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Recirculation Systems

Recirculation systems or RAS (recirculatig aquaculture systems) are the future of intensive fish breeding. RAS is the only environment suitable for intensive fish breeding in a fully controlled environment. The process is eco-friendly and sustainable, with minimum impact on the surrounding ecosystems.

10 main benefits of breeding fish in RAS

High breeding intensity on a small area
Elimination of external diseases
Elimination of predators
Low water usage
RAS can be established almost anywhere
Minimum effect of the breeding on surrounding ecosystems
Optimal breeding conditions, 365 days a year
Stable and controlled environment, efficient operation
Significant subsidy support from EU funds
Sustainable production not affected by climate change, unlike traditional aquaculture

„By investing in RAS you invest in your future.“

Technology schema

The goal of the breeding technology is to create the best possible conditions for intensive fish breeding in reusable water.

Basic arrangement of indoor recirculation systems

Breeding tanks

They are the core and essential part of the system; the design of the tanks must respect a number of biotechnological and strictly technological parameters

Biotechnological parameters (size, depth, volume, shape and colour) are dependent on:

  • species of bred fish
  • age of bred fish
  • size category of fish
  • production intensity
  • spatial capacities of the breeding station

Technological parameters:

  • Stable and durable design, resistant to corrosion and the effects of chemicals (especially NaCl)
  • Effective flow in the tank
  • Effective sedimentation and removal of sediments
  • Separation of dead fish
  • Simple installation, use and maintenance
  • Easy fish handling (easy to collect)

Indoor RAS provides clearly the best round tanks for the breeding of fry and market fish; alternatively, it is possible to use square tanks with rounded corners, a sloping bottom and with central drainage at the tank bottom. A surface collector of dead fish and feed residues can also be installed.

For breeding in early stages, there are troughs or circular tanks of smaller sizes available.

Mechanical filtration

It serves for the removal of suspended solids from the system, such as faeces and feed residues. Efficient mechanical filtration also greatly helps the biofilter, which is thus no longer exposed to suspended waste materials.

Drum filter: with automatic rinsing and sediment clearing; for the removal of contaminants larger than 50µm; currently the most efficient and used.

Sieve filter: capable of removing contaminants larger than 200µm; suitable for smaller systems (nurseries, fry breeding, etc.)

Sedimentation filter: different types of horizontal sedimentation tanks, with or without filter cartridges. The simplest type of mechanical filter, with the need for frequent sediment removal.

Biological filtration

The biofilter is the heart of RAS. It is the most significant part of it. The principle of bio filtration is to convert the toxic products of fish metabolism into non-toxic ones, using autotrophic bacteria = Nitrification

Nitrification = aerobic conversion of ammoniacal nitrogen, via nitrites, to nitrates


  • nitritation = NH₄⁺+ O₂ + HCO₃⁻ →NO₂⁻ + CO₂ + H⁺ + H₂O
  • nitration = NO₂⁻ + O₂ + HCO₃⁻→NO₃⁻ + CO₂ + H⁺ +H₂O

In addition to nitrification, the bio filtration process may also include the aerobic decomposition of the filtered organic matter using heterotrophic bacteria (especially on submerged, static biofilter cartridges).

Organic material (BOD) + O₂ → Sediment biomass + CO₂ + H₂O

Bacteria, both autotrophic and heterotrophic, produce a so-called biofilm, i.e. a layer of bacteria on the system’s surfaces. The aim of bio filtration is to create the largest possible surface for the colonization of bacteria. This is achieved using special filtration materials to which bacteria are attached.

Reactors for denitrification are becoming more and more popular with RAS, serving for the decomposition of nitrates (NO₃) into nitrogen gas (N₂). The degradation of NO₃ is achieved using heterotrophic bacteria and with the supply of CO₂ as a nutrient for bacteria. Denitrification or sorption on special materials is the only technological option to remove NO₃ from the system.

Filtration materials

In RAS, they are primarily the carriers of biomass of the autotrophic nitrifying or heterotrophic bacteria. It is important to make sure that filtration materials with the best properties are used in RAS, because the performance of the entire system depends on them.

What must a good filter material have?

  1. Specific surface for bacteria, approx. 750 m²/m³ and more
  2. Protected surface inside the element, approx. 650 m²/m3 and more
  3. TAN coefficient (NH₃ + NH₄) g/m³/day, approx. 550 g/m³/day, with run-in biofilter, pH 7.2 and 20 °C
  4. Density in g/cm³, (0.93 for sprinkled filters, 1 for MBBR filters and 1.2 for submerged charges)
  5. Easy or zero maintenance 
  6. Easy to apply and replace
  7. Bacteria friendly material

Types of biofilters

MBBR (moving bed bio reactor): biofilter with a moving charge; the movement of elements is based on aeration or water flow. The advantages include minimal maintenance and high efficiency.

Shover biofilter: a biofilter with a percolating charge, water drips through the filter materials in a thin layer, which is beneficial for nitrifying bacteria. The process also includes degassing. Probably the most efficient type of biofilter.

Submerged biofilter: biofilter with a submerged static charge. Heterotrophic bacteria often predominate. Requiring more intensive maintenance, more suitable for systems with a lower breeding density or in combination with an MBBR or sprinkled filter.

Pressure biofilter (Bead filter or Polygeyser): a highly efficient type of biofilter, where the filter medium is stored in a pressure vessel, with easy sediment clearing and an excellent mechanical filtering capacity. The filter charge is PVC or PE granulate. The advantages include lower space requirements, but, on the other hand, the devices have higher energy consumption (powerful pumps are required). They are popular on US farms

Denitration bioreactor: used for the removal of nitrates

How to determine the biofilter capacity?

The capacity of the biofilter, or the volume of filter materials, depends on the biofilter type, the type of the filter materials and the hydraulic load. It will differ between a sprinkled filter and in a pressure-type bead filter, which reflects the design and the usage of individual types of biofilters. Below is an example of an approximate calculation for the floating bed filter (MBBR biofilter), which is currently the most widely used type.

  • 1 kg of consumed feed = approx. 250 m² of filter material area
  • 1m³ of RK-Bioelements = 750 m² of filter area
  • Therefore, approximately 340 litres of RK-Bioelements is needed for 1 kg of consumed feed.

The volume of filter materials requires the use of filtration tanks with double the volume.

This indicative calculation also covers a 25% performance reserve and the possible negative exposures affecting the performance of the biofilter. In reality, the need for the filter material may be lower by up to a third. However, underestimating the biofilter capacity during the construction of RAS would be very detrimental.

UV radiation and ozonization

UV-C radiation is used in RAS to reduce the overall bacterial load and to prevent the pathogenic contamination of the system, especially by bacteriosis and fungal diseases. In addition, RAS also includes ozonation, where it eliminates hazardous residual ozone and, at the same time, releases large amounts of free radicals with a strong disinfecting effect into the water.

Tubular UV-C lamps

  • Lamps are enclosed in tubes for the effective irradiation of the entire flow
  • Can be installed both on gravity and pressure lines
  • Installation on separate branches (bypasses) needed at high flow rates
  • Outputs from 50 to 3500W

Tubular UV-C lamp

Immersion UV-C lamps

  • Can be installed in open channels, special UV-chambers, etc.
  • Can also be installed in piping
  • Available at different lengths and power ranges
  • Can be installed to create unlimited series
  • Simple installation, servicing and maintenance
  • Lamp power between 40 and 120W

Immersion UV-C lamps

The required output of UV lamps in RAS is 5–10W/m3 by vol., depending on the flow rate, water transparency and production intensity.


Ozonization is the most effective way of disinfecting water in RAS. Ozone O₃ can be produced in three ways, either by UV radiation in vacuum, using a corona discharge in conditioned (dried and filtered) air or via so-called cold plasma (dielectric barrier discharge method), where the oxygen is directly the input. Ozone generators or ozonizers are used to produce ozone.

It is important to mix the ozone with water in a reaction chamber as much as possible and then remove the residual ozone using UV radiation to prevent damage to the stocking or to protect the health of the RAS users.

Advantages of ozonization  

  1. Microflocation of fine and colloidal solids, for their easier removal from the system.
  2. Removal of dissolved organic matter by oxidative precipitation
  3. Oxidation of nitrites to nitrates
  4. High oxidation potential potentially useful for disinfection (5 times higher than O₂ and 2 times higher than chlorine)
  5. High level of lethal efficacy for bacteria, viruses and fungi in far shorter time compared to other types of disinfection
  6. Reduction of organic odour and water colouring (clarification)

Ozonizer – PRIMEOZON


Sufficient oxygen supply is a prerequisite for successful breeding. This is vital not just for optimal growth, good health and fitness of the fish, but also on the stability and efficiency of the biofilter.

The optimal oxygen saturation of the water flowing to the tank in high intensity breeding is 130–140%

Sources of oxygen for RAS

  1. Liquid oxygen silo: the most convenient source, centrally monitored by the supplier
  2. Nest of pressure vessels: a more expensive alternative in the long term, suitable only for smaller RAS
  3. Oxygen generator: demanding in terms of power consumption; suitable as a supplement to a silo to compensate for outages in the supplies of liquid oxygen

It is essential to ensure the optimal dissolution of O₂ in the RAS breeding technology. There are basically two possible methods: (i) pumping water into a pressure vessel to which oxygen is also supplied and mixed with water using nozzles, discs or mixing spirals, or (ii) dissolution of oxygen in shafts approx. 6m deep, where the water column pressure enables good oxygen saturation of water. Usually, the first alternative is used, together with the following devices:

  • Low head oxygenator (LHO): 2–3 m high cylinder made of PP or PE, the design of which allows for low pressure mixing and dissolution of O₂ into water by means of a perforated plate. Water is supplied to the cylinder using a pump. Suitable for central or partial oxygenation. The flow through the mixer is 4–500 l/sec

  • Oxygen cone: the oxygenation takes place under the pressure of 1–2 bar, which leads to a 200–300% supersaturation (therefore these are installed on bypasses). The water is mixed with water from the biofilter before it is fed into the breeding tanks. The water flow is 15–150 m3/h, depending on the type of the device.
  • Emergency oxygen: each tank must be provided with an emergency oxygen supply in the form of simple oxy-elements (plates, cylinders) to compensate for any outage in the main oxygen supply.

The oxygen content in the system and its distribution must be monitored and regulated automatically by the RAS control system and associated automatic fittings (solenoids, flow meters, control valves, probes, etc.)

Aeration and degassing

Aeration means either blowing air into the water (using a compressor) or the mechanical splitting of a water stream in the air, using special devices (Bioblocks). In Bioblocks, both applications are used especially in the aeration of MBBR biofilters, in the movement of water (Airlift) or the so-called degassing of recirculated water (removal of CO₂, N₂)

Aeration of MBBR biofilters

  • Using blowers and aeration elements (discs, cylinders)
  • By moving the filter medium
  • By improving the oxygen balance for nitrifying bacteria
  • Partial degassing of recirculated water

Aeration using Bioblocks

  • sprinkling bioblocks or similar materials in sprinkled biofilters
  • effective oxygenation and degassing together with biofiltration


  • Device that moves water by blowing compressed air into it
  • For vertical and/or horizontal movement of water, based on the airlift design
  • At the same time, water is oxygenated and degassed (low-pressure airlift)
  • Unless the height difference is too great, it can be used for pumping large volumes of water at low cost (hydropneumatic pump)


Pumps are the basic means of water circulation in RAS. They provide the necessary water flow to which other system components are adjusted, such as distribution fittings, UV lamps, mechanical and biological filters, etc. The water supply needs are expressed by the required flow rate through the breeding tank per 1 kg of stocking. The optimum is 0.38 l per kg of stocking per minute, the minimum is 0.2 l per kg of stocking per minute. In practice, the following simple flow calculation is often used:

Minimum flow rate in RAS = total volume RAS x 2/1 hour.

Requirements regarding pumps

  1. Reliability
  2. Optimal output/input ratio
  3. Possibility of output regulation (speed)
  4. Availability of servicing
  5. Easy removal and replacement of defective pumps

In the event of a failure, it is necessary to have in the system a backup pump which starts automatically and replaces the defective original pump.

Pumps in RAS must be permanently monitored and faults immediately reported by the control system.

Control system

Due to the intensity of production in RAS, it is necessary to continuously monitor and regulate a number of parameters and processes which are often interrelated and are critical for the success of the breeding process. For this purpose, there are monitoring and regulation technologies, which together with the relevant software ensure permanent control of the recirculation system.

Monitoring: the following are the main criteria monitored with the help of probes and pressure, electronic or flow sensors:

Physical and chemical parameters of water:

  1. O₂, in each breeding tank, at the biofilter inlet and outlet point
  2. pH, in each tank
  3. Temperature, in each tank
  4. ORP (oxidation reduction potential) in the whole system, one check point is sufficient
  5. NH₃/NH₄⁺ in the whole system, one check point is sufficient
  6. NO₂⁻ in the whole system, one check point is sufficient
  7. CO₂ in the whole system, one check point is sufficient

Technological parameters of the system:

  1. Operation of pumps, flow and voltage detection
  2. Operation of blowers, output pressure and voltage detection
  3. Drum filter speed and number of rinsing cycles, operation of the motor and rinse pump
  4. Operation of UV lamps and number of hours of lighting, voltage detection
  5. Detection of feeder filling
  6. Volume of water change in the system
  7. Flooding and level sensors for the detection of emergency situations

Data collection and the regulation of individual operations can be done using configured PLC modules, which, together with the relevant software, provide an efficient system management. They are supplied together with a complete switchboard and wiring.

Another option is to use ready-made hardware, developed for aquaculture purposes, which can be connected to your PC or your smartphone app. Thanks to this you can be in permanent contact with your RAS from anywhere. These control units are very easy to connect to the existing wiring and do not require any specific expertise.

Feeding system

Feeding represents a critical part of intensive aquaculture as regards the breeding of fry, small fish and market fish. For larger RAS, the feedstuff is pneumatically fed from a central container, while smaller RAS include feeders which are usually filled manually.

Feeder types

  1. With a timer
  2. Touch (pendulum system)
  3. Automatic, electronic, connected to the RAS control system

In automatic feeders controlled by an IT system, a number of parameters can be selected, such as the feedstuff volumes, feeding frequency, the beginning and end of feeding. The system also checks the volume of feedstuff in the container and can send text messages to notify operators about the need to refill. The system is also able to reflect the average daily increment, removal of fish for sale or the loss by mortality when calculating the quantities of the feeding needed, and adjust the daily rations accordingly. The IT system is able to feed each tank separately, based on the specific tank situation.



Waste management

The volume of the wastewater produced by the RAS depends on the breeding intensity. Wastewater management is dealt with in accordance with the relevant legislation. In RAS, wastewater can be used for further processing as a source of nutrients for plant cultivation, so-called Aquaponics


Aquaponics is a process that combines aquaculture fish breeding with hydroponics, where the excreta from fish breeding serves as a source of nutrients for cultivated plants.

Aquaponia serves for the growing of vegetables, herbs, aquatic plants, etc.

In standard RAS so far this is only a supplement to the fish production, and adding an extra aspect to the product range offered to customers.

RAS bývá často doplněn o podpůrná zařízení a technologie, jako jsou:

úprava vstupní vody do systému

ohřev/chlazení vody v recirkulaci

odpěňovače, protein skimmery

zpracování odpadů

centrální slovování, automatizované třídění a počítání ryb

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