SRAC Publication No. 454

November 2006 VI Revision PR

Recirculating Aquaculture Tank Production Systems: Aquaponics—Integrating Fish and Plant Culture

James E. Rakocy1, Michael P. Masser2 and Thomas M. Losordo3

Aquaponics, the combined culture of many times, non-toxic nutrients and Aquaponic systems offer several ben- fish and plants in recirculating sys- organic matter accumulate. These efits. Dissolved waste nutrients are tems, has become increasingly popu- metabolic by-products need not be recovered by the plants, reducing dis- lar. Now a news group (aquaponics- wasted if they are channeled into charge to the environment and [email protected] — type sub- secondary crops that have economic extending water use (i.e., by remov- scribe) on the Internet discusses value or in some way benefit the pri- ing dissolved nutrients through plant many aspects of aquaponics on a mary fish production system. uptake, the water exchange rate can daily basis. Since 1997, a quarterly Systems that grow additional crops be reduced). Minimizing water periodical (Aquaponics Journal) has by utilizing by-products from the pro- exchange reduces the costs of operat- published informative articles, con- duction of the primary species are ing aquaponic systems in arid cli- ference announcements and product referred to as integrated systems. If mates and heated greenhouses where advertisements. At least two large the secondary crops are aquatic or water or heated water is a significant suppliers of aquaculture and/or terrestrial plants grown in conjunc- expense. Having a secondary plant hydroponic equipment have intro- tion with fish, this integrated system crop that receives most of its required duced aquaponic systems to their is referred to as an aquaponic system catalogs. Hundreds of school districts (Fig. 1). are including aquaponics as a learn- Plants grow rapidly with dissolved ing tool in their science curricula. At nutrients that are excreted directly least two short courses on aquapon- by fish or generated from the micro- ics have been introduced, and the bial breakdown of fish wastes. In number of commercial aquaponic closed recirculating systems with operations, though small, is increas- very little daily water exchange (less ing. than 2 percent), dissolved nutrients Aquaponic systems are recirculating accumulate in concentrations similar aquaculture systems that incorporate to those in hydroponic nutrient solu- the production of plants without soil. tions. Dissolved nitrogen, in particu- Recirculating systems are designed lar, can occur at very high levels in to raise large quantities of fish in rel- recirculating systems. Fish excrete atively small volumes of water by waste nitrogen, in the form of ammo- treating the water to remove toxic nia, directly into the water through waste products and then reusing it. their gills. Bacteria convert ammonia In the process of reusing the water to nitrite and then to nitrate (see SRAC Publication No. 451, “Recirculating Aquaculture Tank 1 Agricultural Experiment Station, University of the Production Systems: An Overview of Virgin Islands Critical Considerations”). Ammonia 2 Department of Wildlife and Fisheries Sciences, and nitrite are toxic to fish, but Texas A&M University 3 nitrate is relatively harmless and is Biological and Agricultural Engineering Figure 1. Nutrients from red tilapia Department, North Carolina State University the preferred form of nitrogen for growing higher plants such as fruit- produce a valuable crop of leaf let- ing vegetables. tuce in the UVI aquaponic system. nutrients at no cost improves a sys- tem’s profit potential. The daily application of fish feed provides a steady supply of nutrients to plants and thereby eliminates the need to Rearing Solids Hydroponic Biofilter Sump discharge and replace depleted nutri- tank removal subsystem ent solutions or adjust nutrient solu- tions as in hydroponics. The plants remove nutrients from the culture water and eliminate the need for Combined separate and expensive biofilters. Combined Aquaponic systems require substan- tially less water quality monitoring Figure 2. Optimum arrangement of aquaponic system components (not to than separate hydroponic or recircu- scale). lating aquaculture systems. Savings are also realized by sharing opera- goldfish, Asian sea bass (barramun- tional and infrastructural costs such can be located after the biofilter and as pumps, reservoirs, heaters and water would be pumped up to the di) and Murray cod, most commer- alarm systems. In addition, the troughs and returned by gravity to cial systems are used to raise tilapia. intensive, integrated production of the fish-rearing tank. Most freshwater species, which can fish and plants requires less land The system can be configured so tolerate crowding, will do well in than ponds and gardens. Aquaponic that a portion of the flow is diverted aquaponic systems (including orna- systems do require a large capital to a particular treatment unit. For mental fish). One species reported to investment, moderate energy inputs perform poorly is hybrid striped and skilled management. Niche mar- example, a small side-stream flow may go to a hydroponic component bass. They cannot tolerate high lev- kets may be required for profitabili- els of potassium, which is often sup- ty. after solids are removed, while most of the water passes through a biofil- plemented to promote plant growth. System design ter and returns to the rearing tank. To recover the high capital cost and The biofilter and hydroponic compo- operating expenses of aquaponic sys- The design of aquaponic systems tems and earn a profit, both the fish- closely mirrors that of recirculating nents can be combined by using plant support media such as gravel rearing and the hydroponic veg- systems in general, with the addition etable components must be operated of a hydroponic component and the or sand that also functions as biofil- ter media. Raft hydroponics, which continuously near maximum pro- possible elimination of a separate duction capacity. The maximum bio- biofilter and devices (foam fractiona- consists of floating sheets of poly- styrene and net pots for plant sup- mass of fish a system can support tors) for removing fine and dissolved without restricting fish growth is solids. Fine solids and dissolved port, can also provide sufficient biofiltration if the plant production called the critical standing crop. organic matter generally do not Operating a system near its critical reach levels that require foam frac- area is large enough. Combining biofiltration with hydroponics is a standing crop uses space efficiently, tionation if aquaponic systems have maximizes production and reduces the recommended design ratio. The desirable goal because eliminating the expense of a separate biofilter is variation in the daily feed input to essential elements of an aquaponic the system, an important factor in system are the fish-rearing tank, a one of the main advantages of aquaponics. An alternative design sizing the hydroponic component. settleable and suspended solids There are three stocking methods removal component, a biofilter, a combines solids removal, biofiltra- tion and hydroponics in one unit. that can maintain fish biomass near hydroponic component, and a sump the critical standing crop: sequential (Fig. 2). The hydroponic support media (pea gravel or coarse sand) captures solids rearing, stock splitting and multiple Effluent from the fish-rearing tank is and provides surface area for fixed- rearing units. treated first to reduce organic matter film nitrification, although with this Sequential rearing in the form of settleable and sus- design it is important not to overload pended solids. Next, the culture the unit with suspended solids. Sequential rearing involves the cul- water is treated to remove ammonia As an example, Figures 3 and 4 show ture of several age groups (multiple and nitrate in a biofilter. Then, water cohorts) of fish in the same rearing flows through the hydroponic unit the commercial-scale aquaponic sys- tem that has been developed at the tank. When one age group reaches where some dissolved nutrients are marketable size, it is selectively har- taken up by plants and additional University of the Virgin Islands (UVI). It employs raft hydroponics. vested with nets and a grading sys- ammonia and nitrite are removed by tem, and an equal number of finger- bacteria growing on the sides of the Fish production lings are immediately restocked in tank and the underside of the poly- the same tank. There are three prob- styrene sheets (i.e., fixed-film nitrifi- Tilapia is the fish species most com- lems with this system: 1) the period- cation). Finally, water collects in a monly cultured in aquaponic sys- ic harvests stress the remaining fish reservoir (sump) and is returned to tems. Although some aquaponic sys- and could trigger disease outbreaks; the rearing tank. The location of the tems have used channel catfish, 2) stunted fish avoid capture and sump may vary. If elevated hydro- largemouth bass, crappies, rainbow accumulate in the system, wasting ponic troughs are used, the sump trout, pacu, common carp, koi carp, space and feed; and 3) it is difficult The UVI Aquaponic System ing crop of the initial rearing tank is reached. The fish are either herded Effluent line through a hatch between adjoining Fish rearing tanks Degassing Hydroponic tanks tanks or into “swimways” connect- Base addition ing distant tanks. Multiple rearing units usually come in modules of two to four tanks and are connected to a common filtration system. After the largest tank is harvested, all of the remaining groups of fish are moved to the next largest tank and the smallest tank is restocked with Sump fingerlings. A variation of the multi- Clarifier Return line ple rearing unit concept is the divi- Filter tanks sion of a long raceway into compart- Tank dimensions ments with movable screens. As the Rearing tanks: Diameter: 10 ft, Height: Sump: Diameter: 4 ft, Height: 3 ft, fish grow, their compartment is 4 ft, Water volume: 2,060 gal each Water volume: 160 gal increased in size and moved closer Clarifiers: Diameter: 6 ft, Height of Base addition tank: Diameter: 2 ft, to one end of the raceway where cylinder: 4 ft, Depth of cone: 3.6 ft, Height: 3 ft, Water volume: 50 gal they will eventually be harvested. Slope: 45º, Water volume: 1,000 gal Total system water volume: 29,375 These should be cross-flow race- Filter and degassing tanks: Length: 6 gal ways, with influent water entering ft, Width: 2.5 ft, Depth: 2 ft, Water vol- Flow rate: 100 GPM 1 the raceway through a series of ports ume: 185 gal Water pump: ⁄2 hp 1 down one side of the raceway and Hydroponic tanks: Length: 100 ft, Blowers: 1 ⁄2 hp (fish) and 1 hp (plants) 1 Width: 4 ft, Depth: 16 in, Water volume: Total land area: ⁄8 acre effluent water leaving the raceway 3,000 gal, Growing area: 2,304 ft2 through a series of drains down the other side. This system ensures that Pipe sizes water is uniformly high quality Pump to rearing tanks: 3 in Between hydroponic tanks: 6 in throughout the length of the race- Rearing tanks to clarifier: 4 in Hydroponic tanks to sump: 6 in way. Clarifiers to filter tanks: 4 in Sump to pump: 3 in Between filter tanks: 6 in Pipe to base addition tank 0.75 in Another variation is the use of sever- Filter tank to degassing tank: 4 in Base addition tank to sump: 1.25 in al tanks of the same size. Each rear- Degassing to hydroponic tanks: 6 in ing tank contains a different age group of fish, but they are not Figure 3. Layout of UVI aquaponic system with tank dimensions and pipe sizes (not to scale). moved during the production cycle. This system does not use space effi- ciently in the early stages of growth, to maintain accurate stock records ed. An alternative method is to but the fish are never disturbed and over time, which leads to a high crowd the fish with screens and the labor involved in moving the degree of management uncertainty pump them to another tank with a fish is eliminated. and unpredictable harvests. fish pump. A system of four multiple rearing Stock splitting Multiple rearing units tanks has been used successfully with tilapia in the UVI commercial- Stock splitting involves stocking very With multiple rearing units, the scale aquaponic system (Figs. 3 and high densities of fingerlings and peri- entire population is moved to larger 5). Production is staggered so one of odically splitting the population in rearing tanks when the critical stand- half as the critical standing crop of the rearing tank is reached. This method avoids the carryover prob- lem of stunted fish and improves stock inventory. However, the moves can be very stressful on the fish unless some sort of “swimway” is installed to connect all the rearing tanks. The fish can be herded into the swimway through a hatch in the wall of a rearing tank and maneu- Figure 5. The UVI aquaponic system at vered into another rearing tank by the New Jersey EcoComplex at Rutgers movable screens. With swimways, Fig. 4. An early model of the UVI University. Effluent from four tilapia- dividing the populations in half rearing tanks circulates through eight involves some guesswork because aquaponic system in St. Croix show- ing the staggered production of leaf raft hydroponic tanks, producing toma- the fish cannot be weighed or count- lettuce in six raft hydroponic tanks. toes and other crops. the rearing tanks is harvested every harvested and partially restocked eliminate the need for nutrient sup- 6 weeks. At harvest, the rearing tank every 6 weeks. However, this opera- plementation if fish stocking and is drained and all of the fish are tion requires additional labor, which feeding rates are increased relative removed. The rearing tank is then is a recurring cost and makes man- to plants. Another benefit of solids is refilled with the same water and agement more complex. In the long that the microorganisms that decom- immediately restocked with finger- run, having several smaller tanks in pose them are antagonistic to plant lings for a 24-week production cycle. which the fish are not disturbed root pathogens and help maintain Each circular rearing tank has a until harvest (hence, less mortality healthy root growth. water volume of 2,060 gallons and is and better growth) will be more cost SRAC Publication No. 453 heavily aerated with 22 air diffusers. effective. (“Recirculating Aquaculture Tank The flow rate to all four tanks is 100 Production Systems: A Review of gallons/minute, but the flow rate to Solids Component Options”) describes individual tanks is apportioned so some of the common devices used to that tanks receive a higher flow rate Most of the fecal waste fish generate remove solids from recirculating sys- as the fish grow. The average rearing should be removed from the waste tems. These include settling basins, tank retention time is 82 minutes. stream before it enters the hydro- tube or plate separators, the combi- Annual production has been 9,152 ponic tanks. Other sources of partic- nation particle trap and sludge sepa- pounds (4.16 mt) for Nile tilapia and ulate waste are uneaten feed and rator, centrifugal separators, micro- 10,516 pounds (4.78 mt) for red organisms (e.g., bacteria, fungi and screen filters and bead filters. tilapia (Table 1). However, produc- algae) that grow in the system. If this Sedimentation devices (e.g., settling tion can be increased to 11,000 organic matter accumulates in the basins, tube or plate separators) pri- pounds (5 mt) with close observation system, it will depress dissolved oxy- marily remove settleable solids of the ad libitum feeding response. gen (DO) levels as it decays and pro- duce carbon dioxide and ammonia. (>100 microns), while filtration In general, the critical standing crop If deep deposits of sludge form, they devices (e.g., microscreen filters, in aquaponic systems should not will decompose anaerobically (with- bead filters) remove settleable and exceed 0.50 pound/gallon. This densi- out oxygen) and produce methane suspended solids. Solids removal ty will promote fast growth and effi- and hydrogen sulfide, which are devices vary in regard to efficiency, cient feed conversion and reduce very toxic to fish. solids retention time, effluent charac- crowding stress that may lead to dis- teristics (both solid waste and treated ease outbreaks. Pure oxygen is gener- Suspended solids have special signifi- water) and water consumption rate. ally not needed to maintain this den- cance in aquaponic systems. Sand and gravel hydroponic sub- sity. Suspended solids entering the hydro- ponic component may accumulate strates can remove solid waste from The logistics of working with both on plant roots and create anaerobic system water. Solids remain in the fish and plants can be challenging. zones that prevent nutrient uptake system to provide nutrients to plants In the UVI system, one rearing tank by active transport, a process that through mineralization. With the is stocked every 6 weeks. Therefore, requires oxygen. However, some high potential of sand and gravel it takes 18 weeks to fully stock the accumulation of solids may be bene- media to clog, bed tillage or periodic system. If multiple units are used, ficial. As solids are decomposed by media replacement may be required. fish may be stocked and harvested microorganisms, inorganic nutrients The use of sand is becoming less as frequently as once a week. essential to plant growth are released common, but one popular aquaponic Similarly, staggered crop production to the water, a process known as system uses small beds (8 feet by 4 requires frequent seeding, trans- mineralization. Mineralization sup- feet) containing pea gravel ranging 1 1 planting, harvesting and marketing. plies several essential nutrients. from ⁄8 to ⁄4 inch in diameter. The Therefore, the goal of the design Without sufficient solids for mineral- hydroponic beds are flooded several process is to reduce labor wherever ization, more nutrient supplementa- times daily with system water and possible and make operations as sim- tion is required, which increases the then allowed to drain completely, ple as possible. For example, pur- operating expense and management and the water returned to the rear- chasing four fish-rearing tanks adds complexity of the system. However, ing tank. During the draining phase, extra expense. One larger tank could it may be possible to minimize or air is brought into the gravel. The be purchased instead and partially high oxygen content of air (com-

Table 1. Average production values for male mono-sex Nile and red tilapia in the UVI aquaponic system. Nile tilapia are stocked at 0.29 fish/gallon (77 fish/m3) and red tilapia are stocked at 0.58 fish/gallon (154 fish/m3). Harvest weight Initial Final Growth Harvest weight per unit weight weight rate Survival Tilapia per tank (lbs) volume (lb/gal) (g/fish) (g/fish) (g/day) (%) FCR Nile 1,056 (480 kg) 0.51 (61.5 kg/m3) 79.2 813.8 4.4 98.3 1.7 Red 1,212 (551 kg) 0.59 (70.7 kg/m3) 58.8 512.5 2.7 89.9 1.8 pared to water) speeds the decompo- D approximately 50 percent of the total sition of organic matter in the gravel. C particulate solids produced by the The beds are inoculated with red system and primarily removes large worms (Eisenia foetida), which A settleable solids. Although fingerlings improve bed aeration and assimilate are needed for effective clarifier per- organic matter. B formance, their grazing and swim- E ming activities are also counterpro- Solids removal ductive in that they resuspend some The most appropriate device for solids, which exit through the clarifi- solids removal in a particular system er outlet. As fingerlings become larg- depends primarily on the organic er (>200 g), clarifier performance Figure 6. Cross-sectional view (not to diminishes. Therefore, clarifier fish loading rate (daily feed input and scale) of UVI clarifier showing drain feces production) and secondarily on lines from two fish rearing tanks (A), must be replaced with small finger- the plant growing area. For example, central baffle (B) and discharge baffle lings (50 g) periodically (once every 4 if large numbers of fish (high organic (C), outlet to filter tanks (D), sludge months). loading) are raised relative to the drain line (E) and direction of water With clarification as the sole method plant growing area, a highly efficient flow (arrows). of solids removal, large quantities of solids removal device, such as a solids would be discharged to the microscreen drum filter, is desirable. baffle that is perpendicular to the hydroponic component. Therefore, Microscreen drum filters capture fine incoming water flow (Fig. 6). The another treatment stage is needed to organic particles, which are retained lower conical portion has a 45-degree remove re-suspended and fine solids. by the screen for only a few minutes slope and is buried below ground. A In the UVI system, two rectangular before backwashing removes them drain pipe is connected to the apex of tanks, each with a volume of 185 gal- from the system. In this system, the the cone. The drain pipe rises verti- lons, are filled with orchard/bird net- dissolved nutrients excreted directly cally out of the ground to the middle ting and installed after each of the by the fish or produced by mineral- of the cylinder and is fitted with a two clarifiers (Fig. 7). Effluent from ization of very fine particles and dis- ball valve. Rearing tank effluent each clarifier flows through a set of solved organic matter may be suffi- enters the clarifier just below the two filter tanks in series. Orchard cient for the size of the plant growing water surface. The incoming water is netting is effective in removing fine area. If small amounts of fish (low deflected upward by a 45-degree pipe solids. The filter tanks remove the organic loading) are raised relative to elbow to dissipate the current. As remaining 50 percent of total particu- the plant growing area, then solids water flows under the baffle, turbu- late solids. removal may be unnecessary, as more lence diminishes and solids settle on The orchard netting is cleaned once mineralization is needed to produce the sides of the cone. The solids accu- or twice each week. Before cleaning, sufficient nutrients for the plants. mulate there and form a thick mat a small sump pump is used to care- However, un-stabilized solids (solids that eventually rises to the surface of fully return the filter tank water to that have not undergone microbial the clarifier. To prevent this, approxi- the rearing tanks without dislodging decomposition) should not be allowed mately 30 male tilapia fingerlings are the solids. This process conserves to accumulate on the tank bottom required to graze on the clarifier water and nutrients. The netting is and form anaerobic zones. A recipro- walls and consolidate solids at the cleaned with a high-pressure water cating pea gravel filter (subject to base of the cone. Solids are removed spray and the sludge is discharged to flood and drain cycles), in which from the clarifier three times daily. lined holding ponds. incoming water is spread evenly over Hydrostatic pressure forces solids the entire bed surface, may be the Effluent from the UVI rearing tanks through the drain line when the ball is highly enriched with dissolved most appropriate device in this situa- valve is opened. A second, smaller tion because solids are evenly distrib- organic matter, which stimulates the baffle keeps floating solids from growth of filamentous bacteria in the uted in the gravel and exposed to being discharged to the filter tanks. high oxygen levels (21 percent in air drain line, clarifier and screen tank. as compared to 0.0005 to 0.0007 per- The fingerlings serve another pur- The bacteria appear as translucent, cent in fish culture water) on the pose. They swim into and through gelatinous, light tan filaments. Tilapia drain cycle. This enhances microbial the drain lines and keep them clean. consume the bacteria and control its activity and increases the mineraliza- Without tilapia, the 4-inch drain lines growth in the drain line and clarifier, tion rate. would have to be manually cleaned but bacteria do accumulate in the fil- nearly every day because of bacterial ter tanks. Without the filter tanks, UVI’s commercial-scale aquaponic growth in the drain lines, which con- the bacteria would overgrow plant system relies on two cylindro-conical stricts water flow. A cylindrical roots. The bacteria do not appear to clarifiers to remove settleable solids. screen attached to the rearing tank be pathogenic, but they do interfere The fiberglass clarifiers have a vol- drain keeps fingerlings from entering with the uptake of dissolved oxygen, ume of 1,000 gallons each. The cylin- the rearing tank. water and nutrients, thereby affect- drical portion of the clarifier is situat- The cylindro-conical clarifier removes ing plant growth. The feeding rate to ed above ground and has a central the system and the flow rate from more. Nitrification is an acid-produc- ing process. Therefore, an alkaline base must be added frequently, depending on feeding rate, to main- tain relatively stable pH values. Some method of removing dead biofilm is necessary to prevent media clogging, short circuiting of water flow, decreasing DO values and declining biofilter performance. A discussion of nitrification princi- ples and a description of various biofilter designs and operating proce- dures are given in SRAC Publication Nos. 451, 452 and 453. Four major biofilter options (rotating biological contactors, expandable media filters, fluidized bed filters and packed tower filters) are dis- cussed in SRAC Publication No. 453. Figure 7. Components of the UVI aquaponic system at the New Jersey EcoComplex If a separate biofilter is required or if at Rutgers University. a combined biofilter (biofiltration and hydroponic substrate) is used, the rearing tank determine the extent sludge can be separated from water the standard equations used to size to which filamentous bacteria grow, and used with other waste products biofilters may not apply to aquapon- but they can be contained by provid- from the system (vegetable matter) to ic systems, as additional surface area ing a sufficient area of orchard net- form compost. Urban facilities might is provided by plant roots and a con- ting, either by adjusting screen tank have to discharge solid waste into siderable amount of ammonia is size or using multiple screen tanks. sewer lines for treatment and dispos- taken up by plants. However, the In systems with lower organic load- al at the municipal wastewater treat- contribution of various hydroponic ing rates (i.e., feeding rates) or lower ment plant. subsystem designs and plant species water temperature (hence, less bio- to water treatment in aquaponic logical activity), filamentous bacteria Biofiltration systems has not been studied. Therefore, aquaponic system biofil- diminish and are not a problem. A major concern in aquaponic sys- ters should be sized fairly close to The organic matter that accumulates tems is the removal of ammonia, a the recommendations for recirculat- on the orchard netting between metabolic waste product excreted ing systems. cleanings forms a thick sludge. through the gills of fish. Ammonia Anaerobic conditions develop in the will accumulate and reach toxic lev- Nitrification efficiency is affected by sludge, which leads to the formation els unless it is removed by the pH. The optimum pH range for nitri- of gases such as hydrogen sulfide, process of nitrification (referred to fication is 7.0 to 9.0, although most methane and nitrogen. Therefore, a more generally as biofiltration), in studies indicate that nitrification effi- degassing tank is used in the UVI which ammonia is oxidized first to ciency is greater at the higher end of system to receive the effluent from nitrite, which is toxic, and then to this range (high 8s). Most hydroponic the filter tanks (Fig. 7). A number of nitrate, which is relatively non-toxic. plants grow best at a pH of 5.8 to air diffusers vent the gasses into the Two groups of naturally occurring 6.2. The acceptable range for hydro- atmosphere before the culture water bacteria (Nitrosomonas and ponic systems is 5.5 to 6.5. The pH reaches the hydroponic plants. The Nitrobacter) mediate this two-step of a solution affects the solubility of degassing tank has an internal stand- process. Nitrifying bacteria grow as a nutrients, especially trace metals. pipe well that splits the water flow film (referred to as biofilm) on the Essential nutrients such as iron, into three sets of hydroponic tanks. surface of inert material or they manganese, copper, zinc and boron Solids discharged from aquaponic adhere to organic particles. Biofilters are less available to plants at a pH systems must be disposed of appro- contain media with large surface higher than 7.0, while the solubility priately. There are several methods areas for the growth of nitrifying of phosphorus, calcium, magnesium for effluent treatment and disposal. bacteria. Aquaponic systems have and molybdenum sharply decreases Effluent can be stored in aerated used biofilters with sand, gravel, at a pH lower than 6.0. Compromise ponds and applied as relatively dilute shells or various plastic media as between nitrification and nutrient sludge to land after the organic mat- substrate. Biofilters perform optimal- availability is reached in aquaponic ter in it has stabilized. This method is ly at a temperature range of 77 to systems by maintaining pH close to advantageous in dry areas where 86 °F, a pH range of 7.0 to 9.0, satu- 7.0. sludge can be used to irrigate and fer- rated DO, low BOD (<20 mg/liter) Nitrification is most efficient when tilize field crops. The solid fraction of and total alkalinity of 100 mg/liter or water is saturated with DO. The UVI commercial-scale system maintains tem. A significant amount of nitrifi- microbial growth and the roots that DO levels near 80 percent saturation cation occurs on the undersides of remain after harvest. The resulting (6 to 7 mg/L) by aerating the hydro- the polystyrene sheets, especially in reduction in water circulation, ponic tanks with numerous small air the areas exposed to strong currents together with the decomposition of diffusers (one every 4 feet) distrib- above air diffusers where the biofilm organic matter, leads to the forma- uted along the long axis of the tanks. is noticeably thicker. tion of anaerobic zones that impair Reciprocating (ebb and flow) gravel Aquaponic systems using nutrient or kill plant roots. The small, plastic systems expose nitrifying bacteria to film technique (NFT) as the hydro- tubes used to irrigate gravel are also high atmospheric oxygen levels dur- ponic component may require a sep- subject to clogging with biological ing the dewatering phase. The thin arate biofilter. NFT consists of nar- growth. Moving and cleaning gravel film of water that flows through NFT row plastic channels for plant sup- substrate is difficult because of its (nutrient film technique) channels port with a film of nutrient solution weight. Planting in gravel is also dif- absorbs oxygen by diffusion, but flowing through them (Fig. 8). The ficult, and plant stems can be dam- dense plant roots and associated water volume and surface area of aged by abrasion in outdoor systems organic matter can block water flow NFT are considerably smaller than in exposed to wind. Gravel retains very and create anaerobic zones, which raft culture because there is just a little water if drained, so a disruption precludes the growth of nitrifying thin film of water and no substantial in flow will lead to the rapid onset of bacteria and further necessitates the side wall area or raft underside sur- water stress (wilting). The sturdy installation of a separate biofilter. face area for colonization by nitrify- infrastructure required to support Ideally, aquaponic systems should be ing bacteria. gravel and the potential for clogging designed so that the hydroponic sub- limits the size of gravel beds. system also serves as the biofilter, Hydroponic subsystems One popular gravel-based aquaponic which eliminates the capital cost and A number of hydroponic subsystems system uses pea gravel in small beds operational expense of a separate have been used in aquaponics. that are irrigated through a distribu- biofilter. Granular hydroponic media Gravel hydroponic subsystems are tion system of PVC pipes over the such as gravel, sand and perlite pro- common in small operations. To gravel surface. Numerous small holes vide sufficient substrate for nitrifying ensure adequate aeration of plant in the pipes distribute culture water bacteria and generally serve as the roots, gravel beds have been operated on the flood cycle. The beds are sole biofilter in some aquaponic sys- in a reciprocating (ebb and flow) allowed to drain completely between tems, although the media has a ten- mode, where the beds are alternately flood cycles. Solids are not removed dency to clog. If serious clogging flooded and drained, or in a non- from the culture water and organic occurs from organic matter overload- flooded state, where culture water is matter accumulates, but the beds are ing, gravel and sand filters can actu- applied continuously to the base of tilled between planting cycles so that ally produce ammonia as organic the individual plants through small- some organic matter can be dis- matter decays, rather than remove it. diameter plastic tubing. Depending lodged and discharged. If this occurs, the gravel or sand on its composition, gravel can pro- Sand has been used as hydroponic must be washed and the system vide some nutrients for plant growth media in aquaponic systems and is design must be modified by (e.g., calcium is slowly released as an excellent substrate for plant installing a solids removal device the gravel reacts with acid produced growth. In an experimental system, before the media, or else the organic during nitrification). sand beds (25 feet long by 5 feet loading rate must be decreased by Gravel has several negative aspects. wide by 1.6 feet deep) were con- stocking fewer fish and reducing structed on slightly sloped ground feeding rates. The weight of gravel requires strong support structures. It is subject to covered by polyethylene sheets adja- Raft hydroponics, which consists of clogging with suspended solids, cent to in-ground rearing tanks, with channels (with 1 foot of water depth) the tank floors sloping to one side. A covered by floating sheets of poly- pump in the deep end of the rearing styrene for plant support, also pro- tank was activated for 30 minutes vides sufficient nitrification if solids five times daily to furrow irrigate the are removed from the flow before it adjacent sand bed. The culture water reaches the hydroponic component. percolated through the sand and The waste treatment capacity of raft returned to the rearing tank. A hydroponics is equivalent to a feed- coarse grade of sand is needed to ing ratio of 180 g of fish feed/m2 of reduce the potential for clogging over plant growing area/day. (Note: 1 m2 time and some solids should be = 10.76 ft2 and 454 g = 1 lb.) This is removed before irrigation. equivalent to about 1.2 pounds of Perlite is another media that has feed for each 8-foot x 4-foot sheet of been used in aquaponic systems. polystyrene foam. After an initial Figure 8. Using nutrient film technique, Perlite is placed in shallow alu- acclimation period of 1 month, it is basil is produced in an aquaponic sys- minum trays (3 inches deep) with a not necessary to monitor ammonia tem at Bioshelters, Inc. in Amherst, baked enamel finish. The trays vary and nitrite values in the UVI raft sys- Massachusetts. from 8 inches to 4 feet wide and can be fabricated to any length, with 20 etables. The UVI system uses three ponic tanks, they consume plant feet the maximum recommended sets of two raft hydroponic tanks roots and severely stunt plant length. At intervals of 20 feet, adjoin- that are 100 feet long by 4 feet growth, although it is relatively easy ing trays should be separated by 3 wide by 16 inches deep and contain to keep fish from entering by placing inches or more in elevation so that 12 inches of water. The channels a fine mesh screen at the entry point water drops to the lower tray and are lined with low-density polyeth- of water into the degassing tank. becomes re-aerated. A slope of 1 ylene liners (20 mil thick) and cov- Similarly, blooms of zooplankton, inch in 12 feet is needed for water ered by expanded polystyrene especially ostracods, will consume flow. A small trickle of water enters sheets (rafts) that are 8 feet long by root hairs and fine roots, retarding at the top of the tray, flows through 4 feet wide by 1.5 inches thick. Net plant growth. Other pests are tad- the perlite and keeps it moist, and pots are placed in holes in the raft poles and snails, which consume discharges into a trough at the lower and just touch the water surface. roots and nitrifying bacteria. These end. Solids must be removed from Two-inch net pots are generally problems can be surmounted by the water before it enters the perlite used for leafy green plants, while 3- increasing water agitation to prevent tray. Full solids loading will clog the inch net pots are used for larger root colonization by zooplankton and perlite, form short-circuiting chan- plants such as tomatoes or okra. by stocking some carnivorous fish nels, create anaerobic zones and lead Holes of the same size are cut into such as red ear sunfish (shellcrack- to non-uniform plant growth. the polystyrene sheet. A lip at the ers) in hydroponic tanks to prey on Shallow perlite trays provide mini- top of the net pot secures it and tadpoles and snails. mal area for root growth and are bet- keeps it from falling through the ter for smaller plants such as lettuce hole into the water. Seedlings are Sump and herbs. nursed in a greenhouse and then Water flows by gravity from gravel, placed into net pots. Their roots Nutrient film technique (NFT) has sand and raft hydroponic subsystems grow into the culture water while been successfully incorporated into a to a sump, which is the lowest point their canopy grows above the raft number of aquaponic systems. NFT in the system. The sump contains a surface. The system provides maxi- consists of many narrow, plastic pump or pump inlet that returns the mum exposure of roots to the cul- troughs (4 to 6 inches wide) in which treated culture water to the rearing ture water and avoids clogging. The plant roots are exposed to a thin film tanks. If NFT troughs or perlite trays sheets shield the water from direct of water that flows down the are located above the rearing tanks, sunlight and maintain lower than troughs, delivering water, nutrients the sump would be positioned in ambient water temperature, which and oxygen to the roots of the plants. front of them so that water could be is a beneficial feature in tropical The troughs are lightweight, inex- pumped up to the hydroponic com- systems. A disruption in pumping pensive and versatile. Troughs can ponent for gravity return to the rear- does not affect the plant’s water be mounted over rearing tanks to ing tanks. There should be only one supply as in gravel, sand and NFT efficiently use vertical greenhouse pump to circulate water in an subsystems. The sheets are easily space. However, this practice is dis- aquaponic system. couraged if it interferes with fish and moved along the channel to a har- The sump should be the only tank plant operations such as harvesting. vesting point where they can be in the system where the water High plant density can be main- lifted out of the water and placed level decreases as a result of over- tained by adjusting the distance on supports at an elevation that is all water loss from evaporation, between troughs to provide opti- comfortable for workers (Fig. 9). transpiration, sludge removal and mum plant spacing during the grow- A disadvantage of rafts in an splashing. An electrical or mechan- ing cycle. In aquaponic systems that aquaponic system is that roots are ical valve is used to automatically use NFT, solids must be removed so exposed to harmful organisms asso- add replacement water from a stor- they do not accumulate and kill ciated with aquaculture systems. If age reservoir or well. Municipal roots. With NFT, a disruption in tilapia fry gain access to the hydro- water should not be used unless it water flow can lead quickly to wilt- is de-chlorinated. Surface water ing and death. Water is delivered at should not be used because it may one end of the troughs by a PVC contain disease organisms. A water manifold with discharge holes above meter should be used to record each trough; it is collected at the additions. Unusually high water opposite, down-slope end in an open consumption indicates a leak. channel or large PVC pipe. The use of microtubes, which are used in The sump is a good location for the commercial hydroponics, is not rec- addition of base to the system. ommended because they will clog. Soluble base such as potassium The holes should be as large as prac- hydroxide causes high and toxic pH tical to reduce cleaning frequency. levels in the sump. However, as water is pumped into the rearing A floating or raft hydroponic sub- Figure 9. Leaf lettuce being harvested tank, it is diluted and pH decreases system is ideal for the cultivation of from a raft hydroponic tank in the UVI to acceptable levels. leafy green and other types of veg- aquaponic system in St. Croix. The UVI system has a separate base for floating hydroponic subsystems, eter) is recommended for recipro- addition tank located next to the but they are expensive. A good alter- cating (flood and drain) gravel sump. As water is pumped from the native is the 20-mil polyethylene lin- aquaponic systems. This ratio sump to the fish-rearing tanks, a ers that are placed inside concrete- requires that tilapia be raised to a small pipe, tapped into the main block or poured-concrete side walls. final density of 0.5 pound/gallon water distribution line, delivers a They are easy to install, relatively and fed appropriately. With the small flow of water to the base addi- inexpensive and durable, with an recommended ratio, no solids are tion tank, which is well aerated with expected life of 12 to 15 years. A soil removed from the system. The one large air diffuser. When base is floor covered with fine sand will pre- hydroponic beds should be culti- added to this tank and dissolves, the vent sharp objects from puncturing vated (stirred up) between crops resulting high pH water slowly flows the liners. Lined hydroponic tanks and inoculated with red worms to by gravity into the sump, where it is can be constructed to very large help break down and assimilate rapidly diluted and pumped to fish- sizes—hundreds of feet long and up the organic matter. With this sys- rearing tanks. This system prevents a to 30 feet wide. tem, nutrient supplementation rapid pH increase in the fish-rearing may not be necessary. tank. Component ratios As a general guide for raft aquapon- Aquaponic systems are generally ics, a ratio in the range of 60 to 100 Construction materials designed to meet the size require- g of fish feed/m2 of plant growing Many materials can be used to con- ments for solids removal (for those area per day should be used. Ratios struct aquaponic systems. Budget systems requiring solids removal) within this range have been used limitations often lead to the selection and biofiltration (if a separate biofil- successfully in the UVI system for of inexpensive and questionable ter is used) for the quantity of fish the production of tilapia, lettuce, materials such as vinyl-lined, steel- being raised (see SRAC Publication basil and several other plants. In the walled swimming pools. Plasticizers No. 453, “Recirculating Aquaculture UVI system all solids are removed, used in vinyl manufacture are toxic Tank Production Systems: A Review with a residence time of <1 day for settleable solids (>100 micrometers) to fish, so these liners must be of Component Options”). After the removed by a clarifier, and 3 to 7 washed thoroughly or aged with size requirements are calculated, it is prudent to add excess capacity as a days for suspended solids removed water for several weeks before fish safety margin. However, if a separate by an orchard netting filter. The sys- can be added safely to a tank of clean biofilter is used, the hydroponic tem uses rainwater and requires water. After a few growing periods, component is the safety factor supplementation for potassium, cal- vinyl liners shrink upon drying, because a significant amount of cium and iron. become brittle and crack, while the ammonia uptake and nitrification Another factor to consider in steel walls gradually rust. Nylon-rein- will occur regardless of hydroponic determining the optimum feeding forced, neoprene rubber liners are technique. rate ratio is the total water volume not recommended either. Tilapia eat Another key design criterion is the of the system, which affects nutri- holes in rubber liners at the folds as ratio between the fish-rearing and ent concentrations. In raft hydro- they graze on microorganisms. hydroponic components. The key is ponics, approximately 75 percent Moreover, neoprene rubber liners are the ratio of daily feed input to plant of the system water volume is in not impervious to chemicals. If herbi- growing area. If the ratio of daily the hydroponic component, cides and soil sterilants are applied feeding rate to plants is too high, whereas gravel beds and NFT troughs contain minor amounts of under or near rubber liners, these nutrient salts will accumulate rapid- system water. Theoretically, in chemicals can diffuse into culture ly and may reach phytotoxic levels. Higher water exchange rates will be systems producing the same quan- water, accumulate in fish tissue and required to prevent excessive nutri- tity of fish and plants, a daily kill hydroponic vegetables. ent buildup. If the ratio of daily feeding rate of 100 g/m2 would Fiberglass is the best construction feeding rate to plants is too low, produce total nutrient concentra- material for rearing tanks, sumps and plants will develop nutrient deficien- tions nearly four times higher in filter tanks. Fiberglass tanks are stur- cies and need more nutrient supple- gravel and NFT systems (e.g., dy, durable, non-toxic, movable and mentation. Fortunately, hydroponic 1,600 mg/L) than in raft systems easy to plumb. Polyethylene tanks are plants grow well over a wide range (e.g., 400 mg/L), but total nutrient also very popular for fish rearing and of nutrient concentrations. mass in the systems would be the same. Nutrient concentrations out- gravel hydroponics because of their The optimum ratio of daily fish side acceptable ranges affect plant low cost. NFT troughs made from feed input to plant growing area growth. Therefore, the optimum extruded polyethylene are specifically will maximize plant production design ratio varies with the type designed to prevent the puddling and while maintaining relatively stable of hydroponic component. Gravel water stagnation that lead to root levels of dissolved nutrients. A vol- and NFT systems should have a death and are preferable to makeshift ume ratio of 1 ft3 of fish-rearing feeding rate ratio that is approxi- structures such as PVC pipes. Plastic tank to 2 ft3 of pea gravel hydro- 1 1 mately 25 percent of the recom- troughs are commercially available ponic media ( ⁄8 to ⁄4 inch in diam- mended ratio for raft hydroponics. Other factors in determining the supplied by water (H2O) and carbon metabolites to reach their full optimum feeding rate ratio are the dioxide gas (CO2). The remaining growth potential. water exchange rate, nutrient levels nutrients are absorbed from the cul- Maintaining high DO levels in the in the source water, degree and ture water. Other macronutrients culture water is extremely important speed of solids removal, and type of include nitrogen (N), potassium (K), for optimal plant growth, especially plant being grown. Lower rates of calcium (Ca), magnesium (Mg), in aquaponic systems with their water exchange, higher source-water phosphorus (P) and sulfur (S). The high organic loads. Hydroponic nutrient levels, incomplete or slow seven micronutrients include chlo- plants are subject to intense root res- solids removal (resulting in the rine (Cl), iron (Fe), manganese (Mn), piration and draw large amounts of release of more dissolved nutrients boron (B), zinc (Zn), copper (Cu) and oxygen from the surrounding water. through mineralization), and slow- molybdenum (Mo). These nutrients If DO is deficient, root respiration growing plants would allow a lower must be balanced for optimum plant decreases. This reduces water feeding rate ratio. Conversely, higher growth. High levels of one nutrient absorption, decreases nutrient water exchange rates, low source- can influence the bioavailability of uptake, and causes the loss of cell water nutrient levels, rapid and com- others. For example, excessive tissue from roots. The result is plete solids removal, and fast-grow- amounts of potassium may interfere reduced plant growth. Low DO lev- ing plants would allow a higher with the uptake of magnesium or els correspond with high concentra- feeding rate ratio. calcium, while excessive amounts of tions of carbon dioxide, a condition The optimum feeding rate ratio is either of the latter nutrients may that promotes the development of influenced by the plant culture interfere with the uptake of the plant root pathogens. Root respira- method. With batch culture, all other two nutrients. tion, root growth and transpiration plants in the system are planted and Enriching the air in an unventilated are greatest at saturated DO levels. harvested at the same time. During greenhouse with CO2 has dramati- Climatic factors also are important their maximum growth phase, there cally increased crop yields in north- for hydroponic plant production. is a large uptake of nutrients, which ern latitudes. Doubling atmospheric Production is generally best in requires a higher feeding rate ratio CO2 increases agricultural yields by regions with maximum intensity during that period. In practice, how- an average of 30 percent. However, and daily duration of light. Growth ever, a higher feeding rate ratio is the high cost of energy to generate slows substantially in temperate used throughout the production CO2 has discouraged its use. An greenhouses during winter because cycle. With a staggered production aquaponic system in a tightly solar radiation is low. Supplemental system, plants are in different stages enclosed greenhouse is ideal because illumination can improve winter of growth, which levels out nutrient CO2 is constantly vented from the production, but is not generally cost uptake rates and allows good pro- culture water. effective unless an inexpensive ener- duction with slightly lower feeding There is a growing body of evidence gy source is available. rate ratios. that healthy plant development Water temperature is far more In properly designed aquaponic sys- relies on a wide range of organic important than air temperature for tems, the surface area of the hydro- compounds in the root environment. hydroponic plant production. The ponic component is large compared These compounds, generated by best water temperature for most to the surface area of the fish-rearing complex biological processes involv- hydroponic crops is about 75 °F. tank (stocked at commercially rele- ing microbial decomposition of However, water temperature can go vant densities). The commercial- organic matter, include vitamins, as low as the mid-60s for most com- scale unit at UVI has a ratio of 7.3:1. auxins, gibberellins, antibiotics, mon garden crops and slightly lower The total plant growing area is 2,304 enzymes, coenzymes, amino acids, for winter crops such as cabbage, 2 ft and the total fish-rearing surface organic acids, hormones and other brussel sprouts and broccoli. 2 area is 314 ft . metabolites. Directly absorbed and Maintaining the best water tempera- assimilated by plants, these com- ture requires heating during the Plant growth requirements pounds stimulate growth, enhance winter in temperate greenhouses yields, increase vitamin and mineral For maximum growth, plants in and year-round cooling in tropical content, improve fruit flavor and aquaponic systems require 16 essen- greenhouses. In addition to evapora- hinder the development of tial nutrients. These are listed below tive cooling of tropical greenhouses, pathogens. Various fractions of dis- in the order of their concentrations chillers are often used to cool the solved organic matter (e.g., humic in plant tissue, with carbon and oxy- nutrient solution. In tropical outdoor acid) form organo-metallic complex- gen being the highest. The essential systems, complete shading of the es with Fe, Mn and Zn, thereby elements are arbitrarily divided into fish-rearing and filtration compo- increasing the availability of these macronutrients, those required in nents lowers system water tempera- micronutrients to plants. Although relatively large quantities, and ture. In raft hydroponics, the poly- inorganic nutrients give plants an micronutrients, those required in styrene sheets shield water from avenue to survival, plants not only considerably smaller amounts. Three direct sunlight and maintain temper- use organic metabolites from the of the macronutrients—carbon (C), atures that are several degrees lower environment, but also need these oxygen (O) and hydrogen (H)—are than those in open bodies of water. Crop varieties may need to be removal component) or enlarging excess vegetative growth when adjusted seasonally for both temper- the plant-growing areas. nitrate levels are high. The filter ate and tropical aquaponic produc- The major ions that increase conduc- tanks in the UVI commercial-scale tion. Plants cultured in outdoor - system have a mechanism for con- tivity are nitrate (NO3 ), phosphate aquaponic systems must be protect- -2 -2 + +2 trolling nitrate levels through denitri- (PO4 ), sulfate (SO4 ), K , Ca and ed from strong winds, especially +2 - -2 -2 fication, the reduction of nitrate ions Mg . Levels of NO3 , PO4 and SO4 after transplanting when seedlings are usually sufficient for good plant to nitrogen gas by anaerobic bacte- are fragile and most vulnerable to growth, while levels of K+ and Ca+2 ria. Large quantities of organic mat- damage. are generally insufficient. Potassium ter accumulate on the orchard net- is added to the system in the form of ting between cleanings. Denitrifica- Nutrient dynamics potassium hydroxide (KOH) and Ca tion occurs in anaerobic pockets that develop in the sludge. Water moves Dissolved nutrients are measured is added as calcium hydroxide through the accumulated sludge, collectively as total dissolved solids [Ca(OH)2]. In the UVI commercial- which provides good contact (TDS), expressed as ppm, or as the scale system, KOH and Ca(OH)2 are between nitrate ions and denitrifying capacity of the nutrient solution to added in equal amounts (usually 500 bacteria. The frequency of cleaning conduct an electrical current (EC), to 1,000 g). The bases are added the netting regulates the degree of expressed as millimhos/cm alternately several times weekly to denitrification. When the netting is (mmho/cm). In a hydroponic solu- maintain pH near 7.0. Adding basic cleaned often (e.g., twice per week), tion, the recommended range for compounds of K and Ca serves the sludge accumulation and denitrifica- TDS is 1,000 to 1,500 ppm (1.5 to 3.5 dual purpose of supplementing tion are minimized, which leads to mmho/cm). In an aquaponic system, essential nutrients and neutralizing an increase in nitrate concentrations. considerably lower levels of TDS acid. In some systems Mg also may When the netting is cleaned less (200 to 400 ppm) or EC (0.3 to 0.6 be limiting. Magnesium can be sup- often (e.g., once per week), sludge mmho/cm) will produce good results plemented by using dolomite accumulation and denitrification are because nutrients are generated con- [CaMg(CO3)2] as the base to adjust maximized, which leads to a tinuously. A concern with aquaponic pH. The addition of too much Ca decrease in nitrate levels. Nitrate- systems is nutrient accumulation. can cause phosphorous to precipitate nitrogen levels can be regulated High feeding rates, low water from culture water in the form of within a range of 1 to 100 mg/L or exchange and insufficient plant dicalcium phosphate [CaHPO4]. more. High nitrate concentrations growing areas can lead to the rapid Sodium bicarbonate (NaHCO3) promote the growth of leafy green buildup of dissolved nutrients to should never be added to an vegetables, while low nitrate concen- potentially phytotoxic levels. aquaponic system for pH control trations promote fruit development Phytotoxicity occurs at TDS concen- + because a high Na level in the pres- in vegetables such as tomatoes. trations above 2,000 ppm or EC ence of chloride is toxic to plants. +2 +2 above 3.5 mmho/cm. Because The Na+ concentration in hydropon- The micronutrients Fe , Mn , +2 +3 +6 aquaponic systems have variable ic nutrient solutions should not Cu , B and Mo do not accumu- environmental conditions such as exceed 50 mg/L. Higher Na+ levels late significantly in aquaponic sys- daily feed input, solids retention, will interfere with the uptake of K+ tems with respect to cumulative feed +2 mineralization, water exchange, and Ca+2. In lettuce, reduced Ca+2 input. The Fe derived from fish nutrient input from source water or uptake causes tip-burn, resulting in feed is insufficient for hydroponic supplementation, and variable nutri- an unmarketable plant. Tip-burn vegetable production and must be +2 ent uptake by different plant species, often occurs during the warmer supplemented with chelated Fe so +2 it is difficult to predict the exact level months. Salt (NaCl) is added to fish that the concentration of Fe is 2.0 +2 of TDS or EC and how it is chang- feed during manufacture. A produc- mg/L. Chelated Fe has an organic ing. Therefore, the culturist should er who orders large quantities of compound attached to the metal ion purchase an inexpensive conductivi- feed could request that salt not be to prevent it from precipitating out ty meter and periodically measure added if this does not affect fish of solution and making it unavailable TDS or EC. If dissolved nutrients are health. If Na+ exceeds 50 mg/L and to plants. The best chelate is Fe- steadily increasing and approach the plants appear to be affected, a DTPA because it remains soluble at 2,000 ppm as TDS or 3.5 mmho/cm partial water exchange (dilution) pH 7.0. Fe-EDTA is commonly used as EC, increasing the water exchange may be necessary. Rainwater is used in the hydroponics industry, but it is rate or reducing the fish stocking in UVI’s systems because the less stable at pH 7.0 and needs to be +2 rate and feed input will quickly groundwater of semiarid islands gen- replenished frequently. Fe also can reduce nutrient accumulation. erally contains too much salt for be applied in a foliar spray directly However, because these methods aquaponics. to plant leaves. A comparison of either increase costs (i.e., more water Mn+2, B+3 and Mo+6 levels with consumed) or lower output (i.e., less The accumulation of too much standard nutrient formulations for fish produced), they are not good nitrate in aquaponic systems is lettuce shows that their concentra- long-term solutions. Better but more sometimes a concern as fruiting tions in aquaponic systems are sev- costly solutions involve removing plants set less fruit and produce eral times lower than their initial lev- more solids (i.e., upgrade the solids els in hydroponic formulations. market the final product. tered would pose a threat to fish and Deficiency symptoms for Mn+2, B+3 would not be permitted in a fish cul- and Mo+6 are not detected in Crop production systems ture system. Similarly, therapeutants aquaponic systems, so their concen- There are three strategies for pro- for treating fish parasites and dis- trations appear to be adequate for ducing vegetable crops in the hydro- eases should not be used because normal plant growth. Concentrations vegetables may absorb and concen- +2 ponic component. These are stag- of Cu are similar in aquaponic sys- gered cropping, batch cropping and trate them. The common practice of tems and hydroponic formulations, adding salt to treat fish diseases or +2 intercropping. A staggered crop pro- while Zn accumulates in aquapon- duction system is one in which reduce nitrite toxicity is detrimental ic systems to levels that are four to groups of plants in different stages to plant crops. Nonchemical meth- sixteen times higher than initial lev- of growth are cultivated simultane- ods of integrated pest management els in hydroponic formulations. must be used. These include biologi- +2 ously. This allows produce to be har- Nevertheless, Zn concentrations vested regularly and keeps the cal control (resistant cultivars, preda- usually remain within the limit that uptake of nutrients from the culture tors, pathogens, antagonistic organ- is safe for fish. water relatively constant. This sys- isms), physical barriers, traps, and manipulation of the physical envi- Vegetable selection tem is most effective where crops can be grown continuously, as in the ronment. There are more opportuni- Many types of vegetables have been tropics, subtropics, or temperate ties to use biological control meth- grown in aquaponic systems. greenhouses with environmental ods in enclosed greenhouse environ- However, the goal is to culture a veg- control. At UVI, the production of ments than in exterior installations. etable that will generate the highest leaf lettuce is staggered so that a Parasitic wasps and ladybugs can be level of income per unit area per crop can be harvested weekly on used to control white flies and unit time. With this criterion, culi- the same day, which facilitates mar- aphids. In UVI’s systems, caterpil- nary herbs are the best choice. They keting arrangements. Bibb lettuce lars are effectively controlled by grow very rapidly and command reaches market size 3 weeks after twice weekly spraying with Bacillus high market prices. The income transplanting. Therefore, three thuringiensis, a bacterial pathogen from herbs such as basil, cilantro, growth stages of Bibb lettuce are that is specific to caterpillars. Fungal chives, parsley, portulaca and mint is cultivated simultaneously, and one- root pathogens (Pythium), which are much higher than that from fruiting third of the crop is harvested week- encountered in summer at UVI and crops such as tomatoes, cucumbers, ly. Red leaf lettuce and green leaf reduce production, dissipate in win- eggplant and okra. For example, in lettuce require 4 weeks to reach ter in response to lower water tem- experiments in UVI’s commercial- marketable size. The cultivation of perature. scale system, basil production was four growth stages of these lettuce The prohibition on the use of pesti- 11,000 pounds annually at a value of varieties allows one-fourth of the cides makes crop production in $110,000, compared to okra produc- crop to be harvested weekly. In 3 aquaponic systems more difficult. tion of 6,400 pounds annually at a years of continuous operation, UVI However, this restriction ensures value of $6,400. Fruiting crops also has harvested 148 crops of lettuce, that crops from aquaponic systems require longer culture periods (90 which demonstrates the system’s will be raised in an environmentally days or more) and have more pest sustainability. Leafy green vegeta- sound manner and be free of pesti- problems and diseases. Lettuce is bles, herbs and other crops with cide residues. A major advantage of another good crop for aquaponic sys- short production periods are well aquaponic systems is that crops are tems because it can be produced in a suited for continuous, staggered pro- less susceptible to attack from soil- short period (3 to 4 weeks in the sys- duction systems. borne diseases. Plants grown in tem) and, as a consequence, has rela- A batch cropping system is more aquaponic systems may be more tively few pest problems. Unlike appropriate for crops that are grown resistant to diseases that affect plants fruiting crops, a large portion of the seasonally or have long growing grown in standard hydroponics. This harvested biomass is edible. Other periods (>3 months), such as toma- resistance may be due to the pres- suitable crops are Swiss chard, pak toes and cucumbers. Various inter- ence of some organic matter in the choi, Chinese cabbage, collard and cropping systems can be used in culture water that creates a stable watercress. The cultivation of flow- conjunction with batch cropping. growing environment with a wide ers has potential in aquaponic sys- For example, if lettuce is inter- diversity of microorganisms, some of tems. Good results have been cropped with tomatoes and cucum- which may be antagonistic to plant obtained with marigold and zinnia in bers, one crop of lettuce can be har- root pathogens (Fig. 10). UVI’s aquaponic system. Traditional vested before the tomato plant Approaches to system design medicinal plants and plants used for canopy begins to limit light. the extraction of modern pharmaceu- There are several ways to design an ticals have not been cultivated in Pest and disease control aquaponic system. The simplest aquaponic systems, but there may be Pesticides should not be used to con- approach is to duplicate a standard potential for growing some of these system or scale a standard system plants. All plant production has to be trol insects on aquaponic plant crops. Even pesticides that are regis- down or up, keeping the compo- coupled to the producer’s ability to nents proportional. Changing aspects the sump and allows the desired priate number and size of hydroponic flow rate to go from the pump to the tanks? What would the weekly let- next stage of the system. If more tuce harvest be? space is available than the standard 1. Each UVI system contains four design requires, then the system fish-rearing tanks (Fig. 3). Fish could be scaled up within limitations production is staggered so that or more than one scaled-down sys- one fish tank is harvested every 6 tem could be installed. weeks. The total growing period Design for fish production. If the pri- per tank is 24 weeks. If 500 mary objective is to produce a cer- pounds of fish are required tain amount of fish annually, the first weekly, six production systems Figure 10. Healthy roots of Italian pars- step in the design process will be to (24 fish-rearing tanks) are need- ley cultured on rafts in a UVI aquapon- determine the number of systems ed. ic system at the Crop Diversification required, the number of rearing Center South in Alberta, Canada. 2. Aquaponic systems are designed tanks required per system, and the to achieve a final density of 0.5 optimum rearing tank size. The num- pound/gallon. Therefore, the ber of harvests will have to be calcu- of the standard design is not recom- water volume of the rearing lated based on the length of the cul- mended because changes often lead tanks is 1,000 gallons. ture period. Assume that the final to unintended consequences. How- density is 0.5 pound/gallon for an 3. In 52 weeks, there will be 8.7 ever, the design process often starts aerated system. Take the annual pro- harvests (52 ÷ 6 = 8.7) per sys- with a production goal for either fish duction per system and multiply it tem. Annual production for the or plants. In those cases there are by the estimated feed conversion system, therefore, is 4,350 some guidelines that can be fol- ratio (the pounds of feed required to pounds (500 pounds per harvest lowed. produce 1 pound of fish). Convert × 8.7 harvests). Use an aquaponic system that is the pounds of annual feed consump- 4. The usual feed conversion ratio already designed. The easiest tion to grams (454 g/lb) and divide by is 1.7. Therefore, annual feed approach is to use a system design 365 days to obtain the average daily input to the system is 7,395 that has been tested and is in com- feeding rate. Divide the average daily pounds (4,350 lb × 1.7 = 7,395 mon use with a good track record. It feeding rate by the desired feeding lb). is early in the development of rate ratio, which ranges from 60 to 5. The average daily feed input is aquaponics, but standard designs will 100 g/m2/day for raft culture, to 20.3 pounds (7,395 lb/year ÷ 365 emerge. The UVI system has been determine the required plant produc- days = 20.3 lb). well documented and is being stud- tion area. For other systems such as ied or used commercially in several NFT, the feeding rate ratio should be 6. The average daily feed input con- locations, but there are other systems decreased in proportion to the water verted to grams is 9,216 g (20.3 with potential. Standard designs will volume reduction of the system as lb × 454 g/lb = 9216 g). include specifications for layout, tank discussed in the component ratio sec- 7. The optimum feeding rate ratio sizes, pipe sizes, pipe placement, tion. Use a ratio near the low end of for raft aquaponics ranges from pumping rates, aeration rates, infra- the range for small plants such as 60 to 100 g/m2/day. Select 80 structure needs, etc. There will be Bibb lettuce and a ratio near the high g/m2/day as the design ratio. operation manuals and projected pro- end of the range for larger plants Therefore, the required lettuce duction levels and budgets for vari- such as Chinese cabbage or romaine growing area is 115.2 m2 (9,216 ous crops. Using a standard design lettuce. The solids removal compo- g/day ÷ 80 g/m2/day =115.2 m2). will reduce risk. nent, water pump and blowers 8. The growing area in square feet Design for available space. If a limited should be sized accordingly is 1,240 (115.2 m2 × 10.76 ft2/m2 amount of space is available, as in an Sample problem: = 1,240 ft2). existing greenhouse, then that space This example illustrates only the will define the size of the aquaponic 9. Select a hydroponic tank width main calculations, which are simpli- system. A standard design can be of 4 feet. The total length of the fied (e.g., mortality is not considered) scaled down to fit the space. If a hydroponic tanks is 310 feet for the sake of clarity. Assume that 2 scaled-down tank or pipe size falls (1,240 ft ÷ 4 ft = 310 ft). you have a market for 500 pounds of between commercially available live tilapia per week in your city and 10. Select four hydroponic tanks. sizes, it is best to select the larger that you want to raise lettuce with They are 77.5 feet long (310 ft ÷ size. However, the water flow rate the tilapia because there is a good 4 = 77.5 ft). They are rounded should equal the scaled-down rate market for green leaf lettuce in your up to 80 feet in length, which is for best results. The desired flow rate area. The key questions are: How a practical length for a standard can be obtained by buying a higher many UVI aquaponic systems do greenhouse and allows the use of capacity pump and installing a you need to harvest 500 pounds of ten 8-foot sheets of polystyrene bypass line and valve, which circu- tilapia weekly? How large should the per hydroponic tank. lates a portion of the flow back to rearing tanks be? What is the appro- 11. Green leaf lettuce produces good results with plant spacing fish production. The required water 68.75 feet (137.5 ft ÷ 2 = 68.75 of 48 plants per sheet (16/m2). volume can be partitioned among ft). Since polystyrene sheets The plants require a 4-week multiple systems and multiple tanks come in 8-foot lengths, the total growth period. With staggered per system with the goal of creating number of sheets per hydropon- production, one hydroponic tank a practical system size and tank ic tank will be 8.59 sheets (68.75 is harvested weekly. Each hydro- array. Divide the desired individual ft ÷ 8 ft/sheet = 8.59 sheets). ponic tank with ten polystyrene fish weight at harvest by 0.5 To avoid wasting material, sheets produces 480 plants. With pound/gallon to determine the vol- round up to nine sheets. six aquaponic production sys- ume of water (in gallons) required Therefore, the hydroponic tanks tems 2,880 plants are harvested per fish. Divide the number of gal- will be 72 feet long (9 sheets × 8 weekly. lons required per fish by the water ft per sheet = 72 ft). In summary, the weekly production volume of the rearing tank to deter- 5. The total plant growing area of 500 pounds of tilapia results in mine the fish stocking rate. Increase will then be 1,152 ft2 (72 ft × 8 the production of 2,880 green leaf this number by 5 to 10 percent to ft per tank × 2 tanks = 1,152 lettuce plants (120 cases). Six allow for expected mortality during ft2). This is equal to 107 m2 aquaponic systems, each with four the production cycle. The solids (1,152 ft2 ÷ 10.76 ft2/m2). removal component, water pump 1,000-gallon rearing tanks (water vol- 6. At a planting density of 29.3 ume), are required. Each system will and blowers should be sized accord- ingly. plants/m2, a total of 3,135 plants have four raft hydroponic tanks that will be cultured in the system. are 80 feet long by 4 feet wide. Sample problem: The extra plants will provide a Design for plant production. If the pri- Assume that there is a market for safety margin against mortality mary objective is to produce a cer- 1,000 Bibb lettuce plants weekly in and plants that do not meet tain quantity of plant crops annually, your city. These plants will be sold marketing standards. the first step in the design process individually in clear, plastic, 7. Assume that a feeding rate of 60 will be to determine the area clamshell containers. A portion of g/m2/day provides sufficient required for plant production. The the root mass will be left intact to nutrients for good plant growth. area needed will be based on plant extend self life. Bibb lettuce trans- Therefore, daily feed input to spacing, length of the production plants are cultured in a UVI raft sys- the system will be 6,420 g (60 cycle, number of crops per year or tem for 3 weeks at a density of 29.3 g/m2/day 107 m2 = 6,420 g). 2 × growing season, and the estimated plants/m . Assume that tilapia will This is equal to 14.1 pounds of yield per unit area and per crop be grown in this system. The key feed (6,420 g ÷ 454 g/lb = 14.1 cycle. Select the desired feeding rate questions are: How large should the lb). ratio and multiple by the total area plant growing area be? What will be to obtain the average daily feeding the annual production of tilapia? 8. Annual feed input to the system rate required. Multiply the average How large should the fish-rearing will be 5,146 pounds (14.1 daily feeding rate by 365 days to tanks be? lb/day × 365 days = 5,146 lb) determine annual feed consumption. 1. Bibb lettuce production will be 9. Assume the feeding conversion Estimate the feed conversion ratio staggered so that 1,000 plants ratio is 1.7. Therefore, the feed (FCR) for the fish species that will be can be harvested weekly. conversion efficiency is 0.59 (1 cultured. Convert FCR to feed con- Therefore, with a 3-week grow- lb of gain ÷ 1.7 lb of feed = version efficiency. For example, if ing period, the system must 0.59). FCR is 1.7:1, then the feed conver- accommodate the culture of 10. The total annual fish produc- sion efficiency is 1 divided by 1.7 or 3,000 plants. tion gain will be 3,036 pounds 0.59. Multiply the annual feed con- 2. At a density of 29.3 plants/m2, (5,146 lb × 0.59 feed conversion sumption by the feed conversion efficiency = 3,036 lb). efficiency to determine net annual the total plant growing area will fish yield. Estimate the average fish be 102.3 m2 (3,000 plants ÷ 11. Assume that the desired harvest weight at harvest and subtract the 29.3/m2 = 102.3 m2). This area weight of the fish will be 500 g anticipated average fingerling weight is equal to 1,100 square feet (1.1 lb) and that 50-g (0.11-lb) at stocking. Divide this number into (102.3 m2 × 10.76 ft2/m2 = 1,100 fingerlings will be stocked. the net annual yield to determine the ft2). Therefore, individual fish will total number of fish produced annu- 3. Select a hydroponic tank width gain 450 g (500 g harvest weight ally. Multiply the total number of of 8 feet. The total hydroponic - 50 g stocking weight = 450 g). fish produced annually by the esti- tank length will be 137.5 feet The weight gain per fish will be mated harvest weight to determine (1,100 ft2/8 ft = 137.5 ft). approximately 1 pound (454 g). total annual fish production. Divide 4. Multiples of two raft hydroponic 12. The total number of fish har- total annual fish production by the tanks are required for the UVI vested will be 3,036 (3,036 lb of number of production cycles per system. In this case only two total gain ÷ 1 lb of gain per fish year. Take this number and divide by hydroponic tanks are required. = 3,036 fish). 0.5 pound/gallon to determine the Therefore, the minimum length 13. Total annual production will be total volume that must be devoted to of each hydroponic tank will be 3,340 pounds (3,036 fish × 1.1 promising based on studies with the lb/fish = 3,340 lb) when the ini- UVI system in the Virgin Islands and tial stocking weight is considered. in Alberta, Canada. 14. If there are four fish-rearing The UVI system is capable of produc- tanks and one tank is harvested ing approximately 11,000 pounds of every 6 weeks, there will be 8.7 tilapia and 1,400 cases of lettuce or harvests per year (52 weeks ÷ 6 11,000 pounds of basil annually weeks = 8.7). based on studies in the Virgin 15. Each harvest will be 384 pounds Islands. Enterprise budgets for tilapia production combined with either let- (3,340 lb per year ÷ 8.7 harvests Figure 11. Basil production in the UVI per year = 384 lb/harvest). tuce or basil have been developed. The U.S. Virgin Islands represent a aquaponic system. 16. Final harvest density should not small niche market with very high exceed 0.5 pound/gallon. prices for fresh tilapia, lettuce and Therefore, the water volume of basil, as more than 95 percent of veg- demand. The population (108,000 each rearing tank should be 768 etable supplies and nearly 80 percent people) of the U.S. Virgin Islands gallons (384 lb ÷ 0.5 lb/gal = of fish supplies are imported. The cannot absorb 66,000 pounds of fresh 768 gal). The tank should be larg- budgets were prepared to show rev- basil annually, although there are er to provide a 6-inch freeboard enues, costs and profits from six pro- opportunities for provisioning ships (space between the top edge of duction units. A commercial enter- and exporting to neighboring islands. the tank and the water levels). prise consisting of six production A more realistic approach for a six- 17. Each fish requires 2.2 gallons of units is recommended because one unit operation is to devote a portion water (1.1 lb ÷ 0.5 lb of fish/gal fish-rearing tank (out of 24) could be of the growing area to basil to meet = 2.2 gal per fish). harvested weekly, thereby providing local demand while growing other crops in the remainder of the system. 18. The stocking rate is 349 fish per a continuous supply of fish for mar- tank (768 gal ÷ 2.2 gal/fish = ket development. The break-even price for the 349 fish). The enterprise budget for tilapia and aquaponic production of tilapia in the Virgin Islands is $1.47/pound, com- 19. To account for calculated mortali- lettuce shows that the annual return to risk and management (profit) for pared to a sale price of $2.50/pound. ty, the stocking rate (349 fish per The break-even prices are $6.15/case tank) should be increased by 35 six production units is US$185,248. The sale prices for fish ($2.50/lb) and for lettuce (sale price = $20.00/case) fish (349 fish × 0.10 = 34.9) to lettuce ($20.00/case) have been estab- and $0.75/pound for basil (sale price attain an actual stocking of 384 = $10.00/pound). The break-even fish per tank. lished through many years of market research at UVI. Most of the lettuce prices for tilapia and lettuce do not In summary, two hydroponic tanks consumed in the Virgin Islands is compare favorably to commodity (each 72 feet long by 8 feet wide) will imported from California. It is trans- prices. However, the cost of construc- be required to produce 1,000 Bibb ported by truck across the United tion materials, electricity, water, labor lettuce plants per week. Four fish- States to East Coast ports and then and land are very high in the U.S. rearing tanks with a water volume of shipped by ocean freighters to Virgin Islands. Break-even prices for 768 gallons per tank will be required. Caribbean islands. Local production tilapia and lettuce could be consider- The stocking rate will be 384 fish per capitalizes on the high price of ably lower in other locations. The tank. Approximately 384 pounds of imports caused by transportation break-even price for basil compares tilapia will be harvested every 6 costs. Locally produced lettuce is also favorably to commodity prices weeks, and annual tilapia production fresher than imported lettuce. because fresh basil has a short shelf will be 3,340 pounds. Although this enterprise budget is life and cannot be shipped great dis- unique to the U.S. Virgin Islands, it tances. Economics indicates that aquaponic systems can A UVI aquaponic system in an envi- The economics of aquaponic systems be profitable in certain niche mar- ronmentally controlled greenhouse at depends on specific site conditions kets. the Crops Diversification Center and markets. It would be inaccurate The enterprise budget for tilapia and South in Alberta, Canada, was evalu- to make sweeping generalizations basil shows that the annual return to ated for the production of tilapia and because material costs, construction risk and management for six produc- a number of plant crops. The crops costs, operating costs and market tion units is US$693,726. Aquaponic were cultured for one production prices vary by location. For example, systems are very efficient in produc- cycle and their yields were extrapo- an outdoor tropical system would be ing culinary herbs such as basil (Fig. lated to annual production levels. less expensive to construct and oper- 11) and a conservative sale price for Based on prices at the Calgary whole- ate than a controlled-environment fresh basil with stems in the U.S. sale market, annual gross revenue greenhouse system in a temperate cli- Virgin Islands is $10.00/pound. was determined for each crop per mate. Nevertheless, the economic However, this enterprise budget is unit area and per system with a plant potential of aquaponic systems looks not realistic in terms of market growing area of 2,690 ft2 (Table 2). Table 2. Preliminary production and economic data from the UVI aquaponic system at the Crop Diversification Center South, Alberta, Canada.1 (Data courtesy of Dr. Nick Savidov) Annual production Wholesale price Total value Crop lb/ft2 tons/2690 ft2 Unit $ $/ft2 $/2690 ft2 Tomatoes 6.0 8.1 15 lb 17.28 6.90 18,542 Cucumbers 12.4 16.7 2.2 lb 1.58 8.90 23,946 Eggplant 2.3 3.1 11 lb 25.78 5.33 14,362 Genovese basil 6.2 8.2 3 oz 5.59 186.64 502,044 Lemon basil 2.7 3.6 3 oz 6.31 90.79 244,222 Osmin basil 1.4 1.9 3 oz 7.03 53.23 143,208 Cilantro 3.8 5.1 3 oz 7.74 158.35 425,959 Parsley 4.7 6.3 3 oz 8.46 213.81 575,162 Portulaca 3.5 4.7 3 oz9.17 174.20 468,618 1Ecomonic data based on Calgary wholesale market prices for the week ending July 4, 2003.

Annual production levels based on current market suppliers will also quality. Operating small aquaponic extrapolated data from short produc- lead to price reductions. systems can be an excellent hobby. tion cycles are subject to variation. Systems can be as small as an Similarly, supply and demand will Overview aquarium with a tray of plants cov- cause wholesale prices to fluctuate Although the design of aquaponic ering the top. Large commercial during the year. Nevertheless, the systems and the choice of hydropon- operations comprised of many pro- data indicate that culinary herbs in ic components and fish and plant duction units and occupying several general can produce a gross income combinations may seem challenging, acres are certainly possible if mar- more than 20 times greater than that aquaponic systems are quite simple kets can absorb the output. The edu- of fruiting crops such as tomatoes to operate when fish are stocked at a cational potential of aquaponic sys- and cucumbers. It appears that just rate that provides a good feeding tems is already being realized in one production unit could provide a rate ratio for plant production. hundreds of schools where students livelihood for a small producer. Aquaponic systems are easier to learn a wide range of subjects by However, these data do not show operate than hydroponic systems or constructing and operating aquapon- capital, operating and marketing recirculating fish production systems ic systems. Regardless of scale or costs, which will be considerable. because they require less monitoring purpose, the culture of fish and Furthermore, the quantity of herbs and usually have a wider safety plants through aquaponics is a grati- produced could flood the market and margin for ensuring good water fying endeavor that yields useful depress prices. Competition from products—food.

The information given herein is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by the Southern Regional Aquaculture Center or the Cooperative Extension Service is implied.

SRAC fact sheets are reviewed annually by the Publications, Videos and Computer Software Steering Committee. Fact sheets are revised as new knowledge becomes available. Fact sheets that have not been revised are considered to reflect the current state of knowledge.

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 2003-38500-12997 from the United States Department of Agriculture, Cooperative State Research, Education, and Extension Service.