There are more than 100 known system, particularly water quality, on particulate and biological fil- fish diseases, most of which do and diagnose for diseases if feed- ters. Therefore, it is recommended not seem to discriminate between ing behavior suddenly diminish- that feed pellets be sifted or species. Other diseases are very es. screened to remove fines before host specific. Organisms known to feeding. cause diseases and/or parasitize Fish can be fed once or several fish include viruses, bacteria, times a day. Multiple feedings fungi, protozoa, crustaceans, flat- Off-flavor spread out the waste load on the worms, roundworms and seg- biofilter and help prevent sudden Off-flavor in recirculating systems mented worms. There are also decreases in DO. Research has is a common and persistent prob- non-infectious diseases such as shown that small fish will grow lem. Many times fish have to be brown blood, no blood and bro- faster if fed several times a day. moved into a clean system, one ken back syndrome. Any of these Feeding several times a day seems with clear, uncontaminated water, diseases can become a problem in to reduce problems of feeding where they can be purged of off- a recirculating system. Diseases dominance in some species of fish. flavor before being marketed. can be introduced into the system Many recirculating system man- Purging fish of off-flavor can take from the water, the fish, and the agers feed as often as every 30 from a few days to many weeks system’s equipment. Diseases are likely to enter the s Cessation of feeding systems, chemical treatments can system from hauling water, on the s Mortalities severely disrupt the biofilter. fish themselves, or on nets, bas- Biofilter bacteria are inhibited to kets, gloves, etc., that are moved Whenever any of these symptoms some degree by formalin, copper from tank to tank. Hauling water appear the manager should check sulfate, potassium permanganate, should never be introduced into water quality and have a few fish and certain antibiotics. Even sud- the system. Fish should be quar- with symptoms diagnosed by a den changes in salt concentration antined, checked for diseases, and qualified fish disease specialist. will decrease biofilter efficiency. If treated as necessary. Equipment The most common diseases in the system is designed properly, it should be sterilized (e. g., chlorine recirculating systems are caused may be possible to isolate the dip) before moving it between by bacteria and protozoans. Some biofilter from the rest of the sys- tanks. If possible, provide sepa- diseases that have been particular- tem, treat and flush the fish tanks, rate nets and baskets for each tank ly problematic in recirculating and then reconnect the biofilter so they will not contaminate other systems include the protozoal dis- without exposing it to chemical tanks. Disease can spread rapidly eases Ich (Ichthyophthirius) and treatment. However, there is a from one tank to another if equip- Trichodina, and the bacterial dis- danger that the biofilter will re- ment is freely moved between eases columnaris, Aeromonas, introduce the disease organism. tanks or if all the water within the Streptococcus and Mycobacterium. It Whenever a chemical treatment is system is mixed together as in a appears that Trichodina and applied, be prepared to exchange common sump, particulate filter Streptococcus diseases are prob- the system water and monitor the or biofilter. lematic in recirculating systems DO concentration and other water with tilapia, while Mycobacterium quality factors closely. Fish usual- A manager needs to be familiar has been found in hybrid striped ly reduce their feed consumption with the signs of stress and dis- bass in intensive recirculating sys- after a chemical treatment; there- ease which include: tems. fore, feeding rates need to be s Excitability monitored carefully. It may be possible to treat dis- s Flashing or whirling eases with chemicals approved for Tables 7 and 8 give possible caus- s Skin or fin sores or discol- fish (see SRAC Publication No. es and management options based orations 410, Calculating Treatments for on the observation of the fish or Ponds and Tanks), although few water quality tests. s Staying at the surface therapeutants are approved for s Erratic swimming use on food fish species other Conclusions s Reduction in feeding rate than catfish and rainbow trout. Treatment always has its prob- Recirculating systems have devel- s Gulping at the surface oped to the point that they are lems. In the case of recirculating being used for research, for orna- mental/tropical fish culture, for maturing and staging brood fish, Examples of fish diseases for producing advanced fry/fin- gerlings, and for producing food fish for high dollar niche markets. They continue to be expensive ventures which are as much art as science, particularly when it comes to management. Do your homework before deciding to invest in a recirculating system. Investigate the efficiency, compati- A–Columnaris B–Aeromonas bility and maintenance require- ments of the components. Estimate the costs of building and operating the system and of mar- keting the fish without any return on investment for at least 2 years. Know the species you intend to grow, their environmental require- ments, diseases most common in their culture, and how those dis- eases are treated. Know your potential markets and how the fish need to be prepared for that C–Streptococcus D–Mycobacterium (cataract and pop-eye) market. Be realistic about the (granular liver and spleen) Table 7. Possible options in managing a recirculating tank system based on observations of the fish. Observation Possible cause Possible management Fish: Excitable/darting/erratic swimming s excess or intense reduce sound level/pad sides of tank/reduce sounds/lights light intensity s parasite examine* fish with symptoms s high ammonia check ammonia concentration Flashing/whirling s parasite examine fish with symptoms Discolorations/sores s parasite/bacteria examine fish with symptoms Bloated/eyes bulging out s virus or bacteria examine fish with symptoms s gas bubble disease check for supersaturation and examine fish with symptoms Lying at surface/not swimming off s parasite examine fish with symptoms s low oxygen check dissolved oxygen in tank s high ammonia or nitrite check ammonia and nitrite concentrations s bad feed check feed for discoloration/clumping and check blood of fish s high carbon dioxide check carbon dioxide level Crowding around water inflow/aerators s low oxygen check dissolved oxygen in tank s parasite/disease examine fish with symptoms s high ammonia or nitrite check ammonia and nitrite concentrations s bad feed check feed for discoloration/clumping and check blood of fish Gulping at surface s low oxygen check dissolved oxygen in tank s parasite/disease examine fish with symptoms s high ammonia or nitrite check ammonia and nitrite concentrations s high carbon dioxide check carbon dioxide level s bad feed check feed for discoloration/clumping and check blood of fish Reducing feeding s low oxygen check dissolved oxygen in tank s parasite/disease examine fish with symptoms s high ammonia or nitrite check ammonia and nitrite concentrations s bad feed check feed for discoloration/clumping and check blood of fish Stopping feeding s low oxygen check dissolved oxygen in tank s parasite/disease examine fish with symptoms s high ammonia or nitrite check ammonia and nitrite concentrations Discolored blood – s high nitrite examine fish with symptom; add 5 to 6 ppm Brown chloride for each 1 ppm nitrite; purchase new feed and discard old feed Clear (no blood) s vitamin deficiency examine fish with symptom; check feed for discoloration/clumping; purchase new feed and discard old feed Broken back or “S” shaped backbone s vitamin deficiency examine fish with symptom; purchase new feed and discard old feed *Have fish examined by a qualified fish diagnostician. money, time and effort you are Finally, design the system with an Exclude diseases at stocking. willing to invest while you are in emergency aeration system, back- Perform routine diagnostic checks the learning curve of managing a up power sources, and backup and be prepared to treat diseases. recirculating system. system components. Monitor Reduce stress whenever and how- water quality daily and maintain ever possible. STAY ALERT! it within optimum ranges. Table 8. Possible management options based on water quality and feed observations. Observation Possible management Low dissolved oxygen (less than 5 ppm) s increase aeration s stop feeding until corrected s watch for symptoms of new parasite/disease High carbon dioxide (above 20 ppm) s add air stripping column s increase aeration s watch for symptoms of new paraside/disease Low pH (less than 6.8) s add alkaline buffers (sodium bicarbonate, etc.) s reduce feeding rate s check ammonia and nitrite concentarations High ammonia (above 0.05 ppm as un-ionized) s exchange system water s reduce feeding rate s check biofilter, pH, alkalinity, hardness, and dissolved oxygen in the biofilter s watch for symptoms of new parasite/disease High nitrite (above 0.5 ppm) s exchange system water s reduce feeding rate s add 5 to 6 ppm chloride per 1 ppm nitrite s check biofilter, pH, alkalinity, hardness, and dissolved oxygen in the biofilter s watch for symptoms of new parasite/disease Low alkalinity s add alkaline buffers Low hardness s add calcium carbonate or calcium chloride Discolored/clumped feed s purchase new feed and discard old feed s watch for symptoms of new parasite/disease The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045 from the United States Department of Agriculture, Cooperative States Research, Education, and Extension Service. SRAC Publication No. 453 April 1999 VI PR Revised Recirculating Aquaculture Tank Production Systems A Review of Component Options Thomas M. Losordo1, Michael P. Masser2 and James E. Rakocy3 There is a great deal of interest in tion site infrastructure, production water through special filtration recirculating aquaculture produc- management expertise, and other and aeration or oxygenation tion systems both in the United factors. Prospective users of recir- equipment. Each component must States and worldwide. Most fish culating aquaculture production be designed to work in conjunc- grown in ponds, floating net pens, systems need to know about the tion with other components of the or raceways can be reared in com- required water treatment process- system. For more information on mercial scale recirculating sys- es, the components available for water quality requirements and tems, but the economic feasibility each process, and the technology management of recirculating sys- of doing so is not certain. Recircu- behind each component. This tems, see SRAC publications 451 lating systems are generally publication is intended as a start- and 452. expensive to build, which increas- ing point for such a study. es production cost. (For more A recirculating system maintains Waste solids removal information see SRAC publication an excellent cultural environment 456 on the economics of recirculat- The decomposition of solid fish while providing adequate feed for waste and uneaten or indigestible ing systems). The challenge to optimal growth. Maintaining designers of recirculating systems feed can use a significant amount good water quality is of primary of oxygen and produce large is to maximize production capaci- importance in aquaculture. While ty per dollar of capital invested. quantities of ammonia-nitrogen. poor water quality may not be There are three categories of waste Components should be designed lethal to the crop, it can reduce and integrated into the complete solids—settleable, suspended, and growth and cause stress that fine or dissolved solids. system to reduce cost while main- increases the incidence of disease. taining or even improving reliabil- Critical water characteristics ity. Settleable solids include concentrations of dis- Research and development in solved oxygen, un-ionized ammo- Settleable solids are generally the recirculating systems has been nia-nitrogen, nitrite-nitrogen, and easiest to deal with and should be going on for nearly three decades. carbon dioxide. Nitrate concentra- removed from the culture tank There are many alternative tech- tion, pH, alkalinity and chloride water as rapidly as possible. This nologies for each process and levels also are important. is easiest when bottom drains are operation. The selection of a par- properly placed. In tanks with cir- The by-products of fish metabo- ticular technology depends upon cular flow patterns (round, octag- lism include carbon dioxide, the species being reared, produc- onal, hexagonal, square with ammonia-nitrogen, and particu- rounded corners) and minimal late and dissolved fecal solids. 1Department of Zoology, North Carolina agitation, settleable solids can be Water treatment components must removed as they accumulate in State University be designed to eliminate the 2Department of Wildlife and Fisheries the bottom center of the tank, in a adverse effects of these waste Sciences, Texas A&M University separate, small flow-stream of products. In recirculating tank 3University of the Virgin Islands, water, or together with the entire systems, proper water quality is Agricultural Experiment Station, U.S. flow leaving the tank. Center maintained by pumping tank Virgin Islands drains with two outlets are often used for the small flow-stream settle out within the pipe while In rectangular raceways with plug process. This double drain divides the clearer water overflows the flow (flow that moves along the the flow leaving the tank into a standpipe. The external stand long axis of the raceway tank), small pipe carrying the settleable pipe is routinely removed to solids are more difficult to remove solids, and a larger pipe with a increase the water velocity in the as the velocity at the bottom of the higher flow rate carrying the sus- pipe and the settled solids are tank is generally slower than in pended solids from the upper flushed from the line. round tanks. If the water velocity water column of the tank (Fig. 1). at the tank bottom can be Another example of a double increased to move the settled drain is a particle trap developed solids along the bottom of the A A at the Center for Scientific and tank, then solids can be removed Industrial Research (SINTEF), using a sediment trap. The sedi- Norwegian Hydrotechnical ment trap should span the bot- Standpipe Laboratory, in Trondheim, tom, across the short axis of the Norway. raceway, perpendicular to the In this design, settleable solids direction of water flow. Two flow under a plate, spaced just reviews of tank flow and B B slightly off the bottom of the tank, hydraulic analysis can be found in in a flow of water that amounts to Burley and Klapsis (1988) and Solids only 5 percent of the total flow Tvinnereim (1988). collection leaving the center of the tank An alternative to plug flow within bowl B (Flow B, Fig. 2). The larger flow a raceway is to create a complete- A (95 percent of the total) exits the ly mixed (horizontally and verti- tank through a large discharge cally) tank by installing a water Figure 1. Typical double drain for strainer mounted at the top of the inlet and outlet manifold along removing settleable solids from a fish particle trap (Flow A, Fig. 2). the long axis of the tank. As seen Outside of the tank, the settleable culture tank; A = suspended solids in Figure 4, water enters uniform- solids flow-stream from the parti- flow stream, B = settleable solids flow ly along the bottom of one side of cle trap enters a sludge collector stream. (after Losordo, 1997). the raceway and is removed along (Flow B, Fig. 3). The waste parti- the other side. Water must enter at cles settle and are retained in the a high enough velocity to create a Settled solids should be removed sludge collector, and the clarified rotational flow along the short from the center of the tank on a water exits the sludge collector at axis of the raceway (Fig. 4). The continuous or semi-continuous the top and flows by gravity for solids will move across the bot- basis. The flow rate at which the further treatment. The sludge in tom of the raceway and into the settleable solids are carried will the collector, which has an aver- effluent manifold. determine the method used to col- age dry weight solids content of 6 lect and concentrate them for fur- percent, is drained from the bot- Another method of dealing with ther treatment or disposal. In sys- tom of the collector. settleable solids is to keep them in tems with a high settleable solids flow rate (20 to 50 percent of the A A total tank flow), swirl separators, settling basins, or drum screen fil- ters are used to collect these B B solids. At lower flow rates, small- er settling components can be used. An example is a double Tank drain developed by Waterline, floor Inc.1 (Prince Edward Island, B Canada). In this patented design, Settleable the flow containing settleable solids flow solids moves slowly though a pipe (under the tank) leading to an external standpipe (water level control structure). The flow veloci- Main discharge A ty is slow enough that the solids Suspended solids flow 1Mention of a specific product or trade- name does not constitute and endorsement Figure 2. The ECO-TRAP™ particle trap is an advanced double drain design that by the authors or the USDA Southern concentrates much of the settleable solids in only 5 percent of the water flow leav- Regional Aquaculture Center, nor does it ing the fish culture tank (B). (after Hobbs et al., 1997). (ECO-TRAP is a trade- imply approval to the exclusion of other mark of AquaOptima AS, Pir Senteret, 7005 Trondheim, Norway, U.S. Patent suitable products. No. 5,636,595.) suspension with continuous agita- Top view tion until they enter an external Flow B settling tank. In settling tanks (or from basins), water flow is very slow so particle that solids settle out by gravity. trap Settling tanks may or may not Clarified water to drum screen include tube or lamella sedimen- tation material. This material is filter constructed with bundles of tubes or plates, set at specific angles to the horizontal (usually 60o), that Side view reduce both the settling distance and circulation within the settling Clarified water Flow B tank. Using settling plates reduces to drum screen from the size requirement of a settling filter particle basin, thus saving space within a trap facility. However, the plates make routine cleaning of settling basins more time-consuming. The benefits of using external set- tling basins outside of the rearing tank are simplicity of operation, low energy requirements, and the generally low cost of construction. The disadvantages include the rel- atively large size of settling basins, the time used in routine cleaning, and the large quantity of water that is wasted in the clean- ing process. If settling basins are not cleaned regularly, waste solids can break down within the basin and contribute to the ammonia- nitrogen production and oxygen demand of the system. Sludge discharge Another way to remove settleable Figure 3. The sludge collector that works in conjunction with the ECO-TRAP™ solids, external to the culture tank, to remove settled solids from the flow stream B (Fig. 2) (after Hobbs et al., 1997). is to use a centrifugal settling component known as a hydrocy- clone or swirl separator. In this Short design, water and particulate axis solids enter the separator tangen- Long tially, creating a circular or axis swirling flow pattern in a conical shaped tank. The heavier solids move towards the walls and settle to the bottom where they are removed continuously. The main advantage of these units is the compact size. A major disadvan- tage is the large volume of replacement water required because of the continuous stream of wastewater. A Influent Suspended solids From an engineering viewpoint, B the difference between suspended Effluent solids and settleable solids is a Figure 4. Cross-section of a “cross-flow” raceway. Water flows in through an practical one. Suspended solids inlet manifold with jets (A) and out through a similar drain manifold (B) on the will not easily settle out of the opposite side of the tank (after Colt and Watten, 1988). water column in the fish culture tank. Suspended solids are not The screening material has been a high pressure jet of water (from always dealt with adequately in used in a disk configuration (Fig. the outside of the drum) washes recirculating systems. Most cur- 5A), drum screen configuration the solids off the screen and into rent technologies for removing (Fig. 5B), and incline belt configu- an internal collection trough lead- suspended solids generally ration (Fig. 5C). ing to a waste drain. The advan- involve some form of mechanical In rotating disk filters, water to be tage of the drum screen filter con- filtration. Two types of mechanical treated enters one end of the filter figuration over the single plate filtration are screen filtration and unit and must pass through disk filter is the larger surface area expandable granular media filtra- sequential vertical disks within of the drum for comparably sized tion. the filter. A problem with this units. Screen filtration: Screen filters design is the small amount of The main advantage of using use some form of fine mesh mate- screen surface on which to capture screen filter technology rather rial (stainless steel or polyester) solids. In heavily fed production than settling basins and swirl sep- through which effluent passes systems, solids can build up so arators is their small size and rel- while the suspended solids are heavily on one side of the filter atively low water loss during retained on the screen. Solids are that the screens collapse from the backwashing. Libey (1993) report- usually removed from the screen water pressure. ed that, on average, in a tilapia by rotating the clogged screen sur- The most common screen filter is system, only 13.4 percent of the face past high pressure jets of the drum filter (Figs. 5B and 6). water used with a settling basin water. The solids are carried away With this configuration, water was needed with a drum screen from the screen in a small stream enters the open end of a drum filter. of waste water. The feature that and passes through a screen The main disadvantage of com- makes each screen filter different attached to the circumference of mercial screen filters is cost, espe- and the challenge in designing the drum. cially for smaller units. The these units is the process of col- smallest commercially available lecting the solids on the mesh sur- In most installations, the drum rotates only when the filter mesh units can process approximately face. 475 liters per minute (125 gpm) becomes clogged with solids, and loaded with 25 mg/L of suspend- ed solids, and cost about $6,000. A 100 percent Backwash water increase in processing Wastewater capacity increases the cost of a unit by about 50 per- Backwash cent (a unit to process 950 Inflow water liters/minute costs about Cleaned Wastewater water $9,000). So, larger units are more cost effective. To take advantage of this, Cleaned the flow streams from water Inflow several production tanks can be combined into one Disk screens treatment stream that is Drum screen A. Disk screen filter (top view) B. Drum screen filter (top view) cleaned by a larger drum screen filter. However, the advantage of the econo- Waste trough my of scale must be weighed against the risk of spreading disease and Backwash spray water quality problems Belt screen within linked fish pro- duction tanks. Vacuum cleaned drum Inflow screen filters are now in Cleaned use. These units have water Flow limited capacity (375 to 1,800 L/ minute, 100 to 475 gpm) and their per- C. Incline belt screen filter (side view) formance in commercial facilities has not been Figure 5. Three screen filter configurations used in recirculating tanks to capture and remove well documented. Incline suspended solids. screen or belt screen fil- ters also are beginning to Propeller drive motor Return flow to culture tank Floating plastic bead medium Propellers Flow from culture tank Settled solids Pressure To waste backwash Outflow to tank Figure 7. The propeller washed bead filter traps waste solids between the Water filters beads and backwashes by expanding Waste through screen the bed of beads with a propeller. discharge on drum (U.S. Patent No. 5,126,042 by Dr. Ronald Malone, Dept. of Civil Inflow from tank Engineering, Louisiana State University) Figure 6. Typical drum screen filter (shown with a cut-away and expanded midsec- tion) for waste solids removal from aquacultural recycle flow streams. (Drawing pro- These low density, floating plastic vided by and used with permission of PRA Manufacturing, Nanaimo, B.C.) beads trap and remove suspended solids from the flow-stream as the be used in the aquaculture indus- water, swimming pools), the most water passes up through a bed of try (Fig. 5). These units resemble common filtration medium is beads (Fig. 7). conveyor belts placed on an sand. Pressurized down-flow The solids are removed by activat- incline. Water passes through the sand filters have been widely ing a motor that turns a propeller screen where suspended solids are used in hatchery operations. located within the bed of beads. retained; solids are lifted out of While these filters can remove The propeller expands the bed of the water on the incline screen much of the suspended solids in a beads and releases the waste and sprayed off with high pres- flow-stream, when fish are fed solids that are trapped within it. sure water in a cleaning process heavily the filter must be back- After the bed expansion period, a similar to that of disc and drum washed frequently, which wastes short settling period allows the screen filters. The units manufac- a lot of water. Backwashing these beads to re-float and the solids to tured currently have flow capaci- filters is accomplished by revers- settle to the bottom of the filter ties in excess of 7,500 liters per ing the flow of water through the chamber. A valve is then opened minute (1980 gpm). There is little filter medium, causing the bed to and the settled solids are data on the operational character- expand or “boil.” This releases removed. This sequence of events istics of these filters. trapped solids and scrapes bacter- can be automated with electronic Expandable granular media fil- ial growth off the filter medium. circuits and automated valves. tration: Expandable granular However, bacterial growth on the Another bead filter design, media filters remove solids by sand eventually creates gelatinous referred to as the “bubble passing water through a bed of masses within the filters that are washed” bead filter, eliminates the granular medium (sand or plastic impossible to clean with simple requirement for a propeller to beads). The solids either adhere to backwashing. Then it is necessary backwash the filter bed. This filter the medium or are trapped within to open and manually clean the resembles an “hour glass” with the open spaces between the filter. Down-flow sand filters two chambers connected by a nar- medium particles. Over time, the reduce or stop the flow of water row “washing throat” (Fig. 8). filters will become clogged with when they clog. Even short-term interruptions of water flow can be In the filtration mode, water pass- solids and require cleaning, or es up through the beads while backwashing. Backwashing these disastrous to intensive recirculat- ing systems. they are in the upper filtration filters requires that the filter bed chamber. When the beads need be expanded (from a compacted An alternative design, used suc- cleaning, the flow is stopped and state) to release the solids. For cessfully in the U.S., uses floating the filter is drained so that the fil- other applications (e.g., drinking plastic beads instead of sand. ter medium drops through the Return to nitrogen produced must be treat- ture enters the body of the foam tank ed. fractionator tangentially. Floating bead filter bed Fine and dissolved solids Ammonia and nitrite- Many of the fine suspended solids nitrogen control Filtration and dissolved organic solids that chamber Controlling the concentration of build up within intensive recircu- un-ionized ammonia-nitrogen lating systems cannot be removed (NH3) in the culture tank is a pri- Washing with traditional mechanisms. A throat mary design consideration in process called foam fractionation recirculating systems. Ammonia- (also referred to as air-stripping or nitrogen (a by-product of the protein skimming) is often metabolism of protein in feeds) employed to remove and control must be removed from the culture Pumped effluent the build-up of these solids. tank at a rate equal to the rate it is Sludge from tank Foam fractionation is a general produced to maintain a stable and settling term for a process in which air chamber acceptable concentration. In sys- introduced into the bottom of a tems with external ammonia- Filter closed column of water creates support nitrogen treatment processes, the foam at the surface of the column. efficiency of the ammonia-nitro- Waste sludge Foam fractionation removes dis- gen removal process will dictate solved organic compounds (DOC) the recirculating flow rate (e.g., a Figure 8. The bubble bead filter oper- from the water column by physi- less efficient removal system will ates much like the propeller washed cally adsorbing DOC on the rising require a higher recycle flow rate bubbles. Fine particulate solids from the tank through the filter). bead filter, except that it backwashes are trapped within the foam at the There are a number of methods by dropping the filtration medium by top of the column, which can be gravity through a washing throat. for removing ammonia-nitrogen collected and removed. The main from water: air stripping, ion (U.S. Patent No. 5,232,586 by Dr. factors affected by the operational exchange, and biological filtra- Ronald Malone, Dept. of Civil design of the foam fractionator tion. Air stripping of ammonia- Engineering, Louisiana State are bubble size and contact time nitrogen through non-flooded (no University) between the air bubbles and the standing water in the reactor) DOC. A counter-current design packed columns requires that the “washing throat” into the sludge (bubbles rising against a down- pH of the water be adjusted to settling chamber. When the flow ward flow of water) improves above 10 and readjusted to safe is re-started, the filter medium efficiency by lengthening the con- levels (7 ro 8) before the water re- floats back into the filter chamber tact time between the water and enters the culture tank. Ion and the waste sludge settles to the the air bubbles (Fig. 9). In this exchange technology is costly and bottom of the settling chamber design, water is injected into the requires a mechanism for “wast- ready for discharge to a waste foam fractionator through a ven- ing” ammonia-laden salt water. A drain. turi. The venturi mixes air with salt-brine is used to “regenerate” the water and the air/water mix- the filter by removing ammonia- The advantage of bead filters is the compact size of the unit and nitrogen from the resin (filter Foam medium) once it becomes saturat- low water use during backwash- collection ing. Once biologically active, the and ed with ammonia-nitrogen. concentration beads become sticky and can Foam Biological filtration is the most remove even fine suspended removal widely used method. In biological solids. The bacteria that make the filtration (or biofiltration), there is Water filter sticky are a combination of inflow a substrate with a high specific autotrophic and heterotrophic surface area (large surface area Air bacteria. The autotrophic bacteria inflow per unit volume) on which the contribute to nitrification. The nitrifying bacteria can attach and heterotrophic bacteria break down Water grow. Ammonia and nitrite-nitro- inflow the organic solids that are trapped gen in the recycled water are oxi- within the bead bed. This can be a Venturi dized (converted) to nitrite and Water disadvantage, because during the outflow nitrate by Nitrosomonas and time between backwashings (1 to Nitrobacter bacteria, respectively. 48 hours), solids undergoing bac- Figure 9. A pump-driven, venturi- Commonly used biofilter sub- terial degradation use oxygen type foam fractionator design. A strates include gravel, sand, plas- from the system water and release water/air mixture is injected tangen- tic beads, plastic rings, and plastic ammonia-nitrogen. The oxygen tially into the foam fractionator (after plates. The most common biofil- consumed by these bacteria needs Losordo, 1997). tration technologies are discussed to be replaced and the ammonia- below. Rotating biological contactor RBC Biofilter media Rotating biological contactors (RBC) have been used in the treat- ment of domestic wastewater for decades, and are now widely used as nitrifying filters in aquaculture Drive applications. RBC technology is motor based on the rotation of a biofilter Water medium attached to a shaft, par- level tially submerged in water. Flow from Approximately 40 percent of the culture tank substrate is submerged in the Flow to recycle water (Fig. 10). Nitrifying culture tank bacteria grow on the medium and rotate with the RBC, alternately RBC filter tank contacting the nitrogen-rich water and the air. As the RBC rotates, it Figure 10. A rotating biological contactor unit powered by an electric gear motor. exchanges carbon dioxide (gener- ated by the bacteria and fish) for of 3.6 kg feed/day/m3 of medium treatment. This type of filter con- oxygen from the air. The tangen- sists of a water distribution sys- should be used (0.189 tial velocity of the outer edge of pounds/day/ft3 of medium). tem at the top of a reactor filled the RBC should be about 35 to 50 with a medium that has a relative- feet per minute. For example, an The filter medium increases in ly low specific surface area, gener- RBC with a diameter of 4 feet weight as much as 10 fold during ally less than 330 m2/m3 (100 ft2/ would rotate at 3 to 4 revolutions operation, so the support struc- ft3). This creates large void (air) per minute (rpm). The advantages ture must accommodate the addi- spaces within the filter medium of RBC technology are simplicity tional weight. (Fig. 11). As these filters are oper- of operation, the ability to remove ated in a non-flooded configura- carbon dioxide and add dissolved Trickling filters tion, they provide nitrification, oxygen, and a self-cleaning capaci- Trickling filters used in aquacul- aeration, and some carbon dioxide ty. Major disadvantages are the ture systems have evolved from removal in one unit. (The term high capital cost and mechanical those used in domestic sewage non-flooded is used to indicate instability. Poorly designed or built RBCs can break down Rotating Water inflow mechanically with the weight of from culture the biological growth on the filter water distribution tank medium. RBCs also have been designed to be turned by water arm (similar to a water wheel) and compressed air. In early aquaculture applications, RBCs had simple discs cut from Biofilter medium-plastic corrugated fiberglass plate. Now blocks or they use media with high specific plastic rings surface area, such as plastic blocks or a polyethylene tubular medium (resembling hair curlers). These newer plastic media remove more ammonia, nitrite-nitrogen and car- bon dioxide in small RBC units. Low The plastic media have specific pressure surface areas of up to 200 m2/m3 air (69 ft2/ft3). In aquaculture applica- inflow tions, volumetric nitrification rates of approximately 76 g TAN/m3 Water return per day can be expected with this to culture type of biological filter (Wheaton Filter tank et al., 1994). When including these support filters in a recirculating system as legs a nitrifying filter component (assuming 2.5 percent of the feed Figure 11. Trickling filters are non-submerged biological filters in which the water becomes TAN), a design criterion is evenly distributed over the medium. that the biological filter medium is expected with this type of biologi- sheared from the medium so that not completely submerged in cal filter (Beecher et al., 1997). the filter is self-cleaning. The main water). The flow rate through When designing these filters into advantage of fluidized bed tech- trickling filters is limited by the a recirculating system as a nitrify- nology is the high nitrification void space through which water ing and solids removal compo- capacity in a relatively compact can pass. In general, packing nent (assuming 2.5 percent of the unit. The sand also is extremely media with more void space can feed becomes TAN), a design cri- low cost. Fluidization (pumping) pass a higher rate of flow per terion of 13 kg of feed/day/m3 of requirements depend upon the square meter of (top) cross sec- medium should be used (0.81 size and weight of the medium tional surface area. The main dis- pounds/day/ft.3 of medium; the being used. Keep in mind that the advantage of trickling filters is manufacturer recommends a buoyancy of the medium changes that they are relatively large and design rate of 1.0 pound/day/ft3). with the amount of biological biofilter media are expensive. growth on the medium. This, in Also, if the recycled water is not Fluidized bed filters turn, depends upon the water prefiltered to remove suspended temperature, nutrient loading solids, trickling filters can become Fluidized bed filters are essential- rate, and degree of bed fluidiza- clogged over time. As with RBC ly sand filters operated continu- tion. media, the weight of the biologi- ously in the expanded (backwash) mode. Water flows up through a Unless there is a system for recov- cal growth on the filter media ering sand as water leaves the fil- should be considered in designing bed of sand at a rate sufficient to lift and expand (fluidize) the bed ter, the medium will need to be the support structure. replaced routinely. Depending of sand and keep the sand parti- Volumetric nitrification rates of cles in motion so that they no upon the temperature, nutrient approximately 90 g TAN/m3 per longer are in continuous contact concentration and size of the day can be expected with this with each other (Fig. 12). medium (and assuming 2.5 per- type of biological filter (Losordo, Fluidized bed filters use sand of cent of the feed becomes TAN), a unpublished data). When design- smaller diameter than that used in design criterion of 20 to 40 kg of ing these filters into a recirculat- particulate solids removal applica- feed/day/m3 of medium should ing system as a nitrifying filter tions. Plastic beads with densities be used (1.25 to 2.5 component (assuming 2.5 percent slightly greater than water also pounds/day/ft3 of medium). of the feed becomes TAN), a have been used successfully in design criteria of 3.6 kg feed/ fluidized bed filters. A fluidized Mixed bed reactors day/m3 of medium should be bed filter is an excellent environ- used (0.225 pound/day/ft3 of Mixed bed reactors are a new and ment for the growth of nitrifying interesting cross between upflow medium). bacteria, and bacteria can colonize plastic bead filters and fluidized the entire surface area of the filter bed reactors. These filters use a Expandable media filters medium. The turbulent environ- plastic medium kept in a continu- The expandable media floating ment also keeps the bacteria ous state of movement (Fig. 13). bead filters described in the previ- The diameter ous section (Figs. 7 and 8 are also of the plastic used as biofilters in some aquacul- medium is ture applications. Generally oper- usually much ated as upflow filters, the beads Water return to culture larger than have a high specific surface area sand, so it has on which nitrifying bacteria can tank Biofilter a lower spe- colonize. The major advantage of medium- cific surface this technology is the combination fluidized sand area (800 to of nitrification and the solids 1,150 m2/m3; removal processes into one com- 240 to 350 ponent. The disadvantage, as ft2/ft3). The noted before, is that solids are beads are usu- held in a place where they can ally neutrally degrade and affect the system’s buoyant or water quality. In general, using just slightly these filters will require the heavier than designer to provide for more oxy- water. The genation and biofiltration capaci- Water Perforated inflow plate for water plastic beads ty. The plastic bead medium used from distribution are usually in these filters has a specific sur- culture mixed by face area of 1,150 to 1,475 m2/m3 tank mechanical or (350 to 450 ft2/ft3). Volumetric hydraulic nitrification rates of approximate- Figure 12. A simplified view of a fluidized sand bed biological means. Mixed ly 325 g TAN/m3/day can be filter. bed filters are Water aerating only the water cent of the feed rate (that is, 0.5 kg inflow flowing into the culture O2/kg of feed fed). In a system from Water tank will not usually fed 4.5 kg (10 pounds) of feed culture return supply an adequate over an 18-hour period,the esti- tank to culture amount of oxygen for mated oxygen consumption rate tank fish production. The would be approximately 0.125 kg Biofilter amount of oxygen that O2/hour (0.28 pounds/hour). medium- mixed can be carried to the With an actual oxygen transfer plastic fish in this way is limit- efficiency of 0.455 kg O2/kW-h beads ed by the flow rate and (0.75 pounds/hp-h), the diffused the generally low con- aeration system would require a centration of oxygen in blower of approximately 0.275 kw water. Therefore, most (1/3 hp) to provide an adequate aeration in recirculating amount of oxygen. If the fish are systems occurs in the going to be fed over a shorter culture tank. The most period of time, then peak oxygen efficient aeration demand should be estimated and devices are those that the blower capacity should be Figure 13. A common configuration for a mixed bed move water into contact increased. reactor biological filter. with the atmosphere The density of fish production (paddlewheels, pro- with aeration alone is usually lim- designed as up-flow or down- peller-aspirators, vertical-lift ited to 30 to 40 kg of fish/m3 of flow filters and, like fluidized bed pumps). However, these methods culture tank volume (0.25 to 0.33 filters, they generate biological usually create too much turbu- pounds of fish/gal.). In green- solids but will not clog because of lence within a culture tank to be house systems where algal blooms the continuous movement of the useful. The most common way to are common, oxygen is generated medium. The plastic medium aerate in a recirculating tank sys- during the daylight hours, and moves through a pipe within the tem is called diffused aeration. culture densities of up to 60 kg of main reactor to vertically mix the Diffused aeration systems provide fish/m3 of culture volume (0.50 bead bed. Depending upon the low pressure air from a “regenera- pounds of fish/gal.) can be nutrient concentration and medi- tive” type of blower to some form achieved. um size (and assuming 2.5 percent of diffuser near or on the bottom of a culture tank. These diffusers Packed column aerators: An ideal of the feed becomes TAN), a location for aerating and degass- design criterion of 16 to 23 kg of produce small air bubbles that rise through the water column and ing water (i.e., removing carbon feed/day/m 3 of medium should transfer oxygen from the bubble dioxide) is in the recycle flow- be used (1.0 to 1.4 stream just before it re-enters the pounds/day/ft 3 of medium). to the water. culture tank. As mentioned previ- Studies have determined that dif- ously, however, this method does Dissolved gas fused aeration systems can trans- not usually supply enough oxy- fer oxygen at an average rate of gen. With submerged biological Recirculating systems should 1.3 kg O2/kW-h (2.15 lbs./hp - maintain adequate dissolved oxy- filtration, the concentration of dis- hour) under standard (20o C, solved oxygen will most likely be gen (DO) concentrations of at O mg/L DO, clean water) test least 6 mg/L and keep carbon lowest and carbon dioxide highest conditions (Colt and Tchoba- at the outflow of this component. dioxide (CO2) concentrations at noglous, 1979). However, these less than 25 mg/L for best fish Packed column aerators (PCA) are values must be corrected to an effective and simple means of growth. Colt and Watten (1988) account for the actual fish culture and Boyd and Watten (1989) dis- aerating water that is already in a conditions. To achieve acceptable flow-stream. A packed column cuss aeration and oxygenation fish growth rates, the DO concen- systems used in aquaculture; a aerator can be identical in design tration should be kept at 5 mg to a trickling nitrifying filter summary of the component O2/L or higher. At water tempera- options follows. (Fig.11). Water is introduced into a tures of 28o C, according to Boyd reactor filled with medium. The term aeration is used here to (1982), the diffuser system’s oxy- Proper design criteria include refer to the dissolution of oxygen gen transfer rate would be only 35 non-flooded operation and free air from the atmosphere into water. percent of the rate at standard exchange through the reactor. The transfer of pure oxygen gas to conditions. In this case, the oxy- Given a PCA influent DO concen- water is referred to as oxygena- gen transfer rate would be tration of 4 mg O2/L, an effective tion. reduced to 0.455 kg O2/kW-h oxygen transfer rate of 0.75 kg (0.75 lbs./hp - hour). In a well O2/kw-h (1.25 pounds O2/hp-h) Aeration designed recirculating system can be attained. While this is a (one in which solids are removed low transfer rate, the true energy Diffused aeration: Adding oxy- quickly), the oxygen consumption gen to a recirculating system by cost for using a PCA in combina- rate can be estimated as 50 per- tion with an existing flow-stream Water is only the energy required to with Off-gas recycle Oxygen Inflow pump water 1.0 to 1.25 meters (3 low water oxygen gas inlet low DO to 4 feet) to the top of the PCA. If content the PCA is to be used for carbon Outflow dioxide stripping, a low pressure Off gas water high DO air blower should be used to force at least five times as much air as Water with water (by volume) up through the high PCA medium. oxygen content Oxygenation Pure oxygen is used in recirculat- Figure 14. Down-flow bubble contact ing systems when the intensity of aerator (after Colt and Watten, 1988). production causes the rate of oxy- Figure 15. Typical u-tube oxygen dif- gen consumption to exceed the equals the upward velocity of the fusion design. maximum feasible rate of oxygen bubbles. This allows a long con- transfer through aeration. Sources tact time between the water and up to 250 percent of atmospheric of oxygen gas include compressed bubbles and nearly 100 percent saturation (15 to 20 mg/L). oxygen cylinders, liquid oxygen absorption of the injected gas. The (often referred to as LOX), and on- Low head oxygenation system: dissolved oxygen concentration of The multi-staged low head oxy- site oxygen generators. In most water leaving a DFBC can be as applications, the choice is between genator (LHO) oxygenates flow- high as 25 mg/L given a system ing water where there is only a bulk liquid oxygen and an oxygen pressure of approximately 1 bar generator. The selection of the small elevation difference between (14.7 PSI). the source of the water and the oxygen source will be a function of the cost of bulk liquid oxygen U-tube diffusers: At high operat- culture tank. This situation is in your area (usually dependent ing pressures, more oxygen can be often found in raceway systems on your distance from the oxygen absorbed by water. A u-tube oxy- set up in series. That is, the out- production plant) and the reliabil- gen diffusion system is an energy flow of one raceway is just slight- ity of the electrical service needed efficient method of adding pres- ly (1 to 3 feet) above the inflow of for generating oxygen on-site. sure to a flow-stream. A typical an adjacent raceway. This tech- u-tube consists of a contact loop, nology is a patented component Adding gaseous oxygen directly usually a pipe within a pipe (Fig. (U.S. Patent No. 4,880,445; Water into the culture tank through dif- 15), buried in the ground to at Management Technologies, P.O. fusers is not the most efficient least 10 meters (33 feet), the height Box 66125, Baton Rouge, LA) and way to add pure oxygen gas to of water required to add one is made up of a perforated, hori- water. At best, the efficiency of atmosphere of pressure (1 bar, zontal distribution plate and mul- such systems is less than 40 per- 14.7 PSI). The contact loop is tiple, adjacent, vertical contact cent. A number of specialized placed below tank level to mini- chambers (Fig.16). Pure gaseous components have been developed mize energy requirements, rather oxygen enters one (end) contact for use in aquaculture applica- than pumping water up hill to chamber and oxygen with off- tions. For an extensive review of gain the extra hydrostatic pressure gases (nitrogen and CO2) exits the component options, see Boyd and created by a column of water. adjacent contact chamber. Watten (1989). A review of the Oxygen is mixed with the water at more commonly used components The oxygen transfer capability of the entrance to the u-tube and this system is determined by the follows. travels with the flow to the bot- length of water fall, gas and water Down-flow bubble contactor: A tom of the water column. The flow rates, the DO concentration properly designed low pressure additional pressure from the of the influent water, and the oxygen diffusion system can water column accelerates the rate number of contact chambers transfer more than 90 percent of of oxygen absorption into the (Watten 1994). Including packing the oxygen injected through the water. The principal advantages of medium in the contact chambers component. One such system is a this system are the low energy can improve performance. down-flow bubble contact aerator requirements for oxygenating (DFBC), also referred to as a large flow-streams and the resis- Pressurized packed columns: bicone or a Speece cone. The tance to clogging with particulate Pressurized packed columns are DFBC system consists of a cone- solids. The major disadvantage is usually operated in a flooded shaped reactor with a water and the construction cost of drilling mode (water fills the reactor). oxygen input port at the top (Fig. the shaft and installing the u-tube. Water enters the top of a pressur- 14). As the water and oxygen bub- Oxygen transfer efficiencies are ized chamber that contains a bles move down the cone, the generally below 70 percent, with medium with a high specific sur- flow velocity decreases until it effluent oxygen concentrations of face area (much like packed tow- ers). Oxygen gas is usually intro- Gaseous ation, and the level of penetration O3 in the water with the microor- oxygen of the radiation into the water. To ganisms. Ozone must be generat- inflow be effective, microorganisms must ed on-site because it is unstable Low oxygen come in close proximity to the UV and breaks down in 10 to 20 min- influent radiation source (0.5 cm, 0.2 inch- utes. Ozone is usually generated es or less). Turbidity reduces its with either a UV light or a corona effectiveness. For a UV radiation electric discharge source. There Off-gas outflow system to be effective, the water are many commercial ozone gen- should be pre-filtered with some eration units available. form of particulate filtration Ozone is usually diffused into the device. water of a recirculating system in Oxygenated The most popular and effective an external contact basin or loop. effluent type of UV sterilization unit is one Water must be retained in this Figure 16. Multi-staged low head oxy- with a submerged UV radiation side-stream long enough to ensure source. In this type of unit, recy- that microorganisms are killed genator with front plate removed to cled water passes by an elongated and the ozone molecules are show component detail (after Losordo, UV lamp (much like a neon light destroyed. Residual ozone enter- 1997). bulb). The lamp is inside a quartz ing the culture tank can be very glass, watertight jacket and does toxic to crustaceans and fish. duced at the bottom of the col- not come in direct contact with Ozone in the air is also toxic to umn and travels upward, counter the water. The UV lamp and humans in low concentrations. to the water flow. Oxygen transfer quartz tube are held within a Great care should be taken in efficiency can range from 50 to 90 small diameter pipe through venting excess ozone from the percent with effluent dissolved which the treated water flows. As generation, delivery, and contact oxygen concentrations in excess of water passes along and around system to the outside of the build- 100 mg/L. The major disadvan- the UV lamp, microorganisms are ing. Ozonation systems should be tages of this system are high ener- exposed to the UV radiation. designed and installed by experi- gy requirements (to provide the Keeping the quartz jacket clean is enced personnel. pressure) and the buildup of bio- imperative to the proper opera- logical growth on the packing tion of the unit. UV sterilization Summary medium, which makes periodic units are usually rated by their cleaning necessary. manufacturers according to their This publication has outlined the water flow rate capacity. Increased major components and options efficiency can be achieved by used in recirculating aquaculture Disinfection production systems. This is by no reducing the flow rate through a Diseases can spread quickly given unit. The main disadvan- means a complete listing, new because of the density of fish in tage of UV sterilization is the need technologies are continually being recirculating systems. Some chem- for clean water with low suspend- developed. One should not icals used to treat diseases have a ed solids concentrations. Clear attempt to simply link the compo- devastating effect on the nitrifying water is not always economically nents discussed here and expect bacteria within the biofilter and achievable in heavily fed recircu- to have a properly operating sys- culture system. Alternatives to tra- lating systems. Additionally, the tem. Any system you buy should ditional chemical or antibiotic expensive UV lamp must be be the result of years of develop- treatments include the continuous replaced periodically. The main ment, with each component prop- disinfection of the recycled water advantage of UV sterilization is erly sized and integrated for opti- with ozone or ultraviolet irradia- that it is safe to operate and is not mal performance. When review- tion. For more information on dis- harmful to the cultured species. ing your options, always seek the ease treatment in recirculating assistance of a knowledgeable, systems, see SRAC publication Ozonation experienced person, one who has 452 on the management of recircu- designed a currently operating lating systems. Ozone (O3) gas is a strong oxidiz- and economically viable recircu- ing agent in water. Ozone has lating fish production system. Ultraviolet irradiation been used for years to disinfect drinking water. However, because References and Microorganisms (including dis- of the high levels of dissolved and ease-causing bacteria) are killed suspended organic materials in suggested readings when exposed to the proper recirculating systems, the effect of Boyd, C.E. 1982. Water quality man- amount of ultraviolet (UV) radia- ozone on bacterial populations is agement for fish pond culture. tion. Spotte (1979) notes that the questionable (Brazil et al., 1996). Elsevier Scientific Publishing effectiveness of UV sterilization The efficiency of the disinfecting Company, Amsterdam, the depends upon the size of the action depends upon the contact Netherlands. organism, the amount of UV radi- time and residual concentration of Boyd, C.E. 1991. Types of aeration Grace, G.R. and R.H. Piedrahita. Malone R.F. and D.G. Burden. 1988. and design considerations. In: L. 1989. Carbon dioxide removal in a Design of recirculating soft craw- Swann (editor), Proceedings of the packed column aerator. Presented fish shedding systems. Louisiana Second Annual Workshop: paper at the International Summer Sea Grant College Program, Commercial Aquaculture Using Meeting of Am. Soc. Ag. Eng. and Louisiana State University, Baton Water Recirculating Systems. Can. Soc. Ag. Eng., June 25-28, Rouge, LA. Illinois State University, Normal, 1989, Quebec, PQ, Canada. Spotte, S. 1979. Fish and invertebrate Illinois. Nov. 15-16, 1991. pp. 39- Hobbs, A., T. Losordo, D. DeLong, J. culture: Water management in 47. Regan, S. Bennett, R. Gron and B. closed systems. John Wiley & Boyd, C.E. and B.J. Watten. 1989. Foster. 1997. A commercial, public Sons, New York, NY. Aeration systems in aquaculture. demonstration of recirculating Timmons M.B. and T.M. Losordo CRC Critical Reviews in Aquatic aquaculture technology: The (editors). Aquaculture water reuse Sciences 1: 425 - 472. CP&L/EPRI Fish Barn at North systems: Engineering , design and Brazil, B.L., S.T. Summerfelt and G.S. Carolina State University. In: M.B. management. Developments in Libey. Applications of ozone to Timmons and T.M. Losordo (edi- Fisheries Sciences 27. Elsevier recirculating aquaculture systems. tors). Advances in aquacultural Scientific Publishing Company, In: G.S. Libey and M.B. Timmons engineering. Proceedings from the Amsterdam, The Netherlands. (editors), Successes and Failures in aquacultural engineering society technical sessions at the fourth Tvinnereim, K. 1988. Design of water Commercial Recirculating inlets for closed fish farms. In: Aquaculture. Proceeding from the international symposium on tilapia in aquaculture. NRAES-105. Proceedings of the Conference: Successes and Failures in Aquaculture Engineering: Commercial Recirculating Northeast Regional Agricultural Engineering Service, 152 Riley- Technologies for the Future. Aquaculture Conference. Roanoke, Sterling Scotland. IChemE VA. July 19-21, 1996. NRAES-98. Robb Hall, Ithaca, NY. pp. 151-158. Symposium Series #111, EFCE Northeast Regional Agricultural Huguenin, J.E. and J. Colt. 1989. Publications Series # 66, Rugby, Engineering Service, 152 Riley- Design and operating guide for UK. pp. 241-249. Robb Hall, Ithaca, NY., pp. 373-389. aquaculture seawater systems. Elsevier Scientific Publishers, Watten, B.J. 1994. Aeration and oxy- Burley, R. and A. Klapsis. 1988. genation. In: M.B. Timmons and Making the most of your flow (in Amsterdam, The Netherlands. T.M. Losordo (editors), Aquacul- fish rearing tanks). In: Proceedings Libey, G.S. 1993. Evaluation of a drum ture water reuse systems: of the Conference: Aquaculture filter for removal of solids from a Engineering, design and manage- Engineering: Technologies for the recirculating aquaculture system. ment. Developments in Fisheries Future, Sterling Scotland. IChemE In: J.K. Wang (editor), Techniques Sciences 27. Elsevier Scientific Symposium Series #111, EFCE for Modern Aquaculture. Publishing Company, Amsterdam, Publications Series # 66, Rugby, Proceedings of an Aquacultural The Netherlands. UK. pp. 211-239. Engineering Conference. Spokane, WA, June 1993. American Society Wheaton, F.W., J.N. Hochheimer, G.E. Colt, J.E. and G. Tchobanoglous. 1979. Kaiser, R.F. Malone, M.J. Krones, Design of aeration systems for of Agricultural Engineers, St. Joseph, MI. pp. 519.532. G.S. Libey and C.C. Estes. 1994. aquaculture. Department of Civil Nitrofication filter design methods. Engineering, University of Losordo, T.M. 1997. Tilapia culture in In: Timmons, M.B. and T. M. California, Davis, CA. intensive recirculating systems. In: Losordo (editors), Aquaculture Colt, J. and B. Watten. 1988. Costa-Pierce, B. and Rakocy, J. (edi- water reuse systems: Engineering, Applications of pure oxygen in fish tors), Tilapia Aquaculture in the design and management. Develop- culture. Aquacultural Engineering Americas, Volume 1. World ments in Fisheries Sciences 27. 7:397-441. Aquaculture Society, Baton Rouge, Elsevier Scientific Publishing LA. pp. 185-208. Company, Amsterdam, The Netherlands. pp. 125-171. The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045 from the United States Department of Agriculture, Cooperative States Research, Education, and Extension Service. SRAC Publication No. 456 VI PR November 1998 The Economics of Recirculating Tank Systems: A Spreadsheet for Individual Analysis Rebecca D. Dunning1, Thomas M. Losordo2 and Alex O. Hobbs3 A well-designed recirculating the same format can be used to Proper sizing of all system com- aquaculture system offers a num- monitor costs and returns once ponents is very important. If ber of advantages over pond sys- systems are operating. The Excel equipment is oversized for the tems. Designed to conserve both spreadsheet can be downloaded application, the system will func- land and water resources, recircu- from the following Internet tion but will be very costly. If lating systems can be located in address: http://www.agr.state. equipment is undersized, the sys- areas not conducive to open pond nc.us/aquacult/rass.html. tem will not be able to maintain culture. Operators have a greater The spreadsheet in this publica- the proper environment to sustain degree of control of the fish cul- tion uses tilapia for the example fish production. ture environment and can grow species. However, the resulting Operators should size equipment fish year-round under optimal figures on costs and returns are according to the maximum daily conditions. The crop can be har- not meant to be used as an eco- amount of feed placed into the vested at any time, and inventory nomic analysis of tilapia produc- system. The estimated daily feed can be much more accurately tion. Each individual using the rate is based on the system carry- determined than in ponds. This spreadsheet should input equip- ing capacity, which does not usu- latter characteristic is particularly ment and supply costs and the ally exceed 1 pound of fish per beneficial when trying to gain appropriate market price for the gallon of water for even the most financing or insurance for the specific system being analyzed. efficient system. Once carrying crop. capacity and feed rate are defined, Because of these advantages, System design the operator estimates the size of interest in water recirculating sys- equipment components by calcu- tems for fish production continues There is no single recommended lating the required flow rate. The to grow, despite the lack of eco- design for growing fish in a recir- flow rate of each component must nomic information available on culating aquaculture system be sufficient to flush out and treat their use. This publication and (RAS). In general, a system any wasted feed and by-products accompanying spreadsheet are includes tanks to culture fish, of fish metabolism, while supply- designed to help prospective pumps to maintain water flow, ing a uniform concentration of recirculating system operators and some form of water treatment oxygen. examine the economics of pro- to maintain water quality. Follow- ing are a few general considera- Because equipment is sized to posed systems. With modifica- maximum feeding rates, the most tions to the example spreadsheet, tions on system design and how design can affect profitability. For inefficient stock management a more complete explanation of method is to stock fingerlings at 1North Carolina Department of component options and manage- low densities in a tank and grow Agriculture ment issues see SRAC publica- them to market size within the 2North Carolina State University same tank. Most RAS operators 3Carolina Power and Light Company tions 450, 451 and 452. try to make maximum use of each tankÕs carrying capacity by stock- 15,000-gallon (56.78-cubic meter) tions are calculated from this ing fish at increasingly lower growout tanks (G1, G2, G3 and information. Shaded areas in the numbers as the fish grow in size. G4). The quarantine and nursery tables indicate needed informa- The more efficient the use of sys- tanks have their own water filtra- tion and are represented as bold tem carrying capacity, the more tion systems, while each pair of type in the spreadsheet. ÒSpread- fish can be moved through the growout tanks shares a water sheet Cell RangeÓ and cell num- system annually, which generally treatment system. A more detailed bers refer to the location of infor- lowers the cost per pound har- description of the system and mation within the Excel spread- vested. The trade-off is that the equipment can be found in Hobbs sheet. more often fish are restocked, the et al., 1997. higher the labor cost and greater Fish are initially stocked in the Section 1: Specify the Initial the chance of mortality if fish Q tank, screened for diseases for become stressed from the move. Investment Spreadsheet Cell 35 days, then harvested and Range B13:E25 Operators also face a trade-off restocked into the N tank. After when determining both the size 35 days, the fish are transferred to of tanks and the configuration of one of the four G tanks where The initial investment cost is sup- equipment for filtering and oxy- they remain an additional 140 plied by the user in cells E16:E20. genating water. The use of fewer, days until harvest. This 140-day The total is calculated in cell E21. larger tanks, or several tanks period is broken down into four The investment includes the total sharing water treatment equip- distinct production units of 35 value of purchased land, a settling ment, is usually much less expen- days each (defined as g1, g2, g3 pond, building, equipment, and sive than having a number of and g4 in the spreadsheet). Each construction labor, as well as the smaller tanks that do not share of these units has a different feed current value of any owned assets water or components. Managing rate, oxygen demand, and pump- used in the business. quality and disease prevention, ing need. (An alternative to this Annual depreciation on building and however, is typically more effec- configuration would be to move equipment (E22) is the amount of tive where water is not shared the fish into a different tank for money that must be earned each between tanks. There is less risk each of the 35-day periods). year by the business to eventually of losing large portions of the fish Once the system is fully stocked, replace equipment when it wears crop when each tank has its own one of the four G tanks is harvest- out. set of treatment equipment. ed for sale every 35 days. The sys- Interest rate on operating capital There are economies of scale for tem has a maximum culture den- (E24) is used to calculate a cost of individual tank size and for the sity of 0.8 pounds of fish per gal- interest on variable inputs (oxy- size of the entire system. Up to a lon of water (103 kgs of fish per gen, energy, bicarbonate, finger- point, the increase in system size cubic meter of water) in each lings, chemicals, maintenance and generally results in a lower cost growout tank, and each harvest labor). The interest charge could per pound produced, because the yields approximately 12,400 be interest owed to a bank for the fixed costs associated with the pounds (5,636 kgs) of fish. With financing of the purchase of these building and equipment can be 10.43 harvests annually (one every inputs, or the charge could be for spread over more pounds har- 35 days once the facility is fully the cost of using the ownerÕs own vested. stocked), total production for the funds to purchase variable inputs. facility is approximately 130,000 A cost of using ownerÕs funds is The example system pounds (59,091 kgs) per year. used because the investment of The data used for this publication funds in the recirculating system Using the spreadsheet means that the owner foregoes are taken from experiences in a small unit at the North Carolina The Recirculating Aquaculture potential earnings from an alter- State University Fish Barn Project System Spreadsheet (RASS) must native investment. (NC Fish Barn). be supplied with accurate and Interest rate on building and equip- The NC Fish Barn system grows realistic input data based on a ment (E25) is used to calculate an fish in nursery tanks, then grades properly designed system. Proper annual interest charge based on and splits the population into design means that the equipment the average investment. Again, larger growout tanks as the fish components work together to pro- this could be interest owed on a gain weight. The system consists duce the amount of fish in the bank loan used to finance the ini- of six tanks: one 1,500-gallon time period stated. tial investment, or it can represent (5.68-cubic meter) quarantine tank The spreadsheet is divided into earnings that could have been (Q); a 4,000-gallon (15.14-cubic five sections. The user supplies made on an alternative invest- meter) nursery tank (N), and four information for the first three sec- ment. tions. Data in the final two sec- Section 1. System parameters Specify the Initial Investment The remainder of this section Spreadsheet Cell Range = B13:E25 (E48..E54) contains system para- Initial investment meters that will be needed for cal- culations related to costs and land $8,000 returns. Annual production (E48), settling pond $5,000 Average size at harvest (E49), and equipment $172,500 the Survival rate (specified in the building $60,000 next section) are used to calculate the initial stocking density. construction labor & overhead $30,000 There are six production units in Total initial investment $275,000 this example (Number of production Annual depreciation on building and equipment $19,100 units [E50] = 6). As discussed above, a production unit refers to Interest rate on operating capital 9% a specific tank or life stage of the Interest rate on building and equipment 11% fish. Here, three tanks are used: a Q tank, an N tank and a G tank. Fish remain in the Q tank and N tank for 35 days each. Within the Section 2: Specify the Cost Section 2. of Inputs, Sale Price, and Specify the Cost of Inputs, Sale Price, and System Parameters System Parameters Spreadsheet Cell Range = B27:E54 Spreadsheet Cell Range = unit or description cost or amount B27:E54 Variable Costs: Liquid oxygen $/100 cu. ft. $0.30 Variable costs Energy $/kwh $0.065 Variable costs are those directly Bicarbonate $/lb. $0.190 related to production. In the cell Fingerlings $/fingerling $0.090 range E31:E38 the user specifies the cost per unit of oxygen, ener- Chemicals $/cycle $100.00 gy, bicarbonate, fingerlings, Maintenance $/month $637.00 chemicals, maintenance and Labor: management $/month $2,000.00 labor. The quantity used of each of these inputs is defined in Labor: transfer & harvest $/hour $6.50 Section 3. Fixed Costs: Fixed costs Liquid oxygen tank rental $/month $250.00 Fixed costs are incurred regard- Electrical demand charge $/month $100.00 less of whether or not production Building Overhead $/month $100.00 occurs. They are Liquid oxygen tank rental (E41), Electrical demand charge (E42), and Building over- Average overall sale price $/lb. $1.25 head (E43). Each of these is speci- fied as a cost per month. System Parameters Annual production lb. 129,107 Sale price Average size at harvest lb. 1.25 Average overall sale price (E45) is Number of production units number 6 the weighted average sale price per pound, taking into account Days per production unit days 35 the size distribution at harvest Kwh per lb. of production kwh/lb. of prod. 2.30 and differing prices for various System volts volts 230 sizes of fish. The example uses Transfer/harvest labor hrs. per cycle 64 $1.25 so that the system will break even (with $0 profit and $0 losses). G tank, the fish go through four Section 3. 35-day stages. Note that the Days Specify Operating Parameters per Production Unit per production unit (E51) must be Spreadsheet Cell Range = B56..J64 the same for each unit in order for the spreadsheet to accurately Growout tank calculate costs and returns in Q tank N tank g1 g2 g3 g4 Section 5. Water volume, gallons 1,500 4,000 15,000 15,000 15,000 15,000 The Kwh per lb. of production (E52) Size stocked (grams) 1 15 60 135 250 385 is used to calculate energy costs Size harvested (grams) 15 60 135 250 385 567 for the total system and each pro- duction unit. This variable is cal- Survival rate 85% 99% 99% 99% 99% 99% culated by adding up the total Feed cost, per pound $0.52 $0.38 $0.21 $0.21 $0.21 $0.21 KW usage of the systemÑinclud- Feed conversion 1 1.1 1.3 1.6 1.6 1.6 ing energy usage for pumps, blowers and other equipment as well as heating, ventilation and Water volume, gallons (E59:J69) is bonateÑused over one cycle, and air-conditioningÑconverting this used to calculate the Maximum extrapolates this information to an to kwh used per year, and then standing biomass, lbs. per gal. of annual basis. No user input is dividing by the number of water (E73:I73) for any one tank, required in this section. pounds produced. (For the exam- discussed in Section 4. ple, the total energy demand is 34 In the example, once the fish cul- KW. Multiply by 24 hours per day Size stocked (E60:J60) is the average ture system is fully stocked after and 365 days per year, then divide size of fish stocked into that pro- 210 days, the system will have by annual production of 129,107 duction unit. Size harvested 10.43 harvests per year (365 days/ pounds to arrive at 2.30 kwh per (E61:J61) is their average size 35 days). Thus, each number in pound of production). when transferred or harvested the Cycle Total (column L) is multi- from the system. In the example, plied by 10.43 to calculate the System volts (E53) is used to calcu- fish are initially stocked at 1 gram Annual Total (column M). late required amperage in Section into the Q tank, and transferred 5. This is a useful number for Beginning number of fish (E69:J69) into the N tank when they reach begins with the original stocking planning energy requirements for 15 grams. the facility. density and adjusts that number Survival rate (E62:J62 ) is the per- according to the Survival rate Transfer/harvest labor (E54) is the centage of survival for that pro- (E62:J62). number of hours of labor required duction unit. In the example, the per cycle in addition to Labor: Ending number of fish (E70:J70) is lower survival rate for the Q tank based on density and survival for management (defined in E37). includes the discarding of runts each production unit. when the fish are graded before Section 3: Specify restocking into the N tank. Beginning biomass, lbs. of fish Operating Parameters (E71:J71) is based on the number Feed cost, per lb. (E63:J63) is the of fish and average weight per Production Unit average cost per pound for feed stocked into that production unit. Spreadsheet Cell Range fed to that production unit. Feed cost, per lb. and Feed conversion Ending biomass, lbs. of fish (E72:J72) B56:J64 is based on the number of fish (E64:E64) are used to calculate the cost of feed for each production and weight transferred or harvest- Each column in this section repre- unit, for each cycle, and annually. ed from that unit. sents a production unit, which Feed usage is also used to calcu- Maximum standing biomass, lb. per could be a tank or group of tanks late the amount of energy used, as gal. of water (E73:J73) gives the managed in the same manner, or discussed in the following section. pounds of fish per gallon of tank it could refer to a particular life water at the end of that produc- stage. For example, two tanks Spreadsheet calculation of tion period. stocked at the same time with the intent to transfer and harvest fish costs and returns Feed used (E74:J74) is calculated at the same time, and in which from the specified Feed conversion Section 4: Use of Primary ratio (E63:J63) and the difference fish are fed and managed in the same manner, could be treated as Inputs and Costs per between the Beginning biomass one production unit. Or, as in the Production Unit (E71:J71) and Ending biomass table below and spreadsheet Spreadsheet Cell Range (E72:J72). example, two of the six columns B66:J87 The Kwh used is calculated for (Q & N) refer to particular tanks, each production unit as a weight- while the remaining four (g1, g2, This section summarizes the ed percentage of the feed usage g3, g4) refer to a production stage quantity and cost of primary for that unit multiplied by the for fish that remain within the operating inputsÑfingerlings, total amount of kwh used for the same tank. feed, energy, oxygen, and bicar- cycle. The total kwh for the cycle is based on estimated energy this is system specific) x 12.05 (a Days per production unit (D91) usage of 2.30 kwh per pound of conversion factor). repeats information given in cell production. For example, one Bicarbonate used (E77:J77) allows E51. cycle yielding 12,354 pounds for 0.175 pound of sodium bicar- The Number of cycles per year (D92) (5,615 kg) of fish requires an esti- bonate used per pound of feed is simply 365 days divided by mated 28,414 kwh of energy. The fed. Days per production unit. g1 production unit consumes 11.72% of feed used during the Costs by production unit (E80:J87) Required system amps (D93) is cal- cycle (2,172 pounds feed/18,524 are calculated using the cost per culated from System volts (E53) pounds feed), so the estimated input specified in Section 2. and kwh usage assuming a power energy use during that 35-day unit factor of one. Section 5: Summary of is 3,330 kwh (11.72% x 28,414), Overall survival (F91) is calculated given in cell G75. The cost of Annual Costs and Returns using survival given in E62:J62, energy for that period, given in to System in Full Production and Cycle FCR (F92) from feed G82 as $217, is calculated using Spreadsheet Cell Range = conversion ratios in E64:J64. the user-specified cost of $0.065 B89:J122 The cell range C96:J122 calculates per kwh (E45). system costs per cycle, annually, Oxygen used, cubic feet (E76:J76) is This section summarizes the costs and per pound based on informa- calculated as follows: pounds of and returns per cycle and annual- tion specified previously in the feed (E74:J74) x 30% (the amount ly for this system once it is in full spreadsheet. of oxygen used per pound of feed, production (after 210 days). Net returns are calculated before tax. Section 4. Use of Primary Inputs and Costs per Production Unit Spreadsheet Cell Range = B66:J87 Growout tank Cycle Yearly Inventory & Input Use: Q tank N tank g1 g2 g3 g4 total total Beginning number of fish 12,252 10,415 10,310 10,207 10,105 10,004 12,252 127,775 Ending number of fish 10,415 10,310 10,207 10,105 10,004 9,904 9,904 103,286 Beginning biomass (lbs. of fish) 27 344 1,361 3,032 5,558 8,474 27 281 Ending biomass (lbs. of fish) 344 1,361 3,032 5,558 8,474 12,354 12,354 128,838 Max. standing biomass (lbs./gal.) 0.23 0.34 0.20 0.37 0.56 0.82 -- -- Feed used, lbs. 317 1,119 2,172 4,042 4,665 6,209 18,524 193,179 Kwh used 486 1,717 3,331 6,200 7,156 9,525 28,415 296,328 Oxygen used, cubic ft. 1,145 4,045 7,851 14,612 18,864 22,447 66,964 698,342 Bicarbonate used, lbs. 55 196 380 707 816 1,087 3,242 33,806 Costs: Fingerlings $1,103 $1,103 $11,500 Feed $165 $425 $456 $849 $980 $1,304 $4,178 $43,575 Energy $32 $112 $217 $403 $465 $619 $1,847 $19,261 Oxygen $3 $12 $24 $44 $51 $67 $201 $2,095 Bicarbonate $11 $37 $72 $134 $155 $206 $616 $6,423 Total of above costs for this unit $1,313 $586 $768 $1,430 $1,651 $2,197 $7,945 $82,855 Cumulative cost for cycle $1,313 $1,899 $2,667 $4,098 $5,748 $7,945 $7,945 $82,855 Cumulative cost per lb. $3.82 $1.40 $0.88 $0.74 $0.68 $0.64 $0.64 $0.64 Section 5. Summary of Annual Costs and Returns to System in Full Production Spreadsheet Cell Range = B89:J122 Days per production unit 35 Overall survival 81% Average number of cycles/yr. 10.43 Cycle FCR 1.5 Req. system amps 147 unit cost/unit quantity/ $/cycle $/year $/per lb. % of cycle of fish total Gross Receipts lb. $1.25 12,354 $15,443 $161,048 $1.25 Variable Cost fingerlings unit $0.09 12,252 $1,103 $11,500 $0.09 7% feed lb. $0.23 18,524 $4,178 $43,575 $0.34 27% energy kwh $0.07 28,415 $1,847 $19,261 $0.15 12% oxygen 100 cubic feet $0.30 670 $201 $2,095 $0.02 1% bicarbonate lb. $0.19 3,242 $616 $6,423 $0.05 4% chemicals $ per cycle $115.07 1 $115 $1,200 $0.01 1% maintenance $ per cycle $732.99 1 $733 $7,644 $0.06 5% labor: management $ per cycle $2,301.37 1 $2,301 $24,000 $0.19 15% labor: transfer & harvest hour $6.50 64 $416 $4,338 $0.03 3% interest on variable costs dol. 9% 6,307 $327 $3,406 $0.03 2% Subtotal, Variable Cost $11,837 $123,442 $0.96 77% Fixed Cost Oxygen tank rental dol. $288 $3,000 $0.02 2% Electrical demand charge dol. $115 $1,200 $0.01 1% Building overhead dol. $173 $1,800 $0.01 1% Interest on initial investment dol. $1,226 $12,788 $0.10 8% Depr. on bldg. & equip. dol. $1,832 $19,100 $0.15 12% Subtotal, Fixed Cost $3,633 $37,888 $0.29 23% Total Cost $15,470 $161,330 $1.25 100% Net Returns above Var. Cost $3,606 $37,606 $0.29 Net Returns above Total Cost -$27 -$282 $0.00 Interpreting the approximately calculate the point This spreadsheet can be used to spreadsheet results at which the system becomes self- test the effect on costs and returns supporting (can pay all fixed and of changes in sale price, feed con- This publication is not an evalua- variable costs), divide the total version, survival, or the cost of tion of the economics of tilapia costs per cycle by the net returns energy and other inputs. Users production. A sale price of $1.25 per cycle. For example, if the sale can also examine the change in was chosen so that the example price were $1.65 per pound, Total profitability based on a change in system would have annual costs Costs per Cycle would be $15,470 the stocking and transfer of fish nearly equal to annual returns. and Returns above Total Costs or overall size of the system. For It is important to keep in mind would be $4,957. This is equal to example, more frequent moves of that before the end of the first 3.1 cycles ($15,470/$4,957) or 651 fish between tanks could make cycle on day 210, costs are days (3.1 cycles x 210 days per better use of tank carrying capaci- incurred while no fish are har- cycle). The system would not ty, increasing the amount of fish vested and sold. Until that time, become self-supporting until that could be harvested annually. the cost of operations must either approximately 2 years from Or, a more energy intensive sys- be paid by additional owner startup. tem might support a higher carry- funds or bank financing. To ing capacity per tank. Either of these may result in increased prof- that will be suitable for every sys- References it if the costs associated with each tem. Operators with existing or (higher labor cost, stress that may proposed systems similar to the Hobbs, A., T. Losordo, D. DeLong, result in lower survival in the case example presented here can use J. Regan, S. Bennett, R. Gron of more frequent moves, and a this spreadsheet. Radically differ- and B. Foster. 1997. ÒA com- higher energy cost if the system ent systems may require extensive mercial, public demonstration were reconfigured) do not out- modifications of the spreadsheet of recirculating aquaculture weigh the increase in production. structure by the user. The example technology: The CP&L /EPRI Larger systemsÑmore tanks and spreadsheet is simple in design Fish Barn at North Carolina larger tanksÑalso often increase and does not contain any macro- State University.Ó Pages 151- the profitability of recirculating programming. It can be modified 158 In: M.B. Timmons and systems. once cells are unprotected. When T.M. Losordo, editors. working with the original spread- Advances in aquacultural sheet or a modified version, keep engineering. Proceedings from Caveats (a warning) the aquacultural engineering in mind that it can only evaluate There is no single recommended the economics of a properly society technical sessions at design for recirculating aquacul- designed system, and can not cor- the fourth international sym- ture systems. Therefore, it is rect for flaws in design. posium on tilapia in aquacul- impossible to supply a ready- ture. NRAES-105. Northeast made cost/returns spreadsheet Regional Agricultural Engineering Service, Ithaca, NY. For additional suggested reading, see the Internet site. The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045 from the United States Department of Agriculture, Cooperative States Research, Education, and Extension Service.
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