Recirculating Aquaculture Tank Production Systems by fdh56iuoui

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									                                                                                                          SRAC Publication No. 451

                                         September 1998                                                      VI

                     Recirculating Aquaculture Tank
                          Production Systems
                        An Overview of Critical Considerations
                                Thomas M. Losordo1, Michael P. Masser2 and James Rakocy3

Traditional aquaculture produc-                   ally use tanks for aquaculture pro-      Critical production
tion in ponds requires large quan-                duction, substantially less land is
tities of water. Approximately 1                  required.
million gallons of water per acre                                                          All aquaculture production sys-
                                                  Aquatic crop production in tanks
are required to fill a pond and an                                                         tems must provide a suitable
                                                  and raceways where the environ-
equivalent volume is required to                                                           environment to promote the
                                                  ment is controlled through water
compensate for evaporation and                                                             growth of the aquatic crop.
                                                  treatment and recirculation has
seepage during the year.                                                                   Critical environmental parameters
                                                  been studied for decades.
Assuming an annual pond yield                                                              include the concentrations of dis-
                                                  Although these technologies have
of 5,000 pounds of fish per acre,                                                          solved oxygen, un-ionized ammo-
                                                  been costly, claims of impressive
approximately 100 gallons of                                                               nia-nitrogen, nitrite-nitrogen, and
                                                  yields with year-round production
water are required per pound of                                                            carbon dioxide in the water of the
                                                  in locations close to major markets
fish production. In many areas of                                                          culture system. Nitrate concentra-
                                                  and with extremely little water
the United States, traditional                                                             tion, pH, and alkalinity levels
                                                  usage have attracted the interest
aquaculture in ponds is not possi-                                                         within the system are also impor-
                                                  of prospective aquaculturists. In
ble because of limited water sup-                                                          tant. To produce fish in a cost-
                                                  recent years, a variety of produc-
plies or an absence of suitable                                                            effective manner, aquaculture pro-
                                                  tion facilities that use recirculating
land for pond construction.                                                                duction systems must maintain
                                                  technology have been built.
Recirculating aquaculture produc-                 Results have been mixed. While           good water quality during peri-
tion systems may offer an alterna-                there have been some notable             ods of rapid fish growth. To
tive to pond aquaculture technolo-                large-scale business failures in this    ensure such growth, fish are fed
gy. Through water treatment and                   sector, numerous small- to medi-         high-protein pelleted diets at rates
reuse, recirculating systems use a                um-scale efforts continue produc-        ranging from 1.5 to 15 percent of
fraction of the water required by                 tion.                                    their body weight per day
ponds to produce similar yields.                                                           depending upon their size and
                                                  Prospective aquaculturists and           species (15 percent for juveniles,
Because recirculating systems usu-
                                                  investors need to be aware of the        1.5 percent for market size).
                                                  basic technical and economic risks
1Department of Zoology, North Carolina            involved in this type of aquacul-        Feeding rate, feed composition,
 State University, North Carolina                 ture production technology. This         fish metabolic rate and the quanti-
2Department of Fisheries and Allied               fact sheet and others in this series     ty of wasted feed affect tank water
 Aquaculture, Auburn University,                  are designed to provide basic            quality. As pelleted feeds are
 Alabama                                          information on recirculating aqua-       introduced to the fish, they are
3University of the Virgin Islands,                                                         either consumed or left to decom-
                                                  culture technology.
 Agricultural Experiment Station, U.S.                                                     pose within the system. The by-
 Virgin Islands
products of fish metabolism            body weight per day, then 37.5        cost-effective water treatment sys-
include carbon dioxide, ammo-          pounds of feed would produce          tem components. All recirculating
nia-nitrogen, and fecal solids. If     approximately 1.1 pounds of           production systems remove waste
uneaten feeds and metabolic by-        ammonia-nitrogen per day.             solids, oxidize ammonia and
products are left within the cul-      (Approximately 3 percent of the       nitrite-nitrogen, remove carbon
ture system, they will generate        feed becomes ammonia-nitrogen.)       dioxide, and aerate or oxygenate
additional carbon dioxide and          Additionally, if the ammonia-         the water before returning it to
ammonia-nitrogen, reduce the           nitrogen concentration in the tank    the fish tank (see Fig. 1). More
oxygen content of the water, and       is to be maintained at 1.0 mg/l,      intensive systems or systems cul-
have a direct detrimental impact       then a mass balance calculation on    turing sensitive species may
on the health of the cultured          ammonia-nitrogen indicates that       require additional treatment
product.                               the required flow rate of new         processes such as fine solids
In aquaculture ponds, proper           water through the tank would be       removal, dissolved organics
environmental conditions are           approximately 5,600 gallons per       removal, or some form of disinfec-
maintained by balancing the            hour (93 gpm) to maintain the         tion.
inputs of feed with the assimila-      specified ammonia-nitrogen con-
tive capacity of the pond. The         centration. Even at this high flow    Waste solids constraints
pondÕs natural biological produc-      rate, the system also would
                                       require aeration to supplement        Pelleted feeds used in aquaculture
tivity (algae, higher plants, zoo-                                           production consist of protein, car-
plankton and bacteria) serves as a     the oxygen added by the new
                                       water.                                bohydrates, fat, minerals and
biological filter that processes the                                         water. The portion not assimilat-
wastes. As pond production                                                   ed by the fish is excreted as a
intensifies and feed rates increase,   Recirculating systems                 highly organic waste (fecal solids).
supplemental and/or emergency          design                                When broken down by bacteria
aeration are required. At higher                                             within the system, fecal solids and
rates of feeding, water must be        Recirculating production technol-
                                       ogy is most often used in tank        uneaten feed will consume dis-
exchanged to maintain good                                                   solved oxygen and generate
water quality. The carrying capac-     systems because sufficient water
                                       is not available on site to ÒwashÓ    ammonia-nitrogen. For this rea-
ity of ponds with supplemental                                               son, waste solids should be
aeration is generally considered       fish wastes out of production
                                       tanks in a flow-through configura-    removed from the system as
to be 5,000 to 7,000 pounds of fish                                          quickly as possible. Waste solids
per acre (0.005 to 0.007 pound of      tion or production system that
                                       uses water only once. In most         can be classified into four cate-
fish per gallon of pond water).                                              gories: settleable, suspended,
                                       cases, a flow-through requirement
The carrying capacity of tank sys-     of nearly 100 gallons per minute      floatable and dissolved solids. In
tems must be high to provide for       to maintain one production tank       recirculating systems, the first two
cost-effective fish production         would severely limit production       are of primary concern. Dissolved
because of the higher initial capi-    capacity. By recirculating tank       organic solids can become a prob-
tal costs of tanks compared to         water through a water treatment       lem in systems with very little
earthen ponds. Because of this         system that ÒremovesÓ ammonia         water exchange.
expense and the limited capacity       and other waste products, the         Settleable solids control:
of the ÒnaturalÓ biological filtra-    same effect is achieved as with the   Settleable solids are generally the
tion of a tank, the producer must      flow-through configuration. The       easiest of the four categories to
rely upon the flow of water            efficiency with which the treat-      deal with and should be removed
through the tanks to wash out the      ment system ÒremovesÓ ammonia         from the tank and filtration com-
waste by-products. Additionally,       from the system, the ammonia          ponents as rapidly as possible.
the oxygen concentration within        production rate, and the desired      Settleable solids are those that will
the tank must be maintained            concentration of ammonia-nitro-       generally settle out of the water
through continuous aeration,           gen within the tank determine the     within 1 hour under still condi-
either with atmospheric oxygen         recirculating flow rate from the      tions. Settleable solids can be
(air) or pure gaseous oxygen.          tank to the treatment unit. Using     removed as they accumulate on
The rate of water exchange             the example outlined above, if a      the tank bottom through proper
required to maintain good water        treatment system removes 50 per-      placement of drains, or they can
quality in tanks is best described     cent of the ammonia-nitrogen in       be kept in suspension with contin-
using an example. Assume that a        the water on a single pass, then      uous agitation and removed with
5,000-gallon production tank is to     the flow rate from the tank would     a sedimentation tank (clarifier),
be maintained at a culture density     need to be twice the flow required    mechanical filter (granular or
of 0.5 pound of fish per gallon of     if fresh water were used to flush     screen), or swirl separator. The
tank volume. If the 2,500 pounds       the tank (93 gpm/0.5 = 186 gpm).      sedimentation and swirl separator
of fish are fed a 32% protein feed     A key to successful recirculating     processes can be enhanced by
at a rate of 1.5 percent of their      production systems is the use of      adding steep incline tubes (tube
          Fine & Dissolved                                                                     Carbon Dioxide
           Solids Removal                                                                         Removal

          Foam fractionation                                                                  Air stone diffuser
                                                                                               Packed column
                                                    Fish Culture Tank
                                                    Round, Octagonal,
                                                      Rectangular or
             Aeration or                                 D-ended                                 Disinfection
                                                                                               Ultraviolet light
          Air stone diffuser                                                                   Ozone contact
           Packed column
        Down-flow contactor
        Low head oxygenator

                              Waste Solids Removal                   Biological Filtration
                                 Swirl separators                     Fluidized bed filters
                                   Screen filters                      Mixed bed filters
                                    Bead filters                        Trickling filters
                                  Double drain                       Rotating bio contactor

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

settlers) in the sedimentation tank        devices see SRAC 453,                      that creates foam at the top
to reduce flow turbulence and              Recirculating Aquaculture Tank             air/water interface. As the bub-
promote uniform flow distribu-             Production Systems: A Review of            bles rise through the water col-
tion.                                      Component Options.                         umn, solid particles attach to the
                                                                                      bubblesÕ surfaces, forming the
Suspended solids control: From             Fine and dissolved solids                  foam at the top of the column.
an aquacultural engineering point          control: Fine suspended solids             The foam build-up is then chan-
of view, the difference between            (< 30 micrometers) have been               nelled out of the fractionation unit
suspended solids and settleable            shown to contribute more than              to a waste collection tank. Solids
solids is a practical one.                 50 percent of the total suspended          concentration in the waste tank
Suspended solids will not settle to        solids in a recirculating system.          can be five times higher than that
the bottom of the fish culture tank        Fine suspended solids increase the         of the culture tank. Although the
and cannot be removed easily in            oxygen demand of the system and            efficiency of foam fractionation is
conventional settling basins.              cause gill irritation and damage in        subject to the chemical properties
Suspended solids are not always            finfish. Dissolved organic solids          of the water, the process generally
dealt with adequately in a recircu-        (protein) can contribute signifi-          can be used to significantly reduce
lating production system. If not           cantly to the oxygen demand of             water turbidity and oxygen
removed, suspended solids can              the total system.                          demand of the culture system.
significantly limit the amount of          Fine and dissolved solids cannot
fish that can be grown in the sys-         be easily or economically                  Nitrogen constraints
tem and can irritate the gills of          removed by sedimentation or
fish. The most popular treatment                                                      Total ammonia-nitrogen (TAN),
                                           mechanical filtration technology.
method for removing suspended                                                         consisting of un-ionized ammonia
                                           Foam fractionation (also referred
solids generally involves some                                                        (NH3) and ionized ammonia
                                           to as protein skimming) is suc-
form of mechanical filtration. The                                                    (NH4+), is a by-product of protein
                                           cessful in removing these solids
two types of mechanical filtration                                                    metabolism. TAN is excreted from
                                           from recirculating tank systems.
most commonly used are screen                                                         the gills of fish as they assimilate
                                           Foam fractionation, as employed
filtration and granular media fil-                                                    feed and is produced when bacte-
                                           in aquaculture, is a process of
tration (sand or pelleted media).                                                     ria decompose organic waste
                                           introducing air bubbles at the bot-
For more information on these                                                         solids within the system. The un-
                                           tom of a closed column of water
                                                                                      ionized form of ammonia-nitro-
gen is extremely toxic to most          concentrations should not exceed          ner in which it comes into contact
fish. The fraction of TAN in the        10 mg/l for long periods of time          with wastewater define the water
un-ionized form is dependent            and in most cases should remain           treatment characteristics of the
upon the pH and temperature of          below 1 mg/l.                             biological filtration unit. The most
the water. At a pH of 7.0, most of      Nitrates are not generally of great       common configurations for bio-
the TAN is in the ionized form,         concern to the aquaculturist.             logical filters include rotating bio-
while at a pH of 8.75 up to 30 per-     Studies have shown that aquatic           logical contactors (RBC), fixed
cent of TAN is in the un-ionized        species can tolerate extremely            film reactors, expandable media
form. While the lethal concentra-       high levels (> 200 mg/l) of               filters, and mixed bed reactors.
tion of ammonia-nitrogen for            nitrate-nitrogen in production sys-       For more information on biologi-
many species has been estab-            tems. Nitrate-nitrogen concentra-         cal filters and components see
lished, the sub-lethal effects of       tions do not generally reach such         SRAC 453, Recirculating
ammonia-nitrogen have not been          high levels in recirculating sys-         Aquaculture Tank Production
well defined. Reduction in growth       tems. Nitrate-nitrogen is either          Systems: A Review of Component
rates may be the most important         flushed from a system during sys-         Options.
sub-lethal effect. In general, the      tem maintenance operations (such
concentration of un-ionized             as settled solids removal or filter       pH and alkalinity constraints
ammonia-nitrogen in tanks should        backwashing), or denitrification
not exceed 0.05 mg/l.                                                             The measure of the hydrogen ion
                                        occurs within a treatment system          (H+) concentration, or pH, in
Nitrite-nitrogen (NO2- ) is a prod-     component such as a settling tank.        water indicates the degree to
uct of the oxidation of ammonia-        Denitrification occurs when anaer-        which water is either acidic or
nitrogen. Nitrifying bacteria           obic bacteria metabolize nitrate-         basic. The pH of water affects
(Nitrosomonas) in the production        nitrogen to produce nitrogen gas          many other water quality parame-
system utilize ammonia-nitrogen         that is released to the atmosphere        ters and the rates of many biologi-
as an energy source for growth          during the aeration process. For          cal and chemical processes. Thus,
and produce nitrite-nitrogen as a       more information on the effects of        pH is considered an important
by-product. These bacteria are the      water quality on fish production,         parameter to be monitored and
basis for biological filtration. The    see SRAC 452, Recirculating               controlled in recirculating aqua-
nitrifying bacteria grow on the         Aquaculture Tank Production               culture systems. Alkalinity is a
surface of the biofilter substrate      Systems: Management of                    measure of the waterÕs capacity to
although all tank production sys-       Recirculating Systems.                    neutralize acidity (hydrogen ions).
tem components will have nitrify-       Ammonia and nitrite-nitrogen              Bicarbonate (HCO3-) and carbon-
ing bacteria present to some            control: Controlling the concen-          ate (CO3-) are the predominant
extent. While nitrite-nitrogen is       tration of un-ionized ammonia-            bases or sources of alkalinity in
not as toxic as ammonia-nitrogen,       nitrogen (NH3) in the culture tank        most waters. Highly alkaline
it is harmful to aquatic species        is a primary objective of recircu-        waters are more strongly buffered
and must be controlled within the       lating treatment system design.           against pH change than less alka-
tank.                                   Ammonia-nitrogen must be                  line waters.
Nitrite-nitrogen binds with hemo-       ÒremovedÓ from the culture tank           Nitrification is an acid-producing
globin to produce methemoglo-           at a rate equal to the rate of pro-       process. As ammonia-nitrogen is
bin. Methemoglobin is not capable       duction to maintain a safe concen-        transformed to nitrate-nitrogen by
of binding and transporting oxy-        tration. While there are a number         nitrifying bacteria, hydrogen ions
gen and the affected fish become        of technologies available for             are produced. As hydrogen ions
starved for oxygen. The toxicity of     removing ammonia-nitrogen from            combine with bases such as
nitrite-nitrogen is species specific.   water, biological filtration is the       hydroxide (OH-), carbonate and
However, a common practice for          most widely used. In biological           bicarbonate, alkalinity is con-
reducing nitrite-nitrogen toxicity      filtration (also referred to as biofil-   sumed and the pH decreases.
is to increase the chloride concen-     tration), there is a substrate with a     Levels of pH below 4.5 are dan-
tration of the culture water. Main-     large surface area where nitrifying       gerous to fish; a pH below 7.0 will
taining a chloride to nitrite-nitro-    bacteria can attach and grow. As          reduce the activity of nitrifying
gen ratio of 10:1 generally will        previously noted, ammonia and             bacteria. If the source water for a
protect against methemoglobin           nitrite-nitrogen in the recycle           recirculating system is low in
build-up and nitrite-nitrogen toxi-     stream are oxidized to nitrite and        alkalinity, then pH and alkalinity
city. Fortunately, Nitrobacter bacte-   nitrate-nitrogen by Nitrosomonas          should be monitored and alkalini-
ria, which also are present in most     and Nitrobacter bacteria, respec-         ty must be maintained with addi-
biological filters, utilize nitrite-    tively. Gravel, sand, plastic beads,      tions of bases. Some bases com-
nitrogen as an energy source and        plastic rings, plastic tubes, and         monly used include hydrated lime
produce nitrate as a by-product.        plastic plates are common biofil-         [Ca(OH)2] quick lime (CaO), and
In a recirculating system with a        tration substrates. The configura-        sodium bicarbonate (NaHCO3).
mature biofilter, nitrite-nitrogen      tion of the substrate and the man-
Dissolved gas constraints              20 mg/l to maintain good grow-         are an effective and simple means
                                       ing conditions.                        of aerating water that is already in
Although ammonia-nitrogen
                                       The build-up of dissolved nitro-       a flow-stream. In a PCA, water
build-up can severely limit a recir-
                                       gen gas is rarely a problem in         low in oxygen is introduced into a
culating systemÕs carrying capaci-
                                       warm water aquaculture systems.        small tower filled with plastic
ty, maintaining adequate dis-
                                       However, caution is advised            medium. A perforated plate or
solved oxygen (DO) concentra-
                                       when pressurized aeration or oxy-      spray nozzle evenly distributes
tions in the culture tank and filter
                                       genation systems are used              the incoming water over the
system also is of critical impor-
                                       because atmospheric nitrogen can       medium. The packed column is
tance. In most cases, a systemÕs
                                       become supersaturated in water if      operated under non-flooded con-
ability to add dissolved oxygen to
                                       air is entrained into the pressur-     ditions so that air exchange
water will become the first limit-
                                       ized flow stream. Aquatic organ-       through the tower is maintained.
ing factor in a systemÕs fish carry-
                                       isms subjected to elevated concen-     If the PCA is to be used for carbon
ing capacity. To maintain ade-
                                       trations of dissolved nitrogen gas     dioxide stripping, a low pressure
quate DO levels in the culture
                                       can develop Ògas bubblesÓ in           air blower will be required to pro-
tank, oxygen must be added to
                                       their circulatory systems and die.     vide a large quantity of air flow
the tank at a rate equal to that of
                                                                              through the packed medium.
the rate of consumption by fish        Maintaining adequate dissolved
and bacteria. The consumption          oxygen levels and minimizing           A number of recirculating system
rate of dissolved oxygen in a recir-   carbon dioxide concentrations in       designs use air-lift pumps (verti-
culating system is difficult to cal-   the culture tank cannot be over-       cal pipes with air injection) to
culate, yet an estimate is essential   looked in recirculating system         recycle water through treatment
for proper system design. The          design. In a typical intensively       processes and back to the culture
overall rate of oxygen consump-        loaded recirculating system, aera-     tank. Air lifts agitate the water
tion for a system is the sum of the    tion or oxygenation system failure     with air bubbles in the process
respiration rate of the fish, the      can lead to a total loss of the fish   and remove CO2 and add dis-
oxygen demand of bacteria break-       crop in 1/2 hour or less.              solved oxygen.
ing down organic wastes and                                                   Pure Oxygen Injection: In inten-
                                       Aeration and Degassing: The
uneaten food (also referred to as                                             sive production systems, the rate
                                       addition of atmospheric oxygen
Biochemical Oxygen Demand or                                                  of oxygen consumption by the
                                       to water or the release of excess
BOD), and the oxygen demand of                                                fish and bacteria may exceed the
                                       carbon dioxide from water can be
nitrifying bacteria in the filter.                                            capabilities of typical aeration
                                       accomplished in recirculating sys-
The amount of oxygen required                                                 equipment to diffuse atmospheric
                                       tems through a variety of devices
by the system is largely dictated                                             oxygen into the water. In these
                                       such as air diffusers, surface agi-
by the length of time waste solids                                            cases, pure gaseous oxygen diffu-
                                       tators, and pressurized or non-
remain within the system and the                                              sion is used to increase the rate of
                                       pressurized packed columns.
biofilter configuration. In systems                                           oxygen addition and allow for a
                                       System aeration is commonly car-
with non-submerged biofilters,                                                higher oxygen utilization rate.
                                       ried out in the culture tanks,
where solids are removed quickly,                                             The saturation concentration of
                                       although this is not a particularly
as little as 0.3 pound of oxygen                                              atmospheric oxygen in water
                                       good place to add dissolved oxy-
can be consumed for every pound                                               rarely exceeds 8.75 mg/l in warm
                                       gen. This is because the oxygen
of feed added. In systems with                                                water applications (> 20o C).
                                       transfer efficiency of aerators
submerged biological filters,                                                 When pure oxygen is used with
                                       drops as the concentration of dis-
where solids are retained within                                              gas diffusion systems, the satura-
                                       solved oxygen increases to near
the system between backwashings                                               tion concentration of oxygen in
                                       saturation levels in the tank
of solid-removing filters, as much                                            water is increased nearly five fold
                                       water. Because saturated condi-
as 0.75 pound of oxygen will be                                               to 43 mg/l at standard atmos-
                                       tions are desirable in the culture
consumed for every pound of                                                   pheric pressure. This condition
                                       tank, aeration in this location is
feed added.                                                                   allows for more rapid transfer of
                                       extremely inefficient.
Carbon dioxide (CO2) is a by-                                                 oxygen into water even when the
                                       In recirculating systems, a better     ambient tank dissolved oxygen
product of fish and bacterial respi-
                                       place to aerate and degas water is     concentration is maintained close
ration and it can accumulate with-
                                       in the recycled flow-stream just       to atmospheric saturation (> 7
in recirculating systems. Elevated
                                       prior to re-entry into the culture     mg/l).
carbon dioxide concentrations in
                                       tank. At this location, in systems
the water are not highly toxic to                                             A measure of success in using
                                       using submerged biological filtra-
fish when sufficient dissolved                                                pure oxygen in aquaculture is the
                                       tion, the concentration of dis-
oxygen is present. However, for                                               oxygen absorption efficiency of
                                       solved oxygen should be at its
most species, free carbon dioxide                                             the injection or diffusion equip-
                                       lowest and carbon dioxide con-
concentrations in the culture tank                                            ment. The absorption efficiency is
                                       centration will be at its highest.
should be maintained at less than                                             defined as the ratio of the weight
                                       Packed column aerators (PCAs)
of oxygen absorbed by the water                 live or on ice to local niche mar-               nomic evaluation. Construction
to the weight of oxygen applied                 kets. These high-priced markets                  costs of pond production systems
through the diffusion or injection              appear to be necessary for finan-                in the Southeast are approximate-
equipment. Properly designed                    cial success due to the high cost of             ly 90 cents per pound of annual
oxygen diffusion devices can pro-               fish production in recirculating                 production. Recirculating sys-
duce an oxygen absorption effi-                 systems. In fact, the variable costs             tems, on the other hand, cost
ciency of more than 90 percent.                 (feed, fingerling, electricity and               between $1 and $4 per pound of
However, as with tank aeration                  labor) of producing fish in recir-               annual production. A $1 increase
(with air), the culture tank is not             culating systems is not much dif-                in investment cost per pound of
the best location for oxygen diffu-             ferent than other production                     annual production can add more
sion with common Òair stoneÓ                    methods. Where pond culture                      than 10 cents per pound to the
diffusers. Because of the short                 methods require a great deal of                  production cost of fish.
contact time of bubbles rising                  electricity (at least 1 kW per acre
                                                                                                 Given these conditions, producers
through a shallow (< 6 feet) water              of pond) for aeration during the
                                                                                                 using recirculating technology
column in tanks, air stone dif-                 summer months, recirculating
                                                                                                 generally do not attempt to com-
fusers have oxygen absorption                   systems have a steady electrical
                                                                                                 pete in the same markets as pond
efficiencies of not greater than 40             load over the entire year. While it
                                                                                                 producers. However, in specialty
percent. Efficient oxygen injection             may appear that recirculating sys-
                                                                                                 high-value niche markets, such as
systems are designed to maxi-                   tems require more labor in system
                                                                                                 gourmet foods, tropical or orna-
mize both the oxygen/water con-                 upkeep and maintenance than
                                                                                                 mental fish, or year-round supply
tact area and time. This can be                 ponds, when the long hours of
                                                                                                 of fresh product, recirculating sys-
achieved through the use of a                   nightly labor for checking oxygen
                                                                                                 tem products are finding a place.
counter-current contact column, a               in ponds and moving emergency
                                                                                                 The key to niche market success is
closed packed-column contact                    aerators and harvest effort are
                                                                                                 to identify the market size and
unit, a u-tube column or a down-                considered, the difference is mini-
                                                                                                 meet commitments before market
flow bubble contactor. For more                 mal. Recirculating systems can
                                                                                                 expansion. In most cases, niche
information on aeration and oxy-                actually have an advantage in
                                                                                                 markets will limit the size of the
genation equipment see SRAC                     reducing feed costs. Tank produc-
                                                                                                 production units.
453, Recirculating Aquaculture Tank             tion systems generally yield better
Production Systems: Component                   feed conversion ratios than pond                 Before investing in recirculating
Options.                                        systems.                                         systems technology, the prospec-
                                                                                                 tive aquaculturist should visit a
                                                Why, then, are production costs
Other production                                                                                 commercial system and learn as
                                                generally higher for recirculating
                                                                                                 much about the technology as
considerations                                  systems? The answer usually can
                                                                                                 possible. As in all aquaculture
                                                be found when comparing the
There have not been many well-                                                                   enterprises, the decision to begin
                                                capital cost of these systems.
documented successes in large-                                                                   production and the size of the
scale fish production in recirculat-            Comparing the investment costs                   production unit one chooses
ing systems. Most reports of suc-               of recirculating systems with                    should be based on the market.
cessful production have been                    other production methods is criti-
from producers who supply fish                  cal in making an informed eco-

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045 from
the United States Department of Agriculture, Cooperative States Research, Education, and Extension Service.
                                                                                                      SRAC Publication No. 452

                                            March 1999

                      Recirculating Aquaculture Tank
                           Production Systems
                          Management of Recirculating Systems
                                     Michael P. Masser1, James Rakocy2 and Thomas M. Losordo3

Recirculating systems for holding                Production Systems: Component          bacteria and algae, which prolifer-
and growing fish have been used                  Options.                               ate in response to high levels of
by fisheries researchers for more                Recirculating systems are mechan-      nutrients and organic matter. This
than three decades. Attempts to                  ically sophisticated and biological-   can cause increases or decreases in
advance these systems to com-                    ly complex. Component failures,        tank water levels, reduce aeration
mercial scale food fish production               poor water quality, stress, dis-       efficiency, and reduce biofilter effi-
have increased dramatically in the               eases, and off-flavor are common       ciency. Flow rate reduction can be
last decade. The renewed interest                problems in poorly managed             avoided or mitigated by using
in recirculating systems is due to               recirculating systems.                 oversized pipe diameters and con-
their perceived advantages,                      Management of these systems            figuring system components to
including: greatly reduced land                  takes education, expertise and         shorten piping distances. The
and water requirements; a high                   dedication.                            fouling of pipes leaving tanks (by
degree of environmental control                                                         gravity flow) is easily observed
allowing year-round growth at                    Recirculating systems are biologi-     because of the accompanying rise
optimum rates; and the feasibility               cally intense. Fish are usually        in tank water level. If flow rates
of locating in close proximity to                reared intensively (0.5 pound/gal-     gradually decline, then pipes
prime markets.                                   lon or greater) for recirculating      must be cleaned. A sponge, clean-
                                                 systems to be cost effective. As an    ing pad or brush attached to a
Unfortunately, many commercial                   analogy, a 20-gallon home aquari-
systems, to date, have failed                                                           plumber’s snake works well for
                                                 um, which is a miniature recircu-      scouring pipes. Air diffusers
because of poor design, inferior                 lating system, would have to
management, or flawed econom-                                                           should be cleaned periodically by
                                                 maintain at least 10 pounds of fish    soaking them in muriatic acid
ics. This publication will address               to reach this same level of intensi-
the problems of managing a recir-                                                       (available at plumbing suppliers).
                                                 ty. This should be a sobering
culating aquaculture system so                   thought to anyone contemplating        Flow blockage and water level
that those contemplating invest-                 the management of an intensive         fluctuations also can result from
ment can make informed deci-                     recirculating system.                  the clogging of screens used to
sions. For information on theory                                                        retain fish in the rearing tanks.
and design of recirculating sys-                                                        Screen mesh should be the largest
tems refer to SRAC Publication
                                                 System operation                       size that will retain the fish (usu-
No. 451, Recirculating Aquaculture               To provide a suitable environment      ally 3/4 to 1 inch). The screened
Tank Production Systems: An                      for intensive fish production,         area around pipes should be
Overview of Critical Considerations,             recirculating systems must main-       much larger than the pipe diame-
and SRAC Publication No. 453,                    tain uniform flow rates (water and     ter, because a few dead fish can
Recirculating Aquaculture Tank                   air/oxygen), fixed water levels,       easily block a pipe. Screens can be
                                                 and uninterrupted operation.           made into long cylinders or boxes
                                                                                        that attach to pipes and have a
1Auburn University;                              The main cause of flow reduction
                                                                                        large surface area to prevent
2University of the Virgin Islands;               is the constriction of pipes and air
                                                                                        blockage. Screens should be tight-
3North Carolina State University                 diffusers by the growth of fungi,
ly secured to the pipe so that they   Biological filters (biofilters) can                   particulates are too small to be
cannot be dislodged during feed-      fail because of senescence, chemi-                    removed by conventional particu-
ing, cleaning and harvesting oper-    cal treatment (e. g., disease treat-                  late filters and cause or compli-
ations.                               ment), or anoxia. It takes weeks to                   cate many other system problems.
An essential component of recir-      months to establish or colonize a
culating systems is a backup          biofilter. The bacteria that colonize                 Water quality management
power source (see SRAC                a biofilter grow, age and die.
                                      These bacteria are susceptible to                     In recirculating systems, good
Publication No. 453). Electrical                                                            water quality must be maintained
power failures may not be com-        changes in water quality (low dis-
                                      solved oxygen [DO], low alkalini-                     for maximum fish growth and for
mon, but it only takes a brief                                                              optimum effectiveness of bacteria
power failure to cause a cata-        ty, low or high pH, high CO2,
                                      etc.), chemical treatments, and                       in the biofilter (Fig. 1). Water qual-
strophic fish loss. For example, if                                                         ity factors that must be monitored
a power failure occurred in a         oxygen depletions. Biological fil-
                                      ters do not take rapid change                         and/or controlled include temper-
warmwater system (84o F) at sat-                                                            ature, dissolved oxygen, carbon
urated oxygen concentrations          well!
                                                                                            dioxide, pH, ammonia, nitrite and
containing 1/2-pound fish at a                                                              solids. Other water quality factors
density of 1/4 pound of fish per      Particulates
                                                                                            that should be considered are
gallon of water, it will take only    Particulate removal is one of the                     alkalinity, nitrate and chloride.
16 minutes for the oxygen con-        most complicated problems in
centration to decrease to 3 ppm, a    recirculating systems. Particulates                   Temperature
stressful level for fish. The same    come from uneaten feed and from
system containing 1-pound fish at     undigested wastes. It has been                        Temperature must be maintained
a density of 1 pound of fish per      estimated that more than 60 per-                      within the range for optimum
gallon would plunge to this           cent of feed placed into the sys-                     growth of the cultured species. At
stressful oxygen concentration in     tem ends up as particulates. Quick                    optimum temperatures fish grow
less than 6 minutes. These scenar-    and efficient removal of particu-                     quickly, convert feed efficiently,
ios should give the prospective       lates can significantly reduce the                    and are relatively resistant to
manager a sobering feeling for        biological demand placed on the                       many diseases. Biofilter efficiency
how important backup power is         biofilter, improve biofilter efficien-                also is affected by temperature but
to the integrity of a recirculating   cy, reduce the overall size of the                    is not generally a problem in
system.                               biofilter required, and lower the                     warmwater systems. Temperature
Certain components of backup          oxygen demand on the system.                          can be regulated with electrical
systems need to be automatic. An      Particulate filters should be                         immersion heaters, gas or electric
automatic transfer switch should      cleaned frequently and main-                          heating units, heat exchangers,
start the backup generator in case    tained at peak efficiency. Many                       chillers, or heat pumps. Tempera-
personnel are not present. Auto-
matic phone alarm systems are
inexpensive and are essential in                            CLOSED RECIRCULATING SYSTEM
alerting key personnel to power                                                                   N2
failures or water level fluctua-
tions. Some phone alarm systems                                                                NO3                NITRIFICATION
allow in-dialing so that managers                                                                                 ION BALANCE
can phone in and check on the
status of the system. Other com-                                                       H+                         GAS STRIPPING
ponent failures can also lead to
disastrous results in a very short                                                                        CO2
time. Therefore, systems should                                                                                   ADDITION
be designed with essential backup                               TAN
                                                                BOD                                               BOD REDUCTION
components that come on auto-                                  SOLIDS            BACTERIA
matically or can be turned on                                                                                     DISSOLVED
quickly with just a flip of a                                                                                     REFRACTORY
switch. Finally, one of the sim-                                                                             O2
plest backups is a tank of pure                                                                                   AERATION
oxygen connected with a solenoid                                                           INERT
valve that opens automatically                                                             SOLIDS
during power failures. This oxy-           RFM 6/6/90                                                             SOLIDS REMOVAL
gen-solenoid system can provide
                                      Figure 1. Diagram of fish wastes and their effects on bacterial and chemical
sufficient dissolved oxygen to                  interactions in a recirculating system.
keep the fish alive during power
                                      Courtesy of Ronald F. Malone, Department of Civil Engineering, Louisiana State University, from
failures.                             Louisiana Aquaculture 1992, “Design of Recirculating Systems for Intensive Tilapia Culture,”
                                      Douglas G. Drennan and Ronald F. Malone.
ture can be manipulated to reduce          Water said to be “saturated” with      sent). Lethargic behavior and a
stress during handling and to con-         oxygen contains the maximum            sharply reduced appetite are com-
trol certain diseases (e.g., Ich and       amount of oxygen that will dis-        mon symptoms of carbon dioxide
ESC).                                      solve in it at a given temperature,    stress.
                                           salinity and pressure (Table 1).       Carbon dioxide can accumulate in
Dissolved oxygen                           Pure oxygen systems can be incor-      recirculating systems unless it is
                                           porated into recirculating systems.    physically or chemically removed.
Continuously supplying adequate            These inject oxygen into a con-
amounts of dissolved oxygen to                                                    Carbon dioxide usually is
                                           fined stream of water, creating        removed from the water by
fish and the bacteria/biofilter in         supersaturated conditions (see
the recirculating system is essen-                                                packed column aerators or other
                                           SRAC Publication No. 453).             aeration devices (see SRAC
tial to its proper operation.
Dissolved oxygen (DO) concentra-           Supersaturated water, with DO          Publication No. 453).
tions should be maintained above           concentrations several times high-
60 percent of saturation or above 5        er than saturation, is mixed into      pH
ppm for optimum fish growth in             the rearing tank water to maintain
                                           DO concentrations near satura-         Fish generally can tolerate a pH
most warmwater systems. It is                                                     range from 6 to 9.5, although a
also important to maintain DO              tion. The supersaturated water
                                           should be introduced into the          rapid pH change of two units or
concentrations in the biofilter for                                               more is harmful, especially to fry.
maximum ammonia and nitrite                rearing tank near the bottom and
                                           be rapidly mixed throughout the        Biofilter bacteria which are impor-
removal. Nitrifying bacteria                                                      tant in decomposing waste prod-
become inefficient at DO concen-           tank by currents generated from
                                           the water pumping equipment.           ucts are not efficient over a wide
trations below 2 ppm.                                                             pH range. The optimum pH range
                                           Proper mixing of the supersaturat-
Aeration systems must operate              ed water into the tank is critical.    for biofilter bacteria is 7 to 8.
continuously to support the high           Dissolved oxygen will escape into      The pH tends to decline in recir-
demand for oxygen by the fish              the air if the supersaturated water    culating systems as bacterial nitri-
and microorganisms in the sys-             is agitated too vigorously. If the     fication produces acids and con-
tem. As fish approach harvest size         water is mixed too slowly, zones       sumes alkalinity, and as carbon
and feeding rates (pounds/sys-             of supersaturation can cause gas       dioxide is generated by the fish
tem) are near their maximum lev-           bubble disease. In gas bubble dis-     and microorganisms. Carbon
els, oxygen demand may exceed              ease, gases come out of solution       dioxide reacts with water to form
the capacity of the aeration system        inside the fish and form bubbles       carbonic acid, which drives the
to maintain DO concentrations              in the blood. These bubbles can        pH downward. Below a pH of 6,
above 5 ppm. Fish show signs of            result in death. Fry are particular-   the nitrifying bacteria are inhibit-
oxygen stress by gathering at the          ly sensitive to supersaturation.       ed and do not remove toxic nitro-
surface and swimming into the                                                     gen wastes.
current produced by the aeration           Carbon dioxide
device (e. g., agitator, air lift, etc.)                                          Optimum pH range generally is
where DO concentrations are                Carbon dioxide is produced by          maintained in recirculating sys-
higher. If this occurs, a supple-          respiration of fish and bacteria in    tems by adding alkaline buffers.
mental aeration system should be           the system. Fish begin to stress at    The most commonly used buffers
used or the feeding rate must be           carbon dioxide concentrations          are sodium bicarbonate and calci-
reduced.                                   above 20 ppm because it interferes     um carbonate, but calcium
                                           with oxygen uptake. Like oxygen        hydroxide, calcium oxide, and
Periods of heavy feeding may be                                                   sodium hydroxide have been
sustained by multiple or continu-          stress, fish under CO2 stress come
                                           to the surface and congregate          used. Calcium carbonate may dis-
ous feedings of the daily ration                                                  solve too slowly to neutralize a
over a 15- to 20-hour period rather        around aeration devices (if pre-
                                                                                  rapid accumulation of acid.
than in two or three discrete
meals. As fish digest food, their
respiration rate increases dramati-        Table 1. Oxygen saturation levels in fresh water at sea level
cally, causing a rapid decrease in                  atmospheric pressure.
DO concentrations. Feeding small              Temperature          DO               Temperature            DO
amounts continuously with auto-               oC       oF                            oC      oF
matic or demand feeders allows                                  mg/L (ppm)                              mg/L (ppm)
DO to decline gradually without               10      50.0        10.92              24     75.2           8.25
reaching critical levels. During              12      53.6        10.43              26     78.8           7.99
periods of heavy feeding, DO                  14      57.2         9.98              28     82.4           7.75
should be monitored closely, par-
ticularly before and after feedings.          16      60.8         9.56              30     86.0           7.53
Recirculating systems require con-            18      64.4         9.18              32     89.6           7.32
stant monitoring to ensure they               20      68.0         8.84              34     93.2           7.13
are functioning properly.                     22      71.6         8.53              36     96.8           6.95
Calcium hydroxide, calcium oxide       Table 2. Percentage of total ammonia in the un-ionized form at
and sodium hydroxide dissolve
                                                differing pH values and temperatures.
quickly but are very caustic; these
compounds should not be added                                                        Temperature (oC)
to the rearing tank because they
                                       pH             16      18           20       22        24           26    28        30         32
may harm the fish by creating
zones of very high pH. The pH of        7.0        0.30      0.34          0.40    0.46      0.52      0.60      0.70     0.81        0.95
the system should be monitored          7.2        0.47      0.54          0.63    0.72      0.82      0.95      1.10     1.27        1.50
daily and adjusted as necessary to
maintain optimum levels. Usually,       7.4        0.74      0.86          0.99    1.14      1.30      1.50      1.73     2.00        2.36
the addition of sodium bicarbon-        7.6        1.17      1.35          1.56    1.79      2.05      2.35      2.72     3.13        3.69
ate at a rate of 17 to 20 percent of    7.8        1.84      2.12          2.45    2.80      3.21      3.68      4.24     4.88        5.72
the daily feeding rate is sufficient    8.0        2.88      3.32          3.83    4.37      4.99      5.71      6.55     7.52        8.77
to maintain pH and alkalinity
within the desired range (Fig. 2).      8.2        4.49      5.16          5.94    6.76      7.68      8.75     10.00    11.41       13.22
For example, if a tank is being fed     8.4        6.93      7.94          9.09   10.30     11.65     13.20     14.98    16.96       19.46
10 pounds of feed per day then          8.6       10.56     12.03         13.68   15.40     17.28     19.42     21.83    24.45       27.68
approximately 2 pounds of bicar-
                                        8.8       15.76     17.82         20.08   22.38     24.88     27.64     30.68    33.90       37.76
bonate would be added daily to
adjust pH and alkalinity levels.        9.0       22.87     25.57         28.47   31.37     34.42     37.71     41.23    44.84       49.02
Alkalinity, the acid neutralizing       9.2       31.97     35.25         38.69   42.01     45.41     48.96     52.65    56.30       60.38
capacity of the water, should be        9.4       42.68     46.32         50.00   53.45     56.86     60.33     63.79    67.12       70.72
maintained at 50 to 100 mg as cal-      9.6       54.14     57.77         61.31   64.54     67.63     70.67     73.63    76.39       79.29
cium carbonate/L or higher, as
                                        9.8       65.17     68.43         71.53   74.25     76.81     79.25     81.57    83.68       85.85
should hardness. Generally, the
addition of alkaline buffers used      10.0       74.78     77.46         79.92   82.05     84.00     85.82     87.52    89.05       90.58
to adjust pH will provide ade-         10.2       82.45     84.48         86.32   87.87     89.27     90.56     91.75    92.80       93.84
quate alkalinity, and if the buffers
also contain calcium, they add to                                                               daily. If total ammonia concentra-
                                       tissue damage in several species
hardness. For a more detailed dis-                                                              tions start to increase, the biofilter
                                       of warmwater fish. However,
cussion of alkalinity and hardness                                                              may not be working properly or
                                       tilapia tolerate high un-ionized
consult a water quality text.                                                                   the feeding rate/ammonia nitro-
                                       ammonia concentrations and sel-
                                       dom display toxic effects in well-                       gen production is higher than the
Nitrogen wastes                                                                                 design capacity of the biofilter.
                                       buffered recirculating systems.
Ammonia is the principal nitroge-      Ammonia should be monitored
nous waste released by fish and is
mainly excreted across the gills as
ammonia gas. Ammonia is a                   8.5                                         Discontinue
byproduct from the digestion of                                                    supplemental aeration
protein. An estimated 2.2 pounds
of ammonia nitrogen are pro-                8.0
duced from each 100 pounds of
feed fed. Bacteria in the biofilter                                               Optimum
convert ammonia to nitrite and              7.5         sodium                                                        Reduce daily
nitrite to nitrate, a process called                  bicarbonate                                                     bicarbonate
nitrification. Both ammonia and                                                                                         addition
nitrite are toxic to fish and are,
therefore, major management                                                               Increase
problems in recirculating systems                                                         aeration
(Fig. 2).
                                            6.5            Add
Ammonia in water exists as two                           sodium
compounds: ionized (NH4+) and                          bicarbonate
un-ionized (NH3) ammonia. Un-                            & aerate
ionized ammonia is extremely
                                                  0                 100             200              300          400            500
toxic to fish. The amount of un-
ionized ammonia present depends                                             Alkalinity, mg/L as CaCO3
on pH and temperature of the
water (Table 2). Un-ionized            Figure 2. The pH management diagram, a graphical solution of the ionization constant
ammonia nitrogen concentrations                  equation for carbonic acid at 25oC.
as low as 0.02-0.07 ppm have been      Courtesy of Ronald F. Malone, Department of Civil Engineering, Louisiana State University, from
                                       Master’s Thesis of Peter A. Allain, 1988, “Ion Shifts and pH Management in High Density Shedding
shown to slow growth and cause         Systems for Blue Crabs (Callinectes sapidus) and Red Swamp Crawfish (Procambarus clarkii),”
                                       Louisiana State University.
                                                                                                  maintain a pH of 7.5. After the
Table 3. Nutrient solution for pre-activation of biofilter.
                                                                                                  activation period the nutrient
                              Nutrient                          Concentration (ppm)               solution is discarded.
Dibasic ammonium phosphate, (NH4)2HPO4                                      40                    Many fish can die during this
Dibasic sodium phosphate, Na2HPO4                                           40                    period of biofilter activation.
                                                                                                  Managers have a tendency to
Sea salts “solids”                                                          40                    overfeed, which leads to the gen-
Sea salts “liquids”                                                          0.5                  eration of more ammonia than the
Calcium carbonate, CaCO3                                                   250                    biofilter can initially handle. At
                                                                                                  first, ammonia concentrations
                                                                                                  increase sharply and fish stop
Biofilters consist of actively grow-                 tioning properly. Subdividing or             feeding and are seen swimming
ing bacteria attached to some sur-                   compartmentalizing biofilters                into the current produced by the
face(s). Biofilters can fail if the                  reduces the likelihood of a com-             aeration device. Deaths will soon
bacteria die or are inhibited by                     plete failure and gives the manag-           occur unless immediate action is
natural aging, toxicity from chem-                   er the option of “seeding” active            taken. At the first sign of high
icals (e. g., disease treatment), lack               biofilter sludge from one tank or            ammonia, feeding should be
of oxygen, low pH, or other fac-                     system to another.                           stopped. If pH is near 7 the fish
tors. Biofilters are designed so that                                                             may not show signs of stress
                                                     Activating a new biofilter (i. e.,           because little of the ammonia is in
aging cells slough off to create                     developing a healthy population
space for active new bacterial                                                                    the un-ionized form.
                                                     of nitrifying bacteria capable of
growth. However, there can be sit-                   removing the ammonia and                     As nitrifying bacteria, known as
uations (e. g., cleaning too vigor-                  nitrite produced at normal feed-             Nitrosomonas, become established
ously) where all the bacteria are                    ing rates) requires a least 1                in the biofilter, they quickly con-
removed. If chemical additions                       month. During this activation                vert the ammonia into nitrite. This
cause biofilter failure, the water in                period, the normal stocking and              conversion takes place about 2
the system should be exchanged.                      feeding rates should be greatly              weeks into the activation period
The biofilter would then have to                     reduced. Prior to stocking it is             and will proceed even if feeding
be re-activated (taking 3 or 4                       advantageous, but not absolutely             has stopped. Once again, fish will
weeks) and the pH adjusted to                        necessary, to pre-activate the               seek relief near aeration and mor-
optimum levels.                                      biofilters. Pre-activation is accom-         talities will occur soon unless
During disruptions in biofilter                      plished by seeding the filter(s)             steps are taken. Nitrite concentra-
performance, the feeding rate                        with nitrifying bacteria (available          tions decline when a second group
should be reduced considerably                       commercially) and providing a                of nitrifying bacteria, known as
or feeding should be stopped.                        synthetic growth medium for a                Nitrobacter, become established.
Feeding, even after a complete                       period of 2 weeks. The growth                These problems can be avoided if
water exchange, can cause ammo-                      medium contains a source of                  time is taken to activate the biofil-
nia nitrogen or nitrite nitrogen                     ammonia nitrogen (10 to 20                   ters slowly.
concentrations (Fig. 3) to rise to                   mg/l), trace elements and a buffer           Nitrite concentrations also should
stressful levels in a matter of                      (Table 3). The buffer (sodium                be checked daily. The degree of
hours if the biofilter is not func-                  bicarbonate) should be added to              toxicity to nitrite varies with
                                                                                                  species. Scaled species of fish are
                                                                                                  generally more tolerant of high
                                                                                                  nitrite concentrations than species
                         24   System                                          Ammonia - N         such as catfish, which are very
                                                                              Nitrite - N         sensitive to nitrite. Nitrite nitrogen
   Concentration, mg/L

                                                                                                  as low as 0.5 ppm is stressful to
                         18                                                                       catfish, while concentrations of
      as nitrogen

                         15                                                                       less than 5 ppm appear to cause
                                                                                                  little stress to tilapia. Nitrite toxici-
                         12                                                                       ty causes a disease called “brown
                          9                                                                       blood,” which describes the blood
                                                                                                  color that results when normal
                          6                                                                       blood hemoglobin comes in con-
                          3                                                                       tact with nitrite and forms a com-
                                                                                                  pound called methemoglobin.
                          0                                                                       Methemoglobin does not transport
                                                                                                  oxygen properly, and fish react as
Figure 3. Typical ammonia and nitrite curves showing time delays in establishing                  if they are under oxygen stress.
          bacteria in biofilters.                                                                 Fish suffering nitrite toxicity come
Courtesy of Ronald F. Malone, Department of Civil Engineering, Louisiana State University, from   to the surface as in oxygen stress,
Master’s Thesis of Don P. Manthe, 1982, “Water Quality of Submerged Biological Rock Filters for
Closed Recirculating Blue Crab Shedding Systems,” Louisiana State University.                     sharply reduce their feeding, and
are lethargic. Nitrite toxicity can    a low level of suspended solids            take 3 to 4 weeks. Table 4 summa-
be reduced or blocked by chloride      may serve a beneficial role within         rizes general water quality
ions. Usually 6 to 10 parts of chlo-   the system as long as they do not          requirements of recirculating sys-
ride protect fish from 1 part          irritate the fishes’ gills.                tems.
nitrite nitrogen. Increasing con-      If organic solids build up to high
centrations of nitrite are a sign      levels in the system, they will            Water exchange
that the biofilter is not working      stimulate the growth of microor-
properly or the biofilter is not                                                  Most recirculating systems are
                                       ganisms that produce off-flavor            designed to replace 5 to 10 per-
large enough to handle the             compounds. The concentration of
amount of waste being produced.                                                   cent of the system volume each
                                       solids at which off-flavor com-            day with new water. This amount
As with ammonia buildup, check         pounds develop is not known,
pH, alkalinity and dissolved oxy-                                                 of exchange prevents the build-up
                                       but the system water should                of nitrates and soluble organic
gen in the biofilter. Reduce feed-     never be allowed to develop a
ing and be prepared to flush the                                                  matter that would eventually
                                       foul or fecal smell. If offensive          cause problems. In some situa-
system with fresh water or add         odors develop, increase the water
salt (NaCl) if toxic concentrations                                               tions, sufficient water may not be
                                       exchange rate, reduce feeding,             available for these high exchange
develop.                               increase solids removal, and/or            rates. A complete water exchange
Nitrate, the end product of nitrifi-   enlarge biofilters.                        should be done after each produc-
cation, is relatively nontoxic                                                    tion cycle to reduce the build-up
except at very high concentra-         Chloride                                   of nitrate and dissolved organics.
tions (over 300 ppm). Usually
nitrate does not build up to these     Adding salt (NaCl) to the system           For emergency situations it is rec-
concentrations if some daily           is beneficial not only for the chlo-       ommended that the system have
exchange (5 to 10 percent) with        ride ions, which block nitrite toxi-       an auxiliary water reservoir equal
fresh water is part of the manage-     city, but also because sodium and          to one complete water exchange
ment routine. Also, in many recir-     chloride ions relieve osmotic              (flush). The reservoir should be
culating systems some denitrifica-     stress. Osmotic stress is caused by        maintained at the proper temper-
tion seems to occur within the         the loss of ions from the fishes’          ature and water quality.
system that keeps nitrate concen-      body fluids (usually through the
trations below toxic levels.           gills). Osmotic stress accompanies         Fish production
                                       handling and other forms of
Denitrification is the bacteria-
                                       stress (e. g., poor water quality).
mediated transformation of
nitrate to nitrogen gas, which         A salt concentration of 0.02 to 0.2
                                       percent will relieve osmotic stress.       Stocking
escapes into the atmosphere.
                                       This concentration of salt is bene-        Fish management starts before the
                                       ficial to most species of fish and         fish are introduced into the recir-
                                       invertebrates. It should be noted          culating system. Fingerlings
Solid waste, or particulate matter,    that rapidly adding salt to a recir-       should be purchased from a rep-
consists mainly of feces and           culating system can decrease               utable producer who practices
uneaten feed. It is extremely          biofilter efficiency. The biofilter        genetic selection, knows how to
important to remove solids from        will slowly adjust to the addition         carefully handle and transport
the system as quickly as possible.     of salt but this adjustment can            fish, and does not have a history
If solids are allowed to remain in
the system, their decomposition
will consume oxygen and pro-           Table 4. Recommended water quality requirements of recirculating
duce additional ammonia and                    systems.
other toxic gases (e. g., hydrogen
                                          Component                           Recommended value or range
sulfide). Solids are removed by
filtration or settling (SRAC           Temperature                   optimum range for species cultured - less
Publication No. 453). A consider-                                    than 5o F as a rapid change
able amount of highly malodor-         Dissolved oxygen              60% or more of saturation, usually 5 ppm
ous sludge is produced by recir-
                                                                     or more for warmwater fish and greater than
culating systems, and it must be
disposed of in an environmental-                                     2 ppm in biofilter effluent
ly sound manner (e. g., applied to     Carbon dioxide                less than 20 ppm
agricultural land or composted).       pH                            7.0 to 8.0
Very small (colloidal) solids          Total alkalinity              50 to 100 ppm or more as CaCO3
remain suspended in the water.         Total hardness                50 to 100 ppm or more as CaCO3
Although the decay of this mater-
ial consumes oxygen and pro-           Un-ionized ammonia-N          less than 0.05 ppm
duces some additional ammonia,         Nitrite-N                     less than 0.5 ppm
it also serves as attachment sites     Salt                          0.02 to 0.2 %
for nitrifying bacteria. Therefore,
of disease problems in his/her            growth and near the system maxi-
                                          mum—the highest feeding rates at            Table 5. Estimated food con-
hatchery. Starting with poor quali-
ty or diseased fingerlings almost         which acceptable water quality                       sumption by size of a
ensures failure.                          conditions can be maintained.                        typical warmwater fish.
Fish should be checked for para-          When more feed is required, fish               Average            Body weight
sites and diseases before being           stocks should be split and moved            weight per fish        consumed
introduced into the system. New           to new tanks. This would gradual-            (lbs.)   (g)             (%)
fish may need to be quarantined           ly reduce the stocking rate over
                                          the production cycle.                         0.02       9                 5.0
from fish already in the system so
that diseases will not be intro-          Another approach is to divide the             0.04      18                 4.0
duced. A few fish should be               rearing tank(s) into compartments             0.06      27                 3.3
checked for parasites and diseases        with different size groups of fish            0.25     113                 3.0
by a certified fish diagnostician.        in each compartment. In this
                                          approach, the optimum feeding                 0.50     227                 2.75
Once diseases are introduced into
a recirculating system they are           rate for all the compartments is              0.75     340                 2.5
generally hard to control, and            consistently near the biofilter’s             1.0      454                 2.2
treatment may disrupt the biofil-         maximum performance. As one                   1.5      681                 1.8
ter.                                      group of fish is harvested, finger-
Fish are usually hauled in cool           lings are immediately stocked into
water. As they come into the sys-         the vacant compartment or tank.             Table 6 approximates a feeding
tem they usually have to be tem-          Compartment size within a tank              schedule for a warmwater fish
pered or gradually acclimated to          may be adjusted as fish grow, by            (e.g., tilapia) stocked into an 84o F
the system temperature and pH.            using movable screens.                      recirculating system as fry and
Fish can generally take a 5o F                                                        harvested at a weight of 1 pound
change without much problem.              Feeding                                     after 250 feeding days. Feed con-
Temperature changes of more               Knowing how much to feed fish               version is estimated at 1.5: 1, or
than 5o F should be done at about         without overfeeding is a problem            1.5 pounds of feed to obtain 1
1o F every 20 to 30 minutes. Stress       in any type of fish production.             pound of gain.
can be reduced if the system is           Feeding rates are usually based on          Tables 5 and 6 are estimates and
cooled to the temperature of the          fish size. Small fish consume a             should be used only as guidelines
hauling water and then slowly             higher percent of their body                which can change with differing
increased over a period of several        weight per day than do larger fish          species and temperatures.
hours to days.                            (Table 5). Most fish being grown            Growth and feed conversion are
Recirculating systems must oper-          for food will be stocked as finger-         estimated by weighing a sample
ate near maximum production               lings. Fingerlings consume 3 to 4           of fish from each tank and then
(i. e., maximum risk) capacity at         percent of their body weight per            calculating the feed conversion
all times to be economical. It is not     day until they reach 1/4 to 1/2             ratios and new feeding rates from
cost effective to operate pumps           pound, then consume 2 to 3 per-             this sample. For example, 1,000
and aeration devices when the             cent of their body weight until             fish in a tank have been consum-
system is stocked with fingerlings        being harvested at 1 to 2 pounds.           ing 10 pounds of feed a day for
at only one-tenth of the system’s         A rule-of-thumb for pond culture            the last 10 days (100 pounds
carrying capacity. Therefore, fin-        is to feed all the fish will consume        total). The fish were sampled 10
gerlings should be stocked at very        in 5 to 10 minutes. Unfortunately,          days earlier and weighed an aver-
high rates, in the range of 30 fish       this method can easily lead to              age of 0.33 pounds or an estimat-
per cubic foot. Feeding rates             overfeeding. Overfeeding wastes             ed total of 330 pounds.
should be optimum for rapid               feed, degrades water quality, and
                                          can overload the biofilter.

Table 6. Recommended stocking and feeding rates for different size groups of tilapia in tanks, and
         estimated growth rates.
 Stocking rate                Weight (g)             Growth rate           Growth period                Feeding rate
 (number/ft3)           Initial        Final           (g/day)                (days)                        (%)
      225             0.02              0.5-1             -                      30                       20    -   15
       90             0.5-1                 5             -                      30                       15    -   10
       45               5                  20             0.5                    30                       10    -   7
       28              20                  50             1.0                    30                         7   -   4
       14              50                100              1.5                    30                         4   -   3.5
       5.5            100                250              2.5                    30                       3.5   -   1.5
         3            250                450              3.0                    70                       1.5   -   1.0
A new sample of 25 fish is collect-      minutes. Multiple feedings at the      (depending on the type and sever-
ed from the tank and weighed.            same location in a tank can            ity of off-flavor). If fish remain in
The 25 fish weigh 10 pounds or an        increase dominance because a few       the purging tanks for an extended
average of 0.4 pounds per fish. If       fish jealously guard the area and      period, their feeding rate may
this is a representative sample,         do not let other fish feed. In this    need to be reduced, or off-flavor
then 1,000 fish should weigh 400         situation, use feeders that distrib-   may develop within the purging
pounds. Therefore, the change in         ute feed widely across the tank.       system.
total fish weight for this tank is       Fish can be fed by hand, with          See SRAC Publication No. 431,
400 minus 330, or 70 pounds. The         demand feeders, or by automatic        Testing Flavor Quality of Preharvest
fish were fed 100 pounds of feed         feeders, but stationary demand         Channel Catfish, for detailed infor-
in the last 10 days and gained 70        and belt type feeders tend to          mation on off-flavor.
pounds in weight. Feed conver-           encourage dominance. Whichever
sion then is equal to 1.43 to 1 (i.e.,   method is used, be careful to
100 ÷ 70). In other words, the fish      evenly distribute feed and not to
                                                                                Stress and disease control
gained 1 pound of weight for each        overfeed.                              The key to fish management is
1.43 pounds of feed fed. The daily       Always purchase high quality           stress management. Fish can be
feeding rate should now be               feed from a reputable company.         stressed by changes in tempera-
increased to adjust for growth of        Keep feed fresh by storing it in a     ture and water quality, by han-
the fish.                                cool, dry place. Never use feed        dling (including seining and haul-
To calculate the new feeding rate,       that is past 60 days of the manu-      ing), by nutritional deficiencies,
multiply the estimated total fish        facture date. Never feed moldy,        and by exposure to parasites and
weight (400 pounds) by the esti-         discolored or clumped feed.            diseases. Stress increases the sus-
mated percent body weight of             Molds on feed may produce afla-        ceptibility of fish to disease, which
feed consumption for a 0.4-pound         toxins, which can stress or kill       can lead to catastrophic fish losses
fish (from Table 5). Table 5 sug-        fish. Feed quality deteriorates        if not detected and treated quick-
gests that the percent body weight       with time, particularly when           ly. To reduce stress fish must be
consumed per day should be               stored in warm, damp conditions.       handled gently, kept under proper
between 2.75 and 3 percent. If 3         A disease known as “no blood” is       water quality conditions, and pro-
percent is used, then 400 times          associated with feed that is defi-     tected from exposure to poor
0.03 is 12.0. Thus, the new feeding      cient in certain vitamins. In a case   water quality and diseases. Even
rate should be 12 pounds of feed         of “no blood,” the fish appear         sound and light can stress fish.
per day for the next 10 days, for a      pale with white gills and blood        Unexpected sounds or sudden
total of 120 pounds. Using this          appears clear, not red. Another        flashes of light often trigger an
sampling technique the manager           nutritional disease known as “bro-     escape response in fish. In a tank,
can accurately track growth and          ken back syndrome” is caused by        this escape response may send
feed conversion, and base other          a vitamin C deficiency. The only       fish into the side of the tank, caus-
management decisions on these            management practice for “no            ing injury. Fish are generally sen-
factors.                                 blood” disease and “broken back        sitive to light exposure, particular-
                                         syndrome” is to discard the feed       ly if it is sudden or intense. For
Feeding skills                           being used and purchase a differ-      this reason many recirculating
                                         ent batch or brand of feed.            systems have minimal lighting
Feeding is the best opportunity to                                              around the fish tanks.
observe overall vitality of the fish.    Fines, crumbled feed particles, are
A poor feeding response should           not generally consumed by the          Diseases
be an immediate alarm to the             fish but add to the waste load of
manager. Check all aspects of the        the system, increasing the burden      There are more than 100 known
system, particularly water quality,      on particulate and biological fil-     fish diseases, most of which do
and diagnose for diseases if feed-       ters. Therefore, it is recommended     not seem to discriminate between
ing behavior suddenly diminish-          that feed pellets be sifted or         species. Other diseases are very
es.                                      screened to remove fines before        host specific. Organisms known to
                                         feeding.                               cause diseases and/or parasitize
Fish can be fed once or several                                                 fish include viruses, bacteria,
times a day. Multiple feedings                                                  fungi, protozoa, crustaceans, flat-
spread out the waste load on the                                                worms, roundworms and seg-
biofilter and help prevent sudden        Off-flavor in recirculating systems    mented worms. There are also
decreases in DO. Research has            is a common and persistent prob-       non-infectious diseases such as
shown that small fish will grow          lem. Many times fish have to be        brown blood, no blood and bro-
faster if fed several times a day.       moved into a clean system, one         ken back syndrome. Any of these
Feeding several times a day seems        with clear, uncontaminated water,      diseases can become a problem in
to reduce problems of feeding            where they can be purged of off-       a recirculating system. Diseases
dominance in some species of fish.       flavor before being marketed.          can be introduced into the system
Many recirculating system man-           Purging fish of off-flavor can take    from the water, the fish, and the
agers feed as often as every 30          from a few days to many weeks          system’s equipment.
Diseases are likely to enter the        s  Cessation of feeding                systems, chemical treatments can
system from hauling water, on the       s  Mortalities                         severely disrupt the biofilter.
fish themselves, or on nets, bas-                                              Biofilter bacteria are inhibited to
kets, gloves, etc., that are moved      Whenever any of these symptoms         some degree by formalin, copper
from tank to tank. Hauling water        appear the manager should check        sulfate, potassium permanganate,
should never be introduced into         water quality and have a few fish      and certain antibiotics. Even sud-
the system. Fish should be quar-        with symptoms diagnosed by a           den changes in salt concentration
antined, checked for diseases, and      qualified fish disease specialist.     will decrease biofilter efficiency. If
treated as necessary. Equipment         The most common diseases in            the system is designed properly, it
should be sterilized (e. g., chlorine   recirculating systems are caused       may be possible to isolate the
dip) before moving it between           by bacteria and protozoans. Some       biofilter from the rest of the sys-
tanks. If possible, provide sepa-       diseases that have been particular-    tem, treat and flush the fish tanks,
rate nets and baskets for each tank     ly problematic in recirculating        and then reconnect the biofilter
so they will not contaminate other      systems include the protozoal dis-     without exposing it to chemical
tanks. Disease can spread rapidly       eases Ich (Ichthyophthirius) and       treatment. However, there is a
from one tank to another if equip-      Trichodina, and the bacterial dis-     danger that the biofilter will re-
ment is freely moved between            eases columnaris, Aeromonas,           introduce the disease organism.
tanks or if all the water within the    Streptococcus and Mycobacterium. It    Whenever a chemical treatment is
system is mixed together as in a        appears that Trichodina and            applied, be prepared to exchange
common sump, particulate filter         Streptococcus diseases are prob-       the system water and monitor the
or biofilter.                           lematic in recirculating systems       DO concentration and other water
                                        with tilapia, while Mycobacterium      quality factors closely. Fish usual-
A manager needs to be familiar
                                        has been found in hybrid striped       ly reduce their feed consumption
with the signs of stress and dis-
                                        bass in intensive recirculating sys-   after a chemical treatment; there-
ease which include:
                                        tems.                                  fore, feeding rates need to be
s Excitability                                                                 monitored carefully.
                                        It may be possible to treat dis-
s Flashing or whirling                  eases with chemicals approved for      Tables 7 and 8 give possible caus-
s Skin or fin sores or discol-          fish (see SRAC Publication No.         es and management options based
   orations                             410, Calculating Treatments for        on the observation of the fish or
                                        Ponds and Tanks), although few         water quality tests.
s Staying at the surface
                                        therapeutants are approved for
s Erratic swimming                      use on food fish species other         Conclusions
s Reduction in feeding rate             than catfish and rainbow trout.
                                        Treatment always has its prob-         Recirculating systems have devel-
s Gulping at the surface                                                       oped to the point that they are
                                        lems. In the case of recirculating
                                                                               being used for research, for orna-
                                                                               mental/tropical fish culture, for
                                                                               maturing and staging brood fish,
                      Examples of fish diseases                                for producing advanced fry/fin-
                                                                               gerlings, and for producing food
                                                                               fish for high dollar niche markets.
                                                                               They continue to be expensive
                                                                               ventures which are as much art as
                                                                               science, particularly when it
                                                                               comes to management. Do your
                                                                               homework before deciding to
                                                                               invest in a recirculating system.
                                                                               Investigate the efficiency, compati-
            A–Columnaris                            B–Aeromonas                bility and maintenance require-
                                                                               ments of the components.
                                                                               Estimate the costs of building and
                                                                               operating the system and of mar-
                                                                               keting the fish without any return
                                                                               on investment for at least 2 years.
                                                                               Know the species you intend to
                                                                               grow, their environmental require-
                                                                               ments, diseases most common in
                                                                               their culture, and how those dis-
                                                                               eases are treated. Know your
                                                                               potential markets and how the
                                                                               fish need to be prepared for that
           C–Streptococcus                        D–Mycobacterium
        (cataract and pop-eye)
                                                                               market. Be realistic about the
                                              (granular liver and spleen)
Table 7. Possible options in managing a recirculating tank system based on observations of the fish.
                Observation                                Possible cause                   Possible management
Excitable/darting/erratic swimming                  s    excess or intense         reduce sound level/pad sides of tank/reduce
                                                         sounds/lights             light intensity
                                                    s    parasite                  examine* fish with symptoms
                                                    s    high ammonia              check ammonia concentration
Flashing/whirling                                   s    parasite                  examine fish with symptoms
Discolorations/sores                                s    parasite/bacteria         examine fish with symptoms
Bloated/eyes bulging out                            s    virus or bacteria         examine fish with symptoms
                                                    s    gas bubble disease        check for supersaturation and examine fish
                                                                                   with symptoms
Lying at surface/not swimming off                   s    parasite                  examine fish with symptoms
                                                    s    low oxygen                check dissolved oxygen in tank
                                                    s    high ammonia or nitrite   check ammonia and nitrite concentrations
                                                    s    bad feed                  check feed for discoloration/clumping and
                                                                                   check blood of fish
                                                    s    high carbon dioxide       check carbon dioxide level
Crowding around water inflow/aerators               s    low oxygen                check dissolved oxygen in tank
                                                    s    parasite/disease          examine fish with symptoms
                                                    s    high ammonia or nitrite   check ammonia and nitrite concentrations
                                                    s    bad feed                  check feed for discoloration/clumping and
                                                                                   check blood of fish
Gulping at surface                                  s    low oxygen                check dissolved oxygen in tank
                                                    s    parasite/disease          examine fish with symptoms
                                                    s    high ammonia or nitrite   check ammonia and nitrite concentrations
                                                    s    high carbon dioxide       check carbon dioxide level
                                                    s    bad feed                  check feed for discoloration/clumping and
                                                                                   check blood of fish
Reducing feeding                                    s    low oxygen                check dissolved oxygen in tank
                                                    s    parasite/disease          examine fish with symptoms
                                                    s    high ammonia or nitrite   check ammonia and nitrite concentrations
                                                    s    bad feed                  check feed for discoloration/clumping and
                                                                                   check blood of fish
Stopping feeding                                    s    low oxygen                check dissolved oxygen in tank
                                                    s    parasite/disease          examine fish with symptoms
                                                    s    high ammonia or nitrite   check ammonia and nitrite concentrations
Discolored blood –                                  s    high nitrite              examine fish with symptom; add 5 to 6 ppm
    Brown                                                                          chloride for each 1 ppm nitrite; purchase
                                                                                   new feed and discard old feed
     Clear (no blood)                               s    vitamin deficiency        examine fish with symptom; check feed for
                                                                                   discoloration/clumping; purchase new feed
                                                                                   and discard old feed
Broken back or “S” shaped backbone                  s    vitamin deficiency        examine fish with symptom; purchase new
                                                                                   feed and discard old feed
*Have fish examined by a qualified fish diagnostician.
money, time and effort you are       Finally, design the system with an       Exclude diseases at stocking.
willing to invest while you are in   emergency aeration system, back-         Perform routine diagnostic checks
the learning curve of managing a     up power sources, and backup             and be prepared to treat diseases.
recirculating system.                system components. Monitor               Reduce stress whenever and how-
                                     water quality daily and maintain         ever possible. STAY ALERT!
                                     it within optimum ranges.

Table 8. Possible management options based on water quality and feed observations.
              Observation                                           Possible management
Low dissolved oxygen (less than 5 ppm)        s   increase aeration
                                              s   stop feeding until corrected
                                              s   watch for symptoms of new parasite/disease
High carbon dioxide (above 20 ppm)            s   add air stripping column
                                              s   increase aeration
                                              s   watch for symptoms of new paraside/disease
Low pH (less than 6.8)                        s   add alkaline buffers (sodium bicarbonate, etc.)
                                              s   reduce feeding rate
                                              s   check ammonia and nitrite concentarations
High ammonia (above 0.05 ppm as un-ionized)   s   exchange system water
                                              s   reduce feeding rate
                                              s   check biofilter, pH, alkalinity, hardness, and dissolved oxygen
                                                  in the biofilter
                                              s   watch for symptoms of new parasite/disease
High nitrite (above 0.5 ppm)                  s   exchange system water
                                              s   reduce feeding rate
                                              s   add 5 to 6 ppm chloride per 1 ppm nitrite
                                              s   check biofilter, pH, alkalinity, hardness, and dissolved oxygen
                                                  in the biofilter
                                              s   watch for symptoms of new parasite/disease
Low alkalinity                                s   add alkaline buffers
Low hardness                                  s   add calcium carbonate or calcium chloride
Discolored/clumped feed                       s   purchase new feed and discard old feed
                                              s   watch for symptoms of new parasite/disease
The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045
from the United States Department of Agriculture, Cooperative States Research, Education, and Extension Service.
                                                                                                   SRAC Publication No. 453

                                         April 1999                                                   VI

                        Recirculating Aquaculture Tank
                            Production Systems
                                 A Review of Component Options
                        Thomas M. Losordo1, Michael P. Masser2 and James E. Rakocy3

There is a great deal of interest in         tion site infrastructure, production   water through special filtration
recirculating aquaculture produc-            management expertise, and other        and aeration or oxygenation
tion systems both in the United              factors. Prospective users of recir-   equipment. Each component must
States and worldwide. Most fish              culating aquaculture production        be designed to work in conjunc-
grown in ponds, floating net pens,           systems need to know about the         tion with other components of the
or raceways can be reared in com-            required water treatment process-      system. For more information on
mercial scale recirculating sys-             es, the components available for       water quality requirements and
tems, but the economic feasibility           each process, and the technology       management of recirculating sys-
of doing so is not certain. Recircu-         behind each component. This            tems, see SRAC publications 451
lating systems are generally                 publication is intended as a start-    and 452.
expensive to build, which increas-           ing point for such a study.
es production cost. (For more                A recirculating system maintains       Waste solids removal
information see SRAC publication             an excellent cultural environment
456 on the economics of recirculat-                                                 The decomposition of solid fish
                                             while providing adequate feed for      waste and uneaten or indigestible
ing systems). The challenge to               optimal growth. Maintaining
designers of recirculating systems                                                  feed can use a significant amount
                                             good water quality is of primary       of oxygen and produce large
is to maximize production capaci-            importance in aquaculture. While
ty per dollar of capital invested.                                                  quantities of ammonia-nitrogen.
                                             poor water quality may not be          There are three categories of waste
Components should be designed                lethal to the crop, it can reduce
and integrated into the complete                                                    solids—settleable, suspended, and
                                             growth and cause stress that           fine or dissolved solids.
system to reduce cost while main-            increases the incidence of disease.
taining or even improving reliabil-          Critical water characteristics
ity.                                                                                Settleable solids
                                             include concentrations of dis-
Research and development in                  solved oxygen, un-ionized ammo-        Settleable solids are generally the
recirculating systems has been               nia-nitrogen, nitrite-nitrogen, and    easiest to deal with and should be
going on for nearly three decades.           carbon dioxide. Nitrate concentra-     removed from the culture tank
There are many alternative tech-             tion, pH, alkalinity and chloride      water as rapidly as possible. This
nologies for each process and                levels also are important.             is easiest when bottom drains are
operation. The selection of a par-                                                  properly placed. In tanks with cir-
                                             The by-products of fish metabo-
ticular technology depends upon                                                     cular flow patterns (round, octag-
                                             lism include carbon dioxide,
the species being reared, produc-                                                   onal, hexagonal, square with
                                             ammonia-nitrogen, and particu-
                                                                                    rounded corners) and minimal
                                             late and dissolved fecal solids.
1Department of Zoology, North Carolina                                              agitation, settleable solids can be
                                             Water treatment components must
                                                                                    removed as they accumulate in
 State University                            be designed to eliminate the
2Department of Wildlife and Fisheries                                               the bottom center of the tank, in a
                                             adverse effects of these waste
 Sciences, Texas A&M University                                                     separate, small flow-stream of
                                             products. In recirculating tank
3University of the Virgin Islands,                                                  water, or together with the entire
                                             systems, proper water quality is
 Agricultural Experiment Station, U.S.                                              flow leaving the tank. Center
                                             maintained by pumping tank
 Virgin Islands                                                                     drains with two outlets are often
used for the small flow-stream              settle out within the pipe while             In rectangular raceways with plug
process. This double drain divides          the clearer water overflows the              flow (flow that moves along the
the flow leaving the tank into a            standpipe. The external stand                long axis of the raceway tank),
small pipe carrying the settleable          pipe is routinely removed to                 solids are more difficult to remove
solids, and a larger pipe with a            increase the water velocity in the           as the velocity at the bottom of the
higher flow rate carrying the sus-          pipe and the settled solids are              tank is generally slower than in
pended solids from the upper                flushed from the line.                       round tanks. If the water velocity
water column of the tank (Fig. 1).                                                       at the tank bottom can be
                                            Another example of a double
                                                                                         increased to move the settled
                                            drain is a particle trap developed
                                                                                         solids along the bottom of the
              A           A                 at the Center for Scientific and
                                                                                         tank, then solids can be removed
                                            Industrial Research (SINTEF),
                                                                                         using a sediment trap. The sedi-
                                            Norwegian Hydrotechnical
                                                                                         ment trap should span the bot-
                           Standpipe        Laboratory, in Trondheim,
                                                                                         tom, across the short axis of the
                                                                                         raceway, perpendicular to the
                                            In this design, settleable solids            direction of water flow. Two
                                            flow under a plate, spaced just              reviews of tank flow and
     B                         B            slightly off the bottom of the tank,         hydraulic analysis can be found in
                                            in a flow of water that amounts to           Burley and Klapsis (1988) and
 Solids                                     only 5 percent of the total flow             Tvinnereim (1988).
 collection                                 leaving the center of the tank
                                                                                         An alternative to plug flow within
 bowl                                   B   (Flow B, Fig. 2). The larger flow
                                                                                         a raceway is to create a complete-
                                    A       (95 percent of the total) exits the
                                                                                         ly mixed (horizontally and verti-
                                            tank through a large discharge
                                                                                         cally) tank by installing a water
Figure 1. Typical double drain for          strainer mounted at the top of the
                                                                                         inlet and outlet manifold along
removing settleable solids from a fish      particle trap (Flow A, Fig. 2).
                                                                                         the long axis of the tank. As seen
                                            Outside of the tank, the settleable
culture tank; A = suspended solids                                                       in Figure 4, water enters uniform-
                                            solids flow-stream from the parti-
flow stream, B = settleable solids flow                                                  ly along the bottom of one side of
                                            cle trap enters a sludge collector
stream. (after Losordo, 1997).                                                           the raceway and is removed along
                                            (Flow B, Fig. 3). The waste parti-
                                                                                         the other side. Water must enter at
                                            cles settle and are retained in the
                                                                                         a high enough velocity to create a
Settled solids should be removed            sludge collector, and the clarified
                                                                                         rotational flow along the short
from the center of the tank on a            water exits the sludge collector at
                                                                                         axis of the raceway (Fig. 4). The
continuous or semi-continuous               the top and flows by gravity for
                                                                                         solids will move across the bot-
basis. The flow rate at which the           further treatment. The sludge in
                                                                                         tom of the raceway and into the
settleable solids are carried will          the collector, which has an aver-
                                                                                         effluent manifold.
determine the method used to col-           age dry weight solids content of 6
lect and concentrate them for fur-          percent, is drained from the bot-            Another method of dealing with
ther treatment or disposal. In sys-         tom of the collector.                        settleable solids is to keep them in
tems with a high settleable solids
flow rate (20 to 50 percent of the                               A                               A
total tank flow), swirl separators,
settling basins, or drum screen fil-
ters are used to collect these                       B                                                        B
solids. At lower flow rates, small-
er settling components can be
used. An example is a double                 Tank
drain developed by Waterline,                floor
Inc.1 (Prince Edward Island,                                                                                                 B
Canada). In this patented design,                                                                                  Settleable
the flow containing settleable                                                                                     solids flow
solids moves slowly though a
pipe (under the tank) leading to
an external standpipe (water level
control structure). The flow veloci-                                                         Main discharge                 A
ty is slow enough that the solids                                                                                 Suspended
                                                                                                                  solids flow
1Mention of a specific product or trade-
name does not constitute and endorsement    Figure 2. The ECO-TRAP™ particle trap is an advanced double drain design that
by the authors or the USDA Southern         concentrates much of the settleable solids in only 5 percent of the water flow leav-
Regional Aquaculture Center, nor does it    ing the fish culture tank (B). (after Hobbs et al., 1997). (ECO-TRAP is a trade-
imply approval to the exclusion of other    mark of AquaOptima AS, Pir Senteret, 7005 Trondheim, Norway, U.S. Patent
suitable products.
                                            No. 5,636,595.)
                                                                                       suspension with continuous agita-
                                          Top view                                     tion until they enter an external
                                                                         Flow B
                                                                                       settling tank. In settling tanks (or
                                                                         from          basins), water flow is very slow so
                                                                         particle      that solids settle out by gravity.
                                                                         trap          Settling tanks may or may not
 Clarified water
 to drum screen                                                                        include tube or lamella sedimen-
                                                                                       tation material. This material is
                                                                                       constructed with bundles of tubes
                                                                                       or plates, set at specific angles to
                                                                                       the horizontal (usually 60o), that
                                        Side view                                      reduce both the settling distance
                                                                                       and circulation within the settling
 Clarified water                                                         Flow B        tank. Using settling plates reduces
 to drum screen                                                          from          the size requirement of a settling
 filter                                                                  particle      basin, thus saving space within a
                                                                         trap          facility. However, the plates make
                                                                                       routine cleaning of settling basins
                                                                                       more time-consuming.
                                                                                       The benefits of using external set-
                                                                                       tling basins outside of the rearing
                                                                                       tank are simplicity of operation,
                                                                                       low energy requirements, and the
                                                                                       generally low cost of construction.
                                                                                       The disadvantages include the rel-
                                                                                       atively large size of settling
                                                                                       basins, the time used in routine
                                                                                       cleaning, and the large quantity of
                                                                                       water that is wasted in the clean-
                                                                                       ing process. If settling basins are
                                                                                       not cleaned regularly, waste solids
                                                                                       can break down within the basin
                                                                                       and contribute to the ammonia-
                                                                                       nitrogen production and oxygen
                                                                                       demand of the system.
                                      Sludge discharge
                                                                                       Another way to remove settleable
Figure 3. The sludge collector that works in conjunction with the ECO-TRAP™            solids, external to the culture tank,
to remove settled solids from the flow stream B (Fig. 2) (after Hobbs et al., 1997).   is to use a centrifugal settling
                                                                                       component known as a hydrocy-
                                                                                       clone or swirl separator. In this
                                                               Short                   design, water and particulate
                                                               axis                    solids enter the separator tangen-
                       Long                                                            tially, creating a circular or
                       axis                                                            swirling flow pattern in a conical
                                                                                       shaped tank. The heavier solids
                                                                                       move towards the walls and settle
                                                                                       to the bottom where they are
                                                                                       removed continuously. The main
                                                                                       advantage of these units is the
                                                                                       compact size. A major disadvan-
                                                                                       tage is the large volume of
                                                                                       replacement water required
                                                                                       because of the continuous stream
                                                                                       of wastewater.
 Influent                                                                              Suspended solids
                                                                                       From an engineering viewpoint,
                               B                                                       the difference between suspended
                           Effluent                                                    solids and settleable solids is a
Figure 4. Cross-section of a “cross-flow” raceway. Water flows in through an           practical one. Suspended solids
inlet manifold with jets (A) and out through a similar drain manifold (B) on the       will not easily settle out of the
opposite side of the tank (after Colt and Watten, 1988).                               water column in the fish culture
tank. Suspended solids are not                The screening material has been                   a high pressure jet of water (from
always dealt with adequately in               used in a disk configuration (Fig.                the outside of the drum) washes
recirculating systems. Most cur-              5A), drum screen configuration                    the solids off the screen and into
rent technologies for removing                (Fig. 5B), and incline belt configu-              an internal collection trough lead-
suspended solids generally                    ration (Fig. 5C).                                 ing to a waste drain. The advan-
involve some form of mechanical               In rotating disk filters, water to be             tage of the drum screen filter con-
filtration. Two types of mechanical           treated enters one end of the filter              figuration over the single plate
filtration are screen filtration and          unit and must pass through                        disk filter is the larger surface area
expandable granular media filtra-             sequential vertical disks within                  of the drum for comparably sized
tion.                                         the filter. A problem with this                   units.
Screen filtration: Screen filters             design is the small amount of                     The main advantage of using
use some form of fine mesh mate-              screen surface on which to capture                screen filter technology rather
rial (stainless steel or polyester)           solids. In heavily fed production                 than settling basins and swirl sep-
through which effluent passes                 systems, solids can build up so                   arators is their small size and rel-
while the suspended solids are                heavily on one side of the filter                 atively low water loss during
retained on the screen. Solids are            that the screens collapse from the                backwashing. Libey (1993) report-
usually removed from the screen               water pressure.                                   ed that, on average, in a tilapia
by rotating the clogged screen sur-           The most common screen filter is                  system, only 13.4 percent of the
face past high pressure jets of               the drum filter (Figs. 5B and 6).                 water used with a settling basin
water. The solids are carried away            With this configuration, water                    was needed with a drum screen
from the screen in a small stream             enters the open end of a drum                     filter.
of waste water. The feature that              and passes through a screen                       The main disadvantage of com-
makes each screen filter different            attached to the circumference of                  mercial screen filters is cost, espe-
and the challenge in designing                the drum.                                         cially for smaller units. The
these units is the process of col-                                                              smallest commercially available
lecting the solids on the mesh sur-           In most installations, the drum
                                              rotates only when the filter mesh                 units can process approximately
face.                                                                                           475 liters per minute (125 gpm)
                                              becomes clogged with solids, and
                                                                                                loaded with 25 mg/L of suspend-
                                                                                                          ed solids, and cost about
                                                                                                          $6,000. A 100 percent
        water                                                                                             increase in processing
                      Wastewater                                                                          capacity increases the cost
                                                                                                          of a unit by about 50 per-
                                                       Backwash                                           cent (a unit to process 950
                                           Inflow      water                                              liters/minute costs about
 Cleaned                                                                                  Wastewater
 water                                                                                                    $9,000). So, larger units
                                                                                                          are more cost effective. To
                                                                                                          take advantage of this,
                                                         Cleaned                                          the flow streams from
                                                         water                               Inflow       several production tanks
                                                                                                          can be combined into one
         Disk screens                                                                                     treatment stream that is
                                                                                       Drum screen
     A. Disk screen filter (top view)                      B. Drum screen filter (top view)
                                                                                                          cleaned by a larger drum
                                                                                                          screen filter. However, the
                                                                                                          advantage of the econo-
                                       Waste trough                                                       my of scale must be
                                                                                                          weighed against the risk
                                                                                                          of spreading disease and
                                                           Backwash spray                                 water quality problems
                                                            Belt screen                                   within linked fish pro-
                                                                                                          duction tanks.
                                                                                                          Vacuum cleaned drum
                                                                           Inflow                         screen filters are now in
                              Cleaned                                                                     use. These units have
                              water               Flow                                                    limited capacity (375 to
                                                                                                          1,800 L/ minute, 100 to
                                                                                                          475 gpm) and their per-
                               C. Incline belt screen filter (side view)                                  formance in commercial
                                                                                                          facilities has not been
Figure 5. Three screen filter configurations used in recirculating tanks to capture and remove
                                                                                                          well documented. Incline
suspended solids.                                                                                         screen or belt screen fil-
                                                                                                          ters also are beginning to
                                                                                                            drive motor

                                                                                                           Return flow to
                                                                                                           culture tank

                                                                                                                    bead medium

                                                                                                                  Flow from
                                                                                                                  culture tank

                                                                                                                Settled solids

Pressure                                                                                                      To waste
                                                                           to tank     Figure 7. The propeller washed bead
                                                                                       filter traps waste solids between the
                                                     Water filters                     beads and backwashes by expanding
                           Waste                     through screen                    the bed of beads with a propeller.
                           discharge                 on drum
                                                                                       (U.S. Patent No. 5,126,042 by Dr.
                                                                                       Ronald Malone, Dept. of Civil
       from tank                                                                       Engineering, Louisiana State
Figure 6. Typical drum screen filter (shown with a cut-away and expanded midsec-
tion) for waste solids removal from aquacultural recycle flow streams. (Drawing pro-
                                                                                       These low density, floating plastic
vided by and used with permission of PRA Manufacturing, Nanaimo, B.C.)                 beads trap and remove suspended
                                                                                       solids from the flow-stream as the
be used in the aquaculture indus-          water, swimming pools), the most            water passes up through a bed of
try (Fig. 5). These units resemble         common filtration medium is                 beads (Fig. 7).
conveyor belts placed on an                sand. Pressurized down-flow                 The solids are removed by activat-
incline. Water passes through the          sand filters have been widely               ing a motor that turns a propeller
screen where suspended solids are          used in hatchery operations.                located within the bed of beads.
retained; solids are lifted out of         While these filters can remove              The propeller expands the bed of
the water on the incline screen            much of the suspended solids in a           beads and releases the waste
and sprayed off with high pres-            flow-stream, when fish are fed              solids that are trapped within it.
sure water in a cleaning process           heavily the filter must be back-            After the bed expansion period, a
similar to that of disc and drum           washed frequently, which wastes             short settling period allows the
screen filters. The units manufac-         a lot of water. Backwashing these           beads to re-float and the solids to
tured currently have flow capaci-          filters is accomplished by revers-          settle to the bottom of the filter
ties in excess of 7,500 liters per         ing the flow of water through the           chamber. A valve is then opened
minute (1980 gpm). There is little         filter medium, causing the bed to           and the settled solids are
data on the operational character-         expand or “boil.” This releases             removed. This sequence of events
istics of these filters.                   trapped solids and scrapes bacter-          can be automated with electronic
Expandable granular media fil-             ial growth off the filter medium.           circuits and automated valves.
tration: Expandable granular               However, bacterial growth on the            Another bead filter design,
media filters remove solids by             sand eventually creates gelatinous          referred to as the “bubble
passing water through a bed of             masses within the filters that are          washed” bead filter, eliminates the
granular medium (sand or plastic           impossible to clean with simple             requirement for a propeller to
beads). The solids either adhere to        backwashing. Then it is necessary           backwash the filter bed. This filter
the medium or are trapped within           to open and manually clean the              resembles an “hour glass” with
the open spaces between the                filter. Down-flow sand filters              two chambers connected by a nar-
medium particles. Over time, the           reduce or stop the flow of water            row “washing throat” (Fig. 8).
filters will become clogged with           when they clog. Even short-term
                                           interruptions of water flow can be          In the filtration mode, water pass-
solids and require cleaning, or                                                        es up through the beads while
backwashing. Backwashing these             disastrous to intensive recirculat-
                                           ing systems.                                they are in the upper filtration
filters requires that the filter bed                                                   chamber. When the beads need
be expanded (from a compacted              An alternative design, used suc-            cleaning, the flow is stopped and
state) to release the solids. For          cessfully in the U.S., uses floating        the filter is drained so that the fil-
other applications (e.g., drinking         plastic beads instead of sand.              ter medium drops through the
                       Return to         nitrogen produced must be treat-           ture enters the body of the foam
                       tank              ed.                                        fractionator tangentially.
                      Floating bead
                      filter bed         Fine and dissolved solids                  Ammonia and nitrite-
                                         Many of the fine suspended solids          nitrogen control
     Filtration                          and dissolved organic solids that
     chamber                                                                        Controlling the concentration of
                                         build up within intensive recircu-         un-ionized ammonia-nitrogen
                                         lating systems cannot be removed           (NH3) in the culture tank is a pri-
                     Washing             with traditional mechanisms. A
                     throat                                                         mary design consideration in
                                         process called foam fractionation          recirculating systems. Ammonia-
                                         (also referred to as air-stripping or      nitrogen (a by-product of the
                                         protein skimming) is often                 metabolism of protein in feeds)
                                         employed to remove and control             must be removed from the culture
                            effluent     the build-up of these solids.              tank at a rate equal to the rate it is
      Sludge                from tank    Foam fractionation is a general            produced to maintain a stable and
      settling                           term for a process in which air
      chamber                                                                       acceptable concentration. In sys-
                                         introduced into the bottom of a            tems with external ammonia-
                          Filter         closed column of water creates
                          support                                                   nitrogen treatment processes, the
                                         foam at the surface of the column.         efficiency of the ammonia-nitro-
                        sludge           Foam fractionation removes dis-            gen removal process will dictate
                                         solved organic compounds (DOC)             the recirculating flow rate (e.g., a
Figure 8. The bubble bead filter oper-   from the water column by physi-            less efficient removal system will
ates much like the propeller washed      cally adsorbing DOC on the rising          require a higher recycle flow rate
                                         bubbles. Fine particulate solids           from the tank through the filter).
bead filter, except that it backwashes
                                         are trapped within the foam at the         There are a number of methods
by dropping the filtration medium by     top of the column, which can be
gravity through a washing throat.                                                   for removing ammonia-nitrogen
                                         collected and removed. The main            from water: air stripping, ion
(U.S. Patent No. 5,232,586 by Dr.        factors affected by the operational        exchange, and biological filtra-
Ronald Malone, Dept. of Civil            design of the foam fractionator            tion. Air stripping of ammonia-
Engineering, Louisiana State             are bubble size and contact time           nitrogen through non-flooded (no
University)                              between the air bubbles and the            standing water in the reactor)
                                         DOC. A counter-current design              packed columns requires that the
“washing throat” into the sludge         (bubbles rising against a down-            pH of the water be adjusted to
settling chamber. When the flow          ward flow of water) improves               above 10 and readjusted to safe
is re-started, the filter medium         efficiency by lengthening the con-         levels (7 ro 8) before the water re-
floats back into the filter chamber      tact time between the water and            enters the culture tank. Ion
and the waste sludge settles to the      the air bubbles (Fig. 9). In this          exchange technology is costly and
bottom of the settling chamber           design, water is injected into the         requires a mechanism for “wast-
ready for discharge to a waste           foam fractionator through a ven-           ing” ammonia-laden salt water. A
drain.                                   turi. The venturi mixes air with           salt-brine is used to “regenerate”
                                         the water and the air/water mix-           the filter by removing ammonia-
The advantage of bead filters is
the compact size of the unit and                                                    nitrogen from the resin (filter
                                                                    Foam            medium) once it becomes saturat-
low water use during backwash-                                      collection
ing. Once biologically active, the                                  and             ed with ammonia-nitrogen.
beads become sticky and can                       Foam                              Biological filtration is the most
remove even fine suspended                        removal                           widely used method. In biological
solids. The bacteria that make the                                                  filtration (or biofiltration), there is
filter sticky are a combination of       inflow                                     a substrate with a high specific
autotrophic and heterotrophic                                                       surface area (large surface area
bacteria. The autotrophic bacteria                  inflow                          per unit volume) on which the
contribute to nitrification. The                                                    nitrifying bacteria can attach and
heterotrophic bacteria break down        Water                                      grow. Ammonia and nitrite-nitro-
the organic solids that are trapped                                                 gen in the recycled water are oxi-
within the bead bed. This can be a                Venturi                           dized (converted) to nitrite and
disadvantage, because during the                                       outflow      nitrate by Nitrosomonas and
time between backwashings (1 to                                                     Nitrobacter bacteria, respectively.
48 hours), solids undergoing bac-        Figure 9. A pump-driven, venturi-          Commonly used biofilter sub-
terial degradation use oxygen            type foam fractionator design. A           strates include gravel, sand, plas-
from the system water and release        water/air mixture is injected tangen-      tic beads, plastic rings, and plastic
ammonia-nitrogen. The oxygen             tially into the foam fractionator (after   plates. The most common biofil-
consumed by these bacteria needs         Losordo, 1997).                            tration technologies are discussed
to be replaced and the ammonia-                                                     below.
Rotating biological contactor
                                                                     RBC Biofilter media
Rotating biological contactors
(RBC) have been used in the treat-
ment of domestic wastewater for
decades, and are now widely used
as nitrifying filters in aquaculture                                                                               Drive
applications. RBC technology is                                                                                    motor
based on the rotation of a biofilter    Water
medium attached to a shaft, par-        level
tially submerged in water.                                                                                     Flow from
Approximately 40 percent of the                                                                                culture tank
substrate is submerged in the
                                                                   Flow to
recycle water (Fig. 10). Nitrifying                                culture tank
bacteria grow on the medium and
rotate with the RBC, alternately                                       RBC filter tank
contacting the nitrogen-rich water
and the air. As the RBC rotates, it     Figure 10. A rotating biological contactor unit powered by an electric gear motor.
exchanges carbon dioxide (gener-
ated by the bacteria and fish) for
                                        of 3.6 kg feed/day/m3 of medium              treatment. This type of filter con-
oxygen from the air. The tangen-                                                     sists of a water distribution sys-
                                        should be used (0.189
tial velocity of the outer edge of
                                        pounds/day/ft3 of medium).                   tem at the top of a reactor filled
the RBC should be about 35 to 50                                                     with a medium that has a relative-
feet per minute. For example, an        The filter medium increases in
                                                                                     ly low specific surface area, gener-
RBC with a diameter of 4 feet           weight as much as 10 fold during
                                                                                     ally less than 330 m2/m3 (100 ft2/
would rotate at 3 to 4 revolutions      operation, so the support struc-
                                                                                     ft3). This creates large void (air)
per minute (rpm). The advantages        ture must accommodate the addi-
                                                                                     spaces within the filter medium
of RBC technology are simplicity        tional weight.
                                                                                     (Fig. 11). As these filters are oper-
of operation, the ability to remove                                                  ated in a non-flooded configura-
carbon dioxide and add dissolved        Trickling filters
                                                                                     tion, they provide nitrification,
oxygen, and a self-cleaning capaci-     Trickling filters used in aquacul-           aeration, and some carbon dioxide
ty. Major disadvantages are the         ture systems have evolved from               removal in one unit. (The term
high capital cost and mechanical        those used in domestic sewage                non-flooded is used to indicate
instability. Poorly designed or
built RBCs can break down
                                             Rotating                                              Water inflow
mechanically with the weight of                                                                    from culture
the biological growth on the filter          water
                                             distribution                                          tank
medium. RBCs also have been
designed to be turned by water               arm
(similar to a water wheel) and
compressed air.
In early aquaculture applications,
RBCs had simple discs cut from                                                                        Biofilter
corrugated fiberglass plate. Now                                                                      blocks or
they use media with high specific                                                                     plastic rings
surface area, such as plastic blocks
or a polyethylene tubular medium
(resembling hair curlers). These
newer plastic media remove more
ammonia, nitrite-nitrogen and car-
bon dioxide in small RBC units.                  Low
The plastic media have specific                  pressure
surface areas of up to 200 m2/m3                 air
(69 ft2/ft3). In aquaculture applica-            inflow
tions, volumetric nitrification rates
of approximately 76 g TAN/m3
                                                                                                  Water return
per day can be expected with this                                                                 to culture
type of biological filter (Wheaton                Filter                                          tank
et al., 1994). When including these               support
filters in a recirculating system as              legs
a nitrifying filter component
(assuming 2.5 percent of the feed       Figure 11. Trickling filters are non-submerged biological filters in which the water
becomes TAN), a design criterion        is evenly distributed over the medium.
that the biological filter medium is   expected with this type of biologi-         sheared from the medium so that
not completely submerged in            cal filter (Beecher et al., 1997).          the filter is self-cleaning. The main
water). The flow rate through          When designing these filters into           advantage of fluidized bed tech-
trickling filters is limited by the    a recirculating system as a nitrify-        nology is the high nitrification
void space through which water         ing and solids removal compo-               capacity in a relatively compact
can pass. In general, packing          nent (assuming 2.5 percent of the           unit. The sand also is extremely
media with more void space can         feed becomes TAN), a design cri-            low cost. Fluidization (pumping)
pass a higher rate of flow per         terion of 13 kg of feed/day/m3 of           requirements depend upon the
square meter of (top) cross sec-       medium should be used (0.81                 size and weight of the medium
tional surface area. The main dis-     pounds/day/ft.3 of medium; the              being used. Keep in mind that the
advantage of trickling filters is      manufacturer recommends a                   buoyancy of the medium changes
that they are relatively large and     design rate of 1.0 pound/day/ft3).          with the amount of biological
biofilter media are expensive.                                                     growth on the medium. This, in
Also, if the recycled water is not     Fluidized bed filters                       turn, depends upon the water
prefiltered to remove suspended                                                    temperature, nutrient loading
solids, trickling filters can become   Fluidized bed filters are essential-        rate, and degree of bed fluidiza-
clogged over time. As with RBC         ly sand filters operated continu-           tion.
media, the weight of the biologi-      ously in the expanded (backwash)
                                       mode. Water flows up through a              Unless there is a system for recov-
cal growth on the filter media                                                     ering sand as water leaves the fil-
should be considered in designing      bed of sand at a rate sufficient to
                                       lift and expand (fluidize) the bed          ter, the medium will need to be
the support structure.                                                             replaced routinely. Depending
                                       of sand and keep the sand parti-
Volumetric nitrification rates of      cles in motion so that they no              upon the temperature, nutrient
approximately 90 g TAN/m3 per          longer are in continuous contact            concentration and size of the
day can be expected with this          with each other (Fig. 12).                  medium (and assuming 2.5 per-
type of biological filter (Losordo,    Fluidized bed filters use sand of           cent of the feed becomes TAN), a
unpublished data). When design-        smaller diameter than that used in          design criterion of 20 to 40 kg of
ing these filters into a recirculat-   particulate solids removal applica-         feed/day/m3 of medium should
ing system as a nitrifying filter      tions. Plastic beads with densities         be used (1.25 to 2.5
component (assuming 2.5 percent        slightly greater than water also            pounds/day/ft3 of medium).
of the feed becomes TAN), a            have been used successfully in
design criteria of 3.6 kg feed/        fluidized bed filters. A fluidized          Mixed bed reactors
day/m3 of medium should be             bed filter is an excellent environ-
used (0.225 pound/day/ft3 of                                                         Mixed bed reactors are a new and
                                       ment for the growth of nitrifying             interesting cross between upflow
medium).                               bacteria, and bacteria can colonize           plastic bead filters and fluidized
                                       the entire surface area of the filter         bed reactors. These filters use a
Expandable media filters               medium. The turbulent environ-                plastic medium kept in a continu-
The expandable media floating          ment also keeps the bacteria                  ous state of movement (Fig. 13).
bead filters described in the previ-                                                                       The diameter
ous section (Figs. 7 and 8 are also                                                                        of the plastic
used as biofilters in some aquacul-                                                                        medium is
ture applications. Generally oper-                                                                         usually much
ated as upflow filters, the beads                                                      Water return
                                                                                       to culture          larger than
have a high specific surface area                                                                          sand, so it has
on which nitrifying bacteria can                                                       tank
                                         Biofilter                                                         a lower spe-
colonize. The major advantage of         medium-                                                           cific surface
this technology is the combination       fluidized
                                         sand                                                              area (800 to
of nitrification and the solids                                                                            1,150 m2/m3;
removal processes into one com-                                                                            240 to 350
ponent. The disadvantage, as                                                                               ft2/ft3). The
noted before, is that solids are                                                                           beads are usu-
held in a place where they can                                                                             ally neutrally
degrade and affect the system’s                                                                            buoyant or
water quality. In general, using                                                                           just slightly
these filters will require the                                                                             heavier than
designer to provide for more oxy-                                                                          water. The
genation and biofiltration capaci-      Water                                             Perforated
                                        inflow                                            plate for water plastic beads
ty. The plastic bead medium used        from                                              distribution     are usually
in these filters has a specific sur-    culture                                                            mixed by
face area of 1,150 to 1,475 m2/m3       tank                                                               mechanical or
(350 to 450 ft2/ft3). Volumetric                                                                           hydraulic
nitrification rates of approximate-    Figure 12. A simplified view of a fluidized sand bed biological     means. Mixed
ly 325 g TAN/m3/day can be             filter.                                                             bed filters are
 Water                                               aerating only the water      cent of the feed rate (that is, 0.5 kg
 inflow                                              flowing into the culture     O2/kg of feed fed). In a system
 from                                    Water       tank will not usually        fed 4.5 kg (10 pounds) of feed
 culture                                 return      supply an adequate           over an 18-hour period,the esti-
 tank                                    to culture amount of oxygen for          mated oxygen consumption rate
                                         tank        fish production. The         would be approximately 0.125 kg
     Biofilter                                       amount of oxygen that        O2/hour (0.28 pounds/hour).
     mixed                                           can be carried to the        With an actual oxygen transfer
     plastic                                         fish in this way is limit-   efficiency of 0.455 kg O2/kW-h
     beads                                           ed by the flow rate and      (0.75 pounds/hp-h), the diffused
                                                     the generally low con-       aeration system would require a
                                                     centration of oxygen in      blower of approximately 0.275 kw
                                                     water. Therefore, most       (1/3 hp) to provide an adequate
                                                     aeration in recirculating    amount of oxygen. If the fish are
                                                     systems occurs in the        going to be fed over a shorter
                                                     culture tank. The most       period of time, then peak oxygen
                                                     efficient aeration           demand should be estimated and
                                                     devices are those that       the blower capacity should be
Figure 13. A common configuration for a mixed bed    move water into contact      increased.
reactor biological filter.                           with the atmosphere          The density of fish production
                                                     (paddlewheels, pro-          with aeration alone is usually lim-
designed as up-flow or down-              peller-aspirators, vertical-lift        ited to 30 to 40 kg of fish/m3 of
flow filters and, like fluidized bed      pumps). However, these methods          culture tank volume (0.25 to 0.33
filters, they generate biological         usually create too much turbu-          pounds of fish/gal.). In green-
solids but will not clog because of       lence within a culture tank to be       house systems where algal blooms
the continuous movement of the            useful. The most common way to          are common, oxygen is generated
medium. The plastic medium                aerate in a recirculating tank sys-     during the daylight hours, and
moves through a pipe within the           tem is called diffused aeration.        culture densities of up to 60 kg of
main reactor to vertically mix the        Diffused aeration systems provide       fish/m3 of culture volume (0.50
bead bed. Depending upon the              low pressure air from a “regenera-      pounds of fish/gal.) can be
nutrient concentration and medi-          tive” type of blower to some form       achieved.
um size (and assuming 2.5 percent         of diffuser near or on the bottom
                                          of a culture tank. These diffusers      Packed column aerators: An ideal
of the feed becomes TAN), a                                                       location for aerating and degass-
design criterion of 16 to 23 kg of        produce small air bubbles that rise
                                          through the water column and            ing water (i.e., removing carbon
feed/day/m     3 of medium should
                                          transfer oxygen from the bubble         dioxide) is in the recycle flow-
be used (1.0 to 1.4                                                               stream just before it re-enters the
pounds/day/ft     3 of medium).           to the water.
                                                                                  culture tank. As mentioned previ-
                                          Studies have determined that dif-       ously, however, this method does
Dissolved gas                             fused aeration systems can trans-       not usually supply enough oxy-
                                          fer oxygen at an average rate of        gen. With submerged biological
Recirculating systems should              1.3 kg O2/kW-h (2.15 lbs./hp -
maintain adequate dissolved oxy-                                                  filtration, the concentration of dis-
                                          hour) under standard (20o C,            solved oxygen will most likely be
gen (DO) concentrations of at             O mg/L DO, clean water) test
least 6 mg/L and keep carbon                                                      lowest and carbon dioxide highest
                                          conditions (Colt and Tchoba-            at the outflow of this component.
dioxide (CO2) concentrations at           noglous, 1979). However, these
less than 25 mg/L for best fish                                                   Packed column aerators (PCA) are
                                          values must be corrected to             an effective and simple means of
growth. Colt and Watten (1988)            account for the actual fish culture
and Boyd and Watten (1989) dis-                                                   aerating water that is already in a
                                          conditions. To achieve acceptable       flow-stream. A packed column
cuss aeration and oxygenation             fish growth rates, the DO concen-
systems used in aquaculture; a                                                    aerator can be identical in design
                                          tration should be kept at 5 mg          to a trickling nitrifying filter
summary of the component                  O2/L or higher. At water tempera-
options follows.                                                                  (Fig.11). Water is introduced into a
                                          tures of 28o C, according to Boyd       reactor filled with medium.
The term aeration is used here to         (1982), the diffuser system’s oxy-      Proper design criteria include
refer to the dissolution of oxygen        gen transfer rate would be only 35      non-flooded operation and free air
from the atmosphere into water.           percent of the rate at standard         exchange through the reactor.
The transfer of pure oxygen gas to        conditions. In this case, the oxy-      Given a PCA influent DO concen-
water is referred to as oxygena-          gen transfer rate would be              tration of 4 mg O2/L, an effective
tion.                                     reduced to 0.455 kg O2/kW-h             oxygen transfer rate of 0.75 kg
                                          (0.75 lbs./hp - hour). In a well        O2/kw-h (1.25 pounds O2/hp-h)
Aeration                                  designed recirculating system           can be attained. While this is a
                                          (one in which solids are removed        low transfer rate, the true energy
Diffused aeration: Adding oxy-            quickly), the oxygen consumption
gen to a recirculating system by                                                  cost for using a PCA in combina-
                                          rate can be estimated as 50 per-
tion with an existing flow-stream       Water
is only the energy required to          with                                                 Off-gas recycle
                                                              Oxygen            Inflow
pump water 1.0 to 1.25 meters (3        low                                     water
                                        oxygen                gas
                                                              inlet             low DO
to 4 feet) to the top of the PCA. If    content
the PCA is to be used for carbon                                                                               Outflow
dioxide stripping, a low pressure                         Off gas
                                                                                                               high DO
air blower should be used to force
at least five times as much air as                                  Water
water (by volume) up through the                                    high
PCA medium.                                                         oxygen
Pure oxygen is used in recirculat-     Figure 14. Down-flow bubble contact
ing systems when the intensity of      aerator (after Colt and Watten, 1988).
production causes the rate of oxy-                                              Figure 15. Typical u-tube oxygen dif-
gen consumption to exceed the          equals the upward velocity of the        fusion design.
maximum feasible rate of oxygen        bubbles. This allows a long con-
transfer through aeration. Sources     tact time between the water and          up to 250 percent of atmospheric
of oxygen gas include compressed       bubbles and nearly 100 percent           saturation (15 to 20 mg/L).
oxygen cylinders, liquid oxygen        absorption of the injected gas. The
(often referred to as LOX), and on-                                             Low head oxygenation system:
                                       dissolved oxygen concentration of        The multi-staged low head oxy-
site oxygen generators. In most        water leaving a DFBC can be as
applications, the choice is between                                             genator (LHO) oxygenates flow-
                                       high as 25 mg/L given a system           ing water where there is only a
bulk liquid oxygen and an oxygen       pressure of approximately 1 bar
generator. The selection of the                                                 small elevation difference between
                                       (14.7 PSI).                              the source of the water and the
oxygen source will be a function
of the cost of bulk liquid oxygen      U-tube diffusers: At high operat-        culture tank. This situation is
in your area (usually dependent        ing pressures, more oxygen can be        often found in raceway systems
on your distance from the oxygen       absorbed by water. A u-tube oxy-         set up in series. That is, the out-
production plant) and the reliabil-    gen diffusion system is an energy        flow of one raceway is just slight-
ity of the electrical service needed   efficient method of adding pres-         ly (1 to 3 feet) above the inflow of
for generating oxygen on-site.         sure to a flow-stream. A typical         an adjacent raceway. This tech-
                                       u-tube consists of a contact loop,       nology is a patented component
Adding gaseous oxygen directly         usually a pipe within a pipe (Fig.       (U.S. Patent No. 4,880,445; Water
into the culture tank through dif-     15), buried in the ground to at          Management Technologies, P.O.
fusers is not the most efficient       least 10 meters (33 feet), the height    Box 66125, Baton Rouge, LA) and
way to add pure oxygen gas to          of water required to add one             is made up of a perforated, hori-
water. At best, the efficiency of      atmosphere of pressure (1 bar,           zontal distribution plate and mul-
such systems is less than 40 per-      14.7 PSI). The contact loop is           tiple, adjacent, vertical contact
cent. A number of specialized          placed below tank level to mini-         chambers (Fig.16). Pure gaseous
components have been developed         mize energy requirements, rather         oxygen enters one (end) contact
for use in aquaculture applica-        than pumping water up hill to            chamber and oxygen with off-
tions. For an extensive review of      gain the extra hydrostatic pressure      gases (nitrogen and CO2) exits the
component options, see Boyd and        created by a column of water.            adjacent contact chamber.
Watten (1989). A review of the         Oxygen is mixed with the water at
more commonly used components                                                   The oxygen transfer capability of
                                       the entrance to the u-tube and           this system is determined by the
follows.                               travels with the flow to the bot-        length of water fall, gas and water
Down-flow bubble contactor: A          tom of the water column. The             flow rates, the DO concentration
properly designed low pressure         additional pressure from the             of the influent water, and the
oxygen diffusion system can            water column accelerates the rate        number of contact chambers
transfer more than 90 percent of       of oxygen absorption into the            (Watten 1994). Including packing
the oxygen injected through the        water. The principal advantages of       medium in the contact chambers
component. One such system is a        this system are the low energy           can improve performance.
down-flow bubble contact aerator       requirements for oxygenating
(DFBC), also referred to as a          large flow-streams and the resis-        Pressurized packed columns:
bicone or a Speece cone. The           tance to clogging with particulate       Pressurized packed columns are
DFBC system consists of a cone-        solids. The major disadvantage is        usually operated in a flooded
shaped reactor with a water and        the construction cost of drilling        mode (water fills the reactor).
oxygen input port at the top (Fig.     the shaft and installing the u-tube.     Water enters the top of a pressur-
14). As the water and oxygen bub-      Oxygen transfer efficiencies are         ized chamber that contains a
bles move down the cone, the           generally below 70 percent, with         medium with a high specific sur-
flow velocity decreases until it       effluent oxygen concentrations of        face area (much like packed tow-
                                                                                ers). Oxygen gas is usually intro-
                               Gaseous   ation, and the level of penetration     O3 in the water with the microor-
                               oxygen    of the radiation into the water. To     ganisms. Ozone must be generat-
                               inflow    be effective, microorganisms must       ed on-site because it is unstable
oxygen                                   come in close proximity to the UV       and breaks down in 10 to 20 min-
                                         radiation source (0.5 cm, 0.2 inch-     utes. Ozone is usually generated
                                         es or less). Turbidity reduces its      with either a UV light or a corona
                                         effectiveness. For a UV radiation       electric discharge source. There
           outflow                       system to be effective, the water       are many commercial ozone gen-
                                         should be pre-filtered with some        eration units available.
                                         form of particulate filtration          Ozone is usually diffused into the
                                         device.                                 water of a recirculating system in
                          Oxygenated     The most popular and effective          an external contact basin or loop.
                          effluent       type of UV sterilization unit is one    Water must be retained in this
Figure 16. Multi-staged low head oxy-    with a submerged UV radiation           side-stream long enough to ensure
                                         source. In this type of unit, recy-     that microorganisms are killed
genator with front plate removed to
                                         cled water passes by an elongated       and the ozone molecules are
show component detail (after Losordo,    UV lamp (much like a neon light         destroyed. Residual ozone enter-
1997).                                   bulb). The lamp is inside a quartz      ing the culture tank can be very
                                         glass, watertight jacket and does       toxic to crustaceans and fish.
duced at the bottom of the col-          not come in direct contact with         Ozone in the air is also toxic to
umn and travels upward, counter          the water. The UV lamp and              humans in low concentrations.
to the water flow. Oxygen transfer       quartz tube are held within a           Great care should be taken in
efficiency can range from 50 to 90       small diameter pipe through             venting excess ozone from the
percent with effluent dissolved          which the treated water flows. As       generation, delivery, and contact
oxygen concentrations in excess of       water passes along and around           system to the outside of the build-
100 mg/L. The major disadvan-            the UV lamp, microorganisms are         ing. Ozonation systems should be
tages of this system are high ener-      exposed to the UV radiation.            designed and installed by experi-
gy requirements (to provide the          Keeping the quartz jacket clean is      enced personnel.
pressure) and the buildup of bio-        imperative to the proper opera-
logical growth on the packing            tion of the unit. UV sterilization      Summary
medium, which makes periodic             units are usually rated by their
cleaning necessary.                      manufacturers according to their        This publication has outlined the
                                         water flow rate capacity. Increased     major components and options
                                         efficiency can be achieved by           used in recirculating aquaculture
Disinfection                                                                     production systems. This is by no
                                         reducing the flow rate through a
Diseases can spread quickly              given unit. The main disadvan-          means a complete listing, new
because of the density of fish in        tage of UV sterilization is the need    technologies are continually being
recirculating systems. Some chem-        for clean water with low suspend-       developed. One should not
icals used to treat diseases have a      ed solids concentrations. Clear         attempt to simply link the compo-
devastating effect on the nitrifying     water is not always economically        nents discussed here and expect
bacteria within the biofilter and        achievable in heavily fed recircu-      to have a properly operating sys-
culture system. Alternatives to tra-     lating systems. Additionally, the       tem. Any system you buy should
ditional chemical or antibiotic          expensive UV lamp must be               be the result of years of develop-
treatments include the continuous        replaced periodically. The main         ment, with each component prop-
disinfection of the recycled water       advantage of UV sterilization is        erly sized and integrated for opti-
with ozone or ultraviolet irradia-       that it is safe to operate and is not   mal performance. When review-
tion. For more information on dis-       harmful to the cultured species.        ing your options, always seek the
ease treatment in recirculating                                                  assistance of a knowledgeable,
systems, see SRAC publication            Ozonation                               experienced person, one who has
452 on the management of recircu-                                                designed a currently operating
lating systems.                          Ozone (O3) gas is a strong oxidiz-      and economically viable recircu-
                                         ing agent in water. Ozone has           lating fish production system.
Ultraviolet irradiation                  been used for years to disinfect
                                         drinking water. However, because        References and
Microorganisms (including dis-           of the high levels of dissolved and
ease-causing bacteria) are killed        suspended organic materials in          suggested readings
when exposed to the proper               recirculating systems, the effect of    Boyd, C.E. 1982. Water quality man-
amount of ultraviolet (UV) radia-        ozone on bacterial populations is         agement for fish pond culture.
tion. Spotte (1979) notes that the       questionable (Brazil et al., 1996).       Elsevier Scientific Publishing
effectiveness of UV sterilization        The efficiency of the disinfecting        Company, Amsterdam, the
depends upon the size of the             action depends upon the contact           Netherlands.
organism, the amount of UV radi-         time and residual concentration of
Boyd, C.E. 1991. Types of aeration              Grace, G.R. and R.H. Piedrahita.                 Malone R.F. and D.G. Burden. 1988.
  and design considerations. In: L.               1989. Carbon dioxide removal in a                Design of recirculating soft craw-
  Swann (editor), Proceedings of the              packed column aerator. Presented                 fish shedding systems. Louisiana
  Second Annual Workshop:                         paper at the International Summer                Sea Grant College Program,
  Commercial Aquaculture Using                    Meeting of Am. Soc. Ag. Eng. and                 Louisiana State University, Baton
  Water Recirculating Systems.                    Can. Soc. Ag. Eng., June 25-28,                  Rouge, LA.
  Illinois State University, Normal,              1989, Quebec, PQ, Canada.                      Spotte, S. 1979. Fish and invertebrate
  Illinois. Nov. 15-16, 1991. pp. 39-           Hobbs, A., T. Losordo, D. DeLong, J.               culture: Water management in
  47.                                             Regan, S. Bennett, R. Gron and B.                closed systems. John Wiley &
Boyd, C.E. and B.J. Watten. 1989.                 Foster. 1997. A commercial, public               Sons, New York, NY.
  Aeration systems in aquaculture.                demonstration of recirculating                 Timmons M.B. and T.M. Losordo
  CRC Critical Reviews in Aquatic                 aquaculture technology: The                       (editors). Aquaculture water reuse
  Sciences 1: 425 - 472.                          CP&L/EPRI Fish Barn at North                      systems: Engineering , design and
Brazil, B.L., S.T. Summerfelt and G.S.            Carolina State University. In: M.B.               management. Developments in
   Libey. Applications of ozone to                Timmons and T.M. Losordo (edi-                    Fisheries Sciences 27. Elsevier
   recirculating aquaculture systems.             tors). Advances in aquacultural                   Scientific Publishing Company,
   In: G.S. Libey and M.B. Timmons                engineering. Proceedings from the                 Amsterdam, The Netherlands.
   (editors), Successes and Failures in           aquacultural engineering society
                                                  technical sessions at the fourth               Tvinnereim, K. 1988. Design of water
   Commercial Recirculating                                                                         inlets for closed fish farms. In:
   Aquaculture. Proceeding from the               international symposium on tilapia
                                                  in aquaculture. NRAES-105.                        Proceedings of the Conference:
   Successes and Failures in                                                                        Aquaculture Engineering:
   Commercial Recirculating                       Northeast Regional Agricultural
                                                  Engineering Service, 152 Riley-                   Technologies for the Future.
   Aquaculture Conference. Roanoke,                                                                 Sterling Scotland. IChemE
   VA. July 19-21, 1996. NRAES-98.                Robb Hall, Ithaca, NY. pp. 151-158.
                                                                                                    Symposium Series #111, EFCE
   Northeast Regional Agricultural              Huguenin, J.E. and J. Colt. 1989.                   Publications Series # 66, Rugby,
   Engineering Service, 152 Riley-                Design and operating guide for                    UK. pp. 241-249.
   Robb Hall, Ithaca, NY., pp. 373-389.           aquaculture seawater systems.
                                                  Elsevier Scientific Publishers,                Watten, B.J. 1994. Aeration and oxy-
Burley, R. and A. Klapsis. 1988.                                                                   genation. In: M.B. Timmons and
  Making the most of your flow (in                Amsterdam, The Netherlands.
                                                                                                   T.M. Losordo (editors), Aquacul-
  fish rearing tanks). In: Proceedings          Libey, G.S. 1993. Evaluation of a drum             ture water reuse systems:
  of the Conference: Aquaculture                   filter for removal of solids from a             Engineering, design and manage-
  Engineering: Technologies for the                recirculating aquaculture system.               ment. Developments in Fisheries
  Future, Sterling Scotland. IChemE                In: J.K. Wang (editor), Techniques              Sciences 27. Elsevier Scientific
  Symposium Series #111, EFCE                      for Modern Aquaculture.                         Publishing Company, Amsterdam,
  Publications Series # 66, Rugby,                 Proceedings of an Aquacultural                  The Netherlands.
  UK. pp. 211-239.                                 Engineering Conference. Spokane,
                                                   WA, June 1993. American Society               Wheaton, F.W., J.N. Hochheimer, G.E.
Colt, J.E. and G. Tchobanoglous. 1979.                                                             Kaiser, R.F. Malone, M.J. Krones,
  Design of aeration systems for                   of Agricultural Engineers, St.
                                                   Joseph, MI. pp. 519.532.                        G.S. Libey and C.C. Estes. 1994.
  aquaculture. Department of Civil                                                                 Nitrofication filter design methods.
  Engineering, University of                    Losordo, T.M. 1997. Tilapia culture in             In: Timmons, M.B. and T. M.
  California, Davis, CA.                          intensive recirculating systems. In:             Losordo (editors), Aquaculture
Colt, J. and B. Watten. 1988.                     Costa-Pierce, B. and Rakocy, J. (edi-            water reuse systems: Engineering,
  Applications of pure oxygen in fish             tors), Tilapia Aquaculture in the                design and management. Develop-
  culture. Aquacultural Engineering               Americas, Volume 1. World                        ments in Fisheries Sciences 27.
  7:397-441.                                      Aquaculture Society, Baton Rouge,                Elsevier Scientific Publishing
                                                  LA. pp. 185-208.                                 Company, Amsterdam, The
                                                                                                   Netherlands. pp. 125-171.

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045 from
the United States Department of Agriculture, Cooperative States Research, Education, and Extension Service.
                                                                                                   SRAC Publication No. 456

                                     November 1998

                  The Economics of Recirculating
                         Tank Systems:
                        A Spreadsheet for Individual Analysis
                                Rebecca D. Dunning1, Thomas M. Losordo2 and Alex O. Hobbs3

A well-designed recirculating                the same format can be used to         Proper sizing of all system com-
aquaculture system offers a num-             monitor costs and returns once         ponents is very important. If
ber of advantages over pond sys-             systems are operating. The Excel       equipment is oversized for the
tems. Designed to conserve both              spreadsheet can be downloaded          application, the system will func-
land and water resources, recircu-           from the following Internet            tion but will be very costly. If
lating systems can be located in             address: http://www.agr.state.         equipment is undersized, the sys-
areas not conducive to open pond                 tem will not be able to maintain
culture. Operators have a greater            The spreadsheet in this publica-       the proper environment to sustain
degree of control of the fish cul-           tion uses tilapia for the example      fish production.
ture environment and can grow                species. However, the resulting        Operators should size equipment
fish year-round under optimal                figures on costs and returns are       according to the maximum daily
conditions. The crop can be har-             not meant to be used as an eco-        amount of feed placed into the
vested at any time, and inventory            nomic analysis of tilapia produc-      system. The estimated daily feed
can be much more accurately                  tion. Each individual using the        rate is based on the system carry-
determined than in ponds. This               spreadsheet should input equip-        ing capacity, which does not usu-
latter characteristic is particularly        ment and supply costs and the          ally exceed 1 pound of fish per
beneficial when trying to gain               appropriate market price for the       gallon of water for even the most
financing or insurance for the               specific system being analyzed.        efficient system. Once carrying
crop.                                                                               capacity and feed rate are defined,
Because of these advantages,                 System design                          the operator estimates the size of
interest in water recirculating sys-                                                equipment components by calcu-
tems for fish production continues           There is no single recommended         lating the required flow rate. The
to grow, despite the lack of eco-            design for growing fish in a recir-    flow rate of each component must
nomic information available on               culating aquaculture system            be sufficient to flush out and treat
their use. This publication and              (RAS). In general, a system            any wasted feed and by-products
accompanying spreadsheet are                 includes tanks to culture fish,        of fish metabolism, while supply-
designed to help prospective                 pumps to maintain water flow,          ing a uniform concentration of
recirculating system operators               and some form of water treatment       oxygen.
examine the economics of pro-                to maintain water quality. Follow-
                                             ing are a few general considera-       Because equipment is sized to
posed systems. With modifica-                                                       maximum feeding rates, the most
tions to the example spreadsheet,            tions on system design and how
                                             design can affect profitability. For   inefficient stock management
                                             a more complete explanation of         method is to stock fingerlings at
1North Carolina Department of
                                             component options and manage-          low densities in a tank and grow
 Agriculture                                 ment issues see SRAC publica-          them to market size within the
2North Carolina State University                                                    same tank. Most RAS operators
3Carolina Power and Light Company            tions 450, 451 and 452.
                                                                                    try to make maximum use of each
tankÕs carrying capacity by stock-     15,000-gallon (56.78-cubic meter)      tions are calculated from this
ing fish at increasingly lower         growout tanks (G1, G2, G3 and          information. Shaded areas in the
numbers as the fish grow in size.      G4). The quarantine and nursery        tables indicate needed informa-
The more efficient the use of sys-     tanks have their own water filtra-     tion and are represented as bold
tem carrying capacity, the more        tion systems, while each pair of       type in the spreadsheet. ÒSpread-
fish can be moved through the          growout tanks shares a water           sheet Cell RangeÓ and cell num-
system annually, which generally       treatment system. A more detailed      bers refer to the location of infor-
lowers the cost per pound har-         description of the system and          mation within the Excel spread-
vested. The trade-off is that the      equipment can be found in Hobbs        sheet.
more often fish are restocked, the     et al., 1997.
higher the labor cost and greater      Fish are initially stocked in the      Section 1: Specify the Initial
the chance of mortality if fish        Q tank, screened for diseases for
become stressed from the move.                                                Investment Spreadsheet Cell
                                       35 days, then harvested and            Range B13:E25
Operators also face a trade-off        restocked into the N tank. After
when determining both the size         35 days, the fish are transferred to
of tanks and the configuration of      one of the four G tanks where          The initial investment cost is sup-
equipment for filtering and oxy-       they remain an additional 140          plied by the user in cells E16:E20.
genating water. The use of fewer,      days until harvest. This 140-day       The total is calculated in cell E21.
larger tanks, or several tanks         period is broken down into four        The investment includes the total
sharing water treatment equip-         distinct production units of 35        value of purchased land, a settling
ment, is usually much less expen-      days each (defined as g1, g2, g3       pond, building, equipment, and
sive than having a number of           and g4 in the spreadsheet). Each       construction labor, as well as the
smaller tanks that do not share        of these units has a different feed    current value of any owned assets
water or components. Managing          rate, oxygen demand, and pump-         used in the business.
quality and disease prevention,        ing need. (An alternative to this      Annual depreciation on building and
however, is typically more effec-      configuration would be to move         equipment (E22) is the amount of
tive where water is not shared         the fish into a different tank for     money that must be earned each
between tanks. There is less risk      each of the 35-day periods).           year by the business to eventually
of losing large portions of the fish   Once the system is fully stocked,      replace equipment when it wears
crop when each tank has its own        one of the four G tanks is harvest-    out.
set of treatment equipment.            ed for sale every 35 days. The sys-    Interest rate on operating capital
There are economies of scale for       tem has a maximum culture den-         (E24) is used to calculate a cost of
individual tank size and for the       sity of 0.8 pounds of fish per gal-    interest on variable inputs (oxy-
size of the entire system. Up to a     lon of water (103 kgs of fish per      gen, energy, bicarbonate, finger-
point, the increase in system size     cubic meter of water) in each          lings, chemicals, maintenance and
generally results in a lower cost      growout tank, and each harvest         labor). The interest charge could
per pound produced, because the        yields approximately 12,400            be interest owed to a bank for the
fixed costs associated with the        pounds (5,636 kgs) of fish. With       financing of the purchase of these
building and equipment can be          10.43 harvests annually (one every     inputs, or the charge could be for
spread over more pounds har-           35 days once the facility is fully     the cost of using the ownerÕs own
vested.                                stocked), total production for the     funds to purchase variable inputs.
                                       facility is approximately 130,000      A cost of using ownerÕs funds is
The example system                     pounds (59,091 kgs) per year.          used because the investment of
The data used for this publication                                            funds in the recirculating system
                                       Using the spreadsheet                  means that the owner foregoes
are taken from experiences in a
small unit at the North Carolina       The Recirculating Aquaculture          potential earnings from an alter-
State University Fish Barn Project     System Spreadsheet (RASS) must         native investment.
(NC Fish Barn).                        be supplied with accurate and          Interest rate on building and equip-
The NC Fish Barn system grows          realistic input data based on a        ment (E25) is used to calculate an
fish in nursery tanks, then grades     properly designed system. Proper       annual interest charge based on
and splits the population into         design means that the equipment        the average investment. Again,
larger growout tanks as the fish       components work together to pro-       this could be interest owed on a
gain weight. The system consists       duce the amount of fish in the         bank loan used to finance the ini-
of six tanks: one 1,500-gallon         time period stated.                    tial investment, or it can represent
(5.68-cubic meter) quarantine tank     The spreadsheet is divided into        earnings that could have been
(Q); a 4,000-gallon (15.14-cubic       five sections. The user supplies       made on an alternative invest-
meter) nursery tank (N), and four      information for the first three sec-   ment.
                                       tions. Data in the final two sec-
Section 1.                                                                     System parameters
Specify the Initial Investment                                                 The remainder of this section
Spreadsheet Cell Range = B13:E25                                               (E48..E54) contains system para-
Initial investment                                                             meters that will be needed for cal-
                                                                               culations related to costs and
  land                                                          $8,000
                                                                               returns. Annual production (E48),
  settling pond                                                 $5,000         Average size at harvest (E49), and
  equipment                                                   $172,500         the Survival rate (specified in the
  building                                                     $60,000         next section) are used to calculate
                                                                               the initial stocking density.
  construction labor & overhead                                $30,000
                                                                               There are six production units in
Total initial investment                                      $275,000
                                                                               this example (Number of production
Annual depreciation on building and equipment                  $19,100         units [E50] = 6). As discussed
                                                                               above, a production unit refers to
Interest rate on operating capital                                  9%         a specific tank or life stage of the
Interest rate on building and equipment                            11%         fish. Here, three tanks are used: a
                                                                               Q tank, an N tank and a G tank.
                                                                               Fish remain in the Q tank and N
                                                                               tank for 35 days each. Within the

Section 2: Specify the Cost               Section 2.
of Inputs, Sale Price, and                Specify the Cost of Inputs, Sale Price, and System Parameters
System Parameters                         Spreadsheet Cell Range = B27:E54
Spreadsheet Cell Range =                                                 unit or description    cost or amount
B27:E54                                   Variable Costs:
                                            Liquid oxygen                  $/100 cu. ft.             $0.30
Variable costs
                                            Energy                            $/kwh                 $0.065
Variable costs are those directly           Bicarbonate                        $/lb.                $0.190
related to production. In the cell
                                            Fingerlings                    $/fingerling             $0.090
range E31:E38 the user specifies
the cost per unit of oxygen, ener-          Chemicals                        $/cycle              $100.00
gy, bicarbonate, fingerlings,               Maintenance                      $/month               $637.00
chemicals, maintenance and                  Labor: management                $/month             $2,000.00
labor. The quantity used of each
of these inputs is defined in               Labor: transfer & harvest         $/hour                 $6.50
Section 3.
                                          Fixed Costs:
Fixed costs                                 Liquid oxygen tank rental        $/month               $250.00
Fixed costs are incurred regard-            Electrical demand charge         $/month               $100.00
less of whether or not production           Building Overhead                $/month               $100.00
occurs. They are Liquid oxygen
tank rental (E41), Electrical demand
charge (E42), and Building over-          Average overall sale price           $/lb.                  $1.25
head (E43). Each of these is speci-
fied as a cost per month.                 System Parameters
                                            Annual production                    lb.               129,107
Sale price
                                            Average size at harvest              lb.                  1.25
Average overall sale price (E45) is         Number of production units        number                     6
the weighted average sale price
per pound, taking into account              Days per production unit            days                    35
the size distribution at harvest            Kwh per lb. of production     kwh/lb. of prod.            2.30
and differing prices for various            System volts                        volts                  230
sizes of fish. The example uses
                                            Transfer/harvest labor         hrs. per cycle               64
$1.25 so that the system will
break even (with $0 profit and
$0 losses).
G tank, the fish go through four       Section 3.
35-day stages. Note that the Days      Specify Operating Parameters per Production Unit
per production unit (E51) must be      Spreadsheet Cell Range = B56..J64
the same for each unit in order
for the spreadsheet to accurately                                               Growout tank
calculate costs and returns in                                Q tank N tank g1      g2       g3   g4
Section 5.                             Water volume, gallons  1,500 4,000 15,000 15,000 15,000 15,000
The Kwh per lb. of production (E52)    Size stocked (grams)       1      15    60    135      250  385
is used to calculate energy costs
                                       Size harvested (grams)    15      60   135    250      385  567
for the total system and each pro-
duction unit. This variable is cal-    Survival rate           85%    99%    99%    99%      99%  99%
culated by adding up the total         Feed cost, per pound   $0.52 $0.38 $0.21 $0.21 $0.21 $0.21
KW usage of the systemÑinclud-         Feed conversion            1     1.1   1.3    1.6      1.6   1.6
ing energy usage for pumps,
blowers and other equipment as
well as heating, ventilation and       Water volume, gallons (E59:J69) is      bonateÑused over one cycle, and
air-conditioningÑconverting this       used to calculate the Maximum           extrapolates this information to an
to kwh used per year, and then         standing biomass, lbs. per gal. of      annual basis. No user input is
dividing by the number of              water (E73:I73) for any one tank,       required in this section.
pounds produced. (For the exam-        discussed in Section 4.
ple, the total energy demand is 34                                             In the example, once the fish cul-
KW. Multiply by 24 hours per day       Size stocked (E60:J60) is the average   ture system is fully stocked after
and 365 days per year, then divide     size of fish stocked into that pro-     210 days, the system will have
by annual production of 129,107        duction unit. Size harvested            10.43 harvests per year (365 days/
pounds to arrive at 2.30 kwh per       (E61:J61) is their average size         35 days). Thus, each number in
pound of production).                  when transferred or harvested           the Cycle Total (column L) is multi-
                                       from the system. In the example,        plied by 10.43 to calculate the
System volts (E53) is used to calcu-   fish are initially stocked at 1 gram    Annual Total (column M).
late required amperage in Section      into the Q tank, and transferred
5. This is a useful number for                                                 Beginning number of fish (E69:J69)
                                       into the N tank when they reach         begins with the original stocking
planning energy requirements for       15 grams.
the facility.                                                                  density and adjusts that number
                                       Survival rate (E62:J62 ) is the per-    according to the Survival rate
Transfer/harvest labor (E54) is the    centage of survival for that pro-       (E62:J62).
number of hours of labor required      duction unit. In the example, the
per cycle in addition to Labor:                                                Ending number of fish (E70:J70) is
                                       lower survival rate for the Q tank      based on density and survival for
management (defined in E37).           includes the discarding of runts        each production unit.
                                       when the fish are graded before
Section 3: Specify                     restocking into the N tank.             Beginning biomass, lbs. of fish
Operating Parameters                                                           (E71:J71) is based on the number
                                       Feed cost, per lb. (E63:J63) is the     of fish and average weight
per Production Unit                    average cost per pound for feed         stocked into that production unit.
Spreadsheet Cell Range                 fed to that production unit. Feed
                                       cost, per lb. and Feed conversion       Ending biomass, lbs. of fish (E72:J72)
B56:J64                                                                        is based on the number of fish
                                       (E64:E64) are used to calculate the
                                       cost of feed for each production        and weight transferred or harvest-
Each column in this section repre-     unit, for each cycle, and annually.     ed from that unit.
sents a production unit, which         Feed usage is also used to calcu-       Maximum standing biomass, lb. per
could be a tank or group of tanks      late the amount of energy used, as      gal. of water (E73:J73) gives the
managed in the same manner, or         discussed in the following section.     pounds of fish per gallon of tank
it could refer to a particular life                                            water at the end of that produc-
stage. For example, two tanks          Spreadsheet calculation of              tion period.
stocked at the same time with the
intent to transfer and harvest fish    costs and returns                       Feed used (E74:J74) is calculated
at the same time, and in which                                                 from the specified Feed conversion
                                       Section 4: Use of Primary               ratio (E63:J63) and the difference
fish are fed and managed in the
same manner, could be treated as       Inputs and Costs per                    between the Beginning biomass
one production unit. Or, as in the     Production Unit                         (E71:J71) and Ending biomass
table below and spreadsheet            Spreadsheet Cell Range                  (E72:J72).
example, two of the six columns        B66:J87                                 The Kwh used is calculated for
(Q & N) refer to particular tanks,                                             each production unit as a weight-
while the remaining four (g1, g2,      This section summarizes the             ed percentage of the feed usage
g3, g4) refer to a production stage    quantity and cost of primary            for that unit multiplied by the
for fish that remain within the        operating inputsÑfingerlings,           total amount of kwh used for the
same tank.                             feed, energy, oxygen, and bicar-        cycle. The total kwh for the cycle
is based on estimated energy           this is system specific) x 12.05 (a     Days per production unit (D91)
usage of 2.30 kwh per pound of         conversion factor).                     repeats information given in cell
production. For example, one           Bicarbonate used (E77:J77) allows       E51.
cycle yielding 12,354 pounds           for 0.175 pound of sodium bicar-        The Number of cycles per year (D92)
(5,615 kg) of fish requires an esti-   bonate used per pound of feed           is simply 365 days divided by
mated 28,414 kwh of energy. The        fed.                                    Days per production unit.
g1 production unit consumes
11.72% of feed used during the         Costs by production unit (E80:J87)      Required system amps (D93) is cal-
cycle (2,172 pounds feed/18,524        are calculated using the cost per       culated from System volts (E53)
pounds feed), so the estimated         input specified in Section 2.           and kwh usage assuming a power
energy use during that 35-day unit                                             factor of one.
                                       Section 5: Summary of
is 3,330 kwh (11.72% x 28,414),                                                Overall survival (F91) is calculated
given in cell G75. The cost of         Annual Costs and Returns
                                                                               using survival given in E62:J62,
energy for that period, given in       to System in Full Production            and Cycle FCR (F92) from feed
G82 as $217, is calculated using       Spreadsheet Cell Range =                conversion ratios in E64:J64.
the user-specified cost of $0.065      B89:J122                                The cell range C96:J122 calculates
per kwh (E45).
                                                                               system costs per cycle, annually,
Oxygen used, cubic feet (E76:J76) is   This section summarizes the costs       and per pound based on informa-
calculated as follows: pounds of       and returns per cycle and annual-       tion specified previously in the
feed (E74:J74) x 30% (the amount       ly for this system once it is in full
of oxygen used per pound of feed,      production (after 210 days). Net
                                       returns are calculated before tax.

Section 4.
Use of Primary Inputs and Costs per Production Unit
Spreadsheet Cell Range = B66:J87
                                                            Growout tank                        Cycle        Yearly
Inventory & Input Use:                   Q tank N tank   g1     g2       g3     g4               total        total
Beginning number of fish                 12,252 10,415 10,310 10,207 10,105 10,004              12,252      127,775
Ending number of fish                    10,415 10,310 10,207 10,105 10,004 9,904                 9,904     103,286
Beginning biomass (lbs. of fish)             27    344 1,361 3,032 5,558 8,474                       27          281
Ending biomass (lbs. of fish)               344 1,361 3,032 5,558 8,474 12,354                  12,354      128,838
Max. standing biomass (lbs./gal.)          0.23   0.34   0.20   0.37     0.56  0.82                    --           --
Feed used, lbs.                             317  1,119 2,172 4,042 4,665 6,209                  18,524      193,179
Kwh used                                    486 1,717 3,331 6,200 7,156 9,525                   28,415      296,328
Oxygen used, cubic ft.                    1,145 4,045 7,851 14,612 18,864 22,447                66,964      698,342
Bicarbonate used, lbs.                       55    196    380    707      816 1,087               3,242      33,806

Fingerlings                              $1,103                                                 $1,103      $11,500
Feed                                       $165   $425 $456 $849 $980 $1,304                    $4,178      $43,575
Energy                                      $32   $112 $217 $403 $465         $619              $1,847      $19,261
Oxygen                                       $3    $12    $24    $44    $51    $67                $201       $2,095
Bicarbonate                                 $11    $37    $72 $134 $155      $206                 $616       $6,423
Total of above costs for this unit       $1,313  $586 $768 $1,430 $1,651 $2,197                 $7,945      $82,855
Cumulative cost for cycle                $1,313 $1,899 $2,667 $4,098 $5,748 $7,945              $7,945      $82,855
Cumulative cost per lb.                   $3.82 $1.40 $0.88 $0.74 $0.68 $0.64                    $0.64        $0.64
Section 5.
Summary of Annual Costs and Returns to System in Full Production
Spreadsheet Cell Range = B89:J122
Days per production unit              35              Overall survival 81%
Average number of cycles/yr.          10.43           Cycle FCR        1.5
Req. system amps                      147
                                        unit           cost/unit    quantity/   $/cycle       $/year     $/per lb.     % of
                                                                     cycle                                of fish      total
Gross Receipts                          lb.             $1.25        12,354     $15,443     $161,048      $1.25
Variable Cost
 fingerlings                            unit               $0.09     12,252      $1,103      $11,500       $0.09        7%
 feed                                    lb.               $0.23     18,524      $4,178      $43,575       $0.34       27%
 energy                                 kwh                $0.07     28,415      $1,847      $19,261       $0.15       12%
 oxygen                            100 cubic feet          $0.30        670        $201       $2,095       $0.02        1%
 bicarbonate                             lb.               $0.19      3,242        $616       $6,423       $0.05        4%
 chemicals                          $ per cycle          $115.07          1        $115       $1,200       $0.01        1%
 maintenance                        $ per cycle          $732.99          1       $733        $7,644       $0.06        5%
 labor: management                  $ per cycle        $2,301.37          1      $2,301      $24,000       $0.19       15%
 labor: transfer & harvest             hour                $6.50         64        $416       $4,338       $0.03        3%
 interest on variable costs             dol.                 9%       6,307        $327       $3,406       $0.03        2%
Subtotal, Variable Cost                                                         $11,837     $123,442       $0.96       77%
Fixed Cost
  Oxygen tank rental                    dol.                                       $288       $3,000       $0.02        2%
  Electrical demand charge              dol.                                       $115       $1,200       $0.01        1%
  Building overhead                     dol.                                       $173       $1,800       $0.01        1%
  Interest on initial investment        dol.                                     $1,226      $12,788       $0.10        8%
  Depr. on bldg. & equip.               dol.                                     $1,832      $19,100       $0.15       12%
Subtotal, Fixed Cost                                                             $3,633      $37,888       $0.29       23%
Total Cost                                                                      $15,470     $161,330       $1.25     100%
Net Returns above Var. Cost                                                      $3,606      $37,606       $0.29
Net Returns above Total Cost                                                       -$27        -$282       $0.00

Interpreting the                               approximately calculate the point          This spreadsheet can be used to
spreadsheet results                            at which the system becomes self-          test the effect on costs and returns
                                               supporting (can pay all fixed and          of changes in sale price, feed con-
This publication is not an evalua-             variable costs), divide the total          version, survival, or the cost of
tion of the economics of tilapia               costs per cycle by the net returns         energy and other inputs. Users
production. A sale price of $1.25              per cycle. For example, if the sale        can also examine the change in
was chosen so that the example                 price were $1.65 per pound, Total          profitability based on a change in
system would have annual costs                 Costs per Cycle would be $15,470           the stocking and transfer of fish
nearly equal to annual returns.                and Returns above Total Costs              or overall size of the system. For
It is important to keep in mind                would be $4,957. This is equal to          example, more frequent moves of
that before the end of the first               3.1 cycles ($15,470/$4,957) or 651         fish between tanks could make
cycle on day 210, costs are                    days (3.1 cycles x 210 days per            better use of tank carrying capaci-
incurred while no fish are har-                cycle). The system would not               ty, increasing the amount of fish
vested and sold. Until that time,              become self-supporting until               that could be harvested annually.
the cost of operations must either             approximately 2 years from                 Or, a more energy intensive sys-
be paid by additional owner                    startup.                                   tem might support a higher carry-
funds or bank financing. To                                                               ing capacity per tank. Either of
these may result in increased prof-    that will be suitable for every sys-   References
it if the costs associated with each   tem. Operators with existing or
(higher labor cost, stress that may    proposed systems similar to the        Hobbs, A., T. Losordo, D. DeLong,
result in lower survival in the case   example presented here can use             J. Regan, S. Bennett, R. Gron
of more frequent moves, and a          this spreadsheet. Radically differ-        and B. Foster. 1997. ÒA com-
higher energy cost if the system       ent systems may require extensive          mercial, public demonstration
were reconfigured) do not out-         modifications of the spreadsheet           of recirculating aquaculture
weigh the increase in production.      structure by the user. The example         technology: The CP&L /EPRI
Larger systemsÑmore tanks and          spreadsheet is simple in design            Fish Barn at North Carolina
larger tanksÑalso often increase       and does not contain any macro-            State University.Ó Pages 151-
the profitability of recirculating     programming. It can be modified            158 In: M.B. Timmons and
systems.                               once cells are unprotected. When           T.M. Losordo, editors.
                                       working with the original spread-          Advances in aquacultural
                                       sheet or a modified version, keep          engineering. Proceedings from
Caveats (a warning)                                                               the aquacultural engineering
                                       in mind that it can only evaluate
There is no single recommended         the economics of a properly                society technical sessions at
design for recirculating aquacul-      designed system, and can not cor-          the fourth international sym-
ture systems. Therefore, it is         rect for flaws in design.                  posium on tilapia in aquacul-
impossible to supply a ready-                                                     ture. NRAES-105. Northeast
made cost/returns spreadsheet                                                     Regional Agricultural
                                                                                  Engineering Service, Ithaca,
                                                                              For additional suggested reading,
                                                                                  see the Internet site.
The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045 from
the United States Department of Agriculture, Cooperative States Research, Education, and Extension Service.

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