Intensive recirculation of water in a fish production system entails that the water is treated in a comprehensive water treatment system. The treatment involves solid captures by means of sieving, removal of dissolved pollutants in a bioreactor (biofilter), disinfection, degassing (CO2 and nitrogen), as well as oxygen injection. The circulation of the water through the water treatment adds physical energy to the water which is converted into heat, and in certain conditions, it may be necessary to cool down the water in order to obtain the correct temperature.
Removal of large particles
During the hatching period there is a peak in the production of particles (particularly in the hatching stage) which must be removed. KUBE®hatch is equipped with a micro sieve for efficient removal of particles greater than 30μm which are removed together with the flushing water from the sieve.
In KUBE®hatch the disinfection system is based on UV light radiation of the recirculated water which is efficient without producing poisoning by-products. The UV intensity is measured by means of a UV sensor. The disinfection unit employs medium pressure UV lamps, which produce light with a broader spectre of wavelengths than low pressure lamps. So-called medium pressure lamps are more efficient than low pressure lamps in preventing the microorganisms coming alive again after having been subjected to the UV radiation as it produces a broader spectre of UV-C waves. Whereas the low pressure lamp only affects the DNA, the medium pressure lamp destroys DNA, cell walls, proteins and enzymes, making the microorganisms unable to reproduce.
Fine solid and dissolved organic matter capture
The filtered and UV treated water enters into a so-called skimmer which in principle removes tiny particles and partly dissolved matter in that it is ”attached” to small bubbles of air and creates a foam which contains the waste products. In the skimmer, there are diffusers which create small bubbles of air in the flow of water. The foam that is formed – and there may be considerable quantities of it in the hatching stage – is removed on the top of the skimmer by means of a vacuum suction system.
In KUBE®hatch , a biofilter which is designed as a “moving bed bioreactor” is employed. The biofilter medium consists of specially designed plastic units (BF Chip 1200) on which colonies of bacteria are established by growth on the surface of these plastic units. The bacteria growth is called a biofilm, and as the plastic units are constantly moving in the water, there is good contact between the water volume in the reactor and the biofilm. The biofilter is divided into 3 chambers placed in series and separated by a grid between each chamber so that the medium are restricted from floating freely. The MBBR
technology has many advantages compared to stationary biofilter media. This is particularly the case in the exploitation of the filter capacity and the flexibility with reference to operational routines for a system in operation. As an example, cleaning and disinfection of a MBBR system can be considerably easier than for example a stationary system. The reactor in KUBE®hatch is designed for optimization of hydraulic dynamics and avoidance of dead zones. A diffuser situated in the bottom of the reactor adds oxygen to the water and at the same time removes CO2 . The diffuser also ensures continuous movement of the bio carriers so that there is sufficient stirring of the water, a good contact surface between bio carriers and the water, and less accumulation of solids.
Gas exchange and control of the total gas level
In KUBE®hatch, a vacuum degasser is applied to keep optimal TGS-levels (total gas saturation) throughout the hatching process. In the vacuum degasser, some of the dissolved nitrogen (N2 ) in the water is removed so that the saturation of nitrogen in the system will always be below 100%. This secures that no bubbles of gas will be created under grids in the hatching cabinets connected to the water treatment system.
In contrast to the flow- through systems, the waterin a recirculation system has to be cooled down. In KUBE®hatch the heat gain is 0.2o C in every cycle due to excess heat from the various processes of the water treatment. The cooling/heating system is controlled by sensors and integrated with a Programmable Logical Control (PLC) system. The compressor is frequency controlled to regulate its speed according to the temperature difference in the system, thus energy saving. The cooling/heating system can treat make-up water with temperatures between 0- 16o C and is designed to keep a water temperature of 6- 10oC.
The “core” of a recirculation systems is often a biofilter, in which the bacteria convert dissolved waste (ammonium and organic matter) produced by the fish into harmless compounds. In addition to the biological water treatment, one also believe that the properties of the microorganisms in a well functioning biofilter contribute to creating a stable water environment for fish in recirculation facilities. In intensive production systems, “opportunistic” and undesirable bacteria and fungi may establish; a situation whichmay easily result in damage to the eggs and fry. If one succeeds in controlling these and rather add “friendly” microorganisms to the system as these that are found in a biofilter, this will be an advantage. In particular, this may be advantageous in the very first stages of the fish’ life during which it has a limited immune defence. A good start is important for the results later in the production cycle.
KUBE®hatch 8000 is able to supply 7 CompHatch units, which equals a production of 2.5 million eggs/fry. The water flow in the system is 800 l/min, of which 5-10 l/min “new” water is supplied per recirculationcycle. This means that 98.5% of the total water volume in this system is recycled.