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UNIVERSITY OF NAIROBI

SCHOOL OF ENGINEERING

DEPARTMENT OF ENVIRONMENTAL AND BIOSYSTEMS ENGINEERING

PROJECT TITLE: DESIGN OF A RECIRCULATING

AQUACULTURE SYSTEM

CANDIDATE NAME: ANDREW O. AYUKA

CANDIDATE No.: F21/1711/2010

SUPERVISOR’S NAME: DR. OMUTO C. THINE

A Report Submitted in Partial Fulfillment for the Requirements of the

Degree of Bachelor of Science in Environmental and Biosystems

Engineering, of the University Of Nairobi.

MAY, 2015

FEB 540: ENGINEERING DESIGN PROJECT

2014/2015 ACADEMIC YEAR

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DECLARATION I Ayuka Andrew Okeyo, do declare that this project is my original work and has not been submitted for

a degree in any other university to the best of my knowledge.

SIGN: DATE:

NAME: AYUKA ANDREW O KEYO

CERTIFICATION I confirm that the work reported in this project was carried out by the candidate under my supervision

SIGN: DATE:

SUPERVISOR: DR. OMUTO C. THINE

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DEDICATION I dedicate this work to my parents Joseph and Damaris Ayuka for their unceasing support throughout my

life

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ACKNOWLEDGEMENT

Foremost I want to thank The Almighty Father for granting me life and good health to finish this work

successfully.

I wish also to recognize the effort of my supervisor Dr, C. Thine Omuto for his positive and constructive

advice that enabled me achieve this task.

Finally I wish to thank my family members, special friends like Lucia Uhuru and many others who

rendered support in one way or the other towards the success of this project work

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LIST OF ABREVIATIONS RAS-Recirculating Aquaculture System

UV-Ultraviolet light

DO-Dissolved oxygen

MT-Metric ton

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LIST OF FIGURES

Figure 1a map of Jambo Fish farm (from google earth) .......................................................................................... 17

Figure 2 hatchery system with biofilters (captured from the site visit) ...................... Error! Bookmark not defined.

Figure 3 The grow-out farm in a greenhouse system pictures (captured from the site visit) ................................ 18

Figure 4 A photo of the culture tank (from the site visit) ....................................................................................... 19

Figure 5:Paddle wheel aerators ............................................................................................................................... 23

Figure 6 Pump-sprayer aerators .............................................................................................................................. 23

Figure 7 Aquaculture system layout ...................................................................................................................... 31

Figure 8 Common tank shapes. ............................................................................................................................... 33

Figure 9 Down-flow Bubble Contactor .................................................................................................................... 39

Figure 10 Diffused Aerator ...................................................................................................................................... 39

Figure 11Rotating microscreens .............................................................................................................................. 42

Figure 12 moving bed reactors ................................................................................................................................ 45

Figure 13 top view of a RAS System ........................................................................................................................ 70

Figure 14 Isometric view of a RAS System ............................................................................................................... 70

Figure 15 Side view of a RAS System ........................................................................... Error! Bookmark not defined.

Figure 16 particle trap ............................................................................................................................................. 71

Figure 17 the sludge collector ................................................................................................................................. 71

Figure 18 sludge collector ....................................................................................................................................... 72

Figure 19 A cut-away and expanded mid-section of a drum filter .......................................................................... 72

Figure 20 A trickling biological filter ........................................................................................................................ 73

Figure 21 UV sterilizer ............................................................................................................................................. 73

Figure 22General layout of a Recirculating Aquaculture System ............................................................................ 78

Figure 23 Typical Building Layout ................................................................................ Error! Bookmark not defined.

Figure 24A 3D layout of a Recirculating Aquaculture System ................................................................................. 80

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LIST OF TABLES

Table 1: Percentage of total ammonia at differing pH values and temperatures. ................................................. 55

Table 2:Recommended water quality requirements of recirculating systems. ...................................................... 56

Table 3: Field Data values ........................................................................................................................................ 59

Table 4:Recommended stocking and feeding rates for different size groups of tilapia in tanks and estimated

growth rates. ........................................................................................................................................................... 74

Table 5 :Illustrative RAS Core Treatment Configurations. ....................................................................................... 74

Table 6: Bill of Quantities of scale-up RAS farm ...................................................................................................... 75

Table 7:Operation cost and assumption of scale-up RAS farm ............................................................................... 76

Table 8:A table of UV sterilizer sizing (Inc, 2012) .................................................................................................... 77

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EXECUTIVE SUMMARY

The world’s Population has been growing at an alarming rate not commensurate with an increase in food

production. There is a need to come up with a high food density per unit area of land. Aquaculture is one

of the many ways that can be used to satisfy the growing demand for food. Aquaculture is described as

farming of fish under controlled conditions.

The objective of this project was to design a re-circulating aquaculture system that meets the efficiency

and effectiveness of the little resources. This was done by gathering information from books of well-

established literature, through the internet and from many five- year engineering principles learned at the

University of Nairobi

The information gathered will be used in the literature review. The method used to solve the objectives

requires one to apply principles of design of fish pond aerators in a re-circulating aquaculture systems.

Design of RAS components including; the size and the appropriate shape of the tank, bio-filter,

mechanical filter, and UV sterilization system.

The solution will be compared to the existing information available for recirculating aquaculture system

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Contents DECLARATION ............................................................................................................................................................. ii

CERTIFICATION ...................................................................................................................................................... ii

DEDICATION............................................................................................................................................................... iii

ACKNOWLEDGEMENT ............................................................................................................................................... iv

LIST OF ABREVIATIONS ............................................................................................................................................... v

EXECUTIVE SUMMARY ............................................................................................................................................. viii

CHAPTER 1 ............................................................................................................................................................... 12

INTRODUCTION ................................................................................................................................................... 12

1.1 PROBLEM.STATEMENT AND PROBLEM ANALYSIS ......................................................................................... 13

JUSTIFICATION ..................................................................................................................................................... 14

1.2 OVERALL OBJECTIVE ...................................................................................................................................... 14

SPECIFIC OBJECTIVES ........................................................................................................................................... 14

SITE ANALYSIS AND INVENTORY .......................................................................................................................... 15

Location ............................................................................................................................................................... 15

Environmental sensitivity of the site ................................................................................................................... 15

Climatic factors .................................................................................................................................................... 16

Water supply ....................................................................................................................................................... 16

Option for effluent disposal ................................................................................................................................ 16

Data needs ........................................................................................................................................................... 17

HYPOTHESES ........................................................................................................................................................ 19

1.2 STATEMENT OF SCOPE ................................................................................................................................ 19

CHAPTER 2 ............................................................................................................................................................... 20

LITERATURE REVIEW............................................................................................................................................ 20

History development of RAS ............................................................................................................................... 21

Pond Aeration ...................................................................................................................................................... 21

Principles of aeration ........................................................................................................................................... 22

The effect of surface area and turbulence .......................................................................................................... 22

The effect of prevailing dis-solved oxygen concentration .................................................................................. 22

CHAPTER 3 ............................................................................................................................................................... 24

THEORETICAL FRAMEWORK .................................................................................................................................... 24

RECIRCULATION COMPONENTS .......................................................................................................................... 24

Site Components ................................................................................................................................................. 25

System components ............................................................................................................................................ 26

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Nitrogen metabolites level .................................................................................................................................. 29

Benefits of RAS .................................................................................................................................................... 29

Challenges associated with RAS .......................................................................................................................... 30

4.0 METHODOLOGY ................................................................................................................................................. 31

MATERIALS AND METHODS..................................................................................................................................... 31

Overview of aquaculture ..................................................................................................................................... 31

Culture Tank Design ............................................................................................................................................. 32

PUMPS AND MEASUREMENT OF FLOW .............................................................................................................. 35

Centrifugal pumps ............................................................................................................................................... 35

Aeration (oxygen addition) .................................................................................................................................. 35

Blown Air ............................................................................................................................................................. 35

Blown Air Aeration Issues: ................................................................................................................................... 37

DISSOLVED GASSES .............................................................................................................................................. 38

Carbon dioxide..................................................................................................................................................... 38

Oxygenation ........................................................................................................................................................ 38

Diffusers (Diffused Aerator) ................................................................................................................................ 39

Low Head Oxygenation ........................................................................................................................................ 40

Carbon Dioxide Control and Removal ................................................................................................................. 40

Packed Column Aerators: .................................................................................................................................... 40

WASTE SOLIDS REMOVAL: ................................................................................................................................... 40

MECHANICAL FILTRATION ................................................................................................................................... 40

Each requires a different RAS component to eliminate or minimize impact on water quality ........................... 41

Settleable Solids Removal: .................................................................................................................................. 41

Suspended Solids Removal .................................................................................................................................. 42

Rotating microscreens ......................................................................................................................................... 42

Foam Fractionation ............................................................................................................................................. 43

Floating Bead Filters ............................................................................................................................................ 43

Biofiltration .......................................................................................................................................................... 43

Moving bed reactors ........................................................................................................................................... 44

Additional System Components .......................................................................................................................... 45

AMMONIA AND NITRITE-NITROGEN CONTROL: ................................................................................................. 46

BIOLOGICAL FILTRATION ..................................................................................................................................... 48

DISINFECTION ...................................................................................................................................................... 50

Integrated Treatment .......................................................................................................................................... 51

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Stock Management.............................................................................................................................................. 53

To maximize the production capacity of a recirculating system, keeping the stocking density .......................... 53

Feeding Systems .................................................................................................................................................. 53

SIZING A TANK ..................................................................................................................................................... 54

Data collection and research ............................................................................................................................... 55

CALCULATING AMMONIA LOADING .................................................................................................................... 57

Recirculation Rates (turnover times): ................................................................................................................. 58

5.0: CALCULATION, ANALYSIS AND DESIGN ............................................................................................................ 60

5.1 OPTIMUM STOCKING .................................................................................................................................... 62

6.0 CONCLUSION AND RECOMMENDATION ........................................................................................................... 64

6.2 RECOMMENDATION ...................................................................................................................................... 64

7.0 REFERENCES....................................................................................................................................................... 68

8.0 APPENDICES ....................................................................................................................................................... 70

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CHAPTER 1

INTRODUCTION

Aquaculture, also called fish farming under controlled conditions, is a growing industry striving to

satisfy a growing market for protein-rich food. Currently it is one of the fastest growing sectors of

agriculture. Farm-reared fish is growing in popularity and profitability. Catfish, tilapia, trout, oyster,

clams and other aquatic species are fast becoming a new “cash crop” of the 21th century. Growing

public demand for a healthy, tasty and affordable protein-rich food is stimulating the “boom” in this

industry. The decline in the population of the wild fish as a result of overharvesting and water pollution

has promoted the culture of fresh- fish farming that are grown in contaminant-free waters in indoor tank

systems.

Recirculating aquaculture systems are indoor, tank-based systems in which fish are grown at high

density under controlled environmental conditions to maximize fish growth year-round, the flexibility to

locate the production facilities near large markets, complete and convenient harvesting and quick and

effective disease control. These systems can be used to maximize production where suitable land or

water is limited, or where environmental conditions are not ideal for the species being cultured. This

type of aquaculture production system can be used in marine environments; however, it is more

commonly used in freshwater environments. The fish are housed within tanks and the water is

exchanged continuously to guarantee optimum growing conditions. Water is pumped into the tanks,

through biological and mechanical filtration systems and then returned into the tanks. Recirculating

aquaculture systems can operate efficiently by occasionally adding only a relatively small amount of

water on a daily or weekly schedule.

The systems occupy a very small locale and enable the grower to stock fish at high densities and

produce high yields per unit area. Recirculation systems are very intensive and therefore require a high

level in management of stock, equipment and water quality. As a result it is important to comprehend

the principles of recirculating aquaculture systems if the system is to be managed effectively

Recirculating systems can be expensive to purchase and operate. For this reason it is usually only

economically viable to farm high value species in the systems. Cost of production is generally lower for

large production systems. The principle behind recirculating aquaculture systems is relatively simple

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however it important to note that their construction involves some level of technical expertise if they are

to succeed

Recirculation systems are designed to minimize or reduce dependence on water exchange and flushing

in fish culture units. The systems have practical applications in commercial aquaculture hatcheries,

holding tanks and aquaria systems as well as small scale aquaculture projects. Water is specifically

recirculated when there is a specific need to minimize water replacement, to maintain the quality

condition which differ from the supply water or to compensate for an insufficient water supply.

There are innumerable designs for recirculation systems and most will work effectively if they

accomplish:

Aeration

Removal of particulate matter

biological filtration to remove waste ammonia and nitrite and

Buffering of pH.

RAS have been in existence in one form or another since the mid 1950’s. However, in the recent past is

when their potential to grow fish on a commercial scale have been realizes. New water quality

technology, testing and monitoring instrumentation, and computer- enhanced systems design programs,

much of it developed for the waste water treatment industry, have been incorporated and have

revolutionized the ability to grow fish in tank culture.

Nevertheless, despite its apparent potential, RAS should be considered a high-risk experimental form of

agriculture. They are used to culture high densities of fish annually. The system is currently practiced in

many areas in Kenya e.g. Kiambu, Thika, Mwea just to mention but a few

1.1 PROBLEM.STATEMENT AND PROBLEM ANALYSIS

The Kenyan population has been rising tremendously in recent years with a direct proportion in demand

for protein-rich food. There is a need for a suitable protein-rich food supply but contrary to that, the cost

of living has shot higher than the demand of a common citizen. The rising cost of protein-rich food is a

concern in the country due to higher demand with limited supply.

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Traditional aquaculture production in ponds require large quantities of water.in many areas where fish is

reared in large scale, traditional aquaculture in ponds is not possible because of limited water supplies or

the absence of suitable land for pond construction

There is high pollution in the ponds, rivers and lakes where fish production and harvesting is done. This

is perpetuated by low wastes removal rate in those water bodies because there is high amount of wastes

(both solid and dissolved wastes) in water at the inlet point and the nitrogenous wastes released by the

fish in water after the metabolic reaction. On the other hand, the outlets are smaller than the inlets and

will only remove water in controlled volumes or percentages and as a result contributing to the high

amount of wastes consumed by the fish through water and feeds which finally reach the end-users

(consumers) and may cause some serious diseases to the consumers through repeated ingestion of fish

from those water sources over a period of time.

High concentration of nitrogenous wastes, ammonia (NH3) and ammonia nitrogen (NH3-N), nitrites in

water is toxic to fish, resulting in poor growth and lower resistance to diseases

JUSTIFICATION

Aquaculture is the best way to go alongside agriculture or livestock keeping in order to supply the

required amount of food, making it available as a cheap, healthy source of protein-rich food and other

required nutrients. It is very cost effective because it requires a small area of land and uses little but

clean amount of water which can be recycled easily to minimize wastes. Through water treatment and

reuse, and maintaining optimum conditions, a recirculating aquaculture system will use a fraction of

pond water to produce the same output volume of fish as that of the outdoor systems

1.2 OVERALL OBJECTIVE

To design a re-circulating aquaculture system

SPECIFIC OBJECTIVES

To apply principles of design of fish pond aerators in a re-circulating aquaculture systems

Design of RAS components.

.

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SITE ANALYSIS AND INVENTORY

Site location and description is very subjective and has not been determined yet. The project however

targets both rural and urban areas where land and ground water are limited like Nairobi County and its

neighbouring counties which have been studied to have the highest population in the country. Besides

this, there is a limited supply of protein-rich food in these areas. Contrary to that, the available sources

of protein from the animals or fish from rivers and lakes have very many challenges due to unhealthy or

unclean environments in which they are being reared/handled. Therefore RAS is one of the best sources

of clean and healthy protein –rich food since fish from it are reared in water which is always recycled,

cleaned and treated if need be or flushed out if it is not safe for the survival of fish. In aquaculture there

is a clear relationship between physical and biological characteristics of sites and the economic and

environmental costs associated with developing a project. Although the enclosed nature of RASs does

reduce the importance of some physical site features, the selection of a suitable site is of critical

importance in the development of a RAS operation.

Major issues that must be considered during the site selection process are location of the site,

environmental sensitivity of the site, climatic factors, access to water, quality of water supply and

available options for effluent disposal.

Location The most suitable areas for RAS development are rural zones and industrial zones and applications to

develop RAS in these zones may require a less rigorous approvals process. In some rural zones (i.e.

Farming and Rural Activity zones), a permit is not required for aquaculture development. Selection of

an appropriate site should consider the proximity to residential or other areas that may be sensitive to

noise or aesthetic considerations. Other factors such as the proximity of the site to major roads,

transport, markets and population centers should be considered during planning

Environmental sensitivity of the site The site and adjacent areas should be assessed for environmental values such as sensitive flora and

fauna habitats and areas of cultural heritage. Sensitive or protected areas, which could be impacted by

construction activities and farm operations, should be avoided if possible. Authorizations to remove

native vegetation drain wetlands and conduct earthworks must be obtained from relevant agencies

before undertaking work

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Climatic factors

Climate does not generally limit production in RAS as the aquatic environment can be controlled e.g.

temperature, pH. Ammonia levels etc. Selection of sites where the climate approximates the

requirements of the cultured species can reduce heating and cooling costs.

Water supply

While the enclosed nature of RAS provides isolation from the environment, a key risk to systems is that

the quantity and quality of the water supply to the system will be inadequate. During site selection, RAS

developers must consider existing and potential water supply issues including housing and industry

development and the potential effects of climate change.

Water sources include municipal supplies, groundwater and surface waters. An appropriate authorization

issued may be required. Municipal suppliers provide a consistent supply of high quality water but often

disinfect water with chemicals that can kill stock and the beneficial bacteria in biofilter systems. Surface

water supplies may exhibit seasonal variations in temperature and quality related to climatic factors and

rainfall (e.g. silt loadings). Surface water quality can be affected by upstream pollution and, unless

appropriately treated prior to use, may be a source of disease and parasites to the farm.

Groundwater obtained from bore holes offers a more predictable source of water but is often

deoxygenated and can contain elevated levels of metals, particularly iron. Prior to RAS development,

water sources should be assessed against seasonal variations in supply and quality and the requirements

of the cultured species. The potential for pollution including potential upstream pollution sources and

impacts of future developments should also be considered

Option for effluent disposal

Effluent disposal options must be considered during RAS site selection. Discharge greater than 0.2

ML/day to surface waters requires a discharge license and a works permit is required for smaller

volumes. Discharge to surface water is the least preferred option for effluent disposal.

Best practice environmental management requires evaluation of all effluent disposal options including

land irrigation, hydroponics and disposal through the mains sewer system. The suitability of effluent

disposal via irrigation should be assessed against the Guidelines for Wastewater Irrigation.

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Data needs

Data was collected from Jambo fish farm which is located on Parkside, Muthaiga North along coffee

Garden Avenue off Kiambu road. Jambo Fish hatchery majorly does well in the production of African

Catfish and tilapia (Natural Male Tilapia)

The different sections of the hatchery consist of small individual recirculation systems (incubator/hatching

system, fingerling, juvenile and brood stock section) enabling the control of both the quality and

temperature of water. Here borehole water and collected rain water is used to minimize the introduction

of diseases.

Figure 1a map of Jambo Fish farm (from google earth)

Rearing fish in a greenhouse

The farm is located on the outskirts of Kiambu, produces thousands of cat fish fingerlings and they are

sold throughout the country, while the remaining ones are transferred to their farm where they are grown

under greenhouse conditions. The farm has invested both the hatchery as well as in farm, which include

a solar powered water heating system, water recycling system and specialized feeding system. For the

hatcheries, the farm uses borehole water which is constantly recycled through a system called

Recirculation Aquarium System (RAS). This allows the fingerlings to thrive in a clean environment

before they are sold out in the farms. The hatchery rears the fish in tanks which can hold 1500 mature

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catfish. These tanks are placed inside a greenhouse whose temperature is regulated to enhance growth.

The farm has a capacity of producing 1.5 tons of fish per harvest.

Cat fish take approximately six months to mature and a mature cat fish weighs up to a Kilogram

Figure 3 The grow-out farm in a greenhouse system pictures (captured from the site visit)

Figure 2 hatchery system with biofilters (captured from the site visit)

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CULTURE TANK LAYOUT

Figure 4 A photo of the culture tank (from the site visit)

HYPOTHESES

At the implementation phase, the following will be observed:

The whole system will be easily monitored and controlled through recording of sensitive

conditions within the system over time with proper management

Increased fish production under tight security

Easier and controlled fish harvesting because different sizes of fish are reared in different tanks

and only mature ones are harvested according to their demands.

There will be an increase in economic development as a result of opening sites for trade, creating

employment in the society.

1.2 STATEMENT OF SCOPE

The scope of the project is limited to engineering design and functions of aquaculture system

The design can also be done for both marine and fresh water

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CHAPTER 2

LITERATURE REVIEW

Definition of RAS

RAS is basically Recirculation Aquaculture Systems and it captures a new and unique way of rearing

fish instead of using the old-style or outdoors method of rearing fish in open fish ponds. This system

therefore helps in rearing fish at high densities, in indoor tanks with a well monitored environment. RAS

commonly filter and clean the water for reusing back in the fish culture tanks. New water is however

added to the tanks to recover the water that might have been lost through either splashed water,

evaporation, and water that is used to flush out waste materials. Contrary to RAS, fish ponds and

raceway systems pass the entire water through the pond or the tank and then is discarded and hence a lot

of water is wasted in the process. For healthy and grown fish throughout the growing period clean water

at an appropriate temperature, the right quantity of food per day and sufficient dissolved oxygen content

are the fundamental requirements for optimum growth.

Aquaculture in Kenya is a new technology striving to satisfy a growing market for protein-rich food

(fish) and reduce poverty in rural areas. Currently it is one of the fastest growing sectors of agriculture in

the World. Farmed Fish is increasing becoming popular and profitable. The main warm-water fishes

grown in Kenya are Nile perch, tilapia (Oreochromis niloticus) and the African Catfish (Clarias

gariepinus. These species are fast becoming the new "cash crops" of the counties. Thus growing public

demand for a healthy tasty and affordable protein food is stimulating the tremendous rise in this

industry. The decline in wild fish populations as a result of overharvest and water pollution has therefore

promoted the culture of farmed fresh fish that are grown in contaminant-free waters in static pond, tank

and cage systems.

The system filters clean the water for recycling back through culture tanks. Occasionally, new water is

added to the tanks only to make up for evaporation and for water used to flush out waste materials. Fish

grown in RAS need a continuous supply of clean water at a temperature and dissolved oxygen content

that is optimum for growth. A bio-filtering system is necessary to purify the water and remove or

detoxify harmful waste products and uneaten feed. The materials to be used in the system include a

water tank ,pump(submersible, linear air pump or centrifugal pump), filters (bio-filters or polyester), p v

c pipes, foam fractionators, fish tank, tube settlers, drain valves, down bubble reactors and heat supplier.

All these will aid in a performance of the system to achieve the objective. The fish must be fed a

nutritionally complete feed on a daily basis to encourage fast growth and high survival.

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History development of RAS

Development of RAS started in the 1950s in Japan and was later introduced in Europe in the 1970s

though it was first experimentally introduced. Its commercial utilization was however introduced first in

northern Europe, most notably in the Netherlands, Denmark and Germany in 1980s. (Buck, 2014)

In Egypt, the government started to focus on RAS activities in the late 1970s and this was after the

construction of the Aswan high dam in 1961. This was an encouragement to the Egyptian government as

it was from this that it implemented a number of pilot fish culture projects which included developing

fish farms to produce fish near the River Nile. These fish farms achieved some success as from (1980-

1990) the average production were around 800 kg/ha per year based on standard pond fertilization.

Farmed fish production in Egypt increased progressively from about 20,000 metric tons (MT) in 1985 to

about 50,000 MT in 1990 (official statistics) and this was attributed to the technology of producing all-

male tilapia produced in semi-intensive systems. (Leschen, 2011)

Investors started to target people who owned land around Lake Burullus, in the north of the Nile delta to

start fish farming and it was during this period when farmed fish production increased from around

50,000 MT in 1995 to over 200,000 MT by 2000! This technology therefore earned the early adaptors

very good profits. (Leschen, 2011)

In Kenya, RAS has not yet been embraced compared to other countries and this has been attributed to

the lack of the awareness by the citizens, lack of necessary skills and lack of sufficient or reliable fish

feed just but to mention a few. However a notable development have been noted, by the year 2013 as a

number of farms have started to embrace RAS. They includes Thinquobator Hatchery and Farm; near

Kisumu, Jambo Hatchery and Farm; Nairobi, Jewlet farm; Kendu Bay, Holly Will Farm; Kendu Bay,

and Dominion Farms; Siaya just but to mention a few. Therefore, Kenya has a great potential in RAS

based on the demand for fish by the looming population though a lot need to be done for that to be

achieved.

Pond Aeration

Fish, like all animals, must obtain oxygen from the environment for respiration. Oxygen is far less

available to aquatic organisms than it is to air-breathers, and the dissolved oxygen content of water may

limit the activities of fish. In most natural waters, the supply of oxygen to water (diffusion from the

atmosphere and production from underwater photosynthesis) exceeds the amount used in oxygen-

consuming processes, and fish seldom have problems obtaining enough oxygen to meet normal

metabolic demands. In aquaculture ponds, however, the biomass of plants, animals and microbes is

much greater than in natural waters, so oxygen is sometimes consumed faster than it is replenished.

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Depending on how low the dissolved oxygen concentration is and how long it remains low, fish may

consume less feed, grow more slowly, convert feed less efficiently, be more susceptible to infectious

diseases, or suffocate die. Aquaculturists avoid these problems by aerating ponds mechanically to

supplement normal oxygen supplies.

Principles of aeration

The rate of oxygen movement between air and water is described by the gas transfer equation:

dC/dt = KL (A/V) (Cs –Cm).

In the equation: dC/dt =the rate of oxygen transfer between a liquid and a gas; KL =the liquid-film

coefficient; A/V =the ratio of the air-water interfacial area to water volume;

Cs =the dissolved oxygen concentration when water is saturated with oxygen under the prevailing

conditions of water temperature, salinity and atmospheric pressure; and

Cm =the measured dissolved oxygen concentration. The liquid film coefficient, KL, incorporates a

parameter called the surface renewal rate, which is related to turbulence within the liquid. The gas

transfer equation looks complicated, but it is actually simple to interpret. The equation says that the rate

of oxygen transfer between air and water depends on three factors: the amount of turbulence, the ratio of

surface area to water volume, and how far the prevailing dissolved oxygen concentration deviates from

the dissolved oxygen concentration at saturation. This deviation is called the saturation deficit or

surplus, depending on whether the measured concentration is greater than or less than the saturation

concentration.

The effect of surface area and turbulence

Oxygen moves to and from water across the air-water interface. So, a greater amount of oxygen can

enter or leave a given amount of water when the air-water interfacial area is increased. However, even if

the water is initially low in oxygen, the thin film of water at the interface of a calm water surface quickly

becomes saturated with oxygen, which dramatically slows the rate of oxygen diffusion into the water.

Turbulent mixing restores the saturation deficit in the surface film by moving oxygenated waterway

from the surface, increasing the overall rate of oxygen transfer.

The effect of prevailing dis-solved oxygen concentration

Dissolved oxygen moves into or out of water by diffusion. The rate of diffusion depends on the

difference in oxygen partial pressures between the liquid and gas phases—the greater the difference, the

greater the driving force moving oxygen from one phase to the other

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Paddlewheel aerators

Paddlewheel aerators are the most common types used in large ponds. Paddlewheels consist of hub with

paddles attached in a staggered arrangement. The aerator is powered by a tractor power take-off (PTO),

self-contained diesel or gas engine, or electric motor. Electric paddlewheel aerators are usually mounted

on floats and anchored to the pond bank.

Figure 5:Paddle wheel aerators

Pump-sprayer aerators Pump-sprayer aerators have pumps that discharge water at high velocity through pipes or manifolds.

Pumps may be powered by the PTO of attractor or by an electric motor. Pump-sprayers are simple and

require little maintenance.

Figure 6 Pump-sprayer aerators

These types of aerators are very large, more costly in terms of fuel consumption, initial cost since they

draw power from the PTO of the tractor and produces a lot of noise, heat and needs man power to

operate therefore cannot be used in the RAS system. Therefore the first specific objective is put into

consideration or rather achieved in the methodology stage.

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CHAPTER 3

THEORETICAL FRAMEWORK Design and production Every customer is different, based on the client’s specific requirements, production levels and

geographic locations, a design is set up which suits perfectly. All systems are built from scratch using

suitable materials, such as polyester, stainless steel, PVC, poly-propylene etc. which have a very long

life span

Different systems special designed for each life stage

For each stage of fish species specialised systems are needed. Some examples include:

Hatching systems: A special design system to hatch large quantities of African cat fish

Incubation systems: each incubation jar can hold up to 10,000 tilapia eggs

Fry systems: is designed to hold fish at the first stage of fish’s life in the hatchery

Broodstock system: a system to maintain the most optimal conditions like temperature, pH. and

ammonia level for the broodstock

Artemia system: it provides a hatchery of enough life feed for fry

Juvenile systems: is designed to hold fish at the last stage in the hatchery

Design parameters

Culture tank

The size of the culture tank depends on the number of the water availability. This will finally lead to the

choice of the fingerlings or the broodstock required depending on the scientific skills and the ability to

maintain them by proper feeding, adequate oxygen supply, optimum pH control and waste removal.

Removal of impurities

Both suspended and settleable impurities are removed from the culture tanks after a series of tests done

on the control parameters like pH, temperature, and the nitrogenous wastes like ammonium wastes. If

these parameters are found to the extreme and are not good for the survival of fish, then water is flushed

out of the tanks through drains the tank is refilled with a new volume of water and a little quantity of salt

is added to the new water to activate or revive the behaviour of the fish in the new water environment

RECIRCULATION COMPONENTS

There are numerous designs for aquaculture systems utilizing biofiltration, ranging from a simple tank

biofilters to high tech designs with computer controls. However, all systems have certain basic

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components. These components may be separate pieces, or several may be integrated into a single unit.

All systems need a water supply, tanks to rear fish in, a method of removing particulate wastes, the

biofilters, a method to re-oxygenate the water and a method to remove water. In addition, there are

numerous support facilities which must be considered, including, the building to house the facilities, the

heating or cooling systems, food storage facilities, quarantine facilities, pre-market holding facilities,

transport facilities and back-up equipment.in a RAS system, back-up equipment(pumps, air boilers,

electric generators etc.) can never be considered optional

A RAS system comprises of a number of major components that are necessary for the management of

the system, these include:

Site and components

System components

Site Components

The site components include equipment and structures that are not part of the recirculation system. The

following site components are recommended:

a) Building

An isolated building or shed is required to protect the RAS from external climatic conditions.it will also

ensure that the environment in which the fish are cultured is controlled and maintained.

b) Pump House

This houses the pump that will move water and air through the system. Its purpose is to provide

protection to the pump ensuring that it does not come into contact with the moisture created from

humidity or outside environmental conditions that could potentially damage the pump

c) Three phase electricity

Electricity is required to run lighting, pumps, filtration systems, heating etc. Three phase power is

preferable to single phase electricity due to the high energy consumption requirements from the system

and the kind of total heat production.

d) Emergency generator

This is required just in case mains power is disconnected due to faults, overloading or maintenance. Fish

can only survive for very short periods without oxygen or infiltration

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e) Bulk feed storage

As large number of food are required to feed large numbers of fish, storage facilities are required to

protect food from vermin as well as mildew and mould caused by moisture

f) Purging and packing facilities

Fish will need to be purged to expel waste matter before they are sent to the markets. On the other hand,

they will need to be processed and packaged if they are not sold to the live market before a suitable

processing facility that reaches quality control standards will need to be incorporated into the facility

System components

Culture Tank The tank material used in the design was plastic as it suits a number of the desired requirements. The

shape of the culture tank was used was circular by the fact that circular tanks tend to be self-cleaning.

The size of the circular tank was determined using the fish stocking density (90-98 kgm-3) and the

diameter/depth ratios using the recommended values of between 5:1 and 10:1. On the other hand, the

flow velocities to facilitate self-cleaning in the tank was to be in the range of 3 to 40 Cms-1. A center

drain at the bottom of the tank was also a necessity. The fish species that was to grown in the system

was decided to be tilapia. .

g) Mechanical Filtration

Mechanical filtration basically removes both settleable and suspended solid materials. Settleable solids

are removed through the drains that are placed at the bottom in circular tanks with circular flow patterns

having agitation and this helps the solids to accumulate at the bottom and remove in the flow leaving the

tank. Some solids are removed from the surface, while on slower flows on the other hand result in

accumulation at the bottom of the tank. Mechanical filters require regular back-flushing to prevent the

accumulation of sludge.

Suspended solid materials obtained from faeces and non- eaten food are also removed by the use of a

screen filter (the drum filter). This is due to the following advantages over other methods:

it can be adjusted to solids loading

it has a larger surface area than standard disk filters

it does not have the possibility of collapsing under high loading rates of solids

Removal of solids is important to ensure that pipes and equipment components do not become clogged

with waste materials. Decomposing waste matter left in the fish tank will also consume available oxygen

within the water column

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h) Oxygen Generator or Source of Aeration

Fish require oxygen to survive. As fish as usually stocked at high densities within the tanks, simple

aeration using mechanical aeration systems is often not sufficient. Oxygen can be added to the system

via liquid oxygen and/ or oxygen generator, to maintain suitable oxygen levels at high stocking rates.

Aeration pumps will provide the tanks with both oxygen and water recirculation

Biofiltration

After the settleable and suspended materials are removed by mechanical filtration, the next process was

the removal of the dissolved ammonia which came from the waste excreted into the water and uneaten

fish feed particles. The general biofilters requirements for efficient vitrification are as follows:

DO of not less than 2 mg/liter or 3 to 5 mg/liter and this is for maximum efficiency;

PH 7 to 8;

A source of alkalinity for buffer as vitrification produces acid which destroys about 7 mg

of alkalinity for every mg of NH3- N oxidized;

Moderate levels of organic waste (less than 30 mg/ liter and is measured as biochemical

oxygen demand), and therefore good clarification is necessary; and

Water flow velocities that do not remove bacteria.

Biofiltration is carried out using rotating biological contactors (RBC) putting into considerations the

following: media surface area, ammonia loading, and the hydraulic loading. Biofilters are sized by

equating the ammonia production rates with ammonia removal rates. On average, the ammonia

production is about 10 grams/45.3592 kilogram of fish/day (range: 4 to 21) in tilapia culture tanks. On

the other hand, ammonia removal rates ranged from 0.215 to 1.076 grams/m2 of biofilter surface

area/day. One-inch sheet rings had a specific surface area 216.54 m2/m3. Dividing the required biofilter

surface area by the specific surface area gave the biofilter volume needed to remove ammonia

Disinfection

The next water treatment process after biofiltration takes place is the disinfection of the water. This is

necessary to kill harmful microorganisms to fish in the culture tank. Disinfection is carried out by the

use of Ultraviolet radiation sterilizer. The operating principle of a UV sterilizer is that, water that passes

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close to the UV and when bacteria and other microorganisms are exposed to a sufficient amount of

ultraviolet (UV) radiation they are killed.

Pumping

Flow of water in the whole system was by gravity as much as possible. However, pumping are

inevitable in some processes of RAS including mechanical filtration and biofiltration processes so as to

maintain pressures and constant flow of water in the system. Centrifugal pumps are used for the system

and the hydraulic and shaft power requirement of the pump was established using an online pump power

calculator. This was possible having information about the flow rate, gravitation pull, density of the fluid

and the efficiency of the pump.

Oxygen consumption (MO2)

The oxygen consumption (MO2) of fish is variable and depends on many factors such as

Temperature: MO2 increases when temperature increases.

Body mass: MO2 has an inversely exponential proportion when the body mass increases.

Feeding rate: MO2 increases when the feeding rate increases due to the digestion of food.

Growth rate has a directly proportional relationship with MO2.

Swimming velocity and stress levels: increased stress levels may enhance the MO2 of fish.

The above factors are the most important that should be taken into account in any aquaculture system.

The MO2 of fish culture in tanks is calculated by the Fick equation based on the DO concentration of the

inflow and outflow water, the flow rate and the total biomass inside the tank

𝑉𝑂2 = (𝐶𝑂 𝑋 𝐶𝑎) − (𝐶𝑂 𝑋 𝐶𝑣)

𝐶𝑂 =𝑉𝑂2

𝐶𝑎 − 𝐶𝑣

Where CO = Cardiac Output,

Ca = Oxygen concentration of arterial blood and

C v = Oxygen concentration of mixed venous blood.

It is also possible to estimate oxygen requirements of fish based on feed intake and other models such as

body mass, temperature, and water current velocity, time from feeding, water CO2 levels and that when

1.0 mg of oxygen per liter per minute is consumed by the fish, the fish can produce 1.3 mg of CO2, and

these values should be used for estimating expected CO2 production in aquaculture systems; but in the

case of salmonids, per 1.0 mg of DO consumed per liter they can produce 1.0 mg of CO2 per liter

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Nitrogen metabolites level

Ammonia levels

The fish create and expel various nitrogenous waste products through gill diffusion, gill cat-ion

exchange, and urine and faeces excretion; in addition some nitrogenous wastes are accumulated from the

organic debris of dead and dying organisms, uneaten feed, and from nitrogen gas in the atmosphere

Ammonia exists in two forms: unionized ammonia (NH3-N), and ionized ammonia (NH4+-N), the sum

of these two is called total ammonia nitrogen (TAN). The relative concentration of ammonia is primarily

a function of water pH, salinity and temperature. The excretion of TAN by the fish varies depending on

the species in culture. As a general rule, when 1.0 mg of oxygen per liter per minute is consumed by the

fish, the fish can produce 0.14 mg of TAN

Benefits of RAS

RAS offers a variety of benefits to the fish producers in comparison to open pond culture. These include

the following method:

to maximize production on a limited supply of water

low land requirements,

ability to control water temperature

ability to control water quality

independence from adverse weather conditions

nearly complete environmental control to maximize fish growth year-round

complete and convenient harvesting

Quick and effective disease control.

Some other benefit offered by RAS include;

Location Flexibility:

Where land and water are expensive and not readily available, RAS are particularly useful in this area

due to the fact that they require relatively small amounts of land and water.

They can also be practiced in any given climatic region.

RAS can also be located close to large markets which are mostly urban areas and thereby reduce

large distances and transportation costs.

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RAS can also discharge waste into the sewer systems and also use municipal water supplies

though de-chlorination is necessary.

The other good thing about RAS is that nearly all species of fish can be reared using this type of

the system

Challenges associated with RAS

Despite of RAS being preferred compared to open pond culture, they have some challenges:

They are a bit expensive in terms of their development (building, tanks, plumbing, and biofilters)

and operation (pumping, aerating, heating, lighting).

They require skilled technical assistants to manage and supervise complex systems (circulation,

aeration, bio-filtration, and to conduct water quality analysis) successfully.

Fish loss due to mechanical or electrical power failure may occur when fish are reared in high

densities and in small water volumes.

However the disadvantages should not be a point of concern as with a careful observation the

precautions, all the disadvantages can be avoided

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4.0 METHODOLOGY

MATERIALS AND METHODS

Figure 7 Aquaculture system layout

Overview of aquaculture

Traditional aquaculture production requires large quantities of water ― approximately one million

gallons of water is needed to fill a one-acre pond. In contrast, Recirculating Aquaculture Systems

(RAS), through water treatment and reuse utilize less than 10% of the water required by ponds to

produce comparable yields. RAS are designed to provide excellent culture conditions, supporting

high densities of the species being cultured, providing adequate feed, and maintaining good water

quality. Poor water quality, while not necessarily lethal to the crop, and result in reduced growth and

stress related diseases. Critical water characteristics include concentrations of dissolved oxygen, un-

ionized ammonia-nitrogen, nitrite/nitrate-nitrogen, carbon dioxide, and pH, alkalinity, and chloride

levels. The by-products of metabolisms include carbon dioxide, ammonia-nitrogen, and particulate

and dissolved fecal solids, and uneaten food. Therefore, RAS must effectively:

remove solids (settleable and suspended)

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control ammonia and nitrite-nitrogen concentrations, and

Dissolved gasses. Effective RAS design is based upon components that address each of these

water quality issues

The most important consideration in recirculating systems design is the development of an efficient

water treatment system. Recirculating production systems must be designed with a number of

fundamental waste treatment processes. These processes, referred to as "unit processes," include the

removal of waste solids (both feces and uneaten feed), the conversion of ammonia and nitrite-

nitrogen (a non-toxic form of dissolved nitrogen), the addition of dissolved oxygen to the water, and

the removal of carbon dioxide from the water. With less robust species, and depending upon the

volume of new water used, a process to remove fine and dissolved solids, as well as a process to

control bacterial populations, may need to be applied.

Culture Tank Design

Generation of concept design

What is the problem? The need for fish farming method that is suitable in urban areas and areas with

limited land

What is the cause? Insufficient land for practicing existing fish - Farming methods e.g. dugout ponds

which requires much area. Insufficient water for fish farming

What are the effects of the problem?

The adopted method leads to overcrowding of Fish leading to the pollution of the water due to:

Solid wastes from waste feed and fish wastes, Insufficient oxygen, Dissolved substances e.g. ammonium

Microorganism e.g. bacteria

Proposed solutions? Recirculation aquaculture system consisting

Of the following; Mechanical filter, biofilter, Foam fractionator component, pump and UV

Sterilization system

Tanks can be round or rectangular. The minimum water depth should be maintained above 4 feet (1.2

m). It is recommended that hydraulic retention times be maintained at 15 to 30 minutes, with basic

length to width ratios for rectangular tanks of 4:1 to 8:1.

Sizing of fish tanks is based upon the density of fish, the primary controller of system stability. The fish

density also ultimately controls the feed application rate. A very low fish density (<15 kg/m3) is

commonly used for broodstock and display applications where the stock is considered extremely

valuable. Fingerling, baitfish, and ornamental fish applications are typically sized with moderate loading

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(<30 kg/m3). Higher fish densities are common in the production (growout) of food fish. A half a pound

of fish per gallon is a widely accepted design number. Densities of about 1 pound per gallon (120

kg/m3) of water can be achieved, but, often display unstable water quality and are thus more prone to

disease and growth issues. There are three common tank shapes

Figure 8 Common tank shapes.

Circular tanks

Circular designs dominate broodstock and fingerling applications. Many growout facilities use circular

tanks. The walls of a circular tank are maintained in tension by water pressure. In essence, the walls are

self-supporting. This allows circular tanks to be constructed out of relatively thin polyethylene plastic or

sturdier fiberglass materials. The hydrodynamics of a circular tank facilitate the rapid removal of

suspended solids.

A circular tank with a center drain is naturally good at solids removal. Even a small circulation will tend

to accumulate solids in the center where radial velocities are the lowest. Solids removal from a circular

tank can be enhanced by center sloping bottoms or by centering a dual drain system while optimizing

the tank depth to diameter ratios. Circular tanks make good culture vessels for the following reasons:

Improves the uniformity of the culture environment

Allow a wide range of rotational velocities to optimize fish health and condition

Rapid concentration and removal of settleable solids.

The flow inlet and outlet structures and fish grading and/or removal mechanisms should be engineered

to reduce the labor requirements of handling fish and to obtain effective tank rotational characteristics,

mixing, and solids flushing.

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Rectangular tanks

From an engineering perspective, the other extreme in tank design is rectangular. These tanks are often

seen with a 45 degree bevel providing some rounding of the tank corners. The rectangular tank is prone

to poor solids movement. But, are about 20% more efficient in floor space utilization and are more

easily harvested than circular tanks. As a result rectangular tanks are widely used in ornamental fish,

baitfish, soft crab, and tilapia industries. Solids movement in any rectangular tank requires

consideration. Serious water quality problems can occur if solids accumulate in the bottom of a long

rectangular tank. Water movement induced by recirculating water or aeration systems can be used to

accelerate solids movement to the clarifier. In the ornamental fish industry, a secondary species

(typically a Plecostomus or Aeneus catfish) is often used as a sweeper under mid or top water fish to

move solids. In the case of tilapia, the high density of fish tends to re-suspend solids facilitating their

movement.

Raceway Tanks

Raceway tanks blend the advantages of the circular and rectangular tanks and are most often seen in

marine culture. A third wall is centered along the tanks length to facilitate controlled circulation of

water. This circulation is highly effective at movement of solids with natural collection points occurring

just downstream of the center panel ends. The rounded ends are generally compatible with quick moving

species that have difficulty navigating sharp corners. Although raceway tanks would appear to be the

perfect compromise between circular and rectangular, the third wall adds cost and can interfere with the

ease of harvesting.

Circulation

The RAS is connected by water recirculating from the tank through the filtration loop. Recirculation

flow rates vary among design strategies with 5 to 10 gallons per minute per pound of daily feed ration

(42 to 84 lpm/ kg/d) being typical. Generally, the water pump or air blower that drives the circulation

loop is the major source of RAS energy consumption. Failure of the circulation system leads to a rapid

deterioration in RAS tank water quality, thus, the method selected must be cost effective and reliable.

Three common type of pumping systems are centrifugal, axial flow and airlift pumps.

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PUMPS AND MEASUREMENT OF FLOW

Consistent flow of water through recirculating aquaculture systems, and the ability to alter its speed,

pressure, and direction are critical to virtually all of the functions of the components. In some cases, the

flow stream may need to be pressurized. In other cases water retention time in culture tanks may be

different than in side stream disinfection components. In almost all cases, moving water through

gravitational means is the most cost effective, although the water still has to be pumped to some

elevation to begin its journey through the system

Centrifugal pumps

Typically, a centrifugal pump is used to circulate RAS waters. These pumps operate from the thrust

generated when water in the pump head is spun at high speed. The design of most centrifugal pumps is

optimized for moderate to high pressure operation. In most cases, the pump will be placed outside the

tank, but in some smaller systems a submersible pump may be used. Centrifugal pumps are readily

available for virtually any flow range and salinity. In most RAS applications, a centrifugal pump with

high flow and low lift capacity is favored to minimize energy consumption. Older RAS designs were

based upon recirculation pressures of the order of 25 feet (8 m), whereas, most modern RAS designs

target recirculating pressure of about 10 feet (3 m).

Aeration (oxygen addition)

The aeration process deals with the transfer of oxygen into the water. Oxygen is a relatively rare

component in in water with 10 ppm being considered a high concentration. Most warm water

recirculating systems operate with an oxygen level in the range of 5 to 6 parts per million; whereas,

cooler recirculating systems can operate above 8 parts per million. Both the fish and bacteria rapidly

consume oxygen. Under high loads, the RAS aeration system must be capable of replacing all oxygen in

the system every 20 to 30 minutes at peak feeding rates.

The aeration component of the RAS design must be infallible. Even a short term interference with aera-

tion capabilities can lead to complete loss of fish. Virtually all large scale production facilities have an

aeration backup system in the form of a backup electrical generator, liquid oxygen tanks, or a

mechanical blower. Alarm systems with auto-dialers supplement the backup system. Response times for

re-establishing power or blower capacity need to be less than 20 minutes.

Blown Air

The most basic of aeration techniques is blowing air through a submerged air stone that disperses a fine

stream of bubbles through the water at a pressure typically between 1.2 to 1.8 m. Air pressure is

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generated by a linear air pump (280 lpm), a rotary vane pump (280 to 5600 lpm). As a rule of thumb,

187 lpm/kg/day of air is required for each pound of daily food ration under commercial growout

conditions (Sastry et al., 1999; Malone and Beecher, 2000). Blown air is an aeration technique of choice

for smaller RAS, the simplicity of the operation overwhelmingly drives the choice. Blown air systems

are also widely used for commercial production of tilapia where the system's ability to simultaneously

strip carbon dioxide is considered a benefit.

Aerating with Diffuser Hose: Porous stones or hard plastic air diffusers have been traditionally

used to deliver air in the form of small bubbles to enhance oxygen transfer rates. All air delivery devices

are subject to physical clogging as scale (calcium carbonate) occurs adjacent to the bubble formation as

carbon dioxide stripping locally raises the pH. The diffuser hose is flexible and easily cleaned by hand;

whereas, air stones must be routinely soaked in muriatic acid to dissolve scale. Thus, labor savings are

encouraging increasing the use of diffuser hose in blown air systems.

Aerating with Airlifts: An airlift is also a blown air aeration device that is about 80 percent efficient

when used in open tank aeration. Airlifts can provide all aeration needed for broodstock and fingerling

systems where the fish density is less than 30 kg/m3/ day. For growout systems, airlifts provide about

half the oxygen demands for a RAS at full density (60 kg/m3/day). Airlift systems utilize about 4 cubic

feet per minute of air per pound of daily feed ration (249 lpm/kg/day). About half of the air is dedicated

to the airlift operation and the rest is used to drive the diffuser hoses in the tank. Airlift systems are

generally operated without any water pumps, and thus, save on energy and capital investment.

When rearing larvae in hatcheries, air-lifts may be used to keep planktonic larvae, and their food, evenly

distributed in the tank.

The efficacy (or flow rate) of air-lifts to transfer water is dependent upon:

1) the volume of air being provided,

2) the diameter of the air bubbles (the smaller the bubbles, the greater the flow),

3) the diameter of the pipe (greater the pipe diameter the more flow),

4) The degree of submergence of the pipe (the higher the percentage of submergence, the

greater the flow rate).

The effective performance of recirculating aquaculture systems requires that each component within the

system successfully functions in its role to deliver treated, high quality water back to the crop in the

culture tanks.

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Blown Air Aeration Issues:

Conservatively, air should not be injected deeper than about 5 feet (1.5 m) into a tank. Air injection at

depths greater than 5 feet can cause an increase in the total dissolved gas pressure leading to gas bubble

diseases in the fish. The critical depth of injection varies with altitude; higher altitude locations are more

sensitive to this issue. Likewise, atmospheric air should never be injected to a U-tube or pressurized

packed bed as super-saturation of nitrogen gas will cause similar problems

Pure oxygen

The highest rate of oxygen transfer is accomplished using pure oxygen with pressurized delivery

systems. Pure oxygen is available in compressed or a refrigerated liquid form. The compressed form is

often used as an oxygen backup for smaller systems. Compressed oxygen has an unlimited storage life

and can be reliably activated in case of power failures. Refrigerated liquid oxygen is in insulated tanks

that slowly gain heat and are therefore perishable. As the tank warms up liquid oxygen is converted to

gas that creates pressure that can assure delivery without power. Many large commercial systems use

pure oxygen delivery systems to supply or supplement oxygen while providing backup aeration. The

actual delivery of oxygen into the water is accomplished by one of several specialized devices that

attempt to optimize the air-water interface and/or increase the pressure to maximize oxygen transfer

efficiency.

Speece Cone: Water is injected into the neck of the cone at the top and flows downward to the outlet.

Simultaneously, the pure oxygen is injected as a stream of bubbles near the middle of the column. The

downward velocity of the water declines as the cone enlarges so each bubble rises until they are trapped

by the increasing velocity

Aeration Summary

Most recirculating systems employ either a blown air or pure oxygen delivery system to assure oxygen

levels are maintained. The blown air systems are generally simpler since they add oxygen and strip

carbon dioxide at comparable rates. Pure oxygen systems are capable of maintaining higher dissolved

oxygen conditions and may be more cost effective depending on the scale of operation and the pure

oxygen delivery cost. A number of commercial blown air systems combine these technologies using a

smaller pure oxygen system to supplement oxygen delivery and provide backup. This eliminates the

need for a distinct carbon dioxide stripping unit.

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DISSOLVED GASSES

The adequate supply of oxygen (O2) and the timely removal of carbon dioxide (CO2) are critically

important in maintaining healthy animals in aquaculture systems. Typical concentrations for a healthy

environment for most fish is a dissolved oxygen (DO) level of at least 6 mg/L and a (CO2) concentration

below 25 mg/L. There are two terms commonly used to refer to oxygen delivery to the system:

1) Aeration is used for the normal dissolution of oxygen from the atmosphere into the water (a typical

air pump), while

2) Oxygenation refers to the transfer of pure oxygen into the culture water

Carbon dioxide

Carbon dioxide is produced by respiration of fish and bacteria in the system. Fish begin to stress at

carbon dioxide concentrations above 20 ppm because it interferes with oxygen uptake. Like oxygen

stress, fish under CO2 stress come to the surface and congregate around aeration devices (if

present).Lethargic behavior and sharply reduced appetite are common symptoms of carbon dioxide

stress. Carbon dioxide can accumulate in recirculating systems unless it is physically or chemically

removed.

Oxygenation

Intensive rearing systems (high density of culture animals) may consume DO at a greater rate than can

be reasonably provided through conventional aeration. In such cases, pure oxygen is transferred, usually

from compressed oxygen cylinders, liquid oxygen (LOX) or on-site oxygen generators, the latter two

being the most common source. Adding pure oxygen to water through conventional diffusers is only

about 40% efficient, and therefore costly. As such, several specialized components have been developed

to increase oxygen transfer efficiency to over 90%

Down-flow Bubble Contactor ― also referred to as a bicone or Speece cone, introduces both

water and oxygen at the top of the cone

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Figure 9 Down-flow Bubble Contactor

As the water moves down, the velocity is reduced until it is essentially equal to the upward velocity of

the bubbles, resulting in longer contact time and almost 100% oxygen transfer.

Diffusers (Diffused Aerator)

Increasing pressure in a flow stream is a cost effective way to increase transfer efficiency, and diffusers

accomplish this by burying a pipe vertically in the ground to a depth of at most 10 meters (33 feet), the

height of water required to add one atmosphere of pressure (14.7 pounds/in.2). The contact loop is

comprised of a pipe within a pipe, and the oxygen is introduced at the top of the loop.

Figure 10 Diffused Aerator

Aeration is usually provided to fish tanks using air diffusers. Diffused aeration systems provide low

pressure air from an air blower to some form of diffuser device at the bottom of a culture tank. These

diffusers produce small air bubbles within the tank that rise through the water column. Oxygen is

transferred to the water as the bubbles rise.

The smaller the bubbles and the deeper the tank, the more oxygen is transferred. For this reason, most

fish tanks are at least one meter (3.28 ft.) deep. Membrane diffusers seem the most promising for this

application.

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Low Head Oxygenation

Is required where the source water is only slight above the culture tank; it is often used in raceways

placed in series where the outflow of one raceway is just a few feed from the intake of the next. It is

made up of a perforated, horizontal distribution plate and multiple, vertical contact chambers. Oxygen is

introduced through to top of one contact chamber and exits into the adjacent one. The efficiency of this

component is dependent upon the length of water fall, flow rates of both water and oxygen, the DO of

the influent water, and the number of contact chambers. Packing the contact chambers with medium also

will increase transfer efficiency.

Carbon Dioxide Control and Removal

Carbon dioxide (CO2) is produced through the respiration of fish and bacteria and will accumulate

within recirculating systems if not removed at a rate equal to its production.

Elevated carbon dioxide concentrations are not greatly toxic to fish when dissolved oxygen is at

saturated levels in the tank. For most aquacultured fish, free carbon dioxide concentrations should be

maintained at less than 20 mg / L in the tank to maintain good fish growth conditions.

Carbon dioxide is usually removed through some form of gas exchange process. That is, either water is

exposed to air in a “waterfall” type of environment, or air is mixed into the water to remove excess CO2

gas using air stones or some other form of aeration (e.g., a surface aerator)

Packed Column Aerators:

One of the most common methods of CO2 removal in recirculating systems is the use of a packed

column aerator (PCA). This device is similar to a trickling biofilter tower consisting of a plastic media

and a water distribution device at the top of the reactor. The high carbon dioxide level lowers pH and

causes nitrifying bacteria to cease to function; resulting in a rise in nitrite or ammonia levels. A low

pressure air blower is usually used to introduce an upward air-stream from the bottom of the PCA to

remove CO2 from the column. Carbon dioxide is usually removed by blown air or by unpressurized

packed columns.

WASTE SOLIDS REMOVAL:

MECHANICAL FILTRATION

Fish are be fed on pelleted diets. These diets are made of protein, carbohydrates, fat, ash and moisture.

Large quantities of these feeds are not assimilated by the fish and produce an organic waste excreted as

fecal solids. Decomposition of solid wastes and uneaten or indigestible feed produce large quantities of

41

ammonia-nitrogen and consume significant amounts of dissolved oxygen as they decompose (BOD –

Biological Oxygen Demand). There are three categories of waste solids:

a) Settleable,

b) Suspended,

c) Fine or dissolved solids.

Each requires a different RAS component to eliminate or minimize impact on water quality

Settleable Solids Removal:

This is the first line of defense before settleable solids breakdown into less manageable suspended solids

or dissolved organics, and then ammonia. They are removed at the base of the culture tank or in a

settling tank (sump) immediately after water leaves the tank. Water is gravity fed to the sump because

any pumping would further breakdown particles to suspended solids. Properly placed bottom drains in

circular, or hex/octagonal, tanks with circular flow patterns, and minimal agitation will accumulate at

the bottom and removed. Depending on the flow rate in the tank, some solids can be removed from the

surface, while slower flows may result in accumulation at the bottom of the tank. Another method of

removal of settleable solids is to keep them in suspension (high flow or aeration/agitation) with an

exterior settling tank/basin.

An advanced design for removal of solids, is the ECO-TRAPTM, utilizes a plate spaced just above the

bottom of the tank where a small portion of the flow (5%) and settled solids leave via a separate flow

stream, while 95% of the water discharges through a large strainer mounted at the top of the particle

trap. Outside of the tank, the solids flow from the sediment trap enters a sludge collector; the waste

particles settle and are retained, while the clarified water exits the top of the collector. The advantage of

using an external settling basin is simplicity of operation, low cost of construction, low energy

requirements; their disadvantage are space requirements, cleaning requirements and water requirements

for cleaning

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Suspended Solids Removal

The next step in the water treatment process is the final removal of the remaining solids in the

flow stream. This includes various types of screen filters, or granular media filters. In most cases,

a pump is installed between component 1 and 2 to provide pressure to move water through fine

screens and media and to provide head for elevation to the next component. Foam fractionation

also occurs at this point, but it is usually not in this sequence of components .Typically, foam

fractionators are either located as a separate side stream loop or placed directly inside the culture

tank. Suspended solids are usually removed through mechanical filtration utilizing fine-mesh

screens or granular media.

Rotating microscreens

A rotating microscreen uses a drum shaped structure covered with a fine mesh screen. A typical

RAS microscreen will have a 20 to 60 micron mesh size that will catch all the settable solids and

the larger suspended solids. The drum is periodically rotated so the screen can be counter-flowed

with high pressure jets. Although somewhat limited in their ability to capture fine suspended

particles, rotating drum microscreens require very little floor space, do not need daily wash

downs, and operate at very low recirculating heads. This is the predominant solids removal

technology for larger freshwater RAS with moderate (<10 days) to low hydraulic retention times.

Figure 11Rotating microscreens

43

Microscreens can generate a substantial amount of backwash waters. Some RAS address this by

the use of circular tanks with a dual drain system and swirl separators to reduce the solids loads

on the microscreen. Foam fractionators can complement the unit by attacking the finer suspended

solids and colloids. This allows the wash interval to be lengthened so that water loss rates from

the facility can be reduced.

Foam Fractionation

Some of the “super” fine particles and dissolved organic compounds are not easily eliminated

from the culture system through mechanical filtration. A highly effective process for this purpose

is called “foam fractionation” (also air-stripping or protein skimmers). Simply, air is bubbled up

through a closed column resulting in foam at the surface. Dissolved organic compounds (DOC)

are physically absorbed by the bubbles, and fine solid particles become trapped in the surface

foam and can be removed.

Floating Bead Filters

Floating bead filters utilize a bed of static beads to capture solids from a recirculating flow.

Solids are captured as they pass through 12 to 36 inches (30 to 90 cm) of granular plastic beads.

When the bead bed fills with solids it must be backwashed by mechanical means (a propeller) or

pneumatic means (air bubbles). Once the bed is mixed, the solids are allowed to settle so a

concentrated sludge can be removed. They are highly efficient at the removal of particles down

to about 30 microns on a single pass, and in the typical RAS application, will remove all

suspended particles after multiple passes. Although floating bead filters support nitrification,

they are often used as a clarifier supporting moving bed reactors or fluidized beds. In RAS

applications, floating bead filters can be operated at relatively high flowrates (2000 lpm/m3) at

moderate pressure losses (0.5 to 5 pounds per square inch).

Biofiltration

The biofiltration process removes dissolved wastes from the water by bacterial action. Bacteria

can be either grown in suspension (suspended growth) or in a fixed film attached to a physical

substrate (a gravel, sand, or plastic media). The fixed film processes are more reliable and are

most often used to support RAS water treatment objectives. However, suspended growth also

offer successes in the production of shrimp and tilapia where the animals and bacteria are

cultured in the same tank.

44

The most important function of the biofilter is to remove dissolved organic material (sugars,

starches, fats, proteins) that are excreted by the fish. This is a very fast and efficient process that

is must be frequently considered in literature favoring the slower nitrification process. The

nitrification process is the conversion of the toxic nitrogen form, ammonia, to the relatively

nontoxic nitrate. A product of protein metabolism, ammonia, is excreted through the gills of fish.

Ammonia buildup in combination with a slightly elevated pH will rapidly prove fatal to most

fish. The nitrification process is absolutely dependent on the availability of oxygen. It is also

very sensitive to water’s alkalinity and pH.

Low cost of construction and simplicity of operation offset the lower conversion rate, making

this biofilters design popular for all sizes of operation.

Dynamic floating bead biofilters: Dynamic bead filters generally either employ 1/8 inch

shaped polyethylene beads or 3/64 inch (1 mm) styrene beads. The beads are constantly moved,

albeit sometimes at very slow rate, by either hydraulic or by air-induced circulation. Peak nitri-

fication rates are comparable to the fluidized bed reflecting the high surface area of the small

media. In particular, the styrene bead based systems are relatively inexpensive to fabricate, but

these systems have an inherent tendency to biofoul so attention to hydraulic design is critical.

Sizing for these units can be as high as a fluidized bed (1 to 2 pounds per cubic foot per day or

16 to 32 kg/m3/day).

Moving bed reactors

Moving Bed Reactors (MBR) use large (1/4 to 1/2 inch or 6 to 13 mm) plastic media and are

specifically manufactured to provide protected surfaces for fixed film bacteria. The media is

placed in a tank that is constantly aerated. These filters particularly have a robust design and are

relatively easy to design and operate. A significant amount of air must be injected into the bed to

keep it moving, thus the MBRs can make significant contributions to the RAS aeration and

degasification needs. The nitrification capacity of these filters tends to be only ¼ to ½of the

fluidized beds, with filters sized to support about 0.5 to 1 pound of feed ration per cubic foot of

45

media (8 to 16 kg/m3/day).

Figure 12 moving bed reactors

Additional System Components

Once the core processes in a RAS are addressed, additional components can be added to achieve

specific objectives or to compensate for particular issues that the application presents. Some of

the commonly included additions are discussed below.

Heating

There are two basic strategies for heating a RAS facility: heating building air space or heating

the water directly. In either case, the RAS building that receives heating must be equipped with

good insulation and a good water vapor barrier assuring condensation is properly diverted away

from sensitive building structures. Condensation always occurs at an interface between the warm

air in the building and the cold exterior air. This condensation interface will cause a constant

water drip that has been known to cause damage to many improperly designed RAS buildings.

Heating Coils: Direct heating of RAS waters is typically avoided to minimize scaling on

heated lines, a problem in even moderately hard waters. Water heating is generally accomplished

by polypropylene heating coils connected to a boiler or inline heating system. The heating coils

are placed in a sump or in the tank itself. A thermostat in the tank turns on the boiler whenever

the temperature falls below a set minimum and hot water is circulated through the coils. The

chemically treated heating water does not mix with the tank water, so scaling is not a problem.

46

Heating coils are an energy efficient means for thermal input. Although installation of heating

coils can complicate RAS designs, this approach should be given serious consideration.

A Moving Bed Reactor supported by a bank of UV lights on a marine fingerling system.

Air Space Heating: The more common way of heating a RAS is to heat the air space above

the tanks. This strategy uses readily available centralized heaters. The technology is well

understood and broadly serviceable. With some consideration for the high humidity of the

airspace; sizing and implementation is straight forward. However, any heating calculations

should take into consideration the heat loss associated with ventilation or water cooling that may

also be associated with carbon dioxide control strategies.

Greenhouses: In many southern states, greenhouses are a favored way of enclosing a RAS

operation. Greenhouses are well known for their ability to gather and hold solar energy. The

solar energy heats both the air and the water directly, dramatically reducing heating requirements

where winter temperatures are moderate. Greenhouse technology is well developed and the

criteria for heat balances are readily available. Modern plastic designs for dual layered covers

enhance insulation and are not impacted by the moisture. Most greenhouses, however, have to be

equipped with fans and/or swamp coolers to maintain production temperatures in the summer.

Supplemental heating systems may be required for the coldest days of the winter. The heat

balance for greenhouses becomes increasingly unfavorable the further the system is placed to the

north.

AMMONIA AND NITRITE-NITROGEN CONTROL:

Ammonia is the principal nitrogenous waste released by fish and is mainly excreted across the

gills as ammonia gas. Ammonia is a byproduct from the digestion of protein. An estimated 2.2

pounds of ammonia nitrogen are produced from each 100 pounds of feed fed. Bacteria in the

biofilter convert ammonia to nitrite and nitrite to nitrate, a process called nitrification. Both

ammonia and nitrite are toxic to fish and are, therefore, major management problems in

recirculating systems (Fig. 2).Ammonia in water exists as two compounds: ionized (NH4+) and

un-ionized (NH3) ammonia. Unionized ammonia is extremely toxic to fish. The amount of

unionized ammonia present depends on pH and temperature of the water (Table 2). Un-ionized

ammonia nitrogen concentrations as low as 0.02-0.07 ppm have been shown to slow growth and

cause tissue damage in several species of warm water fish. However, tilapia tolerate high un-

47

ionized ammonia concentrations and seldom display toxic effects in well buffered recirculating

systems. Ammonia should be monitored daily. If total ammonia concentrations start to increase,

the biofilter may not be working properly or the feeding rate/ammonia nitrogen production is

higher than the design capacity of the biofilter.

Biofilters consist of actively growing bacteria attached to some surface(s). Biofilters can fail if

the bacteria die or are inhibited by natural aging, toxicity from chemicals (e. g., disease

treatment), lack of oxygen, low pH, or other factors.

Biofilters are designed so that aging cells slough off to create space for active new bacterial

growth. However, there can be situations (e. g., cleaning too vigorously) where all the bacteria

are removed. If chemical additions cause biofilter failure, the water in the system should be

exchanged. The biofilter would then have to be re-activated (taking 3 or 4weeks) and the pH

adjusted to optimum levels. During disruptions in biofilter performance, the feeding rate should

be reduced considerably or feeding should be stopped. Feeding, even after a complete water

exchange, can cause ammonia nitrogen or nitrite nitrogen concentrations (Fig. 3) to rise to

stressful levels in a matter of hours if the biofilter is not functioning properly. Subdividing or

compartmentalizing biofilters reduces the likelihood of a complete failure and gives the manager

the option of “seeding” active biofilter sludge from one tank or system to another. Activating a

new biofilter (i.e., developing a healthy population of nitrifying bacteria capable of removing the

ammonia and nitrite produced at normal feeding rates) requires a least 1month. During this

activation period, the normal stocking and feeding rates should be greatly reduced. Prior to

stocking it is advantageous, but not absolutely necessary, to pre-activate the biofilters. Pre-

activation is accomplished by seeding the filter(s) with nitrifying bacteria (available

commercially) and providing a synthetic growth medium for a period of 2 weeks. The growth

medium contains a source of ammonia nitrogen (10 to 20mg/l), trace elements and a buffer

(Table 3). The buffer (sodium bicarbonate) should be added to maintain a pH of 7.5. After the

activation period the nutrient solution is discarded. Many fish can die during this period of

biofilter activation. Managers have a tendency to overfeed, which leads to the generation of more

ammonia than the biofilter can initially handle. At first, ammonia concentrations increase sharply

and fish stop feeding and are seen swimming into the current produced by the aeration device.

Deaths will soon occur unless immediate action is taken. At the first sign of high ammonia,

feeding should be stopped. If pH is near 7 the fish may not show signs of stress because little of

48

the ammonia is in the un-ionized form. As nitrifying bacteria, known as Nitrosomonas, become

established in the biofilter, they quickly convert the ammonia into nitrite. This conversion takes

place about 2weeks into the activation period and will proceed even if feeding has stopped. Once

again, fish will seek relief near aeration and mortalities will occur soon unless steps are taken.

Nitrite concentrations decline when a second group of nitrifying bacteria, known as Nitrobacter,

become established. These problems can be avoided if time is taken to activate the biofilters

slowly. Nitrite concentrations also should be checked daily. The degree of toxicity to nitrite

varies with species. Scaled species of fish are generally more tolerant of high nitrite

concentrations than species such as catfish, which are very sensitive to nitrite. Nitrite nitrogen as

low as 0.5 ppm is stressful to catfish, while concentrations of less than 5 ppm appear to cause

little stress to tilapia. Nitrite toxicity causes a disease called “brown blood,” which describes the

blood color that results when normal blood hemoglobin comes in contact with nitrite and forms a

compound called Methemoglobin. Methemoglobin does not transport oxygen properly, and fish

react as if they are under oxygen stress.

Fish suffering nitrite toxicity come to the surface as in oxygen stress, sharply reduce their

feeding, and In general at pH values of less than 7.8, most species will tolerate TAN

concentrations of 1 to 2 mg / L.

BIOLOGICAL FILTRATION

It is based on the oxidation of ammonia to nitrite, and finally the less toxic nitrate, Two groups of

bacteria are responsible for this conversion ― Nitrosomonas (ammonia) and Nitrobacter (nitrite

to nitrate). A substrate that has a high specific surface area (large surface area per unit volume)

provides an attachment site for the bacteria. Some common substrates include sand or gravel,

plastic beads, plastic rings, or plates. A primary consideration in the design of water treatment

systems for fish culture is to minimize the tank concentration of total ammonia nitrogen (TAN).

As fish are fed, TAN is continuously generated and must be either flushed from the tank or

removed by some unit process.

Trickling Filters ―This type of filter is comprised of media with a low specific surface area

(less than 330m2/m3 or 100ft2/ft3), allowing for large voids (air spaces) within the media.

Wastewater is delivered at the top of the filter, usually with a rotating distribution bar, and

gravity feeds through the media. Since the filter media in trickling systems are not submerged,

they not only provide biological filtration (nitrification) but also aeration and removal (or

49

degassing) of excess carbon dioxide (CO2). Trickling filters have a slightly higher efficiency 90

grams TAN/day/m3 of medium, but they are relatively large and expensive, given the high cost

of most filter media

Fluidized Bed Filters

These are essential mechanical sand filters operated continuously in the expanded

(backwashing) mode so that the sand media becomes fluidized. An upflow of pressurized water

to keep the sand grains in motion, and not in continuous contact with one another, providing an

excellent substrate for nitrifying bacteria that allows the entire surface for colonization. In most

cases, fluidized beds use a fine-grained sand (finer than typical mechanical sand filters), and in

some cases plastic beads have been used. Usually, fluidized sand filters are tall columns, which

minimize their footprint in the facility. Other advantages include the low cost of sand as a filter

media, compared to plastic beads, rings, etc., and its high efficiency of removing TAN.

Depending on the temperature, nutrient concentration, and size of the unit (and assuming 2.5%

of feed converts to TAN), a fluidized bed filter should have a design criterion of 20 – 40 kg of

feed/day/m3 of medium or 1.25-2.5/lb/ft3.

PH and Alkalinity

Fish generally can tolerate a pH range from 6 to 9.5, although a rapid pH change of two units or

more is harmful, especially to fry. Biofilter bacteria which are important in decomposing waste

products are not efficient over a wide pH range. The optimum pH range for biofilter bacteria is 7

to 8.The pH tends to decline in recirculating systems as bacterial nitrification produces acids and

consumes alkalinity, and as carbon dioxide is generated by the fish and microorganisms. Carbon

dioxide reacts with water to form carbonic acid, which drives the pH downward. Below a pH of

6, the nitrifying bacteria are inhibited and do not remove toxic nitrogen wastes. Optimum pH

range generally is maintained in recirculating systems by adding alkaline buffers. The most

commonly used buffers are sodium bicarbonate and calcium carbonate

The alkalinity of a recirculating fish production system should be maintained above 100 mg / L

as CaCO3 at all times.

50

DISINFECTION

An inherent disadvantage of RAS, as opposed to flow-through aquaculture systems, is the threat

of disease spreading throughout every tank in the system. Use of chemical or antibiotic

treatments can decimate the nitrifying bacteria living within the biofilter and the culture system.

An alternative to chemical treatments and a common disease preventative is continuous

disinfection of the recycled water using ultraviolet irradiation or ozonation.

Ultraviolet Irradiation ― the principle internal disinfection device is UV light. These lights

emit a light spectrum concentrated in the UV wave lengths that are deadly to microorganisms.

Disinfection rates are typically proportional to light intensity, which is related to the wattage of

the bulb used and the flow rate treated. Bacteria and other microorganisms are killed when

exposed to a sufficient amount of ultraviolet (UV) radiation. Therefore, the organisms living in

water that passes in close proximity to UV will die and the water sterilized. Typically, a UV bulb

(similar in design as a florescent light bulb) is housed in a quartz cylinder, which is then placed

inside the flow stream pipe (the bulb does not come into direct contact with the water). The

efficiency of UV irradiation is determined by:

a) The size of the organism

b) Proximity to the UV source (should be around 0.5 cm)

c) Level of penetration of the radiation through the water (influenced by turbidity)

d) Exposure time (flow rate relative to the length of the UV tube.

The advantages of UV disinfection is that it is safe and is not harmful to the cultured species,

nor does it affect the health of the bacteria within the biofilter. The main disadvantages are the

requirement for clear water with low suspended solids, the cost of the UV bulbs, and the need for

periodic replacement.

Ozonation ― Ozone (O3) gas is a strong oxidizing agent that has been used to treat municipal

water supplies for years. In aquaculture systems with high levels of dissolved and suspended

organic materials, the efficacy of ozonation may be limited. The efficiency of ozone to disinfect

is dependent upon contact time with the microorganisms and the residual concentration of ozone

in the water (after oxidizing all of those dissolved and suspended organics). Ozone is supplied by

an on-site ozone generator (due to its short life span – 10 to 20 minutes), and usually through an

51

external contact basin or loop. There, the exposure time can be adjusted to ensure sterilization

and any residual ozone is destroyed. Residual ozone entering the culture tanks is highly toxic to

crustaceans and fish; ozone in the air also is toxic to humans in low concentrations. Therefore,

great care should be taken to vent excess ozone outside the building and generating systems

properly installed.

Foam Fractionation

Foam fractionation is a technique used to remove surfactants from RAS waters. Surfactants are

chemicals that have a molecular end that is hydrophobic forcing the chemicals to the air-water

interface. These surfactants can cause RAS foaming problems. Surfactants are formed as part of

the protein degradation process so it is not uncommon to hear a foam fractionator referred to as a

“protein skimmer”. The foam fractionation process also strips fine solids and some dissolved

organics from the water. In freshwater the effectiveness of a foam fractionator is usually limited

by the availability of surfactants. Foaming action is also effectively suppressed by fish oil found

in many feeds. As a result, the performance of many freshwater foam fractionators is erratic, and

water quality benefits can be marginal. The technology works better in saltwater. In saltwater

small bubbles are easily formed and foam production is dependable. Foam fractionators are more

widely accepted in marine applications ranging from ornamental to food fish production.

Foam can be controlled with a simple foam fractionator design. A basic foam fractionator

consists of a section of PVC pipe with an air stone. The process can be improved by inducing a

counter-flow between the water and the bubbles. The most elegant commercial designs are

fabricated out of clear acrylic columns that are several feet high. These units are capable of

contributing to the removal of fine solids and refractory organic from the RAS waters.

Integrated Treatment

The Table below presents some varieties of RAS configurations that have been successfully

employed. In a typical design exercise, the holding capacity of the system (pounds of fish) is

defined, the peak daily feed ration is calculated, and then all components are sized to support the

peak daily feed load. A prudent designer will then multiple a uniform safety factor (for example

1.5) across all the major component sizing calculations.

With a little knowledge, it is relatively easy to develop a RAS system that will produce fish.

RAS treatment does not have to be complex.

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System Monitoring and Alarms

The proper management of fish production systems is essential to their profitability. For

recirculating production systems, because of the high fish stocking densities, close attention to

management details is critical to the success of the operation. Because of the intensity of

production and potential for rapid changes in water quality, certain parameters and system

operations must be closely monitored e.g. proper functioning of the pumps, pipes, heaters, pH

level, optimum temperature and ammonia level.

Production Management

The economic success of a recirculating system depends on keeping the productivity of the

system at or near the design maximum. However, there are tradeoffs between the maximum

sustainable fish growth rate and the maximum stocking rate for the system. If the system is

stocked beyond the design capacity and the fish are fed at the optimum feeding rate, the water

treatment system will become overwhelmed and the fish will grow slowly due to poor water

quality. On the other hand, under the same stocking conditions, if the operator is limited by the

water treatment capacity of the system, and feeds below the optimum rate for the fish population,

the fish will grow slowly to their market size.

Stocking Number and Density

In evaluating recirculating systems production capabilities, the unit most often used is maximum

tank or system stocking (kg/m3 or lbs. /gallon). However, in terms of production potential, this

unit of measure is meaningless. Fish can be held at very high stocking densities when fed only

enough to maintain their base needs. Underfed fish use less oxygen and produce less waste.

Therefore, the stocking rate of a system (fish/m3) and ultimate maximum fish density (kg / m3)

achieved within a tank should be defined by the maximum feed rate (kg feed / hr. or day) that the

system can accommodate without wasting feed and still maintain good water quality. This

maximum feed rate capacity will be a function of the water treatment system’s design, type of

fish being grown, and type of feed. More research should be performed to an efficient method to

determine the maximum feed rate.

53

Stock Management To maximize the production capacity of a recirculating system, keeping the stocking density

within the system near the maximum design capacity is important. This is attempted in

recirculating fish culture systems with a number of techniques. Fish are rarely stocked as

fingerlings and grown to market size in the same tank. This “batch culture” process does not

utilize the production capacity of the culture tank and water treatment system to its fullest

economic potential. Fish are usually grown to an intermediate size, then removed from the tank,

often graded into size classes, and moved to a larger tank or multiple tanks. Grading fingerlings

(juvenile fish) at least once in the growth process will reduce the tendency for large fish to out

compete small fish for feed.

In almost all cases, especially in systems with common water treatment systems, a

quarantine/nursery system for isolating fingerlings before introduction into the growth system is

recommended. The quarantine/nursery tank (Q tank) and water treatment components should be

physically isolated from the other tanks and water treatment systems. Time in the Q tank can be

used to grow the fish in a nursery environment, while being checked for diseases. The quarantine

period usually lasts three to six weeks while qualified personnel inspect and treat the fish for

diseases

Feeding Systems

Feed costs are usually the single largest variable expense in the production of fish.

Selecting the appropriate diet, pellet size, feed amount and feeding frequency is important to the

productivity and efficiency of the operation. The age and size of the fish will affect the diet

formulation and feed rate. Using high-quality, low-waste diets can improve water quality and

increase the maximum feeding rate that the recirculating system is capable of handling. Feeding

fish two or three times per day as much as they will consume in 15 to20 minutes (satiation) will

produce excellent fish growth. However, in recirculating systems, the addition of this much feed

at any one time may stress the water treatment system’s capacity to process waste and provide

oxygen. Most likely, the rate of oxygen consumption will exceed the system’s ability to add

oxygen to the water, and the DO concentration will decline in the tank. To avoid this, feeding

should be spread over a longer period of time. This usually is done with some type of automatic

feeding device called a “demand feeder” .Demand feeder’s train fish to activate a mechanism to

54

release feed. Fish learn to bump a probe that extends below the surface of the water when they

want to eat. Although demand feeders can provide for good fish growth, they are not problem-

free. If the tank DO concentration declines to dangerous levels, fish will come to the surface of

the tank in search of oxygen.

SIZING A TANK

Grow-out RAS Parameters

Total RAS water flow – 37,800 l/m (18,900 l/m to tanks and 18,900 l/m to biofilter)

Replacement water – 190 l/m

System cycle rate, tanks – 35 minutes

Water reuse rate – 99%

20% system volume replacement/day

Maximum feed load – 800 kg/day

Maximum fish density – 85 kg/m3

TANK VOLUME

Tank volume is a function of [stock of fish+ flow rate+ biomedium size]

As a general rule a minimum water volume of 3.8x 10-3M3-0.012M3is needed for every 454.5g of

fish reared and minimum water flow of 0.03785-0.095 M3per minute and more for tilapia are

needed to grow about 23,000 kilograms of warm water fish per year

Tank volume = (surface area of tank × uniform depth)

BASIC SIZING CRITERIA FOR BIOFILTERS USED IN RECIRCULATING

AQUACULTURE SYSTEMS

Biofiltration is defined as:

A technique for pollution control technique using living material to capture and biologically

degrade process pollutants. A filtration method that uses bacteria to break down waste by means

of the nitrogen cycle or an emission control device that uses microorganisms to destroy volatile

organic compounds and hazardous air pollutants.

In its most basic form, all biofilters perform the same function; the removal of toxic TAN from

system water.

55

Biofilters are typically sized based on either the volumetric TAN conversion rate (g TAN

m-3 filter d-1), or the Areal TAN Conversion Rate (g TAN m-2 filter d-1).

There is no universally accepted methodology for sizing and comparing biofilter

Data collection and research

Data collection on Ammonia, Nitrate, Nitrite and pH is usually done three times a week i.e.

Mondays, Wednesdays and Fridays using Water Test Kits to check on the extreme conditions

which can affect the effective growing conditions of the fish in various culture tanks.

Temperature is regularly checked by using the thermometer and it should be always maintained

at 28oc. any temperature beyond this is referred to as an extreme temperature which should not

be allowed because it can lead to fish death.

Table 1 Percentage of total ammonia at differing pH values and temperatures.

Percentage of total ammonia in the un-ionized form at differing pH values and

temperatures.

Temperature (oC) pH 16 18 20 22 24 26 28 30 32

7.0

0.30 0.34 0.40 0.46 0.52 0.60 0.70 0.81 0.95

7.2 0.47 0.54 0.63 0.72 0.82 0.95 1.10 1.27 1.50

7.4

0.74 0.86 0.99 1.14 1.30 1.50 1.73 2.00 2.36

7.6

1.17 1.35 1.56 1.79 2.05 2.35 2.72 3.13 3.69

7.8

1.84 2.12 2.45 2.80 3.21 3.68 4.24 4.88 5.72

8.0

2.88 3.32 3.83 4.37 4.99 5.71 6.55 7.52 8.77

8.2

4.49 5.16 5.94 6.76 7.68 8.75 10.00 11.41 13.22

8.4

6.93 7.94 9.09 10.30 11.65 13.20 14.98 16.96 19.46

8.6

10.56 12.03 13.68 15.40 17.28 19.42 21.83 24.45 27.68

8.8 15.76 17.82 20.08 22.38 24.88 27.64 30.68 33.90 37.76

56

9.0

22.87 25.57 28.47 31.37 34.42 37.71 41.23 44.84 49.02

9.2

31.97 35.25 38.69 42.01 45.41 48.96 52.65 56.30 60.38

9.4

42.68 46.32 50.00 53.45 56.86 60.33 63.79 67.12 70.72

9.6

54.14 57.77 61.31 64.54 67.63 70.67 73.63 76.39 79.29

9.8

65.17 68.43 71.53 74.25 76.81 79.25 81.57 83.68 85.85

10.0

74.78 77.46 79.92 82.05 84.00 85.82 87.52 89.05 90.58

10.2 82.45 84.48 86.32 87.87 89.27 90.56 91.75 92.80 93.84

Table 2Recommended water quality requirements of recirculating systems.

Table: Recommended water quality requirements of recirculating

systems.

Component Recommended value or range

Temperature

optimum range for species cultured - less

than 5o F as a rapid change

Dissolved oxygen 60% or more of saturation, usually 5 ppm or more for

warm water fish and greater than 2 ppm in biofilter

effluent

Carbon dioxide less than 20 ppm

pH 7.0 to 8.0

Total alkalinity 50 to 100 ppm or more as CaCO3

Total hardness 50 to 100 ppm or more as CaCO3

Un-ionized ammonia-N less than 0.05 ppm

Nitrite-N less than 0.5 ppm

Salt 0.02 to 0.2 %

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CALCULATING AMMONIA LOADING

The amount of ammonia excreted into the tank depends on a number of variables including:

The species

Sizes of fish

Stoking densities

Environmental conditions (temperature, pH.)

Ammonia loading can be roughly estimated from the biomass (weight) of fish in the tank or it

can be based on the weight of feed fed each day.

On the average about 25mg (milligrams) of ammonia per day is produced for every 100grams of

fish in the tank. Therefore:

100grams of fish= 25mg/day

Therefore if the tank containing N striped bass fingerlings each weighing 75g

Total weight of fish=weight of each fingerling ×No. of each fingerling

The total ammonia load produced by all the fish would be

= [𝑇𝑜𝑡𝑎𝑙 𝑤𝑒𝑖𝑔ℎ 𝑜𝑓 𝑓𝑖𝑠ℎ (𝑔) × 25]

100

To remedy excessively high ammonia levels:

Add fresh water

Control of overstocking

Ammonia loading can also be estimated based on the total amount of feed. For manufactured fish

feed with standard protein levels of 30 to 40%

Ammonia loading =total weight of the feed (in grams) × 25

For example, if the fingerling stripers are fed 1 pound (454 grams) of pelleted feed per day, the

amount of ammonia produced per tank would be about 11,350 mg per day.

Biofilter Sizing:

The biofilter in any RAS design must be sized to correspond with the other system components.

Important factors that must be considered in designing a biofilter are:

media surface area (square feet of surface for bacteria attachment),

ammonia leading (ounces of ammonia that need to be converted per day per square foot

of media area), and

58

Hydraulic loading (gallons of water per day per square foot media surface).

Types of Biofilters:

Biofilters can be configured in many ways. The two general categories are (1) submerged bed

filters and (2) emerged bed filters. Submerged bed filters can have fixed (immobile) media in

which the water flow can be upward, downward or horizontally through the media.

Recirculation Rates (turnover times):

Recirculating systems generally replace less than 10 percent of the total system volume per day

with new water. As such, not enough water can be flushed through the tank to control the build-

up of harmful ammonia-nitrogen. To counteract this, the water treatment system of a

recirculating production system must have a unit process designed to either remove or convert

ammonia-nitrogen to a less toxic nitrogen compound.

The recirculation rate (turnover time) is the amount of water exchanged per unit of time.

This can easily be determined by dividing the volume of water in the tank by the capacity of the

pump (M3 / minute).

The turnover rate in volume tank system calculated with a water pump rated at P= M3 per second

(M3 per day) would be:

=V/P tank volume per day

Where V→ volume of the tank

Where P→ pump rating per day

Increasing the number of turnovers per day would provide:

Increased biofiltration,

Greater nitrification (bacterial contact), and

Reduced ammonia levels

For example, the turnover rate in a 2,500 M3 tank system circulated with a water pump rated at

45 M3per minute (64,800 M3per day) would be 25.92 tank volumes per day (a rate of slightly

more than one volume per hour).

Most fish production recirculation systems are designed to provide at least one complete

turnover per hour (24 cycles per day).

A good supply of water, adequate in both quality and quantity, is essential to fish farming

enterprise. Ground water obtained from deep wells or springs is the best source of water for fish

59

farming because it is free from pollutants and relatively high hardness levels, which are

beneficial under some circumstances

Other water sources particularly surface water from rivers, lakes, streams and ponds are not

recommended for fish culture because they may contain fish diseases, parasites, pesticides and

other pollutants which can kill or slow down the growth of fish. Testing the quality and quantity

of the available water supply is one of the first steps for a prospective fish farming to take place

to ensure an adequate supply of high quality water

Because RAS recycles most of their water, they consume considerably less than other types of

culture and are especially well adapted to areas with limited water supplies. The required

quantity of water needed to grow fish varies with species of fish selected, size of the culture

system and investment size

Table 3: Field Data values

RAS

components

Dimensions(m) Other parameters description

Growout tanks R=0.5m,H=1.2m Biofilter sizing Ammonia removal

rate 0.65 gm-2

Nursery tanks R=0.3m,H=1.2m average water velocity 42 cm s-1

Hatchery tanks R=0.3m,H=1.2m culture tanks intake

pipe diameter

6 inch (15.24 cm)

Settling tanks R=0.25m. H=0.8m differential height 3M

Broodstock

tanks

R=0.45m,H=1.2m Pump efficiency of 80%,

stocking rate 98 kgm-3

60

5.0: CALCULATION, ANALYSIS AND DESIGN

Tank size, number of fish and amount of water required Height of the tank, h= 1.2 m

Diameter of the culture tank, d= 1.0 m

Volume of the tank

V =πd2ℎ

4

Using the height of water in the tank= 1.2;

V=0.9425 m3 (volume of a single tank)

Total volume of 12 culture tanks= 11.31 m3

For the determination of the number of tilapia to be in the12 culture tanks:

Assuming a stocking rate 98 kgm-3 and the size of each tilapia fish is 150 g; the mass of fish in

the culture tanks can determined as follows;

If 1 m3= 98 kg.m-3;

Then for 11.31m3= 98 kg.m-3*11.31 m3= 1108kg.

Therefore the number of tilapia fish in the culture tanks; N is;

𝑁 =𝑚𝑎𝑠𝑠 𝑜𝑓 𝑓𝑖𝑠ℎ 𝑖𝑛 𝑡ℎ𝑒 𝑐𝑢𝑙𝑡𝑢𝑟𝑒 𝑡𝑎𝑛𝑘(𝑘𝑔)

𝑚𝑎𝑠𝑠 𝑜𝑓 𝑒𝑎𝑐ℎ 𝑓𝑖𝑠ℎ(𝑘𝑔)

𝑁 =1108(𝑘𝑔)

0.15(𝑘𝑔)

N= 7386.67 tilapia (take as 7386 tilapia)

Biofilter sizing

Assuming an average ammonia production rate 10 g per 45.3592 kg per day and

Ammonia removal rate 0.65 gm-2 of biofilter;

Ammonia production rate;

NH removal can be determined as follows;

NH removal = mass of the fish in the tanks(kg) ∗ ammonia production rate(g/kg)

Where mass of fish= 1108kg

Therefore,

NH removal =244.27 g of fish tanks per day

The required biofilter surface area, BSA can be calculated a follows;

61

BSA =𝑎𝑣𝑒𝑟𝑎𝑔𝑒𝑎𝑚𝑚𝑜𝑛𝑖𝑎 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒

𝑎𝑚𝑚𝑜𝑛𝑖𝑎 𝑟𝑎𝑡𝑒 𝑟𝑒𝑚𝑜𝑣𝑎𝑙

244.27 g

𝟎. 𝟔𝟓 𝐠𝐦 − 𝟐

BSA = 𝟑𝟕𝟓. 𝟖 𝐦𝟐

Taking one-inch sheet rings have a specific surface area 216.54 m2/m3, the biofilter volume; BV

can be determined as follows;

BV =𝑏𝑖𝑜𝑓𝑖𝑙𝑡𝑒𝑟 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎(𝑚2)

𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎(𝑚2/𝑚3)

375.8(𝑚2)

216.54(𝑚2/𝑚3)

Therefore, BV= 1.735m3

Water flow rate

Taking the average water velocity (V) needed to maintain self-cleaning properties to be 42 cm s-

1 or 2520 cm per minute. If the culture tanks intake pipe diameter is 6 inch (15.24 cm) then we

can determine the flow rate (Q) as follows;

Q=AV

Where, A is area of the culture tank intake pipe.

A=π

4d2

A=183.41cm2

Therefore, Q= 183.41cm2 *2520 cm minute-1 =459685cm3 minute-1 or 0.459685m3 min-1 or

27.58m3 per hour.

Pump hydraulic power

Given the flow rate is 27.58m3 per hour, density of water 1000 kg/m3, gravitation pull 9.8 m/s2,

differential height of 3M and assuming an efficiency of 80%, the pump hydraulic and shaft

power can be determined using an online pump power calculator.

H=3m

g=9.8m/s2

Q=27.58m3h-1

Ƞ=80%

Water density (d) =1000 kg/m3

62

Power= d gHQ/ efficiency

= [(1000x9.8x27.58x3)/.8x3600]=281.54w

=.281kw

Pump hydraulic power was found to be .281kw

Pump shaft power on the other hand was found to be 0.08 kW. Two pumps were however

required, one for the biofilter and the other one for mechanical filtration pumping.

5.1 OPTIMUM STOCKING

Calculating Stocking Density

It is important for a culturist to know how many organisms he has to accurately stock culture

systems. Correctly stocked systems can maximize productivity and profit. Too often, a culturist

will overstock ponds or tanks in an effort to maximize productivity. Overstocked systems can

lead to reduced growth, high incidence of disease, and high predation; understocked systems

increase production costs and sometimes lead to overfeeding. Since the culturist cannot

reasonably count the number of animals to stock into a tank or pond, he must rely on

estimating the population using a sub-sample. Sub-sample can be measured

volumetrically, usually by measuring water displacement, or by weight.

EXAMPLE,

You are going to stock seed clams into growout bags at a density of 5,000 clams/bag. Since we

can’t reasonably count that many seed,

1) Count out 50 clam seed (each about the size of a dime),

2) Place 100 ml of water into a 250 ml graduated cylinder,

3) Add the clams to the water and read the new water level (amount of displacement)…let’s say

it is now 145 ml…therefore, 50 clam seed had a volume of 45 ml,

4) Calculate the volume to achieve 5,000 clams.

Using a proportion:

45/50 = X/5,000 50X = 45 x 5,000

50 X = 225,000

63

Therefore X = 4,500 ml or 4.5 liters.

This calculation could also be accomplished using weight of clams measured with an electronic

balance.

A sample question

A wastewater treatment facility has three primary clarifiers available for use. They are all

circular clarifiers with a radius of 3.05M and a depth of 2.44M the design engineer wants you to

maintain a primary clarification detention time of about 3.5 hours. How many tanks will you

need to use if the plant flow rate is approximately M3/day?

Calculation

The best equation to help here is the Detention Time formula. By using that formula, you will be

able to determine the detention time of one tank. Once you know that, you can figure out if you

need only one tank, or two, or all three.

Detention Time of one tank = (Vol in m3) (0.028.31) (24hrs/day)

Flow in M3/day

First, figure out the volume of a single primary clarifier:

Vol in M3 = (π) (R2) (H) = (π) (3.05)2 (2.44) = 71.31 M3

That means that the Detention Time of one tank = (71.31 M3) (0.028.31) (24hours/day)

48.45 M3/day

Detention Time of one tank = 3.52 hrs.

So if the detention time of one tank is 3.52 hours, and the designer wants only 3.5 hours of

primary clarification detention time, that means you only need one tank.

Size of the UV sterilizer Using table 2 and having a flow rate of 27.58m3 per hour and a tank volume of 11.31 m3 we can

conclude the size of a UV sterilizer to be 114watts.

64

6.0 CONCLUSION AND RECOMMENDATION

6.1 CONCLUSION

The general objective of the study was to design a re-circulating aquaculture system. Some

applications of the fish pond aerations were also incorporated in the RAS systems designs. The

various system components were also designed and their specifications come up with, which was

one of the design requirements.

Therefore the objectives of the design were successfully attained as per was the expectation from

the objective section

. Comparison of any fishpond and a RAS shows that RAS has very many advantages:

to maximize production on a limited supply of water

low land requirements,

ability to control water temperature

ability to control water quality

independence from adverse weather conditions

nearly complete environmental control to maximize fish growth year-round

Maintenance practices done in RAS include:

Monitoring temperature, pH, ammonia and oxygen levels

Flushing away mechanical filters to avoid clogging of the filters

Flushing water in tanks whenever the ammonia level is high and the pH. level is to the

extreme beyond control

6.2 RECOMMENDATION

The following recommendations are therefore made, based on the current aquaculture policy

status:

Aquaculture policy

a) Even though the draft National Fisheries Policy has a section on aquaculture, it is

necessary to develop an aquaculture policy in parallel to a more general fisheries

management policy. An aquaculture policy is specifically necessary because it will

directly address issues of food security in line with the Strategy for Revitalization of

Agriculture (SRA). Usually, the Ministry responsible for fisheries is responsible for

65

developing such a policy but it is desirable that a very wide stakeholder’s consultation is

carried out during the policy development. The Aquaculture Policy will address a wide

range of issues including the lack of policy direction of cage culture in natural water

bodies (sea ranching), certification and investment plans. This policy will not only

address policy concerns but also provide a framework for stimulating rapid development

in aquaculture by recognizing the critical input sector, technological sector, extension and

marketing.

b) Once the aquaculture policy is put in place, there would be need to harmonize various

sections of legislation to avoid overlap, contradictions and conflicts. For example, export

of aquarium fish is subject to live fish movement permit and aquarium fish dealers’

license under the current Fisheries Act while when it comes to certification for export, it

is the veterinary department who is responsible.

c) The Public Health Act and the Environmental Management and Coordination Act

(EMCA) are already in conflict when it comes to wetlands, standing pools of water and

their utilization. While an envisaged aquaculture policy and act would encourage the

development of standing waters (ponds and other facilities) for fish farming, the Public

Health Act considers these as a health nuisance and hazards that should be drained and

disinfected and EMCA prohibits the use, drainage or utilization of wetlands for either

personal or commercial purposes.

d) Any public funding of RAS projects should include detailed scrutiny of plans by a

multidisciplinary team of independent (and appropriately experienced) experts.

Aquaculture development

i. It is clear from the existing information the development of aquaculture in Kenya has

been slower than expected due to lack of inputs and financial partnerships. Since there

are new prospects of financing aquaculture projects be microfinancial institutions and

commercial banks such as Equity, this development should be streamlined into the

Aquaculture Policy by the Ministry of Fisheries Development.

ii. However, there could be other major players in the financial sector that include the

Treasury and Central Bank in Kenya that have statutory control over financial institutions

in Kenya. The inclusion of these relevant institutions in drawing the policy would ensure

that there are provisions of exemptions for inputs wherever required.

66

iii. Support for research and pilot-scale projects should be encouraged.

iv. Developing private-public partnerships is an option that is currently gaining popularity in

the development agenda in Kenya. Several options are available to foster this option

v. Involve and encourage nascent young growing commercial aquaculture producers or

could be a community aquaculture development project in joint funding applications for

research development projects with academic and research institutions such as Moi

University, Egerton University and KMFRI among others (e.g. USAID-KBDS Baitfish

Cluster Development). This approach will guarantee not only positive research findings

on key constraint to production and marketing but also for a constructive partnership

between researchers and producers and improve needs driven capacity of research

institutions.

vi. Partner with large scale commercial fish farmers through production agreements in the

form of out-growers such as practiced in the tea, sugarcane and some rice schemes in the

country. This approach requires that contracted out-growers are provided with inputs at a

cost and the cost is recovered at the time of delivery. The large scale farmer would need a

business plan for financing this approach and this could be a possible source under long-

term investment plans other than a simple business plan.

vii. Some of the existing trust lands could be allocated to existing development agencies on

request for the express purpose of aquaculture. This would require a policy framework

and involvement of the Ministry of Lands (Commissioner for Land).

viii. Some existing Government facilities that are essentially used as demonstration could be

upgraded into commercial farm level by a group of entrepreneurs so that they run the

farms on a commercial basis on lease. There would be several conditions to fulfil such as

developing a business plan and obtaining financial security for such an undertaking. This

would guarantee that the centers are used for the intended purpose of demonstration but

the emphasis shifted to large scale commercial production i.e. they pay for themselves.

These facilities could be run on partnership with government agencies for research,

extension and production.

67

Human resource (Extension)

Extension has been identified as one of the constraints in aquaculture development, it would be

appropriate to consider strengthening this area by:

1. Formation of target groups and farmer-to-farmer clusters with the ultimate goal of

developing a critical mass of fish farmers able to move aquaculture to commercial level.

2. Organizing field days for farmers with demonstration centers for better technology

transfer

3. Training clusters of fish farmers in aqua-business in line with the upgrading of

demonstration centers for the same purpose

I would recommend that a pilot project for Aquaculture Recirculation system be conducted so

that there a better understanding of the real conditions of study. An alternative would be to install

Recirculation Aquaculture system for a particular farmer who will be instructed on how to go

about the whole process as it gives researchers an ideal conditions of conducting their research.it

would also show the running cost and the general suitability from farmers’ point of view

68

7.0 REFERENCES 1. Buck, P. D. (2014). Land based recirculation systems. Retrieved hmarc 9, 2014, from

(http://www.awi.de/de/forschung/neue_technologien/marine_aquaculture_maritime_technologies

_and_iczm/research_themes/marine_aquaculture/land_based_recirculation_systems/la

2. Illora, I. M. (2008). Hydrodynamic characterization of aquaculture tanks and design criteria

for improving self-cleaning properties. Castelldefels: technical university of Catalonia.

3. Inc, D. A. (2012). UV Size chart. Retrieved March 21, 2014, from http:// definitive-

aquarium.com/tools/uv_size_chart.html

4. Industries, D. O. (2008). Best practice environmental management guidelines of primary

industries. Victoria: Department of Primary Industries.

5. Leschen, I. A. (2011). Case study on developing financially viable Recirculation Aquaculture

Systems (RAS) for tilapia production in Egypt: Technology transfer from the Netherlands.

Alexandria: Egyptian Aquaculture Centre and Institute of Aquaculture, University of Stirling.

6. Libey, H. a. (2007). Fish farming in recirculating aquaculture systems (RAS). Virginia:

department of fisheries and wildlife sciences, Virginia technology.

7. Quality, A. a. (2000). National water management strategy. Canberra: Australia and New

Zealand Environmental and conservation council.

8. Rakocy, J. E. (1989). Tank Culture of Tilapia. Texas: The Texas A & M University System

service, n. d. (1993). Farming tilapia. Retrieved 3 31, 2014, from http://www. thefishsite.com/

articles/13/farming-tilapia

9. Toolbox, T. E. (2012, may 12). Pump power calculation. Retrieved March 25, 2014, from

http://www.engineeringtoolbox.com/pumps-power-d_505.html

10. Wikipedia. (2013). rotary vacuum- dry filter. Retrieved March 28, 2014, from http://

www.wikipedia.org/wiki/Rotary_vacuum-drum_filter

69

Bibliography Creswell, R.L. 1990, Aquaculture Desk Reference, Van Nostrand Reinhold. New York,

New York.

Biofiltration‐Nitrification Design Overview, James M. Ebeling, Ph.D., Environmental

Engineer Aquaculture Systems Technologies, LLC New Orleans, LA.

Burrows R.E, and Chenoweth H.H, 1955, Evaluation of three types of rearing ponds, US

Department of the Interior, Fish and Wildlife Service, Research Report.

Davidson, J., Summerfelt, S., 2004, Solids flushing, mixing, and water velocity profiles

within large (10 and 150 m3) circular ‘‘Cornell- type’’ dual-drain tanks, Aquaculture Eng.

Kepenyes, J. and A. Ruttkay, 1983, Water requirement of fish production. In International

Conference on water management and production potential in agriculture. Szarvas Hungary, pp.

90-100

Krause, J. et al., 2006. Design guide for REcieculation Aquaculture System, Glassboro, New

Jersey: Rowan University.

Losordo T.M., M.P. Masser, and J.E. Rakocy. 1999, Recirculating Aquaculture Tank

Production Systems: A Review of Component Options, SRAC Publication Number 453, USDA,

Washington, D.C. USA.

Losordo, T.M. 1997, Tilapia Culture in intensive recirculating systems Pages 185-208 in: B.

Costa-Pierce and J. Rakocy (eds.) Tilapia Aquaculture in the Americas, Volume 1. , World

Aquaculture Society, Baton Rouge, Louisiana.

The organic farmer Fish farming requires knowledge and passion, Nr. 91 December, 2012

by Peter Kamau for Agricultural Finance Corporation.

70

8.0 APPENDICES Appendix A

TOP VIEW

Figure 13 top view of a RAS System

ISOMETRIC VIEW

Figure 14 Isometric view of a RAS System

SIDE VIEW

Figure 15 Side view of a RAS System

71

Appendix B

Figure 16 particle trap

ECOTRAP particle trap is a double-drain that concentrates much of the settleable

solids in only 5% of the water flow.

Figure 17 the sludge collector

The sludge collector that works in conjunction with ECOTRAP to remove

settleable solids from the flow stream

72

Figure 18 sludge collector

The sludge collector that works in conjunction with ECOTRAP to remove

settleable solids from the flow stream

Figure 19 A cut-away and expanded mid-section of a drum filter

73

A cut-away and expanded mid-section of a drum filter to remove waste solids from

an RAS.

Figure 20 A trickling biological filter

A trickling biological filter utilizes non-submerged filter media that receives wastewater evenly

distributed through a rotating distribution bar.

Figure 21 UV sterilizer

74

Table 4Recommended stocking and feeding rates for different size groups of tilapia in tanks and estimated growth rates.

Table 5 Illustrative RAS Core Treatment Configurations.

Type Circulation Clarifier Biofilter Aeration CO2

Removal

Marine

Fingerling

Airlift Bead Filter In tank aeration

and airlifts

Cool water

Growout

Axial Flow

Pumps

Microscreen Moving

Bed

Reactor

Pure Oxygen by

hooded

agitation

Moving

Bed

Reactor

Warm

water

Growout

Centrifugal

Pumps

Bead filter Moving

Bed

Reactor

In tank aeration

Coldwater

Growout

Centrifugal

Pumps

Microscreen Fluidized

Sand Bed

Pure Oxygen

Speece Cone

Packed

Column

Marine

Broodstock

Centrifugal

Pumps

Bead Filter Fluidized

Sand Bed

Packed Column

75

Table 6 Bill of Quantities of scale-up RAS farm

Investment Cost Quantity Unit Price

KSH

KSH

1 Building and Utilities

(1200m2)

1 1800000 1800000

2 growout Tanks 50cm

radius

12 2500 30000

3 Settling Tanks 25cm

radius

8 1500 12000

4 nursery Tanks 30cm

radius

8 1800 14400

4 hatchery Tanks 30cm

radius

6 1500 9000

6 Brood stock Tanks

45cm radius

6 2000 12000

4 Storage Tanks

10000lt

3 40000 120000

5 Pumps 1kWh 2 5000 10000

0.75kWh 5 45000 22500

0.5kWh 1 3500 3500

Submersible 0.5kWh 2 2000 4000

6 Oxygenator-Quad 40 2 4000 8000

7 Sandfilter-Triton 60 6 1500 9000

8 Biofilter 10 10000 20000

9 Aerator and LHO 6 3000 18000

10 Pipes and valves

Of various diameters

Lump

sum

120000 120000

11 Generator -Voltmaster

15kW

1 120000` 120000

12 Water Quality

Equipment

Lump

sum

20000 20000

13 Office Equipment Lump

sum

300000 300000

Total 2652400

76

Table 7 Operation cost and assumption of scale-up RAS farm

Investment Cost KSH

monthly

KSH

Annually

1 Fixed Cost *

2 Salary 8000 96000

3 Administration 1000 12000

4 Maintenance 2000 24000

4 Others 500 6000

Total fixed cost 138000

6 Variable Cost*

4 Juvenile 30,000 pcs

x KSH.4/pc

10 300000

5 Feed FCR 1.2 x

RM3.5/kg

120000

6 Electricity-

4800kWh/month 5000 60000

7 Water 1000 12000

8 Marketing and

Transportation

3000 36000

Total variable cost 528000

77

Table 8 A table of UV sterilizer sizing (Inc, 2012)

Freshwater UV Sterilizer Chart

Watts M3 30,000

μw/cm²

(EOL)

m3PH

Pond w/ Partial Sun

cubic meter | m3per

hour

8 0.189-0.757 2.43 5.68 3.79

15 0.757-1.89 2.65 7.57 6.81

25 1.89-4.54 4.45 15.14 7.57

40 4.45-5.68 10.98 22.71 11.37

57 5.68-11.36 12.11 24.61 12.30

80 8.33-16.66 13.92 34.07 17.06

114 8.33-16.66 14.76 35.95 17.08

120 16.66-22.71 15..54 45.42 24.61

160 22.71-32.18 20.44 60.57 34.07

200 32.18-49.21 24.98 75.71 37.85

240 49.21-64.35 27.25 94.63 45.42

78

General layout of a Rrecirculating Aquaculture System

Figure 22General layout of a Recirculating Aquaculture System

79

Figure 23 Typical Building Layout

80

Figure 24A 3D layout of a Recirculating Aquaculture System

81

WORK SCHEDULE (GHANT CHAT)

week Number (15/ September/ 2014 - 28/

May/ 2015)

SEP OCT JAN FEB MAR APR MAY

1 obtaining

project ideas

2 write my first

concept note

3 write second

concept note

4 Cost estimation

5 submit project

proposal

6 proposal

presentation

7 Acquiring

Components

8 Project design

& simulation

9 project

implementation

10 Testing

11 Error handling

12 Project

submission