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.
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:
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.
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.
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
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).
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.
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?
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
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.
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-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.
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
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%
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:
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)
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
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.
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:
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 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.
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.
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.