dr. kumar surahuan

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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

Abstract

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

www.elsevier.nl/locate/aqua-online

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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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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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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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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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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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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Table 2

Convergence achievements (%) of pond models

TurbulenceVelocityArrangement

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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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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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

capability.

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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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.

Acknowledgements

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.

References

Boyd, C.E., Watten, B.J., 1989. Aeration systems in aquaculture. CRC Crit. Rev. Aquatic Sci. 1,

425 – 472.

Boyd, C.E., 1995. Bottom Soils Sediment and Pond Aquaculture. Chapman and Hall, New York.


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Boyd, C.E., 1998. Pond water aeration systems. Aquacultural Eng. 18, 9 – 40.

Chanratchakool, P., 1994. Shrimp pond management — how to keep the feeding area clean. Aquatic

Animal Health Research Institute Newsletter, volume 2, number 3. Department of Fisheries, Jatujak,

Bangkok.

Meijer, L.E., Avnimelech, Y., 1999. On the use of microelectrodes in fish pond sediments. Aquacultural

Eng. 21, 71 – 83.

Peterson, E.L., 1997. Pond aeration study. Queensland Aquaculture News 11. Department of Primary

Industries, Bribie Island, Queensland, Australia, pp. 4 – 5.

Peterson, E.L., 1999. The effect of aerators on the benthic shear stress of a pond. Ph.D. Thesis. James

Cook University of North Queensland, Townsville, Australia. Chapters in volume 1, p. 291.

http:eng.jcu.edu.auaeratorsthesispeterson

Peterson, E.L., 1999b. Benthic shear stress and sediment condition. Aquacultural Eng. 21, 85 – 111.

Peterson, E.L., 2000. Observations of pond hydrodynamics. Aquacultural Eng. 21, 247 – 269.

Peterson, E.L., Indran, G., 2001. Process control of pond sediment redox. In: World Aquaculture 2001

Book of Abstracts. World Aquaculture Society, Orlando, FL.

Peterson, E.L., Pearson, D., 2000. Round peg in a square hole: aeration in a square shrimp pond. Global

Aquaculture Advocate 3 (5), 44 – 46.

Peterson, E.L., Harris, J.A., Wadhwa, L.C., 2000. CFD modelling pond dynamic processes. Aquacul-

tural Eng. 23, 61 – 93.

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