General considerations regarding atmospheric and biological models for aquaponic production systems 1. Atmospheric and water data model for… [621364]

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TABLE OF CONTENTS

Part I
General considerations regarding atmospheric and biological models for
aquaponic production systems
1. Atmospheric and water data model for aquaponics production system …….3
2. Biological model ……………………………………………………………………….15
2.1. General considerations …………………………………………………………15
2.2. The concept of aquaponics modeling for a recirculating aquaculture
system ……………………………………………………………………………… 20
3. References…………………………………………………………………………… 28

Part II
Start -up guide for recirculating integrated systems, based on aquaponics
techniques
1. Aquaculture production systems classification ……………………….33
2. Recirculating aquaculture systems SWOT analysis …………………………..35
3. Examples of configurations for recirculating integrated systems which uses
aquaponics techniques ………………………………………………………………36
4. Processes in recirculating integrated aquaculture systems , based on
aquaponics techniques ……………………………………………………………..38
4.1 Sediments removal ……………………………………………………………………38
4.1.1 Introduction ………………………………………………………………………38
4.1.2 Gravitational separation …………………………………………………………39
4.1.3 Mechanical filtration ……………………………………………………………..42
4.1.4 Physico -chemical processes …………………………………………………..50
4.1.5 Applications for solid particle control systems ……………………………….55
4.1.6 Ratings of dif ferent mechanical filters …………………………………………57
4.2 Biological Filtration …………………………………………………………………58
4.2.1 Introduction ………………………………………………………………………58
4.2.2 Biofiltration and nitrification ……………………………………………………..58
4.2.3 Configuration of the nitrification filter …………………………………………..59

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4.2.4 Factors influencing the biofiltering process …………………………………….65
4.2.5 Ratings of different biological filtration ……………………………………………73
4.3 Aeration and Oxygenation …………………………………………………………74
4.3.1 Introduction ……………………………..………………………………………..74
4.3.2 Gas transfer ……………………………………………………………………….74
4.3.3 Ratings of different Aeration and oxygenation ……………………………………80
4.4 Carbon Dioxide Control ………………………………………………………………81
4.4.1 Introduction ……………………………………………………………………….81
4.4.2 Carbon balance and carbon dioxide control by pH management ……………83
4.4.3 Carbon dioxide control by gas exchange ………………………………………84
4.4.4 CO2 control by gas transfer combined with kinetics of chemical reactions …86
4.4.5 Ratings of different Carbon dioxide control ………………………………………87
4.5 pH Control ………………………………………………………………………………88
4.5.1 Introduction ……………………………………………………………………..88
4.5.2 Alkalinity and pH control ………………………………………………………..88
4.5.3 Nitrification ………………………………………………………………………..89
4.5.4 Management of alkalinity and pH ……………………………………………….90
4.5.5 Ratings of different pH control ……………………………………………………..92
4.6 Water Disinfection M ethods ……………………………………………………….93
4.6.1 Introduction ……………………………………………………………………..93
4.6.2 Chlorination ………………………………………………………………………94
4.6.3 Thermal treatment ……………………………………………………………….95
4.6.4 Ultraviolet Radiation Treatment …………………………………………………96
4.6.5 Ozone ………………………………………………………………………….106
4.6.6 Ratings of different water disinfection …………………………………………113
4.7 Aquaponics ………………………………………………………………………….114
4.7.1 Introduction …………………………………………………………………….114
4.7.2 Media grow beds ………………………………………………………………115
4.7.3 Deep water culture (DWC) ……………………………………………………117
4.7.4 Nutrient film techni que (NFT) …………………………………………………119
4.7.5 Lighting ………………………………………………………………………….120
4.7.6 Ratings of different aquaponics …………………………………………………121
5. Water quality in RAS systems ……………………………………………………122
6. References ……………………………………………………………………………133

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Part I
General considerations regarding atmospheric and biological models for
aquaponic production systems

1. Atmospheric and water data model for aquaponics
production system

Biological significance of environmental parameters

INTRODUCTION
Agricultural and livestock activities are c onsidered the biggest consumers of
fresh water. Estimations reveal that 85% of the global fresh water consumption is for
agriculture (Hoekstra and Chapagain, 2007) and nearly one -third of the total water
footprint of agriculture in the world is used for li vestock products (Mekonnen and
Hoektra, 2012).
In the last 30 years, the increase in the income of the population in developing
countries, led to an increase in fish consumption from 25.0 to 104.3 million ton fish per
year (FAO, 2014). Due to the depletion of marine resources the FAO predicts that in
the future the supply of fish for the population will be entirely dependent on fish
production in aquaculture systems.
The increased demand for fish, water and fertilizer for crop production and the
concerns ab out environment and health are motivations to test innovative farming
systems such as “aquaponics” as viable systems for sustainable fish and crop
production (FAO, 2014).
Aquaponics has ancient roots. Aztec cultivated agricultural islands known as
chinampa s in a system considered by some to be the first form of aquaponics for
agricultural use (Boutwelluc, 2007 and Rogosa, 2013) where plants were raised on
stationary islands in lake shallows and waste materials dredged from the chinampa
canals and surroundin g cities were used to manually irrigate the plants (Boutwelluc,
2007 and Rogosa, 2013). Also, South China, Thailand, and Indonesia who cultivated

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and farmed rice in paddy fields in combination with fish are cited as examples of early
aquaponics systems (FA O, 2001). These aquaponic farming systems existed in many
far eastern countries, in USA, and Canada.
Recent advances by researchers and growers alike have turned aquaponics
into a working model of sustainable food production. The integration of fish and pl ants
results in a polyculture that increases diversity and yields multiple products.
Aquaculture development as a whole in the country in combination with
production technology, favorable socioeconomic condition and culture environment
has already proven s uccessful in terms of increasing productivity, improving
profitability and maintaining sustainability (Toufique and Belton, 2014).
Aquaponics is, farming technique in which water from aquaculture is used to
grow crops and extra water returns back to the fi sh tank. When this water circulated
near root zone, nitrogen fixing bacteria (manly nitrosomonas and nitrobactor) convert
ammonia (NH 4+) into nitrite (NO 2-) and then to nitrate (NO 3-) form. By these, plants get
nutrients as fertilizer and nitrates been less toxic to fish; fish grows better than normal
aqua farming. By this integration of fish farming and agriculture, one can get maximum
output.
Fishes produces nitrogenous compound mainly ammonia which is hazardous
to fish, even in small quantities and to xicity increases in relation to pH and temperature
in the water column. On the other hand, Nitrosomonas bacteria break down ammonia
to NO 2- and Nitrobacter convert the nitrite into nitrate which is food for the plants. By
contrast, NO 3- is less harmful to fish. Decaying organic matters can help to fertilize
ponds, at the same time provides good environment for growing plants which are less
prone to disease unlike soil. Raft aquaponics is one of the ways to use aquaculture
site for vegetable production and c an help to overcome nutritional demand for the
growing population.
Green leafy vegetables with low to medium nutrient requirements are well
adapted to aquaponic systems, including lettuce, basil, spinach, chinese cabbage,
chives, herbs, and watercress ( www.backyardaquaponics.com ).
The selection of plant species in aquaponics system is important. Lettuce,
herbs, okra and especially leafy greens have low to medium nutritional requirements
and are well suitable to aquaponics system. Plants yielding fruits like tomato, bell
pepper and c ucumber have higher nutritional requirement and perform better in a
heavily stocked and well established aquaponics system (Adler et all, 2000).

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Research conducted at University of Florida showed that cucumber crop can be
successfully adopted with aquaponi cs system. This is estimated that 45.300 Kg of
fish will produce sufficient nitrogen for 4050 lettuce or 540 tomato plants when they
are fed with 3 % of their body weight.
Freshwater fish are the most common aquatic animal raised using aquaponics,
although freshwater crayfish and prawns are also sometimes used (Drive, 2006).
A few fish species are adapted to recirculating aquaculture which includes tilapia, trout,
perch, arctic char and bass. Most commercial aquaponics system in North America is
based on ti lapia. Furthermore, tilapia is tolerant of fluctuating water conditions such as
pH, temperature, oxygen and dissolved solids (Rakocy, 1999). Tilapia is the fish
species which is very hardy, can tolerate wide range of environmental parameters, can
live with versatile of feed and are fast growthing fish species (Salam, M.A., 2012).

MATERIALS AND METHODS
The hydroponic greenhouse production system requires a high degree of
environmental control including supplemental lighting and moveable shade to provide
a target amount of light which, in turn, results in a predictable amount of daily growth.
Computer technology is an integral part in the production of hydroponic. A computer
control system should be used to control the abiotic environment. Different sensors a re
used to monitor greenhouse environment parameters. These parameters include
temperature of greenhouse air and nutrient solution, relative humidity and carbon
dioxide concentration of greenhouse air, light intensities from sunlight and
supplemental light ing, pH, Dissolved Oxygen (DO) levels, and Electrical Conductivity
(EC) of the nutrient solution. Sensors will communicate the environmental conditions
to the control computer which will activate environmental control measures such as
heating, ventilation, and lighting.
Atmospheric and water data model for aquaponic production system:
• Temperature
• Relative Humidity
• Carbon Dioxide or CO 2
• Lights
• Dissolved Oxygen
• pH

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• Electrical Conductivity
Temperature controls the rate of plant growth. Generally, as temperature s
increase, chemical processes proceed at faster rates. Most chemical processes in
plants are regulated by enzymes which, in turn, perform at their best within narrow
temperature ranges. Above and below these temperature ranges, enzyme activity
starts to d eteriorate and as a result chemical processes slow down or are stopped. At
this point, plants are stressed, growth is reduced, and, eventually, the plant may die.
The temperature of the plant environment should be kept at optimum levels for fast
and succes sful maturation. Both the air and the water temperature must be monitored
and controlled.
The relative humidity (RH) of the greenhouse air influences the transpiration
rate of plants. High RH of the greenhouse air causes less water to transpire from the
plants, which causes less transport of nutrients from roots to leaves and less cooling
of the leaf surfaces. High humidities can also cause disease problems in some cases.
For example, high relative humidity encourages the growth of botrytis and mildew.
The CO 2 concentration of the greenhouse air directly influences the amount of
photosynthesis (growth) of plants. Normal outdoor CO 2 concentration is around 390
parts per million (ppm). Plants in a closed greenhouse during a bright day can deplete
the CO 2 concentration to 100 ppm, which severely reduces the rate of photosynthesis.
In greenhouses, increasing CO 2 concentrations to 1000 -1500 ppm speeds growth.
CO 2 is supplied to the greenhouse by adding liquid CO 2. Heaters that provide carbon
dioxide as a by -product exist but we do not recommend these because they often
provide air contaminants that slow the growth.
Light measurements are taken with a quantum sensor, which measures
Photosynthetically Active Radiation (PAR) in the units μmol/m2/s. PAR is the li ght which
is useful to plants for the process of photosynthesis. Measurements of PAR give an
indication of the possible amount of photosynthesis and growth being performed by
the plant. Foot -candle sensors and lux meters are inappropriate because they do n ot
directly measure light used for photosynthesis.
Dissolved oxygen (DO) measurements indicate the amount of oxygen available
in the pond nutrient solution for the roots to use in respiration. If no oxygen is added to
the pond, DO levels will drop to nearl y 0 ppm. The absence of oxygen in the nutrient
solution will stop the process of respiration and seriously damage and kill the plant.
Pure oxygen is added to the recirculation system in the ponds. Usually the level is

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maintained at 8 (7 -10, no advantage to 20) ppm. For sufficiently small systems, it is
possible to add air to the solution through an air pump and aquarium air stone but the
dissolved oxygen level achieved will not be as high as can be achieved with pure
oxygen.
The pH of a solution is a measur e of the concentration of hydrogen ions. The
pH of a solution can range between 0 and 14. A neutral solution has a pH of 7. That
is, there are an equal number of hydrogen ions (H+) and hydroxide ions (OH -).
Solutions ranging from pH 0 -6.9 are considered ac idic and have a greater
concentration of H+. Solutions with pH 7.1 -14 are basic or alkaline and have a greater
concentration of OH -.The pH of a solution is important because it controls the
availability of the fertilizer salts.
Electrical conductivity (EC) is a measure of the dissolved salts in a solution. As
nutrients are taken up by a plant, the EC level is lowered since there are fewer salts in
the solution. Alternately, the EC of the solution is increased when water is removed
from the solution through the processes of evaporation and transpiration. If the EC of
the solution increases, it can be lowered by adding pure water, e.g., reverse osmosis
water). If the EC decreases, it can be increased by adding a small quantity of a
concentrated nutrient stock solution.
Water quality parameters such as NH 4+/NH 3+, NO 3-, NO 2-, PO 4-, pH and
dissolved oxygen were measured fortnightly using test kits. Number of flowers, fruits
and fruits weight were recorded. All the sampling data were recorded in the Microsoft
Exce l 7 for analysis.
The production of the lettuce crop is started in a germination area where they
germinate and grow for 11 days. They should be shaded from full sun on the first day
after germination, but can then be exposed to full light (17 mol/m2/d) or slightly greater.
On Day 11, the plants are transported to the greenhouse and transplanted into the
pond area where they are grown until re -spacing on day 21 and finally harvested on
Day 35.
Lettuce will grow satisfactorily at a DO level of at least 4 ppm.
A pH of 5.8 is considered optimum for the described lettuce growing system,
however a range of 5.6 -6.0 is acceptable. Nutrient deficiencies may occur at ranges
above or below the acceptable range.

Letuce atmospheric and water data model for hidroponic pr oduction system:

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• Air Temperature: 24 0C Day/19 0C Night (75 0F/65 0F)
• Water Temperature: No higher than 250C, cool at 260C, heat at 240C
• Relative Humidity: minimum 50 and no higher than70%
• Carbon Dioxide: 1500 ppm if light is available, ambient (~390 ppm) if not
• Light: 17 mol m -2 d-1 combination of solar and supplemental light
• D O: 7 mg/L or ppm, crop failure if less than 3 ppm
• pH: 5.6 -6 upH
To understand the environmental condition of pond, the physico -chemical
parameters of water were needed to be measured. The main parameters including
temperature, pH, dissolved oxygen, nitrate and ammonia were measured before
starting the experiment.
For determine the growth parameters, length, weight and number of leaves and
branches were taken into consideration . The percent gain of growth parameters of the
aquaponic plan were measured using the following formula.

Final stage – Initial stage
%gain = –––––––––––––- x 100
Initial stage

The recorded data were entered into the spreadsh eet in MS Excel 2010 and
then summarized properly before statistical analysis. After entering the data, the
descriptive statistical analyses were done by MS Excel.

RESULTS AND DISCUSSION
Aquaponics water data model
Aquaculture production depends on physical, chemical and biological qualities
of pond water to a greater extent. The successful pond management requires an
understanding of water quality. Intensification of pond makes the water quality
undesirable with a number of water quality parameters. Pond water quality is largely
defined by temperature, transparency, turbidity, water color, carbon dioxide, pH,
alkalinity, hardness, unionized ammonia, nitrite, nitrate, primary productivity, biological
oxygen demand and plankton population (Bhatnagar an d Devi, 2013).

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The accepted level of ammonia should be under the range of 0.05 to 0.10 mg/l
(Shoko et al., 2014) and above range it is toxic to the cultured fish (Francis -Floyd et
al., 2009).
According to Mizanur et al. (2004), intensive aquaculture ponds sediments has
various fertilizing components such as nitrogen, phosphorous, sulphur etc. which are
very useful for growth and production of aquaponic plants. Moreover, water spinach is
an efficient plant having clustered roots that can absorb nutrients fro m the water very
efficiently (Kibria and Haque, 2012).
The length -weight relationship of water spinach depends on the fertility of media
from where nutrients are supplied. The plant’s length -weight relationship is attributed
to a variety and concentration of nutrients, of which nitrogen is the dominating factor.
Waste water of stinging catfish ponds supplied various nitrogenous components of
which ammonia has considerable fertilizing supports to the plant under floating
condition on the pond surface (Kibria and Haque, 2012).

Aquaponics biological data model
Aquaponics is an integrated and intensive fish -crop farming system under
constant recirculation of water through interconnected devices. It is considered a
promising technology, which is highly productiv e under correct set up and proper
management (Lal 2013; Orsini et al., 2013). First, fish feed is eaten by fish and
converted into ammonia (NH 3+). Some ammonia ionizes in water to ammonium (NH 4+).
Then, bacteria ( Nitrosomonas ) convert ammonia into nitrite (NO 2-) and consequently
bacteria ( Nitrobacter ) oxidize nitrite into nitrate (NO 3-) (Tyson et al., 2011). Finally, the
water delivers nutrients and oxygen to promote plant growth. Graber and Junge (2009),
found similar yields b etween hydroponic systems and aquaponics systems. Finally, it
is important to establish systems under “smart water” use and to balance nutrient
concentrations in water to ensure maximum fish and plant growth.
Aquaponics is considered a method where water a nd nutrients are efficiently
used and maintained within the system (Liang & Chien, 2013). In aquaponics it is
possible to reduce daily water loss to 2% of the total water volume of the system. Due
to the constant recirculation of water it is also possible to maintain evenly distributed
high nutrient concentrations in the water (nitrate) as the small addition of water to
compensate the daily loss will not dilute the nutrients (Rakocy, 2006). The “water

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smart” approach makes aquaponics an alternative systems to produce food under
sustainable practices in areas where water is scarce (Essa et al., 2008).
Developing an accurate and practical tool to predict plant and fish growth and
monitor nutrient concentrations in water, will improve the adoption and implement ation
small or commercial scale of aquaponic systems as urban farming or as a business
model for household food security.
The objective of this model is to test and predicts plant and fish growth and net
ammonium and nitrate concentrations in water in an a quaponic system. This is done
by comparing the model outputs with measurements under controlled conditions in
order to assess the accuracy of the tool to simulate nutrient concentrations in water
and fish and plant biomass production of the system.
An accu rate prediction of the model, with low radiation and temperature in our
experiment compared to the experiments with which the model was calibrated, will
enhance the value of the tool to monitor nutrient concentrations in water and the ability
to determine crop and fish production under diverse environmental conditions.
Aquaculture production can potentially cause environmental pollution due to the
nutrients content in the water discharged to the soil, underground water and other
water sources (Edwards, 2015 ). By adding the plant component, the nutrients
concentrated in the water will be taken up through the plant roots and enhance plant
growth, reducing the need of fertilizer. Furthermore, the constant recirculation of water
through interconnected devices, m aintains and delivers resources such as nutrients
and water to all system components. Finally, the fact that aquaponics systems do not
need soils, makes them suitable to be built in small household areas in developing
countries or within the cities as urba n farming (De Bon, 2010).
The nutrients such as nitrogen in particular, start the flow from feed intake by
the fish and excretion into the water. The feaces are rich in ammonia (NH 3) and
dependent on parameters such as pH and temperature, this is partly or completely
converted into ionized ammonium (NH 4+). The combination of both forms is referred to
as total ammonia nitrogen (TAN) (Francis -Floyd et al., 2010). In reality, water is filtered
through bio -filters containing bacteria which nitrify the TAN into nitrite (NO 2-) and
afterwards into nitrate (NO 3-) according to the following equations:
Equation 1: NH 3+ + H 2O ↔ NH 4+ + OH-
Equation 2: NH 4+ + 1.5O 2 => NO 2- + 2H+ + H 2O
Equation 3: NO 2- + 0.5 O 2 => NO 3-

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At high pH the balance in equation 1 lies at the left (NH 3+) and with low pH at
the right (NH 4+). Both active forms of nitrogen (NH 4+ and NO 3-) can be absorbed by the
plant, with preference to nitrate as the active form to enhance plant growth (Andriolo
et al., 2006). Additionally, waste water from fish cont ains macronutrients such as
phosphorous, potassium and micronutrients such as iron that are important through
the growing cycle of the crop (Diver, 2006). Nevertheless, previous studies report
nutrient deficiency in plants grown in the aqua pool after the use of commercial fish
feed over long periods (Roosta, 2014). Therefore, addition of amendments such as
Iron (Fe) is a common practice to supply the nutrient deficit caused by the fish feed.
Additionally, water exchange is adapted according to nutrients concentrations to avoid
any toxicity (ammonia) and salinity such as sodium (Na) above 50 mg/l, and to
minimize denitrification (Ako and Baker, 2009).
In aquaponics systems the ratio between fish feed delivered per day and the
area covered by crops is essential to provide enough nutrients for plants and avoid
toxicity levels from nitrate and ammonia for fish. Additionally, accurate amounts of fish
feed per day will avoid accumulation of organic matter in the systems, reducing
potential denitrification sources by anaerobic conditions (Seawright et al., 1998).
Rakocy et al. (2006) established a ratio between 60 -100 g feed per m2 of crop area f or
leafy crops such as lettuce, spinach, basil and cabbage.
The approach of aquaponics can also be seen as a weakness of the system.
The susceptibility of the fish to chemical compounds such as pesticides, increases the
complexity of aquaponics and forces it to rely on integrated pest management practices
to avoid any negative effect on the crop yield. Finally, according to the definition of
Lehman et al., (1993) aquaponics are considered sustainable food production systems
which do not compromise any natur al resource and are free of any potential harmful
chemical for humans and the environment (Somerville, 2014).
Fish are dependent on the external temperature to regulate their metabolic
functions and rate of activity that affect feed intake, digestion and o xygen consumption.
However, fish have a range of temperatures they can tolerate according to species
(warm and cold water fish). Therefore, assessing the environmental conditions
(temperature) of a region is essential to choose the fish species to be produ ced.
Aquaponics systems are complex and sensible food productions system which
demand daily maintenance and monitoring. Furthermore, parameters such as water
temperature, dissolved oxygen (DO), pH, and nutrient levels must be monitored

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frequently to avoid inefficient performance (Bernstein 2011). Rakocy et al. (2006)
recommended DO concentration in water of 6 mg/L in order to provide enough oxygen
for plants, fish and bacteria. Moreover, a high ammonia concentration in water is toxic
for fish, decreasing th eir growth (feeding and digestion) and eventually can cause
death. Therefore, the suggested maximum co ncentration of ammonia (1mg/L ) should
be maintained in water (Ebeling et al. 2012). Additionally, The European Inland Fishery
Advisory Commission (EIFAC) established a maximum ammonia (NH 3+) concentration
of 0.25 mg/L in water. Nitrate concentrations should be kept lower than 50 mg/L to
avoid negative effect in the fish immune system and prevent algae bloom in the system
in order to avoid the reduction in t he oxygen concentrations in water (Watson and Hill,
2006). Finally, other macro and micronutrients are not often measured individually in
aquaponics. Any concentration above 200 mg/L of total dissolved solid in water should
be avoid (Rakocy et al. 2006).
In order to avoid accumulation of toxic compounds in water such as ammonia
and nitrite, which can cause fish death, it is necessary to decompose these compounds
into more favorable compounds and promote the growth of plant and fish. The main
purpose of the bacteria ( Nitrosomonas and Nitrobacter ) present in the bio filter is to
convert (TAN), essentially the -ionized fraction (NH 3), into nitrate (NO 3-). Additionally,
the larger the amount of oxygen and ammonium present in the water, the higher the
expected ni trification rate (Lucas and Southgate, 2003).
The productivity of water plants and the quality of water for culturing are
influenced, among others, by the stocking density ratio in the culture media, in order to
achieve an ideal combination for a useful bi ological control. The results of fish
metabolism in forms of ammonium (NH 4+), nitrate (NO 3-) and phosphate can be utilized
by the water plants to reduce the percentage of nitrogen in the media (Rakocy 2007),
as well as to increase the growth level of the w ater plants.

Aquaponics growing model
Lettuce is the most common leafy crop grown in aquaponics systems, due to its
low nutrient demand and short growing cycle (5 weeks). Moreover, the constant
recirculation of water in aquaponics systems provides a permanent supply of nutrients
to the root zo ne and therefore no depletion on nutrients is visible (Tyson et al., 2011).
Seginer (2003) suggests to maintain a pH between 7.5 and 8.0 to promote nitrification
and availability of nutrients as phosphorous, calcium and magnesium. Nevertheless,

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Rakocy et a l., (2006) state that such high pH values affect negatively the solubility of
essential micronutrients such as iron, manganese, copper, zinc and boron. Therefore,
a pH between 6.5 and 7.0 is acceptable for the three main components of the
aquaponic system (plant, fish and bacteria).
Lettuce is considered a cold season crop and temperatures below 7˚C and
above 25˚C will result in physiological disorders and unmarketable quality products.
Providing the crop with a proper environment will achieve greater biomass production.
Based on the volume of crops produced, it appears that the productivity of the
plant in the aquaponics system is lower compared to hydroponics. According
Bittsanskzky et al. (2016), most plant nutrients were at significantly lower
conce ntrations in the research aquaponics systems as compared with the standard
hydroponic solutions. The differences were highest in the case of Fe 2+ and Mn 2+ (with
ratios of 68.5 and 138.7, respectively). Fish density also affects the availability of
nutrient s for plants in aquaponics system. The results of the study Villarroel et al (2011)
states that the total amount of feed required per mEq at the low biomass (2 kg fish/m3)
ranged from 1.61 -13.1 kg for the four most abundant ions (NO 3-, Ca 2+, H2PO 4- and K+),
suggesting that it is feasible to integrate fish culture (at low densities) to reduce the
cost of the hydroponic solution supplementation for strawberries. The nutritional
requirements vary with variety, life cycle stage, day length, and weather conditio ns.
However, the use of lettuce and kale in this study had an appropriate selection.
Provided that the system is stocked with enough fish, it is not necessary to add
nutrients for plants with short cropping cycle which do not produce fruits (e.g. lettuce).
In contrast to, for example, lettuce, tomatoes which need to bear fruit, mature and
ripen, need supplemental nutrients.
The results showed that the frequency of crops is higher than the fish harvest.
This is in line with the Love et al (2015) found that f ish production was less profitable
than production plant in the aquaponic units analyzed. That may help to explain why
we found more than fish production plant production in the current study. Also plant
growing cycles are shorter, and the area used for th eir production tends to be larger
than for fish. According to Hepher and Pruginin (1981), this decline of dissolved oxygen
and increase of ammonium in the water are caused by the number and size of the
cultured fish. Boyd and Linchtkoppler (1982) explained that the higher the stocking
density ratio, the higher the demand for oxygen due to the increasing number of fish.

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Along with the increase in fish weight, the oxygen consumption rate and
metabolism waste of each fish also increase. The decrease in the dis solved oxygen
concentration may further lead to fish’s loss of appetite which stunts the fish growth.
Tilapia in farms can tolerate lower concentrations (ranging from 3 to 4 mg/L), the
optimum levels of dissolved oxygen are higher, and so the desirable ran ge is usually
above 5 mg/L (Pillay 2004). Meanwhile dissolved oxygen that sufficient for the growth
of the catfish larvae is above 1 mg/L (Durborow et al. 1985).
Treatment with water plants can heighten the concentration of dissolved oxygen
because, in the daytime, the plants are undergoing assimilation and add oxygen to the
water. At night, the plants use the oxygen available in the water for respiration. As a
result, the increase of dissolved oxygen concentration due to the treatment with water
plants wil l further improve the appetite of the fish that in turn leads to better fish growth.
Dissolved oxygen in water can affect the activity of tilapia and metabolism in the body
of the fish (Ardita et al. 2015).
Water plants ensure the nitrification process run s well due the interaction among
fish, water plants and nitrifying bacteria. Protein coming from the feed is decomposed
into a simple compound by converter bacteria, such as Nitrosomonas that can convert
ammonia into nitrite and Nitrobacter that converts n itrite into nitrate; this nitrate is
further used by the water spinach and lettuce as a nutrient, thus balancing the content
of nitrogen in the aquaponics system (Graber and Junge 2009). This is a good thing
for us because nitrate happens to be the favorit e food of plants. Also the fish will
tolerate a much higher level of nitrate than ammonia or nitrite (Blidariu and Grozea
2011). In this condition, the quality of water remains adequate to support fish growth
through the good use of feed.
In terms of water quality, and the concentrations of salts and minerals needed
for the production of sweet basil (or general guidelines), Racozy (2003) noted: ‘Our
general guideline is to feed fish at a ratio of 57 grams per m2 of plant growing area per
day. This ratio pro vides good nutrient levels. We supplement with equal amounts
calcium hydroxide and potassium hydroxide to maintain pH near 7.0. Every three
weeks we add 2 mg/L of iron in the form of a chelated compound. In a commercial –
scale aquaponic system at UVI (Unive rsity of the Virgin Islands) that was to produce
lettuce continuously for 2.5 years, nutrient concentrations varied within the following
ranges (mg/L) that would have produced excellent sweet basil growth’:

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According to Effendi (2003) phosphorus in the for m of phosphate is a necessary
macronutrient and very essential for aquatic organisms including aquatic plants. Lack
of phosphate may stunt the growth of phytoplankton which may further affect the
aquatic balance in the water (Bahri 2006). At the end of the study, the treatments with
water plants result in a lower concentration of phosphate, since the phosphate has
been used as a nutrient by plants. On the other hand, in the control treatment without
water plants, the phosphate concentration increases due to it not being used by plants.
In a low water exchange, the accumulation of phosphate, As and Cu led to higher
mortality and reduced larvae length and body weight in the culture of carp (Okemwa
2015). Phosphorus is one of the essential nutrients due compoun d will be absorbed by
phytoplankton and get into the food chain (Hutagalung and Rozak 1997).
Orthophosphate is a form of phosphorus that can be directly utilized by aquatic
plants, while polyphosphate orthophosphate should be reduced first before being use d.
Maintain the levels of phosphorus in the aquaponics system is very important because
if its levels are too high can cause algae or microorganisms in the water to grow
uncontrolled in aquaculture ponds. The deficiency of phosphorus will cause some
signs of deficiency include slow growth and leaf stem (Diver 2006).

2. Biological model

2.1 General considerations
A major global trend nowadays is urbanization and its degree reached 70% in
various European countries (dos Santos 2016). The challenges brought by this trend
include population rise, food insecurity and climate change (dos Santos 2016).
According to FAO (2014) aquaculture is one of the fastest -growing food production
sector, which provides approximately 50% of fish and fish products for human
consum ption. However, traditional aquaculture production in natural ponds have a
negative impact on the environment, due to the use of high amount of fresh water and
the load with nutrients of waste water (Suhl et al., 2016). Aquaculture integrated
systems (such as aquaponics) have many advantages like enhanced productivity,
environmental sustainability and water use efficiency (Cerozi and Fitzsimmons, 2017).

16
In developing countries aquaponics may resolve issues such as water scarcity, soil
degradation and climat e-related challenges (Wongkiew et al. 2017; Shete et al. 2016).
Aquaponics is the fusion of recirculating aquaculture and soilless vegetable production,
within a complex closed -loop system (Cerozi and Fitzsimmons, 2017). Being a strictly
controlled system, it combines a high level of biosecurity with a low risk of disease and
external contamination (FAO 2016). Aquaponics integrates aquaculture and
hydroponic techniques in a one merged system. It is a soilless agriculture system that
synergistically combines aquaculture and hydroponics (Wongkiew et al., 2017). The
aquaponics system has the capability to raise fish in high densities, produce profitable
vegetables, reduce the use of pesticides and chemical fertilizers, minimize water
exchange and sustain an ade quate water quality (Cerozi and Fitzsimmons, 2017).
According to dos Santos (2016) aquaponics could represent a new integrated
agricultural system from producers to consumers in an integrated manner, due to the
short supply chains and organic fresh food. A s well, aquaponics could relieve
environmental pressure by the double use of water and nutrients and increased profit
by producing two crops (Suhl et al., 2016). This technological method has the potential
for higher yields of produce and protein with less labor , less land, fewer chemicals and
a fraction of the water usage (FAO 2016). The integration of the aquaculture and
hydroponic systems reduces water discharge into the environment (Bailey and
Ferrarezi, 2017). Therefore, aquaponics is environmental fri endly, suitable for
agriculture practice and it is expected to become widely used for sustainable food
production in the future (Forchino et al., 2017).
Water is one of the most important medium to understand in an aquaponic
system and the main water quali ty parameters are dissolved oxygen, pH, water
temperature, nitrogen compounds and electrical conductivity ( Thorarinsdottir 2015 ).
Temperature
Plants have different temperature requirement, such as 15 -19°C for salads and
higher temperature and humidity need for tropical plants. In terms of optimum
temperature for the nitrifying bacteria, the acceptable temperature range is 17 -34°C.
Nitrobacter group is less tolerant to lower temperature, compared to the Nitrosomonas
group, therefore during colder periods nitrite should be monitored more carefully to
avoid harmful accumulations ( Thorarinsdottir 2015).

17
Dissolved oxygen
To obtain good fish growth, dissolved oxygen levels should be maintained at
saturation and at least above 5 mg/L. Low dissolved oxygen can cause potentially
irreversible damage to fish gills and reduce the efficiency of the nitrifying bacteria
(Thorarinsdottir 2015).
pH
Plants prefer a pH< 6.5 and nitrifying bacteria perform optimally at pH> 7.5 The
ideal pH value for the aquapo nic system ranges between 6.8 -7.0 ( Thorarinsdottir
2015).
Nitrogen compounds
All four forms of nitrogen (NH 3, NH 4+, NO 2-, NO 3-) can be used by plants and
stimulate growth, however the form quickly absorbed by plants is nitrate
(Thorarinsdottir 2015).
Macro – and micronutrients
Plants cultivated in an aquaponic system need several nutrients that ar e
required for the enzymes that facilitate photosynthesis for both growth and
reproduction ( Thorarinsdottir 2015). The nutrients are categorized as macro – and
micronutrients. T he 6 macronutrients are: nitrogen (N), phosphorus (P), potassium (K),
calcium (Ca), magnesium (Mg) and sulphur (S). The range of micronutrients is much
bigger and it includes iron (Fe), copper (Cu), boron (B), manganese (Mn), molybdenum
(Mo) and zinc (Zn) (Thorarinsdottir 2015).
In the young stages of plants, particularly during the plant’s vegetative growth
and before fructification, nitrogen need is high. During maturity, the nitrogen need
decreases in order to avoid difficulties to blooming and fall of young fruits
(Thorarinsdottir 2015). Excess of nitrogen fertilization makes also plants more pron e
to pests and diseases, due to the tenderness of the vegetable tissues ( Thorarinsdottir
2015). The yellowing of older leaves is a main indicator that the system lacks N. In
plants, nitrogen can be reallocated within plant tissues because it is a mobile element.
In case of N deficiency, N gets transferred from older leaves to new growth areas,
which is the reason why N deficiency can be mainly observed in old leaves

18
(Thorarinsdottir 2015). The limit of N in the aquaponic system ranges between 150 –
1000 PPM and the average is 250 ppm.
In the aquaponic system, plants require optimum conditions in comparison to
light, oxygen, carbon dioxide, pH, temperature and nutrients ( Thorarinsdottir 2015).
The aquaponic technology can produce different vegetable crops but in general , leafy
vegetables grow well with the abundant nitrogen in the system, have a short production
period, and are in high demand. Even though fruiting crops have longer production
periods and produce less marketable yield, their value is often higher than the value of
leafy produce ((Bailey and Ferrarezi, 2017). The main plant species grown so far in
aquaponic systems is lettuce, which has been grown under different densities (16 to
44 plants/m2) and crop lengths (21 -28 days) ( Thorarinsdottir 2015).
Phosphorus deficiency causes stunted plant growth, whereas phosphorus
excess may lead to antagonistic interactions with micronutrients, especially zinc
(Cerozi and Fitzsimmons, 2017).
Cerozi and Fitzsimmons (2017) observed in their study that plants are
negatively affected by the high concentration of nitrite in the water. When plant based
diets were fed to fish in aquaponics systems, low amounts of dissolved orthophosphate
were produced in the nutrient solution, which constrained growth and quality of lettuce
(Cerozi and Fitzsimmons, 2017).
In order for plants to fulfill their metabolic requirements, usually aquaponics
nutrient solutions are added in the technological water as dissolution . Cerozi and
Fitzsimmons, 2017 applied the necessary nutrients as foliar spraying in order to avoid
the unwanted interaction between these supplements and dissolved phosphorus, such
as the formation of insoluble salts (magnesium and iron phosphates).
A pe rcent of 50% of the phosphorus input is retained by the fish, while 20%
occurs in the particulate form and 30% is dissolved as orthophosphates (d'Orbcastel
et al., 2008).
According to Cerozi and Fitzsimmons (2017) lettuce growth has a direct effect
on the phosphorus dynamics in aquaponics systems and in the timeline between first
day of transplanting and ten days after the lettuce experienced a growth lag, which
allowed a linear increase of dissolved phosphate in the aquaponics solution. At day 21
after tra nsplanting the lettuce reached exponential phase, therefore a dramatic
decrease in dissolved phosphate was noted due to high phosphorus extraction rate.

19
Aquaponics can maximize phosphorus utilization 71.7% of total P input, with
fish and plants assimilatin g 42.3% and 29.4% of the phosphorus input in the feed,
respectively. 13.1% of the phosphorus input occurred in the unavailable form to fish
and plants (Cerozi and Fitzsimmons, 2017).
In aquaponic systems emissions are mainly constituted by nitrogen and
phosphorous released in the environment, due to suspended solids and dead lettuces
removal and disposal (Forchino et al., 2017). However, these wastes can be easily
recycled within the farm. Low potassium (K), sulfur (S), iron (Fe), and manganese (Mn)
have be en reported in aquaponic plants that received nutrition only from fish waste
(Saha et al., 2016).
Fish in an integrated recirculating aqua -hydroponic system can be reared with
8 times higher density than the recommended fish stocking density (Diem et al., 2017).
As a follow up of the experiment, Diem et al. (2017) proposed an optimal fish density
in an aquaponic system as follows: 114 -125 fish/m3 or 6-7 kg/m3, a feeding rate of 176 –
282 g/m3/day, a 4.2 m2 surface of hydroponic trenches per m3 (with canna and water
spinach plants) and a water recirculation of 200 -400% per day.
Canna is a very suitable plant for the aquaponic system, mostly for the use in
wastewater purification. Besides the potential for nutrient removal canna has
aesthetical values providing nice-looking treatment systems (Diem et al 2017). Diem
et al (2017) demonstrated in their study that canna ( C. glauca L) had the highest growth
rate compared to water spinach and lettuce. Even though water spinach ( Ipomoea
aquatica Forssk.) produced less biomass than canna, it has more economic value as
a leafy vegetable (Diem et al 2017).
Basil ( Ocimum basilicum L.) is an annual herb with high commercial importance,
both for its fresh and dried leaves. It has culinary and medicinal purpose (Saha et al.,
2016). According to Saha et al. (2016) basil produces higher yield in aquaponics
compared with conventional systems.
Lettuce ( Lactuca sativa ) is higly recommended as an aquaponics crop due to its
fast growth and low nutritional requirement (Pinho et al., 2017). Lettuce growth can be
limited (inhibited) by the use of sponge material to hold the plants in position. Diem et
al. (2017) demonstrated that the sponge material has a high water holding capacity
and thus, creating a too moist environment ar ound the plant stems.
Aquaponics may be more efficient in the presence of a diverse microbial
community (Pinho et al., 2017) fact confirmed by Cerozi and Fitzsimmons (2017). In

20
their study, the authors concluded that the addition of a commercial mixture of Bacillus
spp. enhanced plant growth, increased the P accumulation in plant tissues, and
increased the chlorophyll content in the leaves. As well, systems that received the
Bacillus mixture demonstrated higher concentrations of dissolved orthophosphate in
the water than untreated systems.
In aquaponic systems the al kalinity should be above 100 mg/ L for a proper
ammonia assimilation and nitrification process by heterotrophic bacte ria (Pinho et al.,
2017). Pinho et al. (2016) measured in their aquaponic system a concentration of
20.6 ± 1.6 mg/L orthophosphates and noted to be within the recommend values for
leafy vegetables. The pH is a parameter which is difficult to adjust in an a quaponic
system due to the different optimal requirements of fish and plants. In aquaponics the
ideal pH ranges between 6.5 and 7.0, providing the best nutrient absorption by the
plants (ideal pH 5.5 –6.5) and reduction the levels of un -ionized ammonia for fish (Pinho
et al., 2017). Plant production would be higher if pH levels were lower.
In traditional agriculture lettuce has an optimum temperature range between 4
and 27°C. In aquaponic systems the temperature variate from 22°C in the winter
season and 30 °C in the summer season and from the growth performance point of
view, lettuce had higher productivity in the hot season compared to the cold one.
Lettuce tolerance to higher temperatures may be due to the direct contact with water
and thus reducing the ne gative effects (Pinho et al., 2017).

2.2 The concept of aquaponics modeling for a recirculating aquaculture system
Aquaponic systems are characterized as being complex systems, that involves
the presence of two different growing technologies, for fish and respectively, for plants,
with the purpose of increasing the sustainability and to limit the environmental impact.
Thus, as it is a complex system, there are many variables that must be taken into
consideration in order to make a prediction model that wil l estimate the productivity of
an aquaponics system.
Integrated aquaponics systems have two major components, fish and plant
biomass. Therefore, it is important to analyze both those components apart, in order
to observe their connection to other factors that interfere in the multi -trophic system
production cycle. At the end of this activity, the results obtained must be correlated,
finding the links that ensures the direct relation of dependency between fish and
vegetal biomass.

21
Various studies regarding aquaponics integrated systems were conducted in
the past decade (Cristea et. al 2013, Petrea et. al, 2013a, Petrea et. al, 2013b, Blidariu,
2013, Roosta, 2011, Savidov et al. 2007, Licamele, 2009), having different objectives,
from analyzing the growth per formance of fish biomass, till identifying the nutrients
balance inside a multi -trophic integrated aquaponics system. However, a lot of
researches that take into study the growth of different fish species under various
technical and technological condition s were made (Adler et al, 2003, Timmons et al,
2013, Leoni, 2003, Meade, 2002, Lacheta, 2010, Wahome et al, 2011 , Graber et al,
2009, Wilson et al 2006, Leannard et al 2004, Aquaponic Research Project, 2013). On
the other part , studies that imply the growt h of plants under hydroponic conditions are
conducted in order to evaluate the nutritional requirements of different plant species,
growth by using various technologies.
Thus, two approach methods can be distinguished in order to collect the data
present in the scientific literature till nowadays, in order to elaborate a correlation model
for aquaponics integrated systems.
The first way of approach is to consider an aquaponics system as an own
identity and to study the direct and indirect correlations bet ween the main elements
presented in the system, meaning plants and fish. In this case, only the aquaponics
research studies must be taken into consideration for the incipient data processing.
The second way of approach is to consider aquaponics systems as complex
systems, composed of two major parts (plants and fish biomass), each corresponding
to a production technology. Thus, in this case, the research studies that must be taken
into consideration are divided in fish biomass researches and plant biomass
researches. The scientific studies regarding fish biomass, that will be analyzed, must
have as a goal the test of different technological aspects as fish stocking density,
feeding ration, as well as various nutritional aspects. On the other hand, the plant
biomass studies that will be considered as useful are the ones that target the nutritional
requirements of different species of plants, grow in hydroponic systems, emphasizing
especially the nutrients retention rates kinetics and dynamics.
The integrated recirculating aquaculture systems can be analyzed by using a
mathematical model, also by dividing the concept into three parts, as follows: biological
part, microbiological and chemical part and in the end, technical part.
Therefore, by taking into conside ration the biological part, the main
characteristics that must be followed are the ones leading to both fish and plants

22
biomass. Both fish and plants growth rate is depended on atmospheric and technologic
factors. Thus, among the atmospheric factor that ca n influence the plants growth, air
temperature, humidity, light intensity and wavelength are determinant variables that
must be taken into account in order to establish the prediction growth model for plant
biomass. Each plant has its own requirements when it comes to the above mentioned
factors. However, plants requirements may vary, depending on its growth stage. In
order to determine the proper growth prediction model, a large series of variants,
corresponding to the same variable, must be tested.
The m icrobiological and chemical model creates a rather close bond between
fish biomass and plant biomass, comparing with atmospheric model, but it correlates
perfect with the technological factors from the previous biological model.
The microbiological issues are popular nowadays, when it comes to integrated
multi -trophic aquaculture systems and namely integrated recirculating aquaculture
systems, where aquaponics techniques were used. There is a strong debate related to
product security and safety for consume rs, since in aquaponics systems, both fish and
plants are safety from the chemical point of view, but the lack of pesticides can involve
microbiological problems. It is obvious that the accumulation of organic matter inside
the production system can cause microbiological issues as well. This accumulation is
related with applying a proper operative management inside the system, but also with
using a proper technique and production technology.
It is known that the nutrients requirements of plants different significantly, both
in terms of dynamics and quantity. As fish are feed mostly with a fix feeding ration, the
dynamic of their digestive process is relatively constant in time, fact which impl ies a
constant dynamics of the main nutrients in the technological water, on a long term
production cycle. Plants have many upward tendencies, followed by suddenly
downward tendencies, such dynamics being the cause of lack of nutrients required by
them, at a certain moment. This fact makes very difficult the attempts of finding a
perfect microbiological and chemical prediction model in integrated recirculating
aquaculture systems, based on using aquaponics techniques.

23
The correlation between water chemistr y parameters in different research studies made in recirculating aquaculture systems
are presented in table 1.
TABLE 1. The interdependence between fish weight gain and technological water nutrients concentration in different studies made in rec irculating
aquaculture systems
No./
Sourse* Experimental
design Weight
gain (g) Temperature
(0C) pH
(upH) DO
(mg/l) Alkalinity Hardness ammonia
(NH 3)
(mg/L) nitrite
(NO 2)
(mg/L) nitrate
(NO 3)
(mg/L) NH 4

(mg/L) PO 4

(mg/L) Ca
1 Goldfish
Initial stocking density
18.75kg/m3
Testing different flow
regimes:
4, 8, 12, and
24 h/day 1.96 27.05 7.4 5.4 55.8 60.7 0.47 0.04 0.23 N.A. N.A. N.A.
-5.6% 2.5% 2.7% -3.7% 43.7% 4.9% -36.2% 0.0% 13.043% N.A. N.A. N.A.
1.0% 2.6% 1.4% 0.0% 44.1% 5.4% -46.8% 0.0% 8.696% N.A. N.A. N.A.
3.1% 1.4% 1.4% 0.0% 46.6% 11.5% -51.1% -25.0% 21.739% N.A. N.A. N.A.
5.1% 2.7% 1.4% 0.0% 36.2% 18.9% -53.2% -50.0% 21.739% N.A. N.A. N.A.
2 Tilapia
Initial stocking density
0.4 kg/m3 Testing the
aquaponics of tilapia
with different lettuce
varieties, together
with bacteria
inoculation 45.9 29.2 7.3 5.3 20.1 N.A. 0.14 0.49 2.240 3.97 N.A. N.A.
4.2% 1.5% -2.3% -2.4% -6.3% N.A. -7.1% -14.3% -6.250% -15.11% N.A. N.A.
5.7% -0.1% -4.7% -3.1% -7.6% N.A. -14.3% -24.5% -2.679% -15.87% N.A. N.A.
3 Stellate sturgeon
Initial stocking density
0,015 kg/m3
Testing different
feeding intensities:
1.1% and 2.2% 23.7 21.5 N.A. N.A. N.A. N.A. N.A 0.005 21.600 0.07 N.A. N.A.
-5.8% -5.1% N.A. N.A. N.A. N.A. N.A 0.0 -0.926% -17.65% N.A. N.A.
136.8% 2.3% N.A. N.A. N.A. N.A. N.A 0.0% 0.0 % -17.65% N.A. N.A.
163.6% 2.3% N.A. N.A. N.A. N.A. N.A -60.0% 44.4 % -97.06% N.A. N.A.

24
4 Hybrid tilapia
Testing different
stocking densities
1kg/m3 12.2 N.A. 8.2 5.5 N.A. N.A. 0.021 0.13 3.800 N.A. N.A. N.A.
5kg/m3 10.6% N.A. -2.4% -16.4% N.A. N.A. 257.1% 84.6% 2.632% N.A. N.A. N.A.
10kg/m3 9.9% N.A. -3.7% -32.7% N.A. N.A. 685.7% 138.5% 2.632% N.A. N.A. N.A.
15kg/m3 7.8% N.A. -3.7% -40.0% N.A. N.A. 1095.2% 169.2% 2.632% N.A. N.A. N.A.
5 European catfish
Testing different
stocking densities
28 kg/m3 195.0 18.3 7.54 4.29 N.A. N.A. N.A. 0.54 2.7 4.78 1.27 156.55
64kg/m3 -67.7% 0.11% 0.13% -37.1% N.A. N.A. N.A. -35.2% 68.5% -8.2% 24.4% -1.0%
6 Rainbow trout
Testing different
stocking densities
2.64kg/m3 1230 16.1 7.33 6.52 N.A. N.A. N.A. 0.1 4 0.226 1.59 N.A.
5.16kg/m3 148.9% -0.6% -3.1% 4.6% N.A. N.A. N.A. 10.0% -5.0% 25.7% -17.9% N.A.
7,12kg/m3 225.5% 0.2% -3.3% -8.9% N.A. N.A. N.A. 0.0% 0.0% -47.8% -27.0% N.A.
9,42kg/m3 315.2% 0.3% -2.9% -15.3% N.A. N.A. N.A. 10.0% 105.0% -60.2% -25.8% N.A.
7 Rainbow trout
Initial stocking density
12 kg/m3
Testing different
concentrations of
probiotic administrated
in feed
Control
V1:22.4×109 CFU g-1
V2:38.4×109 CFU g-1
and
V3:70.4×10 9 CFU g -1 87.19 13.63 7.1 6.84 N.A. N.A. N.A. 0.034 1.86 0.07 N.A. N.A.
5.0% -0.7% -1.1% 1.2% N.A. N.A. N.A. -29.4% -5.9% -55.7% -17.9 N.A.
-5.5% -0.4% –
1.3% -3.8% N.A. N.A. N.A. 26.5 % -10.2% -32.9% N.A. N.A.
-3.9% -1.0% –
1.7% 2.0% N.A. N.A. N.A. 26.5 % -9.7% -33.6% N.A. N.A.

25
8 Rainbow trout
Initial stocking density
1.2 kg/m3
Testing different
feeding frequencies:
two meals/day for at
F1 and four meals/day
at F2 in duplicate
7.52 19.62 6.9 6.24 N.A. N.A. N.A. 0.0015 0.84 0.0097 N.A. N.A.
10.2 % 0.0% 0.0% 0.2% N.A. N.A. N.A. 0.0% 0.0% 0.0% N.A. N.A.
3.5% 0.1% 0.6% 0.0% N.A. N.A. N.A. 82.7 % -50.3% 0.1% N.A. N.A.
8.6% 0.1% 0.6% 0.0% N.A. N.A. N.A. 82.7 % -50.3% 0.1% N.A. N.A.
9 Hybrid bester Testing
different stocking
densities
14.95kg/m3
10.74 kg/m3
4.56 kg/m3
3.19 kg/m3 42.97 20.33 7.91 7.4 N.A. N.A. N.A. 0.0122 0.903 0.1009 N.A. N.A.
8.8% 0.0% –
0.9% 4.3% N.A. N.A. N.A. 0.0% -1.6% -15.4 % N.A. N.A.
-20.9 % 0.0% 0.4% 7.3% N.A. N.A. N.A. -50.8 % -1.6% -30.8 % N.A. N.A.
-5.9% 0.0% –
1.5% 5.3% N.A. N.A. N.A. -75.4 % -1.8% -46.5 % N.A. N.A.
1. Shete et al 2013, 2. Sri Wahyuningsih et al 2015, 3. Cristea V. et al 2012, 4. A.H. Al -Harbi et al, 2002. 5. Dediu L et al 2010, 6. Cretu M., et al 2014, 7.
Mirela Cretu et al 2013, 8. Dediu L. et al 2011, 9. Andrei R. et al 2016

26

The technical model perspective is the most important data model and implies the
engineering part of the integrated production system. As a start -up data, the configuration of the
production system and also, the type of equipment used and the operational m anagement applied
are required. The start -up data will generate a certain hydraulic inside the production system,
characterized by indicators as hydraulic loading rate and hydraulic retention time. Those
indicators will influence most of the parameters tak en into account in the previous two models
that were described.
Generally, most of the integrated recirculating aquaculture systems, based on aquaponics
techniques, follow certain water treatment processes, as follows: mechanical filtration, biological
filtration (nitrification), denitrification, phytoremediation, oxygenation, pH buffering, sterilization.
Each integrated production system must be projected starting from the amount of maximum
biological material that can be produced, at a certain moment.
Considering that this requirement has been proper accomplished, the prevision model will
therefore analyze the efficiency of the equipment during a long term use. This action is generating
an operational management model that must be used in order to keep t he performances of the
production system at an optimal level.
The biological filtration module performance, on a long term, can be influence by the
hydraulic regime of the production system. Therefore, the fluctuation of hydraulic loading rates
and hydrau lic retention time values will influence the nitrification capacity of the technological
water bio -filtration unit. Also, the specific surface of the bio -balls inside the bio -filter and also their
design, not to mention the bio -filter dimensions, are impor tant variable since they influence the
oxidation process and therefore, the removal rates of ammonia and nitrites.
The technical type of the bio -filter has a very high impact on the production performances
and the dynamics of the biological and chemical m odel. The performances of different bio -filters
are presented in the second part of this report.
Barbu et al. 2016 described a m athematical modeling and analysis of trickling bio -filter and
experimentally noticed that the bio -filter aeration in countercu rrent with respect to the flow of the
processed water has a practical negligible effect, so that the trickling bio -filter does not offer
control means of the nitrification process. In these conditions, the main possibility to control the
nitrification proc ess is the control of recirculated flow.
Regarding the mechanical filtration units, Barbu et al. 2016 mentioned that disturbances
are produced by the washing processes of mechanical, sand and active carbon filters of
recirculating aquaculture systems . Thei r presence makes difficult to discern the effects of control
applied to the recirculating flow by the variations induced through internal disturbances .
Mechanical filtration still remains the main problem inside a recirculating aquaculture system.

27
When ela borating a prediction model for aquaponics integrated recirculating systems, it
must be start from the assumption that we are dealing with close systems, with minimal to none
relation with the environment. Therefore, all the inlets and outlets of the syste m must be quantified
with a high precision. Also, the second step in elaborating a model, after analyzing the production
capacity of the projected system, is to take into account the possible mortalities manifested on
both fish and plants biomasses, during a single production cycle.
When referring to the chemical model mentioned above, it must be accepted the
assumption that all the main nutrients presented in the technological water, especially the
ammonia are due to fish waste discompose. On the other ha nd, the ammonia reduction is
exclusively generated by the oxidation processes that convert it into nitrite.
The oxidation process of ammonia to nitrite and then nitrate will normally generate a nitrate
accumulation in technological water, on the long term . Therefore, it must be considered that plant
biomass is the only source of nitrate uptake from the analyzed closed system.
It is quite important that the system to have a high mixing capacity of the technological
water even from the projection and design phase, fact mentioned also by Bobak and Kunze
(2017). This will ensure an almost equal concentration of each nutrient, in the technological water
volume and will offer equal possibility for the plants to absorb the required elements,
corresponding to thei r nutritional demands.
Loria (2017) mentioned that an aquaponics model presents a way to combine several
components (bacteria, plant and fish) and relate them to environmental factors such as radiation,
temperature, feed given, nitrate and ammonium concent rations, in order to assess the
performance within the system and achieve optimal plant and fish growth. The function in the fish
component relating water temperature to feeding ratio as a percentage of the fish mass had a
subroutine adapted to lower tempe rature factors which was used to fit with the measured water
temperature (Loria, 2017).
Each model should be based also on the characterization of final products. Therefore, the
quality of both fish and plants biomass must be determinants factors for estab lishing new growing
conditions or maintaining the current ones from a certain model. For example, for an atmospheric
model, light intensity and wavelength is considered a main factor that influence the growth
performances an d nitrate and nitrogen concentra tions in the plant, while for a chemical model,
the same parameters must by influence by considering the water nutrients concentration as a
main dependent factor. However, for a technological model, fish to plants ratio is the must
determinant factor for i nfluence growth performances an d nitrate and nitrogen concentrations in
the plant .
Therefore, as a conclusion, it must be mentioned that aquaponics modeling is used as a
tool for a better understanding of the relations between plants – bacteria – fish. Als o, the models

28
give predictions and information that will clarify the way in which different parameters can
positively or negatively affect the nutrients balance in the technological water and the productivity
of the integrated system.

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Part II
Start -up guide for recirculating integrated systems, bas ed on aquaponics techniques

1. Aquaculture production systems classification

Aquaculture production systems and technological facilities can be classified based on a
multitude of aspects, of a technic, technological and ecological nature. It is known that the main
technological facilities encountered in an aquaculture production sy stem are the rearing units,
and these are classified into the following categories:
• Ponds (diked, excavated, dammed);
• “Raceways” – canal -like, long rearing units, with an intense water flow (based on the
design, placement and water flow, these are the fol lowing types: single water passage
“raceways”, parallel or in -series “raceways”, floating/”in pond” “raceways”);
• Tanks of different shapes (circular, octagonal, rectangular) made of concrete, fiber glass,
plastics, metal, etc.;
• Mesh holding structures (flo ating cages, enclosures/pens);
• Shellfish aquaculture specific installations (floating pontoons, floating grates, floating trays,
etc.).
Regarding production systems, the literature mentions numerous criteria for classification.
➢ By type of rearing unit with in the production system:
• Pond aquaculture;
• “Raceway” aquaculture;
• Recirculating aquaculture systems (RAS);
• Floating cages aquaculture;
• Mesh pens aquaculture.

34
➢ Relative to the rearing intensity level:
• Extensive, semi -intensive;
• Intensive;
• Very intensive.
➢ By the placement of the technological facilities:
• Terrestrial;
• Aquatic;
• Transition.
➢ depending on the complexity of technological management and water quality control the
production systems are:
• Opened, semi -controlled (ponds, “raceways”, mesh retention struc tures: cages and
pens);
• Controlled (systems with serial water reuse, systems with partial water reuse,
recirculating systems).
➢ after the production -environment system relationship, respectively the possibility of impact
control of the production system on the environment:
• Opened (mesh retention structures, shellfish aquaculture specific installations);
• Semi -closed (ponds, “raceways”, other tank -type rearing units);
• Closed (recirculating systems).
➢ according to the way of water management in a production syst em:
• flow-through aquaculture systems (the water passes once through the rearing units and
it is entirely discarded in the waterway from whence it was collected);
• Partial water reuse aquaculture systems – PRAS (a part of the rearing units effluent is
retained and recycled to reuse the water);
• Recirculating aquaculture systems – RAS (the production system effluent is entirely
recycled and reused).
➢ By water salinity:
• Freshwater aquaculture systems/continental aquaculture;
• Saltwater aquaculture systems/marine aquaculture;
• Brackish aquaculture systems/brackish aquaculture.
➢ By the reared species:
• Cyprinids aquaculture;
• Salmonids aquaculture;
• Sturgeon aquaculture;
• Mollusks aquaculture;

35
• Crustaceans aquaculture;
• Algae aquaculture, etc.
➢ Considerations related to the profitability of aquaculture and/or reduction of the
environmental impact has led to its integration/association with different plant and animal
systems. As such, the following production systems can be identified:
• Aquaponics systems;
• Multi -trophic integr ated aquaculture systems;
• Partitioned aquaculture systems;
• Plant growing and aquaculture integrated systems;
• Animal rearing and aquaculture integrated systems;
• Substrate -based aquaculture systems;
• Periphyton -based aquaculture systems.
Thus, the industrial aquaculture activity operational management differs mainly depending
on the production system that is being used.

2. Recirculating aquaculture systems SWOT analysis

STRENGTHS WEAKNESSES
• Very low environmental impact;
• Improved product bio -security and food safety;
• Great water conservation;
• Improved control over discharge/effluent;
• No issues with predatory species;
• No issues with escapees;
• Excellent temperature control;
• No (or very low) use of medicine;
• No weather issues;
• Efficient constructed space use;
• Close to market;
• Continuous production, all year round;
• Safer working conditions;
• Ability to sustain very high stocking densities
in a much smaller volume of water. • Higher implementation costs;
• Higher electricity costs;
• Evolving system design;
• Not labeled as “organic”;
• Poor investor confidence;
• Intensive rearing bad image;
• Profits are marginal compared with other
aquaculture systems;
• Issues of flesh tainting;
• Rearing knowledge limited to only a few
species;
• Need of experienced staff to run/maintain the
system;
• Limited marketing experience;
• Higher sensitivity to changing market prices.

36
OPPORTUNITIES THREATS
• Production system automation;
• Reduction of capital costs and
running/operating costs;
• Excellent integration with alternative energy
sources;
• Safe for rearing new species;
• Excellent management and record keeping of
the production system;
• Availability of backup and alarm systems;
• Improved marketing and product placement;
• Stock improvement and genetic selection;
• Reuse of waste as fertilizer/nutrients for
aquaponics systems;
• Consumer demands for healthy products. • Diseases, and the fast spreading of this due to
the higher stocking density in a smaller water
volume;
• Possibility of increasing energy costs;
• Market susceptibility to cheaper imports;
• Fish feed cos ts and availability;
• Human error;
• System component failure;
• Power failures (in case of nonexistent backup
power source);
• Low/poor quality systems/system components;
• Scalability issues;
• Availability of veterinary medicines;
• Availability of experienced perso nal.

3. Examples of configurations for recirculating integrated systems which
uses aquaponics techniques

A. First design (Recommended for hobbyist/family production systems)

Figure 1 . Recirculating integrated aquaculture system with for hobbyist/family use

37
B. Second design (Recommended for small scale production systems)

Figure 2. Recirculating integrated aquaculture system with aquaponics module placed after
the biological filtration unit
C. First design (Recommended for medium scale production systems)

Figure 3. Recirculating int egrated aquaculture system with aquaponics module placed
above fish growing units and communicating directly with them.
D. Third design (Recommended for large scale production systems )

Figure 4. Large scale recirculating integrated aquaculture system with aquaponics module
placed after the biological filtration unit

38

4. Processes in recirculating integrated aquaculture systems, based on
aquaponics techniques

4.1. Sediments removal

4.1.1. Introduction
Suspended solids removal represents a primary objective in the design of water treatment
schemes within recirculating systems. Numerous organic particles that can be found in the rearing
environment strongly influence the noxious substances content and the oxygen content, leading
to serious diseases of the biological material within the rearing systems, especially in the case of
those with a high level of water reuse.
The impact caused by the suspended solids within the water consists in the following:
causing lesions to fish, mechanical clogging of the biological filters, increasing ammonia levels
due to the nitrification processes, and the increase in oxygen consumption needed for organic
matter decomposition. As a consequence, solid wastes are an importa nt factor in limiting
productivity, especially in the case of systems those with a high level of water recirculation. The
EIFAC recommended maximum allowed concentration for solid particles content is 15 mg/L.
There are three ways of approach in regard to the suspended solids control within
recirculating systems:
• Sedimentation and micro -screen filtration – insures the removal of large particles, but are
inefficient for fine particles (< 50 µm); are recommended for systems that function with limited
water re use;
• Granular media filtration – is efficient for the control of suspended solids under 50 µm and is
recommended for systems with a high degree of water reuse or for systems with special
demands regarding water transparency; just like the micro -screens, th e granular media filters
require a small space, but, in general, lead to a much higher loss of hydr aulic load than in the
case of settling tanks (sumps) or micro -screen filters;
• Special physical and chemical processes – consist in the use of special instal lations
(hydrocyclones, activated carbon filters, foam fractionators, ozonizers, porous filters); these
are not used separately, they, usually, constitute complementary filtration to the processes
previously presented.
Within a recirculating system, the to tal suspended solid (TSS) particles with a diameter
greater than 1 µm, are an important parameter for environment quality assessment. The
concentration of these TSS must be maintained, through an adequate management, within

39
optimum limits for each reared s pecies. For example, for salmonids, it is appreciated that fine
particles (5 -10 µm) accumulation can have lethal effects.
From a chemical point of view, the suspended solids are both of an organic and inorganic
nature. The organic part, known as volatile s uspended solids (VSS) determine oxygen
consumption. The inorganic part, contributes to the forming of sludge deposits, with negative
effects on the habitat.
From a physical point of view, the suspended solids can also be categorized as sedimentable
(> 100 µm) and unsedimentable (< 100 µm). As the size of the particles decreases, the efficiency
of the removal processes is reduced.
The removal of the suspended solid particles within an aquaculture production system is
realized through known processes of solid /liquid phase separation, namely gravitational
separation, mechanical filtration and flotation.
The gravitational separation is based on the sedimentation principle in settling tanks (sumps)
or from case to case in hydrocyclones.
The mechanical filtration consists in the retention of the suspended solid phase as water
passes through screens, granular media or porous media.
Through the flotation procedure, the solid particles are attached to the surface of air bubbles
introduced within the water under pressu re and later removed from the system through specific
processes.
In all these processes, the solid phase separation consists in the passing of these particles
through a separation surface. The separation surfaces are represented, according to the
previousl y presented processes, by the bottom of the settling tank, the external surface of the
filtration medium, respectively by the air bubbles.
4.1.2 . Gravitational separation
Gravitational separation consists in the gravitational removal of the solid particles from the
water of an aquaculture production system. There are three gravitational separation procedures,
namely: sedimentation, centrifugation and hydrocyclone separation .
Sedimentation
This represents the simplest method used for solid macroparticles control. The procedure
consists in the passing of the suspended solids loaded water, at a low flow rate, through a
specially set tank (sump) with the purpose of decanting it.
The settling tanks can function in a continuous or discontinuous regime, depending on the
degree of suspended solids accumulated in the water, the granulometric composition of the solid
particles, and the technologic flow rate. For RAS, continuous flow ra te settling tanks are most
common.

40
Continuous flow rate settling tanks, usually rectangular in shape, are com partmented in four
zones (fig. 5 ) each of these having specific functions regarding the access of technological “dirty”
water, assuring the settlin g conditions, accumulation of the settled solids and evacuation of the
technological “clean” water .

Figure 5 . Continuous flow rate settling tank (Timmons et al., 2002)
The inlet zone assures an even distribution of the suspended solids along the entire surface
of the tank. The settling zone, through its morpho -dimensional characteristics, insures the actual
solid phase separation process through settling. From the sludge accumulation zone, solids are
removed periodically. The settled water is collected at the tank outlet zone level, over its entire
cross -section and is discharged through the top.
Centrifugation
Centrifugation is a procedure that, mainly, consists in the s eparation of solid particles from
a system with the help of centrifugal forces induced by the system’s rotation around its own axle.
In this way, the separation process is substantially intensified, the dimensions of the solid
particle control system and t he time needed for suspended solids removal being considerably
reduced.
From a constructive and functional point of view, the centrifugation separation installations
can have a continuous or intermittent flow rate. For the treatment of aquaculture wastewa ter, the
continuous flow rate centrifuges are used (fig. 6) .

Figure 6 . Conical scroll -type continuous centrifuge (Wheaton, 1985)

41
The parameters that influence the functioning and the efficiency of the centrifugal installation
are the rotation speed, solid material (TSS) concentration, the system’s clean water discharge
section size, and the thickness of the water layer maintained along the wall of the installation. The
clean water outlet section is adjustable according to the degree of solid wastewate r loading and
the TSS concentration required for the cleaned water.
Hydrocyclone separation
Hydrocyclones also function on the principle of centrifugal separation of the suspended solid
particles in a liquid medium as a result of amplification of the weigh t difference between liquid and
solid phase under the action of centrifugal force. The separation efficiency is determined by the
difference between solid particle density and water density.
The figure (fig. 7 ) shows the design schematic of a continuous fl owing centrifugal installation
and the principle of its operation. It is noticed that wastewater penetrates tangentially at a certain
speed at the top of the installation. The specific way of the inlet water determines the general
vertical spiral motion of the water in the installation.
The circular motion of the water causes the heavier solid particles to move toward the
hydrocyclone wall. The specific spiral centrifugal motion of the mass of water within the
hydrocyclone causes a downward current in the peripheral area adjacent to the walls and an
upward current in the central area. Thus, the
suspended solids, centrifuged in the wall area, will be
trained towards the base of the hydrocyclone and
discharged.
The efficiency of a hydrocyclone used to control
solid particles in a recirculating system depends on its
constructive characteristics, the speed of inlet water in
the hydrocyclone and the suspended solids (TSS)
concentration in the water.
Hydrocyclones, relatively small size equipment,
are widely used in industry due to their low cost. The
main drawback, which restricts their use, is the high
electricity consumption needed to drive pumps that
have to provide high pressure and high water speeds
in the installation.
Figure 7. Operation of
hydrocyclone (Wh eaton, 1985)

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4.1.3. Mechanical filtration
Mechanical filtration is a basic process used to control solid particles in recirculating systems. The
mechanical filters separate the solid phase from the liquid phase when water passes through a
filter medium based on the difference between the particles size of the solid phase components.
Mechanical filters provide an easy way to operate and are relatively easy to maintain under
the conditions of careful design and exploitation. Mechanical filters can be made in various types
of variants depending on the capacity of the growing systems and the loading degree of solids in
water (TSS).
Mechanical filtration does not ensure total removal of solid particles, very small ones are to
be eliminated by applying specifi c procedures.
Operating costs of mechanical filters are appreciable if the concentration in the TSS is high,
which is why mechanical filtration must be preceded by a gravitational separation.
Depending on the nature of the filter medium, the following mech anical filtration processes
are most commonly used in aquaculture:
• screen filtration;
• granular media (GM) filtration;
• porous media (PM) filtration.
Screen filtration
The process consists in the passage of wastewater through a site system that retains and
eliminates most of the solid particles. The screens are sized according to the solids water loading
(TSS) and granulometric composition.
The main advantage of mechanical screen filter is the recording of low load losses at the
passage of water through the filter medium comparable to those recorded for gravitational
separation.
Mechanical screen filters also show some inconveniences that must be known for a careful
exploitation. Thus, small particles cannot be retained, requiring removal by other processes. Also,
at a high water flow through the filter, large particles fragment, resulting in fine particles not being
retained by the filter. Theoretically, it is possible to remove these fine particles if a proper mesh
size is chosen. However, the use of fine we bs is limited by some impediments, such as high
pressure losses and rapid clogging of filters. In this case, filter cleaning costs can greatly reduce
the efficiency of their use.

43
Depending on the way of operation and the constructive solution, there are th ree types of
screens: stationary, rotatory and vibratory.
Stationary screen filters
They are the simplest mechanical filters. In the simplest form, the screen is placed
perpendicular to the liquid flow ( fig. 8 ). In this way, particles larger than mesh size are retained
and collected by it.
Stationary screens are seldom used for particles smaller than 1.5 mm in diameter, or when
the concentration of water in the TSS is too high because there is a risk of rapid clogging in this
case.
The maintenance of statio nary sites consists of their periodic removal and their
countercurrent washing with a strong water jet; if the degree of clogging is high and the adhesion
of the solid particles to the screen is strong, the screens are mechanically cleaned with the brush
or by other means.
The screens can be made of various materials resistant to corrosive water (steel, brass,
stainless steel, textile fabrics, and plastics) in a wide mesh size dimension ranging of microns or
millimeters.

Figure 8. Stationary screen filter (Wheaton, 1985)
Rotary screen filters
Rotary screen filters are designed to reduce the clogging potential which is a major
disadvantage of stationary screens.
Constructively, such a filter is composed of a drum provided with a screen on the outer
surface. The rotary drum is partially immersed in the wastewater which flows through a prismatic
enclosure.
Continuous rotation of the drum causes the immersed part of the screen to filter solids, and
the upper part periodically passes in front of a mounted flushi ng system. The washing process,

44
permanently and automatically, ensures continuous operation, with minimal hydraulic system
losses and low labor need.
Specific to rotary screen filters is the fact that large amounts of water with a significant
amount of sol ids must be recycled before being discharged from the system.
Depending on the way of water access to the filter section, there are two types of rotary filter
filters, namely axial filters and radial filters.
Axial rotary screen filters
An axial filter with a rotary screen is made up of two chambers, the screen being at the level
of the septum. The wastewater, enters the first chamber, passes axially through the screen and
arrives, in filtered form, in the second chamber where it is disch arged. The rotation of the screen
causes the partially immersed portion to intermittently face a washing mechanism where a
pressurized downstream water jet washes the retained solid particles; the water loaded with
washed material is taken up by a trough p laced on the upstream side of the screen and discharged
(fig. 9 ).

Figure 9. Axial flow rotary screen (Wheaton, 1985)

Rotary screen axial filters are relatively inexpensive, easy to operate and maintain. They
can be washed automatically and efficiently filter out wastewater with a higher concentration of
solid suspension (TSS) than stationary screen filters.
The screens used in an axial rotary filter are circular in shape. This is a major disadvantage
because, given that the water level in the first cham ber cannot exceed, for functional reasons, the
axis of the screen, the active surface available for flow is determined by the diameter of the
screen. In the case of high flows, screens of appreciable diameters and large filtration enclosures

45
are required, also more complicated constructive solutions for washing and discharging solids are
required.
Radial rotary screen filters
The radial rotary screen filter consists of a cylindrical drum that rotates around its horizontal
axis, being partially immersed in f iltered water that is passing through a specially designed tank.
The side surface of the drum is the active part of the installation and is a screen.
The suspended solids (TSS) water stream penetrates axially into the drum and is
discharged, filtered, in a radial direction through the mesh network of the screen.
Depending on the system of washing and evacuating solid particles retained in the screen,
there are several constructive variants of rotary screen radial filters.
Radial rotary screen filters have t he same advantages as axial flow filters. In addition, they
are not so restrictive in terms of capacity, as is the case with other filters.
Screens are made from a wide variety of materials, from galvanized steel to fabrics. The
choice of the material from which the screen will be made is based on the characteristics of the
treated water, the adopted construction solution and the screen mesh size.

Figure 10. Radial flow rotary screen filter with backwash (Wheaton, 1985)
Chain -type rotary screen filters
Constructively, this type of filter consists of a screen in the form of a funicular strip made up
of an assembly of articulated panels ( fig. 11 ).
The screen is made in articulated variant to be rotated by the wheels mounted on the drum
spindle. Since it is difficult to obtain a perfect joint between the screen panels and between the
drive and screen chains, the use of this system is limited, being effective in the case of coarse

46
materials. The advantages of this type of filter consist in the fact that the s ize of the filter surface
is appreciable and the operating costs are reasonable. The functional characteristics of chain –
type rotating screen filters recommend them to equip the water intakes of aquaculture rearing
systems.
In most constructive solutions, the cleaning of rotary screens is accomplished by continuous
flushing with a water jet. For this reason, water losses used for washing can reach significant
costs, which is a major drawback of these types of filters. Rotary screen filters are recommended
for retaining small solid particles up to 3 μm. The minimum size that can be retained under
effective conditions is limited by the complexity of the plant and the amount of water required for
washing. The yield of the filter is determined by the mesh size, the size of the active surface, and
the amount and characteristics of solid particles in wastewater.

Figure 11 – Chain -type movable screen filter (Wheaton, 1985)
Vibratory screen filters
Vibratory screen filters are special installations where liquid and solid phase separation is
carried out on filter surfaces to be printed in order to intensify the filtration process, a vibrational
movement in the horizontal plane.
Depending on the direction of the water inlet into the installation, the vibratory screens are
of two types: axial drainage and radial drainage. The most commonly used in the aquaculture of
the recirculating systems are those with axial discharge (fig. 12 ).

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Figure 12 – Axial flow vibratory screen (Wheaton, 1985)
Wastewater loaded with solid particles moves at a certain speed along the vibratory screen
where the separation of the two phases occurs which are collected and discharged from the
system. The vibration motion is performed with electromagnetic vibrators having amplitudes and
fixed or adjustable frequencies that are determined by the concentration and characteristics of
the suspended solids.
In the case of axial vibratory filters, it is important to ensure an optimal correlation between
the feed rate and the kinematics of the site so th at the moisture content of the retained and
removed solids is small enough to reduce the water loss in the system as much as possible.
Granular media filtration
This type of filtration involves passing the waste water stream through a layer of granular
material (medium) and retaining the solid particles on its contact surface; the most commonly
used granular material is sand, but under certain conditions, other filtering agents (most often
plastic floating beads) may be used. Granular filters work gravitati onally or under pressure and
the direction of water circulation through the filter medium can be upward or downward ((fig.13) .
Sand filters
Sand filters consist of a layer of sand or other mineral granular material (gravel) through
which water passes. Fil tration is a mechanical process that results in retaining solid particles at
the surface or pores of the filter medium, depending on their size .
The maximum particle size that can be retained in the filter is conditioned by the size of sand
grains generall y ranging from 2.0 to 0.02 mm. In order to ensure the retention of smaller particles
in the order of the microns, very fine grained clay, clay or similar materials may be used; In this
case, the flow rate of the water through the filter medium is very low, reducing the efficiency of
the filter. As a rule, sand filters completely remove only particles larger than 30 μm in diameter.

48

Figure 13. Gravity flow sand filter (Wheaton, 1985 )
The discharge rate through the filter and its clogging intensity are dependent on the particle
size of the filter medium and the concentration or characteristics of the solid particles in the
wastewater. Filtering media with fine grain yields low drainage speeds. The higher the
concentration of TSS, the faster the filter wi ll clog, and more frequent washing and, implicitly, high
washing water demand will be required.
Sand filters can work gravitationally or under pressure; the choice of one of the two functional
types is based on the concentration of the TSS in the waste wat er and the t echnologically clean
water flow (fig.14).

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Figure 14 Pressure sand filter (Wheaton, 1985)
Bead filters
The filtration medium for these filters is a layer made up of 3 to 5 mm diameter plastic floating
beads. Due to the subunit specific weight, the plastic, lighter than the water, forms a compact bed
at the top of the filtration chamber which, by complex adsorption processes, retains the solid
particles in the wastewater.
Depending on the TSS water load and the construction features of the growi ng system, the
bead filters can operate free or under pressure and the water flow direction in the filter can be
descending or ascending.
The size and density of the beads are determined by the nature of the solid particles and
the concentration in the TSS .
The plastic bead filters have the advantage of a high reliability and the possibility of applying
some easy washing procedures.
Porous media filtration
Porous media filters are used to remove smaller solid particles that cannot be effectively
retained by other processes.
The operating principle of these filters is similar to that of the screen filters and consists in
passing the waste water through a microporous structure filtering medium in which suspended
solids are retained. Unlike screen filters, poro us agents are characterized by lower filtering speeds
and higher pressure losses.
In porous media filters, the thickness of the filtering layer is higher than that of the screen
filters, and the pore size is smaller than for regular granular filters.
The most commonly used porous agent is diatomaceous earth (DE).

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Figure 15 . Diatomaceous earth filter system (Wheaton, 1985)

Diatomaceous earth filters (fig. 15 ) are mechanical filters that are used for rearing systems
that require a high technological requi rement in terms of clarity and degree of microorganism load
in the water. There are a multitude of diatomite earth categories used as filter media, the finest of
which is capable of retaining solid particles with diameters up to 0.1 μm.

4.1.4 Physico -chemical processes
The physico -chemical processes accomplish the retention of fine solids as well as dissolved
solids, mainly through adsorption processes. Adsorption can be defined as a process of
accumulating or concentrating a substance on a separation between t wo phases. In waste water
treatment, adsorption usually takes place at the level of the separation between a liquid and a
solid or the separation surface between a liquid and a gas.
The most commonly used methods for treating waste water in recirculation s ystems by
physico -chemical processes are: adsorption at the level of the separation surface between a liquid

51
and a solid (active carbon filters and resin ion exchange filters) and adsorption to the separation
surface between a liquid and a gas (foam separa tor).
Active carbon filtration
The process consists in the passage of waste water through an active carbon layer where
adsorption resolves the dissolved substances. The dynamics of the adsorption process is directly
proportional to the decrease in surface tension at the level of the solid -liquid separation surface,
which in turn is directly proportional to the concentration of surfactants represented by the solutes
dissolved in water.
The efficiency of chemical filtration depends mainly on the size of the c ontact surface
available for the adsorption process. Active carbon is the material that satisfies this demand. The
carbon activation process consists in the formation of a very large number of cracks in its structure
by specific (thermal or chemical) proce sses, to which corresponds a significant contact surface .

Factors influencing adsorption on carbon
Adsorption on carbon depends on many factors whose influence is often simultaneous and
difficult to quantify. The main factors that influence this process are: the size of the contact
surface, the solvent's characteristics, the water reaction, the water temperature, the degree of
diversity of the solvents.
• The contact surface is the most important factor influencing the adsorption process. As a
surface pheno menon, the adsorption level is determined, as mentioned, by the size of the
contact surface available to the adsorption process. The size of the contact surface depends
on the starting material from which the activated carbon is obtained, the type of activ ation
process used to produce it and the size of the constituent particles. Thus, activated carbon
can be obtained from various materials: bones, coal, wood, walnut shells (especially coconut
and peanuts), sawdust, processing waste and agricultural waste.
• The solubility properties, the molecular weight and the ionic characteristics are the
characteristics of the solvent that influences the adsorption dynamics (as velocity and
efficiency). The law of Lundelius, the first of the qualitative laws defining the influence of
solvation characteristics on the adsorption process, establishes that the degree of
adsorption of the solution in a solution is inversely proportional to its solubility.
• The water reaction (pH) frequently influences the adsorption of certain i ons, partly because
the hydrogen and hydroxide ions tend to be strongly adsorbed, and on the other hand
because the pH influences the degree of ionization of the acid solutions and basic. The
ionization state of these solutions influences adsorption. In ge neral, organic pollutants in
water are better adsorbed to lower pH.

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• The effects of the temperature on the adsorption process are both direct and indirect. Since
the adsorption process is exothermic, low temperatures tend to favor higher adsorption
rates. I ndirect influences refer to the influence of temperature on viscosity and water density.
However, the temperature in ordinary aquaculture systems has a relatively insignificant
influence on adsorption on activated carbon.
• The degree of diversity of solvate s in water from aquaculture systems is also a factor
influencing the dynamics of carbon adsorption. It has been shown that the presence of more
than one dissolved in water decreases the adsorption rate of each of them, but increases
the total adsorption ca pacity of the active carbon over the value that would have been
achieved in the case of a single solvite substance.
Types of activated charcoal filters
From a constructive and functional point of view, charcoal filters can be intermittent or
continuous.
The activated charcoal filter with intermittent flow regime consists of a granular activated
charcoal tank flooded with wastewater. Typically, the water -coal mixture is agitated to speed up
the adsorption process. For a specified period of time, called conta ct time, the water -charcoal
mixture is left at rest, after which the purified water is drained and the sedimentary coal is either
removed or reactivated. Active charcoal filters with intermittent flow mode function properly for
relatively small systems. It has the disadvantage of requiring high operating expenses to have a
limited flow.
Active charcoal filters with continuous flow mode can be classified according to flow direction
in ascending flow filters and descending flow filters.
The first system uses a granular activated charcoal pool similar to the intermittent system.
Waste water, introduced continuously at the bottom of the basin, passes through the charcoal
bed and is discharged to the top of the tank. In this case, contact time is dependent on the ascent
water speed as well as on the height of the coal bed. Upward flow filters can use either granular
or powdered charcoal.
The second type of continuous flow filter exposes an activated charcoal bed (usually a
column) to a descending stream of wastewa ter. This system eliminates oversized basins and is
frequently used to treat municipal waters.
Filters with ion exchangers
Ion exchange is an electrochemical process that consists of exchanging ions between two
substances, usually a solution and an insoluble solid in that solution. Ion exchange is not only an
adsorption phenomenon on a surface, but also involves the three -dime nsional internal structure

53
of solid phase molecules. Ion exchange is, in these conditions, a complex process of both
adsorption and absorption.
Ion exchange resins can be classified, depending on the mode of production and their
characteristics, into one o f the following four groups: very acidic cations, weak acid cations, very
basic anions, weak basic anions. There are also some materials (clays and zeolites) in which
ionic exchange occurs naturally. Choosing the right resin to retain a particular componen t
depends on its properties.

Foam Fractionation (separation)
Foam fractionation is a process of separating or concentrating the dissolved materials by
adsorbing one or more solvents to the surface of the air bubbles passed through the solution to
be treat ed. Thus foam forms on the surface of which are dissolved both dissolved substances
and solid particles (TSS) which are fixed on the surface of the bubble.
By removing the foam from the surface of the liquid, solvates and solid particles
concentrated in th e respective area are eliminated at the same time.
There are several separation techniques based on foam fractionation, some of which apply
with good results in aquaculture, such as bubbling and flotation.
Bubble fractionation is similar, with respect to t he operating principle, to the foam
fractionation process which differs in that no foaming occurs. Bubble fractionation occurs where
a surfactant is present but, for various reasons, the solution does not foam. As the solution near
the surface of the liqui d is enriched with solvent, removing the surface fluid leads to a decrease
in the solvent content in the waste water introduced at the bottom of the column.
Flotation -based processes are used for waste disposal, ore purification and microorganisms
concentr ation. These processes consist of bubbling, under a certain pressure, the air into the
wastewater which, from a physical point of view, can be considered a liquid -solid mixture. In this
case, the air bubbles attach to the solids, the process continuing unt il the solid – bubble
combination has a specific apparent weight less than that of the water, at which point the solids
become floating and can be easily removed from the system. The physical principle of flotation
consists in the transfer of dissolved sub stances and fine solid particles from wastewater by
adsorption to the gas bubbles.
The operating principle of the installation (fig. 16) consists in pumping the waste water
(solvent and solvite) into a circular section column, where air is introduced as a small bubble
through a diffuser tube at the lower part.

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Figure 16 Basic foam fractionation unit (Wheaton, 1985)
In their ascension move, the bubbles collect the solution they concentrate on the surface of
the water. Thus, at the surface of the liquid the bubbles pass into the foam phase, carrying with
them the solvite charge and fine solid particles together with a small amount of solvent. The
process of continuous foam formation causes it to be lifted into the column until it is forced to pass
into the c ollector from where it is discharged.
The filtered water is collected at the base of the column and the level of the liquid in the
column is controlled by a tap placed on the path of the filtered water outlet .

Ozonation
Ozonation is not a proper procedure for separating solvents from wastewater. Ozone helps
to control solid particles indirectly by modifying particle size. Ozone, a highly unstable reactive
gas, has the ability to fragment high weight molecular organic substances in to simpler substances
that can be readily biodegraded or retained by precipitation or adsorption.
Ozone is used in many aquarium recirculation systems to reduce turbidity and water color.
The effect of ozonation on the change in solid particle size is not yet clearly defined.
Particular attention should be paid to residual ozone in water, which may cause dysfunctions
at the gill level and, in certain situations, even the death of fish or other organisms coming into
contact with ozonated water.
For these rea sons additional research is needed before recommending the use of this
procedure for controlling solid particles in aquaculture recirculation systems .

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4.1.5. Applications for solid particle control systems

The main criteria in choosing a particular process for TSS control in a recirculating system
is to ensure an optimum water quality with respect to the suspended solids content with minimum
capital and running costs.
Typically, the processes and installations presented are integrated into complex
technological schemes that satisfy the above mentioned need.
A complete technology and related facilities for TSS control are shown schematically in
fig.17 . Depending on the particle size and concentration, the applied technology may comprise
all three phase s (pretreatment, main treatment and finishing treatment) or only a part thereof.
The most used pre -treatment installations are sedimentation basins. The main problem to
be considered when applying this process is the limited possibility of removing fine pa rticles and
the relatively high space requirement.
Also, it is necessary to know that with the removal of sludge from the pre -precipitator a
significant solubilization of the suspended solids occurs in the water mass, contributing to the
degradation of the water quality in the system. Therefore, sedimentation pretreatment is a
recommended procedure for resistant species.
Pretreatment of water in tubular decanters has the advantage that it greatly reduces the
technological space and limits the water loss to an appreciable extent, compared to the usual
sedimentation basins. However, the high degree of accumulation of fine particles and, implicitly,
the degradation of recirculated water constitutes an important restrictive factor for the applicability
of these technologies. Biofilter filling, capable of reducing organic matter loading, makes it
possible to use these separators to pre -treat waste water in a recirculating system.
The efficiency of solid particle separation is obtained by associating the pretreatme nt
processes with the actual treatment processes. Among the main treatment processes
encountered in the practice of recirculating systems, most are based on granular media (GM)
filtration.
GM filters are used after the removal of coarse particles by sedime ntation, so that the
wastewater reached in this treatment stage has a relatively low concentration of TSS. This avoids
the clogging of the filters, reduces the frequency of backwashing and, implicitly, the water and
energy consumption associated with these operations. Combining pre -settling with the proper
filtration with granular agents ensures optimal wastewater treatment under conditions of
significant TSS associated with high biomass density in aquaculture rearing systems.

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Figure 17 – Solids removal processes and the particle size range in micron over which the
processes are most effective (Timmons et al., 2002)

The upward flow filters eliminate the clogging and compaction problems that occur in filters
where the flow is descending. The downside of t he upward filters is that it involves a difficult and
long-lasting, more expensive cleaning. Wet sand filters partially solve the systems with moderate
load in the TSS, the deficiencies mentioned above.
For fish species susceptible to TSS concentration, or for some more sensitive stages of their
development, a final treatment of waste water, namely “finishing”, is required.
Filters with porous media (PM) and different physicochemical processes can be used for this
purpose, the quality of treated water justi fying the high cost of these equipment.
Knowing the mechanisms of each solid particle removal process is essential to designing
integrated treatment schemes that meet the requirements of each rearing technology.
The wastewater treatment scheme for a recirculating system is primarily conditioned by its
carrying capacity, implicit by the nature and level of concentration in the TSS.

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4.1.6. Ratings of different mechanical filters
System
technological
process /
component Technology /
Equipment Sub-technology /
Sub-equipment Space
requirements Operational
management
(Ease of use) Operational
performance Acquisition and
implementation
cost Maintenance
and operation
costs
Mechanical filtration Gravitational
separation Sedimentation / Sump     
Centrifugation / Centrifuge     
Hydrocyclone separation /
Hydrocyclone     
Mechanical
filtration Screen filtration / Stationary
screen filter     
Screen filtration / Axial rotary
screen filter     
Screen filtration / Radial rotary
screen filter (Drum filter)     
Screen filtration / Chain -type
rotary screen filter     
Screen filtration / Vibratory
screen filter     
Granular media filtration / Sand
filter     
Granular media filtration / Bead
filter     
Granular media filtration /
Porous media filter     
Physico –
chemical
processes Active carbon filtration /
Diatomaceous earth filter     
Ion exchange filter     
Foam fractionator     
Ozonator     
NOTE: Regarding the scoring system: Space requirements (where  is large and  is small); Operational management (Ease of use – where  is hard and 
is easy); Operational performance (where  is low and  is high); Acquisition and implementation cost (where  is expensive and  is cheap); Maintenance
and operation costs (where  is expensive and  is cheap). All scoring was done by comparing a technology/equipment with another (within the same category).

58

4.2. Biological Filtration

2.1. Introduction
Like all living organisms, fish require a clean environment for optimal growth and survival.
As fish respire and metabolize feed, toxic metabolites are released into the water column.
Metabolite accumulation increasingly degrades system water quality. If i norganic or organic toxins
within the water surpass biologically critical levels, fish growth may become inhibited and mortality
increased.
To maintain a clean environment in recirculating systems, a combination of mechanical and
biological filtration tec hniques must be employed. Although nitrification can occur throughout the
culture system (e.g., in biofilms on pipe and tank walls) (Losordo, 1991), the majority of
biochemical reactions pertaining to heterotrophic and autotrophic bacteria occur within bio filters.
Biofilters are specifically designed for concentrated bacterial attachment and nitrification via fixed –
film processes.

4.2.2. Biofiltration and nitrification
Autotrophic bacteria are credited for performing nitrification (Wedemeyer , 1996). Nitrification
is a two -step process, where Nitrosomonas sp . oxidize ammonia to nitrite, and Nitrobacter sp .
oxidize nitrite to nitrate. Although less toxic than ammonia, nitrites also are considered toxic to
fish, while nitrates (NO 3–N), the fina l oxidized form in nitrification, are considered relatively
nontoxic to fish unless high concentrations are sustained for an extended period of time (Spotte,
1979), (fig.18). Since biofiltration is the principal unit process used for treating fish metaboli tes,
biofilters can be considered major components in intensive recirculating aquaculture systems
(Libey and Miller, 1985).
To ensure prolonged fish survival, high levels of sustained nitrification must be achieved.
Therefore, ecological requirements of th e bacteria (Malone et al., 1993) must be met within
biofilters for effective nitrification to occur. System water quality and filter design characteristics
affect filter environmental conditions. Although a larger number of water quality parameters affect
nitrification kinetics, Kaiser and Wheaton (1983) stated that dissolved oxygen, pH, water
temperature, ammonia -N concentrations, and filter flow rate are the dominant factors affecting a
filter’s nitrification efficacy .

59

Figure 18. Nitrogen cycle
Nitrification Denitrification
1. Heterotrophic bacteria; 2. Nitrosomonas sp .; 3. Nitrobacter sp .; 4. Plants

4.2.3. Configuration of the nitrification filter
There is a variety of nitrification filters in aquaculture. Depending on the construction solution
and the operating mode, the following types of nitrifying filters are distinguished: submersible,
drum, disk, fluidized bed and sand.
Submersed (submerged) filters
The sketch of a submerged filter is shown in fig. 19 . Its operating pr inciple consists in the
passage of water through a filtering layer made up of different materials, the usual mineral
aggregates (sand, gravel, broken stone) or granular plastic structures. A distinctive feature of
these filters is that the filtering agent is permanently immersed.
Water flow through the filter can be upward or downward. Since the population of nitrifying
and heterotrophic bacteria is permanently below the water level, the oxygen required for metabolic
activities is represented by oxygen in t he water, the atmospheric being inaccessible. The
possibility of exclusive use of dissolved oxygen in water is one of the main limits of submerged
nitrification filters. Operation of the submerged filter involves making a column of waste water over
the fil ter layer. Filtration speed and filtered water flow, respectively filter yield, are determined by
the height of the water column that causes water to flow through the filter at a certain speed.
When determining the height of the water column, account shall be taken of the pressure losses
occurring in the filter bed which, in the case of this type of filter, are relatively small (5÷60 cm/m).

60

Figure 19. Submerged filter (Timmons and Losordo, 1994)
Trickling filters
The operating principle of these filters consists in passing water through the filter medium in
the form of drops. Water circulation in the trickling filter is descending, similar to that of the
submerged filter. In contrast, however, the submerged filter where the water flows into forced
mode, t he trickling filter circulates gravitationally under the action of its own weight. As a result of
this specific flow of water, the filter agent is not flooded, being permanently kept wet. Under these
conditions, the filter medium is well aerated, providing enough oxygen for the bacterial population.
Airflow through the filter provides most of the oxygen required for the filter (as opposed to
saturated water containing up to 15 ppm O 2, the air contains 210000 ppm O 2). The energy
required for the operation of these filters is mainly represented by the energy needed to pump the
water at the heights and the required technological flows. There are situations when energy is
also consumed and for the introduction of pressurized air into the filter medium by means o f
compressors.
The thickness of the filter layer on a trickling filter varies between 0.15÷5 m, depending on
the nature of the agent used, the design solution, the flow rate and the degree of waste water
loading.
Periodically, the trickling filters are cou nter-current washed to eliminate the solids deposited
and restore the physical structure of the filtering agent. The washing frequency is as low as
possible, as with submerged filters, as long as the restoration time of the bacterial population
(biological film) is appreciable (20÷30 days).
Figure 20 presents the constructional drawing of a trickling filter in which water is uniformly
distributed over the filter media.

61

Figure 20 – Trickling filter (Losordo et al., 1999)
Drum filters (Biodrums)
They consist of a perforated cylindrical drum disposed in a vat through which waste water
passes ( fig. 21 ). The drum is filled with a certain type of granular agent having high porosity and/or
active surface values. The rotation of the drum is made with an axia l shaft. The water level in the
drum ensures immersion of half the diameter of the drum. As a result of the rotating motion of the
drum, the biological film formed on the granular agent passes alternately, at a certain frequency,
both through air and water .
The rotation speed of the drum is such that the duration of the biological film, represented
by the heterotrophic and autotrophic bacteria populations, does not affect its development due to
lack of oxygen. It is also important that the drum speed does n ot lead to the maintenance of the
biological film longer than necessary to avoid dehydration (drying) of the biological film. In the
drum filter due to the specific mode of operation, the oxygen content of the water is no longer as
important as in the case of the submerged filter, the concentration in ammonia and nitrite being
the one that determines the spin speed of the drum (a lower content in ammonia and nitrite
assumes a lower speed, a stronger load of water in nitrogen compounds requires a higher
rotational speed).
The dynamics of oxygen consumption in the water is different, depending on the speed of
the drum. Thus, in the case of lower rotation speeds, the oxygen in the water is consumed by
bacteria before they are exposed to air. At too high a rate of erosion of the biological film occurs

62
through the entrainment of bacteria due to the hydrodynamic action of the water stream subjected
to filtration. The rotating motion of the drum causes a turbulent water regime inside the filter, which
is why the fil trating agent's clogging rate is quite low. With regard to the energy consumption
required for operation, the drum filters have a specific energy consumption higher than the disk
filters because of the higher turbulence of the water. In a drum filter the l oad losses along it are
usually 1 -5 cm .

Figure 21. Drum filter (Timmons and Losordo, 1994)

Rotating Biological Contactor (RBC)
The operating principle of RBC nitrifying filters is similar to that of drum filters, namely
periodically bringing into contac t with air the biologically active film. The active surface, on which
the nitrifying biological film develops, is represented by the lateral surface of disks disposed
spaced on a rotating shaft at a certain speed, in a continuous rotation motion (Figure 22). The
optimum distance between discs is, as a rule, approx. 20 -30 mm. This distance results in spaces
between discs that are small enough to provide an active filter surface as large as possible. The
aforesaid distance also provides the vital space necess ary for the formation and stabilization of
the biologic film on both sides of each disc and optimum hydraulic conditions for the circulation of
water between the discs covered with bacteria.

Figure 22. Rotating Biological Contactor (RBC) (Losordo et al., 1999 )

63
The rotational speed of the RBC filters depends, as with the drum filters, primarily on the
water loading in ammonia and nitrogen and, to a lesser extent, on the oxygen content of the water.
The turbulence created by the binoculars in the filtering mode of the wastewater is lower than in
the case of the drum filter, and consequently the power required to drive the shaft, or the
specific power consumption that ensures the operation of this type of filter, are lower than in the
case of the rotary drum filter.
For RBC filters, the load losses registered in the water flow direction are almost inexistent,
so RBCs are also more effective from this point of view.
When the design of these filters is judicious, especially with regard to the distance between
the disks and the speed of rotation, the clogging rate is very low and the washing frequency is
low.
Fluidized beds filter
From a constructive point of view, these filters consist of a cylindrical basin whose
dimensions (height and diameter) are determined by the waste water flow and its loading into
toxic nitrogen compounds (ammonium and nitrates). In both the lower and the upper part, the
basin is provided with specific reinforcements that provide access to the wastewater and the
effluent. On a perforated septum placed at the bottom of the unit, the filtering agent is placed, on
which the biological film will be formed and developed ( fig. 23 ).

Figure 23. Fluidized beds filter (Timmons and Losordo, 1994)
Medium specific gravity filtering agents are used t o easily bring in the float and with the finest
granularity so that the active surface is as large as possible. The filter agent that corresponds to
the highest requirements is fine sand. Waste water is pumped into the bottom of the filter by
pumping press ure. In its ascending motion, due to the hydrodynamic drive force, the waste water

64
brings and maintains the floating agent in the form of a fluidized bed. The fluidized bed thickness
depends on the physical -mechanical characteristics of the filter agent an d the water velocity. As
a rule, the fluidized bed occupies a smaller or larger part of the filter height. The water velocity in
the filter is set so that the filter agent is kept permanently suspended in the water mass; at low
water velocities, there is a danger that the filter agent deposits on the perforated septum and at
higher speeds it can be removed from the filter. On the active surface of the fluidized bed a certain
biomass is developed which performs the nitrification process. In the case of fluid ized bed filters,
the oxygen required for the biological film is supplied exclusively by the water stream. Under these
conditions, it is important to ensure a certain water velocity depending on its oxygen content. A
fluidized bed filter operates efficient ly when the dissolved oxygen concentration corresponds to
the saturation limit for a particular temperature.
The fluidized bed filters generally require large flow rates on the surface unit and are small
in size, being compact. It balances hard and once ba lanced they have good efficiency. Most of
the energy required to ensure their functioning is used for the fluidization of the filtering agent and
depends on the flow rate and the amount of load losses.
Bead filter
From a constructive and functional point o f view, plastic ball filters are similar to those
previously described ( fig. 24 ). The difference between them lies in the nature of the filtering agent,
which in this case is represented by plastic balls having a specific subunit weight, thus lighter than
water. For the filter layer to have a specific surface area as large as possible, the size of the
plastic balls is as small as millimeters (usually 2 -4 mm).

Figure 24. Bead filter (Timmons and Losordo, 1994)

65
The water flow in the filter is upward. The s peed of the ascending stream of water to a
plastic ball filter may be lower than that of the fluid bed filter since its floating condition is given by
the specific subunit weight of the beads, and a certain hydrodynamic drive force or a certain speed
is no t required minimum limit. The water flow speed in a plastic ball filter is mainly conditioned by
the wastewater flow.
Both at the bottom and the top, the filter tank has two perforated plate screens, disposed at
different heights. The top perforated plate screen holds the filter balls while running, while the
perforated bottom plate screen forms support for the filter agent when the filter does not work or
when the water flow is discontinuous.
Periodically, plastic ball filters are washed to restore granula rity, or the specific active
surface of the filter medium. There are several washing methods. One of these is to periodically
discontinue the filter operation and drain it; after restarting, for a short period of time, the effluent
is discharged out of the system due to its high suspension content. A second method, which does
not require the interruption of the filter function, consists in periodically shaking the ball bed with
a mechanical device to displace the excessively developed biological film as wel l as the retained
solid particles; as with the previous method, for some time the effluent is eliminated outward, the
filter being connected to the growing system when the water is clean.
It results that plastic ball filters provide mainly biological filtr ation (control of nitrogen
compounds), but also perform a mechanical filtration of solid particles due to the granular
structure of the filtering agent.
The plastic ball bed filters are relatively small in size, so they are more economical under
certain co nditions than other biological filters because the small diameter of the filter agent
provides a large active surface.

4.2.4. Factors influencing the biofiltering process
Chemical factors
Water Reaction (pH). The optimal pH value for the nitrification pro cess is 6÷9. Under certain
conditions, for a particular type of filter, the optimal range may be narrower, depending on the
degree of adaptation of the bacteria from the filters to the water reaction. Nitrification may also
occur at pH values that do not fall within that range, as adaptation of bacterial colonies is slow.
Experimentally, it has been found that at a pH=6, the intensity of the nitrification process is
considerably reduced and at pH=5.5, the nitrifying activity of the biological film usually ceases; for
practical reasons it is recommended to maintain the pH of the water to values higher than 6.
Increasing the pH value to the maximum limit of the optimal range (pH=9) is also contraindicated
since the concentration in non -ionized ammonia (NH 3), an extremely toxic product for fish,
increases directly in proportion to the degree of alkalinity. For these reasons, the efficient

66
operation of a biological filter requires maintaining the pH of the water to the lower limit of the
optimum range for nit rifying bacteria.
Alkalinity . The process of transformation of ammonium into nitrates leads to the decrease of
alkalinity. Carbonates and bicarbonates, compounds that make alkaline water, are the nutrient
support for nitrifying bacteria. Experimentally, it has been shown that lowering the alkalinity of the
carbonate and bicarbonate water can become critical for the development of nitrifying bacteria
and biological filter functions, respectively.
Also, it is recommended to maintain a certain minimum alkalinity threshold taking into
account that due to acid production in the nitrification process, the pH of the water in the filter
tends to decrease.
Oxygen . The rate of nitrification in the filters decreases when the available oxygen for
nitrifying bact eria is insufficient. The oxygen concentration limit is dependent on a plurality of
variables that refer to temperature, concentration in the organic substance of the influent
(wastewater) and the amount of bacteria present in the filter at a given time. T he filters contain
both heterotrophic bacteria and nitrifying bacteria. The first phase of the nitrogen circuit consists
in the decomposition of the organic compounds into ammonia, and in the subsequent steps it is
oxidized successively into nitrites and n itrates respectively. This succession of microbiological
processes determines that in the first stage of nitrification, heterotrophic bacteria are found to be
higher. Subsequently, the Nitrosomonas population participating in the ammonium oxidation will
become dominant in the filter. In the last phase the population of Nitrobacter tends to increase, in
proportion to nitrite concentration. The dynamics of the above mentioned reactions determine that
the oxygen is initially used by the heterotroph and then, successively, by Nitrosomonas and
Nitrobacter . Under these conditions, in the case of filters in which oxygen originates only from the
supply water, it is quite common that Nitrosomonas and Nitrobac ter bacteria no longer have the
necessary oxygen, reducing the rate of nitrification. Scientific information on the minimum limit of
oxygen concentration in nitrifying filters used in aquaculture recirculating systems is insufficient;
the literature indica tes that values higher than 2 mg O 2/L provide an acceptable level of safety for
the operation of nitrifying filters.
In connection with the influence of oxygen on the intensity of the nitrification process, it was
found experimentally that it is signific antly lower than the influence of the concentration in
ammonia and nitrites. So, in the filter activity, the limiting factor will not be the amount of oxygen,
but the concentration in ammonium and nitrites, which is the nutrient support of nitrifying
micro organisms.
Concentrations in ammonium/nitrites. Experimentally, it has been demonstrated that certain
concentrations of ammonium and/or nitrite, considered excessively high, are toxic to the activity
of the microorganisms involved in the nitrification proc ess. Thus, non -ionized ammonia (NH 3)

67
inhibits the activity of the bacteria of the Nitrosomonas genus at concentrations of 10÷150 mg/l
and that of the bacteria of the Nitrobacter genus at much lower concentrations of 0.1÷1 mg/l. As
regards nitric acid (HNO 2), it is considered as inhibitor at concentrations of 0.22÷218 mg/l.
Low concentrations of ammonia and nitrite, specific to the culture water in the growth units
of a recirculating system, are limiting factors of the nitrification rate in the filter since the biological
film has insufficient nutrient support.
Organic macroparticles . Organic macroparticles influence the operation of nitrifying filters in
two aspects, namely, clogging and providing the specific surface for the support of the biological
film.
Particles larger than the pore size of the filter agent will clog the filter over time.
Consequently, there will be pressure losses and flow cuts that may lead to the formation of less
oxygenated, even anaerobic areas, in the filter, which are unsuitable f or the nitrification process.
In the case of submersed or "trickling" filters, it is recommended, in order to avoid clogging them
with organic particles, that the average size of the pellets of the filtering agent is at least 2 cm.
The porosity of the filt er agent in this case ensures a specific active surface, available to form the
biological film, sufficiently large. This recommendation on the granulometric characteristics of the
filter agent does not apply to fluid bed or plastic ball filters, which are not vulnerable to clogging
by their specific mode of operation.
Organic macroparticles are a growth substrate for heterotrophic bacteria that compete with
nitrifying bacteria for growth. For this reason, it is necessary to keep the organic charge within th e
filter at an optimum level.
Dissolved organic substances . It has been noted in the waste water treatment process that
the removal of ammonia is influenced by organic matter loading in the sense that the rate of
ammonia removal decreases with the increase in organic water load. In this regard, it was
emphasized that the nitrification rate was maximum at a BOD5/TAN ratio of 0.25; at higher values
of this ratio, the rate of nitrification is reduced. It is appreciated that the population of nitrifying
organ isms becomes stable when the organic load of water expressed by BOD5 is less than 30
mg/l; at higher BOD 5 index values, nitrifying organisms cannot compete with heterotrophic
organisms whose growth rate is faster.
Salinity . The filters can operate at almo st any salinity regime, from freshwater to salty waters
of 40 ppt, provided that the population of nitrifying bacteria is adapted to the specific salinity
operating conditions. It is known that nitrifying organisms in freshwater are strongly inhibited by
salty waters and vice versa; to a rapid change in water salinity (more than 5 ppt in a few minutes)
the nitrification rate decreases. On the contrary, a constant salinity maintains a high rate of
nitrification. Experimentally, it was established that the ma ximum specific rate of growth of

68
ammonium oxide oxidation decreases when the water salinity gradient is higher than 0.04 ppt/h
in the case of freshwater, respectively, 0.0028 ppt/hr in the case of salt water.
Rate of gas diffusion . The filter medium of an operational nitrification filter is coated with a
biofilm (biological film) as a result of bacterial growth. Bacterial cells develop within this biofilm.
Under these conditions, oxygen, ammonia and nitrites must diffuse inside the film and nitrites,
nitrat es and carbon dioxide must diffuse outward through this film. Adjacent to the biofilm there is
a stagnant water layer through which the substances must diffuse to penetrate or exit the cells.
Thus, the diffusion process may, under certain circumstances, co ntrol the rate of nitrification. The
rate of diffusion increases or decreases in proportion to the temperature.
Other chemicals . The literature offers a variety of information on the influence of other
minerals and chemicals on the nitrification process. T hus, it is considered that a high level of
calcium is necessary for an optimal activity of the bacteria of the genus Nitrosomonas and a high
magnesium concentration is essential for the bacteria of the Nitrobacter genus. Phosphates,
magnesium, molybdenum, iron, calcium, copper and sodium are considered as stimulants in the
nitrification process. On the contrary, copper (over 0.56 mg/l), chromium, nickel, zinc, mercury,
silver and some organic compounds (vitamins, amino acids, alcohols, etc.) are known for t heir
inhibitory effect in the nitrification process. Treatment of fish growth systems for therapeutic
purposes and in judicious concentrations established with formalin, copper sulphate, potassium
permanganate and sodium chloride does not affect nitrificat ion. However, other therapeutic
agents may diminish or even interrupt the nitrification process.
Physical factors
Temperature . Regarding the influence of temperature on the nitrification process, there is a
wealth of information in the literature, some of them contradictory. There is a consensus about
the possibility of gradual adaptation of bacteria to a wide range of temperatures. Thus, there are
data demonstrating that bacteria that oxidize ammonium have been isolated, able to withstand –
5 °C; optimal gr owth for these bacteria occurs at 22 °C, while temperatures above 29 °C are fatal.
In other cases, the bacteria adapted to 25 °C develop optimally at 30 °C and their lethal
temperature is 38 °C. The data suggested suggests that nitrifying bacteria can adap t to a wide
variety of temperatures over appropriate periods of time. To deepen these aspects, the effect of
temperature in the range 7÷35 °C on nitrification in a drum filter was studied (Timmons and
Losordo, 1994).
Reynolds number and RBC rotation speed . It is difficult to characterize and quantitatively
describe the flow regime in any type of nitrification filter. The Reynolds number, which expresses
the ratio of inertial forces to viscosity forces, is frequently used for hydraulic characterization of
flow. The Reynolds numbering relationship contains a characteristic length term that is currently

69
difficult to quantify for the specific flow pattern of a nitrifying filter. Modeling the operation of a
nitrification filter implies a good understanding of the Reynolds number for filter flow.
The rotational speed of RBC filters determines the magnitude of the tangential force acting
on the biofilm, and in this way the contact between the biofilm and the substrate is influenced.
Elimination of ammonia increases as the filter speed increases to a certain point when the relative
peripheral speed (relative to water) of the discs is 0.305 m/s; increasing the speed above this
level does not improve the efficiency of the RBC type nitrifying filters.
Porosity of the fil ter. The porosity of a filter is the ratio of the air volume to the volume of the
filter medium. A high porosity of the filter agent reduces clogging; filter media with a high porosity
coefficient have large, open spaces where solids can easily pass. Also, a large porosity causes
low water pressure losses when passing through the filter. It is concluded that a good hydraulic
operation of the filter, in terms of clogging dynamics and that of load losses, recommends filtering
media with a higher porosity. Fro m the point of view of the specific active surface available, the
surface constituting the support of the biological film, fine -grained filler agents are indicated,
hence the ones with low porosity. In determining the optimal diameter of the granules of th e filter
medium, in addition to the above mentioned considerations, it will also be intended to provide the
best aeration of the filter, which is particularly important in the case of submersed filters.
Nature and granularity of the filter medium . The filt ering agent is a solid material added to
the filter to provide growth support for nitrifying bacteria.
In terms of their nature, there are several types of filtering agents used in the nitrification
process. Typical filtering agents are various mineral agg regates (sand, gravel, crushed stone,
limestone rocks, etc.) as well as various plastic media. The type of filter agent is chosen according
to certain criteria, the most important being granularity, specific surface area, specific cost and
weight. The mine ral aggregate is not recommended as a filter media for drum biofilters, as well
as those that require frequent handling due to washing operations. The use of the mineral
aggregate as a filter medium is advisable primarily because of the generally low cost. An important
argument in the use of carbonate rocks as a filter medium is their buffering capacity, which is
appreciable until full coverage by the biological film. Compared with mineral filtering agents, the
plastics provide some technological advantages , namely reduced specific weight and porosity,
respectively, the specific surface area. Plastic structures are more expensive and have no water
buffering capacity, but the advantages mentioned are important, justifying their widespread use
in aquaculture. In order to ensure proper use during filter operation, plastic filter media should not
be exposed to sunlight.
The size of the granules of the filter media (granularity) determines the clogging rate and the
amount of pressure loss. This requires a minimum limit below which the particle diameter should

70
not decrease so that the clogging rate is as low as possible and the pressure losses are not
excessive.
The nature of the filter medium and its granularity are chosen, differentiated for each type of
filter, a ccording to their mode of operation. Thus, in the fluidized bed filters, sand is commonly
used as a filtering agent; the small size of the granules provides a high specific surface area of
the filter medium and its high specific weight prevents sand from filter removal. With regard to
drum and disc filters (RBC), they can theoretically be used with any type of filtering agent. In their
case, the main practical problem to consider when choosing the filtering agent is to have a specific
weight as small as p ossible to limit the mechanical stresses that occur during operation. This
condition is met in the case of drum filters with plastic shapes of various shapes (balls, plastic
rings and so on); For RBC filters, for the same reason, the biodisks will be made of lightweight
materials (fiberglass, wood, plastic, etc.).
Active specific area . The specific surface area represents the surface area of the filter agent
relative to the volume unit. From a constructive and functional point of view, for a nitrifying fi lter,
the specific surface is an essential parameter as it determines the degree of development of the
biological film. The specific surface area of a filter agent depends on its granularity characteristics,
the most important being the porosity and the diameter of the constituent particles. Filtering media
of low porosity filtering agents, i.e. small granule diameters, provide higher specific surfaces;
under these conditions, more bacteria can be developed per volume unit of the filter medium and,
implic itly, increases the rate of ammonia removal. However, the diameter of the filter media
granules cannot be reduced below a certain amount to limit load losses and clogging rate.
Hydraulic Loading . In the case of submersed, "trickling" or fluidized bed filte rs, hydraulic
loading has the meaning of unit flow being a measure of the amount of water added to the filter
per surface unit and time; is usually expressed as a m3/(m2 x day) ratio. For drum or disk filters,
the hydraulic load most frequently expresses t he volume of water passed through the filter relative
to the area of the filter active surface area.
There are two limit values for the hydraulic charge rate of the nitrifying filters, namely a
minimum value and a maximum value.
Minimum hydraulic load is determined by the lowest flow rate that maintains the entire filter
wet. The lower value of the leakage hydraulic load is more restrictive for submersed filters and
"trickling" filters than for drum or biodisks filters. In "trickling" filters, the filte r medium must always
be wet for bacteria to survive. For submersed filters, the minimum hydraulic charge is imposed
by the smallest water flow that ensures the oxygen demand of the filter. For drum or biodisks
filters, the minimum hydraulic load is given b y the water flow that provides sufficient nutrient intake
for the nitrifying microorganisms.

71
The maximum hydraulic loading of a nitrifying filter is determined by the condition that the
flow rate of the water through the filter does not cause the biodegrad ation of the film or cause
excessive loss of pressure (loss of pressure). When determining the maximum hydraulic load, a
number of additional restrictive conditions imposed by the specific filter mode may occur. Thus,
in fluid bed filters the speed of the ascending stream of water in the filter must not exceed a certain
value in order to maintain the filter agent within the filter; drum, or bio -distribute, the hydraulic
charge will be set according to the rotational speed so that the biofilm is not dislodge d by the
centrifugal force.
Depth and submergence . Depth is a constructive feature specific to submersed and
"trickling" filters, and submersion is a feature that refers to drum filters and discs that express the
depth of immersion in the vat.
For functional reasons, the submergence is considerably less than half the diameter of the
drum or discs; this allows an optimal exposure of the drum or discs to the air and at the same
time ensures the operation of the shaft of the plant under unsupervised co nditions. The literature
indicates that a submergence of 35÷50% of the diameter of the drum or discs ensures efficient
nitrification; for the mentioned interval there are relations of direct proportionality between the
yield of nitrification and the value of the submergence, relations used in the design and
exploitation of biological filters.
The depth of the submersed and "trickling" filters depends on a multitude of variables.
Following the specific mode of operation or water circulation, the submersed fi lters require a
sufficient depth so as to have the necessary time for oxidation of the organic compounds by the
heterotrophic bacteria, a precursor to the nitrification, respectively for the oxidation of ammonia
to the nitrate phase by Nitrosomonas and Nit robacter respectively. Of the plurality of variables
contemplated for determining the depth of a submerged filter so as to provide the necessary
conditions for carrying out said conversions, the most important are: the nature and granularity of
the filteri ng agent, the hydraulic loading rate, the ammonia loading rate and the Reynolds number.
Depending on the variables presented, the depth of the submersed filters and the "trickling" filters
is variable, for the same load in organic matter and ammonia, from a few centimeters when the
filter medium is sand to 4÷5 m when the filter medium is made of plastic rings of 8 cm. As a
general recommendation, it is estimated that the minimum depth for submersed and "trickling"
filters cannot be less than 1 m, except for filters where the fine sand filter medium is used. Filters
whose filter medium has a particle diameter of 7÷10 cm and a high hydraulic charge can have a
depth of 4÷5 m.
Area of cross section . This constructive parameter expresses the cross -sectional are a of
the upper part of the filter in which no filter medium is found. Its value is important to ensure

72
designed hydraulic load. For drum and disc filters, the cross -sectional area is of lesser
importance; for these, the diameter of the drum and discs is important.
Thickness of film . At the level of the biological film there is a thin (thin) film of stagnant water.
The optimal metabolic activity of bacteria requires this layer of water to have a certain thickness.
Film thickness depends on water speed, viscosity and water temperature. By modeling the water
drainage phenomenon through different filtering media, it has been determined that the thickness
of the film may vary between 1÷100 µm, most frequently 50÷60 µm.
Light . The results of the application o f various nitrification equipment in the aquaculture of
the recirculating systems show that bacterial activity is more intense in dark conditions; an
intensity of light of even 1% of sunlight inhibits nitrifying bacteria. The negative effect of light on
the metabolism of nitrifying bacteria is related to the cytochrome C oxidation phenomenon in both
Nitrosomonas and Nitrobacter genes. Nitrobacter bacteria are more sensitive to light than those
of the Nitrosomonas because they contain less cytochrome C. Rega rding the influence of light on
the activity of the decaying bacteria, accumulated experience indicates that complete darkness is
preferable to diurnal cyclical light.
Biological factors
Density of biomass . This parameter is a measure of the mass of bacter ial cells per unit
volume of biofilm. Biomass density depends on availability in nutrients, water speed over
biological film, cell characteristics, and so on. The density of the biomass varies considerably from
one area to another within the same filter. A lso, this parameter is very different from one filter to
another. For the different types of nitrifying filters used in aquaculture, an average biomass density
of 886 g cells/m3 was determined by appropriate modeling.
Cell production . This biological facto r expresses the amount of nitrifying bacterial cells
produced by the ammonia -converted nitrate unit. A biomass of 0.17 g of nitrifying bacterial cells
is formed per gram of nitrate oxide ammonia. The said rate of growth of nitrifying bacteria, relative
to the substrate unit, is lower than the rate of growth of heterotrophic bacteria; this is a major
advantage in the biofilters' operation due to the lower rhythm of clogging. A lower value of cell
production than indicated indicates a decrease in the efficien cy of the nitrification process.

73
4.2.5. Ratings of different biological filtration
System
technological
process /
component Technology /
Equipment Sub-technology /
Sub-equipment Space
requirements Operational
management
(Ease of use) Operational
performance Acquisition and
implementation
cost Maintenance
and
operation
costs
Biological
filtration Submerged filter
Trickling filter
Drum filter (Biodrum)
Rotating biological contactor (RBC)
Fluidized beds filter
Bead filter
NOTE: Regarding the scoring system: Space requirements (where  is large and  is small); Operational management (Ease of use – where  is hard and 
is easy); Operational performance (where  is low and  is high); Acquisition and implementation cost (where  is expensive and  is cheap); Maintenance
and operation costs (where  is expensive and  is cheap). All scoring was done by comparing a technology/equipment with another (within the same cat egory).

74

4.3. Aeration and Oxygenation
4.3.1. Introduction
Dissolved Oxygen (DO) is one of the most important water quality parameters that
determines, in a limitative sense, the level of the carrying capacity and the density of the
population in a recirculating system. The limitation is due to the relatively high oxygen requirement
of aerobic organisms within the system, the relatively low solubility of oxygen in the water, the
absence of photosynthesis, and the reduced water ret ention rate.
In order to avoid oxygen becoming a limiting factor, it is necessary to supplement it with
respect to the feed ration.
Depending on the size of the carrying capacity of the system, there are a variety of oxygen
enrichment methods
Thus, in the case of systems designed for reduced load capacities, water oxygenation is
achieved by processes whose principle of operation is to provide a maximum contact surface
between atmospheric air and water. For this purpose, specific installations and equipment are
used, such as waterfalls in water basins, water jets, paddlewheels surface aerators, etc.
In the case of recirculating systems with a very high load capacity, the supplementation with
DO is frequently carried out by passing water through an oxygen -enriched gas installation. This
is designed to provide a large contact surface between the gaseous phase and the liquid phase.
This type of equipment has as main quality overloading of water with oxygen, while avoiding water
loading with dissolved nitrogen (DN ). The use of supersaturated water with DO allows for
increased stocking density and, implicitly, lower production costs by reducing basin volume, water
flow rate through the system, and the capacity of the water treatment unit. Functionally, these
systems have the advantage of not introducing nitrogen into the water, thereby providing the
possibility of dissolving more oxygen in the water while the total dissolved gas pressure (TGP) is
constant.
The management of dissolved oxygen from water to aquaculture farming systems involves
knowing a multitude of aspects, namely: the properties of dissolved gases in water; the operating
principle, the performance indicators and the design methods of the various types of aeration or
oxygenation equipment and installati ons.
4.3.2. Gas transfer
In the design and operation of aeration – oxygenation equipment it is necessary to know a
series of basic notions regarding gas transfer theory, such as: gas solubility, transfer speed (rate)
and specific performance indicators.
Gas solubility

75
The saturation concentration of a water dissolved in water (C*) influences both the direction
and the rate of gas transfer at the gas -water interface. The value of the saturation concentration
(C*) of a gas is determined by its partial press ure in the gaseous phase, the water temperature
and its composition.
Transfer speed
The driving force to which the transfer of molecular oxygen from the gas phase to the liquid
is mainly determined by the difference between the saturation concentration of the dissolved
oxygen in water (C*) and the oxygen concentration of the environment (C).
In principle, the transfer of oxygen from the gaseous to the liquid phase takes place in
several stages. In a first step, the oxygen molecules pass from the gas phase t o the gas -liquid
separation interface. Further, the oxygen molecules diffuse through the laminar gas and liquid
layers of the interface, and ultimately, their permeation into the liquid phase.
Upon their transfer from the gaseous to the liquid phase, the o xygen molecules (nitrogen,
carbon dioxide, etc.) have a certain resistance; the maximum value of this resistance is recorded
in the gas diffusion phase in the liquid.
Under these conditions, the net transfer rate (absorption or elimination) of the oxygen i n the
gaseous phase in the liquid phase is directly proportional to the difference between the saturation
concentration of the dissolved oxygen in water (C*) and the oxygen concentration of the
environment (C).
Performance indicators of the aeration – oxyg enation systems
In the practice of aquaculture rearing systems, a variety of equipment is used to enrich the
water content of oxygen, depending on the degree of production intensity and the
ecotechnological particularities of the crop species. Depending on these, there are, in principle,
two systems of equipment to ensure optimum oxygen content of water, namely: air -water contact
equipment and pure oxygen input equipment in the water mass; each of these systems is
characterized by specific performance indic es.
Air – water contact aeration equipment
Air-water contact equipment (air -water contactors) are used in rearing systems with a
limited degree of production intensity where the oxygen demand is lower in proportion to the
amount of biomass. The main aerati on equipment used in aquaculture are: floating surface
stirrers, submersible air diffusers, surface stirrers with ejector tube, water jet stirrers, padding
aeration columns, air -lift equipment, waterfall perforated trays.
Air-water contact systems are desi gned to accelerate the absorption of oxygen in the air by
various processes, the most commonly used of which consist principally in increasing the water –
air contact surface and/or the introduction of air under pressure into the mass of water.

76
By contacting the water with the air, the partial gas pressures in the air or the partial stresses
of the same gases dissolved in the water tend to a state of equilibrium that is reached by the gas
transfer between the two phases.
Air stone . Air stones are very ineffic ient oxygen transfer devices (3 –7%), but very
inexpensive in terms of capital and operating costs. At low stocking densities and high exchange
rates, they work very well at maintaining adequate oxygen levels. One disadvantage is the
maintenance requirement s due to clogging and biofouling, especially in very hard water (Timmons
et al., 2002).
Oxygen -water contact equipment
Pure oxygen -water contact equipment (water -oxygen contactors) is used in densely
populated growth systems where the rate of oxygen consumption is very high.
The performance of water -oxygen contactors is assessed using several indicators, namely:
oxygen transfer rate (OTR), oxygen transfer e fficiency (OTE), oxygen absorption (OA), and
oxygen transfer costs (OTC).
When using these types of contactors, it is possible to over -saturate the water with dissolved
oxygen.
U-tubes . The U -tube aerator operates by increasing the gas pressure, thus incr easing the
overall gas transfer rate. It consists of either two concentric pipes or two pipes in a vertical shaft
9 to 45 m deep (fig.25). Oxygen is added at the upper end of the down -leg of the U -tube and as
the water/gas moves downward through the contac t loop, an increase in hydrostatic pressure
increase the oxygen transfer rate. The overall oxygen transfer efficiency is a function of the depth
of the U -tube, inlet gas flow rate, water velocity, diffuser depth and inlet DO concentration.
Concentrations o f dissolved oxygen ranging from 20 –40 mg/L can be achieved, but the overall
oxygen transfer efficiency if only 30 –50%. Off -gas recycling can improve the absorption efficiency
to 55 –80%. Two advantages of the U -tube are the low hydraulic head requirements t hat allow
operation with no external power if sufficient head is available, and that it can be used with water
containing high levels of particulates or organics. Its chief disadvantages are that it does not vent
off gasses such as nitrogen or carbon dioxi de very efficiently and construction costs can be high,
particularly if bedrock is present (Timmons et al., 2002) .
U-tubes are designed for flows where the downflow velocity is between 1.8 to 3.0 m/s. A
particularly unique problem with U -tubes is that if t oo much oxygen is added a gas bubble
blockage can occur that results in flow interruption. This will tend to happen if gas -liquid ratios
exceed 25%. Be careful when adding oxygen (Timmons et al., 2002) .

77

Figure 25 – U-Tube aeration system
Packed Columns . Packed column aerators (PCA) consist of a vertical column filled with
media having a high specific surface area. Water is uniformly distributed over the top of the media
with a perforated plate or through a spray bar, and trickles down through the media. Oxygen is
injected into the column and is transferred into the passing water through the large gas/liquid
interface on the media. The column may be either open or closed at the top. Packed columns are
efficient nitrogen strippers, but require a higher gas /liquid ratio (forced aeration) for carbon dioxide
stripping. Packed columns are simple to build and easy to retrofit into existing facilities.
Performance design characteristics include the water distribution method, media characteristics,
media bed depth , gas/liquid loading rates, inlet DO concentration and operating pressure. Their
main disadvantage is fouling due to the accumulation of organics and particulates on the media
over time (Timmons et al., 2002) .
Packed columns have two additional advantages (Timmons et al., 2002) :
• Provide nitrification;
• Provide CO 2 gas stripping.
LHO’s . Low Head Oxygenators (LHO) are being used more frequently, particularly because
of their adaptability to high flows using minimal hydraulic head, hence their name Low Head
Oxygenator. The original LHO design was developed and patented by Watten (1989). L HO’s vary
in configuration, but all are fundamentally similar in operation. These units consist of a distribution

78
plate positioned over multiple (5 to 10) rectangular chambers (fig. 26). Water flows over the dam
boards at the end of a raceway or is pumped upwards from an indoor fish tank, through the
distribution plate, and then falls through the rectangular chambers. These chambers provide the
gas-liquid interface needed for mixing and gas transfer. The streams of falling water impact a
collection pool at the bottom of each chamber where the effluent water flows away from each
chamber equally in parallel. All of the pure oxygen is introduced into the outer or first rectangular
chamber. The mixture of gases in the first chamber, which now has a diluted oxyge n concentration
passes, sequentially through the remaining chambers. The gaseous mixture will decrease in
oxygen concentration from chamber to chamber as the oxygen is continued to be absorbed.
Finally the gaseous mixture will exit from the last chamber. T his gas is referred to as off -gas. Each
of the rectangular chambers is gas tight and the orifices between the chambers are properly sized
and located to reduce back -mixing between chambers (Timmons et al., 2002) .

Figure 26 – Low head oxygen (LHO) unit. A typical LHO configuration and components are
shown: water flowing into a collection trough or plate (A), through a perforated distribution plate
(B), and is oxygenated in the chambers (C), as gas flows from inlet gas port (D), through holes
between chambe r to chamber (E), to the off gas port (F), where excess gas is bubbled off under
water. Water exits at the bottom of the unit (G) (Timmons et al., 2002) .
Aeration Cone/Downflow Bubble Contactors . The aeration cone, bicone , or downflow
bubble contact aerator consists of a cone -shaped cylinder or a series of pipes with reducing
diameters. Water and oxygen enter at the top of the cone, flow downward, and out. As the cone’s

79
diameter increases, the water velocity decreases, unt il the downward velocity of the water equals
the upward buoyant velocity of the bubbles. Thus, the bubbles are held in suspension, until they
dissolve into the water. The performance of aeration cones is determined by gas and water flow
rates, influent DO concentration, cone geometry and operating pressure. Absorption efficiency
range from 95 –100% with effluent concentrations from 30 to 90 mg/L. Commercial units are
available that transfer from 0.2 to 4.9 kg of oxygen per hour, at 25 mg/L, at flow rates fro m 170 to
2,300 L per minute ( fig. 27 ) (Timmons et al., 2002) .

Figure 27 – Aerator cone (downflow bubble contact aerator) (Stickney, 2000)

Diffused Aeration (Air Stones) . Due to their low absorption efficiency, the use of diffusers
or air stones have been limited mainly to emergency oxygenation and fish live -haul systems.
Although some of the recent fine -bubble diffusers (bubbles 100 to 500 microns) do perform well
in dee p tanks (50% oxygen transfer efficiency), they require a high pressure source of oxygen
(25–50 psi) and are subject to both chemical and organic fouling (Timmons et al., 2002) .
Oxygen Injection . The most widely used form of oxygen injection takes advantage of the
increased pressure available when pumping water. Oxygen is injected though a Venturi nozzle or
orifice, creating a fine bubble suspension in the pressurized line. Pressures of 30 –235 psi (2 to
22 atmospheres) are needed to achieve satisfactory abso rption, with contact times of 6 –12
seconds. Absorption efficiency ranges from 15 to 70% with effluent DO concentration form 30 –
50 mg/L (Timmons et al., 2002) .

80
4.3.3. Ratings of different Aeration and oxygenation
System
technological
process /
component Technology
/
Equipment Sub-technology /
Sub-equipment Space
requirements Operational
management
(Ease of use) Operational
performance Acquisition and
implementation
cost Maintenance
and
operation
costs
Aeration and
oxygenation Air – water
contact
aeration Air pump and air stone     
Submersible air diffusers     
Waterfall perforated trays     
Oxygen –
water
contact U-tube     
Packed column aerator (PCA)     
Low Head Oxygenator (LHO)     
Aeration Cone     
Diffused Aeration / Air Stones     
Oxygen Injection     
NOTE: Regarding the scoring system: Space requirements (where  is large and  is small); Operational management (Ease of use – where  is hard and 
is easy); Operational performance (where  is low and  is high); Acquisition and implementation co st (where  is expensive and  is cheap); Maintenance
and operation costs (where  is expensive and  is cheap). All scoring was done by comparing a technology/equipment with another (within the same category).

81

4.4. Carbon Dioxide Control
4.4.1. Introduction
Carbon dioxide management is an essential issue for intensive aquaculture systems. If its
concentration is not maintained at an optimum level, important ecotechnological changes are
made in the system consisting of the f ollowing: reducing the water reaction to stress levels,
disturbing the fish's respiration mechanism even in the presence of an optimal oxygen
concentration, reducing the efficiency of the nitrifying filters.
In the case of less intensive rearing systems th ere is no tendency for carbon dioxide to
accumulate up to the tolerance limit. In aquaculture farming systems where water is re -used or
recirculated, carbon dioxide can accumulate up to stressful concentrations. In addition, the
increasing use of pure oxyg en as a source of dissolved oxygen in intensive systems has led to a
decline in conventional aeration methods (mechanical agitation, water flow through obstruction
environments: cascades, stuffing columns, etc.) which, due to their specific mode of operati on,
also provided effective water degassing, including the removal of excess carbon dioxide.
Carbon dioxide is a product of the respiratory activity of fish and other organisms that may
be present in a rearing system, the most important of which is bacteria in the filters. An important
difference between carbon dioxide and other dissolved gases, s uch as oxygen and nitrogen, is
that carbon dioxide is part of a chemical balanced system and its concentration in solution is
influenced by pH. A result of carbon dioxide dependence on pH is the possibility of reducing its
concentration by different physic al or chemical processes.
Changing the pH of a solution is the primary chemical method of reducing the concentration
of CO 2 in water, a method whose efficiency is determined by alkalinity, initial pH and temperature.
Physical removal methods are based on t he transfer of carbon dioxide from water to an
adjacent gaseous phase, usually air. The removal of carbon dioxide causes the pH and
concentration of the inorganic carbon to change and thereby to change the share of inorganic
carbon in water as carbon dioxi de.
The mechanism of carbon dioxide removal from the water is complex due to the interaction
between physical gas and chemical transfer processes on the acid -base reaction.
The accuracy of estimating the rate of carbon dioxide removal depends on the extent to
which changes in the composition of the gases and the chemical processes occurring within the
carbon system are taken into account. Thus, a crude estimate of carbon dioxide removal implies
ignoring changes in gas composition as well as chemical reactio ns occurring at the carbon system
level.

82
Due to the diversity of known physical processes for the removal of carbon dioxide from
aquaculture waste waters, the most common, and with very good results, are the transfer of gas
in packed columns.
Mainly, chemi cal processes for carbon dioxide control and carbonate balance consist in
applying a specific pH adjustment to water depending on the transformations that take place
within the system.
In intensive aquaculture systems, carbon dioxide is the result of aero bic breathing of aquatic
organisms, namely the reared biomass and the bacterial biomass. The amount of carbon dioxide
produced is directly proportional to the amount of oxygen consumed. A simplified stoichiometric
description of breathing results from the reaction:

According to this simplified stoichiometry, one mole of carbon dioxide is produced by each
mole of oxygen consumed, and for each gram of oxygen consumed there are 1.27 grams of
carbon dioxide produced (Grace and Pie drahita, 1991 cited by Timmons and Losordo, 1994).
Carbon dioxide in an intensive aquaculture system is mostly produced in growing tanks; In
addition, a lower amount of carbon dioxide is also produced in biological filters.
In less intensive crop systems w ith a low degree of water reuse, the required oxygen is
normally provided by the oxygen -rich influent water stream and the resulting carbon dioxide is
discharged by the effluent water stream; this avoids the accumulation of carbon dioxide in the
system.
In the case of densely populated and high -water reuse systems requiring pure oxygen
supply, the balance between the carbon dioxide production rate and its removal rate in the system
is disrupted and therefore the carbon dioxide concentration increases, requi ring controlling it to
keep it within optimal limits.
The system's biomass loading rate (system load capacity, kg/L/min) that does not cause a
dangerous level of carbon dioxide is dependent on a multitude of ecotechnological parameters
such as: water tempe rature and alkalinity, feeding rate, tolerance of species to CO 2 content, etc.
Thus, the literature (Piedrahita and Grace, 1991 quoted by Timmons and Losordo, 1994) indicates
a carbon dioxide concentration of 20 mg/L at a system capacity of 4.5 kg/L/min.
High concentrations of carbon dioxide influences a recirculating rearing system in the
following two ways:
• dysregulation of the fish's respiratory mechanism by reducing the blood's ability to transport
oxygen;
• decrease in the pH value of the water.

 OnH nCO OCH nO2 2 n 2 2  

83

4.4.2. Carbon balance and carbon dioxide control by pH management
Carbon dioxide control by pH management consists in changing the equilibrium
concentrations of species from the carbonate system.
The balance of carbonated carbon expresses a dependency relationship between pH and
carbon dioxide (CO 2), carbonic acid (H 2CO 3), bicarbonate (HCO 3-) and carbonate (CO 32-).
Carbon dioxide is not a component of alkalinity. Although carbonate and bicarbonate are
reduced when carbon dioxide is removed, the molar su m of the alkaline components remains
constant. Thus, alkalinity remains constant in both the addition of CO 2 in water and its removal
from the solution.
Practically, the reduction of carbon dioxide can be achieved by adding to a water a
concentrated base s olution, such as sodium hydroxide. The basic solution is introduced into the
water with a chemical dispensing pump and with a pH sensor system , the process can be
automated.
Addition of sodium hydroxide or a similar strong base leads to an increase in alka linity
without causing changes in total carbon carbonate concentration. Increased alkalinity,
accompanied by increasing pH and decreasing carbon dioxide levels, causes the rebalancing of
carbonate species to increase carbonate and bicarbonate ions. The tot al carbonated carbon
concentration remains, however, unchanged.
An alternative method of chemical control of carbon dioxide in a growth system consists of
using Na 2CO 3 or calcium -containing substances such as Ca(OH) 2, CaMg(CO 3)2, or CaO. And in
this case, the alkalinity of the solution increases, and in some cases also the total carbonated
carbon.
The analytical determination of the change in carbon dioxide concentration of an aqueous
solution as a result of the addition of one of these substances is based on the carbonate system
equation. With an acceptable accuracy, the concentration in carbon dioxide in an aqueous
solution can also be determined graphically
The chemical control of the carbon dioxide concentration of a growth system causes, over
time, a st eady increase in carbonated carbon concentration and alkalinity. This phenomenon
requires a periodic replacement of water in the system in order to keep the two parameters in the
optimal technological field.
The chemical process apparently ensures an easy control of the carbon dioxide
concentration in the culture water in a recirculating system. The application of this procedure in
practice, however, presents a number of inconveniences among which the most important are:
the high cost of the chemicals used, difficulties in handling the reagents, the induction of negative
technological effects, etc.

84
4.4.3. Carbon dioxide control by gas exchange
Physical processes have a wide use in carbon dioxide management in aquaculture
recirculating systems. The main physi cal process used to remove carbon dioxide from water
consists of exchanging gases between two different physical media, namely water and air.
Removing carbon dioxide through gas exchange or aeration is a very complex process. Unlike
other dissolved gases ( N2, O 2), carbon dioxide is present in the solution as part of a chemical
equilibrium system. Thus, by removing the carbon dioxide from the solution, the configuration of
the chemical equilibrium system, with respect to the concentration of the species in t he carbonate
system, changes, their sum remaining constant. Under these conditions, the bicarbonate, [HCO 3-
], is constituted in a carbon dioxide reservoir which completes, by dihydroxylation, the dissolved
carbon dioxide (CO 2 (water)) as it is removed from the solution. The dihydroxylation reaction of
carbonate in carbon dioxide is not instantaneous. Therefore, there is a delay in the dihydroxylation
of carbonate in carbon dioxide which develops as it is removed from the solution; the magnitude
of the delay is dependent on the kinetics of the carbonate reaction.
Gas exchange .
Supersaturated carbon dioxide from an aqueous solution can be transferred to a gaseous
phase. Exposure of water to the gaseous phase can be accomplished in various ways, namely:
introdu cing air or pure oxygen into the water, passing water through U injector tubes, degassing
columns (PCA) or cascades, using paddles, etc. Since the carbon dioxide concentration in
intensive growth systems is typically 20 times higher than the saturation con centration of the
environment, the carbon dioxide transferred has a significant impact on the adjacent gaseous
phase. Therefore, the gaseous phase of the liquid must be permanently refreshed (replaced) to
prevent the accumulation of carbon dioxide, i.e. to increase its concentration to values that reduce
the efficiency of gas transfer between the two media. For this reason, maintaining a proper rate
of carbon dioxide transfer from water to air requires that the ratio (G/L) between the flow rate of
the gas eous phase (G) and the liquid phase (L) be as high as possible. Under these conditions,
the amount of gas available to transport the carbon dioxide transferred from the water is high,
eliminating the possibility of accumulating it at the interface of the t wo media. The G/L ratio is the
most important variable that causes the removal of carbon dioxide. In the case of degassing
columns (PCAs), most commonly used in aquaculture for carbon dioxide removal, it was
determined experimentally that the minimum value of the G/L ratio, volumetric, is 3.0. Aerating
systems with a G/L ratio of less than 3.0, such as diffusers and injectors, are not indicated for
aquaculture rearing systems characterized by high concentrations of carbon dioxide that require
high rates of removal. It is appreciated that only judiciously designed paddle aerators and
degassing columns (PCA) expose the water to a sufficient amount of air to ensure efficient carbon
dioxide removal. The carbon dioxide removal rate was quantified only for compact degassing

85
columns (Grace, 1992); for other aeration equipment, data on carbon dioxide removal rate are
insufficient.
Carbon dioxide transfer coefficient
The global mass transfer coefficient (K La) is usually determined and, therefore, known for
oxygen. The global mass transfer coefficient for carbon dioxide is determined by the global
oxygen mass transfer coefficient and by the ratio of the molecular diffusivity coefficients of the
two gases. Oxygen diffusivity is 2.5 x 10-5 ± 20% atm-1 while carbon dioxide diffusivity is 1.96 x
10-5 ± 1% atm-1 (Perry, Green and Maloney, 1984 quoted by Timmons and Losordo, 1994). The
value range for the ratio of diffusivity coefficients is 0.65÷0.99. Even though this ratio is less than
1.00, a mass transfer coefficient for c arbon dioxide equal to that of oxygen is recommended when
designing degassing columns (PCAs).
Depending on the availability of the data that characterizes the operation of a degassing
column (PCA), there are more or less accurate methods to estimate carbon dioxide effluent
loading.
An approximate (gross) method for estimating the carbon dioxide removal by aeration in a
degassing column is based on the following simplifying assumptions:
• the transfer of gas from the liquid does not significantly affect the co mposition of the adjacent
gaseous phase nor the value of the saturation concentration;
• no chemical reactions occur in the carbonate system during the transfer.
Characterization of gas flow
It must be taken into account, in addition to the G/L ratio, the ch aracteristics of the gas flow
through the column, which can be in the block (laminar), completely mixed (turbulent) or combined
(mixed).
The flow regime significantly influences the saturation concentration in different sections of
a PCA column. If the gas flow through the column is turbulent, then the saturation concentration
is constant over the entire height of the column. In this case, the rate of carbon dioxide removal
is directly proportional to the height of the degassing column.
When the flow of gas inside the column is laminar, the molar fraction of gas or corresponding
saturation concentration is variable on column height. The gas flow regime in a laminar or
turbulent degassing column (PCA) is conditioned by the column dimensions (height and diamet er)
and the gas flow rate.
For a more accurate determination of mass transfer, assuming a laminar flow regime, it is
necessary to take into account the fact that the saturation concentration is variable on the height
of the column.

86

4.4.4. CO 2 control by gas transfer combined with kinetics of chemical reactions
It was found that the removal of carbon dioxide with a degassing column (PCA) is the
combined result of gas exchange and chemical reactions, both phenomena being prod uced
simultaneously. The operation of the column involves the entry of waste water at the top of the
column, with all carbonate species in balance. In the first section of the column, of the carbonate
species, only the carbon dioxide is removed as a result of the gas exchange. The removal of
carbon dioxide reduces total carbon dioxide and increases the pH by adjusting the carbonate
carbon species balance levels accordingly. While the bicarbonate, carbonate and hydroxyl ions
instantly reach the relative bala nce, the balance between carbon dioxide and bicarbonate is
reached at a lower rate after a certain period of time. During the flow of liquid from the top of the
section to the bottom of it, part of the bicarbonate ions react to form carbon dioxide. Due to the
difference between the rate of carbon dioxide removal by gas exchange and the CO 2 production
by dihydroxylation, the difference between the actual and the carbon dioxide equilibrium
concentration appears in the column, a difference which increases the flow rate of the liquid.

87

4.4.5. Rating s of different Carbon dioxide control
System
technological
process /
component System
technological
process /
component System
technological
process /
component System
technological
process /
component System
technological
process /
component System
technological
process /
component System
technological
process /
component System
technological
process /
component
Carbon
dioxide
control Carbon
dioxide control
by pH
management Chemical dispensing
pump and pH sensor
system     
Carbon
dioxide control
by gas
exchange Degassing column
(within
aeration/oxygenation
equipment)     
NOTE: Regarding the scoring system: Space requirements (where  is large and  is small); Operational management
(Ease of use – where  is hard and  is easy); Operational performance (where  is low and  is high); Acquisition
and implementation cost (where  is expensive and  is cheap); Maintenance and operation costs (where  is expensive
and  is cheap). All scoring was done by comparing a technology/equipment with another (within the same category).

88

4.5. pH Control
4.5.1. Introduction
The functionality of aquaculture growth systems is dependent on the biological nitrification
process by which ammonia and nitrites (toxic compounds), are oxidized into nitrates compounds
with much less toxicity. The nitrification process results in H+ ions consuming the alkalinity of the
water and reducin g the pH of the system. To the extent that the pH is allowed to fall, the
nitrification rate, thus the system's fish productivity, will record lower values. Therefore, the
management of the design and operation of a recirculating system implies a good know ledge of
the fundamental relationship between pH and alkalinity, as well as how to use this relationship in
order to maintain the water reaction at the optimum values required by the nitrification process.
4.5.2. Alkalinity and pH control
In most of the natural freshwater sources, the dominant water buffering system is the
carbonate system. Therefore, in the recirculating systems, the most suitable pH control method
is to buffer the culture water with carbonate products. The carbonation buffer system cons ists of
3 components, namely: carbonic acid (H 2CO 3), bicarbonate (HCO 3-), and carbonate (CO 32-).
Regarding H 2CO 3, this exists only in very low concentrations. The sum of the molar concentrations
of all these components is denoted by C T and represents the t otal concentration of inorganic
carbon in the system.
CT = [H 2CO 3] + [HCO 3-] + [CO 32-]
The carbonate system can be considered "volatile" or "non -volatile" as the carbon dioxide
in the water is allowed or not, as a gas exchange, to balance atmospheric carbon dioxide. Mixing
conditions and hydraulic residence times are criteria for assessing whether a closed cycle
aquaculture system is "volatile" or "non -volatile" in relation to the atmospheric carbon dioxide
balance.
Both in the non -volatile and volatil e systems the pH equilibrium is controlled by the alkalinity
(ALK); the [ALK] -pH correlation for volatile and non -volatile systems is shown in figure 28. The
graphical representation of the [ALK] -pH correlation (fig. 28) indicates that at the same system
alkalinity, [ALK] = 0.0007 ech/L, the pH of the volatile system is 8.2 and for non -volatile is 6.7.
Most systems approach the behavior of volatile systems, but the atmospheric balance is
never reached. The addition or depletion of any component of alkalinit y will result in pH change.

89

Figure 28 – [ALK] -pH correlation for volatile and non -volatile systems
(Timmons and Losordo, 1994)
4.5.3. Nitrification
It is known that biological nitrification of ammonia -nitrogen is primarily accomplished by two
types of bacteria. These genes, Nitrosomonas and Nitrobacter , are classified as autotrophs
because they obtain energy from inorganic compounds. Nitrification com prises two steps; the first
step is mediated by the Nitrosomonas genus, while the second stage, is mediated by the
Nitrobacter genus.

The reactions described by previous equations are accompanied by the production of a
certain amount of energy. The energy produced is used by nitrifying bacteria in cellular synthesis
processes. In both reactions, oxygen is an electron acceptor, the only o ne that can be used by
Nitrosomonas and Nitrobacter . Therefore, the nitrification process requires an aerobic
environment for a continuous and normal deployment.
The biomass synthesis reaction for Nitrosomonas and Nitrobacter is:

where C 5H7O2N represents the biomass of Nitrosomonas and Nitrobacter bacteria.
The synthesis reaction requires continuous power supply. During the nitrification this energy
is obtained from the oxidation of NH 4+ and NO 2-. The energy produced by oxidizing one mole of
NH 4+ is less than the energy required to produce a "mole" of bacterial cells (C 5H7O2N). All of the
  2 2 4 NO H2 O23 NH
 3 2 2 NO O21 NO
2 275 2 2 3 4 O5NOHCOH CO4 HCO NH   

90
three previous equations should be well proportioned so that, after taking into account the energy
transfer eff iciency, energy consumption is equal to energy production.
When this condition is satisfied, the overall reaction of the nitrification process (Timmons
and Losordo, 1994) is expressed by the relati onship:

An important result of this equation is that alkalinity in the form [HCO 3-] is consumed in the
reaction of HCO 3- + H+ = H 2CO 3. The alkalinity consumption rate is 1.98 moles of HCO 3- per mole
of NH 4+ oxidized.
In order to maintain a relatively constant pH of the system, the alkalinity supplementation
rate should be equal to the alkalinity consumption rate; otherwise, the omission of alkalinity will
ultimately lead to a rapid pH decrease, and at the same time, interrupting the system.
4.5.4. Mana gement of alkalinity and pH
The alkalinity and pH management of a closed cycle aquaculture system involves assessing
the rate of alkalinity consumption (nitrification rate) and determining the type and amount of
supplement to be used to restore lost alkali nity.
The rate of alkalinity consumption
The rate of water alkalinity consumption in a recirculating system depends on the rate of
nitrification of the nitrogen compounds.
To evaluate the rate of nitrification in a recirculating system, the following hypot heses are
assumed:
• the nitrification is complete to provide optimum media conditions, corresponding to the
technological requirements of the reared biomass;
• nitrogen from food that has not been assimilated to the fish protein or has not been removed
with the removed macroparticles will be nitrified;
• volatilization of ammonia is minimal;
• nitrification takes place both in the system's growing tanks and outside in the water treatment
plants.
Substances used as alkalinity supplements
The criteria used to choos e an alkalinity supplement are the degree of solubility, cost and
ease of application; Table 2 lists the main substances used as alkaline supplements and their
characteristics.
3 2 2 3 275 3 2 4 COH88,1OH041,1 NO98,0NOHC021,0 HCO98,1O83,1 NH       

91
In terms of solubility, some compounds (MgCO 3) should be avoided due to their low solubility
under the pH conditions specific to aquaculture systems.
Another aspect of solubility refers to the possibility of precipitation of CaCO 3 when calcium –
based alkalinity supplements such as CaO or Ca(OH) 2 are used. Precipitation of calcium
carbonate does not pose a problem because the precipitated CaCO 3 precipitate forms the
alkalinity reserve of the system, a reserve that contributes to pH stabilization in the event of a
sudden increase in the rate of nitrification. In this regard, it is worth mentioning that precipitation
of CaCO 3 is accompanied by a slight decrease in pH.
The potential for overdose is another important consideration taken into consideration when
choosing the alkalinity supplement.

Table 2 – Alkalinity Supplement Properties (Bisogni and Timmons, 1991)
Overdosing causes a rapid, difficult to control, pH of culture water above the desired value.
NaOH and Na 2CO 3, strongly reactive compounds, are highly soluble in water. The risk of
overd osing when using these substances as alkaline supplements is very high. In the case of
NaHCO3, relatively weak and very soluble base, it is difficult to produce an overdose when using
this compound. With regard to the potential for calcium -overdose, it is diminished by the high
precipitation rate of CaCO3 as pH increases.

92

4.5.5 Ratings of different pH control
System
technological
process /
component System technological process /
component System
technological
process /
component System
technological
process /
component System
technological
process /
component System
technological
process /
component System
technological
process /
component
pH control Chemical dispensing pump and pH
sensor system     
Buffering with carbonate products     

NOTE: Regarding the scoring system: Space requirements (where  is large and  is small); Operational management (Ease of use – where  is hard and 
is easy); Operational performance (where  is low and  is high); Acquisition and implementation cost (where  is expensive and  is cheap); Maintenance
and operation costs (where  is expensive and  is cheap). All scoring was done by comparing a technology/equipment with another (within the same cate gory).

93

4.6. Water Disinfection Methods
4.6.1. Introduction
The main criteria for the technological management of aquaculture rearing systems is to
ensure adequate hygiene and sanitation conditions for the reared biomass. The requirement for
monitoring the health status of the fish stock is determined by the degree of intensity of the crop
and the extent to which it can be satisfied depends on the technical complexity of the system and
the value of the initial capital for its r ealization.
More and more severe restrictions on discharging wastewater from open systems in
aquaculture and capitalizing on the natural water ecosystems for their recreational purposes are
key arguments for promoting and developing farming systems with cl osed or semi -closed cycle.
These systems usually involve high population densities and the recirculation of as much as
possible of the technological water, which are the main risk factors for the occurrence of illnesses.
Since closed systems involve high c apital and operational costs, much more rigorous disease
control is needed than open systems.
The most serious diseases that can affect crop biomass in a closed system are caused by
microorganisms (viruses, bacteria, fungi, unicellular organisms). In the culture water of a
recirculating system there may be a multitude of pathogenic organisms at different stages of their
life cycle. Generally, spores are more difficult to destroy than organisms found in their
unsupportive stages.
Controlling diseases caused by microorganisms involves, first of all, the application of
effective methods of disinfection or sterilization of culture water. Sterilization consists in destroying
all living organisms in a culture medium while disinfecting only some of these, namely p athogenic
microorganisms, which are destroyed. Sterilization is rarely applied in the treatment of water in
recirculating systems because this process can destroy some useful microorganisms, such as
crop feed crops or nitrifying bacteria in the biofilter.
In principle, disinfection can be achieved by various methods based on the lethal action
under certain conditions of heat, ultraviolet (UV) radiation or chemical substances on pathogenic
microorganisms. Chlorine and ozone are the main chemicals used to dis infect the culture medium
in the recirculating systems. Other chemicals, such as certain metal ions (silver ions), various
bases and acids, some surfactants and a wide range of oxidizing chemicals (iodine, bromine) can
also be used with goo d results in cer tain situations , potassium permanganate and hydrogen
peroxide).

94

4.6.2. Chlorination
Chlorine, in various forms, has been extensively used for a long time to disinfect water,
especially the waste water. The most common and good results for disinfection of culture water
in the recirculating systems were gaseous chlorine (Cl 2) and hypochlorite (OCl) in the form of
calcium or sodium hypochlorite.
Gaseous chlorine is very soluble in water. Thus, at a temperature of 20 °C and a pressure
of 1013×105 Pa (Rich, 1963 quoted by Wheaton, 1985), the chlorine solubility is 7160 mg/L.
Chlorine rapidly hydrolyzes into water to form hypochlorous acid (HOCl) as follows:

At low concentrations of chlorine (<1000 m g/L) and at pH<3, the entire amount of chlorine
is virtually converted to H OCl.
Hypochlorous acid dissociates in aqueous solutions with the formation of hypochlorite (OCl-
) and hydrogen ion as fol lows:

The dissociation reaction of hypochlorous acid is highly dependent on the reaction of water.
The hypochlorites, such as, for example, calcium hypochlorite, ionize when dissolved in
water, according to the reaction:

The hypochlorite ions are in balance with hypochlorous acid.
Chlorine, hypochlorous acid (HOC l) and hypochlorite ions (OCl-) are strongly oxidizing
agents. One of the most important compounds that react quickly with hypochlorous acid is
ammonia. Reactions between NH 3 and HOCl may give rise to monocloramines, dichloramines or
trichloramines (also c alled nitrogen trichlorides), according to these equations:

Chloramines can also result from reactions of hypochlorous acid with organic amines
present in the solution. The equilibrium constants, depend on the pH, the temperature of the
solution and the relative concentratio ns of ammonia and hypochlorous acid. The balance of
ammonia (NH 3) and ammonium (NH 4+) is also highly dependent on pH.
 Cl H HOCl OH Cl2 2
 H OCl HOCl
  OCl Ca OClCa 22
2
OHClNH HOCl NH2 2 3 
OH NHCl HOCl NH2 2 3 2 2 
OH NCl HOCl NH2 3 3 3 3 

95
Hypochlorous acid (HOCl) and hypochlorite ions (OCl-) are known in the literature as free
chlorine and chloramines as combined chlorine. T he toxicity of the two forms, both for
microorganisms and for evolved aquatic organisms, is different. Thus, it is known that, in its free
form, chlorine is more toxic. The combined form is not eliminated as easily as free form, which is
why it is intentio nally produced, in many situations, in water to generate residual chlorine,
recognized for its high residual capacity.
Water in aquaculture systems contains a multitude of compounds of organic or inorganic
nature. The introduction of chlorine into such wat er leads to the formation of residual chlorine.
The use of chlorine to control the development of micro -organisms in aquatic organisms'
systems presents high risks for crop biomass. Thus, most fish species are susceptible even at
low concentrations of tota l residual chlorine (0.1÷0.3 mg/L, Kelly, 1974 quoted by Wheaton, 1985).
Chloramines, having a degree of toxicity equal to that of chlorine, also require appropriate
precautions when used for water disinfection in aquaculture.
Effective use of chlorine as a disinfectant in aquaculture systems requires sufficient long –
term contact with the culture water, followed by the rapid elimination of chlorine and chloramines
from the growing basin.
The literature (Liu et al., 1971, quoted by Wheaton, 1985) indicates t hat the contact time
required to obtain a 99.99% destruction of viruses at a temperature of 2 °C and a concentration
of 0.5 mg/L residual free chlorine, ranges from 2.7 to 120 minutes.
Chlorine and chloramines can be removed from the water by adsorption te chniques or
chemical processes. Adsorption on activated carbon seems to be effective. Various reducing
agents, such as sodium thiosulphate or iron salts, may also be used to remove chlorine and
chloramines from water (Wheaton, 1985). In this case, the pote ntial toxic effects of the chemical
substances used and / or the resulting reaction products on the culture biomass should be
checked. Chlorine can also be eliminated by aerating the water. A vigorous and complete aeration
ensures efficient chlorine remova l, but there are reservations about the effectiveness of applying
this procedure to eliminate all chloramines, especially from water with significant organic matter
content.
Due to the recognized toxic effects of chlorine and chloramines on aquatic organis ms, their
use as disinfectants in aquaculture requires a great deal of caution. Even if the disinfectant is
subsequently removed, typically by adsorption onto activated carbon, before the water enters the
culture basin, the risk of crop biomass damage is v ery high in closed or semi -closed systems,
especially when the disinfectant is chlorine.
4.6.3. Thermal treatment
The water can be sterilized or disinfected by heating to a sufficiently high temperature and
maintaining this temperature for a period of time . Thus, the process of bacterial destruction is a

96
temperature -dependent function and the time to maintain that temperature, parameters which in
turn are conditioned by the species of microorganisms (bacteria, viruses, etc.) to be removed.
Milk pasteurizati on and thermal processing of preserved foods are such processes. For example,
milk can be pasteurized by maintaining it at 60 °C for 30 min or 71.1 °C for 13 seconds. Heat is
an efficient and convenient means of disinfection, since the rate of elimination of microorganisms
can be adjusted according to the technological requirement by changing the temperature and / or
the time to maintain it.
The main disadvantage of using heat as a water disinfection process is the specific energy
consumption that is quite high due to its high specific heat. An important aspect taken into
consideration when applying the thermal treatment for water disinfection is that the crop biomass
cannot survive the temperature range required for the disinfection process. Therefore, befo re
being introduced into the growing tank, the disinfected water has to be cooled. Water cooling can
be done in a number of ways, depending on the technical characteristics of the technological
facilities; in general, water cooling is carried out in regene rative heat exchangers that allow the
recovery of a significant part of the energy consumed during heating.
The energy expenditure required for disinfection of heating water is a major criteria for the
management of a recirculating system, which is why it needs to be assessed as accurately as
possible. Energy consumption for water heating depends on the amount of recirculation flow, the
proposed thermal gradient and the heat recovery efficiency for preheating.
Obviously, the cost of energy for water heating is quite high, including the initial capital,
repair and maintenance costs, and other heating costs. Therefore, except in exceptional
circumstances, the heating process has proven to be extremely costly to use for water disinfection
in aquatic recirculati ng systems.

4.6.4. Ultraviolet Radiation Treatment
Downs and Blunt (1878, quoted by Wheaton, F., 1985) have found that certain bacteria can
be destroyed by sunlight. In the last decade of the nineteenth century, Ward (1893) confirmed
that the ultraviolet portion of the solar radiation spectrum is responsible for the bactericidal action
of light. Ultraviolet (UV) radiation includes wavelengths in the range 150 Å÷4000 Å (150Å
represents the upper limit of the X -ray wavelength range, while 4000 Å is the lower limit of the
visible wavelength range). The energy of bright (visible, infrared or ultraviolet) radiation exists in
discrete or quantum units. The energy of a quantum depends on the wavelength of the radiation
and is defined by the equation:

where:
chEp

97
– E = the energy of a single quantum (erg);
– hp = Plank's constant (6.62 x 10-27 erg);
– c = light velocity 3 x 1010 cm/s;
– λ = the wavelength of the radiation (cm).
The equation expresses the invers e relationship between the quantum energy and the
wavelength of the light radiation. Thus, the energy of a quantum of UV radiation is greater than
the energy of a quantum of visible radiation since the wavelength of the UV light is less than the
visible li ght wavelength.
The destructive or lethal effect of UV radiation on bacteria, fungi, viruses or other small
organisms depends on the wavelength of the radiation. The correlation between the relative
bactericidal efficiency of UV radiation and its wavelengt h is ill ustrated in the curve of fig. 29 . The
most effective wavelength is 2600 Å, for which the bactericidal effect is 100%. On both sides of
this value, the bactericidal effect of UV radiation drops rapidly. Thus, UV light with a wavelength
of 3200 Å was found to be only effective at 0.4% against UV light with a wavelength of 2600 Å,
and the bactericidal radiation effect of 7200Å (the visible field ) is only 0.002% of the bactericidal
effect of UV radiation of 2600 Å (Koller, 1965 quoted by Wheaton, F., 1 985).

Figure 29 – Relative bactericidal efficacy of UV light versus wavelength
(Wheaton, F., 1985)
The actual mechanism by which UV light determines the destruction of microorganisms is
not yet fully elucidated. There is, however, sufficient evidence to indicate that the bactericidal
effect of UV radiation is the result of certain photochemical interact ions of UV light with nucleic
acids (Phillips and Hanel, 1960 cited by Wheaton, F., 1985).
The destruction of microorganisms, is interpreted as an indication that a single radiation
quantum is sufficient to produce a lethal effect (Koller, 1965 quoted by W heaton, F., 1985).

98
Sometimes, the destruction of microorganisms by treatment with UV radiation can be described
by a sigmoidal curve. In these situations, the sigmoid curve explains the need for multiple strokes
(more than a single light quantum) on a sing le organism or a single hit on several organisms
(targets). Both curves, exponential and sigmoidal, are analyzed according to the target species,
the irradiation technique, the irradiation characteristics and the stage of development of the
organism. Since the volume and accuracy of bacterial physiology data is insufficient, it is often
difficult to assess the suitability of applying one of the two curves to a particular system.
The degree of survival of irradiated microorganisms depends on the multiplicati on between
the UV intensity and the exposure time (I x t). Thus, the same survival ratio can be achieved with
low exposure times and high radiation intensities or high exposure times and reduced radiation
intensity.
Phillips and Hanel (1960, quoted by Whea ton, F., 1985), summarizing information on the
influence of temperature on the bactericidal action of UV radiation with the wavelength of 2540 Å,
concluded for the thermal range 5÷36 °C that once with increasing temperature increases, to a
certain extent, the efficiency of microorganisms destruction. Although the literature reports
different opinions on the influence of the heat factor on the bactericidal action of UV radiation, it
seems that the temperature has a reduced influence on the rate of destructio n within the tolerance
limits of the target species.
Tables 3÷5 show a series of UV radiation data required to kill 90% or 100% of different
microorganisms. Some of the values mentioned in the tables are contradictory. There are several
arguments justify ing these discrepancies, namely: the diversity of sources of bibliographic
information; pigmentation, concentration and / or different age of microorganisms; other variables
that characterize media conditions. Therefore, when designing UV light water disin fection
systems, the highe st amount (indicated in Tables 3 to 5 ) of the energy required to achieve a
certain percentage of microorganisms destruction is adopted.
In order to achieve the same percentage of microorganisms destruction, water disinfection
requires a 5 to 10 -fold higher UV radiation dose than the required dosage for the destruction of
suspended organisms in the air (Phillips and Hanel, 1960 quoted by Wheaton F., 1985).
Also, other researchers have determined that the exposure required to destroy a
microorganism is 40÷50 times higher in water than in dry air (General Electric Company, 1953).
The requirements for the energy requirement of UV radiation to achieve different percentages of
destruction of the Serratia marcescens bacteria are listed in Table 6 and in Table 7 similar data
for different species of microorganisms are presented.

99
Table 3 The UV radiation energy (E) required to destroy 90%
of the microorganisms colonies sown on agar (Wheaton, F., 1985)
Species E (μW x s/cm2)
Bacilus anthracis 4.520
B. megaterium 1.130
B. subtilis 7.100
Escherichia coli 3.000
Micrococcus candidus 6.050
Proteus vulgaris 2.640
Pseudomonas aeruginosa 5.500
Pseudomonas fluorescens 3.500
Salmonella enteritidis 4.000
Sarcina lutea 19.700
Serratia marcescens 2.420
Streptococcus hemolyticus 2.160
Streptococcus lactis 6.150

It is known that water absorbs the energy of most wavelengths of light radiation. The degree
of absorption is dependent on the wavelength of the radiation, first of all, and on the turbidity of
the water. The water is relatively transparent for UV radiation with a wavelength of approx. 2600
Å, recognized for their bactericidal action. Percentages of water transmission of UV radiation with
a wavelength of 2537 Å calculated for di fferent values of water absorption coefficients.
Table 4 – The UV radiation energy (E) with a wavelength of 2537 Å required to
inhibit 90% and 100% colony formation (Wheaton, F., 1985)
Species E (μW x s/cm2)
90% 100%
BACTERIA
Bacilus anthracis 4250 8700
B. megaterium 1300 2500
B. subtilis 5800 11000
Escherichia coli 3000 6600
Micrococcus candidus 6050 12300
Proteus vulgaris 3000 6600
Pseudomonas aeroginosa 5500 10500
Pseudomonas fluorescens 3500 6600
Sarcina lutea 19700 26400
Serratia marcescens 2420 6160
Streptococcus hemolyticus 2160 5500
Streptococcus lactis 6150 8800
FUNGI
Saccharomyces ellipsoideus 6000 13200
Saccharomyces cerevisiae 6000 13200
MOLD SPORES
Penicillium roqueforti 13000 26400
Aspergillus niger 132000 333000
Rhizopus nigricans 111000 220000
Mucor racemosus 17000 35200

100

Table 5. The UV radiation energy (E) required to destroy 100% of the microorganisms
(Wheaton, F., 1985)
Species E (μW x s/cm2)
MOLD SPORES
Penicillium roqueforti 26400
Aspergillus niger 330000
BACTERIA
B. subtilis 11000
Escherichia coli 7000
Proteus vulgaris 7500
Streptococcus hemolyticus 5500
Staphylococcus aureus 6600
VIRUS
Escherichia coli (Bacteriophage ) 6600

The absorption coefficient of a water can only be determined experimentally. Distilled water
allows good transmission of UV radiation with a wavelength of 2537 Å or greater. UV radiation
with wavelengths less than 2537 Å are absorbed to a greater extent by water. Impurities in water
(mineral salts and organic matter) significantly reduce the transmission of radiation. In the case
of similar concentrations, iron and organic matter reduces to a greater extent the transmission of
UV radiation through water tha n alkaline salts (Phillips and Hanel, 1960 quoted by Wheaton, F.,
1985). Thus, seawater absorbs a higher amount of ultraviolet radiation than most freshwaters
because of its higher ionic content.
Increased turbidity due to plankton, alluviums or other mate rials also reduces the
transmission of UV radiation.
Table 6 The effectiveness of destroying the Serratia marcescens bacterium
with an UV Wave Cathode 30W lamp (Wheaton, F., 1985)
E (μW x s/cm2) Destruction efficiency (%)
420 0
540 40
900 50
1 080 68,2
1 500 72,3
2 700 77,7
3 720 94,75
4 500 98,59
10 000 99,989
51 000 99,9999

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Table 7. The UV energy required to achieve different levels of destruction efficiency of
some microorganisms (Wheaton, F., 1985)

Species Destruction efficiency (%) E (μW x s/cm2)
FUNGI 99,99 192 000
99,00 96 000
90,00 48,000
Spores and Escherichia coli
in water 99,99 24 000
99,00 12 000
90,00 6 000
Escherichia coli (dry form)
and viruses 99,99 3 000
99,00 1 500
90,00 750

UV lamps
Devices used for water disinfection in the aquaculture of recirculating systems include a
variety of lamps capable of emitting significant ultraviolet radiation, most of which contain mercury
vapors. The electric current flowing through the vapor excites t he mercury atoms it brings in
various energetic states. As atoms return to lower energy states, they emit very accurate
wavelengths of radiation. Because all atoms have multiple states of excitation, the wavelength of
the radiation emitted is variable depe nding on the energy state at which the transition occurs. The
probability of occurrence of any energy state transition can be increased by varying the mercury
vapor pressure, the amount and the chemical form of mercury contained in the lamp as well as
the electrical emission conditions (Phillips and Hanel, 1960 quoted by Wheaton, F., 1985).
There are three general types of low pressure mercury vapor lamps, namely: cathode lamps,
cold cathode lamps and germicidal high intensity lamps.
Warm cathode lamps operate at a low voltage, similar to standard fluorescent lamps. The
commissioning of a hot cathode lamp is facilitated by the addition of a gas (argon) and the use of
an electrodes preheating device. Electrodes, usually located at the ends of the tube, a re tungsten
filaments coated with a layer of calcium oxide, barium or strontium. The lifetime of a hot cathode
lamp depends on the electrode's reliability and the degree of solar radiation of the lamp glass.
Repeated starts and stops shorten the life of ho t cathode lamps. Solarization is the process of
slow glass darkening, a consequence of prolonged exposure to high -intensity UV radiation. Hot
cathode lamps diminish relatively quickly, have a very low efficiency, and have difficulty starting
when temperatu res are low. The yield of hot cathode lamps is greatly influenced by the
temperature of the water into which they are introduced; the effect of the temperature on the yield
of a wavelength of 2537 Å emitted by a UV lamp when it is in direct contac t with wa ter is shown
in fig. 30 (Wheaton, F., 1985).

102

Figure 30 . The correlation between 2537 Ǻ UV radiation and water temperature
(Wheaton, F., 1985)

Cold cathode lamps produce UV radiation whose yield may be the same magnitude as those
emitted by hot cathode lamps. Compared to hot cathode lamps, cold cathode lamps have some
functional features, which may, on a case -by-case basis, constitute advantages or disadvantages.
Thus, starting at low temperatures is not a problem for cold cathode lamps because, for this type
of lamps, both start -up and operation require high voltages. As the name suggests, their nickel
electrode does not require preheating. Since the electrode is "cold", the lifetime of the lamp is
mainly determined by the solar radiation speed (reductio n of UV transmission) of the lamp. Cold
cathode lamps also contain mercury vapors used in hot, argon and neon cathode lamps (Phillips
and Hanel, 1960 cited by Wheaton, 1985).
High-intensity germicidal lamps , also called “Slimline” germicidal lamps, represe nt a
combination of cathode and cold cathode lamps as a constructive and functional solution. High –
intensity germicidal lamps use high voltages that allow cold cathode starting, and then start
operating with the hot cathode after start -up. Under these cond itions, the lifetime of a high
intensity germicidal lamp depends, first of all, on the life of the electrode that is inversely
proportional to the starting frequency. The main advantage of high intensity germicidal lamps is
their high efficiency, superior to hot or cold cathode lamps. Germicidal lamps produce ozone in
negligible quantities. To limit, however, the production of ozone, these lamps are made of a
special glass.
There are, along with the lamps described above, several types of high pressure merc ury
vapor lamps. Generally, these lamps are not widely used to disinfect water, being considered

103
inferior to low pressure lamps for the following reasons: the efficiency of the conversion of electric
energy into radiant energy is low; the radiation emitted is distributed over a wider band of
wavelengths, thus less used for germicidal purposes; have a relatively reduced life expectancy
and are more expensive; produce considerable amounts of ozone.
UV radiation disinfection systems
Disinfection systems based on water treatment with UV radiation, widely used in the practice
of recirculating systems, can be made in two constructive versions, namely suspended systems
(with or without reflectors) and submersed systems (with or without d irect contact of the water
tube).
Suspended systems
Suspended systems are schematically shown in fig. 31. In this constructive design, UV
lamps are installed above a pool through which water flows. Typically, the height at which the
lamps are suspended above the water level is 10 ÷ 2 0 cm. In the absence of a reflector, the
intensity of UV radiation is inversely proportional to the distance between the lamp and the free
surface of the water. It would be necessary, from this point of view, for lower suspension heights.
Practical and fun ctional considerations, however, limit the height at which the radiation source is
installed. Thus, if the UV lamps are too close to the surface of the water, there are large variations
in the radiation intensity along the pool, and the water that sprays t he lamps creates problems,
which are difficult to solve, for their maintenance.

Figure 31 – Suspended UV disinfection unit for water using reflectors (Wheaton, 1985)

104
The main advantage of this constructive design is that the lamps work in air and therefo re
the water temperature does not significantly affect the emission of the lamp, especially when the
area around the lamp is strongly ventilated with air at room temperature. This also presents an
important opportunity, namely simple and easy access to UV lamps for maintenance, by reclining
the roof of the installation.
The main calculation parameters of a suspension system are water depth, water quality,
lamp height, lamp spacing, and flow rate of water flowing through the gutter. Most of these
parameters are conditioned, first of all, by the degree of disinfection proposed.
The absorption coefficient of UV radiation is mainly influenced by the turbidity of the water.
This parameter determines two main features of the flow, namely water depth and water flow rate.
Thus, a water with a high load of suspended solids requires a lower thickness of the water layer
so that the UV radiation reaches the bottom of the basin. Also, a high germicidal efficiency of UV
radiation requires lower flows in case of high water turbidity. Thus, the literature suggests that for
the same suspended system of UV radiation at a water turbidity of 240 ppm the optimal water
flow is 5 L/min, while at a lower turbidity of 70 ppm the water flow can reach up to at 15 L/min.
(Hill et al., 19 67 quoted by Wheaton, F., 1985).
The location of the lamps, both with regard to the suspension height and their spacing, is a
major criteria considered when designing a suspension system. The distance between the lamp
and the free surface of the water must , as far as possible, be compatible with the lamp
maintenance requirements. The distance between the lamps shall be such that the intensity of
the radiation is constant, within reasonable limits, throughout the length of the treatment unit. With
regard to the two dimensional elements defining the optimal location of the lamps in a Kelly -Purdy
UV radiation disinfection system that does not use reflectors over the lamps, the literature (Furfari,
1966 quoted by Wheaton, F., 1985) signals, in the case of a reci rculating lamellibranchiate growth
system, a suspension height of 15 cm above the water level and a lamp size range of 15 cm. The
germicidal yield of UV radiation can be increased by focusing it with a reflector arranged above
the lamp. Thus, a 30 W reflec tive lamp, installed 25 cm above a pool, develops at the free water
surface a radiation intensity of 610 μW/cm2, sufficient to provide the germicidal effect. The actual
radiation intensity at the free water surface in the case of a suspended structure of U V lamps is
dependent on the reflector construction. Therefore, adopting a reflector, irrespective of its
constructive solution, requires experimental or mathematical determinations of the intensity of UV
radiation, corresponding to the desired distance fro m the lamp. In the case of a cold cathode UV
lamp equipped with a reflector, the radiation distribution or radiation intensity depends on a
multitude of variables, such as: reflector construction material, orientation and physical location
of the lamp rela tive to the reflector and not last row, type of lamp used.

105
With regard to the depth of water in the treatment basin, it shall be so arranged as to ensure
that at least 90% of the incident radiation energy is absorbed. A lower absorption rate of UV
radiation in the water mass implies a higher degree of absorption a t the level of the walls of the
enclosure. Since the energy absorbed by the enclosure walls is not available for bacterial
destruction, the yield of treatment decreases rapidly when the water depth is too low. As UV
penetration enters the water column in t he disinfection chamber, its intensity decreases, which is
why the water layer near the bottom is exposed to lower intensity radiation. Therefore, when
determining the depth and drainage of the water in the disinfection chamber, it will be intended
that th e entire water mass is exposed to the same UV intensity during the same exposure period.
Satisfaction of the two requirements, equal exposure time and constant intensity of UV radiation,
requires the creation of a turbulent drainage regime of the deep wate r layer by placing baffles
(thresholds) on the bottom of the basin, perpendicular to the flow direction of the water.
The flow of water flowing through the treatment basin is determined by several factors, the
most important being the pathogenic microorgan isms, the intensity of UV radiation, the linear
dimensions of the treatment unit and the turbidity of the water. Aspects about the influence of UV
radiation intensity and water turbidity on the germicidal effect of UV lamps have been discussed
previously. Irradiation parameters, namely radiation intensity and exposure time, shown in Tables
2÷6 for different species of microorganisms, should be adjusted according to the turbidity of the
water. The maximum permissible flow rate of a suspended disinfection sys tem is usually
determined experimentally. Under conditions of radiation irradiation required by the species of
microorganisms (radiation intensity and time of exposure) and maximum expected turbidity, the
experiment consists in changing the system flow and monitoring the presence of pathogenic
microorganisms in the effluent of the treatment unit.
Submerged systems
Submerged UV radiation systems use a sunken or water -surrounded lamp. The constructive
solution and operating principle of a submersible system a re shown in fig. 32.

Figure 32 .Submerged UV treatment system (Wheaton, 1985)

106

A tube made of quartz or glass protects the lamp from direct contact with water. The material
from which the tube is made allows the transmission of UV radiation and its absor ption into the
water loaded with pathogenic microorganisms. Although the glass tube could be removed, its
presence provides several advantages. In the case of a protective tube, the water is kept at a
considerable distance from the lamp. Therefore, the wat er temperature has a limited influence on
the emission of the lamp. Changing the lamp is also greatly facilitated in this case, keeping the
lamp tight. Due to the high resistance of the material from which the tube is made, this system
can be placed direct ly into a high pressure water stream.
The manu al tube cleaning device (fig. 32 ) is an indispensable component of the system. Its
role is to effectively remove the solids deposited on the outer surface of the protective tube in
order not to block the emissi on of the UV lamp radiation. In case of large disinfection units, lamp
cleaning is done automatically by a device that slides along the tube in both directions, driven by
the water flow.
The main functional feature of submerged systems is that they use all the radiation produced
by the lamp, with the exception of transmission and reflection losses. Also, the removal of the
reflector and the possibility of direct installation in the water stream are important arguments for
water disinfection with a submerged system. Particularly, suspended systems cannot be easily
adapted to pressurized water flow, require special construction reflectors, and do not allow
automatic lamp cleaning. Therefore, submerged UV radiation treatment plants are used on an
industrial sca le, while suspended systems have a more limited use in experimental research.

4.6.5. Ozone
The qualities of ozone, as a disinfectant, have been recognized for a long time. The first
information on water sterilization with ozone dates back to 1893, and the first commercial use of
ozone for water disinfection in an urban power system was reported in 1906 in France.
Ozone, the triatomic oxygen molecule, is formed when the oxygen molecules are sufficiently
excited to break down into atomic oxygen. Collision s of these atomic oxygen atoms cause ozone
formation. Achieving the degree of excitation of oxygen molecules required for ozone formation
is achieved by passing oxygen through a high voltage corona discharge or exposing oxygen to
ultraviolet radiation with wavelengths between 1000 and 2000 Ǻ (Klein et al., 1975; Koller, 1965
quoted by Wheaton, F., 1985). Most commercial ozone generators use the high voltage corona
discharge system.

107
Several types of ozone generators are available for aquaculture recirculatin g systems. As a
constructive solution, all types of generators consist of two parallel plates spaced at a certain
distance between them. The operating principle consists in connecting the plate system to a high
voltage source, resulting in the formation of an electric current between the two surfaces. Various
dielectric materials can be used to provide uniform electrical current across the entire surface of
the plates. Gas, ordinary air or pure oxygen, is passed through the two surfaces where the oxygen
molecules are sufficiently excited to form ozone. Ozone production in these units is dependent
on several variables. The distance between the plates must be large enough to allow relatively
free flow of gas and ensure a uniform current. The upper limit of the plate spacing must not exceed
a certain value so that the required tension is excessively high. The gas pressure must be high
enough to ensure the desired flow, taking into account the recorded pressure losses. As the gas
pressure increases, the electrica l resistance of the gas changes, a phenomenon that affects the
optimum operating voltage of the ozonator. The rate of ozone decomposition (the return to
molecular oxygen) increases with the increase in temperature. Because approx. 90% of the
energy with wh ich the ozonator is fed is lost in the form of heat, ensuring an optimal operation of
the ozonator requires its cooling (Klein et al., 1975 cited by Wheaton, F., 1985). Depending on
the design of the ozone generator, cooling can be done with water or air. The dielectric material
placed between the surfaces of the electrodes should have the highest electrical resistance and
also a high thermal conductivity, two properties that rarely meet together. Therefore, the
compromise solution is usually the use of a h igh resistivity dielectric material disposed in a thin
layer which solves the problem of thermal conductivity.
The installed power of an ozone generator depends on its requirements regarding the
electrical characteristics of the current formed between the electrodes (voltage, frequency,
intensity). It is known that the efficiency of an ozonator is directly proportional to the frequency of
the electric current. The increase in frequency is accompanied, however, by electrical losses
which, in certain situatio ns, can become economically prohibitive, which is why the choice of
dielectric material for the design of an ozonator is an important problem.
The gas used in the ozonator is air or oxygen. The energy yield of oxygen generators is very
different for the tw o gases. Thus, the same ozonator will consume less than half of the amount of
energy consumed when operating with air when using oxygen. Therefore, even if oxygen is
expensive, it is recommended to use it for large units. It is known that ozone is a good o xidant
and has a strong corrosive effect that is potentiated in a wet environment. Therefore, regardless
of the nature of the gas, it must be dried before use.
Gas flow is also an important parameter that determines the concentration of ozone
measured at t he generator output. Thus, in the case of lower flows, ozone generation is higher.

108
The concentrations of ozone produced by most ozonators vary between 0.5÷10 percent,
expressed massic.
Effectiveness of ozone disinfecting action
The use of ozone as a disin fectant is dependent on contact time and dosage. Ozone reacts
very quickly compared to other compounds, such as chlorine, for several reasons. Thus, chlorine
has to dissociate in water before it reaches the toxicity stage, while ozone is toxic by simple
contact. Ozone efficiency is less determined by pH and temperature than chlorine. However, it
seems that ozone is more efficient at a pH equal to 6 than at a pH equal to 8. Ozone does not
react significantly with ammonia , as is the case with chlorine ( Nebel et al., 1975) . This
phenomenon makes it possible for ozone to react much faster than chlorine. Also, as an oxidizing
action, ozone is twice as powerful as chlorine.
In the secondary ozone treatment of urban effluent, Nebel et al. (19 75) found that between
the number of microorganisms in water and the ozone concentration there is a correlation which,
graphically expressed in a logarithmic coordinate system, it is in the form of a straight line; the
dependence is linear for concentrations of ozone of 0.5÷10 m g/L. The same study indicates that
BOD, ammonia, nitrites, suspended solids, turbidity and color decrease as ozone concentration
increases and nitrate concentration increases with increasing ozone levels.
Pavoni et al., 1975, monitored the effect of ozone in a concentration of 15 ppm on viruses
and bacteria in distilled water. The results show that the survival of microorganisms over time is
described by a sigmoid curve. The disinfection was complete after about 15 seconds under the
initial bacterial count of about 2 x 106 bacteria/mL and that of viruses of 109 units/mL.
Pavoni (1975), Nebel (1975) have shown that both bacteria and viruses are destroyed very
quickly in the case of ozone treatment, a phenomenon confirmed in time by several researchers.
The results of some experiments on the effect of ozone on microorganisms ( E. coli , other
coliforms, polioviruses, T -2 and F -2 bacteriophages, Entamoeba histolytica , Schistosoma
manioni ) were synthesized by Kelly (1974, quoted by Wheaton, F., 1985) showing that , in
conditions of residual ozone concentrations of 0.05÷1.8 mg/L and contact times of 1÷22 minutes,
almost 99% inactivation was achieved and the minimum percentage of 95% was quite rare.
Organic matter also exhibits a certain affinity for ozone. This phen omenon is the basis for
treating wastewater in order to eliminate its color, odor and turbidity. Therefore, purifying a high –
loading organic material requires higher ozone concentrations to achieve a degree of disinfection
similar to that obtained in a wat er with a lower organic matter content. Ozone reacts most rapidly
with organic compounds by ozonolysis of carbon -carbon double bonds (Bailey, 1975) . Ozone has
an oxidizing effect on the following chemical structures: multiple carbon -carbon linkages of olef inic
and acetylene type; aromatic, carbocyclic and heterocyclic molecules; carbon -nitrogen bonds and
other similar unsaturated groups; nucleophilic molecules such as amines, sulfides, sulfoxides,

109
phosphites and phosphorus hydrogen; carbon -hydrogen bonds fr om alcohols, ethers, aldehydes,
amines and hydrocarbons; silicon -carbon, silicon -silicon and silicon -hydrogen bonds; oth er types
of carbon -metal bonds ( Bailey, 1975 .) Inorganic substances in water can also exhibit ozone
affinity. It is well known, for exam ple, that iron and manganese can be oxidized by ozone to forms
of insoluble oxides. By treating a freshwater with ozone, the iron concentration was reduced from
9.54 mg/L to 0.07 mg/L and manganese from 1.21 mg/L to 0.05 mg/L (Kjos and others, 1975 cited
by Wheaton, F., 1985). By using oxygen, instead of ozone, for the treatment of the same water,
the iron concentration dropped to only 3.99 mg/L while the manganese concentration was
reduced to only 0.71 mg/L, much higher than the ozone value. Knowing the oz one oxidation effect
on inorganic compounds, its use in water disinfection in closed -loop marine culture systems
implies rigorous monitoring because some salts, whose presence in water is indispensable, can
be oxidized to insoluble forms, which, in fact, a re thus removed from the system.
Ozone toxicity
Ozone has a pronounced toxic effect for both humans and most aquatic organisms. The
measure of the effect of ozone toxicity on living organisms is given by the volume of the
administered dose, expressed in vo lume, and the duration of exposure.
For humans, the ozone is lethal in the case of an exposure of 0.1 minutes at a dose of 11000
ppm or, for a longer exposure of 1oo minutes, at a much lower dose of about 20 ppm. The ozone
toxicity in humans is felt starti ng at a dose of 3000 ppm associated with an exposure time of 0.1
min, a dose of 4 ppm and a du ration of exposure of 1000 min ( Nebel et al., 1975 ).
Data from the literature on ozone toxicity on aquatic organisms are insufficient and less
precise except for punctual and dispersed information that cannot be linked to conclude on this
issue. However, it seems that any amount of ozone, whether small, present in a culture system
is harmful and should be avoided. For example, Arthur and Mount (1975 cited by Wheato n, F.,
1985) found that an ozone concentration of 0.2÷0.3 ppm is fatal for certain species of fish.
Fortunately, ozone is very unstable, decomposing rapidly into oxygen. Under these
circumstances, the few minutes elapsed between water treatment and ozone i ntroduction into the
culture system are sufficient to avoid toxicity problems. With regard to other aquatic organisms,
for example oyster larvae, the literature (De Manche et al., 1975 cited by Wheaton, F., 1985)
signals that they are extremely sensitive e ven at very low ozone concentrations. In this situation,
it is necessary that, before being introduced into the crop system, the ozone water, especially the
marine, is passed through an activated carbon filter at which the water quality parameters are
rebalanced.
Ozone water disinfection systems

110
Regardless of the constructive solution adopted, an ozone water treatment system must
ensure intimate contact between target organisms and ozone. Therefore, ensuring an effective
mixing of water with the gas is a pr imary requirement for the application of this disinfection system.
Depending on how this is achieved, ozone -water contactors can be grouped into four groups,
namely: spraying columns; columns with filling; columns with trays or screens; gas bubble
dispersi ng systems (fig.33) .
The operating principle of spraying columns consists of injecting water as fine drops into the
ozone atmosphere of the column. In this way, an appropriate contact is made between the two
phases, liquid and gaseous, resulting in the transfer of ozone into water. The ozone transfer
efficiency is directly proportional to the ratio of the gas phase to the liquid phase (G/L) flow rate.
Therefore, the need to achieve higher G/L ratios, to the extent of the proposed efficiency, plus
ozone i nstability, is two main arguments explaining the limited use of spray columns for water
ozone.

Figure 33. Ozone treatment system schematic
for disinfecting 400 –2400 L/min ofsurface water (Timmons et al., 2002)
Filled columns consist of a compact bed of granular material through which water and ozone
pass. The granular medium provides a sufficiently large contact surface at which the gas transfer
occurs. "Trickling" filters, currently used for wastewater treatment, are a good example of a water –
ozone cont actor that falls into the category of filler columns. On the height of the column, gas and
liquid can move in the same direction or counter -current. Styrofoam -type contactors have many
advantages that justify their use in ozonizing water, namely: small siz e, which means costs or

111
reduced technological spaces; appreciable capability of operation over a wide range in terms of
the ratio between the gas and liquid phase flow (G/L); low pressure losses on the passage of gas
and water through the granular medium; proper operation also for liquids producing a large
amount of foam; possibility of realization in constructive variants that allow operation in pressure
lines. The use of filling columns for ozonizing water for disinfection involves the knowledge of
functi onal disadvantages, among which: unequal distribution of liquid and gas inside the column;
the possibility of clogging the filter medium with suspended solids transported by water.
Water -ozone contactors such as tile or screen columns are widely used in la rge systems.
Ozonators of this type are made up of one or more ozone screens/plates. The water flows
over/through these trays/screens and contacts the gas, thereby making the gas transfer. The
functional characteristics of the columns of screens or trays a re similar to those of the columns
with filler. The difference between the two types consists in the fact that for columns with screens
or plates, both phases, liquid and gaseous, are evenly distributed over the height of the column.
Also, trays/screens co lumns can be operated in a larger scale of G/L ratios than in columns with
fillings. The possibility of easier removal of sedimentary solids is also another major advantage of
tile/screen columns compared to those with filler (Stahl, 1975, cited by Wheaton , F., 1985).
The dispersion systems are most commonly used for the transfer of ozone into water. The
dispersion of a gas consists, in its simplest form, in the introduction of the gas in the form of
bubbles into a column of liquid. It is known that mass tr ansfer is potentiated by the increase of the
gas-liquid interface area, which is why small bubbles are preferred. Hence, gas is usually
introduced at the bottom of the column or basin using specific distribution elements capable of
producing bubbles as fin e as possible. The distribution elements are usually represented by
microporous structures made of different materials, such as: stone, ceramic products, silicon
carbide, plastics, etc. Making a contact surface requires, in some situations, vigorous mixing of
water with special devices, most often using the Venturi vacuum cleaner.
The choice of one of the four types of contactors depends on several factors, among which
the most important are: the target micro -organisms, the water quality, the required techn ological
flow and the technical and functional characteristics of the contactors. However, the main
objective to be satisfied by the chosen system is to ensure as much as possible the best possible
contact between ozone and the target micro -organisms. This implicitly implies a larger transfer
area between the two gaseous and liquid phases. Ensuring the necessary contact time,
appropriate to the factors presented above, is an important criteria for assessing the performance
of the chosen contactor. The possi bility of recycling oxygen produced by the contactor is also an
important objective, especially when the ozonator operates with pure oxygen.
The residual ozone concentration along with time and contact surface are the main
parameters determining the effect iveness of disinfection. The value of these parameters varies

112
with target organisms and water quality. Thus, it appears that a concentration of 0.5 mg/ozone in
the effluent of a contactor and a contact time of 5÷10 minutes ensures, in the case of urban
sewerage, a degree of destruction of the microorganisms in water higher than 95%. In most
aquaculture systems, a similar percentage of disinfection of wastewater is usually obtained at
lower values of residual ozone concentration and contact time due to low er water loading in
microorganisms, organic compounds and inorganic compounds. In the literature it is estimated
that contact times of 1÷5 minutes and concentrations of ozone in the contactor of 0.56÷1 mg/L
represent reasonable parameters for water treatme nt in the aquaculture of the recirculating
systems (Sproul and Majumdar, 1955 quoted by Wheaton, F., 1985).

113

4.6.6. Ratings of different water disinfection
System
technological
process /
component System technological process /
component System
technological
process /
component System
technological
process /
component System
technological
process /
component System
technological
process /
component System
technological
process /
component
Water
disinfection Chlorination     
Thermal treatment     
UV
radiation
treatment Suspended UV
radiation disinfection
system     
Submerged UV
radiation disinfection
system     
Ozone generator (Ozonator)     
NOTE: Regarding the scoring system: Space requirements (where  is large and  is small); Operational management (Ease of use – where  is hard and 
is easy); Operational performance (where  is low and  is high); Acquisition and implementation cost (where  is expensive and  is cheap); Maintenance
and operation costs (where  is expensive and  is cheap). All scoring was done by comparing a technology/equipment with another (within the same cate gory).

114

7. Aquaponics
7.1 Introduction
An aquaponic system integrates hydroponic techniques within a recirculating aquaculture
system (RAS). Integrated aquaponic systems have become more and more popular in the last
years, presentin g sustainable new ways to produce food. The main advantage that an aquaponic
integrated system brings to the RAS is the extra biofiltration provided by the plants. As a
consequence, the make -up water daily exchange rate in reduced.
The main components of a n integrated aquaponic system are: the rearing tank, the solid
removal units (sump and mechanical filter), the biofiltration unit, the aeration unit, the degassing
unit, the pumps and the hydroponic culture module ( fig. 34 ).

Figure 3 4. Schematic overview of an integrated aquaponic system (Thorarinsdottir et al.,
2015)
Rakocy et al., 2006, considers that there is an optimum arrangement of these components,
as can be seen in figure 3 5, thus the solid removal unit and the bio filtration unit must precede the
hydroponic culture module.

115

Figure 35. Optimum arrangement of aquaponic system components (not to scale) (Rakocy et
al., 2006)
In this way, the effluent from the rearing unit is treated first by removing its suspended and
settable s olids, then it is biofiltered by removing as much ammonia and nitrite as possible, finally
reaching the hydroponic culture module, where nutrients (nitrate and other micro and macro
elements) are absorbed by the plants and additional ammonia and nitrite ar e removed by the
bacteria growing on the surfaces of the hydroponic module and/or the grow media. After passing
through the hydroponic culture module, the water is collected into a sump from where it is returned
to the rearing unit.
There are different typ es of aquaponic techniques, such as: media grow beds, deep water culture
(DWC), and nutrient film technique (NFT), each presenting advantages and disadvantages over each
other. The design and choosing of the best aquaponic system might seem challenging, bu t in fact it can
be simple when practicing a proper fish stocking density so as to provide a good nutrient level for plant
production.

4.7.2. Media grow beds
A media grow beds uses a tank or container that is filled with a solid medium suitable for plant
cultivation such as: light expanded clay aggregate (LECA) (fig. 3 6A), pumice (fig.3 6B), Growstone
(fig.3 6C), gravel (fig.3 6D), perlite (fig.3 6E), expanded shale (fig.3 6F), peat, coconut, sand,
mineral wool, slag, ceramic balls, bricks and polystyrene or ma rble.

Figure 36 – Different types of growing media

116
This cultivation method uses a hydroponic substrate and includes a drain hole that is 1.5 cm
above the bottom surface. This allows the existence of a thin water mass at the bottom of the
substrate, which serves as a permanent water store (Bradley and Tabar es, 2000).
In order to choose the best material for plant cultivation by this method, must have the
following characteristics (Abou Hadid and El -Behairy, 1999):
• provide solid to support plants;
• be inert "do not contain any chemical elements";
• retain enough water;
• retain enough oxygen at the same time as water retention;
• contain no chemical that could be toxic to plants;
• be free of diseases.
These media provide the plants with a good root fixation substrate, also because of the
porosity (high specific surface area) and a good water and air retention rate, the media provides
extra biofiltration/nitrification and mineralization.
In small scale aquaponic systems that practice a low fish stocking density, the biofilter can
be completely excluded, the nitrif ication process being realized by the grow bed media. Also red
earthworms ( Eisenia fetida ) can be added in the grow bed.
These annelids help break down solid waste and excess roots, they suppress plant diseases
and provide extra nutrients to the plants th rough their excrements – vermicompost. Vermicompost
has been shown to enhance plant growth, crop yield, and improve root structure and development
(Pant et al., 2009).
The media grow bed system can be implemented by choosing one of the two flood regimes:
constant (continuous) flow or reciprocating flow (“ebb and flow”) (fig,37) .

Figure 37 – Bell siphon schematic (A) and examples of bell siphon (B) (Fox et al., 2010)

117
Water flows into the grow bed and it fills it up to a specific level, then it drains out a lmost
completely only to start the process once more. This effect is achieved by narrowing the outlet
pipe, thus creating a suction effect once the water reaches a certain (desired) level.
In the reciprocating flow variant the media and the plant roots get better aeration (Rakocy et
al., 2006), unlike the constant flow variant in which the same media and the roots are always
under the water level. When cultivating a plant with high nutritional requirements, a constant flow
system is recommended, but the rec iprocating flow system has a better biofiltration and solid
removal.
This hydroponic module is quite easy to build and depending on the media type that is used,
the cost vary, making it not the cheapest, but also not the most expensive solution.
Maintenanc e can be difficult if large amounts of solids get through the mechanical filtration
unit and end up in the media, over time clogging the media, the roots and even the piping.
The media grow bed is better suited for plants that bear fruit since the media p rovides better
fixation for the roots and support for the plant.
The crop yield of the media grow bed system is better than that of the DWC system, which
in turn is better than the NFT system (Lennard and Leonard, 2006).

4.7.3. Deep water culture (DWC)
Also known as the floating raft system, the deep water culture system is optimal for both
small and large scale production systems.
The design is simple, the hydroponic tanks are large, not too deep, and usually a minimum
of 30cm water depth level is maintai ned.
The plants grow on polystyrene or plastic sheets that float on the surface of the water,
usually fixed in smal l plastic net pots (fig. 38).

Figure 38 . UVI deep water culture aquaponic production system (Rakocy, J.E.)

118
Just like the media grow bed troughs, the deep water culture tanks, in this case, have a
simple design; rectangular shape, made of plastic, metal, wood or concrete covered with
impermeable lining, etc. Water flows in the hydroponic module through an inlet at one end and it
flows out through an overflow outlet at the other end of the unit. This way a constant water level
is always maintained and in case of a pump malfunction or power failure, the plants do not die.
The rectangular basins have two distinct ad vantages: the nutrient basins are in the form of
conveyor belts for the rafts in which they are planted and for those on which they are harvested,
and the plants are disposed in a single horizontal plane so that the interception of the sunlight on
each pla nt is (Jensen, 1991).
This is a highly productive method, but requires intensive biological and mechanical filtration
to maintain clean water.
The roots of the plant grow directly into the oxygenated water flowing from the fish tanks
with a volumetric exch ange rate of approximately 30% per hour. (Thorarinsdottir et al., 2015). If
the solids filtration and removal is not done properly, sludge accumulating at the bottom of the
hydroponic tank and on the roots of the plant blocking oxygen and nutrients uptake.
In aquaponics it is often encountered the use of these systems as decanting basins. Due to
the fact that these channels on which the plants are grown are long and narrow and contain a
large amount of water, they act as decanters (sumps). This is a good th ing as long as solids are
removed periodically and do not accumulate. However, once covered with floating plates is
difficult, it is consumed for a long time and it is laborious to remove solids from the bottom of the
channels. It is much better for the so lids to settle in a different place from the plant cultivation for
a mild and regular removal (Lennard, 2012).
Floating bed hydroponics also provides sufficient nitrification space if the solids are removed
from the effluent before they reach the hydroponi c component (Rakocy et al., 2006).
Crop management involves transplanting the seedlings into the system and harvesting the
plants once they achieve marketable size. This process can be improved by applying a conveyor
movement of the rafts similar to the co nveyor production system (CPS) used in the NFT systems.
The seedlings are introduced on flo ating trays into the system at inlet end of the hydroponic
module and moved along the length of the tank as it grows, reaching the outlet end of the tank
when it’s r eady to be harvested.
A great improvement to the DWS is the implementing a conveyor production system (CPS)
(fig. 3 9) within the hydroponic modules.

119

Figu re 39 . Schematic of a conveyor production system (CPS) (Adler et al., 2003)
The DWC system is easiest to build and the least expensive of the three types of aquaponic
systems.
4.7.4. Nutrient film technique (NFT)
The nutrient film technique system uses long and narrow plastic tubes with a thin layer (film)
of water continuously flowing through them.
An ab solute requirement in the case of the NFT systems is a good preliminary mechanical
filtering (Thorarinsdottir et al., 2015). Solids accumulating on the roots must be avoided, not only
because it can inhibit plant growth, but also because it can lead to clo gging of the system.
The design of the NFT systems might be more complex than the other two aquaponic
systems, but it certainly is the easiest to manage as long as clogging is avoided. The system uses
long tubes with rectangular or circular section, 10 -15 cm in width or diameter. These tubes have
holes cut into the top part where plants (optionally in plastic pots) are set (fig.40) .
The NFT tubes are placed at a 1% angle, the water entering at the raised end, flowing
through the tube and being gravitatio nally evacuated at the lowered end. Water is introduced into
the NFT hydroponic modules with a low flow pump, aiming for a 1 -2 L/min flow regime
(Thorarinsdottir et al., 2015). Also these pumps require less energy to recirculate the water into
the hydropon ic units, reducing the operational cost.

Figure 40. Nutrient film technique aquaponic system with rectangle troughs (A) and tubes (B)
An absolute advantage of the NFT system over the other two systems is the possibility to
design and build the system even vertically, making excellent use of the available growing area.

120
In order to avoid the oxygen depletion be the plant roots due to the small volume of water
flowing through, the tube must not be longer than 10 m.
NFT systems also have some drawbacks, like the risk of root clogging and the decrease of
the water’s nutrients towards the end of the channel.

4.7.5. Lighting
One important component of the aquaponic system, that must not be overlooked, is the
lighting equipment needed for assuring a fast and proper plant development. In most cases
natural lighting just won’t be sufficient enough for an optimum system yield.
There are several options to choose from, such as: fluorescent bulbs, metal halide bulbs,
and LED lamps. The lights come in various wave lengths, they cover a certain area and need to
be mounted at a certain height above the plants, depending on plant species. When choosing the
lighting solution for an aquaponic system, a very important aspect also needs to be the acquisition
to operating cost ratio. Basically the more expensive the lighting system acquisition is the less
expensive the operation cost over a longer period of time is.
The lighting operation period during a day can vary depending on the crop that is grown and
the time of year , but usually around 10 hours of light : 14 hours of dark (Lennard and Leonard,
2004).

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4.7.6. Ratings of different aquaponics
System
technological
process /
component System
technological
process /
component System
technological
proc ess /
component System
technological
process /
component System
technological
process /
component System
technological
process /
component System
technological
process /
component System
technological
process /
component
Aquaponics Media grow
beds LECA ( Hydroton™)     
Pumice     
Growstone™     
Gravel / Sand     
Perlite     
Deep water culture (DWC)     
Nutrient film technique (NFT)     
NOTE: Regarding the scoring system: Space requirements (where  is large and  is small); Operational management (Ease of use – where  is hard and 
is easy); Operational performance (where  is low and  is high); Acquisition and implementation cost (where  is expensive and  is cheap); Maintenance
and operation costs (where  is expensive and  is cheap). All scoring was done by comparing a technology/equipment with another (within the same cate gory).

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5. WATER QUALITY IN RAS SYSTEMS

Due to the environmental restrictions which appeared in many countries (land restrictions,
water limitations and environment pollution) recirculating aquaculture systems (RAS) are
developed rapidly in many areas of the fish farming sector.
RAS offers the advantage of growing fish in a controlled environment with a big reduction
of water consumption due to waste -management and nutrient recycling, being environmentally
friendly (Timmo ns and Ebeling, 2013; Jacob Bregnballe, 2015; Martins et al., 2010). These kinds
of systems can control the water quality, the water temperature, are not dependent on the weather
and it has low water and land requirements (Ebeling et al., 1995).
Mainly, th e technology uses mechanical and biological filters for water treatment but some
systems contains more treatment units such as: unit for the administration of ozone, for the
wastewater disinfection and organic waste removal; degassing unit, for carbon diox ide removing;
monitoring and control systems (Michaud, 2007; Jacob Bregnballe, 2015).
Water has an important impact at the fish growth and the final quality of the fish (tissue
quality, texture and taste) . Thus, maintaining the physico -chemical parameter s of the water in
the recirculating system, in the optimal range for the raised species, ensures a higher growth rate
of the fish biomass, while poor water quality affects the physiology of the fish, the rate of growth,
causes pathological changes and diso rders of the functioning of internal organs, in some cases
even leading to high mortality. Also, water quality has a decisive influence on the growth of
biological material by direct and/or indirect action of organic or inorganic solvates in water, which,
in optimal quantities corresponding to biological necessities, ensures a normal development. Any
deviation of certain parameters from the optimal interval may be detrimental to the fish.
Mainly, water quality in the growing units depends on two things (fig. 41) , the quality of the
influent water and changes in the water quality inside the rearing units which is influenced by feed
input and biological activity (Colt et al., 2009).

123

Figure 41. Processes that affect water quality in a recirculating aquaculture system
(adapted after Colt et al., 2009).

The primary water quality parameters include temperature, dissolved oxygen (DO), pH,
ammonia, nitrites, nitrates, suspended solids, but it is also important to control aspects like
salinity, alkalinity, biochemi cal oxygen demand (BOD) (Table 8 ). So, in order to be able to reuse
the water in a RAS system it is important to monitor and control variables, since they directly affect
animal health, feed utilization, growth rates and carrying capac ities.

Table 1. Water criteria quality for a recirculation system (Jacob Bregnballe, 2015)

124

Temperature . In a RAS system, water temperature is the most important parameters to
consider when assessing water quality. Because fish are poikilothermic or cold -blooded, their
body temperature is approximately the same as their environment and each species has an
optimum temperature range that maximizes growth and an upper and lower limit beyond which
they cannot survive (table 9) .
Knowing the thermal optimum for each species (table 10) have a great importance in the
intensive growing and exploitation technologies since temperature directly affects the
physiological processes, such as respiration rate, efficiency of feeding and assimilation, growth,
behavior, reproduction (Timmons and Ebeling, 2013).

Table 9 . Optimum Temperature Ranges (°C) for some fish species raised in a RAS system
Species Temp.
Range Source
rainbow trout ( O. mykiss ) 14–16° Aston, 1981
brook trout (Salvelinus fontinalus) 7–13° Piper, et al. 1982
channel catfish ( Ictalurus
punctatus ) 25–30° Tucker & Robinson,
1990
carp ( Cypriums carpio ) 25–30° Aston, 1981
tilapia ( Sarotheorodon, sp.) 28–32° Aston, 1981

Above the optimum temperature, the increased energy requirements for food conversion
and other metabolic processes ensure that the law of diminishing returns applies. Also, at higher
than optimum temperatures, the fish food conversion ratios are lower. Further temperature
increases beyond optimum are of no benefit, and may in fact approach lethal levels (Timmons
and Ebeling, 2013).
Changes of the environmental temperature affect the fishes' rate of biochemical reactions,
which leads to different metabolic and oxygen consumption rates. At the lower ranges of the
species tolerable temperature range, these rates decrease. As water temperatures increase, fish

125
become more active and consume more dissolved oxygen, while simultaneously producing more
carbon dioxide and other excretory products, such as ammonia. The se increasing rates of
consumption of necessary elements and production of detrimental elements can have a direct
effect on overall fish health and survival if these parameters are allowed to exceed nominal
values. If not corrected, the fish will become st ressed to some degree. Even low levels of stress
can have adverse long -term consequences in the form of reduced growth rates or mortality due
to opportunistic organisms that take advantage of the stressed fish. (Timmons and Ebeling, 2013).
Besides that, te mperature affects several other parameters and can influence the physical
and chemical properties of water, such us (Figure 41):
✓ Salinity and conductivity
✓ Metabolic rates and photosynthesis production
✓ Compound toxicity
✓ Dissolved oxygen and other dissolved gas concentrations
✓ Oxidation reduction potential
✓ pH
In a RAS system, the temperature can be regulated with different heaters and coolers
(Losordo et al., 1999) and water exchanges (fig. 42) .

Figure 42. Solution for controlling temperature in a RAS system

Table 10 . Criteria and Effects of Temperature on some Fish Species
Species Lower lethal
temperature
tolerance range Target water quality
range for growth Upper lethal
temperature range
Oncorhynchus mykiss 2 17 – 18 27
Salmo trutta 2 12 – 13 26 – 28
Cyprinus carpio 2 – 4 30 – 32 32 – 41
Clarias gariepinus 8 – 10 28 – 30 40 – 43

Dissolved oxygen (DO). After temperature, dissolved oxygen is the most important and
most critical parameter, which requires continuous monitoring in intensive production systems.
Low oxygen concentrations in water lead to serious adverse effects such as respiratory stress,
hypoxia, lethargy, including fish death.
Controlling temperature in a RAS
system
➢Heaters or coolers (depending of
the species)
➢Water exchange
➢

126
In aquaculture systems there are many competitors for dissolved oxygen .The competition
for oxygen dissolved in water is given on the one hand by the fish biomass and on the other hand
by the chemoautotrophic nitrifyin g bacteria (which carries out the oxidation of ammonia) and the
chemoheterotrophic bacteria that consume the organic carbon.
The required dissolved oxygen level is directly proportional to the density of the fish from
the RAS, so a higher stocking density involves a high biological oxygen demand. Minimum values
of dissolved oxygen as also highly dependent on species and growing conditions (table 11, fig.43) .

Table 11 . Criteria and Effects of Dissolved Oxygen on the Health of Fish (SOUTH
AFRICAN WATER QUALITY GUIDELINES, 1996)

Chronically low DO levels cause stress, resulting in reduced appetite, poor growth and
production and an increase in susceptibility to infectious diseases. Symptoms include gulping of
air at the water surface, stress coloration and an increase in swimming activity

127
Also, high DO concentrations (>20 mg/L) are toxic to fish and can cause physiological
dysfunctions (including gas bubble disease) and developmental abnormalities in fertilized eggs
and larvae ( Department of Water Affairs and Forestry, 1996).

Figure 43 . Solution for controlling DO in a RAS system

pH. In a RAS system, the pH tends to decrease when nitrifying bacteria produce acidity,
consuming alkalinity as well as carbon dioxide resulting from the breathing process of fish and
micro -organisms. Carbon dioxide reacts with water to form carbonic acid that lowers pH. At a pH
of less than 6 pH units, the nitrifying bacteria are inhibited and no longer converts ammoniacal
nitrogen.
The optimum pH value for growth and health of most aquatic animals is 6,5 -9 (Timmons et
al., 2007). If the fish are exposed to ex treme values of pH, growth rate will be significantly reduced
with stressful or lethal effects on the fish biomass.

Table 12 . Criteria and Effects of pH on the Health of Fish (SOUTH AFRICAN WATER
QUALITY GUIDELINES, 1996)

Generally, in recirculating systems, the optimal pH range is maintained by the addition of
alkaline buffer solutions. The most commonly used buffer solutions are sodium bicarbonate and
calcium carbonate. The pH value needs to be constantly monitored and controlled (fig. 44) to
Controll ing DO in a RAS
system
➢Aeration
➢Oxygenation
➢Liquid oxygen injection
➢Water exchange

128
keep it at optimum levels in the rearing tank and the biofilter. If nothing is done the environment
will eventually become toxic (Ebeling et al., 1995; Lekang, 2007; Losordo et al., 1999).

Figure 44 . Technical solution for controlling pH in a RAS system
Nitrogen compounds (Ammonia, Nitrites, Nitrates).
Ammonia is the main source of nitrogen removed from fish and is excreted especially in
the gills in the form of ammoniacal gas. Ammonia is the product of catabolism resulting from
protein digestion and is extremely toxic.
In the aquatic environment, ammonia is p resent in two forms: non -toxic and non -toxic
(NH 4+) dissociated or ionized (NH 4+), which is toxic to most fish at a constant value exceeding
0,03 mg/L. Ammonia in undissociated form easily penetrates the tissue barriers, penetrating
through the branch epithelium in the blood of the fish, acting harmful to the nervous system. The
rate of conversion from NH 4 + to NH 3 depends both on temperature and pH. Also, low
concentrations of dissolved oxygen (DO) increase the toxicity of NH 3 (table 13)

Table 13. Criteria and Effects of ammonia on the Health of Fish (SOUTH AFRICAN
WATER QUALITY GUIDELINES, 1996)
Ammonia Range
(mg NH 3/L) Effects
Cold -water fish
Target Water Quality
Range
0.0- 0.025 At pH > 8.0 the upper range of the TWQR must be < 0.025. Values
slightly higher may not harm fish, if previously acclimatized to this
concentration
0.025 – 0.3 Some sub -lethal effects, especially reduced growth rate for cold -water
species. Adverse physiological and/or histopathological effects may
occur. Blue -sac dise ase in yolk sac fry of rainbow trout. The 96 -hr LC for
50 32 -day- old juvenile rainbow trout is 0.16 mg NH3/L
0.3 – 1.10 Adverse physiological and histopathological effects occur in the range of
0.3 – 0.6 mg NH /L. The 96 -hr 3 LC 50 for post -yolk-sac rainb ow trout is
0.37 mg NH 3 /L. The 96 -hr LC 50 for sub adult and adult rainbow trout is
in the range 0.44 – 1.10 mg NH /L
Warm -water fish
Target Water Quality
Range No health or sub -lethal effects

Controlling pH in a RAS
system
➢Chemical treatments
(based to regulate pH)
➢Water exchange
➢Addition of seawater

129
0.0 – 0.3
0.3 – 0.8
Possible sub -lethal effects in warm -water fish occur in the range of 0.3 –
0.8 mg NH /L
1.0 – 3.0 The 96 -hr LC50 for larvae of common carp 1( Cyprinus carpio ) is in the
range 1.74 – 1.84 mg NH 3 /L. The 96 -hr LC 50 for Mozambique tilapia
(Oreochromis mossambicus ) is in the range 2.08 – 2.53 mg NH 3 /L. The
96-hr LC50 for African catfish larvae ( Clarias gariepinus ) is 2.30 mg NH
/L.
6.5 ± 1.5 96-hr LC for sub -adult African catfish.
9.1 ± 1.4 96-hr LC50 for Clarias gariepinus x Heterobranchus longifilis hybrids.

For most recirculating aquaculture systems, ammonia removal usually takes place in
specialized filtration compartments (biological filters, chemical filters) located outside the growing
basins. Biofilters are formed from active bacterial cultures attached to specific surfaces.
High concentrations of internal ammonia affect intracellular and blood pH, and
osmoregulation. This may result in reduced internal ion concentrations, increased urine flow
(>12% of the lethal threshold concentration) and plasma renin activity (in rainbow tr out) and
acidemia, which adversely affects the oxygen -binding capacity of hemoglobin.
Also, at long -term exposure of fish to reversed water -to-blood ammonia concentrations
leads to various manifestations of chronic toxicity:
– Increases in the primary stre ss indicators in the blood, i.e. increased concentrations of cortisol
and catecholamines;
– Capillary congestion and dilation; renal blood vessel damage; connective tissue inflammation
and hyperplasia;
– Tissue damage of the gills (epithelial cell hyperplasi a and hypertrophy; separation of epithelial
cells from pillar cells which results in inflammation of the gill tissue;
– cellular degeneration; lamellar detachment; congestion; hemorrhage; aneurysms; karyolysis
and karyorrhexis) (Department of Water Affairs and Forestry, 1996).
Nitrites. Generally, nitrites occur in aquatic environments as a product of the activity of
gram -negative bacteria, chemoautotrophs, being intermediates in the nitrification process, which
is the two -step oxidation of ammoniacal nitrogen to nitrates. This nitrification is dependent on the
degree of aeration of the water in the system (Cristea et. al.,2002).
Although nitrite is converted to nitrate relatively quickly by ozone and the nitrifying bacteria in a
properly balanced biofi lter, it is a problem in recirculating systems because it is being created on
a constant basis, so the fish are continually exposed to certain concentration (Timmons and
Ebeling, 2013). Nitrites are very toxic to aquatic organisms because they enter the bo dy of fish
through chlorine cells in the branched epithelium by a mechanism in which chlorine molecules
play a role as mediator.

130
The immediate effect of nitrites on fish is observed in the blood. Plasma can accumulate
nitrites, acting as a means of tra nsport to spread them into tissues. In red blood cells, nitrites
oxidize Fe 2 + hemoglobin into Fe 3+ producing methemoglobin, unable to transport oxygen.
Exposure of fish to high nitrite concentrations causes gills to deteriorate (hypertrophy,
hyperplasia, epithelial separation) and thymus (hemorrhage and necrotic lesions) (Wedemeyer,
1996).
For most recirculating aquaculture systems, ammonia removal usually takes place in
specialized filtration compartments (biological filters, chemical filters) located out side the growing
basins. The toxicity of nitrite can be reduced or stopped by chloride ions. Usually between 6 ÷ 10
parts of chloride protect the 1part nitride fish. In this situation, pH, alkalinity and dissolved oxygen
values must be checked, reduced fee d intensity, or system water may be replaced, and if the
nitrite concent ration increases, NaCl is added (table14).
Table 14. Criteria and Effects of Nitrite on the Health of Fish (SOUTH AFRICAN WATER
QUALITY GUIDELINES, 1996)

Nitrite Concentration
(mg NO2 -N /L) Effects
Target WaterQuality Range
0 – 0.05 No known adverse effects; The TWQR is protective for salmonids and
most other species
0.06 – 0.25 Toxic to salmonids (LC50 for Oncorhynchus mykiss )
7.0 LC 50 for Ictalurus punctatus
10 – 15
LC 50 for Oreochromis and Tilapia spp; Tolerated by Clarias
gariepinus adults
86 LC 50 Lepomis macrochirus
140.2 LC 50 for Micropterus salmoides

Nitrates are the final product of nitrification, they are relatively non -toxic, except for very
high values. Generally, nitrates do not go up to critical levels if there is a change of water in the
system with fresh water in a proportion of 5 ÷ 10% daily. At the s ame time, in many recirculating
systems, it appears that denitrification occurs within the system, which maintains nitrate
concentrations below the toxic level. In systems with low water exchange or high hydraulic
retention times, denitrification has becom e increasingly important (Timmons and Ebeling, 2013)
(table 15) .

Table 15. Criteria and Effects of Nitrate on the Health of Fish (SOUTH AFRICAN WATER
QUALITY GUIDELINES, 1996)

Nitrate Concentration
(mg NO3 -N/L) Effects
Target Water Quality Range No known adverse effects

131
< 300
1 000 Below the 96 -hour LC 50 values for most fish

For the RAS systems a non -specific method of removing nitrate can be: passing the water
stream through an ion exchange column with a selective affinity for nitrates. Unfortunately, the is
expensive because other anions will also be removed, depending on the nature of the resin used.
On a commercial scale the process described requires competent operation, control and
maintenance and is generally not viable under cultur e conditions (fig. 45) .

Figure 4 5. Technical solution for controlling N in a RAS system
Alkalinity. Alkalinity is a measure of the pH -buffering capacity or the acid -neutralizing
capacity of water. Alkalinity is defined as the total amount of titratable bases in water expressed
as mg/L equivalent calcium carbonate (CaCO 3) (Timmons and Ebeling, 2013).
At pH values less than 8.3 the hydrogen carbonate ion concentration is the predominant
form whereas at pH values greater than 8.3 and 9.6 the concentrations of the carbonate and
hydroxide ions, respectively, are of consequen ce. Other ions which may also contribute to the
alkalinity of water are borates, silicates, phosphates and organic bases.
For intensive fish growth, the recommended alkal inity range is 100 ÷ 150 mg /L (table 16)

Table 16. Criteria and Effects of alkalin ity on the Health of Fish (SOUTH AFRICAN
WATER QUALITY GUIDELINES, 1996)

Alkalinity Range
(mg CaCO 3/L) Effects
0 – 20 Below optimal production
Target Water Quality Range
20 – 100 Production is optimal within this range

Water quality problems associated with alkalinity are usually the result of inadequate
alkalinity and therefore poor buffering capacity. Treatment normally entails increasing the
alkalinity of the water by the addition of lime. In a RAS system alkalinity can be adjusted through
the addition of sodium bicarbonate (NaHCO 3), common baking soda. Other materials can be
used, but sodium bicarbonate safe, inexpensive, and easy to be used (fig. 46)
Controlling N compounds in a RAS
system
➢Biofiltration
➢pH control methods integration
with plants or vegetables
➢Aeration
➢Daily water exchange (10%)
➢Feed management

132

Figure 46 . Technical solution for controlling alkalinity in a RAS system
Hardness – represents a measure of the amount of calcium (Ca 2 +) and magnesium (Mg 2
+), salts that are present in water. In freshwater farming systems, fish are hypertonic in their
environment. This means that water tends to balance the differences in osmosis; for this reason,
fish must dispose of large amounts of urine to maintain their internal physiological balance.
Waters have traditionally b een classified as soft (0 –75 mg/L), moderately hard (75 –150
mg/L), hard (150 –300 mg/L), or very hard (> 300 mg/L) (Timmons and Ebeling, 2013). Hard
waters usually exhibit fairly high alkalinities, although during acidification, alkalinity can be rapidly
reduced while the concentrations of calcium and magnesium (i.e. hardness) remains the same
(fig.47)

Figure 47 . Technical solution for controlling hardness in a RAS system
Table 17. Criteria and Effects of hardness on the Health of Fish (SOUTH AFRICAN
WATER QUALITY GUIDELINES, 1996)

Concentration
(mg/L CaCO3) Effects
5 Impairs growth and survival of catfish fry
10 Minimum recommended concentration for catfish
Target Water Quality Range
20 – 100 No known adverse effects; recommended range for most fresh -water
fish
> 175
Production generally less than optimal; osmoregulation of most fish
species may be impaired
> 300 Survival and growth of freshwater prawns affected; much lower
concentrations are preferred
300 – 500 Recommended concentration for the successful hatching of silver carp
eggs. Recommended for high survival, good growth and feed conversion
of red drum, Sciaenops ocellatus , juveniles

Controlling Alkalinity in a RAS
system
➢Lime addition
➢Addition of sodium bicarbonate
(NaHCO 3)
➢Controlling pH
Controlling Hardness in a RAS
system
➢Use mixed -bed ion changes
columns

133
Calcium and magnesium interact with metals, decreasing the toxicity of metals such as
zinc and copper. Similarly, increased calcium concentrations decrease the toxicity of ammonia.
Water is commonly softened either by the addition of lime followed by recar bonation, or by using
ion exchange, sometimes preceded by a precipitation if the feed water is particularly hard.
Adequate concentrations of calcium and magnesium are necessary to ensure growth and
survival of fish. Calcium is necessary for bone formation , blood clotting and other biological
processes. Calcium in culture water reduces the loss of other salts from fish. A loss of salts causes
a reduction in growth, as the fish must use energy supplied through the diet to re -absorb lost salts.
Low levels of calcium also result in reduced disease resistance in fry.
Magnesium and calcium have an important role in muscle contraction and the transmission
of nervous impulses in animals. In a hard water (> 250 mg / L alkalinity) will lose less metabolic
energy for osmoregulation than fish in poor water (<100 mg / L alkalinity) so they have to consume
more metabolic energy for growth.

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