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    Aquacultural Engineering 25 (2001) 51–65

    Arrangement of aerators in an intensive shrimpgrowout pond having a rectangular shape

    Eric L. Peterson *, Lal C. Wadhwa, Jonathan A. Harris

    School of Engineering ,   and the Aquaculture Cooperati e Research Center,   James Cook Uni ersity,

    Townsille   4811,   Australia

    Received 4 September 2000; accepted 9 April 2001


    Simulations have been conducted to suggest general principles for the arrangement of 

    aerators within a rectangular pond used for the growout of marine shrimp such as  Penaeus

    monodon   and   Penaeus japonicus. Computational fluid dynamic models were produced for

    three schemes that were identified in a survey of Australian Prawn Farming Association

    members. These arrangements are ‘in-line’ (series), ‘parallel’ (side by side), and ‘diagonal’

    (diverting apart). Model results were assessed on the basis of benthic shear stress by

    classifying regions of pond bottom as ‘red zone’ (excessive stress), ‘green zone’ (desirable),

    and ‘dead spots’ (sediment traps). A comparison of results indicates that conventional

    aerators should be arranged diagonally or in parallel. It is also apparent that low-speed

    operation would be advantageous. These recommendations are consistent with the long-es-

    tablished practice of establishing pond-wide circulation. © 2001 Elsevier Science B.V. All

    rights reserved.

    Keywords:  Shrimp; Sediment; Aeration; Model; Simulation; Circulation; Pond


    1. Introduction

    Standard practice in Australian intensive shrimp growout ponds has been to

    supply aeration night and day, so that circulation sweeps sediments clear from feed

    areas. Circulation is a very important effect of mechanical aerators when aligned to

    promote circular motion of water around the pond periphery (Boyd and Watten,

    * Corresponding author. Tel.:  +61-7-47814420; Fax:  +61-7-47751184.

    E -mail address:   [email protected] (E.L. Peterson).

    0144-8609/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

    P I I : S 0 1 4 4 - 8 6 0 9 ( 0 1 ) 0 0 0 7 2 - 3

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    52   E .L.   Peterson et al . /   Aquacultural Engineering  25 (2001) 51– 65 

    1989). Primary pond circulation creates a secondary   flow that diverges at the

    surface and converges at the bottom, as illustrated in Fig. 1. The direction of 

    rotation is not important because the Coriolis force is very weak compared with the

    propulsive thrust effect of aerators (Peterson, 1999a). Pond circulation may cause

    soil erosion in intensive shrimp ponds, unless banks are protected with riprap

    (Boyd, 1995). Aeration of marine shrimp ponds should be arranged in such a way

    as to sweep the feed area of the pond, and such that these machines are switched

    off during feeding (Chanratchakool, 1994). Each 1 kW of aeration will support an

    additional 500 kg shrimp production, beyond a base of 2000 kg/ha that may surviveby natural re-aeration (Boyd, 1998). Consequently, a crop of 6500 kg can be

    supported in a 1 ha pond with 9 kW aeration. Square ponds are easier to manage

    because four paddlewheels can be arranged to wash out the corners (Peterson and

    Pearson, 2000) while propeller aspirators can help centralise waste as illustrated in

    Fig. 2. The present research involved the problem of rectangular ponds, where six

    paddlewheel aerators are required to maintain an intensive biomass.

    The focus of the present paper is to compare three alternative deployments of six

    1.5 kW paddlewheel machines. These three arrangements share a common aeration

    intensity of 9 kW/ha. Each case shares the same costs for maintenance, energy and

    equipment, except for the length of electric power cables. The only other variation

    between these three simulations is the positioning and alignment of the aerators.Twenty members of the Australian Prawn Farmers Association returned a

    questionnaire (Peterson, 1997) reporting on marine ponds they use for the growout

    of  P.  monodon  and  P.   japonicus. Survey respondents reported an average pond size

    of 87 m wide by 121 m long with an area of 1.0 ha and an average pond depth of 

    1.6 m. These survey results justify the application of Pond X as a proxy   ‘typical

    pond’. Pond X bathymetry was 85 m×120 m, with a surface area of 1 ha allowing

    for rounding in the corners, and 1.6 m deep at the outlet (Peterson, 2000). The

    pond was stocked with about 30 PL/m2 with   P.   monodon, on the largest marine

    shrimp farm in Australia at the time. Pond X was found to be a completely

    turbulent quasi-steady-state closed system, driven primarily by mechanical aerators.

    The survey (Peterson, 1997) also asked these Australian shrimp farmers tocompare their stocking and aeration strategies. The survey showed that paddle-

    wheels were the dominant aerator type in Australia. The questionnaire also asked

    each farmer to sketch the preferred arrangement of aerators in a pond. After

    reviewing the responses, it was found that each had a unique scheme, but there

    were some common themes. Some pond managers line up aerators one after

    another  ‘in-line’. A few arrange aerators side-by-side working in   ‘parallel’. The use

    of long-arm paddlewheel machines has been included in this classification because

    they are similar to a side-by-side assemblage of smaller units. Half of the survey

    sketches showed various arrangements of aerators that are directed   ‘diagonally’   in

    one way or another. In some instances, the diagonal aerators were located in

    corners and, in other cases, additional aerators were in the middle of reaches. Alldiagonal arrangements involve the spreading apart of two aerator jets so that they

    do not force against one another.

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    53E .L.   Peterson et al . /   Aquacultural Engineering  25 (2001) 51– 65 

         F     i   g .

         1 .

         P   r     i   m   a   r   y   a   n     d   s   e   c   o   n     d   a   r   y

       c     i   r   c   u     l   a    t     i   o   n .

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    Fig. 2. Recommended arrangement of four paddlewheels and four propeller-aspirators in a square pond (after

    paddlewheel is directed into the following corner.

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    55E .L.   Peterson et al . /   Aquacultural Engineering  25 (2001) 51– 65 

    Pond X was observed with a deployment of four propeller-aspirators and two

    paddlewheels, having a total aeration capacity of 9 kW (Peterson, 2000). A

    ‘real-world’   simulation of Pond X was validated by Peterson et al. (2000) by

    assuming that turbulence is generated and dissipated entirely within the system. The

    present paper presents three new simulations of Pond X, each with varied arrange-

    ments of aerators.

    2. Methods

    The computational methods used in present research were described in Peterson

    et al. (2000). The methods were validated in the context of the same   ‘Pond X’

    bathymetry as was used for the present set of simulations. That manuscript made

    reference to  ‘Figure 17’, which was not included in the earlier paper, presented here

    as Fig. 3. The   R2 linear regression correlation coef ficient of 0.7983 indicates that

    the model is good at predicting the direction of  flow within the bottom boundary

    layer. The implication is that our model is a valid predictor of the trajectory of 

    uneaten feed pellets and detritus.

    The methodology employs the   AUTOPOND   program by Peterson (1999a). In the

    interest of solving these large problems with  finite computer resources, it was found

    that convergence criteria needed to be adjusted from the nominal set point of 0.1%

    and liberalised to 1% relative error between succeeding iterations. Questing for

    tighter convergence criteria was found to be elusive, and ultimately may have

    become an exercise in futility due the chaotic  flapping of jets (Peterson et al., 2000).

    Acceptance of results on face value provides the opportunity to illustrate some of 

    Fig. 3. Bottom  flow direction correlation of simulation and observations (Should have been included in

    Peterson et al. (2000) as   ‘Fig. 17’).

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    56   E .L.   Peterson et al . /   Aquacultural Engineering  25 (2001) 51– 65 

    the important processes that contribute to the dynamic interactions at play in a real

    pond microcosm.

    For comparison purposes, the performance index has been taken to be the

    percentage of the pond bottom experiencing various intensities of shear stress

    (N/m2). Peterson (1999b) argued that benthic shear stress is a significant factor

    determining sediment condition. Portions of the pond bottom found to experience

    high shear stress will suffer from erosion, unless armoured with stones, cement, or

    plastic. In the present paper, those regions exposed to high stress are referred to as

    the   ‘red zone’.Soil particles scoured from the red zone follow a   ‘conveyor-belt’   process to fall

    onto surfaces experiencing less shear stress. Sedimentation tends to smother feed in

    a process of anaerobic degradation. Meijer and Avnimelech (1999) found that there

    is an anaerobic layer in the sediments of   fishponds, below the penetration of 

    bioturbation and molecular diffusion.   ‘Dead spots’  are those places where benthic

    shear stress is so weak that sediment oxygen demand exceeds the rate at which

    oxygen can diffuse down from the water column, and so anaerobic conditions may

    exit at the upper surface of the sediment (Peterson 1999b).

    Regions of a pond bottom experiencing moderate stress, bounded by the red zone

    and dead spots’  are referred to herein as the   ‘green zone’. The green zone is that

    area where mineral particles are undisturbed while the overlying water column iswell mixed. This corresponds to a benthic shear stress in the range 0.001 – 0.01

    N/m2, being the area where single cells are kept in suspension and feed is on the

    threshold of resuspension. Clay minerals tend to be cohesive and withstand higher

    stress after they have settled on the bottom, and so stresses up to 0.03 N /m2 have

    been included in the green zone.

    3. Results

    Three alternative simulations were obtained in the course of the present research.

    Each simulation represented the same pond with six 1.5 kW paddlewheels arranged

    in-line, parallel, and diagonally. Graphical presentations of the three simulations

    are given in Figs. 4 – 6, with plot axes that map the co-ordinates of integration

    points where benthic shear stress was calculated. These axes have units of metres in

    the north and west directions, respectively, while contours represent the magnitude

    of shear stress (N/m2) tangential to the alignment in three-dimensional space of the

    bottom and banks of Pond X. The upper-right corner of each simulation plot

    reports the average benthic shear stress. The interpretation of these results involved

    integration of the magnitude of benthic shear stress at each of the bottom and bank

    finite elements of each simulation. These integrated results are tabulated in Table 1

    in terms of the percentage of the pond bottom covered by dead spots, cells, feed,

    clay, silt and sand. The total area of desirable green zone covered 26.4, 34.5, and

    39% of the bottom of Pond X in the in-line, parallel, and diagonal simulations,respectively. Conversely, the adverse red zone covered 61.8, 61.2, and 50.7% of the

    in-line, parallel, and diagonal simulations.

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    57E .L.   Peterson et al . /   Aquacultural Engineering  25 (2001) 51– 65 

         F     i   g .

         4 .

         I   n  -     l

         i   n   e     d   e   p     l   o   y   m   e   n    t  -   s     i   m   u

         l   a    t     i   o   n   r   e   s   u     l    t   s .

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    58   E .L.   Peterson et al . /   Aquacultural Engineering  25 (2001) 51– 65 

         F     i   g .

         5 .

         P   a   r   a     l     l   e     l     d   e   p     l   o   y   m   e   n    t  -   s     i   m   u

         l   a    t     i   o   n   r   e   s   u     l    t   s .

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    59E .L.   Peterson et al . /   Aquacultural Engineering  25 (2001) 51– 65 

         F     i   g .

         6 .

         D     i   a   g   o   n   a     l     d   e   p     l   o   y   m   e   n    t  -   s     i   m

       u     l   a    t     i   o   n   r   e   s   u     l    t   s .

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    Table 1

    Relative zones of sediment quality predicted by the simulations

    Arrangement Dead spots (%) Green zone (desirable) (%)

    Cells, 0.001 – 0.003 Clay, 0.01 – 0.03 N/m2Feed, 0.003 – 0.01Dead, 0.001

    N/m2N/m2 N/m2

    8.2 In-line 12.611.7 5.64.3 5.4 12.3 16.8Parallel

    18.5 Diagonal 7.910.4 12.6

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    61E .L.   Peterson et al . /   Aquacultural Engineering  25 (2001) 51– 65 

    Table 2

    Convergence achievements (%) of pond models


    wDegree of freedom   Pu k     

    0.05 0.77 0.970.21 0.660.34In-line

    0.490.51 0.08 0.88 0.86 0.98Parallel

    Diagonal 0.100.54 0.68 0.72 0.810.56

    Ideally, each simulation should converge such that the integrated shear stress

    vector is equal in magnitude and opposite in direction to the net applied driving

    force (Newton’s law). The integrated shear stress reaction was under 16 N in each

    of the simulations, resolved with a Cartesian coordinate system. This is remarkably

    well balanced considering that the six aerators imparted a total of 1200 N thrust in

    a circular pattern around the centre of the pond. The relative error of velocity of 

    each simulation is presented in Table 2, which is a measure of how closely the last

    two iterations varied. Velocity errors were nearly within 0.5% in each case, while

    the relative error of turbulent kinetic energy,   k , diverged as much as 1%. The

    relative error of velocity of simulations presented in the present paper is compara-

    ble with those achieved in the validated  ‘real-world’ simulation of Pond X (Petersonet al., 2000).

    Some of the aerator jets appear to be distorted in Figs. 4 – 6. In each case, the

    northwest and southeast jets are blocked by the primary pond circulation  flow, so

    they tightly recirculated into corners of the pond. This is not consistent with

    observations of the paddlewheel jet in the northwest corner of Pond X (Peterson

    2000), except that there was real oscillatory   flapping that suggests bifurcation of 

    flow patterns. The simulations of the present research were formulated as a

    steady-state problem because there was not suf ficient disk space to archive and

    analyse transient output data. The practical application of these simulations is

    taken from the percentage of benthos that may be classified as red zone (undesir-

    able) and green zone (preferred), without prejudice as to the exact location andshape of individual patches.

    4. Discussion

    The present paper has considered a   ‘typical’  rectangular pond having an aspect

    ratio of 1.4. Circulation and sediment quality may be better in square and round

    ponds. The present study has simulated the effects of the common Taiwanese

    four-wheel paddlewheel driven at 100 r.p.m. Other aerator types may have other

    effects, and the same paddlewheels would certainly produce better sediment quality

    if driven at lower speeds (Peterson and Pearson, 2000; Peterson and Indran, 2001).Comparing the three alternative deployments of six paddlewheels, the diagonal

    arrangement of Fig. 6 provides a greater area of green zone and a lesser area of red

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    62   E .L.   Peterson et al . /   Aquacultural Engineering  25 (2001) 51– 65 

    zone. The parallel arrangement of Fig. 5 produced a broad front of sweeping effect,

    which reduced the area of dead zone to less than one-half of that resulting from the

    other arrangements. The parallel arrangement would be recommended if minimisa-

    tion of dead zones were the only selection criteria, but it is believed that maximisa-

    tion of the green zone area should be the prime objective. Adversity of the dead

    zone is reduced if sedimentation rates are controlled. Minimising the area of 

    red-zone would control sedimentation most directly. On the basis of the forgoing,

    the diagonal arrangement is superior, the parallel arrangement has been found to be

    mediocre, and the in-line arrangement is the least desirable.

    In-line and parallel arrangements were both situations with aerators positioned

    closely together. The result was an increase in the area of red zone. The concentra-

    tion of excessive stress is most pronounced in the in-line arrangement of Fig. 4,

    where the outlet of one aerator is directed at the inlet of another.

    The trend of the simulations suggests how the area of green zone might be

    increased. The diversion of   flow obtained with the diagonal aerator arrangement

    tends to increase the green zone. Spreading apart of jets is seen as a crucial element

    of any effective deployment of aerators to achieve this end.

    The   findings of the present paper do not conflict with the long-established

    practice of establishing pond-wide circulation. It is important to recognise the

    benefit of any aeration arrangement that tends to clean the peripheral feeding areas

    of a pond and concentrate detritus at the centre. Pond managers should consider

    the conclusions of the present paper and evaluate what actually happens within the

    particular pond geometry and aeration equipment they have to work with.

    Benthic monitoring with a facemask and snorkel is not practical unless there has

    been an algal crash in a pond.   ‘Pond walking’   refers to a wading transit in search

    of dead spots where soft sediment is detected with the feet. Pond managers often

    use this technique to decide where to place aerators. A   floating orange and

    stopwatch can be used to estimate pond surface velocity at various locations, and

    this can indicate the benthic shear stress resulting from pond circulation (Peterson,

    1999b). Surface velocities of 3 – 7 cm/s indicate ideal benthic conditions (green zone)under as much as 2 m water column.

    Surface velocity is not a good indicator of benthic conditions in the vicinity of an

    aerator. Propeller-aspirators may impinge on the bottom and consequently create a

    scour crater. Paddlewheel aerators create a sheet of fast-moving surface water,

    overlaying dead water that may recirculate into the inlet (Peterson, 2000). Drain

    harvesting will ultimately reveal a complete picture of the sources and sinks of 

    sediment transport in a pond, and whether these patterns are uniform and

    manageable. Scoured depressions (puddles) usually appear downstream from pad-

    dlewheels at a distance of greater than seven times the water column depth, which

    is consistent with the normal expansion angle of turbulent jets (Peterson et al.,

    2000). For example, in a 1.5 m water column, scour is expected to start about 10m downstream from a paddlewheel. Scouring is expected to continue downstream

    until the surface velocity drops below 7 cm/s.

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    63E .L.   Peterson et al . /   Aquacultural Engineering  25 (2001) 51– 65 

    5. Conclusions

    The present research has quantified the simulated effects of re-arranging aerators

    in a particular pond. The possible permutations are endless, but only three

    alternatives have been investigated in this paper. Fig. 7 presents a sediment-quality

    pie chart for these arrangements. The following are some possibly general principles

    drawn from the three cases in which we have invested our computer-simulation


    5 .1.  Rearrangement of aerators will not completely sol e the problem

    The red zone (stress exceeding 0.03 N/m2) occupies most of the pond area in all

    of the arrangements investigated. Rearrangement of aerators has not completely

    solved the problems of scour, and other management actions are certainly required.

    Increasing pond depth might help to diffuse aerator jets before they impact on the

    bottom. The red zone should be recognised and armoured with geotextiles, gravel,

    or cement. Sand will withstand most of the red zone stress in the feed area, but

    gravel or pavement is needed on banks and in scour-holes associated with individ-

    ual aerators. The shear stress in the red zone may be temporarily relieved during

    daylight feeding by switching off aeration. One possible strategy is to reduce thethrust of paddlewheels during daylight feeding, temporarily converting the red zone

    to the green zone, which can be achieved with a variable frequency drive (Peterson

    and Pearson, 2000; Peterson and Indran, 2001).

    Fig. 7. Pie-chart comparison of simulation results.

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    64   E .L.   Peterson et al . /   Aquacultural Engineering  25 (2001) 51– 65 

    Provision of continuing pond circulation during daylight is recommended to

    prevent pond stratification. Simply switching the power on and off with a time

    clock is further discouraged because of the high in-rush amperages and mechanical

    shock at start up, and because a shut down of aerators allows moisture to enter

    windings and seals.

    5 .2 .   In-line aerators are the least desirable arrangement

    All indications are that series in-line arrangements are inferior. The possiblemotivation for the widespread use of this strategy may have been to minimise the

    length of electric cabling used in ponds. But the result is that high stress is

    concentrated at the outermost portion of the pond periphery, with erosion of pond

    banks and lowing away feed pellets being likely consequences.

    5 .3 .  Parallel aerators minimise the dead zone

    There was found to be a broad front of sediment sweeping that dramatically

    reduced the area of dead spots in the parallel simulation. Unfortunately, the extent

    of sand and silt scouring consumed nearly as much of the pond bottom as the

    in-line arrangement. The results of the parallel arrangement suggest that, if thespeed were reduced, then a large proportion of pond bottom area could be

    controlled uniformly within desirable shear stress limits.

    5 .4 .   Aerators should be di erted apart

    Normally, at least one aerator is provided in each corner of a rectangular pond.

    More than four aerators may be required to maintain pond oxygen levels in an

    intensively stocked pond. In such cases, the added units should be diverted

    diagonally apart from those that they are placed aside. The objective is to distribute

    stress over a wider region, so that individual jets do not force against one another.


    This paper was made possible with support from James Cook University, under

    a Merit Research Grant administered by the Postgraduate Students Of fice and

    computer time provided by the High Performance Computer Centre, administered

    by Dr Ian Atkinson.


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