28844411 recirculating aquaculture tank production systems an overview of critical considerations

Traditional aquaculture produc-tion in ponds requires large quan-tities of water. Approximately 1million gallons of water per acreare required to fill a pond and anequivalent volume is required tocompensate for evaporation andseepage during the year.Assuming an annual pond yieldof 5,000 pounds of fish per acre,approximately 100 gallons ofwater are required per pound offish production. In many areas ofthe United States, traditionalaquaculture in ponds is not possi-ble because of limited water sup-plies or an absence of suitableland for pond construction. Recirculating aquaculture produc-tion systems may offer an alterna-tive to pond aquaculture technolo-gy. Through water treatment andreuse, recirculating systems use afraction of the water required byponds to produce similar yields.Because recirculating systems usu-

ally use tanks for aquaculture pro-duction, substantially less land isrequired. Aquatic crop production in tanksand raceways where the environ-ment is controlled through watertreatment and recirculation hasbeen studied for decades.Although these technologies havebeen costly, claims of impressiveyields with year-round productionin locations close to major marketsand with extremely little waterusage have attracted the interestof prospective aquaculturists. Inrecent years, a variety of produc-tion facilities that use recirculatingtechnology have been built.Results have been mixed. Whilethere have been some notablelarge-scale business failures in thissector, numerous small- to medi-um-scale efforts continue produc-tion. Prospective aquaculturists andinvestors need to be aware of thebasic technical and economic risksinvolved in this type of aquacul-ture production technology. Thisfact sheet and others in this seriesare designed to provide basicinformation on recirculating aqua-culture technology.

Critical productionconsiderationsAll aquaculture production sys-tems must provide a suitableenvironment to promote thegrowth of the aquatic crop.Critical environmental parametersinclude the concentrations of dis-solved oxygen, un-ionized ammo-nia-nitrogen, nitrite-nitrogen, andcarbon dioxide in the water of theculture system. Nitrate concentra-tion, pH, and alkalinity levelswithin the system are also impor-tant. To produce fish in a cost-effective manner, aquaculture pro-duction systems must maintaingood water quality during peri-ods of rapid fish growth. Toensure such growth, fish are fedhigh-protein pelleted diets at ratesranging from 1.5 to 15 percent oftheir body weight per daydepending upon their size andspecies (15 percent for juveniles,1.5 percent for market size). Feeding rate, feed composition,fish metabolic rate and the quanti-ty of wasted feed affect tank waterquality. As pelleted feeds areintroduced to the fish, they areeither consumed or left to decom-pose within the system. The by-

VIPR

September 1998Revised

SRAC Publication No. 451

Recirculating Aquaculture TankProduction Systems

An Overview of Critical Considerations

Thomas M. Losordo1, Michael P. Masser2 and James Rakocy3

1Department of Zoology, North CarolinaState University, North Carolina

2Department of Fisheries and AlliedAquaculture, Auburn University,Alabama

3University of the Virgin Islands,Agricultural Experiment Station, U.S.Virgin Islands

products of fish metabolisminclude carbon dioxide, ammo-nia-nitrogen, and fecal solids. Ifuneaten feeds and metabolic by-products are left within the cul-ture system, they will generateadditional carbon dioxide andammonia-nitrogen, reduce theoxygen content of the water, andhave a direct detrimental impacton the health of the culturedproduct.In aquaculture ponds, properenvironmental conditions aremaintained by balancing theinputs of feed with the assimila-tive capacity of the pond. ThepondÕs natural biological produc-tivity (algae, higher plants, zoo-plankton and bacteria) serves as abiological filter that processes thewastes. As pond productionintensifies and feed rates increase,supplemental and/or emergencyaeration are required. At higherrates of feeding, water must beexchanged to maintain goodwater quality. The carrying capac-ity of ponds with supplementalaeration is generally consideredto be 5,000 to 7,000 pounds of fishper acre (0.005 to 0.007 pound offish per gallon of pond water).The carrying capacity of tank sys-tems must be high to provide forcost-effective fish productionbecause of the higher initial capi-tal costs of tanks compared toearthen ponds. Because of thisexpense and the limited capacityof the ÒnaturalÓ biological filtra-tion of a tank, the producer mustrely upon the flow of waterthrough the tanks to wash out thewaste by-products. Additionally,the oxygen concentration withinthe tank must be maintainedthrough continuous aeration,either with atmospheric oxygen(air) or pure gaseous oxygen.The rate of water exchangerequired to maintain good waterquality in tanks is best describedusing an example. Assume that a5,000-gallon production tank is tobe maintained at a culture densityof 0.5 pound of fish per gallon oftank volume. If the 2,500 poundsof fish are fed a 32% protein feedat a rate of 1.5 percent of their

body weight per day, then 37.5pounds of feed would produceapproximately 1.1 pounds ofammonia-nitrogen per day.(Approximately 3 percent of thefeed becomes ammonia-nitrogen.)Additionally, if the ammonia-nitrogen concentration in the tankis to be maintained at 1.0 mg/l,then a mass balance calculation onammonia-nitrogen indicates thatthe required flow rate of newwater through the tank would beapproximately 5,600 gallons perhour (93 gpm) to maintain thespecified ammonia-nitrogen con-centration. Even at this high flowrate, the system also wouldrequire aeration to supplementthe oxygen added by the newwater.

Recirculating systemsdesign Recirculating production technol-ogy is most often used in tanksystems because sufficient wateris not available on site to ÒwashÓfish wastes out of productiontanks in a flow-through configura-tion or production system thatuses water only once. In mostcases, a flow-through requirementof nearly 100 gallons per minuteto maintain one production tankwould severely limit productioncapacity. By recirculating tankwater through a water treatmentsystem that ÒremovesÓ ammoniaand other waste products, thesame effect is achieved as with theflow-through configuration. Theefficiency with which the treat-ment system ÒremovesÓ ammoniafrom the system, the ammoniaproduction rate, and the desiredconcentration of ammonia-nitro-gen within the tank determine therecirculating flow rate from thetank to the treatment unit. Usingthe example outlined above, if atreatment system removes 50 per-cent of the ammonia-nitrogen inthe water on a single pass, thenthe flow rate from the tank wouldneed to be twice the flow requiredif fresh water were used to flushthe tank (93 gpm/0.5 = 186 gpm). A key to successful recirculatingproduction systems is the use of

cost-effective water treatment sys-tem components. All recirculatingproduction systems remove wastesolids, oxidize ammonia andnitrite-nitrogen, remove carbondioxide, and aerate or oxygenatethe water before returning it tothe fish tank (see Fig. 1). Moreintensive systems or systems cul-turing sensitive species mayrequire additional treatmentprocesses such as fine solidsremoval, dissolved organicsremoval, or some form of disinfec-tion.

Waste solids constraints

Pelleted feeds used in aquacultureproduction consist of protein, car-bohydrates, fat, minerals andwater. The portion not assimilat-ed by the fish is excreted as ahighly organic waste (fecal solids).When broken down by bacteriawithin the system, fecal solids anduneaten feed will consume dis-solved oxygen and generateammonia-nitrogen. For this rea-son, waste solids should beremoved from the system asquickly as possible. Waste solidscan be classified into four cate-gories: settleable, suspended,floatable and dissolved solids. Inrecirculating systems, the first twoare of primary concern. Dissolvedorganic solids can become a prob-lem in systems with very littlewater exchange. Settleable solids control:Settleable solids are generally theeasiest of the four categories todeal with and should be removedfrom the tank and filtration com-ponents as rapidly as possible.Settleable solids are those that willgenerally settle out of the waterwithin 1 hour under still condi-tions. Settleable solids can beremoved as they accumulate onthe tank bottom through properplacement of drains, or they canbe kept in suspension with contin-uous agitation and removed witha sedimentation tank (clarifier),mechanical filter (granular orscreen), or swirl separator. Thesedimentation and swirl separatorprocesses can be enhanced byadding steep incline tubes (tube

settlers) in the sedimentation tankto reduce flow turbulence andpromote uniform flow distribu-tion. Suspended solids control: Froman aquacultural engineering pointof view, the difference betweensuspended solids and settleablesolids is a practical one.Suspended solids will not settle tothe bottom of the fish culture tankand cannot be removed easily inconventional settling basins.Suspended solids are not alwaysdealt with adequately in a recircu-lating production system. If notremoved, suspended solids cansignificantly limit the amount offish that can be grown in the sys-tem and can irritate the gills offish. The most popular treatmentmethod for removing suspendedsolids generally involves someform of mechanical filtration. Thetwo types of mechanical filtrationmost commonly used are screenfiltration and granular media fil-tration (sand or pelleted media).For more information on these

devices see SRAC 453,Recirculating Aquaculture TankProduction Systems: A Review ofComponent Options.Fine and dissolved solidscontrol: Fine suspended solids (< 30 micrometers) have beenshown to contribute more than 50 percent of the total suspendedsolids in a recirculating system.Fine suspended solids increase theoxygen demand of the system andcause gill irritation and damage infinfish. Dissolved organic solids(protein) can contribute signifi-cantly to the oxygen demand ofthe total system.Fine and dissolved solids cannotbe easily or economicallyremoved by sedimentation ormechanical filtration technology.Foam fractionation (also referredto as protein skimming) is suc-cessful in removing these solidsfrom recirculating tank systems.Foam fractionation, as employedin aquaculture, is a process ofintroducing air bubbles at the bot-tom of a closed column of water

that creates foam at the topair/water interface. As the bub-bles rise through the water col-umn, solid particles attach to thebubblesÕ surfaces, forming thefoam at the top of the column.The foam build-up is then chan-nelled out of the fractionation unitto a waste collection tank. Solidsconcentration in the waste tankcan be five times higher than thatof the culture tank. Although theefficiency of foam fractionation issubject to the chemical propertiesof the water, the process generallycan be used to significantly reducewater turbidity and oxygendemand of the culture system.

Nitrogen constraints

Total ammonia-nitrogen (TAN),consisting of un-ionized ammonia(NH3) and ionized ammonia(NH4

+), is a by-product of proteinmetabolism. TAN is excreted fromthe gills of fish as they assimilatefeed and is produced when bacte-ria decompose organic wastesolids within the system. The un-ionized form of ammonia-nitro-

Fish Culture Tank

Round, Octagonal,Rectangular or

D-ended

Fine & DissolvedSolids Removal

Foam fractionation

Carbon DioxideRemoval

Air stone diffuserPacked column

Disinfection

Ultraviolet lightOzone contact

Aeration orOxygenation

Air stone diffuserPacked column

Down-flow contactorLow head oxygenator

U-tube

Waste Solids Removal

SedimentationSwirl separators

Screen filtersBead filters

Double drain

Biological Filtration(Nitrification)

Fluidized bed filtersMixed bed filtersTrickling filters

Rotating bio contactor

Figure 1. Required unit processes and some typical components used in recirculating aquaculture production systems.

ner in which it comes into contactwith wastewater define the watertreatment characteristics of thebiological filtration unit. The mostcommon configurations for bio-logical filters include rotating bio-logical contactors (RBC), fixedfilm reactors, expandable mediafilters, and mixed bed reactors.For more information on biologi-cal filters and components seeSRAC 453, RecirculatingAquaculture Tank ProductionSystems: A Review of ComponentOptions.

pH and alkalinity constraints

The measure of the hydrogen ion(H+) concentration, or pH, inwater indicates the degree towhich water is either acidic orbasic. The pH of water affectsmany other water quality parame-ters and the rates of many biologi-cal and chemical processes. Thus,pH is considered an importantparameter to be monitored andcontrolled in recirculating aqua-culture systems. Alkalinity is ameasure of the waterÕs capacity toneutralize acidity (hydrogen ions).Bicarbonate (HCO3

-) and carbon-ate (CO3

-) are the predominantbases or sources of alkalinity inmost waters. Highly alkalinewaters are more strongly bufferedagainst pH change than less alka-line waters.Nitrification is an acid-producingprocess. As ammonia-nitrogen istransformed to nitrate-nitrogen bynitrifying bacteria, hydrogen ionsare produced. As hydrogen ionscombine with bases such ashydroxide (OH-), carbonate andbicarbonate, alkalinity is con-sumed and the pH decreases.Levels of pH below 4.5 are dan-gerous to fish; a pH below 7.0 willreduce the activity of nitrifyingbacteria. If the source water for arecirculating system is low inalkalinity, then pH and alkalinityshould be monitored and alkalini-ty must be maintained with addi-tions of bases. Some bases com-monly used include hydrated lime[Ca(OH)2] quick lime (CaO), andsodium bicarbonate (NaHCO3).

gen is extremely toxic to mostfish. The fraction of TAN in theun-ionized form is dependentupon the pH and temperature ofthe water. At a pH of 7.0, most ofthe TAN is in the ionized form,while at a pH of 8.75 up to 30 per-cent of TAN is in the un-ionizedform. While the lethal concentra-tion of ammonia-nitrogen formany species has been estab-lished, the sub-lethal effects ofammonia-nitrogen have not beenwell defined. Reduction in growthrates may be the most importantsub-lethal effect. In general, theconcentration of un-ionizedammonia-nitrogen in tanks shouldnot exceed 0.05 mg/l.Nitrite-nitrogen (NO2

- ) is a prod-uct of the oxidation of ammonia-nitrogen. Nitrifying bacteria(Nitrosomonas) in the productionsystem utilize ammonia-nitrogenas an energy source for growthand produce nitrite-nitrogen as aby-product. These bacteria are thebasis for biological filtration. Thenitrifying bacteria grow on thesurface of the biofilter substratealthough all tank production sys-tem components will have nitrify-ing bacteria present to someextent. While nitrite-nitrogen isnot as toxic as ammonia-nitrogen,it is harmful to aquatic speciesand must be controlled within thetank. Nitrite-nitrogen binds with hemo-globin to produce methemoglo-bin. Methemoglobin is not capableof binding and transporting oxy-gen and the affected fish becomestarved for oxygen. The toxicity ofnitrite-nitrogen is species specific.However, a common practice forreducing nitrite-nitrogen toxicityis to increase the chloride concen-tration of the culture water. Main-taining a chloride to nitrite-nitro-gen ratio of 10:1 generally willprotect against methemoglobinbuild-up and nitrite-nitrogen toxi-city. Fortunately, Nitrobacter bacte-ria, which also are present in mostbiological filters, utilize nitrite-nitrogen as an energy source andproduce nitrate as a by-product.In a recirculating system with amature biofilter, nitrite-nitrogen

concentrations should not exceed10 mg/l for long periods of timeand in most cases should remainbelow 1 mg/l.Nitrates are not generally of greatconcern to the aquaculturist.Studies have shown that aquaticspecies can tolerate extremelyhigh levels (> 200 mg/l) ofnitrate-nitrogen in production sys-tems. Nitrate-nitrogen concentra-tions do not generally reach suchhigh levels in recirculating sys-tems. Nitrate-nitrogen is eitherflushed from a system during sys-tem maintenance operations (suchas settled solids removal or filterbackwashing), or denitrificationoccurs within a treatment systemcomponent such as a settling tank.Denitrification occurs when anaer-obic bacteria metabolize nitrate-nitrogen to produce nitrogen gasthat is released to the atmosphereduring the aeration process. Formore information on the effects ofwater quality on fish production,see SRAC 452, RecirculatingAquaculture Tank ProductionSystems: Management ofRecirculating Systems.

Ammonia and nitrite-nitrogencontrol: Controlling the concen-tration of un-ionized ammonia-nitrogen (NH3) in the culture tankis a primary objective of recircu-lating treatment system design.Ammonia-nitrogen must beÒremovedÓ from the culture tankat a rate equal to the rate of pro-duction to maintain a safe concen-tration. While there are a numberof technologies available forremoving ammonia-nitrogen fromwater, biological filtration is themost widely used. In biologicalfiltration (also referred to as biofil-tration), there is a substrate with alarge surface area where nitrifyingbacteria can attach and grow. Aspreviously noted, ammonia andnitrite-nitrogen in the recyclestream are oxidized to nitrite andnitrate-nitrogen by Nitrosomonasand Nitrobacter bacteria, respec-tively. Gravel, sand, plastic beads,plastic rings, plastic tubes, andplastic plates are common biofil-tration substrates. The configura-tion of the substrate and the man-

Dissolved gas constraints

Although ammonia-nitrogenbuild-up can severely limit a recir-culating systemÕs carrying capaci-ty, maintaining adequate dis-solved oxygen (DO) concentra-tions in the culture tank and filtersystem also is of critical impor-tance. In most cases, a systemÕsability to add dissolved oxygen towater will become the first limit-ing factor in a systemÕs fish carry-ing capacity. To maintain ade-quate DO levels in the culturetank, oxygen must be added tothe tank at a rate equal to that ofthe rate of consumption by fishand bacteria. The consumptionrate of dissolved oxygen in a recir-culating system is difficult to cal-culate, yet an estimate is essentialfor proper system design. Theoverall rate of oxygen consump-tion for a system is the sum of therespiration rate of the fish, theoxygen demand of bacteria break-ing down organic wastes anduneaten food (also referred to asBiochemical Oxygen Demand orBOD), and the oxygen demand ofnitrifying bacteria in the filter.The amount of oxygen requiredby the system is largely dictatedby the length of time waste solidsremain within the system and thebiofilter configuration. In systemswith non-submerged biofilters,where solids are removed quickly,as little as 0.3 pound of oxygencan be consumed for every poundof feed added. In systems withsubmerged biological filters,where solids are retained withinthe system between backwashingsof solid-removing filters, as muchas 0.75 pound of oxygen will beconsumed for every pound offeed added.Carbon dioxide (CO2) is a by-product of fish and bacterial respi-ration and it can accumulate with-in recirculating systems. Elevatedcarbon dioxide concentrations inthe water are not highly toxic tofish when sufficient dissolvedoxygen is present. However, formost species, free carbon dioxideconcentrations in the culture tankshould be maintained at less than

20 mg/l to maintain good grow-ing conditions.The build-up of dissolved nitro-gen gas is rarely a problem inwarm water aquaculture systems.However, caution is advisedwhen pressurized aeration or oxy-genation systems are usedbecause atmospheric nitrogen canbecome supersaturated in water ifair is entrained into the pressur-ized flow stream. Aquatic organ-isms subjected to elevated concen-trations of dissolved nitrogen gascan develop Ògas bubblesÓ intheir circulatory systems and die.Maintaining adequate dissolvedoxygen levels and minimizingcarbon dioxide concentrations inthe culture tank cannot be over-looked in recirculating systemdesign. In a typical intensivelyloaded recirculating system, aera-tion or oxygenation system failurecan lead to a total loss of the fishcrop in 1/2 hour or less. Aeration and Degassing: Theaddition of atmospheric oxygento water or the release of excesscarbon dioxide from water can beaccomplished in recirculating sys-tems through a variety of devicessuch as air diffusers, surface agi-tators, and pressurized or non-pressurized packed columns.System aeration is commonly car-ried out in the culture tanks,although this is not a particularlygood place to add dissolved oxy-gen. This is because the oxygentransfer efficiency of aeratorsdrops as the concentration of dis-solved oxygen increases to nearsaturation levels in the tankwater. Because saturated condi-tions are desirable in the culturetank, aeration in this location isextremely inefficient.In recirculating systems, a betterplace to aerate and degas water isin the recycled flow-stream justprior to re-entry into the culturetank. At this location, in systemsusing submerged biological filtra-tion, the concentration of dis-solved oxygen should be at itslowest and carbon dioxide con-centration will be at its highest.Packed column aerators (PCAs)

are an effective and simple meansof aerating water that is already ina flow-stream. In a PCA, waterlow in oxygen is introduced into asmall tower filled with plasticmedium. A perforated plate orspray nozzle evenly distributesthe incoming water over themedium. The packed column isoperated under non-flooded con-ditions so that air exchangethrough the tower is maintained.If the PCA is to be used for carbondioxide stripping, a low pressureair blower will be required to pro-vide a large quantity of air flowthrough the packed medium. A number of recirculating systemdesigns use air-lift pumps (verti-cal pipes with air injection) torecycle water through treatmentprocesses and back to the culturetank. Air lifts agitate the waterwith air bubbles in the processand remove CO2 and add dis-solved oxygen.Pure Oxygen Injection: In inten-sive production systems, the rateof oxygen consumption by thefish and bacteria may exceed thecapabilities of typical aerationequipment to diffuse atmosphericoxygen into the water. In thesecases, pure gaseous oxygen diffu-sion is used to increase the rate ofoxygen addition and allow for ahigher oxygen utilization rate.The saturation concentration ofatmospheric oxygen in waterrarely exceeds 8.75 mg/l in warmwater applications (> 20o C).When pure oxygen is used withgas diffusion systems, the satura-tion concentration of oxygen inwater is increased nearly five foldto 43 mg/l at standard atmos-pheric pressure. This conditionallows for more rapid transfer ofoxygen into water even when theambient tank dissolved oxygenconcentration is maintained closeto atmospheric saturation (> 7mg/l).A measure of success in usingpure oxygen in aquaculture is theoxygen absorption efficiency ofthe injection or diffusion equip-ment. The absorption efficiency isdefined as the ratio of the weight

of oxygen absorbed by the waterto the weight of oxygen appliedthrough the diffusion or injectionequipment. Properly designedoxygen diffusion devices can pro-duce an oxygen absorption effi-ciency of more than 90 percent.However, as with tank aeration(with air), the culture tank is notthe best location for oxygen diffu-sion with common Òair stoneÓdiffusers. Because of the shortcontact time of bubbles risingthrough a shallow (< 6 feet) watercolumn in tanks, air stone dif-fusers have oxygen absorptionefficiencies of not greater than 40percent. Efficient oxygen injectionsystems are designed to maxi-mize both the oxygen/water con-tact area and time. This can beachieved through the use of acounter-current contact column, aclosed packed-column contactunit, a u-tube column or a down-flow bubble contactor. For moreinformation on aeration and oxy-genation equipment see SRAC453, Recirculating Aquaculture TankProduction Systems: ComponentOptions.

Other productionconsiderationsThere have not been many well-documented successes in large-scale fish production in recirculat-ing systems. Most reports of suc-cessful production have beenfrom producers who supply fish

live or on ice to local niche mar-kets. These high-priced marketsappear to be necessary for finan-cial success due to the high cost offish production in recirculatingsystems. In fact, the variable costs(feed, fingerling, electricity andlabor) of producing fish in recir-culating systems is not much dif-ferent than other productionmethods. Where pond culturemethods require a great deal ofelectricity (at least 1 kW per acreof pond) for aeration during thesummer months, recirculatingsystems have a steady electricalload over the entire year. While itmay appear that recirculating sys-tems require more labor in systemupkeep and maintenance thanponds, when the long hours ofnightly labor for checking oxygenin ponds and moving emergencyaerators and harvest effort areconsidered, the difference is mini-mal. Recirculating systems canactually have an advantage inreducing feed costs. Tank produc-tion systems generally yield betterfeed conversion ratios than pondsystems.Why, then, are production costsgenerally higher for recirculatingsystems? The answer usually canbe found when comparing thecapital cost of these systems.Comparing the investment costsof recirculating systems withother production methods is criti-cal in making an informed eco-

nomic evaluation. Constructioncosts of pond production systemsin the Southeast are approximate-ly 90 cents per pound of annualproduction. Recirculating sys-tems, on the other hand, costbetween $1 and $4 per pound ofannual production. A $1 increasein investment cost per pound ofannual production can add morethan 10 cents per pound to theproduction cost of fish.Given these conditions, producersusing recirculating technologygenerally do not attempt to com-pete in the same markets as pondproducers. However, in specialtyhigh-value niche markets, such asgourmet foods, tropical or orna-mental fish, or year-round supplyof fresh product, recirculating sys-tem products are finding a place.The key to niche market success isto identify the market size andmeet commitments before marketexpansion. In most cases, nichemarkets will limit the size of theproduction units.Before investing in recirculatingsystems technology, the prospec-tive aquaculturist should visit acommercial system and learn asmuch about the technology aspossible. As in all aquacultureenterprises, the decision to beginproduction and the size of theproduction unit one choosesshould be based on the market.

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045 fromthe United States Department of Agriculture, Cooperative States Research, Education, and Extension Service.

Recirculating systems for holdingand growing fish have been usedby fisheries researchers for morethan three decades. Attempts toadvance these systems to com-mercial scale food fish productionhave increased dramatically in thelast decade. The renewed interestin recirculating systems is due totheir perceived advantages,including: greatly reduced landand water requirements; a highdegree of environmental controlallowing year-round growth atoptimum rates; and the feasibilityof locating in close proximity toprime markets.Unfortunately, many commercialsystems, to date, have failedbecause of poor design, inferiormanagement, or flawed econom-ics. This publication will addressthe problems of managing a recir-culating aquaculture system sothat those contemplating invest-ment can make informed deci-sions. For information on theoryand design of recirculating sys-tems refer to SRAC PublicationNo. 451, Recirculating AquacultureTank Production Systems: AnOverview of Critical Considerations,and SRAC Publication No. 453,Recirculating Aquaculture Tank

Production Systems: ComponentOptions.Recirculating systems are mechan-ically sophisticated and biological-ly complex. Component failures,poor water quality, stress, dis-eases, and off-flavor are commonproblems in poorly managedrecirculating systems.Management of these systemstakes education, expertise anddedication.Recirculating systems are biologi-cally intense. Fish are usuallyreared intensively (0.5 pound/gal-lon or greater) for recirculatingsystems to be cost effective. As ananalogy, a 20-gallon home aquari-um, which is a miniature recircu-lating system, would have tomaintain at least 10 pounds of fishto reach this same level of intensi-ty. This should be a soberingthought to anyone contemplatingthe management of an intensiverecirculating system.

System operationTo provide a suitable environmentfor intensive fish production,recirculating systems must main-tain uniform flow rates (water andair/oxygen), fixed water levels,and uninterrupted operation.The main cause of flow reductionis the constriction of pipes and airdiffusers by the growth of fungi,

bacteria and algae, which prolifer-ate in response to high levels ofnutrients and organic matter. Thiscan cause increases or decreases intank water levels, reduce aerationefficiency, and reduce biofilter effi-ciency. Flow rate reduction can beavoided or mitigated by usingoversized pipe diameters and con-figuring system components toshorten piping distances. Thefouling of pipes leaving tanks (bygravity flow) is easily observedbecause of the accompanying risein tank water level. If flow ratesgradually decline, then pipesmust be cleaned. A sponge, clean-ing pad or brush attached to aplumber’s snake works well forscouring pipes. Air diffusersshould be cleaned periodically bysoaking them in muriatic acid(available at plumbing suppliers).Flow blockage and water levelfluctuations also can result fromthe clogging of screens used toretain fish in the rearing tanks.Screen mesh should be the largestsize that will retain the fish (usu-ally 3/4 to 1 inch). The screenedarea around pipes should bemuch larger than the pipe diame-ter, because a few dead fish caneasily block a pipe. Screens can bemade into long cylinders or boxesthat attach to pipes and have alarge surface area to preventblockage. Screens should be tight-

March 1999Revision



Management of Recirculating Systems

Michael P. Masser1, James Rakocy2 and Thomas M. Losordo3

1Auburn University;2University of the Virgin Islands;3North Carolina State University

ly secured to the pipe so that theycannot be dislodged during feed-ing, cleaning and harvesting oper-ations.An essential component of recir-culating systems is a backuppower source (see SRACPublication No. 453). Electricalpower failures may not be com-mon, but it only takes a briefpower failure to cause a cata-strophic fish loss. For example, ifa power failure occurred in awarmwater system (84o F) at sat-urated oxygen concentrationscontaining 1/2-pound fish at adensity of 1/4 pound of fish pergallon of water, it will take only16 minutes for the oxygen con-centration to decrease to 3 ppm, astressful level for fish. The samesystem containing 1-pound fish ata density of 1 pound of fish pergallon would plunge to thisstressful oxygen concentration inless than 6 minutes. These scenar-ios should give the prospectivemanager a sobering feeling forhow important backup power isto the integrity of a recirculatingsystem. Certain components of backupsystems need to be automatic. Anautomatic transfer switch shouldstart the backup generator in casepersonnel are not present. Auto-matic phone alarm systems areinexpensive and are essential inalerting key personnel to powerfailures or water level fluctua-tions. Some phone alarm systemsallow in-dialing so that managerscan phone in and check on thestatus of the system. Other com-ponent failures can also lead todisastrous results in a very shorttime. Therefore, systems shouldbe designed with essential backupcomponents that come on auto-matically or can be turned onquickly with just a flip of aswitch. Finally, one of the sim-plest backups is a tank of pureoxygen connected with a solenoidvalve that opens automaticallyduring power failures. This oxy-gen-solenoid system can providesufficient dissolved oxygen tokeep the fish alive during powerfailures.

Biological filters (biofilters) canfail because of senescence, chemi-cal treatment (e. g., disease treat-ment), or anoxia. It takes weeks tomonths to establish or colonize abiofilter. The bacteria that colonizea biofilter grow, age and die.These bacteria are susceptible tochanges in water quality (low dis-solved oxygen [DO], low alkalini-ty, low or high pH, high CO2,etc.), chemical treatments, andoxygen depletions. Biological fil-ters do not take rapid changewell!

Particulates Particulate removal is one of themost complicated problems inrecirculating systems. Particulatescome from uneaten feed and fromundigested wastes. It has beenestimated that more than 60 per-cent of feed placed into the sys-tem ends up as particulates. Quickand efficient removal of particu-lates can significantly reduce thebiological demand placed on thebiofilter, improve biofilter efficien-cy, reduce the overall size of thebiofilter required, and lower theoxygen demand on the system.Particulate filters should becleaned frequently and main-tained at peak efficiency. Many

particulates are too small to beremoved by conventional particu-late filters and cause or compli-cate many other system problems.

Water quality managementIn recirculating systems, goodwater quality must be maintainedfor maximum fish growth and foroptimum effectiveness of bacteriain the biofilter (Fig. 1). Water qual-ity factors that must be monitoredand/or controlled include temper-ature, dissolved oxygen, carbondioxide, pH, ammonia, nitrite andsolids. Other water quality factorsthat should be considered arealkalinity, nitrate and chloride.

Temperature

Temperature must be maintainedwithin the range for optimumgrowth of the cultured species. Atoptimum temperatures fish growquickly, convert feed efficiently,and are relatively resistant tomany diseases. Biofilter efficiencyalso is affected by temperature butis not generally a problem inwarmwater systems. Temperaturecan be regulated with electricalimmersion heaters, gas or electricheating units, heat exchangers,chillers, or heat pumps. Tempera-

DENITRIFICATION

NITRIFICATION

ION BALANCE

GAS STRIPPING

ALKALINITY ADDITION

BOD REDUCTION

DISSOLVEDREFRACTORYMANAGEMENT

AERATION

SOLIDS REMOVAL

NO2

H+

N2

NO3

CO2

O2

TANBOD

SOLIDS

INERTSOLIDS

BACTERIA

RFM 6/6/90

CLOSED RECIRCULATING SYSTEM

Figure 1. Diagram of fish wastes and their eff ects on bacterial and c hemical interactions in a recir culating system.

Cour tesy of Ronald F . Malone , Depar tment of Civil Engineering, Louisiana State Univer sity , fromLouisiana Aquaculture 1992, “Design of Recir culating Systems f or Intensive Tilapia Culture ,”Douglas G. Drennan and Ronald F . Malone .

ture can be manipulated to reducestress during handling and to con-trol certain diseases (e.g., Ich andESC).

Dissolved oxygen

Continuously supplying adequateamounts of dissolved oxygen tofish and the bacteria/biofilter inthe recirculating system is essen-tial to its proper operation.Dissolved oxygen (DO) concentra-tions should be maintained above60 percent of saturation or above 5ppm for optimum fish growth inmost warmwater systems. It isalso important to maintain DOconcentrations in the biofilter formaximum ammonia and nitriteremoval. Nitrifying bacteriabecome inefficient at DO concen-trations below 2 ppm.Aeration systems must operatecontinuously to support the highdemand for oxygen by the fishand microorganisms in the sys-tem. As fish approach harvest sizeand feeding rates (pounds/sys-tem) are near their maximum lev-els, oxygen demand may exceedthe capacity of the aeration systemto maintain DO concentrationsabove 5 ppm. Fish show signs ofoxygen stress by gathering at thesurface and swimming into thecurrent produced by the aerationdevice (e. g., agitator, air lift, etc.)where DO concentrations arehigher. If this occurs, a supple-mental aeration system should beused or the feeding rate must bereduced. Periods of heavy feeding may besustained by multiple or continu-ous feedings of the daily rationover a 15- to 20-hour period ratherthan in two or three discretemeals. As fish digest food, theirrespiration rate increases dramati-cally, causing a rapid decrease inDO concentrations. Feeding smallamounts continuously with auto-matic or demand feeders allowsDO to decline gradually withoutreaching critical levels. Duringperiods of heavy feeding, DOshould be monitored closely, par-ticularly before and after feedings.Recirculating systems require con-stant monitoring to ensure theyare functioning properly.

Water said to be “saturated” withoxygen contains the maximumamount of oxygen that will dis-solve in it at a given temperature,salinity and pressure (Table 1).Pure oxygen systems can be incor-porated into recirculating systems.These inject oxygen into a con-fined stream of water, creatingsupersaturated conditions (seeSRAC Publication No. 453). Supersaturated water, with DOconcentrations several times high-er than saturation, is mixed intothe rearing tank water to maintainDO concentrations near satura-tion. The supersaturated watershould be introduced into therearing tank near the bottom andbe rapidly mixed throughout thetank by currents generated fromthe water pumping equipment.Proper mixing of the supersaturat-ed water into the tank is critical.Dissolved oxygen will escape intothe air if the supersaturated wateris agitated too vigorously. If thewater is mixed too slowly, zonesof supersaturation can cause gasbubble disease. In gas bubble dis-ease, gases come out of solutioninside the fish and form bubblesin the blood. These bubbles canresult in death. Fry are particular-ly sensitive to supersaturation.

Carbon dioxide

Carbon dioxide is produced byrespiration of fish and bacteria inthe system. Fish begin to stress atcarbon dioxide concentrationsabove 20 ppm because it interfereswith oxygen uptake. Like oxygenstress, fish under CO2 stress cometo the surface and congregatearound aeration devices (if pre-

sent). Lethargic behavior and asharply reduced appetite are com-mon symptoms of carbon dioxidestress. Carbon dioxide can accumulate inrecirculating systems unless it isphysically or chemically removed.Carbon dioxide usually isremoved from the water bypacked column aerators or otheraeration devices (see SRACPublication No. 453).

pH

Fish generally can tolerate a pHrange from 6 to 9.5, although arapid pH change of two units ormore is harmful, especially to fry.Biofilter bacteria which are impor-tant in decomposing waste prod-ucts are not efficient over a widepH range. The optimum pH rangefor biofilter bacteria is 7 to 8. The pH tends to decline in recir-culating systems as bacterial nitri-fication produces acids and con-sumes alkalinity, and as carbondioxide is generated by the fishand microorganisms. Carbondioxide reacts with water to formcarbonic acid, which drives thepH downward. Below a pH of 6,the nitrifying bacteria are inhibit-ed and do not remove toxic nitro-gen wastes. Optimum pH range generally ismaintained in recirculating sys-tems by adding alkaline buffers.The most commonly used buffersare sodium bicarbonate and calci-um carbonate, but calciumhydroxide, calcium oxide, andsodium hydroxide have beenused. Calcium carbonate may dis-solve too slowly to neutralize arapid accumulation of acid.

Table 1. Oxyg en saturation le vels in fresh water at sea le vel atmospheric pressure .

Temperature DO Temperature DOoC oF mg/L (ppm) oC oF mg/L (ppm)10 50.0 10.92 24 75.2 8.2512 53.6 10.43 26 78.8 7.9914 57.2 9.98 28 82.4 7.7516 60.8 9.56 30 86.0 7.5318 64.4 9.18 32 89.6 7.3220 68.0 8.84 34 93.2 7.1322 71.6 8.53 36 96.8 6.95

daily. If total ammonia concentra-tions start to increase, the biofiltermay not be working properly orthe feeding rate/ammonia nitro-gen production is higher than thedesign capacity of the biofilter.

Calcium hydroxide, calcium oxideand sodium hydroxide dissolvequickly but are very caustic; thesecompounds should not be addedto the rearing tank because theymay harm the fish by creatingzones of very high pH. The pH ofthe system should be monitoreddaily and adjusted as necessary tomaintain optimum levels. Usually,the addition of sodium bicarbon-ate at a rate of 17 to 20 percent ofthe daily feeding rate is sufficientto maintain pH and alkalinitywithin the desired range (Fig. 2).For example, if a tank is being fed10 pounds of feed per day thenapproximately 2 pounds of bicar-bonate would be added daily toadjust pH and alkalinity levels. Alkalinity, the acid neutralizingcapacity of the water, should bemaintained at 50 to 100 mg as cal-cium carbonate/L or higher, asshould hardness. Generally, theaddition of alkaline buffers usedto adjust pH will provide ade-quate alkalinity, and if the buffersalso contain calcium, they add tohardness. For a more detailed dis-cussion of alkalinity and hardnessconsult a water quality text.

Nitrogen wastes

Ammonia is the principal nitroge-nous waste released by fish and ismainly excreted across the gills asammonia gas. Ammonia is abyproduct from the digestion ofprotein. An estimated 2.2 poundsof ammonia nitrogen are pro-duced from each 100 pounds offeed fed. Bacteria in the biofilterconvert ammonia to nitrite andnitrite to nitrate, a process callednitrification. Both ammonia andnitrite are toxic to fish and are,therefore, major managementproblems in recirculating systems(Fig. 2). Ammonia in water exists as twocompounds: ionized (NH4+) andun-ionized (NH3) ammonia. Un-ionized ammonia is extremelytoxic to fish. The amount of un-ionized ammonia present dependson pH and temperature of thewater (Table 2). Un-ionizedammonia nitrogen concentrationsas low as 0.02-0.07 ppm have beenshown to slow growth and cause

tissue damage in several speciesof warmwater fish. However,tilapia tolerate high un-ionizedammonia concentrations and sel-dom display toxic effects in well-buffered recirculating systems.Ammonia should be monitored

Table 2. Percenta ge of total ammonia in the un-ioniz ed form at diff ering pH v alues and temperatures.

Temperature ( oC)

pH 16 18 20 22 24 26 28 30 32

7.0 0.30 0.34 0.40 0.46 0.52 0.60 0.70 0.81 0.957.2 0.47 0.54 0.63 0.72 0.82 0.95 1.10 1.27 1.507.4 0.74 0.86 0.99 1.14 1.30 1.50 1.73 2.00 2.367.6 1.17 1.35 1.56 1.79 2.05 2.35 2.72 3.13 3.697.8 1.84 2.12 2.45 2.80 3.21 3.68 4.24 4.88 5.728.0 2.88 3.32 3.83 4.37 4.99 5.71 6.55 7.52 8.778.2 4.49 5.16 5.94 6.76 7.68 8.75 10.00 11.41 13.228.4 6.93 7.94 9.09 10.30 11.65 13.20 14.98 16.96 19.468.6 10.56 12.03 13.68 15.40 17.28 19.42 21.83 24.45 27.688.8 15.76 17.82 20.08 22.38 24.88 27.64 30.68 33.90 37.769.0 22.87 25.57 28.47 31.37 34.42 37.71 41.23 44.84 49.029.2 31.97 35.25 38.69 42.01 45.41 48.96 52.65 56.30 60.389.4 42.68 46.32 50.00 53.45 56.86 60.33 63.79 67.12 70.729.6 54.14 57.77 61.31 64.54 67.63 70.67 73.63 76.39 79.299.8 65.17 68.43 71.53 74.25 76.81 79.25 81.57 83.68 85.85

10.0 74.78 77.46 79.92 82.05 84.00 85.82 87.52 89.05 90.5810.2 82.45 84.48 86.32 87.87 89.27 90.56 91.75 92.80 93.84

8.5

8.0

7.5

7.0

6.5

6.0

0 100 200 300 400 500

Discontinuesupplemental aeration

Reduce dailybicarbonate

addition

Increaseaeration

Addsodium

bicarbonate& aerate

Addsodium

bicarbonate

Optimum

Alkalinity, mg/L as CaCO3

Figure 2. The pH mana gement dia gram, a graphical solution of the ionization constant equation f or carbonic acid at 25 oC.

Cour tesy of Ronald F . Malone , Depar tment of Civil Engineering, Louisiana State Univer sity , fromMaster’ s Thesis of P eter A. Allain, 1988, “Ion Shifts and pH Mana gement in High Density Shed dingSystems f or Blue Crabs (Callinectes sapidus) and Red Swamp Cra wfish (Pr ocambarus c larkii), ”Louisiana State Univer sity .

Biofilters consist of actively grow-ing bacteria attached to some sur-face(s). Biofilters can fail if thebacteria die or are inhibited bynatural aging, toxicity from chem-icals (e. g., disease treatment), lackof oxygen, low pH, or other fac-tors. Biofilters are designed so thataging cells slough off to createspace for active new bacterialgrowth. However, there can be sit-uations (e. g., cleaning too vigor-ously) where all the bacteria areremoved. If chemical additionscause biofilter failure, the water inthe system should be exchanged.The biofilter would then have tobe re-activated (taking 3 or 4weeks) and the pH adjusted tooptimum levels.During disruptions in biofilterperformance, the feeding rateshould be reduced considerablyor feeding should be stopped.Feeding, even after a completewater exchange, can cause ammo-nia nitrogen or nitrite nitrogenconcentrations (Fig. 3) to rise tostressful levels in a matter ofhours if the biofilter is not func-

tioning properly. Subdividing orcompartmentalizing biofiltersreduces the likelihood of a com-plete failure and gives the manag-er the option of “seeding” activebiofilter sludge from one tank orsystem to another. Activating a new biofilter (i. e.,developing a healthy populationof nitrifying bacteria capable ofremoving the ammonia andnitrite produced at normal feed-ing rates) requires a least 1month. During this activationperiod, the normal stocking andfeeding rates should be greatlyreduced. Prior to stocking it isadvantageous, but not absolutelynecessary, to pre-activate thebiofilters. Pre-activation is accom-plished by seeding the filter(s)with nitrifying bacteria (availablecommercially) and providing asynthetic growth medium for aperiod of 2 weeks. The growthmedium contains a source ofammonia nitrogen (10 to 20mg/l), trace elements and a buffer(Table 3). The buffer (sodiumbicarbonate) should be added to

maintain a pH of 7.5. After theactivation period the nutrientsolution is discarded. Many fish can die during thisperiod of biofilter activation.Managers have a tendency tooverfeed, which leads to the gen-eration of more ammonia than thebiofilter can initially handle. Atfirst, ammonia concentrationsincrease sharply and fish stopfeeding and are seen swimminginto the current produced by theaeration device. Deaths will soonoccur unless immediate action istaken. At the first sign of highammonia, feeding should bestopped. If pH is near 7 the fishmay not show signs of stressbecause little of the ammonia is inthe un-ionized form. As nitrifying bacteria, known asNitrosomonas, become establishedin the biofilter, they quickly con-vert the ammonia into nitrite. Thisconversion takes place about 2weeks into the activation periodand will proceed even if feedinghas stopped. Once again, fish willseek relief near aeration and mor-talities will occur soon unlesssteps are taken. Nitrite concentra-tions decline when a second groupof nitrifying bacteria, known asNitrobacter, become established.These problems can be avoided iftime is taken to activate the biofil-ters slowly.Nitrite concentrations also shouldbe checked daily. The degree oftoxicity to nitrite varies withspecies. Scaled species of fish aregenerally more tolerant of highnitrite concentrations than speciessuch as catfish, which are verysensitive to nitrite. Nitrite nitrogenas low as 0.5 ppm is stressful tocatfish, while concentrations ofless than 5 ppm appear to causelittle stress to tilapia. Nitrite toxici-ty causes a disease called “brownblood,” which describes the bloodcolor that results when normalblood hemoglobin comes in con-tact with nitrite and forms a com-pound called methemoglobin.Methemoglobin does not transportoxygen properly, and fish react asif they are under oxygen stress.Fish suffering nitrite toxicity cometo the surface as in oxygen stress,sharply reduce their feeding, and

Table 3. Nutrient solution f or pre-activ ation of biofilter .

Nutrient Concentration (ppm)

Dibasic ammonium phosphate, (NH4)2HPO4 40

Dibasic sodium phosphate, Na2HPO4 40

Sea salts “solids” 40Sea salts “liquids” 0.5Calcium carbonate, CaCO3 250

24

21

18

15

12

9

6

3

0

System Ammonia - N

Nitrite - N

Co

nce

ntr

atio

n, m

g/L

as n

itro

ge

n

Figure 3. Typical ammonia and nitrite cur ves sho wing time dela ys in estab lishing bacteria in biofilter s.

Cour tesy of Ronald F . Malone , Depar tment of Civil Engineering, Louisiana State Univer sity , fromMaster’ s Thesis of Don P . Manthe , 1982, “Water Quality of Submer ged Biological Roc k Filter s forClosed Recir culating Blue Crab Shed ding Systems, ” Louisiana State Univer sity .

are lethargic. Nitrite toxicity canbe reduced or blocked by chlorideions. Usually 6 to 10 parts of chlo-ride protect fish from 1 partnitrite nitrogen. Increasing con-centrations of nitrite are a signthat the biofilter is not workingproperly or the biofilter is notlarge enough to handle theamount of waste being produced.As with ammonia buildup, checkpH, alkalinity and dissolved oxy-gen in the biofilter. Reduce feed-ing and be prepared to flush thesystem with fresh water or addsalt (NaCl) if toxic concentrationsdevelop. Nitrate, the end product of nitrifi-cation, is relatively nontoxicexcept at very high concentra-tions (over 300 ppm). Usuallynitrate does not build up to theseconcentrations if some dailyexchange (5 to 10 percent) withfresh water is part of the manage-ment routine. Also, in many recir-culating systems some denitrifica-tion seems to occur within thesystem that keeps nitrate concen-trations below toxic levels.Denitrification is the bacteria-mediated transformation ofnitrate to nitrogen gas, whichescapes into the atmosphere.

Solids

Solid waste, or particulate matter,consists mainly of feces anduneaten feed. It is extremelyimportant to remove solids fromthe system as quickly as possible.If solids are allowed to remain inthe system, their decompositionwill consume oxygen and pro-duce additional ammonia andother toxic gases (e. g., hydrogensulfide). Solids are removed byfiltration or settling (SRACPublication No. 453). A consider-able amount of highly malodor-ous sludge is produced by recir-culating systems, and it must bedisposed of in an environmental-ly sound manner (e. g., applied toagricultural land or composted).Very small (colloidal) solidsremain suspended in the water.Although the decay of this mater-ial consumes oxygen and pro-duces some additional ammonia,it also serves as attachment sitesfor nitrifying bacteria. Therefore,

a low level of suspended solidsmay serve a beneficial role withinthe system as long as they do notirritate the fishes’ gills.If organic solids build up to highlevels in the system, they willstimulate the growth of microor-ganisms that produce off-flavorcompounds. The concentration ofsolids at which off-flavor com-pounds develop is not known,but the system water shouldnever be allowed to develop afoul or fecal smell. If offensiveodors develop, increase the waterexchange rate, reduce feeding,increase solids removal, and/orenlarge biofilters.

Chloride

Adding salt (NaCl) to the systemis beneficial not only for the chlo-ride ions, which block nitrite toxi-city, but also because sodium andchloride ions relieve osmoticstress. Osmotic stress is caused bythe loss of ions from the fishes’body fluids (usually through thegills). Osmotic stress accompanieshandling and other forms ofstress (e. g., poor water quality).A salt concentration of 0.02 to 0.2percent will relieve osmotic stress.This concentration of salt is bene-ficial to most species of fish andinvertebrates. It should be notedthat rapidly adding salt to a recir-culating system can decreasebiofilter efficiency. The biofilterwill slowly adjust to the additionof salt but this adjustment can

take 3 to 4 weeks. Table 4 summa-rizes general water qualityrequirements of recirculating sys-tems.

Water exchange

Most recirculating systems aredesigned to replace 5 to 10 per-cent of the system volume eachday with new water. This amountof exchange prevents the build-upof nitrates and soluble organicmatter that would eventuallycause problems. In some situa-tions, sufficient water may not beavailable for these high exchangerates. A complete water exchangeshould be done after each produc-tion cycle to reduce the build-upof nitrate and dissolved organics. For emergency situations it is rec-ommended that the system havean auxiliary water reservoir equalto one complete water exchange(flush). The reservoir should bemaintained at the proper temper-ature and water quality.

Fish productionmanagement

Stocking

Fish management starts before thefish are introduced into the recir-culating system. Fingerlingsshould be purchased from a rep-utable producer who practicesgenetic selection, knows how tocarefully handle and transportfish, and does not have a history

Table 4. Recommended water quality requirements of recir culating systems.

Component Recommended v alue or rang e

Temperature optimum range for species cultured - less than 5o F as a rapid change

Dissolved oxygen 60% or more of saturation, usually 5 ppm or more for warmwater fish and greater than 2 ppm in biofilter effluent

Carbon dioxide less than 20 ppmpH 7.0 to 8.0Total alkalinity 50 to 100 ppm or more as CaCO3

Total hardness 50 to 100 ppm or more as CaCO3

Un-ionized ammonia-N less than 0.05 ppmNitrite-N less than 0.5 ppmSalt 0.02 to 0.2 %

of disease problems in his/herhatchery. Starting with poor quali-ty or diseased fingerlings almostensures failure. Fish should be checked for para-sites and diseases before beingintroduced into the system. Newfish may need to be quarantinedfrom fish already in the system sothat diseases will not be intro-duced. A few fish should bechecked for parasites and diseasesby a certified fish diagnostician.Once diseases are introduced intoa recirculating system they aregenerally hard to control, andtreatment may disrupt the biofil-ter. Fish are usually hauled in coolwater. As they come into the sys-tem they usually have to be tem-pered or gradually acclimated tothe system temperature and pH.Fish can generally take a 5o Fchange without much problem.Temperature changes of morethan 5o F should be done at about1o F every 20 to 30 minutes. Stresscan be reduced if the system iscooled to the temperature of thehauling water and then slowlyincreased over a period of severalhours to days. Recirculating systems must oper-ate near maximum production (i. e., maximum risk) capacity atall times to be economical. It is notcost effective to operate pumpsand aeration devices when thesystem is stocked with fingerlingsat only one-tenth of the system’scarrying capacity. Therefore, fin-gerlings should be stocked at veryhigh rates, in the range of 30 fishper cubic foot. Feeding ratesshould be optimum for rapid

growth and near the system maxi-mum—the highest feeding rates atwhich acceptable water qualityconditions can be maintained.When more feed is required, fishstocks should be split and movedto new tanks. This would gradual-ly reduce the stocking rate overthe production cycle. Another approach is to divide therearing tank(s) into compartmentswith different size groups of fishin each compartment. In thisapproach, the optimum feedingrate for all the compartments isconsistently near the biofilter’smaximum performance. As onegroup of fish is harvested, finger-lings are immediately stocked intothe vacant compartment or tank.Compartment size within a tankmay be adjusted as fish grow, byusing movable screens.

Feeding

Knowing how much to feed fishwithout overfeeding is a problemin any type of fish production.Feeding rates are usually based onfish size. Small fish consume ahigher percent of their bodyweight per day than do larger fish(Table 5). Most fish being grownfor food will be stocked as finger-lings. Fingerlings consume 3 to 4percent of their body weight perday until they reach 1/4 to 1/2pound, then consume 2 to 3 per-cent of their body weight untilbeing harvested at 1 to 2 pounds.A rule-of-thumb for pond cultureis to feed all the fish will consumein 5 to 10 minutes. Unfortunately,this method can easily lead tooverfeeding. Overfeeding wastesfeed, degrades water quality, andcan overload the biofilter.

Table 6 approximates a feedingschedule for a warmwater fish(e.g., tilapia) stocked into an 84o Frecirculating system as fry andharvested at a weight of 1 poundafter 250 feeding days. Feed con-version is estimated at 1.5: 1, or1.5 pounds of feed to obtain 1pound of gain. Tables 5 and 6 are estimates andshould be used only as guidelineswhich can change with differingspecies and temperatures.Growth and feed conversion areestimated by weighing a sampleof fish from each tank and thencalculating the feed conversionratios and new feeding rates fromthis sample. For example, 1,000fish in a tank have been consum-ing 10 pounds of feed a day forthe last 10 days (100 poundstotal). The fish were sampled 10days earlier and weighed an aver-age of 0.33 pounds or an estimat-ed total of 330 pounds.

Table 5. Estimated f ood con-sumption b y siz e of a typical warmwater fish.

Average Bod y weightweight per fish consumed

(lbs.) (g) (%)

0.02 9 5.00.04 18 4.00.06 27 3.30.25 113 3.00.50 227 2.750.75 340 2.51.0 454 2.21.5 681 1.8

Table 6. Recommended stoc king and f eeding rates f or diff erent siz e groups of tilapia in tanks, and estimated gr owth rates.

Stoc king rate Weight (g) Growth rate Growth period Feeding rate(number/ft3) Initial Final (g/day) (days) (%)

225 0.02 0.5-1 - 30 20 - 1590 0.5-1 5 - 30 15 - 1045 5 20 0.5 30 10 - 728 20 50 1.0 30 7 - 414 50 100 1.5 30 4 - 3.5

5.5 100 250 2.5 30 3.5 - 1.53 250 450 3.0 70 1.5 - 1.0

A new sample of 25 fish is collect-ed from the tank and weighed.The 25 fish weigh 10 pounds or anaverage of 0.4 pounds per fish. Ifthis is a representative sample,then 1,000 fish should weigh 400pounds. Therefore, the change intotal fish weight for this tank is400 minus 330, or 70 pounds. Thefish were fed 100 pounds of feedin the last 10 days and gained 70pounds in weight. Feed conver-sion then is equal to 1.43 to 1 (i.e.,100 ÷ 70). In other words, the fishgained 1 pound of weight for each1.43 pounds of feed fed. The dailyfeeding rate should now beincreased to adjust for growth ofthe fish.To calculate the new feeding rate,multiply the estimated total fishweight (400 pounds) by the esti-mated percent body weight offeed consumption for a 0.4-poundfish (from Table 5). Table 5 sug-gests that the percent body weightconsumed per day should bebetween 2.75 and 3 percent. If 3percent is used, then 400 times0.03 is 12.0. Thus, the new feedingrate should be 12 pounds of feedper day for the next 10 days, for atotal of 120 pounds. Using thissampling technique the managercan accurately track growth andfeed conversion, and base othermanagement decisions on thesefactors.

Feeding skills

Feeding is the best opportunity toobserve overall vitality of the fish.A poor feeding response shouldbe an immediate alarm to themanager. Check all aspects of thesystem, particularly water quality,and diagnose for diseases if feed-ing behavior suddenly diminish-es. Fish can be fed once or severaltimes a day. Multiple feedingsspread out the waste load on thebiofilter and help prevent suddendecreases in DO. Research hasshown that small fish will growfaster if fed several times a day.Feeding several times a day seemsto reduce problems of feedingdominance in some species of fish.Many recirculating system man-agers feed as often as every 30

minutes. Multiple feedings at thesame location in a tank canincrease dominance because a fewfish jealously guard the area anddo not let other fish feed. In thissituation, use feeders that distrib-ute feed widely across the tank.Fish can be fed by hand, withdemand feeders, or by automaticfeeders, but stationary demandand belt type feeders tend toencourage dominance. Whichevermethod is used, be careful toevenly distribute feed and not tooverfeed. Always purchase high qualityfeed from a reputable company.Keep feed fresh by storing it in acool, dry place. Never use feedthat is past 60 days of the manu-facture date. Never feed moldy,discolored or clumped feed.Molds on feed may produce afla-toxins, which can stress or killfish. Feed quality deteriorateswith time, particularly whenstored in warm, damp conditions.A disease known as “no blood” isassociated with feed that is defi-cient in certain vitamins. In a caseof “no blood,” the fish appearpale with white gills and bloodappears clear, not red. Anothernutritional disease known as “bro-ken back syndrome” is caused bya vitamin C deficiency. The onlymanagement practice for “noblood” disease and “broken backsyndrome” is to discard the feedbeing used and purchase a differ-ent batch or brand of feed. Fines, crumbled feed particles, arenot generally consumed by thefish but add to the waste load ofthe system, increasing the burdenon particulate and biological fil-ters. Therefore, it is recommendedthat feed pellets be sifted orscreened to remove fines beforefeeding.

Off-flavor

Off-flavor in recirculating systemsis a common and persistent prob-lem. Many times fish have to bemoved into a clean system, onewith clear, uncontaminated water,where they can be purged of off-flavor before being marketed.Purging fish of off-flavor can takefrom a few days to many weeks

(depending on the type and sever-ity of off-flavor). If fish remain inthe purging tanks for an extendedperiod, their feeding rate mayneed to be reduced, or off-flavormay develop within the purgingsystem. See SRAC Publication No. 431,Testing Flavor Quality of PreharvestChannel Catfish, for detailed infor-mation on off-flavor.

Stress and disease control The key to fish management isstress management. Fish can bestressed by changes in tempera-ture and water quality, by han-dling (including seining and haul-ing), by nutritional deficiencies,and by exposure to parasites anddiseases. Stress increases the sus-ceptibility of fish to disease, whichcan lead to catastrophic fish lossesif not detected and treated quick-ly. To reduce stress fish must behandled gently, kept under properwater quality conditions, and pro-tected from exposure to poorwater quality and diseases. Evensound and light can stress fish.Unexpected sounds or suddenflashes of light often trigger anescape response in fish. In a tank,this escape response may sendfish into the side of the tank, caus-ing injury. Fish are generally sen-sitive to light exposure, particular-ly if it is sudden or intense. Forthis reason many recirculatingsystems have minimal lightingaround the fish tanks.

Diseases

There are more than 100 knownfish diseases, most of which donot seem to discriminate betweenspecies. Other diseases are veryhost specific. Organisms known tocause diseases and/or parasitizefish include viruses, bacteria,fungi, protozoa, crustaceans, flat-worms, roundworms and seg-mented worms. There are alsonon-infectious diseases such asbrown blood, no blood and bro-ken back syndrome. Any of thesediseases can become a problem ina recirculating system. Diseasescan be introduced into the systemfrom the water, the fish, and thesystem’s equipment.

Diseases are likely to enter thesystem from hauling water, on thefish themselves, or on nets, bas-kets, gloves, etc., that are movedfrom tank to tank. Hauling watershould never be introduced intothe system. Fish should be quar-antined, checked for diseases, andtreated as necessary. Equipmentshould be sterilized (e. g., chlorinedip) before moving it betweentanks. If possible, provide sepa-rate nets and baskets for each tankso they will not contaminate othertanks. Disease can spread rapidlyfrom one tank to another if equip-ment is freely moved betweentanks or if all the water within thesystem is mixed together as in acommon sump, particulate filteror biofilter. A manager needs to be familiarwith the signs of stress and dis-ease which include: ■ Excitability ■ Flashing or whirling ■ Skin or fin sores or discol-

orations ■ Staying at the surface ■ Erratic swimming ■ Reduction in feeding rate ■ Gulping at the surface

■ Cessation of feeding ■ MortalitiesWhenever any of these symptomsappear the manager should checkwater quality and have a few fishwith symptoms diagnosed by aqualified fish disease specialist.The most common diseases inrecirculating systems are causedby bacteria and protozoans. Somediseases that have been particular-ly problematic in recirculatingsystems include the protozoal dis-eases Ich (Ichthyophthirius) andTrichodina, and the bacterial dis-eases columnaris, Aeromonas,Streptococcus and Mycobacterium. Itappears that Trichodina andStreptococcus diseases are prob-lematic in recirculating systemswith tilapia, while Mycobacteriumhas been found in hybrid stripedbass in intensive recirculating sys-tems.It may be possible to treat dis-eases with chemicals approved forfish (see SRAC Publication No.410, Calculating Treatments forPonds and Tanks), although fewtherapeutants are approved foruse on food fish species otherthan catfish and rainbow trout.Treatment always has its prob-lems. In the case of recirculating

systems, chemical treatments canseverely disrupt the biofilter.Biofilter bacteria are inhibited tosome degree by formalin, coppersulfate, potassium permanganate,and certain antibiotics. Even sud-den changes in salt concentrationwill decrease biofilter efficiency. Ifthe system is designed properly, itmay be possible to isolate thebiofilter from the rest of the sys-tem, treat and flush the fish tanks,and then reconnect the biofilterwithout exposing it to chemicaltreatment. However, there is adanger that the biofilter will re-introduce the disease organism.Whenever a chemical treatment isapplied, be prepared to exchangethe system water and monitor theDO concentration and other waterquality factors closely. Fish usual-ly reduce their feed consumptionafter a chemical treatment; there-fore, feeding rates need to bemonitored carefully. Tables 7 and 8 give possible caus-es and management options basedon the observation of the fish orwater quality tests.

Conclusions Recirculating systems have devel-oped to the point that they arebeing used for research, for orna-mental/tropical fish culture, formaturing and staging brood fish,for producing advanced fry/fin-gerlings, and for producing foodfish for high dollar niche markets.They continue to be expensiveventures which are as much art asscience, particularly when itcomes to management. Do yourhomework before deciding toinvest in a recirculating system.Investigate the efficiency, compati-bility and maintenance require-ments of the components.Estimate the costs of building andoperating the system and of mar-keting the fish without any returnon investment for at least 2 years.Know the species you intend togrow, their environmental require-ments, diseases most common intheir culture, and how those dis-eases are treated. Know yourpotential markets and how thefish need to be prepared for thatmarket. Be realistic about the

Examples of fish diseases

A–Columnaris B–Aeromonas

C–Streptococcus(cataract and pop-e ye)

D–Mycobacterium(gran ular liver and spleen)

Table 7. Possib le options in mana ging a recir culating tank system based on obser vations of the fish.

Obser vation Possib le cause Possib le mana gement

Fish:Excitable/darting/erratic swimming ■ excess or intense reduce sound level/pad sides of tank/reduce

sounds/lights light intensity■ parasite examine* fish with symptoms

■ high ammonia check ammonia concentration

Flashing/whirling ■ parasite examine fish with symptoms

Discolorations/sores ■ parasite/bacteria examine fish with symptoms

Bloated/eyes bulging out ■ virus or bacteria examine fish with symptoms

■ gas bubble disease check for supersaturation and examine fish with symptoms

Lying at surface/not swimming off ■ parasite examine fish with symptoms

■ low oxygen check dissolved oxygen in tank

■ high ammonia or nitrite check ammonia and nitrite concentrations

■ bad feed check feed for discoloration/clumping and check blood of fish

■ high carbon dioxide check carbon dioxide level

Crowding around water inflow/aerators ■ low oxygen check dissolved oxygen in tank

■ parasite/disease examine fish with symptoms



Gulping at surface ■ low oxygen check dissolved oxygen in tank



■ high carbon dioxide check carbon dioxide level


Reducing feeding ■ low oxygen check dissolved oxygen in tank




Stopping feeding ■ low oxygen check dissolved oxygen in tank



Discolored blood – ■ high nitrite examine fish with symptom; add 5 to 6 ppm Brown chloride for each 1 ppm nitrite; purchase

new feed and discard old feedClear (no blood) ■ vitamin deficiency examine fish with symptom; check feed for

discoloration/clumping; purchase new feed and discard old feed

Broken back or “S” shaped backbone ■ vitamin deficiency examine fish with symptom; purchase new feed and discard old feed

*Have fish examined by a qualified fish diagnostician.

Table 8. Possib le mana gement options based on water quality and f eed obser vations.

Obser vation Possib le mana gement

Low dissolved oxygen (less than 5 ppm) ■ increase aeration

■ stop feeding until corrected

■ watch for symptoms of new parasite/disease

High carbon dioxide (above 20 ppm) ■ add air stripping column

■ increase aeration

■ watch for symptoms of new paraside/disease

Low pH (less than 6.8) ■ add alkaline buffers (sodium bicarbonate, etc.)

■ reduce feeding rate

■ check ammonia and nitrite concentarations

High ammonia (above 0.05 ppm as un-ionized) ■ exchange system water


■ check biofilter, pH, alkalinity, hardness, and dissolved oxygen in the biofilter


High nitrite (above 0.5 ppm) ■ exchange system water


■ add 5 to 6 ppm chloride per 1 ppm nitrite

■ check biofilter, pH, alkalinity, hardness, and dissolved oxygen in the biofilter


Low alkalinity ■ add alkaline buffers

Low hardness ■ add calcium carbonate or calcium chloride

Discolored/clumped feed ■ purchase new feed and discard old feed


money, time and effort you arewilling to invest while you are inthe learning curve of managing arecirculating system.

Finally, design the system with anemergency aeration system, back-up power sources, and backupsystem components. Monitorwater quality daily and maintainit within optimum ranges.

Exclude diseases at stocking.Perform routine diagnostic checksand be prepared to treat diseases.Reduce stress whenever and how-ever possible. STAY ALERT!

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045from the United States Department of Agriculture, Cooperative States Research, Education, and Extension Service.

There is a great deal of interest inrecirculating aquaculture produc-tion systems both in the UnitedStates and worldwide. Most fishgrown in ponds, floating net pens,or raceways can be reared in com-mercial scale recirculating sys-tems, but the economic feasibilityof doing so is not certain. Recircu-lating systems are generallyexpensive to build, which increas-es production cost. (For moreinformation see SRAC publication456 on the economics of recirculat-ing systems). The challenge todesigners of recirculating systemsis to maximize production capaci-ty per dollar of capital invested.Components should be designedand integrated into the completesystem to reduce cost while main-taining or even improving reliabil-ity. Research and development inrecirculating systems has beengoing on for nearly three decades.There are many alternative tech-nologies for each process andoperation. The selection of a par-ticular technology depends uponthe species being reared, produc-

tion site infrastructure, productionmanagement expertise, and otherfactors. Prospective users of recir-culating aquaculture productionsystems need to know about therequired water treatment process-es, the components available foreach process, and the technologybehind each component. Thispublication is intended as a start-ing point for such a study.A recirculating system maintainsan excellent cultural environmentwhile providing adequate feed foroptimal growth. Maintaininggood water quality is of primaryimportance in aquaculture. Whilepoor water quality may not belethal to the crop, it can reducegrowth and cause stress thatincreases the incidence of disease.Critical water characteristicsinclude concentrations of dis-solved oxygen, un-ionized ammo-nia-nitrogen, nitrite-nitrogen, andcarbon dioxide. Nitrate concentra-tion, pH, alkalinity and chloridelevels also are important.The by-products of fish metabo-lism include carbon dioxide,ammonia-nitrogen, and particu-late and dissolved fecal solids.Water treatment components mustbe designed to eliminate theadverse effects of these wasteproducts. In recirculating tanksystems, proper water quality ismaintained by pumping tank

water through special filtrationand aeration or oxygenationequipment. Each component mustbe designed to work in conjunc-tion with other components of thesystem. For more information onwater quality requirements andmanagement of recirculating sys-tems, see SRAC publications 451and 452.

Waste solids removalThe decomposition of solid fishwaste and uneaten or indigestiblefeed can use a significant amountof oxygen and produce largequantities of ammonia-nitrogen.There are three categories of wastesolids—settleable, suspended, andfine or dissolved solids.

Settleable solids

Settleable solids are generally theeasiest to deal with and should beremoved from the culture tankwater as rapidly as possible. Thisis easiest when bottom drains areproperly placed. In tanks with cir-cular flow patterns (round, octag-onal, hexagonal, square withrounded corners) and minimalagitation, settleable solids can beremoved as they accumulate inthe bottom center of the tank, in aseparate, small flow-stream ofwater, or together with the entireflow leaving the tank. Centerdrains with two outlets are often

VIPR

April 1999Revised



A Review of Component OptionsThomas M. Losordo1, Michael P. Masser2 and James E. Rakocy3

1Department of Zoology, North CarolinaState University

2Department of Wildlife and FisheriesSciences, Texas A&M University

3University of the Virgin Islands,Agricultural Experiment Station, U.S.Virgin Islands

used for the small flow-streamprocess. This double drain dividesthe flow leaving the tank into asmall pipe carrying the settleablesolids, and a larger pipe with ahigher flow rate carrying the sus-pended solids from the upperwater column of the tank (Fig. 1).

Settled solids should be removedfrom the center of the tank on acontinuous or semi-continuousbasis. The flow rate at which thesettleable solids are carried willdetermine the method used to col-lect and concentrate them for fur-ther treatment or disposal. In sys-tems with a high settleable solidsflow rate (20 to 50 percent of thetotal tank flow), swirl separators,settling basins, or drum screen fil-ters are used to collect thesesolids. At lower flow rates, small-er settling components can beused. An example is a doubledrain developed by Waterline,Inc.1 (Prince Edward Island,Canada). In this patented design,the flow containing settleablesolids moves slowly though apipe (under the tank) leading toan external standpipe (water levelcontrol structure). The flow veloci-ty is slow enough that the solids

settle out within the pipe whilethe clearer water overflows thestandpipe. The external standpipe is routinely removed toincrease the water velocity in thepipe and the settled solids areflushed from the line.Another example of a doubledrain is a particle trap developedat the Center for Scientific andIndustrial Research (SINTEF),Norwegian HydrotechnicalLaboratory, in Trondheim,Norway.In this design, settleable solidsflow under a plate, spaced justslightly off the bottom of the tank,in a flow of water that amounts toonly 5 percent of the total flowleaving the center of the tank(Flow B, Fig. 2). The larger flow(95 percent of the total) exits thetank through a large dischargestrainer mounted at the top of theparticle trap (Flow A, Fig. 2).Outside of the tank, the settleablesolids flow-stream from the parti-cle trap enters a sludge collector(Flow B, Fig. 3). The waste parti-cles settle and are retained in thesludge collector, and the clarifiedwater exits the sludge collector atthe top and flows by gravity forfurther treatment. The sludge inthe collector, which has an aver-age dry weight solids content of 6percent, is drained from the bot-tom of the collector.

In rectangular raceways with plugflow (flow that moves along thelong axis of the raceway tank),solids are more difficult to removeas the velocity at the bottom of thetank is generally slower than inround tanks. If the water velocityat the tank bottom can beincreased to move the settledsolids along the bottom of thetank, then solids can be removedusing a sediment trap. The sedi-ment trap should span the bot-tom, across the short axis of theraceway, perpendicular to thedirection of water flow. Tworeviews of tank flow andhydraulic analysis can be found inBurley and Klapsis (1988) andTvinnereim (1988).An alternative to plug flow withina raceway is to create a complete-ly mixed (horizontally and verti-cally) tank by installing a waterinlet and outlet manifold alongthe long axis of the tank. As seenin Figure 4, water enters uniform-ly along the bottom of one side ofthe raceway and is removed alongthe other side. Water must enter ata high enough velocity to create arotational flow along the shortaxis of the raceway (Fig. 4). Thesolids will move across the bot-tom of the raceway and into theeffluent manifold.Another method of dealing withsettleable solids is to keep them in

Standpipe

Solidscollectionbowl

A A

B B

BA

Figure 1. Typical double drain forremoving settleable solids from a fishculture tank; A = suspended solidsflow stream, B = settleable solids flowstream. (after Losordo, 1997).

A A

B B

B

Settleablesolids flow

ASuspendedsolids flow

Main discharge

Tankfloor

Figure 2. The ECO-TRAP™ particle trap is an advanced double drain design thatconcentrates much of the settleable solids in only 5 percent of the water flow leav-ing the fish culture tank (B). (after Hobbs et al., 1997). (ECO-TRAP is a trade-mark of AquaOptima AS, Pir Senteret, 7005 Trondheim, Norway, U.S. PatentNo. 5,636,595.)

1Mention of a specific product or trade-name does not constitute and endorsementby the authors or the USDA SouthernRegional Aquaculture Center, nor does itimply approval to the exclusion of othersuitable products.

suspension with continuous agita-tion until they enter an externalsettling tank. In settling tanks (orbasins), water flow is very slow sothat solids settle out by gravity.Settling tanks may or may notinclude tube or lamella sedimen-tation material. This material isconstructed with bundles of tubesor plates, set at specific angles tothe horizontal (usually 60o), thatreduce both the settling distanceand circulation within the settlingtank. Using settling plates reducesthe size requirement of a settlingbasin, thus saving space within afacility. However, the plates makeroutine cleaning of settling basinsmore time-consuming.The benefits of using external set-tling basins outside of the rearingtank are simplicity of operation,low energy requirements, and thegenerally low cost of construction.The disadvantages include the rel-atively large size of settlingbasins, the time used in routinecleaning, and the large quantity ofwater that is wasted in the clean-ing process. If settling basins arenot cleaned regularly, waste solidscan break down within the basinand contribute to the ammonia-nitrogen production and oxygendemand of the system. Another way to remove settleablesolids, external to the culture tank,is to use a centrifugal settlingcomponent known as a hydrocy-clone or swirl separator. In thisdesign, water and particulatesolids enter the separator tangen-tially, creating a circular orswirling flow pattern in a conicalshaped tank. The heavier solidsmove towards the walls and settleto the bottom where they areremoved continuously. The mainadvantage of these units is thecompact size. A major disadvan-tage is the large volume ofreplacement water requiredbecause of the continuous streamof wastewater.

Suspended solids

From an engineering viewpoint,the difference between suspendedsolids and settleable solids is apractical one. Suspended solidswill not easily settle out of thewater column in the fish culture

Top view

Side view

Flow Bfromparticletrap

Clarified waterto drum screenfilter

Clarified waterto drum screenfilter

Flow Bfromparticletrap

Sludge discharge

Figure 3. The sludge collector that works in conjunction with the ECO-TRAP™to remove settled solids from the flow stream B (Fig. 2) (after Hobbs et al., 1997).

Figure 4. Cross-section of a “cross-flow” raceway. Water flows in through aninlet manifold with jets (A) and out through a similar drain manifold (B) on theopposite side of the tank (after Colt and Watten, 1988).

Longaxis

Shortaxis

InfluentA

BEffluent

a high pressure jet of water (fromthe outside of the drum) washesthe solids off the screen and intoan internal collection trough lead-ing to a waste drain. The advan-tage of the drum screen filter con-figuration over the single platedisk filter is the larger surface areaof the drum for comparably sizedunits.The main advantage of usingscreen filter technology ratherthan settling basins and swirl sep-arators is their small size and rel-atively low water loss duringbackwashing. Libey (1993) report-ed that, on average, in a tilapiasystem, only 13.4 percent of thewater used with a settling basinwas needed with a drum screenfilter.The main disadvantage of com-mercial screen filters is cost, espe-cially for smaller units. Thesmallest commercially availableunits can process approximately475 liters per minute (125 gpm)loaded with 25 mg/L of suspend-

ed solids, and cost about$6,000. A 100 percentincrease in processingcapacity increases the costof a unit by about 50 per-cent (a unit to process 950liters/minute costs about$9,000). So, larger unitsare more cost effective. Totake advantage of this,the flow streams fromseveral production tankscan be combined into onetreatment stream that iscleaned by a larger drumscreen filter. However, theadvantage of the econo-my of scale must beweighed against the riskof spreading disease andwater quality problemswithin linked fish pro-duction tanks.Vacuum cleaned drumscreen filters are now inuse. These units havelimited capacity (375 to1,800 L/ minute, 100 to475 gpm) and their per-formance in commercialfacilities has not beenwell documented. Inclinescreen or belt screen fil-ters also are beginning to

tank. Suspended solids are notalways dealt with adequately inrecirculating systems. Most cur-rent technologies for removingsuspended solids generallyinvolve some form of mechanicalfiltration. Two types of mechanicalfiltration are screen filtration andexpandable granular media filtra-tion.Screen filtration: Screen filtersuse some form of fine mesh mate-rial (stainless steel or polyester)through which effluent passeswhile the suspended solids areretained on the screen. Solids areusually removed from the screenby rotating the clogged screen sur-face past high pressure jets ofwater. The solids are carried awayfrom the screen in a small streamof waste water. The feature thatmakes each screen filter differentand the challenge in designingthese units is the process of col-lecting the solids on the mesh sur-face.

The screening material has beenused in a disk configuration (Fig.5A), drum screen configuration(Fig. 5B), and incline belt configu-ration (Fig. 5C). In rotating disk filters, water to betreated enters one end of the filterunit and must pass throughsequential vertical disks withinthe filter. A problem with thisdesign is the small amount ofscreen surface on which to capturesolids. In heavily fed productionsystems, solids can build up soheavily on one side of the filterthat the screens collapse from thewater pressure.The most common screen filter isthe drum filter (Figs. 5B and 6).With this configuration, waterenters the open end of a drumand passes through a screenattached to the circumference ofthe drum. In most installations, the drumrotates only when the filter meshbecomes clogged with solids, and

Disk screens

A. Disk screen filter (top view)

Inflow

Wastewater

Backwashwater

Cleanedwater

Cleanedwater

Backwashwater

Wastewater

Inflow

Drum screenB. Drum screen filter (top view)

Cleanedwater Flow

C. Incline belt screen filter (side view)

Inflow

Backwash sprayBelt screen

Waste trough

Figure 5. Three screen filter configurations used in recirculating tanks to capture and removesuspended solids.

be used in the aquaculture indus-try (Fig. 5). These units resembleconveyor belts placed on anincline. Water passes through thescreen where suspended solids areretained; solids are lifted out ofthe water on the incline screenand sprayed off with high pres-sure water in a cleaning processsimilar to that of disc and drumscreen filters. The units manufac-tured currently have flow capaci-ties in excess of 7,500 liters perminute (1980 gpm). There is littledata on the operational character-istics of these filters. Expandable granular media fil-tration: Expandable granularmedia filters remove solids bypassing water through a bed ofgranular medium (sand or plasticbeads). The solids either adhere tothe medium or are trapped withinthe open spaces between themedium particles. Over time, thefilters will become clogged withsolids and require cleaning, orbackwashing. Backwashing thesefilters requires that the filter bedbe expanded (from a compactedstate) to release the solids. Forother applications (e.g., drinking

water, swimming pools), the mostcommon filtration medium issand. Pressurized down-flowsand filters have been widelyused in hatchery operations.While these filters can removemuch of the suspended solids in aflow-stream, when fish are fedheavily the filter must be back-washed frequently, which wastesa lot of water. Backwashing thesefilters is accomplished by revers-ing the flow of water through thefilter medium, causing the bed toexpand or “boil.” This releasestrapped solids and scrapes bacter-ial growth off the filter medium.However, bacterial growth on thesand eventually creates gelatinousmasses within the filters that areimpossible to clean with simplebackwashing. Then it is necessaryto open and manually clean thefilter. Down-flow sand filtersreduce or stop the flow of waterwhen they clog. Even short-terminterruptions of water flow can bedisastrous to intensive recirculat-ing systems. An alternative design, used suc-cessfully in the U.S., uses floatingplastic beads instead of sand.

These low density, floating plasticbeads trap and remove suspendedsolids from the flow-stream as thewater passes up through a bed ofbeads (Fig. 7).The solids are removed by activat-ing a motor that turns a propellerlocated within the bed of beads.The propeller expands the bed ofbeads and releases the wastesolids that are trapped within it.After the bed expansion period, ashort settling period allows thebeads to re-float and the solids tosettle to the bottom of the filterchamber. A valve is then openedand the settled solids areremoved. This sequence of eventscan be automated with electroniccircuits and automated valves.Another bead filter design,referred to as the “bubblewashed” bead filter, eliminates therequirement for a propeller tobackwash the filter bed. This filterresembles an “hour glass” withtwo chambers connected by a nar-row “washing throat” (Fig. 8). In the filtration mode, water pass-es up through the beads whilethey are in the upper filtrationchamber. When the beads needcleaning, the flow is stopped andthe filter is drained so that the fil-ter medium drops through the

Pressurebackwash

Outflowto tank

Water filtersthrough screenon drum

Wastedischarge

Inflowfrom tank

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-vided by and used with permission of PRA Manufacturing, Nanaimo, B.C.)

Propellerdrive motor

Return flow toculture tank

Floatingplasticbead mediumPropellers

Flow fromculture tank

Settled solids

To waste

Figure 7. The propeller washed beadfilter traps waste solids between thebeads and backwashes by expandingthe bed of beads with a propeller.(U.S. Patent No. 5,126,042 by Dr.Ronald Malone, Dept. of CivilEngineering, Louisiana StateUniversity)

“washing throat” into the sludgesettling chamber. When the flowis re-started, the filter mediumfloats back into the filter chamberand the waste sludge settles to thebottom of the settling chamberready for discharge to a wastedrain.The advantage of bead filters isthe compact size of the unit andlow water use during backwash-ing. Once biologically active, thebeads become sticky and canremove even fine suspendedsolids. The bacteria that make thefilter sticky are a combination ofautotrophic and heterotrophicbacteria. The autotrophic bacteriacontribute to nitrification. Theheterotrophic bacteria break downthe organic solids that are trappedwithin the bead bed. This can be adisadvantage, because during thetime between backwashings (1 to48 hours), solids undergoing bac-terial degradation use oxygenfrom the system water and releaseammonia-nitrogen. The oxygenconsumed by these bacteria needsto be replaced and the ammonia-

nitrogen produced must be treat-ed.

Fine and dissolved solids

Many of the fine suspended solidsand dissolved organic solids thatbuild up within intensive recircu-lating systems cannot be removedwith traditional mechanisms. Aprocess called foam fractionation(also referred to as air-stripping orprotein skimming) is oftenemployed to remove and controlthe build-up of these solids.Foam fractionation is a generalterm for a process in which airintroduced into the bottom of aclosed column of water createsfoam at the surface of the column.Foam fractionation removes dis-solved organic compounds (DOC)from the water column by physi-cally adsorbing DOC on the risingbubbles. Fine particulate solidsare trapped within the foam at thetop of the column, which can becollected and removed. The mainfactors affected by the operationaldesign of the foam fractionatorare bubble size and contact timebetween the air bubbles and theDOC. A counter-current design(bubbles rising against a down-ward flow of water) improvesefficiency by lengthening the con-tact time between the water andthe air bubbles (Fig. 9). In thisdesign, water is injected into thefoam fractionator through a ven-turi. The venturi mixes air withthe water and the air/water mix-

ture enters the body of the foamfractionator tangentially.

Ammonia and nitrite-nitrogen control Controlling the concentration ofun-ionized ammonia-nitrogen(NH3) in the culture tank is a pri-mary design consideration inrecirculating systems. Ammonia-nitrogen (a by-product of themetabolism of protein in feeds)must be removed from the culturetank at a rate equal to the rate it isproduced to maintain a stable andacceptable concentration. In sys-tems with external ammonia-nitrogen treatment processes, theefficiency of the ammonia-nitro-gen removal process will dictatethe recirculating flow rate (e.g., aless efficient removal system willrequire a higher recycle flow ratefrom the tank through the filter).There are a number of methodsfor removing ammonia-nitrogenfrom water: air stripping, ionexchange, and biological filtra-tion. Air stripping of ammonia-nitrogen through non-flooded (nostanding water in the reactor)packed columns requires that thepH of the water be adjusted toabove 10 and readjusted to safelevels (7 ro 8) before the water re-enters the culture tank. Ionexchange technology is costly andrequires a mechanism for “wast-ing” ammonia-laden salt water. Asalt-brine is used to “regenerate”the filter by removing ammonia-nitrogen from the resin (filtermedium) once it becomes saturat-ed with ammonia-nitrogen.Biological filtration is the mostwidely used method. In biologicalfiltration (or biofiltration), there isa substrate with a high specificsurface area (large surface areaper unit volume) on which thenitrifying bacteria can attach andgrow. Ammonia and nitrite-nitro-gen in the recycled water are oxi-dized (converted) to nitrite andnitrate by Nitrosomonas andNitrobacter bacteria, respectively.Commonly used biofilter sub-strates include gravel, sand, plas-tic beads, plastic rings, and plasticplates. The most common biofil-tration technologies are discussedbelow.

Return totank

Floating beadfilter bed

Filtrationchamber

Sludgesettlingchamber

Washingthroat

Pumpedeffluentfrom tank

Filtersupport

Wastesludge

Figure 8. The bubble bead filter oper-ates much like the propeller washedbead filter, except that it backwashesby dropping the filtration medium bygravity through a washing throat.(U.S. Patent No. 5,232,586 by Dr.Ronald Malone, Dept. of CivilEngineering, Louisiana StateUniversity)

Figure 9. A pump-driven, venturi-type foam fractionator design. Awater/air mixture is injected tangen-tially into the foam fractionator (afterLosordo, 1997).

Foamcollectionandconcentration

Wateroutflow

Foamremoval

Waterinflow

Airinflow

Waterinflow

Venturi

Rotating biological contactor

Rotating biological contactors(RBC) have been used in the treat-ment of domestic wastewater fordecades, and are now widely usedas nitrifying filters in aquacultureapplications. RBC technology isbased on the rotation of a biofiltermedium attached to a shaft, par-tially submerged in water.Approximately 40 percent of thesubstrate is submerged in therecycle water (Fig. 10). Nitrifyingbacteria grow on the medium androtate with the RBC, alternatelycontacting the nitrogen-rich waterand the air. As the RBC rotates, itexchanges carbon dioxide (gener-ated by the bacteria and fish) foroxygen from the air. The tangen-tial velocity of the outer edge ofthe RBC should be about 35 to 50feet per minute. For example, anRBC with a diameter of 4 feetwould rotate at 3 to 4 revolutionsper minute (rpm). The advantagesof RBC technology are simplicityof operation, the ability to removecarbon dioxide and add dissolvedoxygen, and a self-cleaning capaci-ty. Major disadvantages are thehigh capital cost and mechanicalinstability. Poorly designed orbuilt RBCs can break downmechanically with the weight ofthe biological growth on the filtermedium. RBCs also have beendesigned to be turned by water(similar to a water wheel) andcompressed air.In early aquaculture applications,RBCs had simple discs cut fromcorrugated fiberglass plate. Nowthey use media with high specificsurface area, such as plastic blocksor a polyethylene tubular medium(resembling hair curlers). Thesenewer plastic media remove moreammonia, nitrite-nitrogen and car-bon dioxide in small RBC units.The plastic media have specificsurface areas of up to 200 m2/m3

(69 ft2/ft3). In aquaculture applica-tions, volumetric nitrification ratesof approximately 76 g TAN/m3

per day can be expected with thistype of biological filter (Wheatonet al., 1994). When including thesefilters in a recirculating system asa nitrifying filter component(assuming 2.5 percent of the feedbecomes TAN), a design criterion

of 3.6 kg feed/day/m3 of mediumshould be used (0.189pounds/day/ft3 of medium). The filter medium increases inweight as much as 10 fold duringoperation, so the support struc-ture must accommodate the addi-tional weight.

Trickling filters

Trickling filters used in aquacul-ture systems have evolved fromthose used in domestic sewage

treatment. This type of filter con-sists of a water distribution sys-tem at the top of a reactor filledwith a medium that has a relative-ly low specific surface area, gener-ally less than 330 m2/m3 (100 ft2/ft3). This creates large void (air)spaces within the filter medium(Fig. 11). As these filters are oper-ated in a non-flooded configura-tion, they provide nitrification,aeration, and some carbon dioxideremoval in one unit. (The termnon-flooded is used to indicate

RBC Biofilter media

Drivemotor

Flow fromculture tank

Flow toculture tank

RBC filter tank

Waterlevel

Figure 10. A rotating biological contactor unit powered by an electric gear motor.

Water inflowfrom culturetank

Biofiltermedium-plasticblocks orplastic rings

Water returnto culturetankFilter

supportlegs

Lowpressureairinflow

Rotatingwaterdistributionarm

Figure 11. Trickling filters are non-submerged biological filters in which the wateris evenly distributed over the medium.

that the biological filter medium isnot completely submerged inwater). The flow rate throughtrickling filters is limited by thevoid space through which watercan pass. In general, packingmedia with more void space canpass a higher rate of flow persquare meter of (top) cross sec-tional surface area. The main dis-advantage of trickling filters isthat they are relatively large andbiofilter media are expensive.Also, if the recycled water is notprefiltered to remove suspendedsolids, trickling filters can becomeclogged over time. As with RBCmedia, the weight of the biologi-cal growth on the filter mediashould be considered in designingthe support structure.Volumetric nitrification rates ofapproximately 90 g TAN/m3 perday can be expected with thistype of biological filter (Losordo,unpublished data). When design-ing these filters into a recirculat-ing system as a nitrifying filtercomponent (assuming 2.5 percentof the feed becomes TAN), adesign criteria of 3.6 kg feed/day/m3 of medium should beused (0.225 pound/day/ft3 ofmedium).

Expandable media filters

The expandable media floatingbead filters described in the previ-ous section (Figs. 7 and 8 are alsoused as biofilters in some aquacul-ture applications. Generally oper-ated as upflow filters, the beadshave a high specific surface areaon which nitrifying bacteria cancolonize. The major advantage ofthis technology is the combinationof nitrification and the solidsremoval processes into one com-ponent. The disadvantage, asnoted before, is that solids areheld in a place where they candegrade and affect the system’swater quality. In general, usingthese filters will require thedesigner to provide for more oxy-genation and biofiltration capaci-ty. The plastic bead medium usedin these filters has a specific sur-face area of 1,150 to 1,475 m2/m3

(350 to 450 ft2/ft3). Volumetricnitrification rates of approximate-ly 325 g TAN/m3/day can be

expected with this type of biologi-cal filter (Beecher et al., 1997).When designing these filters intoa recirculating system as a nitrify-ing and solids removal compo-nent (assuming 2.5 percent of thefeed becomes TAN), a design cri-terion of 13 kg of feed/day/m3 ofmedium should be used (0.81pounds/day/ft.3 of medium; themanufacturer recommends adesign rate of 1.0 pound/day/ft3).

Fluidized bed filters

Fluidized bed filters are essential-ly sand filters operated continu-ously in the expanded (backwash)mode. Water flows up through abed of sand at a rate sufficient tolift and expand (fluidize) the bedof sand and keep the sand parti-cles in motion so that they nolonger are in continuous contactwith each other (Fig. 12).Fluidized bed filters use sand ofsmaller diameter than that used inparticulate solids removal applica-tions. Plastic beads with densitiesslightly greater than water alsohave been used successfully influidized bed filters. A fluidizedbed filter is an excellent environ-ment for the growth of nitrifyingbacteria, and bacteria can colonizethe entire surface area of the filtermedium. The turbulent environ-ment also keeps the bacteria

sheared from the medium so thatthe filter is self-cleaning. The mainadvantage of fluidized bed tech-nology is the high nitrificationcapacity in a relatively compactunit. The sand also is extremelylow cost. Fluidization (pumping)requirements depend upon thesize and weight of the mediumbeing used. Keep in mind that thebuoyancy of the medium changeswith the amount of biologicalgrowth on the medium. This, inturn, depends upon the watertemperature, nutrient loadingrate, and degree of bed fluidiza-tion.Unless there is a system for recov-ering sand as water leaves the fil-ter, the medium will need to bereplaced routinely. Dependingupon the temperature, nutrientconcentration and size of themedium (and assuming 2.5 per-cent of the feed becomes TAN), adesign criterion of 20 to 40 kg offeed/day/m3 of medium shouldbe used (1.25 to 2.5pounds/day/ft3 of medium).

Mixed bed reactors

Mixed bed reactors are a new andinteresting cross between upflowplastic bead filters and fluidizedbed reactors. These filters use aplastic medium kept in a continu-ous state of movement (Fig. 13).

The diameterof the plasticmedium isusually muchlarger thansand, so it hasa lower spe-cific surfacearea (800 to1,150 m2/m3;240 to 350ft2/ft3). Thebeads are usu-ally neutrallybuoyant orjust slightlyheavier thanwater. Theplastic beadsare usuallymixed bymechanical orhydraulicmeans. Mixedbed filters are

Water returnto culturetank

Perforatedplate for waterdistribution

Waterinflowfromculturetank

Biofiltermedium-fluidizedsand

Figure 12. A simplified view of a fluidized sand bed biologicalfilter.

designed as up-flow or down-flow filters and, like fluidized bedfilters, they generate biologicalsolids but will not clog because ofthe continuous movement of themedium. The plastic mediummoves through a pipe within themain reactor to vertically mix thebead bed. Depending upon thenutrient concentration and medi-um size (and assuming 2.5 percentof the feed becomes TAN), adesign criterion of 16 to 23 kg offeed/day/m3 of medium shouldbe used (1.0 to 1.4pounds/day/ft3 of medium).

Dissolved gas Recirculating systems shouldmaintain adequate dissolved oxy-gen (DO) concentrations of atleast 6 mg/L and keep carbondioxide (CO2) concentrations atless than 25 mg/L for best fishgrowth. Colt and Watten (1988)and Boyd and Watten (1989) dis-cuss aeration and oxygenationsystems used in aquaculture; asummary of the componentoptions follows. The term aeration is used here torefer to the dissolution of oxygenfrom the atmosphere into water.The transfer of pure oxygen gas towater is referred to as oxygena-tion.

Aeration

Diffused aeration: Adding oxy-gen to a recirculating system by

aerating only the waterflowing into the culturetank will not usuallysupply an adequateamount of oxygen forfish production. Theamount of oxygen thatcan be carried to thefish in this way is limit-ed by the flow rate andthe generally low con-centration of oxygen inwater. Therefore, mostaeration in recirculatingsystems occurs in theculture tank. The mostefficient aerationdevices are those thatmove water into contactwith the atmosphere(paddlewheels, pro-

peller-aspirators, vertical-liftpumps). However, these methodsusually create too much turbu-lence within a culture tank to beuseful. The most common way toaerate in a recirculating tank sys-tem is called diffused aeration.Diffused aeration systems providelow pressure air from a “regenera-tive” type of blower to some formof diffuser near or on the bottomof a culture tank. These diffusersproduce small air bubbles that risethrough the water column andtransfer oxygen from the bubbleto the water. Studies have determined that dif-fused aeration systems can trans-fer oxygen at an average rate of1.3 kg O2/kW-h (2.15 lbs./hp -hour) under standard (20o C, O mg/L DO, clean water) testconditions (Colt and Tchoba-noglous, 1979). However, thesevalues must be corrected toaccount for the actual fish cultureconditions. To achieve acceptablefish growth rates, the DO concen-tration should be kept at 5 mgO2/L or higher. At water tempera-tures of 28o C, according to Boyd(1982), the diffuser system’s oxy-gen transfer rate would be only 35percent of the rate at standardconditions. In this case, the oxy-gen transfer rate would bereduced to 0.455 kg O2/kW-h(0.75 lbs./hp - hour). In a welldesigned recirculating system(one in which solids are removedquickly), the oxygen consumptionrate can be estimated as 50 per-

cent of the feed rate (that is, 0.5 kgO2/kg of feed fed). In a systemfed 4.5 kg (10 pounds) of feedover an 18-hour period,the esti-mated oxygen consumption ratewould be approximately 0.125 kgO2/hour (0.28 pounds/hour).With an actual oxygen transferefficiency of 0.455 kg O2/kW-h(0.75 pounds/hp-h), the diffusedaeration system would require ablower of approximately 0.275 kw(1/3 hp) to provide an adequateamount of oxygen. If the fish aregoing to be fed over a shorterperiod of time, then peak oxygendemand should be estimated andthe blower capacity should beincreased.The density of fish productionwith aeration alone is usually lim-ited to 30 to 40 kg of fish/m3 ofculture tank volume (0.25 to 0.33pounds of fish/gal.). In green-house systems where algal bloomsare common, oxygen is generatedduring the daylight hours, andculture densities of up to 60 kg offish/m3 of culture volume (0.50pounds of fish/gal.) can beachieved.Packed column aerators: An ideallocation for aerating and degass-ing water (i.e., removing carbondioxide) is in the recycle flow-stream just before it re-enters theculture tank. As mentioned previ-ously, however, this method doesnot usually supply enough oxy-gen. With submerged biologicalfiltration, the concentration of dis-solved oxygen will most likely belowest and carbon dioxide highestat the outflow of this component.Packed column aerators (PCA) arean effective and simple means ofaerating water that is already in aflow-stream. A packed columnaerator can be identical in designto a trickling nitrifying filter(Fig.11). Water is introduced into areactor filled with medium.Proper design criteria includenon-flooded operation and free airexchange through the reactor.Given a PCA influent DO concen-tration of 4 mg O2/L, an effectiveoxygen transfer rate of 0.75 kgO2/kw-h (1.25 pounds O2/hp-h)can be attained. While this is alow transfer rate, the true energycost for using a PCA in combina-

Waterinflowfromculturetank

Waterreturnto culturetank

Biofiltermedium-mixedplasticbeads

Figure 13. A common configuration for a mixed bedreactor biological filter.

tion with an existing flow-streamis only the energy required topump water 1.0 to 1.25 meters (3to 4 feet) to the top of the PCA. Ifthe PCA is to be used for carbondioxide stripping, a low pressureair blower should be used to forceat least five times as much air aswater (by volume) up through thePCA medium.

Oxygenation

Pure oxygen is used in recirculat-ing systems when the intensity ofproduction causes the rate of oxy-gen consumption to exceed themaximum feasible rate of oxygentransfer through aeration. Sourcesof oxygen gas include compressedoxygen cylinders, liquid oxygen(often referred to as LOX), and on-site oxygen generators. In mostapplications, the choice is betweenbulk liquid oxygen and an oxygengenerator. The selection of theoxygen source will be a functionof the cost of bulk liquid oxygenin your area (usually dependenton your distance from the oxygenproduction plant) and the reliabil-ity of the electrical service neededfor generating oxygen on-site.Adding gaseous oxygen directlyinto the culture tank through dif-fusers is not the most efficientway to add pure oxygen gas towater. At best, the efficiency ofsuch systems is less than 40 per-cent. A number of specializedcomponents have been developedfor use in aquaculture applica-tions. For an extensive review ofcomponent options, see Boyd andWatten (1989). A review of themore commonly used componentsfollows. Down-flow bubble contactor: Aproperly designed low pressureoxygen diffusion system cantransfer more than 90 percent ofthe oxygen injected through thecomponent. One such system is adown-flow bubble contact aerator(DFBC), also referred to as abicone or a Speece cone. TheDFBC system consists of a cone-shaped reactor with a water andoxygen input port at the top (Fig.14). As the water and oxygen bub-bles move down the cone, theflow velocity decreases until it

equals the upward velocity of thebubbles. This allows a long con-tact time between the water andbubbles and nearly 100 percentabsorption of the injected gas. Thedissolved oxygen concentration ofwater leaving a DFBC can be ashigh as 25 mg/L given a systempressure of approximately 1 bar(14.7 PSI).U-tube diffusers: At high operat-ing pressures, more oxygen can beabsorbed by water. A u-tube oxy-gen diffusion system is an energyefficient method of adding pres-sure to a flow-stream. A typicalu-tube consists of a contact loop,usually a pipe within a pipe (Fig.15), buried in the ground to atleast 10 meters (33 feet), the heightof water required to add oneatmosphere of pressure (1 bar,14.7 PSI). The contact loop isplaced below tank level to mini-mize energy requirements, ratherthan pumping water up hill togain the extra hydrostatic pressurecreated by a column of water.Oxygen is mixed with the water atthe entrance to the u-tube andtravels with the flow to the bot-tom of the water column. Theadditional pressure from thewater column accelerates the rateof oxygen absorption into thewater. The principal advantages ofthis system are the low energyrequirements for oxygenatinglarge flow-streams and the resis-tance to clogging with particulatesolids. The major disadvantage isthe construction cost of drillingthe shaft and installing the u-tube.Oxygen transfer efficiencies aregenerally below 70 percent, witheffluent oxygen concentrations of

up to 250 percent of atmosphericsaturation (15 to 20 mg/L).Low head oxygenation system:The multi-staged low head oxy-genator (LHO) oxygenates flow-ing water where there is only asmall elevation difference betweenthe source of the water and theculture tank. This situation isoften found in raceway systemsset up in series. That is, the out-flow of one raceway is just slight-ly (1 to 3 feet) above the inflow ofan adjacent raceway. This tech-nology is a patented component(U.S. Patent No. 4,880,445; WaterManagement Technologies, P.O.Box 66125, Baton Rouge, LA) andis made up of a perforated, hori-zontal distribution plate and mul-tiple, adjacent, vertical contactchambers (Fig.16). Pure gaseousoxygen enters one (end) contactchamber and oxygen with off-gases (nitrogen and CO2) exits theadjacent contact chamber. The oxygen transfer capability ofthis system is determined by thelength of water fall, gas and waterflow rates, the DO concentrationof the influent water, and thenumber of contact chambers(Watten 1994). Including packingmedium in the contact chamberscan improve performance.Pressurized packed columns:Pressurized packed columns areusually operated in a floodedmode (water fills the reactor).Water enters the top of a pressur-ized chamber that contains amedium with a high specific sur-face area (much like packed tow-ers). Oxygen gas is usually intro-

Oxygengasinlet

Off gas

Waterwithhighoxygencontent

Waterwithlowoxygencontent

Figure 14. Down-flow bubble contactaerator (after Colt and Watten, 1988).

Off-gas recycleInflowwaterlow DO

Outflowwaterhigh DO

Figure 15. Typical u-tube oxygen dif-fusion design.

duced at the bottom of the col-umn and travels upward, counterto the water flow. Oxygen transferefficiency can range from 50 to 90percent with effluent dissolvedoxygen concentrations in excess of100 mg/L. The major disadvan-tages of this system are high ener-gy requirements (to provide thepressure) and the buildup of bio-logical growth on the packingmedium, which makes periodiccleaning necessary.

DisinfectionDiseases can spread quicklybecause of the density of fish inrecirculating systems. Some chem-icals used to treat diseases have adevastating effect on the nitrifyingbacteria within the biofilter andculture system. Alternatives to tra-ditional chemical or antibiotictreatments include the continuousdisinfection of the recycled waterwith ozone or ultraviolet irradia-tion. For more information on dis-ease treatment in recirculatingsystems, see SRAC publication452 on the management of recircu-lating systems.

Ultraviolet irradiation

Microorganisms (including dis-ease-causing bacteria) are killedwhen exposed to the properamount of ultraviolet (UV) radia-tion. Spotte (1979) notes that theeffectiveness of UV sterilizationdepends upon the size of theorganism, the amount of UV radi-

ation, and the level of penetrationof the radiation into the water. Tobe effective, microorganisms mustcome in close proximity to the UVradiation source (0.5 cm, 0.2 inch-es or less). Turbidity reduces itseffectiveness. For a UV radiationsystem to be effective, the watershould be pre-filtered with someform of particulate filtrationdevice.The most popular and effectivetype of UV sterilization unit is onewith a submerged UV radiationsource. In this type of unit, recy-cled water passes by an elongatedUV lamp (much like a neon lightbulb). The lamp is inside a quartzglass, watertight jacket and doesnot come in direct contact withthe water. The UV lamp andquartz tube are held within asmall diameter pipe throughwhich the treated water flows. Aswater passes along and aroundthe UV lamp, microorganisms areexposed to the UV radiation.Keeping the quartz jacket clean isimperative to the proper opera-tion of the unit. UV sterilizationunits are usually rated by theirmanufacturers according to theirwater flow rate capacity. Increasedefficiency can be achieved byreducing the flow rate through agiven unit. The main disadvan-tage of UV sterilization is the needfor clean water with low suspend-ed solids concentrations. Clearwater is not always economicallyachievable in heavily fed recircu-lating systems. Additionally, theexpensive UV lamp must bereplaced periodically. The mainadvantage of UV sterilization isthat it is safe to operate and is notharmful to the cultured species.

Ozonation

Ozone (O3) gas is a strong oxidiz-ing agent in water. Ozone hasbeen used for years to disinfectdrinking water. However, becauseof the high levels of dissolved andsuspended organic materials inrecirculating systems, the effect ofozone on bacterial populations isquestionable (Brazil et al., 1996).The efficiency of the disinfectingaction depends upon the contacttime and residual concentration of

O3 in the water with the microor-ganisms. Ozone must be generat-ed on-site because it is unstableand breaks down in 10 to 20 min-utes. Ozone is usually generatedwith either a UV light or a coronaelectric discharge source. Thereare many commercial ozone gen-eration units available.Ozone is usually diffused into thewater of a recirculating system inan external contact basin or loop.Water must be retained in thisside-stream long enough to ensurethat microorganisms are killedand the ozone molecules aredestroyed. Residual ozone enter-ing the culture tank can be verytoxic to crustaceans and fish.Ozone in the air is also toxic tohumans in low concentrations.Great care should be taken inventing excess ozone from thegeneration, delivery, and contactsystem to the outside of the build-ing. Ozonation systems should bedesigned and installed by experi-enced personnel.

SummaryThis publication has outlined themajor components and optionsused in recirculating aquacultureproduction systems. This is by nomeans a complete listing, newtechnologies are continually beingdeveloped. One should notattempt to simply link the compo-nents discussed here and expectto have a properly operating sys-tem. Any system you buy shouldbe the result of years of develop-ment, with each component prop-erly sized and integrated for opti-mal performance. When review-ing your options, always seek theassistance of a knowledgeable,experienced person, one who hasdesigned a currently operatingand economically viable recircu-lating fish production system.

References andsuggested readingsBoyd, C.E. 1982. Water quality man-

agement for fish pond culture.Elsevier Scientific PublishingCompany, Amsterdam, theNetherlands.

Lowoxygeninfluent

Off-gasoutflow

Gaseousoxygeninflow

Oxygenatedeffluent

Figure 16. Multi-staged low head oxy-genator with front plate removed toshow component detail (after Losordo,1997).

Boyd, C.E. 1991. Types of aerationand design considerations. In: L.Swann (editor), Proceedings of theSecond Annual Workshop:Commercial Aquaculture UsingWater Recirculating Systems.Illinois State University, Normal,Illinois. Nov. 15-16, 1991. pp. 39-47.

Boyd, C.E. and B.J. Watten. 1989.Aeration systems in aquaculture.CRC Critical Reviews in AquaticSciences 1: 425 - 472.

Brazil, B.L., S.T. Summerfelt and G.S.Libey. Applications of ozone torecirculating aquaculture systems.In: G.S. Libey and M.B. Timmons(editors), Successes and Failures inCommercial RecirculatingAquaculture. Proceeding from theSuccesses and Failures inCommercial RecirculatingAquaculture Conference. Roanoke,VA. July 19-21, 1996. NRAES-98.Northeast Regional AgriculturalEngineering Service, 152 Riley-Robb Hall, Ithaca, NY., pp. 373-389.

Burley, R. and A. Klapsis. 1988.Making the most of your flow (infish rearing tanks). In: Proceedingsof the Conference: AquacultureEngineering: Technologies for theFuture, Sterling Scotland. IChemESymposium Series #111, EFCEPublications Series # 66, Rugby,UK. pp. 211-239.

Colt, J.E. and G. Tchobanoglous. 1979.Design of aeration systems foraquaculture. Department of CivilEngineering, University ofCalifornia, Davis, CA.

Colt, J. and B. Watten. 1988.Applications of pure oxygen in fishculture. Aquacultural Engineering7:397-441.

Grace, G.R. and R.H. Piedrahita.1989. Carbon dioxide removal in apacked column aerator. Presentedpaper at the International SummerMeeting of Am. Soc. Ag. Eng. andCan. Soc. Ag. Eng., June 25-28,1989, Quebec, PQ, Canada.

Hobbs, A., T. Losordo, D. DeLong, J.Regan, S. Bennett, R. Gron and B.Foster. 1997. A commercial, publicdemonstration of recirculatingaquaculture technology: TheCP&L/EPRI Fish Barn at NorthCarolina State University. In: M.B.Timmons and T.M. Losordo (edi-tors). Advances in aquaculturalengineering. Proceedings from theaquacultural engineering societytechnical sessions at the fourthinternational symposium on tilapiain aquaculture. NRAES-105.Northeast Regional AgriculturalEngineering Service, 152 Riley-Robb Hall, Ithaca, NY. pp. 151-158.

Huguenin, J.E. and J. Colt. 1989.Design and operating guide foraquaculture seawater systems.Elsevier Scientific Publishers,Amsterdam, The Netherlands.

Libey, G.S. 1993. Evaluation of a drumfilter for removal of solids from arecirculating aquaculture system.In: J.K. Wang (editor), Techniquesfor Modern Aquaculture.Proceedings of an AquaculturalEngineering Conference. Spokane,WA, June 1993. American Societyof Agricultural Engineers, St.Joseph, MI. pp. 519.532.

Losordo, T.M. 1997. Tilapia culture inintensive recirculating systems. In:Costa-Pierce, B. and Rakocy, J. (edi-tors), Tilapia Aquaculture in theAmericas, Volume 1. WorldAquaculture Society, Baton Rouge,LA. pp. 185-208.

Malone R.F. and D.G. Burden. 1988.Design of recirculating soft craw-fish shedding systems. LouisianaSea Grant College Program,Louisiana State University, BatonRouge, LA.

Spotte, S. 1979. Fish and invertebrateculture: Water management inclosed systems. John Wiley &Sons, New York, NY.

Timmons M.B. and T.M. Losordo(editors). Aquaculture water reusesystems: Engineering , design andmanagement. Developments inFisheries Sciences 27. ElsevierScientific Publishing Company,Amsterdam, The Netherlands.

Tvinnereim, K. 1988. Design of waterinlets for closed fish farms. In:Proceedings of the Conference:Aquaculture Engineering:Technologies for the Future.Sterling Scotland. IChemESymposium Series #111, EFCEPublications Series # 66, Rugby,UK. pp. 241-249.

Watten, B.J. 1994. Aeration and oxy-genation. In: M.B. Timmons andT.M. Losordo (editors), Aquacul-ture water reuse systems:Engineering, design and manage-ment. Developments in FisheriesSciences 27. Elsevier ScientificPublishing Company, Amsterdam,The Netherlands.

Wheaton, F.W., J.N. Hochheimer, G.E.Kaiser, R.F. Malone, M.J. Krones,G.S. Libey and C.C. Estes. 1994.Nitrofication filter design methods.In: Timmons, M.B. and T. M.Losordo (editors), Aquaculturewater reuse systems: Engineering,design and management. Develop-ments in Fisheries Sciences 27.Elsevier Scientific PublishingCompany, Amsterdam, TheNetherlands. pp. 125-171.


A well-designed recirculatingaquaculture system offers a num-ber of advantages over pond sys-tems. Designed to conserve bothland and water resources, recircu-lating systems can be located inareas not conducive to open pondculture. Operators have a greaterdegree of control of the fish cul-ture environment and can growfish year-round under optimalconditions. The crop can be har-vested at any time, and inventorycan be much more accuratelydetermined than in ponds. Thislatter characteristic is particularlybeneficial when trying to gainfinancing or insurance for thecrop.Because of these advantages,interest in water recirculating sys-tems for fish production continuesto grow, despite the lack of eco-nomic information available ontheir use. This publication andaccompanying spreadsheet aredesigned to help prospectiverecirculating system operatorsexamine the economics of pro-posed systems. With modifica-tions to the example spreadsheet,

the same format can be used tomonitor costs and returns oncesystems are operating. The Excelspreadsheet can be downloadedfrom the following Internetaddress: http://www.agr.state.nc.us/aquacult/rass.html.The spreadsheet in this publica-tion uses tilapia for the examplespecies. However, the resultingfigures on costs and returns arenot meant to be used as an eco-nomic analysis of tilapia produc-tion. Each individual using thespreadsheet should input equip-ment and supply costs and theappropriate market price for thespecific system being analyzed.

System designThere is no single recommendeddesign for growing fish in a recir-culating aquaculture system(RAS). In general, a systemincludes tanks to culture fish,pumps to maintain water flow,and some form of water treatmentto maintain water quality. Follow-ing are a few general considera-tions on system design and howdesign can affect profitability. Fora more complete explanation ofcomponent options and manage-ment issues see SRAC publica-tions 450, 451 and 452.

Proper sizing of all system com-ponents is very important. Ifequipment is oversized for theapplication, the system will func-tion but will be very costly. Ifequipment is undersized, the sys-tem will not be able to maintainthe proper environment to sustainfish production. Operators should size equipmentaccording to the maximum dailyamount of feed placed into thesystem. The estimated daily feedrate is based on the system carry-ing capacity, which does not usu-ally exceed 1 pound of fish pergallon of water for even the mostefficient system. Once carryingcapacity and feed rate are defined,the operator estimates the size ofequipment components by calcu-lating the required flow rate. Theflow rate of each component mustbe sufficient to flush out and treatany wasted feed and by-productsof fish metabolism, while supply-ing a uniform concentration ofoxygen.Because equipment is sized tomaximum feeding rates, the mostinefficient stock managementmethod is to stock fingerlings atlow densities in a tank and growthem to market size within thesame tank. Most RAS operatorstry to make maximum use of each

VIPRNovember 1998


The Economics of RecirculatingTank Systems:

A Spreadsheet for Individual Analysis

Rebecca D. Dunning1, Thomas M. Losordo2 and Alex O. Hobbs3

1North Carolina Department ofAgriculture

2North Carolina State University3Carolina Power and Light Company

tankÕs carrying capacity by stock-ing fish at increasingly lowernumbers as the fish grow in size.The more efficient the use of sys-tem carrying capacity, the morefish can be moved through thesystem annually, which generallylowers the cost per pound har-vested. The trade-off is that themore often fish are restocked, thehigher the labor cost and greaterthe chance of mortality if fishbecome stressed from the move.Operators also face a trade-offwhen determining both the sizeof tanks and the configuration ofequipment for filtering and oxy-genating water. The use of fewer,larger tanks, or several tankssharing water treatment equip-ment, is usually much less expen-sive than having a number ofsmaller tanks that do not sharewater or components. Managingquality and disease prevention,however, is typically more effec-tive where water is not sharedbetween tanks. There is less riskof losing large portions of the fishcrop when each tank has its ownset of treatment equipment.There are economies of scale forindividual tank size and for thesize of the entire system. Up to apoint, the increase in system sizegenerally results in a lower costper pound produced, because thefixed costs associated with thebuilding and equipment can bespread over more pounds har-vested.

The example systemThe data used for this publicationare taken from experiences in asmall unit at the North CarolinaState University Fish Barn Project(NC Fish Barn).The NC Fish Barn system growsfish in nursery tanks, then gradesand splits the population intolarger growout tanks as the fishgain weight. The system consistsof six tanks: one 1,500-gallon (5.68-cubic meter) quarantine tank(Q); a 4,000-gallon (15.14-cubicmeter) nursery tank (N), and four

15,000-gallon (56.78-cubic meter)growout tanks (G1, G2, G3 andG4). The quarantine and nurserytanks have their own water filtra-tion systems, while each pair ofgrowout tanks shares a watertreatment system. A more detaileddescription of the system andequipment can be found in Hobbset al., 1997.Fish are initially stocked in theQ tank, screened for diseases for35 days, then harvested andrestocked into the N tank. After35 days, the fish are transferred toone of the four G tanks wherethey remain an additional 140days until harvest. This 140-dayperiod is broken down into fourdistinct production units of 35days each (defined as g1, g2, g3and g4 in the spreadsheet). Eachof these units has a different feedrate, oxygen demand, and pump-ing need. (An alternative to thisconfiguration would be to movethe fish into a different tank foreach of the 35-day periods).Once the system is fully stocked,one of the four G tanks is harvest-ed for sale every 35 days. The sys-tem has a maximum culture den-sity of 0.8 pounds of fish per gal-lon of water (103 kgs of fish percubic meter of water) in eachgrowout tank, and each harvestyields approximately 12,400pounds (5,636 kgs) of fish. With10.43 harvests annually (one every35 days once the facility is fullystocked), total production for thefacility is approximately 130,000pounds (59,091 kgs) per year.

Using the spreadsheet The Recirculating AquacultureSystem Spreadsheet (RASS) mustbe supplied with accurate andrealistic input data based on aproperly designed system. Properdesign means that the equipmentcomponents work together to pro-duce the amount of fish in thetime period stated.The spreadsheet is divided intofive sections. The user suppliesinformation for the first three sec-tions. Data in the final two sec-

tions are calculated from thisinformation. Shaded areas in thetables indicate needed informa-tion and are represented as boldtype in the spreadsheet. ÒSpread-sheet Cell RangeÓ and cell num-bers refer to the location of infor-mation within the Excel spread-sheet.

The initial investment cost is sup-plied by the user in cells E16:E20.The total is calculated in cell E21.The investment includes the totalvalue of purchased land, a settlingpond, building, equipment, andconstruction labor, as well as thecurrent value of any owned assetsused in the business. Annual depreciation on building andequipment (E22) is the amount ofmoney that must be earned eachyear by the business to eventuallyreplace equipment when it wearsout.Interest rate on operating capital(E24) is used to calculate a cost ofinterest on variable inputs (oxy-gen, energy, bicarbonate, finger-lings, chemicals, maintenance andlabor). The interest charge couldbe interest owed to a bank for thefinancing of the purchase of theseinputs, or the charge could be forthe cost of using the ownerÕs ownfunds to purchase variable inputs.A cost of using ownerÕs funds isused because the investment offunds in the recirculating systemmeans that the owner foregoespotential earnings from an alter-native investment. Interest rate on building and equip-ment (E25) is used to calculate anannual interest charge based onthe average investment. Again,this could be interest owed on abank loan used to finance the ini-tial investment, or it can representearnings that could have beenmade on an alternative invest-ment.

Section 1: Specify the InitialInvestment Spreadsheet CellRange B13:E25

System parameters

The remainder of this section(E48..E54) contains system para-meters that will be needed for cal-culations related to costs andreturns. Annual production (E48),Average size at harvest (E49), andthe Survival rate (specified in thenext section) are used to calculatethe initial stocking density. There are six production units inthis example (Number of productionunits [E50] = 6). As discussedabove, a production unit refers toa specific tank or life stage of thefish. Here, three tanks are used: aQ tank, an N tank and a G tank.Fish remain in the Q tank and Ntank for 35 days each. Within the

Section 2.Specify the Cost of Inputs, Sale Price, and System ParametersSpreadsheet Cell Range = B27:E54

unit or description cost or amountVariable Costs:

Liquid oxygen $/100 cu. ft. $0.30Energy $/kwh $0.065Bicarbonate $/lb. $0.190Fingerlings $/fingerling $0.090Chemicals $/cycle $100.00Maintenance $/month $637.00Labor: management $/month $2,000.00Labor: transfer & harvest $/hour $6.50

Fixed Costs:Liquid oxygen tank rental $/month $250.00Electrical demand charge $/month $100.00Building Overhead $/month $100.00

Average overall sale price $/lb. $1.25

System ParametersAnnual production lb. 129,107Average size at harvest lb. 1.25Number of production units number 6Days per production unit days 35Kwh per lb. of production kwh/lb. of prod. 2.30System volts volts 230Transfer/harvest labor hrs. per cycle 64

Section 2: Specify the Costof Inputs, Sale Price, andSystem ParametersSpreadsheet Cell Range =B27:E54

Variable costs

Variable costs are those directlyrelated to production. In the cellrange E31:E38 the user specifiesthe cost per unit of oxygen, ener-gy, bicarbonate, fingerlings,chemicals, maintenance andlabor. The quantity used of eachof these inputs is defined inSection 3.

Fixed costs

Fixed costs are incurred regard-less of whether or not productionoccurs. They are Liquid oxygentank rental (E41), Electrical demandcharge (E42), and Building over-head (E43). Each of these is speci-fied as a cost per month.

Sale price

Average overall sale price (E45) isthe weighted average sale priceper pound, taking into accountthe size distribution at harvestand differing prices for varioussizes of fish. The example uses$1.25 so that the system willbreak even (with $0 profit and$0 losses).

Section 1.Specify the Initial InvestmentSpreadsheet Cell Range = B13:E25

Initial investmentland $8,000settling pond $5,000equipment $172,500building $60,000construction labor & overhead $30,000

Total initial investment $275,000Annual depreciation on building and equipment $19,100

Interest rate on operating capital 9%Interest rate on building and equipment 11%

bonateÑused over one cycle, andextrapolates this information to anannual basis. No user input isrequired in this section.In the example, once the fish cul-ture system is fully stocked after210 days, the system will have10.43 harvests per year (365 days/35 days). Thus, each number inthe Cycle Total (column L) is multi-plied by 10.43 to calculate theAnnual Total (column M). Beginning number of fish (E69:J69)begins with the original stockingdensity and adjusts that numberaccording to the Survival rate(E62:J62).Ending number of fish (E70:J70) isbased on density and survival foreach production unit.Beginning biomass, lbs. of fish(E71:J71) is based on the numberof fish and average weightstocked into that production unit.Ending biomass, lbs. of fish (E72:J72)is based on the number of fishand weight transferred or harvest-ed from that unit.Maximum standing biomass, lb. pergal. of water (E73:J73) gives thepounds of fish per gallon of tankwater at the end of that produc-tion period. Feed used (E74:J74) is calculatedfrom the specified Feed conversionratio (E63:J63) and the differencebetween the Beginning biomass(E71:J71) and Ending biomass(E72:J72).The Kwh used is calculated foreach production unit as a weight-ed percentage of the feed usagefor that unit multiplied by thetotal amount of kwh used for thecycle. The total kwh for the cycle

G tank, the fish go through four35-day stages. Note that the Daysper production unit (E51) must bethe same for each unit in orderfor the spreadsheet to accuratelycalculate costs and returns inSection 5.The Kwh per lb. of production (E52)is used to calculate energy costsfor the total system and each pro-duction unit. This variable is cal-culated by adding up the totalKW usage of the systemÑinclud-ing energy usage for pumps,blowers and other equipment aswell as heating, ventilation andair-conditioningÑconverting thisto kwh used per year, and thendividing by the number ofpounds produced. (For the exam-ple, the total energy demand is 34KW. Multiply by 24 hours per dayand 365 days per year, then divideby annual production of 129,107pounds to arrive at 2.30 kwh perpound of production). System volts (E53) is used to calcu-late required amperage in Section5. This is a useful number forplanning energy requirements forthe facility.Transfer/harvest labor (E54) is thenumber of hours of labor requiredper cycle in addition to Labor:management (defined in E37).

Each column in this section repre-sents a production unit, whichcould be a tank or group of tanksmanaged in the same manner, orit could refer to a particular lifestage. For example, two tanksstocked at the same time with theintent to transfer and harvest fishat the same time, and in whichfish are fed and managed in thesame manner, could be treated asone production unit. Or, as in thetable below and spreadsheetexample, two of the six columns(Q & N) refer to particular tanks,while the remaining four (g1, g2,g3, g4) refer to a production stagefor fish that remain within thesame tank.

Water volume, gallons (E59:J69) isused to calculate the Maximumstanding biomass, lbs. per gal. ofwater (E73:I73) for any one tank,discussed in Section 4. Size stocked (E60:J60) is the averagesize of fish stocked into that pro-duction unit. Size harvested(E61:J61) is their average sizewhen transferred or harvestedfrom the system. In the example,fish are initially stocked at 1 graminto the Q tank, and transferredinto the N tank when they reach15 grams. Survival rate (E62:J62 ) is the per-centage of survival for that pro-duction unit. In the example, thelower survival rate for the Q tankincludes the discarding of runtswhen the fish are graded beforerestocking into the N tank. Feed cost, per lb. (E63:J63) is theaverage cost per pound for feedfed to that production unit. Feedcost, per lb. and Feed conversion(E64:E64) are used to calculate thecost of feed for each productionunit, for each cycle, and annually.Feed usage is also used to calcu-late the amount of energy used, asdiscussed in the following section.

Spreadsheet calculation ofcosts and returns

This section summarizes thequantity and cost of primaryoperating inputsÑfingerlings,feed, energy, oxygen, and bicar-

Section 3: SpecifyOperating Parameters per Production UnitSpreadsheet Cell RangeB56:J64

Section 4: Use of PrimaryInputs and Costs perProduction UnitSpreadsheet Cell RangeB66:J87

Section 3.Specify Operating Parameters per Production UnitSpreadsheet Cell Range = B56..J64

Growout tankQ tank N tank g1 g2 g3 g4

Water volume, gallons 1,500 4,000 15,000 15,000 15,000 15,000Size stocked (grams) 1 15 60 135 250 385Size harvested (grams) 15 60 135 250 385 567Survival rate 85% 99% 99% 99% 99% 99%Feed cost, per pound $0.52 $0.38 $0.21 $0.21 $0.21 $0.21Feed conversion 1 1.1 1.3 1.6 1.6 1.6

Section 4.Use of Primary Inputs and Costs per Production UnitSpreadsheet 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

is based on estimated energyusage of 2.30 kwh per pound ofproduction. For example, onecycle yielding 12,354 pounds(5,615 kg) of fish requires an esti-mated 28,414 kwh of energy. Theg1 production unit consumes11.72% of feed used during thecycle (2,172 pounds feed/18,524pounds feed), so the estimatedenergy use during that 35-day unitis 3,330 kwh (11.72% x 28,414),given in cell G75. The cost of energy for that period, given inG82 as $217, is calculated usingthe user-specified cost of $0.065per kwh (E45).Oxygen used, cubic feet (E76:J76) iscalculated as follows: pounds offeed (E74:J74) x 30% (the amountof oxygen used per pound of feed,

this is system specific) x 12.05 (aconversion factor). Bicarbonate used (E77:J77) allowsfor 0.175 pound of sodium bicar-bonate used per pound of feedfed.Costs by production unit (E80:J87)are calculated using the cost perinput specified in Section 2.

This section summarizes the costsand returns per cycle and annual-ly for this system once it is in fullproduction (after 210 days). Netreturns are calculated before tax.

Days per production unit (D91)repeats information given in cellE51.The Number of cycles per year (D92)is simply 365 days divided byDays per production unit. Required system amps (D93) is cal-culated from System volts (E53)and kwh usage assuming a powerfactor of one.Overall survival (F91) is calculatedusing survival given in E62:J62,and Cycle FCR (F92) from feedconversion ratios in E64:J64.The cell range C96:J122 calculatessystem costs per cycle, annually,and per pound based on informa-tion specified previously in thespreadsheet.

Section 5: Summary ofAnnual Costs and Returnsto System in Full ProductionSpreadsheet Cell Range =B89:J122

Interpreting the spreadsheet resultsThis publication is not an evalua-tion of the economics of tilapiaproduction. A sale price of $1.25was chosen so that the examplesystem would have annual costsnearly equal to annual returns. It is important to keep in mindthat before the end of the firstcycle on day 210, costs areincurred while no fish are har-vested and sold. Until that time,the cost of operations must eitherbe paid by additional ownerfunds or bank financing. To

approximately calculate the pointat which the system becomes self-supporting (can pay all fixed andvariable costs), divide the totalcosts per cycle by the net returnsper cycle. For example, if the saleprice were $1.65 per pound, TotalCosts per Cycle would be $15,470and Returns above Total Costswould be $4,957. This is equal to3.1 cycles ($15,470/$4,957) or 651days (3.1 cycles x 210 days percycle). The system would notbecome self-supporting untilapproximately 2 years from startup.

This spreadsheet can be used totest the effect on costs and returnsof changes in sale price, feed con-version, survival, or the cost ofenergy and other inputs. Userscan also examine the change inprofitability based on a change inthe stocking and transfer of fishor overall size of the system. Forexample, more frequent moves offish between tanks could makebetter use of tank carrying capaci-ty, increasing the amount of fishthat could be harvested annually.Or, a more energy intensive sys-tem might support a higher carry-ing capacity per tank. Either of

Section 5.Summary of Annual Costs and Returns to System in Full ProductionSpreadsheet Cell Range = B89:J122

Days per production unit 35 Overall survival 81%Average number of cycles/yr. 10.43 Cycle FCR 1.5Req. 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 Costfingerlings 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 CostOxygen 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 $0.29Net Returns above Total Cost -$27 -$282 $0.00

these may result in increased prof-it if the costs associated with each(higher labor cost, stress that mayresult in lower survival in the caseof more frequent moves, and ahigher energy cost if the systemwere reconfigured) do not out-weigh the increase in production.Larger systemsÑmore tanks andlarger tanksÑalso often increasethe profitability of recirculatingsystems.

Caveats (a warning)There is no single recommendeddesign for recirculating aquacul-ture systems. Therefore, it isimpossible to supply a ready-made cost/returns spreadsheet

that will be suitable for every sys-tem. Operators with existing orproposed systems similar to theexample presented here can usethis spreadsheet. Radically differ-ent systems may require extensivemodifications of the spreadsheetstructure by the user. The examplespreadsheet is simple in designand does not contain any macro-programming. It can be modifiedonce cells are unprotected. Whenworking with the original spread-sheet or a modified version, keepin mind that it can only evaluatethe economics of a properlydesigned system, and can not cor-rect for flaws in design.

References Hobbs, A., T. Losordo, D. DeLong,

J. Regan, S. Bennett, R. Gronand B. Foster. 1997. ÒA com-mercial, public demonstrationof recirculating aquaculturetechnology: The CP&L /EPRIFish Barn at North CarolinaState University.Ó Pages 151-158 In: M.B. Timmons andT.M. Losordo, editors.Advances in aquaculturalengineering. Proceedings fromthe aquacultural engineeringsociety technical sessions atthe fourth international sym-posium on tilapia in aquacul-ture. NRAES-105. NortheastRegional AgriculturalEngineering Service, Ithaca,NY.

For additional suggested reading,see the Internet site.

28844411 recirculating aquaculture tank production systems an overview of critical considerations

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