Flow pattern and self-cleaning
A flow pattern will be created inside a production unit having a water inlet and outlet. It is important that this flow pattern encompasses the entire unit so that all the fish can come into contact with flowing water. The flow pattern depends on the design of the production unit.
In a tank with a circulating water flow and correctly designed inlet and outlet, two flow patterns will occur: the primary flow and the secondary flow (Fig. 13.7). The primary flow causes even distribution of the water in the horizontal plane, while the secondary flow will clean the tank walls and bottom.
In well-designed tanks with correctly designed and constructed inlet and outlet, the incoming water may therefore be used to clean the tank walls and bottom. This process is known as hydraulic self-cleaning. To achieve self-cleaning in a tank, a certain amount of water has to be added; the amount depends on the tank construction. The water velocity at the bottom of the tank must be so high that the settled solids are removed. To ensure transport of settled solids in circular tanks, the recommended bottom velocity to ensure self-cleaning is above 6–8 cm/s.7This will also remove algal growth from the tank sides. Inside the tank there will be a velocity profile equal to that in a channel, where the lowest velocity occurs near the bottom due to friction (Fig. 13.8). Bottom water velocities of between 6 and 8 cm/s normally represent a water velocity in the free water mass of between 12 and 15 cm/s.7 Practical experience has also shown that high fish density promotes self-cleaning of the bottom. A lower velocity could therefore be accepted when the fish density is increased because the movement of the fish results in resus-pension of settled solids, so the secondary flow pattern could more easily transport the particles to the drain.
In a correctly designed flow through tank with inlet and outlet, and a circular flow pattern, the water retention time should be between 30 and 100 min for satisfactory self-cleaning. This means a flow through of between 10 and 33 l/m3 farming volume. A retention time of less than 30 min may create a vortex around the centre drain. The peripheral velocity in the tank may also be so high that the fish will have problems staying there. When using low retention times, a specially designed inlet and outlet are necessary. With retention times above 100 min the self-cleaning effect is decreased and additional cleaning is necessary.
To attain hydraulic self-cleaning, a high volume of water is needed to create a high water velocity inside the tank. Even if the water velocity has yielded improved growth results, there is a maximum velocity that not must be exceeded.14 This will vary according to species and growth stage. Examples here are fry of marine or freshwater species, where only very low velocity is tolerated; to maintain satisfactory water quality for these species is therefore a challenge. Settled particles and fouling on the tank bottom and sides will create a sub-optimal environment, and be a good substrate for unwanted bacterial growth. Regular removing of fouling is therefore absolutely essential. If this is done manually, it is labour intensive, and therefore commercially available automatic systems are preferred. Rotating brushes on the bottom of the tank, powered either by electric motors or by the pressure of the incoming water are one solution. Other solutions include a small turtle-like unit moving around on the tank bottom or installing a washing arm half the pipe diameter in length. Addition of chemicals that remove the fouling, such as oxidizing agents, has also been tried by the author and colleagues.