The function of land based aquaculture systems is provided by the re-circulation mechanism. Aquaculture is always based on flow-through technology. Inlets and outlets in the system move opposite each other, which allows the development of water organisms. Such systems treat water only once. Recirculating aquaculture systems may be closed or partially closed. The wastewater generated in these systems is treated for re-use and re-distributed.
Aquaculture systems are generally based on the system of exchanging a relatively small portion of the actual culture medium per unit time. This mechanism is called Recirculating Aquaculture System. It is expected that aquaculture circulation systems will increase in the future in order to comply with water waste restrictions applied in the field.
Aquaculture circulation systems are generally composite systems that combine tanks and water treatment units for fish maintenance. The water in the tanks is renewed every few hours. In this way, the water remains clean at all times and also prevents toxic wastes such as amanioc in the water from damaging the growing fishes. Even if a new water supply is needed continuously, a healthy aquaculture re-circulation system puts 95% of the used water back into use. Another purpose of water treatment is to remove waste and optimize the amount of oxygen in the tanks.
RAS technology also includes the water pre-treatment system. The tanks are supplied with oxygen through the continuous ventilation system. This process then proceeds to another water cleaning system. Larger particles such as food residues and feces pass through mechanical filters. Subsequently, water flows through a biofilter and protein skimmer. Biofilter provides cleaning of tank from differentiating factors such as ammonia.
Bacteria entering biofilter are divided into two groups: bacteria that digest organic matter and bacteria that convert toxic ammonium to nitrate. The first group grows much faster than ammonium-reducing nitrifying bacteria. If the organic load on the biofilter is excessive, heterotrophic bacteria that reduce the number of organics will misplace the nitrification bacteria by the filter. In case the organic load on the biofilter is excessive, then the heterotrophic bacteria that decrease the number of the organics would misplace the nitrifying bacteria from the filter. However, the mechanical filtration removes solids efficiently and minimizes the dissolution of particles, thus minimizing the organic load. This allows for the nitrifying bacteria to be developed and for the performance of the biofilter to be maintained.
When operating a protein skimmer, the water to be treated is mixed vigorously with injected air or liquid oxygen to remove proteins from the water column. Some of the water can be discarded, but the main part is sent back to the culture tanks. The treated clean water is then added to the recirculation flow.
When working in a protein skimmer, ozone is decomposed by direct oxidation properties of residual organic waste that cannot be dissolved by biofilter. In addition, pheromones that can seriously affect fish growth are broken down by ozone.
RAS regardless of climate change, environmental conditions, seasonal changes or locations, it can provide optimum and constant temperature and production throughout the year.
The main applications of Recirculating Aquaculture Systems consist fish production units of several different kinds of species:
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