DISTRIBUTED ANIMAL PRODUCTION IN A …precisionag.sites.clemson.edu/Kirk_Files/2004 Kirk MS...DISTRIBUTED ANIMAL PRODUCTION IN A PARTITIONED AQUACULTURE SYSTEM _____ A Thesis Presented

DISTRIBUTED ANIMAL PRODUCTION IN A

PARTITIONED AQUACULTURE SYSTEM

_______________________________________________

A Thesis

Presented to

the Graduate School of

Clemson University

_______________________________________________

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Biosystems Engineering

_______________________________________________

by

Kendall R. Kirk

August 2004

Advisor: Dr. David E. Brune

ABSTRACT

The objective of this two year study was to adapt the current confined PAS

technology to distributed animal production—specifically, culture of the Pacific white

shrimp (Litopenaeus vannamei)— and to develop and use oxygen, carbon, and nitrogen

mass balances for system optimization. The experimentation performed in 2002 was

targeted at understanding shrimp-tilapia co-culture requirements and water quality

limitations supporting the growth of shrimp. Shrimp production in 2003 averaged 18,685

lb/ac (16,667 kg/ha) with 61% survival and a harvest density of 11.4 shrimp/ft2 (123

shrimp/m2). Peak daily feed rates in excess of 350 lb/ac/day (392 kg/ha/day) were

demonstrated and the average seasonal feed rate to all shrimp units combined was 155

lb/ac/day (174 kg/ha/day). Transfer of water from the shrimp units to the tilapia unit was

necessary in order to maintain water quality. An overall carbon mass balance for the

shrimp units indicated that the organic carbon input to the system by feed application was

7.8 g-C/m2/day, the inorganic carbon input to the system was 0.2 g-C/m

2/day, the algae

assimilated 2.7 g-C/m2/day, the shrimp converted 0.4 g-C/m

2/day, sludge collection

accounted for the removal of 2.9 g-C/m2/day, and CO2 outgassing accounted for the

removal of 4.7g-C/m2/day. An overall nitrogen mass balance for the shrimp units

indicated that the nitrogen input to the system by feed application was 0.89 g-N/m2/day,

algae assimilated 0.48 g-N/m2/day, shrimp converted 0.10 g-N/m

2/day, sludge collection

accounted for the removal of 0.44 g-N/m2/day, and nitrification accounted for the storage

of 0.35 g-N/m2/day as nitrate. Based on pilot-scale observations of total system

performance and water quality dynamics, an overall system design is proposed for

iii

optimization of water quality and sludge management with maximum shrimp

productivity.

ACKNOWLEDGMENTS

This project would not have been possible without the endless hours of support

and commitment from Dr. David Brune. Equally important in the project were the

contributions from Dr. John Collier in the mechanical aspects of the system and Dr. Arnie

Eversole for his extensive knowledge and experience in aquaculture and fisheries

sciences.

The funding for this research was provided by NOAA-Sea Grant and the Clemson

University Newman Endowed Fellowship.

Thanks to those that participated in data collection for this project, including Dr.

Brune, Dr. Eversole, Antonio Aranguren, and Teresa Wilson, participated in collection of

the reams of data collected for this research. Thanks also to Scott Davis for his

indispensable advice on the algal-front, Vickie Byko for going out of her way to help me

out and keep me in the program, and to the crew at the fish farm for your help,

commitment, and guidance; the project would not have been complete (and I might not

be graduating) had it not been for you.

Lastly, I offer thanks to my family and friends, especially my parents, for

providing direction in my life and supporting my academic ventures.

TABLE OF CONTENTS

Page

TITLE PAGE .............................................................................................................. i

ABSTRACT................................................................................................................. ii

ACKNOWLEDGMENTS ........................................................................................... iv

TABLE OF CONTENTS............................................................................................. v

LIST OF TABLES....................................................................................................... viii

LIST OF FIGURES ..................................................................................................... ix

INTRODUCTION ....................................................................................................... 1

Cultural Eutrophication............................................................................................ 1

Aquaculture and the Environment ........................................................................... 3

The Confined PAS ................................................................................................... 7

Advantages of the Confined PAS ............................................................................ 10

Confined PAS vs. Distributed PAS ......................................................................... 12

Distributed PAS Research Objectives ..................................................................... 13

DISTRIBUTED PAS CONFIGURATION AND COMPONENTS ........................... 14

Artificial Marine Environment ................................................................................ 14

Artificial Tropical Environment .............................................................................. 14

Greenhouse Construction and Configuration........................................................... 15

Aeration.................................................................................................................... 18

Water Circulation..................................................................................................... 21

Solids Removal ........................................................................................................ 22

Airlift Pump Construction........................................................................................ 25

Tilapia Co-culture and Algal Management.............................................................. 31

CO2 Delivery............................................................................................................ 33

Temperature Control ................................................................................................ 34

Drainage................................................................................................................... 35

vi

Table of Contents (Continued)

Page

OPERATIONAL PROCEDURES............................................................................... 36

Shrimp Stocking and Harvest .................................................................................. 36

Shrimp Feeding........................................................................................................ 37

Solids Removal ........................................................................................................ 38

Water Transfer to Tilapia Unit................................................................................. 42

pH Control ............................................................................................................... 43

DATA COLLECTION ................................................................................................ 44

Meter-facilitated Measurements .............................................................................. 44

DO ........................................................................................................... 44 Temperature .............................................................................................. 44 pH ............................................................................................................ 45 Salinity ..................................................................................................... 45 Photosynthesis and Respiration ................................................................... 45 Carbon Analysis ........................................................................................ 46

Laboratory and Field Measurements ....................................................................... 47

TAN ......................................................................................................... 47 Nitrite ....................................................................................................... 47 Secchi Depth ............................................................................................. 47 Algal Counts ............................................................................................. 47 Alkalinity.................................................................................................. 48 Solids Concentration in Sludge Removed ..................................................... 48 Shrimp Size Samples.................................................................................. 49 Shrimp Density Samples............................................................................. 49

DATA ANALYSIS AND MANIPULATION ............................................................ 50

Feed Rates................................................................................................................ 50

7-Day Water Quality Averages................................................................................ 51

Unionized Ammonia Concentration ........................................................................ 51

Shrimp Biomass ....................................................................................................... 52

Recarbonation Rate.................................................................................................. 53

Algal Biovolumes .................................................................................................... 57

Carbon Mass Balance .............................................................................................. 62

Nitrogen Mass Balance ............................................................................................ 67

vii

Table of Contents (Continued)

Page

RESULTS AND DISCUSSION.................................................................................. 69

Feed Application Rates ............................................................................................ 69

Water Quality........................................................................................................... 77

Temperature .............................................................................................. 77 Dissolved Oxygen Concentration................................................................. 82 Salinity ..................................................................................................... 87 pH ............................................................................................................ 90 Total Ammonia-Nitrogen............................................................................ 97 Unionized Ammonia .................................................................................. 101 Nitrite ....................................................................................................... 106 Secchi depth .............................................................................................. 111 Alkalinity.................................................................................................. 115

Water Transfer to Tilapia Unit................................................................................. 115

System Productivity ................................................................................................. 116

Shrimp Production ..................................................................................... 117 Algal Production........................................................................................ 124

Solids Removal ........................................................................................................ 134

Carbon Mass Balance .............................................................................................. 136

Nitrogen Mass Balance ............................................................................................ 137

DESIGN PROPOSAL FOR SYSTEM OPTIMIZATION .......................................... 139

SUMMARY................................................................................................................. 145

CONCLUSIONS.......................................................................................................... 153

LITERATURE CITED ................................................................................................ 158

LIST OF TABLES

Table Page

I. Critical water quality levels for feed rate reductions. ...................................... 38

II. Length of time to inject CO2 as a function of pH. ........................................... 43

III. Data included in subdivided recarbonation data sets. ...................................... 55

IV. Phytoplankton taxa, shapes and arbitrary sizes................................................ 59

V. Phytoplankton biovolume calculations summarized for each

taxa............................................................................................................. 61

VI. Particulate organic carbon measurements........................................................ 127

LIST OF FIGURES

Figure Page

1. PAS carrying capacity and production from 1995-2001. ................................ 6

2. PAS feed application rates from 1997-2001. ................................................... 7

3. Confined PAS physical configuration schematic. ........................................... 8

4. Conceptual diagram of nitrogen input, storage, and outputs

in the PAS. ................................................................................................. 10

5. Sketch of general layout of greenhouse. .......................................................... 16

6. Installation of clear greenhouse roof................................................................ 17

7. Oxygen generator used for initial aeration....................................................... 19

8. A 3/4 hp Powerhouse fountain aerator............................................................. 20

9. Plan view of one unit showing configuration and water

recirculation. .............................................................................................. 21

10. Photograph of relative position of PAS components. ...................................... 22

11. Solids delivery to 500 gal settling tank............................................................ 23

12. Shrimp exclusion tunnel in the 500 gal settling tank....................................... 24

13. Schematic of solids removal system. ............................................................... 25

14. Schematic for operation of airlift pumps in sump holes. ................................. 27

15. Photograph showing airlifts positioned in sump holes. ................................... 28

16. Elevated intake for water exchange airlift pumps............................................ 29

17. Corner recirculation airlift pumps.................................................................... 31

18. Airlift water exchange schematic..................................................................... 33

19. Boilers used for water temperature control...................................................... 35

20. Schematic of solids removal operations. ......................................................... 40

x

List of Figures (Continued)

Figure Page

21. Blower sequencing and solenoid operation to control airlifts

in the units.................................................................................................. 42

22. Daily feed rate in unit 1. .................................................................................. 70



25. The 7- and 14-day average feed rates unit 1. ................................................... 72



28. Feed rate as a function of percent body weight in unit 1. ................................ 74



31. The 7-day average feed rate as a function of percent body

weight in unit 1. ......................................................................................... 76


weight in unit 2. ......................................................................................... 76


weight in unit 4. ......................................................................................... 77

34. Daily temperature in unit 1. ............................................................................. 78




39. The 7-day average temperature in unit 2. ........................................................ 81



42. Daily dissolved oxygen concentration in unit 1............................................... 83

xi


Figure Page




46. The 7-day average dissolved oxygen concentration in unit 1.......................... 85




50. Daily salinity in unit 1...................................................................................... 88




54. Daily pH in unit 1. ........................................................................................... 91

55. Daily pH in unit 2. ........................................................................................... 91

56. Daily pH in unit 3. ........................................................................................... 92

57. Daily pH in unit 4. ........................................................................................... 92

58. The 7-day Average pH in unit 1. ..................................................................... 93




62. Carbon input over time in unit 1. ..................................................................... 95



65. Daily total ammonia concentration in unit 1.................................................... 97

xii


Figure Page




69. The 7-day average total ammonia nitrogen concentration in

unit 1. ......................................................................................................... 99


unit 2. ......................................................................................................... 100


unit 3. ......................................................................................................... 100


unit 4. ......................................................................................................... 101

73. Daily unionized ammonia concentration in unit 1. .......................................... 102




77. The 7-day average unionized ammonia concentration in unit 1. ..................... 104




81. Daily nitrite concentration in unit 1. ................................................................ 107




85. The 7-day average nitrite concentration in unit 1. ........................................... 109


xiii


Figure Page



89. Daily Secchi depth in unit 1............................................................................. 111




93. The 7-day average Secchi depth in unit 1........................................................ 113




97. Water transfer to tilapia unit. ........................................................................... 116

98. Shrimp average weight in unit 1. ..................................................................... 118



101. Estimated shrimp biomass in unit 1. ................................................................ 120



104. Feed conversion ratio over time in unit 1. ....................................................... 122



107. Net algal photosynthesis and water column respiration in

unit 1. ......................................................................................................... 125


unit 2. ......................................................................................................... 125

xiv


Figure Page


unit 3. ......................................................................................................... 126


unit 4. ......................................................................................................... 126

111. Algal biovolume by taxon in unit 1. ................................................................ 128




115. Algal biovolume by class in unit 1. ................................................................. 130




119. Rate of volatile solids removal in unit 1. ......................................................... 134



122. Carbon mass balance for shrimp units. ............................................................ 137

123. Nitrogen mass balance for shrimp units........................................................... 138

124. Conceptual drawing of U-tube design. ............................................................ 142

INTRODUCTION

As the world population continues to grow, food demand increases

proportionately. Fisheries industries have passed sustainable ocean harvest. We are now

rapidly depleting our marine resources. In the last two decades, the aquaculture industry

has been the most rapidly growing sector in American agriculture and it continues to

grow exponentially. Advances in aquaculture sciences and engineering supporting the

development of systems for production of freshwater and marine organisms have allowed

the developing aquaculture industry to meet a growing market demand. Conventional

aquaculture techniques potentially lead to release of large quantities of nutrients into the

environment, leading to increased rates of eutrophication in surface waters.

Cultural Eutrophication

Eutrophication is a naturally occurring succession where water bodies age from

oligotrophic water bodies to eutrophic water bodies to wetlands and eventually to dry

land over time due to nutrient and organic deposition. Cultural eutrophication is an

accelerated form of natural eutrophication that occurs when humans interfere with natural

processes and release quantities of nutrients into surface waters in excess of the natural

assimilative capacity. At these high inorganic nutrient levels, algal growth is not limited

in terms of its nutrient requirements and the algal communities will grow to exceptionally

high densities in the water column until they create light-limitations for growth. At this

point, the algal density is so high that the upper strata of algae are shading light from the

algal organisms that are not near the surface of the water. At such high densities, when

2

this light limitation occurs—a function of decrease light penetration into the water

column—the result is in massive die-offs of algae, which sink to the bottom, creating a

large organic loading. The decomposition of the dead algal cells by aerobic bacteria in

the water column and sediments create biological oxygen demands that cannot be

satisfied passively in most water bodies, resulting in a shift to anaerobic or anoxic

conditions.

As large areas of normally aerobic water bodies suffer this rapid depletion in

dissolved oxygen concentration (DO), fish and other aquatic organisms that require

oxygen for survival are killed or displaced. The diversity of aquatic organisms that are

capable of surviving in low DO conditions is less than otherwise and the ultimate result is

a restructuring of the aquatic community towards aquatic animal singularity This

restructuring translates ultimately to food source depletion and therefore reduced

biodiversity (U.S. Environmental Protection Agency, 2000). The availability of

macronutrients, specifically nitrogen and phosphorous, and therefore the extent of

primary production, is inversely related to the diversity of organisms within a given

ecosystem. The end result of accelerated eutrophication comes in the form of water use

restrictions for fisheries, recreation, industry, and drinking water (Sharpley et al., 1999).

It has been reported that about half of the water bodies in the United States suffer from

pollution by excessive nutrient loadings and biological growth (U.S. Environmental

Protection Agency, 2000).

With the development and widespread use of synthetic fertilizers and agricultural

mechanization, the intensification of the agriculture industry in the mid 1900s

inadvertently played a major role in accelerated eutrophication. The synthetic fertilizers

3

and mechanization, both entering the agricultural industry at about the same time, led to a

rapid change in the way food was grown in the U.S. Larger farms were managed and

yields were maximized with excessive fertilizer application rates. The result was that

more nitrogen was released into the environment and that its sources were more heavily

concentrated into localized areas where larger farms continued to operate. Although

agriculture itself is a very old institution, at the time, modern agriculture was a young and

rapidly changing industry. Unfortunately, the changes made led to problems in the

environment that were not anticipated, especially that of eutrophication. As an example

of the lack of foresight and understanding in the 1970s of the implications of accelerated

eutrophication, Pereira (1973) stated the biggest problem associated with eutrophication

was that “excessive growths of algae…cause difficulties in water purification”. Similarly

and currently, aquaculture is an exponentially growing sector of the agricultural industry,

with new systems and practices being studied and employed. Aquaculture, too, has

potential for eutrophication and therefore the practices developed and implemented must

be environmentally conscious.

Aquaculture and the Environment

State and federal regulatory agencies are targeting the aquaculture industry with

discharge limitations. Operating practices for U.S. fish farms are becoming increasingly

stringent every year. As a result, the need exists for a sustainable aquaculture industry,

one that can supply an ample amount of food to satisfy increasing market demands, but

with reduced or eliminated environmental impacts. The research conducted at Clemson

University on the Partitioned Aquaculture System (PAS) has been directed at the

development of 100% closed cycle aquatic animal production facilities.

4

Early trout production used flow-through raceways with discharge to the

environment. In a flow-through system, a farmer wishing to grow large quantities of fish

must correspondingly be able to supply his production units with large quantities of feed.

High feed rates in conventional situations are, however, not sustainable and the water

used to culture the fish becomes so heavily laden with organics and nutrients that it can

no longer support fish growth due to excessive carbonaceous and nitrogenous oxygen

demands. To solve the problem, the farmer may discharge the organic- and nutrient-laden

water and replace it with new water from a local surface or ground water. By managing

the ponds in such a manner, the farmer can increase feed input and expect higher

production rates; however, this release of nutrient enriched water comes not without

deleterious effects on the environment. Although many existing flow-through systems in

the U.S. may continue to operate as such, the installation and operation of new flow-

through systems is not an option for most aquaculture practices because of environmental

regulations. Development of environmentally friendly systems possessing equivalent

production potential to flow-through systems is needed.

The PAS is a recirculating algal system designed to maximize aquatic animal

productivity by incorporating waste treatment within the culture pond. If designed

properly, ponds are ideal wastewater treatment environments because of their simplicity,

economy, and reliability (Oswald, 1990). Inorganic and organic carbon and nitrogen,

especially ammonia and CO2, generated in the system is assimilated into algal biomass by

photosynthesis, allowing for higher feed inputs to the system. These higher feed rates

translate to higher production rates, without the need to discharge water from the system.

Another advantage of stimulating algal growth in the system is the generation of oxygen

5

during photosynthesis, which can be used by the target organisms for respiration. Algal

species transfer oxygen directly into the water column with the capability of producing

supersaturated levels of DO in contrast to other photosynthetic species used for nutrient

removal, e.g. duckweed and water hyacinths (Green et al., 1995). Synthesis of algal

biomass by photosynthesis is given by the classical photosynthetic reaction (King, 1976):

( ) OHOOCHλOHCO 22n222 ++→++ . (1)

At most intensive aquaculture operations, bacterial water treatment systems are

used but research on algal-based systems has been for the most part ignored probably due

to the existing extensive knowledge of bacterial systems used for wastewater treatment

(Wang, 2003). The PAS essentially harvests the energy of the sun to produce oxygen

rather than consuming our limited sources of fossil fuels. Mechanical aeration systems

are reported to produce only 1 kg-O2 per kWh (Green et al., 1995; Oswald, 1996),

whereas the paddlewheel mixing used in the PAS system to stimulate and sustain algal

photosynthesis produced as much as 3 kg-O2 per kWh over 24 hours. So, in the PAS, if

mechanical aeration were used 50% of the time to satisfy the nighttime respiration

requirements and photosynthetic oxygen production relied upon the remaining 50% of

the time for aeration, the 24 hr oxygen production would be 2 kg-O2 per kWh, twice what

it would be using a mechanical system alone. It is not uncommon for bacterial-based

aquaculture systems to be economically unsuccessful mainly due to their complexities

and operational expenses (Wang, 2003).

The PAS concept was developed in 1989 at Clemson University and research

since then has resulted in increased fish carrying capacity and sustainable feed rates

6

(Figure 1 – Figure 2) to the system, with zero discharge (Brune et al., 2004, 2003). Until

recently, the PAS technology developed, applied, and researched at Clemson University

has been a confined PAS technology—where the primary target organism is confined at

relatively high densities (3-8 lb/ft3, 48-128 kg/m

3) to a small portion (5%) of the system’s

total water surface area.

Figure 1. PAS carrying capacity and production from 1995-2001.

7

Figure 2. PAS feed application rates from 1997-2001.

The Confined PAS

The confined PAS is designed to maximize algal productivity because it is the

algae that allows for such high primary target organism production rates. Approximately

95% of the area is therefore dedicated to promote and enhance the growth of algae. The

remaining 5% of the area is represented by raceways for the high-density growth of the

primary target organism, channel catfish (Ictalurus punctatus).

The PAS operates similar to some wastewater pond systems where the promotion

of algal-bacterial symbiosis allows for each to use the metabolic products of the other. In

these systems, bacteria in the system utilize oxygen generated by the algae and in turn,

the algae use the major end products, carbon dioxide and ammonia-nitrogen, of bacterial

metabolism in their photosynthetic reactions (Ward and King, 1976; Almasi and Pescod,

8

1996; Oswald, 1996; Zhao and Wang, 1996). The PAS follows a direct analogy to these

systems, but with bacteria being replaced by catfish.

Catfish are stocked in the fish raceways, which are 1 m deep, and secondary

paddlewheels are used to provide water velocity in the system, moving water through the

fish confinements. As feed is applied to the system and consumed by the catfish, their

waste is excreted into the water column and the water moving past them carries the waste

to the high-rate algal bioreactor (Aranguren et al., 2004) (Figure 3). A primary

paddlewheel is used in the bioreactor to continuously recirculate the water in a closed

loop where the fish waste is treated by algae. Catfish are only capable of incorporating

approximately 25% of the nitrogen available in the feed into fish biomass and the

remaining 75% of the nitrogen in the feed returns to the water column.

Figure 3. Confined PAS physical configuration schematic.

9

As algae grows in the system, nitrogen and carbon (ammonia-nitrogen and CO2)

removed from the water column are fixed into algal biomass. Ammonia-nitrogen, in the

unionized form is highly toxic to most aquatic organisms and is a byproduct of fish

metabolism. Therefore, its removal from the water column is essential in maintaining a

sustainable growth environment at high feed rates. In order to maximize algal

productivity, the algal bioreactors are relatively shallow, about 12 - 18 inches (31 - 46

cm) deep. The shallow depth, along with homogeneous paddlewheel mixing, increases

overall light availability to the algal biomass, ensuring that all of the algal biomass

remains in the photic zone. The amount of photosynthesis occurring in a water body is

dependent on the amount of algae that is present in the photic zone (King, 1976). Greater

depth in the bioreactor would result in light-limiting conditions, ultimately decreasing the

alga productivity per unit volume and per unit area.

Although the system is designed to maximize algal growth, continual algal growth

would become problematic without its removal or harvest. If the algae were allowed to

grow at high rates without the application of any management strategies, its density

would become so high that light-limitations would occur. The extinction coefficient

would increase because the algae in the upper portion of the water column would block

out light from reaching the algae in the lower portion of the water column. When the

algae becomes growth limited, whether it be by a lack of nutrients, carbon, or light, the

response is a decrease in autotrophic productivity. In some instances, the entire algal

community is lost. In order to avoid these sudden changes in algal density, limiting

situations must be avoided and therefore to sustain high-rate algal growth, equal-rate

algal harvest must be employed.

10

Biological and mechanical harvest methods have been employed in the system.

For biological harvest of the algae, Nile tilapia (Oreochromis nilotica), a filter feeder that

consumes algae and other microorganisms, helps keep the algal community cropped.

Several mechanical algal removal mechanisms have also been employed where the algae

is concentrated or flocculated and then physically removed from the PAS. The removal of

algae, whether by mechanical or biological means, creates room for more algal growth

(i.e. the extinction coefficient is decreased, reducing light limitations for growth) and

therefore increased nutrient assimilation, which allows for higher feed rates, ultimately

leading to greater target animal production (Kirk et. Al, 2004) (Figure 4) .

Figure 4. Conceptual diagram of nitrogen input, storage, and outputs in the PAS.

Advantages of the Confined PAS

Because the PAS contains a built-in wastewater treatment facility in the form of

the high-rate algal pond, no discharge is required from the system; the same water that is

11

used in the beginning of a season is the water that is used at the end of the growing

season. This is becoming increasingly important in an industry that is everyday becoming

more highly regulated in terms of waste discharges to surface waters.

Feed inputs to the system can well exceed those sustainable in conventional

earthen pond culture systems because of the assimilative capacity of the algae to remove

ammonia from the water column. As a direct consequence of being able to apply more

feed per unit area, the production capacity per unit area increases. PAS research has

reproducibly demonstrated production of 15,000 - 18,000 lb/ac (16,816 - 20,179 kg/ha) of

channel catfish (Brune et al., 2004), four times the production capacity of conventional

earthen ponds. At such high yields, water usage requirements per unit of fish produced

can be reduced by 90% in the PAS as compared to conventional pond culture (Brune et

al., 2003).

The nitrogen fixed into algal biomass is a valuable resource possessing many

possibilities in regards to its marketability (Oswald, 1996). In other zero-discharge

recirculating aquaculture systems, as opposed to the PAS, nitrogen removal is

accomplished by mechanized systems that facilitate nitrification followed by

denitrification—a process that wastes fixed nitrogen. “Is it not strange that our

agricultural industry spends billions of dollars to fix nitrogen while [others spend]

billions to unfix it?” (Green et al., 1996)

Because the animals are confined to 5% of the total area of the system, the

animals are much more accessible (Brune et al., 2003). Feed management strategies can

be optimized and variability in the growth of the animals is therefore minimized. Harvest

becomes much easier because the animals are already crowded into a relatively small

12

area; use of seines is not required. If the animals should ever require medication, they can

be easily isolated from the remainder of the system, reducing the amount of medication

required in order to reach a predetermined concentration of the drug in the water column.

Aeration can be applied much more efficiently because it is only applied in the pond

where it is needed and less aeration is wasted (Brune et al., 2003). Also, management of

dissolved oxygen concentrations in and waste removal from the fish raceways can be

controlled at variable rates by adjusting the paddlewheel speed and thus, the water flow

rate past the fish.

Confined PAS vs. Distributed PAS

Instead of high-density containment of the target production animal, they were

distributed throughout the entire system. The behavior of shrimp and catfish differ

significantly and therefore their growth methods and culture infrastructures must also be

different. Catfish generally respond positively in terms of production and growth to high-

density stocking, whereas shrimp respond poorly to these conditions mainly due to the

tendency for shrimp to cannibalize their cohorts at molts and the development of “black

spot” disease. Black spot disease is not actually a disease, rather a condition in which the

shrimp held at high densities develop abrasions on their exoskeleton as a result of

collisions with other individuals and system structures. Physiologically, the shrimp are

not greatly harmed; however, their marketability is greatly reduced. In effect, the PAS

adaptations made the entire system both a shrimp growth and a wastewater treatment

environment, which presented new challenges in terms of algal management, oxygen

delivery, pH control, and solids removal.

13

Distributed PAS Research Objectives

The primary objective of this project was to design, develop and operate PAS

technology for the distributed production of shrimp. Specific sub-objectives were to

define the individual operational parameters and overall system performance of a

distributed animal PAS.

DISTRIBUTED PAS CONFIGURATION AND COMPONENTS

The layout of the distributed PAS differs from that of the confined PAS from

combining the waste treatment environment with the culture environment. Although

physical modifications were required in order to adapt the system, the conceptually

underlying themes are similar for the two systems. In order to conduct this research at

Clemson University, with a climate and geography not favorable for tropical shrimp

growth, an artificial tropical marine environment was constructed. The facility was

developed solely for the purposes of research at Clemson University and would not be

economically feasible in a commercial application.

Artificial Marine Environment

The marine environment was created by pumping fresh water into a concrete-

lined system and adding Red Sea brand evaporated salt. As a zero-discharge system with

low permeability, the input of salt is essentially a fixed, capital cost, rather than an

operating cost that must continually be satisfied.

Artificial Tropical Environment

In order to extend the growing season, a greenhouse was constructed with a semi-

transparent plastic, allowing light transmission for photosynthetic productivity, but

withholding heat in the fall and winter months in order to keep the water temperature

within an optimal range, 26 - 30 oC, for shrimp growth and survival. Shrimp growth rates

begin to be affected when the water temperature drops below 28 oC and they are

significantly affected at temperatures less than 26 oC, with no growth at or below 22

oC

15

(Harbor Branch Oceanographic Institution, 2000). Gas-fired boilers and heat exchangers

were installed in each unit and controlled by thermostats so that heating could be applied

to the units as necessary. The sides of the greenhouse were retractable, allowing them to

be raised and lowered as necessary. This proved to be invaluable in the hot summer days

when the water temperature would otherwise have exceeded the maximum allowable

temperature, 32 oC, for shrimp growth.

Greenhouse Construction and Configuration

The greenhouse was completed in the spring of 2002 at the Calhoun Field

Research Laboratory on the Clemson University campus (Figure 5). The system consists

of four 1/16 ac (0.025 ha) recirculating units. The interior dimensions of each unit are

120 ft (36.6 m) in length by 22 ft (6.7 m) in width with 3 ft (0.9 m) high sidewalls. The

units were constructed by laying 8 in concrete block walls along the perimeter of the

structure. Concrete block walls were also used to separate the units and were reinforced

with rebar and filled with concrete to prevent failure under pressure. A 6 in (15 cm)

concrete pad was poured for the bottom of the units and a liner was not used for the 2003

experiment. A black polyethylene plastic liner was used in the 2002 year of

experimentation and determined to be unnecessary and superfluous. The concrete pad

was sloped, 1 in rise per 20 ft length (0.41% slope), so that the units could be more easily

drained.

16

Figure 5. Sketch of general layout of greenhouse.

Anchors were included in the block walls to provide attachment points for the

framing of the greenhouse enclosure. This framing consists mainly of 1.5 in diameter

galvanized steel tubing that is fastened together at joints with 1/4-20 stainless steel bolts

of varying lengths. The roof is divided into four sections, each of which are separated by

gutters and arched in shape, running the length of the greenhouse (Figure 5 – Figure 6).

Each of the five gutters spills rainwater into 6 in pipes that converge into an 8 in pipe,

which is discharged into an empty pond. The material used for the roof (Figure 6) is two

layers of clear plastic, inflated by a 1/16 hp (47 W) centrifugal blower for each of the four

sections of roof. At the time of construction, the solar shading provided by the plastic was

indicated to be about 10%.

17

Figure 6. Installation of clear greenhouse roof.

The front and rear walls of the greenhouse were sheathed with clear, corrugated

plastic; and 3 ft (0.9 m) wide steel storm doors were provided at the rear of each unit.

These doors had removable glass panels in them that were replaced with aluminum

screens in the summertime. In the front of the greenhouse, a 6 ft (1.8 m) wide door was

constructed to allow for easier access for. In addition, a 3 ft (0.9 m) wide storm door was

included in the front of the building next to the electrical breaker box so that the breakers

could be quickly accessed in the case of an emergency.

The 9 ft (2.7 m) high sides of the greenhouse were outfitted with a mechanical

system allowing them to be manually raised and lowered to help control the air

temperature and circulation in the greenhouse. The controls for raising and lowering the

sides are located at the front of the greenhouse and they could be stopped anywhere along

their range of travel, allowing for a range from full enclosure to 75% open. The material

used for the liftable sides was a translucent plastic allowing for a large amount of light

18

transmission. Chicken wire with 1.5 in (3.8 cm) openings was applied to the perimeter of

the structure where the sides could be raised and lowered. The purpose of the chicken

wire was to prevent entry to the greenhouse of predatory birds, as well as that of

dragonflies, whose larvae can be highly predatory to small aquatic animals.

Aeration

In order to provide redundancy, several means of aeration were provided for the

system. During the post-larval stage of growth for the animals, oxygen was supplied with

the use of an OGSI model OG-100 oxygen generator (Figure 7). By supplying pure

oxygen through fine bubble diffusers, the volume of gas required for introduction to the

system was considerably reduced. During this growth stage, it was inferred that more

powerful and turbulent 3/4 hp (559 W) Powerhouse aerators may be detrimental to the

survival and growth of the animals and they were therefore not used. However, as the

oxygen load on the system increased throughout the season (due to increased respiration

by the animals and bacteria at higher feed rates), the delivery and efficiency of delivery

of oxygen via the oxygen generator became insufficient. The oxygen produced by the

oxygen generator was delivered to the units at a maximum flow rate of 12.5 lpm through

10 porous fine bubble diffusers per unit.

19

Figure 7. Oxygen generator used for initial aeration.

About halfway into the season, when the oxygen demand in the system reached a

point that the corner recirculation airlift pumps and the oxygen generator were no longer

capable of maintaining a high enough DO, air diffusers were installed, which were

supplied by 1/2 hp (373 W) regenerative blowers. Two diffusers were installed in each

unit and two blowers were installed for the entire system. The diffusers were plumbed in

such a manner that for any given unit, each of the two diffusers ran off a separate one of

the two blowers. In this manner, some level of redundancy was incorporated; if one of the

blowers were inoperable, the air supply to the unit would not be completely eliminated.

Each diffuser was built using 50 ft (15.2 m) of lawn and garden soaker hose, providing a

20

relatively high volume of small bubbles over a large area. The soaker hose was secured to

the bottom of the units using a weighted frame of 1/2 in PVC pipe and rebar.

Within a week after installation, it was clear that the diffusers and regenerative

blowers were an inadequate supplement to the other aeration devices already installed.

For this reason, and because the animals were thought to be large enough to avoid them,

3/4 hp (559 W) Powerhouse floating axial flow fountain aerators (Figure 8) were

installed. These aerators are capable of moving large amounts of water into the air,

thereby greatly increasing the oxygen transfer for the system. One aerator was installed in

each of the 1/16 ac (0.025 ha) units and each aerator was screened using 1/8 in

polyethylene mesh. Later, to provide redundancy in the instance that an aerator should

fail, an additional Powerhouse aerator was installed and operated in each of the four

units.

Figure 8. A 3/4 hp Powerhouse fountain aerator.

21

Water Circulation

Each unit has a six-blade paddlewheel, 4 ft (1.2 m) diameter, driven by a variable

speed electric motor, one motor per two units, to provide for circulation and mixing of

the culture water (Figure 9 – Figure 10). The paddlewheels span a 4 ft (1.2 m) channel

that was created by constructing an 8 in block wall in the shape of an “L” at the front of

each unit. To one end of the “L” is attached a black plastic polyethylene curtain that runs

almost the entire length of the unit. This curtain allows water propelled by the

paddlewheel to travel in an ovular circuit. The paddlewheels help to keep the algae in

suspension and also provide gentle mixing for a homogeneous water quality environment.

Figure 9. Plan view of one unit showing configuration and water recirculation.

22

Figure 10. Photograph of relative position of PAS components.

Solids Removal

Twelve sump holes were included in each unit at the time of pouring the concrete

pad. The sump holes were equally spaced in three rows of four holes along the length of

each unit. Each sump is constructed with a 3 ft (0.9 m) long section of 6 in PVC pipe

with a cap on the bottom end. The top of the pipe is positioned so that it is flush with the

top of the concrete pad. Airlift pumps were positioned in each of these sump holes,

discharging into a 6 in PVC drainpipe for each unit. The drainpipe discharged into a 500

gal (1893 L) conical-bottom tank for solids settling and removal (Figure 11). The air

supply for all of the airlifts in the sump holes was provided by a 1.5 hp (1,118 W)

regenerative centrifugal blower.

23

Figure 11. Solids delivery to 500 gal settling tank.

The airlift pumps do not appear to harm the shrimp. If shrimp were pumped into

the solids delivery system, they would be carried with the rest of the water towards the

continuous-operated settling tank. A shrimp exclusion tunnel was constructed between

the 6 in PVC solids conveying inlet to the settling tank and the 3 in PVC discharge from

the settling tank so that any shrimp carried through the solids removal system would be

redirected back into the units and excluded from the settling tank (Figure 12). The tunnel

24

was made of 1/4 in polyethylene mesh, allowing water and solids to pass into the settling

tank, but forcing the shrimp back into the units.

Figure 12. Shrimp exclusion tunnel in the 500 gal settling tank.

An airlift pump was also constructed in the bottom of each conical continuous-

operated settling tank in order to remove settled solids into a smaller 110 gal (416 L)

batch-operated settling tank (Figure 13). The 110 gal (416 L) settling tank was fitted with

an adjustable height decanting device, allowing for the clear supernatant to be easily

drained off. A 3/4 hp (559 W) centrifugal irrigation pump was used to pump the

concentrated solids from the 110 gal (416 L) tanks into a temporary graduated storage

tank that was used for convergence of the solids from each of the units and quantification

of the volume of solids removed. From here, the same pump was used to carry the solids

to a sand filter for ultimate disposal.

25

Figure 13. Schematic of solids removal system.

Airlift Pump Construction

Airlift pumps were constructed using Schedule-40 PVC piping. The airlifts used

function by blowing air into a pipe with an inlet resting on the bottom of the tank, in

effect producing a negative pressure system as the air bubbles rise in the pipe into which

it was injected (Figure 14 – Figure 15). The pumps act like vacuums to remove the settled

solids from the system or to exchange water between units. For the airlifts that were used

in the sump holes for solids removal, a foot was constructed using a 2 in PVC tee fitting.

The sides of the fitting were cut at 45o

angles to reduce the restriction of solids from

entering the foot of the pump. A 24 in (61 cm) section of 2 in PVC pipe was connected to

the top of the tee and to the end of this section of pipe was connected a 2 in PVC

coupling. This coupling was drilled and tapped with a 1/2 in NPT tap, in which was

threaded a 1/2 in male pipe thread by 1/2 in PVC slip fitting elbow. This represents the

26

injection point of the air used to operate the airlift pumps. An experiment was conducted

to determine the optimum height of air-injection for potentially varying water levels.

Above this 2 in PVC coupling was positioned a lateral length of 2 in pipe allowing for

gravity fed entry to the 6 in PVC drain line along the length of each unit. The 1/2 in PVC

air supply line was introduced through a series of 2 in PVC pipes hung from the ceiling

of the greenhouse.

27

Figure 14. Schematic for operation of airlift pumps in sump holes.

28

Figure 15. Photograph showing airlifts positioned in sump holes.

The airlift pumps in the 500 gal (1893 L) settling tanks were constructed in the

same manner as those used for solids removal from the units, although it was not

necessary to provide a foot for the pumps. The tank was fitted with a 2 in female pipe

thread fitting at the bottom of the cone, to which was attached a 2 in PVC pipe that

elbowed upwards (Figure 13). Two 1/2 in PVC lines were introduced to this 2 in PVC

pipe coming from the bottom of the 500 gal (1893 L) tank. One of the 1/2 in PVC lines

was provided as a surge line in the case that the bottom of the tank should clog. This

piping was directed to spray a high pressure surge of air or water directly upwards into

the conical tank if necessary, but was never used during the course of the experiment.

29

The second 1/2 in PVC line entering the 2 in PVC pipe from the bottom of the settling

tank was the air line for operation of the airlift and its injection height was positioned 8 in

(20.3 cm) above the invert of the elbow at the bottom of the tank. The 2 in PVC discharge

from this airlift was routed to enter a 110 gal (416 L) tank resting on the edge of the

respective unit.

The airlifts used for water exchange were also constructed with 2 in PVC pipe

(Figure 16). The inlets for these airlifts were through 3 ft (0.9 m) sections of 2 in PVC

pipe into which were drilled many 3/4 in (1.9 cm) diameter holes, creating a screened

inlet. This section of pipe was capped off on one end and wrapped with window screen to

prevent the translocation of animals between the units. The inlet screens were located at

the low water level, 2 ft (0.6 m), for each unit so that water would no longer be pumped

out of a unit if the water level was too low. The air injection was located near the bottom

of a “U” connecting the inlet screen and the discharge side of the airlift pump.

Figure 16. Elevated intake for water exchange airlift pumps.

30

Also installed at the time of stocking were airlifts in the corners of the units

(Figure 17). The intent of these airlift pumps was to recirculate settleables from the

quiescent zones in the units where solids were likely to acccumulate. The water lifted by

these pumps was discharged directly to the surface, whereby settled particles could be re-

suspended into the water column, allowing for additional opportunity for solids to reach

the solids collection devices. These airlifts also provided a small degree of aeration. They

were constructed with 2 in PVC pipe and in order to maximize the area of influence per

unit volume of air introduced; three pump feet were constructed on a manifold with only

two air inlets. The feet on these pumps were constructed with 4 in by 2 in PVC bushings,

where the 2 in PVC pipe could be directly inserted into the top of the fitting. V-notches

were cut into the opposite side of the fitting to allow the foot to sit directly on the floor.

The discharge of the pumps was located at the surface of the maximum water level, 2.5 ft

(0.76m), allowing the pump to function at the highest possible water flow rates for the

available flow rate of air. In addition, the discharges from these airlift pumps were

directed with and not opposing the direction of water flow in the unit to reduce the

chances of short-circuiting. Observations made in the first year of this study indicated

that the animals remained unaffected, even at young ages, by the turbulences produced by

airlift pumps. Unit 1 and 2 were each fitted with three of these corner airlifts and the air

supply for each unit was provided by a 1/2 hp (373 W) regenerative centrifugal blower.

Corner airlifts were not installed in unit 3 and 4 because solids did not accumulate in unit

3—the tilapia unit—and unit 4 was used as a control to determine if the corner airlifts

were necessary in the shrimp units.

31

Figure 17. Corner recirculation airlift pumps.

Tilapia Co-culture and Algal Management

Water exchange between the units was accomplished when necessary through the

use of airlift pumps. The air supply for the pumps was provided by the same 1.5 hp

(1,118 W) regenerative blower used to operate the airlifts in the sump holes. The pumps

were constructed with an elevated intake so that they would no longer continue to remove

water from a unit if the water level goes too low (Figure 16). The sequence of piping and

valves used eliminated the need for more than two lateral lines spanning the units. The

arrangement of valves allowed for each of the two lateral pipes to be sloped in opposing

directions—one pipe with gravity-flow leftward and one with gravity flow in the

rightward direction (Figure 18). By using such an arrangement, inlet to either the leftward

or rightward lateral can be selected based on the units being exchanged, and discharge

32

from either of the laterals may also be selected and controlled with the valves included.

Advantages of using airlift pumps for this application include:

1. The airlift pumps will not be negatively impacted if they quit pumping water

or if they run dry. This allows for unattended operation even when the water

level falls below the point where it can continue to enter the screen at the top

of the unit.

2. As the water level rises in a unit, the head acting on the airlift pump also rises.

The significance of this is that the flow rate of water generated by an airlift

pump is proportional to the pressure head above the air injection point as well

as to the lift height required. Therefore, as the water level in a unit rises, the

pump will speed up, and similarly, as the water level drops, the pump will

slow down and eventually stop. This nature of operation of the airlift pumps is

fortuitous by design in keeping any two given pumps between two units in

sync.

3. Water exchanges between the units are often performed over long stretches of

time. This translates to the necessity of a reliable pump even in the saline

conditions in which they will be operating. Because an airlift pump contains

no moving parts and no parts susceptible to corrosion, it is a very reliable

pump and not likely to fail.

4. Essentially, the airlift pump will continue to operate as planned unless its air

supply is terminated. This is a safe prediction to make because of the

robustness of the pipe connections and the durability of PVC pipe under

normal operating conditions. Because both of the two airlift pumps used

during any given exchange can be operated off of the same blower (same air

supply), they will both quit pumping in the case of blower failure. This is

important because if only one airlift were to stop working and the other

remain pumping, too much water could be removed from one unit while

possibly approaching overflowing levels in the other unit.

33

Figure 18. Airlift water exchange schematic.

CO2 Delivery

As algae remove carbon from the water in the form of CO2 for autotrophic

growth, the pH is increased due to the resulting shift in the carbonate-bicarbonate ratio

(Nurdogan and Oswald, 1995). Carbon dioxide was used for pH control and was

delivered through two 12 in (31 cm) long fine bubble porous diffusers per unit. The rate

of delivery was kept constant throughout the season by maintaining the same pressure of

discharge to the system. The gas was supplied by 50 lb cylinders that were replaced and

refilled weekly as needed. A solenoid for each unit was installed on a manifold

downstream from the regulator on the operating cylinder. The solenoids allowed for the

use of timers to accurately and consistently add a known quantity of carbon dioxide to the

system.

34

Temperature Control

Control of the water temperature was accomplished either indirectly by heating or

cooling the air or directly by heating the water. A 5 ft (1.5 m) fan was installed at the

front of the greenhouse in order to pull cool air through the building when temperatures

were high inside the greenhouse. The operation of the fan was controlled by an air

temperature thermostat inside the greenhouse that was set at varying levels throughout

the season. Additionally, the sides could be raised to allow for cooler air to passively

enter. When the water temperature declined near the end of the season, the curtains were

kept in the down position in order to hold in as much heat as possible. Also, the screens

in the rear access doors to the structure were replaced with glass panels to provide for

better insulation. Each unit was fitted with a titanium heat exchanger that is supplied with

high temperature water from a set of two 1,000,000 Btu (1,055 MJ) boilers operating in

parallel (Figure 19). The boilers provide a constant supply of 180 oF (82.2

oC) water in a

loop. Each unit has a pump (controlled by a thermostat and a thermocouple in the water)

that delivers hot water across the heat exchanger when the temperature falls below the

minimum set-point.

35

Figure 19. Boilers used for water temperature control.

Drainage

Drains for each unit converge into a 12 in pipe that discharges into a cylindrical

concrete basin (6 ft deep by 10 ft diameter, 1.8 m deep by 3.0 m diameter). Each unit

contains a gate valve to control the discharge of water and with a screen over the entry to

the valve to control the transfer of animals from one unit to another. Overflow drains are

also provided in each unit using 4 in PVC standpipes, although were not used during the

study.

OPERATIONAL PROCEDURES

Operational procedures necessary included shrimp stocking and harvest, shrimp

feeding, solids removal, water transfer to tilapia unit, and pH control. Because stocking

and harvest were not performed in all units simultaneously, the season length varied

slightly for each of the units, with an overall average of 201 days. Each of the shrimp

units was treated the same in terms of stocking and feed application.

Shrimp Stocking and Harvest

Approximately 150,000 PL-8 and -9 shrimp were stocked in May 2003 (50,000

animals per unit). Each unit had a surface area of 250 m2, yielding an initial stocking

density of 200 shrimp/m2. The animals were monitored and acclimated for 24 hours in

800 gal (3028 L) tanks, placed at the front of each unit. The tanks were supplied with a

constant supply of air with porous air diffusers from the 1.5 hp (1,118 W) regenerative

centrifugal blower used for the airlifts in the sump holes. In addition to aerating the tanks,

air introduction in this manner reduced settling in the tanks. The initial salinity in the

tanks was about 30 g/L, and the target salinity was about 8 g/L. The animals were

acclimated to the new salinity at a maximum rate of 4 g/L/hr, using the following mixing

mass balance equation:

( )

( )inf

fo

xxt

xxVQ

−

−= , (2)

where Q represents the flow rate in gal/min needed for the given time interval, V

represents the constant volume tank in gal, t represents the time interval for the given

37

flow rate, xo represents the initial salinity in g/L in the tank for the given time interval, xf

represents the final salinity in g/L in the tank for the given time interval, and xin

represents the salinity of the water being added from the unit in g/L. Because the

acclimation could not exceed 4 g/L/hr, (xo – xf) was set equal to 1 for t = 15 min. The

equation was solved for 15 min intervals, with each xf = 1 g/L less than it was in the

previous 15 min interval until the maximum flow rate deliverable by the pump was

reached.

Water was introduced to the tanks using swimming pool sump pumps with ball

valves to control the rate of flow. Water level was controlled by and exited the tanks

through an external standpipe, which had an internal screen with fine mesh to exclude the

animals from premature release into the units. The animals were fed six times over the

course of the initial 24 hr, after which they were released into the system at large.

Immediately prior to release into the system, each unit was pre-fed with 1,000 g of feed,

distributed evenly about the unit.

At the end of the growing season, a harvest was performed for each unit

individually. In order to capture the majority of the animals quickly and efficiently, a

seine was drawn through the unit. After most of the animals had been removed with

several passes of the seine, the unit was drained, accounting for all of the animals present

in the unit at the time of harvest.

Shrimp Feeding

The animals were initially fed a 40% protein #4 feed four times per day. The feed

was uniformly dispersed (broadcast applied) about the units and the amount applied was

based on percent body weight calculations (from 15% to 30% body weight per day).

38

After about the first month, the feed was switched to a 32% protein 3/32 in pellet that was

also uniformly dispersed, but only three times per day. At this point, however, we fed the

maximum amount that the shrimp would consume with heed to critical water quality

criteria that we conservatively set for the system (Error! Reference source not found.).

The bottoms of the units were also scraped with a net in order to determine if there was

an accumulation of uneaten feed as a result of overfeeding, at which point, we would

reduce the feeding rates.

Table I. Critical water quality levels for feed rate reductions.

Feed Decision Parameter Decrease Feed by 50% if: Cease Feed Application if:

Total Ammonia-Nitrogen > 1.2 mg-N/L > 1.5 mg-N/L

Nitrite > 0.15 mg-N/L > 0.25 mg-N/L

Secchi Depth - < 7 cm

Morning DO < 3 mg/L < 2 mg/L

Solids Removal

Solids were continuously removed from the system using the airlift pumps that

were positioned in sumps throughout the bottoms of the units. Each airlift pump, when

operated, delivered about 5 gal/min (19 L/min) of water to the settling tank (Figure 20).

Because four airlifts were operated per unit at any given time that solids removal was

being performed, the settling tank for each unit was introduced with 20 gal/min (76

L/min) of bottom water from the unit at that time. This and the tank’s volume of 500 gal

39

(1,893 L) gives the water in it a detention time of 20 min during continuous-operation

mode, which occurs 3/4 of the time or 18 hr out of each day. The sequential control of the

airlifts and solenoids in the units, as described in the following paragraph, allowed for the

settling tank in each unit to act in batch-operation mode, or static settling, for 90 min out

of each consecutive 3 hr, or 1/4 of the time. Between 60 and 200 gal (227 and 757 L)

concentrated solids in the bottom of the continuous-operated settling tanks were pumped

daily into 110 gal (416 L) batch-operated settling tanks. The supernatant from these tanks

was decanted daily back into its respective unit after 24 hr of static settling was allowed

to occur. The accumulated solids, after decanting were ultimately disposed into a sand

filter after at least 50 gal (189 L) had accumulated in the 110 gal (416 L) settling tank.

40

Figure 20. Schematic of solids removal operations.

41

A four-channel irrigation timer was used to operate 12 normally closed solenoid

valves positioned throughout the system. Each of the solenoid valves, when open,

allowed for air to pass operating four airlifts placed in the solids removal sumps. At any

given time, three solenoids were open, each from a separate unit. This allowed the blower

to operate a total of 12 sumps simultaneously, four in each of three units and one unit on

standby (Figure 21). No more than four sumps were operated simultaneously for any

given unit because the flow rate to the continuous-operated primary settling tank would

decrease the detention time and the settling efficiency losses due to short-circuiting

would be increased. The individual timer cycles ran continuously in 90 min intervals,

returning to the beginning of the four-channel cycle after 6 hr. So, by always alternating

in sequence the unit that was on standby, each 500 gal (1893 L) settling tank was allowed

90 min for each 3 hr, or 3/4 of the time, to operate as a batch settling tank, which allowed

for more enhanced settling.

42

Figure 21. Blower sequencing and solenoid operation to control airlifts in the units.

Water Transfer to Tilapia Unit

The 0.25 ac (0.1 ha) greenhouse was divided into four equal units: three were

stocked with shrimp and one unit was stocked with Nile tilapia. When the water quality

was compromised in the shrimp units, the water was exchanged with the tilapia unit

water for its improvement. The water was exchanged using airlift pumps which provided

reliable and controllable transfer of the saline water between the units. Using the water

quality parameters in Table I, a shrimp unit’s water was exchanged with the tilapia unit’s

if any of the 50% feed reduction criteria were surpassed. Only one unit was exchanged

with the tilapia unit at any given time and therefore if more than one shrimp unit

exceeded the water quality criteria for 50% feed reduction, the unit with what was

43

considered to be the poorest water quality was exchanged with the tilapia unit. Also, if

the water quality in the tilapia unit was worse than that in the shrimp unit desired to be

exchanged for any of the parameters (Table I) then the exchange was not performed, or

halted.

pH Control

The pH was controlled in the units using CO2 injection via air diffusers and the

flow rate of the CO2 into the units was kept constant throughout the season. Interval

timers linked to normally closed solenoid valves were set as needed in order keep the pH

in each of the units at or below about 8.5 pH units (Table II).

Table II. Length of time to inject CO2 as a function of pH.

pH Measurement Time to inject CO2

8.4 < pH < 8.6 30 min

8.6 < pH < 8.9 60 min

8.9 < pH 90 min

DATA COLLECTION

In order to ensure a greater degree of consistency in our measurements, all water

quality analyses were performed in the water column or on samples from the water

column taken just behind the paddlewheel in each unit. Dissolved oxygen concentration

(DO), temperature, pH, and total ammonia nitrogen (TAN) were each measured three

times daily at 8am, noon, and 4pm. Nitrite concentration and salinity were measured once

daily at 8am and the Secchi depth was measured once daily at noon.

Meter-facilitated Measurements

When possible and available, water quality parameters were measured using

digital and analog meters. By using the meters, a greater amount of accuracy and

consistency is acquired, which is important when different people collect the same data at

different times of the day and season. Measurements collected with meters include DO,

temperature, pH, salinity, photosynthesis and respiration, and carbon analysis.

DO

The DO was measured using a YSI model Y55 oxygen meter calibrated for the

salinity at the given time. The range of the meter is from 0 to 20 mg/L with a resolution

of 0.01 mg/L and an accuracy of +/- 0.25 mg/L.

Temperature

The temperature readings were taken a YSI model Y55 oxygen meter. The meter

has an accuracy of +/- 0.4 oC.

45

pH

The pH was measured using a YSI model pH100 pH meter. The meter’s range is

from -2 to 16 with a resolution of 0.01 and an accuracy of +/- 0.1%.

Salinity

Conductivity, mS, in the water column was measured using a Pinpoint model

SM6 conductivity meter and converted to salinities using the manufacturer-supplied

conversion table. The meter has an accuracy of +/- 4%.

Photosynthesis and Respiration

The photosynthesis and respiration (mg-O2/L/day) were measured routinely

throughout the season on sunny days using Wheaton 300 mL BOD light bottles and dark

bottles along with a YSI model 5000 benchtop DO meter and a YSI model 5905 self-

stirring BOD probe. For an individual set of light and dark bottles, the bottles were filled

with samples, the initial DO was measured, and the bottles were then suspended in the

water column. A final DO was measured after a period of time that varied throughout the

season due to respiration and initial DO—care was taken to try to measure the final DO

before reaching below 1 mg/L or above 30 mg/L so that the algae in the bottles did not

become too inhibited. For each run of light and dark bottles, two to five sets of these

individual light and dark bottles measurements were performed consecutively throughout

the day and night over a 24 hr period. All sets of light and dark bottles were normalized

to a 24 hr period for comparison.

The water column respiration over a 24 hr period (RESP24), a negative value, is

represented by the sum of the changes in DO in the dark bottles over the normalized 24

hr period and the net photosynthesis over a 24 hr period (NET24) is represented by the

46

sum of the changes in DO in the light bottles over the normalized 24 hr period. The gross

photosynthesis occurring over a 24 hr period (GROSS24) represents the actual amount of

photosynthesis occurring without regard to respiration. GROSS24 is equal to NET24 –

RESP24, yielding a value that is greater than NET24 because RESP24 is always a negative

value.

Carbon Analysis

A Rosemount Dohrmann model DC-190 total organic carbon (TOC) analyzer was

used to determine the concentrations of carbon present in the water column. TOC

measurements were performed routinely throughout the season, at least once per month.

Water samples were analyzed in the carbon analyzer in duplicate for total carbon and

organic carbon present in the water. The concentration of inorganic carbon in the water

column was calculated as the difference between total carbon concentration and organic

carbon concentration. In order to determine what portion of these values was in a soluble

form, separate duplicate analyses were performed on samples from which the particulates

have been removed. The particulates were removed by spinning the samples down in a

Beckmann model GS-15 centrifuge at 14,500 rpm for 5 minutes and conserving the

supernatant. The supernatant was then vacuum filtered using a glass fiber pre-filter and

further vacuum filtered using a 0.45 µm filter paper. The amount of carbon in particulate

form was determined by subtracting the amount of carbon present in the soluble samples

from the amount of carbon in the total samples with respect to each of total, organic, and

inorganic. The final analysis revealed carbon concentrations in mg/L for total carbon,

total organic carbon, total inorganic carbon, soluble carbon, soluble organic carbon,

47

soluble inorganic carbon, particulate carbon, particulate organic carbon, and particulate

inorganic carbon.

Laboratory and Field Measurements

Meters were not available or necessary for many water quality and production

analyses. It was therefore necessary to rely on laboratory and field procedures as well as

colorimetric measurements to obtain the data collected for TAN, nitrite, Secchi depth,

algal counts, alkalinity, solids concentration in removed sludge, shrimp size samples, and

shrimp density samples.

TAN

The TAN was measured colorimetrically on 5 mL samples using the

Nesslerization method and Rochelle salt solution for a Hach water analysis test kit,

Model FF-1A (HACH Inc., CO, USA).

Nitrite

The nitrite concentration was determined colorimetrically for 5 mL samples using

a Hach water quality analysis test kit, Model FF-1A (HACH Inc., CO, USA), with

NitriVer 6 pillow packs.

Secchi Depth

The Secchi depth was measured using a standard black and white alternating

quadrant Secchi disk, 20 cm in diameter.

Algal Counts

Algal counts were performed routinely throughout the season using an optical

point count method. Water samples of 15 mL were spun down on a centrifuge for five

48

minutes and the suspended solids were concentrated into a volume of 2 mL, giving a

7.5:1 concentration ratio. The concentrated samples were then applied to a

hemacytometer with a known depth of 0.1 mm and known grid spacing. The slide was

observed at 200x on a stereomicroscope and an area of 1 mm2, corresponding to a volume

of 0.1mm3 was counted. Each algal specimen observed was classified using a staged

micrometer based on predetermined size-classes developed specifically for this study.

Identification of the algae was performed using the criteria set by the American Public

Health Association (1989).

Alkalinity

The alkalinity was measured at least monthly on 100 mL samples by using a

digitial titration method for a Hach water analysis test kit, Model FF-1A (HACH Inc.,

CO, USA), with 0.030 N sulfuric acid and a bromcresol green methyl-red pillow pack.

Solids Concentration in Sludge Removed

Solids analyses were performed on the sludge for each sludge removal batch

process. Prior to sludge removal and after decanting, the sludge in the 110 gal (416 L)

settling tank was stirred for 1 min in order to attain a homogeneous slurry and samples

were taken. These samples were dried at 105 oC for 1 hr and burned at 550

oC for 20 min

to determine the total solids content and the inert solids content. The volatile solids

concentration was derived by subtracting the mass of inert solids from that of the total

solids.

49

Shrimp Size Samples

Shrimp size was estimated once per month. Samples of 20 – 30 animals were

captured by rapidly scraping a fine mesh net along the bottom of the units and

individually weighed.

Shrimp Density Samples

The density of shrimp in the unit was estimated once per month using a throw-

trap. The trap, a 1 m2 box, was thrown into the unit and allowed to sink to the bottom.

The frame of the trap was constructed using 1 in square tubing with a height of about 1

m. The four side-walls of the trap were fitted with 0.25 in mesh and the top and bottom

sides of the trap were open. After the trap was cast, the shrimp contained in the trap were

captured using dip nets and counted. The trap was cast three times in each unit, once at

the rear of the unit, once in the middle of the unit, and once at the front of the unit, during

each measurement. The counts were averaged to provide a number of shrimp per square

meter.

DATA ANALYSIS AND MANIPULATION

The raw data for feeding, water quality, shrimp size samples, CO2 addition, solids

removal, and algal counts was manipulated in order to present it in a more useful form.

Feed Rates

The daily feed rates (g/day) were calculated for each water quality measurement

event by taking the sum of the previous feed application, current feed application, and the

following feed application. During beginning of the season when water quality

measurements and feed application was performed four times daily, this sum was

multiplied by 4/3 to reflect the amount that would have been fed over the full course of

the day because only three values are used in the sum. In order to convert these values to

units of lb/ac/day, consideration was only given to the area to which the feed was applied,

0.025 ha (1/16 ac); the area comprised by the tilapia unit was not considered in

calculating this feed rate.

The 7- and 14-day average daily feed rates, kg/ha/day, were calculated in order to

see more clearly the trends that were occurring in terms of feed application. To calculate

these time-averaged feed rates, an average was taken for the days before and the days

after the point in time for which the average is supplied. So, for 14-day average feed

rates, the daily feed rates were averaged for the 7 days prior and the 7 days after the

calculated value. The average seasonal feed rate, lb/ac/day for each unit takes the sum of

all of the feed applied to that unit over the course of season and divides it by the number

of days in the season and the area of the unit.

51

Feed conversion ratios (FCRs) were calculated for each feed application during

the season based on the cumulative amount of feed added to the units and the estimated

shrimp biomass at that time.

7-Day Water Quality Averages

The 7-day averages were calculated for each of the water quality measurements

(temperature, DO, salinity, TAN, nitrite concentration, and Secchi depth) in order to be

able to more clearly see trends that were occurring in the system. When these moving

averages were plotted with respect to time, the peaks and valleys were not as pronounced

as they would be on the plots of the raw data. Because of this, the maximum values in

these charts do not reflect the maximum daily value that was observed. The averages

were calculated like the feed rates by averaging the measurements for the previous 3.5

days and those for the following 3.5 days.

Unionized Ammonia Concentration

Unionized ammonia, which is a percentage of TAN depending on the

environmental factors influencing the ammonia equilibrium, was calculated using:

[ ][ ]

pH273.15T

2729.920.0918

3

10

TANN-NH

−+

+

= , (3)

where [NH3-N] represents the unionized ammonia concentration (mg-N/L), [TAN]

represents the total ammonia nitrogen concentration (mg-N/L), and T represents the

temperature in oC. The 7-day average unionized ammonia concentrations were calculated

using the same equation, but with the variables on the right side of the equation being

replaced by the corresponding 7-day averages.

52

Shrimp Biomass

The average weight samples (g/shrimp) were linearized over the course of time so

that for each data entry or feeding time, an estimated average weight was known. The

average weight values in between any two sampling points in time were calculated using:

∆t∆t

∆AvWAvWtAvWt

tot

1

t+= , (4)

where AvWt (g/shrimp) represents the estimated linearized average weight at the given

point in time, AvWt1 represents the average weight observed (g/shrimp) at the prior

sampling point, ∆AvWt (g/shrimp) represents the difference in the average weight for the

prior and latter sampling points, ∆ttot equals the total amount of time (day) between the

time of the prior average weight sample and the following average weight sample, and ∆t

represents the amount of time that has elapsed since the prior average weight sample. Eq.

(4) can therefore further be broken down to:

( )1

12

121 tt

tt

AvWtAvWtAvWtAvWt −

−

−+= , (5)

where AvWt2 represents the average weight observed for the following sampling point, t2

represents the point in time (Julian day) for when the following sampling occurred, t1

represents the point in time for when the prior sampling occurred, and t represents the

point in time for which the estimated average weight is being calculated.

This linearization of the average weights was performed so that the amount of

shrimp biomass could be estimated and plotted for any point in time. In order to calculate

the shrimp biomass in any given unit (kg/unit) it was assumed that the number of shrimp

53

in a unit at any time in the season was equal to the number of shrimp in that unit at the

end of the season. In other words, it was assumed that all mortality occurred during or

prior to stocking the unit and that the percent survival was the same at any point in the

season. Again, the shrimp biomass per unit area (kg/ha) was calculated using only the

surface area of the unit, with no regard to the tilapia unit. Shrimp biomass was used with

the daily feed rate to estimate the amount of feed applied to a unit in time in terms of

percent body weight per day. Feed conversion ratios (FCRs) were estimated for any point

throughout the season by summing the total amount of feed (kg) applied to a unit prior to

that point in time and dividing this value by the estimated shrimp biomass (kg/unit) at

that point in time.

Recarbonation Rate

The rate of CO2 addition was back-calculated. Although kept constant throughout

the season, the rate was not known at the time of injection—the length of time that CO2

was added was determined more or less by trial and error in order to develop a rough

field relationship between time running and pH units dropped. Despite this, the length of

time for which CO2 was injected was recorded throughout the season and the rate of

addition was kept constant by maintaining a set injection pressure. Analysis of the pH,

alkalinity, and CO2 injection time data reveals a recarbonation rate in terms of

mmol/L/hr. The alkalinity was linearized to estimate the alkalinity for any point in time

between two sampling points using:

∆t∆t

∆AlkAlkAlk

tot

1 += , (6)

54

where Alk (meq/L) represents the estimated linearized alkalinity that is being solved for

at the given point in time, Alk1 represents the alkalinity observed (meq/L) at the prior

measurement, ∆Alk (meq/L) represents the difference in the alkalinity for the prior and

latter measurements, ∆ttot equals the total amount of time (day) between the time of the

prior alkalinity measurement and the following alkalinity measurement, and ∆t represents

the amount of time that has elapsed since the prior alkalinity measurement. The equation

is therefore broken down to:

( )1

12

121 tt

tt

AlkAlkAlkAlk −

−

−+= , (7)

where Alk2 represents the alkalinity observed (meq/L) for the following alkalinity

measurement, t2 represents the point in time (Julian day) for when the following

measurement occurred, t1 represents the point in time for when the prior measurement

occurred, and t represents the point in time for which the estimated alkalinity is being

calculated.

To calculate the recarbonation rate, the data were first subdivided according to

consecutive time of day intervals, giving four data sets for each unit, 16 data sets total:

early afternoon, late afternoon, short night, and long night (Table III). A data set for the

time interval from 8am to noon was not created because rarely, if ever was it necessary to

inject CO2 at the 8am feeding. Also two separate data sets are created for short night and

long night because for the beginning of the season data points exist for four times during

the day and at the end of the season data points only exist for three times per day. At the

beginning of the season feeding and water quality measurements were performed four

times daily, with the last check in the day being at 8pm and the next consecutive check at

55

8am on the following day, representing the short night data subset that was created. At

the end of the season, when feeding and water quality measurements were performed

three times daily, the last check in the day occurred at 4pm and the next consecutive

check occurred at 8am on the following day, representing the long night data subset that

was created.

Table III. Data included in subdivided recarbonation data sets.

Data Set Time of Day pH Data Alkalinity Data CO2 Injection Time

Early Afternoon Noon – 4pm Noon, 4pm Noon Noon

Late Afternoon 4pm – 8pm 4pm, 8pm 4pm 4pm

Short Night 8pm – 8am 8pm, 8am 8pm 8pm

Long Night 4pm – 8am 4pm, 8am 4pm 4pm

For each data point within the data subsets, a value for total carbonate carbon, CT

(mmol/L), was calculated using the equilibrium distribution of solutes in aqueous

carbonate solution.

[ ] [ ] [ ]

21

T2αα

HOHAlkC

+

−−=

+−

, (8)

where [Alk] represents the estimated linearized alkalinity (meq/L) calculated earlier for

that given point in time, [OH-] represents the hydroxide-ion concentration (mol/L) for

that point in time, given by:

56

pH1410]OH[ +−− = ; (9)

[H+] represents the hydrogen-ion concentration (mol/L) for that point in time, given by:

pH10]H[ −+ = ; (10)

α1 represents an inorganic carbon ionization fraction given by:

1

2

1

1][H

K1

K

][Hα

−

+

+

++= , (11)

where K1 and K2 are set equal to the corresponding values for 0 g/L salinity and 30 ºC,

which are 10-6.327

and 10-10.29

, respectively (these values were not interpolated for the

actual temperature and salinity at the time for which CT is being calculated because it was

determined that the changes resulting from interpolation would be negligible.); and α2 is

given by:

1

221

2

2 1K

][H

KK

][Hα

−++

++= , (12)

where all values have been previously defined.

By comparing data within identical time intervals for each unit, a normal change

in total carbonate carbon, ∆CT(normal) (mmol/L), was calculated for each pair of

consecutive data points in the data subsets and a change in total carbonate carbon,

∆CT(inject) (mmol/L), that occurred when CO2 was injected was calculated for each pair of

consecutive data points during which CO2 was added. This ∆CT(inject) was subdivided into

∆CT(inject30), ∆CT(inject60), ∆CT(inject90), and ∆CT(inject120), representing the change in total

57

carbonate carbon that occurred when CO2 was injected for 30 min, 60 min, 90 min, and

120 min. An average was calculated for each of the ∆CT(normal), ∆CT(inject30), ∆CT(inject60),

∆CT(inject90), and ∆CT(inject120) within each data set.

Recarbonation rates (mmol/L/hr) were then calculated for each data set and each

of the four CO2 injection times by taking the difference between the average ∆CT(normal)

and the average ∆CT for the given CO2 injection time and dividing it by the CO2 injection

time (hr). As an example, the calculation for the 30 min recarbonation rate (mmol/L/hr)

for any one of the given time intervals and units follows:

hr5.0

∆C∆CRecarbRate

T(normal))T(inject30

30

−= , (13)

where ∆CT(inject30) and ∆CT(normal) are each for the same unit and time interval for which

the rate is being calculated. Weight was then assigned to each of the 30, 60, 90, and 120

min recarbonation rates by multiplying by the number of pairs of data points used to

arrive at the given value. The sum of the 64 weighted recarbonation rates (four intervals

for each of the four units and four CO2 injection times for each of the four intervals)

divided by the sum of the weights gives the average recarbonation rate for the system

when CO2 is being injected.

Algal Biovolumes

Algal taxa, shapes, and arbitrary size units are found in Table IV. In the table,

“size” is equal to the value for the size classification. The size classifications were

actually based on the units of measurement on the staged micrometer used, where each

unit is equal to 2.5 µm. Hence the length of the dimension used for the size classification

58

for each taxa in the table is equal to 2.5 µm x size. Using the data acquired in the size-

classified algal counts and the arbitrary sizes and shapes developed for each taxa, a

volume was calculated for each algal cell or colony observed (Table V). The sum of the

volumes for any given taxa yielded a total volume for that specific taxon. These volumes

were then converted into volumetric concentrations (µm3-algae/L-water) taking into

account the volume of water observed for the count, 0.1 mm3, and the concentration

factor, 7.5 :1.

Tab

le I

V. P

hyto

pla

nkto

n t

axa,

shap

es a

nd a

rbit

rary

siz

es.

Tax

a S

hap

e

Dim

ensi

on u

sed

for

Siz

e C

lass

Dia

met

er

(µm

)

Len

gth

(µm

)

Wid

th

(µm

)

Hei

ght

(µm

)

Bas

e

(µm

)

Agm

enel

lum

(or

Mer

ism

op

edia

)

Squar

e P

rism

L

ength

or

wid

th

- 2.5

x s

ize

2.5

x s

ize

2.5

-

Anabaen

a

Cyli

nd

er

Hei

ght

0.0

7 x

siz

e -

- 2.5

x s

ize

-

Anacy

stis

(or

Mic

rocys

tis)

Spher

e D

iam

eter

2.5

x s

ize

- -

- -

Anki

stro

des

mus

Cyli

nd

er

Hei

ght

0.2

5·s

ize

- -

3.7

5 x

siz

e -

Chlo

rell

a

Spher

e D

iam

eter

2.5

x s

ize

- -

- -

Cyc

lote

lla

(or

centr

ic d

iato

m)

Cyli

nd

er

Dia

met

er

2.5

x s

ize

- -

1.2

5 x

siz

e -

Navi

lcula

(or

pen

nat

e dia

tom

)

Tw

o C

ones

H

eight

(com

bin

ed)

0.2

5 x

siz

e -

- 1.2

5 x

siz

e

(eac

h)

-

Osc

illa

tori

a (

Sm

all)

C

yli

nd

er

Hei

ght

1

- -

2.5

x s

ize

-

Osc

illa

tori

a (

Lar

ge)

C

yli

nd

er

Hei

ght

2.5

-

- 2.5

x s

ize

-

Ped

iast

rum

C

yli

nd

er

Dia

met

er

2.5

x s

ize

- -

0.6

25 x

siz

e -

Pla

nkt

osp

haer

ia

Spher

e D

iam

eter

2.5

x s

ize

- -

- -

Tab

le I

V. P

hyto

pla

nkto

n t

axa,

shap

es a

nd a

rbit

rary

siz

es. (C

onti

nued

)

Tax

a S

hap

e

Dim

ensi

on u

sed

for

size

cla

ss

Dia

met

er

(µm

)

Len

gth

(µm

)

Wid

th

(µm

)

Hei

ght

(µm

)

Bas

e

(µm

)

Sce

ned

esm

us

A2

Rec

tan

gula

r

Pri

sm

Len

gth

-

2.5

x s

ize

1 x

siz

e 0.5

x s

ize

-

Sce

ned

esm

us

A4

Rec

tan

gula

r

Pri

sm

Len

gth

-

2.5

x s

ize

1.5

x s

ize

0.6

67 x

siz

e -

Sce

ned

esm

us

A8

Rec

tan

gula

r

Pri

sm

Len

gth

-

2.5

x s

ize

0.7

5 x

siz

e 0.3

33 x

siz

e -

Sce

ned

esm

us

B4

Rec

tan

gula

r

Pri

sm

Len

gth

-

2.5

x s

ize

1.5

x s

ize

0.6

67 x

siz

e -

Sce

ned

esm

us

D1

Cyli

nd

er

Hei

ght

1 x

siz

e -

- 2.5

x s

ize

-

Sce

ned

esm

us

D2

Tw

o C

yli

nder

s H

eight

(of

one

cyli

nder

)

1 x

siz

e

(eac

h)

- -

2.5

x s

ize

-

Tet

raed

ron

(Cubic

al)

Cube

Len

gth

, W

idth

,

or

Hei

ght

- 2.5

x s

ize

2.5

x s

ize

2.5

x s

ize

-

Tet

raed

ron

(Pyra

mid

al)

Equil

ater

al

Tri

angula

r

Pyra

mid

Bas

e -

- -

- 2.5

x s

ize

61

Table V. Phytoplankton biovolume calculations summarized for each taxa.

Taxa Volume (µm3)

Agmenellum

(or Merismopedia) ( )( )( )µm5.2sizeµm5.2sizeµm5.2hwl ××=⋅⋅

Anabaena ( )sizeµm5.22

sizeµm07.0hr

2

2 ×

×=⋅ ππ

Anacystis

(or Microcystis)

3

3

2

sizeµm5.2

3

4r

3

4

×⋅=⋅ ππ

Ankistrodesmus ( )size75.32

sizeµm25.0hr

2

2 ×

×=⋅ ππ

Chlorella

3

3

2

sizeµm5.2

3

4r

3

4

×⋅=⋅ ππ

Cyclotella

(or centric diatom) ( )sizeµm25.1

2

sizeµm5.2hr

2

2 ×

×=⋅ ππ

Navicula

(or pennate diatom) ( )

×

×⋅=

⋅⋅ sizeµm25.1

2

sizeµm333.0

3

12hr

3

12

2

2 ππ

Oscillatoria (Small) ( )sizeµm5.22

µm1hr

2

2 ×

=⋅ ππ

Oscillatoria (Large) ( )sizeµm5.22

µm5.2hr

2

2 ×

=⋅ ππ

Pediastrum ( )sizeµm625.02

sizeµm5.2hr

2

2 ×

×=⋅ ππ

Planktosphaeria

3

3

2

sizeµm5.2

3

4r

3

4

×⋅=⋅ ππ

Scenedesmus A2 ( )( )( )sizeµm5.0sizeµm1sizeµm5.2hwl ×××=⋅⋅

62

Table V. Phytoplankton biovolume calculations summarized for each taxa. (Continued)

Taxa Volume (µm3)

Scenedesmus A4 ( )( )( )sizeµm667.0sizeµm5.1sizeµm5.2hwl ×××=⋅⋅

Scenedesmus A8 ( )( )( )sizeµm333.0size.75µ70sizeµm5.2hwl ×××=⋅⋅

Scenedesmus B4 ( )( )( )sizeµm667.0sizeµm5.1sizeµm5.2hwl ×××=⋅⋅

Scenedesmus D1 ( )sizeµm5.22

sizeµm1hr 2 ×

×=⋅ ππ

Scenedesmus D2 [ ] ( )

×

×⋅=⋅⋅ sizeµm5.2

2

sizeµm12hr2 2 ππ

Tetraedron (Cubical) ( )( )( )sizeµm5.2sizeµm5.2sizeµm5.2hwl ×××=⋅⋅

Tetraedron

(Pyramidal) ( )33 sizeµm5.2

3

1b

3

1×=

Carbon Mass Balance

Units of the inputs and outputs of the carbon mass balance were converted to g-

C/m2/day. The feed input rate, FIRfeed (g-feed/m

2/day) was determined for each unit

using:

( )( )( )tA

FEEDFIR Feed = , (14)

where FEED represents the total feed applied (g), A represents the water surface area

(m2), and t represents the season length (days) defined as time elapsed between stocking

63

and harvesting. The assumption that the feed had a 90% volatile solids (VS) content by

mass allows for conversion to a feed input rate of volatile solids, FIRVS (g-VS/m2/day):

( )FeedVS FIR 9.0FIR = . (15)

Assuming that the volatile solids content is 50% carbon by mass allows for the

calculation of feed input rate of carbon, FIRC (g-C/m2/day):

( )VSC FIR 0.5FIR = . (16)

FIRC was determined for each of the shrimp units and averaged to be used in an overall

mass balance for the three units.

The other input considered for the carbon mass balance was by recarbonation,

where the molar based volumetric recarbonation rate from CO2 addition, RR´vol (mmol-

C/L/hr), was converted to a mass based volumetric recarbonation rate, RRvol (g-C/L/hr),

using the molecular weight of carbon:

( )C mmol

C g 012.0RR'RR volvol = . (17)

The mass based recarbonation rate on a per unit area basis, RRC (g-C/m2/day) for each

unit was then calculated:

( )( )( )

( )( )tA

VtRRRR CO2vol

C = , (18)

where tCO2 represents the accumulated time that CO2 injection was performed (hr), V

represents the total water volume of the unit (L), and all other variables have been

64

previously defined. This recarbonation rate was calculated for each unit and averaged to

give the value used for the overall carbon mass balance.

The rate of carbon fixation rate by algae was calculated based on the seasonal

average volumetric 24 hr net photosynthesis rate for each unit, NETvol (mg-O2/L/day).

NETvol was converted to a seasonal average 24 hr net photosynthesis rate on a per unit

area basis, NETarea (g-O2/m2/day) using:

( )( )

=

mg1000

1g

A

VNETNET vol

area , (19)

where all variables have been previously defined. The algal fixation rate of carbon, AFRc

(g-C/m2/day), can then be calculated based on the assumption that the molar ratio of

oxygen produced to carbon in algal biomass is approximately 1:1 as can be seen in eq.

(1).

2

areaCO g32

C g12NETAFR = . (20)

AFRC was calculated for each unit and averaged to be presented in the overall mass

balance. AFRC is an internal parameter to the carbon mass balance—it does not represent

an overall carbon input or output from the system, but a compartment for carbon storage

within the system. Because the carbon is fixed into algal biomass in this process, it is not

really removed from the system until the algae is harvested. Therefore, AFRC is presented

in the mass balance as a reference for carbon storage within the system, but not included

in the calculations for carbon removal from the system.

65

The rate of carbon conversion into shrimp biomass was calculated based on the

harvested shrimp biomass. Because the total shrimp biomass at harvest is orders of

magnitude higher than that at stocking, the total shrimp biomass at stocking is negligible

for these calculations and set equal to 0.0g. So, the carbon present in shrimp biomass was

converted during the course of the season. The wet weight of the total shrimp biomass

harvested, SHRIMPww (g), from each unit was converted to a dry weight of the total

shrimp biomass, SHRIMPdw (g-VS), assuming that the organisms are composed of 90%

water by mass:

( )wwdw SHRIMP 1.0SHRIMP = . (21)

Because SHRIMPdw represents the volatile solids content of the shrimp, the assumption

that volatile solids are 50% carbon can be applied, allowing for calculation of the carbon

present in shrimp biomass for each unit, SHRIMPC (g-C):

( )dwC SHRIMP 5.0SHRIMP = . (22)

The shrimp conversion rate of carbon, SCRC (g-C/m2/day), can then be calculated by:

( )( )tA

SHRIMPSCR C

C = . (23)

SCRC was calculated for each of the shrimp units and then averaged for the overall

carbon mass balance.

The rate of carbon removal from the system via sludge removal was calculated

based on the sludge composition and quantity removed. The mass of volatile solids

66

removed for each sludge removal event, SLUDGEVS (g-VS), in each unit was calculated

using the following:

( )( )VS%SLUDGESLUDGE volVS = , (24)

where SLUDGEvol represents the volume of sludge removed (L), and %VS represents the

volatile solids content of the sludge removed (g-VS/L). A sludge removal rate of volatile

solids, SRRVS (g-VS/m2/day) could then be calculated for each unit, using the sum of

volatile solids removals for each unit:

( )( )

sludge

VS

VStA

SLUDGESRR

∑= , (25)

where tsludge represents the time (days) that the sludge removal operations took place—

sludge removal was not performed for the first 1/4 of the season when the animals were

small. The sludge removal rate of carbon, SRRC (g-C/m2/day), from the system was then

calculated using the assumption that volatile solids are 50% carbon:

( )VSC SRR 5.0SRR = . (26)

SRRC was calculated for each unit and then averaged to arrive at the value used for the

overall carbon mass balance.

The CO2 outgassing rate of carbon removal, OGRC (g-C/m2/day), from the system

can then be calculated based on the assumption that there are no other appreciable carbon

outputs from the system and the fact that the carbon inputs to the system must be equal to

sum of the carbon outputs from the system and accumulation in the system, which is

assumed to be zero. So, from the carbon mass balance,

67

CCCCC OGR-SRR-SCR-RRFIR0 += , (27)

the following equation can be derived:

CCCCC SRR-SCR-RRFIROGR += . (28)

Nitrogen Mass Balance

As with the carbon mass balance, all rates used for the nitrogen mass balance

were converted to units of g-N/m2/day for consistency. The only nitrogen input to the

system was via feed input. The feed input rate of nitrogen, FIRN (g-N/m2/day), to the

system was based on FIRfeed and the C:N ratio in the feed. The protein content of the feed

was 32% and the assumption was made that protein is 16% nitrogen, giving:

( )feedN FIRprotein g

N g16.0

feed g

protein g32.0FIR

= . (29)

FIRN was calculated for each unit and then averaged for the overall nitrogen mass

balance.

The algal fixation rate of nitrogen, AFRN (g-N/m2/day), was calculated based on

AFRC and an assumed C:N ratio of 5.6:1 for algal biomass (Brune et al., 2003):

CN AFRC 5.6g

N g0.1AFR = . (30)

AFRN was calculated for each of the units and then averaged to be presented in the

overall nitrogen mass balance. As in the carbon mass balance, AFRN does not represent

removal of nitrogen from the system—only storage. Because the nitrogen is fixed into

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algal biomass and not actually removed from the system until the algae is harvested,

AFRN does not appear as an output in the nitrogen mass balance equation.

The shrimp conversion rate of nitrogen, SCRN (g-N/m2/day), from the system is

calculated based on SCRC and an assumed C:N ratio for shrimp biomass of 4.3:1, giving:

CN SCRC 4.3g

N g0.1SCR = . (31)

SCRN was calculated for each unit and then averaged for the overall nitrogen mass

balance.

The sludge removal rate of nitrogen, SRRN (g-N/m2/day), is calculated based on

SRRC and the assumption that the C:N ratio of the sludge removed was 6.5:1, giving:

CN SRRC 6.5g

N 1.0gSRR = . (32)

SRRN was calculated for each unit and the average of these values was used for the

overall nitrogen mass balance.

The remainder of nitrogen exiting the system was assumed to all have outputted

by means of nitrification. So, from the nitrogen mass balance, assuming no appreciable

accumulation of nitrogen in the system,

NNNN NR-SRR-SCR-FIR0 = , (33)

where NRN (g-N/m2/day) represents the nitrification rate of nitrogen removal from the

system, the following equation can be derived to define NRN:

NNNN SRR-SCR-FIRNR = . (34)

RESULTS AND DISCUSSION

Feed application rates, water quality, system productivity, algal biovolumes,

solids removal rates, carbon mass balance, and nitrogen mass balance are summarized to

provide a basis for relative comparison of the system researched to other animal

production systems.

Feed Application Rates

The daily feed rates (Figure 22 – Figure 24) show the feed applied by day. The

feeding rates proceed for each of the units in the first month as a stepwise function

reflecting the percent body weight estimates. After one month of stepwise increases in

feeding, the valleys in the charts indicate the feeding response to compromised water

quality and the peaks indicate the maximum rates that appeared to be sustainable in the

system. The peaks in units 1 and 2 were about 10,000 g/d (350 lb-feed/ac/day) and the

peak feeding in unit 4 was about 11,500 g/d (400 lb-feed/ac/day).

70

Figure 22. Daily feed rate in unit 1.


71


The 7- and 14-day averages, (Figure 25 – Figure 27) illustrate the maximum

sustainable feed rates for one and two week periods of time. The peaks and valleys are

less pronounced and trends easier to see. The peaks here for units 1 and 2 are about 7,000

g/d (250 lb-feed/ac/day) and about 6,300 g/d (225 lb-feed/ac/day) for unit 4.

72

Figure 25. The 7- and 14-day average feed rates unit 1.


73


The seasonal average feed rates, or the feed applied over the course of the entire

season divided by the length of the season, for units 1, 2, and 4, are 159, 156, and 151 lb-

feed/ac/day (178, 175, and 170 kg/ha/day), respectively, with an overall average seasonal

feed rate of 155 lb/ac/day (174 kg/ha/day).

These feed rates, combined with the data collected to estimate the total biomass in

each unit, were converted feed rates expressed as percent body weight per day

(%BW/day). Near the beginning of the season, these plots (Figure 28 – Figure 33)

probably do not reflect the actual %BW/day applied to the units. One reason for this is

because average size samples were not conducted until after the first month of growth

when the animals were large enough to sample reliably. A second reason for the initial

%BW/day to be inaccurate is that the shrimp biomass calculations for each unit were

performed with the assumption that all mortality was experienced at stocking. As the

season progressed the accuracy exhibited in the calculated %BW/day increases. The

74

initial 7-day average %BW/day for unit 2 was approximately twice that of units 1 and 4

because of the assumption made that all mortality occurred at stocking for the calculation

of total shrimp biomass. This assumption probably caused an underestimation of the

shrimp biomass in all units, but the greatest error would have occurred in unit 2 because

it demonstrated the lowest survival.

Figure 28. Feed rate as a function of percent body weight in unit 1.

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76

Figure 31. The 7-day average feed rate as a function of percent body weight in unit 1.


77


Water Quality

The water results for each of the units are presented separately and averages are

performed across the entire system for each of the parameters.

Temperature

The temperature trends (Figure 34 – Figure 41) for each of the units are similar.

At the beginning of the season the water temperatures fluctuate, usually remaining

between 28 and 30 °C. Then towards the end of the season the temperatures began to

decline slowly, beginning a new oscillation range between 25 and 27 °C and representing

the period when the system was artificially heated. The sudden drop in water temperature

at the beginning of September occurred during an extended period of overcast and stormy

days. The gradual drop during October until the last week of the month is the period

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immediately prior to commencement of artificial heating. The temperature ranged from

23.3 to 32.8 °C over the entire course of the season with an average of 28.1 °C.

Figure 34. Daily temperature in unit 1.

79



80


Figure 38. The 7-day average temperature in unit 1.

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82


Dissolved Oxygen Concentration

On average, the DO remained above 4 mg/L with larger fluctuations near the

beginning of the season than those that were experienced in the last three quarters of the

season (Figure 42 – Figure 49). The fluctuations in DO reflect algal productivity and the

magnitude of the fluctuations in DO is directly proportional to the algal density and the

photosynthetically available radiation. On July 19, after the first quarter of the season,

0.75 hp fountain aerators were installed in each of the units, which not only increased the

DO during the nighttime, but also decreased it during the daytime via degassing when the

water would otherwise be supersaturated. The respiration in the tilapia unit was high

enough, due to high fish biomass, prior to the installation of the aerators to prohibit

oxygen from becoming as saturated as it was in the shrimp units early in the season,

which had relatively low biomass until later in the season. There were isolated events in

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each of the units due to power outages and aerator failures when the DO reached

critically low levels. The lowest DO events during the season in unit 1, 2, 3, and 4 were

1.32, 1.23, 0.56, and 0.27 mg/L, respectively. The average seasonal DO across the four

units was 6.61 mg/L.

Figure 42. Daily dissolved oxygen concentration in unit 1.

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85


Figure 46. The 7-day average dissolved oxygen concentration in unit 1.

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87


Salinity

The salinities (Figure 50 – Figure 53) at stocking varied somewhat between the

units at 8.9 g/L in units 1 and 2, and 6.5 g/L in unit 4. When the shrimp were stocked the

salinity in the tilapia unit was 0.7 g/L, however the salt concentrations equalized with

time mostly by mass transfer via water exchanges. On average, the salinities in each of

the units were greater than 4 g/L and the seasonal average for the entire system was 5.2

g/L.

88

Figure 50. Daily salinity in unit 1.


89



90

pH

Initially in each of the units, the pH fluctuates between about 7.75 to 8.25 pH

units (Figure 54 – Figure 61). However, this is actually an artificial controlled pH

because at the beginning of the season we were inputting a substantial amount of CO2 in

order to depress it. The pH in each of the units can be seen declining over the course of

the season. This is a function of increased feed rates in the second half of the season

translate to increased organic carbon inputs to the system and an extended period of

overcast days was experienced during the middle of the season, as supported by the

temperature data. The low light levels during the overcast days, along with additional

shading in the system translated into a decrease in algal productivity, which allowed for

the pH to decline because the algae was not extracting as much carbon from the

alkalinity. At any given point in the season, the pH in the tilapia unit was always higher

than that in the shrimp units; this is a direct result of feed—and therefore carbon—inputs

to the shrimp units and not to the tilapia unit. The pH in the system overall ranged from

6.02 to 9.24 with an average of 7.78 pH units.

91

Figure 54. Daily pH in unit 1.


92



93

Figure 58. The 7-day Average pH in unit 1.


94



95

The pH was initially controlled by the addition of CO2, which became

unnecessary as the feed rates increased. The rate of CO2 addition was inversely

proportional to the rate of feed application, although a lesser amount of inorganic carbon

in CO2 was needed to reduce the pH to satisfactory levels than that required by organic

carbon in the feed (Figure 62 – Figure 64). In all of the shrimp units, recarbonation by

CO2 injection became unnecessary at feed rates exceeding about 200 lb/ac/day (10.2 g-

C/m2/day). It can be observed that, generally, the peaks in feed application rate

correspond to valleys in CO2 injection rate and vice-versa. It was not fully understood

why peaks in CO2 injection occurred in conjunction with peaks in feed application in all

units in early September.

Figure 62. Carbon input over time in unit 1.

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97

Total Ammonia-Nitrogen

At our relatively low feed rates during the beginning of the season, the TAN

(Figure 65 – Figure 72) remained below 1 mg/L. As the feed rates to the system

increased, the TAN increased as well. The TAN never averaged higher than 1.4 mg/L for

any given one week period and in the second half of the season it fluctuated around 1.2

mg/L. There were isolated events where the TAN reached maximum values of 2.0, 2.0,

2.1, and 4.0 mg/L in units 1, 2, 3, and 4, respectively.

Figure 65. Daily total ammonia concentration in unit 1.

98



99


Figure 69. The 7-day average total ammonia nitrogen concentration in unit 1.

100



101


Unionized Ammonia

As the two ammonia-equilibrium driving forces—water temperature and pH—

simultaneously decreased during the last half of the growing season with a relatively

stable TAN, the unionized ammonia concentration (NH3-N) decreased (Figure 73 –

Figure 80). TAN was the primary driving factor and at the operating temperatures and pH

levels, pH was the secondary driving factor, with a lesser effect caused by temperature.

This can be seen upon comparison of the 7-day average unionized ammonia plots with

the 7-day average TAN, pH, and temperature plots. Peak observed NH3-N values during

the season fell between 0.39 and 0.43 mg-N/L with seasonal averages in the four units of

0.058 mg-N/L, 0.058 mg-N/L, 0.081 mg-N/L, and 0.060 mg-N/L. The overall average

NH3-N was 0.064 mg-N/L. The data suggests that feed rate reductions at the end of the

season implemented on the basis of excessive TAN concentrations were not necessary for

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shrimp health concerns. Feed rate reductions should have been based on NH3-N rather

than TAN.

Figure 73. Daily unionized ammonia concentration in unit 1.

103



104


Figure 77. The 7-day average unionized ammonia concentration in unit 1.

105



106


Nitrite

Nitrite (Figure 81 – Figure 88) did not appear in our system until after about the

first month. Nitrites appeared in the system in waves, generally persisting for several

days before they were probably further nitrified—it is also possible for nitrite to be used

as the nitrogen source for algal growth, but studies indicate that algae will preferentially

use ammonia when available (Oswald, 1996; Lai and Lam, 1997). The average nitrite

concentration for any given seven day period was generally below 0.30 mg/L, with the

highest observed levels being 0.44, 0.43, 0.35, and 0.30 mg/L in units 1, 2, 3, and 4,

respectively. The overall seasonal average nitrite concentration was 0.11 mg/L.

107

Figure 81. Daily nitrite concentration in unit 1.


108



109

Figure 85. The 7-day average nitrite concentration in unit 1.


110



111

Secchi depth

The Secchi depth (Figure 89 – Figure 96) began relatively high in each of the

units at an average of about 25 cm because the units were not fertilized prior to stocking..

The Secchi depth leveled off at between 9 and 10 cm in each of the four units as the

season progressed. Secchi depths in the shrimp units began to decline rapidly to values as

low as 5 cm during July at feed rates of about 250 lb/ac/day (280 kg/ha/day) but little

water transfer was performed with the tilapia unit prior to August 8. Within two weeks

from this date, alternating which shrimp unit water was exchanged with tilapia unit water,

the Secchi depths improved to values between 9 and 10 cm.

Figure 89. Daily Secchi depth in unit 1.

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113


Figure 93. The 7-day average Secchi depth in unit 1.

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115


Alkalinity

The background alkalinity in the creek water used is relatively low, even after

addition of the salt at about 1 meq. Calcium carbonate was added as needed in each of the

units to try to maintain the alkalinity at about 3 meq, mainly to combat the fluctuations in

pH.

Water Transfer to Tilapia Unit

Water exchanged with the tilapia unit from the shrimp units was necessary, as

defined by the levels set in Table I, almost continuously from one of the three shrimp

units after feed rates exceeded 200 – 300 lb/ac/day (224 – 337 kg/ha/day) on June 21

(Figure 97). The length of the horizontal lines in the chart represent the exchange

duration for the corresponding unit. Because only one unit could be exchanged with the

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tilapia unit at any given time, when more than one unit dictated need for transfer, the unit

with the poorest water quality was selected for exchange. From September 12 – 20, water

transfer with unit 3 was not performed due to a nitrite spike in unit 3. Water exchange

with the tilapia unit water occurred for 40.4 days (20% of the season length), 49.8 days

(25% of the season length), and 30.5 days (15% of the season length) in units 1, 2, and 4,

respectively.

Figure 97. Water transfer to tilapia unit.

System Productivity

The system productivity can be described both in terms of the shrimp production

that occurred over the course of the season and in terms of the algal production, which is

essentially a secondary growth organism in the system. The algal productivity is

generally described in terms of the amount of oxygen produced per unit area per unit

time.

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Shrimp Production

The shrimp average weight (Figure 98 – Figure 100) and biomass (Figure 101 –

Figure 103) increased approximately linearly with respect to time in each of the three

units. For the purposes of generating the biomass plots, all mortality was assumed to

occur at the stocking. Unit 1, which had the highest survival of 87%, reached a peak of

17,899 lb/ac (20,080 kg/ha) at the end of the season. Unit 1 also had the produced the

smallest harvested animal at 11.5 g (39 shrimp/lb). Due to high mortality early in the

season in unit 2 (38% survival), the harvested biomass only reached 11,730 lb/ac (13,160

kg/ha). However, because the survival, and therefore the density, was lower in this unit

than the other units, it also produced the largest animal at a harvest weight of 17.1 g (27

shrimp/lb). The breaks in the plot for unit 4 (Figure 103) represent two partial harvests

that were performed towards the end of the season to look at relationships between

density and growth rate. Although the final harvest weight from this unit was less than

9,000 lb/ac (10,097 kg/ha), the total amount of shrimp produced in this unit at 59%

survival, including the early harvests, was 14,939 lb/ac (16,760 kg/ha) and the average

weight was 16.3 g (28 shrimp/lb). Overall, average survival was 61% and yield was

14,856 lb/ac (16,667 kg/ha) of 34 ct (13.6 g) shrimp for a harvest density of 123

shrimp/m2.

One factor that may have helped increase the survival of shrimp in the system was

the implementation of the paddlewheel mixing. Harbor Branch Oceanographic Institution

(2000) reported that high water velocities within a shrimp culture unit made the shrimp

leave the bottom of the unit to swim in the water column, reducing the frequency of

shrimp to shrimp encounters, therefore reducing aggression and cannibalism. A high

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density of algae and its contribution to reduced visibility may have provided shelter to the

shrimp during molting, increasing survival.

Figure 98. Shrimp average weight in unit 1.

119



120

Figure 101. Estimated shrimp biomass in unit 1.


121


The feed conversion ratios (FCRs), expressed as lb feed to lb wet weight

harvested shrimp, in units 1, 2, and 4 were 1.8:1, 2.7:1, and 2.0:1. The FCR for unit 2 is

relatively high due to the high mortality that occurred early in the season. The overall

average FCR for the three units is 2.1:1. Peaks in FCRs at the beginning of the season

(Figure 104 – Figure 106) for each of the units reflect the excessive feed rates applied at

that time and underestimates for the total shrimp biomass at the time (because all

mortalities were assumed to have occurred at stocking). As the season progresses, FCRs

increased (deteriorated) directly in proportion to shrimp size, feed rates, or a combination

of the two.

122

Figure 104. Feed conversion ratio over time in unit 1.


123


The presence of high densities of primary producers in the system likely improved

the FCRs. There were periods during the study that it was necessary to reduce or cease

feeding for lengths of time in excess of one week. Intestinal tracts of shrimp observed

during these periods appeared green in color, as compared to the normal brown color

present when feed rates were high, indicating that the shrimp were grazing on the algae

present in the water. Shrimp growth is highest in ponds that contain high levels of natural

productivity; phytoplankton and organic detritus are important components of shrimp

diets (Harbor Branch Oceanographic Institution, 2000). Harbor Branch Oceanographic

Institution (2000) also stated that the animals are more likely to cannibalize their cohorts

when they are hungry, implying that a constant supply of some food source will increase

survival. Because feed is generally one of the highest operating costs for shrimp farms,

the availability and utilization of phytoplankton is economically advantageous.

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Algal Production

The light and dark bottle trial data (Figure 107 – Figure 110) illustrate the decline

in algal productivity over the course of the season, which was caused by low light levels

and decreased light penetration into the water column as a result of overcast days and

shading. In general, the net photosynthesis measured was always higher in unit 3, the

tilapia unit, than in any of the other units. The maximum net photosynthesis values

indicate the potential of our system and this can be directly linked to the nutrient

assimilative capacity of our system. These values range from about 50 to 75 mg-

O2/L/day, which translate to the production of 269 lb-O2/ac/day to 403 lb-O2/ac/day.

However, the decline in algal productivity at the end of the season lowered seasonal

averages to 16.36, 11.87, 21.03, and 18.98 mg-O2/L/day in units 1, 2, 3, and 4,

respectively. The seasonal average for the system overall was 17.06 mg-O2/L/day (92 lb-

O2/ac/day). The water column respiration in the system ranged from 5.36 to 20.63, with

an average of 10.61 mg-O2/L/day. Because the tilapia unit remained well-cropped

throughout the season, the algal cell age was lower and therefore the algal growth rates

higher, explaining the higher net photosynthesis values and lower water column

respiration values in unit 3 as compared to the shrimp units.

125

Figure 107. Net algal photosynthesis and water column respiration in unit 1.


126



127

The particulate organic carbon (POC) concentrations (Table VI) measured in the

four units indicate high algal density. Although the range is relatively high for each of the

units, the averages all fell between 71 and 81 mg-POC/L, with a seasonal average across

the system of 77.3 mg-POC/L.

Table VI. Particulate organic carbon measurements (mg/L).

Date Unit 1 Unit 2 Unit 3 Unit 4

6/30/03 75.29 64.62 53.52 40.65

8/13/03 113.57 110.04 70.67 91.90

8/18/03 72.25 74.64 71.01 73.09

8/25/03 91.42 82.01 73.15 75.43

9/2/03 62.33 56.98 48.26 77.07

9/16/03 159.03 172.11 129.68 129.24

10/29/03 73.04 60.97 67.52 10.80

11/8/03 47.31 50.79 30.87 44.33

11/20/03 47.37 61.59 42.93 55.11

11/30/03 59.51 71.35 111.25 97.38

12/6/03 80.26 79.80 87.12 81.83

12/7/03 72.22 81.52 101.95 78.92

12/9/03 93.66 79.27 68.68 73.49

Average 80.56 80.44 73.59 71.48

The algal biovolumes (Figure 111 – Figure 118) indicate that the diversity and

quantity of the algal taxa present in the water column can change dramatically, even on a

daily basis. The abbreviations used (Figure 111 – Figure 114) are as follows: Osc =

Oscillatoria, Chlor = Chlorella, Plank = Planktosphaeria, Micro = Anaycstis (formerly

Microcystis), PennD = Navicula (or pennate diatom), CenD = Cyclotella (or centric

diatom), Tetra = Tetraedron (cubical), TetraT = Tetraedron (pyramidal), Anab =

Anabaena, Meris = Agmenellum (formerly Merismopedia), Ped = Pediastrum, ScenA =

Scenedesmus (multi-cell, ovate cells with spines), ScenB = Scenedesmus (multi-cell,

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pennate cells with spines), ScenD = Scenedesmus (one- or two-cell, ovate cells, no

spines), and Ank = Ankistrodesmus. While green algae (i.e., Ankistrodesmus, Chlorella,

Pediastrum, Planktosphaeria, Scenedesmus, and Tetraedron) and diatoms (i.e.,

Cyclotella and Navicula) were relatively constant on sequential days of counting, the

cyanobacteria (i.e., Agmenellum, Anabaena, Anacystis, and Oscillatoria) biovolume

sometimes changed substantially from day to day. Although the number of zooplankton

in the water column was not quantified and never abundant, it was noted that whenever

ciliates or rotifers were present in conjunction with relatively high concentrations of

cyanobacteria, the concentration of cyanobacteria was diminished by the following day.

Figure 111. Algal biovolume by taxon in unit 1.

129



130


Figure 115. Algal biovolume (µL-algae/L) by class in unit 1.

131



132


Total algal biovolume was lowest in the tilapia unit and second-lowest in unit 2,

which experienced the longest total duration of water transfer with the tilapia unit. Total

algal biovolume (µL-algae/L) ranges and averages were 66 – 725 (average 303), 41 – 906

(average 209), 36 – 523 (average 200), and 25 – 551 (average 223), with an overall range

of 25 – 906 and an overall average of 223. The tilapia unit demonstrated the highest

percent composition of green algae, 29 – 75% higher than that in the shrimp units. Green

algal biovolume (µL-algae/L) ranges and averages, averages expressed as biovolume and

percent composition of the seasonal total algal biovolume, were 20 – 455 (average 198,

65%), 13 – 206 (average 100, 48%), 9 – 356 (average 167, 84%), and 9 – 311 (average

114, 63%), with an overall range of 9 – 455 and an overall average of 142 (64%).

Cyanobacterial biovolume, as a percent of the total algal biovolume, was 70 – 79% lower

in the tilapia unit than in the shrimp units. Cyanobacterial biovolume (µL-algae/L) ranges

133

and averages, were 3 – 353 (average 71, 23%), 5 – 641 (average 71, 34%), 0 – 73

(average 14, 7%), and 2 – 348 (average 43, 24%), with an overall range of 0 – 641 and an

overall average of 50 (22%). Diatom algal biovolume ranges and averages, lowest in the

tilapia unit, were 0 – 98 (average 35, 12%), 0 – 142 (average 38, 18%), 0 – 95 (average

19, 10%), and 0 – 56 (average 24, 13%), with an overall range of 0 – 142 and an overall

average of 29 (13%). The dominant algal taxa in all units was Planktosphaeria,

composing 62%, 43%, 75%, and 55% of the total algal biovolume in each respective unit.

The second-most dominant algal taxa in the shrimp units was Anacystis (formerly

Microcystis), composing 20%, 30%, and 20% of the total algal biovolume in each

respective unit. The second-most dominant algal taxa in the tilapia unit was Cyclotella

(centric diatom), composing 9% of the total algal biovolume.

Water exchange with the tilapia unit influenced the algal population.

Cyanobacteria was rarely present in the tilapia unit in concentrations as high as those that

were exhibited in the three shrimp units, implying that the tilapia may preferentially graze

on the typically smaller cyanobacteria over the green algae and diatoms. Turker et al.

(2003) reported that tilapia filtration rates of cyanobacteria were about 30% higher than

filtration rates of green algae at POC concentrations greater than 25 mg-POC/L. During

this study, a microscopic investigation of the contents of the end portion of a tilapia’s

intestine was performed on a fish that was present in a water column exhibiting

approximately equal biovolumes of green algae and cyanobacteria. In the contents of the

tilapia’s intestinal tract, there were several green algae specimen, particularly

Planktosphaeria and Scenedesmus, that appeared to be unharmed by the animal’s

digestive tract. No recognizable cyanobacteria was observed, perhaps implying that the

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cyanobacteria were fully digested (under the assumption that they were filtered into the

animals digestive system) and the green algae, with a more robust cell composition

including a cell wall, were allowed to pass, unharmed, back into the water column

through the fish’s excrement. This further supports the assumption made based on the

algal biovolume counts that tilapia may be most effective at cyanobacteria removal.

Solids Removal

Only a negligible amount of settleable solids was removed from the tilapia unit

and therefore no data were reported for unit 3. On average, 5.66 lb/day (2.57 kg/day) of

total solids were removed from each unit, with 3.18 lb/day (1.44 kg/day) of this being

volatile solids (Figure 119 – Figure 121). Uneaten feed was sometimes observed in the

solids removed, indicating that the FCR, growth, and yield could have been higher if feed

were excluded from solids removal.

Figure 119. Rate of volatile solids removal in unit 1.

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136

Carbon Mass Balance

The carbon mass balance (Figure 122) was performed as an average for the

shrimp units only. The carbon inputs to the system were 98% (7.8 g-C/m2/day) organic

through feed and 2% (0.2 g-C/m2/day) inorganic by CO2 injection. Algal fixation

accounted for 34% of the total carbon input, storing 2.7 g-C/m2/day into algal biomass.

The net outputs of carbon from the water column were 5% (0.4 g-C/m2/day) by

conversion into shrimp biomass, 36% (2.9 g-C/m2/day) by sludge removal, and 59% (4.7

g-C/m2/day) by CO2 outgassing. It is important to note that at least all of the carbon

formed in algal biomass can be accounted for in the sludge removal system, indicating

that the solids removal system employed was capable of maintaining a rate of carbon

removal equal to the rate of carbon fixation. It is possible that the carbon removed in the

sludge is greater than the carbon fixed into algal biomass because the solids removal also

accounted for some removal of feed applied to the system. A single, centralized solids

removal zone, as opposed to the distributed nature of the many sump holes used in this

research, could decrease the chances of removing feed in the solids removal system. Such

a design would improve the FCRs, average shrimp weight, and yield of the system.

137

Figure 122. Carbon mass balance for shrimp units.

Nitrogen Mass Balance

The only nitrogen input to the system was through feed application, adding an

average of 0.89 g-N/m2/day (Figure 123). Nitrogen was stored in the water column as

algal biomass and nitrate. The algae fixed 54% of the nitrogen input, storing 0.48 g-

N/m2/day in phytomass and nitrification accounted for 39% of the nitrogen storage,

converting an average of 0.35 g-N/m2/day into nitrate. Although nitrification represents

nitrogen storage, it is drawn as an output from the mass balance because nitrate

accumulates and is not recycled in the entirely aerobic system, which disallows

denitrification. The only possible alternative means of nitrate conversion or recycle is by

algal assimilation, which will not occur preferentially to ammonia uptake if ammonia is

available. The net outputs of nitrogen from the water column were via conversion into

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shrimp biomass at 11% (0.10 g-N/m2/day) and sludge removal, which accounted for 50%

(0.44 g-N/m2/day). It is important again to note that the nitrogen content in the sludge

removed from the system accounts for almost all of the nitrogen present in the algal

biomass, indicating that the sludge removal system was successful in reaching an

equilibrium rate of removal with the rate of algal growth.

Figure 123. Nitrogen mass balance for shrimp units.

DESIGN PROPOSAL FOR SYSTEM OPTIMIZATION

Because the PAS is designed to remove nutrient content from the water by

assimilation into algal biomass, one key factor in an optimal system is maximal algal

growth rates. In order to sustainably maintain high-rate growth of algae and maximize

feed rates, algal harvest must be employed. Algae must be removed from the system in

order to reduce the water column respiration and light limitations at high algal growth

rates.

Cyanobacterial populations generally possess more undesirable characteristics for

aquaculture than green and diatom algal populations due mainly to their tendencies to

sometimes produce taste, odor, and toxicity problems and their less robust, more rapidly

degradable physiology. The biggest problem in development and sustained operation of

algal-based zero-discharge systems has been the difficulty in maintaining desirable algal

species (Wang, 2003). It is therefore desirable to be able to be able to maintain a stable

population of green algae and diatoms. Currently, the best apparent method for selecting

for such a population and removing the cyanobacteria is by biological harvest by tilapia.

There may be other filter feeders that can crop the algae more effectively or efficiently,

but there is little research on high-rate systems similar to the PAS that indicate which

organisms may be options. Therefore, it seems clear that the tilapia unit must remain

incorporated within the system in order to maintain a stable algal community.

In the 2002 studies of the distributed animal growth system, it became clear that

shrimp-tilapia co-culture within the same unit was not going to be feasible. High shrimp

mortalities were experienced in the unit stocked with shrimp and tilapia, and possibly

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attributable to tilapia predation. Even when physical, screened barriers are placed

between the shrimp and tilapia within the same unit, the young tilapia are small enough to

move past the screens if the mesh size is not small enough. However, if the mesh size on

the separation devices is too small, water transmission through the screens rapidly

diminishes due to clogging. From a maintenance and production standpoint, the most

practical method of physically separating tilapia from shrimp is by the installation of

physical impermeable barriers—concrete walls.

Because the tilapia are essential and must be physically separated from the shrimp

culture area, water exchange must be performed actively through the use of pumps.

Airlifts used in this research were only capable of exchanging the water between two

units at any given time. There were many instances during the 2003 season when it was

desirable to exchange more than one unit with the tilapia unit to improve water quality

and one design proposal would allow for continuous exchange between each of the

shrimp units and the tilapia unit. By doing so, the algal community in each of the shrimp

units, providing that the tilapia unit is sized large enough to assimilate the flow rates

applied, would remain well-cropped and the water quality would not deteriorate as much.

It is still unclear how large or how densely stocked the tilapia unit needs to be in

relationship to the shrimp units, or the exchange rates required, in order to maintain

optimal water quality conditions in the shrimp units.

Although the solids removal system used in this research performed effectively,

observations indicate that it could be further optimized. The sump holes placed in the

bottoms of the units were effective in removing solids from the system and the airlifts

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were capable of delivering these solids to settling tanks for each unit. However, an

alternative design would be to include a U-tube in the paddlewheel-driven closed water

circuit, designed specifically for solids removal (Figure 124). In essence the device would

act as a primary settling tank within the system and solids could be delivered from here to

a secondary continuously-operated settling tank for further concentration, and finally to a

tertiary batch-operated settling tank for final concentration, decanting the supernatant

back into the system. The high-energy solids removed from the batch-operated settling

tank could then be recycled via anaerobic digestion in order to capture some of the

valuable energy as methane gas rather than wasting it to a sand filter. Anaerobic digestion

would also provide a method of partitioning the nutrients in the sludge, converting them

into a more usable and potentially marketable form. The energy associated with the

fermentation of algae and subsequent electrical generation is more than enough to cover

any costs incurred through mechanical harvesting (Oswald, 1996).

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Figure 124. Conceptual drawing of U-tube design.

The U-tube will accumulate solids that are swept in by the flow of water in the

loop generated by paddle wheels, about 3 ft/s (0.9 m/s). On the exit side of the U-tube,

the cross-sectional area will be sized so that the upflow velocity remains low enough,

about 0.2 ft/s (0.06 m/s), that it is not higher than the settling velocity of the solids

particles. The units for this research were concrete lined and for design purposes, some

type of liner will probably be necessary; concrete or otherwise. Studies on the system

indicate that having a liner, as opposed to an earthen bottom, allow for this passive

“sweeping” of the bottom of the unit, which is necessary for concentration of the solids

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and their ultimate removal. By centralizing the solids collection to one or two locations,

rather than having several sump holes distributed throughout the unit, feed can be applied

strategically to minimize the chances of its removal prior to opportunity for shrimp

uptake, increasing the overall shrimp production efficiency as a function of feed input.

The solids accumulated in the bottom the U-tube will be air-lifted to a primary

settling tank where they may be further concentrated for subsequent transfer to a

secondary settling tank and then ultimate disposal or reuse. Because solids will be

continuously pumped from the U-tube, the primary tank will operate as a continuous

settler. The shrimp exclusion tunnel (Figure 12) used in this research was extremely

effective in excluding shrimp from the primary settling tank and should be incorporated

in the optimized system as well.

The use of air-lifts for the method of pumping is important for several reasons.

Air lifts have no moving parts or impellers that would be harmful to the animals if they

were to be passed through. In addition, because they have no moving parts, they are

relatively robust and maintenance free, qualities that do not exist in most other pumps,

especially those used in saline environments. Thirdly, they provide aeration as an added

functional benefit.

The U-tube will also be used for aeration and pH control. While its exit will be

designed with a low upflow velocity to capture solids particles, the entrance will be

designed with a high downflow velocity. Oxygen and carbon dioxide diffusers will be

placed in the entrance channel of the U-tube. By maintaining a downflow velocity of

water proportional to the upflow velocity of gas bubbles, the efficiency of absorption of

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the bubbles will be greatly increased. In theory, efficiencies of transfer approaching

100% can be attained if the bubble size and flow rates are designed properly. The

addition of carbon dioxide will be used to counteract the extraction of carbon by the

algae.

SUMMARY

A 0.25 ac (0.1 ha) greenhouse was constructed and divided into four equally sized

0.062 ac (0.025 ha) units. Each unit measured 6.7 m (22 ft) in width by 120 ft (36.6 m) in

length. An artificial marine environment was created by adding sea salt to the units and

an artificial tropical environment was created by heating the system with two 1,000,000

Btu (1,055 MJ) boilers when needed. The transparent greenhouse served the purpose of

withholding heat in the winter, extending the growing season, while allowing light

transmission for photosynthesis. Retractable sides on the greenhouse were used to

maintain water temperature in the summer within the range optimal for shrimp growth.

The individual units were constructed with concrete block walls were used to separate the

units; the bottom of the system was lined with a 6 in (15.2 cm) concrete pad. A plastic

curtain was fixed along the length of each unit in the center, creating an oval circuit. A

water velocity field of 0.4 ft/s (12 cm/s) was maintained using 4 ft (1.2 m) diameter six

blade paddlewheels installed in each unit, driven by 1/2 hp (373 W) variable frequency

motors per each two paddlewheel set.

Initially, aeration was delivered at 12.5 L-O2/min into each of the four units with

fine bubble diffusers using an oxygen generator. As the oxygen demand increased in the

system, two 50 ft (15.2 m) long air diffusers were installed in each unit, supplied by 1/2

hp (373 W) regenerative blowers. This was not sufficient to meet the oxygen demand of

the system at the current feed rates between 150 and 200 lb/ac/day (168 and 224

kg/ha/day) and one 3/4 hp (559) floating axial flow fountain aerator was therefore

installed about 60 days into the growing season in each unit, satisfying the oxygen

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demand. At 180 days into the season, an additional 3/4 hp (559 W) fountain aerator was

installed in each unit providing aeration redundancy in the event of aerator failure.

Twelve 6 in (15.2 cm) diameter by 3 ft (91 cm) depth sump holes were installed

in each unit for use in solids removal. Airlifts were positioned in each of these sump

holes, discharging 20 gal/min (76 L/min) ultimately to a 500 gal (1,892 L) settling tank

dedicated to each unit. These airlifts were supplied from a 1.5 hp (1,118 W) centrifugal

blower and activated by the use of solenoid valves and a timer. The twelve sump holes in

each unit were divided into three banks of four sump holes, each bank connected to a

single solenoid valve. Each bank of airlifts was operated for four 90 min intervals during

each 24 hr period, or 25% of the time. Water delivery from the sumps to the settling tank

in each units functioned for four 4.5 hr intervals during each 24 hr period, delivering

water to each settling tank 75% of the time. This allowed the 500 gal (1,892 L) settling

tank to be operated as a continuous settler 75% of the time and as a batch settler 25% of

the time, providing more complete settling. Solids were removed from the 500 gal (1,892

L) settling tank once per day to a 110 gal (416 L) batch-operated concentration tank

assigned to each unit. This solids-laden water was allowed to settle for 24 hours in the

110 gal (416 L) batch settlers; the supernatant was then decanted back into the units, and

the concentrated solids were sampled for solids content and volumetrically quantified

prior to delivery to a sand filter. Solids did not accumulate in the tilapia unit and therefore

were not removed. On average, 5.66 lb/day (2.57 kg/day) of total solids were removed

from the each shrimp unit, with 3.18 lb/day (1.44 kg/day) of this being in the form of

volatile solids. The percent volatile solids in the sludge removed from the system indicate

the degree of concentration accomplished in the sludge removal system. The volatile

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solids concentration in the sludge removed ranged from 1.01 – 4.65%, with an average of

2.41%.

The three 250 m2 distributed PAS units were stocked with PL 8-9 Pacific white

shrimp (Litopenaeus vannamei) at a density of 200 shrimp/m2 during the week of May

22, 2003. Feed rates to the system were limited as a function of water quality in the

system and the amount of feed that the shrimp would consume. Feed application to the

three units reached similar maximum rates of 350 – 400 lb/ac/day (393 – 449 kg/ha/day),

with an overall average seasonal feed rate of 151 – 159 lb/ac/day (170 – 178 kg/ha/day).

Feeding was decreased or halted if the water quality in that unit reached a TAN

concentration greater than 1.2 mg-N/L, nitrite concentration greater than 0.15 mg-N/L,

Secchi depth visibility less than 7 cm, or morning dissolved oxygen concentration less

than 2 mg/L. In retrospect, unionized ammonia would have been a more effective control

parameter than total ammonia nitrogen. The shrimp were harvested on December 11-12,

2003 after an average growing season of 203 days (204 days in unit 1, 203 days in unit 2,

and 197 days in unit 4). Average size of the animals at harvest was inversely related to

the survival, which differed substantially in each unit. Unit 1 produced 87% survival at an

average harvest weight of 11.5 g, unit 2 produced 38% survival at an average harvest

weight of 17.1 g, and unit 4 produced 59% survival at an average harvest weight of 16.3

g, averaging an overall 61% survival and an overall average harvest weight of 13.6 g.

High mortality was experienced in unit 2 at the time of stocking and most of the mortality

experienced in unit 4 can probably be attributed to a severe oxygen event that occurred

early in the season. The average harvest weight is also reported in terms of average

harvest count because this value is generally more commonly used in marketing and sales

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for shrimp production. Average harvest counts were 39 shrimp/lb (87 shrimp/kg) in unit

1, 27 shrimp/lb (59 shrimp/kg) in unit 2, and 32 shrimp/lb (70 shrimp/kg) in unit 4, with

an overall average of 32 shrimp/lb (70 shrimp/kg). FCRs were also inversely proportional

to survival at 1.8:1 in unit 1, 2.7:1 in unit 2, and 2.0:1 in unit 4, respectively, with an

average FCR to the three units of 2.1:1. The yields from the three units were 17,899 lb/ac

(20,080 kg/ha), 11,730 lb/ac (13,160 kg/ha), and 14,939 lb/ac (16,760 kg/ha), with an

overall yield of 14,856 lb/ac (16,667 kg/ha).

A fourth 250 m2 unit was stocked with Nile tilapia (Oreochromis niloticus) for

control of algal density and algal species composition in the shrimp units. The tilapia

were capable of maintaining a well-cropped algal bloom, feeding at a higher rate on

cyanobacteria. It was also observed that the tilapia were successful in reducing the

populations of zooplankton in the system such as ciliates and rotifers. When the water

quality deteriorated below the criteria for feed reductions in any of the shrimp units, the

water in that unit was exchanged with the tilapia unit at a rate of 10 gal/min (37.9 L/min)

using airlift pumps. Water exchange was performed with the tilapia unit from unit 1 for a

total time of 40.4 days (20% of the season), from unit 2 for a total time of 49.8 days (25%

of the season), and from unit 4 for a total time of 30.5 days (15% of the season). No feed

was applied to the tilapia unit during the course of the experiment thereby encouraging

them to filter the water to maintain their energy and growth needs, thereby providing a

biological means of algal harvest.

Water quality in each of the four units was monitored three times daily for the

entire course of the season. The temperature was similar in all units at any given time,

ranging from 23.3 oC to 32.8

oC with a seasonal average of 28.1

oC. DO was generally

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above 4 mg/L in each of the units, with peaks above 20 mg/L prior to installation of the

3/4 hp aerators, which provided for degassing, limiting supersaturated levels, and a

seasonal average across the four units of 6.61 mg/L. During isolated events, the DO

reached minimum values of 1.32, 1.23, 0.56, and 0.27 mg/L. Salinity was maintained in

each of the units between 4 g/L and 8.9 g/L with a seasonal average of 5.2 g/L. Salt was

not added to the tilapia unit; as the season progressed, diffusion and mass transfer via

water exchange with the tilapia unit caused the salinities to equalize. Early in the season,

at feed rates below 200 lb/ac/day (224 kg/ha/day), pH was controlled in each of the units

by injecting CO2 through fine bubble diffusers, with efforts to maintain values below 9.0

pH units. Towards the end of the season, with high feed rates and declining algal

productivity due to low light levels and shading, the pH decreased substantially in each of

the units. The average pH in the shrimp units was approximately equal and somewhat

higher in the tilapia unit, most likely due to the fact that feed was not directly applied to

the tilapia unit. The four units demonstrated similar trends for pH, fluctuating between

7.75 and 8.25 pH units during the first half of the season and decreasing to below 7.0 pH

units by the end of the season. Overall, the pH ranged from 6.02 to 9.24 with an average

of 7.78 pH units. During the first quarter of the season, TAN was below 1.0 mg-N/L in

each of the units, increasing in each of the units to averages of about 1.4 mg-N/L. There

were isolated events in each of the units when the TAN reached as high as 2.0, 2.0, 2.1,

and 4.0 mg-N/L in each of the units. Although peaks in TAN often occurred at different

times in the season, transfer of water to the tilapia unit and feeding adjustments caused

the average values to be similar. The maximum TAN levels in all four units occurred at

the same time as a result of an extremely high TAN (>20 mg-N/L) concentration in the

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creek water added to the system on that day. Unionized ammonia ranged from 0.001 –

0.611 mg-N/L, with an overall seasonal average of 0.064 mg-N/L. Unionized ammonia

was highest during the middle of the season when TAN was stable in all units near 1.4

mg-N/L, temperatures reached maximum averages of about 30 oC, and pH was 8 – 8.5

pH units. Trends in nitrite concentration generally followed the trends for TAN, with a

time delay of about two days for nitrifying bacteria populations to develop in the system

as a result of increased TAN. The average nitrite concentration in the tilapia unit was

probably higher than that in the shrimp units because the algal density was lower there,

allowing for less algal nutrient assimilation. The average nitrite concentration was

generally below 0.30 mg-N/L, with peaks of 0.44, 0.43, 0.35, and 0.30 mg-N/L and a

seasonal average nitrite concentration of 0.11 mg-N/L. The Prior to stocking and feeding

the units, the algal density was not very high due to lack of nutrients and corresponding

to the maximum Secchi visibility depth values in each of the units. As feed inputs

increased to the system, the algal density increased and the Secchi depths therefore

decreased and leveled off between 9 and 10 cm in each of the units. During events when

the Secchi depth reached the minimum values in each of the shrimp units, feeding was

decreased and water was exchanged with the tilapia unit in order to restore a less dense

algal population. Secchi visibility depth in the system averaged 25 cm in each of the units

at stocking and leveled off to average values between 9 and 10 cm in each of the four

units. Alkalinity was adjusted with addition of NaHCO3 to 3 meq in each of the four

units.

Algal productivity was monitored over the course of the season and a decline was

noted during the second half of the season. The maximum net photosynthesis in the units

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ranged from 50 to 75 mg-O2/L/day, with averages of 16.36, 11.87, 21.03, and 18.98 mg-

O2/L/day. The observed 24-hr net photosynthesis was always higher, generally by about

50%, in the tilapia unit than in the shrimp units due to a reduced algal cell age. Water

column respiration in the units increased over the course of the season and ranged from

5.36 at the beginning of the season to 10.61 mg-O2/L/day at the end of the season, with

the respiration always being lowest in the tilapia unit, generally by about 25% also due to

a reduced algal cell age. The average seasonal respiration over the system was 10.61 mg-

O2/L/day. The average POC concentration for each of the units was about the same,

although the extreme values differed substantially. More POC samples should have been

collected in parallel with algal counts and photosynthesis/respiration measurements in

order to try to develop a relationship between algal biovolume and POC and possibly a

ratio of algal biomass to bacterial biomass. The POC averages in each of the units were

between 70 and 85 mg-POC/L. The total algal biovolumes for the units differed

substantially; more algal counts should have been performed in order to be able to better

assess the fluctuations in algal biovolume. Total algal biovolume ranged from 25 – 906

µL-algae/L with an overall system average of 223 µL-algae/L and on average was lowest

in the tilapia unit at 200 µL-algae/L. The green algal biovolume data was not as volatile

as the total algal biovolume, and the green algae composition as a percentage of the total

algal biovolume was 29 – 70% higher in the tilapia unit than in the shrimp units. The

green algal biovolume ranged from 8.94 – 454.72 µL-algae/L with an overall system

average of 142.38 µL-algae/L. The cyanobacterial biovolume fluctuated dramatically in

the shrimp units but remained relatively constant and about 80% lower in the tilapia unit.

As a percent of the total algal biovolume, the cyanobacterial biovolume was 70 – 79%

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lower in the tilapia unit than in the shrimp units. The cyanobacterial biovolume ranged

from 0.46 – 641.22 µL-algae/L with an overall system average of 43.16 µL-algae/L. The

diatom algal biovolume generally did not make up a large portion of the total algal

biovolume but fluctuated considerably over the course of the season. In general, the

diatom algal biovolume was lower in the tilapia unit than in the shrimp units by 35-50%.

The diatom algal biovolume ranged from 0.00 – 142.38 µL-algae/L with an overall

system average of 28.91 µL-algae/L. The dominant algal taxa in each unit was

Planktosphaeria (43 – 75% by biovolume), followed by Anacystis (formerly

Microcysitis) in the shrimp units (20 – 30% by biovolume) and Cyclotella (centric

diatom) in the tilapia unit (9% by biovolume).

A carbon mass balance was developed for the shrimp units, quantifying the

inputs, outputs, and storage of carbon in the system. Feed application accounted for 98%

(7.8 g-C/m2/day) of the carbon input to the system, while recarbonation via CO2 addition

accounted for the remaining 2% (0.2 g-C/m2/day). Approximately 34% (2.7 g-C/m

2/day)

of the carbon input was stored through algal fixation. The outputs of carbon from the

system were 5% (0.4 g-C/m2/day) by shrimp conversion, 36% (2.9 g-C/m

2/day) by sludge

removal, and 59% (4.7 g-C/m2/day) by CO2 outgassing.

A nitrogen mass balance was also constructed, defining the fate of nitrogen in the

system. The only input of nitrogen was by feed application at a rate of 0.89 g-N/m2/day.

Approximately 54% (0.48 g-N/m2/day) of the nitrogen input was stored through algal

fixation. The nitrogen outputs from the system were 11% (0.10 g-N/m2/day) by shrimp

conversion, 50% (0.44 g-N/m2/day) by sludge removal, and 39% (0.35 g-N/m

2/day) by

nitrification and storage as nitrate.

CONCLUSIONS

Data gathered from 203 days of shrimp culture in three 250 m2 distributed animal

production PAS units suggested the following operational parameters and overall system

performance.

1. Shrimp yield in each unit was proportional to the percent survival in that unit.

The yields were 20,080 kg/ha (17,899 lb/ac) in unit 1, 13,160 kg/ha (11,730

lb/ac) in unit 2, and 16,760 kg/ha (14,856 lb/ac) in unit 4, with an average

overall yield of 16,667 kg/ha (14,856 lb/ac).

2. Shrimp individual harvest weight was inversely proportional to the percent

survival in each unit. Average harvest weights of the shrimp were 11.54 g in

unit 1, 17.10 g in unit 2, and 14.20 g in unit 4, with an overall average of

14.28 g.

3. Final harvest densities and survivals were 174 shrimp/m2 (87% survival) in

unit 1, 77 shrimp/m2 (38.5% survival) in unit 2, and 118 shrimp/m

2 (59%

survival) in unit 4, with an overall average of 123 shrimp/m2 (61.5% survival).

4. Shrimp food conversion ratio (FCR, dry feed weight / wet shrimp weight) was

best (lowest) in the units that demonstrated the highest survival at 1.8:1 in unit

1, 2.7:1 in unit 2, and 2.0:1 in unit 4, with an overall system average of 2.1:1.

5. Peak daily feed rates were equal in unit 1 and 2 at 352 lb/ac/day (395

kg/ha/day) and highest in unit 4 at 409 lb/ac/day (459 kg/ha/day).

6. 14-day sustainable feed rates were 250 lb/ac/day (281 kg/ha/day) in unit 1,

257 lb/ac/day (288 kg/ha/day) in unit 2, and 234 lb/ac/day (263 kg/ha/day) in

unit 4.

7. Overall seasonal average feed rates were 159 lb/ac/day (178 kg/ha/day) in unit

1, 156 lb/ac/day (175 kg/ha/day) in unit 2, and 151 lb/ac/day (170 kg/ha/day)

in unit 4, with an average overall rate of 155 lb/ac/day (174 kg/ha/day).

8. System dissolved oxygen concentration ranges and averages were 1.32 –

>20.00 mg/L (average 6.55 mg/L) in unit 1, 1.23 – >20.00 mg/L (average 6.51

mg/L) in unit 2, 0.56 – 16.05 mg/L (average 6.64 mg/L) in unit 3, and 0.27 –

>20.00 mg/L (average 6.75 mg/L) in unit 4, with an overall range of 0.27 –

>20.00 mg/L and an overall average of 6.61 mg/L.

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9. System pH ranges (CO2 controlled) and averages were 6.02 – 8.97 (average

7.68) in unit 1, 6.40 – 8.98 (average 7.74) in unit 2, 6.19 – 9.24 (average 7.93)

in unit 3, and 6.41 – 8.97 (average 7.75) in unit 4, with an overall system

range of 6.02 – 9.24 and an overall system average of 7.78.

10. System total ammonia nitrogen concentration ranges and averages were 0.3 –

2.0 mg-N/L (average 1.13 mg-N/L) in unit 1, 0.2 – 2.0 mg-N/L (average 1.13

mg-N/L) in unit 2, 0.1 – 2.1 mg-N/L (average 1.17 mg-N/L) in unit 3, and 0.5

– 4.0 mg-N/L (average 1.17 mg-N/L) in unit 4, with an overall system range

of 0.1 – 4.0 mg-N/L and an overall system average of 1.15 mg-N/L.

11. System unionized ammonia concentration ranges and averages were 0.001 –

0.611 mg-N/L (average 0.058 mg-N/L) in unit 1, 0.002 – 0.390 mg-N/L

(average 0.058 mg-N/L) in unit 2, 0.001 – 0.589 mg-N/L (average 0.081 mg-

N/L) in unit 3, and 0.004 – 0.426 mg-N/L (average 0.060 mg-N/L) in unit 4,

with an overall system rang of 0.001 – 0.611 and an overall system average of

0.064 mg-N/L.

12. System nitrite-nitrogen ranges and averages for the system were 0.00 – 044

mg-N/L (average 0.12 mg-N/L) in unit 1, 0.00 – 0.43 mg-N/L (average 0.10

mg-N/L) in unit 2, 0.00 – 0.35 mg-N/L (average 0.15 mg-N/L) in unit 3, and

0.00 – 0.30 mg-N/L (average 0.08 mg-N/L) in unit 4, with an overall system

range of 0.00 – 0.44 mg-N/L and an overall system average of 0.11 mg-N/L.

13. System Secchi visibility depth ranges and averages were 5.0 – 28.0 cm

(average 9.9 cm), 5.0 – 30.0 cm (average 9.9 cm), 6.0 – 18.0 cm (average 9.5

cm), 6.0 – 18.0 cm (average 9.5 cm), and 5.0 – 25.0 cm (average 9.4 cm), with

an overall system range of 5.0 – 30.0 cm and an overall system average of 9.7

cm.

14. System particulate organic carbon concentration ranges and averages were

47.4 – 159.0 mg-POC/L (average 83.8 mg-POC/L) in unit 1, 50.8 – 172.1 mg-

POC/L (average 80.4 mg-POC/L) in unit 2, 30.9 – 129.7 mg-POC/L (average

73.6 mg-POC/L) in unit 3, and 10.8 – 129.2 mg-POC/L (average 71.5 mg-

POC/L) in unit 4, with an overall system range of 10.8 – 172.1 mg-POC/L and

an overall system average of 77.3 mg-POC/L.

15. System 24-hr net photosynthesis ranges and averages were 0.00 – 53.31 mg-

O2/L/day (average 16.36 mg-O2/L/day) in unit 1, 0.00 – 58.43 mg-O2/L/day

(average 11.87 mg-O2/L/day) in unit 2, 2.01 – 76.38 mg-O2/L/day (average

21.03 mg-O2/L/day) in unit 3, and 0.00 – 66.79 mg-O2/L/day (average 18.98

mg-O2/L/day) in unit 4, with an overall system range of 0.00 – 76.38 mg-

O2/L/day and an overall system average of 17.06 mg-O2/L/day.

16. System 24-hr water column respiration ranges and averages were 7.97 – 17.98

mg-O2/L/day (average 11.85 mg-O2/L/day) in unit 1, 8.55 – 20.63 mg-

O2/L/day (average 11.41 mg-O2/L/day) in unit 2, 8.55 – 20.63 mg-O2/L/day

155

(average 11.41 mg-O2/L/day) in unit 3, 5.36 – 13.64 mg-O2/L/day (average

8.50 mg-O2/L/day), and 5.69 – 17.16 mg-O2/L/day (average 10.67 mg-

O2/L/day) in unit 4, with an overall system range of 5.36 – 20.63 mg-O2/L/day

and an overall system average of 10.61 mg-O2/L/day.

17. System total algal biovolume ranges and averages were 66 – 725 µL-algae/L

(average 303 µL-algae/L) in unit 1, 41 – 906 µL-algae/L (average 209 µL-

algae/L) in unit 2, 36 – 523 µL-algae/L (average 200 µL-algae/L) in unit 3,

and 25 – 551 µL-algae/L (average 181 µL-algae/L) in unit 4, with an overall

system range of 25 – 906 µL-algae/L and an overall system average of 223

µL-algae/L.

18. System green algal biovolume ranges and averages were 19.67 – 454.72 µL-

algae/L (average 197.52 µL-algae/L, 65% of the total algal biovolume) in unit

1, 13.46 – 205.94 µL-algae/L (average 99.64 µL-algae/L, 48% of the total

algal biovolume) in unit 2, 9.04 – 355.66 µL-algae/L (average 166.82 µL-

algae/L, 84% of the total algal biovolume) in unit 3, and 8.94 – 310.80 µL-

algae/L (average 113.75, 63% of the total algal biovolume) in unit 4, with an

overall system range of 8.94 – 454.72 µL-algae/L and an overall system

average of 142.38 µL-algae/L (64% of the total algal biovolume).

19. System cyanobacterial biovolume ranges and averages were 2.54 – 352.82

µL-algae/L in unit 1 (average 70.74 µL-algae/L, 23% of the total algal

biovolume), 4.79 – 641.22 µL-algae/L in unit 2 (average 71.19 µL-algae/L,

34% of the total algal biovolume), 0.45 – 73.05 µL-algae/L (average 14.11

µL-algae/L, 7% of the total algal biovolume) in unit 3, and 1.89 – 348.15 µL-


4, with an overall system range of 0.46 – 641.22 µL-algae/L and an overall

system average of 43.16 µL-algae/L (22% of the total algal biovolume).

20. System diatom algal biovolume ranges and averages were 0.00 – 98.15 µL-


1, 0.00 – 142.38 µL-algae/L (average 37.86 µL-algae/L, 18% of the total algal

biovolume) in unit 2, 0.00 – 94.52 µL-algae/L (average 18.81 µL-algae/L,

10% of the total algal biovolume) in unit 3, and 0.03 – 55.85 µL-algae/L

(average 24.09 µL-algae/L, 13% of the total algal biovolume) in unit 4, with

an overall system range of 0.00 – 142.38 µL-algae/L and an overall system

average of 28.91 µL-algae/L (13% of the total algal biovolume).

21. The dominant algal taxa in all units was Planktosphaeria, constituting a

seasonal average of 62.1% in unit 1, 42.9% in unit 2, 75.1% in unit 3, and

55.2 % in unit 4, with overall of 59.1%. Anacystis (formerly Microcystis) was

the second most prevalent algal taxa in the shrimp units at a seasonal average

of 19.5 % in unit 1, 30.0% in unit 2, and 19.9% in unit 4, with an overall

156

season average of 19.0%. The second most prevalent algal taxa in the tilapia

unit was Cyclotella (or centric diatoms) at a seasonal average of 9.3%.

22. Shrimp unit total solids removal rate ranges and averages were 0.26 – 12.21

kg-TS/day (average 2.36 kg-TS/day) in unit 1, 0.46 – 20.14 kg-TS/day

(average 3.16 kg-TS/day) in unit 2, and 0.32 – 12.00 kg-TS/day (average 2.19

kg-TS/day) in unit 4.

23. Shrimp unit volatile solids removal rate ranges and averages were 0.15 – 6.66

kg-VS/day (average 1.29 kg-VS/day) in unit 1, 0.26 – 10.31 kg-VS/day

(average 1.83 kg-VS/day) in unit 2, and 0.18 – 6.65 kg-VS/day (average 1.21

kg-VS/day) in unit 4.

24. Shrimp unit removed volatile solids concentration ranges and averages were

1.17 – 4.65% (average 2.54%) in unit 1, 1.19 – 4.12% (average 2.41%) in unit

2, and 1.01 – 3.14% (average 2.27%) in unit 4.

25. Shrimp unit organic carbon input rate ranges and averages from feed

application were 0.00 – 17.8 g-C/m2/day (average 8.0 g-C/m

2/day) in unit 1,

0.00 – 17.8 g-C/m2/day (average 7.9 g-C/m

2/day) in unit 2, and 0.00 – 20.6 g-

C/m2/day (average 7.6 g-C/m

2/day) in unit 4, with an overall range of 0.00 –

18.7 g-C/m2/day and an overall average of 7.8 g-C/m

2/day.

26. Shrimp unit inorganic carbon input rate ranges and averages from CO2

injection were 0.00 – 1.16 g-C/m2/day (average 0.18 g-C/m

2/day) in unit 1,

0.00 – 0.99 g-C/m2/day (average 0.20 g-C/m

2/day) in unit 2, and 0.00 – 1.16

g-C/m2/day (average 0.20 g-C/m

2/day) in unit 4, with an overall range of 0.00

– 1.10 g-C/m2/day and an overall average of 0.19 g-C/m

2/day.

27. Shrimp unit algal carbon fixation rate ranges and averages were 0.00 – 9.1 g-


2/day) in unit 1, 0.00 – 10.0 g-C/m

2/day (average

2.0 g-C/m2/day) in unit 2, and 0.00 – 11.5 g-C/m

2/day (average 3.3 g-C/m

2/)

in unit 4.

28. Shrimp unit overall average shrimp organic carbon conversion rates were 0.49

g-C/m2/day in unit 1, 0.32 g-C/m

2/day in unit 2, and 0.43 g-C/m

2/day in unit

4, with an overall average of 0.41 g-C/m2/day.

29. Shrimp unit organic carbon removal rate ranges and averages were 0.31 –

13.32 g-C/m2/day (average 2.58 g-C/m

2/day) in unit 1, 0.53 – 20.61 g-


2/day) in unit 2, and 0.35 – 13.30 g-C/m

2/day

(average 2.42 g-C/m2/day) in unit 4, with an overall range of 0.31 – 20.61 g-

C/m2/day and an overall average of 2.88 g-C/m

2/day.

30. Shrimp unit CO2 outgassing rates were 5.1 g-C/m2/day in unit 1, 4.1 g-

C/m2/day in unit 2, and 5.0 g-C/m

2/day in unit 4, with an overall average of

4.7 g-C/m2/day.

157

31. Shrimp unit nitrogen input rate ranges and averages from feed input were 0.0

– 2.0 g-N/m2/day in unit 1, 0.0 – 2.0 g-N/m

2/day (average 0.89 g-N/m

2/day)

in unit 2, and 0.0 – 2.3 g-N/m2/day (average 0.87 g-N/m

2/day) in unit 4, with

an overall range of 0.0 – 2.3 g-N/m2/day and an overall average of 0.89 g-

N/m2/day.

32. Shrimp unit algal nitrogen fixation rate ranges and averages were 0.00 – 1.63

g-N/m2/day (average 0.50 g-N/m

2/day) in unit 1, 0.00 – 1.79 g-N/m

2/day

(average 0.36 g-N/m2/day) in unit 2, and 0.00 – 2.04 g-N/m

2/day (average

0.58 g-N/m2/day) in unit 4, with an overall range of 0.00 – 2.04 g-N/m

2/day

and an overall average of 0.48 g-N/m2/day.

33. Average shrimp unit overall shrimp nitrogen conversion rates were 0.11 g-

N/m2/day in unit 1, 0.08 g-N/m

2/day in unit 2, and 0.10 g-N/m

2/day in unit 4,

with an overall average of 0.10 g-N/m2/day.

34. Overall shrimp unit nitrification rates were 0.40 g-N/m2/day in unit 1, 0.25 g-

N/m2/day in unit 2, and 0.40 g-N/m

2/day in unit 4, with an overall average of

0.35 g-N/m2/day.

35. Shrimp unit nitrogen removal rate ranges and averages from sludge harvest

were 0.05 – 2.05 g-N/m2/day (average 0.40 g-N/m

2/day) in unit 1, 0.08 – 3.17

g-N/m2/day (average 0.56 g-N/m

2/day) in unit 2, and 0.05 – 2.05 g-N/m

2/day

(average 2.05 g-N/m2/day) in unit 4, with an overall average of 0.37 g-

N/m2/day for the shrimp units.

Key future research for the system should include further intensification strategies

for or further study of shrimp production, sludge removal system design, system

ammonia detoxification (including a standby system for use during extended low light

events), oxygen and carbon dioxide delivery system design, economic feasibility, staged

growth confinements for early shrimp development, and possibilities for alternative filter

feeding organisms.

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