Development of an ecosystem model MARSHDO for assessing oxygen demand in a recirculating shrimp aqua

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Development of an ecosystem model (MARSHDO) for
assessing oxygen demand in a recirculating shrimp
aquaculture treatment wetland

• David R. Tilley
• Biological Resources Engineering University of
  Maryland
• College Park

Harish Badrinarayanan, Ji Ho Son
Environmental Engineering Texas AM
UniversityKingsville
Paper given at American Society of Agricultural
Engineers 2004, Ottawa, Ontario
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Motivation

• Shrimp Farming Increasing in Popularity
• Needs to be Ecologically Sustainable
• Treatment Wetlands used as Recirculation Filters
• Wetlands remove Suspended Solids and Nutrients
• OXYGEN levels drop!

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Objectives

• Evaluate how a Treatment Wetland altered the
  dissolved oxygen levels of process water
  discharged from a full-scale shrimp farm (8 ha)
• Develop a simulation model of the long-term
  temporal dynamics of dissolved oxygen (D.O.)
  discharged from the Treatment Wetland
• Test affects of organic and nutrient loading and
  management on Treatment Wetland D.O.

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Plan of Study

• Monitor D.O. along flow transect of shrimp farm
  treatment marsh
• H.T. Odums Energy Systems Language used to
  diagram oxygen cycle of treatment marsh.
• Develop computer simulation model using STELLA
  software.
• Model calibration based on literature values and
  validated with field observations.
• Test sensitivity of D.O. to changes in inputs and
  management of live and dead organic matter.

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Loma Alta Shrimp Farm Marsh Recirculation
Filter
Flow Rate 13,600 m3 d-1 (3.6 MGD) Hydraulic
Residence Time 24 h Depth 15 to 45 cm Study
period July 24 - Oct. 9, 2000
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Roseate Spoonbills
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Drop in D.O. across Wetland
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D.O. change along Marsh Transect
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BOD change along Marsh Transect
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pH change along Marsh Transect
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D.O. to BOD ratio lower in Highly Organic
Sections of Marsh
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Model Development
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Systems diagram Marsh-DO simulation model
J. Son
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Systems diagram for Marsh-DO modelnumerical
values for sources, storages and pathways
J. Son
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Determination of MARSHDO coefficients
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STELLA Diagram of MARSH-DO
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Long-term Dynamics of Key State Variables in
Marsh-DO
Biomass
Phosphorus
Org. Matter
D.O.
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10-year Simulation of Sensitivity of D.O. to
Organic Loading
½X Present Load
2X Present Load
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10-year Simulation of Sensitivity of D.O. to
Nutrient Load
½X Present Load
2X Present Load
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Sensitivity of DO to changes in Organic and
Nutrient Loading
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Management Effects
Biomass
Phosphorus Removal
Unmanaged
g/m2

Managed
Dead Organic Matter
Dissolved Oxygen
Unmanaged
Managed
g/m2
ppm
Managed
Unmanaged
Weeks
Weeks
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Model Deficiencies/Improvements

• Separate Organic Matter into suspended and soil
  compartments.
• Be careful in zeal of removing Organic Matter to
  increase Oxygen because
• O.M. supports removal of other pollutants (heavy
  metals, pathogens, nitrogen)

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Conclusions

• An ecosystems model of Dissolved Oxygen levels in
  a Treatment Marsh operated as the Recirculation
  Filter on a Full-scale Shrimp Farm was developed
  with energy systems language and simulated with
  STELLA software.
• Soil oxygen demand is at least as important as
  water column oxygen demand.
• Doubling of Organic Loading decreased long-term
  Dissolved Oxygen levels more than doubling
  Nutrient Loading.
• Annual removal of Biomass and Dead Organic Matter
  increased mean Dissolved Oxygen levels.

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the end