Recirculating Technology
Recirculating Aquaculture Systems (RAS) represent production systems, which partially or completely recirculate the water used for the culture. Therefore the water is treated and can be reused. Commonly only a small fraction of the total system volume will be replaced per day, between 1 and 10% per day.
The water treatment of a recirculating system is in principle the sequential alignment of the individual water treatment steps in the main flow of the system. The first step of the water treatment should be the solid waste removal, which includes uneaten food, faeces and bacterial biomass. Solids within the water affect the health condition of the cultured species, e.g. fish, and decrease the efficiency of the nitrification process. A considerable fraction of the solid waste loads is below a particle size of 50 µm and therefore hardly removable be conventional devices for solid waste removal. Modern recirculating systems should be equipped with a multistep solid waste removal to be able to remove all particle size classes from the systems water.
Following the particle removal process, toxic compounds for the cultured organisms, such as ammonia, will be transferred into less toxic compounds via nitrification, the sequential transfer from ammonia via nitrite to nitrate. The efficiency of the nitrification process can be enhanced by the removal of dissolved carbon compounds and particulate substances from the water. Nitrate can be removed from the system by the anaerobic denitrification process. Denitrification usually is performed as a bypass system.
Further steps for water treatment include oxygenation of the water, the acid-base-equilibrium (pH), temperature and salinity regulation prior to the transfer of the treated water back to the culture tanks.
Water circulation within the system is usually achieved by pumps, which need a constant power supply and simultaneously release energy in the form of heat into the system. Therefore other principles for water circulation are under investigation, e.g. the air-lift-principle, which uses air to move the water and thereby no additional energy is released into the system but oxygen is dissolved into the water.
The development of modern RAS should focus on system stability achieved by biological equilibrium. System control and regulation are important tools to achieve and maintain this system stability.
