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

VIPRApril 1999

Revised

SRAC Publication No. 453

Recirculating Aquaculture TankProduction Systems

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

BSettleablesolids 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 BfromparticletrapClarified water

to 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 screensA. Disk screen filter (top view)

Inflow

WastewaterBackwashwater

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 Outflow

to 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 solidsMany 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 contactorRotating 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/m3per 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 filtersTrickling 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 filtersThe 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 reactorsMixed 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.

OxygenationPure 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 irradiationMicroorganisms (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.

OzonationOzone (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.

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.