Why invest in Recirculation Aquaculture Systems (RAS)?

TABLE OF CONTENTS 1. Introduction to recirculation systems ............................................................................................... 3

2. RAS advantages and disadvantages .............................................................................................. 4

2.1. Advantage 1: Optimal growing conditions ............................................................................... 4

2.2. Advantage 2: Consumption optimisation ................................................................................. 4

2.3. Advantage 3: Biosafety health ................................................................................................. 6

2.4. Advantage 4: Low environmental impact ................................................................................ 6

2.5. Advantage 5: Location flexibility .............................................................................................. 6

2.6. Advantage 6: Production monitoring ....................................................................................... 6

2.7. Advantage 7: Low continental water consumption .................................................................. 7

2.8. Disadvantage 1: System complexity ....................................................................................... 8

2.9. Disadvantage 2: High investment cost .................................................................................... 8

3. Recirculation process ...................................................................................................................... 9

3.1. Mechanical filtration ................................................................................................................. 9

3.2. Biological filtration .................................................................................................................. 10

3.3. Degassing/Airing ................................................................................................................... 11

3.4. Oxygenation........................................................................................................................... 11

3.5. Ultraviolet disinfection ............................................................................................................ 12

3.6. Ozone .................................................................................................................................... 12

4. Bibliography ................................................................................................................................... 12

1. Introduction to recirculation systems Aquaculture, the farming and breeding of aquatic species, is currently an indispensable and necessary

food source. It is essential for the world population, contributing as it does to people's welfare.

Although aquaculture dates back to early civilisations, which already bred their species in rivers and

lakes, it has not been until the last 50 years when this activity has acquired a critical mass from a

production standpoint.

Traditionally, the most widespread production system in aquaculture has been sea cage production,

i.e. marine aquaculture. But technological breakthroughs, along with a growing need for greater public

consumption, has given rise to other alternatives to production systems in marine cages or open

recirculation circuits.

Figure 1 : Marine cages (marine aquaculture)

The method that stands out for its greater potential is the Recirculation Aquaculture System (RAS). It

is essentially a technology for breeding fish or other aquatic organisms through the reuse of water

during the production cycle. The technology is based on the use of various water treatment filters

(mechanical, biological, etc.) that allow controlling species' growth conditions.

Figure 2: Recirculation Aquaculture System (RAS)

2. RAS advantages and disadvantages

2.1. Advantage 1: Optimal growing conditions

Traditional open recirculation systems are dependent on external conditions such as temperature,

water cleanliness and oxygen levels. In a RAS, these external factors are completely or partially

eliminated depending on the system's degree of complexity. The producer has complete control over

all the parameters necessary to operate the plant successfully.

Figure 3 : Some of the parameters affecting the well-being and growth of fish

The control of these parameters results in a stability of optimal culture conditions, where the fish will

feel less stressed and with better growth conditions year-round. Seasonality will make it easier to

obtain predictions of the fish's biological evolution in terms of its culture phase or size.

2.2. Advantage 2: Consumption optimisation

In line with the above, operators can plan production in advance, not depending on natural

environment conditions. Production can be planned in the periods most beneficial to the producer,

totally separate from natural conditions.

Figure 4: Average growth rate of the fish

The producer can thus estimate production costs in advance. This is especially interesting to

determine the amount of feed supplied (the highest cost in cultivation.) In short, the knowledge of

growth rates and feed allow achieving savings in production costs that optimizes plant performance.

Figure 5 : Breakdown of aquaculture production costs

It should also be borne in mind that the energy cost of adjusting the temperature of newly used water

is considered to be very high. SAR systems are thus advised in areas where species to be cultivated

require a habitat temperature similar to that of the nearest coast, the supply source. A very simple

case to explain is the cultivation of the species Seriola Dumerili on the Cadiz coast. The most

favourable growth temperature for the species is from 20 to 24 degrees, and with mild and stable

temperatures throughout the year.

Figure 6: Average seawater temperature in the Mediterranean and South Atlantic area, by month

2.3. Advantage 3: Biosafety health

Traditionally, in open-air recirculation facilities water is taken directly from a river, lake or sea, which

naturally increases the risk of transmitting diseases to species in question. Risk is reduced due to the

limited use of new water in recirculation systems.

Recirculation guarantees the cleanliness of water and the optimal sanitary state of individuals,

lowering the risk of diseases. The system stands out as a barrier against external contaminants and

pathogens that can affect the plant's production, because the RAS, besides using minimal new water,

features a series of filters that sanitise the water. All water entering the system is filtered and then

disinfected.

2.4. Advantage 4: Low environmental impact

Closely related to the above advantage is the low impact on water used in the recirculation system.

Water flow in the closed circuit is only lost to evaporation or residual waters generated in some filters.

Discharges into the sea are free from harmful components, since the water has been properly filtered

and purified. Therefore, the environmental impact generated is considered practically nil.

2.5. Advantage 5: Location flexibility

RAS facilities are particularly useful in places where access to water and land is excessively

expensive, as it requires little space to build. It also represents a production advantage in the most

disadvantaged areas due to extreme weather conditions.

The main advantage lies in the ease of locating the production facility in places that are advantageous

for the producer, close to the market or suppliers. This translates into reducing the CO2 footprint and

transport costs considerably.

Figure 7: Comparison of production costs including transport

2.6. Advantage 6: Production monitoring

Production traceability control is facilitated by means of systems that reflect on screens the current

state of the plant at all times and the possibility of immediate action. Continuous improvement of

SCADA systems is leading up to total monitoring of RAS aquaculture facilities.

Figure 8: SCADA production control system

2.7. Advantage 7: Low continental water consumption

From an environmental standpoint, the limited supply of new water for the recirculation system is a

benefit given limited water resources. This low consumption likewise involves a reduction in the

amount of water that needs to be treated and, therefore, energy costs.

The following table provides a comparison that compares the different types of existing production,

highlighting the low use of water supply sources for a recirculating system.

Type of production Litres of new water consumption per production kg

Traditional fish farming 50,000

Moderate water reuse 25,000

Intensive water reuse 10,000

Partial recirculation 5,000

Moderate recirculation 500

Intensive recirculation 50 - 400

2.8. Disadvantage 1: System complexity

This is an advanced production system consisting of numerous units and subsystems. Moreover,

since it is such a complex and industrial installation, it requires a high degree of safety. Therefore,

energy backups are set up to ensure continued daily production in the event of an outage.

Figure 9: Complexity of the system based on units used

On the other hand, aquaculture requires knowledge, experience and persistence during farming. This

requires a highly qualified, fully available staff equipped with dedication to daily operational tasks. The

complexity of tasks to be performed require increased initial training and education.

2.9. Disadvantage 2: High investment cost

The creation of a new production plant based on RAS technology represents a high initial investment.

The energy requirements of the system are so high that a minimum production capacity is needed to

ensure a return on investment.

Type of farm Litres of new water per kg of production

kWh per kg of production

Total investment per kg of production

Traditional 50,000 0 ?

Moderate water reuse

25,000 1 - 3 €2

Intensive water reuse

10,000 1.5 - 5 €2 – 3

Partial recirculation

5,000 1.5 - 3 €3 – 4

Moderate recirculation

400 - 700 2 - 8 €8 – 10

Intensive recirculation

50 - 400 2 - 8 €10 -12

Figure 10: Energy consumption and investment in different types of farms

In spite of this, the initial investment in RAS is amortised annually and can be recovered in 5 or 10

years, leading to a higher profit with respect to a production investment using marine cages.

3. Recirculation process A recirculation system requires treating water continuously in order to eliminate waste products

generated by fish farming, as well as to ensure the right water conditions. A basic recirculation system

consists of passing the water that comes out of the culture tank through a mechanical filter and then

biological filter before eliminating the CO2 in a degasser and returning the water to the main tank. This

basic installation can be enhanced with an oxygenation process, a PH regulator, heat exchangers,

ultraviolet radiation equipment and ozone disinfection units.

Tanks are normally controlled by water level, water content and temperature sensors to ensure total

water control. Nozzles should also be installed to inject oxygen directly into the tank in case of

emergency.

Figure 11: Basic recirculation process

3.1. Mechanical filtration

Eliminate large organic waste products from 40 to 100 microns. One of the most commonly used

mechanical filters is the drum filter. It works through various stages listed below.

Figure 12: Drum filter

1. Water from the main tank enters the drum.

2. Water is filtered through the drum filter elements. The difference in water level between the

inside and outside of the drum generates filtration.

3. Solids are trapped in the filter elements and dragged backwards by the water by the rotation of

the filter.

4. Organic particles form a sludge when deposited on a tray, where they are sprayed by rinsing

water (under pressure) from outside the filter.

5. The sludge flows with the water by gravity out of the filter through a drainage system.

6.

Figure 13: Drum filter operation

The main advantages of this filter are:

Organic load reduction for the biofilter.

Removal of organic particles blocking pipes, increasing oxygen consumption or saturating

filtration equipment.

Improve nitrification conditions so that the biofilter does not clog.

3.2. Biological filtration

Because not all organic matter is removed in the mechanical filter, finer particles will pass through it

along with other dissolved components. Biological filtration techniques consist of using living

organisms (bacteria) to eliminate some toxic substances in the water such as ammonium (NH4) and

nitrites, generated by the metabolic activity of fish. These nitrites are converted to nitrates, which no

longer pose a hazard in the system.

The Micro filter is based on a deposit area where the water flow speed between the bioelements is

reduced to facilitate the sedimentation of the fine particles. The design of bioelements is geared

toward maximising the contact surface.

Figure 14: Biological tower

The efficiency of this process depends on:

Water temperature in the system (10 - 35 ºC).

pH in the system (7 - 8).

pH regulation is essential to control biofilter efficiency, which at low levels results in a loss of filter

performance. If a high nitrification process is required, increasing the pH of the system is sufficient,

although this will also increase the amount of free ammonia and, therefore, the danger of water

contamination. There are two main factors that affect pH:

CO2 from the fish and the biological activity of the filter.

Acid produced during the nitrification process.

Given these two phenomena, stabilisation and control are sought by means of two methods:

Aeration to eliminate excess CO2.

Adding bases such as sodium hydroxide (NaOH) and counteracting the pH drop.

Figure 15: Tablets used in the biofilter

3.3. Degassing/Airing

Water contains high concentrations of carbon dioxide (CO2) in high concentrations from the respiration

of fish and biofilter bacteria, as well as free nitrogen (N2). This accumulation of gases will have a

negative effect on fish growth and welfare. Under anaerobic conditions (absence of O2) sulphur

hydrogen (H2S) may develop and can kill fish due to its high toxicity.

Aeration can be carried out by means of air pumps aimed directly on the water flow where the contact

of air bubbles with the water releases the gases. However, the process is not as efficient as when a

degasser is used. The water passes through a mesh network that stirs the water and releases CO2.

Figure 16: Trickling system (meshed degasser cylinder)

3.4. Oxygenation

The aeration or degassing process itself will add O2 to the water by the simple fact of the exchange

between gases in the water and gases in the air depending on the level of oxygen saturation in the

water. Oxygen balance is 100% saturation. After passing through the culture tank and previous filters,

the oxygen level will have dropped. That's why the system adds the oxygen it needs to bring the water

back to 100% saturation level, or even higher.

Figure 17: Oxygen diffuser and oxygen reading sensor

3.5. Ultraviolet disinfection

The UV disinfection process works by applying light at wavelengths that destroy DNA in biological

microorganisms, killing any type of bacteria or unicellular organisms. The efficiency of this process

depends on the size and species of the organisms to be eliminated, as well as water turbidity. For this

reason, the best disinfection performance is obtained after the flow of water through mechanical and

biological filters.

Figure 18: UV treatment system

3.6. Ozone

The use of ozone (O3) serves to eradicate any pathogen by strong oxidation of organic matter and

biological organisms. The technology consists of breaking micro particles into molecular structures

that will come together to form longer chains. This flocculation eliminates microscopic suspended

solids to filter them out of the recirculation system. It can also be placed at the entrance of the system.

4. Bibliography

A Guide to Recirculation Aquaculture: An introduction to the new environmentally friendly and highly

productive closed fish farming systems. Author: Jacob Bregnballe, FAO. Edición 2015

Sistemas de Recirculación en Acuicultura. Máster en acuicultura y pesca, UCA. Author: Salvador

Cárdenas, Jefe del departamento de producción de IFAPA, Centro El Toruño.

Recirculated Aquaculture Systems Advantages & Disadvantages. Good Practice Workshop, 2014, Copenhagen (Denmark). Author: Jesper Heldbo, Ph.D., M.Sc., Secretary General AquaCircle