Pond Aeration

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					                                                                                                SRAC Publication No. 3700

                                September 2005

                                         Pond Aeration
                                                    Craig Tucker*

Fish, like all animals, must obtain       described by the gas transfer          surplus, depending on whether
oxygen from the environment for           equation:                              the measured concentration is
respiration. Oxygen is far less                 dC/dt = KL(A/V)(Cs –Cm).         greater than or less than the satu-
available to aquatic organisms                                                   ration concentration.
than it is to air-breathers, and the      In the equation:
dissolved oxygen content of water         dC/dt =the rate of oxygen transfer     The effect of surface area and
may limit the activities of fish.                  between a liquid and a        turbulence
In most natural waters, the sup-                   gas;                          Oxygen moves to and from water
ply of oxygen to water (diffusion             KL =the liquid-film coefficient;   across the air-water interface. So,
from the atmosphere and produc-                                                  a greater amount of oxygen can
                                            A/V =the ratio of the air-water
tion from underwater photosyn-                                                   enter or leave a given amount of
                                                   interfacial area to water
thesis) exceeds the amount used                                                  water when the air-water interfa-
in oxygen-consuming processes,                                                   cial area is increased. However,
and fish seldom have problems                 Cs =the dissolved oxygen con-      even if the water is initially low
obtaining enough oxygen to meet                    centration when water is      in oxygen, the thin film of water
normal metabolic demands. In                       saturated with oxygen         at the interface of a calm water
aquaculture ponds, however, the                    under the prevailing con-     surface quickly becomes satu-
biomass of plants, animals and                     ditions of water tempera-     rated with oxygen, which dramat-
microbes is much greater than in                   ture, salinity and atmos-     ically slows the rate of oxygen
natural waters, so oxygen is                       pheric pressure; and          diffusion into the water.
sometimes consumed faster than               Cm =the measured dissolved          Turbulent mixing restores the sat-
it is replenished.                                 oxygen concentration.         uration deficit in the surface film
Depending on how low the dis-                                                    by moving oxygenated water
                                          The liquid film coefficient, KL,
solved oxygen concentration is                                                   away from the surface, increasing
                                          incorporates a parameter called
and how long it remains low, fish                                                the overall rate of oxygen trans-
                                          the surface renewal rate, which is
may consume less feed, grow                                                      fer.
                                          related to turbulence within the
more slowly, convert feed less            liquid.                                The effect of prevailing dis-
efficiently, be more susceptible to
infectious diseases, or suffocate
                                          The gas transfer equation looks        solved oxygen concentration
                                          complicated, but it is actually sim-
and die. Aquaculturists avoid                                                    Dissolved oxygen moves into or
                                          ple to interpret. The equation says
these problems by aerating ponds                                                 out of water by diffusion. The rate
                                          that the rate of oxygen transfer
mechanically to supplement nor-                                                  of diffusion depends on the differ-
                                          between air and water depends
mal oxygen supplies.                                                             ence in oxygen partial pressures
                                          on three factors: the amount of
                                          turbulence, the ratio of surface       between the liquid and gas phas-
Principles of aeration                                                           es—the greater the difference, the
                                          area to water volume, and how
The rate of oxygen movement               far the prevailing dissolved oxy-      greater the driving force moving
between air and water is                  gen concentration deviates from        oxygen from one phase to the
                                          the dissolved oxygen concentra-        other. The maximum rate of oxy-
                                          tion at saturation. This deviation     gen transfer into water occurs
Mississippi State University.
                                          is called the saturation deficit or    when the dissolved oxygen con-
centration in water is 0 mg/L, the      Under practical aquaculture con-       dard oxygen transfer rate (SOTR)
point at which the maximum dif-         ditions, the culturist cannot mod-     is the amount of oxygen added to
ference in oxygen partial pressures     ify those environmental variables      water in 1 hour under a standard
between water and air occurs.           to increase the partial pressure       set of conditions. The units of
As dissolved oxygen concentra-          differential and improve aerator       SOTR are pounds O2/hour, which
tions increase from 0 mg/L, the         performance. However, measured         can be multiplied by 0.45 to
oxygen partial pressure difference      dissolved oxygen concentration in      derive the metric equivalent in
between air and water steadily          the pond varies diurnally, so the      kg O2/hour. Standard aeration effi-
decreases up to the point where         culturist can control aerator oxy-     ciency (SAE) is the standard oxy-
the dissolved oxygen concentra-         gen transfer rates by selecting        gen transfer rate divided by the
tion equals the saturation concen-      when to begin aerating. This can       power requirement in horsepow-
tration. At that point, there is no     be demonstrated by looking at a        er (hp). Units of SAE are pounds
difference in oxygen partial pres-      couple of extreme examples.            O2/hp⋅hour, which can be multi-
sure between water and air (this        First, if aerators are operated on     plied by 0.61 to derive SAE in
is, in fact, the definition of “satu-   sunny afternoons when water is         metric units of kg O2/kW⋅hour.
ration”). Because there is no driv-     supersaturated, oxygen will be         Boyd (1998) thoroughly describes
ing force compelling oxygen mole-       lost (degassed) from the water.        aerator performance testing and
cules to leave or enter water, no       So, unless the goal is to remove       how to interpret and use SOTR
oxygen can be added to water no         oxygen from the water or simply        and SAE values.
matter how much effort is made          to mix the water, supersaturated       Aerators transfer less oxygen
to increase turbulence or air-          water should not be aerated.           under pond conditions than
water interfacial area.                 Second, the rate of oxygen trans-      under the standard conditions of
When the dissolved oxygen con-          fer is greatest when the mea-          aerator performance tests, so
centration is greater than the sat-     sured dissolved oxygen concen-         SOTR and SAE values are best
uration concentration (the water        tration is very low. In fact, oxy-     used to compare similar styles of
is supersaturated with oxygen),         gen-transfer rates can be maxi-        aerators as an aid in selecting
the oxygen partial pressure in          mized by waiting until dissolved       equipment to purchase rather
water is greater than in air and        oxygen concentration falls to 0        than as design criteria for pond
oxygen moves from water to air.         mg/L before aerating, but this has     use. Also, small differences in
In other words, aeration causes         obvious drawbacks (the fish            SOTR and SAE values are not
the dissolved oxygen concentra-         would be dead by then). On the         meaningful because test condi-
tion to decrease. This process is       other hand, aerating water when        tions may vary and affect results.
called “degassing.”                     dissolved oxygen concentrations        Boyd and Ahmad (1987) com-
                                        are near saturation is wasteful        piled SOTR and SAE values for a
Implications for aeration               because oxygen transfer and aer-       variety of aerators used in pond
                                        ation efficiency are very low          aquaculture.
Each of the three factors in the gas-
transfer equation has important         under those conditions. So there       Good SAE values and durability
implications for pond aeration. The     are important trade-offs between       are most important when select-
effects of surface area and turbu-      biological goals (optimizing aquat-    ing aerators for general day-to-
lence are obvious. Aerators             ic animal health by maintaining        day use. On the other hand, high
increase the air-water interfacial      dissolved oxygen levels above          SOTR values and mobility are
area by breaking water into fine        some critical threshold) and phys-     important for aerators used to
drops or creating bubbles. Aerators     ical constraints (aerator efficiency   save fish in distress. Other fac-
also create turbulence that renews      and oxygen transfer decline as         tors such as cost, durability, spe-
the surface film and moves oxy-         dissolved oxygen concentration         cific application and ease of ser-
genated water away from the aera-       approaches saturation).                vice must also be considered
tor. Implications of the third fac-     Although the oxygen saturation         when selecting an aerator.
tor—the oxygen partial pressure         concentration cannot be manipu-        Durability varies widely among
difference between air and water—       lated to improve oxygen-transfer       aerators, and prospective buyers
are a bit more complicated.             rates of surface aerators, this is     should consult the owners of var-
                                        not true for deep-water diffuser       ious kinds of equipment for
The oxygen partial pressure dif-                                               advice and recommendations.
ferential can be increased (there-      aerators (bubblers) or aerators that
by increasing the oxygen transfer       use pure oxygen as the gas phase.
                                        Those aerators can be very effi-       Common types ot aerators
rate) by increasing the saturation
concentration (Cs), decreasing the      cient because they operate under       Paddlewheel aerators
measured oxygen concentration           conditions where the saturation
                                        dissolved oxygen concentration is      Paddlewheel aerators are the most
(Cm), or both. For surface aerators
                                        higher than in surface waters.         common types used in large
that splash or spray water into
                                                                               ponds. Paddlewheels consist of a
the air, the saturation dissolved
oxygen concentration is set by          Aerator performance                    hub with paddles attached in a
                                                                               staggered arrangement. The aera-
prevailing atmospheric pressure,        There are two ways of describing       tor is powered by a tractor power
water temperature and salinity.         aerator performance. The stan-
take-off (PTO), self-contained diesel   range from 2 to 4 feet; paddles are   designs. For a given design, oxy-
or gas engine, or electric motor.       2 to 10 inches wide and may be        gen transfer can be increased by
Electric paddlewheel aerators are       rectangular, triangular or semicir-   increasing paddle depth and hub
usually mounted on floats and           cular (concave) in cross section.     rotation speed. Increased diame-
anchored to the pond bank.              Paddles are welded to the hub in      ter, paddle depth and speed also
There are many different designs        various spiraled or staggered         increase the power required for
for PTO-driven paddlewheels             arrangements.                         operation.
(Figs. 1 and 2). Gearboxes and          Large-diameter paddlewheels           Tractor PTO-powered paddle-
automobile differentials have been      transfer more oxygen than smaller     wheels can have high SOTRs—
used for gear reduction. Paddle-        diameter aerators, and flat pad-      90 pounds O2/hour or more. Thus,
wheel diameters (paddle tip to tip)     dles are less effective than other    they are particularly useful in
                                                                              emergencies. However, tractors
                                                                              develop more power than is
                                                                              applied to the PTO and aerator
                                                                              drive shaft, and considerable
                                                                              energy is lost through the drive
                                                                              train, so PTO-driven paddle-
                                                                              wheels are not particularly ener-
                                                                              gy efficient. But they are more
                                                                              flexible than other aerator
                                                                              designs. Because they are
                                                                              portable, they can be moved from
                                                                              pond to pond as needed and
                                                                              placed anywhere along the bank.
                                                                              The aerator also can be operated
                                                                              in different modes, depending
                                                                              upon needs.
                                                                              For general pond aeration, PTO-
                                                                              driven paddlewheels are usually
                                                                              operated with paddles submerged
                                                                              3 to 4 inches in the water. Paddle
                                                                              depth can be increased to 5 to 6
                                                                              inches for greater oxygen transfer,
                                                                              but this increases fuel consump-
Figure 1. One of the first tractor PTO-powered paddlewheels. The device was
fabricated from a truck rear end with rectangular paddles welded to the       tion and produces a stronger
wheels.                                                                       water current, which forces fish
                                                                              to expend more energy (and con-
                                                                              sume more oxygen) when swim-
                                                                              ming behind the aerator.
                                                                              Nevertheless, the increased pad-
                                                                              dle depth may be needed when
                                                                              maximum oxygen transfer is
                                                                              important during acute dissolved
                                                                              oxygen depletions.
                                                                              Depending on their diameter,
                                                                              PTO-driven paddlewheel aerators
                                                                              require 15 to 30 hp to operate at
                                                                              paddle depths of 3 to 6 inches.
                                                                              Tractors with PTO power ratings
                                                                              of 45 to 60 hp are typically used
                                                                              as the power source. Larger trac-
                                                                              tors may be needed to move aera-
                                                                              tors around on farms with steep,
                                                                              eroded pond levees, but large
                                                                              tractors are an inefficient power
                                                                              source because most of the fuel
                                                                              consumed is used to run the
                                                                              engine and power train rather
                                                                              than to turn the paddlewheels.
Figure 2. A tractor PTO-powered paddlewheel operated in a catfish pond. A
sidewinder paddlewheel is in the background.                                  When used at paddle depths of 3
                                                                              to 6 inches and driven by a trac-
tor with a PTO power rating of 45
to 60 hp, most PTO-driven paddle-
wheels perform best when the
tractor is operated at about half
throttle (1,200 to 1,500 rpm
engine speed). If the tractor has a
standard 540-rpm PTO shaft and
the aerator gear reduction is about
6 to 1, the PTO shaft speed will
be about 500 rpm and the paddle-
wheel speed 80 to 90 rpm.
Tractor PTO-driven paddlewheels
also can be operated with paddles
almost fully submerged to mix
ponds or provide a current of oxy-
genated water to fish held at high
densities in harvest socks. When
used in this manner, the tractor is
operated just above idle speed
(300 to 400 rpm engine speed) so
that the paddlewheel speed is 20
to 30 rpm.
Some PTO-driven paddlewheels,          Figure 3. A sidewinder paddlewheel showing the orientation of paddles on the
called sidewinders (Figs. 3 and 4),    hub.
use an in-line pinion-and-bullgear
system to reduce PTO shaft rota-
tion speed. Benefits of sidewinders
are that they are durable and pro-
duce a current of oxygenated water
parallel to the pond bank where
stressed fish congregate. Side-
winders also are commonly used to
produce a current of oxygenated
water to sustain fish concentrated
in harvest socks.
Personnel who operate PTO-dri-
ven aerators should be trained in
placing and using them properly
to ensure safe operation, optimal
usage in critical situations, and
long life of the equipment.
For routine, everyday use—where
efficiency is important—most cul-
turists prefer paddlewheel aera-       Figure 4. A sidewinder paddlewheel operated in a catfish pond.
tors powered by electric motors
(Figs. 5 and 6). Electric paddle-
wheels used on commercial cat-
fish farms are usually powered by
10- to 15-hp motors and have hubs
10 to 15 feet long. Electric paddle-
wheels are permanently anchored
in each pond and individually
controlled by switches on the
pond bank. On some commercial
aerators the paddle depth can be
adjusted for optimum energy use
and performance of the motor.
The motor should draw about 90
percent of the full load amperage
rating to provide maximum oxy-         Figure 5. A 10-hp electric paddlewheel aerator.
gen transfer and extend motor life.
                                                                                  Electric paddlewheel design was
                                                                                 systematically studied by Ahmad
                                                                                 and Boyd (1988), who found that
                                                                                 the best design consists of a 3-foot
                                                                                 diameter paddlewheel with pad-
                                                                                 dles that are triangular (135-degree
                                                                                 interior angle) in cross section.
                                                                                 Paddles are about 4 to 6 inches
                                                                                 wide, with four paddles attached
                                                                                 per row and spiraled in a staggered
                                                                                 arrangement around the hub.
                                                                                 Paddle depth is 4 to 6 inches.
                                                                                 Paddlewheel speed should be
                                                                                 about 90 rpm. Commercial designs
                                                                                 similar to that proposed by Ahmad
                                                                                 and Boyd have SAE values of 4.5
                                                                                 to 5.5 pounds O2/hp⋅hour, which is
Figure 6. A series of electric paddlewheel aerators in operation.
                                                                                 very good for surface aerators.
                                                                                 Pump-sprayer aerators
                                                                                 Pump-sprayer aerators have pumps
                                                                                 that discharge water at high velocity
                                                                                 through pipes or manifolds. Pumps
                                                                                 may be powered by the PTO of a
                                                                                 tractor (Fig. 7) or by an electric
                                                                                 motor. Pump-sprayers are simple
                                                                                 and require little maintenance.
                                                                                 Pump-sprayers have a wide range
                                                                                 of effectiveness and efficiency.
                                                                                 Those driven by electric motors
                                                                                 have SAE values of 1.5 to 3.5
                                                                                 pounds O2/hp⋅hour. Pump-sprayers
                                                                                 powered by tractor PTOs have
                                                                                 lower SAE values than electric aera-
                                                                                 tors, but may have very high SOTR
                                                                                 values (up to 160 pounds O2/hour).
                                                                                 Aerators with higher SOTR values
                                                                                 usually require large tractors
                                                                                 (90 PTO hp, or more) or that the
                                                                                 PTO be operated at a high speed
Figure 7. One of the many types of PTO-powered pump-sprayer aerators. A          (up to 1,000 rpm).
horizontal pipe with a series of outlets has been welded to the discharge of a
centrifugal irrigation pump. The device is connected to a tractor PTO and        Vertical pump aerators
backed into the pond with only the horizontal pipe above the water. When
operated, water is sprayed into the air through the holes in the pipe.           A vertical pump aerator consists of
                                                                                 a submersible motor with an
                                                                                 impeller attached to the output
                                                                                 shaft. The motor and impeller are
                                                                                 suspended beneath a float, and
                                                                                 water is sprayed into the air
                                                                                 through an opening in the center
                                                                                 of the float (Fig. 8). Vertical pump
                                                                                 aerators can be relatively effi-
                                                                                 cient—SAE values usually range
                                                                                 from 2 to 4 pounds O2/hp⋅hour—
                                                                                 but most vertical pump aerators
                                                                                 manufactured for aquaculture have
                                                                                 relatively small motors (usually
                                                                                 less than 1 hp) and do not produce
                                                                                 a large area of oxygenated water.
                                                                                 This limits their use to ponds of
                                                                                 less than 1 acre, where they can be
                                                                                 quite effective.
Figure 8. Two vertical pump aerators used in a fish-confinement area.
Diffusers or bubblers                   Fine-pore diffusers operated at low     1 hp/acre in some fish culture
                                        airflow rates are more efficient, but   ponds. In marine shrimp ponds,
These systems use blowers or            these systems foul (clog) easily and    aerators are also used for constant
compressors to supply air to dif-       must be cleaned often to keep them      water circulation, which in many
fusers. The diffusers have many         working properly. In addition to        respects is as important as the
small pores that release bubbles        these problems, most culturists dis-    oxygenation process in the cul-
on the pond bottom. Oxygen is           like diffused aeration systems          ture of this bottom-dwelling ani-
transferred as the bubbles rise         because the network of supply           mal. Aeration practices used on
through the water column.               lines and diffusers interferes with     most commercial catfish farms lie
Diffusers for large-scale aeration      fish harvest.                           between these two extremes.
are usually discs, plates or tubes
constructed of glass-bonded silica,     Diffusers that use pure oxygen in
                                                                                The evolution of aeration
ceramic, porous plastic, or flexible    the gas phase can have high oxy-
                                        gen-transfer rates, but operating       on catfish farms
perforated membranes. Diffusers
are customarily arranged in a grid      costs are too high for routine aera-    Before about 1980, catfish stock-
pattern over the bottom of the          tion of large ponds. Pure-oxygen        ing and feeding rates were low
pond, with the number of individ-       systems may, however, have              and there were few problems
ual diffusers determined by the         important specialty uses in ponds,      with dissolved oxygen, except
oxygen transfer rate of the diffuser    such as providing oxygen to fish        during unusual events such as
and the oxygen consumption rate         held at high densities in live cars     sudden phytoplankton die-offs or
in the water. Oxygen transfer           or socks during harvest (Torrans et     prolonged periods of cloudy
increases with smaller bubble size,     al., 2003).                             weather during the summer. As
deeper bubble release point, and                                                farmers sought to increase pro-
higher oxygen content in the bub-       Aeration practices                      duction by stocking fish at greater
bles.                                   Basic aerator design and the types      densities and increasing the feed-
                                        of aerators used in pond aquacul-       ing rates, episodes of critically
Diffused aeration is common in                                                  low dissolved oxygen concentra-
wastewater treatment, where             ture have become somewhat stan-
                                        dardized. Paddlewheel aerators of       tions became more frequent.
basins 15 to 30 feet deep can be
constructed to optimize oxygen          relatively similar design are used      In the early stages of intensified
transfer. When bubbles are              in most large ponds (>1 acre), and      production, oxygen problems
released in deep water, hydrostatic     vertical pump aerators are com-         remained relatively rare and cat-
pressure from the overlying water       monly used in small ponds.              fish farmers used existing farm
increases the saturation dissolved      Although the choice and design of       equipment (such as irrigation lift
oxygen concentration, so that for       aerators have become somewhat           pumps) or aerators fabricated in
any value of ambient dissolved          standardized, the ways aerators         farm machine shops. These early
oxygen, the saturation deficit is       are used vary widely, probably          aerators used a readily available
increased compared to conditions        because there has been little sys-      power source—a tractor PTO.
at the water surface. Deep water        tematic economic evaluation of          Tractor-powered aerators could be
also creates a long contact time        aeration practices in large com-        moved around the farm to any
between bubble and water, so that       mercial ponds. Although many            pond that required supplemental
more of the oxygen in the bubble        aeration experiments have been          aeration. Quite often, the first
is transferred to the water before      conducted in small ponds, it is dif-    sign of a problem would be fish
the bubble reaches the surface.         ficult to relate the results of those   struggling to obtain oxygen at the
Aeration efficiencies are also high     experiments to conditions in large      pond surface. Farmers would
if very small bubbles are produced      ponds because oxygen dynamics,          then quickly place an aerator in
because they have a higher ratio        mixing characteristics, fish behav-     the pond and fish would congre-
of total surface area to water vol-     ior, and other factors vary so much     gate in the small area of oxy-
ume than large bubbles. Fine-bub-       with pond size. Because there have      genated water near the aerator.
ble diffusers operated in deep          been few appropriate aeration            As farmers continued to increase
water can have very high SAE val-       trials, farmers base their aeration     their catfish stocking and feeding
ues—some over 15 pounds                 practices on the availability of        rates in an effort to grow more
O2/hp⋅hour.                             labor and capital, their individual     fish, most ponds needed aeration
Despite the potential for high SAE      production goals, and the per-          on summer nights. Tractors
values, diffusers are seldom used       ceived effectiveness of various         proved too expensive and difficult
in aquaculture ponds. In shallow        practices.                              to maintain for everyday use.
ponds, diffusers are relatively inef-   The variety of aeration practices       Also, tractor-powered paddle-
ficient because bubbles ascend to       used in pond aquaculture is repre-      wheels are not particularly effi-
the surface too quickly for effec-      sented at one extreme by the con-       cient because only a portion of
tive oxygen transfer. At diffuser       tinuous use of intensive aeration       the energy from the tractor
depths of about 3 to 4 feet, SAE        (up to 10 hp/acre) in marine            engine is transferred to the aera-
values of most diffusers are about      shrimp ponds and, at the other          tor. Floating paddlewheel aerators
1 to 3 pounds O2/hp⋅hour.               extreme, by infrequent, “emer-          powered by electric motors were
                                        gency aeration” with 0.5 to             more efficient, and by the mid-
1980s they had become the most        cally low levels has the disadvan-     pond. Note, however, that the
common type used on catfish           tage of routinely exposing fish to     duration of aeration varies greatly
farms.                                suboptimal concentrations of dis-      among ponds on a given day.
                                      solved oxygen. Despite this draw-      Some ponds may need no aera-
How much to aerate                    back, this is the most common          tion, while others require continu-
In commercial catfish ponds,          aeration practice in commercial        ous aeration throughout the day.
aeration is commonly about 1.5        catfish ponds and is the only          The length of time aeration is
to 2 hp/acre in each pond. For        practice that has proved to be         used also depends on weather
example, two 10-hp electric pad-      economically rational. Maintain-       conditions. For example, during
dlewheel aerators may be used in      ing dissolved oxygen concentra-        periods of warm, cloudy weather
a 10- to 15- acre pond. There is,     tion above a critical threshold        most ponds may need continuous
however, a strong trend in the        throughout the pond has not been       aeration for several days.
catfish industry to increase aera-    shown to be economically justifi-      Some catfish farmers begin aerat-
tion intensity to 2 to 3 hp/acre      able using currently available aer-    ing all ponds on the farm at some
(three 10-hp aerators in a 10- to     ation technology.                      set time early each night rather
15-acre pond). Many farmers also      When to aerate                         than basing aeration on dissolved
maintain a few tractor-powered        The need to aerate varies season-      oxygen levels. This practice
aerators (one for every four or       ally because water temperature         reduces the labor required for fre-
five ponds is common) for emer-       affects the rates of respiration and   quent monitoring, but increases
gency situations where high oxy-      photosynthesis. Problems with          energy costs by starting aeration
gen transfer rates and aerator        low dissolved oxygen concentra-        in some ponds well before it is
mobility are more important than      tions are rare when water temper-      needed.
aeration efficiency.                  ature is consistently below 60 °F      Aerator placement
Mechanical aeration at 1.5 to         (15 °C). Problems are common
2 hp/acre meets only a fraction of    when water temperature is above        Little research has been conduct-
the total oxygen demand of all        80 °F (27 °C). Based on average        ed on this aspect of pond aera-
organisms in the pond. In a typi-     water temperatures, aeration in        tion. If water mixing is important,
cal 15-acre catfish pond, the total   northwest Mississippi catfish          aerators should be placed where
oxygen consumed in respiration        ponds is uncommon from mid-            they will enhance pond circula-
by fish, plankton and sediment        November through February and          tion patterns. For example, water
during the summer may range           frequent from mid-May through          circulation in large, rectangular
from 100 to more than 200             mid-September. Local climates          ponds is optimized by placing
pounds of oxygen per hour. Most       and unseasonable temperatures          paddlewheel aerators off the bank
of the oxygen demand is account-      will, however, alter the need to       near the middle of the long axis
ed for by plankton and sediment       aerate ponds.                          of the pond to direct currents
respiration rather than by fish.      Episodes of low dissolved oxygen       across the short axis (Boyd and
When operated in water with a         concentration usually occur at         Watten 1989). This produces two
dissolved oxygen concentration of     night during the summer. Most          or more (depending on how many
about 2 mg/L, good paddlewheel        catfish farmers manage each pond       aerators are used) water circula-
aerators transfer about 1.5 to 2.5    individually by measuring the dis-     tion cells in the pond. The worst
pounds O2/hp⋅hour under field         solved oxygen concentration at         placement is in a corner, with cur-
conditions. Thus, an energy input     intervals throughout the night and     rents directed diagonally across
of 40 to 130 hp (about 2.5 to 8.5     aerating when it falls below a         the pond. When several aerators
hp/acre) would be required to         level considered critical by the       are placed in a pond, they can be
meet the total respiratory            individual farmer (usually 3 to 5      located where the current of each
demands of fish, plankton and         mg/L). Oxygen is measured manu-        aerator enhances the flow pro-
sediment and maintain dissolved       ally (see SRAC publication 4601)       duced by the others aerators. For
oxygen concentrations of 2 mg/L.      or, far less commonly, with con-       example, an aerator can be placed
Clearly, mechanical aeration at 1     tinuous monitoring systems in          in each corner of the pond to
to 2 hp/acre will not improve the     each pond. Aeration continues          direct currents parallel to the
dissolved oxygen concentration in     until past dawn when measure-          banks in the same clockwise or
the entire pond, but is used only     ments of dissolved oxygen indi-        counterclockwise direction. One
to provide a small refuge of aer-     cate that phytoplankton photosyn-      drawback to this arrangement is
ated water near the aerator.          thesis is producing oxygen.            that aerator currents tend to erode
When dissolved oxygen concen-                                                the pond bottom around the pond
                                      Commercial catfish ponds aerated       margins and deposit the eroded
trations are low, fish congregate     on this “as-needed” basis require
in that area and remain there                                                material in the middle of the
                                      500 to 1,000 hours of supplemen-       pond. The resulting central
until oxygen conditions improve       tal aeration cumulatively during
throughout the pond.                                                         mound of loose sediment can
                                      the summer growing season. This        make it difficult to harvest the
The practice of aerating only a       corresponds to an average of           aquaculture crop by seining. Also,
portion of the pond and only          about 3 to 6 hours per day per         running electrical cables to the
when concentrations fall to criti-
corners of every pond on a large            gen-stressed fish have congregat-                Busch, R. L., C. S. Tucker, J. A.
farm will be expensive.                     ed. For example, when the dis-                       Steeby and J. E. Reames. 1984.
Water mixing is not the primary             solved oxygen concentration varies                   An evaluation of three paddle-
goal of using aerators in catfish           from one end of the pond to the                      wheel aerators used for emer-
ponds, so optimizing water circu-           other, most fish will be in the end                  gency aeration of channel cat-
lation patterns is not the most             where the concentration is highest                   fish ponds. Aquacultural
important consideration in aerator          and the aerator should be placed                     Engineering 3:59–69.
placement. Catfish become condi-            at that end. If a portable aerator is            Moore, J. M. and C. E. Boyd.
tioned to moving near the aerator           used to supplement an existing                       1992. Design of small paddle-
when dissolved oxygen concentra-            aerator or to replace a malfunc-                     wheel aerators. Aquacultural
tions are low, and permanently              tioning permanent aerator, it                        Engineering 11:55–69.
installed electric aerators should          should be placed near the other
                                            aerator so that fish are not forced              Petrille, J. and C. E. Boyd. 1984.
be located mainly for the conve-                                                                 Comparisons of oxygen-
nience of the farmer. Aerators              to swim a long distance to find the
                                            new aerator.                                         transfer rates and water-
should be placed near a graveled,                                                                circulating capabilities of
all-weather road for easy access
during operation and mainte-
                                            References and                                       emergency aerators for fish
                                                                                                 ponds. Aquaculture
nance. They also should be near             additional reading                                   37:377–386.
the main power source to reduce             Ahmad, T. and C. E. Boyd. 1988.
the length of power lines.                                                                   Torrans, E. L., C. D. Hogue, Jr.
                                                Design and performance of                        and S. Pilkinton. 2003. The
If more than one permanent aera-                paddle wheel aerators. Aqua-                     sock-saver: A small trailer for
tor is to be installed in each pond,            cultural Engineering 7:39–62.                    providing liquid oxygen to
many farmers prefer to install              Boyd, C. E. 1998. Pond water aera-                   remote sites on commercial
them near each other (all in one                tion systems. Aquacultural                       channel catfish farms. North
end of the pond, for example)                   Engineering 18:9-40.                             American Journal of
rather than to space them widely                                                                 Aquaculture 65:260–265.
along the pond bank. If several              Boyd, C. E. and T. Ahmad. 1987.
aerators are located far apart and              Evaluation of Aerators for                   Tucker, C. S. and J. A. Hargreaves.
one of them fails at a critical                 Channel Catfish Farming.                         2004. Water quality manage-
time, fish will be forced to swim               Alabama Agricultural                             ment. Pages 215-278 in C. S.
a long distance through oxygen-                 Experiment Station Bulletin                      Tucker and J. A. Hargreaves
deficient waters to find a haven                No. 584, Auburn University,                      (editors). Biology and Culture
near the remaining aerators.                    Alabama.                                         of Channel Catfish. Elsevier,
Putting all the aerators in one end         Boyd, C. E. and C. S Tucker. 1998.                   Amsterdam, The Netherlands.
of the pond also reduces the cost               Pond Aquaculture Water                       Tucker, C. S. and E. H. Robinson.
of the electrical supply and makes              Quality Management. Kluwer                       1990. Channel Catfish
routine maintenance easier.                     Academic Publishers, Norwell,                    Farming Handbook. Van
Portable aerators, such as tractor              Massachusetts.                                   Nostrand Rei         nhold,
PTO-driven paddlewheels, should             Boyd, C. E. and B. J. Watten. 1989.                  New York, New York.
either be placed in the same loca-              Aeration systems in aquacul-
tion each time they are used so                 ture. Reviews in Aquatic
that fish become accustomed to                  Sciences 1:425–472.
the location or placed where oxy-

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

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